Rapid Flame-Annealed CuFe2O4 as Efficient Photocathode for Photoelectrochemical Hydrogen Production

Copper ferrite (CuFe 2 O 4 ) possesses an indirect bandgap in the range of 1.54–1.95 eV. It is used as an attractive p -type photocathode in photoelectrochemical (PEC) water splitting, and theoretically it can yield a maximum photocurrent density of ∼27 mA/cm 2 and a maximum solar-to-hydrogen conversion efficiency of ∼33%. To date, only a few reports have been published on CuFe 2 O 4 photocathodes with very low-photocurrent densities, with a maximum value of 0.4 mA/cm 2 at 0.4 V vs RHE. Herein, we prepared a CuFe 2 O 4 photocathode on FTO glass with the sol–gel method followed by either high-temperature flame annealing or furnace annealing. We found that the flame-annealed CuFe 2 O 4 photocathode generated a photocurrent density of 1.82 mA/cm 2 at 0.4 V vs RHE that is approximately 3.5 times higher than the furnace-annealed CuFe 2 O 4 (0.52 mA/cm 2 ). This photocurrent density is also higher than those of all the reported CuFe 2 O 4 photocathodes, and any Cu containing ternary oxide (Cu–M–O, M: Fe, Bi, V, and Nb) photocathode (0.1–1.3 mA/cm 2 at 0.4 V vs RHE). An improved PEC performance of the flame-annealed CuFe 2 O 4 photocathode is elicited owing to the beneficial effects of flame annealing on the physical, optical, and electrical properties of CuFe 2 O 4 . Flame annealing enhances the light absorption property of the CuFe 2 O 4 photocathode by slightly reducing the bandgap, and by forming a thicker film with increased porosity. Flame annealing also reduces the oxygen vacancy concentration in CuFe 2 O 4, thus facilitating charge transport and interfacial charge transfer processes. Moreover, flame annealing requires only 16 min, which is much shorter than the time required for furnace annealing (∼9 h). These results demonstrate that flame annealing is a rapid and effective means for fabricating metal oxide photoelectrodes with an enhanced PEC water splitting performance.