Synergistic photo-enhanced electrocatalysis of Pt–ZnO–Bi2O3 heterojunction for methanol oxidation under visible light illumination

Abstract

Pt-based bismuth oxide (Bi₂O₃), zinc oxide (ZnO), and Bi₂O₃–ZnO heterojunctions were synthesized and tested in the photo-electrocatalytic oxidation of methanol in alkali under visible light irradiation. The decreased band gaps in Bi₂O₃ (2.05–2.36 eV) and ZnO (2.61–2.94 eV) in the ternary junctions compared to those in Pt–ZnO (2.95 eV), Pt–Bi₂O₃ (2.25 eV), bare Bi₂O₃(2.71 eV) and bare ZnO (3.38 eV) are seemingly responsible for the enhancement of light-absorption causing synergistic catalytic effects. The cyclic voltammetric peak current density of the ternary heterojunction, PZ5B1 (ZnO : Bi₂O₃ = 5 : 1) is improved 5.2, 4.1, and 3.6 times that of Pt–Bi₂O₃, Pt–ZnO and commercial Pt/C, respectively, under visible light. The composite PZ25B1 (ZnO : Bi₂O₃ = 25 : 1) exhibited 1.62 times greater activity under light than the corresponding dark value. The light response gradually increases with the increasing mass percentage of ZnO having a maximum value in PZ25B1 and decreases again. Due to the smallest Pt size (2.51 nm) and high electrochemical surface area (30.9 m² g⁻¹) in PZ25B1, methanol adsorption increases and the small bandgap helps absorb more visible light and creates large number of holes, which makes methanol oxidation faster. The negative shift of the peak potential under illumination in the CO stripping experiment indicates easy removal of adsorbed CO species on the Pt surface of the catalysts. The charge-transfer conductance of the best light-responsive and highly stable catalyst, PZ25B1 is 1.86 times greater in light than that in the dark. The stability of the PZ25B1 catalyst is judged from multiple cycling in cyclic voltammetry and chronoamperometric study. Chromatography experiments help in the identification of MOR product selectivity and mechanistic pathways.