Chemical etching of Ti3AlC2: the chemical etching of Ti3AlC2 with HF and HCl was carried out as reported elsewhere. In brief, 1 g of Ti3AlC2 was slowly added to the chemical etchant and stirred at 300 rpm for 24 hours at 35 °C. The chemical etchant was a mixture of HF (48%, 2 mL) and HCl (37%, 12 mL), diluted by deionized water (volumetric ratio of HCl: H2O: HF= 6:3:1). The resulting suspension was washed with deionized water several times via centrifugation until the pH of the suspension became neutral. The solid was collected via vacuum filtration and stored at 3 °C until further analysis.