Recently, Prof. DING Junfeng and his team from the Institute of Solid State, Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Science (CAS), together with Prof. ZHANG Genqiang from the University of Science and Technology of China, achieved band gap optimization and photoelectric response enhancement of carbon nitride in graphite phase with nitrogen vacancies under high pressure.

The relevant results were published in Physical Review Applied.

Graphitic carbon nitnitide (g-C₃N₄) performed well in many fields like high efficiency photocatalytic hydrogen production and water oxidation. However, the wide band gap of 2.7 eV of the original g-C₃N₄ limits its light absorption in the visible range. High pressure technology is an effective strategy to change properties while remaining composition. Therefore, band gap engineering of g-C₃N₄ by high pressure technology can significantly promote its photocatalytic activity and enhance its application potential.

In this study, scientists prepared N-containing vacancy defect g-C₃N₄ nanosheets with a nitrogen/carbon ratio of 9:10 by alkali assisted thermal polymerization.

A series researches were conducted later about the band gap evolution and photoelectric response behavior of g-C₃N₄ under high pressure. They used diamond anvil cell (DAC) combined with Raman spectroscopy, synchrotron X-ray diffraction, high-pressure absorption spectroscopy, and photocurrent measurement techniques.

An important phenomenon was that g-C₃N₄ underwent a pressure-induced amorphization (PIA) from graphite to amorphous phase at 17 GPa. Under high pressure, the PL of N-deficient g-C₃N₄ changed from yellow to red, and the minimum band gap reaches 1.70 eV.

The narrowed band gap enhanced light absorption in the visible range and increased the photocurrent in visible light from 18 nA to 29 nA. When the applied pressure was higher than the PIA transition pressure, a narrow band gap could still be maintained at ambient conditions, achieving a wide range of g-C₃N₄ band gap engineering from 1.87 eV to 2.42 eV.

This study provided a vacancy/defect assisted PIA strategy that could be used to design and regulate the band gap of materials, and improve photoelectric performance for a variety of applications under environmental conditions.

Color change of g-C3N4 in diamond anvil cell under high pressure. (Image by CHENG Peng)