新穎有序維度氧化銦奈米結構之合成、光學特性及其高性能氣體感測器之應用

Abstract

本研究論文主要聚焦於具新穎奈米結構氧化銦材料之設計、製備、光學特性分析與模擬、及其機能性元件之應用。本研究製備之氧化銦奈米結構具有極為特殊之組裝形貌，從較單純之零維奈米粒子單元結構，乃至高複雜度之有序維度奈米結構，均可被重複製造合成，且均具有特殊之形貌依存吸收光學特性與各式電子及感測元件應用潛力。在零維奈米粒子方面，著重於發展低成本透明電極製程之噴墨列印技術，首先利用共沉法合成高產量之錫參雜氧化銦(tin doped indium oxide, ITO)球狀奈米粒子，再將此奈米粒子製備成ITO奈米粒子噴印漿料，利用噴墨列印技術將ITO奈米粒子漿料，依所設計之電極圖案，直接噴印於軟性基板上，此技術更可以精確控制電極圖案之線寬以及厚度。此外，本研究亦將深入探討噴印奈米粒子薄膜之光學與導電性質。此部分研究相關知識可以增進對其後氧化銦奈米粒子有序維度結構之基礎理解，相關噴印技術更可直接延伸應用於高性能奈米感測元件之製備。 除結構單純之氧化銦奈米粒子外，本研究成功製備多種新穎氧化銦有序維度奈米結構，此有序維度奈米結構是由許多奈米粒子，規則性地組裝排列而成，並可以藉由變動化學合成與後熱處理參數來調控最終之結構形貌。在此獲得兩種具代表性之針狀與花狀氧化銦有序維度奈米結構。相較於傳統利用高溫高壓之化學水熱合成法來合成高複雜度奈米結構，本研究所使用之低溫常壓溶液合成法具有簡易、迅速、高產量之特點。此有序維度奈米結構之紫外-可見光消散光譜，展現出特殊之高度組裝結構依存性。為此，本研究依結構觀察結果，建立經簡化之圓球等效模型，並配合T-matrix模擬定量解釋此有序維度奈米結構之消散、吸收與散射光譜。結果除了釐清針狀與花狀形貌對消光光譜之影響外，更證實此簡化等效模型可快速且準確預測有序維度奈米結構之光學特性，具有高度學術與應用價值。 相對於自由堆積球形奈米粒子之高度緻密化，此特殊氧化銦有序維度奈米結構具有極高之比表面積，結構內外具有無數之細微孔洞與通道，可以作為氣體之吸附與內外擴散通道，大幅提升氣體與材料間之反應機率跟速率，目前已經實驗証明於乙醇氣體感測應用中，展現極為突出之感測敏感度，於400 oC的70 ppm乙醇氣體感測中，針狀與花狀奈米叢束之氣體敏感度可分別達到45與50。另外，為了降低可燃氣體爆炸風險、及因應全球綠色環保之趨勢，室溫型氣體感測元件一直都是此領域之熱門研究主題。為此，我們將此氧化銦有序維度奈米結構進一步發展為室溫型高性能氣體感測元件。由於氣體分子在氧化物表面的吸、脫附作用需由熱能活化，因此氧化物氣體感測器多需於高溫(200 oC到400 oC)下方能發揮作用。目前僅有少數室溫型氧化物氣體感測器相關研究發表，在此些氧化物中，氧化銦感測相關研究更屬稀少。為此，我們成功結合金屬奈米粒子觸媒與花形氧化銦有序維度奈米結構，成為一新穎複合式奈米結構感測材料，作為室溫下一氧化碳感測器之用。藉由低溫化學合成，大量鉑與鈀金屬奈米粒子均勻還原於花形氧化銦有序維度之奈米結構表面，實驗證明此複合式有序維度奈米結構具備室溫一氧化碳感測能力。由於金屬觸媒催化感測氣體與氧化銦間之反應，使得工作溫度大幅下降至室溫，並展現出優異之敏感度與感測速率。

This thesis reports the controlled syntheses, characterizations and applications of numerous novel In2O3 nanostructures from simplest zero-dimensional nanoparticle to complicated high-dimensional hierarchical nanobundles (NBs). For realizing the direct ink-jet printing of transparent tin doped indium oxide (ITO) electrodes on flexible substrates, high-yield Sn doped In2O3 nanoparticles with spherical shapes and designed chemical compositions were successfully synthesized for the first time by co-precipitation and were then applied to the fabrication of nano-ink. The ink-jet printed ITO electrode patterns with a controlled line width and thickness on flexible substrates provides distinct nanoparticle dependent results for the understanding fundamentals and potentials for optical and electronic applications. It is the first time that the In2O3 hierarchical nanostructures have been successfully synthesized in such novel In2O3 needle-like and flower-like NBs composed of numerous parallel assembled nanoparticle chains via a simple solution route and subsequent heat treatments. In contrast with other chemical methods with critical synthetic conditions for obtaining complicated nanostructures, the present solution route has simple, convenient, time-saving, and high-throughput features under relatively low ambient pressure and temperature. In addition to the distinct morphologies, the T-matrix simulation with our special simplified particle models clearly explains the morphology influence on the extinction spectra and further distinguishes the characteristic scattering and absorption spectra for fundamentally understanding the optical difference between the needle-like and flower-like In2O3 NBs. In addition, we have also demonstrated that the In2O3 hierarchical NBs are of great value for applications in chemical gas sensors. In2O3 NBs with large surface areas and extremely porous configurations exhibit outstanding ethanol sensing response in comparison with the randomly packed In2O3 nanoparticles. A very high gas sensitivity of 45 and 50 was obtained for the needle-like NBs and flower-like NBs, respectively, for an ethanol concentration of 70 ppm at 400 oC. Based on these two promising nanostructures, we further developed advanced room-temperature gas sensors. Generally, metal oxide gas sensors can only work at high temperatures (200–400 oC), because both adsorption and desorption of dissociated gas species on the metal oxides are thermally activated processes. Hence, reports related to room-temperature metal oxide carbon monoxide (CO)gas sensors are extremely scarce; for resistance-type In2O3 gas sensors, we cannot find even one successful case presenting gas sensing capability at room temperature. An innovative hybrid nanostructure combining catalytic metal nanoparticles and In2O3 flower-like NBs was designed for room-temperature CO gas sensors, which have not been previously reported. Through the low-temperature solution approach (below 100 oC), well-dispersed Pt and Pd nanoparticles were deposited onto the In2O3 NBs without aggregation. With the assistance of catalytic Pt and Pd nanoparticles, both the Pt- and Pd-In2O3 sensors exhibit very high sensitivities and remarkable response/recovery speeds in sensing CO gas at room temperature.