Single-Site Ni-Grafted TiO₂ with Diverse Coordination Environments for Visible-Light Hydrogen Production

Solar hydrogen production at a high efficiency holds the significant importance in the age of energy crisis, while the micro-environment manipulation of active sites on photocatalysts plays a profound role in enhancing the catalytic performance. In this work, a series of well-defined single-site Ni-grafted TiO₂ photocatalysts with unique and specific coordination environments, 2,2'-bipyridine-Ni-O-TiO₂ (T-Ni Bpy) and 2-Phenylpyridine-Ni-O-TiO₂ (T-Ni Phpy), were constructed with the methods of surface organometallic chemistry combined with surface ligand exchange for visible-light-induced photocatalytic hydrogen evolution reaction (HER). A prominent rate of 33.82 μmol ⋅ g^-1  ⋅ h^-1 and a turnover frequency of 0.451 h^-1 for Ni are achieved over the optimal catalyst T-Ni Bpy for HER, 260-fold higher than those of Ni-O-TiO₂ . Fewer electrons trapped oxygen vacancies and a larger portion of long-lived photogenerated electrons (>3 ns, ~52.9 %), which were demonstrated by the electron paramagnetic resonance and femtosecond transient IR absorption, correspond to the photocatalytic HER activity over the T-Ni Bpy. The number of long-lived free electrons injected from the Ni photoabsorber to the conduction band of TiO₂ is one of the determining factors for achieving the excellent HER activity.