Singular tungsten disulfide core-shell and pure tungsten nanostructures

Abstract

The first potentially scalable synthesis of core-shell tungsten disulfide (WS) nano-onions comprising a pure-tungsten core encased in WS shells (W@WS) is reported. Although W@WS nano-onions were achieved in earlier studies, all methods engendered practical drawbacks such as the need for toxic reagents, inherently customized small-scale processes, long reaction times, complex multi-step processes, and long reaction times, which precluded commercialization and widespread impact. Here, a promising, novel and scalable alternative is depicted which surmounts the limitations of previous procedures: the one-pot, catalyst-free, safe, rapid, high-temperature lamp ablation method, which starts with precursor 2H-WS, and is devoid of toxic chemicals. The major applications are exceptional solid lubricants stable at high temperatures, and excellent catalytic activity. Moreover, pure tungsten web-like nanostructures never before observed or predicted were found among the products - a finding of basic scientific value, awaiting the identification of major applications. Hollow WS nano-onions and nanotubes also emerged, but in far smaller quantities, and for understandable reasons that are addressed. Electron micrographs and a variety of material characterization methods permitted deducing reaction mechanisms that entailed (a) WS decomposing into elemental sulfur and tungsten, and, in parallel, (b) WS exfoliating into nano-platelets that bent and closed upon themselves. Some tungsten nanospheres independently conjoined to form the yet undiscovered pure-tungsten nano-webs. The formation of molten tungsten at temperatures well below the bulk's melting point was accounted for from the substantial melting-point depression as nanosphere diameter decreases.