Structural and Electrical Studies of Bi 2 Se 3 Nanomaterials by Solvothermal Method for Thermo-electric Applications

The Bi 2 Se 3 nanoparticles were synthesized by the solvothermal method. The structural and morphological characterization has been done using XRD, HRSEM and Raman while electrical studies at room temperature were analyzed using impedance spectroscopy and cyclic voltmetry. The phase formation was con�rmed through XRD measurement and average grain size was found to be 25 nm for as-prepared sample and 37 nm for 650 ºC for 12 hours annealed sample. Raman spectrum appears in the higher frequency range, this is due to stronger bonding forces, i.e the peak at 524 cm − 1 or may arise due to the overtones of A 11g and E 2g modes. The redox behavior was due to Bi 3+ converted into Bi 2+ and Bi metallic state. This redox peak was con�ned the formation of Bi 2 Se 3 nanoparticles. The high temperature electrical conductivity studies were performed as-prepared and annealed sample using impedance spectroscopy.


Introduction
Bismuth selenide (Bi 2 Se 3 ) is a gray powder that is a compound of bismuth and selenium also known as bismuth (III) selenide.It is a semiconductor and a thermoelectric material [1].While perfect stoichometric bismuth selenide should be a semiconductor (with a gap of 0.3 eV) naturally occurring selenium vacancies act as electron donors and it often acts as a semimetal [2].Topologically protected surface states have been observed in Bismuth selenide [3] which is the subject of ongoing scienti c research [4].
Studies on thin lms of Bi 2 Se 3 are attracting wide attention for various optoelectronic devices [5].
Semiconducting materials, which are based totally on sul des, selenides and tellurides, have currently obtained a lot activity as materials for optoelectronics technology.Among these, bismuth selenide (Bi 2 Se 3 ), a member of the family of A V B VI semiconducting materials, has been signi cantly studied due to its achievable purposes in infrared photography [6], optical and photosensitive devices [7], present day thermo-electric cooling modules [8], photo electrochemical cells and electro-thermal devices [9,15].Bismuth selenide is a narrow direct band gap semiconductor (E g = 0.16-0.35eV) [16] and noticeable for its signi cant and motivating properties.It has attracted extensive interest due to its feature anisotropic layered structure as well as important applications in the eld of optical recording systems and strain gauges [17].Bismuth selenide crystallizes in the rhombohedral unit cell, where correspondent atoms are approved in two dimensional, layers perpendicular to the c-axis.Five such layers-in the sequence of Se 1 -Bi-Se 2 -Bi-Se 1 , where the Se atoms occupy two different sites, form a tight bound unit, whereas these 'sandwiches' are held together by a much weaker Se 1 -Se 1 interaction.The bonds interior these 'sandwiches' is usually regarded to be pre-dominantly covalent, and these between them, are due to van der Waals forces [18,19].
Thus the structural arrangement leads to an easy cleavage perpendicular to the c-axis displaying a Seterminated surface.Bismuth selenide has been the subject of a massive wide variety of investigations in the bulk structure [20].Effect of Te doping and electron irradiation on thermal diffusivity of bismuth selenide thin lms have been studied via Augustine et al [21].Observed a marked enlarge in thermoelectric performance.Synthesized rhombohedral bismuth selenide nano structures by using solvothermal technique [22].The prepared bismuth selenide lms by way of chemical deposition technique and synthesized hollow nano structured bismuth selenide particles by using chemical route [23,24].Thin lms of bismuth selenide have been prepared by a variety of deposition methods, including thermal evaporation [25,26], chemical vapour deposition [27], chemical bath deposition [28,29], Electrodeposition and successive ionic layer absorption and reaction [30,31].

Experimental Details
The Bi 2 Se 3 nanoparticles were synthesized by the solvothermal method, in which Bi 2 O 3 and SeO 2 were used as a precursor.The rst step was preparing Bi 2 Se 3 nanoparticles: Bi 2 O 3 (2.6 g = 4 mmol), SeO 2 (1.9 g = 12 mmol), added polyvinylpyrrolidone (PVP) (0.2 g) (acts as surfactant), added NaOH solution (0.4g = 8 ml) were dissolved in ethylene glycol (72 ml).The solution was stirred for 2 hours.The solution was transferred to autoclave for 180°C/6 h.After the reaction was completed, the product solution was centrifuged, washed with distilled water, ethanol and acetone repeatedly to purify the product, and nally dried at 80°C/30 min under surrounding atmosphere.The samples were cooled under normal condition (room temperature) and collected gray color as-prepared Bi 2 Se 3 nanoparticles.
The As-prepared sample and 650 ºC annealed for 12 h in argon gas atmosphere sample was used for further analysis.

Characterization Techniques
The X-ray diffraction was used to identify the phase formation and crystallinity of the prepared sample.
The Surface morphology of the Bi 2 Se 3 nanoparticles was studied using HR-SEM and elemental composition analysis were made using EDS measurement.The redox nature of Bi 2 Se 3 nanoparticles was carried out by cyclic voltammetry.The electrical properties were measured by Impedance Spectroscopy and the structural properties of Bi 2 Se 3 nanoparticles were studied by using Raman Spectroscopy.

STRUCTURAL STUDIES
Figure 1 shows the X -ray diffraction pattern of nanocrystalline Bi 2 Se 3 for as-prepared sample and samples annealed at 650 °C for 12 h in argon gas atmosphere.The X-ray diffraction studies were carried out for 2θ angle ranges from 20° to 80°.The target Cu K α1 radiation (λ=1.5406Å) was used for this measurement.The XRD pattern of as-prepared nanocrystalline Bi 2 Se 3 sample diffraction peaks and 650 ºC annealed for 12 h in argon gas atmosphere sample.The diffraction peaks for Bi  The grain size of the prepared samples was calculated using Debye's Scherrer formula Where D is a size of the nanoparticles, θ = diffraction angle, λ = wavelength of X-rays and β = line broadening at Full Width Half Maximum (FWHM).The average grain size was calculated above formula from diffraction pattern was found to be 25 nm for as-prepared sample and 37 nm for 650°C for 12 hours annealed sample.

HIGH RESOLUTION SCANNING ELECTRON MICROSCOPY WITH EDS STUDIES
Figure 2 (a-c) shows the high resolution SEM morphology of as-prepared Bi 2 Se 3 nanocrystalline sample with EDS spectrum analysis.The images indicate that mostly well crystalline nature of as-prepared Bismuth Selenide (Bi 2 Se 3 ) sample, which is con rmed through XRD analysis.
We found that most of the Bi 2 Se 3 nanocrystalline have nano-rod and nano-akes mixture with porous surface morphology nature as shown in Fig. 2.1 (a-b).

RAMAN STUDIES
Figure 3 shows Raman spectra of as prepared and annealed at 650 °C for 12 h in argon gas atmosphere sample.Raman spectroscopy has been an important investigation way as a high-resolution probe technology of crystal structure and vibrational properties of materials.The rhombohedral crystal structure of Bi 2 Se 3 corresponding to the [space group D 5 3d (R3m)] symmetry and there are ve atoms in a primitive unit cell and each unit cell consisting of three few-quintuple layers stacked together by weak van der Waals forces.The atomic arrangement of Bi 2 Se 3 is distinctive (i.e., every repeating unit in the crystal is a quintuple layer of Se-Bi-Se-Bi-Se), which are held collectively through susceptible van der Waals forces.
The Raman spectrum of Bi 2 Se 3 with 785 nm excitation laser was also performed to investigate the four Raman active modes (E g1 , A1 g1 , E g2 and A1 g2 ) of this sample.The letters "E" and "A" indicate the in-plane and out-of-plane (C-axis) lattice vibrations, and subscript "g" denote Raman active.Group theory predicts 12 zone center optical phonon modes in this crystal symmetry, out of 12 optical phonon modes only 4 modes are Raman active, they are E 1 g , A 1 1g , E 2 g and A 2 1g veri ed polarization dependence, out-of-plane A 1 1g and A 2 1g , in-plane E 1 g and E 2 g modes.There are three optical phonon modes are clearly seen in the spectrum.They are ~124 cm -1 , 262 cm -1 and 524 cm -1 .The peak at 124 cm -1 corresponding to the well known E 2  g mode indicate that good quality of the crystal structure.
The peak at 262 cm -1 due to combination of A 1 1g and A 2 1g modes indicate that well crystallinity of this sample.Raman spectrum appears in the higher frequency range, this is due to stronger bonding forces, i.e the peak at 524 cm -1 or may arise due to the overtones of A 1 1g and E 2 g modes.The observed frequencies are well compared with the previously reported experimental and calculated phonon vibration modes of Bi 2 Se 3 .If the polarization of x-direction is present (z (xx)̅ z geometry), one will observe phonons denoted by A 1 1g , A 1 2g and E 2 g .If one measures scattered light with a polarization in the y direction, only E 2 g will appear in the Raman spectrum.This can be understood by considering the nature of the atomic displacements associated with the phonon modes.

CYCLIC VOLTAMMETRY STUDIES
The redox nature of Bi 2 Se 3 nanoparticles was carried out by cyclic voltammetry.This process was done in a three electrode system, Glass Carbon Electrode (GCE) as a working electrode, Ag/AgCl electrode as a reference electrode and platinum wire as a counter electrode.The modi ed Bi 2 Se 3 nanoparticles/GCE was carried out in TBPA (Tetra butyl ammonium perchlorate) used as a supporting electrolyte in acetonitrile (CH 3 CN) condition at scan rate 50mV/s.The modi ed Bi 2 Se 3 nanoparticles/GCE obtained well de ne redox peak (I pa = 0.0114 and I pc = − 0.155) as shown in Fig. 4.This redox behavior was due to Bi 3+ converted into Bi 2+ and Bi metallic state.This redox peak was con ned the formation of Bi 2 Se 3 nanoparticles.Bi 2 Se 3 nanoparticles annealed at 650°C for 12 h sample.This is modi ed on the GCE (Bi 2 Se 3 nanoparticles/GCE) was obtained well known the higher redox peak current with I Pa = 0.021(µA) and I Pc = − 0.144 (µA) as shown in Fig. 4. From this data was clearly indicated that increasing the crystallinity of Bi 2 Se 3 NPs.The crystalline further con rms the XRD data.It gives a broad peak, hence it shows that the three electron (3.4) transfer process was evident in this redox behavior.Here the electron calculating from below equation.
Where i p is the current, R is the Gas Constant (8.314 J/mol K), T is the Temperature (298 K), F is the Faradays Constant (96,485.33A/mol), Q is the Charge and ν is the Scan Rate (50 mV/Sec).

IMPEDANCE SPECTROSCOPY STUDIES OF Bi 2 Se NANOPARTICLES
The complex impedance plots measured at different temperatures ranging from RT to 70 ºC with the frequency range of 1 Hz -1 X 10 6 Hz are shown in Fig. 5.It should be noted from this plots most of the depressed semicircle are exists, whose center is slightly shifted below the real axis (this behavior is commonly considered to be an indication of a non-Debye relaxation process).At low temperatures as well as low measuring frequencies the curve slightly deviates from a perfect semicircle.
The impedance plots are tted with complex non-linear least square t (CNLLS) to acquire the resistance of the sample.With the geometry of the sample, this resistance value has been converted into conductivity using the relation, Here R is the resistance of the sample, A is the area of the cross-section and l is the thickness of the pellet.The high temperature electrical conductivity studies were performed as-prepared and annealed sample using impedance spectroscopy.It shows that very high resistance as well as grain effect curve behavior.

Conclusions
In summary, Bi 2 Se 3 nanoparticles were synthesized using the solvothermal method.The phase formation was con rmed through XRD measurement.The surface morphology and crystal structure of the nano akes were analyzed using HRSEM for Bi 2 Se 3 nanomaterials.From the EDS spectrum analysis of Bi 2 Se 3 (as-prepared and annealed) sample's measurements con rms that Bi and Se elements are presented.A Raman spectrum of the Bi 2 Se 3 sample shows that four Raman active modes (E 1 g , A 1 g1 , E 2 g and A 1 g2 ) were identi ed.From the Cyclic voltammetry measurement of Bi 2 Se 3 nanoparticles, shows that the redox behavior was due to Bi 3+ converted into Bi 2+ and Bi metallic state.This redox peak was con ned the formation of Bi 2 Se 3 nanoparticles.It shows that very high resistance as well as grain effect curve behavior using impedance spectroscopy.This material has been a potential candidate in the area suited for thermo-electric and energy storage applications.
Figure 2.1(c) shows the Energy dispersive spectroscopy (EDS) of Bi 2 Se 3 as-prepared sample and con rms the presence of Bi and Se.Figures 2.2(ac) show the HRSEM images of Bi 2 Se 3 nanocrystalline sample annealed at 650°C for 12 h in argon gas atmosphere with EDS spectrum analysis.The images indicate that mostly Selenide (Se) nanoparticles well incorporated into the Bismuth materials as shown in Fig. 2.2 (a).The white crystalline of Se-nanoparticles is dispersed on the Bi materials at high magni cation 500 nm.The evidences of that formation Bi 2 Se 3 nanoparticle as shown in Fig. 2.2(b).

Figure 2 .
Figure 2.2(c) shows the EDS spectrum of Bi 2 Se 3 sample measurements con rms that Bi and Se elements are presented.

Figure 1 See
Figure 1

Figure 2 See
Figure 2

Figure 3 See
Figure 3

Table
The electrochemical response of Bi 2 Se 3 nanoparticles in 0.1M H 2 SO 4 at 50 mVs− 1