Relationships between the structure of P-doped TiO2 xerogels and their photocatalytic properties

TiO2 heterogeneous photocatalysis has been the subject of numerous recent investigations as it is an attractive technique for the complete destruction of undesirable contaminants both in aqueous and gaseous phase by using solar or artificial light illumination. However, TiO2-based materials present a large band gap and therefore only a small fraction of solar light, in the UV region, can be utilized. Furthermore, anatase-TiO2 transforms to the rutile structure at temperatures relatively lower, which greatly reduces surface areas of the particles resulting in the decrease in photocatalytic ability of TiO2. To counter both disadvantages, several studies have been conducted by doping non-metallic elements, such as phosphor, into the Ti-O framework to form TiO2 solutions. In the present study, a sol-gel method is developed to synthesize P-doped TiO2 xerogels by a cogelation method based on the hydrolysis and the condensation of Ti(OC3H7)4 with functionalized P alkoxides: NH2-(CH2)2-NH-(CH2)2-P(O)-(OC2H5)2, or (C2H5O)2-P(O)-(CH2)7-CH3, in various alcohols. These xerogels are dried at 150°C under vacuum for 24 h, and calcined under air for 6 h at 350°C, 450°C, 550°C and 650°C. The resulting materials were characterized by TG-ATD, TEM, XRD, nitrogen adsorption-desorption isotherms, FT-IR and diffuse reflectance measurements in the UV/Vis region. It was found that the phosphor-doped species could significantly increase the surface area of the materials. Moreover, the phosphor-doping improved the thermal stability of titania and decreased the phase transformation of anatase to rutile. Diffuse reflectance measurements proved that the modification by phosphor shifted the absorption edge of titania to the visible region, making it an effective photocatalyst in visible light. This is shown by the degradation of p-nitrophenol under visible light irradiation. The excellent photocatalytic activity of P-doped TiO2 xerogels compared to pure TiO2 could be explained by its high surface area and small crystallite size