Synthesis and Studies of PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel and Its Possible Applications

Abstract

In this study, a new hybrid hydrogel based on PAM (polyacrylamide)-Ag-g/WS₂/Ti₃C₂T_x was synthesized by radical polymerization using a conductive heterostructural nanocomposite WS₂/Ti₃C₂T_x. The synergy between the polymer matrix and the interface between two-dimensional nanomaterials ensured the production of a hydrogel with high extensibility and conductivity, as well as sensory characteristics. The composite hydrogel exhibited excellent strain-sensing capabilities, with gauge factors of 1.4 at low strain and 2.8 at higher strain levels. In addition, the material showed a fast response time of 2.17 s and a short recovery time of 0.46 s under cyclic stretching, which confirms its high reliability and reproducibility. The integration of Ti₃C₂T_x and WS₂ promoted the formation of a conductive network in the hydrogel structure, which simultaneously increased its mechanical strength and signal stability under variable loads. Measurements confirm some potential of the PAM-Ag-g/WS₂/Ti₃C₂T_x composite hydrogel as a flexible wearable strain sensor. Based on measured numbers, we discussed the impact of the WS₂/Ti₃C₂T_x interface on the Gauge factor and conductivity of the composite. Theoretical modeling demonstrates significant changes in the electronic structure of the WS₂/Ti₃C₂T_x interface, and especially the WS₂ surface, induced by substrate strain. Possible applications of the peculiar properties of PAM-Ag-g/WS₂/Ti₃C₂T_x composite were proposed.

1. Introduction

Transition metal dichalcogenides (TMDs), which have excellent mechanical and electrical conductivity, are among the most promising 2D materials for the creation of a generation of smart and flexible electronic devices [1]. Among TMDs, tungsten disulfide WS₂ is a notable candidate due to its large number of active sites and good electrical properties, and is also less toxic than graphene oxide and can therefore be used as a safer alternative in future applications [2]. The properties of TMDs can be improved by chemical modification. Various methods, such as cocatalyst loading, doping, and heterostructure design, have been used to improve the electrical conductivity of TMDs and expand their application [3,4,5]. WS₂ TMDs possess out-of-plane sulfur atoms that readily interact with surrounding chemical groups, making it easy to form heterostructures with other 2D or 1D nanomaterials, determining the growth and morphology of heterostructured nanocomposites [6]. MXene, a 2D multilayer material, exhibits excellent conductivity and can serve as a good conductive substrate material for the uniform growth of 2D dichalcogenide sheets on top of and between layers. The development of MXene-based hybrid systems is in its infancy compared to other established 2D materials. MXene-based hybrids with interesting hierarchical structures and excellent performances have attracted considerable interest [7,8].

Conductive nanomaterial-filled hydrogels offer excellent conductivity, elasticity, ease of fabrication, and durability, making them ideal for flexible sensors [6]. Driven by Internet of Things (IoT) advancements, the market for flexible electronics like wearable sensors has expanded significantly. Conductive hydrogel-based sensors translate human mechanical motion (pressure, strain, axial displacement) into electrical signals for real-time monitoring [7]. These sensors, combined with AI, enable data analysis and predictive capabilities. However, the limited mechanical strength of traditional conductive hydrogels has hindered their broader use in flexible electronics. For instance, Jia Pan et al. [1] successfully integrated MoS₂ into a flexible hydrogel-based sensor with a quick response time of 150 ms. Integrating WS₂/Ti₃C₂T_x heterostructure into a hydrogel matrix is a promising approach for developing next-generation flexible wearable strain sensors. Mxene has exceptional metallic conductivity, hydrophilicity, and tunable surface end groups (–O, –OH, –F), which facilitate strong interfacial interaction with other nanomaterials and hydrogel networks. The resulting WS₂/Ti₃C₂T_x heterointerface creates a 2D conductive network with an electric field, improving charge carrier mobility and sensor sensitivity. Recent reports demonstrate that interfacing WS₂ with Ti₃C₂T_x can yield 2D-2D heterostructures with improved charge transport and interfacial contact and synergistic sensing/electrocatalytic behaviors [9]. However, most WS₂/Ti₃C₂T_x studies focus on dry films/electrodes embedding a pre-formed WS₂/Ti₃C₂T_x heterostructures nanofiller directly into hydrogel networks remains comparatively underexplored.

PAM-Ag-g hydrogel, a composite of synthetic and natural polymers, is promising for flexible sensors, biomedical devices [10], and wearable technology [11]. Agar-agar (Ag-g) provides structural integrity and prevents dehydration by forming a physical gel network that retains significant water [12]. This combination creates a dual network structure exhibiting high stretchability, elasticity, structural stability, and water retention capacity [13]. In this work, we aimed to synthesize and characterize a PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel strain sensor with enhanced mechanical strength and sensitivity through interfacial engineering of a WS₂/Ti₃C₂T_x heterostructure.

2. Materials and Methods

2.1. Materials

Titanium aluminum carbide 312 MAX Phase (Ti₃AlC₂, ≥90%, ≤40 μm particle size, Sigma-Aldrich, Merck KGaA, Darmstadt, Germany), tangsten hexachloride (WCl₂, ≥99% Sigma-Aldrich, MilliporeSigma, Urbana, IL, USA), L-cysteine (C₃H₇NO₂S, Sigma-Aldrich, Buchs, Switzerland), ethanol (C₂H₅OH, Sigma-Aldrich, Taufkirchen, Germany), phosphoric acid (≥89%, Sigma-Aldrich, Buchs, Switzerland), Acrylamide (AM, CH₂=CHCONH₂, 99%, Sigma-Aldrich, Buchs, Switzerland), N,N′-methylene bisacrylamide (MBA, 99%, Sigma-Aldrich, Wuxi, China), Ammonium persulphate (APS, 98.5%, Sigma-Aldrich, Milwaukee, WI, USA), Agar (Ag-g, (C₁₂H₁₈O₉)_n Sigma-Aldrich, Buchs, Switzerland).

2.2. Materials Characterization

The chemical structure of the samples was analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet iS10 FT-IR Spectrometer, Thermo Fisher Scientific Inc., Ogden, UT, USA). The surface chemical composition of MXene/WS₂ was investigated via X-ray Photoelectron Spectroscopy (XPS, NEXSA Thermo Scientific). Scanning electron microscopy (SEM-EDS, JSM-7500F, JEOL Ltd., Yamagata, Japan) was used to study the morphology of sample surfaces. The crystal structures of samples were analyzed by XRD (SmartLab, Rigaku Co., Takatsuki, Japan, Cu Kα radiation, λ = 0.154056 nm). XRD data were obtained in the 2θ range from 20 to 80 °C at a scan rate of 6 deg. min⁻¹ using X-ray radiation 40 kV, 30 mA. Raman spectroscopy (The Horiba LabRam Evolution, Palaiseau, France) was used to study the structure and phase composition of the hybrid WS₂/Ti₃C₂T_x.

2.3. Synthesis

Synthesis of Ti₃C₂T_x MXene. Ti₃C₂T_x was prepared by selectively etching 1.0 g Ti₃AlC₂ powder was slowly added to 20 mL 48 wt% HF solution in a Teflon beaker under a fume hood with appropriate protective equipment. The mixture was stirred for 48 h at room temperature. The resulting mixture was then washed with deionized (DI) water until a neutral pH was reached. Finally, the Ti₃C₂ MXene was dried at 60 °C overnight in a vacuum oven.

Synthesis of WS₂. During the synthesis, 0.099 g (0.01 M) of WCl₆ and 0.061 g (0.02 M) L-cysteine were dissolved in 25 mL of distilled water under magnetic stirring at 400 rpm for 30 min at room temperature. The above solution was transferred to a 50 mL autoclave chamber. Then, the teflon-lined stainless-steel autoclave was heated at 180 °C for 24 h in a vacuum oven. The product was collected by centrifugation and repeated washing with ethanol and distilled water. The sample was dried at 60 °C overnight in a hot air oven. The powder was collected by centrifugation and repeated washing with ethanol and distilled water. The sample was dried at 600 °C overnight in a hot air oven.

Synthesis of WS₂/Ti₃C₂T_x. The layered hybrid structure WS₂/Ti₃C₂ was synthesized via a one-step hydrothermal method. A solution containing 0.005 M WCl₆, 0.02 M cysteine, and Ti₃C₂ was added to distilled water and stirred for 30 min at room temperature. The resulting mixture was then heated at 180 °C for 24 h in a 50 mL Teflon-lined autoclave. The black precipitate was collected by centrifugation, washed three times with distilled water, and dried in a vacuum oven at 60 °C for 12 h. A schematic illustration is presented in Figure 1a.

Synthesis PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel. PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel was synthesized via free radical polymerization. Acrylamide (1.5 g) and agar (400 mg) were dissolved in 10 mL of deionized water, followed by the addition of N,N′-methylenebisacrylamide (0.03 g) as a crosslinker and ammonium persulfate (0.03 g) as an initiator. After homogenization, WS₂/Ti₃C₂T_x nanofillers (0.2–1 wt% relative to total polymer solids), pre-dispersed in 2 mL of deionized water by bath sonication for 30 min was introduced. The mixture was then polymerized in a mold at 60 °C for 12 h to form the hydrogel (Figure 1b). A schematic illustration is presented in Figure 1b.

2.4. Mechanical Testing of PAM-Ag-hg/WS₂/Ti₃C₂T_x Hydrogel

The mechanical properties of the sample were evaluated using a tensile testing machine (Materials Testing Machine Z010/TN2S, Kennesaw, GA, USA), and the tests were performed at a tensile rate of 10 mm/min. For tensile testing, the hydrogels were made in the form of a rectangle (45 mm long, 20 mm high) and stretched at a strain rate of 5 mm min⁻¹. The strength was calculated by integrating the area under the stress–strain curve.

2.5. Sensor Characteristic of PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel

The sensor characteristics of the hydrogel were evaluated using a custom-made platform with a linear motor (LinMot PS01-37x120F-HP-C, Dongguan, China) operating at 5–11 Hz in contact-opening mode to simulate compression and decompression. Each end of the hydrogel was inserted into a copper wire connected to a TH LCR meter, and the sensors were adjusted to the LCR meter (LCR-106X, IVYTECH, Changzhou, China) at different resistor deformations. The output resistance was measured using Kitstar software (version 5.2, Kitware Inc., Clifton Park, NY, USA) at different strains, and a force sensor (Vernier LabQuest Mini, Beaverton, OR, USA) monitored the applied strain.

2.6. First-Principles Modeling

The atomic structure and energetics of various configurations were studied using DFT with the QUANTUM-ESPRESSO code [14] and the GGA-PBE [15] functional, taking into account van der Waals forces corrections [16]. For all calculations, we used ultrasoft pseudopotentials [17]. The values of energy cutoffs of 35 Ry and 400 Ry for the plane-wave expansion of the wave functions and the charge density, respectively.

Figure 1. Schematic illustration of the synthesis process of (a) WS₂/Ti₃C₂T_x and (b) PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel.

Figure 1. Schematic illustration of the synthesis process of (a) WS₂/Ti₃C₂T_x and (b) PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel.

3. Results and Discussion

3.1. Chemical Characteristics of WS₂/Ti₃C₂T_x

The FTIR spectroscopy analysis (Figure 2a) allowed us to identify the functional groups in the synthesized PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel composite, and to compare its spectral characteristics with those of pristine Ti₃C₂T_x, WS₂ and WS₂/Ti₃C₂T_x nanocomposite. The composite spectrum exhibits enhancement and broadening in the higher-wavenumber region 3200–3500 cm⁻¹, indicating the –NH₂ group of PAM [18], –OH group of Ti₃C₂T_x [19], and –OH group of agar-agar. The asymmetric stretching vibration of –CH₂ for PAM is evident at ~3050 cm⁻¹ [20]. A noticeable shift in this band, as well as shifts in other characteristics peaks, can be attributed to the strong interfacial interactions and hydrogen bonding between PAM chains and the WS₂/Ti₃C₂T_x [18]. The peaks at 2990 cm⁻¹ and 2890 cm⁻¹ were apparently due to the –C–H stretching from residual cysteine originating from the WS₂ synthesis. A slight shift the C=O stretching vibration ~1650 cm⁻¹ and ~1070 cm⁻¹ the C-O-C of agar-agar matrix [20]. The peak at ~1400 cm⁻¹ reflects the O–H deformation vibrations, and the peak at ~1300 cm⁻¹ reflects the C–F stretching vibrations [19] of Ti₃C₂T_x. The W-S stretching band near 600–700 cm⁻¹ that confirms the presence od WS₂ [21]. Taken together, these features indicate incorporation of the WS₂/Ti₃C₂T_x hybrid filler into the hydrogel matrix and the formation of an effective interfacial contact without of degradation of the polymer network.

To study the phase composition and crystal structure, XRD analysis was performed (Figure 2b). The obtained diffraction data for the WS₂/Ti₃C₂T_x composite were compared with the spectra of its individual components. For pure MXene, diffraction peaks were observed at 2θ ≈ 9.5°, 19.1°, 27.5°, 35.8° and 39.1°, corresponding to the (002), (004), (111), (200) and (110) planes of the Ti₃C₂T_x crystalline phase, indicating successful etching and exfoliation of the starting material. These data confirm the successful etching and exfoliation of MXene [22]. WS₂ showed clear diffraction peaks at 2θ ≈ 14.2°, 28.3°, 32.8°, 44.4°, 58.4°, and 63.2°, indexed to the (002), (004), (001), (103), (008), and (112) planes of hexagonal WS₂ (JCPDS Card No. 08-0237), indicating high crystallinity [23]. The WS₂/Ti₃C₂T_x composite exhibited a superposition of the individual components’ diffraction peaks, without any additional peaks, indicating successful composite formation without structural degradation. Minor peak shifts and intensity variations (particularly in the 25–35° region) suggest interphase interactions or partial WS₂ intercalation within the MXene layers. The X-ray diffraction results indicate the preservation of the crystalline structure of both components in the composite, which also points to the possible presence of synergistic interactions between the phases. The PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel spectrum exhibits characteristic peaks of MXene and WS₂, along with a broad amorphous peak at 2θ ≈ 23° from the PAM matrix [24], confirming the preserved crystallinity of the fillers and the polymer’s contribution.

Raman spectra (Figure 2c) showed characteristic vibrations for WS₂. The out-of-plane mode A₁g of sulfur at 458 cm⁻¹ and the in-plane mode E₁g associated with tungsten and sulfur atoms at 336 cm⁻¹, which confirms the presence of the WS₂ phase. The absence of distinct peaks for Ti₃C₂T_x phase could be attributed to the strong presence of Ti₃C₂T_x that reduces the overall crystallinity of the composite and leads to peak suppression in the Raman spectrum [25]. The presence of WS₂/Ti₃C₂T_x2 in the sample is complemented by XRD and FTIR results.

XPS was used to investigate the surface chemical composition and electronic states of the synthesized samples. The spectra (Figure 2d) demonstrate the presence of characteristic peaks of Ti, C, and O elements in the structure of pure Ti₃C₂T_x. All spectra were calibrated using the C 1s peak at 284.8 eV. The spectrum of the WS₂/Ti₃C₂T_x composite additionally contains peaks corresponding to W 4f and S 2p, which confirms the successful incorporation of WS₂ into the Ti₃C₂T_x structure. The XPS data allowed us to clarify the elemental composition and types of chemical bonds in the WS₂/Ti₃C₂T_x hybrid system. The Ti 2p spectrum contains peaks corresponding to Ti–O bonds at ~458.5 eV and ~464.3 eV, as well as S–Ti–C at ~455.1 eV, which indicate the existence of interphase interactions. The presence of the Ti–C peak confirms the preservation of the main MXene crystal structure. The C 1s spectrum contains signals corresponding to C–C, C–O/CH_x, and C–Ti–T_x bonds, which are typical for functionalized Ti₃C₂T_x, corresponding to ~284.8 eV, 286.2 eV, and 288.5 eV, respectively. The O 1s spectrum shows signals indicating the presence of chemical bonds C–Ti–O, O–Ti–O, and C–Ti–OH at ~529.9 eV, ~531.2 eV, and ~532.4 eV, correspondingly, which indicates the presence of –O and –OH functional groups on the surface of the material.

Analysis of the W 4f region reveals characteristic peaks corresponding to W–S bonds (~32.5 eV for W 4f_7/2 and ~34.7 eV for W 4f_5/2), as well as the W 5p_3/2 peak at ~38.6 eV, which confirms the presence of WS₂ in the composite. The S 2p spectrum contains peaks associated with S–W at ~162.3 eV (S 2p_3/2) and ~163.5 eV (S 2p_1/2) [26], as well as a weak peak at about ~165.6 eV, which is associated with the formation of Si–T or S–Ti bonds and indicates interfacial interactions [27]. These data confirm the formation of the WS₂/Ti₃C₂T_x hybrid structure with the preservation of the crystallinity of the original components and the presence of interphase interactions.

The morphology of the obtained Ti₃C₂T_x, WS₂, and WS₂/Ti₃C₂T_x structures was analyzed by SEM, which confirmed the successful growth of both Ti₃C₂T_x and WS₂ layered structures (Figure 3a–d). Figure 3a of pure WS₂ shows a flower-like morphology consisting of numerous individual flower shapes. Ti₃C₂T_x shows an accordion-like morphology, as shown in Figure 3b, where the Ti₃C₂T_x layers can be seen separated from each other. The cross-section of the WS₂/Ti₃C₂T_x heterostructure sample is shown in Figure 3c,d, which depicts the uniform distribution and inclusion of WS₂ nanoflowers within the interlayer spaces of the MXene layers, leading to the formation of wrinkled WS₂/Ti₃C₂T_x heterostructure nanosheets.

The TEM images at different magnifications (Figure 4a–c) show that the flower-shaped WS₂ nanostructures are uniformly distributed both on the surface and between the Ti₃C₂T_x layers. The measured interlayer distance in the heterostructure is 0.26 nm (Figure 4c), which corresponds to the Ti₃C₂T_x crystallographic plane [28].

3.2. Mechanical Properties of PAM-Ag-g/WS₂/Ti₃C₂T_x Hydrogel

Reprehensive uniaxial tensile tests showed that the PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel has improved mechanical properties compared to PAM-Ag-g (Figure 5a). This trend is consistent with hydrogen-bonding interactions formed upon adding Ti₃C₂T_x to the PAM-Ag-g matrix, with can enhance the hydrogels elasticity and ductility. Figure 5b shows that the composition retains high flexibility, strength, and resistance to destruction, which makes it promising for use in smart sensors, soft robotics, and biodegradable wearable devices.

The addition of the WS₂/Ti₃C₂T_x heterostructured nanocomposite to the PAM-Ag-g hydrogel made it possible to obtain an electrically conductive composite [29]. To determine the optimal conductivity, the WS₂/Ti₃C₂T_x content was varied in the range of 0–1 wt.% (0, 0.2, 0.4, 0.6, 0.8, 1 wt.%). Based on the results of four-probe measurements, it was found that the maximum electrical conductivity (1.02 S m⁻¹) was achieved at a content of 0.4 wt.MXene/WS₂ (Figure 6a) exhibits enhanced performance with a further increase in concentration. Note that a similar decrease in conductivity was observed for other MXenes-based composites [30]. Consequently, the PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel with 0.4 wt% WS₂/Ti₃C₂T_x was selected for further investigation.

The strain Gauge factor (GF) of the strain gauge is calculated using the equation:

$$\mathrm{G}\mathrm{F}={\displaystyle \frac{\left(\frac{∆\mathrm{R}}{{\mathrm{R}}_0}\right)}{\mathsf{\epsilon }}}$$

(1)

where ΔR = R − R₀, R₀ is the initial resistance, and R is the resistance at a certain deformation, respectively, ε is the corresponding deformation [29].

The dependence of the relative change in electrical resistance (ΔR/R₀) on the applied strain for the PAM-Ag-g/WS₂/Ti₃C₂T_x composite demonstrates a linear character with calculated values of the sensitivity coefficient (Figure 6b) equal to 1.4 in the region of low strains and 2.8 at high degrees of stretching. The obtained indicators indicate the sensitivity of the material to mechanical effects, which confirms its potential as a strain gauge sensor. However, the magnitude of the Gauge factor is far beyond what has been reported for MXene-based sensors [30].

As shown in Figure 6c, an apparent change in resistance is observed during five extension-relaxation cycles carried out at different strain levels (10%, 30%, 60%, 80%, 100%). The relative resistance (ΔR/R₀) increases gradually with increasing strain, with each loading stage being reproduced with high accuracy. This indicates that the PAM-Ag-g/WS₂/Ti₃C₂T_x -based hydrogel effectively responds to mechanical loads over a wide range and demonstrates stable and reliable operation even under multiple strain cycles. The time plots of ΔR/R₀ (%) change confirm the stability of the sensory response and its repeatability during long-term use.

When stretched to 100%, the sensor element showed a response time of 2.17 s and a rapid recovery of 0.46 s (Figure 6d), indicating the sensitivity of the material to external mechanical influences and its ability to quickly return to its original characteristics after the load is removed.

The working mechanism of the PAM-Ag-g/WS₂/Ti₃C₂T_x strain sensor can be explained by the redistribution of charge carriers at the WS₂/Ti₃C₂T_x heterointerface, driven by electron transfer from the WS₂ layers to the highly conductive MXene surface, which is accompanied by the formation of a built-in electric field. This process is additionally enhanced by the interfacial dipole–dipole interactions between the –O/–OH groups of MXene and the external sulfur atoms of WS₂, leading to some reduction in interface resistance, an increase in the initial conductivity, and a substantial enhancement of the sensor’s sensitivity to mechanical deformation.

3.3. Theoretical Simulations

To unveil the nature of the relatively modest values of the gauge factor and conductivity, we performed the simulation of the changes in the electronic structure upon formation of WS₂/Ti₃C₂T_x composite and following substrate-induced stretching. For this purpose, we built a model interface between the bilayer of WS₂ and the Ti₃C₂(OH)₂ monolayer (Figure 7a). The formation of the interface led to the transfer of the electron density from the MXene substrate to WS₂. Thus, a decrease in the conductivity can be associated with the formation of the interface between highly conductive MXene and a semiconductor.

Next, we checked how the substrate-induced strain affects the electronic structure of the composite. Figure 7b,c depict MXene to WS₂ charge transfer for 5% and 10% in-plane uniaxial strain. As one can see, in-plane stretching leads to an increase in electron transfer from MXene to WS₂. This massive redistribution of the charge density leads to the appearance of a pseudo-gap on the Fermi level (see the area with zero energy in Figure 7d). This explains the relatively modest values of the Gauge factor observed in the experiment. Note that visible changes are observed even in the second layer of WS₂. This doping led to the appearance of distinct states inside the band gap (about −1 eV and +0.5 eV in Figure 7e). These and other states can be the source of distinct optical transitions, which are shown by arrows in Figure 7e. This arising of the particular states on the edges of conductive and valence bands can also be the source of catalytic activity in diselenides as it was shown in our recent work [31]. The combination of mechanical stability, relatively good conductivity, and strain-induced doping can be the source of manipulating the optical, catalytic, and photo-catalytic properties of semiconductive dichalcogenides attached to MXenes incorporated in polymer matrices.

4. Conclusions

In conclusion, the WS₂/Ti₃C₂T_x hybrid structure was successfully synthesized. The combination of the conductive properties of Ti₃C₂T_x and the catalytically active surface of WS₂ allowed the formation of a stable conductive network in the polymer matrix, which ensured the efficient transmission of an electrical signal under mechanical action. The composite hydrogel demonstrated high stretchability, improved mechanical properties and reliability under cyclic loads, and high signal reproducibility, indicating the stability of the sensor function of the hydrogel. The transfer of electrons explains the mechanism of operation of the strain gauge, confirmed by interface modeling, from the WS₂ layers to the highly conductive Ti₃C₂T_x surface, accompanied by the formation of a built-in electric field. Thus, PAM-Ag-g/WS₂/Ti₃C₂T_x hydrogel is a promising material for creating flexible strain gauges applicable in various applications such as motion and chemical sensing, optics and catalysis.