Synthesis Of Interlayer-Expanded MX₂ Nanosheets For Enhanced Electrocatalytic Hydrogen Evolution Performance

Two-dimensional layered transition metal dichalcogenides?MX₂?are considered to have promising applications in electrocatalytic hydrogen evolution reaction?HER?,owing to their advantages of favorable free energy of hydrogen adsorption,good stability in acidic electrolyte and unique electronic structure of dichalcogenides.Few-layered nanosheet structure exposes more active sites in the process of electrocatalytic hydrogen evolution.In this thesis,we develop a simple and universal solvothermal method to synthesize interlayer-expanded and defect-rich?IEDR?MX₂?M=W,Mo,V;X=S,Se?nanosheets.These IEDR MX₂ nanosheets are further demonstrated as active HER electrocatalysts.The main results are as follows:1.We report a one-step solvothermal method to synthesize the MoSe₂ nanotube constructed from numerous thin nanosheets and the WS₂ microrods assembled from layer-by-layer stacked nanosheets.The few-layered MoSe₂ nanosheets have a thickness of 4-7 nm.The spacing between two adjacent monolayers is as large as 1.05nm,indicating the widely expanded interlayer spacing of?002?plane.It shows a⁶2.5%expansion??d=0.404 nm?relative to that?0.646 nm?of the pristine MoSe₂.The WS₂ microrods have a length up to 1?m and a diameter of 300-500 nm.In such a layer-by-layer stacked structure,the edges of the nanosheets are fully exposed,which is beneficial for electrocatalytic hydrogen evolution.The?002?interlayer spacing of the WS₂ nanosheets is as large as 0.98 nm,which is 58.6%expansion??d=0.362 nm?compared to that?0.618 nm?of the pristine WS₂.The octylamine molecules play a key role for the formation of IEDR nanosheers.On one hand,the octylamine molecule can be adsorbed on the surface of primary nanocrystallites,partially hindering the oriented crystal growth and leading to the formation of disordered basal planes with a quasiperiodic configuration.On the other hand,the intercalated octylamine molecules result in the interlayer expansion of MX₂ nanosheets.The enlarged interlayer spacing provides a faster speed for the insertion and release of hydrogen ions.The synergistic effect of the IEDR structureenables the MoSe₂ and WS₂nanosheets to show excellent catalytic performance with samall onset potentials?82and 69 mV?and low Tafel slopes?51 and 55 mV/dec?.2.IEDR metallic 1T-VS₂ sample is also prepared by the simple solvothermal method.The VS₂ sample is general flower-like microstructures and composed of many thin nanosheets.The interlayer spacing between two adjacent VS₂ monolayers is estimated to be 0.95 nm,corresponding to expanded?001?interplanar spacing.It exhibits a⁶5.2%expansion??d=0.375 nm?relative to that?0.575 nm?of the pristine VS₂.And the freshly-prepared VS₂ nanosheets coupled with large amount of V²⁺,realizing the vanadium-doped and the molar ration of V:S is 1:1.65.The plentiful S deficiency should be responsible for the rich defects in the sample.Moreover,the outermost sulfur atoms may be interfered by the inserted vanadium atom,resulting in longer average distance of V-S bond in favour of the more edge position and active sites.The onset potential of the freshly-prepared VS₂ nanosheet catalyst is only 13 mV,and the Tafel slope is merely 35 mV/dec.These values are very close to performance of the commercial Pt/C electrode with excellent catalytic properties.