Photoelectrochemical Properties and Behavior of α-SnWO₄ Photoanodes Synthesized by Hydrothermal Conversion of WO₃ Films

Abstract

Metal oxides with moderate band gaps are desired for efficient production of hydrogen from sunlight and water via photoelectrochemical (PEC) water splitting. Here, we report an α-SnWO photoanode synthesized by hydrothermal conversion of WO films that achieves photon to current conversion at wavelengths up to 700 nm (1.78 eV). This photoanode is promising for overall PEC water-splitting because the flat-band potential and voltage of photocurrent onset are more negative than the potential of hydrogen evolution. Furthermore, the photoanode utilizes a large portion of the solar spectrum. However, the photocurrent density reaches only a small fraction of that which is theoretically possible. Density functional theory based thermodynamic and electronic structure calculations were performed to elucidate the nature and impact of defects in α-SnWO prepared by this synthetic route, from which hole localization at Sn-at-W antisite defects was determined to be a likely cause for the poor photocurrent. Measurements further showed that the photocurrent decreases over time due to surface oxidation, which was suppressed by improving the kinetics of hole transfer at the semiconductor/electrolyte interface. Alternative synthetic methods and the addition of protective coatings and/or oxygen evolution catalysts are suggested to improve the PEC performance and stability of this promising α-SnWO material.

Interfacial Oxide Formation Limits the Photovoltage of α‐SnWO4/NiOx Photoanodes Prepared by Pulsed Laser Deposition

“…The validity of our electronic structure calculation is demonstrated by the relatively good agreement between our HSE06-based computed band gap of 1.5 eV for the pristine material with the reported experimental values (in between 1.6 and 1.9 eV), which is much more accurate than the PBE computed one of 0.8 eV. [17,19,[22][23][24]…”

“…Density Functional Theory: Starting from the 2 × 1 × 2 supercell model, which includes 16 functional units of Sn 16 W 16 O 64 or 96 atoms of the orthorhombic crystal lattice of α-SnWO 4 (Pnna space group), [18,19,[22][23][24] a perfect lattice and five representative self-defective lattices containing intrinsic O-, Sn-, and W-vacancies as well as Sn-antisite on W and W-antisite on Sn were modeled to mimic the possible presence The various generated crystalline structures were fully optimized by means of the spin-polarized DFT using the Vienna Ab initio simulation package (VASP) [44][45][46] with the Perdew-Burke-Ernzerhof (PBE) exchange correlation potential [47] and the frozen-core projector augmented-wave (PAW) approach. [48] The configurations of valence electrons treated explicitly in the plane wave descriptions are 5s 2 5p 2 for Sn, 5d 4 6s 2 for W, and 2s 2 2p 4 for O.…”

“…There are currently only a few comprehensive studies available on this material, but the reported properties are promising. [17][18][19][20][21][22][23][24] It is an n-type semiconductor with an indirect band gap of 1.9 eV, [17] implying a theoretical maximum photocurrent of up to ≈17 mA cm −2 under AM 1.5 solar irradiation. Another attractive property of this material is the flat band potential which is located at ≈0 V versus the reversible hydrogen electrode potential (RHE).…”

“…Another attractive property of this material is the flat band potential which is located at ≈0 V versus the reversible hydrogen electrode potential (RHE). [22][23][24] To date, α-SnWO 4 thin film photoelectrodes have been prepared successfully using various techniques, including hydrothermal conversion, [19,23] magnetron sputtering, [20][21][22] and pulsed laser deposition (PLD). [17,18] A major challenge with this material is the limited stability due to surface oxidation of Sn 2+ to Sn 4+ under (photo)electrochemical α-SnWO 4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting.…”

Recent progress of tungsten- and molybdenum-based semiconductor materials for solar-hydrogen production

“…The VB of α-SnWO 4 is formed by the Sn 5s and O 2p hybrid orbitals, whereas the CB is mainly contributed by W 5d. It is reported that the flat-band potential of α-SnWO 4 is − 0.14 to 0.05 V vs. RHE [203,204], indicating that the CB of α-SnWO 4 is located at a more negative potential than that of 0 V vs. RHE (Fig. 10a).…”