Plasma-processed haematite photoanodes for photoelectrochemical water oxidation

PEC water splitting is one of the most promising methods of harvesting virtually limitless solar power. The major challenge in this field is in the water oxidation half of the water splitting reaction. Haematite photoanodes have been identified as one of the most suitable materials for PEC water oxidation due to its narrow bandgap, high stability, and elemental abundance in Earth. To date, however, there has not been any commercially produced PEC water splitting systems, as creating one proves to be quite an engineering feat. In this work, we attempt to enhance the PEC performance of haematite photoanodes by oxygen, nitrogen and argon plasmas. The as-synthesised haematite photoanodes produced a photocurrent density of 0.014 mA cm−2, which was enhanced to 0.097 mA cm−2 via oxygen plasma, and 0.64 mA cm−2 via argon plasma. This works out to be approximately a 7-fold and a staggering 40-fold enhancement respectively. The onset potentials on the haematite photoanodes were also successfully reduced by oxygen and argon plasmas to 0.79 V and 0.77 V respectively, from 0.84 V. Both photoanodes were able to produce these photocurrent densities for 48 hours without showing any signs of deterioration. The enhancements in PEC performance were achieved by the doping of oxygen vacancies into the crystal structure of the haematite photoanodes. This was demonstrated by the use of XPS and XRD techniques. It was also found that photoanodes with higher concentrations of oxygen vacancies exhibited bandgap widening, a phenomenon that can explained by the Moss-Burstein effect. The Tauc plots derived from the UV-vis spectra of the photoanodes revealed that the bandgap widened from 2.37 eV to 2.56 eV and 2.42 eV after treatment with oxygen and argon plasmas respectively. SEM micrographs were also presented to show the surface morphology of the plasma-treated photoanodes.