Particle size-band gap energy-catalytic properties relationship of PSE-CVD-derived Fe3O4 thin films

Abstract

Abstract This study reports the control of catalytic properties of Fe3O4 thin films through adjusting the particles size and optical properties. Structure analysis of the obtained materials by X-ray diffraction indicated the formation of pure magnetite structure of Fe3O4. X-ray photoelectron spectroscopy showed that the surfaces of the samples were mainly composed of Fe2+, Fe3+, O2−, CO32− and OH−. Scanning electronic microscopy displayed a smooth films surface with an agglomerated crystallite grains. Both XRD and SEM exhibits particles size increases (∼40 to ∼60 nm) with the substrate temperature (Ts), while micro-strain in the sample decreased. The correlation of the Ts with optical energy band gaps (EgOpt) determined from UV visible (UV–vis) measurements indicated the increase of indirect (2.17 ≤ Eg2Opt≤ 2.25 eV) and direct (2.78 ≤ Eg1Opt≤ 2.95 eV) band gap of Fe3O4. Fe3O4 samples have been successfully tested towards the total oxidation of CO. While the change in EgOpt of Fe3O4 has been explained on the basis of the variation in the grain size and likely adsorbed oxygen (OAds) with Ts, the catalytic performance was suggested to be strongly dependent on the films microstructure, catalysts surface composition and more importantly with the EgOpt and OAds variation. Moreover, theoretical calculations based on DFT method of CO oxidation over Fe3O4 film surface catalyst demonstrated that OAds was the most involved oxygen species during the catalytic process, revealing that the LH mechanism is the most appropriate route for the CO catalytic oxidation over MvK and ER mechanisms. This approach of highlighting the interplay among the particle size, optical and catalytic properties with DFT calculations can pave the way to better understand the catalytic behavior of other transition metal oxides.