Influence of metal ions on the charge recombination processes during TiO2 photocatalytic one-electron oxidation reactions

The influence of inorganic salt, such as NaClO4, LiClO4, and Mg(ClO4)(2), on the charge recombination process during the TiO2 photocatalytic one-electron oxidation reaction of stilbene derivatives has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. In the case of trans-4-methoxystilbene (MtSt), a one-electron oxidation process in the submicrosecond time domain was clearly observed. This is the first example of a direct observation of an electron transfer (ET) from the organic compound adsorbed on the TiO2 surface to the photogenerated holes in TiO2. The average lifetime (tau) of the MtSt radical cation (MtSt(.+)), which is generated from the one-electron oxidation reaction by the photogenerated holes at the TiO2 surface, significantly increased with the addition of metal ions, especially, Mg2+, in Ar-saturated acetonitrile (MeCN). On the other hand, a slight enhancement was observed for the stilbenemethanol radical cation (StM(.+)), although tau of Stm(.+) generated from the photosensitized electron transfer reaction with 9, 10-dicyanoanthracene (DCA) in the excited singlet state significantly increased with the addition of metal ions. These results clearly suggest that the adsorption dynamics of substrates (S), such as MtSt and StM, and metal ions on the TiO2 surface is important for the back electron transfer (BET) process between S.+ and the photogenerated electrons in the TiO2 particles. To clarify the influence of metal ions on the decay kinetics of S.+, the driving force dependence of BET rate constants (k(BET)) was examined in terms of Marcus theory. It was found that a significant increase in tau of S.+ with the addition of metal ions results from suppression of the BET process between S.+ and the photogenerated electrons due to the desorption of S.+ from the TiO2 surface.