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Sn doped ZnO layered porous nanocrystals with hierarchical structures and modified surfaces for gas sensors
Accepted Manuscript Title: Sn doped ZnO Layered Porous Nanocrystals with Hierarchical Structures and Modified Surfaces for Gas Sensors Author: Mingshui Yao Fei Ding Yuebin Cao Peng Hu Junmei Fan Chen Lu Fangli Yuan Changyong Shi Yunfa Chen PII: DOI: Reference:
S0925-4005(14)00489-4 http://dx.doi.org/doi:10.1016/j.snb.2014.04.078 SNB 16844
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
9-2-2014 21-4-2014 23-4-2014
Please cite this article as: M. Yao, F. Ding, Y. Cao, P. Hu, J. Fan, C. Lu, F. Yuan, C. Shi, Y. Chen, Sn doped ZnO Layered Porous Nanocrystals with Hierarchical Structures and Modified Surfaces for Gas Sensors, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.04.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sn doped ZnO Layered Porous Nanocrystals with
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for Gas Sensors
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Hierarchical Structures and Modified Surfaces
Mingshui Yaoa, b, Fei Dinga, Yuebin Caod, Peng Hua, Junmei Fana, Chen Lua, b, Fangli Yuana,*,
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering,
an
a
us
Changyong Shic,*, and Yunfa Chena
Chinese Academy of Science (CAS), Zhongguancun Beiertiao 1 Hao, Beijing 100190, China University of Chinese Academy of Sciences (UCAS), No.19A Yuquan Road, Beijing 100049,
M
b
Department of Elements, Beijing Institute of Fashion Technology, NO.2 Yinghua Road
Ac ce pt e
c
d
China
Chaoyang District, Beijing 100029, China d
Department of Chemical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-
gu, Ansan, Kyeonggi-do 426-791, Republic of Korea.
* Corresponding author.
E-mail: [email protected]; Fax: +86-10-62561822; Tel: +86-10-82544974 E-mail: [email protected]; Tel: +86-13651229190
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ABSTRACT: In this paper, a simple solvothermal method is developed for the synthesis of Sn doped ZnO layered porous nanocrystal (Sn-ZLPC). Scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy are used to
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characterize the detailed structures and surface/near-surface chemical composition of the asprepared products. Sn doping is found to be the key factor controlling the layered porous
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structure. The possible growth mechanism of Sn-ZLPC is also discussed. As the stability of
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target gas increases, the optimal operating temperature shifts from 300°C to 500°C and the highest response shifts to the sample with high atomic ratio of Sn. The later shift is attributed to
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the alternate key influence of the activation energy and adsorption/desorption of oxygen ions. 5.0 at% Sn-ZLPC exhibits the best sensing properties to VOCs due to the combined effect of low
structure.
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activation energy, adsorption/desorption of oxygen ions and the mesoporous hierarchical
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d
Keywords: MOX sensors, Hierarchical structure, Modified surfaces, Sn doping, VOCs
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1. Introduction Nanotechnology has generated much attention beyond the realm of science and engineering and can be an ideal candidate to help people see things invisible [1]. One of the interesting
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applications was nano-structured/modified metal oxide (MOX) sensors acting as the nose to detect toxic volatile organic compounds (VOCs).
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For the structural aspect, numerous hierarchical structures possessing enhanced gas diffusion
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(utility factor [2]) than the pristine structures were introduced to MOX sensors, such as branched crystallites [3-5], nanowires assembled polyhedrons [6], mesoporous films [7-9], single/multi-
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shell hollow porous spheres [10-15], complicated structures based on precursors [16-19] or biological systems [20] and so on. Purposed built thin film with controlled particle size and pore
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distribution was also fabricated to improve the sensing response to gases with different Knudsen diffusion coefficients [21]. In addition, large amounts of MOX sensors with controlled crystal
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surface were also synthesized to improve the surface reaction between target gas molecules and
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surface absorbed oxygen ions [22-25].
For the modified surface aspect (receptor function [2]), lattice substituting doping [26-28], new phase doping [29-32], and noble metal decoration [33-36] were experimentally proved to be effective ways to improve the sensing properties mainly by tailoring the activation energy, adsorption/desorption of oxygen ions and reaction rates. In most of the cases, surface modification can also be beneficial to the electron transport (transducer function [2]). Therefore, the combination of hierarchical structure and modified surface might further improve the sensing properties of MOX sensors. Our previous reports have demonstrated that MOX sensors with hierarchical structures [18] and specific exposed crystal facets [25] exhibited high gas sensing activity to benzene, which were mainly due to the enhancement in the gas
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diffusion/molecule capturing. In addition, Sn was found to be an effective dopant that enhanced the sensing properties of ZnO gas sensors [27,37,38]. Herein, we demonstrate the synthesis of novel Sn doped ZnO layered porous nanocrystals (Sn-
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ZLPCs). Sn dopants were introduced to obtain Sn-ZLPCs with hierarchical structures. Each SnZLPC was constructed by multi-layers of ZnO nanosheets with a thickness of 10-20 nm, and
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each layer of the ZnO nanosheet showed a porous structure. The synthesized Sn-ZLPCs
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exhibited excellent sensing properties to VOCs due to the combined effect of low activation energy, adsorption/desorption of oxygen ions and the mesoporous hierarchical structure.
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2. Materials and methods
2.1 Synthesis of Sn doped ZnO layered porous nanocrystal (Sn-ZLPC)
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In a typical synthesis of 5.0 at% Sn-ZLPC, 1.000 g ZnAc2·2H2O and 0.052 g SnCl2·2H2O were added to 10 mL de-ionized water under stirring to form a white mixture containing Sn2+
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hydrolysis products. Then 60 mL glycerol was added slowly to the mixture under vigorous
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stirring. After stirred for 30 min, a semi-transparent white suspension at ambient conditions was formed. Then the resulting solution was transferred to a Teflon-lined 100 mL stainless steel autoclave and maintained at 200°C for 8 h before cooling down to room temperature. After the experiment, the particles were collected from the bottom of the reactor. The products were washed three times with de-ionized water and twice with ethanol by redispersion and centrifugation. The solid materials were then dried in air at 60°C for 12 h. 2.2 Materials characterization
The phase and crystal structure of the product were determined by X-ray diffraction (XRD) patterns, which were recorded with a Philips X’Pert PRO MPD X-ray diffractometer using Cu Kα radiation (λ=1.54178 Å). The morphology and structure of the product were then observed
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by scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscope
(TEM,
JEOL
(Brunner−Emmet−Teller)
JEM-2100).
instrument
N2
using
adsorption a
surface
was
measured
area
analyzer
by
a
BET
(NOVA3200e,
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Quantachrome, USA), and pore size distributions were calculated from the adsorption branch of the Nitrogen adsorption-desorption isotherm using the Barrett Joyner Halenda (BJH) formula.
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The surface chemical analysis was investigated by X-ray photoelectron spectroscopy (XPS) on a
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Thermo Scientific ESCALAB 250 Xi XPS system, where the analysis chamber was 1.5×10-9 mbar and the size of X-ray spot was 500 μm. The particle size distribution of the synthesized
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powder was determined using a Beckman Coulter LS 13 320 laser-diffraction particle size analyzer.
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2.3 Fabrication of gas sensor prototype and sensor characterization The fabrication of gas sensor prototypes and the sensor characterization were reported in our
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previous work [18,25]. Briefly, Ag paste (SA-5121Q, Wuhan Supernano Optoelec Technology
Ac ce pt e
Co. Ltd) was used for the ohmic contact of sensing film and two Pt wires on both ends of the film. Steps to fabricate Ag electrode were as follows: Ag paste was first printed on the Al2O3 substrate and Pt wires; after irradiated by an infrared lamp (250 W, about 150°C) for 15 min, Pt wires, Ag powder and Al2O3 substrate were stuck together with the help of organics (color: golden yellow); then all of them were heated at 550°C for 30 min to burn the organic and sinter them together (color: silvery white). The solution containing sensing material and ethanol was drop-coated onto the Al2O3 substrate. For the drop-coated sensors, preheating at 550°C for 60 min before the achievement of stable process (400°C for 20 h) was needed to ensure good ohmic contact. VOC gas was introduced into the quartz tube by mixing the certified gas ‘‘mixtures’’ and dry air in the proper ratio controlled by the mass flow controllers [4]. The constant flow was
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600 ml min-1. The bias on the sensor was 10 V and the current was recorded using Keithley 2601 Sourcemeter. The response was defined as the ratio of sensor resistance in air and in detected gas (Rair / Rgas -1). The response / recovery time is defined as the time required for the resistance of
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the sensor to change to 90% / 10% of the saturation value after exposure to the test gas / air.
3. Results and discussion
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3.1 Materials preparation and characterization
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3.1.1 Crystallinity, phase and structure
XRD patterns of undoped ZnO, Sn-doped ZnO products and the amorphous mixture of Sn/Zn
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oxides are presented in Fig. 1. These patterns correspond to three main diffraction peaks of hexagonal wurtzite ZnO (JCPDS 01-089-0510), namely (100), (002) and (101). The intensities
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of diffraction peaks declined as Sn concentrations increased, indicating that Sn doping within ZnO caused degeneration of the ZnO crystallinity [39]. It can be seen that Sn doping slightly
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shifts the position of (002) peaks to higher diffraction angles, which due to its smaller radius
Ac ce pt e
[39,40]. The peak shift of Sn doped ZnO also provides an evidence for the substitution of Sn partly into the ZnO lattice [28]. The synthesized product became amorphous when the atomic percentage of Sn up to 50 at% or higher. Fig. 1 also indicates that Sn doped ZnO product was prepared without the formation of a secondary phase. Due to good sensing properties (see section 3.2) and its unique hierarchical structure, 5.0 at% Sn doped ZnO layered porous nanocrystal (Sn-ZLPC) was chose as the sample for detailed structural analysis ( Fig. 2). As shown in Fig. 2a-c, the size of as-prepared 5.0 at% Sn-ZLPC is about 500 nm in diameter and 1 μm in height. Further structural analysis by HRTEM images shows that each particle is constructed by multi-layers of ZnO nanosheet with a thickness of 1020 nm (Fig. 2d-f), and each layer is constructed by nanopores (mainly less than 10 nm) and
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polycrystalline ZnO nanocrystals (Fig. 2g-i). The spacings of 0.28 nm and 0.26 nm from the cross-sectional and plan view of the layer are consistent with the values for (100) and (002) planes of hexagonal wurtzite ZnO phase, respectively (Fig. 2f and i).
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EDS mapping images of 5.0 at% Sn-ZLPC in Fig. 3a indicate that Sn dopants uniformly distributed in the particle and its atomic percentage was calculated to be 4.8% (Fig. 3a). The
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surface/near-surface chemical composition of Sn-ZLPC was examined by X-ray photoelectron
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spectroscopy (XPS). The full-range XPS spectra (Fig. 3b) reveals that it is mainly composed of Zn, Sn, O and C elements, of which C contamination can be mainly attributed to the residual
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solvents. More detailed information on the chemical state of these elements can be obtained from the high resolution XPS spectra of the Zn 2p, Sn 3d and O 1s in Fig. 3c-e. The peaks of Sn 3d3/2
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and Sn 3d5/2 at 495.0 and 486.6 eV can be assigned to the lattice tin (Sn(IV) oxidation state) in tin oxide [20,34,41]. The peak separation of 8.4 eV between these two peaks is in agreement
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with the standard spectrum of SnO2. The O 1s peaks are much asymmetric. The component at
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low binding energy (BE, 530.2 eV) is typical for the lattice O2− ions; and that at high BE (533.2 eV) is usually attributed to chemisorbed or dissociated oxygen or OH species on the surface of the particle, such as CO2, adsorbed H2O or adsorbed O2 [34,42]. The component at medium BE (531.8 eV) was associated with O2− ions in the oxygen deficient regions in the matrix of ZnO by Chen et al [42], while it was attributed to O-H by Coppa and Davis [43]. However, the weak peak of lattice oxygen and strong peaks of non-lattice oxygen indicate that there are large amounts of absorbed oxygen ions on the surface of the crystal. Calculated from the quantified peak area, the atomic ratio of Sn/Zn was estimated to be about 25.2 at% (XPS), which is much higher than that estimated by EDS (4.8 at%). Thus, more active sites on the surface can be expected due to rich absorbed oxygen and high atomic ratio of Sn/Zn. Since the fact that at high
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Sn contents, due to its tendency for octahedral coordination, Sn tends to demix from the ZnO lattice as an amorphous material, together with XRD results and TEM analysis, high ratio of Sn/Zn on the surface might be the result of ultra-thin layer of amorphous Sn on the surface and
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better chemical stability of Sn-O than Zn-O to the etching effects of halogen ions (the dissolve of more Zn ions on the crystal surface).
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The nanoporous structure of 5.0 at% Sn-ZLPC was investigated by conducting nitrogen
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adsorption−desorption measurements. The nitrogen adsorption−desorption isotherm in Fig. 4 exhibits type IV isotherms with type H3 hysteresis loops. In our work, we have found two typical
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phenomenon indicating the risk to determine the BJH pore size distribution from the desorption branch. One was the forced closure of the hysteresis loop at around P/P0 = 0.45 (for N2 at 77 K,
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Fig. 4); the other was the pore size distribution peak at around 4 nm determined by the nature of adsorptive instead of the porous properties of the material (up-left inset of Fig. 4) [44].
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pore size distribution in this work.
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Therefore, it is more appropriate for us to use adsorption branch for the determination of BJH
According to the BJH pore size distribution curve (up-right inset of Fig. 4) calculated from the adsorption isotherm, 5.0 at% Sn-ZLPC exhibited a mesoporous structure with pore diameter mainly less than 10 nm. The cumulative pore volumes were calculated to be 0.083 m3 g-1. With the highly mesoporous structures, the BET surface area of 5.0 at% Sn-ZLPC was calculated to be 78.99 m2 g-1. Pores centered at about 2 and 4 nm might mainly attributed to nanopores on each layer, and pores centered at 10 nm might located at the interfaces of the layers and some large pores on the layer formed by unstable parts. The mesoporous hierarchical structure might be beneficial to the diffusion of target gas. 3.1.2 Reaction time
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The size distributions and the size evolution of particles were also studied. As shown in Fig. 5a, the size of the sample is mainly centered at about 1.0 μm. The median particle diameter (D50) of 5.0 at% Sn-ZLPC increases with the reaction time which is in accordance with Ostwald ripening.
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As shown in Fig. 5b, the corresponding mean size increases sharply at the beginning and then slowly after 8 h.
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In the formation process of 5.0 at% Sn-ZLPCs, the atomic percentages of Sn of the products
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obtained at different reaction times were calculated by EDS and summarized in Table 1. The atomic percentages of Sn varied from 4.28 at% to 6.47 at% and no abrupt changes were
participated in the formation of ZnO products.
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3.1.3 Atomic percentage of Sn dopants
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observed. It indicates that Sn dopants uniformly dispersed in the solution and continuously
Fig. 6 shows that the percentages of Sn dopants significantly affect the morphologies of the
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products (the samples collected from the bottom of the reactor). The size of the particle decreases
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from 5 μm to 0.5 μm and the surface becomes more roughness as the percentage of Sn increases from 0 at% to 10.0 at%. The hollow structure of the particles collected from the bottom of the reactor disappears when the percentage of Sn increased up to 1.0 at% (Fig. 6a and b, the TEM image of particle with small size was chose to show solid core more clearly). When the percentage of Sn is up to 50 at%, only small amorphous particles of Sn/Zn oxides can be observed (Fig. 1 black and Fig. 6f). Without Sn dopants, high crystallinity, large and dense particle assembles of ZnO were beneficial for the formation of hollow and [001] oriented nanorod spheres via Ostwald ripening [45], which showed “core-shell” like structure in optical microscope due to compact outside, loose inside and hollow core. The above analysis reveals that Sn dopants can significantly degenerate the crystallinity of ZnO and prevent small particles
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agglomerate into dense and large particles (detailed discussion can be found in section possible growth mechanism). 3.1.4 Other influence factors
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To investigate the growth mechanism of Sn-ZLPC, we also examined other influence factors
The influences of synthetic parameters were summarized in Fig. 7.
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such as solvents, reactants, reacting temperature, the concentration of the reactants, and so on.
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The water volume strongly affected the morphology of the product. When the volume of water was increased to 35 mL or 70 mL, no particles with mesoporous layered structure were observed
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(Fig. S1, Supporting Information (SI)). The replacement of glycerol by ethanol resulted in the irregular disperse of Sn elements because the solvents with much lower viscosity failed to
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prevent the settlement of white Sn2+ hydrolysis products (Fig. S2, SI). It should be mentioned that when the white mixture of ZnAc2, Sn2+ hydrolysis products, Cl ions, water and glycerol
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were maintained at 70-90°C or higher temperature for a few minutes under stirring, the solution
Ac ce pt e
would turn into clear due to the reaction of Sn2+ hydrolysis products and glycerol. As a result, Sn2+ dispersed uniformly in the solution. Due to high viscosity of glycerol at low temperature, the solvothermal products of preheated at 70- 90°C and non-preheated show not difference in glycerol solvent.
SnCl2 was replaced by other reactants (such as HCl and ZnCl2) with equal concentration of Cl ions to figure out the role of Sn2+ in the formation of mesoporous structure. The results showed that the replacement of SnCl2 to HCl or ZnCl2 had no effect on the size or morphologies of the ZnO hollow spheres, but just made the surface of the ZnO hollow spheres rougher. It indicates the key role of Sn ions in the formation of mesoporous structure.
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Reaction temperature is another important factor for obtaining the mesoporous structure. Particles with mesoporous layered structure can be obtained only when the reaction temperature up to 160°C or higher (Fig. S3, SI). Furthermore, higher temperature can short the time to obtain
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the stable mesoporous particles. In addition, the products formed by lower initial concentration (25% and 50%) were collected
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and characterized. The results showed that the mesoporous layered structures were maintained,
have little effects on the formation of mesoporous structure.
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3.1.5 Possible growth mechanism
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the size of the particles was reduced sharply (Fig. S4, SI). It means that the initial concentration
On the basis of experiments and discussions mentioned above, we deduced the possible growth
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mechanism. In the initial stage, Sn2+ hydrolysis products and glycerol reacted when the temperature up to 70°C. So a clear solution with uniformly dispersed Sn ions was obtained. With
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temperature increasing, more OH- were released due to the reaction of Ac- and glycerol, and
Ac ce pt e
hence a large number of Sn4+ doped ZnO nuclei were produced through the oxidation and hydrolysis process. As the reaction continued, Sn4+ doped ZnO nuclei were assembled into large particles, and the size was affected by the concentration of Sn ions. Then smaller particles and some of the inner parts of larger particles with poor crystalline quality as well as higher surface energies were dissolved due to Ostwald ripening, which then served as the sources for the further growth of larger particles. Under the effects of Ostwald ripening, together with the etching effects of halogen ions (such as Cl- and F-) [46], Sn4+ doped ZnO layered particles with mesoporous hierarchical structure were formed. 3.2 Sensing properties
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As we have mentioned in the introduction and our previous work, gas sensing processes can be divided into three units, which were gas diffusion/molecule capture (utility factor and some case of receptor function), surface reaction (receptor function), and electron transport (transducer
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function) [2,18]. Possible advantages of Sn-ZLPC can be estimated in the following two aspects. One is that the
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modified crystal surfaces (more active sites and high crystal defects, Fig 3b-d, 7a and 7b) and
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reduced activation energy (Fig. 8c) caused by Sn dopants might enhance the surface reaction and electron transport; the other is that the layered mesoporous structure (more accessible crystal
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surfaces, Fig. 2 and 4) and decreasing size of the particles (high surface to volume ratio, Fig. 6 and hollow circles in 7a) facilitate the gas diffusion and molecule capturing of target gases.
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3.2.1 Influences of Sn dopings
As we have been discussed above based on the XPS results, more active sites and crystal defects
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on the surface could be expected due to rich absorbed oxygen and high atomic ratio of Sn/Zn. To
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further probe the crystal defects in doped ZnO material, PL spectra were studied. As shown in Fig. 8b, the intrinsic emission of ZnO (375 nm) only takes a small part in the PL spectra. On the contrary, the luminescence emission from crystal defects (> 390 nm) dominates the entire spectra, whose relative intensity increases with the atomic percentage of the Sn dopant. Therefore, more crystal defects would be expected for the Sn doped ZnO material [27]. At constant surface coverage or constant density of states, the activation energy of semiconductor can be calculated by the Arrehenius plot of the resistance (there are several assumptions, i.e. the mobility of electrons and the ratio of the density of occupied and unoccupied states is insensitive to temperature) [47-50]. 1
σ
=
1
σ0
exp(
ΔE ) kT
(1)
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ln
1
σ
= ln
1
σ0
+
ΔE kT
(2)
Where σ denotes the conductance, k the Boltzmann’s constant, T the absolute temperature and
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∆E is the activation energy. Plotting ln(1/σ) against 1/T, ∆E can be obtained and it is shown in Fig. 8c.
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Fig. 8d shows the responses comparison of particles with different amount of Sn dopants to
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various VOCs at their respective optimal operating temperature. It can be seen that the 5.0 at% sample with the smallest activation energy shows the highest responses to toluene and m-xylene
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and the second but close to the highest responses to acetone and benzene (Fig. 8d). It seems that the activation energy plays the key role in determining the sensing properties here.
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Generally, the surface reaction between the surface adsorbed oxygen species (300-500°C, O2-) and VOCs can be described as the following equations
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C6H6(ads) + 15O2-(ads) → 6CO2(gas) + 3H2O(gas) + 30e- (3)
Ac ce pt e
CH3C6H5(ads) + 18O2-(ads) → 7CO2(gas) + 4H2O(gas) + 36e- (4) (CH3)2C6H4(ads) + 21O2-(ads) → 8CO2(gas) + 5H2O(gas) + 42e- (5) CH3COCH3(ads) + 9O2-(ads) → 3CO2(gas) + 3H2O(gas) + 18e- (6) Then the electrons are released to the conduction band of MOX to improve its conductivity. As we know, benzene is the most stable gas of all the four kinds of gases, while acetone is the most unstable gas. As the stability of target gas increases, the optimal operating temperature shifts from 300°C to 500°C, while the highest response shifts to the sample with high atomic ratio of Sn (red oval in Fig. 8d). For the reaction of the target gases and oxygen ions on surface of the metal oxide, some amount of activation energy has to be provided thermally. Higher operating temperature can provide higher thermal energy so as to stimulate the oxidation/reduction of target gases, which leads to
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the fact that gas responses increase with the operating temperature [51,52]. The point at which the gas response reaches maximum is the actual thermal energy needed for the reaction to proceed. For VOCs with different stability, the gas with higher stability needs a higher thermal
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energy to reach its maximum response. Thus, the optimal operating temperature increases with the stability of the VOCs. However, the response decreases when further increasing the operating
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temperatures, which might due to the desorption of oxygen ions at higher temperature [53].
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It is known that the reduced response at higher operating temperatures was mainly due to the desorption of oxygen ions from the surface of the MOX [53], thus we propose that, the higher
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concentration of Sn dopant in ZnO crystal, the higher desorption temperature of oxygen ions on the crystal surface. At lower temperature (300°C), lower concentration of Sn dopant is more
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beneficial for the adsorption of oxygen ions. Therefore, 1.0 at% Sn-ZLPC shows the highest response to acetone although it performs high activation energy. With the increase of
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temperature (350-400°C), the activation energy starts to play the key role in determining their
Ac ce pt e
responses to gases. At higher temperature, desorption of oxygen ions is more significantly suppressed by higher concentration of Sn dopant. Thus, 10.0 at% Sn-ZLPC shows the highest response to benzene although it performs higher activation energy than 5.0 at% Sn-ZLPC. Therefore, the responses of VOCs with different stabilities are alternately affected by the activation energy and adsorption/desorption of oxygen ions.
3.2.2 Cross sensitivities and repeatability
The cross sensitivities discussed above is benefit for the selective detection of VOCs at typical operating temperature (Fig. 9a). As we can see, the response of 1.0 at% sample to acetone compared to other gases is almost 7-70 times greater (operating temperature: 300°C), indicating good selectivity for acetone detection at 300°C. For m-xylene and toluene, higher operating
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temperature (400°C) and higher Sn dopants (5.0 at%) performed almost 2-12 times greater than other gases. It is hard to selectively detect benzene from other VOCs by a single sensor, even the highest responses obtained by 10.0 at% sample at 475°C only showed a little higher than acetone
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and unfortunately still a little lower than toluene and m-xylene. Sn-ZLPC with different Sn dopants would be potential candidates for the member of an electronic nose.
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Fig. 9b shows the responses repeatability of 5.0 at% Sn-ZLPC to 49.4 ppm benzene gas. The
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responses at ppm-level benzene are remarkable and stable (R49.4 ppm = 28.25 ± 2.4%).
3.2.3 Detailed sensing measurements
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We chose 5.0 at% Sn-ZLPC for detailed sensing measurements due to its overall good responses to VOCs. The typical response-recovery current line to VOCs with different concentration can be
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found in Fig. 10a, which shows good response-recovery properties of the sample. According to the values of responses to benzene gas, the optimal stabilized temperature was chosen to be
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400°C, which might be beneficial for the formation of optimal grain size and grain connections
Ac ce pt e
(Fig. 10b). The temperature-dependent responses to other VOCs are shown in Fig. 10c. Due to different thermal energies is needed to stimulate the oxidation/reduction of target gases, VOCs shows different optimal operating temperatures. According to the response equation of grain based gas sensors and R = Rair / Rgas – 1, we can obtain the following equation (for resistance decrease) [18,54] logR = log(Rair / Rgas – 1) = logAg +βlogpg (4) Where pg is the gas partial pressure, Ag is a prefactor, and the exponent β is the response order. Disordered microstructure is expected to increase β above 0.5 [34,55,56]. The β values of solid 5.0 at% Sn-ZLPC (Fig. 10d) were significantly higher than 0.5, which might due to its wide size distribution and non-spherical shape. The sensor shows good long-term stability to benzene gas
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(R49.4ppm = 26.07 ± 4.1%). The detection limit was also estimated by detecting ppb-level benzene gas. As shown in Fig. 10e, the lowest detection limit for benzene gas can be lower than 30 ppb in a precision detection at 475°C (R=0.1). Response and recovery time decrease significantly with
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increasing operating temperature (Fig. 10f). Because of the fact that the gases need 1.50 min and 2.58 min to fulfill the volume before reaching the sensing film and the whole quartz tube in the
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ideal situation (600 sccm), respectively, the response and recovery of Sn-ZLPC at temperature
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higher than 400°C is very fast. However, it performs poor at temperature lower than 400°C due to low thermal energy and slow adsorption/desorption of gas molecules. Toluene, m-xylene and
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acetone showed similar response and recovery properties (Fig. S5, SI).
The sensing responses to benzene and acetone of 5.0 at% Sn-ZLPC were compared with other
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reports (Table 2) [57-62]. It can be seen that the responses of 5.0 at% Sn-ZLPC are comparable
structure and Sn dopants.
Ac ce pt e
4. Conclusions
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to other gas sensors reported and significantly higher than the sample without hierarchical
In summary, the synthesis of novel Sn doped ZnO layered porous nanocrystals have been demonstrated. Each Sn-ZLPC was constructed by multi-layers of ZnO nanosheet with a thickness of 10-20 nm, and each layer was constructed by nanopores and polycrystalline ZnO nanocrystals. Sn doping was found to be the key factor controlling the layered porous structure. The growth mechanism of Sn-ZLPC has been discussed. 5.0 at% Sn-ZLPC exhibited the best sensing response to VOCs. The key factors affected the sensing response in this work were summarized to be the activation energy, adsorption/desorption of oxygen ions, and the mesoporous hierarchical structure. As the stability of target gas increased, the optimal operating temperature shifted from 300°C to 500°C and the highest response shifted to the sample with
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high atomic ratio of Sn. The later shift was attributed to the alternate influence of the activation energy and adsorption/desorption of oxygen ions. The sensor preserved the superb response and recovery properties to VOCs, which indicated attractive perspective in applications such as
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precision measurements. The study here provides an opportunity to show the potential ability of grain-based sensor to detect VOCs by constructing desired structure, tailoring the crystal surface
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and electronic properties. The interesting particle might be used for a wide range of innovative
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applications (e.g., photo catalysis, the template for complex structure).
Acknowledgment
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This project is financially supported by the National High Technology Research and Development Program of China (863) (No. 2010AA064903) and the Project of Beijing
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Municipal Commission of Education (No. SQKM201210012010). We thank Guolin Hou, a doctoral student at Institute of Process Engineering, CAS, for her contributions on the materials
d
preparation; and Kefei Yan, an undergraduate student in University of Toronto, for his
Ac ce pt e
contributions on the manuscript preparation.
Appendix A. Supplementary Information Supplementary data associated with this article can be found, in the online version, at doi:xx.xxxx/j.snb.20xx.xx.xx.
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Biographics Mingshui Yao is now a doctoral student at State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences. His major field is
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sensing materials and micro gas sensors. Contact: [email protected].
Fei Ding received his Ph.D. in 2011 in Inorganic Chemistry at Peking University, and now he is
organic
light-emitting
diode
(OLED)
and
functional
powers.
Contact:
us
organic
cr
an assistant professor of the Institute of Process Engineering. His major field is the fabrication of
an
[email protected].
Yuebin Cao earned his master degree in Organic Chemistry at China University of Petrolem
M
(East China) in 2007 and Ph. D. degree in Chemical Engineering at Institute of Process Engineering, Chinese Academy of Sciences in 2011. He is now a postdoctor in Hanyang
d
University (Korea). His major filed is Chemical Materials. Contact: [email protected].
Ac ce pt e
Peng Hu received his Ph.D. in Chemical Engineering in Institute of Process Engineering, Chinese Academy of Sciences, PR China in 2008, and now he is an associate professor of the Institute of Process Engineering. His current research fields focus on the synthesis, characterization and self-assembly of inorganic nanoparticles with novel structures, such as 1D nanostructures, hollow structures, and property investigation and functionalization of these novel nanostructures. Contact: [email protected].
Junmei Fan received her Ph.D. in Physics chemistry in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, PR China in 2007, and now she is an associate professor of the Institute of Process Engineering, Chinese Academy of Sciences. Her current research fields focus on the synthesis, characterization and self-assembly of inorganic
26
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nanoparticles with novel structures, such as 1D nanostructures, hollow structures, and property investigation
and
functionalization
of
these
novel
nanostructures.
Contact:
[email protected].
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Chen Lu is now a doctoral student at State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences. His major field is hollow
cr
spheres with high strength. Contact: [email protected].
us
Fangli Yuan earned his master degree and Ph.D. degree in Nuclear Fusion and Plasma
an
Application at Institute of Plasma Physics, Chinese Academy of Sciences in 1996. He is a professor at State Key Laboratory of Multiphase Complex Systems, Institute of Process
M
Engineering, Chinese Academy of Sciences. His major field is preparation and assembly of nanoparticles, functional materials, and industrial application of nanomaterials. Contact:
d
[email protected].
Ac ce pt e
Changyong Shi received his master degree in Nuclear Fusion and Plasma Application at Institute of Plasma Physics, Chinese Academy of Sciences in 1994, , and now he is an associate professor of the Beijing Institute of Fashion Technology. His research interests include the preparation of functional materials, and the application of non-thermal and thermal plasma. Contact: [email protected].
Yunfa Chen received his Ph.D. in Material Science at University Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the Graduate University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic-inorganic composite materials and layered materials.
27
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And
he
is
also
interested
in
industrial
applications
of
nanomaterials.
Contact:
Ac ce pt e
d
M
an
us
cr
ip t
[email protected].
28
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For Table of Contents Use Only
Sn doped ZnO Layered Porous Nanocrystals with
cr
for Gas Sensors
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Hierarchical Structures and Modified Surfaces
us
Mingshui Yaoa, b, Fei Dinga, Yuebin Caod, Peng Hua, Junmei Fana, Chen Lua, b, Fangli Yuana,*,
M
an
Changyong Shic,*, and Yunfa Chena
d
Sn doped ZnO layered porous nanocrystals (Sn-ZLPCs) were synthesized. Each Sn-ZLPC was
Ac ce pt e
constructed by multi-layers of ZnO nanosheet with a thickness of 10-20 nm, and each layer was constructed by nanopores and polycrystalline ZnO nanocrystals. They exhibited high sensing responses to toxic volatile organic compounds (VOCs).
Fig. 1 XRD patterns of ZnO products doped with different atomic percentage of Sn (black is amorphous mixture of Sn/Zn oxides).
Fig. 2 TEM and HRTEM images of 5.0 at% Sn-ZLPC: (a-c) low resolution images; (d-f) the cross-sectional view and (g-i) the plan view of the layers on the solid particle (structure damaged by the ultrasonic treatment).
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Fig. 3 (a) EDS mapping images (Zn-blue, Sn pink, Si green), (b) XPS spectra and (c-e) high resolution XPS spectra of 5.0 at% Sn-ZLPC at binding energies corresponding to Zn 2p, Sn 3d and O 1s, respectively.
from desorption and adsorption branches) of 5.0 at% Sn-ZLPC.
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Fig. 4 N2 adsorption-desorption isotherm (insets are the BJH pore size distribution calculated
cr
Fig. 5 (a) Particle diameter distributions of 5.0 at% Sn-ZLPC that reacted for 8 h; (b) the changes
us
of mean diameter and D50 of 5.0 at% Sn-ZLPC with different reaction time.
Fig. 6 Morphology evolution of ZnO products doped with different atomic percentage of Sn: (a-
an
e) the atomic percentage of Sn are 0, 0.1, 1.0, 5.0, and 10 at%, respectively (each up-right inset is the corresponding SEM image of a single particle, each up-left inset of a-b is the corresponding
M
optical microscope image, and the up-left inset of c is the corresponding TEM image); (f) the amorphous mixture of Sn/Zn oxides when the atomic percentage of Sn is up to 50 at%.
d
Fig. 7 Influences of the synthesis parameters and the possible growth mechanism of Sn-ZLPC.
Ac ce pt e
Fig. 8 (a) Particle size (hollow circles), surface Sn/Zn ratio (solid circles), (b) PL spectra, (c) activation energy, and (d) response comparison to 50 ppm VOCs at their corresponding optimal operating temperature (benzene-500°C, toluene-400°C, m-xylene-350°C and acetone-300°C) of Sn-ZLPCs doped with different atomic percentage of Sn.
Fig. 9 (a) Cross sensitivities of Sn-ZLPCs with different percentage of Sn dopants (operating temperature: 300, 400 and 500°C for 1.0, 5.0 and 10.0 at% Sn-ZLPCs) (b) responses repeatability of 5.0 at% Sn-ZLPC to 49.4 ppm benzene gas.
Fig. 10 Sensing properties of 5.0 at% Sn-ZLPC: (a) the typical response-recovery current line; influences of (b) stabilized condition and (c) operating temperature; and (d) the long-term
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stability; (e) responses to ppb-level benzene gas (30, 50, 100, 250 and 500 ppb, 475°C); and (f) response and recovery time.
Sample
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Table 1 The atomic percentage of Sn for 5.0 at% Sn-ZLPCs with different reaction time. Reaction time / h Sn dopents by EDS / (at%) 1
6.14
5Sn-ZLPC-8h
8
4.28
5Sn-ZLPC-12h
12
4.79
us
cr
5Sn-ZLPC-1h
Table 2. Benzene and acetone sensors based on undoped and doped ZnO materials reported
size
benzene
/nm
/ ppm
ZnO nanoparticle
40-100
100
SnO2 nanoparticle
30-80
Topt a
acetone
/°C
/ ppm
420
100
14
370
[57]
N/A
N/A
50
1.6
290
[58]
R-1
M
materials
an
before and in this work
d
3.2
Ac ce pt e
N/A
R-1
Topt
Ref.
/°C
SnO2 nanowire
80±5
50
0.6
290
50
7
290
[58]
ZnO nanorod array
～8
50
9
370
N/A
N/A
N/A
[59]
ZnO nanowall film
Hier.b
100
7
470
N/A
N/A
N/A
[60]
ZnO nanosheets
Hier.
100
1
420
50
30
420
[61]
ZnO/SnO2 complex
Hier.
N/A
N/A
N/A
50
41
300
[62]
5.0 at% Sn-ZLPC
Hier.
49.4
27.4
475
53.1
210.3
300
a. Topt means optimal operating temperature. b. Hier. means hierarchical structure.
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32
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d
Ac ce pt e us
an
M
cr
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S0925-4005(14)00489-4 http://dx.doi.org/doi:10.1016/j.snb.2014.04.078 SNB 16844
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
9-2-2014 21-4-2014 23-4-2014
Please cite this article as: M. Yao, F. Ding, Y. Cao, P. Hu, J. Fan, C. Lu, F. Yuan, C. Shi, Y. Chen, Sn doped ZnO Layered Porous Nanocrystals with Hierarchical Structures and Modified Surfaces for Gas Sensors, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.04.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sn doped ZnO Layered Porous Nanocrystals with
cr
for Gas Sensors
ip t
Hierarchical Structures and Modified Surfaces
Mingshui Yaoa, b, Fei Dinga, Yuebin Caod, Peng Hua, Junmei Fana, Chen Lua, b, Fangli Yuana,*,
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering,
an
a
us
Changyong Shic,*, and Yunfa Chena
Chinese Academy of Science (CAS), Zhongguancun Beiertiao 1 Hao, Beijing 100190, China University of Chinese Academy of Sciences (UCAS), No.19A Yuquan Road, Beijing 100049,
M
b
Department of Elements, Beijing Institute of Fashion Technology, NO.2 Yinghua Road
Ac ce pt e
c
d
China
Chaoyang District, Beijing 100029, China d
Department of Chemical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-
gu, Ansan, Kyeonggi-do 426-791, Republic of Korea.
* Corresponding author.
E-mail: [email protected]; Fax: +86-10-62561822; Tel: +86-10-82544974 E-mail: [email protected]; Tel: +86-13651229190
1
Page 1 of 32
ABSTRACT: In this paper, a simple solvothermal method is developed for the synthesis of Sn doped ZnO layered porous nanocrystal (Sn-ZLPC). Scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy are used to
ip t
characterize the detailed structures and surface/near-surface chemical composition of the asprepared products. Sn doping is found to be the key factor controlling the layered porous
cr
structure. The possible growth mechanism of Sn-ZLPC is also discussed. As the stability of
us
target gas increases, the optimal operating temperature shifts from 300°C to 500°C and the highest response shifts to the sample with high atomic ratio of Sn. The later shift is attributed to
an
the alternate key influence of the activation energy and adsorption/desorption of oxygen ions. 5.0 at% Sn-ZLPC exhibits the best sensing properties to VOCs due to the combined effect of low
structure.
M
activation energy, adsorption/desorption of oxygen ions and the mesoporous hierarchical
Ac ce pt e
d
Keywords: MOX sensors, Hierarchical structure, Modified surfaces, Sn doping, VOCs
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Page 2 of 32
1. Introduction Nanotechnology has generated much attention beyond the realm of science and engineering and can be an ideal candidate to help people see things invisible [1]. One of the interesting
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applications was nano-structured/modified metal oxide (MOX) sensors acting as the nose to detect toxic volatile organic compounds (VOCs).
cr
For the structural aspect, numerous hierarchical structures possessing enhanced gas diffusion
us
(utility factor [2]) than the pristine structures were introduced to MOX sensors, such as branched crystallites [3-5], nanowires assembled polyhedrons [6], mesoporous films [7-9], single/multi-
an
shell hollow porous spheres [10-15], complicated structures based on precursors [16-19] or biological systems [20] and so on. Purposed built thin film with controlled particle size and pore
M
distribution was also fabricated to improve the sensing response to gases with different Knudsen diffusion coefficients [21]. In addition, large amounts of MOX sensors with controlled crystal
d
surface were also synthesized to improve the surface reaction between target gas molecules and
Ac ce pt e
surface absorbed oxygen ions [22-25].
For the modified surface aspect (receptor function [2]), lattice substituting doping [26-28], new phase doping [29-32], and noble metal decoration [33-36] were experimentally proved to be effective ways to improve the sensing properties mainly by tailoring the activation energy, adsorption/desorption of oxygen ions and reaction rates. In most of the cases, surface modification can also be beneficial to the electron transport (transducer function [2]). Therefore, the combination of hierarchical structure and modified surface might further improve the sensing properties of MOX sensors. Our previous reports have demonstrated that MOX sensors with hierarchical structures [18] and specific exposed crystal facets [25] exhibited high gas sensing activity to benzene, which were mainly due to the enhancement in the gas
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Page 3 of 32
diffusion/molecule capturing. In addition, Sn was found to be an effective dopant that enhanced the sensing properties of ZnO gas sensors [27,37,38]. Herein, we demonstrate the synthesis of novel Sn doped ZnO layered porous nanocrystals (Sn-
ip t
ZLPCs). Sn dopants were introduced to obtain Sn-ZLPCs with hierarchical structures. Each SnZLPC was constructed by multi-layers of ZnO nanosheets with a thickness of 10-20 nm, and
cr
each layer of the ZnO nanosheet showed a porous structure. The synthesized Sn-ZLPCs
us
exhibited excellent sensing properties to VOCs due to the combined effect of low activation energy, adsorption/desorption of oxygen ions and the mesoporous hierarchical structure.
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2. Materials and methods
2.1 Synthesis of Sn doped ZnO layered porous nanocrystal (Sn-ZLPC)
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In a typical synthesis of 5.0 at% Sn-ZLPC, 1.000 g ZnAc2·2H2O and 0.052 g SnCl2·2H2O were added to 10 mL de-ionized water under stirring to form a white mixture containing Sn2+
d
hydrolysis products. Then 60 mL glycerol was added slowly to the mixture under vigorous
Ac ce pt e
stirring. After stirred for 30 min, a semi-transparent white suspension at ambient conditions was formed. Then the resulting solution was transferred to a Teflon-lined 100 mL stainless steel autoclave and maintained at 200°C for 8 h before cooling down to room temperature. After the experiment, the particles were collected from the bottom of the reactor. The products were washed three times with de-ionized water and twice with ethanol by redispersion and centrifugation. The solid materials were then dried in air at 60°C for 12 h. 2.2 Materials characterization
The phase and crystal structure of the product were determined by X-ray diffraction (XRD) patterns, which were recorded with a Philips X’Pert PRO MPD X-ray diffractometer using Cu Kα radiation (λ=1.54178 Å). The morphology and structure of the product were then observed
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Page 4 of 32
by scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscope
(TEM,
JEOL
(Brunner−Emmet−Teller)
JEM-2100).
instrument
N2
using
adsorption a
surface
was
measured
area
analyzer
by
a
BET
(NOVA3200e,
ip t
Quantachrome, USA), and pore size distributions were calculated from the adsorption branch of the Nitrogen adsorption-desorption isotherm using the Barrett Joyner Halenda (BJH) formula.
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The surface chemical analysis was investigated by X-ray photoelectron spectroscopy (XPS) on a
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Thermo Scientific ESCALAB 250 Xi XPS system, where the analysis chamber was 1.5×10-9 mbar and the size of X-ray spot was 500 μm. The particle size distribution of the synthesized
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powder was determined using a Beckman Coulter LS 13 320 laser-diffraction particle size analyzer.
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2.3 Fabrication of gas sensor prototype and sensor characterization The fabrication of gas sensor prototypes and the sensor characterization were reported in our
d
previous work [18,25]. Briefly, Ag paste (SA-5121Q, Wuhan Supernano Optoelec Technology
Ac ce pt e
Co. Ltd) was used for the ohmic contact of sensing film and two Pt wires on both ends of the film. Steps to fabricate Ag electrode were as follows: Ag paste was first printed on the Al2O3 substrate and Pt wires; after irradiated by an infrared lamp (250 W, about 150°C) for 15 min, Pt wires, Ag powder and Al2O3 substrate were stuck together with the help of organics (color: golden yellow); then all of them were heated at 550°C for 30 min to burn the organic and sinter them together (color: silvery white). The solution containing sensing material and ethanol was drop-coated onto the Al2O3 substrate. For the drop-coated sensors, preheating at 550°C for 60 min before the achievement of stable process (400°C for 20 h) was needed to ensure good ohmic contact. VOC gas was introduced into the quartz tube by mixing the certified gas ‘‘mixtures’’ and dry air in the proper ratio controlled by the mass flow controllers [4]. The constant flow was
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600 ml min-1. The bias on the sensor was 10 V and the current was recorded using Keithley 2601 Sourcemeter. The response was defined as the ratio of sensor resistance in air and in detected gas (Rair / Rgas -1). The response / recovery time is defined as the time required for the resistance of
ip t
the sensor to change to 90% / 10% of the saturation value after exposure to the test gas / air.
3. Results and discussion
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3.1 Materials preparation and characterization
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3.1.1 Crystallinity, phase and structure
XRD patterns of undoped ZnO, Sn-doped ZnO products and the amorphous mixture of Sn/Zn
an
oxides are presented in Fig. 1. These patterns correspond to three main diffraction peaks of hexagonal wurtzite ZnO (JCPDS 01-089-0510), namely (100), (002) and (101). The intensities
M
of diffraction peaks declined as Sn concentrations increased, indicating that Sn doping within ZnO caused degeneration of the ZnO crystallinity [39]. It can be seen that Sn doping slightly
d
shifts the position of (002) peaks to higher diffraction angles, which due to its smaller radius
Ac ce pt e
[39,40]. The peak shift of Sn doped ZnO also provides an evidence for the substitution of Sn partly into the ZnO lattice [28]. The synthesized product became amorphous when the atomic percentage of Sn up to 50 at% or higher. Fig. 1 also indicates that Sn doped ZnO product was prepared without the formation of a secondary phase. Due to good sensing properties (see section 3.2) and its unique hierarchical structure, 5.0 at% Sn doped ZnO layered porous nanocrystal (Sn-ZLPC) was chose as the sample for detailed structural analysis ( Fig. 2). As shown in Fig. 2a-c, the size of as-prepared 5.0 at% Sn-ZLPC is about 500 nm in diameter and 1 μm in height. Further structural analysis by HRTEM images shows that each particle is constructed by multi-layers of ZnO nanosheet with a thickness of 1020 nm (Fig. 2d-f), and each layer is constructed by nanopores (mainly less than 10 nm) and
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Page 6 of 32
polycrystalline ZnO nanocrystals (Fig. 2g-i). The spacings of 0.28 nm and 0.26 nm from the cross-sectional and plan view of the layer are consistent with the values for (100) and (002) planes of hexagonal wurtzite ZnO phase, respectively (Fig. 2f and i).
ip t
EDS mapping images of 5.0 at% Sn-ZLPC in Fig. 3a indicate that Sn dopants uniformly distributed in the particle and its atomic percentage was calculated to be 4.8% (Fig. 3a). The
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surface/near-surface chemical composition of Sn-ZLPC was examined by X-ray photoelectron
us
spectroscopy (XPS). The full-range XPS spectra (Fig. 3b) reveals that it is mainly composed of Zn, Sn, O and C elements, of which C contamination can be mainly attributed to the residual
an
solvents. More detailed information on the chemical state of these elements can be obtained from the high resolution XPS spectra of the Zn 2p, Sn 3d and O 1s in Fig. 3c-e. The peaks of Sn 3d3/2
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and Sn 3d5/2 at 495.0 and 486.6 eV can be assigned to the lattice tin (Sn(IV) oxidation state) in tin oxide [20,34,41]. The peak separation of 8.4 eV between these two peaks is in agreement
d
with the standard spectrum of SnO2. The O 1s peaks are much asymmetric. The component at
Ac ce pt e
low binding energy (BE, 530.2 eV) is typical for the lattice O2− ions; and that at high BE (533.2 eV) is usually attributed to chemisorbed or dissociated oxygen or OH species on the surface of the particle, such as CO2, adsorbed H2O or adsorbed O2 [34,42]. The component at medium BE (531.8 eV) was associated with O2− ions in the oxygen deficient regions in the matrix of ZnO by Chen et al [42], while it was attributed to O-H by Coppa and Davis [43]. However, the weak peak of lattice oxygen and strong peaks of non-lattice oxygen indicate that there are large amounts of absorbed oxygen ions on the surface of the crystal. Calculated from the quantified peak area, the atomic ratio of Sn/Zn was estimated to be about 25.2 at% (XPS), which is much higher than that estimated by EDS (4.8 at%). Thus, more active sites on the surface can be expected due to rich absorbed oxygen and high atomic ratio of Sn/Zn. Since the fact that at high
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Page 7 of 32
Sn contents, due to its tendency for octahedral coordination, Sn tends to demix from the ZnO lattice as an amorphous material, together with XRD results and TEM analysis, high ratio of Sn/Zn on the surface might be the result of ultra-thin layer of amorphous Sn on the surface and
ip t
better chemical stability of Sn-O than Zn-O to the etching effects of halogen ions (the dissolve of more Zn ions on the crystal surface).
cr
The nanoporous structure of 5.0 at% Sn-ZLPC was investigated by conducting nitrogen
us
adsorption−desorption measurements. The nitrogen adsorption−desorption isotherm in Fig. 4 exhibits type IV isotherms with type H3 hysteresis loops. In our work, we have found two typical
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phenomenon indicating the risk to determine the BJH pore size distribution from the desorption branch. One was the forced closure of the hysteresis loop at around P/P0 = 0.45 (for N2 at 77 K,
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Fig. 4); the other was the pore size distribution peak at around 4 nm determined by the nature of adsorptive instead of the porous properties of the material (up-left inset of Fig. 4) [44].
Ac ce pt e
pore size distribution in this work.
d
Therefore, it is more appropriate for us to use adsorption branch for the determination of BJH
According to the BJH pore size distribution curve (up-right inset of Fig. 4) calculated from the adsorption isotherm, 5.0 at% Sn-ZLPC exhibited a mesoporous structure with pore diameter mainly less than 10 nm. The cumulative pore volumes were calculated to be 0.083 m3 g-1. With the highly mesoporous structures, the BET surface area of 5.0 at% Sn-ZLPC was calculated to be 78.99 m2 g-1. Pores centered at about 2 and 4 nm might mainly attributed to nanopores on each layer, and pores centered at 10 nm might located at the interfaces of the layers and some large pores on the layer formed by unstable parts. The mesoporous hierarchical structure might be beneficial to the diffusion of target gas. 3.1.2 Reaction time
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Page 8 of 32
The size distributions and the size evolution of particles were also studied. As shown in Fig. 5a, the size of the sample is mainly centered at about 1.0 μm. The median particle diameter (D50) of 5.0 at% Sn-ZLPC increases with the reaction time which is in accordance with Ostwald ripening.
ip t
As shown in Fig. 5b, the corresponding mean size increases sharply at the beginning and then slowly after 8 h.
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In the formation process of 5.0 at% Sn-ZLPCs, the atomic percentages of Sn of the products
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obtained at different reaction times were calculated by EDS and summarized in Table 1. The atomic percentages of Sn varied from 4.28 at% to 6.47 at% and no abrupt changes were
participated in the formation of ZnO products.
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3.1.3 Atomic percentage of Sn dopants
an
observed. It indicates that Sn dopants uniformly dispersed in the solution and continuously
Fig. 6 shows that the percentages of Sn dopants significantly affect the morphologies of the
d
products (the samples collected from the bottom of the reactor). The size of the particle decreases
Ac ce pt e
from 5 μm to 0.5 μm and the surface becomes more roughness as the percentage of Sn increases from 0 at% to 10.0 at%. The hollow structure of the particles collected from the bottom of the reactor disappears when the percentage of Sn increased up to 1.0 at% (Fig. 6a and b, the TEM image of particle with small size was chose to show solid core more clearly). When the percentage of Sn is up to 50 at%, only small amorphous particles of Sn/Zn oxides can be observed (Fig. 1 black and Fig. 6f). Without Sn dopants, high crystallinity, large and dense particle assembles of ZnO were beneficial for the formation of hollow and [001] oriented nanorod spheres via Ostwald ripening [45], which showed “core-shell” like structure in optical microscope due to compact outside, loose inside and hollow core. The above analysis reveals that Sn dopants can significantly degenerate the crystallinity of ZnO and prevent small particles
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Page 9 of 32
agglomerate into dense and large particles (detailed discussion can be found in section possible growth mechanism). 3.1.4 Other influence factors
ip t
To investigate the growth mechanism of Sn-ZLPC, we also examined other influence factors
The influences of synthetic parameters were summarized in Fig. 7.
cr
such as solvents, reactants, reacting temperature, the concentration of the reactants, and so on.
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The water volume strongly affected the morphology of the product. When the volume of water was increased to 35 mL or 70 mL, no particles with mesoporous layered structure were observed
an
(Fig. S1, Supporting Information (SI)). The replacement of glycerol by ethanol resulted in the irregular disperse of Sn elements because the solvents with much lower viscosity failed to
M
prevent the settlement of white Sn2+ hydrolysis products (Fig. S2, SI). It should be mentioned that when the white mixture of ZnAc2, Sn2+ hydrolysis products, Cl ions, water and glycerol
d
were maintained at 70-90°C or higher temperature for a few minutes under stirring, the solution
Ac ce pt e
would turn into clear due to the reaction of Sn2+ hydrolysis products and glycerol. As a result, Sn2+ dispersed uniformly in the solution. Due to high viscosity of glycerol at low temperature, the solvothermal products of preheated at 70- 90°C and non-preheated show not difference in glycerol solvent.
SnCl2 was replaced by other reactants (such as HCl and ZnCl2) with equal concentration of Cl ions to figure out the role of Sn2+ in the formation of mesoporous structure. The results showed that the replacement of SnCl2 to HCl or ZnCl2 had no effect on the size or morphologies of the ZnO hollow spheres, but just made the surface of the ZnO hollow spheres rougher. It indicates the key role of Sn ions in the formation of mesoporous structure.
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Page 10 of 32
Reaction temperature is another important factor for obtaining the mesoporous structure. Particles with mesoporous layered structure can be obtained only when the reaction temperature up to 160°C or higher (Fig. S3, SI). Furthermore, higher temperature can short the time to obtain
ip t
the stable mesoporous particles. In addition, the products formed by lower initial concentration (25% and 50%) were collected
cr
and characterized. The results showed that the mesoporous layered structures were maintained,
have little effects on the formation of mesoporous structure.
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3.1.5 Possible growth mechanism
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the size of the particles was reduced sharply (Fig. S4, SI). It means that the initial concentration
On the basis of experiments and discussions mentioned above, we deduced the possible growth
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mechanism. In the initial stage, Sn2+ hydrolysis products and glycerol reacted when the temperature up to 70°C. So a clear solution with uniformly dispersed Sn ions was obtained. With
d
temperature increasing, more OH- were released due to the reaction of Ac- and glycerol, and
Ac ce pt e
hence a large number of Sn4+ doped ZnO nuclei were produced through the oxidation and hydrolysis process. As the reaction continued, Sn4+ doped ZnO nuclei were assembled into large particles, and the size was affected by the concentration of Sn ions. Then smaller particles and some of the inner parts of larger particles with poor crystalline quality as well as higher surface energies were dissolved due to Ostwald ripening, which then served as the sources for the further growth of larger particles. Under the effects of Ostwald ripening, together with the etching effects of halogen ions (such as Cl- and F-) [46], Sn4+ doped ZnO layered particles with mesoporous hierarchical structure were formed. 3.2 Sensing properties
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Page 11 of 32
As we have mentioned in the introduction and our previous work, gas sensing processes can be divided into three units, which were gas diffusion/molecule capture (utility factor and some case of receptor function), surface reaction (receptor function), and electron transport (transducer
ip t
function) [2,18]. Possible advantages of Sn-ZLPC can be estimated in the following two aspects. One is that the
cr
modified crystal surfaces (more active sites and high crystal defects, Fig 3b-d, 7a and 7b) and
us
reduced activation energy (Fig. 8c) caused by Sn dopants might enhance the surface reaction and electron transport; the other is that the layered mesoporous structure (more accessible crystal
an
surfaces, Fig. 2 and 4) and decreasing size of the particles (high surface to volume ratio, Fig. 6 and hollow circles in 7a) facilitate the gas diffusion and molecule capturing of target gases.
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3.2.1 Influences of Sn dopings
As we have been discussed above based on the XPS results, more active sites and crystal defects
d
on the surface could be expected due to rich absorbed oxygen and high atomic ratio of Sn/Zn. To
Ac ce pt e
further probe the crystal defects in doped ZnO material, PL spectra were studied. As shown in Fig. 8b, the intrinsic emission of ZnO (375 nm) only takes a small part in the PL spectra. On the contrary, the luminescence emission from crystal defects (> 390 nm) dominates the entire spectra, whose relative intensity increases with the atomic percentage of the Sn dopant. Therefore, more crystal defects would be expected for the Sn doped ZnO material [27]. At constant surface coverage or constant density of states, the activation energy of semiconductor can be calculated by the Arrehenius plot of the resistance (there are several assumptions, i.e. the mobility of electrons and the ratio of the density of occupied and unoccupied states is insensitive to temperature) [47-50]. 1
σ
=
1
σ0
exp(
ΔE ) kT
(1)
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Page 12 of 32
ln
1
σ
= ln
1
σ0
+
ΔE kT
(2)
Where σ denotes the conductance, k the Boltzmann’s constant, T the absolute temperature and
ip t
∆E is the activation energy. Plotting ln(1/σ) against 1/T, ∆E can be obtained and it is shown in Fig. 8c.
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Fig. 8d shows the responses comparison of particles with different amount of Sn dopants to
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various VOCs at their respective optimal operating temperature. It can be seen that the 5.0 at% sample with the smallest activation energy shows the highest responses to toluene and m-xylene
an
and the second but close to the highest responses to acetone and benzene (Fig. 8d). It seems that the activation energy plays the key role in determining the sensing properties here.
M
Generally, the surface reaction between the surface adsorbed oxygen species (300-500°C, O2-) and VOCs can be described as the following equations
d
C6H6(ads) + 15O2-(ads) → 6CO2(gas) + 3H2O(gas) + 30e- (3)
Ac ce pt e
CH3C6H5(ads) + 18O2-(ads) → 7CO2(gas) + 4H2O(gas) + 36e- (4) (CH3)2C6H4(ads) + 21O2-(ads) → 8CO2(gas) + 5H2O(gas) + 42e- (5) CH3COCH3(ads) + 9O2-(ads) → 3CO2(gas) + 3H2O(gas) + 18e- (6) Then the electrons are released to the conduction band of MOX to improve its conductivity. As we know, benzene is the most stable gas of all the four kinds of gases, while acetone is the most unstable gas. As the stability of target gas increases, the optimal operating temperature shifts from 300°C to 500°C, while the highest response shifts to the sample with high atomic ratio of Sn (red oval in Fig. 8d). For the reaction of the target gases and oxygen ions on surface of the metal oxide, some amount of activation energy has to be provided thermally. Higher operating temperature can provide higher thermal energy so as to stimulate the oxidation/reduction of target gases, which leads to
13
Page 13 of 32
the fact that gas responses increase with the operating temperature [51,52]. The point at which the gas response reaches maximum is the actual thermal energy needed for the reaction to proceed. For VOCs with different stability, the gas with higher stability needs a higher thermal
ip t
energy to reach its maximum response. Thus, the optimal operating temperature increases with the stability of the VOCs. However, the response decreases when further increasing the operating
cr
temperatures, which might due to the desorption of oxygen ions at higher temperature [53].
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It is known that the reduced response at higher operating temperatures was mainly due to the desorption of oxygen ions from the surface of the MOX [53], thus we propose that, the higher
an
concentration of Sn dopant in ZnO crystal, the higher desorption temperature of oxygen ions on the crystal surface. At lower temperature (300°C), lower concentration of Sn dopant is more
M
beneficial for the adsorption of oxygen ions. Therefore, 1.0 at% Sn-ZLPC shows the highest response to acetone although it performs high activation energy. With the increase of
d
temperature (350-400°C), the activation energy starts to play the key role in determining their
Ac ce pt e
responses to gases. At higher temperature, desorption of oxygen ions is more significantly suppressed by higher concentration of Sn dopant. Thus, 10.0 at% Sn-ZLPC shows the highest response to benzene although it performs higher activation energy than 5.0 at% Sn-ZLPC. Therefore, the responses of VOCs with different stabilities are alternately affected by the activation energy and adsorption/desorption of oxygen ions.
3.2.2 Cross sensitivities and repeatability
The cross sensitivities discussed above is benefit for the selective detection of VOCs at typical operating temperature (Fig. 9a). As we can see, the response of 1.0 at% sample to acetone compared to other gases is almost 7-70 times greater (operating temperature: 300°C), indicating good selectivity for acetone detection at 300°C. For m-xylene and toluene, higher operating
14
Page 14 of 32
temperature (400°C) and higher Sn dopants (5.0 at%) performed almost 2-12 times greater than other gases. It is hard to selectively detect benzene from other VOCs by a single sensor, even the highest responses obtained by 10.0 at% sample at 475°C only showed a little higher than acetone
ip t
and unfortunately still a little lower than toluene and m-xylene. Sn-ZLPC with different Sn dopants would be potential candidates for the member of an electronic nose.
cr
Fig. 9b shows the responses repeatability of 5.0 at% Sn-ZLPC to 49.4 ppm benzene gas. The
us
responses at ppm-level benzene are remarkable and stable (R49.4 ppm = 28.25 ± 2.4%).
3.2.3 Detailed sensing measurements
an
We chose 5.0 at% Sn-ZLPC for detailed sensing measurements due to its overall good responses to VOCs. The typical response-recovery current line to VOCs with different concentration can be
M
found in Fig. 10a, which shows good response-recovery properties of the sample. According to the values of responses to benzene gas, the optimal stabilized temperature was chosen to be
d
400°C, which might be beneficial for the formation of optimal grain size and grain connections
Ac ce pt e
(Fig. 10b). The temperature-dependent responses to other VOCs are shown in Fig. 10c. Due to different thermal energies is needed to stimulate the oxidation/reduction of target gases, VOCs shows different optimal operating temperatures. According to the response equation of grain based gas sensors and R = Rair / Rgas – 1, we can obtain the following equation (for resistance decrease) [18,54] logR = log(Rair / Rgas – 1) = logAg +βlogpg (4) Where pg is the gas partial pressure, Ag is a prefactor, and the exponent β is the response order. Disordered microstructure is expected to increase β above 0.5 [34,55,56]. The β values of solid 5.0 at% Sn-ZLPC (Fig. 10d) were significantly higher than 0.5, which might due to its wide size distribution and non-spherical shape. The sensor shows good long-term stability to benzene gas
15
Page 15 of 32
(R49.4ppm = 26.07 ± 4.1%). The detection limit was also estimated by detecting ppb-level benzene gas. As shown in Fig. 10e, the lowest detection limit for benzene gas can be lower than 30 ppb in a precision detection at 475°C (R=0.1). Response and recovery time decrease significantly with
ip t
increasing operating temperature (Fig. 10f). Because of the fact that the gases need 1.50 min and 2.58 min to fulfill the volume before reaching the sensing film and the whole quartz tube in the
cr
ideal situation (600 sccm), respectively, the response and recovery of Sn-ZLPC at temperature
us
higher than 400°C is very fast. However, it performs poor at temperature lower than 400°C due to low thermal energy and slow adsorption/desorption of gas molecules. Toluene, m-xylene and
an
acetone showed similar response and recovery properties (Fig. S5, SI).
The sensing responses to benzene and acetone of 5.0 at% Sn-ZLPC were compared with other
M
reports (Table 2) [57-62]. It can be seen that the responses of 5.0 at% Sn-ZLPC are comparable
structure and Sn dopants.
Ac ce pt e
4. Conclusions
d
to other gas sensors reported and significantly higher than the sample without hierarchical
In summary, the synthesis of novel Sn doped ZnO layered porous nanocrystals have been demonstrated. Each Sn-ZLPC was constructed by multi-layers of ZnO nanosheet with a thickness of 10-20 nm, and each layer was constructed by nanopores and polycrystalline ZnO nanocrystals. Sn doping was found to be the key factor controlling the layered porous structure. The growth mechanism of Sn-ZLPC has been discussed. 5.0 at% Sn-ZLPC exhibited the best sensing response to VOCs. The key factors affected the sensing response in this work were summarized to be the activation energy, adsorption/desorption of oxygen ions, and the mesoporous hierarchical structure. As the stability of target gas increased, the optimal operating temperature shifted from 300°C to 500°C and the highest response shifted to the sample with
16
Page 16 of 32
high atomic ratio of Sn. The later shift was attributed to the alternate influence of the activation energy and adsorption/desorption of oxygen ions. The sensor preserved the superb response and recovery properties to VOCs, which indicated attractive perspective in applications such as
ip t
precision measurements. The study here provides an opportunity to show the potential ability of grain-based sensor to detect VOCs by constructing desired structure, tailoring the crystal surface
cr
and electronic properties. The interesting particle might be used for a wide range of innovative
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applications (e.g., photo catalysis, the template for complex structure).
Acknowledgment
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This project is financially supported by the National High Technology Research and Development Program of China (863) (No. 2010AA064903) and the Project of Beijing
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Municipal Commission of Education (No. SQKM201210012010). We thank Guolin Hou, a doctoral student at Institute of Process Engineering, CAS, for her contributions on the materials
d
preparation; and Kefei Yan, an undergraduate student in University of Toronto, for his
Ac ce pt e
contributions on the manuscript preparation.
Appendix A. Supplementary Information Supplementary data associated with this article can be found, in the online version, at doi:xx.xxxx/j.snb.20xx.xx.xx.
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ip t
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cr
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Biographics Mingshui Yao is now a doctoral student at State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences. His major field is
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sensing materials and micro gas sensors. Contact: [email protected].
Fei Ding received his Ph.D. in 2011 in Inorganic Chemistry at Peking University, and now he is
organic
light-emitting
diode
(OLED)
and
functional
powers.
Contact:
us
organic
cr
an assistant professor of the Institute of Process Engineering. His major field is the fabrication of
an
[email protected].
Yuebin Cao earned his master degree in Organic Chemistry at China University of Petrolem
M
(East China) in 2007 and Ph. D. degree in Chemical Engineering at Institute of Process Engineering, Chinese Academy of Sciences in 2011. He is now a postdoctor in Hanyang
d
University (Korea). His major filed is Chemical Materials. Contact: [email protected].
Ac ce pt e
Peng Hu received his Ph.D. in Chemical Engineering in Institute of Process Engineering, Chinese Academy of Sciences, PR China in 2008, and now he is an associate professor of the Institute of Process Engineering. His current research fields focus on the synthesis, characterization and self-assembly of inorganic nanoparticles with novel structures, such as 1D nanostructures, hollow structures, and property investigation and functionalization of these novel nanostructures. Contact: [email protected].
Junmei Fan received her Ph.D. in Physics chemistry in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, PR China in 2007, and now she is an associate professor of the Institute of Process Engineering, Chinese Academy of Sciences. Her current research fields focus on the synthesis, characterization and self-assembly of inorganic
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nanoparticles with novel structures, such as 1D nanostructures, hollow structures, and property investigation
and
functionalization
of
these
novel
nanostructures.
Contact:
[email protected].
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Chen Lu is now a doctoral student at State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences. His major field is hollow
cr
spheres with high strength. Contact: [email protected].
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Fangli Yuan earned his master degree and Ph.D. degree in Nuclear Fusion and Plasma
an
Application at Institute of Plasma Physics, Chinese Academy of Sciences in 1996. He is a professor at State Key Laboratory of Multiphase Complex Systems, Institute of Process
M
Engineering, Chinese Academy of Sciences. His major field is preparation and assembly of nanoparticles, functional materials, and industrial application of nanomaterials. Contact:
d
[email protected].
Ac ce pt e
Changyong Shi received his master degree in Nuclear Fusion and Plasma Application at Institute of Plasma Physics, Chinese Academy of Sciences in 1994, , and now he is an associate professor of the Beijing Institute of Fashion Technology. His research interests include the preparation of functional materials, and the application of non-thermal and thermal plasma. Contact: [email protected].
Yunfa Chen received his Ph.D. in Material Science at University Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the Graduate University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic-inorganic composite materials and layered materials.
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And
he
is
also
interested
in
industrial
applications
of
nanomaterials.
Contact:
Ac ce pt e
d
M
an
us
cr
ip t
[email protected].
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For Table of Contents Use Only
Sn doped ZnO Layered Porous Nanocrystals with
cr
for Gas Sensors
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Hierarchical Structures and Modified Surfaces
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Mingshui Yaoa, b, Fei Dinga, Yuebin Caod, Peng Hua, Junmei Fana, Chen Lua, b, Fangli Yuana,*,
M
an
Changyong Shic,*, and Yunfa Chena
d
Sn doped ZnO layered porous nanocrystals (Sn-ZLPCs) were synthesized. Each Sn-ZLPC was
Ac ce pt e
constructed by multi-layers of ZnO nanosheet with a thickness of 10-20 nm, and each layer was constructed by nanopores and polycrystalline ZnO nanocrystals. They exhibited high sensing responses to toxic volatile organic compounds (VOCs).
Fig. 1 XRD patterns of ZnO products doped with different atomic percentage of Sn (black is amorphous mixture of Sn/Zn oxides).
Fig. 2 TEM and HRTEM images of 5.0 at% Sn-ZLPC: (a-c) low resolution images; (d-f) the cross-sectional view and (g-i) the plan view of the layers on the solid particle (structure damaged by the ultrasonic treatment).
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Fig. 3 (a) EDS mapping images (Zn-blue, Sn pink, Si green), (b) XPS spectra and (c-e) high resolution XPS spectra of 5.0 at% Sn-ZLPC at binding energies corresponding to Zn 2p, Sn 3d and O 1s, respectively.
from desorption and adsorption branches) of 5.0 at% Sn-ZLPC.
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Fig. 4 N2 adsorption-desorption isotherm (insets are the BJH pore size distribution calculated
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Fig. 5 (a) Particle diameter distributions of 5.0 at% Sn-ZLPC that reacted for 8 h; (b) the changes
us
of mean diameter and D50 of 5.0 at% Sn-ZLPC with different reaction time.
Fig. 6 Morphology evolution of ZnO products doped with different atomic percentage of Sn: (a-
an
e) the atomic percentage of Sn are 0, 0.1, 1.0, 5.0, and 10 at%, respectively (each up-right inset is the corresponding SEM image of a single particle, each up-left inset of a-b is the corresponding
M
optical microscope image, and the up-left inset of c is the corresponding TEM image); (f) the amorphous mixture of Sn/Zn oxides when the atomic percentage of Sn is up to 50 at%.
d
Fig. 7 Influences of the synthesis parameters and the possible growth mechanism of Sn-ZLPC.
Ac ce pt e
Fig. 8 (a) Particle size (hollow circles), surface Sn/Zn ratio (solid circles), (b) PL spectra, (c) activation energy, and (d) response comparison to 50 ppm VOCs at their corresponding optimal operating temperature (benzene-500°C, toluene-400°C, m-xylene-350°C and acetone-300°C) of Sn-ZLPCs doped with different atomic percentage of Sn.
Fig. 9 (a) Cross sensitivities of Sn-ZLPCs with different percentage of Sn dopants (operating temperature: 300, 400 and 500°C for 1.0, 5.0 and 10.0 at% Sn-ZLPCs) (b) responses repeatability of 5.0 at% Sn-ZLPC to 49.4 ppm benzene gas.
Fig. 10 Sensing properties of 5.0 at% Sn-ZLPC: (a) the typical response-recovery current line; influences of (b) stabilized condition and (c) operating temperature; and (d) the long-term
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stability; (e) responses to ppb-level benzene gas (30, 50, 100, 250 and 500 ppb, 475°C); and (f) response and recovery time.
Sample
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Table 1 The atomic percentage of Sn for 5.0 at% Sn-ZLPCs with different reaction time. Reaction time / h Sn dopents by EDS / (at%) 1
6.14
5Sn-ZLPC-8h
8
4.28
5Sn-ZLPC-12h
12
4.79
us
cr
5Sn-ZLPC-1h
Table 2. Benzene and acetone sensors based on undoped and doped ZnO materials reported
size
benzene
/nm
/ ppm
ZnO nanoparticle
40-100
100
SnO2 nanoparticle
30-80
Topt a
acetone
/°C
/ ppm
420
100
14
370
[57]
N/A
N/A
50
1.6
290
[58]
R-1
M
materials
an
before and in this work
d
3.2
Ac ce pt e
N/A
R-1
Topt
Ref.
/°C
SnO2 nanowire
80±5
50
0.6
290
50
7
290
[58]
ZnO nanorod array
～8
50
9
370
N/A
N/A
N/A
[59]
ZnO nanowall film
Hier.b
100
7
470
N/A
N/A
N/A
[60]
ZnO nanosheets
Hier.
100
1
420
50
30
420
[61]
ZnO/SnO2 complex
Hier.
N/A
N/A
N/A
50
41
300
[62]
5.0 at% Sn-ZLPC
Hier.
49.4
27.4
475
53.1
210.3
300
a. Topt means optimal operating temperature. b. Hier. means hierarchical structure.
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d
Ac ce pt e us
an
M
cr
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