SnO₂ Quantum Dot-Decorated g-C₃N₄ Ultrathin Nanosheets: A Dual-Function Photocatalyst for Pollutant Degradation and Hydrogen Evolution

Abstract

The development of advanced functional composite materials for degrading industrial pollutants and achieving photocatalytic hydrogen (H₂) production using abundant solar energy is pivotal in new and renewable energy research. This study presents the synthesis of a nanostructure comprising SnO₂ quantum dots (QDs) randomly dispersed on the surface of graphitic carbon nitride (C₃N₄) nanosheets (Sn-C₃N₄), achieved through the thermal decomposition of melamine and a tin precursor. The synthesized materials were extensively characterized using various analytical techniques, with HRTEM analysis confirming the strong interaction between SnO₂ QDs and C₃N₄. The influence of SnO₂ QDs on the nanocomposite’s photocatalytic performance was evaluated, particularly regarding H₂ production and the degradation of crystal violet (CV) dye under simulated solar-light irradiation. The SnO₂-loaded C₃N₄ nanostructure exhibited a marked enhancement in photocatalytic activity, attributed to the synergistic effects of the quantum-sized SnO₂ nanoparticles. The optimized photocatalyst, 3-Sn-C₃N₄, demonstrated superior photocatalytic efficiency, achieving 95% degradation of CV dye within 45 min under simulated sunlight, significantly outperforming bare C₃N₄. Furthermore, the 3-Sn-C₃N₄ nanostructure attained the highest H₂ yield of 1305.4 μmol/h/g, a 4.6-fold increase compared with bare C₃N₄ (281 μmol/h/g). Enhanced photocatalytic performance was corroborated by photocurrent and EIS studies, which highlighted reduced charge carrier recombination as a critical factor in the improved activity. The underlying photocatalytic mechanisms were also examined.

1. Introduction

As society progresses, there is growing emphasis on resource conservation and environmental protection. The pursuit of green and renewable energy has become a vital strategy for addressing energy shortages and promoting ecofriendly solutions. Photocatalysis has emerged as a promising green technology for tackling two critical global challenges: environmental pollution and sustainable energy production. Specifically, photocatalytic water splitting to produce hydrogen (H₂) offers a clean energy solution by harnessing sunlight to generate renewable fuel. Simultaneously, photocatalysis can play a crucial role in degrading harmful pollutants in water, contributing to environmental remediation [1,2]. Dual-function photocatalysts, which combine both of these functions in a single material, hold the potential to address pollution and energy demands simultaneously [3,4]. To date, a wide variety of photocatalysts have been investigated for photocatalytic hydrogen evolution applications, including metal oxides, C₃N₄, and metal sulfides. Among these materials, C₃N₄ has garnered considerable interest due to its moderate band gap, which enables visible light absorption, as well as its notable thermal and chemical stability [5]. However, the relatively low inherent photocatalytic efficiency of g-C₃N₄ constrains its practical applications across a broader range of fields.

To overcome this limitation, several strategies have been proposed, such as modifying the electronic structure, enhancing electron transfer rates, expanding visible light absorption, and forming heterostructures or nanocomposites [6]. Techniques like element doping, heterojunction formation, co-catalyst addition, and microstructural modifications have been widely adopted to optimize the electronic properties of C₃N₄ [7]. Extensive research has demonstrated the effectiveness of these approaches in improving photocatalytic performance.

Fareed et al. [8] reported a significant improvement in photocatalytic performance using a C₃N₄/SnS₂ nanostructure. Their result demonstrated that this composite material achieved 90% degradation of MB dye within just 90 min under solar irradiation. This high degradation efficiency was attributed to the Type II heterojunction mechanism in the C₃N₄/SnS₂ system, which facilitated efficient charge transfer between the two components. In this Type II heterostructure, the conduction band of SnS₂ is positioned below that of C₃N₄, allowing the electrons to flow directionally from C₃N₄ to SnS₂. Concurrently, holes are transferred in the opposite direction, promoting effective charge separation and minimizing charge carrier recombination—a key factor that typically limits photocatalytic efficiency. The reduced recombination of charge carriers not only accelerates the degradation of organic pollutants but also shows promise for enhancing other photocatalytic applications, including H₂ generation. In a study by Alsalme [9], a C₃N₄-based composite was further optimized by incorporating 15 wt% SnS₂, resulting in a notable improvement in photocatalytic performance. This C₃N₄/SnS₂ nanostructure achieved 98% degradation of RhB dye within 120 min under solar irradiation, whereas bare C₃N₄ only managed to degrade 35% of the RhB dye in the same period. This limitation in bare C₃N₄ is largely due to its poor light absorption and rapid recombination of electron–hole pairs caused by uncontrolled electrostatic interactions. Alsalme’s study underscores the effectiveness of tuning photocatalysts’ composition and structure to enhance performance. The dramatic increase in RhB dye degradation highlights the potential of combining C₃N₄ with metal sulfides like SnS₂ as a strategy to overcome the limitations of single-component photocatalysts. This approach opens avenues for developing more efficient multifunctional materials for environmental remediation and solar energy applications.

Lu et al. [10] reported a significant enhancement in photocatalytic H₂ production using a novel carbon dot-loaded black-C₃N₄ nanostructure. This composite demonstrated impressive photothermal-assisted photocatalytic activity, achieving a H₂ yield of 3.4 mmol/g/h, far surpassing conventional C₃N₄-based materials. This superior performance was attributed to several key factors that set the carbon dot-loaded black-C₃N₄ apart from its unmodified counterpart. The introduction of carbon dots not only enhances the hydrophilicity of the black-C₃N₄ nanostructure, promoting better interaction with water molecules—which is critical for efficient photocatalytic water splitting—but also contributes to improved charge transfer and increased surface area. This combination offers a promising approach to overcoming the limitations of traditional photocatalysts, making this material a strong candidate for future renewable energy applications.

Metal QDs have attracted considerable attention in photocatalytic applications due to their unique quantum confinement effects, which confer distinct optical and electronic properties. These effects enable QDs to play a pivotal role in enhancing photocatalytic processes. It is well established that integrating QDs with semiconductor materials can substantially improve the efficiency of both photocatalytic pollutant degradation and H₂ production from aqueous solutions [11,12]. In such systems, QDs act as efficient electron acceptors, facilitating charge separation by capturing photogenerated electrons from the semiconductor. This enhanced electron transfer is essential for driving photocatalytic reactions, as it helps suppress charge recombination—a key factor limiting the efficiency of bare semiconductor photocatalysts. By acting as electron sinks, QDs not only extend the lifetime of charge carriers but also promote the formation of reactive radicals, which play a critical role in degrading water pollutants. Additionally, captured electrons can be directed toward the H₂ evolution centers, further boosting H₂ production efficiency. Thus, the inclusion of QDs in photocatalytic systems offers a dual benefit: improving pollutant degradation through radical generation and enhancing H₂ production by optimizing the charge transfer mechanisms [12]. These properties make QD-supported semiconductor materials highly promising for various applications, including environmental remediation and sustainable energy production. Continued exploration of QD–semiconductor composites is expected to yield more efficient, versatile photocatalytic materials capable of addressing critical environmental and energy challenges. In recent studies, Bhattacharjee et al. [13] demonstrated the exceptional photocatalytic performance of a SnO₂ QDs/C₃N₄/biochar nanostructure synthesized via a one-pot pyrolysis method. This composite material achieved impressive degradation efficiencies, with up to 96% degradation of Rose Bengal in 80 min and 95% degradation of MB in 120 min under irradiation. The high photocatalytic activity was attributed to the advanced oxidation process enabled by the synergistic effects of SnO₂ QDs, C₃N₄, and biochar components. Khan et al. [14] reported a significant enhancement in photocatalytic activity using an ultrasound-assisted coprecipitated ZnS QDs/C₃N₄ nanostructure, which achieved up to 98% degradation of RhB within just 54 min, with a rate constant of 0.077/min. The outstanding photocatalytic efficiency can be explained by the quantum confinement effects of ZnS QDs, which were dispersed on the surface of C₃N₄, dramatically improving the system’s overall performance. The incorporation of ZnS QDs onto C₃N₄ nanosheets enhanced light absorption and facilitated efficient electron–hole separation. The quantum effect of ZnS QDs increased the number of reactive centers on the nanostructure’s surface, accelerating the degradation of organic pollutants like RhB. Additionally, ZnS QDs acted as electron reservoirs, reducing the recombination rate of photoexcited electrons and holes—a primary limitation of pure C₃N₄. Despite these advancements, challenges remain, particularly in developing efficient synthesis methods that ensure well-matched interfaces and precise band structure alignment within heterostructures. These factors are crucial for optimizing photoinduced charge separation and transfer, which are essential for maximizing photocatalytic efficiency.

In this study, we developed a SnO₂ QDs/C₃N₄ nanostructure through a straightforward single-step pyrolysis of a tin (Sn) precursor and melamine. This nanocomposite was utilized as an effective photocatalyst for both CV dye decolorization and H₂ production under solar light irradiation. Varied concentrations of SnO₂ QDs were used to assess its effect on photocatalytic performance, specifically regarding dye degradation and H₂ generation. Moreover, the underlying photocatalytic mechanisms driving these processes were thoroughly examined and discussed. The results revealed that the optimized 3-Sn-C₃N₄ nanostructure exhibited significantly enhanced photocatalytic activity compared with bare C₃N₄, primarily attributed to the synergistic interaction between the SnO₂ QDs and C₃N₄ nanosheets. The QDs not only facilitated more efficient charge separation, reducing electron–hole recombination, but also contributed to an increased surface area, providing additional active sites for photocatalytic reactions. These combined factors led to a significant rise in the degradation rate of CV dye and a notable improvement in H₂ production efficiency. This work underscores the potential of SnO₂ QD-C₃N₄ nanostructures as versatile and highly effective photocatalysts, with promising applications in environmental remediation and renewable energy production. The findings also highlight the importance of nanostructure design and material optimization in achieving superior photocatalytic performance under solar light irradiation.

2. Results and Discussion

The XRD patterns of bare C₃N₄ and the Sn-C₃N₄ nanostructures (1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄) are shown in Figure 1. For C₃N₄, a prominent peak at 2θ = 27.76° corresponds to the (002) crystal plane, consistent with previous reports [15,16]. The diffraction peaks observed in the Sn-C₃N₄ samples closely resemble those of pure C₃N₄, suggesting that the structure of C₃N₄ remains largely unchanged during synthesis. Additionally, the Sn-C₃N₄ nanostructures exhibit a distinct SnO₂ peak at 2θ = 26.6°, associated with the (110) plane of SnO₂ and matching JCPDS No. 01-088-0287, alongside the C₃N₄ peaks, indicating successful incorporation of SnO₂ into the nanostructure [17]. The lack of significant differences between the diffraction patterns of C₃N₄ and Sn-C₃N₄ suggests that the addition of SnO₂ did not alter the overall crystal structure of C₃N₄, confirming that SnO₂ QDs were effectively integrated into the C₃N₄ matrix to form the desired nanocomposite materials. Furthermore, the presence of SnO₂ QDs in the nanostructure was confirmed by HRTEM mapping.

Figure 2 presents the FTIR spectra of the C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ photocatalysts, highlighting their critical structural features. The characteristic peak of pristine C₃N₄ was detected at 805 cm⁻¹, which can be attributed to the breathing mode of triazine rings, a hallmark of the basic C₃N₄ framework. The peaks within the 1200–1650 cm⁻¹ range are associated with C-N stretching vibrations from the aromatic loops, likely originating from the C≡N stretching modes in conjugated systems [18]. Moreover, the broad peak in the 3000–3400 cm⁻¹ region is typically attributed to the stretching vibrational modes of -NH groups and surface-adsorbed H₂O molecules. For the SnO₂ QD-decorated C₃N₄ nanostructures (Sn-C₃N₄), no significant shifts in the FTIR peaks were observed, suggesting that the core C₃N₄ structure was preserved after the incorporation of SnO₂ QDs. However, a noticeable decrease in the FTIR peak intensity was evident as SnO₂ QDs were introduced into the C₃N₄ matrix. This reduction in peak intensity suggests interactions between the SnO₂ QDs and the C₃N₄ nanosheets, implying that the presence of SnO₂ QDs influences the surface chemistry of C₃N₄. The weakening of these characteristic peaks may be attributed to the partial coverage of C₃N₄ by SnO₂ QDs, which could reduce the exposure of the functional groups responsible for the FTIR signals. These findings, along with the XRD results, confirm the successful integration of SnO₂ QDs with C₃N₄, supporting the presence of a strong interaction between the two components in the nanocomposite. This interaction is essential for enhancing the photocatalytic properties of the Sn-C₃N₄ nanostructures, as it likely promotes more efficient charge separation and transfer, thereby improving the overall photocatalytic performance.

HRTEM is a powerful technique for examining the structural properties of thin nanosheets. Figure 3a–e shows the HRTEM images of the 3-Sn-C₃N₄ nanostructure, revealing a distinct 2D mesoporous network composed of thin C₃N₄ layers only a few nanometers thick. This structure incorporates quantum-sized SnO₂ nanoparticles, which are not easily distinguishable at lower magnifications. However, these SnO₂ QDs become more apparent in higher-magnification images (Figure 4b). The SAED pattern of the 3-Sn-C₃N₄ nanostructure (Figure 3f) further supports these observations, highlighting the amorphous nature of the C₃N₄ sheets, while the faint diffraction rings correspond to the crystalline SnO₂ QDs. This contrast between the amorphous carbon nitride and crystalline SnO₂ QDs underscores the successful formation of a hybrid nanostructure. Figure 4a presents a HRTEM image of the 3-Sn-C₃N₄ nanostructure, where single-atom SnO₂ QDs (visible as white dots) are randomly distributed across the surface of the C₃N₄ nanosheets. In the higher-magnification TEM images (Figure 4b), the QDs, only a few nanometers in size, are observed to be attached to the surface edges of the C₃N₄. These SnO₂ QDs play a critical role in promoting radical formation during photocatalytic reactions, thereby enhancing the system’s overall reactivity. The HAADF image at an 80 nm scale (Figure 4c) further confirms the anchoring of SnO₂ QDs to the C₃N₄’s surface. This intimate attachment between the SnO₂ QDs and C₃N₄ nanosheets is essential for enhancing photocatalytic activity by improving charge separation and transfer efficiency. Additionally, Figure 4d displays the combined elemental mapping of the 3-Sn-C₃N₄ nanostructure, while individual element maps for C, N, Sn, and O provide clear evidence of the uniform distribution of SnO₂ QDs within the C₃N₄ matrix. This uniformity confirms the successful formation of a heterostructure, which is crucial for enhancing photocatalytic reactions by facilitating efficient charge carrier dynamics and increasing the active sites for photocatalytic processes.

XPS analysis was conducted to investigate the chemical composition of the 3-Sn-C₃N₄ nanostructure. Figure 5a presents the full-scan XPS survey, confirming the successful integration of SnO₂ and C₃N₄ within the composite material. The presence of characteristic peaks for both components indicates that the nanostructure consists of well-defined SnO₂ QDs anchored to the C₃N₄ nanosheets. Figure 5b shows the XPS spectra of C 1s, revealing two prominent deconvoluted peaks. The peak at 284.5 eV corresponds to sp²-hybridized carbon atoms linked to two N atoms within the aromatic ring structure of C₃N₄. The additional peak at 287.9 eV is attributed to carbon atoms associated with the terminal -NH₂ groups [19], confirming the integrity of the C₃N₄ structure within the composite. The N 1s XPS spectra (Figure 5c) provide further insight into the nitrogen species present in the nanostructure, displaying three distinct peaks at 398.4, 400.3, and 404.1 eV. The peak at 398.4 eV corresponds to the nitrogen atoms attached to three carbon atoms (N-(C)₃) within the C₃N₄ framework [20]. The peak at 400.3 eV is attributed to surface amino groups (-NH₂), while the bump at 404.1 eV represents π-excitation within the C/N heterocyclic system. These findings confirm the chemical environment of nitrogen in the C₃N₄ nanosheets and the retention of its structural features upon incorporation of SnO₂. Figure 5d shows the Sn 3d XPS spectra of the 3-Sn-C₃N₄ nanostructure, with characteristic binding energies at 486.6 eV and 494.9 eV corresponding to the Sn 3d_5/2 and Sn 3d_3/2 peaks, respectively. These peaks indicate the presence of Sn⁴⁺ oxidation states, confirming that Sn exists as SnO₂ within the nanostructure [21]. The XPS analysis conclusively demonstrates that SnO₂ QDs were effectively combined with C₃N₄, forming a stable composite material.

UV–vis–NIR absorbance spectroscopy was used to evaluate the light-harvesting capabilities of the C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ photocatalysts, as shown in Figure 6a. The C₃N₄ sample displays an absorption edge extending beyond 460 nm, indicating its ability to absorb visible light, which corresponds to its band gap and allows it to utilize visible light for photocatalytic activity. However, the absorption is relatively weak in the longer visible wavelengths, which limits its efficiency. In the SnO₂-modified nanostructured photocatalysts, absorbance peaks were observed within the visible light range (above 420 nm), indicating enhanced light absorption in this region. Compared with bulk C₃N₄, these nanostructured photocatalysts exhibited a broader light absorption capacity, particularly within the visible spectrum. This extended light-harvesting capability is crucial, as it facilitates the generation of additional charge carriers, thereby boosting photocatalytic efficiency. Figure 6b presents the energy band gaps of the C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ photocatalysts analyzed by Tauc plots. The calculated band gaps were found to be 2.69 eV for C₃N₄, 2.68 eV for 1-Sn-C₃N₄, 2.65 eV for 2-Sn-C₃N₄, and 2.62 eV for 3-Sn-C₃N₄.

The surface area and mesoporous characteristics of the C₃N₄ and 3-Sn-C₃N₄ nanostructures were examined using N₂ adsorption–desorption loops, as shown in Figure 7a. The observed isotherms exhibit H3-type hysteresis loops, indicating the presence of mesoporous structures within the photocatalysts [22]. The BET surface areas of the C₃N₄ and 3-Sn-C₃N₄ nanostructures were measured to be 8.04 m²/g and 3.96 m²/g, respectively, demonstrating that the inclusion of SnO₂ QDs significantly increases the surface area of the nanostructures. Additionally, the BJH pore-size distribution curves (Figure 7b) show that the average pore sizes for the C₃N₄ and 3-Sn-C₃N₄ nanostructures are 21.85 nm and 34.75 nm, respectively. This increase in pore size for the SnO₂-modified structure further supports its mesoporous nature. The larger surface area and enhanced pore size in the 3-Sn-C₃N₄ nanostructure, attributed to the quantum effects of SnO₂, are advantageous for improving photocatalytic performance. The increased surface area provides more active sites for photocatalytic reactions, while the mesoporous structure enhances mass transport and light penetration, both of which contribute to the higher photoactivity of the Sn-C₃N₄ nanostructures compared with pure C₃N₄. This enhancement is particularly beneficial for applications such as pollutant degradation and H₂ generation.

The photodegradation efficiency of C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ photocatalysts was evaluated by examining the degradation of CV dye in aqueous solutions. Figure 8a–d show the UV–visible absorption spectra of CV dye over time for each photocatalyst. Before photocatalytic testing, the photocatalysts were suspended in the dye solution and kept in the dark for 30 min at room temperature with stirring to establish adsorption–desorption equilibrium. Among the tested photocatalysts, 3-Sn-C₃N₄ demonstrated the highest photocatalytic activity for CV dye degradation compared with the other samples, including bare C₃N₄. This enhanced performance can be attributed to the larger surface area of 3-Sn-C₃N₄ (8.04 m²/g), which enables a higher dye adsorption capacity than C₃N₄ (3.96 m²/g) (Figure 9a). A larger surface area facilitates greater dye adsorption, which, in turn, enhances photoactivity by increasing the number of active sites available for the reaction.

The reaction kinetics for CV dye degradation were analyzed using the logarithmic relationship ln(C/C₀) to further compare the photocatalytic efficiency (Figure 9b). The rate constants for the C₃N₄ and 3-Sn-C₃N₄ photocatalysts were determined to be 0.002/min and 0.0052/min, respectively. The significantly higher rate constant for 3-Sn-C₃N₄ can be attributed to its increased surface area and enhanced light absorption capacity, both resulting from the inclusion of SnO₂ QDs. This phenomenon facilitates more efficient charge separation and transfer during the photocatalytic process. The reusability of the 3-Sn-C₃N₄ nanostructure was also evaluated through four consecutive cycles of CV dye degradation (Figure 10a). The results indicate no notable decrease in photocatalytic activity after multiple uses, confirming the stability and durability of the 3-Sn-C₃N₄ photocatalyst. Moreover, a comparison of the photocatalytic performance across various photocatalysts demonstrated that the inclusion of SnO₂ QDs significantly enhances the photocatalytic activity of C₃N₄ through synergistic and plasmonic effects, resulting in more effective CV dye degradation. This improvement highlights the potential of SnO₂-QD-modified C₃N₄ as an effective photocatalyst for environmental pollutant removal.

Under solar light irradiation, the formation of O₂^•− is hindered because the CB potential of SnO₂ QDs is 0.1 eV, which is more positive than the redox potential of O₂/O₂^•− (−0.33 eV) [23,24]. Despite this, previous studies have shown that O₂^•− is a key reactive species driving the photocatalytic process in such nanostructure systems [23]. Consequently, a Z-scheme mechanism is proposed to best match our experimental observations (Figure 10b). Upon irradiation, electrons generated in the conduction band of SnO₂ QDs transfer to the VB of C₃N₄, and from there, these electrons migrate to the CB of C₃N₄. At this point, they interact with O₂ molecules, generating O₂^•−, the primary active species responsible for dye degradation. Concurrently, holes remain in the VB of SnO₂ QDs, where they contribute to the oxidation of H₂O, producing •OH radicals that further enhance the photocatalytic process. This Z-scheme mechanism not only facilitates the effective separation of photoinduced charge carriers, reducing recombination, but also preserves the strong redox potential within the nanostructure. This leads to improved photocatalytic efficiency, as both O₂^•− and •OH radicals serve as potent oxidants that decompose dye molecules, driving the degradation process. This mechanism highlights the synergistic interaction between SnO₂ QDs and C₃N₄ that is crucial to enhancing photocatalytic performance. The possible reactions involved in the photocatalytic degradation of CV dyes by SnO₂ QDs-C₃N₄ nanostructures are as follows:

SnO₂ QDs-C₃N₄ → SnO₂ QDs-C₃N₄ (h_VB⁺ + e_CB⁻)

(1)

C₃N₄ (e_CB⁻) + SnO₂ QDs → SnO₂ QDs (e_CB⁻) + C₃N₄

(2)

e_CB⁻ + O₂ → O₂^•−

(3)

O₂^•− + CV dye → Degraded products

(4)

•OH + CV dye → Degraded products

(5)

The photocatalytic H₂ evolution efficiency of the synthesized nanostructured photocatalysts—the C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ photocatalysts—was systematically evaluated, using TEOA as the sacrificial agent to promote H₂ generation. As shown in Figure 11a,b, the H₂ yields from C₃N₄, 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄ were measured at 281, 1187.3, 1254.6, and 1305.4 μmol/h/g, respectively. Remarkably, the incorporation of SnO₂ QDs into the C₃N₄ matrix led to a substantial increase in H₂ yield, reaching up to 4.64 times that of pure C₃N₄. This significant improvement in photocatalytic efficiency is attributed to the presence of SnO₂ QDs, which enhance the separation and capture of photoinduced charge carriers, thereby reducing electron–hole recombination.

Figure 11c shows the impact of various scavengers on the photocatalytic H₂ yield using the 3-Sn-C₃N₄ nanostructure. Among the tested scavengers, TEOA produced the highest H₂ yield, confirming its effectiveness in enhancing photocatalytic activity. Consequently, TEOA was chosen as the model scavenger for this study. The stability and reusability of the 3-Sn-C₃N₄ photocatalyst were further assessed over three consecutive H₂ evolution cycles under identical photocatalytic conditions (Figure 11d). After the third cycle, the H₂ production rate decreased slightly to 1214 μmol/h/g, suggesting minimal degradation in performance. This stability demonstrates that the 3-Sn-C₃N₄ photocatalyst is highly robust for repeated use, benefiting from the synergistic interaction between SnO₂ QDs and C₃N₄. Enhanced interfacial charge transfer between SnO₂ QDs and C₃N₄ plays a vital role in maintaining efficient photocatalytic H₂ generation over multiple cycles.

To better understand the charge carrier transport behavior of the photocatalysts, photocurrent response measurements were performed for C₃N₄ and 3-Sn-C₃N₄ under light irradiation in a 0.5 M Na₂SO₄ solution (Figure 12a). The 3-Sn-C₃N₄ photocatalyst demonstrated a significantly higher photocurrent density of 0.068 μA cm², compared with 0.033 μA cm² for the C₃N₄ photocatalyst. This evaluated photocurrent response indicates that 3-Sn-C₃N₄ achieves more efficient electron–hole pair separation, reducing recombination and thus enhancing the photocatalytic performance. Additionally, EIS analysis was conducted to investigate the charge transfer properties of both photocatalysts, with the Nyquist plots shown in Figure 12b. A smaller semicircle in the Nyquist plot typically reflects improved interfacial charge mobility. Here, 3-Sn-C₃N₄ displayed a noticeably smaller semicircle than C₃N₄, indicating superior charge transfer across the photocatalyst interface. These results suggest that the 3-Sn-C₃N₄ nanostructure provides enhanced interfacial charge migration and a lower recombination rate, likely due to the strong interaction and close integration between SnO₂ QDs and C₃N₄. This synergy promotes more efficient photocatalytic processes.

The exceptional performance of the 3-Sn-C₃N₄ nanostructure can be attributed to several key factors. The heterostructure formed between C₃N₄ and SnO₂ QDs facilitates efficient charge separation by reducing electron–hole recombination, a common limitation in pure C₃N₄ systems. This improved charge separation extends the lifetime of reactive species, including hydroxyl radicals and superoxide anions, which play crucial roles in degrading organic dye molecules. Additionally, the incorporation of SnO₂ QDs increases the number of active sites on the nanocomposite’s surface, enhancing interactions with dye molecules and accelerating their breakdown. The synergistic interaction between C₃N₄ and SnO₂ QDs not only boosts their overall photocatalytic efficiency but also highlights the potential of these composites to overcome the limitations of traditional photocatalysts, presenting a promising solution for environmental applications.

3. Materials and Methods

Synthesis of SnO₂ QDs/C₃N₄ Nanostructures

For this, 7 g of melamine was mixed with varying amounts of anhydrous tin chloride (0.9, 0.95, and 1 g) using an agate mortar to ensure a uniform mixture. The mixtures were then transferred into a ceramic crucible and subjected to heat treatment at 530 °C for 12 h. The samples were cooled to 25 °C after the reaction, and the resulting precipitates were thoroughly washed multiple times with H₂O and ethanol to remove any impurities. The cleaned products were then dried at 100 °C to yield the final nanostructures. The samples were labeled as 1-Sn-C₃N₄, 2-Sn-C₃N₄, and 3-Sn-C₃N₄, corresponding to the different concentrations of tin precursor used. A detailed characterization of the synthesized nanostructures, including photocatalytic and electrochemical performance tests, is provided in the Supplementary Materials. These tests were conducted to evaluate the efficiency of the SnO₂ QD-C₃N₄ composites in photocatalytic degradation and H₂ evolution, as well as to analyze the charge transfer dynamics.

4. Conclusions

In summary, this study successfully developed SnO₂ QDs-C₃N₄ nanostructures based on a single-step pyrolysis method, achieving significant enhancements in photocatalytic activity for CV dye degradation and H₂ production under solar light. The 3-Sn-C₃N₄ nanostructure exhibited a notably higher H₂ evolution rate (1305.4 μmol/h/g) compared with bare C₃N₄ (281 μmol/h/g). The unique 0D/2D architecture of the Sn-C₃N₄ nanocomposites, incorporating SnO₂ QDs, increased the surface area and enabled efficient charge separation between SnO₂ and C₃N₄. This expanded surface area, combined with improved charge separation and enhanced light absorption, contributed to its outstanding photocatalytic performance. Electrochemical studies, including photocurrent responses and impedance spectroscopy, further validated the efficient charge transport and reduced recombination rates in the 3-Sn-C₃N₄ structure. Additionally, the nanostructure demonstrated strong photostability and reusability, making it a promising candidate for applications in environmental and energy. This work offers an effective strategy for designing advanced photocatalysts with enhanced light utilization and charge carrier dynamics, paving the way for future innovations in photocatalytic materials.