Surface modification of nano-SiO2 particles using polyaniline

Surface & Coatings Technology 197 (2005) 56 – 60 www.elsevier.com/locate/surfcoat

Surface modification of nano-SiO2 particles using polyaniline Xingwei Lia,*, Gengchao Wanga, Xiaoxuan Lib a

School of Materials Science and Engineering, East China University of Science and Technology, Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai 200237, PR China b Chemistry Department, Nanjing University of Science and Technology, Nanjing 210094, PR China Received 4 March 2004; accepted in revised form 22 November 2004 Available online 5 January 2005

Abstract An electrical conducting polyaniline/nano-SiO2 composite was obtained by surface modification of nano-SiO2 particles using polyaniline, and was characterized via Fourier transform infrared (FTIR) spectra, UV–vis absorption spectra, wide-angle X-ray diffraction (WXRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM), as well as conductivity. Surface area, total pore volume, and pore size distribution of both nano-SiO2 particles and polyaniline/nano-SiO2 composite particles were also measured. The results of spectroanalysis show that the composite is not a simple blend of polyaniline with nano-SiO2 particles. An interaction exists at the interface of nano-SiO2 particles and polyaniline macromolecule. Polyaniline/nano-SiO2 composite contains 15% conducting polyaniline by mass, with a conductivity of 0.32 S cm 1 at 25 8C. The dimension of polyaniline/nano-SiO2 composite particles is about 20–30 nm. D 2004 Published by Elsevier B.V. Keywords: Polyaniline; Silicon oxides; Conductivity; Interface

1. Introduction Conducting polymer/inorganic nanocomposites, which possess unique physical and chemical properties, have attracted more and more attention. This is primarily because they combine the merits of conducting polymers and inorganic nanoparticles, and have wide potential applications in diverse areas such as chemistry, physics, electronic, optics, materials, and biomedical science [1–4]. Recently, several studies have been reported on the composite of conducting polymers and inorganic materials. A number of different metals and metal oxide particles have so far been encapsulated into the shell of conducting polymers, giving rise to a host of nanocomposites [5–12]. These composite materials have shown better mechanical, physical, and chemical properties.

* Corresponding author. Tel.: +86 21 64253527; fax: +86 21 64251372. E-mail address: [email protected] (X. Li). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.11.021

Polyaniline is generally recognized to be one of the most important conducting polymers. It has a great potential for commercial applications because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and excellent environmental stability [13–15]. In this paper, we report the properties of polyaniline/ nano-SiO2 composite, which is prepared by surface modification of nano-SiO2 particles using polyaniline. The synthesis method we used here is very simple in comparison with others, and has a great potential for commercialisation of the technology. The results of spectroanalysis indicate that the composite is not a simple blend of polyaniline with nano-SiO2 particles, but rather nano-SiO2 particles serve as the reaction core, and polyaniline macromolecules interacted on the surface of nano-SiO2 particles. The conductivity of polyaniline/nano-SiO2 composite containing 15% polyaniline has reached 0.32 S cm 1 at 25 8C. At this level, it can potentially be used in commercial applications as fillers for electromagnetic shielding materials and conductive coatings.

X. Li et al. / Surface & Coatings Technology 197 (2005) 56–60

2. Experimental 2.1. Materials Aniline (Shanghai Chemical Works, China) was used after twice distillation. Nano-SiO2, with an average particle size of approximately 20 nm (Jiang Su Hehai Nano-ST, China), was used without further purification. Other chemicals were of AR grade. Deionized water was used in this investigation.

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Conductivity was measured on compressed pellets of the powder sample by using the conventional four-probe technique at 25 8C. Surface area, total pore volume, and pore size distribution of nano-SiO2 particles and polyaniline/nano-SiO2 composite particles were measured by COULTERk SA3100k Series Surface and Pore Size Analyzers.

3. Results and discussion

2.2. Preparation

3.1. Fourier transform infrared spectra

Polyaniline/nano-SiO2 composite was prepared as follows. Aniline (5 ml) was injected to 70 ml of 2 M HCl containing 5 g of nano-SiO2 particles under ultrosonication to break the aggregation of nano-SiO2 particles. After 2 h, 4.5 g of (NH4)2S2O8 (in 20 ml of deionized water) was added dropwise into the above solution while stirring was applied constantly. The polymerization was allowed to proceed for 3 h at room temperature. The solid product was obtained by filtration, then washed with 2 M HCl and deionized water to remove residual aniline hydrochloride and ammonium sulfate. Finally, it was dried at 60 8C for 24 h under vacuum. The final product is a fine tint green powder. Doped polyaniline, in contrast to polyaniline/nano-SiO2 composite, was prepared in the procedure as described below. Aniline (5 ml) was mixed with 70 ml of 2 M HCl, then 4.5 g of (NH4)2S2O8 (in 20 ml of deionized water) was added dropwise into the solution while stirring was applied constantly. The polymerization was allowed to proceed for 3 h at room temperature. A fine dark green powder was obtained after filtration, washed, and dried under the same condition as in the composite preparation procedure.

Fig. 1 shows FTIR spectra of nano-SiO2, polyaniline/ nano-SiO2 composite, and doped polyaniline, respectively. The main characteristic peaks of doped polyaniline are assigned as the following. The peak at 3449 cm 1 is attributable to N–H stretching mode, the peaks at 1569 cm 1 and 1473 cm 1 are attributed to CMN and CMC stretching modes for the quinoid and benzenoid rings, the peaks at about 1292 cm 1 and 1232 cm 1 are attributed to C–N stretching mode for benzenoid ring, and the peak at 1110 cm 1 is assigned to the plane bending vibration of C– H (modes of NMQMN, QMN+H–B and B–N+H–B), which is formed during protonation [16]. It is evident from Fig. 1b that the FTIR spectrum of the composite contains contributions from both the nano-SiO2 particles (Fig. 1a) and the polyaniline (Fig. 1c). However, it is difficult to assign the absorption peaks of the composite because the nano-SiO2 particles and doped polyaniline absorb at similar wavenumbers. It is also noted, by comparing Fig. 1b and c, that some polyaniline peaks are shifted due to interactions with nano-SiO2 particles. For example, the stretching modes of CMN, CMC, and C–N at 1569 cm 1, 1473 cm 1, and 1292 cm 1 all shift to higher wavenumbers. Similarly, the peak at 1110 cm 1, formed upon protonation, also shifts to 1120

2.3. Measurements

(a) Transmittance (%)

Fourier transform infrared (FTIR) spectra of the powder sample in the range of 400– 4000 cm 1 were recorded with a fully computerized Bruker VECTOR22 spectrometer. FTIR samples were prepared as KBr pellets. UV–vis absorption spectra of powder sample were made on a Shimadzu UV-2401PC spectrophotometer. Using an integrating sphere and BaSO4 as a white standard, diffuse reflectance spectra were recorded as R stand/R sample vs. wavelength, where R is the absolute reflection intensity. Measurements of wide-angle X-ray diffraction (WXRD) were taken on a Shimadzu XD-3A instrument using CuKa radiation (k=0.154 nm). Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50 instrument at the heating rate of 20 8C min 1 in air. The sample morphology was examined by using transmission electron microscopy (TEM; JEM-100S).

(b) 1603 1300 1491 1120 3439

(c) 1569 14731292 1232 1110

3449

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 1. Fourier transform infrared spectra of nano-SiO2 (a), polyaniline/ nano-SiO2 composite (b), and doped polyaniline (c).

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X. Li et al. / Surface & Coatings Technology 197 (2005) 56–60

cm 1. However, N–H stretching peak at 3449 cm 1 shifts to lower wavenumber. These IR absorption changes suggest that the CMN, CMC, and CUN bonds become stronger in polyaniline/nano-SiO2 composite, but the NUH bond becomes weaker. This is probably because of the hydrogen bonding between the surfaces of the electronegative nanoSiO2 particles and the N–H group in the polyaniline macromolecule [17].

(a)

3.2. UV–vis absorption spectra

(b)

Fig. 2 gives UV–vis absorption spectra of doped polyaniline, polyaniline/nano-SiO2 composite, and nanoSiO2, respectively. Fig. 2a clearly indicates that doped polyaniline has three characteristic peaks, which are at about 300 nm, 450 nm, and 900 nm. The 300 nm peak arises from electron transition within the benzenoid segments. The absorption peak at 450 nm represents the protonation of polyaniline chains, and the absorption around 900 nm is assigned to the presence of polaron resulting from the doping process [4,18]. Fig. 2b shows that the characteristic peaks of polyaniline and nano-SiO2 appear in polyaniline/ nano-SiO2 composite. However, the peak at around 900 nm obviously shifts to shorter wavelength. This implies that encapsulation of nano-SiO2 has an effect on the doping level of conducting polyaniline, with this arising from the interaction between the polyaniline macromolecule and the nano-SiO2 particles. 3.3. Wide-angle X-ray diffraction patterns Wide-angle X-ray diffraction patterns of the nano-SiO2 particles, polyaniline/nano-SiO2 composite particles, and doped polyaniline are shown in Fig. 3. Fig. 3c reveals that doped polyaniline also has some degree of crystallinity. A

(a)

(c)

10

20

30

40

50

60

70

80

2θ Fig. 3. Wide-angle X-ray diffraction of nano-SiO2 (a), polyaniline/nanoSiO2 composite (b), and doped polyaniline (c).

maximum peak at 258 can be assigned to the scattering from polyaniline chains at interplanar spacing [19]. From curve b in Fig. 3, it can be seen that peaks of doped polyaniline do not show up in the polyaniline/nano-SiO2 composite. This indicates that the nano-SiO2 particles influence the crystalline behaviour of polyaniline. Therefore, the crystallinity degree of polyaniline decreases, and the diffraction peaks disappear gradually. The observation also suggests that an interaction exists at the interface of polyaniline and nanoSiO2 particles, which restricts aggregation of polyaniline to form bulk polymer. Comparing curves a and b, it can be seen that the diffraction pattern of the polyaniline/nano-SiO2 composite is the same as nano-SiO2. It also implies that polyaniline deposited on the surface of nano-SiO2 particles has no effect on crystallization performance of nano-SiO2 particles.

R stand/R sample

3.4. Transmission electron microscopy

(b)

(c)

200

300

400

500

600

700

800

900

Wavelength (nm) Fig. 2. UV–vis absorption spectra of doped polyaniline (a), polyaniline/ nano-SiO2 composite (b), and nano-SiO2 (c).

The TEM of nano-SiO2 particles and polyaniline/nanoSiO2 composite particles is shown in Fig. 4. Both micrographs are in the same magnification. It can be seen from Fig. 4a that the original spherical commercial nano-SiO2 particles were aggregated in aqueous solution with an irregular shape. This can be attributed to the high surface energy of nanoparticles. However, a different appearance was observed for the polyaniline/nano-SiO2 composite particles, which were nearly spherical in shape with a few sandwich-like particles, as shown in Fig. 4b. Comparing the spherical nano-SiO2 particles and polyaniline/nano-SiO2 composite particles, it can be found that nano-SiO2 particles have a higher surface energy and a stronger tendency toward aggregation. This result is also

X. Li et al. / Surface & Coatings Technology 197 (2005) 56–60

(a)

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(b)

Fig. 4. Transmission electron microscope of nano-SiO2 (a) and polyaniline/nano-SiO2 composite (b).

supported by the fact that polyaniline/nano-SiO2 composite particles are more easily dispersed than nano-SiO2 particles. Within 10 min, polyaniline/nano-SiO2 composite particles can be dispersed well in aqueous solution, but nano-SiO2 particles need about 30 min. 3.5. Thermogravimetric analysis Fig. 5 illustrates the results of thermogravimetric analysis of nano-SiO2, polyaniline/nano-SiO2 composite, and doped polyaniline, respectively. Curve a in Fig. 5 reveals that pure nano-SiO2 particles are stable in air, and only gives a small weight loss in the range of low temperature, which is presumably the water content of nano-SiO2 substrate. From curve c in Fig. 5, it can be seen that thermal degradation of polyaniline occurs at 480 8C. The initial mass loss at lower temperatures is mainly due to the release of water and dopant anions from the polyaniline. A sharp loss in mass is observed at 320 8C and continues to 700 8C, possibly due to a large scale of thermal degradation of the polyaniline chains.

100

(b)

Weight (%)

60

(c) 40

20

0 100

200

300

400

500

600

3.6. Conductivity The conductivity of polyaniline/nano-SiO2 composite containing 15% polyaniline is 0.32 S cm 1 at 25 8C. Apparently, polyaniline molecules are the most important carriers in polyaniline/nano-SiO2 composite. Although its conductivity is lower than pure polyaniline prepared at the same condition (1.3 S cm 1), it is still useful to enlarge the applied fields of nano-SiO2 and improve the processability of polyaniline. 3.7. Surface area, total pore volume, and pore size distribution Surface area, total pore volume, and pore size distribution of nano-SiO2 particles and polyaniline/nano-SiO2 composite particles are shown in Tables 1 and 2. Table 1 reveals that the surface area of polyaniline/nanoSiO2 composite particles is smaller than that of nano-SiO2 particles, and the total pore volume of polyaniline/nanoSiO2 composite particles increases. This implies that the synthesized polyaniline was deposited on the surface of nano-SiO2 particles, and formed a core–shell structure in which polyaniline encapsulated the nano-SiO2 particles. Polyaniline deposition on the surface of the nano-SiO2

(a)

80

Curve b in Fig. 5 represents the thermal decomposition of polyaniline/nano-SiO2 composite. Curve b shows that the temperature of thermal decomposition of polyaniline/nanoSiO2 composite is about 435 8C, which is lower than that of pure polyaniline. The drop in temperature may be associated with the effect of nano-SiO2 particles upon polyaniline macromolecule. Polyaniline/nano-SiO2 composite exhibits a polyaniline layer mass of 15% in comparison with curves a and b in Fig. 5.

700

800

Table 1 Surface area and total pore volume of nano-SiO2 particles and polyaniline/ nano-SiO2 composite particles

Temperature (oC) Fig. 5. Thermogravimetric analysis curves of nano-SiO2 (a), polyaniline/ nano-SiO2 composite, (b) and doped polyaniline (c).

BET surface area (m2/g) Total pore volume (ml/g)

Nano-SiO2

Polyaniline/nano-SiO2

168.58 0.4718

96.84 0.5448

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X. Li et al. / Surface & Coatings Technology 197 (2005) 56–60

Table 2 Adsorption BJH pore size distribution of nano-SiO2 particles and polyaniline/nano-SiO2 composite particles Pore diameter range (nm)

Under 6 6–8 8–10 10–12 12–16 16–20 20–80 Over 80 BJH total

Nano-SiO2 particles

Polyaniline/nano-SiO2 composite particles

Pore volume (ml/g)

%

Pore volume (ml/g)

%

0.04909 0.02680 0.02163 0.02239 0.02854 0.03177 0.20160 0.19426 0.57536

8.53 4.53 3.76 3.89 4.96 5.52 35.04 33.76 100.00

0.01763 0.01157 0.01163 0.01386 0.02277 0.02909 0.37203 0.16082 0.63939

2.76 1.81 1.82 2.17 3.56 4.55 58.18 25.15 100.00

the two. An interaction exists at the interface of nano-SiO2 particles and the polyaniline macromolecule, and it is highly likely that this interaction is the hydrogen bonding between the surfaces of the electronegative nano-SiO2 particles and polyaniline macromolecules.

Acknowledgement We are grateful for the financial support from the Development Project of Shanghai Priority Academic Discipline and Shanghai Municipal Science and Technology Commission (0352 nm052).

References particles increases the size and decreases the surface area of the particles. The data of the pore size distribution of both nano-SiO2 particles and polyaniline/nano-SiO2 composite particles also support this conclusion. Table 2 shows that the pore size distribution of the nanoSiO2 particles decreases on deposition of polyaniline, except for pore diameters of 20–80 nm. Particles with pore diameters of 20–80 nm and with a pore volume of 0.37203 ml/g are more likely to be formed and dominate the distribution in the polyaniline/nano-SiO2 composite. It appears that more polyaniline is deposited on the surface of nano-SiO2 particles with smaller diameters and higher surface area.

4. Conclusions A polyaniline/nano-SiO2 composite containing 15% conducting polyaniline has been obtained. Synthesized polyaniline was deposited on the surface of nano-SiO2 particles, forming a core–shell structure. The dimension of polyaniline/nano-SiO2 composite particles is about 20–30 nm. The electrical conductivity of the composite reaches 0.32 S cm 1 at 25 8C. Spectroanalyses demonstrate that the polyaniline/nano-SiO2 composite is not a simple blend of

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