Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes has emerged as a promising strategy for sustainable hydrogen production and solar energy storage. However, conventional semiconductor materials face critical limitations, such as rapid recombination of photogenerated electron-hole pairs and wide bandgaps, resulting in low solar-to-hydrogen conversion efficiencies. To address these issues, this study reports the synthesis and performance evaluation of microcapsule-structured α-Fe₂O₃ (hematite) photoanodes designed to enhance surface area and charge separation efficiency. Furthermore, the effect of doping with Ge or Si (X = Ge, Si) on the photoelectrochemical properties of α-Fe₂O₃ microcapsules (denoted as X-Fe₂O₃) was investigated, aiming to improve the charge transport and photocarrier separation behavior.

The α-Fe₂O₃ microcapsules (M-Fe₂O₃) were synthesized via a spray pyrolysis method using FeCl₃ aqueous solution. For the doped samples, Ge or Si precursors were added directly to the FeCl₃ solution prior to the spray pyrolysis process, yielding Ge-Fe₂O₃ and Si-Fe₂O₃ microcapsules. To fabricate the photoelectrodes, a seed layer of α-Fe₂O₃ was first prepared on fluorine-doped tin oxide (FTO) glass substrates by spin-coating a solution of FeCl₃ and Ti(OBu)₄ in ethanol, followed by thermal treatment. Subsequently, the synthesized Fe₂O₃ or metal doped Fe₂O₃ microcapsules were deposited onto the seed layer via electrophoretic deposition and then annealed to complete the electrode fabrication.

Electrochemical characterization was conducted in 1 M KOH aqueous solution using a standard three-electrode setup, where the fabricated photoelectrodes served as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode. Linear sweep voltammetry (LSV) was performed under simulated sunlight irradiation (100 mW cm⁻²) using a xenon lamp. Material characterizations including scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS) were also conducted.

SEM observations revealed that the synthesized Fe₂O₃ particles exhibited a spherical microcapsule morphology with a diameter of approximately 1000 nm, which is advantageous for increasing the surface area and improving electrolyte contact. UV-Vis absorption spectra indicated that all photoelectrodes exhibited strong light absorption up to ~600 nm, consistent with the intrinsic absorption behavior of α-Fe₂O₃, and the estimated bandgap was approximately 2.09 eV. Notably, the metal doped-Fe₂O₃ electrodes showed enhanced absorption in the wavelength region above 550 nm, suggesting improved visible light harvesting due to the doping effect.

Photoelectrochemical performance data demonstrated that metal doped Fe₂O₃ electrodes exhibited higher photocurrent densities compared to undoped Fe₂O₃. Specifically, photocurrent onset was observed around 1.0 V_RHE for all electrodes, and at 1.23 V_RHE, the X-Fe₂O₃ photoelectrodes delivered a photocurrent density of 0.31 mA cm⁻², representing a ~1.1-fold improvement over the undoped Fe₂O₃ electrode. This enhancement is attributed to improved charge separation and transport facilitated by Ge or Si doping, which may act to reduce bulk recombination or modulate band structure favorably. In conclusion, the incorporation of Ge and Si dopants into microcapsule-type α-Fe₂O₃ electrodes successfully enhanced PEC performance by improving light absorption and promoting charge carrier separation. Further improvements are expected by modifying the electrode surface, for example, through the electrodeposition of cobalt phosphate (Co-Pi), which may further suppress surface recombination and boost overall water-splitting efficiency.