Layered MoS₂ Nanostructures For Efficient Hydrogen Generation

Layered transition metal dichalcogenides MoS₂ nanomaterials are widely studied in photoelectrochemical and electrochemical hydrogen evolution performance owing to abundant catalytically active sites,suitable Gibbs free energy and adjustable band gaps.In this dissertation,1T and 2H hybrid phase of ZnO/MoS₂ nanocables array and MoS₂nanotubes array were prepared as photoelectrochemical water splitting photoanode materials and electrochemical catalytic electrodes.Their hydrogen evolution performance was systematically investigated.The main results are as follows:1.In this dissertation,ZnO/MoS₂ nanocables array were synthesized by a simple solvothermal method using ZnO as a template.A core-shell structure was formed on the ZnO nanorods by stacking MoS₂ nanosheets.Compared with the ZnO nanorods array and the MoS₂ nanotubes array,the nanocables array showed good performance in photoelectrochemical water splitting.At a bias voltage of 0.9 V（reference electrode is saturated calomel）,the photocurrent density of the ZnO/MoS₂ nanocable array is 1.39 mA cm^-2,which is significantly higher than that of the MoS₂ nanotubes array(0.66 mA cm^-2)and the ZnO nanorods array(0.11 mA cm^-2).The core-shell nanocable array as a photoanode material has the following characteristic s:（i）the single-crystalline ZnO cores serve as an efficient pass way for fast electron transport,（ii）the nanostructure forms a heterojunction on the contact surface.Under light irradiation,photogenerated electron-hole pairs would be preferably transferred across the interface in opposite directions,which promotes photo-generated charge carrier separation and reduces electron-hole recombination.2.The above nanocable array was treated with dilute sulfuric acid to dissolve the core layer of ZnO nanorods and formed a MoS₂ nanotubes array.The freshly prepared MoS₂nanotube arrays with 1T phase and 2H phase mixing characteristics,the nanotubes are assembled from MoS₂ nanosheets.The nanotubes array as a highly efficient electrocatalytic hydrogen evolution material has the following advantages:（i）the metallic 1T phase in freshly prepared MoS₂ nanotubes array may improve the electrical conductivity in the electro-catalytic process,（ii）the MoS₂ nanotubes array works as the binder-free electrode,where the use of insulating Nafion is avoided,（iii）interlayer expansion of MoS₂ induces modification of electronic structure and achieves an optimized hydrogen adsorption free energy（?G_H）,which are beneficial for faster proton/electron adsorption and hydrogen release processes.The experimental results show that freshly prepared MoS₂ nanotubes array with hybrid phases has superior hydrogen evolution properties.The MoS₂ nanotubes array has a lower onset voltage（103 mV）and a lower Tafel slope(45 mV dec^-1).At overvoltage of 300 mV,the current density of the freshly prepared of MoS₂ nanotubes array was 33.8 mA cm^-2 and the current density of the annealed MoS₂ nanotubes array was 10.7mA cm^-2 and the current density of the free standing MoS₂ nanotubes grown at the bottom of the reacton only was 5.5 mA cm^-2.