Magnesium compound nanoarchitectures with controlled shape were prepared through carbonation reaction of magnesium hydroxide and carbon dioxide in supercritical carbon dioxide–ethanol mixtures and their formation mechanism were discussed. The leaf-like brucite and the floral-like hydromagnesite nanosheets with a monoclinic structure were formed at the reaction temperature of less than 80°C and 100°C, respectively. The floral architecture started to transform the nanosheets with stacked arrangements at the elevated temperature above 130°C, and compeletely transformed to cube-like magnesite with a trigonal crystal structure at 150°C in progress of the carbonation reaction of hydromagnesite and carbon dioxide in a supercritical CO2 with ethanol cosolvent. The morphologies of the product architectures were artificially controlled by the phase transition driven morphological changes. With further increasing temperature to 600°C, the micro–meso porous magnesium oxide architectures with high surface area were finally formed by decarbonation of the magnesite cubes.
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