Surface Modification And Photoelectrochemical Performance Of Nanostructured ZnO

It is well known that ZnO has been applied in the area of photocatalytic andphotoelectrochemical water splitting due to its high electron mobility,wide band gap（Eg=3.3eV）and high binding energy（60 meV）etc.Also,these performances enable the redox process to carry out at fast rate compared to other metal oxide semiconductors.However,ZnO only absorbs ultraviolet（UV）irradiation which restricts the application prospect for photoelectrochemical（PEC）.Used as photoanode,photochemical corrosion of ZnO is a serious consequence caused by the defects on ZnO surface,such as hydroxylgroups and oxygen vacancies inevitably brought about serious degradation.Therefore,interfacial engineering on the modification of ZnO has remained an appealing attention.In this thesis,we concentrate on the optimization of the properties of ZnOphotoanode for PEC via the surface modification and structural composite.The photoanodes of ZnO/WO₃,ZnO/WO₃/Carbon,and ZnO/FeOOH were prepared based on the relatively simple and low-cost method of solution impregnation and immersion.The effects of surface modification on the photoelectrochemical performance of ZnO based photoanodes were analyzed by optimizing the structure control and synthesis conditions.Also,the structures of the composites were characterized by scanning electron microscope,transmission electron microscope,X-ray diffraction,Raman spectra,X-ray photoelectron spectroscope and electrochemical workstation.The major contents can be summarized as follow:1.ZnO electrode on an AZO glass substrate were synthesized by two-stepelectrodeposited method.Then ZnO/WO₃ composites were prepared by impregnating 0.5mmol/40 mL WCl₆ alcohol solution to ZnO electrode.The modification of WO₃ on the surface of ZnO nanorods has a drastic enhancement in visible light absorption.The photoanode of ZnO loaded with WO₃ exhibited around 5 times higher photocurrent density（0.25 mA/cm²）at 0.57 V vs.SCE than pristine ZnO photoanode（0.05 mA/cm²）in 0.5 M K₃PO₄ with H₂O₂ electrolyte under the illumination of simulated solar light（λ>420nm）.Moreover,the ZnO/WO₃ photoanode displays a largest current density at loading amount 20μl of WO₃ and heating time 20 min of ZnO/WO₃ sample for our samples.This result indicates that incorporating additional semiconductor is one of the effective ways to improve performance of ZnO photoanode.2.ZnO/WO₃/C（ZWC）film photoanode were prepared by thermal decomposition ofglucose solution.The ZWC film composite will cause a red shift in the UV-vis absorption spectra and band gap narrowing emerged.Meanwhile,it can produce large photocurrent density about 0.71 mA/cm² compared to the unmodified ZnO/WO₃ photoanode（0.22 mA/cm²at 0.57 V vs.SCE）,much higher 3.2 times.The applied bias photon-to-current efficiency（ABPE）of the ZWC photoanode reaches 0.81%.Different carbon sources and impregnation concentration have effects on its photoelectric properties.When glucose and sucrose are used as carbon sources,the photocurrent densities of the optimized ZWC electrodes are 0.55 and0.41 mA/cm²,respectively,When glucose concentration are 0.05、0.1、0.2 M,it can produce photocurrent densities about 0.27、0.54、0.43 mA/cm².The data from the EIS and M-S measurements shows that nonmetal carbon reduce hole-transfer resistance,limited surface recombination of charge carrier and enhanced stability ZnO/WO₃.3.Based on substrates of electrodeposited ZnO,ZnO/FeOOH was synthesized bycalcination and impregnation method.In comparison to pure ZnO photoanode（0.049mA/cm²）,the decorated sample shows a photocurrent as high as 0.1 mA/cm² at 0.57 V（vs.SCE）,which is twice as great as pristine ZnO.Besides,the results show that ZnO/FeOOH prepared by immersing ZnO in FeCl₃·6H₂O for 10 s presented a higher photocurrent.The existence of FeOOH accelerates the catalysis process of photoanode surface and increased photoanod eelectrical conductivity,and thus improved interfacial charge transfer and charge separation.