Oxygen Vacancy and Adsorbed Superoxides Dependent Photocatalytic Activity of TiO2 Quantum Dot Thin Films for Degradation of Methylene Blue with Variation of Precursor Concentration

Abstract

Growth of TiO2 quantum dot thin films on ITO glass substrate by hydrothermal method at 180 °C with variations of titanium precursor concentration (0.3, 0.5, 0.7, and 1 ml) is reported. UV–vis absorption spectrum shows a strong absorbance peak in the UV region. The bandgap of samples ranged from 3.2 to 3.6 eV. FESEM analysis shows a layered sieve-like porous morphological structure. The particle size is less than 10 nm. The FTIR analysis reveals the presence of isolated adsorbed Ti–OH and –OH groups which take part in photocatalysis reactions to oxidize dye molecules. The superoxides and hydroxyl radicals were yielded by the protonation process. The oxygen vacancies, confirmed from XPS analysis, play an important role in the formation of impurity level states below the conduction band with the accumulation of a large number of electrons. These electrons further take part in the hydrogen production process. This investigation demonstrates that thin film with 0.5 ml precursor volume shows enhanced MB dye degradation activity under UV light. The PL emission is quenched in the UV region for a sample having 0.5 ml precursor volume, enhancing the probability of charge carrier separation. Thus significant results in photocatalytic MB dye degradation activity are obtained.