Inhibition of AlF₃·3H₂O Impurity Formation in Ti₃C₂T_x MXene Synthesis under a Unique CoF_x/HCl Etching Environment

Abstract

MXenes, most commonly transition metal carbides, are a family of two-dimensional (2D) materials with promising potential in, among other applications, supercapacitors and batteries. MXenes are synthesized by etching of aluminum or gallium layers in its parent MAX phase directly by HF or by HF in situ formation using a fluoride salt and strong acid. A commonly undesired byproduct of MXene synthesis is AlF3·3H2O. To relieve MXenes from AlF3·3H2O impurity, it is important to elucidate the factors that drive its formation. Here, we dually deduce the conditions that lead to AlF3·3H2O formation while exploring etching with cobalt fluorides (CoF2/CoF3). Previously uncharacterized, etching with cobalt fluorides offers a forthright method to etch MXenes while intercalating cobalt cations. The influence of this etching environment and AlF3·3H2O formation on MXene’s structure, morphology, and surface bonding is investigated. Ionic strength of solution used for etching is found to be a critical driving factor in the formation of AlF3·3H2O impurity formation. Specifically, when the ionic strength falls between ∼8.5 and 10 M, AlF3 complexation is stable. As a result, Ti3C2T x MXene phase with AlF3·3H2O impurity is obtained. In contrast, near-pure MXene is the only solid state product when I is smaller than 8.5 M or larger than 10 M. Hence, high purity MXene phase can be synthesized by subtle compositional tuning to manipulate the ionic strength of the etching environment.

Investigation and Optimization of Mxene Functionalized Mesoporous Titania Films as Efficient Photoelectrodes

“…Furthermore, a new peak at 27.7 • (008) was observed; this was due to the formation of Ti 3 C 2 (OH) 2 , in agreement with results reported by Li et al[48]. Additionally, the appearance of other, less intense peaks (marked with *) was attributed to the presence of residual Al in the Ti 3 C 2 T x ; this is difficult to remove completely with a 24 h soaking time and leads the formation of AlF 3[49][50][51]. These results suggest that the Ti 3 C 2 T x surfaces were decorated with functional entities of −O and −F after HF treatment, as supported by the EDS spectra[52,53].…”

Engineering Aggregation‐Resistant MXene Nanosheets As Highly Conductive and Stable Inks for All‐Printed Electronics

“…[ 24,27 ] AlF 3 can transform to AlF 3 ·3H 2 O in a hydrated environment, and small amorphous AlF 3 particles scattered within the MXene interlayers are hard to detect using XRD. [ 28 ] However, they can be verified through their contribution to the F 1s peak alongside the appearance of an undesired Al 2p peak in the XPS spectrum of an MXene. [ 24 ] In the case of the currently produced pristine MXene, an Al 2p peak was observed at 75.39 eV (Figure S6 and Table S2, Supporting Information) and a subordinate peak at 686.05 eV was derived from deconvolution of an observed asymmetrical F 1s peak (Figure S7 and Table S2, Supporting Information).…”

One MAX phase, different MXenes: A guideline to understand the crucial role of etching conditions on Ti3C2Tx surface chemistry

“…Moreover, LiF can be replaced by other salts such as NaF, KF, CsF [8]. More recently transition metal-fluorides were used, such as FeF 3 and CoF 2 [36,37]. Non-fluorinated agents, such as alkali based compounds have also been used [38][39][40].…”