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Morphology-modulation of SnO
2
Hierarchical Architectures by Zn Doping
for Glycol Gas Sensing and
Photocatalytic Applications
Qinqin Zhao
1
, Dianxing Ju
2
, Xiaolong Deng
1
, Jinzhao Huang
1
, Bingqiang Cao
2
& Xijin Xu
1
1
School of Physics and Technology, University of Jinan, 336 West Road of Nan Xinzhuang, Jinan, 250022, Shandong Province,
Peoples Republic of China,
2
School of Materials Science and Engineering, University of Jinan.
The morphology of SnO
2
nanospheres was transformed into ultrathin nanosheets assembled architectures
after Zn doping by one-step hydrothermal route. The as-prepared samples were characterized in detail by
various analytical techniques including scanning electron microscopy (SEM), transmission electron
microscopy (TEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption technique. The Zn-doped
SnO
2
nanostructures proved to be the efficient gas sensing materials for a series of flammable and explosive
gases detection, and photocatalysts for the degradation of methyl orange (MO) under UV irradiation. It was
observed that both of the undoped and Zn-doped SnO
2
after calcination exhibited tremendous gas sensing
performance toward glycol. The response (
S5R
a
/R
g
) of Zn-doped SnO
2
can reach to 90 when the glycol
concentration is 100 ppm, which is about 2 times and 3 times higher than that of undoped SnO
2
sensor with
and without calcinations, respectively. The result of photocatalytic activities demonstrated that MO dye was
almost completely degraded (,92%) by Zn-doped SnO
2
in 150 min, which is higher than that of others (MO
without photocatalyst was 23%, undoped SnO
2
without and with calcination were 55% and 75%,
respectively).
As one of the most important classes of materials, metal oxide semiconductors are presenting themselves in
various areas of science and technology, due to their desirable morphologies and distinct structures
1,2
.
SnO
2
, a typical n-type semiconductor with a wide band gap (3.6 ev)
3–5
, has been widely investigated with
many applications such as gas sensors
6–9
, solar cells
10
, lithium batteries
11
, catalysis
12–15
, and transparent conduct-
ive electrodes
16
, because of its unique optical, catalytic, and electrical properties and high chemical stability. It has
been reported that the performances of SnO
2
nanomaterials are greatly affected by their morphology, structure
and surface area
17
. Therefore, it is believed that their advanced nanostructured materials with different dimen-
sionalities would offer good opportunities to explore new physical and chemical applications including gas
sensors and photocatalysis. Recently, Guo et al.
18
. reported that the sensor, fabricated by 3D SnO
2
microstructures
assembled by porous nanosheets through hydrothermal synthesis, exhibited good response-recovery perform-
ance, high sensitivity and excellent long-term stability for ethanol gas. Dai et al. synthesized flower-like SnO
2
microstructures which exhibited higher photoactivity (about 2.2 times) than granular SnO
2
for the degradation of
RhB dye
19
. These structural features can improve both the sensing performance and the photocatalytic degra-
dation rate. In this view, the more attention should be paid to the design and synthesis of hierarchical SnO
2
nanomaterials.
In order to satisfy the increasing demands for the photocatalysts or gas sensors working under harsh and
complicated conditions, many efforts have been made to improve the performance of SnO
2
materials such as
doping
20–23
, adding catalysts
24–26
, and constructing the heterojunctions
27,28
. Among these methods, doping has
been proposed to be a facile efficient way to modulate their properties. In particular, the previous reports
demonstrated that the morphologies and properties of SnO
2
could be modified by Zn doping
29–32
. This could
be attributed to the facile substitution of Sn ions by Zn ions due to the similar ion radius of Zn
21
and Sn
41
(Zn
21
0.074 nm, Sn
41
0.071 nm), which may produce more oxygen vacancies for charge compensation and further
enhance the performance of SnO
2
. Thus, it is expected that Zn-doped SnO
2
hierarchical nanostructures will
provide excellent gas sensing and photocatalytic properties.
OPEN
SUBJECT AREAS:
STRUCTURAL PROPERTIES
SENSORS
Received
15 August 2014
Accepted
15 December 2014
Published
19 January 2015
Correspondence and
requests for materials
should be addressed to
B.C. (mse_caobq@ujn.
edu.cn) or X.X.
(sps_xuxj@ujn.edu.cn,
phys_xu@hotmail.
com)
SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 1
Figure 1
|
SEM images of undoped (a, b) and Zn-doped SnO
2
(c, d). (e) EDS spectra of Zn-doped SnO
2
shown in (c, d). (f) XRD patterns of undoped and
Zn-doped SnO
2
. Undoped (S1 600
o
C) and Zn-doped SnO
2
(S2 600
o
C).
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 2
Here, the Zn-doped SnO
2
hierarchical nanostructures were synthesized
by one-step hydrothermal process, which are composed of a number of
nanosheets with the thickness of about 30 nm. The as-prepared samples
were applied to the glycol detection and the photocatalytic degradation of
methyl orange (MO) under UV irradiation. The results indicated that Zn-
doped SnO
2
hierarchical nanostructures had better glycol gas response
and photocatalytic activity than undoped SnO
2
nanostructures.
Methods
Synthesis of undoped and Zn-doped SnO
2
hierarchical nanostructures.All
chemicals were of analytical grade regents and used without any further purification.
A typical synthesis process was as follows: 6 mM of NH
4
F was dissolved in 50 ml of
deionized water under vigorous magnetic stirring, followed by adding 2 mM of
SnCl
2
?2H
2
O and 2 mM of ZnCl
2
into the above solution with the assistance of
ultrasonication. The mixed solution was then transferred into a 100 ml Teflon-lined
stainless steel autoclave, and kept at 180
o
C for 16 h in order to obtain the precipitates.
Figure 2
|
(a) and (e) TEM images of undoped and Zn-doped SnO
2
; (b, f) HRTEM images of undoped and Zn-doped SnO
2
with their SAED pattern
(c, g)); (d, h) EDX spectra of undoped and Zn-doped SnO
2
.Undoped (S1 600
o
C) and Zn-doped SnO
2
(S2 600
o
C). The peaks of Cu and
C observed in the EDX spectrum are from carbon-coated copper grids.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 3
Then, the autoclave cooled to room temperature naturally. The obtained products
were collected and washed with deionized water and ethanol several times,
respectively, and then dried in air at 60
o
C for 12 h. Finally, the products were
annealed at 600
o
C for 2 h, named as S2. The undoped SnO
2
was obtained by the same
process except the addition of ZnCl
2
, named as S1. The undoped SnO
2
sample
without calcination was named as S0.
Material Characterizations.The compositions and morphologies of the as-prepared
samples were characterized by a field emission scanning electron microscope
(FESEM, FEI QUANTA FEG250) equipped with an energy dispersive x-ray
spectroscopy (EDS, INCA MAX-50) and a high-resolution transmission electron
microscope (Tecnai G2 F20). The crystal structures of the samples were analyzed by
X-ray diffraction (XRD, D8-Advance, Bruker) with Cu K
a
radiation and Raman
spectrometer (Renishaw inVia) in the range from 200 cm
21
to 1000 cm
21
at room
temperature. The average pore size, pore volume, and specific surface area of the
samples were examined through measuring N
2
adsorption-desorption isotherm with
a Micromeritics ASAP2020 apparatus.
Fabrication and Gas-Sensing measurements.Gas sensors were fabricated as follows:
The as-prepared sample was mixed with the deionized water to form a paste, and then
it was coated onto an Al
2
O
3
tube by a small brush to form a thick film between two
parallel Au electrodes, which had been previously printed on the tube. A heater of Ni-
Cr coil was inserted into the Al
2
O
3
tube to keep the sensor at the operating
temperature. The gas sensor properties were measured by a gas sensing test system
(WS-30A, Weisheng Electronics, Zhengzhou, China) with a test voltage of 5 V under
laboratory conditions (30% 610% RH, 23 61uC). The devices were put into an
airproof test box and tested by a static process in a test chamber. A given amount of
the target gas was injected into the test chamber, when the response reached a
constant value, the upper cover of the test chamber was removed and the sensor began
to recover in air. The response of the sensor was defined as S5R
a
/R
g
for target gas,
where R
a
and R
g
are the sensor resistance in air and in target gas, respectively.
Photocatalytic Measurements.The photocatalytic activity of the samples was
investigated by measuring the degradation of methyl orange (MO) under UV light
(generated by a 500 W high pressure mercury lamp). Typically, 30 mg of as-prepared
sample was added to the aqueous solution of MO dye (50 ml, 20 mg/L) with stirring
in the dark for 30 min to ensure the absorption/desorption equilibrium. Then, the
suspension was exposed to UV light irradiation whilst stirring. At given irradiation
time intervals, concentrations of MO were monitored with a TU-1901 UV-vis
spectrophotometer by measuring the absorbance at 464 nm during the degradation
process.
Results and discussion
Representative morphologies and structures of the as-obtained SnO
2
nanostructures, revealed by field-emission scanning electron micro-
scopy (SEM), are shown in Figure 1. The surface morphologies of
undoped SnO
2
(S1) are shown in Figure 1(a, b). It can be seen that the
products are mostly spherical-like structures with a relatively rough
surface, which is consisted of SnO
2
nanorods. The morphologies of
the Zn-doped SnO
2
(S2) are shown in Figure 1(c, d). Compared with
the undoped sample, the products are composed of SnO
2
nanosheets,
which may be attributed to the induced growth of Zn
21
ions, as
shown in Figure 1(e). The peak of Zn can be clearly observed
in the spectrum and the content ratio of Zn and Sn is about
1.60510.93, shown in the insert table. Figure 1(f) shows the typical
XRD patterns of the as-prepared undoped (S1) and Zn-doped SnO
2
(S2) architectures. All the diffraction peaks could be well indexed to
the tetragonal rutile structure of SnO
2
, which was consistent with the
standard data file (JCPDS file no. 41–1445). No obvious character-
istic peaks are observed for the impurities. Additionally, the diffrac-
tion peaks of the Zn-doped SnO
2
tend to become slightly broader
from Figure 1(f), which could be attributed to the size effect of the
crystals. The mean grain size of undoped and the Zn-doped SnO
2
were calculated to be 29.3 nm and 22.6 nm, respectively, using the
Debye-Scherer formula, D~0:89l=(bcos h)
33
, where lis the X-ray
wavelength, his the Bragg diffraction angle and bis the peak full-
width at half maximum.
The morphologies and microstructures of the samples S1 (SnO
2
)
and S2 (Zn-doped SnO
2
) were further studied by TEM, as shown in
Figure 2. From Figure 2(a), we get to know that the undoped SnO
2
(S1) is mainly comprised of spherical-like structures constructed by
nanorods with a diameter of 20–30 nm and a length up to one
hundred nanometers. This is in consistent with the SEM images in
Figure 1(a, b). The HRTEM image is shown in Figure 2(b), in which
the interplanar distances are 0.34 nm and 0.27 nm, respectively,
matching well with (110) and (101) planes of rutile SnO
2
. The
selected area electron diffraction (SAED) pattern taken from the
corresponding microspheres suggests a polycrystalline structure of
as-prepared sample, as shown in Figure 2(c). The peak of Sn and O
can be clearly observed in the spectrum from EDX spectrum
(Figure 2(d)) for the undoped SnO
2
revealing a purity phase. The
peaks of Cu and C observed in the spectrum are attributed to the
carbon-coated copper grids.
Compared with the undoped SnO
2
, the Zn-doped SnO
2
are mainly
composed of SnO
2
nanosheets, as shown in Figure 2(e). The HRTEM
image (Figure 2(f)) shows that the lattice fringes of the Zn-doped
SnO
2
nanostuctures are 0.34 and 0.27 nm, which correspond to the
(110) and (101) lattice planes of SnO
2
, respectively, as shown in
Figure 2(g). The SAED pattern indicates that the nanosheet is
of polycrystalline structure composed of nanoscaled particles.
Figure 2(h) shows the EDX spectrum of the Zn-doped SnO
2
,
in which the peak of Zn can be clearly observed in the spectrum,
which is corresponding to the XRD pattern of Zn-doped SnO
2
.
Furthermore, it can be seen from the SEM and TEM images that
SnO
2
nanorods change into nanosheets after the doping of Zn ele-
ment. This evolution is ascribed to the successfully substitutional
doping of Zn
21
ions into SnO
2
lattice with modification of local
Figure 3
|
Raman spectrum of undoped (S1 600
o
C) and Zn-doped SnO
2
(S2 600
o
C).
Figure 4
|
Nitrogen adsorption-desorption isotherms and pore size
distribution curves (inset) of S1 (SnO
2
600
o
C) and S2 (Zn-doped SnO
2
600
o
C).
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 4
structure
34
. It is presumed that the modification of local structure
varies the surface energy of some crystallographic planes in the pre-
pared nanosheets, which changes the growth rate of these crystal-
lographic planes. As can be seen in our experiment, Zn
21
ions in
SnO
2
lattice inhibit the growth along [110] direction but permit the
growth along the axis of nanorod, promoting the anisotropic growth
of the nanorods
34
. Therefore, it is believed that Zn
21
serves as a
structure-directing agent in the growth of nanosheets.
Raman scattering is generally used to investigate the crystallinity,
structural defects, and size effects of nanoscale crystallites
35,36
.
Figure 3 presents the room temperature Raman spectra of pure
SnO
2
(S1) and Zn-doped SnO
2
(S2) architectures. The peaks at 630
and 774 cm
21
correspond respectively to A1g and B2g modes, which
are related to the symmetric and asymmetric stretching of Sn-O
bonds, respectively
36,37
. The peak at 475 cm
21
represents Eg doubly
degenerate mode, arising from the vibration mode of oxide ions. The
appearance of these ‘‘classical modes’’ confirms the tetragonal rutile
structure of Sn
1–x
Zn
x
O
2
nanostuctures
37
. Compared with undoped
SnO
2
, the peaks located at 630 and 774 cm
21
of the Zn-doped SnO
2
Raman spectrum become broadened, which might be ascribed to the
effects of oxygen deficiency and crystal distortion, caused by the
introduction of Zn ions
36,38
. In addition, such broadening of A1g
mode is referred to the decrease of grain size, which is consistent
with the XRD results
39
.
In order to further confirm the inner architectures, nitrogen
adsorption and desorption measurements of the as-prepared pro-
ducts (S1 and S2) were carried out to estimate the properties, as
shown in Figure 4. The specific surface area of the samples was
calculated to be 51.2 and 61.8 m
2
g
21
, respectively, indicating an
uptrend of the active surface after the doping of Zn. Pore size distri-
bution curves (inset of Figure 4) are important factors for mass
transport and effective surface area. The average pore size of the
two samples (S1 and S2) was calculated to be 8.798 nm and
7.676 nm, respectively. The pores distributed are also observed in
TEM image (Figure 2(a, e)) between the adjacent nanoparticles. It
clearly indicates that the Zn-doped SnO
2
shows a higher specific
surface area than that of undoped SnO
2
, which might be advantage-
ous for enhancing gas sensing performance.
Some papers have reported that Zn-doped SnO
2
would form the
nanorods or nanoneedles in the ethanol-water solution using KOH
or NaOH as the alkali source
30,34
. This morphology may be attributed
to the regulating function of Zn and alcohol which has been men-
tioned by many papers
29,39,40
. However, when the ethanol-water solu-
tion and alkali source were replaced by water and NH
4
F solution as
used in our case, the morphology of the products may be changed.
According to the previous reports, a possible formation mechanism
is proposed, as illustrated in Figure 5
34,41
. The tin precursor under a
high temperature can be fast hydrolyzed to form Sn(OH)
2
, which
further decomposes to form the primary SnO
2
crystallites. After that,
the crystal nuclei of SnO
2
aggregated and grew into SnO
2
nanorods
along the c-axis direction, regulated by the morphology controlling
agents (NH
4
F)
41
. Afterwards, the nanorods self-assemble into the
final nanospheres to minimize their large net surface energy as illu-
strated in Figure 5(a). Furthermore, it has been demonstrated that
the SnO
2
morphology could be successfully modulated by Zn dop-
ing
34,42,43
. For example, Ding et al.
34
have demonstrated that when the
molar ratio of Zn
21
/Sn
41
exceeds 0.133 in the precursor solution,
the morphology of the products can evolve into nanosheets from
the nanorod. Thus, Sn
21
cations can be substituted when Zn
21
cations are introduced into the tin precursor with the same molar
ratio (151), accompanying with the generation of a double ionized
oxygen vacancy (V?O). Then, the charge density and the surface
energy of each crystal face are changed, indicating the existence of
larger polarity in the grown process of Zn-doped SnO
2
nanocrys-
tals
34
. This leads to a different growth rate of the various crystal faces.
With the Zn
21
ions introduced into the solution, some Sn
21
ions are
substituted, forming the compound nuclei under the hydrothermal
conditions. Then, these nuclei grow into nanosheet structures
through aggregation mechanism
44
and Ostwald ripening process
45
under the influence of the Zn
21
. Finally, the nanosheets-assemblied
nanostructures are formed as shown in Figure 5(b).
The gas sensing properties of SnO
2
and Zn-doped SnO
2
sensors
were first measured at different working temperature. Figure 6 exhi-
bits the response of the sensors to 100 ppm glycol, as a function of the
operating temperature in a range of 220–360uC. The optimal oper-
ating temperature for the sensors is around 240uC and the response is
enhanced gradually after the process of calcination and further
doped with Zn element. When the temperature is 220uC, the sensors
Figure 5
|
Schematic formation mechanisms of undoped (S1 600
o
C) and Zn-doped SnO
2
(S2 600
o
C).
Figure 6
|
The relationship between working temperature and response
of three sensors to 100 ppm glycol gas.
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 5
response can reach 50 (Zn-doped SnO
2
sensor) to 100 ppm glycol
which is higher than that of others. With the increase of working
temperature, the response further increases. Until the temperature is
up to 240uC, all the sensors exhibit the maximum response and then
decrease as the temperature further increase, which is due to the
competing desorption of the chemisorbed oxygen
44,46
.
Figure 7(a) shows the response curve of undoped SnO
2
and Zn-
doped SnO
2
sensors to glycol of different concentrations from 5 ppm
to 100 ppm at 240uC. It can be found that all the sensors responses
increase with the increasing concentration of glycol, especially for the
Zn-doped SnO
2
sensors. The response increases gradually and can
reach to 90 when the glycol concentration is 100 ppm, which is about
2 times higher than that of undoped SnO
2
sensor with calcinations
and 3 times higher than that of the sensor without calcinations. It also
indicates that the detection limit could be down to 5 ppm-level with a
response about 5. Compared with the performance of the sensors to
ethanol, as shown in Figure 7(b), glycol gas is more active than
ethanol, which leads to a higher response than ethanol. To the best
of our knowledge,only Zhang et al.
7
fabricated porous SnO
2
nano-
tubes sensor to detect glycol while operating at 300uC. Compared
with the above, the optimal operating temperature of our sensor is
down to 240
o
C, reduced 60
o
C than that of porous SnO
2
nanotubes
sensor. The further comparison between the sensing performances of
our sensor and reported literature is summarized in Table 1.
The response-recovery behavior is also vital characteristic para-
meters for gas sensors. It was defined as the time needed for the
sensor-resistance to change by 90% of the difference from the max-
imum after injecting and removing the detected gas. It can be clearly
observed that the response of Zn-doped SnO
2
sensor increased
abruptly after the injection of ethanol and decreased rapidly, then
recovered to its initial value after the test gas was released. The
response and recovery time are about 14 and 25 s (ethanol), as shown
in Figure 8(a). However, when glycol was injected into the box, the
response and recovery time was prolonged to 66 s and 97 s
(Figure 8(b)), which is much longer than that of ethanol in
Figure 8(a). This may be attributed to the lower operating temper-
ature (240
o
C) and higher boiling point of glycol (197.3
o
C).
Selective detection of the target gas remains a challenge for the
application of metal oxide semiconductor based gas sensor
50
.To
identify the selectivity of the three sensors, the cross response prop-
erties of the sensors were examined by exposing the sensors to
100 ppm glycol and other gases like ethanol, acetone, benzene, para-
xylene,water and hydrogen at 240uC, as summarized in Figure 9. It
was seen that the response of Zn-doped SnO
2
sensor to 100 ppm
glycol is much higher than that of other sensors. The response can
reach to 80, which was about four times higher than the response of
ethanol and more than eight times than that of other gases, indicating
an excellent selectivity to glycol.
As a typical n-type metal oxide semiconductor, SnO
2
based sensor
belongs to the surface-controlled type, and the most widely accepted
model is the formation of a charge depletion layer in the near-surface
region of each grain, relative to the interior parts, due to electron
trapping on adsorbed oxygen species
31,51
. The conductance of the n-
type semiconductor is determined by the amount of electrons in its
conduction band. Thus, according to a standard model
52
, when SnO
2
sensor is exposed to air, oxygen molecules will be adsorbed on its
surface and further capture electrons from the conduction band to
form oxygen ions (O
2
2
,O
22
,O
2
). It will form a depletion region,
resulting in the increase of the sensors resistance. When the sensors
are exposed to a reducing gas such as glycol or ethanol, the oxygen
negative ions will react with the target gas molecules and release the
trapped electrons back to the conduction band of SnO
2
, which reduce
the amount of surface adsorbed oxygen species, and lead to an
increase of the carrier concentrations of the sensors. Consequently,
the depletion layer on the surface of the sensors becomes thin, which
exhibit the decrease of sensor resistance
7,53
. Based on the formula of
response (S5R
a
/R
g
), the sensor shows a high response. Moreover,
after the doped of Zn, the response of the sensors is enhanced
Figure 7
|
Response comparisons of three sensors to (a) glycol and (b) ethanol gas of different concentrations at 240
o
C and 3206C, respectively.
Table 1
|
Sensing properties of Zn-doped SnO
2
hierarchical nanostructures and other reported gas sensors working under different
operating temperatures
Sensing materials (preparation) [ethanol/glycol](ppm) R
a
/R
g
T
sens
(
o
C)
SnO
2
–ZnO thin film
47
200 (ethanol) 4.69 300
Zn-doped SnO
2
nanorods
30
200 (ethanol) 17 270
ZnO-doped porous SnO
2
nanospheres
48
200 (ethanol) 20 150
SnO
2
/ZnO hierarchical nanostructures
49
100 (ethanol) 6.2 400
Porous SnO
2
Nanotubes
7
200 (ethanol) 20 (glycol) 16.7 17.2 300
Our work 200 (ethanol) 100 (glycol) 33 90 320 240
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 6
obviously. The high response of Zn-doped SnO
2
observed here can
be attributed to the following factors. Firstly, the grain size of SnO
2
will decrease with the doped of Zn. Due to the above mentioned, the
surfaces of the SnO
2
nanosheets become more reactive and absorb
more oxygen molecules to form ionized oxygen species
32,54
. Secondly,
the specific surface area of Zn-doped SnO
2
is increased. This means
that the amount of oxygen can be absorbed and ionized on the
surface of Zn-doped SnO
2
. In addition, according to the solid state
chemistry theory, the quantity of oxygen vacancies in Zn-doped
SnO
2
nanostructures is also increased due to the substitution of
Zn
21
for Sn
41
. Thus, owing to the more oxygen species adsorbed
on the surface of SnO
2
and more surface oxygen vacancies in the
Zn-doped SnO
2
nanostructures, the Zn-doped SnO
2
sensor shows a
higher response.
Figure 10(a) shows variations in adsorption spectra of MO organ-
ics dye solution in the presence of the Zn-doped SnO
2
(S2) photo-
catalyst irradiated by a UV lamp for different time. It indicates that
the concentration of MO decreases as the irradiation time increasing
by measuring the intensity of characteristic absorption peak
(465 nm). Different degradation efficiencies for MO, S0, S1 and S2
can be observed directly in Figure 10(b). It can be seen obviously that
S2 (Zn-doped SnO
2
) exhibits more excellent photocatalytic activity
toward MO with the degradation efficiency of 92% than that of
others (MO, 23%; S0, 55%; S1, 75%). Moreover, the photocatalytic
performance is also much higher than those reported. For example,
Yang et al.
55
synthesized ZnO-SnO
2
composite to degrade MO dye,
and the degradation efficiency only reached to 60% in 100 min.
Kowsari et al.
56
also fabricated ZnO/SnO
2
nanocomposites to
degrade MO dye, and the degradation time was longer than
200 min. The higher degradation efficiency may be ascribed to the
specific morphology of S2, which provides larger specific surface area
than others, as shown in Figure 4. Compared with S0, the degrada-
tion efficiency of S1 was enhanced to 75% from 55% (S0), which
indicates an increase of photocatalytic activity after the process of
calcinations, leading to better crystallinity. Generally, the crystallin-
ity of the photocatalysts plays a crucial role in the enhancement of the
separation of the e
2
/h
1
pairs.
In general, the specific surface area plays a main role during photo-
catalytic degradation of dye molecules. On the basis of the photo-
catalytic mechanism
57–59
, it is known that the highly reactive ?OH
and O
2
-
are generated on the surface of photocatalysis under UV
radiation
1
. Therefore, the surface characteristic of nanoparticles will
greatly influence its photocatalytic activity. On the basis of the above
analysis, the Zn-doped SnO
2
catalyst showed the best photocatalytic
activity, this result can be attributed to the special hierarchical struc-
ture, and better crystallinity.
Conclusion
In summary, a facile and simple method has been developed for
preparing undoped and Zn-doped SnO
2
nanostructures. The as-
prepared Zn-doped SnO
2
hierarchical architectures were con-
sisted of interconnected ultrathin nanosheets. It was found that
the Zn doping plays an important role in controlling the
morphologies and structures of the products. The as-prepared
nanostructures were used as the efficient gas sensing materials
to detect a series of flammable and explosive gases, and photo-
catalysts for the degradation of methyl orange (MO) under UV
irradiation. The Zn-doped SnO
2
gas sensor exhibits highly sens-
itive and selective sensing properties to glycol gas. The response
can reach to 90 for 100 ppm glycol which is much higher than
that of undoped SnO
2
. The as-prepared Zn-doped SnO
2
hierarch-
ical nanostructures were also used as efficient photocatalyst and
exhibited excellent degradation for MO. The photocatalytic MO
degradation rate of Zn-doped SnO
2
catalyst is much higher than
that of undoped SnO
2
at the same UV irradiation time. This
work demonstrates that the simply prepared Zn-doped SnO
2
Figure 8
|
Response and recovery time of the S2 (Zn-doped SnO
2
600
o
C) sensor to 100 ppm ethanol and glycol gas at the operating temperature of
320
o
C and 240
o
C, respectively.
Figure 9
|
Selectivity of three sensors for different target gases with same
concentration at 2406C.
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 7
nanostructures have a potential application in glycol gas sensor or
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Acknowledgments
Thanks University of Jinan (UJn) for the support on new staff, and the project supported by
the Taishan Scholar (No. TSHW20120210), the National Natural Science Foundation of
China (Grant No. 11304120, 11174112, 61106059), the Encouragement Foundation for
Excellent Middle-aged and Young Scientist of Shandong Province (Grant No.
BS2012CL005).
Author contributions
Z.Q.Q. and J.D.X. designed the experiments. Z.Q.Q. performed the experiments. H.J.Z. and
D.X.L. performed the SEM observations. C.B.Q. performed TEM observations. Z.Q.Q.,
J.D.X. and X.X.J. discussed and commented on the experiments and results, and wrote the
paper.
Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Zhao, Q. et al. Morphology-modulation of SnO
2
Hierarchical
Architectures by Zn Doping for Glycol Gas Sensing and Photocatalytic Applications. Sci.
Rep.
5
, 7874; DOI:10.1038/srep07874 (2015).
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SCIENTIFIC REPORTS | 5 : 7874 | DOI: 10.1038/srep07874 9
