Nanostructures Based on Layered Transition Metal Chalcogenides
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The discovery of carbon nanotubes by Ijima initiated a new area of materials science. Following Ijima's discovery, Tenne et al. later found layered sulfides (MoS2 and WS2) to form nanotubes and other fullerene-type structures. Nanoparticles based on layered transition metal sulfides (LTMS) are of special interest because of their diverse applications. Layered transition metal sulfides have been extensively studied because of their crucial role in the petroleum and chemical industry. MoS2-based and WS2-based catalysts have been used for sulfur and nitrogen removal from petroleum feedstock. In addition, MoS2 is used as a lubricant additive. Related compounds of TiS2, as well as MoS2, can work as cathodes in lithium nonaqueous batteries and possess interesting and useful intercalation chemistry. WS2, WSe2, TiS2, MoS2, MoSe2, and MoTe2 are all semiconductors with unusual properties and potential electronic applications.
The structure of MoS2, WS2, and ReS2 consists of a two-dimensional layered closed-packed sulfur arrangement with a transition metal sandwiched between each layer. Weak van der Waals interlayer forces occur between stacked “sandwiches” that allow easy, low-strength shearing. These materials are highly folded and distorted in nature, forming commonly known “rag” and “tubular” structures. Layered transition metal sulfide materials appear as morphological analogs of fullerenes exhibiting structures described as inorganic fullerenes (IFs), single sheets, folded sheets, nanocrystals, and nested IFs, also known as “onion crystals” or “Russian dolls.”
Fullerene-related nanoparticles of MoS2 can be obtained by electron beam irradiation, laser ablation of MoS2 bulk powder, and arc discharge. Scanning tunneling microscope electrical pulses over amorphous MoS3 nanoparticles lead to the formation of closed MoS2 shell IF (a few layers thick) and amorphous MoS3. Inorganic fullerene nanoparticles have been synthesized recently by a variety of methods. Others have reported the gas-phase synthesis of fullerene-like structures of WS2 and MoS2 and nested fullerenes of MoS2. Complex WS2 nanostructures have been obtained by template growth by Whitby et al. The preparation and optical characterization of crystalline nanoparticles prepared by intercalation and ultrasonic fragmentation of bulk crystals have also been studied. Divigalpitiya et al. have reported the synthesis and preliminary characterization of composites based on single sheets of MoS2. Wilcoxon et al. Wilcoxon and Samara, and Parsapour et al. have reported the growth of MoS2 nanoparticles using micelle techniques.