In Situ Photoconductivity Kinetic Study of Nano-TiO2 during the Photocatalytic Oxidation of Formic Acid: Effects of New Recombination and Current Doubling

Abstract

In the present research, in situ photoconductivity (σ) was used to study the electron kinetics of nano-TiO2 films during photocatalysis of formic acid under UV light irradiation. Some interesting features of the in situ σ were observed: (a) when the light was turned on, the in situ σ showed a relatively slow decrease just after a fast increase; (b) when the light was stopped, the in situ σ decayed much faster than that in pure water; (c) the in situ σ presented an abnormal increase when decaying toa dark value due to the reinjection of electrons to TiO2 CB. We comprehensively studied the effects of formic acid amounts, UV light intensity, UV light irradiation time, and dark preadsorption time on the in situ σ, indicating the presence of the new recombination and the current-doubling effect. It was seen that the new recombination and the current-doubling effect can be weakened by soft water washing, and the presence of water also is important for the appearance of the new recombination and the current-doubling effect. Combined with the first-principles calculation, it was confirmed that the weakly adsorbed formic acid groups near the TiO2/water interface should mainly contribute to the new recombination and the current-doubling effect. A kinetic model was proposed to simulate the time dependence of the in situ σ during the formic acid photocatalysis. The simulation shows that the inclusion of the new recombination and the current-doubling effect agrees well with the experimental results. Lastly, the effects of Au deposition on the in situ σ of TiO2 film during the photocatalysis of formic acid were studied. The interfacial transfer of electrons from TiO2 to Au can be identified by the in situ σ, which wakens the new recombination and the current-doubling effect.