LETTER doi:10.1038/nature13970 Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance Michael Ghidiu1*, Maria R. Lukatskaya1*, Meng-Qiang Zhao1, Yury Gogotsi1 & Michel W. Barsoum1 19 Safe and powerful energy storage devices are becoming increasingly thin Ti3AlC2 films with ammonium bifluoride . The change in MXene important. Charging times of seconds to minutes, with power den- properties upon intercalation and the compositional variability of fluo- sities exceeding those of batteries, can in principle be provided by ride salts suggested the possibility of a one-step procedure for the syn- electrochemical capacitors—in particular, pseudocapacitors1,2. Recent thesis of many MXenes, with tunable structures and properties. research has focused mainly on improving the gravimetric perform- The MXenes reported in this study were prepared by dissolving LiF ance of the electrodes of such systems, but for portable electronics in 6 M HCl, followed by the slow addition of Ti3AlC2 powders and heat- and vehicles volume is at a premium3. The best volumetric capaci- ing of the mixture at 40u C for 45 h.After etching,the resulting sediments tances of carbon-based electrodes are around 300 farads per cubic were washed to remove the reaction products and raise the pH (several centimetre4,5; hydrated ruthenium oxide can reach capacitances of cycles of water addition, centrifugation and decanting). The resulting 1,000 to 1,500 farads per cubic centimetre with great cyclability, but sediment formed a clay-like paste that could be rolled, when wet (Fig. 1a), only in thin films6. Recently, electrodes made of two-dimensional between water-permeable membranes in a roller mill to produce flex- titanium carbide (Ti3C2, a member of the ‘MXene’ family), produced ible, free-standing films (Fig. 1c) in a matter of minutes, in contrast to by etching aluminium from titanium aluminium carbide (Ti3AlC2,a those previously produced by the laborious technique of intercalation, ‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to delamination, and filtration18. have volumetric capacitances of over 300 farads per cubic centimetre7,8. A graphical depiction of the processing is provided in Extended Data Here we report a method of producing this material using a solution Fig. 1. Further, scaling was not limited to the size of the filtration appa- of lithium fluoride and hydrochloric acid. The resulting hydrophilic ratus; films of any dimensions could readily be produced. Additionally, material swells in volume when hydrated, and can be shaped like clay when wet, the ‘clay’ could be moulded and dried to yield various shapes and dried into a highly conductive solid or rolled into films tens of that were highly conductive (Fig. 1d). Diluted, it could also be used as an micrometres thick. Additive-free films of this titanium carbide ‘clay’ ink to deposit (print) MXene on various substrates. Like clay, the mate- have volumetric capacitances of up to 900 farads per cubic centimetre, rial could be rehydrated, swelling in volume, and shrinking when dried with excellent cyclability and rate performances. This capacitance is (Fig. 1b). almost twice that of our previous report8, and our synthetic method Energy-dispersive spectroscopy confirmed that aluminium (Al) was also offers a much faster route to film production as well as the avoid- removed, and X-ray diffraction (XRD) revealed the disappearance of ance of handling hazardous concentrated hydrofluoric acid. Ti3AlC2 peaks (traces can be seen in the case of incomplete transforma- In the search for new electrode materials, two-dimensional solids are tion). Multilayer particles did not show the accordion-like morphology of particular interest owing to their large electrochemically active surfaces9. seen in HF-etched MXenes reported to date14,20; rather, particles appeared For example, activated graphene electrodes have capacitances of 200– tightly stacked, presumably as a result of water and/or cationic interca- 350 F cm23 compared to 60–100 F cm23 for activated porous carbons10,11. lation (see Extended Data Fig. 2a). Fluorine and oxygen were observed Yet graphene is limited to the chemistry of carbon, does not tap into in energy-dispersive spectroscopy; this, coupled with X-ray photoelec- metal redox reactions as in ruthenium oxide (RuO2) (ref. 6), and its tron spectroscopy showing evidence of Ti–F and Ti–O bonding, sug- conductivity is substantially decreased by the addition of redox-active gests O- and F-containing surface terminations, as has been discussed 12 14,21 functional groups . MXenes (of the formula Mn 1 1XnTx, where M is a at length for HF-produced MXenes . The yield of MXene after etch- 14 transition metal, X is C and/or N, and Tx denotes surface functionali- ing, calculated as described previously , is around 100%, which is com- zation) are a relatively young class of two-dimensional solids, produced parable with the HF-etching method. Our new method thus does not by the selective etching of the A-group (generally group IIIA and IVA lead to material losses, although an accurate yield determination is dif- elements) layers fromthe MAX phases,which comprise a .70-member ficult owing to the variability of surface groups and amount of inter- family of layered, hexagonal early-transition-metal carbides and nitrides13. calated water. To date, all MXenes have been produced by etching MAX phases in XRD patterns of the etched material, in its air-dried multilayered concentrated hydrofluoric acid (HF)14–16. state, showed a remarkable increase in the intensity and sharpness of the MXenes have already proved to be promising candidates for elec- (000l) peaks (Fig. 2a, pink); in some cases the full width at half maxi- trodes in lithium (Li)-ion batteries17,18 and supercapacitors8, exhibiting mum (FWHM) was as small as 0.188u, as opposed to the broad peaks volumetric capacitances that exceedmostpreviously reportedmaterials. typical of HF-etched MXene7, and more typical of intercalated MXenes18. However, the path to electrode manufacturing required the handling of Further, compared to a lattice parameter of c < 20 A˚ for HF-produced concentrated HF and a laborious multi-step procedure. Here we sought Ti3C2Tx, the corresponding value in this work was 27–28 A˚ . XRD pat- a safer route by exploiting the reaction between common, inexpensive terns of still-hydrated sediment showed shifts to even higher spacings: hydrochloric acid (HCl) and fluoride salts, leading to dissolution of alu- lattice parameters as high as c < 40 A˚ have been measured. These large minium and the extraction of two-dimensional carbide layers. Further- shifts are suggestive of the presence of water, and possibly cations, between more, given the ability of MXenes to preferentially intercalate cations the hydrophilic and negatively charged MXene sheets. From these sub- (post-synthesis)8, a related question was whether etching and intercala- stantial increases in c and the clay-like properties (see below), it is reason- tion might be achieved in a single step, as was observed for etching of able to assume that—as in clays22,23—the swelling is due to the intercalation 1Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA. *These authors contributed equally to this work. 78 | NATURE | VOL 516 | 4 DECEMBER 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH a Etching Washing Rolling Shaping Figure 1 | Schematic of MXene clay synthesis and electrode preparation. a, MAX phase is etched in a solution of acid and fluoride salt (step 1), then washed with water to remove reaction products and raise the pH towards neutral (step 2). The resulting sediment behaves like a clay; it can Painting be rolled to produce flexible, freestanding films (step 3), moulded and dried to yield conducting objects of desired shape (step 4), or diluted and painted onto a substrate to yield a conductive Ti3AlC2 Ti3C2Tx ‘Clay’ Electrode coating (step 5). b, When dried samples (left, showing cross-section and top view) are hydrated b c d (right) they swell; upon drying, they shrink. 1,500 S cm–1 c, Image of a rolled film. d, ‘Clay’ shaped into the letter M (,1 cm) and dried, yielding a conductive solid (labelled with the experimental conductivity of ‘clay’ rolled to 5 mm thickness). The etched material is referred to as Ti3C2Tx, where the T denotes surface terminations, such as OH, O and F. of multiple layers of water and possibly cations between the MXene The c parameter expansion also resulted in the weakening of inter- sheets. Interfacial water has a more structured hydrogen-bonding net- actions between the MXene layers, as evidenced by the easy delamina- work than bulk H2O (ref. 24). The MXene surface, holding a negative tion of multilayered particles by sonication, as is done for van der Waals electric charge, may act to align the dipoles of water molecules between solids9. In our previous work, typical sonication times for delamination MXene layers. (after post-synthesis intercalation withdimethyl sulphoxide) were of the When the‘clay’ was rolled into freestandingfilms, XRD patterns again order of 4 h (ref. 18). Here, sonication times of the order of 30–60 min showed strong ordering in the c direction (Fig. 2a, blue). Films, ranging resulted in stable suspensions with concentrations as high as 2 g per litre, in thicknesses from submicrometre to about 100 mm, were readily pro- higher than observed previously. Remarkably, the yield from multilayer duced by this method. The most compelling evidence for particle shearing to dispersed flakes was about 45% by mass. Freestanding films were also is the marked intensity decrease of the (110) peak around 61u, indi- readily fabricated by filtering these suspensions, as reported previously8. cating a reduction of ordering in the non-basal directions while order The fact that the LiF 1 HCl etchant was much milder than HF resulted in the c direction was maintained (see blue XRD pattern in Fig.