Preparation and application of defective graphite phase carbon nitride photocatalysts
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-04-14 |
| Journal | Chinese Science Bulletin (Chinese Version) |
| Authors | Shanshan Ye, Chengyang Feng, Jiajia Wang, Lin Tang |
| Institutions | Hunan University |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe following is a high-density summary of the research review on defective graphite phase carbon nitride (g-C3N4) photocatalysts, tailored for an engineering audience:
- Core Value Proposition: Defective g-C3N4 (ND-g-C3N4) is a highly stable, non-metal semiconductor designed to overcome the limitations of pristine g-C3N4, specifically its narrow visible light response and high photo-excited charge carrier recombination rate.
- Mechanism of Improvement: Nitrogen defects (vacancies) are intentionally introduced, creating mid-gap states that overlap with the valence or conduction bands. This narrows the effective band gap, extending light absorption (wavelengths > 450 nm).
- Charge Carrier Dynamics: The defects act as active centers for electron-hole excitation and serve as effective traps, significantly enhancing charge separation efficiency and reducing recombination loss (improving overall quantum efficiency).
- Synthesis Versatility: Defective materials can be synthesized via three main routes: pre-polymerization adjustment (e.g., alkali/acid assistance), during-polymerization adjustment (e.g., H2 or NH3 atmosphere), and post-polymerization modification (e.g., molten salt treatment or magnesiothermic denitriding).
- High Performance Metrics: Optimized ND-g-C3N4 achieved H2 evolution rates up to 8910.7 ”mol/g (9.9 times the pristine material) and CO2 reduction rates up to 6.21 ”mol/h (4.2 times the pristine material).
- Key Applications: The material demonstrates superior performance in environmental remediation, including the degradation of persistent organic pollutants (antibiotics, pesticides), reduction of toxic heavy metals (Cr(VI) to Cr(III)), and efficient solar fuel production (H2 and CH4).
Technical Specifications
Section titled âTechnical SpecificationsâThe table below summarizes key performance metrics and material properties extracted from the review, focusing on defective g-C3N4 systems.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Pristine g-C3N4 Band Gap (Eg) | 2.7 | eV | Standard value. |
| Reduced Band Gap (Eg) | 2.56 | eV | Achieved via urea/oxamide co-condensation in molten salt. |
| Theoretical Triazine Eg (Mid-gap) | 2.17 | eV | Localized defect state energy (Figure 2a). |
| H2 Evolution Rate (Highest) | 8910.7 | ”mol/g | HNO3-acidified g-C3N4 (9.9 times pristine). |
| H2 Evolution Rate (H2 Atmosphere) | 64.39 | ”mol/h | CN-525 sample (18 times pristine). |
| CO2 Reduction Rate (Highest) | 6.21 | ”mol/h | CN-525 sample, reduction to CO (4.2 times pristine). |
| CO2 Reduction Rate (CH4) | 5.14 | times increase | Defect-modified g-C3N4 conversion to CH4. |
| Tetracycline Degradation Rate | 1.58 | times increase | KOH-assisted synthesis (compared to pristine g-C3N4). |
| NO Removal Rate Constant | 2.6 | times increase | N-vacancy porous microtubes (CNT-12 sample). |
| Pristine g-C3N4 Surface Area (BET) | 28.35 | m2/g | Untreated material (CN). |
| Enhanced Surface Area (BET) | 84 | m2/g | Secondary thermal treatment (g-C3N4(550-650)). |
| Interlayer Stacking Distance | 0.292 | nm | Optimized distance in molten salt synthesis, aiding charge transport. |
| N Vacancy Mid-gap State Energy | 1.17-1.32 | eV | Energy range relative to the conduction band edge. |
Key Methodologies
Section titled âKey MethodologiesâThe introduction of nitrogen defects into g-C3N4 is achieved through three primary engineering strategies, manipulating the precursor, atmosphere, or post-synthesis structure:
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Adjustment Before Polymerization (Precursor Modification):
- Alkali-Assisted Synthesis: Nitrogen-rich precursors (e.g., urea) are mixed with alkaline compounds (e.g., KOH, NaOH, Ba(OH)2) and thermally polymerized. The alkali selectively promotes the formation of nitrogen vacancies during the initial condensation phase.
- Acid-Assisted Synthesis: Precursors (e.g., melamine) are treated with strong acids (e.g., HNO3, CH3COOH) to form acid salts. Subsequent high-temperature calcination (e.g., 550 °C) of these salts yields defective g-C3N4 with enhanced activity.
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Adjustment During Polymerization (Atmosphere Control):
- Reducing Atmosphere: Polymerization is conducted under a reducing gas environment (e.g., H2 or NH3).
- Example: Melamine is heated at 550 °C for 4 h under H2 flow to produce CN-525 nanosheets, where the H2 atmosphere controls the density of nitrogen vacancies.
- Inert Atmosphere (High Temperature): Heating precursors (e.g., ammonium thiocyanate) at high temperatures (e.g., 550 °C) under an inert atmosphere (e.g., N2) can also induce controlled vacancies.
- Reducing Atmosphere: Polymerization is conducted under a reducing gas environment (e.g., H2 or NH3).
-
Adjustment After Polymerization (Post-Treatment):
- Molten Salt Treatment: Pristine g-C3N4 is mixed with eutectic molten salts (e.g., KCl/LiCl) and calcined (e.g., 550 °C). The molten salt acts as a solvent, enhancing polymerization degree, reducing stacking, and promoting defect formation.
- Secondary Thermal Treatment: Synthesized g-C3N4 is re-calcined at a higher temperature (e.g., 650 °C) under N2 to induce thermal decomposition and create nitrogen vacancies, simultaneously increasing the materialâs specific surface area.
- Magnesiothermic Denitriding: Pristine g-C3N4 is reacted with magnesium powder at high temperatures (e.g., 750 °C) under an inert atmosphere (Ar) to chemically strip nitrogen atoms, followed by acid washing to remove byproducts.
Commercial Applications
Section titled âCommercial ApplicationsâDefective g-C3N4 photocatalysts are highly relevant for several critical engineering and industrial sectors due to their stability, low cost, and visible-light activity:
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Wastewater Treatment and Environmental Remediation:
- Advanced Oxidation Processes (AOPs): Degradation of non-biodegradable and persistent organic pollutants (POPs), including antibiotics (e.g., tetracycline) and complex organic pesticides.
- Heavy Metal Management: Photocatalytic reduction of highly toxic heavy metal ions (e.g., Cr(VI)) to less harmful states (Cr(III)).
- Trace Contaminant Sensing: Utilization as a high-capacity, recyclable solid-phase extraction material for pre-concentration and analysis of trace pollutants.
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Sustainable Energy and Fuel Production:
- Solar Hydrogen Generation: Use in photocatalytic water splitting systems to produce H2 fuel, leveraging the enhanced charge separation for high quantum efficiency.
- Carbon Capture and Utilization (CCU): Efficient photoreduction of atmospheric CO2 into valuable chemical feedstocks and fuels, such as CH4 and CO, offering a pathway for solar fuel synthesis.
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Air Quality Control:
- NOx Abatement: Application in air purification systems for the photocatalytic removal and oxidation of nitrogen oxides (NOx), utilizing the high adsorption and activation capacity of surface nitrogen vacancies.
View Original Abstract
<p indent=0mm>With the development of industry and agriculture, the problems of environmental pollution and energy shortage have become increasingly severe. Semiconductor photocatalysis technology is one of the effective ways to solve environmental pollution and energy crisis. The principle of photocatalysis is based on the oxidation-reduction ability of photocatalysts under light conditions, which can achieve the purposes of purification of pollutants, material synthesis and transformation. Graphite phase carbon nitride (g-C<sub>3</sub>N<sub>4</sub>), as a new high-efficiency catalyst, has good stability and shows great engineering application potential in photocatalytic technology. However, the unmodified g-C<sub>3</sub>N<sub>4</sub> has a limited visible light response range, and the photo-excited charge carrier recombination rate is high, resulting in low photocatalytic activity. Nitrogen defects are introduced into the g-C<sub>3</sub>N<sub>4</sub> framework. These defects can manipulate the electronic structure, and the interstitial state produced can be used as a band-tail state, which can overlap with the valence band or the conduction band. The mid-gap state of semiconductors can extend the light response and act as an active center for electron-hole excitation. Introducing defects into g-C<sub>3</sub>N<sub>4</sub> can improve the photocatalytic activity of g-C<sub>3</sub>N<sub>4</sub>. This paper systematically reviews the physical, chemical and electrochemical properties of g-C<sub>3</sub>N<sub>4</sub> on the basis of experimental and theoretical research progress. The synthesis methods of defect g-C<sub>3</sub>N<sub>4</sub> are summarized, including adjustment before polymerization, adjustment during polymerization, and adjustment after polymerization. The adjustment before polymerization is to introduce defects by changing the precursor, such as adding hydroxide, sodium borohydride and other substances to the precursor. The adjustment during polymerization is to provide a reducing atmosphere during polymerization can prepare g-C<sub>3</sub>N<sub>4</sub> with different nitrogen-vacancy densities, such as hydrogen, ammonia and so on. The adjustment after polymerization is to modify the synthesized defect-free g-C<sub>3</sub>N<sub>4</sub>, such as recalcining or acid treatment to achieve the purpose of synthesizing nitrogen vacancies. The effect of defect sites on g-C<sub>3</sub>N<sub>4</sub> is also discussed. The intermediate state induced by nitrogen defects can be transformed into active centers excited by electron holes, and the optical response of defect g-C<sub>3</sub>N<sub>4</sub> is broadened due to the narrowing of the band gap. In the range, nitrogen defects can trap photo-generated carriers and prevent their recombination, thereby increasing the overall quantum efficiency. However, excessive introduction of nitrogen defects will produce deeper interstitial states. These deeper interstitial states can not only capture photo-generated electrons, but also photo-generated h<sup>+</sup>, which then become the recombination sites of photo-generated carriers. In addition, we separately summarized the application of the defect g-C<sub>3</sub>N<sub>4</sub> in water treatment, such as the degradation of antibiotics and organic pesticides and the reduction of the toxicity of heavy metals, as well as water decomposition, carbon dioxide conversion and photocatalytic denitrification. Defect g-C<sub>3</sub>N<sub>4</sub> has achieved good results in these applications. Although considerable progress has been made in the research of g-C<sub>3</sub>N<sub>4</sub> in recent years, there are still many challenges in preparing g-C<sub>3</sub>N<sub>4</sub> with high-efficiency catalytic performance. Finally, in view of the challenges faced by the application of defective g-C<sub>3</sub>N<sub>4</sub>, key discussions and future prospects are proposed from the aspects of mechanism exploration and material development.