Visible-light-active type-II heterojunction CdS@Cu_0.5Mg_2.5SnS₄ composites for the efficient removal of brilliant green dye

Abstract

Copper magnesium tin sulfide (CMTS) is a multifunctional material that has recently generated a lot of interest as a potential new photocatalyst for organic pollutant degradation. Some of the desirable characteristics of CMTS are its narrow band gap energy, good chemical and photochemical stability, and high relative abundance of its constituent elements. In this work, CMTS (with the empirical formula Cu_0.5Mg_2.5SnS₄) and type-II heterojunction CdS@CMTS composites with varying CdS contents have been synthesized using a cost-effective and energy-efficient microwave-assisted method. X-ray diffraction analysis confirmed the presence of kesterite CMTS and hexagonal CdS in the composites. After the addition of CdS, the effective band gap energy of CMTS increased from 2.07 to 2.40 eV. The composites exhibited a uniform distribution of CdS on the CMTS surface. Photocatalytic studies reveal that CdS@CMTS exhibits higher photocatalytic activity for the degradation of brilliant green (BG) dye under visible light irradiation in comparison to pristine CMTS. Among the synthesized materials, the 30% CdS@CMTS exhibited the highest photocatalytic activity, with up to 95.8 ± 1.45% degradation of BG within 5 h. Thus, CdS@CMTS prepared by microwave-assisted synthesis has shown potential to degrade organic pollutants for wastewater remediation.

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1. Introduction

Quaternary chalcogenides contain four different ions: three metal cations and a chalcogen anion.^1,2 Quaternary chalcogenides Cu₂XSnS₄ (X = Zn, Fe, Mg, Co, or Mn) are particularly attractive for photocatalytic applications such as wastewater remediation due to their light absorption characteristics and because they comprise only earth-abundant elements with low toxicity.^3–7 Zhong and co-workers have studied the structural and thermodynamic stabilities of Cu₂MSnX₄ (M = Mg, and Ca, X = S and Se) and reported that Mg can serve as a good substitute for Zn due to their thermodynamically stable nature in comparison to Cu₂CaSnS₄ and Cu₂CaSnSe₄.⁸ To date, there are very few reports available on the fabrication of copper magnesium tin sulfide (CMTS). The methods that have been employed for CMTS synthesis include pulsed laser deposition,⁹ sol–gel spin-coating followed by sulfuration,¹⁰ spray pyrolysis,¹¹ co-precipitation,¹² hot-injection,¹³ and the sol–gel method.³

In a study carried out by Wei and co-workers, Cu₂MgSnS₄ was synthesized using the hot injection method.¹³ They reported that the synthesized CMTS exhibited the kesterite phase with a band gap energy of 1.63 eV. Mg²⁺ and Zn²⁺ have similar ionic radii; therefore, they can take the place of Zn in Cu₂ZnSnS₄ (CZTS) without much alteration in the crystal structure to form CMTS. CMTS may become a new earth-abundant element material for solar cells. Yang and co-workers have synthesized CMTS via a sol–gel spin-coating route followed by sulfuration.¹⁰ The synthesized material yielded a zinc-blende CMTS structure with a band gap energy of 1.5 eV.

In a different study, Ali and co-workers showed that kesterite CMTS modified with tetragonal BaTiO₃ exhibits remarkably strong bacterial inhibition for both Escherichia coli and Staphylococcus aureus.¹⁴ Computational studies have also provided insights into the structural, elastic, and thermodynamic properties of CMTS, as carried out by Bekki and co-workers.¹⁵ Based on their calculations, CMTX (X = S, Se, or Te) can exhibit stannite, kesterite, primitive-mixed CuAu, and mixed-phase wurtzite-stannite structures, and the stannite phase is the most stable among them.

Compared with other synthesis methods, the use of microwave irradiation for CMTS synthesis offers distinct advantages over conventional heating methods.^16–18 The microwave irradiation accelerates the reaction kinetics by enhancing the nucleation process and overcoming energy barriers, leading to reduced synthesis times.¹⁹ Microwave heating also ensures uniform temperature distribution throughout the reaction vessel, minimizing thermal gradients.²⁰ Consequently, microwave heating proves to be more cost-effective, energy-efficient, faster, and more economical compared to traditional methods.²¹ CMTS has garnered attention as a potential candidate for photocatalysis in energy conversion and environmental remediation. However, the photocatalytic performance of CMTS is limited and not widely explored. On the other hand, pristine CdS has drawn a lot of attention due to its suitable 2.4 eV direct band gap, which can be excited by visible light to produce charge carriers.²² However, it is unable to satisfy the requirements of practical applications due to photo-corrosion and the rapid recombination of photogenerated charge carriers.^23,24 This issue can be effectively tackled by forming heterojunction composites with photocatalysts with matched energy bands.²⁵ This strategy is expected to significantly improve its photocatalytic performance.

In this study, a rapid and energy-efficient microwave-assisted method has been utilized to synthesize CMTS and CdS@CMTS using ethylene glycol as a solvent for the first time. To the best of the authors' knowledge, there are no reported studies on type-II heterojunction CdS@CMTS composites. The influence of CdS on the structural, optical, and morphological properties of CMTS was investigated. The photocatalytic degradation of brilliant green (BG) dye using CMTS and CdS@CMTS was achieved with a low dosage of photocatalyst. More importantly, an in-depth study on the scavenging/trapping of active species was carried out for the first time using CdS@CMTS to determine the major species involved in the BG degradation process.

2. Experimental section

2.1. Chemicals used

All reagents were used without further purification. For the synthesis, copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) was purchased from Alfa Aesar as the Cu source. Cadmium sulfate (CdSO₄·8/3H₂O), thiourea (NH₂CSNH₂), and ethylene glycol were purchased from Merck, Germany. The distilled water was purified using a water still from Aquatron, England. Ethanol was purchased from Duksan Pure Chemicals Co. Ltd, South Korea. Magnesium nitrate (Mg(NO₃)₂) tin tetrachloride pentahydrate (SnCl₄·5H₂O) and BG dye were obtained from Sigma-Aldrich. For the free radical scavenging/trapping experiments, isopropanol and benzoquinone were obtained from Acros, and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) was purchased from Fluka.

2.2. Characterization

CMTS and type-II heterojunction CdS@CMTS composites were synthesized using an Anton-Paar microwave reactor (Monowave 400, Austria). A Malvern PANalytical Aeris Research Edition (United Kingdom) benchtop X-ray diffraction (XRD) instrument was utilized to determine the crystal structure of the CMTS, CdS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS composites. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Analytical AXIS Nova XPS instrument to determine the elemental composition and chemical states of the CMTS and CdS@CMTS composites. Fourier transform infrared (FT-IR) spectroscopy was used to identify the vibrational modes present in the as-synthesized materials. The FT-IR spectra of these materials were recorded using an IRspirit Fourier transform infrared spectrometer (Shimadzu, Japan) in the range of 400–4000 cm⁻¹ through the ATR method. The Raman spectra of CMTS and type-II heterojunction CdS@CMTS composites were obtained using a Micro Raman spectrometer (JASCO NRS-5100) equipped with a 532.06 nm laser. The optical band gaps of the synthesized materials were determined using ultraviolet–visible diffuse reflectance spectroscopy (UV-visible DRS, Shimadzu UV-2600i, Japan) and BaSO₄ was used as a reference. The morphology and crystallographic information of the synthesized materials were analyzed using field-emission transmission electron microscopy (FE-TEM), energy dispersive X-ray spectroscopy (EDS) elemental mapping, and selected area electron diffraction (SAED) conducted with a JEM-F200 (JEOL Ltd., Tokyo, Japan) instrument. Indexing of the SAED patterns was performed using ImageJ software. The surface areas of CMTS, 10% CdS@CMTS, and 30% CdS@CMTS were measured using a surface area analyzer (Quantachrome autosorb-iQ, Austria). The photocatalytic degradation of brilliant green (BG) dye using pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS composites was carried out in a Toption (TOPT-V) photochemical reactor equipped with a water-jacketed 300 W Xe lamp with a wavelength >350 nm (light intensity at the position of the reaction vessel ∼14 mW cm⁻²) at room temperature.

2.3. Synthesis of CMTS

CMTS was prepared using a simple microwave-assisted synthesis method. Typically, appropriate amounts of Cu(NO₃)₂·3H₂O, Mg(NO₃)₂, SnCl₄·5H₂O, and thiourea were added into 20 mL of ethylene glycol and the mixture was loaded into a 30 mL quartz vessel (G30 vial). The vessel was purged with N₂ gas and sealed with a septum then rapidly heated to 200 °C with 850 W microwave irradiation for 30 min with continuous stirring. After the reaction was complete, the product was obtained by centrifugation (3500 rpm, 5 min per wash), washed three times with distilled water and ethanol, and dried at 80 °C for 4 h to yield a black powder of CMTS.

2.4. Synthesis of type-II heterojunction CdS@CMTS composites

Appropriate amounts of CdSO₄·8/3H₂O and thiourea were added into a suspension of 0.1 g of CMTS in ethylene glycol for the preparation of type-II heterojunction CdS@CMTS composites. The mixtures were purged with N₂ gas and heated at 200 °C with a microwave power and reaction time of 850 W and 30 min, respectively. The obtained products were washed three times with distilled water and ethanol then dried at 80 °C for 4 h and labelled as either 10% CdS@CMTS, 20% CdS@CMTS, or 30% CdS@CMTS. The composites were synthesized with the CdS content in increments of ten revealing that the percentage degradation peaked at 30%, surpassing the performance of pure CdS (100%). The composites were only synthesized up to 30% of CdS to minimize toxicity while maintaining the high efficiency of the material.

2.5. Photocatalytic degradation of brilliant green dye

The photocatalytic performance of the CMTS and type-II heterojunction CdS@CMTS composites was studied via the photocatalytic degradation of 10 ppm BG (pH 6.10) in an aqueous solution under UV-visible light irradiation. Typically, 10 mg of the as-synthesized material was dispersed and sonicated in 50 mL of BG aqueous solution. Then, to achieve adsorption–desorption equilibrium between the BG dye and the synthesized material, the suspension was kept in the dark for 3 min with constant stirring. Upon light irradiation, a specific amount of suspension (3 mL) was collected regularly every 60 min for 5 h and centrifuged to separate the photocatalyst from the BG dye. The clear aliquots were analyzed using a UV–vis spectrophotometer in the range of 200–800 nm, and the photocatalytic activities of pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were estimated by measuring the percentage of dye degradation using the following relation (1):where A₀ denotes the initial absorbance (the absorbance at time t = 0 h) and A_t denotes the absorbance of aqueous BG solution after time t of treatment. Both A₀ and A_t are recorded at λ_max of BG at 620 nm. The experiments were carried out in triplicate.

2.6. Free radical scavenging/trapping experiments

As photocatalysis involves the formation of different reactive species, it is crucial to determine the role of these species in the degradation process. Free radical scavenging/trapping experiments were carried out in order to understand the main reactive species responsible for the photocatalytic degradation of BG using CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS composites. Different scavenging/trapping agents were used in the experiments to investigate the inhibitory effect of these scavenging/trapping agents during the photocatalytic reaction under the same experimental conditions as described in sub-heading 2.5. These experiments were carried out in the presence of three typical scavenging/trapping agents: isopropanol, benzoquinone, and EDTA, which are utilized as scavenging/trapping agents of hydroxyl (˙OH), superoxide radicals (O₂˙⁻), and holes (h⁺), respectively. These agents were added to the aqueous BG dye solution at the beginning of the photocatalytic reaction.

3. Results and discussion

3.1. Powder X-ray diffraction analysis

The formation and the crystal structure of pristine CMTS, CdS 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were confirmed by XRD analysis. The XRD pattern of CMTS does not match with any pattern in the ICDD database and was simulated based on the kesterite CZTS crystal structure. As shown in Fig. 1, the XRD pattern of CMTS with 2θ peaks at 28.3, 50.0, and 56.9° can be assigned to the (112), (200), and (312) planes of kesterite CMTS (indexed in black).²⁶ The formation of a secondary phase can be observed from the appearance of an additional peak around 16.1°, which is attributed to the (110) plane of Cu₂SnS₃ (ICDD 01-086-5306: indexed in red).²⁷ As the CdS decoration increases from 10% to 30%, the peak intensity decreases. The weak diffraction peaks of CMTS are in good agreement with previous reports and may be weak due to its amorphous nature (incomplete crystallization) because of the absence of a post-annealing process under inert conditions.²⁶ The diffraction peaks of CdS are located at 24.9, 26.5, 28.2, 36.7, 43.8, 47.9, and 51.8°, which correspond to the (100), (002), (101), (102), (110), (103), and (112) planes of hexagonal CdS (ICDD 01-074-9665: indexed in orange), as shown in Fig. S1 (ESI†).²⁸ The diffraction peaks around 25 to 30° for 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were observed to broaden (shown in Fig. S2, ESI†) as the amount of CdS increases from 10% to 30%. Moreover, the peaks at 44.5, 47.4, and 50.2° began to appear in 30% CdS@CMTS, which suggests the successful decoration of CdS on CMTS.

Fig. 1 XRD patterns of CMTS, CdS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS.

3.2. X-ray photoelectron spectroscopy

XPS analysis of CMTS and type-II heterojunction CdS@CMTS composites was conducted to investigate the elemental composition and chemical states of the elements present in the materials. The survey scan spectrum of CMTS shown in Fig. S3 (ESI†) confirmed the presence of Cu, Mg, Sn, and S.¹³ Similar peaks were observed in the 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS survey spectra with the addition of a Cd peak.²⁹

In the Cu 2p spectrum of CMTS shown in Fig. 2(a), two peaks at 951.4 eV (Cu 2p_1/2) and 931.6 (Cu 2p_3/2) with a splitting of 19.8 eV is the characteristic of Cu⁺.¹⁰ While the peaks are observed at 950.2/930.3 eV, 949.8/929.9, and 949.7/929.8 eV for 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS, respectively. The Cu 2p peaks shifted to lower binding energy in the composites, and this may be due to the presence of CdS on the surface of CMTS. Fig. 2(b) shows the spectrum of Mg 1s of CMTS with a peak located at 1303.5 eV, which is associated with the presence of Mg.¹² With CdS decoration, the Mg 1s peak appeared at 1302.5, 1302.2, and 1301.5 eV for 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS, respectively. The lower binding energy portion of the Mg XPS scan is shown in the inset of Fig. 2(b), which clearly provides convincing evidence for the presence of Mg 2p at 451.6 eV in CMTS. The Mg 2p peaks of 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS appeared at 46.7 eV, which is consistent with the literature values reported for an Mg²⁺ state.³⁰

Fig. 2 XPS spectra of CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS; (a) Cu 2p, (b) Mg 1s (inset: Mg 2p), (c) Sn 3d, (d) S 2p, (e) Cd 3d, and (f) C 1s.

The two characteristic Sn 3d peaks in CMTS appeared at 494.2 and 485.8 eV, which can be assigned to Sn 3d_3/2 and Sn 3d_5/2 of Sn⁴⁺, respectively, as shown in Fig. 2(c).³¹ The binding energy peaks of Sn appeared at 493.2/484.8, 492.8/484.4, and 492.9/484.5 eV for 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS, respectively. The peak splitting of Sn 3d was 8.4 eV, which suggested the presence of Sn⁴⁺ in the synthesized materials.⁴ The S 2p peaks of CMTS observed at 162.0 and 160.8 eV can be assigned to the S 2p_1/2 and S 2p_3/2 of S²⁻, respectively (Fig. 2(d)).³² Moreover, the binding energies of S 2p in 10% CdS@CMTS are located at 160.8 and 159.6 eV with a peak splitting of 1.2 eV. However, the peaks appeared at 160.4/159.3 and 160.3/159.3 eV for 20% CdS@CMTS and 30% CdS@CMTS, respectively. The two peaks at 410.0 and 403.2 eV in the Cd 3d spectrum of 10% CdS@CMTS were attributed to Cd 3d_3/2 and Cd 3d_5/2, respectively, as shown in Fig. 2(e). Moreover, the peaks were located at 409.6/402.9 and 409.6/402.8 eV with a binding energy difference of 6.8 eV for 20% CdS@CMTS and 30% CdS@CMTS, respectively. These peaks are characteristic of Cd²⁺.³³

Furthermore, the XPS analysis revealed interesting changes in the chemical states of the elements upon the addition of CdS. The high-resolution XPS spectra showed a shift in the binding energies of the Cu, Mg, Sn, and S peaks compared to the pure CMTS sample. The shifts suggest a potential interaction between the CdS and CMTS. In conclusion, the XPS analysis of the CMTS and type-II heterojunction CdS@CMTS composites not only confirmed the elemental composition and chemical states of the constituent elements but also provided evidence of the successful decoration of CdS onto CMTS.

3.3. Fourier transform infrared spectroscopy

FT-IR was carried out to examine the different vibrational modes present in the pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS. As shown in Fig. 3(a), CMTS exhibits a band located at ∼530–550 cm⁻¹ which can be assigned to metal–S stretching vibrations,³⁴ while the bands around 1040 and 2340 cm⁻¹ can be ascribed to the C=S and S–H vibrations of thiourea, respectively.³⁵ For pristine CdS, the bands around 600, 850, and 1640 cm⁻¹ can be attributed to the characteristic Cd–S bonding^36,37 and the band around 1400 cm⁻¹ belongs to the C–N of thiourea.³⁸ The broadening of the band around 530 cm⁻¹ was observed in the FTIR spectra after the addition of CdS in 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS. The composites also display the characteristic bands corresponding to CMTS and CdS. This confirms the presence of both CMTS and CdS. Thus, the FT-IR measurements also support the successful formation of CdS and CMTS in the type-II heterojunction composites.

Fig. 3 (a) FT-IR spectra and (b) Raman spectra of pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS.

3.4. Raman spectroscopy

Raman spectroscopy was carried out to further confirm the synthesis of the CMTS and type-II heterojunction CdS@CMTS composites. As shown in Fig. 3(b), the Raman spectrum of pristine CMTS exhibited a strong peak at 287 cm⁻¹ and another one with a lower intensity at 218 cm⁻¹, which corresponds to the kesterite phase of CMTS.³⁹ Moreover, CdS exhibited characteristic peaks located at around 300, 600, and 900 cm⁻¹,⁴⁰ which are associated with the phonon vibrational peaks of the 1 longitudinal optical (LO), 2LO, and 3LO modes of hexagonal CdS, respectively.⁴¹ The peak at around 900 cm⁻¹ confirmed the presence of CdS in the type-II heterojunction CdS@CMTS composites. The intensity of the CdS peak increases as the amount of CdS decoration increases from 10% to 30%. The Raman spectra of the CdS@CMTS composites showed the characteristic features of CMTS and CdS. Therefore, Raman analysis also confirms the coexistence of CMTS and CdS in the composites and the formation of type-II heterojunction CdS@CMTS composites.

3.5. UV-visible diffuse reflectance spectroscopy

The diffuse reflectance spectra of CMTS and type-II heterojunction CdS@CMTS composites were measured in the wavelength range of 200 to 1000 nm. The optical band gap energies of pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were estimated from Tauc plots constructed from the diffuse reflectance spectra transformed using the Kubelka–Munk function. Fig. 4 shows the Tauc plot of [F(R)hv]²vs. photon energy. The band gap energy of CMTS was found to be 2.07 eV, which is higher compared to the previously reported values of about ∼1.5 eV.¹¹ This may be influenced by the amount of Cu. As reported by Souli and co-workers, the band gap energy of CMTS increases as the concentration of Cu added decreases.³⁹ As shown in Fig. S4 (ESI†), the band gap energy of CdS was estimated to be 2.50 eV. Moreover, the estimated effective band gap energies of the 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS composites were found to be 2.23, 2.34, and 2.40 eV, respectively. This signifies their potential to harvest visible light.

Fig. 4 Tauc plots constructed from Kubelka–Munk transformed diffuse reflectance spectra for pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS.

3.6. Transmission electron microscopy

TEM was employed to determine the morphologies of CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS. Fig. 5(a₁) shows that the CMTS adopted a sheet-like morphology. The TEM images of 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS shown in Fig. 5(b₁)–(d₁), respectively, show that the spherical CdS was uniformly dispersed on the surface of the CMTS. Moreover, the close contact of CdS and CMTS resulted in the formation of a type-II heterojunction CdS@CMTS composite, which is helpful for fast interfacial charge carrier transfer.⁴² Thus, the photogenerated charge carriers can be utilized effectively, thereby enhancing the photocatalytic performance.

Fig. 5 TEM images and SAED patterns of (a) CMTS, (b) 10% CdS@CMTS, (c) 20% CdS@CMTS, and (d) 30% CdS@CMTS, respectively.

The selective area electron diffraction (SAED) pattern of CMTS in Fig. 5(a₂) confirmed the presence of the (112) and (220) planes of kesterite CMTS.^13,26 The SAED patterns of the 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS composites in Fig. 5(b₂)–(d₂) showed concentric rings corresponding to the (112) and (220) diffraction planes of kesterite CMTS in addition to the (101) and (103) planes of hexagonal CdS.^26,43 Additional (100) and (110) planes of hexagonal CdS were observed in 30% CdS@CMTS, probably due to the presence of a higher amount of CdS compared to the low percentage composites (10% and 20%). These results are in accordance with the XRD results, which also confirm the successful decoration of CdS onto CMTS.

3.7. EDS mapping

The spatial distribution of elements in the CMTS, 10% CdS@CMTS, and 30% CdS@CMTS was determined using EDS mapping. As shown in Fig. 6, all the synthesized materials contained uniformly distributed Cu, Mg, Sn, and S elements.¹³ While the 10% and 30% CdS@CMTS composites exhibited additional uniform Cd mapping. Thus, EDS mapping also confirmed the formation of CMTS and type-II heterojunction CdS@CMTS composites.

Fig. 6 The respective elemental mapping patterns of Cu, Mg, Sn, S, and Cd in the (a₁)–(e₁) CMTS, (a₂)–(f₂) 10% CdS@CMTS, and (a₃)–(f₃) 30% CdS@CMTS.

3.8. Brunauer–Emmett–Teller surface area analysis

Fig. 7 shows the Brunauer–Emmett–Teller (BET) analysis using N₂ adsorption–desorption isotherms of the synthesized materials. As is seen, the synthesized CMTS and type-II heterojunction CdS@CMTS composites exhibit a type IV curve (according to IUPAC classification) with a hysteresis loop.⁴⁴ The surface areas of 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were measured and are found to be 142.23, 76.11, and 73.45 m² g⁻¹, respectively which are all higher than that of the pristine CMTS (40.94 m² g⁻¹). As the CdS decoration increases from 10% to 30%, the BET surface area of the CdS@CMTS composite decreases. At a higher percentage of CdS (30%), the particles may have a higher tendency to agglomerate. These agglomerates may form larger particles that further reduce the overall surface area, reducing the available sites for N₂ molecules to adsorb. This suggests that the enhanced photocatalytic activity of 30% CdS@CMTS does not depend on the surface area but results from the effective separation of the photogenerated e⁻ and h⁺.

Fig. 7 N₂ adsorption and desorption isotherms of CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS.

4. Photocatalytic activity of CMTS and type-II heterojunction CdS@CMTS composites

4.1. Photocatalytic degradation of brilliant green dye

The photocatalytic activities of pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were investigated by degradation of BG as a model dye pollutant under visible light irradiation. All measurements were performed under the same conditions. Fig. 8(a) shows control experiments carried out in the dark, which tracks the adsorption of the BG dye onto the surface of the synthesized materials by monitoring the absorbance at λ_max of BG (620 nm) over a period of 5 h. In the absence of light, the adsorption capacities of CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS reached up to 10.3 ± 0.81%, 31.5 ± 2.15%, 35.9 ± 0.65%, and 16.8 ± 2.85%, respectively, after 5 h. This shows that the materials were able to adsorb BG dye for the sequential photocatalytic degradation process, which is crucial because if the dye molecules cannot be adsorbed on the surface of the synthesized materials, the photocatalytic activity would be poor and not efficient.⁴⁵

Fig. 8 (a) Percentage adsorption, (b) photocatalytic degradation of BG using CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS under visible light irradiation, and (c) photocatalytic activity of 30% CdS@CMTS with different scavenging/trapping agents to determine the main reactive species responsible for photocatalytic degradation of the BG.

Fig. 8(b) shows the photocatalytic degradation of pristine CMTS, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS at different irradiation times. Moreover, CMTS exhibited poor photocatalytic performance of only about 18.9 ± 1.96% under irradiation by visible light, which may be ascribed to the rapid recombination of charge carriers. Within 5 h, 10% CdS@CMTS, 20% CdS@CMTS, and 30% CdS@CMTS were able to degrade about 42.5 ± 3.69%, 81.1 ± 0.26%, and 95.8 ± 1.45% of BG, respectively, under visible light irradiation. The photocatalytic performance of CMTS is significantly enhanced as the amount of CdS decoration increases from 10% to 30%. The photocatalytic activity of 30% CdS@CMTS was found to be 95.8 ± 1.45%, which is considerably higher than those of pristine CMTS, 10% CdS@CMTS, and 20% CdS@CMTS. The synthesis of composites stopped at 30% CdS because this is when the CdS@CMTS performance matches (and slightly exceeds) that of pure CdS (100%). A higher CdS content was not tested to minimize the amount of toxic Cd in the photocatalyst. The improved photocatalytic performance of 30% CdS@CMTS is likely to be due to the optimal band gap energy of 2.4 eV, which enables efficient utilization of light for photocatalytic processes and the effective transfer and separation of the charge carriers between CdS and CMTS, resulting in a higher degradation efficiency.⁴⁶

Generally, organic pollutants can be degraded by photocatalytic processes where a series of reactive species including h⁺, O₂˙⁻, and ˙OH radicals are responsible for the degradation reactions. To further investigate the main reactive species involved in the degradation of the aqueous BG dye, the highest performing composite, 30% CdS@CMTS was used to carry out the free radical scavenging/trapping experiments in the presence of different scavengers including isopropanol, benzoquinone, and EDTA to trap ˙OH, O₂˙⁻, and h⁺, respectively. The significance of a particular active species in the degradation process is proportional to the corresponding inhibition (reduction in percentage photocatalytic degradation) in the presence of a scavenging/trapping agent. As shown in Fig. 8(c), the photocatalytic degradation of BG using 30% CdS@CMTS was only slightly affected by the addition of isopropanol (94.3 ± 1.51%) when compared to no scavenger (95.8 ± 1.45%). While in the presence of EDTA and benzoquinone, the photocatalytic degradation of BG was reduced from 95.8 ± 1.45% to 74.6 ± 2.81% and 68.2 ± 4.68%, respectively. Based on these inhibitions, it can be observed that on average benzoquinone was more inhibiting than EDTA, although there is only a slight difference between these scavengers. This suggests that O₂˙⁻ and h⁺ were the main reactive species during the photocatalytic degradation process.

Based on the aforementioned characterization studies and the free radical scavenging/trapping experiments of the type-II heterojunction CdS@CMTS composite, a proposed mechanism for photocatalytic degradation is illustrated in Fig. 9. The addition of CdS could significantly enhance the photocatalytic degradation performance of CMTS. Although the target synthesis was pure, stoichiometric CMTS, the presence of a Cu₂SnS₃ secondary phase is nearly unavoidable (despite being detected only in XRD and not in other characterization studies). As a result, we have taken the role of Cu₂SnS₃ into account in the photocatalytic process. Based on the estimation of E_CB and E_VB using the atomic electron affinities and ionization energies as reported by previous literature,^47–49 CMTS is estimated to have E_CB = −0.63 and E_VB = 1.43 V, while CdS has E_CB = −0.63 V and E_VB = 1.78 V. The reported band gap energy of Cu₂SnS₃ is 1.16 eV,⁵⁰ and the estimated E_CB and E_VB values are found to be 0.14 and 1.30 V, respectively. Both CMTS and CdS could easily absorb and utilize visible light because of their narrow band gap energies. Upon visible light irradiation, CdS simultaneously generates e⁻ and h⁺, in which photoinduced e⁻ are excited to the conduction band (CB), leaving behind h⁺ in the valence band (VB). Subsequently, the photoinduced e⁻ of CMTS migrates to the CB of CdS to take part in the reduction process. The photogenerated e⁻ on the CB of CdS can reduce the dye directly or react with electron acceptors adsorbed on the surface of the photocatalyst, such as dissolved O₂ in water to produce O₂˙⁻ (O₂/O₂˙⁻ = −0.33 V) which is in accordance with the scavenging/trapping experiment results. In addition, since the VB of CMTS is higher than that of CdS, the photogenerated h⁺ can migrate from the VB of CdS to CMTS and further to the VB of Cu₂SnS₃ in the composite. The VB potential of Cu₂SnS₃ (1.30 V) is lower than the potentials of ˙OH/OH⁻ (+2.38 V) and ˙OH/H₂O (+2.72 V).⁵¹ Thus, the photogenerated h⁺ in the VB of Cu₂SnS₃ cannot oxidize OH⁻ or H₂O to ˙OH. However, the photogenerated h⁺ can oxidize the organic dyes directly into harmless end products. In principle, the presence of Cu₂SnS₃ as an impurity can also prolong the lifetime of these charge carriers by acting as a trapping center,⁵² delaying recombination and facilitating better separation of photoinduced e⁻ and h⁺, thus improving the photocatalytic activity of the type-II heterojunction CdS@CMTS composites. Based on the results of the free radicals scavenging/trapping experiment, both benzoquinone and EDTA showed the highest inhibitions when compared to no scavenger. However, there is no significant difference between the inhibition caused by benzoquinone and EDTA, which indicates that both O₂˙⁻ and h⁺ play equally important roles in the photocatalytic degradation process. Moreover, isopropanol shows the lowest inhibition, implying the least importance of ˙OH taking part in BG degradation.

Fig. 9 Proposed photocatalytic degradation of BG using type-II heterojunction CdS@CMTS composites.

While this study has provided valuable insights into CdS@CMTS composites for the efficient removal of the BG dye, several avenues exist to further develop this promising photocatalyst system. For instance, further optimization of the synthesis process could be pursued to mitigate the formation of impurity phases, potentially through variation in sulfur source, solvent, microwave power, or reaction time. Additional studies could also investigate the point of zero charge of the synthesized materials to enhance the understanding of the surface charge, the effect of pH on the degradation performance, and the recycling stability of the composite for long-term applications. These extensions are recommended to improve the fundamental understanding of the degradation mechanism and to further optimize the photocatalytic performance of the CdS@CMTS system.

5. Conclusion

An efficient CdS@CMTS heterojunction was successfully prepared via a rapid and efficient microwave-assisted method. The structural, optical, and morphological properties of the synthesized materials were analyzed using different techniques. X-ray diffraction analysis confirmed the preparation of the kesterite phase of CMTS and the hexagonal phase of CdS. The effective bang gap energy was greatly influenced by the CdS content, in which an increase of the band gap energy from 2.07 to 2.40 eV was observed. Transmission electron microscopy images showed a uniform distribution of spherical CdS on the sheet-like CMTS in the type-II heterojunction CdS@CMTS composites. The photocatalytic degradation of BG by CMTS and CdS@CMTS photocatalysts was investigated. Based on the results, 30% CdS@CMTS showed the highest photocatalytic response of up to 95.8 ± 1.45% degradation of BG within 5 h. Based on the free radical scavenging/trapping experiment, the results revealed that O₂˙⁻ and h⁺ were the primary reactive species during the photocatalytic degradation process. Therefore, this study shows that CdS@CMTS has the potential to degrade organic pollutants and could be applied for environmental remediation.

Author contributions

Ashmalina Rahman: methodology, investigation, data curation, and writing – original draft. Fazlurrahman Khan: data curation and formal analysis. James Robert Jennings: supervision, writing, and review & editing. Young-Mog Kim: resources, funding, and formal analysis. Mohammad Mansoob Khan: supervision, conceptualization, funding acquisition, writing, and review & editing.

Data availability

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

Conflicts of interest

The authors declare that there are no conflicts of interest.

Acknowledgements

This research was funded by Universiti Brunei Darussalam through grant UBD/RSCH/1.4/FICBF(b)/2023/059 and was also supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea grants funded by the Ministry of Education (2022R1A2B5B01001998 and RS-2023-00241461).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj03782k

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