奈米碳管 (CNTs)因具有奈米級的結構尺寸，比表面積大、可供給的氣體吸附位置眾多，因而引發優良的氣體吸附特性及許多的應用。例如：當氣體分子吸附在 CNTs表面時將造成電性的改變，同時其室溫下的氣體響應率相當快速，因此，可望成為 CNTs之氣體感測器。為了解 CNTs氣體感測器作用的機制以及研發現狀，本論文將針對目前所蒐集到之CNTs氣體感測器文獻做簡要的回顧與整理。同時，因 CNTs氣體感測前表面上已吸附了許多的氣體分子，使得其氣體感測機制複雜，因此有必要了解 CNTs之氣體吸附現象。本論文第三章節即闡述以壓電石英晶體微量天平 (Piezoelectric Quartz Crystal Microbalance; PQCM)裝置進行 CNTs等溫吸脫附曲線 (Adsorption isotherms)的實驗結果，並探討 CNTs與各種氣體間 (O2, N2, Ar等)之基礎氣體吸附參數 (如：Binding energy及activation energy of desorption)。第四章節介紹 CNTs-coated PQCMs sensor進行 CO, NO2, H2及 N2氣體感測的結果，並提出可能的感測機制。考慮 CNTs未來的應用，本論文第五章節將簡述我們以含錫之有機金屬溶液 (Organometallic solution)有(或無)添加單壁奈米碳管 (SWCNTs)所製備之 SnO2基氣體感測器的製程，並文獻回顧傳統之 SnO2基氣體感測器製程。室溫下評估這些氣體感測器進行 NO2 氣體感測的效果，提出室溫下此混成 SnO2基 (Hybrid CNTs/SnO2)氣體感測器之可能的感測機制。
本研究結果顯示：應用本研究 CNTs-coated PQCM實驗裝置，可進行不同待測氣體之濃度監測，為一新型之 CNTs氣體感測器。此 CNTs-coated PQCM裝置亦可進行 adsorption isotherms實驗，配合程溫脫附 (Temperature programmed desorption)之實驗結果，可估算 O2, N2, Ar等氣體分子吸附於 CNTs上時之基礎物理參數。藉由將奈米碳管與 SnO2混成，本研究以少量之奈米碳管即可提供室溫下操作之氣體感測器且此 hybrid CNTs/SnO2氣體感測器於室溫下具有高靈敏度及快速回復等特性，為一新穎且簡易之氣體感測器製造方法。

Carbon nanotubes (CNTs) have the nanostructured size and large surface area that can provide the excellent absorption properties of gases. These extreme absorption properties make CNTs advantage to use in many areas of applications. For example, the gas absorption of CNTs at room temperature will change its electric properties with fast response time, which can enable CNTs as a good candidate of gas sensing material. For purpose to develop the CNTs gas sensors, the literature reviews will emphasize understanding the gas sensing mechanisms and recent researches of CNTs. When the CNTs are applied to a gas sensor, the gas sensing behaviors of CNTs are included the absorption of several gases in the same time that complicates the sensing mechanism. Therefore, to simplify the absorption properties of CNTs, the piezoelectric quartz crystal microbalance (PQCM) is used to examine the gas adsorption of CNTs, which is detailed in Chapter 3 of this thesis. Adsorption isotherm curves and desorption energies will be applied to evaluate the properties of CNTs. Furthermore, in Chapter 4 of this thesis, the gas-sensing behaviors of CNTs-coated PQCM sensor are also investigated using CO, NO2, H2 and N2 in air and the detecting temperature was from room temperature to 200 ℃. The mechanism of gas sensing is also proposed. For CNTs application, in fifth Chapter of this thesis, three types of sensors, marked as blank sensor, hybrid sensor A and hybrid sensor B, respectively, were fabricated to investigate the effects of the addition of single-walled carbon nanotubes (SWCNTs) on the properties of hybrid CNTs/SnO2 sensors, and examined their gas sensing properties on NO2 gas at room temperature. The sensing mechanism of the hybrid CNTs/SnO2 sensors is also discussed. Moreover, the fabrication-process and sensing-mechanism of conventional SnO2 gas sensors is reviewed.
The results show that the SWCNT-coated PQCM can be feasible used to study the gas-sensing properties of SWCNTs, because of their stability and sensitivity. This study has demonstrated that the PQCM provides a simple and elegant method for directly measuring adsorption isotherms and isosteric heats of adsorption of O2, N2, and Ar gases on SWCNTs near room temperature. From the TPD (Temperature Programmed Desorption) results reveal that the Ed values of these gases are very close to their isosteric heats of adsorption. These results imply that these gases are physisorbed onto SWCNTs. A new hybrid CNTs/SnO2 gas sensor has been developed in this study with high sensitivity and good recovery properties in detecting NO2 gas at room temperature. Moreover, the preparation process of the hybrid CNTs/SnO2 sensor is a simple method.

CONTENTS
中文摘要 …………………………………………………………………………………..I
ABSTRACT III
ACKNOWLEDGEMENTS V
CONTENTS VI
LIST OF TABLES IX
LIST OF FIGURES XI
Chapter 1 Introduction 1
Chapter 2 Literature Review 4
2.1 Introduction of carbon nanotubes 4
2.1.1 The forms of carbon nanotubes 4
2.1.2 Growth mechanism of carbon nanotubes 7
2.1.3 Production of carbon nanotubes 8
2.2 Adsorption binding energy and activation energy of gaseous molecules on carbon nanotubes 11
2.2.1 Binding energy obtained by theoretical calculation 11
2.2.2 Binding energy obtained by experimental method 16
2.2.2.1 The adsorption isotherm 17
2.2.2.2 Measuring adsorption isotherms by static volumetric methods 18
2.2.3 Literature review about measuring isotherms by TPD method 21
2.3 Application of CNTs in gas sensor 21
2.3.1 CNT gas sensors functioning through their electrical properties 21
2.3.2 CNT PQCM gas sensors 40
Chapter 3 Binding Energy and Activation Energy of Desorption of Gaseous Molecules on SWCNTs 42
3.1 Basic theory 42
3.1.1 Measuring binding energy by PQCM 42
3.1.1.1 Sauerbrey theory of PQCM 42
3.1.1.2 BET and related isotherms 44
3.1.1.3 Determination heat adsorption and binding energy from adsorption isotherms 45
3.1.2 Measuring activation energy of desorption by TPD 47
3.1.2.1 Activation energy 47
3.1.2.2 Activation energy of desorption determined from TPD method 49
3.2 Experiments 50
3.2.1 PQCM tests 50
3.2.2 BET tests 52
3.2.3 TPD tests 54
3.3 Results and discussion 56
3.3.1 Binding energy & PQCM 56
3.3.1.1 Testing the stability of the PQCMs before measuring the adsorption isotherms 56
3.3.1.2 Relating frequency response of PQCMs to measurement of isotherms 57
3.3.1.3 Adsorption isotherms obtained by PQCMs 59
3.3.2 Isotherm & BET 65
3.3.2.1 Comparison between adsorption characteristics obtained using BET at —196 ℃ and PQCM at around room temperature 65
3.3.3 Activation energy & TPD 69
3.3.3.1 Cleaning the surface of SWCNTs 69
3.3.3.2 Measuring the gas-desorption properties of SWCNTs 70
3.4 Conclusion 75
Chapter 4 Examining Gas-sensing Behaviors using PQCMs 77
4.1 Practically important sensor parameters 77
4.2 Experiments 77
4.3 Results and discussion 79
4.3.1 Stability of sensor devices 79
4.3.2. Adsorption weight analysis using PQCMs 81
4.3.3. Frequency versus time evolution curve 82
4.3.4. Responses of SWCNT-coated PQCM sensors in various gaseous atmospheres 84
4.4 Conclusion 95
Chapter 5 Preparation and Properties of a Novel Hybrid CNTs/SnO2 Gas Sensor 96
5.1 Motivation 96
5.2 Literature review 97
5.2.1 Preparation of the SnO2 gas sensors 97
5.2.2 Basic sensing mechanisms of the SnO2 gas sensors 101
5.3 Experiments 103
5.3.1 Preparing of gas sensors 103
5.3.2 Gas sensing tests 106
5.4 Results and discussion 108
5.4.1 Phase and morphology examination of the sensors 108
5.4.1.1 Phase identification 108
5.4.1.2 Morphology observations 110
5.4.1.3 Efforts on finding out where is the CNTs 111
5.4.2 Sensor response and performance 115
5.4.2.1 Sensors responses to NO2 115
5.4.2.2 Sensors performance to NO2 116
5.4.3 Gas sensing mechanism of the hybrid CNTs/SnO2 gas sensors 119
5.5 Conclusion 122
Chapter 6 Prospect of Future Study 124
References ……………………………………………………………………………….125

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