Photoelectrochemical properties of butane flame-treated niobium-doped hematite thin films grown by the liquid-phase deposition method

Abstract In this study, thin films of undoped and Nb-doped hematite were grown on FTO-coated glass substrates via the liquid-phase deposition method as photoanodes for PEC water splitting applications. The samples were then annealed and flame-treated for 90 s with a butane flame. At all doping concentrations, the thinnest films (~ 200 nm) yielded the best properties. FESEM images showed that film morphology is not dependent on Nb-doping concentration. Compared with the undoped hematite film, the optimum Nb-doped sample showed an approximately five-fold increase in photocurrent density at 1.6 V vs. RHE and a ~ 0.3 eV decrease in energy band gap value. The charge carrier density value of the optimum Nb-doped sample was shown to increase from 1.38 × 1017 to 3.54 × 1018 cm-3 after flame treatment due to the introduction of oxygen vacancies. The number of times the reducing butane flame is applied was shown to affect the PEC performance, both positively and negatively, giving rise to an approximately sixteen-fold increase in photocurrent density at 1.6 V vs. RHE in comparison with the untreated sample after four times of application. Butane flame treatment was also shown to facilitate the process of charge transfer in Nb-doped hematite and its interface with the electrolyte.