Growth of Ba_1-:XSr_xZrO₃ (0 ≤ x ≤ 1) nanoparticles in supercritical water

We studied the formation of Ba_1-xSr_xZrO₃ (0 ≤ x ≤ 1) nanoparticles under highly super-saturated conditions, using supercritical water. It is known that B-site Zr in the perovskite structure plays a dominant role at the nucleation stage, with high nucleation rates under supercritical conditions; this is due to the significantly lower solubility of Zr, compared with A-site ions (i.e., Zr precipitates faster, and A-site ions are taken up into particle after the Zr nucleation). However, in this study, it was found that A-site Ba and Sr significantly influenced the particle size, the A-site deficiency rate, and the surface-OH density of the nanoparticles. The differences in particle size suggested that the ripening or coalescence that occurred after the nucleation stage was dominant in determining the particle size, even under highly super-saturated conditions such as those realized in the supercritical hydrothermal synthesis. The characteristic nanostructure formed in the supercritical water was analyzed in detail; variables such as the A-site deficiency rate and the surface-OH density were investigated. The existence of vacancies at the A-site was confirmed using X-ray absorption fine structure, and a highly defective structure was obtained, particularly when the Ba content was high. The surface state of the nanoparticles was also studied using X-ray photoelectron spectroscopy and first-principles calculations, with the aim of understanding the differences in particle size, and the effects of the A-site deficiencies; the amount of surface-OH corresponded to the A-site deficiency rate, and had an inverse relationship with the particle size.