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

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 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 ∼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 Eg = 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.