Physical and photoelectrochemical properties of Sb-doped SnO2 thin films deposited by chemical vapor deposition: application to chromate reduction under solar light

Sb-doped SnO2 thin films (Sb-SnO2) are prepared by chemical vapor deposition. The X-ray diffraction indicates a rutile phase, and the SEM analysis shows pyramidal grains whose size extends up to 200 nm. The variation of the film thickness shows that the elaboration technique needs to be optimized to give reproducible layers. The films are transparent over the visible region. The dispersion of the optical indices is evaluated by fitting the diffuse reflectance data with the Drude–Lorentz model. The refractive index (n) and absorption coefficient (k) depend on both the conditions of preparation and of the doping concentration and vary between 1.4 and 2.0 and 0.2 and 0.01, respectively. Tin oxide is nominally non-stoichiometric, and the conduction is dominated by thermally electrons jump with an electron mobility of 12 cm2 V−1 s−1 for Sb-SnO2 (1 %). The (C2–V) characteristic in aqueous electrolyte exhibits a linear behavior from which an electrons density of 4.15 × 1018 cm−3 and a flat-band potential of −0.83 VSCE are determined. The electrochemical impedance spectroscopy shows a semicircle attributed to a capacitive behavior with a low density of surface states. The center lies below the real axis with a depletion angle (12°), due to a constant phase element, i.e., a deviation from a pure capacitive behavior, presumably attributed to the roughness and porosity of the film. The straight line at low frequencies is attributed to the Warburg diffusion. The energy diagram reveals the photocatalytic feasibility of Sb-SnO2. As application, 90 % of the chromate concentration (20 mg L−1, pH ~3) disappears after 6 h of exposure to solar light.