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Article

In Situ Reinforced g-C3N4/CoO/CoP Ternary Composite for Enhanced Photocatalytic H2 Production

by
Yanan Han
†,
Zhaohui Wang
†,
Xiuyuan Yang
,
Zhongjun Li
and
Yike Li
*
College of Chemistry, Zhengzhou University, Zhengzhou 450001, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(4), 315; https://doi.org/10.3390/catal15040315
Submission received: 24 February 2025 / Revised: 14 March 2025 / Accepted: 17 March 2025 / Published: 26 March 2025
(This article belongs to the Special Issue Catalysis for Hydrogen Storage and Release)
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Versions Notes

Abstract

:
To meet the growing demand for renewable energy, developing efficient and cost-effective photocatalytic materials is crucial. Specifically, designing photocatalysts with high charge separation efficiency and abundant hydrogen production active sites remains a key challenge for practical applications. In this study, a carbon nitride (g-C3N4)-based ternary photocatalyst has been constructed for enhanced photocatalytic H2 production without the need for precious metal cocatalysts. CoO nanoparticles were loaded onto the surface of g-C3N4 via in situ thermal decomposition. Subsequently, a series of g-C3N4/CoO/CoP ternary composites were successfully prepared using a direct one-step phosphorization method. Under optimized conditions, the g-C3N4/CoO/CoP catalyst exhibits a hydrogen evolution activity of 1277.9 μmol·g−1·h−1, which is 4 times higher than that of g-C3N4/CoO (with g-C3N4 alone showing no hydrogen evolution activity). Its performance is comparable to that of the commonly used Pt cocatalyst. The performance improvement may be attributed to the tight bonding of N-P bonds, which effectively promotes the transport of photogenerated carriers, while the increased loading of CoP provides more active sites. The results offer a promising strategy for designing efficient and low-cost photocatalytic materials.

Graphical Abstract

1. Introduction

A sustainable strategy for converting solar energy into hydrogen via photocatalytic water splitting is currently an effective approach for addressing energy shortages and environmental pollution [1]. In this process, photocatalysts play a crucial role in determining hydrogen production efficiency [2]. Among semiconductor photocatalysts, carbon nitride (g-C3N4) stands out due to its excellent chemical stability and suitable band structure [3]. However, bulk g-C3N4 and most g-C3N4-based materials—modified through element doping [4], morphology adjustments [5], or coupling with other semiconductors [6]—still rely on expensive and scarce platinum (Pt) cocatalysts to accelerate surface reaction kinetics and enhance photocatalytic hydrogen production [7]. This significantly increases costs and severely limits the large-scale application of photocatalytic water splitting for hydrogen generation.
Transition metal oxides have become a research hotspot in the field of photocatalysis, both domestically and internationally, due to their environmental friendliness, low cost, abundant reserves, and unique physicochemical properties [8,9]. Liao et al. first demonstrated that CoO nanocrystals possess a suitable band structure, good photocatalytic activity for water splitting, and a solar hydrogen production efficiency of around 5% [10]. Additionally, nanoscale CoO may offer additional active sites and demonstrate promising electron capture characteristics, which could potentially serve as an alternative to Pt-based cocatalysts [11]. However, the interaction between CoO and g-C3N4 was weak, and the CoO nanoparticles tended to aggregate and become deactivated [10]. Although some significant progress has been made, further improvement in its photocatalytic activity is still needed.
In recent years, transition metal phosphides have gained attention in the field of photocatalysis due to their high conductivity and low overpotential. CoP, in particular, stands out for its exceptional electrochemical hydrogen evolution reaction (HER) efficiency, as phosphorus (P), with its strong electronegativity, can capture positively charged protons and promote charge separation [12]. Zhang et al. synthesized CoP/g-C3N4 by phosphating a cobalt metal glycolic acid layer on g-C3N4, creating a tight contact interface between CoP and g-C3N4 through N-P bonds, which exhibited excellent photostability under continuous illumination for 70 h [13]. Guo et al. synthesized CoP/CoO via a one-step phosphorization method [14]. Building on this, using CoP as a bridge to reinforce g-C3N4/CoO/CoP by forming N-P bonds could enhance interface connectivity and is expected to improve the photostability of g-C3N4/CoO.
Herein, we prepared g-C3N4/CoO materials as precursors using an in situ thermal decomposition method. The g-C3N4/CoO/CoP ternary composite photocatalyst was then constructed through a one-step phosphorization process using NaH2PO2 as the phosphating agent, where a part of the CoO in the precursor was converted to CoP in situ, resulting in low-cost, stable, and efficient hydrogen production (Scheme 1). The N-P bonds formed a tight interface between g-C3N4/CoO/CoP. The g-C3N4/CoO/CoP composite demonstrated good hydrogen evolution stability under continuous irradiation for 12 h. By optimizing the ratio and loading of CoO/CoP, the photocatalytic performance of g-C3N4/CoO/CoP (1277.9 μmol·g−1·h−1) was increased fourfold compared to g-C3N4/CoO and is comparable to that of g-C3N4/Pt. This design establishes a new approach for ternary catalyst engineering by creating CoO/CoP active sites bridged by N-P bonds on g-C3N4. In this system, CoO synergistically enhances charge transfer while CoP provides abundant active sites, enabling efficient hydrogen evolution without the need for precious metal cocatalysts.

2. Results and Discussion

2.1. Morphology and Structure of g-C3N4/CoO/CoP

The morphology and microstructure of g-C3N4 and g-C3N4/CoO/CoP were investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Figure 1a,c show typical two-dimensional folding structures of g-C3N4 nanosheets. g-C3N4/CoO/CoP exhibits a curled thin layer structure, which is similar in morphology to g-C3N4 (Figure 1b). As shown in Figure S1, CoO nanoparticles are uniformly dispersed between the folded lamellae of g-C3N4, with an average particle size of approximately 2 nm. It can be observed from Figure 1d that the CoO/CoP nanoparticles (diameter of ca. 10 nm) are anchored on the surface of g-C3N4. As illustrated in Figure 1e, a lattice spacing of 0.21 nm was measured, which was consistent with the interplanar spacing of the (200) plane of CoO [15,16]. The lattice spacing measured in High-Resolution Transmission Electron Microscopy (HRTEM) is 0.17 and 0.19 nm, corresponding to the (103) and (211) crystal planes of CoP [17]. Moreover, the elemental mapping of g-C3N4/CoO/CoP illustrates the even distribution of C, N, O, P, and Co (Figure 1f). The above results confirmed that CoO and CoP nanoparticles were successfully loaded onto the surface of g-C3N4.
X-Ray Diffraction (XRD) measurements were performed to study the purity and crystallography of the samples. As shown in Figure 2a, g-C3N4/CoO/CoP and g-C3N4/CoO exhibit two typical peaks at 13.0° and 27.5°, corresponding to the (100) and (002) diffraction planes of g-C3N4, respectively [18]. This indicates that the samples retain the structure of g-C3N4 [19]. The diffraction peaks observed at 36.4°, 41.5°, and 61.5° are attributed to face-centered cubic CoO (PDF# 65-2902) [20]. The peaks match the lattice spacings seen in the HRTEM images (Figure 1e) of the CoO nanoparticles, with a measured spacing of 0.21 nm corresponding to the (200) plane of CoO. As CoO loading increases, the intensity of the two characteristic peaks of g-C3N4 in the composite catalyst gradually decreases (Figure S2). This may be attributed to a reduction in the crystallinity of g-C3N4 during the thermal treatment process. It should be noted that the typical CoO peaks in g-C3N4/CoO/CoP become weaker compared to those in g-C3N4/CoO, which can be attributed to the in situ generation of CoP from a portion of CoO after phosphorylation. The diffraction peaks for CoP (PDF# 29-0497) were detected at 31.6°, 46.2°, and 48.1°. Among these, the lattice spacings of 0.17 and 0.19 nm observed in the HRTEM images (Figure 1e) correspond to the (103) and (211) planes of CoP, indicating the success of the preparation of g-C3N4/CoO/CoP.
To further investigate the molecular structure of as-prepared g-C3N4/CoO/CoP, Fourier Transform Infrared Spectroscopy (FT-IR) measurements were performed. As described in Figure 2b, for g-C3N4/CoO/CoP and g-C3N4/CoO samples, the FT-IR spectra are almost identical to that of pristine g-C3N4, where all the characteristic peaks of g-C3N4 at 810, 1200–1600, and 3200 cm−1, corresponding to the s-triazine ring, C-N, C=N heterocycles, and the stretching vibration of -NH [21], are observed. Increasing the CoO content does not compromise the structure of g-C3N4 (Figure S3). This observation indicates that introducing CoO/CoP preserves the chemical backbone of the g-C3N4 network, which is essential for the transport of photogenerated carriers and the process of redox reactions [22].
The N2 adsorption–desorption isotherms shown in Figure 3a display the IV-type hysteresis loops, indicating that g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP are all typical mesoporous structures. The values of the specific surface area and pore volume of the samples are summarized in Table 1. As shown in Figure 3b and Table 1, the specific surface area and pore volume decrease from the original 91.9 m2·g−1 and 0.18 cm3·g−1 to 46.9 m2·g−1 and 0.09 cm3·g−1 after the phosphating of g-C3N4/CoO. This may be due to the formation of CoO/CoP nanoparticles within the mesopores of g-C3N4/CoO. The in situ generation of CoP from CoO results in an increase in nanoparticle size from the original CoO (2 nm) to CoO/CoP (10 nm), which subsequently leads to a reduction in the specific surface area. Despite this decrease, the introduction of CoP nanoparticles provides a large number of active sites, which may potentially enhance the photocatalytic activity.
XPS was utilized to analyze the surface species and chemical states of g-C3N4 and g-C3N4/CoO/CoP. XPS survey spectra reveal the coexistence of C, N, and O elements in g-C3N4, while C, N, O, P, and Co elements are present in g-C3N4/CoO/CoP (Figure 4a). The signal peaks of P and Co elements are weak, which may explain the lower content. The C 1s XPS spectra of g-C3N4 and g-C3N4/CoO/CoP are shown in Figure 4b. For g-C3N4, the peak at 284.8 eV is attributed to the C–C bond [23]. The peak at 287.8 eV is associated with sp2-bonded carbon in the s-triazine ring (N=C–N), whereas the peak centered at 286.1 eV is assigned to sp2 C atoms in the aromatic ring attached to the primary and secondary amines (C–NH₂, C–NH). For the g-C3N4/CoO/CoP composite, these peaks shift to higher binding energies, which can be attributed to the electron density being transferred toward the CoO/CoP components. Notably, the C 1s spectrum shows a significant increase in the C–C peak after CoO loading, which is attributed to the introduction of amorphous carbon during MOF decomposition, as shown in Figure S4. High-resolution N 1s spectra of g-C3N4 and g-C3N4/CoO/CoP are shown in Figure 4c. The peaks located at 398.3 eV, 399.2 eV, and 400.4 eV are ascribed to sp2-hybridized aromatic nitrogen (C−N=C), tertiary nitrogen (N−(C)3), and quaternary nitrogen (C−NHx), respectively [24]. Compared to g-C3N4, the C 1s peaks of C−N=C and N 1s in g-C3N4/CoO/CoP both show a positive shift to higher energy, which may be due to the strong interfacial interaction. The delocalized electrons of g-C3N4 are transferred to CoP/CoO, resulting in an enhanced binding energy of g-C3N4. The XPS high-resolution Co 2p spectrum of g-C3N4/CoO/CoP shows the Co 2p₁/₂ peak at 793.3 eV and the Co 2p2/3 peak at 778.2 eV (Figure 4d) [25]. The characteristic peaks at 782.1 eV and 795.6 eV, as well as the satellite peaks at 786.5 eV and 800.6 eV, indicate the presence of CoO [26]. As seen in the high-resolution O 1s spectrum (Figure 4e), the peaks at 530.5 eV, 531.8 eV, and 533.2 eV are attributed to Co–O, C–O, and H2O on the surface, respectively. The P 2p spectra of g-C3N4/CoO/CoP shown in Figure 4f reveal two peaks at 128.8 eV and 130.3 eV due to the introduction of CoP [27]. Meanwhile, the peaks at 133.0 eV and 134.3 eV are attributed to P–N and P–O bonds [28], indicating the strong coupling interface between CoP and g-C3N4 connected by P–N bonds [29].

2.2. Narrowed Band Gap and Extended Light Absorption of g-C3N4/CoO/CoP

UV–vis diffuse reflectance spectroscopy was performed to investigate the absorption and band gap of the composite materials. As shown in Figure 5a, compared with g-C3N4, the visible light absorption edge of the g-C3N4/CoO/CoP composite catalyst shows a significant red shift, and the formation of CoP expands the visible light absorption. The band gaps of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP are 2.76 eV, 2.99 eV, and 3.02 eV, respectively (Figure 5b). Furthermore, the Mott–Schottky plots in Figure 5c–e reveal that g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP exhibit typical n-type semiconductor characteristics due to the positive slopes. The flat band potentials of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP were calculated to be −1.36 V, −1.79 V, and −1.67 V (vs. Ag/AgCl), or −1.16 V, −1.59 V, −1.47 V (vs. NHE). The flat band potential is closely related to the conduction band (ECB) position and is typically about 0.2 V below the ECB for n-type semiconductors [30], so the CB of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP are −1.36 V, −1.79 V, and −1.67 V (vs. NHE), respectively. The valence band (VB) potentials of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP can be calculated as 1.40, 1.20, and 1.35 eV [31,32]. According to the above results, the band structures of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP are illustrated in Figure 5f. Compared with g-C3N4, the ECB of g-C3N4/CoO/CoP shifts upward. Photogenerated electrons have a strong reducing ability, which is beneficial for improving the performance of photocatalytic hydrogen production.
For photocatalyst applications, electron-hole separation characteristics are crucially important. The transient photocurrent response is used to assess the number of photoexcited charge carriers, with higher photocurrent intensity indicating a higher number of photogenerated charge carriers [33]. Photocurrent directly reflects the mobility of the photogenerated electrons. The photocurrent rapidly increased to its maximum value when the light was turned on, due to the separation of electron-hole pairs at the heterostructure-electrolyte interface. On the contrary, when the light was turned off, the photocurrent rapidly decreased back to its initial value. Since the recombination rate of electrons and holes in the g-C3N4 electrode is high, this electrode displays a relatively weak photocurrent (Figure 6a). The photocurrent intensity generated by g-C3N4/CoO/CoP is significantly higher than that of g-C3N4 and g-C3N4/CoO, indicating that the introduction of CoP cocatalysts can effectively promote the separation of photogenerated carriers. Charge transport resistance was analyzed using Electrochemical Impedance Spectroscopy (EIS). EIS Nyquist plots can effectively reflect the charge transfer kinetics [34]. The EIS plots of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP are shown in Figure 6b. The smallest arc radius is observed for g-C3N4/CoO/CoP, which reveals a higher charge transfer, separation efficiency, and electrical conductivity under light irradiation.
Photoluminescence (PL) analysis is a useful tool for studying the migration, transfer, and separation of photogenerated electrons and holes in semiconductors. The intensity of the PL spectra depends on the rate of recombination of photogenerated electron-hole pairs. Figure 6c displays the PL spectra of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP at the excitation wavelength of 370 nm at room temperature. All catalysts have emission peaks at around 450 nm. The emission intensity of the g-C3N4/CoO/CoP sample is obviously lower than that of pristine g-C3N4 at the same emission position, indicating that electron-pair recombination in g-C3N4/CoO/CoP is suppressed. The above results suggest that effective separation and transportation of the photogenerated carriers were achieved after phosphating, further favoring the improved H₂ generation.

2.3. Enhancement and Mechanism of Photocatalytic Hydrogen Activity

To assess the photocatalytic performance of the as-prepared ternary photocatalyst, the HER was carried out under visible light irradiation. In Figure 7a, the hydrogen generation of all samples increases with illumination time. As shown in Figure 7b, g-C3N4/CoO/CoP exhibits the most significant photocatalytic performance (1277.9 μmol·g−1·h−1) at a CoO/CoP ratio of 0.21. When n = 0.07, most of the CoO is phosphated to CoP. The hydrogen evolution rate of g-C3N4/CoO/CoP-0.07 (714.4 μmol·g−1·h−1) is lower than that of g-C3N4/CoO/CoP-0.21, indicating that the optimal CoO to CoP ratio promotes the synergistic effect of the CoO and CoP dual cocatalysts, achieving efficient photocatalytic hydrogen evolution. In addition, by varying the loading amount of CoO (5, 7.5, 10, 12.5, 15%) while maintaining the optimal CoO/CoP ratio, it can be seen from Figure 7c,d that as the loading amount of CoO increases, the total amount of CoO/CoP formed after phosphating also increases, and the photocatalytic activity of the composite photocatalyst reaches a maximum before gradually decreasing. When the loading amount of CoO is 10%, the g-C3N4/CoO/CoP-0.21 ternary composite photocatalyst exhibits the best hydrogen evolution performance. This indicates that excessive CoO/CoP may hinder photon absorption by g-C3N4, thereby affecting the performance of photocatalytic hydrogen evolution.
To compare with g-C3N4 loaded with Pt as a precious metal cocatalyst, g-C3N4/Pt-1.5 wt% was prepared via in situ photo-deposition at room temperature. As shown in Figure 7e,f, the hydrogen evolution rate of g-C3N4 is nearly zero, which is due to the rapid electron-hole recombination in g-C3N4 under light excitation. After loading CoO, the hydrogen evolution rate significantly increased to 324.6 μmol·g−1·h−1. The g-C3N4/CoO/CoP-0.21 displays a four-fold increase in photocatalytic efficiency, even surpassing g-C3N4/Pt-1.5 wt% (1056.4 μmol·g−1·h−1). The apparent quantum efficiency (AQE) of g-C3N4/CoO/CoP-0.21 is 3.23% at 420 nm, 3.4 times higher than that of g-C3N4/CoO-10% (0.96%). These results indicate that the formation of CoP provides more active sites for photocatalytic reactions. g-C3N4, CoP, and CoO have a strong interface connection due to the formation of N-P bonds, promoting electron transfer and exhibiting enhanced hydrogen evolution activity.
The stability of the photocatalysts was further evaluated through cyclic testing (Figure 8a). The g-C3N4/CoO/CoP composite exhibited good stability, retaining 80% of its hydrogen evolution rate after four consecutive cycles. Compared to g-C3N4/CoO (Figure S5), the cycling stability was well improved, which is attributed to the formation of N-P bonds, enhancing the interaction between g-C3N4 and CoO/CoP. The XRD patterns before and after the photocatalytic experiment show consistent results (Figure 8b). This further demonstrates the strong binding between g-C3N4/CoO/CoP, addressing the issue of CoO detachment from the surface of g-C3N4 due to weak interfacial interactions. This results in excellent photocatalytic activity and good cycling stability for the g-C3N4/CoO/CoP-0.21 ternary composite material.
Based on the above results, a tentative photocatalytic mechanism for g-C3N4/CoO/CoP under visible light irradiation has been proposed (Scheme 2). Under visible light, both g-C3N4 and CoO can be excited to generate electrons and holes. Due to the appropriate band structure alignment between g-C3N4 and CoO [35], the excited electrons transfer from g-C3N4 to CoO, while the holes migrate from CoO to g-C3N4 and are subsequently consumed by triethanolamine. The photogenerated electrons on CoO are further captured by the cocatalyst CoP, providing active sites for proton reduction in the hydrogen evolution reaction. Additionally, the conduction band position of CoO is −0.25 eV, which is more negative than the reduction potential of H⁺/H2 (0 eV), making it favorable for hydrogen evolution reactions.
Compared to the g-C3N4/CoO system, the g-C3N4/CoO/CoP composite exhibits significantly enhanced photostability and hydrogen evolution activity. The primary reasons for these improvements are: (1) The introduction of CoP significantly enhances the separation efficiency of electrons and holes. Acting as an electron capture center, CoP effectively accelerates the transfer of photogenerated electrons from CoO to CoP, reducing electron-hole recombination and thereby improving photocatalytic performance. (2) CoP provides abundant active sites and features a low hydrogen evolution overpotential, enabling a more efficient proton reduction reaction and significantly enhancing the material’s hydrogen evolution capability. (3) Through N-P bonds, CoP forms a strongly coupled interface between g-C3N4 and CoO/CoP, effectively resolving the issue of CoO detachment, and further improving the structural stability and photocatalytic efficiency of the material.

3. Experimental

3.1. Material Synthesis

The reagents used in the experiments are detailed in the Supporting Information.

3.1.1. Synthesis of g-C3N4

Bulk g-C3N4 was synthesized by the well-reported thermal polymerization method [15]. Typically, 2 g of urea was calcined in a covered alumina crucible at 550 °C in air for 2 h with a ramping rate of 5 °C min−1.

3.1.2. Synthesis of g-C3N4/CoO

A total of 0.3 g of as-prepared g-C3N4 was well dispersed in 36 mL of a mixed solvent consisting of N, N-dimethylformamide (DMF) and DI water in a volume ratio of 1:2 and sonicated. Subsequently, a certain amount of cobalt(II) acetate tetrahydrate and sodium terephthalate (molar ratio 1:1.5) were added to the above suspension. After the mixed solution was stirred at 80 °C for 3 h, the system was naturally cooled to room temperature. The pale-pink precipitates were collected, washed with DI water and ethanol several times, and then dried in a vacuum oven at 60 °C. Finally, 0.15 g of the obtained powders were calcined at 475 °C in an argon atmosphere for 2 h with a ramping rate of 5 °C min−1. The products were denoted as g-C3N4/CoO-x (where x represents the weight percentage of CoO, x = 5%, 7.5%, 10%, 12.5%, 15%).

3.1.3. Synthesis of g-C3N4/CoO/CoP

A total of 50 mg of g-C3N4/CoO-10% and a certain amount of NaH2PO2·H2O were mixed and finely ground with a mortar for 30 min. The mixture was then heated at 300 °C for 2 h in an Ar atmosphere with a ramping rate of 2 °C min−1. After cooling to room temperature naturally, the resulting g-C3N4/CoO/CoP was washed with water and ethanol several times to remove unreacted NaH2PO2, then dried under vacuum at 60 °C for 12 h. The products were denoted as g-C3N4/CoO/CoP-n (where n represents the molar ratio of Co and P, n = 0.07, 0.14, 0.21, 0.28, 0.35)

3.2. Characterization

The X-ray diffraction (XRD) patterns of the as-prepared samples were collected using a powder X-ray diffractometer (Bruker, Billerica, MA, USA) (Cu-Kα radiation source). The morphology of the samples was examined by scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, TECNAI G2 F20, Thermo Fisher Scientific, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) was performed using a KRATOS Ultra DLD system (Manchester, UK). Ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) was recorded on a UH 4150 UV-Vis spectrophotometer (Hitachi, Tokyo, Japan) and converted into absorption spectra using the Kubelka–Munk transformation. Gas adsorption analysis was performed to measure the Brunauer–Emmett–Teller (BET) surface area. Fluorescence spectrometry was conducted using a Hitachi F-7100 (Tokyo, Japan) to obtain photoluminescence (PL) emission spectra. Fourier-transform infrared (FT-IR) spectra were obtained using a Tensor 27 spectrometer (Bruker, Billerica, MA, USA). Electrochemical measurements were carried out using a standard three-electrode system (working electrode, reference electrode, and counter electrode).
The relevant performance testing methods and photoelectrochemical characterization techniques are detailed in the Supporting Information.

4. Conclusions

In summary, a novel g-C3N4/CoO/CoP ternary photocatalytic composite has been successfully synthesized. CoO/CoP facilitates rapid electron transfer, minimizing electron-hole recombination and boosting photocatalytic efficiency. Additionally, CoP provides plentiful active sites with low overpotential, enabling efficient proton reduction, while the strongly coupled interface created by N-P bonds reinforces g-C3N4/CoO/CoP within the composite structure, ensuring its durability. By optimizing the ratio and loading of CoO/CoP, the photocatalytic performance of g-C3N4/CoO/CoP increased by four times compared to g-C3N4/CoO, which is equivalent to g-C3N4/Pt. After four cycles of reaction, it still shows good hydrogen evolution activity and photostability. The proposed strategy provides an effective approach to improve the photocatalytic performance of g-C3N4 and offers a promising protocol for the design of semiconductor photocatalysts.

Supplementary Materials

The following Supporting Information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15040315/s1. Photocatalytic hydrogen evolution test. Photoelectrochemical measurements. Figure S1. (a) TEM image of g-C3N4; (b) TEM image of g-C3N4/CoO-10%; and (c) STEM dark-field image with corresponding elemental mappings of C, N, O, and Co. Figure S2. XRD spectra of g-C3N4/CoO-n. Figure S3. FT-IR spectra of g-C3N4/CoO-n. Figure S4. (a) XPS survey spectrum of g-C3N4/CoO-10%; (b) C 1s; (c) N 1s; (d) O 1s; (e) Co 2p spectra of g-C3N4/CoO-10%. Figure S5. Cycling stability test of g-C3N4/CoO-10%.

Author Contributions

Conceptualization, Y.H., X.Y. and Y.L.; Methodology, Y.H., X.Y., Z.L. and Y.L.; Software, X.Y.; Validation, Z.W.; Formal analysis, Y.H. and Z.W.; Investigation, X.Y. and Y.L.; Resources, Z.L. and Y.L.; Writing—original draft, Y.H.; Writing—review and editing, Z.W. and Y.L.; Visualization, Z.W.; Project administration, Z.L.; Funding acquisition, Z.L. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (Nos. 21671176 and 21541011) and the Natural Science Foundation of Henan Province (No. 232300421361).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. Schematic illustration of the preparation of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP.
Scheme 1. Schematic illustration of the preparation of g-C3N4, g-C3N4/CoO, and g-C3N4/CoO/CoP.
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Catalysts 15 00315 g001
Figure 1. SEM images of (a) g-C3N4; (b) g-C3N4/CoO/CoP-0.21; TEM images of (c) g-C3N4; (d) g-C3N4/CoO/CoP-0.21; HRTEM (e) and elemental mapping images of (f) g-C3N4/CoO/CoP-0.21.
Figure 1. SEM images of (a) g-C3N4; (b) g-C3N4/CoO/CoP-0.21; TEM images of (c) g-C3N4; (d) g-C3N4/CoO/CoP-0.21; HRTEM (e) and elemental mapping images of (f) g-C3N4/CoO/CoP-0.21.
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Figure 2. (a) XRD spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21. (b) FT-IR spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-n.
Figure 2. (a) XRD spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21. (b) FT-IR spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-n.
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Figure 3. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
Figure 3. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
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Figure 4. (a) XPS spectra of g-C3N4 and g-C3N4/CoO/CoP-0.21; (b) C 1s; (c) N 1s spectra of g-C3N4 and g-C3N4/CoO/CoP-0.21; (d) Co 2p; (e) O 1s; (f) P 2p spectra of g-C3N4/CoO/CoP-0.21.
Figure 4. (a) XPS spectra of g-C3N4 and g-C3N4/CoO/CoP-0.21; (b) C 1s; (c) N 1s spectra of g-C3N4 and g-C3N4/CoO/CoP-0.21; (d) Co 2p; (e) O 1s; (f) P 2p spectra of g-C3N4/CoO/CoP-0.21.
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Figure 5. (a) UV–vis diffuse reflectance spectra; (b) Tauc plots of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21. Mott–Schottky plots of (c) g-C3N4, (d) g-C3N4/CoO-10%, and (e) g-C3N4/CoO/CoP-0.21; (f) the band structures of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
Figure 5. (a) UV–vis diffuse reflectance spectra; (b) Tauc plots of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21. Mott–Schottky plots of (c) g-C3N4, (d) g-C3N4/CoO-10%, and (e) g-C3N4/CoO/CoP-0.21; (f) the band structures of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
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Figure 6. (a) Photocurrent-time curves; (b) EIS spectra; (c) PL spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
Figure 6. (a) Photocurrent-time curves; (b) EIS spectra; (c) PL spectra of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
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Figure 7. Hydrogen production and average hydrogen evolution rate of (a,b) g-C3N4/CoO/CoP-n; (c,d) g-C3N4/CoO-x/CoP-0.21; (e,f) g-C3N4, g-C3N4/CoO-10%, g-C3N4/CoO/CoP-0.21, and g-C3N4/Pt-1.5 wt%.
Figure 7. Hydrogen production and average hydrogen evolution rate of (a,b) g-C3N4/CoO/CoP-n; (c,d) g-C3N4/CoO-x/CoP-0.21; (e,f) g-C3N4, g-C3N4/CoO-10%, g-C3N4/CoO/CoP-0.21, and g-C3N4/Pt-1.5 wt%.
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Figure 8. (a) Cycling stability test; (b) XRD patterns of g-C3N4/CoO/CoP-0.21 before and after the photocatalytic experiment.
Figure 8. (a) Cycling stability test; (b) XRD patterns of g-C3N4/CoO/CoP-0.21 before and after the photocatalytic experiment.
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Scheme 2. Schematic illustration of the photocatalytic mechanism of g-C3N4/CoO/CoP.
Scheme 2. Schematic illustration of the photocatalytic mechanism of g-C3N4/CoO/CoP.
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Table 1. Specific surface area, pore volume, average pore size of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
Table 1. Specific surface area, pore volume, average pore size of g-C3N4, g-C3N4/CoO-10%, and g-C3N4/CoO/CoP-0.21.
SampleSurface Area (m2·g−1)Pore Volume (cm3·g−1)
g-C3N489.80.16
g-C3N4/CoO-10%91.90.18
g-C3N4/CoO/CoP-0.2146.90.09
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Han, Y.; Wang, Z.; Yang, X.; Li, Z.; Li, Y. In Situ Reinforced g-C3N4/CoO/CoP Ternary Composite for Enhanced Photocatalytic H2 Production. Catalysts 2025, 15, 315. https://doi.org/10.3390/catal15040315

AMA Style

Han Y, Wang Z, Yang X, Li Z, Li Y. In Situ Reinforced g-C3N4/CoO/CoP Ternary Composite for Enhanced Photocatalytic H2 Production. Catalysts. 2025; 15(4):315. https://doi.org/10.3390/catal15040315

Chicago/Turabian Style

Han, Yanan, Zhaohui Wang, Xiuyuan Yang, Zhongjun Li, and Yike Li. 2025. "In Situ Reinforced g-C3N4/CoO/CoP Ternary Composite for Enhanced Photocatalytic H2 Production" Catalysts 15, no. 4: 315. https://doi.org/10.3390/catal15040315

APA Style

Han, Y., Wang, Z., Yang, X., Li, Z., & Li, Y. (2025). In Situ Reinforced g-C3N4/CoO/CoP Ternary Composite for Enhanced Photocatalytic H2 Production. Catalysts, 15(4), 315. https://doi.org/10.3390/catal15040315

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