Cu ₂ O NPs as Visible-Light Photosensitizer Impregnated on Self-Doped TiO ₂ –PTiW ₁₁ O ₄₀ p – n 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 [PTiW 11 O 40 ] 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 TiO 2 -based systems. To this end, 10% self-doping and the introduction of oxygen vacancies (∼14%) on anatase TiO 2 nanocrystals were employed, followed by the impregnation of p -type Cu 2 O nanoparticles onto n -type TiO 2 -[PTiW 11 O 40 ] 5– (TiO 2 –POM) nanocomposites. The resulting Cu 2 O/TiO 2 –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 TiO 2 –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 m 2 g –1, providing abundant active sites for redox reactions. The optimized Cu 2 O/TiO 2 –POM catalyst demonstrated excellent photocatalytic hydrogen evolution activity of 7.75 mmol g –1 h –1 using Na 2 S/Na 2 SO 3 as sacrificial agents under 450 W visible-light irradiation, whereas undoped TiO 2 –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 H 2 SO 4 (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 Cu 2 O/TiO 2 –POM p – n intrinsic heterojunction and the redox-active POM moieties. The Cu 2 O/TiO 2 –POM hybrid thus represents a promising platform for visible-light-driven hydrogen generation and related sustainable energy applications.