Synthesis Of Ternary Metal Oxide Photoanodes And Characterized For Photoelectrochemical Water Splitting

Due to the increasing energy crisis and environmental pollutions,developing clean energy technology has received the widespread attention around the world.As one of the main strategies for the utilization of solar energy the most abundant energy resource,hydrogen production from photoelectrochemical?PEC?water splitting technology is able to directly convert the solar energy into the hydrogen energy.It makes the expect using solar energy to relieve the energy crisis and environmental pollutions possible.However,the efficiency of solar to hydrogen is still relatively low and unable to realize the large-scale application,eventhough it has took more than 40 years of research.This is mainly because there are many shortcomings in the semiconductor materials served as the core components of the photocatalytic water splitting hydrogen production technology.The development of semiconductor photocatalysts with long-term photocatalytic stability,wide response range for sunlight,efficient photogenerated electron-hole pair separation and transfer,and favorable kinetics for water decomposition on the surface plays an important role to solve these problems.Current research in the field of photocatalytic water splitting are mainly focused on the development of new photocatalyst and material preparation technology,the investigation of the mechanism of photocatalysis,the integration of photocatalytic technology with other energy technologies,and so on.Ternary metal oxide semiconductor materials which have good photocatalytic stability and visible light absorption ability are selected as the research object of this thesis.Through exploring the novel ternary metal oxide photoanode,the strategy for improving the carrier separation and transfer of the existing photoanodes,and new facile preparation methods of metal oxide thin films,to further improve the PEC performance of ternary metal oxide,explore the mechanism of photocatalysis,and broaden the preparation method library of metal oxide thin films.The main contents and conclusions are as follows:?1?Thin films of two copper-based metal vanadates?CuV₂O₆ and Cu₂V₂O₇?were synthesized by a facile drop-casting method.The primary photoelectrochemical?PEC?and physical properties of these two materials including photocurrent response,band gap,flat band potential,incident photon to current conversion efficiency,chemical stability,and oxygen evolution faradaic efficiency were researched.The photocurrent density of CuV₂O₆ and Cu₂V₂O₇ films at 1.23 V vs.RHE in 0.1 M sodium borate buffer solution was about 25 and 35 ?A/cm2,respectively.At 1.58 V vs.RHE,however,the photocurrent density reached approximately 220 and 120 ?A/cm2,respectively.Although the photocurrents observed for these two materials at 1.23 V vs.RHE were relatively low,the photocurrents were much higher when tested with sodium sulfite as a hole scavenger.Suitable oxygen evolution catalysts are therefore expected to improve the PEC performance of these materials.?2?Thin films of copper tungstate?CuWO₄?have been prepared through electrodeposition and tested as the anode material for photoelectrochemical?PEC?water oxidation.We found that hydrogen-treatment over the synthesized Cu WO₄ at high temperatures led to enhancement in the PEC performance for water oxidation.The origin of this activity enhancement is attributed to the enhanced kinetics of electron-hole pair separation due to the increased carrier concentration induced by oxygen vacancy formation upon hydrogen-treatment.The incorporation of oxygen vacancies also resulted in enhanced light absorbance in the visible light region because they introduce shallow donors as predicted by first-principles calculations.Nevertheless,incident photon to electron conversion efficiency?IPCE?measurements indicate that light absorbed at long wavelength region after hydrogen-treatment is not successfully utilized to oxidize water.This report provides insights on the effect of hydrogen-treatment on the structural,optical,and electronic properties of CuWO₄.?3?Since monoclinic bismuth vanadate?BiVO₄?has been considered to be a promising anode material for photoelectrochemical?PEC?water splitting,the synthesis of high-quality BiVO₄ photoanodes for use in PEC cells is beneficial.Here,we report a facile simplified successive ionic layer adsorption and reaction?s-SILAR?method to synthesize porous BiVO₄ films.Evaluation of the porous BiVO₄ film as a photoanode yielded photocurrents of ⁰.70 and ¹.20 mA cm-2 at 1.23 V vs RHE under AM1.5G irradiation for water and sulfite oxidation,respectively.After coupling with a cobalt–phosphate?Co-Pi?catalyst,its photocurrent for water oxidation increased to ⁰.97 mA cm-2 at 1.23 V vs RHE.The high performance of the resultant BiVO₄ films and simplicity of operation make this s-SILAR technique a promising strategy to prepare BiVO₄ photoanodes.