Accelerating the charge separation of ZnFe2O4 nanorods by Cu-Sn ions gradient doping for efficient photoelectrochemical water splitting

Spinel zinc ferrite (ZnFe2O4) with an appropriate band gap (2.1 eV) is a promising photoanode in the photoelectrocatalysis field, however, the photoelectrochemical (PEC) performance of ZnFe2O4 is confined due to poor charge separation. Hence, improving charge separation efficiency is essential to modify the PEC performance of ZnFe2O4. Herein, the novel Cu-Sn ions doped ZnFe2O4 nanorods are fabricated for the first time. Cu2+ ions are doped into ZnFe2O4 nanorods from surface to inside with a degressive concentration, it is helpful to enhance the charge separation efficiency of ZnFe2O4 nanorods along horizontal direction. Sn4+ ions originated from fluorine-doped tin oxide (FTO) layer are doped into Cu-ZnFe2O4 nanorods with a decreasing concentration from bottom to top, it is conducive to reducing surface trapping states and accelerating charge separation along vertical direction. Therefore, Cu-Sn dual ions gradient doping synergistically boosts the PEC activity of ZnFe2O4. The Cu.Sn-ZnFe2O4 nanorods have an excellent photocurrent density of 0.46 mA⋅cm−2 at 1.23 V vs RHE, which is 4.18 times than that of ZnFe2O4. The bulk charge separation efficiency (ηbulk) and surface charge separation efficiency (ηsurface) of Cu.Sn-ZnFe2O4 are 17.52% and 55.22%, which are 5.84 and 3.78 times than that of ZnFe2O4, respectively. This work provides inspirations for promoting charge separation efficiency of photoanodes, thus achieving efficient PEC water splitting.