Comparative band gap determination of photocatalytic Bi2Fe4O9

KIRSCH A.; CURTI M.; MURSHED M.M.; MENDIVE C.; SCHOWALTER M.; GESING TH. M.

Sttutgart

Congreso; 24th Annual Meeting of the German Crystallographic Society (DGK); 2016

Universitaet Stuttgart

Bismuth ferrates, BiFeO3 and Bi2Fe4O9,have attracted great attention due to their interesting physical and catalytic properties[1]. The synthesis of these compounds inthe nano-size regime is of special interest due to enhanced or modified properties.In this study, a mullite-type Bi2Fe4O9 precursorwas synthesized using a polyol-mediated method and post-heated under differentconditions. Analysis of X-Ray powder diffraction (XRD) data indicates that theas-synthesized sample is nano-crystalline. During heating, the X-ray amorphouspowder transformed into a rhombohedral perovskite-type BiFeO3 followedby a second transformation into mullite-type Bi2Fe4O9.In-situ XRD measurements demonstratethat the nucleation of both BiFeO3 and Bi2Fe4O9might simultaneously commence, however, their growth and ratios depend ontemperature. Temperature-dependent Fourier transform infrared spectra helpedunderstand some local features of the samples. The locally-resolved high-resolutiontransmission electron micrographs revealed that the surface of some heatedsamples is covered by 4 ? 13 nm sized particles which were identified (from thefringes) as crystalline Bi2Fe4O9. Diffuse UV/Visreflectance spectra showed fundamental absorption edges between 1.80 eV and2.75 eV. A comparative study between the recently developed ?derivation ofabsorption spectrum fitting ? (DASF) [2] and the mostly used Tauc method wasperformed for the calculation of both direct and indirect band gaps. Using theTauc method it is indispensable to know the nature of transition before thecalculation whereas the DASF method provides the opportunity to overcome this limitation.The band gaps obtained by DASF method are in good agreement to the valuesobtained by the Tauc?s method (2.13(3) eV) considering the directtransitions for the samples calcined above 873 K. The obtained band gaps have beencompared to that of calculated by the density functional theory. This study ona mullite type compound bears particular importance to calculate the band gapusing both the DASF and Tauc method, which help determine the band gap as wellas the nature of transition.[1]      Sun,S.; Wang, W.; Zhang, L.; Shang, M. J. Phys. Chem. C 2009, 113,12826?12831.[2]      Souri,D.; Tahan, Z. E. Appl. Phys. B 2015, 119, 273?279.