Coupling Cationic g-C₃N₄ with Anionic Metal–Oxygen Clusters for Photocatalytic H₂O₂ Production

Abstract

With growing global emphasis on green and low-carbon development, energy-related challenges have attracted heightened scholarly and public attention. Polyoxometalates (POMs)—well-defined anionic metal–oxygen clusters—exhibit exceptional capacity for rapid, reversible multi-electron transfer while maintaining structural integrity. In this study, graphitic carbon nitride (g-C₃N₄) was protonated using hydrochloric acid to yield the cationic derivative g-C₃N₄–H⁺Cl⁻. Subsequently, three nanostructured g-C₃N₄-based composite photocatalysts—denoted as g-C₃N₄-H-MMo₆ (M = Ni, Fe, Co)—were synthesized via electrostatic assembly between the protonated support and POM anions. Comprehensive characterization was conducted to elucidate phase purity, morphology, surface properties, elemental composition, and chemical states. Under visible-light irradiation, photocatalytic H₂O₂ production performance was significantly enhanced: g-C₃N₄-H-NiMo₆ achieved a 3.91-fold increase in activity relative to pristine g-C₃N₄. Catalytic efficiency followed the trend g-C₃N₄-H-NiMo₆ > g-C₃N₄-H-FeMo₆ > g-C₃N₄-H-CoMo₆. Electrochemical tests confirmed the role of MMo₆ (M = Ni, Fe, Co). The photogenerated electrons of g-C₃N₄ are transferred to the anchored POMs and reduce dissolved O₂ to H₂O₂ (O₂ + 2H⁺ + 2e⁻ → H₂O₂), while the photogenerated holes oxidize H₂O to H₂O₂ (2H₂O + 2h⁺ → H₂O₂ + 2H⁺), achieving sacrificial-agent-free H₂O₂ photosynthesis with an overall reaction of O₂ + 2H₂O → 2H₂O₂. Collectively, electrostatic immobilization of POMs onto protonated g-C₃N₄ affords robust, high-performance photocatalysts with simultaneously improved activity, stability, and structural fidelity.

1. Introduction

Currently, the excessive exploitation of traditional fossil fuels such as oil and natural gas is exacerbating environmental pollution and global warming, posing increasingly severe challenges to global emission reduction efforts. Reducing carbon emissions and developing a low-carbon economy have become formidable tasks for the future development of the world. In 2020, China proposed the “30·60” dual carbon goals to actively address climate issues [1].

H₂O₂ possesses characteristics such as high chemical reactivity, strong oxidizing-ability, and high energy density, making it widely applicable in numerous fields—including chemical synthesis, sterilization and disinfection, wastewater treatment, paper bleaching, and fuel cells [2]. Current H₂O₂ production faces two challenges: (i) The anthraquinone process employs noble metals such as Pd or Au-Pd as catalysts and involves multi-step reactions—including hydrogenation, oxidation, extraction, and solvent circulation—characterized by high energy consumption and susceptibility to contamination, rendering it a non-green production method. (ii) Direct synthesis of hydrogen peroxide from H₂ and O₂ poses an explosion hazard due to the formation of flammable H₂/O₂ mixtures. Therefore, developing feasible catalytic systems and efficient, low-cost, stable catalysts for H₂O₂ production under mild conditions is highly imperative [3]. The photocatalytic oxygen reduction reaction (ORR) for H₂O₂ synthesis requires only H₂O and O₂ as reactants and solar energy as the sole energy input in the presence of a photocatalyst. It offers advantages such as mild reaction conditions, environmentally benign operation, operational simplicity, and low energy consumption, aligning closely with sustainability objectives. As a clean chemical platform, H₂O₂ can serve as a key enabler for the global low-carbon energy transition and represents a viable pathway toward achieving the “dual carbon” goals [4].

The demand for green H₂O₂ production has driven extensive research into photocatalytic synthesis. Graphitic carbon nitride (g-C₃N₄) is a graphite-like layered polymer whose two-dimensional layer is built from tri-s-triazine (heptazine, C₆N₇) units covalently linked through nitrogen bridges (tertiary/secondary N atoms), forming an in-plane C–N network with periodic triangular nanopores; adjacent layers are stacked through weak van der Waals interactions [5]. g-C₃N₄ is a promising metal-free photocatalyst distinguished by its intrinsic visible-light absorption, economical synthesis, abundant raw material availability, and exceptional chemical inertness and thermal robustness under operational conditions. In 2014, Shiraishi et al. first reported the use of g-C₃N₄ for photocatalytic H₂O₂ synthesis [6]. Since then, research on g-C₃N₄ materials in the field of photocatalytic H₂O₂ production has begun to flourish. However, due to its inherent material properties, pure g-C₃N₄ still faces several challenges as a photocatalyst for H₂O₂ generation, including a relatively wide bandgap, low quantum efficiency during photocatalysis, poor separation and transfer efficiency of photogenerated electron–hole pairs, and the facile decomposition of H₂O₂. These drawbacks result in a persistently low efficiency of H₂O₂ production using pure g-C₃N₄, making it difficult to achieve satisfactory performance [7].

To address the aforementioned challenges, researchers have proposed various modification strategies—including elemental doping, surface functionalization, and heterojunction construction—to design highly efficient and stable g-C₃N₄ photocatalysts. For instance, Choi et al. in situ incorporated potassium and phosphate species into the polymeric carbon nitride framework, significantly enhancing H₂O₂ production while suppressing its self-decomposition [8]. Chen et al. reported a porous g-C₃N₄ photocatalyst doped with Na and functionalized with cyano groups; the introduction of cyano groups modulated the band structure of g-C₃N₄ to achieve high activity, whereas Na⁺ facilitated O₂ reduction to generate more ·O₂⁻, thereby improving two-electron (2e⁻) oxygen reduction reaction (ORR) selectivity [9]. Ouyang et al. designed and synthesized a novel Z-scheme heterojunction photocatalyst, which suppressed recombination of photogenerated charge carriers via interfacial electron transfer, enabling efficient photocatalytic H₂O₂ synthesis under ambient conditions [10]. Furthermore, employing cocatalysts—such as those catalyzing the two-electron ORR or water oxidation—provides effective active sites on the catalyst surface, enhances charge carrier migration efficiency, lowers the activation energy of catalytic reactions, improves reaction kinetics, and strengthens interfacial redox processes, thereby promoting charge separation and transfer [11]. For example, Zhang et al. grafted cationic polyimine molecules onto modified g-C₃N₄ via intermolecular electrostatic attraction, altering the surface charge distribution and consequently enhancing the photocatalytic activity of g-C₃N₄ for H₂O₂ generation [12]. These studies demonstrate that both structural modification of g-C₃N₄ and cocatalyst integration are beneficial for improving catalytic activity and selectivity. However, further research is required to elucidate the optimal cocatalyst types, their binding modes with g-C₃N₄, and the underlying enhancement mechanisms.

Polyoxometalates (POMs) are a class of anionic metal–oxide cluster compounds typically constructed from polyhedral metal oxide units (MO_x) in their highest oxidation states (M = W⁷⁺, Mo⁶⁺, V⁵⁺, Nb⁵⁺, and Ta⁵⁺), connected via shared vertices, edges, or faces [13]. POMs offer advantages in integrating diverse molecules and are widely regarded as an excellent platform for developing functional materials. Due to their high solubility in various solvents, once dissolved, POM clusters interact electrostatically with cationic species [14], providing an intrinsic driving force that enables interactions between POMs and cations, molecules, complexes, polymers, and positively charged solid surfaces.

Anderson-type polyoxometalates (POMs) of the MMo₆ class possess anionic structures described by the general formula [MMo₆O₂₄H_x]ⁿ⁻, in which six edge-sharing MoO₆ octahedra form a planar hexagonal ring surrounding a central MO₆ octahedron; this arrangement confers D_3d point-group symmetry to the entire cluster [15]. As stoichiometrically defined bimetallic building blocks, MMo₆ POMs deliver a fixed Mo:M molar ratio of 6:1—facilitating the rational design and reproducible fabrication of bimetallic nanocomposites with precisely controlled composition, crystallinity, and morphology. Furthermore, the central heterometal M can be systematically substituted with various first-row transition metals (e.g., Fe, Co, Ni, Cu, or Zn), thereby enabling deliberate modulation of electronic structure and significantly broadening the scope of electrocatalytic applications for MMo₆-derived nanomaterials.

Based on the above, this work constructed a g-C₃N₄-H⁺Cl⁻ intermediate via hydrochloric acid protonation of tubular g-C₃N₄, and subsequently introduced three types of anionic polyoxometalates (POMs) through electrostatic interaction to fabricate three nanoscale g-C₃N₄-based composite photocatalysts, g-C₃N₄-H-MMo₆ (M = Ni, Fe, Co). The development of this hybrid photocatalytic system provides a novel strategy for designing high-performance catalyst architectures tailored for H₂O₂ evolution.

2. Experimental Section

2.1. Preparation of g-C₃N₄

A quantity of 6 g of melamine and 8 g of urea were weighed and mixed in 75 mL of deionized water, stirred uniformly on a magnetic stirrer for 1 h, then placed into a clean liner, transferred to a 100 mL autoclave, and heated at 180 °C for 12 h in an oven. After cooling to room temperature, the product was washed with ultrapure water, centrifuged at 8000 r/min for 5 min, dried at 60 °C overnight, and then calcined in a muffle furnace at 500 °C for 4 h to obtain g-C₃N₄, which was ground and sealed for storage.

2.2. Preparation of g-C₃N₄-H⁺Cl⁻

The prepared 1.8 g of g-C₃N₄ was dispersed in 180 mL of 5 M hydrochloric acid solution. The mixture was sealed with plastic wrap and stirred at room temperature for 24 h. The resulting solid was collected by centrifugation, washed with ultrapure water until neutral pH, dried in an oven at 60 °C overnight, and ground to afford g-C₃N₄-H⁺Cl⁻.

2.3. Preparation of g-C₃N₄-H-MMo₆ (M = Ni, Fe, Co)

The Anderson-type polyoxometalates (POMs) used, denoted as MMo₆ (M = Fe, Co, Ni), were synthesized according to the published literature [15] and are represented by the ammonium salts (NH₄)₃[FeMo₆O₂₄H₆]·7H₂O, (NH₄)₄[CoMo₆O₂₄H₆]·xH₂O, and (NH₄)₄[NiMo₆O₂₄H₆]·5H₂O. Quantities of 0.5 g of g-C₃N₄-H⁺Cl⁻ and 0.05 g of the corresponding MMo₆ POM (M = Ni, Fe, or Co) were dispersed in 50 mL of ultrapure water, heated in an oil bath at 100 °C for 10 h, and then cooled to room temperature. The resulting solid was collected by centrifugation, washed thoroughly with ultrapure water, dried in an oven at 60 °C overnight, ground into a fine powder, and stored in a sealed container.

2.4. Photocatalytic H₂O₂ Production Activity Test

Photocatalytic H₂O₂ production activity tests were conducted as follows: 15 mg of catalyst was placed in 15 mL of ultrapure water. An LED light source (Beijing Perfectlight, Beijing, China, PLS-LED100C, 100 mW cm⁻², λ = 420 nm) was used, and the suspension was exposed to ambient air under stirring. The irradiation time was varied, while the light power was kept constant at 100 W. At hourly intervals, 1 mL aliquots were withdrawn and immediately sealed for subsequent analysis. Each sample was centrifuged, and 500 µL of the supernatant was mixed with 4 mL of detection solution. The mixture was allowed to stand for 10 min to complete the colorimetric reaction. The absorbance of the liberated iodine was measured at 352 nm using a Shimadzu UV-2700 UV–Vis spectrophotometer (Kyoto, Japan), and the H₂O₂ concentration was quantified by reference to a calibration curve, thereby reflecting the photocatalytic activity of the catalyst.

The quantification of H₂O₂ is based on the well-established iodometric colorimetric method. In the presence of ammonium molybdate as the catalyst, H₂O₂ oxidizes the iodide ions (I⁻) provided by KI to iodine (I₂): H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O; the generated I₂ immediately combines with the excess I⁻ to form the triiodide complex (I₃⁻): I₂ + I⁻ ⇌ I₃⁻ (overall: H₂O₂ + 3I⁻ + 2H⁺ → I₃⁻ + 2H₂O) [16]. The triiodide ion exhibits a characteristic absorption maximum at 352 nm, and its absorbance follows the Beer–Lambert law.

3. Results and Discussion

3.1. Catalyst Structure Characterization

As schematically outlined in Scheme 1, the synthesis of MMo₆ POM/g-C₃N₄-H⁺Cl⁻ composites proceeds via a multi-step protocol. First, hydrothermal treatment of melamine and urea at 180 °C affords a hydrogen-bonded supramolecular (melamine–cyanuric acid) rod-like precursor. During the subsequent calcination at 500 °C, urea decomposes to release gases (NH₃, CO₂) that act as a porogen, while the rod-like precursor is converted into porous tubular g-C₃N₄ [17,18]. The resulting g-C₃N₄ is then protonated by dispersing it in 5 M aqueous HCl; under these strongly acidic conditions, protonation occurs preferentially at terminal amino groups and interlayer sites, while Cl⁻ anions simultaneously intercalate into the interlamellar spaces. After centrifugation, thorough washing with deionized water until neutral pH (~7), and vacuum drying, the protonated material—designated g-C₃N₄-H⁺Cl⁻—is obtained. Finally, pre-synthesized, water-soluble MMo₆ POMs (M = Ni, Fe, Co) are introduced into an aqueous dispersion of g-C₃N₄-H⁺Cl⁻, and the mixture is subjected to mild hydrothermal crystallization at 100 °C. This step enables electrostatic self-assembly between the positively charged g-C₃N₄-H⁺Cl⁻ matrix and the anionic MMo₆ POM species, resulting in a homogeneous heterogeneous composite material.

Scheme 1.

Schematic illustration for the formation process over g-C₃N₄-H-MMo₆ catalysts.

As shown in Figure 1, the phase and crystal structures were characterized by X-ray diffraction (XRD). All materials exhibit a typical diffraction peak at ~13°, assigned to in-plane structural ordering associated with the triazine ring motif. The diffraction peak at ~27° is attributed to interlayer stacking of aromatic systems [19]. In the XRD patterns of g-C₃N₄-X (X = HCl, FeMo₆, CoMo₆, NiMo₆), in addition to the characteristic (100) reflection at ~13.1° (d ≈ 0.68 nm) and the (002) interlayer-stacking reflection at ~27.5° (d ≈ 0.32 nm) of g-C₃N₄ (JCPDS No. 87-1526), a new reflection at 2θ ≈ 6° (d ≈ 1.47 nm) appearing after HCl protonation is attributed to the expansion of the interlayer periodicity induced by protonation/intercalation during the acid treatment. This assignment is consistent with the observation of a new low-angle diffraction peak at 6.2° (d = 1.421 nm) in g-C₃N₄ nanosheets, indexed to the (001) lattice plane [20]. The slightly larger d-spacing in our sample (1.47 nm vs. 1.42 nm) is ascribed to a higher degree of proton intercalation/hydration under our treatment conditions. Furthermore, several weak fine reflections superimposed on the broad (002) peak were observed for the POM-modified samples; they are assigned to the characteristic reflections of the Anderson-type POM crystallites (e.g., FeMo₆ at 2θ ≈ 27.7°, CoMo₆ at 2θ ≈ 26.8° and NiMo₆ at 2θ ≈ 28.6°), whose low intensity is due to the small POM loading and the high dispersion of the nanoscale POM crystallites. Meanwhile, the rest of the non-peak regions showed an XRD pattern similar to that of g-C₃N₄. This demonstrates the successful preparation of composite catalysts combining graphitic phase g-C₃N₄ and POMs. Moreover, since the crystal structure of the composite material showed no significant change, it suggests that the composite material still retains the structural characteristics of g-C₃N₄ [21].

Figure 1.

XRD patterns of g-C₃N₄, g-C₃N₄-H⁺-Cl⁻, g-C₃N₄-H-FeMo₆, g-C₃N₄-H-CoMo₆, and g-C₃N₄-H-NiMo₆.

As shown in Figure 2, the Fourier-transform infrared spectroscopy (FT-IR) spectra of g-C₃N₄, g-C₃N₄-H⁺Cl⁻, g-C₃N₄-H-FeMo₆, g-C₃N₄-H-CoMo₆, g-C₃N₄-H-NiMo₆, and FeMo₆, CoMo₆, and NiMo₆ POMs are presented. In Figure 2a, compared to g-C₃N₄, the protonated sample g-C₃N₄-H⁺Cl⁻ does not exhibit any distinct peaks. In Figure 2b–d, all composite samples, except for the FeMo₆, CoMo₆, and NiMo₆ POMs, display the characteristic absorption peaks of g-C₃N₄. The peak at 803 cm⁻¹ and the peaks in the range of 1150–1700 cm⁻¹ correspond to the stretching vibrations of the triazine ring and the C≡N heterocycle, respectively [21]. The broad peak in the range of 3000–3500 cm⁻¹ is attributed to surface-adsorbed water and the stretching vibration of N-H bonds [22]. The FT-IR spectra show the characteristic bands of the Anderson-type anion: terminal Mo=O stretching at 1000–800 cm⁻¹, bridging Mo–O–Mo stretching at 750–550 cm⁻¹, Mo–O (H)–M stretching below 450 cm⁻¹, together with O–H/N–H stretching at 3600–2800 cm⁻¹ [15]. The characteristic peaks of FeMo₆, CoMo₆, and NiMo₆ POM materials overlap to some extent with those of the catalyst materials, making it difficult to observe significant changes in peak shape when the loading amount is small.

Figure 2.

FT-IR spectra of (a) g-C₃N₄ and g-C₃N₄-H⁺Cl⁻; (b) g-C₃N₄-H-FeMo₆ and FeMo₆ POM; (c) g-C₃N₄-H-CoMo₆ and CoMo₆ POM; (d) g-C₃N₄-H-NiMo₆ and NiMo₆ POM.

The formation of pore channels helps increase the specific surface area, indicating that the catalyst’s activity is likely improved compared to the raw g-C₃N₄ material. Additionally, the image also shows the typical layered structures of the graphitic-phase g-C₃N₄, indicating that some portion of the g-C₃N₄-H-NiMo₆ was not completely transformed into the columnar structure.

The g-C₃N₄-H-NiMo₆ material was selected for scanning electron microscopy (SEM) testing, and Figure S1 shows the results. It can be observed that part of the g-C₃N₄-H-NiMo₆ material has formed a columnar structure, with white spots attached, which are speculated to be the attachment of POMs. The outer surface of the columnar structure exhibits some tubular collapse, suggesting potential pore channels within the columnar structure, where the walls might be too thin, leading to damage and the formation of voids during preparation [23]. The formation of pore channels helps increase the specific surface area, indicating that the catalyst’s activity is likely improved compared to that of the raw g-C₃N₄ material.

Additionally, the image also shows typical layered structures of graphitic phase g-C₃N₄, indicating that some portion of the g-C₃N₄-H-NiMo₆ was not completely transformed into the columnar structure. The coexistence of the two morphologies originates from the different self-assembly behaviour of the two precursors during the hydrothermal step [24]. The melamine and the cyanuric acid generated from urea form hydrogen-bonded supramolecular aggregates that grow into rod-like precursors, which are converted into the rod/tube-like g-C₃N₄ upon calcination. The urea-derived fraction that does not participate in this rod-like supramolecular assembly directly condenses into bulk sheet-like g-C₃N₄ domains.

As shown in Figure 3a–d, g-C₃N₄-H-NiMo₆ exhibits a unique nanorod morphology. Figure 3a,b indicate that g-C₃N₄ forms a nanorod structure. In Figure 3c,d, at higher magnification, numerous NiMo₆ POM nanoparticles are clearly visible, uniformly dispersed on the surface of g-C₃N₄ nanorods, confirming successful NiMo₆ POM incorporation and the formation of the target composite material. High-resolution transmission electron microscopy (HRTEM) images in Figure 3e,f further verify the precise construction of the g-C₃N₄-H-NiMo₆ homogeneous–heterogeneous composite electrocatalyst. Energy-dispersive X-ray spectroscopy (EDX, Figure 3g) and elemental distribution mapping (Figure 3h) collectively demonstrate that C, N, Ni, Mo, and O are uniformly distributed throughout the catalyst structure, while the overall content of NiMo₆ POM is relatively low, which explains the weak signals observed in the XRD and FT–IR patterns.

Figure 3.

Morphological characteristics of g-C₃N₄-H-NiMo₆ catalysts: (a–d) TEM images and (e,f) the corresponding high-resolution TEM images; (g) Energy dispersive X-ray analysis; (h) Element mapping.

To investigate the adsorption performance of the catalyst materials, this experiment utilized Brunauer–Emmett–Teller (BET) analysis to determine the specific surface area and pore structure of the materials. The adsorption isotherm under nitrogen was employed to characterize the pore size distribution and specific surface area of the photocatalysts. The initial graphitic phase g-C₃N₄ material and the NiMo₆ POM composite catalyst material were selected as representatives. As shown in Figure 4, the isotherms for both materials are typical Type IV with H₃ hysteresis loops, indicating interactions between the adsorbent and adsorbate, accompanied by capillary condensation and confirming the presence of mesopores. In the pore size distributions of both materials, distinct peaks are observed, with pore sizes concentrated in the ranges of 13–70 Å and 13–61 Å, respectively. After measurement and calculation, the specific surface area, total pore volume, and average pore diameter were obtained. The specific surface area of g-C₃N₄-H-NiMo₆ was 13.052 m²/g, larger than that of g-C₃N₄ (5.623 m²/g). The single-point total pore volume of g-C₃N₄-H-NiMo₆ was 0.1322 cm³/g, larger than that of g-C₃N₄ (V = 0.1069 cm³/g), indicating enhanced adsorption capacity under identical conditions. Based on the test results, the graphitic-phase g-C₃N₄ material combined with POMs exhibits an increased specific surface area, which is beneficial for exposing active sites and enhancing catalytic performance [25,26].

Figure 4.

N₂ adsorption–desorption isotherms of the prepared (a) g-C₃N₄ and (b) g-C₃N₄-H-NiMo₆ catalysts. The inset shows the corresponding pore-size distribution curves.

X-ray photoelectron spectroscopy (XPS) characterization was used to investigate the chemical states and elemental composition of g-C₃N₄-H-NiMo₆. Figure 5 shows the XPS survey spectrum of g-C₃N₄-H-NiMo₆, as well as the C 1s, N 1s, Ni 2p, and Mo 3d core-level spectra. The survey spectrum confirms the presence of the main elements C, N, Ni, and Mo in the material. It also reveals O and Cl; the former is attributed to adsorbed water and carbon dioxide on the surface of the material, while the latter originates from incomplete conversion of the protonated precursor to the POM composite material.

Figure 5.

(a) Survey XPS spectrum of g-C₃N₄-H-NiMo₆; (b) C 1s XPS spectrum; (c) N 1s XPS spectrum; and (d) Mo 3d XPS spectrum.

The C 1s XPS was deconvoluted into three distinct peaks, assigned to C–C/C=C (284.80 eV), C–NH_x (286.25 eV), and N–C=N (288.19 eV), the latter corresponding to the carbon atoms in the tri-s-triazine framework and exhibiting the highest intensity [27,28]. The N 1s X-ray photoelectron spectrum (XPS) was deconvoluted into four well-resolved peaks, assigned to C–N=C (398.64 eV), N–(C)₃ (399.51 eV), C–N–H (400.76 eV), and a π-excitation satellite peak at 404.51 eV [29,30]. Various nitrogen configurations may play distinct roles in catalytic processes. Pyridinic nitrogen introduces defects and vacancies into the electronic structure of carbon-rich materials, increasing the charge carrier density and thereby enhancing electronic conductivity, which facilitates rapid electron transfer. Pyrrolic nitrogen improves hydrophilicity and electrical conductivity, and serves as an anchoring site for constructing more efficient composite catalysts.

In the Mo 3d spectrum (Figure 5d), two fitted peaks at binding energies of 231.63 eV and 234.60 eV correspond to Mo 3d_5/2 and Mo 3d_3/2, respectively, confirming Mo incorporation into the same precursor. The XPS analysis confirms that both the pristine g-C₃N₄ and the g-C₃N₄-H-NiMo₆ catalysts contain their intended elements and exhibit characteristic chemical bonding, demonstrating the feasibility of integrating the NiMo₆ POM with g-C₃N₄. Since hydrogen atoms bond with other elements in the material and lack core-level electrons suitable for XPS excitation, they cannot be detected by XPS [31]. The measured content of elemental analysis (ICP-OES) is Ni: 0.24 wt%/Mo: 2.4 wt%. The measured molar ratios of Ni and Mo contents agree well with the theoretical value of 1:6.

3.2. Photocatalytic Activity Performance

Activity tests were conducted at room temperature under irradiation from a 100 W LED lamp with a sampling interval of 1 h. The photocatalytic H₂O₂ synthesis performance of g-C₃N₄; g-C₃N₄-H⁺Cl⁻; g-C₃N₄-H-FeMo₆; g-C₃N₄-H-CoMo₆; and g-C₃N₄-H-NiMo₆ was evaluated over 3 h. The absorbance data obtained from the experiments were converted to H₂O₂ concentrations using a calibration curve (Table S1 and Figure S2). As shown in Figure 6a,b, under visible light irradiation, the photocatalytic H₂O₂ synthesis rates for g-C₃N₄ and g-C₃N₄-H⁺Cl⁻ were 64.5 μmol·g⁻¹·h⁻¹ and 59.4 μmol·g⁻¹·h⁻¹, respectively. The rates for g-C₃N₄-H-FeMo₆, g-C₃N₄-H-CoMo₆, and g-C₃N₄-H-NiMo₆ were 108.3 μmol·g⁻¹·h⁻¹, 85.2 μmol·g⁻¹·h⁻¹, and 252.3 μmol·g⁻¹·h⁻¹, respectively. Compared to pristine g-C₃N₄, the H₂O₂ production rates of the Fe-, Co-, and Ni-doped POM composite catalysts were 1.7-fold, 1.3-fold, and 3.9-fold higher, respectively. The photocatalytic H₂O₂ synthesis rate of hydrochloric acid-treated g-C₃N₄ showed a slight increase, while those of g-C₃N₄ doped with FeMo₆ POMs, CoMo₆ POMs, or NiMo₆ POMs were further enhanced. The data in the figure indicate that combining POMs with g-C₃N₄ effectively improves catalyst activity [32]. Among the g-C₃N₄ materials modified with different POMs, g-C₃N₄-H-NiMo₆ exhibited the highest photocatalytic H₂O₂ synthesis rate and the highest activity.

Figure 6.

(a) Photocatalytic H₂O₂ production; and (b) Bar graph of reaction rates for g-C₃N₄, g-C₃N₄-H⁺Cl⁻and g-C₃N₄-H-FeMo₆; g-C₃N₄-H-CoMo₆; g-C₃N₄-H-NiMo₆ catalysts; (c) Kinetic curve of H₂O₂ production changes in g-C₃N₄-H-NiMo₆ within 3 h under different catalyst dosages; (d) Kinetic curve of H₂O₂ production changes in g-C₃N₄-H-NiMo₆ within 3 h under different intensities of LED lamp.

The experiment investigating the effect of catalyst loading on the photocatalytic H₂O₂ production over g-C₃N₄-H-NiMo₆ was conducted at room temperature, using a 100 W LED lamp and 15 mL of ultrapure water. Catalyst loadings were varied at 10 mg, 15 mg, and 20 mg to evaluate their influence on the reaction rate. The average photocatalytic H₂O₂ synthesis rates over 3 h were 201.3 μmol·g⁻¹·h⁻¹, 252.3 μmol·g⁻¹·h⁻¹, and 363.9 μmol·g⁻¹·h⁻¹ for catalyst loadings of 10 mg, 15 mg, and 20 mg, respectively. Compared with the 10 mg loading, the 3 h average rates increased by approximately 25% and 80% for 15 mg and 20 mg, respectively. Relative to the 15 mg loading, the rate at 20 mg increased by 44%. It can be observed that the increment in catalytic rate per additional 5 mg of catalyst is non-uniform. Moreover, as shown in Figure 6c, the H₂O₂ concentration profile is non-linear. This deviation from linearity is attributed to unavoidable experimental variables, including: (1) real-time fluctuations in dissolved O₂ concentration; (2) minor variations in LED irradiance due to power supply instability; and (3) slight ambient temperature drifts. Nevertheless, it is evident that catalyst loading significantly influences the photocatalytic reaction rate.

As shown in Figure 6d, the experiment investigating the effect of LED irradiance on the photocatalytic H₂O₂ production over g-C₃N₄-H-NiMo₆ was conducted at room temperature using 15 mg of catalyst and 15 mL of ultrapure water. LED lamp powers were varied at 100 W, 75 W, and 50 W to evaluate their influence on the reaction rate. The results show that at irradiance levels corresponding to 100 W, 75 W, and 50 W LED input power, the average photocatalytic H₂O₂ synthesis rates over 3 h were 252.3 μmol·g⁻¹·h⁻¹, 205.8 μmol·g⁻¹·h⁻¹, and 179.7 μmol·g⁻¹·h⁻¹, respectively. Compared with the 100 W condition, the 3 h average rates decreased by approximately 19% and 29% at 75 W and 50 W, respectively. Relative to the 75 W condition, the rate at 50 W decreased by 13%. It can be observed that the reduction in catalytic rate per 25 W decrease in input power is non-uniform. Additionally, as shown in Figure 6d, the H₂O₂ concentration profile is non-linear, with contributing factors identical to those identified in the catalyst loading study. Nevertheless, it is evident that LED lamp input power significantly influences the photocatalytic reaction rate.

3.3. Mechanism Investigation

To further understand the significant differences in H₂O₂ evolution activity among NiMo₆; FeMo₆; and CoMo₆ (Figure 7a–c), we measured the energy levels of these molecular POMs using electrochemical cyclic voltammetry. As shown in Figure 7a, both NiMo₆; FeMo₆; and CoMo₆ exhibit reduction peaks, with calculated LUMO (Lowest Unoccupied Molecular Orbital) levels of 0.04 V (vs. NHE (Normal Hydrogen Electrode)); −0.05 V (vs. NHE); and −0.15 V (vs. NHE), respectively. At the same concentration, NiMo₆ shows the highest peak intensity, indicating that NiMo₆ has the best electron-capturing ability.

Figure 7.

Cyclic voltammetry of (a) NiMo₆, (b) FeMo₆; and (c) CoMo₆. Test conditions: 5 mg POMs dissolved in 50 mL water, scan rate of 50 mV/s, working electrode as glassy carbon electrode, reference electrode as Ag/AgCl, counter electrode as Pt foil.

UV–vis diffuse reflectance spectroscopy (DRS) reveals a high absorption response of g-C₃N₄ (Figure 8a). According to the Kubelka–Munk equation [33], the band gap of g-C₃N₄-H⁺Cl⁻ was calculated to be 2.62 eV (vs. NHE) (Figure 8b). The conductivity type of g-C₃N₄-H⁺Cl⁻ was evaluated using a three-electrode system at frequencies of 1000 Hz, 2000 Hz, and 3000 Hz via Mott–Schottky plots [34] (Figure 8c). The positive slope of the C⁻²–E curve indicates that g-C₃N₄-H⁺Cl⁻ is an n-type semiconductor. The flat-band potential (E_fb) of g-C₃N₄-H⁺Cl⁻ was determined to be −1.03 V (vs. Ag/AgCl) by Mott–Schottky analysis. Therefore, the conduction band (CB) potential of g-C₃N₄ is −1.03 eV, corresponding to −0.83 V vs. the standard hydrogen electrode (NHE) (E_NHE = E_Ag/AgCl + E^θ_Ag/AgCl; E^θ_Ag/AgCl = 0.1976 V) [35].

Figure 8.

(a) UV-vis absorption spectrum; (b) Tauc plots of (αhν)² versus hν; and (c) Mott–Schottky plots of the g-C₃N₄-H⁺Cl⁻.

Based on the above data, we have drawn the electron transfer mechanism diagram shown in Scheme 2. It can be observed that NiMo₆ possesses the most suitable LUMO level [36], which facilitates the transfer of photogenerated electrons from the conduction band of g-C₃N₄ to protons in solution, thereby exhibiting high H₂O₂ evolution activity. Upon light irradiation, g-C₃N₄ generates electron–hole pairs. The electrons in the CB are transferred to the Anderson-type POMs (NiMo₆, LUMO ≈ +0.04 V vs. NHE), which then act as electron mediators to reduce O₂ to H₂O₂: O₂ + 2H⁺ + 2e⁻ → H₂O₂ (E(O₂/H₂O₂) = +0.27 V vs. NHE, pH 7). Meanwhile, the holes accumulated in the VB of g-C₃N₄ (VB edge ≈ +1.79 V vs. NHE) oxidize H₂O to H₂O₂: 2H₂O + 2h⁺ → H₂O₂ + 2H⁺ (E(H₂O/H₂O₂) = +1.35 V vs. NHE, pH 7). This oxidation is thermodynamically feasible because the VB edge of g-C₃N₄ is more positive than the H₂O/H₂O₂ redox potential. Therefore, the reduction of O₂ and the accompanying oxidation of H₂O constitute two coupled half-reactions, giving the overall reaction O₂ + 2H₂O → 2H₂O₂ without any sacrificial electron donor.

Scheme 2.

Schematic diagram of electron transition energy levels over g-C₃N₄-H-NiMo₆ catalyst.

4. Conclusions

As green and low-carbon development gains increasing attention, energy issues have gradually attracted more and more public concern. Polyoxometalates (POMs), as anionic metal–oxygen clusters, can undergo rapid and reversible multi-electron transfer reactions without altering their structure. In this work, g-C₃N₄ was protonated with hydrochloric acid to construct g-C₃N₄-H⁺Cl⁻ material. Subsequently, POMs were immobilized onto the protonated substrate via electrostatic interactions to prepare three nanosized g-C₃N₄-based composite catalysts [g-C₃N₄-H-MMo₆ (M = Ni, Fe, Co)]. Their phase structures and morphologies were characterized, and their photocatalytic activities—as well as the key influencing factors—were systematically investigated. SEM and TEM analyses revealed that the g-C₃N₄-H-NiMo₆ composite exhibits a nanorod morphology. Their phase structures and morphologies were characterized, and their catalytic activities along with influencing factors were investigated. SEM and TEM analysis revealed that the g-C₃N₄-H-NiMo₆ composite catalyst exhibits a columnar structure. FTIR results confirmed that the prepared composites retain the characteristic structural features of g-C₃N₄. BET measurements demonstrated that g-C₃N₄-H-NiMo₆ possesses a larger specific surface area than pristine g-C₃N₄, suggesting enhanced adsorption capacity. XPS was employed to probe the elemental composition and chemical states of the materials. Photocatalytic H₂O₂ production experiments showed that the activity of g-C₃N₄-H-NiMo₆ (252.3 μmol·g⁻¹·h⁻¹) was 3.91-fold higher than that of pure g-C₃N₄ (64.5 μmol·g⁻¹·h⁻¹). The catalytic performance of the composite catalysts varied with the metal identity in the POMs, following the order: g-C₃N₄-H-NiMo₆ > g-C₃N₄-H-FeMo₆ > g-C₃N₄-H-CoMo₆. NiMo₆, with its optimal LUMO energy level, superior reduction capability, and strong adsorption affinity, functions as an efficient cocatalyst for photocatalytic H₂O₂ evolution. In summary, the composite catalysts fabricated via electrostatic coupling of protonated g-C₃N₄ with POMs exhibit both enhanced photocatalytic performance and improved operational stability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma19204430/s1, Figure S1: SEM images of g-C₃N₄-H-NiMo₆ at magnifications corresponding to (a) 5 μm, (b) 2 μm, (c) 1 μm, and (d) 500 nm; Figure S2: Calibration plot of H₂O₂ Production; Table S1: Preparation of the H₂O₂ standard solutions (final volume 10 mL; stock concentration ≈ 20 mmol L⁻¹).

Author Contributions

Conceptualization, P.J., X.M. and Y.D.; writing—original draft preparation, Q.C. and Z.W., data curation; Q.C., Z.W. and Y.L.; writing—review and editing, P.J., X.M. and Y.D.; visualization, Q.C., Z.W., C.Y. and L.L.; supervision, X.M. and Y.D. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Authors Qiang Cui, Peng Jin and Long Li are employed by the China Pingmei Shenma Holding Group Co. Ltd.; Peng Jin is also employed by the Pingdingshan Tian’an Coal Industry Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding Statement

This work was financially supported by the Young Talent Support Program of Henan Association for Science and Technology (No. 2025HYTP053), the Zhengzhou Basic and Applied Basic Research Project (No. ZZSZX202410), the Scientific Research Startup Project for Introduced Talents of Chengdu University of Information Technology (No: 2023KYTZ037).