Advanced Nanomaterials for Inexpensive Gas Microsensors: Synthesis, Integration and Applications 0128148276, 9780128148273

Citation preview

ADVANCED NANOMATERIALS FOR INEXPENSIVE GAS MICROSENSORS

Micro and Nano Technologies

ADVANCED NANOMATERIALS FOR INEXPENSIVE GAS MICROSENSORS Synthesis, Integration and Applications Edited by

EDUARD LLOBET

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-814827-3 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Simon Holt Editorial Project Manager: Peter Adamson Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by SPi Global, India

Contributors

Haitham Amal Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States Francisco J. Arregui Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain Marcel Bouvet Institute of Molecular Chemistry of the University of Burgundy (ICMUB), UMR CNRS 6302, Universit
e Bourgogne Franche-Comt
e, Dijon Cedex, France Danick Briand Ecole Polytechnique F
ed
erale de Lausanne (EPFL), Soft Transducers Laboratory, Neuch^atel, Switzerland Carles Can
e CNM-CSIC, Bellaterra, Spain Jesus M. Corres Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain H
ele`ne Deb
eda Universit
e de Bordeaux, Laboratoire IMS, Talence, France Isabelle Dufour Universit
e de Bordeaux, Laboratoire IMS, Talence, France Cesar Elosua Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain Luis Fonseca CNM-CSIC, Bellaterra, Spain Julian William Gardner School of Engineering, University of Warwick, Coventry, United Kingdom Prasanta Kumar Guha E&ECE Department, IIT Kharagpur, Kharagpur, India

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Contributors

Hossam Haick Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion, Israel Institute of Technology, Haifa, Israel Francisco Hernandez-Ramirez MIND—Departament d’Enginyeria Electro`nica i Biome`dica, Universitat de Barcelona, Barcelona, Spain Saleem Khan Ecole Polytechnique F
ed
erale de Lausanne (EPFL), Soft Transducers Laboratory, Neuch^atel, Switzerland Eduard Llobet MINOS-EMaS, Universitat Rovira i Virgili, Tarragona, Spain Ignacio R. Matias Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain Michele Penza Department for Sustainability, Division of Sustainable Materials, Laboratory of Functional Materials and Technologies for Sustainable Applications, ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Brindisi, Italy Joan Daniel Prades MIND—Departament d’Enginyeria Electro`nica i Biome`dica, Universitat de Barcelona, Barcelona, Spain Mariluz Rodriguez-Mendez Group UVaSens; BioecoUVA Institute, Engineers School, University of Valladolid, Valladolid, Spain Albert Romano-Rodriguez MIND—Departament d’Enginyeria Electro`nica i Biome`dica, Universitat de Barcelona, Barcelona, Spain Jos
e Antonio de Saja Group UVaSens, Engineers School, Universidad de Valladolid, Valladolid, Spain Tilman Sauerwald Lab for Measurement Technology, Saarland University, Saarbr€ ucken, Germany Andreas Sch€ utze Lab for Measurement Technology, Saarland University, Saarbr€ ucken, Germany Janusz Smulko Gda
nsk University of Technology, Gda
nsk, Poland

CHAPTER 1

Introduction Eduard Llobet MINOS-EMaS, Universitat Rovira i Virgili, Tarragona, Spain

1.1 Introduction The last years have seen a sustained and ever-increasing interest in the development of ubiquitous sensing. Sensors are present nowadays in different platforms such as portable devices (e.g., tablets or smartphones), domestic appliances (e.g., washing machines, dishwashers or ovens), and cars, only to cite a few. New developments in consumer electronics, especially in the rapidly evolving field of wearable electronics require the use of different types of sensor devices. Most of the microsensors that are currently routinely integrated into commercially available platforms as the ones mentioned before are physical sensors (accelerometers, gyroscopes, magnetic sensors, temperature sensors, optical sensors for measuring water turbidity, etc.). In contrast, the use of gas microsensors within the ubiquitous sensing paradigm, if any, remains incipient. However, should gas microsensors meet the stringent performance and cost requirements of this new paradigm, then a wide spectrum of applications and enormous associated markets would develop. These would include, but are not limited to, indoor and outdoor air quality control, security in key infrastructures, intelligent cooking, food quality and safety monitoring, remote monitoring of the elderly, point-of-care diagnostics, and personal healthcare via breath analysis or perspiration. This explains why intense research efforts have been devoted to the continuous amelioration of gas microsensors. Many of these research efforts are oriented toward the quest for new nanomaterials that can meet both the specifications of widespread, continuous gas detection and the industrial demands for device integration. Nanomaterials comprise low-dimensional (i.e., 0D, 1D, 2D, or 3D) inorganic, carbon, and molecular materials and their hybrids. Gas-sensitive nanomaterials will find commercial application provided they are produced employing scalable techniques that enable the mass production of high-quality materials at affordable costs. In that sense, solution processing methods and additive fabrication techniques are seen as interesting enabling technologies for coupling nanomaterials to their application substrates. Additionally, nanomaterials can often be operated at low temperatures above the ambient or even at room temperature, and their response and recovery dynamics can be ameliorated by using light irradiation. Achieving ultralow power consumption gas sensors is a key aspect for realizing their enormous potential

Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00001-3

Copyright © 2020 Elsevier Inc. All rights reserved.

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Advanced nanomaterials for inexpensive gas microsensors

for being integrated in a new generation of ubiquitous, portable, or personal sensor systems. The organization of this introductory chapter is as follows. It will start by giving a focused minireview on gas-sensitive nanomaterials, will continue by reviewing some of the methods for synthesizing these and their integration in different transducer platforms, and will end by giving a general overview of the content of the different chapters that integrate this book.

1.2 Gas-sensitive nanomaterials The research in gas-sensitive nanomaterials is mostly targeted at achieving high sensitivity and stability and improved (i.e., reduced) limit of detection and selectivity, in such a way that traces of target gas molecules (e.g., pollutants) can be detected even when in the presence of significantly higher concentrations of other gases or vapors (interfering species). Nanomaterials possess unique morphological properties that make them very attractive for achieving very high sensitivity. In some cases, virtually all their atoms are exposed to the surrounding chemical environment. Some of the most researched gas-sensitive nanomaterials and nanostructures consist of the following: • Nanoparticles (NPs) with few nanometers in diameter, which are often referred to as zero-dimensional nanomaterials. In either simple or core-shell structures, metal oxide NPs can become fully depleted of charge carriers when exposed to clean air, and NP films may experience dramatic changes in electrical conductivity upon the adsorption of gas molecules on their surface. Few-nanometer NPs experience quantum confinement effects (they are also referred to as quantum dots) and present localized plasmon resonance, which can be quenched or enhanced upon the adsorption of gas molecules. Therefore, the use of NPs has been reported in conductometric, optical, and plasmonic gas sensing [1]. • Nanowires (NWs), nanofibers (NFs), nanotubes (NTs), nanoneedles (NNs), or nanobelts (NBs) are nanomaterials with diameters ranging between a few to 100 nm and lengths easily reaching two or three orders of magnitude higher than their diameters (from tens of microns to few millimeters). This is why these are often referred to as one-dimensional nanomaterials. NWs, NNs, or NBs are often single-crystalline metal oxides in which the width of their central conduction channel is modulated by the adsorption of gas molecules. Similarly to NPs, low-diameter NWs can become fully depleted of charge carriers when in clean air, experiencing dramatic changes in electrical conductivity when exposed to traces of pollutant gases [2–8]. Besides inorganic NWs, the use of conducting polymer NWs in which electrical conductivity results from the existence of charge carriers, due to doping, and from the ability of those charge carriers to move along the π bonds of the polymer chains has been reported.

Introduction

Polyaniline (PANI) NWs exhibit a p-type semiconductor behavior, and their sensitivity was significantly higher than that of traditional PANI thin films, due to their higher surface-to-volume ratio [9]. NFs generally consist either of polycrystalline metal oxide nanoparticles arranged in fiber form [10] or of a calcined polymer strand that results in a carbon NF (CNF) [11,12]. NTs are long cylindrical structures related to the fullerenes. Carbon NTs (CNTs) are by far the most researched nanotube structure for gas sensing. While single-walled CNTs consist of a two-dimensional hexagonal lattice of carbon atoms bent and joined in one direction to form a hollow cylinder, multiwalled CNTS comprise two or more nested single-walled CNTs. In polycrystalline metal oxide NFs, the electrical conductivity is modulated by changes in the height of potential barriers that develop at the grain interfaces. These changes are caused by gas adsorption. The electrical conductivity of CNFs and CNTs is also affected by charge transfer and associated p or n doping resulting from the adsorption of gas molecules on their surface or outer wall, respectively. All these mechanisms explain that these nanomaterials have been exploited in conductometric sensors [13–14]. However, the remarkable gas adsorption capacity of CNFs and CNTs explains why these nanomaterials have been employed as well in resonant, gravimetric gas sensors such as bulk acoustic wave (BAW), surface acoustic wave (SAW), or cantilever beams [14]. • Nanoplatelets, nanoflakes, and few-layer and single-layer materials are twodimensional materials. The most researched two-dimensional material in gas sensing has been graphene and its derivatives [14,15,16]. Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex. Graphene-related materials are graphene oxide (GO) and reduced graphene oxide (rGO). GO is a chemically modified graphene containing oxygen functional groups such as epoxides, alcohols, and carboxylic acids. The carbon-tooxygen ratio in GO is approximately three to one. GO can be a semiconductor or insulator, depending on the degree of oxidation. GO results from the chemical exfoliation of graphite. When oxidizing agents react with graphite, the interplanar spacing between the layers of graphite is increased. The completely oxidized compound can then be dispersed in a base solution such as water, and GO is then produced. rGO is a product of the reduction of GO, and different techniques may be used for achieving this; however, the quality and properties of rGO always differ from those of pristine, mechanically exfoliated graphene. By controlling the degree of remnant oxidation in rGO, its electronic and optical properties can be tuned in large scope. Besides graphene nanomaterials, transition metal dichalcogenides (TMDs) such as WS2 or MoS2 have been explored more recently for gas sensing. TMD monolayers are atomically thin semiconductors of the type MX2, where M is a transition metal atom (e.g., Mo and W) and X is a chalcogen atom (S, Se, or Te). One layer of M atoms is sand˚ thick. These wiched between two layers of X atoms. A MoS2 monolayer is only 6.5 A

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materials show a nanoplatelet or nanoflake morphology when they are about 10 monolayer thick (6.5–10 nm). While TMD stacks show indirect bandgap, TMD monolayers show direct bandgap, making them suitable semiconductors for transistors. The electronic properties of these two-dimensional nanomaterials are affected when gases form their environment get adsorbed on their surface. Edges have been found to have increased amount of adsorption sites [17]. • Zeolites, metal-organic frameworks, and hierarchical nanomaterials are known as three-dimensional materials. Zeolites are the aluminosilicate members of the family of microporous solids and mainly consist of Si, Al, O, and metals including Ti, Sn, or Zn. They are often referred to as molecular sieves because they possess the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels, and this can be controlled by the amount Si/Al ratio and the metal employed in a particular zeolite formulation. In addition, their hydrophobicity/ hydrophilicity can be tuned, which helps tuning their affinity toward polar or nonpolar molecules [18]. Metal-organic frameworks (MOFs) are a class of compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. The choice of metal and linker dictates the structure and, therefore, the properties of the MOF. The coordination preference of the metal influences the size and shape of pores by dictating how many ligands can bind to the metal and in which orientation [19]. Both zeolites and MOFs have been used in resonant gravimetric gas sensors. In three-dimensional hierarchical nanomaterials, initial nanostructures (e.g., NPs or NWs) are self-assembled into 3D structures such as hollow microspheres, nanodisks, or dendritic structures. Employing such structures has been reported to be of interest in conductometric gas sensors, due to the increased proportion of exposed active planes and the formation of many nanojunctions at the interface between the initial nanostructure and the secondary ones [20,21]. • Molecular materials have been extensively studied for developing conductometric gas sensors. Any molecule used in organic electronics can be potentially used as gassensitive material. They are conjugated molecules, among which are macrocyclic molecules, pentacene, perylene derivatives, oligomers, and conjugated polymers. Among macrocyclic molecules, the most important families used in chemosensing are phthalocyanines and porphyrins. They exist not only as monomacrocyclic molecules but also as double- or triple-decker complexes, mainly with rare earth metal ions as coordination centers. Films of these molecular materials behave as semiconductors, and their properties are affected by their chemical environment [22–24]. Among other molecules used as gas-sensing materials, cavitand compounds have been explored because they offer the possibility to form inclusion complexes with volatile organic guests. In particular, cavitands, using phosphine oxide-containing resorcinarene

Introduction

derivatives, allow detecting alcohols using BAW transduction. The H-bond acceptor character of P]O groups in the cavitands is responsible for such an affinity toward alcohols [25,26]. Many of the nanomaterials reported earlier are single crystalline, present a very high thermal stability, and a good enough sensitivity for detecting target gases at trace levels when operated at moderate temperatures (ranging from room temperature to under 250°C). This enables achieving gas-sensing devices with superior long-term stability. However, selectivity remains a largely unsolved problem. Different approaches have been reported for ameliorating selectivity and here follow some of the most successful or promising: • Combining different nanomaterials in hybrids has become one of the most exploited approaches for ameliorating selectivity, for example, loading one-dimensional (e.g., NWs and NTs) or two-dimensional (e.g., graphene and graphene-like) nanomaterials with NPs. Metal or metal oxide NPs show catalytic properties and enable the formation of nanoheterojunctions with NWs or NTs that support them. For example, the use of p-type CuO or PdO NPs loading n-type WO3 NWs has been reported for the selective detection of H2S and H2, respectively. The outer sidewalls of CNTs or the surface of graphene can be functionalized via the covalent or noncovalent bonding or large molecules such mercaptans of different chain lengths and terminal functional groups, DNA strands or cavitands. In these approaches, the molecules grafted to carbon nanomaterials act as specific recognition sites for target gases [27–29]. The adsorption of the target molecules triggers changes in the electrical conductivity of the carbon nanomaterial, which acts more as a transducing rather than a sensing element. Another example of such an approach is the combinations of macrocycle molecules to polymers. Different metal phthalocyanines can be combined to either conductive or nonconductive polymers for achieving films with controlled hydrophobicity/ hydrophilicity characteristics, which results in improved sensitivity to target gases and reduced moisture cross sensitivity. • Tailoring surface chemistry via control of defects. The surface chemistry of singlecrystalline nanomaterials can be tuned by controlling the amount of defects. In metal oxides, this can be achieved via a tight control of the conditions during the synthesis process of nanomaterials or by postgrowth treatments. For example, adjusting the conditions of chemical vapor deposition can be used to alter the amount of oxygen vacancies and metal interstitials in metal oxide nanowires [30]. Acidic or basic postgrowth treatments have been reported useful for significantly increasing the amount of oxygen vacancies in ZnO NWs, enabling the room-temperature detection of ammonia vapors. Wet chemistry acidic or basic treatments and reactive cold-plasma treatments have been employed for generating controlled defects on the walls of CNTs or the surface of graphene. Controlled defects not only increase the reactivity of carbon nanomaterials to their chemical environment but also can be used as reactive sites for performing successive functionalization steps (e.g., grafting of functional groups

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or anchoring of metal NPs) in view of increasing selectivity [31]. Cold plasmas have been reported useful as well for the fluorination of CNTs and graphene, a strategy for increasing their hydrophobicity [32]. • Doping of traditional bulk semiconductors has enabled technological applications in electronics by tailoring their chemical, optical, and electronic properties. Substitutional doping in two-dimensional semiconductors (e.g., GO, rGO, and TMDs) would lead to interesting reconfigurations in their morphological and electronic structures, thus affecting surface chemistry. However, substitutional doping is at a comparatively early stage, and the resultant effects are less explored [33]. • Combining a layer of zeolites or MOFs deposited on top of a gas-sensitive sublayer. Zeolites and MOFs act here as molecular sieves, filtering out interfering species and allowing only the target species to diffuse through their pores and actually reach the gas-sensitive film [34–35]. • Designing molecularly imprinted polymers (MIPs). MIPs are polymers that have been processed using the molecular imprinting technique, which leaves cavities in the polymer matrix with an affinity for a chosen “template” molecule [36,37]. The process usually involves initiating the polymerization of monomers in the presence of a template molecule that is extracted afterward, leaving behind complementary cavities. These polymers have affinity for the original molecule and, therefore, help developing molecular sensors. Even though a very high selectivity is potentially achievable, there is always a trade-off between the full removal of the original template and damaging of the substrate binding cavity. Damage is caused by the removal methods and includes collapsing of the cavity, distorting the binding points, incomplete removal of the template, and rupture of the cavity. All this seriously affects yield. Some of the nanomaterials and morphologies discussed in this section are illustrated in Fig. 1.1. These include single-crystalline, low-dimensional metal oxides; nanoplatelets of TMDs either pure or hybridized to CNTs; and nanometer-diameter metal or metal oxide NPs supported on metal oxide NW hybrids. The discussion of the synthesis techniques for these nanomaterials can be found in the next section.

1.3 Synthesis and integration of gas-sensitive nanomaterials Gas-sensitive nanomaterials will mature and reach the market of gas-sensing devices not only because the excellent and highly improved properties they may offer in comparison with the materials currently used by gas sensor manufacturers but also if such nanomaterials can be produced in large quantities, with stable, reproducible characteristics at affordable production costs. Therefore, the synthesis techniques employed and their integration into transducer platforms should be considered for scalability, easiness, and yield, especially if inexpensive gas nanosensors are targeted.

Introduction

Fig. 1.1 Top row: indium oxide octahedra synthesized via vapor transport (pure and decorated with Pd (a) or Pt (b) NPs). Middle row: n-type tungsten oxide nanowires decorated with p-type nickel oxide NPs, grown by AA-CVD, imaged by SEM and TEM. Bottom row: example of 2D nanomaterials. Molybdenum disulfide nanoplatelets obtained by the sulfurization of ultrathin Mo films onto CNT forests and tungsten disulfide nanoplatelets obtained by the sulfurization on tungsten oxide nanowires.

Many of the nanomaterials discussed in the previous section can be synthesized via hydrothermal or solvothermal methods [38,39]. In particular, most of the discussed hierarchical nanomaterials can be prepared from sequential nucleation and growth following a hydrothermal processing in which crystal growth is performed in an apparatus consisting of a steel pressure vessel (autoclave). During growth, a nutrient is supplied along with water, and a temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end, the nutrient solute dissolves, while at the cooler end, it is deposited on a seed crystal. As a result, the desired crystal is grown. The solvothermal synthesis is very similar to the hydrothermal method but does not require an aqueous precursor solution. It allows for the precise control over the size, shape distribution, and crystallinity of metal oxide nanoparticles or nanostructured products. These characteristics can be tuned by altering the process parameters such as reaction temperature, reaction time, solvent, surfactant, and precursor types. The growth of nanomaterials employing these techniques is time and energy consuming, since it generally takes from

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several hours to a few days to complete. The as-grown nanostructures need to undergo a few washing steps before they are suspended in an appropriate solvent to form a solution that can be deposited onto the application substrate. This generally consists of an inert substrate in which interdigitated electrodes have been patterned. Chemical vapor deposition (CVD) is probably the most promising synthesis method for the mass production of nanomaterials. CVD is typically a vacuum deposition method used to produce high-quality, high-performance, solid materials. The growth substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Volatile by-products produced during the chemical reaction are removed from the reaction chamber by a gas flow. The process is used by the semiconductor industry to produce thin films of single-crystalline, polycrystalline, amorphous, or epitaxial materials. CVD appears in many different formats, and gaseous, hot-filament CVD has been used for growing high-quality, large singlecrystalline domains of graphene on dielectric substrates [40]. CVD has become nowadays the most used growth method by the industry manufacturing single-walled and multiwalled CNTs [41]. The aerosol-assisted CVD is a method that is run at atmospheric pressure in which nonvolatile precursors are transported to the reaction chamber by means of an ultrasonically generated aerosol. This technique allows the bottom-up growth of metal oxide NPs and NWs in a matter of minutes at moderate temperatures (i.e., below 500°C) [42]. This enables the direct growth of nanomaterials onto a wide spectrum of application substrates, avoiding the need of a transfer step from the growth substrate to the transducer during the fabrication of gas sensors. Some CVD systems are illustrated in Fig. 1.2. Metal oxide and carbon NFs are often obtained via an electrospinning of metal oxide NPs and/or polymer solutions followed by a calcination step. Electrospun NFs are typically 100 nm in diameter and a few 10 mm in length [43,44]. Solution growth and processing comprise a set of methods that show high promise for the mass production and integration of nanomaterials in gas-sensitive devices. While porphyrin and phthalocyanine films can be coated onto application substrates via vacuum evaporation at high temperatures, the engineering of side chains in porphyrins or the use of substituted phthalocyanines allows for solution processing of these macrocyclic molecules that become soluble [45]. Solution processing is generally run at room temperature and thus enables preparing thin-film coatings onto flexible polymeric substrates. As soon as the molecules are soluble enough, various solution processes can be used. In particular, spin coating, Langmuir-Blodgett [46], and quasi-Langmuir-Sh€afer (QLS) techniques [47,48] have been reported to transfer densely packed films of these molecules to a substrate. These processes can be iterated a few times to adjust film thickness. Besides the solution processing methods mentioned earlier, other techniques such as drop casting, dip coating, spray coating, inkjet printing, and doctor blade or roll-to-roll printing have been regularly employed for transferring films of gas-sensitive nanomaterials (NPs, NWs, NTs, and

Introduction

Fig. 1.2 Upper left panel shows the aerosol ultrasonic generator for an aerosol-assisted CVD. The aerosol generator is connected to a cold-wall CVD reactor made of Teflon. In cold-wall reactors, the growth substrate comprises an integrated heating element, and only the area in which the growth occurs reaches the appropriate temperature. The upper-right panel shows three hot-wall reactors for aerosol-assisted CVD. The reactor on the left has its coverlid open, and the one in the middle is closed, but the upper refractory brick has been removed. Refractory bricks are used to homogenize temperature within a reactor. The aerosol-assisted CVD reactors shown are used to grow pure or metal/metal oxide NP-decorated metal oxide NWs. Finally, the lower panel shows a CVD furnace for the growth of TMDs.

even flakes of two-dimensional nanomaterials) onto a wide spectrum of substrates. These techniques are described later: • Drop casting is possibly one of the simplest ways to coat applications substrates with gas-sensitive nanomaterials. Nanomaterials are dissolved or suspended in a solvent, and this is dropped onto a substrate. The solvent is left to evaporate naturally at room temperature or by applying mild heating to the substrate. This technique has some drawbacks such as the low control of film uniformity and thickness and is limited to small-area coatings. • In dip coating, the substrate is dipped into the solution and then withdrawn at a controlled speed. Film thickness is determined by the balance of forces at the liquidsubstrate interface. It is an easy process, enabling the coverage of large areas with very thin layers and good uniformity. It results in the waste of material; it is slow and results in double-side coating.

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• Spin coating consists of dropping a solution or suspension onto a spinning substrate. It leads to reproducible results with good control of film thickness (down to 10 nm), but it wastes materials and does not allow the coating of large areas. • Doctor blading consists of spreading a solution through a moving blade onto a stationary substrate (or via a stationary blade onto a moving substrate). Even though this method is not suitable for achieving very thin films (200 nm is about the lower thickness limit), it achieves high uniformity and good reproducibility and is suitable for large-area, roll-to-roll, fast printing. • In spray coating, a heated substrate is hit by a vaporized solution cone. The vaporized solution can be obtained by a flow of a carrier gas (similar to aerography), and an ultrasonic nozzle may be used to generate an aerosol of fine droplets. The morphology and thickness of the coating can be adjusted by playing with the process parameters (solution viscosity, solvent properties, airflow rate, substrate temperature, tip geometry, and distance from tip to substrate). In general, this technique leads to large-area coverage and is then amenable to fast roll-to-roll printing. Due to film homogeneity and thickness problems in the outer area of the deposition cone, spray coated films are often defined through a shadow mask, which results in the waste of material. • In Langmuir-Blodgett coating, the transfer of a Langmuir film to a substrate preserving density is performed. The Langmuir film consists of amphiphilic molecules (they have a hydrophilic head and a hydrophobic tail) on water. By reducing the area, pressure increases, and a solid-state monolayer forms, which floats on the water surface. A hydrophilic or hydrophobic substrate is then dipped and gets coated with a monolayer. This method enables coating the substrate with extreme thickness control (ideally a monolayer is achieved). Stacks of different nanomaterials are achievable via a layer-by-layer assembly process. The process is suitable for covering large areas. The methodology works only with amphiphilic molecules and is rather involved but has been extensively employed for coating phthalocyanine films. • Inkjet printing enables the building of films from aqueous nanoparticulate dispersions (e.g., of inorganic nanomaterials, polymers, or carbon nanomaterials). For conductometric gas-sensing applications, these films are generally printed over patterned silver or gold interdigitated electrodes that have been inkjet printed previously onto an inert substrate (e.g., a flexible polymeric substrate such as Kapton or Upiliex). Unlike in screen printing, inkjet printing does not require stencils (masks), and therefore, it allows rapid design and prototyping. Fig. 1.3 illustrates the development of nanomaterial-based gas sensors employing inkjet printing and the aerosol-assisted CVD. In the particular case illustrated in this figure, metal oxide nanowire materials can be grown at atmospheric pressure and relatively low temperatures (near 380°C) in less than 15 min. Kapton can withstand perfectly

Introduction

Fig. 1.3 Upper row, from left to right. A Kapton substrate in which several heating elements and transducing electrodes have been inkjet printed (employing a commercially available silver ink). The enlarged picture shows a close-up view of one of such transducers, and the picture on the right shows the inkjet printer used. The lower row shows one of such devices wire-bonded to a PCB, after being coated via aerosol-assisted CVD with Pt-decorated tungsten oxide nanowires. The SEM and TEM micrographs show the nanomaterials grown on top of the flexible substrate. The picture on the right shows the hot-wall reactor used.

the thermal constraints, and therefore, the direct growth of nanomaterials onto a flexible, polymeric transducer substrate is demonstrated. So far, an introduction to nanomaterials and the ways to grow and integrate them in application substrates for gas-sensing applications has been given. Special stress has been put on methods that hold the promise for an easily scalable, bottom-up growth of nanomaterials and on fast printing or roll-to-roll manufacturing techniques for producing gas sensors in high numbers at affordable costs. All these aspects will be treated and expanded further in the following chapters of this book together with the insightful discussion of well-established and emerging applications. While gas-sensing devices and analyzers used in industrial applications are very expensive, nowadays, indoor and outdoor air quality monitoring concerns are driving the use of gas sensors not only in commercial building automation but also in personal environmental monitoring. Therefore, there is a need for less expensive and compact-sized equipment to hit the consumer sensor market. This desire for ubiquitous, continuous, and real-time monitoring is driving the efforts toward miniaturization, networking/communication, power management, and reliability. The incipient trend of miniaturized systems, printed sensor manufacturing, and flexible electronics for consumer markets is expected to boost the gas sensor market to double its 2019 value by 2025. One of the reasons for this market boost is the adoption of wireless, distributed sensors with IoT. This book aims at giving a critical overview about how the science and technology of inexpensive gas microsensors is evolving now and identifying future trends.

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1.4 Organization of the book The book is organized in 3 sections and 15 chapters. After this first, introductory chapter, the first section reviews advanced nanomaterials for the development of gas microsensors. Therefore, it deals with the synthesis, functionalization, and characterization and discusses the properties and performance of advanced nanomaterials for inexpensive gas micro- and nanosensors. This includes polycrystalline or single-crystalline, nanosized metal oxides (nanoparticles, nanorods, nanowires, and hierarchical materials), carbon nanomaterials (carbon nanofibers, carbon nanotubes, graphene, and graphene-like materials), polymers (including MIPs), molecular materials like porphyrins and phthalocyanines, and other advanced 2D materials like transition metal dichalcogenides. Hybrid nanomaterials will be considered as well. Special emphasis is given to synthesis techniques that ease the integration of nanomaterials in transducers in view of achieving inexpensive devices (e.g., solution processing). Each chapter includes a discussion on the gas-sensing fundamentals for the nanomaterials considered. The chapter breakdown is as follows: Chapter 2 is targeted at inorganic nanomaterials, focused on metal oxide nanowires and nanorods. Special emphasis is given to the integration of these nanomaterials onto micromachined platforms (e.g., integrated silicon-based microheater/electrode arrays). The use of self-heating strategies for achieving dramatic savings in power consumption during operation is discussed as well. Chapter 3 discusses organic nanomaterials for developing gas sensors. The use of polymers, porphyrins, phthalocyanines, carbon nanomaterials, and their hybrid materials is reviewed, and different transducing schemes such as chemoresistors, field effect transistors (FET), and gravimetric or optical sensors are discussed in detail. Chapter 4 is centered on carbon nanomaterials and critically reviews the use of carbon black, carbon nanofibers, carbon nanotubes, and graphene-like materials. A discussion on the gas-sensing fundamentals of the different nanomaterials considered is given, which is followed by a short discussion on the research efforts needed for inexpensive gas sensors making use of carbon nanomaterials to become successfully marketed. This first section is closed with Chapter 5 in which the use of hybrid and 2D nanomaterials is discussed for achieving inexpensive sensors. The hybrid materials considered combine at least two types of the materials reported in the three previous chapters. The chapter ends by identifying the challenges faced by gas sensors that employ hybrid and 2D materials. In Section 2, different transducing platforms for realizing inexpensive gas microsensors are reviewed. Therefore, the five chapters that belong to this section deal with the coupling of sensitive nanomaterials to different types of transducer elements adapted to the applications sought. Chapter 6 is devoted to the integration of nanomaterials into functional transducers with special emphasis on either direct growth or additive fabrication techniques as a way to ensure that inexpensive gas microsensors can be obtained. The following three

Introduction

chapters of the section review the main transduction schemes and discuss the interfacing between transducers and sensing layers and the associated conditioning, read-out, and signal processing electronics. Therefore, Chapter 7 discusses resistive, impedance and FET devices, while Chapter 8 is centered on optical devices and Chapter 9 discusses resonant transducing schemes for gas sensing. Finally, Chapter 10 critically reviews advanced operating methods (e.g., temperature modulation, self-heating, light-activated response, and noise methods), which are devised for enhancing stability, sensitivity, selectivity, and reducing power consumption. The third and last section of this book discusses emerging applications for inexpensive gas nanosensors. As already stated earlier, it is the desire for ubiquitous, continuous, and realtime monitoring that is driving the research efforts toward affordable, miniaturized, communicating, power-efficient, and reliable gas sensors. Therefore, Chapters 11 and 12 are devoted to air quality monitoring indoors and outdoors, respectively. Chapter 13 presents an emerging application in food quality and safety. Since decay tell-tale species are often of gaseous or volatile nature, different gas-sensing schemes can be used leveraging on different optical or electrical transduction mechanisms that can be integrated in cost-effective smallfootprint devices. These are critically discussed in this chapter. Finally, the last chapter of this section discusses a biomedical application of gas microsensors that consists of developing point-of-care and personal breath analysis systems for the noninvasive diagnosis of diseases. The book finishes by a last chapter (Chapter 15) in which general concluding remarks and outlook are given. It provides a summarizing overview of the achievements and the problems that remain to be solved for a widespread use of gas micro-/nanosensors employing advanced nanomaterials. The complete chain from nanomaterial synthesis to integration in functional devices and systems is considered and makes recommendations about where more efforts are needed to overcome current problems and suggests future lines for research and development.

Acknowledgment I am indebeted to Prof. Marcel Bouvet from the Universite Bourgogne Franche-Comte, who provided me with information about molecular and hybrid materials.

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[4] F.E. Annanouch, Z. Haddi, S. Vallejos, P. Umek, P. Guttmann, C. Bittencourt, E. Llobet, Aerosolassisted CVD-grown WO3 nanoneedles decorated with copper oxide nanoparticles for the selective and humidity-resilient detection of H2S, ACS Appl. Mater. Interfaces 7 (2015) 6842–6851. [5] P. Wang, Y. Fu, B. Yu, Y. Zhao, L. Xing, X. Xue, Realizing room-temperature self-powered ethanol sensing of ZnO nanowire arrays by combining their piezoelectric photoelectric and gas sensing characteristics, J. Mater. Chem. A 3 (2015) 3529–3535. [6] F.E. Annanouch, Z. Haddi, M. Ling, F. Di Maggio, S. Vallejos, T. Vilic, Y. Zhu, T. Shujah, P. Umek, C. Bittencourt, C. Blackman, E. Llobet, Aerosol-assisted CVD-grown PdO nanoparticle-decorated tungsten oxide nanoneedles extremely sensitive and selective to hydrogen, ACS Appl. Mater. Interfaces 8 (2016) 10413–10421. [7] T. Van Dang, N. Duc Hoa, N. Van Duy, N. Van Hieu, Chlorine gas sensing performance of on-Chip grown ZnO, WO3 and SnO2 nanowire sensors, ACS Appl. Mater. Interfaces 8 (2016) 4828–4837. [8] E. Navarrete, C. Bittencourt, P. Umek, E. Llobet, AACVD and gas sensing properties of nickel oxide nanoparticle decorated tungsten oxide nanowires, J. Mater. Chem. C 19 (2018) 5181–5192. [9] H. Liu, J. Kameoka, D.A. Czaplewski, H.G. Craighead, Polymeric nanowire chemical sensor, Nano Lett. 4 (2004) 671–675. [10] J. Moon, J.-A. Park, S.-J. Lee, T. Zyung, I.-D. Kim, Pd-doped TiO2 nanofiber networks for gas sensor applications, Sens. Actuators B: Chem. 149 (2010) 301–305. [11] W. Li, L.-S. Zhang, Q. Wang, Y. Yu, Z. Chen, C.-Y. Cao, W.-G. Song, Low-cost synthesis of graphitic carbon nanofibers as excellent room temperature sensors for explosive gases, J. Mater. Chem. 22 (2012) 15342–15347. [12] J.S. Lee, O.S. Kwon, S.J. Park, E.Y. Park, S.A. You, H. Yoon, J. Jang, Fabrication of ultrafine metaloxide-decorated carbon nanofibers for DMMP sensor application, ACS Nano 5 (2011) 7992–8001. [13] Z. Xiao, L.B. Kong, S. Ruan, X. Li, S. Yu, X. Li, Y. Jiang, Z. Yao, S. Ye, C. Wang, T. Zhang, K. Zhou, S. Li, Recent development in nanocarbon materials for gas sensor applications, Sens. Actuators B: Chem. 274 (2018) 235–267. [14] E. Llobet, Gas sensors using carbon nanomaterials: a review, Sens. Actuators B: Chem. 179 (2013) 32–45. [15] K. Vikrant, V. Kumar, K.-H. Kim, Graphene materials as a superior platform for advanced sensing strategies against gaseous ammonia, J. Mater. Chem. A 6 (2018) 22391–22410. [16] E. Singh, M. Meyyappan, H.S. Nalwa, Flexible graphene-based wearable gas and chemical sensors, ACS Appl. Mater. Interfaces 9 (2017) 34544–34586. [17] S.-J. Choi, I.-D. Kim, Recent developments in 2D nanomaterials for chemiresistive-type gas sensors, Electron. Mater. Lett. 14 (2018) 221–260. [18] Y. Zheng, X. Li, P.K. Dutta, Exploitation of unique properties of zeolites in the development of gas sensors, Sensors 12 (2012) 5170–5194. [19] Z.-G. Gu, J. Zhang, Epitaxial growth and applications of oriented metal–organic framework thin films, Coord. Chem. Rev. 378 (2019) 513–532. [20] H. Wang, A.L. Rogach, Hierarchical SnO2 nanostructures: recent advances in design, synthesis and applications, Chem. Mater. 26 (2014) 123–133. [21] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B: Chem. 140 (2009) 319–336. [22] A.A. Vaughan, M.G. Baron, R. Narayanaswamy, Optical ammonia sensing films based on an immobilized metalloporphyrin, Anal. Commun. 33 (1996) 393–396, https://doi.org/10.1039/ac9963300393. [23] C. Di Natale, An electronic nose for food analysis, Sens. Actuators B: Chem. 44 (1997) 521–526. [24] C. Di Natale, D. Monti, R. Paolesse, Chemical sensitivity of porphyrin assemblies, Mater. Today 13 (2010) 46–52. [25] R. Paolesse, C. Di Natale, S. Nardis, A. Macagnano, A. D’Amico, R. Pinalli, et al., Investigation of the origin of selectivity in cavitand-based supramolecular sensors, Chem. Eur. J. 9 (2003) 5388–5395. [26] R. Pinalli, F.F. Nachtigall, F. Ugozzoli, E. Dalcanale, Supramolecular sensors for the detection of alcohols, Angew. Chem. Int. Ed. 38 (1999) 2377–2380. [27] P. Clement, S. Korom, C. Struzzi, E.J. Parra, C. Bittencourt, P. Ballester, et al., Deep cavitand selfassembled on au NPs-MWCNT as highly sensitive benzene sensing interface, Adv. Funct. Mater. 25 (2015) 4011–4020.

Introduction

[28] A. Thamri, H. Baccar, C. Struzzi, C. Bittencourt, A. Abdelghani, E. Llobet, MHDA-functionalized multiwall carbon nanotubes for detecting non-aromatic VOCs, Sci. Rep. 6 (2016) 35130. [29] J. Casanova-Cha´fer, C. Bittencourt, E. Llobet, Hydrophilicity and carbon chain length effects on the gas sensing properties of chemoresistive, self-assembled monolayer carbon nanotube sensors, Beilstein J. Nanotechnol. 10 (2019) 565–577. [30] M. Ling, C. Blackman, Growth mechanism of planar or nanorod structured tungsten oxide thin films deposited via aerosol assisted chemical vapour deposition (AACVD), Phys. Status Solidi C 12 (2015) 869–877. [31] R. Leghrib, A. Felten, F. Demoisson, F. Renier, J.J. Pireaux, E. Llobet, Room-temperature, selective detection of benzene at trace levels using plasma-treated metal-decorated multiwalled carbon nanotubes, Carbon 48 (2010) 3477–3488. [32] C. Struzzi, M. Scardamaglia, J. Casanova-Chafer, R. Calavia, J.-F. Colomer, A. Kondyurin, M. Bilek, N. Britun, R. Snyders, E. Llobet, C. Bittencourt, Exploiting sensor geometry for enhanced gas sensing properties of fluorinated carbon nanotubes under humid environment, Sens. Actuators B: Chem. 281 (2019) 945–952. [33] Z. Hu, Z. Wu, C. Han, J. He, Z. Ni, W. Chen, Two-dimensional transition metal dichalcogenides: interface and defect engineering, Chem. Soc. Rev. 47 (2018) 3100–3128. [34] M.H. Yap, K.L. Fow, G.Z. Chen, Synthesis and applications of MOF-derived porous nanostructures, Green Energy Environ. 2 (2017) 218–245. [35] R.-B. Lin, S.-Y. Liu, J.-W. Ye, X.-Y. Li, J.-P. Zhang, Photoluminescent metal–organic frameworks for gas sensing, Adv. Sci. 3 (2016)1500434. [36] L. Chen, X. Wang, W. Lu, X. Wu, J. Li, Molecular imprinting: perspectives and applications, Chem. Soc. Rev. 45 (2016) 2137–2211. [37] Q. Zhu, Y.M. Zhang, J. Zhang, Z.Q. Zhu, Q.J. Liu, A new and high response gas sensor for methanol using molecularly imprinted technique, Sens. Actuators B: Chem. 207 (2015) 398–403. € [38] M. Andersson, L. Osterlund, S. Ljungstr€ om, A. Palmqvist, Preparation of nanosize anatase and rutile TiO2 by hydrothermal treatment of microemulsions and their activity for photocatalytic wet oxidation of phenol, J. Phys. Chem. B 106 (2002) 10674–10679. [39] R.-C. Xie, J.K. Shang, Morphological control in solvothermal synthesis of titanium oxide, J. Mater. Sci. 42 (2007) 6583–6589. [40] S. Tang, H. Wang, H. Wang, Silane-catalysed fast growth of large single-crystalline graphene on hexagonal boron nitride, Nat. Commun. 6 (2015) 6499. [41] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes, Science 306 (2004) 1362–1365. [42] S. Vallejos, T. Stoycheva, P. Umek, C. Navio, R. Snyders, C. Bittencourt, E. Llobet, C. Blackman, S. Moniz, X. Correig, Au nanoparticle-functionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices, Chem. Commun. 47 (2011) 565–567. [43] M. Imran, N. Motta, M. Shafiei, Electrospun one-dimensional nanostructures: a new horizon for gas sensing materials, Beilstein J. Nanotechnol. 9 (2018) 2128–2170. [44] I.-D. Kim, A. Rothschild, Nanostructured metal oxide gas sensors prepared by electrospinning, Polym. Adv. Technol. 22 (3) (2011) 318–325. [45] P. Ma, J. Kan, Y. Zhang, C. Hang, Y. Bian, Y. Chen, The first solution-processable n-type phthalocyaninato copper semiconductor: tuning the semiconducting nature via peripheral electronwithdrawing octyloxycarbonyl substituents, J. Mater. Chem. 21 (2011) 18552–18559. [46] M.J. Cook, I. Chambrier, Phthalocyanine thin films: deposition and structural studies, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Elsevier, Boston, 2003, pp. 37–127. [47] Y. Chen, M. Bouvet, T. Sizun, Y. Gao, C. Plassard, E. Lesniewska, Facile approaches to build ordered amphiphilic tris(phthalocyaninato) europium triple-decker complex thin films and their comparative performances in ozone sensing, Phys. Chem. Chem. Phys. 12 (2010) 12851–12861. [48] J. Gao, G. Lu, J. Kan, Y. Chen, M. Bouvet, Solution-processed thin films based on sandwich-type mixed (phthalocyaninato)(porphyrinato) europium triple-deckers: structures and comparative performances in ammonia sensing, Sens. Actuators B: Chem. 166-167 (2012) 500–507.

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CHAPTER 2

Inorganic nanomaterials Francisco Hernandez-Ramirez, Albert Romano-Rodriguez, Joan Daniel Prades MIND—Departament d’Enginyeria Electro`nica i Biome`dica, Universitat de Barcelona, Barcelona, Spain

2.1 Introduction Among the different options used for the realization of inexpensive gas sensors, bare inorganic nanomaterials are considered one of the forerunners thanks to their simplicity and unique properties. In fact, inorganic nanomaterials have turned into perfect test beds of new sensing concepts, paving the way toward advanced operating methods that have been fully widespread in affordable and simple devices afterward [1, 2]. This chapter reviews the use of inorganic nanomaterials as building blocks of gas micro- and nanosensors, covering from the fabrication stage to the operation mode. The discussion of the underlying working principles for these materials is also briefly outlined with a special focus on the applications of new devices setups. The first gas sensors based on inorganic materials have already come a long way with the extensive use in Taguchi sensors of metal oxide semiconductor thin films [3–5]. Moving on from there, nanoscale inorganic materials and specifically one-dimensional (1D) structures, such as nanowires, nanobelts, and nanotubes, have gained tremendous attention within the last two decades due to their potential applications in optoelectronic and electronic devices [1, 2]. Inorganic materials confined in several dimensions at the nanometer scale exhibit properties different to their bulk counterparts [6]. Therefore, the interest of using them for practical applications increases with the deeper understanding of the properties and tailoring of their key parameters. With this in mind, the applicability of 1D inorganic materials for gas and biochemical detection, photonics, and energy harvesting production and storage has been extensively demonstrated and validated in the last few years [1, 7–10]. Here, the use of 1D metal oxide nanomaterials for the realization of inexpensive conductometric gas sensors is specifically presented and described. Despite the fact that other inorganic materials have been used for the same purpose, the formers are the most representative at the time of this writing, and they can be considered a prime example of the state-of-the-art technologies.

Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00002-5

Copyright © 2020 Elsevier Inc. All rights reserved.

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2.2 Operating sensing principles The fundamental principle of the gas-sensing mechanism in inorganic materials relies on changes of the active layer at the surface induced by the analytes, which at the same time result in variations of the measurable physical properties of the working device [11]. In this interaction process, the need for temperature control and the challenges related to achieving it in an efficient manner has usually become paradigmatic [12]. In particular, the operation of conductometric sensors is based on the interaction between adsorbed (physi- or chemisorbed) species and the surface and the effective change of the local charge density in the material, which can be easily monitored by the modulation of the electrical conductivity. Differences in the tendency of physi- and chemisorption processes are dependent on the environmental conditions [11, 13–15], and in general, for an effective and reversible gas response, the activation energy necessary for the complete desorption of analyte species must clearly overcome a specific temperature threshold. This is normally achieved by providing thermal energy at temperatures above 100°C. In the sensing process, ionosorbed oxygen also plays a major role, which interacts with detectable species or competes for the same adsorption sites at the semiconductor surface following complex dynamic mechanisms [16, 17]. Alternatively, the necessary activation energy of these processes can be provided by impinging high-energy photons on the sensing material, which triggers the desorption of gaseous species in a similar way that in the case of the thermal scenario [18]. The working principles of these two operating modes (thermally activated and lightactivated) have been extensively reviewed in the literature despite the fact that, quoting Noboru Yamazoe, the full details of inorganic gas sensors still remain far from having fully understood satisfactorily [13].

2.3 General overview of gas sensors based on inorganic nanomaterials From a practical point of view, conductometric gas sensors made up with nanomaterials are always thermally activated or light-activated devices. The former ones have been a subject of research for more than two decades [1, 9]: gas sensors made up with either bundles of nanomaterials or individual 1D structures have been extensively studied despite the fact that most of them were not fabricated by means of cost-effective and simple routes. The first and simplest attempts to develop thermally activated gas sensors with 1D nanomaterials were based on a simple idea: bundles of nanowires are bridging the gap between two electrodes, while they were warmed up with the help of an external heater. From a theoretical point of view, the sensing mechanisms of these devices can be described with the same model used for Taguchi sensors, and as a result, the contact areas between nanowires are the main responsible for the sensor response [11]. However, the

Inorganic nanomaterials

random distribution of the nanowires may lead to poor reproducibility of the response, not showing these nanodevices at first glance a clear advantage compared with standard thin-film gas sensors. Besides, the electrical quality of the electrical contacts is not always optimal, which is considered a major drawback to extend the use in industrial applications involving massive deployments [19] (Fig. 2.1).

Fig. 2.1 Schematic diagrams of different types of conductometric gas sensors based on inorganic semiconductors: (A) commercial thin-film metal oxide device formed by a layer of nanoparticles. Here, electrons must go through a network of nanocrystals with different size and shape. From an energy point of view, electrons are to overcome different barriers, which modulate the detection output. (B) Multinanowire sensor. The same conduction model is valid here. (C) Individual-nanowire sensor fabricated with FIB lithography. (Reproduced from F. Hernandez-Ramirez, J.D. Prades, R. Jimenez-Diaz, T. Fischer, A. Romano-Rodriguez, S. Mathur, J.R. Morante, On the role of individual metal oxide nanowires in the scaling down of chemical sensors, Phys. Chem. Chem. Phys. 11(33) (2009) 7105–7110 with permission from the PCCP Owner Societies.)

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To circumvent these first barriers, devices based on individual 1D nanomaterials like nanowires have progressively attracted the major attention of the scientific community. They define a controlled test ground for semiconductors since the geometry and morphology are well-known and in principle controlled. Obviously, the fabrication complexity of one-nanowire devices is significantly higher, and it requires advanced fabrication tools like focused ion beam (FIB) or electron beam lithography (EBL), which exhibits a limited yield and as a result hampers the transfer of these devices to the mass production level [20, 21] (Fig. 2.1). With the help of these devices, it has been possible to establish a direct relationship between the gas response and the nanowire radii as well as modeling and evaluation many other fundamental phenomena occurring during the sensing process (i.e., role of the oxygen diffusion in the response) [17, 22, 23]. Despite the fact that most of these devices were operated with an external heater, their reduced dimensions have allowed the integration in MEMS, reducing the energy consumption in a significant way. Actually, some of these prototypes have successfully merged the advantages of using individual nanowires and industry-ready substrates, but unfortunately, the fabrication process is relatively complex to extend the use on a massive scale [24, 25] (Fig. 2.2). For this reason, self-assembly techniques for the manipulation and positioning of nanowires have been evaluated as a suitable fabrication alternative, showing promising results at the present time [1]. In particular, dielectrophoresis (DEP) or the motion of polarizable particles in a fluid subject to an electric field has been successfully tested with metal oxide nanowires clearly

Fig. 2.2 (A) SEM image of a suspended microhotplate with an integrated heater and an interdigitated electrode. (B) Tin oxide nanowire electrically contacted with FIB. Nanocontacts are made of platinum. (© IOP Publishing. Reproduced with permission from F. Hernandez-Ramirez, J.D. Prades, A. Tarancon, S. Barth, O. Casals, R. Jimenez-Diaz, E. Pellicer, J. Rodriguez, M.A. Juli, A. Romano-Rodriguez, J.R. Morante, S. Mathur, A. Helwig, J. Spannhake, G. Mueller, Portable microsensors based on individual SnO2 nanowires, Nanotechnology 18(49) (2007) 495501. All rights reserved.)

Inorganic nanomaterials

leading to a higher yield in the production of nanosensors. DEP has been applied to manipulate different types of 1D nanomaterials, such as SnO2 nanobelts and nanowires; GaN, ZnO, InP, and Ga2O3 nanowires; and carbon nanotubes, enabling the fabrication of sensing and electronic devices like current rectifiers; field-effect transistors; and photonic, thermal, and gas sensors [26–33]. It is noteworthy to remark that DEP is compatible with standard CMOS manufacturing process and wafer-scale assembly, validating that this technique is suitable for the mass production of devices [1]. On the other hand, light-activated gas sensors have also attracted attention since their study has made a nice contribution to further enhance our knowledge of the fundamental properties of the semiconductors’ surfaces [18, 34]. Besides, they can extend the use of gas sensors in applications with explosive and inflammable atmospheres. Unfortunately, their configuration is far from being simple despite some promising attempts to design affordable systems, and for this reason, the use remains mainly restrained for academic and research purposes: standard inorganic nanomaterials like metal oxide semiconductors are wide-bandgap materials (i.e., SnO2, Eg > 3.9 eV), which makes necessary UV light to perform sensing experiments at room temperature. This is a major drawback due to the energy consumption constraints, the need for coupling a light source to the sensor, and very specific control requirements during the operation.

2.4 Toward cost-effective gas sensors based on inorganic materials The massive use of gas sensors based on inorganic nanomaterials will become a tangible reality only when simple, reliable, and easy-to-operate devices are finally developed. On this basis, two different strategies are usually followed to reduce the final cost and the complexity of the new devices: (i) the adoption of automated fabrication routes and (ii) the simplification of the operation methods. The first ones focus on innovative strategies specifically designed to automate the growth and assembling of the nanomaterials in the new devices, skipping the use of complex fabrication techniques that may be affected by a limited yield. The goal is, therefore, moving toward an industrial production level keeping the cost low during the fabrication stage. In detail, in situ controlled growth of nanomaterials on substrates and electrodes [35–37] and self-assembly techniques, such as DEP, have shown promising results in the last years, as commented before. On the other hand, the smart use of newly discovered phenomena at the nanoscale can be employed to work with sensors relying on nanomaterials in a simplified way that results in lowering the operational costs during the device life span. In this sense, selfheating effect has become paradigmatic after a decade of active research in the field [38]. This section details the two previously mentioned strategies followed to obtain costeffective sensors, paying special attention to self-heated miniaturized devices since they have proven to be competitive enough in terms of gas detection performance. It goes

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without saying that in the real world, both cost-cutting strategies should be successfully combined to attain mature industry-ready technologies.

2.4.1 Automated fabrication routes The possibility to obtain cost-effective gas sensors always begins with an adequate fabrication route. In this sense, any methodology with options for becoming an industrialization solution needs at least to be automated, give reproducible results, and provide a high production yield. In this sense, the major challenge associated with the fabrication of devices based on nanomaterials rests on how to position them in the proper locations. To accomplish this, two different strategies are usually followed: (i) in situ controlled growth at some specific parts of the device and (ii) the use of postgrowth alignment techniques, such as DEP. As far as the first one is concerned, the literature is full of examples involving the application of templates and etching or epitaxial metal-promoted techniques [1], among other methodologies specifically designed to attain the nanomaterial production at well-defined positions of the devices. This has allowed some interesting results such as the systematic Ti-catalyzed growth of Si nanowires to form bridges across several micron-wide trenches [39]. In the pursuit of developing inexpensive gas sensors that integrate semiconductor nanomaterials, the site-selective direct growth of inorganic nanowires on top of CMOS-compatible MEMS [i.e., suspended microheaters with integrated additional interdigitated electrodes (IDE) on the top] has however been shown as one of the most promising approaches to attain this ambitious goal. Going a step further of the works in which the indiscriminate growth of semiconductor elements over the whole substrates was reported [40], the localized production of nanomaterials on the top of MEMS in a similar way as in the thin-film case [41] has been successfully reported in the literature with quite promising results for silicon nanowires [36], germanium nanowires [35], carbon nanotubes [42], tin oxide nanotubes [43], and cupric oxide nanowires [37], as an example. The main advantage of the here-presented technique relies on achieving localized growth on top of the membranes (heated area) by using the power dissipation of the heater only to that end. Actually, it is the effect of the heater that directly activates the semiconductor growth mechanism, typically based on CVD or thermal oxidation mechanisms [35, 37], removing the need for huge reactors with high-energy consumption requirements [35]. This involves a dramatic reduction of the energy demands during the synthesis process and by extension leads to a drop of the total production costs. It is noteworthy that the sensing performance of gas sensors obtained with the in situ growth controlled strategy has shown promising outputs, such as CO monitoring for concentrations as low as 1 ppm [37], even though they are not optimized yet (Fig. 2.3).

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Time (x1000 s) Fig. 2.3 (A) Schematic drawing of a microhotplate with an integrated buried heater and an external circuit containing interdigitated electrodes (IDE) used to the in situ growth of semiconductor nanowires. (B) SEM images of (left) suspended microhotplates used to grow nanowires. (Right) The micromembrane shows nanowires grown in the device (brighter area in the center). (C) Response of the device shown in (B) to pulses of different concentrations of CO in synthetic air at 260°C. The heating power was 48 mW, the same value required to heat micromembrane to 700°C under vacuum. (Reproduced from B. Sven, J.-D. Roman, S. Jordi, D.P. Joan, G. Isabel, S. Joaquin, C. Carles, R.-R. Albert, Localized growth and in situ integration of nanowires for device applications, Chem. Commun. 48 (2012) 4734–4736 with permission from The Royal Society of Chemistry.)

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This suggests that there is still room for improvement in terms of the final devices’ functionalities. Nevertheless and in spite of the intrinsic advantages of the in situ growth approach, the technology has still a limited range and impact beyond the academic ecosystem, and it has been basically restrained to study the fundamentals of gas sensors based on very specific materials. Regarding the postgrowth alignment and position of nanomaterials, there are several methods that have been described so far to successfully control the fabrication of nanodevices. Apart from some exotic approaches, such as the “blown-bubble film technique” [44], which involves expanding a bubble from a homogeneous suspension of stable epoxy solutions containing surface-modified nanomaterials, resulting in their alignment for the later transfer to both rigid and flexible substrates, the most mature and commonly used technique is DEP, as stated in the previous section. DEP has been successfully used to assemble nanowires from a solution without the need for surface modification. Actually, the electrokinetic motion of dielectrically polarized materials in nonuniform electric fields can be used for the self-alignment of nonsymmetric nanostructures and in particular nanowires and nanotubes. In this respect, an external field is induced between metal microelectrodes by applying an AC voltage, resulting in a well-defined space-charge region. This nonmechanical manipulation technique allows the assembly of different quantities of 1D structures, depending on the concentration of the nanostructures in the solvent, the magnitude and frequency of the applied field, and the gap distance between the electrodes. The alignment process can be optimized for single-nanostructure bridging devices leading to reliable singlenanostructure diodes [45], multiwire FETs [46], and gas sensors [31]. Appropriate bus bar spacing and very high frequencies allow almost exclusive alignment of single nanostructures between electrodes [47]. On the other hand, medium frequencies attract the most wires; however, they partially adhere to the buses or the aligned wires are accompanied by additional wires. This high dependency of the DEP result on the experimental conditions is usually studied with the help of theoretical tools such as finite element simulation that facilitate to understand the role of each key parameter and improve the geometry design of the sensor substrate (i.e., microelectrodes) to minimize undesired effects like the attraction of unaligned elements. Nevertheless and after all, DEP is a technique marked by the need for trial-and-error procedures: the optimization of this nanofabrication strategy involves controlling many different parameters with a direct impact on the position and alignment yields, such as (i) density of nanomaterials in the solution, (ii) electrical properties of the materials, (iii) viscosity of the solvent, (iv) properties of the applied electric field (i.e., amplitude and frequency), (v) flow of the solution, and (vi) geometry of the electrodes. It goes without saying that reaching the full control of all of them is not straightforward taking into account that there are cross dependencies with some of them. For this reason, DEP

Inorganic nanomaterials

usually requires a systematic calibration work to optimize the process prior to the sensor production stage, which can be easily nullified with slight changes of any of the experimental parameters. From a practical point of view, this is a major drawback hampering the application of this technique beyond lab-class devices. Besides, the electrical quality of the contacts between aligned nanomaterials and prepatterned microelectrodes on the substrates is not always optimal after the entire process, and it usually requires the postprocessing of the device, involving an additional optical lithography step in some cases. Despite these shortcomings, DEP has allowed the fabrication of gas sensors based on nanowires with very good performance characteristics, being some of them integrated into functional CMOS operating circuitry [28].

2.4.2 Simplified operation methods: Self-heated nanosensors The final cost of new technologies is linked not only to the fabrication process but also to the operating conditions. Gas sensors require controlling the latter accurately to thus obtain repeatable and long-term stable readouts. In the case of systems based on nanomaterials, at first glance, this means complex working methodologies and the use of increasingly miniaturized heaters. However, the coupling of inorganic semiconductors and MEMS microhotplates for the realization of affordable gas sensors had successfully started a long time ago with metal oxide thin films [48]. Actually, microhotplates can follow standard IC fabrication procedures, allowing series production of portable and reliable devices at low cost. Moreover, due to their low thermal mass, fast dynamics and low power consumption are simultaneously achieved in an easy way [49]. Capitalizing these opportunities, intensive research efforts have been devoted to reach microhotplates fully compatible with SOI CMOS technologies during more than two decades [50], since the combination of metal oxide sensing layers operated between 300°C and 500°C with standard silicon processes could be defined at the very least as troubling. From this point, the race to integrate nanomaterials and microhotplates became accelerated leading to operational hydrogen [51], carbon monoxide [24], and other pollutant sensors [52]. Quoting Meier et al. [43], these devices have resulted on paper in excellent building blocks of “electronic noses” with potentially different sensors integrated into a single microchip, all of them morphologically different and with complementary sensing characteristics. As a result of this activity, the interest to commercially exploit the new generation of microhotplates also grew with the emergence of new companies like AMS Sensors [53]. All in all, this optimization at the expense of putting together nanomaterials and MEMS has allowed reducing the power consumption up to tens of milliwatts in advanced

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devices [24]. Unfortunately, despite being a remarkable jump ahead, these values are still excessive for power-limited systems, if continued operation is required. Therefore, there is not a clear advantage in moving from standard inorganic thin films to nanomaterials, especially considering the problems related to the fabrication steps when we talk about nanodevices. Alternatively, nanotechnology enables the use alternative methods for conductometric-like sensing operation that feature zero power consumption [54–56]. These systems are mostly based on integrating a source of energy collection/harvesting together with the sensing element in a way that no further energy is needed to activate the gas-sensor interactions, providing even an energy surplus that can be used to read the sensor signal. However, their architectures are far from being simple, making the broad adoption difficult. In this context, research with inorganic nanomaterials has revealed an additional and unforeseen advantage when electrically driven. They can reach relatively high temperatures going through electrical tests (i.e., electrical resistance measurements), even with the small amounts of electrical power dissipated during the electrical probing [20, 21]. This so-called self-heating effect makes it possible to reduce the power consumption of nanoscale devices down to the microwatt regime. In the case of conductometric gas sensors, this is a factor 1000 lower than state-of-the-art microsensors mentioned earlier. From a fundamental point of view, the self-heating effect is just the consequence of the Joule dissipation of power at a very small scale. Since 1847, it has been well-known that the electrical power dissipated in a resistive component leads to a temperature increase of the same [57]. The self-heating effect is nothing else than this effect, brought to such small scale that the temperature increase is remarkable, even when only small amounts of electrical power are applied. Simple figures about the power dissipated per unit volume (i.e., the power density) can help to realize the dramatic differences when the device scale is reduced. As a matter of fact, one nanowire in self-heating operation can easily reach power densities 10,000 times larger than a conventional domestic heater (assuming 1 μW in a (100 nm)2
10 μm nanowire and 1 kW in a (1 mm)2
1-m heater filament). The reason why self-heating is so efficient relays on the large power densities directly dissipated in the material that needs to be heated (i.e., a gas-sensitive nanowire). This is a huge difference with conventional device architectures based on an independent heating element that warms up the sensor material. Also, the heater needs to be electrically insulated from the sensor material, which adds even more material to heat. Therefore, the advantage of the self-heating approach is double: (i) removing the need for an external heating element and (ii) reduction of the power needs by eliminating the energy consumption associated with the external heater. A decade of research on self-heated devices has shown that this is a quite general phenomenon in nanowire-like structures with huge potential for efficient heating in miniaturized devices, which paves the way for many different applications in several fields of sensing and actuation.

Inorganic nanomaterials

It is evident that the self-heating effect in nanomaterial-based devices can damage or destroy them, similarly to well-known equivalent effects in microelectronic components (Fig. 2.4). That is the reason why the effect was first described and regarded as a threat or as an experimental challenge [20, 21]. In fact, suppliers of electronic instruments for research offer the know-how and the developed specific tools to avoid the problem (i.e., pulsed-operation modules) [23, 58–60]. In 2007, it was demonstrated for the first time that the self-heating effect could be controlled, not only with costly lab-class equipment but also with inexpensive commercial electronic components [24], providing an opportunity to use nanowire devices in consumer electronic products. In that work, self-heating was controlled and minimized in constant current operation. Today, we also know that a similar level of control can be achieved in constant voltage operation. In any case, it was shown that special care must always be

E-Beam 5.00 kv

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Fig. 2.4 SEM image of a tin oxide nanowire after a few hours under working conditions. The contact area (inset) is destroyed due to self-heating. Melting temperature of tin oxide is close to T ¼ 1100°C. (Reprinted with permission from F. Hernandez-Ramirez, A. Tarancon, O. Casals, E. Pellicer, J. Rodriguez, A. Romano-Rodriguez, J.R. Morante, S. Barth, S. Mathur, Electrical properties of individual tin oxide nanowires contacted to platinum electrodes, Phys. Rev. B 76 (2007) 085429. Copyright (2007) by the American Physical Society.)

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taken during device manipulation, connection, and start-up stages to avoid static discharge effects and power peaks, which may cause irreparable damage to the nanomaterials. One year later, works of Strelcov et al. [61] and Prades et al. [62] reported the first chemical gas sensors based on self-heating effect, both working with individual SnO2 nanowires. The former demonstrated that the bias conditions influenced the response to gases and suggested that it was related to the self-heating mechanism. The latter proved that different electrical biases lead to gas responses fully equivalent to those obtained at different temperatures controlled with an external heater, showing that it was a temperature-related effect practicable to be controlled. This second work also demonstrated that the electrical power required to reach thermal conditions suitable for sensing was remarkably small: already in the range of tens of microwatts to temperature increases of hundreds of Kelvin. The result was a significant step forward to validate the advantage of working with nanomaterials instead of traditional thin-film layers for gas-sensing purposes. From the onset of these pioneering works, major strides have been made: the first reports on self-heating in nanomaterials, basically nanowires, lead to the misconception that the benefits of this operating method (i.e., low power consumption, fast thermal response time, and no need of an additional heater [38]) could only be achieved in devices based on a single nanowire. Consequently, self-heating in nanowires became closely associated to nanofabrication and to the challenges of gaining electrical access to individual nanostructures. Later, Chinh et al. thoroughly investigated the self-heating effect in multiple wire systems [63]. Interestingly, they observed that self-heating also appeared in devices based on a few nanowires but being slightly less efficient. This kind of system is relatively simple to fabricate as it is based on the deposition of random wires followed by a metallization step with conventional lithography methods. Due to the random nature of the few nanowires involved, the dispersion in the electrical properties and in the self-heating behavior was very large, and to palliate this aspect, Guilera et al. reported the use of the alreadyexplained DEP methodology to orienting the wires between a pair of electrodes. In principle, this measure should contribute to increase the degree of order and reduce the dispersion between samples while keeping efficient the self-heating properties [64]. Recent works have finally shown that the self-heating effect can be also extended to devices based on large random networks of 1D nanostructures exhibiting relatively good efficiencies [65, 66], considering the simplicity of both the fabrication and operating methodologies (Fig. 2.5). Actually, these systems are extremely easy to produce (i.e., by drop casting of nanostructures dispersed in a solvent [65] or just by the direct growth of the 1D nanomaterials on the top of microelectrodes [66]) and exhibit good reproducibility. As a matter of fact, in contrast with small arrangements of a few nanowires, these networks contain thousands of wires, and they are large enough to offer a macroscopic average of the distribution and properties of the individual elements [67].

Inorganic nanomaterials

Self-heating 1.5 V Heater Rcorrected=R+Ro·a ·T

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Fig. 2.5 Resistance record of a carbon nanofiber (CNF) film operated with self-heating and with an external heater. The variations in temperature due to the resistance variations caused by the gas pulses were estimated to be in the order of 1°C. Therefore, the coupling between sensing and heating effects in self-heating operation was nearly negligible in this kind of material. (Reprinted from C. Fàbrega, O. Casals, F. Hernández-Ramírez, J.D. Prades, A review on efficient self-heating in nanowire sensors: prospects for very-low power devices, Sens. Actuators B: Chem. 256 (2018) 797–811. Copyright (2018), with permission from Elsevier.)

According to further investigations, the reason why the temperature of large networks of nanowires can be elevated with just a few milliwatts [66, 68] is that heat dissipation concentrates in certain regions of the network: the so-called hot spots. In fact, the most effective regions for this effect seem to be the most resistive segments of the entire structure. In other words, the use of 1D nanostructures leads to mesh-like structures that define current paths in which efficient heating is locally possible (Fig. 2.6). Therefore, despite the macroscopic dimensions of the system, efficiency values fully comparable with those obtained in difficult-to-fabricate one-nanowire systems are possible and, above all, easy to obtain and operate. From the point of view of the sensing signal, these works have also demonstrated that the hot spots are the ones that generate most of the sensor output, leading to a consistent control of the sensor temperature, precisely in the points relevant for sensing.

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Fig. 2.6 (A) Experimental observations (thermal micrographs) of hot spots in random networks of nanofibers (CNF) operated (left) in self-heating mode and (right) with an external heater. Clearly, the temperature pattern in self-heating mode concentrates in some central regions of the nanowire network. This is also seen as disperse high-temperature spikes in the blue histogram of the micrographs earlier, corresponding to the self-heating operation. (B) 3D model of the simulated temperature profile in a random network of nanowires (d ¼ 60 nm, σ ¼ 100 S m1, and κ ¼ 1 W m1 K1) operated at 6.8 μW. The nanowire portions in direct contact with the substrate remain cooler than those located further away from the substrate. (Reprinted from C. Fàbrega, O. Casals, F. Hernández-Ramírez, J.D. Prades, A review on efficient self-heating in nanowire sensors: prospects for very-low power devices, Sens. Actuators B: Chem. 256 (2018) 797–811. Copyright (2018), with permission from Elsevier.)

Inorganic nanomaterials

Be that as it may, these latter multinanowire devices are an important first step in pursuit of cost-effective gas sensors based on inorganic nanomaterials, which combines a simple fabrication step and cost-effective operation leading to ultralow power needs. Despite how to optimize the performance of these sensors is still under research and needs to be systematized [38], self-heated bundle nanowire sensors are among the most promising alternatives to attain at the mid-term industrial gas detectors based on nanomaterials only. Anyhow, to build up a real sensor system, other components in demand of power must be considered. Typically, at least processing and communication units are always needed [69]. Even in the most austere configurations (e.g., with aggressive duty cycling), these components demand at least a few tens of microwatt [70]. Therefore, from a fullsystem perspective, the advantages of using nanomaterials instead of thin films for gassensing purposes should always be analyzed from the proper perspective, and self-heating is just one optimization route of the new generation of gas sensors relying on inorganic nanomaterials.

2.5 Conclusions Inorganic nanomaterials are among the forerunners for the development of new devices and in particular gas nanosensors. After more than two decades of active research in this field, the acquired knowledge about the fundamentals of the sensing mechanisms in 1D nanomaterials has exponentially grown, showing that these devices have clear advantages compared with their microcounterparts. The fabrication processes and running conditions of most of them are unfortunately complex, demanding in some cases highly skilled operators and controlled environments, which may incur elevated costs of the final systems. This undoubtedly results in a major drawback that hampers the transition of the new technologies from the lab to the real world. When it comes down to it, the gas sensor industry is a mature sector quite reluctant to accept any innovative concept that involves the loss of reproducibility and the rising of costs. For this reason, the search of new strategies to increase the production yield and simplify the operating conditions has become a high priority since the advent of the first gas nanosensors based on semiconductor nanomaterials a few years ago. In this context, two parallel strategies have steadily evolved with the aim to improve the throughput of the devices, making them affordable and ultimately simpler. The first one focuses on the fabrication stage of the devices and seeks to obtain them in an automated way to keep the production costs low. To that end, two different approaches have shown promising results up to now: (i) the in situ growth of 1D nanomaterials in welldefined locations of the sensor substrate, or alternatively (ii) the postgrowth positioning in well-defined geometry through self-assembly techniques. Regardless of the fact that both once have allowed obtaining quite promising results and the almost-automated fabrication of nanosensors has become a living reality, these new technologies still remain in their infancy, and a huge optimization effort is pending

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before the industry adoption. Actually, once the conceptual stage of these new technologies has been successfully completed with quite interesting proof-of-concept devices, it is time to carry out more systematic studies to attain transparent methodologies independent of individual cases that could lead to real cost reductions in the coming years. On the other hand, the attractiveness of gas sensors relying on semiconductor nanomaterials is significantly constrained by the demanding requirements of the running conditions. The first devices needed well-controlled experimental parameters difficult to transfer to real applications, but surprisingly, the use of the well-known Joule effect has opened an extremely interesting path toward the reduction of energy needs and the configuration specifications of these devices. In fact, self-heating in nanoscale materials becomes extremely important at low-bias conditions with dramatic influence on their final electrical behavior. It has been demonstrated that gas response fully equivalent responses to those monitored with the help of an external heater are monitored with devices excited only through this phenomenon, while the last results have successfully validated that even very easy-to-fabricate device geometries containing messy nanomaterials are suitable to operate in such conditions. This is a major step forward that allows combining nanodevices with consumer-class electronics keeping the power consumption extremely low without risk of damaging the sensing system. All in all, the self-heating effect has revealed itself as a direct route toward the cost drop of sensor operation. As discussed in this chapter, the different techniques presented herein are the first tangible results toward the systemization of gas sensors based on nanomaterials. From a practical point of view, they should not be examined individually. Once they are mature enough, it is conceivable that they will be combined to attain the expected cost reduction and the simplification level.

Acknowledgments This work has been partially supported by the Spanish Ministerio de Economı´a y Competitividad, through project TEC2013-48147-C6 (AEI/FEDER, European Union), and by the European Research Council, under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 336917. J.D. Prades acknowledges the support of the Serra Hu´nter Program.

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[50] F. Udrea, J.W. Gardner, D. Setiadi, J.A. Covington, T. Dogaru, C.C. Lu, W.I. Milne, Design and simulations of SOI CMOS micro-hotplate gas sensors, Sens. Actuators B: Chem. 78 (1–3) (2001) 180–190. [51] S.Z. Ali, S. Santra, I. Haneef, C. Schwandt, R.V. Kumar, W.I. Milne, F. Udrea, P.K. Guha, J.A. Covington, J.W. Gardner, Nanowire hydrogen gas sensor employing CMOS micro-hotplate, in: IEEE Sensors 2009 Conference, 2009. [52] Z. Dario, B. Angela, C. Elisabetta, H. Martin, P. Nicola, S. Giorgio, Tungsten oxide nanowires on micro hotplates for gas sensing applications, Procedia Eng. 120 (2015) 439–442. [53] AMS Sensors, http://www.ccmoss.com/eng. [54] M.W.G. Hoffmann, A.E. Gad, J.D. Prades, F. Hernandez-Ramirez, R. Fiz, H. Shen, S. Mathur, Solar diode sensor: sensing mechanism and applications, Nano Energy 2 (2013) 514–522. [55] M.W.G. Hoffmann, L. Mayrhofer, O. Casals, L. Caccamo, F. Hernandez-Ramirez, G. Lilienkamp, W. Daum, M. Moseler, A. Waag, H. Shen, J.D. Prades, A highly selective and self-powered gas sensor via organic surface functionalization of p-Si/n-ZnO diodes, Adv. Mater. 26 (2014) 8017–8022. [56] A. Gad, M.W.G. Hoffmann, O. Casals, L. Mayrhofer, C. Fa`brega, L. Caccamo, F. Herna´ndezRamı´rez, M.S. Mohajerani, M. Moseler, H. Shen, A. Waag, J.D. Prades, Integrated strategy toward self-powering and selectivity tuning of semiconductor gas sensors, ACS Sensors 1 (2016) 1256–1264. [57] J.P. Joule, On the effects of magnetism upon the dimensions of iron and steel bars, London Edinburgh Dublin Philos. Mag. J. Sci. 78–87 (1847) 225–241. [58] M. Meyyappan, Nanotechnology Measurement Handbook: A Guide to Electrical Measurements for Nanoscience Applications, first ed., Keythley Instruments, 2007. [59] J.D. Prades, F. Herna´ndez-Ramı´rez, T. Fischer, M. Hoffmann, R. M€ uller, N. Lo´pez, S. Mathur, J.R. Morante, Quantitative analysis of CO-humidity gas mixtures with self-heated nanowires operated in pulsed mode, Appl. Phys. Lett. 97 (2010) 243105. [60] O. Monereo, O. Casals, J.D. Prades, A. Cirera, Self-heating in pulsed mode for signal quality improvement: application to carbon nanostructures-based sensors, Sens. Actuators B: Chem. 226 (2016) 254–265. [61] E. Strelcov, S. Dmitriev, B. Button, J. Cothren, V. Sysoev, A. Kolmakov, Evidence of the self-heating effect on surface reactivity and gas sensing of metal oxide nanowire chemiresistors, Nanotechnology 19 (2008) 355502. [62] J.D. Prades, R. Jimenez-Diaz, F. Hernandez-Ramirez, S. Barth, A. Cirera, A. RomanoRodriguez, S. Mathur, J.R. Morante, Ultralow power consumption gas sensors based on self-heated individual nanowires, Appl. Phys. Lett. 93 (2008) 123110. [63] N.D. Chinh, N. Van Toan, V. Van Quang, N. Van Duy, N.D. Hoa, N. Van Hieu, Comparative NO2 gas-sensing performance of the self-heated individual, multiple and networked SnO2 nanowire sensors fabricated by a simple process, Sens. Actuators B: Chem. 201 (2014) 7–12. [64] J. Guilera, C. Fa`brega, O. Casals, F. Herna´ndez-Ramı´rez, S. Wang, S. Mathur, F. Udrea, A. De Luca, S.Z. Ali, A. Romano-Rodrı´guez, J.D. Prades, J.R. Morante, Facile integration of ordered nanowires in functional devices, Sens. Actuators B: Chem. 221 (2015) 104–112. [65] O. Monereo, J.D. Prades, A. Cirera, Self-heating effects in large arrangements of randomly oriented carbon nanofibers: application to gas sensors, Sens. Actuators B: Chem. 211 (2015) 489–497. [66] H.M. Tan, C. Manh Hung, T.M. Ngoc, H. Nguyen, N. Duc Hoa, N. Van Duy, N. Van Hieu, Novel self-heated gas sensors using on-chip networked nanowires with ultralow power consumption, ACS Appl. Mater. Interfaces 9 (2017) 6153–6162. [67] O. Monereo, O. Casals, J.D. Prades, A. Cirera, A low-cost approach to low-power gas sensors based on self-heating effects in large arrays of nanostructures, Procedia Eng. 120 (2015) 787–790. [68] O. Monereo, S. Illera, A. Varea, M. Schmidt, T. Sauerwald, A. Sch€ utze, A. Cirera, J.D. Prades, Localized self-heating in large arrays of 1D nanostructures, Nanoscale 8 (2016) 5082–5088. [69] M. Belleville, H. Fanet, P. Fiorini, P. Nicole, M.J.M. Pelgrom, C. Piguet, R. Hahn, C. Van Hoof, R. Vullers, M. Tartagni, E. Cantatore, Energy autonomous sensor systems: towards a ubiquitous sensor technology, Microelectron. J. 41 (2010) 740–745. [70] B. Martinez, M. Monton, I. Vilajosana, J.D. Prades, The power of models: modeling power consumption for IoT devices, IEEE Sens. J. 15 (2015) 5777–5789.

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CHAPTER 3

Molecular materials for gas sensors and sensor arrays☆
Antonio de Sajaa Mariluz Rodriguez-Mendeza,b, Jose a

Group UVaSens, Engineers School, University of Valladolid, Valladolid, Spain BioecoUVA Institute, Engineers School, University of Valladolid, Valladolid, Spain

b

3.1 Introduction An electronic nose (e-nose) is a multisensor system, which consists of an array of lowselective sensors combined with advanced mathematical procedures for signal processing based on pattern recognition and/or multivariate data analysis [1–6]. In all cases, the sensing elements have partial specificity. So, they respond to a range of compounds, rather than to a specific chemical species. The sensors are at the hearth of the e-noses. This is the reason why many efforts have been dedicated to the development of sensors with improved specifications mainly in terms of selectivity, reproducibility, and lifetime. A gas sensor is formed by two main parts: The main element is a sensing material that reacts with the volatile molecules causing a change in a certain property; the second part is a transducer, which detects those changes and transforms them in an electronic signal. The nature of the sensing layer is responsible of the selectivity and sensitivity of the sensors, and a large variety of sensing materials have been used in e-noses. The most commonly used sensing materials are catalytic metals and metal oxide semiconductors. However, organic thin layers have also attracted considerable attention as sensing materials because the interaction between some reactive gases and organic thin layers can cause variations in the physical properties of the reactive sensing layers. Organic materials have the advantage of their versatility, and many different families of organic materials such as polymers, porphyrins, or phthalocyanines can be used to obtain sensing layers. In addition, their reactivity can be tuned by modifying chemically their structures or by doping the sensing layers with a variety of materials. An additional advantage is that organic layers can be deposited by simple methods such as drop casting, spraying, spin coating, and printing or by more sophisticated methods such as self-assembling, layer by layer, or Langmuir-Blodgett. Each technique produces films with different structures or porosity, and these differences can also be used ☆

This chapter was written in the memory of Prof. Jos
e Antonio de Saja who passed away in November 2017. He left an immense legacy.

Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00003-7

Copyright © 2020 Elsevier Inc. All rights reserved.

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to modulate the responses. All these techniques can be applied to deposit layers on different types of substrates (conductive, transparent, piezoelectric, flexible, etc.) and used to develop sensors based on different transduction principles [7]. In most of the cases, measures are carried out at room temperature, and organic sensors can be classified as cold sensors. As mentioned before, the transducer transforms the physical changes occurring in the sensing material into an electronic signal. Transduction mechanisms include measures of resistance, mass, and optical properties. In the next paragraphs, the organic materials used in different types of sensors employed in e-noses will be revised.

3.2 Resistive sensors Resistive sensors are usually metal oxide semiconductors (MOS), which are based on inorganic materials. Tin oxide sensors doped with Pt or Pd are the most commonly used resistive sensors in e-noses. In spite of their advantages, their selectivity is low, and they operate at high temperatures. Many organic materials are well known for their conductive properties. A part from the versatility and the flexibility, they have the advantage of being conductors or semiconductors at room temperature. For this reason, the search of molecular semiconducting materials suitable for being used in e-noses is very active. Among the most interesting organic materials exploited in e-noses, we will mention conducting polymers, phthalocyanines and porphyrins, molecular imprinted polymers (MIPs), nanotubes, and their composites. These materials are deposited as thin films on the surface of interdigitated electrodes, and the changes in the resistivity when exposed to gases are measured at room temperature.

3.2.1 Polymers The first polymeric sensors were obtained by dispersing conducting nanofillers (metal or carbon) into insulating polymers. These sensors are called conductive polymer composites (CPC) and have demonstrated their effectiveness for vapor sensing. Arrays of sensing elements are prepared from inexpensive, commercially available polymers, such as polystyrene, polysulfone, polyvinylbutyl, polycaprolactone, polyvinylacetate, polyethyleneimine, and polymethylmethacrylate. Each CPC sensor has a different response depending on the partition coefficient of the analyte. Some examples of this approach can be found in the literature [8–11]. In the last years, nanomaterials such as carbon nanotubes (CNT) or graphene have been incorporated in polymeric nanocomposites with excellent results [12]. Conducting polymers are among the most popular molecular materials used in e-noses due to their unique conducting properties. The changes in the electrical conductance upon exposure to volatile compounds are the basis of their use in e-noses, and many

Molecular materials for gas sensors and sensor arrays

different instruments (including commercial devices) have been developed based on these versatile materials [13]. Conducting polymers are heterocyclic compounds with alternating single and double bonds along the backbone (polypyrrole, polyaniline, poly 3-methyl thiophene, PEDOT, POSS, etc.). They can be n- or p-doped, and this doping generates charge carriers and alters the band structure, thus increasing the mobility. Different types of counterions can be used as dopant agents to obtain polymeric films with diverse physicochemical properties such as conductive or redox properties. In addition, they can be deposited as thin films onto interdigitated electrodes using inkjet deposition, electrodeposition, or electrospinning (among many others) giving rise to films with different structures, hydrophobicity, thickness, and roughness. The electrochemical techniques are very popular to obtain conducting polymers because monomers can be polymerized by applying a certain voltage or a certain current by means of chronoamperometry, chronopotentiometry, or cyclic voltammetry. The experimental conditions used in electrodeposition (time, voltage, and current intensity) can produce films with different porosity and sensitivity. Finally, a variety of doping agents can be introduced in the conducting polymer films during the electropolymerization process [14]. Inject printing is also widely used due to the simplicity, but it has to be taken into account that various factors can affect the quality of the films (materials, substrate treatments, viscosity, etc.) [15]. Other methods such as electrospinning can produce nanofibers with high surface-to-volume ratio [16]. The variety of monomers, counterions, and deposition techniques facilitate to obtain polymeric sensors with cross sensitivity. This versatility has been used to develop e-noses formed by an array of different gas sensors that work at room temperature. They have two disadvantages: first, that the chemoresistive response is strongly influenced by the temperature [17] and, second, their strong response to humidity. In the presence of water vapors, polymeric layers suffer swelling and the consequent change in resistivity. E-noses based on conducting polymers must take into account these facts. Conducting polymer-based e-noses have been used to analyze complex odors. Many works have been devoted to the analysis of foods. For instance, a homemade electronic nose based on Ppy, PANI, and 3-MET deposited by electropolymerization was able to analyze the quality of olive oils [18] or to discriminate olive oils from different varieties of olives [19]. Discrimination of wines using an array of conducting polymer sensors requires the use of solid-phase microextraction (SPME) techniques to eliminate water and alcohol [20]. Polymeric e-noses have been used for many other applications such as the detection of off-odors in automobiles [21]. Gas sensors can detect combustible, explosive, and toxic gases and have been widely used in safety monitoring and process control in residential buildings, industries and mines [11], etc. The great success of conducting polymers in e-noses leads to the development of commercial e-noses. A pioneer e-nose was AromaScan A32S, a conducting polymer electronic nose formed by a 32-sensor array, designed for general-use applications.

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Neotronics was developed by the University of Warwick and was based on 12 conducting polymers [22, 23]. Cyranose, based on conducting polymers, has been the most successful e-nose in the market. It has been used in a variety of medical applications such as the detection of respiratory diseases (lung cancer, asthma, etc.) or digestive diseases [24, 25]. Many works have been dedicated to quality and food control (incoming inspection and verification of bulk chemicals, confirmation of raw materials, ingredients, batch confirmation, contamination, organoleptic analysis, monitoring production, aging, etc.). Studies included fruit juices, wines, and honeys [26–28]. A detailed list of papers published using Cyranose e-nose can be found in [29]. Sensor array platforms based on cross-reactive conducting polymeric sensors can be improved using nanostructured conducting polymers. For instance, a polyaniline polymer nanowire-based chemiresistive sensor array combined with a pattern recognition algorithm was applied for the simultaneous classification and quantification of three chemical species: ascorbic acid, dopamine, and hydrogen peroxide [30]. Similarly, an array of nano-/microstructured-conducting polypyrrole sensors prepared by means of




amperometry and formed by doping with ClO
4 , pTs , Cl , TCA , DS , and DBS was able to detect acetone, methanol, ethanol, 1-propanol, 2-propanol, nitromethane, propylamine, pyridine, and gas mixtures of aliphatic alcohols. Quantitative determinations of the composition of gas mixtures were also successfully achieved [31]. In a very interesting work, a bio-inspired nanofibrous artificial epithelium was combined with the e-nose principles. An array of nine microchemoresistors covered with electrospun nanofibrous structures was prepared by blending doped polyemeraldine (a form of polyaniline) with three different polymers (polyethylene oxide, polyvinylpyrrolidone, and polystyrene), which acted as carriers for the conducting polymer. Such e-nose included a plurality of nanofibers whose electrical parameters were dependent on the tested gases (NO2 and NH3) and on the spatial distribution of the electrospun fibers [32].

3.2.2 Phthalocyanines and porphyrins Phthalocyanines (Pc) and porphyrins (Ppy) are tetrapyrrolic compounds where an aromatic ring is coordinated with transition metals or rare earth metals. They are among the most important organic sensing materials due to their amazing semiconducting, optical, and redox properties. These properties are sensitive to the presence of gas molecules, and changes can be monitored by different transduction methods [33–36]. The possible application of Pc (and in lesser extent PPy) as the sensitive layers in resistive sensors is due to their pi-type semiconducting properties (10
10–10
12 S/cm at 300 K) [37]. Lanthanide bisphthalocyanines (LnPc2) are particularly interesting members of the family of Pcs because they show high intrinsic conductivities (10
6–10
3 S/cm at T ¼ 300 K) [38, 39]. The exposure to gaseous pollutants with strong electron-acceptor properties (O2, NOx, halogens, ozone, etc.) causes an increase in the phthalocyanine

Molecular materials for gas sensors and sensor arrays

conductivity, whereas electron-acceptor gases produce a decrease in the conductivity. Volatile organic compounds (VOCs) such as alcohols, aldehydes, and aromatic compounds, which do not possess a strong electron-donor or electron-acceptor character, can also be detected using phthalocyanines [40, 41]. These excellent sensing properties toward gases and VOCs can be tuned by changing the central metal ion or by introducing substituents in the aromatic ring. In addition, different processing methods can be used to obtain well-controlled structures and nanostructures (drop casting, spin coating, evaporation, or Langmuir-Blodgett among other techniques). Both approaches can help to improve the sensibility and reproducibility of the sensors [41–43]. Using these interesting and varied sensing properties, Pcs have been successfully employed in resistive e-noses dedicated to the analysis of wines [44] or olive oils [45, 46] among other applications.

3.2.3 CNT and graphene resistive sensors In the last years, carbon nanotubes (CNTs) or graphene have been introduced in the formulations of gas-sensing materials [47]. CNTs and graphene are interesting active materials for chemical sensors due to their high carrier mobility, their unique geometry, and their capability to adsorb gases. CNT and graphene solutions can be easily deposited by drop casting, spin coating, or printing producing uniform films with easy scalability [12]. These compounds have a high affinity toward a variety of gases that can be adsorbed at their surface. In addition, CNTs and graphene can establish electrostatic interactions with biomolecules (enzymes, DNA, antibodies, etc.) making sensitive and selective biosensors. As mentioned previously, CNTs or graphene can be combined with polymeric matrices such as poly(caprolactone) (PCL), poly(lactic acid) (PLA), poly(carbonate) (PC), poly(methyl methacrylate) (PMMA), and a biobased polyester to obtain CPC arrays of resistive sensors with enhanced properties. The electrical response of these chemical sensors can be explained by the behavior of polymer swelling upon penetration of volatiles or gases into the subsurface of the CNT/polymer film. These arrays of CNT-CPC transducers have been combined with classical pattern recognition methods, producing interesting properties for vapor recognition [48, 49]. For instance, an e-nose system based on polymer/carboxylic-functionalized single-walled carbon nanotubes (SWNT-COOH) was developed and used to detect volatile amines and sun-dried fish odors. Polymers included polyvinyl chloride (PVC), cumene terminated polystyrene-co-maleic anhydride (cumene-PSMA), poly(styrenecomaleic acid) partial isobutyl/methyl mixed ester (PSE), and polyvinylpyrrolidon (PVP) [50]. An additional advantage of CPC sensors is that they can be deposited on flexible substrates or in fabrics. This has allowed developing an innovative wearable electronic nose based on CNT-polymer composites integrated in a Sigsbee wireless system that was able to analyze armpit odor [51, 52].

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CNTs have been combined with other matrices such as chitosan (CHI). These systems have great potential in the e-nose field because chitosan matrix is sensitive not only to polar vapors such as water and methanol (difficult to obtain with synthetic polymers being mostly nonpolar) but also to nonpolar ones like toluene [53]. E-noses based on carbon nanostructures are treated in depth in Chapter 4.

3.2.4 Combinations of materials in the same layer One of the most fruitfully strategies in the development of sensors with improved characteristics is to combine to more sensing materials in the same layer. In previous sections, we have already presented the idea of introducing nonconducting materials in a layer of a conductive material that can form easily layers on interdigitated electrodes. Examples of conjugated polymer composite (CPC) were presented and mentioned as combinations of CNTs with nonconducting polymers. In other works, it has been established that the association of two conducting materials can induce new sensing properties due to synergistic effects [54]. This strategy has been widely used to develop new improved sensors, and in some cases, these sensors have been included in e-noses. Some examples are shown in next paragraphs. Selectivity and sensitivity in gas detection can be enhanced by using CNT or graphene-based hybrids, where nanocarbons are functionalized covalently or noncovalently with other conducting materials such as conducting polymers, phthalocyanines, or metal nanoparticles to improve the sensitivity or selectivity for a specific analyte. For instance, CNTs have been easily functionalized with different oligomeric silsesquioxanes (POSS) and used to detect lung cancer VOC biomarkers [55]. CNTs can also be functionalized by means of noncovalent interactions, often through π-π interactions with phthalocyanines and porphyrins. These weak interactions allow for facile functionalization with minimal reduction of the CNT conductivity that usually accompanies covalent functionalization. A chemoresistive sensor array fabricated from SWCNTs noncovalently functionalized with metalloporphyrins combined with statistical analyses could accurately classify VOCs [12]. Graphene-based hybrids with noble metals, metal oxides, and conducting polymers have been widely investigated as chemoresistive gas sensors with high sensitivity and selectivity. These systems have not been used in multisensor systems yet, but they have a promising future in e-noses [56]. Reduced graphene oxide (rGO) is an interesting platform for highly sensitive gas sensors. However, the poor selectivity of rGO-based gas sensors remains a major problem. One attempt of developing e-noses using an array of reduced graphene oxide (rGO)-based integrated sensors has been published recently. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale [57]. Hybrids of silver nanoparticle-decorated reduced graphene oxide

Molecular materials for gas sensors and sensor arrays

(Ag-RGO) have been prepared with the use of poly(ionic liquid) (PIL) as a versatile capping agent to develop volatile organic compound (VOC) sensors. These results are promising to design e-noses based on high stability chemoresistive sensors for emerging applications such as anticipated diagnostic of food degradation or diseases by the analysis of VOCs considered as biomarkers [58]. Composite sensors for e-nose applications have also been described consisting on combinations of other conductive sensing materials. Conducting polymers doped with different porphyrin derivatives led to a huge variation in response to VOCs [59]. Tobacco types and cigarette brands were discriminated using an e-nose formed by only three sensors based on a novel derivative of thiophene conducting polymer doped with different porphyrins [60].

3.3 Field effect transistors (FET) FET-based sensors have been used for the fabrication of cross-reactive sensor arrays. There are many different FET sensor structures and sensing materials that could result in a myriad of different sensor system combinations [61]. The most common FET sensor arrays are formed by MOSFET sensors using catalytic metal oxide materials [62] other inorganic materials, and nanomaterials such as metal or silicon nanowires are also quite common [63]. Organic field-effect transistors (OFETs) are the focus of increasing attention in organic electronics. This interest is stimulated by the qualities of organic semiconductors including the variety of molecular structures and functionalizations, morphology of the sensing layers, and solution processability. Organic thin-film transistor (OTFT) are a subset of OFETs fabricated using thin films as the sensing layer that is deposited by techniques such as layer by layer. The discrimination capability of OFET (or OTFT) gas sensors can be improved by combining a number of transistors and/or measurement variables in an array. The combinatorial responses of the whole array provide a unique fingerprint pattern to discriminate analytes [64]. Advanced systems for gas discrimination based on OFETs sensors arrays have been described [65]. Recently, OFET transistors using different polycyclic aromatic hydrocarbon (PAH) derivatives as the semiconductor layers were combined to form OFET gas sensor arrays. Individual OFET sensors displayed unique responses to volatile organic compounds (VOCs) of different polarity and aromaticity in terms of a couple of variables such as IDS, μ, VTh, and Ion/Ioff. Thus, the combination of sensors generated a fingerprint for each analyte [66]. A ChemFET sensor array that used three different polymer composite films—poly(ethylene-co-vinyl acetate), poly(styrene-co-butadiene), and poly(9-vinylcarbazole) each

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mixed with a 20% carbon black loading by weight—sprayed onto the device for the detection of organic vapors was used to detect organic vapors [67]. Using polytriarylamines as the active layer in OFETSs could work as effective vapor sensors with high sensitivity and specificity toward acetone, DMMP, methanol, and propan-1-ol [68]. A back-gated OTFT array using pentacene, poly(3-hexylthiophene) (P3HT), and poly(3-octylthiophene) (P3OT) as the active layer was able to discriminate between milk and water [69]. Arrays based on nanostructured polythiophene OTFTs [70] and on polythiophene films with different side chain length and film thickness [71] have been described. Other materials other than polymers have been used in OTFTs; the best examples are phthalocyanines whose charge transport properties make them suitable for use in organic thin-film transistors (OTFTs). Efforts have been made to modulate the MPc solubility and the surface structure, to obtain self-assembled films [72]. OFET arrays can also be constructed using different semiconductors. For instance, an OFET array was constructed by combining phthalocyanines (which are p-type semiconductors) and naphthalene diimides (which are n-type semiconductors) [73]. As expected, both types of semiconductors exhibited opposite responses toward gases. These multi-OFET-multiparameter-based arrays are promising, but to construct such arrays is complicated. A different strategy to develop e-noses is to use the multiple parameters of the transistors, to develop a multiparametric-but-single-OFET device. This approach has been presented as a proof of concept [74]. In summary, OFET and OTFT sensory arrays have the advantage of the inherent multiparametric feature of the transistors and also by the variety of organic semiconductors. However, multidimensional data are difficult to analyze. Moreover, it is technically difficult to integrate an array of different OFET sensors in the same sensory panel. Indeed, efforts need to be made to combine ad hoc-designed OFET arrays and software able to analyze the data.

3.4 Mass sensors Mass gas sensors measure the variation in mass of a thin film of a sensing material when exposed to volatile molecules. The most common gravimetric e-noses are based on microbalances that consist of piezoelectric crystals (usually quartz) coated with sensing materials that can absorb or adsorb analytes. They belong to the category of bulk acoustic wave (BAW) devices where the wave propagates through the substrate. In quartz crystal microbalances (QCM), the changes in their resonant frequency are recorded during exposure to a certain gas. As analytes adsorb to the sensing layer, the added mass reduces the resonant frequency. A second approach uses arrays of microcantilevers, which are similar to those used in atomic force microscopes, covered with a sensing layer. The system registers changes in oscillations of the microelectromechanical (MEM) devices as a

Molecular materials for gas sensors and sensor arrays

measure of the gas adsorption. The third approach is to work with arrays of surface acoustic wave sensors (SAW). They are composed of a piezoelectric substrate with an input (transmitting) and output (receiving) interdigital transducer deposited on top of the substrate. The sensitive membrane is placed between the transducers, and an AC signal is applied across the input transducer generating an acoustic two-dimensional wave that propagates along the surface of the crystal [75]. The number of sensitive organic materials used in e-noses based on gravimetric sensors is very large.

3.4.1 Polymeric absorbing materials Polymers are widely used as sensing materials because they can absorb vapors producing a swelling effect. Polymeric materials with different polarity include polydimethylsiloxane (PDMS), CarboWax (CW), divinyl benzene (DVB), or their combinations (Car/PDMS or DVB/Car/PDMS). A number of siloxane polymers or polythiophene (PHT) derivatives have also been tested in mass sensors. Polymers have been used in an array of QCM polymeric sensors covered with the regioregular poly (3-hexyl thiophene) (rr-P3HT), which was used to detect VOCs evolved from food spoiled with Salmonella typhimurium [76]. Cantilever arrays have also been successfully used in e-noses. For instance, MEMS coated with polymers could be used to monitor spoilage of fishes due to the modification of the cantilever parameters (mass, stiffness, and surface stress). The number of polymers available is too high, and in this work, a minimal set of polymers from a large list of prospective polymers was selected by means of fuzzy subtractive and fuzzy c-means clustering (FSC and FCM) methods [77]. E-noses based on SAW sensors are quite popular and have been used in a variety of applications such as the detection of explosives and analysis of foods [78, 79]. In an interesting work, Fuzzy c-means clustering algorithm was used to obtain an optimal set of polymers [80]. An array of SAW devices coated with commercially available polymers (PDMS, Car/PDMS, or DVB/Car/PDMS) (SAW) was combined with solid-phase microextraction (SPME). This combination has demonstrated to be a very promising strategy for highly sensitive and selective gas detection in the field of food quality control. Using this system, differentiation between apple varieties or ripe and unripe pineapple was achieved [81].

3.4.2 Molecular imprinted polymers (MIPs) Molecular imprinted polymers (MIP) are one of the most promising recognition materials for e-noses using mass sensors. MIPs are highly selective receptors with specific binding sites for a molecule. They are prepared by cross-linking polymers in the presence of the molecule that is used as a template. In this way, the polymeric 3-D cavities are

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complementary to the size and shape of the target analyte. MIPs also provide interaction points around the template molecule. Despite the interest of these compounds, MIPs have some limitations—mainly related to manufacture—that need to be overcome before they spread in analytical market [82]. MIP-QCM sensor arrays have been applied to a large variety of analytical problems such as sensing terpenes in fresh and dried herbs [83] and monitoring composting processes [84].

3.4.3 Mass sensors based on porphyrins and phthalocyanines Porphyrins and phthalocyanines have been successfully introduced as coating materials in mass sensors. This is due to their flexible synthesis and ability to interact with a large number of organic vapors. These interactions are related to the coordination capabilities of the central metal ion and to the establishment of π interactions between aromatic rings. The main feature of such sensors is the dependence of the sensing properties (in terms of selectivity and sensitivity) on the nature of the central metal and on the peripheral substituents. Efforts have been carried out to exploit the chemical properties of phthalocyanine or porphyrin films to develop QCM e-noses [85]. Sensitivity and selectivity can be modulated by the nature of the Pc or Ppy (metal ion and peripheral substituents) and by the physical properties of the sensitive films (structure, morphology, porosity, or thickness) [86]. A QCM e-nose based on porphyrins was developed by the University of Rome Tor Vergata. LibraNose 2.1 sensor array consists of eight 20-MHz AT-cut quartz crystal microbalance sensors coated with either metalloporphyrines or polypyrrole polymer films. This e-nose has been used in many different applications such as in food analysis including musts from off-vine dried grapes [87], chocolate [88], or strawberry flavors [89] and in biomedical applications including diagnosis of cancer [90]. Phthalocyanines have also proven to be promising recognition elements in QCMbased sensor arrays due to properties afforded by this class of tunable materials [91]. In spite of their interesting properties, they have been studied in lesser extent as QCM elements than porphyrins. In an interesting work, the anionic sulfonate copper phthalocyanine was combined with different cations to obtain different sensors that were used to discriminate VOCs [92].

3.4.4 Alkanethiol self-assembled monolayers Self-assembled monolayers (SAM) are promising materials for thin-film-based sensors. Alkanethiol-based SAMs provide reproducible and ordered thin films to support a range of chemical tail groups. The affinities and kinetics of VOC adsorption on diverse functionalized films have been analyzed using BAWs [93]. An innovative e-nose was developed by modifying microcantilevers with SAMs of 4-mercaptobenzoic acid (4-MBA), 6-mercaptonicotonic acid (6-MNA), and 2-mercaptonicotonic acid (2-MNA) that was able to detect explosives [94]. One of the main interest of SAMs modified with

Molecular materials for gas sensors and sensor arrays

different tail groups is that they are ideal supports to immobilize biologic probes (peptides, enzymes, DNA, etc.) [95].

3.4.5 Host-guest materials In the last years, many families of adsorbing materials have been introduced in gravimetric e-noses. The interactions between the sensing material and the gas are based in the principles of the host-guest chemistry, where a host molecule with well-defined cavities is immobilized on the device surface. In many cases, these host molecules are deposited in a polymeric uniform film. One interesting family of host molecules is cyclodextrins, which are barrel-shaped ring structures of glucose units with a hydrophobic cavity. Cyclodextrins can be formed by a different number of glucose units, and these units can be functionalized to tune both the size and the polarity of the cavity. Cyclodextrins mixed with polymers have been used as the sensing materials in SAW devices dedicated to the detection of explosives [96]. Calixarenes, cup-shaped cyclic oligomers, can be adequately functionalized to form effective SAMs on Au multiarrayed microcantilevers. Such multisensors can detect a variety of cations, and the binding properties can be modulated by chemically changing the number of units in the oligomer and the nature of the substituents [97]. In a recent work, a microfabricated sensor array has been developed in which each resonator was coated with different supramolecular monolayers including a calixarene, a porphyrin, a cyclodextrin, and a cucurbituril. Supramolecular monolayers fabricated by Langmuir-Blodgett techniques could be used as multiparameter fingerprint patterns for highly selective detection and discrimination of VOCs [98]. Metal-organic frameworks (MOFs) are interesting materials for use in e-noses due to their high surface areas, reproducibility, and tunability. An interesting example of an e-nose formed by SAW sensors covered with MOFs has been recently published [99]. Due to the number of MOFs to choose, it is a challenge to select the right combination of materials for any given sensing application.

3.5 Optical sensors The interaction between some ambient reactive compounds and organic thin layers can cause variations in the optical properties of the sensing materials. Optical gas sensors have several advantages such as the stability and reproducibility of the optical signals, the high signal-to-noise ratio, and the low energy consumption. Sensing materials used in optical e-noses must be optically active, and changes in absorbance, fluorescence, evanescent wave, etc. can be suitable methods to be applied in optical e-noses. One of the pioneer works in this field consisted in analyzing the changes in the optical properties of a surface coated with catalytic metals when exposed to gases. Then, a light pulse scanned the surface transforming the optical data into electrical signals. Next

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breakthrough was the development of a chemical sensor array formed by optical-fiber bundles. Each bundle was coated with fluorescent materials that reacted with gases, emitting light of different wavelengths. The pattern of colored circles could be processed to yield an “olfactory image” of the sample [100].

3.5.1 Porphyrins and phthalocyanines Metalloporphyrins (MP) and metallophthalocyanines (MPc) are one of the most attractive materials for optical detection. The UV-vis-NIR spectra of these compounds are characterized by well-defined peaks with high molar absorptivity in the Q band that appear in the 500–800-nm region [101]. The large changes that occur in these absorption spectra (particularly in the region of the Q band) during oxidation and reduction have been used to develop optical sensors. A simple UV-vis spectrophotometer can be easily modified to be the transducer for the optical e-nose [102]. The large variety of phthalocyanines and porphyrins derivatives is a clear advantage for this type of e-nose because compounds with different colors, reactivity, and color changes can be used to form the array. Moreover, the presence of substituents can change the solubility, and this expands the choice of methods to form films. The molecular structure of the films and the thickness also modulate the color of the devices, the position of the Q band, and the optical response. A pioneer work demonstrated the possibility of using the color changes that occur in porphyrins when exposed to gases in e-noses [103]. Since then, most of the works in optical e-noses were carried out using porphyrins and phthalocyanines as sensing elements. For instance, using a colorimetric array of sensors obtained by printing nine porphyrins and three pH indicators on silica-gel flat plate was used to discriminate Chinese green teas [104]. Similarly, an optical e-nose has been obtained using five sensors where sol-gel films containing phthalocyanines and porphyrins have been deposited by inkjet printing [105]. In a final example, sensors prepared with nanostructured Langmuir-Blodgett films of zinc porphyrin and zinc phthalocyanine deposited on quartz substrates have been used to form an optical e-nose used to discriminate VOCs [106].

3.6 Conclusions Organic materials are an interesting alternative for e-noses. There is a large variety of materials that can be used as sensing materials with different transduction units. The main results have been obtained with porphyrinoid materials (phthalocyanines and porphyrins), with polymers (adsorbing polymers, MIP, or conducting polymers) and with nanocarbons (CNT and graphene). It has been evidenced that mixtures of materials combined with the utilization of organic and inorganic analog in nanocomposites may allow for improvement of the

Molecular materials for gas sensors and sensor arrays

sensor performance due to synergetic/complementary effects. There is still a long way to improve the performance of sensors, including combination with biomaterials.

Acknowledgments Financial support by MINECO and FEDER (AGL2015-67482-R) and the Junta de Castilla y Leo´n-FEDER (VA-032U13) is gratefully acknowledged.

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[84] P.A. Lieberzeit, A. Rehman, B. Najafi, F.L. Dickert, Real-life application of a QCM-based e-nose: quantitative characterization of different plant-degradation processes, Anal. Bioanal. Chem. 391 (2008) 2897–2903. [85] C. Di Natale, R. Paolesse, A. D’Amico, Metalloporphyrins based artificial olfactory receptors, Sensors Actuators B Chem. 121 (2007) 238–246. [86] A. Kumar, J. Brunet, C. Varenne, A. Ndiaye, A. Pauly, Phthalocyanines based QCM sensors for aromatic hydrocarbons monitoring: role of metal atoms and substituents on response to toluene, Sensors Actuators B Chem. 230 (2016) 320–329. [87] C. Di Natale, R. Paolesse, A. Macagnano, A. Mantini, C. Goletti, A. D’amico, Characterization and design of porphyrins-based broad selectivity chemical sensors for electronic nose applications, Sensors Actuators B Chem. 52 (1998) 162–168. [88] D. Compagnone, M. Faieta, D. Pizzoni, C. DiNatale, R. Paolesse, T. Van Caelenberg, B. Beheydt, P. Pitti, Quartz crystal microbalance gas sensor arrays for the quality control of chocolate, Sensors Actuators B Chem. 207 (2015) 1114–1120. [89] D. Pizzoni, D. Compagnone, C. Di Natale, N. D’Alessandro, P. Pittia, Evaluation of aroma release of gummy candies added with strawberry flavours by gas-chromatography/mass-spectrometry and gas sensors arrays, J. Food Eng. 167 (2015) 77–86. [90] M. Bernabei, G. Pennazza, M. Santonico, C. Corsi, C. Roscioni, R. Paolesse, C. Di Natale, A. D’Amico, A preliminary study on the possibility to diagnose urinary tract cancers by an electronic nose, Sensors Actuators B Chem. 131 (2008) 1–4. [91] C. Garcia-Hernandez, C. Medina-Plaza, C. Garcia-Cabezon, F. Martin-Pedrosa, I. del Valle, J.A. de Saja, M.L. Rodrı´guez-M
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erez, Y. LiaIsia, M. Warner, Class specific discrimination of volatile organic compounds using a quartz crystal microbalance based multisensor array, Talanta 188 (2018) 423–428. [93] Y. Chang, N. Tang, H. Qu, J. Liu, D. Zhang, H. Zhang, W. Pang, X. Duan, Detection of volatile organic compounds by self-assembled monolayer coated sensor array with concentration-independent fingerprints, Sci. Rep. 6 (2016) 23970. [94] S.J. Patil, N. Duragkar, V.R. Rao, An ultra-sensitive piezoresistive polymer nano-composite microcantilever sensor electronic nose platform for explosive vapor detection, Sensors Actuators B Chem. 192 (2014) 444–451. [95] Y.K. Yoo, M.S. Chae, J.Y. Kang, T.S. Kim, K.S. Hwang, J.H. Lee, Anal. Chem. 84 (2012) 8240–8245. [96] X. Yang, X.X. Du, J. Shi, B. Swanson, Molecular recognition and self-assembled polymer films for vapor phase detection of explosives, Talanta 54 (2001) 439–445. [97] A.N. Alodhayb, M. Braim, L.Y. Beaulieu, G. Valluru, S. Rahman, A.K. Oraby, P.E. Georghiou, Metal ion binding properties of a bimodal triazolyl-functionalized calix[4]arene on a multi-array microcantilever system. Synthesis, fluorescence and DFT computation studies, RSC Adv. 6 (2016) 4387–4396. [98] Y. Lu, Y. Chang, N. Tang, H. Qu, J. Liu, W. Pang, H. Zhang, D. Zhang, X. Duan, Detection of volatile organic compounds using microfabricated resonator array functionalized with supramolecular monolayers, ACS Appl. Mater. Interfaces 7 (2015) 17893–17903. [99] J.A. Gustafson, C.E. Wilmer, Computational design of metal–organic framework arrays for gas sensing: influence of array size and composition on sensor performance, J. Phys. Chem. C 121 (2017) 6033–6038. [100] I. Lundstr€ om, Artificial noses: picture the smell, Nature 406 (2000) 682–683. [101] H. Isago, Optical spectra of phthalocyanines and related compounds, in: A Guide for Beginers, Springer, 2015. [102] T. Kerdcharoen, S. Kladsomboon, Optical chemical sensor and electronic nose based on porphyrin and Phthalocyanine, in: A. Tuantranont (Ed.), Applications of Nanomaterials in Sensors and Diagnostics, Springer Series on Chemical Sensors and Biosensors (Methods and Applications), vol 14, Springer, Berlin, Heidelberg, 2013.

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[103] N. Rakow, K. Suslick, A colorimetric sensor array for odour visualization, Nature 406 (2000) 2–5. [104] L. Li, S. Xie, F. Zhu, J. Ning, Q. Chen, Z. Zhang, Colorimetric sensor array-based artificial olfactory system for sensing Chinese green tea’s quality: a method of fabrication, Int. J. Food Prop. 20 (2017) 1762–1773. [105] J.P. Mensing, A. Wisitsoraat, A. Tuantranont, T. Kerdcharoen, Inkjet-printed sol–gel films containing metal phthalocyanines/porphyrins for opto-electronic nose applications, Sensors Actuators B Chem. 176 (2013) 428–436. [106] W. Yang, J. Xu, Y. Mao, Y. Yang, Y. Jiang, Detection of volatile organic compounds using LangmuirBlodgett films of zinc-porphyrin and zinc-phthalocyanine, J. Synth. React. Inorg. Metal-Org. NanoMetal Chem. 46 (2016) 735–740.

CHAPTER 4

Carbon nanomaterials Eduard Llobet MINOS-EMaS, Universitat Rovira i Virgili, Tarragona, Spain

4.1 Introduction In the last years, carbon nanotubes have become by far the most studied carbon nanomaterial for developing gas sensors. However, graphene, graphene oxide, and reduced graphene oxides are challenging the dominance of carbon nanotubes in gas sensing. Yet, carbon nanomaterials are not limited to nanotubes and graphene. They also exist as nanoparticles, diamonds, fibers, cones, scrolls, whiskers, graphite polyhedral crystals, and nanoporous carbon. Part of the reason for the explosion of interest from the gas sensor community in carbon nanomaterials is that while ranging from well-defined nanosized molecules to tubes with lengths of hundreds of microns, they do not exhibit the instabilities of other nanomaterials as a result of the very high activation barriers to their structural rearrangement. As a consequence, they are highly stable even in their nonfunctionalized forms. Despite the wide range of carbon nanomaterials possible, they exhibit common reaction chemistry—that of organic chemistry. This chapter deals with the synthesis, functionalization, characterization, and discussion on the properties and performance of carbon nanomaterials for achieving inexpensive gas sensors. The discussion is centered on the use of carbon nanoparticles (carbon black), carbon nanofibers, carbon nanotubes, and graphene, because these clearly stand out as the most studied carbon nanomaterials for gas sensing. Especial emphasis is given to the use of scalable synthesis and integration methods. A discussion on the gas sensing fundamentals of the different nanomaterials considered is given as well. Finally, this chapter concludes by identifying what research efforts are needed for inexpensive gas sensors making use of carbon nanomaterials to reach the market.

4.2 Carbon black 4.2.1 Synthesis of carbon black Carbon black consists of pure elemental carbon in the form of colloidal particles that are produced by the incomplete combustion or thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions. The physical appearance of carbon black is that of a finely divided powder. Two carbon black manufacturing processes produce Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00004-9

Copyright © 2020 Elsevier Inc. All rights reserved.

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nearly all of the world’s carbon blacks. These are furnace black and thermal black, with the former method being the most commonly employed nowadays. The furnace black process uses heavy aromatic oils as feedstock. The production furnace uses a closed reactor to atomize the feedstock oil under carefully controlled temperature and pressure conditions. The primary feedstock is introduced into a hot gas stream, which is achieved by burning a secondary feedstock (e.g., natural gas or oil) where it vaporizes and then pyrolyzes in the vapor phase to form microscopic particles. In most furnace reactors, the reaction rate is controlled by steam or water sprays. The carbon black produced is conveyed through the reactor, cooled, and collected in bag filters in a continuous process. Residual gas, or tail gas, from a furnace reactor includes a variety of gases such as carbon monoxide and hydrogen. Most furnace carbon black plants use a portion of this residual gas to produce heat, steam, or electric power [1]. The thermal black process uses natural gas, either methane or heavy aromatic oils, as feedstock material. The process uses a pair of furnaces that alternate approximately every 5 min between preheating and production of carbon black. Natural gas is injected into the hot refractory-lined furnace, and in the absence of air, the heat from the refractory material decomposes the natural gas into carbon black and hydrogen. The aerosol material stream is quenched with water sprays and filtered in a bag. The exiting carbon black may be further processed to remove impurities, pelletized, screened, and then packaged for shipment. The hydrogen off-gas is burned in air to preheat the second furnace [1]. Carbon black is chemically and physically distinct from soot and black carbon, with most types containing greater than 97% elemental carbon arranged as grape-like cluster particulate matter. Carbon black is available with surface areas that are higher than 1000 m2/g, particle size lower than 50 nm, and density much lower than the theoretical value for graphite (2.25 g/cm3).

4.2.2 Gas sensing mechanism in carbon black gas sensors Sensor elements are constructed from films comprising carbon black particles dispersed into insulating organic polymer-coating interdigitated electrodes. The carbon black endows electrical conductivity to the films (carbon black nanoparticles are dispersed in the polymer matrix ensuring that some conducting paths exist). The different organic polymers in which carbon black may be dispersed are the source of chemical diversity between sensor elements. Swelling of the polymer upon exposure to a gas/vapor increases the resistance of the film (as some of the previously existing conducting paths are disrupted), thereby providing an extraordinarily simple means for monitoring the presence of gases [2–5]. The viscosity of the polymer-carbon black composite is adjusted using appropriate solvents, and the resulting paste is deposited on a flat substrate (e.g., via spin coating or screen printing followed by a drying step) with patterned interdigitated electrodes. The resistivity of the resulting carbon black-organic polymer composites versus carbon black content is well described by percolation theory [2,6]. At low carbon

Carbon nanomaterials

black loadings (i.e., below the percolation threshold), the composites are insulators because no connected pathway of conductive particles exists across the material. As the carbon black content is increased, a sharp transition occurs in which the resistivity of the composite can decrease dramatically (by up to 10 orders of magnitude) with a small variation in the carbon black concentration. At this transition point, designated as the percolation threshold, few connected pathways of carbon black particles are formed. An explanation of the differential resistance response of carbon black-polymer composite elements to gases is that swelling disrupts some of the existing conduction pathways, which results in an increased resistance of the composite film. Lewis and coworkers employed an array of carbon black sensor elements to detect a wide variety of vapors (of volatile organic compounds) in an identifiable way [7,8]. They employed a commercially available carbon black and different polymer compositions on each sensor element. The electrical resistance signals that were output from the 17-element array were further processed employing standard chemometrics. However, the sensitivity of such sensors was rather moderate (e.g., the limit of detection for benzene vapors was near a thousand of ppm). Carbon black-polymer composite sensors may suffer from baseline and response drift due both to the ageing of the polymer matrix and to rearrangement of carbon black particles within the polymer leading to changes in the percolation paths. These rearrangements progressively occur as a result of the many swelling/shrinking processes undergone by the polymer matrix upon repeated detection and recovery cycles. In that sense, the use of carbon nanofibers instead of carbon black particles has been suggested as a way to obtain more stable gas sensors employing the same detection principle.

4.3 Carbon nanofibers 4.3.1 Synthesis of carbon nanofibers Vapor-grown carbon nanofibers (VGCNF) are hollow nanosized carbon fibers formed at the surface of catalytic metal particles (Fe, Ni, and Co) in hot (900–1500°C) hydrocarbon gases [9,10]. An initial filament is formed composed of well-organized graphitic planes in a “stacked-cup” morphology with diameters ranging between 20 and 60 nm. These fibers can then be thickened further by the deposition of an outer chemical vapor deposition layer, consisting of undulating graphitic planes, lying parallel to the fiber surface, in which some sp3-hybridized carbons are also present [11]. Many fiber grades are available with diameters ranging from 60–70 to 200 nm or more and as-formed lengths from 1 to 100 mm. An alternative, low-cost process for the production of carbon nanofibers with various controlled structures at relatively high rates [12–14] is electrospinning. A polymer such as polyacrylonitrile (PAN) can be used as carbon source. The polymer is mixed and stirred with a suitable solvent with low boiling point and sufficiently high conductivity to obtain a polymer solution. The polymer solution is ejected from a syringe tip onto a rotating collector. A voltage (typically few kV) is applied between the syringe tip and the

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collector, which are kept a few cm away. Electrospun materials are stabilized via annealing for a few hours in air at temperatures ranging from 300°C to 400°C. Finally, a carbonization step is performed by calcinating the sample at temperatures ranging from 700°C to 1000°C in the absence of oxygen (e.g., in N2 or Ar). Employing this approach, carbon nanofibers of 40–400 nm in diameter and above 70 μm in length can be obtained [14–16], efficiently and at low cost.

4.3.2 Gas sensing mechanisms in carbon nanofibers Fu and coworkers [17] suggested the dispersion of carbon nanofibers within a polymer matrix as a way to overcome the instability experienced with carbon black-polymer composites. This instability occurs because nanosized carbon black particles tend to aggregate when the composite absorbs vapors, which lowers the matrix viscosity and increases its volume. In contrast, dispersing carbon nanofibers in the polymer results in improved vapor sensing stability, because high aspect ratio fibers are not prone to aggregation within their polymer matrix when vapors are absorbed and desorbed. Thus, the original electrical percolation pathways present in these composites are maintained more stably after absorbed vapors have desorbed from the matrix. Additionally, bare carbon nanofibers (i.e., not embedded in a polymer matrix) can be employed for developing gas sensors in a chemoresistor configuration. In this approach, mats of carbon nanofibers are deposited over interdigitated electrodes. In carbon nanofiber mats, the electrical conductivity is mobility limited by the potential barriers developing at the nanofiber-to-nanofiber junctions. The height of these potential barriers is modulated by the absorption of gases into the porous structure of nanofibers. The modification/tailoring of the porous structure of carbon nanofibers has been explored as an approach for increasing their sensitivity. Lee and coworkers have reported the chemical activation of electrospun carbon nanofibers by employing KOH solutions [18]. Electrospun fibers are first thermally treated to obtain carbon fibers, which is followed by the chemical activation step to improve the active sites for gas adsorption. The improved porous structure shows a 100-fold increased specific surface area. In chemically activated fibers, the amount of adsorbed gas is significantly increased, and gas response is improved accordingly. Fig. 4.1 illustrates this mechanism. The electrical resistance depends on the relation between adsorbed molecules and pores in activated carbons. The electrical change was transferred effectively by the electrical conductive network as shown in Fig. 4.1A. The diminished electrical resistance is attributed to the effects of electron localized CO or NO molecules compared with electron nonlocalized N2 gas as shown in Fig. 4.1C. Before exposing the gas sensing material to CO or NO, the electrical resistance of porous carbon nanofibers was stabilized in N2. N2 introduction increases the electrical resistance due to its stable electron configuration, but the introduction of easily electron polarized molecules such as CO and NO reduced the electrical resistance as compared with N2 gas. This result seems to support the occurrence

Carbon nanomaterials

(A)

(C)

Insulator effect

Electron flow F

Electrical conductive network

Pores

Target gas

Micro pore structure

(B) Electron hopping effect

Mesopore Electron flow

F≡ δ −

δ+ + δ− δ

Micropore

δ− δ−

*

: NO or CO δ− : N2

O≡N O−C C≡O

δ+

Fig. 4.1 Suggested mechanism of gas sensing: (A) suggested structural diagram of the prepared gas sensor in [18], (B) structure of micro- and mesopores, and (C) induced fluorine effects for high sensitivity of gas sensor. (Reproduced with permission from J.S. Im, S.C. Kang, S.H. Lee, Y.S. Lee, Improved gas sensing of electrospun carbon fiber based on pore structure, conductivity and surface modification, Carbon 48 (2010) 2573–2581. ©Elsevier, 2010.)

of electron hopping effects in pores. It can be derived that activated carbon nanofibers, especially with seriously destroyed graphite structure, can lose their semiconductor characteristics and then work as just efficient adsorbents for gas targets.

4.4 Carbon nanotubes 4.4.1 Synthesis of carbon nanotubes The allotropes of carbon include diamond in which carbon atoms are bonded together in a tetrahedral lattice arrangement; graphite where the carbon atoms are bonded in sheets of a hexagonal lattice, graphene (single sheets of graphite); and fullerenes in which the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations.

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Carbon nanotubes (CNTs) are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by rolled graphene sheets. These sheets are rolled at specific and discrete chiral angles. The combination of the rolling angle and radius determines whether the individual nanotube shell is metallic or semiconducting. CNTs are categorized as single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). Multiwalled nanotubes (MWNTs) consist of multiple rolled layers (concentric tubes) of graphene. Individual CNTs naturally align themselves into “ropes” held together by van der Waals forces, more specifically, pi-stacking. The chemical bonding of CNTs is composed entirely of sp2 bonds, similar to those of graphite. These bonds are stronger than the sp3 bonds found in alkanes and diamond and provide CNTs with their unique strength. Such strong bonds are characterized by a low chemical reactivity with their gaseous environment. Therefore, the functionalization of carbon nanotube sidewalls is necessary to improve both the sensitivity and the selectivity of CNT-based gas sensors [19]. CNTs have been synthesized employing different techniques such as arc discharge, laser ablation, or chemical vapor deposition. CNTs were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge [20]. During this process, the carbon contained in the negative electrode sublimates because of the high discharge temperatures. Because CNTs were initially discovered using this technique, it soon became the most widely used method of nanotube synthesis. The yield for this method is rather low (i.e., up to 30% by weight), and it produces both single- and multiwalled CNTs with lengths of up to 50 mm with few structural defects. In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor, while an inert gas is bled into the chamber. CNTs grow on the cooler surfaces of the reactor as the vaporized carbon condenses. This process was initially developed by Smalley and coworkers to create multiwalled CNTs [21]. Later on, the same team further refined their initial approach by employing composites of graphite and cobalt and nickel catalyst particles to synthesize single-walled CNTs [22]. The yield of the laser ablation method is about 70% and produces primarily single-walled CNTs with a controllable diameter determined by the reaction temperature. The catalytic vapor-phase deposition of CNTs was reported in 1993 [23]. During chemical vapor deposition (CVD), a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination of these [24]. CVD has become a common method for the commercial production of CNTs. The diameters of the CNTs are related to the size of the catalyst particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of CNTs, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen, or hydrogen) and a carboncontaining gas (such as acetylene, ethylene, ethanol, or methane). CNTs grow at the sites of the metal catalyst via a vapor-liquid-solid mechanism. The carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the

Carbon nanomaterials

edges of the particle, where it supersaturates and forms the CNTs. The catalyst particles can stay at the tips of the growing nanotube during the growth process or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate. A yield higher than 90% is generally achieved. If plasma is generated by the application of a strong electric field during the growth process (plasma-enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field [25]. By adjusting the geometry of the reactor, it is possible to synthesize forests of vertically aligned CNTs (i.e., perpendicular to the substrate). Without the plasma, the resulting CNTs are often randomly oriented. Under certain reaction conditions, even in the absence of plasma, closely spaced CNTs will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. Of the different means for nanotube synthesis discussed here, laser ablation is the most expensive, and arc discharge results in the lowest yield. Therefore, CVD has become a standard for the industrial-scale growth of CNTs, because of its lower cost and high yield and because CVD is capable of growing CNTs directly on a desired application substrate. For the other growth techniques discussed, CNTs must be collected from the growth substrate and deposited onto the application substrate. In the CVD approach, the growth sites are controllable by careful deposition of the catalyst.

4.4.2 Purification and processability of carbon nanotubes No matter of the growth method used, there is always a significant amount of impurities present in the nanomaterial (graphitic debris, catalyst particles, and fullerenes). These impurities may interfere with the properties of CNTs. Therefore, considerable efforts have been devoted to achieve a highly effective removal of impurities. These efforts have resulted in remarkable progress, and several purification methods that result in minimal damage to CNTs are now available. Soft methods comprise microfiltration [26] flocculation [27], chromatographic procedures [28], and centrifugation. Harsher purification methods involve oxidation with a strong oxidizing acid, hydrothermal treatment along with extraction and oxidation [29], and annealing at high temperature in the presence of oxygen. Different studies show that some combinations of these protocols are required for achieving an effective removal of impurities from as-produced carbon nanotubes [30]. Oxidative purification protocols have the side effect of increasing aqueous solubility due to formation of oxygenated sidewall defects (i.e., the treatment turns nanotubes more hydrophilic). This side effect offers a breakthrough for the mass production of CNT-based devices: circumventing the low solubility of pristine CNTs. Their insolubility is due to van der Waals interactions between nanotubes, which results in their aggregation in bundles and ropes, limiting their use in many applications. The acid treatment typically involves refluxing CNTs with a strong oxidizing agent such as nitric acid, sulfuric acid, hydrogen peroxide, or various mixtures of these. Results vary according to

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the parameters of these treatments such as duration, concentration, and number of repeated cycles. It is well known that acid treatments cause defects (e.g., formation of carboxylate group) and/or shortening of CNTs [31]. Alternatively, oxygen plasma treatments have been shown to be useful for achieving similar results, avoiding the use of wet chemistry routes and, possibly, generating fewer residues. Treating carbon nanotubes with oxygen plasmas helps in removing amorphous carbon or catalyst nanoparticles from their outer walls (these derive from the synthesis method employed) and generates oxygenated vacancies [32]. The presence of carbonyl, carboxyl, and hydroxyl functional groups has been revealed by x-ray photoelectron spectroscopy studies [33]. The presence of these functional groups grafted to CNTs makes them more hydrophilic, easing their dispersion in aqueous solutions. The solubility of CNTS may be further increased, while preserving their structures and properties, by adding a surfactant to the solution. Many surfactants have shown promise in this process, including DNA [34] polymers [35] or soaps [36]. Sodium dodecyl sulfate (SDS) is often employed to suspend CNTs in aqueous solutions. SDS coats CNTs with micelles, forming a hydrophobic core and hydrophilic surface, thus assisting in homogenization in water. Once the surfactant has been added, sonication is used to disperse CNTs in the solution. Achieving good dispersions of CNTs in standard solvents and aqueous solutions is an essential step for achieving the solution processing of CNT materials and for the formulation of stable and reproducible CNT inks that can be easily employed in the inexpensive, big scale fabrication of gassensitive devices employing standard methods such as spin coating, air brushing, or inkjet printing.

4.4.3 Gas sensing mechanisms in carbon nanotubes A direct correlation exists between the physical rolling vector of a single-walled carbon nanotube and its electronic properties. In an as-grown sample of carbon nanotube material, roughly one-third will be metallic and two-third p-type semiconducting [37]. A strict control of SWNT properties is necessary for devices based on a single SWNT as bandgap tuning occurs in semiconducting SWNTs [38]. It is not surprising that many research efforts have been directed toward obtaining a higher level of control over the properties of SWNTs. One method involves the sorting and separation of semiconductive from metallic SWNTs. Thus far, these investigations have resulted in methods that work well for small masses of SWNTs [39]. However, separating metallic from semiconducting SWNTs does not result in achieving total control over nanotube properties, as semiconducting properties depend on helicity and diameter [40]. For example, the bandgap of a semiconducting SWNT varies inversely with its diameter. Therefore, bigger diameter nanotubes have a smaller bandgap (lower semiconductor behavior). Thus, reproducibility in SWNT devices is limited not only by the presence of metallic SWNTs but also by the difficulty to control all physical properties of the nanotubes during the

Carbon nanomaterials

growth process. Multiwalled carbon nanotubes (MWCNTs) can either be metallic or p-type semiconducting depending on the axial chirality of the individual shells and depending on the intershell interaction. A detailed description of their conductance is rather complex, but the main contribution to charge conduction near the Fermi energy level is given by the outer tube [41]. Bare CNTs may be used as chemiresistors, where the change in resistance is attributed to shifts in the valence band with respect to the Fermi level. This change in conductance is commonly plotted as ΔR/R. An electron-donating adsorbate (acetone) would deplete the number of charge carriers (composed of holes in the p-type semiconductor), and an electron-accepting adsorbate (nitrogen dioxide) would increase the number of charge carriers. Sensors using a single CNT may be very effective for developing fundamental research but are totally unsuitable for a reproducible mass production as, in addition to the dispersion observed in the semiconducting properties of CNTs, such devices frequently require serial fabrication steps. The main difficulties found in individual CNT gas sensors are the problematic precise positioning of CNTs in large numbers and the variability in chirality, which result in poor device performance reproducibility. Alternative configurations of CNTs have been investigated to overcome these problems. Two such methods consist of using bundles (or ropes) of CNTs or two-dimensional networks of CNTs. The metallic CNTs present in a bundle mitigate the electrical response of the semiconducting CNTs, and therefore, networks of unbundled CNTs present a good compromise between ease of fabrication and improved responsiveness. The formation of two-dimensional networks of interconnected CNTs produces an electronic material, the qualities of which result from the average properties of many individual CNTs. As a result, device properties are dependent on the density of the CNT network. While lowdensity networks behave as thin-film semiconductors, high-density networks exhibit metallic behavior. The approach of using two-dimensional networks highly increases reproducibility in device manufacture, enabling the mass production of CNT-based devices. A two-dimensional network composed of a mixture of metallic and semiconducting CNTs behaves as a semiconductor above the percolation threshold for semiconducting nanotubes. Percolation theory describes how a random network of CNTs will behave as a semiconducting film in the limit between the percolation thresholds for the semiconducting and metallic CNTs [42]. In this range, semiconducting pathways dominate the film, and metallic pathways are not favored until the percolation threshold for the metallic CNTs is exceeded. Similarly to SWCNT networks, mats of MWCNTs consist of a mixture of metallic and semiconducting tubes. Macroscopically, these mats behave as mild p-type semiconductors since their conductance increases or decreases upon adsorption of electronaccepting or electron-donating molecules, respectively [43,44]. Finally, since a MWCNT mat consists of defective nanotubes [45], its resistance is mostly influenced by the resistance of individual nanotubes and not by the internanotube or the electrode-nanotube junctions [46].

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4.4.4 Selectivity enhancement in carbon nanotube gas sensors Goldoni and coworkers investigated the gas sensing properties of CNTs related to the presence of defects and residual contaminants [47]. The changes in the photoemission spectra of SWNT mats were studied in the presence of oxygen, nitrogen, carbon monoxide, moisture, nitrogen dioxide, sulfur oxide, and ammonia before and after the nanotube mats had underwent thermal treatment in ultrahigh vacuum. It was found that such thermal treatment in high vacuum respected the integrity of CNTs, drastically diminished the number of defects introduced by the purification treatments, sealed nanotube caps, and removed catalytic particles employed in the CVD growth of CNTs and the contamination from the standard lithography techniques employed to electrically contact CNT mats. After being thermally treated, the electronic spectra of SWNT were insensitive to oxygen, nitrogen, carbon monoxide, and moisture, while a strong sensitivity to ammonia, nitrogen dioxide, and sulfur oxide was confirmed. As a result, it can be derived that many properties measured on as-prepared or mildly annealed, purified CNTs are not intrinsic to the tubes, but attributable to the presence of catalyst particles, contaminants, and defects coming from the purification procedures and the techniques to contact them. Therefore, the cleaning of the surface of carbon nanotubes and the control of surface defects seem essential for reaching consistent and reproducible gas sensing results. Many authors have shown that the functionalization of carbon nanotube sidewalls helps achieving better chemical bonding between a specific chemical species and the nanotube, enhancing the selectivity of the adsorption process [48,49]. For instance, Pd-coated CNTs are highly sensitive to hydrogen [50]. The concept of CNT-metal cluster hybrid nanomaterials, where the metal cluster surfaces act as reactive sites for the adsorption of target molecules, was first introduced in a theoretical study that considered Al clusters attached to CNTs [51]. It was shown that upon the adsorption of ammonia, a substantial polarization and accumulation of charge in the region between the Al cluster and the nanotube were developed. This charge transfer affected the ionic component of the bonding and altered the position of the Fermi level and the band alignment. It was derived that the variations in the electrical conductance of the CNT-Al system could be used as a measure of the sensitivity of chemical sensors employing this material [51]. Extending further these theoretical results, CNT-metal cluster hybrids could be tailored for the recognition of chemical species with high sensitivity and enhanced selectivity. The key concept was to select suitable nanoclusters (small size helps maximizing the effect of adsorbates on metal clusters) able to donate or accept a significant amount of charge upon adsorption of a target molecule, so as to affect electron transport in the nanotube. Kumar and coworkers employed a wet chemistry route to obtain CNTs decorated with Pt clusters, which resulted highly sensitive to hydrogen [52]. Star and coworkers selected electroplating for decorating CNTs with different metals (i.e., Pt, Pd, Au, Rh, Sn, Mg, Fe, Co, Ni, Zn, Mo, W, V, or Cr) in an attempt to selectively detect carbon

Carbon nanomaterials

monoxide, nitrogen dioxide, methane, hydrogen sulfide, ammonia, or hydrogen [53]. One problem associated to the decoration of CNTs with metals is to obtain metal nanoparticles with monomodal, narrow size distribution and well anchored to CNT sidewalls. Avoiding the presence of mobile nanoparticles on the surface of CNTs is very important to prevent their coalescence during operational life and the associated instability in gas sensing performance. This problem can be tackled by using cold reactive plasma treatments (e.g., oxygen plasma) of CNTs [32,54–56]. Reactive plasma treatments provide the engineering of the interfacial properties of hybrid nanostructures in a single step. They enable, in a single step, the cleaning, activation, functionalization, and metal decoration of CNTs, with good control of metal nanoparticle shape, size, (under)coordination, and diffusion. In addition, oxidative treatments affect the density of states (DOS) of valence band and increase the work function of purified CNTs [57], making it closer to that of metals such as Pt, Au, Pd, Ni, or Rh. This helps electrons to travel between the metal nanoparticles and CNTs, with the direction of charge transfer depending on the gaseous environment. An effective electronic interaction between metal nanoparticles and the CNT enables the detection of gases by measuring the change in the electrical conductivity of these nanomaterials. Llobet and coworkers employed this approach to detect benzene vapors using arrays of plasma-treated, metal-decorated multiwalled carbon nanotube mats [58–60]. The limit of detection for molecular benzene was found to be below 50 ppb even in changing humidity backgrounds and in the presence of interfering species such as carbon monoxide, hydrogen sulfide, and nitrogen dioxide [61]. Penza and coworkers were the first to introduce magnetron sputtering as a way to decorate CNTs with metal nanoparticles (e.g., Au, Pt, Pd, Ru, or Ag) [62–64]. The method was successfully applied to the in situ functionalization of carpets of CNTs, CVD grown onto gas sensor transducers, thus avoiding the need for transferring CNTs to obtain a functional device [64]. Enhanced response toward nitrogen dioxide and ammonia, hydrogen, hydrogen sulfide, and carbon monoxide was reported; however, selectivity remains an issue. Less studied than metal decoration, the use of metal oxide nanoparticles has also been exploited to improve gas sensitivity of CNTs [65,66]. There seems to be an appropriate ratio between the amount of plasma-treated CNTs and metal oxide nanoparticles employed for achieving higher sensitivities. Such hybrid sensors show superior performance in the detection of nitrogen dioxide in the parts-per-billion range and carbon monoxide in the low parts-per-million (ppm) range, even when operated at room temperature. Optimized sensors show a significantly reduced moisture cross sensitivity [66]. Substitutional doping has also been considered, with limited success, to enhance sensitivity and selectivity of carbon nanotube gas sensors. The introduction of B and/or N enhances electrical conductivity of carbon nanotubes [67]. N-doped CNTs are efficient at detecting some gaseous species because of the presence of pyridine-type sites on their surface. Villalpando-Paez and coworkers [68] found that nitrogen-doped carbon nanotube sensors exhibited response and recovery times of the order of a few

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seconds to ammonia, along with strong, reversible responses to ethanol. In addition, while different studies [69,70] suggest that no charge transfer occurs between carbon monoxide and pristine carbon nanotubes, calculations suggest the possibility of using substitutional doping in CNTs for making them responsive to carbon monoxide [71]. In view of ameliorating the selectivity of CNT sensors, some authors have introduced the modification of carbon nanotube sidewalls with organic molecules. This is the case of Gelperin and coworkers [72] who devised chemical sensors based on single-stranded DNA as the chemical recognition site and single-walled CNTs as the electronic readout component or transducer element. They found that DNA-coated CNT devices responded to explosive vapors or nerve agent simulants that did not cause a detectable response change when bare CNT devices were employed. Their selectivity could be crudely tuned (basically the sign and magnitude of response) by choosing the base sequence of the single-stranded DNA. More recently, Penza and coworkers investigated the sensing properties of hybrid carbon nanotube-porphyrin films [73]. They explored the possibility of transducing the adsorption events occurring in a thin porphyrin layer into resistance changes of a CNT layer. Porphyrins increased the sensitivity of the CNT hybrid film, possibly due to the efficiency of metalloporphyrins in conveying the charge transferred from the adsorbate molecule to CNTs. By using different metalloporphyrin-CNTs in a sensor array configuration, an amelioration in selectivity can be expected as well. The covalent functionalization of CNTs (particularly of SWNTs) with polymers has also been investigated as a way to improve responsiveness and tailor selectivity. The use of poly-(m-aminobenzene sulfonic acid) (PABS) has resulted in enhanced response to ammonia [74], while employing polypyrrole (PPY) nanocomposites enhanced the response of SWNTs to nitrogen dioxide [75]. The covalent modification of CNTs with polymers typically requires highly reactive reagents and high temperatures, which may damage the integrity of CNTs. However, Zhang and coworkers pioneered an electrochemical method of covalent polymer functionalization [76], achieving polyanilinefunctionalized SWNTs with improved response to NH3. The noncovalent chemical modification is also a method to functionalize CNTs with polymers. It has been shown that the selectivity to different chemical vapors can be tuned by using different polymers. For instance, polyethyleneimine (PEI) and Nafion are the two compunds commonly used in the noncovalent functionalization of CNTs [77]. Polymer deposition could be performed by drop casting. SWNTs coated with PEI or Nafion showed enhanced selectivity to NO2 [77] and CO2 [78], respectively. Nafion-coated SWNTs have also been found sensitive to ambient moisture [78,79]. The noncovalent modification of CNTs with chlorosulfonated polyethylene and hydroxypropyl cellulose was studied by Li and coworkers [80]. Such devices showed high sensitivity to hydrochloric acid and chlorine gas. Sastry and coworkers presented in [81] a good account on the noncovalent

Carbon nanomaterials

functionalization of CNTs and graphene and the implications in their gas adsorption and electronic properties. One of the main problems that hinder the development of commercial sensors employing CNTs is the variation of humidity levels in the environment to be analyzed. A development by Haick and coworkers [82] offers an interesting breakthrough to tackle this problem. Initially, mats of purified SWNTs are drop casted onto a transducer substrate in which several interdigitated electrodes have been printed in an array configuration. The SWNT layers are then coated with different (e.g., up to seven) polycyclic aromatic hydrocarbon (PAH) compounds having different aromatic coronae and side groups. These PAH contain hydrophobic mesogens that are terminated with different alkyl chains and functional substituents. These molecules self-assemble into long molecular stacks with a large, electron-rich, semiconducting core, which ensures that a good charge carrier transport along the molecular stacking direction and a relatively insulating periphery are achieved [83]. In addition, the nanometer-thick PAH columns can easily form 3-D, micrometer-sized, sponge-like structures with high specific surface for better absorbing target gas molecules [84]. By carefully choosing appropriate combinations of PAH/SWNT sensors, the authors could show that high sensitivity and accuracy can be achieved in the discrimination of polar and nonpolar volatile organic compounds, even at variable humidity levels in a very wide range (i.e., 5%–80% RH). This represents an advancement for the development of a cost-effective, lightweight, low-power, and noninvasive equipment for a widespread detection of volatile organic compounds in realworld environmental, security, food, health, and other applications. Swager and coworkers synthesized a diverse array of multiwalled carbon nanotube sensory materials capable of identifying volatile organic compounds (VOCs) on the basis of their functional groups [85]. Covalently functionalized MWCNTs with a series of cross sensitive recognition groups were successfully synthesized via zwitterionic and posttransformation synthetic procedures. The incorporated chemical functional groups on MWCNT surfaces introduced greatly increased sensitivity and selectivity to the targeted analytes. These included functional groups showing H-bond acidity (for hydrogen bond acceptors such as ketones and ethers), H-bond basicity (for hydrogen bond donors such as acids and alcohols), polarity (for vapors with high polarity such as ketones and ethers), polarizability (for aromatic and chlorinated hydrocarbons), and nonpolar adsorption (for aliphatic hydrocarbons). This covalent functionalization process is illustrated in Fig. 4.2. The process was conducted in different steps. At first, propargyl or allyl groups are introduced onto MWCNTs under mild zwitterionic reaction conditions (see Fig. 4.2A), and then the resulting nanomaterials were further functionalized employing either 1,3-dipolar cycloaddition, thiol-ene addition, or olefin cross metathesis reactions (two examples of this approach are illustrated in Fig. 4.2B and C).

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O

O

O

O

O

O O

O

1) DMAP, THF 60°C, 48h; 2) propargyl alcohol 60°C, 12h

O

1 Propargyl-MWCNT

O

O O

O

O O

O

1) DMAP, THE 60°C, 48h; 2) allyl alcohol 60°C, 12h

2 Allyl-MWCNT

(A) N O

O

O O

N

O

C12H25N3 Cul, DIPEA DMF, 90°C, 24h

N N C12H25

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N N O

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(B)

C12H25

3

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R1S

O

SR1

O O

O

R1SH, THF, hn, 24h O

O

O O

2 Allyl-MWCNT

(C)

R1 =

OH

4

5

Fig. 4.2 Covalent functionalization of carbon nanotubes. The initial step consists of obtaining (1) propargyl and (2) allyl-carbon nanotubes (A). A second functionalization step may be then performed to attach functional groups such as (3) alkyltriazoles (B) or (4) thiochains or (5) thioacids (C). (Reproduced with permission from F. Wang, T.M. Swager, Diverse chemiresistors based upon covalently modified multiwalled carbon nanotubes, J. Am. Chem. Soc. 133 (29) (2011) 11181–11193. © American Chemical Society 2011.)

Carbon nanomaterials

In a similar approach, Llobet and coworkers employed self-assembled monolayers of carboxylic acid-terminated long-chain mercaptan thiols onto Au-decorated MWCNT mats for selectively detecting alcohols [86]. Recently, the approach of employing macrocyclic compounds such as cyclodextrins, calixarenes, or cavitands grafted to CNTs for developing gas sensors has been reported. In such an approach, CNTs play the role of transducing element (able to collect and transport efficiently electronic charge), and the grafted macrocyclic compounds are the selective molecular receptors (i.e., implement a receptor function in the gas sensor). When employing cyclodextrins, the selectivity in the solid-gas interface is mainly driven by London dispersion interactions, size, and shape fit. Duarte and coworkers [87] developed a conductive polymer nanocomposite (CPC) chemoresistor based on linear and branched polyamides synthesized from bifunctional and heptafunctional β CD monomers and (Z) octadec-9-enedioic-N-hydroxysuccinimide ester bearing a multiwalled carbon nanotube (MWCNT) conducting architecture. The latter sensor was formed through a spray deposition of the CNTs and the CD polymers (dispersed separately in an organic solvent) layer by layer on interdigitated ceramic substrates. The same group has demonstrated the ability of CPC-based gas sensor to reversibly detect polar and nonpolar VOCs with an expected limit of detection to lay in the low parts-per-billion range. Furthermore, polyamide synthesized from β-CD(NH2)2(OHw)19 is shown to be selective toward propanol in nitrogen gas carrier. This happens due to the strong hydrophilic character that the 19 hydroxyl moieties offer to the compound making it able to generate many hydrogen bonds with polar protic solvents. Calixarenes possess variable cavity dimensions with the possibility to functionalize their upper and/or lower rim to tailor their affinity with a target guest molecule through different noncovalent interactions such as π-π stacking, cation-π and CH-π interaction, and hydrogen bonding. Baysak and coworkers [88] reported the use of SWCNTs, the sidewalls of which were noncovalently functionalized with pyrene-bearing calix[4]pyrrole. Sensors were implemented as chemoresistors by coating a filter paper with calixarene-functionalized SWCNTs contacted with two planar electrodes. Fast response and higher affinity for acetone (20–500 ppm) compared with other VOCs were reported. Derived from resorcinarene scaffolds, cavitands have been widely studied for their synthetic versatility and selective complexation with target molecules. Notably, cavitands can be designed by respectively tuning their bridging group connected to the phenolic moieties of the resorcinarene. As a result, it is possible to control the dimensions, the shape, and the binding groups of the formed cavity. Cram and coworkers were the pioneers to study cavitands as potential molecular receptors via the host-guest strategy [89]. Dalcanale and coworkers did a subsequent work by modifying the bridging group of the resorcinarene to monitor VOCs in air. They have recently published a review highlighting their progress [90]. Briefly, in their last study, they found out that rigidifying the cavity of the quinoxaline cavitand (QxCav) introducing four ethylenedioxy bridges at the upper rim (EtQxBox) improves the interaction with aromatic

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Fig. 4.3 Representation of the preparation of the cav-Au-MWCNT material: (1) oxygen plasma treatment generates oxygenated defects on the outer wall of MWCNTs, (2) Au-RF-sputtering (formation of Au nucleus anchored to defects), (3) Au-RF sputtering (formation of Au cluster), (4) self-assembly of the cavitand monolayer on the Au-nanoparticle surface by dipping the material in a chloroform (S) solution of the cavitand, and (5) solvent removal (an air molecule, e.g., nitrogen, replaces chloroform (S) molecule from the cavitand interior). (Reproduced with permission from P. Cl
ement, et al., Deep cavitand self-assembled on Au NPs-MWCNT as highly sensitive benzene sensing interface, Adv. Funct. Mater. 25 (2015) 4011–4020. © John Wiley and Sons, 2015.)

guests compared with the conformational mechanism of the QxCav [91]. Recently, Llobet and coworkers studied the possibility to couple the promising gas sensing properties of cavitands with MWCNTs as resistive gas sensors [92]. They first grafted gold nanoparticles on oxygen plasma-treated MWCNTs where the thioether-legged-QxCav is further tethered on gold by a self-assembled monolayer approach (QxCav-Au-MWCNTs). This is illustrated in Fig. 4.3. Upon a sensing event, a charge transfer is observed between the cavitand and the Au-MWCNTs changing the general conductivity of the system. The sensor showed clearly higher sensitivity for benzene than for other aromatic and nonaromatic VOCs, with a limit of detection of 600 ppt in dry air. Nevertheless, a nonnegligible cross sensitivity with NO2 and ambient humidity was observed.

4.4.5 Toward more reproducible CNT devices As already stated earlier, high-performance gas sensors would benefit from CNTs of single conductive type and of pure chirality. Recently, Zhang and coworkers [93] described a rational strategy to construct complex architectures, selectively enrich semiconducting or metallic SWNTs, and control their chirality. During their CVD growth, the complex architectures of CNTs come from the synergetic effect of lattice and gas flow-directed

Carbon nanomaterials

modes. Specifically, the aligned orientations of SWNTs on graphite are chirality selective, and their chiral angles, handedness, and (n,m) index have been conveniently and accurately determined. They explored the use of UV irradiation and sodium dodecyl sulfate (SDS) washing-off methods for selectively removing metallic SWNTs, leaving only semiconductor SWNT arrays on the surface. Moreover, the UV-assisted technique takes the advantages of low cost and high efficiency, and it directly produces a high ratio of semiconductor SWNT arrays.

4.5 Graphene 4.5.1 Synthesis of graphene Graphene consists of a two-dimensional array of carbon atoms covalently attached via sp2 bonds to produce a honeycomb sheet. Graphene was considered no more than part of a graphite crystal structure until 2004, when Novoselov and coworkers first presented some of the surprising electrical properties of graphene layers they had isolated [94]. It is due to its high-quality crystalline structure that graphene shows high mobility and ballistic conduction. Its electronic bandgap, carrier type, and densities can be tailored by, for example, stacking two sheets of graphene and applying different gate voltages to the substrate [95,96]. Some of its properties make this material highly promising for developing gas sensors. Being a strictly two-dimensional material, all graphene atoms are exposed to the environment, which results in the highest surface area per unit volume [97]. It is a highly conductive material exhibiting metallic conductivity and, hence, low Johnson noise even in the limit of full depletion [95,98,99], where few electrons can result in notable relative changes in carrier concentration. Pristine graphene has very few crystal defects [95,96,99] and thus exhibits low levels of 1/f noise caused by their thermal switching [100]. Different methods have been reported to fabricate graphene, which consist of the mechanical cleaving of graphite, chemical cleaving or exfoliation of graphite, epitaxial growth, and chemical vapor deposition. The mechanical cleaving of graphite involves repeated stripping of a graphite fragment with adhesive tape to eventually obtain isolated single layers. Single layers can be resolved from all other graphitic fragments by optical microscopy, atomic force microscopy, or Raman spectroscopy. This process is labor intensive and not scalable but results in nearly defect-free graphene [94]. Chemical exfoliation of graphite implies employing a strongly acidic solution to introduce oxygencontaining moieties into graphene sheets, thus creating graphene oxide [101]. Graphene oxide being hydrophilic can be easily separated into individual sheets and dispersed into an aqueous solution for further processing. Graphene oxide is then reduced, either chemically or thermally in an attempt to eliminate as many as possible of these oxygen functional groups and restore the original properties of graphene. However, reduced graphene oxide still contains a significant amount of oxygen-based moieties and structural defects and is, therefore, different from pristine graphene [102,103]. For graphene oxide

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to be usable as an intermediary in the creation of monolayer or few-layer graphene sheets, it is important to develop an oxidization and reduction process that is able to separate individual carbon layers and then isolate them without modifying their structure. So far, while the chemical reduction of graphene oxide is currently seen as the most suitable method for the low-cost, mass production of graphene, it is still difficult for scientists to complete the task of producing graphene sheets of the same quality as, for example, mechanically exfoliated graphene, but on a much larger scale. Should this issue be conveniently addressed, it can be expected that graphene will become widely used in commercial and industrial applications. Epitaxial growth of graphene involves heating hexagonal silicon carbide crystals to temperatures of more than 1200°C, which allows the evaporation of silicon and then formation of graphene on the basal planes [104,105]. This technique can produce large areas of continuous graphene layers, but their electronic properties are often disturbed by disorder introduced by the substrate [106]. Finally, chemical vapor deposition allows the growth of graphene on metal substrates such as copper or nickel from hydrocarbon vapors at temperatures that typically range from 700°C to 1000°C. This method produces large areas of high-quality graphene, which can then be transferred to application substrates [107,108]. The main challenges for CVD growth of graphene are to accurately control the number of layers grown and to avoid contamination from the metallic substrate. An important aspect for achieving inexpensive gas sensors employing graphene consist of implementing reliable, scalable, and reproducible methods for coating standard transducer substrates with graphene layers (or flakes). In that sense, the functionalization of graphene for obtaining suspensions in aqueous media or inks that can be spin coated or inkjet printed is an essential step that deserves further research. Some of the problems found and solutions researched for carbon nanotube materials seem to be a good starting point for addressing these issues in graphene, yet specific solutions for dealing with stacking, folding, and border defects are needed.

4.5.2 Gas sensing with graphene The first study of graphene use in gas sensing was reported in 2007 by Novoselov’s group [97]. They demonstrated the detection of gas molecules adsorbed on multiterminal Hall bars by monitoring changes in electrical resistance. These were fabricated by conventional lithographic methods from single-layer or few-layer, high-quality graphene that had been mechanically exfoliated from graphite. Adsorption of parts per million of gases caused the devices to show concentration-dependent changes in resistivity, and after a detection event, the resistance baseline could be regained by heating at 150°C under vacuum. The gas-induced changes in resistivity showed different magnitudes for different gases, and the sign of the change indicated whether the gas was an electron acceptor (e.g., nitrogen dioxide, and moisture) or an electron donor (e.g., carbon monoxide,

Carbon nanomaterials

ethanol, and ammonia). Considering that electrical conductivity is proportional to the product of carrier density and mobility, it seems clear that changes in carrier density, mobility, or both are responsible for the experimental results observed. Soon after the realization of graphene gas sensors, different computational chemistry studies were performed to theoretically explain the adsorption of different molecules (moisture, nitrogen dioxide, nitric oxide, ammonia, carbon monoxide, carbon dioxide, oxygen, and nitrogen) on graphene [109,110]. These studies show that nitrogen dioxide behaves as a strong dopant and that moisture or ammonia should produce milder effects. It is the interaction of ammonia with water adsorbed on the devices what probably contributes to the large response observed [111]. Similarly to the work of Goldoni for CNTs [47], Johnson and coworkers [112] showed that conventional lithography or nanolithography employed to contact graphene for making gas sensors left resist residues on the graphene surface. This contamination chemically dopes graphene, enhances carrier scattering, and may act as an absorbent layer that concentrates gas molecules on the surface of graphene, enhancing gas response. By cleaning graphene employing annealing under high vacuum or in H2/Ar, these contaminants were removed (at least partially), and the intrinsic graphene responses to gases could be measured. These responses were found to be very small, something already observed for pristine, high-quality carbon nanotubes. Therefore, the need for functionalizing graphene to reach sub-ppm sensitivity seems mandatory.

4.5.3 Functionalization of graphene for increased sensitivity and selectivity Some of the functionalization approaches developed for other types of gas sensors, especially for single-walled carbon nanotubes [49,113], have found applications in graphenebased gas sensors. At the early stages, the primary purpose of functionalization was to increase the ease of exfoliating graphite to produce modified graphene or to make functionalized graphene for other applications, such as polymer nanocomposites. This is why many of the chemical functionalization methods employed so far have used covalent bonding, at the cost of destroying the sp2 bonding of the graphene lattice. In contrast, methods employing noncovalent bonding exploit the extensive opportunities for π-bonding on the basal plane of graphene and are more suitable to retain its unique electronic properties, than those methods employing covalent functionalization. This is advantageous for gas sensing, since high charge carrier mobility and ballistic transport associated to good-quality graphene often translates into low-noise devices with improved limit of detection. Gianozzi and coworkers [114] suggested the use of substitutional doping in graphene to enhance its gas sensing properties (an equivalent strategy had already been implemented in CNTs [68]). Their computational studies conducted on a B-, N-, Al-, and S-substituted graphene sheets suggested that B- or S-doped graphene would be advantageous for detecting nitric oxide and nitrogen dioxide. A useful review on heteroatom doping of graphene materials and their applications can be found

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in [115]. Reduced graphene oxide has also been reported as a useful material for developing gas sensors. Graphene oxide is much easier to process than graphene and offers the possibility to tailor the amount of functional groups by controlling the degree of reduction. Robinson and coworkers [116] demonstrated reduced graphene oxide as the active material for high-sensitive gas sensors. Graphene oxide (GO) was used to spin coat a Si substrate, and standard photolithography was used to create interdigitated arrays of Ti/Au electrodes. Processed samples were reduced back toward graphene by exposure to hydrazine hydrate vapors while being heated at 100°C. The degree of reduction was changed by controlling the time of exposure to hydrazine. These devices achieved parts-perbillion sensitivity to different warfare agent simulants. The partial reduction of graphene oxide to graphene is necessary because the process leaves a functionalized surface (i.e., with active oxygen defects) that shows stronger reactivity to the target analytes. However, the noise level increases for short reduction times as GO and reduced GO (RGO) show poorer electronic properties compared with high-quality graphene. Therefore, a trade-off exists to reach an optimal lower limit of detection with good sensitivity. Fig. 4.4 illustrates this approach and shows the dependency of 1/f noise on film treatment. A similar approach was reported by Kaner and coworkers [117] in which explosive vapors were detected at parts-per-billion levels. While GO has been used to detect H2O (e.g., ambient moisture), NO2, H2, H2S, NH3, acetone, ethanol, and methanol, RGObased sensors have been shown to detect also NO, CO2, Cl2, and liquefied petroleum gas (LPG) [118]. All this is indicative of the lack of selectivity experienced with these nanomaterials. GO flakes present the advantage of being easily suspended in aqueous solutions that can be drop casted, spin coated, or inkjet printed on standard interdigitated transducers for gas sensing, which avoids the use of photolithographic steps that may contaminate the film. Furthermore, as-deposited films can be treated under high vacuum annealing or partially reduced employing extreme UV lithography to tune their gas sensing properties [119]. The fact that RGO has a functionalized surface with active oxygen defects was exploited by Liu and coworkers [120] to decorate RGO with Pd nanoparticles via a solution chemistry method. The Pd-RGO was then drop coated onto a standard silicon substrate with patterned Ni-/CVD-grown graphene electrodes, and a dielectrophoretic technique that made use of alternate current was employed to assembly the Pd-RGO into ordered conducting channels on the hydrophobic surface of the wafer substrate. The resulting sensors showed parts-per-billion response to nitric acid. Fig. 4.5 illustrates the procedure for the synthesis of nanomaterials and fabrication of these sensors. Different wet chemistry routes have been reported as well to decorate RGO sheets with metal oxide nanoparticles such as SnO2, WO3, In2O3, ZnO, Cu2O, NiO, Co3O4, or MoO2. Similarly to metal oxide-decorated CNTs, the resulting hybrids show overlapping selectivity toward NO2, NH3, H2, C2H2, H2S, ethanol, and acetone [121]. Beyond metal oxide nanoparticles, the use of 1-D, 2-D, or 3-D metal oxides and graphene hybrids has been reported as well [122]. The functional groups on graphene oxide

Carbon nanomaterials

1E-12 1E-13 1E-14 1E-15

rGO film thickness

Noise density (V2/Hz)

1E-11

1E-16 rGO film

SWNT film

1E-17 0.1

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Frequency (Hz)

Fig. 4.4 Graphene oxide films are obtained by dipcoating of a suspension of GO flakes onto a silicon oxide substrate with patterned interdigitated electrodes (top left). The AFM micrograph shows that the film consists of partially overlapping GO flakes (top right). 1/f noise measurements indicate that thin films of reduced GO (after treatment of the sensor surface in hydrazine vapors) approach to the thermal noise limits, indicative of the great potential for achieving very low limits of detection employing reduced GO gas sensors. (Reproduced with permission from J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137–3140. © American Chemical Society 2008.)

provide a wide range of opportunities for further functionalization to create new types of carbon sheets with different surface chemistry, like graphene amine; new hybrid materials, such as DNA tethered to graphene oxide [123]; or through immobilization of metallic nanoparticles, antibodies, and polymeric compounds [124,125]. Some authors have used phosgenation of graphene oxide for a subsequent covalent attachment of, for example, octadecylamine [126] or porphyrin [127] molecules. Still, others have used diazonium salts to attach different types of phenyl groups to surfactant-wrapped graphene oxide [128]. The main strategy for the noncovalent functionalization of graphene makes use of π-bonding or π-stacking between the π-orbitals of the basal plane of graphene and those of aromatic functional molecules. For example, this type of bonding, though weaker than covalent bonding, has been used to accomplish the following: functionalize reduced graphene oxide with sulfonated polyaniline [129] or with 1-pyrenebutyrate [130], assemble monolayers of perylene-3,4,9,10-tetracarboxylic dianhydride on epitaxial graphene [131], or load the drug doxorubicin hydrochloride onto graphene oxide for potential drug delivery applications [132].

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Pd-RGO (2)

(1)

Graphite layer

Pd NPs

RGO

Pd-RGO

(A) Ni electrode SiO2

CVD graphene (2)

(1)

Si

NO

Pd-RGO sheets

(4)

Pd-RGO drop (3)

(B) Fig. 4.5 (A) Scheme of the process for the preparation of Pd-RGO composites: (1) RGO synthesis and (2) Pd decoration of RGO. The inset photograph is the diluted Pd-RGO nanosheet suspension used for the ac-dielectrophoretic deposition (DEP). (B) Scheme of the graphene-Pd-RGO device fabrication and gas sensing test: (1) Ni electrode fabrication, (2) chemical vapor deposition (CVD) growth of graphene, (3) ac-DEP of Pd-RGO nanosheets, and (4) sensor measurement while exposed to nitric oxide. (Reproduced with permission from W. Li, X. Geng, Y. Guo, J. Rong, Y. Gong, L. Wu, X. Zhang, P. Li, J. Xu, G. Cheng, M. Sun, L. Liu, Reduced graphene oxide electrically contacted graphene sensor for highly sensitive nitric oxide detection, ACS Nano 5 (2011) 6955–6961. © American Chemical Society 2011.)

4.6 Conclusions and outlook The commercial production of inexpensive gas sensors employing carbon nanomaterials remains some way off, and there is still a need for important breakthroughs. Achieving device-to-device reproducibility in large numbers is one of the main challenges, and research is needed to find cost-effective, scalable production methods that retain the essential properties of such materials. If the use of high-purity CNTs or graphene as gas-sensitive materials is sought (e.g., because of their controlled surface chemistry), then CVD has established itself as the growth method of choice. While some results have shown the possibility of growing and contacting ultrapure, suspended, single CNTs, suppressing unwanted hysteresis caused by ambient humidity in field effect devices [133], a much needed major breakthrough would be the in situ growth of CNTs with predictable

Carbon nanomaterials

semiconducting or metallic properties. Even beyond controlling the metallic or semiconductor nature of carbon nanotubes, achieving an effective control of chirality and diameter during their growth would surely pave the way for highly reproducible devices. Some results in this direction have already been reported [93], and these will surely evolve in the years to come. The outcome may completely avoid the current need of lengthy and most of the times not scalable, separation/identification and contacting methods, for fabricating devices with optimal sensitivity. On the other hand, if other lower-quality carbon nanomaterials such as carbon nanofibers, multiwalled carbon nanotubes, or reduced graphene oxide continue to build on its promise for gas sensing, then electrospinning, standard CVD, and chemical exfoliation of graphite, respectively, followed by drop or spin casting or printing would be scaled-up. With the current research efforts directed to solve these issues, it seems that reaching large-scale fabrication of carbon nanomaterial gas sensing devices is just a matter of time. Although it has been claimed that singlemolecule adsorption/desorption events are detectable using graphene devices [97], so far, the electrical detection of gas adsorption on CNTs or graphene has had detection limits at parts-per-billion levels, in laboratory conditions. Functionalization of the carbon nanomaterial surface (e.g., decorating with metal or metal oxide nanoparticles, implementing substitutional doping, or by grafting functional groups) is a practical way to increase sensitivity, minimize unwanted effects (e.g., moisture interference) [82], and tune selectivity [61]. The use of reactive cold plasmas [32,54–56] seems to be an advantageous way to clean the surface, create controlled defects, and functionalize the surface of carbon nanomaterials in a single step (e.g., more environmentally friendly than wet chemistry routes and can be scaled-up). Recently, the cold plasma-mediated fluorination of carbon nanotubes (forests and carpets) and graphene (flakes) is under study for achieving simple gas sensors with lower cross sensitivity to ambient moisture. It is a fact that selectivity remains an important issue as many studies show the difficulty of making carbon nanomaterials absolutely selective, and the standard solution of using sensor arrays with partially overlapping sensitivity seems to remain a good option. However, the last years have seen the development of approaches in which complex molecules are grafted to the surface of carbon nanomaterials via covalent or noncovalent interactions. In such an approach, carbon nanomaterials act as support and charge transport transducing elements, while the recognition or receptor function is performed by grafted molecules. Very particularly, we have seen the rise of noncovalent functionalization of CNTs and graphene or RGO as a suitable way to preserve the excellent electronic properties of these nanomaterials while implementing complex receptor functions for enhancing selectivity. An important challenge, somewhat related to the previously mentioned ones, is avoiding the presence of unwanted contaminants at the sensor surface. These contaminants may result adsorbed during the normal function of the sensor because they are present in the environment or derive from the sensor preparation methods (e.g., polymer photoresist residues and solvents used for casting). Periodical temperature cycling

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[60,97] and continuous or pulsed UV light exposure [134] have been reported as useful to regenerate the surface. Possibly, the best approach would be to implement appropriate surface engineering, and therefore, the functionalization layer could be used to protect the device high surface area from adsorption of impurities. However, apart from ameliorated sensitivity and selectivity, the effective detection of gaseous species in the environment requires gas sensors with other specific properties, such as stability, simplicity, low cost, and fast response. Therefore, a well-designed functionalization should be targeted at balancing the strength of adsorption of analytes (needed for high sensitivity) against the reversibility of the detection process, which is a fundamental for gas sensors to perform continuous measurements. This should be further accompanied by simple yet effective baseline correction strategies to counteract the drift often experienced with gas sensors employing carbon nanomaterials. After more than two decades of carbon nanomaterial research for gas sensing, it is still very difficult to determine which material holds more promise for reaching the gas sensor market. Considering the characteristics of the different carbon nanomaterials, graphene offers similar interaction properties with target molecules than that of large-diameter CNTs (generally multiwalled carbon tubes) and similar flexibility for functionalizing its surface but with significantly lower noise levels, which would be preferable for sensors with improved lower detection limits. However, lower-quality carbon nanomaterials such as carbon black, carbon nanofibers, multiwalled carbon nanotubes, or reduced graphene oxide offer in general a looser control of their surface chemistry compared with their high-quality counterparts. This should imply higher detection limits, lower selectivity, and reproducibility for these materials than in SWCNTs or graphene. Yet, low-quality carbon nanomaterials are good candidates for inexpensive detectors in mass-market applications. Some examples of this approach comprise the recent development of fully printed gas sensing tags for detecting highly oxidizing species such as ozone or nitrogen dioxide, in which gas sensors are passive (i.e., do not require a battery), operated at room temperature, and, therefore, virtually zero power.

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CHAPTER 5

Hybrid and 2D nanomaterials Marcel Bouvet Institute of Molecular Chemistry of the University of Burgundy (ICMUB), UMR CNRS 6302, Universit
e Bourgogne FrancheComt
e, Dijon Cedex, France

5.1 Macrocycle-polymer hybrid materials In conductometric and optical sensors, all the molecules used in organic electronics can be potentially used as sensing materials. They are conjugated molecules, among them macrocyclic molecules, such as porphyrins and phthalocyanines, pentacene, perylene derivatives, oligomers, and conjugated polymers. Additionally, with other transducers, as acoustic and optical transducers, very different types of molecules can be used. Among macrocyclic molecules, the most important families used in chemosensing are phthalocyanines and porphyrins. They exist not only as monomacrocyclic molecules, with a weak conductivity, but also as double or triple decker complexes, mainly with rare earth metal ions as coordination centers. The latter are highly more conductive. From the first report on phthalocyanines as sensing materials by Bott and Jones [1], lots of sensor studies dealt with phthalocyanine or porphyrin-containing sensing materials. They were used with different transduction modes, but mainly conductometric transducers, namely, resistors, field-effect transistors (FET), and heterojunctions [2]. They allowed detecting strong oxidizing agents as ozone and nitrogen dioxide in the parts-per-billion (ppb) range and less redox active gases as ammonia in the parts-per-million (ppm) range as well. A useful approach to overcome some of the physical limitations of phthalocyanines, including their poor processability and their too high crystallinity, involves combining these materials with nonconducting polymers, such as polyvinyl alcohol (PVA), polystyrene (PS), and polymethyl methacrylate (PMMA) [3]. Phthalocyanines were incorporated in the polymer by dissolution in a common solvent and deposited by solution processing, namely, solvent-cast or spin-coating techniques. Even though they seem very basic techniques, if the deposition conditions are well controlled, such as the concentration of the solution, the acceleration, the speed, and the rotation time, spin coating can lead to highly reproducible films, in thickness and homogeneity, as exemplified by Cu(tBu)4Pc/polystyrene films [4]. Due to their optical properties, porphyrins have been used in optical sensors to detect ammonia and VOCs. Incorporated in polymers, in particular silicone elastomers and thermoplastics, like ethyl cellulose, polyvinyl chloride (PVC), and nitrocellulose, films were obtained and revealed to be able to detect NH3 down to about 1 ppm in 10 s, with Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00005-0

Copyright © 2020 Elsevier Inc. All rights reserved.

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a good reversibility, by measurement of the absorbance in the visible region [5]. The stability of the films was very short in silicone elastomers, due to photochemical oxidation of the ring and to their reaction with traces of ozone, currently present at 10–30 ppb indoors. The highest stability was observed for PVC modified by a plasticizer, but the devices remain sensitive to relative humidity variations. A particular case in the porphyrins series is this of indium octaethylporphyrin complex (HOInOEP), which exhibits a dimer-monomer equilibrium in the presence of a donating VOC capable to coordinate the indium(III) cation (Eq. 5.1): ½OEP
In
OH
In
OEP + + RNH2 ⇆OEP
In
OH + ½RNH2
In
OEP + (5.1) A redshift of the maximum wavelength of the Soret band, from 390 nm in the dimer, by more than 15 nm, was observed in the monomer [6]. Thus, butylamine, the more lipophilic tested amine, was detected at sub-ppm concentrations, whereas the less lipophilic amine, namely NH3, was detected down to 10 ppm. The hydrophilicity of sensing materials can be adjusted by the composition of the polymer. Thus, the acrylic acid-acrylamide (70/30 in mol/mol) copolymer exhibits an electrical resistance highly stable, up to 80% of relative humidity (RH), whereas this of pure polyacrylic acid decreases rapidly at RH higher than 50% [7]. When a monophthalocyanine is mixed with an insulating polymer, the resulting hybrid material is too resistive for conductometric measurements but can be used with other transduction modes. Thus, the hydroxy gallium phthalocyanine, HOGaPc, was incorporated in cellulose, via its soluble trimethylsilylated form, leading to an amorphous material (Fig. 5.1) [8]. This hybrid material showed a higher sensitivity to ozone (at 100 ppb) than the pure crystalline HOGaPc films, with a better reversibility, as measured by the Kelvin-Zisman vibrating capacitor probe. A particular case is that of lutetium bisphthalocyanine, LuPc2. Because of its high conductivity, it can be incorporated in an insulating polymer while remaining suitable for conductometric measurements. Thus, resistors made from LuPc2-polymethyl

Fig. 5.1 AFM images of HOGaPc and cellulose-HOGaPc mixture as deposited on ITO substrates and view of the two components.

Hybrid and 2D nanomaterials

Fig. 5.2 Schematic view of the radical anion Pc
, LuPc2, and PMMA.

methacrylate (PMMA) blends deposited by spin coating were used to detect ozone in the ppb range (Fig. 5.2) [9]. The best results were obtained with 80/20% (w/w) LuPc2/PMMA blends. It allows keeping homogeneous conducting films, with a high sensitivity to redox active species due to the radical nature of LuPc2, but with smaller crystallinity than with pure LuPc2 films. This makes easier the diffusion of gaseous molecules inside the sensing material. On the contrary, the association of poor conducting molecules like monophthalocyanines with polymers must be carried out with conducting polymers, if we want to keep sensing materials suitable for conductometric or electrochemical measurements. In addition, it is known that the response of conducting polymers to gases can be modulated by the initial conductivity of the sensing polymer and by the nature of counteranions, as shown with polypyrrole for the detection of NH3 [10]. All phthalocyanine-polymer hybrid materials showed enhanced sensitivity to gases relative to pure conducting polymers, as mentioned by Hatchett and Josowicz in their review devoted to composites of conducting polymers as sensing materials [3]. Thus, hybrid materials combining polypyrrole (PPy) with an ionic phthalocyanine, namely, a sulfonated cobalt phthalocyanine (s-CoPc), as counteranion, were electrosynthesized at the surface of Pt electrodes [11]. The small interelectrode distance, typically 5 μm, allowed the polymer growing from one electrode to the other one during the electrodeposition, leading to a resistor suitable for conductometric sensing. It was shown that ionic macrocycles modify the morphology of films, with smaller crystallites and lower roughness compared with the materials obtained with rather small anions, like perchlorate or naphthalene sulfonate, as shown by optical topomicroscopy images (Fig. 5.3) [12]. s-CoPc provided hydrophilic properties to the blend, and it induces conductivity variation upon humidity but only at very low relative humidity (RH) levels, between 0% and 20% RH. At higher RH, the conductivity of PPy/s-CoPc varied only slightly (Fig. 5.4 left). Then, the response to ammonia (NH3), in the range 20–100 ppm, remained almost constant in a broad range of RH (20%–80%), whereas the response of PPy synthesized with ClO4
as counterions exhibited a stronger variation at high RH values. The interference between NH3 and H2O on the response of conjugated polymers can also be modified by their association in blends with nonconducting polymers, like PMMS and PS [13].

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N H

n

22 0

2.22 µm

0.2 0.25 0

Fig. 5.3 Schematic view of the coulombic interactions between the sulfonated CoPc and polypyrrole and an optical topomicroscopy image of an electrodeposited film of the hybrid material.

Fig. 5.4 Response to ammonia of a PPy/s-CoPc hybrid material-based resistor (above) and of a LuPc2/ Cu(F16Pc) molecular semiconductor-doped insulator (MSDI) heterojunction (bottom), with exposure/ recovery cycles (1/4 min), under various relative humidity levels.

Hybrid and 2D nanomaterials

When chemical sensors are used in real environment, the effect of humidity has to be considered. Water was believed to act as donor species on molecular materials, even in the presence of rather strong donor adsorbates, such as ammonia [14]. As a result, water is expected to induce a decrease of conductivity of LuPc2. Thus, the effects of NH3 and H2O should be qualitatively the same. It seems the case in LuPc2 resistors, even though the water contain changes the sensing feature versus NH3 [15]. A competition phenomenon in the adsorption-desorption process occurs, besides the contribution coming from the electron-donating nature of both the molecular material and gases, which makes the sensor response dependent on the applied experimental protocol, in particular the gas flow and the duration of exposure periods. However, NH3 and H2O molecules can induce opposite effects. Thus, n-type molecular semiconductor-doped insulator heterojunctions (MSDIs), which combine a low conducting sublayer, for example, Cu(F16Pc), with a high conducting top layer, such as LuPc2, exhibit a conductivity decrease under water vapors but an increase under ammonia (Fig. 5.4, right) [2]. This means that, in n-MSDIs, NH3 and H2O do not act in the same way. Water molecules act as traps for positive charge carriers in the highly conductive p-type top layer, leading to a conductivity decrease of this material, due to a mobility decrease, but keeping constant the energy barrier at the interface between the two materials [16]. In contrast, NH3 molecules neutralize positive charge carriers by electron transfer leading to a decrease of both the conductivity of the top layer and the energy barrier at the interface between the two materials. This second effect being the most important results in a current increase in the device, contrarily to what is observed for a resistor made from LuPc2, in which both NH3 and H2O molecules induce a current decrease. Organic semiconductor-based Schottky diodes were widely reported as gas sensors [17], but heterojunctions combining two molecular materials and used as gas sensors are very rare. Additionally to MSDIs, we can cite phthalocyanine-based p-n heterojunctions by Inta Muzikante et al. [18], which behave electrically as classical p-n diodes with an important rectification ratio between the forward and reverse currents. The highest relative response was observed in the inverse polarization regime. Recently, a new device combining two materials, namely, a double lateral heterojunction made from the electrodeposited poly(tetrafluoroaniline) covered by LuPc2, showed a high sensitivity to NH3, with a sub-ppm limit of detection (LOD) [19]. Another topic that recently opens is that of ambipolar materials that can exhibit a positive or a negative response to a given gas depending on a targeting parameter [20, 21].

5.2 Macrocycle-carbonaceous compound hybrid materials Even though this part has to be considered as a complement of Chapter 4, we can cite the reviews of M. Prato et al. on covalent functionalization of single-walled CNTs

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Fig. 5.5 Schematic view of SWCNTs functionalized by (from left to right and top to bottom) alkyl, fluorine, pyrrolidine ring, carboxylic acid, ester, amine, amide, and aromatic ring.

(SWCNTs) [22, 23]. It includes amidation and esterification of oxidized SWCNTs in which oxygenated functions (carbonyl, carboxyl, hydroxyl, and so on) appear on opened tubes. However, more elaborate methods have been developed, including cycloadditions, electrophilic and nucleophilic or radical additions. Examples of chemical groups introduced in SWCNTs are given in Fig. 5.5. Electrochemical methods developed for electrodes modification, such as the diazonium salt route, have been applied on CNTs to generate a series of aromatic ring substituted materials. Because of their poor intrinsic solubility, often, CNTs need to be associated with a surfactant or functionalized, if we want to prepare thin films by a solution processing technique [24]. The covalent grafting on CNTs is the first possibility, starting from carboxylic acid or amino-modified CNTs, for example, making them capable to react with amine or carboxylic acid group-containing macrocycles, respectively [25, 26], as illustrated on Fig. 5.6 with SWCNTs [27].

Fig. 5.6 View of ZnPc chemically linked to a SWCNT obtained by reaction of a carboxyphenoxysubstituted ZnPc with amine-functionalized SWCNTs.

Hybrid and 2D nanomaterials

However, it highly modifies the electrical properties of materials. It is the reason why noncovalent functionalization is often preferred [28]. It occurs mainly via π-π interactions that preserve the structure and properties of CNTs. Thus, apart from pyrene-containing molecules well known to strongly interact with CNTs [29, 30], extended aromatic π-system such as porphyrins and phthalocyanines are good candidates for such a functionalization. Another nice example of selectivity improvement of CNT-based sensing materials is that of covalently attached quinoxaline-bridged resorcin[4]-arene cavitands to gold nanoparticles anchored on oxygen plasma-treated carbon nanotubes [31]. A high selectivity toward benzene was obtained, compared with other studied VOCs, including toluene and xylene, with a response toward benzene about 10 times higher than for toluene and 100 times than for o-xylene, at concentrations of 100 ppb for all the gases (Fig. 5.7). Keeping in mind the lower saturation vapor pressure of xylenes compared with benzene, leading to an easier adsorption at a given molar concentration in air of xylenes compared with benzene, most of BTEX sensors are more sensitive to xylenes. So, the present selectivity is a huge performance. The simplest way to incorporate phthalocyanines in carbonaceous material-based electrodes is to mix a phthalocyanine in powder with carbon paste, resulting after compression in a syringe in a working electrode that can be used in electrochemical sensors [32, 33]. Such modified electrodes allowed to evaluate the bitterness in foods and beverages, giving rise to the so-called electronic tongues [32, 34]. For air quality monitoring (AQM) applications, phthalocyanines were also incorporated in carbon black and deposited as sensing films by spray cast, from suspensions containing both components. Thus, a series of resistors, with nonsubstituted MPc (M ¼ Co, Cu, Fe, Sn, Zn) and substituted phthalocyanines,

Fig. 5.7 Schematic view of a SWCNT decorated by a covalently attached quinoxaline-bridged resorcin [4]-arene cavitand to a gold nanoparticle anchored and the relative response of the resistor to VOCs. (Modified from P. Cl
ement, S. Korom, C. Struzzi, E.J. Parra, C. Bittencourt, P. Ballester, et al., Deep cavitand self-assembled on Au NPs-MWCNT as highly sensitive benzene sensing interface, Adv. Funct. Mater. 25 (2015) 4011–4020. https://doi.org/10.1002/adfm.201501234, permission requested from Wiley.)

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ZnPcR4 (R ¼ NO2, tBu) and MPc(O-n-octyl)8 (M ¼ Cu, Zn) were exposed to ammonia and VOCs [35]. ZnPc(NO2)4 exhibited the highest response to NH3 (30 ppm), whereas hybrid materials with phthalocyanines bearing alkoxy-donating groups were unsensitive to NH3. Since the noncovalent functionalization of CNTs preserves the electrical properties of nanotubes, hybrid materials were used as sensing materials associated with conductometric transducers, in particular by association with macrocyclic molecules. Thus, Penza et al. showed that resistors made from MWCNT films, grown by chemical vapor deposition technique onto alumina substrates previously coated with cobalt nanosized catalysts, covered by metallotetraphenylporphyrins, MTPP (M ¼ Mn, Zn), and deposited by solvent cast, exhibit an increased sensitivity toward VOCs, compared with naked CNT-based resistors [24]. The resistance of MTPP-modified CNTs was only slightly lower than this of pristine CNTs, but the response to gases was modified with a relative response higher for MnTPP toward all the tested VOCs, namely, ethyl alcohol, ethylacetate, acetone, and toluene, in the 30–100 ppm range, but triethylamine, for which the response order was CNTs > ZnTPP  MnTPP. The selectivity change from naked to modified CNTs was sufficient to discriminate between these VOCs, from a three-sensor array, after a principal component analysis. It was shown that the π-π interactions were strong enough to ensure the tacking of metalloporphyrins on CNTs [36]. As an example of phthalocyanine-containing hybrid materials, we can cite those prepared from a suspension of SWCNTs in DMF and a solution of a CoPc decorated with hexafluoroisopropanol substituents [37]. After sonication and stirring, the solution was filtered through a 0.22-μm Teflon membrane, then the solid was washed with DMF and rinsed with ethyl alcohol and dried. Thermogravimetric analyses allowed to determine that the hybrid material contained about one-third of CoPc derivative in mass, equivalent to 1 CoPc molecule per 240 C atoms. These analyses show that the washing steps do not remove the macrocycles out of the CNTs surface, indicating again strong interactions between the two components of the material. The sensor was prepared by drop casting of a suspension of CoPc containing SWCNTs between gold electrodes. The resistance increased under exposition to dimethyl methylphosphonate (DMMP), a simulant of the sarin. The sensitivity was high and the detection at 0.5 ppm easy, with a good S/N ratio, indicating that the LOD was still better. More interesting is the selectivity to DMMP, compared with other VOCs, like methanol, dichloromethane, hexane, and even water, due to specific strong hydrogen bonding between DMMP and hexafluoroisopropanol substituents beared by the phthalocyanine ring. Again, macrocyclic molecules bring processability and selectivity to CNTs while maintaining their electrical properties. SWCNTs decorated by tetratertiobutyl copper phthalocyanine, octaethyl porphyrin (H2OEP), and tetraphenylporphyrin (H2TPP) by drop casting a dispersion of the materials were used to detect BTX [38]. It is believed that ethyl moieties of H2OEP favor interaction with alkyl-containing analytes, but the first effect of the functionalization is the increase of the specific surface area.

Hybrid and 2D nanomaterials

5.3 Polymer-carbonaceous compound hybrid materials CNT-containing hybrid materials can be easily obtained by incorporating small quantities (few % in weight) of CNTs into a polymer, as exemplified by the incorporation of 1% of MWCNTs in polyepichlorhydrin, increasing the sensitivity of SAW sensors toward toluene, compared with the pure polymer, but without modifying the detection of octane, because of π-π interactions [39]. Carboxylic acid-functionalized MWCNTs were also used to decorate monodispersed PMMA microbeads (PMMAμB 2 μm in diameter) leading to a segregated network of MWCNTs bridging PMMAμB, which showed a certain selectivity toward methanol compared with water, toluene, and chloroform [40]. Even though covalent grafting on CNTs generally reduces their conductivity, it is not the case when conducting polymers are involved. Thus, PPy was used as a cationic polyelectrolyte with SWCNTs functionalized with carboxylate groups [41]. This PPyn + =SWCNT
CO2 n
material was used as sensing material in QMB sensors toward 1-butanol, in the ppb range. However, the same material combined with SWCNTs bearing tetraphenylporphyrin molecules (TPPH2) exhibited a higher response to 1-butanol, both being highly more sensitive than porphyrin-substituted polypyrrole without CNTs (Fig. 5.8). PANI-/MWCNT-based nanocomposite synthesized by in situ chemical oxidative polymerization of aniline monomer with MWCNT
CO2 n
showed a higher sensitivity to NH3, with a sub-ppm LOD. This response increases with the RH value [42]. Starting from a suspension of SWCNTs and hexafluoroisopropanol-substituted polythiophene, a percolative network was obtained as chemiresistive sensors for chemical warfare agents [43]. The electrodeposition of camphorsulfonic acid (CSA)-doped PANI on carboxylated SWCNTs was optimized to minimize the sensitivity to humidity, down to no sensitivity at all for a particular polymer thickness, allowing the response to NH3 to be almost unaffected by RH variations [44]. Without NH3, it is believed that the resistance increase of carboxylated SWCNTs under H2O is exactly compensated by ClO4 N

H2TPP

O

O

H2TPP

N CH2

N

N 5

O NH

N

O

HN N

H2TPP

ClO4

Fig. 5.8 Schematic view of a H2TPPH2-substituted polypyrrole in its partially oxidized form (left) and of the corresponding starting monomer (right).

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the resistance decrease of CSA-doped PANI. This was explained by a proton exchange-assisted electronic conduction mechanism. However, it tends to also diminish the response to NH3. In the case of PANI electrodeposited on MWCNTs used to detect aromatic hydrocarbons, the increase of conductivity under the target species was attributed to dipoledipole interactions that uncoil the polymer chain and decrease the interchain hopping distance for charge carriers [45].

5.4 Hybrid materials including inorganic materials Inorganic materials can be associated with conducting polymers or organic semiconductors. The simplest way is the addition of metal oxide nanoparticles into a polymer matrix and their deposition by solvent casting or spin coating [46], but the morphology of the film highly depends on the solubility of both components with, in general, a high tendency to aggregation. When the polymerization is carried out in the presence of MOx particles, hybrid materials are obtained, with a more intimate interaction between both materials [47]. With such electrochemical method, the use of a cross-linked PANI, associated with CeO2 nanoparticles, led to a better stability of the response to NH3 [48]. Such localized n-p heterojunctions can also be obtained by covering a nanostructured inorganic material by an organic semiconductor, for example, with n-type ZnO nanowires or ZnS nanoparticles covered by a p-type MPc [49, 50]. This method can be generalized, since it allows using every method to depose a nanostructured inorganic material, generally with a high-temperature process (see Chapter 2), then covering by an organic material using vacuum evaporation or some solution processing technique, depending on its solubility. The top layer can also be deposited by electrodeposition on the conducting sublayer, to form a diode, as in the case of n-ZnO/PANI and n-CdS/PANI, used to detect liquefied petroleum gas [51, 52]. Electrodeposited PANI was also used to decorate electrospun n-type TiO2 fibers leading to sensors that work as electric current switches when NH3 gas was absorbed by PANI nanoparticles, again due to the existence of localized heterojunctions, with a sub-ppm LOD [53]. Pure organic core-shell PANI-based composites with a poly(butyl acrylate) or poly(vinylidene fluoride) core and a composite based on PANI nanofibers embedded in a polyurethane (PU) matrix showed also a high sensitivity to NH3 [54]. For a recent review on PANI-based nanocomposites, we can cite the review by Pandey [55]. Other common conducting polymers, such as polypyrrole (PPy) and polythiophene (PT), were also associated with nanostructured SnO2, ZnO, MoO3, or WO3 to detect a series of VOCs, NH3, NO2, CO, CO2, or H2S, with generally better performances than with the pristine components. Starting from nanobuilding blocks, a large variety or organic-inorganic hybrid materials of tunable properties can be synthesized via various pathways that involve different types of interactions,

Hybrid and 2D nanomaterials

namely, coordinative, ionic, hydrogen, π-π, and van der Waals interactions, as reviewed by Kaushik [56]. In the heterojunctions, the improvement of the sensing performances, including an increase selectivity toward gases, is related to the key role played by the interfaces. This is the reason why reducing the size of domains increases the effect of heterojunctions. The advantages of heterojunctions are well described in the review of Akbar devoted to nanoscale metal oxide-based heterojunctions [57]. Other conductometric devices combine organic and inorganic materials, such as an hybrid n-p-n WO3/LuPc2 heterojunction, with a configuration analog to the aforementioned MSDI, used to detect NH3 in humid atmosphere [58]. An ambipolar hybrid organic-inorganic thin-film transistor, consisting of pentacene and ZnO as semiconductors, was also reported as gas sensor [59]. Interestingly, most of the hybrid organic-inorganic material-based sensors can operate at room temperature. Besides metal oxides and sulfides, polymers were also covalently bound to metal nanoparticles as sensing materials in resistors, via a 4-aminothiophenol linker. The poly (3,4-ethylenedioxythiophene-co-thiophene-3-acetic acid), a copolymer of PEDOT, linked to Ni and Pd particles were used to detect selectively toluene and acetone, respectively [60]. It works by a change of the work function of metals that interact with a certain degree of selectivity with different VOCs. Besides these inorganic-containing materials, we should also cite the metal-organic frameworks (MOF) that allow the separation of gases, because of their porosity and the specific interactions they developed with VOCs. It is out of the scope of this chapter to show the numerous MOFs used as sensing materials, but we can cite a review that shows that MOF-based sensors are mostly designed with optical and acoustic transducers [61]. However, conductive 2D MOFs have been recently obtained using 2,3,6,7,10,11hexahydroxytriphenylene and 2,3,6,7,10,11-hexaimino-triphenylene as ligands associated with Cu(II) and Ni(II) as metal centers [62]. A sensor array built with a family of structurally analogous 2D MOFs provided a clear discrimination between different categories of VOCs [63].

5.5 2D component-containing hybrid materials Since the first detection of individual molecules by graphene reported by Novoselov [64], graphene, graphene oxide, and hybrid materials based on these 2D compounds were highly used as sensing materials, firstly because of their high surface-to-volume ratio. Thus, reduced graphene oxide (rGO) was associated to Co3O4 [65] or to polyaniline (PANI) [66] to get a high sensitivity to NH3. The rGo-Co3O4 composite nanofibers were prepared via electrospinning, starting from a GO-Co3O4-polyvinyl pyrrolidone suspension, followed by thermal annealing up to 800°C [65]. The resulting rGO-Co3O4 composite appeared as nanofibers embedded into amorphous carbon. The encapsulation of oxide nanocrystals avoids their aggregation. The rGO/PANI material was obtained by in situ reduction of

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GO by the oxidative polymerization of aniline [66, 67]. A particular graphene-PANI nanocomposite including PANI nanoparticles and nanofibers deposited on a flexible substrate revealed a subparts-per-million LOD [68]. Such in situ reduction of GO was also achieved in the presence of SnCl2, leading to a SnO2/rGO composite [69]. Another synthetic way to obtain rGO-based nanostructures was the use of microwaves. GO was first coated with SnO2 nanoparticles in a microwave-assisted synthesis in benzyl alcohol, in the presence of SnCl4 as SnO2 precursor, followed by the deposition of Pt nanoparticles by the reduction of H2PtCl6 in ethylene glycol, again assisted by microwave radiation [70]. The Pt-SnO2/rGO nanocomposite showed enhanced hydrogen sensing performance relative to that of the corresponding pure and binary systems. In another example, rGO films were detached from a quartz substrate and transferred onto AgNP and AgNW films obtained by spray coating, then covered by interdigitated gold electrodes [71]. The sensors prepared with AgNWs were more sensitive to NH3 than those containing AgNPs. The recent emergence of a new variety of 2D materials created new opportunities not only in the field of electronics and optics but also in the field of chemical sensors. Transition metal chalcogenides, of general formula MX2 (M ¼ Mo, W and X ¼ S, Se or Te), due to their semiconducting properties, are well suitable for conductometric transducers. For an overview on the synthesis, structure, spectroscopy and applications of 2D metal chalcogenides, see a review by Dubertret et al. [72] and a special issue in Acc. Chem. Res. [73–75]. Associated with molecular materials, they can form what are called van der Waals p-n heterojunctions, for example, with pentacene and MoS2 [76]. As for graphene, their high surface-to-volume ratio makes these 2D materials suitable for chemical sensing applications, where perturbations to the surface resulting in charge redistribution are readily manifested in the transport characteristics, as exemplified by the detection of trimethylamine by MoS2 [77]. Because MoS2-based gas sensors still suffer from long response and recovery times, especially at room temperature, they have been photoactivated under UV light [78] and also combined with other materials. Thus, a 2D graphene/ MoS2 heterostructure where exfoliated MoS2 flakes bridged graphene lines exhibited a negative response to NO2 and a positive response to NH3 [79]. However, the fabrication of the device remained rather sophisticated. MoS2 was also associated with GO as a hybrid composite obtained by simply mixing MoS2 and GO suspensions and spin coating on a substrate before thermal annealing [80]. An ultrasensitive NO2 detection, with a LOD of 50 ppb, even at room temperature, was obtained with a MoS2/graphene hybrid aerogel. An electrically conductive graphene aerogel with ultrahigh surface area was prepared from GO, then coated by ammonium thiomolybdate and the MoS2 formation initiated by annealing at 450°C under hydrogen, the final material being obtained after a second annealing in the presence of sulfur [81]. MoS2 can also be decorated by metals, for example, by vacuum deposition of Pd on drop-casted nanosheets of MoS2, Cr/Au electrodes being deposited on the top of the hybrid material. The obtained hydrogen sensors exhibited a LOD of 50 ppm [82].

Hybrid and 2D nanomaterials

However, even though MoS2 offers a high sensitivity toward various gases, depending on the second component associated with, it was also reported as a good sensing material for humidity. Thus, a MoS2-/GO-based resistor exhibited a very high sensitivity to humidity, the current being multiplied by 55 at 35% RH and by 1.6103 at 85% RH [83]. In the case of Pt-decorated MoS2, with a ratio of Pt/MoS2 of 1/4 (w/w), the current was even multiplied by 4103 at 85% RH [84]. With gold, the decoration of MoS2 was achieved chemically, by adding a HAuCl4 solution to an aqueous dispersion of chemically exfoliated MoS2 at room temperature and stirring for 30 min [85]. The obtained suspension was filtered on a porous alumina membrane; then, alumina was dissolved in a 3-M NaOH solution; finally, the MoS2 films floating at the solution surface were transferred on electrodes. The resistors showed a current increase under ethanol and acetone, but a current decrease under toluene and hexane, whereas the pristine MoS2 resistor showed a current decrease in the four cases. Clearly, the incorporation of Au NPs brings some selectivity.

5.6 Challenges in hybrid material-based gas sensing The combinations of macrocycles, polymers, carbonaceous materials, and inorganic materials are infinite, in particular if we combine more than two of them. As an example, we can cite the case of an electrochemical sensor in which hemin, an iron porphyrin complex, deposited on the surface of a Pt/ZnO/PPy structure, acts as an electrocatalyst to detect NOx at the submicromolar level in aqueous solution [86]. Another example is that of CNT/PPy chemiresistors in which the polymer bears calix[4]arenes that ensure the selective recognition of aromatic VOCs like BTX [87]. Elsewhere, whatever the efforts made on the characterization of materials, the response obtained with a given hybrid material still depends on the nature of electrodes and on the interface between the electrodes and the sensing material. So, sensing performances can be tuned by various electrode treatments, such as plasma treatment and temperature annealing, or by chemical modification of electrodes, for example, by self-assembly monolayers or by electrochemical methods [88]. Finally, as soon as applications in real atmospheres are envisaged, a key attention need to be paid to the cross sensitivity with humidity, which has to guide us in the choice of the different components of the hybrid material used as sensing material. Finally, the choice of a sensing material has to be thought simultaneously with the choice of the type of transducer, as a function of the target application, and the physical parameters modified by the analytes need to be considered.

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CHAPTER 6

Fabrication techniques for coupling advanced nanomaterials to transducers Saleem Khan, Danick Briand Ecole Polytechnique F
ed
erale de Lausanne (EPFL), Soft Transducers Laboratory, Neuch^atel, Switzerland

6.1 Introduction Monitoring of toxic gases is central to a wide range of applications such as industrial processes control, health and environmental monitoring, agriculture, and automobiles. Since the advent in 1970s [1], gas sensors have passed through drastic developments in the device structure, materials, and sensing mechanisms [2, 3]. Recently, great efforts have been devoted to implement new strategies, ranging from developments on silicon-based and on unconventional substrates by employing advanced nanomaterials [3, 4]. Developing gas sensors on polymeric foils using advanced nanomaterials and manufacturing techniques is an emerging approach believed to enhance the cost-effectiveness and functionality. Besides the lower cost of polymeric films, the range of applications is widened by harnessing the benefits of lightweight, flexible, foldable, conformable, transparent, and portable substrates [2]. Performance such as sensitivity and selectivity of the gas sensors is directly related to the surface volume; therefore, nanomaterials having larger surface-tovolume ratio exhibit greater opportunity for enhanced responses. The performing gas-sensing properties of metal oxide semiconductors (MOX) make them favored for the gas-sensing layer [5, 6]. Other class of nanomaterials such as organic conductors, carbon nanotubes (CNTs), and graphene and nanocomposites of these are also extensively reported [7–10]. Conventionally, clean room processes are used to deposit or grow nanostructured materials on rigid silicon or alumina substrates. However, some of these processes are also applied for the development of foil-based gas sensors. In these processes, the nanomaterials are developed either directly as a sensitive layer on the prefabricated gassensing structure or as a colloidal solution that is synthesized and applied through solution-based manufacturing technologies. The recently developed additive manufacturing (AM) involves processing nanomaterials in either solid or liquid phase by depositing them directly on the substrates. Among AM techniques, solution-based printing is attractive and commonly used for rapid prototyping in much simpler way as compared with photolithography procedures [11]. Printing enables manufacturing of large-area devices on unconventional substrates at higher speeds and with reduced infrastructure that greatly reduce the fabrication cost. Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00006-2

Copyright © 2020 Elsevier Inc. All rights reserved.

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As the scope of this chapter is to explore the fabrication technologies involving nanomaterials for gas sensing, therefore, clean room processes and AM (especially printing) techniques will be the focal topics of discussion. In the first section, a brief overview of clean room processes is provided. AM including different types of materials and printing technologies is explored in the second part, followed by concluding remarks and future perspective.

6.2 Clean room processing of nanomaterial for gas sensors Gas sensors are developed following different transduction mechanisms and device architectures [2, 3]. The prominent types are based on resistor, diode, metal-insulatorsemiconductor (MIS) capacitors, and metal-insulator-semiconductor field-effect transistor (MISFET) for monitoring resistance change, bias current, bias potential shift, and threshold voltage shift, respectively, upon detection of gas analytes [3, 4, 7, 12]. Among these, chemoresistance-/conductometric-based sensing has been dominating the field, especially employing oxide-based semiconductors for their good chemical and physical stability [6, 12]. Wafer-based bulk micromachining in clean room has been extensively reported for transducer fabrication [13–16]. Clean room processes such as low-pressure chemical vapor deposition (LPCVD), plasma-enhanced CVD (PECVD), aerosol-assisted CVD (AACVD), sputtering, atomic layer deposition (ALD), vapor liquid solid (VLS) growth, electroplating, and hydrothermal process have been in use for many years [14–18]. A variety of nanomaterials involved in the fabrication cycle of complete or partial device structure are exercised by the aforementioned processes [14–18]. Higher temperatures and harsh chemicals are often needed for the growth of nanoscale materials; therefore, the usual substrates employed are Si wafers. When integrated heating is involved, the common practice is the development of a thin membrane of silicon nitride (Si3N4) for its good thermal stability and electrical insulation, including an embedded resistive heater. LPCVD is usually applied for the development of a low-stress Si3N4 membrane of a silicon-micromachined gas sensor [19] For gas-sensing layers’ development, AACVD is used for the growth of palladium oxide (PdO) nanoparticle-decorated tungsten oxide (WO3) nanoneedles [14]. PdO-decorated TiO2 (titanium oxide) layer is reported using radio-frequency sputtering method. Pd is deposited on top of sputtered TiO2 and annealed the multilayer structure at 550°C to convert Pd into PdO for enhancing the gas-sensing properties [15]. Tin oxide (SnO2) alone and Pd or Pt-doped SnO2 thin films made of nanocrystallites can be deposited using RF sputtering, and their gas-sensing characteristics to different gases are investigated [20, 21]. LPCVD is also applied for deposition of graphene, where a thin layer is grown and decorated further by ZnO and SnO2 [18]. Clean room processes are extended toward deposition of MOX, 2-D, and carbonbased nanomaterials on polymeric substrates [22–25]. Among the list of polymeric

Fabrication techniques for coupling advanced nanomaterials to transducers

substrates, polyimide (PI) has relatively good thermal stability and chemically inert; therefore, it is mostly used for the gas sensor development involving clean room processes [26]. PECVD is reported on PI for deposition of indium-gallium-zinc oxide (IGZO) as NO2 sensing layer in a thin film transistor (TFT) structure. [24]. AACVD is also employed to integrate WO3 nanoneedles functionalized with ferric oxide (Fe2O3) nanoparticles on a flexible substrate [25]. Functionalization with Fe2O3 enhanced the electronic and sensing properties of WO3 by sixfold toward different gaseous compositions [25]. Twodimensional (2-D) materials, that is, niobium diselenide (NbSe2)- and tungsten diselenide (WSe2)-based flexible, wearable, and launderable gas sensors, are prepared through CVD on prepatterned WO3 and Nb2O5 nanostructures [27]. Vapor-phase deposition is applied for the development of gas sensor on flexible substrates by direct integration of highly crystalline WO3 nanostructures functionalized with gold (Au) and platinum (Pt) nanoparticles [28]. Large-area tungsten disulfide (WS2) nanosheets are synthesized by ALD and functionalized by using silver (Ag) nanowires for NO2 detection [29]. Thin films of TiO2 developed through reactive DC magnetron sputtering on a PI substrate are used for H2 gas sensing [23]. Comparison of indium oxide (In2O3)-based sensors developed through thermal oxidation process on rigid alumina and PI substrates is presented. Sputtering is used to functionalize the sensors with Ag nanoparticles and tested against NO2 [30]. A novel approach for developing wearable gas sensor is proposed on textile. The surface of carbon cellulosic fabric is modified by sol-gel and sputter seed layer-coated ZnO nanostructures for detecting various gases [31]. Another interesting approach developed recently is the dry transfer of Si thin membranes onto polymeric substrates. This has provided the opportunity to enable high-temperature processes on the donor (Si) wafer, and the final structures are transferred through soft polydimethylsiloxane (PDMS) stamp. One such approach is presented by developing high-performance and low-power flexible Schottky diode-based hydrogen sensor [32]. The sensor is fabricated by releasing Si nanomembrane (SiNM) and transferring onto a plastic substrate [32]. Carbonaceous materials, especially CNT and graphene, have emerged as the most demanding and are also processed through clean room technologies. SWCNTs (single-walled) have potential for creating high-performance gas sensors, and large-area processability on flexible substrates makes them more interesting. Directly synthesized SWCNTs and molybdenum disulfide (MoS2) by CVD are reported on flexible substrates and applied for sensing of NH3, NO, and NO2 gases [33, 34]. A comprehensive summary is provided about graphene integration onto polymeric substrates and applications for gas and chemical sensing [35]. CVD-developed graphene with hybrid Ag nanowire gas sensors are reported on highly stretchable substrates for sensing toxic gases [36]. Development of gas sensors on most of the polymeric substrates via clean room processes is challenging due to their lower glass transition temperatures and instability upon exposure to various chemicals. Therefore, focus has been shifted toward low-temperature and solution-based AM processes of nanomaterials in recent years.

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6.3 Additive manufacturing AM is a rapidly growing field that involves the “bottom-up” approach for developing multilayer structures using various techniques. AM is fundamentally different from conventional “top-down” approach, where the designed structures are added layer by layer using mainly subtractive processes [37]. The simple and cost-effective methods make AM more attractive, versatile, and highly customizable for a wide range of applications. Materials in different forms such as liquid, wire, or powder can easily be processed for locationspecific depositions of 2-D and 3-D structures at varied dimensions [37–39]. Advantages such as direct manufacturing, lower processing cost, reduced waste, shorter processing time, and developing complex structures distinguish AM from conventional top-down approaches [39, 40]. The AM technologies developed so far are divided into seven different categorized (Fig. 6.1) including binder jetting (BJ), directed energy deposition (DED), material extrusion (ME), material jetting (MJ), powder bed fusion (PBF), sheet lamination (SL), and vat photopolymerization (VP). Processing approach of each technique is different (summarized in Table 6.1), presenting a different set of benefits and challenges [37, 38, 40]. Details of these technologies with their processing approaches, pros and cons, and the type of materials are summarized elsewhere [39, 40]. Materials can be processed both from solid and liquid phase where the specific type of AM is

Binder jetting (BJ) Directed energy deposition (DED)

Vat polymerization (VP)

Additive manufacturing Materials extrusion (ME)

Sheet lamination (SL)

Powder bed fusion (PBF)

Materials jetting (MJ)

Fig. 6.1 Classification of additive manufacturing (AM) techniques.

Fabrication techniques for coupling advanced nanomaterials to transducers

Table 6.1 Summary of additive manufacturing (AM) techniques AM process

Operation principle

Process form

Materials state

VP BJ MJ PBF DED ME SL

Polymerization Direct deposition using ink/colloidal suspension Melting and freezing

Solution

Chemical/colloidal homogeneous solution

Powder and filament

Solid

Joining and fusion

Sheets

involved. For instance, nanomaterials in polymeric or colloidal solutions are desired for the VP and BJ/MJ techniques. On the other hand, solid materials in the form of powder or filament are treated through melting and freezing in the PBF, DED, and ME manufacturing processes [37, 38, 40]. Among the list of AM techniques, MJ is the most attractive for quick patterning and thin film deposition of functional materials. The rapidly growing printing technologies are based on the concept of MJ technique. Printing has enabled a cost-effective route for the fabrication of thin film electronics on diverse substrates [11]. Printing utilizes nanomaterials in the form of colloidal dispersions or nanocomposites for all or partially printed gas sensors [41–43]. Printing technologies have been utilized initially for the development of gas sensors on rigid substrates such as Si and alumina. Materials of choice are potentially MOXs and their composites for their good compatibility with highertemperature processing and annealing conditions. The most common substrates are silicon-micromachined and ceramic-based materials. Low-temperature cofired ceramics (LTCC) is one such example used for developing gas sensors [44]. A two-layer device of printing interdigitated electrodes (IDEs) and sensitive layer is developed by using hybrid screen and inkjet printing technologies. Ag-based screen-printed IDEs are used with polyaniline-chloride for detection of hydrogen sulfide (H2S) gas [45]. SnO2 nanowires have been screen printed on alumina substrates for detection of various gases such as C2H5OH, CH3COCH3, C3H8, CO, and H2 [46]. Similarly, SWCNT-based gas sensor is inkjet printed on alumina substrate for NH3 detection [47]. Several other materials, that is, ZnO-doped MnO2 and SnO2-doped MoO3 nanofibers, are printed directly for gassensing applications [48, 49]. Rheological properties such as viscosity, surface tension, conductivity, particles’ content, and average nanoparticle sizes of the colloidal solution are tuned according to the specific requirements of the printing process. Despite the fact that sensors printed on rigid substrates present good sensitivities, the real benefits of printing can be harvested by processing on large-area polymeric substrates. Therefore, the following sections are focused on nanomaterial-based gas sensors manufactured through printing technologies.

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6.4 Additive manufacturing of gas sensors on foils AM has simplified the fabrication of multilayered structures and significantly reduced the manufacturing cost by minimizing the materials’ waste. Development of gas sensors on polymeric foils is attractive for their exciting properties such as low cost, mechanical flexibility, conformability to uneven surfaces, and lightweight for portability [2]. The lower thermal conductivity losses of plastic substrates are ideal for integrated microhotplates, which are particularly required for MOX gas sensors. Thermally stable polymeric substrates have already been utilized to produce gas sensors but through complex clean room processes [22, 50]. To avoid these complexities, a more robust and simple fabrication such as digital printing of functional materials is of great interest. Multilayer structures can easily be developed on the same side of substrate by using compatible solutions. Printing enables the development of electronic components on areas larger than standard wafer-scale commonly practiced in clean rooms [11]. Micromachining in clean rooms involve subtractive techniques, where layers of desired materials are deposited and structured using photolithography masks and unwanted material is stripped off as residual waste. This directly increases the cost of the products due to the many processing steps and material consumption/wastage that contribute partly to the electronic waste, a major issue these days [51]. In this scenario, location-specific printing of functional materials is more advantageous. Various printing techniques have evolved in line with the rapid growth of printed electronics. All these systems are distinguished based on the operating principle; process ability of materials having different rheological properties, that is, viscosities and surface tension; and potentially the desired printed feature sizes [11]. Gas sensors are fabricated partly or fully, sensing and transducing materials, through printing technologies. A hybrid approach is sometimes adopted, where different printing/deposition techniques are combined for full sensor development. This discussion covers all the approaches, that is, partially or fully printed gas sensors, where nanomaterials are processed through the following most favored printing technologies.

6.4.1 Screen printing Screen printing is a well-established technology in the field of printed electronics, as it has been used extensively for printing thick metallic films and interconnects on printed circuit boards. The simple fabrication process, fast speed, and affordability make screen printing a distinguishing tool as compared with other printing technologies [11]. Screen printer has simple setup comprising stencil, squeegee, press bed, and substrate, as shown in Fig. 6.2A and B. The desired structures are designed in the screen mesh and printed on the target substrate positioned beneath the stencil mask. Ink/paste is poured on the stencil and moved across the screen with a squeegee. [52]. Less amount of ink is deposited depending on the desired structures, while residual ink is collected after finishing the printing cycle. Although a very simple process, the print quality and characteristics are

Fabrication techniques for coupling advanced nanomaterials to transducers

Ink delivery

Ink delivery Jetting waveform Voltage

Ink Piezoelectric actuator

Substra

te

Ink Microdroplets generation

(v)

Droplets ejection

Time

(A)

Pulsed high voltage

(B)

Substra

te

Fig. 6.2 Schematic of (A) screen-printing tools and mechanism and (B) manual screen-printing setup.

affected by many variables including solution viscosity, printing speed, angle and geometry of the squeegee, standoff between screen and substrate, mesh count, size, and material. Therefore, proper tuning of the solution properties and surface conditions of the target substrate are of prime importance. Screen-printing technique is usually compatible with high viscosity, ideally 200 cP [11]. Screen printing has been well established on ceramic-based substrates and is usually used to print electrodes, heaters, and thick sensing layers. Notably, metal oxide gas sensors have been commercialized for several years by screen-printing Pt transducers on alumina substrate with the metal oxide gas-sensing films [53, 54]. Screen printing on silicon-micromachined substrates is challenging as it involves a printing plate in contact with substrate, which is a problem for deposition of gas-sensing films on thin fragile membranes. Screen printing has been successfully implemented for gas sensors’ fabrication using nanoparticle-based solutions/pastes. A two-layer device by printing IDEs and sensitive layer is developed by using hybrid screen and inkjet printing technologies. Ag-based screen-printed IDEs are used with polyaniline-chloride for detection of hydrogen sulfide (H2S) gas [45]. Thick films of doped and undoped SnO2 are screen printed [55] while investigating sensitivity, optimum working temperature, and responsivity in relation to the dopants and preparation route. Further, SnO2 nanopowder mixed with glass frits as a binder and palladium or platinum as catalyst are also reported [56, 57]. Different compositions, that is, 15% alkoxide and 24% binder, were added to SnO2 for optimization of the screen printability [57]. NO2 sensing of a screen-printed SnO2 thick film device are reported with good sensitivity down to 0.5ppm [58]. An investigation of screen printable SnO2 and conductive polymer polypyrrole (PPy) is carried out for detection of CO2. A comparative study is performed by printing the two materials separately and finally through a multilayer structure of SnO2/PPy. The thick multilayer structure presented enhanced stability and good sensing response as compared with the discrete layers of the respective materials [59]. An integrated sensor array comprising four sensors developed by screen printing and sputtering of SnO2 films for detection and discrimination of single analyte (ethanol, acetone, and ammonia vapors) and mixtures is implemented [60].

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Surface modification of SnO2 layers by zeolites was reported to enhance sensitivity and selectivity toward different gases. Admixtures were prepared by screen-printing composites of the control material with 10% (w/w) and 30% (w/w) of zeolite [61]. Another modification of printed SnO2 layer to enhance the selectivity is done through functionalization with 3-aminopropyltriethoxysilane (APTES) [62]. Similarly, tungsten trioxide (WO3)-based thick film chemiresistors are fabricated using screen-printing technology and tested against various gases such as NH3, H2S, and LPG. The effect of microstructure on sensitivity, response, and recovery time of the sensor in the presence of various gases is presented [63, 64]. The possibility of depositing relatively thick layers has enabled printing of lowresistance structures especially with conducting polymers by compensating the high volume resistivity with a thicker layer. Granular and porous structures are printed using tin-doped indium tin oxide (ITO) thick films [65]. This specific morphology is suitable for various gas sensing, and the ITO crystalline grain size dimension is a key parameter influencing the gas response characteristics [65]. Screen-printed mesoporous SnO2CuWO4 is obtained by using tripropylamine(TPA) as template and polyvinylpyrrolidone (PVP) as dispersant/stabilizer for selective detection of H2S [66]. Screen-printed nanocomposites based on SnO2 nanoparticle-reduced graphene oxide (SnO2-RGO) are also investigated [67]. The nanocomposite solution is prepared by a facile method via hydrothermal treatment of aqueous dispersion of GO in the presence of Sn salts. Sensors are tested at room temperature where SnO2-rGO nanocomposites exhibited high response at 5 ppm of NO2 compared with rGO with rapid response, good selectivity, and reproducibility. Zinc oxide (ZnO) nanostructures synthesized by hydrothermal method and thick layers were deposited using screen printing for detection of H2S. The thixotropic paste was formulated by mixing the synthesized ZnO powder with ethyl cellulose in a mixture of organic solvents. The ratio of inorganic to organic part was kept as 75:25 in formulating the pastes, which were in the suitable range of screen-printing paste requirements [68]. The glass nanocomposite materials (V2O5-MoO3-ZnO) in the form of fine granular powders, synthesized by conventional melt quenching technique, are also applied for gas-sensing measurements. Thick films of the nanocomposite were fabricated by screen-printing technique and analyzed to study the gas response and selectivity of the sensor in the presence of ethanol and other gases [69]. Doped ZnO nanocomposites (ZnO and Al3%- and Ca5%-doped ZnO nanoparticles) are printed, showing enhanced selectivity and sensitivity toward CO and CO2 gases [70]. The recently developed perovskites are also printed for gas sensors’ development related to environmental monitoring [71].

6.4.2 Inkjet printing Inkjet printing is the predominant and commonly practiced technique for patterning of functional materials on diverse substrates. It has become a trademark of printing

Fabrication techniques for coupling advanced nanomaterials to transducers

technologies and remains at the core of printed electronics field [72]. Materials in the form of colloidal or chemical solution with viscosity in the range of 10–12 cP are ejected through micrometer-sized nozzles. Nozzle diameters are selected based on the average nanoparticle sizes and other rheological properties of the solution to achieve a given drop volume. Droplet ejection occurs as a result of different actuation mechanism such as piezoelectric, thermal, or electrohydrodynamic (EHD) [11]. Deposition occurs in the shape of microdroplets often termed as drop on demand (DoD), at corresponding actuation sequences. Ink is contained in a reservoir that is connected to the inkjet nozzle head equipped with corresponding actuators. For instance, a piezoelectric source is used to generate a microdroplet at the peak of each waveform pulse as shown in schematics of Fig. 6.3A. EHD works on a different principle, where an alternating current (AC) is applied to the ink solution and droplets are ejected at each rising peak (Fig. 6.3B). Piezoelectric-based inkjet printing is broadly used because of the simple process and ability to handle a wide range of materials. Proper tuning of the solution properties is needed for continuous jetting and unclogged nozzles. Uniform spreading of the droplets on properly treated substrates, right drop-to-drop spacing, and the drying conditions of the droplets have to be appropriately adjusted for achieving good printing resolutions and films with uniform morphologies. Printing of gas sensors using nanomaterials are frequently reported through piezoelectric inkjet systems. A fully printed approach by utilizing both sides of polyimide (PI) substrates is presented for the development of gas sensors [73]. The integrated microhotplate is printed on the back side, whereas IDEs and sensing layers are printed on the front side of the substrate. Au is used for the conductive patterns, whereas SnO2 nanoparticles’ solution is printed for the gas-sensitive layer [72, 73]. A new approach to print a coplanar architecture of gas sensor is also presented by designing two electrodes and three contacts [74]. The sensor is developed on a PI foil by printing Au for metallic part, and Pt-loaded WO3 is used as the sensing layer. The preliminary results of the sensors against H2 are promising as compared with the classical topology [74]. Further, Cu2O core-shell microgels with a nanocube-shaped core structure are utilized for printing gas-sensing layers [75].

Fig. 6.3 Schematic of inkjet printing. (A) Piezoelectric-based inkjet system. (B) Electrohydrodynamic (EHD)-based inkjet system.

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The hybrid microgels showed improvement in colloidal stability as compared with native Cu2O nanocubes, which is suitable for inkjet printing at quite low particles’ loading (i.e., 1.5 wt%) [75]. The effects of surface reactions of nitric oxide (NO) and nitrogen dioxide (NO2) on copper(II) oxide (CuO) are also investigated [76]. The surface reactions are deeply explored by showing gas-induced changes of the electrical conductivity of CuO upon NOx exposure that can be tuned from oxidizing to reducing behavior, by showing that the interaction may be effectively turned off. This is based on the demonstration that the metal oxide reaction is solely governed by the temperature-dependent chemical equilibrium of NO/NO2 [76]. Inkjet-printed copper acetate-Au nanocomposites have shown greater sensitivity at subparts per million level detection of H2S gas [77]. Inkjet printing of ZnO nanosheets as a gas-sensing film is deposited followed by deposition of Al2O3 loaded with Ru nanoparticles (Ru/Al2O3) as catalyst. Sensor has a good sensitivity toward selective detection of SO2, which is of particular interest in the application fields such as environmental protection and food manufactory [78]. EHD is also reported for printing Pd-loaded SnO2 nanofibers precisely on the suspended central part of a microhotplate with an area of 100  100 μm2. Small droplets with diameters of 50–80 μm are produced at each ejection by providing a high voltage to the metallic needle. A wide needle of internal diameter of 110 μm is used to avoid nozzle clogging [79]. Micropatterning of metal oxide nanofibers by electrohydrodynamic (EHD) printing toward highly integrated and multiplexed gas sensor applications is reported by utilizing various MOXs such as SnO2, In2O3, WO3, and NiO. Sensors made from these four kinds of metal oxides could detect down to 0.1 ppm of NO2, 1 ppm of H2S and 20 ppm of CO [80]. One-dimensional SnO2 lines are also synthesized from precursor solutions using EHD printing for NO gas sensor [81]. A large variety of nanomaterials other than MOX are recently developed for printing gas sensors on polymeric substrates. Among these, carbon-based nanomaterials are the most promising and explored widely for gas-sensing beside others. CNTs and graphene and nanocomposites of these two in polymeric blends result into a stable ink for printing [82]. Attractive feature of these materials is their operation at low temperatures, thus providing an opportunity to print sensors on low Tg (glass transition temperature) substrates. For instance, gas sensor properties of polyaniline-functionalized multiwalled carbon nanotube (PANI/MWCNT) nanocomposite are explored for trace level detection of NH3 [83]. PANI/MWCNTs are also compared with carboxylated MWCNTs, exhibiting enhancement in sensor response and response/recovery characteristics toward NH3. An innovative idea to make cost-efficient ammonia gas (NH3) sensor is presented on paper-based substrate using inkjet printing. Poly(m-aminobenzene sulfonic acid) functionalized (SWCNT-PABS) are printed on Ag showing excellent short response time to NH3 while remained stable for several months [84]. Graphene has shown magical properties in the recent trend of electronic materials, and its printability is extensively studied for gas-sensing characteristics [85–87]. Inkjet printing of water-soluble single-layered

Fabrication techniques for coupling advanced nanomaterials to transducers

graphene oxide (GO) and few-layered graphene oxide (FGO) is reported on diverse flexible substrates, including paper, poly(ethylene terephthalate) (PET), and PI [42]. A room-temperature sensor developed by making a composite of graphene/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and inkjet printing is used for deposition on large area [88]. The graphene-PEDOT:PSS exhibited high response and selectivity to NH3 in a low concentration range of 25 ppm. The attained gas-sensing performance is attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH3 molecules [88]. The digital formation of gas sensors arrays through inkjet printing is of great interest and especially expanding the possibility toward integrating sensors on smart wireless labels [89].

6.4.3 Spray coating/printing Spray coating/printing is attractive for its simple processing and direct deposition of solutions on diverse substrates. The process is predominantly driven by a spraying nozzle equipped either with a pressurized air or a high electric field. The pressurized air-driven system often called “aerospray” contains an ink reservoir or continuous supply through microfluidic channel. The solution is injected into the delivery tubes via syringe or microfluidic pumps. A concentric nozzle is used (shown in Fig. 6.4A), where the central nozzle carries the solution and the outer nozzle is supplied with controlled air pressure. The solution is dispersed into wider spray at the exit of the nozzle orifice and is directed toward the target substrate. Alternatively, the electric field-driven process is usually termed “electrospray,” and the system configuration is similar to EHD inkjet system (Fig. 6.4B). In electrospray, the standoff distance between the nozzle tip and counter electrode is increased, and a higher electric field between the two electrodes is applied [90]. A metallic nozzle is used for the ink delivery and top electrode, whereas the counter Ink supply Ink supply system

DC power supply Compressed air/N2

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Light source

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Fig. 6.4 (A) Schematic of aerospray printing. (B) Schematic of electrospray system.

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electrode is a metallic plate used also to hold the target substrate. Ink flow rate and solution’s conductivity play important role in achieving stable cone-jet and are central to the electrospray deposition process. Electrospray generates a very fine spray as compared with the aerospray deposition, and film thicknesses in the range of nanometers can easily be achieved [90]. Electrospray causes less impact on substrate as compared with the aerospray mechanism. Therefore, very thin and uniform layers of conductor, semiconductor, and insulating materials can readily be deposited by adjusting the conductivity, viscosity, and flow rates of the solutions. Flame spray pyrolysis is another interesting technique to directly deposit films or synthesized nanoparticles for gas-sensing layers. Gas sensors are obtained through spray pyrolysis by depositing TiO2 and α-MoO3 for NO2 and ammonia-sensing applications, respectively [91, 92]. Further, sensors are reported by modeling the growth of thin SnO2 films using spray pyrolysis deposition [93]. Detailed analysis, that is, electrical, optical, and structural properties of the spray-coated SnO2 layer, is also explored by observing the effects of Li doping in SnO2 layers [94]. Comparative study of the undoped and cobalt-doped SnO2 spray coating is done on different substrates and tested against ozone and hydrogen gases [95]. Different characteristics of the doping levels and sensor parameters dependent on the fabrication process are briefly investigated. Electrospray deposition of precursor solution of SnCl4
5H2O in ethanol is used to achieve a thin film of SnO2 by exploring different processing parameters [96]. Similarly, ZnO nanoparticles’ solution is coated using electrospray deposition. A porous homogeneous film is obtained showing a good sensitivity toward NO2 and H2S at 1 and 12 ppm respectively [97]. Nanostructured coating of zinc acetate Zn(CH3COO)2 precursor is presented by processing through plasma-assisted spray deposition and tested against NO2 sensing [98]. WO3 is deposited from its precursor solution through spray coating and modified with graphene oxide (GO) suspension to obtain WO3-GO nanocomposite [99]. CNTs alone or blending with metal oxides are the usual approach for developing gas sensors [100, 101]. Spray coating of CNT-based gas sensors functionalized with different metallic nanoparticles (NPs) (Au, Pd, and Ag) with exceptionally high responses toward test gases (i.e., NH3, CO2, CO, and NOx) are reported [101]. Heterojunctions of CNTs and SnO2 nanowires are formed by spray coating CNTs on preprepared SnO2 nanowires [102]. A platform for chemiresistive gas detectors is developed by making surfaceanchored poly(4-vinylpyridine) (PVP)/SWCNT/Ag nanocomposite and deposited using spray coating technique [9]. Reduced graphene oxide (RGO) in pristine or composite form is processed with spray coating for developing the gas-sensing film [103]. The spray-coated RGO is sensitive to gases at room temperature without requiring any post heat treatment, chemical reduction, or doping [103]. Spray deposition of RGO/ZnO bilayer thin films are used to enhance surface roughness and more spacing compared with single RGO layer [104]. Sensitivity of RGO/ZnO film is enhanced by 30% as compared with RGO film due to the improved film structure [8]. NO2 sensor based on metal-

Fabrication techniques for coupling advanced nanomaterials to transducers

semiconductor interface of electrolytically exfoliated graphene and SnO2 nanocomposite is developed that operates at low temperatures [105]. Spray coating of colloidal quantum dot (i.e., PbS) porous film is another approach for developing low-cost gas sensors [41, 106]. The porosity of the coated layer is assumed to be responsible for the superior gas-sensing behavior [41]. All these results show that spray coating has the potential to produce low cost and ultrathin layers of gas sensors in much easier and simple ways.

6.4.4 Aerosol jet printing (AJP) Aerosol jet printing (AJP) is the recently developed and fast-growing AM technology for processing large variety of materials at wider viscosity ranges (1–1000 cP) [26]. The capability to print high-resolution patterns 10 μm and wide as few millimeters makes AJP an attractive tool for printed electronics. The AJP works on the principle of microdroplets generation by atomization of functional material through pressure waves [26]. AJP is divided into two classes based on different atomization procedures, that is, pneumatic and ultrasonic (Fig. 6.5A and B). Pneumatic atomizer generates aerosol, when a pressurized gas/air is supplied to the ink chamber through a vertically inserted dual-hole tube (stem) in the ink jar. The gas emitted at top outer hole causes the liquid level to rise inside the tube through the bottom hole. By reaching at the holes’ junction, solution interacts with the incoming gas and is ejected in the form of aerosol through the upper hole. This process continues, and the microdroplets are entrained in the gas inside the chamber. An exhaust gas is supplied to drive the mist toward a virtual impactor, where the gas and droplets are regularized. The dense aerosol mist is directed toward the nozzle printhead through carrier gas in a Teflon tubing. Ultrasonic aerosol jet is usually used to print highresolution patterns especially for printing metallic structures; however, printing tools can be manipulated to print on wide areas. Atomization occurs as a result of ultrasonication by

Fig. 6.5 Schematic diagram of aerosol jet printing process.

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applying pressure waves with specific frequency (normally in megahertz). The aerosol mist is driven by the gas directed toward the nozzle printhead. Another gas flow, that is, sheath gas is supplied at the printhead to further converge the aerosol and minimize the jet diameter. The stream of droplets is ejected in the form of an intact jet, eventually deposited on the target substrate. The high-resolution printing feature of AJP is ideal for minimizing the overall sensor area. MOX gas sensors based on integrated miniaturized microhotplates result into power-efficient devices. Printing miniaturized microhotplates on polymeric substrates is a step further toward development of cost- and power-efficient sensing devices. One such approach is recently reported by printing Au nanoparticle-based microhotplates with effective area down to 150  150 μm2 at record low power consumption, that is, 22 mW at 250°C temperature [26, 107]. A fully printed MOX gas sensor is being developed using AJP for microhotplate, dielectric, and IDEs, whereas inkjet printing is used to deposit SnO2 nanoparticle solution as sensing layer [107]. Additionally, SnO2 nanowire-based aerosol jet printed electronic nose as fire detector is developed to determine the preburning smell of various gaseous components [16]. AJP of SWCNTs are reported for the development of NO2 gas sensors [108]. Sensor exhibited good sensitivity (ΔR/Ro  96%), fast response (30 s), good stability, and the recovery time of 30 s when exposed to 60 ppm NO2 at room temperature. SWCNTs decorated with platinum for enhancing the sensing performance are produced by AJP [109]. Similar approach of aerosol printing of sorting semiconductor SWCNTs by using poly(9,9-dioctylfluorene) derivatives is produced and is applied for detection of ammonia gas sensors [110]. Upon exposure to 0.6% NH3 at room temperature, the ammonia sensors based on sorted SWCNTs showed sensitivity (ΔR/Ro  54.4%), fast response (30 s), and good stability. Aerosol being recently developed is expected to be adopted rapidly due to the ease in requirements of solutions’ rheological properties especially the viscosity and surface tension.

6.4.5 Sol-gel and drop casting Sol-gel is a special technique for the formation of oxide networks through polycondensation reactions of solid contents in a liquid medium. Solid contents, that is, nanoparticles, are of particular interest for the preparation of homogenous solutions and uniform mixtures (Fig. 6.6). Agglomeration of nanoparticles in a colloidal solution is a serious concern greatly affecting the stability of solution and shelf life. These properties are not restricted only for sol-gel processing but a potential requirement for solution-based AM technologies. Sol-gel is the predominant technique for processing MOXs toward gas-sensing applications from the very beginning. For instance, detailed processing of MOX materials, that is, SnO2-, ZnO-, TiO2-, WO3-, and MoO3-based gas sensors, is presented by comparing the solution concentrations, deposition parameters, gelling time, annealing

Fabrication techniques for coupling advanced nanomaterials to transducers

Hydrolysis

MOX precursor

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trate

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Condensation

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Subs

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Drop casting

Fig. 6.6 Schematics of sol-gel process and drop coating.

time, temperature, etc. [111–114]. Properties of the solution mixtures prepared with solgel technique can be tuned based on the respective deposition technique. For example, SnO2-based ink is prepared with sol-gel and deposited on polymeric substrate by using an inkjet printing technology [73]. Drop casting/coating is the simplest and immensely used technique to embed gassensitive nanomaterials on diverse substrates [50, 115]. It involves preparation of right solutions and dispensing system to drop coat the respective gas-sensing area. Usually, precursor solution (or sol-gel) of MOX nanomaterials having respective dopants is used with drop casting [116]. Drop casting of SnO2 nanopowder mixed with polydiallyldimethylammonium chloride (PDDAC) as dopant and adhesion promoter is used to develop gas sensors on flexible substrates [117]. Quantum dots (QD) of ZnO have been drop coated for testing toward NO2, acetone, and methanol [118]. Functionalization of WO3 nanowires using saturated palladium chloride (PdCl2) solution is achieved and drop coated for detection of hydrogen and volatile organic compounds [119]. A comparative study is performed on the sensing performance, before and after the Pd functionalization of the WO3-NWs. Two-dimensional materials such as molybdenum disulfide (MoS2) has been attractive for application in chemiresistive gas sensors owing to its moderate bandgap energy and high specific surface area [120]. MoS2 nanosheets catalyzed with Pd nanoparticles are drop casted and applied for detection of hydrogen gas [121, 122]. Ammonia gas sensors are developed using carbon nanoflake (CNFL)/SnO2 nanocomposite through drop casting method [123]. A novel fabrication strategy is deployed based on the synergetic effect from highly sensitive SnO2 colloidal (CQDs) and excellent conductive properties of MWCNTs to overcome the transport barrier in CQD gas sensors [124]. The

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attachment and coverage of SnO2 CQDs on the MWCNT surfaces were achieved by simply mixing and coating the presynthesized SnO2 CQDs and MWCNTs at room temperature. Pristine GO is also used as NO2 gas-sensing layer through drop casting to overcome the sluggish responses presented by using RGO alone [125, 126]. A comparative study of the gas response is performed by using nanocomposite SnO2/RGO and pure RGO film. SnO2/RGO resulted into twofold enhanced performance, assumed possibly due to the large surface area, three-dimensional porous nanostructure, and special interaction between RGO sheets and SnO2 nanoparticles[126].

6.4.6 Roll-to-roll printing techniques Roll-to-roll (R2R) printing is the ultimate goal to produce high volume of large-area devices at higher speed and lower cost. R2R is a common platform where different approaches such as engraved cylinders, gravure, and flexographic and offset printing techniques are combined in a single production line. R2R systems are often customized by including also digital fabrication techniques such as inkjet, spray, or slot-die printing. Details of the R2R system and processing parameters are discussed in more detail elsewhere [11]. R2R has been reported for developing gas sensors on polymeric substrates. For instance, a chemiresistive ammonia sensor with sensitive polyaniline layer has been fabricated by gravure printing on flexible poly(ethylene terephthalate) substrate [127]. A stable dispersion by mixing polyaniline with copper chloride has been produced for R2R printing to develop H2S gas sensor [128]. Gravure printing has been used to develop gas sensing utilizing WO3 nanoparticle-based solution for NO detection [129]. Printing sensors alone would result in large volume of sensors for short production time. This technique would be adapted for the integration of sensors on low-cost flexible sensing tags including other printed and discrete electronic components. Despite the attractive features of R2R system, merging different printing technologies into a single production line is challenging. For instance, stable transducers of electrodes would require to print noble metals such as gold or platinum instead of established silver materials. as very precise control of the process parameters and materials’ properties need to be tuned especially when the substrate is moving with high speeds as 5–50m/min [11]. Therefore, a more robust approach is desired to get reproducible results with minimum structural variations in device geometry while printing at such higher speeds.

6.5 Conclusion and perspective A brief overview of the AM technologies practiced for integrating nanomaterials on polymeric substrates for gas-sensing application is presented. Solution-based printing technologies are at the core of these new techniques. Printing offers the opportunity to deposit nanoparticle-based colloidal solution in a single step as against the multiple processing steps of standard microfabrication technologies. The location-specific

Fabrication techniques for coupling advanced nanomaterials to transducers

deposition and reduced material waste make printing technologies cost-effective. The possibility to process area larger than wafer size enables rapid manufacturing at depreciated cost. Detailed overview of the printing technologies of interest, that is, screen printing, inkjet printing, spray coating, AJP, and drop casting methods, is summarized here, and representative examples of nanomaterials are highlighted for gas-sensing applications. Development of new nanomaterials that can be processed by these technologies for developing all-printed gas-sensing platforms is of great importance. Further improvement in the printing processes toward R2R batch manufacturing of miniaturized gassensing devices would have a major impact on the future of gas-sensing technologies. Gas sensors could be integrated in emerging printed electronics and digitally manufactured smart systems produced in large volumes.

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CHAPTER 7

CMOS-based resistive and FET devices for smart gas sensors Julian William Gardnera, Prasanta Kumar Guhab a

School of Engineering, University of Warwick, Coventry, United Kingdom E&ECE Department, IIT Kharagpur, Kharagpur, India

b

7.1 Introduction to CMOS gas sensors There is an increasing demand for low-cost, low-power, handheld, compact gas or volatile organic compound (VOC) sensors that can be implemented within the Internet of Things (IoT). There exist many different transduction principles to detect hazardous gases and VOCs, for example, surface or bulk acoustic wave (SAW/BAW), electrochemical (EC), infrared (IR), calorimetric, and resistive. Among these, traditional IR gas sensors are perhaps the most accurate ones but limited to higher concentrations of methane (1%–4%) and carbon dioxide (100s ppm). They work on the principle of infrared absorption. They contain an infrared source (i.e., hot-wire bulb or diode) that passes through the target gas (within a gas chamber system) and an infrared filter and detector (usually pyroelectric). The gas attenuates certain infrared wavelengths (based on molecular vibrational bands) as the light passes through it, while other wavelengths pass through it unattenuated. Current IR gas sensors are relatively expensive today ($50–$200) and cannot detect toxic gases such as CO, NO2, and formaldehyde at the low levels (parts per billionparts per million) needed. It is not easy to make IR gas sensors using low-cost complementary metal-oxide-semiconductor (CMOS) platform. However, there have been recent reports of microhotplate-based IR emitters (i.e., IR source), which will reduce the cost drastically and improve compactness of the entire system [1]. Among traditional gas sensors, electrochemical (EC) sensors currently occupy the majority of the market share for accurate and stable monitors. The basic components of such sensors are two electrodes ([i] working electrode, also known as the sensing electrode and [ii] counter electrode) and an ion conductor electrolyte (e.g., mineral acid and organic electrolyte) in between them. When a target gas comes in contact with the working electrode, a reduction/oxidation (REDOX) reaction takes place. This electrochemical reaction results in an electrical current that passes through the external circuit and sensed hence creating an amperometric sensor. The current is proportional to the concentration of the gas. As the electrodes have a finite catalytic activity, it is necessary to limit the rate of diffusion of target gas into the sensor (using a barrier) to ensure the Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00007-4

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gas is efficiently reacted. This barrier takes the form of a small hole or capillary in the sensor housing. EC sensors usually have a shelf life of 6 months to 1 year, depending on the gas to be detected and the environment in which they are used. When such sensors are used to detect low concentrations (parts per billion), they are sensitive to changes in ambient temperature and also cross sensitive to humidity and other gases. EC sensors are much cheaper than that of IR sensors; however, a typical electrochemical sensor will still cost around $20, and they need a separate interface board with a high-gain amplifier that will take the cost closer to $100 or more. This high cost prevents the penetration into truly mass markets that are needed today for the IoT. The high cost is primarily because of the use of a semiautomated manufacturing process and requirement of expensive catalytic material, and it is difficult to integrate them with CMOS platforms due to the presence of the electrolyte. For the reasons outlined earlier, resistive gas sensors have a major advantage over IR and EC gas sensors, because they can be integrated with CMOS platforms and manufactured in very high volumes (>10 M units per annum) at ultralow cost (1 year

Oslo (NO)

CO, NO, NO2, O3, PM10, PM2.5, T, RH

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