Unconventional Dexter-Silverton Type Manganese Heteropolytungstate [Mn-7(MnW12O42(OH)(4)center dot 8H(2)O)] Hollow Microsphere: Synthesis, Crystal Structure, Growth Mechanism, and Optical Property Study

摘要

An uncommon yet highly symmetric crystal form of a Mn-based heteropolytungstate (Mn-HPT), Mn7(MnW12O42(OH)(4)).8H(2)O, has been synthesized by a unique solvothermal method at 180 degrees C for 6 h. The nature of the solvent (water/ethanol = 50/50 by volume) and the presence of citric acid were the key reasons for the formation of the Mn-HPT phase and its hollow spherical morphology under the said experimental conditions. Combined powder X-ray diffraction analysis and field emission scanning electron miroscopy (FESEM) analysis reveals that the structure (cubic, space group Im3) consisted of [MnW12O42](10)- polyanion with Mn in a central 12-coordinated cavity, which cross-linked three dimensionally with other MnO6 octahedra, to form the characteristic rhombododecahedron shaped particles of size similar to 150 nm. Each polyanion was made of six pair of face shared WO6 dimers (W2O9) with mean W W separation of 2.98 angstrom. The WO6 octahedrons were tilted to an O-W-O angle of 99.1 degrees with four different types of W-O bonds. The structure was further characterized by thermogravimetric (TG), infrared (IR), Raman, and X-ray photoelectron spectroscopy (XPS) study. UV-vis diffuse reflectance spectroscopy (DRS) analysis suggests that the material essentially was an indirect band gap semiconductor with Eg = 2.33 and 2.93 eV corresponding to the transitions from two different sets of nonbonding HOMO (O 2p) to the p* LUMO (W 5d). Room temperature photoluminescence (PL) analysis further delineates the HOMO-LUMO transition in the blue (412 nm) to green (525 nm) region with characteristic mean decay lifetime similar to 8 ns.