Disparate roles of doped metal ions in promoting surface oxidation of TiO2 photocatalysis

•Ag+ and Cu2+ doped TiO2 significantly outperformed other doped materials.•Disparate roles of doped metal ions other than electron captors were proposed.•Ag+ doping enhanced DO adsorption and ROS generation upon TiO2 surface.•Cu2+ doping greatly facilitated surface oxidation by hvb+ and OHads.

The key standpoint favors the role of doped metal ions as electron traps in enhancing surface-charge separation during TiO2 photocatalysis. Besides that, this study demonstrated two disparate impacts of metal ion doping on reactive species generation during TiO2 photocatalysis. Ag+ and Cu2+ outperformed other dopants in photooxidation of oxalic acid (OA), while further increasing their doping amount from the optimum value produced a negligible impact on the photocatalytic performance. This implied other dominant roles of doped metal ions other than electron captors, because an excessive amount of traps can serve as charge-recombination centers and largely decrease the photocatalytic activity. N2 purging and NaF (a scavenger for surface-radicals) markedly reduced the mineralization rate after Ag+ doping, verifying the significance of dissolved oxygen (DO) in reactive species generation upon TiO2 surface. Non-OH reactive oxygen species (ROS) were found to account for 75% in OA mineralization, attributed to enhanced DO adsorption towards TiO2 surface by Ag+ doping. However, Cu2+ doping could facilitate the oxidation by hvb+ plus OHads, which took as dominant a part as 53%. Ag+-Cu2+ co-doping integrated these two positive sides and more inclined to the role of Ag+. Correspondingly, surface oxidation dominated whereas liquid-phase oxidation weakened.

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