Controlled Grafting of Colloidal Nanoparticles on Graphene through

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Functional Nanostructured Materials (including low-D carbon)

Controlled grafting of colloidal nanoparticles on graphene through tailored electrostatic interaction Seongheon Baek, Jinu Kim, Han Kim, Sangmin Park, Hyeong Woo Ban, Da Hwi Gu, Hyewon Jeong, Fredrick Kim, Joonsik Lee, Byung Mun Jung, Yong-Ho Choa, Ki Hyeon Kim, and Jae Sung Son ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01519 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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ACS Applied Materials & Interfaces

Controlled grafting of colloidal nanoparticles on graphene through tailored electrostatic interaction Seongheon Baek,† Jinu Kim,‡ Han Kim,§ Sangmin Park,† Hyeong Woo Ban,† Da Hwi Gu,† Hyewon Jeong,† Fredrick Kim,† Joonsik Lee,∥ Byung Mun Jung,∥ Yong-Ho Choa,*,§ Ki Hyeon Kim,*,‡ and Jae Sung Son*,†

†School

of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea,

‡ Department

of Physics, Yeungnam University, Gyeongsan 38541, Republic of Korea

§Department

of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, Republic of Korea,

∥Functional

Composites Department, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea.

ACS Paragon Plus Environment

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KEYWORDS: nanoparticles, exfoliated graphene, composite, electrostatic interaction, electromagnetic interference shielding


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ABSTRACT

Nanoparticle/graphene hybrid composites have been of great interest in various disciplines due to their unique synergistic physicochemical properties. In this study, we report a facile and generalized synthesis method for preparing nanoparticle/exfoliated graphene (EG) composites by tailored electrostatic interactions. EG was synthesized by an electrochemical method, which produced selectively oxidized graphene sheets at the edges and grain boundaries. These EG sheets were further conjugated with polyethyleneimine to provide positive charges at the edges. The primary organic ligands of the colloidal nanoparticles were exchanged with Cl- or MoS42- anions, generating negatively charged colloidal nanoparticles in polar solvents. By simple electrostatic interactions between the EG and nanoparticles in a solution, nanoparticles were controllably assembled at the edges of the EG. Furthermore, the generality of this process was verified for a wide range of nanoparticles, such as semiconductors, metals and magnets, on the EG. As a model application, designed composites with size-controlled FeCo nanoparticles/EG were utilized as electromagnetic interference countermeasure


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materials that showed a size-dependent shift of the frequency ranges in the electromagnetic absorption properties. The current generalized process will offer great potential for the large-scale production of well-designed graphene nanocomposites for electronic and energy applications.


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1. Introduction Graphene, which is a two-dimensional monoatomic layer carbon material, has attracted tremendous interest in various research areas due to its various unique and outstanding electrical, mechanical, and thermal properties,1-2 which are suitable for a diverse range of applications such as in electronic,3-4 energy,5-7 and biomedical8-9 fields. Currently, rather than using graphene alone, researchers have devoted significant efforts to integrate functional nanostructures onto graphene, because various unexpected synergistic physicochemical properties can be obtained in these hybrid composites,10-13 that the constituent materials do not possess on their own. For example, graphene has actively been utilized as a support for nanoparticle catalysts, as it plays roles of efficient electron shuttle as well as mechanical support with extremely high surface-to-volume ratio.14-15 Furthermore, graphene-magnetic nanoparticle composites are important candidates for electromagnetic interference (EMI) shielding and absorption materials due to the combination of electrical conduction from graphene and magnetic loss from the nanoparticles.16-20 In addition, graphene exhibits various desired properties for industrial use in EMI shielding materials due to its controlled anti-corrosion capability, ease of


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processing, low-cost, and lightweight.21-22 So far, various synthetic strategies such as the in-situ formation of nanoparticles or the ex-situ decoration of nanoparticles on graphene have been suggested to prepare graphene/nanoparticle composites.10-20 However, the selective grafting of nanoparticles for the specific sites of graphene has rarely been reported due to the difficulty in controlling their interaction. In general, mechanical exfoliation and epitaxial growth with chemical vapor deposition (CVD) are the processes for the synthesis of high-quality graphene. However, these processes require harsh synthetic conditions, a sacrificial metal, and a post-transferring process, which hinders the large-scale production of graphene.1, 23-25 Another prominent route is the chemical exfoliation of graphite by the Hummers method to generate watersoluble graphene oxide (GO) sheets, which allows for large-scale production of graphene and excellent processability for obtaining homogeneous composites with other materials. However, the oxidation of graphene leads to significant degradation of its intrinsic electrical properties, requiring a post-reduction process to produce reduced GO (rGO) to partially recover the primary properties of graphene. In this reduction process, rGO sheets tend to aggregate due to layer-to-layer attraction, which significantly deteriorates the


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solution processability of rGO.4, 26-28 Recently, the electrochemical exfoliation process has been of interest because this enable for a simple and high-yield production of high-quality graphene in solution.29-33 In this process, an exfoliated graphene (EG) sheet is selectively oxidized at its edges and grain boundaries, while the basal plane is less damaged by oxidation, conserving the intrinsic properties of graphene. Parvez et al.34 reported that graphene sheets synthesized by the electrochemical exfoliation process exhibited hole mobilities of 310 cm2 V-1 S-1, which is superior to those obtained from typical rGO sheets.34 Colloidal synthesis of nanoparticles has been regarded as an unique chemical route to synthesize size-, shape-, and composition-controlled nanoparticles of various materials such as semiconductors, metals, and magnetic materials.35-38 These nanoparticles have extensively been used for a variety of applications due to their size- and shape-dependent functional properties.39 Moreover, the surface chemistry of nanoparticles is well established and various chemical routes to modify their surfaces have been developed for target applications.40-42 For example, the modification of nanoparticle with inorganic ligands of anions (S2-, Cl-, MoS42-, etc.) can create particles with negatively charged surfaces, composed of all-inorganic phases, and with complete solubility in polar solvents


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without aggregation.43-46 This chemical surface treatment of nanoparticles provides a new method for studying the interactions of nanoparticles with a wide range of species, such as molecules and nano/micro-structures, and can be used to form homogeneously mixed composites with graphene through tailored interactions. Here, we report the generalized synthesis of homogeneous composites of nanoparticles/EG sheets through controlled electrostatic interactions. We introduced Cland MoS42- anions on the surfaces of metals, semiconductors, and magnetic nanoparticles by a two-phase ligand exchange reaction. Negatively charged nanoparticles were selectively assembled at the positively charged edges of EG treated with polyethyleneimine (PEI) without aggregation.47 As a model application, we synthesized size-controlled FeCo nanoparticle/EG composites and explored their sizedependent EM wave absorption properties, since FeCo nanoparticles have high saturation magnetization (MS) values and permeability compared to ferrites, such as Fe3O4, which are the conventional EM absorption materials used in composites.18-19, 48 These composites showed FeCo size-dependent EM absorption and shielding behaviors.


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The current approach provides a method to design nanoparticle-graphene composites through tailored interactions.

2. Experimental Section Materials.

Iron

(III)

acetylacetonate

(Fe(acac)3,

99.9%,

Aldrich),

cobalt

(II)

acetylacetonate (Co(acac)2, 99.9%, Aldrich), 1,2-hexadecanediol (90%, Aldrich), oleic acid (90%, Aldrich), oleylamine (70%, Aldrich), 1-octadecene (ODE, 90%,

Aldrich),

ammonium chloride (NH4Cl, 99.5%, Samchun chemical), ammonium tetrathiomolybdate ((NH4)2MoS4, 99.97%, Aldrich), polyethyleneimine (PEI, average Mw ~25,000 by LS, average

Mn~10,000

by

GPC,

branched),

cadmium

oxide

(99.99%,

Aldrich),

trioctylphosphine oxide (99%, Aldrich), octadecylphosphonic acid (97%, Aldrich), trioctylphosphine (90%, Aldrich), selenium (powder, 99.99%, Aldrich), cadmium chloride (99.99%, Aldrich), sulfur (99.99%, Aldrich), octylamine (99%, Aldrich), lead (II) chloride (reagent grade 99%, Alfa aesar), gold (III) chloride trihydrate (HAuCl4·3H2O, 99.9%, Aldrich), 1,2,3,4-tetrahydronaphthalene (tetralin, 97%, Alfa aesar), borane tert-butylamine complex (97%, Aldrich), platinum (II) acetylacetonate (97%, Aldrich), dioctyl ether (99%,


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Aldrich), bismuth neodecanoate (Aldrich), 1-dodecanethol (98%, Aldrich), graphite foil (Alfa aesar), ammonium sulfate ((NH4)2SO4, Aldrich), and exfoliated graphene (EG) was supplied from Mexplorer Co., Ltd. All listed materials were used as purchased.

Synthesis of organics-capped nanoparticles

FeCo nanoparticles. All syntheses for nanoparticles were conducted using a conventional schlenk line and N2-filled glovebox. The synthesis procedure of FeCo nanoparticles was slightly modified the developed method by Chaubey et al.48 Typically, for the synthesis of 20.6 nm-sized FeCo nanoparticles, 0.265 g (0.75 mmol) of Fe(acac)3, 0.129 g (0.5 mmol) of Co(acac)2, 0.387 g (1.5 mmol) of 1,2-hexadecanediol, 6.4 ml (20 mmol) of oleic acid, and 3.4 ml (10 mmol) of oleylamine were mixed in 50 ml flask at room temperature. The degassing process of the reaction mixture was proceeded at room temperature for 20 min. After the degassing process, a mixed gas (H2 7% + Ar 93%) was directly flowed through the reaction mixture. The mixture was heated to 100 ℃ and maintained for 10 min. The mixture was further heated to 300 oC and refluxed for 2 h. And then, the mixture was cooled down to room temperature by flowing the mixed gas and removing the heating


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mantle. The synthesized FeCo nanoparticles were collected from the magnetic bar. To obtain the precipitated FeCo nanoparticles, the non-solvent of ethanol (30ml) was added into the mixture and it was centrifuged (5 min, 7000 rpm). The washing process was repeated by three times with the mixed solvents of hexane and ethanol (1:3 in the volume ratio). Finally, FeCo nanoparticles were dispersed in hexane. For the synthesis of 12.9 nm-sized FeCo nanoparticles, 0.265 g of Fe(acac)3, 0.129 g of Co(acac)2, 0.387 g of 1,2-hexadecanediol, 4.8 ml of oleic acid, and 5.1 ml of oleylamine were mixed in 50 ml flask. The subsequent procedure was identical to those for 20.6 nmsized FeCo nanoparticles synthesis method. For the synthesis of 5.0 nm-sized FeCo nanoparticles, 0.265 g of Fe(acac)3, 0.129 g of Co(acac)2, 0.387 g of 1,2-hexadecanediol, 1.07 ml of oleic acid, 2.27 ml of oleylamine, and 10 ml of 1-octadecene were mixed in 50 ml flask. The subsequent procedure was identical to those for 20.6 nm-sized FeCo nanoparticles synthesis method. For the washing process, n-butanol was used as a non-solvent instead of ethanol.

CdSe nanoparticles. The synthesis procedure of 2.5 nm- and 5.5 nm-sized CdSe nanoparticles was same as the method reported by Ban et al.44


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1.2 nm-thick CdS nanoplates. The synthesis method of CdS nanoplates with 1.2 nm thickness was identical to the recipe in literature by Son et al.49

6.7 nm-sized PbS nanoparticles. The synthesis process of 6.7 nm-sized PbS nanoparticles was modified the method reported by Weidman et al.50 A heating mantle was used for preparing a sulfur precursor solution instead of an oil bath.

6.0 nm-sized Au nanoparticles. The synthesis procedure of 6.0 nm-sized Au nanoparticles was slightly modified the developed method by Zhu et al.51 Under N2 flow condition, a HAuCl4·3H2O (0.2 g) was added into 20 ml of the mixture of tetralin and oleylamine (1:1 in the volume ratio).44

2.5 nm-sized FePt nanoparticles. The synthesis method of 2.5 nm-sized FePt nanoparticles was modified based on the procedure in the literature by Liu et al.52 Instead of octyl ether, dioctyl ether was used as a solvent.

11.0 nm-sized Bi nanoparticles. The synthesis method of 11.0 nm-sized Bi nanoparticles was same as the recipe reported by Gu et al.53

Synthesis of exfoliated graphene (EG).


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In the exfoliation procedure, as an anode and cathode, graphite and Pt foil were used. The electrochemical exfoliation was operated at working voltage 10 V in (NH4)2SO4 (0.1M) electrolyte solution. After exfoliation, as-synthesized EG was purified using the distilled water (DI water) and further filtered under vacuum to remove the residual electrolyte. This filtered wet powder of the EG was dispersed in DI water and sonicated for 10 min, subsequently being centrifuged (10 min, 4000 rpm). After then, the supernatant of the solution was collected to be used as the EG solution, which typically have a concentration of 20 g/L.33-34

Synthesis of nanoparticles/exfoliated graphene (EG) composites.

Synthesis of EG conjugated with polyethyleneimine (PEI). PEI-conjugated EG was obtained by same experimental procedure reported by Zhang et al.47 except using EG instead of graphene oxide. 5 mg of EG dissolved in 50 ml of DI water and 1 vol% of polyethyleneimine (PEI) dissolved in 50 ml of DI water were mixed in a 200 ml round flask. It was heated up to 60 ℃ for 7 min and kept at this temperature for 12 h. After then, the mixture was cooled to room temperature slowly. PEI-conjugated EG was precipitated by


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centrifugation (20 min, 7830 rpm). The precipitate of the PEI-conjugated EG was dispersed in 10 ml of polar solvent such as DI water, N-Methyl formamide (NMF) (0.5 mg/ml).

Ligand exchange of nanoparticles with Cl- or MoS42- anions. Typically, 90 mg of NH4Cl or 90 mg of (NH4)2MoS4 was dissolved in 3 ml of polar solvent such as DI water, and NMF. 2ml of nanoparticles solutions in hexane (20mg/ml) was added into this inorganic ligand stock solution. This mixture was stirred vigorously using a magnetic bar until nanoparticles in the upper organic phase were completely transferred to the lower polar phase. The upper organic layer was discarded to remove organic residues. Then, nanoparticles in the lower polar phase were obtained by addition of 9 ml of isopropanol (IPA) and centrifugation (5 min, 7000 rpm). This washing process was repeated by three times with DI water or NMF and IPA (1:3 in the volume ratio) to remove the unreacted anions. The final precipitate was re-dispersed in 3 ml of DI water or NMF.

Synthesis of nanoparticles/EG composites. For preparation of nanoparticles/EG composites, inorganic ligand capped nanoparticles and PEI-conjugated EG were mixed into a vial (8:1 in the weight ratio). The mixture was stirred using a magnetic bar for 24 h.


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Finally, nanoparticles/EG composite was obtained by addition of large excess volume of IPA and centrifugation (5 min, 7000 rpm). The final product was re-dispersed in polar solvent such as DI water or NMF.

Materials characterization. The

structure

and

morphology

characterization

of

nanoparticles,

EG,

and

nanoparticle/EG composites was conducted using a transmission electron microscope (TEM, JEM-2100, JEOL) operated at 200 kV. X-ray diffraction (XRD) patterns of FeCo nanoparticles and FeCo nanoparticles/EG composites were obtained by using a high power XRD (D/Max2500V/PC, Rigaku) with a Cu Kα x-ray sources, which operated at 40 kV and 200 mA. The atomic force microscopy (AFM) images of EG were obtained by using an AFM instrument (XE-100, Park system). Raman spectra of EG were measured using Raman Microscopy (UniRAM, Uni-nanotech.) at room temperature by excitation wavelength of 532 nm. Fourier transform infrared (FT-IR) spectra of nanoparticles was collected using a FT-IR spectrometer (Varian 670). The UV-absorption spectra of EG and PEI-conjugated EG were obtained by a Cary 5000 UV-vis-NIR spectrophotometer at room


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temperature. ζ-potential data of inorganics capped nanoparticles and PEI-conjugated EG were obtained using a Malvern Zetasizer Nano-ZS. Thickness and sheet resistance of the graphene film were measured by focused ion beam scanning electron microscopy (FIB-SEM, Quanta 3d FEG, FEI) and 4-point probe measurement (CMT-SR1000N, Advanced instrument technology).

Electromagnetic wave absorption properties measurements.

The magnetic properties of FeCo nanoparticles and FeCo/EG composites were measured by using magnetic property measurement system (MPMS, Quantum Design, Inc.). To evaluate the EM properties such as complex permittivity, complex permeability, reflection loss (RL) and shielding effectiveness (SE), the FeCo nanoparticles and FeCo/EG composites were blended with 50 wt.% of paraffin and then pressed into the toroidal shape with 2mm thickness for dimension of coaxial waveguide (outer diameter of 7.0 mm, inner diameter of 3.04 mm). To measure the complex relative permeability and


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permittivity, vector network analyzer (VNA, 8510C, HP) with a coaxial waveguide was used in frequency range of 0.1 GHz to 18 GHz.

3. Results and discussion Figure 1 illustrates the synthesis processes of nanoparticles/EG composites where all processes were conducted in solution. Negatively charged colloidal nanoparticles of semiconductors, metals, and magnets were assembled on positively charged EG sheets in solution by electrostatic attraction, which led to edge-selective grafting of nanoparticles on graphene. This assembly selectivity of nanoparticles is attributed to the unique structural characteristics of the EG, which was selectively oxidized at its edges and grain boundaries, rather than at the basal plane. The EG was synthesized from a graphite foil by the electrochemical exfoliation method and it was fully dispersed in DI water. The mechanism can be understood by three step reactions.33-34 First, the nucleophilic attack of OH- ions formed by applying bias voltage in electrolyte solution preferentially occurs at the edges and grain boundaries of graphene.


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Second, such an oxidation of graphite leads to depolarization and expansion of the graphitic layers, enabling the intercalation of SO42− ions into graphitic layers. Also, at this stage, H2O may co-intercalate with the sulfate ions. Third, the gas revolution due to the reduction of anion and oxidation of water lead to the exfoliation of graphene. Accordingly, the edges or grain boundaries on graphene were the major reaction sites in this process, leading to the higher oxidation degree than the basal plane. The structural characteristics of the EG were characterized by transmission electron microscopy (TEM), electron diffraction (ED), and atomic force microscopy (AFM) analyses. The AFM image and the size histogram of the synthesized EG (Figure S1a-S1b) reveal that the EG generally has lateral sizes in the 1-5 μm range and an estimated thickness of 3.2 nm (Figure 2a-2b), indicating that a few layers of graphene are present. The estimated average size of EG flakes is 2.065 μm and the size deviation is 0.15 μm. The low-magnification TEM image (Figure S1c) shows a graphene layer with a lateral size of several micrometers, agreeing with those estimated by the AFM analysis. Moreover, the edge shown in the highmagnification TEM image of the folded EG (Figure S1d) shows five or six graphene walls, indicating that the EG is consisted of multiple layers of graphene sheet. The selected area


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ED pattern at the inner basal plane of the EG (Figure S1e) reveals the single-crystalline nature of graphene, as evidenced by the 6-fold symmetry of the hexagonal structure in the ED pattern, suggesting that the inner basal plane of the EG was well preserved during the electrochemical exfoliation process. To further characterize the defects at the edges of the EG, we performed Raman spectroscopy analysis on the edges and basal plane of the EG separately. The spectra of the edge and basal plane of the EG (Figure 2c-2d) show three typical peaks of few-layer graphene, which can be indexed to D, G, and 2D bands at 1354, 1593, and 2700 cm-1, respectively. Generally, the defects in graphene are characterized by the ratio of the D and G peaks intensity (ID/IG, where I is the peak intensity), since the D peak come from the breathing mode of the sp2 carbon atoms arising from the structural defects and partially disordered structures, while the G peak is attributed to the E2g vibrational mode of the sp2 carbon atoms, being indicative of the degree of graphitization. The estimated ID/IG ratio from the Raman spectra obtained from the edges and basal planes were 1.10 and 0.9, respectively. The reported ID/IG of the chemically or thermally rGO range from ~1.1 to ~1.5, and thus our results demonstrate that the edges of the EG synthesized in this study were more significantly oxidized than


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the basal plane.33-34 Moreover, the ID/IG value of 0.9 in the basal plane was much lower than those of the reported rGO, indicating the quality of the EG. This structural quality of the EG was clearly reflected in its electrical properties. The estimated electrical conductivity of the 500 nm-thick EG film (Figure S2) determined using a 4-point probe measurement was as high as ~23000 S/m. To explore the assembly of nanoparticles with the EG, we synthesized a wide range of different nanoparticles of semiconductor materials, including CdSe, CdS, and PbS, metals, including Au and Bi, and magnetic materials, including FePt and FeCo (Figure S3). All as-synthesized nanoparticles were passivated with long-chain organic ligands, such as tri-n-octyl phosphine oxide, oleic acid, oleylamine, etc. To replace the organic ligands on the nanoparticles surface with inorganic anions, we conducted a typical twophase ligand exchange process using a non-polar upper phase containing organiccapped nanoparticles and a polar lower phase containing inorganic anions of Cl- or MoS42(Figure S4). Our group reported the use of MoS42- and WS42- thiometallates as ligands for various nanoparticles of semiconductor, metal, and oxide materials.44 As expected, all the nanoparticles in the upper phase were successfully transferred to the lower phase by


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simple stirring at room temperature, indicating that the surfaces of nanoparticles became hydrophilic due to the anions. The TEM images (Figure 3a-3d) of the inorganic ligandcapped nanoparticles show that the structural characteristics of the nanoparticles were well preserved during the ligand exchange reaction. The FT-IR spectra of Cl- and MoS42-capped nanoparticles (Figure S5) show the disappearance of the absorption band at 2700~3000 cm-1, which is attributed to C-H stretching modes of organic ligands, suggesting that the complete replacement of organic ligands with inorganic anions occurred. Further, the surface states of the inorganic ligand-capped nanoparticles were characterized by zeta (ζ) potential measurements. Regardless of the types of nanoparticles and anionic ligands, all the nanoparticles after ligand exchange exhibited negative ζ potentials, indicating the attachment of Cl- or MoS42- on the surfaces of the nanoparticles (Figure 3e). For the controlled grafting of nanoparticles on the EG, the cationic polymer of polyethyleneimine (PEI) was conjugated at the EG edges by the reaction of the carboxylic groups of the EG with amino groups of PEI. Since it is widely accepted that GO can be reduced by reacting with amino groups, the PEI-conjugated EG was characterized by UV-


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Vis, Raman spectroscopy, and ζ-potential measurement. The UV-Vis spectra (Figure S6a) show a peak shift from 265 to 271 nm after the PEI conjugation, indicating longer electronic conjugation by the reduction of EG. Moreover, the slight shifts of the D and G bands observed in the Raman spectrum (Figure S6b) of the EG after the PEI conjugation further demonstrates the reduction of the EG.47 TEM analysis (Figure S6c) indicated that the 2D structure of the EG was well preserved during the reaction with PEI. More importantly, the positive ζ-potential confirms the PEI conjugation on the EG (Figure 3e). Negatively charged nanoparticles capped with inorganic anions were simply mixed with the positively charged PEI-conjugated EGs in solution, leading the selective grafting of nanoparticles at the edges of EG sheets, as shown in the TEM images (Figure 4a-4f). This selectivity is attributed to the higher density of positive charges arising from PEI at the edges of the EG and is consistent with the results obtained from Raman analysis of the EG. A small portion of nanoparticles located on the basal plane of the EG might be due to PEI conjugated with carboxylic groups at the grain boundaries of the EG. This edge-selective functionalization of nanoparticles on the EG is advantageous for conserving the intrinsic properties of high-quality graphene by minimizing the interactions


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between the nanoparticles and graphitized carbon atoms at the basal planes. Zhang, et

al.

47

reported a similar approach of Fe3O4 nanoparticles assembly on graphene oxide

(GO). In this paper, the Fe3O4 conjugated with meso-2,3-dimercaposuccinnic acid (DMSA) are assembled at the PEI-grafted GO via 1-ethyl-3-(3-dimethyaminopropyl) carbodiimide (EDC)-N-hydroxysuccinnimide (NHS) coupling reaction, forming the covalent amide bond between carboxylic groups of DMSA molecules and amine groups of PEI. In contrast, we reported that, Cl-- or MoS42--capped nanoparticles were physically assembled to positively charged PEI-grafted EG in the electrostatic manner, which did not require any types of coupling agents. More importantly, the generality of the current method to produce nanoparticle/EG composites were clearly demonstrated for a wide range of colloidal nanoparticles such as semiconductors, metals, and magnets. As the usage of electronic devices, such as smartphones, networks, and communication devices, has rapidly increased, EMI and pollution have emerged as significant worldwide issues. Graphene-magnetic nanoparticles composites are good candidate materials for providing the desired properties for EMI shielding, e.g., metallic electrical conductivities, extremely high surface-to-volume ratios, lightweight, and anti-


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corrosion properties at high temperatures.16-20 Moreover, the EMI shielding can provide an appropriate model application to demonstrate the applicability of our FeCo nanoparticle/EG composites because the EMI shielding requires both of the dielectric loss from high electrical properties of graphene and the magnetic loss of FeCo nanoparticles. Accordingly, we synthesized size-controlled FeCo nanoparticles/EG composites and investigated their size-dependent EMI shielding properties at room temperature as a model application. Size-controlled FeCo nanoparticles were synthesized by the reduction of Fe(acac)3 and Co(acac)2 through H2/Ar gas bubbling in the presence of co-surfactants of oleic acid and oleylamine. In this chemical reaction, we identified the critical reaction parameters that affect the particle sizes, the concentration of surfactants and the molar ratio of oleic acid/oleylamine. Generally, the size of the FeCo nanoparticles increased with the increase of the concentration of surfactants and the molar ratio of oleic acid/oleylamine. The detailed synthesis conditions are described in Table S1 in the Supporting Information. The TEM images of various sized FeCo nanoparticles (Figure 5a-5c and Figure S7) show spherically shaped nanoparticles with high size uniformity (Figure S8). The XRD patterns (Figure 5d) of 5.0, 12.9, and 20.6 nm-nanoparticles


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correspond to the cubic FeCo phase (JCPDS 44-1433), and peaks related to ferrites were not detected. For the preparation of the FeCo nanoparticles/EG composites, we chose the sizes of 5.0, 12.9, and 20.6 nm for the FeCo nanoparticles to examine the wide range of size-dependence on the magnetic property. The TEM images of FeCo nanoparticles/EG

composites

(Figure

6a-6c)

show

selectively

attached

FeCo

nanoparticles at the edges of EG sheets. The XRD pattern of FeCo nanoparticles/EG composite (Figure 6d) shows an identical pattern to that of the as-synthesized organiccapped FeCo nanoparticles, demonstrating that the ligand exchange and composite preparation processes did not have an influence on the structural characteristics of the nanoparticles. The magnetization curves of the composites with corresponding FeCo nanoparticles also show typical hysteresis loops of magnetic nanoparticles (Figure 6e). The magnetization (M) values under an external applied magnetic field of 5 T increased from 56 to 114 emu/g with increasing the particle sizes, as expected. Furthermore, all nanoparticle composites exhibited ferromagnetic properties with small coercive fields of 1-30 mT. Since 5 nm-sized FeCo nanoparticles showed superparamagnetic behavior in the hysteresis loop (Figure S9), the small coercive field less than 1 mT, observed in 5.0


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nm-sized FeCo/EG composite, might be attributed to a partial agglomeration of nanoparticles in the composite. To characterize the EM properties, a coaxial waveguide was employed to determine the complex permittivity, permeability, reflection loss (RL), and shielding effectiveness (SE) of FeCo nanoparticles and FeCo/EG composites. Moreover, we measured the properties of pure EGs and commercial GOs as control samples. The RL of the sample was estimated from its relative complex permittivity and permeability at the given sample thickness,54 based on the following equations: 𝑍𝑖𝑛 = 𝑍0(𝜇𝑟/𝜀𝑟)1/2tanh [(𝑗2𝜋𝑓𝑑/𝑐)(𝜇𝑟𝜀𝑟)1/2 𝑍0 =

𝜇0

(1) (2)

𝜀0

|

𝑅𝐿(𝑑𝐵) = 20𝑙𝑜𝑔

𝑍𝑖𝑛 + 𝑍0

|

(3)

𝑍𝑖𝑛 ― 𝑍0

where Zin, Z0, 0, 0, r (= - j), r (= - j), f, d, and c are the input impedance of specimen, the characteristic impedance of free space, permittivity (free space), permeability (free space), complex permittivity (sample), complex permeability (sample),


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the frequency of the EM waves, thickness of the specimen, and the velocity of the EM wave, respectively. The real part of permittivity and permeability of the FeCo nanoparticles are around 3 and 1, respectively, regardless of the particle size, as shown in Figure 7a and b. The FeCo/EG composites showed values of permittivity that were 4 times larger than those of the FeCo nanoparticles, although the values of the permeability were not changed, as shown in Figure 7c and d. Furthermore, the real part of the permittivity of GOs (Figure 7c) was lower than that of pure EGs by three times and those of FeCo/EG composites by five times. This indicates that both EGs and FeCo nanoparticles in the composites contributed to the increase in the permittivity. These permittivity behaviors can be used to design the input wave impedance, RL, and operating frequency. The RLs values were estimated by the measured complex permittivity and permeability (Figure 8). The maximum RL (RLmax) values of the FeCo/EG composites with 2 mm thicknesses were around -17 dB in the 1011 GHz region, regardless of the size of FeCo nanoparticles, although the RLmax values of the FeCo nanoparticles were below -3.3 dB at 18 GHz. The frequency at RLmax of the FeCo/EG composites shifted from 11 to 10 GHz with the increase of the FeCo


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nanoparticle size. Figure 8c and d show the dependence of the RL values of the FeCo nanoparticles and FeCo/EG composites obtained using 20.6 nm FeCo nanoparticles on the composite thickness. The RLmax values of the FeCo nanoparticles were below -1.7 dB at 18 GHz and decreased with the decrease of the composite thickness. The RLmax values of FeCo/EG composites decreased form -25 dB at 14 GHz (1.5 mm) to -12.7 dB at 4.7 GHz (4 mm) with the increase of the composite thickness. The SE is the materials properties to shield EM radiation, estimated by following equation.55 SE (dB) = 10 log(PI/PT) = SER+SEA

(4)

,where PI and PT are the incident and the transmitted power. The SE is the sum of the reflection (SER) and the absorption (SEA) net shielding, expressed with the scattering parameters: SER = 10 log [1/(1-|S11|2)]

(5)

SEA = 10 log [(1-|S11|2)/|S21|2]

(6)

The total SE can be deduced from eq. (5) and (6) to have the following form: SE (dB) = −20 log |S21|

(7)


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The maximum SE values of the FeCo/EG composites were observed around 7 dB at 11.5 GHz, although FeCo nanoparticles showed a maximum value around 1.6 dB (Figure 9a-9b). On the other hand, the low permittivity of GOs has directly reflected the minimal reflection loss (RL) in the entire frequency range (Figure 8c and d), compared with those of EGs and FeCo/EG composites. More importantly, FeCo/EG composites exhibited more than two times higher RL and SE than those observed in pure EGs. These results clearly demonstrate the synergistic effect coming from nanoparticles and EGs by the conservation of both graphene and nanoparticles. Moreover, the clear size-dependence of the FeCo nanoparticles at the operating frequency allow the possibility for target-driven preparation of EMI shielding materials.

4. Conclusion In summary, we developed a generalized synthesis route for EG sheets functionalized with inorganic anion-capped all-inorganic nanoparticles. The electrochemically exfoliated graphene sheets have been demonstrated to fulfill all the desired functions of high-quality graphene solutions such as dispersibility and good electrical properties, which were


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achieved by edge-selective oxidation. This EG was further conjugated with PEI to provide positive charges at the edges. Moreover, the ligand exchange reaction for the nanoparticles generated negatively charged nanoparticles with Cl- or MoS42- surface ligands without the degradation of their primary structural and electronic properties. These negatively charged nanoparticles were selectively decorated at the edges of the EG by simple mixing due to electrostatic interactions. Furthermore, the generality of the current process was demonstrated for a wide range of colloidal nanoparticles including semiconductors, metals, and magnets. We further studied the EM RLs and SE of EG composites decorated with magnetic FeCo nanoparticles with sizes of 5.0, 12.9, and 20.6 nm. These composites showed the frequency shift of RL with varying the FeCo nanoparticle sizes and the enhanced RL, and SE by the EG. The current approach offers a new method to design and prepare nanoparticle/graphene composites, shedding a light on graphene research for various application in catalysts, electronics, and biomedical engineering.


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■ ASSOCIATED CONTENT Supporting Information AFM images and size histogram of EG, Low- and high-magnification TEM images of EG, Selected area ED pattern at the basal plane of EG, FIB-SEM image of 500nm-thick EG film, TEM images of organics-capped nanoparticles, Photographs for the ligand exchange reaction of nanoparticles, FT-IR spectra of organics- and inorganics- capped nanoparticles, UV-Vis and Raman spectra, and TEM image of PEI-conjugated EG and EG, Synthetic conditions for FeCo nanoparticles, TEM images of FeCo nanoparticles, Size histograms of FeCo nanoparticles, and magnetic hysteresis loop of 5.0 nm-sized FeCo nanoparticle.

■ AUTHOR INFORMATION

Corresponding Author E-mail: [email protected] (Y-H. Choa), [email protected] (K. H. Kim) and [email protected] (J. S. Son)


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■ ACKNOWLEDGMENT This research was supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (No. 2016M3A7B4900044 and NRF-2018M3A7B8060697) and the 2018 Research Fund (1.180028.01) of UNIST (Ulsan National Institute of Science and Technology).

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Figure 1. Schematic illustration of the synthetic processes of nanoparticles/EG composites.


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Figure 2. (a) AFM images of EG and (b) its height profile. Raman spectra of (c) the edges and (d) the basal plane of EG.


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Figure 3. TEM images of (a) Cl--capped FeCo, (b) MoS42--capped Au, (c) PbS nanoparticles, and (d) MoS42--capped CdS nanoplates. (e) ζ-potentials of MoS42--capped Au, PbS, CdSe nanoparticles, MoS42--capped CdS nanoplates, Cl--capped FeCo nanoparticles, and PEI-conjugated EG in DI water and NMF.


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Figure 4. TEM images of EG composites with (a) Au, (b) PbS, (c) CdSe, (d) CdS nanoplates, (e) FePt, and (f) Bi nanoparticles.


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Figure 5. TEM images of (a) 5.0 nm-, (b) 12.9 nm-, and (c) 20.6 nm-sized FeCo nanoparticles. (d) XRD patterns of FeCo nanoparticles with various sizes. The vertical lines in the panel (d) indicates the cubic FeCo phase (JCPDS 44-1433).


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Figure 6. TEM images of (a) 5.0 nm-, (b) 12.9 nm-, (c) 20.6 nm-sized FeCo nanoparticles/EG composites. (d) XRD patterns of organics-capped FeCo nanoparticles and FeCo nanoparticles/EG composites. (e) Magnetic hysteresis loops of 5.0 nm-, 12.9 nm-, and 20.6 nm-sized FeCo nanoparticles/EG composites at room temperature.


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Figure 7. Complex permittivity and permeability of (a), (b) FeCo nanoparticles and (c), (d) FeCo/EG composites, EG and GO.


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Figure 8. Reflection loss (RL) with (a), (b) FeCo nanoparticles size, and (c), (d) with composites thickness of FeCo nanoparticles, FeCo/EG composites, EG, and GO, respectively.


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Figure 9. Shielding effectiveness (SE) of (a) FeCo nanoparticles and (b) FeCo/EG composites, EG, and GO.


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TOC


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Controlled Grafting of Colloidal Nanoparticles on Graphene through

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