Fabrication And Characterization Of Low-Dimensional Polyoxometalate Nano-Materials

Ultrathin multilayer films based on giant-wheel nanoporous polyoxometalate Na₁₅[Mo₁₂₆^VIMo₂₈^VO₄₆₂H₁₄(H₂O)₇₀]_0.5[Mo₁₂₄^VIMo₂₈^VO₄₅₇H₁₄ (H₂O)₆₈]_0.5ca.400 H₂O (Mo₁₅₄) and Poly(ethyenimine) (PEI), and "Keplerate" nanoporous polyoxometalate (NH₄)₄₂ [Mo₁₃₂O₃₇₂ (CH₃COO)₃₀(H2O)₇₂] (Mo₁₃₂) and Poly(ethyenimine) (PEI) have been first fabricated by layer-by-layer self-assembly technique (LBL), respectively. UV-vis spectra show that the absorbance values at characteristic wavelengths of the multilayer films increase almost linearly with the number of Mo₁₅₄/PEI and Mo₁₃₂/PEI bilayers, respectively, suggesting that each adsorption cycle contributes an equal amount of {Mo₁₅₄} or {Mo₁₃₂} into the thin films, which provides persuasive evidence for the regular growth of the multilayer and for high reproduction of the layer-by-layer assembly.Polarized UV-vis spectra suggest that the giant-wheel nanoporous polyoxometalate molecular in the multilayer were arranged with the orientation angle to the substrates and the molecular were paralleled to each other for the electrostatic repulsion between the polyoxometalate anion. The third-order nonlinear optical properties of {Mo₁₃₂} aqueous solution and {Mo₁₃₂} mulilayer films were determined by the well-known Z-scan technique at 532 nm with 5 ns duration. The ultrathin film depicted excellent self-defocusing performance under the specified experimental conditions.In addition, we successfully fabricated polyoxometalate nanotubes by replication of pore templates with a special filter-pressure-template technique. SEM images of (PEI/W₁₀)₆ nanotubes with the diameter about 300 nm are recordered. The walls of the nanotubes consist of polyelectrolyte and polyoxometalates multilayers, which are formed by means of the LbL assembly of oppositely charged polyelectrolytes and polyoxometalates based on electrostatic interactions. They have wall thicknesses of 50 nm. The walls are comparatively thick and thus mechanically stable.