Understanding In-Plane Sliding of Functionalized
Ti3C2Tx MXene by In Situ Microscale Analysis of
Electrochemical Actuation

Harpreet Singh, Shaohua Chen, Grégory Francius, Liang Liu, Pooi See Lee, Mathieu Etienne

► To cite this version:

Harpreet Singh, Shaohua Chen, Grégory Francius, Liang Liu, Pooi See Lee, et al.. Understanding In-Plane Sliding of Functionalized Ti3C2Tx MXene by In Situ Microscale Analysis of Electrochemical Actuation. Chemistry of Materials, 2024, 36 (19), pp.9575-9583. 10.1021/acs.chemmater.4c01597 .
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<figure><img src="image_1.png" /></figure>
# Understanding In-Plane Sliding of Functionalized Ti₃C₂Tₓ MXene by In-Situ Microscale Analysis of Electrochemical Actuation

Harpreet Singh<fnref n="1" />, Shaohua Chen<fnref n="2" />, Grégory Francius<fnref n="1" />, Liang Liu<fnref n="1" />,*, Pooi See Lee<fnref n="2" />, Mathieu Etienne<fnref n="1" />,∗

**Abstract:** Electrochemical actuation is based on the deformation of electrodes upon applying a mild potential (usually in the range of ±5V in a single cell). In this work, *in-situ* local analysis of the electrochemical actuation of Ti₃C₂Tₓ (T = -O, -OH, -F) MXene at microscale is achieved by Raman spectroscopy, atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) while applying potential in an electrochemical cell. First, Raman spectra recorded at steady state with different potentials confirm the intercalation/deintercalation of Li⁺ ions, resulting in the change of out-of-plane vibrations due to the interaction of Li⁺ with the -O and -OH surface group atoms of MXene. A methylcellulose functionalization of MXene films results in more freedom for in-plane vibrations. Second, SECM is applied to visualize the deformation of two-end-fixed MXene films both at steady state (0V vs. Ag/AgCl QRE) and at scanning potential. The results show a buckling-type actuation, which is more pronounced for methylcellulose-functionalized samples. Electrochemical AFM provided the evidence of a significant in-plane sliding of the MXene flakes. Out-of-plane changes appears to be negligible in the actuation mechanism, as supported by the AFM approach curve analysis. Overall, the methylcellulose-functionalized MXene shows better actuation performance, especially in the in-plane direction and at high frequency. The mechanism is then discussed.

## INTRODUCTION

The electrochemical actuators are based on the electrochemically induced deformation of electrodes, typically in a two-electrode configuration separated by an electrolyte layer. Compared with other type of actuations (electrical energy to mechanical energy<fnref n="1" />, Magnetic field induced actuation<fnref n="2" />, spin transitions³, Magnetic field dependent shape memory retention⁴, Piezoelectric mechanism⁵, Photoinduced molecular realignment⁶⁻⁸, and thermally driven shape memory retention⁹) the electrochemical actuators are advantageous because of their compactness¹⁰, lightweight, resistance to humid in case of ionic liquids¹¹ and corrosive¹² conditions, high deformation and displacement under low-voltage applications.¹³,¹⁴ These may significantly improve the safety of operation and reduce the power consumption, which are suitable for various applications such as artificial muscles¹⁵,¹⁶, flapping motions¹⁷, tactile displays, bionics, switches, and converters.¹⁸⁻²⁰

Different active (or electrode) materials have been explored for electrochemical actuators, including graphene²¹,²², carbon nanotubes²³⁻²⁵, black phosphorus²⁶, graphdiyne²⁷, carbon nitride²⁸, MOF²⁹, MoS₂³⁰, etc. Recently, MXene³¹ has shown a high potential as electrode material for electrochemical

actuators because of its high conductivity³², flexibility³³, and tunable interlayer spacing³⁴. The material is easy to process as it can form stable dispersions in various solvents including water. This colloidal solution can then be easily functionalized, aligned, and stacked into layered film structures by following simple vacuum filtration method.⁵⁶ Several types of MXenes (Ti₂CTₓ,³⁵ Ti₃C₂Tₓ,³⁶ Nb₂CTₓ,³⁷ Nb₄C₃Tₓ,³⁸,³⁹ V₂CTₓ,⁴⁰⁻⁴² Mo₂CTₓ,⁴³ Mo₄VC₄Tₓ,⁴⁴ and Mo₂Ti₂C₃⁴⁵) have been studied for energy storage applications⁴⁶. For the choice of electrochemical actuation, the capacitance, conductivity and Young's modulus of the material are considered, and Ti₃C₂Tₓ is selected as a suitable candidate for the application.

Various studies have demonstrated that the structure of MXene plays an important role in the actuation performance. This may further be improved by forming composites, such as with PEDOT:PSS/Ag nanowires⁵⁷ and polystyrene microspheres⁵⁸. Recently, Chen et al. has reported the functionalization of MXene by methylcellulose and tetrabutylammonium for enhancing the electrochemical actuation performance⁵⁶,⁵⁹. It is seen that the 15% functionalized film achieves a peak tensile strength of 53.2 MPa and a Young's modulus over 10 GPa, surpassing many

<footnote n="1">¹Université de Lorraine, CNRS, LCPME, 54000 Nancy, France</footnote>

<footnote n="2">²School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore</footnote>
other materials like pure MC (0.3 MPa)⁶⁰ and PEDOT/MXene (23.3 MPa)⁶¹ films. The material is also mechanically robust under repeated folding.⁵⁶

Apart from the global strain analysis performed in the abovementioned studies for the evaluation of actuation performance, in-situ characterization methods are powerful tools for understanding the electrochemical behavior of MXene as an actuator electrode. Raman spectroscopy is widely used for probing the structural changes of electrodes during electrochemical reactions.⁶²,⁶³ It has also been used for studying MXene/polypyrrole composite upon oxidation/reduction by following the changes on the C=C peak (1571 cm⁻¹: reduction state and 1607 cm⁻¹: oxidation state) of polypyrrole.⁶⁴ Scanning electrochemical microscopy (SECM) was originally proposed for imaging the topography of electrodes⁶⁵, before it was more commonly used for studying the charge transfer kinetics.⁶⁶,⁶⁷ The topographic imaging by SECM was at microscale, which was complemented by electrochemical atomic force microscopy (EC-AFM) and electrochemical scanning tunneling microscopy (EC-STM) with nanometer and atomic resolution, respectively. Notably, SECM and EC-AFM may offer not only steady-state images of the electrode under a fixed applied potential, but also the dynamic deformation and actuation in a potential scan. For example, Deng and co-workers⁶⁸ used EC-AFM to in-situ study the formation of polycrystalline Pt during cyclic voltammetry (CV) of Pt electrode.

In this work, we attempt to carry out a comprehensive in-situ study of the electrochemical actuation behavior of pristine and methylcellulose functionalized MXene films. Raman spectroscopy is applied for analyzing the in-plane and out-of-plane vibrational modes of MXene upon applying potential. SECM is used for quantifying the deformation of MXene sample during actuation. EC-AFM is carried out for observing the in-plane sliding and out-of-plane expansion/contraction of MXene flakes. The obtained information offers better understanding of the role of functionalization on the in-plane and out-of-plane actuation of MXene, which may guide the material optimization. Moreover, all the measurements reveal the actuation of MXene at microscale. This may be helpful for designing MXene-based electrochemical actuator devices towards desired directions of actuation.

## EXPERIMENTAL METHODS

**Materials used:** The synthesis of Ti₃C₂Tₓ (Tₓ = -O, -OH, -F) MXene films with different weight content of methylcellulose (0% (MX100MC00), 15% (MX85MC15), and 20% (MX80MC20)) is detailed in supplementary information⁵⁶. The thickness of the films is ca. 30 µm as measured from profilometry (Figure S2(a-c). 1-1' Ferrocenedimethanol (purity 97%) and Lithium sulphate (purity ≥ 98.5%) were purchased from Sigma-Aldrich for preparing the electrolyte solution with concentration of 1M Lithium sulphate/1mM 1-1' Ferrocenedimethanol. All the other chemicals were analytical grade and used without further purification. DI water was used as the solvent in all experiments.

Platinum wire (25 µm diameter, purity 99.9%, ChemPUR) and glass capillaries (Blaubrand intraMARK) were used for fabricating microelectrodes. The glass shield had a diameter of ca. 250 µm and Rg (ratio between the radius of insulation sheet and the radius of the conductive surface) of ~10. The electrodes were polished to 4000 grits and cleaned by sonication in ethanol before use. The quality of microelectrodes was ensured by measuring CV in 1 mM ferrocenedimethanol solution (Figure S3).

**Electrochemical cell:** The electrochemical cell used for in-situ studies of MXene was in a glass Petri dish (40 mm diameter, 12 mm height) fixed by 3D-printed holders, with MXene sheets as working electrode (WE), an Ag/AgCl wire as quasi-reference electrode (QRE), and a carbon felt as counter electrode (CE). The CE had significantly larger surface area than the MXene WE. The electrolyte was 1 M Li₂SO₄ and the potential was controlled by a PalmSens4 potentiostat in all measurements. The fixation of MXene electrode, the addition of a second WE, and the redox mediator (Ferrocenedimethanol) depend on the technique, thus will be described in details later.

**In-situ Raman spectroscopy:** The Raman analysis was performed in a three-electrode configuration (Figure 1(a,d)). The MXene films were cut in strips of 2 mm × 10 mm, fixed by two carbon plates pressed by two carbon rods in short electrical connection, keeping the exposed length of MXene at 5 mm. In this way, the MXene film can deform freely both in-plane and out-of-plane. The Raman spectra were recorded with 785 nm excitation laser and objective lens of L50 (N.A. 0.5) that showed all the vibrational modes related to the material (Figure S4a). The thickness of electrolyte was maintained at ~1 mm so that it was negligible as compared with the working distance of the objective lens (8 mm). Each spectrum was measured from 50 cm⁻¹ to 1400 cm⁻¹ with an average of 12 scans with exposure time of 12 s, which takes approximately 144 s. For this reason, the measurements were performed only at steady-state, i.e. at fixed potentials (0, -0.2, -0.4, -0.6, -0.8, and -1 V vs. Ag/AgCl QRE). The spectra acquisition was started 200 s after switching the potential (Figure S4b), and the optical focus was readjusted following the actuation of the sample.

**Scanning electrochemical microscopy (SECM):** The SECM analysis was carried out in four-electrode system (Figure 1(b,c)) with bipotentiostat mode. The MXene sample was fixed in the same way as for Raman experiment, and 1 mM ferrocenedimethanol was added in the electrolyte as redox mediator. For approaching the Pt microelectrode to the MXene film, the microelectrode was connected as WE1 and poised at fixed potential of 0.4 V vs. Ag/AgCl QRE, and the MXene film was connected as WE2, biased at different potentials along the experiment (0, -0.5 and -1 V vs. Ag/AgCl QRE). The approach was stopped when the normalized current on WE1, as compared to that measured from bulk solution, reached 1.8 (no sample-tip touching condition). The lateral scan was performed between the two fixation points at an interval of 500 µm. At each sampling point, the
microelectrode was approached from the same z position, and the stop position with the aforementioned criteria was recorded to retrieve the shape of the film. Furthermore, the transient deformation of MXene was measured with the film connected as WE1 and the microelectrode connected as WE2. The potential of MXene (WE1) was scanned between 0 and -1 V vs. Ag/AgCl QRE at 5mV/s while the potential of Pt microelectrode (WE2) was kept at 0.4 V vs. Ag/AgCl QRE. The current response of both electrodes was recorded during the cycling.

**In-situ EC-AFM:** The EC-AFM experiment was performed in a three-electrode configuration as shown in **Figure 1(c,f)**. The 2 mm × 5 mm MXene film was used as sample and entirely fixed on the base using double-sided tape. The electrical connection was made using thin strip of carbon paper, which was verified by CV before starting each experiment (**Figure S5**). The approach curve was measured using non-conductive silicon nitride Tip-D (f0: 10-20KHz, k: 0.03 N/m). A constant potential (0, -0.5, and -1V vs. Ag/AgCl QRE) was applied on the MXene sample, and the approach was terminated when the force reached 1.5 nN. For imaging, the same tip was approached with the sample at open circuit potential, then the images were recorded in an area of 40 µm × 40 µm with different potential of the sample (0, -0.25, -0.5, -7.5 and -1V vs. Ag/AgCl QRE).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic representations and images of the (a,d) RAMAN setup, (b,e) SECM setup and (c,f) AFM setup. WE; Working electrode, RE; Reference electrode, CE; counter electrode, and Pt-µE; Platinum microelectrode.</figcaption></figure>

## RESULTS AND DISCUSSION

The chemical and structural changes of MXene upon applying potential are analyzed by Raman spectroscopy. From

**Figure 2 (a-c)**, it is seen that all the samples show similar peaks in the range of 100 – 800 cm⁻¹. These peaks can be assigned to different vibration modes (**Table 1**) as described in literature.⁶⁹⁻⁷¹ The A₁g peak corresponds to in-plane vibrational modes and E₉ peak is associated to out-of-plane vibrational modes. Note that ω₁ and ω₆ peaks are only visible using 785 nm excitation laser. The peak at 980 cm⁻¹ is related to the electrolyte (SO₄²⁻), that is thus absent in dry samples. The intensity of SO₄²⁻ remains constant after applying different potentials. This insures that there is no drift in the signal and quality of measurement is well controlled.

<table><caption>**Table 1. Assignment of Raman peaks of MXene in the range of 100 – 800 cm⁻¹.**</caption><thead><tr><td>Peak assignment</td><td>Associated frequency (cm<sup>-1</sup>)</td><td>Vibration mode</td></tr></thead><tbody><tr><td>ω₁</td><td>122</td><td>plasmonic resonance/E<sub>g</sub>(Ti,C)</td></tr><tr><td>ω₂</td><td>200</td><td>A₁g(Ti,C,O)</td></tr><tr><td>ω₃</td><td>730</td><td>A₁g(C)</td></tr><tr><td>ω₄</td><td>620</td><td>E<sub>g</sub>(C)</td></tr><tr><td>ω₅</td><td>280</td><td>E<sub>g</sub>(Ti,C, (OH)₂)</td></tr><tr><td>ω₆</td><td>510</td><td>A₁g(Ti,C,(OH)₂)</td></tr><tr><td>ω₇</td><td>430</td><td>E<sub>g</sub>(Ti,C, (OH)₂)</td></tr></tbody></table>

Two major changes are observed on MXene samples when the potential is shifted from 0 to -1 V vs. Ag/AgCl QRE. First, the ω₁ peak (E₉(Ti,C)) shows blue shift as compared to the dry samples(**Figure 2(d-f)**). For neat MXene, the shift is *ca.* 4 cm⁻¹, wheareas for methylcellulose functionalized samples it is more pronounced (*ca.* 4.8 cm⁻¹ for MX85MC15 and 5 cm⁻¹ for MX80MC20, respectively). This may suggest more freedom in the in-plane E₉(Ti,C) movement as a result of functionalization. Second, the ω₂ (A₁g (Ti,C,O)) and ω₆ (A₁g(Ti,C,(OH)₂)) peaks gradually weakening upon applying cathodic potential (**Figure 2(g-i)**). This is likely related to the intercalation of Li⁺ ions in the MXene films. The interaction of Li⁺ ions with -O in ω₂ peak and with -OH in ω₆ peak limits the vibrations of these surface groups.⁷²,⁷³ The overall increase in the intensity of the spectra might be related to the flake decoupling due to high Li⁺ flux inside the layers at negative potential. The interlayer interactions becomes weak after insertion of Li⁺ ions that strenghtness the out-of-plane vibrations in the 230-470 cm⁻¹ and 550-700 cm⁻¹ regions. Simillarly, significant increase is observed in ω₃ (A₁g(C)) with more applied negative potential.⁷⁴
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Full Raman spectra of Ti₂C₃Tx at different applied potentials (0 to -1V vs. Ag/AgCl QRE) with (a) MX100MC00, (b) MX85MC15, and (c) MX80MC20. Zoom in range of 110-150 cm⁻¹ showing the frequency shift affecting ω₁ peak with (d) MX100MC00, (e) MX85MC15, and (f) MX80MC20. Zoom in range of 150-550 cm⁻¹ showing the changes in ω₂ and ω₆ peaks with (g) MX100MC00, (h) MX85MC15, and (i) MX80MC20.</figcaption></figure>

In the Raman experiments, apart from the chemical structural changes shown in the spectra, we have also clearly observed macroscopic deformation of the films upon applying potential. This makes the readjustment of optical focus indispensable for each measurement. In order to measure this deformation, scanning electrochemical microscopy (SECM) is applied in feedback mode (**Figure 3a**). Considering that the actuation of MXene occurs in the range of 0 to -1 V vs. Ag/AgCl QRE, ferrocenedimethanol with formal potential of 0.4 V vs. Ag/AgCl QRE is chosen as redox probe to ensure fast and full electrochemical regeneration on the MXene sample. In this way, the feedback current measured at the microelectrode depends only on the distance of the tip to the MXene sample and reflects quantitatively the deformation of the sample. **Figure 3b** shows the approach curves of Pt microelectrode (0.4 V vs. Ag/AgCl QRE) to the two ended fixed neat MXene sample with different applied potential (0, -0.5 and -1V vs. Ag/AgCl QRE). We emphasize that the approaching starts at exactly the same position in (x, y, z) for the microelectrode. Positive feedback was seen for all the three curves, but the current starts to increase and reaches the same value (e.g. 1.5 normalized to the bulk) at different z positions. This is caused by the deformation of MXene film upon applying cathodic potential.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) SECM mechanism and (b) approach curves on MXene surface (MX100MC00) with different potentials applied to the tip (0, -0.5 and -1V vs. Ag/AgCl QRE).</figcaption></figure>

To obtain an overview of the actuation performance of the samples, approach curves are measured at different lateral positions between two fixation points with Δx = 500 µm, as illustrated in **Figure 4a**. The approach curves are shown in **Figure S6** (MX100MC00), **S7** (MX85MC15), and **S8** (MX80MC20). As discussed in **Figure 3b**, the shift of the approach curve in z position reflects the deformation of the MXene sample. Here, the normalized current of 1.1 is chosen to calculate the relative shift for applied potential of -0.5 V (**Figure 4b**) and -1 V (**Figure 4c**) vs. 0 V (vs. Ag/AgCl QRE) on MXene surface for each sampling point. This relative height change is then plotted versus the relative position of the probe in x axis (x=0; intersection of fixed carbon plate and sample). For all samples, the probe reaches normalized current
of 1.1 at higher position as a result of volume expansion of MXene under cathodic polarization. Such buckling type bumping is more significant for the methylcellulose functionalized films than for neat MXene. Taking the highest point of deformation which is at around the middle point between two fixation points, it can be seen that at -0.5 V (vs. Ag/AgCl QRE), MX85MC15 and MX80MC20 samples have a deflection of ~52 µm and ~38 µm, respectively, which is

much higher than that of neat MXene (~5.5 µm). At -1 V (vs. Ag/AgCl QRE), MX85MC15 and MX80MC20 samples deformed by ~82 µm and ~76 µm, respectively, whereas neat MXene film only have ~25 µm deformation. Considering that all the samples have the same geometrical dimensions, one may conclude that the methylcellulose functionalized MXene has more actuation upon applying cathodic potential, which agrees with our previous observations.⁵⁶

<figure><img src="image_5.png" /><figcaption>**Figure 4.** (a) Principle of line scan over MXene surfaces (MX100MC00, MX85MC15 and MX80MC20) and relative deformations at (b) -0.5V, and (c) -1V with respect to 0V vs. Ag/AgCl QRE.</figcaption></figure>

An advantage of SECM is that it can be used not only for characterizing the steady state behavior, but also for studying the dynamic actuation of MXene samples. Indeed, the potential applied to MXene can be scanned at different rate while measuring the current response of both the microelectrode and the sample. Technically, this requires switching the connections, i.e. the sample connected as WE1 and the microelectrode connected at WE2. All the measurements were carried out with the microelectrode positioned near the center (2500 µm) between two fixation points. The experimental configuration is illustrated in **Figure 5a**. **Figure 5b** shows the current response from the sample between 0 to -1V (vs. Ag/AgCl QRE) at different scan rates from 5 mV/s to 2000 mV/s. As the scan rate increases, the current generally increases due to the capacitive effect but the resistance is also more significant. Meanwhile, the current from the microelectrode is also recorded and normalized by the diffusion limit current of the same electrode in the bulk solution (**Figure 5c**). As discussed before, this normalized current reflects the distance between the microelectrode and the sample, like in the approach curve of SECM⁷⁵. Higher current indicates that the sample is closer to the microelectrode, and *vice versa*. Since the microelectrode position is always fixed, this is an indication of dynamic

actuation of the sample. It is seen that as the potential of MXene gets negative, the current on the microelectrode increases, suggesting that the MXene expands closer to the microelectrode. During the reverse scan, it contracts away from the microelectrode. The contraction appears to be faster than expansion, as seen from the hysteresis with generally lower current. Moreover, the actuation is more significant at low potential scan rate of the sample, as supported by higher current response on the microelectrode. **Figure 5d** compares the dynamic actuation behavior of methylcellulose functionalized and neat MXene. In general, the methylcellulose functionalized sample show higher actuation than the non-functionalized one. For both samples, the actuation is more significant at lower scan rate, especially <50 mV/s (<25 mHz). This can be explained by having more time for Li⁺ ions to intercalate in the sample, which is in line with the charge decreasing as the scan rate increases (**Figure 5e**). The performance measured by SECM is also consistent with that measured at macroscale, where the strain decreases from 0.541% at 4 mV/s to 0.298% at 40 mV/s for methylcellulose functionalized MXene, although still generally higher than that for neat MXene (0.162% at 4 mV/s and 0.037% at 40 mV/s).
<figure><img src="image_6.png" /><figcaption>**Figure 5.** (a) Principle of transient behavior analysis, (b) cyclic voltammetry of MX80MC20 sample at different scan rates, (c) simultaneous current responses of the Pt microelectrode ($I_{\text{ME}}$) measured over the MX80MC20, (d) frequency responses extracted from the experiments performed with the MX100MC00 and MX80MC20, and (e) Capacitive charge of MX100MC00 and MX80MC20 as a function of scan rate.</figcaption></figure>

Note that the exact conversion of the microelectrode current to the microelectrode-sample distance is not yet quantified. This is because the MXene sample especially after intercalation of Li⁺ ions cannot be simply regarded as a planar electrode. First, the sample is rough as seen from the SEM images (**Figure S9 (a-c)**) and profilometry (**Figure S2(d-f)**), and the roughness measured with profilometry is not negligible as compared to the microelectrode-sample distance. Second, the buckling type actuation would make the sample locally tilted. This might be a reason for the saturation of current in some approach curves observed in the experiments (**Figure S8 (e)**). All these make the classical SECM approach curve fitting based on planar electrodes invalid, even though the kinetic effect can be reasonably neglected in our experiments considering that the potential applied on MXene is always much lower than the formal potential of ferrocenedimethanol (by at least 0.4 V). Therefore, in **Figure 5c** we plot the curves with normalized current and use it as a qualitative measure for comparison. Another point of attention is that the microelectrode current is more sensitive to distance when it is high, or in another word, when the microelectrode is close to the sample. This non-linear relationship between normalized current and distance makes it not meaningful to over-explain the difference in the normalized current in **Figure 5d**.

The above results of SECM are addressing the buckling type actuation of MXene films, which is a mixed effect of in-plane and out-of-plane deformations of the sample. In order to separate the analysis of the two effects, atomic force microscopy (AFM) was performed with a different

configuration for fixing the MXene samples. Here, the whole sample is adhered to the base, and the electrical connection is made only at one end. In this way, the out-of-plane deformation could be measured from the touching position of the tip, while the in-plane deformation could be seen from lateral shift of topography images, as illustrated in **Figure 6a**. Since the measurement position and area (40 µm × 40 µm) are fixed, the lateral shift of the image corresponds to the lateral movement of the sample surface. In practice, this is quantified by the distance between a characteristic feature and the top frame of the image, and the change is relative to the image measured 0 V applied potential. **Figure 6c** shows that upon applying cathodic potential, the flakes move away from the electric connection, indicating that the sample has in-plane expansion. This change is more pronounced for methylcellulose functionalized MXene (MX80M20) as compared to neat MXene sample (MX100M00) (**Figure 6b**).

These results also allow calculating the in-plane strain of the samples knowing the length of the sample (**Figure S10**). It is expressed as $\Delta l/l * 100$, where *l* represents the length from the monitored point to the fixed end and $\Delta l$ is the relative displacement of the flake as shown in **Figure 6a**. **Figure 6b** shows that the strain is almost linear to the applied potential and highly reversible upon multiple scans. The MX80MC20 sample shows more strain generation as compared to MX100MC00. Also, the hysteresis type of behavior was seen in MX80MC20 that indicates that below -0.5 V applied potential the intercalation step follows the same behavior as MX100MC00.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** (a) Principle of EC-AFM imaging done at different applied potential, (b) percentage of strain variation in MX100M00 and MX80M20 samples at 0, -0.25, -0.5, -0.75, -1V vs. Ag/AgCl QRE, and (c) AFM imaging of MX100M00 and MX80M20 samples at OCP, 0, -1 and -0.25 V vs. Ag/AgCl QRE. Error bar represents the standard deviation of 3 cycles.</figcaption></figure>

Apart from in-plane displacement as shown in **Figure 6**, AFM was also used to measure the out-of-plane actuation of MXene. The probe is always approaching to the sample from a fixed $Z_0$, then the point of contact as evaluated from the approach curve would reflect the vertical displacement of the sample. However, one has to remember that the sample also has in-plane displacement, therefore it is essential to make sure that the analysis corresponds to the same lateral position on the sample, so that the heterogeneity in topography would not interfere the interpretation. To achieve this goal, the approach curve mapping was done over 20 µm × 20 µm area (256 approach curves per sample). In actuated states, the

lateral displacement was first evaluated in the same way as **Figure 6**. This value is then taken for lateral calibration (X, $\Delta$Y). The X value is not changing because the surface was scanned in the center of the width direction. For example, Pixel number (02, 04) at 0 V vs. QRE corresponds to Pixel number (02, 08) at -0.5 V vs. QRE and Pixel number (02, 12) at -1 V vs. QRE (MX100MC00). The detail illustration is in **Figure S11 and S12**. It is seen that for both MX100MC00 and MX80MC20 samples, there is no significant change in the touching Z position, indicating that the out-of-plane actuation is insignificant (Figure 7).

<figure><img src="image_8.png" /><figcaption>**Figure 7.** (a) Principle of approach curve on the samples. Approach curve analysis on (b) MX100MC00 and (c) MX80MC20 samples.</figcaption></figure>

Following the aforementioned results, the mechanism of MXene actuation is explained in **Figure 8**. The basic principle follows the intercalation/deintercalation of Li⁺ ions upon application of potential. DFT calculations show that the Li⁺ ion adsorbs on the oxygen (O (1)) atom surface by maintaining the distance of around ~1.931 Å and generates the strain in the MXene structure. The Ti(1)-O(1) bond interacting with the Li⁺ ion showed the maximum expansion with change in bond length from 1.964 Å to 2.226 Å (~ 0.1334 % strain). This is in agreement with experimental observations in SECM and AFM. The details about the calculations are provided in Supplementary Information.

The neat MXene shows less in-plane expansion thus less strain than methyl cellulose functionalized samples, as supported by Raman and AFM results. This could be because of the presence of polymer that may weaken the interlayer interaction and may allow larger in-plane sliding of MXene sheets, as discussed in previous work⁵⁶. However, in the case of buckling type actuation measured by SECM, this enhanced in-plane actuation may yield more significant apparent out-of-plane deformation of the sample for functionalized samples.
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Illustration of the in-plane and buckling type actuation of MXene samples.</figcaption></figure>

## CONCLUSION

In this work, we characterized the electrochemical actuation of functionalized MXene films through *in-situ* techniques at the microscale. First, Raman spectroscopy was used to study the chemical structural changes happening in MXene films upon reversible intercalation/deintercalation of ions. It was seen that the increase in methylcellulose functionalization results in high in-plane vibrations (ω₁: E<sub>g</sub>(Ti,C)) at applied cathodic potential. The out-of-plane vibrations (ω₂ and ω₆) showed suppression in peak intensities due to the interaction of Li⁺ ions with -O and -OH terminal groups. This interlayer decoupling resulted in increase in intensity at other wavelengths of the spectra. Secondly, the buckling type deformation of MXene samples was studied by scanning electrochemical microscopy (SECM). The line scan between two fixations of the sample helped in understanding the role of functionalization for this deformation. It was seen that 20 and 15 % methylcellulose functionalized samples showed highest deformation of about by ~82 µm and ~76 µm at -1 V (vs. Ag/AgCl QRE) compared to neat MXene (~25 µm). Further dynamic behavior studies by SECM showed that samples go under significant cyclic expansion/contraction actuation at low scan rate (<50 mV/s). Furthermore, AFM imaging and approach curve analysis helped to analyze this expansion/contraction in more details. It was seen that both neat and methylcellulose functionalized samples goes under mainly in-plane deformation. This deformation is more significant in case of functionalized samples that leads to more strain generation and results in high actuation.

## SUPPORTING INFORMATION

Data including the experiments of profilometry, cyclic voltammetry of microelectrode and MXene samples, Raman experiments, SECM approach curves, AFM imaging and approach curves, DFT calculations of the structure of MXene before and after intercalation of Li⁺ ions. Videos of AFM imaging at different potentials (MX100MC00.mp4 and MX80MC20.mp4). This material is available free of charge via the Internet at http://pubs.acs.org.

## AUTHOR INFORMATION

Corresponding Author

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Funding Sources

This work is funded by National Research Foundation of Singapore under project no. NRF2020-NRF-ANR102 MEACT (ANR-20-CE09-0028). This research was also supported by the DrEAM program under the Lorraine Université d'Excellence Initiative (LUE) for international mobility of its PhD students.

## Acknowledgement

The platform of spectroscopy and microscopy of interfaces (SMI) of LCPME is acknowledged. Dr. Manuel Dossot and Dr. Jérôme Grausem are acknowledged for the training and technical support for Raman spectroscopy. Dr. Mariela Alicia Brites Helú is acknowledged for SEM measurements.

## ABBREVIATIONS

EC-AFM, electrochemical atomic force microscopy; 2; SECM, scanning electron microscopy; CV, cyclic voltammetry; QRE, quasi-reference electrode.

## REFERENCES

(1) Zhou, J.; Mulle, M.; Zhang, Y.; Xu, X.; Li, E. Q.; Han, F.; Thoroddsen, S. T.; Lubineau, G. High-Ampacity Conductive Polymer Microfibers as Fast Response Wearable Heaters and Electromechanical Actuators. *J Mater Chem C Mater 2016*, *4* (6), 1238–1249. https://doi.org/10.1039/C5TC03380B.

(2) Yang, Z.; Zhang, L. Magnetic Actuation Systems for Miniature Robots: A Review. *Advanced Intelligent Systems* **2020**, *2* (9). https://doi.org/10.1002/aisy.202000082.

(3) Gural’skiy, I. A.; Quintero, C. M.; Costa, J. S.; Demont, P.; Molnár, G.; Salmon, L.; Shepherd, H. J.; Bousseksou, A. Spin Crossover Composite Materials

<footnote>* Liang Liu - liang.liu@cnrs.fr</footnote>

<footnote>* Mathieu Etienne – mathieu.etienne@cnrs.fr</footnote>
for Electrothermomechanical Actuators. *J. Mater. Chem. C* **2014**, *2* (16), 2949–2955.
https://doi.org/10.1039/C4TC00267A.

(4) Ze, Q.; Kuang, X.; Wu, S.; Wong, J.; Montgomery, S. M.; Zhang, R.; Kovitz, J. M.; Yang, F.; Qi, H. J.; Zhao, R. Magnetic Shape Memory Polymers with Integrated Multifunctional Shape Manipulation. *Advanced Materials* **2020**, *32* (4), 1906657.
https://doi.org/10.1002/adma.201906657.

(5) Mohith, S.; Upadhya, A. R.; Navin, K. P.; Kulkarni, S. M.; Rao, M. Recent Trends in Piezoelectric Actuators for Precision Motion and Their Applications: A Review. *Smart Mater Struct* **2021**, *30* (1), 013002. https://doi.org/10.1088/1361-665X/abc6b9.

(6) Priimagi, A.; Barrett, C. J.; Shishido, A. Recent Twists in Photoactuation and Photoalignment Control. *J. Mater. Chem. C* **2014**, *2* (35), 7155–7162.
https://doi.org/10.1039/C4TC01236D.

(7) Yu, L.; Cheng, Z.; Dong, Z.; Zhang, Y.; Yu, H. Photomechanical Response of Polymer-Dispersed Liquid Crystals/Graphene Oxide Nanocomposites. *J. Mater. Chem. C* **2014**, *2* (40), 8501–8506.
https://doi.org/10.1039/C4TC01097C.

(8) Hua, D.; Zhang, X.; Ji, Z.; Yan, C.; Yu, B.; Li, Y.; Wang, X.; Zhou, F. 3D Printing of Shape Changing Composites for Constructing Flexible Paper-Based Photothermal Bilayer Actuators. *J Mater Chem C* **Mater** **2018**, *6* (8), 2123–2131.
https://doi.org/10.1039/C7TC05710E.

(9) Oh, J.; Kozlov, M. E.; Carretero-González, J.; Castillo-Martínez, E.; Baughman, R. H. Thermal Actuation of Graphene Oxide Nanoribbon Mats. *Chem Phys Lett* **2011**, *505* (1–3), 31–36.
https://doi.org/10.1016/j.cplett.2011.02.005.

(10) Neagu, C. R. (Cristina R. A Medical Microactuator Based on an Electrochemical Principal; s.n.], 1998.

(11) Nakshatharan, S. S.; Vunder, V.; Pōldsalu, I.; Johanson, U.; Punning, A.; Aabloo, A. Modelling and Control of Ionic Electroactive Polymer Actuators under Varying Humidity Conditions. *Actuators* **2018**, *7* (1), 7. https://doi.org/10.3390/act7010007.

(12) Deng, Q.; Jia, H.; An, C.; Wu, S.; Zhao, S.; Hu, N. Progress and Prospective of Electrochemical Actuator Materials. *Compos Part A Appl Sci Manuf* **2023**, *165*, 107336.
https://doi.org/10.1016/j.compositesa.2022.107336.

(13) Bar-Cohen, Y. *Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential, and Challenges*; 2004; Vol. 136.

(14) O'Halloran, A.; O'Malley, F.; McHugh, P. A Review on Dielectric Elastomer Actuators, Technology, Applications, and Challenges. *J Appl Phys* **2008**, *104* (7), 071101. https://doi.org/10.1063/1.2981642.

(15) Li, W.; Li, F.; Li, H.; Su, M.; Gao, M.; Li, Y.; Su, D.; Zhang, X.; Song, Y. Flexible Circuits and Soft Actuators by Printing Assembly of Graphene. *ACS Appl Mater Interfaces* **2016**, *8* (19), 12369–12376.
https://doi.org/10.1021/acsami.6b04235.

(16) Deng, H.; Zhang, C.; Su, J.-W.; Xie, Y.; Zhang, C.; Lin, J. Bioinspired Multi-Responsive Soft Actuators Controlled by Laser Tailored Graphene Structures. *J Mater Chem B* **2018**, *6* (34), 5415–5423.
https://doi.org/10.1039/C8TB01285G.

(17) Zhu, S.-E.; Shabani, R.; Rho, J.; Kim, Y.; Hong, B. H.; Ahn, J.-H.; Cho, H. J. Graphene-Based Bimorph Microactuators. *Nano Lett* **2011**, *11* (3), 977–981.
https://doi.org/10.1021/nl103618e.

(18) Aliev, A. E.; Oh, J.; Kozlov, M. E.; Kuznetsov, A. A.; Fang, S.; Fonseca, A. F.; Ovalle, R.; Lima, M. D.; Haque, M. H.; Gartstein, Y. N.; Zhang, M.; Zakhidov, A. A.; Baughman, R. H. Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles. *Science* **1979** **2009**, *323* (5921), 1575–1578.
https://doi.org/10.1126/science.1168312.

(19) Brochu, P.; Pei, Q. Dielectric Elastomers for Actuators and Artificial Muscles. In *Electroactivity in Polymeric Materials*; Springer US: Boston, MA, 2012; pp 1–56. https://doi.org/10.1007/978-1-4614-0878-9_1.

(20) Zhao, Y.; Song, L.; Zhang, Z.; Qu, L. Stimulus-Responsive Graphene Systems towards Actuator Applications. *Energy Environ Sci* **2013**, *6* (12), 3520.
https://doi.org/10.1039/c3ee42812e.

(21) Zhang, Y.-L.; Li, J.-C.; Zhou, H.; Liu, Y.-Q.; Han, D.-D.; Sun, H.-B. Electro-Responsive Actuators Based on Graphene. *The Innovation* **2021**, *2* (4), 100168. https://doi.org/10.1016/j.xinn.2021.100168.

(22) Xie, X.; Qu, L.; Zhou, C.; Li, Y.; Zhu, J.; Bai, H.; Shi, G.; Dai, L. An Asymmetrically Surface-Modified Graphene Film Electrochemical Actuator. *ACS Nano* **2010**, *4* (10), 6050–6054.
https://doi.org/10.1021/nn101563x.

(23) Weng, M.; Ding, M.; Zhou, P.; Ye, Y.; Luo, Z.; Ye, X.; Guo, Q.; Chen, L. Multi-Functional and Integrated Actuators Made with Bio-Inspired
Cobweb Carbon Nanotube-Polymer Composites.
*Chemical Engineering Journal* **2023**, *452*, 139146.
https://doi.org/10.1016/j.cej.2022.139146.

(24) Yang, X.; Miao, M. Carbon Nanotube Yarn-Based Actuators. In *Carbon Nanotube Fibers and Yarns*; Elsevier, 2020; pp 271–291.
https://doi.org/10.1016/B978-0-08-102722-6.00011-0.

(25) Goswami, S. K.; McAdam, C. J.; Lee, A. M. M.; Hanton, L. R.; Moratti, S. C. Linear Electrochemical Actuators with Very Large Strains Using Carbon Nanotube-Redox Gel Composites. *J Mater Chem A Mater* **2013**, *1* (10), 3415.
https://doi.org/10.1039/c3ta01202f.

(26) Li, Q.; Wu, J.-T.; Liu, Y.; Qi, X.-M.; Jin, H.-G.; Yang, C.; Liu, J.; Li, G.-L.; He, Q.-G. Recent Advances in Black Phosphorus-Based Electrochemical Sensors: A Review. *Anal Chim Acta* **2021**, *1170*, 338480.
https://doi.org/10.1016/j.aca.2021.338480.

(27) Lu, C.; Yang, Y.; Wang, J.; Fu, R.; Zhao, X.; Zhao, L.; Ming, Y.; Hu, Y.; Lin, H.; Tao, X.; Li, Y.; Chen, W. High-Performance Graphdiyne-Based Electrochemical Actuators. *Nat Commun* **2018**, *9* (1), 752. https://doi.org/10.1038/s41467-018-03095-1.

(28) Lu, F.; Xiang, K.; Wang, Y.; Chen, T. Electrochemical Actuators Based on Nitrogen-Doped Carbons Derived from Zeolitic Imidazolate Frameworks. *Mater Des* **2020**, *187*, 108405.
https://doi.org/10.1016/j.matdes.2019.108405.

(29) Shi, Y.-X.; Wu, Y.; Wang, S.-Q.; Zhao, Y.-Y.; Li, T.; Yang, X.-Q.; Zhang, T. Soft Electrochemical Actuators with a Two-Dimensional Conductive Metal–Organic Framework Nanowire Array. *J Am Chem Soc* **2021**, *143* (10), 4017–4023.
https://doi.org/10.1021/jacs.1c00666.

(30) Hung, N. T.; Nugraha, A. R. T.; Saito, R. Two-Dimensional MoS₂ Electromechanical Actuators. *J Phys D Appl Phys* **2018**, *51* (7), 075306.
https://doi.org/10.1088/1361-6463/aaa68f.

(31) 2D Metal Carbides and Nitrides (MXenes); Anasori, B., Gogotsi, Y., Eds.; Springer International Publishing: Cham, 2019.
https://doi.org/10.1007/978-3-030-19026-2.

(32) Zhang, J.; Kong, N.; Uzun, S.; Levitt, A.; Seyedin, S.; Lynch, P. A.; Qin, S.; Han, M.; Yang, W.; Liu, J.; Wang, X.; Gogotsi, Y.; Razal, J. M. Scalable Manufacturing of Free-Standing, Strong Ti₃C₂Tₓ MXene Films with Outstanding Conductivity.

*Advanced Materials* **2020**, *32* (23), 2001093.
https://doi.org/10.1002/adma.202001093.

(33) Lipatov, A.; Lu, H.; Alhabeb, M.; Anasori, B.; Gruverman, A.; Gogotsi, Y.; Sinitskii, A. Elastic Properties of 2D Ti₃C₂Tₓ MXene Monolayers and Bilayers. *Sci Adv* **2018**, *4* (6).
https://doi.org/10.1126/sciadv.aat0491.

(34) Hart, J. L.; Hantanasirisakul, K.; Lang, A. C.; Anasori, B.; Pinto, D.; Pivak, Y.; van Omme, J. T.; May, S. J.; Gogotsi, Y.; Taheri, M. L. Control of MXenes' Electronic Properties through Termination and Intercalation. *Nat Commun* **2019**, *10* (1), 522.
https://doi.org/10.1038/s41467-018-08169-8.

(35) Liang, X.; Garsuch, A.; Nazar, L. F. Sulfur Cathodes Based on Conductive MXene Nanosheets for High-Performance Lithium-Sulfur Batteries. *Angewandte Chemie International Edition* **2015**, *54* (13), 3907–3911.
https://doi.org/10.1002/anie.201410174.

(36) Alhabeb, M.; Maleski, K.; Anasori, B.; Lelyukh, P.; Clark, L.; Sin, S.; Gogotsi, Y. Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti3C2Tx MXene). *Chemistry of Materials* **2017**, *29* (18), 7633–7644.
https://doi.org/10.1021/acs.chemmater.7b02847.

(37) Mashtalir, O.; Lukatskaya, M. R.; Zhao, M.; Barsoum, M. W.; Gogotsi, Y. Amine-Assisted Delamination of Nb₂C MXene for Li-Ion Energy Storage Devices. *Advanced Materials* **2015**, *27* (23), 3501–3506.
https://doi.org/10.1002/adma.201500604.

(38) Lipatov, A.; Alhabeb, M.; Lu, H.; Zhao, S.; Loes, M. J.; Vorobeva, N. S.; Dall'Agnese, Y.; Gao, Y.; Gruverman, A.; Gogotsi, Y.; Sinitskii, A. Electrical and Elastic Properties of Individual Single-Layer Nb₄C₃Tₓ MXene Flakes. *Adv Electron Mater* **2020**, *6* (4).
https://doi.org/10.1002/aelm.201901382.

(39) Zhao, S.; Chen, C.; Zhao, X.; Chu, X.; Du, F.; Chen, G.; Gogotsi, Y.; Gao, Y.; Dall'Agnese, Y. Flexible Nb₄C₃Tₓ Film with Large Interlayer Spacing for High-Performance Supercapacitors. *Adv Funct Mater* **2020**, *30* (47), 2000815.
https://doi.org/10.1002/adfm.202000815.

(40) Shan, Q.; Mu, X.; Alhabeb, M.; Shuck, C. E.; Pang, D.; Zhao, X.; Chu, X.-F.; Wei, Y.; Du, F.; Chen, G.; Gogotsi, Y.; Gao, Y.; Dall'Agnese, Y. Two-Dimensional Vanadium Carbide (V2C) MXene as Electrode for Supercapacitors with Aqueous Electrolytes. *Electrochem commun* **2018**, *96*, 103–107.
https://doi.org/10.1016/j.elecom.2018.10.012.
(41) VahidMohammadi, A.; Mojtabavi, M.; Caffrey, N. M.; Wanunu, M.; Beidaghi, M. Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities. *Advanced Materials* **2019**, *31* (8). https://doi.org/10.1002/adma.201806931.

(42) Zhang, Y.; Lu, W.; Zhou, J.; Sun, D.; Li, H. The Multiple Synthesis of Layered V2CTx-MXene Composites with Enhanced Electrochemical Properties. *J Alloys Compd* **2022**, *929*, 167276. https://doi.org/10.1016/j.jallcom.2022.167276.

(43) Halim, J.; Kota, S.; Lukatskaya, M. R.; Naguib, M.; Zhao, M.; Moon, E. J.; Pitock, J.; Nanda, J.; May, S. J.; Gogotsi, Y.; Barsoum, M. W. Synthesis and Characterization of 2D Molybdenum Carbide (MXene). *Adv Funct Mater* **2016**, *26* (18), 3118–3127. https://doi.org/10.1002/adfm.201505328.

(44) Hussain, I.; Rehman, F.; Saraf, M.; Zhang, T.; Wang, R.; Das, T.; Luo, Z.; Gogotsi, Y.; Zhang, K. Electrochemical Properties of Mo₄ VC 4 T₃ MXene in Aqueous Electrolytes. *ACS Appl Mater Interfaces* **2024**. https://doi.org/10.1021/acsami.4c06519.

(45) Saraf, M.; Chacon, B.; Ippolito, S.; Lord, R. W.; Anayee, M.; Wang, R. (John); Inman, A.; Shuck, C. E.; Gogotsi, Y. Enhancing Charge Storage of Mo₂Ti₂C₃MXene by Partial Oxidation. *Adv Funct Mater* **2024**, *34* (1). https://doi.org/10.1002/adfm.202306815.

(46) Jun, B.-M.; Kim, S.; Heo, J.; Park, C. M.; Her, N.; Jang, M.; Huang, Y.; Han, J.; Yoon, Y. Review of MXenes as New Nanomaterials for Energy Storage/Delivery and Selected Environmental Applications. *Nano Res* **2019**, *12* (3), 471–487. https://doi.org/10.1007/s12274-018-2225-3.

(47) Lipatov, A.; Goad, A.; Loes, M. J.; Vorobeva, N. S.; Abourahma, J.; Gogotsi, Y.; Sinitskii, A. High Electrical Conductivity and Breakdown Current Density of Individual Monolayer Ti3C2T MXene Flakes. *Matter* **2021**, *4* (4), 1413–1427. https://doi.org/10.1016/j.matt.2021.01.021.

(48) Ying, G.; Kota, S.; Dillon, A. D.; Fafarman, A. T.; Barsoum, M. W. Conductive Transparent V 2 CT x (MXene) Films. *FlatChem* **2018**, *8*, 25–30. https://doi.org/10.1016/j flatc.2018.03.001.

(49) Zhu, K.; Jin, Y.; Du, F.; Gao, S.; Gao, Z.; Meng, X.; Chen, G.; Wei, Y.; Gao, Y. Synthesis of Ti2CT MXene as Electrode Materials for Symmetric Supercapacitor with Capable Volumetric Capacitance. *Journal of Energy Chemistry* **2019**, *31*, 11–18. https://doi.org/10.1016/j.jechem.2018.03.010.

(50) Ghidiu, M.; Lukatskaya, M. R.; Zhao, M.-Q.; Gogotsi, Y.; Barsoum, M. W. Conductive Two-Dimensional Titanium Carbide ‘Clay’ with High Volumetric Capacitance. *Nature* **2014**, *516* (7529), 78–81. https://doi.org/10.1038/nature13970.

(51) Ling, Z.; Ren, C. E.; Zhao, M.-Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y. Flexible and Conductive MXene Films and Nanocomposites with High Capacitance. *Proceedings of the National Academy of Sciences* **2014**, *111* (47), 16676–16681. https://doi.org/10.1073/pnas.1414215111.

(52) Khanal, R.; Irle, S. Effect of Surface Functional Groups on MXene Conductivity. *J Chem Phys* **2023**, *158* (19). https://doi.org/10.1063/5.0141589.

(53) Berdiyorov, G. R.; Mahmoud, K. A. Effect of Surface Termination on Ion Intercalation Selectivity of Bilayer Ti 3 C 2 T 2 (T = F, O and OH) MXene. *Appl Surf Sci* **2017**, *416*, 725–730. https://doi.org/10.1016/j.apsusc.2017.04.195.

(54) Tang, Q.; Zhou, Z.; Shen, P. Are MXenes Promising Anode Materials for Li Ion Batteries? Computational Studies on Electronic Properties and Li Storage Capability of Ti 3 C 2 and Ti 3 C 2 X 2 (X = F, OH) Monolayer. *J Am Chem Soc* **2012**, *134* (40), 16909–16916. https://doi.org/10.1021/ja308463r.

(55) Yang, Q.; Eder, S. J.; Martini, A.; Grützmacher, P. G. Effect of Surface Termination on the Balance between Friction and Failure of Ti3C2Tx MXenes. *Npj Mater Degrad* **2023**, *7* (1), 6. https://doi.org/10.1038/s41529-023-00326-9.

(56) Chen, S.; Ciou, J.; Yu, F.; Chen, J.; Lv, J.; Lee, P. S. Molecular-Level Methylcellulose/MXene Hybrids with Greatly Enhanced Electrochemical Actuation. *Advanced Materials* **2022**, *34* (29), 2200660. https://doi.org/10.1002/adma.202200660.

(57) Liu, L.; Wang, C.; Wu, Z.; Xing, Y. Ultralow-Voltage-Drivable Artificial Muscles Based on a 3D Structure MXene-PEDOT:PSS/AgNWs Electrode. *ACS Appl Mater Interfaces* **2022**, *14* (16), 18150–18158. https://doi.org/10.1021/acsami.2c00760.

(58) Wang, T.; Wang, T.; Weng, C.; Liu, L.; Zhao, J.; Zhang, Z. Engineering Electrochemical Actuators with Large Bending Strain Based on 3D-Structure Titanium Carbide MXene Composites. *Nano Res* **2021**, *14* (7), 2277–2284. https://doi.org/10.1007/s12274-020-3222-x.
(59) Chen, S.; Tan, S. F.; Singh, H.; Liu, L.; Etienne, M.; Lee, P. S. Functionalized MXene Films with Substantially Improved Low-Voltage Actuation. *Advanced Materials* **2024**, *36* (9). https://doi.org/10.1002/adma.202307045.

(60) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Carbon Nanotube Actuators. *Science* (1979) **1999**, *284* (5418), 1340–1344. https://doi.org/10.1126/science.284.5418.1340.

(61) Umrao, S.; Tabassian, R.; Kim, J.; Nguyen, V. H.; Zhou, Q.; Nam, S.; Oh, I.-K. MXene Artificial Muscles Based on Ionically Cross-Linked Ti₃C₂Tₓ Electrode for Kinetic Soft Robotics. *Sci Robot* **2019**, *4* (33). https://doi.org/10.1126/scirobotics.aaw7797.

(62) Zheng, W. Beginner's Guide to Raman Spectroelectrochemistry for Electrocatalysis Study. *Chemistry–Methods* **2023**, *3* (2), e202200042 (1-12). https://doi.org/10.1002/cmtd.202200042.

(63) Moldovan, R.; Vereshchagina, E.; Milenko, K.; Iacob, B.-C.; Bodoki, A. E.; Falamas, A.; Tosa, N.; Muntean, C. M.; Farcău, C.; Bodoki, E. Review on Combining Surface-Enhanced Raman Spectroscopy and Electrochemistry for Analytical Applications. *Anal Chim Acta* **2022**, *1209*, 339250. https://doi.org/10.1016/j.aca.2021.339250.

(64) Li, L.; Xu, J.; Shi, M.; He, J.; Jiang, J.; Dai, K.; Jiang, Z.; Yan, C. In-Situ Raman Investigation and Application of MXene-Stabilized Polypyrrole Composite for Flexible Aqueous Batteries. *Mater Des* **2022**, *217*, 110606. https://doi.org/10.1016/j.matdes.2022.110606.

(65) Bard, A. J.; Fan, F. R. F.; Kwak, Juhyoun.; Lev, Ovadia. Scanning Electrochemical Microscopy. Introduction and Principles. *Anal Chem* **1989**, *61* (2), 132–138. https://doi.org/10.1021/ac00177a011.

(66) Cornut, R.; Lefrou, C. New Analytical Approximation of Feedback Approach Curves with a Microdisk SECM Tip and Irreversible Kinetic Reaction at the Substrate. *Journal of Electroanalytical Chemistry* **2008**, *621* (2), 178–184. https://doi.org/10.1016/j.jelechem.2007.09.021.

(67) *Scanning Electrochemical Microscopy*; Bard, A. J.; Mirkin, M. V., Eds.; CRC Press, 2012. https://doi.org/10.1201/b11850.

(68) Deng, X.; Galli, F.; Koper, M. T. M. In Situ Electrochemical AFM Imaging of a Pt Electrode in Sulfuric Acid under Potential Cycling Conditions. *J Am Chem Soc* **2018**, *140* (41), 13285–13291. https://doi.org/10.1021/jacs.8b07452.

(69) Sarycheva, A.; Gogotsi, Y. Raman Spectroscopy Analysis of the Structure and Surface Chemistry of Ti3C2T XMXene. *Chemistry of Materials* **2020**, *32* (8), 3480–3488. https://doi.org/10.1021/acs.chemmater.0c00359.

(70) Hu, T.; Wang, J.; Zhang, H.; Li, Z.; Hu, M.; Wang, X. Vibrational Properties of Ti3C2 and Ti3C2T2 (T = O, F, OH) Monosheets by First-Principles Calculations: A Comparative Study. *Physical Chemistry Chemical Physics* **2015**, *17* (15), 9997–10003. https://doi.org/10.1039/c4cp05666c.

(71) Placke, T.; Gonzalez-Julian, J.; Bärmann, P.; Nölle, R.; Siozios, V.; Ruttert, M.; Guillon, O.; Winter, M. Solvent Co-Intercalation into Few-Layered Ti3C2Tx MXenes in Lithium Ion Batteries Induced by Acidic or Basic Post-Treatment. *ACS Nano* **2021**, *15* (2), 3295–3308. https://doi.org/10.1021/acsnano.0c10153.

(72) Come, J.; Black, J. M.; Lukatskaya, M. R.; Naguib, M.; Beidaghi, M.; Rondinone, A. J.; Kalinin, S. V.; Wesolowski, D. J.; Gogotsi, Y.; Balke, N. Controlling the Actuation Properties of MXene Paper Electrodes upon Cation Intercalation. *Nano Energy* **2015**, *17*, 27–35. https://doi.org/10.1016/j.nanoen.2015.07.028.

(73) *Intercalation Induced Reversible Electrochromic Behavior of Two-Dimensional Ti 3 C 2 T x MXene in Organic Electrolytes*.

(74) Sarycheva, A.; Gogotsi, Y. Raman Spectroscopy Analysis of the Structure and Surface Chemistry of Ti3C2T XMXene. *Chemistry of Materials* **2020**, *32* (8), 3480–3488. https://doi.org/10.1021/acs.chemmater.0c00359.

(75) Santana, J. J.; Izquierdo, J.; Souto, R. M. Uses of Scanning Electrochemical Microscopy (SECM) for the Characterization with Spatial and Chemical Resolution of Thin Surface Layers and Coating Systems Applied on Metals: A Review. *Coatings* **2022**, *12* (5), 637. https://doi.org/10.3390/coatings12050637.
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