Exploiting the dynamic Sn diffusion from deformation of FTO to boost the photocurrent performance of hematite photoanodes

•Hematite activation was done from Sn-diffusion doping from FTO deformation at 800 °C.•Pre-formed oxide acts as a barrier for dynamic Sn diffusion during two-step activation.•One-step activation from Fe/FTO enriches Sn and preserves FTO conductivity.•Advantages: higher donor density, cathodic shift in onset potential, ordered lattice.•Enhanced interfacial charge-transfer and lower recombination improved photoactivity.

Herein, we report on exploiting effective Sn diffusion from the underlying FTO (F:SnO2) substrate during post-growth annealing treatment to boost the photocurrent performance of α-Fe2O3 photoanodes. Conventionally, hematite photoanodes are activated at high temperature (800 °C>T>650 °C) after initial low temperature annealing. The purpose of such activation is to utilize the diffusion of Sn ions leached from FTO deformation for improving the electronic conductivity and hence the photocurrent response of hematite photoanodes. However, the pre-formed oxide layer (in the first annealing) on the surface of FTO creates obstacle for diffusion of Sn ions during FTO deformation. To overcome this difficulty and to exploit Sn diffusion, we employed a direct single-step annealing of as-grown iron-coated FTO electrodes at 800 °C for short duration followed by quenching in air. Such activation greatly improved the water oxidation photocurrent response of hematite by 37% on account of increased Sn diffusion which was confirmed from X-ray photoelectron spectroscopy. Moreover, the onset of photocurrent is shifted cathodically meaning that the water oxidation reaction proceeds at lower applied bias. Higher Sn content increased the electron donor concentration and hence improved the charge transfer kinetics of hematite as studied from EIS and Mott–Schottky measurements. Interestingly, despite higher Sn diffusion, the loss of FTO conductivity was minimal and the structural ordering was highest in one-step-activated hematite compared to conventionally activated hematite photoanodes as studied from XAFS analysis.

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