Long-Lived Hole Accumulation in Al:SrTiO₃/Rh-Cr Photocatalyst Systems under Continuous Irradiation and Its...

Photocatalytic water splitting offers a scalable and potentially low-cost route for the production of renewable hydrogen. Recently, a state-of-the-art system based on flux-mediated Al³⁺-doped SrTiO₃, modified with Rh-Cr-based proton reduction and CoOOH water oxidation cocatalysts, achieved apparent quantum yields for unassisted water splitting of up to 93%. Herein, we focus on the role of Al³⁺ doping and Rh-Cr-based cocatalyst deposition on the accumulation and reaction dynamics of the long-lived holes required to drive water oxidation. We employ in situ and operando photoinduced absorption spectroscopy (PIAS) under water splitting conditions complemented by X-ray photoelectron spectroscopy (XPS). XPS data indicate that Al³⁺ doping suppresses surface Ti³⁺ defect states, coinciding with a 5-fold increase in the accumulation of long-lived SrTiO₃ holes observed by PIAS. Rh-Cr-based cocatalyst addition is observed to further enhance the yield and lifetime (s-10 s time scales) of these photoaccumulated holes, assigned to the efficient electron extraction by this cocatalyst. These photoaccumulated holes exhibit fast (ca. 1 s) and slow (ca. 10 s) decay phases. While the dominant fast phase is assigned to the desired water oxidation reaction, the slow phase is assigned to deeply trapped unreactive holes; the yield of these unreactive holes is suppressed by facet-selective photodeposition of cocatalysts or preillumination. These results provide key insights into how Al:SrTiO₃ functionalized by Rh-Cr-based cocatalysts accumulates oxidizing holes with lifetimes long enough to drive the kinetically challenging water oxidation reaction, thus achieving remarkably high quantum efficiencies for overall water splitting, insights which can be applied in the design of future photocatalytic materials.