Effect of Ti3AlC2 MAX Phase on Structure and Properties of Resultant

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Effect of Ti3AlC2 MAX Phase on Structure and Properties of Resultant Ti3C2Tx MXene Christopher Eugene Shuck, Meikang Han, Kathleen Maleski, Kanit Hantanasirisakul, Seon Joon Kim, Junghoon Choi, William Reil, and Yury Gogotsi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00286 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Nano Materials

ACS Appl. Nano Mater.

Effect of Ti3AlC2 MAX Phase on Structure and Properties of Resultant Ti3C2Tx MXene Christopher E. Shuck†, Meikang Han†, Kathleen Maleski†, Kanit Hantanasirisakul†, Seon Joon Kim†,a, Junghoon Choi‡, William E. B. Reil†, Yury Gogotsi†* †A.J.

Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, 19104, USA ‡Department

of Chemical and Biomolecular Engineering (BK-21 plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea *Corresponding author: [email protected]

Abstract Ti3C2Tx MXene is an attractive two-dimensional (2D) material for a wide variety of applications, however measured properties vary widely from study to study. A potential factor to the property differences relates to variability in the MAX phase precursors. To illustrate this, Ti3AlC2, the precursor for Ti3C2Tx MXene, was synthesized using three carbon sources (graphite, carbon lampblack, and titanium carbide (TiC)) at 1650 °C for 2 h. Thermal analysis was utilized to examine the reaction mechanism, indicating that the three carbon sources experience different reaction pathways. The Ti3AlC2 MAX powders were then converted into Ti3C2Tx MXene and delaminated. The products revealed differences with respect to the lateral flake size, chemical composition, chemical stability in deionized water, and electrical conductivity. Graphite-produced Ti3C2Tx showed the highest conductivity (~4400 S/cm) and is the most stable (time constant of 10.1 days), while TiC-produced MXene has comparable conductivity (~3480 S/cm), but the lowest colloidal stability (4.8 days), and carbon lampblack has the lowest conductivity (~1020 S/cm) and low chemical stability (5.1 days). Furthermore, gas sensors were fabricated from all three MXenes to probe differences in their performance. The TiC-produced Ti3C2Tx showed the highest response, followed by graphite-produced, and lastly Ti3C2Tx produced from carbon lampblack. This illustrates that synthesis of the MAX precursor material leads to significant difference within the resultant MXene and provides another pathway for further control over their properties. Keywords: MAX, MXene, two-dimensional materials, gas sensing, stability, electrical conductivity

aPresent

address: Materials Architecturing Research Center, Korea Institute of Science and

Technology (KIST), Seoul 02792, South Korea

ACS Paragon Plus Environment

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Introduction MXenes are a new class of two-dimensional (2D) materials that have been found to have a wide variety of applications, including electrochemical energy storage,1-7 electromagnetic interference shielding,8-11 gas sensing,12-14 antennas and RFID tags,15 electrochromic devices,16 and many others.1, 17-21 These materials have the general structure Mn+1XnTx, where M is an early transition metal (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, or Ta), X is C and/or N, n = 1, 2 or 3, and Tx represents surface functional groups, such as -F, OH, or -O.1,

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Many of the MXenes discovered have been produced from selective

etching of Mn+1AXn phase materials,22-23 where A can be Al, Si, P, S, Ga, etc.;24 however, it has only been shown possible to selectively etch Al or Si from the MAX structure to produce MXenes.25 Previous research has shown that MXenes have varying properties depending on the composition, structure, and processing conditions.26-31 For example, the size of MXene flakes played an important role in the electrical conductivity, capacitance, and rate performance of free-standing film electrodes.26 Moreover, it was shown that the processing conditions affect the resulting surface terminations, producing MXenes with different properties.27-30 Additionally, it was recently shown that the thermal stability of MXenes is affected by both the chemical composition, etching conditions, and posttreatment.31 Further understanding into how to control the MXene structure and chemistry is vital to tailor these materials toward the desired applications. In previous studies, Ti3AlC2 has been produced using a variety of precursor materials and techniques. In the first MXene article, Ti3AlC2 was synthesized using Ti2AlC and TiC via high-temperature annealing at 1350 °C for 2 h.2-3 A variety of


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ACS Appl. Nano Mater. titanium and carbon precursors have been utilized, such as Ti,32-36 TiH2,37-38 or TiO23940

as the titanium source and TiC,35,

37-38

graphite,32-34,

40

Al4C3,32-33,

36

or carbon

lampblack36, 39 as the carbon source. Synthesis of the Ti3AlC2 phase has been conducted through hot isostatic pressing,32-33, 37 high-temperature annealing,17,34, 37 self-propagating high-temperature synthesis,39-41 or spark-plasma sintering.35-36,

38

The number of

variables that affect synthesis conditions lead to drastically different structures, phase compositions, grain sizes and properties. This effect was known and studied with the MAX phase materials.24, 32, 38 Following this, these various approaches have led to different MXene properties, such as conductivity values varying from below 1,000 to 10,000 S/cm, different electrochemical performance, stability, and others.1-2,

9, 42-44

For a

number of these studies, combinations of techniques and precursors were utilized, followed by variable etching conditions, making it challenging to directly compare results. The primary goal of this work is to provide direct evidence that the differences in MAX synthesis, resulting from different carbon sources, lead to differences in the final MXene properties when identical synthesis and processing conditions are utilized, isolating the difference to purely MAX precursor synthesis. While it is widely known in the MXene community that the etching and processing protocols affect these properties,22 there have been no significant studies showing that the precursor Ti3AlC2 and even the precursors to synthesize the Ti3AlC2, play an equally important role in the final Ti3C2Tx performance. Understanding how different precursor materials affect the Ti3AlC2 structure allows for further levels of control over the resulting Ti3C2Tx MXene properties. In this study, three different carbon sources (graphite, TiC, and carbon lampblack) were used


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ACS Appl. Nano Mater. for synthesis of Ti3AlC2. These three carbon sources were chosen because they are the most commonly utilized carbon precursors for MAX synthesis. The MAX powders were further converted into Ti3C2Tx, delaminated into 2D sheets and studied with respect to flake size, conductivity, stability/lifetime tests in aqueous media, and as gas sensors.

Experimental Synthesis of MAX phases Titanium (99.5 %, -325 mesh, Alfa Aesar, USA) and aluminum powder (99.5 %, 325 mesh, Alfa Aesar, USA) were mixed with graphite (99 %, -325 mesh, Alfa Aesar, USA), carbon lampblack (99.9 %, Alfa Aesar, USA), or TiC (99.5 %, -325 mesh, Alfa Aesar, USA), leading to a Ti:Al:C atomic ratio of 3:1.1:1.8. For the annealing reaction, 5 g of each of the powder mixtures were placed into cylindrical alumina crucible, and then placed into a high-temperature furnace (Carbolite Gero, UK). The tube was purged with Ar at 50 mL/min. The furnace was heated to 1650 °C at a constant rate of 5 °C/min, then was held at the maximum temperature for 2 h. Finally, the tube cooled naturally to room temperature.

Topochemical Synthesis of MXene Here, the three Ti3AlC2 powders (produced using graphite, TiC, and carbon lampblack) were topochemically converted into MXene (Scheme 1). Typically, an etchant containing 1.6 g of LiF (98.5%, Alfa Aesar, USA) and 15 mL of hydrochloric acid (37 wt.%) was added into 5 mL deionized water.22 After that, the as-synthesized Ti3AlC2 powder (1 g) was added gradually to a premixed etchant solution and stirred for 40 h at room temperature. The reacted solution was washed with deionized water and centrifuged


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ACS Appl. Nano Mater. at 3500 rpm for 3 min; the supernatant was decanted, and the MXene was re-dispersed by manual shaking. This washing procedure was repeated until the pH of the mixture was higher than 6. Then the colloidal solution was centrifuged at 3500 rpm for 30 mins and the delaminated Ti3C2Tx supernatant was obtained. The MXene films were prepared by vacuum filtration a known volume (for example, 10 or 15 mL) of the delaminated Ti3C2Tx supernatant.

Scheme 1: Synthesis of Ti3AlC2 from different carbon sources (TiC, graphite, and carbon lampblack) followed by Ti3C2Tx MXene synthesis.

Characterization The phase composition of the MAX materials was determined by X-ray diffraction (XRD) analysis with Ni-filtered Cu–Kα radiation (Miniflex, Rigaku) operated at 40 kV and 15 mA. Step-scan data (step size of 0.04° and counting time of 1 sec) were recorded over a range of 5–90° (2θ). Phase composition of the resulting XRD patterns was determined by Rietveld refinement using GSAS-II.45 The reaction was studied using thermogravimetric analysis and differential scanning calorimetry (Discovery SDT 650, TA Instrument). For all experiments, ~20 mg of the reactant powder was placed into a 90 μL alumina crucible. Each sample was heated


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ACS Appl. Nano Mater. from 50 °C to 1500 °C at heating rates of 1, 5, and 10 °C/min under 100 mL/min flow of Ar. Scanning electron microscopy (SEM) (Zeiss Supra 50VP, Germany) was used to observe the morphologies of MAX and MXene flakes. To image flakes, a dilute solution (1100 °C) is related to the reaction between the carbide and Ti2AlC, leading to the formation of Ti3AlC2.49 For the carbide mixture, the formation of this phase appears to be slightly shifted, up to 1300 °C. As the heating rates are increased to 5 °C/min, these peaks begin to coalesce, leading to multiple phase transformations occurring simultaneously. Comparing the graphite and lampblack reaction pathways at low heating rates (1 and 5 °C/min), the graphite sample experiences sharper and well-defined peaks at higher temperatures, while the lampblack pathway is smoother and more coalesced even at the slowest heating rate used (1 °C/min); this effect is similar to moving from lower to higher heating rates. This occurs due to the difference in the precursors. The amorphous nature of the carbon lampblack sample suggests there is a lower barrier to each of these diffusive steps.

Figure 2: Differential scanning calorimetry results of mixed Ti3AlC2 precursor powder containing the three carbon sources: a) graphite, b) TiC, and c) carbon lampblack at three different heating rates (1, 5, and 10 °C/min). At the studied heating rates, there is a significant difference between the heat release rates, implying that none of the systems are truly at equilibrium and that the carbon source alters the reaction pathway.

It is important to note that changes in heating rates will lead to different product microstructures.50 At high temperatures (~1350 °C), the graphite-containing samples (Fig. 2a) display a sharp exothermic process related to the reaction between the formed carbides and the Ti2AlC, leading to the formation of Ti3AlC2. Likewise, this occurs in the lampblack mixtures (Fig. 2c) as well, but the magnitude of the thermal effect is lower. The TiC carbide


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ACS Appl. Nano Mater. mixture (Fig. 2b) begins this transformation at higher temperatures, with the 1 °C/min graph trailing behind both the graphite and lampblack samples (Fig. S1a). Even with minor changes in the heating rate, from 1 to 5 °C/min or from 5 to 10 °C/min, vastly different TGA-DSC curves result, demonstrating how the heating rate can play a pivotal role in forming specific reaction products and distinct phase compositions. TGA-DSC results (Fig. S1b) confirm that the three separate systems proceed in three different ways (e.g., the heat capacities change depending both on the composition and the heating rate) indicating that the reaction can progress through multiple pathways. Due to these differences, the produced Ti3AlC2 will have different morphologies, crystal sizes, etc., and may affect the resultant Ti3C2Tx flake characteristics.


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ACS Appl. Nano Mater. By examining the Ti3AlC2 powders before etching, differences in grain size were realized through SEM images (Figs. 3a-c). The graphite and TiC sources both produced large crystals with the expected layered morphology (Fig. 3a and b), in opposition to the crystals produced through lampblack synthesis, which produced smaller crystals (Fig. 3c). To further understand the effect of the precursors, the MAX phases were selectively etched to produce Ti3C2Tx MXene and investigate how the MAX phase composition, production method, etc. affects the Ti3C2Tx properties.

Figure 3: SEM images of Ti3AlC2 synthesized by) graphite, b) TiC, and c) carbon lampblack as carbon source; Ti3C2Tx MXene produced by a MILD-like process (40 h etching in 12 M HCl and 9 M LiF) using d) graphite, e) TiC, and f) carbon lampblack-derived MAX phases and cast from solution on an anodic aluminum oxide membrane.

Using a mixture of LiF and HCl, similar to the MILD method,22 the MAX powders were topochemically converted into MXene flakes. Ti3C2Tx produced through graphitebased MAX had flake sizes reaching 19.2 m, with the average being 4.2 m (Fig. 3d and S3a), however, the MXene produced from TiC has a maximum size of 9.3 m, with the average being 2.6 m (Fig. 3e and S2b). For the amorphous carbon lampblack samples, the nucleation of MAX phase has a lower energy barrier and thus occurs on many sites


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ACS Appl. Nano Mater. simultaneously, leading to smaller MAX crystals (Fig. 3c) and MXene flakes with a maximum size of 2.4 m and an average size of 0.5 m (Fig. 3f and S2c) are obtained. This contrasts with the graphite-produced Ti3AlC2, which has a higher energy barrier for nucleation, leading to fewer initial nucleation sites, resulting in larger MAX crystals (and consequently MXene flakes).51 The differences in the precursor Ti3AlC2, resulting from the different reaction pathways brought about by the different carbon sources, lead to Ti3C2Tx MXene flakes with different sizes and morphologies.

Figure 4: XRD patterns of the produced MXene films produced using different carbon sources, showing the significant difference in the quality of the films. Depending on the film quality and flake size, the Ti3C2Tx films indicate significant difference in constructive interference brought on by preferential stacking orientations.

The XRD patterns of the films are shown in Fig. 4. The three films have comparable d-spacing values, graphite (12.69 Å), TiC (13.02 Å), and lampblack (12.71 Å), indicating that all films have similar quantities of water between the MXene flakes. Apart from water content, the three produced MXene free-standing films have different characteristics. The


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ACS Appl. Nano Mater. graphite-produced MXene, the films had a density of 2.35 g/cm3, the TiC-produced MXene had a density of 1.86 g/cm3, and the lampblack had a density of 1.57 g/cm3. The XRD patterns show the MXene film stacking quality; samples with larger crystal size tend to orient along the (002) plane more coherently. As the flake size decreases, the films did not show as much constructive interference, which reduces the intensity of the higher ordered planes, (004)-(0012). The graphite-produced MXene has the largest crystal size and the highest degree of constructive interference in the (00l) direction (Fig. 4). This could be due to a few variables. First, the larger flakes may orient better within the film resulting in lower destructive interference and second, impurities or particles may be present in the TiC and lampblack films resulting in misorientation and higher destructive interference. The TiC-produced flakes, while significantly smaller than the graphite-produced, still orient constructively along the (002) family, which is why the (004) and (008) peaks are visible. However, for the lampblack-produced films, the (002) is the only visible peak, with low intensity relative to the other films. This implies that the films are of lower quality, with a higher degree of flake misorientation potentially caused by both flake size and impurities, leading to a higher destructive interference along the (00l) direction (Fig. 3f)


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Figure 5: XPS spectra of the Ti 2p (450-468 eV binding energy) region of the a) graphite-produced, b) TiC-produced, and c) carbon lampblack-produced Ti3C2Tx. The XPS spectra for the C 1s (280290 eV binding energy) region of the d) graphite-produced, e) TiC-produced, and f) carbon lampblack-produced Ti3C2Tx.

X-ray photoelectron spectroscopy (XPS) was used to analyze the degree of oxidation of the free-standing MXene films produced from different MAX precursors (Fig. 5). From the survey and high-resolution Al 2p region scans (Fig. S3-5), no Al is present in any of the samples, confirming successful etching of the MAX phases. High-resolution spectra of Ti 2p region (Fig. 5a-c) can be deconvoluted to 4 sets of doublets corresponding to C-Ti-Ox, C-Ti-Ox,Fx, C-Ti-Fx, and TiO2 at 455.2 eV (461.4 eV), 455.9 eV (461.5), 457.2 (462.9), and 459.6 eV (465.2 eV), respectively.52 Moreover, the C 1s region (Fig. 5d-f) shows a clear separation between the C-Ti carbide bond at 282 eV and other types of carbon species at higher binding energies. Based on the TiO2 composition in the Ti 2p region, the degree of oxidation is the highest (20%) for MXene produced from TiC, followed by


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ACS Appl. Nano Mater. lampblack (6.7%) and finally graphite (5%), respectively. The surface of all the samples are terminated by O- and F-containing surface species (see Fig. S3-5). Given that the MAX phases were etched following the same protocol, there should not be a distinct difference in their surface chemistry. Therefore, we do not further consider the effect of surface terminations on MXene properties. Yet, we note that the surface terminations play a crucial role in MXene electronic properties.53 Table 1. The electrical conductivity of MXene films synthesized from different MAX powders

Carbon Source

Graphite

TiC

Lampblack

Conductivity (S/cm)

4400±110

3480±60

1020±50

The electrical conductivity of Ti3C2Tx films produced from different Ti3AlC2 powders is shown in Table 1. The characterization was performed on samples vacuum dried at room temperature to eliminate the effects of other factors (e.g., stability of functional groups and structural water) on the measured values. Higher conductivity values can be achieved by annealing samples at higher temperatures.53 Graphite-produced MXene films displayed the highest conductivity of ~4400 S/cm, while those of TiC-produced and lampblack-produced MXene films are ~3480 and ~1020 S/cm, respectively. This is in agreement with the quality of Ti3C2Tx flakes produced and the amount of impurities, specifically Ti2CTx, that are present in the MXene solutions. When the flake size is large (graphite-produced film), the stacking in the filtered film is denser with more efficient inter-flake contacts. For the films fabricated from lampblack-produced MAX phase, the higher degree of misorientation leads to loose stacking of MXene flakes, which further decreases the conductivity of the film, in addition to the smaller flake size.26 In addition,


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ACS Appl. Nano Mater. the higher degree of oxidation observed in the TiC- and carbon lampblack-produced Ti3C2Tx (Fig. 5 b,c) may contribute to the low electronic conductivity measured.

Figure 6. Solution stability of the 0.15 mg/mL MXene colloidal solutions dispersed in deionized water calculated by the change in solution absorbance (Δ Absorbance) over 12 days. The lines represent fitting results calculated from the exponential decay function (f(x)=y0+Ae(-x/τ) where y0 is the offset value, A is the amplitude, and τ is the time constant) for aqueous solutions stored in air. All samples were stored at ambient temperature in a lab drawer away from sunlight. The digital image (inset) displays the differences in concentration of lampblack, graphite, and TiC produced Ti3C2Tx samples (left to right) after 12 days.

Similar to previous reports, delaminated Ti3C2Tx colloidal solutions produced in this study show a decrease in concentration over time due to oxidation of Ti3C2Tx to TiO2.54 We kept the samples in water under the conditions known to induce degradation, so that it could be monitored by measuring the change in absorbance over time. The changes were quantified by fitting to an exponential decay function f(x)=y0+Ae(-x/τ) where y0 is the offset value, A is the amplitude, and τ is the time constant (Fig. 6). By extracting the time constant


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ACS Appl. Nano Mater. (τ) of the decay, delaminated MXene solutions produced from etching different MAX phases can be compared. The results show that Ti3C2Tx produced from graphite revealed a larger time constant, 10.1 days, compared to Ti3C2Tx produced from TiC or lampblack (Table S2, Fig. 6). TiC and lampblack-produced Ti3C2Tx samples exhibited time constants on the order of 4-5 days (Table S2). While solutions tested in this study had low concentrations (0.15 mg/mL) to expedite degradation, it has been shown that by increasing the sample concentration, the particle’s lateral size, or the environment by which it is stored, slower degradation rate of Ti3C2Tx solution can be achieved.54-58 In comparison to the flake size and quality of flakes demonstrated above, the stability of flakes in solution follows the same trend, with graphite samples showing the highest quality with slowest degradation revealing the importance of selecting a high-quality precursor. Furthermore, as was previously shown,59 relative defect density affects the oxidation rate. This implies that, in addition to size effects, it is possible that there are differences in the defect density of the MXene flakes, leading to differing stability and oxidation rates. Furthermore, considering that Ti2CTx is less stable than Ti3C2Tx,54 and there is some Ti2AlC impurities present in some of the initial samples (Table S1), which may also contribute to differences in stability of the MXene colloidal solutions in water.


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Figure 7: Gas sensing properties of the Ti3C2Tx produced from different MAX precursors. The three MXenes show different sensitivities to ethanol, acetone, and ammonia.

To further investigate whether Ti3C2Tx MXenes derived from different carbon sources influence device performance, we employed them as gas sensors. Ti3C2Tx aqueous solutions produced from different Ti3AlC2 powders were spin casted to created thin films on sensing electrodes. Figure 7 shows the gas response of Ti3C2Tx films toward 100 ppm ethanol, 100 ppm acetone, and 5 ppm ammonia at room temperature. Gas response values were defined as the relative change in electrical resistance compared to the baseline resistance. Results show that gas responses toward ethanol and acetone was in the range of 0.1~0.2 % while the response toward ammonia was in the range of 0.4~0.6 %. Interestingly, Ti3C2Tx derived from graphite and TiC-based Ti3AlC2 showed a similar gas response while Ti3C2Tx derived from lampblack-based Ti3AlC2 shows a lower gas response. One could expect the smallest flakes to be the best due to the fastest and strongest gas adsorption.


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ACS Appl. Nano Mater. However, apparently its lower electrical conductivity resulted in less impressive sensing properties, although further studies are needed to fully confirm this behavior. Table 2. Summary of the differences between the MXenes produced from graphite, TiC, and lampblack.

MAX Ti3AlC2 Purity (at. %) Average Flake Size (m) MXene Film Density (g/cm3) Conductivity (S/cm) Stability Time Constant (days) Gas Response (%) (100 ppm Ethanol) Gas Response (%) (100 ppm Acetone) Gas Response (%) (5 ppm Ammonia)

Graphite-Ti3C2Tx 95.2

TiC-Ti3C2Tx 72.4

Lampblack-Ti3C2Tx 93.5

4.2

2.6

0.5

2.35

1.86

1.57

4400 10.1

3480 4.8

1020 5.1

0.13

0.16

0.11

0.20

0.23

0.15

0.55

0.62

0.38

With the characterization methods utilized in this study, we can see that the reaction mechanisms themselves are different for the three carbon sources (Fig. 2), and thus lead to different phase compositions (Fig. 1), in addition to different MAX crystal sizes and MXene flake sizes (Fig. 3). These structural differences lead to different MXene film morphologies (Fig. 4) characterized by variance in the constructive interference in the higher ordered reflections (00l). These fundamental differences in flake size and composition lead to drastically different material properties, including electrical conductivity (Table 1), chemical stability in solution (Fig. 6), gas sensing ability (Fig. 7), and likely other properties that were not investigated here. A summary of the differences between the three MXene samples is described in Table 2. Not surprising, the use of MAX phases from various commercial sources or produced in different research labs leads to


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ACS Appl. Nano Mater. different properties of the same MXene, e.g. Ti3C2Tx, even when the same etching and processing protocol is used. Understanding the mechanisms which lead to these differences will give fundamental insight on MXenes themselves, and thus on how to more accurately tailor them for specific applications. Considering that more than 30 MXenes are already available,1 dozens more have been predicted to exist and a countless number of solid solutions on M and X sites can be formed, appreciation of the effect of MAX phase precursors on structure and properties of MXenes is of tremendous importance for the entire field of MXenes, because majority of these 2D materials are produced be selective etching of MAX phases.60 Understanding the effect of MXene precursor allows for another level of control over MXene properties. While most studies have focused on control over MXenes by different etching conditions, composition of the MXenes, or by modification of the surface functional groups; it is also possible to affect the material properties by changing the precursor synthesis conditions. Further study of the relationship between synthesis conditions and the resulting properties will lead to enhanced understanding and control over MXenes for the targeted applications.

Conclusions Ti3AlC2 was synthesized by high temperature annealing (1650 °C for 2 h) using three carbon sources: graphite, carbon lampblack, and TiC. The three MAX phases were converted to MXene, and showed different flake sizes, morphologies and compositions. Because of differences in flake size and alignment, the three MXene films had different properties, with graphite-produced ones having the highest conductivity of ~4400 S/cm, with the TiC-produced MXene at ~3480 S/cm, and carbon lampblack-produced MXene at


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ACS Appl. Nano Mater. ~1020 S/cm. The three Ti3C2Tx materials showed different colloidal stabilities in water, with the graphite-produced ones being most stable with τ of 10.1 days in deaerated water, while the TiC- and lampblack-produced MXene surviving on the order of 4-5 days. Finally, it was shown that the gas sensing ability of resistive sensors utilizing MXene films depends on the MAX precursor as well, with the TiC-produced MXene films having the highest response, followed by graphite carbon source, with material from carbon lampblack having the lowest sensitivity. This study illustrates that the MAX synthesis process affects the MXene structure, composition and properties. Furthermore, the material properties are not inherently better or worse depending on the precursor synthesis route, but instead controllable.

Acknowledgements The authors would like to thank Natalia Noriega (Drexel University) for assistance with solution stability testing and Tyler Mathis (Drexel University) for SEM of the MAX phase precursors. The MAX and MXene synthesis 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 2018- 18071700007. 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. Sensors work was supported by the Global Research Development Center Program through the National Research Foundation (NRF) of Korea via the NNFC-KAIST-Drexel FIRST Nano2 Co-op Center


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ACS Appl. Nano Mater. (2016K1A4A3945038, 2015K1A4A3047100). XRD, SEM, and XPS analysis were performed at the Centralized Research Facilities (CRF) at Drexel University

Supporting Information The supporting information contains additional information about the composition of the synthesized mixtures, representative thermal gravimetric curves and comparisons between the three DSC samples, additional information on MXene stability, flake images and size distributions, as well as additional XPS data.

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ToC Figure


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Effect of Ti3AlC2 MAX Phase on Structure and Properties of Resultant

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