Multivalent Cu catalytic sites on TiO2 for efficient photocatalytic hydrogen evolution and mechanistic insights from solid-state operando photochemical analysis

Abstract

The global energy crisis has driven research into earth-abundant copper (Cu)-modified TiO₂ photocatalysts for the solar hydrogen evolution reaction (HER). Current high-performance TiO₂-based catalysts demonstrate optimal performance under UV light, which constitutes ∼5% of the solar spectrum. However, they exhibit negligible activity under illumination with longer wavelengths (λ > 380 nm), which represent ∼50% of solar spectra, hindering their practical application. Understanding dynamic catalyst/co-catalyst interfacial changes and charge transfers is critical for optimization, yet progress is hindered by the need for expensive, specialized techniques lacking broad accessibility. We address these challenges by developing highly active photocatalysts featuring highly dispersed multivalent Cu active sites on surface-reduced TiO₂. Systematically controlling the amount of Cu loaded, we achieve one of the highest reported activities for transition metal-modified TiO₂, reaching 9.35 mmol_H2 g_cat⁻¹ h⁻¹ under simulated solar light conditions and 1.23 mmol_H2 g_cat⁻¹ after 3 h under λ > 380 nm illumination. The AQY values were calculated to be 40.81% at 340 nm and 3.22% at 390 nm indicating the efficient utilization of simulated solar light especially in the UV-region. XPS, UV-DRS and HAADF-STEM confirm coexisting Cu¹⁺/Cu²⁺ species and ∼1.5 nm Cu_xO (x = 1, 2) nanoclusters, highly dispersed on the TiO₂ support. We further propose a solid-state operando UV-DRS approach enabling the direct observation of the dynamic, reversible reduction of Cu^x to Cu⁰ under illumination, triggered by the charge transfer between TiO₂ and Cu^x. The photocatalyst displays remarkable stability, maintaining full activity for at least 26 hours of solar irradiation and retaining significant activity after 18 months in an aqueous/methanol dispersion. This work provides a design strategy for robust, high activity HER photocatalysts and an accessible operando platform for mechanistic studies in heterogeneous photocatalysis.