A Study On The Enhancement Of Photo(Electro) Catalytic Activity Of Titanium Oxide Under Visible Light

Semiconductor heterogeneous photocatalysis has been widely studied and recognized as a highly efficient and promising advanced oxidation technology for the degradation and/or elimination of environmental contaminants for two decades. Among all the used semiconductor photocatalysts, TiO₂, is the prior because of its high reactivity, chemical stability under ultraviolet light, nontoxicity, and low cost. However, the efficiency for photocatalytic reactions at least should be largely increased before the technology can be used on a large scale. In addition, the utilization of solar energy is greatly impaired by the intrinsic wide band gap (3.2 eV) of TiO₂, which requires ultraviolet irradiation for photocatalytic activation, since the UV light part accounts for only a small fraction, less than 5 %, of the sun's energy. Therefore, in this paper we devoted to search new methods to enhance the efficiency for photocatalytic reactions as well as preparation of the photocatalysts active to visible light by doping with one or two foreign elements into TiO₂ for the degradation of pollutants.Firstly, with TiO₂ photoanode, the photoctalytic degradation of methyl red and methylene blue were investigated in a short-circuit photoelectrochemical (PEC) cell. The results showed that by this approach the degradation rate of the two dyes were accelerated due to a reduced recombination of photogenerated carriers by the seperation of the anodic and cathodic reactions. The large the magnitude of the photocurrent is, the larger the degradation rate of methyl red under the same experimental conditions. Moreover, the seperation efficiency of photogenerated carriers, thus, the degradation rates could be further improved by setting an appropriate SnO₂ interlayer between the substrate and the TiO₂ film. Meanwhile, the photocatalytic activities of the electrodes were estimated by electrochemical impedance spectroscopy (EIS).Secondly, several doped TiO₂ photocatalysts with visible photoresponse had been prepared and their photo(electro)chemical performance were evaluated. 1. Co-doped TiO₂ powder photocatalysts. Three cobalt-precursors, i.e. CoCl₂, Co(NO₃)₂, and CoSO₄, with two doping levels (nominally 1 and 10 at.%) and two annealing temperatures (400 and 800℃) were adopted to prepare the doped titania by sol-gel method. The powder samples were well characterized by XRD, SEM, TEM, EDAX, BET, grain size distribution analysis, TG-DSC and UV-vis absorption spectroscopy to examine the effect of cobalt precursor on its microstructure and light absorption. Besides, their photocatalytic activities were evaluated by the degradation of aniline under visible light irradiation. The results showed that the distribution of titania phases, particle size, morphology, surface area and the optical absorption of the catalysts were greatly dependent on the cobalt precursors used. The samples prepared from Co(NO₃)₂, especially for that doped with 1% and calcined at 400℃, showed the highest photocatalytic activity towards the degradation of aniline, and the possible reason was discussed briefly.2. The Cr-doped TiO₂ thin film photocatalysts. The kinetics and mechanism for the photocatalytic oxidation (PCO) of NO₂^- under visible light were studied over the Cr-doped TiO₂ under the assistance of an anodic bias potential. The results showed that without applied anodic potential (E_appl , vs. saturated calomel electrode, SCE), the PCO reaction could not be initiated due to the rapid recombination rate of photogenerated carriers, but when the potential was larger than 1.2 V, the removal efficiency of NO₂^- was increased up to 90 % within 3 hours for an initial concentration of 8 mg l^-1. Apparent zero-order kinetics were observed at E_appl= 0.4 V, while pseudo-first-order kinetics were found at E_appl≥0.8 V. The effects of initial concentration of NO₂^-, initial solution pH, the gas atmosphere and the cathodic reaction on the PCO of NO₂^- were studied as well. Furthermore, the PCO mechanism of NO₂^- was investigated by using tert-butyl alcohol or benzoic acid as a diagnostic probe. It showed that the PCO of NO₂^- under visible light most probably proceeded indirectly via OH radicals, but not directly via the valence-band holes.3. The (Co,N)- and (Ru,N)-codoped TiO₂ thin film photocatalysts. The (Co,N)- and (Ru,N)-codoped TiO₂ thin film photocatalysts were obtained by introducing nitrogen into the cobalt- and ruthenium-doped TiO₂, respectively. Employing XRD, SEM, XPS and UV-vis absorption spectroscopy, the as-prepared thin films were characterized and their band structure, incident photon to current efficiencies (IPCE) and charge transfer rate were studied by PEC methods and EIS. The results showed that a remarkable increase in photocurrent was found for the codoped TiO₂ compared with those single doped, and a large red shift to c.a. 580 nm and 560 nm in PEC response was observed for the (Co,N)- and (Ru,N)-codoped TiO₂, respectively. The possible reason for such an enhancement was proposed in light of the experimental results. It suggests codoping with nitrogen be an effective and promising method to obtain a more active photocatalyst with visible light response.