Increase of Co solubility with decreasing grain size in ZnO

Nanograined (grain size 10 nm) ZnO films with various Co contents (0–52 at.% Co) were synthesized by a novel liquid ceramics method. The solubility limit for Co was determined at 550 °C. The lattice parameter c of the ZnO-based solid solution with würzite structure ceases to grow at 33 at.% Co. The peaks of the second phase (Co2O3 or with cubic lattice) become visible in the X-ray diffraction spectra at 40 at.% Co. The same second phase appears in the bulk ZnO already at 2 at.% Co [Bates CH, White WB, Roy R. J Inorg Nucl Chem 1966;28:397]. A few years ago it was predicted theoretically that ZnO could become ferromagnetic at room temperature and above by doping with Co and other transition metals. Recently published papers on the structure and magnetic behaviour of Co-doped ZnO allowed us to obtain the size dependence of Co solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Co solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of a Co-enriched layer is several monolayers in grain boundaries and at least two monolayers in the free surfaces.