Converting CO2 into fuels by graphitic carbon nitride-based photocatalysts

g-C3N4-Based Photocatalytic Materials for Converting CO2 Into Energy: A Review

Environmental pollution management and renewable energy development are humanity's biggest issues in the 21st century. The rise in atmospheric CO2, which has surpassed 400 parts per million, has stimulated research on CO2 reduction and conversion methods. Presently, photocatalytic conversion of CO2 to valuable hydrocarbons enables the transformation of solar energy into chemical energy and offers a novel avenue for energy conversion while regulating the greenhouse effect. This is an ideal strategy for simultaneously addressing environmental issues and the energy crisis. Photocatalysts are essential to photocatalytic processes. Photocatalyst is the core of photocatalytic technology, and graphite carbon nitride (g-C3N4) has attracted much attention because of its nonmetallic characteristics, and it has the characteristics of low cost, tunable electronic structure, easy manufacture and strong reducibility. However, its activity is not only affected by external reaction conditions, but also by the band gap structure, physical and chemical stability, surface morphology and specific surface area of the photocatalyst it. In this paper, the application progress of g-C3N4-based photocatalytic materials in CO2 reduction is reviewed, and the modification strategies of g-C3N4-based catalysts to obtain better catalytic efficiency and selectivity in CO2 photocatalytic reduction are summarized, and the future development of this material is prospected.

Contemporary advances in photocatalytic CO2 reduction using single-atom catalysts supported on carbon-based materials

The persistent issue of CO2 emissions and their subsequent impact on the Earth's atmosphere can be effectively addressed through the utilization of efficient photocatalysts. Employing a sustainable carbon cycle via photocatalysis presents a promising technology for simultaneously managing the greenhouse effect and the energy dilemma. However, the efficiency of energy conversion encounters limitations due to inadequate carrier utilization and a deficiency of reactive sites. Single-atom catalysts (SACs) have demonstrated exceptional performance in efficiently addressing the aforementioned challenges. This review article commences with an overview of SAC types, structures, fundamentals, synthesis strategies, and characterizations, providing a logical foundation for the design and properties of SACs based on the correlation between their structure and efficiency. Additionally, we delve into the general mechanism and the role of SACs in photocatalytic CO2 reduction. Furthermore, we furnish a comprehensive survey of the latest advancements in SACs concerning their capacity to enhance efficiency, long-term stability, and selectivity in CO2 reduction. Carbon-structured support materials such as covalent organic frameworks (COFs), graphitic carbon nitride (g-C3N4), metal-organic frameworks (MOFs), covalent triazine frameworks (CTFs), and graphene-based photocatalysts have garnered significant attention due to their substantial surface area, superior conductivity, and chemical stability. These carbon-based materials are frequently chosen as support matrices for anchoring single metal atoms, thereby enhancing catalytic activity and selectivity. The motivation behind this review article lies in evaluating recent developments in photocatalytic CO2 reduction employing SACs supported on carbon substrates. In conclusion, we highlight critical issues associated with SACs, potential prospects in photocatalytic CO2 reduction, and existing challenges. This review article is dedicated to providing a comprehensive and organized compilation of recent research findings on carbon support materials for SACs in photocatalytic CO2 reduction, with a specific focus on materials that are environmentally friendly, readily accessible, cost-effective, and exceptionally efficient. This work offers a critical assessment and serves as a systematic reference for the development of SACs supported on MOFs, COFs, g-C3N4, graphene, and CTFs support materials to enhance photocatalytic CO2 conversion.

Recent Progress of Three-dimensionally Ordered Macroporous (3DOM) Materials in Photocatalytic Applications: A Review

In recent years, three-dimensionally ordered macroporous (3DOM) materials have attracted tremendous interest in the field of photocatalysis due to the periodic spatial structure and unique physicochemical properties of 3DOM catalysts. In this review, the fundamentals and principles of 3DOM photocatalysts are briefly introduced, including the overview of 3DOM materials, the photocatalytic principles based on 3DOM materials, and the advantages of 3DOM materials in photocatalysis. The preparation methods of 3DOM materials are also presented. The structure and properties of 3DOM materials and their effects on photocatalytic performance are briefly summarized. More importantly, 3DOM materials, as a supported catalyst, are extensively employed to combine with various common materials, including metal nanoparticles, metal oxides, metal sulfides, and carbon materials, to enhance photocatalytic performance. Finally, the prospects and challenges for the development of 3DOM materials in the field of photocatalysis are presented.

Synthesis and applications of graphitic carbon nitride (g-C3N4) based membranes for wastewater treatment: A critical review

Semiconducting membrane combined with nanomaterials is an auspicious combination that may successfully eliminate diverse waste products from water while consuming little energy and reducing pollution. Creating an inexpensive, steady, flexible, and diversified business material for membrane production is a critical challenge in membrane technology development. Because of its unusual structure and high catalytic activity, graphitic carbon nitride (g-C3N4) has come out as a viable material for membranes. Furthermore, their great durability, high permanency under challenging environments, and long-term use without decrease in flux are significant advantages. The advanced material techniques used to manage the molecular assembly of g-C3N4 for separation membrane were detailed in this review work. The progress in using g-C3N4-based membranes for water treatment has been detailed in this presentation. The review delivers an updated description of g-C3N4 based membranes and their separation functions and new ideas for future enhancements/adjustments to address their weaknesses in real-world situations. Finally, the ongoing problems and promising future research directions for g-C3N4-based membranes are discussed.

Tailoring 2D carbides and nitrides based photo-catalytic nanomaterials for energy production and storage: a review

2D carbides and nitrides-based nanomaterials because of their unusual physical and chemical properties and a vast range of energy-storage applications have attracted tremendous attention. However, 2D carbides and nitrides-based nanomaterials and their corresponding composites have many intrinsic constraints in terms of energy-storage applications. The nano-engineering of these 2D materials is widely investigated, to improve their performance for practical application. In this Review article, the current progress and research on 2D carbides and nitrides-based nanostructures are presented and debated, concentrating on their methods of preparation, and energy conservation applications for example Lithium-ion-battery, supercapacitors, and Sodium-ion-battery. In conclusion, the problems, and recommendations essential to be discussed for the progress of these 2D nanomaterials for energy-storage applications based on carbides and nitrides are displayed.

Single-layer carbon nitride: synthesis, structure, photophysical/photochemical properties, and applications

This Perspective provides a critical summary of the current state of the art in the synthesis and properties of polyheptazine single-layer carbon nitride (SLCN). The summary combines the authors' research and literature reports on SLCN concerning the synthesis of single-layer polyheptazine sheets, light absorption and emission by SLCN, photochemical and photocatalytic properties of SLCN as well as examples of applications of SLCN sheets as "building blocks" in heterostructures with nanocrystalline semiconductors and metals. The Perspective is concluded with an outlook discussing the most promising directions for further studies and applications of SLCN and related composites.

Fabricating intramolecular donor-acceptor system via covalent bonding of carbazole to carbon nitride for excellent photocatalytic performance towards CO2 conversion

Photocatalytic conversion of CO₂ into hydrocarbon fuels is an ideal technology of mitigating greenhouse effect caused by excessive emission of CO₂. However, the high recombination rate of electron-hole pairs and limited charge carriers transport speed constrained the catalytic performance of many semiconductor catalysts. In this contribution, a series of carbon nitride (g-CN) samples with intramolecular donor-acceptor (D-A) system were successfully prepared by introducing organic donor into their structures. Characterization results confirmed that carbazole was successful connected to the structure of g-CN via chemical bond. The formation of intramolecular D-A system greatly enlarged the light response region of g-CN-xDbc. In addition, a new charge transfer transition mode was formed in g-CN-0.01Dbc due to the incorporation carbazole, which enable it to use light with energy lower than the intrinsic absorption of g-CN. Meanwhile, the D-A structure led to the spatial separation of electrons and holes in g-CN-xDbc and significantly decreased the recombination rate of electron-hole pairs. The g-CN-0.01Dbc presented the best catalytic performance and the CO evolution rate was 9.6 times higher than that of g-CN. Moreover, the reaction was performed in water without any additive, which made it green and sustainable. DFT simulation confirmed the D-A structure and charge carrier migration direction in the prepared samples.

Low-Dimensional Nanostructured Photocatalysts for Efficient CO2 Conversion into Solar Fuels

The ongoing energy crisis and global warming caused by the massive usage of fossil fuels and emission of CO2 into atmosphere continue to motivate researchers to investigate possible solutions. The conversion of CO2 into value-added solar fuels by photocatalysts has been suggested as an intriguing solution to simultaneously mitigate global warming and provide a source of energy in an environmentally friendly manner. There has been considerable effort for nearly four decades investigating the performance of CO2 conversion by photocatalysts, much of which has focused on structure or materials modification. In particular, the application of low-dimensional structures for photocatalysts is a promising pathway. Depending on the materials and fabrication methods, low-dimensional nanomaterials can be formed in zero dimensional structures such as quantum dots, one-dimensional structures such as nanowires, nanotubes, nanobelts, and nanorods, and two-dimensional structures such as nanosheets and thin films. These nanostructures increase the effective surface area and possess unique electrical and optical properties, including the quantum confinement effect in semiconductors or the localized surface plasmon resonance effect in noble metals at the nanoscale. These unique properties can play a vital role in enhancing the performance of photocatalytic CO2 conversion into solar fuels by engineering the nanostructures. In this review, we provide an overview of photocatalytic CO2 conversion and especially focus on nanostructured photocatalysts. The fundamental mechanism of photocatalytic CO2 conversion is discussed and recent progresses of low-dimensional photocatalysts for efficient conversion of CO2 into solar fuels are presented.

Graphitic carbon nitride nanotubes: a new material for emerging applications

We provide a critical review of the current state of the synthesis and applications of nano- and micro-tubes of layered graphitic carbon nitride. This emerging material has a huge potential for light-harvesting applications, including light sensing, artificial photosynthesis, selective photocatalysis, hydrogen storage, light-induced motion, membrane technologies, and can become a major competitor for such established materials as carbon and titania dioxide nanotubes. Graphitic carbon nitride tubes (GCNTs) combine visible-light sensitivity, high charge carrier mobility, and exceptional chemical/photochemical stability, imparting this material with unrivaled photocatalytic activities in photosynthetic processes, such as water splitting and carbon dioxide reduction. The unique geometric GCNT structure and versatility of possible chemical modifications allow new photocatalytic applications of GCNTs to be envisaged including selective photocatalysts of multi-electron processes as well as light-induced and light-directed motion of GCNT-based microswimmers. Closely-packed arrays of aligned GCNTs show great promise as multifunctional membrane materials for the light energy conversion and storage, light-driven pumping of liquids, selective adsorption, and electrochemical applications. These emerging applications require synthetic routes to GCNTs with highly controlled morphological parameters and composition to be available. We recognize three major strategies for the GCNT synthesis including templating, supramolecular assembling of precursors, and scrolling of nano-/microsheets, and outline promising routes for further progress of these approaches in the light of the most important emerging applications of GCNTs.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Bio-inspired honeycomb-like graphitic carbon nitride for enhanced visible light photocatalytic CO2 reduction activity

Graphitic carbon nitride (g-C₃N₄) is paying attention lately owing to its interesting characteristics and substantial application in improving environmental and energy concerns. Nevertheless, the photocatalytic activity of g-C₃N₄ is constrained by the inertness of the surface and particle aggregation during photocatalytic activity. Herein, we report the preparation of g-C₃N₄ with honeycomb-like morphology (HC-C₃N₄) via thermal condensation of prepared SiO₂ templates and dicyandiamide. The etching out of the SiO₂ templates by NH₄HF₂ created hollow or macropores in the C₃N₄ matrix resulting in its structural changes. Similar, to the bulk C₃N₄, the HC-C₃N₄ exhibited higher photocatalytic CO₂ reduction in hydrocarbons. This improved photocatalytic achievement is associated with higher specific surface area, excellent visible light absorption capability, higher electron donor density, easy mass diffusion of materials for surface reaction, and effective segregation of photogenerated charge carriers. Furthermore, the HC-C₃N₄ honeycomb structure was deposited with Ni(OH)₂ clusters which showed remarkable CO₂ reduction activity of 1.48 μmolh^-1 g^-1 of CH₄ and 0.73 μmolh^-1 g^-1 of CH₃OH generation which is 3.5 and 4.3 times higher CO₂ reduction activity compared with bulk C₃N₄ clustered with Ni(OH)₂ particles. This comprehensive study demonstrated that HC-C₃N₄ nanostructured polymeric semiconductor is envisaged to have great potential in the application of a variety of fields such as photocatalysis, sensor technology, and nanotechnology.

A simple method to prepare g-C3N4-TiO2/waste zeolites as visible-light-responsive photocatalytic coatings for degradation of indoor formaldehyde

The indoor air quality should be highly addressed because people spend more time staying in indoor environments. Photocatalytic degradation of indoor pollutants (e.g., formaldehyde) is one of the most promising and environmental friendly technologies. In this work, a heterostructured photocatalyst combining graphitic carbon nitride (g-C₃N₄), TiO₂ and waste zeolites (g-C₃N₄-TiO₂/waste zeolites) is developed by a facile calcination and sol-gel method. The prepared photocatalysts exhibit the superior visible-light-responsive activities toward formaldehyde degradation (k = 0.0127 min^-1) which is higher than g-C₃N₄-TiO₂ (k = 0.0123 min^-1) and P25 (k = 0.0056 min^-1). Over 90% of low-concentration formaldehyde can be oxidized by g-C₃N₄-TiO₂/waste zeolites under a commercial LED light within 300 min. The electron spin resonance spectra indicate that the superoxide radical anions (O₂-) photogenerated on the g-C₃N₄-TiO₂/waste zeolites under visible light irradiation are responsible for the decomposition of formaldehyde. The enhancement in the photocatalytic decomposition of formaldehyde in the air is possibly due to the heterojunction between g-C₃N₄ (the enhanced absorption of visible light) and TiO₂ (fast transfer of photogenerated electrons from g-C₃N₄) as well as assisted adsorption of gas-phase formaldehyde via waste zeolites. This work also exemplifies the valorization of industrial silicate wastes to efficient photocatalytic coatings for indoor air purification.

Reduced graphene oxide supported C3N4 nanoflakes and quantum dots as metal-free catalysts for visible light assisted CO2 reduction

The visible light photocatalytic reduction of CO₂ to fuel is crucial for the sustainable development of energy resources. In our present work, we report the synthesis of novel reduced graphene oxide (rGO)-supported C₃N₄ nanoflake (NF) and quantum dot (QD) hybrid materials (GCN) for visible light induced reduction of CO₂. The C₃N₄ NFs and QDs are prepared by acid treatment of C₃N₄ nanosheets followed by ultrasonication and hydrothermal heating at 130-190 °C for 5-20 h. It is observed that hydrothermal exposure of acid-treated graphitic carbon nitride (g-C₃N₄) nanosheets at low temperature generated larger NFs, whereas QDs are formed at higher temperatures. The formation of GCN hybrid materials was confirmed by powder X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy (TEM), and UV-vis spectroscopy. High-resolution TEM images clearly show that C₃N₄ QDs (average diameter of 2-3 nm) and NFs (≈20-45 nm) are distributed on the rGO surface within the GCN hybrid material. Among the as-prepared GCN hybrid materials, GCN-5 QDs exhibit excellent CO₂ reductive activity for the generation of formaldehyde, HCHO (10.3 mmol h^-1 g^-1). Therefore, utilization of metal-free carbon-based GCN hybrid materials could be very promising for CO₂ photoreduction because of their excellent activity and environmental sustainability.

Anatase TiO₂-Decorated Graphitic Carbon Nitride for Photocatalytic Conversion of Carbon Dioxide

Three types of graphitic carbon nitride (gCN) nanosheets were derived from direct thermal condensation of urea, melamine, and dicyandiamide, respectively. As the sample (uCN) synthesized from urea exhibited porous morphology and highest surface area among other gCN, anatase TiO₂ nanoparticles were then in-situ deposited on uCN via solvothermal process without further calcination. The resultant Ti/uCN_x samples remained with higher surface area and exhibited visible-light activity. The derived band structure of each sample also confirmed its ability to photoreduce CO₂. XPS results revealed surface compositions of each sample. Those functional groups governed adsorption of reactant, interfacial interaction, electron transfer rate, and consequently influenced the yield of products. Carbon monoxide and methanol were detected from LED-lamp illuminated samples under appropriate moisture content. Samples with higher ratio of terminal amine groups produced more CO. The presence of hydroxyl groups promoted the initial conversion of methanol. The obtained Ti/uCN_0.5 and Ti/uCN_1.5 samples exhibited better quantum efficiency toward CO₂ conversion and demonstrated stability to consistently produce CO under cycling photoreaction.