Electrospinning of α fe2o3 and znfe2o4 nanofibers loaded with reduced graphene oxide (RGO) for h2s gas sensing application

46 operating temperatures a, sensor resistances b, sensor response c, response time and recovery time d as a function of operating temperatures... Sensor response to H2S gas as a functi

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Nguyen Van Hoang

ELECTROSPINNING OF α -Fe 2 O 3 AND ZnFe 2 O 4 NANOFIBERS LOADED WITH REDUCED GRAPHENE OXIDE (RGO)

FOR H 2 S GAS SENSING APPLICATION

DOCTORAL DISSERTATION OF MATERIALS SCIENCE

Hanoi – 2020

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Nguyen Van Hoang

ELECTROSPINNING OF α-Fe 2 O 3 AND ZnFe 2 O 4 NANOFIBERS LOADED WITH REDUCED GRAPHENE OXIDE (RGO)

FOR H 2 S GAS SENSING APPLICATION

M ajor: Materials Science

Hanoi – 2020

DECLARATION OF AUTHORSHIP

This dissertation has been written in the basic of my researches carried out at Hanoi University of Science and Technology, under the supervision of Prof PhD Nguyen Van Hieu All the data and results in the thesis are true and were agreed to use in my thesis by co authors The presented results have never been published by -others

ACKNOWLEDGMENTS

First, I would like to express my deep gratitude to my supervisor, Prof Nguyen Van Hieu, for his devotion and inspiring supervision I would like to thank him for all his advice, support and encouragement throughout my postgraduate course

I am grateful to Assoc Prof PhD Nguyen Duc Hoa, Assoc Prof PhD Nguyen Van Duy, PhD Dang Thi Thanh Le, PhD Chu Manh Hung, and PhD Nguyen Van Toan for their useful help, suggestions and comments I also would like to express

my special thanks to PhD and Master Students at iSensors Group for their support and shared cozy working environment during my PhD course

I am thankful to the leaders and staffs of International Training Institute for Materials Science (ITIMS), Graduate School for their help and given favorable working conditions

I would like to thank my colleagues at Department of Materials Science and Engineering at Le Quy Don Technical University for their support during my PhD course

I gratefully acknowledge the fund from Vietnam National Foundation for Science and Technology Development (NAFOSTED) under code 103.02-2017.25 and the

911 Scholarship of Ministry of Education and Training for the financial support for

CONTENTS

CONTENTS i

ABBREVIATIONS AND SYMBOLS v

LIST OF TABLES vii

LIST OF FIGURES viii

INTRODUCTION 1

CHAPTER 1 OVERVIEW ON SMO NFs AND THEIR LOADING WITH RGO FOR GAS-SENSING APPLICATION 6

1.1 Electrospinning for NFs fabrication 6

1.1.1 Background on electrospinning 6

1.1.2 Processing structure relationships of electrospun NFs 7 –

1.2 NFs for gas sensing application 10

-1.2.1 Electrospun SMO NFs for gas sensing application 10

-1.2.2 Electrospun SMO NFs for H2S gas-sensing application 13

1.2.2.1 H2S gas 13

1.2.2.2 Electrospun SMO NFs for H2S gas-sensing application 13

1.3 NFs loading with RGO for gas sensing application 14

-1.3.1 Overview on RGO and its application in gas sensing field 14

-1.3.1.1 Overview on RGO 14

1.3.1.2 RGO in gas sensing application 17

-1.3.2 RGO loaded SMO NFs in gas-sensing applications 19

-1.3.2.1 RGO loaded SMO gas sensor 19

-1.3.2.2 RGO loaded SMO NFs gas sensor 22

-i

1.4 Gas sensing mechanism 24

-1.4.1 Gas sensing mechanism of SMO NFs 24

-1.4.2 Gas sensing mechanism of RGO loaded SMO NFs 25 -

-1.4.3 H2S gas-sensing mechanism of SMO NFs and their loading with RGO… 27

Conclusion of chapter 1 28

CHAPTER 2 EXPERIMENTAL APPROACH 29

2.1 Synthesis 29

2.1.1 RGO preparation 29

2.1.2 α-Fe2O3NFs preparation 30

2.1.3 ZFO NFs preparation 31

2.1.4 Preparation of α-Fe2O3, ZFO NFs loading with RGO 32

2.2 Characterization Techniques 32

2.2.1 Raman spectroscopy 32

2.2.2 Thermal analysis 33

2.2.3 X-ray diffraction 33

2.2.4 SEM and EDX 34

2.2.5 TEM and SAED 34

2.3 Gas sensing measurement 35

-Conclusion of chapter 2 36

CHAPTER 3 α-Fe2O3 NFs AND THEIR LOADING WITH RGO FOR H2S GAS-SENSING APPLICATION 37

3.1 Introduction 37

3.2 H2S gas sensors based on α-Fe2O3NFs 39

3.2.1 Morphologies and structures of α-Fe2O3NFs 39

ii

3.2.2 H2S gas-sensing properties of α-Fe2O3 NFs sensors 46

3.2.2.1 Effects of operating temperature 46

3.2.2.2 Effects of solution contents 48

3.2.2.3 Effects of annealing temperature and electrospinning time 50

3.2.2.4 Selectivity and stability 53

3.3 H2S gas sensors based on α-Fe2O3 NFs loaded with RGO 54

3.3.1 Morphologies and structures of α-Fe2O3NFs loaded with RGO 54

3.3.2 H2S gas-sensing properties of RGO loaded α-Fe- 2O3 NFs sensors 58

3.3.2.1 Effects of RGO contents 58

3.3.2.2 Effects of working temperature 61

3.3.2.3 Effects of annealing temperatures 62

3.3.2.4 Selectivity and stability 64

Conclusion of chapter 3 65

CHAPTER 4 ZFO NFs AND THEIR LOADING WITH RGO FOR H2S GAS-SENSING APPLICATION 66

4.1 Introduction 66

4.2 H2S gas sensors based on ZFO NFs 68

4.2.1 Microstructure characterization 68

4.2.2 Gas sensing properties 74

-4.2.2.1 Effects of the operating temperature 74

4.2.2.2 Effects of the annealing temperature 76

4.2.2.3 Effects of annealing time and heating rate 79

4.2.2.4 Selectivity and stability 81

4.3 H2S gas sensors based on ZFO NFs loaded with RGO 82

4.3.1 Microstructure characterization 82

iii

4.3.2 Gas sensing properties 86

-4.3.2.1 Effects of RGO contents 86

4.3.2.2 Effects of operating temperature 88

4.3.2.3 Effects of annealing temperatures 89

4.3.2.4 Selectivity, stability and RH effects 91

Conclusion of chapter 4 94

CONCLUSIONS AND RECOMMENDATIONS 95

LIST OF PUBLICATIONS 97

REFERENCES 98

APPENDIX 117

iv

ABBREVIATIONS AND SYMBOLS

v

28 SAED Selected Area Electron Diffraction

vi

Table 3.2 -α Fe2O3loaded with RGO for gas sensing application 38

nanomaterials and nanostructures 93

with different contents of RGO from 0 to 1.5 wt% RGO at 350°C 117

Table A3.2 Calculation table of DL to H2S of α -Fe2O3 NFs sensors calcined at annealing temperatures from 400°C to 800°C at 350°C 118

sensors calcined at annealing temperatures from 400°C to 800°C at 350°C 119

Table A4.1 Average nanograin sizes determined by Scherrer formula and

integrated intensity of (311) diffraction peak of ZFO NFs calcined at different conditions 120

operating temperature of 350°C of the ZFO NFs sensors calcined at different annealing temperatures (400−700°C), annealing time (0.5−48 h), heating rates (0.5−20°C/min), electrospinning time (10−120 min) 121

annealing temperature from 400°C to 700°C at 350°C 122

sensors calcined at annealing temperatures from 400°C to 700°C at 350°C 123

vii

LIST OF FIGURES

-fibers, 3 Precursor solution, 4 Syringe, 5 Needle, 6 DC voltage power supply.- - - - 7

Figure 1.2 Kind of collectors and needles: (a) plate collector (b) Multiple

spinnerets (c) Coaxial spinneret (d) Bicomponent spinneret (e) Disc collector , (d) Rotating drum [33] 8

Figure 1.4 Number of annual publications on “graphene” and “graphene and

sensors” according to Scopus Database The dashed lines are exponential fitting of the number of publications Inset is where “graphene and sensors” publications have appeared [64] 15

Figure 1.5 Scotch tape method [65] 16 –

Figure 1.6 Histogram detailing the number of graphene-based gas/vapor sensors

publications per year for the period from 2007 to 2014 (data obtained from ISI Web

of Knowledge, January 28, 2015) ) [72] 17

Figure 1.7 (a) RGO device (b) SEM image of a sensing device composed of RGO

platelets that bridge neighboring Au fingers, Representative dynamic behavior of RGO sensors for (c) 100 ppm NO2 and (d) 1% NH3detection [74] 18

decorated with SnO2nanocrystal (b) Schematic of the sensor testing system [83].20

-loaded SnO2; Sensor response of (e) RGO-loaded ZnO NFs and (f) RGO-loaded SnO2; (g) Comparision of of pure SnO2 NFs, pure ZnO NFs, rGO-loaded SnO2 NFs and rGO-loaded ZnO NFs to 10 ppm H2 gas [91] 22

-[11] 26

viii

Figure 1.12 Schematic illustration of sensing mechanisms with respect to NO2 gas RGO-loaded SnO2 NFs [94] 27

Figure 2.1 (a) Schematic of on-chip fabrication of NF sensors by electrospinning:

(1) collector, (2) Pt electrodes, (3) DC high voltage power supply, (4) as-spun nanofibers, (5) needle, (6) syringe; (b-d) FESEM images of -on chip NFs 30

Figure 2.2 Schematic diagram of the gas sensing system [113] 35

-Figure 3.1 Crystal structure of α Fe- 2O3 [119] 37

annealing temperatures (400 − 800°C) for 3 h in air 40

concentrations: 7 (a, d), 11 (b, e), and 15 wt% (c, f), respectively Insets are magnification images 41

ferric salt concentrations: 2 wt% (a, d), 4 wt% (b, e), and 8 wt% (c, f), respectively Insets are low magnification images.- 42

time of 10 (a), 30 (b), 60 (c), and 120 min (d) 43

different annealing temperatures: 400°C (b), 500°C (c), 600°C (d), 700°C (e), and 800°C (f) Inset figures are low magnification images.- 44

-corresponding fast Fourier transform (FFT) inset image, and EDX spectrum (d) of

α-Fe2O3NFs calcined at 600°C for 3 h in air 46

operating temperatures (a), sensor resistances (b), sensor response (c), response time and recovery time (d) as a function of operating temperatures 47

and in H2S gas (b) 48

ix

Figure 3.11 H2S sensing transients of α-Fe2O3 NF sensors with various PVA concentrations (a−c) and different ferric salt concentrations (d−f) Sensor response

to H2S gas as a function of PVA concentrations (g) and ferric salt concentrations (h) 50

temperatures (400−800°C) (a−e) and different electrospinning time (10−120 min) (f−i) Sensor response to H2S gas as a function of annealing temperatures (k) and electrospinning time (l) 51

at 350°C (b) of the sensors based on α-Fe2O3 NFs calcined at 600°C 53

Figure 3.14 XRD patterns (a), Raman spectrum (b) SEM image (c) and TGA

curves (d) of synthesized RGO 54

concentrations: 0 (a), 0.5 (b), 1.0 (c), and 1.5 wt% (d) 55

calcined at 400 (b), 500 (c), 600 (d), 700 (e), and 800°C (f) for 3 h in air 56

600°C for 3 h in air 57

-HRTEM image (d) with corresponding fast Fourier transform (FFT) inset image of 1%wt RGO loaded α-Fe2O3annealed at 600°C for 3 hours in air 58

RGO concentrations: 0 (a), 0.5 (b) 1.0 (c) and 1.5 wt% (d) Sensor resistance (e), gas response (f), and response time and recovery time (g) as a function of RGO concentrations at working temperature of 350°C 59

NFs; band diagram of RGO and SMO (a) at equilibrium (b) in air exposure (c) and

in H2S gas exposure (d) 61

ppm H2S at various operating temperatures (a), sensor resistances (b), sensor

x

response (c), response time and recovery time (d) as a function of operating

temperatures 62

Figure 3.22 H2S sensing transients of sensors based on α -Fe2O3 NFs loaded 1.0

wt% RGO with different annealing temperatures 400 (a), 500 (b), 600 (c), 700 (d)

and 800°C (e) Gas response to H2S various concentrations (f) of sensors based on

α-Fe2O3 NFs loaded 1.0 wt% RGO as a function of annealing temperatures at

working temperatures of 350°C 63

various gases at 350°C (b) of the sensors based on 1%wt RGO loaded α-Fe2O3 NFs

calcined at 600°C 64

RGO loaded α-Fe2O3 NFs to various gases at 350°C 64

Figure 4.1 Crystal structure of ZFO [182] 66

Figure 4.2 TGA and DTG curve for decomposition of as-spun ZFO fibers 68

Figure 4.3 XRD patterns, average grain sizes and integrated intensity of the (311)

diffraction peaks of ZFO NFs calcined at different annealing temperatures (a, d),

-annealing time (b, e), and heating rates (c, f) Insets are profiles of (311) Bragg

diffractions 69

-NFs calcined at 400°C (b), 500°C (c), 600°C (d), and 700°C (e) for 3 h in air; EDX

spectrum of ZFO-NFs calcined at 600°C for 3 h in air (f) 70

Figure 4.5 FESEM images of ZFO-NFs calcined at various annealing time:

0.5 (a), 3 (b), 12 (c), and 48 h (d) 71

Figure 4.6 FESEM images of ZFO-NFs calcined at various heating rates:

0.5 (a), 2 (b), 5 (c), and 20°C/min (d) 72

-HRTEM image (d) with corresponding fast Fourier transform (FFT) inset image of

ZFO-NFs calcined at 600°C for 3 h in air 73

xi

Figure 4.8 The transients of the ZFO NFs sensors to 1 ppm H2S at various operating temperatures (a); sensor response (b), sensor resistances (c), response time and recovery time (d) as a function of the operating temperatures 75

temperatures (400 700°C) measured at 350°C (a d) Gas response as functions of – –

H2S gas concentration at 350°C (e) Gas response (f), DL (g), response time and recovery time (h) as a function of the annealing temperature 78

concentration for different annealing time (a) and heating rate (d) Response and response-recovery time as a function of annealing time (b, c) and heating rate (e, f) 79

at 350°C (b) of the sensors based on ZFO NFs calcined at 600°C 80

NFs sensors to various gases at 350°C 81

concentrations: 0 (a), 0.5 (b), 1 (c), and 1.5 wt% (d) 83

Figure 4.14 FESEM images of 1%wt RGO loaded ZFO NFs calcined at 400°C (a),

500°C (b), 600°C (c), and 700°C (d) for 3 h in air 84

-NFs annealed at 600oC for 3 h in air 84

-HRTEM image (d) with corresponding fast Fourier transform (FFT) inset image of 1%wt RGO-loaded ZFO NFs annealed at 600°C for 3 hours in air 85

concentrations: 0 (a), 0.5 (b), 1.0 (c), and 1.5 wt% (d) Gas response (e), sensor resistance (f) and response time and recovery time (g) as a function of RGO concentrations at the working temperature of 350°C 86

xii

Figure 4.18 Sensing transients of 1% wt RGO loaded ZFO- NFs sensors to 1 ppm

H2S at various operating temperatures (a), sensor resistances (b), sensor response (c), response time (d) and recovery time (e) as a function of operating temperatures 87

wt% RGO with different annealing temperatures 400 (a), 500 (b), 600 (c), and 700°C (d) Gas response (e), sensor resistance (f), and response time and recovery time (g) as a function of annealing temperatures at the working temperature of 350°C 88

Figure 4.20 Comparison of response (a), change level of response (b), detection

limit (DL) (c)and response-recovery time (d) of bare-ZFO and 1%wt RGO loaded ZFO NFs sensors to 1 ppm H2S gas at 350°C as a function of annealing temperatures 90

gases at 350°C (b) of the sensors based on the 1 wt% loaded ZFO NFs calcined at 600°C 91

RGO loaded ZFO NFs to various gases at 350°C 92

Figure 4.23 The transient response (a) and sensor response (b) of the 1 wt%

RGO-loaded ZFO NFs sensors to 1 ppm H2S at the working temperature of 350°C at various RH values 92

Figure A3.1 Fitted values of RSS and slope for DL calculation of sensors based on

α-Fe2O3 NFs loaded with different contents of RGO of 0 (a, e), 0.5 (b, f), 1.0 (c, g), and 1.5 wt.% (d, h), respectively The sensors are tested with H2S gas at operating temperature of 350°C 117

Figure A3.2 Fitted values of RSS and slope for DL calculation of α-Fe2O3 NFs sensors calcined at various annealing temperatures of 400 (a, f), 500 (b, g), 600 (c, h), 700 (d, i), and 800°C (e, j), respectively The sensors are tested with H2S gas 118

at operating temperature of 350°C 118

-loaded α-Fe2O3 NFs sensors calcined at various annealing temperatures of 400 (a, f),

xiii

500 (b, g), 600 (c, h), 700 (d, i), and 800°C (e, j), respectively The sensors are tested with H2S gas at operating temperature of 350°C 119

Figure A4.1 Fitted values of RSS and slope for DL calculation of the sensors

based on ZFO NFs calcined at various annealing temperatures of 400 (a,e), 500 (b,f),

600 (c,g), and 700°C (d,h), respectively The sensors are tested with H2S gas at the operating temperature of 350°C 122

Figure A4.2 Fitted values of RSS and slope for DL calculation of the sensors based

on 1 wt% RGO loaded ZFO NFs calcined at various annealing temperatures of 400 (a,e), 500 (b,f), 600 (c,g), and 700°C (d,h), respectively The sensors are tested with

H2S gas at the operating temperature of 350°C 123

xiv

INTRODUCTION

1 Background

Recently, one dimension (1D) nanostructures including nanowires NWs , ( )nanorods (NRs , ) nanotubes (NTs , and nanofibers NFs have attracted much ) ( ) attention for a wide application including optical catalysis, electronic devices,

optoelectronic devices, storage devices, and gas sensors due to their high

surface-to-volume ratio Especially, NFs are[1] used in many fields such as catalysis, sensor,

and energy storage because of their outstanding properties like large surface

area-to-volume ratio and flexible surface functionalities [1,2] There are several approaches

for NFs fabrication, for example, drawing, template, phase separation,

self-assembly, and electrospinning [3–5], among which electrospinning is a simple, cost

-effective and versatile method for NFs production [1,3– 6]

Regarding gas-sensing applications, semiconductor metal oxides (SMO) NFssensors have a lot of promise due to their advantages of SMO materials, e.g low cost, simple fabrication, and high compatibility with microelectronic processing [7–10] Furthermore, NFs consist of many nanograins, therefore, grain boundaries are large, surface-to-volume ratio is very high, and gases easily diffuse along grain boundaries As a result, an exceptionally high response was observed in SMO NFs gas sensors by electrospinning [11,12] Among various SMO NFs prepared by electrospinning, α-Fe2O3 has become a potential gas sensing material because of its -low cost and thermal stability and ability to detect many gases such as NO2, NH3,

H2S, H2, and CO [13,14] Besides, inc ferrite ZnFez 2O4 (ZFO , Fe) a 2O3-based

ternary spinel compounds,has been a promising material for detecting gases thanks

to its good chemical and thermal stability, low toxicity, high specific surface area and excellent selectivity [15–18] Otherwise, H2S is a colorless, corrosive, inflammable and extremely toxic gas which can be rapidly absorbed by human lungs and easily causes diseases in respiratory and nervous system, even deaths [19 21] However until now very few studies on H2S gas sensing properties of α

1

[19–21] However, until now, very few studies on H S gas-sensing properties of

α-Fe2O3 and ZFO NFs, especially effects of fabrication parameters (i.e solution composition, heat treatment, and electrospun time) on morphology, structure and

H2S gas-sensing properties have been carried out although there have been some reports about H2S gas sensitivity of α-Fe2O3or ZFO with other nanostructures (e.g micro ellipsoids - [22], nanochains [23], porous nanospheres [24], and porous nanosheets (NSs) [25])

Furthermore, reduced graphene oxides (RGO), a type of GP reduced from GO produced from graphite by Hummer method [26], has recently received world-wide

attention owing to its exceptional physicochemical properties [27,28] The combination between SMO NFs and RGO to enhance gas-sensing performance through the formation of heterojunction was mentioned in many works [8,11,29]

For instance, the RGO-loaded ZnO NFs sensors showed a bell-shaped behaviour

response to NO2 gas for different weight ratio of RGO (0 1.04%) [11] The sensor s –containing 0.44 wt% RGO showed the highest response to 5 ppm NO2 at 400°C, higher than that of pure ZnO NFs Enhanced gas-sensing properties by heterojunction between RGO and SMO NFs of the RGO-loaded α-Fe2O3 and ZFO have also mentioned [30,31] For instance, Zhang et al [32] stated that the composite containing 1.0 wt% RGO exhibited an enhanced gas response of 13.9 to

100 ppm acetone at the operating temperature of 225 , which was approximately °C

2.5-fold as high as that of pure α-Fe2O3 (5.5) Guo et al [30] reported that the

response of α-Fe2O3 NFs loaded with 1.0 wt% RGO to 100 ppm acetone at

operating temperature of 375°C was about 8.9 (4.5 times higher than that of pure

α-Fe2O3) Liu et al [31] mentioned that the response of the 0.125 wt% RGO-loaded

ZFO composite to 1000 ppm acetone at 275°C was higher than that of the bare ZFO sensor However, up to present, there have been no reports on the loading of RGO

in α-Fe2O3 and ZFO NFs for enhanced H2S gas sensing- performance

Therefore, the thesis titled “Electrospinning of α-Fe2O3 and ZnFe2O4 nanofibers loaded with reduced graphene oxide (RGO) for H2S gas sensing application” was carried out to answer the concerns mentioned above

2 The study objective

The study objective of the thesis are listed as follows:s

2