Structural and Thermal Characterization of Hydrothermally Synthesized Nb4C3Tx MXene

Nb-based MXenes, especially Nb4C3, exhibit excellent electrical conductivity, higher interplanar spacing, larger surface area, and elasticity, making them suitable for many applications, ranging from energy storage devices and gas storage systems to electro- or photocatalysts and sensors [1–4]. The usual route to synthesize Nb4C3 is to etch the Al layers from the parent 413 MAX phase (Nb4AlC3) with HF [1,2]. Other synthesis methods include the molten-salt technique and in situ HF generation from halogen-mediated methods [5,6]. Here, we use hydrothermal etching of Al layers from Nb4AlC3 MAX phase via in situ HF generation to prepare Nb4C3Tx (T = –F, –O, or –OH). Figure 1a shows the schematic of the synthesis of Nb4C3 from Nb4AlC3 by in situ HF generation using the hydrothermal method. In this method, 1.508 g LiF salt was stirred for 15 minutes in 20 mL of 9 M HCl to generate in situ HF. This is followed by the slow addition of 1.002 g of Nb4AlC3 (Sigma-Aldrich, powder) and stirring continued for an additional 15 minutes. The resulting solution was transferred to the hydrothermal reaction vessel and heated for 72 hours at 180 °C in an oven, then cooled to room temperature naturally. The etched product was first washed with 2 M H2SO4, then with DI water approximately six times by centrifugation (4500 rpm, 5 minutes) until the pH reached 7. The sediment was then dried in an oven at 40 °C for 24 hours to obtain the final Nb4C3Tx powder. The surface morphologies of Nb4AlC3 and Nb4C3Tx powders were observed by SEM (Scanning Electron Microscopy, JEOL Field Emission, 20 kV) and are shown in Figures 1b and 1c, respectively. These images verify that the tightly held layers in Nb4AlC3 became distinctly identifiable after etching in Nb4C3Tx. To confirm the structural changes after etching, we conducted X-ray diffraction (XRD) using a Rigaku Miniflex 600 XRD Machine, as shown in Figure 2a. The XRD clearly shows that the Al-related peak near 40° in Nb4AlC3 is sharply reduced in Nb4C3Tx, indicating Al removal by selective etching during the hydrothermal process [2,6]. At the same time, the (002) peak at 7.38° of Nb4AlC3 shifted to a lower 2θ at 5.18°, with broadening in Nb4C3Tx, indicating an increase in interplanar spacing [2,6]. This case is verified again in the Raman spectroscopy (Horiba XploRA), as shown in Figure 2b. Interestingly, the Raman peak for Nb-Al bond in Nb4AlC3 and the Nb-O bond in Nb4C3Tx lie around the same range of (200-300 cm-1) but the Nb-C bond at 662 cm-1 in Nb4AlC3 is red-shifted to 642 cm-1 in Nb4C2Tx [2,7] because of the lattice expansion and softening of the bonds, due to the Al etching and surface functional groups [8]. The other higher-wavenumber Raman peaks, which are more prominent in Nb4C3Tx compared to Nb4AlC3, are due to surface functional groups.[8] The thermal behavior of Nb4AlC3 and Nb4C3Tx in ambient conditions is measured by Thermogravimetric analysis (TGA) using a SHIMADZU DTG-60 and is shown in Figure 2c and Figure 2d, respectively. Nb4AlC3 simply starts oxidizing above 300 °C, whereas Nb4C3Tx first loses mass due to the loss of Tx functional groups, then oxidizes above 400 °C, forming stable oxides (see the exothermic Differential thermal analysis (DTA) curve) [9]. The systematic characterization of Nb4C3Tx is important for expanding its applications in energy storage systems and photo sensors in future studies [10]. (a) Schematic of Nb4AlC3 (MAX) phase and final Nb4C3 (MXene) phase after hydrothermal etching; SEM of (b) Nb4AlC3 and (c) Nb4C3Tx. Characterization of Nb4C3Tx phase compared to its parent Nb4AlC3 phase; (a) XRD pattern, (b) Raman spectra; TGA and DTA of (c) Nb4AlC3 and (d) Nb4C3Tx.