Construction of CuS/g-C₃N₄ heterojunction composite by in-situ synthesis for enhanced photocatalytic degradation of malachite green

ABSTRACT

The poor efficiency caused by the quick charge recombination limited the most prominent photocatalyst g-C₃N₄’s photocatalytic activity. This work effectively synthesized highly efficient g-C₃N₄, and (CuS & CuS/g-C₃N₄ heterojunction composites) by utilizing simple thermal treatment and microwave-assisted methods, respectively. The synthesized materials were analyzed by XRD, UV-Vis DRS, FTIR, FESEM, PL, XPS, and HRTEM analysis. The CuS/g-C₃N₄ composites was validated by XRD analysis, which showed changes in peak intensity as the g-C₃N₄ content increased. When compared to the individual components, the CuS/g-C₃N₄ composites had a smaller band gap, which improved their absorption of visible light, according to UV-Vis DRS. The produced materials’ distinctive stretching vibrations were detected by FTIR spectra, and their emission characteristics were revealed by PL spectra. CuS nanospheres implanted on g-C₃N₄ nanosheets were visible in FESEM pictures. The components C, N, O, S, and Cu, as well as their chemical states, were verified by XPS analysis of the 1:3 composite. The intricate microstructure and interface morphology of the composites were further clarified by HRTEM. Using a 250W halogen lamp in a handmade photocatalytic reactor, the composites’ photocatalytic effectiveness was assessed for the degradation of malachite green (MG) dye in aqueous solution when exposed to visible light. By creating CuS/g-C₃N₄ heterojunction composites, this work seeks to improve the photocatalytic efficiency of g-C₃N₄, which is generally hampered by rapid charge carrier recombination. The outcomes showed that the CuS/g-C₃N₄ heterojunction performed better photocatalytically than either CuS or g-C₃N₄ alone. After 120 minutes, the 1:3 composite showed the best degrading efficiency of all the CuS/g-C₃N₄ ratios, higher than both CuS/g-C₃N₄ (1:2) at 73.5% and CuS/g-C₃N₄ (1:1) at 58.9%. On the other hand, the efficiency of pure MG, g-C₃N₄, and CuS were just 0.8%, 22.7%, and 60.1%, respectively. Rapid charge carrier recombination considerably reduces the photocatalytic efficacy of g-C₃N₄. CuS/g-C₃N₄ heterojunction composites were created as a solution, which improved photocatalytic efficiency. In comparison to individual components and various composite ratios, the optimized 1:3 CuS/g-C₃N₄ composite demonstrated improved degradation of malachite green (MG) dye under visible light. This improvement is ascribed to the development of a heterojunction interface between CuS and g-C₃N₄, which leads to enhanced charge separation and prolonged visible-light absorption. The research study indicates that heterojunction engineering can effectively modify photocatalytic characteristics, with encouraging prospects for wastewater treatment and environmental remediation applications.

Keywords:
Photocatalysis; Cupper sulfide; Graphitic carbon Nitride; CuS/g-C₃N₄; Malachite green

1. INTRODUCTION

Water is a vital resource for both humans and the environment. However, only 0.02% of the world’s water is accessible to humans, plants, and animals. This availability is significantly influenced by the cycles of precipitation and evaporation [¹]. The rapid expansion of industry worldwide has resulted in widespread pollution, posing significant threats to human health and the natural environment. While chemicals used in agriculture, medicine, and various industries have enhanced our lives, they have also caused considerable environmental damage [²]. The major component of aquatic pollution is the contamination of dye effluents in water. Up to 50% of dyes used in textile dying are not absorbed by the fibers and remain contaminants, which presents a serious risk to the environment. Because of their turbidity and high pollution intensity, dye discharge creates a vibrant color even at low concentrations. The aquatic environment can be greatly impacted by dyes due to their hazardous breakdown products [³]. An estimated one million tons of dyes are generated annually worldwide, of which more than 15% are pollutants in industrial effluents. These wastewater colors pose serious risks to living things since they are extremely poisonous, carcinogenic, and xenobiotic [⁴]. The existing physical and chemical treatment methods employed for dye effluent management are often characterized by their time-intensive nature, high costs, and limited effectiveness [⁵]. The most common of these is photocatalysis, which breaks down organic pollutants by producing reactive oxidative species [⁶]. The utilization of catalysts and light sources in photocatalysis accelerates the degradation of organic contaminants [⁷]. There are several categories of photocatalysis mechanisms: (1) The process known as homogeneous photocatalysis occurs when the catalyst and reactant are in the same phase (i.e., solution). Reactive species are produced when the catalyst, which is frequently a transition metal complex or a metal-containing complex, absorbs light and goes through a photochemical change. The organic molecules in the solution are subsequently treated to chemical reactions by radicals. The major advantage of homogeneous photocatalysis is the direct and quick interaction between the catalyst and the reactants in the same phase [⁷, ⁸]. (2) Heterogeneous photocatalysis is a process in which the reactants and the photocatalyst are in distinct phases. This photocatalyst is usually a solid, such zinc oxide (ZnO), titanium dioxide (TiO₂), or other metal oxides, and the reactants are liquid or gaseous. In this kind of photocatalysis, light is absorbed by the solid photocatalyst, which creates electron-hole pairs and then undergoes redox reactions with the adsorbed species on the catalyst [⁸, ⁹]. Furthermore, because of the direct contact between the solid catalyst and dye molecules, heterogeneous photocatalysis shows improved efficiency and selectivity, resulting in lower side reactions and larger yields of degradation products. Solid photocatalysts’ advantage is further enhanced by their stability and longevity, which allow them to withstand severe reaction conditions without suffering appreciable deterioration [⁷, ¹⁰, ¹¹, ¹², ¹³]. Photocatalysis provides low energy consumption, cost-effectiveness, and positive environmental effects. However, the inability to discover a single material that absorbs all solar energy while retaining low recombination rates and excellent photostability limits the effectiveness of semiconductor photocatalysts in solar energy conversion. Because of their synergistic effects, co-photocatalysts are thought to help somewhat overcome these difficulties when used in composites [¹⁴]. On the other hand, nanoparticles, due to their remarkable surface characteristics and chemical reactivity, have surfaced as an exceptional approach for removing and degrading dyes [⁵]. However, the use of semiconductor-assisted materials and renewable solar energy has emerged as a promising solution to address the problem [¹⁵]. TiO₂’s photocatalytic effectiveness is limited because of its broad bandgap, which only absorbs UV light. Metal sulfides, on the other hand, have the ability to absorb visible and infrared light because of their smaller bandgaps, which provides benefits including improved charge separation and effective photo reducing capabilities. CuS, CdS, In₂S₃, MoS₂, ZnIn₂S₄, and Zn_xCd1–S_x are examples of metal sulfides, a class of compounds made up of metals and sulfur that are favored for use as heterojunction photocatalysts. Single-metal-sulfide photocatalysts must contend with issues such fast electron-hole recombination and restricted light absorption. By integrating metal sulfides with other substances, scientists have created heterojunctions that improve light absorption, lower recombination, and preserve potent redox properties to overcome these problems [¹⁶].

Copper sulfide is one of the many metal chalcogenides that has been studied extensively in recent years because of its non-toxic nature and semiconducting properties. CuS is a great photocatalytic material because of its extraordinary plasmonic effect, appropriate bandgap energy values (1.2–1.5 eV), and outstanding electrical and optical features. With varying bandgap energy values of 1.88, 2.06, 2.08, and 2.16 eV for CuS nanoparticles, nanotubes, microspheres, and nanoflakes, respectively, several morphological forms of CuS have been found [¹⁷]. CuS is widely used in photocatalysis, drug delivery, solar cells, biosensors, and lithium-ion batteries. Despite its numerous applications, it remains mostly unexplored in the field of catalysis. CuS has been used as a catalyst in very few catalytic transformations, including H₂ evolution [¹⁸], electrochemical CO₂ reduction [¹⁹, ²⁰], and NH₃ synthesis [²¹, ²²]. Carbon-based materials, metals, polymers, ceramics, and other material groups have all been investigated as helpful materials for catalyst nanoparticle encapsulation. Enhanced charge carrier mobility, conductivity, stability, and large surface area are some benefits of adding catalytic elements to carbon-based materials [²³]. Graphitic carbon nitride (g-C₃N₄) has engrossed a lot of attention due to its desirable 2.7 eV band gap, which allows it to be used in visible light. Since a large amount of solar electromagnetic radiation falls within this range, visible lights are preferred for photocatalytic activities because they allow for the execution of these processes in direct sunlight. As a result, scientists are investigating the creation of visible light to meet the necessary conditions for practical feasibility [²⁴]. The photocatalytic efficiency of g-C₃N₄ has been effectively increased by researchers using techniques such as heterojunction building, element doping, morphological management, and defect engineering. The photocatalytic effectiveness of many g-C₃N₄-based nanocomposites for dye degradation under visible and UV lights is analyzed by DHARANI et al. [²⁵, ²⁶, ²⁷, ²⁸]. A BaFe/g-C₃N₄ nanocomposite has enhanced light absorption and reduced charge recombination, removed 90% of the Congo Red in 150 minutes, and maintained over 88% efficiency over three cycles [²⁵]. Due to increased hydroxyl radical production, ZnO/g-C₃N₄ composites with different ratios showed the greatest MB degradation, 92% under UV light and 65% under visible light for the 1:0.5 ratio [²⁶]. In 180 minutes, 65% of the crystal violet dye was broken down by the NiFeO₄/g-C₃N₄ nanocomposite, with the main reactive species being h+, ∙OH, and ∙O₂− [²⁷]. Under visible light, the sol-gel-synthesized CoFeO₄/g-C₃N₄ showed 57% Rhodamine-B degradation, surpassing that of its pure constituents [²⁸]. The formation of p-n heterojunctions by g-C₃N₄, which has a band gap energy that is an excellent candidate for CuS, is important because it increases surfaces for efficient photogenerated electron-hole pair transfer, which enhances photocatalytic activity [²⁹].

CAO et al. [²⁹] created g-C₃N₄ modified by CuS using a straightforward precipitation technique for effective bifunctional activation of peroxymonosulfate (PMS) and tetracycline (TC) degradation in water. CuS and g-C₃N₄ recombination dramatically increased the separation efficiency of electron-hole pairs under visible light, and electrons efficiently moved to PMS, for producing more reactive oxygen species. In the ideal arrangement, the removal effectiveness of TC was 97.46%. Furthermore, the 35%CuS/g-C₃N₄/PMS/Light system demonstrated exceptional TC degrading capabilities in several water matrices [²⁹]. In a related study, FAN et al. [³⁰] developed heterojunctions using CuS and g-C₃N₄ to suppress the electron-hole pair recombination and GO to speed up electron transmission. The hydrothermally synthesized CuS flower-like microspheres were examined, as were the synthesis conditions of CuS/g-C₃N₄ binary materials and CuS/g-C₃N₄/GO (CCG) ternary materials, as well as the factors influencing their photocatalytic performance [³⁰]. Similarly, the degradation of Rhodamine B (RhB) under visible light was used by DOĞAN et al. [³¹] to assess the photocatalytic activity of ZnS/g-C₃N₄ nanocomposites. To find the ideal circumstances, different photocatalyst doses and starting RhB concentrations were investigated. ZnS/g-C₃N₄ nanocomposites demonstrated nearly twice the photocatalytic performance of pure g-C₃N₄ and ZnS nanoparticles, according to the results. scavenger studies showed that, superoxide radicals were crucial to the photodegradation process [³¹]. A simple process was used by GHAFURI et al. [³²] to create a superior visible-light-responsive photocatalyst, CuS-g-C₃N₄/Ag. The ternary photocatalyst 10wt% CuS-g-C₃N₄/Ag 6wt% had a higher photocatalytic efficiency than g-C₃N₄ and g-C₃N₄/Ag under visible light radiation. These findings demonstrate that the efficient separation and simple transmission of excited electrons and holes are caused by the synergistic interaction of g-C₃N₄, Ag, and CuS nanoparticles [³²].

SHYAGATHUR et al. [³³] used the hydrothermal technique to create CuS nano-flowers and the ternary composite catalysts CuS/rGO, CuS/g-C₃N₄, and CuS/g-C₃N₄/rGO for improved dye degradation. Under visible light, the photocatalysis of the dyes murexide (MX) and malachite green (MG) was investigated in these samples. The CuS/g-C₃N₄/rGO ternary composite, out of the four samples, demonstrated 97.05% and 72.16% degradation for MG and MX dyes, respectively, over 70 min. The outcomes are achieved with a greater dye concentration of 25 ppm and a small catalytic load of 10 mg. Improved light absorption, effective charge separation, and increased catalytic activity are just a few of the synergistic effects between the various components that contribute to the ternary composite’s improved photocatalytic properties. The results show that CuS/g-C₃N₄/rGO, a new ternary compound, is a potential catalyst for MG and MX dye degradation in wastewater treatment applications [³³]. By employing sophisticated ternary composites, SnS₂/GCN/rGO and CuS/GCN/rGO, SHYAGATHUR et al. [³⁴] suggest a unique technique for breaking down organic contaminants, particularly the dyes Brilliant Green (BG) and Indigo Carmine (IC), when exposed to visible light. SnS₂/GCN/rGO combo achieved remarkable degradation efficiencies of 80.9% for IC and 98.0% for BG. Similarly, the CuS/GCN/rGO composite demonstrated impressive BG and IC degradation rates of 78.5% and 92.7%, among others. By mixing graphitic carbon nitride with reduced graphene oxide, the composites solve the main disadvantages of metal sulfide photocatalysts, such as low electron mobility, quick charge recombination, and poor degradation efficiency under visible light. Measurements of chemical oxygen demand (COD) were utilized to verify that the dye pollutants were breaking down. These findings were further supported by impedance spectroscopy, which revealed a significant drop in photoluminescence intensity and a decrease in charge transfer resistance. All of the results point to enhanced charge separation and a faster rate of degradation in the ternary composites [³⁴]. The 2025 study by SHYAGATHUR et al. [³⁵] explores the hydrothermal synthesis of hierarchical SnS₂-based composites, namely SnS₂/rGO and SnS₂/g-C₃N₄. The photocatalytic activity of these composites was assessed in the degradation of an anionic dye, MX, and a cationic dye, MG, when exposed to visible light. The SnS₂/rGO/g-C₃N₄ ternary composite showed the highest photocatalytic efficiency of all the produced materials, degrading MG by 99.29% and MX by 78% in under 70 mins. The synergistic interactions between SnS₂, rGO, and g-C₃N₄ are responsible for the increased photocatalytic performance. These interactions minimize the recombination of photogenerated charge carriers by facilitating a Z-scheme electron transfer route. Effective electron-hole pairs are produced by SnS₂ nanosheets with a bandgap of 2.17 eV, and charge separation and transport are improved by the addition of g-C₃N₄ and rGO. Studies and scavenger tests verified that the most common elements causing dye degradation are H+ ions. The entire mechanism is provided, when each component contributes to the improved photocatalytic process. Furthermore, the study offers a novel method for building Z-scheme SnS₂/rGO/g-C₃N₄ ternary photocatalysts, which shows promise for effectively eliminating both cationic and anionic dyes when exposed to visible light [³⁵].

The major objective of the current work is to synthesize g-C₃N₄ using thermal method and CuS, & CuS/g-C₃N₄ was prepared by microwave-assisted. The CuS/g-C₃N₄ heterojunction composites were prepared using a microwave-assisted method, which offers several advantages, such as rapid synthesis, uniform heating, and enhanced material properties. CuS nanoparticles that are successfully integrated with the g-C₃N₄ framework are formed as a result of the efficient and uniform reaction conditions that the microwave energy fosters. By ensuring that CuS is evenly distributed across the g-C₃N₄ surface, this technique makes it easier to create a strong heterojunction structure that boosts photocatalytic activity. In addition, the synthesized materials’ properties were categorized by different techniques like X-ray diffraction (XRD), UV-vis absorption spectrophotometer (UV-vis), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), Photoluminescence spectroscopy (PL), x-ray photoelectron spectroscopy (XPS), and High-resolution scanning transmission electron microscope (HRSTEM) for in-depth insights into the properties, structure, and behavior of materials. The degradation of malachite green (MG) using photocatalytic reactions when exposed to visible light is important for several reasons, particularly in environmental remediation and material science. Therefore, the synthesized materials were also used to degrade malachite green by a photocatalytic reaction when exposed to visible light.

2. MATERIALS AND METHODS

2.1. Materials

Thiourea (CH₄.N₂S) was purchased from Fisher Scientific, Copper (II) acetate monohydrate ((CH₃COO)₂Cu. H₂O)) was purchased from Merck, and melamine was purchased from Sigma Aldrich and used without further purification. All solutions used in the studies were prepared by using double-distilled deionized water.

2.2. Methods

2.2.1. Preparation of g-C₃N₄

Graphitic carbon nitride was prepared by thermal treatment of melamine. 5g of Melamine were heated in a sealed alumina crucible to 550°C at a rate of 3°C min–1 for 4 hours. Finally, yellowish bulk g-C₃N₄ powder was obtained. To exfoliate bulk g-C₃N₄ powder 50 ml of water is added and stirred for 30 mins. The above solution was sonicated 2h with sulphuric acid at room temperature. The yellow solution was obtained after the sonication, which was washed with DI water and ethanol. Then dried in a hot air oven at 80⸰C temperature. The resulting products were gathered, ground to powder, and labeled when they had cooled down [³⁶].

2.2.2. Preparation of CuS

In the production of CuS nanoparticles, a microwave-assisted method was employed. This procedure involved the combination of a 100 ml solution containing Copper (II) acetate monohydrate at a concentration of 0.1 M with an equivalent 100 ml solution containing thiourea. The resulting mixture was subjected to magnetic stirring for 1 hour to ensure thorough homogenization. Subsequently, the prepared solution was placed inside a 480W microwave oven for 15 minutes. To further enhance the powder’s characteristics, it was subjected to grinding for 15 minutes using mortar and pestles. As a result of these treatments, CuS powder was successfully obtained.

2.2.3. Preparation of CuS/g-C₃N₄

In a typical microwave-assisted method was used for preparation of the CuS/g-C₃N₄ nanoparticles, prepared CuS (1g) and bulk g-C₃N₄ (1g) were dissolved in 50 ml of distilled water under stirring at room temperature for 1h. Subsequently, the prepared solution was placed inside a 480W microwave oven for 15 minutes. To further enhance the powder’s characteristics, it was subjected to grinding for 15 minutes using mortar and pestles. As a result, CuS/g-C₃N₄ (1:1) was obtained. Similarly, the same method was used for CuS/g-C₃N₄ (1:2) and CuS/g-C₃N₄ (1:3).

2.3. Materials characterization

XRD analysis was used to determine the synthesized materials’ average crystallite size, phase purity, and crystal structure. A Bruker D8 Advance instrument, which uses Cu Kα radiation with a wavelength of 1.5418 Å (λ = 1.5418 Å), was used to perform XRD measurements. The sample was scanned in the 2θ range between 20° and 80° at a rate of 0.02°/0.3S. UV-vis DRS spectrophotometry was used to examine the synthetic materials’ optical characteristics. The amount of light absorbed in the visible and ultraviolet portions of the spectrum was measured using a JASCO UV VIS Model V-750. This method gives details on the materials’ band gaps and electrical transitions. FTIR was used to determine which functional groups were present in the synthesized compounds. A Shimadzu device was used to obtain FTIR spectra in the wavelength range of 4000 to 400 cm–1. This method makes it possible to identify certain chemical bonds by their distinctive vibrational frequencies. The above measurements were made at the KPR Institute of Engineering and Technology in Coimbatore, India, using a Shimadzu instrument. FESEM was used to visualize the synthesized materials’ surface morphology and particle size distribution. A SIGMA HV-Carl Zeiss apparatus with a Bruker Quantax 200-Z10 EDS detector was used for the FESEM experiment at the Coimbatore Institute of Technology in Coimbatore, India. Photoluminescence properties, including excitation and emission spectra, were analyzed using a JASCO fluorescence spectrophotometer. These measurements were conducted at the Advanced Materials Characterization Centre, KPR Institute of Engineering and Technology, Coimbatore, India. To investigate the chemical composition and bonding states of elements on the surface of the synthesized materials, XPS was employed. The measurements were carried out using a Thermo Scientific K-Alpha system. XPS analysis offers detailed insights into the elemental composition, chemical bonding, and oxidation states of surface atoms. These measurements were performed at Pondicherry University, located in Pondicherry, India. The TEM images were captured using a Thermo Scientific Talos F200S G2 high-resolution scanning transmission electron microscope (HRSTEM) at the Sathyabama Institute of Science and Technology, Chennai, India.

2.4. Photocatalytic study

Malachite Green was used as a model pollutant to examine the photocatalytic effectiveness of CuS, g-C₃N₄, and CuS/g-C₃N₄. A photocatalytic chamber exposed to visible light was used for the studies. In particular, 100 mL of aqueous solution (1 × 10–5 M of MG) was mixed with 30 mg of the photocatalyst. To bring the photocatalyst and MG dye into adsorption-desorption equilibrium, the resultant mixture was magnetically stirred for 30 minutes in the dark. The suspension was then exposed to visible light irradiation while being continuously stirred magnetically in a homemade photocatalytic reactor setup using a 250W Halogen lamp. 3ml of the suspension was taken at 30-minute intervals, and the supernatant solution was separated following centrifugation. The photodegradation efficiency (η) was determined using the formula

(1) $$\text{η =}\left(\text{1 –}Ct/Co\right)\text{× 100\%}$$

where C0 and C represent the concentrations of the solution before illumination (at t = 0) and after exposure to light for ‘t’ minutes, respectively, as determined from absorption spectra [³⁷].

3. RESULTS AND DISCUSSION

3.1. X-ray diffraction (XRD)

The graphitic stacking structure of g-C₃N₄ is confirmed through XRD analysis, as shown in Figure 1a. The strong peak at 2θ = 27.5⸰ shows that the layered stacking is consistent with the reported g-C₃N₄ (JCPDS Card No: 00-066-0813). The XRD pattern is demonstrated in Figure 1 and it shows the catalyst’s crystalline nature.

Figure 1
XRD pattern of (a) g-C₃N₄, (b) CuS/g-C₃N₄ (1:3), (c) CuS/ g-C₃N₄ (1:2), (d) CuS/ g-C₃N₄ (1:1), and (e) CuS.

Peaks at various 2θ values, such as ~ 27.7°, ~ 29.3°, ~ 31.9°, ~ 32.7°, ~ 47.9°, ~ 52.7°, and ~ 59.2°, were identified as having CuS material properties and were correspondingly assigned to indices (101), (102), (103), (006), (110), (108), and (116) (JCPDS Card No: 01-079-2321). The development of the covellite CuS’s hexagonal phase is primarily responsible for these diffraction peaks. In comparison to the other peaks, the peak at about 47.9° (110) was discovered to be as narrow and intense. This suggests the crystalline structure of the copper sulfide. The synthesized CuS is free from any notable contaminants, as seen by the lack of additional peaks that relate to the precursor material. Figures 1b, 1c, and 1d demonstrate the effective synthesis of the (CuS/g-C₃N₄) binary nanohybrids. Notably, the intensity of the g-C₃N₄ diffraction peak in these binary nanohybrids indicates the CuS content [³⁸]. The diffraction intensity of g-C₃N₄ in the CuS/g-C₃N₄ heterojunctions steadily improved as the g-C₃N₄ concentration increased, but the diffraction intensity of CuS dropped. This is an interesting finding. In the CuS/g-C₃N₄ heterojunctions, this pattern demonstrates how well the hydrothermal process regulates the mass ratio of the components [³⁹].

The size of the crystallites in the synthesized photocatalysts was calculated using the Scherrer formula (D)

(2) $$D=\left(k\lambda \right)\text{/}\beta \cos \theta$$

where D (nm) is the prepared crystallite size of the photocatalysts, k - the shape factor that is dependent on the photocatalysts’ dimensions, λ (Å) - the incident wavelength of the X-rays, β (Rad) - the peak broadening to the full-width half maximum of the diffraction peak and θ (°) - the Bragg’s angle of the diffraction peak [⁴⁰]. The average crystallite size of the catalysts (CuS, g-C₃N₄, CuS/g-C₃N₄ (1:1), CuS/g-C₃N₄ (1:2), and CuS/g-C₃N₄ (1:3)) are 16.62 nm, 6.36 nm, 9.24 nm, 8.59 nm, and 7.49 nm, respectively. This confirms that the catalysts had been made at the nanoscale. These crystallites’ nanoscale dimensions are essential for increasing the materials’ photocatalytic activity because smaller crystallites have an improved surface area-to-volume ratio, which means there are higher active sites for the adsorption of pollutants and the production of reactive species. CuS/g-C₃N₄ heterojunction composites were measured for crystallite size to learn more about the structural features that affect photocatalytic activity. Recombination rates, charge carrier mobility, and surface area are all directly impacted by crystallite size. While suitable particle sizes promote improved charge separation and transport across the heterojunction interface, smaller crystallite sizes generally offer larger surface areas, supplying more active sites for photocatalytic processes. Consequently, it is scientifically significant to examine crystallite size to correlate the composites’ improved photocatalytic activity with their structural characteristics. Additionally, materials at the nanoscale frequently have distinct electronic and optical characteristics that set them apart from their bulk counterparts and can further enhance their photocatalytic performance. Moreover, the XRD investigation confirmed the development of the binary structure of CuS nanocomposite system and indicated that the generated catalysts were pure with no further contaminations [³⁷].

3.2. UV–Vis DRS analysis

The spectral characteristic feature of the synthesized CuS, g-C₃N₄, and CuS/g-C₃N₄ composite photocatalysts was investigated by UV–Vis DRS study and the results are shown in Figure 2 The absorption edge of the synthesized composites (CuS/g-C₃N₄) has been found to increase from 307 nm to 409 nm with an increase in the loading content of g-C₃N₄, whereas for pure CuS and g-C₃N₄ the absorption edge was found at 428 and 395 nm, respectively.

Figure 2
(a) The UV–Vis DRS absorption spectra of g-C₃N₄, CuS, and the CuS/g-C₃N₄ with different contents of g-C₃N₄, (b)Tauc plots of g-C₃N₄ and CuS/g-C₃N₄ composites, and (c) Tauc plots of CuS.

Synthesized CuS, g-C₃N₄, and CuS/g-C₃N₄ nanomaterials showed significant wavelength absorption in the visible spectrum. These features aid in the use of visible light for irradiation throughout the degradation period since, in the current study, the greatest number of produced materials trust visible irradiation. The absorption band edge of synthesized substances is located between about 200 and 800 nm, as shown in Figure 2a. The band gap of the produced nanomaterials was calculated from the absorption spectra using the Tauc equation (Equation 3).

(3) $$\left(\alpha h\nu \right)=A\left(h\nu –Eg\right)n$$

In this equation, α, Eg, hυ, and A represent the absorption coefficient, bandgap energy, photon energy, and constant, respectively, with n equal to 2 or 1/2. Figure 2b shows the Tauc plot to estimate the band gap for many important CuS-based samples. With the increase in concentration of g-C₃N₄, the band gap of the synthesized CuS/g-C₃N₄ decreases from 2.32 eV to 2.71 eV [³⁷]. The graph of (αh۷)2 vs photon energy h۷ for pure CuS is depicted in Figure. 2c, where the tangent lies at 2.21 eV, indicating the bandgap energy of pure CuS [⁴¹]. The produced samples could function effectively under visible light, according to the bandgap energy-estimated results. Using visible light irradiation was the best option for optimizing the photocatalytic activity of these materials because of the strong absorption in the visible region. This study utilized the advantage of the fundamental properties of the synthesized photocatalysts to achieve effective pollutant degradation while consuming the least amount of energy.

3.3. Fourier transform infrared (FTIR)

FTIR spectroscopy was used to examine the catalyst samples’ chemical bonding interactions as they were obtained. The peak reported at 3700 cm−1 is related to the –OH group of adsorbed H₂O molecules [⁴²]. The large peaks between 3000 and 3400 cm–1 represent the stretching modes of uncondensed amino groups (including –NH₂ and NH) and peaks that appeared at 1237, 1314, and 1395 cm−1 are related to the C–N stretching vibration, while the other two peaks noticed around 1538 and 1635 cm−1 can be contributed to C=N stretching vibration [⁴³]. An s-triazine ring confirms the peak in the FTIR spectrum of bare g-C₃N₄ at 805 cm−1 [⁴⁴] shown in Figure 3a. Figure 3b separately shows the peaks that appeared in the CuS sample. The peaks appeared between 3906–3485 cm–1 and 1806–1317 cm–1 show the –OH stretching vibration of water molecules and C=O ­stretching vibration, respectively. The peak at 1003 cm–1 is related to S-S vibrations, and the broader peaks between 870 – 444 cm–1 correspond to Cu–S bonds [⁴⁵, ⁴⁶]. The FTIR analysis displayed useful insights into the chemical composition and bonding interactions within the produced photocatalysts [⁶, ³⁵, ⁴⁴, ⁴⁷].

Figure 3
a) The FTIR spectra of g-C₃N₄, CuS/g-C₃N₄ (1:3), CuS/g-C₃N₄ (1:2), CuS/g-C₃N₄ (1:1), and CuS. b) FTIR spectrum of CuS.

3.4. Field emission scanning electron microscopy (FESEM)

SEM images of g-C₃N₄ nanosheets’ layered morphology and sheet-like structure are shown in Figure 4a. Meanwhile, CuS nanorods with a diameter of less than 100 nm are seen in Figure 4b SEM pictures. The morphology of CuS/g-C₃N₄ (1:1), CuS/g-C₃N₄ (1:2), and CuS/g-C₃N₄ (1:3) are shown in Figures 4c, 4d, & 4e respectively. From this will see the different ratios of g-C₃N₄ and CuS nanosphere on the g-C₃N₄ nanosheet. CuS nanorods can be dispersed on g-C₃N₄ nanosheets to improve the interfacial contact between the two components, which may lead to better charge transfer and photocatalytic activity. The nanohybrid morphology will improve the photocatalytic properties and can be optimized by controlling the CuS/g-C₃N₄ ratio.

Figure 4
SEM images of a) g-C₃N₄, b) CuS, c) CuS/g-C₃N₄ (1:1), d) CuS/g-C₃N₄ (1:2), and e) CuS/g-C₃N₄ (1:3).

3.5. Photoluminescence (PL)

The PL spectra were analyzed to discover the luminescence properties and defects presented in CuS/g-C₃N₄ in the range of 300 nm to 500 nm. However, the maximum emission was observed under excitation of 370 nm wavelength. The PL spectrum of nanoparticles exhibits strong emission peaks appearing from 425 nm to 460 nm for these five samples. This result differs from that of other studies on CuS nanoparticles, which report that PL is sensitive to particle shape [⁴⁸]. However, the whole PL emission spectrum covers the 300-500 nm of the visible region of the electromagnetic spectrum [⁴⁹]. The broad visible photoluminescence band is shown in Figure 5.

Figure 5
PL spectra of g-C₃N₄ and CuS/g-C₃N₄composites (excitation wavelength of 370 nm).

These results suggest that the inclusion of g-C₃N₄ might restrict the rapid recombination of electron-hole pairs. Additionally, the emission intensity slightly shifted to the higher angle side (Red shift). It might be accounted for by the photocatalysts’ significantly enhanced ability to absorb visible light [⁵⁰]. The peak magnitude increases with increasing content of g-C₃N₄ in the composite till the composition of 1:3 which shows the increasing e- – h+ pair recombination rate [⁵¹]. However, for higher concentration ratio i.e. 1:1 peak intensity was less compared to 1:3 composition which may be due to the fact that higher content of g-C₃N₄ in the composite might covers the surface of CuS more as compared to other composition thus leading to decrease in emission [⁵²]. This pattern is consistent with the UV-Vis absorption spectra, which likewise showed a modest red shift (toward longer wavelengths) in the absorption edge as the g-C₃N₄ content increased. The PL observations are supported by the narrower band gap and prolonged light absorption, which also validate the creation of an efficient heterojunction that modifies the composite’s optical and electrical characteristics.

3.6. X-Ray photoelectron spectroscopy (XPS)

The elemental states and chemical makeup of the CuS/g-C₃N₄ (1:3) composite were examined using X-ray photoelectron spectroscopy (XPS). The results of this study are shown in Figure 6a-f. The presence of O, C, N, S, and Cu elements in the sample is confirmed by the survey spectra (Figure 6a), which were collected in the 0–1400 eV range. Figure 6b-f displays the high-resolution spectra for every element. Two separate peaks are seen at binding energies of 287.90 eV and 284.59 eV in the C 1s spectra (Figure 6b). These are indicative of g-C₃N₄ and correspond to sp2-hybridized C–C bonds and sp2 carbon bound to nitrogen in aromatic rings (N–C=N), respectively [⁴¹, ⁵³, ⁵⁴]. Graphitic carbon nitride is further confirmed by the N 1s spectra (Figure 6c), which shows a significant peak at 398.3 eV that is due to sp2-hybridized nitrogen (N–C=N). The N 1s spectrum (Figure 6c) displays a prominent peak at 398.3 eV, attributed to sp2-hybridized nitrogen (N–C=N), further confirming the presence of graphitic carbon nitride [⁵⁵]. One symmetrical peak at ~531.12 eV in the O 1s spectrum (Figure 6d) can be attributed to weakly adsorbed oxygen species (such O₂, H₂O, or OH groups) on the surface. No oxygen doping takes place in g-C₃N₄ during the calcination process, as indicated by a lack of peaks that correspond to C–O or N–C–O bonds. CuS and g-C₃N₄ interactions are suggested by the broad nature of the peaks [⁵⁴]. The Cu 2p spectra has peaks at 932.52 eV and 952.02 eV, which correspond to Cu 2p_3/2 and Cu 2p_1/2, respectively, as seen in Figure 6e. According to the XRD results, these values are in line with those of pure crystalline CuS. The existence of sulfur species inside the composite is shown by the S 2p spectra (Figure 6f), which show peaks at 162.75 eV and 168.20 eV that are attributable to the S 2p_3/2 and S 2p_1/2 states [⁴¹]. CuS and g-C₃N₄ have been successfully integrated, according to the overall XPS results.

Figure 6
Typical XPS analysis of CuS/g-C₃N₄, (a) survey spectrum, (b) C 1s, (c) N 1s, (d) O 1s, (e) Cu 2p and (f) C 1s and S 2p.

3.7. High-resolution transmission electron microscopy (HR-TEM)

Transmission electron microscopy was used to examine the morphology and microstructure of CuS/g-C₃N₄(1:3) (HR-TEM). Their SAED pattern is displayed in Figure 7a. There are three different diffraction rings and the interplanar space between them. The TEM images of CuS/g-C₃N₄(1:3) are displayed in Figure 7b, revealing a crumpled sheet-like structure. The CuS/g-C₃N₄(1:3) composite showed that thin sheets of g-C₃N₄ were uniformly adorned with crystallized CuS nanoparticles, as seen in Figure 3b, c. The (002) plane of the polymeric g-C₃N₄ is represented by the d-spacing of g-C₃N₄ crystallites, which is indicated in Figure 3d and was determined to be 0.32 nm. The observed results were in good agreement with XRD [⁵⁶]. The (006), (103), and (102) planes of CuS were also detected and measured as shown in Figure 7c, d; their respective lattice spacings were 0.27 nm, 0.28 nm, and 0.30 nm. To facilitate the efficient separation of electron-hole pairs during the electron migration process necessary for the photocatalytic hydrogen evolution and degradation reactions, the HR-TEM images showed that CuS nanoparticles were closely bonded to the g-C₃N₄ nanostructure with close interfacial contact [⁵⁷].

Figure 7
HR TEM images, a) SAED pattern for the CuS/g-C₃N₄ (1:3) nanoparticles. b) TEM Image of the CuS and g-C₃N₄ in 50 nm resolution, c) HRTEM fringe image of CuS and g-C₃N₄ at 10 nm, and d) Different lattice fringes of CuS and g-C₃N₄ at 10nm.

3.8. Photocatalytic activity

The absorbance mode of UV-visible spectroscopy was utilized to measure the absorption rate of different concentrations of malachite green to determine the absorption values at 617 nm versus concentration (mg/L or ppm).

The intensity of the maximum absorption peak of MG (617 nm) was monitored over time. Figure 8 depicts the change in optical absorption spectra of MG, g-C₃N₄, CuS, and CuS/g-C₃N₄ catalysts exposed to visible light during various time intervals.

Figure 8
UV–vis absorption spectra of a) MG, b) g-C₃N₄, c) CuS, d) CuS/g-C₃N₄ (1:1), e) CuS/g-C₃N₄ (1:2), and f) CuS/g-C₃N₄ (1:3) for different exposure times under visible light illumination.

The disappearance of the band at 617 nm indicates that CuS/g-C₃N₄ photodegraded the MG. Figures 9a,b show that the absorbance of MG increased by approximately 58.9%, 73.5%, and 91.5% in the presence of g-C₃N₄ after 120 min, based on the ratio of g-C₃N₄. The absorption without catalysts (Figure 9a) was also explored to understand the role of g-C₃N₄ during the photodegradation of the dyes. Both the electrons in the CB of g-C₃N₄ and the holes in the VB of CuS, which have strong redox potentials, are left behind by this charge transfer pathway. Whereas the holes in CuS oxidize water or hydroxide ions to generate hydroxyl radicals (•OH), the electrons in g-C₃N₄ convert dissolved O₂ to form superoxide radicals (•O₂−). CuS-modified g-C₃N₄ composites have been shown in recent research to be effective for improved photocatalytic elimination of contaminants. A CuS/g-C₃N₄ catalyst was created by CAO et al. [²⁹] to activate peroxymonosulfate (PMS) and degrade tetracycline (TC) in two ways. The catalyst achieved a 97.46% TC removal efficiency by using both PMS activation through Cu+/Cu2+ redox cycling and enhanced charge separation under visible light. Across a range of water matrices, the optimized system (35% CuS/g-C₃N₄/PMS/Light) also demonstrated good performance [²⁹]. Using a hydrothermal method, SHYAGATHUR et al. [³³] synthesized and characterized CuS-based composites, including a ternary CuS/g-C₃N₄/rGO catalyst, which demonstrated the synergistic effects of CuS, g-C₃N₄, and rGO in improving photocatalytic performance by achieving 97.05% degradation of Malachite Green (MG) and 72.16% degradation of Murexide (MX) under visible light in 70 minutes with only 10 mg of catalyst and 25 ppm dye concentration [³⁴]. Malachite green dye deterioration is mostly caused by these reactive oxygen species. According to PL investigations and photocatalytic performance testing, the heterojunction structure reduces the recombination of photoinduced charge carriers and increases visible light absorption because of CuS’s shorter bandgap. In addition to optimal crystallite size and interfacial contact, the synergistic interaction between CuS and g-C₃N₄ also enhances photocatalytic degradation and facilitates effective charge separation.

Figure 9
a) Degradation (C/C0) and b) degradation efficiency under visible light.

Before and after the photocatalytic degradation of malachite green, the CuS/g-C₃N₄ (1:3) nanocomposites’ XRD patterns are shown in Figure 10a. During the photocatalytic process, the crystal structure of the nanocomposites is stable if there are no appreciable changes in peak locations or intensities. Before and after degradation, the FESEM images are shown in Figure 10b. The morphology of the CuS/g-C₃N₄ (1:3) nanocomposites shows no significant alteration in particle shape or surface texture, further supporting the material’s structural and morphological stability after the reaction.

Figure 10
a) XRD patterns and b) FESEM image of CuS/g-C₃N₄ (1:3) nanocomposites before and after photocatalytic experiment.

4. CONCLUSIONS

In the end, the microwave-assisted approach was employed to produce pure CuS as well as CuS/g-C₃N₄ nanomaterials at a low cost. The materials were analyzed using advanced tools such as XRD, UV-vis DRS, FT-IR, SEM, PL, XPS, and TEM. The X-ray diffraction results showed the presence of CuS/g-C₃N₄ peaks and different ratios of g-C₃N₄ also identified by increasing the intensity of peaks. The UV-vis DRS spectroscopy demonstrated that CuS/g-C₃N₄ has a narrower band gap than pure materials, which have larger band gap positions. SEM scans indicated a nanosphere embedded on the nanosheet of CuS/g-C₃N₄ materials. FT-IR spectra were utilized to determine the stretching band locations of the produced materials. The PL spectrum reveals the emission wavelength of CuS and g-C₃N₄. The elemental states and chemical makeup of the CuS/g-C₃N₄ (1:3) composite were examined by XPS. The results of this study revealed the presence of O, C, N, S, and Cu elements. HRTEM was examine the morphology and microstructure of CuS/g-C₃N₄ (1:3). The photocatalytic results under visible light exposure demonstrated that pure materials had low degradation efficiency that is g-C₃N₄ has 22.7% and CuS has 60.1% only, but the CuS/g-C₃N₄ (1:3) material had 91.5% efficiency. By increasing the g-C₃N₄ ratio the photodegradation efficiency was increased, which is due to the decrease in the band of CuS. Furthermore, additional catalyst doses will be created for toxicological research using BOD, COD, TOC, and HPLC systems.

5. ACKNOWLEDGMENTS

Authors GR and PAK sincerely acknowledge the KPR Institute of Engineering and Technology for funding through an Institutional research fellowship and for providing XRD, FTIR, PL, and UV-Vis spectroscopy characterization facilities.

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Publication Dates

• Publication in this collection
  05 Sept 2025
• Date of issue
  2025

History

• Received
  18 Feb 2025
• Accepted
  09 July 2025