Photoactivity of Transparent Nanocrystalline Fe2O3 Electrodes Prepared via Anodic Electrodeposition

Abstract

Highly transparent nanocrystalline α-Fe2O3 films were prepared via anodic electrodeposition using a slightly acidic aqueous medium (pH 4.1). The deposition mechanism involved oxidation of Fe2+ ions to Fe3+ ions followed by precipitation of Fe3+ ions as amorphous γ-FeOOH films. The as-deposited films were then converted to transparent nanocrystalline α-Fe2O3 films by annealing at 520 °C in air. The as-deposited and annealed films were characterized by Raman and UV−vis spectroscopy, X-ray diffraction, and scanning electron microscopy. The photoactivity of the as-deposited and annealed films was studied by measuring short-circuit photocurrents in a 60:40 solution of propylene carbonate:acetonitrile containing 0.5 M tetrabutylammonium iodide and 0.04 M iodine. Both films show n-type behavior generating anodic photocurrent. α-Fe2O3 films with various thicknesses were prepared to study the effect of film thickness on photon absorption and photocurrent. The short-circuit photocurrent of α-Fe2O3 films increased gradually as the film thickness increased to 400−500 nm because of the corresponding increase in photon absorption and surface area of the films. However, when the film thickness exceeded 400−500 nm, aggregation of Fe2O3 particles at the film/substrate interface became severe. This increased recombination losses near the collector electrode, and with no significant gain in photon absorption, led to an overall decrease in photocurrent. α-Fe2O3 films were also prepared by annealing FeOOH films produced via anodic deposition in a neutral medium (pH 7.5). The film prepared from the neutral medium was two times thicker and possessed a surface roughness factor two times higher than the film prepared from the acidic medium when the two films contained the same amount of α-Fe2O3. Comparing photocurrent of these films allowed for better understanding the effect of electrode structures (i.e., surface area, film thickness) on photocurrent generation in α-Fe2O3 electrodes with poor charge transport properties.