Effect of Cr doping on the photoelectrochemical performance of hematite nanorod photoanodes

Hematite (α-Fe ₂O ₃) nanorod arrays modified by surface doping of chromium (III) ions (Cr ³⁺) for photoelectrochemical (PEC) water splitting are fabricated using a general process involving a combination of aqueous chemical growth and spin coating. The PEC activity of Cr-doped α-Fe ₂O ₃ nanorod films first increases then decreases with increasing dopant content. At the optimal content of Cr, Cr-doped α-Fe ₂O ₃ nanorod films exhibit about 3.5 and 6 times higher PEC activity than the undoped material under solar light (AM 1.5, 100mWcm ^-2) and visible light (λ>430nm) irradiation, respectively. A comprehensive characterization of the chemical, morphological, PEC, structural, and optical properties of the doped α-Fe ₂O ₃ films are presented to assess the mechanisms by which the dopants influence photoelectrode performance. The relationship between dopant content, photoluminescence intensity, and PEC performance suggests Cr doping alters charge transfer in the films under irradiation. At low Cr doping contents, Cr dopants act as electron (or hole) traps and retard photoinduced charge recombination, leading to enhanced PEC activity. Whereas, at high Cr doping contents, Cr dopants act as charge recombination sites and lower the charge separation efficiency, leading to decreased PEC activity. High temperature annealing proves to be effective for further improvement of the PEC activities of doped and undoped hematite films, by Sn diffusion.