Hydrogenated Oxide As Novel Quasi-Metallic Co-catalyst For Efficient Visible-light Driven Photocatalytic Water Splitting

The application of photocatalytic splitting of water for hydrogen production can make rational use of solar energy and reduce growing environmental problems and energy shortage.In addition,the application of this technology can help promote social industrial and economic progress.Semiconductor-based photocatalytic materials play an important role in water splitting process,which can absorb solar energy and produce photogenerated electron-hole pairs through inter-band transition and participate in the subsequent oxidation-reduction reaction.However,the solar energy conversion efficiency of a single.component of photocatalyst is limited by the rapid recombination of photogenerated charge carriers.The photocatalytic performance can be improved by surface modification with a cocatalyst material.Molybdenum compounds is an important cocatalyst for photocatalytic H2 evolution reaction,including MoS2,MoOx,MoP,MoN and MoC,the surface metalloid properties and suitable hydrogen adsorption energy of the cocatatlyst material can effectively receive photogenerated eletrons,thus greatly improving the electron-hole separation efficiency,and provide enough active sites to reduce the hydrogen evolution reaction(HER)kinetic barrier.Our group has recently proposed that hydrogenation can cause metal oxide materials to undergo a transition from semiconductive to quasi-metallic,which will result in extremely high free electron concentration and electrical conductivity close to noble metals.Among all investigated metal oxide materials,the MoO3 is attracting more attention from researchers.Since hydrogenation can significantly increase the concentration of free electrons in MoO3,and the extremely high electrical.conductivity of the hydrogen-doped MoO3(HMO)also makes its physical and chemical properties similar to noble metals.Therefore,the quasi-metallic HMO is studied as a potential cocatalyst for photocatalytic H2 production.And up to now,its application as a cocatalyst has not been fully explored according to the best of our knowledge.In this paper,we report an economical,efficient and stable photocatalytic hydrogen production system.In this system,ZnIn2S4(ZIS)is used as the photocatalyst,triethanolamine(TEOA)is used as hole scavenger,and quasi-metallic HMO is used as cocatalyst.The hydrogen production rate of the photocatalytic system was analyzed under visible light irradiation.The main function of HMO as a co-catalyst in the whole photocatalytic system was studied by advanced characterization techniques.The following is the main research scopes and conclusions:(1)The quasi-metallic HMO cocatalyst was prepared by one-step method with metal zinc and commercial MoO3 as raw material under the guidance of a novel electron-proton co-doping strategy previously reported by our group.Firstly,HMO was synthesized by the metal-acid treatment,and its composition was H1.68MoO3.The product is dark black apperance,in sharp contrast to the light yellow of MoO3,which implies the quasi-metallic properties of HMO.Then,the ZnIn2S4(ZIS)nanosheets array with a thickness of 6 nm were synthesized on the surface of HMO by wet-chemical method to construction of hierarchical HMO/ZIS nanostructure.The hydrogen production rate of the optimized composite is 1564 μmol h-1 g-1 under visible light irradiation(λ≥420 nm).It was not only significantly superior to pure ZnIn2S4(327 μmol h-1 g-1),but also superior to 1%wt Pt loaded ZnIn2S4(1308 μmol h-1 g-1).This result provides a new design clue and stratergy for the development of inexpensive and effecient cocatalyst materials to replace traditional noble metals for high-performance photocatalytic water splitting.(2)The crystal structure of quasi-metallic HMO was analyzed by XRD,SEM,TEM,XPS and other characterizations.The promotion mechanism of HMO cocatalyst was further studied by UV-Vis DRS,PL,EIS and other photoelectrochemical characterizations,ultra-fast fs-TA spectroscopy and density functional calculations(DFT).It was found that there were two reasons for HMO as co-catalyst in the improvement of photocatalytic hydrogen production performance:(a)The quasi-metallic MoO3 cocatalyst surface after hydrogenation can effectively reduce the proton reduction overpotential to reach an optimized value;(b)The photogenerated electron-hole pairs in ZIS are effectively separated at the heterostructural Schottky interface and the photogenerated electrons in ZIS migrate rapidly to the HMO co-catalyst surface.