General synthesis of MXene by green etching chemistry of fluoride-free Lewis acidic melts
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General synthesis of MXene by green etching chemistry of fluoride-free Lewis acidic melts Lu-Yang Xiu
, Zhi-Yu Wang*
, Jie-Shan Qiu*
Ó The Nonferrous Metals Society of China and Springer-Verlag GmbH Germany, part of Springer Nature 2020
The MXene has emerged as a rising star on the horizon of material science due to widespread applications ranging from energy and catalysis to biomedicine [1, 2]. These twodimensional (2D) transition metal carbides/nitrides well collect a variety of attractive properties such as rich surface chemistry, high hydrophily, and metallic conductivity that are difficultly attained for all in conventional 2D atomic crystals [3, 4]. The state-of-the-art strategies for making MXene mainly rely on the selective etching of the layered structure consisting of A elements in the MAX phase by fluoride-involved chemistry [5]. Although dozens of MAX phases are commercially available, this way is mostly work for the Al-containing MAX phase with high reactivity with fluoride ions [1–5]. Meanwhile, the use of aqueous fluoridated etchants further makes the process highly hazardous with the fatal risk of toxicity and corrosivity, hindering the extensive studies and use of MXene. These agents also show a negative impact on the specific capacitance of etched MXene in supercapacitors by introducing a high concentration of fluoride groups. Electrochemical etching route has been developed for etching MAX precursors to fluoride-free MXene [6, 7]. Nevertheless, the high effectiveness is still limited to a handful of MXene from the Al-
L.-Y. Xiu, Z.-Y. Wang*, J.-S. Qiu* State Key Laboratory of Fine Chemicals, Liaoning Key Laboratory for Energy Materials and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China e-mail: [email protected] J.-S. Qiu e-mail: [email protected] J.-S. Qiu College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
containing MAX phase. The development of safer and efficient strategies for yielding high-quality MXene from the expended family of the MAX phases is imperatively necessary to full exploitation of the potential of MXene. Recently, Huang. et al. proposed a generic Lewis acid etching route for preparing various MXene from a large family of MAX phases with different A elements (such as Al, Zn, Si, Ga, etc.) [8]. Selectively etching these MAX phases is realized in terms of the oxidation of the A elements by the cation of fluorine-free Lewis acid molten salts with a higher electrochemical redox potential (e.g., ZnCl2, FeCl2, CuCl2, AgCl). Remarkably, the redox coupling between the A elements in MAX phase and the cation of the Lewis acid Cl melts can be predicted by a Gibbs free energy mapping according to the redox potentials of A elements in MAX phases and the cations in molten salt melts (Fig. 1a). It guides the rational selection of appropriate Lewis acid melts for MXene synthesis, and demonstrates the generalization of this route
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