Nanostructured transitional metal oxides: applications in smart windows, optical gas sensors and dye-sensitized solar cells
Creators: name: Ou, Jianzhen
Publisher: name: RMIT University
Year: 2024

description: In this thesis, the PhD candidate pursued the development of high performance smart windows, optical gas sensors and dye-sensitized solar cells (DSSCs) based on three selected nanostructured transitional metal oxides with excellent chemical stabilities and corrosion resistance, which included tungsten trioxide (WO3), molybdenum trioxide (MoO3) and niobium pentoxide (Nb2O5).<br><br>According to the literature review, the PhD candidate found that there were many unknown issues regarding the fundamentals of WO3 and MoO3 gasochromism. This had limited the developments of gasochromic smart windows and optical gas sensors. The use of Raman spectroscopy for the in-situ investigation of the H2 gas molecules interactions with the WO3 and MoO3 surfaces enabled the PhD candidate to examine their gasochromic mechanisms. The in-situ Raman spectra changes of Pd/WO3 films were for the first time investigated and the PhD candidate concluded that the interaction with H2 gas resulted in the formation of oxygen vacancies and water molecules on the WO3 grains. The PhD candidate also employed in-situ Raman spectroscopy to investigate the H2 gas interaction with nanostructured Pd/MoO3. The Raman spectroscopy studies revealed that the H2 gas interaction caused the crystal structure of MoO3 transformed from the original α-MoO3 into the mixed structure of hydrogen molybdenum bronze and sub-stoichiometric MoO3, eventually forming oxygen vacancies and water molecules. Based on these conclusions, the PhD candidate developed highly sensitive fiber optic H2 gas sensors based on nano-platelet Pd/WO3 films. It was found that these gas sensors demonstrated impressive reflectance responses of 6% and 12% in the presence of 0.06% and 1% H2 gas in synthetic air at 100 C, respectively. <br><br>The current reported electrochromic smart windows based on crystalline WO3 systems suffered from low coloration efficiencies. The PhD candidate hypothesized that the utilization of three dimensional (3D) crystalline WO3 nanoporous networks can compensate this serious drawback. The PhD candidate synthesized such nanostructures at room temperature by anodizing the RF-sputtered W films in a delicately chosen electrolyte under unprecedented low applied anodic voltages. For the crystalline nanoporous networks with thicknesses ranging from 0.6 to 1 µm, impressive coloration efficiencies of up to 141.5 cm2 C1 were achieved by applying a low coloration voltage of 0.25 V.<br><br>Currently DSSCs based on random networks of titanium dioxide (TiO2) nanoparticles offer the highest power conversion efficiencies but it is still limited for commercial applications. The PhD candidate chose Nb2O5 as a more suitable photoanode material to tackle this problem. 3D Nb2O5 nanoporous networks were developed by using an elevated temperature anodization process. The PhD candidate showed that for the same thicknesses of ~4 µm, the DSSC based on Nb2O5 layer had a significantly higher efficiency (~4.1%) when compared to that incorporating a TiO2 nanotubular layer (~2.7%). To date, this is the highest efficiency among all the reported photoanodes for such a thickness when utilizing back side illumination. The PhD candidate ascribed this to a combination of reduced electron scattering, greater surface area, wider bandgap and higher conduction band edge, as well as longer effective electron lifetimes in the anodized Nb2O5 nanoporous networks.<br>