Design And Preparation Of Narrowband Semiconductor Composite Photoanodes For Photoelectrochemical Water Oxidation

Photoelectrocatalytic water splitting is mainly based on the research of hydrogen evolution reaction and oxygen evolution reaction on semiconductor photoelectrodes.The oxygen evolution reaction occurring in photoanodes has become the most critical bottleneck problem for photoelectrocatalytic water splitting due to the thermodynamic aspects.The most widely studied photoanode materials are usually n-type semiconductors including?-Fe₂O₃,BiVO₄,and WO₃.However,they still have their own shortcomings,such as the recombination of photogenerated carriers,slow surface oxidation kinetics and poor stability.In order to solve the problems of these semiconductor photoanodes,this thesis presents the works of design and construction of composite/hybrid photoanodes with different homogeneous and heterogeneous water oxidation catalysts.These works are as follows:1.The nanorod?-Fe₂O₃ photoanode was synthesized by hydrothermal method,and then we synthesized a polyoxometalate,[(A-?-SiW₉O₃₄)₂Co₈?OH?₆?H₂O?₂?CO₃?₃]^16-?Co8POM?,to study its effect on the photoelectrocatalytic water oxidation performance of hematite photoanodes.During the research process,we found that Co₈POM decomposed into a layer of CoO_x cocatalyst in the presence of photogenerated carriers of hematite.At 1.23 V_RHE,our CoO_x?POM?/Fe₂O₃composite photoanode has a photocurrent density of 1.1 mA/cm²,which is more than twice that of the Fe₂O₃ photoanode.We also prepared the CoO_x?Salt?/Fe₂O₃ photoelectrode using simple cobalt salt,which got a photocurrent density of only 0.7 mA/cm².By the transient photoresponse experiments,Mott-Schottky tests and open circuit voltage results,the CoO_x derived from Co₈POM effectively suppressed the surface state of?-Fe₂O₃,improved the photovoltage of hematite,and promoted charge separation.2.The nanoporous bismuth vanadate photoanode was prepared by electrodeposition-metal organic degradation method.The Fe-tannic acid complex film?FTA?was coated on the surface of BiVO₄ material.The complex film was prepared by mixing tannic acid containing polyphenolic hydroxyl groups and Fe ions under weak alkaline condition.The FTA film can effectively suppress photocorrosion of BiVO₄ material and help reduce surface recombination of carriers.On the surface of the FTA/BiVO₄ photoanode,we have subsequently assembled a molecular catalyst,[Co₄?H₂O?₄?HPMIDA?₂?PMIDA?₂]^6-?1?.The molecular catalyst 1 made full use of the photogenerated holes transferred from the FTA layer for water oxidation reaction,and greatly improved the surface water oxidation kinetics of the composite photoanode.The 1/FTA/BiVO₄photoanode obtained a photocurrent density of 5.5 mA/cm² at 1.23 V_RHE.3.The preparation of WO₃ photoanode material with nanosheet array structure used seed layer-hydrothermal-calcination method.Meanwhile,the preparation of carbon quantum dot?CQD?was by the calcination of citric acid and the preparation of NiFe-LDH was by electrodeposition method.CQD can be used as a photosensitizer to improve the light absorption capacity of WO₃,leading to the effectively improved its photoelectrochemical performance.Moreover,the loading of NiFe-LDH can simultaneously improve the surface oxygen generation reaction kinetics and enhance the stability under neutral electrolyte condition.At 1.23 V_RHE,the photocurrent density of the NiFe/CQD/WO₃ composite photoanode reached 1.44 mA/cm²,which is 2.8 times than that of the WO₃ electrode.The photocurrent can maintain from 44%to 90%after modifying with NiFe-LDH and CQD in the three-hour stability test.