Crystallization of Transition-Metal Oxides in Aqueous Solution beyond Ostwald Ripening

The crystallization mechanism of transition-metal oxides (TMOs) in a solution was examined based on ZnO crystallization using in-situ x-ray absorption fine structure (XAFS) measurements at the Zn K edge and semi-empirical quantum chemistry (SEQC) simulations. The XAFS results quantitatively determine the local structural and chemical properties around a zinc atom at successive stages from Zn(NO3)2 to ZnO in an aqueous solution. The results also show that a zinc atom in Zn(NO3)2 ions dissolves in a solution and bonds with approximately three oxygen atoms at room temperature (RT). When hexamethylenetetramine (C6H12N4) is added to the solution at RT, a stable Zn–O complex consisting of six Zn(OH)2s is formed, which is a seed of ZnO crystals. The Zn–O complexes partially and fully form into a wurtzite ZnO at 60 and 80 °C, respectively. Based on the structural properties of Zn–O complexes determined by extended-XAFS (EXAFS), SEQC simulations clarify that Zn–O complexes consecutively develop from a linear structure to a polyhedral complex structure under the assistance of hydroxyls (OH–s) in an aqueous solution. In a solution with a sufficient concentration of OH–s, ZnO spontaneously grows through the merging of ZnO seeds (6Zn(OH)2s), reducing the total energy by the reactions of OH–s. ZnO crystallization suggests that the crystal growth of TMO can only be ascribed to Ostwald ripening when it exactly corresponds to the size growth of TMO particles.