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), Mn 7 (MnW 12 O 42 (OH) 4 )• 8H 2 O, 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 [MnW 12 O 42 ] 10− polyanion with Mn in a central 12coordinated cavity, which cross-linked three dimensionally with other MnO 6 octahedra, to form the characteristic rhombododecahedron shaped particles of size ∼150 nm. Each polyanion was made of six pair of face shared WO 6 dimers (W 2 O 9 ) with mean W•••W separation of 2.98 Å. The WO 6 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 E 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.

Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH

“…The X‐ray diffraction (XRD) pattern of the Ni M ‐POM/Ni (Figure a) showed the standard diffraction patterns of the Dexter–Silverton POM of [M 8 W 12 O 42 (OH) 4 (H 2 O) 8 ] (database no. ICDD‐PDF‐80–1525) . Furthermore, a representative sample of NiCo‐POM/Ni was studied by high‐resolution transmission electron microscopy (HRTEM, Figure b) and showed lattice fringes with interplanar distances of 0.34 and 0.38 nm, respectively, corresponding to the (1 2 3) and (2 2 2) planes of NiCo‐POM.…”

“…Notably,aweak peak at 2.6 was assigned to Co(2) coordination with OH anions( bond distance 3.08 ) ( Figure 4a). [32,39,40] Furthermore, the XANES fitting of Dexter-Silverton NiCo-POM/Ni indicated that the Co centres were six-coordinate and occupied the nearly octahedral environment in the NiCo-POM/Ni.B ased on the XPS, XANES and ICP-AES data ( The NiM-POM/Ni composites were investigated as electrocatalysts for HER in alkaline aqueous KOH as electrolyte. In order to evaluatethe HER performance of the Ni-POM/Ni, NiCo-POM/ Ni, NiZn-POM/Ni,a nd NiMn-POM/Ni,atypicalt hree-electrode system in an N 2 -saturated 1.0 m KOH aqueous solution was applied by using the correspondingc omposites as working electrodes, as aturated calomel electrode as reference electrode and ac arbon rod as counter electrode.…”

“…The NiM-POM (M = Co, Zn, Mn) electrocatalysts was synthesized on commercial Ni foam electrodes by hydrothermalr eaction, as illustrated in Figure 1 ICDD-PDF-80-1525). [32] Furthermore, ar epresentative sample of NiCo-POM/Ni was studied by high-resolution transmission electron microscopy (HRTEM, Figure 3b)a nd showed lattice fringes with interplanar distances of 0.34 and 0.38 nm, respectively, corresponding to the (1 23)a nd (2 22)p lanes of NiCo-POM. Ramans pectra of NiM-POM/Ni (Figure 3c)p rovided further structurali nformation.…”

“…The peak at 1.6 Å corresponded to the first octahedral coordination of Co(1)−O shell, while the peak at 3.3 Å was attributed to the scattering between the Co and the W in the second nearest neighbour shell. Notably, a weak peak at 2.6 Å was assigned to Co(2) coordination with OH anions (bond distance 3.08 Å) (Figure a) . Furthermore, the XANES fitting of Dexter–Silverton NiCo‐POM/Ni indicated that the Co centres were six‐coordinate and occupied the nearly octahedral environment in the NiCo‐POM/Ni.…”

Controlled Synthesis and Properties of 3d–4f Metals Co-doped Polyoxometalates-Based Materials

“…After that, star-shaped Keggin-type heteropolytungstate was obtained as catalyst for preparing quinoline derivatives [ 14 ]. From then on, Chattopadhyay’s and co-workers discovered the Dexter-Silverton type molybdenum tungstate of hollow microspheres [ 15 ]. For the past years, ours group has been working on the control-synthesis and functionalization of POM-based nano/micromaterials by chemical precipitation or hydrothermal methods [ 16 – 18 ].…”