Paraffin Composite Phase Change

Research Article pubs.acs.org/journal/ascecg

Fabrication of Graphene/TiO2/Paraffin Composite Phase Change Materials for Enhancement of Solar Energy Efficiency in Photocatalysis and Latent Heat Storage Huan Liu, Xiaodong Wang,* and Dezhen Wu State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: To enhance the solar energy utilization efficiency of microencapsulated phase change materials (PCMs), a novel composite system was designed by combination of graphene nanosheets and the microencapsulated n-eicosane with a brookite TiO2 shell. A series of neicosane@TiO2@graphene microcapsules were fabricated through interfacial polycondensation in an emulsion templating system, and their microstructures, chemical compositions and crystallinity were investigated extensively. The composite system presented a spherical core−shell structural morphology, where graphene nanosheets were attached onto the microcapsule surfaces through hydrogen bonding. The composite system achieved phase-change enthalpies over 160 J/g, and its thermal conductivity was also improved from 0.64 to 0.98 W· m−1·K−1 due to highly thermally conductive graphene nanosheets. This study confirmed that the introduction of graphene nanosheets was an effective way not only to improve the structural stability and serving durability of the composite system but also to enhance its solar photocatalytic activity by promoting the electrons transfer and charges separation of TiO2. The composite system developed by this work exhibits a great potential for direct solar energy utilizations such as solar thermal energy storage in a natural environment, solar photodegradation and detoxification for the water containing organic pollutants, and solar thermal energy collection and decontamination for industrial hot wastewater. KEYWORDS: Phase change materials, Graphene nanosheets, Microcapsules, Solar photocatalysis, Solar thermal energy storage

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INTRODUCTION Nowadays, the climate change and green house emission have become two of the largest concerns in the world, which are evidently associated with the use of fossil fuels. However, the future global economy is likely to rely on even more energy. Therefore, such an energy demand is expected to deploy a broad range of renewable and sustainable energy sources such as wind, solar, wave and biomass for reducing the growing fossil fuels demand.1 Among these types of sustainable energy sources, solar energy accounts for a great proportion.2 Solar energy can be utilized for broad applications in solar photovoltaic, water desalination, solar lighting and space heating.3−5 However, it is necessary to collect solar energy during sunshine hours and then to release energy during nonsolar hours with a high efficiency due to the inconsistent and sporadic nature of solar energy. As a type of sustainable energy materials, phase change materials (PCMs) used for solar thermal energy storage were highlighted with desirable performance: high phase-change enthalpies, suitable phasechange temperatures, small volume change and moderate cost.6,7 There are numerous natural substances that could be employed as solid−liquid PCMs for solar thermal energy© 2017 American Chemical Society

storage applications, including organic fatty acids and paraffin waxes, inorganic salt hydrates and other compounds.8 Although inorganic salt hydrates have a relatively high phase-transition enthalpy and a thermal conductivity around 0.4−0.7 W·m−1· K−1, the major restriction of their applications is a high degree of supercooling, serious phase segregation and a poor long-term stability.9 On the other hand, organic materials such as paraffin waxes show a low thermal conductivity around 0.15−0.3 W· m−1·K−1, but they present a chemical stability, well-defined phase-change temperatures and no trend to segregate. Therefore, organic PCMs are considered as the most promising ones for solar thermal energy storage. To enhance the thermal energy-storage performance of PCMs in practical applications, some attempts such as blending PCMs with some high thermal conductivity materials and microencapsulating PCMs into a compact shell have been made.10−13 As a two-dimensional carbon material, graphene nanosheets have a remarkable thermal conductivity.14 There are Received: February 1, 2017 Revised: March 13, 2017 Published: May 9, 2017 4906

DOI: 10.1021/acssuschemeng.7b00321 ACS Sustainable Chem. Eng. 2017, 5, 4906−4915

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ACS Sustainable Chemistry & Engineering

crystals has the best photocatalytic capability among the three mineral forms of TiO2.32,33 However, the photocatalytic activity obtained from the microcapsules was very limited due to an uncontrollable crystalline form in the sol−gel process which only led to the formation of a brookite TiO2 shell but not the anatase one. Moreover, the utilization of TiO2 is still limited for solar photocatalysis because of its broad-bandgap semiconductive nature.34 In the case of brookite TiO2 with a bandgap of 3.4 eV, its photocatalytic activity can be excitated only by ultraviolet (UV) light with a wavelength shorter than 365 nm.35 However, there is only 3−5% of UV light energy in the total range of sunlight spectrum, which results in a low utilization efficiency for solar photocatalysis.36 In this work, we attempted to enhance the solar photocatalytic effectiveness of microencapsulated PCMs with a TiO2 shell by incorporation of graphene nanosheets. It is reported that graphene nanosheets can act as an electron transfer channel to reduce the recombination of photogenerated holes and electrons, thus enhancing the photocatalytic activity for semiconductor materials.37 Meanwhile, the high thermal conductivity of graphene nanosheets can effectively enhance the thermal performance of microencapsulated PCMs. It is postulated that the combination of microencapsulated PCMs and graphene nanosheets can not only promote the utilization of natural sunlight to photodegrade the organic pollutants in water and to obtain clean energy like hydrogen and oxygen through the photodecomposition of water38−40 but also enhances the collection and reutilization of solar thermal energy through the phase changes of PCM core. The purpose of this study is to develop a new system based on microencapsulated PCMs and graphene nanosheets with the enhanced performance of solar thermal energy storage and solar photocatalysis.

some successful cases in the improvement of thermal conductivity of PCMs by a combination of PCMs and graphene nanosheets.15−17 Microencapsulation is recognized as an important technique for the preparation of form-stable PCMs due to its prominent characteristics such as the required morphology, uniform diameter, considerable thermal stability, relatively high shell strength and low permeability for PCMs. A number of publications showed that both polymers and inorganic materials could be employed as shell materials for microencapsulated PCMs.18,19 It was reported that over 50 polymers had been successfully used to encapsulate PCMs, which effectively prevented the leakage of PCMs from the microcapsules and also provided a structural stability for the microcapsules.20 However, the thermal conductivity of microencapsulated PCMs was not improved due to a low thermally conductive nature of polymeric materials. Considering the features of high thermal conductivity and nonflammability for inorganic materials, the thermal performance is possibly upgraded by encapsulating PCMs into inorganic shells. As reported in open publications, some inorganic materials were successfully used to encapsulate PCMs, which included CaCO3,21 SiO2,22 Al(OH)323 and TiO2.24 The aforementioned work indicated that the microencapsulation of PCMs with inorganic materials not only overcame the flowing problem of PCMs but also imparted a high thermal conductivity to them. However, these studies only concerned the function of PCM core for thermal energy storage and temperature regulation. Photocatalysis is a process by using light energy conversion into a chemical reaction. Almost all of the harmful organic substances for human and environment can be decomposed by photocatalysts without wasting of resources and causing additional pollution, and many applications of photocatalysts have been reported such as the oxidation of carbon monoxide,25 solar water splitting26 and photovoltaic or photoconducting devices.27 Although a lot of semiconductors can be used as a catalyst to photodecompose organic pollutants, TiO2 is one of the most widely investigated photocatalysts due to its low cost, good chemical stability and unique electronic property.28,29 It is generally recognized that TiO2 could generate electrons and positive holes after band gap excitation. These charge carriers may recombine or migrate to the catalyst surface and initiate redox reactions with suitable substrates. For instance, positive holes can oxidize water or hydroxyl at the surface to produce extremely powerful oxidants like OH radicals. The hydroxyl radicals can subsequently oxidize organic species. However, the fast recombination of electrons and positive holes results in a low photocatalytic activity of TiO2.30 Therefore, enhancing the separation of photogenerated electrons and holes by incorporation of additional components in the TiO2 structure could effectively improve the photocatalytic activity of TiO2. In our previous work, we successfully synthesized the microencapsulated paraffin PCMs with a brookite TiO2 shell and found that the obtained microcapsules not only exhibited good latent-heat storage performance and enhanced thermal conductivity but also possessed an extra photocatalytic effectiveness derived from their crystalline TiO2 shell.31 Such a combination of photocatalytic effectiveness and thermal energy storage can greatly extend the applicable area of microencapsulated PCMs. However, the formation of TiO2 shell has to depend on some expensive organic titanium sources like tetrabutyl titanate (TBT) and tetraethyl titanate according to the current synthetic techniques, which leads to a high cost for the resultant microcapsules. It is well-known that anatase

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EXPERIMENTAL SECTION

Materials. n-Eicosane used as a paraffin-type PCM was purchased from Acros Organics Company, USA. Poly(ethylene oxide-bpropylene oxide-b-ethylene oxide) (Pluronic P104) was commercially obtained from BASF Corporation, Germany. Graphene nanosheets prepared by a redox method were commercially available from Hengqiu Tech., Inc., China. TBT, formamide, sodium fluoride (NaF), methylene blue (MB), ethanol and petroleum ether were purchased from Beijing Chemical Reagent Co. Ltd., China. All of the chemicals and reagents were used as received. Synthesis of n-Eicosane@TiO2@Graphene Microcapsules. The microcapsules containing the crystalline TiO2 shell and n-eicosane core and attached with graphene nanosheets (marked as n-eicosane@ TiO2@graphene microcapsules) were prepared through interfacial polycondensation and surface self-assembly. In a typical synthetic process: TBT (5 g), n-eicosane (5 g) and formamide (50.0 mL) were mixed in a three-necked round-bottom flask with stirring at 50 °C for 30 min. Graphene nanosheets (0.05 g) were dispersed in formamide (50.0 mL) at 50 °C under ultrasonication for 30 min. Subsequently, P104 (1.0 g) as a nonionic emulsifier and graphene suspension were added into the flask and mixed with stirring for 4 h, and then the formamide (50.0 mL) containing H2O (2 g) was added dropwise into the emulsion prepared above with mild agitation for 2 h. The reactant mixture was stirred continuously for 5 h to obtain a gray suspension. In succession, NaF (1.0 g) as a crystallization promoter was added into the flask with agitation at 75 °C for 24 h to cause the formation of a crystalline TiO2 shell. Ultimately, some gray powders were obtained by filtration as n-eicosane@TiO2@graphene microcapsules. These microcapsules were washed by deionized water, ethanol and petroleum ether for several times and then dried in a vacuum for further uses. Characterizations. The microstructures of n-eicosane@TiO2@ graphene microcapsules were examined by a Hitachi S-4700 scanning electron microscopy (SEM) instrument and Hitachi H-800 trans4907


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Figure 1. SEM micrographs of (a) graphene nanosheets, (b, c) n-eicosane@TiO2 microcapsules and n-eicosane@TiO2@graphene microcapsules containing (d, e) 1 wt %, (f, g) 3 wt % and (h, (i) 5 wt % graphene nanosheets. TESTO875-1i infrared thermographic camera on a hot plate, and the infrared thermographic images were taken at different heat time.

mission electron microscopy (TEM) instrument . X-ray photoelectron spectroscopy (XPS) was conducted to detect the surface elemental distribution on a Thermo Fisher ESCALAB 250 XPS spectrometer. Fourier transform infrared (FTIR) spectroscopy was performed on a Nicolet iS5 FTIR spectrometer. X-ray powder diffraction (XRD) was conducted to determine the crystallinity of the microcapsule shell on a Rigaku D/Max 2500 X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å). To avoid interference with the pattern of TiO2 shell from the diffraction of n-eicosane crystals, the core materials should be extracted by petroleum ether before XRD measurements. The phase-change properties were characterized by differential scanning calorimetry (DSC) on a differential scanning calorimeter of TA Instruments Q20 at a scanning rate of 10 °C/min. The thermal conductivity was measured by an HS-DR-5 thermal conductivity tester (Shanghai HE SHENG Instrument Technology Co., Ltd., China). Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer of TA Instruments Q50 under a nitrogen flow at a heating rate of 10 °C/min. The solar photocatalytic activity was evaluated through the photodegradation of MB as an organic dye. In a typical experimental process, the microcapsules (100 mg) and MB solution (20 mg/L, 100 mL) were mixed with magnetic agitation in a beaker under a dark environment to obtain an absorption equilibrium. Then the solution was irradiated by a xenon arc lamp (CHF-XM-500W, Beijing Changtuo Technology Co., Ltd., China) as a solar simulator for 240 min. The concentration of MB solution with a variation of irradiation time was measured by a UV−visible spectrophotometer (Shimadzu UV-2550). The solar energy-storage performance of microcapsule samples was evaluated by using a custom-designed experimental setup as depicted in Figure S1. This setup can provide a continuous light solar simulator to produce the full sunlight intensity of 1000 ± 50 W/ m2 and thus can execute thermal energy absorption and release. During this simulated process of solar thermal energy absorption/ release, a transient response to temperature was recorded by the temperature collection recorder every 10 s with an accuracy of ±0.5 °C. The temperature-regulating performance of pure n-eicosane and neicosane@TiO2@graphene microcapsules was evaluated by use of a

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RESULTS AND DISCUSSION The preparation methods for the microencapsulated organic PCMs with inorganic shells have been extensively developed in recent years, and many successful cases have been reported.8,41 According to the relevant references, the interfacial polycondensation of titania precursors in an oil-in-water (O/W) emulsion templating system is regarded as an effective way to obtain the microencapsulated PCMs with a TiO2 shell (marked as n-eicosane@TiO2 microcapsules). However, the hydrolysis and condensation rates of titania precursors like TBT are uncontrollable in an aqueous system, which results in a poor microstructure and low yield accordingly. In this work, formamide was selected as a medium instead of water to construct a nonaqueous O/W emulsion templating system, and then the condensation rates could be easily controlled by adding trace amounts of water to initiate the hydrolysis of TBT. This can lead to a high encapsulation rate as well as a perfect microstructure for the resultant microcapsules. Such a synthetic strategy has been well described in Figure S2. In the synthetic process, the mixture of n-eicosane/TBT as an oil phase was first dispersed in a formamide medium to form a system composed of the dispersed phase and continuous phase, and then graphene nanosheets and P104 as a nonionic surfactant were added into the mixture to construct a nonaqueous O/W emulsion templating system with well-dispersed graphene nanosheets. The hydrolysis of TBT was initiated with the addition of trace amounts of water. Titanic hydroxide was generated in this stage and then was enriched on the surfaces of n-eicosane micelles through the hydrogen bonding between the titanic hydroxide and hydrophilic groups of surfactant. 4908


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Figure 2. TEM micrographs of (a, b) graphene nanosheets, (c) n-eicosane@TiO2 microcapsules and n-eicosane@TiO2@graphene microcapsules containing (d) 1 wt %, (e) 3 wt % and (f) 5 wt % graphene nanosheets.

be determined as approximately 0.2 μm. On the other hand, as shown in Figure 1d−i, the n-eicosane@TiO2@graphene microcapsules all exhibit a similar morphology with the neicosane@TiO2 one, and, however, a few thin slices can be observed on the surfaces of microcapsules due to the adherence of graphene nanosheets. The TEM micrographs in Figure 2d−f clearly disclose the microstructures of the microcapsules attached with graphene nanosheets. These results indicate a successful fabrication of n-eicosane@TiO2@graphene microcapsules. The surface elemental distributions of graphene nanosheets and the n-eicosane@TiO2@graphene microcapsules were examined by XPS spectroscopy, and the XPS spectra obtained for the O 1s, C 1s and Ti p2 spectral lines are presented in Figure S3. It is observed that the C 1s spectrum of graphene nanosheets can be devolved into four peaks, which are assigned to the CC, CO, CO and OCO bonds.43,44 This indicates an abundant of oxygen-containing functional groups like hydroxyl and carboxyl on the graphene nanosheets. The C 1s XPS spectrum of n-eicosane@TiO2@graphene microcapsules also displays the same binding energy with that of graphene nanosheets, which demonstrates that the chemical state of C 1s is not influenced by the introduction of graphene nanosheets. Furthermore, the O 1s XPS spectrum obtained from n-eicosane@TiO2@graphene microcapsules exhibits an abundant surface elemental distribution with four components corresponding to oxygen atoms in different functional groups: the TiO bonds at 531.0 eV for TiO2 shell, the O−H bonds at

Accompanying with the interfacial polymerization of titanic hydroxide, the TiO2 shell was gradually formed on the surfaces of n-eicosane micelles, thus leading to core−shell structural microcapsules. Meanwhile, graphene nanosheets were also attached onto the surfaces of microcapsules through hydrogen bonding between the carboxyl and hydroxyl groups during the long-term aging. Furthermore, the incorporation of NaF electrolyte could promote the crystallization of TiO2 shell effectively after a long-term aging. As a result, the photocatalytic effectiveness was achieved due to the formation of a crystalline TiO2 shell for microencapsulated n-eicosane. The morphologies and microstructures of graphene nanosheets and microcapsule samples were examined by SEM and TEM, and the resulting micrographs are given in Figures 1 and 2. It is notable in Figures 1a and 2a that the graphene nanosheets used in this work exhibit a transparent ultrathinlayered structure, in which some corrugations and ripples can be observed. This may facilitate the thermodynamic stability of these two-dimensional graphene nanosheets.42 Moreover, the thickness of graphene nanosheets could be determined as about 3 nm by a high-resolution TEM image (Figure 2b), indicating that these graphene nanosheets have been exfoliated effectively. It is observed in Figure 1b,c that n-eicosane@TiO2 microcapsules present some regular spheres with a perfect core−shell microstructure and smooth surface, and their particle size shows a narrow distribution in the range of 3−5 μm. The core− shell microstructure of n-eicosane@TiO2 microcapsules is confirmed by TEM (see Figure 2c), and their thickness can 4909


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Figure 3. (a, b) DSC thermograms, (c) characteristic phase-change temperatures, (d) phase-change enthalpies, (e) thermal conductivity and (f) TGA thermograms of pure n-eicosane and the microcapsule specimens, in which SP1 represents the n-eicosane@TiO2 microcapsules, and SP2, SP3 and SP4 represent the n-eicosane@TiO2@graphene microcapsules containing 1, 3 and 5 wt % graphene nanosheets, respectively.

distinguished clearly at 476 cm−1. This indicates that the neicosane has been encapsulated with a TiO2 shell successfully. In addition, the hydroxyl characteristic bands can be observed due to abundant hydroxyl groups on the surface of TiO2 shell.45 It is noteworthy that there is no absorption peak found for graphene nanosheets in the characteristic spectra of neicosane@TiO2@graphene microcapsules because of a very low loading of graphene nanosheets in this microcapsule system. The crystalline structure of TiO2 shell was identified by XRD. Figure S4b shows the XRD patterns obtained for all of the microcapsule samples after removal of the n-eicosane core by solvent extraction, and the pattern of graphene nanosheets is also presented as a reference. The n-eicosane@TiO2 microcapsules are observed to show a set of diffraction peaks of brookite TiO2 according to the standard card data of JCPDS No. 29-1360.46 This indicates that a crystalline TiO2 shell is well formed with the aid of a crystallization promoter, NaF. It is noteworthy that the pristine graphene nanosheets exhibit a sharp diffraction peak at 26.50° in their XRD pattern, which is indexed as the (002) plane of graphene nanosheets. This

531.8 eV for hydroxyl groups on the TiO2 shell and graphene nanosheets, and the CO and OC bonds at 532.6 and 533.2 eV for carboxyl groups on graphene nanosheets. These results implicate that the n-eicosane@TiO2@graphene microcapsules are fabricated through the strong physical adsorption and hydrogen bonding between the graphene nanosheets and TiO2 shell. In addition, the XPS spectrum of n-eicosane@ TiO2@graphene microcapsules in Figure S3d shows two signals attributed to the Ti 2p3/2 and 2p1/2 electrons. These results confirm a fabrication of TiO2 shell onto the n-eicosane core and also indicate a good combination of graphene nanosheets and n-eicosane@TiO2 microcapsules. The chemical composition of n-eicosane@TiO2@graphene microcapsules were characterized by FTIR spectroscopy, and Figure S4a displays the infrared spectra obtained for pure neicosane and microcapsule samples. There are a series of characteristic absorption peaks of pure n-eicosane observed in the infrared spectra of n-eicosane@TiO2 and n-eicosane@ TiO2@graphene microcapsules as marked. Furthermore, a characteristic absorption band of TiO stretching vibration is 4910


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Figure 4. (a) Multicycle DSC thermograms of n-eicosane@TiO2@graphene microcapsules containing 5 wt % graphene nanosheets; (b) phasechange enthalpies as a function of cycle number obtained from multicycle DSC scans; (c) comparative FTIR spectra and (d) TGA thermograms of the microcapsule specimen before and after multicycle DSC scans.

ment of n-eicosane derived from a small internal space within microcapsules. Moreover, the crystallization confinement also causes the formation of imperfect n-eicosane crystals and thus results in a slight reduction of Tm. These two factors result in an increase in supercooling degree (ΔT) for n-eicosane@TiO2 microcapsules as observed in Figure 3c. It was extensively reported that a high supercooling degree was evidently disadvantageous to the use of PCMs because it could result in a hysteresis response to heat. It is noteworthy that the presence of graphene nanosheets makes the Tc shifts to a relatively high temperature, but a reverse trend is observed in Tm. This result may be attributed to the fabrication of high thermally conductive graphene nanosheets onto the surfaces of n-eicosane@TiO2 microcapsules, leading to an improvement in thermal transfer efficiency. Such a deduction can be supported by the data of thermal conductivity listed in Figure 3e. It is expected that, compared to a low thermal conductivity of 0.184 W·m−1·K−1 for pure n-eicosane, n-eicosane@TiO2 microcapsules achieved a significant increase up to 0.64 W·m−1·K−1 due to the encapsulation of a high thermally conductive inorganic shell. The introduction of graphene nanosheets promotes a further improvement in thermal conductivity, and the thermal conductivity increased by 53% when 5 wt % of graphene nanosheets was incorporated into the microcapsules. These results indicate that graphene nanosheets actually impart much higher thermal conductivity to the microcapsules. Consequently, the supercooling degree of the microcapsules is suppressed effectively. The melting enthalpy (ΔHm) and crystallization enthalpy (ΔHc) of pure n-eicosane can be determined as 245.7 and 246.5 J/g by DSC, respectively, as shown in Figure 3d. However, there is a significant decrease in both of them for n-eicosane@ TiO2@graphene microcapsules due to the introduction of inert

phenomenon can be attributed to the chemical reduction that reconstructed the crystal structure in graphene nanosheets after removal of the oxidation effect.47,48 Additionally, the XRD patterns of n-eicosane@TiO2@graphene microcapsules not only show all of the diffraction peaks from the brookite TiO2, but the diffraction peak of graphene nanosheets can also be identified. This indicates good crystallinity maintained for the microcapsule shell after a combination of n-eicosane@TiO2 microcapsules and graphene nanosheets. A comparative investigation was performed on the phasechange performance of n-eicosane@TiO2@graphene microcapsules by dynamic DSC cooling and heating scans. The obtained DSC thermograms are displayed in Figure 3a,b, where the DSC thermograms for pure n-eicosane are presented as a reference, and furthermore, the phase-change data derived from DSC analysis are summarized in Figure 3c,d. It is noticeably observed that both pure n-eicosane and microcapsule samples exhibit a bimodal crystallization behavior by presenting a main endothermic peak along with a shoulder at higher temperatures in the solidification process. This phenomenon is ascribed to the appearance of a metastable rotator phase prior to completing the full crystallization as a result of the heterogeneous nucleation during the cooling process, which was extensively reported for most of the paraffin waxes.49 On the other hand, the pure n-eicosane and microcapsule samples only show a single exothermic peak in the melt process, indicating an isomorphous crystalline form of n-eicosane either in a pristine state or in the encapsulated one. According to the DSC results as shown in Figure 3c, the crystallization peak temperature (Tc) and melting peak temperature (Tm) of pure n-eicosane are determined as 33.29 and 40.98 °C, respectively. However, n-eicosane@TiO2 microcapsules are found to show a considerable decline in Tc due to the crystallization confine4911


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shown in Figure S5f,g. Such a phase transition from solid to liquid makes pure n-eicosane lose its stability in shape completely. In the case of n-eicosane@TiO2@graphene microcapsules, it is found that the microcapsules containing 1 wt % graphene nanosheets also show slight shrinkage due to a heating-impact effect, however, the shrink level decreases with increasing the loading of graphene nanosheets. Figure S5h,i shows the digital images of the specimens before and after infrared thermography measurements. The microcapsules containing 5 wt % graphene nanosheets are found to keep their shapes stable without any shrinkage. These results indicate that the incorporation of graphene nanosheets can significantly improve the structural stability of the microcapsules and thus enhance their resistance to heat impact. Figure 5a shows the

inorganic materials like TiO2 and graphene nanosheets. It should be mentioned that the mass ratio of n-eicosane/TBT was set to 50/50 in the synthetic process in this work, because with such a formulation, the microencapsulated PCMs with a TiO2 shell could achieve a good balance between the encapsulation efficiency and shell thickness on the basis of our previous study.50 According to the equations mentioned in our previous publication,31 the encapsulation and thermal energy-storage efficiencies of n-eicosane@TiO2 microcapsules could be calculated as 65.59% and 65.23%, respectively. Although these two parameters decrease slightly with the addition of graphene nanosheets, n-eicosane@TiO2@graphene microcapsules still maintained high encapsulation and energystorage efficiencies due to a low loading of graphene nanosheets in the microcapsules. Therefore, the n-eicosane@TiO2@ graphene microcapsules also obtained a good solar thermal energy-storage capability. The thermal stabilities of n-eicosane@TiO2@graphene microcapsules were evaluated by TGA, and the resulting thermograms were presented in Figure 3f. All of the samples show a typical one-step degradation behavior with the similar mass-loss trend. As an important indicator of thermal stability, the maximum-rate degradation temperature (Tmax) of pure neicosane was measured to be about 212 °C. However, the neicosane@TiO2 and n-eicosane@TiO2@graphene microcapsules all exhibit an improvement in Tmax. This can be explained by the fact that the inorganic TiO2 shell can prevent the encapsulated n-eicosane from thermal degradation effectively. Moreover, the TiO2 shell remained as residual char at the end of TGA measurements, and the char yield increased slightly with increasing the loading of graphene nanosheets within the microcapsules as shown in Figure 3f. The thermal reliability and durability of n-eicosane@TiO2@ graphene microcapsules was evaluated by DSC with 200-cycle heating−cooling scans, and then the specimen after DSC measurement was examined by FTIR spectroscopy and TGA. Figure 4a shows the resulting DSC thermograms of neicosane@TiO2@graphene microcapsules containing of 5 wt % graphene nanosheets as a representative sample. It is observed that the melting and crystallization curves keep a good coincidence from the first cycle to the last one, whereas the Tm and Tc show a slight fluctuation around 1.2 °C. The ΔHm and ΔHc listed in Figure 4b indicate fairly stable values during the 200-cycle phase-change process. Meanwhile, the FTIR spectra of Figure 4c and TGA thermograms of Figure 4d demonstrate a high similarity before and after the 200-cycle DSC scans. These results indicate that the n-eicosane@TiO2@graphene microcapsules not only have a good phase-change reversibility and resilience but also display an excellent structural and thermal stability under the multicycle heating−cooling processes. The thermoregulatory performance of pure n-eicosane and neicosane@TiO2@graphene microcapsules was evaluated by an infrared camera during the heating process in a hot plate. Figure S5a−g illustrates the infrared thermographic images, which clearly reflect the surface temperature distributions of these specimens at different heating stages. It is interesting to observe a noticeable temperature difference between the background and specimens due to latent-heat absorption derived from the phase change of n-eicosane. The three specimens exhibit a blue color as a low-temperature indicator with an increase of temperature, suggesting a good thermoregulatory effectiveness for PCMs. Nevertheless, pure n-eicosane is found to shrink and flow away quickly in the heating process as

Figure 5. (a) Plots of objective temperature as a function of heating time obtained from infrared thermography; (b) plots of temperature versus time obtained from solar thermal energy-storage and release experiments for microcapsule samples and pure TiO2 shell.

surface temperature as a function of heating time for different specimens obtained from the infrared thermography analysis. The microcapsule specimens are found to undertake a slow increase in surface temperature within 50 s, and then their surface temperatures exhibit a plateau region in the period of 50−80 s during the heating process. Such a unique plateau region is attributed to the heat absorbed by the n-eicosane core in its melting temperature range, which generates a buffering 4912


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ACS Sustainable Chemistry & Engineering effect on temperature jump for microcapsule specimens. However, the surface temperature of pure n-eicosane rose up rapidly in the range of its phase-change temperature because of its overflow in the liquid state. These results confirm a good thermoregulatory capability obtained for n-eicosane@TiO2@ graphene microcapsules. The solar thermal energy-storage performance of neicosane@TiO2@graphene microcapsules was evaluated by using the custom-designed experimental system which simulates the solar energy storage along with a releasing process in the natural environment with a variation of ambient temperatures. Figure 5b shows the time−temperature plots obtained from the solar energy-storage/release experiment. It is noteworthy that there are two temperature plateaus found in the three microcapsule samples at 35−39 °C and 37−30 °C during the simulated sunlight irradiation and cooling processes, respectively, whereas pure TiO2 shell shows a continuous increase and subsequent decrease in temperature. The two temperature plateaus were ascribed to the temperature hysteresis derived from the adsorption of simulated solar thermal energy and the release of latent heat as a result of phase changes of the n-eicosane core. In addition, the microcapsules containing 5 wt % graphene nanosheets exhibit the fastest temperature rise among three microcapsule samples during the simulated sunlight irradiation process as show in Figure 5b. This may be due to the highest thermal conductivity of this sample, resulting in the fastest heat transfer. It is widely reported that graphene has a two-dimensional planar structure with a large π-conjugated system can enhance the charge-separation of TiO2 significantly and, consequently it can lead to an outstanding improvement in photocatalytic activity for the decomposition of organic materials and photocatalytic splitting of water.51,52 Zhang et al. prepared a series of TiO2-based composites with graphene nanosheets and reported that the TiO2 composite containing 5% graphene sheets exhibited a much higher photocatalytic activity toward hydrogen evolution from water splitting than the famous Degussa P25 TiO2 catalyst.53 Perera and his co-workers demonstrated that the TiO2 nanotubes/graphene composite shows much higher photocatalysis for decomposition of malachite green than pure TiO2.54 Therefore, to explore the potential applicability of n-eicosane@TiO2@graphene microcapsules in solar photocatalysis, we investigated their photocatalytic activity by conducting the photocatalytic degradation of an organic dye, MB, under the simulated sunlight irradiation. The photocatalytic effectiveness of these microcapsule samples was determined by the absorption intensity of MB at 664 nm. Figure 6 shows the UV−visible spectra and degradation rates of n-eicosane@TiO2 and n-eicosane@TiO2@graphene microcapsules at different irradiation time. It is notable in Figure 6a that the intensity of absorption peaks corresponding to MB presents a gradual decline as the irradiation time increases, suggesting a stepwise photodegradation process of MB as a function of sunlight irradiation time. This result was also confirmed by a series of color change of the MB solution as illustrated in Figure 6a. As observed from Figure 6b, the MB solution only shows a slight decrease in degradation rate over 240 min simulated sunlight irradiation under the catalysis of neicosane@TiO2 microcapsules, indicating a weak catalytic effect resulting from the TiO2 shell. However, after the graphene nanosheets were attached onto the surfaces of n-eicosane@ TiO2 microcapsules, the degradation rates of MB were improved significantly as displayed in Figure 6b. Moreover, it

Figure 6. (a) UV−visible absorptance spectra of n-eicosane@TiO2@ graphene microcapsules containing 5 wt % graphene nanosheets; (b) plots of degradation rate as a function of illumination time for neicosane@TiO2 and n-eicosane@TiO2@graphene microcapsules.

is noticeable that the degradation rate tends to increase with increasing the loading of graphene nanosheets in the microcapsules. At the same irradiation time, the microcapsules containing 5 wt % graphene nanosheets can result in a higher degradation rate by a factor of 5 than n-eicosane@TiO2 microcapsules, indicating a remarkable enhancement in solar photocatalytic effectiveness of TiO2 shell. Such a photocatalytic effectiveness is similar to the results reported by Zhang et al.53 and Perera et al.54 but superior to the commercial product Degussa P25. As a material with a high electrical conductivity, graphene nanosheets could establish electronic conductive channels to improve the electrochemical performance of the photocatalytic materials. In a n-eicosane@TiO2@graphene microcapsule system, the graphene nanosheets not only have a good interfacial contact with the TiO2 shell surface by physical adsorption, intermolecular force or chemical bonding, but also act as a support material for the dispersion of microcapsules, which effectively promotes the photoconversion efficiency of TiO2. In this case, the fabrication of n-eicosane@ TiO2@graphene microcapsule system can facilitate the electron transfer and charge separation from TiO2 shell to graphene nanosheets during the sunlight irradiation and thus enhances the photocatalytic activity of TiO2 shell. These results confirm that the n-eicosane@TiO2@graphene microcapsules developed 4913


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by this work have a great potential for applications of solar photocatalysis.

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CONCLUSIONS A novel bifunctional microcapsule system was designed for solar energy utilization and fabricated by combining neicosane@TiO2 microcapsules with graphene nanosheets through a self-assembly method. The morphological and microstructural investigations disclosed that graphene nanosheets were attached onto the surfaces of the core−shell structural microcapsules composed of an n-eicosane core and a brookite TiO2 shell. The prepared n-eicosane@TiO2@ graphene microcapsules achieved high phase-change enthalpies over 160 J/g, and their thermal conductivity was also improved from 0.64 to 0.98 W·m−1·K−1 due to the introduction of 5 wt % of highly thermally conductive graphene nanosheets. The presence of graphene nanosheets not only effectively improved the structural stability and serving durability of n-eicosane@ TiO2 microcapsules but also evidently enhanced their solar thermal energy-storage capability and solar photocatalytic activity. The composite system developed by this work indicates a significant enhancement in solar energy efficiency compared to the n-eicosane@TiO2 microcapsules synthesized previously and thus exhibits a great potential for direct solar energy utilizations such as solar thermal energy storage in a natural environment, solar photodegradation and detoxification for the water containing organic pollutants, and solar thermal energy collection and decontamination for industrial hot wastewater.

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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/acssuschemeng.7b00321 Scheme of the custom-designed experimental setup for evolution of solar energy storage and release, schematic synthetic method and reaction mechanism for graphene@TiO2@n-eicosane microcapsules, XPS spectra, FT-IR spectra and XRD patterns, and infrared thermographic images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 6441 0145. Fax: +86 10 6442 1693. E-mail: [email protected] (X. Wang). ORCID

Xiaodong Wang: 0000-0002-8787-1268 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by the National Natural Science Foundation of China with a grant number of 51673018.

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REFERENCES

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