(A) study on the photocatalytic activity under visible light of N-doped titania synthesized at supercritical water condition

Abstract

Although titania is known to be a good photocatalyst due to its inherent photocatalytic activity, high stability, and low toxicity, its photocatalytic activity under visible light is rather very low as observed in view of its wide band gap energy. Various metal-doped titania or nonmetal-doped titania were suggested to overcome such limitations. Metal-doped titania has rather lower thermal stability and the doped metal could act as a recombination site of charge carriers, while the property of nonmetal-doped titania is greatly dependent on the preparation conditions. In this study N-doped titania was continuously prepared by one-step synthetic method under supercritical and subcritical water conditions using titanium(Ⅳ)tetraisopropoxide (TTIP) and nitric acid (and ammonium nitrate) as titania precursor and nitrogen source, respectively. The effects of the crystallite size and the surface area of N-doped titania on the photocatalytic activity under visible light were investigated in comparison with commercial titania (P25). The synthesized N-doped titania was characterized by XRD, $N_2$-adsorption, SEM, TEM, XPS and UV-Vis DRS. The photocatalytic activity was evaluated by the degradation degree of methyl orange under visible light. The synthesized N-doped titania had pure anatase phase with high surface area. The crystallite size increased and surface area decreased when higher synthesis temperature was employed during continuous hydrothermal synthesis. In UV-Vis DRS spectra, the absorption edge of synthesized N-doped titania shifted into the visible light region compared with P25 through reduced band gap energy. The amount of ammonium nitrate used as N source did not significantly influence the UV-Vis DRS spectra of N-doped titania. The photocatalytic acvitiy of all synthesized N-doped titania for methyl orange degradation was much higher than that of P25 under visible light. Among them, N-doped titania synthesized under supercritical condition had the high...