Changes in the solid-state properties of bismuth iron oxide during the photocatalytic reformation of formic acid

Abstract

BiFeO3 nanoparticles were synthesized using a sol gel method and its photocatalytic ability for formic acid reformation is reported, for the first time, focusing on the BiFeO3 stability after photocatalytic reaction at ∼ pH 3. Reformation of formic acid in O2-free aqueous suspensions was found to result in the evolution of CO2 and H2 gases. However, the expected molar ratio of 1:1 was not evolved. Significantly lower amounts of H2 than expected were detected. The position of the conduction band of BiFeO3 resides well below the reduction potential of the protons to form molecular H2, hence it is anticipated that the photoexcited electrons react mainly with other species being present in the reactor, probably the photocatalyst itself. To assess the changes occurring in the photocatalyst, bulk and surface analysis before and after the photocatalytic reaction were performed using different techniques. XRD revealed changes occurring in the bulk of the photocatalyst such as leaching of most of the impurity phases accompanied by the relaxation of the strained lattice to its ideal position. From XPS, the ratio between Fe3+: Fe2+ before and after photocatalytic reaction with formic acid was found to be 86:14 and 64:36, respectively, thus evincing a reduction process of Fe3+ by the photo generated electrons in the conduction band that were unable to reduce H+ into H2. Leaching of Fe and, to less extent, Bi ions into the solution after reaction was recorded. From the fitted XPS, the nominal composition of BFO was calculated to be Bi1Fe0.9O2.9 and Bi1Fe0.71O2.7 before and after the photocatalytic reaction. The results presented in this work signify the importance of addressing bulk and surface stability of iron-based photocatalysts to ensure long-term usability.