Influence of Crystalline Structure and Ti-Doping on Bulk and Surface Properties of ZnFe₂O₄ Thin Film Photoanodes

Pulsed laser deposition (PLD) was used to grow undoped and Ti-doped ZnFe₂O₄ compact polycrystalline thin film photoanodes on glass substrates coated with fluorinated tin-oxide (FTO). The film crystallinity was found to have a substantial effect on both the bulk and surface photoelectrochemical (PEC) properties of the films. Films deposited at low substrate temperatures consisted of nanometer sized grains with boundaries present along film the out-of-plane direction resulting in poor charge separation efficiency. Increasing the deposition temperature led to formation of a columnar film structure without grain boundaries in the film out-of-plane direction resulting in an enhancement of both charge separation and charge transfer efficiency. Despite these improvements, bulk losses still dominate the overall PEC performance of ZnFe₂O₄ and were attributed to limited charge carrier transport lengths as well as nonunity, excitation-wavelength-dependent mobile charge carrier generation. Ti-doping of ZnFe₂O₄ was investigated and found to be a promising way to improve PEC performance without any significant crystallographic or morphological variations in the thin films. While Ti-doping was shown to have minor effect on bulk charge separation efficiency, a substantial increase in charge transfer efficiency of the ZnFe₂O₄ photoanodes was observed and attributed to surface state passivation resulting from Ti segregation to the surface.