# Long-Term Stability Studies and Applications of Ti3C2Tx MXene

Authors: Sunil Kumar; Hyun Min Park; Tej Singh; Manjeet Kumar; Yongho Seo
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The novel 2D materials, MXenes, have many remarkable properties, but their stability against oxidation is a major bottleneck in their applications. In this study, we have investigated the stability of Ti3C2Tx MXene for up to 365 days under different conditions, e.g., in liquid form in different solvents, in dried form when coated at polyethylene terephthalate (PET) substrate, and during the application of Ti3C2Tx MXene in polymer-dispersed liquid crystal- (PDLC-) based smart windows as conducting electrodes. In liquid form, the MXenes were dispersed in different solvents including an antioxidant in DI water and other organic solvents, and the corresponding color change was analyzed. To estimate the stability in dried form, the MXenes were coated on PET substrates and their relative sheet resistance change, i.e., R−R0/R0, where R0 is the initial resistance, was investigated under different conditions including MXene passivated with polymers and coated films stored under different environments. In the case of the MXene application-based stability study, the stability was investigated by using the MXenes as conducting electrodes in a switchable smart window application. The switchable behavior was observed by applying voltage (0-50 V) after different durations of up to 365 days. The MXene used in smart windows was preserved for a long time without additional treatment to MXene.


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## Body

## 1. Introduction

Everlasting increasing demands in different industrial segments have provoked scientists all around the world to discover novel materials. Recently, there has been a focus on different kinds of nanomaterials, especially 2D materials like graphene, transition metal dichalcogenides (TMDs), and many other new materials. Among these materials, MXenes are among the most explored materials in the last decade. These materials have many remarkable properties like high hydrophilicity [1], high electrical conductivity [2], tunable band gap [3], surface area [4], and high electrochemical properties [5]. Due to these properties, these materials have found applications in energy storage, conductive ink, fuel cells, smart windows based on flexible transparent conducting electrodes (TCE), 3D printing, EMI shielding, supercapacitors, etc. [6–14]. These materials are obtained from the hexagonal carbides and/or nitrides with the general formula, Mn+1AXn, popularly known as MAX phases, which are etched by the use of strong acids like HF or LiF/HCl [1]. In Mn+1AXn, M is an early transition metal element, A is group 13 and 14 elements (mostly Al), X is C and/or N, and n is number from 1 to 4 [1]. After etching, the Al is removed and MXenes are subsequently washed with DI water thoroughly. MXenes have terminal groups, like -OH, -O, -Cl, or -F at their surface; hence, MXenes are generally represented as Mn+1XnTx, where Tx represents the terminal groups [2, 3]. Among various MXenes, Ti3C2Tx MXene has unique properties like excellent metallic conductivity, high EMI shielding, and ease of processing owing to its hydrophilic nature, etc. making Ti3C2Tx MXene the front runner in the MXene family [15]. Although Ti3C2Tx MXene has many unique properties [16–18], it is easily oxidized under ambient conditions. There are some reports indicating the highly oxidative nature of the Ti3C2Tx MXene.To exploit the Ti3C2Tx MXene applicability, the stability against oxidation needs to be improved.Earlier, some attempts have been made to improve the stability of MXene and stability also depends upon the synthesis process. The stability of MXenes can be improved when these are stored at low temperatures in an Ar environment [19] or stored in eutectic solvents [20], sodium L-ascorbate [21], etc. Another technique used for preservation is mixing these with polymers [22]. The oxidation of the MXene starts from the edges; hence, if these edges are capped with polyanions, the oxidation probability can be improved significantly [23]. The MXene preserved using antioxidants can also be used in energy storage applications even after 80 days under ambient conditions [24]. The passivation of MXenes by polymer is another method to maintain their stability against oxidation as the MXenes are preserved from being exposed to air; however, the electrical conductivity is reduced due to polymer passivation. In an earlier report, the poly(4-vinylphenol) (PVPh) was applied as a protective polymer layer on Ti3C2Tx MXene, and these electrodes were used in various flexible electronic devices [25]. However, after 25 days, the resistance of the polymer-coated MXene-based electrodes was found to increase up to 38%. The higher thickness of the polymer layer on Ti3C2Tx MXene has little impact on the stability of the MXene. The polymer passivated MXene was used in light-emitting displays, touch sensors, triboelectric nanogenerators, etc. Also, there are reports on the stability of MXene dispersed in different organic solvents; however, the observation time is limited [26]. In another report, the Al-Ti3C2 MAX phase was modified during synthesis to prepare the stable and most conductive Ti3C2 MXene till date, and the oxidation of MXene can be improved by controlling the defects [27]. The oxidation of MXene also depends upon the pH and temperature. At high pH and temperature, a high oxidation rate was observed. It was reported that under acidic conditions, the protonated hydroxyl terminal groups present at Ti3C2Tx flake surface, in the presence of water or oxygen, make Ti atoms prone to oxidative nucleophilic addition reaction which leads to the faster oxidation [28].Besides, as another aspect, the stability of MXene has not been explored when these are used for some applications. Our group has reported earlier the fabrication of Ti3C2Tx MXene-based transparent conducting flexible electrodes in smart window fabrication [14], and the results encouraged us to analyze the stability studies without any special treatment for a duration of one year to estimate any commercial prospects in the future.All these studies indicate the possibilities of Ti3C2 MXene phase oxidation and its prevention; however, the time frame is small. Still, there is a need to understand the oxidation conditions under one umbrella for a longer duration under which Ti3C2 MXene phase is oxidized so that preventative measures could be adopted accordingly.In this report, we have presented 365 days study on MXene oxidation in a series of systematic studies in aqueous and dried environments after coating at PET substrate. In liquid form, the absorbance spectra and visual color change were studied, whereas, in the dried form, the stability of the Ti3C2 MXene phase was studied by calculating the relative resistance change (R−R0/R0%) and transmittance change after regular intervals of time. Additionally, the stability of MXene was also studied during the application of MXene in polymer-dispersed liquid crystal- (PDLC-) based smart windows. These studies were made by observing the change in transmittance in on/off states at the 0-50 V voltage range.


## 2. Methods

The Ti3C2 phase MXene was synthesized from the Ti3AlC2 MAX phase (Carbon-Ukraine) as described in the supporting information (Section 1). The schematic structures of Ti3AlC2 MAX phase and Ti3C2 MXene phase are presented in Figure 1(f). The etched MXene was exfoliated by shaking and sonication techniques which were later centrifuged to get the exfoliated MXene. The exfoliated MXene was dispersed and stored in different solvents, e.g., DI water, acetone, isopropyl alcohol (IPA), ethanol, sodium L-ascorbate (SA), capping agent sodium dodecyl sulfate (SDS), f-CNTs (10%), and reduced graphene oxide (rGO, 10%) dispersions. In the dried form, the MXene was spin-coated on PET substrates and heated for 12 h at 100°C. The process of polymer passivation with NOA65 for MXene stability study has been discussed in supporting information (Section 2).Figure 1
MXene flake study. (a) FESEM and (b) AFM images of MXene flakes. (c) AFM line profiles exhibiting the thickness of the MXene flakes. (d) XRD spectra of Ti3C2 MXene and Ti3AlC2 MAX phase. (e) Raman spectra of Ti3AlC2 MAX phase and Ti3C2 phase. (f) Schematic structures of Ti3AlC2 MAX phase and Ti3C2 MXene phase.
(a)(b)(c)(d)(e)(f)


### 2.1. Characterizations

The morphology of Ti3C2 phase flakes and MXene film thickness were determined using a field emission scanning electron microscope (FESEM, SU8010 Hitachi, Japan) and AFM atomic force microscope (AFM, Nano focus Inc.). The elemental composition of MXene film was carried out using energy-dispersive X-ray spectroscopy (EDX). The crystalline phase of MXene was analyzed using an X-ray diffraction (XRD) system (PANalytical X’Pert Pro). The Raman microscopy (Renishaw, InVia, UK) was used to analyze the phonon modes of MXene. The transmittance of the substrate-coated MXene films’ electrooptical studies of the resultant smart windows was carried out using UV-Vis spectroscopy (Cary 5000, Varian, USA) with the help of a homemade AC voltage driver (0-50 V). The Brunauer-Emmett-Teller (BET) measurement (BELSORP-max, BEL Japan Inc.) was used to estimate the specific surface area of the MXene. The sheet resistance and electrical conductivity were estimated using a 4-point probe-based transport system (Ecopia, HMS-3000). The switching time (on/off time) of smart windows was recorded using a photodiode and laser diode assembly using an oscilloscope (Tektronix 1052B EDU, USA).


## 2.1. Characterizations

The morphology of Ti3C2 phase flakes and MXene film thickness were determined using a field emission scanning electron microscope (FESEM, SU8010 Hitachi, Japan) and AFM atomic force microscope (AFM, Nano focus Inc.). The elemental composition of MXene film was carried out using energy-dispersive X-ray spectroscopy (EDX). The crystalline phase of MXene was analyzed using an X-ray diffraction (XRD) system (PANalytical X’Pert Pro). The Raman microscopy (Renishaw, InVia, UK) was used to analyze the phonon modes of MXene. The transmittance of the substrate-coated MXene films’ electrooptical studies of the resultant smart windows was carried out using UV-Vis spectroscopy (Cary 5000, Varian, USA) with the help of a homemade AC voltage driver (0-50 V). The Brunauer-Emmett-Teller (BET) measurement (BELSORP-max, BEL Japan Inc.) was used to estimate the specific surface area of the MXene. The sheet resistance and electrical conductivity were estimated using a 4-point probe-based transport system (Ecopia, HMS-3000). The switching time (on/off time) of smart windows was recorded using a photodiode and laser diode assembly using an oscilloscope (Tektronix 1052B EDU, USA).


## 3. Results and Discussion

Prior to the stability studies, preliminary examinations of the Ti3C2Tx phase flakes, used in different stability studies, were carried out.


### 3.1. Morphological Studies

The size and thickness of synthesized MXene flakes under study were examined using FESEM and AFM analyses, and corresponding images are shown in Figures1(a) and 1(b), respectively. The FESEM and AFM images indicate the MXene flakes of random size ranging from 100 nm to 1 μm. The thickness of the MXene flake is <2 nm as estimated from the AFM line profile shown in Figure 1(c).


### 3.2. Crystal Structure (X-Ray Diffraction) Analysis

The crystal structure of the Ti3C2 phase before and after etching was analyzed using XRD. The XRD patterns of the Ti3AlC2 MAX phase and Ti3C2 phase are shown in Figure 1(d). The XRD patterns for the Ti3AlC2 MAX phase and Ti3C2 phase accord well with the JCPDS card (JCPDS-52-0875 and JCPDS: 04-004-2919), respectively [29]. After etching, the (002) peak indicates a sharp shift towards a lower angle, i.e., from 2θ angle of 9.95° to 5.78° indicating the highly exfoliated MXene layers and the increase in the “c” lattice parameter [30, 31]. Additionally, the peak broadening suggests the reduced MXene layer thickness and increased d-spacing [21]. The spectrum indicates the disappearance of Al-associated (104) peak in the Ti3C2 phase [32].


### 3.3. Raman Analysis

The Raman spectra of the Ti3AlC2 MAX phase and Ti3C2 phase shown in Figure 1(e) were used to investigate the phonon-related vibrations and confirmation of terminal groups in MXene after etching. The Raman peaks were observed at around 200, 281, 365, 392, 565, 620, and 720 cm-1 (indicated by purple dotted lines). These peaks confirm the formation of Ti3C2 having terminal groups with -F, -O, and -OH functionalities. These terminal groups can be collectively represented by Tx; hence, the Ti3C2 phase will be represented as Ti3C2Tx in the rest of the studies. These results accord well with those reported earlier [14, 33]. Additionally, the Raman spectrum of the Ti3AlC2 MAX phase exhibits peaks at around 250, 395, and 600 cm-1 corresponding to E2g, E1g, and A1g phonon vibrational modes (indicated by green dotted lines), which match with an earlier study [34].


### 3.4. Long-Term Stability Studies of Ti3C2Tx MXene

#### 3.4.1. MXene Stability in Different Solvents

As the Ti3C2Tx is hydrophilic in nature, so the stability study was carried out in DI water also. The stability of MXene was studied for 365 days by dispersing the MXene in DI water at different concentrations, e.g., 1 mg/ml and 2 mg/ml. The real-time digital images of the samples are shown in Figure 2(a).Figure 2
MXene stability in different solvents. Real-time digital images of the MXene dispersed in (a) DI water at different concentrations and (b) MXene dispersed in different organic solvents. I, acetone; II, IPA; and III, ethanol.
(a)(b)The images indicate the black color of the dispersed MXene in DI water on day 1. On day 30, there is no specific change in color. However, from day 60, there is a clear change in MXene color as the black-colored dispersion started appearing slightly milky color indicating the start of conversion of MXene to TiO2 due to oxidation. However, the MXene with 2 mg/ml was found slightly darker than that at 1 mg/ml concentration, which is obvious as at low concentrations; MXene flakes are more exposed to DI water environment as evident from MXene at higher concentration (3 mg/ml) in DI water shown in Figure S1 (Supporting info.). On day 180, the color was completely turned to milky and slightly transparent in both cases. Till day 365, there is no significant change in the color as compared to day 180 as the Ti3C2Tx MXene is completely converted into the TiO2. Under ambient conditions, there is a possibility of conversion of MXene into TiO2 as predicted in the following reaction:
(1)Ti3C2O2+4O2⟶3TiO2+2CO2.If the MXene has defects, then even the purest Ti3C2 can be oxidized to TiO2 as
(2)Ti3C2+3O2⟶3TiO2+2C.The major factor which leads to the oxidation is the atomic defects in Ti3C2Tx MXene. It has Ti-vacancies, which are probably associated with the Ti3C2Tx MXene during the process of chemical synthesis [35].The degradation of MXene dispersed in DI water was also studied using absorbance spectroscopy, and the absorbance spectra recorded at different times are presented in Figure3. The degree of oxidation of MXenes can be estimated from the decrease in the absorbance peak intensity at ~760 nm [36]. The spectra show the highest absorbance for MXene on day 1 as it has dark black color, and the intensity decreases with time. After day 60, the peak at around 750 nm on day 1 was observed to shift towards 780 nm, whereas after day 180, the peak almost disappeared indicating the degradation of MXene flakes possibly due to the formation of TiO2 [27]. After 365 days, the MXene dispersion has negligible absorbance supporting the MXene conversion into white TiO2 [37]. Also, Ti3C2Tx MXene can preserve its structural integrity up to x≈3.5 before converting it into TiO2 [38]. As per an earlier report, MXene flakes retain their structural integrity until the factor x, which is the ratio of Ti atoms to O atoms in MXene flakes, reaching close to 3.5, i.e., ~1.75 oxygen atoms per Ti atoms; then there is a possibility that Ti3C2Tx MXene is converted into TiO2. After day 60, the stability curve is exhibiting a mixed behavior of oxidized and nonoxidized MXene which indicates that some MXene flakes are oxidized or unstable and others are still stable as the intensity of the UV visible spectrum after day 60 was also found to be reduced as compared to that observed for fresh Ti3C2Tx MXene on day 1; simultaneously, the peak at 780 nm is not completely absent as in case TiO2 on days 180 and 365. This trend can be predicted using
(3)A=Astable+Aunstable×e−t/τ,where A is the normalized absorbance, Astable is the absorbance of stable MXene, Aunstable is the absorbance of unstable MXene flakes at a particular wavelength, and τ is the time (t)-dependent decay constant [36].Figure 3
Absorbance spectra of MXene dispersed in DI water at different durations.The stability of the MXene stored in different organic solvents, e.g., acetone, ethanol, and isopropyl alcohol (IPA), was also observed at the same MXene concentration (2 mg/ml). The real-time pictures of the MXene dispersed in these solvents are shown in Figure2(b). There were no signs of oxidation as estimated from the color of MXene dispersions in organic solvents, and even after 365 days, the color of the MXene remained black. The MXene dispersions in these solvents were easily precipitated after a few seconds as MXene is not dispersible in these solvents. These results were similar as observed as reported earlier [26]; however, MXene dispersions are stable for even longer durations.Another possible way to protect the oxidation of MXene is the use of antioxidants. The stability of the MXene (2 mg/ml) was studied using 5 mM, 10 mM, and 20 mM concentrations of antioxidant (SA) in DI water. It was found that at low SA concentration, the MXene was oxidized slowly as compared to DI water after 365 days whereas, at higher SA concentration, the color of MXene remains almost unchanged as indicated in Figure4(a). It indicates that a small SA concentration (~10 mM) can preserve MXene for one year. Of course, higher SA concentration can also preserve Ti3C2Tx MXene for a longer duration but at the expense of the conductivity of the MXene because on day 1, as the MXene+SA mixture was spin-coated on PET film, the conductivity of MXene+SA-coated film in dried form is relatively lesser than that of MXene-only coated film (at same MXene concentration). It may be ascribed to the fact that SA is nonconducting, hence leading to an overall decrease in conductivity. However, for a longer duration, the SA preserves the MXene from being oxidized in solution form.Figure 4
MXene stability in SA antioxidant. Digital images of the MXene (2 mg/ml) dispersed in (a) DI water and SA at different concentrations. A, DI water; B, 5 mM SA; C, 10 mM SA; and D, 20mM SA. (b) MXene (2 mg/ml) dispersed in SDS, rGO, and f-CNTs. X, SDS; Y, rGO; and Z, f-CNTs.
(a)(b)The stability of MXene was also studied with surfactant, sodium dodecyl sulfate (SDS, 0.1 M), 10% functionalized CNTs (f-CNTs), and reduced graphene oxide (rGO). All the dispersions include DI water and the images recorded on day 1 to 365 days are shown in Figure4(b). It was observed that the MXene color was changed to turbid in SDS, whereas in rGO, the color changed to pale, indicating that MXene is oxidized in both media. The SDS is a popular capping agent for nanomaterials; however, it only partially prevented oxidation, probably because it failed to interact with the Ti3C2Tx surface [39]. A pale color in the MXene+rGO sample may be associated with the oxidation of rGO to GO [40]. However, in f-CNT, the black color of dispersion was maintained almost similar after 365 days.


#### 3.4.2. MXene Stability Studies in Dried Form

In the above study, it was observed that the Ti3C2Tx MXenes are easily oxidized in DI water, indicating that DI water is a major factor in instigating the oxidation of the Ti3C2Tx MXene. To investigate the stability of Ti3C2Tx MXene in the dried form under ambient conditions, the Ti3C2Tx MXene films prepared by MXene dispersion in DI water were coated at PET substrates, and stability was estimated by measuring the relative resistance change. Additionally, the MXene dispersions prepared with different solvents were used to coat at PET film, and the sheet resistance was analyzed for 365 days under different conditions. The actual image of the MXene-coated PET film prepared from MXene dispersion in DI water is shown in Figure 5(a).Figure 5
(a) The actual image of the Ti3C2Tx MXene coated at PET film. (b) Top view FESEM image. (c) Cross-sectional FESEM image. (d) AFM image. (e) Corresponding line profiles. (f) Transmittance spectrum of MXene-coated PET film.
(a)(b)(c)(d)(e)(f)The film appears highly transparent as the background is clearly visible. The FESEM image showing the top surface view of the coated MXene film is shown in Figure5(b), and the corresponding FESEM image with the cross-sectional view is shown in Figure 5(c). The thickness of MXene film appears around 10 nm which was also supported by AFM images and corresponding line profiles shown in Figures 5(d) and 5(e), respectively. The transparency of the coated MXene film was confirmed with the transmittance spectrum shown in Figure 5(f). The transmittance of film was found ~83% indicating high transparency which corroborates the visual image shown in Figure 5(a). The electrical parameters of MXene-coated PET film are presented in Table S1 (Supporting info.). The conductivity of the MXene film is much lower than the highest conductivity reported for Ti3C2Tx MXene (~20,000 S/cm) which may be associated with defects induced during the synthesis process [27].(1) Surface and Electrical Study of MXene-Coated PET Films. The MXene dispersed in DI water was coated and analyzed for 365 days. The FESEM images of the MXene films under ambient conditions after 60, 180, and 365 days are shown in Figures 6(a)–6(c), respectively. It was observed that the edges of the MXene flakes have a boundary of powder-like material which is probably due to the oxidation of the MXene as the oxidation starts from the edges [23].Figure 6
FESEM images of MXene films coated at PET after (a) 60 days, (b) 180 days, and (c) 365 days.
(a)(b)(c)To analyze the oxidation, the elemental spectra were recorded and compared on day 1 spectrum with that on day 365 as shown in FigureS2(a) and Figure S2(b) (Supporting info.) and corresponding elemental composition as presented in Table S2 (Supporting info.). After day 365, the oxygen content increases more than double as compared to that of day 1. The oxidation of coated MXene film was also evidenced from transmittance spectra recorded on days 1, 30, 60, 180, and 365 as shown in Figure 7(a). It was found that over time, the transmittance slightly increases. The transmittance after day 30 is almost similar to day 1. After day 60, the transmittance difference is around 1-2%; however, after days 180 and 365, the transmittance increases by ~3% and ~5%, respectively. The increase in transmittance may be associated with the formation of more transparent TiO2 particles. These results again justify the conversion of Ti3C2Tx MXene into TiO2 particles [37].Figure 7
(a) The transmittance of MXene film from day 1 to day 365. The relative resistance change, (ΔR/R0%) of MXene film. (b) Under different storage conditions (inset: magnified curves). (c) Ambient conditions stored film prepared with MXene dispersed in DI water, SA (10 mM), and SDS (0.1 M). (d) MXene dispersed in DI water, MXene+f-CNT (10 wt.%), and MXene+rGO (10 wt.%) under ambient conditions (dotted line indicates nonconducting behavior).
(a)(b)(c)(d)Additionally, the stability was also observed by storing the coated MXene films, prepared by MXene dispersion in DI water, under different conditions like under a vacuum, MXene films in a petri dish covered with paraffin, NOA65 (1%) coating and UV curing, and MXene films stored under nitrogen (N2) environment. The relative resistance change (ΔR/R0%), where ΔR is the difference between the final and initial resistance and R0 is the initial resistance of MXene films stored under these conditions, is shown in Figure 7(b). MXene dispersions were spin-coated at PET substrates with subsequent drying at 100°C and corresponding ΔR/R0% was also analyzed. It was observed that MXene film under ambient conditions indicates a ΔR/R0% variation of ~3000%, whereas the MXene films stored under vacuum and coated with UV-cured NOA65 exhibited a small resistance change as compared to ambient conditions. MXene stability is improved with polymer passivation because it acts as a protective layer against ambient O2 or moisture. The higher oxidation rate in MXene under ambient conditions may be ascribed to the presence of functional groups -O and other functional groups at the MXene surface which make these more reactive [36]. Similarly, the samples stored under the N2 environment indicate low resistance change, whereas the MXene films in a petri dish wrapped with paraffin indicate moderate changes in resistance. All these storage experiments indicate that if the air or moisture is prevented, then the MXene can be preserved for a relatively long time. The ΔR/R0% of MXene dispersion in SDS (0.1 M) and SA (10 mM) coated on PET film were also studied under ambient conditions as shown in Figure 7(c). As expected, the SA-dispersed samples exhibited higher stability as compared to MXene only and MXene dispersed in SDS. Similarly, the ΔR/R0% of MXene, MXene+rGO, and MXene+f-CNT are shown in Figure 7(d). It was observed that relative resistance change for MXene+rGO-coated PET film is the highest among these three films, indicating that MXene+rGO-coated PET film oxidizes more rapidly than even MXene-only coated PET film. After 60 days, MXene+rGO-coated PET film became completely nonconducting (no measurable resistance); hence, ΔR/R0% plot was represented by the dotted line after 60 days. The MXene+f-CNT-coated film exhibited relatively better stability after 365 days as indicated by the lowest relative resistance change plot, which may be associated with the more stable CNTs.


#### 3.4.3. MXene Stability during Application in Smart Windows

Many attempts are made to increase the stability of Ti3C2Tx MXene, but the final goal is to use the stabilized MXene in some applications. But with any additional treatment given to MXene, surely one must compromise on the conductivity of the MXene. But, if the MXene remains stabilized while being used in any application, then there is no need for additional treatment. The use of Ti3C2Tx MXene in polymer-dispersed liquid crystal- (PDLC-) based smart windows is one such application, in which the MXene while being used as a transparent conducting electrode, coated at transparent PET or glass substrates, is preserved from being exposed to air. We have seen in the above studies that MXene is oxidized rapidly under ambient conditions, but these are stabilized if protected by a polymer layer. During PDLC fabrication, the UV-curable polymer in the PDLC mixture is cured by the UV light which preserves the MXene from being exposed to air. The PDLC mixture was prepared by mixing NOA65 monomer with liquid crystals (1 : 1 wt.%) and sandwiched between MXene-coated PET transparent films. The prepared PDLC cell was cured under UV light for 10 minutes. The switching behavior (on/off) was studied at 0-50 V voltage range. The actual images of the MXene and PDLC-based smart window at the on state (50 V) and off state (0 V) are shown in Figure 8(a), and corresponding images after day 365 are presented in Figure 8(b). The off state is perfectly opaque as the background is not visible, and as the voltage is increased, the background becomes visible. The switching mechanism of the PDLC-based smart window is depicted in Figure S3 and discussed in section 6 in supporting information.Figure 8
On and off states of PDLC MXene smart window on (a) day 1 and (b) day 365.
(a)(b)After 365 days, there seems to be no significant change in the PDLC cells, and these appear same as on day 1; however, the actual analysis was done using UV-visible spectra by plotting transmittance against voltage as presented in Figures9(a) and 9(b).Figure 9
Transmittance spectra of MXene electrode-based PDLC smart windows on (a) day 1 and (b) day 365 at different voltages.
(a)(b)On day 1, the off-state transmittance was ~4% (at ~580 nm), whereas the on-state transmittance is ~74%, maintaining a high contrast ratio (Figure9(a)). These results are close to those with silver or ITO electrodes based on PDLC smart windows [41]. After 365 days, the on-state transmittance was ~70%, i.e., ~4% reduction than that on day 1, whereas the off-state transmittance remains almost unaltered (Figure 9(b)). Even at ~30 V, it maintains a >60% transmittance, which is closer to that on day 1 at the same voltage.These results indicate that the PDLC smart window is still exhibiting high contrast ratio. It means that MXenes are conducting enough to generate a high electric field exhibiting good stability even after 365 days. It is because the MXene is preserved by the UV-cured NOA65 polymer, which is almost inert to air and moist atmosphere. The switching time, i.e., the on/off time, was also measured on days 1 and 365 as presented in FigureS4 (a-b) (Suppl. Info.). The plots indicate the switch on time on day 1 and day365<1 millisecond. The switch off time has a slight lag in both cases which is an inherent feature of PDLC-based smart windows, and it is also observed in the case of indium tin oxide (ITO) or other electrode-based PDLC smart windows. Again, this also supports the stability of MXene in this application.Hence, the use of Ti3C2Tx MXene in PDLC-based smart windows is one of the best ways to utilize MXene without giving additional treatment to the Ti3C2Tx MXene, and it could be a suitable alternative to costly and less flexible ITO.


## 3.1. Morphological Studies

The size and thickness of synthesized MXene flakes under study were examined using FESEM and AFM analyses, and corresponding images are shown in Figures1(a) and 1(b), respectively. The FESEM and AFM images indicate the MXene flakes of random size ranging from 100 nm to 1 μm. The thickness of the MXene flake is <2 nm as estimated from the AFM line profile shown in Figure 1(c).


## 3.2. Crystal Structure (X-Ray Diffraction) Analysis

The crystal structure of the Ti3C2 phase before and after etching was analyzed using XRD. The XRD patterns of the Ti3AlC2 MAX phase and Ti3C2 phase are shown in Figure 1(d). The XRD patterns for the Ti3AlC2 MAX phase and Ti3C2 phase accord well with the JCPDS card (JCPDS-52-0875 and JCPDS: 04-004-2919), respectively [29]. After etching, the (002) peak indicates a sharp shift towards a lower angle, i.e., from 2θ angle of 9.95° to 5.78° indicating the highly exfoliated MXene layers and the increase in the “c” lattice parameter [30, 31]. Additionally, the peak broadening suggests the reduced MXene layer thickness and increased d-spacing [21]. The spectrum indicates the disappearance of Al-associated (104) peak in the Ti3C2 phase [32].


## 3.3. Raman Analysis

The Raman spectra of the Ti3AlC2 MAX phase and Ti3C2 phase shown in Figure 1(e) were used to investigate the phonon-related vibrations and confirmation of terminal groups in MXene after etching. The Raman peaks were observed at around 200, 281, 365, 392, 565, 620, and 720 cm-1 (indicated by purple dotted lines). These peaks confirm the formation of Ti3C2 having terminal groups with -F, -O, and -OH functionalities. These terminal groups can be collectively represented by Tx; hence, the Ti3C2 phase will be represented as Ti3C2Tx in the rest of the studies. These results accord well with those reported earlier [14, 33]. Additionally, the Raman spectrum of the Ti3AlC2 MAX phase exhibits peaks at around 250, 395, and 600 cm-1 corresponding to E2g, E1g, and A1g phonon vibrational modes (indicated by green dotted lines), which match with an earlier study [34].


## 3.4. Long-Term Stability Studies of Ti3C2Tx MXene

### 3.4.1. MXene Stability in Different Solvents

As the Ti3C2Tx is hydrophilic in nature, so the stability study was carried out in DI water also. The stability of MXene was studied for 365 days by dispersing the MXene in DI water at different concentrations, e.g., 1 mg/ml and 2 mg/ml. The real-time digital images of the samples are shown in Figure 2(a).Figure 2
MXene stability in different solvents. Real-time digital images of the MXene dispersed in (a) DI water at different concentrations and (b) MXene dispersed in different organic solvents. I, acetone; II, IPA; and III, ethanol.
(a)(b)The images indicate the black color of the dispersed MXene in DI water on day 1. On day 30, there is no specific change in color. However, from day 60, there is a clear change in MXene color as the black-colored dispersion started appearing slightly milky color indicating the start of conversion of MXene to TiO2 due to oxidation. However, the MXene with 2 mg/ml was found slightly darker than that at 1 mg/ml concentration, which is obvious as at low concentrations; MXene flakes are more exposed to DI water environment as evident from MXene at higher concentration (3 mg/ml) in DI water shown in Figure S1 (Supporting info.). On day 180, the color was completely turned to milky and slightly transparent in both cases. Till day 365, there is no significant change in the color as compared to day 180 as the Ti3C2Tx MXene is completely converted into the TiO2. Under ambient conditions, there is a possibility of conversion of MXene into TiO2 as predicted in the following reaction:
(1)Ti3C2O2+4O2⟶3TiO2+2CO2.If the MXene has defects, then even the purest Ti3C2 can be oxidized to TiO2 as
(2)Ti3C2+3O2⟶3TiO2+2C.The major factor which leads to the oxidation is the atomic defects in Ti3C2Tx MXene. It has Ti-vacancies, which are probably associated with the Ti3C2Tx MXene during the process of chemical synthesis [35].The degradation of MXene dispersed in DI water was also studied using absorbance spectroscopy, and the absorbance spectra recorded at different times are presented in Figure3. The degree of oxidation of MXenes can be estimated from the decrease in the absorbance peak intensity at ~760 nm [36]. The spectra show the highest absorbance for MXene on day 1 as it has dark black color, and the intensity decreases with time. After day 60, the peak at around 750 nm on day 1 was observed to shift towards 780 nm, whereas after day 180, the peak almost disappeared indicating the degradation of MXene flakes possibly due to the formation of TiO2 [27]. After 365 days, the MXene dispersion has negligible absorbance supporting the MXene conversion into white TiO2 [37]. Also, Ti3C2Tx MXene can preserve its structural integrity up to x≈3.5 before converting it into TiO2 [38]. As per an earlier report, MXene flakes retain their structural integrity until the factor x, which is the ratio of Ti atoms to O atoms in MXene flakes, reaching close to 3.5, i.e., ~1.75 oxygen atoms per Ti atoms; then there is a possibility that Ti3C2Tx MXene is converted into TiO2. After day 60, the stability curve is exhibiting a mixed behavior of oxidized and nonoxidized MXene which indicates that some MXene flakes are oxidized or unstable and others are still stable as the intensity of the UV visible spectrum after day 60 was also found to be reduced as compared to that observed for fresh Ti3C2Tx MXene on day 1; simultaneously, the peak at 780 nm is not completely absent as in case TiO2 on days 180 and 365. This trend can be predicted using
(3)A=Astable+Aunstable×e−t/τ,where A is the normalized absorbance, Astable is the absorbance of stable MXene, Aunstable is the absorbance of unstable MXene flakes at a particular wavelength, and τ is the time (t)-dependent decay constant [36].Figure 3
Absorbance spectra of MXene dispersed in DI water at different durations.The stability of the MXene stored in different organic solvents, e.g., acetone, ethanol, and isopropyl alcohol (IPA), was also observed at the same MXene concentration (2 mg/ml). The real-time pictures of the MXene dispersed in these solvents are shown in Figure2(b). There were no signs of oxidation as estimated from the color of MXene dispersions in organic solvents, and even after 365 days, the color of the MXene remained black. The MXene dispersions in these solvents were easily precipitated after a few seconds as MXene is not dispersible in these solvents. These results were similar as observed as reported earlier [26]; however, MXene dispersions are stable for even longer durations.Another possible way to protect the oxidation of MXene is the use of antioxidants. The stability of the MXene (2 mg/ml) was studied using 5 mM, 10 mM, and 20 mM concentrations of antioxidant (SA) in DI water. It was found that at low SA concentration, the MXene was oxidized slowly as compared to DI water after 365 days whereas, at higher SA concentration, the color of MXene remains almost unchanged as indicated in Figure4(a). It indicates that a small SA concentration (~10 mM) can preserve MXene for one year. Of course, higher SA concentration can also preserve Ti3C2Tx MXene for a longer duration but at the expense of the conductivity of the MXene because on day 1, as the MXene+SA mixture was spin-coated on PET film, the conductivity of MXene+SA-coated film in dried form is relatively lesser than that of MXene-only coated film (at same MXene concentration). It may be ascribed to the fact that SA is nonconducting, hence leading to an overall decrease in conductivity. However, for a longer duration, the SA preserves the MXene from being oxidized in solution form.Figure 4
MXene stability in SA antioxidant. Digital images of the MXene (2 mg/ml) dispersed in (a) DI water and SA at different concentrations. A, DI water; B, 5 mM SA; C, 10 mM SA; and D, 20mM SA. (b) MXene (2 mg/ml) dispersed in SDS, rGO, and f-CNTs. X, SDS; Y, rGO; and Z, f-CNTs.
(a)(b)The stability of MXene was also studied with surfactant, sodium dodecyl sulfate (SDS, 0.1 M), 10% functionalized CNTs (f-CNTs), and reduced graphene oxide (rGO). All the dispersions include DI water and the images recorded on day 1 to 365 days are shown in Figure4(b). It was observed that the MXene color was changed to turbid in SDS, whereas in rGO, the color changed to pale, indicating that MXene is oxidized in both media. The SDS is a popular capping agent for nanomaterials; however, it only partially prevented oxidation, probably because it failed to interact with the Ti3C2Tx surface [39]. A pale color in the MXene+rGO sample may be associated with the oxidation of rGO to GO [40]. However, in f-CNT, the black color of dispersion was maintained almost similar after 365 days.


### 3.4.2. MXene Stability Studies in Dried Form

In the above study, it was observed that the Ti3C2Tx MXenes are easily oxidized in DI water, indicating that DI water is a major factor in instigating the oxidation of the Ti3C2Tx MXene. To investigate the stability of Ti3C2Tx MXene in the dried form under ambient conditions, the Ti3C2Tx MXene films prepared by MXene dispersion in DI water were coated at PET substrates, and stability was estimated by measuring the relative resistance change. Additionally, the MXene dispersions prepared with different solvents were used to coat at PET film, and the sheet resistance was analyzed for 365 days under different conditions. The actual image of the MXene-coated PET film prepared from MXene dispersion in DI water is shown in Figure 5(a).Figure 5
(a) The actual image of the Ti3C2Tx MXene coated at PET film. (b) Top view FESEM image. (c) Cross-sectional FESEM image. (d) AFM image. (e) Corresponding line profiles. (f) Transmittance spectrum of MXene-coated PET film.
(a)(b)(c)(d)(e)(f)The film appears highly transparent as the background is clearly visible. The FESEM image showing the top surface view of the coated MXene film is shown in Figure5(b), and the corresponding FESEM image with the cross-sectional view is shown in Figure 5(c). The thickness of MXene film appears around 10 nm which was also supported by AFM images and corresponding line profiles shown in Figures 5(d) and 5(e), respectively. The transparency of the coated MXene film was confirmed with the transmittance spectrum shown in Figure 5(f). The transmittance of film was found ~83% indicating high transparency which corroborates the visual image shown in Figure 5(a). The electrical parameters of MXene-coated PET film are presented in Table S1 (Supporting info.). The conductivity of the MXene film is much lower than the highest conductivity reported for Ti3C2Tx MXene (~20,000 S/cm) which may be associated with defects induced during the synthesis process [27].(1) Surface and Electrical Study of MXene-Coated PET Films. The MXene dispersed in DI water was coated and analyzed for 365 days. The FESEM images of the MXene films under ambient conditions after 60, 180, and 365 days are shown in Figures 6(a)–6(c), respectively. It was observed that the edges of the MXene flakes have a boundary of powder-like material which is probably due to the oxidation of the MXene as the oxidation starts from the edges [23].Figure 6
FESEM images of MXene films coated at PET after (a) 60 days, (b) 180 days, and (c) 365 days.
(a)(b)(c)To analyze the oxidation, the elemental spectra were recorded and compared on day 1 spectrum with that on day 365 as shown in FigureS2(a) and Figure S2(b) (Supporting info.) and corresponding elemental composition as presented in Table S2 (Supporting info.). After day 365, the oxygen content increases more than double as compared to that of day 1. The oxidation of coated MXene film was also evidenced from transmittance spectra recorded on days 1, 30, 60, 180, and 365 as shown in Figure 7(a). It was found that over time, the transmittance slightly increases. The transmittance after day 30 is almost similar to day 1. After day 60, the transmittance difference is around 1-2%; however, after days 180 and 365, the transmittance increases by ~3% and ~5%, respectively. The increase in transmittance may be associated with the formation of more transparent TiO2 particles. These results again justify the conversion of Ti3C2Tx MXene into TiO2 particles [37].Figure 7
(a) The transmittance of MXene film from day 1 to day 365. The relative resistance change, (ΔR/R0%) of MXene film. (b) Under different storage conditions (inset: magnified curves). (c) Ambient conditions stored film prepared with MXene dispersed in DI water, SA (10 mM), and SDS (0.1 M). (d) MXene dispersed in DI water, MXene+f-CNT (10 wt.%), and MXene+rGO (10 wt.%) under ambient conditions (dotted line indicates nonconducting behavior).
(a)(b)(c)(d)Additionally, the stability was also observed by storing the coated MXene films, prepared by MXene dispersion in DI water, under different conditions like under a vacuum, MXene films in a petri dish covered with paraffin, NOA65 (1%) coating and UV curing, and MXene films stored under nitrogen (N2) environment. The relative resistance change (ΔR/R0%), where ΔR is the difference between the final and initial resistance and R0 is the initial resistance of MXene films stored under these conditions, is shown in Figure 7(b). MXene dispersions were spin-coated at PET substrates with subsequent drying at 100°C and corresponding ΔR/R0% was also analyzed. It was observed that MXene film under ambient conditions indicates a ΔR/R0% variation of ~3000%, whereas the MXene films stored under vacuum and coated with UV-cured NOA65 exhibited a small resistance change as compared to ambient conditions. MXene stability is improved with polymer passivation because it acts as a protective layer against ambient O2 or moisture. The higher oxidation rate in MXene under ambient conditions may be ascribed to the presence of functional groups -O and other functional groups at the MXene surface which make these more reactive [36]. Similarly, the samples stored under the N2 environment indicate low resistance change, whereas the MXene films in a petri dish wrapped with paraffin indicate moderate changes in resistance. All these storage experiments indicate that if the air or moisture is prevented, then the MXene can be preserved for a relatively long time. The ΔR/R0% of MXene dispersion in SDS (0.1 M) and SA (10 mM) coated on PET film were also studied under ambient conditions as shown in Figure 7(c). As expected, the SA-dispersed samples exhibited higher stability as compared to MXene only and MXene dispersed in SDS. Similarly, the ΔR/R0% of MXene, MXene+rGO, and MXene+f-CNT are shown in Figure 7(d). It was observed that relative resistance change for MXene+rGO-coated PET film is the highest among these three films, indicating that MXene+rGO-coated PET film oxidizes more rapidly than even MXene-only coated PET film. After 60 days, MXene+rGO-coated PET film became completely nonconducting (no measurable resistance); hence, ΔR/R0% plot was represented by the dotted line after 60 days. The MXene+f-CNT-coated film exhibited relatively better stability after 365 days as indicated by the lowest relative resistance change plot, which may be associated with the more stable CNTs.


### 3.4.3. MXene Stability during Application in Smart Windows

Many attempts are made to increase the stability of Ti3C2Tx MXene, but the final goal is to use the stabilized MXene in some applications. But with any additional treatment given to MXene, surely one must compromise on the conductivity of the MXene. But, if the MXene remains stabilized while being used in any application, then there is no need for additional treatment. The use of Ti3C2Tx MXene in polymer-dispersed liquid crystal- (PDLC-) based smart windows is one such application, in which the MXene while being used as a transparent conducting electrode, coated at transparent PET or glass substrates, is preserved from being exposed to air. We have seen in the above studies that MXene is oxidized rapidly under ambient conditions, but these are stabilized if protected by a polymer layer. During PDLC fabrication, the UV-curable polymer in the PDLC mixture is cured by the UV light which preserves the MXene from being exposed to air. The PDLC mixture was prepared by mixing NOA65 monomer with liquid crystals (1 : 1 wt.%) and sandwiched between MXene-coated PET transparent films. The prepared PDLC cell was cured under UV light for 10 minutes. The switching behavior (on/off) was studied at 0-50 V voltage range. The actual images of the MXene and PDLC-based smart window at the on state (50 V) and off state (0 V) are shown in Figure 8(a), and corresponding images after day 365 are presented in Figure 8(b). The off state is perfectly opaque as the background is not visible, and as the voltage is increased, the background becomes visible. The switching mechanism of the PDLC-based smart window is depicted in Figure S3 and discussed in section 6 in supporting information.Figure 8
On and off states of PDLC MXene smart window on (a) day 1 and (b) day 365.
(a)(b)After 365 days, there seems to be no significant change in the PDLC cells, and these appear same as on day 1; however, the actual analysis was done using UV-visible spectra by plotting transmittance against voltage as presented in Figures9(a) and 9(b).Figure 9
Transmittance spectra of MXene electrode-based PDLC smart windows on (a) day 1 and (b) day 365 at different voltages.
(a)(b)On day 1, the off-state transmittance was ~4% (at ~580 nm), whereas the on-state transmittance is ~74%, maintaining a high contrast ratio (Figure9(a)). These results are close to those with silver or ITO electrodes based on PDLC smart windows [41]. After 365 days, the on-state transmittance was ~70%, i.e., ~4% reduction than that on day 1, whereas the off-state transmittance remains almost unaltered (Figure 9(b)). Even at ~30 V, it maintains a >60% transmittance, which is closer to that on day 1 at the same voltage.These results indicate that the PDLC smart window is still exhibiting high contrast ratio. It means that MXenes are conducting enough to generate a high electric field exhibiting good stability even after 365 days. It is because the MXene is preserved by the UV-cured NOA65 polymer, which is almost inert to air and moist atmosphere. The switching time, i.e., the on/off time, was also measured on days 1 and 365 as presented in FigureS4 (a-b) (Suppl. Info.). The plots indicate the switch on time on day 1 and day365<1 millisecond. The switch off time has a slight lag in both cases which is an inherent feature of PDLC-based smart windows, and it is also observed in the case of indium tin oxide (ITO) or other electrode-based PDLC smart windows. Again, this also supports the stability of MXene in this application.Hence, the use of Ti3C2Tx MXene in PDLC-based smart windows is one of the best ways to utilize MXene without giving additional treatment to the Ti3C2Tx MXene, and it could be a suitable alternative to costly and less flexible ITO.


## 3.4.1. MXene Stability in Different Solvents

As the Ti3C2Tx is hydrophilic in nature, so the stability study was carried out in DI water also. The stability of MXene was studied for 365 days by dispersing the MXene in DI water at different concentrations, e.g., 1 mg/ml and 2 mg/ml. The real-time digital images of the samples are shown in Figure 2(a).Figure 2
MXene stability in different solvents. Real-time digital images of the MXene dispersed in (a) DI water at different concentrations and (b) MXene dispersed in different organic solvents. I, acetone; II, IPA; and III, ethanol.
(a)(b)The images indicate the black color of the dispersed MXene in DI water on day 1. On day 30, there is no specific change in color. However, from day 60, there is a clear change in MXene color as the black-colored dispersion started appearing slightly milky color indicating the start of conversion of MXene to TiO2 due to oxidation. However, the MXene with 2 mg/ml was found slightly darker than that at 1 mg/ml concentration, which is obvious as at low concentrations; MXene flakes are more exposed to DI water environment as evident from MXene at higher concentration (3 mg/ml) in DI water shown in Figure S1 (Supporting info.). On day 180, the color was completely turned to milky and slightly transparent in both cases. Till day 365, there is no significant change in the color as compared to day 180 as the Ti3C2Tx MXene is completely converted into the TiO2. Under ambient conditions, there is a possibility of conversion of MXene into TiO2 as predicted in the following reaction:
(1)Ti3C2O2+4O2⟶3TiO2+2CO2.If the MXene has defects, then even the purest Ti3C2 can be oxidized to TiO2 as
(2)Ti3C2+3O2⟶3TiO2+2C.The major factor which leads to the oxidation is the atomic defects in Ti3C2Tx MXene. It has Ti-vacancies, which are probably associated with the Ti3C2Tx MXene during the process of chemical synthesis [35].The degradation of MXene dispersed in DI water was also studied using absorbance spectroscopy, and the absorbance spectra recorded at different times are presented in Figure3. The degree of oxidation of MXenes can be estimated from the decrease in the absorbance peak intensity at ~760 nm [36]. The spectra show the highest absorbance for MXene on day 1 as it has dark black color, and the intensity decreases with time. After day 60, the peak at around 750 nm on day 1 was observed to shift towards 780 nm, whereas after day 180, the peak almost disappeared indicating the degradation of MXene flakes possibly due to the formation of TiO2 [27]. After 365 days, the MXene dispersion has negligible absorbance supporting the MXene conversion into white TiO2 [37]. Also, Ti3C2Tx MXene can preserve its structural integrity up to x≈3.5 before converting it into TiO2 [38]. As per an earlier report, MXene flakes retain their structural integrity until the factor x, which is the ratio of Ti atoms to O atoms in MXene flakes, reaching close to 3.5, i.e., ~1.75 oxygen atoms per Ti atoms; then there is a possibility that Ti3C2Tx MXene is converted into TiO2. After day 60, the stability curve is exhibiting a mixed behavior of oxidized and nonoxidized MXene which indicates that some MXene flakes are oxidized or unstable and others are still stable as the intensity of the UV visible spectrum after day 60 was also found to be reduced as compared to that observed for fresh Ti3C2Tx MXene on day 1; simultaneously, the peak at 780 nm is not completely absent as in case TiO2 on days 180 and 365. This trend can be predicted using
(3)A=Astable+Aunstable×e−t/τ,where A is the normalized absorbance, Astable is the absorbance of stable MXene, Aunstable is the absorbance of unstable MXene flakes at a particular wavelength, and τ is the time (t)-dependent decay constant [36].Figure 3
Absorbance spectra of MXene dispersed in DI water at different durations.The stability of the MXene stored in different organic solvents, e.g., acetone, ethanol, and isopropyl alcohol (IPA), was also observed at the same MXene concentration (2 mg/ml). The real-time pictures of the MXene dispersed in these solvents are shown in Figure2(b). There were no signs of oxidation as estimated from the color of MXene dispersions in organic solvents, and even after 365 days, the color of the MXene remained black. The MXene dispersions in these solvents were easily precipitated after a few seconds as MXene is not dispersible in these solvents. These results were similar as observed as reported earlier [26]; however, MXene dispersions are stable for even longer durations.Another possible way to protect the oxidation of MXene is the use of antioxidants. The stability of the MXene (2 mg/ml) was studied using 5 mM, 10 mM, and 20 mM concentrations of antioxidant (SA) in DI water. It was found that at low SA concentration, the MXene was oxidized slowly as compared to DI water after 365 days whereas, at higher SA concentration, the color of MXene remains almost unchanged as indicated in Figure4(a). It indicates that a small SA concentration (~10 mM) can preserve MXene for one year. Of course, higher SA concentration can also preserve Ti3C2Tx MXene for a longer duration but at the expense of the conductivity of the MXene because on day 1, as the MXene+SA mixture was spin-coated on PET film, the conductivity of MXene+SA-coated film in dried form is relatively lesser than that of MXene-only coated film (at same MXene concentration). It may be ascribed to the fact that SA is nonconducting, hence leading to an overall decrease in conductivity. However, for a longer duration, the SA preserves the MXene from being oxidized in solution form.Figure 4
MXene stability in SA antioxidant. Digital images of the MXene (2 mg/ml) dispersed in (a) DI water and SA at different concentrations. A, DI water; B, 5 mM SA; C, 10 mM SA; and D, 20mM SA. (b) MXene (2 mg/ml) dispersed in SDS, rGO, and f-CNTs. X, SDS; Y, rGO; and Z, f-CNTs.
(a)(b)The stability of MXene was also studied with surfactant, sodium dodecyl sulfate (SDS, 0.1 M), 10% functionalized CNTs (f-CNTs), and reduced graphene oxide (rGO). All the dispersions include DI water and the images recorded on day 1 to 365 days are shown in Figure4(b). It was observed that the MXene color was changed to turbid in SDS, whereas in rGO, the color changed to pale, indicating that MXene is oxidized in both media. The SDS is a popular capping agent for nanomaterials; however, it only partially prevented oxidation, probably because it failed to interact with the Ti3C2Tx surface [39]. A pale color in the MXene+rGO sample may be associated with the oxidation of rGO to GO [40]. However, in f-CNT, the black color of dispersion was maintained almost similar after 365 days.


## 3.4.2. MXene Stability Studies in Dried Form

In the above study, it was observed that the Ti3C2Tx MXenes are easily oxidized in DI water, indicating that DI water is a major factor in instigating the oxidation of the Ti3C2Tx MXene. To investigate the stability of Ti3C2Tx MXene in the dried form under ambient conditions, the Ti3C2Tx MXene films prepared by MXene dispersion in DI water were coated at PET substrates, and stability was estimated by measuring the relative resistance change. Additionally, the MXene dispersions prepared with different solvents were used to coat at PET film, and the sheet resistance was analyzed for 365 days under different conditions. The actual image of the MXene-coated PET film prepared from MXene dispersion in DI water is shown in Figure 5(a).Figure 5
(a) The actual image of the Ti3C2Tx MXene coated at PET film. (b) Top view FESEM image. (c) Cross-sectional FESEM image. (d) AFM image. (e) Corresponding line profiles. (f) Transmittance spectrum of MXene-coated PET film.
(a)(b)(c)(d)(e)(f)The film appears highly transparent as the background is clearly visible. The FESEM image showing the top surface view of the coated MXene film is shown in Figure5(b), and the corresponding FESEM image with the cross-sectional view is shown in Figure 5(c). The thickness of MXene film appears around 10 nm which was also supported by AFM images and corresponding line profiles shown in Figures 5(d) and 5(e), respectively. The transparency of the coated MXene film was confirmed with the transmittance spectrum shown in Figure 5(f). The transmittance of film was found ~83% indicating high transparency which corroborates the visual image shown in Figure 5(a). The electrical parameters of MXene-coated PET film are presented in Table S1 (Supporting info.). The conductivity of the MXene film is much lower than the highest conductivity reported for Ti3C2Tx MXene (~20,000 S/cm) which may be associated with defects induced during the synthesis process [27].(1) Surface and Electrical Study of MXene-Coated PET Films. The MXene dispersed in DI water was coated and analyzed for 365 days. The FESEM images of the MXene films under ambient conditions after 60, 180, and 365 days are shown in Figures 6(a)–6(c), respectively. It was observed that the edges of the MXene flakes have a boundary of powder-like material which is probably due to the oxidation of the MXene as the oxidation starts from the edges [23].Figure 6
FESEM images of MXene films coated at PET after (a) 60 days, (b) 180 days, and (c) 365 days.
(a)(b)(c)To analyze the oxidation, the elemental spectra were recorded and compared on day 1 spectrum with that on day 365 as shown in FigureS2(a) and Figure S2(b) (Supporting info.) and corresponding elemental composition as presented in Table S2 (Supporting info.). After day 365, the oxygen content increases more than double as compared to that of day 1. The oxidation of coated MXene film was also evidenced from transmittance spectra recorded on days 1, 30, 60, 180, and 365 as shown in Figure 7(a). It was found that over time, the transmittance slightly increases. The transmittance after day 30 is almost similar to day 1. After day 60, the transmittance difference is around 1-2%; however, after days 180 and 365, the transmittance increases by ~3% and ~5%, respectively. The increase in transmittance may be associated with the formation of more transparent TiO2 particles. These results again justify the conversion of Ti3C2Tx MXene into TiO2 particles [37].Figure 7
(a) The transmittance of MXene film from day 1 to day 365. The relative resistance change, (ΔR/R0%) of MXene film. (b) Under different storage conditions (inset: magnified curves). (c) Ambient conditions stored film prepared with MXene dispersed in DI water, SA (10 mM), and SDS (0.1 M). (d) MXene dispersed in DI water, MXene+f-CNT (10 wt.%), and MXene+rGO (10 wt.%) under ambient conditions (dotted line indicates nonconducting behavior).
(a)(b)(c)(d)Additionally, the stability was also observed by storing the coated MXene films, prepared by MXene dispersion in DI water, under different conditions like under a vacuum, MXene films in a petri dish covered with paraffin, NOA65 (1%) coating and UV curing, and MXene films stored under nitrogen (N2) environment. The relative resistance change (ΔR/R0%), where ΔR is the difference between the final and initial resistance and R0 is the initial resistance of MXene films stored under these conditions, is shown in Figure 7(b). MXene dispersions were spin-coated at PET substrates with subsequent drying at 100°C and corresponding ΔR/R0% was also analyzed. It was observed that MXene film under ambient conditions indicates a ΔR/R0% variation of ~3000%, whereas the MXene films stored under vacuum and coated with UV-cured NOA65 exhibited a small resistance change as compared to ambient conditions. MXene stability is improved with polymer passivation because it acts as a protective layer against ambient O2 or moisture. The higher oxidation rate in MXene under ambient conditions may be ascribed to the presence of functional groups -O and other functional groups at the MXene surface which make these more reactive [36]. Similarly, the samples stored under the N2 environment indicate low resistance change, whereas the MXene films in a petri dish wrapped with paraffin indicate moderate changes in resistance. All these storage experiments indicate that if the air or moisture is prevented, then the MXene can be preserved for a relatively long time. The ΔR/R0% of MXene dispersion in SDS (0.1 M) and SA (10 mM) coated on PET film were also studied under ambient conditions as shown in Figure 7(c). As expected, the SA-dispersed samples exhibited higher stability as compared to MXene only and MXene dispersed in SDS. Similarly, the ΔR/R0% of MXene, MXene+rGO, and MXene+f-CNT are shown in Figure 7(d). It was observed that relative resistance change for MXene+rGO-coated PET film is the highest among these three films, indicating that MXene+rGO-coated PET film oxidizes more rapidly than even MXene-only coated PET film. After 60 days, MXene+rGO-coated PET film became completely nonconducting (no measurable resistance); hence, ΔR/R0% plot was represented by the dotted line after 60 days. The MXene+f-CNT-coated film exhibited relatively better stability after 365 days as indicated by the lowest relative resistance change plot, which may be associated with the more stable CNTs.


## 3.4.3. MXene Stability during Application in Smart Windows

Many attempts are made to increase the stability of Ti3C2Tx MXene, but the final goal is to use the stabilized MXene in some applications. But with any additional treatment given to MXene, surely one must compromise on the conductivity of the MXene. But, if the MXene remains stabilized while being used in any application, then there is no need for additional treatment. The use of Ti3C2Tx MXene in polymer-dispersed liquid crystal- (PDLC-) based smart windows is one such application, in which the MXene while being used as a transparent conducting electrode, coated at transparent PET or glass substrates, is preserved from being exposed to air. We have seen in the above studies that MXene is oxidized rapidly under ambient conditions, but these are stabilized if protected by a polymer layer. During PDLC fabrication, the UV-curable polymer in the PDLC mixture is cured by the UV light which preserves the MXene from being exposed to air. The PDLC mixture was prepared by mixing NOA65 monomer with liquid crystals (1 : 1 wt.%) and sandwiched between MXene-coated PET transparent films. The prepared PDLC cell was cured under UV light for 10 minutes. The switching behavior (on/off) was studied at 0-50 V voltage range. The actual images of the MXene and PDLC-based smart window at the on state (50 V) and off state (0 V) are shown in Figure 8(a), and corresponding images after day 365 are presented in Figure 8(b). The off state is perfectly opaque as the background is not visible, and as the voltage is increased, the background becomes visible. The switching mechanism of the PDLC-based smart window is depicted in Figure S3 and discussed in section 6 in supporting information.Figure 8
On and off states of PDLC MXene smart window on (a) day 1 and (b) day 365.
(a)(b)After 365 days, there seems to be no significant change in the PDLC cells, and these appear same as on day 1; however, the actual analysis was done using UV-visible spectra by plotting transmittance against voltage as presented in Figures9(a) and 9(b).Figure 9
Transmittance spectra of MXene electrode-based PDLC smart windows on (a) day 1 and (b) day 365 at different voltages.
(a)(b)On day 1, the off-state transmittance was ~4% (at ~580 nm), whereas the on-state transmittance is ~74%, maintaining a high contrast ratio (Figure9(a)). These results are close to those with silver or ITO electrodes based on PDLC smart windows [41]. After 365 days, the on-state transmittance was ~70%, i.e., ~4% reduction than that on day 1, whereas the off-state transmittance remains almost unaltered (Figure 9(b)). Even at ~30 V, it maintains a >60% transmittance, which is closer to that on day 1 at the same voltage.These results indicate that the PDLC smart window is still exhibiting high contrast ratio. It means that MXenes are conducting enough to generate a high electric field exhibiting good stability even after 365 days. It is because the MXene is preserved by the UV-cured NOA65 polymer, which is almost inert to air and moist atmosphere. The switching time, i.e., the on/off time, was also measured on days 1 and 365 as presented in FigureS4 (a-b) (Suppl. Info.). The plots indicate the switch on time on day 1 and day365<1 millisecond. The switch off time has a slight lag in both cases which is an inherent feature of PDLC-based smart windows, and it is also observed in the case of indium tin oxide (ITO) or other electrode-based PDLC smart windows. Again, this also supports the stability of MXene in this application.Hence, the use of Ti3C2Tx MXene in PDLC-based smart windows is one of the best ways to utilize MXene without giving additional treatment to the Ti3C2Tx MXene, and it could be a suitable alternative to costly and less flexible ITO.


## 4. Conclusions

In this study, we have performed a complete study on the long-term stability of Ti3C2Tx MXene under different conditions over a period of one year. The stability was studied in liquid form in different solvents including DI water. In the dried form, the stability of MXene film was studied under different storage conditions after being coated at transparent PET film. Additionally, stability was also studied during the use of Ti3C2Tx MXene-based transparent conducting electrodes in PDLC-based switchable smart windows. It was found that MXene is easily oxidized if exposed to DI water under ambient conditions, and after a certain time, it is converted into TiO2 as the color of MXene dispersion was turned to white from dark black. In liquid form, the oxidation of MXenes can be controlled to some extent by using antioxidants. In dried form at PET substrate, the MXene oxidation rate is slow, and it can be further controlled by storing the MXene films under vacuum conditions or passivating with polymer as under these conditions MXene maintains high electrical conductivity even after 365 days. The oxidation can also be controlled to some extent by combining it with other materials, e.g., CNTs. The stability of MXene-coated PET films can be maintained for a long time if these are used as transparent conducting electrodes in PDLC-based smart windows, without any additional treatment to the MXene. MXene-based conducting electrodes maintain the switching behavior after 365 days in PDLC smart windows. MXene can be a suitable alternative to the more expensive and brittle ITO glass in smart window applications. This study not only investigates the stability of MXenes but also provides insight into the improvement of MXene stability under various conditions.



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*Source: 5275439-2023-02-03.xml*