Photothermal Hydrogen Production Using Noble-Metal-Free Ti@TiO₂ Core–Shell Nanoparticles under Visible–NIR Light Irradiation

Abstract

Limiting resources of fossil fuels and environmental issues inevitably require more efficient utilization of solar energy. Photocatalytic production of hydrogen is identified as one of the most promising routes for developing clean and sustainable energy; however, engineering low-cost materials exhibiting high catalytic activity in the entire range of the solar spectrum is still a challenge. Here, for the first time, we report simple, easily scalable, and environmentally friendly synthesis of stable Ti@TiO2 core–shell nanoparticles exhibiting photocatalytic activity in hydrogen production under vis/NIR light irradiation without any noble metals. Stable to oxidation core–shell Ti@TiO2 nanoparticles have been obtained by the simultaneous actions of ultrasound and hydrothermal treatment on air-passivated titanium metal nanoparticles in pure water. The obtained material is composed of quasi-spherical Ti particles (20–80 nm) coated by 5–15 nm crystals of defect-free anatase with small amounts of rutile. In contrast to pristine TiO2, the Ti@TiO2 nanoparticles extend the photo response from the UV to the NIR light region as a result of the light absorption by a nonplasmonic Ti core. In MeOH–H2O solutions, the Ti@TiO2 nanoparticles exhibit the strongest catalytic activity in H2 formation under joint effect of vis/NIR light and heat. Isotopic study using MeOH–D2O solutions suggests a reaction mechanism involving electron holes’ accumulation in a semiconducting TiO2 shell via charge separation and multiple charge-transfer steps that follow Ti interband transition. The electron transport from the Ti core presumably occurs through the junctions between the TiO2 crystals at the surface of core–shell nanoparticles.