TiO2 modified with polyoxotungstates should induce visible-light absorption and high photocatalytic activity through the formation of surface complexes

Abstract

TiO2 nanospherical particles prepared by the sol-gel method were modified with tungstophosphoric (TPA) and tungstosilicic acid (TSA) at 30% (w/w) and then annealed at 500°C for 2h. Materials were characterized by multitechniques, such as scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), FT-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), 31P, 1H and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR). Acidity and isoelectric point measurements were also performed. DRS results showed evidence of visible light absorption of TiO2-TPA and TiO2-TSA. MAS-NMR, XPS and FT-Raman characterization revealed evidence of a strong interaction between the Keggin anion of TPA or TSA and TiO2 surfaces, possibly promoted by the formation of surface heteropolyacid-TiO2 complexes; the latter should be responsible of its visible light absorption. Photocatalytic activity was evaluated in the degradation of malachite green (MG), a cationic triphenylmethane dye, and 4-chlorophenol (4-CP) aqueous solutions under UV-A and visible light irradiations. Under UV-A light, TPA-TiO2 and TSA-TiO2 materials showed high photocatalytic activity towards MG degradation, even higher than that of TiO2 P-25. When 4-CP was degraded, TiO2 P-25 exhibited higher photocatalytic activity. Under visible light irradiation and using the modified photocatalysts, malachite green oxidation was achieved, while 4-chlorophenol reached almost 20% of degradation. Mechanisms of photocatalytic oxidation either under UV-A or visible irradiation are discussed.