*2.2. In Situ HF-Forming Method*

The in situ HF-forming strategy commonly employs fluoride-containing acidic solutions (e.g., lithium fluoride (LiF), ammonium hydrogen fluoride (NH4HF2), sodium bifluoride (NaHF2), and potassium hydrogen fluoride (KHF2)) [43–50] to indirectly produce HF etchants for stripping Al atoms, the reaction for the etching process can be described as the following:

$$\text{LiF(aq)} + \text{HCl(aq)} = \text{HF(aq)} + \text{LiCl(aq)}\tag{4}$$

The synthetic reaction usually occurs at 40 °C and the obtained samples need to be rinsed to remove the by-products, such as AlF3, LiCl. However, the lower concentration of hydrogen fluoride and fluoride salts compared with HF etchants and long etching time lead to poor etching effect and the accordion-like morphology is hard to obtain in the reaction system.
