Perspective pubs.acs.org/JACS
Inorganic Nanotubes and Fullerene-like Nanoparticles at the Crossroads between Solid-State Chemistry and Nanotechnology Bojana Višić,*,† Leela Srinivas Panchakarla,‡ and Reshef Tenne*,† †
Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel Department of Chemistry, IIT Bombay, Mumbai 400076, India
‡
the adjacent layers cannot be tightly bonded; namely, they are stacked together via weak van der Waals forces. This mode of packing explained their large anisotropy with respect to many physiochemical properties and the facile cleavage along the basal (0001) plane. The structure of chloritesnaturally occurring 2D aluminum (alumo) and magnesium silicates, which were employed for asbestos cementwas studied in 1930 by both Warren and Bragg3 and Pauling4 using XRD. By analyzing the detailed packing of these 2D materials, Pauling noticed that some of the magnesium silicates (like chrysotile) and alumosilicates (such as kaolinite, Al2Si2O5(OH)4) possess an asymmetric structure along the c-axis, while others (including brucite and mica) are symmetric.4 Consequently, he surmised that the top silica layer is under tensile strain, whereas the bottom alumina layer is under compression strain. This observation led him to propose that asymmetric layered compounds like chrysotile and kaolinite will bend and form folded structures or, in contemporary language, “nanoscrolls” (nanotubes (NTs)). Chrysotile NTs were first observed using transmission electron microscopy (TEM) by Turkevich and Hillier5 and Bates et al.6 Ultimately, electron diffraction (ED)7 and high-resolution transmission electron microscopy (HRTEM)8 were used to elucidate the detailed structure of chrysotile NTs (nanoscrolls). In his original work, Pauling went further to advocate the idea that layered compounds with symmetric structure along the caxis, such as brucite, mica, molybdenite (MoS2), cadmium chloride (CdCl2), etc., are not expected to fold and hence form NTs.4 The landmark discovery of C60 in 1985 by Kroto, Smalley, Curl, and their co-workers established a new paradigm in nanosciences.9 This supramolecule was the first example of a stable closed-cage nanostructure derived from graphitic carbon. Kroto reasoned that flat graphite nanoclusters cannot tolerate the large chemical energy stored in the dangling bonds of the rim atoms of a small graphene sheet.10 Like many other layered (2D) compounds, bulk graphite consists of molecular slabs (graphene) stacked together via weak van der Waals interactions. In the honeycomb lattice of graphene, each carbon atom is bonded to three neighboring carbon atoms (sp2 bonds). However, the rim atoms of graphite are only two-foldbonded, each one possessing a dangling bond pointing outward. In bulk graphite, the number of surface atoms is outnumbered by the three-fold-bonded “bulk” atoms. Therefore, the relative chemical energy stored by the rim atoms is negligibly small. This situation is reversed in nano-sized
ABSTRACT: Inorganic nanotubes (NTs) and fullerenelike nanoparticles (NPs) of WS2 were discovered some 25 years ago and are produced now on a commercial scale for various applications. This Perspective provides a brief description of recent progress in this scientific discipline. The conceptual evolution leading to the discovery of these NTs and NPs is briefly discussed. Subsequently, recent progress in the synthesis of such NPs from a variety of inorganic compounds with layered (2D) structure is described. In particular, we discuss the synthesis of NTs from chalcogenide- and oxide-based ternary misfit layered compounds, as well as their structure and different growth mechanisms. Next we deliberate on the mechanical, optical, electrical, and electromechanical properties, which delineate them from their bulk counterparts and also from their graphene-like analogues. Here, different experiments with individual NTs coupled with firstprinciples and molecular dynamics calculations demonstrate the unique physical nature of these quasi-1D nanostructures. Finally, the various applications of the fullerene-like NPs of WS2 and NTs formed therefrom are deliberated. Foremost among the possibilities are their extensive uses as superior solid lubricants. Combined with their nontoxicity and their facile dispersion, these NTs, with an ultimate strength of about 20 GPa, are likely to find numerous applications in reinforcing polymers, adhesives, textiles, medical devices, metallic alloys, and even concrete. Other potential applications in energyharvesting and catalysis are discussed in brief.
1. PREFACE Shortly after X-ray diffraction (XRD) was discovered, it was used to investigate the structure of different inorganic crystals, including inorganic layered (two-dimensional (2D)) compounds, like CdI2 and MoS2. While Bozorth1 studied the structure of CdI2, Dickinson and Pauling2 elucidated the layered structure of MoS2. In both 2D compounds, a layer of metal atoms is sandwiched between two layers of anions. Each metal atom is six-fold-bonded, i.e., three anion atoms each in the top and bottom layers. However, in the case of CdI2, the six iodine atoms surround the cadmium atom in octahedral coordination (symmetry group R3m), while in MoS2 the six sulfur atoms are linked to the molybdenum atom in trigonal biprismatic coordination (the P63/mnn group). The unit cell is made of two MoS2 units arranged in hexagonal symmetry (2H), i.e., in total Mo2S6. Those authors found that, in both compounds, the c-axis is larger than 6 Å, and consequently © 2017 American Chemical Society
Received: February 16, 2017 Published: August 4, 2017 12865
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reinforcement, etc. For instance, it was recently demonstrated that lubricants formulated with small amounts of Re-doped IFMoS2 NPs exhibit superior tribological characteristics, not only with respect to bulk (2H) MoS2 particles but also compared to undoped IFs.23,24 This advantageous behavior of the doped NPs could be useful in variety of medical technologies, as coatings for catheters, endoscopes, stents, etc.25 These kinds of applications can also benefit from the fact that the IFs/INTs were recently found to exhibit much lower cytotoxicity than other NPs, such as silica, carbon black, and many others.26−28 Recent progress in the high-temperature syntheses of WS2 nanotubes (INT-WS2) and its scale-up has permitted detailed studies of their properties and their various applications.29 Most intriguingly, quasi-1D superconductivity of these NTs at low temperatures, gated by ionic liquids, was recently demonstrated.30 Oscillations of the supercurrent akin to the Little− Parks mechanism were recorded when a magnetic field was applied along the tube axis. Moreover, the second harmonic of the AC supercurrent showed non-reciprocal behavior, reflecting the breaking of inversion symmetry due to the chiral nature of the NT. An enticing application of such NTs in concrete reinforcement was also recently reported.31 A hydrothermal synthesis of single-wall INTs, like those of MoO332 and phosphate,33 was also recently described. In analogy to graphene-like (single-layer) MoS2, such single-wall NTs could have optoelectronic properties entirely different from those of multi-wall INTs. While NTs from binary layered compounds have been studied extensively, the synthesis and properties of NTs and fullerene-like NPs from ternary and quaternary compounds remain a relatively unexplored field. However, ternary and quaternary misfit layered compounds (MLCs) have been studied for quite some time now.34,35 In particular, MLCs of the type MX-TX2, where M = Sn, Pb, Sb, Bi, Ln (Ln = lanthanide atoms); T = Nb, Te, Cr, V, etc.; and X = S, Se, Ta, were investigated in the past. One way to visualize the MLC compounds is as an intercalation compound where a molecular (001) slab of a distorted cubic crystal, such as PbS, is intercalated between each two layers of a (layered) compound, like TaS2. The stability of the MLC structure benefits, in addition to the weak van der Waals forces, from electrostatic interactions induced by partial charge transfer between the host and guest. Alternatively, MLCs can be regarded as repeating sequences of (001) layers of the MX compound (O) with distorted rocksalt structure and hexagonal TX2 (T) layer. Thus, a superstructure with the sequence O-T, or even more complex ones, such as O-T-O-T-T, are formed. A large number of MLC NTs were recently synthesized and their structures elucidated in some detail.36,37 This important class of NTs are discussed in greater detail in section 2. This Perspective is dedicated to the field of NTs and fullerene-like nanostructures from inorganic layered (2D) compounds. Some of the important findings in this field in recent years are highlighted. Finally, the expanding range of applications of these nanostructures are discussed in brief, and challenges and future directions in this field are presented.
graphite, where two-fold-bonded rim atoms are abundant. This extra energy elicits instability of the graphitic nanocluster, forcing the carbon atoms to spontaneously reorganize into hollow-cage seamless structures containing 60 carbon atoms each (C60). By introducing 12 disjoint pentagons into the otherwise hexagonal network, the graphene nanocluster folds along two axes, thereby forming the (0D) fullerenes. With 20 hexagons and 12 pentagons, all the carbon atoms in C60 become three-fold-bonded via distorted sp2 bonds. The elastic energy of the distorted bonds is more than compensated by seaming the dangling bonds in the closed-cage fullerenes. However, due to the large distortion of the curved sp2 bond, the fullerenes lose their aromaticity. The C60 molecule was the first case in a series of fullerenes, highlighting the inherent structural instability of layered structures in the nano-size range. In 1991, Iijima11 proposed that flat graphene nanoribbons are also unstable and spontaneously transform into carbon nanotubes (CNTs) under appropriate conditions. To minimize the energy of the dangling bonds at the edges of the tube, they are sealed by hemispherical fullerenes containing six pentagons each. In the case of NTs, the graphitic sheet is folded along a single axis only, which costs less elastic energy, allowing them to accommodate a larger radius of curvature (smaller diameter) compared to the fullerenes. Thus, the instability of graphitic nanoclusters is manifested in two closed-cage generic forms, i.e., (0D) fullerenes and (1D) NTs. The next question was if this inherent instability is unique to carbon or there is a larger class of materials which can accommodate these hollow closed nanostructures. Inorganic layered (2D) materials are abundant among elements (such as B, black phosphorus), binary compounds (such as MoS2 and boron nitride (BN)), and numerous ternary compounds (vide inf ra). While Mo is six-fold-bonded within the MoS2 layer, the atoms are only four-fold-bonded at the rim of the slab. Similarly, rim sulfur atoms are only two-fold-bonded instead of being three-fold-bonded as in bulk MoS2. Inspired by the analogy between graphite and inorganic layered compounds and the discovery of closed-cage carbon nanoparticles (NPs), Tenne et al. proposed in 1992 that such inorganic layered compounds will suffer the same kind of instability and form closed-cage nanostructures.12,13 Here, the extra stored energy in the rim atoms leads to their folding and seaming into closedcage (hollow) nanostructures, which were designated as inorganic fullerene-like nanoparticles (IFs) and inorganic nanotubes (INTs). Prior to the above work, several publications reporting rolled MoS2 sheets14 analogous to nanoscrolls or closed-shell MoS2 NPs15 appeared in the literature. However, no systematic investigation of these nanostructures was pursued any further in the ensuing years. Interest in research into these layered materials plummeted with the reports of their ability to form single layers.16 While the scope of this Perspective is aimed toward the INTs and IFs, advances in the field of 2D counterparts of these compounds are enormous.17 Nevertheless, the IFs/INTs continued to be studied thoroughly. Over the years, numerous experimental and theoretical studies were dedicated to elucidating the structure, growth mechanism, and properties of the IF/INT phases. Much of this work was reviewed in recent years18−22 and will not be reiterated here. In the present Perspective, the most recent progress in this field will be elaborated. Numerous potential applications have been proposed for these NPs, such as in nanoelectronics, sensors, energy research, tribology, polymer
2. SYNTHESIS Doping of inorganic fullerene-like nanoparticles (IFs) and the synthesis of few-wall IFs and inorganic nanotubes (INTs) are among the important accomplishments in this field. Realization of IFs and INTs from ternary and quaternary misfit layered compounds (MLCs) has been another direction of major 12866
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Journal of the American Chemical Society importance in this field. This section briefly describes the different synthetic strategies developed to prepare and characterize these novel nanomaterials. Fullerene-like NPs and NTs of WS2 were investigated quite intensively since the first report in 1992.12 Most of the synthetic techniques developed so far yield large-diameter, multi-wall NTs. On the other hand, methods for the synthesis of smalldiameter NTs and single-wall fullerene-like NPs are rather rare, mainly due to the high elastic energy budget and consequently the great difficulty in preparing small-diameter NTs.38 Promising optical and optoelectronic properties could be anticipated for single-wall NTs, as they become direct bandgap semiconductors in zigzag (n,0) configuration. Recently, the synthesis of WS2 NTs with small diameter (5−7 nm) and very small number of layers (1−3 layers) was accomplished by inductively coupled radiofrequency plasma treatment of multiwall WS2 NTs.38 The exotic condition (electron temperature of 104 K) generated by this technique helps to overcome the high strain of small-diameter NTs. Interaction of the plasma with either point or line defects on the surface of the multi-wall tubes leads to unzipping of their outermost nanosheets. The freshly unzipped nanosheets are believed to release their inherent elastic energy by the inverted umbrella effect (analogous to “Walden inversion” of the SN2 reaction in organic chemistry), resulting in small-diameter “daughter” NTs. Typical TEM images of few-layer (daughter) WS2 NTs synthesized by this technique are shown in Figure 1a,b. The
ambient conditions, it offers an environmentally friendly approach to the synthesis of these nanostructures. Although multi-wall WS2 NTs have been produced in relatively large amounts, synthetic strategies for producing bulk quantities of MoS2 NTs were lacking, until recently. MoS2 is appreciably lighter than WS2, and its Young’s modulus is some 40% larger. Therefore, MoS2 NTs could be potentially useful for reinforcing structural nanocomposites, as well is in rechargeable lithium batteries, etc. Recent progress in the realization of bulk synthesis of MoS2 NT has been reported by a number of groups, which paves the way for a systematic investigation of their properties as well as exploring possible applications.41−43 Pristine single-layer fullerenes have not yet been realized. Nonetheless, two- to three-layer nano-octahedra of MoS2 have been studied and are considered to be the smallest closed-cage moieties, i.e., analogues of C60, in that compound. A recent first-principles calculation of single-layer (hollow) nanooctahedra of different compounds containing 6 four-membered rings concluded that these nanostructures are more stable than the closed-cage icosahedra with 12 five-membered rings.44 In contrast to the five-membered rings, the four-membered rings do not force bonding between adjacent like elements. Core−shell NPs where multi-wall45 (Au@MoS2) and singlelayer46 MoS2 (Au@1L-MoS2) coat the gold NPs conformably were recently reported. The physical and chemical properties of these hybrid NPs are distinct from those of each of its constituents. Single-layer MoS2 nanosheets exhibit considerable stability in the ambient and are biocompatible.47 Therefore, the hybrid core−shell Au@1L-MoS2 NPs could endow extra stability to the reactive Au NPs and simultaneously promote their catalytic reactivity. The composite core−shell Au@1LMoS2 (WS2) was synthesized in three steps. First, Au NPs were formed in the solution by reducing chloroauric solution. Subsequently, the Au NPs were reacted in situ with ammonium thiomolybdate, forming a conformal coating of thiomolybdate (MoS42−) on top of the Au NPs. In the third step, the NPs, which were isolated from the solution and vacuum-dried, were heated in a quartz ampule along a temperature gradient of 88 °C (516−428 °C).46 Careful control of the reaction parameters, such as temperature, time, and ratio of Au to MoS42−, is imperative to yield the conformal coating of a single MoS2 layer on top of the Au NPs. The Au@1L-MoS2 were found to exhibit optical extinction entirely different from that of either constituent. The surface plasmon peak of the Au NP is extinguished, and a strong extinction in the near-IR is observed. Further investigation is needed in order to elucidate the physiochemical properties of these hybrid NPs and their possible applications in catalysishyperthermia, cancer treatment, etc. WS2/MoS2 fullerenes have been proven to be good solidstate lubricant additives that yielded different commercial products (see section 5, “Applications”). However, inherent agglomeration of the NPs reduces its tribological efficacy. To address this problem, WS2/MoS2 IFs have been doped with minute amounts of rhenium atoms and more recently with niobium. Substituting Mo by Re (ReMo) atom, having one extra electron, induces negative charge on the NP and oppositely so for the NbMo. The reaction of H2S with Re-doped MoO3‑x yields Re-doped IF-MoS2 NPs with 0.002−0.7 at.% (20−7000 ppm) rhenium.24 Re doping in IFs/INTs could also be achieved by high temperature heating of MoS2 IFs/INTs with ReO3 (ReCl3).24 X-ray absorption fine structure (XAFS) measurements confirmed the substitutional Re doping (ReMo)
Figure 1. HRTEM images of formation of (a) a two-layer daughter nanoscroll on a multi-wall WS2 nanotube [(I) single-layer scrolling of exfoliated sheet and (II) two and (III) three layers before the formation of nanotube by plasma treatment] and (b) isolated daughter nanotubes. (c) Schematic mechanism of formation of daughter nanotubes by plasma treatment. Adapted with permission from ref 38. Copyright 2014 American Chemical Society.
proposed reaction mechanism of their formation is schematically illustrated in Figure 1c. Alternatively, single- to few-layer NTs of WS2 were obtained by high-temperature sulfurization of narrow WO3 nanowhisker precursors.39 In addition to plasma treatment, the short-pulse laser synthesis of IF-MoS2 and INTWS2 was reported recently.40 As this method does not require the presence of hazardous precursor gases and is performed in 12867
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X = chalcogen atom (S, Se, Te). The MX layer could be thought of as two atom thick, half a unit cell of, distorted NaCl structure whereas the three-atom thick TX2 adopts pseudohexagonal structure.36 The structure of chalcogenide MLC is schematically represented in Figure 2a. Generally, the b- and c-
of the IFs/INTs. HRTEM in the high-angle annular dark-field (HAADF) mode helped direct visualization of the substitutional Re atoms in the MoS2 lattice. Density-functional-based tight-binding (DFTB) calculations showed that the energy of the Re atoms is some 150 meV below the conduction band edge at ambient conditions, i.e., only 1−2% of the Re atoms are ionized. Furthermore, these calculations concluded that the Redoping induces a local 2H to 1T-polytype transformation in close vicinity to the Re atom.48 Since the 1T polytype is semimetallic, this local transformation could lead to the release of a large number of extra free carriers in the NPs, which is substantially larger than the nominal free electrons produced by the ionized dopant atoms. It was shown that the Re-substituted NP acquire negative charge on their surface, becoming thereby self-repelling (Re). Therefore, the (Re) doped NP produce relatively stable suspensions and exhibit appreciably slower agglomeration and sedimentation compared to the undoped IF NP.24 These results suggest also that the doped NPs are more immune to the electrostatic charges that form during the tribolocial testing (tribo-charging). Lubricating fluids containing these NPs exhibited extremely low friction and wear rendering them suitable for variety of medical and other applications. Nb-doped IF-MoS 2 NPs have also been synthesized by high-temperature (∼800 °C) annealing of Nb−Mo oxide in the presence of H2S.49 The exquisite control of the dopant concentrations (50 nm) is appreciably smaller and increases evenly with their diameter. Molecular dynamics calculations showed that, while water molecules do not wet the outer surface of the NTs, CCl4 does. It was concluded that strong capillary forces induce water take-up into the hollow stem of the narrow tubes, leading to high pull-out forces (>1 GPa) up to the point of breaking the slenderest NTs during pull-out.
their carbon counterparts and can be more easily dispersed in various suspensions. In analogy to CNTs, the one-layer molecular structure of BN NTs renders them both flexible and mechanically strong. The W−S bond in WS2 NTs is appreciably weaker than the C−C bond. It is no wonder that the Young’s modulus is about 6 times smaller (160 vs 1200 GPa). However, their relative structural perfectness makes them very strong (6−21 GPa).75,76 Furthermore, being made of three-layer S−W−S structure, they cannot be easily bent, are not flexible, and are always straight, preventing their entanglement. They can therefore be easily dispersed in different polymers and other matrices. Combined with their nontoxic nature, this offers WS2 NTs a plethora of potential applications in diverse technologies, not least among them are for medical devices like prostheses. Titanate NTs are produced by low-temperature solution-based synthesis. Therefore, their structure is not as perfect, and they are mechanically not as robust as NTs produced via high-temperature solid-state processes. Nonetheless, they can serve in various applications where the mechanical strength is not critical (see, e.g., refs 77 and 78). Tensile measurements of individual WS2 NTs allowed a direct comparison with ab initio calculations.75 Using such calculations, the Young’s modulus of different single-wall metal dichalcogenide NTs was studied.79 The authors found that, in analogy to the bulk material, the Young’s modulus of the NTs decreases in the order sulfides, selenides, and tellurides. Furthermore, molybdenum dichalcogenide NTs exhibit higher Young’s modulus compared with their respective tungsten counterparts. A concise summary of the mechanical measurements of individual multi-wall BN NTs by in situ TEM measurements was given in ref 80. These measurements yielded Young’s modulus and ultimate tensile strength of 924 and 18.8 GPa, respectively, for a typical BN NT. More recently, the same group studied the bending of slender (1200 MPa. Such NTs could find applications in toughening structural ceramics, e.g., in armor and ultra-high strength metallic alloys. Torsion measurements of individual WS282 and BN73 NTs were undertaken, offering a direct link between their morphology and detailed structure. Beyond a threshold torsion angle, the outermost wall of the WS2 NTs followed the applied torque, while the inner walls relaxed, revealing a stick−slip mechanism. The BN NTs, on the other hand, were much stiffer and revealed a much stronger mechanical interlayer coupling than either CNTs or WS2 NTs. This behavior was attributed to the interlocking of the faceted layers in BN NTs, which arises from the polarity of the B−N bond. The ramifications of these findings on potential electromechanical torsion devices based on such NTs are discussed in section 4.4. Most structural elements, like concrete, polymers, metallic, and even ceramic components, are processed and shaped via a viscous liquefied state. The proper dispersion of minute amounts of NPs within the viscous fluid is critical for imparting superior mechanical properties to the matrix. Therefore, the wetting behavior of the NTs in various solvents, polymer blends, sols, and molten alloys is of great scientific and technological interest. Recently, the wetting of individual WS2 NTs by water and CCl4 was investigated.83 It was found that open-ended NTs of small diameter (
