A multitude of modifications strategy of ZnFe2O4 nanorod photoanodes for enhanced photoelectrochemical water splitting activity

Abstract

Numerous modifications strategies are applied to spinel ZnFe₂O₄ nanorods with a band gap energy of ∼2.0 eV to enhance their activity as a photoanode for photoelectrochemical (PEC) water splitting. First, hybrid microwave annealing (HMA) imparts high crystallinity to ZnFe₂O₄ nanorods, while preserving the formed nanostructure and maintaining high electric conductivity of F:SnO₂ (FTO) substrate. This is in contrast to conventional thermal annealing (CTA) at 800 °C that causes aggregation of ZnFe₂O₄ and degradation of FTO. Second, insertion of a TiO₂ underlayer blocks charge recombination at the FTO/electrolyte interface and serves as a source of Ti doping. Third, hydrogen treatment yields oxygen vacancies that increase charge carrier density and cause surface passivation. Last, a NiFeO_x co-catalyst promotes hole injection into the electrolyte to improve catalytic water oxidation activity. These synergistic modifications lead to enhanced photocurrent density from 0.025 mA cm⁻² at 1.23 V_RHE for pristine ZnFe₂O₄ nanorods prepared by CTA to 0.92 mA cm⁻² for a fully modified HMA photoanode: a 37-fold increase in photocurrent density. There is also a cathodic shift of the onset potential down to 0.62 V_RHE. The multiple modifications enhance bulk charge separation efficiencies from mere 2% to 30% and surface charge separation efficiency from 40% to 80%.