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Increasing oxygen vacancies at room temperature in SnO2 for enhancing ethanol gas sensing
Materials Science in Semiconductor Processing 111 (2020) 104962
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Increasing oxygen vacancies at room temperature in SnO2 for enhancing ethanol gas sensing Qinghao Zeng a, Yanfa Cui a, Lianfeng Zhu b, Youwei Yao a, * a b
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China Shenzhen Dovelet Sensors Technology Co. Ltd., China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 Ethanol Oxygen vacancy Gas-sensing properties
Partially reduced SnO2 was prepared by using NaBH4 as reducer under room temperature. After NaBH4 treat ment, the oxygen vacancy density of SnO2 was increased greatly. X-ray photoelectron spectroscopy, photo luminescence and electron paramagnetic resonance indicates that the obtained materials are characteristic of a high concentration of oxygen vacancies. Owing to increased oxygen vacancy concentration, these materials showed significantly high gas-sensing response to ethanol. Compared with the ethanol-sensing response value of 36.9 for pristine SnO2 towards 300 ppm ethanol, the response value for the partially reduced SnO2 is 120.3. This ethanol-sensing response enhancement may be attributed to the fact that the increased surface oxygen vacancy facilitates oxygen adsorption, and promotes chemical reaction on the surface.
1. Introduction Volatile organic compounds (VOCs) are not only harmful to the at mospheric environment, but also harmful to human health due to their toxicity, mutagenicity, teratogenicity and carcinogenicity [1]. Ethanol, one of the important VOCs, is widely used in biomedical, food industries and breath analyzer [2]. The Occupational Safety Health and Adminis tration (OSHA) established the threshold limit value (TLV) of ethanol to be 1000 ppm [3]. Prolonged exposure to ethanol vapor can cause health problems such as difficulty in breathing, drowsiness, irritation of the eyes, headaches, liver damage [4]. Therefore, the improvement of ethanol sensor with high-performance is of great significance. Compared with other gas monitoring methods [5–8], resistive gas sensors based on metal oxide semiconductors (MOS) attract great interest because of their ease of fabrication, low cost and miniaturization [9,10]. In the last few years, the metal oxide semiconductors including SnO2, ZnO, CuO, NiO, In2O3, TiO2, and so on have been reported for the use of gas sensors [11, 12]. Since the first commercial gas-sensor device was developed in the 1960s [13], worldwide efforts have been explored to improve the response and selectivity in designing semiconductor gas sensors. In order to improve the sensing performance, doping metal elements such as Ag, Au, Rh, Pd and so on, is usually adopted [14]. In addition, con trolling material morphology is another effective way to further improve gas-sensing performance [15–18].
It is well known that oxygen vacancies are elemental point defects in oxides, which have been demonstrated to play a crucial role in many applications such as photocatalysis [19], supercapacitors [20], and gas sensors [21]. In semiconductor oxide materials, it is widely accepted that increasing the proportion of oxygen vacancies is an efficient method to improve the sensing performance of oxide materials. The existence of oxygen vacancies not only regulates the electronic structure of metal oxides, but also provides more active sites and controls adsorption processes [22,23]. Therefore, increasing oxygen vacancy concentration is considered as a promising method to fabricate high-performance SnO2-based gas sensors. Wu et al. demonstrated that oxygen vacancies on the surface of SnO2 not only enhance the adsorption of oxygen molecules, but also play a vital role in formaldehyde oxidation [24]. Wei et al. reported that the SnO2 obtained in vacuum contained more oxygen vacancies than the SnO2 obtained in air and the sensor showed much enhanced sensing properties to NO2 than the SnO2 sensor obtained in air [25]. Traditionally, annealing oxides in a reducing atmosphere such as hydrogen was an effective method to modify the concentration of oxy gen vacancy [26]. However, this high temperature hydrogen annealing method is not practical for applications due to cost and safety issues. To overcome the above problems, some effective strategies have been developed for construction of more oxygen vacancies onto metal oxides. For instance, solution reduction methods [19,27], annealing in vacuum
* Corresponding author. E-mail address: [email protected] (Y. Yao). https://doi.org/10.1016/j.mssp.2020.104962 Received 5 November 2019; Received in revised form 16 January 2020; Accepted 22 January 2020 Available online 14 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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[25], doping metals [28], plasma treatment [29]. Apart from these, Liu and co-workers successfully modulated the oxygen vacancies of SnO2 through a novel organometallic chemistry-assisted method [22]. Addi tionally, Dou and co-workers prepared oxygen vacancies enriched SnO2 nanoparticles by controlling hydrolysis rate of liquid SnCl4 [30]. Furthermore, Wang and co-workers prepared SnO2-RGO-OVs hybrids with abundant vacancies by a combined hydrothermal synthesis and chemical solution deposition method [31]. However, these strategies require expensive facilities, high temperature environment, complex preparation process, low yields, which limit their practical application. So, it is urgent to explore facile and universal methods to introduce oxygen vacancies into metal oxides-based sensing materials. In this work, SnO2 samples with different oxygen vacancy concen tration were successfully prepared by NaBH4 treatment at room tem perature, which is simple, safe, low-cost and without damaging the morphology of SnO2. In the meantime, the ethanol sensing character istics of sensors based on SnO2 samples with different oxygen vacancies were systematically investigated. The gas sensing results indicate that the sensors based on SnO2 with high oxygen vacancy concentration exhibits better ethanol sensing performance than non-modified SnO2. The possible mechanism of enhanced gas sensing towards ethanol was also discussed.
Fig. 1. XRD patterns of SnO2.
2. Experimental 2.1. Preparation All chemicals were of analytic reagent grade and used without further purification. SnO2 (99.5%, Macklin), NaBH4 (98%, Aladdin). A typical synthesis process, 0.5 g of SnO2, an appropriate amount of NaBH4 and 5 ml deionized water were mixed by a magnetic stirrer for 48 h at room temperature. After centrifuging and washing by deionized water five times, the obtained oxide powders were dried at 80 � C in air for 12 h. Finally, SnO2 samples with different oxygen vacancies were obtained. In this work, the SnO2 powders, which were pristine and reduced by different amount of NaBH4 (1 g and 4 g) were numbered as S0, S-1 and S-2, respectively.
Fig. 2. SEM micrographs of pristine and partially reduced SnO2. a) S-0, b) S-1, c) S-2.
and recovery times are defined as the time taken for the resistance to reach 90% of its total change. Standard testing conditions: 50–70% relative humidity and 20–25 � C.
2.2. Material characterization
3. Results and discussion
The crystal phase of samples was characterized by X-ray powder diffraction (XRD, D8 Advance, Bruker Corp) using Cu Kα1 (λ ¼ 1.5406 Å) radiation. The morphologies and microstructures were observed by scanning electron microscopy (SEM, ZEISS SUPRA®55). The binding energies of SnO2 were characterized using an X-ray photoelectron spectrometer (XPS, PHI-5000 Versaprobe II). The photoluminescence (PL) emission spectra were investigated at room temperature by using a Labram HR800 (Horiba Ltd.) spectrophotometer with an excitation wavelength of 325 nm. The oxygen vacancies were detected by EPR spectroscopy (JES-FA200). The Brunauer–Emmett–Teller (BET) surface area of the samples were measured by N2 adsorption–desorption and the pore size distribution plot was determined via the Bar rett–Joyner–Halenda (BJH) theories (ASAP 2020 M þ C).
3.1. Composition and morphology As shown in Fig. 1, the four strong diffraction peaks centered at around 26.6� , 33.9� , 37.9� , and 51.8� are corresponding to the (110), (101), (200) and (211) crystal planes of rutile SnO2 (PDF#41–1445). All the diffraction peaks of obtained SnO2 samples can be indexed to stan dard rutile SnO2 and no any other phases or impurities are detected. It means that the intrinsic crystal structure of the SnO2 nanoparticles was not destroyed after NaBH4 treatment. Fig. 2a shows the SEM image of pristine SnO2, which reveals that its particle size ranges from a few nanometers to several hundred nanometers. After NaBH4 treatment, the SEM images of partially reduced SnO2 samples are shown in Fig. 2b and c. It is found that the particle size of partially reduced SnO2 is almost the same as that of pristine SnO2, ranging from a few nanometers to several hundred nanometers. Therefore, NaBH4 treatment didn’t change the morphology and size of SnO2. A schematic diagram showing the oxygen vacancies (VO) in the SnO2 structure is illustrated in Fig. 3. During the reduction process, the active hydrogen produced by the decomposition of NaBH4 will react with the lattice oxygen, resulting in the formation of oxygen vacancies in SnO2. We carried out XPS analysis to investigate the oxygen vacancy of partially reduced SnO2. The survey XPS spectra show Sn, O, and C sig nals in Fig. 4a, the peak of C 1s at 284.8 eV. The O 1s and Sn 3d peaks were further characterized by the fine-scanned O 1s and Sn 3d XPS
2.3. Gas-sensing measurement The gas-sensing property was examined using a homemade instru ment as shown in the Supplementary Fig. S1 and Fig. S2. Briefly, the gassensing samples were ground with water and the obtained slurry prod ucts were prepared on alumina substrate. The substrates consisted of two gold electrodes on the top surface and a Ru2O3 microheater on the back side of the substrate. The sensor is housed in a 16 L container with a fan. Different concentrations of ethanol gas are provided by injecting an accurate amount of rapidly evaporating ethanol (liquid). The gas response was defined as Ra/Rg, where Ra and Rg are the resistance of the sensor when exposed to air and the target gas, respectively. The response 2
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Fig. 3. Schematic process of creating oxygen vacancies in SnO2.
Fig. 4. a) XPS survey spectra and high-resolution spectra of the O 1s of SnO2. b) S-0, c) S-1, d) S-2.
spectra (Fig. 4b-d). The O 1s peak was coherently fitted with three peaks located at ~530.5, 531.9 and 533.0 eV corresponding to lattice oxygen (OL), oxygen vacancy (OV), and chemisorbed oxygen (OC) species, respectively [32,33]. The binding energy value and the peak area ratio of each peak in each sample are summarized in Table 1. The relative percentages of OL, OV, and OC components in the pristine SnO2 were about 79.2%, 10.8% and 10.0%, respectively. After NaBH4 treatment, the ratio of OL, OV, and OC components in S-2 were about 67.4%, 25.4% and 7.2%, respectively. It was obviously that the ratio of oxygen va cancies of SnO2 was increased significantly after treatment with NaBH4. Due to spin-orbit splitting, the Sn 3d consists of doublet peaks assigned to Sn 3d5/2 and Sn 3d3/2. In pristine SnO2, the peaks obtained were at 486.5 and 494.9 eV, which were representative signature of
Table 1 XPS characteristics in the O 1s of the SnO2. Sample
Peak name
Peak position (eV)
Area (%)
S-0
OL OV OC OL OV OC OL OV OC
530.5 531.9 533.0 530.4 531.8 532.7 530.3 531.9 533.3
79.2 10.8 10 73.6 16.7 9.7 67.4 25.4 7.2
S-1 S-2
3
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Fig. 5. XPS spectra of Sn 3d signal. a) S-0, b) S-2.
Fig. 6. High-resolution XPS spectra of the Sn 3d.
Fig. 8. EPR spectra.
(Supplementary Fig. S4) were detected in the partially reduced SnO2. At the same time, NaBH4 was replaced with Na2B4O7⋅10H2O, H3BO3 and Na2SnO3⋅3H2O with all the other conditions the same, respectively, and the gas-sensing properties of the treated SnO2 were not significantly changed, which further excluded the influence of B and Na element on gas sensitivity. Apart from XPS spectra, the PL spectroscopy was carried out to o detect oxygen vacancies in the SnO2 samples. Fig. 7 shows the PL spectra at room temperature excited by a 325 nm laser of the SnO2 samples. It is observed that three peaks located at ~595, 625 and 780 nm, corre sponding to the vacancies of SnO2 [38–40]. According to literature [41], the more the defects are, the stronger the PL signal is. Hence, the peak intensity and area of partially reduced SnO2 is far larger than that of pristine SnO2, which means that rich oxygen vacancies generated in the reduction process. To further investigate the oxygen vacancies, electron paramagnetic resonance (EPR) was performed at room temperature. As shown in Fig. 8, in comparison with the pristine SnO2, the partially reduced SnO2 exhibits a much stronger signal at g ¼ 2.003 (25 � C), which is identified as the presence of single-electron oxygen vacancies [27,42]. This result indicated more oxygen vacancies in partially reduced SnO2, which was consistent with the XPS and PL spectra analysis. In order to obtain the surface area of all samples, N2 adsorption and desorption isotherms were carried and shown in Fig. 9. A mixing of type II and IV isotherms is found [43], indicating that there is a mesoporous and nonporous or microporous structure existing in the samples. The
Fig. 7. Room temperature photoluminescence spectrum of the SnO2 samples.
Sn4þ in tetragonal SnO2 [28,34]. However, it consisted of two pairs of Sn 3d5/2 and Sn 3d3/2 peaks for the sample S-2 (Fig. 5). There are two small peaks at low binding energy 486.0 and 494.5 eV. Because the chemical shift between the charge states of Sn2þ and Sn4þ is ~0.5 eV [35,36]. Therefore, the peaks located at high binding energies were assigned to Sn4þ, and located at low binding energies were assigned to Sn2þ. The content of the Sn2þ component was increased to almost 21.0% of the total Sn 3d peak in the sample S-2. In addition, a negative shift of Sn 3d was observed on partially reduced SnO2 (Fig. 6), which can be ascribed to that Sn2þ was produced from the generation of oxygen vacancies [37]. In addition, no peaks related to B (Supplementary Fig. S3) and Na 4
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Fig. 9. a) N2 adsorption/desorption isotherm, b) BJH pore-size distribution, c) SSA and mean pore diameter of SnO2.
Fig. 10. a) response of the sensors to 300 ppm ethanol at different operating temperature; b) base resistance in air of the sensors at different operating temperature; c) concentration dependent response curves of S-0, S-1 and S-2 at 190 � C.
specific surface area of pristine SnO2 nanoparticles is 6.19 m2 g 1. After NaBH4 reduction treatment, partially reduced SnO2 show similar spe cific surface area of 6.81 m2 g 1, indicating the morphology of the SnO2 was preserved. In addition, no significant change in pore size was observed.
ethanol testing gas at operating temperature from 120 to 240 � C. It is well known that the sensor operating temperature is one of the impor tant factors determining the gas sensitivity of materials. It can be clearly seen that the response of all samples showed an “increase-maximumdecrease” characteristic. This phenomenon can be explained as follows: At low temperature, the relatively low response is mainly due to the small amount of oxygen adsorbed on the surface of the material. As the temperature increases, the reaction kinetics of chemisorbed oxygen on the surface of the sensing material becomes faster, and the target gas molecules will acquire sufficient energy to overcome the activation energy barrier to react with the surface adsorbing oxygen atoms. At the optimum operating temperature, the gas adsorption and desorption processes reach equilibrium, resulting in maximum response. However, when the operating temperature continues to increase, the gas
3.2. Sensor performance Gas sensors were fabricated by coating the prepared SnO2 slurry on a commercial alumina substrate and aged 24 h at 150 � C to improve the stability of the sensors. The heating temperature is abstract, which is varies with the heating voltage and Fig. S5 shows the relationship be tween heating voltage and heating temperature. Fig. 10a shows the response of the sensors as a function of temperature exposed to 300 ppm 5
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Fig. 14. Long-term stability of the sensor based on S-2 at 190 � C. Fig. 11. Dynamic resistance curves of the SnO2 sensors to different ethanol concentration at 190 � C.
Fig. 15. Responses of the sensor based on S-2 exposed to 300 ppm different target gases at 190 � C.
Fig. 12. The dynamic response of sensor based on S-2 to 300 ppm ethanol at 190 � C.
based on S-0-S-2 to ethanol concentration of 20–1000 ppm at 190 � C. The response of S-2 based sensor was about 37.2, 55.1, 64.7, 120.3, 168.2 and 205.1, while S-0 based sensor was only about 8.1, 23.3, 27.2, 36.9, 46.3 and 53.5 to 20, 100, 200, 300, 500 and 1000 ppm ethanol, respectively. Fig. 11 showed transient resistance plots of the SnO2 sen sors to ethanol with different concentrations at 190 � C. It can be observed that the response of all the sensors increased with ethanol concentration. Furthermore, the response of sensor S-2 to ethanol was significantly higher than S-0. The response time (Tres) and recovery time (Trec) of sensor based on
desorption rate increases higher than the adsorption rate, resulting in a decrease in response [44]. The maximum gas responses of S-0-S-2 to 300 ppm ethanol was 36.8, 70.3, and 120.3, respectively. Obviously, the gas sensitivity of the materials treated by NaBH4 is higher than that of pristine. It could be concluded that as the oxygen vacancies increase, the gas sensitivity of the pristine SnO2 could be effectively improved. Fig. 10b showed the base resistance of the SnO2 sensors in air. As the oxygen vacancies increase, the electrical resistance of the SnO2 sensors in air significantly increased. Fig. 10c displayed the response of a sensor
Fig. 13. a) Response time and b) recovery time of S-0, S-1 and S-2 to 20–1000 ppm ethanol at 190 � C. 6
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Fig. 16. A schematic diagram of the sensing mechanism of the SnO2 based sensor to ethanol: a) in air, b) in ethanol.
S-2 to 300 ppm ethanol at 190 � C were evaluated from the dynamic sensing curves. As shown in Fig. 12, the response and recovery times for 300 ppm ethanol were about 63 s and 20 s, respectively. As shown in the inset of Fig. 12, the three cycles of the response curve indicated the stable and repeatable characteristics of sensor S-2. In Fig. 13a and b, it could be observed that the response and recovery speeds of the partially reduced SnO2 were fast compared with the pristine SnO2 in the same ethanol concentration from 20 ppm to 1000 ppm, respectively. Fig. 14 showed the long-term stability of the sensor based on SnO2. As shown in the curve, the response of S-2 remains around 110, and the response of S0 is only around 25. The sensing selectivity of S-0-S-2 to ethanol was also studied in this work. As shown in Fig. 15, the selectivity was examined by exposing sensors under 300 ppm different test gases at the optimal operating temperature (190 � C), such as methanol, CO gas and H2 gas. It could be seen that the sensor based on S-2 showed the highest sensitivity to ethanol (120), higher than that of methanol (55.5) and much higher than that of H2 (4.3) and CO (1.3). Obviously, the sensor based on S-2 showed enhanced selectivity toward ethanol opposed to the detected other gases at the same concentrations.
Compared with the ethanol-sensing performances of pristine SnO2, the enhancement of the ethanol-sensing behaviour of partially reduced SnO2 may be attributed to the oxygen vacancies. According to the OV model, oxygen vacancy promotes the adsorption of oxygen molecules to form more active sites and the number of adsorbed oxygen molecules and target gas molecules increases with increasing oxygen vacancy concentration [24,48–50]. Hence, oxygen vacancy enhances the elec trons transfer from the SnO2 surface to the O2 molecules which results more negative oxygen ions. This will further facilitate the reaction with ethanol molecules. Based on the above analysis and discussion, we suggest that oxygen vacancy could greatly promote the gas sensing properties of SnO2. 4. Conclusions We have synthesized SnO2 of high oxygen vacancy concentration by NaBH4 treatment at room temperature. In addition, the crystal structure, morphology and specific surface area of oxides did not change. Mean while, the SnO2 with a partially reduced surface exhibited enhanced gassensing properties. After NaBH4 treatment, the response of SnO2 to 300 ppm ethanol increased from 36.9 to 120.3. NaBH4 treatment increased surface oxygen vacancy, which promoting oxygen adsorption and chemical reaction on the surface.
3.3. Sensing mechanisms It is well known that the sensing mechanism of MOS gas sensors is mainly based on gas adsorption and chemical reactions on the surface of sensing materials [21,45,46]. When the sensors based on SnO2 are exposed to air, oxygen molecules adsorbed on the SnO2 surface and trapped electrons from the conduction band to form oxygen ions (O2 , O , and O2 ). As a result, an electron depletion layer is formed in its surface region, which decreases concentration of electrons and obtains a relatively high resistance. When the sensor is exposed to the reducing gases (such as ethanol in this case), the ethanol molecules react with the adsorbed oxygen irons, releasing the trapped electrons back into the conduction band and causing the decrease of resistance. In this work, the sensing schematic presentation for SnO2 towards ethanol is given in Fig. 16. The possible reaction processes for SnO2 response to ethanol can be described by the following equations [47]:
Declaration of competing interest 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. CRediT authorship contribution statement Qinghao Zeng: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Yanfa Cui: Formal analysis. Lianfeng Zhu: Supervision, Writing - review & editing. Youwei Yao: Formal analysis, Supervision, Validation, Writing - review & editing. Acknowledgements
O2 (gas) ↔ O2 (ads)
(1)
O2 (ads) þ e ↔ O2 (ads)
(2)
O2 (ads) þ e ↔ 2O (ads)
(3)
This work was supported by the Special Development Program of Strategic Emerging Industries of Shenzhen city (JSGG20160331104855391).
↔ O2 (ads)
(4)
Appendix A. Supplementary data
O (ads) þ e
C2H5OH (gas) þ 6O (ads) → 2CO2 (gas) þ 3H2O (gas) þ 6e C2H5OH (gas) þ 6O
2
(ads) → 2CO2 (gas) þ 3H2O (gas) þ 12e
(5)
Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.104962.
(6) 7
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Increasing oxygen vacancies at room temperature in SnO2 for enhancing ethanol gas sensing Qinghao Zeng a, Yanfa Cui a, Lianfeng Zhu b, Youwei Yao a, * a b
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China Shenzhen Dovelet Sensors Technology Co. Ltd., China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 Ethanol Oxygen vacancy Gas-sensing properties
Partially reduced SnO2 was prepared by using NaBH4 as reducer under room temperature. After NaBH4 treat ment, the oxygen vacancy density of SnO2 was increased greatly. X-ray photoelectron spectroscopy, photo luminescence and electron paramagnetic resonance indicates that the obtained materials are characteristic of a high concentration of oxygen vacancies. Owing to increased oxygen vacancy concentration, these materials showed significantly high gas-sensing response to ethanol. Compared with the ethanol-sensing response value of 36.9 for pristine SnO2 towards 300 ppm ethanol, the response value for the partially reduced SnO2 is 120.3. This ethanol-sensing response enhancement may be attributed to the fact that the increased surface oxygen vacancy facilitates oxygen adsorption, and promotes chemical reaction on the surface.
1. Introduction Volatile organic compounds (VOCs) are not only harmful to the at mospheric environment, but also harmful to human health due to their toxicity, mutagenicity, teratogenicity and carcinogenicity [1]. Ethanol, one of the important VOCs, is widely used in biomedical, food industries and breath analyzer [2]. The Occupational Safety Health and Adminis tration (OSHA) established the threshold limit value (TLV) of ethanol to be 1000 ppm [3]. Prolonged exposure to ethanol vapor can cause health problems such as difficulty in breathing, drowsiness, irritation of the eyes, headaches, liver damage [4]. Therefore, the improvement of ethanol sensor with high-performance is of great significance. Compared with other gas monitoring methods [5–8], resistive gas sensors based on metal oxide semiconductors (MOS) attract great interest because of their ease of fabrication, low cost and miniaturization [9,10]. In the last few years, the metal oxide semiconductors including SnO2, ZnO, CuO, NiO, In2O3, TiO2, and so on have been reported for the use of gas sensors [11, 12]. Since the first commercial gas-sensor device was developed in the 1960s [13], worldwide efforts have been explored to improve the response and selectivity in designing semiconductor gas sensors. In order to improve the sensing performance, doping metal elements such as Ag, Au, Rh, Pd and so on, is usually adopted [14]. In addition, con trolling material morphology is another effective way to further improve gas-sensing performance [15–18].
It is well known that oxygen vacancies are elemental point defects in oxides, which have been demonstrated to play a crucial role in many applications such as photocatalysis [19], supercapacitors [20], and gas sensors [21]. In semiconductor oxide materials, it is widely accepted that increasing the proportion of oxygen vacancies is an efficient method to improve the sensing performance of oxide materials. The existence of oxygen vacancies not only regulates the electronic structure of metal oxides, but also provides more active sites and controls adsorption processes [22,23]. Therefore, increasing oxygen vacancy concentration is considered as a promising method to fabricate high-performance SnO2-based gas sensors. Wu et al. demonstrated that oxygen vacancies on the surface of SnO2 not only enhance the adsorption of oxygen molecules, but also play a vital role in formaldehyde oxidation [24]. Wei et al. reported that the SnO2 obtained in vacuum contained more oxygen vacancies than the SnO2 obtained in air and the sensor showed much enhanced sensing properties to NO2 than the SnO2 sensor obtained in air [25]. Traditionally, annealing oxides in a reducing atmosphere such as hydrogen was an effective method to modify the concentration of oxy gen vacancy [26]. However, this high temperature hydrogen annealing method is not practical for applications due to cost and safety issues. To overcome the above problems, some effective strategies have been developed for construction of more oxygen vacancies onto metal oxides. For instance, solution reduction methods [19,27], annealing in vacuum
* Corresponding author. E-mail address: [email protected] (Y. Yao). https://doi.org/10.1016/j.mssp.2020.104962 Received 5 November 2019; Received in revised form 16 January 2020; Accepted 22 January 2020 Available online 14 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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[25], doping metals [28], plasma treatment [29]. Apart from these, Liu and co-workers successfully modulated the oxygen vacancies of SnO2 through a novel organometallic chemistry-assisted method [22]. Addi tionally, Dou and co-workers prepared oxygen vacancies enriched SnO2 nanoparticles by controlling hydrolysis rate of liquid SnCl4 [30]. Furthermore, Wang and co-workers prepared SnO2-RGO-OVs hybrids with abundant vacancies by a combined hydrothermal synthesis and chemical solution deposition method [31]. However, these strategies require expensive facilities, high temperature environment, complex preparation process, low yields, which limit their practical application. So, it is urgent to explore facile and universal methods to introduce oxygen vacancies into metal oxides-based sensing materials. In this work, SnO2 samples with different oxygen vacancy concen tration were successfully prepared by NaBH4 treatment at room tem perature, which is simple, safe, low-cost and without damaging the morphology of SnO2. In the meantime, the ethanol sensing character istics of sensors based on SnO2 samples with different oxygen vacancies were systematically investigated. The gas sensing results indicate that the sensors based on SnO2 with high oxygen vacancy concentration exhibits better ethanol sensing performance than non-modified SnO2. The possible mechanism of enhanced gas sensing towards ethanol was also discussed.
Fig. 1. XRD patterns of SnO2.
2. Experimental 2.1. Preparation All chemicals were of analytic reagent grade and used without further purification. SnO2 (99.5%, Macklin), NaBH4 (98%, Aladdin). A typical synthesis process, 0.5 g of SnO2, an appropriate amount of NaBH4 and 5 ml deionized water were mixed by a magnetic stirrer for 48 h at room temperature. After centrifuging and washing by deionized water five times, the obtained oxide powders were dried at 80 � C in air for 12 h. Finally, SnO2 samples with different oxygen vacancies were obtained. In this work, the SnO2 powders, which were pristine and reduced by different amount of NaBH4 (1 g and 4 g) were numbered as S0, S-1 and S-2, respectively.
Fig. 2. SEM micrographs of pristine and partially reduced SnO2. a) S-0, b) S-1, c) S-2.
and recovery times are defined as the time taken for the resistance to reach 90% of its total change. Standard testing conditions: 50–70% relative humidity and 20–25 � C.
2.2. Material characterization
3. Results and discussion
The crystal phase of samples was characterized by X-ray powder diffraction (XRD, D8 Advance, Bruker Corp) using Cu Kα1 (λ ¼ 1.5406 Å) radiation. The morphologies and microstructures were observed by scanning electron microscopy (SEM, ZEISS SUPRA®55). The binding energies of SnO2 were characterized using an X-ray photoelectron spectrometer (XPS, PHI-5000 Versaprobe II). The photoluminescence (PL) emission spectra were investigated at room temperature by using a Labram HR800 (Horiba Ltd.) spectrophotometer with an excitation wavelength of 325 nm. The oxygen vacancies were detected by EPR spectroscopy (JES-FA200). The Brunauer–Emmett–Teller (BET) surface area of the samples were measured by N2 adsorption–desorption and the pore size distribution plot was determined via the Bar rett–Joyner–Halenda (BJH) theories (ASAP 2020 M þ C).
3.1. Composition and morphology As shown in Fig. 1, the four strong diffraction peaks centered at around 26.6� , 33.9� , 37.9� , and 51.8� are corresponding to the (110), (101), (200) and (211) crystal planes of rutile SnO2 (PDF#41–1445). All the diffraction peaks of obtained SnO2 samples can be indexed to stan dard rutile SnO2 and no any other phases or impurities are detected. It means that the intrinsic crystal structure of the SnO2 nanoparticles was not destroyed after NaBH4 treatment. Fig. 2a shows the SEM image of pristine SnO2, which reveals that its particle size ranges from a few nanometers to several hundred nanometers. After NaBH4 treatment, the SEM images of partially reduced SnO2 samples are shown in Fig. 2b and c. It is found that the particle size of partially reduced SnO2 is almost the same as that of pristine SnO2, ranging from a few nanometers to several hundred nanometers. Therefore, NaBH4 treatment didn’t change the morphology and size of SnO2. A schematic diagram showing the oxygen vacancies (VO) in the SnO2 structure is illustrated in Fig. 3. During the reduction process, the active hydrogen produced by the decomposition of NaBH4 will react with the lattice oxygen, resulting in the formation of oxygen vacancies in SnO2. We carried out XPS analysis to investigate the oxygen vacancy of partially reduced SnO2. The survey XPS spectra show Sn, O, and C sig nals in Fig. 4a, the peak of C 1s at 284.8 eV. The O 1s and Sn 3d peaks were further characterized by the fine-scanned O 1s and Sn 3d XPS
2.3. Gas-sensing measurement The gas-sensing property was examined using a homemade instru ment as shown in the Supplementary Fig. S1 and Fig. S2. Briefly, the gassensing samples were ground with water and the obtained slurry prod ucts were prepared on alumina substrate. The substrates consisted of two gold electrodes on the top surface and a Ru2O3 microheater on the back side of the substrate. The sensor is housed in a 16 L container with a fan. Different concentrations of ethanol gas are provided by injecting an accurate amount of rapidly evaporating ethanol (liquid). The gas response was defined as Ra/Rg, where Ra and Rg are the resistance of the sensor when exposed to air and the target gas, respectively. The response 2
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Fig. 3. Schematic process of creating oxygen vacancies in SnO2.
Fig. 4. a) XPS survey spectra and high-resolution spectra of the O 1s of SnO2. b) S-0, c) S-1, d) S-2.
spectra (Fig. 4b-d). The O 1s peak was coherently fitted with three peaks located at ~530.5, 531.9 and 533.0 eV corresponding to lattice oxygen (OL), oxygen vacancy (OV), and chemisorbed oxygen (OC) species, respectively [32,33]. The binding energy value and the peak area ratio of each peak in each sample are summarized in Table 1. The relative percentages of OL, OV, and OC components in the pristine SnO2 were about 79.2%, 10.8% and 10.0%, respectively. After NaBH4 treatment, the ratio of OL, OV, and OC components in S-2 were about 67.4%, 25.4% and 7.2%, respectively. It was obviously that the ratio of oxygen va cancies of SnO2 was increased significantly after treatment with NaBH4. Due to spin-orbit splitting, the Sn 3d consists of doublet peaks assigned to Sn 3d5/2 and Sn 3d3/2. In pristine SnO2, the peaks obtained were at 486.5 and 494.9 eV, which were representative signature of
Table 1 XPS characteristics in the O 1s of the SnO2. Sample
Peak name
Peak position (eV)
Area (%)
S-0
OL OV OC OL OV OC OL OV OC
530.5 531.9 533.0 530.4 531.8 532.7 530.3 531.9 533.3
79.2 10.8 10 73.6 16.7 9.7 67.4 25.4 7.2
S-1 S-2
3
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Fig. 5. XPS spectra of Sn 3d signal. a) S-0, b) S-2.
Fig. 6. High-resolution XPS spectra of the Sn 3d.
Fig. 8. EPR spectra.
(Supplementary Fig. S4) were detected in the partially reduced SnO2. At the same time, NaBH4 was replaced with Na2B4O7⋅10H2O, H3BO3 and Na2SnO3⋅3H2O with all the other conditions the same, respectively, and the gas-sensing properties of the treated SnO2 were not significantly changed, which further excluded the influence of B and Na element on gas sensitivity. Apart from XPS spectra, the PL spectroscopy was carried out to o detect oxygen vacancies in the SnO2 samples. Fig. 7 shows the PL spectra at room temperature excited by a 325 nm laser of the SnO2 samples. It is observed that three peaks located at ~595, 625 and 780 nm, corre sponding to the vacancies of SnO2 [38–40]. According to literature [41], the more the defects are, the stronger the PL signal is. Hence, the peak intensity and area of partially reduced SnO2 is far larger than that of pristine SnO2, which means that rich oxygen vacancies generated in the reduction process. To further investigate the oxygen vacancies, electron paramagnetic resonance (EPR) was performed at room temperature. As shown in Fig. 8, in comparison with the pristine SnO2, the partially reduced SnO2 exhibits a much stronger signal at g ¼ 2.003 (25 � C), which is identified as the presence of single-electron oxygen vacancies [27,42]. This result indicated more oxygen vacancies in partially reduced SnO2, which was consistent with the XPS and PL spectra analysis. In order to obtain the surface area of all samples, N2 adsorption and desorption isotherms were carried and shown in Fig. 9. A mixing of type II and IV isotherms is found [43], indicating that there is a mesoporous and nonporous or microporous structure existing in the samples. The
Fig. 7. Room temperature photoluminescence spectrum of the SnO2 samples.
Sn4þ in tetragonal SnO2 [28,34]. However, it consisted of two pairs of Sn 3d5/2 and Sn 3d3/2 peaks for the sample S-2 (Fig. 5). There are two small peaks at low binding energy 486.0 and 494.5 eV. Because the chemical shift between the charge states of Sn2þ and Sn4þ is ~0.5 eV [35,36]. Therefore, the peaks located at high binding energies were assigned to Sn4þ, and located at low binding energies were assigned to Sn2þ. The content of the Sn2þ component was increased to almost 21.0% of the total Sn 3d peak in the sample S-2. In addition, a negative shift of Sn 3d was observed on partially reduced SnO2 (Fig. 6), which can be ascribed to that Sn2þ was produced from the generation of oxygen vacancies [37]. In addition, no peaks related to B (Supplementary Fig. S3) and Na 4
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Fig. 9. a) N2 adsorption/desorption isotherm, b) BJH pore-size distribution, c) SSA and mean pore diameter of SnO2.
Fig. 10. a) response of the sensors to 300 ppm ethanol at different operating temperature; b) base resistance in air of the sensors at different operating temperature; c) concentration dependent response curves of S-0, S-1 and S-2 at 190 � C.
specific surface area of pristine SnO2 nanoparticles is 6.19 m2 g 1. After NaBH4 reduction treatment, partially reduced SnO2 show similar spe cific surface area of 6.81 m2 g 1, indicating the morphology of the SnO2 was preserved. In addition, no significant change in pore size was observed.
ethanol testing gas at operating temperature from 120 to 240 � C. It is well known that the sensor operating temperature is one of the impor tant factors determining the gas sensitivity of materials. It can be clearly seen that the response of all samples showed an “increase-maximumdecrease” characteristic. This phenomenon can be explained as follows: At low temperature, the relatively low response is mainly due to the small amount of oxygen adsorbed on the surface of the material. As the temperature increases, the reaction kinetics of chemisorbed oxygen on the surface of the sensing material becomes faster, and the target gas molecules will acquire sufficient energy to overcome the activation energy barrier to react with the surface adsorbing oxygen atoms. At the optimum operating temperature, the gas adsorption and desorption processes reach equilibrium, resulting in maximum response. However, when the operating temperature continues to increase, the gas
3.2. Sensor performance Gas sensors were fabricated by coating the prepared SnO2 slurry on a commercial alumina substrate and aged 24 h at 150 � C to improve the stability of the sensors. The heating temperature is abstract, which is varies with the heating voltage and Fig. S5 shows the relationship be tween heating voltage and heating temperature. Fig. 10a shows the response of the sensors as a function of temperature exposed to 300 ppm 5
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Fig. 14. Long-term stability of the sensor based on S-2 at 190 � C. Fig. 11. Dynamic resistance curves of the SnO2 sensors to different ethanol concentration at 190 � C.
Fig. 15. Responses of the sensor based on S-2 exposed to 300 ppm different target gases at 190 � C.
Fig. 12. The dynamic response of sensor based on S-2 to 300 ppm ethanol at 190 � C.
based on S-0-S-2 to ethanol concentration of 20–1000 ppm at 190 � C. The response of S-2 based sensor was about 37.2, 55.1, 64.7, 120.3, 168.2 and 205.1, while S-0 based sensor was only about 8.1, 23.3, 27.2, 36.9, 46.3 and 53.5 to 20, 100, 200, 300, 500 and 1000 ppm ethanol, respectively. Fig. 11 showed transient resistance plots of the SnO2 sen sors to ethanol with different concentrations at 190 � C. It can be observed that the response of all the sensors increased with ethanol concentration. Furthermore, the response of sensor S-2 to ethanol was significantly higher than S-0. The response time (Tres) and recovery time (Trec) of sensor based on
desorption rate increases higher than the adsorption rate, resulting in a decrease in response [44]. The maximum gas responses of S-0-S-2 to 300 ppm ethanol was 36.8, 70.3, and 120.3, respectively. Obviously, the gas sensitivity of the materials treated by NaBH4 is higher than that of pristine. It could be concluded that as the oxygen vacancies increase, the gas sensitivity of the pristine SnO2 could be effectively improved. Fig. 10b showed the base resistance of the SnO2 sensors in air. As the oxygen vacancies increase, the electrical resistance of the SnO2 sensors in air significantly increased. Fig. 10c displayed the response of a sensor
Fig. 13. a) Response time and b) recovery time of S-0, S-1 and S-2 to 20–1000 ppm ethanol at 190 � C. 6
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Fig. 16. A schematic diagram of the sensing mechanism of the SnO2 based sensor to ethanol: a) in air, b) in ethanol.
S-2 to 300 ppm ethanol at 190 � C were evaluated from the dynamic sensing curves. As shown in Fig. 12, the response and recovery times for 300 ppm ethanol were about 63 s and 20 s, respectively. As shown in the inset of Fig. 12, the three cycles of the response curve indicated the stable and repeatable characteristics of sensor S-2. In Fig. 13a and b, it could be observed that the response and recovery speeds of the partially reduced SnO2 were fast compared with the pristine SnO2 in the same ethanol concentration from 20 ppm to 1000 ppm, respectively. Fig. 14 showed the long-term stability of the sensor based on SnO2. As shown in the curve, the response of S-2 remains around 110, and the response of S0 is only around 25. The sensing selectivity of S-0-S-2 to ethanol was also studied in this work. As shown in Fig. 15, the selectivity was examined by exposing sensors under 300 ppm different test gases at the optimal operating temperature (190 � C), such as methanol, CO gas and H2 gas. It could be seen that the sensor based on S-2 showed the highest sensitivity to ethanol (120), higher than that of methanol (55.5) and much higher than that of H2 (4.3) and CO (1.3). Obviously, the sensor based on S-2 showed enhanced selectivity toward ethanol opposed to the detected other gases at the same concentrations.
Compared with the ethanol-sensing performances of pristine SnO2, the enhancement of the ethanol-sensing behaviour of partially reduced SnO2 may be attributed to the oxygen vacancies. According to the OV model, oxygen vacancy promotes the adsorption of oxygen molecules to form more active sites and the number of adsorbed oxygen molecules and target gas molecules increases with increasing oxygen vacancy concentration [24,48–50]. Hence, oxygen vacancy enhances the elec trons transfer from the SnO2 surface to the O2 molecules which results more negative oxygen ions. This will further facilitate the reaction with ethanol molecules. Based on the above analysis and discussion, we suggest that oxygen vacancy could greatly promote the gas sensing properties of SnO2. 4. Conclusions We have synthesized SnO2 of high oxygen vacancy concentration by NaBH4 treatment at room temperature. In addition, the crystal structure, morphology and specific surface area of oxides did not change. Mean while, the SnO2 with a partially reduced surface exhibited enhanced gassensing properties. After NaBH4 treatment, the response of SnO2 to 300 ppm ethanol increased from 36.9 to 120.3. NaBH4 treatment increased surface oxygen vacancy, which promoting oxygen adsorption and chemical reaction on the surface.
3.3. Sensing mechanisms It is well known that the sensing mechanism of MOS gas sensors is mainly based on gas adsorption and chemical reactions on the surface of sensing materials [21,45,46]. When the sensors based on SnO2 are exposed to air, oxygen molecules adsorbed on the SnO2 surface and trapped electrons from the conduction band to form oxygen ions (O2 , O , and O2 ). As a result, an electron depletion layer is formed in its surface region, which decreases concentration of electrons and obtains a relatively high resistance. When the sensor is exposed to the reducing gases (such as ethanol in this case), the ethanol molecules react with the adsorbed oxygen irons, releasing the trapped electrons back into the conduction band and causing the decrease of resistance. In this work, the sensing schematic presentation for SnO2 towards ethanol is given in Fig. 16. The possible reaction processes for SnO2 response to ethanol can be described by the following equations [47]:
Declaration of competing interest 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. CRediT authorship contribution statement Qinghao Zeng: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Yanfa Cui: Formal analysis. Lianfeng Zhu: Supervision, Writing - review & editing. Youwei Yao: Formal analysis, Supervision, Validation, Writing - review & editing. Acknowledgements
O2 (gas) ↔ O2 (ads)
(1)
O2 (ads) þ e ↔ O2 (ads)
(2)
O2 (ads) þ e ↔ 2O (ads)
(3)
This work was supported by the Special Development Program of Strategic Emerging Industries of Shenzhen city (JSGG20160331104855391).
↔ O2 (ads)
(4)
Appendix A. Supplementary data
O (ads) þ e
C2H5OH (gas) þ 6O (ads) → 2CO2 (gas) þ 3H2O (gas) þ 6e C2H5OH (gas) þ 6O
2
(ads) → 2CO2 (gas) þ 3H2O (gas) þ 12e
(5)
Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.104962.
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