Visible-Light Photocatalysis with Mullite-Type Bi 2 (Al 1– x Fe x ) 4 O 9 : Striking the Balance between Bandgap Narrowing and Conduction Band Lowering

CURTI, MARIANO; KIRSCH, ANDREA; GRANONE, LUIS I.; TARASI, FACUNDO; LÓPEZ-ROBLEDO, GERMÁN; BAHNEMANN, DETLEF W.; MURSHED, M. MANGIR; GESING, THORSTEN M.; MENDIVE, CECILIA B.

ACS Catalysis

American Chemical Society

Año: 2018 vol. 8 p. 8844 - 8855

2155-5435

The rich crystal chemistry of mullite-type Bi2M4O9 (M = Fe, Al, Ga) offersmultiple potential applications. In particular, the strong absorption of visible light shownby Bi2Fe4O9 has led to an influx of research on its photocatalytic properties. However,most of the published studies involve the decolorization of dyes and take it as proof of itsphotocatalytic activity; furthermore, there are no reports on its conduction and valenceband edges, and, thus, the actual redox characteristics of the photogenerated chargecarriers have not been determined. Here, we evaluate the photocatalytic activity towardmethanol oxidation under monochromatic visible light (λ = 450 nm) irradiation of 12 different members of the Bi2(Al1−xFex)4O9 (x = 0−1) series of compounds. The reactionrate reaches its highest value at an iron fraction of x = 0.1, while the compounds with thehighest iron fractions present negligible activity. Based on an extensive characterization,which included the Rietveld refinement of the XRD patterns, the measurement of the specific surface areas by the BET method,and the determination of the flat-band potentials by the Mott−Schottky method, we rationalize the results on the basis of twoopposing factors: the incorporation of iron narrows the fundamental bandgap and thus improves light capture, but at the sametime it lowers the conduction band edge, hindering the oxygen reduction half-reaction and thus promoting electron−holerecombination. Our work highlights the importance of a proper band edge engineering for photocatalytic applications andunderlines the inadequacy of dye decolorization tests for visible light-active materials.