silica nanosheet composites

silica nanosheet composites

Current Opinion in Solid State and Materials Science 12 (2008) 9–13 Contents lists available at ScienceDirect Current Opinion in Solid State and Mat...

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Current Opinion in Solid State and Materials Science 12 (2008) 9–13

Contents lists available at ScienceDirect

Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms

Preparation and characterization of conducting polyaniline/silica nanosheet composites Peng Liu * State Key Laboratory of Applied Organic Chemistry, Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China

a r t i c l e

i n f o

Article history: Received 11 November 2008 Accepted 6 January 2009

Keywords: Polyaniline Silica nanosheets Nanocomposite Electrical conductivity

a b s t r a c t A series of polyaniline/silica nanosheet composites (PANI/SNS) with different contents of the silica nanosheets derived from vermiculite via acid-leaching were prepared via the in situ chemical oxidation polymerization. The PANI/SNS composites were characterized with Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrical conductivity measurement. It is interesting that the electrical conductivities of the PANI/SNS composites increased with the increasing of the contents of the silica nanosheets added because of the moisture absorption. It was confirmed by the TGA analysis. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Polyaniline (PANI) has been known as one of the most technologically important conducting polymers because of its high electrical conductivity, easy producibility, environmental stability, easy preparation, and relatively low cost [1]. It has attracted much attention in recent years due to its potential applications in various hi-tech aspects, for example, in electrochemical displayers, sensors, catalysis, redox capacitors, electromagnetic shielding as well as in secondary batteries [2–6]. In the past few years, nanocomposites based on polyaniline have been harvesting several intriguing properties within themselves due to the mutual influence of the individual constituents and synergism of their properties [7]. In the reported polyaniline based nanocomposites, the inorganic nanoparticles (oxides [8–11] and metals [12–15]), nanosheets [16–25], nanotubes [26–30], nanobelts [31,32], and nanorods [33,34] were also used for the polyaniline based nanocomposites. Compared to other inorganic nanomaterials (nanoparticles, nanotubes, nanobelts and nanorods), the nanosheets have only one dimension (their thicknesses) in nanoscale. The most used inorganic nanosheets, the layered silicates and oxides, were nonconductor. So the research and developments on the electrical conductivities of their polyaniline based nanocomposites are attractive. In the present work, the silica nanosheets (SNS), obtained via acid-leaching of vermiculite, were used for the polyaniline based nanocomposites. The products, PANI/SNS composites were charac* Tel.: +86 931 8912516; fax: +86 931 8912582. E-mail address: [email protected] 1359-0286/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2009.01.001

terized with FT-IR, SEM, XRD, TGA, and electrical conductivity measurement. The effect of the amounts of the silica nanosheets added on the electrical conductivities of the PANI/SNS composites was investigated. 2. Experimental 2.1. Raw materials and reagents Vermiculite used was purchased from Xinjiang, China. Vermiculite was pretreated with hydrochloric acid according to a reported patent [35]. 25 g of 250-mesh crude vermiculite was added into a 1-L polypropylene beaker containing 800 mL of 2 M HCl solution at room temperature. The resulting slurry was magnetically stirred for 12 h. The product was separated by filtration and then washed thoroughly with distilled water several times until the filtrate had a pH value of 7.0. Then the silica nanosheets obtained were stored as aqueous suspension for the further use. Aniline, ammonium persulfate (APS), and other solvents used were all analytical grade reagents of Tianjin Chemicals Co. Ltd., China, and used without any pre-treatment. Distilled water was used throughout. 2.2. Preparation of the PANI/SNS composites Certain amount of the silica nanosheets suspension (including 0.23 g silica nanosheets/10 ml), aniline (9.313 g, 0.10 mol), and 10 ml conc. HCl were added into 200 ml water with stirring in ice-water bath. The mixtures were stirred for further 30 min to ensure that the mixtures were cooled to 0 °C. Then 100 ml aqueous

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solution of APS (containing APS 22.820 g, 0.10 mol) was dropped into the emulsions in 60 min. The mixture was stirred for another 4 h in ice-water bath. The products, PANI/SNS composites with different amounts of the silica nanosheets (1.0–4.0% weight ratios to aniline) (Table 1) were filtered and washed with water and ethanol each for three times in turn. Finally they were dried at 40 °C for 24 h under vacuum. 2.3. Analysis and characterization Elemental analysis (EA) of C and N was performed on Elementar vario EL instrument. The chemical structure of the PANI/SNS powders were conducted by recording infrared spectra using NEXUS 670 (Nicolet) in the range of 400–4000 cm 1 with the resolution of 4 cm 1. The KBr pellet technique was adopted to prepare the

Table 1 Preparation conditions of the PANI/SNS composites. Samples

PANI

PANI/SNS 1

PANI/SNS 2

PANI/SNS 3

PANI/SNS 4

Water (ml) Aniline (mol) HCl (mol) APS (mol) SNS (g) Products (g)

200 0.10 0.10 0.10 0 8.24

200 0.10 0.10 0.10 0.093 7.32

200 0.10 0.10 0.10 0.186 7.14

200 0.10 0.10 0.10 0.279 7.42

200 0.10 0.10 0.10 0.372 7.57

1.16

2.08

7.14

9.09

16.67

r(X

1

cm

1

)

sample for recording the IR spectra. The XRD patterns were recorded in the range of 2h = 10–80° by step scanning with a Shimadzu XRD-6000 X-ray diffractometer. Nickel-filter Cu Ka radiation (k = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 30 mA. The morphology of the silica nanosheets was characterized with a JEM-1200 EX/S transmission electron microscope (TEM). The silica nanosheets suspended colloid solution was deposited on a copper grid covered with a perforated carbon film. The Zeta potentials of the silica nanosheets at different pH values were determined with Zetasizer Nano ZS (Malvern Instruments Ltd, UK). The thermal stabilities of the PANI/SNS powders were determined with Shimadzu ZRY-2P at heating rate of 10 °C/ min from room temperature to 700 °C under N2 atmosphere. The surface morphologies of the PANI/SNS powders were observed using scanning electron microscopy (SEM) (XL-20, Philips Corporation, the Netherlands), operating at 25 kV. The electrical conductivities of the PANI/SNS powders were measured using SDY-4 FourPoint Probe Meter at ambient temperature employing the method on a pressed pellet. 3. Results and discussion 3.1. Silica nanosheets The preparation of the silica nanosheets via acid-leaching of vermiculite was reported in our previous work [36]. After a contact

Fig. 1. SEM images.

P. Liu / Current Opinion in Solid State and Materials Science 12 (2008) 9–13

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Fig. 2. TEM images of the silica nanosheets.

the silica nanosheets were negative charged. So the monomer, aniline, might adsorbed onto their surfaces as anilinium chloride salt [34] before the addition of the oxidant. 3.2. PANI/SNS composites

Fig 1. (continued)

time of 12 h with 2.0 M HCl, the diffraction peaks at 7.3° of the vermiculite was missing. This indicated that the silicate was delaminated and the platelets of vermiculite were less than tens cells or layers of single crystals. The silica nanosheets (Fig. 1 b) showed ordered layered structures compared with the pristine vermiculite (Fig. 1a) from SEM analysis. The terrace structure of the silica nanosheets was observed by TEM (Fig. 2). The silica nanosheets showed the negative zeta potential in the studied pH range (pH 2.0–12.0). This indicated that the surfaces of

The preparation parameters, including the amount of the silica nanosheets, aniline, and the concentration of hydrochloric acid in solution, are given in Table 1. One could found that the conversion of the monomer decreased with the increasing of the amount of the silica nanosheets. From the SEM images (Fig. 1d–g), the morphologies of the PANI/SNS composites containing the silica nanosheets 1.0–3.0% were similar to the pure PANI (Fig. 1c). This indicates that the silica nanosheets had been covered and encapsulated by the polyaniline matrices. However, some uncovered silica nanosheets were found when 4.0% weight ratio of the silica nanosheets added. Fig. 3 shows the FT-IR spectrum of PANI/SNS 4 composites containing 4.0% of the silica nanosheets. The characteristic peaks at about 1460 and 1600 cm 1 are assigned to the [email protected] stretching modes of the benzenoid ring and the quinoid ring, respectively. The peak at 1250 cm 1 is corresponded to CAN stretching vibration of the secondary aromatic amine. These results are in good agreement with previous spectroscopic characterizations of PANI. The band at 3425 and 3230 cm 1 is attributable to NAH and OAH stretching mode. The peak at 1100 cm 1 is attributable to SiAO vibration of the silica nanosheets. This confirms the incorporation of the silica nanosheets in polyaniline matrix using in situ polymerization method for nanocomposite preparation. Fig. 4 shows the XRD patterns of PANI and PANI/SNS composites. The patterns of PANI exhibit two broad peaks at 2h = 21° and 26°, which can be ascribed to periodicity parallel and periodicity perpendicular to the polymer chain, respectively [37]. There was no new peaks appeared. Therefore no additional crystalline order is introduced into the nanocomposites, and the crystalline behavior of PANI is not affected much by the amorphous silica nanosheets. 3.3. Electrical conductivities and TGA The conductivities of nanocomposites were found to increase from 1.16 to16.67 X 1 cm 1 with the increasing of the silica nano-

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100

Transmittance (%)

98 96 94 92 90 88 86 84 82 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

I(CPS)

Fig. 3. FT-IR spectrum of the PANI/SNS 4.

280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Silica/PANI 4 Silica/PANI 3

4. Conclusion

Silica/PANI 2

Novel polyaniline based nanocomposites combined with the silica nanosheets obtained via acid-leaching of vermiculite were prepared via the in situ chemical oxidative polymerization. The addition of the silica nanosheets did not damage the backbone structure of PANI. The conductivities of PANI/SNS composites are higher than that of pure PANI, and are enhanced with the increase in the SNS/monomer mass ratio because of the moisture absorption. It is expected the PANI/SNS composites could be used as sensors for moisture.

Silica/PANI 1 PANI

0

20

40

60

80

Theta-2Theta (deg) Fig. 4. XRD patterns of PANI and PANI/SNS composites.

100

References

PANI PANI/SNS 1 PANI/SNS 2 PANI/SNS 3 PANI/SNS 4

90 80

Weight (%)

TGA analysis (Fig. 5). The weight losses around 100 °C increased with the increasing of the silica nanosheets added in the composites. The weight losses were attributed to the release of the moisture absorption. So the increase of the electrical conductivities could be attributed to the moisture absorption which content was higher when the more silica nanosheets added. However, the conductivities of the PANI/SNS nanocomposites prepared in this work were lower than some PANI/layered nanocomposites reported. For example, the conductivities of the PANI/ layered clay nanocomposites were higher than that of the pure PANI [38]. It had been explained that the PANI chains intercalated into the layers were more extended. So the conductivities were enhanced although the nonconductors were added. In the present work, the silica nanosheets were dispersed intricately in the nanocomposites, as shown in Fig. 1. The PANI chains were not intercalated into the layers of the SNS so they were not extended as in the PANI/layered clay nanocomposites. And the silica nanosheets only disrupted the three-dimensional organization of the polymer chains. Furthermore, the dopant, HCl, might escape during the post-treatments of the samples, such as washing and drying. So the lower conductivities were resulted. In the TGA analysis of the nanocomposites, it was found that the presence of the silica nanosheets had not improved the thermal stability of polyaniline obviously. Similar results were obtained by the PANI/red mud nanocomposites [39]. It might due to the influence of the silica nanosheets on the polymerization and structures of the conducting polymer.

70 60 50 40 30 100

200

300

400

500

600

700

Temperature (deg) Fig. 5. TGA curves.

sheets weight ratios from 0% to 4.0% (Table 1). It is interesting that the electrical conductivities increased with the increasing of the nanosheets in the polyaniline based nanocomposites combined with nonconductor nanosheets. This could be explained with the

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