Carbon Nitride and Its Hybrid Photocatalysts for CO₂ Reduction C1 Product Selectivity

Abstract

The transformation of abundant and cost-effective CO₂ molecules into valuable chemical feedstocks or fuels represents an appealing yet challenging research objective. Artificial photosynthesis offers a promising pathway for CO₂ reduction reactions (CO₂RR) under mild and environmentally friendly conditions. Graphitic carbon nitride (g-C₃N₄) has attracted significant attention for its potential to enhance the efficiency and selectivity of CO₂RR through synthesis and modification strategies. This review explores recent advancements in g-C₃N₄ and its hybrid photocatalysts for selective CO₂ conversions. We examine key factors influencing CO₂RR product selectivity, including electron count and reaction dynamics, CO₂ and reduction intermediates adsorption/desorption, and proton regulation affecting competitive hydrogen evolution. By summarizing various strategies to enhance CO₂ photoreduction performance, this work provides a comprehensive analysis of CO₂RR selectivity mechanisms for each approach. This review aims to inspire research endeavors towards developing efficient artificial systems for enhanced CO₂RR efficiency and product selectivity.

1. Introduction

The energy crisis [1,2,3,4], food security [5,6,7,8], and extreme weather resulting from climate change [9,10,11,12,13,14] pose severe challenges to the sustainable development of global human society. Environmental issues caused by the greenhouse effect, particularly CO₂ emissions [15,16,17], have exacerbated global warming trends, frequent extreme weather events, and significant threats to ecosystem balance and agricultural production [18,19,20,21]. In this context, exploring and developing efficient, clean, and sustainable energy conversion and storage technologies, as well as achieving effective CO₂ reduction and resource utilization, have become focal points of global research. Photocatalysis technology [22,23,24], a green technology based on semiconductor materials driving chemical reactions under light conditions, offers a highly promising solution to these environmental and energy challenges. Various photocatalysts have been developed, including TiO₂ [25,26,27], CdTe [28], ZnO [29,30], zeolitic imidazolate frameworks (ZIFs) [31,32,33,34], metal organic frameworks (MOFs) [35,36], g-C₃N₄ [37,38,39], and other materials [40,41].

With the goal of peaking CO₂ emissions before 2030 and achieving carbon neutrality by 2060, there is an urgent need to accelerate research on photocatalytic CO₂RR to produce value-added chemicals or hydrocarbon fuels such as methane, carbon monoxide, formic acid, ethanol, and ethylene [42,43,44]. Recent advancements in optimizing photocatalyst structures, reaction conditions, and co-catalysts have led to improvements in the activity and stability of semiconductors for CO₂ photoreduction. However, understanding the key factors governing CO₂RR product selectivity and the underlying reaction mechanisms remains a crucial and challenging task. The basic principle of the photocatalytic CO₂RR process involves three main steps (Figure 1a). Firstly, under sufficient light energy, electrons in the valence band (VB) of the photocatalyst are excited to the conduction band (CB), creating holes in the VB [45,46,47,48,49]. The subsequent migration of electrons and holes to the photocatalyst surface, where they may recombine, can impact quantum efficiency and CO₂RR selectivity. The intrinsic stability of CO₂ molecules (ΔG = −394.39 kJ mol⁻¹) and the high energy required to break the C=O bonds pose significant challenges. Moreover, during photocatalytic CO₂ reduction to CO₂, achieving a high redox potential is challenging for most semiconductors. The reduction of CO₂ can proceed via alternative pathways to produce various valuable chemical products, as shown in Figure 1b, which have similar thermodynamic potentials, indicating the need for selective catalysts. The initial protonation steps for CO₂ can form either the OCHO or *COOH intermediate, which will be further reduced to other C1-based fuels, with CO being the most commonly reported two-electron reduction product catalyzed by carbon nitride apart from HCOOH. The effective release of redox products or intermediates from the photocatalyst during the CO₂RR process can determine the type of product formed, as strong intermediate adsorption can lead to exclusive reduction products. The adsorption/desorption strength of CO₂, intermediates, or final products on the catalyst surface plays a critical role in influencing product selectivity. On the oxidation side, surface holes react with water or sacrificial agents to initiate oxidation reactions, generating H⁺ that can be reduced to H₂. The competing hydrogen evolution reaction (HER) is another crucial factor that can impact the selectivity of CO₂RR in many current systems.

To date, numerous efforts have been undertaken to modify and construct various semiconductors to enhance CO₂RR efficiency and product selectivity [50,51,52]. Among semiconductor photocatalytic materials, g-C₃N₄ stands out as a remarkable metal-free semiconductor with superior chemical and thermal stability, abundant precursor resources, non-toxicity, and low cost [53]. These attributes have positioned g-C₃N₄ as a promising candidate for CO₂RR applications. Many studies have focused on enhancing g-C₃N₄’s light absorption range, extending the lifetime of photogenerated carriers, and improving its CO₂ adsorption capacity to boost photocatalytic performance. Consequently, several review articles have explored the fabrication of various g-C₃N₄-based composites, the design of different microstructures of g-C₃N₄, and the general aspects and procedures during CO₂RR progress. However, reviews specifically focusing on the mechanism of selectivity conversion of CO₂ using g-C₃N₄-based photocatalysts are relatively scarce.

The history of g-C₃N₄ can be traced back to the 1830s when Berzelius and Liebig discovered the “melon” embryonic form, which consists of tri-s-triazines interconnected via tertiary amines. However, it was not until 2006 that g-C₃N₄ was introduced into the field of heterogeneous catalysis. In 2009, g-C₃N₄ gained wider recognition when Wang et al. reported its application in visible-light photocatalytic hydrogen evolution. Since then, g-C₃N₄ has become a popular material for photocatalysis, with thousands of articles published on this topic, including numerous studies on CO₂ conversion and various modification strategies aimed at enhancing g-C₃N₄’s selective CO₂ photoreduction capabilities. This section aims to provide a comprehensive discussion of the state-of-the-art of g-C₃N₄ for CO₂RR selectivity. To distinguish it from existing reviews, this article focuses on highlighting key aspects of the latest research progress on g-C₃N₄ and its hybrid photocatalysts for CO₂RR product selectivity. By examining critical aspects, this work delves into the main methods used to enhance g-C₃N₄’s photocatalytic CO₂RR and summarizes the working mechanisms of each enhancement approach in terms of their effects on CO₂RR product selectivity.

The discussion primarily focuses on recent experimental and theoretical advancements in three main areas: charge carrier transmission and reaction dynamics, CO₂ adsorption and reduction intermediate desorption, and surface regulation to control the competitiveness of the hydrogen evolution reaction (HER). Detailed analyses are conducted on how these aspects influence the production of C1 products from CO₂ reduction. This review concludes with insights into future prospects, including challenges, opportunities, and directions for the development of g-C₃N₄-based catalysts. This work serves as a valuable resource for emerging researchers looking to develop highly efficient photocatalysts for selective CO₂RR. Due to variations in catalyst fabrication protocols, reactor system designs for photocatalytic CO₂RR, and evaluation methods for product analysis, inconsistencies in experimental results may arise, even within the same modification strategy for achieving target product selectivity in photocatalytic systems. Figure 2 is the three aspects we have summarized for CO₂RR product selectivity over g-C₃N₄-based photocatalysts.

2. The g-C₃N₄ and Its Composite for Photocatalytic CO₂RR Selectivity

2.1. In the Context of Charge Carrier Transmission and Reaction Dynamics

Surface charge transfer and reaction kinetics play a critical role in determining the quantities and distribution of products in CO₂ reduction reactions. Adequate electron transfer to adsorbed CO₂ molecules or intermediates is essential for driving multi-electron reactions. This section delves into various modification strategies aimed at regulating the charge transfer pathways and reaction dynamics to impact CO₂RR selectivity.

2.1.1. Heteroatom Doping (Electron Property)

g-C₃N₄ has the ability to host a vast variety of impurity atoms into its backbone in order to modify the electronic structure. The introduction of dopants can contribute to the π-conjugated structure by sharing an empty orbital or an occupied orbital with extra electrons. Moreover, the dopants themselves can serve as appealing active sites for the photoreduction of CO₂ molecules due to the increased electron transfer required for subsequent reduction steps.

Element doping serves as an effective approach to adjust the electrical and structural properties as well as charge carrier dynamics of g-C₃N₄ by introducing active impurities. Non-metallic elements such as halogens and chalcogens are examples of heteroatoms that can substitute carbon or nitrogen atoms in g-C₃N₄. On the other hand, metallic elements like transition metals [54,55] and non-metals [56,57,58] have been shown to be capable of inserting into the triangular interstitial cavities of g-C₃N₄, influencing its properties and behavior in CO₂RR processes.

In the realm of non-metallic element doping in g-C₃N₄, Liu et al. conducted a study on boron (B)-doped g-C₃N₄. They observed that the B atoms, when doped, form strong interactions with the nitrogen atoms in the large cavities between adjacent tri-s-triazine units [59]. Theoretical calculations demonstrated that the excitation of electrons from nitrogen (2px, 2py) to boron (2px, 2py) in B/g-C₃N₄ is more favorable compared to the excitation from nitrogen (2px, 2py) to carbon 2pz in pure g-C₃N₄ due to the alignment of the orbitals (Figure 3a–c). This enhancement in charge transfer dynamics was corroborated by fluorescence lifetime measurements, highlighting the importance of appropriate B doping for optimizing charge transfer and localization. The B/g-C₃N₄ catalyst exhibited a significant improvement in CH₄ yield for the 8-electron reduction reaction, showing a 32-fold increase compared to pristine g-C₃N₄ (Figure 3d,e). Other common non-metallic dopants such as oxygen (O) and sulfur (S) have also been shown to modify the physicochemical properties of g-C₃N₄, leading to enhanced photocatalytic performance. For instance, Yu et al. developed hierarchical porous O-doped g-C₃N₄ nanotubes (OCN-Tube) through a series of thermal oxidation and exfoliation processes [60]. The introduction of O atoms resulted in a narrower band gap and improved interfacial charge transfer in the OCN-Tube, leading to high methanol selectivity with an evolution rate of approximately 0.88 µmol g⁻¹ h⁻¹. In a separate study, Zhao et al. successfully prepared an S-doped g-C₃N₄ photocatalyst with high CO₂RR activity and carbon monoxide (CO) selectivity, with 0.05 g catalyst in five hours [61]. The mechanism behind the selectivity enhancement was investigated using various techniques such as UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), photoluminescence (PL), fluorescence (FL), and density functional theory (DFT). The introduction of S impurities in g-C₃N₄ resulted in an upward shift of the conduction band position and modification of the S-CN band gap, leading to enhanced charge rearrangement and improved electron transmission dynamics, ultimately enhancing the photocatalytic CO₂RR selectivity. Furthermore, Huo et al. developed a carbon and oxygen co-doped g-C₃N₄ photocatalyst (OCCNx) [62]. The donor and acceptor system in this catalyst provided an additional electron transfer pathway, significantly improving the delocalization and lifetime of charge carriers. Without additional co-catalysts or sacrificial agents, the OCCNx catalyst achieved a CO evolution rate of 34.97 μmol g⁻¹.

In the domain of metal doping in g-C₃N₄, Wang et al. synthesized potassium-doped g-C₃N₄ (K-CN) for CO₂ reduction reactions [63]. Through DFT theoretical calculations and X-ray photoelectron spectroscopy (XPS), it was confirmed that interlayer doping of potassium alters the electronic structure of g-C₃N₄. The presence of doped potassium influences the band edge positions and enhances the oxidation ability of the VB. Additionally, surface defects induced by potassium doping can trap electrons and inhibit the recombination of charge carriers. The K-CN-7 (the 7 represents the weight percentage of potassium fluoride dihydrate in the mixtures of precursors.) variant exhibited a redshift and broadening of the PL spectrum, indicating that potassium doping facilitated the delocalization of π electrons on g-C₃N₄, thereby improving CO₂ adsorption. Consequently, K-CN achieved a high CO production yield of 8.7 µmol g⁻¹ h⁻¹ without the need for any co-catalysts, representing a 25-fold increase compared to pristine g-C₃N₄. Furthermore, Kumar et al. demonstrated the introduction of Fe and Cr into the in-planes of boron-doped g-C₃N₄ (B-CN) using a wet impregnation method [64]. The total consumed electron number increased by 2.14 times for Cr and B co-doped CN compared to pristine CN, leading to enhanced selectivity for CO with Cr and Fe doping. The mechanism illustrated that photoexcited electrons preferentially migrate to the doped chromium in Cr, B-CN. These electrons then contribute to the formation of the active intermediate CO₂, as well as engage in reversible charge transformation for Cr³⁺ [Cr³⁺ + e⁻ → Cr²⁺]. The CO₂ intermediate subsequently reacts with protons to generate CO and CH₄ through a multi-step process. In the case of Fe-doped B-CN, trapped holes prolong the lifetime of photogenerated electrons and prevent potential re-oxidation of CO to CO₂. Conversely, trapped electrons in Cr, B-CN suppress the formation of CH₄ by restricting the availability of photoexcited electrons on the surface. In situ electron spin resonance (ESR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies identified the dual role of trapped charge carriers in enhancing charge separation and influencing the distribution of CO and CH₄ products.

2.1.2. Metal Co-Catalysts

Co-catalysts typically play a crucial role in enhancing the efficiency of photocatalytic processes for CO₂RR. By acting as electron traps, co-catalysts facilitate charge separation and transport, extracting photogenerated charges from the semiconductor. They also lower the activation energy for gas evolution, thereby improving catalytic kinetics and modulating product selectivity [65].

Wang et al. successfully developed bismuth (Bi)-decorated g-C₃N₄ hybrids using a solvothermal method [66]. The resulting Bi/g-C₃N₄ composite exhibited a remarkable enhancement in CH₄ yield. This improvement can be attributed to the formation of a Schottky junction between g-C₃N₄ and the Bi metal. This junction not only accelerates the separation of charge carriers and inhibits their recombination but also enables the accumulation of electrons on the CB of g-C₃N₄. This electron accumulation provides the essential conditions for multi-electron reactions to occur, leading to the satisfactory CH₄ selectivity observed in the Bi/g-C₃N₄ composite. In 2022, Tonda et al. developed a non-noble Cu-Ni core–shell bimetallic co-catalyst decorated on CN [67]. This catalyst exhibited remarkable CO₂ reduction activity, particularly for selective CH₄ production. The synergy between Cu-Ni and CN played a key role in enhancing the separation efficiency of charge carriers and optical responses, leading to the excellent activity and selectivity observed in the Cu-Ni@CN catalyst for CO₂RR. Yu et al. deposited platinum nanoparticles (Pt NPs) as a co-catalyst on the surface of g-C₃N₄ using NaOH-assisted impregnation and NaBH₄-assisted reduction methods [68]. By adjusting the loading content of Pt NPs, they effectively regulated the selectivity and activity of CO₂RR. Increasing the Pt content from 0 to 1 wt% on g-C₃N₄ led to higher yields of CH₄ and methanol (CH₃OH), while the yield of formaldehyde (HCHO) increased with Pt content up to 0.75 wt% but decreased with further deposition. Moreover, the generation rates of CH₄ and HCHO increased with higher Pt loading content. The loaded Pt co-catalysts facilitated efficient collection of photoexcited electrons for CO₂ reduction and promoted the oxidation of the reduction products. Lv et al. synthesized an ultrathin 2D Ti₃C₂/g-C₃N₄ photocatalyst through the direct calcination of Ti₃C₂ and urea [69]. The optimal sample exhibited higher CO yields compared to methane production, with CO selectivity attributed to the efficient spatial separation of photoexcited charge carriers facilitated by the intimate contact between ultrathin Ti₃C₂ co-catalyst and g-C₃N₄. Recently, Li et al. constructed an Fe₂P co-catalyst decorated on nitrogen vacancies (NVs) enriched g-C₃N₄ hybrid photocatalyst (Fe₂P/NVsCN). The Fe₂P co-catalyst acted as the active site to promote charge carrier separation and migration [70], while the NVs enhanced visible light absorption, leading to improved activity and selectivity for converting CO₂ to CO with a high selectivity of 97.5%. Pan et al. prepared a g-C₃N₄ modified by NiS₂ quantum dots (QDs) co-catalyst using a hydrothermal method. The composite displayed superior CO evolution rate activity in CO₂RR [71]. The smaller size and quantum confinement effect of NiS₂ QDs provided more active sites and facilitated an intimate interface contact, enhancing electron transfer from g-C₃N₄ to NiS₂ and effectively accelerating the CO₂RR process.

2.1.3. Single-Atom

Downsizing metals into a single-atom scale has been considered an effective strategy to improve photocatalytic CO₂RR efficiency [72]. In recent years, various single metal atoms such as Co, Ni, Cu, Pd, La, and so on have been successfully anchored on g-C₃N₄ to prepare photocatalysts for CO₂RR.

Li et al. prepared a single Co²⁺ sites modified g-C₃N₄ photocatalyst by the deposition method [73]. The Co²⁺ sites in Co@C₃N₄ are likely connected with g-C₃N₄ through Co-N bonds. The catalyst exhibited quantum yields up to 0.4% for selective CO₂RR, as shown in the schematic of photocatalytic CO₂ reduction in Figure 4a. Cheng et al. successfully synthesized unsaturated edge confinement Ni anchored g-C₃N₄ via a self-limiting process [74], achieving a CO generation rate of 8.6 µmol g⁻¹ h⁻¹, which is 7.8 times higher than that of pure g-C₃N₄. STEM and EDS mapping confirmed the morphology of Ni₅-CN and Ni single atoms. The isolated Ni clusters on the edge of g-C₃N₄ surroundings can stabilize the single-atom site, and a high density of active monoatomic sites was achieved by trapping single-atom Ni with vacancy ligands in the g-C₃N₄ structure. The cationic coordination environment of the single-atomic-site Ni center is formed by Ni-N doping intercalation, promoting the superiority in synergistic N-Ni-N connection and interfacial carrier transfer. In addition, the interaction between Ni and g-C₃N₄ enhances the separation of charge carriers, as supported by the photocatalytic mechanistic prediction. The introduction of unsaturated Ni-N coordination improves the rate-determining step of intermediates for CO generation. Du et al. studied the impact of single atoms of Pd and Pt anchored on g-C₃N₄ for photocatalytic CO₂RR performance using DFT. Pd/g-C₃N₄ favors the production of HCOOH with a barrier of 0.66 eV, while Pt/g-C₃N₄ reduces CO₂ to CH₄ with a barrier of 1.16 eV. The choice of reduction products is closely linked to the metal atoms’ active sites and charge carrier transmission [75]. Additionally, Dong et al. developed La single atoms on g-C₃N₄ for photocatalytic CO₂RR [76]. Experimental and DFT results illustrate the p–d orbital hybridization of La-N atoms and the role of 4f and 5d orbitals of La in forming charge-transfer channels (Figure 4b). The charge bridges enhance carrier transport dynamics, leading to La-CN achieving a high CO yielding rate of 92 μmol·g⁻¹ h⁻¹ and CO selectivity of 80.3%.

2.1.4. Defects and Crystalline Regulation

The low photocatalytic performance of g-C₃N₄ can be attributed to factors such as unordered domains, low specific surface area, and defect density, which lead to a low charge transfer rate and limited active sites. However, defect engineering, such as creating C/N vacancies on the heptazine units, has been shown to effectively enhance the photocatalytic activity and selectivity for CO₂ reduction reactions. By introducing surface defects and controlling crystallinity, the band structure and electronic structure of g-C₃N₄ can be altered, resulting in uneven electron distribution and the generation of a polarization effect, ultimately improving charge transfer efficiency.

The study by Cao et al. introduced a new material denoted as CCN, which was synthesized under acetonitrile-promoted solvothermal conditions. This CCN material exhibited improved charge transfer, electron storage, and enhanced CO₂ binding properties, leading to effective CO₂ photoreduction into CO with a selectivity of 91.5% [77]. In a separate work by Zhang et al., a novel type of GCN material was developed by intentionally introducing C vacancies through heat treatment in an NH₃ atmosphere [78]. The resulting GCN had a thinner and more nanometer-scale porous layer structure. Characterization techniques such as ESR and XPS confirmed the presence of C vacancies in the GCN. Further analysis using EIS and FL indicated that the concentration of charge carriers in the GCN was enhanced by the C vacancies. The presence of C vacancies in the GCN led to improved exciton dissociation and enhanced charge carrier concentration, resulting in a longer lifetime of the material. This enrichment of C vacancies elevated the charge carrier concentration, leading to more than a twofold increase in CO₂-to-CO selectivity compared to pristine GCN. Zhang et al. conducted a study where they prepared N-deficient CN using a high-temperature crystalline phase transformation process [79]. This method led to the creation of mid-gap states by the N defect, favoring the thermodynamic generation of CH₄, while the broken crystal/amorphous structure facilitated the kinetic generation of CH₄. XPS analysis confirmed the position of the N defect. The crystallized platform plane provided a pathway for electron migration to the edge. The amorphous edge, rich in N defects, offered more active sites with higher electron density for the reduction of CO₂ and adjustment of the band gap. The electron-based selectivity of g-CN-750 (means that melamine was placed in a crucible wrapped in aluminum foil and calcined at 750 °C) for CH₄ reached 96.4% due to the dual synergistic effect of thermodynamics and kinetics. This work is expected to provide valuable insights for the future development of CO₂ reduction photocatalysts with enhanced CH₄ selectivity through the regulation of thermodynamics and kinetics. On the other hand, Liang et al. prepared ultrathin mesoporous g-C₃N₄ nanosheets (CNNSs) enriched with nitrogen vacancies and decorated with cyano groups using a molten salt route [80]. The CNNS exhibited outstanding CO₂RR activity, with a CH₄ yield rate of 23.0 μmol g⁻¹ h⁻¹ and a selectivity of 97.9%. This improved performance can be attributed to the combined effect of nitrogen vacancies and cyano group decoration, which enhance electron storage, charge carrier mobility, and CO₂ affinity, and optimize the band structure. The average fluorescence lifetime of CNNS (5.87 ns) was 1.4 times longer than that of bulk-g-C₃N₄ (4.20 ns), indicating higher CO₂ adsorption affinity.

2.1.5. Morphological Adjustment

The morphological adjustment of g-C₃N₄ has been shown to significantly impact the photocatalytic CO₂RR and the selectivity of products. Researchers have recognized that altering the structure and geometry of g-C₃N₄ can influence the migration distance of charge carriers and the number of photogenerated electrons on the surface, ultimately leading to enhanced CO₂RR selectivity [81,82,83].

Jiang et al. successfully prepared g-C₃N₄ nanosheets (CNS) by calcining sodium borohydride and g-C₃N₄ [84], resulting in a 2D CNS structure (Figure 5a,b). The R-CNS-400 exhibited exceptional CO₂RR activity with a CH₄ yield rate of 55.3 μmol g⁻¹ h⁻¹ and a high CO selectivity of 98.9% without sacrificial agents or co-catalysts. The charge transfer kinetics study revealed that CNS-400 had the longest lifetime of 12.05 ns compared to bulk-CN and CNS, indicating improved carrier dynamics and charge transport efficiency due to the structural reconstruction of CNS. The charge densities difference of Figure 5c,d illustrate that the increased charge depletion in R-CNS can be observed around the C atom of adsorbed CO₂; simultaneously, the increased charge accumulation can be observed between adsorbed CO₂ and the heptazine ring framework, suggesting more opportunity for carrier separation and more occurrence for CO₂ photoreduction. Jiang and colleagues also constructed a fusiform-shaped capsule hollow nanostructure g-C₃N₄ (Hf-g-C₃N₄) with ultrathin wall thickness and adjusted C/N ratios [85]. This unique nanostructure offered a higher specific surface area, enhanced light scattering, and improved conductivity, resulting in reduced charge carrier recombination rates and increased utilization efficiency for CO₂ reduction. The negative conduction band position of Hf-g-C₃N₄ facilitated the selective generation of CO during photocatalytic CO₂RR. Xiang et al. developed nanosheet-assembled hierarchical flower-like g-C₃N₄ (CMN) through self-assembly and ethanol insertion strategy [86]. The reduced fluorescence lifetime of CMN suggested a higher likelihood of non-radiative excited state reduction, while transient photocurrent response and Nyquist plot analysis indicated improved carrier migration and separation, leading to enhanced CO₂ reduction efficiency. Additionally, Wang et al. prepared 3D macropore C-vacancy g-C₃N₄ (3DMC/g-C₃N₄) using a one-step template route [87]. The modified electron structure of 3DMC/g-C₃N₄ resulted in a lowered conduction band minimum (CBM) and a strong driving force for CO₂ conversion. The carbon vacancies in 3DM C/g-C₃N₄ acted as electronic traps to locate photogenerated electrons, increasing surface carrier utilization and facilitating efficient electron–hole separation and transfer. These studies highlight the importance of morphological adjustment in g-C₃N₄ for optimizing photocatalytic CO₂RR performance and selectivity towards specific energy products through tailored charge carrier dynamics, structural features, and band alignments.

2.1.6. Construction of Heterostructures

In the context of constructing heterostructures for enhancing photocatalytic CO₂RR product selectivity, the coupling of g-C₃N₄ with various semiconductors, conductors, or polymers can be a promising approach [88,89,90,91]. By creating heterojunction interfaces between CN and other materials, it is possible to establish space charge regions and internal electric fields that can either assist or suppress the separation of photogenerated charge carriers. This strategy can lead to the prolongation of electron lifetimes and the regulation of electron transport dynamics, ultimately influencing the selectivity of CO₂RR products. In the upcoming section, we will delve into the discussion of different CN-based heterojunctions that have shown potential in improving the selectivity of products in photocatalytic CO₂RR processes.

Wang et al. successfully coated Bi₃NbO₇ on g-C₃N₄ to develop a 2D/2D Bi₃NbO₇/g-C₃N₄ S-scheme heterojunction [92]. This structure enables highly selective photocatalytic reduction of CO₂ to CH₄. Both DFT calculations and experimental results confirmed that the built-in electric field drives electrons from Bi₃NbO₇ to g-C₃N₄, illustrating the presence of an S-scheme charge transfer path during CO₂RR in the heterostructures, resulting in a 90% selectivity for CH₄.

Subsequently, Guo et al. constructed an S-scheme heterojunction by combining α-Fe₂O₃ with In-doped CN [93]. This formation of an S-scheme heterojunction promotes charge separation, leading to a CO production rate of 24.3 μmol g⁻¹ h⁻¹ with up to 94% selectivity. Currently, various types of S-scheme heterojunctions, such as g-C₃N₄/TiO₂/Ti₃AlC₂, Cu₃P/S/g-C₃N₄, g-C₃N₄/Bi₁₂O₁₇C_l2, and CoO/g-C₃N₄ [94,95,96,97], have been developed, demonstrating satisfactory CO₂RR product selectivity. Additionally, the Z-scheme has emerged as a promising approach to achieve spatial charge separation while maintaining high redox activity for CO₂RR [98].

The research conducted by Song et al. and Fu et al. demonstrates the potential of van der Waals (VDW) heterojunction catalysts in enhancing the efficiency of CO₂ conversion to CO without the need for sacrificial agents. Song et al. successfully coupled a covalent organic framework (COF-TD) on a 2D-CN surface, leading to intense interface VDW force interactions. This interaction enlarged the light-responsive region, reduced charge carrier recombination, and provided more active sites for catalyzing CO₂ to CO. The CO yield of COF-TD-2D-CN is 2.03 μmol g⁻¹ h⁻¹, which is 3.2 times that of pure g-C₃N₄ [99]. On the other hand, Fu et al. created ultrathin 2D/2D Sb/g-C₃N₄ VDW heterostructures by self-assembling Sb and g-C₃N₄ [100]. Theoretical calculations indicated that Sb has high carrier mobility, and the charge transfer and in-plane structure distortion at the Sb/g-C₃N₄ interface enhanced CO₂ activation ability. PL and TRPL measurements confirmed better charge carrier separation efficiency in Sb/g-C₃N₄ VDW heterostructures compared to pure g-C₃N₄. As a result, the Sb/g-C₃N₄ VDW heterostructures achieved a higher CO yield of 2.03 μmol g⁻¹ h⁻¹, showcasing their potential for efficient CO₂ conversion.

Mohamed et al. prepared the Z-scheme mesoporous CdS-g-C₃N₄ heterostructure [101]. The Z-scheme not only displays a high surface area for promoting CO₂ adsorption but also exhibits a short electrons transformation track, which is beneficial for selective reduction of CO₂ to generate CH₃OH (1735 μmol g⁻¹). Chen et al. made Au deposited onto ZCS followed by wrapping CN over the ZCS surface and successfully constructed an all-solid–solid Zn_0.5Cd_0.5S/Au@g-C₃N₄ (ZCS/Au@CN) Z-scheme heterojunction [102]. The photocatalyst showed a high CH₃OH evolution rate and selectivity during the photocatalytic CO₂RR. Due to the close contact interface of Au with ZCS and CN, it not only improved the charge separation of ZCS and CN but also advanced the reduction ability of photoinduced electrons in the CB of CN. In a similar study conducted by Bhosale et al. [103], the direct Z-scheme g-C₃N₄/FeWO₄ heterojunction not only showed an enhanced electron–hole pairs separation rate, but also improved the reduction ability of g-C₃N₄. That is why the photocatalyst displayed high CO₂RR performance and CO selectivity. In addition, CeO₂/g-C₃N₄ and SnFe₂O₄/g-C₃N₄ Z-schemes showed high CO₂RR activity and CO selectivity [104,105].

Chai et al. described the preparation of two types of heterojunctions: Ag/AgCl/pCN type I and Ag/AgBr/pCN type II [106]. These heterojunctions were found to exhibit high selectivity for the reduction of CO₂ to CH₄ under normal temperature and pressure conditions. The improved photocatalytic CO₂ reduction activity was attributed to the surface plasmon resonance (SPR) effect generated by Ag, as well as the alignment of electronic band potentials between pCN (polymerized carbon nitride) and AgBr/AgCl. This alignment facilitated charge transfer and inhibited recombination processes.

The research by Zhang et al. on the g-C₃N₄/Bi₄NbO₈Cl type II heterojunction demonstrated the intimate interface between g-C₃N₄ and Bi₄NbO₈Cl, which facilitated the transfer of charge carriers and exhibited high CO selectivity during CO₂ reduction reactions [107]. On the other hand, Li et al. and their colleagues synthesized a core–shell LaPO₄/g-C₃N₄ type II heterojunction using a hydrothermal method [108]. The well-matched band structures of LaPO₄ and g-C₃N₄ in this nanocomposite extended the light absorption range and enhanced the separation and transfer of photogenerated charge carriers. The g-C₃N₄ shell not only aids in carrier separation through the type II heterojunction but also protects the LaPO₄ core, resulting in significantly improved CO₂ reduction activity for producing CO.

Tao et al. developed a novel p–n heterojunction composed of g-C₃N₄ nanowires and NiO nanosheets [109]. Their research confirmed the presence of a p–n junction between NiO and g-C₃N₄, which effectively reduces the rapid recombination of electrons and holes, providing ample electrons for the conversion of CO₂ to CH₄. Guo et al. synthesized oxygen vacancy-rich TiO₂ quantum dots confined within g-C₃N₄ nanosheets (TiO₂₋x/g-C₃N₄) 0D/2D heterostructure using an in situ pyrolysis method [110]. Charge dynamics analysis revealed an ultrafast sub-picosecond timescale at the 2D-g-C₃N₄ and 0D-TiO₂ heterojunction interface. This ultrafast charge transfer facilitated the trapping of electrons in shallow sites by CO₂, resulting in superior CO evolution rates and selectivity. Additionally, Li et al. created a 2D/2D Co₂P@BP/g-C₃N₄ heterojunction. This catalyst exhibited a high CO₂ reduction rate and CO selectivity, attributed to lower Gibbs free energies of COOH, CO, and *⁺CO intermediates, combined with rapid charge transfer at the interfaces of Co₂P/BP and BP/g-C₃N₄ [111]. These characteristics promote the generation of CO through a two-electron reaction in the CO₂ reduction process.

2.2. CO₂ Adsorption and Intermediate Desorption

The adsorption of CO₂ is indeed a crucial initial step in the CO₂RR process. The challenges associated with CO₂ adsorption include the high stability of the CO₂ molecule and the energy required to break the C=O bond (ΔG = ~394.39 kJ mol⁻¹). Additionally, the one-step photocatalytic reduction of CO₂ to CO has a high redox potential (E = −1.85 V vs. NHE), making it a complex process. First-principles calculations can provide insights into the adsorption configuration and strength of CO₂ on surfaces like g-C₃N₄ [112]. The protonation of CO₂ can lead to the formation of different intermediates like OCHO or COOH, which can further influence the final product selectivity. Controlling the adsorption/desorption strength of CO₂ and its intermediates on the catalyst surface is crucial for determining the product selectivity in CO₂RR. The total amounts of CH₄ and CO generated by the CND/pCN-3 photocatalyst after 10 h of visible-light activity were found to be 29.23 and 58.82 μmol·g_catalyst⁻¹, respectively. These values were 3.6 and 2.28 times higher, respectively, than the amounts generated when using pCN alone [113]. Strategies such as designing active sites, creating intimate interfacial contacts, and incorporating heterojunctions and co-catalysts on g-C₃N₄ can help enhance the efficiency and selectivity of the CO₂ reduction process.

2.2.1. Heteroatom Doping

Introducing heteroatoms into g-C₃N₄ can indeed lead to significant changes in its properties. By incorporating active impurities, the band structure, electrical conductivity, optical properties, and structural characteristics of g-C₃N₄ can be modified. These changes are often advantageous for enhancing the surface properties of the material, improving its photocatalytic quantum efficiency, and altering its electronic properties. Furthermore, the introduction of heteroatoms can also impact the adsorption/desorption capacity of CO₂ and the photocatalytic reduction of intermediates. This can potentially enhance the overall performance of g-C₃N₄ in applications related to environmental remediation and renewable energy technologies.

The research conducted by Xi et al. demonstrates the synthesis of Cl-doped CN through a multi-step calcination process, resulting in Cl-CN with remarkable CO₂RR activity, particularly in producing CO with high selectivity (39.89 μmol g⁻¹) [114]. The Cl-CN exhibited superior CO₂ adsorption capabilities compared to pure g-C₃N₄, and these findings were further supported by DFT results. In addition, in situ FTIR spectroscopy revealed that COOH* intermediates were strongly adsorbed on the catalyst surface and easily converted to CO. Furthermore, Chen et al. synthesized P-doped g g-C₃N₄ (P-g-C₃N₄) by calcining a mixture of NaH₂PO₂·H₂O, potassium fluoride, and melamine, leading to the formation of P-g-C₃N₄ nanotubes with an increased BET surface area and an amino-rich surface [115]. The phosphorus doping induced a downshift in the CB and VB positions, narrowing the band gap of g-C₃N₄. The CO₂ adsorption capacity of P-g-C₃N₄ was enhanced by 3.14 times, resulting in a significant increase in the production of CO and CH₄ through CO₂RR, by 3.10 and 13.92 times, respectively, compared to g-C₃N₄. The total CO/CH₄ evolution ratio decreased from 6.02 to 1.30, indicating a higher CH₄ selectivity for P-g-C₃N₄. This improvement was attributed to the unique nanotube structure and amino-rich surface of P-g-C₃N₄, which led to a lower ζ potential and enhanced acid–base interactions beneficial for CO₂ adsorption.

Zhou et al. focused on the impact of Cu and Mo doping on the g-C₃N₄ (001) surface in the context of the CO₂RR mechanism using DFT simulations [116]. The study revealed that co-doping Cu and Mo altered the electronic structure and band gap of g-C₃N₄, leading to reduced energy barriers for CO₂RR. The computational modeling of Cu/Mo doping positions and CO₂ adsorption sites indicated that Cu/g-C₃N₄ (001) favored direct CO₂ dissociation to CO, while Mo/g-C₃N₄ (001) favored the formation of cis-COOH as the primary product. Based on the analysis of activation barriers and reaction route selectivity, Mo-doped g-C₃N₄ (001) was identified as a promising catalyst for CO₂ conversion. Cao et al. introduced K and O atoms into the molecular structure of g-C₃N₄ through thermal polymerization using K₂S₂O₈ and urea [117]. The heteroatom doping of K and O reduced the band gap of g-C₃N₄, enhancing its visible-light absorption capabilities. The presence of a higher concentration of COO* intermediate products, as detected by in situ FT-IR, facilitated subsequent reactions with electrons and protons, leading to a higher CO yield and selectivity in the photocatalytic CO₂ photoreduction process. Baeg et al. developed a novel Se-doped g-C₃N₄ photocatalyst using a one-pot two-step synthesis approach [118]. This Se-doped photocatalyst exhibited a high production rate of HCOOH of 1.001 × 10⁵ μmol g⁻¹ h⁻¹. The exceptional CO₂RR activity of this catalyst was attributed to the presence of Se-doped conjugated systems and the porous nanosheet morphology, which enhanced CO₂ adsorption capacity.

2.2.2. Metal Co-Catalysts

The utilization of metal-based co-catalysts in photocatalytic CO₂RR has been a common practice to enable selective CO₂ reduction [119,120]. Despite this, there is a significant disparity in the types of products obtained even when using the same co-catalyst/g-C₃N₄ system. This inconsistency underscores the necessity for a thorough investigation into the reaction mechanisms and product distribution to better understand the process. Therefore, a detailed summary and exploration of how different metal co-catalysts such as Cu, Au, Pt, and others can modify g-C₃N₄ to enhance the selectivity of CO₂ reduction are essential. By delving into the specific effects of various metal co-catalysts on the selectivity of CO₂ reduction, researchers can gain insights into optimizing the photocatalytic CO₂ reduction process for improved efficiency and selectivity. The study by Chu and colleagues investigated the use of Cu and Au single atoms loaded on CN for the selective CO₂RR [121]. The researchers conducted extensive theoretical calculations to understand the reaction kinetics and adsorption/desorption thermodynamics of Au/CN and Cu/CN in relation to CO₂RR selectivity. In the case of Au/CN, the reaction kinetics revealed that the intermediate COOH tended to be reduced to CO during CO₂RR. The similar values of the desorption energy (Edes) for CO (0.10 eV) and the activation energy (Ea) for CO reduction (0.11 eV) indicated that desorbed CO could undergo further reduction to CO along the 2e⁻ pathway. Subsequently, adsorbed CO was further reduced to CH₃OH along the 6e⁻ pathway. The negative Edes (−0.32 eV) of CH₃OH suggested that it would desorb from Au/CN rather than undergo additional reduction, resulting in the co-generation of CO and CH₃OH with poor selectivity for CO₂RR on Au/CN. On the other hand, for Cu/CN, the COOH intermediate favored the generation of HCOOH over CO, impeding the 2e⁻ pathway for CO formation. As the reaction progressed, intermediates such as CH₃OH were formed, and the high E_des of CH₃OH* on Cu/CN (0.41 eV) enabled its reduction to CH₄ as the final product along the 8e⁻ pathway. This led to the selectivity of CO₂RR towards CH₄ formation on Cu/CN. Moreover, the researchers used ab initio molecular dynamics to simulate the adsorption/desorption thermodynamics of final products on Au/CN and Cu/CN. The results indicated the irreversible desorption of CO from the Au/CN surface, and the collapse of CH₃OH@Au/CN, leaving dangling CH₃OH molecules to move around the lattice boundary. As a result, CO and CH₃OH were identified as the final products over Au/CN in CO₂RR. Conversely, the intensive adsorption of CH₃OH on Cu/CN led to the formation of CH₄ as the final product during the CO₂RR process on Cu/CN. Overall, the study provided valuable insights into the mechanisms and selectivity of CO₂RR on single-atom catalysts supported on CN, highlighting the distinct pathways and final product distributions on Au/CN and Cu/CN surfaces.

The research conducted by Xiang et al. on the preparation of a crystalline g-C₃N₄ supported Cu single atoms (Cu-CCN) photocatalyst using molten salts and reflux method is fascinating [122]. The introduction of Cu single atoms enhances the electron exchange between the d orbital of Cu ions and the CO₂ anti-bond (C=O) orbital, thereby facilitating CO₂ adsorption and mass transport. The in situ FTIR analysis demonstrated the sequential conversion of CO₂ into HCO₃, HCOO, and CO, ultimately leading to CO reduction. Additionally, the DFT calculations revealed that the reduction of CO₂ to CH₄ on Cu-CCN samples is an entropy-increasing process, while the reduction to CO is an entropy-decreasing process. Consequently, Cu-CCN exhibits nearly 100% selectivity in the photocatalytic conversion of CO₂ to CO. The proposed photocatalytic mechanisms and reaction pathway of Cu-CCN in CO₂RR are depicted in Figure 6a. This study sheds light on the potential of Cu-CCN as an efficient catalyst for the selective conversion of CO₂ into CO.

The research by Xue et al. demonstrated the successful synthesis of a Ru and Cu co-decorated PCN (PCN-RuCu) using a preassembly–coprecipitation–pyrolysis process [123]. The PCN-RuCu catalyst exhibited superior selectivity (95%) for CH₄ production compared to individual Ru- or Cu-decorated PCN. The study highlighted that the presence of Ru-N₄ and Cu-N₃ sites in the catalyst effectively modulates the electronic structure of PCN, with Ru sites facilitating photogenerated electron–hole pairs and Cu sites promoting CO₂ hydrogenation. The synergistic interaction between Ru and Cu single atoms significantly enhanced the consecutive hydrogenation processes of *CO species, leading to increased CH₄ production. On the other hand, Yang et al. reported on the utilization of g-C₃N₄ as a scaffold with embedded half-metallic C(CN)₃ as co-catalysts for CO₂RR [124]; the structure diagram is shown in Figure 6b. The unique 2D π-conjugated hybrid structure formed through covalent bonding between g-C₃N₄ and C(CN)₃ facilitated obstacle-free electron transfer from g-C₃N₄ to C(CN)₃, driven by intrinsic forces. DFT calculations and in situ FTIR spectra analysis revealed that C(CN)₃ exhibited exceptional capture and chemical activation capacity towards CO₂, resulting in high CO yield (16.5 μmol g⁻¹ h⁻¹) and selectivity (>98%). Furthermore, Bai et al. loaded PtCu alloy nanocubes with (730) facets onto C₃N₄ nanosheets, which enhanced the photocatalytic CO₂RR activity and selectivity towards CH₄. Experimental characterization coupled with DFT calculations indicated that the (730) high-index facet of PtCu alloy nanocubes provided more low-coordinated metal active sites [125], enhancing CO₂ adsorption and activation. The synergistic effects between Cu and Pt in the catalyst promoted high selectivity towards CH₄ production.

Yu’s group successfully anchored cubic and tetrahedral Pd co-catalysts with exposed (100) and (111) facets onto a g-C₃N₄ surface. This resulted in the formation of two catalysts: Pd nanocubes/g-C₃N₄ (C-CN) and Pd nanotetrahedrons/g-C₃N₄ (T-CN) [126]. Interestingly, the T-CN catalyst showed higher CH₃OH production compared to the C-CN catalyst. The in situ FTIR results revealed that intermediate products such as HCOOH, HCHO, CH₄, and CH₃OH are involved in the multi-step CO₂ reduction process. Calculated adsorption energies and deformation energies indicated that CO₂ can be more strongly adsorbed and activated at the Pd (111) surface than the Pd (100) surface. Additionally, DFT results demonstrated that the Pd (111) surface is more favorable for the desorption of CH₃OH compared to the Pd (100) surface, despite both surfaces having similar probabilities for CH₄ desorption. This suggests that the Pd (111) surface acts as an electron sink, capturing photoexcited electrons from the conduction band of g-C₃N₄. This metal–semiconductor interaction enhances CO₂ adsorption and CH₃OH desorption, leading to more efficient CO₂ photoreduction compared to the Pd (100) surface.

Figure 6. Schematic of the tentative photocatalytic mechanisms and reaction pathway of Cu-CCN [122] (a). Copyright 2020, Wiley-VCH. Structural diagram of g-C₃N₄@hm-C(CN)₃ [124] (b). Copyright 2019, Elsevier.

Figure 6. Schematic of the tentative photocatalytic mechanisms and reaction pathway of Cu-CCN [122] (a). Copyright 2020, Wiley-VCH. Structural diagram of g-C₃N₄@hm-C(CN)₃ [124] (b). Copyright 2019, Elsevier.

2.2.3. Single Atom

Single-atom catalysis is indeed a fascinating area of research due to its high efficiency, robustness, and selectivity in various catalytic processes. Anchoring active metals like Co, Cu, Pt, and Au as single atoms on surfaces such as g-C₃N₄ has shown promise in creating efficient photocatalysts for CO₂RR. Studies have shown that the single-atomic metal centers, coordinated by multiple N or C atoms from the g-C₃N₄ framework, serve as the active sites for catalyzing the CO₂RR. This approach holds great potential for developing sustainable and efficient catalytic systems for converting CO₂ into valuable products.

Liu et al. synthesized atomically dispersed single-atom Co embedded into g-C₃N₄ (Co-CN) using thermal-polymerization of urea and cobalt phthalocyanine [127]. Theoretical calculations predicted that the single-atom Co sites have enhanced CO₂ adsorption capacity and lower activation barriers for CO₂ hydrogenation. Experimental results confirmed the theoretical model Figure 7a,b, with the optimal 1% Co-CN catalyst exhibiting high CO selectivity and a yield of 94.9 μmol g⁻¹h⁻¹. The strong interaction between Co 3d and C 2p electrons activated the C=O bonds of CO₂. In addition, Xiong et al. prepared the Pt-decorated defective CN (Pt-CN) [128]. As revealed by experimental characterization and DFT simulations, the synthesis of Pt atoms creates defects in CN along with the formation of -OH proximal to the coordinated Pt atoms, and the O-H was confirmed by FTIR. Thanks to the O-H, a large amount of CO₂ adsorbed on the catalyst surface, as well as single Pt atoms, could localize the electrons to activate the adsorbed CO₂, while desorption of *CO on Pt@CN surface is a highly endergonic process. This well-explained introduction of Pt single atoms gave the product of CH₄ high selectivity (99%). After that, Chen and his team successfully synthesized a Cu single-atom modified g-C₃N₄ photocatalyst by supramolecular preorganization and subsequent condensation method [129]. The C-Cu-N₂ multi-center bond activated CO₂ and reduced the energy barrier for CO₂ reduction; Figure 7c shows the optimal reaction path on intermediate product generation during CO₂RR. Isotopic labeling experiments demonstrated that the reduction product originated from CO₂ reduction rather than the CN matrix (Figure 7d). These studies highlight the importance of single-atom catalysts in promoting efficient and selective CO₂ conversion processes through unique interactions and active sites on the catalyst surfaces.

2.2.4. Morphological Adjustment

The manipulation of the morphology and microstructure of g-C₃N₄, such as through the use of quantum dots, nanotubes, nanosheets, and 3D structures, has been shown to have a significant impact on the surface reactive sites, CO₂ adsorption, light absorption, mass diffusion of intermediates, and the selectivity of CO₂ reduction products [130]. By altering the morphology and microstructure, it is possible to increase the specific surface area, expose more reactive sites, enhance CO₂ and light adsorption, and, ultimately, regulate the selectivity of the products formed during the photocatalytic reduction of CO₂. This approach shows promise in improving the efficiency and selectivity of CO₂ reduction processes using g-C₃N₄.

The strategy proposed by Fang and co-workers for preparing highly porous g-C₃N₄ microtubes (MCNMs) involves a double-solvent-induced self-assembly process to create well-packed nanoleaf-like frameworks [131]. These special MCNM structures exhibit impressive surface adsorption capabilities for CO₂ and are mechanically stable. The 1D microtubes with mesoporous interconnected microchannels enable a large amount of CO₂ to be trapped within the hierarchical structure, enhancing the adsorption rate. Furthermore, in situ FTIR results provide favorable evidence of CO₂ adsorption and activation on the surface of MCNMs. The results confirm that the electrons from MCNMs play a role in facilitating CO₂ activation and selective conversion to CO.

Huo et al. proposed an effective high-temperature exfoliation strategy to prepare different thicknesses of 2D g-C₃N₄ photocatalysts [132]. Characterization results indicate that the CB of g-C₃N₄ shifts to a more negative position with decreasing thickness. This shift results in a more negative CB, which gives g-C₃N₄ a higher reduction potential that favors CO₂RR activity. Furthermore, thinner g-C₃N₄ sheets expose more edge amino groups, which are beneficial for CO₂ adsorption and aid in charge carrier migration. As the thickness of g-C₃N₄ is reduced, the selectivity for CO during CO₂RR is improved.

Xu et al. reported the synthesis of porous nitrogen-rich g-C₃N₄ nanotubes (TCN-NH₃) using a supramolecular self-assembly method [133]. The TEM images depict the TCN-NH₃ nanotubes. This catalyst demonstrates a remarkable visible-light-induced CO₂-to-CO conversion rate of 103.6 μmol g⁻¹ h⁻¹. The presence of amino groups on the surface of g-C₃N₄ enhances the adsorption of CO₂ due to the strong Lewis basicity of these amino groups. DFT calculations reveal that the energy for CO₂ adsorption on TCN-NH₃ (−0.358 eV) is higher than that on TCN without -NH₃ (−0.298 eV). Additionally, the CO desorption energy on TCN-NH₃ is significantly lower than on TCN, indicating that the amino groups facilitate CO₂ adsorption and promote CO desorption. The weaker CO desorption energy on TCN-NH₃ can be attributed to the N-H…C interaction in the C-terminated adsorption being weaker than the N-H…O interaction. This leads to increased CO₂RR activity and enhanced CO selectivity.

Chen et al. developed a novel approach to design Ag-nanoparticles-decorated 3D ordered CN using a synergistic route involving Ag-induced supramolecular tailoring and assembling followed by thermal polymerization [134]. Experimental and density functional theory (DFT) calculations showed that the electron-rich surface of Ag/CN reduces the activation energy barrier by providing sufficient electrons for enhanced CO₂ adsorption. This setup also facilitates efficient charge separation and a more localized charge density distribution, leading to a decreased energy barrier for the COOH intermediate and enhanced CO desorption. As a result, the CO generation rate is significantly improved to 145.5 μmol g⁻¹h⁻¹ with a CO selectivity of 89%. This strategy sheds light on the multi-scale modulation of g-C₃N₄ photocatalysts for enhanced CO₂RR selectivity. On the other hand, An et al. employed a one-step calcination method to obtain 2D g-C₃N₄ with surface defects (NS-g-C₃N₄) [135]. The NS-g-C₃N₄ exhibits an ultrathin nanosheet structure of 10 nm, as shown in the TEM images. This material demonstrates enhanced light adsorption, charge carrier separation, and CO₂ chemisorption. In contrast to the system with B-g-C₃N₄, where CO₂ is converted into CO, CH₃OH, and CH₄, NS-g-C₃N₄ selectively reduces CO₂ to CO only. The mechanism indicates that the CO₂ molecule adsorption mode for NS-g-C₃N₄ is N-O-C=O rather than N-CO₂⁻, as seen in B-g-C₃N₄. This adsorption mode ultimately contributes to the selective photoreduction of CO₂ to CO. This study not only presents a novel strategy for achieving high selectivity and efficiency in the photocatalytic conversion of CO₂ to CO but also aims to elucidate the interactions between the adsorption model of CO₂ on g-C₃N₄ and the selectivity and efficiency of CO₂ photoreduction.

2.2.5. Fabricating Heterostructures

Hence, we reviewed the Z-scheme heterojunction, VDW heterojunction, S-scheme heterojunction, and p–n heterojunction based on g-C₃N₄ for CO₂RR, and discussed CO₂ and intermediate product adsorption/desorption capacity at different heterojunctions to the effect of photocatalytic CO₂RR selectivity.

The study by Xu et al. involved the fabrication of a van der Waals (VDW) heterojunction between atomically thin CN and Bi₉O_7.5S₆ (BOS) nanoplates, denoted as BOS/CN [136]. Through various analyses such as in situ FTIR data and DFT calculations, the researchers determined the rate-determining steps for the photocatalytic reduction of CO₂ over CN and BOS/CN. In the case of CN, the rate-determining step was identified as the reaction between the OCOH intermediate and H, with an energy barrier of 1.47 eV. Conversely, for BOS/CN, the formation of the COOH intermediate on BOS was found to be the rate-determining step, characterized by an energy barrier of 0.89 eV. Additionally, the study revealed that the desorption of the formed CO species from CN had a relatively low energy barrier of 2.35 eV, facilitating the formation of CHO* and subsequently leading to the production of CH₄ and CH₃OH instead of CO. The VDW heterojunction between CN and covalent organic frameworks (COFs) prepared using a sonication-assisted solvent-evaporation-induced self-assembly method, as reported by Yu et al., exhibited high CO selectivity during CO₂RR [137]. The enhanced CO selectivity was attributed to the strong interactions between CO₂ and CN/COF, which increased CO₂ adsorption and facilitated the transfer and separation of photogenerated carriers. The affinity of COF precursors for CO₂ and the favorable π–π interactions between COF and CN were highlighted as key factors contributing to the improved charge transfer and migration processes in this VDW heterojunction system.

The study by Jiang et al. focused on the development of an urchin-like hierarchical α-Fe₂O₃ modified g-C₃N₄ direct Z-scheme heterojunction [138]. The α-Fe₂O₃/g-C₃N₄ heterojunction exhibited increased CO₂ adsorption capacity and stronger adsorption binding energy (−3.072 eV) compared to g-C₃N₄ alone. This enhancement was attributed to the diffusion of CO₂ within the heterojunction, leading to improved utilization of basic sites for CO₂ capture. Additionally, the desorption of CO from g-C₃N₄ (Eads = −0.056 eV) was found to be easier than that from α-Fe₂O₃/g-C₃N₄, indicating improved CO desorption properties in the heterojunction system. As a result of these characteristics, the α-Fe₂O₃/g-C₃N₄ heterojunction demonstrated enhanced photocatalytic CO₂ reduction activity without the need for additional co-catalysts or sacrificial reagents, achieving a CO evolution rate of 27.2 µmol g⁻¹ h⁻¹. In a separate study, Guo et al. fabricated a Au@g-C₃N₄/SnS Z-scheme heterojunction using a template-assisted strategy [139]. This heterojunction exhibited high selectivity for CO generation over CH₃OH and CH₄ during CO₂ reduction. The presence of amine groups on the g-C₃N₄ surface enhanced CO₂ adsorption capacity, while the yolk–shell structure facilitated light trapping within the system, increasing the availability of light and contributing significantly to the high CO generation observed.

2.2.6. Metal–Organic Frameworks/g-C₃N₄

It has been well documented that the adsorption of CO₂ onto the photocatalyst surface was the first essential step affecting the reaction pathway and dynamics of CO₂RR [140]. The CO₂RR reduction activity and selectivity can be dramatically increased by increasing local CO₂ concentration near the catalytic active sites of g-C₃N₄. Metal–organic frameworks (MOFs), constructed by tunable metal nodes and various organic ligands, have been well demonstrated as a promising material for CO₂ storage and separation [141]; the combination of g-C₃N₄ and MOF can enhance CO₂ adsorption, boosting charge transfer and improving reduction products’ selectivity due to the abundant pore structures [142,143,144].

The study by Liu et al. demonstrated the successful synthesis of a tubular g-C₃N₄ modified with a zeolitic imidazolate framework-8 (TCN/ZIF-8) composite [145]. This hybrid structure inherited broadened optical absorption properties, enhanced photocatalytic redox ability, and excellent CO₂ adsorption capacity due to the incorporation of ZIF-8 nanoclusters (Figure 8a). The TCN/ZIF-8 composite showed significant improvement in CH₃OH production efficiency and selectivity for the CO₂ reduction reaction, achieving a rate of 0.75 mol h⁻¹ g⁻¹ (Figure 8b).

Ye et al. conducted a study where they modified g-C₃N₄ with boron imidazolate framework-20 (BIF-20) using an electrostatic self-assembly approach [146]. The research involved DFT simulations and energy transfer investigations, which revealed that the B-H bonding in BIF-20 can trap electrons, enhance charge carrier transmission, and activate adsorbed CO₂. This activation led to significantly increased CO productivity compared to g-C₃N₄ nanosheets and ZIF-8@g-C₃N₄ (Figure 8c). The high concentration of localized electrons near the surface and the B-H bonding sites were attributed to the activation of CO₂ by trapping O atoms (Figure 8d). The spatial distribution of charge density illustrated that adding a one-electron charge could modify the charge density of B substantially, leading to effective CO₂ reduction. The B-H bonding in BIF-20 was easily triggered for CO₂ reduction upon receiving photoexcited electrons from g-C₃N₄. Additionally, the CO₂ capture ability of BIF-20 was enhanced by receiving electrons, as evidenced by DFT simulations showing increased adsorption energy for CO₂ and changes in adsorption bonding with electron charging. These findings highlighted the crucial role of B-H bonding in activating CO₂ and facilitating electron–hole separation, ultimately achieving high CO photoreduction performance. Zhong et al. described the synthesis of NH₂-UiO-66 anchored on a porous g-C₃N₄ photocatalyst using a combination of calcination and hydrothermal methods [147]. Additionally, Sun and colleagues prepared NH₂-MIL-101(Fe)/g-C₃N₄ [148]. In both studies, the CO₂-to-CO yield saw a significant increase compared to the pristine MOF and g-C₃N₄. This enhanced activity and CO selectivity were primarily attributed to effective interface charge transfer and CO₂ adsorption capacity.

2.3. Proton Regulation to Control the Competitiveness of HER

The hydrogen required for the CO₂RR typically comes from water, leading to competition with the competing HER from direct water reduction [149,150,151]. This competition often results in low selectivity and activity for CO₂RR in many current systems. To address this challenge, recent research efforts have focused on suppressing the HER and enhancing the selectivity of CO₂RR. Some strategies include introducing a hydrophobic layer on the CN surface to enable a high concentration of CO₂ to reach the catalyst surface directly, thereby overcoming the mass-transfer limitations of CO₂. Another approach involves immobilizing g-C₃N₄ onto a hydrophobic substrate to enhance CO₂ solubility and mass transport in aqueous solutions. Additionally, the simultaneous incorporation of H⁺ ions into the lattice of metal co-catalysts can help prevent water splitting to produce H₂, thereby promoting the CO₂RR selectivity.

2.3.1. Hydrophobic Surface Modification

Gong and colleagues synthesized the hydrophobic PCN (o-PCN) [152]. Compared to bulk PCN and Pt/PCN, the o-PCN exhibited significant hydrophobicity, as evidenced by a contact angle of 120°. Upon loading Pt onto the o-PCN surface, the hydrophobicity slightly decreased. The presence of Pt on the hydrophobic surface enhanced the catalytic activity towards carbon derivative generation over H₂, leading to a significant improvement in CO₂RR competitiveness, with a selectivity of 87.9%, approximately 34 times higher than Pt/PCN. The hydrophobic patches were observed to repel water and promote increased gas adsorption on the surface, creating three-phase contact interfaces with highly concentrated CO₂. This coating of carboxylates on the negatively charged Pt particles resulted in lower H⁺ surface concentration, suppressing the HER, overcoming CO₂ mass-transfer limitations, and increasing electron availability for CO₂RR, thereby enhancing CO₂RR product selectivity. Additionally, Huang et al. synthesized amine-functionalized g-C₃N₄ by heating urea and treating it with a monoethanolamine solution [153]. The introduction of amines altered the surface hydrophobic properties of g-C₃N₄, leading to higher CO₂ adsorption capacity and improved photocatalytic CO₂RR selectivity. Liu and colleagues successfully synthesized hydroxyl-grafted (OH) oxygen-linked g-C₃N₄ (HGONTP) by polycondensation of hydrothermally pretreated dicyandiamide [154]. The content of oxygen-containing functional groups (C-O-C) and OH can be adjusted by controlling the hydrothermal temperature, resulting in varying degrees of hydrophilicity for HGONTP (Figure 9). This photocatalyst demonstrates excellent light absorption ranging from UV to near-IR, with a narrow band gap of 2.18 eV, different surface hydrophilicities, and a large specific surface area. The CO₂RR efficiency for producing CO reaches up to 3.3 μmol g⁻¹ h⁻¹ without the need for sacrificial agents or co-catalysts, and the selectivity of CO₂ to CO reaches 88.4%.

2.3.2. Hydrophobic Substrate Modification

Cao and co-workers utilized a straightforward approach involving ultraviolet irradiation and calcination to immobilize CN onto a hydrophobic carbon fiber (CF) substrate [155]. As depicted in Figure 10a–c, the contact angle of the hydrophobic CF substrate is approximately 112°. Figure 10d,e show FESEM images of CN coating on the surface of the hydrophobic CF at low and high magnifications. During the CO₂RR process with CN/CF, the concentration of CO₂ at the interface can be regulated by adjusting the surface adsorption on the CF substrate. The hydrophobic nature of the substrate facilitates the localization of CO₂ from the gas phase and aids in the rapid delivery of CO₂ to the contact area of CO₂, water, and CN. This reaction system enables the continuous transfer of CO₂ from the gas phase to the reaction interface through its hydrophobic channels, rather than relying on slow diffusion through the liquid phase. Consequently, the accessibility of CO₂ to the photocatalyst is significantly enhanced. The behavior of charge carriers and CO₂ supply for the hydrophobic substrate-immobilized system is illustrated in Figure 10f. This study demonstrates a notable improvement in the activity and selectivity of photocatalytic CO₂ conversion, with a CO₂ conversion selectivity as high as 97.7%, indicating that almost all electrons end up in carbon products.

2.3.3. Metal and Organic Co-Catalysts

Bai and collaborators described the simultaneous incorporation of Cu and H atoms into Pd lattice co-catalysts to modify C₃N₄ [156]. The g-C₃N₄-Pd₉Cu₁Hx photocatalyst achieved an average CH₄ evolution rate of 0.018 μmol h⁻¹ with 100% selectivity for the production rate of C₃N₄-based photocatalysts and their selectivity in CH₄ production. The photocatalytic mechanism for CO₂RR suggests that isolated Cu atoms in the Pd lattice serve as active sites in the initial reduction of CO₂ to CO, while H atoms play a role in the rate-limiting step of further reducing CO to CH₄, thereby preventing the reduction of H₂O to H₂. Additionally, Chao and colleagues reported the use of organic terpyridine (Tpy) as a co-catalyst to modify mesoporous g-C₃N₄ (mpg-C₃N₄) for CO₂RR [157]. As depicted in Figure 11a–d, efficient electron transfer from the light-absorbing photosensitizer to Tpy was facilitated by mpg-C₃N₄, and the synergistic effect of mpg-C₃N₄ and Tpy suppressed H₂ evolution, thereby enhancing CO₂RR. This synergistic effect led to a selectivity for CO generation of nearly 99.8% for Tpyx@mpg-C₃N₄.

3. Conclusions and Outlook

g-C₃N₄ has demonstrated exceptional potential as a photocatalyst for CO₂ reduction, attributed to its advantageous band structures, chemical and thermal stability, and cost-effectiveness. This review highlights the strategic advancements in g-C₃N₄-based photocatalysts aimed at enhancing selective CO₂RR. These strategies focus on improving the separation of photogenerated carriers, increasing the absorption capacity for CO₂ and its intermediates, and minimizing the competitive HER during water reduction. Achieving these improvements is crucial for the sustainable and selective conversion of CO₂ into valuable feedstock materials. A key challenge in optimizing g-C₃N₄ lies in balancing charge transfer efficiency and CO₂ adsorption capacity. While increased crystallinity can enhance charge transfer, it may concurrently reduce CO₂ adsorption, necessitating a delicate equilibrium between these properties. Although noble-metal-decorated g-C₃N₄ composites exhibit superior photocatalytic activity, their practical application is limited by the scarcity and high cost of noble metals. Therefore, the development of non-noble metal/g-C₃N₄ composites is strongly encouraged. Recent research has focused on synthesizing heterojunction composite materials based on g-C₃N₄, yet detailed mechanistic studies on performance enhancements remain insufficient.

Despite notable advancements in g-C₃N₄ and its composites for photocatalytic CO₂ reduction, several challenges hinder their practical application, as follows. Photocatalytic activity: Current g-C₃N₄-based photocatalysts often exhibit insufficient efficiency, failing to meet practical application standards. Product selectivity: The complexity of multi-level reactions on g-C₃N₄ complicates product selectivity. While theoretical models offer insights, experimental validation is crucial. Operational stability: Catalyst stability is essential for industrial applications, but research on the long-term stability of g-C₃N₄ photocatalysts lags behind efforts to improve efficiency. Mechanistic understanding: There is a lack of comprehensive research on the atomic-level mechanisms governing photocatalyst performance. Integrating theoretical and experimental approaches can enhance our understanding and practical application of g-C₃N₄ -based photocatalysts. Addressing these challenges requires focused efforts to develop g-C₃N₄-based photocatalysts that achieve high activity, selectivity, and stability. Striking a balance among these critical properties is essential to advancing their applicability in real-world scenarios.