Cation non-stoichiometry in multi-component oxide nanoparticles by solution chemistry: a case study on CaWO4for tailored structural propertiesElectronic supplementary information (ESI) available: Elemental analysis before and after Ar ion bombardment using XPS; XRD patterns of Zn2+doped CaWO4samples with a varied initial doping level of Zn2+compared with an undoped sample; TEM and HRTEM images of Zn2+doped CaWO4nanocrystals with aninitial Zn2+ratio of 0.15; XRD patterns of the sample calcination on Zn2+doped CaWO4nanocrystals at 800 °C in air for 2 h; high-resolution XPS spectra of O1s for Ca1−xZnxWO4nanocrystals with a given dopant content; Raman spectroscopy and XRD patterns of samples prepared with and without citric acid. See DOI: 10.1039/c0cp02153a

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 CaWO4nanocrystals. CaWO4nanocrystals 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 CaWO4was obtained at room temperature, showing a pure-phase of tetrahedral scheelite structure. The molar ratio of Ca2+to W6+was found to be 1.2 : 1, apparently deviating from the unity expected for the stoichiometric CaWO4. Such nonstoichiometry was further modulated viaiso-valent incorporation of smaller Zn2+to the Ca2+-sites in CaWO4. It is found that with increasing the Zn2+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 Zn2+and/or Ca2+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. [ABSTRACT FROM AUTHOR]