Visible-Light Photocatalysis with Mullite-Type Bi₂(Al_1–xFe_x)₄O₉: Striking the Balance between Bandgap Narrowing and Conduction Band Lowering

Abstract

The rich crystal chemistry of mullite-type Bi 2 M 4 O 9 (M = Fe, Al, Ga) offers multiple potential applications. In particular, the strong absorption of visible light shown by Bi 2 Fe 4 O 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 (λ = 450 nm) irradiation of 12 different members of the Bi 2 (Al 1−x Fe x ) 4 O 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.

Hyper oxygen incorporation in CeF3: a new intermediate-band photocatalyst for antibiotic degradation under visible/NIR light

“…BQ, EDTA 2Na and IP are used to scavenge cO 2 À , h+ and cOH species, respectively. [39][40][41] The addition of IP scavenges cOH and has little effect on the TC-HCl degradation. (Fig.…”

Bionanocomposite Hydrogel for the Adsorption of Dye and Reusability of Generated Waste for the Photodegradation of Ciprofloxacin: A Demonstration of the Circularity Concept for Water Purification

“…The E fb for C-TiO 2 NC was determined by extrapolating the tangent of the 1/C 2 vs V plot to the x -axis; the slope of this plot provides the carrier-charge density ( N d ). The Mott–Schottky plot for C-TiO 2 NC exhibits a positive slope that is characteristic of an n-type semiconductor . The charge density for C-TiO 2 NC is found to be 0.15 × 10 18 cm –3 .…”

“…The Mott− Schottky plot for C-TiO 2 NC exhibits a positive slope that is characteristic of an n-type semiconductor. 58 The charge density for C-TiO 2 NC is found to be 0.15 × 10 18 cm −3 . As defect sites always contribute to charge density, the high charge density suggests the presence of a high number of defect sites in C-TiO 2 NC.…”

Aluminum to germanium inversion in mullite‐type RAlGeO5: Characterization of a rare phenomenon for R = Y, Sm–Lu

“…No references were found on modeling of the PIP process for CMCs, though experimental studies on this manufacturing process have been reported. Studies on polycarbosilanederived SiC ceramic 25 and SiC/SiC composite 26,27 have shown that porosity decreases and mechanical properties improve with the increasing number of PIP cycles. Zhu et al 28 show that increasing pyrolysis temperature and the concentration of SiC fillers in polycarbosilane polymer result in improved mechanical properties in two-dimensional Carbon/ SiC (C/SiC) composites fabricated using the PIP process.…”