Encyclopedia Of Nanoscience And Nanotechnology Inorganic Nanotubes Structure, Synthesis, And Properties

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Inorganic Nanotubes: Structure, Synthesis, and Properties

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Reshef Tenne Weizmann Institute, Rehovot, Israel

INTRODUCTION Nanoparticles of inorganic layered compounds, such as WS2, were shown to be unstable in the planar form and, in analogy to carbon nanoparticles, they form closed-cage structures with polyhedral or nanotubular shapes. These findings extend the paradigm of carbon fullerenes and nanotubes to the field of inorganic chemistry, where numerous examples for its validity have been documented in recent years. Templated growth of nanotubes from isotropic (3-D) compounds such as metal oxides and chalcogenides was reported as well, but these nanotubes are generally not perfectly crystalline. Various issues in the structure, synthesis, and properties of such inorganic nanotubular and fullerene-like structures are reviewed, together with some possible applications.

CARBON FULLERENES AND NANOTUBES When the first generation of high-resolution transmission electron microscopes (TEM) became available during the late 1960s, nanoparticles of fired soot with unique character were revealed. These multilayer polyhedra (‘‘onions’’) were essentially closed graphitic sheets with an empty core, which were thought to consist of pieces of small graphitic platelets ‘‘stitched’’ together by an unknown ‘‘glue.’’ In 1980, Iijima proposed that such structures were in fact made of graphitic sheets ‘‘glued’’ together by pentagonal rings. He suggested that according to the Euler rule each polyhedron in the onion consists of 12 carbon pentagons and a large number of hexagons. Each pentagon forces the flat hexagonal network of graphitic sheets to distort from the planar structure, and disposing such 12 pentagons leads to closure of the entire plane into a hollow caged structure. The stunning discovery of the C60 molecule by Kroto, Smalley, and Curl[1] led to a new era in the science of nanomaterials. This molecule was shown to consist of 20 hexagons and 12 symmetrically disposed pentagons. Following this discovery, many other fullerenes and related forms of carbon were identified, in particular multiwall and single-wall nanotubes, discovered by Iijima.[2] Carbon nanotubes are made of rolled graphene sheets capped with half a fullerene molecule of the same Dekker Encyclopedia of Nanoscience and Nanotechnology DOI: 10.1081/E-ENN 120009133 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved.

diameter at each of the two ends. Therefore, these structures can be viewed as an elongated form of a fullerene. Both fullerenes and nanotubes are seamless structures having threefold bonded carbon atoms only. The absence of dangling bonds renders these structures energetically very stable. Nonetheless, the deviation from planarity induces a nonnegligible amount of stress into the fullerenes, which explains many of their chemical and physical properties.

CAN THE PARADIGM OF FULLERENES AND NANOTUBES BE EXTENDED BEYOND CARBON? The driving force for the formation of carbon fullerenes and nanotubes stems from the abundant reactive atoms on the periphery of the quasi 2-D graphitic nanostructure. These rim atoms are only twofold bonded rather than being threefold (sp2) bonded as in the bulk (Fig. 1). It was concluded, therefore, that in spite of the elastic strain of the folded nanoparticles, the planar topology of graphitic nanoparticles is unstable with respect to folding into the seamless fullerenes. Using similar reasoning, it has been proposed[3,4] that the formation of fullerenes is not unique to carbon and in fact is a genuine property of 2-D (layered) compounds. Inorganic layered compounds are abundant, in particular among the transition-metal chalcogenides (sulfides, selenides, and tellurides), halides (chlorides, bromides, and iodides), oxides, and numerous ternary (quaternary) compounds. However, in contrast to graphite, each molecular sheet consists of multiple layers of different atoms chemically bonded to each other. If one considers MoS2 as an example (Fig. 1), each molecular sheet is made of a layer of molybdenum atoms sandwiched between two outer sulfur layers. The Mo atoms are sixfold bonded to sulfur atoms, forming a trigonal biprism. In analogy to graphite, the S–Mo–S layers are stacked together through weak van der Waals forces. Like graphite, such compounds are highly anisotropic with respect to many of their physical and chemical properties. The (00.1) surfaces (van der Waals surfaces) of the crystal, which are perpendicular to the c-axis, consist of threefold bonded sulfur atoms, which are chemically not reactive. In contrast to the fully bonded bulk atoms, 1447

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tiate this initially intuitive hypothesis and show that this new curved and hollow phase of layered compounds, designated as inorganic fullerene-like (IF) material, is the thermodynamically stable form, given the constraint that the particles cannot grow beyond, say 0.2 mm. Initially, however, energy must be provided to overcome the activation barrier ensuing from the elastic energy of bending of the otherwise planar sheet. In most cases, heating (thermal energy) was provided for this purpose, but other energy sources, such as irradiation, microwave, sonochemical, and electrical energy were found to activate the nanoparticles’ folding process. Fig. 2 shows a TEM image of typical MoS2 hollow nanoparticles (onions) consisting of more than 10 molecular MoS2 layers arranged in a concentric seamless form. A few kilograms of nested (fullerene-like) WS2 structures were synthesized by the Weizmann Institute laboratory over the last year and efforts to scale up this production

Fig. 1 Schematic drawings of graphite (a) and MoS2 (b) nanoclusters. Note that in both cases, the surface energy, which destabilizes the planar topology of the nanocluster, is concentrated in the rim atoms, which are situated on the prismatic edges parallel to the c-axis. (View this art in color at www.dekker.com.)

the sulfur and molybdenum atoms on the rim of the nanocluster, i.e., on the prismatic (10.0) faces (parallel to the c-axis), are not fully bonded and are therefore chemically very reactive. Indeed, each molybdenum atom in the (10.0) prismatic edge is bonded to only two sulfur atoms of the upper plane and two sulfur atoms of the lower plane. Therefore, each Mo atom has two dangling bonds per atom on the (10.0) face and is consequently chemically reactive. On the opposite face ( 10.0) of the MoS2 nanosheet, sulfur atoms are only twofold bonded and they possess one dangling bond per atom. Therefore, the Mo atoms of the (10.0) face would tend to recombine with the S atoms of the opposite side of the platelet. Because the ratio between peripheral (partially bonded) and bulk (fully bonded) molybdenum (sulfur) atoms increases with shrinking size of the platelet (sheet), nanoparticles of MoS2 are not stable in the planar form and they fold into closed-cage nanostructures. This hypothesis has been invariably confirmed by both experiment and theory. Extensive experimental data available now tend to substan-

Fig. 2 TEM image of typical MoS2 nanoparticles with fullerene-like structure. Each dark line represents an atomic layer of the basal plane (0001). The distance between each two ˚ . The c-axis is always normal to the surface of layers is 6.18 A the nested fullerene-like structure.

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are well under way. Fig. 3 shows typical TEM images of multiwall WS2 nanotubes. Nanotubes of this kind are currently produced by the grams, albeit not as a pure phase, yet. Theoretical considerations show that the bending energy of isotropic (3-D) materials such as silicon or TiO2 is exceedingly higher than that of 2-D compounds. This energy cannot be fully compensated for by remediation of the dangling bonds,[5] which makes their folding into crystalline nanotubes unlikely. Furthermore, for such nanostructures to become stable, their surface must be passivated, or else they will react with the surrounding and form new entities with a different chemical composition. Therefore, isotropic (3-D) materials are not likely to form stable IF nanostructures. Synthesis of semicrystalline nanotubes from such isotropic compounds has been devised, mostly through templated growth (see, e.g, Ref. [6]). In spite of this fact, or perhaps because their surface atoms are not fully bonded, semicrystalline nanotubes can be very useful. Their large surface area, combined with their nanometer-scale pore sizes and reactive chemical moieties, may lead to the development of highly reactive and selective catalysts, sensors, and a host of other applications. Therefore, from the chemical perspective a question of great importance concerns the borderline between those

Fig. 3 TEM micrographs of three very long WS2 nanotubes.

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materials that can form fully crystalline IF nanostructures and isotropic materials that cannot afford fully crystalline nanostructures, or can result only in semicrystalline nanotubes. Stated differently, it is possible to ask whether IF structures are generic to all 2-D layered compounds, a group of a few thousands materials, or else only to a subset of this large canon. One way to address this issue is to try to synthesize IF nanostructures from layered compounds of completely different chemical nature. For instance, by synthesizing NiCl2 nanotubes[7] it became apparent that even 2-D compounds of highly ionic character, which are very hygroscopic and are quite unstable in the ambient, are able to form stable IF nanostructures. Incidentally, the kinetics of water uptake by the seamless IF NiCl2 nanostructures is appreciably slower than the water uptake by nanoplatelets of the same compound, making the former much less hygroscopic and amenable to detection by electron microscopy.

SYNTHESIS OF INORGANIC NANOTUBES To produce pure IF phases, and nanotubes in particular, it is imperative to prevent the nanocrystallites from growing beyond a certain size during the process (arrested growth). Numerous methods have been devised for this purpose. A few grams of IF WS2 were initially synthesized from the respective oxide phase.[8] However, lately a few kilograms of this phase were synthesized using the fluidized bed reactor.[9] In the first step of this reaction, which lasts about a second, heated WO3 nanoparticles react with H2S gas at 840°C and are sulfidized at the surface. This fast reaction produces one closed layer of WS2, enfolding the entire WO3 nanoparticle. The sulfide-encapsulated oxide nanoparticles become surface passivated and they cannot grow further in size. Once this protective layer has been completed, a slow diffusion-controlled process leads to the conversion of the oxide core into a metal disulfide in a highly uniform and regular fashion. Notably, although the inward diffusion of the (H2S) gas is isotropic, the growth of the WS2 layers occurs along one growth front only, which explains the almost perfect crystallinity of the IF nanoparticles. In early reports, only minor amounts of WS2,[3] MoS2,[10] and BN[11] nanotubes could be produced. However, recently, different strategies were successfully developed for the synthesis of various nanotubes. Thus, using the same fluidized bed reactor that is used for the synthesis of fullerene-like WS2 nanoparticles, under slightly modified operating conditions, a few grams of very long and slender WS2 nanotubes are being synthesized.[12] In another study, a two-step growth process, in which an oxide nanowhisker phase is first obtained and is subsequently converted into WS2 nanotubes phase in H2S

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atmosphere, was demonstrated.[13] Uniform WS2 nanotubes were obtained also by the chemical vapor transport (CVT) technique,[14] in which the WS2 powder is transported from the hot to the cold zones of a quartz ampoule using a transporting agent, such as iodine. Extremely uniform and very long single-wall MoS2 nanotubes with diameter of less than 1 nm were synthesized.[15] These nanotubes all come in the armchair (3,3) configuration. Their perfectness can be also gauged by the fact that they self-assemble into a hierarchy of higher-order structures, from the single-nanotube level to macroscopic crystallites. Interestingly, these nanotubes were obtained via a CVT process using C60 as a catalyst. Nanotubes of the disulfides (and some diselenides) of the transition metals W, Mo, Nb, Ta, Zr, Hf, and Ti were synthesized by high-temperature annealing of the respective metal trisulfides (see, e.g., Ref. [16]). This effort was recently summarized in a comprehensive review article.[17] Double-wall boron nitride (BN) nanotubes of high uniformity were produced via the arc-discharge method.[18] V2O5 nanotubes were synthesized using organic amines as templating molecules in a sol–gel process followed by hydrothermal treatment.[19] Whereas some of the nanotubes are open-ended and have scroll-like shape, others were made of concentric layers and with a closed tip. The inorganic–organic superstructure, obtained from the rather ionic oxide and the aliphatic amine, leads to softening of the layers making them more amenable to folding. The use of ‘‘chimie douce’’ processes for the preparation of new kinds of nanotubes from a number of oxide compounds has recently gained appreciable attention. Thus, scroll-like nanotubules have been obtained from potassium hexaniobate, K4Nb6O17, by acid exchange and careful exfoliation in basic solution.[20] The exfoliation process results in monomolecular layers, which are unstable against folding even at room temperature, and consequently form the more stable scroll-like structures. Moreover, although the binary oxides of these transition metals do not possess the lamellar structure, and consequently they cannot form crystalline nanotubular structures, the reduced oxides of these phases can. A typical example belonging to this category is the compound H2Ti3O7, which is a member of the class of lamellar H2TinO2n + 1 phases. This nanotubular phase was prepared by the usual sol–gel process and subsequent treatment with a concentrated NaOH solution at 130°C.[21] Numerous other strategies were employed for the synthesis of these and other inorganic nanotubes in recent years. These works demonstrate the richness of the chemical apparatus in the context of inorganic nanotubes and fullerene-like nanostructures. Hydrothermal synthesis has recently been used extensively for the synthesis of various nanotubes. Thus, bismuth nanotubes were obtained by hydrothermal treatment of a basic bismuth nitrate solution and hy-

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drazine at 120°C.[22] Tellurium nanotubes were obtained by hydrothermal reduction of Na2TeO3 in ammonia solution at 180°C.[23] The chiral motif of the nanotubes can be attributed to the spiral structure of the trigonal unit cell of tellurium. InS nanotubes were obtained by refluxing tert-butyl indium with H2S gas at 203°C.[24] Interestingly, the layered form of this compound was not known before. Obviously, therefore, the seamless nanotubular structure endows extra kinetic stability to the lamellar phase. Numerous other nanotubular and fullerene-like structures have been reported in the literature over the last few years.

STRUCTURE OF INORGANIC FULLERENE-LIKE NANOPARTICLES C60 is made of hexagons and pentagons only, exhibiting icosahedral symmetry. Neither graphite nor MoS2 have pentagonal elements within their native structure. However, the trigonal network of MoS2 possesses triangles and rectangles (rhombi) as genuine elements of lower symmetry. It was therefore proposed[4] and later on experimentally verified[25] that MoS2 octahedra (bucky octahedra) made of six symmetrically disposed rectangular (rhombohedral) corners can be obtained. Similarly, in one case a MoS2 ‘‘bucky tetrahedron,’’ made of symmetrically disposed four triangles in its corners, was observed. These structures are likely to be the most stable form of IF MoS2 nanoparticles, i.e., the analog of C60 in MoS2. Presently, the synthesis of bucky octahedra from MoS2 and other compounds remains a most challenging task. Recently, rather small IF MoS2 nanoparticles have been prepared by discharging graphite and MoS2 elecrodes embedded in deionized water.[26] Most of the nanoparticles were found to have rectangular (rhombi)-shaped corners, alluding to the great stability of such nanoparticles. Boron nitride nanotubes and fullerene-like nanoparticles were among the early noncarbon systems to be studied. Boron is situated to the left of carbon in the periodic table while nitrogen is to its right. In analogy to carbon, BN exists in two main phases: hexagonal (graphitelike) and cubic (diamond-like). Moreover, the stable form of BN at room temperature is the graphitic (hexagonal) polytype. Therefore, nanoparticles of this phase are expected to be unstable in the planar form and afford a fullerene-like structure. Because B–B or N–N pairs are not favorable nearest neighbors, pentagonal rings cannot form. Instead, the folding can be obtained by introducing six squares into the hexagonal network. The smallest stable fullerene-like structure was calculated to be the B12N12 octaheder,[27] consisting of six hexagonal rings and six squares. Indeed, BN nanooctahedra with

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symmetrically disposed six squares[28] and nanotubes with flat tops having three squares[29] were indeed synthesized by various methods. Using an elastic continuum model, the topology of WS2 (MoS2) cage structures was studied.[5] A first- order phase transition from an evenly curved (quasi-spherical) structure into a polyhedral cage was predicted for nanoparticles with shell thickness larger than about 1/10. The same model foresaw a similar transition for nanoparticles two layers thick and with radius of curvature smaller than 10 nm. A transition of this kind was observed during the synthesis of IF WS2 particles from 50-nm WO3 nanoparticles.[8] The conversion of WO3 nanoparticles to IF WS2 proceeds from the surface of the nanoparticles inward.[8] When the number of WS2 layers is three or less, the nanoparticles appear to be evenly folded and therefore have quasi-spherical shape. However, when the conversion process continues and the thickness of the sulfide shell in the nanoparticles exceeds a few nanometers, i.e., four layers and more, the nanoparticles are transformed into a faceted structure. Further agreement with the above model was obtained in a number of other experimental studies. Fullerene-like MoS2 nanoparticles 3–4 nm in diameter and two to three layers thick, prepared by laser ablation, were found to adopt a highly faceted octahedral shape.[25] Moreover, in accordance with this theory, NbS2 nanoparticles, which have larger Young’s modulus than their WS2 counterparts, are found to be appreciably more faceted.[7] In contrast to the caged fullerene-like nanoparticles, WS2 nanotubes with 20-nm diameters are rarely faceted. Obviously, the elastic strain involved in folding along one axis (nanotubes) is appreciably smaller than that of nanoparticles folded along two axes (fullerene-like nanoparticles). This idea is further vindicated by comparing the stability of phosphorous nanotubes and fullerenelike structures as predicted from ab initio density functional theory tight-binding calculations. While the 1-D nanostructures are found to be perfectly stable,[30] the phosphorous fullerenes are unstable and they decompose into P4 clusters.[31] The ionic character of layered metal chlorides, such as NiCl2 or CdCl2, is appreciably higher than that of the layered metal dichalcogenides. Consequently, the bending modulus of the former compounds is about twice as large as that of the latter compounds. Furthermore, the free slippage of the molecular layers on top of each other is crucial in maintaining low elastic strain during folding of the nanoparticles. However, the shearing energy in the ionic NiCl2 is sixfold larger than for MoS2.[32] Not surprisingly therefore, whereas one-layer-thick fullerene-like nanoparticles of metal dihalides afford either quasispherical or faceted polyhedral structure,[7,32,33] threelayer-thick polyhedra are invariably highly faceted. The most common motif for fullerene-like nanoparticles of

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this family is a hexagonal polyhedron. On the other hand, NiCl2 nanotubes are generally cylindrical.[7,32] The topology of polyhedra made of one layer has been investigated virtually for hundreds of years. Contrarily, the mathematics of polyhedra made of a number of interconnected layers with fixed lattice points in each layer, which is commonplace in IF structures, has been barely studied. This challenging issue is clearly demonstrated in the case of IF CdCl2, where only partial information regarding the structure of the hexagonally shaped polyhedron is available. Furthermore, realistic models of IF structures, like MoS2 octahedra consisting of some 1800 atoms, require highly developed ab initio calculations, which makes this task presently quite intractable. Some progress in this direction was discussed in Ref. [34]. Future developments in handling these mammoth calculations is likely to have tremendous impact on our ability to decipher the structural details and the physical properties of IF structures. In conclusion, the synthesis and elucidation of the structure of IF nanoparticles remains a most challenging task for years to come.

PROPERTIES Early theoretical work was concerned with BN, BC2N, and BC3 nanotubes.[35] More recently, nanotubes of other compounds, such as phosphorous,[30] GaSe,[36] WS2,[37] MoS2,[34] and others, were studied. It follows that nanotubes of semiconducting compounds remain so also after folding. However, the bandgap was found to shrink with decreasing diameter of the nanotubes. Therefore, the bandgap of semiconducting nanotubes can be tuned all the way from the UV spectrum (ca. 3 eV) down to the infrared (ca. 0.2 eV) by varying the diameter of the nanotubes. This behavior stands in sharp contrast with the generic behavior of semiconductor quantum dots, where the bandgap expands with decreasing diameter of the nanoparticles because of the quantum size effect. This opposite shift was experimentally confirmed by measuring the absorption spectrum of IF MoS2 and IF WS2 of different sizes and numbers of shells.[38] More recently, however, I–V curves for single WS2 nanotube was recorded using scanning tunneling microscopy, and consequently the bandgap of the nanotubes could be determined.[39] Fig. 4 shows variation of the normalized bandgap as a function of the nanotube diameter as determined in this study. As expected, shrinkage of the energy gap with the nanotube diameter was observed. This observation was confirmed by first-principle theoretical calculations.[39] The same theory shows[34,37] that armchair nanotubes possess an indirect bandgap for the lowest electronic transition, whereas zigzag nanotubes exhibit a

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Fig. 4 Comparison between theoretically calculated and STM measured normalized bandgap (g/g0) of WS2 nanotubes with varying diameters. g0 is the energy gap of the bulk materials (1.2 eV). Notwithstanding the scattering in the experimental data, the dependence of the bandgap on the nanotube diameter is clearly demonstrated. (From Ref. [33].)

direct transition. This surprising observation suggests that zigzag WS2 (MoS2) nanotubes could show strong optical absorption and luminescence, or even stimulated emission (laser action). First-principle theory also shows that independent of their diameter and chirality, NbS2 nanotubes are metallic,[40] which suggests that they will be very good field emitters. Indeed, the superconducting behavior of NbS2 and NbSe2 nanotubes was recently confirmed.[17,41]

APPLICATIONS 2H-MoS2 (2H-WS2) are used as additives to heavy-duty tribological fluids, as tribological coatings, or as lubricating powders in ultra-high-vacuum instrumentation. The spherical shape of the fullerene-like WS2 nanoparticles and their inert sulfur-terminated surfaces offer enhanced tribological behavior for solid lubricant nanoparticles of this kind. The usefulness of IF powder as a superior solid lubricant has been worked out through a long series of experiments in the laboratory of Rapoport.[42] Further experiments show that IF WS2 and IF MoS2 powders exhibit superior tribological applications in different modalities,[43,44] suggesting almost countless number of possible applications. Fig. 5 shows the results of a

tribological test in which bronze–graphite partly prepared by powder metallurgy and impregnated with 5% IF WS2 outperforms the nonimpregnated part. This work has stimulated substantial commercial interest recently. Recently, substantial interest has been paid to metal and hydrogen intercalation in inorganic nanotubes of various kinds. Thus, Li intercalation and deintercalation in V2O5 and MnxV2O5 +x nanotubes were studied. Potassium and sodium intercalation in MoS2 (WS2) fullerene-like nanoparticles was also investigated. Reversible hydrogen[45] and lithium[46] intercalation in MoS2 nanotubes was demonstrated. The charge/discharge cycles were found to be reversible and relatively facile, which can be attributed to the high surface area and the open tips of the nanotubes. The reversibility of these systems can be attributed to the perfect crystallinity of the nanotubes, allowing one to accomplish numerous charge/discharge cycles of the rechargeable electrode without losing its loading capacity. Catalytic conversion of CO +H2 into methane and water using MoS2 nanotubes as catalyst was recently demonstrated.[47] The mechanism of the catalytic action of the nanotubes is quite a puzzle, because the fully bonded sulfur atoms in the van der Waals surfaces and in the galaries between the MoS2 layers (van der Waals gap) of the nanotubes are not expected to be chemically very reactive. This surprising result is nevertheless promising

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Fig. 5 Friction coefficient as a function of load (in newtons) for a heavy -duty grease. 2H is the curve for a grease mixed with 2HMoS2 platelets, while IF is the curve for the same grease mixed with 5% fullerene-like WS2 nanoparticles. (View this art in color at www.dekker.com.)

to incite a new research effort into the catalytic properties of inorganic nanotubes.

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CONCLUSION The advent of nanotubes and fullerene-like particles from inorganic layered compounds has opened new avenues in the solid-state chemistry of inorganic compounds and new opportunities for application of such nanostructures in the emerging field of nanotechnology as well as in numerous other areas. Fundamental questions remain to be solved to permit a more judicious approach to the synthesis and study of the properties of these new nanophase materials. The elucidation of the detailed structure of IF nanoparticles, which is a most demanding and important issue in this field, is progressing by combining theory and experiment.

ACKNOWLEDGMENTS I am grateful to Dr. Y. Feldman, Dr. A. Zak, A. Margolin, Dr. R. Rosentsveig, Dr. Y. Rosenfeld Hacohen, and Dr. R. Popovitz-Biro, all from the Weizmann Institute of Science; Prof. L. Rapoport (Holon Academic Institute of Technology); and Prof. G. Seifert (TU Dresden). Support of the Israeli Ministry of Science (Tashtiot), Israeli Academy of Sciences (First), the Israel Science Foundation, and Minerva Foundation (Munich) are greatly acknowledged.

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