Yellow TiO2 from titanium peroxo complexes: Verification of the visible light activity and a rational enhancement of its photocatalytic efficiency

Abstract

It is widely believed that colored TiO2 materials (yellow, blue, black, violet, etc.) are visible-light active photocatalysts. In this article, we verify this claim with respect to the yellow TiO2 photocatalyst that has been synthesized using peroxo-titanium complexes formation route. The surface oxygen species, the electronic structure, the photoelectrochemical properties, and the photoactivity of yellow TiO2 have been investigated. FT-IR and XPS analysis confirms the presence of surface peroxo-species that caused the yellow color. Spectroelectrochemical measurements confirmed the absorption of violet light (400–430 nm). The reduced band gap energy (3.05 eV) has resulted from the anodic shift of both conduction and valence band edges by 0.30 and 0.15 eV, respectively. However, the wavelength-dependent photocurrent generation by yellow TiO2 in the presence of oxygen and under deaerated conditions proved that it does not contribute to the photoactivity enhancement in the visible region (λ > 400 nm). The proposed mechanism has shown that although the driving force at the oxidation side is theoretically improved, the reduction counterpart is parallelly weakened and the photoinduced electrons cannot efficiently reduce regular electron acceptors in aqueous systems (e.g., O2, H2O). Adding a low amount of a stronger electron acceptor (e.g., H2O2) into the system provides extra HO• radicals through the reductive mechanism. At higher concentrations of H2O2, the oxidation of H2O2 is more favorable. The hydroxylation of terephthalic acid via the oxidative and reductive pathway along with the bleaching of Congo Red indicates improved photocatalytic activity due to enhanced hydrogen peroxide activation. Although yellow TiO2 is active only upon UV light irradiation, this paper reports how to boost the photocatalytic activity of this material.