Slow and Sustained Release of Carbonate Ions from Amino Acids for Controlled Hydrothermal Growth of Alkaline-Earth Carbonate Single Crystals

[Image: see text] Alkaline-earth metal carbonate materials have attracted wide interest because of their high value in many applications. Various sources of carbonate ions (CO(3)(2–)), such as CO(2) gas, alkaline-metal carbonate salts, and urea, have been reported for the synthesis of metal carbonate crystals, yet a slow and sustained CO(3)(2–) release approach for controlled crystal growth is much desired. In this paper, we demonstrate a new chemical approach toward slow and sustained CO(3)(2–) release for hydrothermal growth of large alkaline-earth metal carbonate single crystals. Such an approach is enabled by the multiple hydrolysis of a small basic amino acid (arginine, Arg). Namely, the amino groups of Arg hydrolyze to form OH(–) ions, making the solution basic, and the hydrolysis of the guanidyl group of Arg is hydrothermally triggered to produce urea and ammonia, followed by the hydrolysis of urea to produce CO(2) and ammonia and then the release of CO(3)(2–) because of the reaction between CO(2) and the OH(–) ions hydrolyzed from ammonia. Such a CO(3)(2–) release behavior enables the slow and controlled growth of various carbonate single crystals over a wide range of pH values. The growth of uniform rhombohedron MgCO(3) single crystals with variable morphologies and crystal sizes is studied in detail. The influences of reaction temperature, solution pH, precursor type, and concentration on the morphology and size of the resulting MgCO(3) crystals are elucidated. The crystal evolution mechanism is also proposed and discussed with various supportive data.