Ni离子替位掺杂TiO2增强光热化学循环还原CO2研究

摘要

利用溶胶凝胶法(Sol-gel)制备纯TiO2 (ST)及Ni掺杂TiO2 (NT)纳米薄膜,将其应用在光热化学循环(PTC)还原CO2中进行机理研究,并与商用P25 (PT)进行对比.结果表明利用NT进行CO2还原,每个循环得到的平均CO产率最高为5.30 μmol/g-cat,分别是利用ST与PT得到CO产率的3.13倍与2.28倍.利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线能量色散谱(EDXS)及X射线衍射技术(XRD)研究样品的物质组成、晶体结构与形貌.稳态与时间分辨固体光致发光光谱(PL)、紫外可见漫反射光谱(UV-vis DRS)和X射线光电子能谱(XPS)被用来进行材料光学性能及表面态检测,从而得到反应中激发电子转移机制.利用第一性原理计算研究材料表面氧空位形成能、态密度(DOS)及分态密度(PDOS).结合实验表征与理论计算,理清控制PTC反应进行的关键因子.%To study the mechanism of the photo-thermochemical cycle (PTC),titanium dioxide (ST) and Ni-doped TiO2 (NT) films were produced using a sol-gel method and applied in the PTC for CO2 reduction.And commercial P25 (PT) has been used as a compared sample.A comparison of CO production shows that Ni-doped TiO2 performed better than undoped TiO2 and P25.Average CO production of NT PTCs was 5.30 μmol/g-cat and it was nearly 3.13 times of ST PTCs' CO production and 2.28 times of PT PTCs'.Scanning electron microscopy (SEM),transmission electron microscopy (TEM),energy-dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) were used to assess the crystal structure and morphology of the films.Ni element could be found in NT by EDXS and inductively coupled plasma-atomic emission spectrometry (ICP-AES),and the mass fraction of Ni was 0.54％ and 0.436％ which were agreed with experiments.This result of XRD indicated that Ni2+ may have high dispersion without significant change in TiO2 and Ni2+ ions were doped into the TiO2 lattice so that Ni-O-Ti bonds were formed.Photoluminescence (PL),time-resolved PL,UV-visible diffuse reflectance spectra (UV-visible DRS) and X-ray photoelectron spectroscopy (XPS) analyses were also conducted to investigate the charge transfer and reaction mechanisms on the sample surface.The incorporation of Ni into TiO2 resulted in a weaker PL intensity than that of bare TiO2,which suggests that the introduction of Ni into TiO2 effectively suppressed the undesirable recombination of electrons and holes.According to UV-visible DRS results,the Eg of PT is approximately 3.07 eV,which is smaller than that of ST (Eg=3.23 eV) and PT (Eg=3.20 eV).The narrower band gap of NT indicates that NT absorbed light with a wider wavelength range than that absorbed by ST and PT.By XPS patterns,the increase of Ni+/0 and Ti3+ indicated that the Vo may have been produced on the bond of Ni-O-Ti after UV illumination,and oxygen vacancies (Vo) have been consumed after thermal step in PTC.Density functional theory (DFT) calculations related to the anatase (101) surface of TiO2 and Ni doped TiO2 was performed to verify and provide guidance for enhancing the PTC mechanism.Single and second Vo formation energy have been calculated.The Ni-doped surface exhibits better performance than the undoped surface in the first step in PTC,because of its lower Vo formation energy which produce more Vo sites.Density of states (DOS) and partial density of states (PDOS) results indicated that narrow energy gap and impurity energy level of Ni-doped TiO2 may lead to a wider wavelength range of NT.As a result,several key factors of the mechanism have been clarified.