Ternary heterogeneous Z-scheme photocatalyst TiO₂/CuInS₂/OCN incorporated with carbon quantum dots (CQDs) for enhanced photocatalytic degradation efficiency of reactive yellow 145 dye in water

Abstract

This study delves into the advanced integration of a ternary heterogeneous Z-scheme photocatalyst, TiO₂/CuInS₂/OCN (OCN: O-g-C₃N₄), with carbon quantum dot (CQD) to improve the degradation efficiency of reactive yellow 145 (RY145) dye in water. Through a systematic examination, we elucidated the photocatalytic mechanisms and the role of radicals, electrons, and holes in the treatment process. Our findings revealed that this novel catalyst integration significantly boosted RY145 degradation efficiency, achieving 98.2%, which is markedly higher than the efficiencies which could be achieved using TiO₂/CuInS₂/OCN alone. Moreover, the TiO₂/CuInS₂/OCN/CQD photocatalyst demonstrated superior rate performance over its components. Comprehensive evaluations, including photoelectrochemical and radical tests, further confirmed the efficiency of the integrated system, adhering to Z-scheme principles. The catalyst showcased remarkable stability, with over 94% reusability after five reaction cycles. These findings pave the way for the potential use of the TiO₂/CuInS₂/OCN/CQD photocatalyst as an innovative solution for water pollutant treatment via photocatalytic technology.

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1. Introduction

Water is crucial for the sustenance and growth of humans, plants and animals. By 2050, it's estimated that dwindling freshwater resources could impact 6 billion people.¹ A significant contributor to this crisis is the improper treatment or release of wastewater from the textile industry.^2–4 Around 2 80 000 tons of dye are used each year, with 10–15% ending up in waste water.⁵ RY145 dye, which is resistant to natural decomposition, is of particular concern.^6–8 Exposure to water tainted with RY145 dye has been linked to a plethora of health issues including neurological disorders, cancer, and kidney failure, and poses environmental risks.^9–11 Photocatalytic technology has emerged as a solution and is being used in diverse fields.^12–16 In water treatment, photocatalysts effectively treat contaminants such as antibiotics, dyes, and pesticides, as well as heavy metals.^14,17–19 However, traditional photocatalysts like ZnO and TiO₂ suffer from limited surface area and quick electron–hole recombination, compromising their efficacy.^20–22 This has spurred the development of advanced photocatalytic materials like AgBr, g-C₃N₄, CdS, and notably, CuInS₂, which boasts strong sunlight absorption and low toxicity.^23–28 However, the biggest drawback of CuInS₂ is its low surface area, which leads to rapid recombination of electrons and holes, thereby affecting the photocatalytic efficiency of this semiconductor material.²⁹ To address this, CuInS₂ has been combined with other semiconductors like ZnS, TiO₂, and g-C₃N₄, yielding a direct Z-scheme composite which excels in hazardous compound decomposition and energy conversion.^30–33 In particular, the combination of CuInS₂ and TiO₂ shifts the absorption wavelength towards visible light and reduces electron–hole recombination.³⁴ Additionally, pairing CuInS₂ with a non-metallic carbon nitride (CN) semiconductor can also enhance visible light absorption reduce and electron–hole recombination efficiency.^28,35–38

Recent research supports the promise of these combinations. For instance, Feng Guo et al. found that CuInS₂/g-C₃N₄ exhibited an enhanced decomposition rate for tetracycline that is 11 and 15 times higher, respectively, compared to CuInS₂ and g-C₃N₄ used separately.³⁹ Xu et al. used a photocatalyst with a Z-scheme heterojunction between CuInS₂ and TiO₂ to directly convert CO₂ into CH₃OH and CH₄ with respective yields of 0.86 and 2.5 μmol g⁻¹ h⁻¹, which is higher than CuInS₂ and TiO₂ used individually.³¹ Zhang et al. reported a Z-scheme heterogenous g-C₃N₄/CuInS₂ photocatalyst that degraded 52.16% of tetracycline in 2 hours under visible light irradiation, with a degradation rate nearly 3.4 times higher than the g-C₃N₄ sample.²⁸ This research demonstrated that the Z-scheme photocatalysts are capable of expanding the visible light absorption range and increasing electron–hole separation efficiency.²⁸ Yet, they have inherent limitations, notably poor active phase contact, resulting in slower charge transfers.^40–42 To enhance semiconductor contact and boost photocatalysis efficiency, recent studies have explored the use of metal–organic frameworks (MOFs) and carbon quantum dots (CQDs). CQDs, with their low cost, biocompatibility, and broad light absorption range, present a promising solution.^43–48 Our study introduces a TiO₂/CuInS₂/OCN heterogeneous photocatalyst combined with a CQD bridge. We synthesized OCN via a thermal condensation method to separate the CN layer and doped with O to reduce the rate of electron–hole recombination. This was then integrated with TiO₂ and CuInS₂, and the resulting TiO₂/CuInS₂/OCN/CQD photocatalysts were tested for RY145 dye degradation. We also examined the factors influencing RY145 degradation such as RY145 concentration, photocatalyst dosage, pH value, and water source and proposed an LC-MS-based degradation pathway for RY145.

2. Experimental methods

2.1. Synthesis of carbon quantum dot (CQD)

Carbon quantum dots were synthesized using the hydrothermal method from a chitosan precursor. First, 0.1 g of chitosan was dissolved in 20 mL of 1% CH₃COOH solution, stirred until dissolved, and then heated at 180 °C for 12 hours. After cooling, the mixture was centrifuged to yield a yellowish solution (CQD), which was utilized in subsequent syntheses.

2.2. Synthesis of OCN material

Following a previously reported protocol, 9 g of urea was dissolved in 80 mL of 30% H₂O₂ and stirred for an hour.^49,50 The solution, when crystallized for 12 hours at 60 °C, yielded white solids. These were mixed with ammonium chloride and heated in a melamine foam cup at 550 °C for 2 hours to obtain the OCN sample.

2.3. Synthesis of CuInS₂ material

Using the thermal decomposition method, 2.884 g of sodium dodecyl sulfate was dissolved in 50 mL of ethylene glycol and stirred at 70 °C for 15 minutes. A separate solution containing 20 mL of distilled water, 0.586 g of InCl₃·4H₂O, 0.483 g of Cu(NO₃)₂, and 0.375 g of thioacetamide was then added and stirred vigorously at 600 rpm for 30 minutes at 70 °C. Next, the mixture was transferred to a Teflon flask and stirred at 180 °C for 1 day to obtain a black colloidal mixture. Finally, the mixture was separated by centrifugation, washed several times with ethanol and distilled H₂O, and dried at 80 °C for 12 hours to obtain the CuInS₂ material.

2.4. Synthesis of TiO₂/CuInS₂/OCN and TiO₂/CuInS₂/OCN/CQD materials

The TiO₂/CuInS₂/OCN material was synthesized according to our previous report. Specifically, 0.4 g of CuInS₂ and 1.0 g of OCN were added to 120 mL of C₂H₅OH and dispersed evenly in ethanol by ultrasonic treatment for 1 hour (mixture A). At the same time, 2.28 mL of titanium isopropoxide (TIP) was added to an 80 mL mixture of acetic acid and ethanol (1 : 1) and vigorously stirred to form a homogeneous mixture (mixture B). Then, mixture B was added dropwise to mixture A and vigorously stirred for 30 minutes before introducing the resulting mixture of A and B into the Teflon reaction system. This mixture was reacted in a microwave device (power 700 W) for 0.5 hours at 100 °C. The formed precipitate was isolated using centrifugation and repeatedly rinsed with distilled water to ensure all unreacted materials were removed. Finally, the product was dried at 80 °C for 12 hours and ground in a mortar before being heated at 450 °C for 4 hours to obtain the TiO₂/CuInS₂/OCN material.

The TiO₂/CuInS₂/OCN/CQD material was synthesized by dispersing 10 mL of the CQD solution synthesized above in a mixture of water and ethanol (volume ratio 4 : 1). Next, 1 g of TiO₂/CuInS₂/OCN material was added to the above mixture and subjected to ultrasonic treatment for 0.5 hours and stirred at 120 °C for 5 hours. The mixture was allowed to cool naturally to room temperature and the solid was separated by centrifugation and washed with distilled water. Finally, the solid was dried at 70 °C for 12 hours to obtain the TiO₂/CuInS₂/OCN/CQD material (see Fig. S1†). The weight ratios between TiO₂, CuInS₂, OCN and CQD in TiO₂/CuInS₂/OCN and TiO₂/CuInS₂/OCN/CQD materials are 3/2/5 and 3/2/5/0.2, respectively.

2.5. Characterization methods

The phase structure, surface valence state, elemental composition, specific surface area, morphology and optical properties of ZnO, TiO₂, OCN and 20%ZnO–TiO₂/OCN material samples were analyzed by X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany), X-ray photoelectron spectroscopy (XPS, Thermo VG Multilab 2000), Energy Dispersive X-ray Spectroscopy (EDS, JED-2300), transmission electron microscopy (TEM, Leica IEO 906E), photoluminescence (PL, Varian), Mott–Schottky (MS), electrochemical impedance spectroscopy (EIS) and UV-vis diffuse reflectance spectra (UV-vis DRS, UV-2600, Shimadzu).

2.6. Evaluation of photocatalyst activity

The TiO₂, CuInS₂, OCN, TiO₂/CuInS₂/OCN, and TiO₂/CuInS₂/OCN/CQD materials were tested for photocatalytic activity using the RY145 photodegradation reaction. To evaluate the efficiency of the photocatalytic process for RY145 degradation, the reaction system was designed with a glass reaction vessel, with circulating water outside to maintain a reaction temperature of 25 °C throughout the reaction. In addition, a light source consisting of 4 solar lamps with a power of 15 W/lamp was placed at a distance of 15 cm from the lamp to the transparent surface of the reaction system, providing a light intensity of 2280 Lux to the reaction system.

First, 30 mg (0.3 g L⁻¹) of different photocatalysts (TiO₂, CuInS₂, OCN, TiO₂/CuInS₂/OCN, and TiO₂/CuInS₂/OCN/CQD) were added to a 100 mL RY145 solution with a concentration of 50 mg L⁻¹ and vigorously stirred in the dark for 60 minutes to achieve adsorption–desorption equilibrium of the materials. Then, simulated solar irradiation was introduced to the system, to initiate the RY145 photocatalytic degradation reaction. To determine the RY145 concentration at different time points, after different reaction times of about 10 minutes, 2 mL aliquots of the reaction mixture were taken and the solid phase was separated by centrifugation for UV-vis spectroscopy. Based on the intensity at 421 nm and using the calibration curve y = 0.0118x + 0.0053, the RY145 concentration at different time points was determined. The photocatalytic degradation efficiency of RY145 on the TiO₂/CuInS₂/OCN/CQD photocatalyst was determined based on eqn (1):

H = (C_o − C_t) × 100/C_o (1)

where %H, C_o, and C_t are the RY145 conversion, initial RY145 concentration, and RY145 concentration at time t, respectively.

The factors affecting the RY145 degradation process on the TiO₂/CuInS₂/OCN/CQD photocatalyst, such as concentration, catalyst mass, pH value, water source, anions, and participating radicals, were investigated according to the evaluation procedure.

3. Result and discussion

3.1. Physicochemical and morphological properties of CuInS₂, OCN, TiO₂ and TiO₂/CuInS₂/OCN/CQD materials

The XRD patterns of CuInS₂, OCN, TiO₂ and TiO₂/CuInS₂/OCN/CQD are presented in Fig. 1. Notably, the CuInS₂ sample reveals diffraction peaks at 2θ of 27.74°, 29.34°, 31.81°, and 48.07°, which are assigned to the (112), (101), (200), and (204) planes of chalcopyrite-type CuInS₂ (JCPDS card number 85-1575).^39,51 Similarly, TiO₂ exhibits peaks at 2θ of 25.40°, 37.84°, 47.94°, 54.10°, and 62.35°, corresponding to the (101), (004), (200), (105), and (204) planes of TiO₂ anatase phase.^52–54 The OCN sample shows diffraction peaks at 2θ ∼27.4° and 13.2°, corresponding to the (002) and (100) planes of graphitic carbon nitride.^55–57 The combined TiO₂/CuInS₂/OCN samples display all expected peaks but with overlap between OCN (peak at 27.6°) and CuInS₂ (peak at 27.5°). Due to the amorphous structure and low content of CQD, peaks relating to this phase were not visible.^41,47 The TiO₂/CuInS₂/OCN/CQD maintains its phase structure through the synthesis process with slight modifications in the 2θ angle due to inter-material interactions.⁴⁷ Employing the Debye–Scherrer equation, the crystal sizes for CuInS₂ and TiO₂ were estimated at 5 nm and 8.3 nm, respectively.

Fig. 1 XRD patterns of CuInS₂, OCN, TiO₂, and TiO₂/CuInS₂/OCN/CQD samples.

The XPS method was used to analyze the surface chemical state and chemical composition of all samples, as shown in Fig. S2 and Table S1.† The OCN sample has binding energies at 284.24 eV (sp² C–C), 285.34 eV (C–O), 287.64 eV (sp² N–C=N), 398.97 eV (sp² C–N=C), 399.64 eV (sp³ N), 401.18 eV (C₂–NH), 530.47 eV (N–C–O), and 531.69 eV (physically adsorbed –OH) (Fig. 2).⁵⁸ The formation of C–O and N–C–O bonds indicates the partial substitution of N atoms with O in the CN aromatic rings.⁵⁹ In the TiO₂ sample, binding energies at 458.80 eV (Ti⁴⁺, Ti 2p_3/2), 464.56 eV (Ti⁴⁺, Ti 2p_1/2), 529.98 eV (Ti–O), and 531.94 eV (–OH) were observed from the high-resolution XPS spectrum of Ti 2p and O 1s.^31,60,61 The binding energy peaks at 163.36 eV (S²⁻, S2p_1/2), 162.04 eV (S 2p_3/2), In²⁺ (446.14 and 453.64 eV), In³⁺ (448.37 and 455.84 eV), Cu^o (929.52 and 949.22 eV), Cu⁺ (932.13 and 952.01 eV), and Cu²⁺ (933.42 and 953.78 eV) were observed from the high-resolution spectra of CuInS₂ (Fig. S3†).⁶² After combining with OCN and TiO₂, a shift in binding energy to lower energy levels of Cu 2p (Cu⁺: 931.94 and 951.74 eV; Cu²⁺: 933.19 and 953.78 eV), In 3d (In²⁺: 445.67 and 452.73 eV; In³⁺: 446.25 and 453.62 eV), and S 2p (161.64 and 162.95 eV) was observed for the TiO₂/CuInS₂/OCN/CQD sample.

Fig. 2 High-resolution Ti 2p, Cu 2p and In 3d XPS spectra of CuInS₂, OCN, TiO₂, TiO₂/CuInS₂/OCN/CQD samples.

On the contrary, the bonding energies of Ti 2p (458.99 and 464.71 eV), C (284.40, 285.55 and 287.66 eV), N 1s (399.01, 400.05 and 401.51) and O 1s (530.92 and 532.01 eV) in the TiO₂/CuInS₂/OCN/CQD sample show a shift to higher energy levels compared to the pure TiO₂ and OCN samples (Fig. 2). This result demonstrates the formation of a ternary bond between CuInS₂, TiO₂ and OCN, as opposed to a mere mechanical mixture of CuInS₂, TiO₂, and OCN.³⁹ There was also evidence of electron transfer between phases, denoting changes in charge density due to the establishment of a hybrid interface when combined with CQD.³¹ This result demonstrates that OCN and TiO₂ are electron donors in the TiO₂/CuInS₂/OCN/CQD sample (due to their decreased electron density) while CuInS₂ acts as an electron acceptor (due to its increased electron density). The surface –OH groups are expected to be pivotal for producing active radicals such as ˙OH and ˙O₂⁻ under visible light.^47,63

The EDS spectrum of the OCN sample (Fig. S4†) verifies successful O integration into the CN framework. Fig. 3 showcases the EDS spectrum and EDS mapping of the TiO₂/CuInS₂/OCN/CQD material, which indicates an even distribution of its elements. The respective weight percentages of elements in the samples are tabulated in Table S2.† The chemical composition of the OCN sample is 3.68% O, 52.32% C, and 44% N, respectively (Table S2†). The TiO₂/CuInS₂/OCN/CQD material shows the presence of elements C (23.47%), N (28.75%), O (17.62%), Cu (4.65%), In (6.88%), S (4.92%), and Ti (13.71%) in the EDS spectrum and EDS-mapping.

Fig. 3 (A) EDS spectrum and (B) SEM image of the TiO₂/CuInS₂/OCN/CQD sample; EDS elemental mapping images of the TiO₂/CuInS₂/OCN/CQD sample, showing elemental distributions of (C) C, (D) N, (E) O, (F) S, (G) Cu, (H) In and (I) Ti.

Fig. 4 presents the N₂ adsorption–desorption isotherms for the discussed materials, measured at 77 K. These isotherms provide insights into the samples' porosity. The TiO₂ isotherm is categorized as type IV according to IUPAC classification, and combined with the evidence of the hysteresis loop, indicates a mesoporous material, which is consistent with the BET surface area measurement (S_BET ∼139 m² g⁻¹). The OCN also appears to have some mesoporosity (as evidenced by the presence of the hysteresis loop), and has a pronounced pore volume (0.649 cm³ g⁻¹) but lower surface area (S_BET ∼88 m² g⁻¹). The CuInS₂ isotherm can be identified as type III (indicating a non-porous material), which is further confirmed by the low measured surface area (12 m² g⁻¹) and low pore volume (0.094 cm³ g⁻¹). The TiO₂/CuInS₂/OCN/CQD isotherm can be considered to be a combination of all of these features, with a surface area and pore volume of 79 m² g⁻¹ and 0.554 cm³ g⁻¹, respectively.

Fig. 4 N₂ adsorption–desorption isotherms of TiO₂, CuInS₂, OCN, and TiO₂/CuInS₂/OCN/CQD materials.

SEM and TEM images reveal that the TiO₂ particles are spherical and uniformly sized between 10 and 20 nm (see Fig. 5A and S5A†). In contrast, OCN particles exhibit a plate-like shape, and CuInS₂ particles resemble flower formations (as depicted in Fig. 5B and S5†). TEM imaging of the TiO₂/CuInS₂/OCN/CQD composite indicates that nano-sized TiO₂ particles, approximately 5–10 nm in size, adhere to the CuInS₂ petals and distribute uniformly over the OCN plates. In higher-resolution images, the darker spherical areas correlate to the nano TiO₂ and CuInS₂ particles, whereas the brighter peripheries correspond to the OCN plates. The TEM visuals confirm a close interaction between the nano TiO₂, CuInS₂, and OCN particles, amalgamating to form a distinct heterostructure.

Fig. 5 TEM images of (A) CuInS₂, (B) TiO₂, (C) OCN and (D) TiO₂/CuInS₂/OCN CQD samples.

The optical and photoelectrochemical properties of TiO₂, CuInS₂, OCN, and TiO₂/CuInS₂/OCN/CQD materials were analyzed using UV-vis DRS, photoluminescence (PL), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods. Fig. 6A shows that TiO₂ primarily absorbs ultraviolet light with a bandgap energy exceeding 3.30 eV. In contrast, OCN and CuInS₂ exhibit absorption in the visible light spectrum, having bandgap energies of 2.76 eV and 1.09 eV, respectively. Notably, the heterojunction in the TiO₂/CuInS₂/OCN/CQD photocatalyst extends its visible light absorption range, reaching a bandgap energy of 2.57 eV (see Fig. S6†). This shift arises due to the intensified quantum confinement effect of CQD combined with the establishment of a dual Z-scheme heterojunction.^41,47,64 This enhanced visible light absorption fosters the creation of reaction intermediates, augmenting photocatalytic activity.³¹ Mott–Schottky plots (Fig. S7†) provide the conduction band (CB) levels of the photocatalysts. Using this method, the flat band energy levels of TiO₂, CuInS₂, and OCN were identified as −1.06, −1.57, and −1.32 eV, respectively. By correlating the Ag/AgCl electrode with the standard hydrogen electrode, we discerned the CB energy levels of these materials as −0.45, −0.96, and −0.71 eV respectively. Their corresponding valence band (VB) energies stand at 2.85, 0.13, and 2.05 eV.

Fig. 6 (A) UV-vis DRS spectra, (B) photoluminescence spectra (PL), (C) transient photocurrent response and (D) cyclic voltammetry (CV) of TiO₂, CuInS₂, OCN and TiO₂/CuInS₂/OCN/CQD samples.

Electron–hole recombination rates are assessed using the PL method. As per Fig. 6B, the order of PL intensity, which relates to recombination of electrons and holes, is (in descending order): CuInS₂ > OCN > TiO₂ > TiO₂/CuInS₂/OCN > TiO₂/CuInS₂/OCN/CQD. The CuInS₂ sample, with the highest PL intensity and the lowest energy level,⁴⁷ exhibits the greatest recombination propensity. After combining TiO₂, CuInS₂ and OCN together (TiO₂/CuInS₂/OCN), the PL intensity decreased significantly compared to the individual semiconductors. The electron transfers from the CB of OCN and TiO₂ to the VB of CuInS₂ elevate the separation efficiency and curb recombination in the semiconductor. The TiO₂/CuInS₂/OCN/CQD composite showcases the lowest PL intensity, pointing to reduced recombination and improved separation. The presence of CQD enables dual Z transfer, further suppressing electron–hole recombination and supporting faster charge transfer.⁴⁷ As Fig. 6C reveals, CuInS₂ and OCN respectively demonstrate the best and poorest charge transfer capabilities, reflected in their Nyquist EIS diagrams. After integrating TiO₂ and CuInS₂ with OCN, the TiO₂/CuInS₂/OCN/CQD material's semicircle diameter reduces notably, an indicator of altered photoelectric properties. This change, due to CQD-mediated heterojunction formation, underlines the enhanced optical attributes of the TiO₂/CuInS₂/OCN composite over OCN. The introduction of CQD also appears to boost charge carrier transport. Cyclic voltammetry (Fig. 6D) further characterizes charge transfer capabilities. Here, the CuInS₂ and TiO₂ samples excel due to the oxidation and reduction of metal ions (Cu²⁺, Cu⁺, In³⁺, In²⁺ and Ti⁴⁺) at 0.32 and 0.12 V, respectively.⁶⁵ In contrast, OCN, a non-metallic semiconductor, presents a starkly inferior charge transfer ability.⁶⁶ However, after its amalgamation with other semiconductors, this ability doesn't markedly improve, attributable to inadequate semiconductor phase contact. Remarkably, the introduction of CQD enhances the TiO₂/CuInS₂/OCN/CQD semiconductor's oxidation–reduction potential, thanks to bolstered inter-semiconductor interactions.^46,67–69

3.2. Photocatalytic activity

The photocatalytic activity of the different materials against RY145 dye degradation was assessed. As illustrated in Fig. 7, under only visible light, RY145 dye exhibited a minimal degradation of about 2.2%. However, the introduction of various photocatalysts accelerated the dye's removal. After 60 minutes of light exposure, the removal efficiencies achieved by TiO₂, CuInS₂, OCN, and the composite TiO₂/CuInS₂/OCN/CQD were 56.81%, 40.31%, 67.64%, and 95.44%, respectively. In individual sample evaluations, TiO₂ displayed the lowest efficiency in degrading RY145 (43.25%) due to its significant bandgap energy and ultraviolet light absorption (3.3 eV). It should be noted that nano TiO₂ showed significant RY145 adsorption (25% under dark conditions) due to its relatively high surface area of 139 m² g⁻¹ (Table 1). By contrast, CuInS₂ with its limited surface area (12 m² g⁻¹), led to modest RY145 removal efficiencies of 5.2% via adsorption under dark conditions and 50.12% post-100 minute photocatalysis, demonstrating strong visible light absorption. OCN outperformed both TiO₂ and CuInS₂, achieving a removal efficiency of 82.1%. Remarkably, the concurrent integration of TiO₂, CuInS₂, and OCN, connected through the CQD bridge into a heterostructure (TiO₂/CuInS₂/OCN/CQD), boosted photocatalytic efficiency by factors of 5.1, 6.09, and 3.06 relative to the individual TiO₂, CuInS₂, and OCN constituents. The highest removal efficiency of RY145 after 60 minutes was achieved by the TiO₂/CuInS₂/OCN/CQD catalyst (95.44%). The enhanced performance is credited to an increased surface area, improved visible light absorption, diminished electron–hole recombination, and the heterostructure's efficient electron transport. Notably, the combined TiO₂/CuInS₂/OCN/CQD system exhibited a 2.13-fold higher efficiency than the TiO₂/CuInS₂/OCN system, highlighting the contribution of the CQD bridge. Reaction rate details for RY145 degradation by each sample are presented in Fig. 7B. Factors influencing RY145 degradation include, photocatalyst concentration, pH, dye concentration and the initial water source. Fig. 7C highlights that reducing the concentration of photocatalyst from 0.6 to 0.3 g L⁻¹ barely affects the removal efficiency of RY145 by the TiO₂/CuInS₂/OCN/CQD photocatalyst. However, a further decrease to 0.2 g L⁻¹ sees a drop from 98.80% to 82.17% efficiency. Thus, for subsequent studies, an optimal photocatalyst concentration of 0.4 g L⁻¹ was chosen. pH variation influences the TiO₂/CuInS₂/OCN/CQD's surface properties.

Fig. 7 (A) C/C_o as a function of reaction time and (B) first-order reaction kinetics over TiO₂, CuInS₂, OCN, TiO₂/CuInS₂/OCN and TiO₂/CuInS₂/OCN/CQD samples; C/C_o as a function of reaction time over the TiO₂/CuInS₂/OCN/CQD photocatalyst, showing dependence on (C) the concentration of photocatalyst used; (D) pH value; (E) different RY145 concentrations; (F) different water sources; conversion of RY145 by theTiO₂/CuInS₂/OCN/CQD photocatalyst, as affected by: (G) different anions and (H) radical scavengers. As Fig. 7D indicates, a pH drop from 10 to 4 elevates RY145 degradation from 66.18% to 98%. This result is because, at low pH values, the TiO₂/CuInS₂/OCN/CQD material and RY145 have opposite charges, encouraging electrostatical interactions which increase reaction rates, resulting in increased adsorption efficiency.⁵⁰ Conversely, at pH values >7, the efficiency drops to 68.18% due to TiO₂/CuInS₂/OCN/CQD and RY145 having similar negative charges, leading to a significant decrease in the RY145 reaction rates, lowering degradation efficiency from 98% to 68.18% after 60 minutes of visible light irradiation. These results are similar to those reported by our previous research where we used Co–Fe-BTC/CN and ZnO–Ag@AgBr/SBA-15 catalysts to treat dyes and phenol red in an aqueous environment.^50,63

Table 1 BET surface area (S_BET), pore volume (V_pore), BJH pore diameter (D_BJH) and bandgap energy (E_g) of TiO₂, CuInS₂, OCN and TiO₂/CuInS₂/OCN/CQD materials

Samples	SBET	Vpore	DBJH	Eg
TiO2	139	0.193	5.58	3.30
OCN	88	0.649	22.30	2.76
CuInS2	12	0.094	31.12	1.09
TiO2/CuInS2/OCN/CQD	79	0.554	19.77	2.57

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To probe the effect of RY145 dye concentrations on removal efficiencies, we investigated dye concentrations from 10 to 70 mg L⁻¹, using a catalyst concentration of 0.4 g L⁻¹ at pH 5. This range was based on the study by Yaseen and Scholz, which stated that dyeing plants discharge industrial wastewater containing RY145 dye concentrations ranging between 10 and 50 mg L⁻¹.⁷⁰ As illustrated in Fig. 7E, the photocatalytic efficiency of the TiO₂/CuInS₂/OCN/CQD material is contingent on the starting RY145 concentration. Notably, there's a direct correlation between dye concentration and treatment duration. At dye concentrations of 10, 30, 50, and 70 mg L⁻¹, we observed RY145 treatment efficiencies surpassing 98% over irradiation periods of 40, 50, 60, and 100 minutes, respectively. Consequently, higher RY145 dye concentrations necessitate an extended treatment duration with the TiO₂/CuInS₂/OCN/CQD photocatalyst to achieve the desired efficiencies. With a reaction time of 60 minutes, the TiO₂/CuInS₂/OCN/CQD material achieves 98.2% RY145 removal efficiency, surpassing the performance of other previously reported TiO₂/activated carbon, g-C₃N₄–SrTiO₃, CuO–ZnO, and Cu–NiO/ZnO photocatalyst materials (Table S3†).

To replicate real-world conditions, we prepared the RY145 dye solution using water samples sourced from rivers and lakes, around Hanoi, Vietnam, specifically the To Lich River, Red River, Tay Lake, and Hoan Kiem Lake, as depicted in Fig. 7F. This figure reveals a notable decline in RY145 treatment efficiency for samples from the To Lich River, Tay Lake, and Hoan Kiem Lake when compared to those prepared with distilled water. After 60 minutes of light exposure, the RY145 dye treatment efficiency using the TiO₂/CuInS₂/OCN/CQD photocatalyst for the aforementioned water sources were 52.13%, 95.32%, 73.96%, and 76.42%, in sequence. This diminished efficiency seen in the water from To Lich River can be attributed to the high concentrations of organic materials and suspended particles in the water. These compounds hinder and react with reactive species such as ˙O₂⁻ and ˙OH, thereby considerably reducing the efficiency of RY145 treatment. This observation aligns with findings reported by our previous studies.^50,71

We also explored the effects of anion including Cl⁻, CO₃²⁻, NO₃⁻, SO₄²⁻ and HCO₃⁻ at a concentration of 10 mM on the RY145 treatment process, as these are ions that may well be found in wastewater streams. As shown in Fig. 7H, these anions were found to influence RY145 treatment efficiency. They engage with the photocatalyst's surface and counteract reactive species, leading to diminished reactive species formation.⁷¹ Among the investigated anions, CO₃²⁻ and HCO₃⁻ considerably affected the RY145 treatment, as they interact with reactive species and simultaneously elevate the pH of the reaction environment, shifting it to alkaline and reducing the degradation efficiency.⁵⁰

The stability of the TiO₂/CuInS₂/OCN/CQD photocatalyst was demonstrated through the repeated oxidation reactions of RY145, indicating >94% oxidation efficiency after five reaction cycles (Fig. S8†). No marked differences appear in the XRD, TEM, and XPS results following these cycles (Fig. S9 and S10,† as compared to Fig. 2 and 3). However, high-resolution spectra of O 1s, C 1s, N 1s, Cu 2p, In 3d, and S 2p show bond energy shifts to higher levels, confirming electron loss during reactive species formation involved in RY145 degradation (see Fig. 2 and 3).⁵⁰ After five reaction cycles, the catalyst remains highly stable and reusable, thus the photocatalyst has a stable structure, tightly bound together by carbon quantum dots and the photocatalytic reaction is carried out under environmental temperature and pressure conditions. Furthermore, this Z-scheme photocatalyst has dual charge transfer, which protects the conduction bands of higher energy semiconductors.⁷²

3.3. Photocatalytic mechanism

Fig. 7H reveals the significant influence of ˙O₂⁻, ˙OH radicals, electrons (e⁻), and holes (h⁺) on the efficiency of RY145 treatment. Specifically, after 60 minutes of visible light exposure, the treatment efficiencies for O₂⁻ radicals, ˙OH radicals, electrons (e⁻), and holes (h⁺) are 54.71%, 72.86%, 68.12%, and 61.63%, respectively. These results indicate that the ˙O₂⁻ radical has the most profound effect on the reaction rate.

In Fig. 8A, due to the negative redox potentials of TiO₂ (−0.45 eV), CuInS₂ (−0.96 eV), and OCN (−0.68 eV) relative to the O₂/˙O₂⁻ potential (−0.33 eV), ˙O₂⁻ radicals are produced across all three photocatalysts.⁷³ In CuInS₂ and OCN's conduction band (CB), the redox energy is lower than H₂O/˙OH (2.4 eV), preventing formation of ˙OH radicals in the valence band.⁷⁴ Conversely, the redox energy of TiO₂ (2.85 eV) exceeds the energy of formation of H₂O/˙OH, leading to ˙OH radical formation. When integrating the third-generation heterogeneous catalysts TiO₂/CuInS₂/OCN through CQD bridges at each interface, conditions favour electron and hole movement, as shown in Fig. 8B. After visible light excites the electrons, they transition from the valence to the conduction band. Electrons from OCN and TiO₂ then transition to CuInS₂'s valence band via the CQD bridges (OCN → CQD → CuInS₂ and TiO₂ → CQD → CuInS₂), increasing the electron density on CuInS₂. Surface vacancies and oxygen defects can trap these electrons or holes, reducing their recombination rate. Additionally, the reactive species participate in the decomposition of RY145 dye, which is expected to result in a range of degradation products.

Fig. 8 Schematic of the separation transfer before contact (A) and possible Z-scheme transfer (B) in the TiO₂/CuInS₂/OCN/CQD photocatalyst.

3.4. Degradation mechanism of RY-145 dye

LC-MS analyzed RY145 dye samples to understand the photocatalytic reaction under optimal conditions. Fig. 9 presents the degradation mechanism of RY145 on the TiO₂/CuInS₂/OCN/CQD photocatalyst, based on LC-MS results at different intervals. Following the formation of reactive species, they contribute to the cleavage of the –SO₃⁻ group and the C–N bond, yielding intermediate products T1 (m/z 396.30), T2 (m/z 292.54), T3 (m/z 186.07), T4 (m/z 208.12), T5 (m/z 163.23), T6 (m/z 145.52) and T7 (m/z 110.21).^63,75 These intermediates undergo further oxidation, leading to lower molecular weight products during the cleavage processes with m/z values of T8 (m/z 274.05), T9 (m/z 126.23), T10 (m/z 148.50) and T11 (m/z 126.23). The LC-MS results show that low molecular weight products such as T12, T13 (m/z 158.22), T14 (m/z 118.13), T15 (m/z 134.16), malonic acid (T16, m/z 104.06), butyric acid (T17, m/z 88.15) and acetic acid (T18, m/z 60.12) are formed during the cleavage process of the intermediate products T8–T11. After 60 minutes, the TOC, BOD, and COD removal efficiencies are 50.38%, 80.36%, and 75.82%, respectively, underscoring the TiO₂/CuInS₂/OCN/CQD photocatalyst's high mineralization efficiency (Fig. S12†).

Fig. 9 Possible degradation pathways of RY145 on TiO₂/CuInS₂/OCN/CQD sample.

4. Conclusion

This research synthesized a cutting-edge heterogeneous Z-scheme photocatalyst, TiO₂/CuInS₂/OCN combined with CQD. The CQD's ultra-small particles amplify the interaction between active phases, enhancing RY145 oxidation process in water, reaching 98.2% degradation efficiency. The degradation rate outpaces standalone TiO₂, CuInS₂, and OCN. The Z-scheme principles underline the photocatalyst's efficiency, with all reactive species significantly impacting RY145 treatment. Factors like RY145 concentration, catalyst concentration, pH, anions, and water source majorly influence the process. The catalyst achieves TOC, BOD, and COD removal efficiencies of 50.38%, 80.36%, and 75.82%, respectively, maintaining over 94% efficiency after five cycles. These results highlight the synergistic effect of combining these components, and the resulting TiO₂/CuInS₂/OCN/CQD photocatalyst's potential for treating water pollutants.

Author contributions

Manh B. Nguyen: investigation, formal analysis, data curation, writing – original draft. Pham Thi Lan: conceptualization, investigation, formal analysis, data curation. Nguyen Tuan Anh: conceptualization, investigation, formal analysis, data curation. Nguyen Ngoc Tung: conceptualization, investigation. Shaoliang Guan: investigation, formal analysis, data curation. Valeska P. Ting: conceptualization, investigation, formal analysis, data curation. T.-Thanh-Bao Nguyen: formal analysis, data curation. Huan V. Doan: conceptualization, investigation, data curation, writing – reviewing and editing. Mai Thanh Tung: formal analysis, data curation. Tran Dai Lam: investigation, formal analysis, data curation, writing – original draft and reviewing and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by The Vietnam Academy of Science and Technology (VAST) under the grant number NCXS 01.01/22-24.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra07546j

‡ The authors contributed equally to this work.

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