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a. R. Bian, R. Lin, G. Wang, Prof G. Lu, Dr D. Cai and Prof W. HuangKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM)Nanjing Tech University30 South PuZhu Road, Nanjing, Jiangsu, 211816, ChinaE-mail: [email protected] (D. Cai); iam whuang @njtech.edu.cn (W. Huang)

b. Prof W. HuangShanxi Institute of Flexible Electronics (SIFE) Northwestern Polytechnical University (NPU) 127 West Youyi Road, Xi’an 710072, China

c. W. Zhi, S. Xiang, Prof T. WangCollege of Materials Science & EngineeringNanjing Tech UniversityNo.5 Xinmofan Road, Nanjing, 21009, China

d. Dr P.S. CleggSchool of Physics and AstronomyUniversity of EdinburghPeter Guthrie Tait Road, Edinburgh, EH9 3FD, UK

†Electronic Supplementary Information (ESI) available: Experimental section and supporting figures. See DOI: 10.1039/x0xx00000x

Nanoscale

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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

3D assembly of Ti3C2-MXene directed by water/oil interfaces†Renji Bian,a Ruizhi Lin, a Guilin Wang, a Gang Lu, a Weiqiang Zhi, c Shanglin Xiang, c Tingwei Wang, c Paul S. Clegg, d Dongyu Cai,* a and Wei Huang* a,b

MXene is an emerging class of 2D materials exfoliated from ternary carbide and nitride ceramics. The exfoliation process, which is an acid etching approach, functionalizes the MXene surface with –OH, -O and –F groups. These functional groups offer significant opportunities for tuning the colloidal properties of the MXene nanoblocks; importantly, this tuneability points the way towards a facile route for assembling these nanoblocks into 3D architectures that are in demand for many applications. This route, presented for the first time here, uses water/oil interfaces for assembling Ti3C2-MXene in 3D architectures. It shows that cetyl trimethylammonium bromide (CTAB) can be used to tune hydrophilic-hydrophobic balance of Ti3C2-MXene via the interaction of positively charged –N(CH3)3 and -O group on the MXene surface. Crucially, it is found that this interaction can be controlled via the hydrogen ion concentration in the aqueous phase. Stable oil-in-water emulsions are the only product when the aqueous phase is neutral or basic. This understanding led us to fabricate a high internal phase Pickering emulsion with more than 70vol% oil droplets and also a solid porous monolith based on this emulsion template.

The MAX phases are a group of layered ceramics (ternary carbide and nitride) with metallic properties, which have a general formula Mn+1AXn with n=1-31. Here, M represents an early transition metal, A is an A-group (typically IIIA and IVA) element and X is either carbon or nitrogen1. In 2011, researchers from Drexel University synthesized a new 2D material, called MXene (early transition metal carbides, nitride and carbonitrides), through selectively etching the A element from the MAX phases using strong acids2. MXene demonstrates a hydrophilic surface terminated by oxygen-containing groups3, 4. The rich chemistry and outstanding conductivity make MXene particles attractive nanoblocks for many potential applications such as rechargeable batteries5-9, supercapacitors3, 10-12, fuel cells13, sensors14, 15 and magnetic shielding16, 17.

Assembly of MXene into 3D structure is key for realizing these potential applications. Owing to its hydrophilic characteristics, a mixture of MXene and water behaves like Play-Doh with the ability to be flexibly shaped3. Stacking MXene sheets layer-by-layer demonstrates a typical route for the formation of large-scale 3D structures (thin films). Nickel foams18 or poly (methyl methacrylate) colloids19 have been used as the templates for assembling MXene into 3D structures. Interfacial assembly is a universal and scalable approach for organizing a variety of nanoblocks (e.g. graphene oxide) into 3D structures20-22. This route demands careful tuning of the surface of nanoblocks. To the best of our knowledge, the interfacial assembly of MXene for the construction of 3D structures has not previously been attempted. In this communication, we report an interfacial route for forming MXene-based 3D structures directed by water/dodecane interfaces.

Ti3C2-MXene was prepared following a published procedure, in which the mixture of hydrogen chloride (HCl) and lithium fluorine (LiF) was used to treat Ti3AlC2 MAX phase3. The SEM image (Fig. S1†) shows the typical structure of multi-layered Ti3C2-MXene with the removal of the Al layer after etching. XPS (Fig. S2†) and FTIR (Fig. 2d) results show that the surface of Ti3C2-MXene contains hydroxyl (Ti-OH), oxygen (Ti-O-Ti) and fluorine (Ti-F) groups as suggested previously23. Shaking a mixture of water/oil/solid particles leads to

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the formation of a Pickering emulsion if the solid particles are partially wet and trapped at water/oil interface24, 25. This rarely occurs for a mixture of water, dodecane and pristine Ti3C2-MXene since the MXene nanoblocks are strongly hydrophilic.

It has been reported that positively charged ions can be exchanged with the hydrogen in [Ti-O]-H+ groups26, 27 or interact with electronegative oxygen atoms in Ti-O-Ti (oxo) groups23, 28. As show in Fig. 1, we propose the use of cationic surfactant (cetyltrimethylammonium bromide (CTAB)) for altering the hydrophilic characteristics of Ti3C2-MXene. Once the positively charged head (–N(CH3)3) of CTAB interacts with Ti3C2-MXene, the resulting attachment of carbon tails is expected to increase the hydrophobicity of Ti3C2-MXene. However, we find that the surface modification is not straightforward: it is very sensitive to the pH value of the aqueous phase. Fig. 2a shows that no stable emulsion can be formed when the MXene solutions is acidic (pH<7), and MXene aggregates eventually reside on the interface of the macroscopically separated water and dodecane phases. The video in Fig. S3† shows the process of the phase separation. This failure mode is very likely to happen when using as-prepared Ti3C2-MXene since it is difficult to completely remove the acid used in the etching process. Stable emulsions are achieved when the MXene solution is neutral or even basic. At a pH of 8.45, Fig. 2b shows the fluorescent optical image of the emulsion with dodecane droplets dyed with Nile Red. Furthermore, Fig. 2c reveals that the emulsions can be prepared when the pH is tuned from 5.50 to 8.80, and the emulsion become unstable when adjusting the pH of MXene solution back to 5.68. This suggests that the effect of pH on the stability of emulsions is reversible. The video in Fig. S5† clearly shows the destabilization of the emulsion induced by adding a few droplets of hydrochloric acid solution. Our results suggest that the existence of H+ promotes the protonation of the MXene surface and this likely results in displacement of the positively charged head of CTAB(–N(CH3)3). This is described by a possible scheme in Fig. 2e. Comparing the FTIR spectrum (Fig. 2d) of the pristine MXene and CTAB doped MXene, the peak owing to hydroxyl groups at 3441 cm-

1 remains unaffected, but the peak of the Ti-O bond shifts from 570 cm-1 to 559 cm-1, indicating that oxygen groups take up CTAB

molecules on the surface of MXene29. The peaks at 2919 cm-1 and 2852 cm-1 are associated with –CH2 groups in CTAB molecules.

For an alkaline dispersion of MXene nanoblocks (pH=8.45), we show in Fig. 3a that the pristine MXene surfaces are hydrophilic and prefer the water phase rather than the dodecane phase or the interface. For a fixed mass of MXene (7.5mg) doped with CTAB (0.045mM), we demonstrate that the colour of the aqueous phase becomes light grey with black sediment (MXene) on the bottom of

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Fig.1 A diagram illustrating the preparation of oil-in-water Pickering emulsions and high internal phase Pickering emulsions (HIPEs) stabilized by modified Ti3C2-MXene. (Blue: water; Yellow: dodecane)

presented at the top of each vial); (b, c and d) showing the fluorescent optical images of the emulsion when the addition of CTAB is 0.045, 0.18 and 0.9mM, respectively. Scale bar=200µm.

Fig.2 (a) The observation of emulsion stability as a function of the pH of the aqueous phase at a fixed ratio of CTAB (0.5mg) to MXene (7.5mg) by weight (note: pH is presented on the top of each vial) ; (b) a fluorescent optical image of an emulsion at a pH 8.45 (note: the oil is labelled by Nile Red and scale bar is 200µm); (c) the reversible effect of pH value on the stability of the emulsion; (d) FTIR spectra of MXene and CTAB-doped MXene; (e) a possible scheme describing the effect of pH on the interaction between CTAB and Ti3C2-MXene.



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the vial and dodecane droplets (Fig. 3b) formed at the top. For comparison, Fig. S4 (a)† shows that the addition of 0.045mM CTAB does not permit the formation of any droplets at concentration lower than cmc (0.9mM). Fig. S4 (b)† also shows that the droplets cannot be stabilized by MXene alone under different pH values. This suggests that doping with CTAB renders MXene particles partially hydrophilic and hence able to stabilize dodecane droplets. Increasing the doping with CTAB gives rise to lighter color in the aqueous phase and less black sediments in the vials from left to right, indicating more MXene is transferred to the water/dodecane interface. This is supported by Fig. 3 (c and d) which show that the droplets are smaller as a result of more MXene being trapped at the

interface.

Having achieved this level of control, we can now prepare high internal phase emulsions (HIPEs) stabilized by CTAB-doped MXene. HIPEs are a class of highly concentrated emulsions with more than 70 vol% dispersed phase30. Their structure is analogous to a gas-liquid foam with a large volume of droplets separated by thin films of continuous phase31. Polymerization of the continuous phase is of being particular interests in designing porous materials32, 33. HIPEs are both kinetically and thermodynamically unstable, but they can be made metastable systems via the use of interfacial stabilizers such as organic surfactants and solid particles31. As demonstrated in Fig. 1, we are able to prepare an o/w HIPE stabilized by CTAB-doped MXene in a straightforward manner. With fixed MXene/CTAB ratio, the diagram in Fig. 4 shows that a stable oil-in-water HIPE with 70vol% oil droplets forms when MXene concentration with respect to the total volume of water and dodecane is more than 24mg/ml. The resulting HIPEs are semi-solid and exhibit a foam-like microstructure imaged by fluorescence microscopy, in which the continuous phase (red) is labelled with Rhodamine B. We also observe that dodecane droplets are constantly formed from low to high volume fraction of dodecane phase without catastrophic inversion. This suggests that CTAB-doped MXene highly prefers the water phase to the oil phase at the interface, while the MXene

concentration is the key factor in arresting HIPEs in our case. The emulsion becomes a free-flowing liquid when the volume fraction of dodecane droplets is reduced to 60%.

Finally, we demonstrate that this HIPE is a robust template for fabricating porous materials with cellular structure. This process is mainly concerned with the partition of a water-soluble monomer (2-hydroxyethyl methacrylate) and initiator into the continuous phase. Temperature-initiated polymerization results in the formation of porous materials with a good replica of the HIPE template. Fig. 5 (a and b) shows that the size of the porous materials is up to 1 cm. Our method has the potential to be scaled


presented at the top of each vial); (b, c and d) showing the fluorescent optical images of the emulsion when the addition of CTAB is 0.045, 0.18 and 0.9mM, respectively. Scale bar=200µm.

Fig.4 A diagram showing the effect of MXene concentration on the formation of HIPEs-stabilized by CTAB-doped MXene. The X-axis represents the MXene concentration with respect to the total volume of water and dodecane. The Y-axis represents the volume fraction of dodecane phase (the total volume of dodecane and water is 3 ml). Inserted: the fluorescent optical images of the emulsions with the water phase labelled using Rhodamin B (Scale bar=200µm).

Fig.5 (a,b) a resulting porous material with a size up to 1cm; (c) the robustness of the porous material under a weight of 50g; (d and e)



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up. Fig. 5 c with the support of a video in Fig. S6† shows that the porous materials relatively stong to an external force with negligible deformation. The detailed data about mechanical compression and thermal stability are provided in Fig. S7†. It shows a typical compression behaivor of a polymer with a yield stress of around 0.07MPa. Thermal gravimetric analysis shows that the material is thermally stable when the temperature is up to around 200 oC. The weight loss is around 70% between 200 and 400 oC that is mainly attributed to the burning of polymer phase in the material. The residual mass owes to the inorganic MXene phase. Fig. 5 (d and e) show the cellular or foam-like structure of the porous material supported by a polymeric wall. The SEM image in Figure 5f demonstrates that the polymeric wall is embedded with Ti3C2-MXene particles.

ConclusionsWe have revealed a straightforward approach for the 3D assembly of Ti3C2-MXene at water/oil interfaces. The hydrophilic nature of MXene nanoblocks could be tuned via the adsorption of CTAB onto the oxygen groups on MXene surface. We have shown that pH is a key factor controlling whether CTAB-doped MXene stabilizes the oil-in-water emulsions. For pH<7, the emulsions are unstable, suggesting that existing H+ ions undermine the bonding between CTAB and Ti3C2-MXene. In contrast, stable water-in-oil emulsions are obtained when the MXene dispersion is neutral or basic. This leads us to successfully demonstrate HIPE production with 70vol% oil droplets; such HIPEs can then be used as a template for fabricating porous materials with a foam-like structure supported by a polymeric wall. Via this study we have presented the design principles for the preparation of MXene-based 3D structured materials.

Conflicts of interestThere are no conflicts to declare.

Acknowledgements

We thank National Natural Science Foundation of China (Grant No. 21503110) for supporting the work of D.C, fundamental Studies of Perovskite Solar Cells (2015CB932200) for supporting the work of W.H, Marie-Skłodowska-Curie European Training Network COLLDENSE (H2020-MSCA-ITN-2014 Grant No. 642774) for support the work of P.S. We also thank Key University Science Research Project of Jiangsu Province (No. 17KJA150005) and Six Talent Peaks Project in Jiangsu Province (No. XCL-038) for supporting the work of G.L.

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Fig.5 (a,b) a resulting porous material with a size up to 1cm; (c) the robustness of the porous material under a weight of 50g; (d and e)



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