Influence of ZnO Magnetron Sputtering on Controlled Buildout of Zirconium-Doped ZnFe₂O₄/Fe₂O₃ Heterojunction Photoanodes for Photoelectrochemical Water Splitting

Abstract

The development of efficient photoanodes for solar fuel generation via photoelectrochemical (PEC) water splitting is becoming a bottleneck. These limitations necessitate the design of iron-containing metal oxides, like “ferrites-based electrode materials” with improved oxygen evolution kinetics, light absorptivity, and intrinsic stability, yet at a low cost. Herein, we report the in-situ formation of Zr–ZnFe2O4/Fe2O3 heterojunction (ZZFO/HT) photoelectrodes using a facile magnetron sputtering and hydrothermal processes. First, the ZnO is systematically sputtered on in-situ Zr-doped FeOOH electrodes and then the ZnO-sputtered electrodes are quenched at 800 °C, 13 min to form ZZFO/HT. Furthermore, the effect of ZnO sputtering and roles of Zr–ZnFe2O4 and Fe2O3 in the ZZFO/HT heterojunction as well as their structural and photoelectrochemical properties were studied in detail. The optimum biphasic 25.6 nm ZnO-sputtered ZZFO/HT (ZZFO/HT-2) photoelectrode exhibited a photocurrent density of 0.430 mA/cm2 at 1.23 V vs RHE with an appropriate fraction of Zr–ZnFe2O4 and Fe2O3. The enhanced PEC performance is attributed to the optimum fraction of Zr–ZnFe2O4 and Fe2O3 in ZZFO/HT-2 heterojunction, which provides efficient charge transport across the bulk and at heterojunction interface. Lastly, the integration of Al2O3 passivation layer and Co–Pi cocatalyst on the optimized ZZFO/HT-2 photoelectrode exhibited a high photocurrent density of 0.780 mA/cm2 at 1.23 V vs RHE and generated 11.8 and 6.2 μmol/cm2 of hydrogen and oxygen, respectively during PEC water splitting. Further, it is expected that by fine-tuning of Zr–ZnFe2O4 and Fe2O3 NC ratio, the photocurrent density can be improved for establishing a benchmark for ZnFe2O4-based photoelectrodes.

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

“…These ternary oxide compounds are made of trivalent iron and divalent metal ions that occupy tetrahedral and octahedral sites in lattice built of 8 face-centered cubic lattices of oxygen atoms. Spinels have found wide use in materials for energy applications due to their tunable material properties. − While ZFO has advantageous properties for PEC water splitting, it has been suggested to possess a few inherent limitations including low light absorption coefficient, poor charge transport properties, and sluggish surface reaction kinetics …”

Magnetron Sputtered Al Co-Doped with Zr-Fe2O3 Photoanode with Fortuitous Al2O3 Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

“…Magnetron sputtering is a simple technique because it does not necessitate the use of complicated chemical reactions and is also scalable and commercially available. Further, sputtering is a preferred method due to the controlled thickness, film homogeneity, high purity of products and low substrate temperature needed [24,25]. Despite the fact that Al-doping has been investigated using a variety of different preparation methods, to the best of our knowledge, no reports of Al-doping using the magnetron sputtering approach have been described to date.…”