S–Modified MXene as a Catalyst for Accelerated Tetracycline Hydrochloride Electrocatalytic Degradation via ·OH and Active Chlorine Triggering Promotion

Abstract

S–modified MXene (Ti₃C₂T_x@S–5) was prepared to improve the catalytic activity of MXene in the electrocatalytic degradation of tetracycline hydrochloride (TC). Here, S groups in the form of Ti–S and S–O species were anchored onto MXene, resulting in superior conductivity and surface activity. Ti₃C₂T_x@S–5 exhibited an excellent performance of 100% TC degradation under the conditions of 25 °C, a pH of 6, a TC concentration of 10 mg L⁻¹, and an applied current of 20 mA. Radical quenching and EPR analyses revealed that ·O₂⁻ and ¹O₂ played dominant roles in Ti₃C₂T_x@S–5 and Ti₃C₂T_x systems. Furthermore, S modification promoted the triggering of ·OH and active chlorine, which contributed to the acceleration of TC degradation. The involvement of these active substances in degradation pathways was further proven. This research advances the S modification of MXene and improves TC degradation by promoting the triggering of ·OH and active chlorine, broadening the applicability of MXene material.

1. Introduction

Anode–dependent electrocatalytic technology, an effective approach to pollution control, is widely used in the advanced treatment of toxic and refractory organic wastewater. In general, the crucial process of the generation of highly oxidative active substrates [1], including hydroxyl radicals, singlet oxygen, and superoxide, determines the performance of electrocatalytic technology and relies on the polarity, surface active sites, and electron transfer capacity of anode materials.

MXenes with conductivities up to 10,000 s cm⁻¹ [2] and high surface areas are distinct from MAX (M_n+1AX_n, where M represents a transition metal; A stands for IIIA or IVA elements; and X indicates C or N) and are regarded as effective anode materials [3]. The terminal edge groups of –F, –O, and –OH, which could adjust the distortion of the MXene structure and consequently influence catalytic performance, are introduced via the chemical etching of IIIA or IVA elements (typically Si, Al, Ge, or Sn). The literature states that Ti₃C₂T_X with an electrophilic terminal –F or –OH group exhibits high surface area, considerable chemical stability, high conductivity, hydrophilicity, and environmental compatibility [4,5,6]. Tang, Q. et al. [7] reported that Ti₃C₂F₂ and Ti₃C₂(OH)₂ monolayers possess various conductive properties that are largely dependent on the spatial arrangement of surface –F and –OH groups. Thus, in consideration of the urgent concerns regarding performance improvement, the modification of terminal edge groups is desired to adjust the properties of MXenes at the molecular level.

Currently, MXenes introduced with –N and –S terminal edge groups exhibit tunable electronic functions and promising capacity for the promotion of properties. The introduction of –N and –S terminal edge groups can not only increase the interlayer spacing of Ti₃C₂T_X but can also effectively improve electrochemical performance. N–doped MXene and S@Ti₃C₂T_x films [8] have all been recently proven to present high conductivity and capacity (489 mA h g⁻¹), excellent cycling performance (415 mA h g⁻¹ after 280 cycles), and desirable Coulombic efficiency (>99%) [9].

Inspired by the attractive contribution of terminal edge group modification, S–modified MXene was constructed as a novel anode catalyst for electrocatalytic degradation. X-ray diffraction (XRD), Raman, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) characterizations were performed with unmodified MXene for comparison to evaluate the changes in the properties of MXene modified via S introduction. The improvement in conductivity and surface activity accounted for the superior degradation performance of Ti₃C₂T_x@S–5. Then, radical quenching experiments and EPR analysis were further conducted to deepen the mechanistic discussion. ·O₂⁻, ¹O₂, ·OH, and active chlorine were identified as participating substances, and the pathway of TC degradation behavior was further proposed.

2. Results and Discussion

2.1. Characterization Results

The literature states that the typical peaks of Ti₃AlC₂ include the (002), (004), (101), (104), (105), (109), and (110) planes at 9.5°, 19.1°, 33.9°, 38.8°, 41.6°, 56.0°, and 60.6°, respectively [10]. Figure 1a shows that after the corrosion of Al layers, Ti₃C₂T_x presented a dramatic leftward shift to 6.5° and the broadening of the (002) plane, implying that the Ti₃C₂T_x structure became distorted with Al etching. The Bragg equation [11] showed that the layer spacing of Ti₃C₂T_x was 1.36 nm, which was greater than Ti₃AlC₂ (0.93 nm). This result further proved that the etching of Al layers induced the expansion of layer spacing. However, the persistence of the characteristic peak (104) of the Al plane indicated that residual Al was present. The Al layers were further etched after modification with S, as proven by the reduction in the (104) Al plane. The slight rightward shift of the (002) peak from 6.5° to 6.9° indicated the differentiation of lattice structure between Ti₃C₂T_x and Ti₃C₂T_x@S–5. Compared with Ti₃C₂T_x, Ti₃C₂T_x@S–5 presented more obvious peaks at 17.8° and 27.3°. These peaks were related to the transformation of Ti₃C₂T_x into TiO₂ [12]. The smaller layer spacing (1.28 nm) of Ti₃C₂T_x@S–5 than Ti₃C₂T_x suggested that S elements had been introduced between layers. As indices of Ti₂S, the small peaks at 36.4° and 41.0° further proved the successful S modification of Ti₃C₂T_x [13].

The defects in the materials caused by S doping were analyzed by using Raman spectra with a wavelength of 523 nm and a power of 75 mW. The results are presented in Figure 1b. Notably, Ti₃C₂T_x retained obvious characteristic a and b peaks after etching, indicating that C–Ti bonds [14] were retained, whereas the Al element was etched away. After S modification, the characteristic a and b peaks intensified and became increasingly obvious. The changes in the strengths of the a and b peaks indirectly proved the etching of Al layers and the decrease in terminal functional groups (–F and –OH). Meanwhile, the peak intensity, at approximately 153 cm⁻¹, belonged to Ti₃C₂T_x MXene oxidation and the formation of the TiO₂ [15]. The two broad peaks at 1350 and 1580 cm⁻¹ were assigned to the D and G bands of graphitic carbon, respectively. The intensity ratio between the D–peak and G–peak (I_D/I_G) can reflect the degree of the defects and graphitization of carbon materials [16]. The I_D/I_G values of Ti₃C₂T_x, Ti₃C₂T_x@S–5, Ti₃C₂T_x@S–25, and Ti₃C₂T_x@S–50 were 0.87, 1.03, 1.02, and 0.98, respectively. The I_D/I_G ratio of Ti₃C₂T_x@S–5 was larger than pure Ti₃C₂T_x. The doping of S atoms created additional defects in Ti₃C₂T_x, resulting in the creation of additional active sites and conductive carbide for rapid charge transfer [17]. However, as the doping amount of S further increased, the I_D/I_G ratios of the materials reduced, likely due to the stacking of S on the surface of Ti₃C₂T_x [18]. To further determine this, the nitrogen adsorption–desorption isotherms (Figure 1c) of Ti₃C₂T_x, Ti₃C₂T_x@S–5, and Ti₃C₂T_x@S–50 were tested, and the pore size distribution (Figure 1d) was calculated from N₂ desorption isothermals. It was found that the BET surface area increases while the pore size decreases with an increase in S doping, suggesting that Ti₃C₂T_x@S–5 can exhibit a larger contact area, presenting a better performance than Ti₃C₂T_x. However, the excessive S (Ti₃C₂T_x@S–50) led to the collapse of the Ti₃C₂T_x, which exhibits a much larger BET surface area and reduced pore size, resulting in fewer channels capable and active sites of performing the reaction, which causes a deterioration of the performance. Therefore, it is believed that the high performance of Ti₃C₂T_x@S–5 is due to the optimal doping of S.

CV and EIS were conducted to investigate the conductivity of the materials further. The results are shown in Figure 1c,d. Ti₃C₂T_x@S–5 presented the largest CV area and the highest peak current among all materials, indicating that the redox and electron transfer capability of Ti₃C₂T_x@S–5 was superior to Ti₃C₂T_x [19]. EIS was further performed to analyze the electron conductivity of the composite materials, as shown in Figure 1d. The Nyquist plots consisted of half arcs in the high–frequency region, which represent the diffusion and migration of ions inside the electrode. A small radius in the high–frequency region is indicative of low charge transfer resistance. The fitting results of the low–frequency region were in a straight line, which reflected internal resistance from the electrolyte and electrode [20]. Ti₃C₂T_x@S–5 had the smallest arc radius among the test materials, further demonstrating its excellent internal resistance [21]. However, extensive S introduction could result in the coverage of conductive carbide for rapid charge transfer, causing a decrease in the arc radii of Ti₃C₂T_x@S–25 and Ti₃C₂T_x@S–50. All of the above CV and EIS results suggested that the redox electron transfer capability of Ti₃C₂T_x@S–5 was superior to the other materials.

The morphologies of Ti₃C₂T_x and Ti₃C₂T_x@S–5 were compared to observe the introduction of S in further detail. Figure 2a,b depict the typical SEM images of the Ti₃C₂T_x and Ti₃C₂T_x@S–5 composites, which all exhibited multilayered structures. In Figure 2a, Ti₃C₂T_x presented stacked layers and a fragmented structure. However, with S modification, the separated layer structure of Ti₃C₂T_x@S–5 became highly obvious, and the agglomeration of Ti₃C₂T_x was significantly inhibited. Thinner and more obvious layers [22] of Ti₃C₂T_x@S–5 were obtained. The TEM image of Ti₃C₂T_x (Figure 2c) presented a clear layered structure with fuzzy edges. By contrast, Ti₃C₂T_x@S–5 appeared as an ultrathin sheet without wrinkles (Figure 2d) and exhibited a larger interlayer spacing of 1.02 nm [23] after S doping than typical Ti₃C₂T_x (0.98 nm). The thin and distinctly layered structure of Ti₃C₂T_x@S–5 confirmed that Al was further etched via S modification, and the expansion of interlayer spacing proved that S had been successfully introduced into Ti₃C₂T_x. The EDS mapping in Figure 2e shows that the Ti, Cl, Al, F, and S elements had been uniformly distributed throughout the Ti₃C₂T_x@S–5 composite, as reflected by the specific data provided (Figure 2f), in which a distribution chart of the total spectrum atomic percentage of Ti₃C₂T_x and Ti₃C₂T_x@S–5 is shown in

Table 1. Distribution chart total spectrum of Ti₃C₂T_x and Ti₃C₂T_x@S–5.

Element	Ti3C2Tx (At%)	Ti3C2Tx@S–5 (At%)
F	49.44	30.90
Al	4.68	1.94
S	0.00	0.99
Cl	1.82	2.06
Ti	44.06	64.11
Total	100.00	100.00

. The content of F and Al elements decreased from 49.44% and 4.68% to 30.90% and 1.94%, respectively, while S increased from 0% to 0.99% with the introduction of S, indicating the successful synthesis of Ti₃C₂T_x@S–5 with an S content of 0.99%.

The materials were subjected to XPS to characterize the changes in elements and chemical bonds in Ti₃C₂T_x with or without S modification. The peaks at 280, 565, 456, 530, and 684 eV in the survey of the XPS spectra of Ti₃C₂T_x and Ti₃C₂T_x@S–5 (Figure 3a) corresponded to C 1s, Ti 2s, Ti 2p, O 1s, and F 1s, respectively. The weaker response of the Al element in Ti₃C₂T_x@S–5 than Ti₃C₂T_x confirmed that Al had been replaced with S and was consistent with the XRD results. The response of F and O was attributed to the terminal edge groups of –F and –OH. However, the response of S in Ti₃C₂T_x@S–5 was unobservable because of the low loading amount of S. The Ti 2p, C 1s, and S 2p spectra were discussed to obtain additional details on S modification. C–C (284.8 eV), C–Ti (281.4 eV), C–O (286.5 eV), C–Al (282.1 eV), and C–F (288.9 eV) were clearly observed [24] in the high–resolution C 1s spectra (Figure 3b) of Ti₃C₂T_x, whereas the C–Al bond diminished in Ti₃C₂T_x@S–5. This result was also proven by XRD. Furthermore, the obvious shifts of the C–C, C–Ti, C–O, and C–F bonds to 284.2, 281.2, 285.6, and 288.4 eV, respectively, indicated that the lattice structure had been modified via S introduction. In addition, the downshifting of the C 1s spectra demonstrated that the proper modification of S was beneficial to attracting free electrons in C [25].

Figure 3c shows the high–resolution Ti 2p spectra of Ti₃C₂T_x. The Ti–O and Ti–C bonds were located at the binding energies of 464.8/458.9 and 461.5/455.5 eV, respectively. The Ti–O and Ti–C bonds had slightly shifted to 464.9/459.0 and 461.0/455.0 eV after S modification, respectively. Furthermore, the appearance of a new Ti–S bond at 462.2 and 456.2 eV confirmed the introduction of S. In the S 2p XPS spectra of Ti₃C₂T_x@S–5 (Figure 3d), S–O and S–Ti–C bonds [26] were observed at 169.8/168.7 and 164.6/163.5 eV, respectively, further validating the XRD results and Ti 2p spectra. Based on the XPS results, S was predicted to react with the exposed Ti sites of Ti₃C₂T_x and form Ti–S bonds, inducing the replacement of some functional groups (–OH and –F) on the surface of Ti₃C₂T_x by –S [27]. The Ti–S bond played a supporting role in maintaining the original structure of Ti₃C₂T_x and prevented the superposition of layers between materials.

2.2. Catalytic Capacity

TC electrocatalytic degradation experiments were conducted with Ti₃C₂T_x, Ti₃C₂T_x@S–5, Ti₃C₂T_x@S–25, and Ti₃C₂T_x@S–50 to demonstrate the function of S groups. Figure 4 shows that after S modification, TC degradation by Ti₃C₂T_x@S–5 had significantly improved compared with Ti₃C₂T_x. This result confirmed the positive contribution of the Ti–S bond to the degradation of TC. After appropriate S modification, Ti₃C₂T_x@S–5 accelerated electron transfer. This effect was conducive to the improvement in catalytic capacity [28]. However, with extensive S loading, the remaining S covered the surface of Ti₃C₂T_x, resulting in a significant decrease in Ti₃C₂T_x@S–50. In other words, excessive S and its uneven distribution hindered TC degradation. This phenomenon was consistent with the CV and EIS results (Figure 1c,d). Therefore, for subsequent experiments on degradation efficiency, Ti₃C₂T_x@S–5 was selected, and the most suitable environmental impact parameters were identified to evaluate the stability and performance of Ti₃C₂T_x@S–5 in TC degradation under various conditions.

The effects of initial concentration, pH, temperature, applied current, and different water bodies were discussed to expand the practical application of the tested materials in the electrocatalytic degradation of TC. TC, with an initial concentration of 5 mg L⁻¹, was completely removed within 20 min (Figure 5a). TC removal efficiency reached 100% and 94.5% at the initial TC concentrations of 10 and 15 mg L⁻¹, respectively. TC degradation efficiency decreased with an increase in temperature due to active chlorine production (Figure 5b). During degradation, Cl⁻ was directly oxidized to generate soluble Cl₂ (aq) (Equation (1)), which can be quickly hydrolyzed into hypochlorous acid (HClO) (Equation (2)). In addition, hypochlorite was unstable and decomposed into hypochlorite ions (ClO⁻) (Equation (3)). The electrode potentials of the three types of chlorine were ranked in descending order as Cl₂ (aq) (1.36 V) > HClO (1.49 V) > ClO⁻ (0.89 V) [29]. TC degradation was significantly promoted by the strongly oxidizing Cl₂ (aq) and HClO but was weakly promoted by ClO⁻. Degradation efficiency decreased when the electrolysis temperature exceeded 25 °C due to the significant decrease in the solubility of Cl₂ (aq) with the increase in temperature. Most Cl₂ escaped in the form of gas [30].

$$2C{l}^{-}\stackrel{ }{\to }C{l}_2\left(aq\right)+2e^{-}$$

(1)

$$C{l}_2\left(aq\right)+H_{2}O\stackrel{ }{\to }HClO+C{l}^{-}+H^{+}$$

(2)

$$HClO\stackrel{ }{\to }Cl{O}^{-}+H^{+}$$

(3)

Figure 5c shows the influence of initial pH on TC degradation. The final TC removal efficiency of Ti₃C₂T_x@S–5 changed slightly with the change in pH. Approximately 100% degradation was obtained at pH 3–9, indicating the applicability of Ti₃C₂T_x@S–5 under acidic and alkaline conditions [31]. When the current was adjusted from 10 mA to 20 mA (Figure 5d), the degradation efficiency of TC increased from 97.4% to 100%. The increase in applied current initially promoted degradation efficiency. Figure 5d illustrates that as the applied current increased, the electron transfer rate increased [32]. The degradation efficiency remained stable (100%) when the applied current was further increased to 30 and 40 mA because the equilibrium between the anode side reaction [33] and reactive species generation under high current maintained the stability of degradation. The efficiencies of TC degradation in various water bodies are reflected in Figure 5e. The efficiencies of TC degradation in distilled water, tap water, and lake water reached 100%, 98.9%, and 86.2%, respectively, within 50 min. The acceptable efficiency exceeded 85%, indicating that Ti₃C₂T_x@S–5 has broad application prospects for TC degradation in actual water bodies. Long–term stability was further estimated, as illustrated in Figure 5f. Cycle tests on the TC system showed that its efficiency was reduced by 20% after the second experiment. Slight differences were observed in the next three cycles. The reduction in catalytic performance after repeated use may be due to the loss of S from the Ti–S bond in Ti₃C₂T_x@S–5.

Ti₃C₂T_x@S–5 and Ti₃C₂T_x before and after the reaction were analyzed via XPS to clarify their mechanism during cycling. As the number of cycles increased, the peaks of F 1s, Ti 2s, and Ti 2p significantly decreased, whereas C 1s drastically increased (Figure 6a,b). Ti 2p, C 1s, and S 2p spectra were further discussed to clarify these changes. The C 1s spectra results show that Ti₃C₂T_x@S–5 still had a Ti–C bond (Figure 6c) compared to Ti₃C₂T_x (Figure 6d) in the second cycle, indicating that the layered supporting effect of the Ti–C bond may enhance the degradation of TC, while Ti₃C₂T_x had already lost the Ti–C bond by the second cycle, which further explained that the degradation efficiency of Ti₃C₂T_x@S–5 remained higher than Ti₃C₂T_x after the second cycle. However, the Ti–C bond disappeared from the C 1s spectrum at the fifth cycle, illustrating the disconnection of the Ti–C bond in the layered structure during degradation. This phenomenon was also confirmed based on the Ti 2p spectra of Ti₃C₂T_x@S–5 (Figure 6e) and Ti₃C₂T_x (Figure 6f). Furthermore, after the second reaction, S species decreased, as reflected in the Ti 2p (Figure 6e) and S 2p spectra (Figure 6g). This finding was consistent with the reduction in degradation efficiency observed after the second cycle and also implied the positive and crucial function of S groups in Ti₃C₂T_x@S–5.

To further demonstrate the high efficiency of Ti₃C₂T_x@S–5 in degrading TC, Ti₃C₂T_x@S–5 and other types of catalysts were compared and shown in

Table 2. Published works of electrocatalytic degradation of TC.

Materials	Reaction Conditions	Initial Concentration of TC	Efficiency	Ref.
rGO/AgCl QDs	0.1 mol/L Na2SO4	20 mg L−1	20.73%@120 min	[34]
EO/PS–MnFe2O4	J = 20 mA cm−2, pH = 4.5, 2 mmol L−1 PS	25 mg L−1	86.23%@60 min	[35]
TiO2 Manotubes	V = 1.0 V, air aeration	15 mg L−1	50%@180 min	[36]
Pd/AG/ITO	I = 8 mA, 0.2 mol L−1 NaCl	12 mg L−1	85.21%@120 min	[37]
TiO2−x–10	J = 10 mA cm−2, 0.075 mol L−1 Na2SO4	50 mg L−1	92.6%@60 min	[38]
Spinel CuxCo1−xMn2O4	J = 20 mA cm−2, pH = 3, 0.05 mol·L−1 Na2SO4	20 mg L−1	91.3%@120 min	[39]
Bi–Sn–Sb/γ–Al2O3	J = 0.1 A·cm−2, pH = 5.9	100 mg L−1	85.9%@180 min	[40]
Ti3C2Tx@S–5	J = 20 mA cm−2, pH = 6, 17 mmol L−1 NaCl	10 mg L−1	100%@30 min	This work

. It can be seen that the Ti₃C₂T_x@S–5 material can achieve 100% degradation efficiency in a shorter time without adjusting the pH in advance, indicating that Ti₃C₂T_x@S–5 has broad application prospects for TC degradation.

2.3. Active Substance Determination

Quenching experiments were conducted, as shown in Figure 7a, to identify the radicals participating in TC degradation triggered by Ti₃C₂T_x and Ti₃C₂T_x@S–5. TBA, benzoquinone, and L–histidine were used as free radical scavengers of ·OH, ·O₂⁻, and ¹O₂, respectively [41,42,43]. The results in Figure 7a illustrate that negligible removal efficiencies were observed in the Ti₃C₂T_x and Ti₃C₂T_x@S–5 systems under interference by benzoquinone and L–histidine. This finding revealed that ·O₂⁻ and ¹O₂ had dominant contributions as main active substances. By contrast, the Ti₃C₂T_x@S–5 system retained approximately 97.0% TC removal efficiency in the presence of TBA, whereas TBA had a negligible inhibitory effect in the Ti₃C₂T_x system. This result indirectly proved the positive contribution of S groups to ·OH generation. The generation of additional ·OH with the introduction of S increased TC removal by approximately 3.0%. Furthermore, the quenching results were confirmed by EPR measurements with DMPO as a spin–trapping reagent of ·OH and ·O₂⁻ and TEMPO as a spin–trapping reagent of ¹O₂ [44,45]. Figure 7b shows that typical sextet peaks [46] of ·O₂⁻ with similar intensities were presented by the Ti₃C₂T_x and Ti₃C₂T_x@S–5 systems, demonstrating the close contribution of ·O₂⁻ in the Ti₃C₂T_x and Ti₃C₂T_x@S–5 systems. However, the weak response of ·OH was covered by ·O₂⁻ and DMPOX signals [47]. In addition, the presence of the typical triple peaks [48] of ¹O₂ with similar intensities confirmed that ¹O₂ was generated from Ti₃C₂T_x and Ti₃C₂T_x@S–5 (Figure 7c). All above quenching and EPR observations demonstrated that ·O₂⁻ and ¹O₂ were produced and were the dominant radicals of TC degradation. These findings, combined with the characterization results of Ti₃C₂T_x and Ti₃C₂T_x@S–5, indicated that the existence of ·O₂⁻ and ¹O₂ was induced by the Ti₃C₂T_x substrate. S groups contributed small amounts of ·OH.

The active species produced from the electrolyte were also taken into account to provide full information on the active species involved in the reaction. The TC removal efficiency in the presence of 1 g L⁻¹ Na₂CO₃, Na₂SO₄, and NaCl electrolytes was examined. Figure 7d shows that within 40 min, the TC removal efficiency reached 100% in the presence of the NaCl electrolyte and reached only approximately 10% within 50 min in the presence of Na₂CO₃ and Na₂SO₄ electrolytes. The superior influence of NaCl on TC removal suggested the generation of secondary active chlorine (such as ClO⁻) from Cl⁻ (Equations (1)–(3)) [49]. The quantitative analysis of active chlorine is presented in Figure 7d. Compared with Ti₃C₂T_x, Ti₃C₂T_x@S–5 consumed active chlorine within 30 min at a faster rate. In addition, the accumulation of active chlorine in the Ti₃C₂T_x@S–5 system was higher than the Ti₃C₂T_x system. These results show that active chlorine is an active species involved in the electrocatalytic degradation of TC [50], and Ti₃C₂T_x@S–5 produces more active chlorine than Ti₃C₂T_x.

2.4. Identification of the Degradation Pathway

The main degradation intermediates were analyzed by LC–MS to identify the specific pathway of the electrocatalytic degradation of TC by Ti₃C₂T_x@S–5. Their detailed data are included in the Supplementary Materials, and the possible pathways of TC degradation are shown in Figure 8.

TC was easily attacked by ·OH, ·O₂⁻, ¹O₂, and active chlorine due to its three high–electron–density functional groups, namely double bonds, phenolic groups, and amino groups [51]. After the TC molecule was attacked by free radicals, the amide end group was removed, and demethylation and dehydration occurred simultaneously, generating intermediates P1 and P5 (m/z = 370 and 382). P2 (m/z = 303) was produced by P1 through dehydration. The central carbon chain of P2 was continuously broken to yield intermediates P3 and P4 (m/z = 193 and 173). Finally, P2 was degraded into inorganic compound compounds, such as CO₂ and H₂O [52,53,54]. In the other path, ·OH can attack the ortho or para position of the phenol ring [55] in P5 (m/z = 382), and demethylation occurred to yield the intermediate P6 (m/z = 278). Subsequently, P7 (m/z = 224) was obtained through simple reactions (ring opening and hydroxyl removal) and ultimately degraded into CO₂ and H₂O.

3. Experimental

3.1. Reagents and Materials

Carbon titanium aluminum (Ti₃AlC₂), tetracycline hydrochloride (TC, 96%), lithium fluoride (LiF), and dibasic potassium phosphate (K₂HPO₄, purity > 99%) for high–performance liquid chromatography (HPLC) were brought from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China) Hydrochloric acid (HCl), sodium hydroxide (NaOH), benzenehexathiol (C₆S₆H₆), sodium chloride (NaCl), sodium sulfate (Na₂SO₄), sodium thiosulfate (Na₂S₂O₃), ethanol (C₂H₅OH), methanol (CH₃OH), acetonitrile (C₂H₃N), tert–butyl alcohol (TBA), L–histidine (C₆H₉N₃O₂), and benzoquinone (BQ) were purchased from Shantou Xilong Scientific Co., Ltd. (Shantou, China).

3.2. Material Synthesis

3.2.1. Synthesis of Ti₃C₂T_x MXene without Hydrothermal Nanosheets

Ti₃C₂T_x was fabricated by acid–etching Al with Ti₃AlC₂ as the raw material. LiF (1.0 g) was added to HCl (9 mol L⁻¹, 40 mL), and the resulting mixture was stirred for 30 min. The mixture was stirred, added to Ti₃AlC₂ MAX powder gradually, and magnetically agitated at 45 °C for 24 h. The obtained mixture was washed with distilled water and centrifuged until the resulting supernatant had a pH of 6.0. Ti₃C₂T_x MXene powder without hydrothermal was obtained by filtering and drying the product.

3.2.2. Synthesis of Ti₃C₂T_x@S Nanosheets

The Ti₃C₂T_x@S complex was synthesized by the hydrothermal method. Ti₃C₂T_x MXene powder was added to distilled water and subjected to ultrasonic treatment for 1 h under nitrogen. A total of 0, 5, 25, or 50 mg of C₆S₆H₆ was dissolved in C₂H₅OH and mixed with the Ti₃C₂T_x MXene suspension under ultrasonication for 3 h under nitrogen. The mixture was placed in an autoclave and reacted at 180 °C for 2 h. Subsequently, the mixture was centrifuged at 1000 rpm for 3 min. The supernatant was then collected and further centrifuged at 3500 rpm for 15 min. The product was filtered and dried to obtain Ti₃C₂T_x, Ti₃C₂T_x@S–5, Ti₃C₂T_x@S–25, and Ti₃C₂T_x@S–50 nanosheets.

3.3. Characterization

Field–emission scanning electron microscopy (SEM, Regulus 8100) and transmission electron microscopy (TEM, Tecnai–G2–F30) combined with energy–dispersive X-ray spectroscopy (EDS) were used to examine the surface morphologies of Ti₃C₂T_x and Ti₃C₂T_x@S. The etching of the catalyst structure was recorded with XRD (D8 Advance) by using Cu Kα radiation. Raman spectra (Renishaw in Via) were applied to characterize the degree of defects in the catalysts. Electrical conductivity was measured by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on electrochemical stations (CHI660E, Chen Hua) with a standard three–electrode cell (with a Pt plate, Ag/AgCl electrode, and carbon rod). Chemical bonds in the catalyst were detected by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250X, Waltham, MA, USA).

3.4. Catalytic Experiment and Analyses

The electrocatalytic experiment used three electrodes: the catalyst as the anode, copper as the cathode, and Ag/AgCl as the reference electrode. The distance between the cathode and anode was 1 cm. A 2 cm × 2 cm carbon cloth with the catalyst was placed in a 60 °C oven for 30 min, and a conductive adhesive was used as the adhesive. The electrolyte was 10 mg L⁻¹ TC and contained 1 g L⁻¹ NaCl. Direct current power was supplied by a PS–12 constant–current rectifier (Eurasian ZTE Technology, Beijing, China). Parallel experiments were conducted to ensure the accuracy of the data of the electrocatalytic experiments.

When the three electrodes were electrified, 2 mL from the 200 mL reaction solution was collected at a certain time point (0, 1, 2, 3, 5, 7, 10, 15, 20, 30, 40, and 50 min), mixed with a quenching agent (50 μL of Na₂S₂O₃), and then loaded into a liquid–phase bottle through a 0.22 μL filter. TC concentration was measured via HPLC (Waters ACQUITY Arc–2695) by using a 2998PDA detector. The reaction conditions were as follows: a mobile phase of 25% C₂H₃N and 75% K₂HPO₄; a C18 column (Waters COR–TECS); a column temperature of 25 °C; a flow rate of 1 mL min⁻¹; and a peak time of within 4 min. C/C₀ was used to represent the degree of TC degradation, whereas C and C₀ represent the concentration at a certain time and the initial concentration, respectively.

Electron paramagnetic resonance (EPR, Bruker EMX–10/12) was used to detect and analyze free radicals with 10 mmol L⁻¹ 5,5–dimethyl–1–pyrrolidine–N–oxide (DMPO) as the ·OH and ·O₂⁻ trapping agent and 2,2,6,6–tetramethyl–4–piperidol (TEMPO) as the ¹O₂ trapping agent. Meanwhile, the quenching experiment was performed to identify active free radicals by adding benzoquinone, L–histidine, and TBA. The concentration of active chlorine at different time points was determined by N, N–diethyl–1,4–phenylenediamine spectrophotometry.

Liquid chromatography–mass spectrometry (LC–MS, Shimadzu) was used to analyze the intermediate degradation products. The pretreatment process of the experiment was the same as the electrocatalytic experiments and had the following specific conditions: a mobile phase of 65% CH₃OH and 35% deionized water; a negative ion scanning mode; an XDB–C18 column (2.1 mm × 150 mm, 5 μm); a column temperature of 30 °C; and a flow rate controlled to 1 mL min⁻¹.

4. Conclusions

In this research, S–doped MXene for the electrocatalytic oxidation degradation of TC was successfully prepared with the hydrothermal method. The electron transfer efficiency and surface activity of Ti₃C₂T_x@S–5 improved with the formation of S–O and Ti–S bonds. The Ti₃C₂T_x@S–5 catalyst achieved 100% degradation efficiency within 50 min under the conditions of 25 °C, pH 6, 10 mg L⁻¹ TC, and 20 mA, and maintained approximately 80% of its degradation efficiency after five cycles. ·OH, ·O₂⁻, ¹O₂, and active chlorine were identified as the main species involved in electrocatalysis. The introduction of S resulted in the positive promotion of ·OH and active chlorine generation. The degradation pathways identified by HPLC–MS analysis further demonstrated the contribution of ·OH, ·O₂⁻, ¹O₂, and active chlorine to TC treatment.