Exploring the Etching Mechanism of Monoatomic Aluminium Layers During MXene Synthesis

<b>MXenes are synthesized by selective etching of <i>A</i>-group element from bulk MAX phases via wet etching process. Because the <i>A</i>-group elements between <i>M</i> layers has atomic-scale layer thickness and wide lateral dimension (~ 15 μm) with strong chemical bonds between <i>A</i> and <i>M</i> elements in MAX phases,</b> <b>selective removal process of A layer should be very complicate</b>. <b>Thus, understanding where the etching of A<i>-</i>element layer starts and how it proceeds depending on types of etchants is the first step, and essential to obtaining high quality of MXenes. In spite of intensive efforts, exploring the etching mechanism of the atomic Al layer is still uncertain.</b> <b>Herein, we report for the first time the etching mechanism of Al atomic layers from Ti₃AlC₂ MAX phases by observing the structural and morphological changes of Ti₃C₂T_x MXenes as a function of etching time via high-resolution electron microscopy. We demonstrate that the atomic Al layers are etched by two distinct mechanisms depending on the types of wet-etchants: For LiF/HCl etchant, the etching begins from either defect edges or/and edges of Al atomic layers exposed at side surface of Ti₃AlC₂ MAX top (or bottom) layers, which are relatively weak Ti-Al bonding sites. The etching proceeds from the edges into inner Al atoms in a single Al layer, and at the same time etches Al atoms at under layers by attacking weak Ti-Al bonding sites generated during the etching. For HF and NH₄HF₂ etchants, however, excess amount of HF starts to attack not only edges of atomic Al layers exposed at side surface of Ti₃AlC₂ MAX top (or bottom) layers, but also grain boundary of poly-crystalline Ti₃AlC₂ MAX multi-layer. Hence, more atomic Al layers are exposed to etching solutions for further etching, leading to fully etched multi-layer MXene particles.</b>