Inorganic Nanotubes Synthesized by Chemical Transport Reactions
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Nanotubes of transition metal dichalcogenides have raised much scientific attention since the first report on the synthesis and structure of MoS2 and WS2 nanotubes in 1992. The two most efficient methods producing tenths of grams of nanotubes consist in annealing the oxidized transition metal films or particles in a stream of H2S gas or in thermal decomposition of (NH4)2MoS4 or MoS3 in hydrogen. Other growth techniques with lower efficiencies make use of electron beam irradiation in transmission electron microscope and in scanning tunneling microscope, sonoelectrochemical bath reaction, and electrochemical deposition from ethylene glycol solution.
The nanotubes of other transition metal dichalcogenides (NbS2, TaS2) have been recently synthesized. The reduction of NbS3 and TaS3 powder in a stream of H2 at 1000°C for 30–60 min led to the growth of thick-walled NbS2 and TaS2 nanotubes with inside diameters ranging from 4 to 15 nm. The silver alloyed NbS2 nanotubes have been grown by partial decomposition of the Ag-(NbS4)xI (x ≈ 4) precursor crystals using electron beam irradiation or microwave irradiation. They represent the first case of nanotubes grown by self-assembly of nanocrystallites.
The MoS2 and WS2 nanotubes were also synthesized by the chemical transport reaction, which is the standard method for the growth of transition metal dichalcogenide-layered crystals. In the present report, the overview of growth mechanisms, their structural properties, and self-assembly at different range scales are presented. Besides pure MoS2 and WS2 nanotubes, the alloyed nanotubes with silver and gold are shown, evidencing the geometrical stabilization of new compounds, which are not known in the usual plane geometry. The subnanometer-diameter MoS2 − x nanotubes, which self-organize to the first case of molecular crystals composed of inorganic nanotubes, represent the extreme case of inorganic nanotubes. Because of their metallic conductivity, we can rank them to the promising family of molecular wires.