Thermal and photo stability of tungsten polyoxometalate-surfactant hybrid compounds
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Inorganic-organic polyoxometalate-hybrid materials have attracted increased interest from researchers in recent years due their favourable photo-redox properties. These compounds have the potential to serve in a wide range of applications including photo-catalysis, gas-sensing and medicine. However, limited thermal and photo-chemical stability of these systems has restricted further development into other applications, such as photochromic technologies. This thesis focuses on the synthesis and thermal and photochemical stability of the polyoxometalate-hybrid designated CTA-W₁₂, formed from the metatungstate anion, [H₂W₁₂O₄₀]⁶⁻ and the cationic surfactant cetyltrimethylammonium, (C₁₆H₃₃)N(CH₃)₃⁺ (CTA⁺). Only a narrow window of synthesis conditions actually leads to the production of the studied material because the products produced are very sensitive to pH, temperature, sequence of reactions steps and time. The CTA-W₁₂ exhibits a lamellar bilayer structure, consisting of 2D sheets of hexagonally arranged polyoxometalate anions separated by interdigitated surfactant alkyl-tails. The thermal stability of CTA-W₁₂ was studied using a battery of techniques, including in situ synchrotron x-ray diffraction. It was found that the salt went through seven phase and/or chemical transitions from room-temperature to 800°C within the enclosed environment of the quartz capillary. The lamellar structure persisted for the first three transitions and was destroyed by the fifth at 230°C, when the polyoxometalates fragmented and assumed a hexagonally-close-packed (HCP) arrangement. By ~350°C, the fragments reorganized into the bulk tungsten-suboxide W₁₇O₄₇ and by ~550°C all organic material was removed from the sample. At ~600°C the sample underwent a final transition to monoclinic WO₂. The HCP fifth phase was further studied due to its high crystallinity and was found to be comprised of two types of polyanion fragments, alluding to the complex decomposition kinetics of polyoxometalates and inorganic-organic hybrids. The photochromism of CTA-W₁₂ highlighted the photochemical instability of the inorganic-organic polyoxometalate hybrid. Multiple cycles of irradiation followed by recovery were applied to elucidate the behaviour of the material. The first four photochromic cycles coincided with a slight discoloration of the bleached state, detrimentally affecting photochromic performance slightly. This was ascribed to irreversible oxidation to organic CTA⁺ which caused a yellowing of the material, as well the production of long-lived W⁵⁺ sites deep within the material which could not be re-oxidized by atmospheric O₂. The material maintained reasonable photochromic performance beyond four cycles, which was attributed to the production of more reversible proton-transfer groups, as compared to CH_2/3, and an equilibrium between the production of long-lived W⁵⁺ sites and the diffusion of O₂ into the material. Lattice expansion and amorphization (which was partially reversed during bleaching) was observed to mutually occur with photo-colouration, as evidenced by XRD. The accumulation of strain in the sample, as indicated by the expansion in lattice parameter along the  direction, may represent a new photomechanical phenomenon. The photochemical instability of CTA-W₁₂ was further revealed during XPS measurements, which caused progressive reduction of tungsten centres with each successive measurement due to the ionizing effect of the X-ray radiation in combination with the high vacuum environment of the instrument. The project has provided detailed insight into the synthesis, thermal stability and photochemical properties of CTA-W₁₂. The mechanism of photochromism, and the reasons for its partial irreversibility, were found. A new photomechanical phenomenon was uncovered and investigated. The new insights provided by the project will facilitate future attempts to develop applications for these and related inorganic-organic hybrid compounds.
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