New insights on the enhanced non-hydroxyl radical contribution under copper promoted TiO2/GO for the photodegradation of tetracycline hydrochloride

TiO₂/graphene oxide (GO) as photocatalyst in the photo-degradation of multitudinous pollutants has been extensively studied. But its low photocatalytic efficiency is attributed to the high band gap energy which lead to low light utilization. Cu-TiO₂/GO was synthesized via the impregnation methods to enhance the catalytic performance. The Cu-TiO₂/GO reaction rate constant for photo-degradation of pollutants (tetracycline hydrochloride, TC) was about 1.4 times that of TiO₂/GO. In 90 min, the removal ratio of Cu-TiO₂/GO for TC was 98%, and the maximum degradation ratio occurred at pH 5. After five cycles, the removal ratio of Cu-TiO₂/GO still exceeded 98%. UV-visible adsorption spectra of Cu-TiO₂/GO showed that its band gap was narrower than TiO₂/GO. Electron paramagnetic resonance (EPR) spectra test illustrated the generation rate of •O₂^- and •OH was higher in Cu-TiO₂/GO system than TiO₂/GO and TiO₂ system. The contribution sequence of oxidative species was •O₂^- > holes (h⁺) > •OH in both TiO₂/GO and Cu-TiO₂/GO system. Interestingly, the contribution of •OH in Cu-TiO₂/GO was less than that in TiO₂/GO during the photo-degradation process. This phenomenon was attributed to the better adsorption performance of Cu-TiO₂/GO which could reduce the accessibility of TC to •OH in liquid. The enhanced non‑hydroxyl radical contribution could be attributed to that the more other active species or sites on (nearby) the surface of Cu-TiO₂/GO generated after doping Cu. These results provide a new perspective for the tradition metal-doped conventional catalysts to enhance the removal of organic pollutants in the environment.