A highly sensitive gas sensor employing biomorphic SnO2 with multi-level tubes/pores structure: Bio-templated from waste of flax

Article abstract of DOI:10.1039/c9ra02064k

Metal oxide gas sensors with porous structures are widely used in numerous applications ranging from health monitoring and medical detection to safety; in this study, we report a highly sensitive SnO₂ gas sensor with a multi-level tube/pore structure prepared via biomimetic technology using flax waste as a bio-template and a simple wet chemical process combined with subsequent annealing. Indeed, MLTPS not only maintained and improved the excellence of porous structure gas sensing materials with abundant active sites and large surface-to-volume ratios, but also overcame the deficiency of the lack of gas diffusion channels in porous gas sensing materials. Thus, this novel multi-level tube/pore SnO₂ gas sensor exhibited significantly enhanced sensing performance, e.g. an ultra-low response concentration (250 ppb), a high response (87.9), a fast response (9.2 s), a low operating temperature (130 °C) and good stability, for formaldehyde. On the basis of these results, via the reuse of agricultural waste, this study provides a new concept for the low-cost synthesis of environmentally friendly and effective multi-level tube/pore gas sensor materials.

Full text of DOI:10.1039/c9ra02064k

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A highly sensitive gas sensor employing biomorphic
SnO2 with multi-level tubes/pores structure: bio-
templated from waste of flax
Cite this: RSC Adv., 2019, 9, 19993
Xilin Jia,†a Ning Wang,†c Junlong Tian,
a Yong Zhang,a Donglin Lu,a Junjiang Tan,a
*
Ruyi Qiao,a Lulu Chen,a Wang Zhangb and Jianxin Zhonga
Metal oxide gas sensors with porous structures are widely used in numerous applications ranging from
health monitoring and medical detection to safety; in this study, we report a highly sensitive SnO2 gas
sensor with a multi-level tube/pore structure prepared via biomimetic technology using flax waste as
a bio-template and a simple wet chemical process combined with subsequent annealing. Indeed, MLTPS
not only maintained and improved the excellence of porous structure gas sensing materials with
abundant active sites and large surface-to-volume ratios, but also overcame the deficiency of the lack of
gas diffusion channels in porous gas sensing materials. Thus, this novel multi-level tube/pore SnO2 gas
sensor exhibited significantly enhanced sensing performance, e.g. an ultra-low response concentration
(250 ppb), a high response (87.9), a fast response (9.2 s), a low operating temperature (130 ꢀC) and good
stability, for formaldehyde. On the basis of these results, via the reuse of agricultural waste, this study
provides a new concept for the low-cost synthesis of environmentally friendly and effective multi-level
tube/pore gas sensor materials.
Received 18th March 2019
Accepted 9th May 2019
DOI: 10.1039/c9ra02064k
rsc.li/rsc-advances
a fundamental optimized route. For example, Yang et al. doped
Cu and Zn into SnO2 and obtained a highly sensitive and low-
operating temperature gas sensor.12 Sun et al. decorated SnO2
1. Introduction
Semiconducting metal oxide-based gas sensors have aroused
signicant research interest owing to their low cost, easy
production and high sensitivity, stability, and scalability.1–5
They are broadly applied in elds ranging from health moni-
toring and medical detection to safety.3,6–8 Among the currently
available metal oxide semiconductors, SnO2, as a typical n-type
semiconductor with a 3.6 eV band gap, has been extensively
studied in the eld of gas sensors and applied to the detection
of a number of different gases such as HCHO, CH3OH, NH3,
C2H6O, CO, SO2, CH4 and H2S.6,9–14 However, the practical
applications of SnO2 have been limited due to its low selectivity,
high operating temperature and unsatisfactory sensing
response. Recently, to overcome these limits, most efforts to
optimize its performance have been based on composition
modication, structure optimization and process improve-
with CeO2, which presented enhanced gas sensing properties.16
Choa et al. synthesized Cux(x¼1,2)O:SnO2 thin lm nano-
composites, which exhibited a fast response (21 s) and recovery
speed.17 Xu et al. decorated SnO2 with bimetal PdAu, and
exhibited dual selectivity for detecting formaldehyde and
acetone.6 However, composition modication faces challenges
in terms of control of the component quantity and complicated
synthesis processes; this adds to the cost of the materials.
Moreover, because the sensing mechanism of SnO2 is surface-
control type, a large number of researchers have made signi-
cant efforts to develop novel structures to improve its gas
sensing performance.18 For example, Huang et al. prepared
porous ower-like SnO2 nanostructures that exhibited good
response and reversibility to some organic vapors.18 Liu et al.
synthesized hierarchical meso-macroporous SnO2 that exhibi-
ted higher gas response in the detection of indoor air pollut-
ants.19 Wang et al. synthesized 3D hierarchical SnO2
nanostructure sensors that exhibited excellent gas sensing
performance.13 Kim et al. easily synthesized Pt-functionalized
meso/macroporous SnO2 hollow spheres with outstanding
ment.12,15
Obviously,
composition
modication
is
aHunan Key Laboratory of Micro-Nano Energy Materials and Devices, Laboratory for
Quantum Engineering and Micro-Nano Energy Technology, School of Physics and
Optoelectronic, Xiangtan University, Hunan 411105, P. R. China. E-mail:
jltian666@xtu.ede.cn
bState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800
response and selectivity toward H2S at 250 C.20 In particular,
ꢀ
Dongchuan Road, Shanghai 200240, P. R. China
among diverse morphologies and structures, porous structures
have attracted substantial research interest due to their abun-
dant active sites and large surface-to-volume ratios. However,
porous structures lack gas diffusion channels for gas sensing.19
cInstitute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center
for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South
Puzhu Road, Nanjing 211816, P. R. China
† Xilin Jia and Ning Wang contributed equally to this work.
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However, due to the limits of the chemical synthesis process, it the pretreated cylinders were dipped into a prepared SnCl2
is difficult to prepare optimized and complex ne structures to ethanol solution (SnCl2 0.65 g, 30 ml of ethanol solution with
ꢀ
further improve gas sensing performance. To overcome this a volume fraction of 65%, stirred for 8 h at 25 C) for 36 h at
challenge, in our previous work, we combined chemosynthesis 40 ꢀC in a sealed vessel, rinsed with deionized water and dried
with biomimetic techniques to fabricate SnO2 tubes with hollow in air for 12 h at 70 ꢀC. Lastly, they were sintered in a tube
and porous structures on their walls which exhibited high gas furnace at a rate of 1 ꢀC minꢁ1 to 550 ꢀC and maintained for 2 h
sensitivity.9,10
(the hemicellulose, cellulose and lignin were burnt off, and
Among the above-discussed detection gases, HCHO is one of crystallized SnO2 remained). Then, the SnO2 cylinders were
the most common harmful gases among volatile organic cooled to room temperature and MLTPS was obtained.
compounds that appear in indoor pollutants and the food
industry; it causes multifarious diseases and was classied as
a human carcinogen by the World Health Organization (WHO)
2.2. Characterization
in 2017.21,22 Furthermore, due to the different HCHO content
levels in the exhaled breath of healthy humans (48 ppb) and
lung cancer patients (83 ppb), HCHO has been proposed to be
a biomarker for lung cancer detection.8,23,24 Therefore, the
detection of HCHO is necessary, and its great signicance has
aroused wide interest. However, most semiconducting metal
oxide-based HCHO sensors still contain defects. For example,
they generally demand higher operating temperatures, from
MLTPS was characterized by X-ray diffraction (XRD, Ultima IV,
Cu K radiation, l ¼ 0.15406 nm, 40 kV, 400 mA). The micro-
structure and morphology of MLTPS were observed by scanning
electron microscopy (SEM, VEGA3SBH, JSM-IT300) operated at
20 kV. The transmission electron microscopy (TEM) images,
high resolution transmission electron microscope (HRTEM)
images and selected area electron diffraction (SAED) patterns
were obtained on a Tecnai G2 F20 S-TWIN TMP instrument
operated at an acceleration voltage of 200 kV. The gas sensing
performance of MLTPS was studied on a CGS-8 intelligent gas
sensing system (resistive type) (Beijing Elite Tech Co., Ltd.). N2
adsorption measurements were taken with an ASAP2460
Autosorb-iQ2-MP adsorption analyzer, and the pore size distri-
butions (PSD) of the samples were calculated using the BJH
algorithm.
200 C to 600 ꢀC,22,25–27 and show low selectivity.26–28
ꢀ
In order to avoid the challenges of composition modication
and overcome the limits of structure optimization, in this work,
we combined chemosynthesis with biomimetic techniques to
fabricate novel multi-level tubes/pores SnO2 (MLTPS). Herein,
we used low-cost agricultural ax waste as a sacricial template.
Compared with traditional SnO2 nanomaterials, MLTPS not
only shows abundant active sites and a large surface-to-volume
ratio but also possesses effective gas diffusion channels, which
exhibit better HCHO response and lower operating tempera-
tures. Consequently, this work provides new insight to fabricate
high-performance and low cost HCHO gas sensors.
2.3. Fabrication of the gas sensor and the gas sensing
measurement system
The fabrication process of the gas sensor was as follows: rst,
the as-prepared MLTPS was mixed with terpineol to form
a homogeneous paste. Then, using a small brush, the homo-
geneous paste was coated on a ceramic tube attached to a pair of
gold electrodes to form a thin lm. Next, the coated ceramic
tube was dried at 60 ꢀC for 24 h. Furthermore, a Ni–Cr alloy coil
2. Experimental
2.1. Preparation of MLTPS
In this work, combining chemosynthesis with biomimetic was inserted into the ceramic tube to form a heating resistor,
techniques, MLTPS was fabricated using a modied method in which provided the operating temperature of the gas sensor.
our previous work.9,10,29 Fig. 1 shows the strategy employed for The operating temperature was controlled by tuning the heating
fabricating MLTPS. Herein, ax waste, which possesses multi- current. In the last step, in order to improve the long-term
level tubes/pores structures, was chosen as a bio-template. stability of the as-prepared gas sensor, the aging process of
The fabrication route consists of three steps. First, ax waste the sensor was performed for 2 h on a gas sensitivity instrument
was cut into cylindrical pieces. The length of these cylinders was (Beijing Elite Tech Co., Ltd.).
about 1 cm and their diameter was about 0.2 cm. Next, to
The sensing measurements were performed using a CGS-8
aminate the ax waste cylinders, the cylinders were heated in intelligent gas sensing system (resistive type) (Beijing Elite
ꢀ
5% dilute ammonia at 100 C for 10 h, then washed in deion- Tech Co., Ltd.) under laboratory conditions (30 RH%, 25 ꢀC).
ized water and dried in air at 70 ꢀC for 8 h. In the second step, Herein, a static system was used to test the gas response, where
Fig. 1 Schematic of the preparation of MLTPS.
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the target gas was induced by injecting its corresponding liquid of interconnected nanoparticles with an average diameter of
and vaporizing it by a heater. The gas response (S) of the sensor about 7.39 nm. Nanoparticles spontaneously aligned with one
in this paper was dened as the resistance ratio Ra/Rg, where Ra another and attached to the surface of the multi-tubular walls to
is the resistance in air and Rg is the resistance in air mixed with form a porous sponge-like multi-level tubes/pores structure
the tested gas. The response or recovery time was expressed as with interior cavities and loose structures, which is benecial to
the time needed for the sensor output to reach 90% saturation inward and outward gas diffusion and gas sensing.28,30 Conse-
aer switching off the gas in a step function.
quently, for gas sensors, MLTPS not only has abundant active
sites and a large surface-to-volume ratio, but also possesses
numerous diffusion channels.
3. Results and discussion
3.1. Characterization of MLTPS
The HRTEM images of MLTPS (Fig. 3(d)) evidently show that
the interconnected nanoparticles that form the loose structures
are SnO2 nanoparticles. The lattice fringes have interplanar
distances of dSnO (101) ¼ 0.264 nm and dSnO (110) ¼ 0.343 nm
In this work, combining a simple and low cost chemosynthesis
method with biomimetic techniques, biologically novel multi-
level tubes/pores SnO2 (MLTPS) was obtained. The
morphology and structure of MLTPS are shown in Fig. 2 and 3.
From the SEM images of MLTPS (Fig. 2), the multi-level tube
array and pore morphology can be observed. As shown in
Fig. 2(a), MLTPS preserved the multi-tubular pore array from
the ax waste, and the diameter of these multi-tubular pores
ranges from approximately 2 mm to 3 mm. In addition, pores are
regularly distributed on the walls of the multi-tubular pores
array (Fig. 2(b)–(e)), and the average diameter of these pores is
about 2 mm. It is clear that the multi-tubular array and pores
formed a hierarchically multi-level tubes/pores structure. The
multi-level tube array and pores structure shows excellent
performance for gas diffusion and successfully overcame the
deciency of porous structures which lack gas diffusion chan-
nels. Moreover, in order to further conrm the hierarchically
multi-level tubes/pores structure, TEM observations were used.
Fig. 3(a)–(c) also show the morphology of the multi-tubular and
porous walls. The high-magnication TEM images (Fig. 3(b)–
(d)) indicate that the multi-level tubes/pores structure consists
2
2
(Fig. 3(d)). From Fig. 3(e), it can be seen that the SAED image is
a typical SnO2 polycrystallinity diffraction pattern, and the rings
can be indexed to the (110), (101), (200), (220) and (310) crystal
planes, respectively. The XRD result is shown in Fig. 3(f). The
XRD pattern shows that the diffraction peak of MLTPS corre-
sponds to the standard SnO2 PDF card (#41-1445), which
demonstrates that the nanoparticles are tetragonal phase SnO2.
The lattice plane corresponding to the diffraction peak has been
marked in the image (Fig. 3(f)). The clear rings of the SAED
image (Fig. 3(e)) match well with the XRD result (Fig. 3(f)).
The chemical states and elemental composition of MLTPS
were determined by XPS. Fig. 4 shows the typical XPS spectra.
The survey spectrum (Fig. 4(a)) indicates that MLTPS consists of
Sn and O elements.31 As shown in Fig. 4(b), the peaks in the Sn
3d spectrum located at 486.63 eV and 495.03 eV originate from
the Sn 3d5/2 and Sn 3d3/2 core level states, respectively.30 The
values correspond to the binding energies of Sn4+ ion.27 The O
1s high resolution XPS spectrum is shown in Fig. 4(c). As shown
in Fig. 4(c), the O 1s peak is asymmetric, and the broadened
Fig. 2 SEM images of the tubes/pores structure of MLTPS. (a) Cross section image of MLTPS, (b) large field image of the rectangular area in (a), (c)
magnified image of the figure in (b), (d) image of the porous structure of MLTPS and (e) magnified image of the selection in (d).
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Fig. 3 (a)–(c) TEM images of the MLTPS tubes/pores structure, (d) HRTEM image of MLTPS, (e) SAED image of MLTPS, (f) XRD pattern of MLTPS.
peak can be Gaussian divided into two sub-peaks centered at channels, which contribute to the transmission of gas.
530.48 eV and 531.89 eV, respectively. The low binding energy According to the BET surface area plot (inset Fig. 5(a)), the BET
(530.48 eV) can be assigned to the lattice O2ꢁ ions in the SnO2 surface area of MLTPS is 41.2129 m2 gꢁ1. Fig. 5(b) shows that
species. The high binding energy (531.89 eV) is usually attrib- the BJH adsorption cumulative volume of the pores is 0.166639
uted to the oxygen species of Oꢁ and O2ꢁ absorbed on MLTPS.30 cm3 gꢁ1. Moreover, the pore size distribution (PSD) curve of
The chemisorbed oxygen species play an important role in gas biological SnO2 (inset Fig. 5(b)) demonstrates that the biological
sensing performance. Therefore, greater amounts of absorbed SnO2 possesses a hierarchical pore structure in which the pore
oxygen species contribute to greater improvement of the gas size is distributed between 1.78 nm and 168.97 nm and most of
sensing properties.32
the pores have pore sizes of 3.7 nm to 20 nm. This is consistent
The architecture of the microporous and mesoporous with the porous sponge-like multi-level tubes/pores structure
structures of MLTPS was determined by nitrogen adsorption with interior cavities and loose structures which is shown in
and desorption measurements. The nitrogen adsorption and Fig. 2 and 3. In conclusion, these results prove that the MLTPS
desorption isotherms curve of MLTPS, the BJH adsorption not only possesses abundant active sites and a large surface-to-
cumulative pore volume and MLTPS pore size distribution volume ratio, but also contains numerous gas diffusion chan-
(PSD) plot (inset) are shown in Fig. 5. MLTPS showed an obvious nels; these greatly improve the gas adsorption, gas sensing
hysteresis loop (Fig. 5(a)), and the hysteresis loop was not steep; reactions, gas desorption and transport of gas.
this indicates the existence of the multi-level tubes/pores
structure.30 This result was consistent with the SEM and TEM
images (Fig. 2 and 3), where the multi-level tube array and pores
morphology can be observed. The tubes form gas diffusion
3.2. Gas sensing measurements
It is known that the gas response of a semiconductor sensor is
usually dependent on the operating temperature. However, as
Fig. 4 XPS images of MLTPS. (a) Survey spectrum of multi-level tubes/pores SnO2 (MLTPS), (b) Sn 3d high resolution XPS spectrum, (c) O 1s high
resolution XPS spectrum.
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dꢁ
dꢁ
ꢁ
activated to Ox (Ox ¼ O2 , Oꢁ).6,33,34 The response then
decreased rapidly with further increase of the operating
temperature, which may occur because large amounts of reac-
tants were desorbed.6,33 Therefore, 130 C is the optimal oper-
ꢀ
ating temperature of the MLTPS sensor; this optimal operating
temperature is obviously lower than those in numerous re-
ꢀ
ported studies (higher operating temperatures from 200 ꢀ C to
600 C).22,25–27 Here, the low operating temperature (130 C) is
ꢀ
due to the fact that MLTPS has abundant active sites, a large
surface-to-volume ratio and numerous gas diffusion channels,
which results in a faster adsorption/desorption rate. Reason-
ably, the subsequent gas sensing measurements were con-
ducted at the optimal operating temperature (130 ꢀC). Gas
sensing repeatability is an important parameter to evaluate the
sensing ability of semiconductor materials. Here, the repeat-
ability of the MLTPS gas sensor was studied by testing the
changes in the resistance response sensitivity to 50 ppm HCHO
with time at 130 ꢀC in three continuous test cycles. From
Fig. 6(b), it can be seen that the response and recovery curves
showed negligible changes in the continuous test cycles. The
reproducibility tests demonstrate that MLTPS has excellent
reversibility and repeatability for HCHO detection.
In order to study the response and recovery characteristics of
MLTPS, this work analysed the response and recovery times of
ꢀ
MLTPS to 50 ppm HCHO at 130 C. As shown in Fig. 6(c), the
response time and recovery time of MLTPS are 9.2 s and 151.0 s,
respectively. The quick response time (9.2 s) is faster than those
in previously reported work.26,35,36 The quick response and
recovery can be attributed to the numerous gas diffusion
channels of MLTPS, which enable rapid adsorption, desorption
and gas diffusion.
Fig. 5 (a) The nitrogen adsorption and desorption isotherm curves of
MLTPS, inset: BET surface area plot, (b) BJH adsorption cumulative
pore volume, inset: MLTPS pore size distribution plot.
The compositions of indoor home gas and human exhaled
breath are complicated. Hence, selectivity is of great importance
a typical gas sensing material, pure SnO2 with a wide band gap to practical applications, such as indoor toxic gas detection and
(3.6 eV) has a higher operating temperature, which restricts its biomarkers.6 Here, in order to clarify its potential gas sensing
application for gas detection.22,25–27 Therefore, the optimal applications for indoor toxic gas detection and biomarkers, the
operation temperature of a sensor for detecting gases is selectivity of the MLTPS gas sensor was tested with 50 ppm
signicant. In this work, it can be observed in Fig. 6(a) that for potential interference gases (ammonia (NH3), methanol
HCHO detection, the response increased with operating (CH3OH), ethanol (C2H6O) and formaldehyde (HCHO)) at
temperature until it reached a maximum at 130 ꢀC; this may 130 ꢀC. These gases are typical components of indoor toxic gas
occur because as the operating temperature increases, more and biomarkers. The selectivity of the MLTPS gas sensor to
ꢀ
oxygen molecules are adsorbed by the material and are 50 ppm of the abovementioned gases at 130 C is displayed in
Fig. 6 (a) Response and recovery curves of MLTPS to 50 ppm HCHO at different operating temperatures, (b) repeatability of MLTPS for sensing
50 ppm HCHO at 130 ꢀC. Resistance response sensitivity changes with time in three continuous test cycles. (c) Response and recovery char-
acteristics of MLTPS to 50 ppm HCHO at the operating temperature of 130 ꢀC.
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Table 1 Target gas bonds and bond energies37
Fig. 7. From Fig. 7(a), it can be found that the MLTPS gas sensor
showed obviously high response and excellent sensitivity to
HCHO, whereas it exhibited relatively low responses to CH3OH,
C2H6O and NH3 under the same test conditions. As demon-
strated in Fig. 7(b), the response of the MLTPS gas sensor to
HCHO is the highest (87.9); meanwhile, the responses to
CH3OH, C2H6O and NH3 are 17.6, 5.5 and 1.3, respectively. The
response to HCHO was about 5.0, 16.0 and 67.6 times higher
than the responses to CH3OH, C2H6O and NH3, respectively.
Consequently, the MLTPS gas sensor showed excellent selec-
tivity to HCHO, which is one of the most common harmful
gases among volatile organic compounds appearing in indoor
pollutants and the food industry and is also a biomarker for
lung cancer detection. The mechanism of high HCHO selec-
tivity can be explained through the following two aspects. As
one reason, the molecular structures and bond dissociation
energies are signicant inuences. As shown in Table 1, the
HCHO band energy is smaller than those of other gases at
a lower temperature, where it is only 364.0 kJ molꢁ1. The
smaller band energy of HCHO can be broken more easily to
realize the reaction between the gas molecules and the sensing
material during chemical absorption. On the other hand, the
effective diffusion is another important factor. In addition to
Gas type
Bond
NH3
CH3OH
C2H6O
HCHO
H–NH2
H–OCH3
H–CH2
H–CH
436.8
H-OC2H5
H–CH2
H–CH
436.0
473.0
452.0
H–CHO
Bond energy (kJ molꢁ1
)
435.0
364.0
473.0
452.0
the pore size distribution and surface area, it is deduced that
Knudsen diffusion will play a key role in the gas diffusion
during the sensor process, according to the classic mean free
paths (MFP) of the gas molecule formula:
.
pffiffi
l ¼ kBT
2pd2 p
(1)
where kB is the Boltzmann constant in J Kꢁ1, T is the tempera-
ture in K, p is the pressure in Pascals, and d is the diameter of
the gas particles in meters.
The MFP values of the four target gases (HCHO, CH3OH,
C2H6O and NH3) were estimated as 36.56 nm,38 139.00 nm,39
25.39 nm,38 and 46.37 nm,38 respectively. Because the MFP
values of the four target gases are far less than the diameters of
the multi-tubular pores array, molecular diffusion was the main
diffusion method when the target gas rst began to diffuse into
the MLTPS. When the target gases diffused into a deeper area
inside the MLTPS, the MFP values of the four target gases were
larger than average pore size (12.95 nm) of MLTPS; at this time,
Knudsen diffusion played a key role in the gas diffusion during
the sensing process. The diffusion coefficient Dk is shown in
eqn (2):
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4r
Dk ¼
ð2RT=pMÞ
(2)
3
where r is the pore width, R and T are the universal gas constant
and temperature, respectively, and M is the molecular mass of
the gas molecule. In fact, compared with CH3OH and C2H6O,
the molecular mass of HCHO is smaller. Therefore, the diffu-
sion coefficient of HCHO is higher, which will result in a better
response. However, although the molecular mass of NH3 is
smaller than the molecular mass of HCHO and, thus, the
diffusion coefficient of NH3 is higher, the band energy of NH3 is
much higher than that of HCHO; thus, the reaction between the
gas molecules and the sensing material during chemical
absorption is more difficult. Consequently, at low temperature,
the MLTPS gas sensor showed excellent selectivity to HCHO.
Furthermore, this work examined the response of the MLTPS
gas sensor to three different HCHO gas concentrations ranging
from 250 ppb to 1000 ppb at the optimum working temperature
(130 ꢀC). Fig. 8 shows the dynamic response curve and the
relationship between the response sensitivity and gas concen-
tration to low HCHO concentrations (250 to 1000 ppb) at 130 ꢀC.
As shown in Fig. 8(a), the dynamic response–recovery curve of
the MLTPS gas sensor obviously changed stepwise upon expo-
sure of the gas sensor to different concentrations of HCHO,
which also illustrates that the MLTPS gas sensor exhibits
Fig. 7 Selectivity of the MLTPS gas sensor to 50 ppm HCHO, CH3OH,
C2H6O and NH3 at 130 ꢀC. (a) The response and recovery curves of the
different gases. (b) The responses of MLTPS to different gases.
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lower (130 ꢀC). In brief, although it contains a single SnO2
component, the MLTPS gas sensor exhibits excellent response
and fast response/recovery times at a low operating tempera-
ture; this indicates that the MLTPS gas sensor is a promising
HCHO sensor for indoor toxic gas detection and biomarkers.
3.3. Sensing mechanism
Furthermore, here, the gas sensing mechanism of MLTPS is
discussed from the following aspects. On one hand, SnO2 is one
of the most representative n-type oxide semiconductor gas
sensing materials. The sensor mechanism of SnO2 is explained
as a change in electrical conductivity which is caused by the
chemical interaction of gas molecules with the surface of the
gas sensor material.44 In this mechanism, the electrical
conductivity change of MLTPS is related to the amount and
species of chemisorbed oxygen during the adsorption and
desorption processes. Firstly, when the MLTPS sensor is
exposed to air, oxygen molecules can be absorbed on the surface
of MLTPS by capturing electrons from the conduction band of
SnO2 to form chemisorbed oxygen ions; this decreases the
concentration of electrons, which results in increased resis-
tance and decreased electrical conduction (Fig. 9).6,26 In this
work, the optimal operating temperature of MLTPS was 130 ꢀC.
At low operating temperatures (<200 C),34,45 the chemisorbed
ꢀ
oxygen ion process can be expressed as follows:26
O2(gas) 4 O2(ads)
O2(ads) + eꢁ 4 O2ꢁ(ads)
O2ꢁ(ads) + eꢁ 4 2Oꢁ(ads)
(3)
(4)
(5)
Fig. 8 The gas sensing properties of the sensor based on MLTPS at an
operating temperature of 130 ꢀC. (a) The dynamic response curve of
the MLTPS gas sensor to low HCHO concentrations (250 to 1000 ppb).
(b) The relationship between response sensitivity and gas
concentration.
Then, when the MLTPS gas sensor is exposed to HCHO,
which is a reducing gas, the gas molecules react with the
adsorbed oxygen ions and release the trapped electrons back to
the conduction band of the SnO2 gas sensing material, leading
to decreased thickness of the space-charge layer and potential
barrier, as shown in Fig. 9.26,30 Finally, it was shown that the
resistance of the MLTPS gas sensor decreases and the electrical
conduction increases. The process by which formaldehyde gas
molecules are adsorbed onto the surface of MLTPS and react
excellent reliability even upon exposure to low gas concentra-
tions. Also, the corresponding responses of the MLTPS gas
sensor gradually increased with increasing concentration of
HCHO from 250 ppb to 1000 ppb (Fig. 8(b)). The responses of
the MLTPS gas sensor to the low HCHO concentrations (250
ppb, 500 ppb and 1000 ppb) at 130 ꢀC were 2.20, 2.40 and 3.65,
respectively. Consequently, these results demonstrate that the
MLTPS gas sensor is reliable and exhibits excellent response
even upon exposure to low gas concentrations (250 to 1000 ppb),
which is suitable for potential gas sensing applications for
indoor toxic gas and biomarker detection.
dꢁ
dꢁ
ꢁ
with Ox (Ox ¼ O2 , Oꢁ) can be expressed as follows:46,47
HCHO(gas) / HCHO(ads)
(6)
(7)
HCHO(ads) + O2ꢁ(ads) / CO2(gas) + H2O + eꢁ
Here, Table 2 shows a comparison of the HCHO gas sensi-
tivity performance of nanostructured materials (based on SnO2)
with different structures. As shown in Table 2, the sensor in this
work (SnO2 (biomorphic MLTPS)) exhibited a higher response
value (87.9/50 ppm) than other HCHO gas sensors containing
HCHO(ads) + 2Oꢁ(ads) / CO2(gas) + H2O(gas) + 2eꢁ (8)
On another hand, as shown in Fig. 2 and 9, the gas molecules
absorbed on the surface of the porous sponge-like multi-level
tubes/pores structure of MLTPS and reacted with SnO2 nano-
particles. Then, the gas molecules diffused effectively in the
a
single SnO2 component,26,36 decorated SnO2,35,40–42 and
composite SnO2.43 In addition, while fast response and recovery
times (2 s/7 s) were reported, their operating temperatures were multi-tubular structures of MLTPS.
higher than that of the MLTPS gas sensor. Compared with the
Consequently, the excellent gas sensing performance to
ꢀ
operating temperatures of these HCHO gas sensors (160 C to HCHO exhibited by MLTPS can be explained in two aspects. In
ꢀ
300 C), the operating temperature of the MLTPS gas sensor is
one respect, its excellent gas sensing performance can be
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Table 2 Comparison of the formaldehyde gas sensitivity performance of nanostructured materials with different structures
Response material (material
structure)
Sensor response (S)/target
gas concentration (ppm)
Response/recovery times (s)
Operating temperature (ꢀC)
SnO2 (microspheres)
17.0 s/25.0 s (100 ppm)
30.0 s/30.0 s (200 ppm)
10.0 s/20.0 s (10 ppm)
200 ꢀC
300 ꢀC
280 ꢀC
38.3/100 ppm (ref. 26)
9.0/200 ppm (ref. 36)
30.0/50 ppm (ref. 40)
SnO2 (hollow microspheres)
Sb-doped SnO2 (hierarchical peony-
like)
In/Pd–SnO2 (microspheres)
NiO-doped SnO2 (nanobers)
Pd decorated SnO2 (nanobers)
NiO/SnO2 (microspheres)
SnO2 (biomorphic MLTPS)
5.0 s/— (100 ppm)
50.0 s/80.0 s (10 ppm)
2 s/7 s (100 ppm)
5.0 s/15.0 s (50 ppm)
9.2 s/151.0 s (50 ppm)
160 ꢀC
200 ꢀC
160 ꢀC
240 ꢀC
130 ꢀC
24.6/100 ppm (ref. 41)
14.0/50 ppm (ref. 35)
24.0/1000 ppm (ref. 42)
27.6/100 ppm (ref. 43)
87.9/50 ppm (this work)
sensing performance can be highly enhanced.26,48 Compared
with the Ld (about 3 nm) for SnO2 materials in air,26 the average
SnO2 grain size of MLTPS (about 7.39 nm) was close to 2Ld. This
indicates that the almost full depletion of electrons promoted
the action rate between chemisorbed oxygen ions with HCHO
molecules, which improved the response of the sensing
materials.
4. Conclusions
In summary, MLTPS with a multi-level tubes/pores structure
was successfully prepared by combining facile and low cost
chemosynthesis with biomimetic techniques. The SnO2 nano-
particles spontaneously aligned with each other and attached to
the surface of the multi-tubular walls to form a porous sponge-
like multi-level tubes/pores structure with interior cavities and
loose structures. Moreover, MLTPS exhibited remarkably
enhanced sensing performance to formaldehyde, e.g. ultra-low
response concentration (250 ppb), high response (87.9), fast
ꢀ
response (9.2 s), low operating temperature (130 C) and good
Fig. 9 Schematic of the gas sensing mechanism.
stability. Indeed, MLTPS not only persisted and improved the
excellence of porous structure gas sensing materials with
abundant active sites and large surface-to-volume ratios, but
also overcame the deciency of the lack of gas diffusion chan-
nels in porous structure gas sensing materials. In conclusion,
this work introduced a facile, low-cost and environmentally
friendly way to prepare porous gas sensing materials with multi-
level tube arrays for effective diffusion channels. Furthermore,
this work presented a promising HCHO sensor with great
potential for indoor toxic gas detection and biomarkers.
attributed to the porous sponge-like multi-level tubes/pores
structure with interior cavities and loose structures. Firstly, its
porous sponge-like multi-level tubes/pores structure showed
a large specic surface area (41.2129 m2 gꢁ1) and abundant
active sites which can supply an adequate surface for absorbing
and sensing reactions of gas molecules. Secondly, the inter-
connected multi-tubular structure provided numerous gas
diffusion channels; thus, the gas diffusion was more effective.
Indeed, MLTPS not only maintained and improved the perfor-
mance of porous structure gas sensing materials with abundant
active sites and large surface-to-volume ratios, but also over-
came the deciency of the lack of gas diffusion channels in
porous structure gas sensing materials. Thus, MLTPS exhibited
high HCHO response, fast response–recovery characteristics
and low gas concentration response at a low operating
temperature. In another aspect, the excellent gas sensing
performance of MLTPS can be attributed to the suitable grain
sizes of its SnO2 nanoparticles. Because the response and
recovery processes are controlled by the surface-charge model,
when the grain size is decreased to a scale comparable to the
thickness of the surface electron depletion layer (Ld), the gas
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (no. 11647062, no. 51702274), the
Provincial Natural Science Foundation of Hunan (no.
2017JJ3288, no. 2018JJ2404), the China Postdoctoral Science
Foundation funded project (no. 2017M622591), the Scientic
Research Project of the Education Department of Hunan
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