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Ag decorated SnO2 nanoparticles to enhance formaldehyde sensing properties
Journal of Physics and Chemistry of Solids 124 (2019) 36–43
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Ag decorated SnO2 nanoparticles to enhance formaldehyde sensing properties
T
Dan Liua, Junli Pana,b, Jianghong Tanga,∗, Weiqiao Liua, Shouli Baib, Ruixian Luob,∗∗ a b
School of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou, 213001, China Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing, 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 Ag decoration Formaldehyde sensor
Silver, as a cheaper and good conductivity noble metal, has successfully been decorated on SnO2 nanoparticles to structure the Ag-SnO2 composites by a facile hydrothermal and in situ reduction method. The difference of structure and properties between SnO2 and composite was characterized by XRD, FESEM, elemental mapping, UV–vis spectroscopy, BET and XPS surface analysis. The sensing performance of SnO2 and composite to formaldehyde were also examined. The composite of 3.7 wt% Ag decorated SnO2 not only exhibits the high response to 10 ppm of formaldehyde but also has better stability and selectivity at an operating temperature of 125 °C. Moreover, the detection limit of the composite was determined to be 0.53 ppm, which is lower than that of pure SnO2 nanoparticles. The response mechanism of the composite to formaldehyde was also discussed in detail, which is attributed to the catalytic effect and the spillover effect of Ag nanoparticles.
1. Introduction
of formaldehyde [3]. One drawback of the fabricated sensor is the high operating temperature and poor selectivity [7] Subsequently, many strategies to try for improvement of sensing performance of SnO2 to HCHO including heterojunction construction, metal-element doping, noble metal loading and oxygen defects creation. For example, Lin et al. first reported the SnO2 sensing material with high sensitivity to formaldehyde by hydrothermal treatment and the enhanced sensing performance was ascribed to the coral-like morphology and the quick passages for gas adsorption [12]. Yali Cao et al. synthesized SnO2–graphene nanocomposite and the sensitivity to formaldehyde reached 35 towards 100 ppm formaldehyde at 260 °C [13]. Lin and Wei et al. prepared the hollow SnO2 nanofibers decorated with Pd nanoparticle and showed the response of 18–100 ppm of formaldehyde at 160 °C [4]. Among them, Ag is the best conductive and the cheapest noble metal, it as an effective catalyst to promote oxide sensing performance and does not damage the structure of oxide. So, it is an ideal additive for enhancing response or lowering the working temperature of the sensing material [14]. Wang and Xiao et al. reported the response be increasing to 12.3 compared to that of pure In2O3 after Ag-loaded on sunflower-like In2O3 to 20 ppm HCHO, and lowered the operating temperature from 240 °C to 200 °C after Ag decorated [15]. Chen et al. synthesized hierarchically porous Sn-Rh codoped ZnO nanocomposite via a simple hydrothermal reaction at 180 °C and found that the codoped sample exhibited a lower detection limit of 5 ppm, ultrashort
Formaldehyde (HCHO) is the most extensively applied chemicals in the many industrial manufactures and daily consumer products [1,2]. As we all know, formaldehyde, is a kind of volatile organic compounds (VOCs) that generates from building material, plastics, cosmetics, carpet etc.. The formaldehyde threatens human health, such as bronchial asthma, atopic dermatitis, and sick building syndrome [3–6]. Although some detecting approaches for formaldehyde have been reported or utilized, including colorimetric approach, electrochemical approach, and optical methods, etc., these methods require expensive instruments and well-trained operators [7]. Thus, a portable and easier operating gas sensor is highly expected to detect trace formaldehyde online. Solid state resistive-type metal oxide gas sensors have been widely used in air quality monitoring and health safety of human. The sensors are attractive because of their high sensitivity, structure simplicity, low cost, and compatibility with microelectronic processing. SnO2, as an n-type semiconductor of the wide band gap, has widely been used as a gas sensing material to monitor toxic oxidizing and reducing gases due to its interior non-stoichiometry, good thermal and chemical stability as well as sensing response to most volatile organic compounds (VOCs) [8–11]. Hai Yu et al. fabricated SnO2 nanosheets based sensor and reported the sensing response being 7 under the experimental conditions of operating temperature of 240 °C and 100 ppm ∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Tang), [email protected] (R. Luo).
∗∗
https://doi.org/10.1016/j.jpcs.2018.08.028 Received 25 April 2018; Received in revised form 30 July 2018; Accepted 22 August 2018 Available online 23 August 2018 0022-3697/ © 2018 Published by Elsevier Ltd.
Journal of Physics and Chemistry of Solids 124 (2019) 36–43
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2, 3 and 4.
Table 1 Sensing response of SnO2 and their composites to formaldehyde reported in literatures and this work. materials
SnO2 Zn-doped SnO2 SnO2–graphene Pd decorated hollow SnO2 Ag-loaded sunflower-like In2O3 Ag coated ZnO–SnO2 Ag decorated SnO2
formaldehyde concentration (ppm)
operating temperature (°C)
response
100 100 100 100
240 160 260 160
7 15.2 35 18
[3] [12] [13] [4]
20
200
12.3
[15]
5
210
21.36
[16]
10
125
14.4
this work
2.3. Characterization
Ref.
Crystalline structure and phase composition of the materials were analyzed using XRD diffractometer (Rigaku D/MAX-2500) at 45 kV and 40 mA with Ni-filtered Cu Kα radiation (λ = 0.154 nm), 2θ range from 10° to 80° and with a scanning speed of 10° min−1. The morphology and element distribution of the sample was observed by field emission scanning electron microscopy (FESEM; Hitachi S-4700, Japan on 20.0 kV) attached an energy dispersive X-ray spectrometer (EDS). The ultraviolet-visible (UV-vis) absorption spectra (Shimadzu UV-2550 type UV–Vis spectrophotometer) were recorded at room temperature in the range from 245 to 700 nm. The X-ray photoelectron spectrum (XPS) surface analysis was recorded on an X-ray photoelectron spectrometer (VG ESCALAB-MK) using Al Ka X-ray as the excitation source (1486.6 eV). The specific surface of the sample was measured using Brunauer-Emmett-Teller (BET) N2 adsorption/desorption method, pore volume and pore size distribution were also determined using BarrettJoyner-Halenda (BJH) formula on a Micromeritics Surface Area & Porosity Gemini VII 2390 system.
response and recovery time to 100 ppm ethanol at 300 °C compared with that of pure ZnO and Sn-doped ZnO [16]. In this work, the Ag as a decoration reagent has been successfully loaded on SnO2 nanoparticles by AgNO3 reduction under mild conditions and obtained the expected effect for improving sensing properties of SnO2 to formaldehyde. For comparison, the response data of gas sensing materials reported in the literature and in the work are listed in Table 1.
2.4. Fabrication of sensor and measurements of sensing properties Certain amounts of sample were dissolved into ethanol to form a paste. The paste was coated on the alumina ceramic tube with Au electrodes and a Ni–Cr heating wire to fabricate the sensor element. The fabricated sensors were aged at 150 °C for 2 days before the gas sensing measurement in order to improve the structural stability of the sensor. The sensing responses of the sensor to formaldehyde were measured using a JF02E gas sensor test system. The resistance (Ra) of the sensor in the air and the resistance of the sensor in a mixture of air and test gas (Rg) were examined, a sudden decrease in the resistance was exhibited when the Ag decorated SnO2 based sensor was exposed to formaldehyde, which indicates that the composite exhibits n-type semiconductor behaviors. Therefore, the response of n-type semiconductor to reducing gas of formaldehyde is defined as the ratio of Ra/Rg. Fig. 1 shows the measurement system of sensor resistance. The test loop includes three parts: load resistance (Rreference), measured voltage (Voutput) and sensor resistance (Rsensor). The sensor resistance can be calculated by Eq. (1):
2. Experiment 2.1. Synthesis of SnO2 nanoparticles All chemical reagents are analytical-pure and used as received. Deionized water was used throughout the experiments. The SnO2 nanoparticles were prepared by hydrothermal treatment followed by calcination as reported in the literature [16,17]. Typically, 7.01 g of SnCl4·5H2O and 7.2 g of glucose were dissolved in 100 mL of distilled water under strong stirring. Then transferred the solution to 100 mL of the Teflon-lined autoclave and heated to 200 °C. After reaction for 6 h, the autoclave was naturally cool to room temperature the obtained white production was centrifuged and washed for several times with water and alcohol, and then dried at 60 °C overnight. Finally, the white product was annealed at 450 °C in air for 2 h with a heating rate of 5 °C min.−1.
Rsensor = (Vworking − Voutput )/ Voutput⋅Rreference 2.2. Synthesis of Ag-SnO2 composite
(1)
The appropriate load resistance was selected in terms of the sensor resistance value, and the working temperature of the sensor should be set by adjusting the voltage of the heating system.
0.159 g of SnO2 was dissolved in 20 mL of deionized water, and 0.0050 g of AgNO3 and 0.6 mL of sodium borohydride (NaBH4) solution (12 mM) were added and sufficiently mixed for 1 h. Finally, the precipitate was filtrated by centrifugation and dried at 60 °C overnight. The obtained Ag-SnO2 composite with 1 wt% Ag was named as sample 1. A series of sample 2–4 were synthesized in the same method by added different AgNO3 and NaBH4 amounts. The added amounts of AgNO3 and NaBH4 solution (12 mM) are 0.0200 g and 1.2 mL, 0.0400 g and 2.4 mL, and 0.0800 g and 4.8 mL respectively corresponding to sample
3. Results and discussion 3.1. Structure and morphology The FESEM images with elemental mapping for pure SnO2 and 3.8 %Ag-SnO2 composites are shown in Fig. 2. As shown in Fig. 2a, a
Fig. 1. Schematic diagram of preparation process for Ag decorated SnO2 nanoparticles. 37
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Fig. 2. SEM images of (a) SnO2, (b) 3.7 %Ag-SnO2, (c–f) elemental mapping of the 3.7 %Ag-SnO2.
Fig. 3. The EDS spectra of samples 2–5.
observed, indicating the Ag nanoparticles (NPs) have successfully decorated on the surface of SnO2. The crystalline structures of the pure SnO2 and 3.8 %Ag-SnO2 sample were characterized by XRD analysis. It can be seen from the XRD pattern of Fig. 5a that all the diffraction peaks of pure SnO2 can be associated to the tetragonal structure with lattice constants of a = b = 4.738 Å and c = 3.187 Å (JCPDS card of 41–1445) [18–21]. The mean crystallite size of SnO2 was estimated to be 20.50 nm that is corresponding to the three principal planes of 19.69(110), 18.46(101), and 23.3 (211) nm by using the Debye–Scherrer formula of d = 0.9λ/ βcosθ, where d, λ, β and θ respectively show crystallite size, X-ray wavelength, full width at half max (FWHM) and Bragg angle [19,22]. The surface area (S) was estimated to be 16.32 m2/g by using the formula S = 6/σρRX, where σ is the particle size and ρRX the X-ray density [23]. From the diffraction pattern of 3.8%Ag-SnO2 sample, the (200) facet of Ag crystalline phases (JCPDS card No. 04–0783), corresponding to 44.3° (2θ), was observed. Comparing pattern of Fig. 5a with b, it is found that peak intensity centered at 37.9°(200) and 64.7°(112) of SnO2
number of nanoparticles with a few nanometers were clearly observed. Fig. 2b displays the morphology of 3.8 %Ag-SnO2 composites. To further confirm the existence of Ag, Sn, and O elements on the composite surface, the elemental mapping was carried out as shown in Fig. 2c–f and Fig 3 shows the EDS spectra. The main peaks can be assigned to O, Sn and Ag for Ag-SnO2 sample with weight ratios of Ag to Ag-SnO2 as 0.81, 1.51, 3.66 and 5.73, respectively, which shows that the practical Ag amounts coated on composite are less than that of added amounts in synthesized process. The composition of the composite with an optimum weight ratio (sample 3) was further determined by ICP-MS to be 4.1 wt%, which is much closer to EDS results. In order to obtain detailed structural information of composite, HRTEM observation was carried out on the Ag-coated SnO2. From Fig. 4 it can be seen that the sample are consist of abundant particles. The lattice fringes of d = 0.24 and 0.33 nm respectively well corresponding to the SnO2 crystalline planes of (110) and (110). From the HR-TEM image of the 3.8% Ag-SnO2 composite, cubic Ag with an interplanar distance of d = 0.20 nm well matching with the (200) plane can also be clearly 38
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which can be attributed to the introducing of impurity level in the bandgap of the composite, leading to the reduction of electron transition energy. The specific surface area is regarded to be an important influencing factor to gas sensing properties because sensing response of semiconductor metal oxide based sensor results from gas surface adsorption and surface reaction. The nitrogen adsorption-desorption isotherms were measured to investigate the specific surface area. The BET specific surface area of the 3.7 wt%Ag-SnO2 composite was calculated to be 24.48 m2 g−1 from nitrogen adsorption-desorption isotherms as shown in Fig. 7. Fig. 7 shows a type IV isotherm with type H3 hysteresis loops and indicates the presence of the mesoporous structure, which causes more chance to adsorb oxygen and test gases, resulting in a fast response and recovery kinetics as well as a high sensitivity. To further confirm the composition of the sample and chemical state of the element, XPS measurement for 3.7 wt%Ag-SnO2 was performed, and Fig. 8 shows the experimental results. In the XPS full survey spectrum of 3.7%Ag-SnO2 (Fig. 8a), the peaks of Sn, Ag, O, and C elements can be observed clearly. For the Sn 3d spectrum, two strong peaks at around 486.4 eV and 494.9 eV (Fig. 8b) are assigned to Sn3d5/2 and Sn 3d3/2. Compared with pure SnO2 (Sn3d5/2 at 486.9 eV and Sn 3d3/2 at 495.4 eV), the Sn 3d peaks shift to lower energy, which indicates the interaction and electron transformation may exist between the SnO2 and Ag [4,18,25]. Fig. 8c displays the high-resolution Ag 3d XPS spectrum. Ag 3d5/2 and Ag 3d3/2 peaks are observed at 367.6 and 373.5 eV, respectively. Furthermore, the peak at 367.6 eV is characteristic of the existence of Ag0 in the Ag-SnO2 composite, which is agreement with the results of XRD and HR-TEM analysis. The O 1s includes chemical states of three components, that is, the oxygen ions of Olatt, Ox− and OOH. It is verified that the crystal lattice oxygen ion of Olatt located at 530 eV has no contribution to the gas sensing response, while the adsorbed oxygen ion of Ox− located at 531 eV has great influence on gas sensing response. OOH centered at 532 eV is assigned to the hydroxyl ions on the surface of the oxide [26,27]. It should be noted that the content of Ox-in Ag-SnO2 composite is much more than that of SnO2, which is responsible for enhancing gas sensing properties of AgSnO2 composites due to Ag decoration on SnO2 nanoparticles [27,28].
Fig. 4. HR-TEM image of 3.7% Ag-SnO2 sample.
3.2. Gas sensing performance Fig. 9a shows the transient response of different sensors exposed to 10 ppm of HCHO at the working temperature of 125 °C. It is obvious that the 3.7%Ag-SnO2 based sensor has the highest response of 14.4, which is 7 times higher than that of pristine SnO2 nanoparticles. The linear tendency of response is observed from Fig. 9 (b) for both SnO2 and Ag-SnO2 composite, the straight line slope of Ag-SnO2 composite is larger than that of SnO2, which indicates that the sensitivity of the composite-based sensor is higher than that of SnO2 based sensor. The detection limit (DL) is one of the important indexes reflecting sensitivity, and a high sensitivity usually has a lower detection limit. The detection limit (DL) is defined as the lowest concentration in which the response generally is significantly different from the 3 times standard deviation of the noise. The measuring relative response of the sensor in the baseline can be used to calculate the sensor noise. On the other hand, the DL can be estimated by following Eq. (2), where N is the number taken data point, the standard deviation (S) was calculated to be 0.733 by the root-mean-square deviation (RMSD) in terms of average value of 10 consecutive data [29,30].
Fig. 5. XRD patterns of (a) SnO2, and (b) 3.7% Ag-SnO2 samples.
increases after loading Ag, which may results from the diffraction peaks of Ag (111) plane at 2θ = 38.1° and the Ag (220) plane at 64.4ºof Ag overlapped the diffraction peaks of SnO2. These results indicate the successful preparation of Ag-SnO2 composite. To further investigate the change of structure for decorated Ag sample, the UV-Vis absorption spectra of pure SnO2 and the 3.8%AgSnO2 composite are shown in Fig. 6. The pure SnO2 sample exhibits an absorption band at ∼260 nm in the UV region. However, the absorption band of Ag-SnO2 composite is red-shifted to 280 nm compared with pristine SnO2, which can be attributed to the charge transition between silver and SnO2, leading to the enhancement of gas sensing performance [24]. The indirect band gap (Eg) of pure SnO2 and the 3.8%Ag-SnO2 composite can be estimated by UV–vis absorption spectra using simplified formula of αhυ = A (hυ − Eg)2, where, α, hυ, Eg and A respectively represent the absorption coefficient, photon energy with frequency υ, band gap and constant. The band gaps of SnO2 and AgSnO2 were found to be 3.64 and 3.19 eV, respectively. The result indicates that the band gap of SnO2 is reduced due to Ag decoration,
RMSnoise =
S2 N
(2)
From the above Eq. (2) the calculated value of RMSnoise is 0.22. According to the definition of detection limit (DL is 3 times standard deviation of noise) and the slope of 1.233 from Fig. 9b, so that: 39
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Fig. 6. (a) UV-Vis absorption spectra of SnO2 and 3.7%Ag-SnO2 composite, and (b) the plot of (αhν)2 versus (hν) of the SnO2 and 3.7%Ag-SnO2.
good selectivity makes the sensor can exclusively a response to a target gas. Herein, the responses of the 3.7%Ag-SnO2 composite to other gases such as 10 ppm of CO, ether, toluene, methanol, ethanol, and acetone were examined at an optimum temperature of 125 °C and the experiment results are shown in Fig. 10b. The results show that the 3.7%AgSnO2 composites based sensor exhibits the highest selectivity to formaldehyde. We know, the selectivity of metal oxide sensor is affected by temperature, adsorption amount of gas on the surface of material and lowest unoccupied molecule orbit (LUMO) energy of gas molecule, because the gas-sensing performance of semiconductor metal oxide depends on gas surface adsorption and reaction between adsorbed oxygen ion and test gas on the surface of material. If the LUMO energy of a gas molecule is lower, gas sensing reaction requires less energy, leading to the enhancement of gas sensing response and reducing of the sensor operating temperature. Moreover, the adsorption states of gas molecule also influence the selectivity of the sensor, because the adsorption states of gas molecules are affected by the orbital energies of the gas molecule, which will result to different electron affinity, thus results to the different resistances of the sensor after gas adsorption. To further clarify the effect of gas molecule orbital energy on adsorption states of gas molecule needs through quantum chemistry calculation [33]. The stability of the sensor is extremely important for the practical gas detection. In the test process, the response of sensor maintains around 14 after 5 test cycles for 35 days, which indicates the sensor has rather good stability as shown in Fig. 10c. So, the Ag decorated SnO2 composite is recognized to be a promising sensing material for efficient and selective detects to formaldehyde at a lower operating temperature.
Fig. 7. Nitrogen adsorption–desorption isotherm for 3.7%Ag-SnO2 composite.
DL = 3
RMSnoise = 3∗0.31/1.233 = 0.533 ppm Slope
(3)
In a similar way, the DL of SnO2 was calculated to be 1.16 (the slope is 0.57112 in Fig. 9b). So, the Ag surface decoration can also reduce the detection limit of SnO2. Fig. 10a displays the effect of Ag contents and operating temperature on sensing response for pure SnO2 and composites in 10 ppm formaldehyde. The responses are 14.4 and 3.2 for 3.7%Ag-SnO2 and pure SnO2, respectively at the optimum operating temperature of 125 °C and 150 °C. The results also indicate that the sensing response firstly increases and then decreases with the increase in operating temperature from 80 to 180 °C. Because the gas sensing response of semiconductor metal oxide is related to the adsorption of oxygen molecules in air and reaction between the oxygen species and the target gas on the surface of the material. The enhancing operating temperature is to provide energy to overcome the activation energies needed gas chemisorption and surface reaction, so, the response increases with the operating temperature at a temperature range from 80 to 125 °C for 3.7 wt% Ag-SnO2 sample. However, once the operating temperature over 125 °C the response decreases with the temperature increase. The temperature (125 °C) is called the optimum operating temperature because at which the gas adsorption on the surface of material reaches saturation, leading to the highest response. With the temperature further increases the desorbed gas amounts will increase, thus the response decreases [31,32]. The selectivity shows the anti-interference ability of a sensor, the
3.3. Mechanism of gas sensing The sensing response of the semiconducting metal oxide based sensor depends on the change in resistance, which results from the oxygen molecules adsorption and the reaction with test gas on the surface of the material [33–35]. When the SnO2 was exposed to air, the oxygen molecules in the air will adsorb on the surface of SnO2 and capture electrons from the oxide to generate oxygen species (O2−). As a result, an electron depletion layer at the surface of oxide is formed where the electron concentration is lower than that of the SnO2 bulk, leading to the increase of the sensor resistance. Once the sensor was exposed to HCHO, the gas molecules are oxidized by the oxygen species adsorbed on oxide surface and the released electrons back to the conduction band of SnO2. The result causes the decrease of sensor resistance, i.e. decrease of electron depletion layer thickness, thus enhanced the sensor response [36–40]. When Ag nanoparticles were decorated on the SnO2 surface, spillover and catalytic effects are generally considered [19,25,26,41]. Ag nanoparticles provide more active sites for adsorption of oxygen 40
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Fig. 8. X-ray photoelectron spectra of 3.7%Ag-SnO2: (a) survey spectrum; (b) Sn 3d; (c) O1s; (d) Ag 3d.
4. Conclusions
molecules and detect gas, and as a catalyst will promote the dissociation of adsorbed oxygen molecules to create more oxygen species of O2−, O−, or O2− [42]. Then transport and distribute these oxygen species to the surface of SnO2 due to the spillover effect of noble metals. The result above forms a deeper depletion layer with a higher resistance than pure SnO2 [4]. When the composite was exposed to formaldehyde, the gas molecules are also firstly adsorbed on the supported Ag nanoparticles and are activated, the Ag nanoparticles play the role of active sites for the oxidizing reaction between HCHO and oxygen ions and then the Ag particles will transport the electrons released by oxidizing reaction to the SnO2 conduction band, leading to reducing of depletion layer thickness, i.e. the resistance decreasing of sensor in formaldehyde [41–43]. Hence, the sensor response is significantly enhanced according to the response definition of Rair/Rgas in reducing gas of HCHO. The schematic diagram of the gas sensing mechanism is shown in Fig. 11.
In summary, Ag decorated SnO2 composites were successfully prepared by in-situ reduction of AgNO3 on the surface of SnO2. The response of the 3.7 wt% Ag decorated SnO2 nanoparticles achieves nearly 7 times higher than that of pristine SnO2 to 10 ppm formaldehyde at operating temperature of 125 °C and has good selectivity to formaldehyde The enhancement of sensing properties is ascribed to the spillover and catalytic effects of decorated Ag nanoparticles. The results prove that the Ag decorated SnO2 composite is a promising sensing material to detect trace harmful VOCs over other frequently encountered in indoor environments.
Acknowledgments This work was supported by the Science Foundation of Jiangsu University of Technology (KYY16024) and the National Natural Science Foundation of China (Grant No. 51772015).
Fig. 9. (a) Response of Ag decorated SnO2 samples to 10 ppm formaldehyde at 125 °C; (b) Fit curve relationship between the sensor response and formaldehyde concentration. 41
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Fig. 10. (a) Responses of pure SnO2 and Ag decorated SnO2 samples at different operating temperature to 10 ppm HCHO; (b) Response of sensor based on 3.7%AgSnO2 composite to different test gases at optimum operating temperature; (c) Stability of the sensor based on 3.7% Ag-SnO2 to 10 ppm HCHO at 125 °C.
Fig. 11. Schematic diagrams of the sensing mechanism for SnO2 and Ag-SnO2 based sensors to HCHO: (a) oxygen molecules in air trap electrons into chemisorbed oxygen species on the surface of sensors; (b) Reaction of HCHO with chemisorbed oxygen species on surface of sensors.
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[35] L.T. Hoa, D.C. Nguyen, T.T. Hoa, D.Q. Khieu, H.T. Long, D.T. Quang, et al., Synthesis, characterization, and comparative gas sensing properties of tin dioxide nanoflowers and porous nanospheres, Ceram. Int. 41 (2015) 14819–14825. [36] F. Qu, C. Feng, C. Li, W. Li, S. Wen, S. Ruan, et al., Preparation and xylene-sensing properties of Co3O4 nanofibers, Int. J. Appl. Ceram. Technol. 11 (2014) 619–625. [37] Q. Huang, D. Zeng, H. Li, C. Xie, Room temperature formaldehyde sensors with enhanced performance, fast response and recovery based on zinc oxide quantum dots/graphene nanocomposites, Nanoscale 4 (2012) 5651–5658. [38] X. Kuang, T. Liu, W. Wang, S. Hussain, X. Peng, Controlled synthesis of SnO2 hierarchical architectures made of ultrathin nanoflakes for enhanced ethanol gas sensing properties, Appl. Surf. Sci. 351 (2015) 1087–1093. [39] W. Zhang, W. Zeng, BinMiao, Z. Wang, Effect of the sheet thickness of hierarchical SnO2 on the gas sensing performance, Appl. Surf. Sci. 355 (2015) 631–637. [40] Y. Li, Q. Wei, P. Song, Q. Wang, Synthesis of porous SnO2 nanocubes via selective leaching and enhanced gas-sensing properties, Appl. Surf. Sci. (2015) (Ahead of Print). [41] C. Liu, Q. Kuang, Z. Xie, L. Zheng, The effect of noble metal (Au, Pd, and Pt) nanoparticles on the gas sensing performance of SnO2-based sensors: a case study on the {221} high-index faceted SnO2 octahedra, CrystEngComm 17 (2015) 6308–6313. [42] C. Suo, C. Gao, X. Wu, Y. Zuo, X. Wang, J. Jia, Ag-decorated ZnO nanorods prepared by photochemical deposition and their high selectivity to ethanol using conducting oxide electrodes, RSC Adv. 5 (2015) 92107–92113. [43] F. Wei, H. Zhang, M. Nguyen, M. Ying, R. Gao, Z. Jiao, Template-free synthesis of flower-like SnO2 hierarchical nanostructures with improved gas sensing performance, Sensor. Actuator. B Chem. 215 (2015) 15–23.
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Ag decorated SnO2 nanoparticles to enhance formaldehyde sensing properties
T
Dan Liua, Junli Pana,b, Jianghong Tanga,∗, Weiqiao Liua, Shouli Baib, Ruixian Luob,∗∗ a b
School of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou, 213001, China Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing, 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 Ag decoration Formaldehyde sensor
Silver, as a cheaper and good conductivity noble metal, has successfully been decorated on SnO2 nanoparticles to structure the Ag-SnO2 composites by a facile hydrothermal and in situ reduction method. The difference of structure and properties between SnO2 and composite was characterized by XRD, FESEM, elemental mapping, UV–vis spectroscopy, BET and XPS surface analysis. The sensing performance of SnO2 and composite to formaldehyde were also examined. The composite of 3.7 wt% Ag decorated SnO2 not only exhibits the high response to 10 ppm of formaldehyde but also has better stability and selectivity at an operating temperature of 125 °C. Moreover, the detection limit of the composite was determined to be 0.53 ppm, which is lower than that of pure SnO2 nanoparticles. The response mechanism of the composite to formaldehyde was also discussed in detail, which is attributed to the catalytic effect and the spillover effect of Ag nanoparticles.
1. Introduction
of formaldehyde [3]. One drawback of the fabricated sensor is the high operating temperature and poor selectivity [7] Subsequently, many strategies to try for improvement of sensing performance of SnO2 to HCHO including heterojunction construction, metal-element doping, noble metal loading and oxygen defects creation. For example, Lin et al. first reported the SnO2 sensing material with high sensitivity to formaldehyde by hydrothermal treatment and the enhanced sensing performance was ascribed to the coral-like morphology and the quick passages for gas adsorption [12]. Yali Cao et al. synthesized SnO2–graphene nanocomposite and the sensitivity to formaldehyde reached 35 towards 100 ppm formaldehyde at 260 °C [13]. Lin and Wei et al. prepared the hollow SnO2 nanofibers decorated with Pd nanoparticle and showed the response of 18–100 ppm of formaldehyde at 160 °C [4]. Among them, Ag is the best conductive and the cheapest noble metal, it as an effective catalyst to promote oxide sensing performance and does not damage the structure of oxide. So, it is an ideal additive for enhancing response or lowering the working temperature of the sensing material [14]. Wang and Xiao et al. reported the response be increasing to 12.3 compared to that of pure In2O3 after Ag-loaded on sunflower-like In2O3 to 20 ppm HCHO, and lowered the operating temperature from 240 °C to 200 °C after Ag decorated [15]. Chen et al. synthesized hierarchically porous Sn-Rh codoped ZnO nanocomposite via a simple hydrothermal reaction at 180 °C and found that the codoped sample exhibited a lower detection limit of 5 ppm, ultrashort
Formaldehyde (HCHO) is the most extensively applied chemicals in the many industrial manufactures and daily consumer products [1,2]. As we all know, formaldehyde, is a kind of volatile organic compounds (VOCs) that generates from building material, plastics, cosmetics, carpet etc.. The formaldehyde threatens human health, such as bronchial asthma, atopic dermatitis, and sick building syndrome [3–6]. Although some detecting approaches for formaldehyde have been reported or utilized, including colorimetric approach, electrochemical approach, and optical methods, etc., these methods require expensive instruments and well-trained operators [7]. Thus, a portable and easier operating gas sensor is highly expected to detect trace formaldehyde online. Solid state resistive-type metal oxide gas sensors have been widely used in air quality monitoring and health safety of human. The sensors are attractive because of their high sensitivity, structure simplicity, low cost, and compatibility with microelectronic processing. SnO2, as an n-type semiconductor of the wide band gap, has widely been used as a gas sensing material to monitor toxic oxidizing and reducing gases due to its interior non-stoichiometry, good thermal and chemical stability as well as sensing response to most volatile organic compounds (VOCs) [8–11]. Hai Yu et al. fabricated SnO2 nanosheets based sensor and reported the sensing response being 7 under the experimental conditions of operating temperature of 240 °C and 100 ppm ∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Tang), [email protected] (R. Luo).
∗∗
https://doi.org/10.1016/j.jpcs.2018.08.028 Received 25 April 2018; Received in revised form 30 July 2018; Accepted 22 August 2018 Available online 23 August 2018 0022-3697/ © 2018 Published by Elsevier Ltd.
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2, 3 and 4.
Table 1 Sensing response of SnO2 and their composites to formaldehyde reported in literatures and this work. materials
SnO2 Zn-doped SnO2 SnO2–graphene Pd decorated hollow SnO2 Ag-loaded sunflower-like In2O3 Ag coated ZnO–SnO2 Ag decorated SnO2
formaldehyde concentration (ppm)
operating temperature (°C)
response
100 100 100 100
240 160 260 160
7 15.2 35 18
[3] [12] [13] [4]
20
200
12.3
[15]
5
210
21.36
[16]
10
125
14.4
this work
2.3. Characterization
Ref.
Crystalline structure and phase composition of the materials were analyzed using XRD diffractometer (Rigaku D/MAX-2500) at 45 kV and 40 mA with Ni-filtered Cu Kα radiation (λ = 0.154 nm), 2θ range from 10° to 80° and with a scanning speed of 10° min−1. The morphology and element distribution of the sample was observed by field emission scanning electron microscopy (FESEM; Hitachi S-4700, Japan on 20.0 kV) attached an energy dispersive X-ray spectrometer (EDS). The ultraviolet-visible (UV-vis) absorption spectra (Shimadzu UV-2550 type UV–Vis spectrophotometer) were recorded at room temperature in the range from 245 to 700 nm. The X-ray photoelectron spectrum (XPS) surface analysis was recorded on an X-ray photoelectron spectrometer (VG ESCALAB-MK) using Al Ka X-ray as the excitation source (1486.6 eV). The specific surface of the sample was measured using Brunauer-Emmett-Teller (BET) N2 adsorption/desorption method, pore volume and pore size distribution were also determined using BarrettJoyner-Halenda (BJH) formula on a Micromeritics Surface Area & Porosity Gemini VII 2390 system.
response and recovery time to 100 ppm ethanol at 300 °C compared with that of pure ZnO and Sn-doped ZnO [16]. In this work, the Ag as a decoration reagent has been successfully loaded on SnO2 nanoparticles by AgNO3 reduction under mild conditions and obtained the expected effect for improving sensing properties of SnO2 to formaldehyde. For comparison, the response data of gas sensing materials reported in the literature and in the work are listed in Table 1.
2.4. Fabrication of sensor and measurements of sensing properties Certain amounts of sample were dissolved into ethanol to form a paste. The paste was coated on the alumina ceramic tube with Au electrodes and a Ni–Cr heating wire to fabricate the sensor element. The fabricated sensors were aged at 150 °C for 2 days before the gas sensing measurement in order to improve the structural stability of the sensor. The sensing responses of the sensor to formaldehyde were measured using a JF02E gas sensor test system. The resistance (Ra) of the sensor in the air and the resistance of the sensor in a mixture of air and test gas (Rg) were examined, a sudden decrease in the resistance was exhibited when the Ag decorated SnO2 based sensor was exposed to formaldehyde, which indicates that the composite exhibits n-type semiconductor behaviors. Therefore, the response of n-type semiconductor to reducing gas of formaldehyde is defined as the ratio of Ra/Rg. Fig. 1 shows the measurement system of sensor resistance. The test loop includes three parts: load resistance (Rreference), measured voltage (Voutput) and sensor resistance (Rsensor). The sensor resistance can be calculated by Eq. (1):
2. Experiment 2.1. Synthesis of SnO2 nanoparticles All chemical reagents are analytical-pure and used as received. Deionized water was used throughout the experiments. The SnO2 nanoparticles were prepared by hydrothermal treatment followed by calcination as reported in the literature [16,17]. Typically, 7.01 g of SnCl4·5H2O and 7.2 g of glucose were dissolved in 100 mL of distilled water under strong stirring. Then transferred the solution to 100 mL of the Teflon-lined autoclave and heated to 200 °C. After reaction for 6 h, the autoclave was naturally cool to room temperature the obtained white production was centrifuged and washed for several times with water and alcohol, and then dried at 60 °C overnight. Finally, the white product was annealed at 450 °C in air for 2 h with a heating rate of 5 °C min.−1.
Rsensor = (Vworking − Voutput )/ Voutput⋅Rreference 2.2. Synthesis of Ag-SnO2 composite
(1)
The appropriate load resistance was selected in terms of the sensor resistance value, and the working temperature of the sensor should be set by adjusting the voltage of the heating system.
0.159 g of SnO2 was dissolved in 20 mL of deionized water, and 0.0050 g of AgNO3 and 0.6 mL of sodium borohydride (NaBH4) solution (12 mM) were added and sufficiently mixed for 1 h. Finally, the precipitate was filtrated by centrifugation and dried at 60 °C overnight. The obtained Ag-SnO2 composite with 1 wt% Ag was named as sample 1. A series of sample 2–4 were synthesized in the same method by added different AgNO3 and NaBH4 amounts. The added amounts of AgNO3 and NaBH4 solution (12 mM) are 0.0200 g and 1.2 mL, 0.0400 g and 2.4 mL, and 0.0800 g and 4.8 mL respectively corresponding to sample
3. Results and discussion 3.1. Structure and morphology The FESEM images with elemental mapping for pure SnO2 and 3.8 %Ag-SnO2 composites are shown in Fig. 2. As shown in Fig. 2a, a
Fig. 1. Schematic diagram of preparation process for Ag decorated SnO2 nanoparticles. 37
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Fig. 2. SEM images of (a) SnO2, (b) 3.7 %Ag-SnO2, (c–f) elemental mapping of the 3.7 %Ag-SnO2.
Fig. 3. The EDS spectra of samples 2–5.
observed, indicating the Ag nanoparticles (NPs) have successfully decorated on the surface of SnO2. The crystalline structures of the pure SnO2 and 3.8 %Ag-SnO2 sample were characterized by XRD analysis. It can be seen from the XRD pattern of Fig. 5a that all the diffraction peaks of pure SnO2 can be associated to the tetragonal structure with lattice constants of a = b = 4.738 Å and c = 3.187 Å (JCPDS card of 41–1445) [18–21]. The mean crystallite size of SnO2 was estimated to be 20.50 nm that is corresponding to the three principal planes of 19.69(110), 18.46(101), and 23.3 (211) nm by using the Debye–Scherrer formula of d = 0.9λ/ βcosθ, where d, λ, β and θ respectively show crystallite size, X-ray wavelength, full width at half max (FWHM) and Bragg angle [19,22]. The surface area (S) was estimated to be 16.32 m2/g by using the formula S = 6/σρRX, where σ is the particle size and ρRX the X-ray density [23]. From the diffraction pattern of 3.8%Ag-SnO2 sample, the (200) facet of Ag crystalline phases (JCPDS card No. 04–0783), corresponding to 44.3° (2θ), was observed. Comparing pattern of Fig. 5a with b, it is found that peak intensity centered at 37.9°(200) and 64.7°(112) of SnO2
number of nanoparticles with a few nanometers were clearly observed. Fig. 2b displays the morphology of 3.8 %Ag-SnO2 composites. To further confirm the existence of Ag, Sn, and O elements on the composite surface, the elemental mapping was carried out as shown in Fig. 2c–f and Fig 3 shows the EDS spectra. The main peaks can be assigned to O, Sn and Ag for Ag-SnO2 sample with weight ratios of Ag to Ag-SnO2 as 0.81, 1.51, 3.66 and 5.73, respectively, which shows that the practical Ag amounts coated on composite are less than that of added amounts in synthesized process. The composition of the composite with an optimum weight ratio (sample 3) was further determined by ICP-MS to be 4.1 wt%, which is much closer to EDS results. In order to obtain detailed structural information of composite, HRTEM observation was carried out on the Ag-coated SnO2. From Fig. 4 it can be seen that the sample are consist of abundant particles. The lattice fringes of d = 0.24 and 0.33 nm respectively well corresponding to the SnO2 crystalline planes of (110) and (110). From the HR-TEM image of the 3.8% Ag-SnO2 composite, cubic Ag with an interplanar distance of d = 0.20 nm well matching with the (200) plane can also be clearly 38
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which can be attributed to the introducing of impurity level in the bandgap of the composite, leading to the reduction of electron transition energy. The specific surface area is regarded to be an important influencing factor to gas sensing properties because sensing response of semiconductor metal oxide based sensor results from gas surface adsorption and surface reaction. The nitrogen adsorption-desorption isotherms were measured to investigate the specific surface area. The BET specific surface area of the 3.7 wt%Ag-SnO2 composite was calculated to be 24.48 m2 g−1 from nitrogen adsorption-desorption isotherms as shown in Fig. 7. Fig. 7 shows a type IV isotherm with type H3 hysteresis loops and indicates the presence of the mesoporous structure, which causes more chance to adsorb oxygen and test gases, resulting in a fast response and recovery kinetics as well as a high sensitivity. To further confirm the composition of the sample and chemical state of the element, XPS measurement for 3.7 wt%Ag-SnO2 was performed, and Fig. 8 shows the experimental results. In the XPS full survey spectrum of 3.7%Ag-SnO2 (Fig. 8a), the peaks of Sn, Ag, O, and C elements can be observed clearly. For the Sn 3d spectrum, two strong peaks at around 486.4 eV and 494.9 eV (Fig. 8b) are assigned to Sn3d5/2 and Sn 3d3/2. Compared with pure SnO2 (Sn3d5/2 at 486.9 eV and Sn 3d3/2 at 495.4 eV), the Sn 3d peaks shift to lower energy, which indicates the interaction and electron transformation may exist between the SnO2 and Ag [4,18,25]. Fig. 8c displays the high-resolution Ag 3d XPS spectrum. Ag 3d5/2 and Ag 3d3/2 peaks are observed at 367.6 and 373.5 eV, respectively. Furthermore, the peak at 367.6 eV is characteristic of the existence of Ag0 in the Ag-SnO2 composite, which is agreement with the results of XRD and HR-TEM analysis. The O 1s includes chemical states of three components, that is, the oxygen ions of Olatt, Ox− and OOH. It is verified that the crystal lattice oxygen ion of Olatt located at 530 eV has no contribution to the gas sensing response, while the adsorbed oxygen ion of Ox− located at 531 eV has great influence on gas sensing response. OOH centered at 532 eV is assigned to the hydroxyl ions on the surface of the oxide [26,27]. It should be noted that the content of Ox-in Ag-SnO2 composite is much more than that of SnO2, which is responsible for enhancing gas sensing properties of AgSnO2 composites due to Ag decoration on SnO2 nanoparticles [27,28].
Fig. 4. HR-TEM image of 3.7% Ag-SnO2 sample.
3.2. Gas sensing performance Fig. 9a shows the transient response of different sensors exposed to 10 ppm of HCHO at the working temperature of 125 °C. It is obvious that the 3.7%Ag-SnO2 based sensor has the highest response of 14.4, which is 7 times higher than that of pristine SnO2 nanoparticles. The linear tendency of response is observed from Fig. 9 (b) for both SnO2 and Ag-SnO2 composite, the straight line slope of Ag-SnO2 composite is larger than that of SnO2, which indicates that the sensitivity of the composite-based sensor is higher than that of SnO2 based sensor. The detection limit (DL) is one of the important indexes reflecting sensitivity, and a high sensitivity usually has a lower detection limit. The detection limit (DL) is defined as the lowest concentration in which the response generally is significantly different from the 3 times standard deviation of the noise. The measuring relative response of the sensor in the baseline can be used to calculate the sensor noise. On the other hand, the DL can be estimated by following Eq. (2), where N is the number taken data point, the standard deviation (S) was calculated to be 0.733 by the root-mean-square deviation (RMSD) in terms of average value of 10 consecutive data [29,30].
Fig. 5. XRD patterns of (a) SnO2, and (b) 3.7% Ag-SnO2 samples.
increases after loading Ag, which may results from the diffraction peaks of Ag (111) plane at 2θ = 38.1° and the Ag (220) plane at 64.4ºof Ag overlapped the diffraction peaks of SnO2. These results indicate the successful preparation of Ag-SnO2 composite. To further investigate the change of structure for decorated Ag sample, the UV-Vis absorption spectra of pure SnO2 and the 3.8%AgSnO2 composite are shown in Fig. 6. The pure SnO2 sample exhibits an absorption band at ∼260 nm in the UV region. However, the absorption band of Ag-SnO2 composite is red-shifted to 280 nm compared with pristine SnO2, which can be attributed to the charge transition between silver and SnO2, leading to the enhancement of gas sensing performance [24]. The indirect band gap (Eg) of pure SnO2 and the 3.8%Ag-SnO2 composite can be estimated by UV–vis absorption spectra using simplified formula of αhυ = A (hυ − Eg)2, where, α, hυ, Eg and A respectively represent the absorption coefficient, photon energy with frequency υ, band gap and constant. The band gaps of SnO2 and AgSnO2 were found to be 3.64 and 3.19 eV, respectively. The result indicates that the band gap of SnO2 is reduced due to Ag decoration,
RMSnoise =
S2 N
(2)
From the above Eq. (2) the calculated value of RMSnoise is 0.22. According to the definition of detection limit (DL is 3 times standard deviation of noise) and the slope of 1.233 from Fig. 9b, so that: 39
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Fig. 6. (a) UV-Vis absorption spectra of SnO2 and 3.7%Ag-SnO2 composite, and (b) the plot of (αhν)2 versus (hν) of the SnO2 and 3.7%Ag-SnO2.
good selectivity makes the sensor can exclusively a response to a target gas. Herein, the responses of the 3.7%Ag-SnO2 composite to other gases such as 10 ppm of CO, ether, toluene, methanol, ethanol, and acetone were examined at an optimum temperature of 125 °C and the experiment results are shown in Fig. 10b. The results show that the 3.7%AgSnO2 composites based sensor exhibits the highest selectivity to formaldehyde. We know, the selectivity of metal oxide sensor is affected by temperature, adsorption amount of gas on the surface of material and lowest unoccupied molecule orbit (LUMO) energy of gas molecule, because the gas-sensing performance of semiconductor metal oxide depends on gas surface adsorption and reaction between adsorbed oxygen ion and test gas on the surface of material. If the LUMO energy of a gas molecule is lower, gas sensing reaction requires less energy, leading to the enhancement of gas sensing response and reducing of the sensor operating temperature. Moreover, the adsorption states of gas molecule also influence the selectivity of the sensor, because the adsorption states of gas molecules are affected by the orbital energies of the gas molecule, which will result to different electron affinity, thus results to the different resistances of the sensor after gas adsorption. To further clarify the effect of gas molecule orbital energy on adsorption states of gas molecule needs through quantum chemistry calculation [33]. The stability of the sensor is extremely important for the practical gas detection. In the test process, the response of sensor maintains around 14 after 5 test cycles for 35 days, which indicates the sensor has rather good stability as shown in Fig. 10c. So, the Ag decorated SnO2 composite is recognized to be a promising sensing material for efficient and selective detects to formaldehyde at a lower operating temperature.
Fig. 7. Nitrogen adsorption–desorption isotherm for 3.7%Ag-SnO2 composite.
DL = 3
RMSnoise = 3∗0.31/1.233 = 0.533 ppm Slope
(3)
In a similar way, the DL of SnO2 was calculated to be 1.16 (the slope is 0.57112 in Fig. 9b). So, the Ag surface decoration can also reduce the detection limit of SnO2. Fig. 10a displays the effect of Ag contents and operating temperature on sensing response for pure SnO2 and composites in 10 ppm formaldehyde. The responses are 14.4 and 3.2 for 3.7%Ag-SnO2 and pure SnO2, respectively at the optimum operating temperature of 125 °C and 150 °C. The results also indicate that the sensing response firstly increases and then decreases with the increase in operating temperature from 80 to 180 °C. Because the gas sensing response of semiconductor metal oxide is related to the adsorption of oxygen molecules in air and reaction between the oxygen species and the target gas on the surface of the material. The enhancing operating temperature is to provide energy to overcome the activation energies needed gas chemisorption and surface reaction, so, the response increases with the operating temperature at a temperature range from 80 to 125 °C for 3.7 wt% Ag-SnO2 sample. However, once the operating temperature over 125 °C the response decreases with the temperature increase. The temperature (125 °C) is called the optimum operating temperature because at which the gas adsorption on the surface of material reaches saturation, leading to the highest response. With the temperature further increases the desorbed gas amounts will increase, thus the response decreases [31,32]. The selectivity shows the anti-interference ability of a sensor, the
3.3. Mechanism of gas sensing The sensing response of the semiconducting metal oxide based sensor depends on the change in resistance, which results from the oxygen molecules adsorption and the reaction with test gas on the surface of the material [33–35]. When the SnO2 was exposed to air, the oxygen molecules in the air will adsorb on the surface of SnO2 and capture electrons from the oxide to generate oxygen species (O2−). As a result, an electron depletion layer at the surface of oxide is formed where the electron concentration is lower than that of the SnO2 bulk, leading to the increase of the sensor resistance. Once the sensor was exposed to HCHO, the gas molecules are oxidized by the oxygen species adsorbed on oxide surface and the released electrons back to the conduction band of SnO2. The result causes the decrease of sensor resistance, i.e. decrease of electron depletion layer thickness, thus enhanced the sensor response [36–40]. When Ag nanoparticles were decorated on the SnO2 surface, spillover and catalytic effects are generally considered [19,25,26,41]. Ag nanoparticles provide more active sites for adsorption of oxygen 40
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Fig. 8. X-ray photoelectron spectra of 3.7%Ag-SnO2: (a) survey spectrum; (b) Sn 3d; (c) O1s; (d) Ag 3d.
4. Conclusions
molecules and detect gas, and as a catalyst will promote the dissociation of adsorbed oxygen molecules to create more oxygen species of O2−, O−, or O2− [42]. Then transport and distribute these oxygen species to the surface of SnO2 due to the spillover effect of noble metals. The result above forms a deeper depletion layer with a higher resistance than pure SnO2 [4]. When the composite was exposed to formaldehyde, the gas molecules are also firstly adsorbed on the supported Ag nanoparticles and are activated, the Ag nanoparticles play the role of active sites for the oxidizing reaction between HCHO and oxygen ions and then the Ag particles will transport the electrons released by oxidizing reaction to the SnO2 conduction band, leading to reducing of depletion layer thickness, i.e. the resistance decreasing of sensor in formaldehyde [41–43]. Hence, the sensor response is significantly enhanced according to the response definition of Rair/Rgas in reducing gas of HCHO. The schematic diagram of the gas sensing mechanism is shown in Fig. 11.
In summary, Ag decorated SnO2 composites were successfully prepared by in-situ reduction of AgNO3 on the surface of SnO2. The response of the 3.7 wt% Ag decorated SnO2 nanoparticles achieves nearly 7 times higher than that of pristine SnO2 to 10 ppm formaldehyde at operating temperature of 125 °C and has good selectivity to formaldehyde The enhancement of sensing properties is ascribed to the spillover and catalytic effects of decorated Ag nanoparticles. The results prove that the Ag decorated SnO2 composite is a promising sensing material to detect trace harmful VOCs over other frequently encountered in indoor environments.
Acknowledgments This work was supported by the Science Foundation of Jiangsu University of Technology (KYY16024) and the National Natural Science Foundation of China (Grant No. 51772015).
Fig. 9. (a) Response of Ag decorated SnO2 samples to 10 ppm formaldehyde at 125 °C; (b) Fit curve relationship between the sensor response and formaldehyde concentration. 41
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Fig. 10. (a) Responses of pure SnO2 and Ag decorated SnO2 samples at different operating temperature to 10 ppm HCHO; (b) Response of sensor based on 3.7%AgSnO2 composite to different test gases at optimum operating temperature; (c) Stability of the sensor based on 3.7% Ag-SnO2 to 10 ppm HCHO at 125 °C.
Fig. 11. Schematic diagrams of the sensing mechanism for SnO2 and Ag-SnO2 based sensors to HCHO: (a) oxygen molecules in air trap electrons into chemisorbed oxygen species on the surface of sensors; (b) Reaction of HCHO with chemisorbed oxygen species on surface of sensors.
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