Unconventional Dexter–Silverton Type Manganese Heteropolytungstate [Mn₇(MnW₁₂O₄₂(OH)₄·8H₂O)] Hollow Microsphere: Synthesis, Crystal Structure, Growth Mechanism, and Optical Property Study

Abstract

An uncommon yet highly symmetric crystal form of a Mn-based heteropolytungstate (Mn-HPT), Mn₇(MnW₁₂O₄₂(OH)₄)·8H₂O, has been synthesized by a unique solvothermal method at 180 °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 <i>Im</i>3) 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 ∼150 nm. Each polyanion was made of six pair of face shared WO₆ dimers (W₂O₉) with mean W···W separation of 2.98 Å. The WO₆ octahedrons were tilted to an O–W–O angle of 99.1° 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 <i>E</i>_g = 2.33 and 2.93 eV corresponding to the transitions from two different sets of nonbonding HOMO (O 2p) to the π* 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 ∼8 ns