Synergistic Integration of g-C₃N₄ with SnS: Unlocking Enhanced Photocatalytic Efficiency and Electrochemical Stability for Dual-Functional Applications

Abstract

The synthesis of graphitic carbon nitride (g-C₃N₄) coupled with tin sulfide (SnS) has been identified as an effective method for improving the photocatalytic and electrochemical performance of SnS, a promising material for environmental and energy-related applications. In this study, we focused on how g-C₃N₄ influences the structural, optical, electrochemical, and functional properties of SnS. XRD and FTIR confirmed the formation of SnS/g-C₃N₄ heterostructure, while surface morphology analysis by SEM showed proper dispersion of SnS particles over g-C₃N₄ with a good interface contact. The SnS/g-C₃N₄ composite itself demonstrated improved photocatalytic performance, with the degradation rate of methylene blue reaching approximately 94% under visible light irradiation compared to the moderate activity of SnS. This enhancement can be credited to the successful charge carrier separation enabled by the type II heterojunction created between SnS and g-C₃N₄. Moreover, the composite presented improved electrochemical activity with a specific capacitance of 1340 F·g⁻¹ at a scan rate of 10 A·g⁻¹ and good cycling stability, where the capacitance was 92% after 5000 cycles. As such, these SnS/g-C₃N₄ composites suggest the specific application of this class of material in photocatalytic degradation as well as energy storage, putting forward new effective resolutions to environmental and energy issues.

1. Introduction

The Industrial Revolution triggered rapid industrial expansion, which produced tremendous wastewater containing heavy metals and organic pollutants, threatening both environmental sustainability and the societal development of humans [1,2]. Semiconductor-based photocatalysts demonstrate great promise as efficient tools to solve these problems because they reduce heavy metals and degrade organic pollutants simultaneously, thus creating dual protection against industrial wastewater [3,4]. TiO₂ functions as a semiconductor known for photocatalysis, which researchers have thoroughly studied for environmental pollutant degradation through its effective breakdown of organic and inorganic contaminants [5,6,7]. Practical use of this material faces two key barriers because the separation of electrons and holes remains slow, and its capacity to absorb visible light spectra is insufficient [8,9]. Contemporary photocatalysis research has established novel visible-light-responsive photocatalysts with high activity as its central focus for overcoming present challenges.

Semantic materials receive extensive research interest in renewable energy technology because they serve fundamental functions within photocatalytic and electrochemical processes [10,11]. Tin monosulfide (SnS) has proven itself as an outstanding candidate semiconductor material because of its narrow bandgap abilities, which match with solar-driven hydrogen production, environmental remediation, and energy storage demands [12,13]. The practical usage of SnS is restricted by fast charge carrier recombination, as well as restricted light absorption from solar radiation and unstable crystal structures when operating [14,15]. Research has attempted to address performance issues in SnS by combining it with relevant materials which improve its functional properties [16]. Scientists consider graphitic carbon nitride (g-C₃N₄) to be an exceptional metal-free polymer semiconductor because of its adaptable electronic structure, together with its chemical durability and visible-light activating capability [17,18]. g-C₃N₄ demonstrates excellent properties for simultaneous charge separation and expanded light absorption, which makes it suitable for cooperative application with SnS [19,20]. Some studies exist regarding the combined effect of SnS and g-C₃N₄ on their photocatalytic and electrochemical performance capabilities. Research examines the bonding patterns and physical transformations that occur between SnS elements and g-C₃N₄ networks in order to develop both energy conversion and energy storage applications.

Photocatalytic applications making use of SnS have received extensive study because of its earth-abundant chemical arrangement and suitable bandgap range of 1.3–1.7 eV [14,15,21]. The limiting factors for SnS performance are its weak electron–hole separation ability and its tendency to undergo photocorrosion [22]. Research has investigated the use of SnS as a lithium-ion battery anode material through electrochemistry, yet structural collapse during cycling, with poor electrical conductivity, stands as a significant challenge [23,24].

The extensive application of g-C₃N₄ in photocatalysis happens because it shows effective visible-light activity together with chemical resistance. The photophysical properties of charge carriers improved through the combination of ZnO or MoS₂ with g-C₃N₄ appeared in recent research to boost photocatalytic activity [25,26,27]. The combination of g-C₃N₄ materials with other elements helps develop supercapacitor electrodes that possess better capacitance performance with improved cycling stability. Investigations mostly study metal oxides and transition metal dichalcogenides instead of metal sulfides, including SnS, although they show promise for research in this area. Some studies unite g-C₃N₄ with other sulfides (including CdS), yet they primarily optimize photocatalytic activity rather than electrochemical performance. No investigation evaluates how SnS/g-C₃N₄ combinations function as integrated solutions for light-driven and charging applications.

Previous research has not investigated the performance of this composite material for hydrogen evolution reactions or supercapacitive storage applications, yet these tests would enable its use in integrated energy systems. Design strategies remain limited by the lack of thorough investigations about how charges transfer between components under combined operational stress conditions involving light and electrochemical cycling. This research develops SnS/g-C₃N₄ composites through controlled in-situ synthesis, followed by optoelectronic characterization and tandem energy evaluation to build a complete framework for multiple-functional semiconductor development.

2. Results and Discussion

2.1. X-Ray Diffraction (XRD)

The XRD patterns of pristine SnS and SnS coated with different concentrations of g-C₃N₄ composites are presented in Figure 1a. The diffraction peaks of SnS correspond to the orthorhombic phase (JCPDS No. 14-0620), with prominent reflections observed at 2θ = ~32.03°, 37°, and 39.2°, assigned to the (111), (131), and (221) planes, respectively, as demonstrated in Figure 1a. For g-C₃N₄, a characteristic peak at 27.4° is evident, corresponding to the interlayer stacking of conjugated aromatic systems. In the SnS/g-C₃N₄ composite, both sets of peaks are present, indicating successful integration of g-C₃N₄ with SnS. Notably, a slight shift in the SnS peaks is observed, suggesting strong interfacial interaction between the two components, as shown in Figure 1b. The reduction in the crystallinity of SnS in the composite indicates that g-C₃N₄ suppresses particle growth, favoring the formation of nanoscale structures that enhance catalytic activity. The inverse relation between g-C₃N₄ dosage and SnS crystal size and strain behavior arises from the controlled nucleation process, together with interfacial strain dissipation mechanisms and defect reduction strategies, according to

Table 1. The calculated crystalline size and strain of SnS coated with different concentrations of g-C₃N₄.

Sample	Crystalline Size (d)	Strain (ε) × 10−3	Dislocation Density (δ) × 10−4
SnS	44.405	1.5525	5.07
SnS@0.025wt% g-C3N4	46.69	1.1575	4.58
SnS@0.05wt% g-C3N4	37.5	0.8275	7.11
SnS@0.075wt% g-C3N4	28.72	0.4975	12.1

. The modified composite structures improve functional characteristics through g-C₃N₄’s multipurpose role in semiconductor systems.

The FTIR spectra of the samples reveal characteristic features of both SnS and g-C₃N₄. For pristine SnS, peaks in the range of 451–600 cm⁻¹ correspond to the Sn–S stretching vibrations, as demonstrated in Figure 2a. In g-C₃N₄, the peaks at 1254 and 1633 cm⁻¹ are associated with the stretching vibrations of C–N and C=N bonds, while the broad peak at ~3250 cm⁻¹ corresponds to N–H stretching in terminal amine groups. In the SnS/g-C₃N₄ composite, these peaks are preserved with slight shifts, indicating successful coupling of the materials. The intensity reduction of the N–H peak suggests an interaction between the amine groups of g-C₃N₄ and the surface of SnS, which contributes to enhanced charge transfer at the interface, as shown in Figure 2b. In the composite materials, the XRD peaks attributed to g-C₃N₄ progressively intensify from pure SnS to 0.075 g-C₃N₄, indicating an increased incorporation of g-C₃N₄ within the matrix. This trend corroborates the earlier XRD analysis. For the SNS sample, a distinct yet moderate absorption peak is observed at 510 cm⁻¹, which corresponds to the vibrational mode of the Sn–S bond.

The g-C₃N₄/SnS nanocomposite features were examined using field-emission scanning electron microscopy (FESEM). The microscopic evaluation of SnS demonstrated an irregular particulate formation with well-defined crystalline structures as depicted in Figure 3a. The heterostructure g-C₃N₄/SnS nanocomposites presented themselves through uniformly anchored irregular SnS nanoparticles measuring 60–120 nm in size across a g-C₃N₄ surface (Figure 3b–d). The g-C₃N₄/SnS nanocomposite features were investigated using field-emission scanning electron microscopy (FESEM). The microscopic evaluation of SnS demonstrated an irregular particulate formation with well-defined crystalline structures as depicted in Figure 3a. The heterostructure g-C₃N₄/SnS nanocomposites presented themselves through uniformly anchored irregular SnS nanoparticles measuring 60–120 nm in size across a g-C₃N₄ surface (Figure 3b–d). EDS analysis provided validation of the elemental substances that make up the nanocomposites. The marker signals in the EDS spectra (Figure 4a–f) proved the existence of Sn, S, C, and N elements, which matched the composition of g-C₃N₄/SnS hybrid materials.

2.2. Optical Properties

The bandgap energy of semiconductors was determined using the Tauc equation [28,29,30]:

$${\left(\alpha hv\right)}^n=A\left(hv-E_{\mathrm{g}}\right)$$

(1)

The absorption coefficient term α contains Planck’s constant h, while the photon frequency is represented by ν, and the bandgap energy appears as E_g in the relation alongside constants A and n. The bandgap energy for SnS and SnS coated with different concentrations of g-C₃N₄ was determined from Tauc plots showing (αhν)² versus hν to have values of 2.24 eV and 2.13 eV (Figure 5a–d). Incorporating g-C₃N₄ into SnS leads to a decrease in the energy band gap relative to pure SnS. The absorption spectra of pure SnS and its SnS/g-C₃N₄ nanocomposites exhibit a noticeable redshift, demonstrating a reduction in their respective band gaps. Specifically, pure SnS has a band gap of 2.24 eV, while the SnS/g-C₃N₄ composite shows a narrowed band gap of 2.17 eV, and the SnS/0.075 g-C₃N₄ composite further decreases to 2.13 eV. This progressive reduction in band gap energy highlights that the integration of g-C₃N₄ effectively lowers the electronic barrier of the SnS matrix, enhancing its optical absorption properties.

2.3. Photocatalytic Performance

The photocatalytic performance of the materials was tested for the degradation of methylene blue (MB) under visible light. The pristine SnS had moderate degradation efficiency because of a high rate of recombination of the charge carriers. The g-C₃N₄ alone had low activity in terms of degradation due to its wide bandgap. Compared with SnS and g-C₃N₄ in the form of a physical mixture, the SnS/g-C₃N₄ composite exhibited higher photocatalytic activity and removed about 94% of MB within 90 min. This enhancement is due to the addition of a type II heterojunction, which increases the ability to separate photogenerated charges by a large margin. The photocatalytic mechanism of the g-C₃N₄/SnS composite depends on the transfer of electrons in the conduction band of g-C₃N₄ to SnS and holes in the valence band of SnS for oxidation processes. It is also said that more surface area and better light absorption are other advantages of the composite, as shown in Figure 6a–e. Among these, the SnS@0.075 g-C₃N₄ sample possessed higher photocatalytic capability, possibly because of the larger specific surface area.

All SnS coated with g-C₃N₄ composites exhibit significantly higher photodegradation performance compared to pristine SnS under visible light irradiation, indicating that the addition of a small amount of g-C₃N₄ enhances the photocatalytic activity of SnS. As the g-C₃N₄ content increases from 0 to 0.075 wt%, the MB degradation efficiency rises sharply, with the 0.075 g-C₃N₄ sample achieving the highest performance, degrading 94% of the MB solution within 90 min, as demonstrated in Figure 7b.

A linearized representation of MB photocatalytic degradation occurs with SnS@g-C₃N₄ photocatalysts, as shown in Figure 8a. The degradation process shows pseudo-first-order behavior because the plots remain linear throughout irradiation time. Analysis of Figure 8a data enabled the determination of apparent rate constants (k) for SnS@ g-C₃N₄ composites with different g-C₃N₄ contents presented in

Table 2. The efficiency and constant (K rate) of pure SnS and coated with different amounts of g-C₃N₄.

Sample	Efficiency	K, min−1	R2
SnS	87.5%	0.021	0.98
SnS@0.025 g-C3N4	88.1%	0.024	0.97
SnS@0.05 g-C3N4	92.5%	0.026	0.92
SnS@0.075 g-C3N4	94%	0.028	0.93

. The rate constant achieved by SnS@0.075 g-C₃N₄ reached 0.028 min⁻¹, which indicates a 1.33-fold increase over the rate constant of pure SnS (0.021 min⁻¹), as shown in Figure 9. When heterojunctions are built using proper g-C₃N₄ amounts, photocatalytic efficiency experiences a noteworthy boost. A photocatalyst’s ability to reuse constitutes a fundamental method for determining maintenance duration alongside implementation potential. The durability of SnS@0.075 g-C₃N₄ became measurable after engaging the catalyst in five subsequent degradation cycles while using visible light irradiation. The separated photocatalyst was reused in subsequent cycles without the addition of new catalyst materials. Under visible light illumination, the SnS@0.075 g-C₃N₄ photocatalyst displayed stable performance in five consecutive cycles, which led to a 94% degradation of MB in the last run. Prolonged exposure to light results in marginal performance loss of the catalyst because nanoparticle agglomeration occurs together with g-C₃N₄ structural damage from photon bombardment.

2.4. Electrochemical (Supercapacitor) Performance

The classical three-electrode system with 6 mol L⁻¹ KOH as an electrolyte is used to probe the performance of SnS coated with x wt.% g-C₃N₄ (x = 0, 0.025, 0.05, and 0.075) as anode materials. As shown in Figure 10a–d, cyclic voltammetry (CV) curves for SnS coated with x wt.% g-C₃N₄ (x = 0, 0.25, 0.5 and 0.75) were measured at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV·s⁻¹. The current density and voltametric curve area increase proportionally with the scan rate. The anodic peak shifts in a positive potential direction while the cathodic peak shifts in a negative potential direction when the scan rate increases. The potential separation enables better efficiency in electronic charge transfer and improves electron transport kinetics. The material’s electrochemical stability, alongside its reversible behavior and enhanced capacitive properties, is reflected through the symmetric profiles shown in all curves.

The galvanostatic charge–discharge (GCD) measurements for SnS with different x wt.% g-C₃N₄ (x = 0, 0.025, 0.05, and 0.075) nanocomposite electrodes follow a current density of 10 A·g⁻¹ to determine their anode capacitance level, as shown in Figure 11a–d. All electrodes exhibited dome-shaped curves while charging and discharging, thus proving that the electrode materials behave capacitively in agreement with CV curves. The charging/discharging tests of pristine SnS and SnS/g-C₃N₄ active electrodes took place across diverse densities from 10 A·g⁻¹ to 20 A·g⁻¹, as demonstrated in Figure 10a–d. When increasing the density of the electrode material, the charging/discharging time shortens, thereby decreasing the specific capacitance of the electrode material through interchanges between electrodes and ions. The capacitance of the anode material reached its maximum level at lower densities because cations could easily penetrate its surface.

Electrochemical impedance spectroscopy (EIS) was performed to analyze the charge transfer dynamics and electron diffusion at the electrode–electrolyte interface. Figure 12a presents Nyquist plots for pristine SnS₂ and SnS₂/g-C₃N₄ composites (with varying g-C₃N₄ loadings), measured across a frequency range of 10 mHz–100 kHz. The high-frequency intercept on the real axis, termed equivalent series resistance (Rs), reflects the combined resistance of the electrolyte, active material, and electrode–electrolyte contact. Notably, SnS₂@0.05 g-C₃N₄ exhibited the lowest Rs among all samples, highlighting its potential for high-power-density applications. In the low-frequency region, the vertical Warburg impedance (W) correlates with ion diffusion kinetics. The steeper slopes of composite materials compared to pure SnS₂ indicate reduced diffusion resistance and a diffusion-dominated charge transfer mechanism. Furthermore, the charge transfer resistance (Rct) decreased significantly in SnS₂/g-C₃N₄ composites, confirming enhanced interfacial charge transport efficiency. Specific capacitance values for SnS₂ and SnS₂/g-C₃N₄ were derived from CV and GCD measurements. At a current density of 20 A·g⁻¹, SnS₂@0.075 g-C₃N₄ retained a capacitance of 1145 F·g⁻¹, far exceeding the 315 F·g⁻¹ of pure SnS₂ (Figure 12b,c). Figure 12d compares energy and power densities, revealing a trade-off: energy density decreased slightly with increasing power density. The exceptional cycling stability of SnS₂/g-C₃N₄—attributed to the heterostructure formed during composite synthesis—mitigates the volume fluctuations in SnS₂ during charge–discharge cycles, thereby extending its operational lifespan.

3. Experimental Technique

3.1. Materials and Reagents

Tin(II) (99.999%, Sigma Aldrich, St Louis, MO, USA), sulfide (S), (99.999%, Sigma Aldrich, St Louis, MO, USA), thiourea (CH₄N₂S), (99.999%, Loba Co., Mumbai, India), graphitic carbon nitride (g-C₃N₄) powder, ethanol (C₂H₅OH), and distilled water were used in these experiments.

3.2. Procedure Synthesis

Tin(II) sulfide (SnS) was synthesized under controlled laboratory conditions using high-purity tin (Sigma Aldrich, St Louis, MO, USA) and sulfur (Sigma Aldrich, St Louis, MO, USA). The stoichiometric quantities of tin and sulfur were weighed, with approximately 2 g of the mixture loaded into a pre-cleaned quartz ampoule (70 mm length and 9 mm diameter). The ampoule was flame-sealed under a high vacuum of 1 × 10⁻⁵ Torr to ensure an oxygen-free environment. The sealed ampoule was placed in a custom-built horizontal rotary furnace and heated incrementally at a rate of 100 °C·h⁻¹ to 950 °C. The temperature was maintained at 950 °C for 48 h with continuous rotation to achieve a homogeneous melt. Subsequently, the ampoule was cooled gradually to room temperature over 24 h [31]. The synthesized SnS was recovered by carefully breaking the quartz ampoule. Analytical-grade urea served as the precursor for synthesizing graphitic carbon nitride (g-C₃N₄) via a single-step pyrolysis approach. In a standard procedure, 10 g of urea was transferred into a covered crucible and subjected to thermal treatment at 550 °C for 3 h in a muffle furnace, with a ramp rate of 10 °C·min⁻¹. Following the reaction, the furnace was then naturally cooled to ambient temperature. The resulting powder exhibited a pale yellow hue. This template-free, additive-free synthesis method is self-sustaining, eliminating the need for external agents or structural templates. To fabricate SnS coated with different concentrations of g-C₃N₄, specific amounts of g-C₃N₄ (0.025, 0.05, and 0.075 g) were admixed with 0.5g of SnS and 50 mL of ethanol via an ultrasonic probe. The dispersion was sonicated for 30 min to obtain a good dispersion of g-C₃N₄ nanosheets under constant stirring to form a homogeneous solution. Then, the prepared SnS solution was slowly poured onto the g-C₃N₄ dispersion while sonicating. A high-power ultrasonic homogenizer (power = 700 W) with a 0.6 mL horn was employed for 1 h to ensure that the SnS was uniformly deposited on the g-C₃N₄ surface. During the ultrasonic process, the temperature was maintained below 50 °C so that the materials did not aggregate or undergo thermal degradation. After the ultrasonic treatment, the suspension was allowed to settle and then the mixture was spun at 5000 rpm for 10 min. After precipitation, it was washed using ethanol and distilled water several times to eliminate other impurities and unreacted precursors. After the preparation of the SnS/g-C₃N₄ composite, the collected composite was dried in an oven at 60 °C for 12 h.

3.3. Characterization

The synthesized SnS/g-C₃N₄ composite can be characterized using techniques such as X-ray diffraction (XRD) for structural analysis, scanning electron microscopy (SEM) for morphology, FTIR, electrochemical performance “CS305”, and UV-Vis spectroscopy for optical properties. The ratio of SnS to g-C₃N₄ can be varied to optimize the performance of the composite for specific applications. Ultrasonic energy enhances the interaction between SnS and g-C₃N₄, ensuring uniform coating and strong interfacial bonding. This ultrasonic-assisted synthesis is a simple and efficient method to prepare SnS/g-C₃N₄ composites with enhanced photocatalytic and electrochemical properties.

3.4. Evaluation of Photocatalytic Activity in SnS Coated with g-C₃N₄

Photocatalytic tests were carried out in a specially designed photoreactor equipped with a 500 W tungsten halogen linear lamp emitting 9500 lumens of visible light. A dye solution was prepared in deionized water, and 0.025 g of either pure SnS or SnS coated with g-C₃N₄ photocatalysts were added to the mixture. To ensure uniform dispersion of the catalysts, the suspension underwent ultrasonication in the dark for 30 min. During irradiation, the reaction mixture was continuously stirred and aerated with ambient air. At defined time intervals, 5 mL aliquots were extracted, and the photocatalyst particles were removed via centrifugation. The degradation of methylene blue (MB) was tracked by measuring the absorbance at its maximum wavelength (λₘₐ) using a UV–Visible spectrophotometer. A decrease in absorbance at 665 nm correlated with the decolorization of the dye, indicating degradation efficiency. The removal efficiency of the dye through photocatalytic action was calculated using a standard formula for degradation percentage:

$$Removal prcentage={\displaystyle \frac{\left(C_0-C_t\right)}{C_0}}\times 100$$

(2)

Following a comprehensive assessment of the photocatalytic degradation mechanism, the initial (C₀) and residual (C_t) pollutant concentrations were precisely quantified. After catalyst recovery through centrifugation, the material was reintroduced into the dye solution for subsequent reuse testing.

3.5. Electrochemical Performance

The electrode materials were analyzed using a three-electrode system, where the working electrode comprised the active material, an Ag/AgCl electrode served as the reference electrode, and a platinum foil functioned as the counter electrode. For electrode fabrication, a uniform black paste was prepared by blending PVDF (10%), the active material (80%), and carbon black (10%). This paste was uniformly coated onto nickel foam substrates and subsequently dried in a vacuum oven at 60 °C for 10 h. Electrochemical characterization, including impedance spectroscopy (EIS), galvanostatic charge–discharge (GCD), and cyclic voltammetry (CV), was conducted using a Corrtest CS350 electrochemical workstation (manufactured in China) within a 6 mol L⁻¹ KOH electrolyte environment.

4. Conclusions

The production of SnS/g-C₃N₄ composite materials for photocatalysts and supercapacitors occurred through a simple melt mixing method. The photocatalytic and capacitive properties of composite materials exceed those of pure SnS, especially when SnS@0.075 g-C₃N₄ functions as a photocatalyst and the SnS@0.05 g-C₃N₄ provides a specific capacitance of 1340 F·g⁻¹ at 10 A·g⁻¹ because of SnS/g-C₃N₄ synergistic action. The conductivity enhancement of the SnS/g-C₃N₄ composite results from the high-quality heterostructured system and the presence of g-C₃N₄ nitrogen, which promotes the composite material’s conductive properties. The g-C₃N₄ functions as both a conducting support to disperse SnS and prevents its aggregation, which normally causes cycle life degradation. The SnS/g-C₃N₄ composite reveals excellent potential for future energy-storage applications.