Influences of Fe[sbnd]Mn ratio on the photocatalytic performance of wolframite (FexMn1-xWO4)

Abstract

Natural semiconducting minerals are widely distributed in supergene environments and are famous for their solar-driven redox activities, in which wolframite (FexMn1-xWO4) as a fascinating member got less attention. In this work, the semiconducting photocatalytic activities of FexMn1-xWO4 series samples (x = 1, 0.74, 0.48, 0.24, 0) influenced by their crystal chemistry were investigated. The bandgaps of MnWO4, Fe0.24Mn0.76WO4, Fe0.48Mn0.52WO4, Fe0.74Mn0.26WO4 and FeWO4 as measured by UV–vis diffuse reflection (DRS), were 2.7, 2.4, 2.3, 2.2, and 2 eV, respectively, which were linearly decreased (R2 = 0.971) when x increased from 0 to 1. The density functional theory (DFT) calculations further indicated with the increasing content of Fe, the contribution of valence-band maximum (VBM) was stepwise occupied by Fe 3d orbits and the bandgap was thus gradually decreased. The photocatalytic activities of FexMn1-xWO4 samples were examined on the degradation of methylene blue (MB, 5 mg/L). The MB removal rate was the highest in Fe0.74Mn0.26WO4 system, which was 3.2, 1.9, 1.2, and 1.5 times faster than that of MnWO4, Fe0.24Mn0.76WO4, Fe0.48Mn0.52WO4, and FeWO4, respectively. The concentration of produced hydroxyl radical (•OH) by FexMn1-xWO4, detected in electron paramagnetic resonance (EPR) measurement, had a positive correlation with the degradation rate of MB. The degradation rate slowed down when •OH was removed, demonstrating that •OH was the major reactive oxygen species in the photocatalytic oxidative degradation of MB. The best-performing Fe0.74Mn0.26WO4 photocatalytically produced the most •OH, which was closely linked with its most abundant oxygen-vacancy defects, revealed by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. It thus can be concluded that narrow bandgap and appropriate oxygen vacancies can give rise to synergistic effect on the improvement of photocatalytic performance. This study helps get insight into the role of crystal chemistry in semiconducting properties and photocatalytic activities of wolframite, which also puts forward a new strategy to control environmental pollution by using natural minerals.