Synthesis and electrochromic properties of niobium oxide nanocrystals




Lu, Hsin-Che

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Niobium oxide (Nb₂O [subscript 5-x]) nanocrystals hold promise for improving the performance of conventional electrochromic smart windows due to their tunable electrochromic properties within various polymorphs and ideal electrochemical and optical stabilities. By tuning the nanocrystal structure, this study aimed at providing experimental tools to control the electrochromic spectral range and switching kinetics of Nb₂O [subscript 5-x] nanocrystals for electrochromic applications. Alongside the experimental exploration, theoretical background that elucidates the change of electrochromic spectral range and switching kinetics brought by Nb₂O [subscript 5-x] nanocrystals was also investigated. Experimentally, the colloidal synthesis of Nb₂O [subscript 5-x]nanocrystals that produces monoclinic Nb₁₂O₂₉ nanoplatelets was achieved by precisely arranging the structure of niobium precursors. Upon progressively reducing the nanoplatelets, increasing absorbance in the near-infrared region is attributed to a surface-dominated mechanism, whereas the secondary absorbance mode in the visible region is brought by Li⁺ intercalation, establishing the dual-mode electrochromism of the monoclinic Nb₁₂O₂₉ nanoplatelets. The colloidal synthesis was further modified to produce both nanorods and nanoplatelets of monoclinic Nb₁₂O₂₉. This synthetic endeavor allows the investigation on the influence of shape anisotropy on the electrochromic spectral range. Both experimental analysis and calculations based on density functional theory were utilized to show that, in nanoplatelets, the presence of both square planar and crystallographic shear sites enables a higher degree of charge localization during Li⁺ intercalation, leading to absorbance increase in both visible and near-infrared regions, while in nanorods, the Li⁺ only intercalates into the square planar sites with lower degree of charge localization and the absorbance is limited within the near-infrared region. Lastly, nanocrystals of orthorhombic Nb₂O₅, monoclinic Nb₁₂O₂₉, and Sn-doped In₂O₃ were utilized to demonstrate the influence of various charge storage mechanisms on the switching kinetics of electrochromic nanocrystals. The absorbance change over time was collected experimentally and modeled by an exponential-growth equation to quantitatively elucidate the key parameters that control the switching kinetics. We concluded that, for the surface-dominated mechanisms, dual-stage switching kinetics were observed regardless of the materials, suggesting that the switching kinetics are efficient at early stage but becoming slower over time. As for the intercalation mechanism, single-stage switching kinetics controlled by the Li⁺ diffusion was observed.


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