Surface-engineered Mo₂TiC₂T_x MXene for moisture-resilient high-performance energy storage

Abstract

Two-dimensional MXenes are promising electrode materials for electrochemical energy storage; however, their practical deployment is limited by moisture-induced degradation arising from hydrophilic surface terminations. Here, we report a low-energy ion-beam engineering strategy that converts intrinsically hydrophilic Mo₂TiC₂T_x MXene into a moisture-repellent and structurally stable material while preserving its layered architecture. Selective modification of surface terminations yields a robust water contact angle of about 130°, effectively suppressing moisture adsorption and mitigating environmental degradation without inducing structural damage. The ion-beam-treated MXene exhibits nearly a twofold increase in specific capacitance (187 F g⁻¹ at 1 A g⁻¹) and superior long-term cycling stability, with the electrode retaining 80% of its initial capacitance after 10 000 charge–discharge cycles at 5 A g⁻¹, compared with only 55% for the pristine MXene, thereby demonstrating the durability advantage rendered by surface engineering. Post-cycling characterization confirms the retention of the MXene phase identity and structural integrity after long-term charge–discharge cycling, validating the robust electrochemical stability of the irradiated electrode. The improved electrochemical performance originates from irradiation-induced defect formation and electronic structure modulation, which enhance charge transport and pseudocapacitive behaviour. Density functional theory calculations support these findings by revealing reduced adsorption of polar species and an increased density of states near the Fermi level, indicative of enhanced electrical conductivity and quantum capacitance. This work establishes ion-beam surface engineering as an effective route to stabilize Mo₂TiC₂T_x MXene against moisture-driven degradation while concurrently improving their electrochemical robustness.

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1 Introduction

Electrochemical energy-storage technologies are central to modern electronics, sensing platforms, and renewable-energy integration, where long-term stability under realistic environmental conditions is as critical as high charge-storage capability.^1–3 Among these technologies, supercapacitors have attracted sustained interest owing to their high power density, rapid charge–discharge characteristics, and exceptional cycling durability, positioning them as promising complements to, or hybrids with, conventional batteries. While low-dimensional materials have recently emerged as promising electrode materials, the operational stability of such materials; particularly under humid or moisture-rich conditions; remains a persistent bottleneck limiting device longevity and reliability.⁴ Wettability is an emerging important component in the design of devices with advanced energy storage capacities (i.e. supercapacitors). As energy storage devices become more sophisticated to accommodate flexible, wearable, and durable technology, the compromise between wetting with water and charge storage is an interesting one to study. Hydrophobic surfaces repel water, inhibit corrosion and undesirable side reactions, and are commonly incorporated into energy storage technologies (particularly those in aqueous or humid environments) to enhance longevity and stability.^5–7 Two-dimensional (2D) materials have emerged as key candidates for next-generation energy-storage electrodes because of their high surface area, tunable electronic properties, and short ion-transport pathways.^8–12 In particular, transition-metal carbide and nitride MXenes, derived from layered MAX phases, have gained prominence due to their metallic conductivity, redox-active surfaces, and chemically adjustable terminations. MXenes are typically represented by the formula M_n+1X_nT_x, where surface terminations (–O, –OH, –F) are introduced during selective etching of the A layer from the parent MAX phase. While these terminations are essential for MXene formation and dispersion, they also render the materials intrinsically hydrophilic and chemically reactive.^13–16 MXenes are classified into two primary types based on the number of transition metals in their structure: single transition metal carbide MXenes, such as Ti₃C₂T_x and Nb₂CT_x, etc.¹⁷ and double transition metal carbide MXenes, such as Mo₂TiC₂T_x and Mo₄VC₄T_x, etc.^18–20

Among the MXene family, double transition-metal carbides such as Mo₂TiC₂T_x are of particular interest because their heterometallic composition promotes enhanced pseudocapacitive behavior and structural robustness compared with single-metal MXenes.^21,22 Despite these advantages, Mo₂TiC₂T_x remains susceptible to moisture-induced degradation, restacking, and interfacial instability arising from abundant surface –OH and –F groups.²³ Both experimental and theoretical studies have shown that fluorine terminations, in particular, impede ion diffusion and slow electronic transport, thereby limiting electrochemical efficiency.^24,25 Consequently, stabilizing MXene surfaces against moisture while preserving their layered architecture and electronic conductivity is a critical challenge.

Recent strategies to address these limitations have focused on chemical routes for modifying or replacing surface terminations, including thermal treatments and reactive substitution processes. While effective, such approaches often involve harsh conditions that risk structural degradation or lack precise control over defect density and termination chemistry. In contrast, physical surface-engineering methods offer an alternative pathway for selectively tailoring surface states without compromising crystallographic integrity.²⁶ For instance, X. Hu et al., used sodium metal at elevated temperatures to remove –F terminal groups almost entirely.²⁷ This pioneering advancement can permit the innovative substitution of these F-terminal groups for N-terminal groups with an exceptional increase in specific capacitance and a greatly extended potential window.²⁸ Such improvements are essential for the rapid improvement of supercapacitor devices.

Here, we report a surface-engineering strategy based on low-energy ion irradiation to modulate the terminal chemistry and defect landscape of Mo₂TiC₂T_x MXene. This approach enables controlled suppression of moisture-attracting functional groups while maintaining the intrinsic layered framework. Rather than directly linking surface wettability to electrochemical performance, the hydrophobic character induced here is interpreted as a marker of enhanced environmental stability and resistance to moisture-driven degradation. By stabilizing the surface chemistry and introducing conductive defect states, the engineered MXene exhibits improved charge-transport kinetics and sustained electrochemical performance. This study further elucidates the interplay between irradiation-induced defects, surface terminations, and electronic structure through first principles calculations, providing fundamental insight into stabilizing MXenes for durable energy-storage applications.

2 Experimental data

2.1 Material synthesis

The Mo₂TiC₂T_x MXene was synthesised following previously reported work with some modifications.^21,29,30 The MXene powder was produced by chemically etching the precursor Mo₂TiAlC₂ MAX phase powder, which was procured from Sigma-Aldrich. 10 mL of a concentrated hydrofluoric acid (48–51%) solution was mixed with 0.8 g of Mo₂TiAlC₂ MAX phase powder. The mixture was stirred for 96 hours at 60 °C in a Teflon beaker at 350 rpm. After stirring, it was washed with de-ionized water at 5000 rpm to obtain a neutral pH. The final product, Mo₂TiC₂T_x MXene powder was collected by filtration and dried in a vacuum oven for 12 h at 60 °C.

Approximately 0.5 g of the as-synthesized Mo₂TiC₂T_x MXene powder was added with 5 mL of absolute ethanol (purity 99.8% from Sigma Aldrich) and sonicated for 30 minutes to attain a homogeneous suspension. The resulting diluted MXene solution was spin-coated onto a pre-cleaned glass substrate to ensure an even distribution across the substrate. The coated glass substrates were then placed on a hot plate at 50 °C for 10 minutes to gently evaporate the ethanol, ensuring stable surface conditions for contact-angle measurements and subsequent irradiation studies.

2.2 Supercapacitor electrode preparation

The as-prepared Mo₂TiC₂T_x MXene (85 mg) was mixed with poly vinylidene fluoride (PVDF, 5 mg) and carbon black (10 mg) to form the electrode composite. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added, and the mixture was ground for 5–6 h to obtain a homogeneous slurry. The resulting slurry was uniformly coated onto nickel foam substrates (3 cm × 1 cm), which were pre-cleaned in 1 M HCl. The mass loading was determined by measuring the weight difference of the nickel foam before and after coating. The coated electrodes were dried at room temperature and subsequently compressed using a hydraulic press to ensure good electrical contact and mechanical integrity.³¹

2.3 Ion beam irradiation

A low-energy (10 keV) Ar⁺ ion beam was used to irradiate the as-prepared electrodes and the spin-coated glass samples. The irradiations were done at the IJCLab at the University Paris-Saclay. Two distinct ion fluences of 1 × 10¹⁶ and 3 × 10¹⁶ ions per cm² were used for systematic study. A few electrodes were also irradiated at the implanter at IIT Bhubaneswar with 5 keV argon ions. The beam current was kept at 500 nA during all the irradiation processes to avoid Joule heating effects.³²

2.4 Characterization

Field-emission scanning electron microscopy (FESEM; Zeiss Merlin Compact, Gemini) was used to examine the surface morphology of the MXene samples. The crystal structures of pristine and irradiated MXenes were analyzed by X-ray diffraction (XRD) using a PANalytical X'Pert Pro diffractometer. Raman spectroscopy was performed with a Jobin Yvon T64000 micro-Raman spectrometer to probe the chemical bonding and structural features. X-ray photoelectron spectroscopy (XPS) measurements were carried out using Al Kα radiation (photon energy 1486.6 eV) with an overall energy resolution of 1 eV at room temperature. Surface wettability was evaluated by static water contact-angle measurements using a contact-angle goniometer (Apex Instrument Pvt. Ltd, model ACAM D1) with 10 µL water droplets. For each sample, measurements were taken at three different locations to ensure reproducibility. Electrochemical measurements were performed using an electrochemical workstation (Origalys Pvt. Ltd, model OGF500+EIS).

2.5 TRI3DYN simulation

A simulation technique, namely TRI3DYN, which assesses and describes the modifications occur in three-dimensional systems when exposed to ion beams is explored in this study. The present application utilizes the binary collision approximation (BCA) inside the framework of the Monte Carlo approach. TRI3DYN models the computational volume as a collection of uniformly sized cubes, also known as voxels, each allocated a certain atomic density.^33,34 Ion-beam irradiation induces collisional interactions between adjacent voxels, enabling material transport and intermixing within the simulated volume. The model accounts for sputtering, atomic recoil, redeposition, and interfacial atomic mixing processes. Simulations can be performed under both static and dynamic irradiation conditions, allowing the evolution of target properties to be tracked over time. A computational domain of 85 × 85 × 85 nm³ discretized into 100 × 100 × 100 voxels was employed. To represent the layered MXene architecture, three nanosheets with comparable thicknesses (∼15 nm) were arranged in a stacked configuration. The central layer (6 nm thick), positioned between the two outer Mo–Ti–C nanosheets, was assigned surface termination species (–O and –F), thereby capturing approximately the essential features of the Mo₂TiC₂T_x system under investigation.

2.6 Density functional theory (DFT) calculation

The first principles based simulations were performed using the Vienna Ab initio Simulation Package (VASP).^35,36 PAW-GGA exchange correlation functional³⁷ was employed to perform all the simulations. The cut-off for the plane wave basis was chosen to be 520 eV. The energy and force convergence criteria were set as 10⁻⁵ eV and 0.01 eV Å⁻¹ respectively. The Brillouin zone was sampled using a Γ-centered k-point mesh of 7 × 7 × 1 for atomic optimization and 11 × 11 × 1 for density of states calculations respectively.

The dispersion corrections were incorporated in all the calculations by implementing Grimme's DFT-D2 scheme.³⁸ The AIMD simulations in this work, were carried out in two stages. First stage involved the heating of system from 0 to 300 K in an NVE ensemble for a time-period of 5 ps. Then, the system was further equilibrated for another 5 ps in an NVT ensemble with the temperature maintained at 300 K using the Nose–Hoover thermostat.³⁹

3 Results and discussions

Multilayered Mo₂TiC₂T_x MXene was synthesised by etching out aluminium from the Mo₂TiAlC₂ (MAX phase). FESEM was used to study the surface morphology and microstructure of pre-irradiated and post-irradiated samples. Fig. 1(a and b) represents the plan-view images of pristine Mo₂TiC₂T_x microstructure, measuring 5–12 micrometres in length, characterised by a distinct sheet-like layered structure with an average layer spacing of 8–15 nanometres.

Fig. 1 Planview FESEM image of pristine Mo₂TiC₂T_x MXene (a and b), irradiated Mo₂TiC₂T_x MXene sample at two different fluences of 1 × 10¹⁶ ions per cm² (c) and 3 × 10¹⁶ ions per cm² (d), respectively.

Fig. S1 presents the energy dispersive spectra (EDS) of the pristine Mo₂TiC₂T_x film, along with the relative composition (Table S1). Following ion irradiation, a noticeable change in composition is observed. With increasing ion fluence, adjacent MXene sheets become interconnected, forming a welded and mechanically robust structure without a measurable change in interlayer spacing at a fluence of 3 × 10¹⁶ ions per cm². The EDS spectrum of the irradiated sample (Fig. S2) reveals a reduction in the atomic fractions of oxygen and fluorine, which correspond to weakly bound surface termination groups.¹⁹ The decreased oxygen and fluorine contents after irradiation (Table S2) indicate preferential sputtering of terminal species from the MXene surface. X-ray diffraction (XRD) was employed to examine the crystallographic phases of the pristine and irradiated MXene samples. Fig. 2 compares the XRD patterns of pristine MXene, irradiated MXene, and the parent MAX phase (Mo₂TiAlC₂).

Fig. 2 XRD pattern of MAX Mo₂TiAlC₂ powder (black line), pristine Mo₂TiC₂T_x MXene (red line), and the irradiated MXene at an ion fluence of 3 × 10¹⁶ ions per cm² (blue line).

After 72 hours of HF (48–50%) etching, the characteristic MAX-phase reflections diminish as expected, while the emergence of the MXene phase becomes evident through the intensified and downshifted (002) peak from 2θ = 10.3° to 6.1°. This shift corresponds to a substantial expansion of the c-lattice parameter and interlayer spacing, increasing from 16.9 Å and 8.45 Å in the parent MAX phase to 28.5 Å and 14.25 Å in the resulting Mo₂TiC₂T_x MXene, respectively. These values are consistent with trends reported for Ti₃C₂ and Nb₂C MXenes.^40–42 The elimination of the aluminium layer from MAX phase and the existence of intercalated water molecules are linked to the rise in the c-lattice parameter. After irradiation with argon ions at a fluence of 3 × 10¹⁶ ions per cm², there is a reduction in intensity for (002) and (004) peaks due to structural deformation and a shift in peak position to higher angles due to the defect-induced compressive stress following Ar⁺ ion irradiation⁴³ (SI Fig. S3). Thus, while Al removal and surface termination/intercalation initially expand the c-lattice, subsequent ion irradiation sputters terminations and removes interlayer species, inducing defect-assisted relaxation and compressive stress that partially contracts the interlayer spacing.

3.1 Raman analysis

Fig. 3(a) illustrates the Raman scattering spectra for both pristine as well as irradiated samples subjected to the fluence of 3 × 10¹⁶ ions per cm². The analysis of Mo₂TiC₂T_x MXene revealed distinct Raman modes at 151, 271, 395, 586, 602, 666, 770, and 976 cm⁻¹. These observations are consistent with previously documented data on Mo₂TiC₂T_x MXene,^44–46 thereby affirming the successful synthesis of the phase. Specifically, the peak observed at approximately 151 cm⁻¹ is equivalent to E_g in-plane vibrations associated with the molybdenum and titanium constituents of Mo₂TiC₂T_x MXene. The peak located at approximately 395 cm⁻¹ is directly related to the E_g vibration of hydroxyl (–OH) groups, while the peak at 602 cm⁻¹ is predominantly assigned to the A_1g (M–T_x) vibrational mode.

Fig. 3 (a) Raman spectroscopy results for both pristine (black) and post irradiated MXene at a fluence of 3 × 10¹⁶ ions per cm² (red). (b) FTIR spectrum for the pristine (black) and irradiated (red) Mo₂TiC₂T_x MXene.

Another notable feature at 586 cm⁻¹ matches to E_g (in-plane) carbon atoms' vibration.⁴⁴ Furthermore, a higher frequency mode at 770 cm⁻¹ is primarily characterised by carbon atoms vibrating in both parallel (E_g) and perpendicular (A_g) orientations relative to the basal plane.⁴⁵ Additionally, the Raman peak near 271 cm⁻¹ is associated with E_g modes involving oxygen atoms.⁴⁶ The presence of Mo–O bonds is corroborated by the detection of a significant band at 666 cm⁻¹ (A_g), aligning with findings from previous studies.⁴⁷ Raman peak, at 976 cm⁻¹, signifies the stretching vibrational modes of O–Mo.⁴⁶ Upon exposure to low-energy irradiation with a fluence of 3 × 10¹⁶ ions per cm², a notable decrease in the peak intensities of these E_g (–OH), A_1g (M–T_x), A_g (Mo–O), and Mo=O bending modes was observed. This reduction is attributed to the removal of functional groups attached to the MXene surface (–O/–F, –OH), highlighting the impact of irradiation on the structural integrity of the material.¹⁹ FTIR reveals the composition and functional groups of MXene before and after ion irradiation. The FTIR spectrum of the pristine (black line) and that of the irradiated (red line) sample are displayed in Fig. 3(b). Pristine Mo₂TiC₂T_x MXene nanosheets have characteristic stretching vibration peaks at around 3437, 2336, 2190, 2021 cm⁻¹, corresponding to hydroxyl (–OH), stretching vibrational bonds of C=O, C≡C, and C=C, respectively, while 837, 631, 562 cm⁻¹ are related to deformed molecular vibration of Mo–O and Ti–O.^22,48 The unique absorption peaks observed at 3437 and 837 cm⁻¹ diminish following ion irradiation, suggesting that the functional groups attached to the MXene are removed as a result of the irradiation.

3.2 XPS analysis

X-ray photoelectron spectroscopy (XPS) was used to examine the chemical composition of Mo₂TiC₂T_x MXene. Fig. 4(a–d) and (e–h) present XPS spectra for both the pristine and post-irradiated Mo₂TiC₂T_x MXenes, respectively. The binding energy of the C 1s state (284.6 eV) was used as a reference for sample calibration. The deconvolution of the carbon peak revealed several distinct bonding environments: C–C at 284.62 eV, C–O at 286.30 eV, O–C=O/C–F at 288.34 eV, and Mo–C at 282.44 eV.⁴⁹ Fig. 4(a and e) indicate the XPS spectra of the Mo 3d level for both the pristine and irradiated MXenes at an ion fluence of 3 × 10¹⁶ ions per cm². The peaks at 229.12, 232.34, and 235.73 eV correspond to Mo–C (3d_5/2), Mo–C (3d_3/2), and Mo–O bonds, respectively. Irradiation of the Mo–O peak shifted to 236.13 eV, accompanied by a concomitant reduction in intensity. Fitting the Ti 2p region indicates the presence of Ti–C (sp³), Ti(V) (2p_3/2), Ti–C (sp¹), and Ti(IV) (2p_1/2), with peaks at 455.51, 458.93, 461.41, and 464.80 eV, respectively.^17,50 Notably, the intensities of the Ti–C (sp³) and Ti–C (sp¹) peaks diminished following irradiation, likely due to the disintegration of loosely bound carbon atoms associated with Ti. The spectrum of pristine Mo₂TiC₂T_x exhibits a prominent asymmetric peak, consistent with previous studies on this MXene, indicating the presence of various oxygen-containing species. The deconvoluted oxygen peaks identified the bonding environments of C–Mo–O at 531 eV and C–Mo–OH at 532 eV, along with peaks at 530 eV and 533.32 eV corresponding to molybdenum oxides (MoO_x), TiO₂, or surface-terminated oxygen, and absorbed water, respectively.⁵¹ After irradiation, the intensities of the peaks related to C–Mo–OH, C–Mo–O, and absorbed water decreased, likely due to the sputtering out of O-atoms from the MXene surface as a result of high fluence exposure. Finally, Fig. 4(d and h) presents the XPS spectra of F 1s for the pristine as well as the irradiated Mo₂TiC₂T_x MXenes. The peak positions at 685 and 686.07 eV were attributed to C–Mo–F_x and Al–(OF)_x, respectively.^51,52 XPS analysis reveals a pronounced reduction in the C–Mo–F_x component after ion irradiation, accompanied by the suppression of residual Al(O,F)_x species, confirming preferential sputtering of fluorine terminations from the MXene surface (see also SI Fig. S4(c)). Diminution of fluorine-linked peaks after irradiation makes the MXene surface more hydrophobic.

Fig. 4 XPS spectra of pristine Mo₂TiC₂T_x MXene and irradiated MXene, (a and e) Mo 3d, (b and f) Ti 2p, (c and g) O 1s, (d and h) F 1s.

Furthermore, UV-Vis absorption spectroscopy and Tauc analysis have been studied for both pristine and irradiated MXene (see Fig. S5). A clear red shift in the absorption edge and a reduction in optical bandgap (from 1.30 to 1.15 eV) are observed after irradiation, indicating irradiation-induced electronic structure modulation through termination alteration and defect-related sub-band states. Importantly, this optical band gap reflects defect- and termination-related optical transitions rather than a fundamental semiconductor band gap, which is consistent with the metallic electronic structure revealed by density functional theory calculations in this work. Overall, the combined experimental and theoretical results provide a unified mechanism linking irradiation-induced surface chemistry and defect engineering with improved ion transport, enhanced capacitive contribution, and superior electrochemical performance, as discussed in the following sections.

3.3 TRI3DYN study

To investigate the impact of irradiation on morphology and atomic composition, a Monte Carlo-based ion–solid interaction simulation, i.e., TRI3DYN, was performed.³³ The Fig. S6 illustrates the pristine structure featuring two layers of oxygen and one layer of fluorine functional groups sandwiched between two consecutive MXene nanosheets. In the simulation, a uniform broad beam of 10 keV argon ions with two different fluences (1 × 10¹⁶ and 3 × 10¹⁶ ions per cm²) was directed from above at an azimuthal angle of 90° relative to the Y axis, along with a polar angle of 20° concerning the X axis. This configuration was chosen to ensure non-convergence and consistency with the arbitrary orientation of the nanosheets. After irradiation, well-separated nanosheets converge through mechanisms involving atomic mixing, sputtering, and the redeposition of atoms.³⁴ During irradiation, the forward recoiled atoms are redeposited into the interstitial spaces between the nanosheets.

To analyse the fractional variation in atomic density of the nanosheet, we meticulously extracted a central segment from the computational volume along the vertical axis. This approach enables a focused analysis of atomic distribution within the nanosheet structure, allowing for a clearer understanding of its properties and behaviour. The collisional effects induced by irradiation at two distinct fluences (such as 1 × 10¹⁶ and 3 × 10¹⁶ ions per cm²) cause the creation of interstitials and vacancies. Fig. 5 illustrates the relative changes in the atomic fractions of elements present in the MXene structure after argon irradiation with an energy of 10 keV at two distinct fluences (1 × 10¹⁶ and 3 × 10¹⁶ ions per cm²). Fig. 5(l and o) show a decrease in the atomic fraction to 55% and 82% for oxygen and fluorine, respectively, with respect to the pristine (Fig. 5(j and m)), in the irradiated systems at the elevated ion fluence (i.e., 3 × 10¹⁶ ions per cm²). This reduction in oxygen and fluorine percentages indicates the elimination of surface terminal groups from the nanosheets due to sputtering and recoils, which becomes significant with increasing ion fluence. Fig. 5(a–f) present the variations in the atomic fractions of Mo and Ti under two distinct ion fluences. These figures emphasize the sputtering redeposition and forward recoils of Mo and Ti atoms at both the junction and outermost surface of the nanosheets. Notably, there was an approximately 27% increase in the Mo atomic fraction on the upper surface of MXene nanosheets at the higher fluence (i.e., 3 × 10¹⁶ ions per cm²). In comparison, the atomic fraction of Ti atoms exhibits a slight variation of approximately 4–5%. Such variation is attributed to atomic mixing occurring at the same elevated fluence of 3 × 10¹⁶ ions per cm². Fig. 5(h and i) represent a decrease in the atomic fraction of carbon atoms to 32% as the ion fluence increases, a result of recoiling out of carbon atoms that are re-deposited at the adjacent areas of the nanosheets of MXene. As more atoms are recoiled out and redeposited at the junction, a welded structure forms between the nanosheets. The results of the TRI3DYN simulation clearly indicate that, after irradiation with different fluences, fluorine atoms are preferentially sputtered out than any other atoms, such Ti, Mo, C etc. This indicates the extent to which the ion irradiation is capable of modifying the functional groups as well as other atomic composition of MXene.

Fig. 5 TRI3DYN simulation result of Mo atomic density for pristine (a), after 10 keV Ar⁺ irradiation at ion fluence of 1 × 10¹⁶ ions per cm² (b) and 3 × 10¹⁶ ions per cm² (c); for Ti atomic density for pristine (d), after 10 keV Ar⁺ irradiation at ion fluence of 1 × 10¹⁶ ions per cm² (e) and 3 × 10¹⁶ ions per cm² (f); for C atomic density for pristine (g), after 10 keV Ar⁺ irradiation at ion fluence of 1 × 10¹⁶ ions per cm² (h) and 3 × 10¹⁶ ions per cm² (i) and for O atomic density for pristine (j), after 10 keV Ar⁺ irradiation at ion fluence of 1 × 10¹⁶ ions per cm² (k) and 3 × 10¹⁶ ions per cm² (l) for F atomic density for pristine (m), after 10 keV Ar⁺ irradiation at an ion fluence of 1 × 10¹⁶ ions per cm² (n) and 3 × 10¹⁶ ions per cm² (o), respectively.

3.4 Wettability

The wettability of Mo₂TiC₂T_x MXene was examined using contact angle measurements under both static and dynamic operational modes at ambient temperature. A stationary water droplet (deionized) of about 10 µL in volume on the pristine Mo₂TiC₂T_x MXene surface formed a water contact angle (WCA) of 42.99 ± 1.52°, indicating that the pristine Mo₂TiC₂T_x MXene exhibits hydrophilic properties. However, this behaviour changes after irradiation, making the surface more hydrophobic due to partial removal of surface terminal groups.¹⁹ With irradiation at 1 × 10¹⁶ ions per cm², the WCA increases to 107.23 ± 1.15°. When we reach 3 × 10¹⁶ ions per cm², the Fig. 6: (a) the graph shows the correlation between WCA and ion fluence, along with optical images in the inset. (b) The change in contact angle over time is displayed for pristine MXene (black) and irradiated MXene at two distinct ion fluences: 1 × 10¹⁶ (red line) and 3 × 10¹⁶ ions per cm² (green line).

Fig. 6 Static water contact angle of pristine and Ar⁺ ion-beam-irradiated Mo₂TiC₂Tx MXene as a function of fluence (a), showing progressive hydrophilic-to-hydrophobic transition from ≈42° (pristine) to ≈130° (3 × 10¹⁶ ions per cm²); and dynamic contact angle stability over 600 s (b), confirming superior moisture repellency and minimal wetting in irradiated samples.

WCA further rises to 129.07 ± 2.31°. Fig. 6(b) illustrates the time-dependent WCA measurements for both pristine and irradiated samples at two distinct ion fluences. For the pristine MXene sample, the WCA is approximately 42°. Within 10 minutes, the pristine surface fully absorbs a 10 µL water droplet. In contrast, the suspended water droplet remains on the irradiated surface, showing a WCA of around 130° for over 10 minutes. The variation in WCA for irradiated samples at two different fluences of 1 × 10¹⁶ and 3 × 10¹⁶ ions per cm² is about 18° and 6° over ten minutes, respectively, which indicates a stable and moisture-repelling surface post-irradiation.

3.5 Electrochemical analysis

Electrochemical tests were conducted on Mo₂TiC₂T_x samples in their pristine condition and after irradiation with argon ions at a fluence of 3 × 10¹⁶ ions per cm² using a three-electrode configuration to thoroughly evaluate their charge storage properties in a 3 M KOH aqueous solution. An Ag/AgCl reference and platinum were employed as the counter electrode in this analysis. CV analysis was useful for confirming the storage characteristics of the samples in terms of charges and identifying the range of their working capacity and the type of capacitance. The determination of the storage capacity of the material was done using the CV curves. Cyclic curves of the pure Mo₂TiC₂T_x nanosheets in a potential range of 100–500 mV versus the Ag/AgCl reference electrode were recorded at various scan rates (5, 10, 20, 40, 100, and 200 mV s⁻¹) as illustrated in Fig. 7(a). The anodic and cathodic currents had their higher peaks associated with oxidation and reduction processes in Mo₂TiC₂T_x MXene and electrode reactions. The symmetrical curves of the CV indicate that MXene exhibits high reversibility, with distinct oxidation and reduction peaks.⁵³

Fig. 7 Cyclic voltammetry curves of (a) pristine Mo₂TiC₂T_x MXene, (b) irradiated Mo₂TiC₂T_x MXene, charge–discharge (CD) curves of (c) pristine Mo₂TiC₂T_x MXene, (d) irradiated Mo₂TiC₂T_x MXene.

In the investigation of irradiated Mo₂TiC₂T_x MXene nano-sheets (Fig. 7(b)), the electrochemical behaviour was demonstrated to be analogous to that observed in pristine Mo₂TiC₂T_x MXene samples. However, a notable increase in the integrated area under the cyclic voltammetry curve for the irradiated samples was evident at each scan rate, indicating a significant enhancement in storage capacity relative to the pristine counterparts. This increase in storage capacity can be attributed to the introduction of defects and vacancies resulting from the irradiation process, which are likely to enhance the electrical conductivity and improve the material's charge transfer properties, as explained using DFT calculations.²⁶ Additionally, the observed joining of nanosheets may help in better percolation, which may further contribute to the enhancement in storage capacity. Fig. 8(a) presents a comparative examination of cyclic voltammetry observations at a scan rate of 50 mV s⁻¹ for both pristine and irradiated MXene samples. The data illustrated a substantial expansion in the area for the irradiated sample, accompanied by a shift in the oxidation and reduction peaks towards lower potentials due to surface modification and an increase in electrocatalytic activity resulting from the creation of defects and vacancies after irradiation.⁵⁴ These alterations are indicative of improved electrochemical activity, which correlates with the observed increase in the specific capacitance.

Fig. 8 (a) A comparison of the CV curves for pristine and irradiated Mo₂TiC₂T_x MXene at a scan rate of 50 mV s⁻¹, (b) a comparison of the CD curves for pristine and irradiated Mo₂TiC₂T_x MXene is shown at a current density of 1 A g⁻¹. (c) A comparison of specific capacitance values for pristine and irradiated Mo₂TiC₂T_x MXene is provided at different current densities. (d) A comparison of the stability between pristine and irradiated Mo₂TiC₂T_x MXene. The irradiations were performed using 5 keV argon ions at an ion fluence of 3 × 10¹⁶ ions per cm².

The constant current charge–discharge (CCCD) curve, also referred to as the CD curve, is invaluable in the analysis of storage devices. These curves provide critical insights into the charge storage mechanism, operating potential window, and specific capacitance. Fig. 7(c and d) present the charge–discharge curves for the pristine and irradiated MXene samples, indicating that the operational potential range was consistently between 100 mV and 500 mV, for both samples. In addition, the increased discharge time indicates that, in this instance at least, the specific capacitance of the irradiated MXene sample exceeded that of the pristine sample. The unirradiated sample achieves a maximum specific capacitance of about 96.42 F g⁻¹ at a normalized current density of 1 A g⁻¹. It is essential to note that as the current density increased, the specific capacitance decreased. This phenomenon can be assigned to the limitations of the charge transfer and diffusion processes that arise with increasing potential steps. The expected trends for the irradiated sample will be explored further, reinforcing our understanding of its enhanced performance. In the irradiated MXene sample, we observed a remarkable increase in specific capacitance, which was approximately 94.2% higher than that of the pristine counterpart across various normalized current densities. The irradiated sample achieved a much higher specific capacitance of approximately 187.37 F g⁻¹ at a normalized current density of 1 A g⁻¹, which is nearly two-fold enhancement compared to pristine and is competitive with reported values for Mo₂TiC₂T_x-based electrodes in the literature (see Table S3 in the SI). Cyclic stability is a critical factor for evaluating storage devices. We conducted comprehensive evaluations of the stability of the supercapacitor electrodes (Fig. 8(d)) by assessing the rate capability of both pristine and irradiated samples. The percentage of specific capacitance retained following a specified number of charge/discharge cycles (up to 10 000 cycles), denoted as rate capability, reveals that the pristine sample retains approximately 55%, whereas the irradiated sample retains about 80% up to10 000 cycles. This remarkable enhancement in storage capacity and stability is attributed to the substantial role of defect formation and partial elimination of functional groups such as –F from the MXene nanosheets created by Ar⁺ ion irradiation. Decreasing the percentage of fluorine (–F) terminal groups in MXene significantly enhances its specific capacitance, mainly by enhancing electrolyte ion transit and increasing the number of active sites. Further details on this is discussed in the DFT results section. Additionally, the interconnection of adjacent MXene sheets on the surface, without restacking of the sheets during irradiation, is likely to enhance the conductivity due to reduced contact resistance. We thoroughly analysed the conductivity of both samples, (see SI Fig. S7), demonstrating an impressive 19 times rise in the conductivity of the irradiated MXene sample at fluence of 3 × 10¹⁶ ions per cm² in comparison to that of the pristine, which enables a rapid charge discharge cycle by reducing resistance at the electrolyte–electrode interface and also enhances the ion transport.⁵⁵ Crucially, this improvement is achieved through a single-step, physical surface-engineering route, low-energy ion beam irradiation without chemical intercalation, composite formation, or structural compromise.

To investigate the effect of excessive ion fluence on electrochemical performance, Mo₂TiC₂T_x MXene was additionally irradiated at a higher fluence of 4 × 10¹⁶ ions per cm² and characterized in 3 M KOH aqueous electrolyte. Fig. S8(a) in SI presents the CV curves of the 4 × 10¹⁶ ions per cm² irradiated electrode at scan rates of 10–100 mV s⁻¹, showing a reduction in the integrated CV area compared to the optimally irradiated (3 × 10¹⁶ ions per cm²) electrode, indicative of diminished charge storage capacity at excessive fluence. This is further confirmed in Fig. S8(b), where the peak current of the 4 × 10¹⁶ ions per cm² sample is measurably lower than that of the 3 × 10¹⁶ ions per cm² sample at 100 mV s⁻¹, reflecting degraded electrochemical activity arising from irradiation-induced structural disorder beyond the optimal defect density threshold. The GCD profiles of the 4 × 10¹⁶ ions per cm² irradiated electrode at current densities of 0.5–4 A g⁻¹ are presented in Fig. S8(c). The comparative specific capacitance values derived from these GCD curves are shown in Fig. S8(d). A significant reduction in specific capacitance from 187.37 F g⁻¹ (3 × 10¹⁶ ions per cm²) to 122.73 F g⁻¹ (4 × 10¹⁶ ions per cm²) is observed at 1 A g⁻¹, representing approx. 34% decrease in charge storage performance. This deterioration is attributed to excessive defect accumulation at higher fluence, which disrupts the continuous electron transport pathways, increases charge-transfer resistance, and compromises the structural integrity of the Mo₂TiC₂T_x layered framework collectively outweighing the beneficial effects of moderate defect-assisted pseudocapacitance observed at 3 × 10¹⁶ ions per cm². These results confirm that 3 × 10¹⁶ ions per cm² represents the optimal irradiation fluence for Mo₂TiC₂T_x MXene, beyond which excessive structural damage leads to progressive electrochemical performance degradation.

This finding indicates that post-irradiation, the Mo₂TiC₂T_x MXene, which becomes a hydrophobic state due to the removal of terminal groups, can be linked to the better stability of the electrode. Such a hydrophobic surface reduces the ion diffusion barrier, enabling high capacitance retention at a fast scan rate. This transformation via a low-energy ion beam reduces restacking and water molecule interaction, which minimizes oxidative and structural degradation of the electrode, thereby enhancing stability in aqueous electrolytes.^56,57 Notably, this process augments the overall storage capacity, underscoring the efficacy of our method.

To quantitatively deconvolute the charge storage contributions, the method proposed by Dunn et al. was employed, which separates the total stored charge into two components: the capacitive contribution (electric double-layer effect) and the diffusion-controlled contribution (faradaic/intercalation processes).⁵⁸ The relative contributions of these two mechanisms to the overall charge accumulation in both pristine and irradiated Mo₂TiC₂T_x electrodes are presented in Fig. S9(a and b). As the scan rate increases from 20 to 100 mV s⁻¹, the capacitive contribution progressively dominates while the diffusion-controlled contribution diminishes, which is a trend consistent with the kinetic limitations imposed on ion intercalation at higher sweep rates. In the irradiated Mo₂TiC₂T_x electrode, ion beam irradiation selectively modifies the surface terminations (–O, –OH, and –F), generating termination defects and local structural disorder, as confirmed by XPS and Raman analysis. These surface defects serve as electrochemically active sites that enhance capacitive charge storage by improving ion accessibility at the electrode–electrolyte interface and accelerating redox kinetics, thereby shifting the charge storage mechanism toward a predominantly capacitive response.

Zeta potential measurements of pristine and ion-irradiated Mo₂TiC₂T_x MXene are presented in Fig. S10. The pristine Mo₂TiC₂T_x exhibits a zeta potential of −31.6 mV, which decreased to −25.0 mV following ion beam irradiation. This reduction in surface negativity reflects the irradiation-induced modification and partial removal of electronegative surface terminations (–O, –OH, and –F), which are the primary contributors to the highly negative zeta potential of as-synthesized MXenes. The shift toward a less negative value indicates the formation of surface sites with reduced electronegativity, consistent with the XPS evidence of termination restructuring discussed earlier.

From an electrochemical standpoint, this moderation of surface charge has two significant consequences. First, it weakens the electrostatic screening effect at the electrode–electrolyte interface, facilitating faster ion transport and improved interfacial ion mobility, both of which are favourable for high-rate supercapacitor operation. Second, the irradiation-induced defect sites and structural disorder introduce additional electrochemically active centers that promote defect-assisted pseudocapacitance. The synergistic combination of enhanced electric double-layer capacitance enabled by improved ion accessibility and accelerated surface redox kinetics collectively accounts for the superior electrochemical performance of the irradiated Mo₂TiC₂T_x electrode over its pristine counterpart.

Electrochemical Impedance Spectroscopy (EIS) is a key method for the characterization of the electrode. It helps to measure the capacitative and resistive characteristics of the as-prepared cell and the movement of ions at the electrode–electrolyte boundary. The analysis was performed without current flow using frequencies from 0.01 Hz to 100 kHz and a 1 mV AC signal. Fig. 9(a and b) present the Nyquist plots of the pristine and irradiated Mo₂TiC₂T_x samples, respectively. The equivalent circuits are shown in the inset of Fig. 9(a and b). The Nyquist plot for pristine demonstrates a semicircle at high frequencies and a straight line at low frequencies. The Nyquist plot for the irradiated sample exhibited small semicircles and nearly vertical lines, indicative of its capacitive characteristics.

Fig. 9 (a) Nyquist plot of pristine Mo₂TiC₂T_x MXene before and after 2000 cycle stability performance, (b) Nyquist plot of irradiated Mo₂TiC₂T_x MXene before and after 2000 cycle stability performance. The irradiations are done with 10 keV argon ions at an ion fluence 3 × 10¹⁶ ions per cm².

The presence of smaller semicircles in the high-frequency region and steeper inclines in the low-frequency region suggests a reduction in diffusion resistance resulting from irradiation.²⁸ The high-frequency region is related to charge transfer and is used to determine the effective series resistance, consisting of resistance due to the solution (R_S) and the resistance related to charge transfer (R_CT) at the interface between the electrode and electrolyte.⁵⁹ As tabulated in Table S4, the pristine sample exhibits an R_CT of 4.9803 Ω and R_S of 0.65867 Ω, while the irradiated sample shows a significantly reduced R_CT of 3.1853 Ω and a marginal increase of R_S to 0.68226 Ω. The lower R_CT of the irradiated electrode confirms enhanced charge-transfer kinetics, attributable to irradiation-induced surface defects that promote better electronic coupling between MXene sheets and reduce the insulating effect of surface terminations (–F/–OH). After cycling, only a marginal change is observed, indicating reasonable stability but limited improvement in interfacial kinetics. On the other hand, the irradiated electrode shows a noticeably smaller semicircle, signifying a reduced and faster electron transport. This improvement is attributed to ion-beam-induced defect formation, partial removal of surface terminations, and enhanced inter-layer connectivity. The low-frequency region becomes more vertical, approaching ideal capacitive behaviour, which suggests improved ion diffusion and reduced Warburg impedance.

Thus, low-energy ion beam irradiation is shown to induce controlled modification of surface terminations and generate near-surface defects without disrupting the layered framework. Partial removal of electronegative –O/–F terminations reduce surface polarity, as evidenced by zeta-potential measurements and enhanced hydrophobicity, while irradiation-induced vacancies introduce defect states that improve electronic conductivity and electrochemically accessible active sites. These combined effects lower the interfacial energy barrier for ion migration, leading to enhanced ion transport kinetics and reduced charge-transfer resistance, as confirmed by electrochemical impedance spectroscopy.

To evaluate the practical applicability of the ion-irradiated Mo₂TiC₂T_x MXene electrode, a two-electrode symmetric supercapacitor device was fabricated using 3 M KOH aqueous electrolyte within an operating potential window of 0–0.5 V. The electrochemical performance of the symmetric device is presented in Fig. S11. Fig. S11(a) shows the cyclic voltammetry (CV) curves recorded at scan rates ranging from 10 to 100 mV s⁻¹. The CV curves retain a quasi-rectangular shape across all scan rates, confirming predominantly capacitive charge storage behavior with good rate capability. The galvanostatic charge–discharge (GCD) profiles at current densities of 0.5–4 A g⁻¹ are presented in Fig. S11(b). The GCD curves exhibit good symmetry between the charge and discharge segments, further confirming the capacitive nature of the device. Fig. S11(c) shows the variation in specific capacitance as a function of current density. The device with pristine electrode delivers a maximum specific capacitance of 37.40 F g⁻¹ at 0.5 A g⁻¹, with capacitance decreasing progressively at higher current densities due to the kinetic limitations of ion diffusion at faster charge–discharge rates.

Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 0.1 Hz to 100 kHz. The Nyquist plot (Fig. S10(d)) yields a solution resistance R_S = 0.88 Ω and a charge-transfer resistance R_CT = 3.72 Ω, indicating low interfacial resistance and efficient charge transfer within the symmetric device. The equivalent circuit used for impedance fitting is shown in the inset of Fig. S10(d). The long-term cycling stability of the symmetric device was evaluated over 1500 consecutive charge–discharge cycles at 4 A g⁻¹, as shown in Fig. S10(e). The energy and power density of the device were calculated from the GCD data and are presented as a Ragone plot in Fig. S10(f). The device achieves a maximum energy density of 1.29 Wh kg⁻¹ at a power density of 94.78 W kg⁻¹, and retains an energy density of 0.65 Wh kg⁻¹ at a high-power density of 848.30 W kg⁻¹, demonstrating a favorable energy-power balance for practical energy storage applications. To demonstrate real-world applicability, the symmetric device was connected in series. The series-connected device exhibited a doubled operating voltage window, successfully powering a commercial LED, as shown in Fig. S12 (SI). To further assess the structural stability of the electrode material, post-cycling characterization was performed after long-term charge–discharge cycling. As shown in Fig. S13–S15, the Mo₂TiC₂T_x MXene phase largely remains intact, with all key structural features preserved, confirming the good stability of the irradiated electrode.

4 DFT analysis

4.1 Structural properties

The computed lattice parameters of MXene are a = b = 2.95 Å and α = β = 90°, γ = 120°, matching the reported value.⁶⁰ A vacuum of 30 Å is provided to avoid interlayer interaction. The structures of functionalized MXenes were first optimized, after which the terminal groups (–O, –F, –OF) were removed and the resulting bare MXene structure was further optimized. The optimized structures of the MXene along with the terminal functional group are shown in Fig. 10.

Fig. 10 Optimized structure of MXenes (a) bare (b) –O functionalized (c) –F functionalized (d) –OF functionalized.

4.2 DOS calculation

Total density of states (TDOS) plots for functionalized MXene and the bare MXene are plotted in Fig. 11 to get the insights for the electronic properties. The gain in the electronic states near the Fermi level for bare MXene indicates higher electrical conductivity as compared to the functionalized MXene.

Fig. 11 Total density of states plot for both bare and functionalized MXene.

4.3 Thermal stability of irradiated MXene at room temperature

To get the insights about the thermal stability of bare MXene, we have performed the ab initio molecular dynamics simulations. From the simulation, the evolution of the total energy of bare MXene after equilibration (at 300 K, for 5 ps) has been plotted in Fig. 12(a). The structure suffers minor distortion after heating. The time evolution of bond length variation of Mo–C and Ti–C is plotted in Fig. 12(b). Small variation in energy (about 0.48%) and bond length (2.02–2.71%) suggests that the MXene without functional groups is a stable structure.

Fig. 12 (a) Evolution of total energy of bare MXene with time. (b) Bond length fluctuation of Mo–C and Ti–C.

4.4 Quantum capacitance

The following formula⁶¹ was used to determine the quantum capacitance of bare MXene and functionalized MXene:where D(E) and ϕ_G respectively stands for the density of states of the system and electrode potential, and F_T(E) denotes the thermal broadening function, which can be expressed as,

F_T(E) = (4k_BT)⁻¹ sech²(E/2k_BT) (2)

where k_B is Boltzmann's constant and T represents temperature (298.5 K).

The following relationship⁶² can be used to relate the computed quantum capacitance and total capacitance of any system:

where the electric double-layer capacitance, C_EDL, is determined by the interfacial interaction between the electrode and electrolyte. Fig. 13 shows the variation of quantum capacitance with the electrode potential for bare and functionalized MXene. The quantum capacitance is higher for most of the electrode potential for bare MXene, suggesting higher total capacitance and better charge storage performance than the functionalized MXene.

Fig. 13 Plot showing variation of quantum capacitance with electrode potential for both bare and functionalized MXene.

4.4.1 Adsorption of a water molecule. To check the effect of functional groups on the MXene surface on the wetting behavior, water adsorption energies were studied. The adsorption energy is higher for the functionalized MXene, while it decreases in the bare system, suggesting that the system moves towards the hydrophobic nature on removal of the terminal functional group. The water adsorption energies on the functionalized MXenes and the bare MXene are tabulated in Table 1, and the optimized structures of water-adsorbed MXenes are shown in Fig. 14.

Table 1 Adsorption energy of water molecule on bare and functionalized MXene

System	Bare	–O functionalized	–F functionalized	–OF functionalized
Adsorption energy of H2O (eV)	−0.942	−1.319	−1.146	−1.286

---

Fig. 14 Optimized structure after adsorption of a water molecule.

5 Conclusions

In summary, this study demonstrates a low-energy ion-beam induced surface engineering strategy that effectively stabilizes Mo₂TiC₂T_x MXene by selectively modifying its surface chemistry while preserving the layered crystal framework. Ion irradiation reduces weakly bound surface terminations, resulting in a transition from an intrinsically hydrophilic surface to a moisture-repellent state with a water contact angle of ∼130°, which is expected to mitigate degradation under moist environments rather than directly influence charge storage. Electrochemically, the irradiated MXene exhibits nearly a twofold enhancement in specific capacitance and excellent cycling stability, retaining 80% of its capacitance after 10 000 cycles. Impedance analysis reveals a substantial reduction in charge-transfer resistance and improved low-frequency capacitive behavior, indicating faster ion diffusion and enhanced interfacial charge transport induced by irradiation-generated defects and improved inter-layer connectivity. Post-cycling characterization confirms the retention of the Mo₂TiC₂T_x MXene phase identity and structural integrity after long-term charge–discharge cycling, validating the robust electrochemical stability of the irradiated electrode. Density functional theory calculations qualitatively support these observations, showing reduced adsorption affinity for polar species and an increased electronic density of states near the Fermi level following terminal group removal, which enhances conductivity, reduces diffusion barrier and increases quantum capacitance. Overall, this work establishes ion-beam surface engineering as an efficient and scalable route to improve the durability, interfacial kinetics, and electrochemical performance of MXene electrodes for next-generation energy storage systems.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Supplementary information (SI): additional characterizations before and after electrochemical measurements, TRI3DYN simulation results, conductivity measurements, electrochemical measurements at higher fluence, zeta potential measurements, device performance data, and tables. See DOI: https://doi.org/10.1039/d6ta01660j.

Acknowledgements

SC is thankful to Department of Biotechnology for funding support through grant # BT/PR49803/MED/32/941/2023 and funding support from the Indian Council of Medical Research for project grant # IIRPSG-2025-01-03628. SC is further grateful to the University Paris-Saclay for the Scientific Missionary Fellowship support. OP acknowledges support from Silvin Herve at IJCLab for ion beam irradiation. DMP would like to thank the Indian Institute of Technology Bhubaneswar for providing the fellowship and funding for research work. SC is thankful to Prof. Wolfhard Moeller for the TRI3DYN program and Dr Dinesh Topwal for XPS measurements. The FESEM, XRD and ion implantation facilities at IIT Bhubaneswar, as well as the Raman spectroscopy and FTIR facilities at CSIR IMMT Bhubaneswar, are gratefully acknowledged. The author(s) used ChatGPT (OpenAI) to improve English clarity and grammar in parts of the manuscript. The authors take full responsibility for the content.

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