Template-free Synthesis And Properties Of Inorganic Hollow Microspheres

Inorganic hollow micro/nanostructures have been one of the research frontiers and focuses for nanomaterials field due to their low density, high specific surface area, monodispersity, stability, and novel multifunctional properties arising from combining different materials into the hollow interiors and potential applications in micro/nanoscale chemical reactors, drug-delivery carriers, photonic building blocks, efficient catalysts, energy-storage media and so forth. The key scientific questions in hollow micro/nanostructures include i) controlled synthesis of hollow micro/nanostructures with multicompositions and materials which are difficult to prepare by template-free methods; ii) investigating the formation mechanism of hollowing process, which providing theoretic evidence for the controlled synthesis; iii) realizing the functionality of hollow micro/nanostructures for the applications. In this work, we have studied the above three questions and obtained the following conclusions.We have developed a template-free hydrothermal process for CaWO4 and CaMoO4 hollow microspheres. Experimental results show that the morphology, size and homogenization are fine controlled by changing the reaction conditions."Sodium dodecyl sulfate (SDS) induced Ostwald ripening"and"dissolution-nucleation-growth-Ostwald ripening"mechanisms are the main reason for the formation of CaWO4 and CaMoO4 hollow microspheres, respectively. Structural instability of intermediates formed at initial reaction process is the key step for the subsequently hollowing process. The results demonstrate that Ostwald ripening can be used for preparing ternary metal oxides with hollow interiors, which provide an efficient pathway for the synthesis of multicompositional hollow structures.A solution-phase hot-injection-based route has been developed for the synthesis of CaMoO4 microstructures with well-defined shapes and sizes at 100â—‹C. The CaMoO4 doughnuts are composed of numerous nanosheets along thickness direction. Shapes such as microspheres, spindles, and complex three-dimensional structures can be produced by controlling the reaction conditions including the concentration of reactants, molar ratio between reactants, molybdenum source, pH, and the volume ratio of the mixed solvents. CaMoO4 doughnuts with hollow interiors can be obtained by etching in acetic condition.CdMoO4 hollow microspheres have been prepared via a template-free aqueous solution method at room temperature. The shell is composed of nanorods along diameter direction. Using CdCl2 and Na2MoO4 as the starting reaction reagents without surfactants and heating, this method provides a suitable way to investigate the formation mechanism of the hollowing process. The results of intermediates obtained at different reaction times show that"NaCl induced Ostwald ripening"is account for the hollow structures. It is believed that core-shell CdMoO4 microspheres are formed first by a two-step precipitation process: precipitation of solid cores and subsequent precipitation of the outer shell onto ionic species (Na+ and Cl-)-stabilized cores. The core has a strong tendency to dissolve because of high surface energies compared to those in the outer shells, providing the driving force for Ostwald ripening. By adding a suitable amount of SDS, homogenous CdMoO4 core-shell microspheres are prepared. A possible three-step growth mechanism is presented for the formation of homogenous core-shell CdMoO4 microspheres. The mutual effect between SDS and CdMoO4 is the main reason for core-shell microspheres.UV-Vis spectrum of CdMoO4 hollow microspheres shows that there is a strong absorption peak in the wavelength of 200-370 nm. The Eg is 3.48eV. PL spectrum shows a strong and broad emission band centered at 354 nm, originating from the charge-transfer transitions within the [MoO42-] complex. A new Eu-doped CdMoO4 red phosphor was prepared by a similar aqueous solution process at room temperature. The phosphor exhibits three emissions at 535, 590 and 613 nm, while a dominant red emission at 613 nm at room temperature.We report for the first time the photocatalytic activity of the hollow CdMoO4 microspheres in the degradation of Rhodamine B (RhB) under UV light irradiation. Calcination the CdMoO4 microspheres at 500 oC for 2 h significantly enhance the photocatalytic activity. The photooxidation of the pollutant over CdMoO4 photocatalyst occurs from the charge transfer from the O 2p orbitals to empty Mo 4d orbitals. The CdMoO4 hollow microspheres after calcinating at 500 oC for 2 h exhibit higher photocatalytic efficiency than that of corresponding solid microspheres under the same conditions. The effective photocatalytic activity of hollow microspheres is related to their hollow structures, which possess plenty of meso- and macro-pores in the shells. "Chemical conversion induced Ostwald ripening"is developed for the synthesis of hollow microstructures which are difficult to prepare by general methods. CaF2 and Cd(OH)2 hollow microspheres are prepared by using CaWO4 and CdMoO4 solid microspheres as precursor, respectively. It has been shown that the formation process includes three important steps: the first formation of CaWO4 or CdMoO4 solid microspheres by direct precipitation process; the chemical conversion of CaWO4 or CdMoO4 solid microspheres to CaF2 or Cd(OH)2 solid microspheres; and finally hollowing process via Ostwald ripening in the CaWO4 or Cd(OH)2 solid microspheres to form corresponding hollow spheres."Chemical conversion induced Ostwald ripening"is a universal method, providing a new strategy and pathway for the synthesis of hollow micro/nanostructures which are difficult to prepare by general methods. Further study shows that Eu-doped CaWO4 solid microspheres can be tranferred to Eu-doped CaF2 hollow microspheres, which exhibit excellent optical properties. These results indicate that the"chemical conversion induced Ostwald ripening"method opens new insights for the formation mechanism, properties and applications of hollow micro/nanostructures.