氧化鋅奈米柱為基礎三元奈米複合材料氣體感測器之大氣與成長參數效應

Effects of Atmospheric and Growth Parameters on Zinc Oxide Nanorod Based Ternary Nanocomposite Gas Sensors

薩欽(Kumar, Sunkuru Sachin)

指導教授 ： 林鶴南

國立清華大學/工學院/材料科學工程學系/碩士(2020年)

摘要

本研究以不同成長參數與材料堆疊順序，將氧化鋅、銅氧化物、金組成三元奈米複合材料，作為化學電阻式NO2氣體感測器，目標是在室溫下達到高穩定性及高靈敏度的氣體感測應用，並對環境參數如溫溼度對氣體感測影響進行探討。
先以低溫水熱法於基板上製備ZnO奈米柱(nanorod)，並進行高溫退火，然後透過光還原法，將 CuxO奈米團簇(nanoclusters)與Au奈米粒子(nanoparticles, NPs)成長在ZnO奈米柱上。依照不同光還原次序，製備Au/Cu_x O/ZnO (Au在最外)與CuxO/Au/ZnO (CuxO在最外)兩種三元奈米複合材料，同時CuxO有三種不同還原時間，共製備六種樣品。
掃描式電子顯微鏡(SEM)用來觀察奈米複合材料的表面形貌，ZnO奈米柱呈現六角形，平均直徑約100 nm。CuxO奈米團簇在Au/CuxO/ZnO與CuxO/Au/ZnO上的大小分別約為400 nm與200 nm。Au奈米粒子只在Cu_x O奈米團簇上可見，大小約30 nm，而CuxO奈米團簇在CuxO/Au/ZnO的分布密度較高。並以螢光光譜(PL)分析電荷分離現象、能隙與缺陷的存在，能量分散式能譜(EDS)來了解試片中的元素組成，X射線光電子能譜(XPS)來分析Cu2O與CuO在 CuxO奈米團簇的組成比。
氣體感測以NO2作為測試氣體，在100 ppb NO2，還原6小時的Au/CuxO/ZnO樣品具有最高響應值為680%。對濕度效應，電阻會先隨濕度增加而下降，但在達到臨界濕度後，電阻反而隨濕度增加上升。臨界濕度取決於存在的缺陷數量，大量受體缺陷會得到較高臨界濕度。響應會先隨著濕度上升而上升，達到臨界濕度後，響應隨濕度增加而下降。隨著溫度上升，試片的電阻與響應都隨之下降。此外也發現Au/CuxO/ZnO樣品，因溫濕度造成的電阻變化較大，在43天長期測試，CuxO/Au/ZnO樣品具有較佳穩定性。由於樣品含有CuxO試片，在連續紫外光照射下，產生自我氧化及還原，因此會有不穩定狀況。最後並使用自製的可攜式氣體感測裝置，進行試片的長期測試。
總之，我們製作了CuxO/Au/ZnO與Au/CuxO/ZnO奈米複合材料，並應用於NO2氣體感測。CuxO/Au/ZnO奈米複合材料展示較高穩定性，在溫溼度條件變化時有較低的變化。而溫溼度影響並非線性，高溫下濕度造成的變化較低，高濕度下溫度造成的變化較低。

關鍵字

氣體感測 ； NO2 感測 ； 三元奈米複合材料 ； 氧化鋅 ； 奈米柱 ； 濕度 ； 溫度

並列摘要

Ternary nanocomposites comprising of zinc oxide, copper oxide and gold are prepared with different growth parameters and different sequence of material stacking for realizing a chemiresistive NO2 gas sensor. The goal of this work is to achieve highly stable and highly sensitive sensors for room temperature practical application. The effects of environmental parameters like temperature and humidity on the resistance and gas sensing performance are also investigated.
ZnO nanorods (NRs) are grown using a low-temperature hydrothermal method. CuxO nanoclusters (NCs) and Au nanoparticles (NPs) are then reduced on annealed ZnO NRs using photoreduction method. Au/CuxO/ZnO and CuxO/Au/ZnO ternary nanocomposite are prepared with three different reduction times for CuxO deposition, six samples in total.
Scanning electron microscopy (SEM) is used for morphology analysis of the fabricated sensors. The NRs have a hexagonal shape with an average diameter of ~100 nm. CuxO NCs ~400 nm and ~200 nm are found in Au/CuxO/ZnO and CuxO/Au/ZnO respectiveley. Au NPs are found only on CuxO NCs with a size of ~30 nm. The density of CuxO NCs distribution is higher in case of CuxO/Au/ZnO. The photoluminescence (PL) is used for analyzing the charge separation, bandgap and presence of defects qualitatively. Energy Dispersion Spectroscopy (EDS) is used to better understand the elemental composition in the sample. In addition, X-ray Photoelectron Spectroscopy (XPS) is used to analyze amounts of Cu2O and CuO that constituent the CuxO NCs in fabricated samples.
The sensing response in this work is performed using NO2 as a target gas. The highest response we got is 680% for 100 ppb NO2incase of Au/CuxO/ZnO reduced for 6 hrs. In all the six sensors, resistance first decreased with increase in relative humidity (RH) and after reaching a threshold RH, the resistance increased with further increase in RH. This threshold RH depends on the number of defects present, high amount of donor defect results in lower threshold RH. In the case of response, with increase in RH it first increases and after a threshold RH, it decreases with further increase in RH. The threshold RH here becomes lower if higher amount of acceptor defects is present. With the increase in temperature, the resistance and response both decreases. The variation in resistance due to humidity and temperature is higher in the case of Au/CuxO/ZnO samples. A 43-day long stability analysis shows that CuxO/Au/ZnO sample is more stable. The reason behind the instability in these samples containing CuxO turned out to be the self-oxidization/self-reduction of CuxO under continuous ultra-violet irradiation. In the final segment of this work, long-term real-life measurement is done for fabricated samples using the portable device developed from our group.
In conclusion, we produced CuxO/Au/ZnO and Au/CuxO/ZnO ternary nanocomposites for NO2 gas sensing. The CuxO/Au/ZnO nanocomposite showed higher stability and lower variation to change in environment parameters like temperature and humidity. Effect of humidity and temperature are not linear. Variation due to RH is found to be lower in higher temperature and variation due to temperature is found to be lower in higher RH.

並列關鍵字

Gas Sensing ； NO2 sensing ； Ternary Nanocomposite ； Zinc Oxide ； Nanorods ； Humidity ； Temperature

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