Plasma–Liquid Synthesis of PLA/MXene Composite Films and Their Structural, Optical, and Photocatalytic Properties

Abstract

This study addresses the need for sustainable, high-performance photocatalytic materials by developing novel polylactide (PLA)/MXene composites. A one-step plasma-liquid synthesis method was employed, utilizing a direct current discharge between metal electrodes (Ti, Mo) in a carbon tetrachloride and PLA solution. This single-step process simultaneously exfoliates MXene nanosheets (Ti₂CCl_x, Mo₂CCl_x, Mo₂TiC₂Cl_x) and incorporates them into the polymer matrix. The resulting composite films exhibit a highly porous morphology and significantly enhanced optical absorption, with band gaps reduced to 0.62–1.15 eV, enabling efficient visible-light harvesting. The composites demonstrate excellent photocatalytic activity for degrading a mixture of organic dyes (Methylene Blue > Rhodamine B > Reactive Red 6C) under visible light. The developed plasma-liquid technique presents a streamlined, efficient route for fabricating visible-light-driven PLA/MXene photocatalysts, offering a sustainable solution for advanced water purification applications.

1. Introduction

Polylactide (PLA) is a biodegradable aliphatic polyester synthesized from renewable resources, which positions it as a sustainable alternative to conventional petroleum-based plastics [1,2]. For photocatalytic applications, PLA offers a unique combination of advantages as a catalyst support matrix: it is derived from renewable resources, compostable, and can be processed into flexible, self-supporting films [3,4]. These characteristics are crucial for developing sustainable environmental remediation technologies that avoid the secondary pollution associated with persistent plastic waste.

However, the inherent properties of pristine PLA present significant challenges for photocatalysis. Its poor barrier properties against ultraviolet (UV) radiation, with transmittance exceeding 80% in the UV range (280–315 nm), allow most light to pass through unused, rendering it inefficient for light-driven processes [5]. Furthermore, the material’s exceptionally low thermal conductivity (~0.13–0.25 W/m·K) can impede heat dissipation during light irradiation, potentially leading to localized overheating and reduced catalyst stability [6]. Perhaps most critically, its electrically insulating nature prevents it from participating in the crucial charge separation and transport processes that underpin photocatalytic activity.

To transform PLA from a passive support into an active component of a photocatalytic system, researchers have focused on incorporating functional nanofillers to create composites [7,8]. Among these, MXenes—a family of two-dimensional transition metal carbides, nitrides, and carbonitrides—have emerged as exceptionally promising candidates [9,10]. First discovered in 2011, MXenes are produced by selectively etching the A-layer from MAX phase precursors, resulting in materials with the general formula M_n+1X_nT_x, where M is an early transition metal (e.g., Ti, Mo), X is carbon and/or nitrogen, and T denotes surface terminations (e.g., –O, –F, –OH) [11]. MXenes possess a unique combination of properties highly sought after for photocatalysis: metallic electrical conductivity (up to 20,000 S/cm) to facilitate charge separation and transport and tunable optical absorption across the visible and near-infrared spectrum [12,13]. Their hydrophilic nature also facilitates dispersion in polar solvents and polymer matrices like PLA.

The integration of MXenes into PLA has been shown to directly address the polymer’s limitations. For instance, MXene incorporation can provide nearly 100% UV shielding [14] and enhance thermal conductivity [15], mitigating PLA’s transparency and heat dissipation issues. More importantly, the conductive MXene network can serve as a platform for harvesting light and shuttling photogenerated charges, thereby introducing photocatalytic functionality to the composite. However, conventional fabrication of PLA/MXene composites remains a complex, multi-step process often involving pre-synthesized MXenes, hazardous etchants (e.g., HF), and separate mixing stages. These steps can lead to oxidation, agglomeration, and high production costs [16,17].

Film photocatalysts based on polylactide represent a significant advancement in sustainable environmental remediation technology. By embedding photocatalytic nanoparticles, such as TiO₂ or ZnO, into a biodegradable PLA polymer matrix, researchers create flexible, self-supporting films capable of harnessing light energy to degrade organic pollutants and pathogens. These composite films show great promise for applications in water and air purification, as demonstrated in studies on pollutant degradation [18], as well as for developing antibacterial surfaces [19]. Ongoing research focuses on enhancing photocatalytic efficiency and stability while fully maintaining the biodegradable nature of the polylactide matrix.

Previous research has demonstrated that low-temperature plasma in contact with liquid can effectively synthesize polylactide-based composites [20]. Separately, it has been shown that a direct current discharge between metal electrodes in carbon tetrachloride (CCl₄) enables the one-step synthesis of MXene particles [21]. Building upon these findings, this study introduces a novel, streamlined one-step plasma-liquid synthesis method for producing PLA/MXene composites. This innovative approach simultaneously exfoliates MXene nanosheets (Ti₂CCl_x, Mo₂CCl_x, Mo₂TiC₂Cl_x) from metal electrodes and incorporates them directly into the dissolving PLA matrix within a single reactor. This eliminates the need for complex chemical processing, expensive precursors, and multiple steps, offering a more efficient and scalable route to fabricate high-performance photocatalytic films. The structural, optical, and photocatalytic properties of the resulting composites were thoroughly investigated for the degradation of organic dyes under visible light.

2. Results and Discussions

2.1. Structural Properties of Composite Films

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

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

Name	Experimental Conditions	Chemical Composition	Content of MXenes in the Composite Film, wt%
Sample 1	Underwater plasma in CCl4 polylactide solution (2 wt%) at discharge current of 0.25 A; Ti—anode, Ti—cathode	PLA/Ti2CClx	1.75 ± 0.08
Sample 2	Underwater plasma in polylactide solution in CCl4 (2 wt%) at discharge current of 0.25 A; Mo—anode, Mo—cathode	PLA/Mo2CClx	1.54 ± 0.11
Sample 3	Underwater plasma in polylactide solution in CCl4 (2 wt%) at discharge current of 0.25 A; Ti—anode, Mo—cathode	PLA/Mo2TiC2Clx	1.69 ± 0.04

). The MXene content in the composites was determined based on the measured sputtering rates of the electrodes during the discharge burning.

The X-ray diffraction patterns of PLA and three PLA/MXene composites—PLA/Ti₂CCl_x, PLA/Mo₂CCl_x, and PLA/Mo₂TiC₂Cl_x—display a combination of diffraction peaks corresponding to the semi-crystalline structure of PLA and the characteristic (002), (008), and (103) planes of MXene (Figure 1a). For PLA, a broad amorphous halo is observed centered on 2θ ≈ 16.5°, which is typical of semi-crystalline PLA. Additionally, weaker crystalline peaks appear near 2θ ≈ 19° and 2θ ≈ 21°, corresponding to the (110) and (200) planes of PLA’s α-form. For composites, the prominent peaks located at 9°, 26 and 33° correspond to the diffractions of the (002), (008), and (103) planes of MXenes, respectively [22]. The (002) peak appears at low angles (2θ ≈ 7–9°), indicating a large interlayer spacing due to Cl termination. It should be noted that the (002) peak of MXene shift slightly due to polymer intercalation. The amorphous halo of PLA may partially obscure the weaker MXene peaks. At the same time, the crystallinity of PLA slightly increases because MXene acts as a nucleating agent, as evidenced by sharper PLA peaks in the composites. The (002) peak is intense and appears at low angles, confirming the retention of the layered structure; however, the absence of major shifts in PLA peaks suggests weak interfacial bonding. The d-spacings of the composites were calculated using the following formula:

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

(1)

In this equation, n represents the order of reflection, λ denotes the wavelength of CuKα (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 PLA/Ti₂CCl_x to PLA/Mo₂TiC₂Cl_x, changing from 25.47 Å to 28.33 Å. The d parameter varies significantly between 19 Å and 59 Å across different studies [23], as the interlayer spacing can differ due to the presence of various ionic species. In our case, the chloride anion occupies the interlayer space in MXenes. Compared to previously synthesized MXenes produced using the plasma-liquid method under similar conditions [22], the presence of the polymer increases the interlayer spacing.

Fourier-transform infrared (FTIR) spectroscopy was employed to characterize the interactions between MXenes and PLA, as shown in Figure 1b. A clear distinction was observed between the spectra of PLA before and after the incorporation of discharge-treated MXene particles, indicating chemical interactions between the two components. Key vibrational modes were identified in the FTIR spectra. The peaks at 1178 cm⁻¹ and 2992 cm⁻¹ were assigned to the C–O and C–H stretching vibrations, respectively, associated with the –CH(CH₃)–OH terminal group of PLA [24]. The C=O carbonyl stretching vibration, typically observed at 1747 cm⁻¹, exhibited splitting, which may be attributed to the presence of the –CH–CO–O– group in the PLA backbone. The peaks at 1449 cm⁻¹ and 1357 cm⁻¹ correspond to the bending vibrations of –CH₃ and –CH– groups, including both symmetric and asymmetric deformations. The absorption band at 860 cm⁻¹ was assigned to C–COO bond stretching [25]. The interaction between PLA chains and MXenes was primarily associated with shifts in the peaks at 1747 cm⁻¹ (C=O) and 1449 cm⁻¹ (–CH₃), suggesting weak interfacial bonding. A redshift in these peaks indicates possible hydrogen bonding or van der Waals interactions between the –C=O and –CH₃ groups of PLA and the surface functional groups of MXenes [20,26]. Additionally, a weak peak corresponding to the C–Cl bond was detected, which may originate from residual chloride-terminated groups on the MXene layers [27]. However, its low intensity suggests minimal presence. Furthermore, the presence of Me–O bonds was confirmed in the spectra, likely due to surface oxidation or hydroxylation of MXenes during synthesis. The FTIR analysis confirms successful interaction between PLA and MXenes, with spectral shifts indicating non-covalent interactions at the interface. These findings support the potential for improved interfacial adhesion in MXene-reinforced PLA composites.

Figure 1c presents the DSC thermograms of neat PLA and PLA-MXene composites obtained at a slow heating rate of 0.1 °C/min to enhance the resolution of thermal transitions. Key thermal transitions—glass transition (T_g) and melting (T_m)—were analyzed to investigate the effects of MXene incorporation and plasma treatment on the thermal behavior of PLA. Neat PLA exhibited a T_g at approximately 60 °C, characteristic of its semi-crystalline nature. The composites showed a 5–10 °C increase in T_g, suggesting restricted polymer chain mobility due to interfacial interactions with MXenes (e.g., hydrogen bonding between PLA’s –C=O/–OH groups and MXenes’ surface groups) and potential crosslinking effects induced by plasma treatment [28].

Neat PLA exhibited a single endothermic peak at 172.5 °C, corresponding to the melting of α-crystalline domains. The composites showed a higher melting temperature (T_m) of 175–178 °C, indicating enhanced crystallinity due to MXenes acting as nucleating agents and plasma-induced stabilization of crystalline regions [29]. Additionally, a broader melting peak was observed, suggesting a heterogeneity in the crystal size distribution. The larger endothermic peak area in the composites implies increased crystallinity (up to approximately 10–12%, calculated using ΔH_m/ΔH°_m, where ΔH°_m = 93 J/g for crystalline PLA [29]). Therefore, plasma treatment may introduce polar groups (e.g., –COOH, –OH) that facilitate adhesion between MXenes and PLA.

SEM analysis, presented in Figure 2, reveals a stark contrast in surface morphology between the pristine polylactide PLA film and the PLA/MXene composite films. The micrograph of the unmodified PLA film (Figure 2a) exhibits a characteristically smooth, homogeneous, and virtually featureless surface. This uniform topography is typical of solvent-cast PLA films and indicates a dense, non-porous internal structure. In direct contrast, the composite films incorporating MXene nanosheets (Figure 2b–d) display a highly porous and architecturally complex surface morphology. The pronounced microporosity likely results from the solution-casting process, where the introduction of MXene nanoplatelets disrupts polymer chain packing and alters solvent evaporation dynamics. The MXene sheets themselves are clearly visible as distinct, flake-like inclusions embedded within the polymer matrix and protruding from the pore walls. This transformation in surface structure has significant implications for the material’s properties, as the developed porosity is expected to increase the specific surface area.

2.2. Optical Properties of Composite Films

In the UV-Vis absorption spectra ranging from 250 to 850 nm, the PLA exhibits no pronounced peaks, with only a slight increase in absorption between 450 and 850 nm (Figure 3a). In contrast, the polylactide/MXene composite demonstrates absorption that is approximately 100 times higher across the entire wavelength range, with a notable increase in the 250–400 nm region. This phenomenon can be attributed to the incorporation of MXene, which fundamentally alters the optical properties of the PLA matrix. The hundredfold increase in absorption throughout the 250–900 nm range primarily results from the exceptional, broad-spectrum light-absorbing capabilities of MXene. Unlike the insulating and largely optically transparent PLA, MXenes are conductive two-dimensional materials composed of transition metal carbides. Their electronic structure enables strong light interactions through a combination of mechanisms: free-electron intraband transitions, which contribute to the high baseline absorption across visible and near-infrared wavelengths, and interband transitions, responsible for the significant enhancement in the ultraviolet to blue light region (250–400 nm). This specific increase suggests that the bandgap energy of this MXene corresponds to photons in this higher-energy range, producing a pronounced absorption peak. Additionally, the large surface area and potential plasmonic effects of the MXene nanosheets dispersed within the PLA create an effective percolation network that traps light, minimizing reflection and transmission while maximizing absorption throughout the composite film.

The band gap energies were derived from the absorption spectra by applying the Tauc plot method [30]:

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

(2)

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.

For all samples, the dependence of (αhν)^1/2 on photon energy (hν) was extrapolated to (αhν)^1/2 = 0 to determine E_g, corresponding to an indirect band transition (n = 1/2) (Figure 2b). This approach is justified by density functional theory calculations confirming that MXenes exhibit indirect band gaps [31]. The calculated band gap values, presented in Figure 3b, decrease in the sequence: PLA/Ti₂CCl_x (1.15 eV) > PLA/Mo₂TiC₂Clₓ (0.92 eV) > PLA/Mo₂CCl_x (0.62 eV).

The pure polylactide PLA near-IR spectrum exhibits only two small peaks, whereas the PLA/MXene composites display the richer spectra with numerous and more intense peaks (Figure 3c). This suggests that most of the new peaks do not originate from PLA itself but are associated with the presence of MXene and, importantly, the interaction between PLA and MXene.

Peaks observed in pure PLA at 2253 nm and 2286 nm are attributed to combination bands of C-H stretching and deformation vibrations (e.g., CH₃ asymmetric stretch plus deformation) [32]. Their small intensity is typical, as these are forbidden transitions that acquire only weak intensity.

The peaks at 1680 nm and 1720 nm correspond to the region of the first overtone of C–H stretching vibrations [32]. The pure PLA signal in this region is likely very weak. The presence of these peaks suggests that the MXene surface has organic terminators (T_x) containing C–H bonds or, more likely, that the MXene enhances the signal of the PLA’s C–H groups through a surface-enhanced effect or by improving the dispersion and scattering properties of the sample.

The peak at 1905 nm corresponds to the O–H combination band from water (O–H stretch + O–H bend) [32]. Its strong presence is a direct indicator of water adsorbed on the hydrophilic MXene surface. The peak at 2120 nm is a combination band region that can involve several vibrations, such as O–H stretching combined with C–O stretching or C–H stretching combinations. Its appearance suggests new vibrational modes at the interface between PLA (C=O, C–O–C, C–H) and the MXene surface groups (O–H). The enhanced intensity of the peak at 2324 nm in the composite suggests that the local environment of PLA’s methyl groups is altered by the MXene, affecting the transition probability. The bands at 2415 nm and 2447 nm fall within the region of O–H combination bands (symmetric stretch + bend) and the second overtone of the C=O stretch. The strong O–H signal again highlights the hydrophilic nature of MXene and the presence of surface hydroxyl groups. Thus, the rich NIR spectrum of the PLA/MXene composite, compared to the featureless spectrum of pure PLA, provides strong evidence for the successful incorporation of MXene. The enhancement of peaks related to PLA’s functional groups (e.g., C–H at 2324 nm) and the appearance of new combination bands (e.g., 2120 nm) suggest physical or chemical interactions between the PLA polymer chains and the MXene surface [33].

The photoluminescence (PL) spectra of the synthesized samples, recorded at an excitation wavelength of 330 nm, are presented in Figure 3d. The measured optical band gaps of the composites are quite narrow, ranging from 0.62 eV to 1.15 eV. The corresponding emission from band-edge recombination would occur in the IR region, specifically between 1070 nm and 2000 nm. However, photoluminescence arising from defect states or surface states is observed in the visible to near-IR range for MXenes [34]. The PLA/Ti₂CCl_x sample has the widest band gap (1.15 eV) and exhibits the strongest photoluminescence signal, with a peak at 750 nm, indicating the highest rate of radiative recombination. In the PLA/Mo₂TiC₂Cl_x composite, the peak position is slightly redshifted due to a narrower band gap. This sample shows quenched PL intensity compared to PLA/Ti₂CCl_x, indicating improved charge separation. Meanwhile, the PLA/Mo₂CCl_x composite demonstrates the furthest redshift of the peak, corresponding to its narrowest band gap of 0.62 eV. PL spectrum exhibits the most severe quenching of the emission peak. The low intensity indicates that most photo-generated charges are being separated and utilized for photocatalysis rather than recombining radiatively. All composite spectra display a similar broad, asymmetric peak shape, which is characteristic of defect-related or trap-state emission in nanomaterials.

2.3. Photocatalytic Activity of Samples

Figure 4 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 composite films as photocatalysts. The absorption spectra of the dye mixture solution after photocatalytic degradation and irradiation of different durations are shown in Figure S1 in the Supplementary Material File. The process of photocatalysis results from the synergistic combination of the properties of MXene and the porous structure of the PLA film. MXenes, such as Ti₂CCl_x, Mo₂CCl_x, and Mo₂TiC₂Cl_x, are not only conductors but are also emerging as excellent photocatalysts. Unlike traditional TiO₂, these MXenes possess a narrow band gap, enabling them to absorb energy directly from visible light photons.

When a photon of visible light with energy equal to or greater than the MXene’s band gap strikes its surface, it excites an electron (e⁻) from the valence band (VB) to the conduction band (CB), leaving behind a positively charged hole (h⁺) in the valence band [35].

MXene + hν (visible light) → e⁻ (CB) + h⁺ (VB)

(3)

The SEM images reveal a porous structure of the composites. This feature is a key element of the composite that facilitates high photocatalytic efficiency.

The high porosity significantly increases the total surface area of the composite film. This provides numerous sites for dye molecules to adsorb onto the surface [36], facilitating close contact with the catalytic MXene flakes. The interconnected pore network facilitates the free diffusion of dye molecules from the bulk solution into the film, as well as the diffusion of reaction products out of the film. Without pores, only the very top surface would be active, significantly limiting performance.

The PLA matrix serves as a solid, flexible support that immobilizes the MXene flakes, preventing their aggregation—which reduces the active surface area—and leaching into the solution. This design makes the catalyst reusable and easy to handle.

Once the electron-hole pairs are generated, they initiate redox reactions that degrade the dye molecules.

The dye molecules first diffuse through the pores and adsorb onto the surface of the MXene flakes. The metallic conductivity of MXenes facilitates the rapid transport of photogenerated charges, increasing the likelihood of their participation in reactions.

The electron (e⁻) in the conduction band is transferred to dissolved oxygen (O₂) in the water, forming a superoxide radical anion (•O₂⁻) [37].

e⁻ + O₂ → •O₂⁻

(4)

The hole (h⁺) in the valence band can oxidize water molecules (H₂O) or hydroxide ions (OH⁻) to generate hydroxyl radicals (•OH) [37].

h⁺ + H₂O/OH⁻ → •OH

(5)

These highly reactive oxygen species (•O₂⁻ and •OH) are extremely potent oxidants. They non-selectively attack the complex organic structures of dye molecules, breaking apart the chromophore group—the component responsible for color—resulting in decolorization.

The active species involved in the photocatalytic process were identified through scavenger experiments. The following trapping agents were employed: isopropyl alcohol (IPA) to quench hydroxyl radicals (•OH), *p*-benzoquinone (p-BQ) to scavenge superoxide anion radicals (•O₂⁻), and ethylenediaminetetraacetic acid (EDTA) to capture photogenerated holes (h⁺). Figure S2 presents the corresponding photocatalytic efficiencies with the addition of these scavengers. A pronounced decrease in degradation activity was observed in all cases, confirming that •OH, •O₂⁻, and h⁺ are generated and participate in the reaction. •OH radicals are identified as the primary agents responsible for oxidizing RR6C and RhB. The more pronounced inhibitory effect of isopropanol compared to EDTA suggests that •OH radicals are generated not only through hole-mediated oxidation of H₂O/OH⁻ but also potentially via alternative pathways, such as the protonation of superoxide radicals (•O₂⁻ + H⁺ → HOO•), followed by further reduction and decomposition. Meanwhile, •O₂⁻ and h⁺ are the principal species responsible for the degradation of MB.

The active species further mineralize the molecules, breaking them down into smaller, harmless inorganic compounds such as CO₂, H₂O, NO₃⁻, SO₄²⁻, and others. The composite containing Mo₂CCl_x exhibits enhanced photocatalytic properties.

Mo₂CCl_x has a band gap sufficient to generate electron-hole pairs under visible light, while also being large enough to ensure that the photo-generated electrons and holes possess adequate energy to drive the desired reactions. Its band structure is probably better aligned with the redox potentials required to form •O₂⁻ and •OH radicals. Molybdenum-based MXenes (Mo₂CT_x) exhibit exceptionally high metallic conductivity [16]. This superior charge transport capability significantly reduces the rate of electron-hole recombination, thereby increasing the availability of charges to generate reactive oxygen species and degrade dyes.

In summary, the porous PLA film serves as an efficient matrix, maximizing exposure to and accessibility of the active MXene sites. The MXene, particularly Mo₂CCl_x, functions as an excellent visible-light harvester and catalyst, generating highly reactive species that mineralize organic dyes into harmless end products.

Figure 5 illustrates the degree of dye decomposition resulting from visible light irradiation after each cycle. The data indicate that the efficiency of the photocatalysts slightly decreases with each successive cycle. This decline may be attributed to residual dye adsorbed within the pores of the material, which is not entirely removed by standard washing with distilled water.

Figure 6 shows the pseudo-first-order model kinetics of the photocatalytic degradation of dyes in an aqueous solution of the samples exposed to visible light radiation, based on the following equation [38]:

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

(6)

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

Table 2. Kinetic parameters of photocatalysis for samples.

Dye	k, min−1	R2
	PLA/Ti2CClx
RR6C	0.01249 ± 0.00151	0.91
RhB	0.01269 ± 0.00152	0.91
MB	0.02274 ± 0.00117	0.96
	PLA/Mo2CClx
RR6C	0.01419 ± 0.0012	0.97
RhB	0.01662 ± 0.0011	0.96
MB	0.02416± 0.0022	0.94
	PLA/Mo2TiC2Clx
RR6C	0.01345 ± 0.0011	0.95
RhB	0.01442 ± 0.0012	0.95
MB	0.02313 ± 0.0019	0.96

presents the rate constants for the photocatalytic degradation of individual dyes in a ternary mixture. A detailed analysis reveals that the degradation kinetics are influenced by a complex interplay between the physicochemical properties of the target pollutants and the composition of the photocatalyst. The observed degradation rates consistently followed the order: MB > RhB > RR6C. This trend can be attributed to a complex interplay of factors, including the molecular structure, size, and charge of the target pollutants. The cationic dye MB has a relatively small, planar structure that likely facilitates stronger adsorption onto the catalyst surface. In contrast, the anionic dye RR6C features a bulkier, multi-component azo structure, which probably impedes its diffusion and access to active sites, resulting in the slowest degradation kinetics. The degradation rate of RhB, a zwitterionic dye whose net charge depends on pH, falls between those of MB and RR6C. Furthermore, the data clearly indicate that for each dye, the rate constant increases with higher MXene loading in the composite, underscoring the role of MXene in enhancing charge separation and photocatalytic efficiency. This structure-activity relationship provides critical insights for designing advanced photocatalysts.

3. Materials and Methods

All reagents used in this study were of analytical grade and were utilized in their original form without any purification. In particular, polylactide PLA with a density of 1.24 g/mL and a purity of over 98% was supplied by NatureWorks 4060D (Plymouth, MN, USA). Tetrachloromethane (99.5%) was obtained from Chimmed (Nizhny Novgorod, Russia). Titanium and molybdenum wires, a diameter of 1.0 mm and a purity of 99.99%, were sourced from Shenzhen Tangda Technology Co., Ltd. in Shenzhen, China.

The schematic of the setup is presented in Figure 7. The underwater direct current discharge was initiated between two metal rods (titanium or molybdenum). The electrodes were positioned within a ceramic tube with a diameter of 7 mm, which facilitated a constant interelectrode distance of 1.5 mm. polylactide pellets (Mw = 40,000) were dissolved in 120 mL of tetrachloromethane to achieve a concentration of 2 wt%. The resulting solution was placed in the discharge cell. The experiments were conducted in a glass cell with a fixed volume of 140 mL with a flat optical quartz window. A DC power supply (BP-0.25–2, LLC TD ARS THERM, Novosibirsk, Russia), capable of providing an output voltage of up to 10 kV, was employed in conjunction with a 1000 Ω ballast resistor to stimulate the discharge process. The average discharge current was measured at 0.25 A. The discharge emission spectra (λ = 200 − 950 nm) were recorded using the AvaSpec ULS3648 spectrometer (Avantes, Apeldoorn, The Netherlands), which has a resolution of 0.3 nm. The diffraction grating served as the dispersing element. During the experiment, the solution was agitated using a magnetic stirrer. The temperature of the solution during and after the discharge treatment was monitored using a chromel-alumel thermocouple connected to a digital multimeter (DT9208A, RESANTA, Moscow, Russia). The treatment duration was 5 min. To determine the quantitative yield of MXenes, the electrodes were weighed before and after the discharge ignition. An analytical balance (HR-150AZ, A&D Company, Tokyo, Japan), with a measurement error of 5%, was utilized. The experiments were repeated multiple times. Subsequently, the processed polylactic acid solution containing MXenes was poured onto a clean glass substrate measuring 10 × 10 cm. After casting the films, they were dried in a drying oven overnight (24 h) at 25 °C.

The resulting films were characterized using various analytical techniques, including scanning electron microscopy (Quattro S, Thermo Fisher Scientific, Prague, Czech Republic), X-ray phase analysis (D2 Advance X-ray diffractometer with CuKα source, Bruker, Billerica, MA, USA), and Fourier-transform infrared (FTIR) spectroscopy (VERTEX 80v spectrometer, Bruker Optics, Ettlingen, Germany). Samples for X-ray diffraction (XRD), weighing 1 g, were prepared using a planetary ball mill. The interpretation of diffractograms was performed with the aid of the open crystallographic database. The absorption spectra of the composite films were analyzed using an SF-56 spectrophotometer, operating in the wavelength range of 250–850 nm and equipped with a thin-film sample holder (SDB Spectr, Saint Petersburg, Russia). The spectra in the near-infrared region (950–2700 nm) were measured via an SF-256 BIK spectrophotometer (SBS Lomo Photonika, Saint Petersburg, Russia). The fluorescence spectra of the samples were obtained using a FluoTime 300 fluorometer (PicoQuant Berlin, Germany). The thermal properties of the samples were investigated using differential scanning calorimetry (DSC) with a DSC 204 F1 t-sensor (NETZSCH, Selb, Germany). The analysis was conducted over a temperature range of 25 to 250 °C at a heating rate of 10 K min⁻¹ in an argon atmosphere. The degree of crystallinity of the samples was calculated using the following formula [39]:

$$x={\displaystyle \frac{∆H_m}{∆H_m^0(1-n)}}\times 100\%,$$

(7)

where ΔH_m represents the measured enthalpy of melting obtained from DSC analysis, ΔH_m⁰ is the enthalpy of melting for 100% crystalline polylactic acid (PLA), which is 93 J·g⁻¹, and n denotes the mass fraction of the filler structures.

The photocatalytic activity of the obtained composite films was evaluated by measuring the rate of dye decomposition in an aqueous solution under both dark and visible light irradiation conditions. A mixture of dyes (

Table 3. Chemical structures of used dyes.

Name	Chemical Structure	Class of Dye	λmax, nm
Rhodamine B		zwitterionic xanthene	554
Methylene Blue		cationic thiazine	667
Reactive Red 6C		anionic	533

) was used for the tests, including Reactive Red 6C (RR6C, an anionic dye, λ_max = 533 nm), Rhodamine B (RhB, a zwitterionic xanthene dye, λ_max = 554 nm), and Methylene Blue (MB, a cationic thiazine dye, λ_max = 667 nm), each at a concentration of 1.2 mg/L. The reaction was conducted in a 500 mL cylindrical vessel. The visible light source was a Falcon Eyes QL-500BW halogen lamp (LLC FY, Kowloon, Hong Kong, China) with a power of 500 W, a color rendering index of approximately 100%, and an emission spectrum corresponding to sunlight in the wavelength range of 300–800 nm. The light source was positioned above the reaction cylinder, while the composite film, with a diameter of 10 cm, was fixed to the bottom of the cylinder on a support 1 cm in height. The dye mixture was constantly stirred in the reaction vessel; photocatalytic reactions were performed in air. Concentration changes of each dye were determined using an SF-56 spectrophotometer, covering the wavelength range of 400–700 nm (OKB Spektr, Saint Petersburg, Russia). The percentage removal of each dye was calculated according to Equation:

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

(8)

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

4. Conclusions and Outlook

This study successfully demonstrates the efficacy of a novel, one-step plasma-liquid synthesis method for fabricating advanced PLA/MXene composite films. The direct current discharge between metal electrodes (Ti, Mo) in a solution of carbon tetrachloride and PLA simultaneously generates MXene nanosheets (Ti₂CCl_x, Mo₂CCl_x, Mo₂TiC₂Cl_x) and incorporates them into the biodegradable polymer matrix, eliminating the need for complex multi-step procedures and costly precursors.

The comprehensive characterization of the composites confirmed their successful formation and revealed enhanced properties essential for photocatalysis. The incorporation of MXene transformed the smooth PLA surface into a highly porous, architecturally complex morphology, significantly increasing the specific surface area available for pollutant adsorption. Furthermore, the MXene nanosheets markedly altered the optical properties of PLA, inducing a hundredfold increase in broad-spectrum light absorption and reducing the band gap to values as low as 0.62 eV (for PLA/Mo₂CCl_x), thereby enabling efficient harvesting of visible light.

The composite films exhibited remarkable photocatalytic activity in degrading a ternary dye mixture under visible light irradiation. The degradation kinetics followed a pseudo-first-order model, with efficiency consistently in the order of Methylene Blue > Rhodamine B > Reactive Red 6C. This trend is influenced by the molecular structure of the dyes and their affinity for the catalyst surface. Among the synthesized composites, PLA/Mo₂CCl_x demonstrated superior performance, attributed to its optimal band gap for visible light absorption and potentially higher metallic conductivity, which facilitates rapid charge separation and minimizes electron-hole recombination.

In summary, this study presents a groundbreaking and streamlined approach for manufacturing high-performance, visible-light-driven photocatalysts. The plasma-liquid-synthesized PLA/MXene composites combine the exceptional catalytic properties of MXenes with the sustainability and practicality of a biodegradable PLA film matrix. These findings pave the way for the development of efficient, reusable, and environmentally friendly photocatalytic systems for advanced water purification and environmental remediation applications. Future work will focus on optimizing plasma parameters to achieve higher MXene loadings and exploring the composites’ efficacy against a broader range of pollutants.