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

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⁻, CO₃ ²⁻ 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 (T_s), while micro-strain in the sample decreased. The correlation of the T_s with optical energy band gaps (Eg^Opt) determined from UV visible (UV–vis) measurements indicated the increase of indirect (2.17 ≤ E_g2 ^Opt≤ 2.25 eV) and direct (2.78 ≤ E_g1 ^Opt≤ 2.95 eV) band gap of Fe₃O₄. Fe₃O₄ samples have been successfully tested towards the total oxidation of CO. While the change in E_g ^Opt of Fe₃O₄ has been explained on the basis of the variation in the grain size and likely adsorbed oxygen (O_Ads) with T_s, the catalytic performance was suggested to be strongly dependent on the films microstructure, catalysts surface composition and more importantly with the E_g ^Opt and O_Ads variation. Moreover, theoretical calculations based on DFT method of CO oxidation over Fe₃O₄ film surface catalyst demonstrated that O_Ads 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.