TY - JOUR

T1 - Visible-Light Photocatalysis with Mullite-Type Bi2(Al1–xFex)4O9: Striking the Balance between Bandgap Narrowing and Conduction Band Lowering

AU - Curti, Mariano

AU - Kirsch, Andrea

AU - Granone, Luis I.

AU - Tarasi, Facundo

AU - López-Robledo, Germán

AU - Bahnemann, Detlef W.

AU - Murshed, M. Mangir

AU - Gesing, Thorsten M.

AU - Mendive, Cecilia B.

N1 - Funding information: The authors acknowledge MINCYT-FONCYT (No. PICT 1456-2013) and UNMDP (No. EXA794/16) for the financial support. C.B.M. is a member of the research staff of Consejo Nacional de Investigaciones Cientifí cas y Te?nicas (CONI-CET). M.C. is grateful to CONICET for his doctoral and postdoctoral fellowships, and to the MAPEX Center for Materials and Processes (University of Bremen) for the Research stay. The authors would like to thank Iva? Berengeno (UNMdP) for his help setting up the photocatalytic reaction, and Joaqui? H. Ubogui, Paula Cecilia dos Santos Claro, and Antonela Ca?neva (YPF Tecnologiá S. A.) for the XPS spectra and HRTEM micrographs. L.I.G. acknowledges the Nieder-sachsisches? Ministerium für Wissenschaft und Kultur (NTH-research group “ElektroBak”) for the financial support. Finally, we would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support in the large scientific instrument program (University of Bremen, No. INST 144/435-1 FUGG).

PY - 2018/9/7

Y1 - 2018/9/7

N2 - The rich crystal chemistry of mullite-type Bi 2M 4O 9 (M = Fe, Al, Ga) offers multiple potential applications. In particular, the strong absorption of visible light shown by Bi 2Fe 4O 9 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 its photocatalytic activity; furthermore, there are no reports on its conduction and valence band edges, and, thus, the actual redox characteristics of the photogenerated charge carriers have not been determined. Here, we evaluate the photocatalytic activity toward methanol oxidation under monochromatic visible light (= 450 nm) irradiation of 12 different members of the Bi 2(Al 1-xFe x) 4O 9 (x = 0-1) series of compounds. The reaction rate reaches its highest value at an iron fraction of x = 0.1, while the compounds with the highest 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 two opposing factors: the incorporation of iron narrows the fundamental bandgap and thus improves light capture, but at the same time it lowers the conduction band edge, hindering the oxygen reduction half-reaction and thus promoting electron-hole recombination. Our work highlights the importance of a proper band edge engineering for photocatalytic applications and underlines the inadequacy of dye decolorization tests for visible-light-active materials.

AB - The rich crystal chemistry of mullite-type Bi 2M 4O 9 (M = Fe, Al, Ga) offers multiple potential applications. In particular, the strong absorption of visible light shown by Bi 2Fe 4O 9 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 its photocatalytic activity; furthermore, there are no reports on its conduction and valence band edges, and, thus, the actual redox characteristics of the photogenerated charge carriers have not been determined. Here, we evaluate the photocatalytic activity toward methanol oxidation under monochromatic visible light (= 450 nm) irradiation of 12 different members of the Bi 2(Al 1-xFe x) 4O 9 (x = 0-1) series of compounds. The reaction rate reaches its highest value at an iron fraction of x = 0.1, while the compounds with the highest 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 two opposing factors: the incorporation of iron narrows the fundamental bandgap and thus improves light capture, but at the same time it lowers the conduction band edge, hindering the oxygen reduction half-reaction and thus promoting electron-hole recombination. Our work highlights the importance of a proper band edge engineering for photocatalytic applications and underlines the inadequacy of dye decolorization tests for visible-light-active materials.

KW - band-edge tuning

KW - bandgap narrowing

KW - heterogeneous photocatalysis

KW - methanol oxidation

KW - mullite-type materials

UR - http://www.scopus.com/inward/record.url?scp=85052292008&partnerID=8YFLogxK

U2 - 10.1021/acscatal.8b01210

DO - 10.1021/acscatal.8b01210

M3 - Article

VL - 8

SP - 8844

EP - 8855

JO - ACS catalysis

JF - ACS catalysis

SN - 2155-5435

IS - 9

ER -