Coal Waste to Two-Dimensional Materials: Fabrication of α-Fe₂O₃ Nanosheets and MgO Nanosheets from Brown Coal Fly Ash

High-purity 2-D α-Fe₂O₃ nanosheets and MgO nanosheets have been successfully fabricated with a high extraction yield of 45 and 25 wt %, respectively, from a brown coal fly ash, an otherwise valueless waste. A two-stage counter-current solid-liquid extraction method was first employed under the mild conditions of 25-60 °C and atmospheric pressure to selectively extract Fe³⁺ and Mg²⁺, which were then precipitated out separately upon gradual addition of NaOH and subsequent hydrothermal treatment. Finally, 2-D nanosized α-Fe₂O₃ nanosheet was obtained with a lamellar morphology, while the synthesized Mg(OH)₂ precursor undertook an extra multistep calcination to convert into the respective 2-D MgO nanosheet.