氧化鋁披覆一維氧化鋅奈米複合材料之製備與光學性質研究

Abstract

以低溫水溶液法合成氧化鋁披覆之一維氧化鋅奈米材料，經由熱處理溫度和氣氛的調控，可得到不同晶相之氧化鋁披覆層，進而對氧化鋅奈米材料之發光特性產生不同影響。 奈米級氧化鋁粉體是以溶液析出法(precipitation in aqueous solution)製備，得到以pseudo-boehmite和bayerite兩相為主的前趨物，經由不同溫度、環境之熱處理條件，可轉變為γ、δ、θ、α等不同晶相之氧化鋁粉體，並發現產物呈現極佳的發光特性；另一方面，也由解膠、稀釋後配置不同濃度之氧化鋁溶液，以進行後續披覆實驗。 一維氧化鋅奈米材料則利用水溶液法在鍍有氧化鋅薄膜之矽基版上生成。在極低的濃度(0.001 M)下於95 oC持溫反應24小時，得到直徑約為25 nm之氧化鋅奈米管，同條件下增加反應濃度至0.005 M則產物轉變為氧化鋅奈米線。氧化鋅晶體之生長可藉由結構導引劑(SDA)之添加來改變其生長特性，本實驗利用檸檬酸(citric acid)和1,3-二氨基丙烷(1,3-diaminopropane, DAP)之添加來控制氧化鋅之生長模式；微量的檸檬酸會使氧化鋅在<0001>方向之生長受到阻礙，而形成短柱狀甚至六角盤狀結構，然而在0.001M之低濃度氧化鋅溶液中加入檸檬酸，則會對氧化鋅之生長產生抑制，在95 oC下反應24小時，可得到分散的氧化鋅奈米管；而DAP之添加則有助於氧化鋅側向生長的可能，本實驗在0.001 M之氧化鋅溶液中加入微量DAP，將覆蓋氧化鋅奈米柱之矽基板置入溶液中進行第二段成長，得到特殊的氧化鋅奈米線之螺旋生長結構。 進行氧化鋁之披覆實驗時，調配不同的氧化鋁溶液濃度，即可得到不同厚度(1∼8 nm)之氧化鋁薄膜，而不同熱處理條件下可得到不同晶相的氧化鋁薄膜。氧化鋁薄膜的厚度大小會對一維氧化鋅奈米材料之光學性質有不同的影響，當披覆厚度約為8 nm時，其PL量測結果以氧化鋁之發光為主；若將氧化鋁披覆厚度降至1∼2 nm，則氧化鋁之放光不明顯，此時氧化鋁之披覆會使氧化鋅奈米管之本質光明顯增強，有助於氧化鋅奈米材料在紫外光發光元件上之應用。

A novel aqueous solution method has been developed for growing well-aligned alumina coated ZnO nanomaterials. The ZnO nanowires and ZnO nanotubes are synthesized by solution method on the Si wafer coated with ZnO film, and the organic structure-directing agents (SDAs), citric acid and diaminopropane (DAP), are found to play different roles in controlling the morphologies through the selective adsorptions on different crystal facets of ZnO. Nano-sized pseudo-boehmite and bayerite mix powder is obtained through precipitation method and characterized using X-ray diffraction, HRTEM, PL and FTIR spectroscopy after thermal annealing. The as-synthesized powder will transfer into γ-phase by annealing process at 400 oC, and then change to α-phase over 1000 oC. Furthermore, it can be observed a strong blue emission from PL result. On the contrary, transparent conductive alumina film could be obtained after peptized growth process. Alumina-coated ZnO nanowires are synthesized by aqueous solution method at low temperature. When the ZnO nanostructure is immersed into peptized alumina solution, the alumina shell would be form on the surface of ZnO nanostructure. The thickness of alumina film could be controlled by modulating the concentration of alumina solution; When the thickness about 8 nm, the alumina-coated nanowires show both the blue emission of alumina and the UV emission of ZnO after thermal annealing. However, ZnO nanotubes show obviously visible emission with increasing annealing temperature, while alumina-coated ZnO nanotubes exhibit a strong UV emission after thermal annealing.