Nanostructured α-Fe2O3Thin Films for Photoelectrochemical Hydrogen Generation
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Abstract
α-Fe2O3 thin film photoelectrodes were fabricated by aerosol-assisted chemical vapor deposition (AACVD) using a new hexanuclear iron precursor [Fe6(PhCOO)10(acac)2(O)2(OH)2]·3C7H8 (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 α-Fe2O3 on fluorine-doped SnO2 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 α-Fe2O3 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 Fe3+ 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 α-Fe2O3 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 α-Fe2O3 was 2.914 × 1023 m−3, and the flatband potential is approximately −0.86 V vs VAg/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 α-Fe2O3 photoelectrode to date.
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