Humidity sensor based on solution processible microporous silica nanoparticles

Accepted Manuscript Title: Humidity Sensor Based on Solution Processible Microporous Silica Nanoparticles Authors: Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei, Geyu Lu PII: DOI: Reference:

S0925-4005(18)30542-2 https://doi.org/10.1016/j.snb.2018.03.052 SNB 24337

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

25-10-2017 27-12-2017 12-3-2018

Please cite this article as: Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei, Geyu Lu, Humidity Sensor Based on Solution Processible Microporous Silica Nanoparticles, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.03.052 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.

Humidity

Sensor

Based

on

Solution

Processible

Microporous

Silica

Nanoparticles

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Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei*, Geyu Lu

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science

*Corresponding

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and Engineering, Jilin University, Changchun 130012, P.R. China

author:

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Tel.: +86 431 85166504; Fax: +86 431 85168270

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E-mail address: [email protected] (T. Fei)

The organic-inorganic hybrid materials MSNs-DMCs were prepared and applied for humidity

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monitoring.

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Highlights

The small size and the modification of polyelectrolyte endow the solution processibility for

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sensitive film preparation.

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The channels formed by particle stacking in the sensitive film leads to a rapid response and

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recovery.

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Abstract Composites fabricated by mesoporous silica loading hydrophilic elements have been widely investigated as humidity sensitive materials. The physical mixing process for loading hydrophilic elements is difficult to guarantee the uniformity and structural stability of the composites. Furthermore, the poor dispersibility of the silica skeleton

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restricts its solution processability. Herein, solution processible organic-inorganic

hybrid materials were synthesized via a thiol-ene click reaction between sulfhydryl

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functionalized microporous silica nanoparticles (MSNs) and hydrophilic alkenyl monomer. The optimal hybrid material could disperse in water stably with a concentration of 50 mg/mL owing to the small particle size (about 105 nm) and the

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hydrophilic characteristic. Humidity sensors were fabricated by solution process and

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the humidity sensing properties were investigated. The sensor exhibits high sensitivity

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(about three orders impedance modulus change), small hysteresis (~2% RH) and rapid

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range of 11%-95% RH.

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response (the response and recovery time are 5 and 40 s, respectively) in the humidity

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reaction

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Keywords: humidity sensor; microporous silica nanoparticles; solution process; click

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1. Introduction Humidity is a mutable parameter among our peripheral environment and has significant effects on many fields, such as agriculture, electronic industry, storage and medical, etc [1, 2]. In recent years, various kinds of materials, such as electrolytes [3], metal oxides [4] and polymers [5] have been applied to fabricate humidity sensors. In

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addition, composites based humidity sensors are also attractive [6, 7]. In the last decade, mesoporous silica-based composites by loading hydrophilic electrolytes,

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metal oxides or polymers have been widely investigated [8-10]. The order pore

structure and large pore volume could avoid the agglomeration of hydrophilic elements and enhance the contact area with targeted molecules. However, composites

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fabricated by a physical mixing process always suffer from the poor adherence

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between host and guest materials, and the poor solubility of silica is not beneficial to

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the uniform dispersion of hydrophilic materials.

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In our previous work, organic-inorganic hybrid materials synthesized by

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anchoring hydrophilic organic units onto the sites of functionalized mesoporous silica were developed [11]. Through the chemical modification, stable bonds were formed

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between host and guest materials. But the mesoporous silica-based sensitive materials were bulk like with the particle size of several micrometers which is difficult to

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disperse well in solutions. In this work, the sulfhydryl functionalized microporous silica nanoparticles (MSNs-SH) were synthesized and modified with hydrophilic

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methacrylatoethyl trimethyl ammonium chloride (DMC) via click chemistry. By controlling the particle size and modifying hydrophilic organic group, the resultant hybrid materials exhibited good dispersibility in water. Thus, sensitive film could be prepared via the solution-process. Meanwhile, channels formed by particle stacking are available for water molecules transport in the sensitive film which could 3

accelerate the response and recovery process. Moreover, the modification of the hydrophilic units inside the hybrid particles are expected to lead high sensitivity and wide sensing range of the humidity sensors. 2. Experimental 2.1 Chemicals

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Benzoin dimethyl ether (DMPA) and (3-mercaptopropyl) trimethoxysilane (MPTMS) were purchased from Aladdin. DMC was purchased from Tokyo Chemical

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Industry. Cetyltrimethylammonium chloride (CTAC), tetraethoxysilane (TEOS), Diethanolamine (DEA), tetrahydrofuran (THF), hydrochloric acid (HCl), methanol and ethanol were purchased from Beijing Chemical Corp (Beijing, China). All

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chemicals were used as received without further purification. The water used

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throughout all experiments was purified through a Millipore system.

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2.2 Preparation of sulfhydryl functionalized MSNs

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The MSNs-SH was synthesized according to the literature with a slight alteration

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[12]. In a typical preparation, the mixture of 64 mL of water, 9.0 g of ethanol, 10.4 g of CTAC solution (25 wt%) and 0.2 g of DEA were stirred at 60 ºC for 20 min, then,

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6.1 g of TEOS was added dropwise. Half an hour later, 0.6 g of MPTMS was added and the mixture was stirred for another one and a half hours. The mixture was cooled

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to room temperature, centrifuged at 11000 rpm, washed with deionized water for three times and dried under vacuum at 40 ºC overnight. The surfactant was removed by

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refluxing the product in the mixture solution of 150 mL of ethanol and 3 mL of HCl (36-38 wt%) for 24 h. 2.3 Preparation of MSNs-DMCs The MSNs-DMCs were fabricated by a thiol-ene click reaction with the different feed ratios of MSNs-SH and DMC. Take MSNs-DMC (1:1) for example, 88 mg of 4

DMC was dissolved in 2 mL of methanol and 8 mL of THF. Then 200 mg of MSNs-SH and 8.5 mg of DMPA were added in the above solution. The mixture was stirring under the ultraviolet light (365 nm, 120000 µJ/cm2) with nitrogen protection for 2 h. The final product was washed with deionized water, collected by centrifuging at 11000 rpm and dried under vacuum at 40 ºC.

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2.4 Characterizations

The FT-IR spectra of polymers were obtained on a WQF-510AFTIR spectrometer,

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using KBr pellets as the reference. Elemental analyses (EA) of carbon, hydrogen, and

nitrogen were performed by Flash EA 1112, CHNS-O elemental analysis instrument. The scanning electron microscopy (SEM) images were taken by a JSM-6700F

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electron microscope (JEOL, Japan). Powder XRD patterns were obtained with a

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Rigaku D/Max-2550 diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The high

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resolution transmission electron microscopy (TEM) images were recorded on the

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HRTEM JEM-2011F operating at 200 kV. The N2 adsorption-desorption isotherm

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measurements were performed on a JW-BK 132F volumetric adsorption analyzer at 77 K.

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2.5 Fabrication and measurement of humidity sensors The resultant materials were dispersed in deionized water by sonication with a

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concentration of 50 mg/mL to form a uniform aqueous dispersion. Then, 6 μL of aqueous dispersion was drop-coated on an aluminum oxide substrate (9 mm long, 4

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mm wide and 0.5 mm in thick) with four pairs of graphitic interdigitated electrodes. The sensitive film was formed by slow evaporation at room temperature with a thickness of ~6.3 μm. The sensors were aged in 95% RH atmosphere with a sinusoidal voltage of 1 V at 1 kHz for 24 h before the test. The mode measurement by the impedance analyzer is applied in all the tests. The measurement of humidity sensing 5

properties was carried on a Keysight E4990A impedance analyzer at 298 ± 2 K. The ambient temperature was controlled by air-condition. The RH environments were produced by different saturated salt solutions [13]. The response and recovery time in this work is defined as the time taken by the impedance modulus achieved 90% of the total change during adsorption and desorption process, respectively [14]. Humidity

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hysteresis is defined as the maximum difference of the humidity sensor between the adsorption and desorption processes [15].

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3. Results and discussions

MSNs-DMCs were designed and synthesized as the humidity sensitive material, as shown in Fig. 1. The MSNs-SH was obtained by a simultaneous condensation of

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TEOS and MPTMS. Different contents of hydrophilic DMC were then modified on

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remove the unreacted DMC monomer.

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the MSNs skeleton. The resultant products were washed with deionized water to

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The FT-IR spectroscopy and EA were carried on to determine the chemical

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structure of resultant materials. From the FT-IR spectroscopy (shown in Fig. 2), a broad peak could be observed at 2576 cm-1 on all the curves, which stands for the

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asymmetric stretching of Si-O-Si. Meanwhile, the peak corresponding to the stretching vibrations of the Si-OH at 952 cm-1 also appears on all curves. Before

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modifying with DMC, there is a weak peak coming from the stretching of S-H could be observed at 2577 cm-1 on the curve of MSNs-SH. The peak of S-H disappears after

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DMC modification, and a new peak corresponding to the C=O stretching of ester group appears at 1729 cm-1 on the MSNs-DMCs curves. Above results indicate the DMC has been successfully modified on the MSNs. To determine the content of DMC in the resultant hybrid materials, the EA data were measured and shown in Table 1. The content of carbon, hydrogen and nitrogen element in the resultant 6

materials increase with the rising DMC feed ratio, while the sulfur content decreases with the increase of DMC feed ratio. Meanwhile, the molar ratio of the nitrogen and sulfur was determined as about 0.996, 1.172, 1.569, and 2.061 when the feed ratio of MSNs-SH and DMC is 1:1, 1:2, 1:3 and 1:4, respectively. The phenomenon indicates the modification of DMC was realized by two steps: firstly, the DMC was anchored

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on the surface of MSNs by a thiol-ene click reaction, a point to point thiol-ene click

reaction occurs between the MSNs-SH and DMC when the feed ratio is 1:1; secondly,

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with the feed ratio of DMC goes up, the excess DMC continue to anchor on the MSNs

through a free radical polymerization process, but it is difficult compared with the thiol-ene click reaction.

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The morphology was characterized by SEM, and the images are shown in Fig. 3.

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The particle size of the MSNs-SH is about 105 nm (Fig. 3a), and the morphology does

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not vary obviously after with the modification of DMC (Fig. 3b-3e). The particle size

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contributions of MSNs-DMCs shown in Fig. 3f are also similar. It is considered that

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the hydrophilic unit is chemically modified on the whole silica particle (surface and inside), and this process has not destroyed the structure of the nanospheres. The small

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particle size and the modification of the hydrophilic group lead to an excellent dispersibility of the hybrid materials in water, which provides the film forming ability

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by a solution process method. As shown in Fig. 4, the dispersion of MSNs-DMCs with a concentration of 50 mg/mL could remain stable after standing for one day.

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The pore structure of the resultant materials was investigated by TEM characteristic

and N2 adsorption-desorption isotherm measurement. In TEM images of MSNs-SH and MSNs-DMCs shown in Fig. 5, the porous structure could be observed in all the materials with the pore size of ~1.7 nm. The pore structure does not exhibit obvious difference after modifying with DMC. From the N2 adsorption-desorption isotherm 7

(shown in Fig. 6), the adsorption-desorption curve of MSNs-SH exhibits a significant increase under low pressure and shows the shape of type I isotherms which indicates MSNs-SH owns microporous structure [16]. Meanwhile, the appearance of the step between 0.2 and 0.8 relative pressure and the hysteresis loop at high pressure area indicates the appearance of the mesopores. The Brunauer-Emmett-Teller (BET)

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surface area and pore volume of MSNs-SH is 863.1 m2/g and 0.333 cm3/g, respectively. The large specific surface area is beneficial for the contact between

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MSNs-SH and other reactor in the synthesis [17, 18]. After modifying with DMC, the significant increase of adsorption-desorption curve under low relative pressure disappeared, and the BET surface area and pore volume of MSNs-DMC (1:3)

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decrease to 29.1 m2/g and 0.009 cm3/g, which are owing to the modification of DMC

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in the micropores of MSNs. The pore size distribution curves show that the pore size

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of resultant materials remains ~1.3 nm before and after modifying with DMC, but

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with an obvious reduction in pore volume.

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To investigate the humidity sensing properties of the resultant materials, sensors based on the MSNs-SH and MSNs-DMCs were fabricated and measured by keeping

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the applied voltage at AC 1 V and the frequency at 1 kHz. Fig. 7a shows the impedance modulus vs. RH relationship of the obtained sensors. The MSNs-SH

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sensor almost shows no response to the low RH environment, with about an order of magnitude change over the humidity range of 54%-95% RH, which is owing to the

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hydrophilic property of sulfhydryl unit. Differently, the response of MSNs-DMCs sensors at low RH atmosphere turns more and more noticeable due to the stronger hydrophilic property of DMC unit. Meanwhile, the sensitivity exhibits a significant increase with the enhancing DMC content, because the Cl- ions could ionize from the DMC group and participate in the electric conduction. Among the MSNs-DMCs 8

sensors, the impedance modulus vs. RH curve of MSNs-DCM (1:3) sensor exhibits good sensitivity and the best linearity (R2 = 0.987) in a semi-logarithmic scale. Hence, the MSNs-DMC (1:3) sensor was selected for further investigation. To investigate the hysteresis property, the MSNs-DMC (1:3) sensor was first measured from 11% to 95% RH, and then in the opposite direction. The adsorption and desorption curve was

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shown in Fig. 7b, the hysteresis of MSNs-DMC (1:3) sensor is ~2% RH. Meanwhile, the hysteresis of MSNs-SH was measured as 15% RH.

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Response and recovery speed is one of the most important parameters of humidity sensors. Four cycles of response and recovery processes of MSNs-DMC (1:3) sensor between 11% and 95% RH were recorded and shown in Fig. 7c. During the

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continuous measurement, the baseline of the response and recovery curve does not

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exhibit obvious drift, indicating the good repeatability of the MSNs-DMC (1:3) sensor

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in short-term dynamic measurement. In the single cycle of response and recovery

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curve, the response and recovery time was calculated as 5 s and 40 s, respectively.

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The rapid response and recovery speed is attributed to the channels formed by particle stacking inside the sensitive film, which is available for water molecules transport.

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The reproducibility and repeatability are very important for practical application. In order to evaluate the reproducibility of the optimal sensor, five MSNs-DMC (1:3)

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sensors were fabricated and measured by the same method which mentioned above. The impedance modulus vs. RH curve with error bar is shown in Fig. 8a. The

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repeatability of the MSNs-DMC (1:3) sensors was also investigated by measuring one sensor for five times as shown in Fig. 8b. The good repeatability property is owing to the excellent structure stability of the sensitive material. In recent years, many kinds of silica based composites and hybrid materials have been designed and applied for humidity monitoring. The humidity sensing 9

performance of some reported sensors are shown in Table 2. The sensitivity shown in Table 2 was defined as the ratio of the impedance modulus of sensors under the lowest RH to the highest RH. Compared with previous sensors, our sensor exhibits decent hysteresis and relatively rapid response and recovery speed. The good response and recovery performance is mainly due to the numerous channels formed by particle

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stacking in the sensitive film as shown in Fig. 9, which is available for water molecules transport in the sensitive film and reach the interface of the electrodes and

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sensitive film [25]. Meanwhile, the good sensitivity and the wide sensing range are owing to the hydrophilic property of the DMC unit inside the hybrid particles.

The mechanism of the sensitive film was investigated by the complex impedance of

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MSNs-DMC (1:3) sensor under different RH with the frequency range from 100 Hz

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to 20 MHz. The equivalent circuit (EC) was fitted with the complex impedance plots

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(CIPs) by the Zview software. As shown in Fig. 10, the CIP at 11% RH is a part of a

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semicircle with a large radius of curvature, the EC of sensitive film was modeled as a

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parallel circuit of a resistance and a capacitance, which is consistent with reported work [26]. As the result shown in Fig. 11a, the phase angle (θ) of MSNs-DMC (1:3)

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sensor is about -70º at 1 kHz, indicating the sensor mainly works as a capacitor-type device [27] and conducts by charging and discharging electric charge without any

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current path inside. As the humidity rises to 33% RH, the shape of CIP remains a part of a semicircle, but the radius of curvature decreases obviously. The phase angle turns

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to -10º at 1 kHz, which corresponds with a resistor device. The variation of the main conductive path of the resistance and capacitance parallel circuit from capacitor to resistor indicates a current path has been built inside the sensitive film. The conductive path is formed by the water adsorption process of the hydroxyl and DMC group, which could build discontinuous water layer under relative low RH situation. 10

Protons are able to migrate by hopping from site to site across the surface of film leading to electric conduction [28]. While the humidity rises beyond 54% RH, the CIP becomes a semicircle with a straight line, meanwhile, the θ increases with the RH at the working frequency (1 kHz). This phenomenon is caused by the appearance of the Warburg impedance (Zw), which is due to the diffusion process of ions or charge

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carriers at the sensitive film and electrode interface [29]. At this time, the influence of

parallel capacitor (Cf) and resistor (Rf) on EC of sensitive film turns weaker and the

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effect of Zw gets conspicuous with the increasing humidity. Under the relative high humidity condition (54%-95% RH), the water layer formed by chemical adsorption process becomes continuous, and thickness grows with the RH by physical adsorption

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process. At this time, the Cl- ions could ionize from the sensitive film and participate

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in the electric conduction, which cause the significant decrease of the impedance [30].

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In order to verify above EC modes, the impedance modulus vs. RH curves of

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MSNs-DMC (1:3) sensor were measured under 100 Hz, 1 kHz, 10 kHz, 100 kHz and

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1 MHz (shown in Fig. 11b). The points measured at different frequencies distribute equidistantly under 11% RH are in a semi-logarithmic scale, which agrees with the

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behavior of a capacitance device [31]. When the humidity raises to 33% RH, the frequency exhibits a significant influence on impedance modulus at high frequency

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range but much less affect at low frequency range. The phenomenon is just consistent with the characteristic of the parallel circuit of a resistance and a capacitance. With

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the increase of humidity, the effect of frequency on impedance modulus turns weaker and weaker, which is due to the appearance of Zw. 4. Conclusion In summary, the DMC modified MSNs hybrid materials have been synthesized as the humidity sensitive materials. The small particle size (about 105 nm) and the 11

modification of the hydrophilic organic units lead the hybrid materials could disperse in water stably with a concentration up to 50 mg/mL, which is beneficial for preparing film via the solution process. The modification of the electrolytes in MSNs enhances the adsorption ability to moisture. Meanwhile, channels formed by particle stacking is available for water molecules transport in the sensitive film and reach the interface of

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the electrodes and sensitive film, which could facilitate the response and recovery process.

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Acknowledgements

This work was supported by the Natural Science Foundation Committee (NSFC, No. 61773178), Projects of Science and Technology Development Plan of Jilin

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Province (No. 20160520093JH).

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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

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is functional sensing materials and devices.

Tong Zhang completed her M.S. degree in semiconductor materials in 1992 and her

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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

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Science and Engineering, Jilin University in 2001. Her research interests are sensing

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functional materials, gas sensors, and humidity sensors.

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Rongrong Qi received her B.S. degree from the College of Electronic Science and

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Engineering, Jilin University, China in 2016. As a M.S. student, her research interest

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is functional sensing materials.

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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

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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 17

University and his current research is focused on the carbon-based functional materials and chemical sensors.

Teng Fei received his B.S. degree in 2005 in chemical engineering and technology

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and Ph.D. degree in 2010 in polymer chemistry and physics from Jilin University, China. He is currently an associate professor in the College of Electronics Science and

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Engineering, Jilin University. His research interests include sensing functional

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materials and devices.

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Geyu Lu received the B.Sci. degree in electronic sciences in 1985 and the M.S.

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degree in 1988 from Jilin University in China and the Dr. Eng. degree in 1998 from

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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

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application of the function materials.

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Figure captions Figure 1. Synthetic routes to MSNs-DMCs. Figure 2. FT-IR spectra of MSNs-SH and MSNs-DMCs. Figure 3. SEM images of (a) MSNs-SH, (b) MSNs-DMC (1:1), (c) MSNs-DMC (1:2),

distribution curves.

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(d) MSNs-DMC (1:3) and (e) MSNs-DMC (1:4); (f) the corresponding particle size

Figure 4. Photographs of the MSNs-DMC (1:3) dispersion in water fabricated with a

SC R

concentration of 50 mg/mL before (a) and after (b) one day standing.

Figure 5. TEM images of (a) MSNs-SH, (b) MSNs-DMC (1:1), (c) MSNs-DMC

U

(1:2), (d) MSNs-DMC (1:3) and (e) MSNs-DMC (1:4).

N

Figure 6. Nitrogen adsorption-desorption isothermals of (a) MSNs-SH and (b)

A

MSNs-DMC (1:3). The inset shows the corresponding pore size distribution curves.

M

Figure 7. (a) Impedance modulus vs. RH curves of sensors based on MSNs-SH and MSNs-DMCs; (b) humidity hysteresis characteristic of sensor based on MSNs-DMC

ED

(1:3); (c) continuous response and recovery curve of MSNs-DMC (1:3) sensor; (d) response and recovery characteristic of MSNs-DMC (1:3) sensor between 11% and 95%

PT

RH.

Figure 8. Impedance modulus vs. RH curve with error bar of MSNs-DMC (1:3)

CC E

sensors generated by descriptive statistical analysis based on (a) several sensors and (b) multiple measurements for one sensor.

A

Figure 9. The schematic of the water adsorption and transportation processes in the MSNs-DMCs sensitive film. Figure 10. The complex impedance plots and equivalent circuits of MSNs-DMC (1:3) sensor at different RH. ReZ and ImZ are the real part and imaginary part of the complex impedance, respectively. 19

Figure 11. (a) The phase angel-frequency relationship of MSNs-DMC (1:3) at different RH. (b) The impedance modulus vs. RH curve of MSNs-DMC (1:3)

A

CC E

PT

ED

M

A

N

U

SC R

IP T

measured under different frequencies.

20

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 1

21

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 2

22

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 3

23

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 4

24

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 5

25

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 6

26

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 7

27

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 8

28

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 9

29

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 10

30

A ED

PT

CC E

IP T

SC R

U

N

A

M

Figure 11

31

Table Captions Table 1. Elemental analysis data of MSNs-SH and MSNs-DMCs. Table 2. Humidity sensing properties of reported humidity sensors based on silicic

N (%)

C (%)

H (%)

S (%)

MSNs-SH

0.00

6.18

2.27

3.79

MSNs-DMC (1:1)

1.27

14.74

3.33

2.92

MSNs-DMC (1:2)

1.42

15.37

3.34

2.77

MSNs-DMC (1:3)

1.74

17.55

3.86

2.53

MSNs-DMC (1:4)

2.08

19.77

4.14

2.31

A

CC E

PT

ED

M

A

N

SC R

Material

U

Table 1

IP T

composites/hybrid materials.

32

Table 2

Sensitive material

Sensing

Sensitivity

range

Hysteresis

Response

Recovery

References

(% RH)

time (s)

time (s)

--

100

150

[19]

11-95% RH

NaCl/KIT-6

11-95% RH

105

--

47

150

[20]

SnO2/SBA-15

11-98% RH

105.5

1.5

15

21

[21]

Fe2O3/MCM-41

11-95% RH

104

3

20

40

Ag/SBA-15

11-92% RH

105

--

100

125

SNs/poly(AMPS)

30-90% RH

104

--

60

120

MSNs-DMC

11-95% RH

103

2

[22] [23] [24]

SC R U

5

N A M ED PT CC E A

33

IP T

LiCl/MCM-41

104

40

this work