Photoelectrochemcal Performance Study Of Quantum Dots Sensitized Oxide Photoanode

TiO₂ and ZnO have been demonstrated as promising candidates for photoanode due to their appropriate energy band position and good corrosion resistance in aqueous solution.However,the band gaps are too wide to absorb visible light efficiently,one way to improve the utilization of sunlight is sensitizing semiconductor quantum dots on TiO₂ or ZnO by successive ion layer adsorption and reaction?SILAR?method.In this thesis,we will focus on three main aspects to improve the photoelectrochemical performance:superficial area improvement of oxide semiconductor,surface modification of oxide semiconductor and SILAR deposition steps adjustment.The main contents are as follows:?1?A forest-like TiO₂ nanotubes array film was formed on the Ti foil by one-step hydrothermal synthesis.The surface area of the film can be adjusted by changing the hydrothermal reaction time.When the reaction lasts for 8 hours,the diameter of the TiO₂ nanotubes is about 10 nm.The TiO₂ nanotubes array film just like forest when the surface area reaches a maximum.The photocurrent of 8h photoanode?0.38mA/cm²?is 2.4 times as large as the one of 4h photoanode?0.16 mA/cm²?.?2?The TiO₂ nanotubes array films in different reaction times were sensitized with CdS quantum dot by SILAR.We found that the absorption range of the film is greatly widened due to CdS.The photocurrent density of the composite film increases first and then decreases with the increase of SILAR cycles.We got the largest photocurrent density?2.30 mA/cm²?when the 8h TiO₂ nanotubes was sensitized 5cycles with CdS in SILAR,which is 6 times as large as the pure TiO₂ nanotubes array film.This is att the larger surface area has more active centers and can adsorb more CdS quantum dots.As a result,the photoelectrochemical performance of the composite film was increased.?3?ZnO nanosheets?ZnONS?array films were soaked with NaOH solution.The effects of alkali treatment on the properties of ZnO nanosheets were investigated by SEM,Raman,EDX and UV-visible.The results showed that the alkali treatment had little effect on the morphology and vibration mode of pure ZnO.EDX pattern indicates that alkali treatment increases the oxygen deficiency of the sample.The time-resolved fluorescence test data showed that the alkalization treatment had a significant effect on the fluorescence lifetime.The time-resolved mid-infrared absorption spectrum showed that the FWHM of the absorption spectrum after surface alkalization treatment first increased and then decreased.The maximum spectral change was observed at 1 h of alkalization,which proved that the decrease of carrier lifetime due to oxygen vacancy defects is the essential cause of the unique optical and electrical properties of alkali treated ZnONS array film samples.?4?CdS QDs were deposited on the surface of the alkali treatment ZnONS array films by using the SILAR method,and their photoelectrochemical properties were investigated carefully.The J-V curves display that the effect of alkali treatment on pure ZnONS is weak while it's great for CdS sensitized composite thin film.The optimal photocurrent density of ZnONS,ZnONS?OH?,ZnONS/CdS and ZnONS?OH?/CdS simples are 0.13 mA/cm²,0.15 mA/cm²,4.82 mA/cm² and 7.25mA/cm²,respectively.Alkali treatments nearly enhance the photocurrent of ZnONS/CdS for 50%.This can be attributed to two points,firstly,the effect of that alkali treatment boosted the oxygen vacancy concentration of ZnONS which is beneficial to CdS QDs nucleate;secondly,alkali treatment extended the lifetime of photo-generated carriers.?5?We adopt ZnO NS,ZnO NR and alkali treatment ZnONS array films as substrates to investigate the effect of SILAR deposition order on the photoelectrochemical property of CdS sensitized composite thin film.The experimental data display that both the optical absorption properties and photoelectrochemical properties obtained from the deposition step of S-Cd are superior to that of Cd-S deposition order in all cases.