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

Abstract

α‐SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO4/NiOx interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.

Awakening the Photoelectrochemical Activity of α‐SnWO4 Photoanodes with Extraordinary Crystallinity Induced by Reductive Annealing

“…In comparison with all previous reports, the optimum crystallization temperature here is notably lower. [23] Moreover, the (111) and (121) peaks of 500 C-H 2 shifted to smaller angles, indicating the larger d-spacing in the lattice. According to the structure of α-SnWO 4 , they are the only two strong peaks corresponding to the crystal planes having their normal vectors with a nonzero c-direction component of the unit cell, which corresponds to the interconnected [WO 6 ] layers spaced by Sn 2þ .…”

“…The observation agrees with previous findings based on α-SnWO 4 thin film photoanodes prepared by magnetron sputtering and pulsed laser deposition that the surface modification is necessary even in the presence of hole scavengers. [23,25] This is in contrast with the surface charge accumulation on BiVO 4 or CdS, which can be readily eliminated with suitable sacrificial agents. [2,41] Based on the aforementioned understanding, we further evaluated the α-SnWO 4 photoanodes in the water oxidation reaction (OER), which is the anodic half-reaction of solar water splitting.…”

“…[19][20][21] In contrast, there are reports of α-SnWO 4 thin-film preparation via magnetron sputtering and pulsed laser deposition techniques. [22][23][24] Interestingly, it seems only the dry deposition approaches could produce α-SnWO 4 with observable PEC activities. [25] All previous attempts made with the wet-chemistry prepared materials only exhibited barely observable photocurrents, even including thin films converted from WO 3 .…”

The surface reconstruction induced enhancement of the oxygen evolution reaction on α-SnWO4 (010) based on a density functional theory study

“…Experimentally, α-SnWO 4 is promising for overall photoelectrochemical water-splitting. 11,24,61 To explore its mechanism, we also examined the OER performance of the α-SnWO 4 (010) surface for enhanced water-splitting performance. The O–Sn, O–W, and R–OOSn terminations of α-SnWO 4 (010) are modelled by a 2 × 2 surface unit cell with the top ten layers fully relaxed in all directions (the other layers are fixed).…”

Extending the Absorption Limit of BiVO4 Photoanodes with Hydrogen Sulfide Treatment

“…The NiO x protection layers (20 nm thick) were deposited using PLD. [44,45] In short, a Ni target (99.99%, Alfa Aesar) was ablated in a custom-built PLD system from PREVAC with a KrF excimer laser with a wavelength of λ ¼ 248 nm (LPXpro 210, Coherent). Target ablation was performed with a repetition rate of 10 Hz and a laser fluence of 2 J cm À2 .…”