Facilitating hole transfer on electrochemically synthesized p-type CuAlO2 films for efficient solar hydrogen production from water

Abstract

Delafossite CuAlO₂ photoelectrodes are synthesized via the electrodeposition of Cu(II) and Al(III) onto fluorine-doped tin oxide (FTO) substrates in water and dimethylsulfoxide (DMSO) solvents, followed by annealing in air and Ar. The surface properties, crystalline structure, and photoelectrochemical (PEC) performance of the as-synthesized samples are significantly affected by the synthetic conditions. Optimized CuAlO₂ electrodes (synthesized in DMSO and annealed in air) possess suitable energetics for H₂ production under sunlight (an optical bandgap of ∼1.4 eV and a conduction band level of −0.24 V_RHE). They exhibit a photocurrent onset potential of ∼+0.9 V_RHE along with a faradaic efficiency of ∼70% at +0.3 V_RHE in an alkaline solution (1 M KOH) under simulated sunlight (AM 1.5; 100 mW cm⁻²). The addition of sacrificial hole scavengers (sulfide and sulfite) significantly improves the PEC performance of CuAlO₂ by a factor of eight, along with providing a faradaic efficiency of ∼100%. This indicates that the hole transfer limits the overall PEC performance. This issue is addressed by employing a ∼150 nm-thick Au film-coated FTO substrate for the CuAlO₂ deposition. In the absence of hole scavengers, the H₂ production with the Au-underlain CuAlO₂ photoelectrode (Au/CuAlO₂) is three-fold higher than that with bare CuAlO₂, while the faradaic efficiencies at +0.3 and +0.55 V_RHE are ∼100%. The time-resolved photoluminescence emission decay spectra of the CuAlO₂ and Au/CuAlO₂ confirm the facilitated charge transfer in the latter.