PPy composites

Solid State Communications 150 (2010) 1807–1811

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Synthesis, characterization and electrochemical impedance spectroscopy of VOx-NTs/PPy composites Chaojun Cui, Guangming Wu ∗ , Huiyu Yang, Shifeng She, Jun Shen, Bin Zhou, Zhihua Zhang Pohl Institute of Solid State Physics, Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, PR China

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Article history: Received 16 December 2009 Received in revised form 1 July 2010 Accepted 6 July 2010 by F. Peeters Available online 16 July 2010 Keywords: A. VOx-NTs A. Polypyrrole A. Hybrid composites E. Electrochemical impedance spectroscopy

abstract Composites consisting of vanadium oxide nanotubes (VOx-NTs) and polypyrrole (PPy) were synthesized by a two-steps method. VOx-NTs were firstly prepared by a combined sol–gel reaction and hydrothermal treatment procedure, in which V2 O5 powder and H2 O2 were used as raw materials and hexadecylamine as a structure-directing template. Then VOx-NTs/PPy composites were fabricated by a cationic exchange reaction between hexadecylamine and polypyrrole. The structure and morphology of the samples were investigated by SEM, TEM, XRD and FTIR techniques. The results confirmed that the template molecules were successfully substituted by the conducting polymers PPy without destroying the previous tubular structure. Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical kinetics of the samples. The results indicated that VOx-NTs/PPy composites had a lower charge transfer resistance and a faster lithium-ion diffusion speed than those of VOx-NTs, and the enhanced electrochemical kinetics could be attributed to the excellent electronic conductivity of polypyrrole. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Since carbon nanotubes were discovered by Iijima [1], onedimensional nanotubes including organic and inorganic nanotubes have attracted considerable interests due to their unique onedimensional tubular structure and high specific surface area [2–5]. Among these nanotubes, vanadium oxides nanotubes (VOx-NTs) are one of the most interesting materials to be applied as catalysts, sensors and cathode materials [6–10]. In general, VOx-NTs were synthesized by using vanadium alkoxides, ammonium vanadate, or vanadium pentoxide as precursors and the long chain amines as a structure-directing template under hydrothermal conditions. However, the electronic conductivity of VOx-NTs is rather poor due to the low electronic conductivity of the organic template and transition metal oxides [8]. Therefore, it is important to study how to enhance the electronic conductivity of VOx-NTs. Some reports revealed that a large proportion of the organic template can easily be substituted by metal cations due to the structural versatility of oxide network [8,11–15]. These methods can not only enhance the electronic conductivity of the materials, but also preserve the previous tubular structure. Recently, polypyrrole (PPy), a key member of organic conducting polymers with conjugated double bonds, has received

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Corresponding author. Tel.: +86 21 65980886; fax: +86 21 65986071. E-mail addresses: [email protected] (C. Cui), [email protected] (G. Wu). 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.07.008

considerable attention because of its high electronic conductivity, excellent electrochemical and environmental stability. These properties make it an extremely useful application in different fields such as gas sensors and electronic devices [16,17]. In this paper, we report a convenient and controllable route to fabricate the VOx-NTs/PPy composites by exchanging hexadecylamine with polypyrrole. SEM, TEM, XRD and FT-IR are employed to investigate the structure and morphology of the samples. Electrochemical impedance spectroscopy (EIS) as a powerful tool for studying interfacial charge transfer and lithium-ions diffusion is carried out to evaluate the electrochemical kinetics of the samples. 2. Experimental 2.1. Preparation of materials The synthesis of VOx-NTs was performed following the method described by Chandrappa et al. [18]. Firstly, 1 g crystalline V2 O5 was dissolved in a solution of hydrogen peroxide (H2 O2 , 50 ml, 30%). The exothermic reaction was occurred during the synthetical process, which led to the release of oxygen gas and the formation of V (V) peroxo complexes. A clear orange solution was formed after about 20 min then gradually turned into a red gel after about 1 day, this gel has been shown to correspond to V2 O5 ·nH2 O. Secondly, 1.35 g hexadecylamine (C16 H33 NH2 ) was added to the V2 O5 ·nH2 O gel and the green mixture were stirred for 16 h. Finally, the resulting composites were transferred into a Teflon-lined autoclave with a stainless steel shell. The autoclave was kept at

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Fig. 1. SEM and TEM images of VOx-NTs (a and c) and VOx-NTs/PPy (b and d).

180 °C for 7 days. The product was filtered and washed repeatedly with distilled water and absolute ethanol, dried at 80 °C under a vacuum for about 5 h and a black powder was then obtained, these were VOx-NTs. To obtain VOx-NTs/PPy composites, VOx-NTs (0.5 g) were dispersed in deionized water (100 ml) within an ultrasound sonication bath for 30 min at room temperature. After that, the mixture was removed to a vessel with vigorous stirring at 0 °C by ice-water bathing. Then, 9.1 g FeCl3 ·6H2 O was first dissolved in 10 ml of deionized water and then slowly added into the stirred solution. After 30 min, 1 ml freshly distilled pyrrole monomer was slowly injected into the stirred solution and stirred continuously. The polymerization was carried out for 12 h with constant mechanical stirring, and the whole synthesis process was bubbled with N2 gas. The synthesized VOx-NTs/PPy composites were filtered and rinsed several times with distilled water and ethanol. Finally, the product was dried under a vacuum at 80 °C for 10 h. 2.2. Characterization and measurements Scanning electron microscopy (SEM, Philips-XL-30FEG) and transmission electron microscopy (TEM, JEOL-1230) were employed to observe the structure and morphology of the samples. X-ray powder diffraction (XRD) was carried out using a Rigata/maxC diffractometer with Cu-Kα radiation source (λ = 1.5406 Å). The infrared spectroscopy measurements were carried out in the range of 400–3000 cm−1 using a Bruker-TENSOR27 FTIR spectrometer. The working electrodes were prepared by pasting the mixture of active materials, carbon black and polyvinylidene fluoride (PVDF) binder (80:10:10 wt%) on the aluminum foils. The coated electrodes were dried at 120 °C for 12 h and then pressed, a microporous film (Celgard 2500) was used as the separator, lithium metal was used as the counter and reference electrode, and the electrolyte was 1 M LiPF6 dissolved in ethylene carbonate

(EC)/ dimethyl carbonate (DMC) (1:1, v/v), then the coin cells were assembled in an argon-filled glove box with moisture content and oxygen levels less than 1 ppm. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI660C electrochemical workstation at open circuit potential and a discharge potential of 1.5 V, the amplitude of the AC signal was 5 mV over the frequency range between 100 kHz and 0.001 Hz. The impedance spectra were analyzed using Z-view software. 3. Results and discussion The SEM images of the two samples are presented in Fig. 1(a) and (b), respectively. It can be seen that both VOx-NTs and VOxNTs/PPy present a uniform tubular-like morphology which are mostly grown-together in the form of bundles. From the TEM images (shown in Fig. 1(c) and (d)), both samples exhibit an openended and multiwalled tubular structure with inner diameters of 20–40 nm and outer diameters of 50–80 nm. It can be seen that the tubular shape is retained after exchanging between hexadecylamine and PPy, but the inner walls of VOx-NTs/PPy are no more parallel and straight than those of VOx-NTs. Both the SEM and TEM images disclose that the substitution of hexadecylamine by polypyrrole does almost nothing destroy the previous tubular shape. As shown in Fig. 2(a), the FTIR spectrum of VOx-NTs exhibits four characteristic absorption peaks at 2922, 2850, 1465 and 721 cm−1 , which are attributed to the axial stretching vibrations of aliphatic C–H, the symmetric angular deformation in the plane of CH2 species and the symmetric out of plane angular deformation of N–H species in the hexadecylamine molecules, respectively [19]. Compared to VOx-NTs, the corresponding four characteristic peaks of VOx-NTs/PPy decrease remarkably and even disappear, indicating that a large part of organic template are extracted from the interlamellar space of vanadium oxide layers.

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Fig. 2. FT-IR spectra (a) and XRD patterns (b and c) of VOx-NTs and VOx-NTs/PPy.

From the FTIR spectrum of VOx-NTs/PPy composites, most of the characteristic peaks associated with oxidized polypyrrole are observed, the peak located at 1566 cm−1 is associated with the C–C and C=C backbone stretching vibration of the pyrrole ring, the vibration bonds at 1202 and 933 cm−1 can be assigned to the stretching vibration of the doped polypyrrole, the peaks at 1290 and 1055 cm−1 are attributed to C–N stretching vibration and C–H deformation vibration, respectively [20–22]. Furthermore, the weak peak at 1710 cm−1 is attributed to the carbonyl group, indicating that polypyrrole is slightly overoxidized during the growth process [22]. In addition, the three intense peaks located at 1010, 570 and 496 cm−1 are assigned to the stretching vibration of V=O bond, asymmetric and symmetric stretching vibrations of V–O–V bonds from the vanadyl groups, respectively [13]. In contrast with the FTIR spectra of the two samples, it can be seen that the three peaks of V–O bonds are almost unaffected during the exchange reaction process. These results confirm that the template molecules are mostly substituted by polypyrrole in the inner-layer space without damaging the previous tubular structure. The XRD patterns of VOx-NTs and VOx-NTs/PPy are shown in Fig. 2(b) and (c), respectively. Fig. 2(b) exhibits the 00l set of diffraction peaks with high intensity at low diffraction angle (2θ < 15°), which correspond to well-ordered lamellar structure of the samples. Fig. 2(c) presents the hk0 set of diffraction peaks with lower intensity at larger diffraction angle (2θ > 15°), which correspond to two-dimensional structure of the vanadium oxide layers. In contrast with the 00l series peaks of the two samples (see Fig. 2(b)), the slight right shift of the 00l peaks for VOx-NTs/PPy correspond to a slight reduction of the interlayer distance. The dvalue of the 001 peak decreases from 3.43 nm of VOx-NTs to 3.28 nm of VOx-NTs/PPy. In addition, the relative intensity of the 00l peaks for VOx-NTs/PPy is lower than those of VOx-NTs, indicating that it is not as well-ordered as before. It can be seen from Fig. 2(c) that there are no significant differences between the hk0 series peaks of the two samples, indicating that the hk0 series peaks reflect only the two-dimensional structure of vanadium oxide layers and have nothing to do with the intercalating species. Therefore, these results indicate that the tubular structure of

VOx-NTs/PPy is retained after exchanging between hexadecylamine and PPy, which is in consistent with the analytical results obtained by TEM and IR techniques. In order to further understand the improved performance, electrochemical impedance spectroscopy (EIS) was employed to evaluate the electrochemical kinetics of the both samples. The typical Nyquist plots recorded for VOx-NTs and VOx-NTs/PPy at open circuit potential and a discharge state of 1.5 V are presented in Fig. 3(a) and (c), respectively. All plots display a depressed semicircle in the high-frequency region and a sloped line in the low-frequency region. In general, the semicircle in the highfrequency region is associated with the charge transfer reaction at the electrolyte/electrode interface and the sloped line in the lowfrequency region is related to the Warburg impedance associated with the ions diffusion process in the electrode materials [23,24]. According to Refs. [8,25], the diameters of the semicircles are related to the charge transfer resistance, the smaller the diameters of the semicircles, the smaller the charge transfer resistance and the higher the electronic conductivity. As can be seen from Fig. 3(a) and (c), the semicircle diameters of VOx-NTs/PPy composites are much smaller than those of VOx-NTs in the high-frequency region, indicating that VOx-NTs/PPy composites have lower charge transfer resistance or higher electronic conductivity compared to VOx-NTs. A simple equivalent circuit shown in the inset of Fig. 3(a) is built to analyze the impedance spectra of the both samples. In this circuit, Re presents the electrolyte resistance, Rct stands for the charge transfer resistance, CPE is the double layer capacitance, and W is the Warburg impedance. The electrochemical parameters of the two samples are simulated by using Z-view software, and a good agreement between experimental results and the parameters obtained from the equivalent circuit can be seen from the EIS spectra. The fitted electrochemical parameters are listed in Table 1. As can be seen from Table 1, the charge transfer resistances (Rct ) for VOx-NTs and VOx-NTs/PPy composites at open circuit potential are 340.8  and 181.3 , respectively. When discharged to 1.5 V, the corresponding resistances for the two samples are 565.9  and 202.5 , respectively. It is clear that the Rct values of VOx-NTs/PPy composites are much smaller than that of

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Fig. 3. EIS spectra of VOx-NTs and VOx-NTs/PPy: Nyquist plots (a) and Bode plots (b) at open circuit potential; Nyquist plots (c) and Bode plots (d) at a discharge potential of 1.5 V. The inset in Fig. 3(a) is the equivalent circuit.

Table 1 The simulated charge transfer resistances and the errors of the both samples at the different conditions. Samples

Open circuit potential Rct / Error/(%)

Discharged to 1.5 V Rct / Error/(%)

VOx-NTs VOx-NTs/PPy

340.8 181.3

565.9 202.5

0.76 1.46

1.72 1.57

VOx-NTs, indicating that VOx-NTs/PPy composites have a much higher electronic conductivity than that of VOx-NTs. The Bode plots can be used to estimate the effectiveness of lithium-ions diffusion in the electrode materials. According to Refs. [26,27], the lithium-ions diffusion is related to the phase angle at the low-frequency region, the smaller the phase angle, the better the capacitive performance and the faster the lithiumions diffusion. As shown in Fig. 3(b) and (d), the phase angle of VOx-NTs/PPy composites is much smaller than that of VOxNTs at the low-frequency region, indicating that VOx-NTs/PPy composites have more rapid lithium-ions diffusion speed than that of VOx-NTs. Therefore, EIS tests demonstrate that VOx-NTs/PPy composites have a lower charge transfer resistance and a faster lithiumions diffusion speed than those of VOx-NTs, the improved electrochemical kinetics may be attributed to the synergism of the conducting polymer polypyrrole. 4. Conclusions

Acknowledgements This research is financially supported by National Natural Science Foundation of China (50752001 and 50802064), the funds (07JC14052, 10JC1414800, 0852nm01300, 0952nm00700 and 0952nm00900) from the Science and Technology Commission of Shanghai Municipality, Innovation Program of Shanghai Municipal Education Commission (08ZZ22) and Shanghai Key Lab Program (07DZ22302). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

In summary, VOx-NTs/PPy composites were successfully synthesized by the cationic exchange reaction between hexadecylamine and polypyrrole. The morphological and structural characterization confirmed that the template hexadecylamine was successfully substituted by the polypyrrole without destroying the previous tubular structure. EIS tests demonstrated that VOxNTs/PPy composites had a lower charge transfer resistance and a faster lithium-ion diffusion speed than those of VOx-NTs. Therefore, VOx-NTs/PPy composites possess potential applications in sensors and electronic devices.

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