Electrospinning of α-Fe2O3 and ZnFe2O4 nanofibers loaded with reduced graphene oxide (rgo) for H2S gas sensing application

MINISTRY OF EDUCATION AND TRAINING 
HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY 
Nguyen Van Hoang 
ELECTROSPINNING OF α-Fe2O3 AND ZnFe2O4 NANOFIBERS 
LOADED WITH REDUCED GRAPHENE OXIDE (RGO) 
FOR H2S 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-Fe2O3 AND ZnFe2O4 NANOFIBERS 
LOADED WITH REDUCED GRAPHENE OXIDE (RGO) 
FOR H2S GAS SENSING APPLICATION 
 Major: Materials Science 
 Code: 9440122 
DOCTORAL DISSERTATION OF MATERIALS SCIENCE 
SUPERVISOR: PROF. PhD. NGUYEN VAN HIEU 
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. 
. 
 Hanoi, 16th January 2020 
Supervisor PhD. Student 
Prof. PhD. Nguyen Van Hieu 
Nguyen Van Hoang 
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 
my research. 
Last but not least, I am deeply thankful to my family for their endless love and 
unconditional support. Without them, the work would have been impossible. 
 PhD. Student 
 Nguyen Van Hoang 
 i 
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 
 ii 
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-Fe2O3 NFs 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-Fe2O3 NFs ...................................................... 39 
3.2.1. Morphologies and structures of α-Fe2O3 NFs ...................................... 39 
 iii 
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-Fe2O3 NFs loaded with RGO ......... 54 
3.3.2. H2S gas-sensing properties of RGO-loaded α-Fe2O3 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 
 iv 
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 
 v 
 ABBREVIATIONS AND SYMBOLS 
Number 
Abbreviations and 
symbols 
Meaning 
1 1D One Dimension 
2 2D Two Dimension 
3 CVD Chemical Vapor Deposition 
4 DI Deionized Water 
5 DL Detection Limit 
6 DMF Dimethylformamide 
7 DTG Derivative Thermogravimetric 
8 EDX Energy Dispersive X-ray spectroscopy 
9 FE-SEM 
Field Emission Scanning Electron 
Microscope 
10 FFT Fast Fourier Transform 
11 GO Graphene Oxides 
12 GP Graphene 
13 HRTEM 
High Resolution Transmission Electron 
Microscope 
14 IUPAC 
International Union of Pure and Applied 
Chemistry 
15 JCPDS 
Joint Committee on Powder Diffraction 
Standards 
16 NFs Nanofibers 
17 NPs Nanoparticles 
18 NRs Nanorods 
19 NSs Nanosheets 
20 NTs Nanotubes 
21 NWs Nanowires 
22 ppb Parts Per Billion 
23 ppm Parts Per Million 
24 PVA Poly(vinyl alcohol) 
25 RGO Reduced Graphene Oxides 
26 RH Ambient Relative Humidity 
27 RT Room Temperature 
 vi 
28 SAED Selected Area Electron Diffraction 
29 sccm Standard Cubic Centimeters Per Minute 
30 SEM Scanning Electron Microscope 
31 SMO Semiconductor Metal Oxides 
32 TEM Transmission Electron Microscope 
33 TGA Thermogravimetric Analysis 
34 WF Work Function 
35 XRD X-ray Diffraction 
36 ZFO Zinc Ferrite, ZnFe2O4 
37 Ra Sensor resistance in dry air 
38 Rg Sensor resistance in tested gas 
39 S Sensor Response 
40 τres Response time 
41 τrec Recovery time 
 vii 
LIST OF TABLES 
Table 1.1. SMO NFs for gas-sensing application .................................................... 12 
Table 2.1. Processing parameter of of α-Fe2O3 NFs, ZFO NFs and their loading 
with RGO .................................................................................................................. 31 
Table 3.1. Different nanostructures of α-Fe2O3 for H2S gas-sensing application ... 38 
Table 3.2. α-Fe2O3 loaded with RGO for gas-sensing application .......................... 38 
Table 4.1. Different nanostructures of ZFO for H2S gas-sensing application ......... 67 
Table 4.2. Comparison of the H2S gas sensitivity of the sensor based on other 
nanomaterials and nanostructures ............................................................................. 93 
Table A3.1. Calculation table of DL to H2S of sensors based on α-Fe2O3 NFs loaded 
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 
Table A3.3. Calculation table of DL to H2S of 1.0 wt.% RGO-loaded α-Fe2O3 NFs 
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 
Table A4.2. Response and response-recovery time to 1 ppm H2S gas at the 
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 
Table A4.3. Calculation table of DL to H2S of the ZFO NFs sensors calcined at the 
annealing temperature from 400°C to 700°C at 350°C. ......................................... 122 
Table A4.4. Calculation table of DL to H2S of the 1 wt% RGO-loaded ZFO NFs 
sensors calcined at annealing temperatures from 400°C to 700°C at 350°C. ........ 123 
 viii 
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 117 
APPENDIX 
Table A3.1. Calculation table of DL to H2S of sensors based on α-Fe2O3 NFs loaded with 
different contents of RGO from 0 to 1.5 wt% RGO at operating temperature of 350°C. 
Samples RSS rmsnoise Slope DL (ppb) 
0 3.66E-05 0.001913113 5.53 1.04 
0.5 3.03E-05 0.00174069 6.76 0.78 
1 2.40E-05 0.001549193 8.66 0.54 
1.5 1.34E-05 0.001157584 2.44 1.42 
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. 
0 2 4 6 8 10
0.98
1.00
1.02
1.04
1.06
1.08
0.96
0.98
1.00
1.02
1.04
Time (s)
 1.5 wt.% RGO@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 1.34E-5
 1.0 wt.% RGO@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 2.4E-5
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
g
)
 0.5 wt.% RGO @ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.03E-5
 Pure - Fe
2
O
3
 NFs@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.66E-5
(a)
(b)
(c)
(d)
0.00 0.25 0.50 0.75 1.00
0
5
0
5
10
0
5
10
0
5
10
H
2
S conc. (ppm)
 1.5 wt.% RGO@ air & 350 
o
C
 Linear Fit
 Slope = 2.44
 1.0 wt.% RGO@ air & 350 
o
C
 Linear Fit
 Slope = 8.66
R
e
s
p
. 
(R
a
/R
g
)
 0.5 wt.% RGO @ air & 350 
o
C
 Linear Fit
 Slope = 6.76
 Pure - Fe
2
O
3
 NFs@ air & 350 
o
C
 Linear Fit
 Slope = 5.53
(f)
(g)
(h)
(i)
 118 
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 operating temperature of 350°C. 
Samples RSS rmsnoise Slope DL (ppb) 
400 oC 1.31E-05 0.001144552 4.15 0.83 
500 oC 5.12E-06 0.000715542 2.76 0.78 
600 oC 3.66E-05 0.001913113 5.53 1.04 
700 oC 2.34E-05 0.001529706 4.48 1.02 
800 oC 1.77E-05 0.001330413 0.83 4.81 
0.00 0.25 0.50 0.75 1.00
0
5
0
5
0
5
0
5
0
5
H
2
S conc. (ppm)
 Cal. 800 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 0.83
 Cal. 700 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 4.48
R
e
s
p
. 
(R
a
/R
g
)
 Cal. 600 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 5.53
 Cal. 500 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 2.76
 Fe
2
O
3 
Cal. 400 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 4.15 (f)
(g)
(h)
(i)
(j)
0 2 4 6 8 10
1.00
1.02
1.04
0.96
0.98
1.02
1.04
1.00
1.02
1.04
1.00
1.02
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
g
)
Time (s)
 Cal. 800 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 1.77E-5
 Cal. 700 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 2.34E-5
 Cal. 600 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.66E-5
 Cal. 500 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 5.12E-6
 - Fe
2
O
3
 Cal. 400 
o
C@ air&350 
o
C
 Fifth-order polynomial fit
 RSS = 1.31E-5
(a)
(b)
(c)
(d)
(e)
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 
at operating temperature of 350°C. 
 119 
Table A3.3. Calculation table of DL to H2S of 1.0 wt.% RGO-loaded α-Fe2O3 NFs sensors 
calcined at annealing temperatures from 400°C to 800°C at operating temperature of 
350°C. 
Samples RSS rmsnoise Slope DL (ppb) 
400 oC 1.67E-05 0.001292285 6 0.65 
500 oC 1.21E-05 0.0011 3.68 0.90 
600 oC 2.40E-05 0.001549193 8.66 0.54 
700 oC 4.05E-05 0.002012461 3.12 1.94 
800 oC 9.00E-06 0.000948683 0.14 20.33 
Figure A3.3. Fitted values of RSS and slope for DL calculation of 1.0 wt.% RGO-loaded 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 at 
operating temperature of 350°C. 
0 2 4 6 8 10
1.00
1.02
1.04
1.00
1.02
1.04
1.06
0.98
1.00
1.02
1.02
1.04
Time (s)
 Cal. 800 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 9.0E-6
 Cal. 700 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 4.0E-5
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
g
)
 Cal. 600 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 2.40E-5
 Cal. 500 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 1.21E-5
 1 wt% RGO Cal. 400 
o
C@ air&350 
o
C
 Fifth-order polynomial fit
 RSS = 1.67E-5
(a)
(b)
(c)
(d)
(e)
0.00 0.25 0.50 0.75 1.00
0
5
10
0
5
10
0
5
10
0
5
10
0
5
10
H
2
S conc. (ppm)
 Cal. 800 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 0.14
 Cal. 700 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 3.15
R
e
s
p
. 
(R
a
/R
g
)
 Cal. 600 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 8.66
 Cal. 500 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 3.68
 1 wt.% RGO Cal. 400 
o
C@H
2
S&350
o
C
 Linear Fit
 Slope = 6.00
(f)
(g)
(h)
(i)
(j)
 120 
Table A4.1. Average nanograin sizes determined by Scherrer formula and integrated 
intensity of (311) diffraction peak of ZFO-NFs calcined at different conditions. 
Samples 
β (FWHM) 
(radian) 
Crystallite 
sizes (nm) 
Integrated 
intensity 
400 oC 0.62 13.53 20.24 
500 oC 0.55 15.16 22.44 
600 oC 0.46 18.08 35.39 
700 oC 0.35 23.92 84.24 
0.5 h 0.51 16.32 24.68 
3 h 0.46 18.08 35.39 
12 h 0.38 21.82 69.01 
48 h 0.36 23.28 108.82 
0.5 oC/min 0.46 18.08 35.39 
2 oC/min 0.68 12.25 29.19 
5 oC/min 0.61 13.63 35.29 
20 oC/min 0.55 15.23 46.58 
 121 
Table A4.2. Response and response-recovery time to 1 ppm H2S gas at the 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). 
Samples 
S 
1ppm 
τres. 
(s) 
τrec. 
(s) 
400 oC 8.5 47 423 
500 oC 61 9 261 
600 oC 102 8 206 
700 oC 21.8 7 129 
0.5 h 34 9 217 
3 h 102 8 206 
12 h 42.3 5 63 
48 h 15.4 6 40 
0.5 oC/min 102 8 206 
2 oC/min 19.9 7 53 
5 oC/min 53.4 5 68 
20 oC/min 7.4 12 122 
 122 
Table A4.3. Calculation table of DL to H2S of the ZFO NFs sensors calcined at the 
annealing temperature from 400°C to 700°C at the operating temperature of 350°C. 
Samples RSS rmsnoise Slope DL (ppb) 
400 oC 1.37E-05 0.00117161 8.32664 0.422 
500 oC 3.69E-05 0.00192032 67.70817 0.085 
600 oC 3.28E-05 0.001810392 113.96252 0.048 
700 oC 4.78E-06 0.002185514 22.34538 0.093 
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. 
0 2 4 6 8 10
1.00
1.02
1.00
1.02
1.00
1.02
1.04
1.06
Time (s)
 Cal. 700 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 4.78E-6
(d)
 Cal. 600 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.28E-5
(c)
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
a
)
 Cal. 500 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.69E-5
(b)
 Cal. 400 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 1.37E-5
(a)
0.00 0.25 0.50 0.75 1.00
0
10
20
0
50
100
0
25
50
0
5
10
H
2
S conc. (ppm)
 Cal. 700 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 22.34538 (h)
 Cal. 600 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 113.96252
(g)
R
e
s
p
. 
(R
a
/R
g
)
 Cal. 500 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 67.70817 (f)
 Cal. 400 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 8.32664
(e)
 123 
Table A4.4. Calculation table of DL to H2S of the 1 wt% RGO-loaded ZFO NFs sensors 
calcined at annealing temperatures from 400°C to 700°C at the operating temperature of 
350°C. 
Samples RSS rmsnoise Slope DL (ppb) 
400 oC 5.05E-05 0.002247221 48.94 0.14 
500 oC 5.57E-05 0.002360085 86.58 0.08 
600 oC 3.28E-05 0.001811077 164.15 0.03 
700 oC 5.20E-06 0.002280351 15.25 0.44 
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. 
0 2 4 6 8 10
1.04
1.06
1.00
1.02
1.04
0.98
1.00
1.02
0.98
1.00
Time (s)
 Cal. 700 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 5.2E-5
 Cal. 600 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 3.28E-5
(d)
(c)
(b)
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
a
)
 Cal. 500 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 5.57E-5
 Cal. 400 
o
C@ air & 350 
o
C
 Fifth-order polynomial fit
 RSS = 5.05E-5
(a)
0.00 0.25 0.50 0.75 1.00
0
10
20
0
50
100
150
0
25
50
75
0
25
50
H
2
S conc. (ppm)
 Cal. 700 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 15.25
 Cal. 600 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 164.15
(h)
(g)
(f)
R
e
s
p
. 
(R
a
/R
g
)
 Cal. 500 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 86.58
(e)
 Cal. 400 
o
C@ H
2
S & 350 
o
C
 Linear Fit
 Slope = 48.94