Amorphous <i>a</i> -Cr ₂ O ₃ Nanosheets for NO ₂ and NH ₃ Gas Sensing

The design of bidimensional materials with unique catalytic properties has recently focused on the synthesis and applications of layered (2D) amorphous (a) metal oxide (2D a-MOx) interfaces. Herein, we report an original strategy for synthesizing bidimensional (2D) amorphous Cr2O3 (2D a-Cr2O3) via “low-temperature” annealing of liquid-phase exfoliated anhydrous CrCl3 transition metal trihalide (TMTH), yielding p-type semiconductor interfaces with excellent NO2 and NH3 gas-sensing capabilities. By tuning the annealing time at temperatures below the recrystallization of a-Cr2O3, the removal of chlorine atoms and the complete oxidation/amorphization of CrCl3 into a-Cr2O3 are achieved. Moreover, the oxidation/amorphization proceeds from the flake’s edge toward the core, following a “Shrinking core” kinetic oxidation model, which preserves the shape of pristine 2D CrCl3. Gas-sensing characterization of a-Cr2O3 to NO2 and NH3 exhibits enhanced signal gains of 18% (1 ppm of NO2) and 45% (100 ppm of NH3) compared to pristine CrCl3. Our research offers valuable insight for extending the “low-temperature” amorphization strategy to the synthesis of 2D amorphous metal oxides (a-MOx where M = Ni, Co, Fe, Bi, Ti), departing from anhydrous transition metal chlorides, such as NiCl2, CoCl2, and oxychlorides, including FeOCl, BiOCl, and TiOCl, broadening the scope and applications of a-MOx interfaces.