Guidelines for Synthesis and Processing of Two-Dimensional

Methods/Protocols

pubs.acs.org/cm

Guidelines for Synthesis and Processing of Two-Dimensional Titanium 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 different compositions have been synthesized by the selective etching of MAX phase and other precursors and many more theoretically predicted. They offer a variety of different properties, making the family promising candidates in a wide range of applications, such as energy 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 of ways. Due to this promise, the amount of research on MXenes has been increasing, and methods of synthesis and processing are expanding quickly. The fast evolution of the material can also be noticed in the wide range of synthesis and processing protocols that 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 the ff ff most studied MXene, titanium carbide (Ti3C2Tx), using di erent etchants and delamination methods. We also explain e ects of synthesis parameters on the size and quality of Ti3C2Tx and suggest the optimal processes for the desired application.

■ INTRODUCTION mechanical exfoliation hardly possible. To date, more than Since the isolation of single layer graphene in 2004,1 20 members of the MXene family have been synthesized, and dozens more are predicted, making it one of the fastest growing two-dimensional (2D) materials have gained tremendous atten- 7 tion because of their distinctive properties relative to their bulk 2D material families. Additionally, the MXene family comes in form. The isolation of graphene has become a reference for all three atomic structures, ranging from M2XtoM3X2 and M4X3, yielding tunability and opportunity to discover and mold materials 2D materials and opened the possibility to discover even more. 7,10,11 Today, there are dozens of new 2D materials, including hexag- based on necessary demands. In contrast to most other 2D materials, including graphene, onal boron nitride, transition metal dichalcogenides, transition 7 metal oxides, clays, etc.2 MXenes possess hydrophilic surfaces and high metallic conductivities (∼6000−8000 S/cm),12,13 showing promising − Downloaded via NORTH DAKOTA STATE UNIV on October 8, 2018 at 18:08:40 (UTC). In 2011, a new family of 2D materials, named MXenes, were 14 17 3 performance in energy storage devices, water desalina- discovered by Drexel University scientists. The MXene family 18 19 20 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. is comprised of transition metal carbides, carbonitrides, and tion, catalysis, electromagnetic interference shielding, transparent, conducting thin films,12,21,22 and many other nitrides with a general formula of Mn+1Xn, where M represents 7 transition metals (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, applications. 3,4 “ ” Since their discovery, production of multilayered MXene etc.) and X is carbon and/or nitrogen. The name MXene fl was given to describe the similarities between this 2D material akes was possible through wet-chemical etching with HF, and different MXene compositions were synthesized in this form, family and graphene but also to recognize the parent ternary 3,23,24 such as Ti2CTx,Ti3CNTx,Nb2CTx,andV2CTx (Figure 1). carbide and nitrides, MAX phases, which MXenes are fl synthesized from.5 MAX phases are layered ternary carbides However, it was not until 2013 when single-layer MXene akes were isolated by intercalating large organic molecules and and nitrides with a general formula Mn+1AXn, where A delaminating the sheets from each other, opening the door to represents elements from the group 13 and 14 of the periodic 25 table.5,6 exploration of the truly 2D nature of MXenes. In 2014, MXenes reported to date are synthesized by wet-chemical Ti3C2Tx MXene was produced using a HF-containing etchant fl 21 etching in hydrofluoricacid(HF)orHF-containingor such as ammonium bi uoride (NH4HF2) salt, and later in the − HF-forming etchants,7 9 which add surface functionalities − − − such as O, F, or OH, represented by Tx in this formula Received: July 8, 2017 as Mn+1XnTx. Etching is required because of strong chemical Revised: August 25, 2017 bonds between A and M elements in MAX phases that make Published: August 25, 2017

fi Figure 1. Timeline of MXene: a journey from 2011 to now. In 2011, the rst MXene multilayered powder (Ti3C2Tx) was made by selective etching fl 3 ff of Al from Ti3AlC2 using HF acid. A schematic of Ti3C2Tx ake structure is shown as an example. In 2012, a variety of di erent MXenes, such as Ti2CTx, (Ti,Nb)2CTx, (V,Cr)3C2Tx,Ti3CNTx, and Ta4C3Tx, were also synthesized, and MXenes became a family of transition metal carbides and carbonitrides.21 In 2013, single layers of MXenes were isolated by intercalation and delamination with organic molecules.23 In 2014, in situ HF 24 15 etchants such as NH4HF2 or LiF/HCl mixtures were used. The latter resulted in Ti3C2Tx with clay-like behavior and is called the clay method. fl 15 With sonication of Ti3C2Tx clay in water, submicrometer lateral-size single akes of Ti3C2Tx were isolated. 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, ff ff ordered-double-transition metal carbides, where two di erent transition metals occupy di erent atomic layers, were discovered (such as Mo2TiC2Tx, ff 27 fl Cr2TiC2Tx,Mo2Ti2C3Tx), adding more than 25 di erent possible MXenes to the family. In 2016, delamination and isolation of large single akes μ of Ti3C2Tx (>2 m) were achieved without sonication through optimization of the LiF/HCl etching method. The optimized method is referred to as fl 28 MILD, making single- ake characterization possible. In 2017, by synthesizing in-plane double transition metal MAX phase ((Mo2/3Sc1/3)2AlC) and 8 etching one of the transition metals, MXene with ordered divacancies (Mo1.3CTx) was synthesized. SEM images in 2014 and 2016 show Ti3C2Tx flakes on an alumina membrane. Scale bars are 2 μm. “ ” same year, Ti3C2Tx clay was synthesized by taking advantage etch aluminum (Al) out of Ti3AlC2 with concentrated HF of making in situ HF through addition of lithium fluoride (LiF) (∼50 wt %) at room temperature.25 Based on the type of salt to hydrochloric acid (HCl), significantly simplifying the MXene and the synthesis conditions, the intercalants may vary synthesis method and improving MXene performance in energy as well as the delamination yield.7 We recently summarized storage applications (Figure 1).15 With the need for methods to etching and delamination conditions reported in literature for increase the delamination yield, in 2015 additional organic most MXenes known to date, and we encourage the readers to intercalants were used, such as isopropylamine26 and tetra- view the referenced review on etching, delamination, and 27 7 butylammonium hydroxide (TBAOH). Moreover, in 2015, processing protocols for MXenes besides Ti3C2Tx. two new MXene subfamilies, ordered double-transition metal Here, we discuss the best practices used in our lab and ′ ″ ′ ″ ′ ″ MXenes, M 2M C2, and M 2M 2C3, where M and M are two provide step-by-step guidance for room temperature synthesis different early transition metals, were discovered, further expand- of titanium carbide (Ti3C2Tx), the most studied and widely ing the 2D family.28 Each of these evolutionary discoveries has used MXene. We categorize currently used etchants for added tunability and diversity to MXene family. selective removal of Al from Ti3AlC2 structure into two fl In 2016, larger single Ti3C2Tx akes were isolated by using types: synthesis protocols using HF and protocols using in situ the minimally intensive layer delamination (MILD) meth- formed HF. Moreover, we provide a guideline on how to od,20,29 where manual shaking (no sonication) was used and delaminate as-synthesized materials into colloidal solutions resulted in larger and less defective single flakes of MXene.13 even without sonication. We also discuss storage, processing, The MILD method expanded the opportunities for research and deposition techniques to make different types of con- fi on electronic, optical, and size-dependent material properties ductive Ti3C2Tx MXene lms. as well as further enhanced the scalability and production of ■ DISCUSSION OF METHODS Ti3C2Tx MXene. A timeline progress of MXenes is shown in Figure 1. There are many different protocols for synthesizing MXene The exact synthesis conditions used to produce MXenes materials reported in literature,7 and one synthesis route may influence the resulting properties and thus are directly related be better for one application but not suitable for another to the performance of MXenes in their applications. In general, application. When synthesizing any kind of MXene, one must the synthesis of any type of MXene can take from a few hours be aware of the end goal, the properties desired, and the to a few days, and it depends on at least a few factors: con- materials warranted, to develop the best material for the job. centration of HF and temperature of etching. For example, in Synthesis of MXene begins with etching the A-element the case of Ti3C2Tx, we used as low as 3 wt % HF containing atomic layers (for example, aluminum) in a MAX phase etchant and showed the higher the HF content, the higher the (for example, Ti3AlC2) from the Mn+1Xn layers adjacent to the fl fl concentration of defects in Ti3C2Tx akes, which in uences the A-element layers. The etching process results in terminated quality, environmental stability, and properties of as-produced multilayered MXene powders (for example, Ti3C2Tx) with the MXene.13,29 Our group showed that 2 h was enough time to 2D layers held together by hydrogen and van der Waals bonds.3

7634 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

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 the ff ff ffi HF route, three di erent concentrations with di erent durations were used, all su cient to synthesize Ti3C2Tx. In situ HF is the alternative way compared to the direct use of hazardous HF in which the resulting Ti3C2Tx multilayered powder behaves like clay in comparison to powders produced 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 fl + (TMAOH) followed by sonication, if required. In the case of lithium uoride/hydrochloric acid (LiF/HCl), the resulting Li intercalated Ti3C2Tx clay can be delaminated by sonication (clay) or without sonication (MILD) depending on the concentration of LiF and HCl used to prepare the etchant.

After the etching is finished (that is, the complete removal of Supporting Information. However, based on the previously ff the A-element layers), washing is required to remove residual reported studies, Ti3AlC2 can be made from di erent starting ∼ 30 30 acid and reaction products (salts) and achieve a safe pH ( 6). mixtures, for example, Ti, Al, and C; Ti, Al4C3, and C; or 31,32 Washing is normally done by repeated centrifugation to sep- even TiO2, Al, and C under proper conditions. In the arate multilayered MXene from acidic solution and decantation earlier studies of MAX phases, before the discovery of MXenes, of the acidic supernatant (Figure S1a, Supporting Information). pressure was used (for example, hot pressing or hot isotactic After the pH is increased to ∼6, the multilayered flakes can be pressing)30,31 to obtain fully dense MAX phase samples. When fi collected, e.g., via vacuum-assisted ltration (Figure S1b, Ti3AlC2 powder is needed, no pressure is required, since a less Supporting Information) and dried in vacuum (Figure S1c, dense block of MAX phase is easier to mill into powder. Supporting Information). For the rest of this study, we will In general, Ti3C2Tx MXene can be made from Ti3AlC2 regard- focus on synthesis and processing of one type of MXene, less of the MAX phase sintering route. However, the sintering ff Ti3C2Tx. conditions of each MAX phase can result in di erent impurities 32 General Synthesis and Processing of Ti3C2Tx MXene. in Ti3AlC2, such as TiC, Ti2AlC, and Al2O3. ff Figure 2 describes a general map for the synthesis of Ti3C2Tx at Powder X-ray di raction (XRD) can reveal the existence of room temperature with HF or in situ HF and delamination of the impurity phases. A common impurity in Ti3AlC2 is Ti2AlC. fi multilayered MXene powders produced by each technique, One method to distinguish Ti3AlC2 from Ti2AlC is its rst XRD ∼ ° with choice of intercalants, where sonication can be optionally peak (002) because Ti3AlC2 has a (002) peak at 9.5 which is ff ∼ ° used to meet certain application requirements (e.g., the need di erent from that of Ti2AlC at 13 (Figure S2a, Supporting fl for higher concentrations or smaller akes). In the following Information). The coexistence of Ti2AlC MAX phase in Ti3AlC2 sections, we report the best practices, to our knowledge, to can lead to a mixed phase MXene of Ti3C2Tx with Ti2CTx, synthesize Ti3C2Tx MXene with emphasis on using minimal which often leads to changes in properties (conductivity, optical concentration of HF based etchants as well as describing absorbance spectra, etc.) of the resulting MXene and most delamination and processing techniques used to fabricate thin importantly can influence the interpretation of the reported fi 33,34 coatings or free-standing lms of Ti3C2Tx. results. Also, diluted colloidal solutions of Ti3C2Tx optically Choice of Ti AlC Precursor. Choice of Materials. 3 2 To date, exhibit green color while Ti2CTx solutions have a purple/magenta − the main starting ingredient of any MXene is the precursor. color. The UV vis spectra of Ti2CTx,Ti3C2Tx,andmixedphase Ti3C2Tx is made from Ti3AlC2 MAX phase. Ti3AlC2 has been Ti2CTx/Ti3C2Tx are distinguishable from one another, and details synthesized via a variety of routes using different raw materials, are presented in Figure S2b−d, Supporting Information.The ff compositions, and sintering conditions. Ti3AlC2 is stable at e ect of sintering conditions of MAX phase on the resulting 1350 °C or even higher temperatures;14 therefore, when the MXene is yet to be studied, and it is out of the scope of this required stoichiometric Ti:Al:C ratio is available at the proper paper. We only used one batch of Ti3AlC2 in this study, which 6 sintering conditions, Ti3AlC2 can form. For example, in the was synthesized from TiC:Ti:Al with the 2:1:1 stoichiometric first few years of MXene synthesis, we were using a mixture ratio (for synthesis details, see Supporting Information). 3 Choice of Etchants and Intercalants. fl of Ti2AlC and TiC as precursors to synthesize Ti3AlC2. Hydro uoric acid However, we changed the synthesis path recently to use (HF, 49.5 wt %) was purchased from Acros Organics. ammo-  fl less expensive commercially available raw materials TiC, Ti, nium hydrogen uoride (NH4HF2, 95% grade) powder, tetra- and Al. We explain our Ti3AlC2 synthesis method in the methylammonium hydroxide (TMAOH, 25 wt % in water),

7635 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols and tetrabutylammonium hydroxide (TBAOH, 40 wt % in 30, 10, and 5 wt % HF, respectively. All three MXene powders water) were purchased from Sigma-Aldrich. Lithium fluoride were stored in vacuum before any further processing or (LiF, 98.5% grade) powder and hydrochloric acid (HCl, 37 wt %) analysis. were purchased from Alfa Aesar, and all the above chemicals were As shown in Figure S3, the resulting Ti3C2Tx MXene used as received. Deionized water (DI H2O) used in this study powders exhibited distinctive color differences (dark gray fi was puri ed using Elix Essential 3 UV, Millipore). to black) from the graphitic-gray color of Ti3AlC2 powder. Choice of the Synthesis Method. For Ti3C2Tx, the The 30F-Ti3C2Tx powder exhibited similar color to Ti3C2Tx choice of a synthesis method directly influences the resulting previously synthesized with 50 wt % HF;3 however, it looked 35 29 properties in terms of surface terminations, size, and quality darker than both 10F-Ti C T and 5F-Ti C T which exhibited 13 3 2 x 3 2 x of the flakes (for example, number and type of defects). a similar color (Figure S3, Supporting Information). HF Etching Protocol. When handling hazardous HF, We compare the scanning electron microscopy (SEM) independent of the concentration, it is extremely important to images of all the powders in this study in Figure 3.Ti3AlC2 be aware of the risk assessment and required safety protocols (see safety section, Supporting Information). Most synthesis protocols reported in literature use 10 or 50 wt % HF,3,7,25 although a lower concentration of HF, as discussed later, is ffi su cient to remove Al from Ti3AlC2. Here, we compare three different HF concentrations of 30, 10, and 5 wt % and show that removing Al can be achieved with as low as 5 wt % and that an etching time of 5 h is enough in the case of using a concentration ≥30 wt % HF (Figure 2). For all three HF concentrations, 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 while Figure 3. SEM images of MAX and MXene powders studied here. fl stirring with the help of a Te on magnetic bar (Movie S1, SEM of (a) Ti3AlC2 (MAX) powder showing the compact layered Supporting Information). The gradual addition of structure. Multilayered Ti3C2Tx powder synthesized with (b) 30 wt %, (c) 10 wt %, and (d) 5 wt % HF. Accordion-like morphology only Ti3AlC2 powder is necessary to minimize excessive bubbling formed due to the exothermic nature of observed for the 30 wt % HF etched powder (b). (e) Multilayered NH -Ti C T powder synthesized with ammonium hydrogen fluoride this reaction (For detailed instruction see Movie S1, 4 3 2 x and (f) MILD-Ti3C2Tx powder etched with 10 M LiF in 9 M HCl, Supporting Information). both showing negligible opening of MXene lamellas, similar to what is (2) The reaction was then allowed to proceed at room observed in 5F-Ti3C2Tx. temperature (∼23 °C) for 5, 18, and 24 h for 30, 10, and 5 wt % HF, respectively. These etching times for different HF concentrations are sufficient to achieve complete powders show compact, layered morphology (Figures 4a and S4 in Supporting Information) as expected for a bulk layered etching, as discussed below. 5 (3) Each powder was washed separately five times (that is ternary carbide. However, the multilayered Ti3C2Tx powders look different depending on the etchant concentration (compare equivalent to the total volume of 1000 mL of deionized − Figure 3b d). 30F-Ti3C2Tx powders exhibit an accordion-like H2O) via centrifugation (5 min per cycle at 3500 rpm). morphology (Figures 3b and S5 in Supporting Information) The washing process was carried as follows: even when etching in 30 wt % HF was used for low etching (a) After each centrifuge cycle, Ti3C2Tx powder times (5 h). settled at the bottom of the centrifuge tube as Morphology such as this was previously observed as a feature sediment separated from water-like supernatant. 3,7 fi of Ti3C2Tx MXene synthesized with 50 wt % HF. For lower (b) The supernatant after the rst washing cycle HF concentrations such as 10 wt % HF (Figures 3c and S6 in exhibited acidic pH (Figure S1a, Supporting Supporting Information), the accordion-like structure was not Information) and was decanted into waste. as prevalent as in the case of using 30 wt % HF. Additionally, (c) The sedimented Ti C T was redispersed with an 3 2 x the accordion-like morphology was not observed in Ti3C2Tx additional 150 mL of deionized H O followed by 2 powder synthesized with 5 wt % HF, as the resulting Ti3C2Tx is another centrifugation cycle. The washing cycles barely distinguishable from MAX powder (Figures 3d and S7 in were repeated until the pH of the supernatant Supporting Information). This expanded accordion-like struc- ∼ fi became 6. A total of ve centrifugation cycles ture can possibly be ascribed to the larger amount of escaped were enough for washing Ti C T in the case of 3 2 x gases (H2 in this case) because of exothermic nature of HF washing 0.5 g of powder and using a 175-mL reaction with Al.3 graduated conical tube (see Figure S1a, Supporting Interestingly, low concentrations of HF (5 wt %) for 24 h Information). were still sufficient for selectively etching aluminum, similar to (4) The Ti3C2Tx sediments were rewashed with 1000 mL of fi the higher concentrations (10 wt % HF for 18 h and 30 wt % deionized H2O via vacuum-assisted ltration using fi fl fi HF for 18 h), as con rmed by energy dispersive X-ray (EDX) a polyvinyl di uoride (PVDF) lter membrane with analysis (Table S1). This was further confirmed by a shift of the 0.22 μm pore size (Durapore, Millipore) before collec- ° ° (002) peak of Ti3AlC2 at 9.5 to 9.0 for the Ti3C2Tx in XRD tion of the remaining materials. patterns (Figure 4a) and no residual peaks of Ti3AlC2 after The MXene powders (Figure S3, Supporting Information) etching for 5F-, 10F-, and 30F-Ti3C2Tx. The lower peak shift of were dried in vacuum at 80 °C for 24 h. The resulting powders the basal planes (such as (002) peak) is due to the removal of were labeled as 30F-Ti3C2Tx, 10F-Ti3C2Tx, and 5F-Ti3C2Tx, for Al in the Ti3AlC2 and introduction of surface terminations

7636 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

Figure 4. XRD patterns of all the powders in this study and photographs of centrifuge tubes of MILD-Ti3C2Tx. XRD pattern of (a) Ti3AlC2 powder and resulting Ti3C2Tx MXene powders synthesized with 5, 10, and 30 wt % HF and in situ HF by using NH4-Ti3C2Tx routes. (b) The same amount fi of MILD-Ti3C2Tx powder after the rst and eighth centrifuge cycle during washing, showing that the sediment swells after the eighth wash in a 50-mL centrifuge tube due to the water intercalation. The swollen sediment after the eighth wash has two layers, the top labeled as Ti3C2Tx (slurry) fi which is black and the bottom layer, labeled as Ti3AlC2/Ti3C2Tx, having a gray color. (c) XRD pattern of dried lm of Ti3C2Tx slurry showing only Ti3C2Tx and dried powder of Ti3AlC2/Ti3C2Tx mixture showing a mixture of MAX and MXene patterns. − − − (represented as Tx)inTi3C2Tx (e.g., F, O, OH). Vacuum reaction was left to run for 24 h at room temperature ° ∼ ° drying of the HF etched Ti3C2Tx powders at 80 C removed ( 23 C). the intercalated water molecules in between MXene flakes. (2) Similar to earlier described washing processes, the Usually, if XRD is performed on wet or partially dried multi- mixture product was washed with deionized H2O via layered MXene powders, larger shifts in the (00l) peaks are centrifugation at 3500 rpm (5 times per 5 min per cycle). observed due to the presence of water molecules trapped Further washing was performed using vacuum-assisted 36,37 fi fi between MXene layers. ltration by ltering the Ti3C2Tx product over a PVDF Therefore, the accordion-like expanded structure usually filter membrane with 0.22 μm pore size and rinsing with observed in SEM should not be considered as the only 1000 mL of deionized water through the filtration setup. ° indication of successful etching of Al out of Ti3AlC2, and both (3) The powder was dried at 120 C for 24 h in a vacuum XRD and EDX analyses should be performed to confirm oven. MXene produced by this route is referred to as selective etching. Low concentration of HF (≤10 wt % HF) NH4-Ti3C2Tx. presented in this section as well as etching by in situ formed The morphology of multilayered NH4-Ti3C2Tx powders 15 HF result in Ti3C2Tx with less open structure. In the following shown in Figure 3e(seealsoFigureS8,Supporting section, we discuss the in situ HF etching with concentration Information) resembles Ti3C2Tx multilayered powders pro- ≤5 wt % HF. duced with lower concentrations of HF (Figure 3c,d). Here, the In situ HF Formation. As reported in most MXenes selective removal of Al can be confirmed by complete dis- ° literature, concentrated HF has been often the etchant of appearance of Ti3AlC2 peaks (for example, no (002) at 9.5 )in 7,38 choice to selectively remove Al from Ti3AlC2. However, the XRD pattern (Figure 4a). This was also supported by EDX one way to avoid using unnecessarily high HF concentrations results showing the removal of Al (Table S1, Supporting is to use dilute etchant with 5 wt % HF, as shown in the Information). NH4-Ti3C2Tx showed basal plane peaks shift to a θ  ∼ ° ° previous section. The alternative method can be through lower 2 -angle from 9.5 in the Ti3AlC2 precursors to 8.5 in situ HF formation from hydrogen fluoride or fluoride salts and 7.15°, which correspond to d-spacings of 10.35 and 12.37 Å, such as NH HF 21,39 or LiF13,15 to prepare etchants containing respectively. The larger d-spacing after drying can be explained 4 2 fi + 3−5 wt % HF. by the presence of con ned/intercalated (NH4 )ionsandwater fl molecules in between the MXene layers.21,40 NH -Ti C T , Bi uoride-Based Etchants. HF salts, such as NH4HF2, 4 3 2 x have been used at room temperature either for a few hours to unlike the HF-etched Ti3C2Tx, needs a longer duration or higher 21 temperature vacuum drying to completely remove the inter- etch thin coatings of Ti3AlC2 or for a longer time of 5 days 40 calated water molecules from the powder. Similar behavior was to etch Ti3AlC2 powder at room or elevated temperature (60 °C).39 Here we describe this approach and delamination observed for other in situ HF etched Ti3C2Tx samples, as protocols (discussed later in detail) of MXene powders syn- discussed in the following section. Fluoride-Based Salt Etchants. When performing synthesized using NH4HF2. The synthesis of NH4HF2 etched fl Ti C T can be done as follows: thesis with uoride-based salts, it is crucial to understand that 3 2 x the concentration of LiF and HCl used to prepare in situ HF (1) A total of 0.5 g of Ti AlC powder was added to 10 mL 13,29 3 2 will change the quality, size, and ability to process Ti3C2Tx. fl of 2 M NH4HF2 under continuous stirring, and the For example, the quality and the size of Ti3C2Tx akes were

7637 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols improved when the LiF:Ti3AlC2 molar ratio was increased from top right inset). A thick film was formed after drying in ≥ 5to 7.5 (that is equivalent to change of the LiF:Ti3AlC2 mass vacuum at 120 °C for 24 h. All these steps are shown in ≥ ratio from 0.67 to 1, respectively) as well as the concentration Figure S10a-d, Supporting Information.Itisworthnoting of HCl from 6 to 9 M.15,29 Although a LiF/HCl mix was used fi here that the lm made from the Ti3C2Tx sediment in both (5 M LiF/6 M HCl) and (7.5 M LiF/9 M HCl) (Figure S10d)wasflexible. methods, the concentration optimization of these two ingre- dients (LiF and HCl) in the latter method enhanced the quality The swelling of Ti3C2Tx sediment after washing was only fl observed in our previous report29 as well as shown in the (fewer defects) as well as the size of produced Ti3C2T akes which enabled studying and quantification of defects in single- optimized MILD described above. This swelling behavior of fl 13,29 fi Ti3C2Tx during washing cycles after synthesis was not observed layer akes of Ti3C2Tx. Freestanding lms produced from fl in the earlier “clay” synthesis using LiF/HCl (5 M of larger size Ti3C2Tx akes showed high electrical conductivity 15,29,44 (6000−8000 S/cm−higher than other solution-processed LiF:Ti3AlC2) with both 6 M HCl and 12 M HCl. ff inorganic materials) and enabled use of MXene in many This is another important di erence between the two methods, fi + important applications.20,41 and it con rms that there is a minimum concentration of Li As previously reported,15 multilayered Ti C T produced by required, besides a certain concentration of HCl, when prefor- 3 2 x ≤ the clay method (5 M LiF/6 M HCl) (Figure 2) were isolated ming etching at room temperature for 24 h. Cross sectional fi into single flakes via sonication, which often created small and SEM images of the lm (Figure S9d) made by vacuum-assisted 15,28 fi defective MXene flakes (Figure 1). However, hand-shaking ltration of the black layer of Ti3C2Tx slurry (Figure 4b, top inset) are shown in Figure S11, Supporting Information. The was enough to isolate Ti3C2Tx multilayered powder, produced by (7.5 M LiF/9 M HCl), into single and obviously large XRD pattern of this black slurry sediment MILD-Ti3C2Tx thick flakes.20 To distinguish this optimized method from the clay film after vacuum drying at 120 °C exhibited only the (00l) 15 ° LiF/HCl method, we called this method minimally intensive peaks of Ti3C2Tx MXene with the (002) peak centered at 8.6 layer delamination (MILD) to stress the fact that a minimal and (Figure 4c, top pattern). The SEM image of the thick film made fi less milder route was used to produce single and larger flakes by ltering Ti3C2Tx slurry sediment Figure S9d is very similar 13,29 fi 15 with fewer defects (Figure 1). to that of the rolled/sheared lm of the Ti3C2Tx clay, and the MILD method has been the choice for studies/applications XRD pattern is very similar to that of the films made from 13,42,43 15 where larger flake size is favorable. Here we report on an delaminated single- to few-layer Ti3C2Tx reported before. optimized MILD synthesis method using (12 M LiF/9 M HCl) In other words, the as-separated/collected Ti3C2Tx slurry instead of (7.5 M LiF/9 M HCl) at room temperature as sediment does not require any type of mechanical vibration or follows: shearing and only addition of deionized H2O to the Ti3C2Tx slurry, followed by gentle shaking, is needed to make the (1) The etchant was prepared by adding 0.8 g of LiF to fi 10 mL of 9 M HCl and left under continuous stirring for delaminated MILD Ti3C2Tx lm possibly due to presence 5 min. of solvated Li ions between the MXene layers. The XRD pattern of the dried gray material from the bottom of the tube (2) A total of 0.5 g of Ti AlC powder was gradually added 3 2 (Figure 4b, bottom inset) showed a mixture of nonetched (over the course of 5 min) to the above etchant, and Ti AlC and Ti C T (Figure 4c, bottom pattern). SEM image the reaction was allowed to run for 24 h at room 3 2 3 2 x of this layer shows that it contains Ti C T particles (Figure 3f), temperature. 3 2 x similar to that of Ti C T produced by etching in low- (3) The acidic mixture was washed with deionized H O via 3 2 x 2 concentrations of HF (Figure 3d). Thus, we recommend centrifugation (5 min per cycle at 3500 rpm) for multiple separation of the black Ti3C2Tx slurry sediment from the gray cycles. After each cycle, the acidic supernatant was sediment containing a mixture of nonetched and etched decanted as waste followed by the addition of fresh particles, as explained in step 4 of this section (Figure 4b). deionized H2O before another centrifuging cycle. These Choice of Intercalation Method. Using intercalating washing cycles were repeated until pH 4−5 was achieved. fl compounds to expand the interlayer spacing of Ti3C2Tx akes This range of pH was observed after two cycles (when a has proven to be a key step for weakening the interactions 175-mL centrifuge tube was used) or seven cycles (when between 2D layers and delaminating MXene into separate 2D a 50-mL centrifuge tube was used). sheets.27 The intercalation and delamination process typically ≥ (4) When pH 5, there are two important observations: requires a suitable solvent for the material and intercalant, a fi rst, a dark-green supernatant is observed, which is stable mixing step where the intercalant is introduced between 2D even when centrifugation time is increased from 5 min to sheets, occasionally a sonication step depending on desired 1 h (see Figure S9, Supporting Information) indicating flake size and concentration, and a centrifugation step to isolate delamination has already started during washing with no the delaminated material from the material which is multi- 25 further processing steps. Second, the Ti3C2Tx sediment layered (nondelaminated). The final colloidal solution will settled at the bottom of the centrifuge tube, after collec- contain dispersed 2D sheets of electrostatically stabilized ting the supernatant, is found to be swollen to almost MXene, which are stable against aggregation, or clumping, twice the volume compared to the previous washing and are processable and functional. Moreover, the sonication cycles, when no delamination occurred. The swelled step is highly dependent on the etching method and the desired sediment exhibits a distinctive black layer of Ti3C2Tx application, as well as the required concentration. Sonicating for slurry above a grayish layer of nonetched Ti3AlC2/ longer times and higher powers will exhibit smaller flakes with Ti3C2Tx mixture (Figure 4b). more defects and may yield concentrations different than those (5) The black Ti3C2Tx slurry was carefully collected, which are not sonicated. The concentration of MXene sheets ff separately from Ti3AlC2/Ti3C2Tx, with a spatula and in solution also depends on di erent parameters such as the vacuum-filtered producing a thick black cake (Figure 4b, synthesis methods and type of intercalants used to weaken the

7638 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

ff Figure 5. (a) Optical images of aggregated Ti3C2Tx powder and stable colloidal aqueous solutions of Ti3C2Tx prepared with di erent intercalants and fi (b) the respective particle size analysis as well as (c) XRD patterns of dried lms with TMA-5F-Ti3C2Tx,TMA-NH4-Ti3C2Tx, and MILD-Ti3C2Tx methods. interlayer interaction between MXene sheets. In this section, with the help of TMAOH can be described as follows: we report on the advantages and disadvantages in processing (1) A total of 100 mg of each MXene powder (5F-Ti C T , ff 3 2 x of Ti3C2Tx MXenes using di erent intercalants that cause 10F-Ti3C2Tx, 30F-Ti3C2Tx or NH4-Ti3C2Tx) was mixed delamination with or without sonication. separately with 10 mL of deionized H O containing Dimethyl Sulfoxide (DMSO). Large, organic molecules, 2 fi 100 mg of TMAOH in a 20-mL glass vial, and the such as dimethyl sulfoxide (DMSO), were some of the rst inter- mixture was stirred for 12 h at room temperature. calants used for expanding the interlayer spacing of Ti3C2Tx (2) The basic mixture (∼pH 10) was washed twice via MXenes synthesized with HF.25 Additionally, DMSO or other centrifugation (3500 rpm, 5 min per cycle) using 50 mL organic solvents can be used as dispersants for Ti C T .45 When 3 2 x tubes to bring the pH to ∼7. using DMSO or other organic solvents, sonication is required to (3) The stable MXene colloidal solutions were collected after disperse the Ti3C2Tx sheets in solution, and without sonication, the multilayered sheets will fall to the bottom of the vial as 1 h of centrifugation at 3500 rpm, and the sediments fi were collected via vacuum assisted filtration and dried in sediment ( rst vial from the left in Figure 5a). Flake sizes ° produced by these methods are usually on the order of a few vacuum at 120 C. hundred nanometers. Using this method, all three HF-etched samples can be Tetraalkylammonium Hydroxides. Tetraalkylammonium delaminated, and stable colloidal solutions are formed. The + compounds, such as tetrabutylammonium hydroxide (TBAOH) TMA delaminated colloidal solution of 5F-Ti3C2Tx MXene and tetramethylammonium hydroxide (TMAOH), have (labeled as TMA-5F-Ti3C2Tx) is black and stable (Figure 5a) ∼ been used to exfoliate 2D layered oxides.46 This delamination with concentration of 0.5 mg/mL. When the solution was technique is based on ion exchange between protons (H+) analyzed via dynamic light scattering (DLS), the size of the fl and the bulky tetraalkylammonium ions (TMA+ or TBA+) akes collected after an hour of centrifugation at 3500 rpm was in the range of 0.2 to 0.7 μm(Figure 5b), which is similar to the leading to swelling and delamination with manual shaking, fl 41 when used on different materials, such as layered oxides.46 ake size reported for the sonicated LiF/HCl clay method. TBAOH has also been used to exfoliate MXenes, such This shows that delamination of MXene with stirring in TMAOH can break the flakes into smaller sizes. The breakage as Ti CN, Mo C, V C, and Nb C. However, TBAOH could 3 2 2 2 of flakes due to intercalation of TMAOH was previously not delaminate Ti C T , even with sonication.27 In another 3 2 x observed in layered 2D oxides even with stirring, and larger study, Ti C T was claimed to be synthesized and delam- 46 3 2 x flakes were only obtained with gentle shaking. inated using TMAOH after pre-treatment of Ti3AlC2 surface + fi − 47 The intercalation of TMA into 5F-Ti3C2Tx was con rmed using 20 30 wt % HF for a short time. However, we have by the (002) peak shift from 9.0° to 6.0° 2θ-angle (Figure 5c) not been able to reproduce the results by only using TMAOH for 5F-Ti3C2Tx which corresponds to a d-spacing shift of 9.7 Å as etchant and we were only able to delaminate Ti C T into + 3 2 x to 14.7 Å. This shows that TMA intercalated the Ti3C2Tx single- and few-layers when HF was used as an etchant flakes and stayed in between the layers even after 120 °C drying and TMAOH was used an intercalant, regardless of HF in vacuum. A higher temperature is needed to remove the concentration. intercalated molecules, as the decomposition temperature of Here, we used TMAOH to delaminate all the HF-synthesized TMAOH exceeds 200 °C.48,49 and NH4HF2-synthesized Ti3C2Tx samples in this study. Similarly, a stable but less concentrated colloidal solution of Delamination of 100 mg of each material resulted in darker Ti3C2Tx MXene can be made by intercalation of TMAOH into and more concentrated colloidal solutions for HF-etched NH4-Ti3C2Tx powder followed by sonication. The inter- Ti3C2Tx in comparison to NH4HF2-synthesized Ti3C2Tx. The calated NH4-Ti3C2Tx powder (labeled as TMA-NH4-Ti3C2Tx) detailed process of delaminating multilayered Ti3C2Tx powders showed similar (00l) peak positions as the TMA-5F-Ti3C2Tx.

7639 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

The (002) peak is at 5.9°, which is slightly shifted further to the supernatant for 1 h at 3500 rpm resulted in a black left and corresponds to 14.86 Å interlayer spacing (Figure 5c). clay sediment (Figure S10a, Supporting Information) This successful delamination of Ti3C2Tx due to intercalation and stable dark supernatant (Figure 5b) mainly con- + fl of TMA , regardless of HF content, suggests that intercalation sisting of single-layer akes of Ti3C2Tx. The average con- of larger ions like TBA+ is possible with more prolonged centration of this colloidal solution is about 1−2 mg/mL, stirring time. To test our hypothesis, we stirred 0.2 g of which is higher than that of TMA delaminated solutions 30F-Ti3C2Tx in 20 mL of deionized H2O containing 0.2 g of (<0.5 mg/mL). TBAOH for 2.5 days. This long time of stirring resulted in a It is noteworthy that the colloidal solutions of MILD-Ti3C2Tx stable and concentrated colloidal solution after sonication often contain larger flakes (>2 μm) as shown in Figure 5b. (Figure S12, Supporting Information), collected after 1 h of We also drop casted diluted delaminated solutions of both centrifugation at 3500 rpm. MILD-Ti3C2Tx and TMA-5F-Ti3C2Tx on a porous alumina Lithium Ions. When etchants containing lithium are used, membrane and compared the flake size distributions in SEM. such as LiF/HCl, in both clay and MILD methods, fl The akes of MILD-Ti3C2Tx have larger sizes (Figure 6a,b) in delamination is driven by the intercalation of solvated Li+ ions. However, sonication of the intercalated Ti3C2Tx was required in the case of the clay method and was optional in the case of the MILD method (Figure 2). In the following, we describe the effect of intercalant (Li+) used in the MILD fl method for delamination of Ti3C2Tx akes (MILD-Ti3C2Tx). Because no sonication is needed, these flakes are larger with fewer defects. However, to prepare high-quality, large flakes in high 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 major role in the differences between the clay and MILD methods 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 LiF fl can be present) when added to HCl, not vice versa. Figure 6. SEM images of Ti3C2Tx MXene akes collected from This reaction produces lithium chloride (LiCl), and colloidal solution. (a, b) MILD-Ti3C2Tx and (c, d) TMA-NH4- Ti3C2Tx on porous alumina membrane showing electron-beam copious washing with deionized H2O is required to fl fl completely wash the LiCl salt away. transparent akes of MXene. MILD-Ti3C2Tx has larger akes with distinctive and straight edges possibly similar to the original shape of (3) The excess HCl remaining in the etchant results in a crystalline grains of their Ti AlC precursor. higher concentration of protons (H+) and enables ion 3 2 exchange with Li+, resulting in swollen clay-like Ti C T 3 2 x fl (Figure 4b) which can be delaminated by manual shaking. comparison to MXene akes in colloidal solution prepared via This is confirmed by higher concentration of the delam- TMAOH intercalation (Figure 6c,d). Additionally, the MILD- fl inated solution of MXene when 9 M HCl was used instead Ti3C2Tx akes have distinctive and straight edges, mimicking of 6 M HCl. the original shape of the Ti3AlC2 crystalline grains. In contrast, in the case of TMAOH delamination, flakes are broken Considering all these parameters, here is the best method, to into small fragments, and their edges appear to contain more our knowledge, to delaminate MILD-Ti3C2Tx and obtain a defects, showing that stirring or intense shaking when using stable colloidal solution: organic base intercalants, such as TMAOH, leads to fractured (1) As explained in the etching section, after the third flakes. This strong or excessive shaking after intercalation of centrifuge wash for 1 h using a 175-mL graduated conical TMAOH or TBAOH, even for a short time, has been shown to tube (Figure S9), a stable dark-green supernatant is cause breakage of flakes in 2D layered oxides.46,50 Thus, it is produced. This supernatant can be collected as a stable advised to perform gentle shaking if larger flakes are desired colloidal solution with MXene concentration of ∼1mg/mL, when using TMAOH or TBAOH, similar to the protocol without any further processing. previously adapted for delamination of 2D layered oxides.46,50 (2) As mentioned earlier, because this method results in As explained, synthesis of MXene is a top-down method and swollen black Ti3C2Tx slurry with some nonetched therefore the flake size produced in the MILD method is Ti3AlC2 (Figure 4b), separation of Ti3C2Tx from directly related to the grain size and shape of the starting MAX nonetched Ti3AlC2 must be performed before delamina- powder. Thus, large crystals of MAX phases are needed to tion. The separation process was either done as men- achieve larger MXene flakes. tioned above in step 5 in the MILD method or by the It is important to note that the synthesis protocols described addition of 50 mL of deionized H2O to the whole above are applicable not just to small laboratory samples sediment (Ti3C2Tx slurry and the nonetched Ti3AlC2) characterized in this study but also to large scale manufacturing. followed by manual shaking until all the sediment was We can produce up to 100 g of MXene per batch using a redispersed. Then, we separate the nonetched Ti3AlC2 reactor designed for us by the Materials Research Center (gray sediment in Figure 4b) via centrifugation at 3500 rpm (Kiev, Ukraine). Control of the synthesis conditions becomes for 2 min followed by collection of the dark concentrated even more important when large amounts of MXenes are fl supernatant of Ti3C2Tx akes. Centrifuging the collected manufactured.

7640 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

Processing, Deposition, and Storage. MXenes can be processed, deposited, and stored in a variety of ways. Once the material has been processed by delamination, size selection, or sonication, it can be deposited as freestanding film or thin film, painted, or sprayed on diverse substrates depending on the application. However, MXene flakes have been shown to degrade in the presence of a water containing dissolved oxygen, especially at elevated temperatures and under sunlight, making storage in aqueous solutions challenging.51,52 So far we have presented different routes to etch and delaminate Ti3C2Tx, as well as prepare stable high-concentration colloidal 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 available published information. Sonication and Size Selection. The size of the flakes can be controlled by performing cascading centrifugation to isolate fl fi and fractionate akes based on lateral size or by decreasing the Figure 7. Fabrication of Ti3C2Tx MXene electrically conducting lms flakes by bath or probe/tip sonication, and some of these via vacuum assisted filtration (left), spray-coating (middle), and processing techniques have been used in protocols for other 2D painting (right) of MXene ink over various flexible substrates. materials.53 Figure S13 shows flakes of colloidal solution of MILD-Ti3C2Tx can be separated by size via centrifugation for 2 Büchner funnel and the pore size is small enough to prevent min at 2500, 5000, and 10000 rpm, and the collected colloidal MXene flakes from passing through the filter. Using this solution contained flake sizes as large as 2 μm after 2500 rpm, method, freestanding films with controlled thickness can be ∼1 μm after 5000 rpm and as small as ∼200 nm after 10000 rpm. made of delaminated MXenes and their hybrids with other When performing sonication, it is critical to monitor the nanomaterials, such as carbon nanotubes.58 power output of the sonicator as well as the time spent These colloidal solutions of delaminated MXenes can also be sonicating. Each of these parameters will change the resulting deposited onto substrates by spin or spray coating for thinner concentration, the quality of flakes, and their lateral size distri- coatings compared to vacuum-assisted filtration. Spray deposition bution. For example, it has been shown that bath sonication of has obvious advantages; it is easy to use, can cover large areas, fl ∼ μ a MILD-Ti3C2Tx can change the ake size from 4 m to less and can be done on any surface, also outside the lab. However, 39 than 1 μm. When performing bath sonication, it is desired to the coating is less uniform and often produces rougher films22,41 keep the bath at a constant temperature and avoid exposing compared to spin coating.12 Spin coating has the potential to the material to elevated temperatures, to prevent oxidation. create even, uniform, and optical quality films, for applications We monitor the water in the bath to ensure the temperature where thickness and transparency are important.12 is constant, and we may periodically add ice to the bath. Substrate Functionality. Titanium-based MXenes have Furthermore, to prevent oxidation during sonication, we bubble been successfully deposited on simple substrates such as argon (Ar) through the solution. When performing sonication paper,56 plastic (PET) substrates,20,41 glass55 and silicon.12 by a tip or probe sonicator, we use pulse sonication where the Because MXenes are inherently hydrophilic and negatively probe is on for 8 s and off for 2 s, and we also use a cooling charged, the nature of the chosen substrate in terms of hydro- system surrounding the sample, keeping the solution temper- phobicity versus hydrophilicity and surface charges is important. ature ∼ 5 °C, similar to the temperature of a refrigerator. Based on our experience, hydrophilic surface treatments such as Deposition of Flakes. Solution-processed MXenes can be UV/ozone or O2 plasma may be used to make the substrate deposited using a variety of methods, including vacuum-assisted surface hydrophilic, or one may use liquid treatments such as filtration,20,54 spin coating,12,34,55 spray coating,22,41 or roll- Piranha solution on glass or silicon.22,55 ing15,56 as shown in Figure 7. Each technique can be tailored for Storage of Material. Once synthesized, the material may a specific application. For example, spin coating may produce need storage before use. Elimination of dissolved oxygen is the even, uniform, thin coatings for optics or electronics,12,55 first step to suppress oxidation, and this can be done by storing whereas vacuum-assisted filtration and rolling are more suitable the solution in argon-sealed vials. Second, refrigeration can for making battery or electrochemical capacitor electrodes,7,57 also suppress the oxidation by decreasing the temperature and while spray coating is convenient for producing large-area films eliminating UV radiation. A detailed study on the shelf life of 20 52 for electromagnetic interference shielding. Ti3C2Tx and Ti2CTx has been published recently. Other techniques, such as screen printing, inkjet printing or Based on our experience, the rate of degradation and quality slurry rolling, can be employed. Rolling of MXene slurry with a of the solution can be monitored by ultraviolet−visible absor- clay-like rheology can provide a way to quickly deposit material bance (UV−vis) spectroscopy over time and noting the and produce thick films for electrode applications.54 Printing change in absorbance of the main peak (e.g., 700−800 nm 52 MXene allows for precise control over the pattern, increasing for Ti3C2Tx). Aqueous solutions that are kept in open vials the amount of material by increasing the number of layers; at room or elevated temperatures start degrading almost however, it requires ink formulation for optimal results. immediately, with large amounts of TiO2 forming a week In the most simplistic manner, nanosheets can be vacuum- after synthesis. By placing the aqueous colloidal solution in the filtered to create freestanding films.25 A vacuum-filtration setup refrigerator, the oxidation process can be slowed dramatically. of any size can work, assuming a filtration membrane (typically When the solution is kept refrigerated in vials sealed under Ar, a porous polymer such as Celgard or PVDF) can fit over the the degradation is minimal over the course of 24 days.52

7641 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols

Additionally, because the aqueous solutions may cause degra- In the case of LiF/HCl, which is the least harsh etchant, some dation of the samples, storing MXenes in organic solvents can residual nonetched particles may remain. However, the MILD- be advantageous, due to the stabilizing solvation shell which is LiF/HCl method offers the largest flake size and best quality provided by the organic solvent. A dark supernatant remained (least defects) among all etching methods. Moreover, many fi stable even after months of dispersing the material in protic properties, such as electrical conductivity of Ti3C2Tx lms, are organic solvents, suggesting that this may be the best way when synthesis and processing dependent. For example, Ti3C2Tx long-term storage of the material in solution is needed.45 films produced from colloidal solutions made via TMAOH Additionally, since MXenes can be redispersed in different intercalation showed high resistivity in comparison to films solvents, another practice to store MXenes is to filter the produced by the MILD method. Therefore, we recommend colloidal solution and store the resulting film under vacuum. using MILD synthesis in applications where high electrical con- When colloidal solution is needed, MXene film can be redis- ductivity, larger flake sizes, environmental stability, or mechan- persed in the solvent of choice by either manual shaking or ical properties are important and alternative methods, such as 45,52 “ ” sonication, depending on the solvent/material combination. clay or HF etched Ti3C2Tx, where smaller or more defective Note that drying the film at elevated temperature may lead to flakes are necessary (for example, for catalysis and selected strong bonding between the layers, making redispersion difficult. electrochemical or biomedical applications). Characterization Methods. Despite the differences in Furthermore, we summarized the best practices to process, methods and protocols reported in many papers throughout deposit, and store MXene films and colloidal solutions. the 2D materials community, basic characterization protocols We have explained our methods for characterization of these are necessary to ensure reproducibility.53 To characterize materials by performing microscopy, diffraction, performance MXene flakes, many methods are used to probe the structure testing (for example, conductivity), and light scattering. and morphology of the flakes, such as XRD, SEM, transmis- sion electron microscopy (TEM), and Raman spectroscopy. ■ ASSOCIATED CONTENT Elemental composition of Ti C T MXene powder/films can 3 2 x *S Supporting Information be characterized by EDX. We use a Zeiss Supra 50VP SEM, and EDX spectra were collected from 5 different locations at The Supporting Information is available free of charge on the ACS 250× magnification (an area of almost 0.5 × 0.5 mm2) for 30 s. Publications website at DOI: 10.1021/acs.chemmater.7b02847. A powder diffractometer (Rigaku Smart Lab, U.S.A.) with Cu Additional figures, details, and photographs (PDF) Kα radiation was used to collect the XRD patterns of MXene Movie S1: Synthesis of Ti C T MXene showing and fi ° 3 2 x powders/ lms and MAX powders at a step size of 0.03 with describing the process that should be followed when × 0.5 s dwelling time. MXene powders are placed on a 2 cm adding Ti AlC powder to an etchant (MP4) 2 cm engraved or flat glass substrates for XRD analysis. 3 2 We pressed the powder with a glass slide manually to intensify the (00l) peaks and monitored if the (002) peak of nonetched ■ AUTHOR INFORMATION fl Ti3AlC2 appears in the XRD pattern. The lateral size of akes Corresponding Author can be measured using SEM by depositing colloidal solution of *E-mail: [email protected] (Y.G.). MXenes on porous alumina substrate (Anodisc, 0.1 μm pore ORCID size, Whatman) or by using dynamic light scattering (DLS) on the colloidal solution to confirm the particle size (Zetasizer Mohamed Alhabeb: 0000-0002-9460-8548 Nano ZS, Malvern Instruments, U.S.A.). Ideally, more than one Kathleen Maleski: 0000-0003-4032-7385 technique should be used to obtain reliable data. For example, Babak Anasori: 0000-0002-1955-253X it is necessary to use DLS in conjunction with SEM to confirm Yury Gogotsi: 0000-0001-9423-4032 particle size distributions, as shown in Figures 5 and 6. Mea- Author Contributions fi suring the properties derived from a speci c synthesis route can The manuscript was written through contributions of all be used to quantify differences between the methods and select fi fi authors. All authors have given approval to the nal version of the synthesis method for a speci c application. For example, the the manuscript. electrical conductivities of spray/spin coated, vacuum-filtered, or painted films of Ti C T MXene can be measured using a Notes 3 2 x fi four-point probe (ResTest v1, Jandel Engineering Ltd., Bed- The authors declare no competing nancial interest. fordshire, U.K.) with a probe distance of 1 mm. Free-standing films produced by the MILD method exhibit high electrical ■ ACKNOWLEDGMENTS ∼ fi conductivity of 8000 S/cm in comparison to lms produced M.A. and synthesis of MXenes were supported by the Fluid ∼ through TMAOH intercalation ( 200 S/cm). Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department ■ SUMMARY of Energy, Office of Science, Office of Basic Energy Sciences. In this Methods and Protocols paper, we have described best K.M. was supported by the Leading Foreign Research Institute practices used to synthesize and process Ti3C2Tx MXene at Recruitment Program through the National Research Foundation room temperature, as well as various techniques for making of Korea (NRF) funded by the Ministry of Education, Science thin coatings or free-standing films of MXene. We showed that and Technology (MEST), via the NNFC-KAIST-Drexel Nano as low as 5 wt % HF can etch aluminum out of the Ti3AlC2 Co-Op Center (NRF-2015K1A4A3047100). B.A. and Y.G. were MAX phase, but the accordion-like morphology of particles supported by the King Abdullah University of Science and is usually observed when ≥10 wt % HF solutions are used. Technology under the KAUST-Drexel University Competitive When in situ HF formation was used (NH4HF2 and LiF/HCl Research Grant. XRD and SEM analysis were performed at the etchants), similar MXene particle morphologies were observed. Centralized Research Facilities (CRF) at Drexel University.

7642 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644 Chemistry of Materials Methods/Protocols ■ REFERENCES Barsoum, M. W. Transparent Conductive Two-Dimensional Titanium Carbide Epitaxial Thin Films. Chem. Mater. 2014, 26, 2374−2381. (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, (22) Hantanasirisakul, K.; Zhao, M. Q.; Urbankowski, P.; Halim, J.; Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in Anasori, B.; Kota, S.; Ren, C. E.; Barsoum, M. W.; Gogotsi, Y. atomically thin carbon films. Science 2004, 306, 666−669. Fabrication of Ti3C2Tx MXene Transparent Thin Films with Tunable (2) Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano Optoelectronic Properties. Adv. Electron. Mater. 2016, 2, 1600050. 2015, 9, 9451−9469. (23) Naguib, M.; Mashtalir, O.; Carle, J.; Presser, V.; Lu, J.; Hultman, (3) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; L.; Gogotsi, Y.; Barsoum, M. W. 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7644 DOI: 10.1021/acs.chemmater.7b02847 Chem. Mater. 2017, 29, 7633−7644