Photoelectrochemical And Photocatalytic Properties Of Modified Nanostructured TiO₂Photoanode

Photoelectrochemical and photocatalytic technique of semiconductor has great potential for the utilization of solar energy to solve problems of energy shortage and environmental crisis. Among reported oxide semiconductors, TiO2has attracted much attention due to its low cost, non-toxicity and stability. However, its full potential use was hampered by activation only under ultraviolet light irradiation due to the wide band gap and the fast recombination rate of photogenerated electron/hole pairs in TiO2. Thus, the modification of TiO2for the improvement of photoelectrochemical and photocatalytic propeties was currently hot research areas. In this paper, we mainly used new preparation methods, doping and coupling with carbonous materials technique to improve the performance of TiO2. Furthermore, the3D structure for light harvesting was also adopted to fully utilize solar light. The prepared samples were studied by series characterization methods and the properties of modified TiO2were investigated by phtoelectrochemical and photocatalytic technique. Dye-sensitized solar cells (DSSCs) with higher efficiency were also detected with the modified TiO2photoanodes. In this dissertation, the main and interesting results are summarized as following:1. A novel "solid-state oxide bath" method was directly used to prepare the Si-doped TiO2thin film by annealing titanium metal sheet embedded in SiO2powders, XPS results showed that the content of silicon in the Si-doped TiO2films was proportional to the thermal treatment time and temperature. The photoelectrochemical response for the as-prepared samples was investigated and the highest photocurrent value of31.8μAcm-2was obtained using samples prepared at450℃for2h. In addition, higher photocurrent value in the visible light region, more negative flat band potential and higher carrier density were demonstrated for the doped samples compared to the undoped ones. Such a novel and simple fabrication technology demonstrated in this work is also feasible approach for the preparation of other doped TiO2based on the corresponding oxides.2. Si-W codoped TiO2nanotubes were fabricated in-situ by a one-step anodization process with the presence of silicotungstic acid in the electrolyte. XPS results indicated the successful doping of Si and W. The content of Si and W in the TiO2nanotubes was proportional to the concentration of the silicotungstic acid in the solution. UV-Vis diffuse reflectance spectra showed more absorption in the visible light region for the resulted samples. An enhanced photoelectrochemical visible response was demonstrated for the codoped samples and the photocurrent was2.5times larger than that of pure TiO2nanotubes under visible light irradiation. More negative flat band potential and a higher carrier density were confirmed with the codoping of Si and W into TiO2nanotubes. The energy conversion of DSSCs based on Si-W codoped TiO2phtotoanode was higher than TiO2nanotubes.3. N doped and C-N codoped TiO2nanoparticles were fabricated by calcining TiN nanoparticles under air and carbon monoxide (CO) atmosphere at four different temperatures between400℃and600℃. The as-prepared samples were characterized with XRD, FE-SEM, XPS and UV-Vis DRS. Enhanced light absorption in both the UV and visible light region was observed for the resulted TiO2nanoparticles. Improved photocatalytic activity towards the degradation of methylene blue over C-N codoped TiO2nanoparticles was demonstrated under UV and visible light, respectively. The highest degradation rate was achieved for C-N codoped TiO2(13%) compared to N doped TiO2(10%) and the commercial P25(5%) under visible light illumination for40min. Dye-sensitized solar cells were fabricated using P25, N doped TiO2and C-N codoped TiO2photoanodes, respectively. The highest conversion efficiency of3.31%was achieved by the DSSCs based on the C-N codoped TiO2photoanodes in comparison with the commercial P25(1.61%) and N doped TiO2(2.44%) photoanodes.4. C-dots were successfully fabricated by electrochemical-etching graphite electrodes and investigated in tunning the performance of TiO2. TiO2nanotubes/C-dots nanocomposites were prepared via immersing TiO2nanotubes in C-dots solution and characterized by FE-SEM, XRD, UV-Vis DRS, photoelectrochemical and photocatalytic measurements. XPS measurement shows the presence of carbon species which arise from C-dots. A red shift of light absorption edge and more absorption in the visible light region were observed for the resulted samples from the UV-Vis diffuse reflectance spectra. An enhanced visible light photocurrent and photopotential were demonstrated for the C-dots sensitized TiO2nanotubes and the photocurrent density was2.7times larger than that of pristine TiO2nanotubes. A solar cell was assembled for further verifying the sensitization of C-dots over TiO2nanotubes. Furthermore, the sensitizing function of C-dots on the photocatalytic activity of TiO2nanotubes towards the degradation of methyl blue was demonstrated and13%enhanced degradation efficiency was observed with the presence of C-dots.5. A novel nanocomposites P25/C-dots were successfully prepared via a hydrothermal method using P25and carbon nanodots (C-dots) as the starting materials. The characterization of X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectra indicated the chemically bonded loading of C-dots on the surface of P25. The as-prepared P25/C-dots exhibited enhanced light absorption in both UV and visible region. The extended light absorption range to visible region was also observed. Their effective photocatalytic activity for water splitting under UV-vis light irradiation was investigated compared to P25. P25/C-dots nanocomposites also exhibited higher photocatalytic activity for the photodegradation of methylene blue under visible light irradiation.6. Needle-shaped dye-sensitized solar cells were successfully developed using anodized Ti wire and Pt/carbon fiber which. The as-prepared DSSC looks like a needle leaf of pine tree. Improved photovoltaic performance was demonstrated by the TiCl4post-treatment of photoanode. The energy conversion efficiency is2.21%which can be up to3.07%after TiCl4post-treatment of photoanodes. Pt nanoparticle/carbon fiber is firstly used as counter electrode and the cyclic voltammetries investigation show that Pt nanoparticle/carbon fiber possess higher electrocatalytic activity than Pt wire. In addition, its potential application in parallel connected situation was preliminarily explored. Such type DSSC is TCO free and may be capable of achieving a long distance transport of photocurrent and harvesting all light from any direction in surroundings to generate electricity.