The approaching Zn+2 ion first bind

The approaching Zn+2 ion first binds to the bridging hydroxyl (Figure 3). This process is observed to occur easily due to the hydrogen-bonding interactions between the surface hydroxyls and the first hydration shell species around the Zn+2 ion. We investigated the energy barriers of this binding process by applying a bond-restraint approach (Supporting Information, Figure S1). When Zn+2 approaches the surface, the required energy for Zn+2 to divest itself of the hydration shell and bind to the bridging hydroxyl was calculated as 3 kcal/mol. The adsorption energy is very small because Zn+2 can directly bind to the oxygen of the bridging hydroxyl due to the interaction between oxygen lone pairs and divalent Zn+2 ion empty orbitals; therefore, there is no ligand exchange process for adsorption. Thus, hydroxylation of the surface does not reduce grain growth as typically expected. Instead, it very much aids the initial adsorption, which prevents recrystallization from limiting grain growth in CSP. However, it is observed to be different in the presence of an excess amount of HAc (4.6m and 9m) in the system. In the simulations, HAc molecules dissociate on the ZnO surface and bind to the positively charged surface sites, and when in excess, acetate molecules cover a significant part of the surface and tend to stay on the surface. Thus, this strong adsorption makes the exchange between the Zn+2 ions and acetate more difficult and delays recrystallization until elevated temperatures are reached. The number of HAc molecules that could be modeled in the simulation with 2m acid concentration was limited; therefore, in order to examine the difference between 0m and 2m HAc conditions, we extended our simulation sizes for these two specific cases (Figure 2D). As can be seen from the figure, no significant difference is observed in crystallization, because most of the HAc molecules bind to Zn+2 ions in the solution and do not cover the surface as happened in the 4.6m and 9m cases.