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研究生：	徐煥炫 Hsuan, Huan
論文名稱：	氧化鋅薄膜感測VOCs之研究 Sensing of VOCs with ZnO Thin Films
指導教授：	王鴻博 Wang, H. Paul
學位類別：	碩士 Master
系所名稱：	工學院 - 環境工程學系 Department of Environmental Engineering
論文出版年：	2007
畢業學年度：	95
語文別：	英文
論文頁數：	100
中文關鍵詞：	半導體薄膜、氧化鋅、氣體感測器、EXAFS 、XANES 、Cl-VOCs
外文關鍵詞：	Self-assembling, ZnO, gas sensors, Cl-VOCs, XANES, EXAFS
相關次數：	點閱：114 下載：3
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含氯揮發性有機物(Cl-VOCs)對人體及環境均會造成嚴重的危害，因此發展一套即時、連續而且利於攜帶之感測設備，可以防止Cl-VOCs所可能帶來的危害。雖然ZnO薄膜感測器已被廣泛研發及應用，但針對Cl-VOCs之感測特性相對未被重視，而且ZnO薄膜感測Cl-VOCs時的機制並未完全了解，因此，本研究之主要目的包括：(1)合成氧化鋅(ZnO)薄膜；(2)合成Fe/ZnO (Fe-doping ZnO) 及 Al/ZnO (Al doped ZnO) 薄膜並研究其對C2H5OH之靈敏度及再現性；(3)研究ZnO薄膜感測Cl-VOCs (CCl4、CHCl3、CH2Cl2、chlorophenols及chlorobenzenes)及C2H5OH之靈敏度及再現性；(4)分析感測CCl4及 C2H5OH時薄膜之鋅之即時精細結構分析；(5)設計即時感測系統。
實驗結果顯示，自組合產生之ZnO、Fe/ZnO及Al/ZnO薄膜，表面大部分為排列良好之奈米粒子。對C2H5OH之感測時間(response time)皆低於2分鐘，而且具有可接受之靈敏度(Rair/Rethanol) (>10)及再現性。ZnO薄膜對C2H5OH 之感測電阻之變化與Cl-VOCs相反。ZnO薄膜對其他少氯之Cl-VOCs (CHCl3或CH2Cl2)之感測度類似CCl4，靈敏度隨含氯量之減少而減少。即時X射線吸收近邊緣結構(X-ray absorption near edge structural (XANES))光譜顯示ZnO薄膜中鋅之主要物種為奈米ZnO(95%)及ZnO(5%)，當通入200 ppm C2H5OH時，發現少量Zn(OH)2 (7%)生成；當通入1000 ppm CCl4後發現少量Zn(5%)，兩者均消失於停止通入氣體後。X射線吸收光譜之延伸區微細結構(extended X-ray absorption fine structural (EXAFS))光譜顯示ZnO薄膜中鋅之Zn-O鍵距為1.92 Å，當通入200 ppm C2H5OH 後Zn-O鍵距微幅增加至1.93 Å；當通入1000 ppm CCl4後Zn-O鍵距微幅減少至1.90 Å，兩者也於停止通入氣體後均恢復至1.92 Å。依據上述實驗數據也初步設計一套Cl-VOCs感測系統，成為一種可以在可能具Cl-VOCs存在風險之場所之個人防護器具。

The hidden danger of Cl-containing volatile organic compounds (Cl-VOCs) may exacerbate the environment and human health. ZnO thin films have been widely used in sensing of Cl-VOCs. However, speciation of thin films during sensing gases, especially for the Cl-VOCs, is still lacking in the literature. Thus, the main objectives of this work were (1) Synthesis of ZnO, Fe/ZnO and Al/ZnO thin films, (2) Determination of sensitivity, response time, and reproducibility of the ZnO, Fe/ZnO, and Al/ZnO thin films with C2H5OH, (3) Determination of sensitivity, response time, and reproducibility of the ZnO thin films with Cl-VOCs, (4) Speciation studies of zinc in the ZnO, Fe/ZnO, and Al/ZnO thin films, and (5) Conceptual design of a ZnO thin film based sensor for sensing of Cl-VOCs as well as C2H5OH and phenols.
Experimentally, it is found that the ZnO, Fe/ZnO, and Al/ZnO thin films are consisted of nanosize particles that are packed closely and well-distributed. The mean particle size in the thin film is about 100 nm. The nanosize ZnO, Fe/ZnO, and Al/ZnO thin films have a good sensitivity (Rair/RC2H5OH) (>10) to C2H5OH with a short response time (about 2 min). A good reproducibility for the ZnO thin film in sensing of C2H5OH vapor has also been observed. During sensing of Cl-VOCs at 333 K, the ZnO thin films also posses a high sensitivity, low response time and good reproducibility.
By XANES (X-ray absorption near edge structure) spectroscopy, during sensing of 1000 ppm of CCl4, nanosize ZnO (90%) and a small amount of metallic zinc (5%) have been observed on the ZnO thin film. When sensing of 200 ppm C2H5OH, in addition to nanosize ZnO (88%), Zn(OH)¬2 (7%) was found on the thin film. The EXAFS data also shows that as CCl4 is introduced onto the thin film, the bond distance of Zn-O is decreased slightly from 1.92 to 1.91 Å, which may be due to the fact that electrons of oxygen on the ZnO surfaces are withdrawn by the absorbed CCl4 (Zn-O…>Cl…>Cl4). On the contrary, the Zn-O bond distance is increased slightly from 1.92 to 1.93 Å in the presence of 200 ppm of C2H5OH. Interaction of C2H5OH with zinc (C2H5(H)O…>ZnO) on the surfaces of ZnO may occur. A simple, cheap, miniaturized and portable device has, therefore, been designed based on experimental data for sensing of Cl-VOCs as well as C2H5OH and phenols.

中文摘要		I
ABSTRACT	II
致謝	III
CONTENT	IV
LIST OF TABLES	VI
LIST OF FIGURES	VII

CHAPTER 1 INTRODUCTION	1

CHAPTER 2 LITERATURE SURVEY	3
2.1 Cl-VOCs	3
  2.1.1 CH2Cl2	3
  2.1.2 CH3Cl3	3
  2.1.3 CCl4	4
2.2 The Transparent Conducing Oxide Thin-Films	5
  2.2.1 Basic Properties of TCO Thin Films	5
2.3 Zinc Oxide Thin Films	8
  2.3.1 Doping	8
    2.3.1.1 Doping theory	8
    2.3.1.2 Quantum mechanics of doping	9
2.4 Gas Sensors	11
  2.4.1 Semiconducting Transparent Thin-Film Gas Sensors	11
  2.4.2 Adsorption theory	13
  2.4.3 Sensing Mechanisms	13
  2.4.4. Factors Affect the Sensing Behaviors	19
  2.4.5 Chlorinated Methanes Detected with TCO Thin Films	19
2.5 Self Assembling Method	24
  2.5.1 Sol gel	24
  2.5.2 Dip coating	24
CHAPTER 3 EXPERIMENTAL METHODS	28
3.1 The Experimental Design	28
3.2 Preparations of Thin Films	30
  3.2.1 Preparation of ZnO sol gels	30
  3.2.2 Preparation of Fe/ZnO sol gels	30
  3.2.3 Preparation of Al/ZnO sol gels	30
  3.2.4 Dip coating	30
3.3 Characterization of the Thin Films	33
  3.3.1 X-Ray Diffraction Spectroscopy (XRD)	33
  3.3.2 Surface Morphology Measurements	33
3.4 Sensitivity Measurements	33
3.5 In-situ X-ray Absorption Spectroscopy (XAS)	35

CHAPTER 4 RESULTS AND DISCUSSION	39
4.1Chemical structure of zinc in the ZnO thin films during sensing of C2H5OH	39
4.2 Sensing of C2H5OH on Fe/ZnO and Al/ZnO Thin Films	48
  4.2.1 Sensing of C2H5OH on the Fe/ZnO Thin Films	48
     4.2.2 Sensing of C2H5OH on the Al/ZnO Thin Films	55
4.3Sensing of Cl-VOCs with nanosize ZnO Thin Films	62
4.4Sensing of Chlorophenols with Nanosize ZnO Thin Films	69
4.5A Preliminary Design of Large-scale Real-Time Cl-VOCs Sensing System	76

CHAPTER 5 CONCLUSIONS AND SUGGESTIONS	81

REFERENCES	83

APPENDIXES	90
Appendix A Speciation of Zinc in Nano Phosphor Particulars Abstracted in an Ionic Liquid	90










TABLES

Table 2.1 	Some properties of transparent conducting oxides at room temperature.	6
Table 2.2 	Typical deposition techniques used for the preparation of semiconducing gas sensor.	7
Table 2.3 	Resistance responses expected for oxidizing and reducing gases on n- and p- type semiconducing oxides.	16
Table 2.4 	List of studies on semiconducting metal oxides for detecting chlorinated methanes.	23
Table 4.1.1 In-situ EXAFS data of zinc in the thin film during sensing of 300 ppm C2H5OH at 300 K	47
Table 4.2.1 In-situ EXAFS data of zinc and iron in the Fe/ZnO thin film during sensing of 300 ppm of C2H5OH at 300K ( mL/min)	54
Table 4.2.2 In-situ EXAFS data of zinc in the Al/ZnO thin film during sensing of 300 ppm C2H5OH at 300K ( mL/min)	60
Table 4.3.1 In-situ EXAFS data of zinc in the ZnO thin film during sensing of 1000 ppm CCl4 at 333 K	66
Table 4.4.1 Sensitivities of the ZnO thin film in the presence of Cl-VOCs, phenols, and C2H5OH	73
Table 4.5.1 Key factors that can be used in sensing of Cl-VOCs, C2H5OH, and phenols	79




 
FIGURES

Figure 2.1 		Structure of ZnO: (a)Wurtzite and (b) Zinc blende structure	10
Figure 2.2 	Classification of sensors according to the principle of operation	12
Figure 2.3 	Relative energy level for (a) adsorption without dissociation and (b) dissociative adsorption as a function of distance between adsorbate and absorbent.	12
Figure 2.4 	Barriers at intergranular contacts on a pressed pellet. (a) Three grains with adsorbed oxygen providing surface depletion layers cause a high contact resistance. (b) The corresponding band model for a more quantitative analysis where, for conductance, electrons must cross over the surface barriers.	17
Figure 2.5 	A model of a potential barrier to electronic conduction at a grain boundary.	18
Figure 2.6 	Schematic models for grain-size effects. Hatched parts shows core region (low resistance), while the unhatched part refers to the space-charge region (high resistance).	20
Figure 2.7 	Illustration of layered nano architectures: (a) Reversed “Duckweed” LB film and (b) Self-assembling film with interdigitated structure.	25
Figure 2.8 	Simplified molecular picture of film deposition starting with a positively charged substrate.  The polyion conformation and layer interpenetration are an idealization of the surface charge reversal with each adsorption step[Decher, 1991].	26
Figure 3.1 	Flow chat of experiments.	29
Figure 3.2 	A flowchart for the preparation of the thin films.	31
Figure 3.3 	Dip coater.	32
Figure 3.4 	A schematic diagram of the Cl-VOCs sensing system.	34
Figure 3.5 	Schematic apparatus for in-situ XANES and EXAFS experiments.	37
Figure 3.6　 A schematic illustration of typical X-ray absorption spectroscopic experiment	38
Figure 4.1.1 	X-ray diffraction pattern of the ZnO thin film	41
Figure 4.1.2 	SEM of the ZnO thin film.	42
Figure 4.1.3 	Resistances of the ZnO thin film in the presence or absence of 200 ppm of ethanol at 300 K	43
Figure 4.1.4 	Sensitivity of the ZnO thin film in the presence of 200-500 ppm of C2H5OH at 300 K	44
Figure 4.1.5 	Component fitted XANES spectra of zinc in the (a) ZnO thin film in the (b) presence and (c) absence of C2H5OH (200 ppm) at 300 K.	45
Figure 4.2.1 X-ray diffraction patterns of the Fe/ZnO thin film	49
Figure 4.2.2 	SEM of the Fe/ZnO thin film	50
Figure 4.2.3 	Component fitted XANES spectra of (a) zinc and (b) iron species in the Fe/ZnO thin film	51
Figure 4.2.4 	Resistances of the Fe/ZnO thin film in the presence or absence of 300 ppm C2H5OH at 300 K	52
Figure 4.2.5 	X-ray diffraction patterns of the Al/ZnO thin film.	56
Figure 4.2.6 SEM of the Al/ZnO thin film	57
Figure 4.2.7 Component fitted XANES spectra of zinc species in the Al/ZnO thin film when sensing of C2H5OH	58
Figure 4.2.8 Resistances of the Al/ZnO thin film in the presence or absence of 300 ppm C2H5OH at 300 K	59
Figure 4.3.1 Fig. 4.3.1 Component fitted XANES spectra of zinc species in the ZnO thin film when (a) prior to sensing, (b) in the presence of 1000 ppm CCl4, (c) in the absence of ethanol	64
Figure 4.3.2 	Resistances of the ZnO thin film in the presence of 1000 ppm (a) CCl4, (b) CHCl3, and (c) CH2Cl2 at 333 K.	65
Figure 4.3.3 	Relationship between gas sensitivity and gas concentration for a ZnO thin film ( at 333 K).	68
Figure 4.4.1 	Resistances of the ZnO thin film in the presence of 1000 ppm of (a) chlorophenol, (b) chlorobenzene, and (c) phenol at 333 K.	74
Figure 4.4.2 	Sensitivity of ZnO thin film in the presence of 300-1000 ppm of (a) phenol, (b) chlorophenol, and (c) chlorobenzene	75
Figure 4.5.1	Schematic diagram of a designed real-time Cl-VOCs sensor.	80
                                    

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