In situ synthesis of MXene@Iron oxide nanoparticle composites via pulsed laser ablation in liquid applied for electrochemical H₂O₂ reduction

Abstract

We propose to circumvent the lack of intrinsic magnetism in MXenes by in situ decorating the samples with magnetic nanoparticles during laser ablation in liquid by deposition of iron species on three types of MXenes (nanocomposites). The basic properties of such nanocomposites were characterized by a number of techniques including Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectrometry (SIMS), X-ray Photoelectron Spectroscopy (XPS), electrochemical investigation and the optical Faraday effect. The results indicate significant differences in the content of Fe species deposited on MXenes (Ti₃C₂T_x; Nb₂CT_x and Nb₄C₃T_x) and the amount of Fe species deposited on MXenes is triggered by the density of plasmons present within MXenes. Moreover, we confimed a correlation between the capability of MXene-based Fe nanocomposites to electrochemically reduce hydrogen peroxide and electrochemical activity of Fe species present on MXenes. The highest electrochemical current density obtained for H₂O₂ reduction was obtained on the Ti₃C₂T_x MXene-based Fe nanocomposite with the value of 4399 µA cm⁻². We further discuss how to improve the current density obtained for H₂O₂ reduction on the other MXene-based Fe nanocomposites (Nb₂CT_x and Nb₄C₃T_x) using alternative synthesis protocols.

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Introduction

Over the last decade, MXenes^1,2 have become prominent two-dimensional nanomaterials due to their advantageous physical and chemical properties. Their application scope^3–13 has expanded considerably, driven by ease of processing and compositional versatility that yield favorable electronic,^14,15 optical,^16,17 mechanical,^18,19 antimicrobial,^20,21 electrocatalytic,^22,23 thermal,^24,25 and magnetic properties.^26,27 Their attractiveness is further reinforced by inherent hydrophilicity, tunable surface chemistry, high flake availability, and favorable synthesis yields.²⁸ Recent advances have focused on patterning techniques to create functional MXene-based materials with enhanced performance.^29–31

However, most MXenes lack spontaneous magnetism due to strong covalent M–X and M–T bonds,²⁶ as evidenced by the limited intrinsic magnetic response of Ti₃C₂ and Nb₂C MXenes.^32,33 The former exhibited paramagnetic behavior while the latter showed superconductivity-like diamagnetism. This limitation currently restricts their use in spintronics³⁴ and related fields.³⁵ Two strategies have emerged to address this issue: incorporating magnetic transition metal elements such as Co, Cr, Mn, Ni, V, and Fe,^36–41 or decorating MXenes with magnetic nanoparticles-an approach validated by numerous studies.

Tan et al.⁴² synthesized Ti₃C₂T_x MXene from carbon soot extracted from fossil fuel combustion waste and decorated it with Fe₃O₄ via hydrothermal treatment for vanadium redox flow batteries. Similarly, iron oxide nanodots were self-assembled on MXene surfaces for energy storage applications.⁴³ Hydrothermal methods have been widely employed to fabricate Fe₃O₄-based MXene composites for applications including photocatalysis,^44,45 electrochemical sensing,⁴⁶ and pollutant adsorption.^47,48 Zhang and co-workers⁴⁹ introduced hierarchical pore structures in MXene/Fe₃O₄/polyimide aerogels through directional freezing and thermal annealing for electromagnetic interference (EMI) shielding. Various fabrication approaches-including layer-by-layer assembly,^50,51 electrospinning,⁵² hydrothermal synthesis,^53–55 and precipitation methods^56,57-have been utilized to develop magnetic MXene hybrids for EMI shielding applications. The dielectric-magnetic synergistic effect for effective electromagnetic wave absorption was demonstrated in MXene/Fe₃O₄@Fe₃C@carbon composites.⁵⁵ Other notable applications include tannic acid-Fe₃O₄@MXene hydrogels with advanced conductivity and flexibility,⁵⁸ multichamber Fe₃O₄/Ti₃C₂T_x@rGO structures for broadband microwave absorption,⁵⁹ Fe₃O₄@Ti₃C₂T_x composites for antibiotic degradation,⁶⁰ covalently bonded Fe₃O₄-MXene hybrids for microwave absorption,⁶¹ and gradient-structured cellulose/MXene/Fe₃O₄ films for EMI shielding.⁵¹ Magnetic N-TiO₂/Fe₃O₄@MXene showed high photo-Fenton degradation capacity for phenol,⁶² while magnetically recyclable Fe₃O₄/Ti₃C₂ composites were developed for tetracycline adsorption.⁶³ Hierarchically porous Fe₃O₄/SiO₂/MXene/rGO⁶⁴ and Fe₃O₄/MoS₂/MXene⁶⁵ aerogels were prepared for microwave absorption. Magnetically enhanced photothermal antibacterial activity was observed for Fe₃O₄ grown in situ on MXene nanosheets with polydopamine coating.⁶⁶ Additionally, sandwich-type electrochemical biosensors based on Fe₃O₄–NH₂ and Ti₃C₂ MXene were developed for exosome detection with high sensitivity.⁶⁷

Most preparation methods involve at least a two-step procedure: separate synthesis of MXenes and nanoparticles, followed by their combination to obtain hybrid composites. In this work, we demonstrate that benchmark MXenes—Ti₃C₂, Nb₄C₃, and Nb₂C—can be directly decorated in situ with magnetic nanoparticles fabricated via laser ablation in liquid (LAL).⁶⁸ To our knowledge, only one report describes LAL-mediated Fe nanocluster formation on Ti₃C₂T_x for ammonia sensing.⁶⁹ LAL enables production of colloidally stable, ligand-free nanoparticles with tunable surface chemistry under ambient conditions.^70,71 Nanoparticles produced by LAL exhibit surface defects providing electrostatic stabilization, and the absence of surface ligands results in enhanced catalytic performance compared to chemically synthesized nanoparticles.⁷⁰

Here we propose synthesis of iron oxide (Fe_yO_z) nanoclusters on three types of MXenes (Ti₃C₂T_x, Nb₂CT_x and Nb₄C₃T_x). We characterized the resulting magnetic nanocomposites by applying scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), electrochemical performance in a plain buffer and an optical Faraday effect. We also investigated performance of hybrid nanocomposites for the reduction of H₂O₂.

Materials and methods

Reagents and materials

The phosphate buffer (PB) components (i.e. KH₂PO₄ and K₂HPO₄, pH 7.0), dimethyl sulfoxide (DMSO) were of ≥99% purity or p.a. grade and were purchased from Sigma-Aldrich (USA). 50% aq. HF was purchased from Fisher chemicals. The PB solution was freshly prepared in 0.055 µS ultrapure deionized water (DW). DW was obtained from Milli-Q® HX 7000 SD all-in-one system.

Synthesis of Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes

Ti₃C₂T_x MXene was prepared by selective Al etching by in situ-formed HF (reaction of LiF with HCl) using a previously described protocol.⁷² The multilayered (ML) Nb₄C₃T_x and Nb₂CT_x MXenes were prepared by selective Al etching from Nb₄AlC₃ and Nb₂AlC MAX phase, respectively, by hydrofluoric acid (HF) to remove Al layer as it was previously reported.^73,74 The stirred dispersions of Nb₂AlC or Nb₄AlC₃ powders immersed in 50% HF aqueous solution were heated at temperature of 40 °C for 96 h. After cooling, the resulted dispersion was thoroughly washed by DW six times and separated by centrifugation at 3500 rpm to obtain the multilayered-MXenes (ML-MXenes) as remaining residue from the supernatants. The ML-MXenes were finally washed using ethanol, and dried at 30 °C under argon atmosphere.

The subsequent delamination of ML-Nb₄C₃T_x was performed by intercalation and ultrasonic treatment at 60% amplitude using 750 W by Hielscher Ultrasonics instrument. The dispersion of ML-Nb₄C₃T_x (500 mg) in DMSO (10 mL) was stirred for 24 h. Then the residue was isolated by centrifugation followed by decantation of the supernatant. Finally, the delaminated product (DL) Nb₄C₃T_x was achieved by probe sonication (Hielscher Ultrasonics instrument)⁷⁵ of dispersion of ML-Nb₄C₃T_x (20 mg mL⁻¹) in DW at 20 °C in Ar atmosphere for 1 h. ML-Nb₂C MXene was similarly processed. The resulting solution was centrifuged at 5000 rpm for 10 min and decanted followed by freeze drying to get delaminated Nb₂CT_x or Nb₄C₃T_x nanosheets (DL-Nb₂CT_x or DL-Nb₄C₃T_x).

Preparation of MXene@Iron oxide nanoparticle composites by laser ablation in liquid

An amplified Ti:Sapphire system (COHERENT Legend DUO) produced pulse trains with a repetition rate of 3 kHz at central wavelength of 800 nm and with pulse energies of 2.7 mJ at 120 fs. The beam was routed to a cuvette containing 1 mL of the dispersion (deionized water + MXene) and focused by a lens with a focal length of 175 mm onto the Fe target (purchased from Goodfellow Cambridge Ltd.) immersed 1 mm below the surface of the dispersion. The peak laser density at the focus was thus on the order of 10¹⁴ W cm⁻². The Fe target was exposed to laser radiation for 30 s. Thus, the laser ablation in liquid proceeded at a rate of 3000 pulses per s. Note that we were dynamically removing water vapor from above the cuvette and thus improving the laser beam coupling onto target. The as prepared MXenes patterned by Fe oxides (Fe_yO_z) nanoparticles (FeNPs) were carefully centrifuged and rinsed three times with deionized water to remove the unattached iron particles composites and to obtain stable nanocomposites (MXene@FeNPs).

Instrumentation for characterization of MXenes and MXenes@Fe oxides nanoparticle composites

Scanning electron microscopy (SEM). The surface morphology and chemical composition of synthesized Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes and their composites were studied using a scanning electron microscope (JEOL JSM-7600F equipped with energy dispersive X-ray spectrometer, EDS from Oxford Instruments X-Max 50 mm²). Energy dispersive spectroscopy (EDS) was used to analyze the chemical composition with INCA analysis software. All SEM-EDS measurements were performed at an operating voltage of 15 kV with a 70 nm aperture. The standardized magnesium reference was used as the calibration element for optimization of chemical composition measurements.

Secondary ion mass spectrometry (SIMS). Secondary ion mass spectrometry (SIMS)^76,77 analysis was performed using a TOF-SIMS IV (ION-TOF, Muenster, Germany), a reflectron type of Time-of-Flight (TOF) mass spectrometer equipped with a Bi ion source providing data with a high sensitivity on the level of ppm–ppb. Pulsed 25 keV Bi⁺ were used as primary ions with ion current of 1.1 pA. SIMS experiments were performed in both positive and negative polarity with details provided in our previously published works.

X-ray photoelectron spectroscopy (XPS). XPS measurements were carried out on an ESCALAB 250Xi X-ray Photoelectron Spectrometer (Thermo Fisher Scientific) at a take-off angle of 90°. The system is equipped with a 500-mm Rowland circle monochromator with a microfocused Al Kα X-ray source. An X-ray beam with a power of 200 W (650 µm spot size) was used. The survey spectra were acquired with a pass energy of 50 eV and an energy step of 1 eV. High-resolution Fe 2p scans were acquired with a pass energy of 20 eV and an energy step of 0.1 eV. The base pressure in the analysis chamber was in the 10⁻⁹ mbar range. An electron flood gun was used to compensate for the charge accumulation on the surface. Data calibration, processing, and fitting were performed with Avantage software.

Electrochemical behaviour. The laboratory potentiostat/galvanostat Autolab PGSTAT302N with an impedimetric module (Ecochemie, Utrecht, The Netherlands) was employed to study electrochemical performance of MXenes and MXene composites. The three-electrode cell system consisting of glassy carbon working electrode (GCE, d = 3 mm) used as a working electrode), an Ag/AgCl/3 M KCl reference electrode and a counter Pt electrode (Bioanalytical systems, USA) was applied. The surface area of GCE was pretreated and aqueous dispersions of MXenes (3 mg mL⁻¹) were prepared, as we mentioned in our previous study.⁷⁶ All electrochemical measurements were run under Nova Software 1.10, and data acquired were evaluated using OriginPro 9.1.

Optical Faraday effect. Our approach closely follows already published approach.^78,79 A 640 nm laser diode module (CPS635, THORLABS) was used as the light source. The beam passed through a Glan-Taylor polarizer (GT-10, THORLABS) and subsequently through the core of a solenoid. Here the beam passed through a glass cuvette filled with the sample solution. The polarization state has been resolved by passing the beam through a combination of a Wolaston polarizer (WP10, THORLABS) and a balanced detector (modified PDB210A, THORLABS). The output from the detector was coupled to a lock-in amplifier (SRS830, Stanford Research Systems). We have sampled the response at the fundamental (153 Hz) and third-harmonic frequency (459 Hz). The solenoid was driven by the trigger output of the lock-in amplifier coupled to a power amplifier through a series capacitance and hence by observing the RC resonant condition (at 153 Hz). The in-core magnetic field was calibrated by means of a Teslameter (GS-100D2). We have been working at low magnetic fields in order to limit the thermal loads on the coil.

Note that at the applied MXene concentrations, scattering was severely limiting our measurements as the MXene fraction would quickly sediment to the bottom of the cuvette thus creating a concentration gradient. Hence, we have applied a following protocol. After each measurement, the sample was taken out of the solenoid core and inserted into a shaker for 1 min and subsequently reinserted into the core. As the typical measurement only takes a couple of seconds, this approach allowed us to obtain consistent results.

Results and discussion

Microscopic characterization

The SEM imaging was performed on pristine samples of Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes and subsequently on the in situ decorated samples (Fig. 1). The complex, multilayered structure of pristine Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes is shown in Fig. 1a, c and e. As confirmed by Fig. 1b, d and f all three samples had shown substantial coverage by iron oxide (Fe_yO_z) nanoparticles (FeNPs), thereby proving our approach. Moreover, we have evaluated the NP size distribution and obtained median diameters of 65 nm, 85 nm and 115 nm for Ti₃C₂T_x, Nb₄C₃T_x, Nb₂CT_x, respectively (see SI, Fig. S1–S3) and a relatively broad distribution function characteristic of LAL.⁶⁸ The 2D structure of MXenes was still preserved, after the nanocomposites were formed.

Fig. 1 Scanning electron micrographs showing the multilayered structure of MXenes, i.e. Ti₃C₂T_x (a), Nb₄C₃T_x (c), Nb₂CT_x (e) and MXene@FeNPs composites, i.e. Ti₃C₂T_x@FeNPs (b), Nb₄C₃T_x@FeNPs (d), Nb₂CT_x@FeNPs (f). The scale bar corresponds to 1 µm.

Secondary ion mass spectrometry characterization

The characteristic ions and molecules identified in the samples of Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes and MXenes@FeNPs composites were obtained in the form of mass spectra in both positive and negative polarities. Fig. S2, S4, S6, S8, S10 and S12 show the mass spectra in a positive polarity and the identified ions and fragments with the corresponding isotopic distribution for MXenes and MXenes@FeNPs composites. Fig. S3, S5, S7, S9, S11 and S13 show the mass spectra in a negative polarity with identified ions and fragments originating from MXenes and MXenes@FeNPs composites molecules, as well as the iron oxide NPs themselves. SIMS spectra show a plethora of iron oxides, including FeO⁺, Fe₂O⁺, Fe₃O₂⁺, Fe₃O₃⁺.

SIMS 2D distributions for the selected fragments of Ti₃C₂T_x@FeNPs, Nb₄C₃T_x@FeNPs and Nb₂CT_x@FeNPs composites in both polarities are shown in Fig. 2 and 3, and additional data is shown in the supplementary information file, Fig. S14–S19. Time-of-Flight-SIMS imaging was performed to evaluate the lateral distribution of Fe species. The 2D SIMS images show a clear spatial correlation between the transition metal signal (Ti⁺/Nb⁺) and characteristic Fe oxide fragments (e.g., FeO⁺, Fe₂O⁺, Fe₃O₃⁺). The images were recorded over area of 250 × 250 µm.

Fig. 2 SIMS 2D distribution for the selected fragments of Ti₃C₂T_x@FeNPs (upper row), Nb₄C₃T_x@FeNPs (middle row), Nb₂CT_x@FeNPs (lower row) in positive polarity.

Fig. 3 SIMS 2D distribution for the selected fragments of Ti₃C₂T_x@FeNPs (upper row), Nb₄C₃T_x@FeNPs (middle row), Nb₂CT_x@FeNPs (lower row) in negative polarity.

Fig. 2, upper row represents the 2D distribution of the selected species in positive polarity: Ti⁺, Fe⁺ and the sum of FeO⁺, FeOH⁺, Fe₂O⁺, Fe₃O₂⁺, Fe₃O₃⁺, Fe₃O₃H⁺ and Fe₃O₄H⁺ for Ti₃C₂T_x MXene@FeNPs. Fig. 2, middle row represents the 2D distribution of the selected species in positive polarity: Nb⁺, Fe⁺ and the sum of Fe₂O⁺, Fe₃O₃⁺ for Nb₄C₃T_x MXene@FeNPs. Fig. 2, lower row represents the 2D distribution of the selected species in positive polarity: Nb⁺, Fe₊ and sum of FeO⁺, FeOH⁺, Fe₃O⁺, Fe₃O₂⁺, Fe₃O₂⁺, Fe₃O₃⁺, Fe₃O₃H⁺ for Nb₂CT_x MXene@FeNPs.

Fig. 3, upper row represents the 2D distribution of the selected species in negative polarity: sum of TiO₂⁻, CTi⁻, TiF⁻ and OTiF⁻, sum of FeO⁻, FeOH⁻, FeO₂⁻, Fe₂O₃⁻, Fe₃O₄⁻ and Fe₄O₆⁻ and sum of TiFeO₃⁻, TiFeO₄⁻ and Ti₃Fe₃O₅⁻ for Ti₃C₂T_x MXene@FeNPs. Fig. 3, middle row represents the 2D distribution of the selected species in negative polarity: sum of NbO⁻, NbO₂⁻, NbO₂H⁻ and NbO₃⁻, sum of FeO⁻, FeOH⁻, FeO₂⁻ and Fe₃O₄⁻ and sum of NbFe₃O₅H₅⁻, Nb₂Fe₂O₃H₉⁻ and Nb₄FeO₃H₃⁻ for Nb₄C₃T_x MXene@FeNPs. Fig. 3, lower row represents the 2D distribution of the selected species in negative polarity: sum of NbO₂⁻ and NbO₃⁻, sum of FeO⁻, FeOH⁻, FeO₂⁻, Fe₂O₃⁻ and Fe₃O₄⁻ and sum of Nb₂FeO₆⁻, Nb₃FeOH₄⁻, Nb₄FeO₃H₃⁻ and Nb₃Fe₃O₆H₆⁻ for Nb₂CT_x MXene@FeNPs. Unlike in positive polarity, we were also able to identify the M_xFe_yO_z cluster ions.

The values of normalized intensities for identified ions in Ti₃C₂T_x, Nb₄C₃T_x and Nb₂CT_x MXenes with iron oxide nanoparticles in positive and negative polarity are summarized in Table S1.

In summary, the SIMS imaging shows a clear correlation between the distribution of the respective transition metal and the characteristic iron cations. Hence, it can be concluded that the original terminations at the transition metal site of the parent MXene are now predominantly occupied by the iron oxide nanoparticles. This can be understood in terms of MXenes possessing surface terminations such as oxygen (O), hydroxyl (OH), and fluorine (F) inherited from their synthesis from MAX phases and these further act as primary binding sites.^80,81 For instance, it has been shown that hydroxyl groups provide activated Ti sites that can strongly interact with metal ions (like Zn²⁺).⁸²

X-ray photoelectron spectroscopy (XPS) analysis

The formation of Fe nanoparticles (NPs) was further confirmed by XPS, a powerful surface-sensitive technique capable of identifying both elemental composition and oxidation states. As shown in Table 1, XPS analysis confirmed that the LAL method successfully produced MXenes enriched with Fe NPs. The highest efficiency was observed for Ti₃C₂, yielding a solution containing 15% Fe, of which 9% remained after water rinsing. For Nb₂C and Nb₄C₃ MXenes, the initial Fe NP contents were 9% and 5%, respectively, decreasing to 8% and 3% after rinsing.

Table 1 Relative fractions of Fe₃O₄, FeO, and Fe₂O₃ derived from Fe²⁺/Fe³⁺ ratios obtained by XPS for Fe-dopped MXenes (Ti₃C₂, Nb₂C, Nb₄C₃). The data highlight the influence of MXene type and rinsing on Fe oxide phase stability

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High-resolution XPS spectra were used to determine the Fe²⁺/Fe³⁺ ratios in the metal oxides.⁸³ Deconvolution of the Fe 2p₃/₂ peak revealed two main components: one at a binding energy (BE) of 710.9 eV corresponding to Fe²⁺, and another at 713.4 eV representing Fe³⁺ (Fig. 4). Relative areas of these components are summarized in Table 1. A satellite peak located 7.9 eV higher than the main Fe 2p_3/2 peak was also observed, providing a clear evidence for the presence of Fe²⁺, since Fe³⁺ does not exhibit a satellite peak. The relative fractions of Fe₃O₄, FeO, and Fe₂O₃ were estimated from the deconvoluted Fe 2p_3/2 spectra by quantifying Fe²⁺ and Fe³⁺ contributions (Table 1). This calculation assumed that all Fe is present exclusively in these three stoichiometric oxides. This assumption is supported by previous studies, which indicate that the LAL method frequently produces mixed FeO, Fe₂O₃, and Fe₃O₄.^84–86 Stoichiometric Fe₃O₄ can also be written as FeO·Fe₂O₃; therefore, the Fe²⁺/Fe³⁺ ratio was set to 1 : 2 (FeO contains only Fe²⁺, whereas Fe₂O₃ contains only Fe³⁺). A set of linear equations was established to relate the experimentally determined Fe²⁺/Fe³⁺ ratio to the contributions from each oxide phase. Solving these equations yielded the relative phase composition, providing a straightforward means of estimating the oxide distribution directly from XPS measurements.⁸³

Fig. 4 High-resolution Fe 2p XPS spectra of Ti₃C₂T_x@FeNPs (a), Nb₄C₃T_x@FeNPs (b) and Nb₂CT_x@FeNPs (c).

For Ti₃C₂ and Nb₂C, Fe₃O₄ constitutes 50% of the total Fe oxides regardless of rinsing, indicating high stability of the mixed-valence phase. In Ti₃C₂, rinsing decreases FeO (from 42% to 36%) while increasing Fe₂O₃ (from 8% to 14%), suggesting partial oxidation of Fe²⁺ to Fe³⁺. Conversely, Nb₂C exhibits a slight increase in FeO (from 40% to 45%) and a decrease in Fe₂O₃ upon rinsing, implying enhanced stabilization of Fe²⁺ species. For Nb₄C₃, however, a more pronounced change is observed. The Fe oxide distribution shifts from Fe₃O₄ = 55%, FeO = 35%, and Fe₂O₃ = 10% in the non-rinsed sample to Fe₃O₄ = 30%, FeO = 60%, and Fe₂O₃ = 10% after rinsing. The overall Fe content simultaneously decreases from 5 at% to 3 at%, indicating that rinsing leads primarily to the removal of weakly bound Fe₃O₄ nanoparticles rather than to chemical transformation of Fe species. The apparent increase in the Fe²⁺/Fe³⁺ ratio (70/30) also reflects the preferential leaching of Fe³⁺-containing phases.

These observations demonstrate that Fe oxide stability on MXene surfaces is composition-dependent. Ti₃C₂ and Nb₂C retain nearly constant oxide distributions upon rinsing, confirming strong surface adhesion and chemical stability of the deposited Fe phases. In contrast, Nb₄C₃ shows partial loss of Fe₃O₄ during rinsing, suggesting that, Fe nanoparticles are more weakly anchored to this substrate and more susceptible to removal in aqueous media.

Presence of three different types of Fe oxides on MXene i.e. magnetite (Fe₃O₄), Fe₂O₃ and FeO generated from Fe target using LAL is in an excellent agreement with the literature.⁸⁷

Based on SIMS analysis, the Fe-related species are distributed across the analysed surface without evidence of macroscopic segregation or localized clustering. These results indicate that Fe oxide nanoparticles are laterally distributed over the MXene surface at the micrometer scale. Although minor nanoscale variations cannot be excluded, the combined XPS and ToF-SIMS results demonstrate that the Fe loading is sufficiently homogeneous for reliable catalytic performance evaluation.

Electrochemical performance of pristine MXenes and MXene-based Fe nanocomposites

All types of MXenes exhibit a significant anodic peak, which decreases with a number of scans during CVs (Fig. 5b, d and f). While an anodic peak for Ti₃C₂T_x appears at a potential of 367 mV (Fig. 5b), for Nb₂CT_x it appears at a potential of 545 mV (Fig. 5d) and for Nb₄C₃T_x at a potential of 479 mV (Fig. 5f). Thus, while for oxidation of Ti₃C₂T_x the lowest potential is required for its oxidation such MXene material is the least electrochemically/redox stable. On the other hand, the highest potential of 545 mV was required for the oxidation of Nb₂CT_x; thus, Nb₂CT_x is the most electrochemically/redox-stable MXene among those investigated in the study, with Nb₄C₃T_x MXene being moderately stable. Electrochemical/redox stability in the order Nb₂CT_x > Nb₄C₃T_x > Ti₃C₂T_x is also confirmed by the kinetics of MXene oxidation with increasing number of scans, as shown in Fig. 6a.

Fig. 5 Electrochemical performance of Ti₃C₂T_x, Nb₂CT_x, and Nb₄C₃T_x MXenes at negative (a, c and e) or positive potentials (b, d and f). CVs were run in 0.1 M PB pH 7.0 at a sweep rate of 100 mV s⁻¹ with 20 scans collected. Anodic (plasmon) peaks are visible for all three MXenes i.e. Ti₃C₂T_x (b); Nb₂CT_x (d) and Nb₄C₃T_x (f).

Fig. 6 Stability of an anodic peak current (i.e. the highest anodic peak due to presence of surface plasmons) for Ti₃C₂T_x (current read at 367 mV), Nb₂CT_x (current read at 545 mV), and Nb₄C₃T_x (current read at 479 mV) MXenes during several cycles in CVs run in 0.1 M PB pH 7.0 at a sweep rate of 100 mV s⁻¹ (a). Correlation between the content of Fe within the MXene-based nanocomposites and an anodic (plasmon) peak current observed in the 1st scan of CVs (Fig. 5b, d and e) using all types of MXenes (b).

Interestingly, the Nb₂CT_x MXene exhibits rather wide oxidation peak, most likely due to two different redox processes at a potential of ∼310 mV and 545 mV (the main or maximal peak) (Fig. 5d), suggesting a more complex redox/electrochemical behaviour of Nb₂CT_x compared to Nb₄C₃T_x and Ti₃C₂T_x MXenes.

A decrease of an anodic peak with a number of scans during CVs was well described for Ti₃C₂T_x MXene⁷⁶ and was attributed to the presence of plasmons i.e. presence of free electrons and upon oxidation of Ti₃C₂T_x MXene, such plasmons are channels to the electrode, resulting in oxidized Ti₃C₂T_x MXene with significantly altered electrochemical performance.⁷⁶ Thus, Nb₂CT_x and Nb₄C₃T_x MXenes should not be exposed to conditions exceeding such an oxidation potential in order to not significantly alter electrochemical/redox performance of Nb-based MXenes, similarly to the case of Ti₃C₂T_x MXene. Decrease of an anodic peak for Nb₂CT_x and Nb₄C₃T_x MXenes was also described recently.⁸⁸

All of the explored MXenes have metallic like bands at E_F with large density of states that is due to Ti 3d bands in Ti₃C₂T_x and Nb 4d bands in the Nb based MXenes. Under normal conditions, they would all support collective electron oscillations and hence plasmons with the plasmon strength given by the following order: Nb₄C₃T_x > Nb₂CT_x > Ti₃C₂T_x. However, under strong oxygen and hydroxyl termination the order can be fully reversed. i.e. Ti₃C₂T_x > Nb₂CT_x > Nb₄C₃T_x. We are arguing this is indeed the case due to the following: (1) the order given by the plasmon response as identified from Fig. 5d, e and f and (2) SIMS data, specifically the images S18, S19 and Table S2.

It is worth mentioning that the electrochemical behavior of all types of MXenes is more stable in the cathodic potential window (Fig. 5a, c and e), with Nb-based MXenes being more stable compared to Ti₃C₂T_x MXene.

The redox/electrochemical behavior of Nb₄C₃T_x MXene in the cathodic potential window with a cathodic peak at ∼–700 mV and an anodic peak at a potential of ∼−530 mV (Fig. 5e) is in an excellent agreement with published data i.e. peaks at −700 mV or −490 mV, respectively, for Nb₃C₄T_x MXene.⁸⁹ Two redox peaks at approximately the same potential were present on Nb₂CT_x (Fig. 5c), but more hidden in the capacitive currents.

Since the current of an anodic (plasmon) peak represents density of plasmons present in MXenes it is reasonable to assume that the density of free electrons (plasmons) can have a detrimental effect on the Fe loading capacity on MXenes during LAL process. The rationale behind is electrostatic attraction of positively charged Fe cations generated during LAL process and negatively charged MXenes. We found that the current intensity of anodic (plasmon) peak observed on pristine MXenes is increasing with an increasing the amount of Fe species deposited on MXenes during formation of MXene nanocomposites (Fig. 6b). Thus, we can conclude that the density of free electrons (plasmons) within the pristine MXenes affects effectivity of Fe deposition during LAL on pristine MXenes and thus also Fe surface coverage on the MXene nanocomposites.

Although, plasmon density expressed as anodic (plasmon) peak current is significantly lower on Nb₂CT_x and Nb₄C₃T_x MXenes, it can be effectively enhanced by the procedure of MXene synthesis from MAX phases. In the past we showed that density of plasmons expressed as anodic (plasmon) peak current was significantly lower on to Ti₃C₂T_x MXene when prepared using HF etching compared to LiF + HCl etching.⁹⁰ Since Nb₂CT_x and Nb₄C₃T_x MXenes in this study were prepared by HF etching from their MAX phases, altered synthesis of Nb₂CT_x and Nb₄C₃T_x MXenes from their MAX phases (i.e. using LiF + HCl etchants) can enhance density of plasmons within Nb₂CT_x and Nb₄C₃T_x MXenes and thus also efficiency of Fe deposition on Nb₂CT_x and Nb₄C₃T_x MXenes using LAL.

It is noteworthly, that of an anodic peak at +550 mV on all nanocomposite electrodes (Fig. 7a, c and e), i.e. in an agreement for redox potential of Fe^2+/Fe³⁺ redox couple (770 mV vs. NHE i.e. 570 mV vs. Ag/AgCl).⁹¹ Thus, we can attribute this peak to the redox/electrochemical activity of Fe species deposited on MXenes during LAL process. We were tempted to correlate the peak current observed at +550 mV on the nanocomposites with the electrochemical ability of nanocomposites to electrochemically reduce hydrogen peroxide by the electrochemically active Fe species present on the nanocomposites. When the peak current observed at +550 mV on nanocomposites was correlated to current observed for H₂O₂ reduction using nanocomposites (i.e. read at potential of −1000 mV) a good correlation was observed with R² = 0.990 (Fig. 8). Thus, the intensity of the peak at 550 mV can be attributed to the density of electrochemically active Fe species responsible for H₂O₂ reduction.

Fig. 7 Electrochemical behaviour of Ti₃C₂T_x, Nb₂CT_x, and Nb₄C₃T_x MXene-based nanocomposites with deposited Fe species in 0.1 PB pH 7.0 run at a sweep rate of 100 mV s⁻¹ (a, c and e). Electrochemical ability of Ti₃C₂T_x, Nb₂CT_x, and Nb₄C₃T_x MXene-based nanocomposites with deposited Fe species to reduce hydrogen peroxide run in 0.1 PB pH 7.0 run at a sweep rate of 100 mV s⁻¹ (b, d and f).

Fig. 8 Correlation between the electrocatalytic peak current observed for electrochemical reduction of hydrogen peroxide read at −1000 mV from Fig. 7b, d and f and an anodic peak observed at +550 mV on the nanocoposites (read from Fig. 7a, c and e) with R² = 0.990.

The peak currents obtained at three different MXene-based nanocomposites can be converted to current densities using geometric surface area of 7.1 mm^2 90 as follows: 4399 µA cm⁻² (Ti₃C₂T_x), 434 µA cm⁻² (Nb₂CT_x) and 782 µA cm⁻² (Nb₃C₄T_x). The value of current density of 4399 µA cm⁻² for H₂O₂ reduction obtained using Ti₃C₂T_x MXene-based nanocomposite is significantly higher compared to the value of current density of 2020 µA cm⁻² for H₂O₂ reduction obtained using pristine Ti₃C₂T_x MXene.⁹⁰ Thus, LAL deposition of Fe species on MXenes is a promising way to achieve highly electrochemically active nanocomposites for H₂O₂ reduction using non-precious metals. We assume that synthesis of Nb₂CT_x and Nb₄C₃T_x MXenes using LiF + HCl synthetic route can enhance density of Fe species deposited on such MXenes and also increase density of electrochemically active Fe species, responsible for efficient H₂O₂ reduction.

Optical Faraday effect

Optical Faraday effect can be considered a benchmark technique for characterization of (optically) transparent magnetically active materials, see supplementary information, Fig. S20. Here, sample response at slowly alternating magnetic fields at a frequency of 153 Hz was observed. We started by exploring the Faraday response of LAL-prepared NP solutions in water with no MXenes added, see supplementary information, Fig. S21. They show a regular response well within the model described by Patterson et al.⁷⁸ developed for characterization of superparamagnetic NPs.

Next, we have switched to sample solutions containing nanocomposites. The obtained Faraday response is depicted in Fig. 9. All three MXenes under study do show magnetic properties but none of them qualitatively resembles the response of pure NPs. Most strikingly, while the NP-decorated Ti₃C₂T_x and Nb₂CT_x both show a monotonically rising Faraday response the Nb₄C₃T_x shows an activated response with the polarization rotation angle being negligible up until approximately 2.2–2.5 mT after which we do observe a steep rise.

Fig. 9 Faraday optical response of the Ti₃C₂T_x@FeNPs, Nb₂CT_x@FeNPs, and Nb₄C₃T_x@FeNPs nanocomposites. The solid lines are only to guide an eye.

Also note that of the three MXenes under study the Ti₃C₂ exhibited the strongest Faraday response within the experimentally attainable range of 0–4 mT. Given the similar coverage of all three MXenes by NPs the difference cannot be simply explained by higher NP concentration for the given sample. Direct comparison between XPS data given in Table 1 and the Faraday response shows that, at low magnetic fields, up to approximately 2.5 mT, the signal magnitude for respective MXene samples follows the ordering given by either Fe or Fe₃O₄ content, i.e. Ti₃C₂T_x ≥ Nb₂CT_x > Nb₄C₃T_x. Consequently, we may speculate that at low enough mag. fields, the response is still linear and representing the actual Fe loading, while at higher fields we are entering a nonlinear regime and a direct correspondence cannot be drawn anymore.

In fact, a quantitative modeling would be needed in order to get a deeper insight into observed phenomenology. Note that our main goal here was to prove that the composite system MXene + NPs would inherit/retain magnetic properties of the NPs and not to provide a thorough quantitative analysis.

Conclusions

We have introduced a novel method for synthesis of MXene-based Fe nanocomposites using LAL allowing to prepare Fe-based nanoclusters on the surface of MXenes without any ligands, which are needed during deposition of nanoclusters on surfaces using traditional ways. The coverage of Fe on the MXene surfaces varied in the range from 5 to 15 at% of Fe depending on MXene used for deposition. Interestingly we showed that the current of plasmon peak representing density of free plasmons observed on pristine MXenes is the main driving force besides deposition of Fe species on MXenes. Furthermore, we observed that the anodic peak at +550 mV on MXene-based Fe nanocomposites represent electrochemically active Fe species with intensity of such anodic peak at +550 mV in a direct correlation with electrochemical activity of MXene-based Fe nanocomposites towards H₂O₂ reduction. The highest current density of electrochemical reduction of H₂O₂ was observed on Ti₃C₂T_x MXene-based Fe nanocomposite with a value of 4399 µA cm⁻². We also proposed that significantly higher electrochemical reduction of H₂O₂ could be observed also on Nb₂CT_x and Nb₄C₃T_x MXene-based Fe nanocomposites, when using HF + HCl method for synthesis of MXenes from MAX phases. Furthermore, we have proven that the MXene-based Fe nanocomposites are magnetically active with potential to use them for some separation processes. Moreover, presence of plasmons within Nb₂CT_x and Nb₄C₃T_x MXenes could be used for spontaneous grafting of (bio)molecules using diazonium grafting-based process as was described in our earlier work on Ti₃C₂T_x MXene.⁹² Finally, it may be of great merit to test MXene-based nanocomposites using different nanoclusters deposited on MXenes using LAL for other specific purposes such as glycan enrichment and such study are underway in our laboratories.⁹³

Author contributions

Lenka Lorencová – writing – original draft, methodology, investigation, formal analysis, conceptualization. Eva Noskovičová – investigation. Monika Jerigová – investigation. Monika Stupavská – writing – original draft, investigation, formal analysis. Ľubomír Orovčík – investigation. Lucia Kopčanová – investigation. Peter Kasák – investigation. Andrej Vikartovský – investigation. Dušan Lorenc – investigation, writing – original draft. Khaled A. Mahmoudi – investigation. Ján Tkáč – writing – original draft, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI) i.e. the file “Supporting information file – “raw data.xlsx”. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra00426a.

Acknowledgements

The authors would like to acknowledge the financial support received from the projects of Slovak Research and Development Agency (APVV-22-0345 and APVV-23-0083). This work was jointly supported by Qatar University and Institute of Chemistry, Slovak Academy of Sciences-IRCC-2024-429. The findings achieved herein are solely the responsibility of the authors. The work was funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project no. 09105-03-V02-00070. This research has been also supported by the project LM2023039 funded by the Ministry of Education, Youth and Sports of the Czech Republic.

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