High-quality CuBi₂O₄ photoelectrodes with increased stability by rapid thermal processing of Bi₂O₃/CuO grown by pulsed laser deposition

A new approach for fabricating high-quality ternary photoelectrodes such as CuBi₂O₄ (an emerging p-type semiconductor for solar hydrogen production) will be presented. Pulsed laser deposition (PLD) is used to deposit the binary oxides Bi₂O₃ and CuO on FTO substrates, and rapid thermal processing (RTP) is used to achieve an efficient solid-state reaction between the two oxide films. It is shown that when Bi₂O₃ is deposited first, the thin film photoelectrodes are comprised of single-phase, compact, crystalline, and well-defined CuBi₂O₄ particles. A comparative study with conventional furnace heating reveals the significance of radiative heating in the processing of complex metal oxides photoelectrodes. Additionally, the much shorter heating times enable the use of FTO substrates at temperatures up to 650 °C, higher than their thermal stability, also resulting in a low thermal budget (the product of process temperature and processing duration at elevated temperatures). In a comparative study with CuBi₂O₄ photoelectrodes produced by spray-pyrolysis, a benchmark synthesis technique of photoactive materials and complex metal-oxide photoelectrodes, the CuBi₂O₄ layer fabricated by PLD + RTP photoelectrodes featured superior electronic properties, which were sustained even in cases of a secondary oxide co-existence in the film by deliberately altering the Bi:Cu stoichiometry ratio during the PLD stage. The photoelectrodes have shown an efficient charge separation, low tail energies, and band-gap energy virtually identical to that of a CuBi₂O₄ single crystal, shown by modulated surface photovoltage analysis and confocal Raman microscopy. Furthermore, bare photoelectrodes, without the addition of electron scavengers, protection layers, or catalysts, exhibited exceptional photoelectrochemical stability, substantially improving the prospects of implementing CuBi₂O₄ photoelectrodes in photoelectrochemical devices towards high solar-to-hydrogen efficiencies.