Synthesis and Enhanced Ethanol Gas Sensing Properties of the g-C3N4 Nanosheets-Decorated Tin Oxide Flower-Like Nanorods Composite

Abstract

Flower-like SnO2/g-C3N4 nanocomposites were synthesized via a facile hydrothermal method by using SnCl4·5H2O and urea as the precursor. The structure and morphology of the as-synthesized samples were characterized by using the X-ray powder diffraction (XRD), electron microscopy (FESEM and TEM), and Fourier transform infrared spectrometer (FT-IR) techniques. SnO2 displays the unique 3D flower-like microstructure assembled with many uniform nanorods with the lengths and diameters of about 400–600 nm and 50–100 nm, respectively. For the SnO2/g-C3N4 composites, SnO2 flower-like nanorods were coupled by a lamellar structure 2D g-C3N4. Gas sensing performance test results indicated that the response of the sensor based on 7 wt. % 2D g-C3N4-decorated SnO2 composite to 500 ppm ethanol vapor was 150 at 340 °C, which was 3.5 times higher than that of the pure flower-like SnO2 nanorods-based sensor. The gas sensing mechanism of the g-C3N4nanosheets-decorated SnO2 flower-like nanorods was discussed in relation to the heterojunction structure between g-C3N4 and SnO2.

Full text

nanomaterials
Article
Synthesis and Enhanced Ethanol Gas Sensing
Properties of the g-C3N4Nanosheets-Decorated Tin
Oxide Flower-Like Nanorods Composite
Yan Wang 1,2 ID , Jianliang Cao 3,*ID , Cong Qin 3 ,*, Bo Zhang 3, Guang Sun 3and
Zhanying Zhang 3
1The Collaboration Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454000, China;
yanwang@hpu.edu.cn
2State Key Laboratory Cultivation Bases Gas Geology and Gas Control (Henan Polytechnic University),
Jiaozuo 454000, China
3School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000, China;
zhb@hpu.edu.cn (B.Z.); mcsunguang@hpu.edu.cn (G.S.); zhangzy@hpu.edu.cn (Z.Z.)
*Correspondence: caojianliang@hpu.edu.cn (J.C.); qincongxy@163.com (C.Q.);
Tel.: +86-391-398-7440 (J.C. & C.Q.)
Received: 23 August 2017; Accepted: 18 September 2017; Published: 22 September 2017
Abstract:
Flower-like SnO
2
/g-C
3
N
4
nanocomposites were synthesized via a facile hydrothermal
method by using SnCl
4·
5H
2
O and urea as the precursor. The structure and morphology of the
as-synthesized samples were characterized by using the X-ray powder diffraction (XRD), electron
microscopy (FESEM and TEM), and Fourier transform infrared spectrometer (FT-IR) techniques. SnO
2
displays the unique 3D flower-like microstructure assembled with many uniform nanorods with
the lengths and diameters of about 400–600 nm and 50–100 nm, respectively. For the SnO
2
/g-C
3
N
4
composites, SnO
2
flower-like nanorods were coupled by a lamellar structure 2D g-C
3
N
4
. Gas
sensing performance test results indicated that the response of the sensor based on 7 wt. % 2D
g-C
3
N
4
-decorated SnO
2
composite to 500 ppm ethanol vapor was 150 at 340
◦
C, which was 3.5 times
higher than that of the pure flower-like SnO
2
nanorods-based sensor. The gas sensing mechanism
of the g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods was discussed in relation to the
heterojunction structure between g-C3N4and SnO2.
Keywords:
nanocomposites; microstructure; gas sensor; flower-like SnO
2
nanorod; graphitic
carbon nitride
1. Introduction
As an n-type metal-oxide semiconductor, tin oxide (SnO
2
) has wide applications in many fields,
such as lithium-ion batteries [
1
], photocatalysis [
2
], and gas sensors [
3
]. SnO
2
has been investigated as
a typical semiconductor gas sensor to ethanol because of its unique chemical properties and crystal
structure [
4
]. As is known, the gas-sensing performance of SnO
2
-based sensors can be improved by
means of morphology and size control. Hence, diverse shape-controlled SnO
2
nanostructures have been
synthesized, such as nanoflower [
5
], nanoarray [
6
], nanoplate [
7
], and nanowire [
8
]. These SnO
2
-based
sensors exhibited good sensing properties, including low-cost and fast response and recovery.
However, there are some limitations which prevent the direct application of these sensors, such
as poor electrical characteristics, high work temperature, and a low response [
9
]. Coupling SnO
2
with other semiconductors to construct the heterojunction structure could be an efficient way to
overcome these disadvantages. Therefore, many SnO
2
-based composites such as
SnO2/r-GO [10–15],
SnO
2
/ZnO [
16
–
19
], SnO
2
/Fe
2
O
3
[
20
–
23
], and SnO
2
/NiO [
24
–
26
] have been synthesized as
Nanomaterials 2017,7, 285; doi:10.3390/nano7100285 www.mdpi.com/journal/nanomaterials

Nanomaterials 2017,7, 285 2 of 14
high-efficiency gas sensors. Graphitic carbon nitride (g-C
3
N
4
) is a two-dimensional (2D) semiconductor
with a 2.7 eV band gap, which possesses good chemical stability and a large surface area. It is available
to form an n/n junction structure with SnO
2
[
27
]. For example, SnO
2
/g-C
3
N
4
nanocomposites with
a strong heterojunction structure were designed and fabricated. The photocatalytic activity of the
SnO
2
/g-C
3
N
4
nanocomposites exhibited enhanced catalytic activity and stable cycle property [
28
].
Zhang et al. prepared the
α
-Fe
2
O
3
/g-C
3
N
4
heterostructural nanocomposites as an ethanol gas sensor,
and the composites exhibited a high response value (S = 7.76) to 100 ppm ethanol under a working
temperature of 340
◦
C [
29
]. Zeng et al. successfully fabricated the
α
-Fe
2
O
3
/g-C
3
N
4
composites for the
cataluminescence sensing of H
2
S [
30
]. An efficient dielectric barrier discharge (DBD) plasma-assisted
method for the fabrication of the g-C
3
N
4
-Mn
3
O
4
composite was investigated by Hu et al., which
displayed a highly selective, sensitive, and linear cataluminescence (CTL) response towards H
2
S
gas [
31
]. Sanjay Mathur et al. synthesized SnO
2
nanowires via the CVD method, and the SnO
2
nanowires exhibited an excellent photoresponse performance [
32
]. Kuang et al. have synthesized
high-yield SnO
2
nanowires via an Au catalytic vapor-liquid-solid (VLS) growth process and the
SnO
2
nanowire-based humidity sensor displayed a fast response and high sensitivity to relative
humidity changes at room temperature [
33
]. The three-dimensional network’s SnO
2
nanowire was
prepared via a flame-based thermal oxidation process (FTS) and applied for ethanol sensing [
34
].
Oleg Lupan et al.
investigated the hybrid networks of heterogeneous shell-core Ga
2
O
3
/GaN:O
x
@SnO
2
nano- and micro-cables with a shell in mixed phases for improving sensor properties [
35
]. However, to
our best knowledge, there is still no research focused on the design and gas sensing application of the
g-C3N4nanosheets-decorated SnO2flower-like nanorods.
Herein, the hydrothermal method was utilized for the first time to synthesis the g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods for the ethanol sensing application. It was found that
the g-C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods composite possesses a much higher
response value, repeatability, and stability to ethanol vapor than pure flower-like SnO2nanorods.
2. Results and Discussion
2.1. Sample Characterization
The pure SnO
2
and SnO
2
/g-C
3
N
4
composites with 5, 7, and 9 wt % g-C
3
N
4
contents were
synthesized by a facile hydrothermal method. Also, the as-prepared samples were marked as
SnO
2
/g-C
3
N
4
-5, SnO
2
/g-C
3
N
4
-7, and SnO
2
/g-C
3
N
4
-9, respectively. Figure 1displays the XRD patterns
of the synthesized SnO
2
, g-C
3
N
4
, and g-C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods
(SnO
2
/g-C
3
N
4
) composites with different g-C
3
N
4
contents. One can see from the XRD pattern that two
diffraction peaks at 13.1
◦
and 27.5
◦
can be observed for pure g-C
3
N
4;
these two peaks were accorded
to the (100) plane and (002) plane of g-C
3
N
4
,which could be due to the inter-layer structure of the
tri-s-triazine unit with interplannar spacing and the conjugated aromatic system, respectively [
36
].
The XRD patterns of SnO
2
/g-C
3
N
4
composites show some diffraction peaks at 26.61
◦
, 33.89
◦
, 37.94
◦
,
and 51.78
◦
, which could be assigned to the (110), (101), (200), and (211) planes of the tetragonal rutile
structure SnO
2
(JCPDS Card No. 41-1445). However, the diffraction peaks of g-C
3
N
4
are not observed
in the SnO2/g-C3N4composites. This could be due to the small content of g-C3N4.
The microstructure and morphology of the synthesized samples were verified by using FESEM
and TEM. One can see clearly from Figure 2a that the morphology of the as-prepared g-C
3
N
4
possesses
many wrinkles with overlaps at the edges, demonstrating the existence of the two dimensional (2D)
nano-lamellar structure. It can be observed from Figure 2b that the pure SnO
2
product displays
the unique 3D flower-like microstructure assembled with many uniform nanorods. The lengths
and diameters of a single nanorod are about 400–600 nm and 50–100 nm, respectively. For the
g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods composite, as shown in Figure 2c, the
SnO
2
flower-like nanorods were closely coupled by g-C
3
N
4
nanosheets. A proposed growth
mechanism of SnO
2
flower-like nanorods can be summarized by crystal growth and nucleation

Nanomaterials 2017,7, 285 3 of 14
theory. The SnO
2
nanocrystals nucleation points are generated in different orientations. Therefore, the
SnO
2
nanorods grow in irregular directions and finally formed into 3D flower-like structures. Figure 2f
displays the typical TEM image of pure g-C
3
N
4
, and g-C
3
N
4
possesses two dimensional nanosheets
structure with many wrinkles. The TEM images of g-C
3
N
4
nanosheets-decorated tin oxide flower-like
nanorods composites are displayed in Figure 2d,e, and SnO
2
flower-like nanorods were coupled by
a lamellar structure, which is 2D g-C
3
N
4
. Thus, we can conclude that g-C
3
N
4
nanosheets-decorated
SnO
2
flower-like nanorods composites were successfully synthesized by the hydrothermal method
combining the above analysis results offered by XRD, FESEM, and TEM.
Nanomaterials 2017, 7, 285 3 of 13
structure with many wrinkles. The TEM images of g-C
3
N
4
nanosheets-decorated tin oxide flower-like
nanorods composites are displayed in Figure 2d,e, and SnO
2
flower-like nanorods were coupled by
a lamellar structure, which is 2D g-C
3
N
4
. Thus, we can conclude that g-C
3
N
4
nanosheets-decorated
SnO
2
flower-like nanorods composites were successfully synthesized by the hydrothermal method
combining the above analysis results offered by XRD, FESEM, and TEM.
20 40 60 80
SnO2/g-C3N4-9
SnO2/g-C3N4-7
Intensity (a.u.)
2 Theta (deg.)
g-C3N4
SnO2
SnO2/g-C3N4-5
(100) (002)
(110) (101)
(200) (211)
Figure 1. X-ray powder diffraction (XRD) patterns of the synthesized SnO
2
, g-C
3
N
4
, and g-
C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods composites (SnO
2
/g-C
3
N
4
-5, SnO
2
/g-C
3
N
4
-
7, and SnO
2
/g-C
3
N
4
-9).
Figure 2. Field-emission scanning electron microscopy (FESEM) images of pure g-C
3
N
4
(a); SnO
2
flower-like nanorods (b); SnO
2
/g-C
3
N
4
nanocomposite (c); and transmission electron microscopy
(TEM) images of the SnO
2
/g-C
3
N
4
composite (d,e) and pure g-C
3
N
4
(f).
Figure 1.
X-ray powder diffraction (XRD) patterns of the synthesized SnO
2
, g-C
3
N
4
,
and g-C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods composites (SnO
2
/g-C
3
N
4
-5,
SnO2/g-C3N4-7, and SnO2/g-C3N4-9).
Nanomaterials 2017, 7, 285 3 of 13
structure with many wrinkles. The TEM images of g-C
3
N
4
nanosheets-decorated tin oxide flower-like
nanorods composites are displayed in Figure 2d,e, and SnO
2
flower-like nanorods were coupled by
a lamellar structure, which is 2D g-C
3
N
4
. Thus, we can conclude that g-C
3
N
4
nanosheets-decorated
SnO
2
flower-like nanorods composites were successfully synthesized by the hydrothermal method
combining the above analysis results offered by XRD, FESEM, and TEM.
20 40 60 80
SnO2/g-C3N4-9
SnO2/g-C3N4-7
Intensity (a.u.)
2 Theta (deg.)
g-C3N4
SnO2
SnO2/g-C3N4-5
(100) (002)
(110) (101)
(200) (211)
Figure 1. X-ray powder diffraction (XRD) patterns of the synthesized SnO
2
, g-C
3
N
4
, and g-
C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods composites (SnO
2
/g-C
3
N
4
-5, SnO
2
/g-C
3
N
4
-
7, and SnO
2
/g-C
3
N
4
-9).
Figure 2. Field-emission scanning electron microscopy (FESEM) images of pure g-C
3
N
4
(a); SnO
2
flower-like nanorods (b); SnO
2
/g-C
3
N
4
nanocomposite (c); and transmission electron microscopy
(TEM) images of the SnO
2
/g-C
3
N
4
composite (d,e) and pure g-C
3
N
4
(f).
Figure 2.
Field-emission scanning electron microscopy (FESEM) images of pure g-C
3
N
4
(
a
); SnO
2
flower-like nanorods (
b
); SnO
2
/g-C
3
N
4
nanocomposite (
c
); and transmission electron microscopy
(TEM) images of the SnO2/g-C3N4composite (d,e) and pure g-C3N4(f).

Nanomaterials 2017,7, 285 4 of 14
Figure 3exhibits the FT-IR spectra of g-C
3
N
4
, SnO
2
, and SnO
2
/g-C
3
N
4
-7 samples. As can be
seen from Figure 3b, the broad absorption peaks could be observed at wave-numbers of 570 cm
−1
and 660 cm
−1
, which could be assigned to the Sn–O characteristic peaks. In Figure 3a,c, the peaks in
the range of 1240–1637 cm
−1
are ascribed to the C–N and C=N stretching vibration modes, and the
peak at 808 cm
−1
corresponds to the triazine units. These two sets of characteristic vibration peaks are
characteristic of g-C
3
N
4
. As is shown in Figure 3c, all the characteristic peaks of SnO
2
and g-C
3
N
4
can be observed clearly. These results make up for our XRD analysis, in which g-C
3
N
4
and SnO
2
are
coexisting in the SnO
2
/g-C
3
N
4
composites. Compared with pure g-C
3
N
4
, there is a slight red shift in
the bands of g-C
3
N
4
in the composite. This result indicates that there is an interaction between SnO
2
and g-C3N4[27], which is beneficial to the gas sensing application.
Nanomaterials 2017, 7, 285 4 of 13
Figure 3 exhibits the FT-IR spectra of g-C3N4, SnO2, and SnO2/g-C3N4-7 samples. As can be seen
from Figure 3b, the broad absorption peaks could be observed at wave-numbers of 570 cm−1 and 660
cm−1, which could be assigned to the Sn–O characteristic peaks. In Figure 3a,c, the peaks in the range
of 1240–1637 cm−1 are ascribed to the C–N and C=N stretching vibration modes, and the peak at 808
cm−1 corresponds to the triazine units. These two sets of characteristic vibration peaks are
characteristic of g-C3N4. As is shown in Figure 3c, all the characteristic peaks of SnO2 and g-C3N4 can
be observed clearly. These results make up for our XRD analysis, in which g-C3N4 and SnO2 are
coexisting in the SnO2/g-C3N4 composites. Compared with pure g-C3N4, there is a slight red shift in
the bands of g-C3N4 in the composite. This result indicates that there is an interaction between SnO2
and g-C3N4 [27], which is beneficial to the gas sensing application.
500 1000 1500 2000 250
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Transmittance (%)
Wavenumber (cm-1)
(a) g-C3N4
(b) SnO2
(c) SnO2/g-C3N4-7 (a)
(b)
(c)
Sn-O C-N C=N
1242 cm-1
1240 cm-1 1637 cm-1
808 cm-1
Figure 3.Fourier transform infrared spectrometer (FT-IR) spectra of g-C3N4 (a); SnO2 (b); and SnO2/g-
C3N4-7 (c) nanocomposite.
TG analysis was investigated by heating up from room temperature to 800 °C at a heating rate
of 5 °C·min−1 to reveal the weight change of g-C3N4. It can be seen from Figure4 that the weight of g-
C3N4 is set constant at temperature below 500 °C. When the temperature increases to 510 °C, the
weight of g-C3N4 starts to decrease (the combustion of g-C3N4 in air). The weight stays at the same
level when the temperature exceeds 655 °C. It can be concluded that g-C3N4 is stable at low
temperature and burn at high temperature. This phenomenon demonstrates that g-C3N4 could stably
exist in the composite under the operating temperature in the range of 200–400 °C in the gas-sensing
test process.
100 200 300 400 500 600 700 800
0
20
40
60
80
100
Weight Loss(%)
Temperature (oC)
Figure 4. Thermogravimetry (TG) analysis for heating the g-C3N4from room temperature to 800 °C.
Figure 3.
Fourier transform infrared spectrometer (FT-IR) spectra of g-C
3
N
4
(
a
); SnO
2
(
b
); and
SnO2/g-C3N4-7 (c) nanocomposite.
TG analysis was investigated by heating up from room temperature to 800
◦
C at a heating rate
of 5
◦
C
·
min
−1
to reveal the weight change of g-C
3
N
4
. It can be seen from Figure 4that the weight
of g-C
3
N
4
is set constant at temperature below 500
◦
C. When the temperature increases to 510
◦
C,
the weight of g-C
3
N
4
starts to decrease (the combustion of g-C
3
N
4
in air). The weight stays at the
same level when the temperature exceeds 655
◦
C. It can be concluded that g-C
3
N
4
is stable at low
temperature and burn at high temperature. This phenomenon demonstrates that g-C
3
N
4
could stably
exist in the composite under the operating temperature in the range of 200–400
◦
C in the gas-sensing
test process.
Nanomaterials 2017, 7, 285 4 of 13
Figure 3 exhibits the FT-IR spectra of g-C3N4, SnO2, and SnO2/g-C3N4-7 samples. As can be seen
from Figure 3b, the broad absorption peaks could be observed at wave-numbers of 570 cm−1 and 660
cm−1, which could be assigned to the Sn–O characteristic peaks. In Figure 3a,c, the peaks in the range
of 1240–1637 cm−1 are ascribed to the C–N and C=N stretching vibration modes, and the peak at 808
cm−1 corresponds to the triazine units. These two sets of characteristic vibration peaks are
characteristic of g-C3N4. As is shown in Figure 3c, all the characteristic peaks of SnO2 and g-C3N4 can
be observed clearly. These results make up for our XRD analysis, in which g-C3N4 and SnO2 are
coexisting in the SnO2/g-C3N4 composites. Compared with pure g-C3N4, there is a slight red shift in
the bands of g-C3N4 in the composite. This result indicates that there is an interaction between SnO2
and g-C3N4 [27], which is beneficial to the gas sensing application.
500 1000 1500 2000 250
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Transmittance (%)
Wavenumber (cm-1)
(a) g-C3N4
(b) SnO2
(c) SnO2/g-C3N4-7 (a)
(b)
(c)
Sn-O C-N C=N
1242 cm-1
1240 cm-1 1637 cm-1
808 cm-1
Figure 3.Fourier transform infrared spectrometer (FT-IR) spectra of g-C3N4 (a); SnO2 (b); and SnO2/g-
C3N4-7 (c) nanocomposite.
TG analysis was investigated by heating up from room temperature to 800 °C at a heating rate
of 5 °C·min−1 to reveal the weight change of g-C3N4. It can be seen from Figure4 that the weight of g-
C3N4 is set constant at temperature below 500 °C. When the temperature increases to 510 °C, the
weight of g-C3N4 starts to decrease (the combustion of g-C3N4 in air). The weight stays at the same
level when the temperature exceeds 655 °C. It can be concluded that g-C3N4 is stable at low
temperature and burn at high temperature. This phenomenon demonstrates that g-C3N4 could stably
exist in the composite under the operating temperature in the range of 200–400 °C in the gas-sensing
test process.
100 200 300 400 500 600 700 800
0
20
40
60
80
100
Weight Loss(%)
Temperature (oC)
Figure 4. Thermogravimetry (TG) analysis for heating the g-C3N4from room temperature to 800 °C.
Figure 4. Thermogravimetry (TG) analysis for heating the g-C3N4from room temperature to 800 ◦C.

Nanomaterials 2017,7, 285 5 of 14
2.2. Sensing Performance Tests
In order to investigate the gas sensing performance of the synthesized samples-based
sensor to ethanol, a series of tests were performed. The response values (R
a
/R
g
) of the
g-C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods composite and the pure flower-like
SnO
2
-based gas sensors toward 500 ppm ethanol vapor were measured under different operating
temperature. With the increase of operating temperature, one can see from Figure 5a that all of the
samples exhibited the similar variation tendency. Also, the response values of the SnO
2
/g-C
3
N
4
-based
sensors reached the maximum value at 340
◦
C, while the maximum value of pure SnO
2
was 70 at 360
◦
C.
This result shows that the optimum operating temperature of SnO
2
/g-C
3
N
4
decreased compared with
that of pure SnO
2
. This result may be due to the fact that the chemisorbed oxygen species can achieve
the required energy and effectively react with ethanol vapor molecules on sample surface varying
the resistance significantly [
37
]. The response value of the SnO
2
/g-C
3
N
4
composite-based sensor is
much higher than pure SnO
2
. The response values increased with adding the content of g-C
3
N
4
from
5 wt. % to 7 wt. % and decreased with further increase of g-C
3
N
4
content. The response values of the
pure SnO
2
, SnO
2
/g-C
3
N
4
composites with g-C
3
N
4
contents of 5, 7, and 9 wt. % to 500 ppm ethanol
vapor at 340
◦
C are 43, 125, 150, and 135, respectively, indicating that the addition of g-C
3
N
4
has a
great influence on enhancing the gas sensing performance. When the mass percentage of g-C
3
N
4
in
the composites is 7 wt. %, the response reaches the maximum value. A suitable content of g-C
3
N
4
in the composite is beneficial to form the preferable heterojunction structure in the interface region
between flower-like nanorods SnO
2
and 2D g-C
3
N
4
. However, much higher addition of g-C
3
N
4
may
result in the formation of the connection of bulk. This will further decrease the specific surface area of
the sample and reduce the active sites for oxygen and ethanol gas adsorption, further leading to the
degradation of sensing performance. Hence, the optimum operating temperature is 340
◦
C and the
optimum g-C
3
N
4
content is 7 wt. % for this g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods
nanomaterial. Therefore, all of the further research was carried out by using SnO
2
/g-C
3
N
4
-7 composite
sensor at 340
◦
C. Figure 5b displays the response values of the four samples to different ethanol vapors
in the concentration range of 100–3000 ppm at 340
◦
C. With the increase of ethanol concentration, the
response values increased for all of these four sensors. The curves increased rapidly in the range of
100–500 ppm and increased slowly with the increasing concentrations from 500 ppm to 3000 ppm.
Nanomaterials 2017, 7, 285 5 of 13
2.2. Sensing Performance Tests
In order to investigate the gas sensing performance of the synthesized samples-based sensor to
ethanol, a series of tests were performed. The response values (Ra/Rg) of the g-C3N4nanosheets-
decorated tin oxide flower-like nanorods composite and the pure flower-like SnO2-based gas sensors
toward 500 ppm ethanol vapor were measured under different operating temperature. With the
increase of operating temperature, one can see from Figure 5a that all of the samples exhibited the
similar variation tendency. Also, the response values of the SnO2/g-C3N4-based sensors reached the
maximum value at 340 °C, while the maximum value of pure SnO2 was 70 at 360 °C. This result shows
that the optimum operating temperature of SnO2/g-C3N4 decreased compared with that of pure SnO2.
This result may be due to the fact that the chemisorbed oxygen species can achieve the required
energy and effectively react with ethanol vapor molecules on sample surface varying the resistance
significantly [37]. The response value of the SnO2/g-C3N4 composite-based sensor is much higher than
pure SnO2. The response values increased with adding the content of g-C3N4 from 5 wt. % to 7 wt. %
and decreased with further increase of g-C3N4 content. The response values of the pure SnO2, SnO2/g-
C3N4 composites with g-C3N4 contents of 5, 7, and 9 wt. % to 500 ppm ethanol vapor at 340 °C are 43,
125, 150, and 135, respectively, indicating that the addition of g-C3N4 has a great influence on
enhancing the gas sensing performance. When the mass percentage of g-C3N4 in the composites is 7
wt. %, the response reaches the maximum value. A suitable content of g-C3N4 in the composite is
beneficial to form the preferable heterojunction structure in the interface region between flower-like
nanorods SnO2 and 2D g-C3N4. However, much higher addition of g-C3N4may result in the formation
of the connection of bulk. This will further decrease the specific surface area of the sample and reduce
the active sites for oxygen and ethanol gas adsorption, further leading to the degradation of sensing
performance. Hence, the optimum operating temperature is 340 °C and the optimum g-C3N4 content
is 7 wt. % for this g-C3N4 nanosheets-decorated SnO2 flower-like nanorods nanomaterial. Therefore,
all of the further research was carried out by using SnO2/g-C3N4-7 composite sensor at 340 °C. Figure
5b displays the response values of the four samples to different ethanol vapors in the concentration
range of 100–3000 ppm at 340°C. With the increase of ethanol concentration, the response values
increased for all of these four sensors. The curves increased rapidly in the range of 100–500 ppm and
increased slowly with the increasing concentrations from 500 ppm to 3000 ppm.
(a) (b)
Figure 5. The response values of the SnO2, SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9 to 500
ppm ethanol (a) under different operating temperatures; (b) for different concentrations of ethanol at
340 °C.
Figure 6a shows the real time response curves of the pure SnO2 and SnO2/g-C3N4-7 to ethanol in
the range of 100–3000 ppm at 340 °C. All of the response-recovery cycles were measured about 300 s
with a response interval and a recovery interval of 150 s, respectively. We can observe from Figure
6a that the two samples show a similar trend: the response values increase with increasing ethanol
concentration. To the same concentration of ethanol, SnO2/g-C3N4-7 exhibited much higher response
Figure 5.
The response values of the SnO
2
, SnO
2
/g-C
3
N
4
-5, SnO
2
/g-C
3
N
4
-7, and SnO
2
/g-C
3
N
4
-9 to
500 ppm ethanol (
a
) under different operating temperatures; (
b
) for different concentrations of ethanol
at 340 ◦C.
Figure 6a shows the real time response curves of the pure SnO
2
and SnO
2
/g-C
3
N
4
-7 to ethanol
in the range of 100–3000 ppm at 340
◦
C. All of the response-recovery cycles were measured about
300 s with a response interval and a recovery interval of 150 s, respectively. We can observe from

Nanomaterials 2017,7, 285 6 of 14
Figure 6a that the two samples show a similar trend: the response values increase with increasing
ethanol concentration. To the same concentration of ethanol, SnO
2
/g-C
3
N
4
-7 exhibited much higher
response value than that of pure SnO
2
. The response value of the SnO
2
/g-C
3
N
4
-7 composite-based
sensor towards 500 ppm ethanol vapor was about 150, about four times higher than pure SnO
2
. As is
known, response-recovery time is another very important influential factor on the application of gas
sensor. Figure 6b shows the response-recovery time curve of the SnO
2
/g-C
3
N
4
-7 composite toward
500 ppm ethanol vapor. As seen from the curve, when the sensor was exposed and separated to
ethanol, the response increased rapidly (31 s) and also decreased rapidly (24 s), respectively.
Nanomaterials 2017, 7, 285 6 of 13
value than that of pure SnO
2
. The response value of the SnO
2
/g-C
3
N
4
-7 composite-based sensor
towards 500 ppm ethanol vapor was about 150, about four times higher than pure SnO
2
. As is known,
response-recovery time is another very important influential factor on the application of gas sensor.
Figure 6b shows the response-recovery time curve of the SnO
2
/g-C
3
N
4
-7 composite toward 500 ppm
ethanol vapor. As seen from the curve, when the sensor was exposed and separated to ethanol, the
response increased rapidly (31 s) and also decreased rapidly (24 s), respectively.
(a) (b)
Figure 6. (a) The real time response curves of the SnO
2
and SnO
2
/g-C
3
N
4
-7 composite sensors toward
ethanol vapor; (b) response-recovery time characteristics of the SnO
2
/g-C
3
N
4
-7 based sensor to 500
ppm ethanol vapor at 340 °C.
The repeatability and stability are both crucial influence factors of gas sensing performances. As
is shown in Figure 7a, the response values of these four response-recovery cycles of SnO
2
/g-C
3
N
4
-7
sensor stay almost the same (165, 160, 167, and 155) toward 500 ppm ethanol at 340 °C. This result
indicated that the synthesized SnO
2
/g-C
3
N
4
-7 composite possesses an admirable repeatability for
ethanol vapor detection. Figure 7b displays the stability test result of SnO
2
/g-C
3
N
4
-7 composite sensor
toward 500 ppm ethanol vapor at 340 °C, and the response values were kept at a stability of around
155 after 30 days test, indicating that the synthesized g-C
3
N
4
nanosheets-decorated SnO
2
flower-like
nanorods composite possesses an excellent stability.
(a) (b)
Figure 7. (a) Repeatability and (b) stability measurements of the SnO
2
/g-C
3
N
4
-7-based sensor to 500
ppm ethanol at 340 °C.
As is well known, the selectivity of the sensor for the different gases is one of the most important
factors for its practical application. Figure 8 displays the selectivity test results of SnO
2
flower-like
nanorods and SnO
2
/g-C
3
N
4
-7 composite to methanol, ethanol, toluene, formaldehyde, and acetone
with the concentration of 500 ppm. The test results indicated that the sample possesses the superior
Figure 6.
(
a
) The real time response curves of the SnO
2
and SnO
2
/g-C
3
N
4
-7 composite sensors
toward ethanol vapor; (
b
) response-recovery time characteristics of the SnO
2
/g-C
3
N
4
-7 based sensor
to 500 ppm ethanol vapor at 340 ◦C.
The repeatability and stability are both crucial influence factors of gas sensing performances. As
is shown in Figure 7a, the response values of these four response-recovery cycles of SnO
2
/g-C
3
N
4
-7
sensor stay almost the same (165, 160, 167, and 155) toward 500 ppm ethanol at 340
◦
C. This result
indicated that the synthesized SnO
2
/g-C
3
N
4
-7 composite possesses an admirable repeatability for
ethanol vapor detection. Figure 7b displays the stability test result of SnO
2
/g-C
3
N
4
-7 composite sensor
toward 500 ppm ethanol vapor at 340
◦
C, and the response values were kept at a stability of around
155 after 30 days test, indicating that the synthesized g-C
3
N
4
nanosheets-decorated SnO
2
flower-like
nanorods composite possesses an excellent stability.
Nanomaterials 2017, 7, 285 6 of 13
value than that of pure SnO
2
. The response value of the SnO
2
/g-C
3
N
4
-7 composite-based sensor
towards 500 ppm ethanol vapor was about 150, about four times higher than pure SnO
2
. As is known,
response-recovery time is another very important influential factor on the application of gas sensor.
Figure 6b shows the response-recovery time curve of the SnO
2
/g-C
3
N
4
-7 composite toward 500 ppm
ethanol vapor. As seen from the curve, when the sensor was exposed and separated to ethanol, the
response increased rapidly (31 s) and also decreased rapidly (24 s), respectively.
(a) (b)
Figure 6. (a) The real time response curves of the SnO
2
and SnO
2
/g-C
3
N
4
-7 composite sensors toward
ethanol vapor; (b) response-recovery time characteristics of the SnO
2
/g-C
3
N
4
-7 based sensor to 500
ppm ethanol vapor at 340 °C.
The repeatability and stability are both crucial influence factors of gas sensing performances. As
is shown in Figure 7a, the response values of these four response-recovery cycles of SnO
2
/g-C
3
N
4
-7
sensor stay almost the same (165, 160, 167, and 155) toward 500 ppm ethanol at 340 °C. This result
indicated that the synthesized SnO
2
/g-C
3
N
4
-7 composite possesses an admirable repeatability for
ethanol vapor detection. Figure 7b displays the stability test result of SnO
2
/g-C
3
N
4
-7 composite sensor
toward 500 ppm ethanol vapor at 340 °C, and the response values were kept at a stability of around
155 after 30 days test, indicating that the synthesized g-C
3
N
4
nanosheets-decorated SnO
2
flower-like
nanorods composite possesses an excellent stability.
(a) (b)
Figure 7. (a) Repeatability and (b) stability measurements of the SnO
2
/g-C
3
N
4
-7-based sensor to 500
ppm ethanol at 340 °C.
As is well known, the selectivity of the sensor for the different gases is one of the most important
factors for its practical application. Figure 8 displays the selectivity test results of SnO
2
flower-like
nanorods and SnO
2
/g-C
3
N
4
-7 composite to methanol, ethanol, toluene, formaldehyde, and acetone
with the concentration of 500 ppm. The test results indicated that the sample possesses the superior
Figure 7.
(
a
) Repeatability and (
b
) stability measurements of the SnO
2
/g-C
3
N
4
-7-based sensor to
500 ppm ethanol at 340 ◦C.

Nanomaterials 2017,7, 285 7 of 14
As is well known, the selectivity of the sensor for the different gases is one of the most important
factors for its practical application. Figure 8displays the selectivity test results of SnO
2
flower-like
nanorods and SnO
2
/g-C
3
N
4
-7 composite to methanol, ethanol, toluene, formaldehyde, and acetone
with the concentration of 500 ppm. The test results indicated that the sample possesses the superior
selectivity to ethanol vapor at the operating temperature of 340
◦
C. The high selectivity to ethanol
maybe come from the fact that when reacted with the absorbed oxygen, ethanol is more likely to lose
electrons, and hydroxyl group (–OH) is easy to oxidize under the optimum operating condition.
Nanomaterials 2017, 7, 285 7 of 13
selectivity to ethanol vapor at the operating temperature of 340 °C. The high selectivity to ethanol
maybe come from the fact that when reacted with the absorbed oxygen, ethanol is more likely to lose
electrons, and hydroxyl group (–OH) is easy to oxidize under the optimum operating condition.
Figure 8. Comparision of the response values of the flower-like SnO
2
nanorods and the SnO
2
/g-C
3
N
4
-
7 composite toward 500 ppm different gases at 340 °C.
Table 1 lists the sensing performances of different materials to ethanol vapor. One can observe
from Table 1 that the RGO/hollow SnO
2
nanoparticles [11], hollow ZnO/SnO
2
spheres [16], tubular α-
Fe
2
O
3
/g-C
3
N
4
[29], and Au/3D SnO
2
microstructure [38] samples possess the response values to
ethanol of 70.4, 78.2, 7.76, and 30, respectively. In our research, the 7 wt. % 2D g-C
3
N
4
decorated SnO
2
flower-like nanorods composite possesses the response value of 85 to 100 ppm ethanol vapor at 340
°C, indicating a great potential application of the synthesized sample to ethanol detection.
Table 1. The ethanol sensing performance of the previous reported results and this work.
Materials Ethanol Vapo
r
Concentration (ppm)
Temperature
(°C)
Response
(R
a
/R
g
) Ref.
RGO/hollow SnO
2
100 300 70.4 [11]
Hollow ZnO/SnO
2
spheres 100 225 78.2 [16]
α-Fe
2
O
3
/g-C
3
N
4
100 340 7.76 [29]
Au/3D SnO
2
microstructure 150 340 30 [38]
SnO
2
/g-C
3
N
4
100 340 85 this work
2.3. Mechanism Discussion
SnO
2
flower-like nanorods have been synthesized via a hydrothermal reaction method, and the
schematic diagram of the synthesis of the g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods is
displayed in Figure 9. Many researchers hold the point that the diameter of the SnO
2
nanorods is
changed by varying the Sn
+
/OH
−
ratio in solution [39]. However, Vuong et al. declared that the
diameter of SnO
2
nanorods decreased with the increase of the stannic chloride amount. A proposed
growth mechanism of SnO
2
flower-like nanorods can be summarized by crystal growth and
nucleation theory. The synthesis process includes two sections of nucleation and crystal growth. In
the hydrothermal condition, the Sn(OH)
62−
nucleus is formed via the following chemical reactions in
the process of nucleation stage [40]:
Sn +4OH
↔Sn
󰇛OH󰇜 (1)
Sn󰇛OH󰇜+2OH
↔Sn
󰇛OH󰇜
 (2)
Figure 8.
Comparision of the response values of the flower-like SnO
2
nanorods and the SnO
2
/g-C
3
N
4
-7
composite toward 500 ppm different gases at 340 ◦C.
Table 1lists the sensing performances of different materials to ethanol vapor. One can observe
from Table 1that the RGO/hollow SnO
2
nanoparticles [
11
], hollow ZnO/SnO
2
spheres [
16
], tubular
α
-Fe
2
O
3
/g-C
3
N
4
[
29
], and Au/3D SnO
2
microstructure [
38
] samples possess the response values to
ethanol of 70.4, 78.2, 7.76, and 30, respectively. In our research, the 7 wt. % 2D g-C
3
N
4
decorated SnO
2
flower-like nanorods composite possesses the response value of 85 to 100 ppm ethanol vapor at 340
◦
C,
indicating a great potential application of the synthesized sample to ethanol detection.
Table 1. The ethanol sensing performance of the previous reported results and this work.
Materials Ethanol Vapor
Concentration (ppm) Temperature (◦C) Response (Ra/Rg) Ref.
RGO/hollow SnO2100 300 70.4 [11]
Hollow ZnO/SnO2spheres 100 225 78.2 [16]
α-Fe2O3/g-C3N4100 340 7.76 [29]
Au/3D SnO
2
microstructure
150 340 30 [38]
SnO2/g-C3N4100 340 85 this work
2.3. Mechanism Discussion
SnO
2
flower-like nanorods have been synthesized via a hydrothermal reaction method, and the
schematic diagram of the synthesis of the g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods
is displayed in Figure 9. Many researchers hold the point that the diameter of the SnO
2
nanorods
is changed by varying the Sn
+
/OH
−
ratio in solution [
39
]. However, Vuong et al. declared that the
diameter of SnO
2
nanorods decreased with the increase of the stannic chloride amount. A proposed

Nanomaterials 2017,7, 285 8 of 14
growth mechanism of SnO
2
flower-like nanorods can be summarized by crystal growth and nucleation
theory. The synthesis process includes two sections of nucleation and crystal growth. In the
hydrothermal condition, the Sn(OH)
62−
nucleus is formed via the following chemical reactions in the
process of nucleation stage [40]:
Sn4++4OH−↔Sn(OH)4(1)
Sn(OH)4+2OH−↔Sn(OH)2−
6(2)
Sn(OH)2−
6→SnO2+H2O+2OH−(3)
Nanomaterials 2017, 7, 285 8 of 13
Sn󰇛OH󰇜
 →SnO
+H
O+2OH
 (3)
Figure 9. The schematic diagram of the synthesis of the g-C3N4 nanosheets decorated SnO2 flower-
like nanorods.
In the process, nucleation plays an important role not merely in the morphology formation but
also in the quantity of the final product. At the initial stage of the chemical reaction, the Sn4+ ions start
to react with the redundant OH− ions and further form the [Sn(OH)6]2− coordination ions. Meanwhile,
a small quantity of SnO2nanocrystals can be generated due to the decomposition of the [Sn(OH)6]2−
coordination ions. These initial SnO2nanocrystals play an important role as seeds in the next
hydrothermal stage. The [Sn(OH)6]2− coordination ions could be accelerated to decompose and form
plenty of SnO2nanocrystals in the later hydrothermal reaction condition, which can be aggregated
into SnO2 nanoparticles. At the same time, these SnO2 nanocrystals nucleation points around
[Sn(OH)6]2− can be oriented to grow into the rod-like structures. The SnO2 nanocrystals nucleation
points are generated in different orientations. As a result, the SnO2 nanorods grow in irregular
directions and finally form into 3D flower-like structures. This phenomenon can be explained by the
fact that the surface-free energy of the rutile structure of crystalline SnO2 faced an increase in the
order of (110) < (100) < (101) < (001), which lead to the crystal growth on the faces of (001) or (101).
However, the other faces have no exceptions [41,42].
2D g-C3N4 nanosheets-decorated SnO2 flower-like nanorods composites were synthesized via a
facile hydrothermal method as a high-property gas sensor for detecting ethanol. In order to
understand the gas-sensing process, the schematic diagram of the test gas that reacted with SnO2/g-
C3N4 composite was shown in Figure 10a. As is known, the similar principle of gas sensor is the
surface-adsorbed oxygen theory. When the sensor was exposed in the air condition, the oxygen
molecules were adsorbed on the SnO2 surface and capture electrons from the conduction band of
SnO2. Furthermore, the adsorbed oxygen molecules were ionized into O2−, O−, and O2− (Equation (4)),
and formed a depletion layer with a certain width (Ws) of the hole accumulation (h+) on the SnO2
surface [43,44]. When the sensor was exposed in ethanol gas, the reduced gas ethanol molecules were
oxidized into acetaldehyde and finally turned into carbon dioxide and water by these oxygen
anions(Equations (5) and (6)) [10]. As a result, the trapped electrons were released back to the SnO2,
depletion layer, where the width (Ws) of depletion area of the hole accumulation (h+) became thinner
and led to the decrease of the resistance by the transfer of electrons between ethanol molecules and
oxygen anions. As is well known, the electrons transfer may affect the great change of the composite
resistance. In addition, the improved ethanol-sensing performances of the g-C3N4 nanosheets-
decorated flower-like SnO2 nanorods composite could be attributed to the structure of SnO2 nanorods
coupled by 2D g-C3N4nanosheets and the heterojunction of interface region between flower-like SnO2
nanorods and 2D g-C3N4.
Figure 9.
The schematic diagram of the synthesis of the g-C
3
N
4
nanosheets decorated SnO
2
flower-like nanorods.
In the process, nucleation plays an important role not merely in the morphology formation but
also in the quantity of the final product. At the initial stage of the chemical reaction, the Sn
4+
ions
start to react with the redundant OH
−
ions and further form the [Sn(OH)
6
]
2−
coordination ions.
Meanwhile, a small quantity of SnO
2
nanocrystals can be generated due to the decomposition of the
[Sn(OH)
6
]
2−
coordination ions. These initial SnO
2
nanocrystals play an important role as seeds in
the next hydrothermal stage. The [Sn(OH)
6
]
2−
coordination ions could be accelerated to decompose
and form plenty of SnO
2
nanocrystals in the later hydrothermal reaction condition, which can be
aggregated into SnO
2
nanoparticles. At the same time, these SnO
2
nanocrystals nucleation points
around [Sn(OH)
6
]
2−
can be oriented to grow into the rod-like structures. The SnO
2
nanocrystals
nucleation points are generated in different orientations. As a result, the SnO
2
nanorods grow in
irregular directions and finally form into 3D flower-like structures. This phenomenon can be explained
by the fact that the surface-free energy of the rutile structure of crystalline SnO
2
faced an increase in
the order of (110) < (100) < (101) < (001), which lead to the crystal growth on the faces of (001) or (101).
However, the other faces have no exceptions [41,42].
2D g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods composites were synthesized
via a facile hydrothermal method as a high-property gas sensor for detecting ethanol. In order
to understand the gas-sensing process, the schematic diagram of the test gas that reacted with
SnO
2
/g-C
3
N
4
composite was shown in Figure 10a. As is known, the similar principle of gas sensor
is the surface-adsorbed oxygen theory. When the sensor was exposed in the air condition, the
oxygen molecules were adsorbed on the SnO
2
surface and capture electrons from the conduction
band of SnO
2
. Furthermore, the adsorbed oxygen molecules were ionized into O
2−
, O
−
, and O
2−
(Equation (4)), and formed a depletion layer with a certain width (W
s
) of the hole accumulation (h
+
)
on the SnO
2
surface [
43
,
44
]. When the sensor was exposed in ethanol gas, the reduced gas ethanol

Nanomaterials 2017,7, 285 9 of 14
molecules were oxidized into acetaldehyde and finally turned into carbon dioxide and water by these
oxygen anions (Equations (5) and (6)) [
10
]. As a result, the trapped electrons were released back
to the SnO
2
, depletion layer, where the width (W
s
) of depletion area of the hole accumulation (h
+
)
became thinner and led to the decrease of the resistance by the transfer of electrons between ethanol
molecules and oxygen anions. As is well known, the electrons transfer may affect the great change
of the composite resistance. In addition, the improved ethanol-sensing performances of the g-C
3
N
4
nanosheets-decorated flower-like SnO
2
nanorods composite could be attributed to the structure of
SnO
2
nanorods coupled by 2D g-C
3
N
4
nanosheets and the heterojunction of interface region between
flower-like SnO2nanorods and 2D g-C3N4.
O2+e−→O−
2(4)
2CH3CH2OH +O−
2→2CH3CHO +2H2O+e−(5)
2CH3CHO +5O−
2→4CO2+4H2O+5e−(6)
Nanomaterials 2017, 7, 285 9 of 13
O+e
→O

 (4)
2CHCHOH + O
→2CH
CHO + 2HO+e
 (5)
2CHCHO + 5O
→4CO
+4H
O+5e
 (6)
(a)
(b)
Figure 10. (a) The schematic diagram of air and ethanol react with the synthesized composite; and (b)
the band diagram of SnO
2
/g-C
3
N
4
before and after the combination.
In general, the large specific area of 2D g-C
3
N
4
nanosheets can provide more active sites to
adsorb oxygen molecules and ethanol molecules. The interconnecting network structure created by
SnO
2
nanorods and 2D g-C
3
N
4
nanosheets could supply more channels for the gas adsorption and
diffusion and further enhance the interaction between SnO
2
and ethanol molecules. The energy band
model (Figure 10b) was used to explain the energy change of SnO
2
/g-C
3
N
4
for ethanol detection.
Figure 10b shows that the g-C
3
N
4
and SnO
2
have the structure of valence band and conduction band
(E
v
and E
c
) and the Fermi level (E
f
) is between these two bands. When flower-like SnO
2
nanorods and
2D g-C
3
N
4
nanosheetswere combined together, a heterojunction structure was formed. When ethanol
molecules pass through the interface between g-C
3
N
4
and SnO
2
, the electrical property at the
heterojunction was changed. SnO
2
and g-C
3
N
4
are all n-type semiconductors with band gaps of 3.6
eV and 2.7 eV, respectively. Since the work function of g-C
3
N
4
is smaller than that of SnO
2
, the
electrons will inflow from the conduction band of g-C
3
N
4
to the conduction band of SnO
2
, leading to
a higher potential barrier. The fermi level is aligned when the electronic transmission achieves a new
dynamic balance. The electrons may go over the low energy barriers and the schottky barrier is 0.4
Figure 10.
(
a
) The schematic diagram of air and ethanol react with the synthesized composite;
and (b) the band diagram of SnO2/g-C3N4before and after the combination.
In general, the large specific area of 2D g-C
3
N
4
nanosheets can provide more active sites to
adsorb oxygen molecules and ethanol molecules. The interconnecting network structure created by

Nanomaterials 2017,7, 285 10 of 14
SnO
2
nanorods and 2D g-C
3
N
4
nanosheets could supply more channels for the gas adsorption and
diffusion and further enhance the interaction between SnO
2
and ethanol molecules. The energy band
model (Figure 10b) was used to explain the energy change of SnO
2
/g-C
3
N
4
for ethanol detection.
Figure 10b shows that the g-C
3
N
4
and SnO
2
have the structure of valence band and conduction band
(E
v
and E
c
) and the Fermi level (E
f
) is between these two bands. When flower-like SnO
2
nanorods
and 2D g-C
3
N
4
nanosheetswere combined together, a heterojunction structure was formed. When
ethanol molecules pass through the interface between g-C
3
N
4
and SnO
2
, the electrical property at
the heterojunction was changed. SnO
2
and g-C
3
N
4
are all n-type semiconductors with band gaps of
3.6 eV and 2.7 eV, respectively. Since the work function of g-C
3
N
4
is smaller than that of SnO
2
, the
electrons will inflow from the conduction band of g-C
3
N
4
to the conduction band of SnO
2
, leading
to a higher potential barrier. The fermi level is aligned when the electronic transmission achieves a
new dynamic balance. The electrons may go over the low energy barriers and the schottky barrier
is 0.4 eV. As a result, the electrons and holes are separated [
27
,
43
]. Meanwhile, the heterojunction
structure may suppress the recombination of electron-hole and urge electrons to transfer quickly from
ethanol vapour to the surface of SnO
2
/g-C
3
N
4
. Therefore, this leads to a higher response because of
the increased conductivity of the heterojunction structure [29].
3. Materials and Methods
3.1. Chemicals
Analytical-grade purity SnCl
4·
5H
2
O (99.0%), NaOH, and absolute ethyl alcohol were
purchased from Shanghai Macklin Biochemical Co., Ltd, Shanghai, China and were used without
further purification.
3.2. Sample Preparation
Graphitic carbon nitride (g-C
3
N
4
) was synthesized by our previous reported method [
45
].
Typically, 7 wt. % g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods (SnO
2
/g-C
3
N
4
-7) were
synthesized by the hydrothermal method: 0.17 g g-C
3
N
4
powder was dispersed into 200 mL ethanol
under ultrasonic treatment for 2 h. 5.259 g SnCl
4·
5H
2
O was added into 200 mL of NaOH solution
(0.81 M). Subsequently, the g-C
3
N
4
solution was added into this mixture solution with magnetic stirring
until it formed a white suspension. Finally, the mixture was transferred into a 500 mL stainless-steel
Teflon-lined autoclave, then put into an oven and further heated at 200 ◦C for 48 h. The final product
was washed with deionized water and ethanol several times and dried at 60
◦
C. According to this
method, the SnO
2
/g-C
3
N
4
composites with 5 and 9 wt. % g-C
3
N
4
content were also synthesized and
marked as SnO
2
/g-C
3
N
4
-5 and SnO
2
/g-C
3
N
4
-9, respectively. The pure flower-like SnO
2
nanorods
were also synthesized by the same method.
3.3. Characterizations
X-ray diffraction (XRD) analysis was carried out on Bruker-AXS D8 (Bruker, Madison, WI, USA)
with CuK
α
radiation at 40 kV and 25 mA. Fourier Transform Infrared Spectrometer (FT-IR) was
recorded on a Bruker Tensor 27 (Bruker, Madison, WI, USA). Thermogravimetry (TG) analysis was
completed on a NETZSCH STA449C Simultaneous Thermal Analyzer (NETZSCH, Selb, Germany)
at a heating rate of 10
◦
C
·
min
−1
under air atmosphere. Field-emission scanning electron microscopy
(FESEM) (Quanta
™
250 FEG, FEI, Eindhoven, The Netherlands) was used to observe the structure and
morphology of the sample. Transmission electron microscopy (TEM) analysis was done on a JEOL
JEM-2100 microscope (JEOL, Tokyo, Japan) operating at 200 kV.
3.4. Gas Sensor Fabrication and Analysis
Gas-sensing performance test of the synthesized sample was carried out on an intelligent
gas-sensing analysis system of CGS-4TPS (Beijing Elite Tech. Co., Ltd., Beijing, China). Figure 11 shows

Nanomaterials 2017,7, 285 11 of 14
the schematic diagram of the system, the sensor structure, and the working principle. In the sensor
fabricate process, the synthesized sample was mixed with several drops of distilled water to form a
paste. Then, a ceramic substrate (13.4 mm
×
7 mm, screen-printed with Ag-Pd comb-like electrodes)
was coated onto the paste to obtain the resistance-type sensor. Before the gas sensing test, the sensor
was aged at 200
◦
C for 12 h to improve its stability and repeatability. In the sensing performance test
process, the test gas was first injected into the closed 0.018 m
3
volume chamber by a microinjector
with the relative humidity of 40%. The operating temperature was set in the range of 200
◦
C to 400
◦
C.
The gas response (S) was defined as the ratio of Ra/Rg, where Ra and Rg were the resistances of sensor
in air and in the test gas, respectively. The response and recovery times were defined as the time
required for a change in response to reach 90% of the equilibrium value.
Nanomaterials 2017, 7, 285 11 of 13
Figure 11. (a) The schematic diagram of the CGS-4TPS gas-sensing test system and (b) the gas sensor
structure.
4. Conclusions
In summary, the g-C3N4 nanosheets-decorated tin oxide flower-like nanorods (SnO2/g-C3N4)
composite was successfully synthesized by using a facile hydrothermal method. The as-prepared
sample possesses flower-like nanorods and a lamellar structure. Compared with pure SnO2, the g-
C3N4 nanosheets-decorated SnO2 flower-like nanorods exhibited an obvious improvement of gas
sensing performance to ethanol, and the response value was 150 to 500 ppm ethanol at 340 °C. The
improved sensing properties are mainly attributed to the high surface area of the sample and the
heterojunction between SnO2 and g-C3N4. Considering the effective synthesis approach and the high
sensing performance, the as-prepared SnO2/g-C3N4 composite could be an ideal candidate for ethanol
detection application.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (51404097,
51504083, U1404613), Natural Science Foundation of Henan Province of China (162300410113), Program for
Science & Technology Innovation Talents in Universities of Henan Province (17HASTIT029, 18HASTIT010),
Project funded by China Postdoctoral Science Foundation (2016M592290), the Research Foundation for Youth
Scholars of Higher Education of Henan Province (2016GGJS-040), the Fundamental Research Funds for the
Universities of Henan Province (NSFRF1606,NSFRF1614), Program for Innovative Research Team in University
of Ministry of Education of China (IRT_16R22), Foundation for Distinguished Young Scientists of Henan
Polytechnic University (J2016-2, J2017-3), and the State Key Laboratory Cultivation Base for Gas Geology and
Gas Control (Henan Polytechnic University) (WS2017A03).
Author Contributions: Yan Wang conceived and designed the experiments; Bo Zhang, Guang Sun, and
Zhanying Zhang performed the experiments and analyzed the data; Jianliang Cao and Cong Qin provided the
concept of this research and managed all the experimental and writing process as the corresponding authors; all
authors discussed the results and commented on the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Shi, Y.H.; Ma, D.Q.; Wang, W.J.; Zhang, L.F.; Xu, X.H. A supramolecular self-assembly hydrogel binder
enables enhanced cycling of SnO2-based anode for high-performance lithium-ion batteries. J. Mater. Sci.
2017, 52, 3545–3555.
2. Akhundi, A.; Habibiyangjeh, A. A simple large-scale method for preparation of g-C3N4/SnO2
nanocomposite as visible-light-driven photocatalyst for degradation of an organic pollutant. Mater. Express
2015, 5, 309–318.
3. Zhang, J.; Liu, X.H.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Adv.
Mater. 2016, 28, 795–831.
4. Das, S.; Jayaraman, V. SnO2: A Comprehensive Review on Structures and Gas Sensors. Prog. Mater. Sci.
2014, 66, 112–255.
5. Wu, M.Y.; Zeng, W.; Li, Y.Q. Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing
properties. Mater. Lett. 2013, 104, 34–36.
Figure 11.
(
a
) The schematic diagram of the CGS-4TPS gas-sensing test system and (
b
) the gas
sensor structure.
4. Conclusions
In summary, the g-C
3
N
4
nanosheets-decorated tin oxide flower-like nanorods (SnO
2
/g-C
3
N
4
)
composite was successfully synthesized by using a facile hydrothermal method. The as-prepared
sample possesses flower-like nanorods and a lamellar structure. Compared with pure SnO
2
, the
g-C
3
N
4
nanosheets-decorated SnO
2
flower-like nanorods exhibited an obvious improvement of gas
sensing performance to ethanol, and the response value was 150 to 500 ppm ethanol at 340
◦
C.
The improved sensing properties are mainly attributed to the high surface area of the sample and the
heterojunction between SnO
2
and g-C
3
N
4
. Considering the effective synthesis approach and the high
sensing performance, the as-prepared SnO
2
/g-C
3
N
4
composite could be an ideal candidate for ethanol
detection application.
Acknowledgments:
This work was supported by the National Natural Science Foundation of China (51404097,
51504083, U1404613), Natural Science Foundation of Henan Province of China (162300410113), Program for Science
& Technology Innovation Talents in Universities of Henan Province (17HASTIT029, 18HASTIT010), Project funded
by China Postdoctoral Science Foundation (2016M592290), the Research Foundation for Youth Scholars of Higher
Education of Henan Province (2016GGJS-040), the Fundamental Research Funds for the Universities of Henan
Province (NSFRF1606,NSFRF1614), Program for Innovative Research Team in University of Ministry of Education
of China (IRT_16R22), Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2016-2,
J2017-3), and the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic
University) (WS2017A03).
Author Contributions:
Yan Wang conceived and designed the experiments; Bo Zhang, Guang Sun, and
Zhanying Zhang performed the experiments and analyzed the data; Jianliang Cao and Cong Qin provided
the concept of this research and managed all the experimental and writing process as the corresponding authors;
all authors discussed the results and commented on the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

Nanomaterials 2017,7, 285 12 of 14
References
1.
Shi, Y.H.; Ma, D.Q.; Wang, W.J.; Zhang, L.F.; Xu, X.H. A supramolecular self-assembly hydrogel binder
enables enhanced cycling of SnO
2
-based anode for high-performance lithium-ion batteries. J. Mater. Sci.
2017,52, 3545–3555. [CrossRef]
2.
Akhundi, A.; Habibiyangjeh, A. A simple large-scale method for preparation of g-C
3
N
4
/SnO
2
nanocomposite as visible-light-driven photocatalyst for degradation of an organic pollutant. Mater. Express
2015,5, 309–318. [CrossRef]
3.
Zhang, J.; Liu, X.H.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors.
Adv. Mater. 2016,28, 795–831. [CrossRef] [PubMed]
4.
Das, S.; Jayaraman, V. SnO
2
: A Comprehensive Review on Structures and Gas Sensors. Prog. Mater. Sci.
2014
,
66, 112–255. [CrossRef]
5.
Wu, M.Y.; Zeng, W.; Li, Y.Q. Hydrothermal synthesis of novel SnO
2
nanoflowers and their gas-sensing
properties. Mater. Lett. 2013,104, 34–36. [CrossRef]
6.
Luo, L.; Jiang, Q.P.; Qin, G.H.; Zhao, K.; Du, G.F.; Wang, H.; Zhao, H.Y. Gas sensing characteristics of novel
twin-layered SnO
2
nanoarray fabricated by substrate-free hydrothermal route. Sens. Actuators B
2015
,218,
205–214. [CrossRef]
7.
Zhang, J.; Wu, J.J.; Wang, X.X.; Zeng, D.W.; Xie, C.S. Enhancing room-temperature NO
2
sensing properties
via forming heterojunction for NiO-rGO composited with SnO
2
nanoplates. Sens. Actuators B
2016
,243,
1010–1019. [CrossRef]
8.
Choi, S.W.; Katoch, A.; Kim, J.H.; Kim, S.S. Striking sensing improvement of n-type oxide nanowires by
electronic sensitization based on work function difference. J. Mater. Chem. C 2015,3, 1521–1527. [CrossRef]
9.
Zhang, H.; Zeng, W.; Hao, J.H.; Li, Y.Q.; Miao, B. Hydrothermal synthesis of flower-like SnO
2
architectures
with superior gas sensing properties. Mater. Lett. 2015,145, 133–136. [CrossRef]
10.
Zhang, D.Z.; Liu, J.J.; Chang, H.Y.; Liu, A.M.; Xia, B.K. Characterization of a hybrid composite of SnO
2
nanocrystal-decorated reduced graphene oxide for ppm-level ethanol gas sensing application. RSC Adv.
2015,5, 18666–18672. [CrossRef]
11.
Zito, C.A.; Perfecto, T.M.; Volanti, D.P. Impact of reduced graphene oxide on the ethanol sensing performance
of hollow SnO2nanoparticles under humid atmosphere. Sens. Actuators B 2017,244, 466–474. [CrossRef]
12.
Feng, Q.X.; Li, X.G.; Wang, J. Percolation effect of reduced graphene oxide (rGO) on ammonia sensing of
rGO-SnO2composite based sensor. Sens. Actuators B 2017,243, 1115–1126. [CrossRef]
13.
Song, Z.L.; Wei, Z.R.; Wang, B.C.; Luo, Z.; Xu, S.M.; Zhang, W.K.; Yu, H.X.; Li, M.; Huang, Z.; Zang, J.F.; et al.
Sensitive room-temperature H
2
S gas sensors employing SnO
2
quantum wire/reduced graphene oxide
nanocomposites. Chem. Mater. 2016,28, 1205–1212. [CrossRef]
14.
Mao, S.; Cui, S.M.; Lu, G.H.; Yu, K.H.; Wen, Z.H.; Chen, J.H. Tuning gas-sensing properties of reduced
graphene oxide using tin oxide nanocrystals. J. Mater. Chem. 2012,22, 11009–11013. [CrossRef]
15.
Zhang, H.; Feng, J.C.; Fei, T.; Liu, S.; Zhang, T. SnO
2
nanoparticles-reduced graphene oxide nanocomposites
for NO2sensing at low operating temperature. Sens. Actuators B 2014,190, 472–478. [CrossRef]
16.
Liu, J.Y.; Wang, T.S.; Wang, B.Q.; Sun, P.; Yang, Q.Y.; Liang, X.S.; Song, H.W.; Lu, G.Y. Highly sensitive
and low detection limit of ethanol gas sensor based on hollow ZnO/SnO
2
spheres composite material.
Sens. Actuators B 2017,245, 551–559. [CrossRef]
17.
Poloju, M.; Jayababu, N.; Manikandan, E.; Ramana Reddy, M.V. Enhancing the Isopropanol gas sensing
performance of SnO
2
/ZnO core/shell nanocomposite gas sensor. J. Mater. Chem. C
2017
,5, 2662–2668.
[CrossRef]
18.
Silva, L.F.D.; M’Peko, J.-C.; Catto, A.C.; Bernardini, S.; Mastelaro, V.R.; Aguir, K.; Ribriro, C.;
Longo, E. UV-enhanced ozone gas sensing response of ZnO-SnO
2
heterojunctions at room temperature.
Sens. Actuators B 2017,240, 573–579. [CrossRef]
19.
Wang, L.W.; Li, J.T.; Wang, Y.H.; Yu, K.F.; Tang, X.Y.; Zhang, Y.Y.; Wang, S.P.; Wei, C.S. Construction of 1D
SnO
2
-coated ZnO nanowire heterojunction for their improved n-butylamine sensing performances. Sci. Rep.
2016,6, 35079. [CrossRef] [PubMed]
20.
Choi, K.S.; Park, S.; Chang, S.P. Enhanced ethanol sensing properties based on SnO
2
nanowires coated with
Fe2O3nanoparticles. Sens. Actuators B 2017,238, 871–879. [CrossRef]

Nanomaterials 2017,7, 285 13 of 14
21.
Wang, T.S.; Huang, Z.S.; Yu, Z.D.; Wang, B.Q.; Wang, H.; Sun, P.; Suo, H.; Gao, Y.; Sun, Y.F.; Li, T.; et al.
Low operating temperature toluene sensor based on novel
α
-Fe
2
O
3
/SnO
2
heterostructure nanowire arrays.
RSC Adv. 2016,6, 52604–52610. [CrossRef]
22.
Sun, P.; Wang, C.; Liu, J.G.; Zhou, X.; Li, X.W.; Hu, X.L.; Lu, G.Y. Hierarchical assembly of
α
-Fe
2
O
3
nanosheets
on SnO
2
hollow nanospheres with enhanced ethanol sensing properties. ACS Appl. Mater. Interface
2015
,7,
19119–19125. [CrossRef] [PubMed]
23.
Yu, Q.X.; Zhu, J.H.; Xu, Z.Y.; Huang, X.T. Facile synthesis of
α
-Fe
2
O
3
@SnO
2
core–shell heterostructure
nanotubes for high performance gas sensors. Sens. Actuators B 2015,213, 27–34. [CrossRef]
24.
Gu, C.P.; Cui, Y.W.; Wang, L.Y.; Sheng, E.H.; Shim, J.J.; Huang, J.R. Synthesis of the porous NiO/SnO
2
microspheres and microcubes and their enhanced formaldehyde gas sensing performance. Sens. Actuators B
2017,241, 298–307. [CrossRef]
25.
Wang, Y.; Zhang, H.; Sun, X.H. Electrospun Nanowebs of NiO/SnO
2
p-n Heterojunctions for Enhanced Gas
Sensing. Appl. Surf. Sci. 2016,389, 514–520. [CrossRef]
26.
Zhang, J.; Zeng, D.W.; Zhu, Q.; Wu, J.J.; Huang, Q.W.; Zhang, W.; Xie, C.S. Enhanced room temperature NO
2
response of NiO-SnO
2
nanocomposites induced by interface bonds at the p-n heterojunction. Phys. Chem.
Chem. Phys. 2016,18, 5386–5396. [CrossRef] [PubMed]
27.
Zang, Y.P.; Li, L.P.; Li, X.G.; Lin, R.; Li, G.S. Synergistic collaboration of g-C
3
N
4
/SnO
2
composites for
enhanced visible-light photocatalytic activity. Chem. Eng. J. 2014,246, 277–286. [CrossRef]
28.
Chen, X.; Zhou, B.H.; Yang, S.L.; Wu, H.S.; Wu, Y.X.; Wu, L.D.; Pan, J.; Xiong, X. In situ construction
of an SnO
2
/g-C
3
N
4
heterojunction for enhanced visible-light photocatalytic activity. RSC Adv.
2015
,5,
68953–68963. [CrossRef]
29.
Zhang, Y.J.; Zhang, D.K.; Guo, W.M.; Chen, S.J. The
α
-Fe
2
O
3
/g-C
3
N
4
heterostructural nanocomposites with
enhanced ethanol gas sensing performance. J. Alloys Compd. 2016,685, 84–90. [CrossRef]
30.
Zeng, B.R.; Zhang, L.C.; Wan, X.Y.; Song, H.J.; Lv, Y. Fabrication of
α
-Fe
2
O
3
/g-C
3
N
4
composites for
cataluminescence sensing of H2S. Sens. Actuators B 2015,211, 370–376. [CrossRef]
31.
Hu, Y.; Li, L.; Zhang, L.C.; Lv, Y. Dielectric barrier discharge plasma-assisted fabrication of g-C
3
N
4
-Mn
3
O
4
composite for high-performance cataluminescence H
2
S gas sensor. Sens. Actuators B
2017
,239, 1177–1184.
[CrossRef]
32.
Mathur, S.; Barth, S.; Shen, H.; Pyun, J.-C.; Werner, U. Size-Dependent Photoconductance in SnO
2
Nanowires.
Small 2005,1, 713–717. [CrossRef] [PubMed]
33.
Kuang, Q.; Lao, C.S.; Wang, Z.L.; Xie, Z.X.; Zheng, L.S. High-sensitivity humidity sensor based on a single
SnO2nanowire. J. Am. Chem. Soc. 2007,129, 6070–6071. [CrossRef] [PubMed]
34.
Paulowicz, I.; Hrkac, V.; Kaps, S.; Cretu, V.; Lupan, O.; Braniste, T.; Duppel, V.; Tiginyanu, L.; Kienle, L.;
Adelung, R.; et al. Nanowire Networks: Three-Dimensional SnO
2
Nanowire Networks for Multifunctional
Applications: From High-Temperature Stretchable Ceramics to Ultraresponsive Sensors. Adv. Electron. Mater.
2015,1, 1500081. [CrossRef]
35.
Lupan, O.; Braniste, T.; Deng, M.; Ghimpu, L.; Paulowicz, I.; Mishra, Y.K.; Kienle, L.; Adelung, R.; Tiginyanu, I.
Rapid switching and ultra-responsive nanosensors based on individual shell–core Ga
2
O
3
/GaN:O
x
@SnO
2
nanobelt with nanocrystalline shell in mixed phases. Sens. Actuators B 2015,221, 544–555. [CrossRef]
36.
Dai, H.; Zhang, S.P.; Xu, G.F.; Peng, Y.R.; Gong, L.S.; Li, X.H.; Li, Y.L.; Lin, Y.Y.; Chen, G.N. Highly photoactive
heterojunction based on g-C
3
N
4
nanosheets decorated with dendritic zinc(II) phthalocyanine through axial
coordination and its ultrasensitive enzyme-free sensing of choline. RSC Adv.
2014
,4, 58226–58230. [CrossRef]
37.
Xiao, Y.; Yang, Q.Y.; Wang, Z.Y.; Zhang, R.; Gao, Y.; Sun, P.; Sun, Y.F.; Lu, G.Y. Improvement of NO
2
gas
sensing performance based on discoid tin oxide modified by reduced graphene oxide. Sens. Actuators B
2016
,
227, 419–426. [CrossRef]
38.
Guo, J.; Zhang, J.; Gong, H.B.; Ju, D.X.; Cao, B.Q. Au nanoparticle-functionalized 3D SnO
2
microstructures
for high performance gas sensor. Sens. Actuators B 2016,226, 266–272. [CrossRef]
39.
Lupan, O.; Chow, L.; Chai, G.; Heinrich, H.; Park, S.; Schulte, A. Synthesis of one-dimensional SnO
2
nanorods
via a hydrothermal technique. Physica E 2009,41, 533–536. [CrossRef]
40.
Vuong, D.D.; Hien, V.X.; Trung, K.Q.; Chien, N.D. Synthesis of SnO
2
micro-spheres, nano-rods and
nano-flowers via simple hydrothermal route. Physica E 2011,44, 345–349. [CrossRef]
41.
Cheng, B.; Russell, J.M.; Shi, W.S.; Zhang, L.; Samulski, E.T. Large-Scale, Solution-Phase Growth of
Single-Crystalline SnO2Nanorods. J. Am. Chem. Soc. 2004,126, 5972–5973. [CrossRef] [PubMed]

Nanomaterials 2017,7, 285 14 of 14
42.
Liu, Y.L.; Huang, J.; Yang, J.D.; Wang, S.R. Pt nanoparticles functionalized 3D SnO
2
nanoflowers for gas
sensor application. Solid State Electron. 2017,130, 20–27. [CrossRef]
43.
Chen, Y.; Zhang, W.; Wu, Q.S. A highly sensitive room-temperature sensing material for NH
3
: SnO
2
-nanorods
coupled by rGO. Sens. Actuators B 2017,242, 1216–1226. [CrossRef]
44.
Zhang, Z.Y.; Zhu, L.P.; Wen, Z.; Ye, Z.Z. Controllable synthesis of Co
3
O
4
crossed nanosheet arrays toward an
acetone gas sensor. Sens. Actuators B 2017,238, 1052–1059. [CrossRef]
45.
Cao, J.L.; Qin, C.; Wang, Y.; Zhang, B.; Gong, Y.X.; Zhang, H.L.; Sun, G.; Hari, B.; Zhang, Z.Y. Calcination
Method Synthesis of SnO
2
/g-C
3
N
4
Composites for a High-Performance Ethanol Gas Sensing Application.
Nanomaterials 2017,7, 98. [CrossRef] [PubMed]
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2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
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