Effect of Co-catalyst (CuO, CoO or NiO) on Bi₂O₃–TiO₂ Structures and Its Impact on the Photocatalytic Reduction of 4-nitrophenol

CoO, NiO or CuO oxides were employed as co-catalysts on Bi₂O₃–TiO₂ (BT) structure in the photocatalytic reduction of 4-nitrophenol under UV light irradiation. The intimate contact between the co-catalysts and base material contributed to an enhancement in the photocatalytic activity due to the formation of the p-n heterojunction that effectively separates charge carriers. CuO–BT, NiO–BT and Co–BT showed a reduction of 4-nitrophenol of 83%, 59% and 52% after 60 min under illumination, respectively, which are higher than that obtained with the BT semiconductor. The co-catalysts increase the donor density favoring the reduction of 4-nitrophenol. The donor density calculated for CuO–BT, NiO–BT and Co–BT was of N_d= 15.5 × 10¹⁸cm⁻³, N_d= 12.4 × 10¹⁸cm⁻³ and N_d= 9.41 × 10¹⁸cm⁻³, respectively. The results were associated to that CuO and NiO oxides are considered as reduction co-catalysts while CoO oxide is used as oxidation co-catalyst.