Precipitation synthesis and characterization of SnO₂@g-C₃N₄ heterojunctions for enhanced photocatalytic H₂ production

Abstract

This study reports the development of SnO₂@g-C₃N₄ heterojunctions, a hybrid semiconductor photocatalyst with varying mass percent ratios using a facile precipitation method for hydrogen (H₂) production. The synergistic effect between the SnO₂ nanoparticles and g-C₃N₄ sheets suppresses the charge recombination and enhances carrier separation, leading to improved photocatalytic activity. The nanocomposites demonstrate increased hydrogen production across all composites, with SC-20 sample (i.e., 80% SnO₂ and 20% g-C₃N₄) achieving the highest H₂ production rate of 287.7 μmol g⁻¹ h⁻¹, that is, 1.87-fold and 1.63-fold higher than that of SnO₂ and of g-C₃N₄ counterparts, respectively. Furthermore, the nanocomposites maintain excellent photostability. Specifically, SC-20 achieves approximately 1500 μmol H₂ evolution per 5 hour-cycle. The facile precipitation-based synthesis and enhanced photocatalytic activity of the SnO₂@g-C₃N₄ nanocomposite position it as a reliable, cost-effective, and sustainable candidate for solar-driven hydrogen production and other clean energy applications.

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Introduction

Population growth and rapid industrialization have significantly increased the global demand for energy, which is predominantly met by finite, non-renewable petroleum resources.^1–3 The continuous depletion of these energy reserves, combined with the environmental damage caused by their combustion, has accelerated global warming and climate change, creating an energy crisis that threatens both energy security and environmental sustainability.^4–8 To address these critical issues, relevant stakeholders have prioritized the development of alternative energy sources that are environmentally friendly, sustainable, and cost-effective. In this context, hydrogen energy has emerged as a promising solution to mitigate the energy crisis and to reduce environmental pollution due to its clean nature and high energy content.⁹

Prominent approaches to produce hydrogen include semiconductor-based photocatalytic water splitting. In particular, such a method has gained significant attention as a renewable and eco-friendly approach, as it not only provides a clean energy source but also reduces ecological pollution by decreasing reliance on fossil fuels.^10,11 Over the past few decades, various semiconductor materials explored for this purpose, specifically metal oxide materials including SrTiO_3, TiO₂, ZrO₂, Ta₂O_5, ZnO, WO₃, and SnO₂, have been systematically studied for their photocatalytic properties.^{1,4,5,12–14} However, these materials face significant limitations. For instance, TiO₂ and SnO₂, with a large bandgap energy, absorb only UV light, utilizing merely 4% of sunlight. On the other hand, materials like ZnO are prone to photo-corrosion under illumination, while WO₃ is inactive for H₂ production due to its low conduction band edge potential.^13–15

In response, several composite photocatalysts have been developed to address these limitations, among which graphitic carbon nitride (g-C₃N₄) emerged as a promising candidate.^16,17 g-C₃N₄ is a metal-free polymeric semiconductor with a suitable bandgap of 2.7 eV, enabling efficient sunlight absorption and charge carrier excitation.^18,19 Additionally, its high chemical and thermal stability, attributed to its polymeric structure and degree of polymerization, render it a robust material for photocatalytic applications.^20,21 Furthermore, g-C₃N₄ is cost-effective and can be readily synthesized through the simple thermal decomposition of urea.²² As a result, it has been extensively explored for photocatalytic hydrogen evolution,^23,24 and a variety of g-C₃N₄-based heterostructures have been reported, including BiOCl/g-C₃N₄,²⁵ and CdS/g-C₃N₄,^26,27 and metal oxide-based ones such as Bi₂WO₆/g-C₃N₄,^28,29 TiO₂/g-C₃N₄,^30,31 ZnO/g-C₃N₄,^32,33 and TaON/g-C₃N₄,³⁴ which have shown promise in suppressing charge recombination and enhancing photocatalytic performance through the formation of heterojunction.^35–39

Among metal oxide-based heterostructures, tin dioxide (SnO₂) is notable due to its non-toxicity, low cost, and excellent optical, physical and photoelectrochemical properties,^40,41 which understandably finds wide range applications in, e.g., energy storage, gas sensing, solar cells, photocatalysis, electronics, and electrochemical cells.^42–44 In the context of photocatalysts, previous studies have shown that SnO₂ outperforms ZnO and TiO₂ as an electron acceptor, making it a more appealing candidate for such a system.⁴⁵ With these properties, SnO₂ thus constitutes a rational choice to be coupled with g-C₃N₄ to enhance its photocatalytic performance.⁴⁶ In recent years, several studies have reported the synthesis of SnO₂/g-C₃N₄ heterostructures for photocatalytic hydrogen production using various methods, including ultrasonic-assisting deposition,⁴⁷ sol–gel,⁴⁸ hydrothermal,^49,50 and solid-phase methods.^51,52 While these techniques offer advantages like crystallinity, good interfacial contact, and promising H₂ evolution performance, they are often time consuming, require high temperature and pressure, and involve toxic solvents and specialized equipment, making them less scalable and environmentally friendly.⁵³ Therefore, developing simple, low-cost, scalable and eco-friendly techniques to synthesize SnO₂@g-C₃N₄ photocatalyst for hydrogen evolution is highly desirable.

As a response, in this study we report the successful synthesis of SnO₂@g-C₃N₄ hybrid photocatalyst via a simple and cost-effective precipitation method, which is widely recognized for producing metal oxide nanoparticles under mild conditions (ambient temperature and pressure) without the use of toxic gases.^54–57 To the best of our knowledge, no previous studies reported the use of precipitation-based synthesis for constructing SnO₂/g-C₃N₄ heterojunctions for photocatalytic H₂ production. To address this gap, we systematically varied the mass ratios of SnO₂ to g-C₃N₄, and we found SC-20 sample (i.e., 80% SnO₂ and 20% g-C₃N₄) to deliver the highest and stable photocatalytic hydrogen production, indicating an optimal interface for charge transfer. Aided with various materials characterization, the improved hydrogen production is attributed to the enhancement in the photo-induced charge carrier separation and suppressed charge recombination. This performance positions our synthesis strategy to be comparable with other complex methods employing noble metal catalysts, highlighting its potential for further optimization and practical applications.

Experimental section

Chemicals

All chemicals, including urea (CH₄N₂O), SnCl₂·2H₂O and NH₃ were purchased from Sigma-Aldrich without further purification. DI water was produced using water distillation apparatus DU-L4 MEDILAB.

Synthesis of g-C₃N₄ nanosheets

g-C₃N₄ nanosheets were prepared using the bath sonication method. First, 10 g of urea (CH₄N₂O) was heated in a muffle furnace at 550 °C for 3 h to produce bulk g-C₃N₄, which was subsequently ground into a powder. To convert this into nanosheets, 0.05 g of g-C₃N₄ powder was sonicated in 50 mL of distilled water for 90 min. The material was washed seven times with distilled water and dried at 70 °C for 24 h, yielding the desired light-yellow g-C₃N₄ nanosheets.

Preparation of SnO₂ nanoparticles

SnO₂ nanoparticles were synthesized via precipitation. First, 2.5 g of SnCl₂·2H₂O was dissolved in 50 mL DI water under stirring. A diluted ammonia solution (4 mL of 35% ammonia in 26 mL DI water) was added dropwise under continuous stirring to the SnCl₂·2H₂O solution until the mixture turned milky at pH 9. The precipitate was washed five times with DI water by centrifugation, dried at 60 °C for 24 h, ground into a powder, and annealed at 400 °C for 2 h.

Preparation of SnO₂@g-C₃N₄ nanocomposites

SnO₂@g-C₃N₄ nanocomposites with varying mass ratios were synthesized via a precipitation method as illustrated in Fig. 1. Initially, a measured quantity of g-C₃N₄ was dispersed in DI water, followed by the dissolution of SnCl₂·2H₂O under continuous stirring. A 35% ammonia solution was added dropwise to the mixture under stirring until a milky precipitate formed at pH 9. The precipitate was washed five times with DI water, dried at 60 °C for 24 h, ground into a fine powder, and annealed at 400 °C for 2 h. The prepared composites were labeled as SC-X, where X denotes the at% of the g-C₃N₄, that is, SC-10 comprises 90% SnO₂ and 10% g-C₃N₄, SC-20 does 80% SnO₂ and 20% g-C₃N₄, SC-30 does 70% SnO₂ and 30% g-C₃N₄, and SC-40 does 60% SnO₂ and 40% g-C₃N₄.

Fig. 1 Step-by-step schematic illustration of synthesis of SnO₂@g-C₃N₄ nanocomposites. First, the pre-synthesized g-C₃N₄ nanosheets are dispersed in 50 mL of deionized (DI) water. Subsequently, SnCl₂·2H₂O and NH₃ are added to the solution, followed by stirring and pH adjustment. After washing and drying, the resulting precipitates are then annealed. The final product, SnO₂@g-C₃N₄ composites, appears as light-yellow sheets with black dots on the surface.

Physiochemical characterizations

Various characterization techniques were employed to analyze the properties of the prepared samples. XRD (JDX-3532, JEOL) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA confirmed the tetragonal structure for SnO₂, hexagonal structure for g-C₃N₄, and the coexistence of both in SnO₂@g-C₃N₄ composites. FTIR (Cary 630, Agilent Technologies) identified Sn–O and C–N vibrational modes in SnO₂ and g-C₃N₄, respectively. SEM (JSM-6490A, JEOL) revealed sheet-like morphology for g-C₃N₄, particle morphology for SnO₂, and heterojunction formation in the composites. UV-vis (PerkinElmer) spectra showed enhanced absorbance in the visible region for the composites. PL (LS45 PerkinElmer) investigated the lower recombination of photo-generated electrons.

Photoelectrochemical measurements

Photocurrent (PC) measurements and electrochemical impedance spectroscopy (EIS) were performed using a CHI-660E electrochemical workstation with platinum counter electrode, Ag/AgCl reference electrode, and 0.5 M Na₂SO₄ electrolyte solution.

Photocatalytic H₂ generation measurements

Photocatalytic hydrogen production was evaluated in a flask irradiated by a 300 W xenon lamp (CEL HXF300), equipped with a cutoff filter (λ > 420 nm). For the experiment, 20 mg of the prepared sample was dispersed in 80 mL of aqueous solution containing 10% methanol as the hole scavenger, via ultrasonication. Prior to irradiation, the system was purged with nitrogen (N₂) for 15 min to eliminate any residual oxygen. The reaction was carried out under continuous stirring and illumination for a period of 5 h, and the amount of hydrogen produced was quantified using gas chromatography (GC-2014, Shimadzu).

Results and discussions

As the first step of our study, we investigate the structural properties and the molecular composition of our prepared samples using X-ray diffraction (XRD). Fig. 2a shows the obtained XRD patterns of pure g-C₃N₄ and SnO₂, their composites, as well as the corresponding JCPDS references for g-C₃N₄ and SnO₂. The diffraction pattern of pure g-C₃N₄ shows a characteristic peak at 2θ = 27.5° corresponding to the (002) reflection of its graphitic stacking structure, which matches perfectly with the corresponding standard reference (ICSD 01-087-1526), confirming the hexagonal structure of pure g-C₃N₄.⁵⁸ In the case of pure SnO₂, observed peaks at 26.4°, 34°, 38°, 51.5°, and 65° correspond to (110), (101), (200), (211) and (301) planes, respectively, corroborating the tetragonal rutile structure of the SnO₂ (JCPDS 41-1445).⁵⁹ Examining the patterns of the composites (SC-10 to SC-40), the coexistence of both hexagonal g-C₃N₄ and tetragonal SnO₂ phases is confirmed, in that the corresponding patterns comprise the characteristic peaks of the constituents.⁶⁰ This finding thus corroborates the successful formation of heterojunction structures, notably without introducing any impurity phase. In detail, the intensity of the SnO₂ diffraction peaks gradually decrease with increasing g-C₃N₄ content from SC-10 to SC-30. Interestingly, among the composites, SC-40 shows a diffraction pattern closely resembling with pure g-C₃N₄, likely due to the higher g-C₃N₄ content in SC-40.

Fig. 2 X-ray diffraction pattern of g-C₃N₄, SnO₂, and their composites, alongside its standard reference. The coexistence of both g-C₃N₄ and SnO₂ signature peaks in the composite samples confirms their successful formation.

Moreover, FTIR spectroscopy was performed to analyze the chemical bonding and functional groups of the samples. As shown in Fig. 3, the pristine g-C₃N₄ exhibit peaks at 1243–1637 cm⁻¹, which correspond to C–N and C=N stretching vibrations, at 808 cm⁻¹, which is attributed to the tri-s-triazine ring structure, and at 3180–3331 cm⁻¹, which belongs to N–H stretching mode.^58,61 For pure SnO₂, a broad peak at 659 cm⁻¹ represents the Sn–O stretching mode in Sn–O–Sn.⁶² To this end, the absence of additional peaks in the spectra of pure g-C₃N₄ and SnO₂ confirms their high purity, consistent with the XRD data above. In the case of the composites (SC-10 to SC-30), the distinct vibrational modes of both g-C₃N₄ and SnO₂ are observed, corroborating the successful formation of the nanocomposites. The FTIR spectrum of SC-40, however, closely resembles that of g-C₃N₄, again indicating a dominant presence of g-C₃N₄ in this composition.

Fig. 3 FTIR spectra of pure g-C₃N₄, SnO₂, and their composites in the range of 500–4000 cm⁻¹. The characteristic C–N, C=N, and Sn–O bonds, along with some additional bonds, are observed in pure g-C₃N₄ and SnO₂, as well as their coexistence in the composites.

Last, Scanning Electron Microscopy (SEM) was employed to analyze the morphology of the synthesized samples. As shown in Fig. 4a and b, pure g-C₃N₄ exhibits a sheet-like morphology, though the sheets are not distinctly visible due to agglomeration. Meanwhile, Fig. 4c and d confirm the formation of well-defined SnO₂ nanoparticles with average diameters around 94.5 nm (Fig. S1†). When integrated into a heterostructure, the SnO₂ nanoparticles are expected to spread uniformly on the g-C₃N₄ sheets (Fig. 4e and f). In addition, energy dispersive X-ray (EDX) analysis confirmed the elemental composition and purity of g-C₃N₄, SnO₂ and SnO₂@g-C₃N₄ nanocomposite (Fig. S2†). Specifically, the g-C₃N₄ spectra displays distinct peaks corresponding to elements C and N, while SnO₂ shows peaks attributed to elements Sn and O. Finally, the SnO₂@g-C₃N₄ composite exhibits peaks Sn, O, C and N, confirming no impurity and creation of highly pure composite.

Fig. 4 Heterostructure morphology. SEM image of (a and b) g-C₃N₄, (c and d) SnO₂ and (e and f) SnO₂@g-C₃N₄ nanocomposites. g-C₃N₄ assumes sheet-like morphology, while SnO₂ does particle-like morphology. In the composite form, both these morphologies are observed.

Next, we proceed to investigate the photoctalytic H₂ production performance of the synthesized heterostructures, along with those of their pure counterparts. To this end, we performed the photocatalytic reaction under visible light irradiation for 5 h (Fig. 5a). First we note that the hydrogen production of pure g-C₃N₄ is higher than that of SnO₂, throughout the reaction. Converting into production rate, pristine g-C₃N₄ exhibits H₂ production of 175.98 μmolg⁻¹ h⁻¹ compared to 160.84 μmolg⁻¹ h⁻¹ of SnO₂ (Fig. 5b). This behavior can be explained by the narrower bandgap, and thus higher visible light absorption, of g-C₃N₄ compared to SnO₂, as we will show later. Notably, and of our interest here, all four composites exhibit greater hydrogen generation than pure SnO₂ and g-C₃N₄, indicating the formation of an effective heterojunction that promotes interfacial charge transfer and suppresses photogenerated charge recombination. Among the composites, the champion sample is SC-20, whose production reaches 287.7 μmolg⁻¹ h⁻¹, which is 1.63 times higher than g-C₃N₄ and 1.79 times higher than SnO₂. This performance enhancement can be attributed to the optimal ratio between SnO₂ and g-C₃N₄, which ensures sufficient interfacial contact for charge while maintaining visible light absorption. From the results in the composites it is evident that as the amount of SnO₂ increases, the hydrogen production increases, reaching a maximum at SC-20. Surprisingly, further increasing the SnO₂ content leads to a decline in hydrogen production at SC-10. This decline can be attributed to the shielding effect, as the g-C₃N₄ nanosheets become fully covered by SnO₂ nanoparticles. Since SnO₂ is a wide-bandgap semiconductor, this coverage inhibits the effective absorption of visible light, thereby reducing hydrogen production.^63,64 Furthermore, the hydrogen production level in SC-40 is comparable to that of pure g-C₃N₄, as the high g-C₃N₄ content dominates. This observation is consistent with the XRD pattern of the SC-40 above. Establishing that SC-20 outperforms all other composites, in the subsequent discussion we will focus on SC-20, alongside pure g-C₃N₄ and SnO₂ for further analysis regarding the enhancement mechanism.

Fig. 5 Photocatalytic H₂ production performance of the as prepared SnO₂, g-C₃N₄, and their composites. (a) H₂ production over 5 h, showing SC-20 achieving the highest performance; (b) H₂ production rate per hour, where SC-20 outperforms pure SnO₂, g-C₃N₄, and other composites, a clear increasing trend is observed from SC-40 to SC-20, reaching a value of 287.7 μmolg⁻¹ h⁻¹. However, a further increase in SnO₂ content (SC-10) results in a decline in H₂ production, indicating SC-20 as the optimal composition.

To better understand the enhanced hydrogen evolution performance of SC-20 compared to its counterparts, additional verification of its optical properties is necessary. To assess the light-harvesting capabilities, UV-vis spectroscopy was conducted in the wavelength range of 200–800 nm. The UV-vis spectra of pure SnO₂, g-C₃N₄, and SnO₂@g-C₃N₄ nanocomposites, along with their corresponding Tauc plots, are presented in Fig. 6a–d, respectively. Pure g-C₃N₄ shows strong absorption in the visible region due to its narrow bandgap (2.62 eV), while SnO₂ exhibit absorption predominantly in the UV region, attributed to its wider bandgap (3.29 eV). Notably, SC-20 sample demonstrates a broader absorption across both visible and UV regions. The Tauc plot of the composite SC-20 reveals an apparent bandgap of approximately 2.75 eV, which represents the integrated optical response of the heterostructure. This apparent bandgap likely arises from the synergistic interaction between SnO₂ and g-C₃N₄, leading to enhanced photoresponse and improved photocatalytic activity.^65,66

Fig. 6 Optical properties of g-C₃N₄, SnO₂, and SC-20 composite. UV-vis spectra of (a) g-C₃N₄, (b) SnO₂ and (c) SC-20 composite, along with their corresponding Tauc's plots shown as in (d). The band gaps were estimated to be 2.62 eV for g-C₃N₄, 3.29 eV for SnO₂, and 2.75 eV for SC-20. The slight increase in band gap of SC-20 compared to g-C₃N₄ is due to incorporation of SnO₂, while maintaining visible-light absorption, indicating successful heterojunction formation.

Furthermore, to evaluate the separation efficiency and photoresponse of the photocatalysts, time-resolved photocurrent measurements were carried out under visible light illumination. In a photocurrent analysis, the sample is sandwiched between electrodes and then excited by a light pulse to generate charges. The photo-generated charges produce current on the electrodes, measured under light-on and light-off. The light-on phase represents charge buildup on electrodes, while light-off phase shows charge decay. As shown in Fig. 7a, SC-20 heterojunction exhibits the highest and most stable photocurrent density compared to pure SnO₂ and g-C₃N₄. The sharp photocurrent spikes under light-on and rapid decay during light-off indicate fast charge generation and relatively good carrier mobility. This enhanced photocurrent response in SC-20 can be attributed to improved charge separation and interfacial coupling between SnO₂ and g-C₃N₄, which facilitate directional charge transfer and reduce recombination losses. To further investigate charge transport dynamics, electrochemical impedance spectroscopy (EIS) was performed. In the Nyquist plots shown in Fig. 7b, SC-20 displays the smallest semicircular arc radius, indicating the lowest charge transfer resistance among the tested samples, followed by g-C₃N₄ and SnO₂. The decreased arc radius for SC-20 validates the formation of an efficient heterojunction, supporting the photocurrent findings. Moreover, photoluminescence (PL) spectroscopy was employed to assess photo-generated charge carrier recombination behavior in photocatalysts. In Fig. 7c SC-20 displayed significantly quenched photoluminescence intensity, compared to pure SnO₂ and g-C₃N₄. This quenching is the evidence of enhanced suppression of charge carrier recombination in the composite. These results collectively indicate that SC-20 heterostructure exhibits enhanced charge carrier separation, faster interfacial electron transport, and reduced recombination rates compared to the pure components.

Fig. 7 Photocurrent, EIS, and PL analyses of SnO₂, g-C₃N₄, and SC-20. (a) Transient photocurrent response of pure SnO₂, g-C₃N₄, and SC-20 composite under chopped visible light irradiaion, shows that SC-20 exhibits a higher photocurrent compared to pure SnO₂ and g-C₃N₄, attributed to the synergistic effect in SC-20. (b) Nyquist plots of SC-20, g-C₃N₄, and SnO₂, illustrating their electrochemical impedance properties. SC-20 exhibits the smallest semi-circle, confirming the low charge transfer resistance in composite as compared to pure counterparts. (c) PL spectra of SC-20, g-C₃N₄, and SnO₂. SC-20 shows the lowest PL, indicating low charge recombination under illumination of visible light.

Last, to evaluate the long-term photostability and practical viability of our SnO₂@g-C₃N₄ (SC-20) nanocomposite, a four-cycle photocatalytic hydrogen evolution test was conducted, with each cycle lasting 5 h under visible light irradiation (Fig. 8). Throughout all four cycles, the H₂ evolution rate remained nearly constant, suggesting excellent structural and photoelectrochemical stability. The consistent performance indicates that the heterojunction interface effectively prevents photocorrosion and supports stable charge separation over extended illumination periods. Specifically, SC-20 achieves approximately 1500 μmol H₂ evolution per 5 hour-cycle, demonstrating negligible performance loss. This robustness surpasses that of many conventional such as TiO₂, ZnO, Fe₂O₃, and WO₃ based photocatalysts, which typically exhibit a noticeable decline in activity due to rapid charge accumulation, poor stability and limited visible light absorption.^67–70 The photocyclic results thus highlight the material's potential for real PEC applications requiring long-term operation without catalyst regeneration or replacement.

Fig. 8 Photostability of SC-20. Time-dependent H₂ evolution of SC-20 over four consecutive 5-hour cycles under visible light irradiation. Our best composite SC-20 maintains nearly constant H₂ production rate across all four cycles, demonstrating its strong photostability.

Comparative study

Having established the enhanced performance of our heterostructures compared to their pure counterparts, it is then interesting to benchmark them against other works reported in the literature employing similar heterostructures, however using different synthesis routes. As shown in Table 1, our composite, synthesized via a simple precipitation method, achieved a hydrogen evolution of 287.7 μmolg⁻¹ h⁻¹, positioning it within the middle range of previously reported works. Notably, the heterostructures incorporating Pt as a co-catalyst or utilizing more advanced nanostructures such as quantum dots or nanodots have shown superior performance, often exceeding 1300 μmolg⁻¹ h⁻¹. However, these methods often need precise control over nanostructures formation and multistep complicated synthesis methods, which can limit large-scale production. In conclusion, while advanced nanostructures and Pt incorporation boost H₂ evolution, our coprecipitation-based SnO₂@g-C₃N₄ nanocomposites offers a competitive balance of photocatalytic performance, synthesis simplicity, and cost-effectiveness. These findings suggest that further enhancements of photocatalytic activity may be possible through the incorporation of co-catalysts like Pt or quantum dots, as well as through refined interface engineering strategies.

Table 1 H₂ evolution performance of SnO₂/g-C₃N₄-based photocatalysts reported in the literature

Photocatalyst	Preparation method	Amount of H2 gas evolved	References
g-C3N4/SnO2	Solvent evaporation followed by calcination	132 μmol h^−1	71
Pt g^−1-C3N4/SnO2	One pot pyrolysis	241 μmol h^−1 g^−1	72
SnO2@g-C3N4nanocomposites	Coprecipitation	287.7 μmol h^−1g^−1	Our work
Pt g^−1-C3N4/SnO2	Simple calcination	627 μmol h^−1	73
g-C3N4/SnO2–Pt	Physical mixing	900 μmol h^−1 g^−1	74
C3N4–SnO2–Pt	Hydrothermal	1060 μmol h^−1 g^−1	75
SnO2 QDs/g-C3N4	Thermal decomposition	1305.4 μmol h^−1 g^−1	76
SnO2 nanodots/g-C3N4	One-step polymerization	1398.2 μmol h^−1 g^−1	77

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Photocatalytic mechanism of SnO₂@g-C₃N₄

To close the discussion, a Z-scheme heterojunction mechanism is proposed based on band structure alignment and charge transfer behavior to explain the enhanced photocatalytic activity of SnO₂@g-C₃N₄ composite. As illustrated in Fig. 9, g-C₃N₄ is a visible light-driven photocatalyst with bandgap ≈ 2.62 eV (cf. Fig. 6d) while the photoresponse of SnO₂ is limited to UV-region with a bandgap ≈ 3.29 eV (cf. Fig. 6d). Furthermore, the conduction band position of g-C₃N₄ is (−1.12 eV vs. NHE) while SnO₂ have (0.05 eV vs. NHE).^78,79 upon solar illumination, both SnO₂ and g-C₃N₄ are excited to generate electron–hole pairs. In the Z-scheme configuration, photogenerated electrons in the conduction band of SnO₂ recombine with holes in the valence band of g-C₃N₄ at the heterojunction interface. This unique charge transfer pathway preserves the highly energetic electrons in the CB of g-C₃N₄ and the strong oxidizing holes in the VB of SnO₂, effectively enhancing the overall redox capability of the heterojunction.⁶⁰ As a result, the photogenerated electrons in the conduction band of g-C₃N₄ take participation in the reduction of H⁺ ions to produce H₂. Meanwhile, photogenerated holes in the valence band of SnO₂ oxidize water molecules, generating O₂. The synergistic effect between these two components enhances overall photocatalytic performance. This mechanism aligns well with the experimental findings, including increased photocurrent response, reduced charge transfer resistance (EIS), and quenched PL intensity all of which confirm efficient charge separation and transport in the SnO₂@g-C₃N₄ heterostructure.

Fig. 9 Probable mechanism of the SnO₂@g-C₃N₄ photocatalyst: sunlight excites electrons in the valence band of g-C₃N₄, which transfer to the SnO₂ conduction band, driving the reduction of H⁺ ions to H₂. Simultaneously, holes oxidize water to generate O₂. The diagram highlights the synergistic effect of combining SnO₂ and g-C₃N₄, to enhance photocatalytic performance.

In addition to improved and charge carrier dynamics, the hydrogen adsorption on the catalyst surface is a critical factor influencing overall HER efficiency. Literature-based Gibbs free energy (ΔG_H*) studies provide further thermodynamic insight into this aspect. ΔG_H* is a key thermodynamic parameter for evaluating a material's catalytic activity toward the hydrogen evolution reaction (HER).⁸⁰ It reflects how favorably hydrogen atoms bind to the catalyst surface, which in turn governs the balance between adsorption and desorption steps. Ideally, an effective HER catalyst should have a ΔG_H* value close to zero, ensuring that hydrogen atoms can adsorb and desorb efficiently. If ΔG_H* is too positive, hydrogen adsorption is unfavorable, leading to sluggish HER kinetics. If it is too negative, hydrogen binds too strongly, hindering H₂ release.⁸¹ For pristine g-C₃N₄, previous theoretical studies have reported a ΔG_H* value of approximately −0.54 eV, indicating relatively strong hydrogen binding and potentially slow desorption.⁸² In contrast, SnO₂ surfaces have shown ΔG_H* values ranging from 0.85 eV to 2.37 eV at different crystallographic orientations and adsorption sites, suggesting unfavorable hydrogen adsorption that may limit the generation of active hydrogen intermediates.^83,84 Although the Gibbs free energy of hydrogen atom adsorption for the SnO₂@g-C₃N₄ heterojunction itself has not yet been theoretically investigated, the significantly improved experimental hydrogen evolution performance observed in this study and previously explored studies suggests that interfacial charge redistribution may help tune ΔG_H* closer to the thermoneutral range. This highlights the need for future theoretical studies to better understand the active sites and mechanisms of hydrogen adsorption in such heterostructures.

Conclusion

In summary, we have successfully synthesized SnO₂@g-C₃N₄ heterojunctions using a cost-effective and environmentally friendly precipitation method. Four composites with varying mass ratios of SnO₂ to g-C₃N₄ (SC-10, SC-20, SC-30, and SC-40) were prepared to evaluate their photocatalytic performance for hydrogen production under solar illumination. Among these, SC-20 (80% SnO₂, 20% g-C₃N₄) exhibited the best performance, achieving the highest H₂ production rate (287.7 μmolg⁻¹ h⁻¹) due to its optimal heterojunction structure, which effectively suppressed charge recombination and enhanced electron transfer to the conduction band. Compared to previously reported works, employing complex synthetic routes or noble metal co-catalysts, our approach offers a practical balance of photocatalytic efficiency, material simplicity, and stability. Importantly, the excellent photostability of our SnO₂@g-C₃N₄ composite, demonstrated over extended cycling, further reinforces the potential of this system for sustainable solar-to-fuel applications. Furthermore, this synthesis platform can be enhanced by introducing additional functional components such as Pt co-catalyst or nanostructure modifiers to further improve charge carrier dynamics and overall efficiency.

Data avaibility

All data generated or analyzed during this study are included in the article and its ESI.†

Author contributions

R. K.: investigation, formal analysis, data curation, validation, visualization, writing – original draft. S. S. A.: investigation. H. I.: investigation. S. S. N.: writing – original draft. S. Z.: conceptualization, data curation, methodology, validation, supervision, resources. F. A. A. N.: data curation, validation, supervision, resources, funding acquisition, writing – review and editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We acknowledge funding by the Faculty of Mathematics and Natural Sciences, Universitas Indonesia, under Publication Grant scheme 2025.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra03721b

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