Efficient photocatalytic removal of RhB, MO and MB dyes by optimized Ni/NiO/TiO2 composite thin films under solar light irradiation

Abstract

In this study, Ni/NiO/TiO2 composite powder and thin films were successfully prepared via sol-gel method. The results indicated that Ni/NiO/TiO2 composite with 2.5% Ni/Ti molar ratio, and the thin film calcined under 400 °C for 2 h with 3 coating layers, demonstrated the highest efficiency for Rhodamine B (RhB) degradation. Over 99.9% of RhB was photocatalytically degraded by Ni/NiO/TiO2 thin film under 60 min of simulated solar light irradiation, whereas merely 61.9% by pure TiO2 film. This remarkable ability is caused by suitable amount of Ni and NiO dopant (2.5%), which made the composite present a smaller crystallite size (5.16 nm), bigger specific surface area (154.72 m2/g), lower band gap energy (2.78 eV) and recombination rate of electron-hole pairs. Additionally, the photocatalytic activity did not exhibit significant loss even after repeated use of ten cycles, suggesting the high stability of the Ni/NiO/TiO2 thin film. The degradation test of Ni/NiO/TiO2 thin film for different dyes (Methyl blue, Methyl orange and Rhodamine B) revealed that the photocatalyst has remarkable activity on dye degradation. Moreover, the radical trapping experiments demonstrated that the major active species including O2− and OH played important roles in the photocatalytic process. The proposed photocatalytic mechanism showed that Ni and NiO could reduce the recombination of photo-generated electron-hole pairs and decrease the band gap energy of composite, resulting in enhanced photocatalytic ability and enlarged solar spectral response range.