Photoelectrochemical Stability Of TiO₂ And Its Photoelectrochemical Anticorrosion Against Metal

In recent decades, the photoelectochemical anticorrosion property of semiconductor has been drawn more attention besides its wide applications in light-electricity conversion and disposal of environmental pollutants. In the process of photoelectrochemical anticorrosion against metal, the excited energy can be derived from solar energy and photoanodes will not sacrifice, so that this method has long lifetime and potential application prospects. However, the recent researches are only on the threshold of this field. Most of the photoanode materials studied are the wide band gap semiconductors such as TiO₂ and the cathode materials are those metals with more positive corrosion potential such as stainless steel and copper. And the focus was concentrated on demonstrating the feasibility of photoelectrochemical anticorrosion against those metals. When it comes to the effect of TiO₂ photoelectrochemical stability, which plays a very important role in the protection process on the successive photocathode protection, we found there were few literatures studied about this.In the first part of this paper, we studied the kinetics of TiO₂ photoanodes attained by different methods using electrochemical impedance spectroscopy under different applied anodic potential and steady state measurements. The later experiments present the photoresponse of electrode P25 (Germany, Degussa) and nanoporous electrode SG derived by sol-gel, was more effective than that of rutile nanoporous electrode RU and electrode TO derived by thermal oxidation. And the EIS results show that all the TiO₂ photoanodes are not ideal semiconductor electrodes; some of the applied potential dropped over the Helmholtz layer, i.e.Fermi level pinning, changing the charge transfer constant, which would affect the photovoltage, instead of dropping over the space charge layer thoroughly to suppress the recombination of photogenerated carriers. The effect of such a pinning of the SG and P25 electrodes was more obvious than that of TO.In the second part, we studied the photoelectrochemical stability of TiO₂ photoanode. Three kinds of TiO₂ electrodes with different crystalline were prepared and the effects of different experimental conditions on the photoelectrochemical stability of TiO₂ such as pH value, concentration of hole scavenger etc were investigated. It turned out that all the photoanodes studied were photoelectrochemically stable in acid and neutral conditions but less stable in alkaline condition, especially for SG photoanodes. The photoelectrochemical stability followed the order: SG < P25 < TO. Such instability is, however, revisable and could be recovered by immersing in acid medium. According to the experimental results, we proposed the most probable mechanism that could explain our results very satisfactorily. This provides a method for photoelectrochemical anticorrosion against metal for long term.In the last part, the photoelectrochemical anticorrosion of SG photoanodes against carbon steel and stainless steel was studied. It was found that the SG photoanodes could protect carbon steel from corrosion under the conditions that corrosion electrolyte was 5 mol dm^-3 NaCl and the light source was 500 W Xe lamp, the electrolyte in anodic compartment was 0.5 mol dm^-3 Na₂SO₄ + 0.1 mol dm^-3 NaOH with or without methanol, but the protection effect was losing gradually. However, the corrosion rate was accelerated in acid, neutral conditions and in pure water of different extent. The possible reason was also discussed briefly in this part.