In Situ Synthesis and Phase Change Properties of Na2

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In Situ Synthesis and Phase Change Properties of Na2SO4 3 10H2O@SiO2 Solid Nanobowls toward Smart Heat Storage J. Zhang, S. S. Wang, S. D. Zhang, Q. H. Tao, L. Pan, Z. Y. Wang,* and Z. P. Zhang* Institute of Intelligent Machines, Chinese Academy of Sciences; Department of Chemistry, University of Science & Technology of China, Hefei, Anhui, 230031, China

Y. Lei, S. K. Yang, and H. P. Zhao Institute of Material Physics, University of Muenster, Muenster, 48189, Germany ABSTRACT: As one of the promising thermal energy storage materials, inorganic hydrate salts have long been suffering from two intrinsic drawbacks including phase segregation and supercooling in their heat storage applications. In this study, Na2SO4 3 10H2O@SiO2 solid nanobowls, structured as Na2SO4 3 10H2O nanoclusters dispersed in SiO2 matrix, were synthesized on a large scale through hydrolysis of tetraethyl silicate and 3-aminopropyltriethoxysilane synchronously in reverse-microemulsion. Microstress imbalance originated from the relative rotation of the growing nanoparticle inside the aqueous droplet leads to a whirlpool-like microreactor and thus results in the formation of brims of the solid nanobowls. Confined by SiO2 matrix, heat storage properties of the hydrate salts are greatly improved. Their phase segregation is inhibited, and their supercooling is mitigated as well, which might originate from the mesoporous confinement effect. Their excellent cycling performance is of great importance in the prospective thermal energy storage application.

’ INTRODUCTION Inorganic hydrate salt is one of the most promising materials for application in heat storage and retrieval because of its low melt point, great latent heat per unit volume, nonflammability, and low cost.1 4 As thermal energy storage materials, however, they usually exhibit many intrinsic drawbacks such as phase segregation, supercooling, and very poor thermal conductivity (∼0.4 to 0.6 W 3 m 1 K 1). These will weaken their capability of heat storage release in practical usage. During the past few decades, various traditional approaches have been explored to overcome these above problems, including the use of nucleating, thickening agents and the addition of extra water to minimize supercooling and phase segregation of the inorganic phase change materials (PCMs) in their melting crystallizing cycles, respectively.5 8 Although the thermal performance of hydrated salts was improved to some degree, the addition of several kinds of extra agents is hard to control in an accurate way and may reduce the latent heat and its conversion efficiency. Recently, the progress in nanotechnology may provide an alternative solution to these problems by changing the structure and form of the material system. Some nonadditive approaches,9 14 by impregnating hydrate salts into various mesoporous matrixes, have been suggested to mitigate effectively their supercooling and phase segregation through mesopores confinement.15,16 From this point of view, in situ assembly of r 2011 American Chemical Society

hydrate salts in mesoporous matrix will no doubt greatly improve their thermal performance because content and uniformity of the hydrate salts dispersing in matrix could thus be controlled conveniently. In particular, confinement of hydrate salts within nanoparticles will avoid problems originated from low thermal conductivity by tremendously reducing the heattransmitting distance in essence.17 On the basis of this understanding, in this study, we report the in situ synthesis of solid nanobowls with Na2SO4 3 10H2O nanoclusters dispersed into SiO2 matrix. Importantly, the heat storage properties and cycling performances of this unique structured solid nanobowl are dramatically improved, which might originate from the mesoporous confinement effect.

’ EXPERIMENTAL METHODS The solid nanobowls were synthesized via synchronous hydrolysis of tetraethyl silicate (TEOS) and 3-aminopropyltriethoxysilane (APTS) in reverse microemulsions assisted with vigorous agitation. A typical preparation procedure is initialized by the addition of 0.95 mL of sodium sulfate anhydrous solution (2 mol/L) to 25 mL of cyclohexane containing 0.18 g sodium Received: March 13, 2011 Revised: August 30, 2011 Published: September 01, 2011 20061

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Figure 1. (a) Scanning electron microscopy images of the solid nanobowls. The inset in the bottom left of panel a is the high-magnification SEM image of the nanobowls. (b) X-ray diffraction spectrum of the as-produced nanostructures. (c) Energy-dispersive X-ray spectra recorded from the nanobowls and (d) their TEM image.

dodecyl sulfate (SDS) at 60 °C under stirring, followed by the addition of 5 mL of n-pentanol into the suspending media and stirring for 15 min, forming a water/oil (w/o) microemulsion. After aging for 2 h, 0.70 mL of TEOS was added to the w/o emulsion drop by drop. Then, the pH value of the mixture was adjusted to 8 by introducing APTS, followed by stirring (1000 rpm) for 24 h. Finally, the resulting nanocapsules were filtered and washed with ethyl alcohol three times and dried at 10 °C for 20 h. The as-produced materials were analyzed by fieldemission scanning electron microscopy (FESEM, FEI 200), energy dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD, PW1710 instrument with Cu Kα radiation). Detailed microstructures of the products were further investigated by transmission electron microscopy (TEM). The thermal properties were characterized by differential scanning calorimeters (Netzsch model DSC 200).

’ RESULTS AND DISCUSSION The size and morphology of the product were examined using scanning electron microscopy (SEM). Figure 1a shows the lowmagnification FESEM image of the as-grown solid nanobowls. It is clearly demonstrated that the solid nanobowls are highly monodispersed. They are ∼120 nm in diameter and have a narrow size distribution. As shown in the inset in Figure 1a, the brim of each solid nanobowl can be clearly observed.

The XRD pattern of the resulting sample is shown in Figure 1b, of which the standard XRD pattern for Na2SO4 (Joint Committee for Powder Diffraction Standards, JCPDs card no. 89-4751) is also given at the bottom. It reveals that all diffraction peaks can be well-indexed to the Na2SO4 phase after being compared with the above-mentioned standard data file. The amorphous character, which is apparent at low diffraction angle, can be ascribed to the SiO2 matrix, as substantiated by EDX result later. According to the EDX spectrum (Figure 1c), Na, S, Si, and O are the main element compositions of the sample. The atom ratio of the Na and S is about 2:1. Obviously, they stem from Na2SO4 crystals confirmed by the above XRD examinations. After the O from Na2SO4 is deducted from their total amount, the remaining elements are Si and O, which reveals that there is SiO2 in the solid nanobowls. It is notable that there is still superfluous O after taking out that contained in Na2SO4 and SiO2 from the total amount according to the stoichiometric ratio. This superfluous O might originate from the crystal water in hydrates of Na2SO4. On the basis of both the XRD tests and the EDX examinations, it could be concluded that the solid nanobowls are composed of crystalline hydrate Na2SO4 and amorphous SiO2. The morphology and microstructure of the as-synthesized Na2SO4 3 10H2O@SiO2 solid nanobowls were further studied using TEM (Figure 1d). During our observation, darker 20062

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Figure 2. Schematic illustration of the growth process of the solid Na2SO4 3 10H2O@SiO2 nanobowls.

Na2SO4 3 10H2O clusters can be seen dispersing uniformly into the brighter amorphous SiO2 nanoparticles. However, these dispersed darker clusters turned bright immediately under radiation of the electron beam. This is probably because the Na2SO4 3 10H2O crystals, with low melt point, are dissolved in their crystal water. According to the XRD, SEM, and TEM observations, the crystal structure features of the solid nanobowls can be described as Na2SO4 3 10H2O nanoclusters dispersed into amorphous SiO2 solid nanobowls. The formation of the resulting Na2SO4 3 10H2O@SiO2 solid nanobowls is quite different from that of the previously reported hollow nanobowls, which are in common initiated with growth of shells around various sphere templates, followed by removal of the templates.18 21 It is worth noting that microstress plays a key role in the synthesis of the resulting solid nanobowls. The preparation of solid nanobowls is initiated with the formation of the Na2SO4 3 10H2O@SiO2 oblate spheres through coprecipitation of Na2SO4 3 10H2O and SiO2 in the droplet microreactors, followed by growth of the brims via the same process in a whirlpool-like aqueous phase. The whole synthesis process can be divided into four stages, as illustrated by the Figure 2. In the following, the formation mechanism of the solid nanobowls will be described in detail by stages. Stage I Nucleation of the SiO2. The SiO2 is produced by hydrolysis of TEOS together with APTS and the subsequent polycondensation of the resulting Si OH during a typical sol gel process.22,23 In the beginning, APTS together with

Figure 3. TEM images of samples taken from a same synthesis process other than the growth time. (a d) Morphological change of the product on stirring the mixture for 0.25, 1, 4, and 16 h, respectively, after the APTS and TEOS were added to the reaction system.

TEOS is introduced into the W/O microemulsion to control the pH value of the emulsion system. Their inherent [ NH2] 20063

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Figure 4. SEM characterization of the comparison sample prepared in almost the same process except that no stirring was adopted after the APTS and TEOS were added to the reaction system.

groups lead to the release of enough OH to favor the nucleophilic substitution reaction of TEOS,22 together with hydrolyzing themselves unconditionally. Then, polycondensation between the hydrolysates Si OH of both TEOS and APTS results in the formation of SiO2 cluster capped with [ NH2] groups,23 which are the preferred nucleation sites of subsequent produced SiO2. Stage II Growth of Na2SO4 3 10H2O@SiO2 Oblate Sphere. After the nucleation of [ NH2] group-capped SiO2 cluster, the subsequent hydrolysis of TEOS together with APTS and polycondensation of the resulting Si OH will produce SiO2 continuously. At the same time, water is constantly consumed in the hydrolysis reaction, which leads to the Na2SO4 3 10H2O precipitation due to its supersaturation in the solution. The coprecipitation of the produced SiO2 and Na2SO4 3 10H2O results in the growth SiO2 particles with Na2SO4 3 10H2O clusters dispersed, corresponding to the shape of the aqueous phase droplets, which act as microreactors during the synthesis. This growth trend will not be changed until a critical large oblate sphere is formed. Stage III Growth of the Brims of the Solid Nanobowls. Until now, under stirring, the grown nanoparticles move nearly synchronously with the aqueous phase because of the interfacial viscous force between them. Once the solid particle grows to a critical size, the viscous force between the liquid phase and the solid phase will be not strong enough to drive them to rotate synchronously. Then, relative rotation of the aqueous droplet to this solid particle will unavoidably occur. Just like an oblate sphere rotating in the aqueous phase, a whirlpool-like droplet will be formed, as shown in the third step of Figure 2. Hereafter, the continuously produced SiO2 and Na2SO4 3 10H2O will start the growth of the brims of the nanoparticles. Stage IV Formation of the Solid Nanobowls. Thereon, as shown in the fourth step of Figure 2, coprecipitation of Na2SO4 3 10H2O and SiO2 proceeds in the whirlpool-like microreactor, and the brim is grown to the formation of Na2SO4 3 10H2O@ SiO2 solid nanobowl (final step in Figure 2). To validate our proposal, we took samples at intervals during the same synthesis process other than the reaction time. After the APTS and TEOS were added to the reaction system, four samples were taken when the mixture was stirring for 15 min,

Figure 5. TEM observation of the morphology change of the Na2SO4 3 10H2O@SiO2 solid nanobowls produced at different SDS concentrations of (a) 0.01 and (b) 0.025 M, respectively.

1 h, 4 h, and 16 h, respectively. Figure 3 shows the corresponding TEM images of the four samples. It is clear that the sample in Figure 3a shows oblate nanoparticle morphology. In Figure 3b, the nanoparticles grow larger, and thin fins come into being on the edge of the oblate spheres, as directed by the red arrow. In Figure 3c, the major axes of the oblate spheres remain almost the same size as those in Figure 3b; other than that, the fins grow into shallow brims and the rudiment of solid nanobowls has been grown. The brims of the solid nanobowls grow deeper and deeper to have formed the solid nanobowls as indicated in Figure 3d. The morphological change of the samples is in good agreement with our proposed formation mechanism of the solid nanobowls. To verify the critical role of microstress in the formation of the solid nanobowls, we prepared samples in almost the same process, other than adopting no stirring after the APTS and TEOS were added to the reaction system. According to SEM examination, the configuration of the sample is not solid nanobowl but nanosphere (as shown in Figure 4). This further proves that the microstress, originated from the relative rotation of the critical nanoparticles, results in the formation of the solid nanobowls. 20064

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Figure 6. (a) DSC thermal spectra of pure Na2SO4 3 10H2O and Na2SO4 3 10H2O@SiO2 solid nanobowls and (b) DSC spectra of Na2SO4 3 10H2O@SiO2 solid nanobowls with cycling of 1, 15, 30, 45, and 60 times.

As anionic surfactant, SDS is no doubt also essential to the formation of the Na2SO4 3 10H2O@SiO2 solid nanobowls because it affords the reverse microemulsion, where the aqueous droplets act as the microreactors for the hydrolysis of TEOS and APTS. For the salt-solution-containing water-inoil system in this work, 0.02 M SDS has been proved to be optimal by our extensive experiments. It is also concluded by these explorations that only SDS with concentration from 0.002 to 0.028 M can afford the reverse microemulsion system. Beyond this SDS concentration scope, the whole system will be either continuous phase or stratification. Within the above mentioned SDS concentration scope, according to our observation as shown in Figure 5, the higher the SDS concentration, the smaller the resulting solid nanobowls and vice versa. Figure 5a,b shows the TEM images of the Na2SO4 3 10 H2O@SiO2 solid nanobowls produced at SDS concentrations of 0.01 and 0.025 M, respectively. For the SDS concentration of 0.01 M, the diameter of the resulting solid nanobowls is ∼180 nm. When increasing the SDS to 0.025 M, the diameter of the sample decreases to ∼100 nm. Furthermore, it is notable that a higher SDS concentration results in a larger ratio of depth to diameter of the solid nanobowls, as shown in Figure 5. This might originate from the larger mole ratio of SDS to water, which will lead to a higher stiffness of the

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interfacial film between oil and water. Thus a deeper whirlpool will be formed under relative rotation of solid particle to aqueous phase. Confined by the SiO2 matrix, the hydrate Na2SO4 3 10H2O displays good heat storage actions, as shown in Figure 6. As indicated by the upper curve in Figure 6a, there is only one set of peaks in both exothermal and endothermal curves, whereas dual sets of peaks appear in both exothermal and endothermal curves for pure Na2SO4 3 10H2O (as shown in the bottom curve in Figure 6a). This indicates that phase segregation of hydrate salts has been inhibited. More importantly, there is no obvious change in both endothermal and exothermal curves of a same sample of solid nanobowls after cycling up to 60 times, as shown in Figure 6b, which is of great importance in their energy storage application. Additionally, compared with the pure Na2SO4 3 10 H2O, the supercooling in Na2SO4 3 10H2O @SiO2 solid nanobowls is also mitigated, as shown in Figure 6a. Meanwhile, during our DSC investigations, enthalpies of both pure Na2SO4 3 10H2O and the resulting Na2SO4 3 10H2O@SiO2 solid nanobowls are found to be 254 and 180.7 J/g, respectively. Whereas the weight ratio of Na2SO4 3 10H2O to the solid nanobowls can be roughly calculated to be only 58% according to the EDX examination, it is of great importance that enthalpy of the resulting Na2SO4 3 10H2O@SiO2 solid nanobowls can be up to 70% of that of pure Na2SO4 3 10H2O, even though the Na2SO4 3 10H2O is no more than 58 wt %. This enthalpy is no doubt better than that of the previously reported micro-/ nanocomposites,24 27 for which the enthalpy is no more than 50% of that of the pure PCM. This improvement in enthalpy might be owed to the fact that there is no phase segregation during the phase change process, which means that the enthalpy of the pure Na2SO4 3 10H2O can be greatly improved by confining them into nanomaterials. Of note, these favorable thermal properties of our resulting Na2SO4 3 10H2O@SiO2 solid nanobowls are associated with mesoporous confinement effect.9,10 As previously mentioned, for our resulting solid nanobowls, the Na2SO4 3 10H2O nanoclusters are uniformly dispersed into the SiO2 matrix. Namely, the Na2SO4 3 10H2O nanoclusters are in situ confined by mesopores correspondingly, which leads to the mesopores suitable for the Na2SO4 3 10H2O nanoclusters in both shape and chemistry. On one hand, the walls of these mesopores might be the ideal heterogeneous nucleation sites to favor the nucleation of Na2SO4 3 10H2O nanoclusters during the crystallization process.15 On the other hand, during heating, water molecules from Na2SO4 3 10H2O could be confined in the mesopores because of capillarity. Meanwhile, during heating, melting of Na2SO4 3 10H2O will generate a higher pressure in the mesopores to shorten their intermolecular spacing and then enhance the weak interactions including H-bonding and electrostatic forces between water molecules and SiO2 mesopore walls, which will further strengthen the confinement of mesopores to water molecules.16 Put concisely, heterogeneous nucleation of Na2SO4 3 10H2O together with mesoporous confinement is most responsible for the inhibition of their phase segregation with cycling and the mitigation of their supercooling. This will ensure their good cyclic performance in potential energy storage application through intelligent heat storage release. Furthermore, brims of the solid nanobowls can retard their possible deposition when they are adopted as heat storage release carriers of functional thermal fluid. 20065

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’ CONCLUSIONS In summary, the Na2SO4 3 10H2O@SiO2 solid nanobowls have been in situ prepared via synchronous hydrolysis reactions of TEOS and APTS in a reverse microemulsion system in which Na2SO4 is dissolved. Microstress inside the droplets is responsible for the formation of the bowl-like nanostructures. Their greatly improved thermal performances in the process of heat storage release are closely related to mesoporous confinement of the SiO2 matrix. As environment friendly and low-cost materials, the as-grown Na2SO4 3 10H2O@SiO2 solid nanobowls are no doubt of potential application importance in energy storage and intelligent thermosolar energy usage as well.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.Y.W.); [email protected] (Z.P.Z.).

’ ACKNOWLEDGMENT The financial support of this work by The National Basic Research Program of China (no. 2009CB939902), innovation project of Chinese Academy of Science (KJCX2-YW-H2O), and National Natural Science Foundation of China (no. 51002159) is gratefully acknowledged. ’ REFERENCES (1) Benli, H. Energy Convers. Manage. 2011, 52, 581–589. (2) Kenar, J. A. Sol. Energy Mater. Sol. Cells 2010, 94, 1697–1703. (3) Guenther, E.; Hiebler, S.; Mehling, H.; Redlich, R. Int. J. Thermophys. 2009, 30, 1257–1269. (4) Kaugusuz, K. Energy Sources 2003, 25, 791–807. (5) Mohammed, M. F.; Amar, K. A.; Siddique, A. K.; Said, A. H. Energy Convers. Manage. 2004, 45, 1597–1615. (6) Biswas, D. R. Solar Energy 1977, 19, 99–100. (7) Telkes, M. Ind. Eng. Chem. 1952, 44, 1308–1310. (8) Jegadheeswaran, S.; Pohekar, S. D. Renewable Sustainable Energy Rev. 2009, 13, 2225–2244. (9) Zhang, S. D.; Zhou, M.; Lu, X.; Wu, C. Z.; Sun, Y. F.; Xie, Y. Cryst. Eng. Commun. 2010, 12, 3571–3577. (10) Wu, C. Z.; Xie, W.; Zhang, M.; Bai, L. F.; Yang, J. L.; Xie, Y. Chem.—Eur. J. 2009, 15, 492–498. (11) Zeng, H.; Bando, Y.; Golberg., D. Nano Lett. 2010, 10, 5049–5055. (12) Zeng, H.; Duan, G.; Li, Y.; Xu, X.; Cai, W. Adv. Funct. Mater. 2010, 20, 561–572. (13) Zeng, H.; Xu, X.; Bando, Y.; Gautam, U. K.; Zhai, T.; Fang, X.; Liu, B.; Golberg, D. Adv. Funct. Mater. 2009, 19, 3165–3172. (14) Zeng, H.; Cai, W.; Xu, X.; Liu, P. ACS Nano 2008, 2, 1661–1670. (15) Chen, S. M.; Wu, G. Z.; Sha, M. L.; Huang, S. R. J. Am. Chem. Soc. 2007, 129, 2416–2417. (16) Liu, Z. W.; Bando, Y.; Mitome, M.; Zhan, J. H. Phys. Rev. Lett. 2004, 93, 095504. (17) Fang, Y. T.; Kuang, S. Y.; Gao, X. N.; Zhang, Z. G. Energy Convers. Manage. 2008, 49, 3704–3707. (18) Marechal, M.; Kortschot, B. J.; Demirors, A. F.; Imhof, A.; Dijkstra, M. Nano Lett. 2010, 10, 1907–1911. (19) Choi, W.; Zhang, Y.; Thomopoulos, S.; Xia, Y. N. Langmuir 2010, 26, 12126–12131. (20) (a) Li, X.; Peng, J.; Kang, J. H.; Choy, J. H.; Steinhart, M.; Knoll, W.; Kim, D. H. Soft Matter 2008, 4, 515–521. (21) Guan, G. J.; Zhang, Z. P.; Wang, Z. Y.; Liu, B. H.; Gao, D. M.; Xie, C. G. Adv. Mater. 2007, 29, 2370–2374. 20066

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