Dark Photocatalysis via Long-Term Storage of Photoinduced Electrons in {Black-TiO_x/Ag0} Heterojunctions

Dark photocatalysts allow storage of photogenerated electrons and their post-irradiation use under no-light conditions. This requires the development of materials able to perform photogeneration of electrons, long-term in situ charge storage, and controlled transfer to acceptors on demand. Herein, we present {Black-TiO_x/Ag⁰} nanohybrids consisting of {Black-TiO_x} decorated with plasmonic Ag⁰ nanoparticles produced by flame spray pyrolysis (FSP). A library of {Black-TiO_x/Ag⁰} nanocomposites was engineered, with precise control over Ag⁰ loading and {Black-TiO_x} particle size. Under dark photocatalysis conditions, pre-irradiated {Black-TiO_x/Ag⁰} exhibits significant capacity to store and transfer electrons to Cr⁶⁺ or Cu²⁺ ions under dark conditions. Specifically, pre-irradiated {Black-TiO_x/Ag⁰} provides 0.6 × and 2.3 × 10²¹ electrons g⁻¹ of {Black-TiO_x/Ag⁰} to Cr⁶⁺ ions and Cu²⁺ ions, respectively, for at least 120 min. Operando electron paramagnetic resonance (EPR) spectroscopy reveals that Magnéli -TiO_x surface sites on {Black-TiO_x/Ag⁰} act as efficient trap/store units of the photoexcited electrons. The interfacial Magnéli islands introduce sub-gap energy states in TiO₂, drastically improving both electron storage and visible-light activity compared to conventional TiO₂/Ag⁰. Photoluminescence and photoelectrochemical analyses show that {Black-TiO_x/Ag⁰} exhibits high electron mobility and low hole/electron recombination rates. This analysis reveals that the interfacial Magnéli islands in contact with Ag⁰ act [i] as efficient electron storage pools of surface electrons under sunlight and [ii] as long-lived electron donors in the dark. This work provides a first proof-of-concept for {Black-TiO_x/Ag⁰} nanocomposites as photochargeable battery-like materials, enabling decoupled dark redox chemistry, while highlighting FSP as a versatile and scalable route for their synthesis.