19390
J. Phys. Chem. C 2008, 112, 19390–19398
Controlled Synthesis of Calcium Tungstate Hollow Microspheres via Ostwald Ripening and Their Photoluminescence Property Wen-Shou Wang, Liang Zhen,* Cheng-Yan Xu, Li Yang, and Wen-Zhu Shao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001 People’s Republic of China ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: October 15, 2008
Nearly monodispersed CaWO4 hollow microspheres with controlled diameters were successfully prepared by a facile one-step hydrothermal method using CaCl2, Na2WO4, and sodium dodecyl sulfate (SDS) as the starting reaction reagents and distilled water as solvent. The crystal structure, morphology, macroporous and photoluminescence properties of the as-synthesized products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction, nitrogen adsorption-desorption isotherm technique, and fluorescence spectrometer. The hollow microspheres have an average diameter of 2.2 µm and hollow interior of 0.5-1 µm. The shell is consisting of numerous nanoparticles 45 nm in diameter. The effects of SDS amount, pH value, concentrations of reaction reagents, and reaction durations have been studied systematically. The results indicate that a certain SDS amount plays a key important role in the final formation of hollow microspheres. It is also found that the average diameters of the hollow spheres can also be tuned by increasing the SDS amount in the range of 0.2-1 g into the reaction system. Based on the detailed investigation on the influences of experimental parameters, the formation mechanism of the hollow interiors by a SDS-induced Ostwald ripening process is proposed for the CaWO4 hollow microspheres. The Brunauer-Emmett-Teller (BET) surface areas and pore volume of the CaWO4 hollow microspheres are 14.4 m2/g and 0.087 cm3/g, respectively. Furthermore, photoluminescence spectrum of the CaWO4 hollow microspheres reveals a strong and broadband from 350 to 450 nm with an emission peak at 413 nm originating from the WO42- groups. This Ostwald ripening process could open novel and efficient pathway for the preparation of ternary metal oxides with hollow interiors. 1. Introduction Inorganic hollow micro/nanostructures with well-defined structure, composition, and tailored properties have received increasing research attentions due to their technological importance in a myriad of applications such as nanoscale chemical reactors, efficient catalysts, drug-delivery carriers, protection of environmentally sensitive biological molecules, lightweight filler materials, and photonic building blocks.1-10 This promise has motivated intense research efforts by many research groups seeking to develop both specific and general strategies for preparing hollow micro/nanostructures. In all cases considered to date, templating against narrow-size-distribution colloidal particles is probably the most effective and general method for the preparation of hollow particles, using monodisperse silica,11-13 latex,14,15 carbon,16,17 metal, or metal oxide nanospheres18,19 and so forth as removable or sacrificial templates. Although this templating process is very versatile and simple for preparing hollow nanostructures, it has two main cumbersome procedures for coating a layer of designed materials on the templates’ surface and subsequently removing the templates.20,21 Because of materials incompatibility, one often encounters difficulty in uniform deposition of desired materials on the templates, and in fact prior time-consuming surface modification is required in the template process. The following templates removing procedure usually need high temperature calcinations or dissolution with proper solvents to form hollow structures, which suffer from the disadvantages of destroying the hollow struc* To whom correspondence should be addressed. Fax: +86-451-86413922. Tel: +86-451-8641-2133. E-mail:
[email protected].
tures. To overcome difficulties associated with the above template process and simplify the synthetic process of these interesting hollow structures, it is essential to devise new template-free methods under one-pot conditions for synthesizing desired hollow micro/nanostructures. Recently, using wellknown physical phenomena, such as Ostwald ripening2,22 and Kirkendall effect,23-25 provides some new insights for the synthesis of inorganic hollow micro/nanostructures in the absent of templates. In addition, among all of the reports of inorganic hollow micro/nanostructures, the successes are mainly limited to element and binary systems, very few works are focused on ternary metal oxides because compositional and structural control becomes difficult for ternary and quaternary compounds. However, from the viewpoint of applications, it is necessary to extend the scopes to ternary metal oxides because these materials exhibit specific functions in comparison to binary oxides, such as superconductivity, ferroelectricity, magnetism, colossal magnetoresistivity, and so forth.26,27 Therefore, it still remains a major challenge to develop facile one-step, template-free methods for the preparation of ternary metal oxide micro/ nanostructures with hollow interiors. Metal tungstates, expressed by the general formula of MWO4, have two crystal structures depending on the size of cations.28,29 Tungstates of relatively large bivalent cations (ionic radium >0.99 Å, M ) Ca, Ba, Pb, and Sr) exist in the so-called scheelite structure form, where the tungsten atom adopts tetrahedral coordination. Tungstates of smaller bivalent cations (ionic radium
