液態臨場穿透式電子顯微鏡觀測氧化鋅奈米線合成之研究

Abstract

氧化鋅奈米線因其半導體特性、光電特性和壓電特性而被廣泛運用在太陽能電池、LED、奈米發電機…等各種奈米元件中，而使用水熱法製備奈米結構有製程簡單和成本低的優點，故有許多學者研究以水熱法合成氧化鋅奈米線，然而以水熱法合成氧化鋅奈米線的生長機制因缺乏直接證據而尚未被證實，故本研究著重在使用液態試片搭配液態載具，對氧化鋅奈米線合成之過程進行液相臨場觀察。 研究中使用的前驅物為醋酸鋅和六亞甲基四胺，其合成不須藉由晶種層亦能成長氧化鋅奈米結構，之後將前驅物溶液封裝在液態試片並放置到液態載具中，以穿透式電子顯微鏡進行臨場觀測，臨場觀測過程中，電子束的加熱效應將做為驅動力促使水熱法發生。而奈米線的成長主要分成兩個部分，一為奈米粒子成核析出，並在{21 ̅1 ̅0}和{01 ̅10}平面族進行等向性成長，或是奈米粒子聚合形成新的奈米粒子，第二部分為氧化鋅奈米粒子在(0001)平面進行非等向性成長，因鋅離子而帶有正電荷的(0001)終端平面會吸引帶負電荷的成長單體[〖Zn(OH)〗_4 ]^(2-)，促使成長單體在此平面沉積，進而成長為氧化鋅奈米線。 而液態試片因空間限制而將合成範圍侷限在奈米尺度下，奈米結構之間因為靜電吸引導致流動性下降，使彼此壓應力上升，產生擴散阻礙效應影響奈米結構生成。此外，入射電子會和水溶液產生輻射分解效應，促使溶液中升成氫氧自由基，氫氧自由基和六亞甲基四胺反應後，在液態試片中生成氫氣氣泡，使液態試片進行臨場觀察的時間有所限制。雖然液態試片有上述限制而難以觀察到水熱法完整的反應過程，但本臨場液相觀察能將理論機制與實際反應提供直接關聯的證據。

ZnO nanostructures have attract much attention due to their semiconducting, photoelectric, and piezoelectric properties which have been applied in solar cell, LED, nanogenerator and so on. There are many synthesis method to produce ZnO nanostructures such as PVD, MOCVD, PLD and liquid-phase synthesis. Among them, hydrothermal synthesis method is more promising because of low cost, simple process and large scale production. However, the growth mechanism remains impalpable. The liquid cells was used for in-situ TEM because it can provides direct evident and extend the study of kinetic in hydrothermal synthesis process. In this work, zinc acetate and HMTA were dissolved in DI water as precursor solution to produce ZnO nanowires without the use of seed layer. The precursor solution was then sealed in liquid cell in order to be observed by in-situ TEM. The driving force for hydrothermal synthesis method results from the heating effect of electron beam. The growth of ZnO nanowires can be classified into two parts. First is the nucleation and growth of ZnO nanoparticles. The ZnO nanoparticles grow as a result of either isotropic monomer attachment on {21 ̅1 ̅0} and {01 ̅10} surface or coalescence of nanoparticles in same crystal arrangement direction. The second is that ZnO nanoparticles grow anisotropically on (0001) surface resulting in the formation of nanowires. Since (0001) surface is Zn-terminated with positive charge that attract the monomers of [〖Zn(OH)〗_4 ]^(2-) which is negatively charged. The monomers will deposit on (0001) surface. However, the small-sized liquid cell confined the growth space in nanometer scale and electrostatic force between nanostructure also confined the movement of nanostructure. That led to the hindrance effect and retared the growth of ZnO nanostructures. In addition, the solution was irradiated by electron beam which cause the formation of hydroxyl radical. Hydroxyl radical will react with HMTA resulted in the formation of hydrogen bubble. The bubble limited the observation time of liquid cell. Although the liquid cell cannot show the exactly same situation for usual synthesis process, it still offer directly evident to hydrothermal synthesis process. The study provide valuable information in growth of ZnO nanostructures.