Design and fabrication of bifunctional microcapsules for solar thermal energy storage and solar photocatalysis by encapsulating paraffin phase change material into cuprous oxide

Solar Energy Materials and Solar Cells 168 (2017) 146–164

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Design and fabrication of bifunctional microcapsules for solar thermal energy storage and solar photocatalysis by encapsulating paraffin phase change material into cuprous oxide

MARK

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Fengxia Gao, Xiaodong Wang , Dezhen Wu State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Bifunctional microcapsules Cu2O shell Phase change materials Solar photocatalysis Solar thermal energy storage Gas-sensing effectiveness

This article reported the design and fabrication of bifunctional microcapsules for solar photocatalysis and solar thermal energy storage by using cuprous oxide (Cu2O) as an inorganic shell to encapsulate a paraffin-type phase change material (PCM), n-eicosane. Such a new type of microcapsules was synthesized successfully by using an emulsion templating self-assembly method along with in-situ precipitation. The chemical structures of the resultant microcapsules were determined by Fourier-transform infrared spectroscopy, and the elemental distributions of microcapsule shell were confirmed by X–ray photoelectron spectroscopy and energy-dispersive X–ray spectroscopy. The scanning and transmission electronic microscopic observations demonstrated that the microstructures and morphologies of microcapsules were influenced significantly by the surfactant and alkali concentrations as well as the portion of cupper source for the synthesis. After the synthetic condition was optimized, the obtained microcapsules exhibited an interesting octahedral morphology and a typical core-shell structure. The thermal analysis results suggested that the microcapsules synthesized at the optimum condition not only obtained high encapsulation and energy-storage efficiencies but also presented a high thermal stability and phase-change reliability. Most of all, the microcapsules obtained a solar thermal energy-storage capability through solar photothermal conversion and also exhibited a high solar photocatalytic activity to organic dyes under the sunlight illumination. In addition, the microcapsules showed a gas-sensitive feature to some harmful organic gases in the presence of Cu2O shell. The microcapsules developed by this work indeed reveal a bifunctional feature derived from both the core and the shell materials and thus show a great potential for industrial and domestic applications due to their extended functions.

1. Introduction Phase change materials (PCMs) are a class of substances which are capable of absorbing and releasing latent heat through phase transitions [1]. Different from conventional thermal energy-storage materials, PCMs can absorb large amounts of thermal energy at a certain temperature without getting hotter. On the other hand, PCMs will solidify with a decline in ambient temperature around them and release their stored latent heat accordingly [2]. With rapid growth of the consumption of fossil fuels followed by a serious environmental impact as well as a shortage of fossil energy resources, a great deal of attention has been paid recently to the improvement of energy utilization efficiency and the development of renewable energy by both scientific societies and industrial communities [3]. PCMs have been recognized as a class of renewable energy materials with a high energy-utilization efficiency, and the latent heat storage by use of PCMs is also considered

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Corresponding author. E-mail address: [email protected] (X. Wang).

http://dx.doi.org/10.1016/j.solmat.2017.04.026 Received 18 October 2016; Received in revised form 9 April 2017; Accepted 13 April 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

as one of the most important thermal energy-storage technologies [4]. Nowadays, PCMs have received wide applications in various areas like energy-saving building materials, collection and reutilization of solar thermal energy, smart fibers and textiles with a temperature-regulating function, cooling systems of electronic components and apparatuses, industrial waste water recovery, refrigerated transportation units, etc [5]. Wu et al. [6] reported the application of PCMs as a thermal storage medium for the biofuel micro trigeneration prototype. Ma et al. [7] presented a detailed literature review on the use of PCMs for the thermal regulation and electrical efficiency improvement of photovoltaic modules. Wang et al. [8] provided a detailed introduction on the application of solar water heating system with PCMs. Sun et al. [9] introduced a free-air cooling system using PCMs as a nature cool source for space cooling in telecommunication bases. Sahoo et al. [10] summarized the applications of PCMs in heat sinks for cooling of electronic components. Taking into account the boom in industrial and

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domestic fields, there is no doubt that PCMs will play a key role in renewable resources for green growth and sustainable development in the future. PCMs can usually be arranged into three categories: organic PCMs, inorganic PCMs and eutectic PCMs. Inorganic PCMs primarily include molten salts and salt hydrates [11]. As the most commonly used PCMs, the organic PCMs tend to be oligomers or polymers with long molecular chains consisting mainly of hydrogen and carbon, including aliphatic acids, fatty acid ester, paraffin waxes and polyglycols. They usually show high orders of crystallinity when solidifying and mostly change the phase above 0 °C [12]. Within the human comfort and electronic device tolerance range of 25–37 °C, organic paraffin waxes are very effective to store large amounts of latent heat energy and then release them controllably in such a narrow temperature range without substantial density change during the phase transition. Therefore, paraffin-based PCMs are promising candidates for thermal energy storage in the low-to-moderate temperature range and have been broadly used for solar domestic hot water heating systems, passive solar space heating–cooling systems for day/night temperature equalization in domestic buildings, thermal enclosures like Basic Telecom Shelters, and smart thermal regulating fibers and textiles [13–15]. For example, a paraffin-PCM-based solar domestic hot water system can provide more cumulative and life cycle savings than the conventional one and will continue to perform efficiently even after 15 years due to application of non-metallic tank [14]. Although the paraffin waxes are cheap comparatively, the packaging and processing necessary to obtain acceptable performance from them is really expensive and complicated. They cannot offer a reliable pattern of releasing latent heat with the chemicals in PCMs separating and stratifying when in the liquid state. Moreover, these PCMs cannot always perform re-solidification in a proper way, or they did not solidify completely with a decline of temperature, thus resulting in the reduction of their capacity to store latent heat. In addition, the molten paraffin waxes have a mobile feature and thus easily flow away when used with other supporting materials [16]. These problems have been addressed by packaging PCMs in thin or shallow containers and by adding clumping agents [17]. Microencapsulation is a physical or chemical process to engulf small liquid or solid particles of 1–100 µm diameter by a solid shell and is considered as a good solution to overcome the defects in organic PCMs. Through the microencapsulation of organic PCMs with an inert material, the resulting microcapsules are able to keep a macroscopic solid form when the PCM core is molten [18]. Furthermore, the microencapsulation of organic PCMs not only enhances the ease of handling but also provides a large specific surface area for PCMs, which allows an effective heat transfer [19]. The microencapsulation of paraffin waxes has been well developed as an important packing technology for the solid–liquid organic PCMs in the past decade. There are a large number of studies reporting the synthetic techniques and performance investigations of microencapsulated paraffin waxes with various polymeric shells through chemical methods such as complex coacervation, in-situ polymerization, interfacial polycondensation and suspension polymerization [20,21]. The typical polymers used as shell materials include polystyrene [22], polyurea-formaldehyde resin [23], poly(methyl methacrylate) [24], melamine-formaldehyde resin [25], poly(butyl acrylate) [26], polyurethane [27], and even the bio-based materials like silk fibroin [28] and gelatin/Arabic gum [29]. Considering the superiority of inorganic materials over the polymeric ones in nonflammability, mechanical strength, thermal conductivity, and thermal and chemical stabilities, an increasing number of studies have been conducted on the synthetic technologies associated with the microencapsulation of organic PCMs with an inorganic shell or inorganic/ organic hybrid shell [30]. The most recent publications indicated that some inorganic chemicals such as SiO2 [31], Al(OH)3 [32], CaCO3 [33], and amorphous TiO2 [34] were employed successfully as wall materials to encapsulate organic PCMs through the self-assembly method. The

resultant microcapsules not only exhibit a good phase-change thermal performance resulting from the enhancement of thermal conduction but also gain a higher thermal stability, longer durability, and better sealing tightness and anti-permeability due to the encapsulation by a rigid and compact inorganic shell [35]. There is no doubt that most of the studies on the development of microencapsulated organic PCMs either with polymeric shells or with the inorganic ones unexceptionally concerned the monofunctional issue of latent heat storage. Taking account of the functional diversity of inorganic materials, it is expected that, if organic PCMs are encapsulated into the specified inorganic shell, some specific physical or chemical functions are possible to be imparted to the resulting microcapsules. In this case, the diverse design and fabrication of bior multi-functional microcapsules were proposed as a new design idea for the preparation of microencapsulated organic PCMs in our previous studies. With such a design concept in mind, we have made great efforts to design and fabricate the microencapsulated paraffin waxes with various inorganic functional shells and have already succeed in the fabrication of microencapsulated PCMs with crystalline ZnO [36], TiO2 [37], ZrO2 [38,39], SiO2/Fe3O4 hybrid [40,41] and SiO2/Ag doubledlayered shells [42]. These microencapsulated PCMs all exhibited a variety of functions such as photocatalysis, magnetic effectiveness, photoluminescence, antibacterial action and electrical conduction in addition to the thermal energy-storage capability, indicating a bi- or multi-functional feature. It should be emphasized that each type of these microencapsulated PCMs was prepared on the basis on the different reaction mechanisms, synthetic pathways, and respective technique know-how as described in the relevant publications. For example, the microencapsulated PCMs with a ZnO shell were formed through interfacial precipitation along with a series of aging reactions [36]. For the microencapsulated PCMs with a TiO2 or ZrO2 shells, a non-aqueous oil-in-water emulsion templating system was designed to conduct a self-assembly polycondensation of inorganic precursors on the PCM core, and a crystallization promoter was utilized to promote the formation of crystalline TiO2 or ZrO2 shells with various functions [37–39]. As regards the microencapsulated PCMs with a SiO2/Fe3O4 hybrid shell, a Pickering emulsion templating system was first established, in which magnetic Fe3O4 nanoparticles were used as a Pickering stabilizer, and then a self-assemble process of silica precursors followed by in-situ polycondensation was conducted to fabricate a SiO2/Fe3O4 hybrid shell on the surface of PCM core [40]. In the case of the microencapsulated PCMs with a SiO2/Ag doubled-layered shell, a thiolfunctional silica inner layer was first fabricated onto the surface of PCM core, and the a silver outer layer was formed through surface assembly of silver ions with the aid of thiol groups followed by a reduction reaction [40]. It is evident that the fabrication of these bi- or multifunctional microencapsulated PCMs required significant breakthroughs from traditional synthetic techniques, although their syntheses were followed with some general principles like emulsion templating and interfacial self-assembly. In this study, we reported the design and synthesis of a new type of bifunctional microcapsules based on the paraffin core and cuprous oxide (Cu2O) shell. As a typical p-type semiconductive material with a direct bandgap of 2.0–2.2 eV, Cu2O has attracted an intense interest for its various characteristic properties in recent years, and it has obtained wide applications for solar photocatalysis, photoelectrolytic cells, solar energy conversion, antifouling coatings, and water-splitting materials [43]. Cu2O is especially characteristic of its high photocatalytic activity under the visible light illumination, low toxicity and good environmental acceptability [44]. Therefore, it is anticipative that, by encapsulating organic PCMs into a Cu2O shell, the resulting microcapsules achieve a solar photocatalytic function as well as a latent heat-storage capability. Such a novel type of microencapsulated PCMs is especially applicable for the disinfection of water supply accompanied by outdoor solar thermal energy collection or the decontamination and waste heat recovery of industrial wastewater [45]. It may also have great potential 147

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electron microscope. The specimen was dispersed in ethanol, and some pieces were collected on carbon-coated 300-mesh copper grids for the TEM characterization. The chemical structures and compositions of microcapsule shell were detected by X–ray photoelectron spectroscopy (XPS) on a Thermo Fisher ESCALAB 250 X–ray photoelectron spectrometer with a focused monochromatized Al–Kα radiation, and the resulting spectra were fitted with a Casa XPS software using a Gaussian–Lorentzian method. Fourier-transform infrared (FTIR) spectroscopy was conducted to characterize the chemical structures of microcapsules on a Nicolet iS5 FTIR spectrophotometer at a scanning number of 32. The powder X–ray diffraction (XRD) measurement was conducted to determine the crystalline structure of microcapsule shell on a Japan Rigaku D/max 2500VB2+/PC X–ray diffractometer using Cu–Kα radiation (λ=0.154 nm). All of the XRD patterns were obtained at 40 kV and 20 mA with a scan rate of 2°/min in the 2θ range of 5–80°. Differential scanning calorimetry (DSC) was conducted to investigate the phase-change behaviors and thermal performance of microcapsules on a TA Instruments Q20 differential scanning calorimeter. DSC measurements were carried out at a scanning rate of 10 °C/min, and thermal history should be diminished by holding the specimen at 60 °C for 5 min when running the first scan. The isothermal and nonisothermal crystallization behaviors of pure and encapsulated neicosane were also characterized by dynamic DSC scans at different crystallization temperatures and scanning rates, respectively, and their crystalline morphologies were observed by an Olympus BX51-P polarized light microscope. The thermal stability of microcapsules was investigated by thermogravimetric analysis (TGA) on a TA Instruments Q50 thermal gravimetric analyzer in nitrogen at a heating rate of 10 °C/min. The thermal conductivity of microcapsules was measured on an HS–DR–5 thermal conductivity tester (Shanghai HE SHENG Instrument Technology Co., Ltd., China) in terms of the transient plane source method. The reported data of thermal conductivity reflected an average of five tests. Osmosis experiments were conducted to evaluate the antipermeability of microcapsules. In a typical measurement, about 10 g of microcapsules were dispersed in 50 mL of acetone as an extraction solvent by mild stirring, and then the transmittance of the resulting suspension was measured by a 723PC–Vis spectrophotometer at different extraction time. All of the transmittance data were conversed into the weight fraction of n-eicosane released from the microcapsules, which was named as the release rate reflecting the osmotic property of microcapsules. To evaluate the solar thermal absorption performance of microcapsules, a custom-designed experimental setup was constructed by a xenon arc lamp as a solar irradiation source, a glass test tube connecting with a thermocouple, and a notebook computer equipped with an SH–X multi-channel temperature collection recorder (Shenzhen Shenhwa Technology Co., Ltd., China). The xenon arc lamp has a power of 250 W and can be used as a continuous light solar simulator to produce the full sunlight intensity of 1000 ± 50 W/m2. The test tubes filled with the microcapsules were illuminated by the xenon arc lamp for 16 min, and then the illumination was stopped. The transient temperature response during the above heating and cooling processes was recorded once every 20 s by the temperature collection recorder with an accuracy of ± 0.5 °C. The measurement was repeated three times for each specimen, and the average data were obtained from these three tests. The thermal energy storage performance of microcapsules was further evaluated with a TESTO®875–1i infrared thermographic camera on a hot plate, and the infrared thermographic images were taken at different heat time. The solar photocatalytic degradation experiments for three organic dyes, i.e. Congo red, malachite green and acid fuchsin, were carried out directly using sunlight as a light source on a bright clear day. A typical testing method was described as follows: 60.0 mg of the microcapsules was dispersed in 150.0 mL of aqueous solution containing 1.0 mg of Congo red in a beaker with agitation for 30 min at a dark room in order to reach the adsorption equilibrium. The aqueous suspension was

for applications in non-concentrated solar thermal energy collection and storage, solar photocatalytic mineralization of organic water pollutants, solar photochemical detoxification of textile dyes and other industrial pollutions, and so on. The objective of this work is to develop synthetic technologies of the bifunctional microcapsules based on a Cu2O shell and n-eicosane core, to investigate their thermal performance and solar photocatalytic effectiveness, and also to exploit the solar thermal energy and solar photochemical utilization for these bifunctional microcapsules. In addition, considering the gas-sensitive characteristics of Cu2O and its potential applications for detection of harmful gases as a gas sensor element [46], the gas-sensing behaviors of the microcapsules were also investigated so that their functions could be further extended. 2. Experimental 2.1. Materials n-Eicosane used as a paraffin-type PCM was purchased from Beijing Innochem Science & Technology Co., Ltd., China, and it has a high purity of approximately 98 wt%. Copper sulfate (CuSO4) was purchased from J & K Chemical Co., Ltd., China and used as a copper source. Cetyl trimethylammonium bromide (CTAB), sodium hydroxide (NaOH), glucose (C5H11O5CHO) and absolute ethanol were purchase from Beijing Chemical Reagent Co. Ltd., China. Three organic dyes, i.e. Congo red, malachite green and acid fuchsin, were kindly provided by Tianjin Yongda Chemical Reagent Co. Ltd., China and used for photocatalytic degradation experiments. All reagents are of analytical grade and used as received without further purification. 2.2. Synthesis of bifunctional microcapsules The bifunctional microcapsules composed of the n-eicosane core and Cu2O shell were prepared via an in-situ precipitation process under different synthetic conditions. A typical synthetic process was described as follows: a 500-mL round flask was charged with 80.0 mL of deionized water, 4.8g of n-eicosane and 4.66g of CTAB surfactant. The obtained mixture was emulsified mechanically at 60 °C using a high-speed mixer, and a stirring rate of 400 rpm was set to accomplish the required emulsification. With stirring for 1.5 h, a transformation from translucence to milky appearance was observed in the mixture, implicating the formation of a stable oil-in-water (O/W) emulsion, in which the water and n-eicosane served as a water phase and oil phase, respectively. Subsequently, 60.0 mL of aqueous solution with 4.8g of CuSO4 was added to the above emulsion and then stirred for 4 h. Then, 60.0 mL of aqueous solution with 8.0g of NaOH and 60 mL of aqueous solution with 3.0g of glucose were simultaneously added dropwise into the flask under vigorous agitation. The mixture was put in an oil bath and kept at 60 °C with stirring for 6 h, producing some brick-red precipitates. The resultant suspension was aged for 8 h at 60 °C to solidify the microcapsule shell. Finally, the targeted microcapsules were obtained as some brick red powders by filtration. The collected powders were washed with deionized water and absolute alcohol several times to remove the residual chemicals and then dried naturally under a ventilating condition for further characterizations and measurements. 2.3. Characterization Scanning electron microscopy (SEM) along with energy-dispersive X–ray (EDX) spectroscopy was conducted to investigate the morphology and surface elemental distribution of microcapsules using a Hitachi S–4700 scanning electron microscope coupled with a TEAM® EDX spectrometer. The specimens were made electrically conductive by sputter coating with a thin layer of gold–palladium alloy. The microstructures of the microcapsules were investigated by transmission electron microscopy (TEM) on a Hitachi JEM–3010 transmission 148

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With addition of a NaOH aqueous solution into this self-assemble system, a precipitation reaction actually occurred between OH–1 and Cu2+ ions to form a Cu(OH)2 layer surrounding the n-eicosane micelle. The Cu(OH)2 layer was further reduced with glucose as a weak reducing agent, followed by a long-term aging process to form a well-defined Cu2O shell encapsulating the n-eicosane core. The formation mechanisms of Cu2O shell in the current work can be described by four reactions as depicted in Fig. 1, where it is also interesting to observe a series of color variations for the reactant solution reflecting these reactions. The Cu2+ ions were first precipitated to Cu(OH)2 in the presence of excessive OH–1 ions, resulting in a color change from blue to green. The formation of Cu2O shell needed to be conducted by a reduction process with glucose. Although some of Cu(OH)2 could be directly reduced to Cu2O by glucose, most of them had to under take a phase transition from Cu(OH)2 to CuO and then perform the reduction from CuO to Cu2O. In this case, it is interesting to note the color changes from green to dark brown then to brick red as shown by Fig. 1. In our investigation on synthetic technology, the concentrations of CTAB and NaOH were found to play a critical role in the formation of the core-shell structural microcapsules, thus determining the successful encapsulation of n-eicosane with Cu2O. Fig. 2 shows the SEM micrographs of the microcapsules synthesized at different concentrations of CTAB and NaOH but at a fixed n-eicosane/CuSO4 weight ratio of 50/50. As observed in Fig. 2a, the microcapsules present an irregularly polyhedral shape with a size of 2–3 µm when synthesized at a low concentration of CTAB. With increasing the surfactant concentration, the shape of microcapsules is found to become more regular and their size seems to be much larger. Finally, the microcapsules exhibit an interesting octahedral morphology (see Fig. 2d) when synthesized at a surfactant concentration of 0.15 mol/L. The magnified SEM micrograph displays that these microcapsules have a compact and smooth surface as seen in Fig. 2e. The morphological observation indicates that the surfactant concentration dominates the formation of the well-defined microencapsulated n-eicosane with a Cu2O shell, because the surfactant can establish a perfect templating system for the self-assemble of cupper ions only at an appropriate concentration. However, for the microcapsules obtained at low surfactant concentrations, the insufficiency of surfactant results in a reduction of cupper ions attracted onto the surfaces of n-eicosane micelles, thus leading to an irregular morphology. Moreover, differing from the conventional microcapsules with a spherical shape, the microcapsules synthesized in this study reveal an interesting polyhedral or octahedral shape, which may result from the growth of Cu2O crystals on the basis of their characteristic crystalline structures [48]. On the other hand, it is noteworthy in Fig. 2f–i that the NaOH concentration is one of the important factors that effect the morphology of microcapsules significantly. The microcapsules are found to show an irregular shape with a few disfigurements on their surface (see Fig. 2f) when synthesized at a low concentration of NaOH. Nevertheless, the microcapsules tend to present a perfect octahedral morphology with an increase of NaOH concentration. It is well known that the Cu2+ ions are precipitated directly with OH– ions to form a Cu (OH)2 layer surrounding the n-eicosane core at the first synthetic stage, and then the Cu(OH)2 layer is reduced to the Cu2O shell. In this case, the adequate amount of NaOH is needed to ensure a prompt and sound precipitation reaction. When the NaOH concentration is low, the Cu (OH)2 nucleus formed at the initial precipitating stage are insufficient to deposit in bulk, thus resulting in their random growths. This may lead to an irregular morphology for the Cu2O shell after the Cu(OH)2 layer is reduced. However, the Cu2O shell is easily eroded by the excessive OH– ions, and therefore, the resulting microcapsules shows a defective surface as observed in Fig. 2h and i. To further confirm a typical core-shell structure for the microcapsule samples synthesized in this work, the microcapsules were intentionally fractured by a proper mechanical pressure, and then the resulting microcapsules were observed by SEM. Fig. 3 shows the SEM micrographs of some representative samples of fractured microcap-

exposed to the outdoor sunlight for 3 h at noon. During the illumination period, an approximate 8 mL suspension was taken out of the beaker at a fixed interval of 20-min time to detect the concentration of Congo red solution after centrifuging. The solar photocatalytic activity of microcapsules was evaluated by characterizing the absorbance of the Congo red solution on a Shimadzu UV–2550 UV–visible spectrophotometer as a function of illumination time. Gas-sensing measurements were conducted on a CGS–8 intelligent gas-sensing analysis system (Beijing Elite Technology Co., Ltd., China). The microcapsule sample was homogeneously dispersed in deionized water to form a paste, and then the resulting paste was coated onto a ceramic tube with a diameter of 1 mm and a length of 4 mm. After dried completely, the tube was jointed with the gas-sensing measurement system through four Pt wires, and a coil heater made from the Ni–Cr alloy passed through the tube to offer an operating temperature control. The gas-sensing response is defined as the ratio of Rg/Ra, where Rg and Ra are the electrical resistance of the sensor in the atmospheres of tested gas and air, respectively. The response time and recovery time are defined as the time taken by the sensor to reach a change rate of 90% for electrical resistance when performing absorption in the tested gas and desorption in air, respective. 3. Results and discussion 3.1. Synthetic strategy and microstructures The core-shell structural microcapsules were designed as bifunctional microencapsulated PCMs by fabricating a Cu2O shell onto an neicosane core. Differing from the syntheses of the microencapsulated PCMs with ZnO, TiO2, ZrO2, SiO2/Fe3O4 hybrid and SiO2/Ag doubledlayered shells we reported previously, the Cu2O shell is hard fabricated directly through a simple reaction, but its formation have to conduct a series of complicated redox reaction [47]. In order to realize such a design objective, we adopted an emulsion templating self-assembly synthetic technique to perform in-situ precipitation, because the emulsion templating self-assembly is a facile method to form an organized structure or pattern. Although the emulsion templating self-assembly method has been adopted widely to synthesize several types of PCMsbased microcapsules either with polymeric shells or with the inorganic ones, two key factors should still be paid more attention for the successful preparation of the well-defined microcapsules composed of the n-eicosane core and Cu2O shell. The first factor is the selection of an appropriate surfactant template that can supply specific and local interactions among the n-eicosane core to attract inorganic precursors for self-assembly themselves, and the second one is the accurate control of a balance between the deposition and precipitation of inorganic precursors at the oil–water interface to ensure the self-assembled system of inorganic precursors formed in advance and the in-situ precipitation occurring later. In this study, CuSO4 was used as a copper source to encapsulate n-eicosane PCM, and CTAB was selected as a cationic surfactant to establish an emulsion templating self-assembly system based on n-eicosane and copper species. Fig. 1 illustrates the schematic synthetic route and reaction mechanism for such a new type of bifunctional microcapsules. As shown by the synthetic route in Fig. 1, an O/W emulsion templating system containing n-eicosane micelles was built first with the aid of surfactant. In such a templating system, the surfactant molecules trimly cover the surfaces of oily n-eicosane globules with hydrophobic segments oriented inward into the oily globules. On the other hand, the hydrophilic quaternary ammonium bromide groups are associated outward with water phase by arranging along the hydrophobic chains of CTAB. After an aqueous solution of CuSO4 was added into the templating system, copper ions were attracted onto the surfaces of n-eicosane micelles by an electrostatic interaction between copper cations and bromide anions in terms of the charge-controlled mechanism [47], thus resulting in the formation of a self-assemble system with copper species at the oil–water interface. 149

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Fig. 1. Scheme of synthetic route and reaction mechanism for the bifunctional microcapsules based on an n-eicosane core and Cu2O shell.

encapsulated into Cu2O through in-situ precipitation in the templating system established in this work, followed by a reduction with glucose, and the well-defined core-shell structure can be achieved by the microcapsules synthesized only at the optimum surfactant and alkali concentrations. In addition, we also attempted to control the shell thickness by altering the weight ratio of n-eicosane/CuSO4 in the reaction mixture. Fig. 5 shows the SEM micrographs of the microcapsules synthesized at

sules. It is interesting to note that these samples all exhibit a distinct core-shell structure, although some of them have poor aspects due to the synthetic conditions unoptimized. Furthermore, the TEM investigation also confirms the well-defined core-shell structure for the microcapsules as seen in Fig. 4. Additionally, the TEM micrographs of Fig. 4c and d clearly reveal some structural disfigurements on the Cu2O shell of the microcapsules obtained at inappropriate concentrations of CTAB and NaOH. These results indicate that n-eicosane has been well

Fig. 2. SEM micrographs of the microcapsule samples synthesized at a fixed weight ratio of n-eicosane/CuSO4 (50/50) and different concentrations: (a) CCTAB=0.08 mol/L and CNaOH=5.00 mol/L; (b) CCTAB=0.10 mol/L and CNaOH=5.00 mol/L; (c) CCTAB=0.13 mol/L and CNaOH=5.00 mol/L; (d, e) CCTAB=0.15 mol/L and CNaOH=5.00 mol/L; (f) CCTAB=0.15 mol/L and CNaOH=3.33 mol/L; (g) CCTAB=0.15 mol/L and CNaOH=4.17 mol/L; (h, i) CCTAB=0.15 mol/L and CNaOH=5.83 mol/L.

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Fig. 3. SEM micrographs of the damaged microcapsules synthesized at a fixed n-eicosane/CuSO4 weight ratio of 50/50 and different concentrations: (a) CCTAB=0.10 mol/L and CNaOH=5.00 mol/L; (b) CCTAB=0.15 mol/L and CNaOH=5.00 mol/L; (c) CCTAB=0.15 mol/L and CNaOH=4.17 mol/L; (d) CCTAB=0.15 mol/L and CNaOH=5.83 mol/L.

Fig. 4. TEM micrographs of the microcapsule samples synthesized at a fixed n-eicosane/CuSO4 weight ratio of 50/50 and different concentrations: (a) CCTAB=0.13 mol/L and CNaOH=5.00 mol/L; (b) CCTAB=0.15 mol/L and CNaOH=5.00 mol/L; (c) CCTAB=0.15 mol/L and CNaOH=3.33 mol/L; (d) CCTAB=0.15 mol/L and CNaOH=5.83 mol/L.

pure n-eicosane as a reference. It is notable that these microcapsule samples exhibit a similar aspect in the infrared spectrum, and a series of characteristic absorption bands can be found in their infrared spectra at 2960, 2919, 2851 and 1639 cm−1 for the C–H and C–C stretching vibrations of n-eicosane chains, at 1471 and 1382 cm−1 for the methylene-bridged C–H stretching vibration, and at 718 cm−1 for the in-plane rocking vibration of methylene groups. These characteristic bands are in good agreement with the characteristic absorption peaks of pure n-eicosane as seen in Fig. 6a. On the other hand, an intensive peak at 626 cm−1 is clearly observed in the infrared spectra for all of the microcapsule samples. Such a characteristic peak represents the Cu (I)–O vibration of Cu2O. It is noteworthy that no characteristic absorption of the Cu(II)–O vibration for CuO or Cu(OH)2 is found at 500 cm−1 in the infrared spectra. The infrared characterization results

the other two weight ratios of n-eicosane/CuSO4. A multiporous structure is observed all over the whole microcapsules synthesized at the n-eicosane/CuSO4 weight ratio of 60/40 (see Fig. 5a and b). However, when the weight ratio of n-eicosane/CuSO4 was set to 40/ 60, it is surprising to observe some small solid particles (see Fig. 5c and d), but there were not any microcapsules found. These results implicate that the self-assembly system in this work is established elaborately and is dependent strongly on reactant formulation. 3.2. Chemical composition and crystalline structure FTIR spectroscopy was conducted to characterize the chemical compositions of the microcapsules synthesized in this work. Fig. 6a shows the resulting infrared spectra of microcapsule samples as well as 151

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Fig. 5. SEM micrographs of the microcapsule samples synthesized at fixed surfactant and alkali concentrations of 0.15 mol/L and 5.00 mol/L but at different n-eicosane/CuSO4 weight ratios of (a, b) 60/40 and (c, d) 40/60.

mine the formation of Cu2O shell. Fig. 6b displays the EDX spectrum of the representative sample synthesized at the optimum condition. This spectrum clearly shows the signals associated with copper and oxygen elements as well as the carbon background, and the atomic ratio of Cu

confirm that these microcapsule samples are composed of n-eicosane and Cu2O absolutely. The chemical compositions and elemental distributions of microcapsule surfaces were further investigated by EDX and XPS to deter-

Fig. 6. (a) FTIR spectra, (b) EDX spectrum, and High-resolution XPS spectra in (c) Cu2p and (b) O1s regions of binding energy for the microcapsule samples synthesized at different conditions; the codes of curves are identical to the sample codes in Table 1, where the synthetic conditions are clearly described.

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XRD measurements were conducted to investigate the crystalline structure of Cu2O shell, and the resulting patterns are presented in Fig. 7. It is emphasized that, in order to avoid the diffraction interference from crystalline n-eicosane, the n-eicosane core must be solvent-extracted from the microcapsules before the XRD measurement. The XRD patterns all reveal a set of diffraction peaks at the 2θ values of 29.58°, 36.46°, 42.36°, 61.52°, 73.68°, and 77.52°, which can be assigned to the (110), (111), (200), (220), (311), and (222) facets of Cu2O, respectively, as marked in Fig. 7. No diffraction peak for Cu (OH)2, CuO, or other impurities was found in the XRD patterns. These diffraction data clearly indicate that the Cu2O shell is completely formed by simple cubic crystals as reported by JCPDS card no.050667 [50]. On the basis of the XRD characterization, it is understandable that the formation of octahedral morphology of the microcapsules is ascribed to the crystal growth along with the (111) and (100) facets of cubic of Cu2O single crystals. It was reported that the morphology of a monocrystal was determined by the relative growth rates of different facets according to the crystal growth theory [51]. In general, the morphology of Cu2O tends to an octahedral shape when the crystals grow faster on the (100) facet than on the (111) one. On the contrary, its morphology tends to cubic or truncated octahedral shapes when the crystal growth on the (100) facet is lower than or closer to that on the (111) one, respectively. Meanwhile, such a growth rate is strongly influenced by the reactant concentrations, surfactant concentrations, temperature, stirring speed, etc. In this case, the octahedral morphology of the microcapsules with a Cu2O shell is evidently derived from the precise control of synthetic conditions.

Fig. 7. XRD patterns of the microcapsule samples synthesized at different conditions; the codes of curves are identical to the sample codes in Table 1, where the synthetic conditions are clearly described.

to O is calculated to be 2:1.2 on the basis of EDX quantitative analysis as presented in Fig. 6b. These results suggest that the microcapsule shell is composed of Cu2O solely. Fig. 6c and d show the high-resolution XPS spectra of the same microcapsule sample in Cu 2p and O 1s regions, respectively. There are two typical Cu p1/2 and Cu p3/2 peaks observed at binding energy of 952.5 and 932.8 eV in the Cu 2p region of the XPS spectrum, respectively. Meanwhile, a single peak located at binding energy of 530.1 eV was found in the O 1s region, which is attributed to the O–Cu bond of Cu2O according to the curve fitting as shown in Fig. 6d. These results are in good agreement with the published data XPS data of Cu2O [49]. Furthermore, the mole ratio of Cu to O was determined to be 1.98:1 by integrating the curve areas under the peaks of Cu 2p and O 1s. The XPS characterization not only provides an evidence for the presence of Cu2O as a wall material but also confirms that Cu(OH)2 and CuO have all been reduced to Cu2O during the synthesis.

3.3. Phase-change characteristics and encapsulation performance DSC measurements were conducted to investigate the phase-change performance of microcapsules. Fig. 8 illustrates the resulting thermograms and the thermal analysis data derived from DSC measurements are listed in Table 1. It is interesting to observe in Fig. 8 that both pure n-eicosane and the microcapsule samples exhibit an exothermic crystallization peak along with a shoulder at a slightly higher temperature, and however, only a single endothermic melting peak is found at on their DSC heating thermograms. A number of publications reported that

Fig. 8. DSC cooling and heating thermograms of pure n-eicosane and the microcapsule samples synthesized at different conditions; the curve codes of samples are identical to the sample codes in Table 1, where the synthetic conditions are clearly described.

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Table 1 The phase-change properties, encapsulation parameters, and thermal performance of the microcapsules based on an n-eicosane core and Cu2O shell obtained at different synthetic conditions. Sample code

1 2 3 4 5 6 7 8 9 10 11 a b c

Synthetic condition

Solidification process

Fusion process

Cth

Cact

Een

Ees

ΔTs

Thermal conductivity

CCTABa (mol/L)

CNaOHb (mol/L)

Wn-Eicosane /WCuSO4c

Tc (°C)

TR (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

(%)

(%)

(%)

(%)

(°C)

(W m−1 K−1)

– 0.08 0.10 0.13 0.15 0.15 0.15 0.15 0.15 0.15 –

– 5.00 5.00 5.00 5.00 3.33 4.17 5.83 5.00 5.00 5.00

100/0 50/50 50/50 50/50 50/50 50/50 50/50 50/50 60/40 40/60 0/100

31.86 30.15 32.40 32.68 32.52 31.92 32.07 31.98 29.41 – –

32.47 32.37 33.40 33.92 33.86 34.02 33.70 32.67 32.17 – –

267.2 40.4 80.2 110.6 163.1 73.7 117.2 86.5 7.5 0 –

38.70 37.65 38.23 38.46 38.71 37.25 37.96 37.88 37.68 – –

268.3 41.3 81.5 111.2 165.3 74.8 118.2 87.8 7.9 0 –

– 68.96 68.96 68.96 68.96 68.96 68.96 68.96 76.92 59.70 –

– 17.25 32.31 44.79 64.21 30.26 48.64 35.17 3.26 0.75 –

– 15.39 30.38 41.45 61.61 27.88 44.05 32.72 2.94 0 –

– 15.26 30.20 41.42 61.32 27.73 43.96 32.55 2.87 0 –

6.84 7.50 5.83 5.78 6.19 5.33 5.89 5.90 8.27 – –

0.143 5.231 4.972 4.258 3.652 4.965 4.352 5.015 7.548 16.839 18.236

The concentration of CTAB in the reactant mixture. The concentration of NaOH for the synthesis. The weight ratio of n-eicosane/CuSO4 for the synthesis.

synthetic condition influences the ΔHc and ΔHm of resulting microcapsules significantly. As observed in Table 1, the microcapsules show very low values in ΔHc and ΔHm when synthesized at an n-eicosane/ CuSO4 weight ratio of 50/50 with inadequate concentrations of NaOH and CTAB. Phase-change enthalpies are also found to increase with the optimization of alkali and surfactant concentrations. The microcapsule sample obtained at the optimum condition is distinctly found to present the highest values for both ΔHc and ΔHm. On the other hand, for the microcapsule samples obtained at n-eicosane/CuSO4 weight ratios of 60/40 and 40/60, almost no phase-change enthalpy is detected in their DSC scans. These results are in coincidence with the previous investigation on morphology and microstructure, suggesting that the microcapsules can achieve high phase-change enthalpies only in good morphological and structural conditions. Several important parameters for encapsulation performance can be deduced by use of several simple equations [36,37]. The theoretical core content (Cth) of a microcapsule samples can be calculated by Eq. (1):

most of the paraffin waxes exhibited such a bimodal crystallization behavior due to the presence of a metastable rotator phase prior to completing the full crystallization [52]. The phase-transition temperature resulting from this metastable state is evidently higher compared to the bulk crystallization one. Therefore, the paraffin waxes generally show two transition temperatures. The first one is associated with a phase transition from homogeneously nucleated liquid to the metastable rotator phase, whereas the second one is due to the transformation from the heterogeneously nucleated rotator phase to the completely crystalline one during the solidification process. Such twice phase transitions lead to the bimodal crystallization behavior of pure neicosane as observed in this work. Although the DSC curve profiles are similar with one another for the n-eicosane encapsulated or not, it is of evidence that the encapsulation of n-eicosane with Cu2O influences its crystallization and melting behaviors as seen in Fig. 8. These microcapsule samples are all observed to present a slight increase in crystallization peak temperature (Tc) in their cooling DSC thermograms but a decline in melting peak temperature (Tm) in the heating ones. It is inevitable that the inner wall of Cu2O generates a heterogeneous nucleation effect on the encapsulated n-eicosane and thus enhances the crystallinity of n-eicosane, leading to an improvement in Tc accordingly. On the other hand, a confined crystallization effect on neicosane may result from microencapsulation. This evidently caused imperfect crystals as well as much smaller crystal grains when the neicosane core was crystallized in a confined space. As a result, the Tm of the encapsulated n-eicosane was reduced in its heating DSC thermogram. The crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) as two important phase-change parameters represent the thermal energystorage capability of a PCM, and both of them can also be derived from DSC measurements. It is note in Table 1 that pure n-eicosane show fairly great values over 265 J/g in ΔHc and ΔHm, Such results are noticeably higher than the data previously reported due to the high purity of n-eicosane [16,17], thus suggesting that this paraffin-type PCM has a good latent heat-storage/release capability during the solid–liquid phase changes. However, the microcapsule samples reveal a significant decline in absolute values of ΔHc and ΔHm, and their phase-change enthalpies are also found to vary case by case with the synthetic conditions. This phenomenon is ascribed to the fact that the Cu2O shell as an inert material cannot perform a solid–liquid phase transition in the temperature range of DSC measurements. Therefore, the ΔHc and ΔHm of microcapsules are mainly dominated by the amount of n-eicosane core encapsulated within microcapsules. In general, the amount of n-eicosane core is associated with the synthetic formulations as well as reaction conditions for the microcapsules, and hence these

Cth =

wcore × 100% wcore + wshell

(1)

where wcore is the weight of n-eicosane charged for the synthesis, and wshell is the weight of Cu2O shell converted from the amount of CuSO4 charged for the synthesis. The actual content (Cact) of n-eicosane encapsulated within microcapsules could be estimated from the weight loss after the n-eicosane core was solvent-extracted from microcapsules according to Eq. (2):

Cact =

w0 − w1 × 100% w0

(2)

where w0 is the weight of as-prepared microcapsules, and w1 is the weight of the microcapsules after solvent extraction, equal to the weight of Cu2O shell. Furthermore, the encapsulation efficiency (Een) can be derived from DSC results for a microcapsule sample by the following equation:

Een =

ΔHm,core × 100% ΔHm,PCM

(3)

where ΔHm,PCM and ΔHm,core are the values of melting enthalpy for pure n-eicosane and the encapsulated one, respectively. Meanwhile, the thermal energy-storage efficiency (Ees) of microcapsules can also be estimated by Eq. (5):

Ees = 154

ΔHm,core + ΔHc,core × 100% ΔHm,PCM + ΔHc,PCM

(4)

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where ΔHc,PCM and ΔHc,core are the values of crystallization enthalpy for pure n-eicosane and the encapsulated one, respectively. These encapsulation parameters obtained from Eqs. (1)–(4) are also listed in Table 1. As seen in Table 1, it seems that the theoretical core content is only determined by the weight ratio of n-eicosane/CuSO4 for the synthetic formulation. Nevertheless, the actual core content tends to be always lower than the theoretical one at the same weight ratio of n-eicosane/ CuSO4. Nevertheless, the actual core content is found to increase gradually with variations of surfactant and alkali concentrations from 0.08 to 0.15 mol/L and from 3.33 to 5.0 mol/L, respectively. This indicates that not all of the Cu2+ ions were conducted by assembly onto the surfaces of n-eicosane micelles to fabricate the Cu2O shell, and some of them were precipitated with OH–1 directly, followed by reduction with glucose to form the solid Cu2O particles. In this case, some of neicosane micelles were not fully encapsulated by Cu2O, thus resulting in a leakage of n-eicosane core and a decrease in actual core content. As discussed previously, the microstructure and morphology of microcapsule samples tended to be perfect with the alkali and surfactant concentrations closed to the optimum values, and the Cu2O shell became compacter and stronger enough to protect the n-eicosane core from leaking. This improves the actual core content effectively. It is noteworthy in Table 1 that the microcapsule samples synthesized at the n-eicosane/CuSO4 weight ratios of 60/40 and 40/60 exhibit exceedingly lower actual core contents compared to their higher theoretical ones. This implicates the unsuccessful encapsulation of n-eicosane into Cu2O due to a failure in the emulsion templating self-assembly. The encapsulation efficiency and thermal energy-storage efficiency are also two main characteristic parameters reflecting the thermal energystorage capability of microencapsulated PCMs. The former characterizes the effective encapsulation of a wall material for the PCM core, and the latter describes the latent heat-storage/release efficiency of the PCM core during the reversible phase transitions. It is seen in Table 1 that the higher actual core content makes the microcapsule samples gain much higher encapsulation and thermal energy-storage efficiencies. The microcapsules are found to achieve the highest encapsulation and thermal energy-storage efficiencies only when synthesized at the conditions of CCTAB=0.15 mol/L, CNaOH=5.00 mol/L and Wn-Eicosane/ WCuSO4=50/50. This result confirms that such a synthetic condition is optimum for the preparation of microencapsulated n-eicosane with the Cu2O shell. However, the other microcapsule samples exhibit considerably low encapsulation and thermal energy-storage efficiencies due to the leakage of n-eicosane core caused by their poor microstructures formed under inappropriate synthetic conditions.

and d.

⎛ dHc ⎞ ⎟ dT ⎝ dT ⎠ XT = × 100% T ⎛ dH ⎞ ∫T ∞ ⎜ c ⎟ dT 0 ⎝ dT ⎠ T

∫T ⎜ 0

(5)

where T0 and T∞ are the crystallization onset and end temperatures, respectively, T is an arbitrary temperature and (dHc/dT) is the heat flow rate of DSC scans. Assuming that the specimen experiences the same thermal history as designated by the DSC furnace, the crystallization time (t) can be obtained from the following equation:

t=

T0 − T × 100% ϕ

(6)

where T0 is the onset crystallization temperature at t=0, T the temperature at crystallization time t, and ϕ is the cooling rate. In this case, the half crystallization time (t1/2) can be calculated by Eq. (6), and the crystallization rate parameter (CRP) can be determined from the slope of the plot of reciprocal of t1/2 against ϕ [53]. Moreover, the activation energy (ΔEa) of nonisothermal crystallization can be determined by the Kissinger's equation as expressed by [54]:

⎛ϕ⎞ ⎛ A⋅R ⎞ ΔE 1 ln ⎜⎜ 2 ⎟⎟ = − a ⋅ + ln ⎜ ⎟ ⎝ ΔEa ⎠ R Tp ⎝ Tp ⎠

(7)

where ϕ is the cooling rate, Tp the crystallization peak temperature, R the gas constant, and A is the frequent factor. In this case, the values of ΔEa can be calculated by the slope of the plot of ln(ϕ/Tp2) against 1/Tp. All of the kinetic parameters of nonisothermal crystallization were summarized in Table 2. It is observed in Fig. 9a and b that both pure neicosane and the microencapsulated n-eicosane present a two-stage crystallization in their nonisothermal DSC thermograms, and their Tonset's, Tp's and Tend's also shift to lower temperatures with an increase of cooling rate as shown in Table 2. Moreover, these two specimens show a decrease in t1/2 with increasing the cooling rate, indicating the crystallization rates become faster with an elevation of cooling rate. These results suggest that the nonisothermal crystallization for both pure n-eicosane and the microencapsulated one occurs in a nucleationcontrolled region. However, these three characteristic temperatures of the microencapsulated n-eicosane are higher than those of the pure one, and meanwhile, their Tp's seem to become closer to one other. This implicates a reduction in temperature dependency of crystallization followed by an improvement in thermal response for the encapsulated n-eicosane. The microencapsulated n-eicosane also shows shorter t1/2 compared to the pure one. It is understandable that the crystallization process of pure n-eicosane is mainly controlled by homogenous nucleation and growth. Microencapsulation of n-eicosane can provide a large number of nuclei by the inner Cu2O shell, and thus generating a heterogeneous nucleating effect for the crystallization of n-eicosane. This can improve the crystallization rate of n-eicosane core significantly. Furthermore, the values of CRP in Table 2 reflect the total crystallization rates of n-eicosane in different situated environments and thus confirm a faster crystallization process for the microencapsulated n-eicosane. The crystallization activation energy, ΔEa, represents the energy barrier for crystallization of an organic PCM under the nonisothermal condition. As seen in Table 2, the microencapsulated neicosane exhibits lower ΔEa compared to the pure one, indicating that it tends to crystallize more easily due to the lower energy barrier resulted by heterogeneous nucleation and also gains a faster growth rate for crystallization. Same with the nonisothermal process, the development of relative crystallinity degree (Xt) as a function of crystallization time (t) can be obtained by Eq. (8) from the DSC scans under the isothermal condition.

3.4. Nonisothermal and isothermal crystallization behaviors Crystallization has been considered as a major thermal energystorage process for PCMs through the phase transition from a disordered state to the ordered one. Such a process is extensively influenced by situated environments as well as alien additives. It is anticipative that the microencapsulation with a Cu2O shell definitely affects the crystallization behaviors of PCMs. Therefore, the investigations relative with the crystallization kinetics of PCMs are of importance due to the fact that the energy storage performance of a PCM depends on its crystallization behaviors strongly during the phasechange processes. In the current study, the nonisothermal and isothermal crystallization kinetics of the microencapsulated n-eicosane with a Cu2O shell as well as pure n-eicosane were intensively investigated so that their temperature dependency and time dependency of crystallization rate during the phase change process could be well understood, respectively. Fig. 9a and b show the nonisothermal DSC thermograms of pure n-eicosane and the microcapsules synthesized at the optimum condition, respectively. Meanwhile, the development of relative crystallinity degree (XT) as a function of temperature (T) can be derived by Eq. (5) and then the resulting plots are illustrated in Fig. 9c 155

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Fig. 9. DSC cooling thermograms of (a) pure n-eicosane and (b) the microencapsulated n-eicosane under nonisothermal conditions; plots of relative crystallinity degree against crystallization time for (c) pure n-eicosane and (d) the microencapsulated n-eicosane.

⎛ dHc ⎞ ⎟ dt ⎝ dt ⎠ Xt = × 100% ∞ ⎛ dH ⎞ ∫0 ⎜ c ⎟ dt ⎝ dt ⎠

lization temperature of 32 °C, and the presence of last crystallization step is ascribed to the fact that a high crystallization temperature can promote recrystallization and secondary crystallization of the n-eicosane core. The isothermal crystallization kinetics of the microencapsulated n-eicosane with a Cu2O shell were studied by use of the wellknown Avrami equation [55], which establishes the relationship between the development of Xt and t by the following equation:

t

∫0 ⎜

(8)

where t is an arbitrary crystallization time and dHc/dt is the heat flow rate of DSC scans. Fig. 10a and b show the plots of Xt against t for pure n-eicosane and the microencapsulated n-eicosane under the isothermal condition, respectively. Both pure n-eicosane and the microencapsulated n-eicosane are found to present a typical two-stage development of Xt with increasing the t at various crystallization temperatures, but the microencapsulation of n-eicosane evidently shortens the total crystallization time. It is notable in Fig. 10b that the microencapsulated n-eicosane undergoes a three-step crystallization process at the crystal-

1 − Xt = exp(−Kt n )

(9)

where K and n are the Avrami crystallization rate constant and the Avrami exponent, respectively. The values of K and n can be obtained from the slope of the plot of log[–lin(1–Xt)] vs. logt by Eq. (10).

log[− ln(1 − Xt )] = n log t + log K

(10)

The linear fitting plots of log[–lin(1–Xt)] against logt for pure n-

Table 2 The crystallization kinetic parameters of the microencapsulated n-eicosane with a Cu2O shell and pure n-eicosane under nonisothermal and isothermal conditions. Sample

Nonisothermal crystallization

Isothermal crystallization

Cooling rate, ϕ (°C/min)

Tonset (°C)

Tp (°C)

Tend (°C)

t1/2 (min)

CRP (K–1)

ΔEa (kJ/mol)

Tc (°C)

K (1/minn)

n

t1/2 (min)

Pure n-eicosane

5 10 15 20

34.48 34.02 33.59 33.28

32.50 31.14 30.16 29.20

27.84 23.61 19.81 16.27

1.328 1.041 0.918 0.851

0.0279

–17.327

26 28 30 32

0.3459 0.3273 0.3319 0.5507

1.531 1.465 1.511 1.509

1.574 1.669 1.628 1.164

Microencapsulated n-eicosane

5 10 15 20

34.64 34.24 33.89 33.59

33.70 32.95 32.36 31.83

29.23 26.32 23.37 20.47

1.082 0.792 0.702 0.656

0.0392

–26.920

28 26 30 32

0.5139 0.4463 0.4450 0.4898

1.584 1.408 1.397 1.329

1.208 1.367 1.373 1.210

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Fig. 11. Polarizing microscopic images of (a) pure n-eicosane and (b) the microencapsulated n-eicosane crystallized under an isothermal condition.

Fig. 10. Development of relative crystallinity degree as a function of crystallization time along with the inserted Avrami plots of log[-ln(1-Xt)] against log t for (a) pure n-eicosane and (b) the microencapsulated n-eicosane under isothermal conditions.

time. This indicates that the n-eicosane core within microcapsules experiences a rapid nucleation and growth at the early crystallization stage followed by a slow post crystallization process at a high temperature. In this case, the microencapsulated n-eicosane shows a weak temperature dependency of crystallization only at lower temperatures as observed in Fig. 10b. The crystalline morphologies of pure n-eicosane and the microencapsulated n-eicosane were observed by polarized light microscope, and the resulting images were presented in Fig. 11. Pure n-eicosane is found to exhibit some lamellar and acicular crystals when crystallized under an isothermal condition. These crystals were formed in a large size around 50–100 µm because of the homogeneous nucleation as seen in Fig. 11a. However, it is surprisingly noted in Fig. 11b that the microencapsulated n-eicosane only presented a large number of fine crystalline grains with a size smaller than 5 µm when the isothermal crystallization was completed. It is understandable that the presence of Cu2O shell enhances the nucleating capability of n-eicosane through a heterogeneous nucleating effect, and therefore, these crystals show a remarkable increase in density. On the other hand, the microencapsulation evidently restricts the growth of n-eicosane crystals due to a small space within microcapsules, thus leading to the formation of many small crystalline grains. These results confirm the deduction from the study of isothermal crystallization kinetics.

eicosane and the microencapsulated n-eicosane are illustrated in the insets of Fig. 10a and b, respectively. It is noteworthy in the inserted figures that there is a serious deviation for linear fits in the post crystallization stage due to the occurrence of re-crystallization and secondary crystallization. Moreover, the half time (t1/2) of isothermal crystallization, defined as the time to reach an Xt of 50%, is commonly used to evaluate the crystallization rate of n-eicosane within different environments, and it can be calculated by Eq. (11). 1

⎛ ln 2 ⎞ n t1/2 = ⎜ ⎟ ⎝ K ⎠

(11)

These kinetic parameters of isothermal crystallization obtained from the analysis of Avrami model are all collected in Table 2. It is observed that pure n-eicosane and the microencapsulated n-eicosane have an Avrami exponent, n, in the range of 1.46–1.53, which reflects a growth geometry of n-eicosane crystals acted by various nucleation mechanisms under the isothermal condition. However, the Avrami exponent of the microencapsulated n-eicosane seems to be less than that of the pure one. This may be due to the decrease of crystal dimensions resulting from the confinement effect of inner space within microcapsules. Moreover, the half crystallization time, t1/2, is found to decrease after n-eicosane is encapsulated with a Cu2O shell, indicating the isothermal crystallization rate is improved because of the heterogeneous nucleating effect of inner Cu2O shell. For the most cases, the higher the crystallization temperature, the faster the overall crystallization rate of a PCM. However, it is interestingly noted that the microencapsulated neicosane exhibits the shortest half crystallization time at the crystallization temperature of 32 °C but has the longest total crystallization

3.5. Thermal performance and reliability Heat transfer is an important thermal behavior for PCMs to conduct phase changes. With a high thermal conductivity, PCMs can gain a prompt heat transfer and quick response to a heat flow when undergoing a phase change. However, most of the organic PCMs have very low thermal conductivities, which often generated a hysteresis effect on 157

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phase changes. The thermal conductivities of pure Cu2O, pure neicosane, and the microcapsule samples synthesized at different conditions were measured by use of transient plane source method, and the resulting data are summarized in Table 1. As anticipated, pure neicosane as an organic substance has a thermal conductivity as low as 0.143 W m−1 K−1, whereas pure Cu2O exhibits a considerably high thermal conductivity of 18.236 W m−1 K−1 because of its nature as a metallic oxide. It is surprising to note that the thermal conductivities of microcapsule samples are greatly increased by a factor of at least 29 in comparison of pure n-eicosane. Such a significant improvement is evidently attributed to the presence of highly thermally conductive Cu2O. Moreover, the thermal conductivity is found to be associated with the actual core content of microcapsules. The lower the actual core content, the higher is the thermal conductivity of microcapsules. This can be explained by the fact that the lower actual core content generally results a high portion of Cu2O shell within microcapsules, thus reducing the thermal resistance of n-eicosane core more effectively. It is of evidence that the microencapsulation of n-eicosane with a highly thermally conductive Cu2O shell is an effective solution for enhancing the heat transfer capability of the resulting microcapsules. Supercooling is another major obstacle for PCMs in industrial and domestic applications. Many organic and inorganic PCMs suffer from supercooling when performing a phase transition from liquid to solid, and this situation is especially serious for microencapsulated PCMs [56]. The degree of supercooling (ΔTs) is usually adopted to evaluate the supercooling level of a PCM caused by the crystallization hysteresis. The degree of supercooling is defined as a difference between the melting and crystallization peak temperatures as expressed by:

ΔTs = Tm − Tc

(10)

It was reported that the degree of supercooling could be improved by 13 °C when an organic PCM was encapsulated within the microcapsules with a size of 5 µm [57]. Such a serious supercooling problem evidently impacts the applicability of microencapsulated PCMs. Table 1 displays the degrees of supercooling for pure n-eicosane and the microcapsule samples. It is amazingly found that, although pure neicosane exhibits a high supercooling degree of 6.84 °C, most of the microcapsule samples present a slight decline in the degree of supercooling except for the special samples obtained under uncertain conditions. Inorganic materials commonly possess a thermal conductivity much higher than the polymeric ones. Therefore, The depression of supercooling degree for microcapsules is due to the highly thermally conductive Cu2O shell, which not only significantly promotes a heat transfer among the microcapsules but also offer effective nuclei for heterogeneous nucleation by its inorganic wall, thus resulting in an enhancement in the crystallinity of n-eicosane core. Such a result is quite beneficial to the suppression of supercooling phenomenon for microencapsulated PCMs. The thermal stability is one of the most important factors for determining the upper operating temperature of a PCM. The thermal stability of microcapsule samples was evaluated by TGA, and the resulting thermograms are shown in Fig. 12. Pure n-eicosane is observed to present a typical one-step thermal decomposition along with a fast weight loss in the temperature range of 140–230 °C caused by the pyrolysis of n-eicosane chains. The thermal decomposition of pure n-eicosane was almost completed at about 230 °C, and nothing as a char was remained over 550 °C. The Tmax is defined as the characteristic temperature, at which the thermal decomposition occurs at a maximum rate. It is generally recognized as an indicator of thermal stability for a material and can be determined by derivative thermogravimetry (DTG). The DTG thermogram reveals that pure n-eicosane has a poor thermal stability by indicating a Tmax at 200.1 °C due to its organic nature (see Fig. 12b). The microcapsule samples also present the same thermal degradation behavior with pure n-eicosane, whereas their Tmax's are found to increase by at least 10 °C as shown in Fig. 12b. The improvement in thermal stability is derived from the encapsulation of

Fig. 12. (a) TGA and (b) DTG thermograms of pure n-eicosane and the microcapsule samples synthesized at different conditions; the curve codes of samples are identical to the sample codes in Table 1, where the synthetic conditions are clearly described.

n-eicosane with the Cu2O shell. There is no doubt that the presence of a rigid inorganic shell can effectively prevent the n-eicosane core from decomposing thermally and leads to an increase in Tmax for the microcapsules accordingly. The microcapsule sample obtained at the optimum condition exhibits the highest Tmax among these samples as a result of the most compact shell formed in this condition. Moreover, a certain amount of residual char was produced at the end of the pyrolysis of microcapsule samples as observed in Fig. 12a. The residual char is considered as the highly thermally stable Cu2O wall material. Therefore, the char yields are relative to the actual content of neicosane core and are found to be in according with the corresponding data shown in Table 1. The reliability and durability of microcapsule samples were investigated first by two-hundred-cycle DSC scans, and then, FTIR and TGA were conducted to analyze the specimen obtained from the DSC measurement. Fig. 13 shows the characterization results for the representative sample synthesized at the optimum condition. It is observed from Fig. 13a that the DSC thermograms almost keep overlapping from the first cycle to the last one followed by few changes in Tc and Tm. Meanwhile, as seen in Fig. 13b, both the ΔHc and ΔHm are almost identical to the initial values during the multicycle solid–liquid phase transitions. This indicates a good phase-change reversibility and resilience for this microcapsule sample. Moreover, the infrared spectra of the specimens before and after multicycle DSC scans present a high similarity in the location and intensity of characteristic absorption bands as shown in Fig. 13c, indicating a good structural stability for the microcapsules after undergoing multicycle phase changes. Furthermore, the TGA thermograms in Fig. 13d demonstrate that the weight 158

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Fig. 13. Test results of durability and reliability of the microcapsules obtained at the optimum condition: (a) DSC thermograms of 200-cycle dynamic scans; (b) phase change enthalpies as a function of the cycle number of DSC scans, (c) FTIR spectra of the specimen before and after multicycle DSC scans, and (d) TGA thermograms of the specimen after multicycle DSC scans.

thus can reflect the long-term antipermeability of microencapsulated PCMs. It is observed from Fig. 14 that the microcapsules only released little core material during the initial 24 h, and then the release of neicosane core was accelerated gradually. The release rate reaches approximately 35% and 80% at the extraction time of 76 h and 148 h, respectively. It was reported that most of the microencapsulated PCMs with polymeric shells definitely released about 90% of core materials at extraction time of 200 min in their osmosis experiments [23,25]. This is indicative that the Cu2O shell has much a better antipermeability to prevent the n-eicosane core from penetrating through it in comparison with polymeric shells. Furthermore, based on our previous studies [31,33], the Cu2O shell exhibits better antipermeability than the SiO2 shell, but it is more easily permeable than the CaCO3 shell. In summary, it is crucial that these results confirm the excellent reversibility and durability for the microcapsules developed by this work. 3.6. Solar thermal energy-storage and solar photocatalytic effectiveness Fig. 14. Plot of release rate as a function of extraction time for the microcapsules obtained at the optimum condition.

The solar energy-storage effectiveness of microcapsules was investigated by use of a custom-designed photothermal conversion system as schematically depicted in Fig. 15a. This system can simulate the solar thermal energy storing and releasing process of PCMs with an artificial sunlight source. Fig. 15b shows the curves of the time-dependency of temperature resulting from the photothermal conversion experiments for two microcapsule samples with different actual core contents as well as pure n-eicosane as a reference. It is observed from Fig. 15b that both the microcapsule samples and pure n-eicosane first show a temperature elevation with illumination time under the xenon arc lamp and then

loss profiles of the microcapsules almost overlap one another before and after multicycle DSC measurements, indicating a good thermal stability for this microcapsule sample after multiple heating–cooling cycles. The durability of microcapsule samples was further evaluated by the osmosis experiment. Fig. 14 shows the release rate as a function of extraction time obtained from the microcapsule sample synthesized at the optimum condition. This curve describes the weight faction of the neicosane core released from microcapsules in an acceleration mode and 159

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present a noticeable plateau region in the temperature range of 30–45 °C. The presence of the plateau region is ascribed to the photothermal energy absorption as a result of the fusion of n-eicosane at a constant temperature. These phenomena clearly indicate an effective solar photothermal conversion for n-eicosane no matter in the pristine state or in the encapsulated one. It is noteworthy that the microcapsule samples exhibit a faster rise in temperature than pure neicosane at the same illumination time due to their much higher thermal conductivities. Moreover, the microcapsule sample with a lower actual core content has a higher thermal conductivity as seen in Table 1, and hence, it reaches a much higher temperature at the end of illumination due to a faster heat transfer. On the contract, pure neicosane exhibits a higher temperature-retention capability by showing a wider plateau region in comparison of the microcapsules. This can be explained by the reason that the inert Cu2O shell of microcapsules is not involved in any phase change. On the other hand, with stopping the illumination of xenon arc lamp, both the microcapsules and pure neicosane undergo a temperature decline with an extension of time as shown in Fig. 15b. Similar with the photothermal storage process, a plateau region can also be observed in the temperature range of 20–35 °C because of the temperature hysteresis caused by latent heat release during the crystallization process of n-eicosane. With the same reason, the microcapsule samples reveal a faster cooling phenomenon and a narrower temperature retention zone than pure n-eicosane. The latent heat-storage performance of microcapsules was further characterized by infrared thermography during the heating and cooling processes. Fig. 16 shows the resulting infrared thermographic images along with the plots of specimen temperatures as a function of heating and cooling time. These infrared thermographic images are found to clearly reflect the surface temperature distributions of specimens at different heating and cooling stages by different colors. As observed in Fig. 16a–d, it is evident to note a distinct temperature difference between the microcapsules and pure Cu2O shell. The microcapsule

Fig. 15. (a) Scheme of the custom-designed setup for photothermal conversion measurement; (b) plots of time vs. temperature obtained from photothermal conversion tests for pure n-eicosane and microcapsule samples.

Fig. 16. Infrared thermographic images taken at the heating time of (a) 0 s, (b) 20 s, (c) 40 s and (d) 60 s, and at the cooling time of (e) 4 min, (f) 6 min, (g) 8 min and (h) 10 min for the microcapsules (Sp1) and pure Cu2O shell (Sp2), Plots of the specimen temperatures as a function of (i) heating time and (j) cooling time obtained from infrared thermography.

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Fig. 17. UV–visible absorptance spectra of photocatalytic suspensions containing the microcapsules and organic dyes at different sunlight illumination time along with the relevant digital photos attached.

heat absorption by the n-eicosane core in its melting temperature range, which generates a buffering effect on temperature jump for the microcapsule specimen. However, the surface temperature of pure Cu2O specimen exhibits a rapid elevation during the heating process because no heat absorption or storage occurs. Similar with the heating process, a temperature-buffering phenomenon is also observed in for the microcapsules during the cooling process as shown in Fig. 16e–h and j, which is attributed to the latent heat release by crystallization of the n-eicosane core. These results confirm an effective heat-storage and thermoregulatory capability obtained for the microencapsulated neicosane with a Cu2O shell.

specimen shows a blue color as a low-temperature indicator with an elevation of temperature at the hot plate, suggesting the thermal energy absorption derived from melting of the n-eicosane core. On the other hand, the specimen of pure Cu2O presents the almost same color with the background during the heating process. This indicates that there is no heat storage and exchange occurring in pure Cu2O shell. Moreover, as observed from the surface temperature distribution with heating time in Fig. 16i, the microcapsule specimen is found to undertake a rapid increase in surface temperature within 30 s, and then its surface temperatures show a plateau region in the period of 30–60 s during the heating process. Such a unique plateau region is attributed to the latent

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Fig. 18. Plots of the degrees of degradation as a function of illumination time for three different organic dyes.

Fig. 20. Response time and recover time of the microcapsules as a gas-sensing element in respect to four different organic gases.

As one of the principal oxides of copper, Cu2O is a typical p-type semiconductive material with a high visible photocatalytic activity. With an extensive investigation on the photocatalytic mechanisms of Cu2O, and a number of publications reported that the solar photocatalytic activity of Cu2O was derived from its p-type semiconducting characteristics, which allow Cu2O to generate holes as majority charge carriers and electrons as majority charge carriers easily under the visible light illumination [58]. The photogenerated holes could be captured by water to generate hydroxyl radicals through oxidation at the valence band, and meanwhile, the photogenerated electrons were

donated to oxygen at the conductance band, thereby generating the superoxide radicals. Both of them can in turn attack and degrade organic contaminations. Such an unusual feature makes Cu2O very beneficial to the utilization of solar energy through a photochemical conversion, and hence Cu2O has been widely applied for the decontamination of industrial wastewater by performing solar photocatalysis in the open air [45]. For this reason, it is expected that the microcapsules developed by this work will be imparted with solar photocatalytic effectiveness through encapsulation by use of Cu2O.

Fig. 19. Isothermal response–recovery curves of the microcapsules as a gas-sensing element in respect to four different organic gases.

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the prompt response time and recovery time are also required necessarily for a gas sensor. These two parameters can be derived from the isothermal response curves and are illustrated in Fig. 20. The response time and recovery time for these four gases are found to be kept within 150 and 56 s, respectively. Such response and recovery data are different with a variation of the tested gas. It is notable that the gas sensor exhibits shorter response time and recovery time for benzene than for the other three gases when using the microcapsules as a gassensing element. It should be recognized that the gas-sensing properties of the microcapsules is still not so satisfactory compared to the results reported by Sui et al. [61] and Zhou et al. [62] This may be due to the crystalline structure and accumulation pattern of Cu2O in the shell layer as well as the test temperature. The gas-sensing enhancement of Cu2O shell can be realized by optimizing the synthetic technique and will be another important subject in our future study. Anyway, on the basis of the above gas-sensing results, it is reasonable to believe that the microcapsules developed by this work can be used to exploit a new bifunctional sensor element with temperature-sensing and gas-sensing features by combination of their thermal storage capability and gassensing effectiveness.

The microcapsules synthesized at the optimum condition were selected as a representative sample to conduct solar photocatalysis for three different types of organic dyes, i.e. Congo red, malachite green and acid fuchsin under natural sunlight. Fig. 17 shows the UV–visible absorptance spectra of three photocatalytic systems at different intervals of sunlight illumination time. The Congo red, malachite green and acid fuchsin systems are observed to exhibit an intensive absorption band at 498, 617 and 523 nm on their original UV–visible spectra, respectively. Such a characteristic absorption behavior is ascribed to the electronic transition in the respective chromogenic groups of organic dyes as a result of UV irradiation, and therefore, the concentration of organic dyes in solutions can be derived from the absorption intensity of the characteristic bands. It is interesting to note a gradual decline in absorption intensity for these characteristic peaks with an increase of sunlight illumination time for three suspensions containing the microcapsules and organic dyes. The decline of absorption intensity suggests a reduction in the concentrations of organic dyes. This is due to the change of molecular structure of organic dyes caused by photodecomposition. The aspect of UV–visible spectra looks flat comparatively when the specimens receive sunlight illumination for 160 min. The discoloring processes of three specimens relevant to the illumination time could also be observed from the digital photos attached in Fig. 17. The above results indicate that these three organic dyes undergo a solar photocatalytic degradation resulting from the Cu2O shell, and furthermore, the solar photocatalytic effectiveness is enhanced gradually with increasing the illumination time, ultimately resulting in a complete decomposition of organic dyes. Fig. 18 shows the degrees of degradation as a function of illumination time for these organic dyes. Congo red is found to show a higher degree of degradation than the other two organic dyes at the early stage of sunlight illumination. However, the photocatalytic degradations of malachite green and acid fuchsin are evidently accelerated with an extension of illumination time. Finally, malachite green and acid fuchsin achieved a high degree of degradation over 90%, whereas the degree of degradation for Congo red only reached 81% as shown in Fig. 18. This is indicative that the Cu2O shell of microcapsules has a better solar photocatalytic effect on malachite green and acid fuchsin than on Congo red.

4. Conclusion The bifunctional microcapsules composed of the n-eicosane core and Cu2O shell were synthesized through in-situ precipitation using the emulsion templating self-assembly technique, and their surface elemental distribution and chemical structure were confirmed by EDX, XPS and FTIR, spectroscopy. The crystalline structure of Cu2O shell was determined as a simple cubic phase by XRD patterns. The microstructures and morphologies of microcapsules were influenced significantly by the surfactant and alkali concentrations as well as the weight ratio of neicosane/CuSO4. The microcapsules exhibit an interesting octahedral morphology and typical core-shell structure when synthesized at the optimum conditions of CCTAB=0.15 mol/L, CNaOH=5.00 mol/L and WnEicosane/WCuSO4=50/50. In this case, the microcapsules not only obtained high encapsulation efficiency, rapid thermal response and thermal energy-storage efficiency but also presented a good thermal stability and high phase-change reliability. The microcapsules also gained a high thermal conductivity and low degree of supercooling due to the encapsulation of n-eicosane with a highly thermally conductive inorganic wall. Most of all, the microcapsules obtained a solar thermal energystorage capability through solar photothermal conversion, and meanwhile, they presented a high solar photocatalytic activity to organic dyes under the sunlight illumination. In addition, the microcapsules achieved good gas-sensing effectiveness to some harmful organic gases due to the presence of Cu2O shell. The microcapsules developed by this work indeed demonstrate the bifunctional characteristics derived from both the core and the shell materials and thus show great potential for industrial and domestic applications due to their extended functions.

3.7. Gas-sensing properties It has been widely reported that Cu2O shows sensitivity to many gases such as H2S, NO2, CO, ethanol, acetone and benzene due to its ptype semiconducting characteristics [59], and it has been recognized as an important gas-sensing material used for gas sensors to detect various harmful gases [60]. Therefore, considering a possible gas-sensitive feature within the Cu2O shell, a gas-sensing characterization was conducted to investigate the gas-sensing properties of the microcapsules synthesized in this work. The microcapsules synthesized at the optimum condition was selected as a tested specimen to be loaded in a solid-stage gas sensor, where the microcapsules was used as a gassensing element to perform gas-sensing behavior. The gas-sensing detecting mechanism is based on the oxidation–reduction reaction of the detected gases taking place on the surface of gas-sensing element, which can result in an abrupt change in electrical resistance for the gassensing element [61]. Fig. 19 shows the typical isothermal response curves of this gas sensor in respect to four types of harmful organic gases including methanol, ethanol, acetone and benzene. It is observed anticipatively from Fig. 19 that the microcapsules exhibit distinct timedependent sensing response to all of the four organic gases. The response sensitivity (S) of gas-sensing element is defined as the ratio of Rg/Ra at a maximum value. As marked in Fig. 19, the microcapsules also presents different response sensitivity to these four gases in an order of acetone > methanol > ethanol > benzene. It is noteworthy that the response sensitivity seems to increase slightly with the number of repetitive tests, indicating good repeatability and reproducibility for the gas-sensing element based on the microcapsules synthesized in this work. Besides a high response capability and high response sensitivity,

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