PdO奈米粒修飾對SnO2之一氧化碳氣體感測行為的影響

Abstract

二氧化錫(SnO2)是一種負型金屬氧化物半導體(metal oxide semiconductor, MOS)材料，被廣泛地應用於商業化的MOS氣體感測器中。為了要改善氣體感測的效果，貴金屬常被利用做為敏化劑來提升氣體感測效果，金屬鈀(Pd)就是一種常用的貴金屬。在高溫環境下，金屬Pd會被氧化成氧化鈀(PdO)；而PdO亦會被一氧化碳(CO)還原，生成金屬Pd。這種金屬Pd氧化與PdO還原共同存在的表面反應過程會影響到SnO2的氣體感測感測行為。本研究以濺鍍方式沉積PdO奈米粒在SnO2薄膜上，探討PdO修飾對SnO2的CO氣體感測行為的影響。SnO2薄膜和氧化鈀在經過退火後皆具有結晶性，實驗中會使用X光繞射分析儀(XRD)、化學分析電子儀(XPS)、掃描式和穿透式電子顯微鏡(SEM、TEM)來分析材料的結晶性、化學態、表面形貌和微結構。 當SnO2薄膜曝露於CO/乾空氣的混合氣體時，吸附在SnO2表面的氧分子離子或過氧離子可CO被還原，到特定高溫時，SnO2晶格氧原子會被CO還原，形成氧空缺，這些表面反應都會改變SnO2薄膜表面層電子濃度，因此CO氣體感測會造成氣體感測響應。SnO2薄膜厚度越薄，電性受表面電子濃度影響所佔整體比例越多，氣體感測的效果會越好。 SnO2薄膜在PdO奈米粒修飾後，SnO2與PdO之間PN接面的電子效應會直接影響CO感測前後SnO2的電性，使CO氣體感測效果更好。根據XPS分析，在低溫50℃-100℃時，PdO表面化學態不會因為CO感測過程發生改變。在高溫150℃以上，PdO會還原成金屬Pd，此化學態的改變會明顯增加CO感測時SnO2試片的電流值，增加感測響應，但當隨著金屬Pd還原量越來越多，氧分子會裂解吸附在金屬Pd上，導致電流值產生反轉降低，此現象從150℃開始溫度越高表現越不明顯，因為溫度越高，金屬Pd被氧化成PdO的反應速率也會越快，導致氧分子裂解吸附變弱，導電率下降行為變不明顯。

Tin oxide is an n-type metal oxide semiconductor (MOS), and widely used as the sensing material of MOS gas sensors. To improve the sensing performance of SnO2 sensors, noble metals are usually used as a sensitizer. Pd is one of the mostly used sensitizer. Metal Pd can be oxidized under a high temperature sensing condition, while PdO can be reduced by reducing target gases in the similar temperature regime. The simultaneous occurrence of Pd oxidation and PdO reduction on the SnO2 sensor surface can significantly affect the gas sensing behavior of SnO2. In this study, we deposited PdO nanoparticles on the SnO2 thin film by sputter deposition followed by high temperature annealing, and study the CO sensing behavior of the Pd-decorated SnO2 sensor. X-ray diffraction spectroscopy (XRD), x-ray photoelectron spectroscopy (XPS), secondary electron microscopy (SEM) an transmission electron microscopy (TEM) were used to study the material properties, including chemical composition, morphology and crystallinity. When the SnO2 thin film is exposed to the gas mixture of CO and dry air, superoxide ion(O2-) and peroxide ion (O22-) adspecies on the SnO2 sensor can be reduced by CO, and the lattice oxygen can also be reduced at high temperatures forming oxygen vacancies in the SnO2 lattice. These surface reactions increase the electron concentration of the SnO2 thin film exposed to toward carbon the CO gas mixture. A thinner SnO2 thin film has a higher sensing response because of a larger volume ratio of the depletion zone to the thin film. PdO decoration greatly increases the sensing response of the SnO2 thin film toward CO. The PN junction formed between SnO2 and PdO modifies the electrical properties of the SnO2 thin film both before and after the CO gas sensing, resulting in an improved sensing performance. According to XPS analyses, the chemical state of the SnO2 thin film varies trivially after the CO sensing. Upon the CO exposure at 150℃and above, oxygen vacancies are formed in SnO2, leading to the increase in the conductivity of the SnO2 sensor. PdO is reduced by CO producing Pd nanoislands, and the conductivity of the SnO2 sensor significantly drops because oxygen can be dissociatively adsorbed on Pd nanoislands. Pd nanoislands can be reoxidized at 150℃as they are grown to a critical size, thereby alleviating the reduction in the sensing current. At temperatures above 150℃, the conductivity reduction as a result of the Pd nanoisland formation becomes insignificant because the higher temperature result in a faster reoxidation rate for Pd nanoislands.