Rutile (TiO2) anodes prepared by arc-plasma spraying of raw futile powder containing Si, Fe, Cr, Ni, and Mn impurities performed similarly in photoelectrolysis experiments to polycrystalline TiO~ electrodes made by chemical vapor deposition and by oxidation of metal foils. The impure arcplasma-sprayed anodes showed photoresponse characteristics and flatband potential comparable to that of single crystal TiO2, but the breakdown voltage was lower. The low breakdown voltage was attributed to electron tunneling through the thin depletion region of the strongly reduced TiO2 via grain boundaries containing impurity atoms and second phases.As a result of the recent interest in photoelectrolysis of water using compound semiconductor electrodes such as TiO2, different forms of electrode materials are being studied (1-6). Electrodes of polycrystalline materials perform similarly to single crystals because the semiconductor-electrolyte interface alleviates the electron-hole recombination problem. Studies of polycrystalline TiO2 electrodes made by chemical vapor deposition (CVD) (3, 4), by oxidation of metal foils (5, 6), and by hot pressing o~ rutile powder (6) showed that; the photoresponses and energy conversion efficiencies were comparable to that of single crystal TiO2. However, while much effort has been directed toward characterizing pure materials on the performance of photoelectrolysis, the effects of impurities in raw materials were little investigated. Additional economic advantages may be gained if the degradation of performance due to impurities in raw materials and impurities introduced via the containment used in the fabrication process is tolerable.Here, we report the results of the investigation of photoelectrolysis of water using arc-plasma sprayed raw futile anodes. Because raw rutile is a low cost, commercial form of TiO2, and arc-plasma spraying is an economical and versatile fabrication process to produce thin or thick coatings with unlimited types of materials, we hope to demonstrate the implication of the mass fabrication process with raw materials. Although the process was not optimized in this study, we found that the arc-plasma sprayed rutile performed comparable to other polycrystalline and single crystal TiO2 electrodes, except for the degradation of the property, such as a low breakdown voltage that may be associated with the microstructure.
Anode PreparationThe raw rutile powder, obtained from Metco, Incorporated, was brownish and contained nearly 1 w/o (weight percent) of Si and Fe as major impurities and less than 1 w/o total of Cr, Ni, and Mn as minor impurities according to x-ray elemental analyses. Plasmasprayed ruti]e coatings can be made that range from electrical conductors to insulators, depending on the power of the plasma and the distance from the torch to the substrate. The typical coating used in this study had a resistivity of ~150 &2-cm with a blue-black color indicating oxygen deficiency. X-ray diffraction showed that the plasma-sprayed TiO2 coatings had the structure of rutile ...