Preparation And Photoelectrochemical Properties Of Modified TiO₂Thin Films

Titanium dioxide (TiO2) is an important n-type semiconductor material with many advantages such as low cost, high stability and relative nontoxicity. It has been considered to a promising material in many application fields such as dye sensitized solar cells, hydrogen production via water splitting, photocatalytic degradation of organic pollutants and so on. However, TiO2can only be excited by ultraviolet (UV) light due to its large band gap, and the photoinduced electrons and holes may rapidly recombine, which hinder the utilization of solar energy. Therefore, numerous attempts have been carried out to improve the photoresponse of TiO2. In this paper, Si doped TiO2(Si-TiO2) thin films were prepared by direct current (DC) reactive magnetron sputtering, and modified anodized TiO2nanotube array thin films were prepared through carbon quantum dots (CQDs) sensitization or hydrothermal treatment in milk. Enhanced photoelectrochemical properties of TiO2thin films were obtained by doping or sensitization. The main results and conclusions can be summarized as following:1. Si doped TiO2thin films were prepared by DC reactive magnetron sputtering using a Ti-Si mosaic target. The Si contents of the films were5%and12.5%, respectively. The as-prepared Si-TiO2thin films were characterized by atomic force microscope (AFM), field-emission scanning electron microscopy (FE-SEM), X-ray photoemission spectroscopy (XPS) and ultraviolet-visible (UV-Vis) absorption spectroscopy. Moreover, the photo-induced hydrophilicity, photocatalytic degradation performance and photoelectrochemical properties were studied. The experimental results revealed that the pristine TiO2and Si-TiO2thin films showed relatively smooth surfaces with well-dispersed nanoparticles. Larger nanoparticles could be observed on the surface of Si-TiO2thin film compared with the pristine TiO2thin film. XPS results showed that Ti-O-Si bond was formed in Si-TiO2thin film. The optical properties revealed a red shift of absorption edge for5%Si-TiO2thin film with the optical band gap of3.36eV, smaller than that of pristine TiO2thin film (3.51eV).5%Si-TiO2thin film exhibited enhanced behavior in forming and keeping hydrophilic surface compared to the pristine TiO2thin film due to more surface hydroxyl groups. Meanwhile,12.5%Si-TiO2thin film showed hydrophobic property. During the degradation process of methylene blue (MB),5%Si-TiO2thin film exhibited enhanced photocatalytic degradation performance. The photocurrent densities of5%Si-TiO2 thin film under UV-Vis and visible light illumination were4.3and4.9times larger than that of pristine TiO2thin film, respectively. Meanwhile,12.5%Si-TiO2thin film showed obviously decreased photocurrent densities compared with the pristine TiO2thin film. Alkaline electrolyte could enhance the photoelectrochemical properties of pristine TiO2and5%Si-TiO2thin films, especially at pH value above12. However, acidic electrolyte was not beneficial for the enhancement. From the EIS results,5%Si-TiO2thin film showed decreased interfacial impedance compared to the pristine one.2. CQDs sensitized TiO2NTs (CQDs-TiO2NTs) thin films were prepared by anode oxidation followed with simply immersing in CQDs solutions. The films were characterized by FE-SEM, X-ray diffraction (XRD), Raman spectrum and UV-Vis diffuse reflectance spectroscopy. The photoelectrochemical properties under visible light illumination were studied. The experimental results revealed that the TiO2NTs prepared by anode oxidation had an average inner pore diameter of60nm and wall thickness of15nm, whereas the lengths were approximately200-300nm. After annealed at450℃for2h, anatase phase could be observed from XRD results. The CQDs-TiO2NTs thin film showed a red shift of optical absorption edge and increased absorbance during visible light region. Compared with the pristine TiO2NTs thin film, the photocurrent densities of these two kinds of CQDs-TiO2NTs thin films under visible light illumination increased about32%and40%, respectively. The CQDs-TiO2NTs thin films are beneficial for the utilization of solar energy thus have potential application prospects.3. A novel strategy was demonstrated to improve the photoelectrochemical properties of TiO2NTs thin film with by hydrothermal treatment in milk. Hydrothermal method with high-temperature and high-pressure environment was used to promote the adsorption of molecules from milk. Thermal treatment for the doped TiO2NTs thin film was expected to enhance the photoelectrochemical properties. The experimental results revealed that this strategy could not enhance the photoelectrochemical properties effectively at low hydrothermal temperature (60℃90℃). However, the photocurrent density of the sample treated at120℃under UV-Vis light illumination increased about34%after annealing though the protein molecules in milk denatured. The property of protein denaturation at high temperature restrains the application of this strategy. However, other attempts to enhance the properties of TiO2with milk treatment are still worthy of study.