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

Area of Science:

• Materials Science
• Photocatalysis
• Renewable Energy

Background:

• Efficient solar hydrogen production is crucial for addressing the energy crisis.
• Manipulating the micro-environment of active sites on photocatalysts significantly enhances catalytic performance.
• Single-site catalysts offer precise control over active site coordination and electronic properties.

Purpose of the Study:

• To construct well-defined single-site Ni-grafted TiO2 photocatalysts with specific coordination environments for visible-light-induced photocatalytic hydrogen evolution reaction (HER).
• To investigate the structure-activity relationship between the coordination environment of Ni active sites and the HER performance.
• To elucidate the underlying mechanisms responsible for the enhanced photocatalytic activity.

Main Methods:

• Surface organometallic chemistry combined with surface ligand exchange to synthesize T-Ni Bpy and T-Ni Phpy photocatalysts.
• Visible-light-induced photocatalytic hydrogen evolution reaction (HER) measurements.
• Electron paramagnetic resonance (EPR) and femtosecond transient IR absorption spectroscopy to analyze charge carrier dynamics and defect states.

Main Results:

• The optimal catalyst, 2,2'-bipyridine-Ni-O-TiO2 (T-Ni Bpy), achieved a prominent HER rate of 33.82 μmol·g⁻¹·h⁻¹ and a turnover frequency of 0.451 h⁻¹ for Ni.
• The T-Ni Bpy catalyst exhibited a 260-fold higher HER activity compared to Ni-O-TiO2.
• Characterization revealed fewer electron-trapped oxygen vacancies and a larger portion of long-lived photogenerated electrons (>3 ns, ~52.9%) in T-Ni Bpy, correlating with enhanced HER activity.

Conclusions:

• Well-defined single-site Ni-grafted TiO2 photocatalysts with specific coordination environments can significantly enhance visible-light-driven HER.
• The T-Ni Bpy catalyst demonstrates superior performance due to optimized electronic properties and reduced charge carrier recombination.
• The number of long-lived free electrons injected from the Ni photoabsorber to the TiO2 conduction band is a key factor determining excellent HER activity.