Abstract
Polymer matrix composites composed of poly(methyl methacrylate) (PMMA) and MXenes (Ti3C2T x ) are synthesized using direct solution blending and casting techniques. MXenes are a new family of two-dimensional materials. Both optical and conductivity properties of the resulting PMMA-MXene nanocomposite thin films are studied as a function of MXene concentration, for the first time. The resulting thin films are in the micrometer range (8.10–8.80 µm) in thickness. As the concentration of MXenes increases, the PMMA embeds MXenes, causing structural disturbance but without any change in the crystal structure. The MXene thickness in single-layered structure is 15–20 nm. Optical investigations such as UV-Vis absorption, absorption coefficient, extinction coefficient, and band gap have been reported to study the light absorption of nanocomposites. Resistivity measurement associated with electrical conductivity is studied. The relationship between optical responses and electrical conductivity is discussed. When compared to pure PMMA (1 × 10−14 to 1 × 10−13 S m−1), nanocomposites have electrical conductivity that is more than 3,000 times higher. The nanocomposites containing 15 wt% MXenes had the highest conductivity of 1.35 × 10−3 S m−1. Both the conductivity improvement and tunable optical findings accelerate the route of integrating MXenes into polymers to create more promising multifunctional composites for optoelectronic applications such as conductive electrodes, thin film transistors, and logic circuits.
1 Introduction
In general, most polymers display good compatibility characteristics with a wide variety of materials owing to their interfacial tension, which enables them to easily construct low-cost polymer matrix composites (PMCs). Polymers are good hosts for incorporating filler substances, particularly 2D compounds like graphene-based materials, silicate clays, metal oxide nanosheets, etc. [1]. Electrical, optical, mechanical, and thermal properties of more versatile composites enable their successful fabrication and use in optoelectronics, photonics, microelectronics, and optical transistor devices [1,2]. The optimal polymer-to-filler ratio, synthesizing techniques, type of solvent used, their resulting structures, inherent characteristics, and filler’s distribution inside the host material, must be studied from a research standpoint [3,4]. Poly (methyl methacrylate) (PMMA) is one of the best organic optical materials due to its transparency, which makes it an ideal host material for the fabrication of various optical devices, such as optical lenses, gratings, waveguides, and so on [5]. By harnessing its strong chemical resistance, mechanical flexibility, hardness, color versatility, thermal stability, biocompatibility, and low processing temperature, it is used to fabricate organic electrical devices, sensors, solar display units, pneumatic actuators, as well as in wide range of biomedical and medicinal applications [4,6,7]. Another important advantage of this polymer is that it has less potentially hazardous components, such as bisphenol-A, which is often present in polycarbonates, polysulfones, and epoxy resins [7].
MXenes are two-dimensional (2D) carbides or nitrides, which were first reported in 2011. However, this whole new big family of 2D materials was born in 2012. MXenes have the general formula M n+1X n T x (n = 1–4), where M indicates a transition metal (Ti, Cr, V, Nb, Hf, Ta, etc.), X denotes carbon and/or nitrogen atoms, and T x represents the terminations (–O, –OH, and –F) on the surface of the outmost transition metal layers. By using wet etching methods, which vary according to the type of etchant used [8,9,10,11], Al is selectively etched away from the Al-containing MAX phases and the freshly exposed transition metal (M) atoms are immediately coordinated by anions (–O, –OH, or –F) in the etchant, forming the surface terminations T x and eventually, leading to the formation of MXenes. The surface termination or functionalization of MXenes results in excellent hydrophilicity and superior solution processing capabilities. The abundance of transition metals, the possibility of in-plane and out-of-plane ordering of the metal atoms, as well as surface and mixed terminations, all contribute to the formation of an almost infinite variety of 2D materials with diverse characteristics. Therefore, these 2D materials can be conveniently tuned by using a suitable mix of transition metals and etchants [8,9]. Hydrofluoric acid (HF) is one of the primary candidates (etchants) to be used predominantly for MXenes synthesis. However, HF exposure is considered severe due to the fact that it can pass through the epidermis and dermis to enter the bloodstream and attack tissues, leading to heart function and liquefactive necrosis, respectively. Hence, safety precautions must be taken [10,11].
MXenes have metallic electrical conductivity that is not present in the majority of other 2D materials, such as semiconductors, semimetals, or dielectrics. The highest conductivity values reported in a past work is up to 20,000 S cm−1 [9]. MXenes also allow for the modification of their electronic characteristics since their Fermi levels can be tuned by surface functionalization. The surface functionalization can significantly reduce the density of states of surface transition metal at the Fermi level, resulting in the behavior of certain MXenes as semiconductors [12]. Since the efficiency of electromagnetic interference (EMI) shielding is directly associated with the electrical conductivity and thickness of materials, MXenes’ high electrical conductivity makes them ideal for EMI shielding applications. The incident electromagnetic wave may be reflected and absorbed several times by a great intensity of free electrons which are located on the surface of MXenes [8,9]. Furthermore, MXenes have been proven to exhibit nonlinear light absorption and plasmonic properties [9]. Their optical properties can be further altered when they serve as intercalation hosts for a variety of important cations [8,9,12].
MXenes are superior to other nanoparticles or 2D materials because of their excellent metallic conductivity and high hydrophilicity, which enable them to possess excellent electronic conductivity, high capacitance, good electrochemical performance, and easy processing properties [8,9]. Their metallic conductivity is due to the presence of transition metal backbones that provide abundance of free electrons. Meanwhile, the surface terminations contribute to their hydrophilicity. In addition, MXenes are an excellent filler in PMCs due to their improved strength and stiffness over other solution-based 2D materials. Currently, only some studies have been conducted on the synthesis of PMCs employing MXenes [13,14,15,16,17,18,19,20,21,22]. The involved methods for preparing MXenes-based PMCs are such as dry mixing, solution blending, in situ polymerization, electrospinning, emulsion mixing, and lamination stacking [23]. Electrical properties of polymers such as polyethylene oxide [18], polyvinyl acetate [17], polystyrene (PS) [20], poly(2-(dimethylamino)ethyl methacrylate) [21], and polyvinylidene fluoride [22] have been improved by MXenes in the past works. In addition, polymeric MXene composites such as MXenes/PS [20], MXenes/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate ultrathin film [24], and MXenes/cellulose nanofiber composite film [25] have displayed high EMI shielding effectiveness values of more than 42 dB, which make them as excellent potential and flexible EMI shielding materials. Other MXenes-based PMCs have been discovered to exhibit enhanced thermal, mechanical, catalytic, and environmentally friendly antibacterial capabilities [1,23,26]. Further study is needed to investigate the nanocomposites’ electrical conductivity and optical properties in depth, especially the role played by MXenes. Both PMMA and MXenes are mixed by stirring and sonication. The solution blending and casting approach is used to fabricate the PMMA–MXenes nanocomposite thin films. In specified formulations, both PMMA and MXenes are designed to achieve a specific range of film thickness. Thin film optical constants such as absorption coefficient, extinction coefficient, and band gap are determined. Additionally, resistivity and impedance measurements are conducted. Both optical and electrical conductivity results are connected to justify the prospective uses of the resulting nanocomposite thin films in optoelectronic devices.
2 Materials and methods
Sigma Aldrich supplied the poly(methyl methacrylate) (PMMA) (MW = 996,000). Wet chemistry approach was used to produce the MXenes (Ti3C2T x ). Both MXenes (1.5 mg/mL) and PMMA (3 mg/mL) solutions were prepared, respectively. By using solution blending and casting techniques, PMMA–MXenes nanocomposite thin films with different MXenes concentration of 2, 5, 8, 10, and 15 wt% were formed. Field emission scanning electron microscope (FESEM) was used to investigate the morphological aspect of the nanocomposites. Fourier transform infrared (FTIR) spectrometer was used to explore distinct chemical bonds between host and filler via the identification of chemical functional groups. An Ultraviolet-visible (UV-Vis) spectrometer was used to examine the relationship between the incident electromagnetic waves and materials. The electrical conductivity nature and electrochemical characteristics of the samples were correlated to each other by probing their resistivity and impedance values with the use of a four-point probe and impedance spectroscopy. More information is provided in the sub-sections that follow.
2.1 Synthesis of MXenes
MAX Phase material (Ti3AlC2) from Y-Carbon Ltd, ammonium hydrogen difluoride (NH4HF2; reagent grade 95%, Sigma Aldrich), sodium hydroxide (97% purity, pellets, Sigma Aldrich), and dimethyl sulfoxide (DMSO; analytical reagent grade, Fisher Chemicals) were used without any further purification. To begin, 20 mL of NH4HF2 solution in 2 M was prepared precisely, which acted as a chemical etchant for the wet-chemistry etching process. The NH4HF2 dilution procedure was carried out using deionized (DI) water and required magnet-stirring at 500 rpm for 1 h at room temperature using a magnet stirrer (RCT BASIC, IKA). A microbalance (Explorer series, EX224, Ohaus) was used to weigh 1 g of Ti3AlC2. The Ti3AlC2 was then added slowly into the well-prepared NH4HF2 solution since the reaction is exothermic. With this approach, HF was not used directly to etch the Al layer from the MAX phase as HF is a dangerous chemical and it must be handled with extreme care. Alternatively, NH4HF2 was used as a mild etchant through in situ HF formation and is less hazardous than HF [10,11]. Both the removal of Al layer via etching as well as intercalation of ammonium species were done simultaneously to form MXenes, while minimizing unnecessary risk. The mixture (NH4HF2 and Ti3AlC2) was continuously stirred at 300 rpm for 48 h at room temperature. When the etching treatment ended, a diluted NaOH solution was added slowly into the mixture to achieve a pH of 6. The solution was washed with DI water, centrifuged at 4,000 rpm for 10 min using an ultrahigh centrifuge (Sorvall LYNX 6000, Thermo Scientific), and eventually followed by solution removal. These washing procedures were repeated four times.
After washing, the resulting multilayered flakes of MXene powder were delaminated by magnetically stirring a mixture of 1 g as-prepared MXenes and 15 mL of DMSO for 15 h at room temperature [27]. Similar washing procedures mentioned earlier were conducted again in order to completely remove DMSO from the MXenes. The resulting powder was stirred in water for 5 h within an inert condition (nitrogen), followed by 1 h of sonication via an ultrasonic probe sonicator (FS-1200N) under a setting of 60% power and 7/3 s on/off time. The final supernatant will be known as the MXenes colloidal solution from now on.
2.2 Synthesis of PMMA–MXenes nanocomposite thin films
The PMMA solution (3 mg/mL) was made by dissolving the PMMA into the versatile solvent tetrahydrofuran (THF) at 40°C using magnetic stirring. The resulting PMMA solution and the as-prepared MXenes solution (1.5 mg/mL) were magnetically mixed for 0.5 h. The mixture was sonicated for 1 hour in a water bath to obtain a homogenous mixture before being cast into a 5 cm diameter Petri dish. A vacuum oven (MEMMERT VO 500, Germany) was used to remove water with a drying time of 12 h. All the preceding steps were repeated to form nanocomposite thin films with varying concentrations of MXenes; 2, 5, 8, 10, and 15 wt% MXenes (Figure 1).
A simplified schematic diagram illustrating the whole synthesis process of PMMA–MXene nanocomposites.
2.3 Morphological, crystallinity, and chemical structural studies
Both the pure MXenes and PMMA–MXene nanocomposites were viewed, and images were captured by using FESEM (Hitachi SU8000, Japan) at a 5 kV accelerating voltage and a working distance in the range of 10,000–11,100 µm. A suitable cross-sectional region of a sample is inspected to estimate its thickness. The crystal phases of all samples were examined by the D/teX Ultra2 X-ray diffractometer (XRD; Rigaku, Japan) with Cu Kα radiation (λ = 0.154 nm) over the range of 3–90° (2θ) at a scan speed of 6° min−1. Atomic force microscopy (AFM) (Bruker JPK NanoWizard 3, Germany) was employed to examine the surface topography profile of a sample. The involved functional groups present in the sample was examined and detected by the FTIR spectrometer. The FTIR spectrum was probed by using PerkinElmer Spectrum Two-UATR integrated with a detector of MIR TGS. All FTIR measurements were performed in the range of 4,000–400 cm−1 at a resolution of 4 cm−1 and 32 scans at a scanning speed of 0.2 cm s−1.
2.4 Optical analysis
UV-Visspectrum was applied to the nanocomposite thin films to evaluate how good the interacting electromagnetic waves were absorbed by them. The PerkinElmer Lambda 750 was used to acquire the absorption data in the wavelength range of 200–900 nm at room temperature. The samples were scanned by an 860 nm monochromatic source.
When electromagnetic waves travel through a material, their intensity is attenuated by a factor known as the absorption coefficient (α). The higher the value of α, the more the light can pass through a material in a shorter length before being absorbed, which can be used to determine with ease which sample absorbs light. This parameter is associated with both equations (1) and (2). Subsequently, α is calculated by using equation (3) [28,29,30].
where α is the absorption coefficient (cm−1); A is the light absorbance; l is the thickness (cm); T is the transmittance.
Another important parameter, extinction coefficient (k), explains the amount of light absorption loss in a material, which is related to how quickly the light disappears owing to absorption or scattering in a substance. It can be calculated according to equation (4) [29,31].
In solid-state chemistry, band gap energy is employed to deduce the nature of a matter’s outer shell electrons. It is a measure of the mobility of these electrons, which are responsible for the inherent characteristics of matter, such as electrical conductivity and light absorption. Therefore, band gap energy is utilized to correlate with the abovementioned features, as it is determined by the movement of the outer shell electrons [32]. The band gap energy of a nanocomposite thin film was determined by using the method described by Tauc [31,33,34]. The Kubelka–Munk function, as defined in equation (5), was used. The band gap energy was estimated by extrapolating the linear component of the Tauc plot of (αE) 1/n vs photon energy from the intercept to the abscissa axis (E).
where E is the photon energy (eV); α o is the transition probability constant; E g is the band gap energy (eV); n is the value of 1/2, 3/2, 2, or 3 that depends on the types of electronic transition.
2.5 Conductivity studies
The electrical conductivity of nanocomposite thin films with varying MXenes concentration was evaluated by using electrochemical impedance spectroscopy (EIS) at room temperature. EIS was used to understand any involved conduction mechanism and complex relaxation occurring in materials as a function of frequency. EIS was performed using a Gamry Interface 1010 E potentiostat under a setting of 10 mV amplitude in the range of 10 mHz to 200 kHz, so that the phase shift of current and impedance were measured. Both the imaginary part, Z Imaginary and the real part, Z Real were measured and presented in a plot, the so-called Nyquist impedance plot, in order to estimate the values of the circuit parameters that correspond to the charge carrier transport properties. Using the two-probes approach, the bulk resistance (R b) of a nanocomposite was calculated in this study [35]. A circular sample with a 32 mm radius was placed between two probes that served as electrodes. To provide a secure physical and electric contact for the interface between the probes and sample, a fixed spring was used while silver paste was applied on a sample prior to any measurement. Both resistivity (ρ) and electrical conductivity (σ) were then calculated by using equation (6) (19, 35, 36) and equation (7), respectively. The thickness of all the samples was previously measured by FESEM.
where ρ is the resistivity (Ω m); R b is the bulk resistance (Ω); A is the surface area (m2); l is the thickness (m); σ is the electrical conductivity (S m−1).
To achieve consistent results, an average of three different measurements was obtained on each thin film sample. The potentiostat also worked as the current source to probe the voltage for the purpose of calculating the resistance of a sample, R.
3 Results and discussion
3.1 Morphologies, crystallinity, and chemical structures
As observed in Figure 2(a), the as-prepared MXenes display both multilayered and delaminated structures, indicating that exfoliation of the MAX phase to MXene sheets occurred by etching treatment. However, several layers of MXene flakes are partially aligned parallel to one another with a certain separation distance, indicating that the MXene sheets are not perfectly delaminated. The presence of MXene flakes reveals the morphological nature of 2D materials. By using direct solution blending and casting techniques, the PMMA–MXenes thin film is successfully formed (Figure 2(f)). The cross-section of all thin film samples is captured, and their thicknesses are measured and tabulated in Table 1. The thickness values for all thin film samples fall in the range of 8.10–8.88 µm. The compatibility between PMMA and MXenes is probably assisted by the delamination treatment done on the MXenes prior to the blending of the two components. The delaminated MXenes consist of a few layers of flakes instead of multilayered flakes, providing more space between the flakes to be conveniently accessed and surrounded by PMMA [14]. The series of stirring and ultrasonication steps makes it easier for PMMA to get into MXenes, which makes for a good mix [37].
FESEM images of MXenes and PMMA–MXenes nanocomposite. (a) The formation of multilayered and delaminated pure MXenes; (b) cross-section of thin film with 2 wt% MXenes; (c) cross-section of thin film with 5 wt% MXenes; (d) thin film with 8 wt% MXenes; (e) thin film with 10 wt% MXenes; (f) top view of thin film with 15 wt% MXenes.
Various characterization data for all samples
| Sample configuration | Thickness, l (µm) | Band gap, E g (eV) | Bulk resistance, R b (Ω) | Resistance, R (Ω) from V–I curve | Resistivity, ρ (Ω m) | Electrical conductivity, σ (S m−1) |
|---|---|---|---|---|---|---|
| PMMA–MXenes 2 wt% | 8.40 | 2.90 | 5.05 × 106 | — | 1.93 × 109 | 5.17 × 10−10 |
| PMMA–MXenes 5 wt% | 8.70 | 2.60 | 8.55 × 105 | — | 3.16 × 108 | 3.16 × 10−9 |
| PMMA–MXenes 8 wt% | 8.50 | 2.70 | 1.36 × 106 | — | 6.08 × 108 | 1.65 × 10−9 |
| PMMA–MXenes 10 wt% | 8.80 | 2.50 | 2.45 | 2.52 | 9.27 × 102 | 1.08 × 10−3 |
| PMMA–MXenes 15 wt% | 8.60 | 2.45 | 1.97 | 2.01 | 7.37 × 102 | 1.35 × 10−3 |
| Pure PMMA | 20–50 | *3.00–5.50 | Undisclosed | — | *1 × 1013 to 1 × 1014 | *1 × 10−13 to 1 × 10−14 |
At the lowest concentration of 2 wt% MXenes within the PMMA, the MXene flakes are fully enclosed by PMMA, in which the layered architecture is less significant, as evidenced in Figure 2(b). The MXenes are surrounded and dominated by PMMA entirely. A similar structural feature has also been observed in the thin film sample with 5 wt% MXenes (Figure 2(c)). The MXene flakes are now more significant as their distribution within the PMMA is more uniform. However, when the concentration of MXenes is further increased to 8 wt% and above, Figure 2(d) and (e) displays the inhomogeneous arrangement of MXenes within the PMMA, where the stack of MXene flakes is only visible in certain regions of the whole image. The degree of disorder in the nanocomposite starts to increase with the MXenes. Both components are not dispersed uniformly, and the PMMA is difficult to access the interspaces between the delaminated MXene flakes. In addition, with the increasing amount of added MXenes, they may not fully delaminate to provide accessibility to the host material. Based on Figure 3, AFM measurement reveals that the MXene sheets are single-layered structures, with a thickness of 15–20 nm. The MXene sheets have been delaminated into single layers and they are all over the surface of PMMA.
AFM image (top) and height profile (bottom) of the PMMA–15 wt% MXenes with an inset blue dotted line crossing a few MXene layers.
XRD was used to establish the presence of MXenes within the PMMA–MXenes nanocomposites and the interaction between the polymer matrix and MXenes. The resulting XRD patterns are shown in Figure 4. MXenes have notable peaks at 2θ = 5.6°, 17.6°, and 25.2°, which correspond well to the crystal planes (002), (004), and (008) of a few layered MXenes, respectively [38]. The sharp (002) peak with the highest intensity validates the ordered stacking of MXene layers in all nanocomposite samples, implying that despite variable MXene concentrations, all PMMA–MXene nanocomposites still contain the similar layered stacking of MXenes. The FESEM images also confirm this characteristic (Figure 2). When PMMA and MXenes are mixed together, there are no obvious changes in the crystallinity of MXenes within the nanocomposites [39]. The Bragg’s law is capable of associating the sharp (002) peak with the d-spacing MXenes. With increased polymer loading (decreasing concentration of MXenes), this (002) peak gradually shifts toward a lower angle, indicating a gradual increase in the interlayer spacing between the MXene sheets. This is because of the intercalation and confinement of PMMA, which is aided by the delamination of MXene sheets and eventually allows the polymer to easily access the interspaces between the MXene sheets [38,39].
XRD patterns of as-prepared MXenes and PMMA–MXenes nanocomposites.
Possible physicochemical interactions between PMMA and MXenes are studied by using FTIR measurement. Figure 5 displays FTIR spectra for all PMMA–MXenes nanocomposite thin film samples with different concentrations of MXenes at room temperature. It is evidenced that they show similar bands that have been observed in pure PMMA [15,40]. There is a sharp peak at 1,727 cm−1, which corresponds to the carbonyl group, C═O. Peaks at 3,000/2,950, 1,436/1,480, and 1,365 cm−1 are attributed to methyl ester C–H stretching vibrations, –CH3 symmetric stretching vibrations, C–H deformation, and –CH3 symmetrical bending vibration or deformation, respectively. In addition, the peaks at 1,268/1,241/1,147/988 cm−1 correspond to C–O stretching. All the aforementioned peak bands are assigned to pure PMMA. All samples also exhibit peaks at 1,064, 1,191, and 1,389 cm−1, which are associated with the –CH3 twisting, –CH3 wagging, and hydroxyl group (–OH), respectively [15]. The detection of the –OH group implies the successful incorporation of MXenes into the composites. Figure 5(a) clearly presents the similarity between pure PMMA and PMMA–MXenes nanocomposites indicating that no strong chemical interaction bond is formed between the host and filler [14,41]. However, based on the normalized FTIR spectra as shown in Figure 5(b), with an increase in the concentration of MXenes, the intensity of peaks gradually decreases, suggesting the occurrence of physical interaction between the host and filler [15,41].
FTIR spectra for all samples in different presentation modes. (a) FTIR spectra are stacked to each another. (b) FTIR spectra with normalized transmittance.
3.2 Optical analysis
UV-Vis absorption test is conducted on the PMMA–MXene samples with different concentration of MXenes (2, 5, 8, 10, and 15 wt%). Figure 6 shows that the pure PMMA spectrum exhibits the lowest absorbance (high transmittance), which is expected because PMMA finds wide applications in a variety of optical devices, such as optical lenses, transparent window materials, waveguides, gratings, etc. [5,7]. However, the absorption capability of nanocomposites is greatly improved with the inclusion of MXenes in all samples [16,42,43], which increases the absorbance with the concentration of MXenes, as indicated by a vertical arrow. The electronic excitation within the carbonyl chromophores (C═O) contained in the PMMA structure causes a dominating peak in the region of 250–260 nm in all samples. This strong absorption band is associated with π–π* transition in the C═O system. The incorporation of MXenes into the PMMA induces a change in the band gap energy in the nanocomposites [16]. Another absorption edge is observed at about 375 nm, as encircled by the oval (Figure 6). It is contributed by the presence of titanium oxide (TiO2) in the MXenes [16,44]. The transition element Ti in MXenes readily oxidizes in the air to form TiO2, which has been proven in some past works [12,45].
The UV-Vis absorption spectra of all samples as a function of wavelength.
An insignificant peak (highlighted by the rectangle in Figure 6) starts to appear at 482 nm for the samples with 5 wt% MXenes and above. This peak is attributed to the creation of intermolecular hydrogen bonds between the PMMA and MXenes. MXenes’ high hydrophilicity associated with surface terminations (–OH, –O, –F) could result in some interactions with PMMA [36,46]. In addition, MXenes display plasmon resonance by displaying another absorption band which is found at 852 nm, as shown within the rounded rectangle [12,47]. The visibility of both absorption peaks (482 and 852 nm) increases with the concentration of MXenes. The insufficient amount of MXenes in PMMA–MXenes 2 wt% sample results in the disappearance of these insignificant peaks. The overall absorbance in both the visible and UV light regions increases with MXenes’ concentration. The increasing amount of MXenes (filler) introduce more localized charge carrier levels within PMMA (host) that act as trapping sites, making more electronic transitions between the filler and host and consequently increase absorbance [43,47]. The TiO2 within the MXenes also renders good absorption capabilities by inducing vibrational-electronic transitions to the primary singlet excited state of the molecular structure of the nanocomposites. The unbound or free electrons in the TiO2 also amplify the incident electromagnetic wave, which consequently intensify the light absorbance as well [16,43,44].
To further validate the absorption capability of nanocomposite thin films, α is calculated by using equation (1) and is presented in Figure 7. All nanocomposite samples display increasing trend for α in the whole region of photon energy (E). The increase is more drastic in the range 1.4–4 eV, then α slightly increases with E, as shown in the range of 4–6.2 eV, regardless of the change in the concentration of MXenes. As compared to the pure PMMA, all nanocomposite thin film samples exhibit higher α values, ranging from 2.2 × 103 cm−1 up to 3.9 × 103 cm−1, as contributed by the addition of MXenes. Overall, the absorption capability of thin films increases with the concentration of MXenes [16,42,43]. The sample PMMA–MXenes 15 wt% achieves the highest α among all samples. However, only the PMMA–MXenes 8 wt% exhibits higher α compared to the PMMA–MXenes 10 wt%. The sample with 8 wt% MXenes has lower thickness than that of sample with 10 wt% MXenes. Apart from the filler (MXenes) content in nanocomposite, the thickness of thin film affects α [14,28,29,30]. All nanocomposite samples display a tiny peak at around 1.5 eV (encircled by the oval in Figure 4), which corresponds to the existence of MXenes within the PMMA. Two other peaks observed at about 2.6 and 3.3 eV are correlated with the intermolecular bonds between PMMA and MXenes and with the presence of TiO2 within the samples, respectively [12,16,44,45,47]. There is also an insignificant and broad hump located in 4–5 eV, as highlighted by the square, which implies that the addition of MXenes alters band gap energy of the nanocomposites [16]. The modification of α permits MXenes to play important role in functioning as a filler to optimize the optical properties of PMMA [42,43].
The absorption coefficient (α) of all samples as a function of photon energy (E).
Figure 8 shows that all samples display decreasing k values with E. With the addition of MXenes, nanocomposite thin films possess higher k, as compared to the pure PMMA. The higher the concentration of MXenes added to PMMA, the greater the loss of electromagnetic wave due to better absorption capability, as what have been explained previously based on Figure 7. Similarly, the PMMA–MXenes 8 wt% has k which is slightly higher than that of PMMA–MXenes 10 wt% due to the thickness factor. The PMMA–MXenes 15 wt% exhibits the highest k among all samples, which indicates the smallest amount of time the light disappeared within the sample, since it has shown the highest α previously. The electric component of the electromagnetic wave is restrained from further propagating deeper into the nanocomposites by the TiO2 contained in MXenes. The upward trend in k value demonstrates that MXenes (filler) optically tuned the PMMA (host) [16,42,43].
The extinction coefficient (k) of all samples as a function of photon energy (E).
The band gap (E g) for all nanocomposite samples is estimated by applying the previously calculated α values together with equation (5). Based on Tauc’s relationship and by using power probability of n = ½ to suppose that all nanocomposite thin films are direct allowed transition type, a suitable Tauc plot against E is constructed and presented in Figure 9 [31,33,34,48]. The E g is then computed and reported in Table 1 by projecting the relevant straight dotted line to cross the E-axis at (αE)2 = 0 [28,48].
Tauc plot for all samples as a function of photon energy (E).
Based on Figure 10, when MXenes are added, all nanocomposite thin film samples display much lower E g in the range of 2.4–3 eV, as compared to pure PMMA, which has an estimated E g of 4.2 eV (Figure 9). According to the literature, the obtained E g values for pure PMMA and PMMA composites are approximately 6 eV and 3.5–5.5 eV, respectively [48]. When the MXenes concentration is increased from 2 to 5 wt%, the E g decreases from 2.9 to 2.6 eV (Figure 9). However, increasing the MXenes concentration by 8 wt% increases the E g, and then it decreases with MXenes concentration all the way. Overall, the band gap of the resulting nanocomposites decreases as the amount of filler increases. The lower the E g value, the fewer energy electrons need to carry charges and move from the valence band to the conduction band, allowing current to conduct smoothly. Therefore, the addition of MXenes improve the electrical conductivity of nanocomposite thin films [19,48,49]. As the FTIR result (Figure 5b) has previously proven the existence of interaction between the host and filler, the formation of some networks between PMMA and MXenes based on a basis of charge transfer complex increase the electrical conductivity [50]. In addition, during the etching and delamination processes, the surface termination of the MXenes enables them to render metal-to-semiconductor transition. The surface termination significantly modifies the density of states of the surface transition metal of Ti at the Fermi level, thereby increasing the electronic conductivity of MXenes and ultimately offering tunable electrical conductivity [8,9,23,51].
The Tauc plot illustrates the band gap (E g) variation in nanocomposites as a function of MXenes concentration.
3.3 Conductivity studies
The EIS for PMMA–MXenes 2, 5, and 8 wt% show a depressed semicircle in the high-frequency end, followed by a tilted spike in the low-frequency end, as shown in Figure 11. Instead of the ideal Debye relationship, their impedance trends exhibit deviated Debye-type behavior [46]. The irregular semicircle and the nearly straight line could be caused by the thin films’ irregular thickness and morphology. [35]. Due to the fact that most polymers can exhibit numerous ionic conductivity mechanisms, increasing the amount of MXenes causes the sample to exhibit more complex impedance behavior, as multiple relaxation times are involved [46]. To investigate the electrical property of thin film, the bulk resistance, R b , is extracted from the intersection of the plot associated with the low-frequency end of the semicircle and the high-frequency end of the straight line [35]. R b values of 5.05 × 106, 8.55 × 105, and 1.36 × 106 Ω were found in samples with 2, 5, and 8 wt% MXenes, respectively. Using equation (6), they obtained resistivities (ρ) of 1.93 × 109, 3.16 × 108, and 6.08 × 108 Ω m, which are lower than the pure PMMA resistivity of about 1 × 1014 Ω m [52,53]. The MXenes were used as a filler to significantly reduce the ρ to a considerable extent [54].
EIS were recorded at room temperature for all samples. An arrow indicates the magnified EIS of the samples seen in a relatively narrow range of Z Real components.
However, neither the 10 wt% MXenes nor the 15 wt% MXenes samples show any deviating Debye curves. They exhibit nearly vertical curves in a narrow range of Z Real, suggesting that these samples behave like perfect resistors, obeying Ohm’s law across the whole range of voltage and current values. [55,56]. During the impedance measurement, both the alternating current and voltage signals flow in phase through the samples. Regardless of frequency, their resulting resistance values are consistent, allowing the resistance to be derived from their respective Z Real component intercept values. Figure 11 and Table 1 show the equivalent R b values of PMMA–MXenes 10 and 15 wt%, which are 2.45 and 1.97 Ω, respectively. To validate these values, a voltage (V) against current (I) measurement is performed, and the acquired V and I values form a linear trend (Figure 12), which is fully compliant with the Ohm’s law. Therefore, the gradients of these V–I graphs are extracted to represent the resistance values (R). The corresponding R is highly correlated with the R b determined from the impedance spectra (Table 1).
Voltage–current (V–I) graphs for samples at room temperature.
For the sake of comparison and discussion, both ρ and σ values, as well as other pertinent parameters for all samples, are included in Table 1. As illustrated in Figure 11, both PMMA–MXenes 15 and 2 wt% reach the maximum and lowest values of 1.35 × 10−3 and 5.17 × 10−10 S m−1, respectively. The introduction of MXenes into PMMA greatly improves electrical conductivity of nanocomposite thin films by more than 3,000 times when compared to pure PMMA in past works, despite the fact that the samples’ thickness is not uniform. Apart from the intrinsic electrical properties of MXenes and their concentrations that increase the electronic conductivity, the ionic conductivity of nanocomposites is significantly dependent on the interaction between the charge carriers and the polymer matrix [23,36,50]. The presence of insignificant peak at 482 nm in UV-Vis spectra (Figure 6), as well as the diminished intensity of the peaks in FTIR spectra (Figure 5), may indicate the successful formation of certain interfaces between PMMA and MXenes, since the MXenes are highly soluble in aqueous solution with their surface being terminated with functional groups (–O, –F, –OH) [36,46]. These hydroxyl and oxy groups on the surfaces of MXenes interact with PMMA through hydrogen bonds [23]. This interaction results in an amplification effect caused by the space charge, which speeds the movement of ions and therefore the electrical conductivity. In addition, MXene flakes with large aspect ratios form a charge transfer percolation network that enhances the electrical conductivity of the nanocomposites. The formation of an interconnected ionic network creates long-range interfacial pathways within the nanocomposites and therefore, contributes to efficient ionic conductivity [23,50]. Overall, the nanocomposites display increasing σ with the amount of MXenes (Figure 13). Accumulation of MXenes stimulates the creation of the conductivity chain or network, which raises the number density of mobile ions, resulting in a dramatic increase in electrical conductivity, particularly when the MXenes concentration is increased from 8 to 10 % [36]. When the concentration is increased from 5 to 8 %, however, σ is slightly reduced with its E g that is slightly higher (Figure 10 and Table 1).
The obtained results of σ and E g are also highly correlated. The PMMA–MXenes 15 wt% exhibits the lowest E g (2.45 eV) yet the highest σ (1.35 × 10−3 S m−1) [19,49]. MXenes modify the electronic structure of PMMA by forming localized electronic states in the optical band gap of PMMA that overlap with the band system. These localized electronic states serve as trapping and recombination centers, which can affect E g [19,50]. MXenes containing TiO2 produce a greater number of delocalized π-electrons, lowering the E g between π and π* and increasing the optical π–π* transitions [16]. Additionally, the decrease in E g that occurs as a result of the increase in σ can be explained by the increased degree of disorder in the nanocomposites [19,58]. Further, the development of the localized electronic states adds new defect states into the PMMA (host), increasing the defect concentration inside the nanocomposites. According to the earlier morphological finding in Section 3.1, MXenes concentration increases the degree of disorder in nanocomposites.
The electrical conductivity (σ) of nanocomposites is shown against the concentration of MXenes at room temperature.
4 Conclusion
A solution blending and casting technique is used to synthesize the PMMA–MXenes nanocomposite thin film. The nanocomposite thin film is composed of a multilayered and partially delaminated MXene structure enclosed in a polymer matrix. The nanocomposites become more disordered in direct proportion to the MXenes concentration. However, when MXenes are integrated into PMMA, their crystallinity remains unchanged. The thickness of the interspacing layer between the MXene sheets strongly depends on the intercalation and confinement of PMMA, which can be enhanced further through the delamination process. The MXenes can be fully delaminated into a single-layered structure with a thickness of 15–20 nm. As demonstrated by the calculated optical absorption and attenuation coefficients, MXenes’ absorption capabilities boost the light absorption capacity of nanocomposites. The electrical conductivity of the nanocomposites is increased by more than 3,000 times when compared to pure PMMA. The MXenes’ intrinsic electrical qualities, and their large aspect ratio flakes that form good interfaces with PMMA, result in a promising charge conduction percolation channel, and the alteration of PMMA’s electrical structure contribute to the electrical conductivity improvement. The band gap values determined from the Tauc plot correspond well with the conductivity data. A maximum of 1.35 × 10−3 S m−1 is obtained in PMMA–MXenes 15 wt%. The significant improvement in electrical conductivity, light absorption capabilities, and tunable band gap of the nanocomposites permits their potential usage in optoelectronic applications, including conductive electrodes (LCD and OLED), electrodes in field-effect and thin film transistors, and photonic devices.
Acknowledgment
The authors would like to thank the Research Center for Nano-Materials and Energy Technology (RCNMET), School of Engineering (SET), and Sunway University for their equipment support. The authors also would like to thank Nanotechnology & Catalysis Research Center (NANOCAT) for its financial support.
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Funding information: The work is financially supported by University of Malaya Research Grant (RU003-2021).
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Author contributions: KimHan Tan devised the whole work, performed experiments, and wrote and revised the whole manuscript. L Samylingam assisted in characterizations. Navid Aslfattahi prepared MXenes. Rahman Saidur and Mohd Rafie Johan reviewed and edited the manuscript. All authors authorized the current version of the work for submission.
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Conflict of interest: Authors declare that there are no conflicts.
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Ethical approval: The conducted research is not related to either human or animal use.
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Data availability statement: All data generated or analyzed during this study are included in this article.
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