High-Temperature Behavior and Surface Chemistry of Carbide

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High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis Mykola Seredych, Christopher Eugene Shuck, David Pinto, Mohamed Alhabeb, Eliot Precetti, Grayson Deysher, Babak Anasori, Narendra Kurra, and Yury Gogotsi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00397 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Chemistry of Materials

High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis

Mykola Seredych, Christopher Eugene Shuck, David Pinto, Mohamed Alhabeb, Eliot Precetti, Grayson Deysher, Babak Anasori, Narendra Kurra, and Yury Gogotsi*

A.J. Drexel Nanomaterials Institute, and Department of Material Science and Engineering, Drexel University, Philadelphia, PA 19104, USA

*Corresponding author. E-mail address: [email protected] (Y. Gogotsi)

ABSTRACT Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have attracted significant attention due to their electronic, electrochemical, chemical, and optical properties. However, understanding of their thermal stability is still lacking. To date, MXenes are synthesized via topdown wet chemical etching, which intrinsically results in surface terminations. Here, we provide detailed insight on the surface terminations of three carbide MXenes (Ti3C2Tx, Mo2CTx and Nb2CTx) by performing thermal gravimetric with mass spectrometry analysis (TA-MS) up to 1500 ºC under He atmosphere. This specific technique enables probing surface terminations including hydroxyl, oxygen and fluoride –OH/=O/–F; intercalated species, such as salts and structural water. The MXene hydrophilicity depends on the type of etching (hydrofluoric acid concentration and/or mixed acid composition) and subsequent delamination conditions. We show that the amount of structural water in Ti3C2Tx increases with decreasing O-containing surface terminations. The 1 ACS Paragon Plus Environment

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thermal stability of Ti3C2Tx is improved by employing low HF concentration or using a mixture of etchant acids, such as H2SO4/HF or HCl/HF instead of only HF, due to the reduced defect density. When tetramethylammonium hydroxide (TMAOH) is used for delamination, new N-containing species appear on the MXene surface. Moreover, free-standing films produced from Ti3C2Tx etched with different HF concentrations and delaminated using TMAOH have similar TA-MS profiles, indicating that post-treatment of Ti3C2Tx controls its surface chemistry. The thermal stability of MXenes strongly depends on their chemical composition and structure; Ti3C2Tx is more thermally stable than the fewer-atomic-layered Mo2CTx or Nb2CTx and Mo2CTx is more/less thermally stable than Nb2CTx.

INTRODUCTION Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have recently attracted significant attention, due to their unique electronic structure and diverse physicochemical properties compared to bulk cubic carbide and nitride counterparts.1 MXenes were first produced at Drexel University in 20111 by selective chemical etching of the A-group atomic layers from bulk MAX phase materials. MAX phases are a class of layered ternary transition metal carbides or nitrides, with the general formula: Mn+1AXn, where M is an early transition metal (Ti, Nb, Ta, V, Mo, etc.), A is an A-group element (mostly group IIIA and IVA, including Al, Si, S, P, etc.), X is C and/or N, and n = 1, 2 or 3.2-4 The general chemical formula of MXenes is written as Mn+1XnTx,2 where M and X are the same as in the MAX phase, and Tx refers to the surface functional groups. The latter, which can be fluoride (–F), oxy (=O), or hydroxyl (–OH) groups, exist on MXenes as a result of the topochemical conversion from MAX to MXene, usually conducted in fluoride-containing aqueous solutions.2,5 MXenes combine the unique properties of

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hydrophilicity with metallic conductivity and some even exhibit remarkably high electrical conductivity up to 10,000 S/cm.6-9 About 30 different MXenes have been synthesized, with several additional varieties predicted computationally, rendering them among the most diverse, tunable, and fastest growing families of 2D materials.2 MXenes have already shown promising performance in many applications including energy storage,10-14 electromagnetic interference shielding,15,16 antennas,9,16 water desalination,17 photothermal therapy,18,19 and optoelectronics.20 However, their surface chemistry has been shown to depend on the synthesis conditions,21 which consequently affects the processing and properties of MXenes.22-25 Moreover, the surface terminations are often key for tuning the material properties further, and are specifically suitable for heterogeneous catalysis, electrochemical energy storage, and formation of composites, among other applications.2,26 For example, MXene-polymer composites can be developed by removing water and –OH groups from MXenes (in the case of hydrophobic polymers). In the context of energy storage applications, surface functional groups are beneficial for aqueous supercapacitors, while non-terminated surface sites are preferred for battery electrodes to suppress parasitic reactions from the functional groups.27 Therefore, understanding MXene surface terminations and their stabilities is crucial for tailoring these materials for specific applications. Previous studies have shown that the composition of surface terminal groups is dependent on synthesis and storage conditions. For example, using nuclear magnetic resonance spectroscopy, it was shown that the etching conditions affect the surface terminations, with 48-51 wt.% HF synthesis leading to 8 at.% OH, 55 at.% F, and 37 at.% O, while LiF-HCl etching led to 5 at.% OH, 22 at.% F and 73 at.% O.21 In another study, based on an atomic pair distribution function analysis (neutron scattering), the composition of functional groups was determined to be



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Ti3C2(O0.13OH1.04F0.83) and Ti3C2(O0.1OH0.8F1.1) for samples synthesized using 10 wt.% and 48 wt.% HF, respectively.28 It was also shown, using X-ray photoelectron spectroscopy, that the MXene surface composition depends on aging and post-treatment conditions.29 Transition metal carbides and nitrides in the bulk state are thermally stable compounds with melting points or decomposition temperatures often exceeding 2000 – 3000°C.30,31 It is imperative to understand the nature of the thermal stability and decomposition of 2D MXenes and their surface terminations.3234

Controlling physicochemical properties through surface chemistry modifications35-37 is key for

a variety of potential applications.23,24,38,39 Previous theoretical work has predicted terminationinduced transitions from metallic to semiconducting behavior in Ti2CO240 and magnetic properties that are dependent on surface chemistry.41 The stability of MXenes at higher temperatures gives an upper boundary for fabrication without structural degradation, which is especially important for ceramic42 and metal-matrix composites. Also, transition metal carbides are used in high temperature applications31 and the limits for MXene use at elevated temperature are largely unknown. In this paper, we describe the thermal stability of three MXenes (Ti3C2Tx, Nb2CTx, and Mo2CTx) and determine their surface moieties using simultaneous thermal gravimetric analysis (TA) with mass spectrometry (MS) from ambient temperature to 1500°C in He. We also discuss the effect of the MXene synthesis route, specifically etchant type and concentration, on the surface functional groups.

RESULTS AND DISCUSSION To study the effect of wet-chemical etching condition on Ti3C2Tx thermal properties and surface functionalizations, we employed different etchant compositions, varying the concentration of hydrofluoric acid (5-30 wt.% HF) or by mixing HF with a secondary acid (hydrochloric acid


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(HCl) or sulfuric acid, (H2SO4)). The etching and delamination protocols are illustrated in Figure 1a. The surface chemistries vary depending on the nature and concentration of acids used during etching.

Figure 1. a) Atomic view of the Ti3AlC2 MAX phase, etching was performed with three different etchants HF, HF/HCl, or HF/H2SO4, leading to multilayer (ML) Ti3C2Tx MXene; b) Schematic of multilayer MXenes with various surface terminations and their thermal stability pathways. The examples are given for Ti3C2Tx. The hydrophilic properties of MXenes promote the formation of hydrogen bonds between their hydroxyl groups and water. The surface functionality of MXenes can be controlled by annealing. Beyond 850°C, partial oxidation and phase transformation of MXenes takes place under He environment. Though there is no external O2 supply in the system, oxidation is caused by the reactive forms of oxygen, such as hydroxyl radicals (HO) and superoxide anions (O2-) generated during heat treatment.

The effect of post-treatment was analyzed (Figure 1b) by comparing multilayer (ML) MXene powders and free-standing films after delamination of ML MXene. As shown in Figure 5 ACS Paragon Plus Environment


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1b, heat treatment above a critical temperature of phase transformation, which can be between 750 to 900°C, depending on the etching process (~850°C on the schematic), results in chemical and structural transformation, while below the critical temperature results in modification of the surface chemistry and thermal decomposition of functional groups. A list of all samples studied and their synthesis conditions is provided in Table S1. Ti3C2Tx etched in HF of different concentration. The results of TG-MS analysis of Ti3C2Tx ML MXene powder etched with 5, 10, and 30 wt.% HF and vacuum-dried at 55ºC are shown in Figure 2. Ti3C2Tx produced using the lowest HF concentration in this study (5 wt.%) showed three stages of weight loss at about 320, 530 and 740°C (Figure 2a and S1a). The first peak, centered at 320°C is due to the release of entrapped structural water, leading to an increase of signal for H2O, –OH, and =O in this range. At 530°C, the onset of a slow dissociation of surface groups occurs, including O2 and H2. The H2 gas released is likely due to a combination of –OH terminations and/or molecular hydrogen trapped between Ti3C2Tx MXene sheets.43 Above 740°C, the material starts to degrade and transform, with CO being released. Degradation accelerates at 830-850°C (Figure 2a-c), with significant release of CO, implying that the structure is converting to the cubic TiC form, with some C reacting with the remaining surface functionalizations (=O and –OH).44 Additionally, at these higher temperatures, fluorine begins to be released in the form of HF and AlF3. The release of –F termination after –OH and =O is not in agreement with previous predictions for Ti3C2Tx45 and reported observations of heat treatments under ultra-high vacuum,46 which may be due to a compact and closed structure of Ti3C2Tx powders produced by etching in 5 wt.% HF.


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Figure 2. Thermal gravimetric (TG) curves with derivatives of the weight losses (DTG) for Ti3C2Tx MXene obtained by etching Ti3AlC2 using three different HF concentrations of 5, 10 and 30 wt.%: Ti3C2Tx-5HF (5 wt.% HF for etching) (a), Ti3C2Tx-10HF (10 wt.% HF for etching) (b), Ti3C2Tx-30HF (30 wt.% HF for etching) (c) and mass spectrometry analysis for the atomic mass unit (amu) of 18/H2O (d), 28/CO (e) and 20/HF (f). By increasing the HF concentration from 5 wt.% to 10 (Fig. 2b and S1b) or 30 wt.% (Fig. 2c and S1c) a larger water loss was observed (Fig. 2d and S2). While all three samples show an H2O peak at 320ºC, by increasing HF concentration, a new peak appears at 200ºC. This doublet increases in intensity with increasing the HF concentration. Additionally, the onset temperature of water release was decreased from 320ºC for the 5 wt.% HF sample to 200ºC and 100ºC for 10 and 30 wt.% HF samples, respectively. The shift to lower temperatures shows an increased water content in Ti3C2Tx with higher HF etchant concentration. The water released at lower temperatures (< 200ºC) is related to multilayer water (weak water-water interaction), not monolayer water bound to –OH groups (strong water-surface interaction) and its amount increases with the HF concentration. This is because MAX phase etched with higher HF concentrations leads to MXene 7 ACS Paragon Plus Environment


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with a more open structure than MAX phase etched with lower HF concentrations.5 The resulting accordion-like morphology of Ti3C2Tx-30HF MXene has more macroscopic sites available for multi-layer water adsorption compared to the compact structures of Ti3C2Tx-5HF MXene. As shown in Fig. 2a-d, for the 5, 10, and 30 wt.% cases, 0.35, 1.9, and 7.4% of the total mass loss is directly related to the release of the water at low temperatures, respectively. As the HF concentration is increased, the relative amount of strongly bonded water increases. This is due to the increased surface defects,47 leading to water bonded with higher coordination, thus being released at higher temperatures. All three samples have similar weight loss at temperatures above 400ºC. Regardless of the HF concentration, Ti3C2Tx remains stable up to ~800°C, but the sample degrades above this point (Figure 2, S1). In all three samples (Fig. 2e), CO is released at > 830ºC with Ti3C2Tx-5%HF showing the largest amount of released CO compared to 10 and 30 wt.% HF. Oppositely (Fig. 2f), as the HF etching concentration is increased, the amount of weight loss due to HF increases. These trends occur due to the relative amounts of oxygen- and fluorine-based terminations. The higher HF etching conditions lead to an increase of -F terminations. Once the samples reach ~800°C, they are gradually converted to cubic titanium carbide (TiC), as shown in Figure S3. At 1000°C, there is still a significant amount of MXene remaining, while at 1500°C the sample is converted completely to TiC. However, Raman spectroscopy (Fig. S4) reveals the presence of XRD-amorphous TiO2 and free carbon at 1000°C; the former is then reduced and forms cubic titanium carbide at 1500°C. Scanning electron microscopy (SEM) of the initial MLTi3C2Tx and after heating to 1000°C and 1500°C under He are shown in Figure 3, where the TiC flakes still retain the layered morphology.


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Figure 3. Scanning electron microscopy (SEM) images of Ti3C2Tx MXene a) pristine, b) after heating to 1000 °C, and c) 1500 °C. Ti3C2Tx etched in mixed acids. Etching with a combination of acids, HF/H2SO4 or HF/HCl, leads to different surface terminations, resulting in the evolution of different species during thermal treatment (Figure S5). Both the HF/H2SO4 and HF/HCl samples contain negligible amounts of water (Fig. S5d). Furthermore, the amount of O released up to 800ºC is negligible for these two samples. For the Ti3C2Tx-HF/H2SO4, significant =O terminations remain, leading to the release of CO at ~900 C. Also, no sulfur terminations (such as sulfoxides, sulfones, and sulfonic acids) have been detected by mass spectrometry (Fig S5b,e and S6b). In contrast, for the sample produced with HF/HCl (Fig. S5c,e and S6c), the amount of CO released is significantly lower which indicates that the =O terminations are reduced compared to Ti3C2Tx-HF/H2SO4. Here, a significant amount of –Cl termination release is observed, with degradation marked by the release of Cl– anions, implying chlorine surface terminations. For the Ti3C2Tx etched with mixed acids, a similar amount of HF is released (Fig. S5f), implying that the samples have comparable –F terminations. Both samples are stable until ~ 900C (Figure S5b,c). The enhanced thermal stability can be due to the reduced defect density compared to samples etched in pure HF.46 Mass spectrometry analysis (Fig. S6) shows the evidence of molecular hydrogen trapped in Ti3C2Tx (HF/H2SO4) and Ti3C2Tx



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(HF/HCl), emphasized by the small amount of hydroxyl groups combined with a non-negligible amount of H2 detected. Ti3C2Tx films. HF-etched Ti3C2Tx was delaminated using tetramethylammonium hydroxide (TMAOH) as an intercalant followed by vacuum-assisted filtration into a free-standing film, which was dried at 55°C for 48 hours (see Methods for details). The TA-MS patterns of three freestanding films from Ti3C2Tx powders etched with 5, 10 and 30 wt.% HF show the same trend (Figure 4). This indicates that post-treatment (here, delamination) of Ti3C2Tx brings its surface chemistry to the same composition. For the sake of simplicity, we only show the TG-MS of Ti3C2Tx film made from the 10 wt.% HF in Figure 4b, the other two films had similar patterns. It is noteworthy that our results do not show any peaks related to water, which can be explained by occupancy of interlayer spacing by TMA+ instead. Similar to water release in ML samples, a multistage release was observed for TMA+. The first peak at 84°C is related to TMA+ that is not bound directly to the MXene surface. The weakly-interacting molecule decomposes into NH2/NH3/NH4+ resulting in the weight loss of 2.1 wt.%. At 300°C, the TMA+ adsorbed at the surface begins to decompose and thermally desorb. Finally, at 460°C the TMA+ with higher coordination (probably, located at defect sites) decomposes. The free-standing MXene films begin to transform to cubic TiC at 840°C, leading to the release of CO2 and CO.


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Figure 4. Thermal gravimetric curves for free-standing Ti3C2Tx films from multilayer Ti3C2Tx powders after delamination with tetramethylammonium hydroxide (TMAOH): Ti3C2Tx-5HF (5 wt.% HF for etching), Ti3C2Tx-10HF (10 wt.% HF for etching) and Ti3C2Tx-30HF (30 wt.% HF for etching) (a), and typical mass spectra of the film synthesized by delamination of Ti3C2Tx-10HF with TMAOH. M2CTx MXenes: Mo2CTx, and Nb2CTx. Figures 5 and S7 show the thermal analysis of Nb2CTx and Mo2CTx ML MXene powders after etching in 50 wt.% HF 50°C for 48 or 120 h, respectively (Table S1). Nb2CTx (Fig. 5a and S7a) releases water at 246°C accounting for 4.2 wt.%. For Mo2CTx powder (Fig. 5b and S7b), water (4.4 wt.%) is initially released at 135°C with a continuous release of both water and –OH groups up to 500°C. It was previously shown that Mo2CTx has more O-terminations and fewer –F than Ti-containing MXenes.48 For Nb2CTx, a release of water (coupled with OH release) is observed between 500°C and 700°C, suggesting stronger bonding of water compared to Mo2CTx. Similar to Ti3C2Tx, a release of H2 (uncorrelated with water or hydroxyl release) is observed for Nb2CTx at moderate temperatures, while a stable release happens in the case of Mo2CTx. For Nb2CTx and Mo2CTx, the main phase transformations occur at 807°C and 780°C, respectively, resulting in a loss of 10-15 wt.%. This loss is concurrent with a significant release of CO and CO2 (for Mo2CTx), which indicates a potential transformation of the MXene to the stable 11 ACS Paragon Plus Environment


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corresponding bulk carbide (similar to Ti3C2Tx described in the previous section). In the case of Nb2CTx, a second loss arises at 1300°C, here again with a release of CO suggesting a second transformation at higher temperatures. It is likely that the first transformation is the MXene transforming to both trigonal Nb2C and Nb with dissolved C inside. At the second significant weight loss, the higher temperatures lead to increased atomic mobility of the dissolved C inside the Nb phase, leading to a reaction with the remaining oxygen groups. Although these two samples we are etched in a high-concentration HF (50 wt.% HF at 55ºC for more than 48 h), a significantly lower amount of HF is released for Nb2CTx and almost no HF is detected for Mo2CTx. This observation suggests a lower energy of Nb-F and Mo-F bonds and primarily oxygen-terminated surfaces. This observation is in good agreement with a previous study suggesting that Mo2CTx contains a significantly lower amount of fluorine terminations after delamination.48 These results suggest that the surface chemistry and thermal properties are linked to both the synthesis conditions (such as, the etchant composition here), and the nature of the transition metal in the MXene (such as less –F content in Mo2CTx and Nb2CTx). 29, 48

Figure 5. Thermal gravimetric analyses with derivatives of the weight losses (DTG) for multilayer Nb2CTx (a) and Mo2CTx (b) MXene powders obtained by etching of Nb2AlC and Mo2Ga2C in 50 wt.% HF for 48 and 120 hours at 50°C, respectively.


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Mo2CTx and Nb2CTx films. For this work, Mo2CTx and Nb2CTx multilayer powders were delaminated by TMAOH and filtered as films, following the same method as described for Ti3C2Tx. The thermal analysis data for Nb2CTx and Mo2CTx films are shown in Figures 6 and S8. In the case of the Nb2CTx, a continuous loss of 8.5 wt.% occurs from an onset temperature of ~ 50°C, until 600°C. MS analysis in this range of temperatures indicates a release of NH2, NH3 and NH4+ species due to decomposition and desorption of TMA+ ions used for intercalant ion and delamination. These results provide evidence that the intercalant binds to the surface of the MXene flakes strong enough to hinder water adsorption (absence of H2O signal). Similar MS results were observed for Ti3C2Tx films. While Nb2CTx exhibits single-step release of NH2, NH3 and NH4+ at 200°C, Mo2CTx shows multi-step release between 100 and 700°C, similar to Ti3C2Tx films. Here, a three-step loss is observed in the low temperature region, with the first at 100°C, second at 250°C, and third at 590°C. MS indicates release of N-species in three steps, combined with the release of CO and CO2 (peaks below 600°C). No CO2 evolution is observed for Nb2CTx in this temperature range. This demonstrates that different groups of tetramethylammonium are weakly (100°C) or strongly (250°C and 590°C) bonded to the surface of Mo2CTx flakes, as explained in detail for Ti3C2Tx. The peak of CO2 at 300°C may indicate degradation of the intercalant. Otherwise, the CO2 release between 600°C and 1000°C was observed in the multilayer powders and is not related to the delamination process. The transformation temperature determined for the ML powders is similar to the Nb2CTx film, with a slight increase to 820°C (+13°C) and reduced mass loss (4 wt.%), suggesting surface reduction and higher stability of the stacked flakes. Similarly, for the Mo2CTx film, a transformation concurrent with the 4 wt.% mass loss happens at 780°C, which is similar to the ML powders.



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Figure 6. Thermal gravimetric analyses with derivatives of the weight losses (DTG) for Nb2CTx (a) and Mo2CTx (b) films obtained by vacuum assisted filtration of corresponding delaminated dispersions. Tetramethylammonium hydroxide (TMAOH) was used as an intercalant to delaminate multilayer Nb2CTx and Mo2CTx powders. Chemical and thermal stability implications. In-depth understanding of the thermal analysis data and effects of the sample history on their thermal stability can better guide the tuning of MXene properties via thermal treatment under specific conditions, to meet application requirements. The surface chemistry and texture (induced by defect formation) control the hydrophilicity of MXenes, which is of great importance in applications where wettability/impermeability plays a major role. Owing to its oxygen terminations and structural water, Ti3C2Tx has hydrophilic character, however, Ti3C2Tx obtained by etching with different HF concentrations has different degrees of hydrophilicity and surface wettability. The degree of hydrophilicity is governed by the ratio of hydrophilic (=O, –OH) to hydrophobic (-F) surface terminations. Also, we have determined the temperatures for release of confined water, as well as thermal desorption of surface functional groups of MXenes. These observations provide insight into the thermal properties of MXenes and may have implications in numerous fields, including but not limited to: 14 ACS Paragon Plus Environment

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During the synthesis of MXene composites with hydrophobic polymers, the MXene water and –OH groups should be removed in order to enhance the MXene-polymer interactions or preference can be given to MXene synthesized in acid mixtures (Figure S5), which have reduced surface bound water. Thermal treatment of MXenes may be a viable option. For ceramic and metal matrix composites, Ti3C2Tx can be incorporated for operation temperatures of up to ~800°C, while Nb2CTx and Mo2CTx tolerate lower temperatures. From the TA-MS analysis data presented above, it is clear that the specific functional groups can be removed thermally without degrading the 2D MXene structure. It may be possible to modify the surface to have equivalent surface sites so that MXenes with controlled and stable potentials for specific metal ion insertion/extraction for battery electrodes can be produced.49 In energy storage applications employing organic electrolytes require completely dry samples, since residual water can be detrimental to the life-time of the device and lead to its degradation due to parasitic reactions and limits the voltage window.10 Alternatively, MXene energy storage devices working with aqueous electrolytes might require Osurface terminations for improved electrochemical performance. Indeed, it was previously demonstrated the pseudocapacitive mechanism of Ti3C2Tx in aqueous acidic electrolytes is dependent on TiOH/TiO redox-active sites as well as the Ti oxidation state.50 Surface terminations are equally important for MXene basal planes provide catalytic action.51

SUMMARY The thermal stability of Ti3C2Tx, Nb2CTx, and Mo2CTx MXene was studied systematically using combined thermogravimetric and mass spectrometry analyses. The thermal properties were strongly dependent on their chemical composition, both the type of transition metal as well as the surface chemistry. It was found that the synthesis conditions during etching and delamination



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greatly affected the surface chemistry. For Ti3C2Tx MXene, increasing the HF concentration from 5 to 30 wt.% resulted in an increase in the amount of water trapped between the MXene layers. This occurred because of large specific surface area and open structure. The surface chemistry of Ti3C2Tx was also affected by the HF concentration; increasing the HF concentration resulted in an increase in the fluorine to oxygen ratio. Due to the increased fluorine content, the amount of fluorine containing reaction products increased, while the amount of CO released up to the decomposition temperature (~860ºC) decreased. However, the HF concentration had little effect on the thermal stability of Ti3C2Tx; the decomposition temperature only slightly decreased at 30 wt.% HF. In addition to the HF concentration, the etchant composition had a significant effect on the surface chemistry of Ti3C2Tx. For example, using a mixture of acids, either HF/HCl or HF/H2SO4, decreased the interlayer water and hydroxyl groups on the surface of MXene. This result shows that Ti3C2Tx MXene produced by the same MAX phase using different etchants will have very different thermochemical properties due to their surface chemistry. Intercalants used for delamination, such as TMAOH, replaced water and controlled the low temperature decomposition products. The decomposition products exhibited the same weight loss, regardless of the specific etching conditions used. After the heat treatment of Ti3C2Tx MXene, the layered structure was maintained but Ti3C2Tx MXene is converted into stoichiometric titanium carbide. Compared to Ti3C2Tx, Nb2CTx and Mo2CTx show substantially different thermal stability and surface chemistry. Nb2CTx and Mo2CTx started to degrade at lower temperatures than Ti3C2Tx. No fluoride terminations were observed on the surface, and more oxygen groups exist on the surface. EXPERIMENTAL Synthesis of Ti3C2Tx and fabrication of Ti3C2Tx film. Ti3C2Tx powder MXene was synthesized from 1 g Ti3AlC2 MAX (Y-Carbon Ltd., Ukraine) using 20 mL of 5, 10, and 30 wt.% hydrofluoric


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acid (HF, 48-51 % aqueous, Acros Organics) separately at ambient temperatures (~ 22°C) for 24, 18, and 4 h, respectively. The etching durations were optimized based on the HF concentration for this Ti3AlC2 powder to achieve complete etching with minimal mass loss. The MXene multilayers were washed by several steps of centrifugation (3500 rpm, 5 min) and decantation of the supernatant, until it reached a pH > 6. Then, powders were collected by vacuum-assisted filtration. Delamination of 1 g Ti3C2Tx powder was achieved via intercalation of tetramethylammonium hydroxide (TMAOH, 25 wt.% aqueous solution, Sigma-Aldrich) followed by the collection of the stable aqueous colloidal solution of Ti3C2Tx used to fabricate films via vacuum-assisted filtration. The synthesis and delamination of Ti3C2Tx is described step-by step elsewhere.5 Synthesis of Ti3C2Tx using HF/H2SO4 or HF/HCl. Similar to the synthesis of Ti3C2Tx using 5 wt.% HF, 1 g of Ti3AlC2 was etched using 20 mL of etchant solution at ambient temperatures for 24 h. The acid mixture etchant made by adding 2 mL of HF (50 wt%, Acros Organics) to either 4 mL of H2SO4 (96 wt%, Fisher) or 12 mL of HCl (37 wt%, Fisher).13 Washing and delamination steps are similar to those employed for Ti3C2Tx etched in pure HF. Ti3C2Tx multilayer powders and films were stored in vacuum and then vacuum-dried at 55°C for 48 hours before TA-MS analysis. Synthesis of Mo2CTx multilayer powder and 2D-Mo2CTx film. Mo2Ga2C MXene precursor was synthesized as per the protocol described elsewhere.52 2 grams of Mo2Ga2C precursor was etched in 20 mL of 50 % HF for 120 hours at 50°C under stirring. After etching, the solution was washed by centrifugation at 3500 rpm for 5 min, decantation of the acidic supernatant and addition of 300 mL of deionized water. This step was repeated 5 times until the supernatant reached pH > 6. Finally, the multilayer Mo2CTx MXene was filtered through a Millipore 0.45 µm cellulose acetate membrane (Millipore). The collected powder was dried at 55°C under vacuum for 48 hours. To



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prepare a film of Mo2CTx from MXene single-layer flakes, 1 g of the multilayer powder was delaminated in a mixture of 4 mL of TMAOH and 6 mL of d-H2O for 18 h under stirring. After delamination, the solution was washed 2 times by centrifugation at 3500 rpm for 20 min, decantation of the basic supernatant and addition of 50 mL of water, until the supernatant reached pH < 8. Finally, the sediment was re-dispersed in 50 mL of d-H2O and bath sonicated for 1 hour in an ice-bath under Ar flow. The final solution was centrifuged for 1 h at 3500 rpm. The supernatant was collected and filtered through a Celgard (3501, 25 nm pore size, Celgard) membrane. Mo2CTx multilayers and films were stored in vacuum and then vacuum-dried at 55°C for 48 h before TG-MS analysis. Synthesis of Nb2CTx multilayer powder and 2D-Nb2CTx film. Nb2AlC MAX was synthesized as previously described.53 Briefly, the elemental powders were mixed in a ratio of 2Nb:1Al:1C and sintered in an alumina crucible for 2 h at 1600°C (5°C/min) under Ar flow. The resulting MAX phase block was ground into a powder with a typical particle size of ≤ 74 µm. 2 g of Nb2AlC MAX was etched in 40 mL of concentrated 50 wt.% HF and stirred for 48 h at 50°C. After etching, the MXene was washed and delaminated, resulting in a colloidal suspension of single flakes (and ultimately filtered as a film), following the same protocol as for Mo2CTx (number of washes was adapted to reach the pH6-7). Nb2CTx multilayers powders and single-flake films were stored in vacuum and then vacuum-dried at 55°C for 48 hours before TA-MS analysis. Thermal Analysis-Mass Spectrometry (TA-MS). TA-MS measurements were carried out using a TA Instruments thermal analyzer (SDT Q 650, Discovery Series) equipped with a Mass Spectrometer (110/220V) from room temperature to 1500°C with a heating rate of 10 C/min under a constant helium flow 100 mL/min. The gas products that evolved during heating were determined by MS and mass/charge (m/z) evolution profiles as a function of temperature were obtained. Parts


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per billion (ppb) sensitivity over the mass range 1-300 amu (atomic mass unit; gas dependent) is achieved with a quadrupole detection system, including a closed ion source, triple mass filter, and dual (Faraday and Secondary Electron Multiplier) detector system. All samples analyzed by TAMS were stored and dried following the same protocol including storing condition (in vacuum)/duration, as well as drying condition (55°C for 48 h in a vacuum oven). The ion current was normalized by the corresponding initial weight of the materials loaded for TA-MS analysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. The supporting material includes thermal gravimetric analyses with derivatives of the weight losses for MXenes, X-ray diffraction patterns and Raman spectra of Ti3C2Tx-10HF obtained by etching Ti3AlC2 with 10 wt.% HF MXene after annealing at 1000°C and 1500°C. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The research is based upon work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via Kristin DeWitt 201818071700007. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon.



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Table of Contents


High-Temperature Behavior and Surface Chemistry of Carbide

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