Preparation and Dye Degradation Performances of Self-Assembled MXene-Co3O4 Nanocomposites Synthesized via Solvothermal Approach

Two-dimensional metal carbides or nitrides (MXenes) demonstrated wide applications in energy storage, water treatment, electromagnetic shielding, gas/biosensing, and photoelectrochemical catalysis due to their higher specific surface area and excellent conductivity. They also have the advantages of flexible and adjustable components and controllable minimum nanolayer thickness. In this study, a cube-like Co3O4 particle-modified self-assembled MXene (Ti3C2) nanocomposite has been prepared successfully by a simple solvothermal method. The Co3O4 particles are well dispersed on the surface and inner layers of the Ti3C2 sheets, which effectively prevent the restacking of Ti3C2 sheets and form an organized composite structure. The physical properties of these nanocomposites were studied by using XRD, SEM, EDX, TEM, and XPS. The performance of the obtained samples was evaluated as new nanocatalysts for degrading methylene blue and Rhodamine B in batch model experiments. The prepared Mxene-Co3O4 nanocomposites can be well regenerated and reused for eight consecutive cycles, indicating potential wide applications in wastewater treatment and composite materials.


INTRODUCTION
Two-dimensional materials have unique electrical, optical, and mechanical properties. MXene is a new type of twodimensional crystalline compound with graphene-like structure and novel properties. 1 It is one of the stars in the field of functional materials research in recent years. It is peeled off by MAX phase etching. 2 The MAX phase is a ternary-layered carbide or nitride material that combines the properties of both metals and ceramics. 3 The MAX phase has the general formula M n+1 AX n , where M is a transition metal element, A is mainly a group IIIA or IVA element, X is carbon and/or nitrogen, and n = 1, 2, or 3. So far, more than 70 MAX phases have been discovered. 4 The MAX phase is a hexagonal crystal structure, the M atomic layer is closely packed, the X atom is filled in the M atom octahedron, and the M atomic layer is interspersed in the A atomic layer. It could be thought that the MAX phase is bonded by a two-dimensional layered carbide/nitride to the A atomic layer. Among them, M−A is a mixture of covalent bond/metal bond/ion bond, and M−X is a covalent bond. 5 Thus, the A atomic layer is removed by etching, and a twodimensional MXene nanosheet is obtained by liquid phase stripping. 6,7 In addition, the thickness of a single MXene layer can reach 1 nm, and the area diameter can reach several tens of micrometers. 8 The corresponding structures of MXene are also different from MAX phases with different n values. Nineteen kinds of MXene have been successfully prepared, and dozens of MXene have predictive stability in theory; this diversified structural composition provides a broad space for its property regulation and derivative material construction. Moreover, the Co 3 O 4 crystal exhibited a normal spinel structure, namely, Co 2+ (Co 3+ ) 2 O 2− 4 , where O 2− is in a densely packed cubic structure, Co 2+ is located in its tetrahedral gap, and Co 3+ is located in its octahedral gap with higher crystal field stabilization energy. When the air is lower than 800°C, the properties are very stable. At room temperature, it is not easily soluble in various concentrated acids and water but could be dissolved in a hot sulfuric acid solution at a lower rate. In addition, it is also a p-type semiconductor material. It also has a very wide range of applications, such as sensors, 9 supercapacitors, 10 catalysts, 11−19 magnetic semiconductors, 20 and rechargeable battery materials. 21−23 For example, Huang et al. synthesized silver nanoparticle-modified MXene composites by self-reduction reactions with enhanced catalytic performances. 11 Yang et al. used a simple pyrolysis method to prepare Co 3 O 4 nanoparticles/nitrogen-doped carbon composites with different structures for oxygen evolution. 12 Chen et al. skillfully used a multistep method to synthesize mesoporous Co 3 O 4 and carbon nanotube composites with a layered pipe structure as a negative electrode material for lithium ion batteries. 13 This unique nanostructure solved the problem of volume expansion and low electron conductivity of metal oxide anodes. 24 Moreover, some similar composites exhibit excellent performance in electrochemistry and wide applications. 25−31 Dyes are widely used in various industries, among which methylene blue (MB) is often used in the printing and dye industry as an important target for wastewater treatment. 32−39 It is reported to be carcinogenic and mutagenic, which may be harmful to plants and animals. Rhodamine B (RhB) is also a kind of widely used dye. Therefore, the removal of MB and RhB from wastewater causes widespread concern. Herein, we proposed to synthesize Co 3 O 4 particle-modified MXene (Mxene-Co 3 O 4 ) nanocomposites by an in situ solvothermal method. The Co 3 O 4 nanocomposites were uniformly anchored on the surface of Ti 3 C 2 sheets, which enhanced the catalytic activity. At the same time, the close interaction between the   Figure 2 shows the microscopic morphology of MXene-Co 3 O 4 , Co 3 O 4 , and MXene. From Figure 2a,b, it could be seen that MXene had a well-fabricated accordion morphology. Pure Co 3 O 4 showed a small cube type, which had an average crystallite size of 20 nm. Figure 2c,d shows the TEM and SEM images of the MXene-Co 3 O 4 composites. It could be clearly seen that the surface of MXene was coated with a large amount of Co 3 O 4 particles, and a small amount of Co 3 O 4 could enter the layer of MXene, which indicated that this is beneficial to the catalytic degradation of methylene blue and Rhodamine B. It could be explained that the formed MXene-Co 3 O 4 composite has a larger specific surface area, which was advantageous for the adsorption of dyes and could be used for catalytic experiments. Energy dispersive X-ray (EDX) spectroscopy (inset in Figure 2c) confirms that the elemental composition of the composite consists solely of carbon, titanium, cobalt, and oxygen, whose proportions were consistent to form MXene-Co 3 O 4 . In addition, Figure 3 describes the elemental mappings of the prepared MXene-Co 3 O 4 composite. According to C, Ti, and Co images, the data further illustrated the good distribution of the Co element on the surface of MXene after the successful solvothermal process.
X-ray powder diffraction (XRD) analysis identifies the formation of MXene-Co 3 O 4 nanocomposites, as shown in Figure 4. A set of (111), (110), (223), and (440) peaks at 17.9, 25.1, 59.8, and 64.6°appeared, respectively. The disappearance of the (006) peak from Ti 3 C 2 MXene suggested the suppressed restacking of MXene sheets by the Co 3 O 4 nanostructure standing on its surface (Figure 2c). The other peaks originated from MXene were quite weak even in pristine MXene, for being easily overlapped by the signals from the Co 3 O 4 nanostructure. After the Co 3 O 4 assembly process, three distinct characteristic peaks of Co 3 O 4 particles could also be found in the formed composite, that is, (111), (511), and (440) crystal planes (JCPDS number 090418), indicating that a large number of Co 3 O 4 particles were adsorbed on the surface of the MXene during the solvothermal treatment process.
The thermogravimetric curve showed that the mass changes with the change of temperature, as shown in Figure 5. While Co 3 O 4 was relatively stable in the test temperature range, the weight loss was attributed to the change in Ti 3 C 2 . It could be concluded from the curve that the weight loss of as-prepared MXene-Co 3 O 4 composites from room temperature to 200°C was mainly due to the adsorption of water. When the temperature was gradually increased, the weight loss was mainly due to the decomposition of the oxygen-containing functional groups in which the Co 3 O 4 and MXene complexes were bonded to each other and the decomposition of Ti 3 C 2 .
Moreover, Figure 6 shows the XPS spectra of MXene and the MXene-Co 3 O 4 composite. Figure 6a shows the characteristic peaks in the curve of the MXene-Co 3 O 4 nanocomposite, such as Co2p, O1s, C1s, and Ti2p. Compared with pure MXene, the peak of O1s of MXene-Co 3 O 4 increased obviously. It could be seen that the results of XPS are consistent with the Co, C, Ti, and O elements in the elemental EDX analysis. In addition, for the Co2p XPS spectrum in Figure 6b, it showed a low energy band (Co2p 3/2 ). This low energy band could be convolved into two peaks: 779.8 and 781.3 eV. 40,41 The Co2p 3/2 peak position (Figure 6b) was in accordance with the presence of Co 3 O 4 . 42 The analysis results were consistent with XRD, which proved the generation of Co 3 O 4 .
2.2. Catalytic Performances of MXene-Co 3 O 4 Nanocomposites. The catalytic performance of the obtained composites was assessed by catalytic reduction of model dyes. In the present study, the catalytic reaction was carried out in a 250 mL glass flask containing 100 mL of MB dye solution (12.5 mg/L) or 100 mL of RhB (5 mg/L) and 10 mg of catalyst in the presence of 15 mL of H 2 O 2 (30%) at room temperature with continuous stirring. The supernatant was centrifuged at given time intervals, and the residual dye concentrations at different time intervals were investigated by UV−visible spectroscopy using different absorption wavelengths (MB 664 nm, RhB 554 nm). In this study, catalytic reductions of MB or RhB from Co 3 O 4 particles and MXene-Co 3 O 4 composites were also investigated. Meanwhile, the dye removal rate was calculated according to the following formula: 43 where K is defined as the dye

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Article removal rate, A 0 is defined as the initial absorbance of the supernatant of the dye solution, and A T is defined as the absorbance of the supernatant of the dye solution collected at different intervals.
In order to compare the performance of the synthesized catalyst, we evaluated the effects of catalytic degradation of dyes (MB and RhB) using two catalysts, respectively. The adsorption kinetic experiments of the as-prepared nanocomposites were carried out using the results of RhB and MB, as shown in Figures 7 and 8 In addition, the adsorption kinetic process could be described by classical kinetic models as follows: The pseudofirst-order model could be represented by eq 1 The pseudo-second-order model could be represented by eq 2

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Article where q e is the equilibrium adsorption capacity (mg/g), q t is the adsorption capacity at time t (mg/g), and k 1 and k 2 values are the kinetic rate constants. 44−47 At the same time, Table 1 clearly shows the kinetic results of the experimental data, and the respective fitting parameters were obtained. As shown in Figure 7, by comparing the linear fitting parameters, the pseudo-first-order and pseudo-secondorder kinetics of MB and RhB were catalyzed by MXene-Co 3 O 4 and Co 3 O 4 . Based on the above data, it could be easily observed that the obtained MXene-Co 3 O 4 composite showed the best adsorption capacity, which could be related to the synergy effect between MXene and Co 3 O 4 .

Reasonable Activation Mechanisms of H 2 O 2 .
The discovery of antibiotics derived from natural products has aroused global attention. A promising and challenging method of antibiotic degradation is the Fenton method. This reaction can be carried out at neutral pH, which was an important advantage. A study on oxidative degradation of organic dyes with H 2 O 2 had been reported previously. 16 However, the exact mechanism for dye decomposition with H 2 O 2 remains unclear. According to a previous study, cobalt-based Fenton/Fentonlike processes could be performed at neutral pH. 18 On the one hand, it was found that homogeneous Co 2+ /H 2 O 2 systems were able to degrade dye contaminants without controlling pH completely. 19 On the other hand, the catalytic decomposition of organic contaminants by heterogeneous Co catalysts under the condition of adding H 2 O 2 was also investigated. In a recent study, Co 2+ adsorbed alumina was used as a heterogeneous catalyst to degrade methylene blue and methyl orange. 17 In those cases, the Co-based heterogeneous catalyst could effectively degrade contaminants at neutral or even alkaline pH. Co-based heterogeneous catalysts could effectively degrade the pollutants at neutral or even basic pH. In contrast, HO • produced by Co 2+ mediated H 2 O 2 activation has been recorded in only a few studies. 18,21 The HO • formation mechanism by Co could be proposed as reactions 3−5. As shown in reaction 3, HO • played an important role in the Co(II)/Co(III) cycle. The ability of MXene-Co 3 O 4 in the catalytic process of MB could be attributed to the positive synergistic effect of MXene, MB, and Co 3 O 4 with the aid of H 2 O 2 . First, the cationic MB molecules were easily adsorbed to the surface of MXene due to electrostatic attraction, and the functional groups such as −F and −OH that were rich on the surface of MXene also endowed the excellent catalytic ability during the chemical reaction process.
This adsorption significantly improved the effective concentration of MB molecules anchored on the surface of MXene-Co 3 O 4 , leading to a high catalytic degradation rate. Second, the hydrophilicity of MXene made the MXene-Co 3 O 4 complex well dispersed in water. Good contact between Co 3 O 4 nanocomposites and MXene prevented Co 3 O 4 from falling off during the catalytic process. The H 2 O 2 was then effectively catalyzed to produce free • OH radical species and ultimately promote degradation of the MB molecules. Hence, in terms of MXene-Co 3 O 4 composites, there was a coupling between adsorption and catalytic reactions in a single process. 23 Compared to bare MXene and Co 3 O 4 , the degradations of MB and RhB were significantly increased.
Compared to other nanoparticle catalysts reported, the present prepared composite catalyst could be recovered readily from the solution. 48 After reaching the adsorption equilibrium in the reaction solution, the recovered MXene-Co 3 O 4 composite was treated by a thorough cleaning procedure to eliminate possible dye residues and regenerate the catalyst. Through repeated adsorption by using the same catalyst and fresh dye solutions, it could be used in eight consecutive cycles

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Article (Figure 9). The results showed that, after eight consecutive cycles, the MB removal rate was maintained at about 92.37%.
The obtained nanocomposites performed well in terms of stability and recyclability. In addition, the reduction of MB degradation could be attributed to the loss and aggregation of the catalyst in the recycling process, as well as the adsorption of MB or intermediates. The above data indicated that the synthesized catalyst could potentially be utilized in pollutant treatment.

CONCLUSIONS
In conclusion, we have successfully synthesized a Co 3 O 4 nanocrystal-loaded MXene composite by simple in situ solvothermal synthesis. Due to its simple and mild reaction conditions, this simple in situ synthesis method could also be applied to the synthesis of other MXene-based transition metal oxides. The present obtained MXene-Co 3 O 4 nanostructure has a very high MB and RhB degradation capacity and well repeatability. After eight consecutive catalytic cycles, the catalytic properties of the sample were still good, showing that the stability and repeatability of Mxene-Co 3 O 4 nanocomposites were satisfying. The possible mechanism for adsorbing methylene blue and Rhodamine B to Mxene-Co 3 O 4 and Co 3 (20 mg) was dissolved in 4 mL of ultrapure water, and the mixture was sonicated for 0.5 h. Then, 2 mL of 0.2 M Co(Ac) 2 solution was added dropwise, 20 mL of ethanol was added, and the solution was magnetically stirred for 2 h. Then, the above reaction solution was poured into a 100 mL Teflon-lined steel autoclave. The autoclave was then heated at 120°C for 8 h. After cooling down to room temperature and centrifugation, the final obtained precipitate was washed three times with ethanol and lyophilized to obtain the desired MXene-Co 3 O 4 composites.
4.3. Preparation of Co 3 O 4 Nanoparticles. The synthesis of pure Co 3 O 4 samples was similar to the preparation of MXene-Co 3 O 4 by reducing the addition of MXene of the same quality. Ethanol (20 mL) was added to 2 mL of 0.2 M Co(Ac) 2 solution and stirred for 2 h. Then, the mixture was transferred to a 100 mL steel autoclave reactor and heated at 120°C for 8 h.
4.4. Characterization. Thermogravimetric analysis (TGA) was tested in an argon environment by a NETZSCH STA 409 PC Luxx simultaneous thermal analyzer (Netzsch Instruments Manufacturing Co., Ltd., Germany). X-ray photoelectron spectroscopy (XPS) was measured by the Thermo Scientific ESCALab 250Xi with an Al Kα X-ray source. The adsorption experiments were monitored by using a Shimadzu UV2550 spectrophotometer. All aqueous solutions were prepared with water purified in a double-stage Millipore Milli-Q Plus purification system. The morphologies were obtained by using a transmission electron microscope (TEM) (HT7700, Hitachi High-Technologies Corporation, Japan) with an accelerating voltage of 20 kV. X-ray diffraction (XRD) analysis was recorded with an X-ray diffractometer (SMART LAB, Rigaku, Japan). The microstructures of the samples were characterized by using a field-emission scanning electron microscope (SEM) (S-4800II, Hitachi, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). Absorption spectra were measured with a LabTech UV-2100 ultraviolet− visible (UV−vis) spectrophotometer.

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Article 4.5. Catalytic Test of MXene-Co 3 O 4 Nanocomposites. In order to evaluate the catalytic performance of the obtained materials, we assessed the performance by degradation of methylene blue and Rhodamine B. 49,50 The prepared Mxene-Co 3 O 4 composites (10 mg) and 15 mL of H 2 O 2 solution (30% in water) were added to the Rhodamine B and methylene blue solution. A small sample amount of the suspension was taken at regular intervals, and the absorbance of the sample was measured by using a UV−vis spectrophotometer. Moreover, cyclic stability of the catalyst was evaluated by eight replicate experiments. At time t (min), the dye adsorption amount q t (mg/g) per unit mass of catalyst was calculated by the following formula where C 0 is the initial concentration of the adsorption solution (mg/L), C t is the concentration of the adsorption solution at time t (mg/L), m is the total amount of the sample (g), and V is the volume of the adsorption solution (L).