Dielectric Nanomaterials for Power Energy Storage: Surface

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Dielectric Nanomaterials for Power Energy Storage: Surface Modification and Characterization Yujuan Niu, and Hong Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01846 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Nano Materials

Dielectric Nanomaterials for Power Energy Storage: Surface Modification and Characterization Yujuan Niua, Hong Wangb* a

Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology,

Shenzhen 518055, China. b

Department of Materials Science and Engineering, Southern University of Science and Technology,

Shenzhen 518055, China. ABSTRACT Nanocomposites exhibit promising performance in the application of dielectric capacitors due to their excellent dielectric properties. However, nanoparticles are easy to aggregate, and difficult to be compatible with polymer matrices, thus requiring surface modification of the nanoparticles with organic ligands. Surface modification has been proposed as a useful method for regulating the surface properties of nanoparticles over a long history. In this review, we have outlined some modifiers and their characteristics, including the reactive mode of each modifier, modification effect, as well as its characteristic advantages and limitations. The modifiers selected in this review contain different functional groups, and they have also been widely used in the surface modification of nanoparticles. We have also summarized the characterization methods for surface composition, the properties of modified nanoparticles, the interfacial structure and the performance of nanocomposites. At the same time, we have conducted a preliminary exploration on the chemical compatibility between the polymer matrix and the terminal functional groups in the modifier molecules. We hope that this review will provide assistance to researchers in the targeted selection of modifiers. KEYWORDS: energy storage, surface modification, interface, dielectric, characterization

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INTRODUCTION

Composites are formed from two or more materials by physical or chemical methods. The components with different properties complement each other in terms of performance and produce synergistic effects.1 Therefore, the overall performance of composites is generally superior to the original component materials, thus broadening the application fields of these composite materials.2-4 Ceramic/polymer composites are widely used in electronics, such as capacitors,5-8 gate insulators,9 printed circuit board (PCB),10 and fabrication of microwave substrate.11 For dielectric capacitors, the energy density is calculated by εEb2/2, where ε is the relative permittivity and Eb is the breakdown strength. Therefore, high energy density composites can be obtained by combining ceramic nanoparticles having high permittivity and polymers having high breakdown strength.12-14 Dielectric capacitors enable a quick accumulation and an almost instantaneous output of a lot of energy, exhibiting an unmatched charge-discharge speed in comparison with other energy components.15-16 This kind of “pulse power” is widely used in military and commercial products，such as smart devices,17 electromagnetic weapons 18, and electric cars.19-20 When one component of a composite reaches a nanometer scale, it is called a nanocomposite.21 In nanocomposites, the interface between the polymer and the nanofiller plays a dominant role in bulk properties owing to the fact that a large number of atoms will gather on the surface of the ultrafine nanoparticles if a homogeneous dispersion can be guaranteed.14,

22

However ， the reality is that

nanoparticles tend to aggregate together and form defect centers resultantly,23-24 leading to the enhancement of the local electric field and the reduction of the breakdown strength.14, 25 In the meantime, aggregates also act as stress concentration points that will initiate a local destruction, and deteriorate the mechanical properties of the composites.26-27 Therefore, the improvement in the dispersion of inorganic ACS Paragon Plus Environment

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nanofillers is of great importance to enhance the comprehensive properties of nanocomposites.28-29 In order to overcome interfacial energy barriers and reduce aggregation, surface modification of nanoparticles is necessary.30 In micro-particle/polymer composites, surface modification is widely used as an effective technique to improve filler dispersion. With the development of nanocomposites, the surface modification for nanoparticles has attracted more and more attention. As early as 2001, Caruso proposed that surface modification was the state-of-the-art strategy for particle surface engineering.31 Later, Huang et al. summarized the research advances of surface modification with polymers and inorganic materials as modifiers for nanoparticles.32 In this review, we outline some modifiers that have been used to modify nanoparticles in dielectric nanocomposites, and then describe the mode of reaction for each modifier, and list its characteristic advantages and limitations. The characterization methods for modified nanoparticles, including surface morphology, chemical composition, microstructure and dispersion in the matrix, are also summarized. In addition, measurements of the physical properties of nanocomposites can also be used to evaluate the effect of surface modification. 2.

INTERFACE THEORY MODEL

The interface formed by polymer matrix and nanoparticles plays a critical role in the performance of nanocomposites.33-34 Since it is difficult to visualize them using conventional methods,35-36 scientists have established many interface models to explain the phenomena that occur when two or more materials are existing in the composite. For example, the performance of nanocomposites can be greatly improved when surface modifiers are introduced, which can be attributed to the reason why modifiers can affect the composition and structure of the interface in these composites. Herein, we make a brief introduction to some related interface models for a better understanding of the effect of modifiers on the interface. ACS Paragon Plus Environment


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Electrical Double Layer Model In 1994, Lewis proposed the “electrical double layer model”, in which he emphasized the importance of the nanometric thickness interface in altering electrochemical and electromechanical behaviors.37 Lewis considered that the incorporation of nanoparticles with an insulating matrix could create interfaces with nanometric dimensions, and in the meantime a Stern layer and a diffuse Gouy-Chapman layer would be formed around each nanoparticle with a changing internal charge activity (Figure 1a).38-40 Multi-core Model The multi-core model is used to describe the chemical/physical and electrical structure of the interface between the nanoscale fillers and the polymer matrix41 (Figure 1b), which was proposed by Tanaka. In this model, the surface structure of the nanoparticle is divided into three layers: a bonded layer that is chemically bonded to the nanoparticle, a bound layer corresponding to a region of ordered polymer chains, and a loose layer that is loosely coupled to the second layer.41 The loose layer has different chain conformation, chain mobility and free volume or crystallinity compared to the polymer matrix.41-43 Polymer Chain Alignment Model The polymer chain alignment model assumed that surface-modified nanofillers could cause the polymer chains to align and, consequently, lead to reorganization of the host polymer (Figure 1c).44-45 After being surface-modified, the polymer can react with the groups of the modifiers and form an aligned layer of the polymer chains, which is perpendicular to the particle surface.41 As the polymer chains often consist of long chains, the polymer region surrounding these aligned zones would also be affected.41


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Figure 1. Interface theory model: (a) Electrical double layer model.38 Copyright 2005, IOP science; (b) Multi-core model.43 Copyright 2006, IEEE; (c) Polymer chain alignment model.45 Copyright 2011, IEEE. 3.

SURFACE MODIFIERS

The concept of surface modification was first proposed by Schmidt when he attempted to synthesize silica nanoparticles using sol-gel method.46 Generally, surface modification involves grafting suitable ligands onto the surface of the particles to prevent them from agglomeration,47-48 and make them compatible with other phases.49 There are several approaches that can be utilized for surface modification,50-53 and surface modifiers can be organic54 or inorganic.55-56 In this review, only organic surface modification is discussed. Normally, an organic surface modifier consists of two major components. One is the functional groups -OH, -NH2, -NR3z+, -COOH, -COO-, -SO3H-, -SO32-, -PO43-, etc., which help the modifier to



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anchor on the surface of the particles through hydroxyl and electrostatic bonds; the other is a soluble macromolecular chain that is suitable for dispersion in different media.57-58 So far, researchers have exploited a lot of modifiers for surface modification.59-61 In this review, modifiers (organosilanes, phosphonic acids, dopamine, and small molecule carboxylic acids) with different types of functional groups and widely used for ceramic nanoparticles are exemplified. Organic macromolecules, a major class of organic modifiers, usually have multiple functional groups, and are also discussed in the last part of this section. 3.1 Organosilanes Organosilanes are often used as coupling agents and investigation on them is the earliest. Around 1945, Union Carbide Corporation (UCC) and Dow Chemical first published some silane coupling agents,62 and then a series of silane coupling agents appeared successively.63-65 Organosilanes can bind to a plurality of functional groups in its molecules, for example, cyano,66 amino,67carboxylic acid,68 epoxy groups,69 etc., and can be easily purchased in the market. Organosilanes have been widely used for surface modification of inorganic particles (such as TiO2, SiO2, SnO2, ZrO2, Al2O3, ceramic, etc).70 The mode of surface modification by organosilanes is the chemical reaction between the modifier and the active groups on particle surfaces.71-72 The reaction includes three parts. Firstly, the organosilanes hydrolyze and form reactive silanol (Si-OH), and then the silanol groups condense with each other and produce silanol oligomers.71,

73

Secondly, the silanol

oligomers and the -OH groups from the surface of the particles can form hydrogen bonds. Finally, the Si-O-M (metal) covalent bonds will be formed after drying through the dehydration reaction (Figure 2).74


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Figure 2.Mechanism of organosilanes as surface modifier for ceramic particles.71 Copyright 2015, Wiley. Peng et al. modified AlN nanoparticles using gamma-aminopropyl triethoxysilane and found that the dispersion of the nanoparticles improved, and that the electrical and thermo-physical properties of the composites were all enhanced.75 Wu et al used an adequate amount of 1H, 1H, 2H, 2HPerfluorooctyltrichlorosilane to modify Ba0.5Sr0.5TiO3 (BST) nanoparticle surface, and found that the poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE))-based nanocomposites exhibited higher permittivity and more uniform microstructure (Figure 3).76 Zhang et al. successfully fabricated Ba0.6Sr0.4TiO3 (BST)/poly(vinylidene fluoride) (PVDF) composites, in which the surfaces of the BST fillers were modified by aminopropyl-triethoxy-silane (KH550).77 They considered that the modifier would bind to the surface of the BST particles via Si-O-M bonds, and the other end dispersing into PVDF could form hydrogen bonds with the matrix.78 Therefore, the breakdown strength was enhanced and the energy density was also improved (Figure 4). Ramajo et al. used glycidoxy-methoxy silane with different silane concentrations to modify BaTiO3 (BT) particles, and found that a low silane concentration could retain the electrical properties of the composites, while increasing the concentration of silane would introduce a greater porosity and more defects in composites.79 ACS Paragon Plus Environment


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Figure 3. SEM images of (a) purchased BST nanoparticles, (b) silane modified BST nanoparticles, and (c) the cross-section of the composite with four layers, 15 wt.% of coupling agent and 30 vol.% BST.76 (d) Schematic of the surface structure of the modified BST powders in polymer-based nanocomposites.76 Performance comparison between the pure matrix and the nanocomposites: (e) displacement-field (D-E) loops at 80MV/m, and (f) energy density as function of electric field.76 Copyright 2010, American Institute of Physics.

Figure 4. (a) XPS spectra of original BST and modified BST using KH550, (b) SEM image of 40 vol% BST/PVDF composites modified with 4 wt% of KH550, (c) breakdown strength and energy density of


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BST/PVDF composites with 40 vol% as a function of KH550 content, and (d) schematic of coupling reaction process between BST, KH550, and the matrix.77 Copyright 2016, Elsevier. Although widely used in surface modification, organosilanes have their own limitations. First, organosilanes are easily self-condensed if there is moisture in the environment, which leads to difficulties in handling and storage.80 Secondly, in surface modification, acids, bases, or buffers are needed to control the rate of hydrolysis by adjusting the pH of the solution, but they also have an uncontrollable effect on the physicochemical properties of the inorganic particles. Thirdly, the hydrolysis of the chloro- or alkoxysilanes will lead to HCl or alcohol byproducts.70 3.2 Phosphonates Since the seminal work of Ries and Cook, phosphonic acids and their phosphonate ester derivatives have become an attractive anchoring group for hydroxylated surfaces.81 Phosphonates and phosphonic acids (PAs) are usually used as surface modifier for ZrO2, TiO2, bio-ceramic particles, and indium tin oxide (ITO) electrode surface.82-86 In PA molecules, the phosphorus atom in the +5 oxidation state is tetra-coordinated,50 bonded with one oxygen (double-bond) and two hydroxyl groups (single bond). As functional groups, they can make PA molecules to be covalently bonded to a particle surface in the modes of monodentate, bidentate, or tridentate (Figure 5).50, 87 The bonds may be either bridging, that is, each acid oxygen bonded to different metal atoms; or chelating, which means multiple acid oxygen atoms bonded to the same metal atom.50 Additionally, chemisorption may also occur via electrostatic or hydrogen bonds.88 Therefore, PAs can form robust modifying layers on many different metal oxides. Generally, there is no condensation reaction between the P-OH groups; hence, under mild conditions, only monolayer can be formed on the surface of the metal oxide.89-90 ACS Paragon Plus Environment


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Figure 5 Mechanism of surface modification by phosphonic acid. Since the property of metal oxides can affect the reaction mode, the mechanism are divided into two types: (a) is for Lewis acidic metal oxides, (1)modifier and metal oxide surface, (2) a active site on the surface coordinating to the phosphoryl oxygen, and hetero-condensation with the more electrophilic phosphorus following, (3) the heterocondensation going on, and (4) final forming tridentate binding state; (b) is for poorly Lewis acidic metal oxides: (5) hetero-condensation taking place between the phosphorus and a hydroxyl group on the surface, (6) repeating the hetero-condensation, (7) forming bidentate bound state, and (8) hydrogen bond formed between phosphoryl group and surface hydroxyl.50, 87 Copyright 2016, Elsevier. In recent years, with the widespread application of nanocomposites in the electronics industry, the use of PAs in surface modification for ceramic nanoparticles is becoming increasingly broad. In 2007, Kim et al. first used pentafluorobenzyl phosphonic acid (PFBPA) to modify the BT nanoparticles. They found that the modifier formed a robust and functional organic shell on the surface of BT nanoparticles, and effectively improved the dispersion of the nanofillers in the matrix. The mBT/poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) composites exhibited a combination of high permittivity and large breakdown strength (Figure 6).91-92 Later, Chon et al. used diethyl 3-


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(trimethoxysilyl)propyl phosphonate (TMSP) as a surface modifier to improve the dispersion of BT nanoparticles. Excellent interfacial performance was achieved due to the reaction between the trimethoxysilane units from TMSP bonded to BT surface and the silane moieties of the matrix.93 On the other hand, the interface charge density also has an important effect on the dielectric properties of composites. Siddabattuni and his colleagues used PAs containing electron-poor or electron-rich groups to modify the surfaces of BT and TiO2 nanoparticles, and fabricated nanocomposites filled with modified nanoparticles. The results showed that the leakage current and the dielectric loss were all reduced, and that the breakdown strength could be improved when electropositive phenyl, an electron-withdrawing functional group, was located at the interface (Figure 7).89

Figure 6 (a) Schematic illustration of PFBPA as surface modifier for BT nanoparticles, and the techniques used to fabricate nanocomposite films, (b) 31P MAS SS-NMR spectra of PFBPA-BT (blue, cross-polarization and red, direct polarization) and free PFBPA (black, cross-polarization), and (c) the breakdown strengths of the nanocomposites with different volume fractions.91 Copyright 2007, WileyVCH. SEM images of the cross-section of (d) BT/P(VDF-HFP), and (e) PFBPA-BT/P(VDF-HFP).92 ACS Paragon Plus Environment


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Copyright 2009, American Chemical Society.

Figure 7. (a) Dielectric losses of polymer and epoxy-based composites with 5 vol % filler at lower frequencies, (b) molecular structures of organophosphate ligands and their electrostatic potential maps obtained from density functional theory (DFT) modeling (red and blue represent negative and positive charge, respectively).89 Copyright 2013, American Chemical Society. As a result of the good hydrolytic stability of the P-O-C bonds, the surface modification by the PAs or their derivatives can be performed in water at moderate temperature. However, P-O-Si bonds are easily hydrolyzed, even under neutral conditions, which will cause PAs to form labile bonds with siliconcontaining substances.89 The difference in stability between PAs and organosilanes makes them quite complementary. Certainly, PAs still have their own limitations in practical use. There is a competitive balance between chemisorption (leading to a monolayer of phosphonate species grafted onto the surface) and dissolution-precipitation (leading to the formation of metal phosphonate or phosphinate phases) in surface modification for metal oxides by PAs.94-95 3.3 Dopamine As is well known, mussels can show strong adhesion to the marine surface in the natural environment. Inspired by the composition of adhesive proteins in mussels, Phillip Messersmith and his colleagues synthesized polydopamine (PDA) by self-polymerization of dopamine (1,2-dihydroxybenzene), and ACS Paragon Plus Environment

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dopamine is a catechol compound that has a functional group of primary amine. PDA can form surfaceadherent films on many different kinds of organic and inorganic materials, including ceramics, polymers, semiconductors, oxides, and noble metals.96-98 The self-polymerization reaction of dopamine is not complicated, and the specific process is as follows. The materials to be surface-modified are immersed in an aqueous dopamine solution at pH=8.5, and form a 50 nm-thick PDA film on the substrates by auto-polymerization.99 However, due to its amorphous characteristic and insolubility in most solvents, the detailed molecular structure of the PDA has not been clarified, and the polymerization mechanism has not been established either. Despite this, the widespread use of dopamine is not affected at all. Following the publication of Messersmith's results, dopamine began to be widely used for the surface modification of ceramic nanoparticles in nanocomposites. Dopamine has been utilized to modify the surface of BT nanoparticles (BTNPs), nanofibers (BTNFs),100-101 and nanotubes (BTNTs),102 Ba0.6Sr0.4TiO3 (BST) nanofibers,103 Bi2O3-doped Ba0.3Sr0.7TiO3 (BSBT),104 BT@Al2O3 nanofibers (BT@AO NFs) (Figure 8),105 Ba(Fe0.5Ta0.5)O3 (BFT),106 and so on. After surface modification of dopamine, the dispersion of the nanofillers was enhanced, and the dielectric properties of the composites were improved.

Figure 8 (a) Schematic illustration of the preparation of BT@AO NFs by electrospinning, (b) SEM image ACS Paragon Plus Environment


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of BT@AO NFs, (c) TEM images of dopamine-coated BT@AO NFs (BT@AO-DA NFs), and (d) crosssection SEM image of BT@AO-DA NFs/PVDF film with 5.1 vol %.Comparison of (e) current density and (f) energy density between nanocomposites BT@AO NFs/PVDF and BT@AO-DA NFs/PVDF.105 Copyright 2017, American Chemical Society. In the surface modification of nanofillers, dopamine has attracted a growing attention due to its astonishing adhesion, simple ingredients, mild reaction conditions, and non-polluting properties.107 However, in most cases, dopamine needs to undergo a self-polymerization to form PDA. As a polymer, PDA will produce a greater steric hindrance on the surface of the filler, which can reduce the agglomeration of the nanofiller and, in the meantime, prevent the matrix polymer from approaching the surface of the filler. The result is the formation of a loose interface structure and the production of a weak interaction between the filler and the matrix, which limits the improvement of the dielectric properties. In comparison with other catechin polymers that have been exploited, the study of PDA as a surface modifier is still in the primary stage. 3.4 Small-Molecule Carboxylic Acids Carboxylic acids, especially long-chain aliphatic carboxylic acids, are one of the oldest organic modifiers studied systematically,108 and they can form a closely-packed and highly organized layer films on the surface of materials.81 However, the research on small-molecule carboxylic acid as modifier was relatively less, although they have their own unique advantages, such as small molecular weight, simple molecular structure, rich variety, easy access etc. Compared with the double-bonded oxygen and the two hydroxyl groups in the PA molecule, a carboxylic acid molecule contains one double-bonded oxygen and one hydroxyl group (Figure 9).109 Therefore, the surface coverage of the carboxylic acid is lower than that of phosphate and silane ACS Paragon Plus Environment

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modifiers.91 However, no study has shown that lower coverage can’t effectively improve material properties.110 Meanwhile, if necessary, the number of carboxyl groups in the modifier molecule can also be adjusted.

Figure 9. Possible interactions between the carbonyl oxygen and the surfaces of metal oxide: a monodentate binding mode (A), the double bond weaken to a carboxylate (asymmetric stretch), and form two different bonding modes (B and C), and the symmetric bonding mode of a carboxylate group (D).109 Copyright 2017, American Chemical Society. Wang’s group did a lot of work in the field of surface modification with small molecules of carboxylic acid for the nanoparticles.57, 111-112 First, they used tetrafluorophthalic acid (F4C2) to modify BT nanoparticles, and found that the nanocomposites exhibited enhanced electric breakdown strength, high energy density, and low dielectric loss. They illustrated that the modifier layer acted as a passivation shell on the filler surface and could improve its compatibility with matrix, thus promoting interfacial interaction.111 However, due to the high symmetry of the molecular structure, the dipole moment of the modifier is small, giving rise to reduced permittivity in the nanocomposites. To solve this problem, they used lower symmetrical 2,3,4,5-tetrauorobenzoic acid (F4C) as a modifier, and found that because it contained only one carboxyl group, the grafting density of the modifier was very small, but the ACS Paragon Plus Environment


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improvement of performance was quite significant.112 Afterwards, they systematically investigated the influence of the molecular structure of the modifier on the performance of the composites.57 The results showed that the types of functional groups presented on the modifier had a great influence on the material properties, especially electric properties, and the easily ionized groups will damage the breakdown strength of the nanocomposites (Figure 10).

Figure 10. (a) Schematic of surface modification of nanoparticle and composite film incorporated by modified filler, and (b) FTIR spectra of BT and the surface modified BT. (c) Permittivity and loss tangent, (d) breakdown strength, and (e) D-E loops of nanocomposite filled with BT and m-BT nanoparticles.57 Copyright 2015, American Chemical Society. 3.5 Organic Macromolecules In addition to the above modifiers, macromolecular polymers are also partly used in surface modification for nanofillers. Organic macromolecules grafting on the surface of inorganic nanoparticles can enhance the surface chemical functionality and alter the surface topology.2 A series of work have been carried out in this field due to the ever growing interest in improving the dispersion of the nanoparticles and their compatibility with polymers. ACS Paragon Plus Environment

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Corresponding to the different formation processes of the modified layers, the surface modification by organic macromolecules can be divided into two categories: (i) monomers are polymerized from active compounds (initiators or co-monomers) that are covalently bonded to the surface of the filler, called “grafting from” (ii) grafting the pre-prepared polymer chains onto the surface of the nanoparticles, called “grafting to”.32 The modification of nanofillers by “grafting from” method has been discussed in other reviews;2, 32 therefore, we will not elaborate it here. The “grafting to” method has also been widely used in recent years. For example, Yu et al. used polyvinylpyrrolidone (PVP) as a modifier to modify the surface of BT nanoparticles, and successfully prepared homogeneous nanocomposite films with increased permittivity and enhanced breakdown strength.113 In the “grafting from” method, monomers with low molecular weights and simple molecular structures can penetrate into the nanoparticle aggregates to react with the activated sites on the surface of the nanoparticles.2 However, the polymerization process is complex and the polymer molecular weight is difficult to control.114 While the “grafting to” method is simple, and the molecular weight of it is easy to control, whereas it is difficult to penetrate into the aggregates.32 In a word, both methods have their own characteristics and complement each other at the same time. Sometimes, both methods will be used simultaneously to achieve a particular effect.114 3.5 Brief Summary Permittivity and breakdown strength are two key factors that determine the energy storage density of nanocomposites. Surface modification will impose an important influence on the permittivity and the breakdown strength of these nanocomposites. Next, we make a comparison of the modified effect of the modifiers that have been widely used (Table 1). Phosphoric acids and carboxylic acids, as organic small molecular modifiers, can effectively ACS Paragon Plus Environment


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improve the breakdown strength of the nanocomposites. The effect on permittivity is determined mainly by its molecular structure and its properties. As shown in Table 1, the phosphoric acid modifiers could increase the breakdown strength of the nanocomposites by approximately 25%; however, the permittivity gets reduced at the same time. The effect of carboxylic acids on the permittivity presented diversity, but they all could contribute to the increase in the breakdown strength by about 40% or more. For dopamine, the changes in permittivity are also diversified, and the increase in breakdown strength is mostly less than 20%. The dopamine derivatives that were obtained by modifying the dopamine molecular chains can result in an increase in breakdown strength by nearly 38%, but reduce the permittivity by approximately 11%.115 Modifiers have different structures that even in the same kind of molecule, there are differences in the functional groups including type, position and quantity, and their impacts on the performance of the nanocomposites are different.57, 116 As a result, it is very difficult to give an optimal amount or an approximate range for the modifiers. In general, excessive modifiers will have a negative impact on material properties.

117

However, such phenomena are not observed when liquid crystal

molecules are used as modifiers.118 As discussed earlier on, there are still many aspects that need to be explored regarding the amount of modifiers in surface modification. In the modifier molecule, the functional groups anchored on the surface of the nanofillers usually possess a certain universal applicability, while the types of functional groups dispersed in the matrix should be adjusted according to the properties of the matrix for the purpose of compatibility. However, due to the limitation of characterization methods, the study of the interaction between the modifier and the polymer matrix is rarely, and a unified theory has not yet been formed. Although the targeted selection for the modifier is still difficult for different material systems, we still believe that with the continuous development of surface modification for nanofillers, it will be increasingly precise. ACS Paragon Plus Environment

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Table 1. Effect of Modifiers on Dielectric Properties of Nanocomposites. Volume

εr

Eb

fraction (%)

Before

After

Δεr/εr

Before

After

ΔEb/Eb

PFBPA-BT/P(VDF-HFP)91

50 %

43

37

-0.140

130

210

0.24

NPP-BT/epoxy89

5%

6.8(10k)

6.3(10k)

-0.074

310

383

0.24

PDA@BT/PVDF119

5%

22.4

21.0

-0.062

149

191

0.28

BST@PDA NPs/PVDF120

2.5%

9.7

10.2

0.052

300

350

0.17

BFT@DA/PVDF106

1%

8.6

8.3

-0.035

170

200

0.18

BT@AO-DA NFs/PVDF105

3.6%

11.2

11.8

0.054

380

420

0.11

Dopa@BTNPs/epoxy100

1.8%

9.3

10.0

0.075

90

125

0.39

Dopa@BTNFs/epoxy100

1.8%

10.8

11.4

0.056

110

202

0.84

f-DOPA@BTNW/P(VDF-HFP)115

15%

20.9

18.7

-0.105

240

330

0.38

F4CBT/PVDF112

10%

14.3

14.8

0.035

280

400

0.43

C2BT/PVDF117

23%

31.8

30.1

-0.05

230

380

0.65

F4C2BT/PVDF117

23%

31.8

23.3

-0.27

230

320

0.39

Nanocomposites

4.

CHARACTERIZATION FOR SURFACE MODIFICATION

The evaluation of the effect of surface modification is an important part of composites research. It consists mainly of two parts, namely the characterization of modified fillers and the measurement of the related performance of composites. 4.1 Characterization of Surface Modified Fillers Particle Size and Distribution



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The size and distribution of the particles can reflect whether the filler agglomeration occurs, especially the hard agglomeration, which is one of the important indicators of the surface modification effect. Typically, after surface modification, the particle size distribution curve of the fillers will become narrower.121 For example, as shown in Figure 11b and 11c, the size of the BT nanoparticles decreases after being grafted by polymers, indicating that surface modification could suppress the aggregation of nanoparticles.122 The main instruments for determining the particle size and distribution of the fillers are the sedimentation particle size analyzer, the laser particle size analyzer, and the Coulter counter. Wettability Generally, nanoparticles possess a very high surface energy and resultant small wetting contact angle in water. Surface modification with organic modifiers can reduce the surface energy, increase surface hydrophobicity, and significantly increase the wetting contact angle in water. Therefore, the contact angle is usually used to test the physical and chemical properties of the surface of the nanoparticles.123-124 It can be seen from Figure 11d and 11e that the contact angle of graphene oxide (GO) pellet is 50.6° and it increases to 109.8° after octadecylamine (ODA) functionalization, which is ascribed to the presence of long C18 hydrocarbon chains that make the GO-ODA film become more hydrophobic.125 There are many methods to measure the contact angle, such as the angle measurement method, the length measurement method, the capillary penetration rate method, and so on. The wettability of powdered materials is often measured by the capillary penetration rate method. Particle Morphology The morphology of the particles is often observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and optical microscopy, among which high-resolution and high-power electron microscopy can directly reflect the microcosmic morphology of the surface of the ACS Paragon Plus Environment

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nanoparticles100. As shown in Figure 11f, 11g, and 11h, the TEM images demonstrate that there is an obvious shell layer on the surface of BT-PDA (polydopamine modified BT nanoparticles) compared to the received BT nanoparticles. After being decorated with Ag nanoparticles, a large amount of Ag nanoparticles with an average size of 3-5 nm were uniformly embedded in the BT-PDA shells.126

Figure 11. Particle size and distribution of (a) as-received BT nanoparticles, (b) BT-g-PS2 (BT nanoparticles grafted with PS), and (c) BT-g-PMMA2 (BT nanoparticles grafted with PMMA) in toluene.122 Copyright 2014, American Chemical Society. Images of the contact angle of (d) graphene oxide (GO) pellet and (e) octadecylamine (ODA) modified GO pellet in static water.127 Copyright 2013, Elsevier. TEM images of the (f) BT, (g) PDA modified BT nanoparticles, BT-PDA, and (h) Ag nanoparticles coated BT-PDA, BT-PDA-Ag nanoparticles.126 Copyright 2015, Wiley.



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Surface Microstructure of Particles The surface microstructure of nanoparticle includes surface chemical composition, atomic valence state, atomic position, and chemical bond characteristics. There are many methods that can be utilized for studying the surface structure, and the most commonly used ones are thermal analysis, XPS and IR spectra. Thermal analysis mainly refers to thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). These methods are often combined with each other and used to characterize the surface composition of the nanoparticles and the type of interaction between the modifier and the surface of the nanoparticles.92, 128 Generally, much information of surface structure can be obtained by comparing the spectra of the filler before and after modifying. In Figure 12b, the TEM image shows that the surface of the Ba(Zr0.3Ti0.7)O3 nanofibers (BZT NF) is uniformly coated with PVP molecules having a thickness of about 8 nm. In Figure 12c, the FTIR spectra show that the modified BZT NF possesses more surface oxygen functional groups compared to the pristine BZT NF,129 which indicates that there are modifiers on the surface of the BZT NF. On the other hand, the diﬀerences in the TGA curves before and after the modification of BZT NF further evidence that the PVP has been grafted onto the surfaces of the fillers (Figure 12d). In the TGA curve, the weight loss under 100 °C is ascribed to the dehydration of the fillers. From 200 to 1000°C there is a dramatic loss caused by thermal decomposition of PVP. Meanwhile, in the XPS spectra of BZT NF (Figure 12e), the peak of N1s can hardly be found. However, after modification with PVP, strong signals of N1s appear at 399.0 eV, which corresponds to free -NH2 of PVP.129 All these methods can provide structural information of the particle surface and help evaluate the quality of the interfacial bond.


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Figure 12. (a) Schematic diagram of surface modification for BZT NF, (b) the HRTEM image of PVP surface layer. (c) FTIR spectra of PVP, untreated BZT NF, and modified BZT NF, (d) TGA curves and (e) XPS spectra before and after modification of BZT NF.129 Copyright 2016, Springer Nature. Coverage Level of Particle Surface (grafting density) Grafting density means the number of chains per square nanometer on the surface of the nanoparticles.117 It has a significant impact on the performance of nanocomposites, especially electrical properties. Higher modifier grafting density can result in decreased permittivity of composites.57 Grafting density is calculated by the following Eq.,130-131 wm  filler   ) 100  w filler   (100  w m  filler   106 (  mol / m 2 )   MS 100    

(1)

Where wm-filler is the weight loss of the filler after surface modification, M is the molecular weight of the modifier, wfiller and S represent the weight loss and the specific surface area of the filler before modifying, respectively.57 4.2 Characterization of Composites



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In this section, we will mainly discuss the measurement methods for the physical properties of the composites filled with modified fillers. Changes in physical properties can also help to evaluate the effect of surface modification on the performance of the composites. Dispersion of Nanoparticles in Matrix Many methods can be used to characterize the distribution of nanoparticles in the polymer matrix, but they are generally classified into two broad categories: (a) direct analysis techniques (directly yielding a two-dimensional or three-dimensional image of the filler and the matrix phases); and (b) indirect analysis techniques (indirectly speculating the distribution of fillers by the specific physical properties of the composites).132 Herein, we will mainly discuss the first category. In polymer-based composites, the dispersion of the nanoparticles can be acquired by surface and cross-sectional micro-morphology of the composites. SEM and TEM methods can offer very high resolution micro-morphology images and maps of elements present in composites. Both of them visually reflect the state of the existence of the filler phase in the matrix. However, both techniques suffer from a common drawback; that is, the analysis results are highly localized.133 Electrical Properties Electrical properties are one of the most important physical properties for energy storage materials. In nanocomposites, the dispersibility of the fillers in the matrix and their compatibility with the matrix both impose significant impacts on the electrical properties of the composites.134 As shown in Figure 13, after surface modification of the BT NWs by fluoro-polydopamine (f-DOPA), the nanocomposites exhibit improved electrical properties, including increased permittivity, enhancement in breakdown strength, suppression of leakage current, and improved electrical resistivity.115


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Figure 13. (a) Schematic of the processes for preparing f-DOPA@BT NWs, and a mussel picture. (b, c) TEM images of f-DOPA@BT NWs. (d) SEM images of the cross-sectional of P(VDF-HFP)-based nanocomposite films with f-DOPA@BT NWs volume fraction of 15%. Electrical properties of the nanocomposites, including (e) leakage current density, (f) electrical resistivity, (g) permittivity and dielectric loss as a function of frequency, and (h) breakdown strength.115 Copyright 2017, American Chemical Society. Thermal Properties Heat dissipation has always been a major challenge in the development of high-power electronic devices and integrated circuits.135 Electrically insulating but thermally conductive materials are attracting more



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and more attention on account of their widespread applications in electrical equipment and electronic devices.136 It has been evidenced that the surface modification of fillers is important for the thermal conductivity of composites.75, 137-139 In Figure 14, the silicon carbide (SiC) nanoparticles were coated with PS and formed p-SiC nanoparticles, and the composites filled with p-SiC exhibited better thermal conductivity.140

Figure14. (a) Illustration of SiC and p-SiC nanoparticles dispersing in PS/PVDF blends, and (b) the thermal conductivity of the PS/PVDF composites as a function of filler volume fraction.140 Copyright 2013, American Chemical Society. Mechanical Properties In dielectric composites, stress-strain performance is rather important for achieving high breakdown strength. The stress-strain curve is usually measured by uniaxial tension. Generally, the addition of nanoparticles to the polymer matrix will inhibit the crystallization process of the polymer chains, causing the composite to rupture at a higher strain.141 If the surface of the nanoparticles is modified by organic modifiers, the stress-strain behaviors of the composites will also change due to the bridging effect of the modifiers between the filler and the matrix.142-143 As indicated in Figure 15, the elongation of cPVDF/PDA@BT (composite with cross-linked P(VDF-CTFE-DB) and poly(dopamine) modified BT)


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remarkably improved compared to those of c-PVDF/BT (Figure 15c and 15d). The crosslinking of the PVDF matrix results in a continuously increased elongation in the composites, where the surface modification of the BT particles also contributes to the improvement in the elongation.119

Figure 15. (a) Schematic diagram of modifying process for BT particles and preparation of composite films. (b) Pictures of tension test of composite film filled with PDA@BT. Stress-strain curves of composites with different particle loadings: (c) c-PVDF/BTs, and (d) c-PVDF/[email protected] Copyright 2017, American Chemical Society. 4.3 Brief Summary The surface modification of the fillers has a significant impact on the structure and performance of composites. At present, the research on the mechanism of action of the organic modifiers at the interface of composites focuses mainly on the characterization of modified particles; that is, the interaction between modifiers and nanoparticles, owing to the fact that there are many characterization methods applicable to make in-depth studies, yet more studies are still needed for clarifying how the modifier layer and the matrix interact with each other. The current available characterization means are difficult to isolate the interfacial section from composites, thus there are many difficulties in the characterization ACS Paragon Plus Environment


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of the interface structure and the physical properties are the only indirect evidences for inferring the approximate interface structure of the composites. With the development of atomic-scale electron microscopy, it can provide new characterization methods for the interface structure. The methods of calculation and simulation can also easily obtain data that is difficult or impossible to obtain through experiments. 5.

OUTLOOK

The depletion of fossil fuels and the increasingly severe environmental problems have forced human beings to seek changes in the energy structure. Renewable energy such as wind energy, solar energy and tidal energy, has become the main candidates for replacing fossil fuels. Nowadays, the main problem that exists in renewable energy is how to realize an effective conversion and storage. The main application of dielectric nanocomposites is the dielectric of capacitors. However, in nanocomposites, agglomeration of nanofillers and phase separation between the two constituent phases are the main factors affecting the energy storage density and discharge efficiency. Therefore, more efforts need to be devoted to improve the energy conversion efficiency of the dielectric composites to reach the benchmark commercial BOPP level and make them applicable for capacitive devices. As the most effective way to solve this problem, surface modification has continued to develop with the development of dielectric materials. In this review, an overview of this field is provided by including the most recent progress and some typical examples. Silane is the earliest used and most studied modifier, but its hydrolytic stability has limited its development to some extent. The environmentally unfriendly nature of phosphorus in phosphonates is also a weakness that cannot be overcome. Small molecule carboxylic acids, as well as catechols, represented by dopamine and polydopamine, open doors for future direction in the development of surface modifiers. Although the application of surface modification methods has been extensive, there ACS Paragon Plus Environment

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are still many fields that require further research. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The author declares no competing financial interest. ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (No. 61471290 and 61631166004) National 973 projects of China (No. 2015CB654603). REFERENCE 1.

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TOC


Dielectric Nanomaterials for Power Energy Storage: Surface

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