Enhanced Photoelectrochemical Performance of Anodic Nanoporous β-Bi2O3

Abstract

Bismuth oxide has well-dispersed valence bands that show enhanced mobility of charge carriers, high refractive index, and large dielectric constant. These properties are attractive for photo catalysis. Thin films of bismuth oxide containing vertically oriented and uniformly distributed nanopores of controllable dimensions were synthesized by a simple electrochemical anodization of a bismuth metal substrate in citric acid solution at 3–60 V. Annealing the anodic nanoporous Bi2O3 at 200°C for 2 h resulted in stabilization of the metastable β-Bi2O3 phase at room temperature. The nanoporous anodic oxides showed an energy bandgap of 2.48 eV, and n-type semiconductivity. Scanning electron microscopy and electrochemical impedance spectroscopy results suggested a dual layered structure of the anodic oxide with a nanoporous outer layer and a planar inner layer. Thickness of the inner layer predominantly influenced the impedance of the anodic oxide. Under simulated 1-sun intensity, a maximum photo current density of 0.97 mA/cm2 was observed in 1 M KOH at a potential of 1.53 VRHE for the sample anodized at 10 V. A positive shift in the flatband potential upon illumination suggested accumulation of photo generated holes due to low catalytic activity of the anodic bismuth oxide for oxygen evolution reaction. The observed photo current density of the nanoporous anodic oxide of bismuth was an order of magnitude higher than that of the planar anodic oxide, and twice that of thin film β-Bi2O3 prepared by reactive sputtering.