1. Introduction
Graphitic carbon nitride (g-C
3N
4) exhibits great application potential in energy conversion and environmental remediation due to its favorable visible-light response, suitable band structure, high chemical stability, low cost and facile synthesis.[
[1-9]] Nevertheless, the deployment of g-C
3N
4 are severely restricted by fast recombination of photogenerated electron-hole pairs,
[10,11] insufficient visible-light absorption capacity,
[12-16] and limited number of surface active sites.
[17-21] To overcome these drawbacks, many modification strategies have been developed, among which biomass engineering emerges as a green, sustainable and highly efficient approach. Biomass, primarily composed of cellulose, hemicellulose and lignin, possesses an inherently hierarchical porous architecture and a rich surface chemistry featuring hydroxyl, aldehyde and carboxyl moieties.
[22] It serves not only as a potential clean energy carrier but also as an ideal precursor for catalytic materials and functional additives.
[23] In 2017, Liu et al.
[24] first pioneered the carbon quantum dot/g-C
3N
4 photocatalytic system and demonstrated excellent synergistic catalytic activity between the two components. Building on this seminal work, Zhang and his co-workers
[25] fabricated biomass-derived carbon quantum dots using orange peel, coupled them with g-C
3N
4 composites, for fluoroquinolone antibiotic degradation. Subsequently, our group systematically investigated the hydrogen-bonding interactions between cellulose and g-C
3N
4 and innovatively proposed the concept of biomass proton donors.
[18,26] More recently, advances in biomass-derived carbon engineering have further accelerated the high-value valorization of biomass (
Fig. 1).
[27] Collectively, these foundational studies have established a robust framework for the rational design of high-performance biomass-based photocatalytic systems.
Research advances in biomass-engineered g-C
3N
4 composite photocatalysts have achieved remarkable progress to date.
[2,28] Diversified design strategies have been successively developed ranging from simple physical blending and covalent bonding construction to morphological regulation and heterojunction fabrication as well as monocomponent hybridization and biomass derived carbon nitrogen co-doping modification. These elaborate modification approaches have substantially enhanced the photocatalytic activity and structural stability of g-C
3N
4-based composites toward diverse reaction systems including water splitting for H
2 evolution, H
2O
2 production, CO
2 reduction, and organic pollutant elimination.
[18,22,24 -26] Nevertheless, this research field still faces several critical bottlenecks that restrict its further advancement. The inherent complexity and structural variability of natural biomass inevitably trigger structural collapse and the loss of active sites during composite fabrication. Moreover, the interfacial interaction mechanism between biomass substrates and g-C
3N
4 is still poorly understood, and reliable strategies for precisely regulating interfacial electron transfer are currently lacking. Accordingly, the rational design of high-efficiency biomass-engineered g-C
3N
4 photocatalysts remains to be further optimized and refined.
Based on the above background introduction, this review centers on the core theme of constructing high efficiency photocatalysts through the coupling of graphitic carbon nitride and biomass. It systematically summarises the design principles, fabrication strategies, performance regulation mechanisms, and representative application advances of biomass-engineered g-C3N4. The work further clarifies the fundamental roles of hydrogen bonds engineering, heterojunction engineering, and defect engineering in optimizing light harvesting capacity charge separation behavior and interfacial reaction kinetics of such biomass-engineered materials. It also comprehensively discusses the dominant bottlenecks and key scientific challenges restricting current research progress. Finally, it delivers a forward-looking review on the future development roadmap of biomass-engineered g-C3N4 photocatalysts from multiple dimensions. This review aims to provide in depth theoretical references and practical guidelines for the rational design of innovative photocatalytic materials with prominent activity robust stability and favorable environmental compatibility. It also promotes the in-depth integration of advanced photocatalytic technology and high value biomass utilization to facilitate the sustainable development of energy and environmental remediation systems.
2. Synthesis strategies of g-C3N4
Various synthetic strategies for graphite nitride carbon (g-C3N4) nanosheets could endow the material with distinctive microstructures, electronic properties and surface features. At present, all synthetic pathways for nanostructured g-C3N4 can be generally grouped into two fundamental strategies, top-down and bottom-up strategies. Top-down strategies employ external driving forces to overcome interlayer van der waals forces within bulk g-C3N4, yielding monolayer and few-layer nanosheets. Benefiting from simple operation and well-retained intrinsic crystal frameworks, representative techniques cover liquid exfoliation and thermal exfoliation. Bottom-up strategies depend on the polymerization and self-assembly of molecular precursors to precisely modulate the growth of g-C3N4 nanostructures at the atomic level, enabling fine regulation of their morphology, pore architecture and electronic configuration. The most widely adopted methods include solvothermal synthesis, supramolecular preassembly and templating routes.
2.1 Top-Down Strategies
Top-down synthetic strategies leverage diverse external driving forces to exfoliate and thicken bulk-layered g-C3N4 into small-sized porous carbon nitride (PCN) particles. Rational top-down engineering can disrupt the internal hydrogen bonding and van der Waals stacking interactions of bulk g-C3N4 (BCN), substantially boosting the specific surface area and dispersibility of the material in reaction media. To date, the predominant top-down approaches for converting BCN into PCN include chemical exfoliation, liquid exfoliation and thermal exfoliation. This section systematically elaborates the applications of these methodologies in the application of PCN.
2.1.1 Chemical Exfoliation Technique
Chemical exfoliation is a relatively mature and efficient top-Down Strategies for preparing ultrathin two-dimensional (2D) g-C
3N
4 nanosheets. This technique relies on intercalation of acid/base ions or oxidative species into bulk g‑C
3N
4 interlayers to disrupt interlayer hydrogen bonds and π-π van der Waals stacking, combined with mild etching and surface functionalization to delaminate stacked sheets.
[38] Liang et al.
[39] successfully fabricate bromine-doped carbon nitride (BrCN) nanosheets via a chemical exfoliation strategy, enabling the construction of high-loading and long-cycle-stable zinc-iodine batteries (
Fig. 2a-d). Compared with pristine carbon nitride, the resultant BrCN nanosheets exhibit optimized structural characteristics and substantially enlarged specific surface area. In another related study, Wang et al.
[40] reported carboxylation-acylation chemical exfoliation strategy to synthesize amino-modified CN with adjustable adsorption functional groups. These treatments retained CN crystal structures, exfoliated bulk CN into non-aggregated thin nanosheets, and introduced abundant ethylenediamine-derived -NH
2 sites on 24-CN-EDA. The controllable amino decoration via EDA functionalization greatly enhanced the photocatalytic activity of CN. Additionally, Feng et al.
[38] developed a nitrate intercalation and decomposition (NID) strategy for the chemical exfoliation of bulk carbon nitride (
Fig. 2e-i). This approach integrates dilute nitric acid-mediated hydrothermal pretreatment with temperature-controlled pyrolysis, enabling the controllable exfoliation of bulk g-CN (BCN). The method affords few-layer g-CN with tunable layer thickness and permits the precise fabrication of bilayer g-CN samples. Compared with exfoliation systems employing phosphoric acid or concentrated sulfuric acid, the NID strategy avoids residual impurity contamination. Moreover, dilute nitric acid can in situ introduce oxygen-containing functional groups, which remodels the surface microstructure of g-C
3N
4, further boosting its oxygen adsorption capacity and catalytic kinetic performance (
Fig. 2j-k).
2.1.2 liquid exfoliation Technique
Liquid exfoliation technique has been validated as a pivotal synthetic strategy for fabricating size-tunable g-C
3N
4 nanostructures. When dispersed in appropriate solvents, solvent molecules interact with g-C
3N
4 via intermolecular forces and intercalate uniformly between adjacent layers. In the work reported by Zhou et al.
[41], bulk yellow g-C
3N
4 was first synthesized through conventional thermal polymerization, followed by liquid-phase exfoliation in N-methylpyrrolidone (NMP) to obtain few-layer g-C
3N
4 nanosheets (
Fig. 3a). Alternatively, Wang et al.
[42] prepared layered poly(triazine imide) (PTI) via an ionothermal method, which was further subjected to one-step liquid exfoliation in water to yield 2D carbon nitride nanosheets. The as-obtained triazine-based nanosheets possess a thickness ranging from 1 to 2 nm and can form stable suspensions under both acidic and alkaline environments (
Fig. 3b-d).
2.1.3 Thermal exfoliation approach
Thermal oxidation is an effective route to exfoliate graphite into monolayer graphene or large-area graphene sheets. Owing to the structural resemblance between g-C
3N
4 and graphene, this approach has also been explored for the fabrication of g-C
3N
4 nanosheets. By modulating the microstructure of bulk pristine g-C
3N
4 (BCN) through thermal and ultrasonic exfoliation, Zheng et al.
[43] successfully synthesized porous ultrathin g-C
3N
4 nanosheets (PuCN), which possessed plenty of anchoring sites to facilitate single-atom loading.(
Fig. 4a) Wu et al.
[44] prepared bulk carbon nitride (Bulk-CN) using low-cost dicyandiamide as the precursor, and successfully transformed it into three types of carbon nitride nanosheets (CNNs) via a facile continuous thermal exfoliation process in air (
Fig. 4b). TEM characterizations verified the transformation of bulk polymeric carbon nitride (PCN) into a 2D layered structure (
Fig. 4c). AFM images further provided solid evidence of the successful exfoliation of Bulk-CN into atomically thin nanosheets (
Fig. 4d). XRD pattern analysis revealed that the thermal-induced etching effect reduced the in-plane grain size of the material, while the planarization of wrinkled monolayers during heat treatment alleviated the interlayer stacking of the Bulk-CN structure (
Fig. 4e). Although the conventional thermal exfoliation method can enlarge the specific surface area of g-C
3N
4, it inevitably aggravates the incomplete polymerization of heptazine molecular chains. To address this issue, Jia et al.
[45] developed a thermal repolymerization strategy under nitrogen atmosphere to repair the defective structure of CN and synthesize bulk g-C
3N
4 with highly polymerized heptazine chains (NCN) (
Fig. 4f). Specifically, bulk g-C
3N
4 with incompletely polymerized structures (CN) derived from urea thermal polymerization was first exfoliated into ultrathin porous nanosheets with a high specific surface area (ACN) via thermal shearing in an ammonia atmosphere. Subsequent thermal repolymerization between amino groups further precisely repaired the broken heptazine chains of ACN, ultimately yielding ultrathin porous g-C
3N
4 nanosheets (NACN) that integrate high specific surface area and highly polymerized heptazine chains. AFM characterization verified that the obtained NACN presents an ultrathin porous nanosheet structure with a thickness of 1.5 nm after thermal shearing treatment (
Fig. 4g). In a recent study, Zhang and co-workers synthesized ultrathin g-C
3N
4 nanosheets through a strategy integrating synergistic thermal exfoliation and ultrasonic exfoliation.
[45] Attributed to the ultrathin structural characteristic, the nanosheet substrate enables the firm anchoring of AuPt nanoparticles by electrostatic adsorption coupled with in-situ NaBH
4 reduction, thereby tuning the electronic structure and greatly promoting the photocatalytic performance of the composite catalyst. Collectively, combined with auxiliary ultrasonic treatment or post thermal repolymerization engineering, thermal exfoliation approach not only effectively mitigates severe interlayer stacking of bulk carbon nitride but also provides a scalable platform for loading metal nanoparticles or single atoms, holding great promise for developing high-efficiency carbon nitride-based photocatalytic materials.
2.2 Bottom-Up Strategies
Bottom-up methodologies underpin g-C3N4 synthesis by enabling the direct conversion of nitrogen-containing precursors into polymeric g-C3N4. As documented in previous studies, such synthetic routes preserve the intrinsic backbone of g-C3N4 to the greatest extent, suppress the formation of undesired defect sites, and thereby facilitate the separation of photogenerated charge carriers. To date, the predominant top-down approaches include solvothermal synthesis, supramolecular preassembly and template-mediated approach. This section systematically elaborates the applications of these methodologies in the application of g-C3N4.
2.2.1 Solvothermal synthesis
Solvothermal synthesis g-C
3N
4 typically adopts sealed autoclaves with anhydrous organic solvents such as acetonitrile and toluene under mild high-temperature and high-pressure conditions. Different from the traditional thermal polycondensation via direct calcination, this method can precisely regulate the microstructure, defect distribution and crystallinity of carbon nitride by adjusting precursors, reaction temperature, holding time and solvent types. A previous study reported a supercritical methanol (ScMeOH) post-treatment strategy conducted at 250-300 °C under 8.1-11.8 MPa to modulate the structural properties of pristine g-C
3N
4 derived from the thermal polycondensation of melamine (
Fig. 5a).
[47] During the ScMeOH post-treatment, methanol molecules diffuse into the interlayer spacing of g-C
3N
4 and subsequently trigger intralayer N-methylation and hydroxylation reactions, partially converting pristine g-C
3N
4 into quinonoid poly(heptazine imide)-based carbon nitride (Q-PHI) abundant in methyl and hydroxyl groups (
Fig. 5b). Molecular dynamics simulations verify that the transformation from g-C
3N
4 to Q-PHI via ScMeOH treatment is thermodynamically spontaneous, and high temperature and high pressure enable more methanol molecules to surmount the kinetic energy barrier for interfacial reaction(
Fig. 5c). Solution thermal modification strategy can effectively regulate the optoelectronic and excitonic behaviors of carbon nitride. This treatment can efficiently suppress the Coulomb interaction of singlet Frenkel excitons, narrow the band gap of carbon nitride, and thereby inhibit the exciton dissociation process. In a representative modification study, Lu et al.
[48] employed a solvothermal protocol to simultaneously introduce π-electron-rich domains and polarizable hydroxyl groups into mesoporous carbon nitride, successfully fabricating ultrathin mesoporous carbon nitride nanosheets (
Fig. 5d). This work demonstrates that precise functionalization of carbon nitride with diverse functional moieties can rationally tailor its structural and electronic characteristics, ultimately enabling the substantial enhancement of photocatalytic performance.
2.2.2 Supramolecular Assembly
Supramolecular assembly of carbon nitride usually employs melamine and cyanuric acid as building blocks to fabricate well-ordered co-crystalline precursors via hydrogen bonding and π-π stacking interactions. Subsequent thermal polycondensation transfers the topological features of these precursors into the final graphitic carbon nitride framework. This strategy enables precise modulation of pore architecture, homogeneous heteroatom doping and controllable defect engineering. The resulting materials possess enlarged specific surface areas, optimized band structures and accelerated separation of photogenerated charge carriers, thereby delivering substantially enhanced photocatalytic activity. Quan’s team incorporated uracil into the supramolecular self-assembly system of carbon nitride to precisely regulate carbon doping in the matrix (
Fig. 6a).
[49] By modulating the electronic structure and dipole moment, a locally asymmetric carbon nitride photocatalyst (KCCNV) with cyano-based electron traps was fabricated. The cyano-group-induced localized electron-rich domains efficiently capture photogenerated electrons and steer directional charge migration, thereby effectively suppressing the non-radiative recombination of photogenerated electron-hole pairs. Li et al.
[50] developed a fluid shear force-assisted supramolecular self-assembly strategy to disrupt the stacked layered structure of bulk carbon nitride, yielding carbon nitride clusters assembled from ultrathin nanosheets (
Fig. 6b). Carbonyl groups and nitrogen vacancies were simultaneously introduced into the conjugated framework of the obtained ASCN. This facile, reagent-free and high-yielding strategy enables the synchronous modulation of the band structure, electronic configuration and morphology of g-C
3N
4. Panangattu et al.
[51] achieved molecular-level control over supramolecular complexes self-assembled from cyanuric acid and 3-amino-1,2,4-triazole (AT) monomers, verifying that this strategy can be extended to directly regulate crystal growth toward specific phases (
Fig. 6c).
Figure 6d-f display distinct morphologies of the as-formed complexes prepared at various molar ratios. Such morphological evolution originates from variations in the hydrogen-bonding environment between the precursor monomers CA and AT.
2.2.3 Template-mediated approach
Template-mediated approach of carbon nitride can be categorized into hard-template and soft-template strategies. Utilizing the spatial confinement effect of templates, nitrogen-containing precursors such as urea and melamine are infiltrated into the pore channels or assembled architectures of templates, followed by high-temperature calcination for thermal polymerization. Subsequent template removal via etching or solvent treatment enables the precise construction of carbon nitride with hierarchical porous morphologies including ordered mesopores, one-dimensional tubular structures and nanosheets. This approach effectively addresses the drawbacks of bulk carbon nitride, such as severe material stacking, low specific surface area and rapid recombination of photogenerated charge carriers, thereby greatly enhancing light harvesting capacity as well as the separation and transport efficiency of photogenerated charges. In the hard template method, Sadanandan et al.
[52] developed a thermal polymerization strategy employing 3-amino-1,2,4-triazole as the nitrogen precursor, sulfur powder as the functionalization agent, and silica nanoparticles as the hard template, successfully fabricating modified carbon nitride catalysts with high specific surface area and tailored surface chemical properties (
Fig. 7a-c). Britto and coworkers synthesized copper-incorporated mesoporous carbon nitride (x-Cu-mCN) using mesoporous SBA-15 silica as a hard template (
Fig. 7d).
[53] Low-angle X-ray diffraction and transmission electron microscopy characterizations verified the well-defined periodic hexagonal porous architecture of the as-synthesized materials. In the soft template method, Zhen’s group reported the fabrication of N-doped porous carbon (NPC) with a hierarchical porous structure via high-temperature calcination and chemical activation, where carbon nitride served as both the nitrogen source and structural template, and glucose acted as the carbon precursor (
Fig. 7e).
[54] The systematic investigation revealed the significant dependence of the specific surface area, nitrogen doping configuration, and electrochemical performance of the resultant NPC materials on the mass ratio of carbon nitride to glucose. Optimally, the NPC sample prepared at a carbon nitride-to-glucose mass ratio of 1:2 (NPC-2) delivered a maximum specific surface area of 2407 mg. Furthermore, Sun et al. proposed a moderate- and low-temperature hydrogen thermal treatment strategy for porous carbon nitride, which enabled the directional deposition of in-situ generated methane on the boundary active sites within the confined porous space, thereby constructing atomic-layer graphene patches on porous carbon nitride (
Fig. 7f).
[55] The fabricated intra-layer heterostructures effectively reduce the interfacial transport resistance of photogenerated charge carriers. Coupled with the enhanced interfacial electric field effect between graphene and carbon nitride, this structural and compositional modulation synchronously boosts the photocatalytic oxidation and reduction capabilities of the composite photocatalysts.
Both top-down and bottom-up construction strategies possess distinct intrinsic advantages as well as unavoidable limitations. Accordingly, the optimal synthetic strategy should be selected based on specific application scenarios and available resource conditions. In the field of photocatalysis, g-C3N4 typically suffers from inherent structural defects including insufficient polymerization degree and poor crystallinity. These drawbacks inevitably lead to sparse active sites, rapid recombination of photogenerated electron-hole pairs, and low visible-light utilization efficiency, which severely restrict the photocatalytic performance of g-C3N4. In this regard, structural engineering modification of g-C3N4 is highly necessary and imperative for advancing its photocatalytic applications.
3. Biomass-engineered g-C3N4
Biomass engineering harnesses the unique structural and chemical characteristics of diverse biomass species to rationally tailor the electronic structure, interfacial interaction, surface morphology, and defect structure of g‑C
3N
4, aiming to significantly enhance its photocatalytic performance.
[56,57] Different biomass components exhibit distinct functional advantages and regulation mechanisms. For example, cellulose is rich in hydroxyl groups and can form strong hydrogen-bond networks with g-C
3N
4 to accelerate interfacial charge transfer; lignin contains aromatic conjugated structures and phenolic hydroxyl groups, which can construct donor-acceptor systems and promote proton-coupled electron transfer; nitrogen-rich biomass such as chitosan can serve as both nitrogen sources and soft templates to realize in-situ doping and porous structure construction; biomass-derived carbon can act as an electron mediator to build high-speed charge-transport channels. Meanwhile, various biomass can also induce carbon doping, nitrogen vacancies, and hierarchical pore structures to optimize active sites and light-harvesting capacity. In this section, we systematically classify modification strategies based on typical biomass components, clarify the interfacial interaction and electronic regulation mechanisms of different biomass units, and elucidate the specific roles of biomass-based modifications in boosting charge separation, light absorption, and surface catalytic reactivity of g-C
3N
4.
3.1 Cellulose-engineered g-C3N4
Cellulose and its derivatives bear a high density of hydroxyl groups (-OH) along their molecular chains, which form dense, stable intermolecular hydrogen-bonding networks with the amino (-NH
2) groups exposed on the surface and edge sites of g‑C
3N
4.
[18] These interfacial interactions effectively reduce charge-transfer resistance, optimize band alignment, and build robust directional charge-transport pathways. The well-connected interface accelerates photogenerated carrier separation and migration, suppresses electron-hole recombination, extends carrier lifetime, and ultimately enhances the quantum efficiency and photocatalytic performance of the composite. For example, Xiao and co-workers
[58] demonstrated that introducing a small amount of cellulose-based carbon nanofibers (CF) into g-C
3N
4 homojunctions serves as an electron mediator and band-structure regulator, suppressing the recombination of photogenerated electron-hole pairs and enhancing photocatalytic H
2O
2 production (
Fig. 8a). Further investigations revealed that the hydrogen-bonding strength between carboxymethyl cellulose (CMC) and g-C
3N
4 increases significantly with a decreasing degree of substitution.
[23] CMC with a low degree of substitution (CMC-L) forms stronger hydrogen bonds, thereby efficiently boosting interfacial charge transfer. Peng and co-workers
[59] employed cellulose nanofibrils (CNF) as both a modifier and soft template to construct one-dimensional porous g-C
3N
4 nanorods (HCN) with nitrogen vacancies and oxygen doping via hydrogen-bonding assembly and molecular shearing. (
Fig. 8b) The optimized HCN catalyst exhibited excellent photocatalytic performance toward biomass monosaccharide oxidation to lactic acid with a yield of 75.5% and a hydrogen production rate of 2.8 mmol h
-1 g
-1, together with good substrate compatibility and cycling stability. Mechanistic studies and theoretical calculations verified that the synergistic effect of nitrogen vacancies and oxygen doping accelerated the generation of superoxide radicals, which dominated the superior photocatalytic activity. This work provides a green and feasible route for designing high-efficiency photocatalysts applied in biomass refining. Additionally, Liu and co-workers
[60] developed a highly compressible, ultralight 3D CNF/rGO/g-C
3N
4 aerogel via a facile bidirectional freeze-casting and thermal reduction route, addressing the long-standing trade-off between mechanical stability, recyclability and photocatalytic performance of bulk g-C
3N
4. In this 3D framework, CNFs acted as a reinforcing skeleton via hydrogen-bonding interactions with rGO and g-C
3N
4, endowing the aerogel with excellent mechanical robustness (over 89% height retention after 100 compression cycles at 40% strain), while rGO served as an electron transport channel to accelerate photogenerated carrier separation. The interconnected porous structure fully exposed the active sites of g-C
3N
4, enabling 87.3% RhB degradation efficiency under visible light and a hydrogen evolution rate of 417.5 μmol·h
-1·g
-1. The monolithic aerogel can be easily recycled with outstanding cyclic stability, providing a practical strategy for fabricating robust, multifunctional 3D g-C
3N
4-based photocatalysts for wastewater treatment and clean energy production.
3.2 Lignin-engineered g-C3N4
Lignin is one of the three core components of lignocellulosic biomass and the only naturally occurring aromatic biopolymer, stands out as a highly promising renewable carbon resource and sustainable alternative to fossil-derived carbon sources, owing to its ultrahigh natural abundance, low cost, and high intrinsic carbon content.
[22,61] For g-C
3N
4, carbon doping is a well-established, powerful strategy to address its intrinsic performance bottlenecks.
[62] As an earth-abundant, eco-friendly non-metal with an atomic size well-matched to the C/N atoms in the g-C
3N
4 framework, low ionization energy, and high structural stability, carbon is an ideal heteroatom for bulk doping of g-C
3N
4. Incorporating carbon into the g-C
3N
4 conjugated skeleton broadens the visible-light response window, improves intrinsic electronic conductivity, and facilitates photogenerated charge carrier separation and migration through electron trap engineering, thus fundamentally alleviating the key intrinsic limitation of fast electron-hole recombination in pristine g-C
3N
4.
[22] For example, biochar from lignin pyrolysis can introduce abundant bridging C sites and controllable N vacancies into the g-C
3N
4 lattice.
[27] These defect sites finely regulate the local electronic structure, optimize charge density distribution, and lower the energy barrier for C-C coupling during catalytic reactions. Benefiting from these synergistic effects, the lignin-engineered g-C
3N
4 photocatalyst achieves highly selective CO
2 reduction toward C
2H
6, with an outstanding product selectivity of 80.33%. Additionally, Xu and co-workers
[63] fabricated lignin-derived carbon-doped g-C
3N
4 via a facile one-step thermal copolymerization method with lignin as the renewable carbon source, achieving markedly improved photocatalytic performance. Comprehensive characterizations verified that lignin incorporation did not alter the intrinsic heptazine framework of g-C
3N
4, while the optimized LCN-1 sample showed enhanced visible-light absorption and more efficient charge separation and transfer compared with pristine MCN (
Fig. 9a). The composite delivered outstanding performance in both direct and peroxymonosulfate (PMS)-assisted photocatalytic tetracycline (TC) degradation, as PMS scavenged photogenerated electrons to suppress carrier recombination and boost reactive oxygen species (ROS) generation. This work provides valuable guidance for the construction of low-cost lignin-derived carbon-doped g-C
3N
4, and validates its excellent efficiency in PMS activation-coupled photocatalytic pollutant degradation. Additionally, Zhao and co-workers
[64] used alkali lignin (AL) as a green carbon precursor to construct porous coral-like g-C
3N
4 with nitrogen vacancies and biochar persistent free radicals (PFRs) for photothermal conversion of 5-hydroxymethylfurfural (HMF) to maleic acid (MA) (
Fig. 9b). The optimized 1.5% PLCN delivered a maximum MA yield of 50.43% under visible-light irradiation at 90°C for 4 h. Alkali lignin played a critical dual role in forming the biochar component and introducing nitrogen vacancies, which greatly boosted in-situ H
2O
2 production (156.68 μmol L
-1 h
-1) (
Fig. 9c-e). Mechanistic studies showed that biochar PFRs induced ·OH generation via a metal-free Fenton-like reaction and provided active sites for
1O
2 formation, which dominated the selective production of MA. These work offers new strategies for lignin valorization and the design of multifunctional g-C
3N
4 photocatalysts for selective biomass conversion.
3.3 Biomass-derived carbon-engineered g-C3N4
Biomass-derived carbon engineering serves as a green and powerful strategy to optimize g-C
3N
4, where various biomass precursors can be converted into functional carbon materials via controlled pyrolysis, acting as high-speed electron mediators to construct π-π conjugated heterojunctions with g-C
3N
4, significantly accelerating the separation and migration of photogenerated carriers and suppressing electron-hole pair recombination. Meanwhile, the thermal decomposition of biomass introduces abundant hierarchical porous structures, carbon doping and nitrogen vacancies into g-C
3N
4, which broaden the visible-light absorption range, increase specific surface area and surface active sites, optimize the local electronic structure and reduce the reaction energy barrier of key intermediates, thereby comprehensively enhancing the photocatalytic performance of g-C
3N
4 in hydrogen evolution, H
2O
2 production, CO
2 reduction and pollutant degradation, and realizing the high-value utilization of biomass resources synchronously. A et al.
[65] used hemp stalks as the renewable carbon precursor and prepared defect-enriched Ag
3BiO
3/ZnO/BC composites by hydrothermal treatment. The in-situ formed n-n heterojunction and incorporated biochar exert a synergistic effect, which effectively promotes the separation and migration of photogenerated electron-hole pairs and suppresses charge recombination. After compositional optimization, the optimal sample, 0.2-Ag
3BiO
3/ZnO/BC, exhibits excellent photocatalytic activity and stability, achieving a 95.8% removal efficiency of levofloxacin (LFX) within 120 minutes. Primitive g-C
3N
4 suffers from large exciton binding energy and severe charge recombination. Peng and co-workers
[66] integrated biomass-derived carbon rings with heptazine units via π-conjugation to construct a biochar-welded donor-acceptor (D-A) architecture in g-C
3N
4. Such a built-in D-A configuration generates an intrinsic driving force that promotes electron delocalization and directional charge transport. Meanwhile, interlayer π-π stacking among carbon rings provides efficient out-of-plane migration pathways for electrons across the layered structure. The resulting g-C
3N
4 with biochar-welded D-A structure enhanced the yield of xylonic acid from biomass monosaccharides to 87.52%. Mechanistic investigations confirmed the dominant role of superoxide radicals (·O
2-) and distinguished singlet oxygen (
1O
2) generation pathways, verifying that
1O
2 contributed via an energy-transfer process. This work establishes a general strategy for designing D-A structured g-C
3N
4-based photocatalysts toward efficient photocatalytic reforming of biomass.
3.4 Others
In addition to cellulose and lignin, a variety of other biomass resources can be employed for the functional modification of g-C
3N
4. For instance, chitosan, as a typical nitrogen-rich biomass, can not only act as an extra nitrogen source to participate in the polymerization process but also function as a soft template to induce in-situ nitrogen doping and facilitate the formation of porous structures during thermal polymerization, thus effectively regulating the electronic structure and surface morphology of g-C
3N
4.
[67] Moreover, algal biomass such as spirulina can serve as a sacrificial template that decomposes at high temperatures to construct interconnected porous frameworks for g-C
3N
4, which can significantly increase the specific surface area, expose more catalytic active sites, and optimize light absorption and charge carrier migration behaviors simultaneously.
[68] This strategy not only remarkably enhances the photocatalytic performance of g-C
3N
4, but also provides a facile and feasible pathway for the low-cost and large-scale preparation of high-performance g-C
3N
4-based photocatalytic materials, promoting the organic integration of biomass resource utilization and the green synthesis of high-efficiency catalytic materials. For example, Yang and co-workers
[69] developed a green, facile biomass-derived strategy to fabricate hydroxylated carbon nitride nanosheets using urea and sucrose as bio-based precursors, followed by the construction of conjugated polyelectrolyte-modified CNOH heterojunctions for efficient photocatalytic hydrogen evolution. Two typical CPEs, cationic PFNBr and anionic PCP-2F-Li were selected to regulate the layer stacking and microstructure of CNOH. The CPEs were tightly coupled with bio-derived CNOH via strong interfacial hydrogen bonding and π-π stacking interactions, achieving well-matched energy level structures, optimized electronic state distribution, and excellent surface hydrophilicity, which significantly promoted the separation and migration efficiency of photogenerated charge carriers.
Different biomass sources should be rationally selected according to their intrinsic chemical features and target photocatalytic functions. As summarized in
Table 1, cellulose and its derivatives are particularly suited for hydrogen-bond construction and proton-transfer regulation, lignin is advantageous for carbon doping and conjugated-domain modulation, chitosan serves as a nitrogen-rich platform for surface functionalization, algal biomass offers heteroatom-rich templating capability, and biochar functions as an efficient mediator for interfacial charge transfer.
Collectively, biomass engineering provides a green, cost-effective, and multifunctional approach to tailor the electronic structure, surface chemistry, and morphology of g-C3N4. These advances not only enhance photocatalytic performance across diverse applications including energy conversion and environmental remediation but also promote the valorization of abundant lignocellulosic biomass, bridging sustainable resource utilization and green catalytic chemistry.
4. Enhancement strategies for biomass-engineered g-C3N4
The remarkable photocatalytic performance of biomass-engineered g-C
3N
4 is mainly derived from the multi-dimensional structural and electronic regulation effects introduced by biomass components.
[2,71] Biomass with abundant functional groups and diverse morphologies can interact with the precursor and framework of g-C
3N
4 during thermal polymerization, thus triggering multiple synergistic modification effects. In this section, we systematically elaborate the enhancement pathways of biomass engineering on the photocatalytic activity of g-C
3N
4. On the one hand, hydroxyl- and amino-rich biomass promotes interfacial interaction and charge separation through hydrogen bond engineering.
[18,26] On the other hand, biomass-derived carbon or heteroatom-doped components can construct tight heterojunctions, optimizing band structure and accelerating carrier transfer.
[69] Meanwhile, the decomposition of biomass templates can induce defect engineering, generating rich vacancy defects and porous structures. These strategies synergistically broaden the light absorption range, increase specific surface area and active sites, suppress charge recombination, and ultimately significantly boost the intrinsic photocatalytic activity of g-C
3N
4.
4.1 Hydrogen bonds engineering
Biomass featuring abundant hydroxyl and amino groups can form a stable, continuous hydrogen-bonding network with the heptazine and triazine repeating units of g‑C
3N
4 during in-situ polymerization or solution-based composite fabrication. This hydrogen-bonding network, as a class of non-covalent intermolecular interaction, enables intimate, reduces interfacial defects interfacial contact between the biomass moieties and the g‑C
3N
4 framework, and in turn establishes a continuous, low-resistance charge transfer channel across the composite interface. Through this well-defined interfacial charge transfer system, the hydrogen bonding interactions effectively suppress the recombination of photogenerated electron-hole pairs, precisely modulate the interfacial electron density distribution, and optimize the separation and migration dynamics of charge carriers, thereby directly enhancing the photocatalytic redox activity of the as-fabricated composite material. For example, our groups
[18] demonstrated that hydroxyl‑rich microcrystalline cellulose (MCC) can form strong intermolecular hydrogen bonds with g‑C
3N
4, which significantly promotes charge separation and suppresses recombination of photogenerated electron‑hole pairs, thus greatly boosting photocatalytic tetracycline degradation and hydrogen evolution activity.(
Fig. 10a-b) Furthermore, in-depth studies found that the construction of a hydrogen bonds network between carboxyl-functionalized cellulose nanofibrils (f-CNF) and CdS/g-C
3N
4 (CdS/PCN) S-scheme heterojunctions enhanced proton-donating capacity, effectively facilitated PCET process (
Fig. 10c), lowered the reaction energy barrier of the *OOH intermediate, and ultimately achieved a photocatalytic activity of 2867 μmol·L
-1·h
-1.
[26](
Fig. 10d)
4.2 Defect engineering
Defect engineering is an important approach to optimize the band structure of g-C
3N
4 and enhance the separation efficiency of photogenerated electron-hole pairs. Nonmetal doping can modify the g-C
3N
4 structure through framework atom substitution or surface grafting, while defect engineering can regulate the carbon nitride framework without introducing extrinsic metal elements; both strategies can disrupt the ordered heptazine ring structure of g-C
3N
4, induce planar polarization and charge redistribution. Through the rational selection of biomass precursors and precise control of synthetic processes, the controllable construction of elemental doping and vacancy defects can be simultaneously achieved, which finely tailors the microstructure and electronic properties of g-C
3N
4, thereby providing new insights and breakthrough directions for the development of photocatalytic technology.
[72] For example, our groups
[27] designed biochar-tailored g-C
3N
4 (PCCN-x) with controllable carbon doping and nitrogen vacancies (
Fig. 11a), which activated CO
2 molecules and promoted C-C coupling to selectively generate C
2H
6. Furthermore, the fabricated PCCN-x with sp
3-hybridized C bridges and tunable N defects, which constructed a strong dipole field to boost charge transport and lowered the energy barrier of *OOH intermediate, realizing a piezo-photocatalytic H
2O
2 production rate of 4.61 mmol g
-1 h
-1 even in pure water system (
Fig. 11b-c).
[70] Peng et al.
[66] fabricated graphitic carbon nitride (GM-CN) integrated with biochar-welded donor-acceptor (D-A) architecture and defect engineering through the self-assembly and thermal copolymerization of melamine and glucosamine.(
Fig. 11d) During the polycondensation process, the carbocyclic units derived from glucosamine were covalently incorporated into heptazine frameworks, which allowed the precise modulation of carbon doping concentration and structural defect density. Characterizations by electron paramagnetic resonance (EPR) and solid-state
13C nuclear magnetic resonance (
13C NMR) demonstrated that defect-related signals were gradually intensified with elevated doping levels, indicating the promoted concentration of delocalized electrons and increased abundance of active defect sites. The introduction of carbon-ring units triggered a downshift of the band structure, which broke the symmetric charge distribution in pristine g-C
3N
4 and accelerated the directional electron transfer toward tri-s-triazine segments. Consequently, the electrons in the lowest unoccupied molecular orbital (LUMO) of GM-CN were concentrated in the conjugated defect regions between carbon rings and heptazine rings, while the holes in the highest occupied molecular orbital (HOMO) were localized around the defect-containing tri-s-triazine rings. Such defect-mediated spatial charge separation effectively optimized the electronic structure of g-C
3N
4, thereby substantially boosting its photocatalytic activity toward biomass conversion. (
Fig. 11e-f)
4.3 Heterojunction engineering
The construction of composite systems is a core strategy to modulate the electronic band structure of g-C
3N
4. These systems are categorized into homojunctions and heterojunctions including type I, type II, Z-scheme, and S-scheme configurations, with most existing studies focusing on g-C
3N
4 composites integrated with metal-based materials (
Fig. 12a).
[3,73 -75] In contrast to high-cost noble metal modification and conventional metal-based heterojunctions that carry inherent metal ion leaching risks, g-C
3N
4 heterojunctions built with sustainable, eco-friendly biomass offer pronounced advantages. These biomass-derived heterojunctions deliver high photocatalytic activity and excellent cycling durability without secondary environmental contamination, representing a key development direction for the green modification of g-C
3N
4.
[76,77] Biomass-derived carbon materials form a π-π conjugated heterojunction with g-C
3N
4, in which the carbon phase acts as an ultrafast electron bridge to directional export photogenerated electrons, reduce interfacial charge-transfer resistance, and activate O
2 for efficient H
2O
2 production. The optimized CN/CFs heterojunction delivers a H
2O
2 generation rate up to 1.31 mmol·L
-1·h
-1, far exceeding pure g-C
3N
4. Furthermore, Zhou et al.
[69] employed an aqueous urea-sucrose precursor to fabricate a type‑II CPEs/CNOH heterojunction mediated by conjugated polyelectrolytes (CPEs). Hydrogen bonds and π-π stacking synergistically strengthen interfacial coupling, driving the exfoliation of hydroxylated g-C
3N
4 into a porous, wrinkled nanostructure (
Fig. 12b). This extended visible-light absorption and accelerated spatial charge separation.
Collectively, these strategies broaden light absorption, increase specific surface area and active sites, and suppress charge recombination, providing a systematic framework for the rational design of high-performance, green g-C3N4-based photocatalysts.
5. Applications of biomass-engineered g-C3N4 photocatalysts
The biomass-engineering strategy described above optimizes the physicochemical properties and electronic structure of g-C3N4, endowing this material with notable advantages in photoreduction applications. By tailoring the bandgap, enhancing interfacial charge separation and transfer, and improving substrate adsorption capacity, biomass-modified g-C3N4 photocatalysts achieve excellent catalytic activity and selectivity in typical photoreduction reactions, including water splitting for H2 evolution, H2O2 production, CO2 reduction, and organic pollutant degradation. Furthermore, the inherent green synthesis characteristics of this approach eliminate the risk of secondary pollution associated with conventional chemical modification methods, thereby conferring both high reduction efficiency and environmental compatibility upon the resulting material.
Collectively, biomass-derived engineering offers a sustainable and high-performance route for designing g-C
3N
4-based photoreduction catalysts. Representative research advances in this field are summarized in
Table 1.
5.1 Photocatalytic water splitting
Photocatalytic water splitting converts solar energy into storable chemical fuels (e.g., H
2 or O
2), offering a sustainable route to address energy and environmental challenges.
[78-84] Since Wang and colleagues first used g-C
3N
4 for photocatalytic water splitting to produce H
2 and O
2 in 2009
[85](
Fig. 13a), the field has attracted considerable attention. g-C
3N
4 is a typical semiconductor with a bandgap of approximately 2.7 eV, a suitable band structure, and good stability for overall water splitting
[86,87] (
Fig. 13b). Under visible-light excitation, its conduction band potential (-1.3 V vs. NHE) is more negative than the reduction potential of H
+/H
2 (-0.41 V vs. NHE), enabling water reduction to H
2[88] (
Fig. 13c). Meanwhile, its valence band potential (+1.4 V vs. NHE) is more positive than the oxidation potential of O
2/H
2O (+1.23 V vs. NHE), allowing water oxidation to O
2. Consequently, g-C
3N
4 shows significant potential for photocatalytic water splitting applications.
By utilizing biomass-derived carbon to modulate the electronic structure, facilitate charge transfer, and introduce photothermal effects, the carrier recombination in g-C
3N
4 is effectively suppressed, thereby significantly enhancing its photocatalytic hydrogen evolution performance. Ullah et al.
[89] developed a facile one-step annealing route to synthesize carbon-doped g-C
3N
4 (CCN-X) using urea and L-aspartic acid as precursors (
Fig. 13d). Carbon doping extended visible-light absorption, narrowed the band gap, and promoted the separation and transfer of photogenerated charge carriers, leading to drastically enhanced photocatalytic H
2 evolution. Under visible-light irradiation (λ ≥ 420 nm), the optimized CCN-8 catalyst delivered a maximum H
2 production rate of 2192.2 μmol·h
-1·g
-1, 6.6-times higher than pristine g-C
3N
4 (333.0 μmol·h
-1·g
-1). CCN-8 also exhibited an apparent quantum efficiency of 6.57% at 420 nm and excellent stability over 16 h of cycling. Gao et al.
[90] prepared a biomass-derived carbon/porous carbon nitride composite (BC/PCN) using an in-situ compositing method (
Fig. 13e). Through close interfacial bonding, the composite modulated the electronic structure and promoted directional migration of photogenerated carriers. Owing to its ultralow light reflectivity, biomass-derived BC enabled near-full-spectrum light harvesting, causing a redshift in the composite's absorption edge and narrowing its bandgap. Meanwhile, BC acted as an efficient charge-transfer bridge, rapidly conveying photogenerated electrons from PCN to active sites and thereby greatly suppressing carrier recombination. The photothermal effect of BC also significantly enhanced the reaction kinetics of the photocatalytic system. The optimal BC/PCN-0.1 composite achieved a HER rate of 4.98 mmol·g
-1·h
-1 under simulated solar irradiation, representing a 2.1 times improvement over pristine PCN (2.35 mmol·g
-1·h
-1). An apparent quantum efficiency of 4.82% at 420 nm confirmed the crucial role of efficient charge separation at the BC/PCN interface in boosting catalytic performance. After five cycles, the HER activity remained 78% of its initial value. Yang et al.
[69] fabricated type-II heterojunctions of conjugated polyelectrolyte (CPE)-assisted hydroxylated g-C
3N
4 (CNOH) (
Fig. 13f). The optimal 3% PFN/CNOH
0.5 and 1% PCP/CNOH (1.5 wt% Pt) delivered photocatalytic H
2 evolution (PHE) rates of 22.75 and 24.01 mmol g
-1 h
-1 under visible light (600 mW cm
-2), 13.87- and 14.64 times higher than bulk g-C
3N
4, respectively (
Fig. 13g-h). The remarkable PHE enhancement arises from well-matched energy alignment between CPEs and CNOH, enlarged surface area via CPE-facilitated exfoliation, extended visible-light absorption from sucrose and CPE doping, strengthened interfacial interactions through hydrogen bonding and π-π stacking, and improved hydrophilicity endowed by sucrose modification.
5.2 Photocatalytic H2O2 production
Hydrogen peroxide (H
2O
2), a green oxidant and bleaching agent, finds extensive applications in water treatment, food processing, medical disinfection, and chemical synthesis.
[91-93] Photocatalytic H
2O
2 synthesis proceeds under mild conditions, combining energy efficiency with environmental benignity, and has thus become a research focus. The conduction band of g-C
3N
4 lies at approximately -1.3 eV, providing an ideal thermodynamic potential for O
2 reduction (-0.33 V vs. NHE); its valence band at about +1.4 eV is considerably lower than that of most metal oxides, facilitating the suppression of H
2O
2 decomposition.
[94-96]Optimizing the light absorption range and band structure of g-C
3N
4 through biomass regulation enhances solar energy utilization, which benefits photocatalytic H
2O
2 synthesis. Chen et al.
[70] developed biochar-tailored carbon nitride nanosheets (PCCN-x) featuring carbon bridges and nitrogen defects for high-efficiency piezo-photocatalytic H
2O
2 production. The introduction of sp
3-hybridized carbon bridges distorted the planar structure of g-C
3N
4 and strengthened π-π delocalization, whereas nitrogen defects broke the structural symmetry to generate a pronounced dipole field, driving spontaneous polarization and oriented charge migration. (
Fig. 14a-c) The optimal PCCN-10 achieved a remarkable H
2O
2 yield of 4.61 mmol g
-1 h
-1 without cocatalysts under light-ultrasound co-irradiation, and maintained a production rate of 2.19 mmol g
-1 h
-1 in pure water with stable activity over five cycles. Experimental and theoretical analyses confirmed that the dipole field significantly boosted piezoelectric polarization and charge separation, lowered the energy barrier for *OOH formation, and enabled efficient H
2O
2 generation via an indirect two-electron O
2 reduction pathway (
Fig. 14d). This work provides a defect-engineering design principle for piezo-photocatalytic materials toward sustainable H
2O
2 production.
5.3 Photocatalytic CO2 reduction
Photocatalytic conversion of CO
2 into value-added hydrocarbons provides an effective strategy to alleviate energy shortages.
[97-101] On g-C
3N
4 surfaces, bridging and edge N atoms serve as primary active sites, where lone-pair electrons coordinate with the C atom of CO
2 molecules, promoting efficient CO
2 adsorption in aqueous environments. Moreover, g-C
3N
4 has a relatively negative conduction band potential (approximately -1.3 V vs. NHE), significantly lower than the standard reduction potentials for converting CO
2 into various carbon products (e.g., CO
2/CO: -0.53, CO
2/CH
4: -0.24, CO
2/CH
3OH: -0.38, CO
2/HCOOH: -0.61 V vs. NHE), indicating sufficient thermodynamic reduction capability (
Fig. 15a-b).
[100,102]Structural optimization and surface functionalization of g-C
3N
4 through biomass incorporation can mitigate its inherent limitations by enhancing visible light harvesting, CO
2 adsorption, and product desorption, thereby boosting CO
2 photoreduction capability.
[103-106] From an application perspective, C
2H
6 is more valuable than C1 products because it is an energy-rich C2 hydrocarbon and an important feedstock for ethylene production. More importantly, C
2H
6 formation from CO
2 requires multi-electron/proton transfer coupled with C-C bond construction. Therefore, selective CO
2-to-C
2H
6 conversion over biomass-engineered g-C
3N
4 should be understood not only as activity improvement but also as selective manipulation of adsorption geometry, local electron density and reaction intermediates.
C-C coupling on carbon-doped/nitrogen-vacancy g-C
3N
4 is generally initiated by CO
2 adsorption and activation at electron-rich N-defective or C-bridged sites. The first proton-electron transfer generates *COOH, followed by dehydration/electron transfer to form *CO. When adjacent *CO or *CHO species are stabilized on neighboring active sites, their surface residence time increases and C-C coupling can proceed through *CO-*CO, *CO-*CHO or *CHO-*CHO coupling pathways, producing key *OCCO, *COCHO or related C
2 intermediates. Subsequent stepwise hydrogenation and deoxygenation can convert these C
2 intermediates into *CH
3CH
3 and finally release C
2H
6.
102 Our groups
[27] fabricated carbon-doped, nitrogen-vacancy-engineered g-C
3N
4 nanosheets (PCCN-x) using a lignin-biomass-derived precision molecular design strategy (
Fig. 15d). The introduction of bridged C sites and N vacancies modulated the electronic structure and local electron density distribution, narrowing the bandgap and red-shifting the light absorption edge while promoting charge separation and suppressing photogenerated carrier recombination. This configuration enhanced CO
2 adsorption and activation significantly lowered the energy barrier for C-C coupling to form the key *OCCO intermediate, and enabled highly selective photoreduction of CO
2 to C
2H
6 in pure water. The optimal PCCN-10 photocatalyst achieved a C
2H
6 production rate of 99.14 μmol·g
-1·h
-1 with 80.33% selectivity, along with CO and CH
4 yields of 9.20 and 15.07 μmol·g
-1·h
-1, respectively (
Fig. 15c, e-f). This work achieved precision molecular engineering of g-C
3N
4 via biomass mediation, establishing a paradigm for rational photocatalyst optimization that aligns with green chemistry principles and sustainable development, and demonstrating significant potential for practical implementation.
5.4 Photocatalytic degradation of organic pollutants
As a green and sustainable technology, photocatalytic pollutant degradation holds great promise for environmental remediation.
[107-109] Sustainable biomass-derived strategies for engineering g-C
3N
4 have also shown remarkable performance in this context.
[110,111] The core mechanism involves the generation of reactive oxygen species (ROS, e.g., OH, O
2-,
1O
2, and H
2O
2) driven by photogenerated carriers, followed by redox reactions. g-C
3N
4 exhibits both photocatalytic activity and notable adsorption capability. Its conduction band potential is sufficiently more negative than the reduction potential of dissolved O₂ to superoxide radicals (·O
2-) (O
2 + e
- → ·O
2-, -0.33 V vs. NHE).
[112] Surface holes can oxidize H
2O to ·OH or directly oxidize functional groups of organic pollutants. Additionally, singlet oxygen (
1O
2) can be generated via energy transfer or disproportionation of ·O
2-.
However, similar to water splitting and CO
2 reduction, g-C
3N
4 exhibits limited active sites and rapid carrier recombination. Biomass-derived strategies can overcome these drawbacks by enhancing charge separation and ROS generation, embodying green principles from synthesis to application. Han et al.
[113] fabricated a wood pulp cellulose biochar/g-C
3N
4 (WPBC/g-C
3N
4) composite via hybridization, which efficiently activated peroxymonosulfate (PMS) to degrade diclofenac (DCF) under visible light (
Fig. 16a). The optimal WPBC
50/g-C
3N
4 showed outstanding stability, retaining high DCF degradation efficiency after five cycles. Incorporation of WPBC narrowed the bandgap, enhanced visible-light absorption, and improved charge separation and PMS activation performance of g-C
3N
4. Both radical species (·OH, h
+, ·O
2-) and nonradical pathways (
1O
2, electron transfer) contributed to DCF degradation, with
1O
2 as the dominant reactive oxygen species. The C=O and N-(C)
3 groups served as active sites for PMS activation, and principal component analysis identified DCF concentration as the most influential factor (
Fig. 16b-e). Tailored biomass sources can markedly enhance photocatalytic degradation efficiency. Such experiments provide new insights and prospects for the value-added utilization of biomass-modified g-C
3N
4 as a photocatalyst for degrading environmental pollutants.
5.5 Photocatalytic nitrogen (N2) fixation
In addition, another photocatalytic application is nitrogen (N
2) fixation. Photocatalytic N
2 fixation is a key technology for energy and environmental remediation.
[114,115] It utilizes solar energy to fix nitrogen under ambient conditions, offering a low-carbon route for ammonia synthesis.
[91,114,116 -118] The mechanism involves photogenerated electrons (e
-) reducing N
2 to NH
3 under catalytic action, while photogenerated holes (h
+) are consumed via oxidation by sacrificial agents or H
2O.
[114]Biomass materials, particularly biomass carbon, possess a porous structure that confers exceptional adsorption advantages. Tang et al.
[71] fabricated diphasic carbon-decorated carbon nitride (DC-CN) photocatalysts via a one-pot co-pyrolysis strategy using lotus root starch as a biomass precursor, simultaneously introducing in-plane fused carbon rings and physically piled carbon particles into the g-C
3N
4 framework. The diphasic carbon synergistically created dual electron transfer pathways, optimized band alignment, broadened visible-light absorption, and greatly enhanced the separation efficiency of photogenerated carriers. Both carbon species acted as active sites to regulate electron transfer and lower the energy barrier for N
2 activation and hydrogenation. The optimal DC-CN0.1 achieved a superior NH
3 production rate of 167.35 μmol·g
-1·h
-1 under visible light, five times higher than that of pure g-C
3N
4, with excellent cycling stability (
Fig. 17a-b). Experimental characterizations and DFT calculations confirmed that the diphasic carbon synergy was responsible for the enhanced photocatalytic N
2 fixation, thereby enriching the metal-free skeleton engineering toolbox for photocatalysts (
Fig. 17c-d).
Collectively, biomass-engineered g-C
3N
4 has demonstrated versatile and exceptional performance across diverse photocatalytic applications (
Table 2). For H
2 evolution, the synergistic integration of C doping, heterostructure construction, and photothermal effects has enabled up to a 14.6 times enhancement in H
2 production rate relative to g-C
3N
4. For H
2O
2 synthesis, cooperative modulation of C bridges and N vacancies delivers a remarkable yield of 4.61 mmol·g
-1·h
-1 in the absence of any cocatalysts. In CO
2 photoreduction, a landmark breakthrough has been achieved in the highly selective conversion of CO
2 to C
2H
6 in pure water, opening a transformative avenue for photocatalytic multi-carbon product synthesis. For organic pollutant degradation and nitrogen fixation, the porous architecture and interfacial regulation conferred by biomass-derived carbon have also substantially improved both reaction kinetics and long-term catalyst stability. These advances underscore the enormous untapped potential of biomass resources for the rational design of high-performance photocatalytic materials.
6. Conclusions and outlook
Biomass-engineered g-C3N4 has emerged as a transformative green platform for addressing the three core intrinsic limitations of pristine g-C3N4: ultrafast photogenerated electron-hole recombination, narrow visible-light absorption range, and scarcity of surface active sites. Leveraging the unique structural and chemical merits of renewable biomass feedstocks including abundant reactive functional groups (hydroxyl, carboxyl, amino), hierarchical porous architectures, and tailorable molecular structures. Researcher have developed versatile modification strategies centered on hydrogen-bond engineering, heterojunction construction, and defect engineering. These strategies substantially boost the photocatalytic performance toward H2 evolution, H2O2 production, selective CO2 reduction, organic pollutant degradation and N2 fixation. Meanwhile, they realize high-value valorization of lignocellulosic biomass, thereby integrating sustainable resource utilization with green catalytic chemistry.
Despite remarkable progress in biomass-engineered g-C3N4 photocatalysts for solar energy conversion and environmental remediation, translating these laboratory achievements into stable, scalable, and industrially viable technologies still faces a series of profound and interrelated challenges. These bottlenecks mainly lie in precise structural control, unclear catalytic mechanisms, insufficient interfacial stability, and difficulties in large-scale preparation.
(i) The lack of precise structural and compositional control over biomass-derived functional motifs severely limits the rational optimization of g-C
3N
4. Natural biomass exhibits inherent structural heterogeneity, batch-to-batch variability, and complex thermal decomposition behaviors during calcination, making it difficult to uniformly tailor surface functional groups, heteroatom doping concentrations, vacancy densities, and hierarchical pore architectures at the molecular level.
[133,134] The random distribution of hydroxyl, carboxyl, and aromatic moieties often leads to uncontrolled structural collapse, partial agglomeration, or loss of active sites during in-situ polymerization, resulting in inconsistent photocatalytic performance and poor reproducibility across different preparations.
[135] Unlike well-defined synthetic modifiers, biomass cannot be easily fine-tuned to deliver targeted electronic modulation, impeding the establishment of clear structure activity relationships essential for predictive catalyst design.
(ii) The fundamental understanding of interfacial charge-transfer dynamics and intrinsic reaction mechanisms remains incomplete and largely phenomenological. While hydrogen-bonding interactions, heterojunction formation, and defect engineering are widely invoked to explain enhanced activity, direct in-situ observations of charge separation, migration pathways, and intermediate evolution at the biomass-engineered g-C3N4 interface are still scarce. Key questions persist. For example, how do hydrogen-bond networks precisely regulate the density of states and band alignment at the atomic scale? What are the exact roles of biomass-derived carbon bridges and nitrogen vacancies in promoting PCET or C-C coupling for CO2 reduction to multi-carbon products? Without quantitative mechanistic insights derived from operando characterization and high-fidelity theoretical simulations, the field relies heavily on empirical optimization rather than knowledge-driven design, slowing the development of next-generation catalysts with maximized quantum efficiency and product selectivity.
(iii) Interfacial stability and long-term durability under realistic operating conditions pose substantial limitations. The non-covalent interactions (e.g., hydrogen bonding, π-π stacking) that dominate biomass-engineered g-C3N4 coupling are susceptible to disruption under prolonged illumination, continuous aqueous immersion, or harsh redox environments, leading to gradual delamination, leaching of active components, and irreversible performance decay. Biomass-derived moieties may also undergo photo-oxidation or hydrolysis over extended cycling, reducing the density of active sites and compromising structural integrity. For industrial implementation, photocatalysts must maintain high activity over hundreds of hours without significant deactivation; yet, systematic studies on aging mechanisms, regeneration strategies, and stability thresholds for biomass-engineered g-C3N4 are largely absent, creating a critical gap between laboratory demonstrations and real-world applicability.
(iv) Limited performance in practical reaction systems and insufficient product selectivity restrict real-world utility. Many high-activity results are obtained under idealized conditions (e.g., pure water, sacrificial agents, concentrated simulated solar irradiation) that poorly mimic complex industrial or environmental matrices containing impurities, varying pH, and natural sunlight fluctuations. For gas-phase reactions such as CO2 photoreduction, low selectivity toward high-value multi-carbon products (e.g., C2H6) remains a persistent challenge, with most systems favoring C1 products or suffering from competitive H2 evolution. Similarly, in pollutant degradation, incomplete mineralization and potential formation of toxic intermediates are rarely addressed, undermining the environmental benefits of photocatalytic remediation.
(v) The environmental footprint of biomass processing should be evaluated from a life-cycle assessment (LCA) perspective. Although biomass is renewable and abundant, its conversion into functional modifiers or carbonaceous components may involve energy-intensive pyrolysis/calcination, acid/base or oxidative pretreatment, solvent consumption, wastewater generation and downstream separation. Therefore, the real sustainability of biomass-engineered g-C3N4 should be judged not only by photocatalytic activity, but also by full-process indicators, including biomass source and transportation, chemical and energy inputs, carbon emissions, solvent recovery, catalyst durability, recyclability and end-of-life treatment. Future studies should prioritize waste or low-value biomass feedstocks, mild aqueous/enzymatic pretreatment, low-temperature or solar-assisted synthesis, closed-loop solvent recovery and quantitative LCA benchmarking to avoid shifting environmental burdens from the application stage to the materials fabrication stage.
Collectively, to overcome these challenges, it is imperative to move beyond empirical trial-and-error and toward knowledge-based rational design of biomass-modified g-C3N4 photocatalysts. To unlock their full potential for large-scale sustainable applications in solar energy conversion and environmental remediation, future research should prioritize three transformative directions: (i) Develop machine learning frameworks to rapidly screen and quantitatively predict the optimal mass/volume biomass-to-g-C3N3 feeding ratios, which can accelerate high-throughput catalyst screening and establish quantitative structure-activity relationships for predictive rational design and stable batch reproducibility; (ii) Customize and advance operando in situ characterization techniques capable of dynamically tracking real-time biomass pyrolysis, thermal decomposition, heteroatom doping evolution, and interfacial bonding rearrangement throughout the calcination process, to uncover hidden intermediate transformation mechanisms; (iii) Establish universal, standardized biomass pretreatment protocols covering raw biomass fractionation, purification, and mild activation workflows, to eliminate batch-to-batch compositional fluctuations and drastically improve the experimental repeatability of biomass-derived photocatalysts.
With an eye to the future, a great number of opportunities and challenges are presented for this booming and hot field (
Fig. 18). We believe that this review article can provide important strategic guidance for the rational design and development of novel biomass-engineered g-C
3N
4 photocatalysts with high catalytic activity, excellent stability, efficient visible light utilization, environmental friendliness and low cost. It is expected that with the continuous innovation and technological breakthroughs in related fields, biomass-engineered g-C
3N
4 materials will break through the limitations of basic laboratory research, move towards large-scale practical applications. They will provide an integrated and coordinated solution for renewable energy development, ecological environment restoration and high-value utilization of biomass, helping to achieve the goals of green and low-carbon development.
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