Impact Of Ferroelectric Polarization Field On Different Semiconductors For Photoelectrochemical Water Oxidation

As a new energy with abundant reserves,high efficiency,cleanness and sustainability,hydrogen energy has been widely concerned because it can alleviate energy shortage and environmental pollution problems.Among many hydrogen production technologies,photoelectrochemical（PEC）hydrogen production technology is one of the most promising hydrogen production technologie in the future,and is regarded as a research hotspot in the field of new energy.Compared with the reduction reaction of the photocathode,the water oxidation reaction of photoanode is a dynamic speed-limiting step in the overall water splitting because it involves a four-electron transfer process.Hence,it is important to exploit photoanodes with high catalytic efficiency for PEC water oxidation applications.Some n-type semiconductor oxides,such as WO₃,TiO₂,Bi VO₄,and CuWO₄,are candidate materials for photoanodes due to their natural abundance,low cost,and good chemical stability.However,they are largely limited by the low mobility of photo-generated charges,which blocks the activity of water oxidation reactions severely.In recent years,the introduction of a built-in electric field into the semiconductor photoanode is an effective strategy to tune interfacial energy band bending and enhance charge transport.Ferroelectric materials with non-centrosymmetric can generate a polarized electric field under the external electric field,which could affect the space charge region of semiconductor electrode to some extent and change the degree of band bending to enhance photoelectrocatalytic activity.However,different semiconductors have different crystal structures and energy band structures,and the impact of polarization field of ferroelectric on their photoelectrocatalytic activity has not been reported.In this paper,the photoelectrochemical water oxidation activities of WO₃,TiO₂ and CuWO₄ semiconductor photoanodes were studied systematically,and ferroelectric Ba TiO₃ particles were added to these three photoanodes respectively.The influences of the polarization field of Ba TiO₃ on the oxidation performance of the above-mentioned different semiconductors were analyzed,and the related mechanism was explored.The main research contents and results of this paper are as follows:（1）Selecting WO₃,TiO₂ and CuWO₄ as the research objects of semiconductor photoanodes,their structures,morphologies,and optical properties were characterized by using XRD,XPS,SEM,and UV-vis spectroscopy.Then,WO₃,TiO₂and CuWO₄photoanode films were prepared by using the drop-casting method,and the differences of their water oxidation activities were investigated from the aspects of charge separation efficiencies,oxidation kinetic efficiencies and charge transport characteristics.The results show that WO₃,TiO₂ and CuWO₄ were all nanoparticle structures with good crystallinity.Moreover,the water oxidation photocurrents of WO₃ and TiO₂photoanodes were 37.8 and 18.9μA cm^-2 at 1.23 V vs.RHE,which were significantly better than that of CuWO₄ photoanode(3.5μA cm^-2).The good catalytic performance of WO₃ and TiO₂ is mainly attributed to their relatively good separation and transport of photo-generated charge.（2）Ba TiO₃（BTO）ferroelectric particles were added to the above three photoanodes respectively to prepare BTO-WO₃,BTO-TiO₂ and BTO-CuWO₄composite photoanodes,and the effective polarization direction and optimal mass ratio of BTO were studied systematically.Then,the influences of ferroelectric BTO polarization field on the water oxidation performances and charge transport properties of composite photoanodes were studied using J-V curve,CIMPS spectroscopy,Mott-Schottky curve and KPFM image.When the BTO mass ratio is 3 wt%and after+20 V voltage polarization,we found that the water oxidation photocurrent of BTO-WO₃,BTO-TiO₂ and BTO-CuWO₄ was respectively increased by 68.5%,51.3%,and 62.9%,compared to the pure photoanodes.At the same time,the efficiencies of charge separation and oxidation kinetics for these composite photoanodes increased by12-19%and 1.34-1.42 fold,and the photogenerated electron transport time reduced by14-16%.The improvements of the performance of the composite photoanodes are mainly due to the fact that the ferroelectric field could increase the band bending at the photoelectrode-electrolyte interface and promote the charge migration.We found that this adjustment is more beneficial to semiconductors with lower Fermi level,and the more upward bending can accelerate the charge migration and reduce the carrier recombination.In short,this paper can provide a reference for optimizing new ferroelectric-semiconductor composite photoanodes and constructing high-efficiency photoanode catalysts.