Synthesis and Characterisation of Metal Dichalcogenide Based Nano Materials
Thesis or dissertation
University of Exeter
WS2, MoS2 and ZrS2 nanomaterials in various forms, such as nanoflakes, inorganic fullerene-like nanoparticles and nanorattles, were synthesised using two modified conventional techniques: solid-gas reaction and chemical vapour deposition. Both of these techniques are essentially based on reactions between metal oxides/chlorides and sulphur at a relatively low temperature in the range of 350-950°C in H2/Ar. Compared with other common techniques, these techniques are cost effective and environmentally friendly and produce well-crystallised WS2, MoS2 and ZrS2 nanomaterials with controllable sizes and morphologies, arising from the involvement of simple equipment and a H2S-free process. With the solid-gas reaction technique, the formation of WS2 and MoS2 inorganic fullerene like (IF) particles follows a so-called "template growth" mechanism, which implies that the sizes of the final products resemble their metal oxide raw materials. Therefore, because of the usage of WO3 nanoparticles and MoO3 submicron particles as precursors, nanosized WS2 (<100 nm) and submicron-sized MoS2 (approximately 500 nm) particles were generated, respectively. Further investigation of the reaction mechanism reveals that H2 is a vital factor in the formation of WS2 IF nanoparticles. Without H2, WS2 nanoflakes are instead produced, which is attributed to that the formation of WS2 IF nanoparticles based on the synergy between H2 reduction and S sulphidation. Using the CVD technique, WS2 IF nanoparticles with sizes below 100 nm were readily produced. However, the initially formed WS2 IF nanoparticles were poorly crystallised with numerous defects and disconnections, which is consistent with the results of other researchers. In this project, an additional annealing process was introduced to eliminate these defects and disconnections. After this process, well-crystallised WS2 IF nanoparticles were formed, which should exhibit improved mechanical properties and stability. In addition to the WS2 IF nanoparticles, ZrS2 was also prepared using the same route from the reaction of ZrCl4 with S. Unlike the WS2, the generated ZrS2 was in the form of nanoflakes with sizes below 30 nm. Consequently, these nanoflakes exhibited a strong quantum confinement effect and good photocatalytic performance for the decomposition of 4-NP. Based on the investigation of the WO3 sulphidation mechanism, novel W@WS2 and WS2@WS2 nanorattles were designed and first synthesised using a simple gas-solid reaction. The as-synthesised nanorattles were composed of tiny, moveable W/WS2 cores and continuous WS2 shells with much larger sizes. By simply tailoring the processing parameters, several types of nanostructures, including WS2 nanoflakes, IF nanoparticles and nanorattles (with desirable core size and shell thickness) were selectively prepared. Moreover, it was observed for the first time that the as-prepared nanorattles exhibited excellent catalytic activities, which were close to or even better than their much more expensive Au- and Pt-based counterparts.