Investigating the Role of Tunable Nitrogen Vacancies in Graphitic Carbon Nitride Nanosheets for Efficient Visible-Light-Driven H₂ Evolution and CO₂ Reduction

Abstract

Vacancy engineering, that is, self-doping of vacancy in semiconductors, has become a commonly used strategy to tune the photocatalytic performances. However, there still lacks fundamental understanding of the role of the vacancies in semiconductor materials. Herein, the g-C3N4 nanosheets with tunable nitrogen vacancies are prepared as the photocatalysts for H2 evolution and CO2 reduction to CO. On the basis of both experimental investigation and DFT calculations, nitrogen vacancies in g-C3N4 induce the formation of midgap states under the conduction band edge. The position of midgap states becomes deeper with the increasing of nitrogen vacancies. The g-C3N4 nanosheets with the optimized density of nitrogen vacancies display about 18 times and 4 times enhancement for H2 evolution and of CO2 reduction to CO, respectively, as compared to the bulk g-C3N4. This is attributed to the synergistic effects of several factors including (1) nitrogen vacancies cause the excitation of electrons to midgap states below the conduction band edge, which results in extension of the visible light absorption to photons of longer wavelengths (up to 598 nm); (2) the suitable midgap states could trap photogenerated electrons to minimize the recombination loss of photogenerated electron–hole pairs; and (3) nitrogen vacancies lead to uniformly anchored small Pt nanoparticles (1–2 nm) on g-C3N4, and facilitate the electron transfer to Pt. However, the overintroduction of nitrogen vacancies generates deeper midgap states as the recombination centers, which results in deterioration of photocatalytic activities. Our work is expected to provide new insights for fabrication of nanomaterials with suitable vacancies for solar fuel generation.

Unprecedented Centimeter‐Long Carbon Nitride Needles: Synthesis, Characterization and Applications

“…For CN 650 , the C1s spectra revealed that the weight percentage of the CNC specie (287.8 eV) decreased compared to the contribution of CC bond (284.7 eV), confirming the slight degradation of the tri‐s‐triazine structure. As well, a new peak appears at 285.6 eV, which belongs to the formation of a new CC π bond . The N1s spectra revealed that the atomic weight percentage of the CNC species (398.5 eV) was considerably decreased compared to N(C) 3 , suggesting that the loss of nitrogen atoms occurs at the two‐coordinated lattice sites .…”

Atomic‐Level Reactive Sites for Semiconductor‐Based Photocatalytic CO2 Reduction

“…2D graphitic carbon nitride has received significant research attention. Nitrogen and carbon vacancies are created in this material by annealing in an H 2 atmosphere, NH 3 atmosphere and in situ method. In nitrogen vacancy, abundant lone‐pair electrons are captured to facilitate adsorption and activation of molecules of CO 2 .…”

Nanostructured Carbon Nitrides for CO2 Capture and Conversion

“…Copyright 2014, Wiley‐VCH. E–H) Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…Nitrogen vacancies in the C 3 N 4 materials play an important role in controlling the optoelectronic and other physicochemical properties, thus lead to the enhancement of catalytic activity. It was realized by Tu et al who systematically investigated the role of nitrogen vacancies in the carbon nitride nanosheets toward solar conversion performance in CO 2 reduction (Figure E–H) . Nitrogen defects in the g‐CN nanosheets were realized through the postannealing of melamine‐derived C 3 N 4 in an H 2 environment and used for the CO 2 photoreduction for the production of 6.2 µmol h −1 of CO with visible‐light irradiation.…”

“…This is considered as one of the key parameters for the photocatalytic CO 2 reduction reactions (Figure I,J) . The S‐doping and introduction of N‐vacancies in g‐C 3 N 4 could form impurity level under the conduction band edge, which maximizes light absorption for photons of longer wavelengths and minimizes the electron–hole charge recombination . Amine or silica‐functionalized g‐C 3 N 4 presents an enhancement in the conversion of CO 2 due to high CO 2 affinity of the functional groups .…”