Structure and Stability of Hydrolysis Reaction Products of MgO Nanoparticles Leading to the Formation of Brucite
journal contributionposted on 12.09.2017, 00:00 by Mingyang Chen, David A. Dixon
The bottom-up formation of MgxOy(OH)z nanoparticles leading to Mg(OH)2 nanoparticles was modeled in two steps by using an evolutionary global optimization approach: (1) the formation of small MgnOm+nH2m clusters via the hydrolysis of (MgO)n and (2) the formation of multilayered (Mg(OH)2)n clusters via monolayer stacking. The sheet-like (Mg(OH)2)n structures were predicted as the energetically favorable reaction products for the (MgO)n + nH2O reaction, whereas more compact structures were found to dominate for the products of (MgO)n + mH2O, m < n. The stepwise hydrolysis reactions most likely follow the compact reaction path until the crossover point is reached. Multistep structural rearrangements are required for the conversion between the compact products and the sheet-like products even after the crossover point. The protective shell formed by the hydroxyl groups may inhibit the further hydrolysis reaction of the compact products, even though the hydrolysis reactions are both exothermic and exergonic; such an effect is less significant in the smaller structures. The hydrolysis of (MgO)n is not suitable for the preparation of the large sized (Mg(OH)2)n nanoparticles but may be used to synthesize the ultrasmall (Mg(OH)2)n nanoparticles. A fragment-based model was used to determine the structure-energy relationship for the monolayered (sheet-like) and multilayered (Mg(OH)2)n nanoparticles. The normalized clustering energy as a function of the size n was obtained for the raw particles and for the particles including solvent effects. The thermodynamically favored (Mg(OH)2)n nanoparticle types are (1) rhombic monolayers for n < 40, (2) hexagonal monolayers for 40 < n < 53, (3) rhombic multilayers for 53 < n < 78, and (4) hexagonal multilayers for n > 78 in vacuum at 0 K. In the presence of solvent, the critical sizes for the transition of the dominating particle shapes are shifted, and the growth rates in each dimension also change. This work provides a basis for controlling nanoparticle morphologies in the selective bottom-up synthesis of brucite-like (Mg(OH)2)n-related nanoparticles.