Design Of Cocatalyst And The Study Of Reaction Mechanism For Photocatalytic Hydrogen Production

Photocatalytic water splitting for hydrogen is regarded as the promising technique for harvesting and storing intermittent solar energy in the form of a clean chemical fuel.However,despite the great advance in photocatalytic hydrogen production during past decades,this technology is still in the proof-of-concept stage because of the low solar-to-hydrogen energy conversion efficiency.One of the biggest obstacles to achieving practical application is the high recombination rate of photogenerated electron-hole pairs.In general,the cocatalysts can not only drop the overpotential or activation energy of redox reaction,but can also boost the electron–hole pairs separation,hence significantly promoting the photocatalytic hydrogen evolution rate.Exploring the efficient photocatalytic cocatalysts is the key to realize the industrialization of photocatalytic hydrogen evolution technology.(1)The reaction mechanisms of MoS₂ in semiconducting phase(2H)and metallic phase(1T)are different in the process of photocatalytic hydrogen production.In order to explore the photogenerated electron transfer process between between Ti O₂and mixed phase MoS₂,a simple hydrothermal synthesis method was adopted.With highly dispersed Mo O₃ as the precursor and conductive glass(FTO)as the substrate,the mixed phase few-layer MoS₂nanosheets were coated on Ti O₂ nanorod arrays(MoS₂@Ti O₂).The results displayed that the designed MoS₂@Ti O₂ exhibited a photocatalytic H₂ evolution rate of 8.43?mol·h^-1·cm^-2,which was two times higher activity than that of Pt@Ti O₂.The photochemical analysis showed that the mixed phase MoS₂ exhibited similar characteristics to Pt,which promoted the separation of charge carriers as the electron conductor and provided an efficient active site for photocatalytic hydrogen production.The photodeposition experiment and in-situ KPFM characterization have been used to clarify the electron transfer process from Ti O₂ to MoS₂,and theoretical calculation has been made to further confirm the role of mixed phase MoS₂,acting as 1T-MoS₂ cocatalyst,rather than acting as a semiconductor behavior.This study not only provides a new direction for MoS₂ to replace precious metal Pt as an efficient photocatalytic co-catalyst for photocatalytic hydrogen production,but also provides a series of solid experimental basis for verifying the electron transfer process of mixed phase MoS₂ co-catalyst system.(2)In order to further improve the photocatalytic performance of mixed phase metal chalcogenides,MoSe₂ cocatalyst with more active sites was synthesized based on density functional theory(DFT).The DFT calculations indicated that both the basal plane and the edge of mixed phase MoSe₂ provide the highly catalytic activity for hydrogen evolution reaction(HER),which is different from prue phase MoSe₂.Mixed phase MoSe₂ not only has good electron transfer ability,but also has more active sites for HER,so it is an efficient co-catalyst.As a guide,photocatalyst with ternary 1T/2H MoSe₂/Cd S-Cd Se structure was prepared by simple hydrothermal method.The hydrazine monohydrate as reductant not only helped to control 1T phase content of MoSe₂,and the maximum content of 1T phase MoSe₂ reached66.5%,but also cause S defect in Cd S,which was conducive to the formation of Cd Se.The results showed that the H₂ evolution rate of the optimized MoSe₂/Cd S-Cd Se reached a value of24.34 mmol·h^-1·g^-1 and a high quantum efficiency(QE)of 28.5%at 500 nm.A volcano-like correlation has been presented for the first time to illustrate the effect of structures of MoSe₂ on photocatalytic activity for HER,which provides the direction of design of MoSe₂ cocatalyst.(3)Compared with one-component cocatalyst,the multicomponent cocatalysts with synergistic effect can effectively accelerate the separation of photogenerated electron-hole pairs in photocatalysis.Hence,a novel bicomponent Co₃S₄/Co cocatalyst on Cd S semiconductor photocatalyst was successfully synthesized by a one-step hydrothermal method.The designed Co₃S₄/Co-Cd S photocatalyst exhibited high-efficiency photocatalytic H₂ evolution rate of15.17 mmol·h^-1·g^-1,which was 2.8 times higher than that of Pt-Cd S.Furthermore,the quantum efficiency of Co₃S₄/Co-Cd S reaches a maximum value of 16.80%at 475 nm.The results showed that the remarkable improvement of the photocatalytic performance was due to the unique"step"energy level of Co₃S₄/Co-Cd S photocatalyst.The Co₃S₄ has a lower Fermi level than Co,which can receive the photogenerated electrons from Co metal,further preventing the recombination of photogenerated electrons and holes,and greatly improving the efficiency of photocatalytic hydrogen production.This work not only presents a new protocol for constructing bicomponent cocatalysts on semiconductor structure,but also proves the bicomponent Co₃S₄/Co to be a promising and low-cost cocatalyst.