Thermochemically evolved nanoplatelets of bismuth selenide with enhancedthermoelectric figure of meritZulfiqar Ali, Sajid Butt, Chuanbao Cao, Faheem K. Butt, Muhammad Tahir, M. Tanveer, Imran Aslam,Muhammad Rizwan, Faryal Idrees, and Syed Khalid Citation: AIP Advances 4, 117129 (2014); doi: 10.1063/1.4902159 View online: http://dx.doi.org/10.1063/1.4902159 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced thermoelectric figure of merit in vertical graphene junctions Appl. Phys. Lett. 105, 133105 (2014); 10.1063/1.4896915 Tuning phonon properties to enhance the thermoelectric figure of merit AIP Conf. Proc. 1590, 95 (2014); 10.1063/1.4870203 Enhancement of thermoelectric figure of merit in bismuth nanotubes Appl. Phys. Lett. 97, 023112 (2010); 10.1063/1.3463473 Bismuth telluride compounds with high thermoelectric figures of merit J. Appl. Phys. 93, 368 (2003); 10.1063/1.1525400 Thermoelectric Figure of Merit in Silver Selenide Powders J. Appl. Phys. 34, 3587 (1963); 10.1063/1.1729262

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AIP ADVANCES 4, 117129 (2014)

Thermochemically evolved nanoplatelets of bismuthselenide with enhanced thermoelectric figure of merit

Zulfiqar Ali,1 Sajid Butt,2 Chuanbao Cao,1,a Faheem K. Butt,1Muhammad Tahir,1 M. Tanveer,1 Imran Aslam,1 Muhammad Rizwan,1Faryal Idrees,1 and Syed Khalid11Research Centre of Materials Science, School of Materials Science and Engineering,Beijing Institute of Technology, Beijing 100081, People’s Republic of China2State Key Laboratory of New Ceramics and Fine Processing, School of Materials Scienceand Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

(Received 26 July 2014; accepted 4 November 2014; published online 17 November 2014)

We firstly present a simple thermochemical method to fabricate high-quality Bi2Se3nanoplatelets with enhanced figure of merit using elemental bismuth and seleniumpowders as precursors. The crystal structure of as synthesized products is char-acterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS)and high resolution transmission electron microscopy (HRTEM) measurements.Morphological and chemical synthetic parameters are investigated through a seriesof experiments; thickness and composition of the platelets are well controlled inlarge scale production. Subsequently spark plasma sintering (SPS) is performed tofabricate n-type nanostructured bulk thermoelectric materials. Raman Spectroscopyof the two selected samples with approximately of 50 and 100 nm thicknesses showsthree vibrational modes. The lower thickness sample exhibits the maximum red shiftof about 2.17 cm-1 and maximum broadening of about 10 cm-1 by in-plane vibrationalmode E2

g. The enhanced value of figure of merit ∼0.41 is obtained for pure phasebismuth selenide to the best of our knowledge. We observe metallic conductionbehavior while semiconducting behavior for nanostructured bismuth selenide isreported elsewhere which could be due to different synthetic techniques adopted.These results clearly suggest that our adopted synthetic technique has profound effecton the electronic and thermoelectric transport properties of this material. C 2014 Au-thor(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4902159]

I. INTRODUCTION

Thermoelectric devices could convert waste energy directly into useful electricity.1 Researchinterest2 in these materials is due to the fact that they find applications in thermoelectric generatorswhich can convert waste heat from engines, aircrafts and power production units. Thermoelectriccoolers such as nanodevice hot spot cooling and Peltier coolers also take advantage of thermo-electric effect as these coolers have no moving parts hence compatible in size and flexible shape.The main disadvantages are the cost of such thermoelectric devices and low power efficiency. Thispower efficiency can be calculated by a dimensionless quantity; figure of merit ZT which is givenby σS2T/k, where σ is electrical conductivity, S the thermopower, T the absolute temperature andk the thermal conductivity.3 The value of ZT can be increased either by decreasing k or increasingpower factor (PF): σS2. The thermal conductivity k can be decreased by tailoring nanostructuresof the materials which show enhanced phonon scattering at surfaces and interfaces while σ canbe enhanced by quantum confinement effect.3–5 The cost can be reduced by employing simple andcheaper fabrication techniques. To reduce the cost and improve the efficiency of thermoelectric

aCorresponding Author Email: cbcao@bit.edu.cn (Chuanbao Cao) Tel: +86 10 68913792. Fax: +86 10 68912001.

2158-3226/2014/4(11)/117129/8 4, 117129-1 ©Author(s) 2014

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117129-2 Ali et al. AIP Advances 4, 117129 (2014)

devices, we propose a simple thermochemical method in which bismuth and selenium elementalpowders are used as precursors in horizontal tube furnace. Our results show that this technique isnot only cost effective but can also improve the efficiency of these thermoelectric devices.

Bismuth chalcogenides like Bi2Te3 and Bi2Se3 comprise some of the best thermoelectric de-vices because nanostructuring these materials produces a layered crystal structure with alternativeBi2Te3 or Bi2Se3 layers which generate good electrical conductivity and perpendicular to whichpoor thermal conductivity. There have been some excellent reports worth mentioning in this fieldregarding thermoelectric performances of these compounds like it is theoretically predicted thata single quintuple layer, has an intrinsic advantage originating from the particular shape of itsvalence band leading to a large power factor and figure-of-merit exceeding bulk Bi2Te3 because ofinteraction between top and bottom topological surface states. These surface states from top andbottom surfaces hybridize (1-6QL) and conventional diffusive transport predicts that the tunablehybridization-induced band gap leads to increased thermoelectric performance at low tempera-tures.6–15 We slightly manipulated CVD technique to fabricate Bi2Se3 by using elemental Se andBi powders instead of bismuth selenide compound itself. In this type of fabrication extra seleniumis present to reduce selenium vacancies as compared to compound itself for nanoscale fabrication.Temperature dependence of thermoelectric properties shows that electrical conductivity decreaseswith increasing temperature representing metallic behavior. A reasonably high ZT value of ∼0.41 isobtained at 533 K which is better than already reported values for this material.16,17

Nanostructuring the layered crystal structure can not only significantly increase the electricalconductivity of the system but decrease the thermal conductivity as the grain size decreases.18 Inour previous work we have shown that synthetic technique can have effect on surface states ofBi2Se3 by reducing selenium vacancies which play important role in controlling the properties ofthis material.19

Raman spectroscopy provides an excellent tool for studying electronic surface states becauseit involves phonon-phonon and phonon-electron interactions and such thickness dependent inter-actions present well defined and unique finger prints of a material. E2

g mode shows considerablebroadening (∼10 cm-1) and red shift.

II. EXPERIMENTAL PROCEDURES

A. Preparation of materials

Bi and Se powders were used as starting materials. These powders were mixed in (1:1) ratioand grinded in mortar for 30 minutes. About 5 g of the mixture was loaded in small alumina cruci-bles. Since Se has lower melting point as compared to Bi, therefore, Se can easily be evaporatedwhen temperature is increased to 350oC. Therefore, the crucible is covered and further wrapped inaluminum foil; hence, the chances of selenium evaporation and going out of the system are reduced.These crucibles were then placed in horizontal tube furnace (HTF) and the furnace was pumped outusing a mechanical rotary pump for removing the residual air contents from the furnace tube andthen also flushed heavily with high purity Argon gas for about 30 minutes. Later on argon gas flowwas adjusted at 50 sccm (standard cubic centimeter per minute) and the furnace was switched on toreach the target temperature (oC) at ramp rate (R), and kept there for (T) minutes, then it was coolednaturally to room temperature supplementary material.27 A dark grey color product was obtainedfrom the alumina boat and preserved in airtight plastic boxes for further investigations.

B. Thermoelectric measurements

The powdered Bi2Se3 was sintered with spark plasma sintering (SPS). For SPS, the powderedBi2Se3 was placed in a graphite module of diameter 20 mm, lined internally with a graphite foiland then heated up to 523 K for 5 minutes under a pressure of 50 MPa in the vacuum. After, it wascooled to room temperature; carbon was removed over the surfaces by grinding. The obtained pelletwas cut into two parts of a rectangular bar of 2.5×3×15 mm3 and a smaller pellet of diameter 10 mmhaving a thickness of 2-3mm.

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FIG. 1. (a) TEM image of the products at 2µm scale showing agglomerated sheets (b) TEM image at 500 nm scale whereseparate sheets can be well distinguished (c) High resolution TEM image showing d-spacing and growth direction along(012) direction (d) Corresponding SAED pattern of the sheets showing rhombohedral crystal structure marked planes.

The electrical conductivity and thermopower of the bar-shaped sample were measured fromroom temperature to 533 K with the LINSEIS LSR-3. The thermal conductivity k was calculated asgiven by the relation k = DCpd, where D and Cp are the thermal diffusivity and the specific heatrespectively, measured simultaneously by the Laser Flash method (Netzsch LFA 457) and d is themass density measured by the Archimedes principle.

III. RESULTS AND DISCUSSIONS

The XRD spectrum and EDS analysis are presented in the supplementary material27 and dis-cussed accordingly.

To determine the crystal structure of the sheets, an individual sheet is investigated using highresolution electron microscopy (HRTEM) and selected-area electron diffraction (SAED). Shown inFig. 1 is the detailed analysis carried out to determine the structure, growth planes and d-spacing.Fig. 1(a) represents the agglomerated sheets at 2µm scale while 1(b) shows the TEM images at500 nm scale where individual sheets can be distinguished.

Fig. 1(c) represents the high resolution TEM image of the samples and atomic spacing can becalculated from the image to be 0.344 nm which corresponds to the d-spacing of (012) plan in theXRD spectrum. Corresponding SAED pattern is shown in Fig. 1(d) which is taken along [121] zoneaxis and shows spot pattern suggesting that nanosheets are single crystalline in nature. Both theHRTEM image and SAED pattern may well be indexed using the rhombohedral phase of Bi2Se3

(space group: R3m (166) with cell constants a=4.1368A and c=28.6342A (JCPDS 33-0214). Thelattice spacing of 0.344 correspond to the (012) plane spacing of rhombohedral phase Bi2Se3.

The photoelectron spectra (XPS) of selected sample are presented in the supplementary mate-rial.27

Fig. 2(a) and 2(b) represents the SEM images of the product synthesized by CVD method usingBi and Se powders as precursors at 600oC and ramp rate of 2oC/min for a reaction time of 2 hrs. Fig.2(c) presents the SEM micrographs of the product obtained at the same conditions but with differentreaction time i.e. 4 hrs. In order to observe morphology clearly, SEM micrographs were taken at

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FIG. 2. (a) SEM images of as synthesized Bi2Se3 nanosheets at 600 oC and ramp rate of 2oC/min and a reaction time of 2hrs (b) SEM at higher resolutions showing average thickness of the sheets (c) SEM of Bi2Se3 for a reaction time of 4 hours(d) SEM at higher resolution showing average thickness of 100nm.

higher resolution scales (Fig. 2(d)) and the average thickness of the product was estimated to be100 nm.

Ramp rate is another very important parameter in controlling the morphology of the products;we have performed a series of experiments with different rates, as 2oC/min, 5oC/min and 10oC/min.We observe that at the rate of 10oC/min, the sheets are thick in the range of 500nm and 1 µm, whilefor the rate of 5oC/min the sheets have the thickness of 200 nm and for the rate of 2oC/min theproduct mainly consists of thin plates in the range of 20 to 50nm. Reaction temperatures mainlyhave influence on the purity of the product but at 650oC for a rate of 2oC/min and reaction time of 4hrs, we have obtained the best product as shown in Fig. 3.

Diffusion barrier of adatoms down multiple-layer surface steps can be substantially larger thanthat down monolayer surface steps. The competition of monolayer and multiple-layer surface stepsgives rise to a new characteristic length scale, the dimension of surface facets. Under any PhysicalVapor Deposition without a non-wetting substrate this dimension is of the order of 100nm.20 Wesummarize the controlling parameters in the supplementary material.27

Raman spectroscopy results are presented in the supplementary material at Ref. 27.

Thermoelectric transport properties

Since Raman Spectroscopy predicted the enhanced interactions of phonons which is a functionof thermal conductivity so lowest thickness sample was selected to measure the thermoelectricfigure of merit. The temperature dependence of thermoelectric transport properties in Bi2Se3 weremeasured from 323 K to 533 K and are presented in Fig. 4.

The electrical conductivity and thermopower of pure Bi2Se3 are presented in Fig. 4(a). Theelectrical conductivity decreased monotonically with increasing temperature which indicates a

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FIG. 3. (a) SEM images of self assembled flower like Bi2Se3 nanosheets at ramp rate of 2oC/min (b) SEM at higherresolutions showing average thickness of the sheets (c) SEM of Bi2Se3 for a reaction time of 4 hours (d) SEM at higherresolution showing average thickness of 50nm.

metallic conduction behavior in the given temperature range, as shown in Fig. 4(a). Generally, theelectrical conductivity is represented by the following relation:

σ = neµ (1)

Where, σ, e, µ and n represents the electrical conductivity, charge on an electron, mobility and car-rier’s concentration, respectively. The highest σ of 551 S/cm was obtained at 323 K which is abouttwo times larger than that of reported for pure Bi2Se3 by Min et al21 and comparable to the resultsreported by Sun et al.22 The increased electrical conductivity is caused by nanometric scale growthwhich allows increased number of grains on the surfaces of platelets as shown in Fig. 3(d) obtainedby optimized CVD parameters. For degenerate metals and semiconductors, electron-phonon scat-tering is increased upon increasing temperature. Hence, mobility and the electrical conductivity isdecreases with an increase in temperature marking a typical metallic conduction behavior whichis contradicting with the results of Kadel et al23 who have reported a semiconducting nature fornanostructured Bi2Se3 synthesized by solvothermal method. Such variations in resistivity are pres-ent in nanocrystalline materials which possess different degrees of disorder and formation of grainboundaries. The transport properties are dominated by grain boundary potential barrier scattering incombination with phonon scattering.

The thermopower increased monotonically with increasing temperature and the negative Svalues represent an n-type semiconductor which infers that electrons are the majority charge car-riers, as shown in Fig. 4(a). The thermopower is largely dependent on the number of charge carriers(electron in our case) reaching at the colder side.24 Above 400 K, the linear temperature depen-dence of thermopower suggests that diffusive transport is the dominant transport mechanism.25 The

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FIG. 4. Temperature dependence of (a) electrical conductivity and thermopower (b) power factor (c) thermal conductivity(d) figure of merit ZT of Bi2Se3 nanoplatelets.

maximum S value of 121µV/K is obtained at 533 K which is even larger than that of reported inliterature.21,23 The improved results might be associated to high quality of sheet-like nanostructures.

Combining the electrical conductivity and thermopower, the power factor PFσS2 was calcu-

lated and the results are presented in Fig. 4(b). PF increases with increasing temperature. Althoughwith increasing temperature the electrical conductivity decreases but at the same time the increasein thermopower is dominant to the enhancement of PF with increasing temperature. The highest PFvalue of 739 µW/mK2 is attained at 533 K and to the best of our knowledge it is the highest valueamong the reported data21,23 for Bi2Se3. The highest value of power factor is attributed to the largeamount of surface atoms as compared to the total volume of these structures.

Fig. 4(c) presents the temperature dependence of thermal conductivity. Thermal conductivity de-creases monotonically upon increasing temperature which shows a typical semiconducting behaviorof a crystalline solid. The total thermal conductivity k is measured implicitly by k = kph + ke, where,ke and kph is the explicit thermal conductivity contributions associated with the electron and phonontransportation, respectively. Thermal conductivity associated with electrons (ke) can be estimatedfrom Wiedemann-Franz law: ke = L0σT , where, L0 is the Lorentz number (1.5×10-8 WΩ/k2)26 andσ is the electrical conductivity. It can be seen in Fig. 4(c) that at higher temperatures the totalthermal conductivity decrease is dominant due to suppressed phonon thermal conductivity. Enhancedphonon-phonon scattering is responsible for this decreased thermal conductivity as depicted byRaman spectroscopy results.

The dimensionless figure of merit ZT as a function of temperature is presented in Fig. 4(d).The simultaneous increase in PF and decrease in thermal conductivity resulted in a higher ZT valueswhich increased monotonically over all the given temperature range. The highest ZT value of ∼0.41is attained at 533 K, which is about two times of that of reported by Min et al21 However, the valueof ZT is lower than Soni et al6 who have reported the value of ZT as 0.61 in Se doped bismuth tellu-ride. Since Tellurium based compounds posses excellent thermoelectric characteristics as compared

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117129-7 Ali et al. AIP Advances 4, 117129 (2014)

to selenium based compounds and doping with selenium also increases the grain boundary scatter-ing hence their thermal conductivity is lower than in our case. Since tellurium is not abundantlypresent as compared to selenium, hence using tellurium certainly increases the fabrication costs.As a result we adopted a facile, cost effective and free of toxic reagents synthetic technique toobtain the highest ZT value among all the previously reported data on pure Bi2Se3 which makes it apotential candidate for n-type thermoelectric (TE) materials for practical applications in the energyharvesting devices.

IV. CONCLUSION

In conclusion, we have successfully controlled the morphology of Bi2Se3 up to approximately50 nm by a facile technique using Bi and Se powders as precursors. First, we optimized theparameters at which there were no oxide peaks and then we controlled the morphology by tuningdifferent parameters like reaction temperature, reaction time and ramp rate. Crystal structure ofthe final products was investigated by XRD and HRTEM analysis. Raman spectroscopy of theselected samples with 100nm and 50nm thicknesses was performed and three vibrational modeswere observed in our samples. The considerable Raman shift and broadening of peaks from 50nmto 100nm thickness suggest enhanced phonon interactions and wave interferences in 50nm sample.Therefore, temperature dependent thermoelectric measurements of this selected sample showedenhanced value of figure of merit. The electrical conductivity decreases with increasing temperatureshowing metallic conduction behavior which is the result of synthetic technique. We conclude thatthis synthetic technique has profound effect on the thermoelectric transport characteristics of Bi2Se3and this work will provide the basis for designing high performance and low cost thermoelectricdevices.

ACKNOWLEDGMENT

This work was supported by National Natural Science Foundation of China (21371023,50972017) and the Research Fund for the Doctoral Program of Higher Education of China (2010110-1110026)1 A. I. Hochbaum, R. Chen, R. D. Delgado, W. J. Liang, E. C. Garnett, M. Najarian, A. Majumdar, and P. D. Yang, Nature

451, 163 (2008).2 M. S. Dresselhaus, G. Chen, M. Y. Tang, R. G. Yang, H. Lee, D. Z. Wang, Z. F. Ren, J. P. Fleurial, and P. Gogna, Adv. Mater.

19, 1043 (2007).3 G. Chen, M. S. Dresselhaus, G. Dresselhaus, J. P. Fleurial, and T. Caillat, Int. Mater. Rev. 48, 45 (2003).4 A. Purkayastha, A. S. Jain, C. Hapenciuc, R. Buckley, B. Singh, and C. Karthik, Chem. Mater. 23, 3029 (2011).5 L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B. 47, 12727 (1993).6 A. Soni, Z. Yanyuan, Y. Ligen, M. K. Aik, M. S. Dresselhaus, and Q. H. Xiong, Nano Lett. 12, 1203 (2012).7 J. Maassen and M. Lundstrom, Appl. Phys. Lett. 102, 093103-1 (2013).8 Y. Zhang, K. He, C. Z. Chang, C. L. Song, L. L. Wang, X. Chen, J. F. Jia, Z. Fang, X. Dai, W. Y. Shan, S. Q. Shen, Q. Niu,

X. L. Qi, S. Z. Zhang, X. C. Ma, and Q. K. Xue, Nat. Phy. 6, 584 (2010).9 M. T. Pettes, J. Maassen, I. S. Jo, M. S. Lundstrom, and L. Shi, Nano Lett. 13, 5316 (2013).

10 P. Y. Ghaemi, R. S. Mong, and J. E. Moore, Phys. Rev. Lett. 105, 166603-1 (2010).11 Y. L. Li, G. Wang, X. G. Zhu, M. H. Liu, C. Ye, X. Chen, Y. Y. Wang, K. He, L. L. Wang, X. C. Ma, H. J. Zhang, X. Dai,

Z. Fang, X. C. Xie, Y. Liu, X. L. Qi, J. F. Jia, S. C. Zhang, and Q. K. Xue, Adv. Mater. 22, 4002 (2010).12 Y. S. Hor, A. Richardella, P. Roushan, Y. Xia, J. G. Checkelsky, A. Yazdani, M. Z. Hasan, N. P. Ong, and R. J. Cava, Phy.

Rev. B. 79, 195208-1 (2009).13 O. V. Yazyev, J. E. Moore, and S. G. Louie, Phy. Rev. Lett. 105, 266806-1 (2010).14 W. S. Liu, K. C. Lukas, K. McEnaney, S. Y. Lee, Q. Zhang, C. P. Opeil, G. Chen, and Z. F. Ren, Energy Environ. Sci. 6,

552 (2013).15 M. X. Wang, C. H. Liu, J. P. Xu, F. Yang, L. Miao, M. Y. Yao, C. L. Gao, C. Y. Shen, X. C. Ma, X. Chen, Z. A. Xu, Y. Liu,

S. Z. Zhang, D. Qian, J. F. Jia, and Q. K. Xue, Science 336, 52 (2012).16 M. K. Jana, K. K. Biswas, and C. N. R. Rao, Chem. Eur. J. 19, 9110 (2013).17 C. Anglin, N. Takas, J. Callejas, and P. F. Poudeu, J. Sol. Stat. Chem. 183, 1529 (2010).18 I. Garate and M. Franz, Phys. Rev. Lett. 104, 146802-1 (2010).19 Z. Ali, C. B. Cao, J. L. Li, Y. L. Wang, T. Cao, M. Tanveer, M. Tahir, F. Idrees, and F. K. Butt, J. Power Sources. 229, 216

(2013).20 H. C. Huang, JOM 64, 1253 (2012).21 Z. L. Sun, S. C. Liufu, X. H. Chen, and L. D. Chen, Cryst. Eng. Comm. 12, 2672 (2010).

All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See:

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117129-8 Ali et al. AIP Advances 4, 117129 (2014)

22 Y. Min, H. J. W. Roh, H. S. Yang, M. W. Park, S. T. Kim, S. W. Hwang, S. M. Lee, K. H. Lee, and U. Y. Jeong, Adv. Mater.25, 1425 (2012).

23 K. Kadel, L. Kumari, W. Z. Li, J. Y. Huang, and P. P. Provencio, Nanoscale Res. Lett. 6, 57–1 (2011).24 G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008).25 D. M. Rowe, in Thermoelectrics Hand Book: Macro to Nano, edited by D. M. Rowe (CRC/Taylor & Francis, Boca Raton

FL, 2006).26 H. S. Kim and S. J. Hong, J. Alloy. Comp. 586, S428–S431 (2014).27 See supplementary material at http://dx.doi.org/10.1063/1.4902159 for [controlling parameters, XRD and EDS analysis,

XPS spectra and Raman spectroscopy results].

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