Comparative study of gas sensing properties between zno sno2 and zno sno2 nanofibers

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY --- PHAN HONG PHUOC COMPARATIVE STUDY OF GAS SENSING PROPERTIES BETWEEN ZnO/SnO2 AND ZnO-SnO2 NANOFIBERS NGHIÊN CỨU CHẾ TẠO VÀ SO SÁNH ĐẶC

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

-

PHAN HONG PHUOC

COMPARATIVE STUDY OF GAS SENSING PROPERTIES BETWEEN

ZnO/SnO2 AND ZnO-SnO2 NANOFIBERS

NGHIÊN CỨU CHẾ TẠO VÀ SO SÁNH ĐẶC TRƯNG NHẠY KHÍ

GIỮA SỢI NANO ZnO/SnO2 VÀ ZnO-SnO2

MASTER THESIS MATERIAL SCIENCE

Hanoi - 2019

PHAN HONG PHUOC

COMPARATIVE STUDY OF GAS SENSING PROPERTIES BETWEEN

ZnO/SnO2 AND ZnO-SnO2 NANOFIBERS

NGHIÊN CỨU CHẾ TẠO VÀ SO SÁNH ĐẶC TRƯNG NHẠY KHÍ

GIỮA SỢI NANO ZnO/SnO2 VÀ ZnO-SnO2

Major: Material Science

MASTER THESIS MATERIAL SCIENCE

SUPERVISOR

Associate professor Ph.D Nguyen Van Duy

Hanoi - 2019

BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ

Họ và tên tác giả luận văn: Phan Hồng Phước

Đề tài luận văn: Nghiên cứu chế tạo và so sánh đặc trưng nhạy khí

giữa sợi nano ZnO/SnO2 và ZnO-SnO2

Tác giả, người hướng dẫn khoa học và Hội đồng chấm luận văn xác nhận tác giả đã sửa chữa, bổ sung luận văn theo biên bản họp Hội đồng ngày 30 tháng 10 năm 2019 với các nội dung sau:

- Tên đề tài luận văn đã được chỉnh sửa từ “Nghiên cứu chế tạo sợi nano tổ hợp bằng phương pháp phun tĩnh điện ứng dụng cho cảm biến khí” thành:

“Nghiên cứu chế tạo và so sánh đặc trưng nhạy khí giữa sợi nano ZnO/SnO2

Giáo viên hướng dẫn

PGS TS Nguyễn Văn Duy

Tác giả luận văn

Phan Hồng Phước

CHỦ TỊCH HỘI ĐỒNG

PGS TS Nguyễn Phúc Dương

PHAN HONG PHUOC i ITIMS 2017-2019

ACKNOWLEDGMENTS -

I would like to take this opportunity to express my gratitude to all persons who have given me their invaluable support and assistance

I am profoundly grateful to Professor Ph.D Nguyen Van Hieu, Associate professor Ph.D Nguyen Van Duy, Associate professor Ph.D Nguyen Duc Hoa, Ph.D Chu Manh Hung for their scientific advice and insightful discussions

I would like to thank Associate professor Ph.D Pham Anh Son (HUS-VNU), MSc

Ta Ngoc Bach (VAST), MSc Nguyen Quang Hoa (HUS-VNU), MSc Pham Thi Nga (HUS-VNU) for their help in material characterization analysis I am very grateful to my colleague, Ph.D student Nguyen Van Hoang, who has dedicated so much time in helping me during all the time I do my thesis I would like to say great thanks to my classmate, MSc student Tran Thi Mai Phuong who was giving me a lot of supports in two years I am doing my Master's degree at ITIMS

Finally, but not least, I am deeply thankful to my family, my parents, for their love and encouragement I am heavily indebted to my younger brother and my maternal grandmother who passed away while I have to live far from home to do this thesis Dedicated to the memory of my younger brother and my maternal grandfather, who have always believed in my ability to be successful in the academic arena You are gone but your belief in me has made this journey possible I love you so much

Hanoi, 30th September 2019 Phan Hong Phuoc

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COMMITTAL IN THE THESIS -

I confirm that this thesis is the result of my personal research and is solely my own work under the guidance of Associate professor Ph.D Nguyen Van Duy I declare that my scientific results are righteous I have responsibilities for my research results in this thesis

Author

Phan Hong Phuoc

PHAN HONG PHUOC iii ITIMS 2017-2019

TABLE OF CONTENTS

ACKNOWLEDGMENTS i

COMMITTAL IN THE THESIS ii

TABLE OF CONTENTS iii

List of Abbreviations v

List of Figures vi

List of Tables ix

INTRODUCTION 1

Object for study 2

Research objective 2

Research method 2

Organization of the Thesis 3

Chapter 1 LITERATURE REVIEW 4

1.1 Metal oxide semiconductor gas sensors 4

1.1.1 Gas sensors construction 4

1.1.2 Gas-sensing mechanisms 5

1.2 Nanofibers for gas sensors 7

1.2.1 Fabrication of nanofibers 7

1.2.2 Gas sensing mechanisms of nanofibers 10

1.2.3 Review of composite nanofibers for gas sensors 11

1.3 Effect of hetero-junction between ZnO and SnO2 nanofibers on gas sensing properties 13

1.3.1 Zinc oxide and Tin oxide properties 13

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1.3.2 Hetero-junction between ZnO and SnO2 in gas sensing 15

1.4 Research orientation 17

Chapter 2 EXPERIMENTAL 18

2.1 Chemicals preparation 18

2.2 Nanofibers synthesis 18

2.2.1 Preparation of the precursor solution for electrospinning 18

2.2.2 Electrospinning process 19

2.3 Material characterization 22

2.4 Gas sensing measurements 22

Chapter 3 RESULTS AND DISCUSSIONS 25

3.1 Materials characterization 25

3.1.1 Thermogravimetric analysis 25

3.1.2 Morphological observation of nanofibers 26

3.1.3 Compositional and crystal properties of the nanofibers 29

3.2 Gas sensing properties of the nanofibers 34

3.2.1 H2S sensing results 34

3.2.2 NO2 sensing results 39

3.3 Gas sensing mechanisms 41

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK 46

LIST OF PUBLICATIONS 47

REFERENCES 48

APPENDIX 56

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List of Abbreviations

3 FESEM Field Emission Scanning Electron Microscopy

4 HRTEM High-Resolution Transmission Electron Microscopy

5 JCPDS Joint Committee on Powder Diffraction Standards

13 TG-DTA Thermal gravimetric and differential thermal analysis

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List of Figures

Figure 1-1 The scheme of a typical resitive sensor system 4

Figure 1-2 Band bending at an n-type semiconductor surface due to oxygen

absorbtion [23] 5Figure 1-3 The schematic of the on-chip fabrication of NF sensors by electrospinning [34] 7Figure 1-4 The schematic illustration of the effect of increasing the surface charge

on drop deformation [36] 8Figure 1-5 (a) The schematic diagram of the path of an electrospinning jet (b) The schematic illustration of the Earnshaw instability leading to bending of an electrified jet [41],[43] 9Figure 1-6 The schematic of the gas-sensing mechanism of NFs: (a) in air, and (b)

in H2S gas [46] 10Figure 1-7 (a) SEM images of SnO2/In2O3 hetero-NFs, (b) Gas responses of SnO2,

In2O3 and SnO2/In2O3 sensors to 10 ppm formaldehyde as a function of operating temperature [13] 11Figure 1-8 (a) SEM images of SnO2-ZnO NFs (the inset shows the corresponding high magnification images), and (b) Comparison the gas response of ZnO, SnO2-ZnO and SnO2 NFs to 100 ppm ethanol at different operating temperatures (200 °C

- 400 °C) [9] 12Figure 1-9 ZnO unit cell with wurtzite structure [49] 13Figure 1-10 Bulk structures of the SnO2 polymorphs (gray and red colors represent

Sn and O atoms, respectively) [57] 14Figure 1-11 The schematic diagrams of the energy band structure of ZnO and SnO2: (a) before contact, and (b) after-contact 16

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Figure 2-1 The schematic diagram for the preparation process of electrospinning solution 19Figure 2-2 The electrospinning system at ITIMS 20Figure 2-3 The schematic diagram of on-chip fabrication using single jet ZnO-SnO2 NFs (a) and ZnO/SnO2 NFs double jets (b) by electrospinning process 21Figure 2-4 The schematic diagram of heat treatment processes (a), Thermo tube furnace at ITIMS (b) 21Figure 2-5 Gas sensing measurement system at ITIMS 23Figure 2-6 Design layout (a) The gas testing chamber (b), and Keithley 2602A source meter (c) at ITIMS [74] 23Figure 3-1 The TGA-DTG curves of the as-spun (a) ZnO NFs, (b) SnO2 NFs, (c) ZnO/SnO2 NFs, and ZnO-SnO2 NFs (d) 25Figure 3-2 The SEM images of the as-spun ZnO NFs (a) and calcined ZnO (b), as-spun SnO2 NFs (c), and calcined SnO2 NFs (d) 27Figure 3-3 The SEM images of the as-spun ZnO/SnO2 NFs (a), calcined ZnO/SnO2(b) and as-spun ZnO-SnO2 NFs (c), and calcined ZnO-SnO2 NFs (d) 28Figure 3-4 The EDX spectra of ZnO/SnO2 NFs (a,b), ZnO NFs (c), SnO2 NFs (d), and ZnO-SnO2 NFs (e) 30Figure 3-5 The XRD patterns of ZnO/SnO2 NFs (a), ZnO-SnO2 NFs (b), ZnO NFs (c), SnO2 NFs (d), ZnO (JCPDS 36-1451) (e), and SnO2 (JCPDS 41-1445) (f) 31Figure 3-6 The TEM and HRTEM images with corresponding fast Fourier transform (inset) of the ZnO NFs (a,b), and SnO2 NFs (c,d) 33Figure 3-7 The TEM (a) and HRTEM (b) images with corresponding fast Fourier transform (inset) of the ZnO-SnO2 NFs 34

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Figure 3-8 The resistance of NFs at various temperatures (a), gas response of ZnO/SnO2 NFs, ZnO-SnO2 NFs ZnO NFs, SnO2 NFs at 1 ppm H2S at various temperatures (b) 35Figure 3-9 (a) The response time and (b) recovery time of ZnO/SnO2 NFs, ZnO-SnO2 NFs ZnO NFs, SnO2 NFs towards 1 ppm H2S at various operating temperatures 36Figure 3-10 H2S sensing characteristics (a-c) and H2S response at various operating temperatures of ZnO/SnO2 NFs, ZnO-SnO2 NFs, SnO2 NFs, and ZnO NFs (d-f) 37Figure 3-11 Response time (a) and recovery time (b) of ZnO/SnO2 NFs, ZnO-SnO2NFs, SnO2 NFs, and ZnO NFs toward various H2S concentration at 350 °C 38Figure 3-12 Stability of ZnO/SnO2 NFs (a), ZnO-SnO2 NFs (b), SnO2 NFs (c), and ZnO NFs (d) to 1 ppm-H2S gas at 350 °C 39Figure 3-13 NO2 sensing characteristic (a-c) and NO2 response (d-f) at various operating temperatures of ZnO/SnO2 NFs, ZnO-SnO2 NFs, SnO2 NFs, and ZnO NFs 40Figure 3-14 Gas response of ZnO/SnO2 NFs, ZnO-SnO2 NFs ZnO NFs, SnO2 NFs

to 10 ppm NO2 at various temperatures 41Figure 3-15 Gas sensing mechanism of (a) ZnO-SnO2 NFs, and (b) double-jets ZnO/SnO2 NFs 42Figure 3-16 Gas response of ZnO-SnO2 NFs, ZnO/SnO2 NFs, ZnO NFs, SnO2 NFs

to 200 ppm CO, 250 ppm H2, 250 ppm NH3, 5 ppm NO2, and 1 ppm H2S at 350 °C 44

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List of Tables

Table 1-1 Parameters of ZnO and SnO2 [60], [61] 15

Table 1-2 A survey of composite NFs between ZnO and SnO2 based-on gas sensors 17

Table 2-1 Chemicals for NFs synthesis 18

Table 2-2 The precursor composition for NFs deposition 19

Table 3-1 The extracted parameters of samples from XRD patterns 32

Table 3-2 Properties of several gas molecules [80], [81] 45

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INTRODUCTION

In practice, gas sensors have been employed for detecting explosive and hazardous gases It has become much more important in many different fields including air quality control, environmental pollution monitoring [1], [2] The existence of flammable, and extremely hazardous gases (NH3, NO2, CO, H2S, etc.),

which is emitted from industrial activities and agricultural processes such as biogas

H2S in the atmosphere, which is emitted from craft paper mills, food processing, tanneries, and coking ovens [3], which irritates the human eyes, nose, throat, and respiratory system even at low concentrations (<10 ppm) [4] The existence of indoor sources of NO2 gas can cause harmful human health effects (even an extremely low level of 3 ppm) [5] This toxic gas can be emitted from tobacco burning, wood stoves, candles

One dimensional (1D) nanostructure of various types including nanowires, nanobelts, nanotubes, nanorods, and nanofibers (NFs) have attracted more attention Due to the large surface-to-volume ratio, 1D nanostructures have been considered

as excellent candidates for ultrasensitive gas detection [6] Among them, the NFs structure consists of string polycrystalline nanograins, which makes it a highly porous, high specific surface, hence, NFs demonstrated its outstanding advantages

Composite NFs show great potential for the further enhanced gas response

By combining two dissimilar materials, the hetero-junction between the two materials would be formed by the charge transfer across the interface until the Fermi level was equivalent As a result, the conduction and valence bands simultaneously bend and the potential barrier height creates between two adjacent grains in the individual NF (so-called internal junction) by the formation of a charge depletion layer This junction block electron flow in each NF from one electrode to another [7] When exposing to a tested gas, this junction would be changed and it could be attributed to the high response of NFs [8] This observation has been

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reported in many studies [9]–[11]

In addition, there are many NF-NF junctions in the spider-net-like morphology of the electrospun NFs due to it is randomly aligned The depletion layers established at these inter-NF contacts (so-called external-junction) block the electron flow among NFs, which serve as sources of resistance modulation of NFs Such a junction is also an effective way to contribute to gas sensing enhancement [12] The NFs composites for gas sensing has been studied, and the results indicated they present a higher sensitivity compared to pure NFs [13]–[15] However, up to present, no research has been performed to compare the effect of internal and external junctions on the gas sensing properties of NFs

Base on the above analysis, we decided to choose the research work with the

title: “Comparative study of gas sensing properties between ZnO/SnO 2 and ZnO-SnO 2 nanofibers”

Object for study

In this study, we focus on synthesizing and investigate the gas sensing characteristic of the ZnO NFs, SnO2 NFs, ZnO-SnO2 NFs, ZnO/SnO2 NFs

Research objective

The main goal of the thesis is to compare the effects of internal-junction and external-junction on gas sensing performance of on-chip ZnO-SnO2 NFs and ZnO/SnO2 NFs sensors

Research method

Experimental research combined with the theoretical method is implemented

in this thesis Herein, the electrospinning method is employed to synthesize the NFs The thermal, morphological, compositional, and crystal properties of the as-spun and calcinated NFs were investigated by thermogravimetric analysis (TG-DTA), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD), high-resolution transmission

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electron microscopy (HRTEM) The gas sensing properties of NFs were investigated by a homemade dynamic gas sensing system

Organization of the Thesis

Based on the aims, this thesis is organized in the following manner

Chapter 1 (Introduction) introduces SMO NFs for gas sensing applications,

the gas sensing mechanism involved in composite NFs gas sensors as well as the materials that we pay attention to

Chapter 2 (Experimental) provides detail about the methodology that has

been used to synthesize samples and collect measured data

In Chapter 3 (Results and discussion), the characteristics of the materials,

gas sensing properties of NFs are presented, the issues which are relating to the themes were discussed in details Besides, the gas sensing mechanisms were proposed to explain the different sensing behavior between two composite structures Overall, we summarize all studies in the thesis and give an outlook for future research directions

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Chapter 1 LITERATURE REVIEW 1.1 Metal oxide semiconductor gas sensors

1.1.1 Gas sensors construction

The construction of a gas sensor mainly comprised of three main parts: receptor (recognition) function, transducer function, and measuring electronics The most important part of the gas sensors is the reception function, which can be interacted with the test gas In the past decades, different kinds of gas sensors have been developed based on various principles such as oxide semiconductors gas sensors, catalytic combustion gas sensors, electrochemical gas sensors, thermal conductivity gas sensors, infrared absorption gas sensors [16] Compared to other types, the MOS gas sensors is normally used since it shows high sensitivity, fast response time, stability, and low cost [17] The measuring circuitry provides the ability to convert the chemical interaction of tested gas and the oxide surface into the electrical signal Figure 1-1 presents a typical resistive sensor configuration

One-dimensional (1D) nanostructures, including nanowires, nanorods, nanotubes, nanobelts, and NFs have recently attracted increasing attention Owning

to peculiar properties originated from their shape, these structures show high porous, large surface area, that could be attributed to the increased active sites It was demonstrated better structures for enhanced sensing capabilities of sensors

MOS sensing layer

Microheater

Figure 1-1 The scheme of a typical resitive sensor system

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Furthermore, the simple configuration of 1D structure makes it easy to integrate between electrodes in the sensors fabrication process [18]

Among them, the NFs comprise many nanograins, which is a different structure than other 1D nanostructures Due to the formation of grain boundaries and potential barriers between nanograins, the gas response of the material under gas exposure would be increased [19], [20]

Of these, the molecular form O2- is dominated at low temperature below 150

°C, the O- species are dominant in the temperature range of 300 °C - 500 °C, which

is the most reactive inflammable gases, when temperature range above 500 °C, the ionic species (O2-) is dominated [21], [22] Figure 1-2 shows the bend banding of n-

type MOS when exposed to oxygen [23]

Oxygen

Figure 1-2 Band bending at an n-type semiconductor surface due to oxygen absorbtion

[23]

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In general, the n-type semiconductor is a material in which the majority carriers are electrons while the p-type semiconductor is material in which the

majority charges are holes [24] In the air atmosphere, the adsorption of oxygen

molecules generates an electron-depletion layer in n-type MOS or a accumulation layer in p-type MOS [25] As a result, the conductivity of n-type MOS is decreased and the conductivity of p-type MOS is increased

hole-When n-type MOS sensors expose to reducing gas, for example H2S, H2S molecules can react with oxygen pre-adsorbed on the surface and release electrons back to the material according to the equations [26], [27]

2H2S(ads) + 3O2-(ads) ↔ 2H2O + 2SO2 + 6e- (1)

H2S(ads) + 3O-(ads) ↔ SO2 + H2O + 3e- (2)

H2S(ads) + 3O2-(ads) ↔ SO2 + H2O + 6e- (3)

As a result, the width of the depletion layer decreases and the conductivity of

sensors increases In contrast, when p-type material exposes to reducing gas (such

as H2S), H2S molecules react with oxygen pre-adsorbed on the surface and release electrons back to the material Thus, the accumulation layer becomes narrower and the resistance increases

When oxidizing gas (such as NO2) is exposed to MOS sensors, it can react with pre-adsorbed oxygen on the surface of the material or directly trapping electrons from the conduction band due to its even higher electron affinity (220 kJ/mol) compared to oxygen (42 kJ/mol) [28] In this case, the adsorbed reaction of

NO2 gas on the surface of the material takes place, which can be described in the following equations [29], [30]:

NO2(gas) + e- ↔ NO2-(ads) (4)

NO2(gas) + 2O-(ads) ↔ NO2-(ads) + O2(gas) + e- (5)

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2NO2(gas) + O2-(ads) + 2e- ↔ 2NO2-(ads) + 2O-(ads) (6)

NO2-(ads) + 2O-(ads) + e- ↔ NO(gas) + ½ O2(gas) + 2O2-(ads) (7)

The adsorption of oxidizing gas on the n-type semiconductor takes electrons away from the conduction band of n-type MOS NFs, which provides more charge

density on the surface of the material Thus, the electron depletion layer further extended leading to an increase in the potential barrier at the material surface Therefore, the sensor resistance increases upon exposure to the oxidizing gas [31],

[32] In contrast, when p-type material exposes to oxidizing gas, more hole carriers

are generated at the surface of the material Thus, the accumulation layer becomes wider and the resistance decreases

1.2 Nanofibers for gas sensors

1.2.1 Fabrication of nanofibers

Electrospinning is an efficient, low-cost, versatile method to produce NFs of various morphologies including composite, multicomponent, core-shell, hollow, and porous fibers from a wide range of materials [33], [34]

Figure 1-3 The schematic of the on-chip fabrication of NF sensors by electrospinning [34]

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A typical electrospinning system consists of three major components: a high

DC voltage power source, syringes with a metallic capillary tip, and the collector In the common procedure, the syringe pump is used to provide a constant flow rate [35], [36] Figure 1-3 presents a schematic of the electrospinning method [34] The electrospinning mechanism is based on the electrostatic principle, in which the electrostatic repulsion forces caused by a high electrical field are used for NFs synthesis [37] The high voltage is applied on the needle tip, the liquid droplet at the capillary tip deforming into a conical shape known as “Taylor cone” When the voltage reaches a critical value, the repulsive force of the charged polymer overcomes the surface tension of the solution, a charged jet of the precursor solution erupts from the tip of the Taylor cone to produce thin fibers In reality, the tip is mounted at a certain angle to the horizontal in order to prevent solution dropping out of the syringe under the gravity of its own weight and improves a uniform spreading of NFs on substrates [38], [39] Figure 1-4 shows the schematic illustration of the effect of increasing the surface charge on drop deformation [36]

At the beginning of the electrospinning process, electrostatic repulsion force

is the main factor affecting the jet than others, the path is almost straight in the external electric field direction, which is the result of the Coulomb forces [40], [41] However, at the critical point, the path of the jet begins bending stability motion, the

Figure 1-4 The schematic illustration of the effect of increasing the surface charge on drop

deformation [36]

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solution evaporates rapidly from the surface of the spinning jet The motion of the jet is affected by two forces The downward component of the jet was driven by the electric field applied between the tip and the collector, while the perpendicular force with the primary axis relevant to the self-repulsion of the charge caused by the redistribute itself to minimize the Coulomb interactions, which comes from the solvent evaporation [36], [42]

The third stage is the unstable segmental motion, which is formed after several whipping turns This segment generates a larger coil with many turns of the smaller coil [42], [43] Thanks to these instabilities, the NFs possess spider-net-like morphology and their length increased enormously [42], [44] Finally, the elongation stops, the fibers deposit randomly on the electrode-attached rotating collector Figure 1-5 exhibits the perturbations causing by the bending instability

A, B, and C represent three discrete, equally-similarly charged parts of the jet where

B has been perturbed from the symmetrical axis

To obtain a metal oxide composite NFs, the electrospinning solution was

Figure 1-5 (a) The schematic diagram of the path of an electrospinning jet (b) The schematic illustration of the Earnshaw instability leading to bending of an electrified jet

[41],[43]

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prepared by mixing two metal salts, which are the metal oxides precursors The NFs were synthesized through the electrospinning method following the annealing treatment process Thus, the composite NFs are composed of nanograins of two metal oxides, which were random dispersion in NFs On the other hand, the mixed NFs were synthesized by contemporaneous dispersed electrospinning the two solutions Thereby, two different types of NFs random dispersed on the electrode

1.2.2 Gas sensing mechanisms of nanofibers

The surface depletion layer and grain boundary model have related the gas sensing mechanisms of NFs [19], [45], [46] Upon exposure to the air, oxygen molecules traps the electrons of material, leading to the formation of the electron depletion layer along NFs Particularly, the junctions form randomly between the NFs (so-called external-junction), which serve as sources of resistance modulation

of the gas sensor [47] Thus, the surface depletion region contributes to the sensitivity of sensors

On the other hand, the formation of potential barriers between adjacent nanograins in NFs also plays an important role in enhancing the sensitivity of sensors (so-called internal-junction) [48] This junction was varied upon exposure to tested gas and dry air, which plays a crucial role in enhanced gas-sensing

gas [46]

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performance Figure 1-6 exhibits the gas sensing mechanisms of n-type MOS fibers

towards reducing gas [46]

1.2.3 Review of composite nanofibers for gas sensors

Recently, composite NFs have been used to improve the gas sensing characteristic The gas response of the NF sensors can be explained by the surface depletion layer and grain boundary mechanisms The potential barrier between similar materials is referred to as homo-junction and the potential barrier between two dissimilar materials is referred to as a hetero-junction By combining two types

of materials to form hetero-junction, the gas response of sensors is expected to be improved

In the first mechanism, the change in resistance related to the depletion layer

at the surface of the fibers In this case, the modulation depth of fibers was varied when the fibers place in gas and air environment At the inter-NF junction between two NFs (external-contact) in the spider-net like morphology of NFs structure, the depletion layers formed on each side of NFs serve as sources of resistance modulation of NFs, which block the electron flow in individual NFs This junction was contributed to improving the gas sensing properties of fibers [12] The hetero-NFs by double jets electrospinning were reported in various works

(b) (a)

[13]

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For instance, Haiying Du et al [13] synthesized the formaldehyde gas sensor

based on SnO2/In2O3 hetero-NFs The sensors exhibited a high response value compared to that of SnO2 NFs and In2O3 NFs (Figure 1-7) Akash Katoch et al [15]

reported the CuO/SnO2 mixed NFs with high sensitivity to H2S in the range of concentrations from 10 to 100 ppm, which is higher than that of pure SnO2 NFs The existence of potential barrier height at the NF-NF junction is a major reason for high response value

Besides, the connecting configuration of primary particles within NFs were generating the potential barriers at the boundaries between the nanograins The NFs

of mixed metal oxide are developed to enhanced gas sensing characteristics In this structure, the junctions at the intergranular were contributed to enhancing the gas

response of sensors In literature, S.H Yan et al [9] fabricated SnO2-ZnO NFs for ethanol gas-sensing, the results illustrate the SnO2-ZnO heterogeneous structure show better ethanol sensing than those of pure ZnO and pure SnO2 NFs (Figure 1-8) The higher sensitivity of the sensor was supposed to the formation of the potential barrier at the interface of ZnO and SnO2 grains

hetero-Sun-Woo Choi et al [10] reported their work on synthesis of CuO-SnO2

NFs to 100 ppm ethanol at different operating temperatures (200 °C - 400 °C) [9]

(b)

(a)

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composite NFs for H2S detection The response of CuO-SnO2 composite sensors was higher than those of the other two NF sensors composition The response of sensors reaches the highest value when the proportional component of CuO-SnO2 is

1:1 ratio J.-H Kim et al [11] also confirmed the optimal composition between

SnO2 and Co3O4 is of 1:1 ratio to achieve better sensing performance

1.3 Effect of hetero-junction between ZnO and SnO 2 nanofibers on gas sensing properties

1.3.1 Zinc oxide and Tin oxide properties

Zinc oxide (ZnO) is well-known as a wide direct bandgap semiconductor (Eg = 3.37 eV at 300 K) There are three crystal structures forms of ZnO: hexagonal wurtzite, cubic zinc blende, and cubic rock-salt Among them, the hexagonal wurtzite phase is the most thermodynamically stable form under ambient conditions The unit cell of ZnO crystal is presented in Figure 1-9 [49]

The lattice constant parameters of wurtzite ZnO are a = 0.32495 nm and c =

Figure 1-9 ZnO unit cell with wurtzite structure [49]

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0.52069 nm with ratio of c/a = 1.602 corresponds to P63mc space group with two interconnecting hexagonal close-packed (hcp) sub-lattices in hexagonal lattice [50]

In nature, ZnO represents n-type MOS due to the presence of intrinsic defects, such

as oxygen vacancies and zinc interstitials The high electron mobility 4.6 × 10-3cm/V.s, high electron concentration 1.3×1019 cm-3 [51], high thermal conductivity, its special electrochromic and photochromic properties make ZnO nanostructure can

be applied for a wide range of applications including optical devices, piezoelectric devices, transparent electrodes, photocatalysts and gas sensors [52]

In literature, ZnO NFs have been used as a gas sensing layer, which shows a high response to both reducing and oxidizing gases For instance, Akash Katoch [7] reported the ZnO NF sensors show highly sensitive to 10 ppm - H2 Wan-Yu Wu [53] produced ZnO NFs for ethanol detection, the sensors exhibited high sensitivity

to ethanol In addition, N V Hoang [54] fabricated ZnO NFs for NO2 detection, the sensors show high sensitivity in the range of 2.5 to 10 ppm

Tin oxide (SnO2) is n-type metal-oxide semiconductors with rutile structure

has a large bandgap of 3.56 eV at 300 K, and a high carrier concentration of up to 5.7 × 1020 cm-3 [55], [56] SnO2 crystallizes in the tetragonal rutile structure with P42/mnm space group and lattice constants of a0 = b0 = 4.7374 Å, c0 = 3.1864 Å

and O atoms, respectively) [57]

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Each unit cell contains two formula units of SnO2 The tin cation is octahedrally coordinated and forms chains along the c-axis, where each Sn atom is surrounded

by a distorted octahedron of O atoms [57] Figure 1-10 shows the structures of the SnO2 [58]

Due to the existence of intrinsic oxygen vacancies in SnO2 lattice, the electron donor can be excited to the conduction band at elevated temperatures to electronic conduction [58], [59] The key parameters of zinc oxide and tin oxide are listed in Table 1-1 [60], [61]

Table 1-1 Parameters of ZnO and SnO2 [60], [61]

(ZnO)

Tin oxide (SnO 2 ) Crystal structure Hexagonal, Wurtzite Tetragonal, Rutile

Because of excellent electrical, high chemical, and thermal stability, SnO2has demonstrated to be suitable sensing material for the detection of both reducing and oxidizing gases [59], [62] For example, Qi Qi [63] synthesized SnO2 NFs for toluene detection, the SnO2 NF sensors exhibit excellent sensitivity to toluene in the range of 10 - 300 ppm at 350 °C Yang Zhang [64] fabricated the sensors based on SnO2 NFs, the sensors shows large response to 10 ppm ethanol Besides, Nam Gyu Cho [32] reported SnO2 NFs-based sensor obtained high sensitivity to 2 ppm NO2gas

1.3.2 Hetero-junction between ZnO and SnO 2 in gas sensing

In general, when two dissimilar semiconducting materials such as ZnO and SnO2, which possess different original Fermi levels are brought into contact in the

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vacuum environment, the electron from SnO2 with a lower work function (Φ = 4.55eV) were transfer across the interface to ZnO with higher work function (Φ = 5.2 eV) until Fermi levels are equilibrated [60] Consequently, the charge redistribution and band bending take place at the interface of the junction As a result, the electron depletion layer is created on the SnO2 side and the accumulation layer is formed on the ZnO (as illustrated in Figure 1-11)

The formation of hetero-junction establish the depletion layer [65] In order to transport from one electrode to another, charge carriers have to overcome this potential barrier [66] The composite between ZnO and SnO2 are listed in Table 1-2

before contact, and (b) after-contact

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Table 1-2 A survey of composite NFs between ZnO and SnO2 based-on gas

 Investigating the gas sensing performance of all fibers towards reducing gas

as well as the oxidizing gas

 Analyzing the influencing factors on gas sensing performance of ZnO-SnO2

NFs, ZnO/SnO2 NFs

 Proposing gas sensing mechanisms and explaining the impact of these

factors on gas sensing properties of ZnO-SnO2 NFs and ZnO/SnO2 NFs

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Chapter 2 EXPERIMENTAL

In this chapter, we describe electrospinning process that was used to synthesize NFs The characterization techniques used to examine the thermal properties, morphological, compositional and crystal properties such as TGA, FESEM, EDX, XRD, TEM as well as the dynamics gas sensing measurement system are detailed

2.1 Chemicals preparation

Table 2-1 presents all chemicals which have been used to synthesized NFs in this thesis All chemicals were used without any other further purification

Table 2-1 Chemicals for NFs synthesis

No Chemical Chemical formula

Molecular weight (g/mole)

Supplier

1 Zinc acetate

dihydrate (CH3COO)2Zn.2H2O 219.49 Merck Co

2 Tin (II) chloride

Xilong Scientific Co

5 Dimethylformamide

Xilong Scientific Co

2.2 Nanofibers synthesis

2.2.1 Preparation of the precursor solution for electrospinning

The electrospinning solutions were prepared following the procedure shown

in Figure 2-1 Firstly, the desired amounts of metal salts were dissolved into a bottle which was containing a mixed solution of 5 g ethanol and 5 g dimethylformamide (1:1 weight ratio) After magnetic stirring for about 2 hours, 1 g PVP was then added into the solution and continued stirring for 24 h to obtain the homogeneous

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solution

The amount of metal salts that have been used to synthesize NFs are listed in Table 2-2 To prepared the composite ZnO-SnO2 precursor solution, the amount of salts was calculated to obtain the molar ratio of Zn:Sn is 1:1

Table 2-2 The precursor composition for NFs deposition

Sample (CH 3 COO) 2 Zn.2H 2 O

(g)

SnCl 2 2H 2 O (g)

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To synthesize ZnO NFs, SnO2 NFs, ZnO-SnO2 NFs, the solution prepared in Table 2-2 were then loaded into a syringe, which equipped with a stainless-steel needle The flow rate of the solutions was controlled at a constant rate at 0.2 ml/h

by syringe pumps The distance between the needle tip and the collector was fixed

at 13 cm In the electrospinning process, the high voltage of 17 kV was applied between the tip of the needle and the collector The collector was rotated at

1500 rpm The fibers were electrospun for 10 min

To synthesize ZnO/SnO2 NFs, ZnO and SnO2 precursors were loaded into two syringes, hence, the electrospinning could take place simultaneously The on-chip fabrication NFs by electrospinning method is illustrated in Figure 2-3 The comb-type interdigitated Pt electrodes deposited Si/SiO2 substrate (Figure A1) were used in these experiments

Figure 2-2 The electrospinning system at ITIMS

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All as-spun fibers were dried at 60 °C for 24 h to remove residual solvents Then as-spun fibers were annealed at 600 °C for 3 h in the air with a heating rate of 0.5 °C/min to decompose completely residual solvent and form nanocrystalline metal-oxide (as can be seen in Figure 2-4)

SnO 2 precursor

ZnO precursor

(b)

T, o C

Annealing time (h) RT

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2.3 Material characterization

Thermogravimetric analysis (TGA) was conducted on the as-spun NFs simultaneous thermogravimetric analyzer to evaluate the decomposition of the precursors and the formation of oxide phase The TGA measurement was performed

in the air from room temperature to 800 °C with a heating rate of 10 °C /min (Labsys TG/DSC-SETARAM, Department of Inorganic Chemistry, Faculty of Chemistry, Hanoi University of Science) The morphology of the samples was characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800), and the elemental composition of the samples was studied by Energy Dispersive X-Ray spectroscopy (EDX, Horiba, attached to FESEM Hitachi S-4800, Vietnam Academy of Science and Technology) X-Ray Diffraction analysis with Cu-Kα radiation source (λ = 1.54056 Å) was performed to determine the crystal structure of the samples (XRD, D5005 diffractometer Bruker, Faculty of Physics, Hanoi University of Science) The Origin 2018 software was used to fit profile along with the experimental data to determine the crystallite sizes of the NFs The microstructures of NFs were examined by high-resolution transmission electron microscopy (HRTEM, Tecnai G2 20 S-TWIN/FEI, Geology, Geoengineering, Geoenviroment, and Climate Change Labs - Faculty of Geology - Hanoi University

of Science - Vietnam National University, Hanoi)

2.4 Gas sensing measurements

Gas sensing properties of the sensor were investigated by using flow through testing system (Figure 2-5) at International Training Institute for Material Science (ITIMS), Hanoi University of Science and Technology (HUST)

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The gas sensing properties of the sensors of NFs were tested to various gases, gas concentrations at different operating temperatures The changing of sensors resistance was recorded by to a Keithley 2602A source-meter system as in Figure 2-6 [74]

Figure 2-5 Gas sensing measurement system at ITIMS

(c)

Figure 2-6 Design layout (a) The gas testing chamber (b), and

Keithley 2602A source meter (c) at ITIMS [74]