Hydrogen Evolution Activity of Nitrogen-Rich g-C_3-N₄₊ Synthesized by Solid-Gas Interface Method

Area of Science:

• Materials Science
• Photocatalysis
• Renewable Energy

Background:

• Efficient solar-to-hydrogen conversion using metal-free photocatalysts is crucial for sustainable energy.
• Developing nitrogen-rich graphitic carbon nitride (g-C3N4) derivatives with enhanced properties remains a challenge.

Purpose of the Study:

• To synthesize nitrogen-rich g-C3N4 derivatives using a solid-gas interface approach.
• To investigate the effect of nitrogen doping on the photocatalytic performance for hydrogen evolution.

Main Methods:

• Synthesis of pure g-C3N4 (CN), g-C3N4+ under ammonia flow (CN-NH3), and g-C3N4+ under nitrogen flow (CN-N2) via a solid-gas interface method.
• Characterization of material properties including surface conductivity, optical absorbance, and electron donor density.
• Photoelectrochemical hydrogen evolution (HER) measurements and photocatalytic water splitting tests.
• Density functional theory (DFT) calculations to understand band gap modifications.

Main Results:

• The CN-NH3 sample exhibited superior surface conductivity, broad visible light absorption, reduced charge recombination, and high electron donor density.
• CN-NH3 achieved a photocurrent density of 2.06 μA cm-2, 2.5 times higher than pure CN (0.85 μA cm-2).
• CN-NH3 demonstrated enhanced photocatalytic water splitting, producing 634 μmol g-1 H2 without a cocatalyst and 1163 μmol g-1 H2 with Pt.
• DFT calculations indicated that increased N-doping progressively reduces the band gap by increasing partial π-occupations, stabilizing the conduction band minima.

Conclusions:

• The solid-gas interface approach successfully produced nitrogen-rich g-C3N4 sheets with enhanced photocatalytic activity.
• Ammonia treatment significantly boosts the performance of g-C3N4 for solar hydrogen evolution due to improved electronic and optical properties.
• Theoretical modeling supports the experimental findings, explaining the band gap tuning mechanism through nitrogen doping.