Here, we present oxygen-deficient black ZrO_2−x as a new material for sunlight absorption with a low band gap around ~1.5 eV, via a controlled magnesiothermic reduction in 5% H₂/Ar from white ZrO₂, a wide bandgap(~5 eV) semiconductor, usually not considered for solar light absorption. It shows for the first time a dramatic increase in solar light absorbance and significant activity for solar light-induced H₂ production from methanol-water with excellent stability up to 30 days while white ZrO₂ fails. Generation of large amounts of oxygen vacancies or surface defects clearly visualized by the HR-TEM and HR-SEM images is the main reason for the drastic alteration of the optical properties through the formation of new energy states near valence band and conduction band towards Fermi level in black ZrO_2−x as indicated by XPS and DFT calculations of black ZrO_2−x. Current reduction method using Mg and H₂ is mild, but highly efficient to produce solar light-assisted photocatalytically active black ZrO_2−x.

Using first-principles density functional theory, we study the effect of particle size and surface structure on the chemisorption energy of OH and O on nanoparticles of Pt. We find that the chemisorption energies of O and OH are strongly affected by the size and structure of the Pt particle varying by up to $1.0\mathrm{eV}$ at different adsorption sites and particle sizes.