Microwave-assisted metal-ion attachment for ex-situ zirconium doping into hematite for enhanced photoelectrochemical water splitting

Ex-situ doping into hematite films is carried out via a short-duration (∼60 s) microwave-assisted metal-ions attachment (MWMA) of tetravalent Zr⁴⁺ ion on the surface of FeOOH/FTO, followed by high temperature annealing (HTA) to fabricate Zr⁴⁺:Fe₂O₃/FTO photoanode for photoelectrochemical (PEC) water splitting. Compared to a simple dipping attachment without microwave irradiation, this MWMA allows a much larger amount of attached Zr⁴⁺-ions on the FeOOH precursor, leading to a properly doped photoanode of much higher PEC activity. The primary effect of Zr⁴⁺ doping is to improve the charge transport characteristics in the bulk of hematite. In addition, it also boosts charge injection efficiency at the semiconductor and electrolyte interface by forming an inadvertent passivation layer and promoting hole transfer via surface states. As a result, the Zr⁴⁺:Fe₂O₃/FTO photoanode shows a higher photocurrent density of 1.54 mA cm⁻² at 1.23 V_RHE under 1 Sun irradiation relative to undoped Fe₂O₃/FTO (1.02 mA cm⁻²) or Zr⁴⁺:Fe₂O₃/FTO (1.19 mA cm⁻²) prepared without MWMA.