title: Cation non-stoichiometry in multi-component oxide nanoparticles by solution chemistry: a case study on CaWO4 for tailored structural properties.
abstract: Chemical composition directly determines the structure and properties of almost all bulk inorganic solids, which are however popularly dismissed in the literature as a cause of property changes when studying multi-component oxide nanostructures by solution chemistries. The current work focuses on this subject through a systematic case study on CaWO(4) nanocrystals. CaWO(4) nanocrystals were prepared using room-temperature solution chemistry, in which a capping agent of citric acid was employed for kinetic grain size control. Sample characterizations by a set of techniques indicated that 5-7 nm CaWO(4) was obtained at room temperature, showing a pure-phase of tetrahedral scheelite structure. The molar ratio of Ca(2+) to W(6+) was found to be 1.2:1, apparently deviating from the unity expected for the stoichiometric CaWO(4). Such nonstoichiometry was further modulated via iso-valent incorporation of smaller Zn(2+) to the Ca(2+)-sites in CaWO(4). It is found that with increasing the Zn(2+) content, there appeared transformation from high to low nonstoichiometry, though a pure scheelite-typed structure was retained. Such a nonstoichiometry was primarily represented by excessive cations like Zn(2+) and/or Ca(2+) within the surface disorder layers, which in turn showed a great impact on the structure and properties as demonstrated by a lattice contraction, band-gap narrowing, luminescence quenching, as well as improved conductivity. The property changes were rationalized in terms of surface structural disorder, electro-negativity discrepancy, and effective activation on the mobile protons. Consequently, systematic control over the non-stoichiometry for single-phase multi-component oxide nanostructures by solution chemistry is proven fundamentally important, which may help to achieve quantitatively the structure-property relationship for materials design and performance optimization.
authors.affiliation: State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter and Graduate School of Chinese Academy of Sciences, Fuzhou 350002, PR China.
authors.forename: Wanbiao
authors.initials: W
authors.lastname: Hu
authors.forename: Wenming
authors.initials: W
authors.lastname: Tong
authors.forename: Liping
authors.initials: L
authors.lastname: Li
authors.forename: Jing
authors.initials: J
authors.lastname: Zheng
authors.forename: Guangshe
authors.initials: G
authors.lastname: Li