Activation of Hematite Photoanodes for Solar Water Splitting: Effect of FTO Deformation

The sintering at 800 °C is found to induce the diffusion of Sn from the F-doped SnO₂ (FTO) into the hematite lattice, enhancing the photoelectrochemical cell (PEC) properties of the hematite photoanodes, but this diffusion also has detrimental effects on the conductivity of the FTO substrate. In the present research we examined the role of FTO deformation during the activation of hematite photoanodes synthesized on FTO substrates. The incorporation of Sn dopants from the FTO substrates in the hematite lattice was confirmed by X-ray photoelectron spectroscopy and was found to increase with sintering time. Further from the extended X-ray absorption fine structure analysis, it was found that the diffused Sn atoms affected the metal sites of the hematite lattice. Increased diffusion of Sn into the hematite lattice caused structural disordering of the FTO, but optimum sintering time compensated for the structural disordering and improved the ordering. Under high-temperature annealing at 800 °C, the FTO substrates underwent a stoichiometric change that directly affected their electrical conductivity; their resistivity was doubled after 20 min of sintering. Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite and the resulting thermal deformation and conductivity loss in the FTO substrates

Trade-off between Zr Passivation and Sn Doping on Hematite Nanorod Photoanodes for Efficient Solar Water Oxidation: Effects of a ZrO₂ Underlayer and FTO Deformation

Herein we report the influence of a ZrO₂ underlayer on the PEC (photoelectrochemical) behavior of hematite nanorod photoanodes for efficient solar water splitting. Particular attention was given to the cathodic shift in onset potential and photocurrent enhancement. Akaganite (β-FeOOH) nanorods were grown on ZrO₂-coated FTO (fluorine-doped tin oxide) substrates. Sintering at 800 °C transformed akaganite to the hematite (α-Fe₂O₃) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrates and surface migration, shallow doping of Zr atoms from the ZrO₂ underlayer. The ZrO₂ underlayer-treated photoanode showed better water oxidation performance compared to the pristine (α-Fe₂O₃) photoanode. A cathodic shift in the onset potential and photocurrent enhancement was achieved by surface passivation and shallow doping of Zr from the ZrO₂ underlayer, along with Sn doping from the FTO substrate to the crystal lattice of hematite nanorods. The Zr based hematite nanorod photoanode achieved 1 mA/cm² at 1.23 V_RHE with a low turn-on voltage of 0.80 V_RHE. Sn doping and Zr passivation, as well as shallow doping, were confirmed by XPS, <i>I</i>_ph, and M–S plot analyses. Electrochemical impedance spectroscopy revealed that the presence of a ZrO₂ underlayer decreased the deformation of FTO substrate, improved electron transfer at the hematite/FTO interface and increased charge-transfer resistance at the electrolyte/hematite interface. This is the first systematic investigation of the effects of Zr passivation, shallow doping, and Sn doping on hematite nanorod photoanodes through application of a ZrO₂ underlayer on the FTO substrate