Oxygen Vacancies as an Efficient Strategy for Promotion of Low

Subscriber access provided by University of South Dakota

Article

Oxygen Vacancies as an Efficient Strategy for Promotion of Low Concentration SO2 Gas Sensing: the case of Au Modified SnO2 Lingyue Liu, and Shantang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03205 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Oxygen Vacancies as an Efficient Strategy for Promotion of Low Concentration SO2 Gas Sensing: the case of Au Modified SnO2 Lingyue Liu, Shantang Liu* Key Laboratory for Green Chemical Process of Ministry of Education and School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan, 430073, PR China * Corresponding author: Prof. S. Liu, E-mail: [email protected] Tel.: (+86) 13618626285; Fax: (+86) 027-87195001.

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract SO2 gas is toxic for humans, long stay in environment with low concentration SO2 gas can trigger bronchitis and other respiratory impairments, thus leads to an increasing mortality. However, high-efficiency gas sensors for detecting SO2 have not been widely developed and applied. Here, an Au nanoparticle-modified SnO2 (AuNPs-SnO2) sensor has been successfully prepared to detect the SO2 gas. The gas sensor functioned at a low temperature of 200 oC can detect SO2 concentrations as low as 500 ppb, has an excellent selectivity of SO2 gas and shows good repeatability and stability through the regulation of oxygen vacancies. Meanwhile, the sensor has a response sensitivity of 10.4 for 20 ppm SO2 and a response/recovery time of 34 and 14 s at 200 oC, which is twice less than those of the products with absence of oxygen vacancies. Additionally, a possible mechanism of SO2 sensing which is depended on the in-situ Fourier transform infrared (in-situ FTIR) spectra and other catalysis results has been proposed. Keywords: AuNPs-SnO2, oxygen vacancy, sulphur dioxide, mechanism, in-situ FTIR.


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction SO2 mainly comes from fossil-fuel combustion and has been deemed culprit of air pollution disasters, notably in the district of Donora (U.S.A), Meuse Valley (Belgium) and London [1-3]. It is common knowledge that SO2 is harmful gas for we human beings. Exposure under the high concentration of SO2 for a very short period can seriously damage the respiratory system, long-term living or working in an environment with a small quantity of SO2 also can cause respiratory disorders, contributing to an increasing mortality [4, 5]. In addition, a low content of SO2 in air can decrease agricultural productivity and damage the overall plant ecosystems [6, 7]. Thus, the detection of SO2 is of great significance, unfortunately, there are few reports of metal oxide semiconductor gas sensors used to detect SO2 gas efficiently [8-10]. Therefore, it is of urgent necessity to research efficient sensor to defect low concentration SO2. The most promising candidate semiconductor material in commercial applications is Tin dioxide (SnO2) ,having been widely studied in the area of electrocatalysts, solar cells as well as sensors due to the good stability, nontoxicity and low cost [11-13]. It is most widely used in gas sensors and mainly for detection of volatile organic compound (VOC)-based gas [14-18]. The purpose of the present work on SnO2 sensors is to determine how their sensitivity and low selectivity can be increased. The uniform loading of noble metal has been extensively used for modification of SnO2 sensors to improve its efficiency. This approach can not only create more gas molecule adsorption sites but can also reduce the activation energy required for



sensing reactions [19-22]. Generally, these improved performance characteristics have been attributed to the facilitation of the chemisorption of molecular oxygen by the catalytic activity of the metal nanoparticles [23-26]. Moreover, formation of Schottky barrier at the metal/oxide interface can yield an improved charge carriers separation and have been considered a meaningful route. Oxygen vacancies feature an important part to activate oxygen molecules react with target gas (e.g., SO2) on the metal oxide surface, give rise to a dramatic difference in the electron structure and thus lead to a variation in the electrical resistance [27-31]. Based on the above considerations, an Au nanoparticles (NPs) modified nanosheet-based SnO2 gas sensor which is hyposensitized and selective, has been reported. The Au NPs loaded onto the surfaces of the nanosheet-based SnO2 nanocomposite can promote the adsorption of SO2 through the formation of Au-S bonds. Additionally, the oxygen vacancy content can be easily tuned by varying the calcination atmosphere. In a comparison, the sensor based on the AuNPs-SnO2 nanocomposite annealed at 500 oC in an air atmosphere exhibited the best SO2 sensing among the other as-obtained AuNPs-SnO2 products. In addition, the sensor showed a high sensitivity of about 10.4 of 20 ppm SO2 of which the detection concentration is 500 ppb at 200 oC. The enhanced SO2 sensing of AuNPs-SnO2 nanocomposite ascribed to the spillover effect of Au NPs loaded onto SnO2 surface and the adequate oxygen vacancy content. Furthermore, a feasible mechanism of SO2 sensing is proposed based on in-situ Fourier transform infrared (FTIR) spectra and other catalysis results.


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experiment Synthesis In the typical synthesis, 1 mmol of SnCl2 and 1 mmol Cetyltrimethyl ammonium bromide (CTAB) were added in 40 ml of deionized water, then 1 mol/L NaOH is added to control the pH=12 when stirring vigorously for 1 hour. The mixture was then shifted to a 100 ml Teflon-lined stainless autoclave and heated in an electric oven at 120 oC for 12 hours. After cooling to room temperature, the precipitates obtained by centrifugation was washed several times with water and ethanol. Then, the as-obtained product was dispersed in 25 mL of lysine (0. 1 mol/L) and 22.5 mL of HAuCl4 (0.01 mol/L) by rapid stirring for 20 min, followed by adding 0.2 mL of NaBH4 (0.1 mol/L) drop by drop into the aforementioned liquid with vigorous stirring. The product was desiccated in the vacuum oven at 80 oC for 12 hours. The samples were annealed for 2 hours in a 400 oC tube furnace with a heating rate of 2 oC/min in air (denoted as AS-A) and oxygen (denoted as AS-O) for further characterization and use. Characterization The X-ray powder diffraction (XRD) using a Shimadzu XRD-6000 diffractometer with CuKα1 irradiation over a 2θ range from 20 to 80o was used to analyse crystal phase of the samples. The inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce) was used to analyse relative content of Au in AuNPs-SnO2(-x) nanocomposites. The scanning electron microscopy (SEM) was used to examine morphologies of the as-obtained products. The X-ray photoelectron spectroscopy



(XPS, K-Alpha) was used to analyse electron configuration of the surface of the as-synthesized products. The electron spin resonance (ESR) measurements of the obtained samples were conducted with the usage of a Bruker EMX 10/2.7 ESR spectrometer (X-band) that is equipped with a double chamber, of which the modulation frequency and the microwave frequency were respectively 100 kHz and 9.4145 GHz. Ultraviolet-visible diffuse reflectance spectra (DRS) were measured by a UV-vis spectrometer (Hitachi UH4150) and the reflection were converted to absorbance through the Kubelka-Munk method. Photoluminescence (PL) spectra were measured with a Shimadzu RF-5301PC fluorescence spectrophotometer. To measure H2-temperature program reduction (H2-TPR), 10% H2/N2 stream flew through the catalyst bed with the rate of 50 sccm while the temperature increased from room temperature to 800 °C. The Brunauer-Emmett-Teller (BET) specific surface area of the as-synthesized products was studied by Nitrogen adsorption with a Micromeritics ASAP 2020 (USA) to determine. Sensor fabrication and gas sensing measurement The paste was prepared by a mixture of the as-synthesized products with ethylene glycol, glycerol and H2O (2 : 2: 1 mass ratio). Then it is followed by spot-coating onto an Al2O3 ceramic plate substrate, (area of 8 mm × 10 mm × 0.65 mm) on which a pair of Au interdigitated electrodes (electrode width of 0.2 mm and gap width of 0.2 mm) were printed on the substrate in advance. The sensor was desiccated at 120 oC for 2 hours, and then dried by annealing at 400 oC for another 2 hours. Finally, the attributes of gas sensing were measured with a static measurement system produced


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by CGS-1TP (Beijing Elite Tech. Co., Ltd., China) with a test chamber of 18 L. We controlled the relative humidity and room temperature of the environment to be approximately 25% and 20 oC, respectively. The measurement results were processed statistically. This sensing device is analogical to the one our research group used before. Firstly, the sensors were placed into the chamber and were preheated to their operational temperature. We then injected a certain amount of measured gas into the test chamber. The upper cover of the test bulb is opened after the response reaching a constant value and air is introduced, and the sensor response will return to the initial value. The response of the sensor in this work was determined as S = Ra/Rg (the reducing gas) or Rg/Ra (the oxidizing gas), where Ra and Rg represents the sensor resistance exposed to the air and the measured gas, respectively. Recovery and response times represented the time required to reach 90% of the total resistance change of adsorption and desorption for the sensor. A controllable humidity environment was obtained by using saturated salt solution in a sealed container: LiCl, MgCl2, NaBr, NaCl, KCl, and K2SO4, which respectively yielded 11%, 33%, 59%, 75%, 85%, and 98% relative humidity (RH). Investigation of the SO2 sensing mechanism The mechanism of SO2 sensing was tentatively studied using in-situ diffuse reflectance FTIR spectra recorded with a Nicolet Magna 560 FT-IR spectrometer comes with an MCT detector. The substrate pre-deposited with a thin layer of the AuNPs-SnO2(-x) film was placed at the centre of the designed reaction cell. The AuNPs-SnO2(-x) film coated on the substrate was firstly dehydrated at 400 °C (heating



rate 20 °C/min) and then dropped to 30 °C at 500 ppm of He-balanced SO2 with a rate of 90 mL/min. Finally, in order to eliminate the effect of the physisorbed SO2, the samples were heated in pure He flow at 200 °C for 10 min so that the small amount of SO2 adsorption can be removed. Along with the reaction, the IR signal was collected through an MCT detector. Results and Discussion Property of Material Figure 1a exhibits crystalline structure and phase purity of the synthetic AS-O and AS-A samples were analysed through powder XRD. The characteristic diffraction peaks of the as-prepared samples which were calcinated in air atmosphere (AS-A) and O2 atmosphere (AS-O) were assigned to tetragonal SnO2 (JCPDS 5-467) and orthorhombic Au (JCPDS 2-1095). There were no other impurity phase discovered in XRD pattern, indicating the samples were highly pure. The field emission scanning electron microscopy (SEM) is used to measure the morphologies of these as-prepared samples, and the SEM images (Figure S1a,b) revealed that AS-A and AS-O have a similar self-assembly nanoflower morphology characterized by amounts of nanosheets with an approximately thickness of 20 nm. The gap between the nanosheets is filled with many particles, which may be Au nanoparticles. The surface area of the AS-A and AS-O samples were analysed using N2 adsorption and desorption experiment to exclude the effect of the BET surface area (Figures S2a,b). The prepared samples exhibited the characteristic of the type IV isotherms and owned a hysteresis cycle in the scope of 0.60-1.00 P/P0, indicating these samples existed


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mesoporous properties. The calculated BET surface areas for AS-A and AS-O are 50.63 and 45.95 m2g−1, respectively.

Figure 1. XRD spectra of the as-obtained AuNPs-SnO2 nanocomposites (AS-A, AS-O) (a1 and a2 are photographs of samples). The chemical composition of these samples were analysed by X-ray photoelectron spectroscopy (XPS). XPS survey spectrum was presented in Figure S3a, the peaks are corresponding to element Sn, O, Au, and C clearly. The presence of C may be due to the hydrocarbons in the AuNPs-SnO2(-x) samples. The Au 4f7/2 and 4f5/2 peaks are located at 83.2 and 87.1 eV in Figure 2a, corresponding to the zero oxidation state of Au. The peaks located at 486.3 and 494.8 eV in Figure 2b were pointed to the Sn 3d5/2 and Sn 3d3/2 states of Sn4+, respectively. Shown in Figure 2c, the O1s peak at the bonding energy of 530.7 eV was

analysed with a Lorentzian distribution fitting

in which can be identified for all AuNPs-SnO2(-x) samples, the peak at 530.9 eV was



ascribed to the coordination of the O2− bound to tin atoms. The slight shift of the peak may be attributed to oxygen vacancies due to the alteration of the chemical environment [31, 32]. The other peak at 531.6 eV (O1s peak) corresponds to oxygen vacancies and is concordant with the above results. Moreover, the amounts of Au in the SnO2 and SnO2-x nanomaterials is 13.18 and 12.27 wt%, as confirmed by XPS, for AS-A and AS-O, respectively (Table S1), which is close to the values of 13.31 wt% and 12.43 wt% determined by ICP elemental analysis (Figure S3b), respectively. Considering the truth that XPS is a non-destructive surface analysis technique with a several nanometres detectable depth and ICP analyses can get the information of the total Au content, it stands to reason to deem that Au nanoparticles are uniformly distributed on the SnO2 (or SnO2-x) nanomaterial surface,. Moreover, we studied the unpaired electrons in the AuNPs-SnO2(-x) nanomaterial by electron spin resonance (ESR) spectroscopy. As shown in Figure 2d, an intense signal at 3357 G (with g value 2.001) is appeared in the ESR spectrum of AS-A, which is due to the single electron Zeeman Effect of oxygen vacancy capture [33, 34]. By contrast, the signal detected in the AS-O ESR spectrum is very weak. This result shows that the oxygen vacancies can be produced by changing calcination atmosphere [35] and that Au is irrelevant to the introduction of oxygen defects. For the further confirmation of generation and amounts of oxygen vacancies, H2-TPR testing of the AS-A and AS-O samples were performed. As given in Figure 2e, the TPR profile of synthesized AuNPs-SnO2-x displays a broad intense peak at 280 oC, which was due to the cut down of oxygen vacancies. The peak was followed by the onset of bulk reduction (which is the


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reduction of AuNPs-SnO2-x) starting from 450 oC. However, consistent with the ESR results, there is no significant TPR in the AS-O sample since the oxygen consumption increases with text temperature. To further examine the surface-related defects of the nanocrystals, UV-Vis diffuse reflectance spectroscopy (DRS) analysis was also hired and the results are presented in Figure 2f. The DRS spectrum of the AS-A sample also confirms the production of oxygen vacancies. The visible response enhancement of AS-A is ascribed to the generated surface oxygen vacancies, and the UV light absorption was mostly due to the intrinsic response of SnO2 [36]. Meanwhile, a stronger absorption encompassing the full range from 445 to 745 nm can be observed with the introduction of Au. This absorption is believed to correspond to the surface plasmon resonance (SPR) of Au NPs [37] and consistent with previous reports.

Figure 2. X-ray photoelectron spectra: (a) Au 4f; (b) Sn 3d; (c) O 1s; (d) ESR, (e) H2-TPR and DRS spectrum of as-prepared AS-A and AS-O. Gas sensing performance testing



A proof of the presence of oxygen vacancies and Au NPs in SnO2 could be beneficial to the gas sensing property of the already tested AS-A and AS-O. A comparison of the SO2 response at diverse operational temperatures, which ranges from 25 to 300 oC, is presented in Figure 3a. The results indicated the sensors response changed with the experiment temperature. For Au-loaded SnO2 sensor, the initially response is increasing with rising of operating temperature till up to 200 oC. The highest response to 20 ppm SO2 was 10.43 and 3.67 for AS-A and AS-O at 200 oC, respectively. And then, the response gradually decreasing with a further increasing of temperature till up to 300 oC for both materials. The adsorption of the SO2 gas molecules onto the AuNPs-SnO2(-x) surface causes a new weak donor energy level in the energy bandgap. Thus, a decreased resistance of AuNPs-SnO2(-x) can be resulted from the existence of the target SO2 gas. The continuous increasing of Ra/Rg value with the increasing of temperature till up to 200 oC, this can be attributable to the donor states gradually ionize aided by the thermal energy. In addition, the sensor response depended on the gas-sensing temperature appears as a shoulder, which is a behaviour that is universal in metal oxide semiconductor gas sensors. Thus, the optimal operating temperature for the above sensors was 200 oC. The sensors responses appeared an approximately linear increasing along as the gas concentration increases, and the slope increases slowly after the SO2 concentration increases to 5 ppm, due to the sensor beginning to be saturated with the SO2 gas (Figure 3b). In addition, the 0.5-20 ppm concentration testing of the SO2 sensor shows that this sensor has a large range of SO2 response. The transients SO2 sensors that owned the SO2 concentrations starting from 0.5 to 20 ppm


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


under optimum operating temperature were shown in Figures 3c and 3d for AS-A and AS-O samples. The results revealed that the AS-A sensor possess a higher response than that of AS-O sensor under the same SO2 gas concentration.

Figure 3. (a) Sensors response to 20 ppm SO2 of AS-A and AS-O as a function of the operating temperature, (b) change and comparison of the sensors for diverse SO2 concentrations (0.5-20 ppm) at their separate optimal operating temperatures, (c and d) dynamic sensor transient response based on the AS-A and AS-O. To confirms the reproducibility of the sensor response to SO2, we conducted the sensing tests at 200 oC and SO2 concentration is 20 ppm for three cycles (Figure 4a). It is well-known that sensor response prepared with either air or O2 calcined samples is reproducible from the testing results. In addition, the stability of the sensor over three months has been tested; this is is of great significance to the practical application of sensors. As shown in Figure 4b, in the long-term gas-sensing tests, the SO2 sensors response is still good despite a slight decrease in the response. To be a great gas



sensor, the sensor must select the measured gas accurately even in the presence of interfering gases. Hence, a systematic test was conducted for the response to other gases containing formaldehyde (HCHO), toluene (C7H8), methanol (CH3OH), ammonia gas (NH3), carbon monoxide (CO), ethanol (C2H6O) and acetone (C3H6O) at the same concentration and temperature (20 ppm, 200 oC). As shown in Figure 4c, dynamic

sensor resistance response based on AuNPs-SnO2(-x) composites to various

gas of 20 ppm at 200 oC. Dramatic decrease is easily observed with the injection of target gas, indicating the n-type response for the sensor. The inset of Figure 4c shows that the AS-A sensor exhibited the best response towards the 20 ppm SO2 (about 10.4), which is four times higher than the response of ethanol, formaldehyde and acetone and ten times higher than those of methanol, toluene, NH3 and CO. According to these results, the AS-A nanomaterial displays highly improved selectivity toward SO2 because of the existence of oxygen vacancies. In addition, AS-O sensors were also tested (Figure 4c), but they did not display selective sensing to SO2. Moreover, we also test the recovery time and response of the AS-A and AS-O toward 200 ppm SO2 (Figure 5). It is obvious from Figure 5a and 5b that both the response and recovery time of AS-A sensor and AS-O sensor are within 60 s. The response time of AS-A was 34 s and recovery time was only 14 s, which is twice less than those of the AS-O. Both the response and recovery time of AS-A are shortest compared to the previously reported SO2 gas sensors. What’s more, the impact of humidity was investigated on the sensor AS-A (Figure 5c). The figure shows that Ra slightly reduces when the relative humidity increases. The impact of humidity on the sensor response based on


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AuNPs-SnO2-x to 20 ppm SO2 at room temperature is clarified in the inset of Figure 5c. The responses is almost linear with relative humidity, and water has obvious negative effect on SO2 response. Meanwhile, the sensor has the similar sensing properties with the one made of metal oxide, hybrids as well as polymer. Table 1 shows the proposed SO2 sensor properties compared to previous work [9-10, 38-43]. The presented AuNPs-SnO2-x sensor indicated better sensing properties than any ever reported sensor, and exceeded the ZnO and GO counterparts in sensing properties.

Figure 4. The reproducibility (a), stability (b) dynamic response curves for three cycles (c) dynamic resistance response (the inset is the response ratio for SO2 to the other gases) of AS-A and AS-O sensors toward 20 ppm target gas concentration and temperature of 200 °C, respectively.



Page 16 of 26

Figure 5. Response and recovery time of AS-A (a), AS-O (b) sensors and variation in resistance (c) and sensing response (inset) of AS-A based sensor versus variational relative humidity at a 20 ppm SO2 concentration and temperature of 200 °C, respectively. Table 1 SO2 sensing properties of the presented sensor in this work in comparison with previous work. Sensor materials

Operating temp. / oC

Concent. / ppm

Resp.

Ref.

TiO2/rGO rGO rGO-SnO2 polyaniline Ru/Al2O3/ZnO GO NiO/SnO2 V2O5/SnO2 AuNPs-SnO2-x

25

5 5 500 10 25 5 500 5 20

1.12 1.06 23 1.04 1.25 1.06 56 1.45 10.43

9 10 38 39 40 41 42 43

27 60 25

350 25

180 350 200

This work

Sensing mechanism of AuNPs-SnO2-x for SO2 To understand the charge transfer behaviour in the presence of oxygen vacancies, we resolved the PL spectrum into several sub-emission bands. By the PL peak-differentiation analysis of AS-A, ten emission bands (S1-S10) were obtained, as shown in Figure 6. The near-UV emission (441 nm) band related to the band-edge emission of SnO2 corresponds to the approximately 2.91 eV optical band gap value [44]. The defect emissions caused long-wavelength emission bands (450-650 nm). The green emission bands (530-600 nm) are often designated as ionized oxygen


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


vacancies (• , •• ) and neutral oxygen vacancies ( ) located at the surface of material in terms of emission bands.[45, 46]. Therefore, in the AuNPs-SnO2(-x) system, the electron trapping and detrapping are considered predominantly happened at the oxygen vacancy sites as described by, . Generally, surface oxygen vacancies are the dissociation products of oxygen molecules (O−, O2− and O4−) adsorbing onto the surfaces of AuNPs-SnO2 in the air, which leads to the forming of electron-depletion layers on the material surface [47, 48]. As is widely known, various of sulphur oxides can be produced by the reaction of SO2 with oxygen atoms (eq. 1) or by the disproportionation (eq. 2) of SO2.

The high potential barrier, which is generated at the grain boundary of AuNPs-SnO2, leads to an increased resistance for gas sensors. However, bulk oxygen vacancies easily form electron traps leading to the blocking of the electron flow, which suppressed the dissociation products of oxygen molecules and resulted the decrease of the sensors response. Furthermore, the improved SO2 response of the Au loaded SnO2(-x) nanomaterials may be attributed to the strong S-Au bonds (forming ‘‘alligator clips’’) [49]. The conclusion that strong gold-sulphur bond on a alkylthiol self-assembled monolayers gold particle surface results in the formation of Au-S with the loss of the thiol oxygen has been widely accepted in the past few years. Meanwhile, while the Au nanoparticles loaded on SnO2 surface, electrons flowing



Page 18 of 26

from SnO2 to the Au nanoparticles, thus leading to the forming of the AuNPs-SnO2(-x) Schottky contact, widening the depletion layers of SnO2 nanomaterials and further increasing the device resistance of SnO2 nanomaterial sensor. When AuNPs-SnO2(-x) nanostructures are generated, the majority of the electrons in SnO2 are

consumed

because of the formation of the Schottky contact, the adsorption of the oxygen molecules and the forming of Au-S bond. This gives rise to the large resistance state of AuNPs-SnO2(-x) sensors.

Figure 6. Gaussian deconvolutions of the PL spectra of (a) AS-A and (b) AS-O. To further confirm the SO2 oxidation process on the AuNPs-SnO2(-x) surface, in situ diffuse reflectance FTIR spectroscopy was employed to research the time-dependent variation of the AuNPs-SnO2-x functional groups at 200 oC. For AS-A, absorption


Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bands accelerated step by step with the extending of irradiation time from 0 min to 25 min in a SO2/He atmosphere. As can be seen from Figure 7, there are two absorption bands, specifically, a sharp band located at 1283 cm−1 and a broad band positioned at 1128 cm−1 were attributed to the asymmetric and symmetric S=O stretches at 0 min, respectively [50]. In addition, the bands positioned at 1059 and 534 cm−1 were considered as the adsorbed SO3 or SO molecular and the band arose at 1484 cm−1 characterised the S=O2 stretch [51]. Furthermore, the characteristic absorption peaks for SO2 gradually decreased, whereas those of SO3 increased with the prolongation of the reaction time. These results strongly confirmed that SO2 oxidation or DE oxidation can occur effectively on the surfaces of the AuNPs-SnO2(-x) products at 200 o

C, which illustrated that the reason for the n-type response of the sensor based on

AuNPs-SnO2(-x).

Figure 7. In-situ FTIR spectra recorded at different SO2 oxidation time of AS-A at a 20 ppm SO2 concentration and temperature of 200 °C. The mechanism of sensing and the role of oxygen vacancies in the AuNPs-SnO2(-x)



system are presented and clarified in detail in Scheme 1 on the basis of the above analysis. The beginning of the gas sensor mechanism was because of electrical conductivity change of SnO2 nanomaterials, which results from the reaction kinetics of the oxygen molecules adsorbed on the surface and the measured SO2 gas. The molecular oxygen in atmospheric is adsorbed onto SnO2 nanomaterial surface and trap the electrons generated from the interior of the SnO2. Accordingly, a space charge layer is formed on the interface of SnO2 and in the air, thus resulting in a potential obstacle that impedes the free electrons flowing, and furthermore, the resistance value SnO2 sensor becomes high. Due to the semiconductor property of SnO2, its resistance reduces with the temperature augments. Nevertheless, a small resistance increasing at 200 oC was observed that can result from the transformation of oxygen molecules (O2−) into oxygen atoms (2O−) and the massive capture of free electrons. The SO2 gas molecules after the mutual effect on the SnO2 nanomaterial surface with the adsorbed oxygen (O2−) can release the trapped charge carriers. When oxygen vacancies were introduced into the AuNPs-SnO2(-x) system, they could mediate the reaction of the electrons, absorb oxygen and decrease the movement of charge carriers, thus lead to the decrease of space charge layers and increase the sensor response. Our results show that the enhanced sensor performance was due to the increased surface oxygen vacancies. However, the oxygen vacancies are not the sole determining factor. This is not surprising, as the presence of vacancies caused the band structure of the products to bend down.


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Illustration of gas sensing mechanisms of AuNPs-SnO2(-x) for SO2. Conclusion To conclude, we report a fabrication of sensitively responded SO2 gas sensor by designing AuNPs-SnO2(-x) nanomaterials at room temperature. Additionally, the sensing mechanism was discussed in detail. By employing a solvothermal and calcination route, the construction process of AuNPs-SnO2(-x) nanomaterials is of highly control and reproduction. The gas sensor functioned at a low temperature of 200 oC and has an excellent selectivity of SO2 gas and the high response value is of 10.4 for 20 ppm compared with other gases. The introduction of oxygen vacancies promotes the reaction between electrons and adsorbed oxygen, forming an electron depletion layer. Additionally, the Schottky contact increases the sensor resistance, and the presence of Au allows the electrons to stream from SnO2 surface to Au nanoparticles, thus results in the fabrication of AuNPs-SnO2(-x) Schottky contact and



extending of the depletion layer of SnO2 nanomaterials, which is the main reason that caused the enhancement of the sensor response to SO2 gas. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21471120), the International Cooperation Foundation of Hubei Province (2012IHA00201), and the Educational Commission of Hubei Province of China (T201306). References: (1) Majernik, O.; Mansfield, T.A. Direct effect of SO2 pollution on the degree of opening of stomata, Nature 1970, 227, 377-378. (2) Granieri, D.; Vita, F.; Inguaggiato, S. Volcanogenic SO2, a natural pollutant: Measurements, modeling and hazard assessment at Vulcano Island (Aeolian Archipelago, Italy), Environ. Pollut. 2017, 231, 219-228. (3) Philip, L.; Deshusses, M. A. Sulfur dioxide treatment from flue gases using a biotrickling filter-bioreactor system, Environ. Sci. Technol. 2003, 37, 1978-1982. (4) Chen, X.; Wang, X.; Huang, J. J.; Zhang, L. W.; Song, F. J.; Mao, H. J.; Chen, K. X.; Chen, J.; Liu, Y. M.; Jiang, G. H.; Dong, G. H.; Bai, Z. P.; Tang, N. J. Nonmalignant respiratory mortality and long-term exposure to PM10 and SO2: A 12-year cohort study in northern China, Environ. Pollut. 2017, 231, 761-767. (5) Froom, P.; Sackstein, G.; Cohen, C.; Lerman, Y.; Kristal-Boneh, E.; Ribak, J. The effect of exposure to SO2 on the respiratory system of power-station workers, Work 1998, 11, 325-329. (6) Bacon, K. L.; Belcher, C. M.; Haworth, M.; Mcelwain, J. C. Increased atmospheric SO2 detected from changes in leaf physiognomy across the Triassic-Jurassic boundary interval of East Greenland, PloS one 2013, 8, 60614-60623. (7) Wang, C.; Xing, D.; Zeng, L.; Ding, C.; Chen, Q. Effect of artificial acid rain and SO2 on characteristics of delayed light emission, Luminescence 2005, 20, 51-56. (8) Singh, E.; Meyyappan, M.; Nalwa, H. S. Flexible Graphene-Based Wearable Gas and Chemical Sensors, ACS Appl. Mater. Interfaces, 2017, 9, 34544-34586. (9) Zhang, D.; Liu, J.; Jiang, C.; Li, P.; Sun, Y. High-Performance Sulfur Dioxide Sensing Properties of Layer-by-Layer Self-Assembled Titania-Modified Graphene Hybrid Nanocomposite. Sens. Actuators, B 2017, 245, 560-567. (10) Kumar, R.; Avasthi, D. K.; Kaur, A. Fabrication of Chemiresistive Gas Sensors Based on Multistep Reduced Graphene Oxide for Low Parts Per Million Monitoring of Sulfur Dioxide at Room Temperature. Sens. Actuators, B 2017, 242, 461-468. (11) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M. B.; Zhang, J.; Marinkovic, N. S.; Liu, P.; Frenkel, A. I.; Adzic, R. R. Ternary Pt/Rh/SnO2 electrocatalysts for oxidizing ethanol to CO2,


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nature Mater. 2009, 8, 325. (12) Das, S.; Jayaraman, V.; SnO2: A comprehensive review on structures and gas sensors, Prog. Mater. Sci. 2014, 66, 112-255. (13) Dong, Z.; Ren, H.; Hessel, C. M.; Wang, J.; Yu, R.; Jin, Q.; Yang, M.; Hu, Z.; Chen, Y.; Tang, Z.; Zhao, H.; Wang, D. Quintuple-shelled SnO2 hollow microspheres with superior light scattering for high-performance dye-sensitized solar cells, Adv. Mater. 2014, 26, 905-909. (14) Liu, L.Y.; Shu, S. M.; Zhang, G. Z.; Liu, S. T. Highly selective sensing of C2H6O, HCHO, and C3H6O gases by controlling SnO2 nanoparticle vacancies, ACS Appl. Nano Mater. 2018, 1, 31-37. (15) Reddy, M. V.; Linh, T. T.; Hien, D. T.; Chowdari, B. V. R. SnO2 Based Materials and Their Energy Storage Studies, ACS Sustainable Chem. Eng. 2016, 4, 6268-6276. (16) Li, Y. X.; Guo, Z.; Su, Y.; Jin, X. B.; Tang, X. H.; Huang, J. R.; Huang, X. J.; Li, M. Q.; Liu, J. H. Hierarchical morphology-dependent gas-sensing performances of three-dimensional SnO2 nanostructures, ACS Sens. 2017, 2, 102-110. (17) Cho, I.; Kang, K.; Yang, D.; Yun, J.; Park, I. Localized liquid-phase synthesis of porous SnO2 nanotubes on MEMS platform for low-power, high performance gas sensors, ACS Appl. Mater. Interfaces 2017, 9, 27111-27119. (18) Lee, S. H.; Galstyan, V.; Ponzoni, A.; Juan, I. G.; Riedel, R.; Dourges, M. A.; Nicolas, Y.; Toupance, T. Finely Tuned SnO2 Nanoparticles for Efficient Detection of Reducing and Oxidizing Gases: The Influence of Alkali Metal Cation on Gas-Sensing Properties, ACS Appl. Mater. Interfaces 2018, 10, 10173-1084. (19) Bulemo, P. M.; Cho, H. J.; Kim, D. H.; Kim, I. D. Facile synthesis of Pt-functionalized meso/macroporous SnO2 hollow spheres through in situ templating with SiO2 for H2S sensors, ACS Appl. Mater. Interfaces 2018, 10, 18183–18191. (20) Zhang, S.; Zhang, P.; Wang, Y.; Ma, Y.; Zhong, J.; Sun, X. Facile fabrication of a well-ordered porous Cu-doped SnO2 thin film for H2S sensing, ACS Appl. Mater. Interfaces 2014, 6 14975-14980. (21) Xiao, L.; Xu, S.; Yu, G.; Liu, S. Efficient hierarchical mixed Pd/SnO2 porous architecture deposited microheater for low power ethanol gas sensor, Sensor Actuat. B: Chem. 2018, 255, 2002-2010. (22) Liu, C.; Kuang, Q.; Xie, Z.; Zheng, L. 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 2015, 17, 6308-6313. (23) Hashmi, A. S.; Hutchings, G. J. Gold catalysis, Angew. Chem. Int. Edit. 2006, 45, 7896-7936. (24) Kim, J. H.; Wu, P.; Kim, H.W.; Kim, S.S. Highly selective sensing of CO, C6H6, and C7H8 gases by catalytic functionalization with metal nanoparticles, ACS Appl. Mater. Interfaces 2016, 8, 7173-7183. (25) Song, H.; Li, C.; Lou, Z.; Ye, Z.; Zhu. L. Effective Formation of Oxygen Vacancies in Black TiO2 Nanostructures with Efficient Solar-Driven Water Splitting, ACS Sustainable Chem. Eng. 2017, 5 10, 8982-8987. (26) Kim, H. S.; Cook, J. B.; Lin, H.; Ko, J. S.; Tolbert, S. H.; Ozolins, V.; Dunn, B. Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x, Nat Mater. 2017, 16, 454-460. (27) Yang, W.; Feng, L.; He, S. H.; Liu, L. Y.; Liu, S. T. Density Gradient strategy for preparation of broken In2O3 microtubes with remarkably selective detection of triethylamine vapor, ACS Appl. Mater. Interfaces, 2018, 10, 27131-27140.



(28) Epifani, M.; Prades, J. D.; Comini, E.; Pellicer, E.; Avella, M.; Siciliano, P.; Faglia, G.; Cirera, A.; Scotti, R.; Morazzoni, F.; Morante, J. R. The role of surface oxygen vacancies in the NO2 sensing properties of SnO2 nanocrystals, J. Phys. Chem. C 2008, 112, 19540-19546. (29) Setvin, M.; Aschauer, U.; Scheiber, P.; Li, Y. F.; Hou, W.; Schmid, M.; Selloni, A.; Diebold, U. Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101), Science 2013, 341 988-991. (30) Rajendran, S.; Manoj, D.; Raju, K.; Dionysiou, D. D.; Naushad, M.; Gracia, F.; Cornejo, L.; Gracia-Pinilla, M. A.; Ahamad, T. Influence of mesoporous defect induced mixed-valent NiO (Ni2+/Ni3+)-TiO2 nanocomposite for non-enzymatic glucose biosensors, Sensors Actuat. B 2018, 264, 27-37. (31) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 2011, 331, 746-750. (32) Rao, H.; Liu, Y.; Zhong, J.; Zhang, Z.; Zhao, X.; Liu, X.; Jiang, Y.; Zou, P.; Wang, X.; Wang, Y. Gold Nanoparticle/Chitosan@N,S Co-doped Multiwalled Carbon Nanotubes Sensor: Fabrication, Characterization, and Electrochemical Detection of Catechol and Nitrite, ACS Sustainable Chem. Eng. 2017, 5, 10926-10939. (33) Gao, Z.; Sasaki, K.; Qiu, X. Structural memory effect of Mg-Al and Zn-Al layered double hydroxides in the presence of different natural humic acids: process and mechanism, Langmuir 2018, 34, 5386-5395. (34) Pan, X.; Xu, Y. Defect-mediated growth of noble-metal (Ag, Pt, and Pd) nanoparticles on TiO2 with oxygen vacancies for photocatalytic redox reactions under visible light, J. Phys. Chem. C 2013, 117, 17996-18005. (35) Drouilly, C.; Krafft, J. M.; Averseng, F.; Casale, S.; Bazer-Bachi, D.; Chizallet, C.; Lecocq, V.; Vezin, H.; Lauron-Pernot, H.; Costentin, G. ZnO oxygen vacancies formation and filling followed by in situ photoluminescence and in situ EPR, J. Phys. Chem. C 2012, 116, 21297-21307. (36) Ding, J.; Dai, Z.; Tian, F.; Zhou, B.; Zhao, B.; Zhao, H.; Chen, Z.; Liu, Y.; Chen, R. Generation of •• • defect clusters for 1O2 production for molecular oxygen activation in photocatalysis, J.

Mater. Chem. A 2017, 5, 23453-23459. (37) Li, H.; Qin, F.; Yang, Z.; Cui, X.; Wang, J.; Zhang, L. New reaction pathway induced by plasmon for selective benzyl alcohol oxidation on BiOCl possessing oxygen vacancies, J. Am. Chem. Soc. 2017, 139, 3513-3521. (38) Tyagi, P.; Sharma, A.; Tomar, M.; Gupta, V. A comparative study of RGO-SnO2 and MWCNT-SnO2 nanocomposites based SO2 gas sensors, Sens. Actuators, B 2017, 248, 980-986. (39) Chaudhary, V.; Kaur, A. Solitary surfactant assisted morphology dependent chemiresistive polyaniline sensors for room temperature monitoring of low parts per million sulfur dioxide, Polym. Int. 2015, 64, 1475-1481. (40) Liu, Y. Y.; Xu, X. Y.; Chen, Y.; Zhang, Y.; Gao, X. H.; Xu, P. C.; Li, X. X.; Fang, J. H.; Wen, W. J. An integrated micro-chip with Ru/Al2O3/ZnO as sensing material for SO2 detection, Sens. Actuators, B 2018, 262, 26-34. (41) Shen, F.; Wang, D.; Liu, R.; Pei, X.; Zhang, T.; Jin, J. Edge-tailored graphene oxide nanosheet-based field effect transistors for fast and reversible electronic detection of sulfur dioxide, Nanoscale 2013, 5, 537-540. (42) Tyagi, P.; Sharma, A.; Tomar, M.; Gupta, V. SnO2 thin film sensor having NiO catalyst for detection of SO2 gas with improved response characteristics, Sens. Actuators, B 2017, 248,


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


998-1005. (43) Das, S.; Chakraborty, S.; Parkash, O.; Kumar, D.; Bandyopadhyay, S.; Samudrala, S. K.; Sen, A.; Maiti, H. S. Vanadium doped tin dioxide as a novel sulfur dioxide sensor, Talanta 2008, 75, 385-389. (44) Zhang, G.; Zhang, S.; Yang, L.; Zou, Z.; Zeng, D.; Xie, C. La2O3-sensitized SnO2 Nanocrystalline Porous Film Gas Sensors and Sensing Mechanism toward Formaldehyde, Sensor Actuat. B 2013, 188, 137-146. (45) Kar, A.; Kundu, S.; Patra, A. Surface defect-related luminescence properties of SnO2 nanorods and nanoparticles, J. Phys. Chem. C 2011, 115, 118-124. (46) Ding, J.; Dai, Z.; Qin, F.; Zhao, H.; Zhao, S.; Chen, R. Z-scheme BiO1-xBr/Bi2O2CO3 photocatalyst with rich oxygen vacancy as electron mediator for highly efficient degradation of antibiotics, Appl. Catal. B: Environ. 2017, 205, 281-291. (47) Mazumder, N.; Sen, D.; Saha, S.; Ghorai, U. K.; Das, N. S. Chattopadhyay, K. K. Enhanced ultraviolet emission from Mg doped SnO2 nanocrystals at room temperature and its modulation upon H2 annealing, J. Phys. Chem. C 2013, 117, 6454-6461. (48) Zou, Y.; Chen, S.; Sun, J.; Liu, J.; Che, Y.; Liu, X.; Zhang, J.; Yang, D. Highly efficient gas sensor using a hollow SnO2 microfiber for triethylamine detection, ACS Sens. 2017, 2, 897-902. (49) Ganbavle, V. V.; Inamdar, S. I.; Agawane, G. L.; Kim, J. H.; Rajpure, K. Y. Synthesis of fast response, highly sensitive and selective Ni:ZnO based NO2 sensor, Chem. Eng. J. 2016, 286, 36-47. (50) Li, Z.; Wang, N.; Lin, Z.; Wang, J.; Liu, W.; Sun, K.; Fu, Y. Q.; Wang, Z.; Room-temperature high-performance H2S sensor based on porous CuO nanosheets prepared by hydrothermal method, ACS Appl. Mater. Interfaces 2016, 8, 20962-20968. (51) Lee, H.; Jung, Y. M.; Lee, K. I.; Kim, H. S.; Park, H. S. Understanding the unique interaction of amine-containing ionic compounds with SO2 for high absorption capacity, RSC Adv. 2013, 3, 25944-25949.

Table of contents (TOC) graphic



The introduction of oxygen vacancies promotes the reaction between electrons and adsorbed oxygen, forming an electron depletion layer, which results in the formation of Au-SnO2(-x) Schottky contact and extending of the depletion layer of SnO2 nanomaterials.


Page 26 of 26

Oxygen Vacancies as an Efficient Strategy for Promotion of Low

Recommend Documents