Guidelines for Synthesis and Processing of Two-DimensionalTitanium Carbide (Ti3C2Tx MXene)Mohamed Alhabeb, Kathleen Maleski, Babak Anasori, Pavel Lelyukh, Leah Clark, Saleesha Sin,and Yury Gogotsi*

A.J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street,Philadelphia, Pennsylvania 19104, United States

*S Supporting Information

ABSTRACT: Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides(MXenes) were discovered in 2011. Since the original discovery, more than 20 differentcompositions have been synthesized by the selective etching of MAX phase and otherprecursors and many more theoretically predicted. They offer a variety of differentproperties, making the family promising candidates in a wide range of applications, such asenergy storage, electromagnetic interference shielding, water purification, electrocatalysis,and medicine. These solution-processable materials have the potential to be highly scalable,deposited by spin, spray, or dip coating, painted or printed, or fabricated in a variety ofways. Due to this promise, the amount of research on MXenes has been increasing, andmethods of synthesis and processing are expanding quickly. The fast evolution of thematerial can also be noticed in the wide range of synthesis and processing protocolsthat determine the yield of delamination, as well as the quality of the 2D flakes produced.Here we describe the experimental methods and best practices we use to synthesize themost studied MXene, titanium carbide (Ti3C2Tx), using different etchants and delamination methods. We also explain effects ofsynthesis parameters on the size and quality of Ti3C2Tx and suggest the optimal processes for the desired application.

■ INTRODUCTION

Since the isolation of single layer graphene in 2004,1

two-dimensional (2D) materials have gained tremendous atten-tion because of their distinctive properties relative to their bulkform. The isolation of graphene has become a reference for all2D materials and opened the possibility to discover even more.Today, there are dozens of new 2D materials, including hexag-onal boron nitride, transition metal dichalcogenides, transitionmetal oxides, clays, etc.2

In 2011, a new family of 2D materials, named MXenes, werediscovered by Drexel University scientists.3 The MXene familyis comprised of transition metal carbides, carbonitrides, andnitrides with a general formula of Mn+1Xn, where M representstransition metals (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,etc.) and X is carbon and/or nitrogen.3,4 The name “MXene”was given to describe the similarities between this 2D materialfamily and graphene but also to recognize the parent ternarycarbide and nitrides, MAX phases, which MXenes aresynthesized from.5 MAX phases are layered ternary carbidesand nitrides with a general formula Mn+1AXn, where Arepresents elements from the group 13 and 14 of the periodictable.5,6

MXenes reported to date are synthesized by wet-chemicaletching in hydrofluoric acid (HF) or HF-containing orHF-forming etchants,7−9 which add surface functionalitiessuch as −O, −F, or −OH, represented by Tx in this formulaas Mn+1XnTx. Etching is required because of strong chemicalbonds between A and M elements in MAX phases that make

mechanical exfoliation hardly possible. To date, more than20 members of the MXene family have been synthesized, anddozens more are predicted, making it one of the fastest growing2D material families.7 Additionally, the MXene family comes inthree atomic structures, ranging from M2X to M3X2 and M4X3,yielding tunability and opportunity to discover and mold materialsbased on necessary demands.7,10,11

In contrast to most other 2D materials, including graphene,MXenes possess hydrophilic surfaces7 and high metallicconductivities (∼6000−8000 S/cm),12,13 showing promisingperformance in energy storage devices,14−17 water desalina-tion,18 catalysis,19 electromagnetic interference shielding,20

transparent, conducting thin films,12,21,22 and many otherapplications.7

Since their discovery, production of multilayered MXeneflakes was possible through wet-chemical etching with HF, anddifferent MXene compositions were synthesized in this form,such as Ti2CTx, Ti3CNTx, Nb2CTx, and V2CTx (Figure 1).

3,23,24

However, it was not until 2013 when single-layer MXene flakeswere isolated by intercalating large organic molecules anddelaminating the sheets from each other, opening the door toexploration of the truly 2D nature of MXenes.25 In 2014,Ti3C2Tx MXene was produced using a HF-containing etchantsuch as ammonium bifluoride (NH4HF2) salt,

21 and later in the

Received: July 8, 2017Revised: August 25, 2017Published: August 25, 2017

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same year, Ti3C2Tx “clay” was synthesized by taking advantageof making in situ HF through addition of lithium fluoride (LiF)salt to hydrochloric acid (HCl), significantly simplifying thesynthesis method and improving MXene performance in energystorage applications (Figure 1).15 With the need for methods toincrease the delamination yield, in 2015 additional organicintercalants were used, such as isopropylamine26 and tetra-butylammonium hydroxide (TBAOH).27 Moreover, in 2015,two new MXene subfamilies, ordered double-transition metalMXenes, M′2M″C2, and M′2M″2C3, where M′ and M″ are twodifferent early transition metals, were discovered, further expand-ing the 2D family.28 Each of these evolutionary discoveries hasadded tunability and diversity to MXene family.In 2016, larger single Ti3C2Tx flakes were isolated by using

the minimally intensive layer delamination (MILD) meth-od,20,29 where manual shaking (no sonication) was used andresulted in larger and less defective single flakes of MXene.13

The MILD method expanded the opportunities for researchon electronic, optical, and size-dependent material propertiesas well as further enhanced the scalability and production ofTi3C2Tx MXene. A timeline progress of MXenes is shown inFigure 1.The exact synthesis conditions used to produce MXenes

influence the resulting properties and thus are directly relatedto the performance of MXenes in their applications. In general,the synthesis of any type of MXene can take from a few hoursto a few days, and it depends on at least a few factors: con-centration of HF and temperature of etching. For example, inthe case of Ti3C2Tx, we used as low as 3 wt % HF containingetchant and showed the higher the HF content, the higher theconcentration of defects in Ti3C2Tx flakes, which influences thequality, environmental stability, and properties of as-producedMXene.13,29 Our group showed that 2 h was enough time to

etch aluminum (Al) out of Ti3AlC2 with concentrated HF(∼50 wt %) at room temperature.25 Based on the type ofMXene and the synthesis conditions, the intercalants may varyas well as the delamination yield.7 We recently summarizedetching and delamination conditions reported in literature formost MXenes known to date, and we encourage the readers toview the referenced review on etching, delamination, andprocessing protocols for MXenes besides Ti3C2Tx.

7

Here, we discuss the best practices used in our lab andprovide step-by-step guidance for room temperature synthesisof titanium carbide (Ti3C2Tx), the most studied and widelyused MXene. We categorize currently used etchants forselective removal of Al from Ti3AlC2 structure into twotypes: synthesis protocols using HF and protocols using in situformed HF. Moreover, we provide a guideline on how todelaminate as-synthesized materials into colloidal solutionseven without sonication. We also discuss storage, processing,and deposition techniques to make different types of con-ductive Ti3C2Tx MXene films.

■ DISCUSSION OF METHODSThere are many different protocols for synthesizing MXenematerials reported in literature,7 and one synthesis route maybe better for one application but not suitable for anotherapplication. When synthesizing any kind of MXene, one mustbe aware of the end goal, the properties desired, and thematerials warranted, to develop the best material for the job.Synthesis of MXene begins with etching the A-element

atomic layers (for example, aluminum) in a MAX phase(for example, Ti3AlC2) from the Mn+1Xn layers adjacent to theA-element layers. The etching process results in terminatedmultilayered MXene powders (for example, Ti3C2Tx) with the2D layers held together by hydrogen and van der Waals bonds.3

Figure 1. Timeline of MXene: a journey from 2011 to now. In 2011, the first MXene multilayered powder (Ti3C2Tx) was made by selective etchingof Al from Ti3AlC2 using HF acid. A schematic of Ti3C2Tx flake structure is shown as an example.3 In 2012, a variety of different MXenes, such asTi2CTx, (Ti,Nb)2CTx, (V,Cr)3C2Tx, Ti3CNTx, and Ta4C3Tx, were also synthesized, and MXenes became a family of transition metal carbides andcarbonitrides.21 In 2013, single layers of MXenes were isolated by intercalation and delamination with organic molecules.23 In 2014, in situ HFetchants such as NH4HF2

24 or LiF/HCl15 mixtures were used. The latter resulted in Ti3C2Tx with clay-like behavior and is called the clay method.With sonication of Ti3C2Tx clay in water, submicrometer lateral-size single flakes of Ti3C2Tx were isolated.15 In 2015, large scale (high yield)delamination of different MXenes using the amine-assisted method or TBAOH were reported.25,26 Additionally, two subfamilies of MXenes,ordered-double-transition metal carbides, where two different transition metals occupy different atomic layers, were discovered (such as Mo2TiC2Tx,Cr2TiC2Tx, Mo2Ti2C3Tx), adding more than 25 different possible MXenes to the family.27 In 2016, delamination and isolation of large single flakesof Ti3C2Tx (>2 μm) were achieved without sonication through optimization of the LiF/HCl etching method. The optimized method is referred to asMILD, making single-flake characterization possible.28 In 2017, by synthesizing in-plane double transition metal MAX phase ((Mo2/3Sc1/3)2AlC) andetching one of the transition metals, MXene with ordered divacancies (Mo1.3CTx) was synthesized.

8 SEM images in 2014 and 2016 show Ti3C2Txflakes on an alumina membrane. Scale bars are 2 μm.

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After the etching is finished (that is, the complete removal ofthe A-element layers), washing is required to remove residualacid and reaction products (salts) and achieve a safe pH (∼6).Washing is normally done by repeated centrifugation to sep-arate multilayered MXene from acidic solution and decantationof the acidic supernatant (Figure S1a, Supporting Information).After the pH is increased to ∼6, the multilayered flakes can becollected, e.g., via vacuum-assisted filtration (Figure S1b,Supporting Information) and dried in vacuum (Figure S1c,Supporting Information). For the rest of this study, we willfocus on synthesis and processing of one type of MXene,Ti3C2Tx.General Synthesis and Processing of Ti3C2Tx MXene.

Figure 2 describes a general map for the synthesis of Ti3C2Tx atroom temperature with HF or in situ HF and delamination ofmultilayered MXene powders produced by each technique,with choice of intercalants, where sonication can be optionallyused to meet certain application requirements (e.g., the needfor higher concentrations or smaller flakes). In the followingsections, we report the best practices, to our knowledge, tosynthesize Ti3C2Tx MXene with emphasis on using minimalconcentration of HF based etchants as well as describingdelamination and processing techniques used to fabricate thincoatings or free-standing films of Ti3C2Tx.Choice of Materials. Choice of Ti3AlC2 Precursor. To date,

the main starting ingredient of any MXene is the precursor.Ti3C2Tx is made from Ti3AlC2 MAX phase. Ti3AlC2 has beensynthesized via a variety of routes using different raw materials,compositions, and sintering conditions. Ti3AlC2 is stable at1350 °C or even higher temperatures;14 therefore, when therequired stoichiometric Ti:Al:C ratio is available at the propersintering conditions, Ti3AlC2 can form.6 For example, in thefirst few years of MXene synthesis, we were using a mixtureof Ti2AlC and TiC as precursors to synthesize Ti3AlC2.

3

However, we changed the synthesis path recently to useless expensive commercially available raw materialsTiC, Ti,and Al. We explain our Ti3AlC2 synthesis method in the

Supporting Information. However, based on the previouslyreported studies, Ti3AlC2 can be made from different startingmixtures, for example, Ti, Al, and C;30 Ti, Al4C3, and C;30 oreven TiO2, Al, and C31,32 under proper conditions. In theearlier studies of MAX phases, before the discovery of MXenes,pressure was used (for example, hot pressing or hot isotacticpressing)30,31 to obtain fully dense MAX phase samples. WhenTi3AlC2 powder is needed, no pressure is required, since a lessdense block of MAX phase is easier to mill into powder.In general, Ti3C2Tx MXene can be made from Ti3AlC2 regard-less of the MAX phase sintering route. However, the sinteringconditions of each MAX phase can result in different impuritiesin Ti3AlC2, such as TiC, Ti2AlC, and Al2O3.

32

Powder X-ray diffraction (XRD) can reveal the existence ofthe impurity phases. A common impurity in Ti3AlC2 is Ti2AlC.One method to distinguish Ti3AlC2 from Ti2AlC is its first XRDpeak (002) because Ti3AlC2 has a (002) peak at ∼9.5° which isdifferent from that of Ti2AlC at ∼13° (Figure S2a, SupportingInformation). The coexistence of Ti2AlC MAX phase in Ti3AlC2

can lead to a mixed phase MXene of Ti3C2Tx with Ti2CTx,which often leads to changes in properties (conductivity, opticalabsorbance spectra, etc.) of the resulting MXene and mostimportantly can influence the interpretation of the reportedresults.33,34 Also, diluted colloidal solutions of Ti3C2Tx opticallyexhibit green color while Ti2CTx solutions have a purple/magentacolor. The UV−vis spectra of Ti2CTx, Ti3C2Tx, and mixed phaseTi2CTx/Ti3C2Tx are distinguishable from one another, and detailsare presented in Figure S2b−d, Supporting Information. Theeffect of sintering conditions of MAX phase on the resultingMXene is yet to be studied, and it is out of the scope of thispaper. We only used one batch of Ti3AlC2 in this study, whichwas synthesized from TiC:Ti:Al with the 2:1:1 stoichiometricratio (for synthesis details, see Supporting Information).

Choice of Etchants and Intercalants. Hydrofluoric acid(HF, 49.5 wt %) was purchased from Acros Organics. ammo-nium hydrogen fluoride (NH4HF2, 95% grade) powder, tetra-methylammonium hydroxide (TMAOH, 25 wt % in water),

Figure 2. General map for Ti3C2Tx synthesis from Ti3AlC2 discussed in this paper. For the etching, there are two routes, HF and in situ HF. In theHF route, three different concentrations with different durations were used, all sufficient to synthesize Ti3C2Tx. In situ HF is the alternative waycompared to the direct use of hazardous HF in which the resulting Ti3C2Tx multilayered powder behaves like clay in comparison to powdersproduced by NH4HF2. Delamination of Ti3C2Tx powder depends on the chosen synthesis routes. In the case of the HF route or a salt (NH4HF2),delamination can be achieved by introduction of large organic molecules such as dimethyl sulfoxide (DMSO) or tetramethylammonium hydroxide(TMAOH) followed by sonication, if required. In the case of lithium fluoride/hydrochloric acid (LiF/HCl), the resulting Li+ intercalated Ti3C2Txclay can be delaminated by sonication (clay) or without sonication (MILD) depending on the concentration of LiF and HCl used to prepare theetchant.

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and tetrabutylammonium hydroxide (TBAOH, 40 wt % inwater) were purchased from Sigma-Aldrich. Lithium fluoride(LiF, 98.5% grade) powder and hydrochloric acid (HCl, 37 wt %)were purchased from Alfa Aesar, and all the above chemicals wereused as received. Deionized water (DI H2O) used in this studywas purified using Elix Essential 3 UV, Millipore).Choice of the Synthesis Method. For Ti3C2Tx, the

choice of a synthesis method directly influences the resultingproperties in terms of surface terminations,35 size,29 and qualityof the flakes (for example, number and type of defects).13

HF Etching Protocol. When handling hazardous HF,independent of the concentration, it is extremely important tobe aware of the risk assessment and required safety protocols(see safety section, Supporting Information). Most synthesisprotocols reported in literature use 10 or 50 wt % HF,3,7,25

although a lower concentration of HF, as discussed later, issufficient to remove Al from Ti3AlC2. Here, we compare threedifferent HF concentrations of 30, 10, and 5 wt % and showthat removing Al can be achieved with as low as 5 wt % and thatan etching time of 5 h is enough in the case of using a con-centration ≥30 wt % HF (Figure 2). For all three HF concen-trations, the synthesis of Ti3C2Tx can be done as follows:

(1) A total of 0.5 g of Ti3AlC2 powder was added gradually(in the course of 5 min) to 10 mL of etchant whilestirring with the help of a Teflon magnetic bar (Movie S1,Supporting Information). The gradual addition ofTi3AlC2 powder is necessary to minimize excessivebubbling formed due to the exothermic nature ofthis reaction (For detailed instruction see Movie S1,Supporting Information).

(2) The reaction was then allowed to proceed at roomtemperature (∼23 °C) for 5, 18, and 24 h for 30, 10, and5 wt % HF, respectively. These etching times for differentHF concentrations are sufficient to achieve completeetching, as discussed below.

(3) Each powder was washed separately five times (that isequivalent to the total volume of 1000 mL of deionizedH2O) via centrifugation (5 min per cycle at 3500 rpm).The washing process was carried as follows:(a) After each centrifuge cycle, Ti3C2Tx powder

settled at the bottom of the centrifuge tube assediment separated from water-like supernatant.

(b) The supernatant after the first washing cycleexhibited acidic pH (Figure S1a, SupportingInformation) and was decanted into waste.

(c) The sedimented Ti3C2Tx was redispersed with anadditional 150 mL of deionized H2O followed byanother centrifugation cycle. The washing cycleswere repeated until the pH of the supernatantbecame ∼6. A total of five centrifugation cycleswere enough for washing Ti3C2Tx in the case ofwashing 0.5 g of powder and using a 175-mLgraduated conical tube (see Figure S1a, SupportingInformation).

(4) The Ti3C2Tx sediments were rewashed with 1000 mL ofdeionized H2O via vacuum-assisted filtration usinga polyvinyl difluoride (PVDF) filter membrane with0.22 μm pore size (Durapore, Millipore) before collec-tion of the remaining materials.

The MXene powders (Figure S3, Supporting Information)were dried in vacuum at 80 °C for 24 h. The resulting powderswere labeled as 30F-Ti3C2Tx, 10F-Ti3C2Tx, and 5F-Ti3C2Tx, for

30, 10, and 5 wt % HF, respectively. All three MXene powderswere stored in vacuum before any further processing oranalysis.As shown in Figure S3, the resulting Ti3C2Tx MXene

powders exhibited distinctive color differences (dark grayto black) from the graphitic-gray color of Ti3AlC2 powder.The 30F-Ti3C2Tx powder exhibited similar color to Ti3C2Txpreviously synthesized with 50 wt % HF;3 however, it lookeddarker than both 10F-Ti3C2Tx and 5F-Ti3C2Tx which exhibiteda similar color (Figure S3, Supporting Information).We compare the scanning electron microscopy (SEM)

images of all the powders in this study in Figure 3. Ti3AlC2

powders show compact, layered morphology (Figures 4a andS4 in Supporting Information) as expected for a bulk layeredternary carbide.5 However, the multilayered Ti3C2Tx powderslook different depending on the etchant concentration (compareFigure 3b−d). 30F-Ti3C2Tx powders exhibit an accordion-likemorphology (Figures 3b and S5 in Supporting Information)even when etching in 30 wt % HF was used for low etchingtimes (5 h).Morphology such as this was previously observed as a feature

of Ti3C2Tx MXene synthesized with 50 wt % HF.3,7 For lowerHF concentrations such as 10 wt % HF (Figures 3c and S6 inSupporting Information), the accordion-like structure was notas prevalent as in the case of using 30 wt % HF. Additionally,the accordion-like morphology was not observed in Ti3C2Txpowder synthesized with 5 wt % HF, as the resulting Ti3C2Tx isbarely distinguishable from MAX powder (Figures 3d and S7 inSupporting Information). This expanded accordion-like struc-ture can possibly be ascribed to the larger amount of escapedgases (H2 in this case) because of exothermic nature of HFreaction with Al.3

Interestingly, low concentrations of HF (5 wt %) for 24 hwere still sufficient for selectively etching aluminum, similar tothe higher concentrations (10 wt % HF for 18 h and 30 wt %HF for 18 h), as confirmed by energy dispersive X-ray (EDX)analysis (Table S1). This was further confirmed by a shift of the(002) peak of Ti3AlC2 at 9.5° to 9.0° for the Ti3C2Tx in XRDpatterns (Figure 4a) and no residual peaks of Ti3AlC2 afteretching for 5F-, 10F-, and 30F-Ti3C2Tx. The lower peak shift ofthe basal planes (such as (002) peak) is due to the removal ofAl in the Ti3AlC2 and introduction of surface terminations

Figure 3. SEM images of MAX and MXene powders studied here.SEM of (a) Ti3AlC2 (MAX) powder showing the compact layeredstructure. Multilayered Ti3C2Tx powder synthesized with (b) 30 wt %,(c) 10 wt %, and (d) 5 wt % HF. Accordion-like morphology onlyobserved for the 30 wt % HF etched powder (b). (e) MultilayeredNH4-Ti3C2Tx powder synthesized with ammonium hydrogen fluorideand (f) MILD-Ti3C2Tx powder etched with 10 M LiF in 9 M HCl,both showing negligible opening of MXene lamellas, similar to what isobserved in 5F-Ti3C2Tx.

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(represented as Tx) in Ti3C2Tx (e.g., −F, −O, −OH). Vacuumdrying of the HF etched Ti3C2Tx powders at 80 °C removedthe intercalated water molecules in between MXene flakes.Usually, if XRD is performed on wet or partially dried multi-layered MXene powders, larger shifts in the (00l) peaks areobserved due to the presence of water molecules trappedbetween MXene layers.36,37

Therefore, the accordion-like expanded structure usuallyobserved in SEM should not be considered as the onlyindication of successful etching of Al out of Ti3AlC2, and bothXRD and EDX analyses should be performed to confirmselective etching. Low concentration of HF (≤10 wt % HF)presented in this section as well as etching by in situ formedHF15 result in Ti3C2Tx with less open structure. In the followingsection, we discuss the in situ HF etching with concentration≤5 wt % HF.In situ HF Formation. As reported in most MXenes

literature, concentrated HF has been often the etchant ofchoice to selectively remove Al from Ti3AlC2.

7,38 However,one way to avoid using unnecessarily high HF concentrationsis to use dilute etchant with 5 wt % HF, as shown in theprevious section. The alternative method can be throughin situ HF formation from hydrogen fluoride or fluoride saltssuch as NH4HF2

21,39 or LiF13,15 to prepare etchants containing3−5 wt % HF.Bifluoride-Based Etchants. HF salts, such as NH4HF2,

have been used at room temperature either for a few hours toetch thin coatings of Ti3AlC2

21 or for a longer time of 5 daysto etch Ti3AlC2 powder at room40 or elevated temperature(60 °C).39 Here we describe this approach and delaminationprotocols (discussed later in detail) of MXene powders syn-thesized using NH4HF2. The synthesis of NH4HF2 etchedTi3C2Tx can be done as follows:

(1) A total of 0.5 g of Ti3AlC2 powder was added to 10 mLof 2 M NH4HF2 under continuous stirring, and the

reaction was left to run for 24 h at room temperature(∼23 °C).

(2) Similar to earlier described washing processes, themixture product was washed with deionized H2O viacentrifugation at 3500 rpm (5 times per 5 min per cycle).Further washing was performed using vacuum-assistedfiltration by filtering the Ti3C2Tx product over a PVDFfilter membrane with 0.22 μm pore size and rinsing with1000 mL of deionized water through the filtration setup.

(3) The powder was dried at 120 °C for 24 h in a vacuumoven. MXene produced by this route is referred to asNH4-Ti3C2Tx.

The morphology of multilayered NH4-Ti3C2Tx powdersshown in Figure 3e (see also Figure S8, SupportingInformation) resembles Ti3C2Tx multilayered powders pro-duced with lower concentrations of HF (Figure 3c,d). Here, theselective removal of Al can be confirmed by complete dis-appearance of Ti3AlC2 peaks (for example, no (002) at 9.5°) inthe XRD pattern (Figure 4a). This was also supported by EDXresults showing the removal of Al (Table S1, SupportingInformation). NH4-Ti3C2Tx showed basal plane peaks shift to alower 2θ-anglefrom ∼9.5° in the Ti3AlC2 precursors to 8.5°and 7.15°, which correspond to d-spacings of 10.35 and 12.37 Å,respectively. The larger d-spacing after drying can be explainedby the presence of confined/intercalated (NH4

+) ions and watermolecules in between the MXene layers.21,40 NH4-Ti3C2Tx,unlike the HF-etched Ti3C2Tx, needs a longer duration or highertemperature vacuum drying to completely remove the inter-calated water molecules from the powder. Similar behavior wasobserved for other in situ HF etched Ti3C2Tx samples, asdiscussed in the following section.

Fluoride-Based Salt Etchants. When performing syn-thesis with fluoride-based salts, it is crucial to understand thatthe concentration of LiF and HCl used to prepare in situ HFwill change the quality, size, and ability to process Ti3C2Tx.

13,29

For example, the quality and the size of Ti3C2Tx flakes were

Figure 4. XRD patterns of all the powders in this study and photographs of centrifuge tubes of MILD-Ti3C2Tx. XRD pattern of (a) Ti3AlC2 powderand resulting Ti3C2Tx MXene powders synthesized with 5, 10, and 30 wt % HF and in situ HF by using NH4-Ti3C2Tx routes. (b) The same amountof MILD-Ti3C2Tx powder after the first and eighth centrifuge cycle during washing, showing that the sediment swells after the eighth wash in a50-mL centrifuge tube due to the water intercalation. The swollen sediment after the eighth wash has two layers, the top labeled as Ti3C2Tx (slurry)which is black and the bottom layer, labeled as Ti3AlC2/Ti3C2Tx, having a gray color. (c) XRD pattern of dried film of Ti3C2Tx slurry showing onlyTi3C2Tx and dried powder of Ti3AlC2/Ti3C2Tx mixture showing a mixture of MAX and MXene patterns.

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improved when the LiF:Ti3AlC2 molar ratio was increased from5 to ≥7.5 (that is equivalent to change of the LiF:Ti3AlC2 massratio from 0.67 to ≥1, respectively) as well as the concentrationof HCl from 6 to 9 M.15,29 Although a LiF/HCl mix was usedin both (5 M LiF/6 M HCl) and (7.5 M LiF/9 M HCl)methods, the concentration optimization of these two ingre-dients (LiF and HCl) in the latter method enhanced the quality(fewer defects) as well as the size of produced Ti3C2T flakeswhich enabled studying and quantification of defects in single-layer flakes of Ti3C2Tx.

13,29 Freestanding films produced fromlarger size Ti3C2Tx flakes showed high electrical conductivity(6000−8000 S/cm−higher than other solution-processedinorganic materials) and enabled use of MXene in manyimportant applications.20,41

As previously reported,15 multilayered Ti3C2Tx produced bythe clay method (5 M LiF/6 M HCl) (Figure 2) were isolatedinto single flakes via sonication, which often created small anddefective MXene flakes (Figure 1).15,28 However, hand-shakingwas enough to isolate Ti3C2Tx multilayered powder, producedby (7.5 M LiF/9 M HCl), into single and obviously largeflakes.20 To distinguish this optimized method from the clayLiF/HCl method,15 we called this method minimally intensivelayer delamination (MILD) to stress the fact that a minimal andless milder route was used to produce single and larger flakeswith fewer defects (Figure 1).13,29

MILD method has been the choice for studies/applicationswhere larger flake size is favorable.13,42,43 Here we report on anoptimized MILD synthesis method using (12 M LiF/9 M HCl)instead of (7.5 M LiF/9 M HCl) at room temperature asfollows:

(1) The etchant was prepared by adding 0.8 g of LiF to10 mL of 9 M HCl and left under continuous stirring for5 min.

(2) A total of 0.5 g of Ti3AlC2 powder was gradually added(over the course of 5 min) to the above etchant, andthe reaction was allowed to run for 24 h at roomtemperature.

(3) The acidic mixture was washed with deionized H2O viacentrifugation (5 min per cycle at 3500 rpm) for multiplecycles. After each cycle, the acidic supernatant wasdecanted as waste followed by the addition of freshdeionized H2O before another centrifuging cycle. Thesewashing cycles were repeated until pH 4−5 was achieved.This range of pH was observed after two cycles (when a175-mL centrifuge tube was used) or seven cycles (whena 50-mL centrifuge tube was used).

(4) When pH ≥ 5, there are two important observations:first, a dark-green supernatant is observed, which is stableeven when centrifugation time is increased from 5 min to1 h (see Figure S9, Supporting Information) indicatingdelamination has already started during washing with nofurther processing steps. Second, the Ti3C2Tx sedimentsettled at the bottom of the centrifuge tube, after collec-ting the supernatant, is found to be swollen to almosttwice the volume compared to the previous washingcycles, when no delamination occurred. The swelledsediment exhibits a distinctive black layer of Ti3C2Tx

slurry above a grayish layer of nonetched Ti3AlC2/Ti3C2Tx mixture (Figure 4b).

(5) The black Ti3C2Tx slurry was carefully collected,separately from Ti3AlC2/Ti3C2Tx, with a spatula andvacuum-filtered producing a thick black cake (Figure 4b,

top right inset). A thick film was formed after drying invacuum at 120 °C for 24 h. All these steps are shown inFigure S10a-d, Supporting Information. It is worth notinghere that the film made from the Ti3C2Tx sediment(Figure S10d) was flexible.

The swelling of Ti3C2Tx sediment after washing was onlyobserved in our previous report29 as well as shown in theoptimized MILD described above. This swelling behavior ofTi3C2Tx during washing cycles after synthesis was not observedin the earlier “clay” synthesis using LiF/HCl (5 M ofLiF:Ti3AlC2) with both 6 M HCl and 12 M HCl.15,29,44

This is another important difference between the two methods,and it confirms that there is a minimum concentration of Li+

required, besides a certain concentration of HCl, when prefor-ming etching at room temperature for ≤24 h. Cross sectionalSEM images of the film (Figure S9d) made by vacuum-assistedfiltration of the black layer of Ti3C2Tx slurry (Figure 4b, topinset) are shown in Figure S11, Supporting Information. TheXRD pattern of this black slurry sediment MILD-Ti3C2Tx thickfilm after vacuum drying at 120 °C exhibited only the (00l)peaks of Ti3C2Tx MXene with the (002) peak centered at 8.6°(Figure 4c, top pattern). The SEM image of the thick film madeby filtering Ti3C2Tx slurry sediment Figure S9d is very similarto that of the rolled/sheared film of the Ti3C2Tx clay,

15 and theXRD pattern is very similar to that of the films made fromdelaminated single- to few-layer Ti3C2Tx reported before.15

In other words, the as-separated/collected Ti3C2Tx slurrysediment does not require any type of mechanical vibration orshearing and only addition of deionized H2O to the Ti3C2Txslurry, followed by gentle shaking, is needed to make thedelaminated MILD Ti3C2Tx film possibly due to presenceof solvated Li ions between the MXene layers. The XRDpattern of the dried gray material from the bottom of the tube(Figure 4b, bottom inset) showed a mixture of nonetchedTi3AlC2 and Ti3C2Tx (Figure 4c, bottom pattern). SEM imageof this layer shows that it contains Ti3C2Tx particles (Figure 3f),similar to that of Ti3C2Tx produced by etching in low-concentrations of HF (Figure 3d). Thus, we recommendseparation of the black Ti3C2Tx slurry sediment from the graysediment containing a mixture of nonetched and etchedparticles, as explained in step 4 of this section (Figure 4b).

Choice of Intercalation Method. Using intercalatingcompounds to expand the interlayer spacing of Ti3C2Tx flakeshas proven to be a key step for weakening the interactionsbetween 2D layers and delaminating MXene into separate 2Dsheets.27 The intercalation and delamination process typicallyrequires a suitable solvent for the material and intercalant, amixing step where the intercalant is introduced between 2Dsheets, occasionally a sonication step depending on desiredflake size and concentration, and a centrifugation step to isolatethe delaminated material from the material which is multi-layered (nondelaminated).25 The final colloidal solution willcontain dispersed 2D sheets of electrostatically stabilizedMXene, which are stable against aggregation, or clumping,and are processable and functional. Moreover, the sonicationstep is highly dependent on the etching method and the desiredapplication, as well as the required concentration. Sonicating forlonger times and higher powers will exhibit smaller flakes withmore defects and may yield concentrations different than thosewhich are not sonicated. The concentration of MXene sheetsin solution also depends on different parameters such as thesynthesis methods and type of intercalants used to weaken the

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interlayer interaction between MXene sheets. In this section,we report on the advantages and disadvantages in processingof Ti3C2Tx MXenes using different intercalants that causedelamination with or without sonication.Dimethyl Sulfoxide (DMSO). Large, organic molecules,

such as dimethyl sulfoxide (DMSO), were some of the first inter-calants used for expanding the interlayer spacing of Ti3C2Tx

MXenes synthesized with HF.25 Additionally, DMSO or otherorganic solvents can be used as dispersants for Ti3C2Tx.

45 Whenusing DMSO or other organic solvents, sonication is required todisperse the Ti3C2Tx sheets in solution, and without sonication,the multilayered sheets will fall to the bottom of the vial assediment (first vial from the left in Figure 5a). Flake sizesproduced by these methods are usually on the order of a fewhundred nanometers.Tetraalkylammonium Hydroxides. Tetraalkylammonium

compounds, such as tetrabutylammonium hydroxide (TBAOH)and tetramethylammonium hydroxide (TMAOH), havebeen used to exfoliate 2D layered oxides.46 This delaminationtechnique is based on ion exchange between protons (H+)and the bulky tetraalkylammonium ions (TMA+ or TBA+)leading to swelling and delamination with manual shaking,when used on different materials, such as layered oxides.46

TBAOH has also been used to exfoliate MXenes, suchas Ti3CN, Mo2C, V2C, and Nb2C. However, TBAOH couldnot delaminate Ti3C2Tx, even with sonication.27 In anotherstudy, Ti3C2Tx was claimed to be synthesized and delam-inated using TMAOH after pre-treatment of Ti3AlC2 surfaceusing 20−30 wt % HF for a short time.47 However, we havenot been able to reproduce the results by only using TMAOHas etchant and we were only able to delaminate Ti3C2Tx intosingle- and few-layers when HF was used as an etchantand TMAOH was used an intercalant, regardless of HFconcentration.Here, we used TMAOH to delaminate all the HF-synthesized

and NH4HF2-synthesized Ti3C2Tx samples in this study.Delamination of 100 mg of each material resulted in darkerand more concentrated colloidal solutions for HF-etchedTi3C2Tx in comparison to NH4HF2-synthesized Ti3C2Tx. Thedetailed process of delaminating multilayered Ti3C2Tx powders

with the help of TMAOH can be described as follows:

(1) A total of 100 mg of each MXene powder (5F-Ti3C2Tx,10F-Ti3C2Tx, 30F-Ti3C2Tx or NH4-Ti3C2Tx) was mixedseparately with 10 mL of deionized H2O containing100 mg of TMAOH in a 20-mL glass vial, and themixture was stirred for 12 h at room temperature.

(2) The basic mixture (∼pH 10) was washed twice viacentrifugation (3500 rpm, 5 min per cycle) using 50 mLtubes to bring the pH to ∼7.

(3) The stable MXene colloidal solutions were collected after1 h of centrifugation at 3500 rpm, and the sedimentswere collected via vacuum assisted filtration and dried invacuum at 120 °C.

Using this method, all three HF-etched samples can bedelaminated, and stable colloidal solutions are formed. TheTMA+ delaminated colloidal solution of 5F-Ti3C2Tx MXene(labeled as TMA-5F-Ti3C2Tx) is black and stable (Figure 5a)with concentration of ∼0.5 mg/mL. When the solution wasanalyzed via dynamic light scattering (DLS), the size of theflakes collected after an hour of centrifugation at 3500 rpm wasin the range of 0.2 to 0.7 μm (Figure 5b), which is similar to theflake size reported for the sonicated LiF/HCl clay method.41

This shows that delamination of MXene with stirring inTMAOH can break the flakes into smaller sizes. The breakageof flakes due to intercalation of TMAOH was previouslyobserved in layered 2D oxides even with stirring, and largerflakes were only obtained with gentle shaking.46

The intercalation of TMA+ into 5F-Ti3C2Tx was confirmedby the (002) peak shift from 9.0° to 6.0° 2θ-angle (Figure 5c)for 5F-Ti3C2Tx which corresponds to a d-spacing shift of 9.7 Åto 14.7 Å. This shows that TMA+ intercalated the Ti3C2Tx

flakes and stayed in between the layers even after 120 °C dryingin vacuum. A higher temperature is needed to remove theintercalated molecules, as the decomposition temperature ofTMAOH exceeds 200 °C.48,49

Similarly, a stable but less concentrated colloidal solution ofTi3C2Tx MXene can be made by intercalation of TMAOH intoNH4-Ti3C2Tx powder followed by sonication. The inter-calated NH4-Ti3C2Tx powder (labeled as TMA-NH4-Ti3C2Tx)showed similar (00l) peak positions as the TMA-5F-Ti3C2Tx.

Figure 5. (a) Optical images of aggregated Ti3C2Tx powder and stable colloidal aqueous solutions of Ti3C2Tx prepared with different intercalants and(b) the respective particle size analysis as well as (c) XRD patterns of dried films with TMA-5F-Ti3C2Tx, TMA-NH4-Ti3C2Tx, and MILD-Ti3C2Txmethods.

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The (002) peak is at 5.9°, which is slightly shifted further to theleft and corresponds to 14.86 Å interlayer spacing (Figure 5c).This successful delamination of Ti3C2Tx due to intercalation

of TMA+, regardless of HF content, suggests that intercalationof larger ions like TBA+ is possible with more prolongedstirring time. To test our hypothesis, we stirred 0.2 g of30F-Ti3C2Tx in 20 mL of deionized H2O containing 0.2 g ofTBAOH for 2.5 days. This long time of stirring resulted in astable and concentrated colloidal solution after sonication(Figure S12, Supporting Information), collected after 1 h ofcentrifugation at 3500 rpm.Lithium Ions. When etchants containing lithium are used,

such as LiF/HCl, in both clay and MILD methods,delamination is driven by the intercalation of solvated Li+

ions. However, sonication of the intercalated Ti3C2Tx wasrequired in the case of the clay method and was optional in thecase of the MILD method (Figure 2). In the following, wedescribe the effect of intercalant (Li+) used in the MILDmethod for delamination of Ti3C2Tx flakes (MILD-Ti3C2Tx).Because no sonication is needed, these flakes are larger withfewer defects. However, to prepare high-quality, large flakes inhigh concentrations the following notes should be considered:

(1) The mass ratio of LiF to Ti3AlC2 should be ≥1(corresponding to molar ratio of ≥7.5) as the Li+

content and the H+ provided by HCl plays a majorrole in the differences between the clay and MILDmethods described in Figure 2.

(2) In the case of in situ HF formation from LiF and HCl,the LiF must be the limiting reagent (no excess of LiFcan be present) when added to HCl, not vice versa.This reaction produces lithium chloride (LiCl), andcopious washing with deionized H2O is required tocompletely wash the LiCl salt away.

(3) The excess HCl remaining in the etchant results in ahigher concentration of protons (H+) and enables ionexchange with Li+, resulting in swollen clay-like Ti3C2Tx(Figure 4b) which can be delaminated by manual shaking.This is confirmed by higher concentration of the delam-inated solution of MXene when 9 M HCl was used insteadof 6 M HCl.

Considering all these parameters, here is the best method, toour knowledge, to delaminate MILD-Ti3C2Tx and obtain astable colloidal solution:

(1) As explained in the etching section, after the thirdcentrifuge wash for 1 h using a 175-mL graduated conicaltube (Figure S9), a stable dark-green supernatant isproduced. This supernatant can be collected as a stablecolloidal solution with MXene concentration of ∼1 mg/mL,without any further processing.

(2) As mentioned earlier, because this method results inswollen black Ti3C2Tx slurry with some nonetchedTi3AlC2 (Figure 4b), separation of Ti3C2Tx fromnonetched Ti3AlC2 must be performed before delamina-tion. The separation process was either done as men-tioned above in step 5 in the MILD method or by theaddition of 50 mL of deionized H2O to the wholesediment (Ti3C2Tx slurry and the nonetched Ti3AlC2)followed by manual shaking until all the sediment wasredispersed. Then, we separate the nonetched Ti3AlC2(gray sediment in Figure 4b) via centrifugation at 3500 rpmfor 2 min followed by collection of the dark concentratedsupernatant of Ti3C2Tx flakes. Centrifuging the collected

supernatant for 1 h at 3500 rpm resulted in a blackclay sediment (Figure S10a, Supporting Information)and stable dark supernatant (Figure 5b) mainly con-sisting of single-layer flakes of Ti3C2Tx. The average con-centration of this colloidal solution is about 1−2 mg/mL,which is higher than that of TMA delaminated solutions(<0.5 mg/mL).

It is noteworthy that the colloidal solutions of MILD-Ti3C2Txoften contain larger flakes (>2 μm) as shown in Figure 5b.We also drop casted diluted delaminated solutions of bothMILD-Ti3C2Tx and TMA-5F-Ti3C2Tx on a porous aluminamembrane and compared the flake size distributions in SEM.The flakes of MILD-Ti3C2Tx have larger sizes (Figure 6a,b) in

comparison to MXene flakes in colloidal solution prepared viaTMAOH intercalation (Figure 6c,d). Additionally, the MILD-Ti3C2Tx flakes have distinctive and straight edges, mimickingthe original shape of the Ti3AlC2 crystalline grains. In contrast,in the case of TMAOH delamination, flakes are brokeninto small fragments, and their edges appear to contain moredefects, showing that stirring or intense shaking when usingorganic base intercalants, such as TMAOH, leads to fracturedflakes. This strong or excessive shaking after intercalation ofTMAOH or TBAOH, even for a short time, has been shown tocause breakage of flakes in 2D layered oxides.46,50 Thus, it isadvised to perform gentle shaking if larger flakes are desiredwhen using TMAOH or TBAOH, similar to the protocolpreviously adapted for delamination of 2D layered oxides.46,50

As explained, synthesis of MXene is a top-down method andtherefore the flake size produced in the MILD method isdirectly related to the grain size and shape of the starting MAXpowder. Thus, large crystals of MAX phases are needed toachieve larger MXene flakes.It is important to note that the synthesis protocols described

above are applicable not just to small laboratory samplescharacterized in this study but also to large scale manufacturing.We can produce up to 100 g of MXene per batch using areactor designed for us by the Materials Research Center(Kiev, Ukraine). Control of the synthesis conditions becomeseven more important when large amounts of MXenes aremanufactured.

Figure 6. SEM images of Ti3C2Tx MXene flakes collected fromcolloidal solution. (a, b) MILD-Ti3C2Tx and (c, d) TMA-NH4-Ti3C2Tx on porous alumina membrane showing electron-beamtransparent flakes of MXene. MILD-Ti3C2Tx has larger flakes withdistinctive and straight edges possibly similar to the original shape ofcrystalline grains of their Ti3AlC2 precursor.

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Processing, Deposition, and Storage. MXenes can beprocessed, deposited, and stored in a variety of ways. Once thematerial has been processed by delamination, size selection, orsonication, it can be deposited as freestanding film or thinfilm, painted, or sprayed on diverse substrates depending onthe application. However, MXene flakes have been shown todegrade in the presence of a water containing dissolved oxygen,especially at elevated temperatures and under sunlight, makingstorage in aqueous solutions challenging.51,52

So far we have presented different routes to etch and delam-inate Ti3C2Tx, as well as prepare stable high-concentrationcolloidal solutions with controlled flake sizes. In the following,we are going to discuss the current practices to process, deposit,and store MXenes, based on our experience and the availablepublished information.Sonication and Size Selection. The size of the flakes can

be controlled by performing cascading centrifugation to isolateand fractionate flakes based on lateral size or by decreasing theflakes by bath or probe/tip sonication, and some of theseprocessing techniques have been used in protocols for other 2Dmaterials.53 Figure S13 shows flakes of colloidal solution ofMILD-Ti3C2Tx can be separated by size via centrifugation for 2min at 2500, 5000, and 10000 rpm, and the collected colloidalsolution contained flake sizes as large as 2 μm after 2500 rpm,∼1 μm after 5000 rpm and as small as ∼200 nm after 10000 rpm.When performing sonication, it is critical to monitor the

power output of the sonicator as well as the time spentsonicating. Each of these parameters will change the resultingconcentration, the quality of flakes, and their lateral size distri-bution. For example, it has been shown that bath sonication ofa MILD-Ti3C2Tx can change the flake size from ∼4 μm to lessthan 1 μm.39 When performing bath sonication, it is desired tokeep the bath at a constant temperature and avoid exposingthe material to elevated temperatures, to prevent oxidation.We monitor the water in the bath to ensure the temperatureis constant, and we may periodically add ice to the bath.Furthermore, to prevent oxidation during sonication, we bubbleargon (Ar) through the solution. When performing sonicationby a tip or probe sonicator, we use pulse sonication where theprobe is on for 8 s and off for 2 s, and we also use a coolingsystem surrounding the sample, keeping the solution temper-ature ∼ 5 °C, similar to the temperature of a refrigerator.Deposition of Flakes. Solution-processed MXenes can be

deposited using a variety of methods, including vacuum-assistedfiltration,20,54 spin coating,12,34,55 spray coating,22,41 or roll-ing15,56 as shown in Figure 7. Each technique can be tailored fora specific application. For example, spin coating may produceeven, uniform, thin coatings for optics or electronics,12,55

whereas vacuum-assisted filtration and rolling are more suitablefor making battery or electrochemical capacitor electrodes,7,57

while spray coating is convenient for producing large-area filmsfor electromagnetic interference shielding.20

Other techniques, such as screen printing, inkjet printing orslurry rolling, can be employed. Rolling of MXene slurry with aclay-like rheology can provide a way to quickly deposit materialand produce thick films for electrode applications.54 PrintingMXene allows for precise control over the pattern, increasingthe amount of material by increasing the number of layers;however, it requires ink formulation for optimal results.In the most simplistic manner, nanosheets can be vacuum-

filtered to create freestanding films.25 A vacuum-filtration setupof any size can work, assuming a filtration membrane (typicallya porous polymer such as Celgard or PVDF) can fit over the

Buchner funnel and the pore size is small enough to preventMXene flakes from passing through the filter. Using thismethod, freestanding films with controlled thickness can bemade of delaminated MXenes and their hybrids with othernanomaterials, such as carbon nanotubes.58

These colloidal solutions of delaminated MXenes can also bedeposited onto substrates by spin or spray coating for thinnercoatings compared to vacuum-assisted filtration. Spray depositionhas obvious advantages; it is easy to use, can cover large areas,and can be done on any surface, also outside the lab. However,the coating is less uniform and often produces rougher films22,41

compared to spin coating.12 Spin coating has the potential tocreate even, uniform, and optical quality films, for applicationswhere thickness and transparency are important.12

Substrate Functionality. Titanium-based MXenes havebeen successfully deposited on simple substrates such aspaper,56 plastic (PET) substrates,20,41 glass55 and silicon.12

Because MXenes are inherently hydrophilic and negativelycharged, the nature of the chosen substrate in terms of hydro-phobicity versus hydrophilicity and surface charges is important.Based on our experience, hydrophilic surface treatments such asUV/ozone or O2 plasma may be used to make the substratesurface hydrophilic, or one may use liquid treatments such asPiranha solution on glass or silicon.22,55

Storage of Material. Once synthesized, the material mayneed storage before use. Elimination of dissolved oxygen is thefirst step to suppress oxidation, and this can be done by storingthe solution in argon-sealed vials. Second, refrigeration canalso suppress the oxidation by decreasing the temperature andeliminating UV radiation. A detailed study on the shelf life ofTi3C2Tx and Ti2CTx has been published recently.52

Based on our experience, the rate of degradation and qualityof the solution can be monitored by ultraviolet−visible absor-bance (UV−vis) spectroscopy over time and noting thechange in absorbance of the main peak (e.g., 700−800 nmfor Ti3C2Tx).

52 Aqueous solutions that are kept in open vialsat room or elevated temperatures start degrading almostimmediately, with large amounts of TiO2 forming a weekafter synthesis. By placing the aqueous colloidal solution in therefrigerator, the oxidation process can be slowed dramatically.When the solution is kept refrigerated in vials sealed under Ar,the degradation is minimal over the course of 24 days.52

Figure 7. Fabrication of Ti3C2Tx MXene electrically conducting filmsvia vacuum assisted filtration (left), spray-coating (middle), andpainting (right) of MXene ink over various flexible substrates.

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Additionally, because the aqueous solutions may cause degra-dation of the samples, storing MXenes in organic solvents canbe advantageous, due to the stabilizing solvation shell which isprovided by the organic solvent. A dark supernatant remainedstable even after months of dispersing the material in proticorganic solvents, suggesting that this may be the best way whenlong-term storage of the material in solution is needed.45

Additionally, since MXenes can be redispersed in differentsolvents, another practice to store MXenes is to filter thecolloidal solution and store the resulting film under vacuum.When colloidal solution is needed, MXene film can be redis-persed in the solvent of choice by either manual shaking orsonication, depending on the solvent/material combination.45,52

Note that drying the film at elevated temperature may lead tostrong bonding between the layers, making redispersion difficult.Characterization Methods. Despite the differences in

methods and protocols reported in many papers throughoutthe 2D materials community, basic characterization protocolsare necessary to ensure reproducibility.53 To characterizeMXene flakes, many methods are used to probe the structureand morphology of the flakes, such as XRD, SEM, transmis-sion electron microscopy (TEM), and Raman spectroscopy.Elemental composition of Ti3C2Tx MXene powder/films canbe characterized by EDX. We use a Zeiss Supra 50VP SEM, andEDX spectra were collected from 5 different locations at250× magnification (an area of almost 0.5 × 0.5 mm2) for 30 s.A powder diffractometer (Rigaku Smart Lab, U.S.A.) with CuKα radiation was used to collect the XRD patterns of MXenepowders/films and MAX powders at a step size of 0.03° with0.5 s dwelling time. MXene powders are placed on a 2 cm ×2 cm engraved or flat glass substrates for XRD analysis.We pressed the powder with a glass slide manually to intensifythe (00l) peaks and monitored if the (002) peak of nonetchedTi3AlC2 appears in the XRD pattern. The lateral size of flakescan be measured using SEM by depositing colloidal solution ofMXenes on porous alumina substrate (Anodisc, 0.1 μm poresize, Whatman) or by using dynamic light scattering (DLS) onthe colloidal solution to confirm the particle size (ZetasizerNano ZS, Malvern Instruments, U.S.A.). Ideally, more than onetechnique should be used to obtain reliable data. For example,it is necessary to use DLS in conjunction with SEM to confirmparticle size distributions, as shown in Figures 5 and 6. Mea-suring the properties derived from a specific synthesis route canbe used to quantify differences between the methods and selectthe synthesis method for a specific application. For example, theelectrical conductivities of spray/spin coated, vacuum-filtered,or painted films of Ti3C2Tx MXene can be measured using afour-point probe (ResTest v1, Jandel Engineering Ltd., Bed-fordshire, U.K.) with a probe distance of 1 mm. Free-standingfilms produced by the MILD method exhibit high electricalconductivity of ∼8000 S/cm in comparison to films producedthrough TMAOH intercalation (∼200 S/cm).

■ SUMMARYIn this Methods and Protocols paper, we have described bestpractices used to synthesize and process Ti3C2Tx MXene atroom temperature, as well as various techniques for makingthin coatings or free-standing films of MXene. We showed thatas low as 5 wt % HF can etch aluminum out of the Ti3AlC2MAX phase, but the accordion-like morphology of particlesis usually observed when ≥10 wt % HF solutions are used.When in situ HF formation was used (NH4HF2 and LiF/HCletchants), similar MXene particle morphologies were observed.

In the case of LiF/HCl, which is the least harsh etchant, someresidual nonetched particles may remain. However, the MILD-LiF/HCl method offers the largest flake size and best quality(least defects) among all etching methods. Moreover, manyproperties, such as electrical conductivity of Ti3C2Tx films, aresynthesis and processing dependent. For example, Ti3C2Txfilms produced from colloidal solutions made via TMAOHintercalation showed high resistivity in comparison to filmsproduced by the MILD method. Therefore, we recommendusing MILD synthesis in applications where high electrical con-ductivity, larger flake sizes, environmental stability, or mechan-ical properties are important and alternative methods, such as“clay” or HF etched Ti3C2Tx, where smaller or more defectiveflakes are necessary (for example, for catalysis and selectedelectrochemical or biomedical applications).Furthermore, we summarized the best practices to process,

deposit, and store MXene films and colloidal solutions.We have explained our methods for characterization of thesematerials by performing microscopy, diffraction, performancetesting (for example, conductivity), and light scattering.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on the ACSPublications website at DOI: 10.1021/acs.chemmater.7b02847.

Additional figures, details, and photographs (PDF)Movie S1: Synthesis of Ti3C2Tx MXene showing anddescribing the process that should be followed whenadding Ti3AlC2 powder to an etchant (MP4)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: gogotsi@drexel.edu (Y.G.).

ORCIDMohamed Alhabeb: 0000-0002-9460-8548Kathleen Maleski: 0000-0003-4032-7385Babak Anasori: 0000-0002-1955-253XYury Gogotsi: 0000-0001-9423-4032Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

M.A. and synthesis of MXenes were supported by the FluidInterface Reactions, Structures and Transport (FIRST) Center, anEnergy Frontier Research Center funded by the U.S. Departmentof Energy, Office of Science, Office of Basic Energy Sciences.K.M. was supported by the Leading Foreign Research InstituteRecruitment Program through the National Research Foundationof Korea (NRF) funded by the Ministry of Education, Scienceand Technology (MEST), via the NNFC-KAIST-Drexel NanoCo-Op Center (NRF-2015K1A4A3047100). B.A. and Y.G. weresupported by the King Abdullah University of Science andTechnology under the KAUST-Drexel University CompetitiveResearch Grant. XRD and SEM analysis were performed at theCentralized Research Facilities (CRF) at Drexel University.

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