Preparation And Photoelectric Properties Of TiO₂ Nanorod Arrays Co-modified With TiN And Cd

With the rapid development of the global economy,the energy crisis and environmental pollution are becoming increasingly serious.Developing low-cost and environmentally-friendly renewable energy technologies will be an important measure to achieve "carbon neutrality".The photoelectrochemical(PEC)water splitting by semiconductor can directly decompose water into hydrogen and oxygen,thus directly converting low-density solar energy into high-density clean hydrogen energy.This technology is expected to become one of the effective ways to solve both energy and environmental issues simultaneously in the future.High-efficiency semiconductor photoanode is the key to photoelectrochemical water splitting technology to produce hydrogen.Among numerous photoanode materials,titanium dioxide(TiO2)as an important semiconductor oxide,is widely used for PEC water splitting because of its excellent physicochemical properties and suitable energy level.One-dimensional TiO2 nanostructure is considered as an ideal photoanode material because of its large specific surface area,which can provide more reactive sites for PEC water splitting.However,TiO2 can only utilize the ultraviolet part of the solar spectrum since its wide bandgap.In addition,the high recombination rate of photogenerated carrers inside the TiO2 photoanode extremely limits water splitting performance.Therefore,improving the efficiency of light energy utilization and photogenerated carrier separation is the key to improve the PEC water splitting performance of TiO2 photoanodes.In this thesis,TiO2 nanorod array were prepared by hydrothermal method.Then the TiO2/CdS heterogeneous nanorod array were prepared by loading CdS nanosheets on the surface of TiO2 nanorod arrays by chemical bath deposition method.Finally,the TiO2/CdS/TiN heterogeneous nanorod array was prepared by loading TiN nanoparticles on the surface of TiO2/CdS heterogeneous nanorod array using spincoating technique.The TiO2/CdS/TiN heterogeneous nanorod array effectively facilitates the separation and transport of photogenerated carriers in the photoanode to achieve efficient and stable PEC water splitting for hydrogen production.Meanwhile,the composition,morphology,optical properties and PEC properties of the prepared TiO2,TiO2/CdS and TiO2/CdS/TiN photoanodes were investigated by various testing and characterization methods.The photogenerated carrier transport process of TiO2/CdS/TiN heterogeneous nanorod array was explored to elucidate the enhancement mechanism of their PEC properties.The main research and conclusions are as follows:(1)Preparation and PEC properties tests of TiO2 nanorod arrays.TiO2 nanorod arrays were prepared on FTO by hydrothermal method,and the structural morphology,optical and PEC properties of the prepared TiO2 nanorod arrays were investigated.The TiO2 nanorods with a tetragonal morphology,a diameter of about 130 nm and a light absorption range of 300-400 nm were prepared,and 0.92 mA·cm-2 the optimal photocurrent density was achieved under simulated solar irradiation.(2)Preparation and PEC properties of TiO2/CdS heterogeneous nanoarrays.The TiO2/CdS heterogeneous nanoarrays were constructed by growing CdS nanosheets on the surface of TiO2 nanorod arrays using chemical bath deposition method.The optical absorption performance tests showed that the visible light absorption performance of the TiO2/CdS heterogeneous nanoarrays was significantly enhanced.The PEC performance study shows that the photocurrent density of TiO2/CdS heterogeneous nanoarrays photoanode under simulated sunlight irradiation reaches up to 4.0 mA·cm-2 under unbiased condition,which is 4.2 times higher than that of TiO2 nanorod array.The results of optical absorption spectra and energy band structure analysis are as follows:On the one hand,the light absorption range of TiO2/CdS heterogeneous nanoarrays is extended from 400 nm to 560 nm after CdS nanosheets deposited,and the light absorption in the visible region is significantly enhanced,which significantly improves the light energy utilization of the heterogeneous structure photoanode;On the other hand,the type-Ⅱ band alignment structure between TiO2 and CdS enables the effective separation and transport of photogenerated carriers in the TiO2/CdS heterogeneous nanoarrays,thus significantly enhancing the PEC performance of the TiO2/CdS heterogeneous nanorod array structure.(3)Preparation and PEC properties of TiO2/CdS/TiN heterogeneous nanoarrays.The TiO2/CdS/TiN heterogeneous nanoarrays was constructed by depositing TiN nanoparticles on the surface of TiO2/CdS heterogeneous nanorod array using spin-coating technique.The light absorption performance tests showed that the light absorption range of the TiO2/CdS/TiN heterogeneous nanoarrays was extended to 800 nm,and the light energy utilization of the photoanode was further improved.The PEC performance showed that the photocurrent density of TiO2/CdS/TiN heterogeneous nanoarrays photoanode under simulated solar irradiation reached up to 4.5 mA·cm-2 under unbiased conditions,which was 1.13 and 4.9 times higher than that of TiO2/CdS and TiO2 nanorod array,respectively.The results of the photochemical decomposition of aquatic hydrogen showed that the hydrogen production rate of TiO2/CdS/TiN heterogeneous nanoarrays photoelectrode was 129.6 μmol·cm-2·h-1 after simulated solar irradiation for 2 h,which was 1.3 and 4.2 times higher than that of TiO2/CdS and TiO2 nanorod array photoelectrodes,respectively.The enhanced performance of TiO2/CdS/TiN heterogeneous nanoarrays for PEC water splitting was elucidated by combining optical absorption spectroscopy,photoluminescence spectroscopy,electrochemical impedance spectroscopy and energy band structure analysis.Firstly,the synergistic effect of excellent visible light response of CdS and SPR effect of TiN in the visible-near-infrared light region which significantly extends the light absorption range of photoanodes,and hence improves the light energy utilization.Secondly,the type-Ⅱ energy band alignment structure formed at the TiO2/CdS interface and the Schottky junction formed at the CdS/TiN interface can effectively promote the separation and transport of photogenerated carriers and inhibit the compounding of photogenerated carriers.Thirdly,due to the SPR effect of TiN nanoparticles,hot electrons are injected into the photoanode,thus increasing the number of photogenerated electrons participating in the PEC splitting of water.