English

• 0.   Introduction

  Rapid economic development and industrialisation have led to the widespread discharge of numerous toxic pollutants into the environment. Particularly, the contamination of water sources by antibiotics is concerning owing to its significant but underestimated environmental impact^[1-4]. These pollutants are not readily biodegradable, exhibit toxic properties, and tend to accumulate over time. Antibiotic residues in the environment pose serious risks to human health and ecosystems by endangering organisms and promoting the emergence of antibiotic-resistant genes^[5]. A notable example is tetracycline (TC) pollution, which primarily originates from wastewater generated by the antibiotic industry, medical applications, and agricultural practices in the livestock and aquaculture sectors^[6].

  Photocatalytic oxidation is an efficient and eco-friendly method for degrading organic pollutants without secondary pollution^[7-8]. However, conventional semiconductors suffer from wide band gaps (3.0-3.2 eV), low sunlight utilization (3%-5%), and high electron-hole recombination rates, limiting their photocatalytic performance^[9]. To overcome these challenges, researchers have explored the integration of advanced materials into semiconductor systems^[10]. Recently, layered ternary transition-metal carbides and nitrides, known as MAX phases, have garnered significant attention due to their distinctive two-dimensional (2D)-like structures and exceptional electrical conductivity^[11]. Ti₃AlC₂, a material that is frequently utilized as a co-catalyst, has been shown to enhance charge separation when employed in conjunction with semiconductors. This enhancement can be attributed to the high conductivity, oxidation resistance, and self-healing capacity inherent to Ti₃AlC₂^[12]. Furthermore, recent studies have demonstrated that Ti₃AlC₂ exhibits characteristics analogous to 2D materials, including a substantial surface area and interlayer diffusion pathways, which facilitate charge carrier migration^[13-14]. Ti₃AlC₂ has also shown catalytic activity in oxidative reactions, improving both the photocatalytic activity and material durability when integrated into composite systems^[15]. Given these findings, integrating CdS with Ti₃AlC₂ is expected to enhance charge separation and structural stability, improving UV-induced photocatalytic efficiency.

  CdS, a visible-light-responsive semiconductor (band gap is approximately 2.37 eV), has been widely used in photocatalysis due to its strong crystallinity, stability under alkaline conditions, and excellent carrier transport properties^[16-18]. However, its high electron-hole recombination rate and susceptibility to photocorrosion limit its practical application^[19]. Recent studies have shown that coupling CdS with co-catalysts, such as Ti₃C₂, enhances charge separation and photocatalytic efficiency^[20]. Given these findings, integrating CdS with Ti₃AlC₂ is expected to enhance charge separation and structural stability, improving UV-induced photocatalytic efficiency.

  In this study, a simple and effective hydrothermal method was employed to synthesize CdS/Ti₃AlC₂ heterostructures, utilizing calcined Ti₃AlC₂ as a functional photocatalyst rather than merely a co-catalyst. This work provides valuable insights into the rational design and optimization of CdS-based photocatalysts for sustainable pollutant degradation.

  1.   Experimental

  1.1   Materials and reagents

  TC, anhydrous ethanol (C₂H₅OH), isopropyl alcohol (C₃H₈O), and cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O) were procured from McLean Biochemical Technology Co., China. Thiourea (CH₄N₂S) was procured from Dahua Weiye Chemical Co., China. Ethylenediamine (C₂H₈N₂), p-benzoquinone (C₆H₄O₂), and ammonium oxalate ((NH₄)₂C₂O₄) were procured from Wande Chemical Co., China.

  1.2   Synthesis of CdS/Ti₃AlC₂ catalyst

  1.2.1   Synthesis of calcined modified Ti₃AlC₂ material

  10 g Ti₃AlC₂ (500 mesh, Scientific Compass Co., China) was calcined at 110 ℃ for 1 h and 550 ℃ for 4 h in a muffle furnace, then naturally cooled to 25 ℃.

  1.2.2   Synthesis of CdS nanoparticles

  Cd(NO₃)₂·4H₂O (1 mmol), thiourea (3 mmol), and ethylenediamine (30 mL) were mixed, transferred to a Teflon reactor, and heated at 160 ℃ for 8 h. After cooling to 25 ℃, the samples were washed with ultrapure water and vacuum-dried at 60 ℃ to obtain CdS nanoparticles.

  1.2.3   Synthesis of CdS/Ti₃AlC₂

  CdS/Ti₃AlC₂ was synthesized using the solvothermal method. First, mix the calcined Ti₃AlC₂ (2 g) with ethylenediamine (15 mL) and ultrasonicate for 30 min. Subsequently, Cd(NO₃)₂·4H₂O (0.174, 0.522, 0.871, and 1.218 g) was added to the thiourea solution (15 mL, 0.1 mol·L^-1) to form a uniform suspension. Subsequently, the suspension was transferred to a polytetrafluoroethylene reactor and heated at a constant temperature of 160 ℃ for 8 h. After the reaction was completed, the xCdS/Ti₃AlC₂ samples prepared with different mass fractions (x=1%, 3%, 5%, and 7%) of CdS in the composite material were obtained after washing multiple times with ultrapure water and vacuum-dried at 60 ℃.

  1.3   Photocatalyst characterisation

  The crystal structure of the samples was determined by X-ray diffraction (XRD) using a Bruker D8 diffractometer equipped with Cu Kα radiation (λ=0.154 06 nm, 2θ=5°-80°) operated at 40 kV and 40 mA. The morphological characteristics were observed via scanning electron microscopy (SEM, Regulus 8100) and transmission electron microscopy (TEM, Talos F200X FEI, 200 kV). Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted in conjunction with SEM to evaluate the elemental composition. Fourier transform infrared spectroscopy (FTIR) spectra (500-4 000 cm^-1) were recorded using a Thermo Nicolet iS5 spectrometer. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) measurements (200-800 nm) were conducted using an EVOLUTION 220 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was performed with a Thermo K-Alpha+ system. Transient photocurrent response was measured using an FLS1000 spectrometer and an electrochemical workstation. In addition, photoluminescence (PL) spectra were recorded using a Shimadzu UV-3600i Plus spectrophotometer. Transient photocurrent response (TPR) and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI760E, Chenhua Instrument Co., Ltd., Shanghai, China). Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMXplus-6) was employed to identify the active species generated during the photocatalytic process.

  1.4   Photocatalytic experiments

  Degradation experiments were performed in a PCX-50 Discover system (Beijing Perfect Light Technology Co.). A 30-minute dark adsorption phase was conducted to establish adsorption-desorption equilibrium, followed by photocatalytic degradation of TC under a custom-designed 500 W xenon lamp equipped with a UV-specific wavelength output. Absorbance was measured every 30 min. The effect of CdS/Ti₃AlC₂ was assessed by varying CdS content, TC mass concentration, and catalyst dosage. The calculation formula used was^[21]:

  $ R=\frac{{\rho }_{0}-{\rho }_{t}}{{\rho }_{0}}\times 100\% $

  (1)

  where R represents the degradation efficiency of TC (%), ρ₀ is the initial mass concentration of TC solution (mg·L^-1), and ρ_t is the mass concentration of TC in solution at time t (mg·L^-1).

  The first-order kinetic model was used to assess the degradation efficiencies of different catalysts. Reaction rate constants were calculated using TC mass concentration changes during photocatalysis^[22]. The following formula was as follows:

  $\ln \frac{\rho_0^{\prime}}{\rho_t}=k_{\mathrm{app}} t$

  (2)

  where k_app is the reaction rate constant (min^-1), ρ₀′ is the initial mass concentration of TC solution (mg·L^-1) after dark adsorption, and t is the reaction time (min).

  2.   Results and discussion

  2.1   Structural and morphological analysis

  To investigate the crystal structure and crystallinity of the materials, XRD and FTIR analyses were conducted, as shown in Fig.1a and 1b. The XRD pattern of pristine Ti₃AlC₂ exhibited distinct diffraction peaks at 9.69°, 19.34°, 34.29°, and other characteristic planes, confirming its high purity and layered structure (PDF No.97-062-0306). After calcination at 550 ℃, a slight diffraction signal appeared near 25.3°, attributes to the (101) plane of TiO₂, suggesting minor surface oxidation^{[9, 12, 23]}. Pure CdS displayed diffraction peaks matching the hexagonal wurtzite structure (PDF No.97-019-0563). The XRD patterns of xCdS/Ti₃AlC₂ heterostructures exhibited the characteristic peaks of both CdS and Ti₃AlC₂, with increasing CdS crystallinity as the doping content increased, confirming successful heterostructure formation. FTIR spectra (Fig.1b) further revealed the functional groups on the material surfaces. Calcined Ti₃AlC₂ exhibited absorption peaks in the 500-1 000 cm^-1 range attribute to Al—O, Ti—O, and Ti—C vibrations, which were absent in pristine Ti₃AlC₂^[24]. Both calcined Ti₃AlC₂ and CdS displayed peaks at 1 650 and 3 400 cm^-1, corresponding to O—H vibrations from adsorbed water^[25-26]. CdS exhibited additional peaks at 1 410, 1 195, and 1 108 cm^-1, associated with C—O stretching vibrations^[27]. The xCdS/Ti₃AlC₂ heterostructures retained the characteristic peaks of both materials, with increasing CdS content enhancing the peak intensity. These results confirm the successful integration of CdS into Ti₃AlC₂, which enhances the material′s optical response and photocatalytic properties.

  Figure 1

  

  Figure 1.  (a) XRD patterns and (b) FTIR spectra of Ti₃AlC₂, calcined Ti₃AlC₂, CdS, and xCdS/Ti₃AlC₂

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  SEM and EDS analyses were conducted to examine the surface morphology and elemental composition of the samples. As shown in Fig.2a, pristine Ti₃AlC₂ exhibited a characteristic layered structure^{[10, 15]}. After calcination (Fig.2b), the morphology remained largely unchanged, indicating structural stability. Pure CdS (Fig.2c) displayed a dendritic structure, offering numerous active sites for photocatalysis^[28]. In the 5%CdS/Ti₃AlC₂ heterostructure (Fig.2d), CdS particles formed a rod-like accumulation on the Ti₃AlC₂ surface, suggesting effective surface modification for enhanced light absorption. TEM (Fig.2e, 2f) confirms the crystalline nature of both CdS and Ti₃AlC₂, with distinct lattice fringes and tight contact, indicating successful heterojunction formation^{[10, 29]}. The EDS spectrum (Fig.2g, 2h) verifies the presence of Ti, Al, C, O, Cd, and S elements, indicating that CdS was successfully incorporated at approximately 5%, which was in line with the designed composition. Furthermore, the single elemental mapping shows uniform distribution of Cd, S, Ti, Al, and C (Fig.2i-2n), confirming homogeneous CdS incorporation while preserving Ti₃AlC₂′s structural integrity^[30-31]. These results highlight the effective structural modification of Ti₃AlC₂ by CdS, which enhances photocatalytic performance.

  Figure 2

  

  Figure 2.  SEM images of (a) Ti₃AlC₂, (b) calcined Ti₃AlC₂, (c) CdS, and (d) 5%CdS/Ti₃AlC₂; (e, f) TEM images of 5%CdS/Ti₃AlC₂; (g) Elemental mapping overlay; (h) EDS spectrum and elemental composition of 5%CdS/Ti₃AlC₂; (i-n) Individual elemental mappings of C, O, Al, S, Ti, and Cd, respectively

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  The chemical composition and valence states of the prepared materials were further investigated by XPS. Fig.3 displays the XPS spectra of pristine CdS, calcined Ti₃AlC₂, and the 5%CdS/Ti₃AlC₂ heterostructure. The Cd3d peaks at 405.15 and 411.91 eV are consistent with the reported values of Cd3d_5/2 and 3d_3/2 peaks, respectively^[32]. In contrast, the peaks at 406.61 and 413.46 eV correspond to the Cd—N bond^[33]. After composite formation, these peaks in the 5%CdS/Ti₃AlC₂ spectrum exhibited a slight positive shift (approximately 0.20 eV), indicating a change in the local electronic environment of Cd, likely due to electron transfer between CdS and Ti₃AlC₂. Similar behavior was observed in the S2p spectrum, pristine CdS showed peaks at 161.38 and 162.56 eV (S^2-), while the heterostructure presented a slight shift toward higher binding energy, further confirming strong interfacial interaction^[29]. Fig.3e shows the Ti2p spectrum of Ti₃AlC₂, in which 531.80 eV peaks can be assigned to Ti—C, while peaks at 530.37 and 533.26 eV belong to Ti—O in the Ti₃AlC₂ sample^[34-35]. These signals were retained in the composite, with no significant shift, suggesting that the Ti₃AlC₂ framework remains structurally stable. For the Al2p region, a peak at 74.31 eV was observed in both Ti₃AlC₂ and CdS/Ti₃AlC₂, consistent with Al in the MAX phase. The O1s spectrum exhibited three fitted peaks located at approximately 530.5, 531.6, and 532.6 eV, corresponding to lattice oxygen in Al₂O₃, surface Ti—O species, and chemisorbed oxygen or hydroxyl groups (—OH), respectively^[36].

  Figure 3

  

  Figure 3.  XPS spectra of CdS, 5%CdS/Ti₃AlC₂, and Ti₃AlC₂: (a) Survey, (b) Cd3d, (c) S2p, (d) Ti2p, (e) Al2p, and (f) O1s

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  UV-Vis DRS analysis (Fig.4) was employed to investigate the light responsiveness of the materials. A comparison of the UV-Vis spectra of different catalysts revealed that pristine Ti₃AlC₂ exhibited negligible light absorption in both UV and visible regions. After calcination, Ti₃AlC₂ exhibited enhanced absorption, with a distinct absorption edge around 350 nm. This suggests the formation of TiO₂ during calcination, improving UV-light absorption. In addition, pure CdS showed an absorption edge in the range of 450-500 nm, which was similar to the result observed when CdS was loaded onto calcined Ti₃AlC₂, thereby enhancing the visible light response of the material. These UV-Vis DRS results indicate that loading calcined Ti₃AlC₂ with CdS enhances the photoresponse of the material in the UV region and improves its overall light utilisation as a catalyst. Moreover, UV-Vis DRS were processed using the Kubelka-Munk function to derive (αhν)²-hν plots for various samples. The band gap values of different catalysts were obtained using Eq.3^[37-40].

  $ (αhν)^{n}=A(hν-E_\text{g}) $

  (3)

  Figure 4

  

  Figure 4.  (a) UV-Vis DRS and (b) (αhν)²-hν curves of the samples

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  where h, α, ν, n, A, and E_g denote the Planck′s constant, absorption coefficient, optical frequency, transition mode exponent of the semiconductor, absorbance constant, and band gap, respectively. The estimated band gaps were 2.41 eV for CdS and 3.09 eV for calcined Ti₃AlC₂, consistent with typical values reported in the literature.

  To gain further insights into the photocatalytic performance of the CdS/Ti₃AlC₂ heterostructures, their charge separation efficiency and carrier dynamics were comprehensively investigated. To evaluate the charge separation efficiency of the samples, their PL spectra were obtained at an excitation wavelength of 255 nm. As shown in Fig.5a, the PL spectra exhibited intense emission peaks at approximately 520 and 380 nm, indicating the rapid recombination of photoinduced charge carriers^[41]. The incorporation of CdS into Ti₃AlC₂ significantly decreased the intensity of PL peaks, indicating a substantial increase in the separation efficiency of the samples^[42]. The TPR of the prepared photocatalysts was measured to elucidate the effect of CdS on carrier separation and transport. As illustrated in Fig.5b, the TPR of 5%CdS/Ti₃AlC₂ heterostructures was notably more pronounced than that of calcined Ti₃AlC₂ samples. Furthermore, the 5%CdS/Ti₃AlC₂ sample exhibited the highest photocurrent density, which aligned well with its superior performance in photocatalytic degradation^[43]. As shown in Fig.5c, the EIS Nyquist plot displays negative values for imaginary and real components, enabling the direct determination of charge-transfer resistance from the radius of the semicircle. A smaller arc or curve indicates a lower charge-transfer resistance, indicative of enhanced electron mobility and conductivity, which leads to an improved photocatalytic performance^[44]. The semicircle radius was lower for 5%CdS/Ti₃AlC₂ than for the calcined Ti₃AlC₂ and CdS samples. This suggests that 5%CdS/Ti₃AlC₂ exhibited a lower charge-transfer resistance during the photodegradation experiment, thereby demonstrating a higher process efficiency than the original material. This implies that CdS/Ti₃AlC₂-based heterostructures exhibit enhanced capacity for charge separation and migration, thereby augmenting the efficiency of photocatalytic degradation.

  Figure 5

  

  Figure 5.  (a) PL spectra under 255 nm excitation, (b) TPR curves, and (c) EIS of calcined Ti₃AlC₂, CdS, and 5%CdS/Ti₃AlC₂

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  2.2   Photocatalytic performance

  The photocatalytic performance of the samples was evaluated under UV irradiation (Fig.6a). Within 30 min, all catalysts exhibited increased TC degradation efficiency, with 5%CdS/Ti₃AlC₂ and 7%CdS/Ti₃AlC₂ heterostructures achieving the highest rates of 75% and 76%, respectively. CdS, pristine Ti₃AlC₂, and calcined Ti₃AlC₂ exhibited lower degradation efficiencies of 26%, 29%, and 44%, respectively. At 180 min, 5%CdS/Ti₃AlC₂ showed the highest degradation efficiency (96.3%), demonstrating the highest reaction rate constant (k=0.012 17 min^-1) (Fig.6b). The improved performance is attributed to enhanced charge separation and prolonged carrier lifetime^[36]. The effect of pH on degradation efficiency was examined (Fig.6c). At pH 3, the degradation efficiency was 52% at 30 min, whereas it ranged from 65% to 70% at pH 5-11. After 180 min, the highest degradation efficiency (96.3%) was achieved at pH 7, indicating optimal photocatalytic activity under neutral conditions. Additionally, increasing catalyst dosage from 25 to 100 mg·L^-1 enhanced degradation efficiency to 89%, but further increasing to 125 mg·L^-1 led to a decline due to catalyst agglomeration (Fig.6d). The optimal initial TC mass concentration was found to be 30 mg·L^-1, with higher mass concentrations negatively impacting degradation. Recycling tests (Fig.6f) confirmed the stability of 5%CdS/Ti₃AlC₂, retaining over 90% of its catalytic activity after four cycles. XRD analysis before and after degradation showed no significant structural changes, confirming the catalyst′s stability.

  Figure 6

  

  Figure 6.  (a) Photocatalytic degradation of TC by different catalysts under UV light irradiation and (b) the corresponding kinetics curves; Impact of (c) initial pH, (d) catalyst dosage, and (e) TC mass concentration on the degradation performance of the 5%CdS/Ti₃AlC; (f) Four repetitive cycles of photocatalytic degradation of TC by 5%CdS/Ti₃AlC₂ under UV light irradiation and XRD patterns of 5% CdS/Ti₃AlC₂ for before and after degradation reaction (Inset)

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  To more effectively illustrate the degradation efficiency of the proposed catalyst, it was compared with catalysts reported in recent studies, as presented in Table 1. The CdS/Ti₃AlC₂ composite achieved a TC degradation efficiency of 96.3%, outperforming many photocatalysts reported in recent studies (Table 1). Its performance was comparable to the highest efficiency observed, such as MnFe-LDO-biochar/K₂S₂O₈ (98%)^[45], but without the need for chemical oxidants, making it more environmentally friendly. Similarly, g-C₃N₄/TiO₂ (96.3%)^[46] and FeNi₃/SiO₂/CuS (96.0%)^[47] demonstrated high efficiencies but involve complex synthesis processes or rare materials. In comparison, the CdS/Ti₃AlC₂ composite offers similar or superior performance while maintaining a simpler synthesis process and greater environmental compatibility. Newer photocatalysts, such as TiO₂@Ti₃C₂ (90.7%)^[35] and CuS/CdS (90%)^[48], achieved lower efficiencies under visible light. The broad UV applicability and high efficiency of CdS/Ti₃AlC₂, combined with its improved light absorption and enhanced charge separation efficiency, make it a promising alternative.

  Table 1

  

  Table 1.  Recent studies related to the degradation of TC

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  Material	Irradiation source	R / %	Ref.
  Calcite/TiO2	UV light	90.0	[49]
  Chitosan/TiO2	UV light	70.0	[50]
  UV/NH2Cl	UV light	90.0	[51]
  MnFe-LDO-biochar/K2S2O8	UV light	98.0	[45]
  W/BaZrO3	UV light	94.9	[52]
  g-C3N4/TiO2	UV light	96.3	[46]
  BiOCl/CdS	Full light	75	[53]
  g-C3N4/silica mesopores	UV light	92.9	[54]
  CuS/CdS	Visible light	90	[48]
  FeNi3/SiO2/CuS	UV light	96.0	[47]
  TiO2@Ti3C2	Visible light	90.7	[35]
  CdS/Ti3AlC2	UV light	96.3	This work

  2.3   Photocatalytic mechanism

  Through the capture agent experiment, a preliminary determination of the active species in the reaction system (Fig.7a). Benzoquinone, isopropanol, and ammonium oxalate were used to capture superoxide radical (·O₂^-), hydroxyl radicals (·OH), and holes (h⁺). respectively. Benzoquinone led to a significant decrease in the photocatalytic degradation of TC, whereas the other capture agents had a relatively minor effect on the degradation efficiency of TC^[55]. This indicates that ·O₂^- is the main active substance in the reaction system. Additionally, to verify the types of free radicals produced in the photocatalytic process, the EPR spectra of the prepared photocatalysts were obtained, as shown in Fig.7b-7d. As evident from Fig.7b, the intensity of the ·O₂^- signal significantly increased after 5 min of light exposure compared to that observed under the dark reaction condition. As shown in Fig.7c and 7d, the signal intensities of ·OH and h⁺ did not change significantly before and after the light exposure. This further indicates that ·O₂^- is the primary active substance responsible for the photocatalytic degradation of TC by CdS/Ti₃AlC₂^[56-57].

  Figure 7

  

  Figure 7.  (a) Effect of different capture agents on catalyst degradation of TC; EPR spectra of (b) ·O₂^-, (c) ·OH, and (d) h⁺

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  Based on the analysis and discussion above, this study proposed a comprehensive mechanism for the photocatalytic degradation of TC by the CdS/Ti₃AlC₂ heterostructure catalyst. The integration of CdS with calcined Ti₃AlC₂ leads to the formation of a well- defined and continuous heterojunction, enabling intimate interfacial contact between the two components. This structural configuration effectively suppresses the recombination of photogenerated electron-hole pairs, promotes more efficient charge carrier separation and migration, and ultimately enhances the overall photocatalytic performance. According to the UV-Vis DRS analysis, the bandgap energies of CdS and Ti₃AlC₂ were calculated to be 2.41 and 3.09 eV, respectively. To validate the energy band structure, XPS was employed to determine the valence band maximum (VBM) of the materials. The results indicate that the VBM of Ti₃AlC₂ and CdS were situated at 2.74 and 1.96 eV, respectively. These values indicate that the VB potential of the material is sufficient to oxidize water or hydroxide ions to generate hydroxyl radicals, and the results are consistent with EPR and quenching experiments. The schematic illustration of the band alignment and heterostructure formation is shown in Fig.8. The photocatalytic degradation process was predominantly driven by the presence of superoxide radicals, indicated by the symbol ·O₂^-. Furthermore, h⁺ and ·OH were identified as contributing factors to the degradation efficiency. Furthermore, UV-Vis DRS results suggest a reduction in the band gap of Ti₃AlC₂ following CdS coupling. Under UV light irradiation, both CdS and Ti₃AlC₂ were excited, generating electron-hole pairs. The transfer of photogenerated electrons from the CdS conduction band (CB) to the Ti₃AlC₂ CB was facilitated by the built-in electric field and energy band alignment. This process suppresses recombination, thereby prolonging carrier lifetimes and enhancing photocatalytic activity. In the case of photocatalytic degradation of TC, the photogenerated electrons on Ti₃AlC₂ can react with electron acceptors (e.g., O₂) to form ·O₂^-, which are strong oxidative agents that can directly degrade organic pollutants. A preliminary examination of the subject reveals that intermediate products may include decarboxylation products, ring-opening structures, and the final low-molecular-weight organic acids and inorganic ions, among other elements^{[20, 39, 58-60]}. The involved photocatalytic reaction processes are described as:

  $\mathrm{CdS} / \mathrm{Ti}_3 \mathrm{AlC}_2+h \nu \rightarrow \mathrm{e}_{\mathrm{CB}}^{-}+\mathrm{h}_{\mathrm{VB}}^{+} $

  (4)

  $ \mathrm{e}^{-}+\mathrm{O}_2 \rightarrow \cdot \mathrm{O}_2^{-}$

  (5)

  $\cdot \mathrm{O}_2^{-}+\mathrm{TC} \rightarrow \text { Degraded products }$

  (6)

  $ \mathrm{h}^{+}+\mathrm{TC} \rightarrow \text { Degraded products }$

  (7)

  $\mathrm{h}^{+}+\mathrm{H}_2 \mathrm{O} / \mathrm{OH}^{-} \rightarrow \cdot \mathrm{OH}$

  (8)

  $ \text { - } \mathrm{OH}+\mathrm{TC} \rightarrow \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}+\text { Degraded products }$

  (9)

  Figure 8

  

  Figure 8.  VB-XPS spectra of (a) Ti₃AlC₂ and (b) CdS; (c) Schematic mechanism of photocatalytic TC degradation

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  3.   Conclusions

  This study employed a hydrothermal method to synthesise CdS/Ti₃AlC₂ heterostructure photocatalysts, modifying Ti₃AlC₂ MAX, and evaluated their photocatalytic activity for TC degradation under UV-light irradiation. Among the prepared materials, 5%CdS/Ti₃AlC₂ heterostructures exhibited the highest photocatalytic performance for TC, which was significantly superior to that of pure CdS, Ti₃AlC₂, and other doped CdS/Ti₃AlC₂ heterostructures. The enhanced photocatalytic performance of Ti₃AlC₂-based heterostructures under UV light can be attributed to several factors: (1) improved light absorption: the introduction of CdS effectively broadens the light absorption range of Ti₃AlC₂, increasing its response to UV light. (2) Increased efficiency for charge carrier separation: the MAX phase of Ti₃AlC₂, distinguished by its thermal stability and layered structure, has been shown to facilitate charge separation and migration, thereby supporting the construction of efficient heterostructures. The integration of the layered structure of Ti₃AlC₂ with the rod-like structure of CdS facilitates the effective separation of photogenerated electrons and holes, thereby reducing their recombination probability and enhancing photocatalytic efficiency. (3) Intrinsic photocatalytic activity of calcined Ti₃AlC₂: unlike its conventional role as a co-catalyst, calcined Ti₃AlC₂ functions as an independent photocatalyst, exhibiting intrinsic photocatalytic activity for pollutant degradation. Moreover, TEM, PL spectra, and electrochemical studies confirmed that the heterojunction between CdS and Ti₃AlC₂ leads to enhanced charge separation and lower charge-transfer resistance, corroborating the observed photocatalytic performance. In summary, Ti₃AlC₂-based heterostructures demonstrate excellent photocatalytic degradation performance through effective modulation of light absorption and charge carrier separation behaviours. This study provides new insights and theoretical foundations for the employment of Ti₃AlC₂-based heterostructures in water treatment and pollutant degradation applications.

  
  
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  Figure 1  (a) XRD patterns and (b) FTIR spectra of Ti₃AlC₂, calcined Ti₃AlC₂, CdS, and xCdS/Ti₃AlC₂

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  Figure 2  SEM images of (a) Ti₃AlC₂, (b) calcined Ti₃AlC₂, (c) CdS, and (d) 5%CdS/Ti₃AlC₂; (e, f) TEM images of 5%CdS/Ti₃AlC₂; (g) Elemental mapping overlay; (h) EDS spectrum and elemental composition of 5%CdS/Ti₃AlC₂; (i-n) Individual elemental mappings of C, O, Al, S, Ti, and Cd, respectively

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  Figure 3  XPS spectra of CdS, 5%CdS/Ti₃AlC₂, and Ti₃AlC₂: (a) Survey, (b) Cd3d, (c) S2p, (d) Ti2p, (e) Al2p, and (f) O1s

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  Figure 4  (a) UV-Vis DRS and (b) (αhν)²-hν curves of the samples

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  Figure 5  (a) PL spectra under 255 nm excitation, (b) TPR curves, and (c) EIS of calcined Ti₃AlC₂, CdS, and 5%CdS/Ti₃AlC₂

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  Figure 6  (a) Photocatalytic degradation of TC by different catalysts under UV light irradiation and (b) the corresponding kinetics curves; Impact of (c) initial pH, (d) catalyst dosage, and (e) TC mass concentration on the degradation performance of the 5%CdS/Ti₃AlC; (f) Four repetitive cycles of photocatalytic degradation of TC by 5%CdS/Ti₃AlC₂ under UV light irradiation and XRD patterns of 5% CdS/Ti₃AlC₂ for before and after degradation reaction (Inset)

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  Figure 7  (a) Effect of different capture agents on catalyst degradation of TC; EPR spectra of (b) ·O₂^-, (c) ·OH, and (d) h⁺

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  Figure 8  VB-XPS spectra of (a) Ti₃AlC₂ and (b) CdS; (c) Schematic mechanism of photocatalytic TC degradation

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  Table 1.  Recent studies related to the degradation of TC

  Material	Irradiation source	R / %	Ref.
  Calcite/TiO2	UV light	90.0	[49]
  Chitosan/TiO2	UV light	70.0	[50]
  UV/NH2Cl	UV light	90.0	[51]
  MnFe-LDO-biochar/K2S2O8	UV light	98.0	[45]
  W/BaZrO3	UV light	94.9	[52]
  g-C3N4/TiO2	UV light	96.3	[46]
  BiOCl/CdS	Full light	75	[53]
  g-C3N4/silica mesopores	UV light	92.9	[54]
  CuS/CdS	Visible light	90	[48]
  FeNi3/SiO2/CuS	UV light	96.0	[47]
  TiO2@Ti3C2	Visible light	90.7	[35]
  CdS/Ti3AlC2	UV light	96.3	This work

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