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Physical Investigation of Gasochromic Phenomena Mechanism in Tungsten Oxide Nanostructure

Tahmasebi Garavand, Nemat | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43236 (04)
  4. University: Sharif University of Technology
  5. Department: physics
  6. Advisor(s): Mahdavi, Mohammad; Iraji Zad, Azam
  7. Abstract:
  8. In the present thesis the fabrication of various tungsten oxide nanostructures (films and colloidal nanoparticles) with coverage of catalyst on their surfaces has been investigated which show gasochromic properties. Tungsten oxide films and nanoparticles were synthesized by pulsed laser abalation technique in vacuum and liquid environment, respectively. To activate the synthesized films and nanoparticles against hydrogen gas, the catalyst nanoparticles (Pd or Pt) were deposited onto their surface through hydrogen reduction of metal salt and electroless methods, respectively. The products were characterized by XPS, UV-Vis., AFM and DLS techniques. The prepared samples show significant color change (gasochromic response) in the presence of hydrogen gas. To indicate the effect of crystalline structure on gasochromic performance the as-prepared amorphous WO3 films were annealed in air at temperature from room temperature up to 550°C for an hour. The results showed the as-prepared and annealed film at 250 °C have an amorphous nature. The amorphous to polycrystalline phase transition could take place by annealing at temperature above 300 °C. At the annealing temperature of 350 °C the monoclinic peaks appear as dominant phase. It is also confirmed that with rising heat treatment above 400 °C, the intensity of gasochromic response has decreased which is likely due to the sodium diffusion from glass substrate into the WO3 structure. The influence of operating temperature on gasochromic performance of amorphous and polycrystalline WO3 films is also presented in this work. The results showed the maximum gasochromic response was achieved at an optimum operating temperature. Then according to the role of operating temperature and water molecules on gasochromic performance of Pd-WO3 we proposed an appropriate model to describe the interaction between H2 molecules and the Pd-WO3 surface. In the final part of this thesis, colloidal tungsten oxide nanoparticles were fabricated by PLAL method at three different laser energy (E1=160, E2=370 and E3=500 mJ/Pulse), respectively. The aim is to study the influence of WO3 nanoparticles on Pd2+ reduction and gasochromic properties of colloidal samples. According to the optical measurements, during hydrogen bubbling into the produced colloidal Pd-WO3 nanoparticles, we proposed an appropriate model to describe the optical absorption of colloidal nanoparticles in colored states. Our results showed a “three band model” is an appropriate model to describe the observable optical absorption peaks. Three observed absorption peaks can be related to the formation of paramagnetic centers (W5+) on the surface and in the bulk of nanoparticles and diamagnetic center (W5+W5+) on the surface of WO3 nanoparticles
  9. Keywords:
  10. Thin Films ; Nanoparticles ; Tungsten Oxide ; Pulse Laser ; Palladium ; Gasochromic

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