The XPS analysis shows that a small

The XPS analysis shows that a small amount of BaCO3 is not detected by XRD and that the local oxygen vacancies in the synthesized material create structural defects in the sample. An XPS analysis is a useful technique to detect the surface chemical composition and material structure. After coating, neither the chemical composition nor the bonding at the particle surface change significantly (Fig. 4–7), indicating that the coating layers are BaTiO3, which is consistent with the TEM results. Furthermore, comparing the C1s peaks in Fig. 7 of the uncoated and coated powders shows that the peak of C-O groups increases significantly. This is probably due to calcination, which reportedly increases surface contamination by the C-O group [28]. The uncoated BaTiO3 powders calcined at 600℃ for 2 h also show a higher peak for the C-O group (Fig. S2, Supporting Information), but that does not help increase the density of the ceramic. Moreover, Table 2 gives the element content on the surface of the uncoated and coated powders. The ratio Ba/Ti < 1, which is common in BaTiO3 nanopowders and is caused by Ba ions leaching out of the surface area [12]. After coating, the coating surface has a much lower ratio Ba/Ti by using the sol with Ba:Ti = 1:1, which is consistent with the presence of Ba vacancies in HRTEM images. This result is likely due to more facile precipitation for Ti than for Ba [21]. To compensate for Ba, the sol of Ba:Ti = 2:1 and 3:1 was used during precipitation, or the Ba(OH)2 solution was added during the CSP. When using the sol of Ba:Ti=2:1 and 3:1, the ratio Ba/Ti increases, but still remains below 1.