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SBA-15 nanohybrid sensors
Accepted Manuscript Fast response with high performance humidity sensing of Ag-SnO2/SBA-15 nanohybrid sensors Vijay K. Tomer, Surender Duhan, Ritu Malik, S.P. Nehra PII:
S1387-1811(15)00446-1
DOI:
10.1016/j.micromeso.2015.08.016
Reference:
MICMAT 7259
To appear in:
Microporous and Mesoporous Materials
Received Date: 8 July 2015 Revised Date:
8 August 2015
Accepted Date: 14 August 2015
Please cite this article as: V.K. Tomer, S. Duhan, R. Malik, S.P. Nehra, Fast response with high performance humidity sensing of Ag-SnO2/SBA-15 nanohybrid sensors, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.08.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Fast response with high performance humidity sensing of Ag-SnO2/SBA-15 nanohybrid sensors Vijay K. Tomera, Surender Duhana, *, Ritu Malik b, S.P. Nehrac a
Nanomaterials Research Laboratory, Department of Materials Science & Nanotechnology b
Department of Physics
Center of Excellence for Energy and Environmental Studies
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c
D.C.R. University of Science & Technology, Murthal (Sonepat) Haryana, 131039 (INDIA) Abstract
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In this work, a simple and straight forward technology of loading Ag nanoparticles in SnO2/SBA15 nanocomposite for measuring room temperature relative humidity (RH) is demonstrated. The
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nanocomposite SnO2/SBA-15 comprising Ag nanoparticles exhibit several fundamental characteristics, such as high specific surface area, open porous structure and good interconnectivity. Therefore, the RH sensors based on Ag-SnO2/SBA-15 hybrid nanocomposite shows very high sensitivity and ultrafast response/recovery time to humidity, which outperform
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current porous structure based sensors. The impedance of synthesized nanocomposite sensor varies from 108 Ω to 102 Ω between 11 RH% and 98 RH%, the humidity range required for a sensor to operate at ambient humidity. Moreover, the resultant sensors also display relatively small
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hysteresis and long-term stability. This study demonstrates that Ag-SnO2/SBA-15 complex hybrid material is a step towards fabricating futuristic high performance humidity sensors.
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Keywords: Nanohybrid; fast response; sensor; SBA-15; humidity * Corresponding author
E-mail: [email protected] Tel.: +91- 9813170944
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ACCEPTED MANUSCRIPT 1. Introduction The ability in detecting and controlling humidity plays an important role in food processing, pharmaceutical/medical industries and environmental fields [1-4]. In the previous
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years, numerous endeavors have been made in development of high performance (excellent sensitivity, linearity, selectivity, stability, fast response/recovery time and small working hysteresis) humidity sensors [5-8]. Different transduction procedures, such as capacitive, resistive,
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acoustic, optical and mechanical techniques, have been embraced for the outline of humidity sensor [7, 9-12]. Recently, materials with high intrinsic surface area such as carbon, silica,
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polymers and metal oxides, have become a focus of intensive research for designing humidity sensors considering the novel surface properties offered by these materials such as small size, high surface-to-volume ratio, high density of surface sites and unique physico-chemical properties [8, 13-15]. SBA-15, an important member of the family of mesoporous silica, is gaining particular attention as an efficient matrix material for realizing highly efficient RH sensors [16-18]. The high
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specific surface area, large pore volume, high thermal stability and long ordered pore channels of SBA-15 increases the chemical reactivity and physical adsorption of water molecules and results
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in enhancing the conductivity of the SBA-15 on exposing to humid conditions [19-20]. SnO2, due to its inherent chemical and physical stability has emerged as an important
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humidity sensing material in the recent times [21-22]. However, the response of SnO2-based RH sensors depends upon the reactions between water molecules and SnO2 surfaces which have stimulated the interest of researchers in tailoring the microstructure and morphology of SnO2 nanostructures [23]. Hence, many scientific and technological efforts have been made to enhance the sensitivity, response-recovery characteristic, selectivity, and stability of SnO2-based RH sensors [24]. Another approach for enhancing the sensing properties of SnO2 lies in its modification with noble metal nanoparticles such as Au, Ag and Pt [25-27]. Among them, metallic 2
ACCEPTED MANUSCRIPT Ag nanoparticles applications in chemical and biological sensors have witnessed a significant growth in past few years due to their extraordinary advantages including low cytotoxicity, high stability, good biocompatibility, conductivity and outstanding ability as activating analyte to
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accelerate the chemical reaction process [28-30]. In this work, with the aim of synthesizing a humidity sensor with improved linearity and quick response/recovery time, we have loaded the SnO2 in pristine SBA-15 materials using one
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step hydrothermal method [31] and further disperse the Ag nanoparticles in SnO2/SBA-15 nanocomposite using wet impregnation process. SBA-15 materials loaded with multi-components
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are known to increase the performance of hybrid nanocomposite [32-33]. In such complex nanohybrid materials, the SBA-15 mesoporous matrices offer plentiful dynamic active sites for the nanoparticle location whereby simultaneously encouraging the large scale molecular diffusion and transportation of charge carriers across the materials surface. SnO2 in the Ag-SnO2/SBA-15 fills the double need of a scattering agent for controlling the aggregation of Ag nanoparticles and also
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plays a key role in improving the sensing performance of the hybrid nanocomposite. To the high of our expectations, the synthesized Ag-SnO2/SBA-15 nanocomposite based RH sensor displays
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an impressive 6 orders change in complete RH% range at 25 °C. In addition, the sensor shows excellent linearity, negligible hysteresis (~ 0.9%), super rapid response and recovery time (5 and 8
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s respectively) and outstanding stability which make the Ag-SnO2/SBA-15 nanohybrid an excellent candidate for the fabrication of futuristic RH sensors. 2. Experimental 2.1 Materials
Tetraethoxy orthosilicate ((C2H5O)4Si, TEOS, Sigma Aldrich), Pluronic P123 (Ethylene Oxide-Propylene Oxide-Ethylene Oxide, (EO20PO70EO20), Mw = 5800, Sigma Aldrich), Tin (II) chloride (SnCl2•2H2O, Merck), Silver Nitrate (AgNO3, Fisher Scientific) and HCl (35%, Fisher Scientific) were used as received. Double distilled water was used throughout the experiments. 3
ACCEPTED MANUSCRIPT 2.2 Material Preparation Synthesis of mesoporous SBA-15: In a typical recipe, 2 g P123 was dissolved in 70 ml distilled water by vigorously stirring (1000 rpm) for 3 h at 45 °C and further followed by the
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addition of 10 ml HCl (2M). To this solution was added 4.6 ml TEOS before keeping the resultant mixture under stirring for 24 h. The white gel products formed were then hydrothermally treated in a Teflon lined stainless steel autoclave at 100 °C for 24 h. The solid products were filtered, washed
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and dried at 70°C and calcined at 550 °C for 6 h in air to remove organic templates and thus pure mesoporous SBA-15 was obtained.
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Synthesis of SnO2/SBA-15: For synthesis of mesoporous SnO2/SBA-15 (5 wt% SnO2) nanocomposite using in-situ method, 2 g P123 was dissolved in 70 ml distilled water at 40 °C with vigorous stirring (1000 rpm), followed by addition of 10 ml HCl (2M). When a clear miceller solution was obtained after 3 h of stirring, 0.1g of Tin chloride salt solution was added to it and stirred for another 2 h. Then, 4.5 g of TEOS was added and resultant solution was kept under
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stirring for 24 h at 45 °C to ensure the homogeneous mixing of silica and tin species. The final SnO2/SBA-15 nanocomposite was obtained by following the same procedure as the one used for
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SBA-15 after 24 h of stirring.
Synthesis of Ag-SnO2/SBA-15: Ag was loaded in SnO2/SBA-15 nanocomposite by wet
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impregnation method as follows: 1 g SnO2/SBA-15 nanocomposite was added in an aqueous solution of 0.1g AgNO3 solution under continuous stirring for 24 h. The stirring was done in dark to prevent the reduction of AgNO3. The dried samples were then recovered and calcined at 600 °C to obtain the complex mesoporous nanocomposite. The influence of Ag loading on the RH response was investigated by varying the concentration of Ag in SnO2/SBA-15. The products were designated as Ag-SnO2/SBA-15(X), where X was the content of Ag in wt%. The values of X in our experiments were 1, 2 and 3 for three different Ag-SnO2/SBA-15 samples. 4
ACCEPTED MANUSCRIPT 2.3 Fabrication and performance test of humidity sensors The powder samples were ground and mixed with ethanol in a weight ratio of 1:20 to form a dilute paste. The paste was drop coated using a 10 µL pipette on the top of ceramic substrate
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(13.4mm x 7mm x 0.5mm) on which a pair of Ag-Pd interdigital electrodes (IDE) had been previously printed. The structure of this electrode is shown in Fig 1. A film of thickness ~5 µm was deposited on the substrate which was finally dried at 80 °C for 12 h. The controlled humidity
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environments were achieved by using fixed point saturated salt solution method. The fixed humidity environments were generated by using 6 different saturated solutions LiCl, MgCl2•6H2O,
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MgNO3•4H2O, NaCl, KCl and K2SO4 which yielded 11%, 33%, 54%, 75%, 84% and 98% relative humidity, respectively at 25 °C [34]. The salt was dissolved in water in such a proportion that 30– 90% of the weighted sample remained as dissolved. These salt solutions were placed in a closed vessel overnight so as to attain equilibrium with the air in the closed chamber. Then these vessels were installed in a temperature controller. The ambient air temperature was set at 25 °C and the
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variation of air temperature was kept within 0.2 °C. The sample coated IDE were placed in the glass chambers containing different salt
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solutions. A two probe LCR Hitester (Hioki 3532-50) with a test voltage of 1V AC was used to obtain characteristics RH response curves by measuring change in impedance while exposing the
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sensor in the closed chambers with different RH% environment. The IDE remains in the humid environment of chamber for the uptake of water molecules until the impedance of the sensing material arrived at stable value. To minimize the obvious effect of exposing the sensor to laboratory climate, we have attempted to complete the chamber change process within 1s. 2.4 Characterization of materials X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance diffractometer using CuKα monochromatic radiation (λ=1.5418 Å) 40 kV and 40 mA with a step size of 0.02°. 5
ACCEPTED MANUSCRIPT Nitrogen adsorption-desorption isotherms were measured on Micrometrices (Tristar 3000) at 77K. Before the measurements, the samples were degassed at 300 °C for 6 h. The specific surface area was estimated by the five point Brunauer-Emmett-Teller (BET) method [35], and the pore size
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distribution was derived from the desorption branch of the isotherms by using the Barrett-JoynerHalenda (BJH) analysis [36]. Morphology and elemental composition of the samples was characterized using scanning electron micrographs (FEI QUANTA 200F) at an acceleration
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voltage of 10 - 15 kV. Fourier-Transform Infrared (FTIR) spectra were recorded at room temperature under atmospheric pressure with a resolution of 4 cm-1 in the range of 400 - 4000 cm-1
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using FTIR spectroscopy (Perkin Elmer- Frontier). High resolution transmission electron microscopy (HRTEM) images along with elemental composition were recorded on TECNAI G20 electron microscope equipped with an Energy-dispersive X-Ray spectroscopy at an acceleration voltage of 200 kV.
3.1 Characterization
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3. Results and Discussion
Fig. 2(A) shows the small-angle XRD patterns of SBA-15, SnO2/SBA-15 and AgSnO2/SBA-15(X) nanocomposites. All samples studied gave well-defined XRD patterns in the
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small-angle range with main peak observed at 2θ = 0.8°, which indicates the presence of
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cylindrical hexagonally arranged mesopores [16-17]. These cylindrical tubes also show a longrange ordering which is demonstrated in LAXRD patterns by two additional peaks in the 2θ region of 0−5°. The Ag-SnO2/SBA-15(X) nanocomposite exhibits a very similar pattern to that of pure SBA-15, with well-resolved diffraction peaks which are indexed to the (100), (110) and (200) reflections of two-dimensional hexagonal mesostructure of space group p6mm [16]. The d-spacing [100] was determined to be 10.24, 9.83, 9.15 and 8.37 nm for SnO2/SBA-15 and Ag-SnO2/SBA15(1), (2), (3) respectively (Table 1). As observed, the d-spacing shifts slightly to higher angle with an increase in Ag concentration, suggesting the filling of pore walls of SBA-15. However, the 6
ACCEPTED MANUSCRIPT intensity of reflection [100] in the obtained samples get lower and lower along with the increase of Ag content, because the more inorganic salt added, the more Ag is attached to the inside wall of pore and decrease the order degree of mesoporous SBA-15. Overall, the result indicates that, even
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after loading of Ag in SnO2/SBA-15, mixing and calcination does not destroy the mesoporous structure of nanocomposite.
The wide-angle XRD patterns of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X)
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samples in Fig. 2(B) display well-resolved diffraction peaks of (110), (101), (200), (211), (220), (002), (310), (112) and (301) that can be indexed to a tetragonal structure of SnO2 (JCPDS no. 03-
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1116) and the broad peak centered at 2θ = 22° represents the characteristic band of amorphous silica walls of the pristine material [16]. The reflection from Ag nanoparticles were not observed for samples containing 1-2 wt% of Ag, however, small diffraction peaks of Ag reflections at 2θ = 44.2° and 64.3° (circled in red color) corresponding to (200) and (220) planes of Ag (JCPDS no.
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04-0783) were observed for 3 wt% Ag loaded in SnO2/SBA-15 nanocomposite. These reflections correspond to pure silver metal with face centered cubic (FCC) symmetry and space group
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The growing pattern of Ag reflection peaks confirms the loading of Ag in SnO2/SBA-15
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nanocomposite.
Nitrogen physisorption measurements and pore size distributions curves of SBA-15,
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SnO2/SBA-15 and Ag-SnO2/SBA-15(X) hybrid materials are shown in Fig. 3. The adsorption and desorption isotherms of both materials display type IV isotherms with H1-type hysteresis loops at the high relative pressure according to the IUPAC classification, which is a characteristic of capillary condensation within uniform pores [16-17]. A sharp inflection in P/P0 > 0.6 associated with capillary condensation was observed for all the samples which denote a steep jump in the N2 adsorption volume, thus confirming their mesoporous structures and narrow pore size distributions. The shape was well maintained for Ag-SnO2/SBA-15(X), providing further proof on 7
ACCEPTED MANUSCRIPT conserving of the mesoporous structure of framework matrices after loading of Ag nanoparticles. The physico-chemical properties of pure SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) composites are summarized in Table 1. Textural properties such as specific surface area, pore
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diameter and pore volume of hybrid nanocomposites systematically decrease with the increased amount of Ag nanoparticles. However, the decreasing amount of surface area and pore volume are not directly proportional to the amount of Ag nanoparticles, which may be attributed to pore
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blockage by Ag particles. The decrease in pore diameter indicates that the pore wall becomes thicker, showing that some SnO2 and Ag nanoparticles has entered into the mesopores.
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The FTIR spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) are shown in Fig. 4. The framework bands at 1092, 808 and 464 cm-1 are due to asymmetric stretching, symmetric stretching and bend vibrations of Si-O-Si bands, respectively. The broad band around 3430 cm-1 represents the surface silanols and adsorbed moisture. The absorbance band which was centered at
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1635 and 968 cm-1 belongs to the stretching vibrations of the Si-OH group. As observed, the absorbance intensity decreases with successive loading of metal nanoparticles in SBA-15 implying that Si-OH groups were changed or consumed in the hybrid nanocomposite. The changes of band
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intensity at 960-970 cm-1 is assigned to stretching vibration of the Si-O-M (Metal) linkage and denotes the loading of Ag into SBA-15 framework. With the loading of Ag and SnO2 in SBA-15
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framework, the Si-O-H replaces with Si-O-M and a decrease in the peak intensity was observed which further justifies the presence of Ag nanoparticles in SnO2/SBA-15 nanocomposite. To further prove that the meso-ordered structure of the samples was retained after
incorporation, the hybrid materials were characterized by using HRTEM. The HRTEM image of the SBA-15 and SnO2/SBA-15 in Fig. 5(a) and Fig 5(b) displays a well-ordered 2D p6mm regular hexagonal mesostructure with ordered, strip-like channels, which are the characteristic (001)
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ACCEPTED MANUSCRIPT direction of the one-dimensional channels templated by the block copolymer P123 [16]. The TEOS and SnCl2•2H2O are mixed evenly due to the “synchronous assembly strategy” whereby the reactant and precursor salts are mixed evenly during continuous stirring. Fig 5(b) shows uniform
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SnO2 particles located in the mesoporous channels of SBA-15. However, the Ag0 nanoparticles were loaded in SnO2/SBA-15 nanocomposite in their metallic state. The loading process initiated during the process of calcination in wet impregnation method. The size of most Ag nanoparticles
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is comparable to the pore diameter of SBA-15 framework whereas some larger ones embedded in channels are also observed. The pore channels were also not destroyed due to the presence of Ag
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nanoparticles.
The EDX spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) are shown in Fig. 5(d), Fig 5(e) and Fig 5(f) respectively. Strong signals of elemental tin and silicon were detected by EDXS, indicating that SnO2 has been supported on the mesoporous SBA-15 silica material. However, the results in Table-1 reveal that the efficiency of Ag loading in SnO2/SBA-15 decreases
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continuously with successive loading of Ag nanoparticles in the SBA-15 matrix. This inverse proportional relation between theoretical and experimental loading concentration of Ag
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nanoparticles could be due to constant pore blockage of SBA-15 leading to the depreciation of effective Si-OH bonding sites for Ag nanoparticles.
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SEM images in Fig. 6 clearly confirms that the wheat like morphology of the pristine SBA15 has been well preserved for all the samples, suggesting no obvious damage to the frameworks during the process of introduction of the guest species. The morphologies are found to exhibit typical entangled structures that are composed of many short rods domains with relatively similar sizes of 0.3-1 µm. This type of morphology has been proven to have long range parallel channels with the 2-D hexagonal mesostructure.
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ACCEPTED MANUSCRIPT 3.2 Humidity sensing properties In this study, the humidity sensing performance based on parameters such as humidity sensitivity, response/recovery time, hysteresis and stability of the hybrid materials, was tested
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under various levels of RH%. The reported data are the mean values obtained from various measurement cycles at room temperature (25 °C). Fig. 7(a) shows the dependence of sensor impedance on relative humidity. In the 11 RH% to 98 RH% range, pure SBA-15 shows a poor
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response, however, a steep drop off in impedance was observed for hybrid samples in complete RH% range which strongly justifies the contribution of Ag and SnO2 species in enhancing the
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humidity sensing performance of mesoporous hybrid materials. In case of only SnO2 loaded in mesoporous silica SBA-15, a change of 5.5 orders was observed, which is quite better than RH response of pure SBA-15 material. However, the RH response crosses the ‘6 orders change’ bar when Ag nanoparticles mark their presence in the SnO2/SBA-15 framework. As shown in Fig 7(b), the RH response increases from 1wt% and 2 wt% Ag loaded in SnO2/SBA-15, where after,
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the sensitivity decreases (Table 1). The lowering of response of the hybrid material above 2 wt% Ag loaded in SnO2/SBA-15 nanocomposite could be due to blockage of pore channels of SBA-15
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with Ag nanoparticles. The pore blockage causes the obstacle in the transmission of charge carriers across the mesoporous channels. Overall, the nanocomposite having 2 wt% of Ag loaded
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in SnO2/SBA-15 shows highest change in impedance and also exhibits best linearity, therefore this nanocomposite was selected for evaluation of further RH sensing characteristics. Fig. 8 shows the impedance variation with RH% at various frequencies (50 Hz, 100 Hz,
1000 Hz, 2500 Hz and 5000 Hz) under an AC test voltage of 0.5 V. As the RH% increases, more amount of water molecules get adsorbed on the sensor surface, which strengthen the polarization and in turn increases the dielectric constant of the material leading to a decreases in the impedance of sensor. The impedance was almost flat at 5000 Hz, showing that the impedance became 10
ACCEPTED MANUSCRIPT independent of humidity with increasing frequency because the electric field direction changes gradually at low frequencies and there shows up the space charge polarization of adsorbed water [17]. At higher frequencies, a quick change in electric field direction was observed which cannot
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be meet up with the polarization of adsorbed water thus causing the dielectric constant small and independent of RH. As the sensitivity was highest at 100 Hz frequency, we have performed experiments related to response/recovery times, hysteresis and stability at 100 Hz.
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Hysteresis is an important characteristic of a RH sensor to determine stability by measuring the time slack in adsorption and desorption processes. The sensing material was exchanged in
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increasing (low RH% to high RH%) and decreasing (high RH% to low RH%) RH% for the process of adsorption and desorption respectively. The hysteresis error the expression,
, where,
was calculated using
is the difference in output of adsorption and
desorption processes and FFS is the full scale output [17]. The hysteresis response in Fig. 9 shows
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excellent reversible characteristics, where the adsorption and desorption curves nearly coincide with each other. Hysteresis error,
was calculated to be 0.9%, much lesser than earlier reported
results (Table 2), indicating a good reliability of the sensor.
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Both response and recovery times significantly affect the performance of RH sensors and are important variables to judge the actual performance of humidity sensing materials. Response
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and recovery times are defined as the time taken by the sensor to obtain ~90% of overall impedance change in case of adsorption and desorption, respectively [30]. According to the observations made in Fig. 10, the sensor response time (humidification from 11 RH% to 98 RH%) was 5 s, and the recovery time (desiccation from 98 RH% to 11 RH%) was 8 s, both better than conventional resistive sensors utilizing hybrid SBA-15 materials and SnO2 supported nanocomposites (Table 2).
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ACCEPTED MANUSCRIPT To determine the dynamic response of Ag-SnO2/SBA-15(2) nanocomposite based sensor towards rapid variations in the 11-98 RH% range, the sensor was switched between closed chambers in four loops (11 RH% → 98 RH% → 11 RH% → 98 RH% → 11 RH%). The
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measurements shows that the sensor response is highly reversible and the impedance values at 11 RH% and 98 RH% were achieved precisely in four reiterated circles of estimations, recommending an excellent reproducibility of the RH response. The standard deviation in process
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of exchanging of sensor reaction was calculated to be 0.8%. Moreover, the RH response of the sensor tested every 5th day (Fig. 11) shows excellent consistency and a minor variation in
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impedance (~ 1.1%) was observed at each humidity level. The Ag-SnO2/SBA-15(2) sensor sensitivity is measured by increasing RH values from 11% to 98% and the results are shown in Fig. 12. As observed, the sensitivities were highly linear with the relative humidity. 3.3 Humidity sensing mechanism
Mesoporous materials because of their high surface area, high pore volume and long,
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uniform pore channels encourages the introduction of dynamic surface active sites for easier absorption of water molecules and also helps in smoother propagation of charge carriers on the internal and external surface of the material. At low RH, water molecules are primarily
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physisorbed onto the available active sites (Si-OH) of the Ag-SnO2/SBA-15(2) surface through
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double hydrogen bonding, which is called the first-layer physisorption of water. In this regime, the water molecules are unable to move freely because of the restriction from double hydrogen bonding. The hopping transfer of protons between adjacent hydroxyl groups in the first-layer physical adsorption of water requires much energy and due to their little presence at low RH%, they are restricted by discontinuous mobile layers causing Ag-SnO2/SBA-15(2) to exhibit strong impedance. However, as the RH% increases, the multilayer physical adsorption of water molecules occurs. From the second physisorbed layer, water molecules are physisorbed through single hydrogen bonding on the hydroxyl groups. Thereafter, the water molecules become mobile 12
ACCEPTED MANUSCRIPT and progressively more identical to those in the bulk liquid. As the multilayer physical adsorption progresses, the physisorbed water can be ionized under an electrostatic field to produce a large number of hydronium ions (H3O+) as charge carriers. With further increase in humidity, the
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physisorbed water layers gradually exhibit liquid-like behavior. In bulk liquid, proton hopping between adjacent water molecules occurs in Ag-SnO2/SBA-15(2), with charge transport taking place via the conductivity generated by a Grotthuss chain reaction (H2O + H3O+ → H3O+ + H2O)
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conductivity, which causes a dramatic increase in conductivity of the sensor [45]. A schematic of adsorption of water molecules on the structure of Ag-SnO2/SBA-15 has been shown in Fig 13.
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This finding explains the excellent humidity sensing characteristics of Ag-SnO2/SBA-15(2) hybrid nanocomposite.
To validate the sorption mechanism of Ag-SnO2/SBA-15(2) at different RH%, complex impedance spectra were plotted at different RH values and measured over a frequency range to understand the polarization and conductivity processes taking place in a nanocomposite material.
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The impedance spectra denotes to different physical phenomena for the electrical conductivity and polarization that occur in nanocomposite surface in the presence of water molecules. At low RH%
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(11 RH%), a portion of half semicircle is observed in the complex impedance spectra (Fig 14a). The semicircle emerged from the intrinsic impedance of the nanocomposite material sensing layer.
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As the RH% increases, the curvature of semicircle disappears (Fig 14b) and is left with a line appearing at higher RH% (84 and 98 RH%). This process of decrease in the curvature of semicircle with increasing RH% denotes the decrease in the intrinsic impedance, which is related to the interaction between the nanocomposite surface and water molecules. The straight line results from the ionic and/or electrolytic conductivity. This line represents the Warburg impedance which is related to the diffusion of the electroactive species at the electrode [46]. The electronic conductivity of complex nanocomposite Ag-SnO2/SBA-15(2) relies upon the surface coverage of 13
ACCEPTED MANUSCRIPT adsorbed water. At low RH levels (11%), the water vapor is insignificant and so is their adsorption of nanocomposite surface. The water molecules are chemically adsorbed on nanocomposite surface by means of hydrogen bond. When RH% increases, successive layers of water molecule
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are adsorbed and formed the first physical adsorption through double hydrogen bonding which resembles like a bulk liquid phase of water. The protons are released from the hydration of H3O+ (H3O+ → H2O + H+) at these layers and become the main source of charge carriers across the
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nanocomposite surface leading to a significant decrease in impedance of the material. The presence of Sn4+ ions provides dynamic locales for the adsorption of water atoms due to their high
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charge density [21] and causes the frequent ionization of physisorbed water molecules at the surface and pores of the nanocomposite resulting in generation of H+ ions for electrical conduction. Furthermore, the oxygen molecules of water atoms like to adsorb on the Ag metal nanoparticles by giving their lone-pair of electrons prompting an increment in the number of e- in the conduction sp band of Ag [30]. This generation of additional electrons on the water layer
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results in quick conduction and a sharp lessening in impedance is observed. The process of ionization increases from 11 RH% to 98 RH% and a significant increase in charge carriers leads to a remarkable ~6 orders change in impedance of the material.
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In general, the presence of Sn4+ ions and Ag metal nanoparticles in mesoporous SBA-15
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account for the excellent sensing response of the nanocomposite synthesized using a sequence of in-situ and wet impregnation process. The dual metal nanoparticle loaded SBA-15 hybrid nanocomposite shows excellent performance along with rapid response/recovery times, relatively negligible hysteresis and outstanding stability. A comparison with other humidity sensors available in market (Table 3) shows that our material possess better RH% sensing characteristics and justifies for being used in commercial applications.
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ACCEPTED MANUSCRIPT 4. Conclusion In summary, Ag-SnO2/SBA-15 nanohybrids were synthesized using a combination of both in-situ and wet impregnation processes. Large mesoporous surface area supported by the presence
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of Sn4+ and Ag0 nanoparticles, endow the nanocomposite with excellent humidity sensing behavior. Electrical testing under different RH environments shows that the nanohybrid sensor based on Ag-SnO2/SBA-15(2) exhibits ultrahigh sensitivity (12.0 x 105 at 100 Hz) over the entire RH range. Furthermore, the sensor shows excellent performance along with rapid response (5 s)
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and recovery times (8 s), relatively low hysteresis (0.9%), and excellent stability (1.1%) in 11-98
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RH% range. We also established a mechanism that can potentially explain the excellent performance of the Ag-SnO2/SBA-15 based nanohybrid sensors. Our material supports the capability of utilizing complex nanohybrid mesoporous materials for novel RH sensing applications.
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Acknowledgements
Authors are grateful to UGC, New Delhi (Grant No. 41-997/2012(SR)) for providing financial assistance. Authors are also thankful to Dr. I.S. Mulla (Emeritus scientist, CSIR, India)
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for his thoughtful and valuable suggestions.
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[17] V. K. Tomer, S. Duhan, P.V. Adhyapak, I.S. Mulla, J. Am. Ceram. Soc., 98 (2015) 741 [18] Q. Yuan, N. Li, W. Geng, Y. Chi, J. Tu, X. Li, C. Shao, Sens. Actuators B, 160 (2011) 334 [19] B. J. Melde, B. J. Johnson, P. T. Charles, Sensors, 8 (2008) 5202. 16
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[34] L. Greenspan, J. Res. Nat. Bur. Stand., 81 (1977) 89. [35] S. Brunauer, P. H. Emmet, E. Teller, J. Am. Chem. Soc., 60 (1938) 309. [36] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. [37] R. Wang, T. Zhang, Y. He, X. Li, W. Geng, J. Tu, Q. Yuan, J. App. Polymer Sci., 115 (2010) 3474. [38] Q. Qi, T. Zhang, X. Zheng, L. Wan, Sens. Actuators B, 135 (2008) 255 [39] R. Wang, X. Liu, Y. He, Q. Yuan, X. Li, G. Lu, T. Zhang, Sens. Actuators B, 145 (2010) 386. 17
ACCEPTED MANUSCRIPT [40] T. Zhang, R. Wang, W. Geng, X. Li, Q. Qi, Y. He, S. Wang, Sens. Actuators B, 128 (2008) 482 [41] M. Anbia, S.E.M. Fard, Sens. Actuators B, 160 (2011) 215– 221
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[46] P. A. Christensen, A. Hammett, Techniques and Mechanisms in Electrochemistry, Springer,
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Netherlands, 1994, DOI: 10.1007/978-0-585-27623-6_2.
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analysis. Table 2: A comparison of humidity sensing performance of hybrid SBA-15 and SnO2 supported nanocomposites.
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Table 3: A comparison of sensing characteristics of commercial humidity sensors.
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d100-spacing (nm) a
ao (nm)b
DP (nm)c
DW (nm)e
VP (cm3/g)d
SBET (m2/g)f
Effective Ag wt% g
Sensitivity, (S)h
SBA-15
10.54
12.17
8.98
3.19
1.25
878
--
3.69 x 102
SnO2/SBA-15
10.24
11.82
7.11
4.71
1.02
661
--
2.1 x 105
X=1
9.83
11.35
6.68
4.75
0.94
X=2
9.15
10.57
5.77
4.8
0.85
X=3
8.37
9.66
4.82
4.84
0.79
d100: d-spacing a0: Unit cell parameter [a0 = 2 x d100/√3] c DP: Pore size d VP: Pore volume e DW: Pore wall thickness [DW = a0 - DP] f SBET: Total surface area g Calculated from EDX studies Sensitivity, S =
0.62
4.7 x 105
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[37] [38] [39] [40] [41]
100 100
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26 8
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[44] This work
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Reference
Table 2
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3 3
Hysteresis
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NiO-PPY/SBA-15 Fe/SBA-15 MgO/SBA-15 Li/SBA-15 La3+ and K+ doped TiO2–10 mol% SnO2 SnWO4-SnO2 K+-doped SnO2– LiZnVO4 MgO-KCl/SiO2 Ag-SnO2/SBA-15
Recovery time (s) 90 50 20 180 18
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1 2 3 4 5
Order of impedance change in Response complete RH% range time (s) 3.5 45 3.5 20 3.5 10 3.5 60 5 11
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C C R R E E E E E
DHT22 SHT15 Polymer HR202 DHT11 HMT3301 HMT3302 HMT3303 HC2-C Fluke 971
Sensitivity Range (%RH) 0-100 0-100 20-95 20-90 0-100 0-100 0-100 0-100 5-95
C: Capacitive; R: Resistive; E: Electronic : With grid filter; 2: With steel netting filter, 3: With sintered filter
Recovery Hysteresis time (s) (%) NA 2 8 2 10 1 10 4 17 1 50 1 60 1 NA NA NA NA
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Response time (s) NA 8 10 10 8 20 40 15 60
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ACCEPTED MANUSCRIPT Figure Captions: Figure 1: Schematic drawing of the interdigitated electrode (IDE) substrate. Figure 2: (A) Low-angle (2θ = 0.5° - 5.0°) XRD and (B) Wide angle XRD spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(1), (2), (3) nanohybrids denoted from (a) to (e) respectively.
SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) nanohybrids.
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Figure 3: (a) N2 adsorption-desorption isotherms curves and (b) Pore size distributions curves for
Figure 4: FTIR spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(1), (2), (3) nanohybrids
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Figure 5: HRTEM image showing uniform channels with long range order of (a) SBA-15, (b) SnO2/SBA-15 and (c) Ag-SnO2/SBA-15(2) nanocomposite, (d-f) corresponding EDX spectra of
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SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(2) nanocomposites.
Figure 6: SEM image of (a) SBA-15, (b) SnO2/SBA-15 and (c) Ag-SnO2/SBA-15(2) nanocomposite,
Figure 7: (a) Humidity sensing curves showing decrease in impedance with increase in RH% for SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) nanocomposites, (b) Sensitivity of Ag-
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Figure 8: Relationship of impedance and relative humidity based on nanocomposite AgSnO2/SBA-15(2) at various frequencies.
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Figure 9: Hysteresis curve showing adsorption-desorption responses measured in the 11-98 RH% range for Ag-SnO2/SBA-15(2) nanocomposite.
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Figure 10: Response/recovery time of the Ag-SnO2/SBA-15(2) sensor for humidity levels between 11 RH% and 98 RH%. Repeated measurement of impedance in four RH loop to determine the stability of the sensor.
Figure 11: The response of Ag-SnO2/SBA-15(2) monitored at different humidity conditions for 30 days.
Figure 12: Relative Sensitivity of Ag-SnO2/SBA-15(2) measured at each RH%. Figure 13: A schematic of adsorption of water layers on the Ag-SnO2/SBA-15 surface. Figure 14: The measured and simulated complex impedance spectra (Nyquist plot) based on AgSnO2/SBA-15(2), where RH varies from 11% to 98%. 23
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Highlights 1. Mesoporous SnO2/SBA-15 was synthesized by in-situ hydrothermal processes. 2. Ag nanoparticles were loaded in SnO2/SBA-15 using wet impregnation process.
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3. Impedance of Ag-SnO2/SBA-15 sensor changes from 108 Ω to 102 Ω in 11-98 %RH range.
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4. The sensor displays superfast response (5 s) and recovery time (8 s).
S1387-1811(15)00446-1
DOI:
10.1016/j.micromeso.2015.08.016
Reference:
MICMAT 7259
To appear in:
Microporous and Mesoporous Materials
Received Date: 8 July 2015 Revised Date:
8 August 2015
Accepted Date: 14 August 2015
Please cite this article as: V.K. Tomer, S. Duhan, R. Malik, S.P. Nehra, Fast response with high performance humidity sensing of Ag-SnO2/SBA-15 nanohybrid sensors, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.08.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Fast response with high performance humidity sensing of Ag-SnO2/SBA-15 nanohybrid sensors Vijay K. Tomera, Surender Duhana, *, Ritu Malik b, S.P. Nehrac a
Nanomaterials Research Laboratory, Department of Materials Science & Nanotechnology b
Department of Physics
Center of Excellence for Energy and Environmental Studies
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c
D.C.R. University of Science & Technology, Murthal (Sonepat) Haryana, 131039 (INDIA) Abstract
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In this work, a simple and straight forward technology of loading Ag nanoparticles in SnO2/SBA15 nanocomposite for measuring room temperature relative humidity (RH) is demonstrated. The
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nanocomposite SnO2/SBA-15 comprising Ag nanoparticles exhibit several fundamental characteristics, such as high specific surface area, open porous structure and good interconnectivity. Therefore, the RH sensors based on Ag-SnO2/SBA-15 hybrid nanocomposite shows very high sensitivity and ultrafast response/recovery time to humidity, which outperform
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current porous structure based sensors. The impedance of synthesized nanocomposite sensor varies from 108 Ω to 102 Ω between 11 RH% and 98 RH%, the humidity range required for a sensor to operate at ambient humidity. Moreover, the resultant sensors also display relatively small
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hysteresis and long-term stability. This study demonstrates that Ag-SnO2/SBA-15 complex hybrid material is a step towards fabricating futuristic high performance humidity sensors.
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Keywords: Nanohybrid; fast response; sensor; SBA-15; humidity * Corresponding author
E-mail: [email protected] Tel.: +91- 9813170944
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ACCEPTED MANUSCRIPT 1. Introduction The ability in detecting and controlling humidity plays an important role in food processing, pharmaceutical/medical industries and environmental fields [1-4]. In the previous
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years, numerous endeavors have been made in development of high performance (excellent sensitivity, linearity, selectivity, stability, fast response/recovery time and small working hysteresis) humidity sensors [5-8]. Different transduction procedures, such as capacitive, resistive,
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acoustic, optical and mechanical techniques, have been embraced for the outline of humidity sensor [7, 9-12]. Recently, materials with high intrinsic surface area such as carbon, silica,
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polymers and metal oxides, have become a focus of intensive research for designing humidity sensors considering the novel surface properties offered by these materials such as small size, high surface-to-volume ratio, high density of surface sites and unique physico-chemical properties [8, 13-15]. SBA-15, an important member of the family of mesoporous silica, is gaining particular attention as an efficient matrix material for realizing highly efficient RH sensors [16-18]. The high
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specific surface area, large pore volume, high thermal stability and long ordered pore channels of SBA-15 increases the chemical reactivity and physical adsorption of water molecules and results
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in enhancing the conductivity of the SBA-15 on exposing to humid conditions [19-20]. SnO2, due to its inherent chemical and physical stability has emerged as an important
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humidity sensing material in the recent times [21-22]. However, the response of SnO2-based RH sensors depends upon the reactions between water molecules and SnO2 surfaces which have stimulated the interest of researchers in tailoring the microstructure and morphology of SnO2 nanostructures [23]. Hence, many scientific and technological efforts have been made to enhance the sensitivity, response-recovery characteristic, selectivity, and stability of SnO2-based RH sensors [24]. Another approach for enhancing the sensing properties of SnO2 lies in its modification with noble metal nanoparticles such as Au, Ag and Pt [25-27]. Among them, metallic 2
ACCEPTED MANUSCRIPT Ag nanoparticles applications in chemical and biological sensors have witnessed a significant growth in past few years due to their extraordinary advantages including low cytotoxicity, high stability, good biocompatibility, conductivity and outstanding ability as activating analyte to
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accelerate the chemical reaction process [28-30]. In this work, with the aim of synthesizing a humidity sensor with improved linearity and quick response/recovery time, we have loaded the SnO2 in pristine SBA-15 materials using one
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step hydrothermal method [31] and further disperse the Ag nanoparticles in SnO2/SBA-15 nanocomposite using wet impregnation process. SBA-15 materials loaded with multi-components
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are known to increase the performance of hybrid nanocomposite [32-33]. In such complex nanohybrid materials, the SBA-15 mesoporous matrices offer plentiful dynamic active sites for the nanoparticle location whereby simultaneously encouraging the large scale molecular diffusion and transportation of charge carriers across the materials surface. SnO2 in the Ag-SnO2/SBA-15 fills the double need of a scattering agent for controlling the aggregation of Ag nanoparticles and also
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plays a key role in improving the sensing performance of the hybrid nanocomposite. To the high of our expectations, the synthesized Ag-SnO2/SBA-15 nanocomposite based RH sensor displays
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an impressive 6 orders change in complete RH% range at 25 °C. In addition, the sensor shows excellent linearity, negligible hysteresis (~ 0.9%), super rapid response and recovery time (5 and 8
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s respectively) and outstanding stability which make the Ag-SnO2/SBA-15 nanohybrid an excellent candidate for the fabrication of futuristic RH sensors. 2. Experimental 2.1 Materials
Tetraethoxy orthosilicate ((C2H5O)4Si, TEOS, Sigma Aldrich), Pluronic P123 (Ethylene Oxide-Propylene Oxide-Ethylene Oxide, (EO20PO70EO20), Mw = 5800, Sigma Aldrich), Tin (II) chloride (SnCl2•2H2O, Merck), Silver Nitrate (AgNO3, Fisher Scientific) and HCl (35%, Fisher Scientific) were used as received. Double distilled water was used throughout the experiments. 3
ACCEPTED MANUSCRIPT 2.2 Material Preparation Synthesis of mesoporous SBA-15: In a typical recipe, 2 g P123 was dissolved in 70 ml distilled water by vigorously stirring (1000 rpm) for 3 h at 45 °C and further followed by the
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addition of 10 ml HCl (2M). To this solution was added 4.6 ml TEOS before keeping the resultant mixture under stirring for 24 h. The white gel products formed were then hydrothermally treated in a Teflon lined stainless steel autoclave at 100 °C for 24 h. The solid products were filtered, washed
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and dried at 70°C and calcined at 550 °C for 6 h in air to remove organic templates and thus pure mesoporous SBA-15 was obtained.
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Synthesis of SnO2/SBA-15: For synthesis of mesoporous SnO2/SBA-15 (5 wt% SnO2) nanocomposite using in-situ method, 2 g P123 was dissolved in 70 ml distilled water at 40 °C with vigorous stirring (1000 rpm), followed by addition of 10 ml HCl (2M). When a clear miceller solution was obtained after 3 h of stirring, 0.1g of Tin chloride salt solution was added to it and stirred for another 2 h. Then, 4.5 g of TEOS was added and resultant solution was kept under
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stirring for 24 h at 45 °C to ensure the homogeneous mixing of silica and tin species. The final SnO2/SBA-15 nanocomposite was obtained by following the same procedure as the one used for
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SBA-15 after 24 h of stirring.
Synthesis of Ag-SnO2/SBA-15: Ag was loaded in SnO2/SBA-15 nanocomposite by wet
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impregnation method as follows: 1 g SnO2/SBA-15 nanocomposite was added in an aqueous solution of 0.1g AgNO3 solution under continuous stirring for 24 h. The stirring was done in dark to prevent the reduction of AgNO3. The dried samples were then recovered and calcined at 600 °C to obtain the complex mesoporous nanocomposite. The influence of Ag loading on the RH response was investigated by varying the concentration of Ag in SnO2/SBA-15. The products were designated as Ag-SnO2/SBA-15(X), where X was the content of Ag in wt%. The values of X in our experiments were 1, 2 and 3 for three different Ag-SnO2/SBA-15 samples. 4
ACCEPTED MANUSCRIPT 2.3 Fabrication and performance test of humidity sensors The powder samples were ground and mixed with ethanol in a weight ratio of 1:20 to form a dilute paste. The paste was drop coated using a 10 µL pipette on the top of ceramic substrate
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(13.4mm x 7mm x 0.5mm) on which a pair of Ag-Pd interdigital electrodes (IDE) had been previously printed. The structure of this electrode is shown in Fig 1. A film of thickness ~5 µm was deposited on the substrate which was finally dried at 80 °C for 12 h. The controlled humidity
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environments were achieved by using fixed point saturated salt solution method. The fixed humidity environments were generated by using 6 different saturated solutions LiCl, MgCl2•6H2O,
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MgNO3•4H2O, NaCl, KCl and K2SO4 which yielded 11%, 33%, 54%, 75%, 84% and 98% relative humidity, respectively at 25 °C [34]. The salt was dissolved in water in such a proportion that 30– 90% of the weighted sample remained as dissolved. These salt solutions were placed in a closed vessel overnight so as to attain equilibrium with the air in the closed chamber. Then these vessels were installed in a temperature controller. The ambient air temperature was set at 25 °C and the
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variation of air temperature was kept within 0.2 °C. The sample coated IDE were placed in the glass chambers containing different salt
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solutions. A two probe LCR Hitester (Hioki 3532-50) with a test voltage of 1V AC was used to obtain characteristics RH response curves by measuring change in impedance while exposing the
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sensor in the closed chambers with different RH% environment. The IDE remains in the humid environment of chamber for the uptake of water molecules until the impedance of the sensing material arrived at stable value. To minimize the obvious effect of exposing the sensor to laboratory climate, we have attempted to complete the chamber change process within 1s. 2.4 Characterization of materials X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance diffractometer using CuKα monochromatic radiation (λ=1.5418 Å) 40 kV and 40 mA with a step size of 0.02°. 5
ACCEPTED MANUSCRIPT Nitrogen adsorption-desorption isotherms were measured on Micrometrices (Tristar 3000) at 77K. Before the measurements, the samples were degassed at 300 °C for 6 h. The specific surface area was estimated by the five point Brunauer-Emmett-Teller (BET) method [35], and the pore size
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distribution was derived from the desorption branch of the isotherms by using the Barrett-JoynerHalenda (BJH) analysis [36]. Morphology and elemental composition of the samples was characterized using scanning electron micrographs (FEI QUANTA 200F) at an acceleration
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voltage of 10 - 15 kV. Fourier-Transform Infrared (FTIR) spectra were recorded at room temperature under atmospheric pressure with a resolution of 4 cm-1 in the range of 400 - 4000 cm-1
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using FTIR spectroscopy (Perkin Elmer- Frontier). High resolution transmission electron microscopy (HRTEM) images along with elemental composition were recorded on TECNAI G20 electron microscope equipped with an Energy-dispersive X-Ray spectroscopy at an acceleration voltage of 200 kV.
3.1 Characterization
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Fig. 2(A) shows the small-angle XRD patterns of SBA-15, SnO2/SBA-15 and AgSnO2/SBA-15(X) nanocomposites. All samples studied gave well-defined XRD patterns in the
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small-angle range with main peak observed at 2θ = 0.8°, which indicates the presence of
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cylindrical hexagonally arranged mesopores [16-17]. These cylindrical tubes also show a longrange ordering which is demonstrated in LAXRD patterns by two additional peaks in the 2θ region of 0−5°. The Ag-SnO2/SBA-15(X) nanocomposite exhibits a very similar pattern to that of pure SBA-15, with well-resolved diffraction peaks which are indexed to the (100), (110) and (200) reflections of two-dimensional hexagonal mesostructure of space group p6mm [16]. The d-spacing [100] was determined to be 10.24, 9.83, 9.15 and 8.37 nm for SnO2/SBA-15 and Ag-SnO2/SBA15(1), (2), (3) respectively (Table 1). As observed, the d-spacing shifts slightly to higher angle with an increase in Ag concentration, suggesting the filling of pore walls of SBA-15. However, the 6
ACCEPTED MANUSCRIPT intensity of reflection [100] in the obtained samples get lower and lower along with the increase of Ag content, because the more inorganic salt added, the more Ag is attached to the inside wall of pore and decrease the order degree of mesoporous SBA-15. Overall, the result indicates that, even
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after loading of Ag in SnO2/SBA-15, mixing and calcination does not destroy the mesoporous structure of nanocomposite.
The wide-angle XRD patterns of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X)
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samples in Fig. 2(B) display well-resolved diffraction peaks of (110), (101), (200), (211), (220), (002), (310), (112) and (301) that can be indexed to a tetragonal structure of SnO2 (JCPDS no. 03-
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1116) and the broad peak centered at 2θ = 22° represents the characteristic band of amorphous silica walls of the pristine material [16]. The reflection from Ag nanoparticles were not observed for samples containing 1-2 wt% of Ag, however, small diffraction peaks of Ag reflections at 2θ = 44.2° and 64.3° (circled in red color) corresponding to (200) and (220) planes of Ag (JCPDS no.
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04-0783) were observed for 3 wt% Ag loaded in SnO2/SBA-15 nanocomposite. These reflections correspond to pure silver metal with face centered cubic (FCC) symmetry and space group
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The growing pattern of Ag reflection peaks confirms the loading of Ag in SnO2/SBA-15
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Nitrogen physisorption measurements and pore size distributions curves of SBA-15,
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SnO2/SBA-15 and Ag-SnO2/SBA-15(X) hybrid materials are shown in Fig. 3. The adsorption and desorption isotherms of both materials display type IV isotherms with H1-type hysteresis loops at the high relative pressure according to the IUPAC classification, which is a characteristic of capillary condensation within uniform pores [16-17]. A sharp inflection in P/P0 > 0.6 associated with capillary condensation was observed for all the samples which denote a steep jump in the N2 adsorption volume, thus confirming their mesoporous structures and narrow pore size distributions. The shape was well maintained for Ag-SnO2/SBA-15(X), providing further proof on 7
ACCEPTED MANUSCRIPT conserving of the mesoporous structure of framework matrices after loading of Ag nanoparticles. The physico-chemical properties of pure SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) composites are summarized in Table 1. Textural properties such as specific surface area, pore
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diameter and pore volume of hybrid nanocomposites systematically decrease with the increased amount of Ag nanoparticles. However, the decreasing amount of surface area and pore volume are not directly proportional to the amount of Ag nanoparticles, which may be attributed to pore
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blockage by Ag particles. The decrease in pore diameter indicates that the pore wall becomes thicker, showing that some SnO2 and Ag nanoparticles has entered into the mesopores.
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The FTIR spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) are shown in Fig. 4. The framework bands at 1092, 808 and 464 cm-1 are due to asymmetric stretching, symmetric stretching and bend vibrations of Si-O-Si bands, respectively. The broad band around 3430 cm-1 represents the surface silanols and adsorbed moisture. The absorbance band which was centered at
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1635 and 968 cm-1 belongs to the stretching vibrations of the Si-OH group. As observed, the absorbance intensity decreases with successive loading of metal nanoparticles in SBA-15 implying that Si-OH groups were changed or consumed in the hybrid nanocomposite. The changes of band
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intensity at 960-970 cm-1 is assigned to stretching vibration of the Si-O-M (Metal) linkage and denotes the loading of Ag into SBA-15 framework. With the loading of Ag and SnO2 in SBA-15
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framework, the Si-O-H replaces with Si-O-M and a decrease in the peak intensity was observed which further justifies the presence of Ag nanoparticles in SnO2/SBA-15 nanocomposite. To further prove that the meso-ordered structure of the samples was retained after
incorporation, the hybrid materials were characterized by using HRTEM. The HRTEM image of the SBA-15 and SnO2/SBA-15 in Fig. 5(a) and Fig 5(b) displays a well-ordered 2D p6mm regular hexagonal mesostructure with ordered, strip-like channels, which are the characteristic (001)
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ACCEPTED MANUSCRIPT direction of the one-dimensional channels templated by the block copolymer P123 [16]. The TEOS and SnCl2•2H2O are mixed evenly due to the “synchronous assembly strategy” whereby the reactant and precursor salts are mixed evenly during continuous stirring. Fig 5(b) shows uniform
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SnO2 particles located in the mesoporous channels of SBA-15. However, the Ag0 nanoparticles were loaded in SnO2/SBA-15 nanocomposite in their metallic state. The loading process initiated during the process of calcination in wet impregnation method. The size of most Ag nanoparticles
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is comparable to the pore diameter of SBA-15 framework whereas some larger ones embedded in channels are also observed. The pore channels were also not destroyed due to the presence of Ag
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nanoparticles.
The EDX spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) are shown in Fig. 5(d), Fig 5(e) and Fig 5(f) respectively. Strong signals of elemental tin and silicon were detected by EDXS, indicating that SnO2 has been supported on the mesoporous SBA-15 silica material. However, the results in Table-1 reveal that the efficiency of Ag loading in SnO2/SBA-15 decreases
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continuously with successive loading of Ag nanoparticles in the SBA-15 matrix. This inverse proportional relation between theoretical and experimental loading concentration of Ag
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nanoparticles could be due to constant pore blockage of SBA-15 leading to the depreciation of effective Si-OH bonding sites for Ag nanoparticles.
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SEM images in Fig. 6 clearly confirms that the wheat like morphology of the pristine SBA15 has been well preserved for all the samples, suggesting no obvious damage to the frameworks during the process of introduction of the guest species. The morphologies are found to exhibit typical entangled structures that are composed of many short rods domains with relatively similar sizes of 0.3-1 µm. This type of morphology has been proven to have long range parallel channels with the 2-D hexagonal mesostructure.
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ACCEPTED MANUSCRIPT 3.2 Humidity sensing properties In this study, the humidity sensing performance based on parameters such as humidity sensitivity, response/recovery time, hysteresis and stability of the hybrid materials, was tested
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under various levels of RH%. The reported data are the mean values obtained from various measurement cycles at room temperature (25 °C). Fig. 7(a) shows the dependence of sensor impedance on relative humidity. In the 11 RH% to 98 RH% range, pure SBA-15 shows a poor
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response, however, a steep drop off in impedance was observed for hybrid samples in complete RH% range which strongly justifies the contribution of Ag and SnO2 species in enhancing the
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humidity sensing performance of mesoporous hybrid materials. In case of only SnO2 loaded in mesoporous silica SBA-15, a change of 5.5 orders was observed, which is quite better than RH response of pure SBA-15 material. However, the RH response crosses the ‘6 orders change’ bar when Ag nanoparticles mark their presence in the SnO2/SBA-15 framework. As shown in Fig 7(b), the RH response increases from 1wt% and 2 wt% Ag loaded in SnO2/SBA-15, where after,
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the sensitivity decreases (Table 1). The lowering of response of the hybrid material above 2 wt% Ag loaded in SnO2/SBA-15 nanocomposite could be due to blockage of pore channels of SBA-15
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with Ag nanoparticles. The pore blockage causes the obstacle in the transmission of charge carriers across the mesoporous channels. Overall, the nanocomposite having 2 wt% of Ag loaded
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in SnO2/SBA-15 shows highest change in impedance and also exhibits best linearity, therefore this nanocomposite was selected for evaluation of further RH sensing characteristics. Fig. 8 shows the impedance variation with RH% at various frequencies (50 Hz, 100 Hz,
1000 Hz, 2500 Hz and 5000 Hz) under an AC test voltage of 0.5 V. As the RH% increases, more amount of water molecules get adsorbed on the sensor surface, which strengthen the polarization and in turn increases the dielectric constant of the material leading to a decreases in the impedance of sensor. The impedance was almost flat at 5000 Hz, showing that the impedance became 10
ACCEPTED MANUSCRIPT independent of humidity with increasing frequency because the electric field direction changes gradually at low frequencies and there shows up the space charge polarization of adsorbed water [17]. At higher frequencies, a quick change in electric field direction was observed which cannot
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be meet up with the polarization of adsorbed water thus causing the dielectric constant small and independent of RH. As the sensitivity was highest at 100 Hz frequency, we have performed experiments related to response/recovery times, hysteresis and stability at 100 Hz.
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Hysteresis is an important characteristic of a RH sensor to determine stability by measuring the time slack in adsorption and desorption processes. The sensing material was exchanged in
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increasing (low RH% to high RH%) and decreasing (high RH% to low RH%) RH% for the process of adsorption and desorption respectively. The hysteresis error the expression,
, where,
was calculated using
is the difference in output of adsorption and
desorption processes and FFS is the full scale output [17]. The hysteresis response in Fig. 9 shows
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excellent reversible characteristics, where the adsorption and desorption curves nearly coincide with each other. Hysteresis error,
was calculated to be 0.9%, much lesser than earlier reported
results (Table 2), indicating a good reliability of the sensor.
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Both response and recovery times significantly affect the performance of RH sensors and are important variables to judge the actual performance of humidity sensing materials. Response
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and recovery times are defined as the time taken by the sensor to obtain ~90% of overall impedance change in case of adsorption and desorption, respectively [30]. According to the observations made in Fig. 10, the sensor response time (humidification from 11 RH% to 98 RH%) was 5 s, and the recovery time (desiccation from 98 RH% to 11 RH%) was 8 s, both better than conventional resistive sensors utilizing hybrid SBA-15 materials and SnO2 supported nanocomposites (Table 2).
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ACCEPTED MANUSCRIPT To determine the dynamic response of Ag-SnO2/SBA-15(2) nanocomposite based sensor towards rapid variations in the 11-98 RH% range, the sensor was switched between closed chambers in four loops (11 RH% → 98 RH% → 11 RH% → 98 RH% → 11 RH%). The
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measurements shows that the sensor response is highly reversible and the impedance values at 11 RH% and 98 RH% were achieved precisely in four reiterated circles of estimations, recommending an excellent reproducibility of the RH response. The standard deviation in process
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of exchanging of sensor reaction was calculated to be 0.8%. Moreover, the RH response of the sensor tested every 5th day (Fig. 11) shows excellent consistency and a minor variation in
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impedance (~ 1.1%) was observed at each humidity level. The Ag-SnO2/SBA-15(2) sensor sensitivity is measured by increasing RH values from 11% to 98% and the results are shown in Fig. 12. As observed, the sensitivities were highly linear with the relative humidity. 3.3 Humidity sensing mechanism
Mesoporous materials because of their high surface area, high pore volume and long,
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uniform pore channels encourages the introduction of dynamic surface active sites for easier absorption of water molecules and also helps in smoother propagation of charge carriers on the internal and external surface of the material. At low RH, water molecules are primarily
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physisorbed onto the available active sites (Si-OH) of the Ag-SnO2/SBA-15(2) surface through
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double hydrogen bonding, which is called the first-layer physisorption of water. In this regime, the water molecules are unable to move freely because of the restriction from double hydrogen bonding. The hopping transfer of protons between adjacent hydroxyl groups in the first-layer physical adsorption of water requires much energy and due to their little presence at low RH%, they are restricted by discontinuous mobile layers causing Ag-SnO2/SBA-15(2) to exhibit strong impedance. However, as the RH% increases, the multilayer physical adsorption of water molecules occurs. From the second physisorbed layer, water molecules are physisorbed through single hydrogen bonding on the hydroxyl groups. Thereafter, the water molecules become mobile 12
ACCEPTED MANUSCRIPT and progressively more identical to those in the bulk liquid. As the multilayer physical adsorption progresses, the physisorbed water can be ionized under an electrostatic field to produce a large number of hydronium ions (H3O+) as charge carriers. With further increase in humidity, the
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physisorbed water layers gradually exhibit liquid-like behavior. In bulk liquid, proton hopping between adjacent water molecules occurs in Ag-SnO2/SBA-15(2), with charge transport taking place via the conductivity generated by a Grotthuss chain reaction (H2O + H3O+ → H3O+ + H2O)
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conductivity, which causes a dramatic increase in conductivity of the sensor [45]. A schematic of adsorption of water molecules on the structure of Ag-SnO2/SBA-15 has been shown in Fig 13.
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This finding explains the excellent humidity sensing characteristics of Ag-SnO2/SBA-15(2) hybrid nanocomposite.
To validate the sorption mechanism of Ag-SnO2/SBA-15(2) at different RH%, complex impedance spectra were plotted at different RH values and measured over a frequency range to understand the polarization and conductivity processes taking place in a nanocomposite material.
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The impedance spectra denotes to different physical phenomena for the electrical conductivity and polarization that occur in nanocomposite surface in the presence of water molecules. At low RH%
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(11 RH%), a portion of half semicircle is observed in the complex impedance spectra (Fig 14a). The semicircle emerged from the intrinsic impedance of the nanocomposite material sensing layer.
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As the RH% increases, the curvature of semicircle disappears (Fig 14b) and is left with a line appearing at higher RH% (84 and 98 RH%). This process of decrease in the curvature of semicircle with increasing RH% denotes the decrease in the intrinsic impedance, which is related to the interaction between the nanocomposite surface and water molecules. The straight line results from the ionic and/or electrolytic conductivity. This line represents the Warburg impedance which is related to the diffusion of the electroactive species at the electrode [46]. The electronic conductivity of complex nanocomposite Ag-SnO2/SBA-15(2) relies upon the surface coverage of 13
ACCEPTED MANUSCRIPT adsorbed water. At low RH levels (11%), the water vapor is insignificant and so is their adsorption of nanocomposite surface. The water molecules are chemically adsorbed on nanocomposite surface by means of hydrogen bond. When RH% increases, successive layers of water molecule
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are adsorbed and formed the first physical adsorption through double hydrogen bonding which resembles like a bulk liquid phase of water. The protons are released from the hydration of H3O+ (H3O+ → H2O + H+) at these layers and become the main source of charge carriers across the
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nanocomposite surface leading to a significant decrease in impedance of the material. The presence of Sn4+ ions provides dynamic locales for the adsorption of water atoms due to their high
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charge density [21] and causes the frequent ionization of physisorbed water molecules at the surface and pores of the nanocomposite resulting in generation of H+ ions for electrical conduction. Furthermore, the oxygen molecules of water atoms like to adsorb on the Ag metal nanoparticles by giving their lone-pair of electrons prompting an increment in the number of e- in the conduction sp band of Ag [30]. This generation of additional electrons on the water layer
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results in quick conduction and a sharp lessening in impedance is observed. The process of ionization increases from 11 RH% to 98 RH% and a significant increase in charge carriers leads to a remarkable ~6 orders change in impedance of the material.
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In general, the presence of Sn4+ ions and Ag metal nanoparticles in mesoporous SBA-15
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account for the excellent sensing response of the nanocomposite synthesized using a sequence of in-situ and wet impregnation process. The dual metal nanoparticle loaded SBA-15 hybrid nanocomposite shows excellent performance along with rapid response/recovery times, relatively negligible hysteresis and outstanding stability. A comparison with other humidity sensors available in market (Table 3) shows that our material possess better RH% sensing characteristics and justifies for being used in commercial applications.
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ACCEPTED MANUSCRIPT 4. Conclusion In summary, Ag-SnO2/SBA-15 nanohybrids were synthesized using a combination of both in-situ and wet impregnation processes. Large mesoporous surface area supported by the presence
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of Sn4+ and Ag0 nanoparticles, endow the nanocomposite with excellent humidity sensing behavior. Electrical testing under different RH environments shows that the nanohybrid sensor based on Ag-SnO2/SBA-15(2) exhibits ultrahigh sensitivity (12.0 x 105 at 100 Hz) over the entire RH range. Furthermore, the sensor shows excellent performance along with rapid response (5 s)
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and recovery times (8 s), relatively low hysteresis (0.9%), and excellent stability (1.1%) in 11-98
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RH% range. We also established a mechanism that can potentially explain the excellent performance of the Ag-SnO2/SBA-15 based nanohybrid sensors. Our material supports the capability of utilizing complex nanohybrid mesoporous materials for novel RH sensing applications.
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Acknowledgements
Authors are grateful to UGC, New Delhi (Grant No. 41-997/2012(SR)) for providing financial assistance. Authors are also thankful to Dr. I.S. Mulla (Emeritus scientist, CSIR, India)
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for his thoughtful and valuable suggestions.
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[10] H. –S. Hong, G. –S. Chung, Sens. Actuators B, 195 (2014) 446 [11] V. K. Tomer, S. Duhan, Sens. Actuators B, 220 (2015) 192–200 [12] N.T.T. Ha, D. K. An, P. V. Phong, P. T. M. Hoa, L. H. Mai, Sens. Actuators B, 66 (2000) 200
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[13] M. Shah, Z. Ahmad, K. Sulaiman, K .S. Karimov, M. H. Sayyad, Solid State Electronics, 69
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[14] S. Duhan, S. Devi, J. Electronics Engg. 2 (2010) 89-92 [15] J. Xu, T. Chen, J. K. Shang, K.-Z. Long, Y.-X. Li, Microporous Mesoporous Mater., 211 (2015) 105-112 [16] V. K. Tomer, R. Malik, S. Jangra, S. Duhan, Colloids Surf. A: Physicochem. Eng. Asp., 466 (2015) 160
[17] V. K. Tomer, S. Duhan, P.V. Adhyapak, I.S. Mulla, J. Am. Ceram. Soc., 98 (2015) 741 [18] Q. Yuan, N. Li, W. Geng, Y. Chi, J. Tu, X. Li, C. Shao, Sens. Actuators B, 160 (2011) 334 [19] B. J. Melde, B. J. Johnson, P. T. Charles, Sensors, 8 (2008) 5202. 16
ACCEPTED MANUSCRIPT [20] V. K. Tomer, S. Duhan, App. Phy. Lett., 106 (2015) 063105 [21] Q. Kuang, C.S. Lao, Z.L. Wang, Z.X. Xie, L.S. Zheng, J. Am. Chem. Soc. 129 (2007) 6070. [22] E. Comini, G. Faglia, G. Sberveglieri, Z. W. Pan, Z. L. Wang, Appl. Phys. Lett. 81 (2002)
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1869. [23] R. Malik, V. K. Tomer, P. S. Rana, S.P. Nehra, S. Duhan, Mat. Lett. 154 (2015) 124 [24] Y. Shimizu, A. Jono, T. Hyodo, M. Egashira, Sens. Actuators B, 108 (2005) 56.
[25] N. S. Ramgir, Y. K. Hwang, S. H. Jhung, H. K. Kim, J. S. Hwang, I. S. Mulla, J. S. Chang,
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[27] K. Y. Dong, J. K. Choi, I. S. Hwang, J. W. Lee, B. H. Kang, D. J. Ham, J. H. Lee, B. K. Ju,
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[34] L. Greenspan, J. Res. Nat. Bur. Stand., 81 (1977) 89. [35] S. Brunauer, P. H. Emmet, E. Teller, J. Am. Chem. Soc., 60 (1938) 309. [36] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. [37] R. Wang, T. Zhang, Y. He, X. Li, W. Geng, J. Tu, Q. Yuan, J. App. Polymer Sci., 115 (2010) 3474. [38] Q. Qi, T. Zhang, X. Zheng, L. Wan, Sens. Actuators B, 135 (2008) 255 [39] R. Wang, X. Liu, Y. He, Q. Yuan, X. Li, G. Lu, T. Zhang, Sens. Actuators B, 145 (2010) 386. 17
ACCEPTED MANUSCRIPT [40] T. Zhang, R. Wang, W. Geng, X. Li, Q. Qi, Y. He, S. Wang, Sens. Actuators B, 128 (2008) 482 [41] M. Anbia, S.E.M. Fard, Sens. Actuators B, 160 (2011) 215– 221
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[44] W. Geng, Q. Yuan, X. Jiang, J. Tu, L. Duan, J. Gu, Q. Zhang, Sens. Actuators B 174 (2012)
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[46] P. A. Christensen, A. Hammett, Techniques and Mechanisms in Electrochemistry, Springer,
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Netherlands, 1994, DOI: 10.1007/978-0-585-27623-6_2.
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ACCEPTED MANUSCRIPT Table Caption: Table 1: Structural and textural properties of mesoporous SBA-15, SnO2/SBA-15 and AgSnO2/SBA-15(X) nanocomposites as determined from XRD, N2 adsorption-desorption and EDX
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analysis. Table 2: A comparison of humidity sensing performance of hybrid SBA-15 and SnO2 supported nanocomposites.
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Table 3: A comparison of sensing characteristics of commercial humidity sensors.
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d100-spacing (nm) a
ao (nm)b
DP (nm)c
DW (nm)e
VP (cm3/g)d
SBET (m2/g)f
Effective Ag wt% g
Sensitivity, (S)h
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10.54
12.17
8.98
3.19
1.25
878
--
3.69 x 102
SnO2/SBA-15
10.24
11.82
7.11
4.71
1.02
661
--
2.1 x 105
X=1
9.83
11.35
6.68
4.75
0.94
X=2
9.15
10.57
5.77
4.8
0.85
X=3
8.37
9.66
4.82
4.84
0.79
d100: d-spacing a0: Unit cell parameter [a0 = 2 x d100/√3] c DP: Pore size d VP: Pore volume e DW: Pore wall thickness [DW = a0 - DP] f SBET: Total surface area g Calculated from EDX studies Sensitivity, S =
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4.7 x 105
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6 5
[37] [38] [39] [40] [41]
100 100
---
[42] [43]
26 8
4% 0.9%
[44] This work
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--2% 3% --
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Reference
Table 2
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3 3
Hysteresis
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NiO-PPY/SBA-15 Fe/SBA-15 MgO/SBA-15 Li/SBA-15 La3+ and K+ doped TiO2–10 mol% SnO2 SnWO4-SnO2 K+-doped SnO2– LiZnVO4 MgO-KCl/SiO2 Ag-SnO2/SBA-15
Recovery time (s) 90 50 20 180 18
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Order of impedance change in Response complete RH% range time (s) 3.5 45 3.5 20 3.5 10 3.5 60 5 11
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C C R R E E E E E
DHT22 SHT15 Polymer HR202 DHT11 HMT3301 HMT3302 HMT3303 HC2-C Fluke 971
Sensitivity Range (%RH) 0-100 0-100 20-95 20-90 0-100 0-100 0-100 0-100 5-95
C: Capacitive; R: Resistive; E: Electronic : With grid filter; 2: With steel netting filter, 3: With sintered filter
Recovery Hysteresis time (s) (%) NA 2 8 2 10 1 10 4 17 1 50 1 60 1 NA NA NA NA
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Response time (s) NA 8 10 10 8 20 40 15 60
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ACCEPTED MANUSCRIPT Figure Captions: Figure 1: Schematic drawing of the interdigitated electrode (IDE) substrate. Figure 2: (A) Low-angle (2θ = 0.5° - 5.0°) XRD and (B) Wide angle XRD spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(1), (2), (3) nanohybrids denoted from (a) to (e) respectively.
SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) nanohybrids.
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Figure 3: (a) N2 adsorption-desorption isotherms curves and (b) Pore size distributions curves for
Figure 4: FTIR spectra of SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(1), (2), (3) nanohybrids
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denoted from (a) to (e) respectively.
Figure 5: HRTEM image showing uniform channels with long range order of (a) SBA-15, (b) SnO2/SBA-15 and (c) Ag-SnO2/SBA-15(2) nanocomposite, (d-f) corresponding EDX spectra of
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SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(2) nanocomposites.
Figure 6: SEM image of (a) SBA-15, (b) SnO2/SBA-15 and (c) Ag-SnO2/SBA-15(2) nanocomposite,
Figure 7: (a) Humidity sensing curves showing decrease in impedance with increase in RH% for SBA-15, SnO2/SBA-15 and Ag-SnO2/SBA-15(X) nanocomposites, (b) Sensitivity of Ag-
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Figure 8: Relationship of impedance and relative humidity based on nanocomposite AgSnO2/SBA-15(2) at various frequencies.
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Figure 9: Hysteresis curve showing adsorption-desorption responses measured in the 11-98 RH% range for Ag-SnO2/SBA-15(2) nanocomposite.
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Figure 10: Response/recovery time of the Ag-SnO2/SBA-15(2) sensor for humidity levels between 11 RH% and 98 RH%. Repeated measurement of impedance in four RH loop to determine the stability of the sensor.
Figure 11: The response of Ag-SnO2/SBA-15(2) monitored at different humidity conditions for 30 days.
Figure 12: Relative Sensitivity of Ag-SnO2/SBA-15(2) measured at each RH%. Figure 13: A schematic of adsorption of water layers on the Ag-SnO2/SBA-15 surface. Figure 14: The measured and simulated complex impedance spectra (Nyquist plot) based on AgSnO2/SBA-15(2), where RH varies from 11% to 98%. 23
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Highlights 1. Mesoporous SnO2/SBA-15 was synthesized by in-situ hydrothermal processes. 2. Ag nanoparticles were loaded in SnO2/SBA-15 using wet impregnation process.
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3. Impedance of Ag-SnO2/SBA-15 sensor changes from 108 Ω to 102 Ω in 11-98 %RH range.
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4. The sensor displays superfast response (5 s) and recovery time (8 s).