Structural Stability and Electronic Transport Properties of Nb2C‐MXenes under High Pressure

The extraordinary electronic properties of MXenes are closely associated with their surface terminations and external strain. Herein, by applying pressure, the structural and electronic properties of Nb2C‐MXenes with two different terminations F (Nb2CFx) and Cl (Nb2CClx) are reported. Through high‐pressure X‐ray diffraction and scanning electron microscopy, it is observed that Nb2CFx loses its layered structure and becomes disordered above 20 GPa, while Nb2CClx remains as layered structures up to 42.5 GPa. Surprisingly, the stable Nb2CClx shows the axial compression coefficients with 1174.5 ± 39.2 GPa along the c axis and 1234.8 ± 25.8 GPa along the a axis, even larger than that of its precursor Nb2AlC MAX phase. It is also found that the resistance of Nb2CFx exhibits a drop of two orders of magnitude from the ambient pressure. Accompanied by a slight increment of the superconducting transition temperature, the superconductivity of Nb2CClx survives up to 50.5 GPa. The results uncover the effects of surface terminations on structural stability and electronic properties of Nb2C‐MXenes under high pressure.


Introduction
As an emerging family of 2D layered materials, [1] MXenes is synthesized by etching its precursor MAX phases, with the chemical formula of M n+1 X n T x . [2] In M n X n+1 T x , the M element represents the early transition metal, X is the carbon or nitrogen, and n = 1, 2, 3, and T is a surface termination. The composition of the terminations depends on their etching conditions, including OH, O, F, and other active terminations such as Cl. Due to their diverse electronic properties manipulated by surface terminations, MXenes have promising applications in many areas such as electronic capacitive, [3] electromagnetic interference shielding, [4] and piezoresistive www.advelectronicmat.de have a larger compression ratio along the c axis than the a axis, because the presence of C stabilizes MX bonds, while Cr 2 AlC shows a contradictory behavior. [15] It may explain why Cr 2 C MXenes have not been synthesized successfully. Some 2D materials showed structural transition under high pressure; for example, few-layer graphene lost X-ray diffraction (XRD) signals and crumpled above 18 GPa. [16] However, no structural transition up to 26.7 GPa was observed in the Menes structure of Ti 3 C 2 T x (T x = O, OH, F). [17] The structural stability of MXenes under high pressure still lacks investigation and how the surface terminations affect the electronic properties also needs to be resolved.
In this work, we present a series of mutually verifiable experiments on Nb 2 C-MXenes with two different surface terminations, selected because they represent two transport behaviors of Nb 2 C-MXenes. Our results figure out the functions of surface terminations on the mechanical and electronic transport properties of Nb 2 C-Mxenes under high pressure.

Results and Discussion
The XRD patterns of Nb 2 CF x etched by HF solutions, Nb 2 CCl x etched by CdCl 2 molten salts and their MAX phase Nb 2 AlC are shown in Figure 1. The (002) diffraction peak around 12.81° of Nb 2 AlC disappears and shifts to a low angle, which indicates that the Nb 2 AlC has been etched completely by those two different methods. [18] All of the XRD patterns are consistent with previous works [9,19] and well-indexed to hexagonal P6 3 /mmc in which d 110 = 1/2 × a, d 100 = √3/2 × a, and d 002 = 1/2 × c. The lattice parameters a and c represent interlayer and intralayer distance, respectively. For Nb 2 CF x , the lattice parameters a is 3.087 Å and c is 10.284 Å, while for Nb 2 CCl x the a is 3.145 Å and c is 8.639 Å. The chemical compositions detected by X-ray photoelectron spectroscopy (XPS) spectrum also illustrate that Nb 2 C-MXenes have surface terminations with F and Cl ( Figure S1, Supporting Information).
Comparing the interlayer peak (002) of Nb 2 CCl x , the Nb 2 CF x shows a larger increment of the (002) peak. Although terminal F's radius is smaller than terminal Cl, which is mainly caused by the process difference wherein the HF acid etching process. There are some water molecules intercalated in the layers of Nb 2 CF x , causing an interlayer expansion. [20] The intralayer peaks (100) and (110) are relevant to the NbC bonds, proving that Nb 2 CCl x has a larger intralayer crystal structure than Nb 2 CF x . Because of the F has stronger electronegativity, it can attract more electrons from Nb, leading to the lower electronic density of NbC bonds. [21] High-pressure synchrotron XRD measurements were conducted to explore the structure evolution in Nb 2 CF x and Nb 2 CCl x , as shown in Figure 2. With the pressure up to 40 GPa, no new diffraction peaks appear or diffraction peaks disappear The results indicate there is no phase transition emerging in Nb 2 CF x under compression same as the behavior of its precursor phase Nb 2 AlC. [13] For Nb 2 CF x , all the peaks including interlayer peaks (002) and intralayer peaks (110) and (100) slightly shift to high angles with the increasing pressure up to 16.8 GPa. The compressive ratios of lattice a/a 0 and c/c 0 as functions of pressure show a and c compressed by 2% and 6.3% at 16.8 GPa (Figure 2b), meaning the P6 3 /mmc structure is more compressible along the c direction. The reason is that interlayer van der Waals interaction is weaker than covalent NbC bonding. However, the diffraction peaks slowly widen in the range of 16.8-22.2 GPa and almost lose the crystalline diffraction characteristics with pressure above 22 GPa. The crystallinity disappearance could be attributed to a pressure-induced interlayer interaction and Nb 2 CF x layer buckling or crumbling. The following scanning electron microscopy (SEM) images will further reveal it. Figure 2c illustrates XRD patterns of the Nb 2 CCl x measured at different pressure until 42.5 GPa. In all the pressure ranges, all the diffraction peaks slightly shift to high angles, indicating lattice parameters shrink with the increased pressure. Unlike Nb 2 CF x , the diffraction signals of Nb 2 CCl x do not have significant disappear. As shown in Figure 2d, the compressive ratios of a and c axis for Nb 2 CCl x keep a coherent compressive behavior below 16.9 GPa and the lattice parameters a and c are compressed by 2.3% and 3.5% at 42.5 GPa, respectively. After pressure increases, there is a bifurcation between the compressive ratios along the a and c axis, implying Nb 2 CCl x begins to emerge into inhomogeneity deformation.
To further reveal the elasticity of both Nb 2 C-MXenes with different terminals, we use the Murnaghan equation to fit the compressive ratios curve [22] / where r represents the lattice constants along the c and a axis, B 0 is the linear compression coefficient reflecting the stiffness of materials and B′ is the pressure derivate of B 0 , which normally equals to 4. [22] For ultrathin 2D materials, B 0 is close to Young's modulus, so it can be used as a substitution to evaluate the elasticity coefficients. [23] As shown in Table 1, the compression coefficient along the c axis of Nb 2 CF x has a smaller value than that of the a axis, indicating that the interlayer has weak connections. Along the a axis direction, the linear compression coefficient of Nb 2 CF x (202 GPa) is larger than that of graphite (35.7 GPa). [24] On www.advelectronicmat.de the other hand, the linear compression coefficient of Nb 2 CF x (530 GPa) is smaller than that of graphite (1250 GPa) along the c direction. For Nb 2 CCl x , all the compression coefficients are beyond 1100 GPa, indicating Cl terminations enhance the stiffness of Nb 2 C-Mxene. For comparison purposes, we also calculated the compression coefficients of its precursor phase Nb 2 AlC by high-pressure synchrotron XRD patterns ( Figure S2, Supporting Information). Surprisingly, the compression co efficients of Nb 2 AlC are 1069 GPa along the a direction and 883 GPa along the c direction. Both of them are smaller than that of Nb 2 CCl x , which is not well understood yet and needs further study.
The synchrotron XRD measurements indicate that high pressure can lead to the structural disorder on Nb 2 CF x , while it does not occur in Nb 2 CCl x . As shown in Figure 3, we used the SEM to check the morphology of Nb 2 CF x and Nb 2 CCl x before and after compression. For Nb 2 CF x , before the high-pressure experiment, Figure 3a shows a clear layered structure. After the pressure is released, small scraps appear and the layered structures disappear (Figure 3b and Figure S3, Supporting Information), giving complementary evidence of the structural disorder of Nb 2 CF x after compression. For Nb 2 CCl x , as shown in Figure 3c,d, there remains a well-defined layered structure after high-pressure compression up to 42.5 GPa, which is in accord with the results of the high-pressure XRD. After highpressure compression, the significant difference between two Nb 2 C-Mxenes may be related to the different radius of terminals. For the Cl terminal, its atomic radius is 79 pm, while   the F terminal's radius is 42 pm. The larger terminal's radius is more strong in supporting the layered structure. Additionally, the high-pressure Raman spectra ( Figure S4, Supporting Information) showed that the G peak of Nb 2 CF x lost its signal above 17 GPa, caused by buckled and crumpled layers under high pressure. [16] To explore the electrical properties of Nb 2 C-MXene under pressure, we carried out the four-point probe method measurements. Figure 4a shows the resistance of Nb 2 CF x with the decreasing temperature in the pressure ranges from 0.2 to 41.9 GPa. When the pressure is increased to 41.9 GPa, the resistance at 300 K of the sample decreases from 1000 to 10 Ω with two orders of magnitude drops, indicating that the resistance of Nb 2 CF x is susceptible to pressure. [25] Zheng et al. have revealed that the band-like mechanism happened in the intralayer electronic transport in MXenes, while the hopping-like transport behaviors governed the interlayer transport. [26] Here, the interlayer distance strongly decreases with the applied pressure because of the relatively weak van der Waals bonding between Nb 2 CF x layers, making the electrons easily hopping from layer to layer. Furthermore, there is no evidence of superconductivity, even at the temperature of 2 K, concluding that pressure cannot lead to superconductivity in Nb 2 CF x .
To further shed light on how pressure affects the conductivity of Nb 2 CF x , we turn to use different conduction models to fit the resistance-temperature (R-T) curves and finally found 3D variable-range hopping (VRH) model is more applicable as the interlayer transport mechanism, in which the resistance (R vrh ) follows [12] exp vrh 0 where R 0 is a resistance prefactor, d is the dimension and here is 3 for 3D VRH models. T 0 is a characteristic temperature, relating to the density of states at Fermi level, D(E F ) by a relation of the form where D(E F )γ −3 is called the localization parameter and a is a geometry constant. By fitting the R-T curves of Nb 2 CF x at each pressure with 3D VRH models, we can obtain specific a T 0 for each pressure as shown in Figure 4b. Below 20 GPa, both T 0 show a monotonically decreasing trend with the increase of pressure. It represents the localization parameters and is positively associated with interlayer distance. However, when the pressure is beyond 20 GPa. The VRH model is unable to fit the R-T curves well and parameters like R 0 and T 0 cannot converge ( Figure S5, Supporting Information). It suggests the basic conduction mechanism in Nb 2 CF x is changed under pressure above 20 GPa, which is coherent to the structural disorder observed by in situ high-pressure XRD.
To understand how pressure impacts on the electronic transport properties of Nb 2 CCl x and its superconductivity, we carried out the similar R-T experiments. As shown in Figure 5a, the resistance decrease from 1 to 0.1 Ω at 300 K with the increased pressure, indicating that Nb 2 CCl x are also sensitive to pressure, especially in the range from 1.2 to 18 GPa. However, the roomtemperature resistance shows no significant changes with the pressure when the pressure is over 21.2 GPa. Compared with the semiconducting behavior of Nb 2 CF x , the Nb 2 CCl x shows metallic behavior and it is virtually independent of temperature at low temperatures before the superconducting transition temperature (T c ).
Coming to the aspect of superconductivity, we plot R-T curves within the temperature range at which superconductivity occurs in Figure 5b and the enlarged curves in Figure 5c.

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As the pressure increases, the superconducting transition width keeps broadening. This might be due to the pressure-induced shrinkage in the size of the crystals, similar to the MgB 2 case. [27] To further demonstrate the effect of pressure on superconducting in Nb 2 CCl x , we marked the zero-resistance temperature T c zero and resistance dropping temperature T c onset on each pressure, as shown in Figure 5d. We found that the T c zero is decreasing with the pressure, while T c onset drops slightly with pressure below 20 GPa and increases above 27 GPa. The decrement of T c zero can be ascribed to the weaker coulomb screening and the localization of Cooper pairs induced by increasing disorder with the pressure. So we suppose that the pressure leads to the reduction of interlayer distance and enhances the interlayer Cooper pairing like the positive relationship between T c and thickness of NbSe 2 . [28] To further investigate how the superconducting transition is affected by pressure, the evolution of superconducting transition widths (∆T c ) with pressure is shown in Figure 6a. As the pressure is increased, the ∆T c is unchanged at the beginning and increases progressively from 0.7 to 2.5 K when pressure is above 20 GPa. Noteworthy, the obvious bifurcation of compressive ratios between the a and c axis also appeared around 21 GPa, which indicates that a pressure inhomogeneity inside the sample space emerged above 20 GPa. So the broadened superconducting transition could be induced by the inhomogeneity of Nb 2 CCl x sample under high pressure.

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Additionally, the temperature dependence of resistance at ambient pressure of Nb 2 CCl x follows the weak localization model [29] where σ 0 is residual conductivity, aT 1/2 is related to quantum correction from the electron-electron interaction and the second term bT 2 describes the high temperature part. Using this weak localization model to fit the temperature-dependent resistance of Nb 2 CCl x at the normal state under each pressure, we extract the parameters a and σ 0 ( Figure S6, Supporting Information). Both of the parameters increase with the pressure at the beginning. While pressure is above 20 GPa, the value keeps relatively unchanged, which coincides with the structural evolution. More importantly, parameter a represents the disorder level of samples and it will suppress the superconductivity. This may explain the slight decrease of T c onset from ambient to 20 GPa (Figure 5d). It should be mentioned that in the overall pressure range T c of Nb 2 CCl x shows slight change, which is not rare in Nb-based superconductors, such as NiTi, [30] NbN, [31] and 2D superconductor Mo 2 C. [32] The commonalities of these superconductors are worthy of future experimental and theoretical research.

Conclusion
We have conducted a series of high-pressure experiments on the structure and electronic transport of Nb 2 C-MXenes with two different terminations Cl and F. The layered structure of Nb 2 CF x becomes disorder above 20 GPa along with remarkably decreased resistance. For Nb 2 CCl x , its layered structure maintains even pressure up to 42.5 GPa and has more great axial compression coefficients even than its precursor Nb 2 AlC. Besides, Nb 2 CCl x behaves as a pressure-tolerant superconductor. Our findings shed new light on the role of surface terminations in the structural and electronic properties of Nb 2 C-MXenes.
Two different etching processes were adopted to synthesize Nb 2 CF x and Nb 2 CCl x , respectively. One is the HF etching method: Nb 2 AlC powder was added to 40 mL of hydrofluoric acid solution and reacted for 96 h in a tetrafluoroethylene reactor at 60 °C. Another is the molten salt method: First, the Nb 2 AlC powder was mixed with the CdCl 2 powder in 1:10 molar ratio and ball milled for 12 h. The mixture was put in a corundum crucible and kept at 300 °C for 8 h in a tube furnace under the flow of argon gas (mixed with 5% hydrogen) to remove moisture and then reacted at 750 °C for 36 h. After the completion of the reaction, the obtained products were stirred and washed with hydrochloric acid for 12 h.
The same postprocessing process was employed to get the MXene products. The reacted acid solution was centrifuged at 7000 rpm for 5 min and washed with deionized water several times ultrasonically until the pH of the supernatant reached 7. The precipitate was taken out from the centrifuge tube with ethanol and dried at 55 °C under vacuum for 12 h to obtain pure Nb 2 C-MXene powder.
Structure Characterization: The ambient pressure XRD spectrum was measured using a Bruker D8 ADVANCE X-ray diffractometer. The SEM images were obtained using a Gemni-500 scanning electron microscope. The XPS spectrum was collected by the Thermo Scientific K-Alpha X-ray photoelectron spectrometer.
Beamline Configuration: In situ high-pressure XRD measurements used a DAC at room temperature, performed at the beamline 4W2 of Beijing Synchrotron Radiation Facility, where the wavelength of the incident monochromatic X-ray beam is 0.6199 Å. Silicone oil is used as the pressure transmitting medium for the quasi-static pressure environment. The sample was placed in a stainless steel gasket hole (130 µm in diameter) with a diamond culet (300 µm in diameter). Ruby was used as an internal pressure standard. [33] Then, the diffraction ring was transferred by Dioptas program. [34] Physical Properties Measurements: The high-pressure electrical measurements of the Nb 2 C-MXenes were measured by a Quantum Design physical properties measurement system EverCool-II in the nonmagnetic Be-Cu high pressure cell without any transmitting medium.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.