Synthesis of Hf-BTB-NH2 nanosheets. To prepare Hf-BTB-NH2 MONs, HfCl4 (34.59 mg, 0.108 mmol) and 2,4,6-tris(4carboxyphenyl)aniline (32.87 mg, 0.072 mmol) were added to a 20 mL screw-cap glass vial. DMF (12.5 mL) was added and the mixture was dissolved with the aid of ultrasonics to obtain a clear solution. Formic acid (2.5 mL) and H2O (0.7 mL) were added, then the vial was sealed and placed in a 120 °C oven for 48 h. The vial was taken out of the oven and cooled to room temperature. The turbid solution was centrifuged to collect Hf-BTB-NH2 as a pale-yellow precipitate. The precipitate was washed multiple times with DMF, followed by ethanol. To prevent stacking of the MON layers, Hf-BTB-NH2 was stored in ethanol until needed for further studies.