Synthesis, Characterization and Electrical properties of a Composite of Topological Insulating Material: Bi2Te3-PANI
aX-ray Research Laboratory, Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur – 440033, India
bPolymer Nanotech Laboratory, Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur – 440033, India
In the present work, we carried out a systematic study of structure, UV-Vis Spectra and surface conductivity of pure Bi2Te3, pure Polyaniline (PANI) and Bi2Te3 (5%)-PANI (95%) composite. Bi2Te3 was synthesized by a method similar to solvothermal method, whereas, pure PANI and Bi2Te3-PANI composite were synthesized by a chemical oxidative method. The materials were structurally characterized and the electrical properties were investigated in the temperature range from room temperature to 100°C. The electrical conductivity of the Bi2Te3-PANI composite is found to be higher than that of its pure constituents at all the temperatures. The enhancement in the surface conductivity may be due to the PANI generated ordered molecular arrangement of Bi2Te3 in the Bi2Te3-PANI composite, as confirmed from powder x-ray diffraction, UV–vis spectral analysis.
© 2015 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014).
Keywords: Topological insulators; Bismuth telluride; thermoelectric material; Polyaniline composites; electrical conductivity.
Topological insulators (TIs) are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface (Hasan and Kane, 2010). Most of the current researches are focused on the materials, like Bismuth telluride (Bi2Te3), Antimony telluride (Sb2Te3) and Bismuth selenide (Bi2Se3) due to the topological insulating properties exhibited by them. Bi2Te3 is one of the best TI materials. It is a semiconducting compound with narrow bulk band gap of 0.2 eV. It is also a good material for thermoelectric applications (Das and Soundararajan, 1988). It has shown a drastic change in its thermoelectric properties when it is doped with Polyaniline (PANI) (Li et al., 2011). It has also given added advantages of polymers like inexpensiveness, intrinsically low thermal conductivity, flexibility and good processability etc. (Marjanovic et al., 2013).
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Various research workers have attempted to synthesize and characterize composites of Bi2Te3 with polyaniline (Zhao et al., 2002; Xu et al., 2005; Hostler et al., 2006; Chatterjee et al., 2009; Toshima et al., 2011). Very recently, mechanical blending method was employed by Li et al. (2011) for the synthesis of Bi2Te3–PANI; they found that the power factor of the composite is less than both of the individual components. However, a report on the synthesis of a Bi2Te3 and PANI hybrid by physical mixing and solution mixing showed a higher power factor in the case of the physical mixture (Toshima et al., 2011). Though there is a vast literature available on the TE properties of Bi2Te3-PANI composites, experimental reports on TI Properties of the composites are not available. Based on this background, in the present work, we carried out a systematic study of structure, UV-Vis Spectra and surface conductivity of Bi2Te3 (5%)-PANI (95%) composite, pure Bi2Te3 and pure PANI. Bi2Te3 was synthesized by a method similar to solvothermal method (Deng et al., 2002), whereas, pure PANI and Bi2Te3-PANI composite were synthesized by a chemical oxidative method (Stejskal and Gilbert, 2002). The materials were structurally characterized and the electrical properties were investigated in the temperature range from room temperature to 100°C. The electrical conductivity of the Bi2Te3-PANI composite is found to be higher than that of its pure constituents at all the temperatures. The enhancement in the surface conductivity may be due to the construction of highly ordered chain structures of PANI on Bi2Te3, as confirmed from powder x-ray diffraction (XRD), UV–vis spectral analysis.
2. Experimental details
2.1 Materials used
Bismuth chloride (BiCl3), Tellurium (Te) metal powder, Potassium hydroxide (KOH), Sodium borohydrate (NaBH4), N-N dimethylformamide (DMF), Hydrochloric acid (HCl), Aniline (C6H5NH2), Acetone [(CH3)2.CO], Methanol (CH3OH) were purchased from Merck Chemicals. Ammonium persulphate [(NH4)2S2O8] was purchased from Hi-media. All the chemicals were of analytical reagent grade and are used without further purification, except aniline which was purified prior to use.
2.2 Preparation of Bi2Te3
A mixture of BiCl3 (10 mmol), Te powder (15 mmol), KOH (80 mmol) and NaBH4 (30 mmol) were put into a beaker of 100 ml capacity. The beaker was then filled with N-N dimethylformamide (DMF) up to 90 ml and was kept into a muffle furnace. Temperature of the furnace was maintained at 100-180°C for 24 hours and was then slowly cooled to the room temperature. The product was filtered, washed with double distilled water and dried in vacuum oven at 80°C for 12 hours.
2.3 Preparation of pure PANI and Bi2Te3– PANI Composite
PANI was synthesized by using chemical oxidative method (Stejskal and Gilbert, 2002) 0.2 mol Aniline with 0.25 mol Ammonium persulphate was oxidized in acidic aqueous medium. Aniline and Ammonium persulphate were dissolved, separately, in 50 ml solution of 1.0 mol HCl in double distilled water. Both the solutions were kept at room temperature for 1h and were mixed together in a beaker. During the process, the colour of the solution changed from colourless to light blue and then to dark green. The solution was briefly stirred and was left to polymerize for 24 h. The precipitate of PANI was collected on a filter paper, washed with distilled water, then with 20 ml of 0.1M HCl and with Methanol. PANI (emeraldine salt) powder was dried in vacuum oven at 80°C. A similar procedure was followed for the synthesis of Bi2Te3-PANI composite, but this time Bi2Te3 was introduced in aniline solution.
3.1 Structural characterization
The samples were structurally characterized by XRD and UV–vis spectral analyses. XRD measurements were performed using a Diffractometer (Rigaku Miniflex II x-ray Diffractometer) with Cu Kα radiation (λ = 1.541838 A°). The UV–vis spectra of the prepared samples were recorded by a spectrophotometer (UV-1800 Shimatzu Spectrophotometer) using samples dissolved in dimethyl formamide in a quartz cuvette.
3.2 Electrical characterization
All the prepared samples were pressed at pressure less than 5 tonnes, at room temperature, to form the compacted pellets for measurement of the electrical conductivity. The measurements were carried out, in the temperature range 20-100 °C, by standard four-probe method with constant current source kept at 2 mA.
4. Results and Discussions
4.1 Powder X-ray diffraction
Figure 1 shows the XRD patterns of Bi2Te3, pure PANI and Bi2Te3-PANI composite. All the peaks of the pattern for Bi2Te3 can be indexed in rhombohedral structure (JCPDS No: 015-0863) with unit cell parameters: a = 4.358A°; b = 4.358A° and c = 30.48A°. Broad peaks at 22° and 25° for pure PANI are observed, which are due to the repeat unit of monomer. Compared with the pure PANI, only one peak is clearly observed at 25°, the same position, in the XRD pattern of Bi2Te3-PANI composite. The observation of a single peak is related to the monodistribution of the periodicity of the repeat unit of the PANI and ordering of the molecular arrangement of the Bi2Te3 in the PANI matrix (Talwar et al., 2014). This suggests that PANI generate an ordered molecular arrangement of Bi2Te3 in the Bi2Te3-PANI composite.
Figure 1 XRD patterns of Pure PANI, Pure Bi2Te3 and Bi2Te3-PANI composite
4.2 UV-vis spectroscopy
Figure 2 shows the UV–vis spectra of the samples used to explore the electronic states of Bi2Te3, pure PANI and the Bi2Te3-PANI composite. A characteristic band centred at 697 nm, a shoulder at 478 nm, corresponding to a polaronic transition (polaron- π*) and a band around at 389 nm, assigned to the π–π* electron orbital transition are observed in Bi2Te3-PANI composite. In comparison with UV–vis spectra of the pure PANI: a band around 385 nm, assigned to the π–π* electron orbital transition, shifts to longer wavelength at 389 nm in Bi2Te3-PANI composite. The polaronic transition (polaron- π*) is absent in the pure PANI but is observed in Bi2Te3-PANI composite. These show an interaction between the quinoid ring of PANI and Bi2Te3 (Xia and Wang, 2003). This is a signature of an ordered molecular arrangement in Bi2Te3-PANI composite and it is the purely surface conducting state.
Figure 2 UV-vis spectra of Pure PANI, Bi2Te3 and Bi2Te3-PANI composite
4.3 Electrical characterization
Figure 3 Variation of the electrical conductivity with temperature of Bi2Te3, PANI and Bi2Te3-PANI composite.
The variations of electrical conductivity as a function of temperature of the prepared samples are shown in figure 3. The value of electrical conductivity for Bi2Te3 slightly decreases with the increase in the temperature. However, both PANI and Bi2Te3-PANI composite show increase in the value of electrical conductivity with temperature. The electrical conductivity of the Bi2Te3-PANI composite is higher than that of its pure constituents over a whole temperature range. The enhancement in the surface conductivity indicates the increase in protected states at the surface compared to that of the pure Bi2Te3. This may be due to the large surface provided to the Bi2Te3 by the PANI generated ordered molecular arrangement of Bi2Te3 in the Bi2Te3-PANI composite.
In conclusion, we carried out a systematic study of structure, UV-Vis Spectra and surface conductivity of Bi2Te3 (5%)-PANI (95%) composite, pure Bi2Te3 and pure PANI. Bi2Te3 was synthesized by a method similar to solvothermal method, whereas, pure PANI and Bi2Te3-PANI composite were synthesized by a chemical oxidative method. The materials were structurally characterized and the electrical properties were investigated in the temperature range from room temperature to 100°C. The electrical conductivity of the Bi2Te3-PANI composite was found to be higher than that of its pure constituents at all the temperatures. The enhancement in the surface conductivity may be due to the PANI generated ordered molecular arrangement of Bi2Te3 in the Bi2Te3-PANI composite, as confirmed from powder x-ray diffraction (XRD), UV–vis spectral analysis.
It is a pleasure to thank Dr. S. B. Kondawar for helpful discussions. This work was supported by the Department of Science and Technology (DST), India, under women scientist scheme-A (WOS-A), File No. SR/WOS-A/PM-1001/2014.
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