Titanium Carbide Composite Hollow Cobalt Sulfide Heterojunction with Function of Promoting Electron Migration for Efficiency Photo-Assisted Electro-Fenton Cathode

Abstract

Constructing heterostructure within electrocatalysts proves to be an attractive approach to adjust the interfacial charge redistribution to promote the adsorption of reactive species and accelerate the charge transfer. Herein, we present the one-pot solvothermal synthesis of Ti₃C₂ supported hollow CoS₂/CoS microsphere heterostructure with uneven charge distribution as the cathodic catalyst, which displays a superior quasi-first-order degradation rate (0.031 min⁻¹) for sulfamethazine (SMT) in photo-assisted electric–Fenton (EF) process. CoS₂/CoS/Ti₃C₂ is proven to favor the 2e⁻ oxygen reduction reaction (ORR), with H₂O₂ selectivity up to 76%. The built-in potential present in the heterojunction helps to accelerate electron transfer, thus promoting the production of H₂O₂. Subsequently, H₂O₂ is rapidly activated to produce ∙OH due to the synergistic effect of Co and S. Notably, CoS₂/CoS/Ti₃C₂ exhibits enhanced photo-assisted EF (PEF) performance under light. The excellent photocatalysis properties of CoS₂/CoS/Ti₃C₂ are attributed to that the unique hollow microsphere structure of catalyst improves the light absorption, and the uneven charge distribution of CoS₂/CoS heterojunctions promotes the separation of photo-generated holes and electrons. Given the above advantages, CoS₂/CoS/Ti₃C₂ cathode delivers a high degradation rate of 98.5%, 91.8%, and 94.5% for SMT, bisphenol A, and sulfadiazine, respectively, with TOC removal efficiency of 76% for SMT with 120 min. This work provides a novel light of the design and construction of efficient PEF cathodes for the treatment of antibiotic wastewater.

1. Introduction

The environmental problems caused by antibiotics and organic pollutants have become more serious with the progress of human activities, however, conventional water treatment methods are difficult to achieve effective removal of such pollutants [1]. As a kind of electrochemical advanced oxidation processes (EAOPs), Electro-Fenton (EF) is a prospective water pollution treatment method [2] that generate H₂O₂ through the 2e⁻-ORR by in-situ reduce O₂ at the cathode [3], and then the ∙OH can be produced by catalysis of H₂O₂. However, the rapid formation of ∙OH is usually limited by the yield, selectivity, and reactivation of H₂O₂ [4]. Therefore, the rapid generation and activation of H₂O₂ is the key technology for efficient EF reaction [5]. Iron-based catalysis, including FeS, FeS₂, etc., have been proven to have EF properties for activating H₂O₂, however, the tepid H₂O₂ yield greatly hinder the efficacy of EF technology [6]. One of the common solutions is combination metal-based materials with carbonaceous carriers because of that the oxyfunctional groups and defect sites of the carbon materials provide sufficient approach for the 2e⁻ ORR [7,8,9,10]. However, the large overpotential and long-term operation of the metal-carbon carrier cathode may cause the metal particles to fall off and carbonaceous carrier poisoning. Additionally, atomic precision doping or defect modulation of carbonaceous carriers may require higher costs. Therefore, it is necessary to find a suitable bifunctional catalyst with efficient 2e⁻ ORR selectivity and Fenton performance [11,12,13,14].

Cobalt sulfide has a variety of chemical formulas (such as CoS₂, CoS, Co₉S₈, Co₄S₃), which have emerged as low-cost and abundant content materials with excellent catalytic activity based on both computational and experimental studies [15,16,17,18]. Among them, the metal cobalt pyrite (CoS₂) exhibits excellent 2e⁻ ORR activity to produce H₂O₂ due to the single porphyrin structure of CoS₂ inhibiting the O–O bond breaking of and promoting the production of H₂O₂, and the high 2e⁻ ORR selectivity of CoS₂ is expected to serve as a high efficiency catalyst in EF reaction [16]. Additionally, hexagonal cobalt sulfide (CoS) is commonly used in Fenton-like fields because of narrow band gap structure and efficient activation of H₂O₂ performance, which is attributed to that CoS can provide suitable Lewis acid strength to adsorb and catalyze H₂O₂ to produce ∙OH [17]. Besides, the synergistic effect of Co and S effectively promotes electron transfer and accelerates the recycling of cobalt active species [18,19]. It is well known that cobalt sulfide with different formulas can interact with each other to form heterojunction with two opposite charge space field and built-in electric field area [20,21,22], which will promote the formation of uneven charge distribution. This uneven charge distribution may be able to facilitate the charge carrier transport in the photo-assisted electric–Fenton (PEF) process, thus exhibiting enhanced photo-assisted electrocatalytic activity [23]. However, the application of cobalt sulfide in electrocatalysis is always limited in its general conductivity. Transition metal carbide (Mxenes), as a novel two-dimensional material, exhibits excellent electrical conductivity (Titanium carbide(Ti₃C₂)~9880 S cm⁻¹) [24,25] and highly exposed metal sites, which has great potential in the field of photocatalysis and electrochemistry, such as li-ion battery, supercapacitors, and photocatalytic hydrogen production [26,27,28,29]. There are reports that Ti metal at the end of titanium carbide exhibits poor oxygen resistance due to the high amount of exposed metal atoms, which promotes the regeneration of active sites [28]. In addition, the Schottky junction formed between Ti₃C₂ and metal semiconductors can suppress the photo-generated holes and electrons recombination, which also helps to enhance the photo-assisted EF performance [30,31,32]. Therefore, the combination of hollow heterostructure CoS₂/CoS with Ti₃C₂ is meaningful for maintaining the 2e⁻ ORR behavior of CoS₂ and Fenton behavior of CoS while optimizing activity, electrical conductivity, and sustainability of CoS₂/CoS.

In this work, heterojunction CoS₂/CoS nanocomposites with Ti₃C₂ were prepared by simple hydrothermal synthesis. CoS₂/CoS/Ti₃C₂ cathode displayed a superior degradation performance for sulfamethazine (SMT) with degradation rates of 0.031 min⁻¹. Rotating disk electrode (RDE) tests exhibit that CoS₂/CoS/Ti₃C₂ has excellent 2e⁻ ORR characteristics, which contributes to promote the rapid production of H₂O₂. In addition, the uneven charge distribution accelerates the migration of charge carriers to facilitate photo-assisted EF process. UV-Vis DRS proves that CoS₂/CoS/Ti₃C₂ has strong light absorption in the visible and UV regions, and the PL spectrum demonstrates that CoS₂/CoS/Ti₃C₂ is provided with excellent charge-hole separation efficiency, which is attributed to the excellent electron-withdrawing characteristics of Ti₃C₂, as well as the heterostructure in CoS₂/CoS. The total organic carbon (TOC) removal rate of SMT within 120 min is as high as 76%, and the possible degradation pathways of SMT are given based on UPLC-M-QTOF. The above work indicates that CoS₂/CoS/Ti₃C₂ as a novel electrode material enriches the choice of available cathode materials and offers a new way for the degradation of persistent pollutants in the PEF process.

2. Results

The synthesis of CoS₂/CoS/Ti₃C₂ involved of CoS₂/CoS and subsequent CoS₂/CoS/Ti₃C₂ (Figure 1). The prepared Ti₃C₂ was distributed in tetramethylammonium hydroxide (TMAOH) solution and processed with ultrasonic waves, and the exfoliated Ti₃C₂ was added to the solvothermal process of CoS₂/CoS/Ti₃C₂. Finally, hollow CoS₂/CoS grown in situ on the surface of Ti₃C₂. For comparison, the CoS₂, CoS₂/CoS, and CoS were obtained under the conditions of V_EG:V_DMF = 0.5, 0.9, and 2, respectively.

As shown in Figure 2a, all characteristic peaks could be well attributed to the diffraction peaks of CoS (PDF#65-3418) and CoS₂ (PDF#89-1492) [33] while V_DMF:V_EG = 0.5 and 2. The mixed phase of CoS₂ and CoS is obtained when the value of V_EG:V_DMF is 0.9 [22,34]. The characteristic peaks of Ti₃C₂ (36.4°) can be found in CoS₂/CoS/Ti₃C₂. As shown in Raman spectra (Figure 2b), the peaks at ~293 cm⁻¹ (E_g), and ~393 cm⁻¹ (A_g) match well with the value of CoS₂ in reference [22,35]. The peaks at ~475, and ~670 cm⁻¹ represent the E_g, and A_1g pattern of CoS [36], respectively. Both the characteristic peaks of CoS₂ (~392.5 cm⁻¹) and CoS (~476.0 and 679.0 cm⁻¹) can be discovered in the pattern of CoS₂/CoS/Ti₃C₂. It is worth noting that the peaks corresponding of CoS₂ and CoS are red-shifted and blue-shifted in heterojunction, respectively. The opposite shifts of CoS and CoS₂ characteristic peaks in Raman indicates the creation of a built-in potential region due to the production of heterojunctions, which help to promote electrons transport and enhance catalytic performance.

The morphology of Ti₃C₂ before HF etching shows a multi-layered bulk structure (Figure S1). SEM images of exfoliated Ti₃C₂ and CoS₂/CoS/Ti₃C₂ are detected in Figure 3. The Ti₃C₂ peeled off by the intercalation method showed a uniform monolithic structure with slight wrinkles on the surface. With CoS₂/CoS compounded with Ti₃C₂, CoS₂/CoS is evenly dispersed on the surface of Ti₃C₂ (Figure 3b,c), which is due to the excellent dispersing effect of lot of terminating groups on Ti₃C₂ surface. More interestingly, the particles on the surface of Ti₃C₂ show typical hollow characteristics at high magnification (Figure 3d). The TEM of the CoS/CoS₂ heterostructure showed the characteristics of hollow microspheres (Figure 4a), and the diameter of the microspheres is about 50 nm, and the particle size is relatively uniform. The wrinkled Ti₃C₂ lamella can be seen in Figure 4b,c, which demonstrated that the successfully combination of CoS/CoS₂ and Ti₃C₂. Plain stripe spacing of 0.248 nm, 0.279 nm can be observed in the high-resolution TEM picture (Figure 4d), which corresponds to the plane (200) and (210) of CoS₂, and interplanar spacing of 0.193 nm and 0.252 nm can be assigned to the plane (102) and (101) of CoS [22,33]. The clear lattice stripes and the absence of obvious amorphous regions at the interface between CoS₂ and CoS confirm the formation of heterojunctions (Figure 4e). The overlapping patterns of diffraction rings of CoS₂/CoS indicated both the formation of heterostructure and its good crystallinity (Figure 4f). Therefore, the heterogeneous structure of CoS₂/CoS was successfully prepared. The element mappings of TEM and SEM demonstrates that Co and S mainly exist on the microspheres, and S and Co are uniformly distributed on the Ti₃C₂ surface, indicating that cobalt sulfide and Ti₃C₂ form a good composite structure (Figure 4g).

Without the addition of Ti₃C₂, CoS₂/CoS exhibits the characteristics of aggregated particles (Figure S2a). Interestingly, the addition of Ti₃C₂ can form uniformly dispersed hollow microspheres. This difference [37] may be caused by the following reasons. Firstly, the functional group of Ti₃C₂ have a clear tendency to coordinate with Co²⁺ ions to generate the stable chelate complexes. The effect of Ti₃C₂ is commanding the release speed of Co²⁺ via the coordinating effect of Ti₃C₂-Co²⁺ chelate composite, thus slowing down deposition rate of CoS₂ and CoS to facilitate the regulation of CoS₂/CoS hollow microparticles. In the initial step, the sulfur powder in solution is converted by the reduction of urea, which contributes to the rapid acquisition of soft-template S-core at ambient temperature. At the same time, Co²⁺ is uniformly dispersed on the surface of S-core. As the reaction proceeds, CoS₂ and CoS with interaction could be generated after crystal growth overcoming the anisotropy, thereby successfully introducing the heterogeneous structure. With the rise of reaction temperature, the S-core fades away, and the internal cavity becomes apparent, resulting in the formation of CoS₂/CoS hollow microspheres with heterogeneous structure. Some examinations have shown that the hollow structure can exhibit a kinetically good open surface structure and a shorter mass and charge transport diffusion path [37]. Therefore, while this unique structure provides more active sites, the shorter mass and charge diffusion paths accelerate the ORR process of H₂O₂ synthesis and the subsequent Fenton process, thereby effectively improving the degradation efficiency. In addition, the multiple reflection of light in the hollow structure can also improve the efficiency of light utilization, which may improve the light absorption performance, thereby enhancing the PEF process.

The corresponding characteristic peaks of S 2p and Co 2p are displayed in CoS, CoS₂, CoS₂/CoS, and the peaks of Ti 2p could also be noticed in CoS₂/CoS/Ti₃C₂ (Figure 5a). The high-resolution Ti 2p spectrum of CoS₂/CoS/Ti₃C₂ exhibits peaks at ~464.8, ~458.5 eV corresponds to Ti-O (Figure 5b), and the peak centered at 472.0 eV could be assign to Ti-F, indicating the existence of Ti₃C₂ [28]. For the Co 2p spectrum, two distinct peaks can be found at binding energy of ~779 and 797.5 eV, indicating the presence of Co²⁺, which can be attributed to Co-S [33,34,37,38]. Two correlative characteristic lines at 782.5 and 802.4 eV can be attribute to satellite peaks of Co²⁺ and Co³⁺ (Figure 5c). In order to gain a clearer comprehending of the formation of CoS₂/CoS heterojunctions as well as the charge distribution at the interface, S 2p was further investigated. The characterized S₂²⁻ peaks at 162.3, 163.5 eV and the S²⁻ at 161.5, 162.7 eV can be observed in CoS₂ and CoS, respectively (Figure 5d). It is not difficult to find that the binding energy of S 2p_3/2 (S²⁻) in CoS₂/CoS structure move mildly to a lower energy about 0.1 eV than that in CoS because of additional negative charges in CoS side, in contrast, the energy of S₂²⁻ is marginally higher than that in CoS₂ [39], therefore, the distribution of opposite charges in CoS₂/CoS and CoS₂/CoS/Ti₃C₂ can be determined because of the electrons shift of S 2p_3/2 in the opposite direction.

The influence of different cobalt sulfide crystal phases and composites on EF property was discussed in Figure 6a. 10 mg/L antibiotic solution in 0.05 mol/L Na₂SO₄ was considered as degraded pollutant and different samples loaded on carbon cloth (CC) as the cathodes. Note that CoS₂/CoS/Ti₃C₂ displays the highest attenuation performance (~67%, 90 min) under the condition of electricity without light, which is higher than that of CoS (~45%,90 min), CoS₂ (~56%, 90 min), and CoS₂/CoS (~60%, 90 min). This result may be assigned to the excellent synergy between CoS₂, CoS and Ti₃C₂, which significantly improves the transfer efficiency of charge carriers in 2e⁻ ORR, thereby helping to accelerate the formation of ∙OH. pH value generally has a significant impact for PEF systems involving transition metal-based catalyst. The catalytic performance of CoS₂/CoS/Ti₃C₂ at different pH is shown in Figure 6b. As the initial pH decreases from 9 to 3, the removal performance of SMT improves from 48% to 80.5%. CoS₂/CoS/Ti₃C₂ at pH = 3, 5, 7, 9 displays completely different attenuation efficiencies for SMT, which is helpful to show new viewpoint of the influence of catalyst on the efficiency of pollutants, as well as the formation of active components. CoS₂/CoS/Ti₃C₂ shows a well documented prevalence of pH values, with similar attenuation of organic compounds at pH of 5–9. The removal efficiency at pH = 3 is significantly higher than that at other pH conditions due to the lowered oxidation potential of ·OH at high pH values (2.6–2.8 eV at pH = 3 and 1.90 eV at pH = 7).

The influence of catalyst load on degradation performance is shown in Figure 6c. With the increase in catalyst dosage from 3 mg/cm² to 7 mg/cm², the degradation rate of SMT improved from 73% to 98.4% (under electricity without light, 120 min, SMT = 10 mg/L). An increase in the catalyst loading capacity brings about an increase in the number of active sites, which contributes to improved degradation performance of SMT. The effect of current is investigated in Figure 6d. When the current intensity reaches 20 mA/cm², the EF performance (89%) of the CoS₂/CoS/Ti₃C₂ is significantly higher than that of 10 mA/cm² (43%) and 5 mA/cm² (24%), which is due to that the increase in current can provide more cathode electrons, thereby enhancing the oxygen reduction activity of the material. However, with the further rise of current intensity, the degradation rate of SMT did not show a significant increase, which may be associated with the occurrence of H₂O₂ decomposition at high currents. The synergistic effect of photo on EF is shown in Figure 6e, compared to the EF process without light, the removal performance of SMT increased by 10% within 120 min, and SMT decay rate can reach 98.5% under PEF conditions, and the reactions rate is as high as 0.031 min⁻¹ (Figure S4), which is better than that of part reported cathodes (Table S1). Moreover, under the condition of light-only, the removal rate of CoS₂/CoS/Ti₃C₂-cathode can reach 34.5% within 120 min, which is higher than that of CoS₂/CoS (28.3%) and CoS (22.0%) under the same conditions. This result indicates that CoS₂/CoS/Ti₃C₂ has better photocatalytic properties, which may be related to the formation of the built-in potential in CoS₂/CoS and Schottky junctions between cobalt sulfide and titanium carbide that facilitates the transmission of photo-generated electrons. The removal capacity of different substrates was also examined, which may be directly related to practical applications. As shown in the Figure 6f, sulfadiazine (SDZ) and bisphenol A (BPA) showed similar removal performance as SMT, and the reaction rate constants for SDZ and BPA are 0.023 and 0.021 min⁻¹, respectively, indicating that the CoS₂/CoS/Ti₃C₂-cathode is suitable for the removal of antibiotic contaminants and has good potential for industrial applications. All of these results show that CoS₂/CoS/Ti₃C₂ has excellent photo-assisted EF performance.

To further illustrate the mechanism of the PEF process, we drop-casted CoS₂, CoS₂/CoS, and CoS₂/CoS/Ti₃C₂ nanomaterials on disk ring-electrode without carbon addition and tested the materials selectivity and activity for two-electron oxygen reduction in alkaline solutions (KOH, 0.1 M) without interference from carbon [16]. The electrocatalytic activity was examined by CV in O₂ saturated electrolyte for all three materials (Figure 7a). Compared with CoS₂ (−1.08 V) and CoS/CoS₂ (−1.05 V), the larger oxygen reduction potential (−1.0 V) and double-layer current value of CoS/CoS₂/Ti₃C₂ indicate its better activity, which may be due to the presence of Ti₃C₂ and built-in potential in CoS₂/CoS accelerate the charge transfer, thereby accelerating the formation of OOH* (Equation (2)), and OOH* reacts with H⁺ to form H₂O₂ as the Equation (3). The oxidation peaks at −0.55 V can be ascribed to the oxidation of Co²⁺, and Co³⁺ was further converted to Co²⁺ when it became electrons at the cathode [13]. As the oxidation of Co(II), the sulfide with metal defects is formed on the catalyst surface, which can be further oxidized to generate sulfur intermediate products (S₂O₃²⁻, S₂O₄²⁻), thus, the peaks at 0.166 V could be due to the formation of intermediate sulfides, and these sulfur oxides could be further activated to produce sulfate radicals [40,41]. These results prove that CoS₂/CoS/Ti₃C₂ has excellent redox activity, which can be used as the EF cathode to facilitate the produce of active species. The electron transfer capability of the material was studied by Nyquist plots. The impedance test results are shown in Figure 7b, compared with pure CoS and CoS₂, and CoS₂/CoS significantly reduces the electron transfer resistance, which is attributed to the built-in potential in CoS₂/CoS heterojunction helps to increase the charge mobility. After compounding with titanium carbide, the conductivity of CoS/CoS₂/Ti₃C₂ material is further improved, which is due to the excellent conductivity of titanium carbide [22].

LSVs of all the catalysts at different speed are depicted in Figure S3a–c. According to the Koutecky-Levich (K-L) formula, the number of electrons transfer (n) for the ORR is calculated (Text S1) [28]. The number of electrons transfer is calculated to be 2.85, 2.12, and 2.22 for CoS₂, CoS₂/CoS, and CoS₂/CoS/Ti₃C₂, respectively, demonstrating that CoS₂/CoS/Ti₃C₂ has a tendency to 2e⁻ ORR process, which is attributed to that built-in potential formed by the heterostructure helps to accelerate the electron migration rate, thus promoting the formation of OOH* on the CoS₂ surface. Although the number (2.22) of electron transfer after composite Ti₃C₂ is reduced according to the K-L equation, CoS₂/CoS/Ti₃C₂ exhibits a positive onset potential of −0.13 V vs. SCE (Figure S3d), which is higher than that of CoS₂/CoS (−0.16 V), indicating that Ti₃C₂ can enhance the electrocatalytic activity of CoS₂. In addition, rotating ring disk electrode (RRDE) tests of CoS₂/CoS/Ti₃C₂-casted disk from 0.2 V to −1 V vs. SCE at 50 mV/s and different rotational speed were recorded in Figure 7c, and corresponding H₂O₂ selectivity (p) and the n value are calculated on the basis of Equations (S3) and (S4). The H₂O₂ selectivity of CoS₂/CoS/Ti₃C₂ up to about 76% and remained above 70% over a wide potential range (Figure 7d), and the polarization curves of the CoS₂/CoS/Ti₃C₂ show about 0.75 V vs. RHE of half-wave potential, which is close to thermodynamic limit of two-electron ORR. Apart from the high activity, the stability tests of the material are shown in Figure 7e, with a ring voltage setting of 1.48 V and disc voltage of 0.5 V, the i-t test of CoS₂/CoS/Ti₃C₂ material showed excellent stability with a stable H₂O₂ selectivity of about 70% within 2 h. The above results indicate that CoS₂/CoS/Ti₃C₂ has excellent 2e⁻ ORR activity and stability, and 2e⁻ ORR is uptaken at low overpotentials.

The photo adsorption properties of different samples were analyzed by UV-Vis DRS (Figure 8a). CoS exhibits strong light absorption in the ultraviolet and visible regions (edge of approximately 450 nm and 280 nm), and CoS₂/CoS and CoS₂/CoS/Ti₃C₂ display stronger absorption in the scope of 280–700 nm. The tauc plot method (Figure 8b) is used to fit the light absorption value as the Equation (1). The band gap of CoS₂/CoS/Ti₃C₂ (~1.95 eV) is slightly lower than that of CoS (~2.25 eV) and CoS₂/CoS (~2.06 eV), which indicates that the heterostructure of CoS₂/CoS and the schottky junction formed under the modification of Ti₃C₂ broaden the light absorption range of the material, which helps to absorb more visible light to further improve the catalytic activity. Furthermore, the photocurrent curve (Figure 8c) is used to analyze the electron-hole separation efficiency of the materials. Compared with CoS and CoS₂/CoS, CoS₂/CoS/Ti₃C₂ show higher photocurrent intensity and stronger photo response performance, which is attributed to the electron withdrawing effect of Ti₃C₂ facilitating the separation of photo-generated holes and electrons [42], moreover, the built-in potential in the heterojunction also helps to accelerate the transfer of electrons. PL spectrum (Figure 8d) also proves this fact, and the lowest PL peak of CoS₂/CoSTi₃C₂ indicates that photo-generated electron recombination is well suppressed, which helps to extend the life of photo-generated carriers and holes, thereby improving photocatalytic activity. There is a significant gap in the removal capacity of SMT under light (98.5%) and dark (89%) conditions, respectively (Figure 6e). Under the condition of only light, the effective removal of SMT is about 34.5% within 120 min. The above facts illustrate that CoS₂/CoS/Ti₃C₂ have excellent photo-assisted electrocatalytic activity.

$${(\mathsf{ɑ}\mathrm{h}\mathsf{\upsilon })}^{1/\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)$$

(1)

$${\mathrm{O}}_2+*+\left({\mathrm{H}}^{+}{+\mathrm{e}}^{-}\right)\to \mathrm{OOH}*$$

(2)

$$\mathrm{OOH}*+\left({\mathrm{H}}^{+}{+\mathrm{e}}^{-}\right)\to {\mathrm{H}}_2{\mathrm{O}}_2+*$$

(3)

Electron paramagnetic resonance (EPR) was conducted by the spin trap agents of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to confirm the existence of various reactive oxide species in this reaction process. The result is shown in Figure 9a, and weak DMPO-∙OH signal can be observed under only light conditions, which is attributed to the interaction of photogenerated electrons with ∙O₂⁻ to produce ·OH as the Equations (4)–(7). Therefore, the removal of SMT under light conditions alone may be attributed to the ∙O₂⁻ (−0.33 eV). After turning on the power, obvious signals of four peaks with the intensity value of 1:2:2:1 (DMPO-·OH) adducts can be found in the EPR spectrum [43], indicating that the existence of ·OH in CoS₂/CoS/Ti₃C₂ system under PEF condition. Furthermore, the weak characteristic peaks of DMPO-SO₄⁻ also could be detected, which is due to the oxidation of S²⁻ and SO₄²⁻ in the solution to sulfur oxides (such as S₂O₃²⁻, S₂O₄²⁻), and then sulfur oxides could be further activated to produce SO₄⁻, and the high oxidation potential (2.7~3.1 eV) of SO₄⁻ can also effectively promote the degradation of the substrates (Figure 9b). Six characteristic peaks of DOMP-·O₂⁻ compounding are observed, suggesting the production of ·O₂⁻ under the action of light and electricity (Figure 9c). Quenching experiments were performed to clarify the contribution of different ROS in this PEF system. TBA and MeOH act as the trapping agents of ·OH and SO₄⁻, respectively. Figure 9d showed that SMT decay rate is significantly inhibited when TBA (100 mM) is added, and the degradation performance is as low as 50.5% in 120 min, indicating the major roles of ·OH in reaction system. In the presence of MeOH (100 mM), the decay rate of SMT still remains around 71.2%, indicating that SO₄⁻ has a relatively low contribution for degradation process, therefore, the ∙OHs are critical to the degradation performance in the CoS₂/CoS/Ti₃C₂ system. Anti-ion interference experiments were also carried out, as shown in Figure S6. It can be seen that NO₃⁻ and Cl⁻ have little effect on the degradation process of SMT, however, the presence of HCO₃⁻ makes the removal efficiency of SMT slightly lower (about 20%), but it is undeniable that even in the presence of anions, the removal rate of SMT is still more than 70%, which indicates that the CoS₂/CoS/Ti₃C₂ cathode system has a better ability for resistance to ion interference. Based on the above results, the CoS₂/CoS/Ti₃C₂ cathode system can effectively produce active radicals under EF conditions, and the degradation performance was further enhanced by the addition of light. Compared with FeS₂NWs/Ti₃C₂ cathode 28, CoS₂/CoS/Ti₃C₂ showed higher 2e⁻ ORR catalytic activity, higher TOC removal rate (76%), and greater PEF catalytic performance owing to the uneven charge distribution of CoS₂/CoS heterojunction. Moreover, due to the excellent 2e⁻ ORR properties of the CoS₂/CoS/Ti₃C₂ material, the addition of reduced graphene oxide (RGO) is effectively avoided.

The stability and long-term runnability of the cathode are essential factors in assessing the ability of the material to perform in real-world applications. The removal efficiency of SMT by the CoS₂/CoS/Ti₃C₂ cathode after five cycle tests is well preserved (Figure 9e), decreasing by about 14.5% compared with that for the first cycle. The main reasons for the performance decay of the CoS₂/CoS/Ti₃C₂ cathode may be that deactivation of functional group of Ti₃C₂ due to the occupation of the active site by contaminants, and quality loss of CoS₂/CoS/Ti₃C₂ due to sulfide being further oxidized to sulfur oxides (such as S₂O₃²⁻, S₂O₄²⁻) [40,41]. Despite this, the performance remained around 85% after five cycles, indicating its excellent stability. Figure 9f shows the variation of total organic carbon content with time in SMT degradation process. As the reaction time increased from 30 to 120 min, the mineralization rate of SMT by the CoS₂/CoS/Ti₃C₂ cathode PEF system increased from 31% to 76%. The high mineralization rate is ascribed to the oxidation of reactive oxygen species (·O₂⁻, ·OH, SO₄⁻). Stability of the catalysts in water treatment system is crucial for their engineering applications. To further reveal the element features of the used catalyst, XRD and XPS of used CoS₂/CoS/Ti₃C₂ were presented in Figure S5. The XRD feature of the used catalyst still has the characteristic peaks of cobalt sulfide, but the corresponding peak intensity is reduced, which may be related to the adsorption of binder polymers and contaminants on the catalyst sites. Moreover, the reusability of the electrode was confirmed through the XPS results of the electrode before and after cycle. The XPS spectrum of Figure S5b shows that the used catalyst has similar characteristics to the fresh CoS₂/CoS/Ti₃C₂ catalyst, and the characteristic peaks corresponding to Co 2p and S 2p are weakened (Figure S6c,d), which may be due to the inevitable ion leaching and sulfide being further oxidized to intermediate sulfur oxides in acidic conditions, and then sulfur oxides could be further activated to produce SO₄⁻ to promote the degradation of the SMT. Thus, CoS₂/CoS/Ti₃C₂ can be considered as an effective PEF cathode material for more thorough removal of pollutants.

$${\mathrm{CoS}}_2{/\mathrm{CoS}/\mathrm{Ti}}_3{\mathrm{C}}_2+\mathrm{h}\mathsf{\upsilon } \to { \mathrm{h}}^{+}{+\mathrm{e}}^{+}$$

(4)

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

(5)

$$·{\mathrm{O}}_2^{-}{+\mathrm{e}}^{-}{+2\mathrm{H}}^{+} \to { \mathrm{H}}_2{\mathrm{O}}_2$$

(6)

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{h}\mathsf{\upsilon } \to 2·\mathrm{OH}$$

(7)

To clarify the degradation pathway of SMT in the CoS₂/CoS/Ti₃C₂ system, the transformation products of SMT were analyzed by QTOF LC/MS (UPLC-M-QTOF). The proposed intermediate compounds are shown in Figure S7. SMT was attacked by by ∙O₂⁻, SO₄⁻ and ∙OH to form intermediate products, and the reaction pathway (Figure 10) included (1) oxidation process of aniline group in SMT to produce N-(4-methylpyrimidin-2-yl)-4-(hydroxyamino)-benzenesulfonamide (P1, m/z, 281.0) and N-(4-methylpyrimidin-2-yl)-4-nitrobenzene-sulfonamide (P1-1, m/z, 296.1). (2) The attack of ROS could lead to the cleavage of S−N bond to form 4-methylpyrimidin-2-amine (P2, m/z, 110.2) and 4-aminobenzenesulfonic acid, which was further oxidized to form 4-nitrobenzenesulfonic acid (P2-1, m/z, 203.1) [44], and this result was frequently reported in AOPs-based water treatment processes for the degradation of sulfonamide antibiotics [45]. (3) Smiles-type rearrangement of SMT to generate 4-(2-imino-4-methylpyrimidin-1(2H)-yl) aniline, which was further oxidized to form 4-methyl-1-(4-nitrosophenyl) pyrimidin-2(1H)-imine (P3, m/z, 214.9) and 4- methyl-1-(4-nitrophenyl) pyrimidin-2(1H)-imine, (P3-1, m/z, 232.9). (4) The SO₂ extrusion of SMT or the oxidation of P3-1 resulted in the formation of nitrobenzene (P4, m/z, 124.9), which could be further oxidized to phenol (P4-1, m/z, 94.9). The intermediate products generated above will become smaller molecules through dealkylation and ring-opening reactions under the action of ROS, and ultimately, the above intermediates could be partially mineralized to carbon dioxide and water.

Given the above results, mechanism of CoS₂/CoS/Ti₃C₂ cathode for removal of micro-pollutants was proposed. The energy of E_VB and E_CB of CoS₂/CoS/Ti₃C₂ are calculated according to Equations (8) and (9) [46]. According to the calculation results, the values of E_VB and E_CB are 1.79 eV and −0.46 eV, respectively. Under visible light irradiation, electrons are generated in CB and holes are generated in VB of CoS. Due to the electron-absorbing properties of Ti₃C₂, the conducting electrons of CoS are further transferred to the surface of it, which facilitates the separation of holes and electrons. The electrons in the Ti₃C₂ could reduce O₂ to generate ·O₂⁻ (−0.33 eV) [42]. At the same time, the continuously generated e⁻ and h⁺ could make the CB and VB bend upward, establishing Schottky barriers through the contact interface and facilitating the separation of photo-generated electrons and holes [28,46]. In addition, O₂ is rapidly converted to produce H₂O₂ on the CoS₂/CoS/Ti₃C₂ surface as Equations (5) and (6). The generated H₂O₂ is then activated on the surface of CoS₂/CoS/Ti₃C₂ and participated in the activated process to generate ·OH as the Equation (10). Due to the synergistic effect of S²⁻ and Co, Co(II) can be regenerated on the surface of the cathode (Equation (11)) [18,19]. In addition, decomposition of H₂O₂ (Equation (7)) can be improved under irradiation. Sulfate in the solution can be oxidized to form persulfate, which is further activated to produce sulfate radicals [13, 41,42]. Therefore, the ROS produced in the PEF process could attack functional groups of contaminants quickly and effectively.

$${\mathrm{E}}_{\mathrm{VB}}=\mathsf{\chi }-{\mathrm{E}}_{\mathrm{e}}+0{.5\mathrm{E}}_{\mathrm{g}}$$

(8)

$${\mathrm{E}}_{\mathrm{CB}}{=\mathrm{E}}_{\mathrm{VB}}-{\mathrm{E}}_{\mathrm{g}}$$

(9)

where χ is the absolute electronegativity of the semiconductor. E_e is the energy of free electrons on the hydrogen scale (4.50 eV). E_g is the band gap of the semiconductor [20].

$$\equiv \mathrm{Co}\left(\mathrm{II}\right){+\mathrm{H}}_2{\mathrm{O}}_2 \to \mathrm{Co}\left(\mathrm{III}\right)+·\mathrm{OH}+{\mathrm{OH}}^{-}$$

(10)

$${2\mathrm{S}}^{2-}{+16\mathrm{Co}\left(\mathrm{III}\right)+8\mathrm{H}}_2\mathrm{O}\to {2\mathrm{SO}}_4^{2-}{+16\mathrm{H}}^{+}+16\mathrm{Co}\left(\mathrm{II}\right)$$

(11)

3. Experimental Section

3.1. Chemicals

Cobalt sulfate heptahydrate (CoSO₄∙7H₂O, 99.99%), urea (CH₄N₂O, 99.5%), sublimed sulfur (S, 99.5%), ethylene glycol ((CH₂OH)₂, 98%), N, N-Dimethylformamide (C₃H₇NO, 99.8%), tetramethylammonium hydroxide solution (C₄H₁₃NO, 25%), titanium aluminum carbide powder (Ti₃AlC₂, 98%), tert-butanol (TBA, 99.5%), and methanol (MeOH, 99.5%) are chemicals, which were all purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China), unless otherwise stated.

3.2. Preparation of CoS₂/CoS and CoS₂/CoS/Ti₃C₂

In the typical procedure, 10 mmol urea, 2 mmol CoSO₄, and 25 mmol sulfur powder were added to 60 mL mixture solution of DMF and EG. The mixture solution was stirred for 30 min and then sonicated for 30 min. The well mixed precursor solution is added to Teflon-lined autoclave. The solution is maintained in an automatic heating oven at 180 °C for 12 h. After the hydrothermal reaction, the sample was naturally cooled to room temperature, centrifuged, and washed three times with ethanol and ultrapure water. The samples were dried under vacuum. The CoS₂, CoS₂/CoS, and CoS were obtained under the conditions of V_EG:V_DMF = 0.5, 0.9, and 2.0.

Ti₃AlC₂ was etched in HF solution to obtain Ti₃C₂. Subsequently, the 25% TMAOH solution was used to intercalate Ti₃C₂ to obtain a monolithic layer of Ti₃C₂. Similarly, to obtain CoS₂/CoS/Ti₃C₂, (50 mg) of stripped Ti₃C₂ was placed in a mixed solution, and the temperature was maintained at 180 °C for 12 h. After centrifugal washing, the sample is vacuum dried.

3.3. Characterization and Experimental Process

Detailed information of preparation of CoS₂/CoS/Ti₃C₂ cathode and degradation test was shown in Supplementary Materials Text S2.

4. Conclusions

In summary, CoS₂/CoS/Ti₃C₂ displayed a superior catalytic performance in photo-assisted-electro-Fenton process compared with other materials (Table S3). The charged surface derived from the built-in potential benefited to facilitate the electron migration. Both the oxygen reduction ability for 2e⁻ process and photogenerated electron-hole separation of CoS₂/CoS/Ti₃C₂ were enhanced owing to the uneven charge distribution of CoS₂/CoS heterojunction. CoS₂/CoS/Ti₃C₂ cathode demonstrated a high PEF performance with 98.5% degradation rate for SMT at 20 mA/cm² within 120 min and excellent performance retention of 85% after five cycles. Moreover, ESR signals revealed that the main ROS in the degradation process of SMT was ∙OH. The design and construction of photoelectric Fenton electrocatalysts with heterojunction offers a promising strategy to enhance the performance and competitiveness. In addition, based on the excellent catalytic performance of this material, further research with various substrates (such as other Mxenes materials) is undergoing to further improve the water splitting process and broaden the application scope of CoS₂/CoS-based materials.