Enhanced photocatalytic performance of CuS/O,N-CNT composite for solar-driven organic contaminant degradation

Abstract

Herein, a hydrothermal etching approach was used to generate an innovative CuS/O,N-CNT composite. The hydrothermal etching of g-C₃N₄ led to the creation of O,N-CNT, with ethanol as the oxygen source. The SEM and TEM characterizations confirmed the formation of CNT, whereas the XPS analysis proved the doping of oxygen and nitrogen in the CNT matrix along with the incorporation of CuS. Under sun irradiation, the produced CuS/O,N-CNT showed outstanding photocatalytic efficiency, eliminating methyl orange and methylene blue dyes with 97.21% and 98.11% efficacy, respectively. Adding hydrothermally etched O,N-CNT increased light absorption and charge migration kinetics, as can be studied from the UV-DRS and PL analysis; hence, the observed improvements in light absorption and charge transfer pathways contributed to the CuS/O,N-CNT composite's enhanced photocatalytic activity, indicating its potential for efficient elimination of organic contaminants under solar irradiation. The catalyst demonstrated high reusability performance up to six cycles and significantly degraded other dyes. Scavenger analysis, along with VB-XPS and UV-DRS analysis, aid in developing a photocatalytic mechanism that confirms the participation of hydroxyl and superoxide radicals in the degradation process.

Introduction

Pollution, particularly water pollution, is a significant problem today. Dyes are used in many sectors, including textile, painting, printing, and fashion design, to make their goods more appealing¹. However, the wastewater generated by these enterprises contains organic pollutants, which are the primary source of water pollution. Innovative and sustainable wastewater treatment methods are urgently needed to respond to the worldwide water pollution problem, notably from the discharge of refractory organic dyes². Removing these dyes from our water sources is critical to avoid various health problems, including skin disorders, respiratory tract infections, and irritated eyes. The removal of hazardous dyes from water sources has arisen as a significant research goal, demanding the exploration of innovative photocatalytic materials to fulfill the limits associated with current water treatment procedures³.

The Advanced Oxidation Process (AOP) has gained popularity recently as a technique for eliminating contaminants from wastewater^4,5. Light creates active species, such as hydroxyl radicals, which initiate the breakdown process of organic compounds. AOPs are affordable, safe, environmentally friendly, and highly effective at eliminating various organic pollutants⁶. The equations show how light-induced e^- and h⁺ combine with H₂O and adsorbed O₂ to produce reactive oxygen species (ROSs). ROSs then decompose complicated hazardous waste into smaller byproducts (Eqs. 1–3)⁷.

$${\text{O}}_2+{\text{e}}^{-}\to {\text{O}}_2^{·-}$$ 1

$${\text{H}}_2\text{O}+{\text{h}}^{+}\to ·\text{OH}+{\text{H}}^{+}$$ 2

$$·\text{OH}+{\text{O}}_2^{·-}{\to }^{Toxicwaste}\text{Degradation}\text{Byproducts}$$ 3

For the AOP-based photodegradation process to occur, three things are required: the release of photons with an appropriate wavelength, a photocatalyst (preferably heterogeneous), and an oxidizing agent (usually H₂O₂)⁸.

Copper sulfide (CuS) emerges as an interesting choice in this context, owing to its well-established photocatalytic characteristics, stability, and ability to assist the degradation of a wide range of organic contaminants⁹. CuS has a 2.29 eV band gap and can absorb visible light efficiently¹⁰. It has superior electrical and optical qualities. When exposed to light, they produce photogenerated electrons and holes¹¹. Its unique characteristics, such as enhanced conductivity at elevated temperatures, superconductivity at 1.6 K, minimal toxicity, and optimal sunlight absorption ability, represent CuS nanocomposite for photodegradation¹². Following this, Hao et al. developed a CuFe₂O₄/CuS photocatalyst for the degradation of Orange II under visible light irradiation with an efficiency of 97%¹³. Another research group fabricated a CuS/ZnS core/shell to eliminate rhodamine B¹⁴.

A strategic strategy, including the synthesis of CuS anchored onto a novel support matrix constituted of oxygen and nitrogen-doped carbon nanotubes (CuS/O,N-CNT) was used to enhance the performance and adaptability of CuS. O,N-CNT, the precursor for these modified carbon nanotubes, was created using a carefully engineered hydrothermal etching technique of graphitic carbon nitride (g-C₃N₄)^15–18. This transition adds oxygen and nitrogen functions to the carbon nanotube structure and increases its surface area and charge transport characteristics, making it more suitable as a CuS support material. Remarkably, Zhang et al.¹⁹ discovered the effect of ultrasonic cavitation on carbon atom-created clusters (CQDs) on g-C₃N₄ due to leftover carbon atoms decaying to nitrogen or in some locations of the compound. We were encouraged by earlier publications to investigate the instability of g-C₃N₄, which can be utilized as a carbon–nitrogen source to create N-CNTs under specific circumstances²⁰. O,N-doping can improve carbon atoms' electron transition capacity, which is advantageous for photocatalysis^21,22. Consequently, it is anticipated that g-C₃N₄-generated O,N-CNTs will function well as a photocatalysis²². The use of O,N-CNT as a support material is justified by its superior properties compared to pristine carbon nanotubes²³. Including oxygen and nitrogen moieties increases the number of active sites for catalysis and the overall affinity for organic pollutants. Furthermore, the hydrothermal etching procedure improves surface morphology and creates a more suitable interface for effective electron transfer, thus increasing the photoactivity of the composite material.

In response to the growing demand for sustainable and effective wastewater treatment solutions, we report a ground-breaking research project centered on the synthesis of a new photocatalytic material, CuS, anchored onto oxygen and nitrogen-codoped carbon nanotubes (CuS/O,N-CNT). The oxygen and nitrogen-codoped carbon nanotubes (O,N-CNT) were created using a precisely controlled technique that involved hydrothermal etching of g-C₃N₄. The photocatalytic destruction of complex organic pollutants, especially methyl orange and methylene blue, is investigated using the resultant CuS/O,N-CNT composite material. This study adds to the growing repertory of sophisticated photocatalytic materials and highlights the potential of customized nanocomposites in tackling modern environmental concerns via sustainable wastewater treatment approaches.

Experimental section

Materials used

Melamine (C₃H₆N₆), Copper acetate monohydrate [Cu(CH₃COO)₂·H₂O], Sodium Sulfide nonahydrate (Na₂S·9H₂O), ethanol, cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Without any further purification, all reagents were used exactly as supplied.

Fabrication of g-C₃N₄

A direct thermal polymerization method was used to create pure g-C₃N₄ from melamine. To do this, 5 g of melamine were calcined in a furnace at a rate of 20 °C per minute for four hours until the temperature reached 550 °C. After it cooled down at room temperature, the resulting yellow product was crushed into a uniform powder²⁴.

Fabrication of CuS

The CuS nanomaterials were manufactured using the hydrothermal method at 220 °C for 24 h. To create the solution, 20 mL of distilled water, 2 mmol of copper acetate monohydrate Cu(CO₂CH₃)₂·H₂O, 0.1 mmol of CTAB, and 2 mmol of sodium sulfide nonahydrate (Na₂S·9H₂O) were mixed and stirred vigorously for 15 min. The mixture was then placed in a 100 mL Teflon-lined stainless-steel autoclave and heated to 220 °C. After the reaction time, the autoclave was taken out and cooled to ambient temperature. After being filtered and cleaned with distilled water and ethanol three times, the result was a black precipitate. Ultimately, the products were dried for two hours at 80 °C.

Fabrication of CuS/O,N-CNT

CuS/O,N-CNTs were synthesized through the hydrothermal method at a temperature of 220 °C for 24 h, as illustrated in Fig. 1²². To prepare the solution, a mixture of 20 mL distilled water, 10 mL of ethanol, 2 mmol copper acetate, 1 g of g-C₃N₄, 0.1 mmol CTAB, and 2 mmol sodium sulfide (Na₂S) was vigorously stirred for 15 min. The solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated to 220 °C. After the reaction time, the autoclave was cooled down to the ambient temperature. The resulting bluish-black precipitate was filtered and cleaned with distilled water and ethanol in a 1:1 ratio and then dried for two hours at 80 °C before undergoing characterization.

Figure 1.

Schematic illustration of synthesis of CuS/O,N-CNT.

Characterization technique

Powder XRD was performed with the Phillips X'pert Pro MPD at a 2°/min scan rate using Cu K radiation (2θ = 10–90°). The Varian Cary eclipse fluorescence spectrophotometer was used to document photoluminescence spectra. SEM-EDAX analysis uses the Jeol 6390LA/OXFORD XMX N, which has a quickening voltage range of 0.5 to 30 kV and a maximum magnification of 30 k. EDAX features an area detector of 30 mm² with a resolution of 136 eV. The Jeol/JEM 2100 (200 kV) electron gun with a LaB6 lattice resolution and point resolution of 0.14 nm and 0.23 nm, respectively, was used to explore HRTEM. Thermo Fischer Scientific's Nexsa base model was used for the XPS examination. The surface functional groups of all the synthesized materials were assessed using Fourier transform infrared spectroscopy (FT-IR, Niagoli IS50, USA) in the range of 500–4000 cm⁻¹. Using a Genesys 10S UV–Vis spectrophotometer, the dye concentration was determined.

Photocatalysis analysis

This study investigated the effectiveness of CuS/N-CNTs in the photocatalytic degradation of methylene blue (MB) and methyl orange (MO) dyes under sunlight. Variations were seen in the concentrations of dye solution, dosages of photocatalyst, and amounts of hydrogen peroxide (0–1 mL of 25% H₂O₂). The resulting mixtures were exposed to sunlight for 60 min to examine the photocatalytic breakdown process (the month of October, 10–12 pm). The experiment, using varied photocatalyst doses (6, 8, 10, 12, and 14 mg), used independent beakers, each containing a 50 mL solution of 10 ppm MB and MO dyes and the optimal amount of H₂O₂. Following the optimal catalyst dosage finding, the ideal dye concentration (between 10 and 40 ppm) was calculated using the optimized catalyst dose and peroxide volume. The dye-photocatalyst reaction solutions were left to equilibrate in the dark while stirring for 30 min. The appropriate amount of peroxide was then added, and the solutions were exposed to sunlight for 30 min.

The breakdown rate was tracked by extracting 2 mL samples of the dye-photocatalyst solution at 5-min intervals. MO and MB's maximum absorbance were determined at 464 nm and 664 nm, respectively, using a UV–Vis spectrophotometer.

Results and discussion

XRD analysis

As seen in Fig. 2, the orientation and crystal structure of produced materials such as CuS and CuS/O,N-CNT were investigated using XRD analysis. Distinct peaks indexed to planes (1 0 1), (1 0 2), (1 0 3), (0 0 6), (1 0 7), (1 1 0), (1 0 8), (2 0 0), and (1 1 6) were located at 27.42°, 29.07°, 31.42°, 32.19°, 46.01°, 47.66°, 52.36°, 54.38°, and 59.21°, respectively. These data matched with JCPDS card no—78-2391, having a hexagonal structure with cell values of a = 3.796 Å and c = 16.360 Å, indicating a primitive lattice with a space group of P63. Compared to the nanocomposite, the CuS peaks showed a backward shift, which meant the presence of a slightly amorphous or disordered phase and confirmed the formation of another nanomaterial, leading to increased strain resulting in a decrease of 2θ values. This inference can further be confirmed by investigating the crystallinity of pure CuS nanoparticles and CuS/O,N-CNT nanocomposite (Eq. 4). The pure CuS showed a crystallinity of 78.6%, whereas the crystallinity of the nanocomposite decreased to 66.29%. Conversely, the O,N-CNT peaked at 52.36° indexed to (1 0 0) plane with higher 2θ values than pristine CNT. Additionally, the appearance of a new (1 0 5) peak indicates structural modification, the disappearance of the (1 1 0) peak suggests altered crystal symmetry or phase transformation upon interaction with O,N-CNT, and the increased intensity of the (1 0 7) peak in the CuS/O,N-CNT composite suggests enhanced crystal growth or preferential orientation. In addition, the crystallite size of both materials was calculated using the Scheerer equation (Eq. 5), and it was found that the CuS/O,N-CNT (7.55 nm) has a smaller crystallite size compared to CuS (8.94 nm).

$$Crystallinity\%=\left[\frac{Areaundercrystallinepeaks}{Areaunderallpeaks}\right]\times 100$$ 4

$$D=\frac{k\lambda }{\beta cos\theta }$$ 5

Figure 2.

XRD plot of fabricated CuS and CuS/O,N-CNT.

FTIR analysis

Fourier Transform Infrared Spectroscopy investigated the materials' functional groups. The FTIR spectrum of CuS, g-C₃N₄, and CuS/O,N-CNT in the range of 4000–500 cm⁻¹ are shown in Fig. 3a. Pure CuS, being an inorganic material, does not show any characteristic peaks of organic functional groups in the IR region. However, the peaks in the region 740–500 cm⁻¹ indicate the stretching of Cu–S bonds. The FTIR spectrum of g-C₃N₄ showed a broad peak in the 3300–3000 cm⁻¹ range, which is attributed to N–H stretchings present in adsorbed moisture and amine groups overlapping in the same region²⁵. The confirmation of the C=N and C–N bonds in the graphitic carbon nitride are observed between 1800 and 1200 cm⁻¹. The doublets at 1526 cm⁻¹ and 1634 cm⁻¹ arise due to C=N stretchings, and the bands of C-N stretchings are observed at 1454 cm⁻¹, 1396 cm⁻¹, 1314 cm⁻¹, and 1234 cm⁻¹. Finally, the existence of a tris-triazine system in the g-C₃N₄ was asserted by a strong peak at 800 cm⁻¹. The FTIR spectrum of CuS/O,N-CNT was different from g-C₃N₄. The peak structure in the fingerprint region from 1500 to 800 cm⁻¹ changed, suggesting that the structure of g-C₃N₄ was destroyed through hydrothermal etching²⁶. The 880–690 cm⁻¹ peaks indicate the bending of C=C bonds in the CNT structure. A strong peak at 1084 cm⁻¹ was due to the C–O stretching, and the peak of N–O stretching was observed at 1533 cm⁻¹, confirming the existence of O,N-CNT²². A peak at 1454 cm⁻¹ indicates C–H bending in the CNT. The peak at 1742 cm⁻¹ ensures the stretching of carbonyl carbon. The peaks in the region 2200–1940 cm⁻¹ were attributed to C-H bending in the CNT and the same was confirmed by the peaks of C-H stretchings observed in the region from 3200–2500 cm⁻¹ formed due to the hydrothermal etching of g-C₃N₄²⁷. Thus, the FTIR analysis was helpful for studying the functional groups and successfully developing O,N-CNT.

Figure 3.

(a) FTIR analysis of CuS, g-C₃N₄, and hydrothermally derived CuS/O,N-CNT; (b) UV-DRS, (c) Tauc’s plot and (d) PL analysis of CuS and CuS/O,N-CNT; (e) VB-XPS analysis of CuS/O,N-CNT.

Optical properties studies

The light harvesting and optical properties of pure CuS and CuS/O,N-CNT nanocomposite were investigated through UV–visible absorbance spectroscopy. The absorbance spectrum of the nanomaterials is illustrated in Fig. 3b. The absorbance spectrum of both samples showed similar peaks in the full UV–visible spectrum owing to the excellent light harvesting properties of CuS. The first shoulder from 260 to 360 nm was due to the bluish-black color of covellite CuS^28,29. The second shoulder form 500–700 nm was ascribed to the d-d transitions in Cu²⁺ ions, which is characteristic of hexagonal covellite CuS^30,31. After forming the nanocomposite, the absorption intensity of both peaks increased, indicating an enhancement in the light-harvesting properties of the nanocomposite. The samples' bandgap was determined from Tauc's plot using the following equation (Eq. 6):

$${\left(\mathrm{\alpha }\text{h}\mathrm{\nu }\right)}^{\text{n}}=\text{A}\left(\text{h}\mathrm{\nu }-{\text{E}}_{\text{g}}\right)$$ 6

where α is the absorption coefficient, h is Planck's constant, ν is the frequency of light radiation, A is a constant, E_g is the optical bandgap of the material, and n is ½ for indirect and 2 for direct allowed transitions, respectively. Pure CuS exhibited a bandgap of 2.2 eV, which is indicative of its visible light's active photocatalytic nature³². The bandgap of the nanocomposite was reduced to 2.0 eV, suggesting improvement in the photocatalytic properties after forming the nanocomposite (Fig. 3c).

The greater electron–hole recombination rate of photocatalysts is known to restrict their photocatalytic efficiency³³. A slow charge recombination rate is preferred when creating a visible light-active photocatalyst. An excellent method for examining charge recombination in semiconductor photocatalysts is photoluminescence (PL) spectroscopy³⁴. A lower intensity of the nanocomposite's PL peak signifies a slower rate of charge recombination, whereas a greater intensity peak points to a quicker charge recombination rate³⁵. Figure 3d shows the PL spectrum of CuS and CuS/O,N-CNT nanocomposite recorded at an excitation wavelength of 400 nm. Pure CuS showed the highest peak at around 465 nm, corresponding to bandgap excitation transition. Moreover, the PL intensity decreased after the formation of the nanocomposite, indicating that the rate of charge recombination was significantly delayed due to charge delocalization properties and higher electrical conductivity of CNT^36,37. Therefore, by π-conjugation of the O,N-CNT's carbon network, photoinduced electrons from the CB of CuS are easily transported to the CNT surface in CuS/O,N-CNT nanocomposite. Consequently, the charge recombination rate decreases in the prepared nanocomposite by an increase in the charge separation between the CB and VB. Therefore, CuS/O,N-CNT nanocomposite could show enhanced photocatalytic activity under visible light irradiation.

From the VB XPS, as illustrated in Fig. 3e, the E_VB was found to be 1.0 eV. Using the below Eq. 7, the E_CB was calculated and found to be − 1.0 eV. The bandgap was determined from UV-DRS analysis and was found to be 2 eV.

$$E_{\mathit{CB}}=E_{\mathit{VB}}-E_g$$ 7

TEM analysis

Further analysis was conducted using CuS/O,N-CNT composite utilizing Transmission Electron Microscopy (TEM). The geomorphology, form, and structural variations between various areas and their interfaces were examined using TEM. The TEM images in Fig. 4a,b indicate hexagonal and rod-shaped images with distinct grain boundaries attributed to CuS and CNT, respectively. The HR-TEM image illustrated in Fig. 4c further confirmed the above findings. The Selected Area Electron Diffraction (SAED) pattern in Fig. 4d specifies the presence of both crystalline (CuS) and amorphous (O,N-CNT) areas in the nanocomposite. The irregular concentric rings and brilliant spots support this theory. The uneven rings may show the amorphous or disordered portions of the sample, while the bright spots represent crystalline zones with clearly defined crystallographic planes. Materials that exhibit both crystalline and amorphous phases, as well as nanomaterials with a variety of crystal sizes and orientations, frequently exhibit this pattern.

Figure 4.

(a, b) TEM images, (c) HR-TEM image, and (d) SAED pattern of manufactured CuS/O,N-CNT nanocomposite.

SEM and EDAX analysis

The surface topography of the designed CuS/O,N-CNT nanocomposite, which has microporous and mesoporous morphologies, was examined employing scanning electron microscopy (SEM). The SEM scans showed a hexagonal sheet-like morphology, confirming the presence of CuS and rod-shaped carbon nanotubes (CNTs). Figure 5a shows the compound's rough and uneven surface, which is caused by surface fracture during hydrothermal etching of g-C₃N₄. Further examination of Fig. 5b,c reveals unique hexagonal-shaped CuS and rod-shaped CNT structures. This uneven and porous structure promotes pollutant adsorption, improving photocatalytic activity. Figure 5d depicts the elemental distribution inside the produced CuS/O,N-CNT nanocomposite. The photocatalyst's elemental composition contained carbon, oxygen, nitrogen, sulfur, and copper at weight percentages of 30.79, 7.87, 6.30, 20.88, and 34.16%, indicating the lack of any extraneous element contaminants.

Figure 5.

(a–c) SEM images and (d) EDAX analysis of fabricated CuS/O,N-CNT nanocomposite.

XPS analysis

The XPS survey spectra confirm the presence of Cu, S, O, N, and C as demonstrated in Fig. 6a. The short scan spectra of C 1 s are displayed in Fig. 6b, and it deconvoluted into 3 peaks signifying C–C/C–H, C–O–C/C–N, and C=O/C=N located at 284.8, 285.81, and 287.93 eV respectively³⁸. The strong C–O–C/C–N bond confirmed the oxygen and nitrogen co-doping onto the CNT matrix³⁹. The high-resolution N 1 s spectra confirmed the existence of pyridinic N and pyrrolic N at 399.52 and 401.5 eV, respectively (Fig. 6c)^40,41. The peak at 400.56 eV indicates that N-sites interconnect the sulfurized Cu and N-doped carbon layers, confirming the interaction of dopants with the nanocomposite^42,43. The sharpness of the N 1 s peaks in Fig. 6c indicates that the nitrogen atoms of the N-CNTs are uniformly distributed and that the range of probable C–N functionalities is limited⁴⁴. The short scan spectra of oxygen, as shown in Fig. 6d, resulted in 2 peaks at 531.16 and 531.97 eV dedicated to C=O and C–O/O–H bonds, respectively⁴⁵. The shift in low binding energies of O 1 s spectra infers strong interaction between CuS nanoparticles and O,N-CNT matrix³⁹. As a result of the oxygen radicals attacking the carbon nanotubes during the doping process, C-O bonds will probably form, activating the CNTs' surface and forming more active sites, leading to enhanced performance⁴⁶. The Cu 2p spectrum in Fig. 6e confirms the existence of Cu²⁺ ion. Cu 2p_3/2 and Cu 2p_1/2 may be ascribed to the two peaks at about 932.04 and 951.95 eV, respectively^13,47. The energies of 162.85 and 161.58 eV, which correspond to the terminal polysulfide and the sulfide anions 2p_1/2 and 2p_3/2, respectively, are where the peaks for S 2p in Fig. 6f are located³⁵. It demonstrates that sulfur is the moiety that is immediately linked with Cu²⁺ and subsequently connected to the O,N-CNT matrix. The XPS analysis and other characterization techniques successfully infer CuS/O,N-CNT nanocomposite formation.

Figure 6.

(a) Survey spectrum of the fabricated nanocomposite; Short scan spectra of (b) C 1 s, (c) N 1 s, (d) O 1 s, (e) Cu 2p, and (f) S 2p.

Photocatalytic application

The use of CuS/O,N-CNT for photocatalytic degradation of methylene blue (MB) and methyl orange (MO) is described in this section. A detailed analysis of the reaction condition optimization is provided, clarifying the process of fine-tuning necessary to increase the photocatalytic efficiency. The evaluation of reusability is also included, providing information on the long-term performance of CuS/O,N-CNT composites across several cycles. A better comprehension of the photocatalytic process may be achieved by looking at the function of scavengers in the degradation process. To further expand on CuS/O,N-CNT composites' potential for environmental remediation, this section also examines how well they degrade different contaminants. This section provides an entry point for a more in-depth investigation of the many uses and characteristics of CuS/O,N-CNT composites in the field of photocatalysis.

Effect of H₂O₂

Numerous studies have established the usefulness of adding hydrogen peroxide (H₂O₂) to wastewater for photocatalytic breakdown of organic contaminants. As a result, the photocatalytic activity of a newly created nanohybrid in the presence of H₂O₂ was investigated. Various concentrations of H₂O₂ (0 mL, 2 mL, 4 mL, 6 mL, 8 mL, and 10 mL) were mixed with 10 mg of the produced nanocomposite in a 50 mL solution containing 10 ppm of methyl orange (MO) and methylene blue (MB) dye (Fig. 7a). The initial injection of H₂O₂ increased photodegradation efficiency up to 8 mL for both dyes, followed by a slight decrease in degradation capacity.

Figure 7.

Effect of (a) H₂O₂, (b) catalyst dosage, and (c) dye concentration on the degradation performance of CuS/O,N-CNT; (d) degradation kinetics for the removal of MO and MB through CuS and CuS/O,N-CNT.

According to current studies, the first increase in degradation efficacy is caused by the self-decomposition of H₂O₂ (Eq. 8).

$$H_2{O}_2+hv\to ·OH+·OH$$ 8

The scavenging effect of H₂O₂ decreases with its increased concentration, resulting in reduced decomposition efficiency and fewer reactive ·O₂H and ·OH free radicals generated (Eq. 9).

$$H_2{O}_2+·OH\to H_{2}O+·O_{2}H$$ 9

The study sought to analyze the efficacy of photocatalytic degradation. The results showed that adding 10 mg of catalyst to 50 ml of 10 ppm MO dye resulted in a breakdown efficiency of 91.7%, with an ideal H₂O₂ dose of 8 mL. Without H₂O₂, decomposition efficiency dropped to 58.6%. Similarly, with 10 ppm MB dye, adding 10 mg of catalyst to a 50 ml solution resulted in a breakdown efficiency of 97.7% with an optimal H₂O₂ dose of 8 mL, but without H₂O₂, the decomposition efficiency dropped to 56.72%.

Effect of catalyst dosage

This study used several photocatalyst doses (6, 8, 10, 12, and 14 mg) to determine the role of photocatalyst amount on the degradation of 50 mL solutions containing 10 ppm of methyl orange (MO) and methylene blue (MB) dyes. The ideal amount of hydrogen peroxide supplied was 0.8 mL. An increase in catalyst dose resulted in a higher conversion of peroxide molecules to hydroxyl radicals, increasing the availability of active surface sites. However, excessive ·OH radicals may endure recombination at higher catalyst doses, as revealed in Eq. 10. Higher catalyst dosage would lead to increased turbidity in the solution, resulting in decreased light penetration and decreased efficiency⁴⁸.

$$·OH+·OH\to H_2{O}_2$$ 10

The optimal catalyst concentrations for MO and MB were 8 mg (0.16 g/L) and 10 mg (0.2 g/L), respectively, resulting in effective dye degradation. Figure 7b shows the degradation efficacy for different catalyst doses. The results showed that 8 mg of catalyst for MO and 10 mg for MB resulted in 94.55% and 97.70% degradation of 10 ppm dye.

Effect of dye concentration

To investigate the impact of the initial contaminant concentration, the photocatalyst doses were kept at 8 mg for MO and 10 mg for MB, with an ideal H₂O₂ dosage of 0.8 mL for both. Samples were collected and tested routinely throughout a concentration range of 10 to 30 ppm for MO and MB, respectively (Fig. 7c). Although such high dye concentrations are uncommon in wastewater settings, their use in degradation research provides precise concentration measurements. A reduction in degradation efficiency was found, most likely due to restricted photon penetration into the solution. Furthermore, increasing dye concentrations increases solution opaqueness and impedes photon paths, decreasing performance. The optimal MO and MB dye degradation concentrations were determined to be 20 and 15 ppm, respectively, with 97.21% and 98.11% efficiencies. The comparison of the degradation performance of these dyes through other nanocomposites has been shown in Table 1. Compared to other works, the degradation of MB and MO by the fabricated photocatalyst utilized less catalyst dosage and time and exhibited higher efficiency.

Table 1.

Comparative analysis of the degradation performance of CuS/O,N-CNT with other reported photocatalysts.

Nanocomposite	Pollutant	Dye Conc. (ppm)	Catalyst dosage (g/L)	Time (min)	Efficiency (%)	Refs.
Titanate nanorods	MO	10	0.1	60	95.6	50
Fe3O4–CuS@SiO2	MO	25	0.4	20	94	51
rGO/Ag3PO4	MO	20	0.5	90	97	52
CNT/TiO2	MO	20	0.4	240	100	53
CuS/O,N-CNT	MO	20	0.16	30	97.21	This work
La,Ce-co-doped ZnO	MB	10	0.1	120	95	54
TiO2/CNT	MB	30	0.2	180	85	55
SnO2 QDs@g-C3N4/biochar	MB	20	0.08	120	94.5	56
CO/Ni-MOFs@BiOI	MB	20	0.2	180	81.3	57
CuS/O,N-CNT	MB	15	0.2	30	98.11	This work

Kinetics analysis

The kinetics of dye degradation were often described using the Langmuir–Hinshelwood model, and the rate constant may be found using the following equation (Eq. 11)⁴⁹:

$$r=\frac{-dC}{\mathit{dt}}=\frac{\mathit{kKC}}{1+KC}$$ 11

where t is the illumination period, k is the rate constant (mg/min), r is the rate of dye degradation (mg/min), C is the dye concentration (mg/L), and K is the dye's adsorption coefficient (L/mg). For a very diluted solution, C would be pretty minor; hence, Eq. 12 might be expressed as

$$ln\left(\frac{C_0}{C}\right)=kKt\approx k_{\mathit{app}}t$$ 12

The apparent rate constant, k_app, is shown here by a pseudo-first-order kinetics graph for the dyes' degradation. Plotting the relationship between ln (C₀/C) and time yields a straight line whose slope equals the pseudo-first-order rate constant. As can be seen in Fig. 7d, the rate constant for degradation of dyes through the fabricated nanocomposite, i.e., CuS@MO, CuS@MB, CuS/O,N-CNT@MO, and CuS/O,N-CNT@MB were found to be 0.02743, 0.02839, 0.11418, and 0.12433 min⁻¹, respectively. Thus, it was observed that the addition of CNT led to enhanced results as the degradation rate increased up to 5 times.

Reusability and impact on other pollutants

The photocatalytic efficiency of CuS/O,N-CNT, which outperformed other produced materials, was comprehensively tested for reusability in the photodegradation of methyl orange (MO) and methylene blue (MB) dyes. The findings showed constant reusability throughout six cycles, with an efficiency drop of around 10–15%, as shown in Fig. 8a. This demonstrates the potential use of the manufactured CuS/O,N-CNT photocatalyst for the breakdown of organic contaminants in water treatment operations. Furthermore, the nanocomposite's adaptability was assessed by expanding the study to include the degradation of several pollutants, including Rhodamine (RhB), Eosin yellow (EY), Methyl red (MR), Pantoprazole (PTZ), and Metronidazole (MTZ) (Fig. 8b). Under constant reaction conditions of 15 ppm contaminant concentration, 0.2 g/L photocatalyst dose, 0.8 mL H₂O₂, and 30 min of solar light irradiation, the CuS/O,N-CNT catalyst displayed outstanding efficacy in the degradation of a variety of pollutants. This demonstrates the extensive application and relevance of the developed catalyst in tackling a variety of aquatic contaminants.

Figure 8.

(a) Reusability studies, (b) removal of other contaminants, and (c) effect of scavengers on performance.

Effect of scavengers

Undoubtedly, there is a strong relationship between the amount of electrons and holes engaged, the quantity of reactive oxygen species (ROS) generated, and the process of photocatalytic decomposition. During the decomposition experiment, several agents were used to investigate the reaction of the reacted species (Fig. 8c). This helped determine the role played by critical reactive species such as ·OH, O₂·⁻, e⁻, and h⁺ in the photocatalytic degradation of MB and MO⁵⁶. Disodium EDTA was utilized as a hole scavenger, potassium persulphate (K₂S₂O₈) as an electron scavenger, ascorbic acid as an O₂·⁻ scavenger, and benzoic acid as an ·OH scavenger⁴⁵. MB was shown to be significantly degraded by ·OH and O₂·⁻ when 1 mM benzoic acid⁵⁸ and ascorbic acid⁵⁹ were present, as evidenced by the degradation efficacy falling to 15.88% and 34.55%, respectively. Similarly, MO degradation efficacy in the presence of benzoic acid and ascorbic acid fell to 25% and 31.62%, respectively. No discernible impediment to the breakdown of MO and MB is seen with disodium EDTA or K₂S₂O₈ (1 mM)⁶⁰. According to Bellán et al.⁶¹, the photocatalytic decomposition of MO and MB dye was facilitated by H₂O₂, which means the impact of e⁻ and h⁺ was negligible in the scavenger experiment described above, whereas the function of ·OH and O₂·⁻ was considerable⁴⁸. Although electron interactions produce superoxide and hydroxyl radicals, these more reactive species frequently obscure electrons' direct involvement in degradation processes. The overall efficiency and routes of oxidative degradation processes are determined by a mixture of reactive species, with electrons supporting them. This is why, in many cases, scavenger study reveals that electrons play no role in degradation. Hence, in this degradation process, the maximum contribution of the charge species is in order of ·OH > O₂·⁻  > e⁻ > h⁺.

Photocatalytic mechanism

Figure 9 depicts the photodegradation method, which involves the transformation of pollutants (MO and MB) under visible light utilizing the CuS/O,N-CNT photocatalyst, based on previous literature^62–64. Illumination causes electron excitation from the valence band (VB) to the conduction band (CB), resulting in holes (h⁺)⁶⁵. As seen in the photoluminescence (PL) research, the dopant, when combined with carbon nanotubes (CNT), acts as an electron reservoir, increasing charge transfer under visible light and reducing electron–hole recombination (Fig. 3d). The outstanding conductivity of CNT leads to the enhanced electron transfer leading to improved performance⁶². Slowing recombination dramatically increases photocatalytic activity, indicating the relevance of inhibiting electron–hole pair recombination for pollutant destruction.

Figure 9.

Plausible photocatalytic mechanism of degradation of MO and MB by synthesized CuS/O,N-CNT.

CuS/O,N-CNT have outstanding light absorption characteristics, generating photoinduced electrons (e⁻) in the conduction band (CB) and holes (h⁺) in the valence band (VB). The accumulated h⁺ in the VB is unable to synthesize ·OH radicals because its potential cannot reach the requisite levels (+ 1.99 eV/NHE for ·OH/H₂O and + 2.40 eV/NHE for ·OH/OH⁻)⁴⁵. CuS/O,N-CNT has a higher negative CB potential than O₂/O₂·⁻ (− 0.33 eV/NHE), allowing direct pollutant oxidation by forming superoxide radicals⁶⁶. Furthermore, h⁺ on the VB can quickly oxidize toxic MO and MB. To increase the concentration of ·OH radicals, H₂O₂ was used as an electron-trapping agent, interacting with electrons from the CB to produce ·OH radicals. Optimal H₂O₂ dose is critical to avoid excess, which can hinder other ROS and impair degradation efficiency for specific pollutants. Thus, the peroxide dose was optimized for the degrading system. The equation below clearly shows the degradation procedure followed by fabricated novel CuS/O,N-CNT (Eqs. 13–18).

$$CuS/O,N-CNT+hv\to e^{-}+h^{+}$$ 13

$$H_{2}O\to O{H}^{-}+H^{+}$$ 14

$$H_2{O}_2+hv\to ·OH+·OH$$ 15

$$H_2{O}_2+e^{-}\to O{H}^{-}+·OH$$ 16

$$O_2+e^{-}\to O_2^{·-}$$ 17

$$O_2^{·-}+·OH+h^{+}+Dyes\to DegradationBy-products+C{O}_2+H_{2}O\prime$$ 17

Conclusion

Finally, the hydrothermal etching of the unique CuS/O,N-CNT composite, as validated by extensive XRD, FTIR, and XPS investigations, displayed outstanding effectiveness in the photocatalytic destruction of methyl orange (MO) and methylene blue (MB) dyes under sun irradiation. UV-DRS and PL investigations confirmed the composite's higher light absorption and decreased charge recombination, contributing to its improved photocatalytic activity. Morphological characterization using SEM and TEM verified the effective production of CuS/O,N-CNTs with different structural characteristics. The fabricated material showed excellent degradation performance in H₂O₂, with an efficiency of 97.21% for MO and 98.11% for MB dye. The material showed remarkable reusability, constant performance throughout six cycles, and around 85% efficiency in the last cycle. This study highlights the manufactured CuS/O,N-CNT composite's promise as a strong and reusable photocatalyst for efficiently remedying organic contaminants in solar-driven photocatalytic applications.

Author contributions

S.R.M. wrote the main manuscript text. B.P. modified, revised and edited the manuscript. V.G. commented, revised the manuscript. N.S. prepared figures and edited the manuscript and M. A. discussed results, commented, revised and corrected the whole manuscript. All authors reviewed the manuscript.

Data availability

All data generated or analyzed during this study are included in this published article.

Competing interests

The authors declare no competing interests.