Effect of oxygen vacancies in non-stoichiometric ceria on its photocatalytic properties

Abstract

Non-stoichiometric ceria nanoparticles were synthesized by three different methods: plasma heating of commercial ceria (c-CeO2), treatment of a precursor with a molten KOH-NaOH salt system, and solid-state reaction of c-CeO2 with reductant NaBH4. These processes increased the concentration of oxygen vacancies and Ce3+ fraction on ceria surface, which was observed from the results of Raman spectroscopy and X-ray photoelectron spectroscopy of the products. Raman results indicated that the plasma treated ceria (p-CeO2) had the largest downshift of 7.9 cm−1, followed by the molten salt treated ceria (m-CeO2), NaBH4-reduced ceria (t-CeO2) and commercial nanoceria (c-CeO2), while results from XPS indicated that p-CeO2 had the largest Ce3+ fraction. The photocatalytic properties of these non-stoichiometric ceria were evaluated using the degradation of methylene blue (MB) under UV–Vis light irradiation, and the kinetic studies indicated that pseudo-first-order kinetics was followed based on the Langmuir–Hinshelwood mechanism. The reduced photoluminescence (PL) intensity in p-CeO2 indicated that the oxygen vacancies effectively suppress the recombination rate of photogenerated electron–hole pairs. Hence, p-CeO2 exhibited an enhanced photocatalytic activity compared to other non-stoichiometric ceria samples and a detailed mechanism for the enhancement in photocatalytic activity is discussed.