Solution-Processed Highly Superparamagnetic and Conductive

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Solution-Processed Highly Superparamagnetic and Conductive PEDOT:PSS/Fe3O4 Nanocomposite Films with High Transparency and High Mechanical Flexibility Yi-Jie Xia, Jie Fang, Pengcheng Li, Bangmin Zhang, Hongyan Yao, Jingsheng Chen, Jun Ding, and Jianyong Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

Solution-Processed Highly Superparamagnetic and Conductive PEDOT:PSS/Fe3O4 Nanocomposite Films with High Transparency and High Mechanical Flexibility Yijie Xia1,2, Jie Fang2, Pengcheng Li2, Bangmin Zhang2, Hongyan Yao2, Jingsheng Chen2, Jun Ding2, and Jianyong Ouyang2,* 1

School of Mechanical Engineering, University of Shanghai for Science and Technology,

Shanghai 200093, People’s Republic of China. 2

Department of Materials and Engineering, National University of Singapore, Singapore 119796

KEYWORDS: PEDOT:PSS, Fe3O4 nanoparticle, superparamagnetic, highly conductive, mechanical flexibility

ABSTRACT: Multifunctional films can have important application. Transparent and flexible films with high conductivity and magnetic properties can be used in many areas, such as electromagnetic interference (EMI) shielding, magnetic switching, microwave absorbing and also biotechnology. Herein, novel highly conductive and superparamagnetic thin films with excellent transparency and flexibility are demonstrated. The films were formed from poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS,

Clevios

PH1000)

aqueous

solution added with iron oxide (Fe3O4) nanoparticles that have a size of ~20 nm by spin-coating.

ACS Paragon Plus Environment

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The PEDOT:PSS/Fe3O4 films have a high conductivity of 1080 S/cm through a treatment with methylammonium iodide (MAI) in organic solvent. The high-conducting PEDOT:PSS/Fe3O4 films can also have a saturation magnetization of 25.5 emu/g and an EMI shielding effectiveness (SE) of more than 40 dB in 8-12.5 GHz (X band) frequency range. The PEDOT:PSS/Fe3O4 films have additional advantages like excellent transparency, good mechanical flexibility, low cost, and light weight. In addition, we fabricate flexible PEDOT:PSS/Fe3O4 silk threads with high magnetism and conductivity.

1. INTRODUCTION Multifunctional materials can have important applications in many areas. Materials with both magnetic and electrical properties have been attracting intense attention because they have important application in many areas such as EMI shielding,1,2 microwave absorbing,3,4 magneticcontrolled switches,5 Schottky diode,6 magnetic and conductive membranes,7 tissue engineering,8 biocatalytic fileld,9 energy storage,10,11 sensors,12 optical devices,13,14 filtration,15,16 and removal of organic and inorganic pollutants or heavy metal ions from waste water.17-20 The common method for developing such materials is to admix conducting polymer composites with magnetic nanoparticles (NPs).1,21 Conducting poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is a suitable material for this application, because it has some unique merits. PEDOT:PSS is the most successful conducting polymer in terms of practical application. PEDOT:PSS films can have high transparency in the visible range, high mechanical flexibility, and excellent thermal stability. Most important, PEDOT:PSS can be dispersed in water and some organic solvents. Thus, high-quality PEDOT:PSS films can be readily coated on substrates through conventional solution processing techniques, such as coating and printing.22-24 Several


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methods have been reported to enhance the conductivity of PEDOT:PSS films, such as treatments with polar organic compounds, ionic liquids, anionic surfactants, salts, zwitterions, organic or inorganic acids, or cosolvents.25-43 The recently reported highest conductivity was 4,600 ± 100 S/cm by Bao et al.44 Therefore, PEDOT:PSS is very suitable to be the polymer matrix of magnetic nanoparticles to form conductive and magnetic nanocomposite. Highly conductive PEDOT:PSS can have important application, and they can have more applications by forming composites with other materials. Polymer composites of magnetic nanoparticles are interesting materials. Among various kinds of nanoparticles, magnetic iron oxide (Fe3O4) nanoparticles are of particular interests due to their unique magnetic properties, biocompatibility, and nontoxicity. Magnetic thin films can be fabricated by dispersing magnetic nanoparticles in polymer matrix. Various polymers have been investigated as the matrix. For instance, insulating polymers were used for the matrix of the composites. Hirano et al. reported using poly(methylmethacrylate) (PMMA) to form composite with Fe3O4 and γ-Fe2O3 (weight ratio of 1:1) nanoparticles through the sol-gel process.45 The saturation magnetization is about 75 emu/g for the composites with a loading of 40 wt% γ-Fe2O3 and Fe3O4. Fe3O4 nanoparticles were also blended into polymers immediately after the wet chemical synthesis of the nanoparticles. The saturation magnetization is 37.8 emu/g for polyisobutylene functionalized with tetraethylenepentamine (PIB-TEPA) blended with 61 wt% γ-Fe2O3/Fe3O4.46 Sutter et al. observed a saturation magnetization of 54.3 emu/g for SU-8 composites with 5 vol% Fe3O4 nanoparticles and demonstrated their application as a cantilever based micro electro mechanical systems (MEMS) actuator.47,48 Recently, Alfadhel et al. reported a magnetic flow sensor by integrating of iron nanowires into polydimethylsiloxane (PDMS).49 The remanent magnetization of the nanocomposite pillars are 2.1 memu with a remanence-to-


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saturation magnetization ratio of 0.7 with a coercivity of 1520 Oe. (This is insulator matrix. It should be presented in the part for insulator composite) Apart from insulator polymers, conductive polymers were also explored for the nanoparticle matrix. Huang et al. prepared Fe3O4/polyaniline nanocomposites by macromolecule-induced self-assembly.21 But the conductivity of Fe3O4/polyaniline (8 wt% Fe3O4) composites is below 10-3 S cm-1 and its saturation magnetization is 5.07 emu/g. Wei et al. fabricated Fe3O4/polyaniline nanocomposites through a sol gel process.50 The composites have a conductivity of 10-4 -10-1 S/cm and a magnetization of 11 emu/g. Therefore, developing a facile and solution processable way to fabricate transparent films with both high conductivity and magnetism is very important in this area. In addition conductive and magnetic composites can usually effectively shield electromagnetic wave. Here we report multifunctional PEDOT:PSS/Fe3O4 nanocomposites prepared by solution processing. The nanocomposite films can have a high conductivity of up to 1082 S/cm and a transparency of above 80% in the visible-light range with a thickness of 50 nm. They are paramagnetic and have a saturation magnetization of 25.5 emu/g. These conductive and magnetic films can effectively shield electromagnetic radiation. The PEDOT:PSS/Fe3O4 films with multiple magnetic and electrical functions are also flexible, low-cost, and light-weight.

2. EXPERIMENTAL SECTION 2.1 Preparation of PEDOT:PSS/Fe3O4 composite PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus. Its concentration was 1.3 % by weight, and the weight ratio of PSS to PEDOT was 2.5. Methylammonium (MAI)


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and dimethylformamide (DMF) were obtained from Sigma-Aldrich. All materials were used as received. Fe3O4 nanoparticles were prepared through the following process. 0.71 g (2 mmol) Fe(acac)3 (Iron(III) acetylacetonate), 1.13 g (4 mmol) oleic acid and 22 g (110 mmol) benzyl ether were mixed and stirred. After degassed with N2 for 1 h, the mixture was heated to 290 °C with 20 °C min-1 heating rate under mild N2 flow. The mixture was refluxed at 290 °C for 1 h and then cooled down to room temperature. The product was precipitated by adding ethanol into the mixture and collected by centrifugation at 8000 rpm for 10 min. The product was then redispersed in 10 ml toluene under sonication. Residual surfactant and solvent were washed away by repeating the precipitation and re-dispersion process for three times. The nanoparticles were obtained after the vaporization of the remaining toluene in air. The solutions dispersed with PEDOT:PSS and Fe3O4 nanoparticles were prepared by adding Fe3O4 nanoparticles into PEDOT:PSS aqueous solution with a vortex mixer. PEDOT:PSS/Fe3O4 films were formed by spin coating the PEDOT:PSS/Fe3O4 aqueous solutions on pre-cleaned glass substrates. They were subsequently annealed at 120 oC for 20 min. The composite films were treated with organic solution by dropping 100 ul of MAI solution in DMF onto each composite film (1.3×1.3 cm) at 140 oC. After the composite films became dry, they were rinsed with de-ionized (DI) water for three times and then dried at 140 oC.

2.2 Characterizations of PEDOT:PSS/Fe3O4 films The conductivities of the PEDOT:PSS/Fe3O4 films were measured by a Keithley 2400 source/meter using four-probe technique. Indium were pressed on the four corners of each PEDOT:PSS/Fe3O4 film as electrical contacts . The UV-Vis-NIR absorption spectra were


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measured by a Varian Cary 5000 UV-vis-NIR spectrometer. The AFM images were collected with a Veeco NanoScope IV Multi-Mode AFM in tapping mode. The scanning electron microscopic (SEM) images were taken using Hitachi S-4100 scanning electron microscope. The thicknesses of the PEDOT:PSS films were measured using an Alpha-Step IQ surface profiler. The field dependent magnetic curves (M vs H) were carried by quantum design PPMS (model 6000). The EMI shieldin were measured by Agilent/HP 8510C Vector Network Analyze (VNA). The X-ray photoelectron spectroscopy (XPS) spectra were collected with an Axis Ultra DLD Xray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV). X-ray diffraction (XRD) patterns were obtained by a Bruker D8 Advance XRD Instrument with Cu Kα radiation (λ=0.154 nm). The dynamic light scattering (DLS) measurements were carried out with a Zetasizer Nano ZS by Malvern with a HeNe laser (633 nm) as the light source.

3. RESULTS AND DISCUSSION PEDOT:PSS is a polyelectrolyte with positive PEDOT chains and negative PSS chains, and it can be dispersed in water. The Fe3O4 nanoparticles capped with oleic acid are hydrophilic so that they can be easily dispersed in PEDOT:PSS aqueous solution. The PEDOT:PSS/Fe3O4 dispersion is homogeneous. Dispersions with two weight ratios of PEDOT:PSS to Fe3O4, 1:1 and 1:5, were prepared. PEDOT:PSS/Fe3O4 films was fabricated by spin-coating PEDOT:PSS/Fe3O4 aqueous solutions onto glass substrate. Table 1 presents the conductivities, thicknesses, and sheet resistance of PEDOT:PSS and PEDOT:PSS/Fe3O4 films before and after the MAI treatment. An as-prepared PEDOT:PSS film has a conductivity of 0.22 S/cm with 63 nm thickness. The conductivities of as-prepared PEDOT:PSS/Fe3O4 films with 1:1 and 1:5 weight ratios are 0.24 and 0.16 S/cm, respectively. Their thicknesses increase to 85 and 142 nm, respectively. The


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addition of Fe3O4 hardly affects the conductivity, while it increases the film thickness. This suggests that Fe3O4 nanoparticles are dispersed in the PEDOT:PSS matrix. The thickness increase by the Fe3O4 NPs cause the sheet resistance decreases.

Table 1. Conductivities, thicknesses, and sheet resistance PEDOT:PSS/Fe3O4 films before and after a MAI treatment. Sample Untreated PEDOT:PSS Untreated PEDOT:PSS/Fe3O4 (1:1) Untreated PEDOT:PSS/Fe3O4 (1:5) MAI-treated PEDOT:PSS/Fe3O4 (1:1) MAI-treated PEDOT:PSS/Fe3O4 (1:5)

of

PEDOT:PSS

Thickness (nm)

Sheet resistance (Ω/sq)

Conductivity (S cm-1)

63 85 142 30 50

702,000 483,000 431,000 308 340

0.22±0.05 0.24±0.05 0.16±0.05 1080±50 580±50

and

The conductivity of the PEDOT:PSS/Fe3O4 composite films was enhanced by post treatments. Although an acid treatment can give rise to the highest conductivity enhancement for PEDOT:PSS, it is not applicable for PEDOT:PSS/Fe3O4 because Fe3O4 can be dissolved by acids. As we reported recently, the conductivity of PEDOT:PSS could be enhance by more than 1000 times through a treatment with DMF solution of MAI.43 The conductivity enhancement is more significant than with polar organic solvents like dimethyl sulfoxide (DMSO) or ethylene glycol. The PEDOT:PSS/Fe3O4 films were treated with 0.1 M MAI/DMF solution at 140 °C. After the treatment, the conductivities of PEDOT:PSS/Fe3O4 with the weight ratios of 1:1 and 1:5 increase to 1080 and 580 S/cm, respectively. These conductivities are significant higher than that of other conductive and magnetic polymer films whose conductivities are below 1 S/cm.21,50 These conductivities are also higher than that of composites of carbon nanomaterials and Fe3O4 nanoparticles. For instance, the conductivities of graphene/Fe3O4 film and Fe3O4/carbon nanofiber composites are about 40 S/cm and 7.1±0.7 S/cm, respectively.5,17,51 The PEDOT:PSS/Fe3O4 (1:5) films have lower conductivity than that of PEDOT:PSS/Fe3O4 (1:1)


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because of the loading increase of non-conductive Fe3O4. The conductivity enhancement by the MAI/DMF solution treatment is attributed to the MAI-induced phase segregation of PSSH chains from PEDOT:PSS and the conformational change of the conductive PEDOT chains.43 The highly conductive PEDOT:PSS/Fe3O4 films after the MAI/DMF treatment are optically transparent in the visible range. Figure 1 presents the transmittance spectra of MAI/DMF-treated PEDOT:PSS/Fe3O4 films. The transmittance of a 30 nm-thick PEDOT:PSS/Fe3O4 (1:1) film, which has a sheet resistance of 308 Ω/sq, is 94.6% at 550 nm. The transmittance is over 90% in the wavelength range from 350 to 800 nm. The transmittance of a 50 nm-thick PEDOT:PSS/Fe3O4 (1:5) film, which has a sheet resistance of 340 Ω/sq, is over 80% at 550 nm. Therefore, the MAI/DMF-treated PEDOT:PSS/Fe3O4 films have both high conductivity and high transparency.

100

Transparence (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60 PEDOT:PSS/Fe3O4 1:1 PEDOT:PSS/Fe3O4 1:5 40

20

400

500

600

700

800

Wavelength(nm) Figure 1 Transmittance spectra of PEDOT:PSS/Fe3O4 films after MAI treatment.


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The XRD patterns of a neat PEDOT:PSS film, Fe3O4 powder, and PEDOT:PSS/Fe3O4 films with different Fe3O4 loadings were presented in Figure 2. The neat PEDOT:PSS film shown amorphous structure, while Fe3O4 powder exhibited crystal structure. The peaks in PEDOT:PSS/Fe3O4 films are the same as those in Fe3O4 powder. Thus, Fe3O4 nanoparticles are well loaded on PEDOT:PSS film and the crystal structure of Fe3O4 nanoparticles are not changed after they assembled on PEDOT:PSS film.

Intensity

1000

PEDOT:PSS 500

0

30

40

50

60

70

80

2θ degree

Intensity

1000

Fe3O4 500

0 30

40

50

60

70

80

2θ degree

Intensity

1000

PEDOT:PSS-Fe3O4 1:1 500

0 30

40

50

60

70

80

2θ degree 3000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PEDOT:PSS-Fe3O4 1:5 2000 1000 0 30

40

50

60

70

80

2θ degree

Figure 2 XPD pattern of a neat PEDOT:PSS film, Fe3O4 powder, PEDOT:PSS/Fe3O4 (1:1), and PEDOT:PSS/Fe3O4 (1:5) films.


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The PEDOT:PSS/Fe3O4 films with different Fe3O4 loadings were studied by XPS. Fig. 3 shows the Fe2p and S2p XPS spectra. The XPS spectra of a PEDOT:PSS film and Fe3O4 powder are presented as well. For the neat Fe3O4 powder, the Fe 2p1/2 and Fe 2p3/2 peaks appear at 724.0 and 710.6 eV, respectively, (Fig. 3a).17,52 The absence of the satellite peak of Fe 2p3/2 at around 720 eV is consistent with the Fe3O4 structure. The Fe 2p1/2 and Fe 2p3/2 XPS peaks of the PEDOT:PSS/Fe3O4 (1:5) film have the binding energies higher than that of the neat Fe3O4 powder by 0.6-0.8 eV. These suggest that there may charge transfer between Fe3O4 and PEDOT:PSS. (a)

(b)

Fe 2p3/2

Fe 2p1/2

PEDOT S2p3/2 S2p1/2

PSS S2p

Intensity (a.u.)

III Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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II IV

III

II

I 740

735

730

725

720

Binding Energy (ev)

715

710

705

172

170

168

166

164

162

160

Binding Energy (ev)

Figure 3 (a) Fe2p and (b) S2p XPS spectra of (I) a neat PEDOT:PSS film, (II) PEDOT:PSS/Fe3O4 (1:1) film, (III) PEDOT:PSS/Fe3O4 (1:5) film, and (IV) neat Fe3O4 powder. All the three films were treated with MAI/DMF solution before the XPS measurements.

The charge transfer between the Fe3O4 nanoparticles and PEDOT:PSS is confirmed by the S2p XPS spectra. As shown in Fig. 3b, the broad peaks between 166 and 170 eV arise from the S atoms in PSS, while the two peaks between 162 and 166 eV originate from the S atoms in PEDOT.35-43 In comparison with neat PEDOT:PSS, the S2p peaks of PEDOT of the PEDOT:PSS/Fe3O4 (1:5) films slightly shift to lower binding energies. This is consistent with the binding energy shift of the Fe2p bands. Ling et al. also found similar changes in the binding


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energies of S2p and W4f peaks for PEDOT:PSS/WO3 composite.53 Such interactions may play an important roles in dispersing Fe3O4 nanoparticles in PEDOT:PSS solution uniformly. The particle sizes distribution in PEDOT:PSS aqueous solution before and after blending with Fe3O4 were investigated by DLS. All solutions were diluted to 0.013 wt% PEDOT:PSS for the DLS study. As shown in Figure 4, there are two size distribution peaks for the pure PEDOT:PSS solution. The majority (88%) has a size distribution of 255-700 nm with the maximum at 429 nm, and the average size is 440 nm. The minority (12%) has a size distribution of 21-51 nm, and the average size is 34 nm. The sizes are consistent with those reported in literature.54 The sizes are related to the globular gel particle structure formed by PEDOT and PSS in water. The addition of Fe3O4 nanoparticles changed size distribution. In PEDOT:PSS/Fe3O4 (1:1) solution, the size distribution of 20-50 nm reduced to 3%. The majority (93%) has a wider size distribution of 80-2700 nm with the maximum at 955 nm. A size distribution at large size range from 4100 to 5600 nm with 5% was appeared. In PEDOT:PSS/Fe3O4 (1:5) solution, the size distribution peak at 20-50 nm was disappeared. The majority (91%) has a larger size distribution of 220-2700 nm with the maximum at 1130 nm. The ratio of size distribution from 4100 to 5600 nm was increased to 9%. The particle size increased after blending with Fe3O4 nanoparticles indicates that Fe3O4 dispersed well into PEDOT:PSS domains.


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20

PEDOT:PSS PEDOT:PSS-Fe3O4 1:1 PEDOT:PSS-Fe3O4 1:5

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

5

0 10

100

1000

10000

Size (d.nm)

Figure 4. Particle size distributions of PEDOT:PSS solutions without and with different Fe3O4 loadings by DLS. The PEDOT:PSS concentration in solutions are 0.013 wt%.

The morphology of the PEDOT:PSS/Fe3O4 films before and after the MAI/DMF treatment were studied by SEM (Fig. 5). Fe3O4 nanoparticles （confirmed by EDX）appeared on the PEDOT:PSS/Fe3O4 (1:1) film surface. They have a size of 20-50 nm. Many more Fe3O4 nanoparticles can be observed for the PEDOT:PSS/Fe3O4 (1:5) film because of the higher Fe3O4 loading. It can be seen that the MAI/DMF treatment can cause the edge of Fe3O4 nanoparticles sharper. This can be attributed to the removal of some PSSH chains from the composites after the MAI/DMF treatment.43 Those PSSH chains may cover the surface of the Fe3O4 nanoparticles. (a)

(b)


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(c)

(d)

Figure 5 SEM images of a PEDOT:PSS/Fe3O4 (1:1) film (a) before and (b) after a MAI/DMF treatment and a PEDOT:PSS/Fe3O4 (1:5) film (c) before and (d) after a MAI/DMF treatment. Figure 6 shows the cross-section SEM images of neat PEDOT:PSS film and PEDOT:PSS/Fe3O4 films after the MAI/DMF treatment. The Fe3O4 nanocomposites are well dispersed in the matrix. (a)

(b)

(c)

Figure 6 Cross-section SEM images of (a) a neat PEDOT:PSS film, (b) a PEDOT:PSS/Fe3O4 (1:1) film, and (c) a PEDOT:PSS/Fe3O4 (1:5) film after a MAI/DMF treatment.


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The region of PEDOT:PSS/Fe3O4 films with few Fe3O4 particles were studied by AFM (Fig. 7), the PEDOT:PSS surface of both PEDOT:PSS/Fe3O4 films before MAI/DMF treatment are quite smooth with RMS roughness of 0.97 and 1.1 nm, respectively. The surface became rougher after MAI/DMF treatment. The RMS roughness both increased to 1.4 nm. Grains and wire-like structure can be observed on the polymer surface. The morphology change is consistent with those reported in literatures55,56, which is due to the removal of PSSH chains. It seems that the addition of Fe3O4 particles does not affect the PEDOT:PSS morphology. Thus, it does not influence the conductivity enhancement of PEDOT:PSS through MAI/DMF treatment. (a)

(b)

(c)

(d)

Figure 7 AFM images of a PEDOT:PSS/Fe3O4 (1:1) film (a) before and (b) after a MAI/DMF treatment and a PEDOT:PSS/Fe3O4 (1:5) film (c) before and (d) after a MAI/DMF treatment.


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The magnetic properties of PEDOT:PSS/Fe3O4 films were investigated by measuring the magnetization versus the magnetic field (M-H curve). The M-H curves of PEDOT:PSS/Fe3O4 (1:5) film at 400, 300, 80 and 20 K are shown in Figure 8. There is no hysteresis in the M-H curves at 400 K, and well fitted by the Langevin model, which indicates that the PEDOT:PSS/Fe3O4 films are superparamagnetic at 400 K. The curves gradually deviate from superparamagnetic behaviour at lower T. The film is almost still superparamagnetic at room temperature 300 K, while significant hysteresis occurs at 80 K, indicating a blocked state where the NPs show a ferromagnetic behaviour. The PEDOT:PSS/Fe3O4 (1:5) films have a high saturation magnetization of 43 emu/g which is comparable with that of other magnetic composite films.45-48 50

PEDOT:PSS-Fe3O4 1:5

40 40

30

20 Moment (emu/g)

Moment (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 K 20 K 80 K 300 K 400 K Langvin fit @400 K

20

10

0 0

10000

0

-20

-40 -2000

-1000

20000 H (Oe)

0 H (Oe)

30000

1000

2000

40000

Figure 8 Magnetization curves of PEDOT:PSS/Fe3O4 (1:5) film at 400, 300, 80, 20 and 10 K, with Langevin fit at 400K. Inset: close-up on the hysteresis at H = 0.

The EMI shielding effectiveness (SE) of the PEDOT:PSS/Fe3O4 films after a MAI/DMF treatment were studied in 8-12.5 GHz (X band) frequency range, and a neat PEDOT:PSS film


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was also studied for comparation. (Fig. 9). The EMI SE stands for the electromagnetic interference reduction, defined as the logarithmic ratio of the incident power (Pi) to the transmitted power (Pt) of an electromagnetic wave and generally expressed in decibels (dB) as defined in equation: 57,58

. A material with a high SE value can effectively shield electromagnetic wave. In practical application, the SE value of an EMI shielding material should be higher than 20 dB, which allows only 1% of the electromagnetic waves to be transmitted.59 In Fig. 9a, the neat PEDOT:PSS film exhibited an EMI SE of about 16 dB. The EMI SE increases when the loading of the Fe3O4 increase. The PEDOT:PSS/Fe3O4 (1:1) film has an EMI SE of above 36 dB. It corresponds to a 99.97% attenuation of the electromagnetic wave, and it is high enough for many practical applications. For the PEDOT:PSS/Fe3O4 (1:5) film, the EMI SE increases to more than 40 dB, and the highest SE value is 46 dB at 9.2 GHz. These EMI SE values of PEDOT:PSS/Fe3O4 is much higher than that (18.2 dB) of the graphene/Fe3O4 foam that is 2.5 mm thick.60 Although the SE of Fe3O4/carbon nanofiber composite is 67 dB at 10.4 GHz, the thickness of the composite is 0.7 mm, much thicker than that of our PEDOT:PSS/Fe3O4 films (10 µm).52 As the EMI SE usually increases along with the material thickness, the EMI SE value of our PEDOT:PSS/Fe3O4 films can be improved further by increasing the film thickness.61,62 The attenuation of the electromagnetic radiation can be caused by the absorption and/or reflection by the EMI shielding material. To further clarify the EMI shielding mechanism in PEDOT:PSS/Fe3O4 films, the total EMI SE, absorption (SEA) and reflection (SER) are plotted in Fig. 9b-d. SEA is greater than SER for PEDOT:PSS/Fe3O4 films, but it is reversal for the neat PEDOT:PSS film. Hence, the radiation absorption is the dominant shielding mechanism for


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PEDOT:PSS/Fe3O4 films, while the radiation reflection is the dominant factor for the neat PEDOT:PSS film. In addition, the SEA value increases with the increasing Fe3O4 loading in the composite films. This can be attributed to the electromagnetic absorption by Fe3O4. As reported in literature, magnetic fillers can improve the radiation absorption more than the radiation reflection.63 50

EMI shielding effectiveness (dB)


(a) 45 40 35

PEDOT:PSS PEDOT:PSS/Fe3O4(1:1) PEDOT:PSS/Fe3O4(1:5)

30 25 20

20

(b)

15

PEDOT:PSS PEDOT:PSS SA PEDOT:PSS SR

10

5

15 8

9

10

11

12

8

13

9

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11

12

Frequency (GHz)

Frequency (GHz)

50

50

(c)

(d)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 PH1000/Fe3O4(1:1) PH1000/Fe3O4(1:1) SA

30

PH1000/Fe3O4(1:1) SR 20

10

40

30 PH1000/Fe3O4(1:5) PH1000/Fe3O4(1:5) SA

20

PH1000/Fe3O4(1:5) SR 10

0

8

9

10

11

Frequency (GHz)

12

8

9

10

11

12

13

Frequency (GHz)

Figure 9 (a) SE of PEDOT:PSS and MAI/DMF treated PEDOT:PSS/Fe3O4 films as a function of frequency (8-12.5 GHz). SEA and SER of (b) PEDOT:PSS film, (c) a PEDOT:PSS/Fe3O4 (1:1) film, and (d) a PEDOT:PSS/Fe3O4 (1:5) film after a MAI/DMF treatment . The thickness of all films is 10 µm. Apart from the magnetic and conductive PEDOT:PSS/Fe3O4 films, we also fabricated magnetic and conductive PEDOT:PSS/Fe3O4 fibers. The fibers were fabricated by immersing silk or cotton threads in PEDOT:PSS/Fe3O4 (1:5) aqueous solution for about 30 min. After the


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silk or cotton threads coated with PEDOT:PSS/Fe3O4 were dried at 70 oC in an oven, they were treated with a MAI/DMF solution. The morphology of PEDOT:PSS/Fe3O4 (1:5) combined silk and cotton threads were studied by SEM (Fig. 10). The SEM images indicate that the silk and cotton threads are coated with PEDOT:PSS and Fe3O4 nanoparticles. In addition, the dispersion of Fe3O4 nanoparticles on cotton is better than on silk.

(a)

(c)

(b)

(d)

Figure 10 SEM images of PEDOT:PSS/Fe3O4 (1:5) combined silk (a-b) and cotton (c-d) threads. The silk or cotton threads coated with PEDOT:PSS/Fe3O4 exhibit good mechanical flexibility. They show sensitive response to magnetic field. As demonstrated in Figure 11, the silk or cotton threads coated with PEDOT:PSS/Fe3O4 can iteratively be attracted to the magnet from its initial upright position and do not drop from the magnet. The silk and cotton threads coated with


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PEDOT:PSS/Fe3O4 exhibit conductivities of 83 and 35 S/cm, respectively. These magnetic and conductive fibers have applications in wearable antistatic devices.

(a)

(b) silk thread

magnet

magnet

cotton thread

Figure 11 Mechanical response of (a) Silk and (b) cotton threads coated with PEDOT:PSS/Fe3O4 to a magnet nearby. 4. CONCLUSIONS We demonstrated a facile and novel way to fabricate magnetic, conductive, and transparent thin film. The films are prepared by spin coating aqueous solution of PEDOT:PSS and Fe3O4 nanoparticles. The PEDOT:PSS/Fe3O4 films have a high conductivity of 1080 S cm-1 through a treatment with MAI/DMF solution and a saturation magnetization of 43 emu/g. They can effective shield electromagnetic radiation. Their EMI shielding effectiveness can be more than 40 dB in the frequency range of 8-12.5 GHz. Moreover, the PEDOT:PSS/Fe3O4 films are transparent, flexible, low-cost, and light-weight. Furthermore, we fabricate flexible silk and cotton threads coated with PEDOT:PSS/Fe3O4. They are magnetic and conductive. Both the PEDOT:PSS/Fe3O4 films and threads coated with PEDOT:PSS/Fe3O4 can have potential application in many areas, such as electromagnetic interference shielding and microwave absorbents applications.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. Notes ACKNOWLEDGMENT This project was financially supported by a research grant from the Ministry of Education, Singapore (R-284-000-156-112). Y. Xia thanks to the Young Eastern Scholar (QD2016012) of Shanghai Municipal Education Commission.

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Solution-Processed Highly Superparamagnetic and Conductive

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