Solar-driven hydrogen peroxide production on designed g-C<sub>3</sub>N<sub>4</sub>: Strategies, mechanisms, and perspectives

Solar-driven hydrogen peroxide production on designed g-C₃N₄: Strategies, mechanisms, and perspectives

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Zhimin Yuan^a, Xingling Zhao^a, Xianglin Zhu^c, Kaili Wang^a, Ya-Qian Lan^b, Zaiyong Jiang^a

Chinese Chemical Letters | 2026, 37(6) : 112572

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Chinese Chemical Letters | 2026, 37(6): 112572

• Review •

Solar-driven hydrogen peroxide production on designed g-C₃N₄: Strategies, mechanisms, and perspectives

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Zhimin Yuan^a, Xingling Zhao^a, Xianglin Zhu^c, Kaili Wang^a, Ya-Qian Lan^b, Zaiyong Jiang^a

Affiliations

^aSchool of Chemistry & Chemical Engineering and Environmental Engineering, Weifang University, Weifang 261061, China

^bSchool of Chemistry, South China Normal University, Guangzhou 510006, China

^cInstitute for Energy Research, Jiangsu University, Zhenjiang 212013, China

About Author:

CRediT authorship contribution statement

Zhimin Yuan: Writing – original draft.

Xingling Zhao: Supervision.

Xianglin Zhu: Formal analysis.

Kaili Wang: Data curation.

Ya-Qian Lan: Conceptualization.

Zaiyong Jiang: Writing – review & editing.

Published: 2026-06-15 doi: 10.1016/j.cclet.2026.112572

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Abstract

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The photocatalytic production of hydrogen peroxide (H₂O₂) utilizing graphitic carbon nitride (g-C₃N₄) offers a sustainable alternative to the conventional, energy-intensive anthraquinone method. Nevertheless, the practical deployment of pristine g-C₃N₄ is constrained by its limited absorption of visible light, rapid recombination of photogenerated charge carriers, and low surface catalytic activity. This review critically examines various modification strategies aimed at enhancing the H₂O₂ generation efficiency of g-C₃N₄-based photocatalysts. Prominent approaches encompass elemental doping, defect modification, and the junction engineering. These modifications synergistically enhance light absorption, facilitate charge separation, and accelerate oxygen reduction reaction kinetics. Consequently, such engineered photocatalysts have achieved H₂O₂ production rates reaching millimolar concentrations per hour under visible-light irradiation, alongside marked improvements in selectivity and apparent quantum efficiency. Despite these significant advancements, challenges persist in realizing broad-spectrum solar energy utilization, ensuring long-term operational stability, and developing scalable synthesis methods for catalyst fabrication. This review delineates prospective research directions aimed at advancing efficient and practical photocatalytic systems for sustainable hydrogen peroxide synthesis.

Key words

g-C₃N₄-based photocatalysts / Hydrogen peroxide production / Photocatalytic / Modification strategies / H₂O₂

Cite this Article

Zhimin Yuan, Xingling Zhao, Xianglin Zhu, Kaili Wang, Ya-Qian Lan, Zaiyong Jiang. Solar-driven hydrogen peroxide production on designed g-C₃N₄: Strategies, mechanisms, and perspectives[J]. Chinese Chemical Letters, 2026 , 37 (6) : 112572 - . DOI: 10.1016/j.cclet.2026.112572

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1　Introduction

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Hydrogen peroxide is recognized as a green oxidant with broad applicability across diverse fields, including wastewater treatment, biomedicine, organic synthesis, chemical sensing, and energy conversion technologies such as fuel cells [1-6]. Its appeal lies in its environmentally benign decomposition pathway, yielding only water and oxygen [7-11]. With the advancement of green chemistry and carbon neutrality goals, H₂O₂ has not only gained prominence as a clean oxidant but is also regarded as a promising energy carrier and electrocatalytic medium within renewable energy systems [12-17]. However, the current industrial production of H₂O₂ predominantly relies on the anthraquinone redox process, which involves organic solvents, anthraquinone carriers, high-pressure hydrogen, and multiple mass-transfer steps [18-20]. This method suffers from complexities in operation, high energy consumption, and significant environmental impact. In response to the demand for green synthesis and decentralized applications, several alternative strategies have been proposed, including the direct synthesis from hydrogen and oxygen, electrocatalytic oxygen reduction, and plasma-assisted processes [21-23]. Nonetheless, these approaches often encounter limitations such as safety concerns, low Faradaic efficiency, or complicated reactor design [24-26]. Hence, there is a pressing need to develop novel H₂O₂ synthesis systems that operate without hazardous gases, under mild conditions, with low energy input, and through clean reaction pathways [27,28].

In recent years, photocatalytic H₂O₂ production has emerged as an attractive alternative, driven by solar energy and utilizing benign reactants such as O₂ and H₂O under ambient conditions [29-32]. In a typical photocatalytic process (Fig. 1a), the photocatalyst absorbs photons with energy equal to or exceeding its band gap, electrons in the valence band are excited to the conduction band, forming electron-hole pairs [33-37]. These unstable pairs then need to separate and migrate to the catalyst’s surface, meaning efficient separation is critical to avoid non-productive recombination that dissipates energy as heat or light. Once on the surface, the electrons react with electron acceptors, while holes oxidize donors; these radical species further drive target redox reactions, with minimizing electron-hole recombination being key to enhancing catalytic efficiency [38-42]. For the H₂O₂ generation, the photoexcited electrons migrate to the conduction band to reduce oxygen molecules into H₂O₂, while the holes can be consumed by sacrificial agents or water oxidation, thereby maintaining charge balance and enabling redox coupling. A critical aspect of this process lies in achieving selective two-electron oxygen reduction (2e⁻ ORR) to form H₂O₂, while avoiding competing pathways such as the four-electron reduction to water or further reduction and decomposition of reactive intermediates.

The photocatalytic synthesis of H₂O₂ can proceed via two principal routes: the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR) [43-46]. Upon exposure to light, the photocatalyst generates electron-hole pairs, where the photogenerated electrons drive the reduction of O₂ to H₂O₂, and the photogenerated holes facilitate the oxidation of H₂O to H₂O₂. For both ORR and WOR to proceed efficiently, the photogenerated charge carriers must possess appropriate redox potentials that meet the thermodynamic criteria of the respective reactions.

The reduction of O₂ in photocatalytic systems typically proceeds via the following pathways [47-50]:

(a) Oxygen reduction reaction (ORR) pathways

Two-electron reduction (2e⁻ ORR):

O 2 + 2 H + + 2 e − → H 2 O 2 (E 0 = + 0.68 V v s . NHE)

(1)

One-electron reduction (1e⁻ ORR):

O 2 + e − → · O 2 − (E 0 = − 0.33 V v s . NHE)

(2)

· O 2 − + e − + 2 H + → H 2 O 2 (E 0 = + 1.44 V v s . NHE)

(3)

The competing reaction in this process is:

O 2 + 4 e − + 4 H + → 2 H 2 O (E 0 = + 1.23 V v s . NHE)

(4)

(b) Water oxidation reaction (WOR) pathways [51,52]

Two-electron water oxidation (2e⁻ WOR):

2 H 2 O + 2 h + → 2 H 2 O 2 + 2 H + (E 0 = + 1.76 V v s . NHE)

(5)

One-electron water oxidation (1e⁻ WOR):

H 2 O + h + → · OH (E 0 = + 2.73 V v s . NHE)

(6)

2 · OH → H 2 O 2 (E 0 = + 1.14 V v s . NHE)

(7)

The main competing reaction in this process is:

2 H 2 O + 4 h + → O 2 (E 0 = + 1.23 V v s . NHE)

(8)

The ORR involved in H₂O₂ synthesis proceeds via a two-electron transfer mechanism, which can be conceptualized either as a direct two-electron process (Eq. 1) or as a sequential two-step single-electron process ((2), (3)). In the latter pathway, the superoxide radical anion (·O₂⁻) serves as a key intermediate species. Notably, the redox potential of the O₂/H₂O₂ couple (0.68 V versus the normal hydrogen electrode, NHE) is more positive than that of the O₂/·O₂⁻ couple (-0.33 V vs. NHE), suggesting, from a thermodynamic standpoint, a greater likelihood for the direct two-electron pathway to predominate. During photocatalytic O₂ reduction, a competing reaction exists wherein O₂ undergoes further reduction to water (H₂O) via photogenerated electrons (Eq. 4). Given that the redox potential of the O₂/H₂O couple (1.23 V vs. NHE) exceeds that of the O₂/H₂O couple, the four-electron ORR pathway is thermodynamically more favorable.

Similarly, the WOR on photocatalysts can proceed through multiple electron transfer routes, including four-electron, two-electron, and one-electron pathways (Eqs. 5-7). The two-electron WOR pathway can be further subdivided into an indirect two-step single-electron transfer mechanism (Eqs. 6 and 7) and a direct one-step two-electron process (Eq. 5). Thermodynamically, the direct two-electron pathway is preferred; however, kinetically, the indirect mechanism tends to dominate. For effective H₂O₂ generation via the two-electron WOR route, the valence band edge of the photocatalyst must be positioned more positively than + 1.76 V versus NHE to satisfy thermodynamic criteria. In contrast, the four-electron WOR pathway is thermodynamically more advantageous, thereby favoring oxygen evolution over H₂O₂ production.

Photocatalysts typically demonstrate low adsorption energies for oxygen, which facilitates the desorption of reaction products [53]. Overall, the two-step single-electron transfer mechanism is kinetically more favorable. Concurrently, O₂ readily interacts with photogenerated holes to form singlet oxygen species (¹O₂, ·O₂^⁻/¹O₂ 0.34 V vs. NHE), resulting in a reduction of H₂O₂ yield. Analogous to the two-electron ORR, the actual efficiency of the two-electron WOR is governed by the interplay between thermodynamic driving forces and kinetic energy barriers. This reaction can proceed via two potential pathways: a direct two-electron transfer or a stepwise single-electron transfer, with the stability of intermediate species critically determining the preferred reaction route.

The formation and stability of key intermediates such as superoxide radicals (·O₂⁻), hydroperoxyl species (^*OOH), and singlet oxygen (¹O₂), alongside oxygen adsorption and activation behaviors, charge separation efficiency, and interfacial reaction kinetics, collectively exert a decisive influence on the rate and selectivity of H₂O₂ production. It is imperative to modulate the electron density at the catalyst’s active sites to ensure that the bond energies align with the energy barriers required for efficient two-electron transfer, thereby minimizing side reactions associated with excessively strong or weak bonding. Furthermore, in photocatalytic systems, promoting the directional transfer of photogenerated charge carriers-electrons to O₂ in ORR or holes to H₂O in WOR-while suppressing charge recombination is essential to maintain sufficient charge availability for two-electron processes. An ideal photocatalyst should exhibit an appropriate band structure, a high density of surface-active sites, favorable oxygen affinity, and controllable electron transfer pathways to optimize performance.

2　The applications and modification strategies of g-C₃N₄ in the photocatalytic H₂O₂ production

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In the domain of photocatalytic research, the efficient photocatalyst is one of the important cores. But the developments of semiconductor photocatalysts exhibit high efficiency, robust stability, and the capability to utilize a broad spectrum of solar radiation remains a significant and ongoing challenge. A variety of photocatalysts have been extensively studied, demonstrating promising performance in specific reactions; these include metal oxides [54-62], sulfides [63-68], oxynitrides [69], metal-organic frameworks (MOF) [70-74], covalent organic frameworks (COF) [75-77], and polymer [78,79], etc. Nevertheless, they still share several fundamental limitations, such as restricted visible-light absorption, rapid recombination of photogenerated charge carriers, low quantum yield, and inadequate long-term chemical stability [60,61,80]. These issues significantly hinder their practical applications on a larger scale.

A prototypical example is titanium dioxide (TiO₂), one of the most extensively investigated photocatalysts. With a bandgap of approximately 3.2 eV, TiO₂ predominantly absorbs ultraviolet light, which accounts for less than 5% of the solar spectrum, and exhibits minimal photocatalytic activity under visible light [81-87]. Although strategies such as elemental doping, composite formation, and surface sensitization have been employed to enhance its light-harvesting capabilities, these modifications cannot fully surmount the fundamental absorption limitations imposed by its wide bandgap. Photostability constitutes another critical issue. Narrow-bandgap materials such as cadmium sulfide (CdS) and tantalum oxynitride (TaON), which possess favorable band alignments for visible-light absorption, are prone to photo-corrosion and chemical degradation [88]. For example, under illumination, photogenerated holes in CdS can oxidize sulfide ions to sulfate, causing irreversible structural damage [89]. Similarly, certain nitride-based photocatalysts experience surface oxidation or lattice instability upon prolonged light exposure, leading to rapid declines in catalytic activity over successive cycles. Additionally, phenomena such as light-induced leaching of active species and surface reconstruction further undermine the structural integrity and long-term durability of these materials.

Multielectron processes, such as overall water splitting, impose more rigorous demands on photocatalysts [90-93]. The photocatalytic material must not only provide active sites for both proton reduction and water oxidation but also effectively reduce the substantial overpotentials associated with hydrogen and oxygen evolution reactions [94-96]. These requirements establish stringent criteria concerning the band structure, charge transport characteristics, and surface reaction kinetics of photocatalysts. At present, only a limited number of systems concurrently fulfill the criteria of broad spectral absorption, efficient charge separation, and exceptional stability [97,98]. Consequently, progress in this domain necessitates the design and synthesis of novel photocatalytic materials that can harness a wider spectrum of solar radiation, promote effective charge separation and transfer, and sustain durable performance under chemically demanding conditions. Pursuing this approach is critical for realizing efficient, stable, and scalable solar energy conversion technologies.

Recently, g-C₃N₄ has garnered considerable interest as a potential photocatalyst for H₂O₂ synthesis and become a hot research topic in recent years (Fig. 2), due to its unique two-dimensional layered structure, metal-free composition, ability to absorb visible light, and adjustable electronic properties [99-102]. The performance comparison of some modified g-C₃N₄-based photocatalysts for H₂O₂ production and relevant references are shown in Table 1. The conduction band potential of g-C₃N₄ is approximately -1.3 V vs. NHE [103], which thermodynamically facilitates the oxygen reduction reaction (Fig. 1b). Additionally, the intrinsic g-C₃N₄ material absorbs mainly in the UV–visible region, yet its visible-light response remains insufficient, restricting its photocatalytic efficiency [104,105]. Therefore, improving photocatalysts capable of effectively utilizing visible light is crucial for enhancing solar-energy conversion. Although nearly 48% of solar irradiation lies below 700 nm, the limit constrains the theoretical conversion efficiency, meaning that extending the absorption range alone does not guarantee higher photocatalytic performance. Furthermore, the practical utilization of g-C₃N₄ is impeded by several inherent drawbacks, including a high exciton binding energy that hampers effective charge carrier separation, limited oxygen adsorption capacity, a low density of states within the conduction band region, a scarcity of catalytically active sites, and the propensity of H₂O₂ to decompose under reaction conditions.

From view of the structure, the layered structure held together by weak van der Waals forces provides poor in-plane conductivity and promotes rapid electron–hole recombination, further exacerbated by abundant intrinsic defect states associated with lone-pair nitrogen atoms that act as charge-trapping centers. The low specific surface area of bulk g-C₃N₄ (< 10 m²/g) leads to a scarcity of catalytically active sites and weak adsorption of O₂ and the OOH^* intermediate, which slows the kinetics of the 2e⁻ oxygen reduction pathway required for selective H₂O₂ formation. In addition, its conduction band is sufficiently negative to trigger competing 4e⁻ ORR or promote further reduction and decomposition of the generated H₂O₂, while the valence band position (~1.4 V vs. RHE) is not positive enough to thermodynamically drive the 2e⁻ water oxidation pathway, preventing a synergistic oxidation contribution to H₂O₂ accumulation [106-109]. The combination of restricted visible-light absorption, short carrier lifetimes, limited active-site density, weak O₂ activation capability, and competing redox pathways collectively results in the inherently low H₂O₂ yield of g-C₃N₄, making rational structural modification essential for achieving high photocatalytic efficiency.

While the fundamental mechanisms provide a theoretical foundation, translating these principles into efficient photocatalytic systems requires deliberate material design. To enhance the photocatalytic performance of g-C₃N₄, researchers have developed multi-dimensional optimization strategies focusing on its electronic structure and interface chemistry (Fig. 1c). Approaches such as defect engineering, heteroatom doping, single-atom loading, π-π interaction modification, junction construction, colloidal dispersion, and multi-interface synergy have been proposed [110-113]. These strategies not only improve the light absorption capacity and charge carrier separation efficiency of g-C₃N₄, but also regulate the O₂ adsorption mode, the formation and stability of ^*OOH intermediates, proton transfer pathways, and H₂O₂ desorption behavior from the intrinsic structural level, thereby significantly boosting catalytic activity and product selectivity [114-117]. Particularly in recent years, the H₂O₂ production rate of high-performance g-C₃N₄ catalysts has increased from several hundred µmol g⁻¹ h⁻¹ in the early stage to 10–50 mmol g⁻¹ h⁻¹, with quantum efficiency exceeding 30%, indicating that photocatalytic H₂O₂ synthesis is becoming increasingly mature for practical application. Nonetheless, current systems continue to face significant challenges, including limited universality in structural regulation, difficulties in in situ observation of reaction intermediates, inadequate long-term stability, and obstacles to large-scale fabrication, all of which impede the commercial advancement of photocatalytic H₂O₂ technologies.

In response, this paper centers on g-C₃N₄ and provides a systematic review and in-depth analysis of the evolution of reaction mechanisms, material design strategies, and performance optimization pathways for photocatalytic H₂O₂ production in recent years. The scope of this work encompasses: (1) Fundamental reaction pathways and surface dynamic processes involved in photocatalytic H₂O₂ generation; (2) the effects of intrinsic and modified structural regulation of g-C₃N₄ on catalytic performance; (3) key challenges and limitations in current research; and (4) prospects for future material design, device integration, and application development. This study aims to offer a theoretical foundation and research reference to facilitate the development of a new generation of efficient, environmentally friendly, and cost-effective photocatalytic H₂O₂ technologies.

2.1　Doping

Doping represents a straightforward yet effective approach for optimizing the performance of g-C₃N₄ in photocatalytic application. Introducing metal or non-metal dopants can significantly modulate the electronic structure of the material, thereby improving its catalytic activity [118-125]. Such modifications induce profound changes in the electronic configuration through interactions between dopant atoms and the carbon/nitrogen framework [126-129]. Specifically, doping narrows the band gap by introducing in-gap states or facilitating orbital hybridization, while also tuning the energies of the valence band maximum (VBM) and conduction band minimum (CBM) to optimize redox potentials. Furthermore, the density of states near the Fermi level is altered, enhancing charge carrier density and conductivity. Notably, the reconstruction of the surface electronic structure promotes the adsorption of reactants and reduces the activation energy barriers for key reaction intermediates. Based on the nature of the incorporated elements, doping strategies for g-C₃N₄ can be categorized into non-metal doping, metal doping, and co-doping. Each approach contributes synergistically to enhance light absorption, facilitate charge separation, and improve interfacial catalytic efficiency. These mechanisms have been corroborated through comprehensive experimental analyses, including spectroscopic characterization, transient photoelectrochemical techniques, and detailed reaction pathway studies.

2.1.1　Non-metal doping

2.1.1.1　Carbon doping

Carbon doping is commonly achieved by increasing the C/N ratio within the g-C₃N₄ framework or introducing carbon-rich organic precursors to partially substitute nitrogen sites. This approach enhances the π-electron density and increases the density of states in the conduction band, thereby facilitating photogenerated electron transfer toward the oxygen reduction pathway. Additionally, the incorporation of carbon can lead to the formation of C-terminal defects and non-conjugated groups, which further modulate the conduction band edge-promoting O₂ adsorption and stabilizing reactive intermediates. These modifications collectively contribute to an extended charge carrier lifetime.

For example, Yang et al. engineered dual defects-namely carbon doping and surface hydroxyl (-OH) modification-by adjusting the acetylacetone to urea precursor ratio [130]. The optimized sample A_0.05CN demonstrated a H₂O₂ production rate of 3.6 mmol g⁻¹ h⁻¹ under visible light irradiation (λ ≥ 420 nm), which is 3.5 times greater than that of pristine g-C₃N₄ (Figs. 3a-d). Additionally, the selectivity increased from 60% to 85%. Integrated experimental and theoretical investigations indicated that carbon doping enhanced oxygen adsorption and activation at N₂C sites, whereas surface hydroxyl groups promoted charge carrier diffusion and inhibited recombination. The observed narrowing of the band gap to 2.56 eV, the enlarged surface area, and a significant electron transfer number (n ≈ 2.0) collectively corroborate the improved two-step single-electron oxygen reduction mechanism.

In another approach, Zhang et al. synthesized carbon-doped, defect-rich g-C₃N₄ (s-B≡CN) via supramolecular pre-assembly and thermal polymerization [131]. Incorporating barbituric acid yielded a nest-like porous structure enriched with N vacancies and –C≡N groups, which broadened visible light absorption and improved charge separation. The material achieved an H₂O₂ production rate of 108.4 µmol/L within 2 h in pure water, attributed to the synergistic effect of strengthened 2e⁻ ORR (via ·O₂⁻) and hole-driven water oxidation. Furthermore, the in-situ generated H₂O₂ enabled efficient degradation of 4-chlorophenol via a photo-self-Fenton process, highlighting potential practical applications. Xu et al. employed a co-modification strategy using lignin and KOH to embed carbon rings and induce nitrogen defects (cyano groups and N vacancies) in KLCN (Figs. 3e-g) [132]. This synergistic incorporation resulted in an H₂O₂ yield 6.93 times greater than that of the unmodified catalyst, with mechanistic studies confirming the critical role of ·O₂⁻ intermediates in the two-step single-electron reduction pathway.

Current studies have demonstrated that carbon doping enhances the photocatalytic activity of g-C₃N₄ through band structure engineering, improved charge carrier kinetics, and enhanced surface adsorption of reactants. A key advantage is the concurrent activation of both oxygen reduction and water oxidation pathways, providing a versatile strategy for designing efficient photocatalytic systems for H₂O₂ production.

2.1.1.2　Oxygen doping

Wei et al. developed an oxygen-enriched carbon nitride polymer (OCN) with enhanced photocatalytic performance for H₂O₂ generation (Figs. 3h-k) [133]. DFT simulations indicated that incorporating C-O-C groups considerably lowered the energy barrier for forming 1,4-endoperoxide intermediates by 10.5 kcal/mol compared to -NH₂ groups, thus favoring selective oxygen reduction. The material, labeled OCN-500, was synthesized through calcination at 500 ℃ in air using dicyandiamide and ammonium paratungstate as precursors. This process introduced C-O-C and -OH functional groups, increased the specific surface area, and narrowed the band gap. Under O₂-saturated conditions in pure water, OCN-500 exhibited an H₂O₂ production rate of 730 µmol over 5 h, representing a 3.5-fold enhancement over unmodified g-C₃N₄. The catalyst also demonstrated high apparent quantum yields (28.5% at 365 nm and 10.2% at 420 nm) and exceptional stability over 20 h. RDE and EPR analyses corroborated the dominance of the two-electron oxygen reduction pathway (n = 2.54) and the presence of radical intermediates (ROO·/RO·), respectively, with the proposed 1,4-endoperoxide mechanism further supported by DFT.

2.1.1.3　Phosphorus doping

A dual-site phosphorus doping strategy was introduced by Yu et al. to markedly improve the photocatalytic H₂O₂ production activity of g-C₃N₄ (Figs. 3l-o) [134]. Through thermal polymerization, P atoms were selectively incorporated at both bay and corner sites of the heptazine framework, effectively breaking electron localization symmetry and reducing exciton binding energy. This resulted in an exciton dissociation efficiency surge from 0.59% to 11.9%, alongside a 13-fold increase in charge carrier concentration and a 7-fold mobility improvement. The optimized catalyst (PCN-D) achieved an H₂O₂ generation rate of 413.85 µmol L⁻¹ h⁻¹ under visible light which is 6 times that of pristine g-C₃N₄ and performed robustly in both pure water and seawater. Mechanistic studies revealed that corner-site P doping enhances O₂ adsorption and activation, while bay-site doping facilitates charge delocalization and separation, collectively promoting an efficient 2e⁻ ORR.

2.1.1.4　Boron doping

Ma et al. employed boron doping to construct frustrated Lewis pairs (FLPs) within the carbon nitride matrix, significantly boosting H₂O₂ production (Figs. 4a-d) [135]. The FLPs consist of boron Lewis acid sites and nitrogen Lewis base sites adjacent to cyano groups. The introduced B atoms created spin-polarized empty orbitals, improving visible light absorption and charge separation. The FLP structure preferentially activated ethanol via a "push–pull" mechanism rather than direct oxygen reduction. DFT studies indicated a reduction of the ethanol dissociation barrier to 0 eV, leading to H₂O₂ formation through synergistic action of ·O₂⁻ and ¹O₂ intermediates. In a 10% ethanol solution, the catalyst (BCN-450) attained a remarkable H₂O₂ production rate of 51008 µmol L⁻¹ g⁻¹ h⁻¹, 12.4 times that of the baseline, with consistent performance over five cycles. In-situ IR spectroscopy confirmed ethanol activation to aldehyde species, underscoring the role of FLPs in altering reaction selectivity and pathway efficiency.

2.1.1.5　Sulfur doping

Zhang et al. concurrently introduced sulfur dopants and nitrogen vacancies into g-C₃N₄ via hydrothermal etching with ammonium persulfate, coupled with morphological control to obtain a porous fluffy architecture (Figs. 4e-g) [136]. This dual-defect system synergistically optimized the electronic structure, and the 27-fold enlargement of specific surface area significantly improved O₂ adsorption capacity. The modified catalyst yielded 293.2 µmol/L H₂O₂ within 4 h, 2.4 times higher than pristine g-C₃N₄, with high selectivity for the two-electron pathway. Sulfur doping induced a valence band positive shift, while nitrogen vacancies enhanced charge separation. The porous framework further facilitated mass transfer and exposure of active sites. Nevertheless, the system’s reliance on ethanol as a sacrificial agent-causing a 53% activity drop in its absence-highlights a limitation for practical application and underscores the need for developing sacrificial-agent-free configurations.

2.1.1.6　Multielement doping

Liu et al. prepared boron/phosphorus/sulfur-doped g-C₃N₄ nanotubes (B/P/S-CNT) via a template-free hydrothermal-thermal polymerization synergistic strategy, enabling efficient photocatalytic H₂O₂ synthesis (Figs. 4h-j) [137]. A melamine-cyanuric acid supramolecular self-assembly was used to form a tubular precursor, which was then thermally polymerized at 520 ℃ to obtain a porous nanotube structure (BET specific surface area was 6.7 times that of bulk g-C₃N₄). Its hierarchical pores significantly promoted mass transfer and light harvesting. Simultaneously, the co-doping of non-metallic elements B, P, and S optimized the electronic structure, shifting the conduction band negatively by 0.15 eV and improving charge carrier separation efficiency. The optimal B-CNT sample achieved a maximum H₂O₂ yield of 1830.98 µmol/L, 4 h, twice that of bulk g-C₃N₄. Experimental results confirmed that the activity difference originated from the regulatory strength of doped elements on the reduction potential and the enhanced light reflection and absorption by the tubular structure. The two-electron oxygen reduction pathway dominated the H₂O₂ generation process, and isopropanol as a sacrificial agent provided key protons.

2.1.2　Metal doping

Metal doping introduces foreign metal atoms into the g-C₃N₄ matrix, effectively modulating its electronic structure and surface properties to enhance photocatalytic H₂O₂ production. Habarugira et al. developed a one-step copolymerization approach using sodium dicyanamide to fabricate sodium-doped carbon nitride with nitrogen vacancies (SD-CN) [138]. The introduced sodium disrupted the triazine unit ordering, leading to a wrinkled morphology. Characterization confirmed the coexistence of cyano groups, nitrogen vacancies, and Na incorporation, with the band gap narrowing to 2.75 eV (Figs. 5a-d). The optimized catalyst exhibited an H₂O₂production rate of 297.2 µmol L⁻¹ h⁻¹, which is 9.8 times higher than pristine g-C₃N₄ and obtained an AQE of 1.29% at 420 nm, and 90% stability retention over 8 cycles. Mechanism studies indicated cyano groups aided charge separation, while nitrogen vacancies promoted O₂ adsorption, collectively favoring the two-electron ORR pathway as evidenced by ESR and RDE (n = 1.77). Despite these merits, the low specific surface area (3.40 m²/g) and limited anti-interference ability in real water environments necessitate further improvements in mass transfer and practicality.

Wu et al. employed a eutectic salt ionothermal etching strategy (NaCl/KCl) to co-incorporate alkali metals (K⁺/Na⁺) and nitrogen vacancies into g-C₃N₄ (Figs. 5e-g) [139]. The dual modification narrowed the band gap from 2.85 eV to 2.63 eV and enhanced charge separation efficiency by 6.2 times. The best-performing ACCN sample achieved a remarkable H₂O₂ yield of 10.2 mmol g⁻¹ h⁻¹, 89.5 times that of the pristine material, with an apparent rate constant increased by 149 times and an AQE of 30.7% at 429 nm. Nitrogen vacancies functioned as electron traps to facilitate charge separation, while alkali metal doping optimized the band structure via charge compensation from -O^⁻ and -C≡N groups. Nevertheless, the system’s dependency on isopropanol as a sacrificial agent and modest cycling stability represent challenges for sustainable application.

Chen et al. designed a cyano-rich, honeycomb-structured carbon nitride (C_v@g-C₃N₄) via a NaCl-assisted calcination route (Figs. 5h-o) [140]. The material concurrently introduced cyano defects and sodium doping, which reduced the band gap through newly formed defect levels and enhanced O₂ adsorption via improved charge polarization. A NaN₃-like coordination environment further promoted carrier separation and stabilized superoxide intermediates. Combined with a porous morphology for improved surface area, the catalyst delivered an H₂O₂ production rate of 7.01 mmol L⁻¹ h⁻¹ under visible light-220 times higher than conventional g-C₃N₄-with an AQE of 9.58% at 420 nm. It also demonstrated broad pH adaptability (3–12) and good cycling stability, maintaining 5 mmol L⁻¹ h⁻¹ after 5 cycles.

Photocatalytic activity is considered the cumulative result of surface reactions between active sites on the catalyst and reactants. However, identifying the dynamic structural evolution of active sites during photocatalytic surface reactions and clarifying the corresponding catalytic enhancement mechanisms remain major challenges. Single-atom photocatalysts (SAPs), featuring well-defined single-atom active sites and high atomic utilization, serve as ideal catalytic models to explore the structural evolution of active sites and reaction mechanisms in depth [141,142].

Zhang et al. anchored Ni single atoms in a porous carbon nitride matrix (Ni-SAPs/PuCN) with a high loading of 12.5 wt% in Ni-N₃ coordination (Figs. 5t-v) [143]. The catalyst delivered an H₂O₂ production rate of 342.2 µmol L⁻¹ h⁻¹ in pure water, with an AQY of 10.9% and an SCC efficiency of 0.82%. In-situ synchrotron X-ray absorption and Raman spectroscopy revealed the dynamic evolution of Ni sites from Ni-N₃ to O₁-Ni-N₂ and finally to HOO-Ni-N₂ during reaction, which reduced the energy barrier for ^*OOH formation and suppressed O-O bond cleavage. The Ni atoms also optimized the host electronic structure, enhancing charge separation and migration.

In a recent advance, Su et al. developed an antimony single-atom catalyst (Sb-SAPC) with Sb³⁺ (4d¹⁰5s²) atoms atomically dispersed in a polymeric carbon nitride framework via Sb-N bonds [144]. The unique electronic configuration prevented midgap states and facilitated charge separation. Under visible light and without sacrificial agents, the catalyst achieved an AQY of 17.6% at 420 nm, an SCC efficiency of 0.61%, and an H₂O₂ production rate 248 times that of the pristine material. The Sb single atoms adsorbed O₂ in an end-on configuration, forming an Sb-OOH intermediate that promotes the 2e⁻ ORR pathway, while simultaneously accelerating the 4e⁻ WOR through hole accumulation on adjacent N atoms. The in-situ produced O₂ from WOR was rapidly consumed via ORR, creating a synergistic cycle that enhanced overall kinetics.

2.1.3　Co-doping of metal and non-metal element

Heteroatom co-doping can create synergistic effects between metal and non-metal elements, further optimizing the electronic and catalytic properties of g-C₃N₄. Zhang et al. constructed a sulfur and potassium co-doped catalyst (AKMT) using thiourea/melamine precursors treated with NaOH/KCl (Figs. 5p-s) [145]. The dopants disrupted the layered structure, generating abundant edge cyano and surface hydroxyl groups. Sulfur was incorporated via C-S and N-S bonds, while potassium formed N-K coordination complexes, collectively red-shifting the absorption edge to 500 nm (band gap 2.52 eV) and positively shifting the valence band to 1.89 eV. Under acidic and O₂-saturated conditions, the H₂O₂ yield reached 4.1 mmol/L within 3 h, with a notable AQE of 100% at 450 nm and 96% electron selectivity. Transient infrared spectroscopy revealed that sulfur doping extended the lifetime of shallowly trapped electrons and directed deeply trapped electrons toward oxygen reduction. The catalyst also demonstrated potential for practical disinfection applications through in situ H₂O₂ generation in tap water, though activity decay in alkaline conditions remains an issue.

In a related study, Wang et al. reported a one-pot synthesis of phosphorus-sodium co-doped -C₃N₄ (Na_1.2-P_0.2-CN) [146]. Phosphorus substitution formed N-P-N bonds, while sodium intercalation induced cyano group formation and valence band shifts. DFT simulations illustrated that charge density redistribution established a built-in electric field, and transition state calculations confirmed a reduced energy barrier for ^*O₂ formation. With isopropanol as a sacrificial agent, the H₂O₂ yield reached 3001.64 µmol g⁻¹ L⁻¹ within 100 min, surpassing the undoped catalyst by 62 times.

2.2　Defect modification

2.2.1　Surface defect engineering

Defects and vacancies, as crucial regulatory units of the electronic structure and surface properties of catalysts, exert a concentration- and type-dependent dual influence on catalytic activity [147]. Typically, precise regulation of defect/vacancy type, concentration, and distribution to balance activity, selectivity, and stability remains a research focus in catalyst engineering. Surface defect engineering is an effective approach to enhance the photocatalytic efficiency of carbon nitride by addressing its inherent limitations, such as restricted visible-light absorption and rapid charge carrier recombination [59,148]. Introducing surface defects can achieve band structure modulation via defect-induced energy levels that extend light absorption into the near-infrared region and facilitate carrier separation. Defect sites serve as active centers can optimize reactant adsorption and electron migration, enhancing reaction kinetic.

2.2.1.1　Carbon defects

Wu et al. employed an oxalic acid etching-assisted self-assembly method to synthesize flower-like carbon nitride with carbon defects (GCN) (Figs. 6a-d) [149]. Using melamine in DMSO followed by etching and calcination at 550 ℃, a porous microstructure with a diameter of 1.2 µm was obtained. The resulting material exhibited a reduced C/N ratio (0.78) and a 1.8-fold stronger EPR signal, confirming enhanced carbon defects that improved O₂ adsorption and promoted the two-electron oxygen reduction pathway, as evidenced by an electron transfer number of 2.50 in RDE tests. The H₂O₂ production rate reached 21.59 mmol L⁻¹ g⁻¹ h⁻¹, five times that of bulk g-C₃N₄. When applied in a self-Fenton system, it efficiently degraded 2,4-dichlorophenol with a rate constant of 0.070 min⁻¹. Mechanism studies revealed that carbon defects increased carrier separation efficiency, raising the photocurrent density by 300% and the photoluminescence quenching rate by 60%. The in-situ generated H₂O₂ reacted with Fe²⁺ to produce ·OH, which was confirmed by ESR, and the system showed over 90% stability in tap water across four cycles, demonstrating practicality for pollutant degradation.

2.2.1.2　Nitrogen defects

Nitrogen defects also play a critical role in modulating the electronic and catalytic properties of g-C₃N₄. Li et al. optimized the electronic structure and spin density distribution of carbon nitride (SCN_x) via a combined strategy of sulfur doping and edge group modification, enabling efficient photocatalytic H₂O₂ synthesis without sacrificial agents (Figs. 6e-h) [150]. The material exhibited a high surface area (163 m²/g), a negatively shifted conduction band (-0.92 V), and a narrowed band gap (2.48 eV). Without sacrificial agents, it achieved an H₂O₂ production rate of 416.7 µmol g⁻¹ h⁻¹ under visible light (λ ≥ 420 nm), 10 times that of pristine g-C₃N₄, with an apparent quantum yield of 31.6% at 365 nm. In-situ IR and theoretical analyses revealed that edge functionalization enhanced O₂ adsorption and reduced the energy barrier of ^*OOH formation, underscoring the role of spin density localization in promoting reaction kinetics. The H₂O₂ solution (624.8 µmol/L) generated by this catalyst in pure water could efficiently degrade rhodamine B (5 mg/L, > 95% removal within 5 min) and inactivate Escherichia coli, providing a new insight for the design of spin-regulated photocatalysts.

Zeng et al. constructed a carbon nitride with multiple active sites (BPMC-Vs) via a synergistic strategy of boron-phosphorus dual doping and nitrogen defects, significantly enhancing the efficiency of photocatalytic H₂O₂ synthesis (Figs. 6i-o) [151]. The cooperation of P/B atoms and defects increased local proton coverage and reduced the ^*OOH energy barrier from 1.121 eV to 0.767 eV. Under visible light and sacrificial agent-free conditions, the catalyst achieved an H₂O₂ production rate of 465 µmol g⁻¹ h⁻¹ driven by visible light (λ ≥ 420 nm), with a solar-to-chemical conversion efficiency of 0.33% and a selectivity of 95.2%. Key contributions included enhanced proton adsorption (via P and -C≡N), improved O₂ activation (via B and N vacancies), and facilitated charge separation, as confirmed by photoelectrochemical and spectroscopy results.

Zhang et al. developed a dual-defect catalyst (Nv-C≡N-CN) incorporating both nitrogen vacancies and cyano groups (Figs. 7a-g) [152]. This combination resulted in an electron-rich structure with localized charge distribution, significantly enhancing light absorption, charge separation, and H₂O₂ production, which reached 3093 µmol g⁻¹ h⁻¹ with an apparent quantum yield of 36.2% at 400 nm. The nitrogen vacancies facilitated O₂ adsorption and activation, while the cyano groups promoted proton adsorption, collectively improving H₂O₂ formation kinetics.

2.2.2　Defect suppression

Enhancing bulk crystallinity represents another essential route to improve photocatalytic performance by reducing internal defects and suppressing charge recombination. Highly crystalline materials possess ordered atomic arrangements and fewer trap states, thereby promoting the migration of photogenerated carriers to surface reaction sites.

Chen et al. proposed an anti-defect engineering strategy to synthesize high-crystallinity carbon nitride (CNT) via a molten salt method (Figs. 7h-m) [153]. The material showed well-defined lattice fringes (1.742 nm) and a defect concentration 1.8 times lower than conventional g-C₃N₄. These structural merits led to a carrier separation rate three times higher, a 76% reduction in radiative recombination, and an extended carrier lifetime of 3.27 ns. The H₂O₂ production rate attained 2.48 mmol g⁻¹ h⁻¹ with an AQE of 22% at 400 nm 5.8 times higher than that of traditional carbon nitride. Rotating Disk Electrode (RDE) tests confirmed enhanced selectivity for the two-electron oxygen reduction pathway (electron transfer number n = 1.73), and the conduction band potential shifted negatively to -1.23 V (vs. NHE), promoting O₂ reduction kinetics. The in-situ generated H₂O₂ further enabled efficient tetracycline degradation via a Fenton-like process, achieving 84% removal within 3 h.

Zhong et al. synthesized a potassium-doped highly crystalline g-C₃N₄ photocatalyst (CN-K) using a KI-assisted recrystallization strategy. The as-prepared CN-K exhibited higher interplanar crystallinity, a narrower band gap, and smaller particle size (Figs. 7n-r) [154]. Potassium atoms acted as electron acceptors, shifting the conduction band negatively by 0.35 eV and enhancing the O₂ adsorption energy to -0.93 eV. The CN-K (1:6) sample achieved a photocatalytic H₂O₂ production rate of 7.8 mmol L⁻¹ h⁻¹, 220 times higher than that of traditional carbon nitride.

In summary, defect engineering-whether through introducing beneficial surface defects or suppressing bulk defects-offers powerful strategies for enhancing charge behavior and surface reactions, providing a robust foundation for designing highly efficient photocatalysts.

2.3　Junction engineering

Heterojunctions boost catalytic activity through a built-in electric field that separates charges efficiently, while band bending guides charge transfer and blocks recombination [155-160]. New interfacial active sites lower reaction barriers, and tailored electronic structures optimize reactant adsorption, collectively enhancing performance [161-166]. In photocatalytic filed the catalyst composed of two or more semiconductors with tailored band alignments will enhance photocatalytic activity primarily through the formed built-in electric field at the interface, which can significantly suppress non-productive charge recombination [105,167-172]. The combining of a narrow-bandgap semiconductor enables more efficient utilization of solar energy compared to single semiconductors. Finally, the separated electrons and holes retain sufficient reducing and oxidizing capabilities respectively, meeting the energy requirements for target reactions that a single catalyst might fail to achieve. These combined effects collectively boost the overall photocatalytic performance.

2.3.1　Heterojunction

Li et al. successfully constructed a Bi₄Ti₃O₁₂/g-C₃N₄ (BTOCN) inorganic/organic S-scheme heterojunction photocatalyst via an electrospinning-assisted ultrasonic assembly strategy; this composite system formed a tightly contacted S-scheme heterojunction through interface engineering, which significantly optimized carrier kinetic behavior, prolonged carrier lifetime, and notably enhanced the generation of ·O₂⁻ active intermediates and the two-electron oxygen reduction pathway (Figs. 8a-e) [173]. Under simulated sunlight irradiation, the optimized BTOCN2 sample achieved an H₂O₂ production rate of 260 µmol g⁻¹ h⁻¹ in pure water, 2.8 times higher than that of single g-C₃N₄, with an apparent quantum yield of 0.36% at 420 nm, and retained 93% of its activity after 10 cycles.

In another work, Zhang et al. in-situ grew copper indium sulfide (CuInS₂) nanosheets on the surface of polymeric carbon nitride (PCN) via a low-temperature hydrothermal method, successfully constructing a CuInS₂/PCN photocatalyst with an S-scheme heterojunction structure (Figs. 8f-i) [174]. The well-dispersed CuInS₂ and tight interfacial contact induced electron transfer from CuInS₂ to PCN due to their Fermi level difference, forming a built-in electric field. Under visible light, recombination between PCN electrons and CuInS₂ holes preserved electrons with high reducing ability, contributing to an H₂O₂ production rate of 1247.6 µmol g⁻¹ h⁻¹ in 5% isopropanol (AQY = 16.0% at 420 nm), which is 11.6 times and 16.0 times that of individual PCN and CuInS₂, respectively.

2.3.2　Homojunction

Homojunctions, composed of the same material with varying phases or morphologies, offer superior lattice matching and lower charge migration barriers than heterojunctions, showing great potential in facilitating charge separation for photocatalytic applications [175-178]. For instance, in a study by Chen et al., a carbon nitride homojunction (homo-CN) was constructed via a hydroxyl-induced strategy (Figs. 9a-d) [179]. By in-situ growing a hydroxyl-rich carbon nitride (HR-CN) nanolayer on hydroxyl-defective carbon nitride (HD-CN) nanosheets, the synergistic optimization of bulk charge separation and surface reverse reactions was achieved. This design adjusted the band structure by regulating hydroxyl concentration, creating a gradient driving force to facilitate electron-hole separation and extending the light absorption edge. Surface hydroxyl groups provided additional electron-accepting bands, enabling efficient extraction of electrons from the bulk to the surface; meanwhile, they enhanced the adsorption energy of ^*OOH intermediates and increased the overpotential of the 4-electron oxygen reduction reaction, making the 2e⁻ ORR kinetically more favorable and thus inhibiting H₂O₂ decomposition. Performance tests showed that the H₂O₂ production rate of this homojunction was 4.5 times that of pristine carbon nitride, with an apparent quantum yield of 2.1% at 500 nm in pure water.

Zhou and his collaborators constructed a novel dual-channel carbon nitride homojunction composite catalyst by incorporating cellulose-derived carbon nanofibers (CF) into the homojunction formed by carbon nitride nanotubes (MCN) and carbon nitride nanosheets (SCN) (Figs. 9e-g) [180]. The MCN/SCN/CF composite exhibited a dual-channel H₂O₂ synthesis mechanism: ORR on the conduction band and WOR on the valence band. The unique structure, coupled with improved light absorption and charge separation, resulted in an H₂O₂ generation rate of 136.9 µmol L⁻¹ h⁻¹ without sacrificial agents, with an AQY of 4.57% and solar-to-chemical efficiency of 0.57%, which was 5 times that of MCN alone.

Zhang’s team first fragmented bulk g-C₃N₄ into smaller pieces (CN-NH₄) using a soft template strategy, then subjected them to molten salt treatment for directional splicing, resulting in a homojunction (CN-NH₄-NaK) with multiple ordered-disordered interfaces (Fig. 9h) [181]. The material featured coexisting crystalline and amorphous regions, abundant cyano groups, and a built-in electric field that enhanced charge separation. It achieved an exceptional H₂O₂ production rate of 16675 µmol g⁻¹ h⁻¹ and over 91% selectivity across a wide pH range, which is 158 times that of pristine g-C₃N₄. The crystalline domains facilitated H₂O₂ production via a two-step single-electron ORR, while the amorphous regions utilized in-situ oxygen from water oxidation through a direct two-electron pathway.

In Khan et al.’s study, an S-scheme homojunction (B-CN/P-CN) based on boron-doped carbon nitride (B-CN) and phosphorus-doped carbon nitride (P-CN) was constructed for efficient photocatalytic H₂O₂ synthesis (Figs. 9i-l) [182]. The work function difference [183] drove interfacial charge transfer, forming a built-in electric field, while B-P bonds acted as charge migration bridges. The optimal composite showed an H₂O₂ production rate of 2199.5 µmol L⁻¹ h⁻¹ under 365 nm irradiation, outperforming pristine CN by 2-8 times.

Foo et al. synthesized a heptazine/triazine-based crystalline homojunction (CCN-X) via one-step ionothermal polymerization (Figs. 9m-p) [184]. KCl and LiCl promoted the formation of cyano/hydroxy groups and triazine units, respectively, while K⁺ and Li⁺ intercalation enhanced interlayer charge transport. The catalyst simultaneously produced H₂O₂ and benzaldehyde with high yields (5838.91 and 7041.32 µmol L⁻¹ h⁻¹, respectively), 100% selectivity, and an AQY of 11.57% at 420 nm.

2.3.3　Cocatalyst modification

In photocatalytic H₂O₂ synthesis, the efficient formation of H₂O₂ relies critically on the activation of O₂ or H₂O molecules through well-designed surface sites that lower reaction energy barriers and steer selectivity toward the desired two-electron pathway [185,186]. Beyond intrinsic strategies such as doping and defect engineering, which introduce electron-rich or electron-deficient centers, recent efforts have focused on constructing extrinsic active sites via anchored clusters or single atoms [187-189]. These approaches preserve the structural integrity of carbon nitride while improving bulk charge migration and interfacial reactivity through synergistic effects.

For instance, Hou et al. constructed a photocatalyst (CoO_x-NvCN) co-modified with surface nitrogen vacancies and CoO_x nanoclusters on polymeric carbon nitride (CN) via a two-step method to enhance the performance of 2e⁻ ORR for H₂O₂ production (Figs. 10a-d) [190]. The nitrogen vacancies optimized the electronic structure to facilitate charge separation, while the CoO_x nanoclusters served as high-activity sites that enhanced O₂ adsorption and reduced the energy barrier for ^*OOH formation. This dual modification resulted in a catalyst capable of producing H₂O₂ at a rate of 244.8 µmol L⁻¹ h⁻¹ in pure water without sacrificial agents, with an apparent quantum yield (AQY) of 5.73% at 420 nm and a solar-to-chemical conversion (SCC) efficiency of 0.47%, ranking among the top-performing carbon nitride-based systems.

In another study, Jing’s team designed a boron-doped carbon nitride (BCN) photocatalyst (CoO_x-BCN-FeOOH) modified with dual active sites: Coordinatively unsaturated FeOOH and CoO_x clusters (Figs. 10e-k) [191]. The boron doping optimized the band structure, while the CoO_x clusters promoted water oxidation and extended charge lifetime, and the unsaturated FeOOH clusters facilitated electron acceptance and O₂ activation. The catalyst achieved an H₂O₂ production rate of 0.34 mmol g⁻¹ h⁻¹ (30 times that of pristine g-C₃N₄, an SCC efficiency of 0.75% and an AQY of 8.36% at 420 nm). Notably, the FeOOH sites enabled Pauling-type O₂ adsorption, stabilized peroxide intermediates, and increased selectivity for the 2e⁻ ORR pathway to 85%.

3　Summary and outlook

Less

Due to its distinctive electronic structure, responsiveness to visible light, and metal-free composition, g-C₃N₄ holds significant promise in the photocatalytic synthesis of H₂O₂. Comprehensive investigations into oxygen reduction pathways-including the two-electron 2e⁻ ORR, ·O₂⁻ mediated pathway, and ¹O₂-mediated pathway-have elucidated that the formation and stability of critical intermediates such as ·O₂⁻, ^*OOH, and ¹O₂ are pivotal determinants of H₂O₂ selectivity. The photocatalytic performance of g-C₃N₄ is collectively influenced by its band structure, surface proton coverage, and the nature of its defects.

Advancements in the mechanistic understanding have facilitated the implementation of various modification strategies, including defect engineering, elemental doping, construction of heterojunctions and homojunctions, and surface-active site modification. These approaches effectively tailor the band structure, enhance light absorption, promote charge carrier separation, optimize oxygen adsorption and activation, stabilize reactive intermediates, suppress H₂O₂ decomposition, improve charge separation efficiency and mass transfer, and reinforce interfacial reaction kinetics and pathway selectivity.

The synergistic integration of these modifications has substantially improved the light-harvesting capability, charge separation efficiency, and surface reaction kinetics of g-C₃N₄, resulting in a marked enhancement of its H₂O₂ production rate under visible light irradiation. Certain systems have demonstrated remarkable stability and applicability across diverse conditions, including pure water, broad pH ranges, and seawater. These advancements have elevated H₂O₂ production rates from trace amounts reported in early studies to the millimole per gram per hour scale, with AQY surpassing 30%, thereby underscoring the potential for transitioning from laboratory-scale research to practical applications.

The band gap of g-C₃N₄, approximately 2.7 eV, constrains its capacity to fully harness visible light, particularly limiting its ability to absorb long-wavelength photons. This limitation impedes further enhancement of solar energy conversion efficiency. Although strategies such as doping, defect engineering, and heterojunction construction have yielded improvements, significant challenges persist due to pronounced electron-hole recombination within the bulk material and at interfaces. Notably, the efficiency of hole utilization in systems devoid of sacrificial agents requires urgent enhancement. Additionally, the competition among multiple reaction pathways-such as 4e⁻ ORR and H₂O₂ decomposition-remains difficult to suppress entirely. The in-situ observation of the dynamic behavior of critical intermediates is also challenging, complicating the rational design of catalysts with high selectivity. Furthermore, most current modification approaches depend on complex precursor designs, high-temperature polymerization processes, and template-assisted synthesis, which collectively hinder the scalable production of g-C₃N₄ materials with uniform structure and stable performance. Moreover, the majority of investigations have been conducted under controlled laboratory conditions, with limited systematic evaluation of the effects of real-world factors such as impurities in natural water sources, pH variability, and fluctuating light conditions on catalytic performance.

Future progress in g-C₃N₄-based photocatalytic hydrogen peroxide synthesis will necessitate interdisciplinary collaboration and systematic design strategies, aiming for synergistic advancements across three domains: Material development, mechanistic understanding, and device engineering. At the material level, integrating theoretical modeling with in situ dynamic characterization techniques is essential to rationally engineer surface structures that exhibit narrowed band gaps, enhanced charge separation, and multi-site synergistic effects. Such design will facilitate precise control over oxygen adsorption, proton transfer, and H₂O₂ desorption processes. Regarding light energy utilization, the development of advanced composite systems is imperative to extend light absorption capabilities into the visible and near-infrared spectra. From the perspectives of synthesis and application, it is critical to establish green, low-temperature, and scalable fabrication methods.

In addition, increasing efforts in computational analysis and materials informatics provide useful guidance for the rational design and scalable preparation of g-C₃N₄-based photocatalysts for hydrogen peroxide production. By systematically examining the relationships among precursor chemistry, polymerization conditions, structural modification strategies, and resulting material properties, it becomes possible to identify key compositional and structural features that are beneficial for efficient H₂O₂ generation. This perspective is particularly relevant to g-C₃N₄ systems, where small variations in synthesis parameters, calcination temperature, atmosphere, or post-treatment methods, can lead to pronounced differences in crystallinity, defect distribution, surface chemistry, and electronic structure. A clearer understanding of these structure-property relationships may help guide targeted material optimization and reduce excessive empirical trial-and-error. With further integration of computational analysis and experimental synthesis, g-C₃N₄ photocatalysts with more controllable structures, improved reproducibility, and better scalability are expected to be developed, which will be beneficial for advancing photocatalytic hydrogen peroxide production toward practical applications. Concurrently, efforts should focus on comprehensive validation and long-term stability assessments of photocatalysis-Fenton coupling systems, flow reactors, and performance under actual water body conditions. Ultimately, these endeavors aim to transition this technology from laboratory-scale research to large-scale, sustainable production and environmental remediation applications.

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doi: 10.1016/j.cclet.2026.112572

Receive Date：2025-12-13
Online Date：2026-10-27
Published：2026-06-15

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Received：2025-12-13
Revised：2026-02-24
Accepted：2026-03-02

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^aSchool of Chemistry & Chemical Engineering and Environmental Engineering, Weifang University, Weifang 261061, China

^bSchool of Chemistry, South China Normal University, Guangzhou 510006, China

^cInstitute for Energy Research, Jiangsu University, Zhenjiang 212013, China

Corresponding:

E-mail addresses: zhuxl@ujs.edu.cn (X. Zhu)

yqlan@m.scnu.edu.cn (Y.-Q. Lan)

zaiyongjiang@qlu.edu.cn (Z. Jiang)

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1 Performance comparison of some modified g-C₃N₄-based photocatalysts for H₂O₂ production.

Authors (year)	Catalyst	Modification strategy	Light source	H₂O₂ production activity	Enhancement factor (vs. pristine CN)	AQY/wavelength	Electron donor	Ref.
Yang et al. (2023)	A_0.05CN	Carbon doping & surface -OH	Visible (λ ≥ 420 nm)	3.6 mmol g⁻¹ h⁻¹	3.5 ×	Not available	Pure water	[130]
Zhang et al. (2024)	s-B_xCN	Carbon doping, N vacancies, -C≡N	Visible light	54.2 µmol L⁻¹ h⁻¹	Not available	Not available	Pure water	[131]
Xu et al. (2025)	KLCN	Lignin-derived carbon ring embedding & N defects	Visible light	266.82 µmol/L	6.93 ×	Not available	Isopropanol	[132]
Wei et al. (2018)	OCN-500	Oxygen doping (C-O-C/—OH)	Visible light	730 µmol / 5h	3.5 ×	28.5%@365 nm, 10.2%@420 nm	Isopropanol	[133]
Yu et al. (2023)	PCN-D	Dual-site P doping (bay & corner)	Visible light	413.85 µmol L⁻¹ h⁻¹	> 6 ×	Not available	Isopropanol	[134]
Ma et al. (2024)	BCN-450	B doping (Frustrated Lewis Pairs)	Visible light	51008 µmol L⁻¹ g⁻¹ h⁻¹	12.4 ×	Not available	Ethanol	[135]
Zhang et al. (2023)	CN-1.0	S doping & N vacancies	Xenon lamp	293.2 µmol/L / 4 h	2.4 ×	Not available	Ethanol	[136]
Liu et al. (2021)	B-CNT	B/P/S co-doping, nanotube	Simulated sunlight	1830.98 µmol/L / 4 h	2 ×	Not available	Isopropanol	[137]
Habarugira et al. (2024)	10SD-CN	Na doping & N defects	Visible light	297.2 µmol L⁻¹ h⁻¹	9.8 ×	1.29%@420 nm	Isopropanol	[138]
Wu et al. (2020)	ACNN	K⁺/Na⁺ doping & N vacancies	Visible (λ ≥ 420 nm)	10.2 mmol g⁻¹ h⁻¹	89.5 ×	30.7%@429 nm	Isopropanol	[139]
Chen et al. (2021)	5Cv@g-C₃N₄	Cyano defects & Na doping, porous	Visible (λ ≥ 420 nm)	7.01 mmol L⁻¹ h⁻¹	220 ×	9.58%@420 nm	Ethanol	[140]
Zhang et al. (2020)	AKMT	S/K co-doping	Visible light	4.1 mmol/L / 3 h	Not available	100%@450 nm	Ethanol	[145]
Wang et al. (2024)	Na_1.2-P_0.2-CN	P/Na co-doping	Visible light	3001.64 µmol g⁻¹ L⁻¹ / 100 min	62 ×	Not available	Isopropanol	[146]
Wu et al. (2022)	GCN	Carbon defects, flower-like	Xenon lamp (λ > 420 nm)	21.59 mmol L⁻¹ g⁻¹ h⁻¹	5 ×	Not available	Isopropanol	[149]
Zhang et al. (2022)	Nv-C≡N-CN	N vacancies & -C≡N groups	Visible (λ ≥ 420 nm)	3093 µmol g⁻¹ h⁻¹	8.7 ×	36.2%@400 nm	Isopropanol	[152]
Li et al. (2023)	SCN2	S doping, -NH₂/-CH₃ edge groups	Visible (λ ≥ 420 nm)	416.7 µmol g⁻¹ h⁻¹	10 ×	31.6%@365 nm	Pure water	[150]
Zeng et al. (2025)	BPMC-Vs	B/P co-doping & N defects	Visible (λ ≥ 420 nm)	650.7 µmol g⁻¹ h⁻¹	9.9 ×	39.1%@420	Isopropanol	[151]
Chen et al. (2023)	CNT	Crystallinity enhancement (Molten salt)	AM 1.5	2.48 mmol g⁻¹ h⁻¹	5.8 ×	22%@ 00 nm	Isopropanol	[153]
Zhong et al. (2024)	CN-K(1:6)	K doping, High crystallinity	LED	7.8 mmol L⁻¹ h⁻¹	220 ×	5.17%@420 nm	Isopropanol	[154]
Li et al. (2024)	BTOCN2	Bi₄Ti₃O₁₂/g-C₃N₄ S-scheme	Simulated sunlight	1650 µmol g⁻¹ h⁻¹	2.8 ×	0.36%@420 nm	Methanol	[173]
Zhang et al. (2023)	3CuInS₂/PCN	CuInS₂/PCN S-scheme	Visible light	1247.6 µmol g⁻¹ h⁻¹	11.6 ×	16.0%@420 nm	Isopropanol	[174]
Chen et al. (2023)	homo-CN	Homojunction (OH-rich/OH-defective)	Xenon lamp (λ ≥ 420 nm)	508.4 µmol L⁻¹ L⁻¹	4.5 ×	2.1%@500 nm	Ethanol	[179]
Zhang et al. (2024)	CN-NH₄-NaK	Homojunction (Order-Disorder)	Visible (λ ≥ 420 nm)	16675 µmol g⁻¹ h⁻¹	158 ×	34.3%@395 nm, 28.4%@420 nm	Isopropanol	[181]
Foo et al. (2025)	CCN-550	Homojunction (PHI/PTI), K⁺/Li⁺, -C≡N	Visible (λ ≥ 420 nm)	5838.91 µmol L⁻¹ h⁻¹	31.7 ×	11.57%@420 nm	Benzyl alcohol	[184]
Hou et al. (2024)	CoO_x-NvCN	N vacancies & CoO_x clusters	Visible (λ ≥ 420 nm)	244.8 µmol L⁻¹ h⁻¹	12.1 ×	5.73%@420 nm	Pure water	[190]
Jing et al. (2024)	CoO_x-BCN-FeOOH	B doping, FeOOH & CoO_x clusters	Visible (λ ≥ 420 nm)	0.34 mmol g⁻¹ h⁻¹	30 ×	8.36%@420 nm	Pure water	[191]
Zhang et al. (2023)	Ni-SAPs/PuCN	Ni single-atom (Ni-N₃)	Visible (λ ≥ 420 nm)	342.2 µmol L⁻¹ h⁻¹	20.7 ×	10.9%@420 nm	Pure water	[143]
Su et al. (2021)	Sb-SAPC15	Sb single-atom (Sb-N)	Visible (λ > 420 nm)	6.2 mg L⁻¹ h⁻¹	248 ×	17.6%@420 nm	Pure water	[144]

Fig.1 (a) Schematic diagram of the basic principle of photocatalysis. (b) Photocatalytic preparation of H₂O₂ reaction. (c) The modification strategies schematic of g-C₃N₄.

Fig.2 The number of articles on photocatalytic H₂O₂ generation searched from web of science from 2015 to 2025.

Fig.3 (a) Schematic diagram of reaction mechanism. (b) The H₂O₂ production under visible light irradiation. (c) Effects of reactive species scavengers. (d) Schematic illustration of ORR and WOR in CN and A_0.05CN. Reproduced with permission [130]. Copyright 2023, American Chemical Society. (e) The preparation technical route. (f) Photocatalytic production of H₂O₂ under simulated sunlight. (g) Free energy diagrams of ORR steps for MCN and KLCN. Reproduced with permission [132]. Copyright 2025, Elsevier B.V. (h) The mechanism of OCN model (C-O-C) photocatalytic O₂ reduction. (i) The photocatalytic generation of H₂O₂ in O₂ saturated pure water under visible light. (j) The photocatalytic generation of H₂O₂ with 10% isopropyl alcohol under visible light irradiation with air atmosphere. (k) The photocatalytic generation of H₂O₂ with 10% isopropyl alcohol under different atmosphere and pH under visible light irradiation. Reproduced with permission [133]. Copyright 2018, Royal Society of Chemistry. (l) Schematic diagram of the PCN-D sample synthesis. (m) Time course of photocatalytic H₂O₂ generation of PCN-D samples in 10% IPA solution, and (n) in pure water. (o) Wavelength-dependent AQY of PCN-D-1 sample. Reproduced with permission [134]. Copyright 2023, Elsevier B.V.

Fig.4 (a) Electron density, spin density, HOMO, and LUMO of BCN. (b) Energy barriers (E_a) of the rate-determining steps on various samples. (c) Time-dependent photocatalytic H₂O₂ production of different CN-based samples. (d) H₂O₂ production rate of BCN-450 with different reductive scavengers. Reproduced with permission [135]. Copyright 2024, American Chemical Society. (e) The possible mechanism for H₂O₂ production over CN-0.5 catalyst, (f) H₂O₂ generation rates of CN, CN-0.1, CN-0.5, and CN-1.0 catalysts. (g) Concentration of the produced H₂O₂ under different conditions. Reproduced with permission [136]. Copyright 2023, Elsevier B.V. (h) Schematic illustration for catalyst fabrication. (i) The optical properties of samples. (j) Photocatalytic H₂O₂ generation performance of different samples. Reproduced with permission [137]. Copyright 2021, Elsevier B.V.

Fig.5 (a) Schematic illustration of SD-CN catalyst structure and H₂O₂ generation. (b) H₂O₂ production rates of different SD-CN and CN samples. (c) Apparent quantum yield (AQY) of 10SD-CN, and (d) the electronic band structure. Reproduced with permission [138]. Copyright 2024, Elsevier B.V. (e) Schematic illustration of the preparation procedure. (f) H₂O₂ production of ACNN and C₃N₄ under visible light irradiation. (g) Formation (k_f) and decomposition (k_d) kinetics constant of H₂O₂ over ACNN and C₃N₄. Reproduced with permission [139]. Copyright 2020, American Chemical Society. (h) Schematic of proposed structure of Cv@g-C₃N₄. (i) The solid-state nuclear magnetic resonances (NMR) spectra ¹³C of g-C₃N₄ and 5Cv@g-C₃N₄. (j) Catalytic performance of Cv@g-C₃N₄. (k) Rate constant over g-C₃N₄ and 5Cv@g-C₃N₄. (l) The apparent quantum yield (AQY). (m) The H₂O₂ production rate with different conditions. (n) H₂O₂ generation rate in the O₂-saturated pure water. (o) The free energy results. Reproduced with permission [140]. Copyright 2021, Wiley. (p) Proposed reaction mechanism of AKMT catalyst. (q) H₂O₂ production performance at pH 3. (r) The AQY of M and AKMT samples. (s) Solid-state nuclear magnetic resonance (NMR) spectra of ¹³C. Reproduced with permission [145]. Copyright 2020, Wiley. (t) Schematic diagram of Ni site structure evolution in the photocatalytic H₂O₂ generation. (u) The time course of H₂O₂ production activities in pure water. (v) Free energy profiles for photocatalytic H₂O₂ evolution reactions. Reproduced with permission [143]. Copyright 2023, Springer.

Fig.6 (a) Possible degradation mechanism. (b) Photocatalytic H₂O₂ generation performance in the presence of IPA. (c) H₂O₂ concertation of BCN and GCN without IPA under visible light irradiation. (d) Photocatalytic decomposition curves. Reproduced with permission [149]. Copyright 2022, Elsevier B.V. (e) UV-visible DRS spectra of CN and SCN_x samples. (f) Estimated band structure. (g) H₂O₂ production under visible light irradiation. (h) AQY for photocatalytic H₂O₂ generation by SCN2. Reproduced with permission [150]. Copyright 2023, Elsevier B.V. (i) Structure model of BPMC-Vs. (j) Free energy results of different samples, time course of H₂O₂ production performance in (k) pure water, and (l) 10% IPA. (m) In situ FT-IR result under light radiation. (n) TPD-O₂ profiles of BCN, PMC, and BPMC-Vs. (o) The wavelength-dependent AQY results. Reproduced with permission [151]. Copyright 2022, American Chemical Society.

Fig.7 (a) UV-vis DRS results and (b) time-resolved photoluminescence of the samples. (c) Solid-state ¹³C MAS NMR spectra. (d) H₂O₂ generation of different samples in pure water. (e) Time course of H₂O₂ production. (f) The wavelength-dependent AQY. (g) Free energy calculation of H₂O₂ evolution reaction. Reproduced with permission [152]. Copyright 2022, Royal Society of Chemistry. (h, i) XRD patterns. (j) ESR g-factor tests. (k) HRTEM image of CNT. (l) Photocatalytic H₂O₂ production performance. (m) Wavelength dependent AQE of CNT. Reproduced with permission [153]. Copyright 2023, American Chemical Society. (n, o) XRD data, (p) photocatalytic H₂O₂ production activities, (q) apparent quantum yield for CN and CN-K. (r) Calculated free energy. Reproduced with permission [154]. Copyright 2024, Chinese Physical Society.

Fig.8 (a) Band structures of BTO and CN. (b) Photocatalytic H₂O₂ production in 10 vol% methanol aqueous solution and (c) pure water. (d) Calculated electron transfer numbers, H₂O₂ selectivity and RRDE curves of BTOCN2. (e) Surface photovoltage results. Reproduced with permission [173]. Copyright 2024, Chinese Physical Society. (f) Photocatalytic H₂O₂ evolution with different catalysts. (g) ESR signals of DMPO-·O₂⁻ under visible light irradiation. (h) Free energy profiles. (i) Schematic illustration of S-scheme. Reproduced with permission [174]. Copyright 2023, Elsevier B.V.

Fig.9 (a) HRTEM of homo-CN. (b) FT-IR spectroscopy, (c) photocatalytic H₂O₂ of homo-CN, HD-CN and HR-CN. (d) AQY of homo-CN in pure water and 10% ethanol. Reproduced with permission [179]. Copyright 2023, Elsevier B.V. (e) Energy band structure diagrams. (f) H₂O₂ evolution rate with water or 0.1 mmol/L PBQ. (g) AQE of MCN/SCN/CF. Reproduced with permission [180]. Copyright 2024, Wiley. (h) Free energy diagrams through an indirect two-step single-electron ORR, Reproduced with permission [181]. Copyright 2024, Wiley. (i) The S-scheme charge transfer mechanism. (j) Band structure of the samples. (k) Photocatalytic H₂O₂ generation. (l) k_f and k_d constant of P-CN/B-CN. Reproduced with permission [182]. Copyright 2025, American Chemical Society. (m) UV-vis measurement and photos of the catalysts. (n) Photocatalytic performance of ACN and CCN-X. (o) AQY results. (p) Schematic of the photogenerated charge transmission. Reproduced with permission [184]. Copyright 2025, Elsevier B.V.

Fig.10 (a) Schematic illustration of the sample synthesis. (b) Photocatalytic H₂O₂ generation rate. (c) Wavelength-dependent AQY of CoO_x-N_vCN. (d) Free energy diagrams of O₂ reduction reaction. Reproduced with permission [190]. Copyright 2024, American Chemical Society. (e) Schematic of the charge transfer and H₂O₂ generation. (f) Photocatalytic H₂O₂ generation performance. (g) H₂O₂ generation rates under different reaction conditions and the influence of different sacrificial agents. (h) H₂O₂ selectivity characterization. (i) In situ DRIFTS of O₂ adsorption over CoO_x-BCN-FeOOH. (j) In-situ DRIFTS of CoO_x-BCN-FeOOH during reaction. (k) Free energy diagrams. Reproduced with permission [191]. Copyright 2024, Springer.

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