Here, we present oxygen-deficient black ZrO2−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 ZrO2−x as indicated by XPS and DFT calculations of black ZrO2−x. Current reduction method using Mg and H2 is mild, but highly efficient to produce solar light-assisted photocatalytically active black ZrO2−x.

Hydrogen adsorption/desorption behavior plays a key role in hydrogen evolution reaction (HER) catalysis. The HER reaction rate is a trade-off between hydrogen adsorption and desorption on the catalyst surface. Herein, we report the rational balancing of hydrogen adsorption/desorption by orbital modulation using introduced environmental electronegative carbon/nitrogen (C/N) atoms. Theoretical calculations reveal that the empty d orbitals of iridium (Ir) sites can be reduced by interactions between the environmental electronegative C/N and Ir atoms. This balances the hydrogen adsorption/desorption around the Ir sites, accelerating the related HER process. Remarkably, by anchoring a small amount of Ir nanoparticles (7.16 wt%) in nitrogenated carbon matrixes, the resulting catalyst exhibits significantly enhanced HER performance. This includs the smallest reported overpotential at 10 mA cm −2 (4.5 mV), the highest mass activity at 10 mV (1.12 A mg Ir −1 ) and turnover frequency at 25 mV (4.21 H 2 s −1 ) by far, outperforming Ir nanoparticles and commercial Pt/C.