- Email: [email protected]
SnO2 porous nanospheres
Journal Pre-proof Enhanced formaldehyde gas sensing performance based on Bi doped Zn2SnO4/ SnO2 porous nanospheres R. Zhang, S.Y. Ma, J.L. Zhang, B.J. Wang, S.T. Pei PII:
S0925-8388(20)30771-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154408
Reference:
JALCOM 154408
To appear in:
Journal of Alloys and Compounds
Received Date: 25 September 2019 Revised Date:
14 February 2020
Accepted Date: 16 February 2020
Please cite this article as: R. Zhang, S.Y. Ma, J.L. Zhang, B.J. Wang, S.T. Pei, Enhanced formaldehyde gas sensing performance based on Bi doped Zn2SnO4/SnO2 porous nanospheres, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154408. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Author contributions R. Zhang: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing. S.Y. Ma: Resources, Supervision, Funding acquisition. B.J. Wang: Validation, Formal analysis, Writing - Review & Editing. J.L. Zhang: Writing - Review & Editing, Project administration. S.T. Pei: Conceptualization, Software, Writing - Review & Editing.
Enhanced formaldehyde gas sensing performance based on Bi doped Zn2SnO4/SnO2
porous nanospheres
R. Zhang, S.Y. Ma∗, J.L. Zhang, B.J. Wang, S.T. Pei
Key Laboratory of Atomic and Molecular Physic & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract
In this work, pure and Bi doped ZTO/SnO2(Zn2SnO4/SnO2) porous nanospheres are synthesized via a hydrothermal process. The phase structure, morphology and elemental composition of samples are analyzed by various techniques. The results show that Bi ions has been introduced into the Bi doped ZTO/SnO2 samples lattice and Bi2Sn2O7 has been found in X-ray diffraction (XRD) pattern. The Bi doped samples exhibited higher response (23.2) to 50 ppm formaldehyde and possessed better selectivity to formaldehyde than pure samples, respectively. The improvement of gas sensing properties of Bi doped ZTO/SnO2 sensor are benefited from the Bi3+ ions doping and the presence of Bi2Sn2O7. Meanwhile, the porous nanostructure of Bi doped ZTO/SnO2 is another reason for improving the gas sensing properties.
Keywords: Bi doped ZTO/SnO2, Bi2(Sn2O7), formaldehyde, sensors, semiconductors
1. Introduction
Formaldehyde, a common air pollutant, has attracted widespread attention due to its toxicity and
∗
Corresponding author: S.Y. Ma
Tel: +86 18893729425
E-mailaddress:[email protected] 1
Fax: +86 9317971503
carcinogenicity. In the past few decades, metal oxide semiconductor (MOS) materials are widely used in the detection of formaldehyde gas, such as ZnO [1], SnO2 [2], Zn2SnO4(ZTO) [3] and so on. However, single metal oxide materials often have some shortcomings, such as low response, high working resistance, so the nanocomposites are studied for improving the gas sensing performance of gas sensor. In recent years, various metal oxide semiconductor composite materials have been investigated to take full use of the synergistic effects among the various components and their respective advantages. For example, Sun et al. has prepared the ZTO/SnO2 nanocomposites for improving the gas sensing performance to formaldehyde [4]. Zhang et al. reported ZTO/SnO2 porous hierarchical nanospheres were prepared for testing triethylamine gas [5]. Shu et al. studied the layered hierarchical octahedral-like structured ZTO/SnO2 showed good gas sensing performance to formaldehyde [6]. In addition, the microstructure of materials and doping of certain elements are also closely related to the improvement of gas sensing performance. For instance, Zhu et al. has synthesized the excellent formaldehyde gas sensor based on Bi doped SnO2 nanoflowers [7]. Guo et al. has fabricated Bi doped SnO2/rGO nanocomposites which have excellent gas sensing properties to benzene [8].
Therefore, pure and Bi-ZTO/SnO2(Bi doped ZTO/SnO2) porous nanospheres have been prepared by a facile hydrothermal treatment. Moreover, Bi doped ZTO/SnO2 sensor exhibits better selectivity and higher response to formaldehyde (HCHO) than pure ZTO/SnO2 sensor, which indicates that Bi doping effectively enhance the formaldehyde gas sensing performance of ZTO/SnO2 sensor.
2. Experimental process
2
2.1 Preparation of samples
All Chemicals reagents used in experiment process no need further purification. The detailed
information of the chemical reagents as shown in Table 1. First of all, 1.06 g Na2SnO3 3H2O, 1.75 g Zn(CH3COO)2·2H2O and 5 wt% of Bi(NO3)3·5H2O were dissolved in 20 ml distilled water under magnetic stirring. Secondly, an amount of sodium alginate and 5 ml NH3 H2O (25 %) were introduced to the above solution and then stirred for 1 h at 35
. Thirdly, the above mixed solution was transferred
to 100 ml Teflon-lined stainless-steel autoclave and reacted for 8 h at 160 autoclave cooling down to the temperature of 30
naturally, the precipitate was washed with
deionized water and ethanol for 3-4 times, collected, and then dried at 70 precipitate was annealed at 700
. Subsequently, after the
for 10 h. Ultimately, the
for 2 h to obtain Bi doped ZTO/SnO2 porous nanospheres. For
comparison, pure ZTO/SnO2 samples were also prepared through the same process but in absence of Bi(NO3)3·5H2O.
2.2 Characterization
X-ray diffraction (XRD, D/Max-2400), Scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, USA FEI Tecnai G2 TF20) were exploited to characterize the structure and morphology of all samples. Gas sensing performance of two sensors was tested by gas sensor test instrument (WS-30A, Wei Sheng Electronics Science and Technology Co. Ltd. China). The response of gas sensor (S) is descripted as Ra / Rg, where Ra and Rg are the resistance of gas sensor in air and in test gas. The response and recovery time are defined as the time which is taken by the sensor to achieve to 90 % of the total resistance change in the gas and air, separately [9].
3. Results and discussion
3
3.1 XRD characterization The crystal structure of all samples was characterized by XRD. Fig. 1(a) display two sets of diffraction peaks are well corresponding to ZTO (JCPDS No.74-2184) and SnO2 (JCPDS No.99-0024). In addition, for Bi doped samples, the diffraction peaks of (440) and (622) are consistent with Bi2Sn2O7 (JCPDS No.87-0284). Compared with pure ZTO/SnO2, the (311) and (222) peaks of Bi doped 3+
ZTO/SnO2 have slightly shift to the higher angle in Fig.1(b). It is due to the fact that Bi
ion(1.03 Å)
has a larger ionic radius than Zn2+ ion (0.74 Å) resulting in a part of Bi ions substitute Zn ions lattice sites and another part of Bi ions maybe stay in the interstitial state [8][10]. It can be seen that Bi element has been successfully doped into the Bi-ZTO/SnO2 samples. The corresponding the ratio of
each compounds in pure and Bi-ZTO/SnO2 samples is shown in Table 2. 3.2 Morphological analysis
The morphology and microstructure of all samples were analyzed by SEM and TEM. The SEM morphology of pure and Bi doped ZTO/SnO2 samples are shown in Fig. 2(a-d). All the samples were composed of a large number of nanospheres with a well-dispersed and rough surface. The diameter of pure and Bi doped ZTO/SnO2 nanospheres is about 700 nm and 500 nm, respectively. The TEM images of pure and Bi doped ZTO/SnO2 samples are showed in Fig. 2(e-f), where obvious contrast between bright and dark areas indicate porous structure of as prepared samples.
3.3 XPS analysis
Fig.3(a) shows the X-ray photoelectron spectroscopy (XPS) survey spectrum of the as-prepared Bi doped ZTO/SnO2 samples. It can be observed that there are four elements including Zn, Sn, O, and Bi in the samples. Fig. 3(b) shows the Zn 2p spectrum, where two features at 1045.2 eV and 1022.2 eV are observed. The peaks are corresponding to the binding energy of the Zn 2p1/2 and Zn 2p3/2 peaks, respectively [11], which indicates Zn element is present in the sample as Zn2+. Fig. 3(c) shows the Sn 3d spectrum. The characteristic peaks at 495.1 eV and 486.6 eV are consistent with the binding 4
energies of the Sn 3d3/2 and Sn 3d5/2 peaks [12], separately. The presence of the Sn element in the sample is Sn4+. Fig .3(d) shows the Bi 4f spectrum. Corresponding to Bi4f
5/2
and Bi4f
7/2
are two
characteristic peaks at 164.4 eV and 159.4 eV, respectively, demonstrating the presence of Bi3+ ions in the material [13]. Fig. 3(e) is the O 1s peak which can be decomposed into two peaks at 530.6 eV and 531.7 eV, and corresponding to the crystal lattice oxygen and adsorption oxygen, respectively [14]. The
corresponding element atomic concentration is shown in the Table 3. 3.4 The gas sensing properties
Operating temperature is an important part of a MOS sensor. Therefore, the responses of both sensors to 50 ppm formaldehyde at different operating temperature were determined. As shown in Fig. 4(a), with the operating temperature changed from 140 increases firstly until reaching their maximum value at 180
to 240
, the response of two sensors
and then decreases. It is obviously seen
that the optimum operating temperature of pure ZTO/SnO2 and Bi doped ZTO/SnO2 sensors is 180 and the response of two sensors to 50 ppm formaldehyde are 14.4 and 23.2 at 180 response to 50 ppm various test gases at 180
, respectively. The
of two sensors is showed in Fig. 4(b). It can be
obviously seen that the response of all sensors to formaldehyde is much higher than other gases, which indicates two sensors have good selectivity to formaldehyde. In Fig. 4(c), the response curve of the two sensors to different concentrations (10-1000 ppm) of formaldehyde at optimum operating temperature is showed. With the concentration of formaldehyde gradually increasing, the response of two sensor is also increasing. The response of the Bi doped ZTO/SnO2 sensor is fairly higher than pure ZTO/SnO2 sensor. Moreover, the response of two sensors augment rapidly before 200 ppm, but after reaching a certain concentration, the adsorption and desorption processes of formaldehyde molecules gradually tend to balance, leading to the increasing slowly response [5]. The concentration ranges from 10 to 100 5
ppm of formaldehyde, the linear relationship between response and concentration is shown in Fig. 4(d). The linear fitting formulas of pure and Bi doped ZTO/SnO2 are as follows: ln (S-1) = 0.74854 ln (C) 0.25213 (R2 = 0.96704) and ln (S-1) = 1.02310 ln (C) - 0.71994 (R2 = 0.97206), respectively. The result shows a good linear relationship in the range of 10-100 ppm, which indicates that these sensors have an advantage in low-concentration formaldehyde detection [15]. As shown in Fig. 4(e), the response and recovery time of pure and Bi doped ZTO/SnO2 porous nanospheres to 50 ppm HCHO at 180
is 16 s
and 9 s, respectively. As depicted in Fig. 4(f), pure and Bi doped ZTO/SnO2 sensors exhibit nearly constant response, indicating that two sensors have good stability. The good repeatability of the Bi-ZTO/SnO2 samples is shown in Fig. 5. Therefore, Bi doping effectively improves the formaldehyde gas sensing performance of the sensor based on the ZTO/SnO2 porous nanospheres. The remarkable improvement may be attributed to the hollow porous structure and the change in electrons concentration caused by Bi ions doping.
3. 5 The gas sensing mechanism
Both ZTO and SnO2 are typical n-type semiconductors. The gas-sensing mechanism is depicted by the changes of resistance caused by the oxidation-reduction reaction of materials and gas molecules (Fig. 6(a)). According to the space charge layer model, firstly, when the material is exposed to air, oxygen molecules will adsorb on the surface of ZTO/SnO2 and capture the free electrons from conduction band to form oxygen anions (O2-, O-, O2-). The specific process is as follows：
O2 (gas)→ O2 (ads)
O2 (ads) + e- → O2- (ads) (T <100 ℃)
O2- (ads) + e- → O- (ads) (100 ℃ ≤ T ≤ 300 ℃)
6
(1)
(2)
(3)
O- (ads) + e- →O2- (ads) (T >300 ℃)
(4)
In this process, a thick electron depletion layer is formed on the surface of ZTO/SnO2, and the carrier concentration is decreased, resulting in an increase in resistance of the sensor.
As long as the sensor is exposed to formaldehyde gas at a suitable temperature, the formaldehyde molecules will react with oxygen anions. The specific process is as follows：
HCHO(gas) → HCHO(ads)
(5)
HCHO（ads）+ 2O-（ads）→CO2 + H2O + 2e-
(6)
During this process, the trapped free electrons are released back to the conduction bands of materials and the thickness of the depletion layer is reduced, which makes the material resistance decreased.
Based on the above gas sensing mechanism, the reasons for the enhanced gas sensing performance of Bi doped ZTO/SnO2 may be as follows：
First, the interface between SnO2 and ZTO will form a heterojunction. As shown in the Fig. 5(b), because the conduction band (CB) position of ZTO is higher than that of SnO2, electrons will transfer from ZTO to SnO2 efficiently, resulting in the low composite probability of electrons and holes. Therefore, the heterojunction between ZTO and SnO2 is beneficial to the improvement of gas sensing performance. Second, when Bi3+ is doped into the crystal lattice of Zn2SnO4, the occupancy of Bi ions at Zn sites would result in the generation of oxygen vacancy related defects. These defects can reduce composite probability of electrons and holes and increase the number of free electrons in the Bi-ZTO/SnO2, which can contribute to improved gas sensing performance. Third, the interface among
7
SnO2 and ZTO and Bi2Sn2O7 will form a heterojunction. Due to the formation of heterojunction, the Bi-ZTO/SnO2 samples have high resistance than pure samples in the air. The sharp change in resistance will lead to better gas sensing performance. Fourth, due to the unique electronic structure of Bi2Sn2O7, it also is beneficial to the improvement of gas sensing performance [13, 16]. Finally, the porous structure of Bi doped ZTO/SnO2 samples is also conducive to gas diffusion, improving the gas sensing performance.
4. Conclusions
Pure and Bi doped ZTO/SnO2 porous nanospheres were synthesized via a hydrothermal reaction. Compared with pure ZTO/SnO2 sensor, Bi doped ZTO/SnO2 sensor exhibits excellent gas sensing properties. The response of Bi doped ZTO/SnO2 sensor to 50 ppm formaldehyde at 180
is 23.2.
Moreover, Bi doped ZTO/SnO2 sensor shows good selectivity to formaldehyde and good long-term stability. The results show the Bi doped ZTO/SnO2 porous nanospheres have a certain potential application in the detection of low concentration formaldehyde.
Acknowledgments
This work was supported by the National Natural Science Foundations of China (Grant No. 11864034 and 11964035), and the Scientific Research Project of Gansu Province (Grant No. 18JR3RA089 and 17JR5RA072).
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8
[2] D. Xue, Z. Zhang, Y. Wang, Enhanced methane sensing performance of SnO2 nanoflowers based sensors decorated with Au nanoparticles, Materials Chemistry and Physics 237 (2019) 121864. [3] Y. Li, N. Luo, G. Sun, B. Zhang, L. Lin, H. Jin, Y. Wang, H. Bala, J. Cao, Z. Zhang, In situ decoration of Zn2SnO4 nanoparticles on reduced graphene oxide for high performance ethanol sensor, Ceramics International 44(6) (2018) 6836-6842. [4] G. Sun, G. Ma, Y. Li, Z. Zhang, Z. Chen, Y. Wang, J. Cao, H. Bala, Improved formaldehyde-sensing performance of SnO2/Zn2SnO4 nanocomposites by structural evolution, Materials Letters 191 (2017) 145-149. [5] S. Zhang, G. Sun, Y. Li, B. Zhang, Y. Wang, Z. Zhang, Enhanced triethylamine gas sensing performance of the porous Zn2SnO4/SnO2 hierarchical microspheres, Journal of Alloys and Compounds 785 (2019) 382-390. [6] S. Shu, M. Wang, W. Yang, S. Liu, Synthesis of surface layered hierarchical octahedral-like structured Zn2SnO4/SnO2 with excellent sensing properties toward HCHO, Sensors and Actuators B: Chemical 243 (2017) 1171-1180. [7] K.M. Zhu, S.Y. Ma, Preparations of Bi-doped SnO2 hierarchical flower-shaped nanostructures with highly sensitive HCHO sensing properties, Materials Letters 236 (2019) 491-494. [8] W. Guo, Q. Zhou, J. Zhang, M. Fu, N. Radacsi, Y. Li, Hydrothermal synthesis of Bi-doped SnO2/rGO nanocomposites and the enhanced gas sensing performance to benzene, Sensors and Actuators B: Chemical 299 (2019) 126959. [9] H.M. Yang, S.Y. Ma, G.J. Yang, Q. Chen, Q.Z. Zeng, Q. Ge, L. Ma, Y. Tie, Synthesis of La2O3 doped Zn2SnO4 hollow fibers by electrospinning method and application in detecting of acetone, Applied Surface Science 425 (2017) 585-593. [10] Q. Chen, S.Y. Ma, H.Y. Jiao, B.Q. Wang, G.H. Zhang, D.J. Gengzang, L.W. Liu, H.M. Yang, Sodium alginate assisted hydrothermal method to prepare praseodymium and cerium co-doped ZnSn(OH)6 hollow microspheres and synergistically enhanced ethanol sensing performance, Sensors and Actuators B: Chemical 252 (2017) 295-305. [11] E.-B. Kim, S. Ameen, M.S. Akhtar, H.S. Shin, Iron-nickel co-doped ZnO nanoparticles as scaffold for field effect transistor sensor: Application in electrochemical detection of hexahydropyridine chemical, Sensors and Actuators B: Chemical 275 (2018) 422-431.
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[12] P.G. Choi, N. Izu, N. Shirahata, Y. Masuda, Improvement of sensing properties for SnO2 gas sensor by tuning of exposed crystal face, Sensors and Actuators B: Chemical 296 (2019) 126655. [13] C. Hu, J. Zhuang, L. Zhong, Y. Zhong, D. Wang, H. Zhou, Significantly enhanced photocatalytic activity of visible light responsive AgBr/Bi2Sn2O7 heterostructured composites, Applied Surface Science 426 (2017) 1173-1181. [14] F. Liu, X. Chen, X. Wang, Y. Han, X. Song, J. Tian, X. He, H. Cui, Fabrication of 1D Zn2SnO4 nanowire and 2D ZnO nanosheet hybrid hierarchical structures for use in triethylamine gas sensors, Sensors and Actuators B: Chemical 291 (2019) 155-163. [15] Q. Chen, S.Y. Ma, X.L. Xu, H.Y. Jiao, G.H. Zhang, L.W. Liu, P.Y. Wang, D.J. Gengzang, H.H. Yao, Optimization ethanol detection performance manifested by gas sensor based on In2O3/ZnS rough microspheres, Sensors and Actuators B: Chemical 264 (2018) 263-278. [16] S. Zhang, G. Sun, Y. Li, B. Zhang, L. Lin, Y. Wang, J. Cao, Z. Zhang, Continuously improved gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sensors and Actuators B: Chemical 255 (2018) 2936-2943.
10
Table 1 The detailed information of the chemical reagents Reagent Zinc acetate dihydrate
chemical formula Zn(CH3COO)2 2H2O
Sodium stannate trihydrate
Na2SnO3
Ammonia solution
NH3
3H2O
H 2O
purity 99.0%
98%
25%
Sodium alginate
(C6H7NaO6)n
99.8%
Bismuth nitrate pentahydrate
Bi(NO3)3·5H2O
≥99.0%
manufacturer Tianjin Baishi Chemical Industry Co. Ltd Tianjin Baishi Chemical Industry Co. Ltd Tianjin Baishi Chemical Industry Co. Ltd
molecular weight 219.51
266.73
35.05
Tianjin Guangfu Fine Chemical Research Institute Shanghai Zhanyun Chemical Co. Ltd
485.07
Table 2 The ratio of each compounds in samples Materials
wt%
Zn2SnO4
57.6
SnO2
42.4
Zn2SnO4
52.8
SnO2
42.8
Bi2(Sn2O7)
4.4
ZTO/SnO2
Bi-ZTO/SnO2
Table 3 The atomic concentration of the Bi-ZTO/SnO2 Element
C 1s
O 1s
Zn 2p3
Sn 3d5
Bi 4f
At/%
15.05
55.96
10.85
17.5
0.65
Fig. 1 (a) XRD pattern of pure and Bi doped ZTO/SnO2 samples, (b) Comparison the shift of (311) and (222) peaks of pure and Bi doped ZTO/SnO2 samples.
Fig.2 (a-d) SEM images of pure and Bi doped ZTO/SnO2 samples, (e, f) TEM images of pure and Bi doped ZTO/SnO2 samples.
Fig. 3 (a) Survey XPS spectrum of Bi doped Zn2SnO4; XPS spectra of (b) Zn 2p, (c) Sn 3d, (d) Bi 4f and (e) O 1s.
Fig. 4 (a) Responses of all sensors at different operating temperature to 50 ppm formaldehyde, (b) Selectivity of pure and Bi doped ZTO/SnO2 hollow porous nanospheres to 50 ppm various gases, (c) The response of all samples to different formaldehyde concentrations ranging from 10 to 1000 ppm, (d) Fitting curve of the response to 10 - 100 ppm formaldehyde, (e) Dynamic sensing transient of all sensors to 50 ppm formaldehyde, (f) The long-term stability of pure and Bi doped ZTO/SnO2 samples to 50 ppm formaldehyde at 180
.
Fig. 5 The repeatability of the Bi-ZTO/SnO2 samples
Fig. 6 Schematic illustration of the gas sensing mechanism.
1
Fig. 1
2
Fig. 2 3
Fig. 3
4
Fig. 4
5
Fig. 5
6
Fig. 6
7
Highlights:
The pure and Bi-Zn2SnO4/SnO2 hollow porous nanospheres were successfully prepared. Bi doping effectively improves the gas sensing performance to detect formaldehyde. Bi2Sn2O7 contributes to the improvement of gas sensing performance to formaldehyde. The response of Bi doped Zn2SnO4/SnO2 is 23.2 to 50 ppm formaldehyde at 180
.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30771-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154408
Reference:
JALCOM 154408
To appear in:
Journal of Alloys and Compounds
Received Date: 25 September 2019 Revised Date:
14 February 2020
Accepted Date: 16 February 2020
Please cite this article as: R. Zhang, S.Y. Ma, J.L. Zhang, B.J. Wang, S.T. Pei, Enhanced formaldehyde gas sensing performance based on Bi doped Zn2SnO4/SnO2 porous nanospheres, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154408. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Author contributions R. Zhang: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing. S.Y. Ma: Resources, Supervision, Funding acquisition. B.J. Wang: Validation, Formal analysis, Writing - Review & Editing. J.L. Zhang: Writing - Review & Editing, Project administration. S.T. Pei: Conceptualization, Software, Writing - Review & Editing.
Enhanced formaldehyde gas sensing performance based on Bi doped Zn2SnO4/SnO2
porous nanospheres
R. Zhang, S.Y. Ma∗, J.L. Zhang, B.J. Wang, S.T. Pei
Key Laboratory of Atomic and Molecular Physic & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract
In this work, pure and Bi doped ZTO/SnO2(Zn2SnO4/SnO2) porous nanospheres are synthesized via a hydrothermal process. The phase structure, morphology and elemental composition of samples are analyzed by various techniques. The results show that Bi ions has been introduced into the Bi doped ZTO/SnO2 samples lattice and Bi2Sn2O7 has been found in X-ray diffraction (XRD) pattern. The Bi doped samples exhibited higher response (23.2) to 50 ppm formaldehyde and possessed better selectivity to formaldehyde than pure samples, respectively. The improvement of gas sensing properties of Bi doped ZTO/SnO2 sensor are benefited from the Bi3+ ions doping and the presence of Bi2Sn2O7. Meanwhile, the porous nanostructure of Bi doped ZTO/SnO2 is another reason for improving the gas sensing properties.
Keywords: Bi doped ZTO/SnO2, Bi2(Sn2O7), formaldehyde, sensors, semiconductors
1. Introduction
Formaldehyde, a common air pollutant, has attracted widespread attention due to its toxicity and
∗
Corresponding author: S.Y. Ma
Tel: +86 18893729425
E-mailaddress:[email protected] 1
Fax: +86 9317971503
carcinogenicity. In the past few decades, metal oxide semiconductor (MOS) materials are widely used in the detection of formaldehyde gas, such as ZnO [1], SnO2 [2], Zn2SnO4(ZTO) [3] and so on. However, single metal oxide materials often have some shortcomings, such as low response, high working resistance, so the nanocomposites are studied for improving the gas sensing performance of gas sensor. In recent years, various metal oxide semiconductor composite materials have been investigated to take full use of the synergistic effects among the various components and their respective advantages. For example, Sun et al. has prepared the ZTO/SnO2 nanocomposites for improving the gas sensing performance to formaldehyde [4]. Zhang et al. reported ZTO/SnO2 porous hierarchical nanospheres were prepared for testing triethylamine gas [5]. Shu et al. studied the layered hierarchical octahedral-like structured ZTO/SnO2 showed good gas sensing performance to formaldehyde [6]. In addition, the microstructure of materials and doping of certain elements are also closely related to the improvement of gas sensing performance. For instance, Zhu et al. has synthesized the excellent formaldehyde gas sensor based on Bi doped SnO2 nanoflowers [7]. Guo et al. has fabricated Bi doped SnO2/rGO nanocomposites which have excellent gas sensing properties to benzene [8].
Therefore, pure and Bi-ZTO/SnO2(Bi doped ZTO/SnO2) porous nanospheres have been prepared by a facile hydrothermal treatment. Moreover, Bi doped ZTO/SnO2 sensor exhibits better selectivity and higher response to formaldehyde (HCHO) than pure ZTO/SnO2 sensor, which indicates that Bi doping effectively enhance the formaldehyde gas sensing performance of ZTO/SnO2 sensor.
2. Experimental process
2
2.1 Preparation of samples
All Chemicals reagents used in experiment process no need further purification. The detailed
information of the chemical reagents as shown in Table 1. First of all, 1.06 g Na2SnO3 3H2O, 1.75 g Zn(CH3COO)2·2H2O and 5 wt% of Bi(NO3)3·5H2O were dissolved in 20 ml distilled water under magnetic stirring. Secondly, an amount of sodium alginate and 5 ml NH3 H2O (25 %) were introduced to the above solution and then stirred for 1 h at 35
. Thirdly, the above mixed solution was transferred
to 100 ml Teflon-lined stainless-steel autoclave and reacted for 8 h at 160 autoclave cooling down to the temperature of 30
naturally, the precipitate was washed with
deionized water and ethanol for 3-4 times, collected, and then dried at 70 precipitate was annealed at 700
. Subsequently, after the
for 10 h. Ultimately, the
for 2 h to obtain Bi doped ZTO/SnO2 porous nanospheres. For
comparison, pure ZTO/SnO2 samples were also prepared through the same process but in absence of Bi(NO3)3·5H2O.
2.2 Characterization
X-ray diffraction (XRD, D/Max-2400), Scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, USA FEI Tecnai G2 TF20) were exploited to characterize the structure and morphology of all samples. Gas sensing performance of two sensors was tested by gas sensor test instrument (WS-30A, Wei Sheng Electronics Science and Technology Co. Ltd. China). The response of gas sensor (S) is descripted as Ra / Rg, where Ra and Rg are the resistance of gas sensor in air and in test gas. The response and recovery time are defined as the time which is taken by the sensor to achieve to 90 % of the total resistance change in the gas and air, separately [9].
3. Results and discussion
3
3.1 XRD characterization The crystal structure of all samples was characterized by XRD. Fig. 1(a) display two sets of diffraction peaks are well corresponding to ZTO (JCPDS No.74-2184) and SnO2 (JCPDS No.99-0024). In addition, for Bi doped samples, the diffraction peaks of (440) and (622) are consistent with Bi2Sn2O7 (JCPDS No.87-0284). Compared with pure ZTO/SnO2, the (311) and (222) peaks of Bi doped 3+
ZTO/SnO2 have slightly shift to the higher angle in Fig.1(b). It is due to the fact that Bi
ion(1.03 Å)
has a larger ionic radius than Zn2+ ion (0.74 Å) resulting in a part of Bi ions substitute Zn ions lattice sites and another part of Bi ions maybe stay in the interstitial state [8][10]. It can be seen that Bi element has been successfully doped into the Bi-ZTO/SnO2 samples. The corresponding the ratio of
each compounds in pure and Bi-ZTO/SnO2 samples is shown in Table 2. 3.2 Morphological analysis
The morphology and microstructure of all samples were analyzed by SEM and TEM. The SEM morphology of pure and Bi doped ZTO/SnO2 samples are shown in Fig. 2(a-d). All the samples were composed of a large number of nanospheres with a well-dispersed and rough surface. The diameter of pure and Bi doped ZTO/SnO2 nanospheres is about 700 nm and 500 nm, respectively. The TEM images of pure and Bi doped ZTO/SnO2 samples are showed in Fig. 2(e-f), where obvious contrast between bright and dark areas indicate porous structure of as prepared samples.
3.3 XPS analysis
Fig.3(a) shows the X-ray photoelectron spectroscopy (XPS) survey spectrum of the as-prepared Bi doped ZTO/SnO2 samples. It can be observed that there are four elements including Zn, Sn, O, and Bi in the samples. Fig. 3(b) shows the Zn 2p spectrum, where two features at 1045.2 eV and 1022.2 eV are observed. The peaks are corresponding to the binding energy of the Zn 2p1/2 and Zn 2p3/2 peaks, respectively [11], which indicates Zn element is present in the sample as Zn2+. Fig. 3(c) shows the Sn 3d spectrum. The characteristic peaks at 495.1 eV and 486.6 eV are consistent with the binding 4
energies of the Sn 3d3/2 and Sn 3d5/2 peaks [12], separately. The presence of the Sn element in the sample is Sn4+. Fig .3(d) shows the Bi 4f spectrum. Corresponding to Bi4f
5/2
and Bi4f
7/2
are two
characteristic peaks at 164.4 eV and 159.4 eV, respectively, demonstrating the presence of Bi3+ ions in the material [13]. Fig. 3(e) is the O 1s peak which can be decomposed into two peaks at 530.6 eV and 531.7 eV, and corresponding to the crystal lattice oxygen and adsorption oxygen, respectively [14]. The
corresponding element atomic concentration is shown in the Table 3. 3.4 The gas sensing properties
Operating temperature is an important part of a MOS sensor. Therefore, the responses of both sensors to 50 ppm formaldehyde at different operating temperature were determined. As shown in Fig. 4(a), with the operating temperature changed from 140 increases firstly until reaching their maximum value at 180
to 240
, the response of two sensors
and then decreases. It is obviously seen
that the optimum operating temperature of pure ZTO/SnO2 and Bi doped ZTO/SnO2 sensors is 180 and the response of two sensors to 50 ppm formaldehyde are 14.4 and 23.2 at 180 response to 50 ppm various test gases at 180
, respectively. The
of two sensors is showed in Fig. 4(b). It can be
obviously seen that the response of all sensors to formaldehyde is much higher than other gases, which indicates two sensors have good selectivity to formaldehyde. In Fig. 4(c), the response curve of the two sensors to different concentrations (10-1000 ppm) of formaldehyde at optimum operating temperature is showed. With the concentration of formaldehyde gradually increasing, the response of two sensor is also increasing. The response of the Bi doped ZTO/SnO2 sensor is fairly higher than pure ZTO/SnO2 sensor. Moreover, the response of two sensors augment rapidly before 200 ppm, but after reaching a certain concentration, the adsorption and desorption processes of formaldehyde molecules gradually tend to balance, leading to the increasing slowly response [5]. The concentration ranges from 10 to 100 5
ppm of formaldehyde, the linear relationship between response and concentration is shown in Fig. 4(d). The linear fitting formulas of pure and Bi doped ZTO/SnO2 are as follows: ln (S-1) = 0.74854 ln (C) 0.25213 (R2 = 0.96704) and ln (S-1) = 1.02310 ln (C) - 0.71994 (R2 = 0.97206), respectively. The result shows a good linear relationship in the range of 10-100 ppm, which indicates that these sensors have an advantage in low-concentration formaldehyde detection [15]. As shown in Fig. 4(e), the response and recovery time of pure and Bi doped ZTO/SnO2 porous nanospheres to 50 ppm HCHO at 180
is 16 s
and 9 s, respectively. As depicted in Fig. 4(f), pure and Bi doped ZTO/SnO2 sensors exhibit nearly constant response, indicating that two sensors have good stability. The good repeatability of the Bi-ZTO/SnO2 samples is shown in Fig. 5. Therefore, Bi doping effectively improves the formaldehyde gas sensing performance of the sensor based on the ZTO/SnO2 porous nanospheres. The remarkable improvement may be attributed to the hollow porous structure and the change in electrons concentration caused by Bi ions doping.
3. 5 The gas sensing mechanism
Both ZTO and SnO2 are typical n-type semiconductors. The gas-sensing mechanism is depicted by the changes of resistance caused by the oxidation-reduction reaction of materials and gas molecules (Fig. 6(a)). According to the space charge layer model, firstly, when the material is exposed to air, oxygen molecules will adsorb on the surface of ZTO/SnO2 and capture the free electrons from conduction band to form oxygen anions (O2-, O-, O2-). The specific process is as follows：
O2 (gas)→ O2 (ads)
O2 (ads) + e- → O2- (ads) (T <100 ℃)
O2- (ads) + e- → O- (ads) (100 ℃ ≤ T ≤ 300 ℃)
6
(1)
(2)
(3)
O- (ads) + e- →O2- (ads) (T >300 ℃)
(4)
In this process, a thick electron depletion layer is formed on the surface of ZTO/SnO2, and the carrier concentration is decreased, resulting in an increase in resistance of the sensor.
As long as the sensor is exposed to formaldehyde gas at a suitable temperature, the formaldehyde molecules will react with oxygen anions. The specific process is as follows：
HCHO(gas) → HCHO(ads)
(5)
HCHO（ads）+ 2O-（ads）→CO2 + H2O + 2e-
(6)
During this process, the trapped free electrons are released back to the conduction bands of materials and the thickness of the depletion layer is reduced, which makes the material resistance decreased.
Based on the above gas sensing mechanism, the reasons for the enhanced gas sensing performance of Bi doped ZTO/SnO2 may be as follows：
First, the interface between SnO2 and ZTO will form a heterojunction. As shown in the Fig. 5(b), because the conduction band (CB) position of ZTO is higher than that of SnO2, electrons will transfer from ZTO to SnO2 efficiently, resulting in the low composite probability of electrons and holes. Therefore, the heterojunction between ZTO and SnO2 is beneficial to the improvement of gas sensing performance. Second, when Bi3+ is doped into the crystal lattice of Zn2SnO4, the occupancy of Bi ions at Zn sites would result in the generation of oxygen vacancy related defects. These defects can reduce composite probability of electrons and holes and increase the number of free electrons in the Bi-ZTO/SnO2, which can contribute to improved gas sensing performance. Third, the interface among
7
SnO2 and ZTO and Bi2Sn2O7 will form a heterojunction. Due to the formation of heterojunction, the Bi-ZTO/SnO2 samples have high resistance than pure samples in the air. The sharp change in resistance will lead to better gas sensing performance. Fourth, due to the unique electronic structure of Bi2Sn2O7, it also is beneficial to the improvement of gas sensing performance [13, 16]. Finally, the porous structure of Bi doped ZTO/SnO2 samples is also conducive to gas diffusion, improving the gas sensing performance.
4. Conclusions
Pure and Bi doped ZTO/SnO2 porous nanospheres were synthesized via a hydrothermal reaction. Compared with pure ZTO/SnO2 sensor, Bi doped ZTO/SnO2 sensor exhibits excellent gas sensing properties. The response of Bi doped ZTO/SnO2 sensor to 50 ppm formaldehyde at 180
is 23.2.
Moreover, Bi doped ZTO/SnO2 sensor shows good selectivity to formaldehyde and good long-term stability. The results show the Bi doped ZTO/SnO2 porous nanospheres have a certain potential application in the detection of low concentration formaldehyde.
Acknowledgments
This work was supported by the National Natural Science Foundations of China (Grant No. 11864034 and 11964035), and the Scientific Research Project of Gansu Province (Grant No. 18JR3RA089 and 17JR5RA072).
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Table 1 The detailed information of the chemical reagents Reagent Zinc acetate dihydrate
chemical formula Zn(CH3COO)2 2H2O
Sodium stannate trihydrate
Na2SnO3
Ammonia solution
NH3
3H2O
H 2O
purity 99.0%
98%
25%
Sodium alginate
(C6H7NaO6)n
99.8%
Bismuth nitrate pentahydrate
Bi(NO3)3·5H2O
≥99.0%
manufacturer Tianjin Baishi Chemical Industry Co. Ltd Tianjin Baishi Chemical Industry Co. Ltd Tianjin Baishi Chemical Industry Co. Ltd
molecular weight 219.51
266.73
35.05
Tianjin Guangfu Fine Chemical Research Institute Shanghai Zhanyun Chemical Co. Ltd
485.07
Table 2 The ratio of each compounds in samples Materials
wt%
Zn2SnO4
57.6
SnO2
42.4
Zn2SnO4
52.8
SnO2
42.8
Bi2(Sn2O7)
4.4
ZTO/SnO2
Bi-ZTO/SnO2
Table 3 The atomic concentration of the Bi-ZTO/SnO2 Element
C 1s
O 1s
Zn 2p3
Sn 3d5
Bi 4f
At/%
15.05
55.96
10.85
17.5
0.65
Fig. 1 (a) XRD pattern of pure and Bi doped ZTO/SnO2 samples, (b) Comparison the shift of (311) and (222) peaks of pure and Bi doped ZTO/SnO2 samples.
Fig.2 (a-d) SEM images of pure and Bi doped ZTO/SnO2 samples, (e, f) TEM images of pure and Bi doped ZTO/SnO2 samples.
Fig. 3 (a) Survey XPS spectrum of Bi doped Zn2SnO4; XPS spectra of (b) Zn 2p, (c) Sn 3d, (d) Bi 4f and (e) O 1s.
Fig. 4 (a) Responses of all sensors at different operating temperature to 50 ppm formaldehyde, (b) Selectivity of pure and Bi doped ZTO/SnO2 hollow porous nanospheres to 50 ppm various gases, (c) The response of all samples to different formaldehyde concentrations ranging from 10 to 1000 ppm, (d) Fitting curve of the response to 10 - 100 ppm formaldehyde, (e) Dynamic sensing transient of all sensors to 50 ppm formaldehyde, (f) The long-term stability of pure and Bi doped ZTO/SnO2 samples to 50 ppm formaldehyde at 180
.
Fig. 5 The repeatability of the Bi-ZTO/SnO2 samples
Fig. 6 Schematic illustration of the gas sensing mechanism.
1
Fig. 1
2
Fig. 2 3
Fig. 3
4
Fig. 4
5
Fig. 5
6
Fig. 6
7
Highlights:
The pure and Bi-Zn2SnO4/SnO2 hollow porous nanospheres were successfully prepared. Bi doping effectively improves the gas sensing performance to detect formaldehyde. Bi2Sn2O7 contributes to the improvement of gas sensing performance to formaldehyde. The response of Bi doped Zn2SnO4/SnO2 is 23.2 to 50 ppm formaldehyde at 180
.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: