Catalyst-doped ZnO nanowire networks for highly selective gas sensors
- 주제(키워드) Gas Sensors , ZnO , Catalyst-doped , Nanowires
- 발행기관 고려대학교 대학원
- 지도교수 이종흔
- 발행년도 2016
- 학위수여년월 2016. 2
- 학위구분 박사
- 학과 대학원 신소재공학과
- 원문페이지 135 p
- 실제URI http://www.dcollection.net/handler/korea/000000065140
- 본문언어 영어
- 제출원본 000045867149
초록/요약
Metal oxide semiconductor gas sensors can be applied to detect harmful, environmental, explosive and toxic gases that cause air pollution and various diseases to human body. However, sensitive and selective detection of a specific gas with highly reproducible characteristics still remains a challenge for real applications. In the present study, highly sensitive and selective gas sensors were designed by doping Mo, Co, and Ni to ZnO nanowire (NW) networks. Highly selective and reversible H2S gas sensor was prepared by doping ZnO NWs with Mo. The Mo doped ZnO NW networks were prepared by coating the MoS2 layer on the ZnO NWs through successive ionic layer adsorption and reaction (SILAR) method and its subsequent heat treatment at 600 °C for 2 h. The resulting Mo-doped ZnO NW network sensor demonstrated a high gas response (S = resistance ratio) of 14.11 to 5 ppm H2S at 300 °C, which is 7.66–11.47 times higher than those of 5 ppm C2H5OH, NH3, HCHO, CO, H2, o-xylene, benzene, toluene, and trimethylamine. The reversibility of the sensor signal, as well as the response and selectivity to H2S, is significantly improved by Mo doping into the ZnO NW networks, which can open various applications to monitor H2S in indoor atmosphere, industrial petroleum/gas, and automobile exhaust gas. Both morphological modification of NWs and doping of catalytic material were carried out by multi-step vapor-phase reactions to design highly selective Volatile-Organic-Compounds (VOCs) sensors. Co-doped branched ZnO nanowires were prepared by multi-step vapor phase reactions for the ultraselective and sensitive detection of p-xylene. Highly crystalline ZnO NWs were transformed into CoO NWs by thermal evaporation of CoCl2 powder at 700 °C. The Co-doped ZnO branched NWs were grown subsequently by thermal evaporation of Zn metal powder at 500 °C using CoO NWs as catalyst and sacrificial template. The response of Co-doped branched ZnO NW network sensor to 5 ppm p-xylene at 400 °C was 19.55, which was significantly higher than those to 5 ppm toluene, C2H5OH and other interference gases. The selective and sensitive detection of p-xylene, particularly distinguishing among benzene, toluene, and xylene with lower cross-responses to C2H5OH, can be attributed to the tuned catalytic activity of Co components, which induces preferential dissociation of p-xylene into more active species. Finally, Branched ZnO NWs doped with Ni were grown by a three-step vapor phase method for the sensitive and selective detection of p-xylene. ZnO NWs were directly grown on sensor substrates with Au electrodes, which were transformed into NiO NWs by the thermal evaporation of NiCl2 powder at 700 °C. Subsequently, brush-like branched ZnO NWs doped with Ni were grown from NiO NWs by the thermal evaporation of Zn metal powder at 500 °C. The stem NiO NWs played the role of catalyst for the growth of ZnO branches through vapor-liquid-solid mechanism. The Ni-doped branched ZnO NWs showed enhanced gas response to methyl benzenes, especially to 5 ppm p-xylene (S = 42.44) at 400 °C. This value is 1.7 and 2.5 times higher than the responses to 5 ppm toluene (S = 25.73) and C2H5OH (S = 16.72), respectively, and significantly higher than the cross-responses to other interfering gases such as benzene, HCHO, C2H5OH, trimethylamine, H2, and CO. The selective detection of xylene was attributed to the catalytic role of the Ni component. This novel method to form catalyst-doped hierarchical ZnO NWs provides a promising approach to accomplish superior gas sensing characteristics by the synergetic combination of enhanced chemiresistive variation due to the increased number of branch-to-branch Schottky barrier contacts and the promotion of gas sensing reaction by catalytic dopants.
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TALBE OF CONTENTS
ABSTRACT i
TABLE OF CONTENTS v
LIST OF FIGURES viii
LIST OF TABLES xv
LIST OF SYMBOLS xvi
Chaper 1. Introduction 1
Chaper 2. Theoretical Background 5
2-1. Basic mechanism of metal oxide semiconductor gas sensors 5
2-2. Gas sensors using metal oxide semiconductor nanowires 10
2-3. Enhancement of gas sensing characteristics 15
2-4. p-type metal oxide catalysts 20
Chapter 3. Experimental 23
3-1. Preparation of sensing materials 23
3-1-1. Preparation of Mo-doped ZnO nanowires 23
3-1-2. Preparation of Co-doped branched ZnO nanowires 26
3-1-3. Preparation of Ni-doped branched ZnO nanowires 29
3-2. Characterization 32
3-3. Gas sensing measurements 33
Chapter 4. Results & Discussion 34
4-1. Highly selective, sensitive, and reversible H2S sensors using Mo-doped ZnO nanowire networks 34
4-1-1. Phase and morphology analyses 34
4-1-2. H2S sensing characteristics 46
4-1-3. Dual role of Mo dopant in gas sensing characteristics: selectivity and full recovery 54
4-2. Co-Doped Branched ZnO Nanowires for Ultraselective and Sensitive Detection of Xylene 57
4-2-1. Structural, composition and morphological properties 57
4-2-2. p-xylene sensing characteristics 68
4-2-3. Role of Co catalyst doping and branch growth 74
4-3. Highly selective and sensitive xylene sensors using Ni-doped branched ZnO nanowire networks 80
4-3-1. Phase, composition and morphology analyses 80
4-3-2. Methyl benzene sensing characteristics 90
4-3-3. Role of Ni dopant and branch growth 97
Chapter 5. Conclusions 101
References 104
