Organic-inorganic hybrid materials based on mesoporous silica derivatives for humidity sensing

Accepted Manuscript Title: Organic-inorganic hybrid materials based on mesoporous silica derivatives for humidity sensing Author: Hongran Zhao Tong Zhang Rongrong Qi Jianxun Dai Sen Liu Teng Fei Geyu Lu PII: DOI: Reference:

S0925-4005(16)31902-5 http://dx.doi.org/doi:10.1016/j.snb.2016.11.104 SNB 21314

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

8-9-2016 16-11-2016 22-11-2016

Please cite this article as: Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei, Geyu Lu, Organic-inorganic hybrid materials based on mesoporous silica derivatives for humidity sensing, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.11.104 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.

Organic-inorganic hybrid materials based on mesoporous silica derivatives for humidity sensing

Hongran Zhao1, Tong Zhang1,2, Rongrong Qi1, Jianxun Dai1, Sen Liu1, Teng Fei*1, Geyu Lu1

1 State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P.R. China 2 State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100083, P.R. China

*Corresponding author: E-mail address: [email protected] (T. Fei) Tel.: +86 431 85168385; Fax: +86 431 85168270

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Highlights 

Novel organic-inorganic hybrid materials SBA-15-PSSx have been prepared via a free radical polymerization.



The dispersibility and stability of hydrophilic groups in humidity sensing materials could be guaranteed by chemical modification method.



The obtained sensor shows particularly rapid response.



The impedance analysis under different frequencies is used to support equivalent circle modes of the sensor at different RH.

Abstract Mesoporous silicas loading hydrophilic materials composites were widely used in humidity sensors during the last decade. However, the physical mixing process used for loading the hydrophilic materials in the channel of mesoporous silicas is hard to guarantee their uniform dispersion, and there is no strong force to bind the host and guest materials together. To solve above problems, novel vinyl functionalized mesoporous silicas (SBA-15-vinyl) were chemically modified with hydrophilic sodium p-styrenesulfonate (SSS) with free radical polymerization. The chemical and porous structures of the obtained hybrid materials (SBA-15-PSSx) were characterized by fourier transform infrared spectroscopy (FT-IR), elemental analyses (EA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and N2 adsorption-desorption. The humidity sensing properties of the sensors based on SBA-15-vinyl and SBA-15-PSS indicate the introduction of PSS could effectively enhance the sensing properties of SBA-15-vinyl. The impedance of the optimized sensor changed by more than three orders of magnitude over the relative humidity 2

(RH) range of 11%-95%, with rapid response to RH change.

Keywords Humidity sensor; mesoporous silica; free radical polymerization; organic-inorganic hybrid materials

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1. Introduction Along with social progress and the development of industry, there is a growing demand for high performance humidity sensor to monitor and detect humidity effectively. Traditional resist-type humidity sensors mainly include the electrolytes humidity sensors [1, 2], metal oxide humidity sensors [3-5] and amphiphilic humidity sensors [6-9], and each of them has their own advantages and disadvantages. Electrolytes humidity sensors exhibit high response to low humidity environment, but the sensitive material is easy to deliquesce in high humidity atmosphere. Metal oxide humidity sensors have good stability, while the extra heating equipment which is requisite in dehydration process increases the power consumption. Amphiphilic humidity sensors are attractive by their low cost and excellent sensing performance, however, the defective stability under high humidity environment limits their application. Mesoporous silica materials have aroused much attention since they were synthesized, because of their merits of large specific surface area and uniform pore size distribution [10,11]. For humidity sensing application, the large surface area could improve the water vapor adsorption properties, but the pure mesoporous silica sensors show poor response at low relative humidity (RH) and suffering from the defect of saturation under high RH [12,13]. In our previous work, a small amount of hydrophilic electrolyte (LiCl) was dispersed in the channel of the mesoporous silica to enhance the humidity sensing [14,15], and through this method, the obtained sensors could work under the whole humidity range with good sensing performance. After that, a lot of effort has been paid to investigate the humidity sensitive properties of the composites fabricated by loading hydrophilic electrolytes or metal oxide in the 4

channel of various mesoporous silicas [16-20], in order to prepare the composites with better humidity sensitive properties. Till now, many optimized sensors with good humidity sensitive properties have been obtained, but the structure of composited sensitive materials is still imperfect. Usually, a physical mixing process is needed to disperse the guest material, because the host material, mesoporous silicas, could not be dissolved in any solvent. So it is hard to guarantee the uniform dispersion of the guest material. Moreover the sensitive materials may partly loss after working under high humidity environment without strong force to bind host and guest materials together. In view of the above-mentioned facts, a novel method of preparing humidity sensitive hybrid materials based on mesoporous silica has been presented in this work. The organosilica precursor was introduced during the synthetic process of the mesoporous silica, the simultaneous condensation of silica and organosilica precursor leads to organic sites anchored covalently to the pore walls homogeneously. The hydrophilic organic unit was, then, introduced to mesoporous silica through a chemically modification. In our work, the vinyl modified SBA-15 (SBA-15-vinyl (20%)) was synthesized as the host material, and sodium p-styrenesulfonate (SSS), the hydrophilic organic unit, was then modified on it through a free radical polymerization. Through this method, the uniform dispersion of the hydrophilic sites could be guaranteed by simultaneous condensation reaction. Meanwhile, the chemically modification forms chemical bonds between host and guest material, which is expected to improve their stability. 2. Experimental 2.1 Chemicals Triblock polymers poly (ethylene glycol)-block-poly (propylene glycol)-block-poly 5

(ethylene glycol) (P123, MW = 5800) and azodiisobutyrodinitrile (AIBN) were purchased from Sigma-Aldrich. Tetraethoxysilane (TEOS), triethoxyvinylsilane (TEVS) and sodium p-styrenesulfonate hydrate were purchased from Aladdin. N, N-Dimethylformamide

(DMF)

was

purchased

from

Sinopharm

Chemicals.

Hydrochloric acid (HCl) and ethanol were purchased from Beijing Chemical Corp. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. 2.2 Preparation of SBA-15-vinyl (20%) The SBA-15-vinyl (20%) was synthesized according to a reported literature [21]. In a typical procedure, 2.00 g triblock copolymer P123 (EO20PO70EO20) was mixed with 60 mL HCl solution (2 M) at room temperature. After stirring for 2 h, 4.58 g TEOS was added dropwise under stirring at 40 ºC. Thirty minutes later, 1.05 g TEVS was added into above solution with stirring for another 20 h. Then, the mixture was transferred to a 40 mL Teflon-lined stainless-steel autoclave and aged at 100 ºC for 24 h. The product was centrifuged at 11000 rpm for 10 min, washed with deionized water for five times, and dried at 60 ºC over night. The surfactant was then removed by refluxing the synthesized material in 100 mL ethanol containing 2 mL HCl (36 wt%) for 24 h. 2.3 Preparation of SBA-15-PSSx The synthetic routes of SBA-15-PSSx (x denotes 1:1, 1:5 and 1:10) is shown in Fig. 1. The three different SBA-15-PSSx materials were prepared from different mole ratios of SBA-15-vinyl (20%) and SSS by the free radical polymerization. Take SBA-15-PSS (1:1) for example, 0.20 g SBA-15-vinyl (20%), 0.15 g SSS and 0.01 g AIBN were dissolved in 10 mL of DMF under stirring. The mixture was transferred to the 40 mL Teflon-lined stainless-steel autoclave, and the reaction system was heated 6

to 100 ºC for 24 h. Then, the final product was centrifuged at 11000 rpm for 10 min, washed with deionized water and dried at 60 ºC. 2.4 Characterizations The fourier transform infrared spectroscopy (FT-IR) spectra of polymers were obtained on a WQF-510AFTIR spectrometer, using KBr pellets as the reference. Elemental analyses (EA) of carbon, hydrogen, and sulfur were performed by Flash EA 1112, CHNS-O elemental analysis instrument. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max-2550 diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The high resolution transmission electron microscopy (HRTEM, JEM-2011F) were operated at 200 kV for the images of SBA-15-vinyl (20%) and SBA-15-PSS (1:5). The N2 adsorption-desorption isotherm measurements were performed on a JW-BK 132F volumetric adsorption analyzer at 77 K. 2.5 Fabrication and measurement of humidity sensors For the fabrication of humidity sensors, each obtained material was dispersed in deionized water to form a mixture at a weight ratio of 4:1. The mixture was then drop-coated on a ceramic substrate (9 mm × 4 mm, 0.5 mm in thick) made of aluminum oxide on which four pairs of graphitic interdigitated electrodes were printed, and the sensing film (about 50 μm) was formed by slow evaporation at room temperature for 2 h. The sensors were placed into 95% RH atmosphere aging with a sinusoidal voltage of 1 V at 1 kHz, without DC-bias, for 24 h before the test. The measurement mode by the impedance analyzer is analogous in all the tests, only at different frequencies. Humidity sensitive properties were measured on a Keysight E4990A impedance analyzer at room temperature. The humidity environments were produced by different saturated salt solutions [22]. The measurement system of the humidity sensors is shown in Fig. 2. The response and recovery times in this work are 7

defined as the time taken by a sensor to achieve 90% of the total logarithmic impedance modulus change in the case of adsorption and desorption process, respectively. Humidity hysteresis is defined as the maximum difference of the humidity sensor between the adsorption and desorption processes [23]. 3. Results and discussions In our work, the SBA-15-vinyl (20%) skeleton with uniform distribution of vinyl sites on pore walls was synthesized through a simultaneous condensation of TEOS and TEVS. Then, the PSS was formed by self-polymerization of SSS and immobilized on SBA-15-vinyl (20%) via a free radical polymerization between PSS and vinyl sites of SBA-15-vinyl (20%). Finally the result products were washed with deionized water to remove unreacted PSS. To investigate the chemical structure of SBA-15-PSSx, the result materials were characterized by FT-IR spectroscopy and EA. The FT-IR spectra of SBA-15-vinyl (20 %) and SBA-15-PSSx are shown in Fig. 3. The bands exhibit at 1066 and 794 cm-1 are due to the asymmetric and symmetric stretching of Si-O-Si, respectively. The bands appear at 952 cm-1 belongs to the stretching vibrations of the Si-OH unit. After modified with PSS, all the curves of SBA-15-PSSx exhibit two peaks corresponding to the stretching of S-O, and two peaks standing for stretching of C-H on phenyl rings also appear. Moreover, from the EA results shown in Table 1, the content of sulfur and carbon increase with the SSS ratio in reactants. The above results demonstrate PSS are modified on SBA-15-vinyl (20%) successfully, and more PSS could be modified on SBA-15 by increasing the SSS ratio in reactants. The co-condensation method used for preparing functionalized mesoporous silicas is beneficial for the homogeneously distribution of organic hydrophilic units, but the degree of mesoscopic order of products decreases with increasing concentration of 8

organosilica precursor [24]. The porous structure of SBA-15-vinyl (20%) and SBA-15-PSSx was characterized by small-angle XRD as shown in Fig. 4. All curves exhibit an intense peak at 2θ of 1.03 degree, indicates the materials have a periodic structure inside. Generally, the small-angle XRD curve of SBA-15 will show three or more peaks attribute to (100), (110), (200), and other planes of the 2D hexagonal space group p6mm [25]. In addition, the intensity of peaks decreases after modifying with vinyl unit, and the peaks with relatively weak intensity disappear when the vinyl amount increase to 20 % [21,26], due to the partly breakage of the ordered pore structure. In this work, the small-angle XRD curve of SBA-vinyl (20%) only rests the peak corresponding to the (100) planes of the 2D hexagonal space group p6mm, and the intensity of this peak decreases with the increasing PSS content. In order to observe the pore structure of SBA-15-vinyl (20%) and SBA-15-PSS (1:5) visually, the HRTEM images were taken and shown in Fig. 5. The 2D worm-like channel with long-range order structure still could be observed in the HRTEM image of SBA-15-vinyl (20%) materials, but the degree of mesoscopic order of the pore structure is imperfect. The phenomenon is attributed to the different hydrolysis rates of silica and organosilica precursor, which is consistent with the previous reports [21,26]. After modifying PSS, the SBA-15-PSS (1:5) still remains the 2D worm-like channel structure, but the degree of long-range order structure decreases. The N2 adsorption and desorption isotherm of SBA-15-vinyl (20%) and SBA-15-PSS (1:5) are shown in Fig. 6. The curve of SBA-15-vinyl (20%) exhibits type-IV isotherms with H1 hysteresis loop, indicating the SBA-15-vinyl (20%) owns channel-like mesoporous structure inside [27]. The Brunauer-Emmett-Teller (BET) surface area and pore volume of SBA-15-vinyl (20%) is 760.1 m2/g and 1.48 cm3/g respectively. After modifying with PSS, the hysteresis loop disappears, and the N2 9

adsorbed volume decreases significantly at all the relative pressure in the isotherm of SBA-15-PSS (1:5), which is due to the reduction of ordering of mesoporous structure. The BET surface area and pore volume of SBA-15-PSS (1:5) reduces to 96.6 m2/g and 0.12 cm3/g, respectively, further indicating the pore structure of SBA-15-vinyl (20%) is partly blocked by the molecular chain of PSS. The similar phenomenon could be observed in pore size distribution curves. The pore size decreases from 6.98 nm for SBA-15-vinyl (20%) to 1.09 nm for SBA-15-PSS (1:5). To investigate the humidity sensing performance of obtained materials, the SBA-15-vinyl (20%) sensors and SBA-15-PSSx sensors were prepared. Fig. 7a shows the impedance modulus and RH relationship of SBA-15-vinyl (20%) and SBA-15-PSSx sensors which were measured by keeping the applied voltage at AC 1 V and the frequency at 1 kHz. The impedance moduli of sensors were recorded from 11% to 95% RH with the gradual increase of the RH. At each RH atmosphere, about ten minutes was given for the sensors to reach equilibrium. The SBA-15-vinyl (20%) sensor exhibits poor response to RH change, especially at lower RH range. The modification of PSS enhances the sensing property of SBA-15-vinyl (20%), and the impedance modulus variation of composites increases with the PSS content. The improvement of sensing property of composites is due to the strong hydrophilicity of sulfonate unit, which is beneficial for the chemisorption of water molecules under low RH range. At high RH range, the Na+ ions ionize from the sodium sulfonate group and participate in electric conduction, so the SBA-15-PSSx sensors exhibit significant response to the RH change. Further, through carrying on the linear fitting to the result data, the SBA-15-PSS (1:5) sensor exhibits the best linearity among the whole humidity range in a semi-logarithmic scale. The humidity hysteresis curve of SBA-15-PSS (1:5) sensor is shown in Fig. 7b. The sensor was first measured from 10

11% to 95% RH, and then in the opposite direction. The maximum humidity hysteresis of SBA-15-PSS (1:5) sensor is about 7% RH. Fig. 7c shows the response and recovery curve of SBA-15-PSS (1:5) sensor under different RH. During the dynamic measurement, the sensor exhibits a rapid response to RH fluctuation, and the curve shows no obvious baseline shift during the five measuring cycles, indicating good repeatability of the sensor. It is noted that the response and recovery processes turn longer with the increasing RH, and the response and recovery times are 20 s and 11 s, respectively, between 11% and 95% RH. The definition of response and recovery times in this work is different with most of other reports about resist type humidity sensor. The semi-logarithmic coordinate system is used to show the response and recovery process, because the other humidity sensing performances are evaluated under a semi-logarithmic scale. In semi-logarithmic coordinate system, it is difficult for the response curve to reach equilibrium, since a tiny change of impedance modulus turns obviously under high RH. Even so, the sensor still exhibits rapid response which is due to the strong hydrophilicity of PSS. In addition, to contrast with other works, the response and recovery times of SBA-15-PSS (1:5) sensor between 11% and 95% RH are also evaluated in a linear coordinate system (Fig. 7d). The sensor exhibits a particularly short response time of 5 s under linear coordinate system, with a recovery time of 106 s. To analyze the conduction mode of sensing film, the complex impedance of SBA-15-vinyl (20%) and SBA-15-PSS (1:5) sensor was measured under different RH with a frequency range from 100 Hz to 20 MHz. The complex impedance plots (CIP) are shown in Fig. 8 and Fig. 9, ReZ and ImZ are real part and imaginary part of the complex impedance, respectively. Among the plots of SBA-15-PSS (1:5) in Fig. 8, the ReZ-ImZ curve presents arc shape with large radius of curvature at 11% RH, which 11

indicates the equivalent circle (EC) of the sensing film can be expressed as a constant phase element (CPE) [28]. The ReZ-ImZ curve forms a semicircle when RH increases to 33% RH, the EC can be modeled as a parallel capacitor (Cf) and resistor (Rf) [29]. As the RH reaches 54% RH, the ReZ-ImZ curve is consisted of a semicircle at the high frequency region and a straight line at low frequency region. The straight line represents the Warburg impedance (Zw) due to the diffusion process of electroactive species. With the RH continues to increase, the EC mode of ReZ-ImZ no longer changes. However, the radius of curvature of the semicircle reduces gradually and the straight line turns longer with the increasing RH, indicating the effect of electroactive species diffusion on conduction becomes more significant. By contrast, the ReZ-ImZ curves of SBA-15-vinyl (20%) remain the arc shape before 54% RH (Fig. 9), the radius of curvatures gradually decreases with the increasing RH, indicating the CPE type ECs with the decreasing impedance modulus. Under 75% RH, the ReZ-ImZ curves turn to a semicircle with the EC of a parallel Cf and Rf. When the RH increases to 95%, a short straight line appears at low frequency region which demonstrates the formation of Zw. The frequency influence on impedance modulus of SBA-15-PSS (1:5) sensor under different RH was measured to prove the EC modes of humidity sensor at various RH. As shown in Fig. 10, the impedance modulus at 11% RH exhibit great frequency dependent property. The points almost equally space vertically, which indicates the logarithmic impedance modulus and logarithmic frequency have a linear relationship. The variation tendency of impedance modulus under different frequency just conforms to the behavior of CPE [28]. The CPE mainly conduct by charging and discharging electric charge under AC condition, without any conductive particle transport inside. Hence, there is almost no conducting path in sensing film. As the RH 12

comes to 33%, the points standing for the test frequency at 100 Hz, 1 kHz, and 10 kHz are close to each other, but the points standing for the test frequency above 10 kHz are still almost equally spaced. The distribution of the points can just fit with the EC at 33% RH. The current mainly flows through the resistor branch under low frequency conditions, because the capacitor branch exhibits greater resistance. Hence, the impedance does not exhibit an obvious change with frequency. When the increasing frequency makes capacitor branch shows smaller resistance than resistor branch, the capacitor branch becomes the main current path. The appearance of the resistor branch in EC demonstrates a conductive path is formed, meanwhile the conductive particles appear and transport in the sensing film. The formation of the conduction path can be explained as the water molecules are adsorbed by hydroxyl and PSS of SBA-15-PSS (1:5) through a chemisorption process and form a discontinuous water layer. According to the ion transfer mechanism of Grotthuss [30], protons are able to migrate by hopping from site to site across the surface of film leading to electric conduction. After the RH rises beyond 54% RH, the points measured at lower frequencies are separated from others with the RH goes up, while the points measured at higher frequencies are closer. In this RH range, the appearance of Zw further weakens the effect of the capacitor branch in EC on electric conduction, so all the points are closer to each other. Under low frequency, the main current path of the parallel circuit is the resistor, Rf and Zw series circuit consists of the main part of the impendence. At high frequency, the capacitor turns to the main current path of the parallel circuit, but the capacitor left only tenuous resistance to current at especially high RH and frequency. Hence, Zw almost become to the only part of impedance. The impedance variation tendency of EC under different frequency at the RH range of 54% to 95% can match our results. The transformation of EC indicates 13

the formation of the new conductive path in sensing film. The water molecules are adsorbed by chemisorption process and form a continuous water layer, then more water molecules are adsorbed through the physisorption process layer by layer. The theory reported by Casalbore-Miceli et al. indicates ion can dissociate and transport on the surface of materials under high RH [31]. Hence, in this work, it is considered the Na+ ions could dissociate from SBA-15-PSS (1:5) material and participate in electric conduction. The transport of Na+ and H+ ions causes significant reduction of the impedance modulus at 54% to 95% RH. In comparison, the difference of ECs between SBA-15-vinyl (20%) and SBA-15-PSS (1:5) under 33% RH indicates the modification of PSS unit is beneficial for the chemisorption process at low RH. At 95% RH, the short straight line appeared at low frequency region in ReZ-ImZ curves of SBA-15-vinyl (20%) is owning to the weak diffusion process of H+ ions. It can be seen the diffusion process of electroactive species is much stronger in the plot of SBA-15-PSS (1:5), which further proves the Na+ could dissociate from SBA-15-PSS and participate in electric conduction. Fig. 11 shows the long-term stability plot of SBA-15-PSS (1:5) sensor, the sensor was exposed at 95% RH for four weeks, and the impedance modulus at different RH was measured every week. It can be seen that the impedance modulus does not exhibit obvious change in four weeks measurements, indicating good durability of SBA-15-PSS (1:5) sensor. The good stability of the host-guest hybrid material is due to the chemical bonds between host and guest material, which immobilize the PSS on the surface of SBA-15. To judge the humidity sensing performance of our sensor, sensing properties of this work contrast with previous works based on SBA-15 loading hydrophilic materials are presented in Table 2. Comparing with sensor based on hybrid composites 14

fabricated by SBA-15 and hydrophilic inorganic materials, our sensor exhibits extremely rapid response to RH increase. It is considered that the hydrophilicity of organic polymer could be regulated with the polymerization degree of hydrophilic monomer neatly, which is difficult for inorganic materials. In this work, chemically modified polymeric PSS exhibits strong hydrophilicity, causing rapid response speed to humidity change. 4. Conclusion In summary, a series of SBA-15-PSSx materials with different proportion of modification PSS were synthesized via free radical polymerization. Humidity sensors based on SBA-15-PSSx materials were fabricated and investigated in detail. The optimum sensor shows good linearity, low hysteresis, rapid response and good stability. Compared with the physical loading method, the chemical modification of the gust material can guarantee the dispersibility and stability of gust materials in the channel of host materials. In addition, it is convenient to regulate the hydrophilicity of the sensitive material by organic units. This work provides a new way to fabricate host-guest humidity sensor.

Acknowledgements This work was supported by the Natural Science Foundation Committee (NSFC, No. 51103053), Projects of Science and Technology Development Plan of Jilin Province (No. 20160520093JH) and Program from Changjiang Scholars and Innovativation Research Team in University (No. IRT13018).

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References [1] F. Tailoka, D. J. Fray, R.V. Kumar, Application of Nafion electrolytes for the detection of humidity in a corrosive atmosphere, Solid State Ion. 161 (2003) 267-277. [2] I. Must, V. Vunder, F. Kaasik, I. Pöldsalu, U. Johanson, A. Punning, A. Aabloo, Ionic liquid-based actuators working in air: the effect of ambient humidity, Sens. Actuators B 202 (2014) 114-122. [3] S. Pokhrel, B. Jeyaraj, K. S. Nagaraja, Humidity-sensing properties of ZnCr2O4-ZnO composites, Mater. Lett. 57 (2003) 3543-3548. [4] R. Sundaram, K. S. Nagaraja, Solid state electrical conductivity and humidity sensing properties of WO3-Y2O3 composites, Pys. Status Solidi (a) 201 (2004) 529-535. [5] C. L. Dai, M. C. Liu, F. S. Chen, C. C. Wu, M. W. Chang, A nanowire WO3 humidity sensor integrated with micro-heater and inverting amplifier circuit on chip manufactured using CMOS-MEMS technique, Sens. Actuators B 123 (2007) 896-901. [6] Y. Sakai, M. Matsuguchi, T. Hurukawa, Humidity sensor using cross-linked poly (chloromethyl styrene), Sens. Actuators B 66 (2000) 135-138. [7] Y. Li, M. J. Yang, Humidity sensitive properties of a novel soluble conjugated copolymer: ethynylbenzene-co-propargyl alcohol, Sens. Actuators B 85 (2002) 73-78. [8] R. Nohria, R. K. Khilan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramyan, Humidity sensor based on ultrathin polyaniline film deposited using layer by layer nano-assembly, 114 (2006) 218-222. [9] M. S. Gong, S. W. Joo, B. K. Choi, Preparation of water-durable humidity sensor 16

by attachment of polyelectrolyte membrane to electrode substrate by photochemical crosslinking reaction, Sens. Actuators B 147 (2010) 539-547. [10] C.T. Kresge, M.E. Leoniwics, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoprous molecular sieves synthesized by a liquid crystal template mechanism, Nature 359 (1992) 710. [11] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, A new family of mesoporous molecular sieves prepared with liquid crystal templates, J. Am. Chem. Soc. 114 (1992) 10834. [12] Y. Xia, H. Zhao, S. Liu, T. Zhang, The humidity-sensitive property of MCM-48 self-assembly fiber prepared via electrosping, RSC Adv. 4 (2014) 2807-2812. [13] Y. Zhu, H. Yuan, J. Xu, P. Xu, Q. Pan, Highly stable and sensitive humidity sensors based on quartz crystal microbalance coated with hexagonal lamelliform monodisperse mesoporous silica SBA-15 thin film, Sens. Actuators B 144 (2010) 164-169. [14] W. Geng, R. Wang, X. Li, Y. Zou, T. Zhang, J. Tu, Y. He, N. Li, Humidity sensitive property of Li-doped mesoporous silica SBA-15, Sens. Actuators B 127 (2007) 323-329. [15] L. Wang, D. Li, R. Wang, Y. He, Q. Qi, Y. Wang, T. Zhang, Study on humidity sensing property based on Li doped mesoporous silica MCM-41, Sens. Actuators B 133 (2008) 622-627. [16] Q. Qi, T. Zhang, X. Zheng, L. Wan, Preparation and humidity sensing properties of Fe-doped mesoporous silica SBA-15, Sens. Actuators B 135 (2008) 255-261. [17] J. Tu, R. Wang, W. Geng, X. Lai, T. Zhang, N. Li, N. Yue, X. Li, Humidity sensitive property of Li-doped 3D periodic mesoporous silica SBA-16, Sens. 17

Actuators B 136 (2009) 392-398. [18] H. Zhao, S. Liu, R. Wang, T. Zhang, Humidity-sensing properties of LiCl-loaded 3D cubic mesoporous silica KIT-6 composites, Matt. Lett. 147 (2015) 54-57 [19] V. L. Tomer, S. Duhan, R. Malik, S. P. Nehra, S. Devi, A novel highly sensitive humidity sensor based on ZnO/SBA-15 hybrid nanocomposite, J. Am. Ceram. Soc. 98 (2015) 3719-3725. [20] V. K. Tomer, S. Duhan, In-situ synthesis of SnO2/SBA-15 hybrid nanocomposite as highly efficient humidity sensor, Sens. Actuator B 212 (2015) 517-525. [21] M. A. Wahab, I. Imae, Y. Kawakami, C. S. Ha, Periodic mesoporous organosilica materials incorporating various organic functional groups: synthesis, structural characterization and morphology, Chem. Mater. 17 (2005) 2165-2174. [22] L. Greenspan, Humidity fixed points of binary saturated aqueous solution, J. Res. Natl. Bur. Stand. 81A (1977) 89-96. [23] L. Wang, X. Duan, W. Xie, Q. Li, T. Wang, Highly chemoresistive humidity sensing using poly(ionic liquid)s, Chem. Commun. 52 (2016) 8417-8419. [24] F. Hoffmann, M. Cornelius, J. Morell, M. Fröba, Silica-based mesoporous organic-inorganic hybrid materials, Angew. Chem. Int. Ed. 45 (2006) 3216-3251. [25] Y. Wang, B. Zibrowius, C. Yang, B. Spliethoff, F. Schüth, Synthesis and characterization of large-pore vinyl functionalized mesoporous silica SBA-15, Chem. Comm. (2004) 46-47 [26] A.S.M. Chong, X.S. Zhao, A.T. Kustedjo, S.Z. Qiao, Functionalization of large-pore mesoporous silicas with organosilanes by direct synthesis, Micropor. Mesopor. Mat. 72 (2004) 33-42. [27] Y. Teng, X. Wu, Q. Zhou, C. Chena, H. Zhao, M. Lan, Direct electron transfer of myoglobin in mesoporous silica KIT-6 modified on screen-printed electrode, Sens. 18

Actuators B 142 (2009) 267-272. [28] W. C. Geng, Q. Yuan, X. M. Jiang, J. C. Tu, L. B. Duan, J. W. Gu, Q. Y. Zhang, Humidity sensing mechanism of mesoporous MgO/KCl-SiO2 composites analyzed by complex impedance spectra and bode diagrams, Sens. Actuators B 174 (2012) 513-520. [29] X. F. Song, Q. Qi, T. Zhang, C. Wang, A humidity sensor based on KCl-doped SnO2 nanofibers, Sens. Actuators B 138 (2009) 368-373. [30] F. M. Ernsberger, The nonconformist ion, J. Am. Ceram. Soc. 66 (1983) 747-750. [31] G. Casalbore-Miceli, M. J. Yang, N. Camaioni, C. M. Mari, Y. Li, H. Sun, M. Ling, Investigations on the ion transport mechanism in conducting polymer films, Solid State Ionics 131 (2000) 311-321. [32] V.K. Tomer, P. V. Adhyapak, S. Duhan, I.S. Mulla, Humidity sensing properties of Ag-loaded mesoporous silica SBA-15 nanocomposites prepared via hydrothermal process, Micropor. Mesopor. Mat. 197 (2014) 140-147. [33] W. Zhang, R. Wang, Q. Zhang, J. Li, Humidity sensitive properties of K-doped mesoporous silica SBA-15, J. Phys. Chem. Solids 73 (2012) 517-522.

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Biographies Hongran Zhao received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2013. As a Ph.D. student, his research interest is functional sensing materials and devices.

Tong Zhang completed her M.S. degree in semiconductor materials in 1992 and her Ph.D. in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors.

Rongrong Qi received her B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2016. As a M.S. student, her research interest is functional sensing materials.

Jianxun Dai received his B.S. degree from the College of Electronic Science and Engineering, Jilin University, China in 2015. As a M.S. student, his research interest is humidity sensors based on organic polymers.

Sen Liu received his B.S. degree in 2005 in Chemistry and Ph.D. degree in 2010 in Inorganic Chemistry from Jilin University. Now he is an associate professor in Jilin University and his current research is focused on the carbon-based functional materials and chemical sensors.

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Teng Fei received his B.S. degree in 2005 in chemical engineering and technology and Ph.D. degree in 2010 in polymer chemistry and physics from Jilin University, China. He is currently a lecturer in the College of Electronics Science and Engineering, Jilin University. His research interests include sensing functional materials and devices.

Geyu Lu received the B.Sci. degree in electronic sciences in 1985 and the M.S. degree in 1988 from Jilin University in China and the Dr. Eng. degree in 1998 from Kyushu University in Japan. Now he is a professor of Jilin University, China. His current research interests include the development of chemical sensors and the application of the function materials.

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Figure and Table Captions Table 1. Elemental analysis data of SBA-15-vinyl (20%) and SBA-15-PSSx. Table 2. Humidity sensing properties of reported SBA-15 based humidity sensors. Figure 1. Synthetic routes to SBA-15-PSSx. Figure 2. Measurement system of the humidity sensors. Figure 3. FT-IR spectra of SBA-15-vinyl (20%) and SBA-15-PSSx. Figure 4. Small-angle XRD patterns of SBA-15-vinyl (20%) and SBA-15-PSSx. Figure 5. HRTEM images of (a) SBA-15-vinyl (20%) and (b) SBA-15-PSS (1:5). Figure 6. Nitrogen adsorption-desorption isothermals of (a) SBA-15-vinyl (20%) and (b) SBA-15-PSS (1:5). The inset shows the pore size distribution curves. Figure 7. (a) Impedance modulus of sensors based on SBA-15-vinyl (20%) and SBA-15-PSSx under different RH; (b) humidity hysteresis characteristic of sensor based on SBA-15-PSS (1:5); (c) continuous response and recovery curve of SBA-15-PSS (1:5) sensor under different RH in a semi-logarithmic coordinate system; (d) response and recovery characteristic of SBA-15-PSS (1:5) sensor between 11% and 95% RH in a linear coordinate system. Figure 8. The complex impedance plots of SBA-15-PSS (1:5) sensor. Figure 9. The complex impedance plots of SBA-15-vinyl (20%) sensor. Figure 10. Schematic conduction mechanism of sensor based on SBA-15-PSS (1:5) under different RH. Figure 11．The long-term stability curves of SBA-15-PSS (1:5) sensor for four weeks.

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Figure 1

23

Figure 2

24

Figure 3

25

Figure 4

26

Figure 5

27

Figure 6

28

Figure 7

29

Figure 8

30

Figure 9

31

Figure 10

32

Figure 11

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Table 1

Material

C (%)

H (%)

S (%)

SBA-15-vinyl (20%)

14.59

2.83

0.00

SBA-15-PSS (1:1)

15.11

2.78

0.62

SBA-15-PSS (1:5)

20.10

3.04

3.19

SBA-15-PSS (1:10)

21.39

3.18

4.10

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Table 2

Material

Measure

Impedance

Response

Recovery

range

modulus change

time (s)

time (s)

Reference

LiCl/SBA-15

11-95 % RH 103

21

51

14

ZnO/SBA-15

11-98% RH

105

17

18

19

SnO2/SBA-15

11-98% RH

105.5

15

21

20

Ag/SBA-15

11-92% RH

105

100

125

32

K/SBA-15

11-95% RH

105

10

25

33

SBA-15- PSS

11-95% RH

103

5

106

this work

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