Vacuum-annealing induces sub-surface redox-states in surfactant-structured α-Fe2O3 photoanodes prepared by ink-jet printing

[Display omitted] •Nanostructured hematite has been produced from a surfactant ink on ITO substrates with an ink-jet printing method.•Photo-activity is observed in aqueous NaOH but substantial improvements in photo-currents are observed after vacuum-annealing.•The formation of sub-surface states is suggested and discussed in terms of the photo-transient responses. Transparent nano-structured hematite (α-Fe2O3) films of approximately 550nm thickness on tin-doped indium oxide (ITO) have been obtained conveniently by ink-jet printing of a Fe(NO3)3/Brij® O10 precursor ink and subsequent annealing at 500°C in air. When illuminated with a blue LED (λ=455nm, ca. 100mWcm−2), the hematite films exhibited photocurrents of up to 70μAcm−2 at 0.4V vs. SCE in 0.1M NaOH electrolyte. Thermal annealing in vacuum at 500°C for 2h increased photocurrents more than three times to 230μAcm−2 in agreement with previous literature reports for pure hematite materials. These results suggest that a simple ink-jetting process with surfactants is viable. The effects of vacuum-annealing on the photoelectrical properties of α-Fe2O3 films are discussed in terms of a sub-surface state templating hypothesis based on data gathered from photo-transients, field emission scanning electron microscopy, X-ray photoelectron spectroscopy analysis, X-ray diffraction, photocurrent spectra, and cyclic voltammetry.