Near‐Atomic‐Scale Perspective on the Oxidation of Ti3C2Tx MXenes: Insights from Atom Probe Tomography

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties, bearing great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography (APT) analysis of as‐synthesized Ti3C2Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalis are enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of APT to image these elements in 3D at the near‐atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

The discovery of MXenes by the Barsoum and Gogotsi groups in 2011 has reignited scientific interest in 2D and layered materials [1].To date, numerous MXenes have been synthesized [2], and are being intensively studied and tested in applications including supercapacitors and batteries [3,4], optoelectronics [5], catalysis [6], and biosensors or antibacterial membranes [7,8].Their properties and performance in various applications are significantly affected by unavoidable surface terminations, denoted by T x in the chemical formula, which saturate the bare MXene surface during synthesis [9].Changes in the electronic properties of the MXenes, for example, have been explained by careful consideration of these functional groups and possible intercalants originating from wet chemical synthesis [10].Using them to fine-tune properties would require establishing relationships between activity and their spatial distribution and concentration, which remain extremely difficult to characterize.
Atom Probe Tomography (APT) is one of the few techniques that can measure composition in 3D at the near-atomic scale [11].In recent years, the application of APT to nanoparticles [12][13][14], nanowires [15,16] and nanosheets [17] has triggered a reckoning that the detailed composition and the presence of trace impurities are of paramount importance when it comes to understanding the activity of nanomaterials [18].Impurity elements in the raw materials used during the synthesis may not react at the same rate and hence be preferentially incorporated into the final product [17], or may be incorporated into the structure during processing or synthesis by wet chemistry [19,20].For MXenes and their surface terminations, most studies to date have relied on X-ray spectroscopy and electron microscopy techniques for compositional measurements [21][22][23][24][25], leaving many questions unanswered that could be potentially be addressed by using APT.However, APT has never been applied to MXenes before.
As the structural and chemical stability of MXenes can also be extremely fragile, APT could also be a tool for better understanding the mechanisms behind the degradation.MXenes degrade particularly fast in an oxidizing environment (H 2 O, O 2 , etc.), leading to their transformation into transition metal oxides [26,27].Oxidation, in its early stage, can affect the surface terminations [28] and thus the properties.For example, an increased concentration of O functional groups results in an increased catalytic activity [29].Several studies have demonstrated how further oxidation severely destroys the integrity of the MXene structure, thereby limiting its lifetime in service and precluding many useful applications due to a decrease in the properties of interest [30][31][32][33].Although efforts have been made to understand the details of their evolving chemistry during oxidation [34,35], some questions are still unanswered, such as the role of non-O surface terminations, intercalants or impurities from synthesis.Designing MXenes with improved oxidation resistance is therefore extremely challenging, and for this reason the improvement of chemical and temperature stability of MXenes is considered in the community to be one of the research challenges of this decade [36].
In order to further the understanding of the chemical evolution of colloidal Ti 3 C 2 T x MXene solution, we introduce here the first APT analyses of as-synthesized MXenes, alongside with the transition metal oxides formed during oxidation.APT revealed the incorporation of alkali (Li, Na) and halogen (Cl, F) elements into the MXene nanosheets, probably inherited from the synthesis.Following oxidation, these elements remained within the newly formed TiO 2 nanowires, indicating that they lowered their free energy and helped to stabilize them.Although most of these elements are considered to be an essential part of the MXenes themselves, these results emphasize that discussions of the activity of MXene-based materials, including throughout oxidation, should take account of them, which are likely to play an important role in the activity [37] and their resistance to degradation.Ti 3 C 2 T x MXenes were synthesized by wet chemical etching of the Al layer from a Ti 3 AlC 2 MAX phase via the HCl-LiF route (synthesis of both materials is described in detail in the Supporting Information).In Figure 1 (a), the as-synthesized MXenes shows very clean edges and a defect-free surface, suggesting no major internal defects.Small black dots on the edges may indicate the formation of oxidation products immediately after synthesis.To observe the oxidation of the nanosheets with electron microscopy, the colloidal Ti 3 C 2 T x MXene solution was kept at a temperature below 5 • C throughout the oxidation experiment, as described in Ref. [38].
Oxidation of the nanosheets in the colloidal solution may have started from the edges due to local structural variations [39], where nanoparticles were observed in Figure 1 (b).Previous research has reported the formation of carbon-supported anatase during the oxidation of Ti 3 C 2 T x MXenes [40], indicating that the nanoparticles formed are TiO 2 .Because atomic vacancies and microstructural defects such as wrinkles are unavoidable in the course of the wet chemical synthesis processes, these preexisting defects on the Ti 3 C 2 T x MXene may have created a local electric field that drives the development of the TiO 2 particles by promoting the migration of both Ti cations and electrons [41].These pinhole defects then gradually evolved into larger voids, resulting in the degradation and accelerated oxidation of the MXene structure.Zhang et al. [42] reported that oxidation starts at the defective edges of the MXene sheets and propagates inwards, causing cracks to nucleate and grow.This so-called 'scissor effect' shreds the MXene sheets into small pieces and leads to a complete loss of the original structure of the MXene into TiO 2 debris, such as those imaged in Figure 1 (c).
Dense and sharp needle-shaped APT specimens containing either the as-synthesized or the oxidized Ti 3 C 2 T x MXenes were prepared from a nanosheet-metal composite (detailed APT specimen preparation is provided in the Supporting Information).The electrodeposition within a metallic matrix, used as an encapsulating material, increases the success rate and data quality [43,44].Here, a Co matrix was used, which, compared to the more commonly used Ni, presents the advantage of having only a single isotope, making it less likely to create peaks that can obscure the signal from the material of interest in the APT mass spectrum.For example, the use of Ni would cause an unavoidable peak overlap with TiO molecular ions in the APT mass spectrum [16].
The reconstructed 3D atom map in Figure 2 (a) shows the Ti 3 C 2 T x MXenes nanosheets with a complex 2D morphology, likely arising from agglomerated or folded nanosheets.These are embedded in Co, shown in yellow.Differences in the evaporation field between the matrix and the nanosheets lead to an irregular curvature of the end surface of the specimen, which results in trajectory aberrations, varying magnification and intermixing zones in the reconstruction [45].An indicative thickness at full width at half maximum of 3.2 nm was obtained by evaluating the Co 1D composition profile in Figure 2 (b).The nominal thickness of a Ti 3 C 2 T x monolayer has been reported to be within the order of 1 nm to 1.5 nm [46,47], whereas the interlayer distance between two separate nanosheets can vary depending on the intercalated species [48].A single nanosheet or stack of up to three Ti 3 C 2 T x nanosheets was hence analyzed.A region of interest, delineated by the dark green-cyan iso-composition surface, encompassing regions in the 3D atom map containing over 6 at.%Ti, in Figure 2 (a), was extracted to exclude the Co matrix and any CoO layer from electrodeposition for compositional analysis.Compared to the nominal value of 1.5, the atomic ratio of Ti to C was measured to be 2.2.Losses of C have been reported in the APT analysis of carbides [49][50][51], depending on the analysis conditions, and partly due to detector saturation associated with two 12 C + ions [52].Importantly, it has been questioned whether most MAX phases and their derived MXenes are actually pure carbides or rather oxycarbides [53].Pure carbides may be more resistant to oxidation than oxycarbides, as recent results suggest [53,54].
The specimen was analyzed immediately after synthesis; however, a significant amount of O was measured in the material, approximately 46.7 at.%.The presence of O can be attributed to synthesis in an aqueous solution, as the highly reactive surface is immediately bonded to, for example, O or OH.Previous reports have shown that under certain controlled high vacuum conditions in the transmission electron microscope the O surface terminations can reach a supersaturation level of 41 at.% while retaining the Ti 3 C 2 T x MXene nanosheet structure [28], and the MXenes directly used from the solution were unlikely to be already oxidized MXenes.However, the measured amount of O was not entirely attributable to O or OH surface terminations, but was also associated with weakly bonded water molecules to these surface terminations.Besides ion intercalation, intercalation of water molecules is a crucial factor for the aqueous MXene delamination [55].Intrinsically hygroscopic cations, such as Li + and Na + , which were also detected as discussed below, also increase the amount of intercalated water molecules [56].Characteristic peaks in the mass spectrum at 17, 18, and 19 Da confirmed the evaporation of H 1 -3 O + molecular ions, while peaks at 81, 82, 83, 84, and 85 Da, partially overlapping with peaks from TiO 2 + , were attributed to the evaporation of TiO(OH 3 ) + .Quantification of the H content was deliberately omitted, as it is known to be extremely challenging from APT to distinguish between H from the analysis chamber and the sample in form of OH surface terminations or intercalated water molecules itself [57].In the future, isotopic labeling using heavy water (D 2 O) during MXene synthesis may clarify the controversial question of the intrinsic existence of OH terminations on the MXene surface [21,58,59].
Not entirely unexpected, several functional surface terminations, intercalated elements, and impurities including Cl, F, Li, Na, and Al were also incorporated and could be quantified within the extracted region of interest of the Ti 3 C 2 T x MXenes.The incorporation of Cl and F results from the wet chemical etching process of Al from the precursor MAX phase and the synthesis of delaminated MXenes.High concentrations of HCl and LiF are used in the wet chemical synthesis [60,61].Here, the Cl content was measured to be 2.49 at.%. Accurate quantification of the amount of F was complicated by the overlapping peaks of F + and H 3 O + in the mass spectrum region at 19 Da.However, TiF 2+ molecular ions were measured (see the mass spectrum in the Supporting Information), so that the F content could be quantified to at least 1.16 at.%. Tuning the termination groups on the surface of the MXene may influence its oxidation stability, for instance, Cl-terminated MXenes have been shown to have higher stability than F-terminated MXenes [62].
Despite vigorous washing protocols, residuals of Li and Na elements were still present in the analyzed material at 0.02 at.% and 0.11 at.%, respectively.The amount of Na was obtained by means of a peak decomposition with the overlapping peak of 46 Ti 2+ at 23 Da.Na likely originated as an impurity in the synthesis chemicals, as was similarly observed for MoS 2 synthesized by wet chemistry [17].For example, Na is a known impurity even in high-purity Li salts for battery applications [63,64], and here LiF (99 %) was used during synthesis.It should be noted that the spatial resolution of APT [65] does not allow to conclude whether these alkali impurities are on the surface or integrated into the structure of a single layer MXene.Due to electrostatic effects, these elements could be absorbed on the surface to stabilize it [66], or they could spontaneously intercalate between two MXene layers [67], as is even desired in the case of Li in the synthesis route used here.The presence of Li or Na atoms on the surface may also directly impact the properties by intercalation doping [68], which should grant further targeted investigations.
The complete removal of residual Al atoms is possible by harsh HF etching, often confirmed by X-ray (photoelectron) spectroscopy [69].Although the wet etching environment removes the Al layers within the MAX phase by mixing HCl and LiF to form in situ HF, and the desired MXenes were rinsed seven times with deionized water, a trace amount of Al residual atoms at 0.17 at.% was still measured within the Ti 3 C 2 T x MXenes.
Figure 3 (a) shows the reconstructed 3D atom map of the MXene oxidized in the colloidal solution.The structures highlighted by red iso-compositional surfaces of 0.5 at.%TiO 2 are similar to those imaged by scanning transmission electron microscopy in Figure 1 (c), which have a nanowire-like morphology.The atomic ratio of Ti to O within the extracted oxide particle, indicated by the black box, was measured to be 0.54, which is close to the stoichiometric ratio of TiO 2 .Changes in the acquisition parameters in APT [70] or a slight O deficiency could be responsible for the off-stoichiometry of TiO 2 .Figure 3 (b) is a close-up of one of these nanowire-like structures with distribution maps of the individually detected elements.The measured impurity contents of C, Cl, F, Li, and Na were 2.09, 2.58, 1.62, 0.79, and 0.71 at.%, respectively.54 ± 30 atomic parts per million of Al was collected in the oxidized MXene.Overall, the oxidation reaction of the MXene led to an enrichment of alkali elements and a depletion of unetched Al, as visualized in Figure 4. Oxidation of Al to Al 2 O 3 [71] may be responsible for the depletion of Al, as the Al 2 O 3 and TiO 2 phases have a positive mixing enthalpy [72], making the incorporation of Al in the TiO 2 phase unfavorable [73].However, fundamental atomistic calculations comparing the stability of the incorporated alkali elements in the MXene and the oxide are missing.While the incorporation of Li into anatase TiO 2 is energetically feasible in general [74], Na is a known impurity of TiO 2 synthesized on a soda-lime glass substrate [75].Since several Li [76] and Na titanate derivatives [77][78][79][80] were synthesized by oxidative treatment of Ti 3 C 2 T x MXenes with these alkali elements, it is reasonable to assume that they promote the formation of the oxide and stabilize it by lowering the free energy.
Because metal-semiconductor heterostructures can provide rapid separation of photogenerated charge carriers [81], Ti 3 C 2 T x MXenes were intentionally oxidized to form Ti 3 C 2 T x /TiO 2 derivatives for photocatalysis [82][83][84][85].Other promising applications for these derivatives include electrodes for supercapacitors [86] and lithium-ion batteries [87].However, in our previous report on TiO 2 hollow nanowires, it was pointed out that the lack of precise characterization of impurities has an impact on the laboratory-synthesized TiO 2 properties, which vary and are inconsistent with reported results [16].For example, incorporated Na influences the growth and crystallization of TiO 2 , and thus the resulting properties [75].From this point of view together with the observations made in this study, the alkali elements may have a significant influence on the oxidation mechanism and its kinetics, and consequently also on the properties of the Ti 3 C 2 T x /TiO 2 derivatives.For easier comparability of the data, the compositions were normalized to one Ti (data given in at.% are provided in the Supporting Information).
In addition, the detected elements may also work as a dopant for both the as-synthesized Ti 3 C 2 T x MXenes and oxidized TiO 2 .Doping is a proven and effective engineering strategy to enhance the performance of TiO 2 , particularly with alkalies and halogens.For instance, Li-doped TiO 2 has shown significantly improved properties by increasing electronic conductivity, and thus faster electron transport [88].Density functional theory calculations predicted an influence of Cl-and F-doping on the band gap of TiO 2 [89] and incorporated Cl shifts the absorption edge to a higher wavelength [90], both effects enabling excellent photocatalytic activity.Doping TiO 2 with C can effectively facilitate photo-generated charge transfer and prevent electron-hole recombination [91].Surface terminations such as halogens are likely to influence the band structure, as functionalization of Ti 3 C 2 T x MXenes with halogens could lead to the formation of Dirac cones near the Fermi level, resulting in semi-metallic behavior [92].Experimentally, it was observed that preintercalated Na leads to a reduced diffusion barrier for Na ions and an increased number of active sites due to the increased interlayer spacing of the MXenes [93].In summary, both alkali and halogen elements can affect the functional properties of Ti 3 C 2 T x MXenes and the oxidized TiO 2 .It is therefore essential to study these incorporated elements on the near-atomic scale, for example using APT.
It has been recognized that the methods used to synthesize high-quality MXenes with chemical and structural stability can be critical.We have demonstrated the capability of APT to study the incorporation of alkalies and halogens as surface terminations, intercalated ions, or impurities introduced by the wet chemical synthesis of the MXenes.The alkali elements tended to further concentrate during the oxidation of the Ti 3 C 2 T x MXenes to TiO 2 nanowires.Their presence may affect the physical properties of the MXenes themselves, but, importantly, this suggests that the presence and concentration of these elements may contribute to the oxidation mechanism and kinetics, by stabilizing the oxide to the detriment of the MXene itself.In the future, a broader understanding of the influence of these elements may enable targeted utilization to tailor the functional properties of the MXenes and their derived transition metal oxides.

Table of Contents
Following oxidation of Ti 3 C 2 T x MXenes, atom probe tomography reveals that alkalies concentrate in TiO 2 nanowires, indicating that their presence influences the oxidation mechanisms and kinetics.These results highlight how atom probe tomography can be utilized to better understand the functional surface terminations, intercalated ions or impurities that are inevitable in the wet chemical synthesis of MXenes.

Figure 1 :
Figure 1: Oxidation processes of the Ti 3 C 2 T x MXenes.(a) As-synthesized Ti 3 C 2 T x MXene nanosheets.(b) Oxidized Ti 3 C 2 T x MXene after storage in a colloidal solution in a refrigerator.(c) TiO 2 nanoparticles as products of Ti 3 C 2 T x MXene oxidation.

Figure 2 :
Figure 2: APT analysis of as-synthesized Ti 3 C 2 T x MXenes.(a) Reconstructed 3D atom map.Agglomerated Ti 3 C 2 T x MXene nanosheets are highlighted by a dark green-cyan iso-compositional surface at 6 at.%Ti and a reddishpurple iso-compositional surface at 4 at.%C. (b) 1D compositional profile (∅15 nm x 15 nm) across the agglomerated MXenes as indicated in (a).

Figure 3 :
Figure 3: APT analysis of oxidized Ti 3 C 2 T x MXenes.(a) Reconstructed 3D atom map.TiO 2 nanowires are highlighted by red iso-compositional surfaces at 0.5 at.%.(b) Molecular distribution map of TiO 2 and elemental distribution maps of impurity elements in the extracted region of interest indicated in (a).

Figure 4 :
Figure 4: Comparison of the composition of the halogens, alkalies, and Al in the assynthesized Ti 3 C 2 T x MXenes and the oxidized TiO 2 .For easier comparability of the data, the compositions were normalized to one Ti (data given in at.% are provided in the Supporting Information).