The Fabrication And Photoelectrochemical Performance Of Perovskite-Type Oxynitrides

Photoelectrochemical（PEC）water splitting can effectively convert solar energy into clean and storable hydrogen energy,which is one of the possible ways to solve the energy demand and environmental pollution problems.An ideal semiconductor material used for PEC water splitting should have a smaller band gap to absorb more sunlight.Meanwhile,it is necessary to have suitable energy band positions to meet the thermodynamic conditions of water splitting.Traditional semiconductor photoanode materials（such as TiO₂,α-Fe₂O₃,etc.）cannot meet the above two requirements at the same time,so it is difficult to achieve high solar to hydrogen energy conversion efficiency.Among various semiconductor materials,the perovskite-type oxynitride semiconductor materials（AB（ON）₃,A=Ba,La;B=Ta,Nb,etc.）have attracted extensive attentions and research owing to their suitable energy band edge positions and the different combinations of A-site and B-site transition metals that can tune the band gap width for widely using the visible-light spectrum.However,perovskite-type oxynitride has poor morphology controllability and high defect density,resulting in severe photogenerated carrier bulk and surface recombination,which limits the conversion efficiency of PEC water splitting.Based on the above problems,the research mainly focuses on the construction of high-quality perovskite-type oxynitride films.Additionally,diverse strategies,such as morphology control,crystal structure regulation and surface modification,have been developed to promote the PEC performances of perovskite-type oxynitride photoanodes.The main contents are as follows:（1）The controllable construction of LaTaON₂ thin films has been achieved by a dual-source electron-beam co-evaporation and nitridation method.The results show when the thickness of the LaTaON₂ films is moderate,the absorption depth of photons matches the thickness of the films,thereby improving the absorption and utilization of the semiconductor films to visible light wavelength range.Using H₂O₂ as the hole sacrificial agent,the photocurrent density of LaTaON₂ photoanode gets a certain improvement,but it is still far below its theoretical maximum photocurrent density.It indicates that the photogenerated carriers have serious bulk and surface recombination,which is also the main reason for limiting the photocurrent density of LaTaON₂ photoanode.On this basis,the surface properties of the LaTaON₂ films can be improved by annealing in an argon atmosphere,which could promote the separation and transmission of photogenerated carriers.Meanwhile,the annealing process can reduce the defect density of low-valence Ta,and effectively suppress the accumulation and recombination of photogenerated electrons and holes at the defects.After annealing in Ar atmosphere,LaTaON₂photoanode achieves a photocurrent density of 100μA·cm^-2 at 1.23 V_RHE,which is twice as high as that of the LaTaON₂ photoanode without Ar annealing.（2）The perovskite-type BaTaO₂N thin films are directly fabricated on the Nb metal substrate through a dual-source electron-beam co-evaporation and nitridation method.By adjusting the Ba/Ta atom ratio in the co-evaporated precursor films,the crystallinity and morphology of the BaTaO₂N films have been significantly improved,realizing effective separation and transfer of photogenerated carriers.The research results show that appropriately increasing the content of Ba²⁺can effectively suppress the defect density of surface Ta⁴⁺and reduce the recombination probability of photogenerated carriers at the semiconductor/electrolyte interface.Under the optimal Ba/Ta ratio,the BaTaO₂N photoanode modified with a Ni Co Fe-B_i oxygen evolution cocatalyst achieves a photocurrent density of 4.7 m A·cm^-2 at 1.23 V_RHE and a maximum applied bias photon to current efficiency of 1.18%at 0.83 V_RHE,indicating its excellent PEC water splitting performance.