銀奈米粒子修飾氧化鋅奈米線於光觸媒應用

Application of ZnO Nanowires Modified with Silver Nanoparticles in Photocatalysis

陳建華

氧化鋅 ； ZnO

清華大學材料科學工程學系學位論文

2016年

碩士

林鶴南

繁體中文

氧化鋅具有無毒、生物可相容性、價格便宜、很強的光生電荷能力，因此氧化鋅為一個良好的光觸媒材料，但是氧化鋅在可見光區吸收強度低與電子電洞再複合率較高，因此本實驗利用光還原法將銀奈米顆粒還原至氧化鋅奈米線上，藉由銀的修飾可以增強光觸媒材料的吸收光能力，另外銀與氧化鋅的異質接面可以降低電子電洞的複合率進而提升光觸媒效率。相較於昂貴的傳統矽基板材料，本實驗以紙作為基底、水溶液法成長氧化鋅，大幅降低光觸媒成本以期未來能應用於工業上。 藉由掃描式電子顯微鏡確認氧化鋅表面形貌與奈米顆粒結構，以5×10^(-2) M 硝酸銀光還原2分鐘可以得到最佳的銀奈米顆粒修飾濃度，推測其有最好的光觸媒效率，而5×10^(-2) M 硝酸銀光還原2.5分鐘或1×10^(-3) M硝酸銀光還原6分鐘後光還原後的奈米顆粒會聚集成團聚物或薄膜於氧化鋅的頂端，此團聚物或薄膜會抑制光觸媒效率；能量色散X-射線光譜分析得到本材料內部並無其他雜質的存在；螢光光譜分析圖顯示修飾後的氧化鋅螢光光譜強度有明顯的下降，其中以5×10^(-2) M硝酸銀光還原的氧化鋅螢光光譜強度下降最多，推斷高濃度且均勻的銀奈米粒子顆粒更能有效抑制電子電洞再複合。 光觸媒分解實驗以面積1.5×1.5 cm^2的紙作為基板並成長氧化鋅或銀/氧化鋅在紫外燈光照下分解10 µM羅丹明B (Rhodamine B)。原生氧化鋅一階反應常數為0.0113 min^(-1)，以5×10^(-2) M 硝酸銀還原銀奈米顆粒兩分鐘於氧化鋅上可得到最佳一階反應常數0.0218 min^(-1)，其效率為原生氧化鋅的1.93倍，成功以簡單的光還原方法成功的製備出效率較好的光觸媒複合材料。 成長於紙上的氧化鋅奈米線對於氣體感測也有良好的反應，在紫外光條件與大氣作為背景氣體下感測1 ppm NO氣體下最高反應可達89%，但以銀奈米顆粒修飾成長於紙上的氧化鋅則無法作為良好的氣體感測材料，這是因為紙吸附了大量的硝酸銀，光還原下銀在紙上形成了連續性的銀薄膜，此銀薄膜大幅地降低了材料的電阻使其偏離了適當的響應電流範圍。

ZnO is a suitable photocatalytic material from non-toxic, biocompatibility, low-cost, and strong photo-induced electron-hole pairs. However, ZnO has the limitation of absorbing visible light and fast recombination rate of electron-hole pairs. In this work, ZnO nanowires were modified with Ag nanoparticles. Owing to the improvement of the Ag/ZnO absorption ability, the photocatalytic performance could be enhanced. Moreover, the superior photocatalytic performance of the Ag/ZnO can be ascribed to the heterostructure which lowers the recombination rate of photo-excited electron-hole pairs. Compared to the traditional Si substrate, we took paper as substrates and grow ZnO nanowires by low-cost solution method to lower the price and applied in the industry. The scanning electron microscopy indicates the morphologies and structures of nanowires and nanoparticles, the ZnO nanowires modified with the best concentration of Ag nanoparticles showed the best photocatalytic performance under the condition of 2-min photoreduction with 5×10^(-2) M silver nitrate. The Ag nanoparticles formed nanoclusters or thin film on the top of ZnO nanowires, which lowers the photocatalytic performance, under the condition of 2.5-min photoreduction with 5×10^(-2) M silver nitrate or after 6-min photoreduction with 1×10^(-3) M silver nitrate. The energy dispersive spectrometer presents that there are no impurities inside the material. The photoluminescence spectra show that the decreasing of Ag/ZnO, especially the Ag/ZnO photoreduced with 5×10^(-2) M silver nitrate, compares to the as-grown ZnO, revealing that the recombination of electron-hole pairs is more likely reduced by high concentration and uniform distribution of Ag nanoparticles. The photocatalytic performance of the ZnO or Ag/ZnO nanowires on paper substrate with 1.5 cm by 1.5 cm surface area was evaluated by degrading a 10 µM rhodamine B solution under the illumination of ultraviolet light. The best first-order kinetic constant of the Ag/ZnO nanowires is 0.218 min^(-1). It is 1.93 times as high as as-grow ZnO nanowires (0.0113 min^(-1)). It showed a high-efficiency photocatalytic material by a simple photoreduction method successfully. The ZnO nanowires on paper substrate also showed a good response of the gas sensor, which has been utilized to sense 1 ppm NO gas under UV light and background ambient gas and showing response up to 89%. However, the ZnO nanowires modified with Ag nanoparticles were not able to be a good gas sensor device. Because the paper sucked up silver nitrate under the photoreduction, the Ag formed a continuous thin film on the paper. This Ag thin film lowers the resistance of the gas sensor material dramatically and diverts the electric current from the proper response region.

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