Fabrication And Water Photoelectrolysis Properties Of TiO₂ Nanotube Arrays By Anodization

Since the discovery of photoelectrochemical splitting of water into hydrogen and oxygen on n-TiO₂ electrodes, semiconductor-based photoelectrolysis of water has received much attention in hydrogen fuel. A variety of reports have indicated that the highly ordered vertically oriented nature of the crystalline TiO₂ nanotube (NT) arrays makes them excellent electron percolation pathways for vectorial charge transfer between interfaces. Furthermore, to increase water splitting efficiency, several attemps have been made to obtain a good visible light adsorption for n-TiO₂ NT arrays by metal and non-metal doping, combinainon with narrow-band gap semiconductor.In the present work, we focused on the development of electrochemical anodic oxidation technique to prepare self-organized TiO₂ NT arrays on titanium foil. The effects of the process parameters on the TiO₂ NT arrays growth were systemically studied. On the other hand, we examined the use of TiO₂ NT arrays as anode for the photocleavage of water under UV and Xe lamp irradiation, with particular emphasis on the enhancement of photoelectric efficiency by modifying the TiO₂ NT arrays, such as introducing vacancies in oxygen lattice sites, depositing Pt or CdS microcrystals onto TiO₂ NT. The main results and progresses of this dissertation are outlined as following:1. Highly density, well ordered and uniform TiO₂ NT arrays were fabricaed by electrochemical anodization of titanium sheets in the HF/water electrolyte. The results confirmed that the length of NT arrays was limited to 500nm due to high chemical dissolution rate of solution to the top of TiO₂ NT arrays. In NH4F/glycol electrolytes, the ordered TiO₂ NT arrays with lengths up to 36μm were achieved because of the low quality barrier layers through which ionic transport may be enhanced. We have demonstrated that TiO₂ nanowires with a diameter of 20nm and a length up to several micron only can be synthesized in NH4F/glycol solution with a small amount of water and high anodic voltage. The nanowires originated from the vertical splitting of anodically grown nanotubes.2. A new NH4F/glycerol system has been developed to prepare the controllable architecture of TiO₂ NT arrays. It was found that uniform surface morphology of the NT arrays was obtained in acidic condition. However, the basic and inhibitor (HMT and NaAc) added environments were much more efficient for relatively longer nanotubes by effectively slowing the chemical dissolution rate at the tube mouth. The TiO₂ NT arrays with 60nm inner pore diameter, 3.3μm length and 20nm wall-thickness, annealed at 550℃, behaved a remarkable water photoelectrolysis efficiency of 23.8% upon UV illumination at intensity of 1.6mW/cm2.3. Optical absorption spectra showed that TiO₂ NT arrays annealed under H2 atmosphere noticeably absorbed the light at visible light by introducing vacancies in oxygen lattice sites, whereas the TiO₂ NT arrays annealed under air and Ar atmosphere did not. So the TiO₂ NT arrays (H2) generated photocurrents double of what the others (air and Ar) sample do in the Xe illustration. Pt nanoparticles were successfully deposited on the surface of TiO₂ NT arrays by ac electrodeposition and dipping method. There was one catalytic peak of the electrodeposition electrode, which was at -0.95 V vs. Ag/AgCl attributed mainly to Pt2+ catalysis. There were two catalytic peaks of the dipped electrodes, one was at -0.95 V vs. Ag/AgCl attributed mainly to Pt2+ catalysis and the other was at -0.7 V vs. Ag/AgCl attributed to Pto catalysis.4. Furthermore, a novel fabrication route for core/sheath heterostructure CdS@TiO₂ NT arrays was proposed using ac electrodeposition for application in photoelectrochemical cells. The deposited material was found to be hexagonal CdS structure annealed at 400℃. The maximum photocurrent density was obtained with the core/sheath heterostructure CdS/TiO₂ nanotube arrays with 2.5μm tube length, which were fabricated by CdS deposition at 5 V for 30 min.