is

ceering360

witelef thuthcrobineg egy arfacto the weight percentage of Bi:Te=62%:38% was conrmed from the data

city hargy cos aretherm

ity [1].value

Materials Letters 64 (2010) 449452

Contents lists available at ScienceDirect

Materials

e lsproposed [4,5] because of the quantum effects [6,7]. For example, thethermoelectric ZT of Bi2Te3 nanocomposite is greater than 1.0 [5]. Biand BiTe nanowires have the ZT of 2.6 [8]. The ZT of nanostructuredthin-lm superlattices of Bi2Te3 is 2.4 [9].

BiTe alloys may be made by high temperature melting methodssuch as the zone-melting technique [10], hydrothermal techniques[11,12], and electrochemical deposition [13,14]. There are someadvantages of electrodepositing thermoelectric materials. For exam-ple, PbTe coating can be made on nanoporous nickel [15] and highthermopower due to the quantum-connement effects of nanostruc-

(Ward Hill, Maryland). The foil was then cut into 2.520 mm platespecimens. The composition of the CuZn alloy was Cu:Zn=70:30 wt.%. A three-electrode system was set up for the electrochemicaldealloying of Zn from the CuZn alloy. The Ag/AgCl reference electrodepurchased fromChem Instrument (Austin, Texas),work electrode (theCuZn alloy foil), and platinum counter electrode were used in thiswork. Removing Zn from CuZn alloy was followed by deposition ofpure copper on the plate specimens in the same electrolyte containing0.5 M CuSO45H2O and 1.0 M H2SO4. Zn was removed from the CuZnalloy at positive potentials. At negative potentials, Cu formed.tures [1618] has been found. In this wonanoparticle formation on porous Cu. Cyclicmicrobalance tests were performed to revealkinetics of the deposition. The surface mo

Corresponding author. Tel.: +1 419 530 6007; fax:E-mail address: yong.gan@utoledo.edu (Y.X. Gan).

0167-577X/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.11.045of gure of merit, ZT [2].at room temperature [3].nd nanocomposites are

foil with thickness of 0.5 mm were obtained from Alfa Aesar.Electrochemical dealloying was performed to selectively remove Znfrom CuZn foil with a thickness of 0.5 mm purchased from Alfa AesarFor example, the ZT of Bi2Te3 is about 0.60.8In order to increase ZT, nanostructures a1. Introduction

Converting waste heat into electriattention. In order to increase the eneperformance thermoelectric materialthermoelectric materials have lowmaintaining high electrical conductivthermoelectric property due to its highs recently caught muchnversion efciency, highneeded. Typically, goodal conductivity whileBiTe alloys have good

nanoparticles was examined by scanning electron microscopy (SEM).The composition of the nanoparticles was analyzed via energydispersive X-ray spectroscopy (EDX).

2. Materials and methods

Chemicals includingHNO3, acetone, TeO2, and Bi(NO3)3, and CuZnrk, we focus on BiTevoltammetry and quartzthe mechanism and therphology of the BiTe

BiTe alloy wwas made by diwas added sobismuth nitrateance was usedmicrographs weanalyzed using aHitachi S4800 sc

+1 419 530 8206.

ll rights reserved. 2009 Elsevier B.V. All rights reserved.Energy dispersive X-ray spectrum

obtained.Morphology results indicated that the sualloy, which is equivalentElectrodeposition and morphology analysnanoparticles on copper substrate

Yong X. Gan a,, James Sweetman a, Joseph G. Lawrena Department of Mechanical, Industrial and Manufacturing Engineering, College of Engineb Department of Bioengineering, College of Engineering, University of Toledo, Toledo, OH 4

a b s t r a c ta r t i c l e i n f o

Article history:Received 23 October 2009Accepted 17 November 2009Available online 20 November 2009

Keywords:ThermoelectricityNanomaterialsElectrodeposition

Nanoscale BiTe particlessubstrate was prepared bysolution. Electrodeposition ocomposed of 0.05 M bismvoltammetry and quartz mishow the mechanism and kwas obtained using scanninrespectively. The morpholo

j ourna l homepage: www.of BiTe thermoelectric alloy

b

, University of Toledo, Toledo, OH 43606, USA6, USA

h thermoelectric properties on copper substrate were investigated. Thectroplating copper layer on a copper zinc alloy plate in a copper sulfatee BiTe alloy particles was then performed in a nitrate bath. The electrolyte isnitrate and 0.01 M tellurium dioxide dissolved in 2.0 M HNO3. Cyclicalance tests associated with the electrodeposition process were conducted totics of the deposition. The morphology and compositional analysis of BiTelectron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS)nalysis suggested that nanoscale BiTe particles were obtained and the EDSe of the copper substrate contained Cu2O. The atomic ratio 1:1 for Bi:Te in the

Letters

ev ie r.com/ locate /mat le tas deposited on the copper. Te-containing solutionssolving TeO2 into nitric acid. Then, bismuth nitratethat the electrolyte contains 0.01 M TeO2, 0.05 Mand 2.0 M HNO3. A CHI400A quartz crystal microbal-to control the potentials during deposition. SEMre taken and the composition of the BiTe wasn Oxford Instruments EDX detector mounted on theanning electron microscope.

3. Results and discussion

3.1. Cyclic voltammetry of BiTe electrodeposition

Fig. 1(A) is the plot showing the current densitypotential (IV)relation. In the deposition process, a triangle waveformwas used. Theexperimental parameters were as follows: The start potential was0.000 V. The end potential was 0.250 V and the scanning rate was0.01 V/s. The potential dependent current density curve shown inFig. 1(A) reveals an electrochemical reaction and diffusion-controlledmechanism during BiTe deposition. This is due to the lowconcentration of Te ion (0.01 M) and Bi ion (0.05 M). The peak for

450 Y.X. Gan et al. / Materials Letters 64 (2010) 449452Fig. 1. Electrochemical measurement results: (A) cyclic voltammogram of BiTe, and

(B) frequency difference vs. scanning potential.Bi deposition on the copper substrate is located at0.01 V (peak inFig. 1(A), vs. Ag/AgCl reference). The potential of Ag/AgCl vs. standardhydrogen electrode is 0.22 V. In Fig. 1(A), peak corresponds to thedeposition of Te. At even lower potentials, a drastic increase in currentdensity due to the hydrogen formation as shown by peak in Fig. 1(A)was found.

3.2. Electrodeposition kinetics

To study the deposition kinetics, time-resolved behavior wasstudied. The frequency difference between the work quartz crystal(with gold as the conductive lm for BiTe alloy deposition) and thereference quartz crystalwas recorded. The reference quartz crystal hasan oscillation frequency of 8.000 MHz. The frequency difference isplotted as shown in Fig. 1(B). The line segment of AB corresponds to Bideposition on the gold coated quartz crystal electrode. Line segmentBC corresponds to the deposition of both Te and Bi. Starting frompoint C tomore negative potential range, the line is at. This is becausethe formation of hydrogen gas does not change the resonant frequencyof the electrode. Since the potential scan rate was 0.01 V/s, thefrequency change due to Bi dominated deposition on the quartzelectrode was about3 kHz/s. The result was obtained from the scanrate and the slope of line segment AB in Fig. 1(B). Likewise, thefrequency change due to both Te and Bi deposition on the quartzelectrode can be calculated as about 5 kHz/s. Consequently, thecontribution from Te dominated deposition is about2 kHz/s.

The results of frequency change can be used to calculate the masschange rate of BiTe deposition using the following equation.

dmdt

= A

p2f 20

dfdt

1

where f0 is the resonant frequency of the fundamental mode of thecrystal, A is the area of the conductive coating (Au), is the density ofthe quartz crystal, and is the shear modulus of the quartz.

In this work, the resonant frequency of the fundamentalmode of thecrystal is 8.000 MHz. The area of the gold coating is 2.05105 m2, thedensity of the quartz crystal is 2.648103 kg/m3, and the shearmodulusis 2.947104 GPa. By substituting these data into Eq. (1), it is found thatfor 1 kHz frequency drop, the mass increases 1.41010 kg. Thus thedeposition rate for Bi is 4.21010 kg/s. For Te is 2.81010 kg/s.

3.3. Morphology and composition analysis

Fig. 2(A) provides a global view of the electrodeposited BiTe alloyparticles on Cu. At higher magnication, the SEM image in Fig. 2(B)shows clusters of BiTe alloy nanoparticles. The size of the clusters isabout 400 nm. The clusters are aggregates of several nanoparticles.The dimension of these particles is about 80 nm. Fig. 2(C) shows theelemental analysis results of the BiTe nanoparticles on the coppersubstrate using energy-dispersive X-ray spectroscopy. The maincompositions of the coating include Bi and Te. Signals from thecopper substrate were also detected, as shown by the three Cu peaks.Small peaks corresponding to Al and Th which come from theimpurity of the materials were found. Oxygen is believed to be theresult of surface oxidation of copper. The atomic ratio of Cu to Oindicates that Cu2O is the major composition of the oxide. For Bi, thehighest peak is located at 2.2 keV. For Te, the highest peak is at3.8 keV. Quantitative analysis indicated that the atomic ratio of Bi toTe is 1 to 1. This atomic ratio corresponds to the weight percent of Bi:Te=62%:38%. This result is in agreement with the electrochemicalquartz microbalance test results as shown in Fig. 1(B) and thedeposition kinetics analysis. Obviously, the composition of the BiTenanoparticles is away from the stoichiometric composition of Bi2Te3.Since there is excessive Te in BiTe obtained in this work, this BiTe

alloy should be an n-type thermoelectric material.

451Y.X. Gan et al. / Materials Letters 64 (2010) 449452Preliminary studies on the thermoelectric behavior of the fabri-cated BiTe nanoparticles on the copper substrate were performed.Depending on how much the particles on the copper, the absolutevalue of the Seebeck coefcient increases at different levels. Thecopper specimen has a Seebeck coefcient of 6.18 V/K. When thesubstrate was partially covered by BiTe nanoparticles as shown inFig. 2(A) or (B), the Seebeck coefcient changes to 6.92 V/K. TheSeebeck value of the BiTe material on copper takes negative sign.This is an indication that the BiTe nanoparticle is indeed an n-typethermoelectric material.

The agglomeration of the particles remains to be a very challengingproblem to be solved in order to obtain very ne electrodepositednanoparticles.We are trying to solve this problem in twoways. The rstway is to increase the cathode potential such that a non-equilibriumelectrodeposition condition holds. Under highly non-equilibriumdeposition condition of 0.8 to 1.0 V, the nanoscale phase growsout-of-the-plane along some preferential directions to form nanoscale

Fig. 2. SEM analysis of BiTe on Cu: (A) global view,fractals or dendrites as shown in Fig. 3(A). This prevents the agglom-eration in the 2-D growth case. The other method we tried is to usealumina nanoporous templates. Fig. 3(B) shows the nanoporoustemplate being sputter-coated by a conductive layer of metal, forexample gold with the thickness of about 4 to 10 . Using the coatedtemplate as the substrate for electrodeposition, conned growth ofnanocrystals have been found and conrmed by the SEM micrographas shown in Fig. 3(C). Again, this could prevent the agglomeration ofnanoparticles.

It should be noted that any other substrate which is electricallyconductive is also suitable for electrodeposition of thermoelectricmaterials. For example, electrodeposition of thermoelectric materialson nickel has been successfully performed [15]. Also, Fig. 3(C) showsthe case that the BiTe material was deposited on alumina substratethrough an intermediate coating process (sputtering gold as theconductive layer for electrodeposition). Therefore this type of exper-iment can be done on substrates other than copper.

(B) nanoparticle clusters, and (C) EDX spectrum.

collected and dispersed into polymers by spin coating to form polymer-based composite materials. Thermoelectric nanoparticles can also bedeposited into nanoporous ceramic templates to make ceramic matrixcompositematerials. These are very important work to be performed inour future research.

4. Conclusions

452 Y.X. Gan et al. / Materials Letters 64 (2010) 449452Finally, how can we use the nanoparticles to prepare functionalcomposites? Indeed, there are many ways to make nanoparticle-containing composite materials. For example, nanoparticles could be

Nanoscale BiTe particles have been successfully prepared using theelectrochemical deposition process from a nitrate bath, which canprovide the raw materials for making thermoelectric nanoparticle-containing composite materials. The morphology of the BiTe materialshows nanoparticle clusters in the size range of 400 nm. The dimensionof individual nanoparticle is about 80 nm. Cyclic voltammetry revealsthat the deposition has a reactiondiffusion mechanism. The quartzmicrobalance test results show that the rate of BiTe deposition on thegold coated quartz crystal is in the range of 1010 kg/s. The EDXspectrum conrms the atomic ratio of the BiTe nanoparticles is 1:1.

Fig. 3. SEM images showing the separationofnanoclusters: (A) BiTedendrite, (B)aluminananoporous template coated with Au, and (C) conned growth of nanocrystals on thetemplate.This ratio corresponds to theweight percentage of 62%Bi and 38%Te. TheEDX results also reveal Cu2O due to the oxidation of the copper.

Acknowledgments

This work was supported in part by the 2008 Summer FacultyResearch Fellowships from the Ofce of Research Development at TheUniversity of Toledo. YXG also acknowledges the support from theresearch start-up funds provided by The University of Toledo.

References

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Electrodeposition and morphology analysis of BiTe thermoelectric alloy nanoparticles on copper.....IntroductionMaterials and methodsResults and discussionCyclic voltammetry of BiTe electrodepositionElectrodeposition kineticsMorphology and composition analysis

ConclusionsAcknowledgmentsReferences