Plasma–Liquid Synthesis of Titanium- and Molybdenum-Containing MXenes and Their Photocatalytic Properties

Abstract

Previous studies have demonstrated that underwater low-temperature plasma is effective for synthesizing nanomaterials by generating plasma discharges between metal electrodes submerged in water. This study extends this approach to the one-step synthesis of MXenes containing titanium, molybdenum, and titanium–molybdenum composites through pulsed discharges in carbon tetrachloride, an oxygen-free, non-flammable solvent characterized by a high boiling point and low permittivity. By employing titanium and molybdenum electrodes in various configurations, three MXene samples were synthesized: Ti₂CT_X, Mo₂CT_X, and Mo₂TiC₂T_X. Characterization techniques, including UV-Vis spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, confirmed the successful synthesis of high-purity MXenes with distinct structural and optical properties. Notably, the bandgap values of the synthesized MXenes were determined as 1.71 eV for Ti₂CT_X, 1.42 eV for Mo₂TiC₂T_X, and 1.07 eV for Mo₂CT_X. The photocatalytic performance of the synthesized MXenes was evaluated, showing a removal efficiency of 65% to 98% for dye mixtures, with methylene blue showing the highest degradation rate. This plasma-assisted method offers a scalable, precursor-free route for the synthesis of MXenes with potential applications in energy storage, environmental remediation, and optoelectronics due to their tunable bandgaps and high catalytic activity.

1. Introduction

The escalating global demand for environmental remediation technologies has spurred significant research into advanced materials capable of addressing pressing challenges such as climate change, pollution, and resource depletion. Among the diverse classes of materials under investigation, two-dimensional (2D) nanomaterials have emerged as promising candidates due to their exceptional physical, chemical, and electronic properties. In particular, MXenes—a family of transition metal carbides, nitrides, and carbonitrides—have attracted considerable attention since their discovery in 2011 [1,2]. These materials exhibit unique characteristics, including high electrical conductivity, tunable bandgaps, large surface areas, and remarkable chemical stability, making them ideal for applications in energy storage, electronics, sensing, and catalysis [3,4,5,6]. Photocatalysis, a process that utilizes light energy to facilitate chemical reactions, presents a renewable and environmentally friendly solution to pressing challenges such as water purification, hydrogen production via water splitting, and carbon dioxide reduction [7,8]. Traditional photocatalysts, including titanium dioxide, zinc oxide, and other semiconductors, have been researched extensively; however, they often exhibit significant limitations, such as the inadequate absorption of visible light, low quantum efficiency, and the rapid recombination of photogenerated electron–hole pairs [9]. These shortcomings underscore the necessity for alternative materials that demonstrate enhanced performance.

MXenes, characterized by the general formula M_n+1X_nT_x (where M represents a transition metal such as Ti, V, or Mo; X denotes carbon or nitrogen; and T refers to surface functional groups like -OH, -Cl, or -O [10]), offer significant advantages due to their tunability. By varying their composition, structure, and surface functionalization, MXenes can be customized for specific applications [11]. For example, their bandgap can be adjusted by modifying the metal in the composition or by introducing heteroatoms [12], which enables their use in various photocatalytic processes, including the degradation of organic pollutants and the reduction of CO₂.

Several studies have demonstrated the effectiveness of MXenes in photocatalytic water treatment. For instance, Ti₃C₂T_x, one of the most extensively researched MXenes, has exhibited remarkable performance in degrading methylene blue (MB), rhodamine B (RhB), and phenol under visible light irradiation [13]. Zhang et al. reported the nearly complete removal of MB and RhB within 60 min using Ti₃C₂T_x MXenes [14]. Similarly, Kim et al. demonstrated that silver-decorated Ti₃C₂T_x MXenes significantly enhanced the degradation of diclofenac, highlighting the potential of noble-metal-modified MXenes in treating pharmaceutical wastewater [15]. Praus et al. developed a composite of Ti₃C₂T_x and g-C₃N₄ for industrial wastewater containing phenolic compounds, achieving high removal efficiencies under simulated solar light [16].

Conventional MXene synthesis relies on hazardous hydrofluoric acid (HF) to etch the “A” layer from MAX-phase precursors. This process generates toxic byproducts and poses significant environmental and safety risks [17]. To address these concerns, alternative “greener” methods have been developed, including fluoride salt/HCl etching (e.g., LiF/HCl), which generates HF in situ but still requires corrosive chemicals and multi-step purification [18]. Electrochemical etching reduces HF usage but often results in incompletely exfoliated or oxidized MXenes [19]. Molten salt etching avoids the use of aqueous HF but operates at high temperatures (300–600 °C), which limits scalability. Microwave-assisted synthesis accelerates the etching process but struggles with uniformity and control over the surface terminations of MXenes. While these methods enhance safety, they often compromise scalability, cost, or material quality. In contrast, plasma–liquid synthesis provides a precursor-free, one-step alternative that operates under ambient conditions, thereby eliminating the need for toxic etchants or high temperatures.

Previous studies have demonstrated that underwater low-temperature plasma can be effectively utilized for the synthesis of oxide nanomaterials. By generating a plasma discharge between metal electrodes submerged in water, it is possible to produce metal oxide nanoparticles. This method offers a promising pathway for nanomaterial production [20]. A key advantage of this technique is its ability to synthesize materials without the need for precursors, such as inorganic salts or organic compounds, thereby avoiding the generation of byproducts.

Additionally, plasma discharges can be initiated between metal electrodes immersed in non-aqueous solvents [21,22]. When using an oxygen-free solvent, the synthesis process is expected to yield products devoid of oxide nanoparticles. Previous research has demonstrated that the synthesis of MXenes in plasma solution systems is feasible [23]. The aim of this study was the synthesis of titanium-, molybdenum- and titanium–molybdenum-containing MXenes in one step by initiating a pulsed discharge in tetrachloromethane (CCl₄). Carbon tetrachloride was selected as an oxygen-free, non-flammable solvent with a relatively high boiling point and low permittivity.

2. Results and Discussion

2.1. Process of Plasma–Liquid Synthesis of MXenes

In a series of experiments, titanium electrodes were utilized for the first set of tests (Sample 1), while molybdenum electrodes were employed for the second series (Sample 2). For the third series (Sample 3), a combination of a titanium anode and a molybdenum cathode was used (

Table 1. Experimental conditions for obtaining samples and their chemical composition.

Name	Experimental Conditions	Chemical Composition
Sample 1	Underwater plasma in CCl4 at discharge current of 0.25 A; Ti—anode; Ti—cathode	Ti2CClx
Sample 2	Underwater plasma in CCl4 at discharge current of 0.25 A; Mo—anode; Mo—cathode	Mo2CClx
Sample 3	Underwater plasma in CCl4 at discharge current of 0.25 A; Ti—anode; Mo—cathode	Mo2TiC2Clx

).

Throughout these experiments, the average current of the pulsed underwater discharge remained consistent at 0.25 A. The progression of a pulsed discharge in carbon tetrachloride differs significantly from that in water due to its lower dielectric constant (ε = 2.24). Initially, the discharge forms as a thin streamer extending from the anode to the cathode, followed by an explosive breakdown of the inter-electrode space, resulting in a bright flash. This process leads to the localized overheating of the solvent, eventually forming pulsed discharges within solvent bubbles. Previous research has indicated that the plasma temperature in the discharge region between electrodes can reach several thousand degrees [24]. Since the synthesis of composite materials takes place in the plasma–solution interface, the emission spectra of the pulsed discharge burning in carbon tetrachloride must be investigated in order to understand this process. The emission spectrum of the discharge in the system with a titanium anode and molybdenum cathode is shown in Figure 1a. The main active species determined by the discharge emission spectra, as well as their wavelengths and relative intensities, are presented in Table S1. Since carbon tetrachloride is destroyed during the burning process of the discharge, the emission spectrum of the discharge contains emission lines of Cl atoms and emission bands of C₂ particles. The “Swan band” emission of C₂ was observed in three sequences: Δv = 1 (461–481 nm), Δv = 0 (501–517 nm), and Δv = −1 (545–560 nm) [22]. The emission spectrum of the discharge contains a large number of lines of molybdenum and titanium atoms, including the emission of an ionized titanium atom (Ti II). The detection of metal atoms in the spectrum indicates the erosion of the electrode materials and suggests reactions in the plasma involving these particles. Under plasma conditions, high-energy solvated electrons (1–20 eV) dissociate CCl₄ via the following:

$$CC{l}_4+e_{solv}^{-}\to CC{l}_3^{·}+{Cl}^{·}+e_{solv}^{-}$$

(1)

$$CC{l}_4+e_{solv}^{-}\to CC{l}_2^{·}+2{Cl}^{·}+e_{solv}^{-}$$

(2)

The following processes may then occur:

$$2CC{l}_2^{·}\to C_2+2C{l}_2$$

(3)

$$Me\left(electorode\right){⟶}^{plasma }Me\left(gas\right){⟶}^{plasma }M{e}^{n+}\left(Me sputtering\right)$$

(4)

The next stage is the formation of metal carbides, such as titanium carbide:

$$Ti+C_2⟶Ti{C}_{x }(x\approx 1-2)$$

(5)

In addition, the formation of titanium subchlorides is possible:

$$Ti+{Cl}^{·}⟶Ti{Cl}_{x }(x\approx 2-3)$$

(6)

Next, a reaction occurs between titanium chlorides and carbon particles:

$$2Ti{Cl}_x+C_2⟶T{i}_2{CCl}_{x }+CC{l}^{·}$$

(7)

Next, the processes of layer stacking occur in the plasma–solution interface. The synthesis of MXene takes place sheet by sheet in the plasma–liquid system. This distinguishes this process from the classical method of MXene extraction by etching.

When a discharge pulse occurs, the voltage can reach a maximum of 2500 V, with a peak current of 0.62 A (as shown in Figure 1b). The discharge pulses have an average frequency of approximately 20 Hz, and the duty cycle is 10%. The waveforms of both the current and voltage clearly illustrate all stages of the discharge process. The sudden rise in current amplitude during a pulse corresponds to the generation of a greater number of conductive particles. Additionally, the formation of a secondary spark may result from the breakdown of the inter-electrode space being facilitated by the overheating of the solution.

2.2. UV-Vis Spectra of Synthesized MXenes

Figure 2 presents the UV-Vis absorption spectra (300−950 nm) of MXene suspensions in CCl₄. The synthesized MXenes exhibit an absorption edge at approximately 400 nm, indicating a small bandgap. However, the plasmon peaks in the UV–visible range are not clearly defined. These peaks are sensitive to various factors, including composition and layer thickness: thinner MXene flakes tend to display weaker plasmon peaks due to a reduced electron density. The bandgap energy was determined from the absorption spectra using the Tauc equation [25]:

$$\left(\alpha hv\right)=B_0(hv-E_g{)}^n,$$

(8)

where B₀ is a constant associated with the type of band-to-band transition, v is the frequency of the incident radiation, n is an index related to the type of optical transition, and E_g is the optical bandgap of the material. The dependence was extrapolated to (αhv)^1/2 = 0 to obtain the bandgap. MXenes (e.g., Ti₂CT_x, Mo₂CT_x) exhibit indirect bandgaps due to their intricate electronic structures. This observation is supported by density functional theory calculations for MXenes with similar compositions [26]. The results, shown in Figure 2, indicate that the bandgap width in the series of studied samples decreases in the order Sample 1 > Sample 3 > Sample 2, with values of 1.71 eV for Sample 1 (Ti₂CT_x), 1.42 eV for Sample 3 (Mo₂TiC₂T_x), and 1.07 eV for Sample 2 (Mo₂CT_x). In previous studies, the bandgap values for Ti₂CT_x and Mo₂TiC₂T_x were found to be approximately 1 eV [27].

The bandgap values for semiconducting MXenes are influenced by the surface groups. Chlorine (-Cl) typically reduces the bandgap due to its strong electronegativity, whereas hydroxyl (-OH) and oxygen (-O) groups can increase the bandgap by introducing additional states near the Fermi level [28].

2.3. XRD Patterns of Synthesized MXenes

Figure 3 presents the X-ray diffraction (XRD) patterns of the MXene powders synthesized using the plasma–liquid system. For Sample 1, the prominent peaks located at 9.11°, 21.02°, 26.29°, and 33.57° correspond to the diffractions of the (002), (006), (008), and (103) planes of Ti₂CT_x, respectively [29]. Additionally, peaks associated with the (111) and (200) planes can be attributed to TiC, which may have formed during the MXene synthesis process. Notably, there are no detectable peaks corresponding to titania, indicating the high purity of the MXene phase in the synthesized powder and confirming the absence of oxide structures. The (002) and (008) planes are also evident in Sample 2, which contains Mo₂CT_x in its composition. Furthermore, Sample 2 exhibits distinct reflections corresponding to α-Mo₂C, a characteristic feature of MXenes with this composition [30]. In the case of Sample 3, peaks associated with both titanium carbide and molybdenum carbide are observed, indicating the presence of these phases in the material. The XRD pattern of Sample 3 exhibits peaks consistent with both Ti₂C and Mo₂C, suggesting the presence of these individual carbide phases. However, based on additional evidence from complementary characterization techniques, such as Raman spectroscopy and SEM analysis, we have indications that support the formation of the Mo₂TiC₂ MXene phase in this sample. Our analysis indicates that the lattice parameters obtained from the X-ray diffraction data closely correspond to those anticipated for the Mo₂TiC₂ structure, rather than representing a mere superposition of the Ti₂C and Mo₂C phases. The d-spacings of the composites were calculated using the following formula:

$$d={\displaystyle \frac{n\lambda }{2sin\theta }}$$

(9)

In this equation, n represents the order of reflection, λ denotes the wavelength of CuK_α (1.541 Å), and θ indicates the angle of incidence of the X-rays. The d-spacing of the most prominent peak (002) decreases from Sample 1 to Sample 3, changing from 19.48 Å to 19.31 Å. The d parameter varies significantly between 19 Å and 59 Å across different studies [31], as the interlayer spacing can differ due to the presence of various ionic species. In our case, the chloride anion most likely occupies the interlayer space in MXenes.

The average crystallite size (D) of the MXene samples was estimated using Scherrer’s equation:

$$D={\displaystyle \frac{k\lambda }{\mathsf{\beta }cos\theta }}$$

(10)

In this equation, k represents the Scherrer constant, and β denotes the full width at half maximum (FWHM) of the XRD peak (in radians). According to the results obtained, Sample 1 (Ti₂CT_x) exhibits larger crystallites measuring 21.5 nm. In contrast, Sample 2 displays smaller crystallites, approximately 16.7 nm, which can be attributed to a higher defect density resulting from the incorporation of molybdenum (Mo). Sample 3 (Mo₂TiC₂T_x) exhibits an intermediate size of 19.1 nm. Sample 2, which has smaller crystallites, exhibits a higher surface area, resulting in an increased number of active sites for oxidation–reduction reactions in photocatalysis. However, the likelihood of recombination processes occurring is significant. The optimal intermediate crystallite size may account for the moderate bandgap of 1.42 eV and enhanced visible light absorption of Sample 3. For Sample 1 with a large crystallite size, the processes of charge carrier recombination will be inhibited.

2.4. Raman Studies of Synthesized MXenes

For MXenes, Raman spectroscopy offers valuable insights into their unique properties, including crystallinity, surface terminations, and structural integrity. Figure 4 displays the Raman spectra of the samples. In Sample 1, six distinct peaks are clearly observed at 154, 228, 315, 390, 720, and 870 cm⁻¹ [32]. A small, sharp peak appears around 154 cm⁻¹, which may be attributed to excessive laser power, resulting in the formation of oxidized TiC [33]. The peak at 228 cm⁻¹ corresponds to the E_2g mode, associated with the in-plane stretching vibrations of the Ti-C bonds, and is one of the most prominent features in the Raman spectrum of Ti₂CCl_x. A redshift of this mode suggests the presence of defects within the material. The A_1g mode at 390 cm⁻¹ arises from the out-of-plane vibrations of titanium atoms in the MXene layers. Furthermore, the presence of chlorine as a surface termination introduces an additional vibrational mode at 315 cm⁻¹. Similar to Ti₂CCl_x, Mo₂CT_x exhibits a prominent E_2g mode corresponding to the in-plane stretching vibrations of the Mo-C-Mo bonds. However, due to the presence of molybdenum, this peak shifts slightly to higher wavenumbers, reaching 302 cm⁻¹ [34]. The modes at 143 cm⁻¹ and 178 cm⁻¹ correspond to the vibrations of carbide groups. The A_1g mode in Mo₂CT_x also shifts compared to Ti₂CCl_x, occurring at approximately 620 cm⁻¹. The A₁g mode in Mo₂CT_x reflects the out-of-plane vibrations of molybdenum atoms, which are sensitive to the type of surface terminations. In the composite, modes are observed that are present in both Sample 1 and Sample 2, but their positions are shifted by 5–10 cm⁻¹ due to the mutual influence of atomic groups on one another. In Mo₂CT_x, the shift in the E₂g mode (from ~228 cm⁻¹ in Ti₂CCl_x to ~302 cm⁻¹ in Mo₂CT_x) is influenced by the substitution of titanium with molybdenum and the nature of surface terminations (e.g., -Cl). Due to its more complex layered structure, Mo₂TiC₂Cl_x may exhibit additional peaks related to interlayer interactions or specific vibrational modes unique to the Mo-Ti-C system [35]. The results obtained are supported by various reports in the literature and can be interpreted as evidence of the successful synthesis of MXenes.

2.5. Microscopic Studies of Samples

Scanning electron microscopy (SEM) analysis confirmed that the obtained MXenes exhibit a characteristic “accordion-like” morphology, as illustrated in Figure 5. This morphology is a hallmark of exfoliated MXenes and results from the layered structure of these two-dimensional (2D) materials. The accordion-like appearance signifies the successful delamination of the bulk material into thin, flexible sheets.

Energy-dispersive X-ray spectroscopy (EDS) showed the primary compositional elements of the synthesized MXenes, specifically titanium (Ti), molybdenum (Mo), and carbon (C), with chlorine (Cl) identified as the terminal functional group (Figure 6A). The halogen atoms predominantly occupy the surface end positions.

Notably, EDS analysis demonstrated the absence of common impurities such as aluminum (Al), oxygen (O), sulfur (S), and copper (Cu). These impurities are frequently observed in MXenes prepared using conventional chemical methods, where incomplete etching or contamination during synthesis can result in the incorporation of unwanted elements [36]. The absence of impurities underscores the effectiveness of the synthesis process, ensuring high material purity and structural integrity.

A slight variation in the elemental percentages detected by EDS mapping can be attributed to the distribution of chlorine within the MXene structure (Figure 6B). Chlorine is predominantly located in the surface end groups but may also be present in the interlayer spaces, particularly at the edges of the MXene flakes. Consequently, the edge regions of the sample tend to exhibit higher concentrations of chlorine compared to the basal planes. This localized enrichment accounts for the observed differences in elemental ratios throughout the sample. In general, the EDS mapping confirms the uniformity of the atomic distributions of the elements.

The Brunauer–Emmett–Teller (BET) surface area, BJH surface area, BJH desorption average pore diameter, and pore volume of the synthesized powder are summarized in

Table 2. Textural properties of samples.

Parameter	Sample 1	Sample 2	Sample 3
SBET, m2g−1	4.45	7.22	6.89
SBJH, m2g−1	3.12	5.48	4.98
Dp, nm	12.04	13.14	12.97
Vp, cm3g−1	0.027	0.036	0.031

. The pore size distribution was calculated by the Barrett–Joyner–Halenda (BJH) method, using the nitrogen adsorption branch of the isotherm. The obtained particles have an average mesopore diameter of about 12 nm. The data indicate a layered structure of the obtained samples containing MXenes.

2.6. Photocatalytic Activity of Samples

Figure 7 illustrates the kinetic curves of the photocatalytic decomposition of a dye mixture (Reactive Red 6C (RR6C), rhodamine B (RhB), and methylene blue (MB)) in the presence of MXenes, highlighting the effectiveness of these materials in removing dyes from aqueous solutions. The synthesized MXene structures demonstrate removal efficiencies ranging from 65% to 98%, depending on the specific class of dye targeted. The highest photocatalytic activity was observed for methylene blue. Methylene blue is a cationic dye, and MXenes, which possess negatively charged surfaces due to the presence of -Cl groups, exhibit a strong electrostatic attraction toward MB molecules. This interaction enhances the adsorption of MB onto the MXene surface, increasing the local concentration of the dye and facilitating its degradation. When MXene is exposed to light with energy equal to or greater than its bandgap, electrons in the valence band are excited to the conduction band, resulting in the formation of electron–hole pairs. The surface functional groups and structural defects of the obtained samples play a crucial role in enhancing charge separation by trapping electrons or holes, thereby reducing recombination rates. The photogenerated electrons and holes participate in redox reactions at the surface of the MXene. Electrons in the conduction band reduce oxygen molecules to form reactive oxygen species (ROS), such as superoxide radicals (O₂⁻·) and hydrogen peroxide (H₂O₂). Simultaneously, holes in the valence band can oxidize water or hydroxide ions to produce hydroxyl radicals (·OH). The ROS and hydroxyl radicals generated during these redox reactions attack the dye molecules, breaking them down into smaller, less harmful products, such as CO₂, H₂O, and mineral acids. Sample 1 (Ti₂CT_x), which has the largest bandgap (1.71 eV), may absorb higher-energy photons, potentially limiting its efficiency under visible light but making it suitable for UV-driven photocatalysis. Sample 2 (Mo₂CT_x), which has the smallest bandgap of 1.07 eV, is more effective under visible light because it can absorb a broader range of wavelengths. However, the smaller bandgap leads to faster electron–hole recombination, which reduces the overall photocatalytic efficiency. Sample 3 (Mo₂TiC₂T_x), which has an intermediate bandgap of 1.42 eV, strikes a balance between light absorption and charge separation, potentially providing the optimal performance for visible-light-driven photocatalysis. Notably, the sorption stage, which represents the initial adsorption of dyes onto the surface of the MXenes before photocatalytic activity commences, accounts for only 20–25% of the total removal efficiency. However, an exception is observed with Sample 2 concerning methylene blue, where the sorption contribution increases to approximately 35%. This enhanced sorption capacity for MB aligns with data obtained from BET analysis, which indicate that Sample 2 has a larger specific surface area compared to other samples, thereby improving its ability to adsorb MB molecules.

Table 3. The efficiency of dye decomposition under ultraviolet radiation during each cycle of photocatalysis.

	Photocatalytic Decomposition Efficiency, %
	Cycle 1			Cycle 2			Cycle 3
	RR6C	RhB	MB	RR6C	RhB	MB	RR6C	RhB	MB
Sample 1	61.8	71.8	88.1	56.2	68.2	84.6	55.1	66.2	82.0
Sample 2	62.6	64.5	82.4	57.6	59.3	75.0	54.7	55.8	72.0
Sample 3	69.1	70.2	97.4	65.6	67.4	93.5	63.0	64.7	90.7

shows the degree of decomposition of dyes as a result of irradiation with ultraviolet light after each cycle. The data obtained show that the efficiency of the investigated photocatalysts decreases with each cycle. This may result from residual dye adsorbed in the material’s pores, which is not fully removed by standard washing with distilled water.

A series of experiments with traps was carried out to detect reactive species during photocatalysis. Isopropyl alcohol (IPA), p-benzoquinone (p-BQ), and ethylenediaminetetraacetate acid (EDTA) were used as the scavengers for OH, O₂⁻·, and h⁺, respectively. The results are presented in Figure 8. As various scavengers are added, the dye destruction decreases. For the RR6C and RhB dyes, OH· played a key role in the photocatalytic process, while in the case of the MB dye, the main reactive species responsible for photodecomposition were O₂⁻· and h⁺.

Figure 9 shows the pseudo-first-order model kinetics of the photocatalytic degradation of dyes in an aqueous solution of the samples exposed to ultraviolet radiation, based on the following equation [37]:

$$\ln \left({\displaystyle \frac{C}{C_0}}\right)=-kt,$$

(11)

where C₀ is the initial concentration, C is the concentration during irradiation over time, and k is the pseudo-first-order rate constant.

Table 4. Kinetic parameters of photocatalysis for samples.

Dye	k, min−1	knorm, min−1·m−2	R2
	Sample 1
RR6C	0.01013 ± 4.8·10−4	0.07588 ± 0.00359	0.98
RhB	0.01613 ± 3.9·10−4	0.12082 ± 0.00292	0.99
MB	0.03191 ± 0.00021	0.23902 ± 0.00149	0.96
	Sample 2
RR6C	0.01071 ± 0.0015	0.04944 ± 0.00692	0.86
RhB	0.01313 ± 5.8·10−4	0.06062 ± 0.00267	0.98
MB	0.01985 ± 0.0011	0.09164 ± 0.00507	0.97
	Sample 3
RR6C	0.01413 ± 9.8·10−4	0.06836 ± 0.00474	0.96
RhB	0.01609 ± 0.0012	0.07784 ± 0.00581	0.95
MB	0.03177 ± 0.0019	0.15371 ± 0.00918	0.97

presents the rate constants and rate constants normalized by surface area for the photocatalytic decomposition of dyes in the mixture. Analysis of the data indicates that the removal rate is influenced not only by the composition of the MXenes but also by the class of dyes used. The photocatalytic decomposition rates followed the order MB > RhB > RR6C. This trend correlates with the structural properties of both the dyes and the MXenes studied. For example, MB, a cationic dye with a planar structure, exhibits stronger interactions with the negatively charged surface of the MXenes, resulting in faster degradation kinetics. In contrast, RR6C, which possesses a more complex molecular structure and lower affinity for the MXene surface, decomposes at a slower rate. Variations in the elemental composition, layer spacing, and surface functional groups of the MXenes significantly affect their photocatalytic performance.

Plasma–liquid synthesis produces MXenes with higher bandgap values compared to those etched with hydrofluoric acid. Additionally, plasma–liquid-synthesized MXenes, particularly Mo₂TiC₂Cl_x, demonstrate significantly higher efficiency in degrading methylene blue (98% degradation) under UV light compared to most HF-etched MXenes (

Table 5. Comparison of the current study outcomes with other reports on dye photocatalytic processes, concerning MXene.

MXene	Synthesis Conditions	Eg, eV	SBET, m2/g	Photocatalysis Conditions	Efficiency	Ref.
Ti3C2Tx	Etching using HF-forming etchants (LiF and HCl)	-	6.138	Visible light, t = 120 min, Cdye = 10 mg/L, mph = 30 mg	MB—81.2% RhB—17.3% MO—2.8%	[14]
Ti3C2Tx	MXene synthesized with HF/tetramethylammonium hydroxide	1.05	-	Visible light, t = 180 min, Cdye = 25 mg/L, mph = 3.2 mg	MB—100% BG—100%	[38]
Ti3C2Tx	Used the HCl/LiF method, known as minimally intensive layer delamination	0.97	-	Visible light, t = 180 min, Cdye = 25 mg/L, mph = 3.2 mg	MB—100% BG—100%	[38]
Ti3C2	HF treatment	-	-	UV light, t = 60 min, Cdye = 10 mg/L, mph = 50 mg	MB—60%	[39]
Ti3C2	Etching using HF	2.01		Visible light, t = 120 min, Cdye = 100 mg/L, mph = 100 mg	CR—80%	[40]
Ti3C2	Etching using HF	-	-	UV light, t = 45 min, Cdye = 20 mg/L, mph = 100 mg	MO—60%	[41]
Ti3C2	Etching using HF	-	0.37	Solar light, t = 480 min, Cdye = 10 mg/L, mph = 100 mg	RhB—30%	[42]
Mo2CTx	Etching using HF-forming etchants (LiF and HCl)	--	-	Visible light, t = 300 min, Cdye = 5 mg/L, mph = 25 mg	MB—75%	[43]
Ti3C2Tx	Plasma–liquid synthesis	1.71	4.45	UV light, t = 60 min, Cdye = 1.2 mg/L, mph = 30 mg	MB—89% RR6C—73% RhB −65%	This work
Mo2CTx	Plasma–liquid synthesis	1.07	7.22		MB—83% RR6C—66% RhB—65%
Mo2TiC2Clx	Plasma–liquid synthesis	1.42	6.89		MB—98% RR6C—69% RhB—66%

MB—methylene blue; RhB—rhodamine B; MO—methyl orange; BG—bromocresol green; CR—Congo red; RR6C—Reactive Red 6C; m_ph—mass of photocatalyst; C_dye—dye concentration.

).

3. Materials and Methods

The DC power supply BP-0.25-2 (LLC TD ARS THERM, Novosibirsk, Russia), which provides an output voltage of up to 10 kV (Figure 10), was employed in conjunction with a 1000 Ω ballast resistor to stimulate the discharge process [23]. The emission spectra were recorded using an AvaSpec-3648 (Avantes B.V., Apeldoorn, The Netherlands) spectrometer equipped with a 1200 mm⁻¹ grating and a slit size of 25 µm. The spectra recordings were taken during the discharge at an average current of 0.3 A. The optical radiation receiver, fitted with a collimating lens, was positioned 5 cm away from the plasma zone. Voltage and current waveforms were captured using the ADS-2072 digital multichannel oscilloscope (JSC NPP ELIKS, Moscow, Russia).

The experiments were conducted in a glass cell with a fixed volume of 180 mL. The TCA thermocouple in a set with a RESANTA DT9208A digital multimeter was used to control the temperature of the solution in the cell. Titanium and molybdenum wires (Shenzhen Tangda Technology Co., Ltd., Shenzhen, China, purity 99.99%), each with a diameter of 1.0 mm, served as the electrodes and were used without any pre-treatment. These electrodes were housed in a refractory ceramic mullite tube, ensuring a consistent inter-electrode distance of 1.0 mm. Carbon tetrachloride (JSC Ekos-1, Moscow, Russia, purity 99.99%) acted as the medium for the pulsed discharge. It is important to note that after synthesis, the solvent was not utilized, the synthesis products were centrifuged, and the solvent was subsequently reused.

Following the experiment, the dispersion was centrifuged at 6000 rpm for 1 h using a UC-6000E centrifuge (ULAB, Nanjing, China). The precipitate was then dried in air at room temperature. The supernatant could be reused for the plasma–liquid synthesis of MXenes. The resulting structures were characterized through various analytical techniques: scanning electron microscopy (Quattro S, Thermo Fisher Scientific, Praha, Czech Republic), X-ray phase analysis (D2 Advance X-ray diffractometer with CuK_α source, Bruker, USA), energy-dispersive X-ray spectroscopy (EDS, Thermo Fisher Scientific, Praha, Czech Republic), and Raman spectroscopy (Confotec NR500 Raman microscope with 532 nm excitation wavelength, SOL instruments, Minsk, Belarus). Measurements of the specific surface area by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were conducted on a NOVAtouch NT LX-1 Quantachrome analyzer at 77 K (Quantachrome Instruments, Boyton Beach, FL, USA).

The photocatalytic activity of the synthesized MXene powders was evaluated by measuring the rate of dye decomposition in an aqueous solution under both dark and UV-irradiation conditions. A mixture of dyes was used for the tests, including Reactive Red 6C (RR6C, anionic dye, λ_max = 533 nm), rhodamine B (RhB, zwitterionic xanthene, λ_max = 554 nm), and methylene blue (MB, cationic thiazine, λ_max = 667 nm), each at a concentration of 1.2 mg L⁻¹. The reaction was conducted in an 800 mL cylindrical vessel, which was placed within a water-cooled quartz jacket. The ultraviolet radiation source was a high-pressure 250 W mercury lamp, with a maximum emission of 365 nm. In each experiment, 0.03 g of the synthesized MXene powder was added to the 800 mL dye solution. The concentration changes of each dye were determined using an SF-56 spectrophotometer, covering the wavelength range of 350–750 nm (OKB Spektr, Saint Petersburg, Russia). The percentage removal of each dye was calculated according to Equation (1):

$$Dye remorval\left(\%\right)={\displaystyle \frac{C_0-C_t}{C_0}}·100\%,$$

(12)

where C₀ and C_t represent the initial concentration of the dye and the concentration at a given irradiation time, respectively.

4. Conclusions

This study successfully demonstrated the one-step synthesis of MXenes containing titanium (Ti), molybdenum (Mo), and Ti-Mo composites through pulsed discharges in carbon tetrachloride. By employing titanium and molybdenum electrodes in various configurations, three distinct samples of MXenes were synthesized. The plasma–liquid synthesis process, characterized by high temperatures and the localized overheating of the solvent, facilitated the decomposition of CCl₄ into reactive species and interactions with the electrode materials, leading to the formation of MXenes layer by layer. Characterization techniques, including UV-Vis spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, confirmed the successful synthesis of high-purity MXenes. Synthesized MXenes are semiconductors characterized by relatively low bandgap values, likely attributable to the specific conditions involved in their synthesis using the plasma–liquid method. The photocatalytic performance of the synthesized MXenes was evaluated using a dye mixture, revealing removal efficiencies of up to 98%. Methylene blue exhibited the highest degradation rate, which can be attributed to its strong interaction with the negatively charged surfaces of the MXenes. Kinetic studies indicated that the removal rate followed the order methylene blue > rhodamine B > Reactive Red 6C, correlating with the structural properties of both the dyes and the MXenes. This study highlights the potential of plasma-assisted synthesis as a safe, efficient, and scalable method for producing tunable MXenes. The synthesized MXenes exhibit promising applications in environmental remediation, particularly in the removal of organic pollutants from aqueous solutions. Future research could concentrate on optimizing the synthesis conditions and investigating the use of these materials in other advanced applications.