The Chemical-composition-dependent Microstructures and Electronic Structures of Sol-gel-derived ZrO2 Films

• Main Authors: Sue-min Chang, 張淑閔
• Other Authors: Ruey-an Doong
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/58754455278107108017

博士 === 國立清華大學 === 原子科學系 === 93 === Abstract Zirconium dioxide (ZrO2) and metal-doped ZrO2 films are promising materials for various advanced applications because of their special physicochemical properties. Microstructures and electronic structures play important roles in governing the physicochemical properties as well as the performance of ZrO2. Calcination conditions have shown to greatly influence the microstructures and electronic structures of ZrO2. Such influence is associated with changes in lattice defects and related chemical compositions. However, the qualitative and quantitative analyses of chemical compositions for elucidating the influence of calcination conditions on the microstructures and electronic structures of pure and doped ZrO2 is little addressed. In this study, the chemical compositions, including O/Zr ratios, contents of Zr-O and Zr-OH, contents and distribution of Zr species, of ZrO2 films with respect to their microstructures and electronic structures after calcination in air and in N2 were thoroughly examined. Moreover, the mechanisms of conversion of the microstructures and electronic structures in the pure ZrO2 and doped ZrO2 films under different calcination conditions are proposed based on the chemical compositions. A sol-gel method was used for preparing ZrO2 films in this study because it is simple and feasible to control the morphologies and thicknesses of films to satisfy the requirement of the samples for this study. The phase transformation of the sol-gel-derived ZrO2 films was amorphous ® m-tetragonal ® monoclinic in air, while the evolution followed the sequence was amorphous ® m-tetragonal ® monoclinic ® m-tetragonal in N2 at 80-950 °C. During crystallization, the decrease in surface and lattice hydroxyl groups, generation of low-valent Zr species, and decrease in bulk O/Zr ratios to non-stoichiometry (< 2) prove that dehydroxylation reduced Zr4+ to low-valent states and introduced oxygen vacancies to stabilize tetragonal phase at temperatures than its thermodynamic temperatures (~ 1200 °C). When the m-tetragonal phase converted to the monoclinic phase, surface hydroxyl groups also decreased. Afterward, the bulk O/Zr ratios approach stoichiometry (~ 2) when the phase transformation was almost completed. These observations clearly show that the compensation of oxygen vacancies from surface hydroxyl groups, which play the role of O2- donor, triggers the m-tetragonal-to-monoclinic phase transformation. Comparatively, the m-tetragonal phase has lower stability against temperatures in air than in N2. The increase in the surface hydroxyl groups on the ZrO2 films calcined in air depicts that the dissociation of water and O2 on the surface accelerates the phase transformation. When ZrO2 films were calcined at 450-600 °C in N2 or 850-950 °C in air, crystallite sizes of the m-tetragonal phase and the monoclinic phase decreased with increasing temperatures. Meanwhile, bulk O/Zr ratios also decreased. These phenomena show that segregation of oxygen vacancies as well as reduced Zr species occurs to stabilize the m-tetragonal phase and the monoclinic phase. The minimal and maximal O/Zr ratios for the stabilization of the m-tetragonal phase were 1.63 and 1.98, respectively, while the monoclinic phase is only stable at stoichiometry. The changed microstructures and O/Zr ratios dominate the band structures and band gaps of the sol-gel-derived ZrO2 films under different calcination conditions. The sol-gel-derived ZrO2 films contain direct band, indirect band and band tail structures. The band tails are detected when O/Zr ratios are almost equal to or larger than stoichiometry (³ 2), indicating that the tails are primarily resulted from imperfect crystalline structures. In addition, the indirect band structures are determined when the O/Zr ratios are smaller than stoichiometry (< 2), suggesting that the indirect band structures are produced by lattice defects. These three kinds of band gaps were determined experimentally. Direct band gaps of the amorphous and m-tetragonal ZrO2 films ranged 5.90-6.12 and 5.32-5.74 eV, respectively, the monoclinic ZrO2 exhibited two direct band gaps ranged 5.87-6.00 and 4.89-5.08 eV, and the indirect band gaps of the amorphous, m-tetragonal and monoclinic ZrO2 films ranged 5.01-5.47, 4.77-5.40 eV, and 4.82-5.10 eV, respectively. The direct band gaps of ZrO2 films decreased with increasing crystallite sizes in air because of a quantum effect. However, O/Zr ratios dominate the band gaps in N2. Both the direct and indirect band gaps decrease upon decreasing O/Zr ratios because of increased contents of the lattice defects. Doping metal ions into ZrO2 lattice has been illustrated to improve the catalytic efficiency and ionic conductivity because introduced lattice defects modify the microstructures and electronic structures of ZrO2. The influence of calcination conditions on the ZrO2 films doped with seven transition metal ions, including Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+, are also examined based on the conversion of chemical states of dopants. Dehydroxylation, deoxygenation, and intake of oxygen ions from surface resulted in the reduction and oxidation of dopants in the bulk ZrO2 films. The depants, including Mn2+, Fe3+, Cu+, and Zn2+, having d5 or d10 configurations exhibited high stability against reduction induced by dehydroxylation. However, deoxygenation induced further reduction of these metal ions at high temperatures. The changes in chemical states of dopants dominate the d-spacings, crystallite sizes, preferred orientations and m-tetragonal-to-monoclinic phase transformation of the doped ZrO2 depending on their metallic or ionic radii relative to Zr4+. Also, the changed chemical states determine the impurity levels between intrinsic bands as well as band gaps of ZrO2, depending on their d-configurations. In summary, the reduction and oxidation induced by calcination change the chemical compositions of ZrO2 and doped ZrO2 films. The changed chemical compositions govern the microstructures and electronic structures of ZrO2 under different calcination conditions.