NO2 gas sensing characteristics of SNO2 nanofiber - based sensors

d rain, which affects human life and plant 
development [2]. Furthermore, it can be reacted and 
destructed the ozone layer [2]. Therefore, it is 
important to detect and monitor the NO2 gas emitted 
into the air. One-dimensional (1D) nanostructures, 
including nanowires, nanorods, nanotubes, nanobelts, 
and NFs were widely used as a gas sensing element. 
Among them, the NFs was exhibited its outstanding 
advantages. Fig. 1 presents a typical resistive sensor 
configuration. 
Electrospinning is a facile, versatile, 
inexpensive method to directly produce NFs with 
highly porous structure, specific surface area ratio 
[3]. These properties show high potential for gas 
sensing applications. 
Tin dioxide (SnO2) is an n-type semiconductor 
with a rutile structure has a large bandgap of 3.6 eV 
[4], which is one of the most promising materials for 
gas sensing application due to their high carrier 
concentration, high chemical, and thermal stability 
[5]. The gas sensors based on SnO2 NFs have also 
* Corresponding author: Tel.: (+84) 988.138.085 
Email: mhchu@itims.edu.vn/ 
hung.chumanh@hust.edu.vn 
been intensively developed for many reducing gases 
such as ethanol [6], H2 [7]. In our previous work, the 
gas sensors based on SnO2 NFs have shown its ability 
for H2S reducing gas [8]. However, the gas sensors 
based on SnO2 NFs for oxidizing NO2 gas are rarely 
reported. 
SiO2/Si
Cr
Pt
NFs
Fig. 1. The scheme of a typical resistive sensor 
system. 
In the present work, sensors based on SnO2 NFs 
synthesized through electrospinning method, were 
tested gas sensing characteristics towards NO2 gas 
with various concentrations from 1 - 10 ppm at 
different temperatures. The results exhibit the SnO2 
NF sensors have high sensitivity to oxidizing NO2 
gas. 
2. Experimental 
The electrospinning solutions were prepared 
following by the procedure shown in Fig. 2(a). 
Firstly, 1.5g Tin (II) chloride dehydrate was 
dissolved in the ethanol (EOH)/dimethylformamide 
(DMF) solvent (1:1 ratio). Then, 1g 
polyvinylpyrrolidone polymer (PVP, Mw=360.000, 
Sigma-Aldrich Corp.) was added into the above-
solution and was continued stirring at room 
Journal of Science & Technology 142 (2020) 023-027 
24 
temperature for 24h to obtain the desired viscous 
solvent. The solution was loaded into a plastic 
syringe, equipped a stainless needle. In 
electrospinning process, the high voltage of 17 kV 
was generated between the needle and the collector. 
The jet ejected from the needle tip has undergone 
evaporation and whipping instability, finally 
deposited on the collector, which attached to the 
Si/SiO2 substrate. The real electrospinning system 
used in the present work was shown in Fig. 2(b). The 
as-spun fibers were undergone a heat treatment 
process at 600 °C for 3h to remove polymer and to 
form crystalline SnO2 NFs. Details of the synthesis 
process can be found elsewhere [8], [9]. 
Stirring
5g EOH
+5gDMF
Viscous solution
1.5g
SnCl2.2H2O
PVP
2h
24h
(a)
(b)
Fig. 2. (a) The procedure of preparation of the 
electrospinning solution. (b) real electrospinning 
system for the NF synthesis 
The morphology and composition, and crystal 
properties of fibers were characterized using FESEM 
(Hitachi S-4800), EDX attached to FESEM (Hitachi 
S-4800), XRD (D8 Advance, Bruker), respectively. 
The gas sensing characteristic of fibers was tested 
using a home-made gas sensing system [10]. The 
oxidizing gas response (R) of sensors was typically 
defined as R = Rg/Ra where Rg and Ra were the 
resistance in the test gas and air environment, 
respectively. The response and recovery times were 
defined as the time to reach 90% change in resistance 
upon the supply and removal of the target gas, 
respectively. 
3. Results and discussion 
The SEM image (Fig. 3) were showed the 
typical morphology of the NFs. It can be seen the 
nanofibers were randomly deposited on the substrate. 
The average diameter of fibers was approximately 
about 150 nm. 
30 nm1 µm
Fig. 3. SEM images of the SnO2 NFs. 
The high-magnification SEM image showed the 
NFs were composed of many nanograins. Fig. 4(a) 
reveals the EDX spectrum, indicating the presence of 
Sn and O which belong to SnO2 NF component. The 
Si composition in the spectrum is due to the fibers 
were directly deposited on Si/SiO2 substrate [8]. 
The XRD results of SnO2 NFs (Fig. 4(b)) 
indicated that all diffraction peaks at 2θ values of 
26.611°, 33.893°, 37.95°, 51.781°, 54.759°, 57.820°, 
61.872°, corresponding to (110), (101), (200), (211), 
(220), (002), (310) planes of tetragonal SnO2 (JCPDS 
41-1445), respectively [8]. The average grain sizes of 
SnO2 NFs were calculated using the Scherrer 
formula: D=0.9λ/βcosθ, where D is the average 
crystalline size, λ is the X-ray wavelength (0.154 
nm), β and θ are the line broadening at half the 
maximum intensity (FWHM) and the Bragg angle of 
the diffraction peak, respectively. Herein, the highest 
peaks of (110) crystal plane of the tetragonal-SnO2 
were used to determine the average grain size of the 
fibers. It could be found that the average grains size 
of SnO2 NFs were about 13 nm. 
Journal of Science & Technology 142 (2020) 023-027 
25 
0 1 2 3 4 5 6 7 8 9
C
o
u
n
ts
 (
a
.u
)
 Spectrum 1
Sn
Sn
Si
O
Energy (keV)
(a)
(b)
20 30 40 50 60 70
(3
0
1
)
(1
1
2
)
(1
1
1
)
In
te
n
s
it
y
 (
a
.u
.)
2q (degree)
 SnO2 nanofibers
 SnO2 (JCPDS 41-1445)(
1
1
0
)
(1
0
1
)
(2
0
0
)
(2
1
1
)
(2
2
0
)
(0
0
2
)
(3
1
0
)
Fig 4. (a) The EDX spectrum and (b) XRD pattern of 
the SnO2 NFs. 
(b)
(a)
0 500 1000 1500
1
10
100
1
10
100
1
10
100
Time (s)
 NO2 @ 450 
oC
 NO2 @ 400 
oC
2.5 ppm 1 ppm
5 ppm
R
e
s
p
o
n
s
e
 (
R
g
/R
a
)
 NO2 @ 350 
oC10 ppm
350 400 450
0
10
20
30
40
50
60
70
R
e
s
p
o
n
s
e
 (
R
a
/R
g
)
Temperature (oC)
 NO2 @ 10 ppm
 NO2 @ 5 ppm
 NO2 @ 2.5 ppm
 NO2 @ 1 ppm
Fig. 5. (a) NO2 sensing transients of the SnO2 NFs at 
various operating temperatures and (b) Gas responses 
as a function of the NO2 concentration at different 
temperatures. 
Fig. 5(a) shows the transient response of SnO2 
NFs towards NO2 gas as a function of concentration 
from 1 - 10 ppm within a temperature range of 350 - 
450 °C. As can be seen in Fig. 5(b), the response of 
sensors was varied with gas concentration. At high 
concentration, more NO2 molecules absorbed on the 
oxide surface, consequently, the gas response of 
sensors was higher compared to low concentration. It 
can be visualized when the temperature decreased 
from 450 to 350 °C, the response of fibers was 
increased. At the temperature of 350 °C, the response 
of sensors was 53 times. 
The detection limit (DL) is one of the key 
parameters of sensors. The DL value can be 
calculated as DL (ppm) = 3(rmsnoise/S) [11], [12], 
where rmsnoise is the root-mean-square standard 
deviation and S is the slope value of the linear fit of 
the gas response versus gas concentration. The DL of 
the SnO2 NF sensors was found to be 0.015 ppm. 
This value is much lower compared to the threshold 
limit value of American health safety standards (3 
ppm) [13]. 
(a)
(b)
0.0 2.5 5.0 7.5 10.0
20
30
40
50
Slope = 3.59813
R
e
s
p
o
n
s
e
 (
R
g
/R
a
)
 SnO2 Nanofibers @ 350
oC
 @ Linear Fit 
NO2 concentration
0 2 4 6 8 10
0.8
0.9
1.0
1.1
1.2
RSS = 0.00348
R
e
s
p
. 
B
a
s
e
 (
R
a
/R
a
)
 Air @ 350oC
Fifth-order polynomial fit
Time (s) 
Fig. 6. (a) Slope for DL calculation of the SnO2 NFs 
sensors. (b) fitted values of residual sum of square 
(RSS). 
It has been known that when SnO2 NFs were 
placed in the air, the oxygen will capture electrons 
from SnO2, which generates ionosorption species of 
O2-, O-, and O2- on the surface [14], leading to form 
the depletion layer at the surface of the material. 
When SnO2 NF sensors exposed to oxidizing NO2 
gas, it can be reacted with oxygen pre-adsorbed on 
Journal of Science & Technology 142 (2020) 023-027 
26 
the surface of the material or directly trapping 
electrons from the conduction band. The competition 
absorbed reaction can take place between oxygen and 
oxidizing gas as anionic ions. The adsorption of NO2 
gas is considerably stronger compared to oxygen 
[15], [16]. In this case, the adsorbed reaction of NO2 
gas on the surface of n-type semiconductor may be 
dominated, which can be described in the following 
equation [17], [18]: 
NO2(gas) + e- ↔ NO2-(ads) 
NO2(gas) + 2O-(ads) ↔ NO2-(ads) + O2(gas) + e- 
2NO2(gas) + O2-(ads) + 2e- ↔ 2NO2-(ads) + 2O-(ads) 
NO2-(ads) + 2O-(ads) + e- ↔ NO(gas) + ½ O2(gas) + 2O2-(ads) 
The adsorption of oxidizing gas on the n-type 
semiconductor takes electrons away SnO2 NFs, which 
provides more charge density on the surface of 
material. Thus, the electron depletion layer further 
extended leading to increase the potential barrier at 
the surface of material as well as the grains 
boundaries created by nanograins in NFs (Fig. 7). 
Therefore, the sensor resistance increases upon 
exposure to the oxidizing gas [16], [19]. 
In air
O-
O-O
-
In NO2
O-
O-
O-
NO2
-
NO2
-
NO2
-
VSVS
Fig. 7. Schematic illustration the NO2 sensing 
mechanism of the SnO2 NFs. 
The response of the sensors decreased with a 
further decrease in the temperature as shown in Fig. 
8(a). Furthermore, the response-recovery times were 
long (Fig. 8(b)). This phenomenon can be explained 
by the activation energy for the reaction between the 
NO2 gas and the surface of the sensors. At lower 
temperature, the activation energy for the adsorption 
of NO2 is insufficient for physical absorbed while at 
higher temperatures, the NO2 molecular tend to 
escape before absorbed on the surface of the material 
due to their high activation. 
(a)
(b)
250 300 350 400 450
10
20
30
 Response time (s)
 Recovery time (s)
NO2 concentration (ppm)
t r
e
s
. (
s
)
@350oC
0
20
40
60
t r
e
c
. (
s
)
250 300 350 400 450
10
20
30
40
50
60
R
e
s
p
o
n
s
e
 (
R
g
/R
a
)
 NO2 @ 10 ppm
Temperature (oC)
Fig 8. (a) Gas response and (b) response-recovery 
time at different temperatures. 
4. Conclusion 
In the present work, we have fabricated the 
SnO2 NF based sensors for effective detection of NO2 
oxidizing gas. The gas sensing properties of the SnO2 
NF sensors were tested to 1-10 ppm NO2 gas in the 
temperature range from 250 °C to 450 °C. The results 
showed highest response of 53 times to 10 ppm NO2 
at optimal operating temperature. The obtained high 
sensitivity was explained by the surface depletion and 
grain boundary in NFs. 
Acknowledgments 
This research is funded by the Hanoi University 
of Science and Technology (HUST) under project 
code number T2018-PC-070. 
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