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研究生：	張家銘 Chang, Chia-Ming
論文名稱：	氧化鋅奈米棒陣列之合成與改質及其氣體感測特性 Preparation of ZnO Nanorod Arrays and Surface Modification for Gas Sensing Application
指導教授：	洪敏雄 Hon, Min-Hsiung
學位類別：	博士 Doctor
系所名稱：	工學院 - 材料科學及工程學系 Department of Materials Science and Engineering
論文出版年：	2013
畢業學年度：	101
語文別：	中文
論文頁數：	158
中文關鍵詞：	水溶液法、氧化鋅奈米棒陣列、表面修飾改質、氣體感測元件、光化學還原法
外文關鍵詞：	Aqueous chemical growth, ZnO nanorod arrays, Surface modification, Gas sensing device, Photochemical reduction
相關次數：	點閱：128 下載：4
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本論文的主軸是以各種化學溶液法(水溶液法、水熱法、光化學還原法)分別進行合成與表面修飾，於低溫下成長之高表面積比與高活性表面之氧化鋅奈米棒陣列，以製作各種還原性氣體感測元件。主要的目標為提高氣體之敏感度(sensitivity)、降低元件操作溫度(operating temperature)與偵測極限(detection limit)、減少反應以及回復時間(response and recovery time)和提高特定氣體之選擇性(selectivity)等。首先利用不同成長特性之化學溶液法或改變預沉積晶種的熱處理溫度，探討所成長氧化鋅奈米棒陣列的表面或內部缺陷特性及其所衍生的相關電特性對於氣體感測特性之影響。以強鹼水溶液溶蝕-成長交互作用的沉積合成富含表面缺陷(或能態密度)之氧化鋅奈米棒陣列，並以光致螢光光譜和其接面電子特性驗證。結果顯示，具有較多缺陷能階之氧化鋅，其表面有較高的載子濃度提供氣體感測過程中氧氣吸附之用，隨後反映在元件的電阻變化量上，因此乙醇氣體的敏感度即可藉由表面缺陷(氧空缺)數量的增進而提升。
接著，以水熱法將金奈米粒子修飾氧化鋅奈米棒以提升元件表面的活性，透過適量奈米顆粒催化分解氧氣幫助其化學性吸附(溢流)作用，有效提高表面吸附氧的數量，進而增進元件對於乙醇氣體的感測敏感度約18倍；再加上由於不同氣體與吸附氧之間感測效率(電子捐輸作用)的差異，提高了氧化鋅奈米棒陣列對於乙醇/一氧化碳氣體感測的選擇性(selectivity)約12倍。過程中不同密度與大小之金奈米顆粒可藉由前驅物濃度加以控制，且可由結晶型態推論水熱法合成金奈米粒子的成長機制，並探討顆粒大小與分散性對於一氧化碳氣體感測的影響。隨後，藉由光化學還原的水溶液製程，在室溫下將鈀奈米顆粒修飾氧化鋅奈米棒陣列，以進一步增進一氧化碳氣體感測性能。金屬鈀的溢流效應提升氧化鋅表面化學吸附氧的數量達3倍以上，此化學敏化作用分別減少不同濃度一氧化碳氣體感測的反應與回復時間約4~12和1~2倍，並降低感測溫度至260 °C。而鈀粒子在氣體感測的過程中會產生氧化還原耦合對，使得鈀與氧化鋅的界面生成一額外的蕭基式能障，顯著地提高元件的背景電阻並增大元件與氣體反應後的電阻變化量，提升一氧化碳氣體的敏感度5.6倍且降低該氣體的偵測極限至10 ppm。
此外，利用具有螯合功能與自組裝特性的聚乙烯吡咯烷酮(PVP)作為載體，發展新穎的PVP輔助光化學還原法，除了能使受到光電子所吸引的鈀離子以較緩慢的速率移向氧化鋅表面進行還原，其疏水性聚乙烯基間的排斥作用避免奈米顆粒聚集形成團簇，有效調控鈀奈米顆粒於一維氧化鋅表面的生長形態與密度。固定前驅物濃度控制PVP的含量，即可調整鈀奈米粒子於氧化鋅上的沉積密度由14.3到3.1(單位面積, 15700 nm2)。而透過適當數量(14.3)的分離型鈀粒子修飾，元件於260 °C下對於500 ppm氫氣的感測敏感度可達1106，是未添加PVP製程所合成之團聚型鈀修飾元件的553倍。在室溫偵測的部分，其感測敏感度仍可達16.9，說明了此元件在室溫下即對於有爆炸安全疑慮的氫氣具有優異的敏感性。最後也藉由數據的分析以及與相關文獻的探討，提出了不同溫度區間的氫氣感測機制，包括一般200-300 °C的氧氣吸/脫附模式、60-120 °C的表面電導模式以及室溫下(約25-30 °C)的氫化鈀(PdHx)轉換模式。

The main target in this dissertation is aimed at fabricating high-performance gas sensors based on the ZnO nanostructuers with high surface area and active surface configuration. For this object, it is devoted to raise the gas sensitivity, sensing kinetics, selectivity to specific analyte and lower the operating temperature via several wet chemical processes composed of alkali solution growth, hydrothermal route and photochemical reduction. The substantial proceedings include fabrication and surface modification of ZnO nanorod arrays (NRs) as well as analysis of relevant physical and chemical properties, and sequentially put them to different gases sensing applications.
The ZnO NRs grown with Zn salt/KOH solution have a larger amount of surface defects (or surface states) than that grown with Zn salt/HMTA solution by its dissolution-growth behavior, which is verified by the photoluminescence analysis and the interfacial electrical characteristics between sputtering Au films and ZnO nanorods. It incurs a superior C2H5OH sensitivity in the former structure because more oxygen vacancy induces more free carriers for gas sensing reaction.
The Au nanonanoparticles with different size and density are loaded on the ZnO NRs by tuning the reactant (HAuCl4) concentration within hydrothermal procedure; moreover, the growth behavior and mechanism of those Au nanocrystals on ZnO surface are proposed and clearly elucidated. The enhancing surface activity may increase the number of chemisorbed oxygen on ZnO NRs by adequate Au decoration, and then the sensitivity to 600 ppm C2H5OH can be promoted higher than 18-fold. Furthermore, the selectivity for 600 ppm C2H5OH/CO (Sethanol/Scarbon monoxide) also can be enhanced more than 12-fold due to the discrepancy in electron donating effect of different analytes. In order to improve the CO gas sensing property, the Pd nanoparticles are attached on the surface of ZnO nanorods by photochemical deposition (PCD) at room temperature with ultraviolet (UV) light. After that, the sensitivity to 600 ppm CO of Pd decorated ZnO NRs (Pd/ZnO NRs) is increased about 6 times as compared with the pristine ZnO NRs, and the detection limit is reduced to 10 ppm. This improvement is ascribed to an enlargement of the difference in Schottky barrier height between Pd-ZnO while the transition between metallic Pd and redox couple is accompanied by work function variation. Besides, the number of chemisorbed oxygen on Pd/ZnO NRs is promoted more than 3-fold because of the catalytic Pd acting as a promoter of oxygen adsorption on the ZnO surface by spillover effect, which facilitates the sensing reaction and consequently decreases the response/recovery time as well as the optimum operating temperature.
For the sake of resolving the issue of Pd aggregation on ZnO surface, the PVP polymer (polyvinylpyrrolidone) possessed of chelating property and self-assembly characteristic (or called spatial effect) is introduced to the PCD. Thus one-step decoration of nearly monodispersed Pd nanoparticles with controllable density (14.3-3.1/unit area) on ZnO nanorods can be achieved by tailoring the mole ratio of reactant (PdCl2) to PVP (from 45 to 3) in this unique PVP-mediated photochemical reduction. It is demonstrated that the electron sensitization related to the transition of redox couple predominates the H2 sensing mechanism of Pd/ZnO NRs at 200-300 °C. As a consequence, the gas sensitivity to 500 ppm H2 of Pd/ZnO NRs is remarkably improved by around 553-fold (Ra/Rg=1106) at 260 °C through decorating 14.3 (/unit area) discrete Pd nanoparticles instead of the Pd clusters. Furthermore, the corresponding sensitivity at room temperature is 16.9 that is superior to some promising devices like Pd decorated multilayer graphene nanoribbon (Pd-MLGN) or Pd nanotube arrays, which manifests the outstanding H2 sensing performance in the specific Pd/ZnO NRs.Therefore, the present Pd/ZnO NRs have a giant potential and advantage for being applicable to the H2 gas sensor even at the room temperature. Eventually, the diverse H2 sensing mechanisms are proposed based on the Pd density dependence of sensitivity variation in those Pd/ZnO NRs at different temperature regions.

總目錄
摘要	I
Abstract	III
致謝	V
總目錄	VI
圖目錄	IX
表目錄	XVII
中英名詞與符號對照表	XVIII
第一章 緒論	1
1-1引言	1
1-2研究動機與目的	4
第二章 理論基礎與文獻回顧	7
2-1氣體吸/脫附理論[14]	7
2-1-1 物理性吸附	7
2-1-2化學性吸附	9
2-1-3吸附等壓線	11
2-2化學阻抗感測器工作原理	14
2-2-1典型氣體感測模式	14
2-2-2氣體感測元件效能參數	20
2-2-3提升氣體感測效能	21
2-2-4電子敏化作用	22
2-2-5化學敏化作用	25
2-2-6表面修飾金屬奈米顆粒之製程	27
2-3 ZnO的性質與製程技術	30
2-3-1 ZnO的晶體結構與性質	30
2-3-2一維ZnO奈米結構	31
2-3-3一維ZnO奈米結構的合成方法	33
2-3-4水溶液合成一維氧化鋅奈米結構	34
2-3-5 ZnO基氣體感測元件	37
第三章 實驗步驟與方法	41
3-1 實驗流程	41
3-2 實驗藥品	42
3-3 基板製備	43
3-4水溶液法合成ZnO 奈米棒陣列結構	45
3-4-1 種晶層製備	45
3-4-2水溶液法的製程設備、實驗步驟與參數設定	45
3-5 水熱法沉積Au奈米顆粒於ZnO奈米棒陣列	46
3-5-1水熱法設備、合成步驟與參數設定	46
3-6 光化學還原法沉積Pd奈米粒子於ZnO奈米棒陣列	47
3-7材料分析與元件特性量測	49
3-7-1 X光繞射分析(XRD)	49
3-7-2掃描式電子顯微鏡(SEM)與穿透式電子顯微鏡(TEM)	49
3-7-3光致發光光譜(PL)與X光光電子能譜儀(XPS)	49
3-7-4紫外光-可見光光譜儀(UV-Visible)與傅立葉轉換紅外線光譜儀(FT-IR)	50
3-7-5 氣體感測元件特性分析	50
第四章 結果與討論	53
4-1 ZnO奈米棒陣列表面缺陷特性及其對乙醇感測之影響	53
4-1-1 ZnO種晶薄膜與奈米棒陣列之形貌與晶體結構	53
4-1-2 ZnO種晶薄膜與奈米棒陣列之乙醇氣體感測特性	58
4-1-3 ZnO奈米棒陣列之光學特性分析	59
4-1-4 ZnO奈米棒陣列之電特性及相關感測機制	63
4-2 ZnO奈米棒陣列之結構與缺陷特性及其對一氧化碳感測的影響	66
4-2-1 種晶熱處理溫度對奈米棒陣列之表面形貌的影響	67
4-2-2 種晶熱處理溫度對奈米棒陣列之結構與發光特性的影響	68
4-2-3 種晶熱處理溫度對奈米棒陣列電特性之影響	73
4-2-4 種晶熱處理溫度對奈米棒陣列一氧化碳氣體感測的影響	74
4-3 Au奈米顆粒修飾ZnO奈米棒陣列之氣體感測特性	80
4-3-1 金氯酸濃度對於Au/ZnO奈米棒陣列表面形貌的影響	81
4-3-2 金氯酸濃度對於Au/ZnO奈米棒之組成與晶體結構的影響	83
4-3-3 金氯酸濃度對於Au/ZnO奈米棒陣列之一氧化碳感測的影響	88
4-3-4 Au/ZnO奈米棒陣列應用於不同還原性氣體的感測特性	93
4-4 Pd奈米顆粒修飾ZnO奈米棒陣列之氣體感測特性	96
4-4-1 Pd奈米顆粒修飾於奈米棒陣列之表面形貌及其成長機制	97
4-4-2 Pd奈米顆粒修飾於ZnO奈米棒之晶體結構與表面化學特性	100
4-4-3 Pd奈米顆粒修飾奈米棒陣列之一氧化碳感測性能與機制	105
4-5 Pd修飾密度對於ZnO奈米棒陣列之氫氣感測特性	110
4-5-1 PVP數量對於Pd/ZnO奈米棒陣列之表面形貌與Pd修飾密度的影響	111
4-5-2 PVP輔助光化學還原製備Pd/ZnO奈米棒之光學特性分析	119
4-5-3 PVP輔助光化學還原製備Pd/ZnO奈米棒陣列之成長機制	121
4-5-4 Pd密度對於Pd/ZnO奈米棒陣列之氫氣感測效能的影響與感測機制探討	125
第五章 結論	136
第六章 參考文獻	139

 
圖目錄
Fig. 2-1 The five types of physisorption isothermas[16]. (a) Langmuir isotherm-adsorption limited to a single monolayer, (b) BET approximation- mutilayer adsorption, (c) interaction between first layer and substrate is much weaker than the interaction between the second and first layer, (d) similar to Langmuir isotherm, but with two limitations of the volume adsorbed and (e) combines low sorbent/adsorbate interaction (III) and limited adsorption (I).	10
Fig. 2-2 The energy of the system versus distance of the adsorbate from the surface：(a) adsorption as molecule, (b) adsorption as two atoms [14].	10
Fig. 2-3 The adsorption isobar (adsorption at constant pressure): (a) physisorption, (b) nonequilibrium chemisorption where the volume adsorbed depends on history, and (c) equilibrium chemisorption [14].	13
Fig. 2-4 Receptor and transducer function of a semiconductor gas sensor: (a) surface, providing the receptor function, (b) microstructure of the sensing layer, providing the transducer function, and (c) element, enabling the detection of the change in output resistance of the sensing layer, here deposited on an interdigital microelectrode [18].	16
Fig. 2-5 Simplified model illustrating band bending in a wide band gap semiconductor after chemisorption of charged species (here the ionosorption of oxygen) on surface sites.  ,  , and and   denote the energy of the conduction band, valence band, and the Fermi level, respectively, while   denotes the thickness of the space-charge layer, and   the potential barrier. The conducting electrons are represented by   and + represents the donor sites [18].	16
Fig. 2-6 Energy diagram of various oxygen species in the gas phase, adsorbed at the surface and bound within the lattice of a binary metal oxide [24].	19
Fig. 2-7 Structural and band model showing the role of intergranular contact regions in determining the conductance over a polycrystalline metal oxide semiconductor: (a) initial state, and (b) effect of CO on   and   for large grains [18].	19
Fig. 2-8 Structural and band model for particles with D ‹‹ 2  leading to the so-called “flat-band”case: (a) the initial state, and (b) the effect of CO on the position of the conduction band  [18].	23
Fig. 2-9 Mechanism of sensitization by metal or metal oxide additives: (a) electronic sensitization, where the additive is an acceptor for electrons and the redox state/chemical potential is changed by reaction with the analyte; (b) chemical sensitization by activation of the analyte (H2) followed by spill over and change of the surface oxygen concentration [18].	23
Fig. 2-10 Crystal structure of wurtzite ZnO [70].	32
Fig. 2-11 Schematic diagram of the ZnO nanowire surrounded by (a) air and (b) ethanol gas [109].	40
Fig. 3-1 The flow chart of experiment for growth of ZnO nanorod arrays and sequential decoration of noble metal in order to fabricate high performance ZnO-based gas sensing device.	41
Fig. 3-2 Schematic diagram of the configuration of the ZnO-based gas sensing device.	44
Fig. 3-3 Schematic illustration of (PVP-mediated) photochemical deposition (PCD) of Pd nanoparticles on ZnO nanorod arrays.	48
Fig. 3-4 Schematic diagram of gas sensing equipment.	52
Fig. 3-5 Schematic diagram of gas detection system.	52
Fig. 4-1 SEM top-view images of (a) ZnO seed layer fabricated by spinning 2.0M sol-gel and annealing at 300 °C for 1hr, (b) ZnO nanorod arrays grown with Zn salt/HMTA solution composed of 0.025 M Zn(NO3)2 and HMTA at 95 °C for 3hr, (c) ZnO nanorod arrays grown with Zn salt/KOH solution by mixing 0.5 M Zn(NO3)2 and 4 M KOH with 1:1 volume ratio at room temperature for 10 min. The insert is cross-sectional view of the ZnO seed layer and ZnO nanorod arrays	55
Fig. 4-2 XRD spectra of the ZnO seed layer, ZnO nanorod arrays grown with Zn salt/HMTA system at 95 °C for 3hr and Zn salt/KOH solution at room temperature for 10 min, respectively.	56
Fig. 4-3 (a) TEM image and (b) the corresponding SAED-pattern of ZnO nanorod fabricated by the Zn salt/KOH solution by mixing 0.5 M Zn(NO3)2 and 4 M KOH with 1:1 volume ratio at room temperature for 10 min.	57
Fig. 4-4 (a) Time dependence of resistance change of ZnO-based sensing devices as the gas ambient is switched from air to air-ethanol (600 ppm) mixed gas and back to air at 300 °C. (b) The variation of resistance in ZnO NRs grown with Zn salt/KOH solution as gas switched from air to various concentrations of ethanol in air (100-600 ppm) as time proceeds at 300 °C.	61
Fig. 4-5 Room-temperature photoluminescent spectra of the ZnO nanorod arrays grown with Zn salt/HMTA and Zn salt/KOH solutions, respectively.	62
Fig. 4-6 I-V curves of M-S contact in the ZnO nanorod arrays grown with Zn salt/HMTA and Zn salt/KOH solutions, respectively. The upper inset is enlarged I–V curves in the low voltage range. The lower insert shows schematic structure configuration of the M-S (Au-ZnO) contact.	65
Fig. 4-7 Schematic diagram of the M-S contacts of the ZnO nanorod arrays grown with Zn salt/HMTA and Zn salt/KOH solutions, respectively. (The  ,  , and and   are respectively the codes for the energy of conduction band, valence band and Fermi level.)	65
Fig. 4-8 HR-SEM images of ZnO nanorod arrays grown with sol-gel seed layer annealed at (a) 300 °C (b) 400 °C (c) 500 °C (d) 600 °C (e) 700 °C for 1 hr. The insert is cross-sectional view of the corresponding ZnO nanorod arrays. (f) XRD spectra of the ZnO seed layer annealed at different temperatures.	70
Fig. 4-9 (a) The variation of diameter, length and density of ZnO nanorods grown on seed layer annealed at various temperatures. (b) Room-temperature PL analysis of ZnO nanorod arrays grown with seed layer annealed at 300 °C~700 °C.	71
Fig. 4-10 The variation of surface area and integrated  /  ratio in ZnO nanorod arrays grown with the seed layer annealed at different temperatures.	75
Fig. 4-11 I-V characteristics of ZnO NRs gas sensors grown on different seed layers annealed at 300 °C~700 °C. (The data are measured by the interdigital electrodes (I. E.) under the sensing elements.)	75
Fig. 4-12 (a) Time-dependent resistance change of different ZnO NRs gas sensors as the ambient gas is switched from air to various concentrations (100 ppm~600 ppm) of air-carbon monoxide mixed gas alternately at 300 °C. (b) The variation in sensitivity with ZnO NRs grown with seed layers annealed at different temperatures (exposed to several concentrations of air-carbon monoxide mixed gas at 300 °C).	76
Fig. 4-13 (a) The XPS of the O 1s core levels obtained from ZnO nanorod arrays prepared with seed layers annealed at 400, 500 and 600 °C. (b) The variation of integrated area ratio of O3 peak and the chemisorbed oxygen area in those ZnO NRs with the seed layer annealed from 300 °C to 700 °C.	79
Fig. 4-14 Top-viewed and cross-sectioned SEM images (BSE inset in the right half) of (a)(b) pristine ZnO nanorod arrays and Au/ZnO NRs produced from (c)(d) 0.125 mM, (e)(f) 0.25 mM, (g)(h) 0.5 mM and (i)(j) 1.5 mM HAuCl4 solution.	82
Fig. 4-15 XRD spectra of the pristine ZnO nanorod arrays and Au/ZnO NRs produced with different concentrations (0.125 mM-1.5 mM) of HAuCl4 solution.	84
Fig. 4-16 Typical TEM and HR-TEM images (insets are the diffraction patterns and Fast Fourier transforms of the corresponding Au or ZnO revealed in the HRTEM images) of the Au/ZnO nanorods prepared at (a)(b) 0.125 mM, (c)(d) 0.25 mM and (e)(f) 0.5 mM HAuCl4 solution.	85
Fig. 4-17 Size distribution histograms of the Au nanoparticles in different Au/ZnO nanorods synthesized via (a) 0.125 mM, (b) 0.25 mM and (c) 0.5 mM HAuCl4 solution, which are extracted from the magnified TEM images.	86
Fig. 4-18 Transient resistance change of pristine ZnO nanorod arrays and various Au/ZnO NRs (fabricated with different HAuCl4 concentrations) under different CO concentrations (100-600 ppm) mixed-air ambient gases at 300 °C. (b) The variation in sensitivity of diverse ZnO-based sensor exposed to different concentrations of CO contained ambient gases.	89
Fig. 4-19 The schematic diagram of spillover effect on the Au/ZnO nanorods fabricated by (a) 0.125 mM, (b) 0.25 mM and (c) 0.5 mM HAuCl4 solution.	91
Fig. 4-20 Transient resistance change of the Au/ZnO NRs fabricated by 0.25 mM HAuCl4 under different (a) H2 and (b) C2H5OH concentrations (100-600 ppm) mixed-air ambient gases at 300 °C. (c) Sensitivity variation of Au/ZnO NRs (0.25mM) exposed to different analytes with various concentrations (100-600 ppm). (d) Transient resistance change (and sensitivity) of pristine ZnO nanorod arrays under different C2H5OH concentrations (100-600 ppm) mixed-air ambient gases at 300 °C.	95
Fig. 4-21 The SEM images of (a) (c) pristine ZnO nanorod arrays and (b) (d) Pd/ZnO NRs fabricated by photochemical reduction in 0.75 mM PdCl2 ethanol solution. The corresponding EDS spectra and BSE images are also shown.	98
Fig. 4-22 The GA-XRD spectra of pristine ZnO nanorod arrays and Pd/ZnO nanorod arrays produced through PCD in 0.75 mM PdCl2 ethanol solution.	99
Fig. 4-23 (a) The low-magnification TEM image and (b) (c) high resolution TEM images of the Pd nanoparticles decorated ZnO nanorod. The insert of (c) is the corresponding electron diffraction pattern of a Pd nanoparticle on the ZnO nanorod produced by PCD with 0.75 mM PdCl2 ethanol solution..	101
Fig. 4-24 The XPS of (a) the O 1s core levels obtained from pristine ZnO nanorod arrays and Pd/ZnO NRs fabricated through PCD in 0.75 mM PdCl2 ethanol solution (b) the Pd 3d core levels acquired from Pd/ZnO nanorod arrays. (The spectra were calibrated using the C 1s core level associated with carbon pollution at 284.6 eV as a reference.)	104
Fig. 4-25 (a) The variation in sensitivity while the pristine ZnO NRs and Pd/ZnO nanorod arrays were exposed to various air-carbon monoxide mixed ambient gases at individual optimal operating temperatures (300 °C and 260 °C). Insert is the transient resistance change of pristine ZnO NRs and Pd/ZnO NRs under different CO concentrations. (b) The response and recovery times to various concentration of CO in pristine ZnO and Pd/ZnO NRs gas sensors. The Pd/ZnO NRs was fabricated through PCD with 0.75 mM PdCl2 ethanol solution.	108
Fig. 4-26 The schematic band diagrams of (a) pristine ZnO NRs and (b) Pd/ZnO nanorod arrays exposed to air and CO gas ambient. (The C. B., V. B. and   are the codes of conduction band, valence band and Fermi level. The D. R. and C. C. denote the depletion region and conduction channel)	109
Fig. 4-27 The SEM images of Pd/ZnO NRs produced by photochemical deposition (PCD) with (a) 0, (b) 4, (c) 16, (d) 30, (e) 60 mg PVP (K30) in 0.25 mM PdCl2-methanol solution. (molar ratio of Pd: PVP = 45 ~ 3) (f) The XRD spectrum of Pd/ZnO NRs fabricated through pristine PCD course.	112
Fig. 4-28 The TEM images of Pd/ZnO nanorods produced by photochemical deposition (PCD) with (a) 0, (b) 4, (c) 16, (d) 30, (e) 60 mg PVP (K30) in 0.25 mM PdCl2-methanol solution. (f) The variation in density and average size of Pd nanoparticles attached on ZnO nanorods with different doses of PVP addition. (The density is defined as the number of Pd nanoparticles attached onto the surface of cylindrical ZnO nanorod with 50 nm diameter and 100 nm length)	115
Fig. 4-29 The histogram of size distribution of Pd nanoparticles attached on ZnO nanorod by PVP- mediated PCD with (a) 4, (b) 16, (c) 30, (d) 60 mg PVP (K30) in 0.25 mM PdCl2-methanol solution.	117
Fig. 4-30 (a) The high-magnification and (b) high-resolution TEM images of Pd nanoparticles decorated on ZnO nanorod by PVP-mediated PCD with 16 mg PVP (K30) with 0.25 mM PdCl2-methanol solution. The insets in figure a and b are the corresponding NBEDs (Nanobeam Electron Diffraction) and FFTs (fast Fourier transform).	118
Fig. 4-31 (a) UV-Vis absorption spectra of the precursor solution (0.25 mM PdCl2-methanol solution) containing different amount of PVP and that consisting of 60 mg PVP is subsequently irradiated with UV for 30 min and 3 hr. (b) FTIR spectra of pure PVP, pristine ZnO nanorods and Pd/ZnO nanorods produced by PCD with different amount (0, 4, 60 mg) of PVP (K30) in precursor solution.	123
Fig. 4-32 Schematic diagrams about the influence of PVP addition on the morphology and dispersion of Pd nanoparticles decorated on ZnO nanorod via photochemical deposition (PCD) processes.	124
Fig. 4-33 (a) The sensitivity variation of distinct Pd/ZnO NRs used for detecting 500 ppm H2 mixed air under different temperature. The D value in the bracket means the average density (per ZnO nanorod with 50 nm diameter and 100 nm length) of Pd nanoparticles decorated on Pd/ZnO NRs fabricated with different amount (4-60 mg) of PVP. (The sensitivity was measured by exposing to 15 min pulses of specific concentration of H2 mixed in air as shown in (b)). (b) The resistance variation of Pd/ZnO NRs fabricated with 4 mg PVP exposed to various concentration (10-600 ppm) of H2 mixed with air at 260 °C. The inset is the corresponding sensitivity variation versus the concentration of H2 mixed with air.	127
Fig. 4-34 The sensitivity variation to the 500 ppm H2 mixed air of the Pd/ZnO NRs fabricated with (a) 0 mg, (b) 4 mg, (c) 16 mg, (d) 30 mg and (e) 60 mg PVP under different operating temperature. (f) The variation of response time in those Pd/ZnO NRs used for sensing 500 ppm H2 from RT to 300 °C (Response time is defined as the time required for the device to reach 90 % of the maximum resistance variation after the sensor is exposed to the given concentration of H2).	128
Fig. 4-35 The resistance variation of the Pd/ZnO NRs (4 mg PVP) versus time when it is exposed to 10 and 600 ppm H2 and air alternately for several cycles at 260 °C.	131


 
表目錄
Table 2-1 Common properties of bulk wurtzite ZnO [70]	32
Table 3-1 The major chemicals used in this study.	42
Table 4-1 The calculated grain sizes of ZnO seed layer annealed at different annealing temperatures	70



                                    

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