Study On The Regulation Of Oxidation Products Of Water Photoelectrocatalytic By Crystal Plane And Grain Boundary Of WO₃ Photoanode

With the aggravation of energy crisis and environmental crisis,strengthening the utilization of renewable energy has gradually become the focus of international research.Photocatalysis technology began in 1976,and was taken as an alternative to traditional energy for it can reduce the consumption of electric energy by capturing photons.However,the photocatalytic oxygen evolution is slow in kinetic,which limits the growing of efficiency of photocatalysis.In recent years,researchers have gradually found that compared with the oxygen evolution reaction of four electron water oxidation（4e-WOR）,single electron transfer water oxidation reaction（1e-WOR）and two electron transfer water oxidation reaction（2e-WOR）have faster reaction kinetics,which can improve the efficiency of the overall reaction.At the same time,their products are hydroxyl radical（·OH）and hydrogen peroxide（H₂O₂）.Compared with the oxygen products of 4e-WOR,they have important application prospects in the fields of environmental treatment,industrial chemistry and organic synthesis.However,the thermodynamic energy barrier of 1e-WOR and 2e-WOR is higher than that of 4e-WOR,which leads to their low selectivity in oxidation reaction of water and the low production efficiency.Because the process of semiconductor photocatalysis is carried out on the surface of the catalyst,the crystal surface and morphology of the catalyst have a direct impact on the catalytic process.Therefore,this paper takes tungsten trioxide（WO₃）as the research object to carry out crystal engineering regulation（using the methods of morphology regulation and crystal surface regulation）to improve the photoelectric conversion efficiency of WO₃ and the selectivity to 1e-WOR and 2e-WOR.Firstly,the WO₃ crystal surface with large surface energy was sealed by chloride ion so that it can’t contact with hydrothermal solution and its growth was inhibit.Then the crystal can only grow along both sides to form a multilayer nano plate structure.Then,the electronegativity of Cl^-is weakened by adding ammonium ions,so that the end capping effect disappears to obtain WO₃ with single-layer structure.The experimental results show that the photoelectrochemical properties of multilayer WO₃ photoanode are greatly improved compared with that of monolayer,and the photocurrent density is 3.1 times that of the monolayer.The EPR results show that the electron resonance of WO₃ increases gradually with the time of illumination.The increase of·OH and photocurrent density is mainly due to the deeper valence band of multilayer structure of WO₃,which can meet 1e-WOR in thermodynamics.And the surface defects and grain boundaries can improve the carrier concentration,lifetime and transmission rate at the same time.On the basis of the above multilayer structure,a series of WO₃ with different（002）and（200）crystal plane exposure ratios were prepared by adjusting the volume and cooling rate of the reaction precursor solution.Experiments show that the change of crystal plane ratio affects the energy band structure of WO₃ photoanode,and the increase of（200）crystal plane strengthens the photogenerated hole oxidation ability of WO₃ photoanode,enhance the adsorption ability of water and the synergistic effect on HCO₃^-.Furthermore·OH bound on the surface of WO₃photoanode from（200）plane has high reaction activity and lower hole density,thus,priority selectivity is provided for 2e-WOR.Under illumination of AM 1.5G,the average Faraday Efficiency（FE）of H₂O₂ production by 2e-WOR reached 60.5%at bias voltage in the range of 0.7-1.6 V vs.RHE.