Adverse changes to our natural environment appear to manifest through extreme weather experienced in almost every corner of the globe, understood to be associated with the harmful effects of environmental pollution. Indeed, anthropogenic emissions are believed a major contributor to the problems the planet is experiencing. Technological advances that will remediate these emissions are of constant interest but rely on significant research breakthroughs.
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With low energy consumption and high efficiency, photocatalysis technology continues to gain traction in this regard, with material selection and property optimization of semiconductor photo-catalysts a particular focus. Here, the attraction of graphite carbon nitride is explained, covering current research pathways aimed at improving its performance as a photocatalyst and potential future investigations.
Photocatalysis and Carbon Dioxide
Photocatalysis is a photo-activated chemical reaction. In absorbing light, the energy created produces excited electron-hole pairs, which, in turn, drive pairs of chemical reactions resulting in the production of new chemical compounds. When considering environmental remediation, pollutant degradation is a priority.
Air pollution, no matter the source, generates pollutants from combustion that need broken up and dispersed. The constant introduction of carbon dioxide into the atmosphere with its disturbing, harmful effects on our ecosystem is one area that needs addressing.
Atmospheric carbon dioxide can be reduced to benign carbon compounds using photocatalytic activity. Interestingly, metal-free graphitic carbon nitride (or g-C3N4) has been found to have unexpected catalytic activity when applied to carbon dioxide.
Obtaining an Effective g-C3N4-based Photocatalyst
Graphitic carbon nitride possesses a series of characteristics that have contributed to its serious candidacy as the photocatalyst of choice for investigating pollutant degradation. These include its non-toxic nature, excellent stability, graphene-like layered structure, and ease of synthesis.
Although it holds promise, graphitic carbon nitride is not without its challenges. Un-doped g-C3N4 suffers from similar traits exhibited by other photocatalysts; rapid electron-hole pair recombination and low mobility regarding charge carriers are common examples. Improving the photocatalytic efficiency of g-C3N4 and wide-scale adoption will require these phenomena to be addressed.
Modification Leading to Enhanced Activity
To date, a broad range of modification methodologies has been investigated to enhance the activity of g-C3N4-based photocatalysts.
Loading co-catalysts have received such as to improve the poor solar-to-chemical (STC) energy conversion that single-component g-C3N4 exhibits. Teams like Bie et al. have been forming two-dimensional (2D)/2D hetero-junctions with g-C3N4, obtaining very encouraging results. The large-area contact interfaces with g-C3N4 provide several advantages. For example, strengthening energy conversation when the interface is between black phosphorus and molybdenum disulfide has been explored.
The large-area contact interfaces with g-C3N4 provide several advantages. For example, strengthening energy conversation when the interface is between black phosphorus and molybdenum disulfide has been investigated.
Ion doping and metal deposition are two other modifications researchers have pursued. Unique metal-semiconductor composites have been designed that can change the photochemical properties of g-C3N4 and expand the physical range of photoabsorption. Other experiments have used nitric acid treatment on g-C3N4, which has been shown to improve photocatalytic activity by removing Cr(III) deposits due to the acid-soaking.
More comprehensive strategies rest in exploring the architectures and designs that can be synthesized from various nanostructures of g-C3N4. A myriad of varying dimensional structures at this scale have been fabricated; nanorods and nanotubes, 2D nano-sheets, and three-dimensional nano-spheres. Although, the majority of investigation seems to be driven into that of zero-dimensional quantum dots and their modification. Coupling g-C3N4 with carbon quantum dots (CQDs) or hybrid nanocomposites, for example, has gained attention. Other groups have enhanced the connection between CQDs and g-C3N4 with hydrogen bonding by converting monomers into polymers under high temperatures.
Drawing Optimal Designs
Although the research and development invested into graphitic carbon nitride photocatalytic technology have yielded significant advances, further exploration is still needed if extensive practical applications are to be realized. Optimized optical and photocatalytic properties, in particular, are currently insufficient for large-scale industrialization. To improve, it will be necessary to ascertain in greater depth the reaction mechanisms at play. The use of molecular simulations is likely to be of help to this.
The Future of g-C3N4-based Photocatalysts
Future material modifications are likely to be driven by environmental and economic concerns due to the need to scale up the technology. Non-toxic, abundant, and cheap elements will be front and center; avoiding noble metals and doping with rare earths will be necessary to mitigate cost escalation if significant adoption is to prevail. Attention will also be placed on more semiconductor coupling and metal-free doping, along with manipulating faster responses from quantum dots with increased sensitization.
In the context of environmental pollution control, g-C3N4-based photocatalysts have a high probability of becoming an integral component in practical applications tackling this vital issue. Though, significant further research is required to understand the full capability of this photocatalyst.
References and Further Reading
Huang R. et al. (2021) Strategies to enhance photocatalytic activity of graphite carbon nitride based photocatalysts Materials & Design 210 110040. Available at: https://doi.org/10.1016/j.matdes.2021.110040
Bie C. et al. (2021) Enhanced solar-to-chemical energy conversion of graphitic carbon nitride by two-dimensional co-catalysts EnergyChem 3(2) 100051. Available at: https://doi.org/10.1016/j.enchem.2021.100051
Chen Y. et al. (2020) A Review on Quantum Dots Modified g-C3N4-Based Photocatalysts with Improved Photocatalytic Activity Catalysts 10 142. Available at: https://doi.org/10.3390/catal10010142
Zhang Y. et al. (2015) Acid-treated g-C3N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light Separation and Purification Technology 142(4) 251. Available at: https://doi.org/10.1016/j.seppur.2014.12.041