Cu ₂ O NPs as Visible-Light Photosensitizer Impregnated on Self-Doped TiO ₂ –PTiW ₁₁ O ₄₀ <i>p</i> – <i>n</i> Heterojunctions for Photo-, Electro-, and Photoelectrochemical Hydrogen Production

Polyoxometalates (POMs) are distinguished by their tunable redox properties, enabling precise modulation of the electronic reactivity when integrated with metal oxide frameworks. Building on previous work demonstrating the reversible electron-donor behavior of [PTiW11O40]5– toward Ni nanoparticle active sites for photo- and electrocatalytic hydrogen evolution (ACS Appl. Energy Mater. 2025, 8, 6320–6329), the present study focuses on extending light-harvesting capability into the visible spectrum through bandgap engineering of TiO2-based systems. To this end, 10% self-doping and the introduction of oxygen vacancies (∼14%) on anatase TiO2 nanocrystals were employed, followed by the impregnation of p-type Cu2O nanoparticles onto n-type TiO2-[PTiW11O40]5– (TiO2–POM) nanocomposites. The resulting Cu2O/TiO2–POM hybrid forms an intrinsic p–n heterojunction with a narrowed band gap of 1.98 eV, significantly lower than 3.16 eV for undoped TiO2–POM. Comprehensive characterization using Tauc analysis, powder X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP–MS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller (BET) surface area measurements confirmed successful hybrid formation and a high specific surface area of 961 m2 g–1, providing abundant active sites for redox reactions. The optimized Cu2O/TiO2–POM catalyst demonstrated excellent photocatalytic hydrogen evolution activity of 7.75 mmol g–1 h–1 using Na2S/Na2SO3 as sacrificial agents under 450 W visible-light irradiation, whereas undoped TiO2–POM showed no detectable activity. Furthermore, the hybrid exhibited enhanced electrocatalytic performance, with a low Tafel slope of 99.6 ± 0.002 mV dec–1 in 0.25 M H2SO4 (pH 0.3) and a 17% increase in photocurrent density during photoelectrochemical testing. These pronounced improvements in photo-, electro-, and photoelectrocatalytic performance are attributed to efficient charge separation and prolonged carrier lifetimes facilitated by the Cu2O/TiO2–POM p–n intrinsic heterojunction and the redox-active POM moieties. The Cu2O/TiO2–POM hybrid thus represents a promising platform for visible-light-driven hydrogen generation and related sustainable energy applications.