Russell R. Chianelli
Department of Chemistry
University of Texas at El Paso
El Paso, TX 79968-0513
As demonstrated in this workshop, the science of nanoparticles and nanophase materials has blossomed over the past fifteen years and is providing the seeds for new applications in optoelectronic and chemical technologies. Nanoparticles based on layered transition metal chalcogenides (LTMCs) have been important in the field of catalysis and lubrication, but it has been only recently recognized that these materials appear as morphological analogs of fullerenes exhibiting structures described as inorganic fullerenes, nested inorganic fullerenes, single sheets, folded sheets, and nanocrystals. Reports of these structures and synthetic approaches to them now regularly appear in the literature, but relatively little has been reported on their fundamental properties and the chemical origins of the structures.
Layered transition metal chalcogenides have found a remarkably diverse set of applications. MoS2 and WS2 catalysts have been used for the removal of sulfur and nitrogen from petroleum feedstocks for over 50 years (Weiser and Landa 1973; Daage and Chianelli 1994). MoS2 is used as a lubricant additive (Fleishauer 1987). TiS2 and MoS2 can act as cathodes in lithium nonaqueous batteries (Whittingham 1976; Tributsch 1981) and have interesting and useful intercalation chemistry (Whittingham and Jacobson 1982). WS2, WSe2, TiS2, MoS2, MoSe2, and MoTe2 are all semiconductors with unusual properties and potential electronic applications (Wilson and Yoffe 1969).
Most of these unusual and useful properties arise from the highly anisotropic physical properties that are due to weak binding between layers where a layer consists of a monolayer of metal atoms clad by covalently-bonded chalcogen atoms. For example, weak binding between layers leads to the useful lubrication properties. Weak bonding between layers also leads to nearly two-dimensional confinement of optically excited electrons and holes, which in turn leads to excitonic optical absorption peaks that can be observed at room temperature. The variety of transition metals (Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) that exist in layered structures, coupled with the choice of chalcogen (S, Se, Te) leads to a rich variability in the physics and chemistry.
LTMCs with different nanoscale structure have been synthesized recently by a variety of approaches. Tenne and collaborators (1992) reported the gas-phase synthesis of fullerene-like structures of WS2 and of fullerenes and nested fullerenes of MoS2 (Feldman et al. 1995). Persans and others (Lu et al. 1990; Persans et al. 1990; Chianelli et al. 1995; Roxlo et al. 1987) have reported on the preparation and optical characterization of crystalline nanoparticles prepared by intercalation and ultrasonic fragmentation of bulk crystals. Divigalpitiya, Frindt, and Morrison (1989; 1990; 1991) have reported the synthesis and preliminary characterization of composites based on single sheets of MoS2. Wilcoxon and collaborators have reported the growth of MoS2 nanoparticles using micelle techniques (Wilcoxon et al. 1995; Wilcoxon and Samara 1995; Parsapour and Wilcoxon 1996). These structures are related to one another and to the well-known "poorly-crystalline" and "rag-like" layered transition metal sulfides (Chianelli 1982; 1984; Chianelli et al. 1985).
Recently, synthesis of these materials has focused on high temperature methods that occur above 650°C. These methods involve such techniques as growth from the gas phase in which MoO3 in the vapor phase is reacted with H2S in a carrier gas producing nested fullerenes and nanotubes designated as inorganic fullerenes (Feldman et al. 1995). These methods suffer from the difficulty of producing large amounts of materials.
A general method for preparing the transition metal dichalcogenides at or near ambient temperatures was reported by Chianelli and Dines (1978). The materials thus produced have physical properties significantly different from those produced at higher temperatures. By appropriate adjustment of parameters, poorly crystalline or amorphous powders, gels, glasses, or homogeneous dispersions of chalcogenides can be prepared. Additionally, normally crystalline compounds can be prepared, and because the preparations take place below 400°C, portions of the transition metal-sulfur phase diagrams not previously studied are accessible.
A consequence of the two-dimensional macromolecular nature of MoS2 and other layered transition metal chalcogenides such as WS2 and ReS2 is the existence of highly folded and disordered structures. These structures have been recognized for a long time. "Rag" and "tubular" structures were first reported by Chianelli et al. (1979) and were studied for their usefulness as catalytic materials.
Figure 7.1 shows folded and disordered sheets of MoS2. Rolled sheets of MoS2 can also be seen which at lower magnification give the appearance of "crystalline needles." These are undoubtedly nanotubes, as has been reported recently (Remskar et al. 1996). Interest in these structures has been renewed with the development and importance of carbon fullerenes and the discovery of large inorganic fullerene structures (M.S. and G. Dresselhaus 1995). Figure 7.2 shows some of the many forms which have been prepared using high temperature methods and which are currently under study for potential application.
Recently, transition metal trichalcogenides have been synthesized at high temperatures in a variety of shapes. The trichalcogenides such as NbSe3, TiS3, and others have layered as well as chain-like structures (Trumbore and ter Haar 1989). These objects exhibit unusual morphologies at larger than atomic scales; Figure 7.3 shows an example. Morphologies observed include "wheels," "tubes," "rods," and numerous other shapes.
Fig. 7.2. Tungsten disulfide forms (Tenne et al. 1992, 444).
Fig. 7.3. Structure of trichalcogenides -- NbSe3 shown (Trumbore and ter Haar 1989, 490).
The ability to synthesize shapes such as these from materials that can be metals, magnets, semiconductors, and insulators have led to speculation regarding the construction of micromachines. For example, the "wheel" shown above if magnetic might be incorporated into a microelectric motor. In another example, a microbattery might be constructed based on the intercalating abilities of the LTMC. Fabrication seems feasible, but reproducible synthesis in large uniform quantities remains a problem; however, possibilities for micromachine devices seem extensive.
It has been suggested that folded and disordered MoS2 structures are similar in nature to large fullerene structures. The cause of the ability of MoS2 and related compounds to form these structures remains unclear and is the subject of intense study. The study of these materials has been hindered by the inability to synthesize large quantities of the material. This situation is similar to early days of research in carbon fullerenes, which progressed slowly until improved synthetic methods led to larger quantities being available. Speculation on the cause of folding and curvature in these falls into three categories:
Heterogeneous catalysts used in the petroleum refining and chemical industries are obvious examples of widely applied nanoparticles. Traditionally, industry has relied successfully on empirical methods for synthesizing these materials but has not applied understanding of nanoparticles to make further improvements. MoS2 may be used as an example to show how understanding of nanoparticles is leading to improvement of catalytic activity and selectivity.
Catalysts based on MoS2 have been widely used in the petroleum refining industry since before World War II. Usually the MoS2 is mixed with a second component such as Co or Ni to improve activity (Weiser and Landa 1973). These very successful catalysts were largely discovered and developed empirically; recently, much progress has been made in understanding their catalytic properties from a nanoparticulate point of view (Chianelli, Daage, and Ledoux 1994).
Several issues have become clear and are probably generally applicable to other heterogeneous catalysts:
The industrially important hydrodesulfurization (HDS) reaction indicated in Figure 7.5 occurs along two pathways that take place on different parts of the crystal.
The first pathway leading to cyclohexylbenzene occurs only on the "rim" sites, and the second pathway leading to biphenyl occurs on the "rim" and "edge" sites. Thus, the aspect ratio of the nanoparticles must be controlled to control the selectivity of the reaction, and the diameter of the crystallite must be made as small as possible to maximize the activity. All this must be accomplished with as much crystalline order as possible being retained.
It is a further challenge to synthesize these complex nanophase materials in an optimum form in commercial quantities.
Nanoparticles are now being recognized as playing a potentially important role in the complex physical and chemical processes that occur above heavily polluted cities (Yacaman and Chianelli 1997). Atmospheric aerosols occurring in these areas are found to be complex materials that have the potential to accelerate important ozone-forming reactions both photocatalytically and thermocatalytically. In addition, because the particles are respirable, they represent a considerable health hazard. The aerosols consist of two intermixed components: the first consists of amorphous carbonaceous materials of variable composition with "fullerene-like" materials dispersed throughout; the second is an inorganic material consisting of nanoparticles of oxides and sulfides "supported" on clay minerals. This inorganic component has all of the characteristics of an airborne photocatalyst. Nanoparticles of Fe203, MnO2, and FeS2 have demonstrated catalytic properties, particularly when occurring in the nanoparticle range as they do in the subject aerosol materials. These materials have band-gaps that occur in the broad solar spectrum, enhancing the photocatalytic adsorption of solar radiation beyond that of the wider band-gap aluminosilicate and titanate materials that also occur in the aerosols. In addition, the materials are acidic and probably are coated with moisture when suspended in air, further enhancing the catalytic ability to crack hydrocarbons and create free radicals. Though this area is just being studied, nanoparticles appear to play an important but as yet undetermined environmental role.
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