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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

MOLYBDENUM DISULFIDE SYNTHESIS, PROPERTIES AND INDUSTRIAL APPLICATIONS

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

MOLYBDENUM DISULFIDE SYNTHESIS, PROPERTIES AND INDUSTRIAL APPLICATIONS

JEREMIAH MCBRIDE EDITOR

New York

Copyright © 2016 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. We have partnered with Copyright Clearance Center to make it easy for you to obtain permissions to reuse content from this publication. Simply navigate to this publication’s page on Nova’s website and locate the “Get Permission” button below the title description. This button is linked directly to the title’s permission page on copyright.com. Alternatively, you can visit copyright.com and search by title, ISBN, or ISSN. For further questions about using the service on copyright.com, please contact: Copyright Clearance Center Phone: +1-(978) 750-8400 Fax: +1-(978) 750-4470 E-mail: [email protected].

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Library of Congress Cataloging-in-Publication Data Names: McBride, Jeremiah, editor. Title: Molybdenum disulfide : synthesis, properties and industrial applications / [edited by] Jeremiah McBride. Description: Hauppauge, New York : Nova Science Publishers, Inc., [2016] | Series: Chemical engineering methods and technology | Includes bibliographical references and index. | Description based on print version record and CIP data provided by publisher; resource not viewed. Identifiers: LCCN 2016017650 (print) | LCCN 2016011277 (softcover) | ISBN 9781634850513(ebook) |ISBN 9781634850322 (hardcover) | ISBN 9781634850513 (ebook) Subjects: LCSH: Molybdenum disulfide. Classification: LCC TP245.M7 (print) | LCC TP245.M7 M65 2016 (ebook) | DDC 661/.0534--dc23 LC record available at https://lccn.loc.gov/2016017650

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface

vii

Chapter 1

Thin Film Growth of MoS2 Takashi Yanase and Toshihiro Shimada

Chapter 2

Recent Progress in Molybdenum Disulfide (MoS2) Synthesis as a Promising Photocatalytic Material S. V. Prabhakar Vattikuti and Chan Byon

21

Molybdenum Disulfide: A Promising Material for Photocatalytic Hydrogen Evolution Yongtao Lu

51

Chapter 3

Chapter 4

Doping to MoS2 Toshihiro Shimada and Takashi Yanase

Chapter 5

Exfoliated Polypyrrole-MoS2 Nanocomposites: Preparation and Characterization Jiabao Hong, Rabin Bissessur and Douglas C. Dahn

Index

1

71

83 107

PREFACE Molybdenum disulfide (MoS2) is a semiconductor which is composed of Mo atoms sandwiched between two layers of hexagonal close packed sulfur atoms in a structure similar to graphene. Traditionally, it has been used as a solid lubricant due to its low friction properties and as a hydrodesulfurization catalyst to lower the sulfur content in natural gas and fuels. Bulk MoS2 were first examined as a possible hydrogen evolution reaction electrocatalyst as early as 1977 by Tributsch et al. However, it was not until about 20 years later that its potential in the hydrogen evolution reaction was fully unveiled. This book discusses the synthesis, properties and industrial applications of molybdenum disulfide. Chapter 1 - This chapter describes thin film growth to synthesize nanosheets (few- or mono- layer) of molybdenum disulfide (MoS2), mainly focusing on chemical vapor deposition (CVD). The CVD system is still being improved in terms of the controllability and reproducibility: from one zone systems, two zone systems, three zone systems to finally two flow systems. The growth conditions of CVD and the surface treatment of the substrate are critically reviewed. The appearance of MoS2 nanosheets on substrates shows various shapes such as star shape, dendritic shape, and round shape due to kinetic effects and off-stoichiometric growth conditions. The mechanism of the shape evolution is explained in this chapter. Impurity-assisted method is recently proposed to enlarge grain size and to lower growth temperature. Methods to evaluate the MoS2 nanosheets synthesized by CVD are also discussed. The crystallinity strongly affects the device performance. Fabrication of heterostructures including MoS2 is now strenuously studied as a fundamental semiconductor technology.

viii

Jeremiah McBride

Chapter 2 - Transition metal dichalcogenides (TMDs) synthesis using simple and facile methods has been considered as the most feasible approach for preparing different types of large-area two-dimensional (2D) layered materials. In this chapter, the authors report the current progress in the synthesis of TMDs and their morphology-dependent characteristics and highlight their photocatalytic behavior in waste water treatment. Degradation of pollutants from industrial waste water and recycling using photocatalysts has been attracting much research interest for using abundant solar sources in environmental remediation. Many materials display good photocatalytic activity under ultraviolet (UV) and visible light irradiation. However, the applications of these materials are limited due to recombination of the electron–hole pairs. MoS2 has an advantage over other semiconductor photocatalyst materials like SnO2, TiO2, and ZnO for photocatalytic applications due to its lower band gap of ~1.9 eV. As a result, MoS2 is capable of absorbing both the UV and visible solar spectrum. In addition, its simple and easy synthesis is low cost and has good chemical stability in an aqueous medium, which makes it a promising material for photocatalytic water treatment and water splitting applications. However, the use of MoS2 has been limited by the usable portion of the visible light spectrum due to low diffusion length and positive valence band with respect to the H+/H2 potential. The authors review the research to overcome these disadvantages and to enhance the photocatalytic activity of MoS2. Chapter 3 - Hydrogen as a zero carbon emission fuel has recently attracted increasing attention because of the depletion of fossil fuel reserves and the severe environmental crisis. Since Fujishima and Honda first reported the photoelectrochemical splitting of water into H2 and O2, wide variety of photocatalytic H2 generation tactics have been exploited over the last four decades. So far, the Pt-based materials were found to be well-performing catalysts for photoinduced H2 production, while high-cost limited their largescale application. Recently, molybdenum disulfide (MoS2) has been demonstrated as a promising material for substituting Pt-based catalysts for the improvement of photocatalytic performance of H2 production. This chapter mainly focuses on the recent progress of photocatalytic H2 evolution systems in which MoS2 worked as supporting materials. It involves the design and the architect of MoS2 functionalized nanomaterials for enhancing photocatalytic performance of H2. At the end of this chapter, the prospects of the new kind of MoS2-based materials for improving photocatalytic H2 production are also addressed briefly.

Preface

ix

Chapter 4 - In this chapter, the authors critically review current research activities on doping to MoS2. First the authors describe the characteristics and functions of MoS2 and related materials as hosts of dopants. The authors deal with electronic structures and physics, electronics, catalysis and tribology. Then the authors explain the electronic states of various dopants. Special attention will be paid about the fact whether the dopant can supply charge carriers, which are n-type or p-type. Each dopants are reviewed: vacancies, Mo-substituting elements and S-substituting elements from the viewpoints of experiments and computations. Finally the authors briefly review the surface related techniques to modify the carrier concentrations in MoS2: surface transfer doping and field effect doping. Chapter 5 - Exfoliated nanocomposites consisting of polypyrrole (PPy) and molybdenum disulfide (MoS2) were synthesized. The MoS2 was first prepared in an exfoliated state by reacting molybdic acid with a huge excess of thiourea at 500oC under nitrogen flow. The PPy-MoS2 nanocomposites were prepared by polymerization of pyrrole with ammonium peroxydisulfate, in the presence of the exfoliated MoS2. The amount of MoS2 in the reaction mixture was systematically varied to produce a range of nanocomposite materials ranging from 1 to 50% by mass of MoS2. The nanocomposites were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, electron microscopy, and van der Pauw electrical conductivity measurements. Powder X-ray diffraction provided evidence that the nanocomposites are exfoliated. The diffractograms of the nanocomposites were completely amorphous, suggesting lack of structural order in these materials and indicating the formation of genuine exfoliated systems. It was intriguing to observe that the nanocomposites exhibited enhanced electronic conductivity when compared to the pure polymer.

In: Molybdenum Disulfide Editor: Jeremiah McBride

ISBN: 978-1-63485-032-2 © 2016 Nova Science Publishers, Inc.

Chapter 1

THIN FILM GROWTH OF MOS2 Takashi Yanase and Toshihiro Shimada Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Japan

ABSTRACT This chapter describes thin film growth to synthesize nanosheets (few- or mono- layer) of molybdenum disulfide (MoS2), mainly focusing on chemical vapor deposition (CVD). The CVD system is still being improved in terms of the controllability and reproducibility: from one zone systems, two zone systems, three zone systems to finally two flow systems. The growth conditions of CVD and the surface treatment of the substrate are critically reviewed. The appearance of MoS2 nanosheets on substrates shows various shapes such as star shape, dendritic shape, and round shape due to kinetic effects and off-stoichiometric growth conditions. The mechanism of the shape evolution is explained in this chapter. Impurity-assisted method is recently proposed to enlarge grain size and to lower growth temperature. Methods to evaluate the MoS2 nanosheets synthesized by CVD are also discussed. The crystallinity strongly affects the device performance. Fabrication of heterostructures including MoS2 is now strenuously studied as a fundamental semiconductor technology.

2

Takashi Yanase and Toshihiro Shimada

1. OVERVIEW OF THE FABRICATION TECHNIQUES OF MOS2 NANOSHEETS Successful fabrication of graphene sheet (monolayer graphite) has triggered the research of the other layered materials to make them thin, ultimately monolayers [1, 2]. In particular, molybdenum disulfide (MoS2), which is one of the transition metal dichalchogenides, is the most widely studied due to the availability as a mineral [3-7]. Monolayer MoS2 consists of single layer of Mo atoms sandwiched between two layers of sulfur atoms by covalent bonds as illustrated in Figure 1. Monolayer MoS2 is direct transition semiconductor with a band gap of 1.8 eV while bulk MoS2 is indirect one with that of 1.2 eV. The band gap can be tuned by changing the number of layers (in the range of several layers) [6]. The finite band gap is much different from that of graphene (zero gap), therefore, it would be expected that MoS2 layer can be applied to field effect transistors (FETs), photodetectors, electroluminescence, and biosensing devices, particularly using monolayer MoS2. The simplest way to obtain monolayer MoS2 is Scotch tape exfoliation method. Bulk MoS2 is easily exfoliated using scotch tape along c-axis which is the direction of van der Waals interaction. The number of MoS2 layer decreases by repeating this exfoliation, then finally monolayer MoS2 is obtained. At the first stage of the research on MoS2 nanosheets, this exfoliation method is widely used to make a few layer or monolayer MoS2. The scotch tape method is still used now for fundamental researches [8]. The champion value of the FET mobility (184 cm2/Vs) was recorded using 10-nm-thick MoS2 fabricated by the Scotch tape exfoliation method [3]. However, from the view point of practical application, the Scotch tape exfoliation method is unsuitable due to the uncontrollability of the size and the difficulty of scale-up. There are three major techniques to synthesize large-scale thin films: dipcoating method [9], thermal vapor sulfurization (TVS) [7, 10, 11], and chemical vapor deposition (CVD) [12-22]. In dip-coating method, after a substrate is immersed into a solvent with molybdenum precursor compound ((NH4)2MoS4 is frequently used [9]), the sample was annealed to convert the precursor into MoS2 thin film. It is very easy to obtain the MoS2 thin films by this method, however, the critical issues are the inclusion of impurities, the difficulty of synthesizing monolayer, and poor crystallinity. TVS is involved with the sulfurization of precursor films (mainly Mo metals) deposited on the substrate. CVD uses gasified Mo and S containing species that react on the

Thin Film Growth of MoS2

3

surface of the substrate. Note that although some articles confuses TVS with CVD, these two method should strictly be distinguished because solid Mo source on the substrate (not gasified Mo) is used in TVS.

Figure 1. Crystal structure of MoS2.

2. EVALUATION OF MOS2 NANOSHEETS The final goal is to synthesize MoS2 monolayer with good crystallinity, which is indispensable for achieving high carrier mobility. The quality of MoS2 nanosheets must be evaluated and discussed by using various means to finely tune the preparation conditions: Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), atomic force microscope (AFM). In Raman spectroscopy, the number of layers can be deduced from the position of peak and peak-to-peak value. The crystallinity is discussed from full width at half maximum (FWHM) (E2g mode is especially used for discussion) [23]. In-plane vibrational mode (E2g) of bulk MoS2 is located at 383 cm-1 and out-of-plane vibrational mode (A1g) is located at 408 cm-1. As the number of layer decreases, the peak shift appears at five-layer and lower [23-25]. Monolayer MoS2 shows the largest peak shift: E2g shift to 386 cm-1 (blue shift), A1g shift to 404 cm-1 (red shift) [23]. Although the position of the peaks and the peakto-peak distance depend on the growth conditions, the tendency of the peak shift is almost unchanged. Such peak shift is due to the reduction of van der Waals interaction caused by decrease of the number of layers.

4

Takashi Yanase and Toshihiro Shimada

Stoichiometry and impurity is often discussed in the literature using XPS [12-19]. However, sensitivity of XPS is 0.1% at best, therefore, careful attention should be paid when exact stoichiometry and a small amount of impurity are discussed. Ref. 26 discusses that a small amount of impurity greatly affect the electric property of MoS2 nanosheets. XRD is used to evaluate the crystal orientation. However, out-of-plane diffraction cannot be observed from a monolayer. Local vacancies, defects, and crystallinity is directly observed using TEM [15, 17]. Selective area electron diffraction using TEM is also powerful to evaluate the crystallinity and orientation of MoS2 nanosheets. AFM is used to directly observe grain boundary and the number of layer of MoS2 nanosheet [12-19]. The thickness of monolayer is approximately 0.65 nm. The result of these evaluation technique is mostly combined with that of FET characteristics (especially mobility) to discuss the quality of MoS2 nanosheets. Although small amount of defects and impurity (even ppm level) degrade the electric property, quantitative analyses of defects and impurity are hardly found in the literature. References 8 and 21 demonstrate the quantitative evaluation of the trap density for MoS2 nanosheets synthesized by CVD using capacitance method. These experiments have clearly demonstrated that there exists trap states of 1014 cm-2 at the interface between SiO2 (widely used gate insulator) and MoS2. These trap states cause severe degradation of FET mobility, therefore, the mobility of FET fabricated using MoS2 synthesized by CVD is much lower than that of theoretical value (acoustic limiting mobility = 410 cm2/Vs [26]). These trap states should be decreased for practical applications, however, effective technology to reduce them is under investigation. Aforementioned champion FET mobility (184 cm2/Vs) is achieved using mechanically exfoliated multilayer MoS2 and scandium contact that strongly reduce the contact resistance [3]. It has been recently reported that there exist many sulfur vacancies on the surface of even exfoliated monolayer MoS2 [27], which greatly affected work function and other characteristics. Moreover, the vacancies is easily oxidized by oxygen and water in air [28, 29]. Oxidized MoS2 shows poor electric characteristics. It is expected that deep discussion including small amount of defects and impurity will be carried out in the future researches to obtain reliable MoS2 semiconductor films. Most discussion includes carrier type of MoS2 nanosheets. When intentional doping is not carried out, most of CVD-synthesized MoS2

Thin Film Growth of MoS2

5

nanosheets show n-type conduction. Only a few articles have reported p-type [11]. Although there are still considerable controversies in details, most researchers believed that n-type is caused by Fermi level pinning due to metal contacts or sulfur vacancies. P-type behavior has not been explained consistently.

3. THERMAL VAPOR SULFURIZATION TVS consists of two processes: (i) Deposition of Mo source film and (ii) sulfurization of the deposited film. Metal Mo or MoO2 is used as a molybdenum source. Elemental sulfur is mainly used to sulfurize the deposited film. In a few cases, H2S gas was used for sulfurization. An advantage of TVS is that it is easy to control the thickness of MoS2 nanosheets because the thickness can be adjusted by the thickness of Mo-containing film because the sticking coefficient of Mo is almost unity. Another advantage is that a dopant element can be incorporated into the Mo-containing film to realize p-type characteristics [29] (as synthesized MoS2 shows basically n-type). The other advantage is the surface can be composed of edges of MoS2 layers (c-axis parallel to the surface), which is beneficial for the application in catalysis. Disadvantages are that the process is more complicated than that of CVD and it is difficult to obtain MoS2 nanosheets with large grain size. Necessity of an expensive high vacuum system is another disadvantage. CVD, that is actually used in industry for synthesis of III-V semiconductors, is one of the realistic methods to make large-scale MoS2 using a relatively simple equipment. Therefore, the main technique to synthesize large-scale MoS2 nanosheets is now CVD described in the following.

4. SYNTHESIS OF MOS2 NANOSHEETS BY CHEMICAL VAPOR DEPOSITION While there are several types of CVD such as thermal CVD, plasma CVD, metal organic CVD, and so on, hot-walled thermal CVD is mainly used to synthesize MoS2 nanosheets [12-22]. In thermal CVD, Mo source and sulfur source are gasified and transported to the surface of a substrate. Transported species are diffused onto the surface of substrate, adsorbed, and decomposed

6

Takashi Yanase and Toshihiro Shimada

into reactive atoms. Adsorbates migrate and is trapped at the edge of MoS2, then react with each other resulting in the formation of covalent bonds that means the growth of MoS2 nanosheets. Byproducts and unreacted species are transported out by the carrier gas. MoO3 is the most frequently used Mo source due to the volatile nature (evaporation temperature: 500-700°C) and ease in handling. MoCl5 is also used because it can be evaporated at low temperature (150-200°C). MoCl5 must be always handled in an inert atmosphere because it is very sensitive to oxygen and water in air. Recently, Mo(CO)6 is also utilized as a Mo source due to the volatility at low temperature and ease in handling [24, 30, 31]. Elemental sulfur and H2S gas are used as a sulfur source. H2S gas is toxic and careful attention must be paid to handle it, therefore, it is preferred to use elemental sulfur for the synthesis. N2 or Ar gas was used as a carrier gas in many cases. In a few articles on the growth of molybdenum or tungsten selenides, H2 gas was used to enhance reducing power [32].

5. OVERVIEW OF CVD SYSTEMS IN LITERATURES: ADVANTAGES AND DISADVANTAGES 5.1. One Zone CVD System The simplest CVD system is one zone CVD system. First demonstration for CVD synthesis of MoS2 nanosheets was carried out using this system [12]. Figure 2 shows the schematic illustrations of one zone CVD system. This system includes only one zone furnace. All of sulfur source, Mo source, and substrate are placed at the same region. It is very easy to set up the equipment, however, it is very difficult to control the growth parameters individually. Another deficit is that the temperature of elemental sulfur is adjusted by placing it at slightly apart from the center of the furnace. No monitoring and control of the temperature of sulfur mean very low reliability for synthesis. The critical disadvantage is that the growth parameters including the temperatures of sulfur source, Mo source, and substrate cannot be separately controlled. Hence, this system is not suitable for practical applications due to the lack of approach to optimize each growth parameter. Yet, it is still used in some articles due to its simplicity [33, 32].

Thin Film Growth of MoS2

7

5.2. Two Zone CVD System The temperature of sulfur can be independently tuned by using two zone CVD system. Figure 3 shows the schematic illustration of the two zone CVD system [13]. Although Mo source and a substrate are placed at the same region, the sulfur can be independently heated by using another heater. This improvement makes it possible to supply the optimized amount of sulfur. However, the temperatures of the Mo source and the substrate still cannot be controlled separately. Hence, the amount and concentration of Mo supply and the growth temperature cannot be optimized at the same time. In the view point of good crystallinity and device application, the system should be more sophisticated to optimize all of the growth parameters.

5.3. Three Zone CVD System Three zone CVD system was developed in order to independently control the temperatures of sulfur source, Mo source, and substrates. Figure 4 shows the schematic illustration of the three zone CVD system [16]. By using this system, all of the important parameters can be optimized. In many cases, sulfur source and Mo source are placed in each small quartz tube to prevent cross contamination [16, 35], resulting in stable supply of Mo source. Owing to the stable supply of Mo source, CVD can be carried out for long time. Recently, it has been reported that combination of three zone CVD system and oxygen addition greatly improved the grain size of MoS2 nanosheet [35]. (Impurity-assisted CVD is briefed in the section X.9) Research on the enlargement of grain size and on scale-up to substrate size would be rapidly progressing using this system.

Figure 2. One zone CVD system.

8

Takashi Yanase and Toshihiro Shimada

Figure 3. Two zone CVD system.

5.4. Two Flow CVD System The present author has proposed the more sophisticated system called two flow CVD system with a three zone furnace and a one zone furnace [36]. Figure 5 shows the schematic illustration of two flow CVD system. In addition to the separate control of the temperatures of sulfur source, Mo source, and substrates, flow rate of each carrier gas for sulfur source and Mo source can be independently adjusted. This system make it possible that the supply of Mo source can be completely stopped during ramping and lowering the temperatures. Hence, the growth time can be rigidly defined as a duration of the Mo source supply. Nucleation and nanosheets growth can be accurately discussed as the growth time is changed. Currently, the two flow CVD system is the most sophisticated (to the best of our knowledge) regarding the controllability of the growth parameters. Although only the present author adopted this system at moment due to the complication of the system, careful attention must be paid to the growth parameter to obtain MoS2 nanosheets with good quality in future study.

6. GROWTH CONDITION OF CVD The important parameters in CVD synthesis are type of source materials, the temperature of sulfur source, the temperature of Mo source, the temperature of substrates, pressure, type of carrier gas, and type of substrate. Currently, each researcher uses own his/her growth condition, in other words, a unified condition for synthesis of MoS2 nanosheets has not been established.

Thin Film Growth of MoS2

9

This may be because there are some differences among the CVD systems used by researchers. The most general Mo source is now MoO3 [12-17], but MoCl5 [18, 37] and Mo(CO)6 [24, 30, 32] are used in several cases. Mo(CO)6 has recently gathered much attention due to the success of the large scale MoS2 film. The major issue of CVD using Mo(CO)6 is to completely remove carbon contamination including graphite that derived from the source molecule. Since the quality of the MoS2 nanosheets fabricated using Mo(CO)6 is improving, we should to wait subsequent reports. It is desirable to establish the optimal growth condition including temperatures of source materials, growth temperature, type of carrier gas, flow rate of carrier gas, and so on after much discussion.

Figure 4. Three zone CVD system.

Figure 5. Two flow CVD system.

10

Takashi Yanase and Toshihiro Shimada Table 1. Summary of growth parameters in the literature

Temp. of Mo Temp. of S 650°C (MoO3) 700°C (MoO3) 700°C (MoO3) 540°C (MoO3) 700°C (MoO3) 790°C (MoO3) R.T. (MoCl5) 800°C (MoO3) 680°C (MoO3) 700°C (MoO3) no monitoring

〜300°C

Temp. of sub. 650°C

200°C

700°C

no monitoring 130°C

700°C

150°C

700°C

210°C

790°C

(H2S gas)

900°C

150°C

no monitoring 680 °C ambient N2 no info. SiO2/Si PTAS

[18]

700°C

ambient N2 10 sccm SiO2/Si none

[21]

850°C

2 Torr

[36]

130°C (Mo(CO)6)

(H2S gas)

no monitoring no monitoring 〜300°C

750°C

850°C

Pressure Carrier gas Substrate Flow rate Promoter ambient N2 1 sccm SiO2/Si rGO

Refs.

50 Torr Ar 150 SiO2/Si none sccm ambient N2 30 sccm SiO2/Si none

[12]

0.67 Ar 130 SiO2/Si none Torr sccm ambient Ar 10 sccm SiO2/Si none

[14]

ambient Ar 100 SiO2/Si none sccm ambient Ar 300 SiO2/Si none sccm 150 Torr He 20 sccm SiO2/Si none

[16]

Ar 50 sccm SiO2/Si, and c-sapphire none 20 Torr Ar 15 sccm c-sapphire and quartz/none

[11]

[13]

[15]

[17]

[19]

[37]

Table 1 shows summary of the growth conditions in CVD for synthesis of MoS2 nanosheets. The temperature of MoO3 ranges from 540-800°C. The temperature of sulfur ranges from 130-300°C. The reaction temperature ranges from 650-1000°C. The flow rate of carrier gas ranges from 1-800 sccm. In addition to such differences of growth parameters, some researchers synthesize MoS2 nanosheets under atmospheric pressure while the other researchers synthesize it under reduced pressure. Such wide range parameter would be attributed to the great degree of freedom of parameters. A common finding in the previous reports is that the shape of synthesized MoS2 nanosheets reflects its crystal structure. Triangle, hexagonal, and starshape are mostly seen in the literatures. The side length of triangle MoS2 nanosheets generally ranges from several hundred nm to several ten m. Surprisingly, MoS2 nanosheets with 325-m-length have been successfully

Thin Film Growth of MoS2

11

synthesized by oxygen-assisted method [35] (that is briefed in section X.9). A few articles have reported dendritic shape [16] and round shape [15]. The reason why MoS2 nanosheets shows such shapes is discussed in section X.8. A typical optical microscopic image of MoS2 nanosheets fabricated by our CVD system is shown in Figure 6. In most of literatures, thickness of MoS2 nanosheets was directly observed by AFM, which revealed the growth of monolayer (or multilayer) MoS2. In Raman spectra, E2g of in-plane vibrational mode and A1g of out-of-plane vibrational mode are seen. The difference between E2g and A1g can be used for deduction of the number of layers, however, direct observation such as AFM should be also used to determine the number of layers because Raman spectra of MoS2 is easily affected by the crystallinity. Strong photoluminescence (PL) is observed for monolayer MoS2. FWHM and the intensities of the PL were used in discussion to assure the quality of the crystal. However, FET mobility is not always as high as that fabricated by the scotch tape exfoliation. TEM is also used to discuss the quality of MoS2 nanosheets. In particular, high resolution TEM can reveal the local sulfur vacancies, defects, and grain boundary.

Figure 6. Optical microscopic image of MoS2 nanosheets.

7. SURFACE TREATMENT OF SUBSTRATE Silicon wafer with thermal oxide layer is commonly used as a substrate. This is because MoS2 nanosheets, even monolayer, can be observed by optical microscope due to interference. As thickness changes, the interference color also change, which enables to deduce the number of layers. Moreover, the

12

Takashi Yanase and Toshihiro Shimada

thermal oxide layer can be utilized for gate insulator in the subsequent process of FET fabrication, which can skip the process of transfer and electrode deposition. c-plane sapphire is also used due to the small mismatch of lattice constants between the sapphire and MoS2 (0.62%) when 3×3 MoS2 supercells on 2×2 sapphire is considered as well as flatness of the substrate [38, 39]. Surface treatment of substrate is very important to obtain MoS2 nanosheets with good crystallinity. The following surface treatments can be seen in the literatures. (i) Graphene sheet is placed on the surface of substrate [11]. (ii) Organic molecules with π conjugated system are coated [12, 20]. (iii) the surface is treated by oxygen plasma [16, 21]. (iv) the surface is cleaned by solutions [13-16, 36]. The aim of the treatment (i) is to use van der Waals epitaxy [40-42], resulting in easy formation of c-axis oriented MoS2 nanosheets. However, it is impossible to evaluate the electric properties of the MoS2 nanosheets due to the high conductivity of graphene sheet. Even if MoS2 nanosheets is transferred to another substrate, it is necessary to remove graphene nanosheets. In the view point from these disadvantages, the treatment (i) is not suitable for practical applications. Insulating h-BN is a good candidate as a buffer layer for van der Waals epitaxy. Practical synthesis of h-BN has been developed [43, 44], therefore, van der Waals epitaxy using h-BN would be a good technique to obtain MoS2 nanosheets. The treatment (ii) increases nucleation center. The conjugate organic molecules works as promoters which induce the nucleation (the growth starting point) of MoS2. It has been reported that graphene and perylene-3,4,9,10-tetracarboxylic acid tetrapotassiuim salt (PTAS) were effective promoters [12]. On the other hand, there are many studies to synthesize MoS2 nanosheets without organic promoters [13-19]. The effect of the promoter should be carefully discussed in future research. The promoters may be incorporated into the MoS2 nanosheets, which may cause the degradation of the device quality. Therefore, it is preferable to synthesize MoS2 nanosheets without any promoters for practical applications. The treatment (iii) makes the surface to be hydrophilic. As hydrophilicity of the surface become higher, the number of MoS2 layers decrease. Monolayer MoS2 can be obtained using superhydrophilic surface. Note that exact mechanism to form monolayer on the superhydrophilic surface is still unknown. The treatment (iv), which is the most common method, is carried out to remove the contaminations on the surface of substrate. When the surface is dirty, nucleation occurs at the contaminations along random orientation. This causes the growth of MoS2 particles, not nanosheets. It is clear that the surface treatment is critically important for the synthesis of large-

Thin Film Growth of MoS2

13

scale MoS2 nanosheets. Careful attention must be paid to the state of the surface.

8. THE SHAPE OF MOS2 NANOSHEET The shape of MoS2 nanosheets fabricated by CVD depends on the growth conditions [16, 19]. The shapes reported in the literatures are triangle, hexagonal, star-shape, round-shape, dendritic shape, and truncated triangle. Ref. 15 deeply discusses the growth mechanism to explain various kinds of the shapes. It is very important to control the ratio of Mo source and sulfur source for obtaining the desired shape. Hexagonal nanosheets can grow under the condition of Mo:S = 1:2. Hexagonal nanosheets is composed of S-zig-zag (Szz) termination side and Mo-zz termination side alternatively. When the large excess of Mo source is supplied the growth rate of Mo-zz termination sides become faster than that of S-zz termination sides. Hence, the nanosheets become triangle. When the small excess of Mo source is supplied, the shape does not become perfect triangle, resulting in truncated triangle. The same situation occurs when the excess of S source is supplied. The authors directly observed the shape evolution as the growth duration changed [36]. The growth duration ranged from 30 sec to 60 min. Immediately after the beginning of nanosheets growth, the shape is hexagonal. As the growth time has passed, the shape changed into round triangle and finally into triangle. This can be explained by the above mentioned mechanism: hexagonal nanosheets forms at the first stage of CVD and it becomes finally triangle due to the excess of S source supply. When the ratio of Mo source and sulfur source reach the critical point, the nanosheets becomes star-shape. In the case of very large excess of sulfur source, for instance, only Mo-zz termination sides in hexagonal nanosheets can grow (no S-zz termination side growth), resulting in formation of starshape. When both flow rates of Mo source and sulfur source becomes very large, dendritic MoS2 nanosheets forms. The large flow rates make thinner boundary layer that is defined as a distance between the substrate and bulk flow, which results in diffusion limiting growth (kinetic limiting), not transport limiting growth (thermodynamic limiting). In other words, Mo and sulfur atoms are incorporated into random sites, not the most stable sites. Some articles have reported the round-shape nanosheets with poor crystallinity. This may be caused by non-optimal growth conditions.

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9. IMPURITY ASSISTED GROWTH Recently, intriguing attempts have been reported in CVD for synthesis of MoS2 nanosheets. Intentional addition of impurity can lower the growth temperature or enlarge the grain size. To the best of my knowledge, tellurium [45] and oxygen [35] are used for additive. Since tellurium can form alloy with molybdenum and tungsten, molybdenum and tungsten can be evaporated at the lower temperature using tellurium. Such tellurium-assist CVD results in the lowering of the growth temperature, which is important for practical process. The key point in tellurium-assist CVD is that instability of MoTexS2-x and WTexS2-x as well as stability of MoS2 and WS2 lead to no contamination of tellurium. In addition, the crystallinity of MoS2 nanosheets is as good as that of MoS2 nanosheets synthesized at high temperature. Oxygen additive can prevent the Mo source from sulfurization during CVD. This means that stable supply of Mo source can be achieved. Oxygenassist CVD greatly improve the grain size: the side length of the biggest triangle MoS2 nanosheet is 325 m. Note that defects and oxygen displacement are not incorporated into MoS2 nanosheets in spite of the addition of oxygen. The mobility of FET using MoS2 nanosheets synthesized by oxygen-assist CVD shows the highest value (90 cm2/Vs) so far among the CVD-synthesized MoS2 FET. It is very curious why the quality of MoS2 nanosheets synthesized by oxygen-assist CVD is so high despite the addition of oxygen [35]. It is expected that the entire growth mechanism of oxygenassist CVD will be clear in future research.

10. FABRICATION OF HETEROSTRUCTURES Fabrication of heterostructures is a core in the semiconductor technology [46-51]. Built-in potential formed near the interfaces between different semiconducting materials makes it possible to separate exciton carriers (photovoltaic effect). Rectifying effect is also obtained by forming built –in potentiall [51], which opens the new stage for applications. It is clear that well-controlled growth of other materials is necessary to fabricate the heterostrucutres with sharp interface. WS2, MoSe2, and WSe2 were commonly selected due to the same crystal structure and semiconducting nature. There exist two type of heterstructrures: lateral and vertical.

Thin Film Growth of MoS2

15

Lateral heterostructures is more valid than vertical heterostructures for carrier separation and rectification due to the covalent bonds. Moreover, it is controllable to fabricate lateral heterostructures due to the growth from the edge (growth point). Lateral heterostructures are fabricated by simple two step CVD: deposition of small MoS2 nanosheets and subsequent deposition of different material (WS2, MoSe2, or WSe2). Surprisingly, the interface of the heterostructures is relatively flat even using such simple deposition technique. TEM images have clarified that interdiffusion of mutual atoms is within several nm [50]. In case of vertical heterostructures, it is necessary to control the stacking structure. Even in case of vertical homostructures such as bi-layer or tri-layer MoS2, it is well known that the properties of MoS2 nanosheets change as the stacking structure change [33] (see Figure 7. left: AB stacking, right: AA’ stacking). If two layers are mechanically stuck, the alignment of the two layers cannot be controlled, resulting in showing unstable properties. When CVD is utilized to make both homo- and hetero- structure, thermodynamically stable stacking structure, AB stacking or AA’ stacking, is always obtained. However, there is no technology to separately make each of them at present. It is a kind of antiphase boundary problem often encountered in heteroepitaxial gowth. Moreover, because lateral growth from the edge of MoS2 nanosheets is thermodynamically more stable than vertical growth, it is still very challenging to obtain pure vertical heterostructures (without lateral heterostrucures).

Figure 7. AB and AA’ stacking of MoS2.

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11. MOLECULAR BEAM EPITAXY Finally, we would like to briefly mention pioneering works of molecular beam epitaxy of layered materials in 1980-90s. The research efforts were initiated by Prof. A. Koma at University of Tokyo (who named “van der Waals epitaxy”) [40-42] and spread to DuPont (Prof. F. S. Ohuchi’s group and Prof. B. A. Parkinson’s group) [52] and Hahn-Meitner-Institute (Prof. W. Jaegermann’s group) [53]. They use molecular beam epitaxy in ultrahigh vacuum using elemental metals and chalcogens as sources. They succeeded to grow monolayers and several layers of MoSe2, NbSe2 and so on. Cleaved faces of MoS2 and other layered materials were used as substrates at first but later the surface termination technique of three dimensional materials were invented such as GaAs by sulfur [54] and Si(111) by GaSe [55] to make “van der Waals” surfaces. Unique electronic structures of atomically abrupt “van der Waals interfaces” were revealed by surface science techniques including charge density waves of TaS2 [56], Moire STM images of MoSe2/MoS2 [57] and characteristic band dispersions [58]. At that time the technique of making devices from the monolayer materials were not matured and we had to wait the breakthroughs we are experiencing now.

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[55] L. T. Vinh, M. Eddrief, C. Sebenne, A. Sacuto, M. Balkanski, J. Cryst. Growth, 135 (1994) 1-10. [56] T. Shimada, F. S. Ohuchi, A. Koma, Surf. Sci., 291 (1993) 57-66. [57] A. Parkinson, F. S. Ohuchi, K. Ueno, A. Koma, Appl. Phys. Lett., 58 (1991) 472-474. [58] A. Klein, O. Lang, R. Schlaf, C. Pettenkofer, W. Jaegermann, Phys. Rev. Lett., 80 (1998) 361-364.

In: Molybdenum Disulfide Editor: Jeremiah McBride

ISBN: 978-1-63485-032-2 © 2016 Nova Science Publishers, Inc.

Chapter 2

RECENT PROGRESS IN MOLYBDENUM DISULFIDE (MOS2) SYNTHESIS AS A PROMISING PHOTOCATALYTIC MATERIAL S. V. Prabhakar Vattikuti and Chan Byon School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea

ABSTRACT Transition metal dichalcogenides (TMDs) synthesis using simple and facile methods has been considered as the most feasible approach for preparing different types of large-area two-dimensional (2D) layered materials. In this chapter, we report the current progress in the synthesis of TMDs and their morphology-dependent characteristics and highlight their photocatalytic behavior in waste water treatment. Degradation of pollutants from industrial waste water and recycling using photocatalysts has been attracting much research interest for using abundant solar sources in environmental remediation. Many materials display good photocatalytic activity under ultraviolet (UV) and visible light irradiation. However, the applications of these materials are limited due to recombination of the electron–hole pairs. MoS2 has an advantage over other semiconductor photocatalyst materials like SnO2, TiO2, and ZnO for photocatalytic applications due to its lower band gap of ~1.9 eV. As a result, MoS2 is capable of absorbing both the UV and visible solar spectrum. In addition, its simple and easy synthesis is low cost and has good chemical stability in an aqueous medium, which makes it a

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S. V. Prabhakar Vattikuti and Chan Byon promising material for photocatalytic water treatment and water splitting applications. However, the use of MoS2 has been limited by the usable portion of the visible light spectrum due to low diffusion length and positive valence band with respect to the H+/H2 potential. We review the research to overcome these disadvantages and to enhance the photocatalytic activity of MoS2.

Keywords: TMDs, photocatalyst, synthesis, morphology, layered materials

1. INTRODUCTION The sun is an abundant source of energy and sustains life on earth. Our surrounding environment continues to become more polluted due to industrialization, and the traditional chemical methods that deal with environmental pollution have been unable to meet the requirements of saving energy and environmental protection. Environmental problems induced by toxic and hard-to-degrade organic pollutants (such as halides, dioxins, pesticides, dyes, etc.) have threatened human well-being and development. Recycling waste water from various industries by photocatalysis has been studied extensively for the removal of organic pollutants using a range of photocatalysts. Photocatalysts for waste water treatment hold promise for meeting the increasing water recycling demands without compromising the quality of our environment. Although photocatalysis successfully addresses environmental concerns at the lab scale, there are some technological issues that hamper the widespread commercial applicability of this technique for environmental remediation. Photocatalysis could achieve optimum use and commercial viability by replacing expensive and technically complicated artificial light sources with cheap and renewable sunlight. Transition metal dichalcogenides (TMDs) like MoS2 and WS2 are photocatalysts with high activities for environmental applications such as air purification, water disinfection, hazardous waste remediation, and water purification. The advanced oxidation process (AOP) uses light irradiation to produce highly reactive species such as OH-, O2•-, and O3. These species react with pollutants and form non-hazardous by-products. Limited availability of fossil fuels has also motived researchers to find different techniques to use solar energy. A well-known method for converting solar energy into electricity is photovoltaics (PV). However, they are limited by poor conversion efficiency. Researchers have recently focused on

Recent Progress in Molybdenum Disulfide (MoS2) Synthesis …

23

producing hydrogen (H2) from photocatalysis of water for fuel cell devices to generate power using sunlight to overcome these problems. Photocatalytic performance in nanoscale materials is one of the most important and interesting research topics due to applications in various areas, such as waste water conversion [1-4], H2 generation [5-8], air pollution control [9, 10], and dye-sensitized solar cells [11, 12].

1.1. Factors Influencing Photocatalytic Activity The photocatalytic activity and efficiencies of photocatalysis are influenced by various parameters, including porosity, surface area, defects, microstructure, surface morphology, and the nature of the exposed planes. The influential experimental parameters include the amount of dye, concentration of pollutants, pH of the reaction medium, reaction temperature, light intensity, calcination temperature, and oxidizing agents [13-16]. Different approaches have been promoted to enhance the photocatalytic ability of a material, including composites, surface modification, doping with metals or nonmetals [17-22], porous structures [23, 24], and hetero or multilayer structures [25, 26]. Heterogeneous coupling is considered as an efficient and promising technique to suppress the recombination effect of electron and hole pairs. Based on the MoS2 band gap for solar spectrum absorption, forming composites of MoS2 with other semiconductors (e.g., TiO2 [27], α-S [28] CdS [29], g-C3N4 [30]) is an effective approach to harvest visible light and enhance photocatalytic efficiency. For example, a wet chemically synthesized α-S/MoS2 composite presented good light absorption ability and much greater photocatalytic efficiency compared with pure MoS2 and α-S for methylene blue (MB) degradation under both UV and visible light [28].

2. MOLYBDENUM DISULFIDE (MOS2) 2.1. Structure and Properties TMD materials such as such as MQ2 (M = Mo, Nb, Re, V, W, and Q = S, Se) have gained much attention due to their unique properties and applications since discovering the nanoscale form of these inorganic materials. Tenne et al. discovered spherical fullerene-like nanoparticles of MoS2 and WS2 nanotubes

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S. V. Prabhakar Vattikuti and Chan Byon

in 1992 [31]. Since then, research on these materials and their tribological properties has intensified. One-dimensional (1D) or two-dimensional (2D) structures of TMD materials have remarkable properties, such as chemical inertness, anisotropy, photo-corrosion resistance, electronic properties [32-36], good catalytic properties, and resistance to sulfur poisoning [37]. MoS2 and WS2 are the most popular types of TMD materials and are most commonly used in layered structured forms. These materials have a layered close-packed hexagonal crystal structure composed of vertically stacked monolayers that are bonded together by weak van der Waals forces. The structural and morphological features of these materials widely depend on the synthesis strategies. MoS2 is a newly emerging layered semiconductor material and has attracted increasing attention due to its electronic and optical properties [35, 36]. MoS2 has an intrinsic layered structure formed by stacking S–Mo–S covalent bonds through weak van der Waals interactions [38]. Bulk MoS2 is an indirect band gap (1.29 eV) semiconductor and becomes a direct band gap (1.90 eV) semiconductor when reduced to a thin film with few layers due to quantum confinement effects [39, 40]. MoS2 is a promising candidate for photocatalytic applications due to its narrow band gap of about 1.9 eV. MoS2 is also more active in the UV light region, it is stable in most aqueous solutions (pH > 2.5), and and it has high yield and low cost. Single-layered MoS2 has a carrier mobility of up to 200 cm2/V, and that of multi-layered MoS2 is up to 517 cm2/V, making them promising for conducting capacitors [41, 42]. Based on thermodynamic equilibrium, the conduction band (CB) and valence band (VB) position of the semiconductor photocatalyst should be positioned within the band gap in such a way that the oxidation potential of the hydroxyl radicals is (Eo (H2)/ •OH) = 2.8 V vs. NHE and the reduction potential of superoxide radicals is (Eo (O2/•O2-) = -0.28 V vs. NHE [43]. The redox potential of the VB holes must be sufficiently positive to generate hydroxyl radicals, and the CB electrons must be sufficiently negative to produce superoxide radicals [10, 44]. Figure 1 shows the band edge position and band gap energies of common semiconductor sulfides and oxides along with their selected redox potentials. These semiconductor materials are suitable for photocatalysis reaction due to their empty CB and inherently filled VB. The electrons are excited from the VB to the CB when these semiconductor materials absorb photons and hʋ > Eg. This is represented in the following equation:

Recent Progress in Molybdenum Disulfide (MoS2) Synthesis … hʋ + semiconductor → h+ + e¯

25 (1)

Then, the electron of the semiconductor can be shifted to an adjacent compound in a binary or ternary system. The major criteria for selection of the best photocatalyst material are photo-corrosion resistance and reusability. Understanding the relationship between the photocatalytic performance and the microstructure is important for practical application. Hence, design and controllable synthesis are being investigated for nanostructured photocatalysts with optimized microstructure and improved photocatalytic performance [45].

Figure 1. The band gap energy position of various semiconductor materials on a potential scale (V) versus the normal hydrogen electrode (NHE).

2.2. Synthesis of MOS2 Nanomaterials In the reaction stage, size and shape control of MoS2 nanomaterials is crucial during the synthesis to obtain well-defined materials with specific properties. The morphology and size of MoS2 plays a significant role in catalysis, sensors, and other applications. Shape control is also important for applications in photochemistry and fuel cell catalysis. Other factors include monodispersity, avoiding agglomeration, and surface functionalization. The wide variety of synthetic methods for obtaining MoS2 nanomaterials can be divided into three main groups: (i) mechanical methods (e.g., grinding, ultrasonic cracking, or milling), (ii) liquid-phase methods (e.g., sol-gel,

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hydrothermal, or wet chemical methods), and (iii) gas-phase technique (e.g., chemical vapor deposition or laser ablation deposition). Liquid-phase technique is used most often due to simplicity, low cost, and wide variety of different sizes, shapes, and surface functionalities that can be obtained throughout a sample. Also, the size and agglomeration are effectively controlled due to functionalization of the nanomaterial surface with surfactants. In some cases, these surfactants are used to control shape and to favor growth in a specific direction by selective binding to some crystalline faces. The combination of size- and shape-dependent physical properties along with their simple fabrication and processing techniques make MoS2 nanomaterials a promising candidate for a wide range of applications. The properties of the individual particles and their mutual interactions determine important features of nanomaterial systems. For example, optical properties are highly dependent on the size, shape, and crystallinity of the MoS2 nanomaterials. However, controlled synthesis with a narrow size distribution and uniform shape remains an important issue in photocatalytic applications. More detailed studies on synthesis methods are discussed in section 3.

2.3. Photocatalytic Properties of MOS2 Photocatalysis involves converting solar energy into chemical energy. The main goal is maximizing the solar energy utilization of a photocatalyst for potential applications in solar energy conversion. However, photocatalytic ability has been limited due to the fast recombination effect of electron–hole pairs and an insufficient absorption coefficient. Enhancing the efficiency of a photocatalyst under visible light still remains a challenge. Many semiconductor oxides, sulfides, and nitrides have been used as photocatalysts for various applications. However, most sulfides and nitrides have lower band gaps and stability issues in an aqueous medium, which are key issues that must be solved to promote their use. The band gap of oxides is higher than that of sulfides, and the absorption edge is only in the UV region, which limits the usage of the solar spectrum. However, they have good stability in an aqueous medium. An ideal photocatalyst would have maximum absorption in the visible solar spectrum, favorable band edges for promoting reactions, environmental friendliness, low cost, good stability, and reusability [46].

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The band gap of MoS2 (1.9 eV) and its much lower VB edge than the water oxidation potential make it promising as a photoelectrode material for water splitting through photo-electro-chemical technique. The theoretical photocurrent density of chemically exfoliated MoS2 is ~17.6 mA/cm2 at 0.0 V vs. RHE under solar irradiation [47], and the solar energy conversion efficiency is ~18.7% for an ideal photo-electro-catalytic cell. However, the limited photocatalytic activity of MoS2 leads to low efficiencies and higher potential required to promote the photo-assisted water oxidation process [48]. Many researchers have tried to overcome these drawbacks by lowering the recombination rate through forming a composite or heterogeneous structure, enhancing conductivity by doping with metal, and promoting the charge carrier transferability [49-52]. MoS2 can also be used as photocatalyst for degrading organic compounds in waste water. Vattikuti et al. reported the mechanism of the degradation of rhodamine B (RhB) dye [53]. Photosensitization of the RhB dye first takes place when charge transfer occurs from the VB of the dye to the CB of the photocatalyst. This is followed by a photocatalytic reaction where MoS2 generates electron–hole pairs under photoirradiation. The electrons transfer from the VB of MoS2 to the CB and recombine with holes in the VB. The photoinduced electrons of MoS2 produce intermediate superoxide radicals (O2•¯) by reacting with chemisorbed oxygen on the photocatalyst surface and oxygen in the aqueous solution. The O2•¯ radicals react with dissociated water (H+) to form •HO2 and H2O2. Hydroxyl groups (OH¯) are also formed on the catalyst surface by the reaction with photoinduced holes (h+) by absorbed water (OH2). These radicals and intermediate species react with RhB dye and degrade it into non-toxic organic compounds as follows: RhB + hʋ → RhB •

(2)

RhB • + MoS2 → RhB • + MoS2 (eCB¯)

(3)

MoS2 + hʋ → MoS2 (eCB¯+ hVB+)

(4)

H2O → H+ + OH-

(5)

eCB¯+ O2 → O2•¯

(6)

O2•¯ + H+aq → HO2•

(7)

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S. V. Prabhakar Vattikuti and Chan Byon HO2• + HO2• ↔ H2O2 + O2

(8)

hVB+ + OH-aq → OH•-

(9)

RhB •/RhB •+ + (O2•¯, HO2•, H2O2, OH•-) →

Degradation products (10)

MoS2 is also a good photocatalyst for oxidative desulfurization [54]. Sulfur compounds in fuel convert to SOx, causing air pollution and acidic rain. Hydrodesulfurization (HDS) is commercially used for removing sulfur species at high temperature (350°C) and pressure (7 MPa). However, photo-oxidative desulfurization has recently received attention for its economic benefits and high efficiency [55, 56]. This method can provide cleaner and more efficient removal of sulfur species from petroleum fuel oils [57]. Lia et al. reported that CeO2/MoS2 and attapulgite show excellent electron transfer in a composite that favors the desulfurization process under solar irradiation [54]. MoS2assisted nanocomposite systems show promise for high activity and low cost photocatalysts in applications such as deep desulfurization. Thurston et al. [58] and Wilcoxon et al. [59] demonstrated MoS2 nanoparticles with 3–4.5-nm diameter as a catalyst for the degradation of phenol, 4-chlorophenol, and pentachlorophenol under visible light irradiation.

3. ONGOING RESEARCH ON THE PHOTOCATALYTIC BEHAVIOR OF MOS2 Based on the applications of MoS2, we emphasize three different forms of MoS2 that have been studied. Ongoing research on MoS2 nanoparticles as a photocatalyst is addressed first, followed by composite MoS2. This section concludes with a discussion on thin coated MoS2.

3.1. MOS2 Nanoparticles High aspect ratio plays a key role in the photocatalytic activity of materials. Researchers have concentrated on reducing the size of photocatalysts and improving the photocatalytic activities by making nanoscale MoS2. Many approaches have been used to synthesize MoS2 in nanocrystalline form with different morphologies, including ultrasonic

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cracking [60], hydrothermal methods [61-63], chemical synthesis [64], combustion methods [65-68], wet chemical methods, and co-precipitation methods [53, 69-71]. We reported synthesized MoS2 multiwall nanotubes (MWNTs) by a wet chemical method with H2O2 solvent as a growth promoter [53]. They were used as photocatalysts for the degradation of RhB, and the MoS2 MWNTs showed excellent performance compared to pure MoS2. The higher photocatalytic activity of MoS2 MWNTs was ascribed to the large number of active sites with high specific surface area. The performance with the optimal amount of 0.5wt% MoS2 MWNTs was attributed to the higher transfer of electrons and holes during the photoreaction, which effectively suppressed the recombination of the electron–hole pairs and enhanced the degradation efficiency. Zhou et al. [72] hydrothermally synthesized porous MoS2 without any sacrificial template using sodium molybdate and thioacetamide as Mo and S sources. Porous MoS2 showed 89.2% degradation efficiency of MB under 150 min of visible light irradiation. The photodegradation of MB with porous MoS2 showed a pseudo-first-order kinetic reaction rate of 0.01484 min-1. Polycrystalline porous MoS2 has attractive photocatalytic activity that is ascribed to the active edge sites. Sheng et al. [73] synthesized flower-like MoS2 spheres via hydrothermal method and studied the effects of excess sulfur source on the flower-like MoS2 structure. MoO3 and potassium thiocyanate (KSCN) were used as Mo and S sources with different ratios to obtain the flower-like MoS2 spheres. The optimal S/Mo ratio of 2.75 produced the highest degradation rate of MB with a rate of 0.03833 min-1 under 90 min of visible light irradiation. The increased photocatalytic performance was ascribed to the increased exposed area of the [100] facets with the optimal S/Mo ratio in the hydrothermal synthesis environment. The sheet thickness of the MoS2 spheres increased with the S/Mo ratio and enhanced the photocatalytic activity. Ye et al. [74] synthesized bilayer MoS2 nanosheets by a hydrothermal method. They studied the influence of molar ratio of the precursors and the hydrothermal time on the purity, crystal structure, and thermal stability of the nanosheets. These nanosheets have higher photo-adsorption capacity of methyl orange (MO) due to the large surface area (specific surface area SBET ~ 40.8 m2 g-1) and good photoluminescence properties. Therefore, these nanosheets are promising for photodevices and photochemical catalysts. James et al. [75] synthesized surfactant-free nanoparticles of MoS2 by thermally decomposing hexacarbonyls with the assistance of co-dissolved

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sulfur. These MoS2 nanoparticles have 10–20-nm diameter and show a sizedependent shift in their threshold UV-visible absorption. This is favorable for photodegradation of acetone in an aqueous solution under 6.5 h of visible light irradiation at 265 nm. However, these MoS2 nanoparticles do not dissolve in the aqueous solution due to their hydrophobic nature. The photodegradation of MB with MoS2 nanoparticles was not obvious due to photo-bleaching of MB. Hence, a high amount of surface silica-titania (1 wt %) nanoparticles was used to assist the MoS2 catalyst and improved the photodegradation of MB under 6.5 h of visible light irradiation (>400 nm). The SiO2 or TiO2 supported MoS2 nanoparticles were more phtocatalytically active than the individual nanoparticles. Liu et al. [76] synthesized MoS2 nanosheets by hydrothermal method with silicic acid (H2SiO3) hydrogel embedded in ammonium molybdate hydrate and thiourea precursors. The nanosheets were obtained after removing the H2SiO3. These nanosheets have a high specific surface area (SBET) of 37.8 m2g-1 and good absorption of MO in the visible region rather than the ultraviolet region in 70 min of irradiation time. Changing the concentration of silicic acid with MoS2 molar ratios of 2.5 and 0.8 led to different shapes of MoS2 nanosheets, such as leaf and flower shapes. These shapes provide steric hindrance for the MoS2 nanosheets growth. The amount of hydroxyl radicals was highest at pH 2 and decreased with increasing pH to 9. Therefore, the OH group plays a major role in MO photodegradation in the catalytic system. The reaction time, initial concentration, catalyst dosage, and local structures are key factors that affect the photocatalytic performance.

3.2. Heterostructured MOS2 The previous section comprises a review of make an effort made to enhance the photocatalytic activity of pure MoS2 in nanocrystalline and other morphological structures. Further, revisions that included the effects of morphology, particle size, pH, holding time, temperature and sintering temperatures on the photocatalytic performance of MoS2 were discussed. This section reviews the effect of heterogeneous structures or composite forms of MoS2 on the photocatalytic properties. Combining MoS2 with metals or nonmetals and semiconductor materials is a common practice for enhancing photocatalytic performance by facilitating and promoting efficient charge transfer at the interfaces. Similar attempts have been made for other classes of materials to improve photocatalytic activity, including Fe2O3, TiO2, and ZnO.

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Thurston et al. [58] reported that MoS2 nanoparticles with 8–10-nm diameter could not photodegrade phenol under visible light due to poor light absorption. Hence, they sensitized TiO2 nanoparticles with MoS2 nanoparticles, which enabled photodegradation under visible light irradiation. This composite structure showed a blue shift in absorbance due to quantum confinement of the charge carriers [58, 77]. We reported improved photocatalytic performance of MoS2 nanosheets decorated with mesoporous SnO2 nanospheres by a facile two-step method [78]. We also observed the photocatalytic effect in the degradation of RhB with less than 50 min of UV light irradiation. The supported mesoporous SnO2 nanoparticles significantly suppressed the recombination of electron–hole pairs compared to pure MoS2 photocatalyst material. The improved photocatalytic performance of the MoS2/SnO2 composite was explained by two mechanisms: (i) the absorption ability of the MoS2 nanosheets with active edges and (ii) enhanced electron transfer from SnO2 to the MoS2 nanosheets. This heterostructured composite facilitated effective electron transfer from the CB of SnO2 to the MoS2 nanosheets and suppressed the recombination effect. Therefore, the SnO2decorated MoS2 nanocomposite showed better photocatalytic performance than pure MoS2. Photocorrosion is the main reason for the lower photocatalytic activity of the pure MoS2. Pourabbas et al. [79] synthesized a hybrid MoS2/TiO2 composite using a modified hydrothermal method. The changes from the normal hydrothermal method included using sodium lauryl sulfate as a surface-active agent with 1octanol as a co-surfactant and varying reaction temperature. The hybrid composite was used as a photocatalyst for the photo-oxidative removal of phenol. The composite enhanced the photocatalytic performance in the phenol degradation under both UV (70 min) and visible light (24 min) compared to pure TiO2 and MoS2. The complete mineralization of phenol during the photooxidation reaction in 145 min of UV irradiation was indicated by HPLC chromatograms. Zhou et al. [80] and Bai et al. [81] did similar work and evaluated the photocatalytic performance of the MoS2/TiO2 composite for photodegradation of MB under visible light irradiation. MoS2-coated TiO2 nanobelt composites showed excellent photocatalyst for RhB degradation under 33 min of visible light irradiation. The matched energies of the TiO2@MoS2 composite are favorable for the charge transfer and suppress the recombination of electron–hole pairs. They also enhanced the photocatalytic hydrogen production. Liu et al. [82] synthesized a composite of TiO2 nanobelts decorated with MoS2 nanoparticles using a two-step hydrothermal method. The photocatalytic degradation of the TiO2/MoS2

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composite was evaluated with RhB under 90 min of visible light irradiation. The sample with 40 wt% MoS2 nanoparticles decorated on TiO2 nanobelts showed the best photocatalytic performance, which was attributed to the prevented recombination of photo-induced electron–hole pairs. This sample showed a high photocatalytic reaction rate constant that is about 4.78 times that of pure TiO2. Cao et al. [83] synthesized MoS2/TiO2 hybrid composites by a two-sept hydrothermal route. MoS2/TiO2 hybrid composite showed excellent photocatalytic performance in the degradation of RhB under 100 min visiblelight irradiation as compared to pure forms. The improvement in photocatalytic activity of MoS2/TiO2 hybrid composite was mainly ascribed to the properly matching CB and VB energy levels as well as the enhanced separation efficiency of photo-induced electron–hole pairs at interfacial contacts of the composite. Wang et al. [84, 85] reported the in situ deposition of Ag3PO4 on graphene-like MoS2 nanosheets were developed via wet chemical route to improve the photocatalytic performance for the degradation of RhB under 20 min of visible light irradiation (>400nm). The improved photocatalytic performance of the heterostructure of Ag3PO4/MoS2 composite is ascribed to the efficient separation of photo-induced electron-hole pairs within the photocatalyst. Ding et al. [86] synthesized MoS2-GO hydrogel composite using hydrothermal method for degradation of MB under 60 min solar light irradiation. This hydrogel-based composite showed enhanced photocatalytic performance in the degradation of MB with a maximum degradation rate of 99% for 60 min under solar light irradiation was attributed to the increased light absorption and suppressed recombination effect of semiconductor photocatalysis. Zhang et al. [87] synthesized MoS2/rGO photocatalyst for fluorescence detection of glutathione by •OH radical elimination system due to the reducing ability of glutathione under the visible light irradiation. Under the visible light irradiation, MoS2/rGO composite efficiently generation of •OH radical and reduction of •OH radicals were occurred by absorption of glutathione, which reflects by reduction of the fluorescent intensity due to the elimination of •OH radicals. This kind of photocatalyst can be effectively implemented for the identification of glutathione in commercial available drugs and human serum. Wang et al. [88] synthesized MoS2/Bi2O2CO3 composites for photodegradation of RhB under 150 min of visible light irradiation by a simple hydrothermal method. The effect of photocatalyst concentration on the photocatalytic efficiency was observed. This composite more active sites of

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MoS2 on Bi2O2CO3, which promoted the photocatalytic performance by absorbing and decomposing more RhB pollutant than pure Bi2O2CO3. The remarkable enhancement in the photocatalytic activity could be ascribed to the synergistic effect between the MoS2 and Bi2O2CO3 in the heterostructured composite. Li et al. [89] reported MoS2/BiVo4 hetero-nanoflower composites as an excellent photocatalyst for MB degradation with less than 120 min of sunlight irradiation. Li et al. [90] successfully synthesized a 2D heterojunction photocatalyst of g-C3N4 coupled with MoS2 nanosheets using a simple impregnation and calcination method. The g-C3N4/MoS2 composite promoted the charge transfer and improved the separation efficiency of photo-induced electron–hole pairs in RhB and MO degradation under 180 min of visible light irradiation. Jo et al. [91] synthesized MoS2 nanosheets loaded with ZnO-gC3N4 ternary photocatalyst for MB photodegradation under 60 min of UV-visible light irradiation. The ternary nanocomposite significantly improved the lifetime of charge carriers and facilitated effective migration and charge separation at the interface. Chen et al. [92] synthesized a hierarchical MoS2/Bi2MoO6 composite using an in-situ controlled method for growing Bi2MoO6 nanoflakes on preexfoliated MoS2 nanoslices as a supporting matrix. The composite showed considerably improved photocatalytic performance for RhB photodegradation under 120 min of visible light irradiation. The hierarchical structure suppressed the photocorrosion of the MoS2 nanoslices and improved the photoinduced charge separation by increasing the number of catalytic active sites and light harvesting. They used photovoltage spectroscopy, electrochemical impedance spectra, and photoluminescence spectrum measurements for analysis. These composites can shed light on structure engineering for novel visible-light-driven semiconductor photocatalysts for environmental friendliness. Core-shell structured materials have attracted attention from researchers due to their potential applications for photocatalytic degradation, electrochemical electrodes, and H2 production. Zhao et al. [93] synthesized nBiVO4/p-MoS2 p-n heterojunction photocatalysts with core-shell structure using a facile in-situ hydrothermal method. In the reaction process, the MoS2 shell thickness was altered by changing the concentration of the MoS2 precursor (sodium molybdate). This core-shell structure was used as photocatalyst for degradation of Cr6+ and oxidation of crystal violet (CV) dye under 60 min of visible light irradiation. The highest improvement in photocatalytic reduction of Cr6+ and photooxidation activity of CV dye was

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achieved with the 10 wt% BiVO4/MoS2 sample with MoS2 shell thickness of 300 nm. The improved photocatalytic performance could be ascribed to the suppression of charge recombination, high specific surface area, suitable absorption capacity towards the dye molecules, and better or tunable light absorption of the core-shell structured composite. The core-shell structure geometry is also favorable for increasing the contact area between MoS2 and BiVO4 and facilitates more charge transfer at their interface. The CB and VB positions of MoS2 are higher than that of BiVO4. The photo-induced electrons move from the CB of MoS2 to the CB of BiVO4, and holes are transferred in the opposite direction and accumulate at the VB of MoS2. This results in more efficient charge separation (Figure 2). Strong interfaces between the MoS2 and BiVO4 with corresponding potentials reduce the recombination of charge carriers more significantly. The electrons at the BiVO4 surface cannot be surrounded by molecular oxygen in solution to form •O2-. This allows more electrons to contribute to the photocatalytic reduction of Cr6+ and improves the photoreduction capability.

Figure 2. The schematic diagram demonstrating the energy band structure and occurrence of electron and hole pair transfer in the TiO2-MoS2 composites. Reprinted with permission from Ref. [82].

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In addition, the VB potential of MoS2 is not positive enough compared with the standard reduction potentials of •OH/H2O or •OH/OH- (2.27 eV and 2.38 eV, respectively) [94]. Therefore, the oxidation degradation of CV could be ascribed to the direct reaction with h+, and H2O and OH- are oxidized into •OH radicals at the VB of BiVO4. Hence, h+ and •OH radicals are the key active species for photocatalytic oxidation of CV dye. The following is proposed for the mechanism of photocatalytic Cr6+ reduction and CV dye oxidation [93]: BiVO4 + hʋ → e−(BiVO4) + h+(BiVO4)

(11)

MoS2 + hʋ→ e−(MoS2) + h+(MoS2)

(12)

e−(MoS2) → e−(BiVO4)

(13)

h+ (BiVO4) → h+ (MoS2)

(14)

e−(BiVO4) + Cr6+→ Cr3+

(15)

h+ (MoS2) + CV → Oxidative products

(16)

h+ (BiVO4) + H2O → •OH + H+

(17)

h+ (BiVO4) + OH− → •OH

(18)

•OH + CV → Oxidative products

(19)

Zhang et al. [95] synthesized a ternary composite system of TiO2/MoS2@Zeolite using a facile ultrasonic-hydrothermal synthesis method with TiCl4 as a Ti source and zeolite as a carrier. The photocatalytic performance was investigated for MO degradation for 60 min under xenon long-arc lamps as a visible light irradiation source. The photo-induced electrons and holes are collected in the CB of MoS2 and the VB of TiO2. Due to the more negative bottom CB energy of MoS2 and more positive top CB energy of TiO2, the photo-induced electrons in the CB of MoS2 can reduce the absorbed O2 into •O2−, and •OH can be easily produced in the VB of TiO2. The •O2− and •OH active species lead to MO degradation. Han et al. [96] synthesized a graphene-based 3D aerogel embedded with TiO2 nanoparticles and loaded with MoS2 nanosheets using a one-pot

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hydrothermal method. This method is suitable for obtaining 3D interconnected networks in aerogel through a self-assembly process. The graphene/MoS2/ TiO2 aerogel acted as a photocatalyst for H2 generation and MO photodegradation with a reaction time of 30 min of under UV light irradiation (Figure 3). This composite showed excellent photocurrent (37.45 mA/cm2) at +0.6 V that was six times higher than that of pure TiO2. Also, the fastest photodegradation of MO was observed within 15 min of UV light irradiation. This improved photocatalytic performance could be ascribed to (i) the porous structure, (ii) the features of active sites of the unique 3D aerogel, (iii) good electrical conductivity, (iv) increases active absorption sites, (v) the creation of photocatalytic reaction centers in the presence of MoS2 nanosheets, and (vi) the positive synergetic effect within the 3D aerogel composite. The composite is promising for potential applications in photochemical hydrogen generation and water purification.

Figure 3. The photodegradation mechanism of the n-BiVO4/p-MoS2 heterojunction photocatalyst. Reprinted with permission from Ref. [93].

Hu et al. [97] synthesized MoS2/Kaolin composites using calcining a MoS3/kaolin precursor in H2 under strong acidic conditions. The composite had a specific surface area of 16 m2g-1 and showed a positive photocatalytic effect on MO degradation under 105 min of visible light irradiation. This was attributed to the good absorption capacity in the visible light region. The photocatalyst has remarkable stability and could be regenerated and reused via

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filtration. The deactivating photocatalyst could be reactivated even after photocatalytic reaction at 450°C for 30 min under H2. The photocatalytic performance of exfoliated MoS2 was also investigated, and the relationship between the morphology of nano-MoS2 and the photocatalytic properties was discussed [98]. The photocatalytic performance of this and heterostructured composites are influenced by the quantity of photocatalyst, initial concentration of pollutant or dye, pH, irradiation time, type of light source, and degradation temperature.

Figure 4. Schematic structure of (a) the 3D graphene/MoS2/TiO2 aerogel networks, (b) illustration of the proposed reaction mechanism for hydrogen production and photodegradation of MO over the graphene/MoS2/TiO2 aerogel under UV irradiation. Reprinted with permission from Ref. [96].

3.3. Modified/Doped MOS2 Peng et al. [99] synthesized Co-doped MoS2 nanoparticles that were 30 nm in diameter using a hydrothermal method. They used the nanoparticles for photo-electro-catalytic conversion of CO2 into methanol with a yield of 35 mmolL-1 at 350 min. They described a strong effect of Co on the oxidation and reduction of reactive species with increasing Co content. In an electrocatalysis experiment, the resistance of the nanoparticles reduced by 85 kΩ, and the reduction over-potential for CO2 decreased from -0.82 V to -0.64V. This is led to a large enhancement in electro-catalytic performance. The Co-doped MoS2 nanoparticles showed significantly improved photocatalytic and electro-catalytic performances. The two-cell reactor was

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kept at a constant temperature of 42°C. In the photo-electro-catalytic reaction, water splitting occurs through water oxidation and produces oxygen and H+ on the anode side and photocatalytic oxidation of produces oxygen and H+ on the cathode side [99]. The H+ form the anode could move to the cathode through a proton exchange membrane due to an electric field, and the H+ produced by both electrodes would participate in the in situ reduction of CO2 at the cathode. Due to the more negative CB (-0.52 eV) than the CO2/CH3OH reduction potential, the photo-induced electrons have enough reductive ability to convert the CO2 to methanol.

SUMMARY AND FUTURE OUTLOOK To date, hierarchical MoS2 materials have continuously attracted increasing interest and demand for preparation techniques that have been successfully developed. We have provided a detailed overview of the photocatalytic performance of MoS2 nanomaterials. Three different types of MoS2 photocatalyst systems were distinguished according to their structural components: single-component, heterostructured, and doped MoS2. There is great interest in preparing various MoS2 photocatalyst systems using novel strategies, as is the desire for hierarchical MoS2 structures with special functionalities. Therefore, effort to develop MoS2 materials with novel structures and studies on their applications are ongoing. The purification of water and air will be an essential technology in this century, and there is currently enormous global effort being applied to solve the problem. This chapter review highlights the importance of MoS2 photocatalysts for pollutant degradation in contaminated waste water through solar light irradiation. There have been a number of advances in this field, including low-cost lower-band gap materials with increased stability and reusability. MoS2 photocatalysts are thus promising for further practical advances in the future. However, degradation rates are still generally low, the materials are somewhat unstable over repeated use, and there is great variability in the reported reduction rate and efficiencies of such systems. It is very important to reproduce the reduction rates from one lab to another, and repeatability and reusability are currently some of the great deficiencies in the field. In the future, scientists should focus materials design and realization of practical applications.

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C3N4 with tunable surface area. Phys. Chem. Chem. Phys. 15, 45104517. Xu, J., Wang, Y., Zhu, Y. 2013. Nanoporous Graphitic Carbon Nitride with Enhanced Photocatalytic Performance. Langmuir. 29, 1056610572. Li, H., Zhou, L., Wang, L., Liu, Y., Lei, J., Zhang, J. 2015. In situ growth of TiO2 nanocrystals on g-C3N4 for enhanced photocatalytic performance, Phys. Chem. Chem. Phys. 17, 17406-17412. Xiao, H., Wang, W., Liu, G., Chen, Z., Lv, K., Zhu, J. 2015. Photocatalytic performances of g-C3N4 based catalysts for RhB degradation: Effect of preparation conditions, Appl. Surf. Sci. 358, 313318. Zhou, W.J., Yin, Z.Y., Du, Y.P., Huang, X., Zeng, Z.Y., Fan, Z.X., Liu, H., Wang, J.Y., Zhang, H. 2013. Synthesis of Few-Layer MoS2 Nanosheet-Coated TiO2 Nanobelt Heterostructures for Enhanced Photocatalytic Activities. Small. 9, 140–147. Hu, Ch., Zheng, Sh. Lian, Ch., Chen, F., Lu, T., Hu, Q., Duo, Sh., Zhang, R., Guan, Ch.S. 2015. Nanoparticles grown on MoS2 nanosheets: A novel sulfur-based photocatalyst with enhanced photocatalytic performance. J. Mol. Catal. A: Chem. 396, 128–135. Liu, Y., Yu, Y.X., Zhang, W.D. 2013. MoS2/CdS Heterojunction with High Photoelectrochemical Activity for H2 Evolution under Visible Light: The Role of MoS2. J. Phys. Chem. C. 117, 12949–12957. Hou, Y., Wen, Z.H., Cui, S.M., Guo, X.R., Chen, J.H. 2013. Constructing 2D Porous Graphitic C3N4 Nanosheets/Nitrogen-Doped Graphene/Layered MoS2 Ternary Nanojunction with Enhanced Photoelectrochemical Activity. Adv. Mater. 25, 6291–6297. Tenne, R., Margulis, L., Genut, M., Hodes, G.1992. Polyhedral and cylindrical structures of WS2. Nature. 360, 444-447. Tacchini, I., Terrado, E., Anson, A., Martınez, M.T. 2011. Preparation of a TiO2–MoS2 nanoparticle-based composite by solvothermal method with enhanced photoactivity for the degradation of organic molecules in water under UV light. Micro Nano Lett. 6, 932–936. Kushmerick, J.G., Weiss, P.S. 1998. Mobile promoters on anisotropic catalysts: nickel on MoS2. J. Phys. Chem. B. 102, 10094–10097. Kuchibhatla, S.V.N.T., Karakoti, A.S., Bera, D., Seal, S. 2007. One dimensional nanostructured materials. Prog. Mater. Sci. 52, 699–913.

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properties of flower-likeMoS2 spheres by hydrothermal synthesis, Mater. Lett. 144, 153–156. Ye, L., Xu, H., Zhang, D., Chen, Sh. 2014. Synthesis of bilayer MoS2 nanosheets by a facile hydrothermal method and their methyl orange adsorption capacity. Mater. Res. Bull. 55, 221–228. James, D., Zubkov, T. 2013. Photocatalytic properties of free and oxidesupported MoS2 and WS2 nanoparticles synthesized without surfactants, J. Photochem. Photobiol. A: Chem. 262, 45– 51. Liu, W., Hu, Q., Mo, F., Hu, J. Feng Y, Tang, H., Yea, Sh., Miao, H. 2014. Photo-catalytic degradation of methyl orange under visible light by MoS2 nanosheets produced by H2SiO3 exfoliation. J. Mol. Catal. A: Chem. 395, 322–328. Wilcoxon, J. P.; Samara, G. A. 1995. Strong quantum-size effects in a layered semiconductor: MoS2 nanoclusters. Phys. Rev. B. 51, 7299-7302. Vattikuti, S.V.P., Byon, Ch., Reddy, Ch.V., Ravikumar, R.V.S.S.N. 2015. Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles, RSC Adv. 5, 86675–86684. Pourabbas, B., Jamshidi, B. 2008. Preparation of MoS2 nanoparticles by a modified hydrothermal method and the photo-catalytic activity of MoS2/TiO2 hybrids in photo-oxidation of phenol. Chem. Eng. J. 138, 55–62. Zhou, H., Gu, T., Yang, D., Jiang, Zh., Zeng, J. 2011. Photocatalytic Degradation of Methylene Blue on MoS2/TiO2 Nanocomposite. Adv. Mater. Res. 197-198, 996-999. Bai, S., Wang, L., Chen, X., Du, J., Xiong, Y. 2015. Chemically exfoliated metallic MoS2 nanosheets: a promising supporting co-catalyst for enhancing photocatalytic performance of TiO2 nanocrystals, Nano Res. 8(1), 175-183. Liu, H., Lv, T., Zhu, Ch., Su, X. Zhu, Zh. 2015. Efficient synthesis of MoS2 nanoparticles modified TiO2 nanobelts with enhanced visiblelight-driven photocatalytic activity. J. Mol. Catal. A: Chem. 396, 136– 142. Cao, L., Wang, R., Wang, D., Li, X., Jia, H. 2015. MoS2-hybridized TiO2 nanosheets with exposed {001} facets to enhance the visible-light photocatalytic activity. Mater. Lett.160, 286–290. Wang, P., Shi, P., Hong, Y., Zhou, X., Yao, W. 2015. Facile deposition of Ag3PO4 on graphene-like MoS2 nanosheets for highly efficient photocatalysis. Mater. Res. Bull. 62, 24–29.

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[85] Wang, L., Chai, Y., Ren, J., Ding, J., Liu, Q., Dai, W. 2015. Ag 3PO4 nanoparticles loaded on 3D flower-like spherical MoS2: a highly efficient hierarchical heterojunction photocatalyst, Dalton Trans. 44, 14625–14634. [86] Ding, B.Y., Zhou, Y. Nie, W. Chen, P. 2015. MoS2–GO nanocomposites synthesized via a hydrothermal hydrogelmethod for solar light photocatalytic degradation of methylene blue. Appl. Surf. Sci. 357, 1606–1612. [87] Zhang, N., Ma, W., Han, D.X., Wang, L., Wu, T., Niu, L. 2015. The fluorescence detection of glutathione by ∙OH radicals’ elimination with catalyst of MoS2/rGO under full spectrum visible light irradiation, Talanta.144, 551–558. [88] Wanga, Q., Yun, G., Bai, Y., An, N., Lian, J., Huang, H., Su, B. 2014. Photodegradation of rhodamine B with MoS2/Bi2O2CO3 composites under UV light irradiation. Appl. Surf. Sci. 313, 537–544. [89] Li, H., Yu, K., Lei, X., Guo, B., Fu, H., Zhu, Z. 2015. Hydrothermal Synthesis of Novel MoS2/BiVO4 Hetero-Nanoflowers with Enhanced Photocatalytic Activity and a Mechanism Investigation. J. Phys. Chem. C. 119, 22681−22689. [90] Li, J., Liu, E., Ma, Y., Huc, X., Wan, J., Sun, L., Fan, J. 2016. Synthesis of MoS2/g-C3N4 nanosheets as 2D heterojunction photocatalysts with enhanced visible light activity. Appl. Surf. Sci.364, 694-702. [91] Jo, W., Lee, Y.J., Selvam, N.C.S. 2016. Synthesis of MoS2 nanosheets loaded ZnO–g-C3N4 nanocomposites for enhanced photocatalytic applications. Chem. Eng. J. 289, 306-318. [92] Chen, Y., Tian, G., Shi, Y., Xiao, Y., Fu, H. 2015. Hierarchical MoS2/Bi2MoO6 composites with synergistic effectfor enhanced visible photocatalytic activity. Appl. Catal. B: Environ. 164, 40–47. [93] Zhao, W., Liu, Y., Wei, Zh., Yang, Sh., He, H., Sun, Ch. 2016. Fabrication of a novel p–n heterojunction photocatalystn-BiVO4@pMoS2with core–shell structure and its excellent visible-light photocatalytic reduction and oxidation activities. Appl. Catal B: Environ. 185, 242–252. [94] Cheng, H.F., Huang, B.B., Dai, Y., Qin, X.Y., Zhang, X.Y. 2010. OneStep Synthesis of the Nanostructured AgI/BiOI Composites with Highly Enhanced Visible-Light Photocatalytic Performances. Langmuir. 26, 6618–6624. [95] Zhang, W., Xiao, X., Zheng, L., Wan, C. 2015. Fabrication of TiO2/MoS2@zeolite photocatalyst and its photocatalytic activity for

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degradation of methyl orange under visible light. Appl. Surf. Sci.358, 468–478. Han, W., Zang, Ch., Huang, Z., Zhang, H., Ren, L. Qi, X. Zhong, J. 2 0 1 4. Enhanced photocatalytic activities of three-dimensional graphenebased aerogel embedding TiO2 nanoparticles and loading MoS2 nanosheets as Co-catalyst. Int. J. Hydrogen Energy. 39, 1 9 5 0 2-1 9 5 1 2. Hu, K.H., Liu, Zh., Huang, F., Hu, X.G., Han, Ch.L. 2010. Synthesis and photocatalytic properties of nano-MoS2/kaolin composite. Chem. Eng. J. 162, 836–843. Hu, K.H., Hu, X.G. 2009. Formation, exfoliation and restacking of MoS2 nanostructures. Mater. Sci. Technol. 25, 407–414. Peng,H. Lu, J. Wu, Ch. Yang, Zh., Chen, H., Song, W., Li, P., Yin, H. 2015. Co-doped MoS2 NPs with matched energy band and low overpotentialhigh efficiently convert CO2 to methanol. Appl. Surf. Sci. 353, 1003–1012.

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BIOGRAPHICAL SKETCH Name: S V Prabhakar Vattikuti Affiliation: Yeungnam University Education: Ph.D Address: 510, School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea-712749 Research and Professional Experience: Vattikuti has completed his PhD degree in Nanomaterials and Coatings from Chung Hua University, Taiwan. He received one year postdoctoral fellowship from Yeungnam University. Currently he is International assistant professor in School of mechanical Engineering, Yeungnam University, South Korea. He has published more than 25 papers in reputed journals and has been serving as a reviewer of repute journals. His research focuses on transition metal Chalcogenide materials for energy and photocatalytic applications. 

Currently working as Assistant Professor in School of Mechanical Engineering, Yeungnam University, South Korea from Sept 2015 to till date.

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Responsibilities: My responsibilities include Research and Teaching to undergraduate and graduate students. Apart from this I am actively involved in writing original research articles, book chapters, communications and projects. 

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Currently working as Post-Doctoral Fellow in School of Mechanical Engineering, Yeungnam University, South Korea from Oct’2014 to Aug 2015. Worked as Associate Professor, Department of Mechanical Engineering, Vardhman College of Engineering (Autonomous), Shamshabad, Hyderabad from Jan’2012 to Sept 2014. Worked as a Research Development Engineer (Nanocoatings Designer) in Production Division, Shining Optical Tech. Corp; Kwei Shan Township; Taoyuan Country.333; Taiwan, from January 2006 to January 2009. Worked as a Quality Inspector, TechoPlasttic Co. Ltd, Porur, Chennai, TN, India from June 2002 to May 2003.

Professional Appointments:        

Regular member- The American Ceramic Society – Ref. No: AQ0ACDF2F376. Life member- Tribology Society of India (TSI) – Membership No: 5251. Life member- Indian Society for Technical Education (ISTE). Member-Institute of Indian Foundry men (IIF). Life Member -Solar Energy Society of India (SESI). Life Member-Material Research Society of India (MRSI). Life Member-International Association of Engineers (IAENG). Member - Institute of social Research and Development (ISARD), India. www.isard.org.in.

Books Published: SV Prabhakar Vattikuti, Hsi-Hsin Chien, B. Venkatesh, “Wettability of Water Based Sol-Gel Coatings for optical Glass Preforms- Solutions for Glass Sticking Problems” ISBN: 978-3-659-39057-9, 2013. B. Venkatesh, SV Prabhakar Vattikuti, “Design of High speed, High Quality Gears for Marine Applications” ISBN: 978-3-659-39888-9, 2013.

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Honors: NA Publications Last 3 Years: [1]

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S.V. Prabhakar Vattikuti*, Young-Jin Baik, Chan Byon, Enhanced photocatalytic activity of MoO3-supported SnO2 composite synthesized by a wet chemical method, ASP-Materials Express (Accepted). IF: 2.28. S.V. Prabhakar Vattikuti*, Chan Byon, Ch. Venkata Reddy, Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method, Materials Research Bulletin 75 (2016) 193-203. doi:10.1016/j.materresbull.2015.11.059 IF: 2.28. S. V. Prabhakar Vattikuti,* Chan Byon Ch. Venkata Reddy and R. V. S. S. N. Ravikumar, Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles, RSC Adv., 2015, 5, 86675–86684. doi: 10.1039/c5ra15159g IF: 3.86. S.V. Prabhakar Vattikuti*, Chan Byon, Effect of CTAB Surfactant on Textural, Structural, and Photocatalytic Properties of Mesoporous WS2, ASP-Science of Advanced Materials, Vol:and, No:12, Dec (2015), pp. 2639-2645(7). doi:10.1166/sam.2015.2584 IF: 2.59. Ch. Venkata Reddy, S.V. Prabhakar Vattikuti, R.V.S.S.N. Ravikumar, SangJunMoon, Jason Shim, Influence of calcination temperature on Cd0.3Co0.7Fe2O4 nanoparticles: Structural, thermal and magnetic properties, Journal of Magnetism and Magnetic Materials 394 (2015) 70–76. http://dx.doi.org/10.1016/j.jmmm.2015.06.054 IF: 1.97. S.V. Prabhakar Vattikuti, Chan Byon, Synthesis and characterization of Al2O3-coated molybdenum disulfide nanospheres with high photocatalytic activity, Journal of nanomaterials, Volume 2015 (2015), Article ID 978409, 9 pages http://dx.doi.org/10.1155/2015/978409 IF: 1.64. S.V. Prabhakar Vattikuti*, Chan Byon, CV Reddy, J Shim, Effect of temperature on structural, morphological and magnetic properties of Cd0.7 Co0.3Fe2O4 nanoparticles, Journal of Magnetism and Magnetic Materials, 393(2015)132–138. DOI: 10.1016/j.jmmm.2015.05.057, IF: 1.97. S V Prabhakar Vattikuti*; Chan Byon; Ch. Venkata Reddy, Synthesis of MoS2 multi-wall nanotubes using wet chemical method with H2O2 as growth promoter, Superlattices and Microstructures, 85 (2015) 124–132. DOI: 10.1016/j.spmi.2015.05.037 IF: 2.09.

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S. V. Prabhakar Vattikuti and Chan Byon S V Prabhakar Vattikuti; Chan Byon; Synthesis and characterization of molybdenum disulfide nanoflowers and nanosheets: nanotribology, Journal of nanomaterials, Volume 2015 (2015), Article ID 710462, 11 pages. http://dx.doi.org/10.1155/2015/710462 IF: 1.64. S.V. Prabhakar Vattikuti*, C Byon, CV Reddy, B Venkatesh, J Shim, Synthesis and structural characterization of MoS2 nanospheres and nanosheets using solvothermal method, Journal of Materials Science 50 (14), 5024-5038, DOI:10.1007/s00339-015-9163-7, IF: 2.371. S.V. Prabhakar Vattikuti*, C Byon, CV Reddy, J Shim, B Venkatesh, Co-precipitation synthesis and characterization of faceted MoS2 nanorods with controllable morphologies, Applied Physics A 119 (3), 813-82, DOI:10.1007/s00339-015-9163-7, IF: 1.704. Ch. Venkata Reddy, Chan Byon, B. Narendra, D. Baskar, Jaesool Shim, S.J. Moon, S.V. Prabhakar Vattikuti*, Effect of calcination temperature on cobalt substituted cadmium ferrite nanoparticles, Journal of Materials Science: Materials in Electronics DOI:10.1007/s10854-0153031-2, IF: 1.56. Ch. Venkata Reddy, Chan Byon, B. Narendra, D. Baskar, G. Srinivas, Jaesool Shim, S.V. Prabhakar Vattikuti*, Investigation of structural, thermal and magnetic properties of cadmium substituted cobalt ferrite nanoparticles, Superlattices and Microstructures, Vol. 82, June 2015, Pages 165–173, doi:10.1016/j.spmi.2015.02.014., IF: 2.09. Ch.Venkata Reddy, K.Vijaya Kumar, S.V.Prabhakar Vattikuti and R.V. S. S. N. Ravikumar “Preparation and photoluminescence of Cr3+ doped β- BaB2O4 nanopowder by co-precipitation Method” Physica BCondensed Matter, Volume 429, 15 November 2013, Pages 18–23, DOI: http://dx.doi.org/10.1016/j.physb.2013.07.028 ISSN: 0921-4526, IF: 1.327. G.V.S.S.Sarma, Ch.VenkataReddy, S.V.Prabhakar Vattikuti, P. Narayana Murthy and R.V.S.S.N. Ravikumar “Spectral investigations of Mn2+doped Zn3 (BO3)2 nanopowder” Journal of Molecular Structure, Volume 1048, 24 September 2013, Pages 64–68, DOI: http://dx.doi.org/10.1016/j.molstruc.2013.05.033 ISSN: 0022-2860, IF: 1.404.

In: Molybdenum Disulfide Editor: Jeremiah McBride

ISBN: 978-1-63485-032-2 © 2016 Nova Science Publishers, Inc.

Chapter 3

MOLYBDENUM DISULFIDE: A PROMISING MATERIAL FOR PHOTOCATALYTIC HYDROGEN EVOLUTION Yongtao Lu1,2 1

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, China 2 i-LAB and Nano-Bionics Division, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, China

ABSTRACT Hydrogen as a zero carbon emission fuel has recently attracted increasing attention because of the depletion of fossil fuel reserves and the severe environmental crisis. Since Fujishima and Honda first reported the photoelectrochemical splitting of water into H2 and O2, wide variety of photocatalytic H2 generation tactics have been exploited over the last four decades. So far, the Pt-based materials were found to be wellperforming catalysts for photoinduced H2 production, while high-cost limited their large-scale application. Recently, molybdenum disulfide (MoS2) has been demonstrated as a promising material for substituting Ptbased catalysts for the improvement of photocatalytic performance of H 2 production. This chapter mainly focuses on the recent progress of photocatalytic H2 evolution systems in which MoS2 worked as supporting materials. It involves the design and the architect of MoS2 functionalized nanomaterials for enhancing photocatalytic performance of H2. At the end

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Yongtao Lu of this chapter, the prospects of the new kind of MoS2-based materials for improving photocatalytic H2 production are also addressed briefly.

Keywords: molybdenum disulfide, hydrogen evolution, photocatalysis, cocatalyst, semiconductor

1. INTRODUCTION Due to the global energy crisis and environmental protection demand, hydrogen as the highest energy density carrier per unit weight and an environmentally friendly energy source has attracted great attention. Since the discovery of hydrogen evolution through the photoelectrochemical splitting of water on TiO2 electrodes [1], the technology of photocatalytic water splitting to produce hydrogen using solar energy has been considered as one of the most important approaches to solving the world energy crisis and the environmental issues. Hence, the development of the necessary photocatalysts has undergone considerable research. Over the past few decades, many photocatalysts reportedly exhibited high photocatalytic activities for splitting water into hydrogen. These photocatalysts are generally distinguished into two types based on their phase difference in reaction system: heterogeneous and homogeneous photocatalysts. In heterogeneous photocatalytic systems, photocatalysts are commonly solid powders, such as TiO2, CdS, C3N4 etc. [26], and while in homogeneous photocatalytic systems, photocatalysts are commonly organic dyes coupled with hydrogen evolution catalyst [7, 8]. In both heterogeneous and homogeneous photocatalysis, photocatalytic progress commonly consists of the light absorption, the charge separation and transportation, the redox catalytic activity and hydrogen evolution reaction. Over last two decades, a significant progress has been made in photocatalytic hydrogen generation experimentally and computationally. However, recent photocatalytic systems generally have two serious limitations: high electronhole recombination rate and the large overpotential for hydrogen production. To overcome the limitation, many characteristic materials were introduced into photocatalytic hydrogen system. Among these materials, molybdenum disulfide is a very promising and effective material for the improvement for the photocatalytic hydrogen production. Molybdenum disulfide (MoS2) is a semiconductor which is composed of Mo atoms sandwiched between two layers of hexagonal close packed sulfur atoms in a structure similar to graphene. Traditionally, it has been used as a

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solid lubricant due to its low friction properties and as a hydrodesulfurization catalyst to lower the sulfur content in natural gas and fuels. Bulk MoS2 were first examined as a possible hydrogen evolution reaction electrocatalyst as early as 1977 by Tributsch et al. [9]. However, it was not until about 20 years later that its potential in the hydrogen evolution reaction was fully unveiled. Recent researches indicate that while the basal plane of MoS2 was catalytically inert, its sulfided Mo-edges were active for the hydrogen evolution. The overpotential of MoS2 is close to that of traditional efficient hydrogen evolution electrocatalyst Pt. Hydrogen evolution is a very important step in the photocatalytic hydrogen production system like that in electrocatalytic hydrogen production. In 2008, Li’s group first introduced electrocatalyst MoS2 as a cocatalyst into photocatalytic hydrogen production system [10]. Their work found that the rate of H2 evolution on CdS is significantly enhanced by loading MoS2 as a cocatalyst on CdS for the H2 production under visible light. Since this pioneering work, considerable progress has been made toward developing MoS2-based photocatalyst that are able to evolve hydrogen from water under solar illumination. Another characteristic of MoS2 is two dimensional layered forms that can be exfoliated or grown into small thicknesses based mono- or multiple layers like graphene. Two dimensional layered forms are beneficial for the electron transfer. More importantly, MoS2 with the layered structures has the geometric similarity with some two dimensional semiconductor. This geometric similarity can facilitate the planar growth of MoS2 on the semiconductor surface and the formation of the imitate interface. Wang’s group [11] in 2013 reported layered nanojunctions composed by MoS2 and two dimensional C3N4. The photocatalytic results illustrated the H2 production performance of C3N4 under visible light is significantly improved by growing thin layers of MoS2 on C3N4. And the characterizations of the photocatalysts indicated that geometric similarity in the layered structures of MoS2 and C3N4 mainly contributed to the promotion of the photocatalytic results. Based on the layered structures, another interesting feature of MoS2 reported in 2014 is its layer-numberdependent electrocatalysis for hydrogen evolution. After this discovery, in 2015 Ye’s group [12] introduces MoS2 with different layer-numbers in to photocatalytic system. They systematically study on controlled synthesis of MoS2 with layer number ranging from ≈1 to 112 and their activities for photocatalytic H2 evolution over commercial CdS. They observed a drastic increase in photocatalytic H2 evolution when decreasing MoS2 layer number. The high hydrogen evolution rate is mainly attributed to the more exposed edges and unsaturated active S atoms in MoS2 with less layer number.

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The past few years has seen a rapid increase by using MoS2 in the design of the photocatalysts in an effort to obtain high performance hydrogen evolution photocatalysts. In particular, MoS2 with ultrathin layer structures have enabled researchers to take many strategies to design effective photocatalysts to make full use of two dimensional structures. A variety of experimental approaches have emerged relating preparation, characterization and photocatalytic hydrogen performance of MoS2 based photocatalysts. In this chapter, we present a brief survey of recent work in this field. Our objective is MoS2 based photocatalysts both in heterogeneous and homogeneous photocatalytic system. Discussions will start with the basic mechanism of heterogeneous and homogeneous photocatalytic system. We will then discuss the preparation and detailed applications of MoS2 based photocatalyst in photocatalytic system. At the end of this chapter we will give a short summary and outlook in this field.

2. FUNDAMENTALS OF PHOTOCATALYTIC HYDROGEN PRODUCTION In Fujishima and Honda’s pioneering work [1], they constructed an electrochemical cell for the decomposition of water into hydrogen and oxygen. When the TiO2 surface was irradiated by UV light, as a result of a water oxidation reaction, oxygen evolution occurred at the TiO2 surface. Concomitant reduction led to hydrogen evolution at the platinum black electrode. This concept, which emerged from the use of photoelectrochemical cells with semiconductor electrodes, was later applied by Bard to the design of a photocatalytic system using semiconductor particles or powders as photocatalysts. These works are the origin of the heterogonous photocatalysis. In heterogonous photocatalysis, a photocatalyst absorbs UV and/or visible light irradiation from sunlight or an illuminated light source. The electrons in the valence band of the photocatalyst are excited to the conduction band, while the holes are left in the valence band. After photoexcitation, the excited electrons and holes separate and migrate to the surface of photocatalyst. Here, in the photocatalytic water-splitting reaction, they act as reducing agent and oxidizing agent to produce H2 and O2, respectively. To facilitate both the reduction and oxidation of H2O by photoexcited electrons and holes, the match of the band gap and the potentials of the conduction and valence bands are important. Both the reduction and oxidation potentials of water should lie

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within the band gap of the photocatalyst. The bottom level of the conduction band has to be more negative than the reduction potential of H /H2 whereas the top level of the valence band has to be more positive than the oxidation potential of O2/H2O (1.23 V). The band structure requirement is a thermodynamic requirement for water splitting. Other factors, such as overpotentials, charge separation, mobility, and lifetime of photogenerated electrons and holes, affect the photocatalytic generation of hydrogen from water splitting as well. Besides heterogeneous photocatalysis, homogeneous photocatalysis is also an important method for hydrogen production. Homogeneous photocatalysis imitate the nature photosynthesis and it commonly contains organic dye, hydrogen evolution catalyst and the electron donor. In a homogeneous system, organic dye absorbs the light energy and generates an excited state. The electron in the excited state transfer to the hydrogen evolution catalyst and the H+ in the water capture the electron to produce the hydrogen. At the same time, the organic dye oxidizes the electron donor. +

3. SYNTHESIS OF MOLYBDENUM DISULFIDE In the past few years, MoS2 has gained intensive attentions for photocatalytic hydrogen production due to its low overpotential for hydrogen production and unique layer structures. In order to effectively use the advantages of MoS2 for photocatalytic hydrogen production, many strategies for the preparation of MoS2 have been proposed. These strategies included thermal annealing, hydrothermal/solvothermal method, in situ photodeposition, the hot-injection method and liquid exfoliation methods etc.

3.1. Thermal Annealing Thermal annealing is the earliest method for the preparation of MoS2based photocatalyst. By impregnating CdS with an aqueous solution of MoS2 precursor (NH4)2MoS4, followed by a treatment in H2S flow at high temperature calcination, Li’s group [5, 13] prepared MoS2/CdS photocatalyst, which showed an increasement up to 36 times when loaded with only 0.2 wt % of MoS2. Using similar method except for in H2S/H2 atmosphere, Li’s group [14] successfully prepared the MoS2/ZnIn2S4 composite with excellent photocatalytic hydrogen evolution performance. Thermal annealing is a very

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facial method for preparation of MoS2. But it is very difficult to get precise morphology MoS2 and it needs H2S which is a toxic gas.

3.2. Hydrothermal/Solvothermal Method The hydrothermal/solvothermal method is a very convenient route for the large scale synthesis of MoS2. It often use sodium molybdate dehydrate as Mo source and thiourea or thioacetamide as S source. The reaction mixture was heated in an oven around 200°C for certain duration and bulk MoS2was obtained. In addition, for this method the size and shape of MoS2 could be controlled with certain range by add appropriate kind and appropriate amount of surfactants. The hydrothermal/solvothermal method has been widely used in the preparation of MoS2-based composites, such as CdS/MoS2, TiO2/MoS2, Cu2ZnSnS4/MoS2, SrZrO3/MoS2 etc. However, the disadvantages of hydrothermal/solvothermal method are also obvious. For example, it is difficult to realize very precise control of the layer numbers of MoS2. Also hydrothermal/solvothermal method for preparation of MoS2 based composites requires the other component of the composite is stable in the hydrothermal/solvothermal reaction conditions. Because of this reason, C3N4/MoS2 composite can’t be synthesized by hydrothermal reaction since C3N4 will decompose completely in hydrothermal condition at around 200°C [15].

3.3. Exfoliation Methods The layers in layered materials are composed with strong interaction which are stacked each other during weak intermolecular Van der Waals force or electrostatic force with interlayer ions bearing opposite charges. Such structural features offer opportunity to destroy the interlayer interaction by physical or chemical approaches, and therefore achieve the so-called nanosheets, individual sheets with single or few host layers. MoS2 is a typical layered material and it has the general character of the layered material. Such layer structural features offer opportunity to destroy the interlayer interaction by physical or chemical methods, and therefore obtain the single or few layers nanosheets. The property for hydrogen production of the nanosheet obtained is quite different from that of bulk materials. Ye’s group [7] demonstrated that single layer MoS2 can increase drastically the photocatalytic hydrogen

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evolution amount of CdS. Due to the excellent performance of thin MoS2 nanosheet, great efforts have been devoted to preparing various kinds of MoS2 nanosheet. To obtain the MoS2 nanosheet, several effective exfoliation methods have been established. These methods mainly include the scotch tape technique, ultrasonic exfoliation and chemical exfoliation. R. Sasikalaa’s group [16] used the scotch tape technique to synthesize a few layered (4-5 layers) MoS2 flakes and introduced it into CdS systems. However the monolayer yield from the “Scotch tape” method is so low that few chemists will consider it viable for doing chemical reactions. Ultrasonic exfoliation is the most widely used method to prepare the MoS2 nanosheet because it is very facial and effective. Generally, MoS2 nanosheets can be obtained from direct sonication commercial MoS2 in different solvents, such as N,N-dimethylformamide, N-methyl-pyrrolidinone and isopropanol for a suitable duration. Ultrasonic exfoliation can avoid the phase transition, but typically yield a range of MoS2 flake thicknesses and size. In 2015, Yang’s group [17] used the ultrasonic exfoliation to prepare the MoS2 nanosheet and the obtained exfoliated MoS2 nanosheets were combined with theZnxCd1-xS. The generated composites have an improved performance for photocatalytic hydrogen production. Li intercalation approach is also a widely used method and it has been explored since the early 1970s. Such technique involves introducing Li inbetween the gaps of the MoS2 and then separating it using various ways. Generally, the MoS2 is immersed into n-butyl lithium for a particular duration to allow consistent intercalation of Li. Water is then added to the Li intercalation material, where the intercalated Li vigorously reacts with water, producing lithium hydroxide and hydrogen gas, and separating the monolayers. In 2010, Osterloh’s group [18] first used this method to prepare the MoS2 nanosheet and introduced it into CdSe. After the introduction of MoS2 nanosheet, the photocatalytic activity of CdSe increases almost four times. In 2014, Xiong’s group [19] used the Li intercalation method to prepare the MoS2 nanosheet and they found MoS2 changed from the 2H phase to 1T phase after the Li intercalation/exfoliation progress. MoS2 nanosheet with 1T phase has higher mobility for charge transport since it is metallic. It is beneficial for the photocatalytic hydrogen production. However, the disadvantage of this approach is that sometimes the intercalation process takes several days. Moreover, the operation is complicated because the reaction needs to be conducted in the glovebox or with the Schlenk line.

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3.4. In Situ Photodedeposition Method In situ photodedeposition is a new technique for preparation of MoS2. It use photoexcited electrons from the semiconductor to reduce the precursor (NH4)2MoS4 (Equation 1) and generated MoS2 deposited on the surface of the semiconductor. [MoS4]2- + 2e- → MoS2 + 2S2-

(1)

In 2011, Tada’s group [20] first used this method to prepare the MoS2/TiO2. Their work shows the MoS2/TiO2 system exhibits a high level of photocatalytic activity for hydrogen generation. After this work, many efforts have been devoted to preparing various kinds of MoS2-based catalysts with excellent photocatalytic hydrogen performance, such as ZnxCd1-XS/MoS2 [21], ZnIn2S4/MoS2 [22], and RGO/CdS/MoS2 [23] by using this method. Comparing to the conventional thermal annealing method relying on decomposition of (NH4)2MoS4 precursor at high temperature and under H2S pressure, in situ photodedeposition method need a mild condition (room temperature, atmospheric pressure and light illumination).

3.5. Other Methods The hot injection method has been used to fabricate MoS2 containing photocatalysts. For example, Zhang’s group [24] uses (NH4)2MoS4 as Mo source and inject it into hot CdS/oleylamine mixtures. CdS/MoS2 composites are obtained and interestingly single layer MoS2 selectively grew on the Cdrich (0001) surface of wurtzite CdS nanocrystals. The generated CdS/MoS2 exhibited excellent catalytic performance under visible light irradiation. Mechanochemistry also can be used to fabricate MoS2 containing photocatalysts. Zhu’s group [25] use ball-milling to synthesize the TiO2/MoS2 photocatalyst. The TiO2/MoS2 composite possesses excellent and stable photocatalytic performance for H2 evolution.

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4. MOS2-BASED COMPOSITES FOR PHOTOCATALYTIC HYDROGEN EVOLUTION 4.1. Oxide/MoS2 Photocatalysts TiO2 is the first reported photocatalyst for hydrogen evolution under UV irradiation. It can produce hydrogen from aqueous solutions containing electron donor. And noble metal platinum is often used to modify the TiO2 to improve the photocatalytic performance. However, the high cost of noble metal greatly limits its practical applications on a large scale. To produce the hydrogen at a low cost, alternative co-catalysts made of earth-abundant elements have been actively pursued. MoS2 is noble metal free and moreover its overpotential for hydrogen evolution is close to that of Pt. So recent years, MoS2 as an alternate cocatalyst was often used to modify the TiO2. In 2011 Kanda et al. first reported the fabrication of MoS2 nanocrystals on TiO2 through in situ photodeposition using (NH4)2MoS4 as a precursor [14]. In their work, the MoS2/TiO2 photocatalyst exhibited a photocatalytic activity with a H2 evolution rate of approximately 73 μmol h-1g-1under UV irradiation (λ > 300 nm) in the presence of HCOOH as sacrificial agent. Their work first demonstrates MoS2 can replace the Pt as a cocatalyst to improve the photocatalytic performance of TiO2 although the photocatalytic activity is relative low. Since the hydrogen evolution reaction activity for MoS2 is correlated with the number of exposed edge sites that have a local stoichiometry, physical structure and electronic structure that differs from the catalytically inert basal planes of MoS2. To improve the activity, increasing the exposed edge sites is an effective strategy. Zhang’s group recently reported few-layer MoS2 nanosheet modified TiO2 nanobelts as an active photocatalyst for photocatalytic hydrogen production [26]. The characterization demonstrated that only few-layered MoS2 nanosheets were uniformly coated on the surface of TiO2 nanobelts and the lattice fringes of MoS2 and TiO2 can be seen clearly in the HRTEM image. The enhanced photocatalytic hydrogen production activity is due to the matched energy band of the TiO2/MoS2 heterostructure. It favors the charge transfer and suppresses the photogenerated electron/hole recombination between MoS2 and TiO2. The highest hydrogen production rate reaches 1.6 mmol h-1g-1 when using the TiO2/MoS2 photocatalyst containning 50 wt% of MoS2. Compared to the above work, the improvement of the photocatalyst is due to the more exposed edge sites of MoS2 nanosheets.

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To further increase exposed edge sites, making single or few-layer MoS2 nanosheets vertically stand on a substrate is a very effective way. Liu’s group reported a novel TiO2/MoS2 nanocomposite in which single or few-layer MoS2 nanosheets vertically standing on porous TiO2 nanofibers [27]. Due to plenty of pores in the electrospun TiO2 nanofibers, the MoS2 nanosheets vertically grow from the inside to the outside, and the growth mode of the MoS2 nanosheets rooting into the TiO2 nanofibers endows not only intimate contact between TiO2 and MoS2 for fast electrons transfer but also high structural stability of TiO2/MoS2 heterostructure. The vertical orientation of MoS2 nanosheets enables the active edge sites of MoS2 to be maximally exposed. The maximum hydrogen evolution rate reaches 1.68 mmol h-1g-1 under UV-vis illumination. Another interesting phenomenon of TiO2/MoS2 composite photocatalyst occurs at the MoS2 prepared by Li intercalation method. After Li intercalation method, the phase of MoS2 changed from 2H to 1T. Xiong’s group [13] demonstrates that MoS2 nanosheets with 1T phase can function as a cocatalyst with multiple merits: (1) only containing earth-abundant elements; (2) high charge mobility; (3) abundant active sites for hydrogen evolution on basal planes and exposed edges; (4) favorable performance stability; (5) high light transparency. The photocatalytic experiments indicated that the developed hybrid structure with TiO2 exhibits excellent performance, in sharp contrast to bare TiO2 and the hybrid counterpart with 2H-MoS2. Majima’s group [28] recently demonstrated that 3D architectures of TiO2 mesocrystals uniformly packed with a Li intercalatin/exfoliated MoS2 shell exhibit promising reactive efficiency and good stability in synergetic hydrogen evolution. The efficient interfacial electron transfer from the excited TiO2 moieties to the decorated ultrathin MoS2 shell was effectively monitored. The maximum H2-evolution rate reaches 0.55 mmol h-1 g-1 and the apparent quantum efficiency reaches 7.4%. Besides the above work, Zhu et al. in 2014 proposed a very simple but effective method to prepare the MoS2/TiO2 composite photocatalysts through mechanochemistry [19]. They use the ball milling method to load the direct precursor MoS2 onto TiO2. The transient photocurrent responses confirmed that the photogenerated electrons of TiO2 could be easily transferred to the MoS2 cocatalyst, which promoted efficient charge separation and improved the photocatalytic performance. When the loading amount of MoS2 is 4.0%, the optimal photocatalytic hydrogen evolution rate is 753 μmol h-1g-1, which is 48.6 times higher than that of pure TiO2.

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All the above mentioned work is binary system containing MoS2 and TiO2. Another strategy to improve the photocatalytic performance is to add some other materials such as graphene to improve the charge separation and transfer. Graphene has excellent electron mobility and large specific surface area, which is useful for photocatalytic hydrogen production. In 2012, Yu et al. synthesized a ternary composite consisting TiO2, MoS2, and reduced graphene oxide as a highly efficient and earth-abundant elements containing photocatalyst for hydrogen generation [29]. This photocatalyst was prepared through a two-step simple hydrothermal process. The photocatalyst containing 0.5 wt% MoS2/graphene as cocatalyst and a molar ratio of MoS2 to graphene of 95:5 had the highest photocatalytic activity with a hydrogen evolution rate of 2066 μmol h-1g-1. In this ternary system reduced graphene oxide acts as an electron acceptor and transporter to separate photogenerated electron/hole pairs. It consequently improves the photocatalytic hydrogen performance. Han and coworkers reported similar work using TiO2/MoS2/graphene as photocatalysts [30]. Besides TiO2, attention has been paid to other oxides such as ZnO [31], SrZrO3 [32] and Cu2O [33] as host photocatalysts. However, these oxides usually suffer from serious photocorrosion under the UV irradiation. Loading MoS2 onto these oxides as a cocatalyst is an effective way to eliminate the photocorrosion and improve the photocatalytic performance. Yuan and coworkers used the hydrothermal method to synthesize a MoS2-nanosheetcoated ZnO heterostructure photocatalyst. The optimum MoS2 loading amount was found to be 1.00wt%, giving rise to a highest hydrogen evolution rate of 768μmol h-1g-1 in the presence of Na2S and Na2SO3 as the sacrificial reagents under UV light irradiation. It is 14.8 times higher than that of pure ZnO. Zhou and coworkers successfully synthesized a novel heterojunction of a MoS2/SrZrO3 photocatalyst via a simple hydrothermal process. The optimal MoS2 content is 0.05 wt% and the heterostructure exhibits the highest hydrogen production rate of 5310μmol h-1g-1. This high performance for hydrogen production is due to the junction between SrZrO3 and MoS2, which suppresses the recombination of photogenerated electrons and holes. Yan’s group prepared a p-type semiconductor of Cu2O decorated with MoS2 nanosheets as cocatalyst for efficient production hydrogen production under visible light (λ > 420 nm). Their work demonstrates a much more efficient hydrogen production was achieved with MoS2@Cu2O compared to pure Cu2O.

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4.2. Sulfide/MoS2 Photocatalysts Although the photocatalytic H2 production activity of TiO2 can be significantly enhanced by using MoS2 as cocatalyst, TiO2 itself is a UV responding semiconductor. It can’t absorb visible light. Sulfides, such as CdS, ZnIn2S4 and ZnxCd1_xS have a narrower band gap and they can absorb visible light. Zong et al. reported that MoS2 as a cocatalyst loaded on CdS for hydrogen production using lactic acid solution as the sacrificial agent under visible light [5]. The photocatalytic performance of the composite demonstrates that the activity of the CdS can be enormously increased by loading MoS2 as a cocatalyst. When the loading amount of MoS2 on CdS is about 0.2wt %, the rate of the hydrogen evolution achieves a maximum, ca. 5.4 mmol h-1 g-1, which is about 36 times that of bare CdS. The activity of MoS2/CdS could be even higher than that of Pt/CdS under the same reaction condition. These results not only suggested that MoS2 could act as a noble metal free cocatalyst in solar energy to hydrogen conversion, but also provided an important strategy to obtain a more efficient interelectron transfer through an intimate junction between the cocatalyst and semiconductor. Similar results were also reported by Meng’s group. Ye’s group [7] recently synthesizes a series of MoS2 with layer number ranging from ≈1 to 112 and explored their activities for photocatalytic H2 evolution over commercial CdS. They observed that the photocatalytic hydrogen evolution rate increased drastically with decreasing MoS2 layer number. Particularly for the single-layer MoS2, the single-layer MoS2 /CdS sample reaches a high hydrogen generation rate of ≈2.01 mmol h−1 in Na2S– Na2SO3 solutions and ≈2.59 mmmol h−1 in lactic acid solutions, corresponding to an apparent quantum efficiency of 30.2% and 38.4% at 420 nm, respectively. Their research demonstrated that because of more exposed edges for single layer MoS2 with unsaturated active S atoms, and matched junction with CdS, single layer MoS2/CdS exhibits a high activity for photocatalytic H2 evolution. Zhang et al. recently used the hot-injection method to prepare the CdS/MoS2 composite photocatalyst [18]. Very interestingly, in their obtained binary hybrids, single layers MoS2 nanosheet selectively grow on the Cd-rich (0001) surface of wurtzite CdS nanocrystals. This hybrid possesses many edge sites in the MoS2 layers, which are active for the hydrogen production. The highest hydrogen production rate 1472μmol h-1g-1 from the optimized CdS/MoS2 nanocomposite is much higher than that of pure CdS. Moreover the

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hybrid shows quite good stability. The excellent performance of the CdS/MoS2 arises from the large number of the active sites of single-layer MoS2 nanosheets and the inherent p/n heterojunction formed between MoS2 and CdS. Also, Zhu’s group utilized Li intercalation method to synthesize the MoS2 nanosheet, and then prepare the CdS/MoS2 using the one pot solvothermal progress [34]. The photocatalytic experiments demonstrate that the nanohybrid has a superior hydrogen rate under visible light irradiation, which reaches 6.85 mmol h-1 g-1. This excellent performance of the nanohybrid is due to the unique p-n junction, large surface area, and decreased band gap. Another interesting phenomenon of CdS/MoS2 composite photocatalyst is that the defect degree of MoS2 ultrathin nanoplates can influence the photocatalytic performance. Wu’s group synthesized successfully MoS2 ultrathin nanoplates with different defect degrees via a simple hydrothermal process [35]. They took CdS as the trigger for the photocatalytic reaction and investigated the effect of the MoS2 defective sites on photocatalytic hydrogen production. Their work confirmed that the rich defects have a positive effect on photocatalytic hydrogen evolution. Although MoS2 is an outstanding catalyst for hydrogen production, it is a fact that its activity is limited by its poor electrical conductivity. Recently, some groups’ work indicated adding some other conductive materials such as graphene, can improve the activity of MoS2 for hydrogen evolution. In 2014, Chang and coworkers reported a composite materials consisting of CdS nanoparticles grown on the surface of MoS2/grapheme [36]. The composites act as a high performance photocatalyst for hydrogen production under the visible light irradiation. The optimized photocatalyst CdS/MoS2/graphene exhibited excellent hydrogen evolution performance up to 9.0 mmol h-1 g-1 when the content of the MoS2/graphene cocatalyst is 2.0 wt % and the molar ratio of MoS2 to graphene is 1:2. Tsang’s group also report a simple but effective composite with CdS and MoS2 dispersed on grapheme [37]. In their work, CdS/MoS2/graphene showed the highest catalytic activity, 3.067 mL h-1, which is about 71 times higher than that of CdS. Another interesting CdS/MoS2/graphene system was reported by Li and coworkers [17]. In their work, MoS2 was controllably loaded on the composite of reduced graphene oxide and CdS by a facile photoreduction method at different pHs. At low pH 7, MoS2 deposits on the surface of the CdS particles of the composite. However, at high pH 11, it loads on the exposed RGO. When MoS2 is on the RGO, the transfer of the photoexcited electron from CdS to RGO is compatible with the hydrogen evolution at MoS2 (synergic effect),

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whereas the transfer is incompatible with the hydrogen evolution when it is on the CdS (antisynergic effect). Moreover, the MoS2 depositedon the CdS decreases the photoabsorption and photoactivity of CdS, and the effect is avoided when MoS2 is on the RGO. The photocatalytic hydrogen production rate under the synergic condition is 4.3 times as high as that under antisynergic condition. Another effective strategy to improve the photocatalytic performance of MoS2/CdS photocatalysts for hydrogen production is modifying MoS2 with metal nanoparticles. Yang and coworkers use the metal nanoparticles (Cr, Ag) to modify the MoS2 nanosheets [38]. They found that both Cr–MoS2 and Ag– MoS2 composites shows much higher catalytic activities for photocatalytic hydrogen production than pure MoS2 nanosheet. The optimized photocatalyst is Ag–MoS2/CdS and its hydrogen evolution performance is up to 107 mmol h1g-1. They also use the carrier dynamics and photoluminescence studies to investigate the mechanism. They found that both Cr and Ag nanoparticles can depress the electron/hole recombination probability in MoS2/CdS. Besides CdS, there are several important sulfide semiconductors which can act as host materials in photocatalytic hydrogen production, such as ZnxCd1-xS, ZnIn2S4, ZnS, and Cu2ZnSnS4. Meng’s group has designed a new MoS2/ZnIn2S4 photocatalyst for hydrogen production under the visible light irradiation [16]. The photocatalyst was obtained by hydrothermal method to afford floriated ZnIn2S4 microspheres, and MoS2 co-catalyst was subsequently loaded on to the ZnIn2S4 surface via a facile in-situ photo-assisted deposition process. The optimum H2 evolution rate reaches 8.047mmol h−1 g−1 which is 28 times higher than that of untreated ZnIn2S4. Fu’s group also designed a MoS2/ ZnIn2S4 photocatalyst which was synthesized in situ by using a facile controlled-growth approach through a solvothermal process [39]. During the solvothermal reaction, ultrathin curled ZnIn2S4 nanosheets grew on the surface of MoS2 slices which could help to form a more-homogeneous mixture, effective interfacial contact, and strong interactions between the ZnIn2S4 nanosheets and the MoS2 slices. The intimate contact between ZnIn2S4 and MoS2 favored the formation of junctions between the two components, thereby improving the charge separation and prolonging the mean life time of the electron–hole pairs. Moreover, growing ZnIn2S4 nanosheets by visible light catalysis on MoS2 slices afforded a higher number of available catalytically active sites. The optimum amount of MoS2 was about 15 wt%, at which the hierarchical MoS2/ZnIn2S4 sample showed the highest activity in H2 production (78 μmol h-1), four-times higher than that of pure ZnIn2S4 (19μmol

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h-1). Li’s group also has reported a similar ZnIn2S4/MoS2 composite photocatalysts and gets a good photocatalytic hydrogen production performance. Qureshi and coworkers recently reported a quaternary semiconductor Cu2ZnSnS4 loaded with 1% MoS2 [40]. It shows a hydrogen production rate of 1.32 mmol g-1 h-1, which is about 1.23 times higher than that of pure Cu2ZnSnS4. Like CdS, photocatalytic hydrogen activity of other sulfides semiconductor also can be improved by adding other conductive materials, such as graphene. Lin and coworkers successfully prepared graphene and MoS2 nanosheets modified ZnS nanoparticles prepared by a simple one pot hydrothermal route in the presence of graphene and MoS2 nanosheets [41]. The resultant ZnS/graphene/MoS2 nanocomposites exhibited significantly enhanced photocatalytic activity and good recurrence stability in H2 evolution from water splitting. When the loading content of graphene was 0.25 wt% and that of MoS2 was 2 atom%, the ZnS/graphene/MoS2 nanocomposite reached a high H2-evolution rate of 2.26 mmol h-1g-1under a 300 W Xe lamp irradiation, which is about 2 times that of ZnS alone. Meng’s group designed a new ZnIn2S4/RGO/MoS2 photocatalytic system [42]. By optimization of solvothermal reaction temperatures, reaction time, and RGO introduction amount, up to 1.62 mmol h-1g-1 of hydrogen evolution rate has been achieved.

4.3. MoS2/C3N4 Photocatalysts In 2009, Wang and coworkers found a metal free polymer C3N4 with a bandgap of 2.7 eV can act a visible light responding photocatalyst for hydrogen production [43]. After this founding, many researchers around the world use numerous ways to improve the photocatalytic performance of C3N4. Among numerous strategies, coupling C3N4 and MoS2 is an unordinary method. This is because C3N4 has a layered structure analogous to MoS2. Combine these two materials can increase the contact resulting the improved photocatalytic hydrogen performance. In 2013, Hou and coworkers use (NH4)2MoS4 as precursor to prepare the C3N4/MoS2 composite photocatalyst with H2S gas at 350°C [6]. The analogous layered structures of C3N4 and MoS2 minimize the lattice mismatch and facilitate the planar growth of MoS2 on the C3N4 surface. The C3N4/MoS2 (0.2wt%) showed the highest catalytic activity (1400μmol h-1g-1). Unfortunately, cycle runs for the photocatalytic hydrogen production over this

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photocatalyst revealed that it became deactivated during prolonged operations. This is due to the photocorrosion of MoS2. Ge et al. also use the impregnation method to synthesize the C3N4/MoS2 composite photocatalysts [44]. However, their results reveal that C3N4/MoS2 (0.5wt%) showed a low catalytic activity with a maximum hydrogen production rate of 231μmol h-1g-1.

4.4. Homogenous MoS2 Catalyst for Solar H2 Generation and Dye-Sensitized MoS2-Based Photocatalysts MoS2 is a semiconductor which is inactive for photocatalytic hydrogen production. However, dye modified MoS2 can act as a suitable photocatalyst for hydrogen production. Zong and coworkers prepared colloidal MoS2 nanoparticles with diameters of less than 10 nm with a simple solvothermal method [45]. They use poly(viny1pyrrolidone) as a protecting polymer and N2H4H2O as a reductant. After the MoS2 colloid was prepared, the dye Ru(2,2’-bipyridine)32+ was introduced into the colloid. They use the Ru(2,2’bipyridine)32+ sensitized MoS2 colloid as photocatalyst and explored the reaction activity of the photocatalyst. They have demonstrated “soluble” colloidal MoS2 nanoparticles could act as an efficient catalyst for hydrogen production. This work demonstrated the possibility of using colloidal MoS2 as a H2 evolution catalyst in a molecular system. The rate of the photocatalytic hydrogen production was high under visible light irradiation; however, it would decrease after a few hours irradiation. Zong also reported a similar system consisting of MoS2 colloid obtained by an in situ photoreduction manner and Erythrosine B [46]. the photocatalyst demonstrated high efficiency for catalyzing H2 evolution in a noble-metal-free organic dye-sensitized system. In 2012 Min and Lu reported a high active cocatalyst, limited-layered MoS2 confined on reduced graphene oxide sheets for hydrogen evolution in Eosin Y sensitized photocatalytic systems [47]. The highest H2 evolution rate (4.190 mmol h-1g-1) was achieved in the Eosin Y-sensitized MoS2/RGO system under visible light irradiation (λ > 420 nm). It has been known that the high electrical conductivity of MoS2 is important factor in determining its activity for hydrogen production. Metallic MoS2 with 1T phase has higher electrical conductivity than semiconductingMoS2 with 2H phase. Matitra and coworkers [48] use the Li intercalation method to prepare the metallic MoS2. The obtained metallic MoS2 showed a high hydrogen rate of 30 mmol h-1g-1 in the presence of Eosin Y as a photosensitizerand TEOA as an electron donor,

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which is 600 times higher than that of few-layer semiconducting MoS2. In 2014 Yuan and coworkers the utilization of colloidal MoS2 for photocatalytic hydrogen production coupled with a series of cyclometalated Ir(III) sensitizers [49]. In this homogenous system, they introduced the carboxylate group as the anchoring group. It allowed the photosensitizers to be adsorbed onto the MoS2 nanoparticles surfaces, improve the electron transfer rate, which is beneficial for the hydrogen production. Under the optimal condition, the total turnover number for hydrogen production was up to 3142. Tsang’s group [50] recently reported a novel and interesting system which consist of single layer MoS2 and the dye molecule, Eosin Y. Single layer MoS2 synthesized by exfoliation with Li is demonstrated to take up the dye molecule, Eosin Y, with strong binding affinity via sulfur vacancies. This dyesensitized single layer MoS2 ensemble exhibits remarkable activity and stability for photocatalytic hydrogen production from water. After this finding, they sequentially introduce graphene into this system and the Eosin Y/MoS2/graphene ensemble shows excellent activity and stability for hydrogen evolution from water. It is attributed to the efficient capture of photons by surface bound EY molecules and faster electron transfer through the direct chemical linkage of conjugated rings of EY with exposed Lewis acidic Mo4+ of MoS2 on the basal plane.

SUMMARY AND OUTLOOK In this chapter, we reviewed recent developments in the utilization of MoS2-based photocatalysts for hydrogen evolution. A large number of progresses have been achieved using MoS2-based photocatalysts for hydrogen production from solar energy. However, there are still many problems in those systems. One problem is the synthesis of ultrathin MoS2 nanosheets in large scale and in a facial method. The MoS2 nanosheets synthesized by percent strategies commonly have many different sizes and layer numbers. Controllable synthesis of MoS2 nanosheets with suitable size and precise layer numbers in a large scale is a great challenge. Another vital problem is how to combine the ultrathin MoS2 nanosheet with other materials. The combination strategy has a great influence on the photocatalytic hydrogen production performances.

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Fujishima, A.; Honda, K. Nature 1972, 238, 37. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chemical Reviews 2010, 110, 6503. Yang, J.; Wang, D.; Han, H.; Li, C. Accounts of Chemical Research 2013, 46, 1900. Qipeng Lu, Y. Y., Qinglang Ma, Bo Chen, and Hua Zhang Advanced Materials 2015. An, X.; Yu, J. C. RSC Advances 2011, 1, 1426. Yuan, Y.-J.; Lu, H.-W.; Yu, Z.-T.; Zou, Z.-G. ChemSusChem 2015, 8, 4113. Tinker, L. L.; McDaniel, N. D.; Bernhard, S. Journal of Materials Chemistry 2009, 19, 3328. Frischmann, P. D.; Mahata, K.; Wurthner, F. Chemical Society Reviews 2013, 42, 1847. H. Tributsch, J. C. B. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1977, 81, 97. Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. Journal of the American Chemical Society 2008, 130, 7176. Hou, Y.; Laursen, A. B.; Zhang, J.; Zhang, G.; Zhu, Y.; Wang, X.; Dahl, S.; Chorkendorff, I. Angewandte Chemie International Edition 2013, 52, 3621. Chang, K.; Li, M.; Wang, T.; Ouyang, S.; Li, P.; Liu, L.; Ye, J. Advanced Energy Materials 2015, 5. Zong, X.; Wu, G.; Yan, H.; Ma, G.; Shi, J.; Wen, F.; Wang, L.; Li, C. The Journal of Physical Chemistry C 2010, 114, 1963. Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z. Applied Catalysis B: Environmental 2014, 144, 521. Hong, J.; Wang, Y.; Wang, Y.; Zhang, W.; Xu, R. ChemSusChem 2013, 6, 2263. Sasikala, R.; Gaikwad, A. P.; Jayakumar, O. D.; Girija, K. G.; Rao, R.; Tyagi, A. K.; Bharadwaj, S. R. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2015, 481, 485. Lu, Y.; Wang, D.; Yang, P.; Du, Y.; Lu, C. Catalysis Science and Technology 2014, 4, 2650. Frame, F. A.; Osterloh, F. E. The Journal of Physical Chemistry C 2010, 114, 10628. Bai, S.; Wang, L.; Chen, X.; Du, J.; Xiong, Y. Nano Res. 2014, 1.

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[20] Kanda, S.; Akita, T.; Fujishima, M.; Tada, H. Journal of Colloid and Interface Science 2011, 354, 607. [21] Mai Nguyen, P. D. T., Stevin S. Pramana, Rui Lin Lee, Sudip K. Batabyal, Nripan Mathews, Lydia H. Wong, Michael Graetzel Nanoscale 2013. [22] Chen, G.; Ding, N.; Li, F.; Fan, Y.; Luo, Y.; Li, D.; Meng, Q. Applied Catalysis B: Environmental 2014, 160–161, 614. [23] Li, Y.; Wang, H.; Peng, S. The Journal of Physical Chemistry C 2014, 118, 19842. [24] Chen, J.; Wu, X.-J.; Yin, L.; Li, B.; Hong, X.; Fan, Z.; Chen, B.; Xue, C.; Zhang, H. Angewandte Chemie International Edition 2015, 54, 1210. [25] Zhu, Y.; Ling, Q.; Liu, Y.; Wang, H.; Zhu, Y. Physical Chemistry Chemical Physics 2015, 17, 933. [26] Zhou, W.; Yin, Z.; Du, Y.; Huang, X.; Zeng, Z.; Fan, Z.; Liu, H.; Wang, J.; Zhang, H. Small 2013, 9, 140. [27] Liu, C.; Wang, L.; Tang, Y.; Luo, S.; Liu, Y.; Zhang, S.; Zeng, Y.; Xu, Y. Applied Catalysis B: Environmental 2015, 164, 1. [28] Zhang, P.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Chemical Communications 2015, 51, 7187. [29] Xiang, Q.; Yu, J.; Jaroniec, M. Journal of the American Chemical Society 2012, 134, 6575. [30] Han, W.; Zang, C.; Huang, Z.; Zhang, H.; Ren, L.; Qi, X.; Zhong, J. International Journal of Hydrogen Energy 2014, 39, 19502. [31] Yuan, Y.-J.; Wang, F.; Hu, B.; Lu, H.-W.; Yu, Z.-T.; Zou, Z.-G. Dalton Transactions 2015, 44, 10997. [32] Tian, Q.; Zhang, L.; Liu, J.; Li, N.; Ma, Q.; Zhou, J.; Sun, Y. RSC Advances 2015, 5, 734. [33] Zhao, Y.-F.; Yang, Z.-Y.; Zhang, Y.-X.; Jing, L.; Guo, X.; Ke, Z.; Hu, P.; Wang, G.; Yan, Y.-M.; Sun, K.-N. The Journal of Physical Chemistry C 2014, 118, 14238. [34] Zhang, J.; Zhu, Z.; Feng, X. Chemistry – A European Journal 2014, 20, 10632. [35] Xiong, J.; Liu, Y.; Wang, D.; Liang, S.; Wu, W.; Wu, L. Journal of Materials Chemistry A 2015, 3, 12631. [36] Chang, K.; Mei, Z.; Wang, T.; Kang, Q.; Ouyang, S.; Ye, J. ACS Nano 2014, 8 (7), 7078. [37] Jia, T.; Kolpin, A.; Ma, C.; Chan, R. C.-T.; Kwok, W.-M.; Tsang, S. C. E. Chemical Communications 2014, 50, 1185.

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[38] Yang, L.; Zhong, D.; Zhang, J.; Yan, Z.; Ge, S.; Du, P.; Jiang, J.; Sun, D.; Wu, X.; Fan, Z.; Dayeh, S. A.; Xiang, B. ACS Nano 2014, 8, 6979. [39] Tian, G.; Chen, Y.; Ren, Z.; Tian, C.; Pan, K.; Zhou, W.; Wang, J.; Fu, H. Chemistry – An Asian Journal 2014, 9, 1291. [40] Gogoi, G.; Arora, S.; Vinothkumar, N.; De, M.; Qureshi, M. RSC Advances 2015, 5, 40475. [41] Zhu, B.; Lin, B.; Zhou, Y.; Sun, P.; Yao, Q.; Chen, Y.; Gao, B. Journal of Materials Chemistry A 2014, 2, 3819. [42] Ding, N.; Fan, Y.; Luo, Y.; Li, D.; Meng, Q. APL Materials 2015, 3, 104417. [43] Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. Nature Materials 2009, 8, 76. [44] Ge, L.; Han, C.; Xiao, X.; Guo, L. International Journal of Hydrogen Energy 2013, 38, 6960. [45] Zong, X.; Na, Y.; Wen, F.; Ma, G. J.; Yang, J. H.; Wang, D. G.; Ma, Y.; Wang, M.; Sun, L.; Li, C. Chemical Communications 2009, 4536. [46] Zong, X.; Xing, Z.; Yu, H.; Bai, Y.; Lu, G. Q.; Wang, L. Journal of Catalysis 2014, 310, 51. [47] Min, S.; Lu, G. The Journal of Physical Chemistry C 2012, 116, 25415. [48] Maitra, U.; Gupta, U.; De, M.; Datta, R.; Govindaraj, A.; Rao, C. N. R. Angewandte Chemie International Edition 2013, 52, 13057. [49] Yong-Jun Yuan, Z.-T. Y., Xiao-Jie Liu, Jian-Guang Cai, Zhong-Jie Guan and Zhi-Gang Zou SCIENTIFIC REPORTS 2013. [50] Jia, T.; Li, M. M. J.; Ye, L.; Wiseman, S.; Liu, G.; Qu, J.; Nakagawa, K.; Tsang, S. C. E. Chemical Communications 2015, 51, 13496.

In: Molybdenum Disulfide Editor: Jeremiah McBride

ISBN: 978-1-63485-032-2 © 2016 Nova Science Publishers, Inc.

Chapter 4

DOPING TO MOS2 Toshihiro Shimada and Takashi Yanase Division of Applied Chemistry, Faculty of Engineering, Hokkado University, Sapporo, Japan

ABSTRACT In this chapter, we critically review current research activities on doping to MoS2. First we describe the characteristics and functions of MoS2 and related materials as hosts of dopants. We deal with electronic structures and physics, electronics, catalysis and tribology. Then we explain the electronic states of various dopants. Special attention will be paid about the fact whether the dopant can supply charge carriers, which are n-type or p-type. Each dopants are reviewed: vacancies, Mosubstituting elements and S-substituting elements from the viewpoints of experiments and computations. Finally we briefly review the surface related techniques to modify the carrier concentrations in MoS2: surface transfer doping and field effect doping.

Keywords: Doping, vacancies, Mo-substitution, S-substitution, surface transfer doping, field effect doping

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1. INTRODUCTION The objective of this chapter is to provide a critical overview of doping to MoS2 and related materials. In semiconductor technology, the doping is an indispensable method to control the dopant concentration. In chemical technology, embedding controlled reaction centers in inhomogenous catalysis support or electrodes is very important target to pursue. In solid state physics, MoS2 and other layered materials provides a unique playground for many body effects and spin-orbit interactions. Various unique properties and functions are predicted and actually being proved by experiments. All of them are involved with doping. This chapter is organized as follows. In Section 2, we will overview the electronic structure, physics, electronics, chemistry, and tribology of MoS2 and their relationship with doping. In Section 3, we focus on vacancies. In Section 4 and 5, we will summarize theoretical prediction and experimental results of Mo-substituting and S-substituting dopants, respectively. In Section 6, we briefly mention intercalation to MoS2. In Section 7, we will review the results of recently developed techniques of surface transfer doping and field effect doping. The number of publications on MoS2 and other atomic layer materials is now very large and still increasing. We are afraid that new results may appear in the near future. We tried to give a concise but comprehensive overview of this field at present.

2. OVERVIEW OF MOS2 AS A HOST OF DOPANT 2.1. Structure and Electronic Structure MoS2 is a layered material. The conduction band and the valence band are mainly made of d-electrons of molybdenum. This feature offers an interesting outcome of large spin-orbit coupling of heavy elements, which is more prominent in tungsten analogue WS2. Layers of S-Mo-S trigonal lattice are connected with weak van der Waals forces. It makes the material cleavable to the thickness of a unit layer.

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2.2. Physics and Electronics Monolayer MoS2 offers a playground of solid state physics that might lead to new energy-saving information handling technology called “valleytronics”. By mechanically cleaving a single crystal, it is possible to make electronic devices with monolayers or a few layers thickness. The monolayer MoS2 has the following properties that are very unique and useful. (1) The band gap becomes a direct gap. This is a distinct difference from bulk MoS2 crystal and makes the electronics structures directly modifiable by circularly polarized photon irradiation. (2) The band gap resides at the boundary of Brilluin zone, which are called K and K’ points, or “valley”, because of the quadratic shape of the band dispersion around this point. This feature is very useful in “valleytronics”. Valleytronics is a promising technology to process information by the charges or excitons at different valleys. (3) Exciton binding energy is exceptionally large (several hundreds meV) [1-5]. It is because the screening of Coulomb interaction between electrons and holes is reduced in two dimension. It will elongate the exciton lifetime which enables observation of the coherence between valleys in WSe2 [6]. Also positively and negatively charged excitons, which are called “trions” [1-5], with various quantum numbers can be observed by optical spectroscopies. (4) The centrosymmetry is broken. This is different from monolayer graphene and is a very useful feature in valleytronics. (5) Spin orbit coupling is very strong, amounts to 160 meV in MoS2 and 430meV in WS2 [7]. It will make the coupling of valley variable with spins, and facilitates the design of the actual devices. The true two dimensional nature of monolayer MoS2 is beneficial in short channel devices [8, 9]. Many reports on the fabrication of transistors have been published. The maximum mobility of the cleaved monolayer transistor is somewhat deviates in experiments maybe due to the remote phonon scattering and Coulombic interactions from the substrates [10], but in theory it is predicted to be 410 cm2/Vs [11], which is unfortunately lower than that of Si. The monolayer transitors show much lower mobilities such as tens of cm2/Vs, which is achieved by covering of the device to prevent the effect of surface adsorption [12]. The properties of monolayer transistors are very sensitive to the environment including various gas molecules in the environment. It makes the monolayers very sensitive sensors like graphene. Light emitting diodes can be fabricated by field effect doping techniques using ionic liquid [13]. It is an important tool for the valleytronics at the moment.

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2.3. Catalysts and Electrocatalysts MoS2 is an important catalyst for various reactions, including hydroprocessing or hydro-converting molecules with heteroatoms such as sulfur and nitrogen [14]. and electrochemical hydrogen evolution reaction. Recently a MoS2 monolayer are found to be an excellent electrocatalyst or cophotocatalyst in hydrogen evolution reaction (HER) [15-17]. In those reactions, the edge of nanosheets and disordered regions of distorted 1T-MoS2 (that is a metallic material produced by Li intercalation, see below) plays important roles. The study on the doping effect to the catlytic activities is only experimental and phenomenological. Some of the remarkable examples from recent literature are as follows. A core shell structure of MoO2 nanobelts with nitogen self-doped MoS2 nanosheet works as an effective HER electrocatalyst [18]. Co/Ni-doped WS2 films were synthesized by electrodepositon using [Co(WS4)2]2- and [Ni(WSS4)2]2- sources and found to be showing excellent electrocatalytic HER activities [19]. Cu4-doped MoS2 is predicted as excellent CO oxidization catalyst [20].

2.4. Tribology Tribology is another important function of MoS2. The effect of doping has been studied by several groups. The samples were prepared by co-sputtering and polycrystalline or amorphous. Carbon-containing WS2 films showed more excellent properties than pristine WS2 but nitrogen-containing one showed deteriorated properties in humid air [21]. Co-sputtering WS2 with Ti improved the lubrication and durability at high temperatures [22]. These effect does not come from electronic properties as examined below, but modification of the chemistry in air or mechanical effect of mixing hard and soft materials.

3. VACANCIES MoS2 often appears to be a n-type semiconductor. It is now being established that the main sources of the n-type carriers are sulfur vacancy, rhenium substitution (in natural crystals) and from the environment and interfaces. Sulfur vacancy is considered as a major source of unintentional n-type carrier doping in MoS2 from experiments [23-27]. They are actually observed

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by atomic resolution STEM but care have to be taken because the high electron beam irradiation during the observation can knock off sulfur atoms [28]. Annealing of bulk or exfoliated MoS2 flakes in ultrahigh vacuum at 200oC or more reduces the S/Mo stoichiometry measured by XPS from 2.00 to 1.95 for bulk. Mo and S species parallelly shifted XPS binding energies, that are possibly corresponding to S vacancy, appears after heating [29]. In electronic properties, monolayer FET showing a mobility of 7.7 cm2/Vs shows two dimensional variable range hopping transport (apparent mobility is proportional to exp(-(const./T)1/3)) [27]. The observation is quantitatively accounted for by the sulfur vacancy density from TEM observation and trap depth (0.46eV) from DFT calculation. The depth of traps from Arrhenius type plots of electrical measurements of monolayer transistor is 0.27eV and 0.05 eV [30]. It is different from 0.7 eV determined by STM of bulk sample [31]. These shallow traps might have come from the interface with the substrate SiO2, not the S vacancy. There are many articles on the calculation of a single sulfur vacancy. The depth of the midgap states is somewhat varying between reports, such as 0.46eV in monolayer [27] and 0.6eV in monolayer [32], 0.65 eV for monolayers and 0.77 eV for bulk [33]. It is rather consistent with the experimental values obtained from STM. Photoluminescence of the sulfur vacancy created by accelerated -particle was reported [34]. It creates an additional broad excitonic states at 1.78eV, which is lower than the states at 1.90 eV of a pristine MoS2 monolayer. The result is explained by the vacancies with a single sulfur atom or two neighboring sulfur atoms. Interesting finding is that the adsorption of N2 or O2 molecules in the environment to the defects enhances the pristine exciton emission in addition to the appearance of defect-related emssion with a slight shift of the energy. On the other hand, Mo vacancy has not been observed experimentally in the normal conditions. The reason is probably its great formation energy (3eV) [35]. The formation energy of defects in MoS2 under various chemical potential (partial pressure) of S and Mo species is analyzed in Ref. 35. The defect density of S vacancy predicted by their analysis (13%) makes the material ferromagnetic [41] and will be an effective method to change the valley polarization in MoS2 [42]. The magnetic moment of Mn substituting Mo (MnMo) is reasonably stable in S rich environment (formation energy < 100meV), and produces magnetic polarization. It will be effective for manipulating valley variable in the devices. MnMo will make a deep mid gap state 0.5eV from conduction band minimum) and need additional dopants to make it p-type, which will be effective to utilize it in valleytronics. (f) V, Cr, Co, Ni, Ru, Rh, Ag, Cd, Zr, Y [43-45]: Other transition metal elements substituting Mo have been examined by computation. The interest in those researches are mainly about magnetism. The magnetic moments (M) per dopant are listed in Table 1. Most of them shows the negative formation energy that is favorable in the synthesis except for Ag. Cr doping makes the material “half metallic”, in which one spin channel is semiconducting and the other metallic.

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5. S-SUBSTITUTING DOPANTS S substitution by other chalcogen elements such as Se and Te has been reported. We could not find a detailed study of the S-substitutional doping in MoS2, but WS2 is studied intensively. From the research, the following things have been established. (a) Selenium (Se): WSe2 can be doped into WS2 with arbitorary ratio. The photoluminescence wavelength of a monolayer can be continuously tuned from 751.9 nm (WSe2) to 626.6 nm (WS2). The carrier polarity of monolayer FET changes from n-type of WSe2-rich samples to p-type of WS2-rich samples [46]. (b) Terullium (Te) is not considered as a high concentration dopant to MoS2, because Te impurity was not detected in Te-assisted CVD of MoS2 [47]. (c) Phosphorus (P) has been doped into a few layer MoS2 by soft ion implantation using PH3 plasma. It clearly shows the hole doping (p-type) behavior but the ionization energy has not been determined [48]. From DFT calculation, P substituting a S atom makes a very shallow p-type doping level (50meV or less) [44, 45, 49]. Table 1. Magnetic moment induced by a single Mo-substituting dopant in MoS2 element V Cr Co Ni Ru Rh Pd Ag Cd Zr Y M/µB 1.00 4.00 3.00 4.00 2.00 3.00 4.00 2.57 1.58 0.00 0.00

(d) Nitrogen (N) and arsenic (As): From DFT calculation, it is reported that N will be a good p-type dopant with the ionization energy of 75 meV. On the other hand, As doping will make almost metallic states absorbed in the valence band [44-49]. (e) Boron (B) substituting sulfur is predicted as an effective p-type dopant to MoS2 monolayers. Concentrated doping will make the material “half metallic” that is spin polarized [50]. (f) halogens (F, Cl, Br, I) [44]. Since halogens are used as metal vaporization reagent in single crystal growth, it is expected that they are incorporated in the single crystals grown by this method. However, reliable measurement of the concentration and properties of halogen dopants by experiments are not found in the literatures and only computational prediction can be found. The result is that halogen

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dopants have spins and makes rather deep donor levels. For example, Cldoped MoS2 monolayer makes a magnetically polarized level 0.4 eV below the conduction band minimum. F and Br show the similar results. In the case of I (iodine) substitutionally doped at S-site gives a non magnetic level 0.3 eV below the conduction band minimum. Such deep levels will not be easily detected by electronic measurement and practically not very important. However, plasma treatment by CF4, CHF3 and SF6 makes MoS2 p-type. The plasma-treated MoS2 frakes (a few nm thick) clearly shows p-type FET chacacteristics and electronically rectifying behavior with untreated n-type regions [50, 51]. The detail mechanism is not clarified yet, but halogen related vacancies might be involved. (g) Oxygen (O) has not been considered as an effective dopant because of the volatile nature of molybdenum trioxide (MoO3). Howerver, oxygen plasma treatment of a part of MoS2 single crystalline frakes also gives retifying behavior and suggests conversion to p-type [51].

6. INTERCALATION It is well established that the layered materials undergo intercalation. Intercalation of Li is known for long time and used for effective exfoliation technique of MoS2 into monolayers in a solution. The intriguing finding is the trigonal prismatic coordination changes from trigonal prismatic (1H-MoS2) to octahedral (1T-MoS2) [53]. 1T-MoS2 shows metallic electronic conduction. The similar behavior is obeserved in MoSe2 and WS2. The mechanism is understood by DFT calculations [54].

7. SURFACE TRANSFER DOPING AND FIELD EFFECT DOPING Charge doping from surface adsorbate is called “surface transfer doping” and widely used to the materials with the difficulty of chemical doping [55] Similar concept of charge transfer across the interface to undoped semiconductors is already used to achieve high mobility in III-V semiconductors and widely applied as high frequency circuits [56]. Charge trapping at the interface between MoS2 and gate dielectric is reported by detailed transport and capacitance measurement at various

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temperatures. For example, substrate hydroxylation and hydration of Al2O3(0001), that happens after air exposure of CVD grown ML films, causes work function shift of 100 meV and charge transfer >1013 cm-2 [57]. Also the interface with amorphous SiO2 (oxide layer on silicon wager) is reported to be a source of carrier traps [58]. Adding strain and hydorgen adsorption (chemisorption) is examined theoretically and ferromagnetism is expected [43]. An attempt to surface transfer doping using the S-vacancy with thiol chemisorption has been successful. MoS2 can become p-type by introducing Svacancies by heating at 250oC followed by the treatment by fluorine containing thiol [59]. Field effect transistors can be used to introduce carriers in MoS2. Ionic liquids have been used to increase the carrier density to ~ 0.2 e- / Mo atom, which has lead to superconductivity [60].

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[11] K. Kaasbjerg, K. S. Thygesen, K. W. Jacobsen, Phys. Rev. B 85, 115317 (2012). [12] A. Rai et al., Nano Lett. 15, 4329 (2015). [13] Y.J. Zhang, T. Oka, R. Suzuki, J.T. Ye, Y. Iwasa, Science 344, 725 (2014). [14] M. Daage, R.R. Chanelli, Journal of Catalysis 149, 414 (1994). [15] T.F. Jeramillo, K.P. Jorgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff, Science 317, 100 (2007). [16] B. Hinnemann, P.G. Moses, J. Bonde, K. P. Jorgensen, S. Horch, I. Chorkendorff, J. K. Norskoff, J. Am. Chem. Soc. 127, 5308 (2005). [17] X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang, C. Li, J. Am. Chem. Soc. 130, 7126 (2008). [18] W. Zhou, D. Hou, Y. Sang, S. Yao, J. Zhou, G. Li, L. Li, H. Liu, S. Chen, J. Mater. Chem. A 2, 11358 (2014). [19] P. D. Tran et al., Energy Environ Science 6, 2452 (2013). [20] Z.W. Chen et al., Scientific Reports 5:11230 (2015). [21] T. Polcar, A. Cavaleiro, Thin Solid Films 519, 4037 (2011). [22] T.W. Scharf, A. Rajendran, R. Banerrjee, F. Sequenda, Thin Solid Films 517, 5666 (2009). [23] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat. Nanotech. 6, 147 (2010). [24] H. Qiu, et al., Appl. Phys. Lett. 100, 123104 (2012). [25] H. Wang et al., Nano Lett. 12, 4674 (2012). [26] A. Ayari, E. Cobas, O. Ogundadegbe, M.S. Fuhrer, J. Appl. Phys. 101, 014507 (2007). [27] H. Qiu et al., Nature Comm. 4:2642 (2013) [28] H.P. Komsa et al., Phys. Rev. B 88 035301 (2013). [29] M. Donarelli, F. Bisti, F. Perrozzi, L. Ottaviano, Chem. Phys. Lett. 588, 198 (2013). [30] J. Y. Kwak et al., Nano Lett. 14, 4511 (2014) [31] C.-P. Lu, G. Li, J. Mao, L.-M. Wang, E. Y. Andrei, Nano Lett. 14, 4628 (2014). [32] K. C. Santosh et al., Nanotechnology 25, 375703 (2014). [33] A.N. Andriotis, M. Menon, Phys. Rev. B 91, 125304 (2014). [34] S. Tongay et al., Sci. Rep. 3, 2657 (2013). [35] H. P. Komsa, A. V. Krasheninnikov, Phys. Rev. B 91, 125304 (2015). [36] Q.C. Sun et al., ACS Nano 7, 3506 (2013). [37] J. Suh et al., Nano Lett. 14, 6976 (2014). [38] H. Shu et al., ACS Appl. Mater. Inter.7, 7534 (2015).

Doping to MoS2 [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]

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In: Molybdenum Disulfide Editor: Jeremiah McBride

ISBN: 978-1-63485-032-2 © 2016 Nova Science Publishers, Inc.

Chapter 5

EXFOLIATED POLYPYRROLE-MOS2 NANOCOMPOSITES: PREPARATION AND CHARACTERIZATION Jiabao Hong1, Rabin Bissessur1, and Douglas C. Dahn2

*

1

Department of Chemistry, University of Prince Edward Island, Charlottetown, Canada 2 Department of Physics, University of Prince Edward Island, Charlottetown, Canada

ABSTRACT Exfoliated nanocomposites consisting of polypyrrole (PPy) and molybdenum disulfide (MoS2) were synthesized. The MoS2 was first prepared in an exfoliated state by reacting molybdic acid with a huge excess of thiourea at 500oC under nitrogen flow. The PPy-MoS2 nanocomposites were prepared by polymerization of pyrrole with ammonium peroxydisulfate, in the presence of the exfoliated MoS 2. The amount of MoS2 in the reaction mixture was systematically varied to produce a range of nanocomposite materials ranging from 1 to 50% by mass of MoS2. The nanocomposites were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, electron microscopy, and van der Pauw electrical conductivity measurements. *

Email: [email protected].

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Jiabao Hong, Rabin Bissessur and Douglas C. Dahn Powder X-ray diffraction provided evidence that the nanocomposites are exfoliated. The diffractograms of the nanocomposites were completely amorphous, suggesting lack of structural order in these materials and indicating the formation of genuine exfoliated systems. It was intriguing to observe that the nanocomposites exhibited enhanced electronic conductivity when compared to the pure polymer.

Keywords: exfoliated conductivity

MoS2,

nanocomposites,

polypyrrole,

electrical

INTRODUCTION Development of new types of energy storage devices has become a fast growing area over the years in order to meet the needs for society and for a greener environment [1-3]. The search for more sustainable, affordable and more efficient energy storage devices has been a strong motivation for scientists to explore new materials and improve existing materials [2]. The goal is to find materials with the following attributes: (1) be able to deliver high levels of electrical power; (2) inexpensive to synthesize; (3) air stable; (4) fast charge and discharge rates; (5) longer life time and good reversibility [13]. In order to realize and optimize the applications of “the dream materials,” researchers have put a lot of efforts into understanding the chemistry and physics of graphene and graphene analogous materials [1]. The rapid rise of interest in graphene has stimulated the exploration of various graphene-analogous materials, such as layered metal oxides, metal dichalcogenides, boron carbide, silicene, transition metal carbides and nitrides, and other 2D layered compounds [4]. Recently, there has been a dramatic growth in research on transition metal dichalcogenides (TMDs). These materials have the generalized formula MX2, where M stands for transition metals such as molybdenum (Mo), tungsten (W), titanium (Ti), zirconium (Zr) and hafnium (Hf), and X represents chalcogens, such as sulfur (S), selenium (Se) or tellurium (Te) [5]. TMDs are layered materials with structures similar to graphite. Similar to graphene (a monolayer of graphite), individual 2D layers of TMDs can be isolated using mechanical cleavage or intercalation methods [5]. A monolayer of a TMD consists of a transition metal atom (M) sandwiched between two layers of chalcogen atoms (X), which are covalently bound to the transition metal [5, 6]. There are more than 40 TMDs [6]. Depending on the combination

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of M and X, and on the oxidation state of the metal atom (M), layered TMDs could be metallic or semiconducting. 2D TMDs offer a wide range of important properties, including large surface area and tunable band gap which can be achieved by different combination of M and X in the TMDs, or by structural changes [5]. The tunable band gap property could change the conductive nature of the material, such as from metallic to semiconducting. As the material changes its conductive nature, it could present altered fluorescence and electrochemical properties [7]. As one of the 2D TMDs, molybdenum disulfide (MoS2) is attractive to both physicists and materials chemists due to its unique morphology, excellent mechanical and electrical properties [3]. MoS2 is a transition metal dichalcogenide (TMD), and a single layer of MoS2 is similar to graphene. Each monolayer in bulk MoS2 is composed of a three-atom layer, i.e., a Mo layer sandwiched between two S layers. The layers of MoS2 in its bulk form are stacked and held together by weak van der Waals forces [8]. Extensive research has been done on bulk MoS2 since it is a semiconductor with a tunable band gap which can be controlled by the particle size of the layered material, and this characteristic has fascinated many researchers [6]. Similar to other 2D TMDs, the exfoliated form of MoS2 presents vastly different properties compared to the bulk material [9-12]. The ease of exfoliation of MoS2 has led to significant interests in the characteristics of the single and few-layers MoS2 after exfoliation of the bulk material [9, 11].

a

b

Figure 1. (a) The structure of layered MoS2, (b) A top view of the MoS2 honeycomb lattice.

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Exfoliated MoS2 can be achieved by several methods, as reported by Ramakrishna Matte et al. [13]. The most common method to produce exfoliated MoS2 is by the ion intercalation method using n-butyllithium [10, 13]. In this method, bulk MoS2 is mixed with n-butyllithium/hexane solution to achieve lithium intercalation, and the resulting LixMoS2 is submerged in water. LiOH and H2 gas are produced as products, and the H2 gas triggers the exfoliation of MoS2 layers [13]. Besides the ion intercalation method, there is a direct method for the synthesis of exfoliated MoS2, as reported in the literature [13]. The direct method involves grinding and mixing molybdic acid with an excess of thiourea, and heating the reaction mixture at 773 K for 3 hours under nitrogen atmosphere [13]. Conducting polymers are attractive candidates for use in energy storage devices due to their high energy density [3]. The physical properties of conducting polymers strongly depend on the synthetic method, their morphology, the dopant used in the synthesis, and the doping level [14]. For most conducting polymers, the doping levels can be easily controlled by redox chemistry, which provides an excellent route to enhance the electrical conductivity of the material [14]. Among the conducting polymers, polypyrrole (PPy) has several advantages over others, and these include high electrical conductivity, high storage capacity, and good environmental stability [15]. It is important to highlight that PPy exhibits electrical conductivities only when it is doped, and the doped PPy behaves like a semiconductor [14]. The electrical conductivity of PPy results from the formation of polarons and bipolarons due to the dopant molecules [14]. The formation of additional energy bands between the valence band and the conduction band reduces the energy band gap and the doping leads to semiconducting properties [14]. The true nature of doped PPy is actually complex and it is affected by many factors, such as the synthetic method employed, the dopant, the structure and morphology of the polymer, as well as defects in the material. The conductivity of doped PPy can range from 10-3 to 103 S/cm [14, 16]. However, the relatively poor cycling stability and temperature dependence, limits the use of PPy in energy storage devices [3]. In addition, the redox sites in the polymer backbone of PPy are not sufficiently stable. Thus, the backbone of the polymer could be broken after a few cycles of charge or discharge [3]. For this reason, PPy is often hybridized with inorganic or organic materials to form composites that can achieve better cycling stability, enhanced specific capacitance and improved mechanical stability [3]. The field of nanotechnology is one of the most popular areas of research. Some applications of nanotechnology include microelectronics, polymer-based

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biomaterials, nanoparticle drug delivery systems, fuel cell electrodes, catalysts, as well as cosmetic products [17]. Recent interest in polymer based nanocomposites has been centred on exfoliated clays, graphene and graphenelike materials, carbon nanotubes, carbon nanofibers, and other nanoparticles [17]. Inorganic-polymer nanocomposites are materials where the polymer and the inorganic component (filler) are blended together at the nanoscale (1-100 nm) [17-22]. One common reason for adding fillers into a polymer matrix is to enhance the polymer properties, such as thermal stability, electrical conductivity, and mechanical strength [17-22]. A variety of nanoparticles such as clays, metal oxides, transition metal dichalcogenides, and semiconducting metallic crystals have been incorporated into polymeric materials [18]. There are several methods to prepare inorganic-polymer nanocomposites, such as blending or mixing the polymer and the inorganic particles, sol-gel processing, melting, and in situ polymerization of the monomer in the presence of the inorganic particles [18]. For most purposes, complete exfoliation of the filler materials, i.e., separation of the inorganic nanoparticles from one another, and complete dispersion within the polymer matrix is the desired goal when synthesizing exfoliated nanocomposites [17]. However, this ideal morphology is frequently not achieved and varying degrees of dispersion generally occur. The literature refers to three types of morphology: immiscible, intercalated, and exfoliated nanocomposites [17] The focus of this book chapter is on the creation of exfoliated PPy-MoS2 nanocomposites via oxidative polymerization of pyrrole in the presence of exfoliated MoS2. The structural and electrical conductivity properties of these nanocomposites are reported in this chapter.

METHODS 1. Experimental 1.1. Materials Pyrrole (Py) was distilled and kept in the refrigerator. Thiourea, molybdic acid, ammonium persulphate (APS) were of analytical grade, and were used without further purification.

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1.2. Synthesis of Exfoliated Molybdenum Disulfide 0.2530 g (1.560 mmol) of molybdic acid was mixed with 5.643 g (74.14 mmol) of thiourea. The mixture was ground with a mortar and pestle in order to produce an intimate blend. The reaction mixture was then placed in a ceramic vessel which was then inserted into a ceramic tube installed in a split furnace. The mixture was heated at approximately 500˚C for 3 hours under nitrogen atmosphere. The gray-black product was allowed to cool in the ceramic vessel under nitrogen atmosphere overnight. 1.3. Synthesis of Polypyrrole Polypyrrole (PPy) was synthesized using the chemical oxidation of the monomer, pyrrole, with ammonium persulfate (APS) as the oxidizing agent. 1.3418 g (0.02 mol) of pyrrole was mixed with 40 mL of 1M HCl in a 500 mL Erlenmeyer flask. The pyrrole solution was cooled to 0˚C in an ice bath. 4.5636 g (0.02 mol) of APS was dissolved in 40 mL of 1M HCl, and the solution was cooled to 0˚C in an ice bath. The cold APS solution was slowly added to the pyrrole solution. The polymerization was carried out for three hours with continuous stirring at ice bath temperature. The product was vacuum-filtered and washed with approximately 500 mL of 1M HCl. It was then air dried under suction. 1.4. Synthesis of Exfoliated PPy- 5% MoS2 Nanocomposite 1.3418 g (0.02 mol) of pyrrole was mixed with 40 mL of 1M HCl in a 500 mL Erlenmeyer flask. The pyrrole solution was cooled to 0˚C in an ice bath. 4.5636 g (0.02 mol) of APS was dissolved in 40 mL of 1M HCl, and the solution was cooled to 0˚C in an ice bath. 0.0706 g of exfoliated MoS2 was placed in a 40 mL beaker containing 30 mL of deionized water, and the mixture ultrasonicated for 20 minutes at 30% amplitude. The aqueous suspension of MoS2 was then added to the acidic solution of pyrrole with stirring, followed by dropwise addition of the cold APS solution. The reaction was carried out for 3 hours, with stirring. The black product was isolated via vacuum filtration and washed with 300 mL of 1M HCl. It was air dried overnight under vacuum. The experimental procedure was repeated to produce exfoliated PPy-MoS2 nanocomposites consisting of 1%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% and 50% by mass of MoS2.

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1.5. Materials Characterization Fourier transform infrared (FTIR) spectroscopy was performed by using a Bruker Alpha A-T over the range of 4000-400 cm-1. The resolution of the equipment is 0.9 cm-1. Samples were prepared as pressed KBr pellets and 64 scans were used. Powder X-ray diffraction (XRD) was run on a Bruker AXS D8 Advance diffractometer which was equipped with a graphite monochromator, variable divergence slit, variable anti-scatter slit, and a scintillation detector. Cu (Kα) radiation (λ = 1.5406 Å) was used. Solid samples were ground to a fine powder and adhered on glass slides using double sided tape. Scanning electron microscopy (SEM) was performed on an LVEM5 benchtop instrument, operating at 5 kV. For each sample, a small amount was dispersed in deionized water. One drop of the suspension was deposited on a copper stub which was then allowed to air dry prior to imaging. High resolution transmission electron microscopy (HRTEM) was performed on a Hitachi 7500 Bio-TEM, using an accelerating voltage of 80 KV. The powdered samples were dispersed in deionized water with the help of ultrasonication, and the dispersed samples were cast on carbon-coated copper grids. Electrical conductivity at room temperature as well as at variable temperatures was measured on samples of the nanocomposites. The van der Pauw method was used. Samples were pressed pellets, 12.7 mm in diameter, with thicknesses ranging from 10 to 45 mm. The current flow through the sample was supplied with a homebuilt constant-current source. The current and voltage readings were measured by two Hewlett Packard 34401A multimeters. For variable-temperature measurements, a homebuilt system cooled by a model 350CP CTI Cryogenics closed cycle helium refrigerator was used. The pellets were mounted on a copper-supported sapphire disk which was then placed in a vacuum chamber. The sample on the sapphire plate had a weak thermal link to the refrigerating unit. The sample was heated to the desired temperature electrically, with temperature control provided by an electronic temperature controller (Lakeshore Cryotronics Model 321) that utilizes a silicon diode temperature sensor.

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2. Results and Discussion 2.1. Power X-Ray Diffraction (XRD) Power X-ray diffraction was used to confirm the successful formation of exfoliated MoS2 and exfoliated PPy-MoS2 nanocomposites. XRD data were collected on the synthesized MoS2, as well as on the pristine layered MoS2 (purchased from Sigma Aldrich), for comparison. The results are displayed in Figure 2. The X-ray diffractogram for pristine layered MoS2 (Sigma Aldrich) shows a clear characteristic (002) reflection, which is indicative of the layered character of the material [23]. The interlayer spacing value of the material is 6.12 Å. The percent crystallinity of the material as determined by the XRD software is 82.3%. By comparing Figure 2(a) with Figure 2(d), it is obvious that the synthesised MoS2 is predominantly exfoliated in nature (single-layer or few-layer) due to the absence of the (002) reflection. The synthetic MoS2 also has a low percent crystallinity of 28.1%, as determined by the XRD software. The diffractogram of the synthetic MoS2 shows three local maxima at approximately 27°, 33° and 59°. These diffraction peaks correspond to the (004), (100), (110) planes, respectively. These data agree very well with the literature [23-24], and demonstrates that the synthetic MoS2 is in an exfoliated state, and the synthetic procedure utilized was successful.

Figure 2. XRD diffractograms for (a) layered MoS2 (Sigma Aldrich), (b) synthetic PPy, (c) PPy-15%MoS2 nanocomposite, (d) synthesized exfoliated MoS2.

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The XRD diffraction pattern for PPy prepared by the chemical oxidation of pyrrole is shown in Figure 2(b). The XRD pattern reveals the amorphous nature of the polymer. The broad peak at about 26° is characteristic of an amorphous material. The diffractogram is in good agreement with the literature [14, 25]. By comparing the XRD patterns of PPy and the nanocomposites (Figures 2(b) and 2(c)), we can see they are very much alike. The lack of the (002) diffraction peak and the presence of a broad peak around 26° suggest that the nanocomposites are amorphous in nature. These observations confirm that exfoliated nanocomposites have been synthesized.

a Figure 3. (Continued)

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b Figure 3. SEM micrograph of (a) layered MoS2 (Sigma Aldrich) and, (b) synthetic MoS2.

2.2. Scanning Electron Microscopy (SEM) Surface morphological studies on synthetic MoS2 and bulk layered MoS2 (Sigma Aldrich) were performed by using scanning electron microscopy (SEM). From the SEM images, we can clearly see the layers of the SigmaAldrich MoS2 sample stacked on top of each other (Figure 3a), consistent with the XRD pattern. In addition, the layers can be observed in different shapes (not perfect honeycomb shape), and the edges of some of the layers are slightly rolled up. SEM on our synthetic MoS2 (Figure 3 b) reveals its amorphous character, with the layers being in a highly disordered state. SEM provides clear cut evidence that the synthetic MoS2 is in an exfoliated state, in agreement with the XRD data.

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The SEM image of PPy shows that it has a uniform granular morphology, with an average grain size of 0.7 μm (Figure 4). This observation is in agreement with the literature [14, 25].

Figure 4. SEM micrograph of PPy.

The granular morphology of PPy in PPy-5%MoS2 is clearly visible (Figure 5). However, in PPy-50%MoS2, we can see the MoS2-like character of the material. From these images, we can distinguish the MoS2 from the PPy, which means that based on SEM characterization the composites may not be fine mixtures at the nanoscale. However, further characterization by TEM proves that the composites are, in fact, genuine nanocomposite materials (see below).

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a

b Figure 5. SEM micrograph of (a) PPy-5%MoS2 and (b) PPy-50%MoS2.

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2.3. Transmission Electron Microscopy (TEM) Characterization TEM was used to shed light into the structure of the materials. As viewed under TEM, layered MoS2 (Sigma Aldrich) depicts its lamellar character (Figure 6 a), and this is consistent with the XRD and SEM observations. In contrast, TEM shows that our synthesized MoS2 (Figure 6 b) is in an exfoliated state, and is in very good agreement with the XRD and SEM results. The pure polymer polypyrrole as observed under TEM is completely amorphous in character (Figure 7). All composites as viewed under TEM show that they are in a structurally disordered state, providing strong evidence that the polypyrrole and MoS2 are intimately mixed at the nanoscale. As an illustration the TEM micrographs of PPy-5%MoS2 and PPy-50%MoS2 are displayed in Figure 8.

a Figure 6. (Continued)

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b Figure 6. (a) TEM micrograph of (a) layered MoS2, and (b) synthesized MoS2.

2.4. Fourier Transform Infrared Spectroscopy (FTIR) The synthesized MoS2 was characterized by FTIR spectroscopy (Figure 9). From the spectrum, the broad peak at 3106 cm-1 is attributed to moisture in air or KBr pellet. The peak at 2300 cm-1 is assigned to CO2 trapped in the KBr pellet. There are a couple of organic vibrations in the region 1239-1635 cm-1, which are not expected from the synthetic MoS2. These peaks are due to the decomposition products of the excess thiourea used in the synthesis of MoS2. Thiourea has been known to thermally decompose into several compounds such as hydrogen sulfide (H2S) and carbodiimide (HN = C = NH) [26-27]. Thus, the peak at 1635 cm-1 in the IR spectrum is consistent with the C = N stretch, and the peak at 1554 cm-1 can be assigned to the NH stretch [27]. According to the literature, the Mo-S stretch should appear in the range 465480 cm-1, however, this is not observed in the IR spectrum [28-29].

Exfoliated Polypyrrole-MoS2 Nanocomposites

Figure 7. TEM micrograph of PPy.

a Figure 8. (Continued)

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b Figure 8. TEM micrograph of (a) PPy-5%MoS2, and (b) PPy-50%MoS2.

The synthesized PPy was analyzed by FTIR spectroscopy as a pressed KBr pellet. The broad band at 3376.71 cm-1 is attributed to the OH group, since the NH group on the PPy backbone can easily pick up water from air via hydrogen bonding (Figure 10). The two peaks at 800-900 cm-1 are attributed to C-H wagging. The characteristic peak at 1542.33 cm-1 corresponds to the C = C stretch. The signal at 2336 cm-1 is due to CO2 gas trapped in the KBr pellet. The peak at 3376 cm-1 is consistent with the N-H stretch in the polymer backbone. The noise above 3600 cm-1 and around 1300-1800 cm-1 is due to water vapour from air [14, 25]. The peaks observed in this spectrum match well with those from the literature, and confirm that PPy was successfully synthesized [25]. The IR spectra of the PPy-MoS2 nanocomposites show similar peaks as in the spectrum of the pure PPy, which indicates the presence of PPy in the

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nanocomposites. As an illustration, the IR spectrum of PPy-35%MoS2 nanocomposite is displayed in Figure 11.

Figure 9. FTIR of synthesized exfoliated MoS2.

Figure 10. FTIR of synthesized PPy. Insert: chemical formula of PPy.

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Figure 11. FTIR spectrum of PPy-35%MoS2 nanocomposite.

2.5. van der Pauw Conductivity Measurements Electrical conductivity measurements were attempted on both the layered and exfoliated MoS2 using pressed pellets of these materials. Conductivity results for a layered MoS2 pellet are shown in Figure 12 as a function of inverse temperature. When conductivity data plot as a straight line in an Arrhenius-type plot like Figure 12, this indicates thermally-activated conduction described by an equation of the form

 (T )   0e E / kT ,

(1)

where σ0 is a constant and E is the activation energy. Typical semiconductors exhibit approximately this sort of behavior, with E related to the semiconductor band gap. Bulk MoS2 is known to be a semiconductor, so the trend shown in Figure 12 is expected. Several attempts were made to determine the electrical conductivity of the synthesized exfoliated MoS2. However, the conductivity of the pressed pellets was too small to measure with our equipment. Synthesis of exfoliated MoS2 from molybdic acid and thiourea is expected to produce the 2H form of MoS2. Single-layer 2H-MoS2 is known to be a semiconductor with a direct band gap of about 1.9 eV, significantly larger than the 1.2 eV indirect gap in bulk MoS2 [9]. Thus, depending on doping levels, there could be far fewer thermallyactivated charge carriers in the exfoliated sample as compared to bulk layered

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MoS2. Another possible explanation for the very low conductivity in our pressed pellets of exfoliated MoS2 is poor electrical contact between the MoS2 particles in the pellet. As mentioned earlier, pure PPy in the undoped form is an insulator, and doped PPy behaves like a semiconductor. Our experimental data for PPy (Figure 13) show that as temperature increases, the conductivity increases, as expected for a semiconductor. However, the temperature dependence is not consistent with equation [1]. In fact, the conductivity increases almost linearly with temperature. Variable-temperature conductivity measurements were also performed on pressed pellets of PPy-5%MoS2, PPy-10%MoS2, and PPy-15%MoS2 composite materials. The electrical conductivity of these materials also increases linearly as temperature rises. As an illustration, the variabletemperature electrical conductivity of the PPy-15%MoS2 composite is shown in Figure 13.

Figure 12. Electrical conductivity of a pressed pellet of bulk layered MoS2 as a function of temperature. The line is a fit to equation [1].

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Figure 13. Electrical conductivity of (a) PPy and (b) PPy-15%MoS2 nanocomposite as a function of temperature.

Conductivity measurements on other nanocomposite samples were made at room temperature only (about 295 K). These conductivity values are shown in Figure 14 as a function of mass % of MoS2. Also included in Figure 14 are the 300 K data from the variable-temperature conductivity runs. Although there is significant variability in the data, perhaps due to differences in PPy doping levels in different samples with the same percentage of MoS2, there are some consistent and reproducible trends. The 1% and 5% samples have conductivity close to or below that of PPy. Nanocomposites in the 10% to 35% range generally have higher conductivity; with values up to 12 S/cm. (The apparent drop at 30% may not be significant, as only one 30% sample was tested.) Above 40% MoS2, the conductivity falls. While we do not completely understand the conductive properties of these nanocomposites, the data suggest that electron conduction is mediated primarily by the PPy component. The reasons for making this conclusion are that exfoliated MoS2 is a very poor conductor compared to either PPy or the nanocomposites; and that the temperature dependence of conductivity is the same for PPy and the nanocomposites. The reason for enhanced conductivity between about 10% and 35% MoS2 is not understood, but could be related to

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interaction between the MoS2 and PPy components of the composite. As mentioned above, the conductivity of PPy depends critically on doping, and during the synthesis of our materials doping is controlled by the concentration of the oxidizing agent APS. If doping is also affected by charge transfer between MoS2 and PPy then doping levels would vary with the mass % of MoS2. At the highest MoS2 percentages (above 40%), poorly-conducting MoS2 layers become so numerous that they cut off the current flow paths through PPy, and conductivity drops.

Figure 14. Room temperature conductivity of the nanocomposites.

It was observed that several nanocomposite samples which had been stored in air for an extended period of time (greater than about 75 days) showed very low conductivity compared to fresh samples of the same composition. Such aged samples are not included in Figure 14. This loss of conductivity with aging is not yet understood, but may be due to changes in PPy doping occurring as a result of long-term exposure to humid air.

CONCLUSION Exfoliated MoS2 and bulk PPy were successfully synthesized, as evidenced by XRD, SEM and TEM. XRD and TEM provide evidence that the PPy and MoS2 are intimately mixed at the molecular level, resulting in the formation of genuine nanocomposite materials. The enhanced electrical

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Jiabao Hong, Rabin Bissessur and Douglas C. Dahn

conductivity of the nanocomposites over the pure polymer suggests that they could potentially be used as electrodes in rechargeable batteries. Further work is underway to better understand the oxidation state of PPy in these nanocomposites, and the effect that MoS2 may have on the doping of PPy.

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INDEX A acetone, 30 acid, ix, 12, 30, 43, 83, 86, 87, 88, 100 acidic, 28, 36, 67, 88 activation energy, 100 active site, 29, 32, 33, 36, 60, 63, 64 adsorption, 29, 45, 73, 75, 79 AFM, 3, 4, 11 Ag, 40, 64, 76, 77 alkylation, 43 ammonium, ix, 30, 83, 87, 88 aniline, 105 anisotropy, 24 annealing, 55, 58 APL, 70 aqueous solutions, 24, 59 aqueous suspension, 88 architect, viii, 51 arsenic, 77 atmosphere, 6, 55, 86, 88 atmospheric pressure, 10, 58 atomic force microscope, 3 atoms, vii, 2, 6, 13, 15, 52, 53, 62, 75, 84

B band gap, viii, 2, 21, 23, 24, 25, 26, 27, 38, 42, 54, 62, 63, 73, 85, 86, 100 binding energy(ies), 73, , 7576

biomaterials, 87 bonding, 98 Boron, 40, 77

C cadmium, 50 calcination temperature, 23, 49, 50 carbides, 84 carbon, viii, 9, 40, 51, 87, 89 carrier gas, 6, 8, 9, 10 catalysis, ix, 5, 25, 64, 71, 72 catalyst(s), vii, 27, 28, 30, 43, 45, 46, 47, 52, 53, 55, 60, 63, 64, 66, 74 catalytic activity, 45, 52, 63, 65 catalytic properties, 24 catalytic system, 30 Cd, 58, 62, 76, 77 centrosymmetry, 73 CF4, 78 charge density, 16 Charge trapping, 78 chemical, vii, viii, 1, 2, 21, 22, 24, 26, 27, 29, 32, 40, 43, 49, 56, 57, 67, 72, 75, 78, 88, 91, 99 chemical inertness, 24 chemical reactions, 57 chemical stability, viii, 21

108

Index

chemical vapor deposition (CVD), vii, 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 26, 77, 79 chemisorption, 79 CHF3, 78 chromatograms, 31 Co, 37, 47, 48, 50, 74, 76, 77 CO2, 37, 38, 47, 96, 98, 105 cobalt, 43, 50 cocatalyst, 52, 53, 59, 60, 61, 62, 63, 66 combustion, 29, 44 composites, 23, 31, 32, 33, 34, 36, 39, 46, 56, 57, 58, 63, 64, 86, 93, 95 compounds, 27, 28, 84, 96 conduction, 5, 24, 54, 72, 76, 78, 86, 100, 102 conductivity, ix, 12, 27, 40, 66, 84, 86, 89, 100, 101, 102, 103, 104 conductor, 102 contamination, 7, 9, 14 controversies, 5 coordination, 78 copper, 76, 89 corrosion, 24, 25 Coulomb interaction, 73 covalent bond, 2, 6, 15, 24 Cr, 64, 76, 77 crystal growth, 77 crystal structure, 10, 14, 24, 29 crystalline, 26, 78 crystallinity, vii, 1, 2, 3, 4, 7, 11, 12, 13, 14, 26, 90 crystals, 42, 74, 87 CTAB, 49

D decomposition, 54, 58, 96 defects, 4, 11, 14, 23, 63, 75, 86 degradation, 4, 12, 23, 27, 28, 29, 31, 32, 33, 35, 36, 37, 38, 39, 41, 45, 46, 47 degradation rate, 29, 32, 38 density functional theory (DFT) , 42, 75, 76, 77, 78 deposition, 12, 15, 26, 32, 45, 64

detection, 32, 46 diffraction, ix, 4, 84, 90, 91 diffusion, viii, 13, 22 dimethylformamide, 57 direct observation, 11 dispersion, 73, 87 displacement, 14 distribution, 26 divergence, 89 DOI, 49, 50 dopants, ix, 71, 72, 76, 77 doping, ix, 4, 23, 27, 40, 71, 72, 73, 74, 76, 77, 78, 79, 86, 100, 102, 103, 104

E electric field, 38 electrical conductivity, ix, 36, 63, 66, 83, 84, 86, 87, 100, 101, 104 electrical properties, 85 electricity, 22 electrocatalysis, 37, 53, 74 electrochemical impedance, 33 electrodes, 33, 38, 39, 52, 54, 72, 87, 104 electroluminescence, 2 electron(s), viii, ix, 3, 4, 21, 23, 24, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 38, 52, 53, 54, 55, 58, 59, 60, 61, 63, 64, 66, 67, 72, 73 75, 83, 89, 102, electron diffraction, 4 electron microscopy, ix, 83, 89 electronic structure, ix, 16, 59, 71, 72 Electronics, 50, 73 environment, 22, 29, 73, 74, 75, 76, 84 environmental crisis, viii, 51 environmental issues, 52 environmental protection, 22, 52 equilibrium, 75 equipment, 5, 6, 89, 100 evaporation, 6, 49 evidence, ix, 84, 92, 95, 103 evolution, vii, viii, 1, 13, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 74 EXAFS, 76

109

Index exciton, 14, 73, 75 Exciton binding energy, 73 exposure, 79, 103

F fabrication, vii, 1,2, 12, 14, 26, 39, 44, 46, 59, 73 Fermi level, 5 ferric ion, 40 ferrite, 50 ferromagnetic, 76 ferromagnetism, 79 films, 2, 4, 74, 79 filtration, 37, 88 flatness, 12 fluid, 43 fluorescence, 32, 46, 85 fluorine, 79 formation, ix, 6, 12, 13, 44, 53, 64, 75, 76, 84, 86, 90, 103 formula, 84, 99 friction, vii, 53 FTIR spectroscopy, 96, 98 fuel cell, 23, 25, 87 fullerene, 23 functionalization, 25, 26

G gate dielectric, 78 geometry, 34 glutathione, 32, 46 graduate students, 48 grain size, vii, 1, 5, 7, 14, 93 graphene sheet, 2, 12 graphite, 2, 9, 84, 89 growth conditions in CVD, 10 growth mechanism, 13, 14 growth rate, 13 growth temperature, vii, 1, 7, 9, 14 growth time, 8, 13

H H2S, 5, 6, 10, 55, 58, 65, 96 hafnium, 84 halogen(s), 77, 78 harvesting, 33 hazardous waste, 22 helium, 89 Heterostructures, 14, 41 hexane, 86 HRTEM, 59, 89 hybrid, 31, 32, 42, 43, 60, 62 hydorgen adsorption, 79 hydrogen, vii, 23, 25, 31, 36, 37, 43, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 74, 96, 98 hydrogen gas, 57 hydrogen sulfide, 96 hydrophilicity, 12 hydrothermal process, 61, 63 hydrothermal synthesis, 29, 35, 44, 45 hydroxide, 57 hydroxyl, 24, 30

I ideal, 26, 27, 87 identification, 32 illumination, 53, 58, 60 image(s), 11, 15, 16, 59, 92, 93 impregnation, 33, 66 Impurity Assisted Growth, 14 infrared spectroscopy (IR spectra), ix, 83, 98 interface, 4, 14, 15, 33, 34, 53, 75, 78 interference, 11 iodine, 78 ion implantation, 77 Ionic liquids, 79 ionization, 77 Iron, 42, 76 irradiation, viii, 21, 22, 27, 28, 29, 30, 31, 32, 33, 35, 36, 38, 46, 54, 58, 61, 63, 64, 65, 66, 73, 75

110

Index

K KBr, 89, 96, 98

L lactic acid, 62 laser ablation, 26 lifetime, 33, 55, 73 light, viii, 21, 22, 23, 26, 28, 29, 30, 31, 32, 33, 35, 36, 38, 39, 42, 44, 45, 46, 47, 52, 53, 54, 55, 58, 60, 61, 62, 63, 64, 65, 66, 95 lithium, 57, 86

M magnetic moment, 76 magnetic properties, 49, 50 magnetism, 76 Manganese, 76,78, 83, 84, 85, 86, 87, 93, 95, 100, 101, 103 matrix, 33, 44 measurement(s), ix, 33, 75, 76, 77, 78, 83, 89, 100, 101, 102 mechanochemistry, 60 metal nanoparticles, 64 metal oxides, 84, 87 metals, 2, 16, 23, 30, 84 methanol, 37, 38, 47 methylene blue, 23, 46 microelectronics, 86 microscope, 3, 11 microspheres, 44, 64 microstructure, 23, 25 mineralization, 31 mixing, 74, 86, 87 Mo(CO)6, 6, 9, 10 MoCl5, 6, 9, 10 molecular beam epitaxy, 16 molecular oxygen, 34 molecules, 12, 34, 41, 67, 73, 74, 75, 86 molybdenum, vii, viii, ix, 1, 2, 5, 6, 14, 43, 44, 49, 50, 51, 52, 72, 76, 78, 83, 84, 85

molybdenum trioxide (MoO3), 78 monolayer, 2, 3, 4, 11, 12, 16, 57, 73, 74, 75, 76, 77, 78, 84, 85 MoO3, 6, 9, 10, 29, 39, 49 morphology, viii, 21, 22, 23, 25, 30, 37, 56, 85, 86, 87, 93 Mo-Substitutional Dopants, 76

N nanobelts, 31, 45, 59, 74 nanocomposites, ix, 46, 65, 83, 84, 87, 88, 89, 90, 91, 98, 102, 103, 104 nanocrystals, 41, 44, 45, 58, 59, 62 nanofibers, 60, 87 nanojunctions, 53 nanomaterials, viii, 25, 26, 38, 42, 49, 50, 51 nanoparticles, 23, 28, 29, 31, 35, 37, 40, 42, 44, 45, 46, 47, 49, 50, 63, 64, 65, 66, 67, 87 nanorods, 43, 44, 50 nanoscale materials, 23 nanostructured materials, 41 nanostructures, 42, 47 nanotechnology, 39, 86 natural gas, vii, 53 nickel (Ni), 41,43, 44, 74, 76, 77 Niobium, 76 nitrides, 26, 84 nitrogen, ix, 74, 83, 86, 88 n-type, ix, 5, 71, 74, 76, 77, 78

O operations, 66 optical properties, 24, 26 optimization, 65 orbit, 72, 73 organic compounds, 27 oxidation, 22, 24, 27, 31, 33, 35, 37, 38, 42, 45, 46, 54, 85, 88, 91, 104 oxygen, 4, 6, 7, 11, 12, 14, 27, 38, 54, 78

111

Index

P permission, 34, 36, 37 petroleum, 28 pH, 23, 24, 30, 37, 63 phenol, 28, 31, 45 Phosphorus, 77 photoabsorption, 64 photocatalysis, 22, 23, 24, 32, 39, 42, 45, 52, 54, 55 photocorrosion, 33, 61, 66 photodegradation, 29, 30, 31, 32, 33, 36, 40 photodetectors, 2 photoirradiation, 27 photoluminescence, 11, 29, 33, 50, 64, 77 photons, 24, 67 photooxidation, 33, 43 photosensitizers, 67 photosynthesis, 43, 55 physical properties, 26, 86 physical structure, 59 Physics, 19, 50, 69, 71, 72, 73, 81, 83, 84, 104, 106 plasma treatment, 78 platinum, 54, 59, 105 polarity, 77 polarization, 76 pollutant(s), viii, 21, 22, 23, 33, 37, 38 pollution, 22, 23, 28 polymer matrix, 87 polymer nanocomposites, 87 polymer properties, 87 polymeric materials, 87 polymerization, ix, 83, 87, 88 polymers, 86 porosity, 23 potassium, 29 precipitation, 29, 50 preparation, 3, 38, 41, 42, 54, 55, 56, 58 promoter, 12, 29, 43, 49 p-type, ix, 5, 61, 71, 76, 77, 78, 79 purification, 22, 38, 87

Q quantum confinement, 24, 31 quartz, 7, 10

R radicals, 24, 27, 30, 32, 35, 46 reaction center, 36, 72 reaction mechanism, 37 reaction medium, 23 reaction rate, 29, 32 reaction temperature, 10, 23, 31, 65 reaction time, 30, 36, 65 reactions, 26, 74 recombination, viii, 21, 23, 26, 27, 29, 31, 32, 34, 52, 59, 61, 64 recycling, viii, 21, 22 remediation, viii, 21, 22, 39 requirement(s), 22, 55 researchers, 5, 9, 10, 22, 27, 33, 54, 65, 84, 85 resistance, 4, 24, 25, 37 resolution, 11, 75, 89 reusability, 25, 26, 38 Rh, 76, 77 rhenium, 74, 76 Ru, 66, 76, 77

S sapphire, 10, 12, 89 scandium, 4 scanning electron microscopy, 92 Scotch tape exfoliation, 2 selenium 77, 84 self-assembly, 36, 49 semiconductor, vii, viii, 1, 2, 4, 14, 21, 24, 25, 26, 30, 32, 33, 42, 43, 45, 52, 53, 54, 58, 61, 62, 65, 66, 72, 74, 85, 86, 100, 101 semiconductors, 5, 23, 64, 78, 100 sensor(s), 25, 73, 89 serum, 32

112

Index

SF6, 78 single crystals, 76, 77 SiO2, 4, 10, 30, 75, 79 society, 84 sodium, 29, 31, 33, 56 solar cells, 23, 39 sol-gel, 25, 87 solution, 27, 30, 34, 40, 55, 62, 78, 86, 88 species, 2, 5, 22, 27, 28, 35, 37, 75 specific surface, 29, 30, 34, 36, 61 spectroscopy, 3, 33, 89 S-Substituting Dopants, 77 stability, 14, 26, 36, 38, 60, 63, 65, 67, 86 STM, 16, 75 stoichiometry, 4, 59, 75 storage, 84, 86 strain, 79 strong interaction, 56, 64 structural changes, 85 structure, vii, 3, 15, 24, 27, 29, 31, 33, 34, 36, 37, 46, 52, 55, 60, 65, 74, 85, 86, 95 substitution, 71, 74, 77 substrate, vii, 1, 2, 5, 6, 7, 8, 11, 12, 13, 16, 60, 73, 75, 79 sulfur vacancies, 4, 5, 11, 67 surface area, 23, 29, 41, 63, 85 surface modification, 23 surface transfer doping, ix, 71, 72, 78, 79 surface treatment, vii, 1, 11, 12 surfactant(s), 26, 29, 31, 45, 56 synergistic effect, 33, 46 synthesis, vii, viii, 5, 6, 8, 10, 12, 14, 21, 22, 24, 25, 26, 29, 40, 44, 45, 50, 53, 56, 67, 76, 86, 96, 103

T techniques, ix, 2, 16, 22, 26, 38, 71, 72, 73 technology, vii, 1, 4, 14, 15, 38, 52, 72, 73 tellurium (Te), 14, 77, 84 temperature, 6, 7, 8, 10, 14, 28, 30, 37, 38, 49, 55, 58, 86, 88, 89, 100, 101, 102, 103 temperature dependence, 86, 101, 102 thermal stability, 29, 87 thermal vapor sulfurization (TVS), 2, 5

thermodynamic equilibrium, 24 thiol, 79 titania, 30 titanium, 84 transition metal, 2, 44, 47, 76, 84, 85, 87 Transmission Electron Microscopy (TEM), 3, 4, 11, 15, 75, 89, 93, 95, 96, 97, 98, 103 transparency, 60 transport, 13, 57, 75, 78 transportation, 52 treatment, viii, 12, 21, 22, 55, 78, 79 tribology, ix, 71, 72 Tribology, 48, 74 tungsten, 6, 14, 72, 84 Two Flow CVD System, 8

U UV irradiation, 31, 37, 39, 59, 61 UV light, 24, 31, 36, 40, 41, 46, 54, 61

V vacancies, ix, 4, 5, 11, 67, 71, 72, 74, 75, 78, 79 vacuum, 5, 16, 42, 75, 88, 89 valence, viii, 22, 24, 54, 72, 77, 86 valleytronics, 73, 76 van der Waals epitaxy, 12, 16

W waste, viii, 21, 22, 23, 27, 38 water purification, 22, 36 well-being, 22

X X-ray diffraction (XRD), ix, 3, 4, 83, 89, 90, 91, 92, 95, 103 X-ray photoelectron spectroscopy (XPS), 3, 4, 75

Index

Z

zig-zag, 13 zirconium (Zr), 76, 77, 84 ZnO, viii, 21, 30, 33, 46, 61

113