Morphology-controlled synthesis of microencapsulated phase change materials with TiO2 shell for thermal energy harvesting and temperature regulation

Accepted Manuscript Morphology-controlled synthesis of microencapsulated phase change materials with TiO2 shell for thermal energy harvesting and temperature regulation Huan Liu, Xiaodong Wang, Dezhen Wu, Shengfu Ji PII:

S0360-5442(19)30167-7

DOI:

https://doi.org/10.1016/j.energy.2019.01.151

Reference:

EGY 14624

To appear in:

Energy

Received Date: 24 November 2018 Revised Date:

16 January 2019

Accepted Date: 29 January 2019

Please cite this article as: Liu H, Wang X, Wu D, Ji S, Morphology-controlled synthesis of microencapsulated phase change materials with TiO2 shell for thermal energy harvesting and temperature regulation, Energy (2019), doi: https://doi.org/10.1016/j.energy.2019.01.151. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

Morphology-controlled synthesis of microencapsulated

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phase change materials with TiO2 shell for thermal energy

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harvesting and temperature regulation

Huan Liu1, Xiaodong Wang1,*, Dezhen Wu2, Shengfu Ji2

State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology,

Beijing 100029, China. 2

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State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology,

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Beijing 100029, China.

* Corresponding authors: Tel & Fax: +86 10 6442 1693

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E-mail: [email protected] (X. Wang).

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ABSTRACT In this work, we designed and fabricated three types of n-eicosane/TiO2-based microcapsules with different morphologies through an emulsion-templated interfacial polycondensation route, followed by the structure-inducing formation of TiO2 shell. As we expected, the resultant microcapsules achieved the

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tubular, octahedral and spherical morphologies as well as a well-defined core-shell microstructure under the structure-directing control with different crystallization promoters. X-ray diffraction confirmed that the tubular microcapsules obtained a crystalline TiO2 (B) shell, whereas a brookite TiO2 shell was formed for

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the octahedral and spherical microcapsules. According to the thermal analysis under the isothermal and nonisothermal conditions, the tubular microcapsules showed the fastest thermal response due to their

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specific internal nanostructures, and the spherical microcapsules presented the largest amounts of charged and discharged heat because of their highest effective encapsulation ratio of n-eicosane core. These three types of microcapsules achieved a good energy storage capability in an order of the spherical microcapsules > the octahedral microcapsules > the tubular microcapsules, and all of them presented a

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diverse range of superior performance including excellent temperature-regulating and photocatalytic capabilities, good thermal reliability and durability, good shape stability and prominent heat charging/discharging performance when used for thermal energy harvesting and temperature regulation.

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Keywords: Phase change materials; TiO2 shell; morphology control; thermal energy harvesting;

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temperature regulation; photocatalytic activity

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1. Introduction Non-renewable energy sources such as coal and petroleum have so far led to spectacular industrialization and social development, but with economic growth accelerating all over the developing

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countries, the demands on non-renewable sources are reaching their limits [1]. In addition, the global warming and climate change caused by increment of greenhouse gas and the uneven global distribution of energy sources have required drastic changes in the way of energy generation and supply. To deal with

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these changes, we need to develop cheaper, greener and scalable technologies for energy production and

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storage. As one of the most promising solutions for these challenges, semiconductor materials have attracted much attention. Semiconductor materials such as ZnO, Cu2O, CdS and TiO2 have the properties of variable conductivity, excited electrons, light emission and thermal energy conversion, and they are able to generate electrons and positive holes after illumination of excitation light energy over the band-gap energy.

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Therefore, semiconductor materials exhibit a sustainable innovation capability for decomposing organic pollutants, producing hydrogen by water splitting and reducing CO2 into renewable hydrocarbon solar fuels [2]. Much interest has been focused on TiO2 in particular since Fujishima and Honda discovered a true

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revolution of TiO2 as a photocatalyst for water splitting in 1972 [3].

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As a classical N-type semiconductor, TiO2 is one of the most widely investigated semiconductor materials due to its low cost and toxicity, good chemical stability, suitable band structures and unique electronic properties [4,5]. TiO2 occurs in nature as three well-known minerals, i.e. rutile, anatase and brookite. Among these three different crystal phases, rutile exhibits lower total free energy than the metastable phases of anatase and brookite and is the most stable polymorph of TiO2 at all temperatures accordingly. Nevertheless, the anatase TiO2 with a band-gap of 3.2 eV has been testified to be the most active phase when used as a photocatalyst. In addition to rutile, anatase and brookite, TiO2 has eight

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ACCEPTED MANUSCRIPT modifications, including three metastable phases (tetragonal, monoclinic and orthorhombic) and five high-pressure forms. With a further research aiming to TiO2, scientists found that the morphologies could greatly affect the optical, electronic and catalytic properties of TiO2. A number of synthetic routes and

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methods have been investigated to control the crystalline structure and microstructure of TiO2, which include hydrothermal method [6], electrodeposition [7], microwave [8], microemulsion and reverse micelles [9], chemical vapor deposition [10] and sol–gel hydrolysis process [11], and various

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nanostructures like spheres [12], fibers [13], nanorods [14], nanotubes [15] and nanosheets [16] were

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achieved for TiO2-based nanomaterials. These nanostructures broaden the applicable ranges of TiO2 from solar-driven applications like photocatalysis, water splitting and self-cleaning and biomedical devices to solar energy-storage systems such as dye-sensitized or quantum dot-sensitized perovskite-based solar cells, supercapacitors and lithium-ion batteries [17–20].

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In addition to the aforementioned cases, TiO2 could be used as a highly thermally conductive additive or a shell material for solar thermal energy-storage application in phase change materials (PCMs) in recent years [21,22]. As a class of reusable clean energy materials, PCMs can adsorb and release large amounts of

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latent heat energy at a certain temperature during fusion and solidification, respectively. Therefore, PCMs

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can utilize existing energy sources more effectively and then build a green and safe energy system for sustainable development of society. To improve the thermal response speed and latent heat-storage capability for various thermal energy storage systems based on PCMs, TiO2 nanoparticles were normally incorporated into a variety of PCMs including inorganic salts and hydrate salts, organic paraffin waxes and ethylene glycol [23–25]. As a result, the thermal conductivity and dispersion stability of PCM nanofluids in these systems were enhanced significantly due to a relatively high thermal conductivity of TiO2 nanoparticles. It was broadly observed that the PCM/TiO2 hybrid nanofluids presented a lower

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ACCEPTED MANUSCRIPT supercooling degree, shorter total freezing time and more effective energy-storage performance compared to pure PCM nanofluids [26]. Motahar et al. [27] synthesized the fluids based on n-octadecane along with dispersed TiO2 nanoparticles, and their experimental results demonstrated that these dispersed TiO2

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nanoparticles enhanced the thermal conductivity of PCM fluids but resulted in an increase in solidified volume. On the other hand, TiO2 has high rigidity and mechanical strength, so it can be used as a wall material to encapsulate PCMs for improving their shape/form stability, thermal transfer performance,

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operation reliability and working durability [28,29]. A literature survey demonstrated that some organic

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PCMs had been successfully encapsulated by pure TiO2, SiO2/TiO2 hybrid and polymer/TiO2 composite shells. Chai et al. [30] designed and synthesized a novel bifunctional microencapsulated PCM by encapsulating n-eicosane core with a crystalline TiO2 shell, and the as-synthesized microcapsules not only presented high thermal reliability for latent-heat storage but also exhibited a photocatalytic activity. Genc et

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al. [31] prepared a shape-stabilized thermal energy-storage material based on a myristic acid core and TiO2 shell in the sol–gel process, and the resulting microencapsulated PCM was reported to have a melt point of 56.95 °C and a latent heat of 96.64 J/g. Li et al. [32] investigated the performance of microencapsulated

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paraffin-type PCM with a SiO2/TiO2 hybrid shell and reported a decrease in latent heat by only 6.58% after

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1000-times melting–refreezing cycles for the obtained microcapsules. Zhao et al. [33] reported a novel approach for the encapsulation of PCMs by a polyurea/TiO2 composite at low temperatures and found the resultant microcapsules exhibited a high encapsulation ratio and high thermal storage capacity with mitigated supercooling. These reports clearly indicated that the microencapsulation of PCMs with TiO2-based shells not only can effectively prevent the leakage of PCM cores during the melting process but also can lead to a faster thermal response and higher reliability for latent-heat storage and release. However, most of the studies related to microencapsulated PCMs with TiO2-based shells have focused

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ACCEPTED MANUSCRIPT on the encapsulation techniques, but few of them were associated with their morphologies. Moreover, these microcapsules all presented a spherical morphology as reported [30,31,34,35]. It is well known that the functional diversity of TiO2 is strongly dependent on its morphology and crystalline structure, and therefore

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the diverse formation of TiO2-based materials with different crystalline structures and morphologies is highly anticipated to gain a much broader range in applications. For example, TiO2 nanotubes are widely applied in photocatalytic areas due to their strong ion-exchange ability and long lifetime of electron/hole

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pairs [36]. Both the membranes based on Ti layers and the ordered porous or defined channel-like TiO2

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were found to be suitable directly for use of size selective protein separation [37]. Huang et al. [38] reported a facile synthesis of SiO2/TiO2 core-shell structural microcapsules with a spherical morphology through liquid-phase deposition and found that their optical transmission spectra were significantly influenced by the thickness of TiO2 shell. The study by Wang et al. [39] indicated that the CuO/TiO2

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core-shell microcapsules exhibited an outstanding electro-chemical performance when they were formed in a mesoporous octahedral morphology. In addition, people will have a higher quality requirement for materials with the progress in science and the living standard. The current PCMs-based microcapsules are

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mainly used for traditional thermal energy-storage applications but are seldom involved in the applications

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for advanced scientific and technological fields, where a precise temperature control is needed. It is highly anticipated that the development of PCMs-based microcapsules as an elementary microstructural unit for the advanced innovative applications will be the next major trend in the fields of in-situ thermal management and thermoregulation technologies [40,41]. On the other hand, most of studies related to the TiO2 shell for the encapsulation of PCMs as we mentioned above is amorphous and inert without any additional functions. Although the microencapsulated PCMs with a crystalline TiO2 shell was successfully synthesized in our previous studies [30], the TiO2 shell still could not provide an enough solar

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ACCEPTED MANUSCRIPT photocatalytic activity for PCMs-based microcapsules to drive a photochemical reaction. To bridge this gap, it is necessary to explore new methodologies to develop high-performance PCMs-based microcapsules with a better thermal energy-storage capability and enhanced photocatalytic activity.

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Taking account of the possible diverse morphologies and microstructure of microencapsulated PCMs with a TiO2 shell, we attempted to synthesize PCMs-based microcapsules with different morphologies and photocatalytic effectiveness by fabricating a structure-controllable TiO2 shell onto the paraffin core in this

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work, and the geometrical effect on thermal response and thermal energy-storage performance of

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microencapsulated PCMs were investigated extensively. Moreover, in this microcapsule system, the different crystalline TiO2 shell can generate a photocatalytic response to sunlight and thus has a relatively high solar photocatalytic effectiveness to degrade water pollutants, hydrogen generation and antibacterial applications, while the n-eicosane core presents a thermal response to solar thermal energy by phase

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changes. Such a type of bifunctional microencapsulated PCMs with different morphologies and photocatalytic activity will provide the tremendous potential to fulfill the dual functions of thermal and solar energy collection and photocatalytic effectiveness for the application in advanced scientific and

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technological fields. The aim of this study is to discover the internal relationship between the

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microstructure/morphology and thermal energy-storage performance as well as the photocatalytic activity of such PCMs-based microcapsules and then to develop a type of novel thermal energy-storage materials based on the microcapsules consisting of n-eicosane as a PCM core and crystalline TiO2 as an inorganic shell (designated as the n-eicosane/TiO2-based microcapsules) with an additional function of photocatalytic activity for green and sustainable applications in diverse areas.

2. Experimental section

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ACCEPTED MANUSCRIPT 2.1 Materials n-Eicosane with a purity of 99% was commercially received from Acros Organics Company, NJ, USA. Tetrabutyl titanate (TBT) was purchased from J&K Scientific Co., Ltd., China. Poly(ethylene

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oxide-co-propylene oxide-co-ethylene oxide) (EO27–PO61–EO27, Pluronic® P104) was commercially obtained from BASF Corporation, Germany. Formamide, ammonium fluoride (NH4F), sodium hydroxide (NaOH), sodium fluoride (NaF), ethanol, Rhodamine B, and petroleum ether were purchased from Tianjin

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Fuchen Chemical Reagent Co. Ltd., China. All chemicals and reagents were used as received without

2.2 Synthesis of microcapsules

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further purification.

The n-eicosane/TiO2-based microcapsules with different morphologies were synthesized to form an expected core-shell structure through interfacial polycondensation under an emulsion-templating condition,

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and the synthetic strategy and mechanism were depicted in Figure 1. As a paraffin-type PCM, n-eicosane was employed as a core material due to its excellent performance with a good chemical stability and well-defined phase-transition points. The core/shell ratio of microcapsules was fixed by synthetic

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formulation according to our previous experimental results for the purpose of an appropriate balance

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between the n-eicosane core loading and TiO2 shell thickness [30]. In a typical synthetic procedure: a 250-mL three-neck round-bottom flask was charged with 5.0 g of n-eicosane and 5.0 g of TBT, and the mixture was stirred at 50 °C for 30 min to form a homogenous oil phase. In a beaker, a homogeneous solution was obtained by mixing 1.0 g of P104 as a nonionic surfactant with 100.0 mL of formamide at 50 °C under magnetic agitation. The homogeneous solution was poured directly into the flask with agitation for 4 h to achieve a stable nonaqueous oil-in-water (O/W) emulsion. Subsequently, 50.0 mL of formamide containing 2.0 g of deionized water was added dropwise into this emulsion with stirring for 1 h.

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ACCEPTED MANUSCRIPT The reactant mixture was agitated vigorously for 4 h at 50 °C to form the as-synthesized microcapsules containing an n-eicosane core and TiO2 shell. The morphology of as-synthesized microcapsules and the crystalline structure of TiO2 shell were

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further evolved controllably to form the tubular, octahedral and spherical microcapsules by using different structure-directing agents according to the following pathway: (1) the tubular microcapsules were obtained by adding 2.5 g of NH4F into the suspension containing as-synthesized microcapsules with mild agitation at

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85 °C for 24 h; (2) the octahedral microcapsules were obtained by adding 2 mL of NaOH solution (2 mol/L)

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into the suspension containing as-synthesized microcapsules with mild agitation at 85 °C for 6 h, followed by adding 2.5 g of NH4F with vigorous agitation at 85 °C for 24 h; (3) the spherical microcapsules were obtained by adding 1.0 g NaF into the suspension containing as-synthesized microcapsules with mild agitation at 85 °C for 24 h. After the reaction was completed, the reactant mixture was filtered to harvest

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some white powders as the microcapsules. The collected white powders were washed with deionized water, ethanol and petroleum ether several times and then dried at 30 °C overnight in a vacuum oven until the detergent solvent evaporated completely.

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2.3 Characterizations

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The morphologies of microcapsule samples were observed by scanning electron microscopy (SEM) using a Hitachi S–4700 scanning electron microscope (SEM) operated at an accelerating voltage of 20 kV, and their microstructures were further determined by transmission electron microscopy (TEM) on a Hitachi H–800 transmission electron microscope operated at 200 kV acceleration voltages. The chemical compositions of microcapsule samples were characterized by a Nicolet iS Fourier-transform infrared (FTIR) spectrometer at a scanning number of 32. The crystalline structures of TiO2 shells from three microcapsule samples were examined by powder X–ray diffraction (XRD) on a Japan Rigaku D/Max 2500

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ACCEPTED MANUSCRIPT VB2 + P/C X–ray diffractometer using Cu–Kα radiation (λ = 1.5405 Å), and the relevant patterns were recorded at a scanning rate of 2 °/min in a 2θ angular range of 5°–90°. The phase-change behaviors and thermal energy-storage/release performance of microcapsule samples

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were investigated by differential scanning calorimetry (DSC) using a TA Instruments Q20 differential scanning calorimeter at a scanning rate of 10 °C/min under a nitrogen atmosphere. Crystallization temperature (Tc) and melting temperature (Tm) were directly recorded by DSC thermograms, and

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crystallization enthalpy (∆Hc) and melting enthalpy (∆Hm) were obtained from the definite integral of DSC

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thermograms. DSC scans were also conducted to estimate the thermal energy charging and discharging performance of the PCM microcapsules under the isothermal and nonisothermal conditions. The development of relative degree of fusion or solidification (Xt) was derived from the isothermal DSC thermograms by using Eq. (4). The heat history was diminished by holding the microcapsule samples at

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60 °C for 4 min before the formal DSC measurements. The DSC measurements were carried out five times on one specimen, and a mean value of the data obtained from five tests was reported for each sample. The thermal stability of microcapsule samples was evaluated by thermogravimetric analysis (TGA) on a TA

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Instruments Q50 thermogravimetric analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere.

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The thermal conductivity of microcapsule samples was measured using an HS-DR-5 thermal conductivity tester (Shanghai HE SHENG Instrument Technology Co., Ltd., China). The temperature regulation performance of microcapsule samples as well as pure TiO2 shell as a control investigated by a Testo® 875–1i infrared thermal imaging camera, and the temperature distribution and mean surface temperature as a function of time was derived from the infrared thermographic analysis during the heating and cooling process. All of the thermal characterizations were carried out five times for each sample, and the reported result for each sample represented an average value of the data obtained from five tests.

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ACCEPTED MANUSCRIPT The photocatalytic activity of three microcapsule samples was evaluated by photodegradation using Rhodamine B as an organic dye. In a typical procedure, 100 mg of microcapsules and RhB solution (20 mg/L, 100 mL) were mixed in a beaker under a dark environment with stirring for 30 min to obtain an

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absorption equilibrium. Afterward, the suspension was illuminated by a UV lamp for 210 min. The concentration of Rhodamine B solution with a variation of illumination time was evaluated by a Shimadzu UV-2550 UV-visible spectrophotometer. Each test was repeated for five times, and the average values were

of Rhodamine B was determined by the following equation:

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obtained from five repeated tests with a relative standard deviation less than 5%. The degradation rate (D)

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I −I  D(%) =  0 t  × 100%  I0 

(1)

where I0 and It are the adsorption intensity of the suspension without UV illumination and illuminated for

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specific time t, respectively.

3. Results and discussion

3.1 Morphology and microstructure

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SEM and TEM were used to identify the morphologies and microstructures of microencapsulated

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n-eicosane with a TiO2 shell, and the obtained micrographs are presented in Figure 2. It is very interesting to note that there are three types of morphologies, i.e. tubular, octahedral and spherical morphologies, observed from the microcapsule samples synthesized in this work. It has been described in Figure 1 that the n-eicosane/TiO2 core-shell structural microcapsules were fabricated through emulsion-templated interfacial polycondensation in the sol–gel process, followed by a structural rearrangement of TiO2 shell with the aid of structure-directing agents. Our previous study demonstrated that the sol–gel process of titanate precursors only resulted in the formation of spherical microcapsules with an amorphous TiO2 shell,

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ACCEPTED MANUSCRIPT and however fluoride ions could act as a crystallization promoter to induce the crystallization of amorphous TiO2, thus resulting in a further structural rearrangement of TiO2 shell [30]. Such a phase transformation from amorphous TiO2 to the crystalline one is expected to occur with an ordering growth of crystals in the

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presence of structure-directing agent. Accordingly, a long-term ordering microstructure and regular morphology are possibly formed for the core-shell structured microcapsules with n-eicosane as a core and TiO2 as a shell. As observed in Figure 2a, the microcapsules exhibit a well-defined tubular morphology

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with a length of 1–5 µm and a diameter of 50–300 nm when synthesized with NH4F as a structure-directing

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agent. The experimental records indicated that such a characteristic morphology was only formed in the presence of high concentration of NH4F. According to the relevant research literature [42], the fluctuations of fluoride ion concentration onto the surface of amorphous TiO2 shell may initiate the formation of nanoloops. These nanoloops can act as the seeds to induce an oriented crystal growth process, thus leading

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to the formation of tubular TiO2. Furthermore, the morphology and microstructure of TiO2 were reported to be highly influenced by the Ti/F ratio [9]. The Ti/F ratio set in the current synthetic system was anticipated to promote the {100} facet of titania crystals, and therefore the spherical morphology was transformed into

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the well-defined tubular one due to the long-term structural rearrangement of TiO2 shell during the

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crystallization process. Nevertheless, a typical octahedral morphology with a particle size of 2–4 µm can be identified in Figure 2b when the microcapsules were synthesized with NaOH and NH4F as combined structure-directing agents. Dambournet et al. [43] found that the morphology of TiO2 particles was not only dependent on the fluoride-ion concentration of reaction medium but also on its pH. Their experimental results indicated the alkali ions could also promote the etching and dissolution mechanisms in the reaction system, which was able to tailor the morphology of TiO2 particles by controlling adsorption capability from different crystal facets, thus resulting in a diversity of microstructure of TiO2. Therefore, there is no doubt

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ACCEPTED MANUSCRIPT that the synergistic effect of alkali ions and fluoride species induce the formation of octahedron morphology [44,45]. The microcapsules are also found to present a well-defined spherical morphology with a diameter of

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0.2–4 µm (see Figure 2c) when using NaF as a structure-directing agent. On the basis of our previous study [30], NaF only plays a role as a crystallization promoter in the synthetic system of n-eicosane/TiO2-based microcapsules, and it can well induce the crystallization of TiO2 shell but shows no influence on the

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morphology of microcapsules. The TEM investigation verified a well-defined core-shell structure for three

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types of n-eicosane/TiO2-based microcapsules as observed in Figure 2d–2f. According to these TEM micrographs, the wall thickness could be determined as approximately 50, 150 and 250 nm for the tubular, octahedral and spherical microcapsules, respectively. It is noteworthy that the internal shell of tubular microcapsules exhibits a disordered mesoporous nanostructure. This may be due to the local fluctuations of

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NH4F concentration during the crystal-growth process of tubular TiO2, which leads to a disordered nanoporous structure with defects [46]. In summary, the SEM and TEM characterizations soundly confirmed that the n-eicosane/TiO2-based microcapsules were successfully fabricated with the expected

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morphologies and microstructures.

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3.2 Chemical and crystalline structures The chemical compositions and crystalline structures of the microcapsules synthesized in this work were characterized by FTIR and XRD, respectively, and the resulting infrared spectra and XRD patterns are illustrated in Figure 3. It is found that the three types of n-eicosane/TiO2-based microcapsules present a similar profile in their infrared spectra as seen in Figure 3a. A series of characteristic absorption peaks can be observed at 2960, 2915 and 2844 cm–1, which are attributed to the alkyl C–H stretching vibrations of methyl and methylene groups. Furthermore, the C–H deformation vibration of methyl and methylene

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ACCEPTED MANUSCRIPT groups is found at 1472 and 1371 cm–1, while a characteristic peak appears at 718 cm–1 due to the in-plane rocking vibration of methylene group. These characteristic absorption bands are in a good agreement with the characteristic peaks in the infrared spectrum of pure n-eicosane as marked in Figure 3a. Most of all,

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two intense characteristic peaks at 615 and 473 cm–1 can be distinguished from the infrared spectra of three microcapsule samples and are attributed to the Ti–O stretching vibration. These observed characteristic absorption peaks are in good agreement with the ones reported by literature [30,35], confirming the

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successful encapsulation of n-eicosane with a TiO2 shell. In addition, the absorption peaks at 3416 and

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1618 cm–1 can be assigned to the stretching and bending vibrations of O–H bond in the microcapsules, respectively. This suggests that there are abundant hydroxyl groups in the TiO2 shell due to the absence of the further dehydration reaction of TiO2.

It is surprising to note that the three types of microcapsules exhibit different XRD patterns as observed

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in Figure 3b. For the tubular microcapsules, a set of diffraction peaks are observed at 2θ = 14.02°, 15.85°, 23.47°, 27.42°, 28.10°, 35.43°, 43.43°, 48.16°, 49.70°, 50.27°, 57.99°and 60.69° in their XRD pattern, which can be assigned to the (001), (200), (201), (111), (202), (402), (003), (020), (203), (021), (421) and

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(710) planes of TiO2 (B), respectively, according to the standard card data of JCPDS No. 74–1940 [47,48].

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On the other hand, a series of diffraction peaks are found at 2θ = 29.74°, 36.68°, 42.47°, 47.77°, 52.69°, 56.16°, 61.65°, 65.80° and 69.81°in the XRD patterns of octahedral and spherical microcapsules, which are attributed to the (121), (012), (221), (231), (240), (151), (052), (161) and (332) reflections of brookite TiO2 (JCPDS 29–1360), respectively [49,50]. These characteristic diffraction results confirmed the formation of TiO2 shell with different crystalline structures as a result of an inductive effect by crystallization promoters, NH4F and NaF [51]. It is evident that the high concentration of NH4F not only can tailor a tubular morphology for the microcapsules but also can promote a crystalline TiO2 (B) shell. Nevertheless, sodium

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ACCEPTED MANUSCRIPT ions and fluoride ions dominated the crystalline structure of TiO2 shell when both NaOH and NH4F were introduced into the synthetic system as reported by Yang et al. [51]. In addition, there are two intensive diffraction peaks observed at 2θ = 17.97° and 20.88° from the octahedral and spherical microcapsules,

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which are assigned to the Na0.5Ti2O4 crystals induced by NaF [52]. These XRD data clearly confirmed that different crystal structures of TiO2 shell were induced when the n-eicosane/TiO2-based microcapsules were fabricated with different structure-directing agents.

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3.3 Phase change behavior and latent heat-storage performance

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To investigate the effect of morphology on the phase change behavior and latent heat-storage performance of the n-eicosane/TiO2-based microcapsules, DSC scans were carried out to detect the exothermic and endothermic thermograms of three types of microcapsule samples as well as pure n-eicosane, the resulting DSC curves and associated thermal parameters are shown in Figure 4 and Figure

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5. As observed in Figure 4a and Figure 4b, both microcapsule samples and pure n-eicosane are found to exhibit a bimodal exothermic behavior due to the crystallization of n-eicosane core in the cooling process but only present one melting peak in their DSC curves during the heating process. This bimodal

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crystallization behavior is attributed to the presence of metastable rotator phase during the phase transitions

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of n-eicosane from liquid to solid in the solidification process, and such a metastable rotator phase can easily be formed at a temperature higher than the conventional crystallization temperature [53]. It seems that there is no significant influence of morphology on the crystallization and melting behaviors of microencapsulated n-eicosane because of no chemical bonding or other special interaction between the TiO2 shell and n-eicosane core. However, the tubular microcapsules show a weak crystallization peak as well as an intensive phase-transition peak for the metastable rotator phase. This may be due to the internal porous nanostructure within the tubular microcapsules, which promotes heterogeneous nucleation for the

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ACCEPTED MANUSCRIPT encapsulated n-eicosane and thus leads to an enhancement in exothermic phase transition of the metastable rotator. To better understand such a bimodal crystallization behavior, a DSC cooling scan was performed for

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both pure n-eicosane and microcapsule samples at a slow scanning rate of 3 °C/min so as to evaluate their crystallization behaviors in a larger time scale, and the resulting DSC curves are presented in Figure 5. It is noted that the bimodal crystallization behaviors of pure n-eicosane become more remarkable compared to

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the result obtained at a scanning rate of 10 °C/min. This phenomenon indicates that the larger time scale

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allows for more complete crystallization of n-eicosane, thus resulting in the stable formation of rotator orthorhombic phase and stable triclinic phase as described by Fu and co-workers [54,55]. However, it is surprisingly observed in Figure 5 that the tubular microcapsules only show a single crystallization peak at the slow scanning rate compared to their bimodal crystallization behavior at the high scanning rate. The

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tubular microcapsules can provide a narrow geometrical confinement for crystallization of n-eicosane core, and such a tubular geometry can stabilize the rotator orthorhombic phase of n-eicosane by surface freezing, which leads to a direct transition from the rotator orthorhombic phase to the stable orthorhombic one in a

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large time scale [56]. As a result, only a unimodal crystallization behavior is observed. On the other hand,

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both the octahedral and spherical microcapsules have a much larger geometrical space and a smaller specific surface area than the tubular ones and therefore allow their encapsulated n-eicosane to experience the dual transitions from liquid to rotator phase and then to stable phase in a large time scale [55]. Therefore, these two types of microcapsules are still observed to show a bimodal crystallization behavior as seen in Figure 5. It is noteworthy that the Tm of the tubular microcapsules decreases more significantly than those of the other two samples. This phenomenon can be explained by the fact that the internal porous nanostructure of

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ACCEPTED MANUSCRIPT tubular microcapsules effectively improves the factual contact areas between the core and shell to enhance the crystallinity of the encapsulated n-eicosane. As a result, its supercooling was depressed intensively [30,57], leading to a decrease in supercooling degree (∆T = Tm – Tc) by 2.5 ºC compared to pure n-eicosane

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as seen in Figure 4c. On the other hand, the octahedral and spherical microcapsules are found to present a slight decrease in Tc but a considerable increase in Tm. The decrease of Tc is due to the crystallization confinement caused by a small inner space within the microcapsules, whereas the increase in Tm is

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considered as a result of heterogeneous nucleation, which leads to more perfect crystallization of

in this case as observed in Figure 4c.

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encapsulated n-eicosane. The supercooling degrees for these two microcapsule samples tended to increase

On the basis of the data obtained from DSC analysis in Figure 4d, the ∆Hc and ∆Hm of pure n-eicosane were determined as 245.4 J/g and 246.3 J/g, respectively, indicating a high latent heat-storage

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capability. Nevertheless, the n-eicosane/TiO2-based microcapsules seem to present a considerable decline in ∆Hc and ∆Hm due to the reason that the inorganic TiO2 shell is not involved in any phase transitions during the cooling and heating processes. The two phase change parameters are observed to highly depend

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on the weight fraction of n-eicosane core within the microcapsules, and the higher the core loading, the

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greater the phase change enthalpy. It is also noted in Figure 4d that there is a great variation in ∆Hc and ∆Hm for these three types of microcapsules, and both of them show a decreasing trend in an order of tubular microcapsules > octahedral microcapsules > spherical microcapsules. This indicates that the spherical microcapsules have the highest loading of n-eicosane core among three microcapsule samples. It is understandable that the spherical microcapsules have a larger capacity to contain much more of n-eicosane core than the octahedral ones. On the other hand, the tubular microcapsules with an internal multiporous nanostructure present a relatively smaller inner space compared to other two samples, which

17

ACCEPTED MANUSCRIPT limits the core loading and results in the lowest ∆Hc and ∆Hm accordingly. As three important characteristic parameters for the n-eicosane/TiO2-based microcapsules, encapsulation ratio (Een), latent heat-storage efficiency (Ees) and thermal energy-storage capability (Ces) can

Ees =

∆H m, PCM

×100%

∆H m, core + ∆H c, core ∆H m, PCM + ∆H c, PCM

× 100%

(∆H m, core + ∆H c, core ) ⋅ ∆H m, PCM

(∆H m, PCM + ∆H c, PCM ) ⋅ ∆H m, core

× 100%

(2)

(3)

(4)

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Ces =

∆H m, core

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Een =

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be calculated by Eqs. (2)–(4) using the DSC data [30,40].

where ∆Hc,core and ∆Hc,PCM are the average crystallization enthalpies of microencapsulated PCM and pure PCM, respectively, and ∆Hm,core and ∆Hm,PCM are the average melting enthalpies of microencapsulated PCM and pure PCM, respectively. These obtained characteristic parameters are all illustrated in Figure 6a. The

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Een values of microcapsule samples are observed to vary with their ∆Hm’s and also be correlative with the weight fraction of n-eicosane in the microcapsules. The tubular, octahedral and spherical microcapsules

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present the Een values of 25.0%, 48.9% and 75.9% respectively. Although the TiO2 shell as an inorganic material has a poor compatibility with organic n-alkanes, these three types of microcapsules, especially for

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the spherical microcapsules, has the relatively similar data of Een as reported in literature for PCMs-based microcapsules with polymer shells as depicted in Table 1 [58–64]. It is an anticipated result (see Figure 6a) for the spherical microcapsules to have the highest Een among three microcapsule systems due to their largest core loading. This result suggests the highest latent heat-storage efficiency gained by the spherical microcapsules. As an indicator for phase-change working efficiency, the Ees is always associated with the total phase change enthalpies in the melting and crystallization processes. It is found that the n-eicosane/TiO2-based microcapsules achieved the highest and lowest values of Ees in the spherical and 18

ACCEPTED MANUSCRIPT tubular morphologies, respectively. In addition, three microcapsule samples all exhibit a high value of Ces over 96.5%, suggesting that more than 96.5% of the encapsulated n-eicosane can effectively perform thermal energy storage and release through phase changes even in the microcapsules with different

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morphologies.

3.4 Thermal properties

It is widely accepted that the thermal energy-storage/release rates of thermal storage devices are

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highly dependent on the thermal conductivities of core and shell materials for a microencapsulated PCM. It

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is expected that the encapsulation of a PCM into the inorganic shell can effectively enhance the thermal conduction of the resulting PCM-based microcapsules. As shown in Figure 6b, the thermal conductivity of pure n-eicosane is determined to be 0.161 W·m-1·K-1. The three types of microcapsules are all found to exhibit a significant enhancement in thermal conductivity, which is attributed to the fabrication of an

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inorganic TiO2 shell with a high thermal conductivity. It is interesting to note that the n-eicosane/TiO2-based microcapsules obtained an improvement in thermal conductivity by 350% in a spherical shape, by 535% in an octahedral shape, and by 672% in a tubular shape. This result confirms that

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the higher weight fraction of TiO2 shell makes the microcapsules a higher thermal conductivity. Moreover,

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the tubular microcapsules achieved the highest thermal conductivity among these three types of microcapsules due to their fine nanostructure as well as the largest weight fraction of shell material, which may favor the heat transfer and thermal response rate for this type of microcapsules during the thermal regulation and thermal management processes. In addition, these three types of n-eicosane/TiO2-based microcapsules have an equal or even higher thermal conductivity compared to the paraffin-type PCMs/carbon composites reported by literature such as paraffin/graphene oxide composites [65], paraffin/graphene aerogel microcapsules [66], paraffin wax/carbon nanotube composites [67],

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ACCEPTED MANUSCRIPT paraffin/nano-graphite composites [68], n-eicosane/graphene nanoplatelets [69] and docosane/spongy graphene [70] as listed in Table 2. This indicated that the use of TiO2 shell as a supporting material for microencapsulation of PCMs could effectively enhance the thermal conductance of microencapsulated

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PCMs. Thermal stability plays an indispensable role in practical applications of PCMs for thermal energy storage and thermal regulation, because it determines the upper service temperature of PCMs. The thermal

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stability of three microcapsule samples was examined by TGA, and the resulting TGA and DTG

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thermograms are shown in Figure 7. It is observed in Figure 7a that both n-eicosane and microcapsule samples exhibit a one-step degradation behavior due to the evaporation of alkane chains in the temperature range of 130–280 ºC. There is no residual substance remained for pure n-eicosane at the end of TGA measurement, whereas the char yields of 69.9, 49.1 and 25.8 wt % are produced by thermal degradation for

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the tubular, octahedral and spherical microcapsules, respectively. On the other hand, pure TiO2 shell only shows a very slight weight loss during the thermal degradation process due to the further condensation of titanic hydroxyls and just gives a char yields of 4.62 wt % at the end of TGA test. It is noteworthy that the

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char yields of microcapsule samples are consistent with the Een data presented in Figure 6a, indicating that

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the residual char is mainly composed of TiO2 shell. In this case, the more the n-eicosane encapsulated into TiO2 shell, the higher the weight loss of microcapsule sample observed in the TGA thermogram. Moreover, the three types of microcapsules are found to present a higher characteristic temperature at the maximum weight-loss rate (Tmax) than pure n-eicosane as seen in Figure 7b, which is attributed to the fact that the inorganic TiO2 shell can act as a good sealing barrier to hinder the evaporation of n-eicosane effectively. Additionally, it is clearly observed that the spherical microcapsules gained the highest value in Tmax among the three types of microcapsules. This implies that the micro-container in a spherical shape can achieve a

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ACCEPTED MANUSCRIPT better sealing effect than those in the other two shapes and therefore provide a more effective barrier to prevent evaporation of alkane chains for the microcapsules.

3.5 Shape stability and temperature-regulating performance

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The shape stability of the n-eicosane/TiO2-based microcapsules was examined by an isothermal heating method on a hot plate at 50 °C. Figure 8 shows the digital photographs of pure n-eicosane and three microcapsule samples at different heating time. It is clearly observed that pure n-eicosane lost its

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initial shape gradually during the heating process, suggesting that the solid–liquid phase transition makes

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pure n-eicosane a very poor stability in shape during the heat charging process. However, the three types of microcapsules can always maintain their original shapes during the whole heating process even if the heating temperature is higher than the melting temperature of n-eicosane. There is no liquid n-eicosane found to leak out of the microcapsules. This result suggests that these three types of microcapsules with

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different morphologies all can effectively prevent the n-eicosane core from leaking out when they are subjected to practical applications. To further investigate the sealing effect of on the microcapsules with different morphologies, an extraction experiment was conducted for three microcapsule samples with

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acetone as an extraction solvent, and the DSC scans were conducted to measure the latent heat of fusion of

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three microcapsules over different extraction time. Figure 9 shows the plots of latent heat of fusion as a function of extraction time obtained from three microcapsule samples. It is observed in these plots that three microcapsules samples all show a decline in latent heat of fusion with extraction time due to the osmosis of n-eicosane core caused by solvent extraction. However, the tubular microcapsules are found to present the smallest amplitude reduction among them, indicating that the tubular microcapsules have a much tighter TiO2 shell to provide a better sealing effect for the PCM core than the other two samples. On the other hand, the octahedral microcapsules seem to have better sealing effect than the spherical ones due

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ACCEPTED MANUSCRIPT to their slower decline in latent heat of fusion. This verified that a tighter crystalline structure was rearranged for the TiO2 shell during the formation of the octahedral microcapsules, thus leading to a better sealing effect than the spherical microcapsules.

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The temperature-regulating performance of the n-eicosane/TiO2-based microcapsules was evaluated by an infrared thermographic method using a thermographic camera. Figure 10 shows the infrared thermal images and associated temperature distribution curves of microcapsule samples and pure TiO2 shell as a

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control during the heating and cooling processes. As illustrated in Figure 10a, the surface temperature of

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pure TiO2 shell is observed to rise continuously with an increase of the temperature of hot plate during the heating process due to a fast heat transfer. However, the surface temperatures of three microcapsule samples are evidently lower than that of pure TiO2 shell, and the temperature hysteresis is distinctly found from the relevant temperature distribution curves in Figure 10a. This phenomenon can be ascribed to the

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latent heat absorption resulting from the phase change of n-eicosane core from solid to liquid during the heating process. Furthermore, an increasing trend in temperature hysteresis is found to follow with an order of the spherical microcapsules > the octahedral microcapsules > the tubular microcapsules as a result of the

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increase of n-eicosane core loading. Similarly to the phenomenon observed in the heating process, a similar

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result can also be found during the cooling process as shown in Figure 10b. The surface temperatures of three microcapsule samples are apparently higher than that of pure TiO2 shell due to a phase transition for crystallization of n-eicosane core. It is understandable that the latent-heat release resulting from the crystallization of n-eicosane core maintains a higher temperature for the microcapsules. According to the temperature distribution curves in Figure 10b, the effect of heat preservation on the three types of microcapsules stands by an order of the spherical microcapsules > the octahedral microcapsules > the tubular microcapsules. These infrared thermographic results confirmed that the n-eicosane/TiO2-based

22

ACCEPTED MANUSCRIPT microcapsules could effectively regulate their ambient temperature by phase changes of the n-eicosane core. Figure 11 shows the surface temperature evolution of three microcapsule samples and pure TiO2 shell obtained from infrared thermographic analysis during the heating and cooling processes. It is anticipated

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that there is no temperature hysteresis observed from pure TiO2 shell as seen in Figure 11, because no PCM is involved in thermal regulation. However, in the case of three microcapsule samples, two temperature hysteresis regions can be clearly distinguished in 30–40 °C and 37–24 °C during the heating

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and cooling processes, respectively, which are associated with the melting- and crystallization-temperature

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ranges of n-eicosane core. There is no doubt that the latent-heat absorption and release by the n-eicosane core cause these two temperature hysteresis regions in the heating and cooling processes. It is noteworthy in Figure 11 that the spherical microcapsules present more significant temperature hysteresis than the octahedral and tubular ones due to the higher loading of n-eicosane core, which leads to a more effective

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thermal management and temperature regulation. On the basis of these infrared thermographic results, the n-eicosane/TiO2-based microcapsules exhibit an excellent thermal management capability to carry out effective temperature regulation.

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3.6 Thermal reliability and operation durability

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The thermal reliability and operation durability of three microcapsule samples were evaluated by consecutive 1000-cycle thermal treatments, and the DSC scans were conducted to measure the phase change behaviors and parameters of microcapsule samples after every 100-cycle thermal treatments. Figure 12 shows the resulting DSC curves and relevant phase-change parameters. The three types of microcapsules not only exhibit a good coincidence in heat flow profile during the multicycle DSC scanning process but also show the almost overlapping crystallization and melting peaks with slight thermal treatments at different cycle numbers. Furthermore, their phase change temperatures and enthalpies for

23

ACCEPTED MANUSCRIPT crystallization and melting are observed to keep relatively stable values with a fluctuation lower than 5% from the first heating-cooling cycle to the last one, indicating that the three types of microcapsules have good phase change reversibility and repetitiveness after multicycle thermal treatments. The collected

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specimens after multicycle DSC measurements were further analyzed by FTIR spectroscopy and TGA methodology, and the resulting FTIR spectra and TGA thermograms are given in Figure 13. It is surprising to note in Figure 13a that the FTIR spectra of three microcapsule samples all present a very similar

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location and intensity for each of characteristic absorption peaks before and after the multicycle DSC scans.

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This suggests that there is no change in chemical structure and composition for these three types of microcapsules after the multicycle heating-cooling process. Meanwhile, the three microcapsule samples also show almost overlapped TGA thermograms before and after the multicycle DSC measurement as seen in Figure 13b, indicating a good chemical stability and thermal performance for the microcapsules to

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undergo a multicycle heating-cooling impact. Summarily, these results confirmed an excellent thermal reliability and operation durability for the n-eicosane/TiO2-based microcapsules developed by this work.

3.7 Heat charging and discharging performance

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The heat charging and discharging performance of the n-eicosane/TiO2-based microcapsules was

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investigated by programmed DSC scans under the nonisothermal and isothermal conditions, and the development of relative degree of fusion or solidification (Xt) was derived from the isothermal DSC scans using Eq. (5).

 dH   dt dt  Xt = × 100% ∞  dH  ∫0  dt  dt t

∫  0

(5)

Where t is the arbitrary solidification or fusion time and dH/dt is the heat flow resulting from melting or crystallization phase transition. Furthermore, the mean heat charging or discharging rate (Rh) of the

24

ACCEPTED MANUSCRIPT microcapsules could be determined by Eq. (6).

Rh =

∆H tend − tonset

(6)

Where ∆H is the fusion or solidification heat, and tonset and tend are the onset and end time of phase

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transition, respectively. These obtained results not only could be used to simulate the latent heat-storage and thermal regulation behaviors but also provide optimum operation parameters for these microcapsules in

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practical applications. Figures 14–16 show the heat charging DSC thermograms and associated thermal diagrams of three microcapsule samples under the isothermal condition. The three types of microcapsules

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are found to exhibit a distinct variation in their endothermic peaks with the heat charging temperature. It is observed that the peak amplitude becomes higher and the peak width becomes narrower with an increase of heat charging temperature. This variation trend can be explained by the fact that a higher temperature promotes a faster melting phase change for microencapsulated n-eicosane due to the decrease of the Gibbs

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free energy associated with the heat charging temperature, thus resulting in a shorter period of fusion time for microencapsulated n-eicosane during the isothermal heat charging process. Moreover, as observed in

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Figures 14c–16c, the heat charging temperature seems to have more significant effect on the development of relative degree of fusion with heat charging time for the spherical and octahedral microcapsules. The

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effect of heat charging temperature on fusion heat also presents a complete variation trend in these three types of microcapsules as seen in Figures 14d–16d. The spherical and octahedral microcapsules exhibited much higher fusion heat at 38 ºC than at other temperatures, whereas the fusion heat of the tubular microcapsules tends to increase with the improvement of heat charging temperature. These results suggest that a low temperature of 38 ºC is the optimum operation temperature for the heat charging of the spherical and octahedral microcapsules, but much higher temperatures may favor the heat charging of the tubular microcapsules. In this case, a high charging temperature of 48 ºC is necessary for the full melting phase 25

ACCEPTED MANUSCRIPT transition of encapsulated n-eicosane in the tubular microcapsules. On the other hand, it seems that a higher temperature can lead to a faster heat charging process for these three types of microcapsules as observed in Figures 14d–16d, although there is a fluctuation in mean heat charging rate at high charging temperatures

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for the tubular microcapsules. This abnormal phase change behavior may be due to the specific confined space within the tubular microcapsules.

Figures 17–19 show the isothermal heat discharging DSC thermograms and associated thermal

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diagrams of three microcapsule samples. Contrary to the isothermal heat charging process, the exothermic

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peak amplitudes of microcapsule samples are found to decrease with an increase of heat discharging temperature as found in Figures 17a–19a and 17b–19b. It is well known that a lower temperature can thermodynamically promote the crystallization process of encapsulated n-eicosane when the ambient temperature is lower than the crystallization temperature of n-eicosane [71]. However, these three types of

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microcapsules are found to present the fastest solidification periods as well as the lowest exothermic peak amplitudes at 34 ºC according to the development of relative degree of solidification shown in Figure 17c–19c, which indicates that such a discharging temperature is too high to promote a full crystallization

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phase transition for the encapsulated n-eicosane. This result is also testified by the data of solidification

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heat shown in Figure 17d–19d. Moreover, both the solidification heat and mean heat discharging rate exhibit a rapid decreasing trend with an increase of heat discharging temperature. There is no doubt that the full crystallization of the encapsulated n-eicosane can normally be promoted by the more negative Gibbs free energy resulting from a lower crystallization temperature. Therefore, the lower the heat discharging temperature, the larger the solidification heat and mean heat discharging rate. As a result, the three types of microcapsules can gain the optimum heat discharging from a low ambient temperature. On the basis of the isothermal heat charging and discharging results of three types of microcapsules, it is obvious to find that

26

ACCEPTED MANUSCRIPT the tubular microcapsules exhibit the fastest heat charging and discharging responses to ambient temperature, which may be attributed to their internal porous nanostructures. The heat charging and discharging performance of the tubular microcapsules are also superior to that of other works as

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investigated by Sun et al. [72] and Gao et al. [73] This suggests that the tubular microcapsules synthesized in this work have an excellent charging/charging response to heat change of environment for the practical use of solar thermal energy storage and release. Meanwhile, the spherical microcapsules present the largest

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mean heat charging and discharging rates due to their highest loading of n-eicosane core among the three

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types of microcapsules.

Figure 20 shows the 6-cycle DSC thermograms of three microcapsule samples under the nonisothermal heating and cooling processes. It is surprising to note that these three types of microcapsules all present a stable and repeatable phase change behavior without any delay during the repetitious

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nonisothermal operations for heat charging and discharging. This result demonstrates that the three types of microcapsules have a highly sensitive thermal response to a change in ambient temperature. Moreover, the heat charging/discharging periods are determined as 1.22/1.63 min for the tubular microcapsules, 1.48/1.19

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min for the octahedral microcapsules and 1.63/1.46 min for the spherical microcapsules. It is evident that

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the tubular microcapsules gain the shortest heat charging period among the three types of microcapsules. This result is ascribed to their internal nanostructures, which provide a large surface area for the microcapsules and thus improve the heat transfer and heat exchange rate of n-eicosane core. However, the internal nanostructures influence the crystal growth of encapsulated n-eicosane within the tubular microcapsules, and such a nanoscale confinement effect increases the crystallization period of n-eicosane core, thus resulting in the longest heat discharging period for the tubular microcapsules among the three types of microcapsules. The octahedral microcapsules show a shorter heat discharging period compared to

27

ACCEPTED MANUSCRIPT the spherical one, which may be due to the lower effective encapsulation ratio of n-eicosane core. These results suggest that the three types of microcapsules developed by this work present prominent heat charging and discharging performance, and all of them have an excellent reliable thermal response

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capability to perform thermal management and temperature regulation in practical applications.

3.8 Photocatalytic activity

It is well known that the crystalline TiO2 has a photocatalytic activity due to the feature of

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broad-bandgap semiconductive nature, which can be excited by a specific UV radiation (λ ≤ 387 nm). To

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evaluate the photocatalytic activity of three types of microcapsules, a standard photocatalytic degradation experiment with Rhodamine B was conducted for the as-synthesized microcapsules in this work. The photocatalytic effectiveness of three types of microcapsules was evaluated by the absorption intensity of UV-visible spectroscopy. Figure 21a displays the UV-visible spectra of the tubular microcapsules as a

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representative sample at different illumination time, while Figure 21b shows the plots of degradation rates of three types of n-eicosane/TiO2-based microcapsules as function of illumination time. It is noted that the absorbance intensity of characteristic band at 552 nm decreases gradually with an increase of irradiation

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time, indicating the decrease of Rhodamine B concentration in the suspension containing the tubular

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microcapsules. This result was also confirmed by a series of color change of Rhodamine B solution as observed from the inserted photograph in Figure 21b. In generally, TiO2 can generate electrons in the valence band and positive holes in the conduction band after irradiated by any light with energy higher than its bandgap energy, and the photogenerated electrons and positive holes then move to the surface of TiO2. Afterwards, the electrons and positive holes are able to facilitate the reduction and oxidation reactions to generate superoxides and hydroxyl radicals. These extremely powerful oxidants can attack the Rhodamine B molecules, then leading to a series of complicated degradation of Rhodamine B [19,36]. Moreover, as

28

ACCEPTED MANUSCRIPT shown in Figure 21b, the photocatalytic efficiency of three types of microcapsules are observed in an order of the tubular microcapsules > the spherical microcapsules > the octahedral microcapsules. The tubular, octahedral and spherical microcapsules obtained the degradation rates of 93.1%, 72.2% and 82.9% after

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illumination for 120 min, respectively. It is understandable that the photocatalytic degradation of microcapsules is in accordance with the surface area, and a higher surface area of microcapsules can lead to a higher contact area for TiO2 shell with Rhodamine B molecules. On the other hand, the unique

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nanostructure of the tubular microcapsules can facilitate the electron transfer and charge separation and

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then reduces the recombination of photogenerated electrons and positive holes, thus enhancing the photocatalytic activity of TiO2 shell [17]. In this case, the tubular microcapsules have the largest photocatalytic efficiency due to its highest surface area in three types of microcapsules. These results confirm that the three types of microcapsules synthesized by this work have great potential for solar

4. Conclusion

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photocatalytic applications.

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We successfully synthesized three types of n-eicosane/TiO2-based microcapsules with the tubular, octahedral and spherical morphologies through emulsion-templated interfacial polycondensation, followed

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by the structure-inducing formation of TiO2 shell. The presence of NH4F generated a structure-directing effect on the crystallization of amorphous TiO2 shell and resulted in the formation of tubular microcapsules as well as a crystalline TiO2 (B) shell. On the other hand, the octahedral microcapsules with a brookite TiO2 shell could be fabricated by structure-directing control with NaOH and NH4F, whereas fluoride ions only acted as a crystallization promoter to induce the formation of brookite TiO2 as well as the spherical microcapsules. The spherical, octahedral and tubular microcapsules achieved the effective encapsulation ratios of 75.7%, 48.9% and 27.1%, respectively, but all of them exhibited an effective thermal response to

29

ACCEPTED MANUSCRIPT ambient temperature in the phase-transition temperature range of n-eicosane core. An improved thermal conductivity of 1.216 W·m-1·K-1 and a reduced supercooling degree of 5.2 ºC were obtained for the tubular microcapsules due to their specific geometrical structure. These three types of microcapsules all presented a

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diverse range of superior thermal performance including excellent temperature-regulating and photocatalytic capabilities, good thermal reliability and operation durability, good shape stability and prominent heat charging/discharging performance. On the basis of the thermal analysis results under the

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isothermal and nonisothermal conditions, the tubular microcapsules showed the fastest thermal response

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among the three types of microcapsules due to their specific internal nanostructures, and the spherical microcapsules presented the largest amounts of charged and discharged heat because of their highest effective encapsulation ratio of n-eicosane core. With diverse external morphologies and perfect internal microstructures, the three types of n-eicosane/TiO2-based microcapsules exhibit a wide range of potential

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application for the thermal energy harvesting and temperature regulation of self-cleaning and anti-fogging materials and the precision thermal management of electrochemical biosensor, lithium-ion batteries and

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supercapacitors.

Acknowledgement

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This work was supported by the National Natural Science Foundation of China (Project Grant Numbers 51673018 and 51873010), the China Postdoctoral Science Foundation (Project Grant Number 2018M631309) and the Fundamental Research Funds for the Central Universities.

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ACCEPTED MANUSCRIPT hierarchical microcapsules containing phase change material for in-situ thermal management of supercapacitors, Appl. Energy 237 (2019) 549–565. [73] F.X. Gao, X.D. Wang, D.Z. Wu, Design and fabrication of bifunctional microcapsules for solar

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thermal energy storage and solar photocatalysis by encapsulating paraffin phase change material into

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Figure Captions Figure 1 Scheme of synthetic route and formation mechanisms of the n-eicosane/TiO2-based microcapsules with different morphologies.

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Figure 2 SEM micrographs of (a) the tubular, (b) octahedral and (c) spherical microcapsules. TEM micrographs of (d) the tubular, (e) octahedral and (f) spherical microcapsules.

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Figure 3 (a) FTIR spectra and (b) XRD patterns of (1) pure n-eicosane and (2) the tubular, (3) octahedral and (4) spherical microcapsules.

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Figure 4 (a,b) DSC cooling and heating thermograms, (c) phase change temperatures and (d) phase change enthalpies of (1) pure n-eicosane and (2) the tubular, (3) octahedral and (4) spherical microcapsules at a scanning rate of 10 ºC/min.

Figure 5 DSC cooling thermograms of pure n-eicosane and microcapsule samples at a scanning rate of 3

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ºC/min.

Figure 6 (a) Thermal energy-storage parameters derived from DSC analysis and (b) thermal

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conductivities of (S0) pure n-eicosane and (S1) the tubular, (S2) octahedral and (S3) spherical

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microcapsule samples.

Figure 7 (a) TGA and (b) DTG thermograms of three microcapsule samples, pure n-eicosane and pure TiO2 shell.

Figure 8 Digital photographs of (a) pure n-eicosane and (b) the tubular, (c) octahedral and (d) spherical microcapsules during the isothermal heating process. Figure 9 Plots of latent heat of fusion as a function of extraction time for three microcapsule samples after solvent extraction with acetone.

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ACCEPTED MANUSCRIPT Figure 10 Infrared thermographic images and associated temperature distribution curves of (S0) pure TiO2 shell and (S1) the tubular, (S2) octahedral and (S3) spherical microcapsule samples during (a) the heating and (b) cooling processes.

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Figure 11 Plots of average surface temperature evolution derived from the infrared thermographic analysis for (1) pure TiO2 shell and (2) the tubular, (3) octahedral and (4) spherical microcapsules during the (a) heating and (b) cooling processes.

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Figure 12 Multicycle DSC curves and plots of phase change parameters as a function of cycle number for

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(a,b) the tubular, (c,d) octahedral and (e,f) spherical microcapsules.

Figure 13 (a) Comparative FTIR spectra and (b) comparative TGA thermograms of the microcapsule samples before and after multicycle DSC measurements.

Figure 14 (a,b) Isothermal DSC profiles, (c) development of relative degree of fusion, and (d) fusion heat

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and mean heat charging rate of the tubular microcapsules.

Figure 15 (a,b) Isothermal DSC profiles, (c) development of relative degree of fusion and (d) fusion heat

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and mean heat charging rate of the octahedral microcapsules. Figure 16 (a,b) Isothermal DSC profiles, (c) development of relative degree of fusion and (d) fusion heat

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and mean heat charging rate of the spherical microcapsules.

Figure 17 (a, b) Isothermal DSC profiles, (c) development of relative degree of solidification and (d) solidification heat and mean heat discharging rate of the tubular microcapsules.

Figure 18 (a, b) Isothermal DSC profiles, (c) development of relative degree of solidification and (d) solidification heat and mean heat discharging rate of the octahedral microcapsules. Figure 19 (a,b) Isothermal DSC profiles, (c) development of relative degree of solidification and (d) solidification heat and mean heat discharging rate of the spherical microcapsules. 41

ACCEPTED MANUSCRIPT Figure 20 Nonisothermal DSC profiles of (a) the tubular, (b) octahedral and (c) spherical microcapsules. Figure 21 (a) UV-visible spectra of Rhodamine B suspension containing the tubular microcapsules at different UV-illumination periods during the photodegradation process. (b) Plots of degradation

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rates of three microcapsule samples as a function of illumination time.

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ACCEPTED MANUSCRIPT Table 1 Comparison of encapsulation ratio and thermal storage capacity of the n-eicosane/TiO2-based microcapsules and PCMs-based microcapsules with a polymer shell. ∆Hm of microencapsulate PCM

Een

(J/g)

(J/g)

(%)

214.6

146.5

Paraffin/polyurea

208.2

92.5

Paraffin wax/cross-linked of poly(methyl

132.5

113.4

163.5

77.3

Paraffin/polyaniline

136.2

108.8

Tetradecane/poly(urea-formaldehyde)

217.5

Fatty acid

164.6

68.2

[58]

44.4

[59]

85.6

[60]

47.3

[61]

79.9

[62]

134.2

61.7

[63]

61.3

37.2

[64]

61.6

25.0

This work

120.5

48.9

This work

187.0

75.9

This work

melamine-formaldehyde

methacrylate) Palmitic-capric acid eutectic

/poly(melamine-urea-formaldehyde) 246.3

The octahedral microcapsules

246.3

The spherical microcapsules

246.3

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The tubular microcapsules

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mixture/polystyrene

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n-Octadecane/resorcinol-modified

References

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∆Hm of pure PCM

PCM-based microcapsule system

ACCEPTED MANUSCRIPT Table 2 Comparison of thermal conductivity of the n-eicosane/TiO2-based microcapsules and paraffin-type PCM/carbon composites. Paraffin-type PCM/carbon

Thermal conductivity of

Thermal conductivity of

Increasing rate

composite system

pure PCM

composite PCM

(%)

-1

-1

(W·m ·K )

0.287

1.32

0.207

0.274

0.28

1.14

0.1264

0.9364

microcapsules Paraffin wax/carbon nanotube composites Paraffin/nano-graphite composites

0.26

The tubular microcapsules

0.161

The octahedral microcapsules

0.161

The spherical microcapsules

0.161

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32%

[66]

307%

[67]

[68]

403%

[69]

0.59

127%

[70]

1.244

663%

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1.023

535%

This work

0.724

350%

This work

2.095

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Docosane/spongy graphene

[65]

641%

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n-Eicosane/graphene nanoplatelets 0.4167

360%

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(W·m ·K )

composites Paraffin/graphene aerogel

-1

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Paraffin/graphene oxide

-1

References

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ACCEPTED MANUSCRIPT

Highlights The microcapsules consisting of n-eicosane core and TiO2 shell were designed in different shapes.




The microcapsules show interesting tubular, octahedral and spherical morphologies as expected.




These microcapsules present excellent thermal regulation and thermal management capabilities.




The tubular microcapsules exhibit a faster thermal response to ambient temperature than the other

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two.