Morphology and doping engineering of sulfur-doped g-C₃N₄ hollow nanovesicles for boosting photocatalytic hydrogen production

Abstract

The rational design and directional synthesis of desirable structural heteroatom-doped graphitic carbon nitride (CN) is of great significance for achieving efficient photocatalytic hydrogen evolution (HER) performance, but challenges remain. Herein, we have successfully developed an attractive sulfur-doped hollow CN nanovesicle (HV-SCN) photocatalyst via a supramolecular self-assembly strategy. The engineered HV-SCN not only possesses a large specific surface area, strong hydrophilicity and high light absorption capacity, but also displays efficient photogenerated carrier excitation and transfer efficiency. Consequently, the resultant HV-SCN achieves an extremely high H₂ generation rate of 9.49 mmol h⁻¹ g⁻¹. Subsequent density functional theory (DFT) calculations and band configuration results confirm that S-doping induces band gap shortening and favorable hydrogen adsorption, which leads to enhanced photocatalytic HER performance of the HV-SCN. Furthermore, the catalytic mechanism and carrier migration dynamics are confirmed by in situ X-ray photoelectron and femtosecond transient absorption spectroscopy (fs-TAS). This study provides valuable experimental and theoretical references for the rational design and directional preparation of high-performance catalysts.

---

1. Introduction

Hydrogen (H₂), as a renewable, efficient and non-polluting energy source, is considered to be one of the most promising new energy sources to replace fossil fuels.^1,2 Photocatalytic water splitting for H₂ production has received extensive attention as a clean and efficient technology.³ Therein, exploring an efficient photocatalyst for the H₂ evolution reaction (HER) is the key to the high-quality development of this technology.^4,5 In the past decade, various attractive semiconductors, including metal oxides, metal sulfides, non-metallic semiconductors, and bismuth-based nanomaterials, have been developed as active HER photocatalysts.^6–8 Among various photocatalysts, graphitic carbon nitride (g-C₃N₄, CN) has broad prospects in the field of photocatalytic HER due to its low cost, environmental friendliness and excellent photoelectrochemical properties.^9–13 However, the intrinsic limitations of inefficient band configuration, low specific surface area, poor light absorption capacity, and rapid charge-hole recombination rate endow CN with unsatisfactory photocatalytic activity.^14–18

Sulfur (S) doping is an effective strategy to manipulate the band gap and electronic structure of CN, thereby optimizing its photoelectrochemical properties.^19–22 For example, Wu et al. successfully synthesized S-doped CN nanosheets by using a sulfuric acid pretreatment precursor strategy, which greatly promotes its near-infrared HER photocatalytic process.²³ Moreover, several previous studies of our group also confirmed the important role of S-doping in regulating band structures and catalytic performances of CN.^24–26 Although S-doping provides a superior band configuration for CN, the low specific surface area, weak light absorption capacity, and long carrier transfer distance still greatly limit its photocatalytic HER activity.²⁷ Based on this, the favorable morphology engineering of S-doped CN is inevitable for constructing remarkable CN-based HER photocatalysts. Typically, a hollow nanovesicle structure with large porosity and high interfacial reaction area is extremely beneficial for the HER that occurs at the interface/surface of the photocatalyst.^28–34 For example, Li et al. constructed a 0D carbon dots/3D porous CN nanovesicle heterostructure for efficient photocatalytic HER. This special structure produces more photoexcited electrons due to the reflection of incident light, shortens the carrier transport distance, and achieves efficient photocatalytic HER activity.³⁵ Similarly, Sun's group designed and prepared carbon dot anchored porous CN vesicles via one-step template-free thermal polymerization. The porous vesicle structure not only expands the specific surface area, but also exposes the internal and external double reaction sites, thus resulting in superior photocatalytic HER activity.³⁶ Although the synergistic effect of heteroatom doping and morphological regulation to improve the photocatalytic HER performance of CN is convincing, the available results of combining morphology and doping engineering for constructing efficient CN-based photocatalysts are still limited. Moreover, the spatial location, charge properties and catalytic mechanism of S dopants also remain elusive. Therefore, clarifying the subtle regulation of the S-doping position and type, as well as its contribution to carrier migration kinetics and HER photocatalytic activity is of great importance, but still challenging.

In this work, a highly active HER photocatalyst, sulfur-doped hollow CN nanovesicle (HV-SCN), is cleverly constructed by supramolecular self-assembly of melamine, cyanuric acid, and trithiocyanuric acid, as well as subsequent pyrolysis. We systematically investigated the important promoting effects of constructing S-doped hollow vesicle nanostructures on surface area, porosity, hydrophilicity, band configuration and photoelectrochemical properties of CN. As expected, the HV-SCN achieves efficient photocatalytic HER activity with an extremely high H₂ generation rate of 9.49 mmol h⁻¹ g⁻¹. Furthermore, DFT computation, in situ XPS and fs-TA spectroscopy are employed for an in-depth analysis of the catalytic HER mechanism and carrier transfer kinetics of the HV-SCN. Importantly, this work provides a valuable reference for the design and preparation of high performance heteroatom-doped catalysts.

2. Experimental section

2.1 Preparation of bulk carbon nitride (CN)

Melamine (MA, 2 g) is dissolved in a mixed solution of 20 mL deionized water and 10 mL N,N-dimethylformamide (DMF). After sonication and agitation, the resulting suspension is washed with deionized water and then dried naturally. Finally, the obtained sample is calcined at 550 °C for 3 h in a N₂ atmosphere to prepare CN.

2.2 Fabrication of porous carbon nitride (PCN)

Different mass ratios of MA and cyanuric acid (CA) are dissolved in a mixed solution of 20 mL deionized water and 10 mL DMF (MA : CA = 1 : 1, 1 : 2, 1 : 3, 2 : 1, 3 : 1). After sonication and agitation, the MA–CA (MC) supramolecular complex precursor is formed. Finally, the washed and dried MC is calcined at 550 °C in a N₂ atmosphere for 3 h to obtain PCN.

2.3 Synthesis of S-doped g-C₃N₄ hollow nanovesicles (HV-SCNs)

Different mass ratios of MA, CA and trithiocyanuric acid (TCY) are dissolved in a mixed solution of 20 mL deionized water and 10 mL DMF (MA : CA : TCY = 1 : 1 : 0.25, 1 : 1 : 0.5, 1 : 1 : 0.75, 1 : 1 : 1, 1 : 1 : 2, 1 : 1 : 3, 1 : 1 : 4, 1 : 1 : 5, 1 : 1 : 6). After sonication and agitation, the MA–CA–TCY (MCT) supramolecular complex precursor is formed. Finally, the washed and dried MCT is calcined at 550 °C in a N₂ atmosphere for 3 h to obtain HV-SCNs.

3. Results and discussion

The fabrication of HV-SCNs is skilfully realized via copolymerization of melamine (MA), cyanuric acid (CA), and trithiocyanuric acid (TCY), as well as a subsequent pyrolysis process (Fig. S1†). Fig. 1a, S2 and S3† show the molecular structures of the supramolecular complex precursors and HV-SCNs formed by monomer (MA, CA and TCY) polymerization or high temperature pyrolysis. Two-dimensional (2D) CN nanosheets (Fig. 1b and f) are prepared by thermal condensation polymerization of MA monomers (Fig. S2†).³⁷ After the addition of CA, a typical self-assembly polymerization occurs between MA and CA monomers, resulting in the formation of a hydrogen-bonded MA–CA (MC, Fig. S3†) supramolecular precursor.³⁸ During the subsequent pyrolysis process, the MC precursor is transformed into porous CN nanosheets (PCN, Fig. 1c and g) due to the release of a large amount of gas (carbon dioxide, nitrogen, ammonia gas, etc.).^39,40 The fabrication of S-doped g-C₃N₄ hollow nanovesicles (HV-SCNs) is similar to that of PCN, the only difference being the introduction of TCY involved in precursor self-assembly, yielding hydrogen-bonded MA–CA–TCY (MCT) supramolecular complexes. Consequently, a few S atoms are smoothly linked into the CN structure during pyrolysis.⁴¹ Moreover, the involvement of TCY in MC polymerization is central to the construction of hollow structures. Specifically, MA self-assembles with CA and TCY through hydrogen bonding to form graft copolymers. As the graft copolymers continue to be generated, the hydrophobic segments attract each other to promote molecular aggregation, thus evolving into vesicular micellar aggregates (Fig. 1a). Therein, the introduction of abundant polar groups in CA and TCY endowed the micellar clusters with strong intermolecular binding, which is conducive to the stable formation of nanovesicle precursors. During the subsequent high-temperature pyrolysis, the vesicular precursor decomposes and generates a large amount of gases (NH₃, H₂O, CO₂, etc.), thereby resulting in the formation of pore-rich hollow nanovesicle structures.

Fig. 1 (a) Schematic illustration of the preparation process of HV-SCNs. (b and f) SEM images of CN, (c and g) PCN and (d and e) HV-SCNs. (h and i) TEM images of HV-SCNs. (j) Elemental mappings and (k) EDX spectrum of HV-SCNs.

The typical scanning electron microscopy (SEM) results (Fig. 1d, e and S4†) suggest the well defined nanovesicle structure of developed HV-SCNs. The corresponding transmission electron microscopy (TEM) images not only confirm the expected hollow structure, but also reveal a shell layer thickness of about 10 nm (Fig. 1h and i). In addition, the elemental mappings of HV-SCNs (Fig. 1j) disclose the successful S doping. Meanwhile, the atomic content of S dopants is found to be 0.23% according to the EDX measurement (Fig. 1k).

X-ray diffraction (XRD) patterns clarify the crystal type. The two characteristic peaks located at 13.0 and 27.5° correspond to the (100) and (002) planes of CN (Fig. 2a), which are related to its intra- and inter-layer stacking, respectively. The almost disappeared (100) diffraction peak of PCN and the HV-SCN suggests the destroyed planar stacking structure of CN introduction after introducing CA.⁴² The HV-SCN possesses the weakest (002) peak intensity in comparison to those of CN and PCN, which results from the reduced size of the CN skeleton and the integration of S-dopants.⁴³

Fig. 2 (a) XRD patterns, (b) N₂ adsorption–desorption isotherms and (c) contact angles of CN, PCN and the HV-SCN. (d) High-resolution XPS spectra of S 2p, (e) C 1s and (f) N 1s for PCN and the HV-SCN.

Fig. 2b displays the N₂ sorption isotherms of CN, PCN and the HV-SCN. The type-II isotherm accompanied by an obvious H4 hysteresis loop suggests the coexistence of micropores and mesopores for PCN and the HV-SCN. Moreover, the corresponding pore size distribution also indicates their favorable pore structure. Noteworthily, the surface area and pore volume of HV-SCN are calculated to be 70.93 m² g⁻¹ and 0.405 cm³ g⁻¹ (Table S1†), which are significantly larger than those of CN (5.08 m² g⁻¹ and 0.029 cm³ g⁻¹), revealing the unique advantages of the hollow structure in exposing the active site and promoting carrier transport.⁴⁴ Considering the important role of the material hydrophilicity in photocatalytic water splitting for H₂ production, we systematically record the contact angle of the CN, PCN and HV-SCN samples. Compared with CN and PCN, the HV-SCN possesses the smallest contact angle (37.5°, Fig. 2c), which demonstrates its enhanced H₂O adsorption behavior over the developed HV-SCN catalyst.^45,46

X-ray photoelectron spectroscopy (XPS) and elemental analyzer (EA) techniques are employed to study the configuration of sulfur doping. Compared with PCN, the increased C/N atomic ratio of the HV-SCN obtained by EA and XPS indicates S-doping at the N-sites of PCN (Tables S2 and S3†). The almost invisible S signal for the XPS spectrum of the HV-SCN is due to a small amount of the S dopant (Fig. S5†).⁴⁷Fig. 2d presents the high-resolution S 2p spectra, where the characteristic peaks appearing at 164.3 and 168.3 eV are attributed to S–C and S–O (Table S4†),⁴⁸ which demonstrates the occurrence of S-doping. The C 1s and N 1s spectra are all deconvoluted into three peaks (Fig. 2e and S7†), in which the binding energies of 284.6, 287.8 and 288.3 eV correspond to C–C, C–NH₂ and N–C=N bonds,⁴⁹ while the peaks centred at 398.6, 399.2 and 400.9 eV are assigned well to C–N=C, N–(C)₃ and C–NH₂ (Fig. 2f), respectively.⁵⁰ The smaller N–C=N and C–N=C relative proportions of the HV-SCN in comparison to those of PCN (Tables S5 and S6†) prove the doping substitution of sp² hybridized N atoms by added S atoms.^51–53 Fig. S6c and S7† depict the O 1s spectra of PCN and the HV-SCN, where the obvious two peaks located at 531.8 and 533.0 eV reveal the existence of two adsorbed oxygen types, which are mainly due to water, oxygen or carbon dioxide adhering to the catalyst surface (Table S7†).

To obtain the suitable S-doping degree and the best photocatalytic HER performance, the precursor ratio, calcination temperature, and pyrolysis retention time are systematically optimized as shown in Fig. S8.† Accordingly, the MA : CA : TCY mass ratio, calcination temperature and holding time are identified as 1 : 1 : 4, 550 °C, and 3.5 h, respectively. Fig. 3a and b compare the H₂ evolution rate of CN, PCN and the HV-SCN. Remarkably, the developed HV-SCN catalyst possesses a H₂ evolution rate of 9.49 mmol h⁻¹ g⁻¹, which is as much as 45 times higher than that of CN, confirming the irreplaceable and significant role of S doping in photocatalytic performance. Moreover, such excellent photocatalytic HER activity of the HV-SCN only displays negligible attenuation during the continuous catalytic process for 24 h, demonstrating its robust long-term catalytic stability (Fig. 3c). The results of XRD patterns before and after 24 h continuous photocatalytic HER testing confirm the good structural integrity of the HV-SCN (Fig. S9†). Fig. S10† displays the photocatalytic stability of the afforded HV-SCN sample after resting in a dry environment for 40 days, in which the attenuation of the H₂ evolution rate is only around 8%, further confirming its outstanding reusability. In addition, the promising applicability of the HV-SCN is also confirmed by its efficient photocatalytic HER performance at different light wavelengths (Fig. S11†). In particular, the apparent quantum efficiency (AQE) of the HV-SCN is even as high as 7.09% at 365 nm (Fig. 3d). More importantly, the H₂ evolution rate of the HV-SCN also exceeds that of most other S-doped CN photocatalysts (Fig. 3e and Table S8†).

Fig. 3 (a and b) Photocatalytic HER performance of CN, PCN and the HV-SCN. (c) Cycling tests of the HV-SCN. (d) AQEs of the HV-SCN. (e) Comparison of HER activities between the HV-SCN and other S-doped CN catalysts.

The photoelectrochemical properties and band configurations are compared between CN, PCN and the HV-SCN to elucidate the effect of S-doping on the photocatalytic HER activities. The strongest ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) intensity of the developed HV-SCN between 300 and 700 nm (Fig. 4a) confirms the enhanced light absorption capability after the construction of sulfur-doped hollow structures. The photoluminescence (PL) spectra of CN, PCN and the HV-SCN are exhibited in Fig. 4b, in which the lowest emission peak intensity at 456 nm for the HV-SCN demonstrates that S-doping introduces electron trapping sites and significantly suppresses carrier recombination.⁵⁴ The longest average fluorescence lifetime of the HV-SCN compared to that of CN and PCN (Fig. 4c and Table S9†) also reveals its beneficial carrier behavior. Fig. 4d and e present the transient photocurrent (TPC) and electrochemical impedance spectroscopy (EIS) results of all samples. Compared with CN and PCN, the developed HV-SCN not only possesses the largest photocurrent density, but also shows the smallest EIS Nyquist curve semicircle. Such a result convincingly proves that tailoring the S-doped hollow vesicle nanoskeleton is beneficial to carrier excitation and transfer.⁵⁵ Thus, the HV-SCN displays the smallest HER overpotential (Fig. 4f) in comparison to that of CN and PCN.

Fig. 4 (a) UV-vis DRS, (b) PL, (c) TRPL, (d) TPC responses, (e) EIS, (f) HER polarization curves, and (g) estimated E_g of CN, PCN and the HV-SCN. (h) M–S plots of the HV-SCN at different frequencies. (i) Energy band diagrams of CN, PCN and the HV-SCN.

Changes in the band configuration of CN after S-doping and morphology modulation are revealed by resolving the results of UV-vis DRS and Mott–Schottky (M–S) curves. The band gap energies (E_g) of CN, PCN and the HV-SCN are presented in Fig. 4g, where the smallest E_g value of NV-SCN (2.45 eV) compared to that of CN and PCN discloses the favorable carrier excitation of constructing the S-doped hollow vesicle nanostructure.⁵⁶ Typical N-type semiconductor features of developed photocatalysts are confirmed by their distinguishably positive M–S plot slopes (Fig. 4h and S12†).⁵⁷ Moreover, the flat band potentials of CN, PCN and the HV-SCN are determined to be −0.52, −0.59 and −0.44 eV (vs. RHE). Generally, the conduction band (CB) position of an N-type semiconductor is ∼0.2 eV more negative than its flat band potentials.^58,59 Accordingly, the CB and valence band (VB) positions of CN, PCN and the HV-SCN are obtained (Fig. 4i) by combining the above E_g results.

DFT is employed to deeply investigate the differences in band and electron structures of CN before and after S-doping of the N-sites. Initially, the Gibbs formation energy of three possible S-doped CN configurations (N1, N2, and N3, Fig. S13†) is simulated by DFT calculations. The smallest formation energy (−0.60 eV) observed in Fig. S11c† suggests the easiest doping substitution of sp² hybridized N atoms by added S atoms, which is consistent with the above XPS results. Subsequently, the Gibbs free energy of adsorbed hydrogen (ΔG_H*) on the three S-doped active sites is calculated. As shown in Fig. 5a, b and S14,† the optimal ΔG_H* value is found to be −0.24 eV after S-doping of the N2 site (HV-SCN-S2, Fig. S12a†), which accurately indicates the important promotion effect of S-doping on the photocatalytic HER activity of CN.⁶⁰Fig. 5c and e exhibit the theoretical band structures and density of states (DOS) of CN and the HV-SCN. Theoretical simulations also confirm that S-doping significantly reduces the band gap of CN. Specifically, the introduction of S-dopants increases the VB potential, thus leading to a shortening of the band gap, which is critical for enhancing light absorption.⁶¹ Additionally, the hollow vesicle nanostructure induces multiple light reflections to further improve the ability of light capture and absorption (Fig. 5d), as convincingly demonstrated by the significantly enhanced UV-vis DRS intensity and TPC response of the HV-SCN compared with those of CN and PCN.

Fig. 5 (a) Optimized structure of H* adsorption at the S site for the HV-SCN. (b) ΔG_H* of the HV-SCN. (c) Band structures and DOS results of CN and (e) HV-SCN. (d) Light reflection mechanism of the HV-SCN. (f) Charge density difference of the HV-SCN (yellow and green regions represent charge accumulation and depletion, respectively). (g) In situ XPS spectra of C 1s, (h) N 1s and (i) S 2p for the HV-SCN.

Fig. 5f and S15† present the calculated charge density difference of the HV-SCN between the H* active intermediate and S adsorption active site. Obviously, the S dopants lead to a significant redistribution of charges at the doping site. As exhibited in Fig. 5f, the electrons tend to cluster around the doped S atoms, resulting in a large number of H* adsorption sites, which provides a strong internal driving force for HER photocatalysis. Such an electron migration mechanism of the developed HV-SCN is further clarified through in situ irradiation XPS measurement (Fig. 5g–i). Distinctly, the C 1s and N 1s peaks of the HV-SCN move to the high binding energy direction under light illumination, while the S 2p peak shifts toward low binding energy. Moreover, after turning off the lights, these peaks shift in the opposite directions, remaining consistent with the initial dark state. The results indicate that the photogenerated electrons of the HV-SCN tend to migrate from the C and N positions to the S-doped sites. Consequently, the negative potential accumulated around the S doped sites facilitates hydrogen adsorption, thereby enhancing the photocatalytic HER activity of the HV-SCN.

Based on the above characterization and experimental results, Fig. S16† elucidates the plausible photocatalytic HER mechanism of the HV-SCN. Under illumination, visible light penetrates the HV-SCN via the hollow pores and is efficiently captured after undergoing multiple reflections within the interior. Subsequently, HV-SCNs are excited to produce abundant photogenerated electron and hole pairs, which are usually randomly transferred. Fortunately, the strong affinity of the S-doping site for photogenerated electrons promotes the rapid separation and directional migration of carriers, which effectively avoids the recombination of electron–hole pairs in the bulk phase or at the surface of the HV-SCN, thereby prolonging the carrier lifetime. In addition, the hollow vesicle structure not only provides a unique internal and external catalytic microenvironment, but also endows the HV-SCN with a large specific surface area, which provides abundant active sites for the adsorption of water molecules, thereby facilitating the evolution of H₂.

The carrier transport dynamics of the HV-SCN is disclosed by femtosecond transient absorption spectroscopy (fs-TAS) technology. Broad positive signals in the range of 600–750 nm are all observed for CN, PCN and the HV-SCN, which are caused by the excited-state absorption (ESA) properties of photogenerated electrons (Fig. 6).^62–64 The absorption intensities of fs-TAS decrease significantly with the prolongation of the detection time, indicating that the number of active photogenerated electrons is negatively correlated with time.⁶⁵ The delayed dynamics of photo-excited carriers of CN, PCN and HV-SCN are investigated via a double exponential function attenuation fitting at 650 nm.⁶⁶Fig. 6c, f, i and Table S10† display the corresponding fitting results, where the two fast decay parameters correspond to the captured photo-induced electrons from CB to the shallow trap (τ₁), and the accumulated electrons in the deep trap state (τ₂).^67,68 Compared with CN (10.17 ps) and PCN (4.24 ps), the HV-SCN catalyst possesses a smaller τ₁ (3.42 ps), which reveals the favorable carrier excitation after constructing the S-doped hollow nanovesicle structure. The τ₂ of the HV-SCN is determined to be 120.58 ps, which is longer than that of PCN, indicating its long carrier lifetime. Therefore, the integration of S doping and the hollow structure accelerates the carrier migration kinetics of CN, thereby endowing the HV-SCN with efficient HER photocatalytic performance.

Fig. 6 (a and b) fs-TAS spectra of CN, (d and e) PCN and (g and h) HV-SCN. Normalized fs-TAS kinetics of (c) CN, (f) PCN and (i) HV-SCN.

4. Conclusions

In summary, a promising HER photocatalyst, sulfur-doped hollow CN nanovesicle (HV-SCN), has been successfully constructed. The HV-SCN displays a well-defined hollow vesicle nanostructure with a large surface area, high light absorption capacity, and strong hydrophilicity. Various advanced characterization techniques confirm that the construction of the S-doped hollow vesicle nanoskeleton not only greatly promotes the excitation and transfer efficiency of photogenerated carriers, but also effectively alleviates the recombination of electron–hole pairs. Accordingly, the photocatalytic H₂ generation rate of the developed HV-SCN is as high as 9.49 mmol h⁻¹ g⁻¹, surpassing that of most recently reported sulfur-doped CN photocatalysts. DFT calculations and in situ XPS results reveal that the excellent catalytic performance of the HV-SCN is due to the shortening of the band gap and the enhancement of hydrogen adsorption by S-doping. Moreover, fs-TAS spectra results confirm the promoting effect of S-doping and hollow nanovesicle structures on carrier migration kinetics. Most importantly, the HV-SCN photocatalyst developed in this study possesses the advantages of low cost, simple preparation, high H₂ evolution efficiency, and excellent stability, which provides valuable prerequisites for the large-scale production of HER photocatalysts and industrial H₂ production.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Natural Science Foundation of Hubei Province (No. 2024AFB890 and 2021CFB133), the Liuzhou Science and Technology Fund project (2023PRJ0103 and 2024AA0204A001), and the Innovation Project of Hubei Three Gorges Laboratory (SC240007).

References

1. D. Gunawan, J. Zhang and Q. Li, et al., Materials advances in photocatalytic solar hydrogen production: integrating systems and economics for a sustainable future, Adv. Mater., 2024, 36, e2404618.
2. W. Shi, Z. Xu and Y. Shi, et al., Constructing S-scheme charge separation in cobalt phthalocyanine/oxygen-doped g-C₃N₄ heterojunction with enhanced photothermal-assisted photocatalytic H₂ evolution, Rare Met., 2024, 43, 198–211.
3. P. Zhou, I. A. Navid and Y. Ma, et al., Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting, Nature, 2023, 613, 66–70.
4. J. Fan, H. Wang and W. Sun, et al., Recent developments and perspectives of Ti-based transition metal carbides/nitrides for photocatalytic applications: A critical review, Mater. Today, 2024, 76, 110–135.
5. L. Lin, Y. Ma and J. Vequizo, et al., Efficient and stable visible-light-driven Z-scheme overall water splitting using an oxysulfide H₂ evolution photocatalyst, Nat. Commun., 2024, 15, 397.
6. B. Yang, L. Lu and S. Liu, et al., Recent progress in perylene diimide supermolecule-based photocatalysts, J. Mater. Chem. A, 2024, 12, 3807–3843.
7. A. Shabalina, E. Fakhrutdinova and A. Golubovskaya, et al., Laser-assisted preparation of highly-efficient photocatalytic nanomaterial based on bismuth silicate, Appl. Surf. Sci., 2022, 575, 151732.
8. L. Kuate, Z. Chen and Y. Yan, et al., Construction of 2D/3D black g-C₃N₄/BiOI S-scheme heterojunction for boosted photothermal-assisted photocatalytic tetracycline degradation in seawater, Mater. Res. Bull., 2024, 175, 112776.
9. D. Zhao, Y. Wang and C.-L. Dong, et al., Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for photocatalytic overall water splitting, Nat. Energy, 2021, 6, 388–397.
10. H. Zeng, Z. Li and G. Li, et al., Interfacial engineering of TiO₂/Ti₃C₂ MXene/carbon nitride hybrids boosting charge transfer for efficient photocatalytic hydrogen evolution, Adv. Energy Mater., 2021, 12, 2102765.
11. F. Li, X. Yue and Y. Liao, et al., Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction, Nat. Commun., 2023, 14, 3901.
12. L. Chen, M. Maigbay, M. Li and X. Qiu, Synthesis and modification strategies of g-C₃N₄ nanosheets for photocatalytic applications, Adv. Powder Mater., 2024, 3, 100150.
13. Z. Zhang, K. Xiang and H. Wang, et al., Advanced carbon nitride-based single-atom photocatalysts, SusMat, 2024, 4, e229.
14. L. Lin, Z. Lin and J. Zhang, et al., Molecular-level insights on the reactive facet of carbon nitride single crystals photocatalysing overall water splitting, Nat. Catal., 2020, 3, 649–655.
15. M. Yang, R. Lian and X. Zhang, et al., Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction, Nat. Commun., 2022, 13, 4900.
16. H. Ou, S. Ning and P. Zhu, et al., Carbon nitride photocatalysts with integrated oxidation and reduction atomic active centers for improved CO₂ conversion, Angew. Chem., Int. Ed., 2022, 61, 202206579.
17. S. R. Nagella, R. Vijitha and B. Ramesh Naidu, et al., Benchmarking recent advances in hydrogen production using g-C₃N₄-based photocatalysts, Nano Energy, 2023, 111, 108402.
18. H. Ou, S. Ning and P. Zhu, et al., Carbon nitride photocatalysts with integrated oxidation and reduction atomic active centers for improved CO₂ conversion, Angew. Chem., Int. Ed., 2022, 61, e202206579.
19. F. Liu, W. Li and L. Wang, et al., Sulfur- and strontium-doped graphitic carbon nitride for efficient photocatalytic hydrogen evolution, ACS Appl. Energy Mater., 2022, 5, 15834–15843.
20. L. Wang, Z. Yang and G. Song, et al., Construction of S-N-C bond for boosting bacteria-killing by synergistic effect of photocatalysis and nanozyme, Appl. Catal., B, 2023, 325, 122345.
21. J. Wang, Q. Zhao and P. Kumar, et al., Solar-driven cellulose photorefining into arabinose over oxygen-doped carbon nitride, ACS Catal., 2024, 14, 3376–3386.
22. Y. Liu, Y. Zheng and W. Zhang, et al., Template-free preparation of non-metal (B, P, S) doped g-C₃N₄ tubes with enhanced photocatalytic H₂O₂ generation, J. Mater. Sci. Technol., 2021, 95, 127–135.
23. X. Wu, D. Li and B. Luo, et al., Molecular-level insights on NIR-driven photocatalytic H₂ generation with ultrathin porous S-doped g-C₃N₄ nanosheets, Appl. Catal., B, 2023, 325, 122292.
24. H. Wang, J. Jiang and L. Yu, et al., Tailoring advanced N-defective and S-doped g-C₃N₄ for photocatalytic H₂ evolution, Small, 2023, 19, e2301116.
25. H. Wang, J. Fan and J. Zou, et al., Sulfur-doped g-C₃N₄/V₂C MXene Schottky junctions for superior photocatalytic H₂ evolution, J. Mater. Chem. A, 2024, 12, 30429–30441.
26. H. Wang, L. Yu, J. Jiang, Arramel and J. Zou, S-doping of the N-Sites of g-C₃N₄ to enhance photocatalytic H₂ evolution activity, Acta Phys.-Chim. Sin., 2023, 40, 2305047.
27. H. Yang, S. Sun, J. Lyu, Q. Yang and J. Cui, Mechanism insight into triple S-Scheme intermolecular carbon nitride homojunction with robust built-in electric field for highly enhanced photocatalytic hydrogen evolution, Chem. Eng. J., 2024, 481, 148297.
28. N. Dharmarajan, D. Vidyasagar and J. H. Yang, et al., Bio-inspired supramolecular self-assembled carbon nitride nanostructures for photocatalytic water splitting, Adv. Mater., 2023, 36, 2306895.
29. A. AbdelHamid, A. Mendoza-Garcia and J. Ying, Advances in and prospects of nanomaterials' morphological control for lithium rechargeable batteries, Nano Energy, 2022, 93, 106860.
30. C. Yang, H. Hu, C. Qian and Y. Liao, Hollow sp²-conjugated covalent organic framework encapsulating thiophene-based photosensitizer for enhanced visible-light-driven hydrogen evolution, J. Mater. Chem. A, 2023, 11, 25899–25909.
31. X. Sun, Z. Chen and Y. Shen, et al., Efficient photothermal-assisted photocatalytic H₂ production using carbon dots-infused g-C₃N₄ nanoreactors synthesized via one-step template-free thermal polymerization, Chem. Eng. J., 2024, 488, 151041.
32. H. Luo, T. Shan and J. Zhou, et al., Controlled synthesis of hollow carbon ring incorporated g-C₃N₄ tubes for boosting photocatalytic H₂O₂ production, Appl. Catal., B, 2023, 337, 122933.
33. Y. Zhang, M. Zhang and Z. Yu, et al., Visual light-driven valorization of biomass-based 5-hydroxymethylfurfural over ZnS/ZnIn₂S₄ heterostructures as hollow nanoreactors, Appl. Catal., B, 2024, 350, 123914.
34. R. Liu, Z. Yu and R. Zhang, et al., Hollow Nanoreactors for Controlled Photocatalytic Behaviors: Fundamental Theory, Structure-Performance Relationship, and Catalytic Advantages, Small, 2024, 20, 2308142.
35. B. Li, W. Peng and J. Zhang, et al., High-throughput one-photon excitation pathway in 0D/3D heterojunctions for visible-light driven hydrogen evolution, Adv. Funct. Mater., 2021, 31, 2100816.
36. X. Sun, Z. Chen and Y. Shen, et al., Efficient photothermal-assisted photocatalytic H₂ production using carbon dots-infused g-C₃N₄ nanoreactors synthesized via one-step template-free thermal polymerization, Chem. Eng. J., 2024, 488, 151041.
37. C. Cheng, L. Mao and X. Kang, et al., A high-cyano groups-content amorphous-crystalline carbon nitride isotype heterojunction photocatalyst for high-quantum-yield H₂ production and enhanced CO₂ reduction, Appl. Catal., B, 2023, 331, 122733.
38. X. Sun, Y. Shi, J. Lu, W. Shi and F. Guo, Template-free self-assembly of three-dimensional porous graphitic carbon nitride nanovesicles with size-dependent photocatalytic activity for hydrogen evolution, Appl. Surf. Sci., 2022, 606, 154841.
39. H. Niu, W. Zhao, H. Lv, Y. Yang and Y. Cai, Accurate design of hollow/tubular porous g-C₃N₄ from melamine-cyanuric acid supramolecular prepared with mechanochemical method, Chem. Eng. J., 2021, 411, 128400.
40. J. Bao, W. Bai and M. Wu, et al., Template-mediated copper doped porous g-C₃N₄ for efficient photodegradation of antibiotic contaminants, Chemosphere, 2022, 293, 133607.
41. T. Fei, C. Qin and Y. Zhang, et al., A 3D peony-like sulfur-doped carbon nitride synthesized by self-assembly for efficient photocatalytic hydrogen production, Int. J. Hydrogen Energy, 2021, 46, 20481–20491.
42. S. Hou, C. Yu and X. Song, et al., Modulating in-plane defective density of carbon nanotubes by graphitic carbon nitride quantum dots for enhanced triiodide reduction, Adv. Funct. Mater., 2023, 33, 2212112.
43. H. Duan, J. Fan and C. Liu, et al., Green synthesis of phosphorus-doped g-C₃N₄ as an advanced photocatalyst for H₂ evolution, NANO, 2025 DOI:10.1142/S1793292025500328.
44. H. Wang, X. Li and Y. Deng, et al., Advances in MXene-based single-atom catalysts for electrocatalytic applications, Coord. Chem. Rev., 2025, 529, 216462.
45. K. Li, L. Bao and S. Cao, et al., Template-assisted surface hydrophilicity of graphitic carbon nitride for enhanced photocatalytic H₂ evolution, ACS Appl. Energy Mater., 2021, 4, 12965–12973.
46. C. Tang, T. Bao and S. Li, et al., Rapid charge transfer endowed by hollow-structured Z-scheme heterojunction for coupling benzylamine oxidation with CO₂ reduction, Adv. Funct. Mater., 2024, 2415280.
47. J. Lu, Z. Chen and Y. Shen, et al., Boosting photothermal-assisted photocatalytic H₂ production over black g-C₃N₄ nanosheet photocatalyst via incorporation with carbon dots, J. Colloid Interface Sci., 2024, 670, 428–438.
48. H. Wang, X. Qiu and Z. Peng, et al., Cobalt-gluconate-derived high-density cobalt sulfides nanocrystals encapsulated within nitrogen and sulfur dual-doped micro/mesoporous carbon spheres for efficient electrocatalysis of oxygen reduction, J. Colloid Interface Sci., 2020, 561, 829–837.
49. W. Wang, J. Gong and Q. Long, et al., Fe-Fe₃N composite nitrogen-doped carbon framework: multi-dimensional cross-linked structure boosting performance for the oxygen reduction reaction electrocatalysis and zinc-air batteries, Appl. Surf. Sci., 2023, 639, 158218.
50. X. Qiu, Y. Yu and Z. Peng, et al., Cobalt sulfides nanoparticles encapsulated in N, S co-doped carbon substrate for highly efficient oxygen reduction, J. Alloys Compd., 2020, 815, 152457.
51. Q. Li, Y. Jiao and Y. Tang, et al., Shear stress triggers ultrathin-nanosheet carbon nitride assembly for photocatalytic H₂O₂ production coupled with selective alcohol oxidation, J. Am. Chem. Soc., 2023, 145, 20837–20848.
52. S. P, J. John and T. Rajan, et al., Graphitic carbon nitride (g-C₃N₄) based heterogeneous single atom catalysts: synthesis, characterisation and catalytic applications, J. Mater. Chem. A, 2023, 11, 8599–8646.
53. B. Qu, P. Li and L. Bai, et al., Atomically dispersed Zn-N₅ sites immobilized on g-C₃N₄ nanosheets for ultrasensitive selective detection of phenanthrene by dual ratiometric fluorescence, Adv. Mater., 2023, 35, e2211575.
54. Z. Liu, J. Ma, M. Hong and R. Sun, Potassium and sulfur dual sites on highly crystalline carbon nitride for photocatalytic biorefinery and CO₂ reduction, ACS Catal., 2023, 13, 2106–2117.
55. Y. Yu, L. Jiang and X. Zhang, et al., MoP nanocrystals encapsulated in N-doped three-dimensional carbon networks for efficient hydrogen evolution electrocatalysis, New J. Chem., 2024, 48, 2707–2714.
56. E. Hu, Q. Chen and Q. Gao, et al., Cyano-functionalized graphitic carbon nitride with adsorption and photoreduction isosite achieving efficient uranium extraction from seawater, Adv. Funct. Mater., 2024, 34, 2312215.
57. S. Gupta, Y. Kwak, R. P. Raj and P. Selvam, Ytterbium–nitrogen co-doped ordered mesoporous TiO₂: an innovative hetero-phase photocatalyst for harnessing solar energy in green hydrogen production, J. Mater. Chem. A, 2024, 12, 6906–6927.
58. Z. Zhou, J. Wang, M. Reheimujiang and Z. Jin, Graphdiyne/hierarchical flower-like Sr₂Co₂O₅ S-scheme heterojunction for enhanced photocatalytic hydrogen evolution, J. Mater. Sci. Technol., 2025, 213, 241–251.
59. Z. Chen, J. Hu and X. Lu, et al., Effect mechanism of Cd on band structure and photocatalytic hydrogen production performance of ZnS, Int. J. Hydrogen Energy, 2024, 88, 1147–1155.
60. Y. Shen, X. Du and Y. Shi, et al., Bound-state electrons synergy over photochromic high-crystalline C₃N₅ nanosheets in enhancing charge separation for photocatalytic H₂ production, Adv. Powder Mater., 2024, 3, 100202.
61. C. Feng, L. Tang and Y. Deng, et al., A novel sulfur-assisted annealing method of g-C₃N₄ nanosheet compensates for the loss of light absorption with further promoted charge transfer for photocatalytic production of H₂ and H₂O₂, Appl. Catal., B, 2021, 281, 119539.
62. Y. Gu, H. Feng and J. Zhao, et al., Rational construction of edge-grafted g-C₃N₄via cross-linking aromatic compounds with C-F bonds for efficient photocatalytic H₂ evolution, Chem. Eng. J., 2023, 476, 146555.
63. W. Wang, X. Bai and Q. Ci, et al., Near-field drives long-lived shallow trapping of polymeric C₃N₄ for efficient photocatalytic hydrogen evolution, Adv. Funct. Mater., 2021, 31, 2103978.
64. Y. Shen, R. Xu and P. Shan, et al., Abundant edge active sites-modified high-crystalline g-C₃N₅ for hydrogen peroxide production from pure-water via a quasi-homogeneous photocatalytic process, Small, 2024, 20, 2401566.
65. D. Jiang, S. Hu and Y. Qu, et al., Infrared irradiation-lattice vibration coupling-initiated N→Π* electronic transition in carbon nitride nanosheets for increased photocatalysis, Adv. Funct. Mater., 2024, 34, 2311803.
66. X. Yang, L. Ren and D. Jiang, et al., Strong interfacial chemical bonding in regulating electron transfer and stabilizing catalytic sites in a metal-semiconductor Schottky junction for enhanced photocatalysis, Small, 2024, 20, e2308408.
67. F. Gao, H. Xiao and J. Yang, et al., Modulation of electronic density in ultrathin g-C₃N₄ for enhanced photocatalytic hydrogen evolution through an efficient hydrogen spillover pathway, Appl. Catal., B, 2024, 341, 123334.
68. D. Zu, Y. Ying and Q. Wei, et al., Oxygen vacancies trigger rapid charge transport channels at the engineered interface of S-Scheme heterojunction for boosting photocatalytic performance, Angew. Chem., Int. Ed., 2024, 63, e202405756.

---

Footnotes

† Electronic supplementary information (ESI) available: Characterization, photoelectrochemical measurements, photocatalytic HER tests, and details of theoretical calculations, as well as supplementary figures and tables associated with this article. See DOI: https://doi.org/10.1039/d4ta09249j

‡ These authors contributed equally to this work.

---