Synthesis of Large‐Area MXenes with High Yields through Power‐Focused Delamination Utilizing Vortex Kinetic Energy

Abstract Evaluating the delamination process in the synthesis of MXenes (2D transition metal carbides and nitrides) is critical for their development and applications. However, the preparation of large defect‐free MXene flakes with high yields is challenging. Here, a power‐focused delamination (PFD) strategy is demonstrated that can enhance both the delamination efficiency and yield of large Ti3C2T x MXene nanosheets through repetitive precipitation and vortex shaking processes. Following this protocol, a colloidal concentration of 20.4 mg mL–1 of the Ti3C2T x MXene can be achieved after five PFD cycles, and the yield of the basal‐plane‐defect‐free Ti3C2T x nanosheets reaches 61.2%, which is 6.4‐fold higher than that obtained using the sonication–exfoliation method. Both nanometer‐thin devices and self‐supporting films exhibit excellent electrical conductivities (≈25 000 and 8260 S cm‐1 for a 1.8 nm thick monolayer and 11 µm thick film, respectively). Hydrodynamic simulations reveal that the PFD method can efficiently concentrate the shear stress on the surface of the unexfoliated material, leading to the exfoliation of the nanosheets. The PFD‐synthesized large MXene nanosheets exhibit superior electrical conductivities and electromagnetic shielding (shielding effectiveness per unit volume: 35 419 dB cm2 g–1). Therefore, the PFD strategy provides an efficient route for the preparation of high‐performance single‐layer MXene nanosheets with large areas and high yields.


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conductivity of MXene member was performed on a four-point probe system (Guangzhou Four Probes Tech. Co. Ltd.,. HAADF-STEM were performed on a FEI Titan Themis with a probe corrector at 300 kV. A static mechanical tester Shanghai Hengyi Precision Instrument Co.,Ltd.) was used to test the tensile mechanical properties of the samples, the tensile plastic was 100 mm·min -1 , and the sample size was 0.5 cm*3 cm. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (Thermo fisher Scientific, K-Alpha, USA) apparatus using an Al Kα X-ray source to investigate their surface electronic states. Raman spectra of the L PFD -Ti 3 C 2 T x MXene and S 60 -Ti 3 C 2 T x MXene were obtained using a Raman spectroscope (Horiba Scientific LabRAM HR Evolution, Japan) equipped with a 532 nm laser source. The N 2 adsorption-desorption isotherms were recorded on a Micromeritics ASAP 2460 analyzer at 77.3 K and the surface area was calculated by BET methods.

Calculation of EMI SE of the Ti 3 C 2 T x films.
(1) The reflection loss (SE R ) and absorption loss (SE A ) of conductive shielding can be calculated by the following equations.

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where and 0 are the impedances of the shield and air, respectively, and are the electrical conductivity and the magnetic permeability of the shield, respectively, is the frequency of the incident electromagnetic waves, and is angular frequency and is dielectric permittivity.
(2) The EMI SE was calculated from the scattering parameters (S11 and S21) by the following formulas: = | 12 | 2 = | 21 | 2 (5) The total shielding effectiveness (SE T ) is known as the sum of contributions resulting from reflection (SE R ), absorption (SE A ) and multiple reflections (SE M ). In many cases, multi-reflection has been considered as an absorption because multiple internal reflections of electromagnetic waves are absorbed or dissipated as heat in the shielding materials. Therefore, the total SE (SE T ) can be rewritten as Where R, T and A are the reflection, transmission and absorption coefficients, respectively. P in and P out are the incident and transmitted power, respectively. SE T , SE R and SE A are the total, reflective and absorptive EMI SE, respectively.
(3) Calculation of the Specific Shielding Effectiveness (SSE/t) of the Ti 3 C 2 T x films.
where " " is the film density and "t" is the film thickness.

Supplementary Figures
Supplementary Figure 1. (a) Schematic of Ti 3 C 2 T x preparation. SEM images of (b) Ti 3 AlC 2 , (c) multilayered structure, (d) few-layer MXene before delamination, and (e) monolayer MXene. The stacked arrangement of Ti 3 C 2 sub-layers and Al atomic layers in the Ti 3 AlC 2 MAX phase allows selective etching, starting from the etching of the initial Al atomic layer by LiF and HCl. As shown in Supplementary Fig. 1-2, acid etching allowed the successful removal of Al (Supplementary Table 1), and the Ti 3 C 2 T x MXene underwent interlayer expansion because of the intercalation of Li + ions. Because of the well-defined two-dimensional structure and flexibility of the Ti 3 C 2 T x MXene, the individual layers of the Ti 3 C 2 T x MXene stacks lift from the edges during the delamination process and can be delaminated by manual shaking. Therefore, when the multilayered MXene (tightly stacked layers) formed at the bottom of the sample tube are held in place, the energy of the water flow is concentrated on the Ti 3 C 2 T x MXenes at the surface. After the application of multiple PFD cycles, a high yield of large, exfoliated monolayers of the Ti 3 C 2 T x MXene can be obtained. A scanning electron microscopy (SEM) image of a single Ti 3 C 2 T x MXene layer prepared by the PFD method is shown in Supplementary Fig. 1e. The Ti 3 C 2 T x MXene is thin and transparent, and the porous alumina grid below the monolayer can be clearly seen. We evaluated the exfoliation efficiency of PFD using a vortex shaker to simulate the hand-shaking process. The concentration of the monolayer Ti 3 C 2 T x MXene solution obtained by the MILD method after 30 min of shaking was only 2.56 mg/mL, slightly higher than the literature-reported value, probably because mechanical shaking is more efficient than hand-shaking. However, with increase in the shaking time, there was no obvious improvement in exfoliation efficiency. This is because the impact of the vortex fluidic on the multilayer Ti 3 C 2 T x MXene is insufficient for delamination because both the monolayer Ti 3 C 2 T x MXene and fluid move together. Surprisingly, the delamination efficiency of ultrasonic treatment alone is about the same as that of the MILD method.
We also found that shaking combined with ultrasound-assisted delamination was also excellent. Further, the delamination yield can be increased by increasing the number of PFD cycles, reaching 61.2% after five cycles. In general, ultrasound, vortex shear caused by hand-shaking, and centrifugal processing can exfoliate multilayer MXenes. Furthermore, both sonication after sufficient shaking and redispersion after centrifugation (i.e., via PFD) can substantially improve the delamination efficiency.  As reported previously, the application of ultrasound causes monolayer Ti 3 C 2 T x MXene nanosheets to break up, as well as the generation of defects. Therefore, in comparison to the ultrasound method, the PFD method is an efficient route to prepare large monolayers of Ti 3 C 2 T x MXenes with few of basal-plane defects.
Notice, sonication decreases the transverse dimension of Ti 3 C 2 T x MXene. However, the relationship between the ultrasound time and the transverse dimensions of MXene nanosheets has not been investigated to date. As shown in Supplementary  Fig. 5 and Table S1, sonication causes the structural integrity of MXene nanosheets to be disrupted, resulting in a substantial reduction in the dimensions of the material. In particular, in the first 30 min of sonication, the size of the MXene nanosheets decrease rapidly. Along with the size reduction, the conductivity of the Ti 3 C 2 T x MXenes also decreases dramatically. Because electron transport inside the MXene nanosheet is almost barrier-free but the irregular accumulation of MXene nanosheets results in contact resistance between the nanosheets, the formation of smaller nanosheets leads to a decrease in conductivity Supplementary Fig. 6. In addition, the larger Ti 3 C 2 T x MXene sheets have greater tensile strength ( Supplementary Fig. 7). Therefore, the PFD method provides an efficient way to prepare high-quality Ti 3 C 2 T x MXene sheets having high conductivity and high strength.  Here, ultra-large Ti 3 C 2 T x nanosheets were screened (as shown in the experimental section) by differential centrifugation and their solids content was calculated by freeze-drying. Among them, the ultra-large Ti 3 C 2 T x nanosheets solid content in L PFD -Ti 3 C 2 T x MXene is 66.1% (wt/wt), and the ultra-large Ti 3 C 2 T x nanosheets content in S 60 -Ti 3 C 2 T x MXene is only 10.8% (wt/wt).  As seen from the high-resolution XPS spectra, the C 1s components centered at 282.0, 284.6, 286.2 and 288.6 eV can be assigned as C-Ti, C-C/C=C, C-O, and C-F bond, respectively [2] . The Ti 2p 3/2 spectrum consists of four kinds of titanium species. Four peaks centered at 454.9, 455.6, 457.2, and 459.0 eV arose from the Ti−C bond, Ti−X from substoichiometric TiC x (x < 1), Ti ions in valence 3 + or 2 + (Ti x O y ), and Ti ions with oxidized charge state (TiO 2 ), respectively [2][3] . The O 1s XPS spectrum consists of four peaks located at 533.1, 531.7, 530.5, and 529.8 eV, which correspond to the C−O, C=O, Ti−O−Ti bond, and the surface adsorbed O species, respectively [2,4] . With respect to the high-resolution F 1s XPS spectra, three peaks at 685.2, 686.4 and 688.3 eV have been deconvoluted, which are ascribed to C−Ti−F x species, Al−F x , and Al(OF) x species, respectively [5] . The composition and purity of L PFD -Ti 3 C 2 T x MXene and S 60 -Ti 3 C 2 T x MXene can be also evaluated by Raman spectroscopy. The Raman peaks located at 195 cm −1 , 583 cm −1 and 704 cm −1 are assigned to the characteristic A1g symmetry out of plane vibration of (Ti, O, C), O atom and C atom, respectively [6] . The peaks at 275 cm −1 , 396 cm −1 and 616 cm −1 are attributed to the in-plane E g vibration of H atom in the surface group (T x ), O atom and C atom, respectively [6] . With increase in the number of PFD cycles, the precipitate became gradually and obviously divided into several parts. The presence of different components in the precipitate is also indicated by the differences in color. Therefore, we freeze-dried and collected the different portions of the precipitate for further analysis. Importantly, the different components are separated spontaneously during the centrifugation process, and impurities and raw materials were not detected in the XRD patterns of the upper layer and the final product.