Photoelectrochemical Impedance Spectroscopy (PEIS) has been used to characterize the kinetics of electron transfer and recombination taking place during oxygen evolution at illuminated polycrystalline α-Fe(2)O(3) electrodes prepared by aerosol-assisted chemical vapour deposition from a ferrocene precursor. The PEIS results were analysed using a phenomenological approach since the mechanism of the oxygen evolution reaction is not known a priori. The results indicate that the photocurrent onset potential is strongly affected by Fermi level pinning since the rate constant for surface recombination is almost constant in this potential region. The phenomenological rate constant for electron transfer was found to increase with potential, suggesting that the potential drop in the Helmholtz layer influences the activation energy for the oxygen evolution process. The PEIS analysis also shows that the limiting factor determining the performance of the α-Fe(2)O(3) photoanode is electron-hole recombination in the bulk of the oxide.

R-Fe 2 O 3 thin film photoelectrodes were fabricated by aerosol-assisted chemical vapor deposition (AACVD) using a new hexanuclear iron precursor [Fe 6 (PhCOO) 10 (acac) 2 (O) 2 (OH) 2 ] 3 3C 7 H 8 (1) (where PhCOO =benzoate and acac=2,4-pentanedionate). The precursor (1) designed for AACVD has a low decomposition temperature and sufficient solubility in organic solvents and was synthesized by simple chemical techniques in high yield. It was characterized by melting point, FT-IR, X-ray crystallography, and thermogravimetry (TGA). The TGA analysis proved that complex (1) undergoes facile thermal decomposition at 475 °C to give iron oxide residue. In-house designed AACVD equipment was used to deposit highly crystalline thin films of R-Fe 2 O 3 on fluorinedoped SnO 2 coated glass substrates at 475 °C in a single step. The material properties were characterized by XRD, XPS, and Raman spectroscopy, and the results confirmed that films were highly crystalline R-Fe 2 O 3 and free from other phases of iron oxide. Further analysis of XRD data of the thin films proved the formation of crystalline hematite with an average diameter of 35 nm. X-ray photoelectron spectroscopy (XPS) confirmed that Fe is present only in the Fe 3þ oxidation state. Scanning electron microscopy (SEM) showed that the needle-like particles having length in the range of 100 to 160 nm with a diameter of 30-50 nm are sintered together to form a compact structure of the 80-nm-thick R-Fe 2 O 3 layer. Optical, electrical, and photoelectrochemical studies were conducted by UV-vis, electrochemical impedance spectroscopy, and steady-state current-voltage plots. The optical bandgap was estimated, and it is about 2.13 eV. The donor density of the R-Fe 2 O 3 was 2.914 Â10 23 m -3 , and the flatband potential is approximately -0.86 V vs V Ag/AgCl . The photoelectrochemical characteristics recorded under AM 1.5 illumination indicated that the photocurrent density of 600 μA cm -2 at 1.23 V vs RHE, which is among the highest reported for an undoped R-Fe 2 O 3 photoelectrode to date.

Si-doped nanostructured hematite (α-Fe2O3) has attracted significant attention as a low-cost, high-efficiency candidate material for photoelectrochemical water splitting. In this work, we investigated the effect of Si incorporation on the preparation and performance of α-Fe2O3 films produced by atmospheric pressure chemical vapor deposition (APCVD). Structural, optical, electrical, and photoelectrochemical characterization of doped and undoped hematite films was performed using XRD, FIB/SEM, Raman spectroscopy, UV−vis absorption spectroscopy, J-V and electrochemical capacitance measurements. It was concluded from the XPS data that Si is incorporated in the hematite structure as Si4+. The results suggest that Si-free additives as well as the use of fluorinated transparent conducting oxide (FTO) substrates can influence the preferred orientation of hematite films. It was also found that the incorporation of silicon at very low levels led to the formation of disorder in the hematite structure. Moreover, it is shown that the optical bandgap of Si-doped film increased with the increase of TEOS flow rate. It contributed to the reduction in the size of the hematite nanoparticles and the size quantization effect. The observed donor densities for the doped samples seemed to be much higher than the true values, mainly because the total capacitance measured was higher than space charge layer capacitance, which resulted from the surface area enhancement in the doped films. Therefore, it is considered that donor densities of doped films were smaller than that of the undoped hematite films.

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