Development of solution processible organic-inorganic hybrid materials with core-shell framework for humidity monitoring

Accepted Manuscript Title: Development of Solution Processible Organic-inorganic Hybrid Materials with Core-shell Framework for Humidity Monitoring Authors: Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei, Geyu Lu PII: DOI: Reference:

S0925-4005(17)31772-0 http://dx.doi.org/10.1016/j.snb.2017.09.106 SNB 23197

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

14-4-2017 17-8-2017 15-9-2017

Please cite this article as: Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei, Geyu Lu, Development of Solution Processible Organic-inorganic Hybrid Materials with Core-shell Framework for Humidity Monitoring, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.09.106 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.

Development of Solution Processible Organic-inorganic Hybrid Materials with Core-shell Framework for Humidity Monitoring Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei*, Geyu Lu State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P.R. China

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

Tel.: +86 431 85168385; Fax: +86 431 85168270

Highlights 

A novel organic-inorganic hybrid material PSS@SNs with the core-shell framework was prepared.



The small SNs size and the modification of hydrophilic polyelectrolyte endow the ability of preparing films with the solution process method.



The obtained sensor is applied for

respiratory rate monitoring due to the particularly rapid

response. 

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

Abstract A novel organic-inorganic hybrid material with the core-shell framework was prepared by modifying the poly(vinylsulfonic acid, sodium salt) (PSS) on vinyl functionalized silica nanoparticles (VSNs) via the free radical polymerization. SNs with different contents of PSS were prepared and the structures were carefully characterized. The hybrid material could enhance the durability of PSS under high humidity environment. Meanwhile, the SNs skeleton provides interstitial channels in the PSS@SNs films, which are available for water molecules transport. The PSS coated SNs (PSS@SNs) films were formed by drop-coating and their sensing properties were investigated. The impedance of the optimal humidity sensor changed more than four orders of magnitude over the relative humidity range of 33-95%, with good linearity under semi-logarithm scale (R2 = 0.950) and a small humidity hysteresis of 2% RH. The obtained sensor also showed rapid response to humidity change (the response and recovery time were 2 and 65 s, respectively). Finally, the resultant sensor was explored for respiratory rate monitoring.

Keywords

Organic-inorganic hybrid materials;

humidity sensor;

core-shell

framework; solution processible nanoparticles

1. Introduction Humidity, one of the most important parameters, has a significant effect on human life and industrial production. Novel humidity sensors with excellent properties are in demand. Among traditional humidity sensitive materials, the inorganic sensitive

materials, such as metal oxides [1,2], carbon-based materials [3,4] and silicas [5] etc., exhibit good stability to high humidity atmosphere, but it is difficult to prepare sensitive film by solution process due to their poor solubility. Organic polyelectrolytes have also been widely investigated and employed as humidity sensitive materials due to their advantages of solution process ability, high sensitivity, and low cost [6,7], but the polyelectrolytes are usually suffering from their poor durability under high humidity environment. In order to further improve the performance of humidity sensors, recently, organicinorganic hybrid materials were regarded as new candidates [8,9]. The humidity performance of hybrid materials could be controlled by adjusting the structure and content of organic unit. Meanwhile, it has been proved that the addition of inorganic structure could enhance the mechanical strength and high humidity stability of the materials [10,11]. In our previous works, polyelectrolytes were united with various silicon-based materials, such as the mesoporous silica and slisesquioxane [12,13], to overcome their poor durability under high humidity environment, but the large particle sizes still limited their solution process ability. In this work, novel small-sized organic-inorganic hybrid spheres with core-shell structure were designed and prepared. Silica nanoparticles (SNs) were employed as a stable hydrophobic core to guarantee the stability of the hybrid materials, while the hydrophilic polyelectrolyte, poly(vinylsulfonic acid, sodium salt) (PSS), was applied to form the shell layer and supplies the solution process and humidity sensing ability. By anchoring on SNs, the stability of PSS under high relative humidity (RH)

environment could be guaranteed. Furthermore, the presence of the SNs skeleton could provide large quantity of channels for water molecules transport, which is beneficial for realizing fast response to humidity change. 2. Experimental Chemicals Vinyltrimethoxysilane (VMTS) was purchased from Energy Chemical. The sodium ethenylsulfonate was purchased from Aladdin. The azodiisobutyrodinitrile (AIBN) was purchase from Sigma-Aldrich. The n-methyl pyrrolidone (NMP) was purchased from Xilong Scientific Co., Ltd. The ammonia, diethyl ether and methanol 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. Preparation of vinyl functionalized SNs (VSNs) The VSNs were prepared according to a reported work [14]. In a typical procedure, 1.29 g VTMS was added in 100 mL deionized water under stirring at room temperature. After VTMS droplets disappeared, 6.7 mL ammonia was added dropwise to the solution until the pH reached 9.0. The solution was kept stirring under 70 ºC for four hours and white suspension was obtained. The resultant mixture was centrifuged at 11000 rpm for 10 min, then, wash three times by deionized water (40 mL for each time). Finally, 316 mg white powder was obtained after drying at 60 ºC over night. Preparation of PSS@SNs

The synthetic scheme of PSS@SNs is shown in Fig. 1. Three materials defined as PSS@SNs1, PSS@SNs2 and PSS@SNs3 were synthesized with different mole ratios of VSNs to sodium ethenylsulfonate as 1:0.5, 1:1 and 1:1.5, respectively. Take PSS@SNs2 as an example, VSNs (100 mg) and sodium ethenylsulfonate (870 mg, 25 wt% aqueous solution) were added in NMP (5 mL), and the mixture was stirred for 10 min. Then, AIBN (27.4 mg) was added in above mixture at 60 ºC. After stirring for another 48 h, the mixture was transferred into a large excess of diethyl ether. The result product was filtered and washed by methanol for three times (40 mL for each time) and finally 115 mg white powder was obtained after drying at 60 ºC over night. 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. The morphologies were observed by field emission scanning electron microscopy (FE-SEM) on a JSM-6700F electron microscope (JEOL, Japan). The high resolution transmission electron microscopy (HRTEM, JEM-2011F) were operated at 200 kV for the images of PSS@SNs 2. Fabrication and measurement of humidity sensors The resultant materials were dispersed in deionized water by sonication with a 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 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 about 18 μm. The sensors were aged in 95% RH atmosphere with a voltage of 1 V at 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 properties was carried on a Keysight E4990A impedance analyzer at 298 K. The ambient temperature was controlled by air-condition. The RH environments were produced by different saturated salt solutions [15]. 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 [16]. Humidity hysteresis is defined as the maximum difference of the humidity sensor between the adsorption and desorption processes [17]. 3. Results and discussions In this work, the VSNs template with abundant vinyl units was synthesized by the sol-gel reaction of VTMS in aqueous solution. Then, PSS was coated on the surface of VSNs via a free radical polymerization between sodium ethenylsulfonate and vinyl sites on VSNs. The resultant products were washed with methanol to remove the unreacted monomer and PSS chains which were not chemically modified on the SNs. The chemical structure of resultant materials was characterized by FT-IR spectroscopy and EA. In the FT-IR spectra (Fig. 2), the characteristic peak of the silica could be observed from all curves, including the peaks corresponding to the asymmetric stretching of Si-O-Si at 1046 and 1135 cm-1, the peak corresponding to symmetric mode of Si-O-Si lattice vibrations at 776 cm-1 and the peak of Si-OH at

1592 cm-1. Meanwhile, all of curves exhibit two peaks corresponding to the stretching of C=C bond at 973 and 1280 cm-1, and the intensity of the peaks decreases with the increasing PSS content, demonstrating the consumption of vinyl units during the free radical polymerization process. The peaks corresponding to S=O bond and S-O(H) bond could not be observed due to the overlap of the peak position of sulfonate group and Si-O-Si bond among 1000 to 1100 cm-1. To further investigate the PSS content, the EA characterization of resultant materials were carried out. From the EA data (Table 1), the sulfur content gradually increases with the sodium ethenylsulfonate amount during the synthesis process, which certificates the successfully modification of PSS on SNs. The morphology of materials has a significant effect on their film forming ability. The SEM images of the materials are shown in Fig. 3 (a-d). The size distribution of the materials was obtained by calculating 200 spheres from the corresponding SEM images (Fig. 3e). As can be seen in the SEM image (Fig. 3a), VSNs are spheres with the diameter of about 300 nm, and there is a rigid contact between the spheres. The morphology of PSS@SNs1 does not exhibit obvious change with VSNs, but the diameter of spheres turns larger (Fig. 3b). As the feed ratio of sodium ethenylsulfonate and VSNs increases to 1:1, amorphous species appear at the contact point of two spheres, and the diameter of spheres increases to about 600 nm (Fig. 3c). The phenomenon becomes much more notable with the further increase of PSS. In the SEM image of PSS@SNs3 (Fig. 3d), the amorphous species almost fills up the interspace of spheres. In the TEM image of PSS@SNs2 (inset of Fig. 3c), the spheres

exhibit obvious core-shell structure, indicating the PSS is successfully coated on the surface of VSNs with a thickness of about 200 nm. The rigid contact between spheres generally turns to a flexible contact by the junction of PSS shell layer. The obtained PSS@SNs could be dispersed in water with a concentration of 50 mg/mL. The good dispersibility is attributed to the small SNs size and the modification of hydrophilic polyelectrolyte, which endows the ability of preparing films with the solution process method. In order to investigate the humidity sensing properties of resultant materials, sensors based on VSNs, PSS@SNs1, PSS@SNs2 and PSS@SNs3 were fabricated and the variation of their impedance modulus to RH change were recorded. The impedance modulus variation of resulted sensors with RH change during the static measurement and dynamic measurement were summarized in Fig. 4. Fig. 4a shows the impedance modulus and RH relationship of obtained sensors from 33% to 95% RH. VSNs sensor exhibits poor response to RH change because there are no strong hydrophilic groups in VSNs. After coated by PSS, the impedance modulus decreases obviously with the increasing RH, and the response increases with the polyelectrolyte content. The modified polyelectrolyte promotes the chemisorption process of water molecules, meanwhile, the Na+ ions could ionize from PSS and transport in the water layer formed by physisorption process, leading a significant decrease of impedance modulus. Among the resulted impedance modulus-RH curves, the curve of PSS@SNs2 sensor shows the best linearity among the whole RH range in a semilogarithmic scale (R2 = 0.950). Meanwhile, the PSS@SNs2 sensor exhibits a tiny

maximum humidity hysteresis (2% RH) during the adsorption and desorption process (shown in Fig. 4b), in which the impedance modulus of sensors were recorded from 33% to 95% RH, and then in the opposite direction. The response and recovery speed and the repeatability were estimated by a dynamic measurement. The sensor exhibits a significant rapid response (only 2 s) to the abrupt RH change, and the recovery time is 65 s (Fig. 4c). The remarkable rapid response is contributed to the plenty of hydrophilic site on the PSS layer. Meanwhile, in the sensitive film, the space formed by rigid VSNs acts as the stable skeletons and provides channels for the rapid transport of water molecules, which would result in fast response. The repeatability of the PSS@SNs2 sensor was evaluated by recording the impedance modulus in five circles of response and recovery process between 33% and 95% RH (Fig. 4d). During the repeatability measurement, the baseline of the repeatability curve does not exhibit obvious drift and the sensor remains rapid response and recovery speed in five circles tests. To evaluate the effect of the ambient temperature on the response, the impedance modulus vs. RH curves of PSS@SNs2 sensor (shown in Fig. 5) were measured at different temperatures. The impedance modulus decreases with the increasing temperature, because the rising temperature would increase the mobility and the quantity of ions, such as H+ and Na+. Meanwhile, the average temperature coefficient in the range of 25-65 ºC was calculated as 0.12% RH/ºC. Fig. 6 shows the response of PSS@SNs2 sensor to humidity (from 33% to 95% RH) and various gases (1000 ppm) at room temperature. The humidity of the atmosphere,

which is applied as the background of interference gases was 49% RH (measured by hygrometer). The response here was defined as Z0/Zg, Z0 is the initial impedance modulus and the Zg stands for the impedance modulus in the target environment. The results imply that the sensor has prominent selectivity and totally insensitive to ethanol, acetone, methylbenzene, NH3, CH4 and NO2. Considering the extremely fast response of PSS@SNs2 sensor, the application of respiratory rate monitoring was explored. The impedance modulus variation of the PSS@SNs2 sensor during several respiratory cycles of a heath adult male was recorded and shown in Fig. 7. The exhaled gas was collected by connecting the tester’s nasal cavity and the humidity sensor with a 30 cm rubber hose (the scheme is shown in Fig. 7a). From the result of human breathing monitoring shown in Fig. 7b, the humidity sensor exhibits a repetitive impedance modulus variation at the range of 2000 to 40000 kΩ with the respiratory rhythm of the tester while exposed to the exhale gas. The respiratory period (about 3 s) could be clearly observed from the enlarged view (Fig. 7c) of respiratory rate monitoring curve. Meanwhile, the impedance modulus of sensor could return to the initial rapidly after measurement. Above results demonstrate that the obtained PSS@SNs2 sensor exhibits good response, small humidity hysteresis, acceptable repeatability and fast response speed. The remarkable performance not only comes from the good hydrophilic property of PSS shell layer, the unique framework of the humidity sensitive film also contributes to the sensing process. As shown in Fig. 8, the VSNs act as the rigid core for the anchoring of PSS. Meanwhile, the holes with a diameter of ~175 nm would be formed

between the spheres. Different from the dense sensitive film formed by normal organic polymers, there are innumerable interstitial channels available for water molecules transport in the sensitive film. Hence, water molecules could pass through sensitive film and reach the electrode interface rapidly, resulting in fast response speed. Till now, many kinds of composites/hybrid materials have been applied for humidity monitoring. In order to evaluate the properties of our sensor, reports of various sensitive composites/hybrid materials were collected and the detail data are presented in Table 2. Compared with present works, the sensor in this work exhibits good sensitivity, small hysteresis and especially fast response, which are owing to the unique structure of hybrid spheres. The hydrophilic PSS shell layer, which contacts with external environment directly, could guarantee the abundant water adsorption. Hydrophobic SNs core is beneficial for the water desorption process. Meanwhile the presence of the SNs skeleton could provide large quantity of channels for water molecules transport, which is beneficial for realizing fast response to humidity change. To further discover the sensing mechanism of the sensitive film, the complex impedance of PSS@SNs2 sensor was measured under different RH with the frequency range from 100 Hz to 20 MHz. The complex impedance plots (CIP) is shown in Fig. 9, ReZ and ImZ are real part and imaginary part of the complex impedance, respectively. The CIP at 33% RH (Fig. 9a) exhibits part of a semicircle with large radius of curvature, indicating the equivalent circuit (EC) of the sensitive film can be modeled as a parallel capacitor (Cf) and resistor (Rf) [18]. The phase angle

(θ)-frequency cure (shown in Fig. 9f) shows that θ increases with the AC frequency and retains at about -90º when the frequency beyond 600 Hz at 33% RH, indicating the capacitor branch acts as the main current path under the working frequency (1 kHz) [19]. There is almost no conducting path inside the sensitive film. When the RH increases to 54%, the ReZ-ImZ curve becomes a semicircle with a short line. The short line appearing at low frequency region represents the Warburg impedance (Zw) due to the diffusion process of ions or charge carriers at the sensitive film and electrode interface [20]. At this RH the θ raises to -90º from 0º with the AC frequency increasing, indicating the sensitive film appears as resistive under low frequencies and capacitive under high frequencies, and the curve exhibits maximum slop when θ reaches 45º, which just matches the characteristic of a parallel capacitor (Cf) and resistor (Rf). At the used working frequency, the resistor branch acts as the main current path and the influence of Zw is unconspicuous. Rf turns to the main current path indicates the formation of the conductive path, generated by the adsorbed water via chemisorption and physisorption processes, in the sensitive film. As the RH continues to increase, the radius of curvature of the semicircle reduces gradually and the straight line turns longer and longer, meanwhile, the θ at high frequency range decreases with the RH increasing but opposite at low frequency range. The above phenomenon demonstrates the influence of parallel capacitor (Cf) and resistor (Rf) on EC of sensitive film turns weaker and the effect of Zw gets conspicuous. When the RH reaches 95%, the semicircle disappears and there is only a straight line, indicating the diffusion process of ions or charge carriers at the sensitive film and electrode interface

becomes the main limitation of the impedance modulus change. By contrast, in the CIP of the VSNs sensor (shown in Fig. 10), ImZ-ReZ curves remain part of semicircle framework from 33% to 95% RH. Meanwhile, from the phase angle-frequency relationship shown in the inset, we can see that the sensitive film exhibits capacitive under 33% and 54 % RH. With the RH continues to increase, the sensitive film gradually turns resistive under low frequency. However, the resistor branch does not become the main current path even RH reaches to an extremely high level (95%), due to the VSNs film could not adsorb enough water molecules to build conductive path inside. Moreover, the straight line corresponding to Zw does not appear in the CIP of the VSNs sensor, demonstrating there are almost no ions or charge carriers diffusion process at the sensitive film and electrode interface without PSS layer. The EC modes of PSS@SNs2 sensor were proved by impedance modulus vs. RH curves measured at different frequencies which shown in Fig. 11. The applied frequency exhibits a significant influence on the impedance modulus at 33% RH, which coincides with the behavior of CPE. When the RH increases to 54% RH, the effect of frequency becomes less distinct at low frequency, owing to the formation of the resistor branch. With the further increase of RH, the points measured at various frequencies turn closer, which is due to the appearance of Zw. Hence, the response of PSS@SNs sensor increases with the decreasing frequency. Finally, 1 kHz was chosen as the optimal frequency by considering the response and the linearity. In order to investigate the stability of the hybrid materials, the PSS@SNs2 sensor was exposed in the 95% RH atmosphere for 20 days. And the impedance moduli

under different RH were measured by every two days. From the long-term stability curves shown in Fig. 12, it can be seen that the impedance modulus does not exhibit obvious variation at all investigated RH levels during the 20 days measurement. The good stability of the PSS@SNs2 sensor is due to the organic-inorganic hybrid structure. The stability of PSS under high relative humidity environment is guaranteed by anchoring on the insoluble SNs. 4. Conclusion In conclusion, high-performance humidity sensors based on organic-inorganic PSS@SNs hybrid materials with core-shell framework were demonstrated. The small SNs size and flexible organic shell layer enhances the dispersibility of the hybrid materials and realizes good film-forming ability. The performance of resultant sensors inherits and even goes beyond the traditional polymer humidity sensors. The channels formed by SNs skeleton is beneficial for water molecules transport, resulting in the ultrafast response which makes the sensors possible for respiratory rate monitoring.

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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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.

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.

Figure and Table Captions Figure 1. Synthetic routes to PSS@SNs. Figure 2. FT-IR spectra of VSNs and PSS@SNs. Figure 3. SEM images of (a) VSNs, (b) PSS@SNs1, (c) PSS@SNs2 and (d) PSS@SNs3, the inset of (c) is the TEM image of PSS@SNs2; (e) particle size distribution curves of VSNs, PSS@SNs1, PSS@SNs2, PSS@SNs3. Figure 4. (a) Impedance modulus of sensors based on VSNs and PSS@SNs under different RH; (b) humidity hysteresis characteristic of sensor based on PSS@SNs2; (c) continuous response and recovery curve of PSS@SNs2 sensor; (d) response and recovery characteristic of PSS@SNs2 sensor between 33% and 95% RH. Figure 5. The impedance modulus vs. RH curves of PSS@SNs2 sensor under different temperatures. Figure 6. Selectivity of PSS@SNs2 sensor measured at 1 kHz. Figure 7. (a) The test method scheme of respiratory rate monitoring; (b) impedance responses to the RH of the breath exhaled from the human nasal cavity; (c) the enlarged view of the respiratory rate monitoring curve at the range of 60 to 80 s. Figure 8.The schematic of the water adsorption and transportation processes in the PSS@SNs sensitive film. Figure 9. (a)-(e) The complex impedance plots and equivalent circuits of PSS@SNs2 sensor at different RH; (f) the phase angel-frequency relationship of PSS@SNs2 at different RH. Figure 10. The complex impedance plots of VSNs sensor at different RH, the inset is the phase angel-frequency relationship of VSNs under different RH.

Figure 11. The impedance modulus vs. RH curves of PSS@SNs2 sensor at different frequencies.

Figure 12. The long-term stability curves of PSS@SNs2 sensor for 20 days.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

30

Figure 11

31

Figure 12

32

Table 1 Table 1. Elemental analysis data of VSNs and PSS@SNs.

Sample

C (%)

H (%)

S (%)

VSNs

25.05

3.93

0.00

PSS@SNs1

27.37

3.86

0.23

PSS@SNs2

28.02

3.50

2.29

PSS@SNs3

28.74

3.35

2.93

33

Table 2 Table 2. Humidity sensing properties of reported humidity sensors based on composites/hybrid materials.

Sensitive element

Category

Sensing

Sensitivity

range

Hysteresis

Response

Recovery

(% RH)

time (s)

time (s)

References

LDH-PANI/SDS

Salt-base/polymer

11-95% RH

105

3.2

4

25

[8]

LiCl/PDVB

Salt-base/polymer

11-95% RH

103

5

6

30

[21]

QC-P4VP/PANI

Polymer/polymer

1-98% RH

103

3

24

35

[22]

TiO2 NPs/PPy

Metal oxide/polymer

30-90% RH

103

--

40

20

[23]

SnO2/POA

Metal oxide/polymer

20-100%RH

101

4

87

13

[24]

MPOSS/DVB/DMC

Silicon-base/polymer

11-95% RH

103

1.5

3

40

[13]

DMC/SBA-15

Silicon-base/polymer

11-95% RH

103

2

11

60

[12]

SNs/poly(AMPS)

Silicon-base/polymer

30-90% RH

104

--

60

120

[25]

PSS@SNs

Silicon-base/polymer

33-95% RH

104

2

2

65

This work

34