Research Article www.acsami.org
Excellent Humidity Sensor Based on LiCl Loaded Hierarchically Porous Polymeric Microspheres Kai Jiang,† Hongran Zhao,† Jianxun Dai,† Da Kuang,† Teng Fei,*,† and Tong Zhang*,†,‡ †
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China ‡ State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing, P. R. China S Supporting Information *
ABSTRACT: A catalyst-free Friedel−Crafts alkylation reaction has been developed to synthesize hierarchically porous polymeric microspheres (HPPMs) with phloroglucin and dimethoxymethane. HPPMs with uniform size were obtained and the size can be tuned by the concentration of raw materials. The chemical structure and hierarchical porous characteristic of HPPMs were characterized in detail. HPPMs were then loaded with humidity sensitive material LiCl to construct composites for humidity sensor. The optimum sensor based on 3 wt % LiCl-loaded HPPMs shows high sensitivity at the relative humidity (RH) atmosphere of 11−95%, small hysteresis, enhanced durability and rapid response. The sensitive mechanism was discussed through the investigation of complex impedance plots. KEYWORDS: humidity sensor, LiCl loaded, hierarchically porous polymeric microspheres, Friedel−Crafts alkylation, cross-linked
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INTRODUCTION The monitoring and controlling of humidity has been paid much attention due to its applications in environment, agriculture, and medicine.1−3 Recently, polymeric sensitive materials have been widely used in humidity sensors among multifarious sensitive materials for their merits of high sensitivity, easy processability, etc.4−7 However, their intrinsic defect of poor stability under high humid environment restricts their further development. Significant effort has been devoted to overcoming their drawbacks, including forming cross-linked or interpenetrated network structures,8−10 grafting or copolymerizing with hydrophobic monomers,11 and constructing inorganic/organic composites.12,13 In general, the formation of cross-linked structures could effectively enhance the durability of humidity sensors under high humid environment. But the procedure of constructing cross-linked polymers is relatively complicated, which needs reactions between polymer and cross-linker at solid state under high temperature (usually over 100 °C), leading to uncontrollability of reactions and unclear cross-linked structures.8,14 Porous polymers are a class of materials with high surface area, well-defined porosity, easy processability and good structural tunability.15 The porous polymers have been used for different applications, such as catalysis,16,17 chemical sensors,18,19 and gas storage etc.20−22 Recently, we developed a humidity sensor with LiCl loaded microporous polymer.23 Porous polymers with abundant pores are capable of physically immobilizing functional molecules, meanwhile small molecules are able to transport into the interior of the polymers freely. © XXXX American Chemical Society
Furthermore, the cross-linked structures of porous polymers can enhance the durability of humidity sensors. Hierarchically porous polymers (HPPs), as a special kind of porous polymers, contain pores across multiple scales.24 For HPPs, micropores can offer the function of adsorption, while meso-pores and macro-pores can enormously increase accessibility of the microporous surface by enhanced diffusion effect. Therefore, when HPPs are used as humidity sensitive materials, the meso-pores or macro-pores can accelerate diffusion rate of water molecules. In this paper, we developed an approach to synthesize hierarchically porous polymeric microspheres (HPPMs) via a catalyst-free Friedel−Crafts alkylation reaction under hydrothermal condition, and LiCl was loaded in HPPMs to prepare humidity sensitive materials. HPPMs and LiCl/HPPMs sensors were prepared and their sensitive properties were discussed in detail. The optimal humidity sensor shows high sensitivity, small hysteresis, enhanced durability, and rapid response/recovery.
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EXPERIMENTAL SECTION
Chemicals and Reagents. Phloroglucin and 1,2-dichloroethane were purchased from Sinopharm Chemicals. Dimethoxymethane was obtained from Jinchun Chemicals. The other reagents were purchased from Xilong Chemicals. All chemicals in this work were analytical grade and used as received.
Received: July 3, 2016 Accepted: September 6, 2016
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DOI: 10.1021/acsami.6b08071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Route to HPPMs
Synthesis of HPPMs. HPPMs-1 was synthesized as the following procedure. Phloroglucin (0.189 g, 1.5 mmol) and dimethoxymethane (0.228 g, 3.0 mmol) were dissolved in dichloroethane (28 mL) and methanol (2 mL). After stirring at room temperature for 1 h, the solution was transferred to a Teflon lined autoclave and placed in an oven at 150 °C for 72 h. Brown product was obtained after filtration, which was then washed with methanol until the filtrate was colorless. HPPMs-2 and HPPMs-3 were synthesized with a similar process, with 3.0 mmol phloroglucin/6.0 mmol dimethoxymethane and 4.5 mmol phloroglucin/9.0 mmol dimethoxymethane, respectively. Apparatus. The IR spectra of the polymers were measured from the PerkinElmer FT-IR spectrometer (reference: potassium bromide). Scanning electron microscopy (SEM) images of HPPMs were completed on the JEOL JSM-6700F instrument. High resolution transmission electron microscope (HRTEM, JEM-2011F) were operated at 200 kV for the images of HPPMs. The nitrogen isotherms of HPPMs were performed by the Micromeritics Tri-star3000 instrument. Humidity Sensors Preparation and Measurements. Humidity sensors were prepared with HPPMs-1 or HPPMs-1 loaded with different contents of LiCl. The information on the detailed preparation process and aging method of the sensors are similar to that described in our previous work.23 The different RH conditions (11%−95%) were achieved with saturated salt solutions. Agilent E4990A impedance analyzer was used for the sensing experiments of the humidity sensors (under AC 1 V). All of the experiments were conducted at ∼20 °C. The schematic measuring diagram is depicted in the Supporting Information (Figure S1).
Figure 1. SEM images of HPPMs: (a−c) HPPMs-1, (d−f) HPPMs-2, and (g−i) HPPMs-3, respectively.
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RESULTS AND DISCUSSION HPPMs were synthesized via Friedel−Crafts alkylation based on phloroglucin and dimethoxymethane as shown in Scheme 1. Usually, Friedel−Crafts alkylation reactions need Lewis acid as a catalyst. In our work, the HPPMs were synthesized by the polymerization of phloroglucin/dimethoxymethane in a mixture solution of methanol and 1,2-dichloroethane without any catalyst under hydrothermal conditions (150 °C for 72 h). A small amount of methanol was used because phloroglucin could not be dissolved well in 1,2-dichloroethane, which is an effective solvent for Friedel−Crafts alkylation reaction.25 It is considered that the application of the mixed solvent contributes to the formation of the hierarchical porous structure of the polymers. Phloroglucin possesses high reactivity of electrophilic substitution due to the electron-donating effect of the three hydroxyl groups substituted on the phenyl ring. The influence of different reactant concentrations (1.5/3.0/4.5 mmol phloroglucin, respectively) on the obtained HPPMs (named HPPMs-1, HPPMs-2, and HPPMs-3, respectively) was researched.
Figure 2. HRTEM images of HPPMs: (a) HPPMs-1, (b) HPPMs-2, and (c) HPPMs-3.
The successful synthesis of HPPMs-1, HPPMs-2, and HPPMs-3 was confirmed by FT-IR spectra and elemental analysis. The three polymers show similar FT-IR spectra (Figure S2 in Supporting Information). The absorption at 2922 cm−1 comes from the stretching of the methylene groups between the phenyl rings. The unsaturated CC bonds show absorption peaks at 1615 and 1460 cm−1, while the C−H bonds in the phenyl rings show a vibration peak at 672 cm−1. Additionally, the absorption at 1278 cm−1 is attributed to the hydroxyl groups substituted on the phenyl rings, and the C−O bonds show a vibration peak at 1108 cm−1. In addition, the elemental analysis of HPPMs-1, HPPMs-2 and HPPMs-3 are very close (C: 57.24%, H: 4.47% for HPPMs-1; C: 57.22%, H: B
DOI: 10.1021/acsami.6b08071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. (a) Nitrogen sorption isotherm of HPPMs-1, HPPMs-2, and HPPMs-3 measured at 77 K and (b) pore size distribution profile of HPPMs-1, HPPMs-2, and HPPMs-3.
Table 1. Porous Characteristics of HPPMs-1, HPPMs-2, and HPPMs-3 sample
SBETa (m2/g)
SLb (m2/g)
PVc (cm3/g)
MPVd (cm3/g)
HPPMs-1 HPPMs-2 HPPMs-3
146 105 82
163 116 102
0.14 0.08 0.07
0.06 0.04 0.03
Figure 4. (a) The impedance modulus of 0, 1, 2, 3, 4 wt % LiCl/ HPPMs sensors in different humidity atmospheres; (b) the relationship of impedance modulus to RH for 3 wt % LiCl/HPPMs-1 sensor with working frequencies of 102, 103, 104, 105, and 106 Hz; (c) the impedance modulus of 3 wt % LiCl/HPPMs-1 sensor in the continuous adsorption and desorption processes; (d) the impedance modulus of 3 wt % LiCl/HPPMs-1 sensor in the RH atmosphere change between 11% and 95% RH.
a
Surface area calculated from nitrogen adsorption isotherms at 77 K using BET equation. bSurface area calculated from nitrogen adsorption isotherms at 77 K using Langmuir equation. cPore volume calculated from nitrogen isotherm at P/P0 = 0.995. dMicropore volume calculated from the nitrogen isotherm at P/P0 = 0.050.
HPPMs-3 at low relative pressure (P/P0 < 0.001), which indicates substantial micropores exist in the samples, and the sorption isotherms of all samples show hysteresis loop at the relative pressure of 0.2−0.9, suggesting the mesoporous structure in the obtained HPPMS.27 In Figure 3b, there are two kinds of main pore sizes (0.65 and 2.57 nm) for the HPPMs, which also demonstrates their hierarchically porous structures. The micropores (0.65 nm) are produced by the sterics of two monomers in the cross-linking polymerization process and the meso-pores (2.57 nm) are considered to be generated by the accumulation of the nanoparticles during the formation process of nanorods.25,28 The surface areas of HPPMs-1, HPPMs-2, and HPPMs-3 by Brunauer−Emmett− Teller (BET) are 146, 105, and 82 m2/g, respectively, which indicates the increased concentration of the reactants would decrease the surface areas of the polymers. HPPMs-1 owning the highest BET surface area was chosen to prepare humidity sensitive materials for humidity sensors. The gas adsorption determines the hierarchically porous structure of the polymers, which are considered beneficial as the host for loading humidity active LiCl. The polymer acts as the stable framework for loading and dispersing small content of LiCl, and both the micropores and meso-pores could act as the channel for transporting water molecules. In addition, from the pore size distribution of the hierarchically porous polymers, it is possible to load LiCl in the meso-pores, which is an ideal model for LiCl based humidity sensitive composites. Composites based on LiCl loaded HPPMs-1 were prepared as the humidity sensitive material. Currently, we have no way of distinguishing LiCl crystallized on surfaces from LiCl
4.37% for HPPMs-2; C: 57.18%, H: 4.25% for HPPMs-3, respectively), thus indicating the polymerization processes of the two monomers with different concentrations are similar. In order to research the morphology of the obtained HPPMs, their SEM images were measured and shown in Figure 1. It is obvious that polymeric microspheres with uniform size have been obtained and these microspheres show rough external surfaces, consist of massive nanorods with hierarchical structures. The nanorods are composed of many small nanoparticles. The sizes of HPPMs were calculated by choosing 300 microspheres from each SEM image. The average sizes of HPPMs-1, HPPMs-2 and HPPMs-3 microspheres are 3.3, 3.9, and 4.5 μm, respectively (Figure S3, Supporting Information). The obtained results demonstrate that the average size of microspheres minifies with the decrease of reactant concentration. This can be easily understood from the synthesis process, because the quantity of nanorods assembled in a certain region decreases with the decrease of the monomer concentration, and the sizes of microspheres would decrease accordingly.26 Furthermore, the HRTEM images of HPPMs-1, HPPMs-2, and HPPMs-3 are shown in Figure 2. From the HRTEM images, there are obvious porous structures in the interior of microspheres, and the details of their porous properties are described as following. The porous characteristics of HPPMs-1, HPPMs-2, and HPPMs-3 were analyzed by nitrogen sorption isotherms. The results are shown in Figure 3 and the related data are summarized in Table 1. A steep nitrogen gas uptake is displayed in the adsorption isotherms of HPPMs-1, HPPMs-2 and C
DOI: 10.1021/acsami.6b08071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. (a) Complex impedance plots measured at different RH with the frequency range of 20 Hz-20 MHz for 3 wt % LiCl/HPPMs-1 sensor (ReZ is the real part and ImZ is the imaginary part); (b) one-to-one match between each complex impedance plot and each equivalent circuit.
Figure 7. Durability of 3 wt % LiCl/HPPMs-1 sensor in 95% RH atmosphere with a period of 30 days. Figure 6. Diagrammatic sketch of hierarchically porous cross-linked framework structure of HPPMs-1.
the impedance module of the sensor shows a high linearity to RH (R2 = 0.995, semilog). Hence, the 3 wt % LiCl/HPPMs-1 sensor was chosen as the optimum sensor for more detailed testing. The impedance modulus of 3 wt % LiCl/HPPMs-1 sensor in different humidity atmospheres with working frequencies of 102, 103, 104, 105, and 106 Hz are shown in Figure 4b. At low RH, the impedance modulus of the 3 wt % LiCl/HPPMs-1 sensor decreases obviously with the increasing frequency, and the curves get closer as the RH increases. For the impedance modulus curve at 106 Hz, it is flat in a wide range and shows a decrease only at high humidities, which demonstrates that the impedance modulus of the sensor is hardly affected by the RH change. This result is owing to the fact that the polarization process of water is lower than the change of the electrical field
incorporated within the meso-pores. The HPPMs-1 sensor was prepared for comparison. Pure HPPMs-1 sensor shows unsatisfied performance with very little impedance modulus (| Z|) change before 50% RH (in Figure 4a). LiCl, as a typical electrolyte material, has been widely used in humidity sensors.2 The small radius of Li+ ion produces big polarization force to attract water molecules with abundant electrons in oxygen atoms.29 Herein the humidity sensors LiCl loaded HPPMs-1 show much better humidity sensitive properties (Figure 4a). The impedance moduli of these sensors decrease as LiCl content increases at 11−95% RH. The 3 wt % LiCl/HPPMs-1 sensor shows the highest sensitivity with nearly 5 orders of magnitude impedance change from 11−95% RH, meanwhile D
DOI: 10.1021/acsami.6b08071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces at high frequencies.30 The impedance modulus vs RH curve at 103 Hz shows the best straight line semilog fit, thus 103 Hz was determined as the optimum frequency. Humidity hysteresis is generally used to estimate the reliability of sensors.31 The impedance modulus of 3 wt % LiCl/HPPMs-1 sensor in the continuous adsorption and desorption processes were measured and shown in Figure 4c. The maximum humidity hysteresis is approximately 4% RH over 11−95% RH, illustrating an improved reliability of the 3 wt % LiCl/ HPPMs-1 sensor. The response speed to RH change is an important feature for humidity sensors. During the process of RH change, the impedance of the sensor changes accordingly, and the interval for 90% impedance change is the response or recovery time.32 The process of RH change was completed quickly (less than 1 s) to avoid the influence of laboratory air atmosphere. Figure 4d shows the impedance modulus of 3 wt % LiCl/HPPMs-1 sensor in the RH atmosphere change between 11% and 95% RH. The sensor possesses a prompt response to RH change, with a response of 2 s for adsorption and a recovery time of 32 s for desorption, respectively. The continuous response and recovery of 3 wt % LiCl/HPPMs-1 sensor for several cycles were measured (Figure S4, Supporting Information), and the sensor shows fast response and recovery to humidity change during the continuous measurements. The idea of preparing humidity sensitive composites based on porous polymer is similar to our previous work based on Li-loaded microporous polymer.23 The main advantage of this work is the hierarchically porous structure of the HPPMs. It is considered that the porous polymer with both micropores and meso-pores could be an ideal choice for preparing humidity sensitive material, because the meso-channels are beneficial for transporting water molecules (the response and recovery time are 2 and 32 s in this work, and 3 and 36 s in our reported work). The humidity sensitive mechanism of the 3 wt % LiCl/ HPPMs-1 composites was discussed with complex impedance plots, which were measured at different RH with the frequency range of 20 Hz-20 MHz (Figure 5a). At 11% RH, the obtained curve is an arc, and the corresponding equivalent circuit is equal to a constant phase element (CPE) as displayed in Figure 5b. The CPE is characteristic of a double-layer capacitor.33 This can be proved from the plot of log (impedance) vs log (frequency) (Figure S5, Supporting Information) In this process, the impedance is high because only limited protons hopping occurs.34 With the increase of RH (33% and 54%), the impedances decrease and the curves become a semicircle indicating the equivalent circuit consists of a parallel capacitor and resistor.35 H3O+ ions could be formed by H+ and H2O, and the transfer of H3O+ (H2O + H3O+ → H3O+ + H2O) is easy,36 which results in decreased impedance. At higher RH (75%), the obvious trailing line with the semicircle is assigned to Warburg impedance (Zw), due to the contribution from diffusion of electroactive species.37 At this stage, LiCl could be ionized by the adsorbed water layers, and Li+ and Cl− ions can take part in the conduction with H3O+. As the RH increases to 95%, more Li+ and Cl− ions could be generated and Zw dominates the impedance, meanwhile the impedance decreases further. The improved humidity sensitive properties discussed above are attributed to the porous characteristic of HPPMs-1, as depicted in Figure 6. The porous HPPMs-1 makes the transport of water molecules into HPPMs-1 much easier in adsorption and desorption processes, leading to a rapid response/recovery to humidity change. Besides, owing to the
cross-linked framework of HPPMs-1, the stability of the composites could be ensured for long-term measurements. From the present results, humidity sensitive composites based on the hierarchical porous polymer show enhanced sensing properties. Currently, we could not explain the detailed mechanisms about the roles of the pores with different sizes. Theoretically, both the micropores and meso-pores could act as the channel for transporting water molecules. In order to research the durability of the sensor, the 3 wt % LiCl/HPPMs-1 sensor was put in 95% RH atmosphere for 30 days, and the impedance was measured every 5 days (Figure 7). There are slight variations on impedance moduli during the long-term measurements, thus indicating enhanced durability of the 3 wt % LiCl/HPPMs-1 sensor. Currently, we have no way to determine whether the amount of encapsulated LiCl remains the same after prolonged operation. The above results on impedance moduli demonstrate the potential practical application of the 3 wt % LiCl/HPPMs-1sensor.
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CONCLUSIONS In this work, a novel method has been promoted to synthesize HPPMs via Friedel−Crafts alkylation reactions under the hydrothermal condition with dichloroethane and methanol as the mixed solvent. The size of HPPMs can be adjusted with the monomers’ concentration. The LiCl/HPPMs-1 sensors were prepared and investigated carefully. The 3 wt % LiCl/HPPMs-1 sensor shows high sensitivity, small hysteresis, enhanced durability and rapid response/recovery. The stable cross-linked porous framework of HPPMs-1 is the main reason for the sensor’s stability and prompt response property. Furthermore, HPPMs-1 possesses large numbers of reactive −OH groups, which are available for further functionalization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08071. The schematic measurement system of humidity sensors. The FT-IR spectra of HPPMs-1, HPPMs-2 and HPPMs3. Size distribution histograms of HPPMs-1, HPPMs-2, and HPPMs-3 from SEM images. Continuous response and recovery curve of 3 wt % LiCl/HPPMs-1 sensor. Linear fit plot of log Z to log f for at 11% RH (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(T.F.) E-mail:
[email protected]. *(T.Z.) E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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). E
DOI: 10.1021/acsami.6b08071 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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