RESEARCH PAPER

Synthesis and thermoelectric characterisation of bismuthnanoparticles

Gianfranco Carotenuto Æ Cornelia L. Hison ÆFilomena Capezzuto Æ Mariano Palomba ÆPietro Perlo Æ Pellegrino Conte

Received: 13 February 2008 / Accepted: 12 October 2008 / Published online: 31 October 2008

Springer Science+Business Media B.V. 2008

Abstract An effective method of preparation of

bismuth nanopowders by thermal decomposition of

bismuth dodecyl-mercaptide Bi(SC12H25)3 and pre-

liminary results on their thermoelectric properties are

reported. The thermolysis process leads to Bi nano-

particles due to the efficient capping agent effect of

the dodecyl-disulfide by-product, which strongly

bonds the surface of the Bi clusters, preventing their

aggregation and significantly reducing their growth

rate. The structure and morphology of the thermolysis

products were investigated by differential scanning

calorimetry, thermogravimetry, X-ray diffractometry,1H nuclear magnetic resonance spectroscopy, scan-

ning electron microscopy, and energy dispersive

spectroscopy. It has been shown that the prepared

Bi nanopowder consists of spherical shape nanopar-

ticles, with the average diameter depending on the

thermolysis temperature. The first results on the

thermoelectric characterization of the prepared Bi

nanopowders reveal a peculiar behavior characterized

by a semimetal–semiconductor transition, and a

significant increase in the Seebeck coefficient when

compared to bulk Bi in the case of the lowest grain

size (170 nm).

Keywords Bismuth nanoparticles Mercaptide thermolysis Semimetal–semiconductor transition Thermoelectric characteristics Nanopowder

Introduction

Thermoelectric materials generate electrical power

from a temperature gradient through Seebeck effect

and use electricity to work as heat pumps through

Peltier effect, providing active cooling (in the

absence of refrigerants) or heating without the need

of moving parts, but based on carrier conduction

(Nolas et al. 2001). These materials, with long life

and maintenance-free, have important economical

applications in systems where the waste heat gener-

ated can be harvested to provide useful power, such

as in microelectronics–microprocessor cooling, opto-

electronics, etc. (Heremans 2005). The high

simplicity and environmental-friendly (no noise, no

pollution) characteristics of the thermoelectric energy

G. Carotenuto (&) C. L. Hison F. Capezzuto M. Palomba

Istituto dei Materiali Compositi e Biomedici,

Consiglio Nazionale delle Ricerche, Piazzale Tecchio 80,

80125 Napoli, Italy

e-mail: giancaro@unina.it

P. Perlo

Centro Ricerche Fiat, Strada Torino 50,

10043 Orbassano (TO), Italy

P. Conte

Dipartimento di Ingegneria e Tecnologie Agro-Forestali

(DITAF), Universita degli Studi di Palermo,

Viale delle Scienze 13, 90128 Palermo, Italy

123

J Nanopart Res (2009) 11:1729–1738

DOI 10.1007/s11051-008-9541-6

conversion principle have cost, until now, lower

efficiency. Consequently, the use of thermoelectric

devices, consisting of a sequence of thermoelectric

materials connected electrically in series and ther-

mally in parallel, has been limited to niche

applications for which the high reliability, compact-

ness, and user-friendly performance overcome the

lack of efficiency.

As well known, the measure of the thermoelectric

efficiency of a material is given by the dimensionless

figure of merit ZT defined as (Goldsmid 1964):

ZT ¼ S2rke þ kL

T ð1Þ

where S is the Seebeck coefficient, defined as the

thermoelectric voltage induced by a temperature

gradient across the material, S ¼ DVDT , r is the electrical

conductivity, T is the temperature (in Kelvin degrees),

ke and kL are the electronic and lattice (phononic)

thermal conductivities, respectively. An efficient ther-

moelectric energy conversion requires large ZT values,

which means large electrical conductivity, high

Seebeck coefficient (large voltage in power generation

and large Peltier coefficient in cooling) and low

thermal conductivity (to allow large temperature

differences and, consequently, large voltage in power

generation or to reduce the heat leakage between the

hot and cold side of the device when used as

refrigerator) (Nolas et al. 2001).

In the recent years, the identification of new

materials with high ZT value has proved to be an

extremely challenging task due to the interdepen-

dence among the Seebeck effect, thermal, and

electrical conductivity. The tailoring of the three

parameters in view of large ZT values is difficult in

conventional bulk crystalline solids because the

modification of one of them adversely affects the

other (e.g., an increase in the electrical conductivity

leads to an additional enhancement in the electronic

contribution to the thermal conductivity) (Ashcroft

and Mermin 1976; Hicks and Dresselhaus 1993a, b).

The recent approach for the thermoelectric efficiency

improvement is based on nanoscale structuring to

benefit from the phonon boundary scattering and

quantum size effects (i.e., charge carriers confine-

ment at nanoscale in one (quantum wires), two

(quantum wells, superlattices) or three (quantum

dots) dimensions), which determine the decoupling

of ZT parameters (Hicks and Dresselhaus 1993a, b);

Dresselhaus et al. 1999; Chen et al. 2000, 2003). In

this way, the thermoelectric energy conversion in

low-dimensional structures could reach the kind of

performances needed for the widespread application

of the thermoelectric technology.

The peculiar electronic transport characteristics of

bulk semimetal bismuth Bi, such as small band

overlap of the conduction and valence bands (Gallo

et al. 1963), very small electron effective masses

(Isaacson et al. 1969), very long carrier mean-free

path (several orders of magnitude greater than for

most metals) (Rogacheva et al. 2003), low carrier

concentration, and highly anisotropic carrier effective

masses (varying as much as 2009) (Gallo et al. 1963;

Issi 1979) make it particularly interesting for ther-

moelectric applications when the size reduction of the

building blocks induces quantum confinement effects

(e.g., semimetal–semiconductor transition) and pho-

non boundary scattering (Gallo et al. 1963; Heremans

et al. 2000; Black et al. 2002). In fact, the nano-

structured Bi exhibits significantly enhanced

thermoelectric efficiency when compared to bulk

Bi. Most of the previous works on nanostructured

bismuth show a long-standing interest in nanotubes,

nanowires and nanowire arrays (Hicks and Dresselhaus

1993a, b); Zhang et al. 1998; Heremans and Thrush

1999; Li et al. 2001; Huber et al. 2003; Heremans

2005), and thin films structures/quantum well super-

lattices (Hoffman et al. 1993; Lu et al. 1996; Cho

et al. 1997; Rogacheva et al. 2003).

Although larger enhancements in the thermoelec-

tric performance are predicted in quantum dot

structures (Heremans et al. 2002; Lin et al. 2003),

essentially confined in all three dimensions, much

less studies on Bi nanoparticles production and

thermoelectric characterization are available in the

literature (Zhao et al. 2004; Balan et al. 2004; Grass

and Stark 2006; Hostler et al. 2007) with respect to

the researches directed toward thin films and nano-

wire/nanorod arrays. It has been reported that Bi

nanoparticles are often contaminated during the

production process (e.g., by oxidation, from remain-

ing surfactants or solvents, etc.) (Balan et al. 2004;

Zhao et al. 2004; Fu et al. 2005; Hostler et al. 2007)

and their large-scale preparation is limited by low-

effectiveness production rates and complicated pro-

cedures (Wegner et al. 2002; Hostler et al. 2007).

1730 J Nanopart Res (2009) 11:1729–1738

123

In this context of just a few studies in the field of

thermoelectric Bi nanoparticles and of their difficult

preparation, the present work reports an effective

synthesis method of zero-valent Bi nanopowder by

thermal decomposition of bismuth dodecyl-mercap-

tide and the first experimental results on the

thermoelectric characterisation of Bi pills sintered

by powder uniaxial compression. The structural and

morphological characterisation of Bi nanopowder

prepared at different thermolysis temperatures is also

provided.

Experimental

The precursor bismuth dodecyl-mercaptide

Bi(SC12H25)3 is not a commercially available product.

Therefore it was synthesised in the laboratory by

reacting stoichiometric amounts of dodecanethiol

C12H25SH (Aldrich, 98.5%) with bismuth (III) chlo-

ride BiCl3, (Aldrich, 99.999%) according to a simple

chemical route already described in a previous article

(Nicolais and Carotenuto 2008). The thermal decom-

position of Bi(SC12H25)3 is expected to give zero-

valent bismuth Bi(0) and dodecyl-disulfide

(SH25C12)2, as organic by-product. The mercaptide

thermolysis was performed in a glass tube by immer-

sion in an oil thermostatic bath with the temperature in

the range 140–180 C, for 3 min under vacuum in

order to prevent bismuth oxidation. The thermolysis

product was dispersed in chloroform, separating the

organic and non-organic products by centrifugation at

8,000 rpm, for 10 min. The purified non-organic

product Bi(0) was isolated as a gray powder, while

the organic by-product (SH25C12)2 was obtained as a

white solid layer after chloroform evaporation. Bulk

samples in the shape of pills with the diameter of

13 mm and about 0.5 mm in thickness were obtained

by synthesising the as-prepared Bi powder at 590 MPa

for 15 min at room temperature by means of an

uniaxial hydraulic press (Retsch PP 25).

The thermal decomposition of bismuth dodecyl-

mercaptide was studied by differential scanning

calorimetry (DSC, TA INSTRUMENTS 2920) and

thermogravimetric analysis (TGA, TA INSTRU-

MENTS 2950). Two consecutive DSC runs were

performed from 0 to 300 C, at a rate of 10 C/min

under fluxing nitrogen and using sealed aluminum

capsules to avoid changes in the thermogram baseline

due to organic by-product evaporation. The TGA

thermograms were acquired by heating the samples

from room temperature up to 850 C, at a rate of

10 C/min, under fluxing nitrogen.

The identification of the organic by-product of Bi-

mercaptide thermal decomposition was performed by

solution-state 1H nuclear magnetic resonance

(1H-NMR) spectroscopy, carried out on a Bruker

Avance 400 MHz instrument, operating at a proton

frequency of 400.13 MHz. The spectrometer was

equipped with a 5-mm Bruker inverse broadband probe

with an actively shielded z-gradient coil. The spectra

were acquired and elaborated by Bruker Topspin 1.3

software. 1H NMR spectra were referenced to the

chemical shift of the solvent (deuterated chloroform),

resonating at 7.26 ppm. Two-dimensional correlation

spectroscopy (2DCOSY) experiments were acquired

with a 16:12:40 gradient ratio (duration, 1 ms), 44

scans, 2,000 points in F2, and 256 points in F1. COSY

spectra were transformed with a sine-bell weighting

function in both dimensions applying a sine-bell shift

(SSB) of 0.

The composition and crystal structure of the as-

prepared gray powder were investigated by X-ray

powder diffractometry (XRD, Rigaku DMAX-IIIC),

using CuKa radiation (k = 0.154056 nm), in a stan-

dard Bragg-Brentano geometry. The detection range

was 2h = 5–80 in steps of 0.02 and with a counting

rate of 8 s/step.

The morphology of both as-prepared powder and

sintered pills was examined by scanning electron

microscopy (SEM, Cambridge-S360). The SEM

specimens were obtained by placing the powder/pill

fragment(s) onto an aluminum stab, on a biadhesive

graphite tape, and performing successively a graphi-

tisation process. The powder composition was further

examined by means of an energy dispersive spec-

trometer coupled with the used scanning electron

microscope (SEM–EDS).

The thermoelectric characterization of the synthe-

sized Bi pills was accomplished by DC electrical

resistance and Seebeck coefficient measurements, as

well as by investigating the electrical resistance

behavior with the temperature. The resistance was

measured by conventional two-point probe DC

measurements performed at room temperature and

at 77 K, in a probe station, with an error not

exceeding ±5%. Practically, the pills resistance was

determined from current measurements over a range

J Nanopart Res (2009) 11:1729–1738 1731

123

of applied voltage (?/–10 V) using a high resolution

picoamperometer (Keithley 6487). The resistance

versus temperature dependence was obtained by

cooling the bismuth pill from 280 to 80 K under

vacuum at 1 V constant voltage using a cryogenic

probe station. The Seebeck coefficient S ¼ DVDT was

determined by means of a modified Z-Meter (Gromov

et al. 2001), as first screening parameter for the

improvement in the thermoelectric efficiency. A

small AC voltage was applied to the pill under test,

inducing a temperature gradient across its thickness,

through the Peltier effect. The current was periodi-

cally disconnected to measure the induced

temperature difference across the pill and the gener-

ated Seebeck voltage simultaneously. The effective

Seebeck coefficient was evaluated by fitting the

Seebeck voltage linearly as a function of the

temperature difference.

Results and discussion

The typical DSC thermogram of pure bismuth

dodecyl-mercaptide presented in Fig. 1a shows: a

quite intensive endothermic peak at 66 C corre-

sponding to the melting point of the mercaptide; a

much less intensive endothermic peak at 105 C

determined by the mercaptide thermal decomposi-

tion; a broad exothermic peak of low intensity in the

range 180–200 C related to the clustering of the Bi

atoms generated from the mercaptide thermal decom-

position; and another endothermic peak at 271 C,

corresponding to Bi melting point. The second DSC

run (Fig. 1b), performed on the same sample, exhibits

two endothermic peaks: the first, at 27 C, is

determined by the melting of the organic by-products

mixture resulted from the thermal decomposition

reaction and the second, at 271 C, corresponds to the

Bi melting point.

A complementary method employed to investigate

the product of the mercaptide thermolysis was the

TGA, which gives information about the reaction

stoichiometry. The TGA thermogram of pure

Bi(SC12H25)3 (Fig. 2) reveals one distinct weight

loss in the range 175–320 C and a residual weight

equal to 26 wt.% at temperatures over 320 C. Since

the organic by-product of the Bi dodecyl-mercaptide

thermolysis is completely removed by evaporation at

temperatures close to 300 C, the residual weight

corresponds to the synthetised inorganic phase. It was

verified that the experimentally found residual weight

corresponds perfectly to the theoretically calculated

Bi percentage in the precursor mercaptide. This result

confirms, therefore, that the mercaptide thermolysis

gives only zero-valent bismuth (apart the organic by-

product), as theoretically predicted.

The 2DCOSY spectrum (Fig. 3), with mono-

dimensional 1H-NMR experiments on the two F1

and F2 axes, of the organic by-product of

Bi(SC12H25)3 thermal decomposition shows only

one spin system, attributable to (SH25C12)2. The

cross peak A (Fig. 3) was assigned to carbon 1 C1

resonating at 2.69 ppm correlated with C2 at

1.69 ppm. The signal at 1.69 ppm (C3) was, in turn,

0 50 100 150 200 250 300-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

)g/W(

wolF tae

H

Temperature (°C)

a

0 50 100 150 200 250 300

-2,0

-1,5

-1,0

-0,5

0,0

0,5

)g/W(

wolF tae

H

Temperature (°C)

b

Fig. 1 Typical DSC thermograms (heating rate 10 C/min) of bismuth dodecyl-mercaptide under fluxing nitrogen: first (a) and

second (b) run

1732 J Nanopart Res (2009) 11:1729–1738

123

correlated to carbon 3 at 1.38 ppm (cross peak B),

whereas C3 and C4–11 (1.33 ppm) were correlated

through cross peak C. Finally, carbons 4–11 appeared

to be correlated to the methyl group at 0.91 ppm

through cross peak D (Fig. 3). The results of the

NMR experiments reveal that the thermal decompo-

sition of Bi(SC12H25)3 produces disulfide as organic

by-product.

The DSC and TGA results in what concern the

formation of only Bi(0) as non-organic product of the

Bi mercaptide thermolysis are confirmed by XRD

measurements. The typical XRD spectrum of the gray

powder resulted after the thermolysis process is

shown in Fig. 4. All the observed diffraction peaks

can be indexed to the rhombohedral crystal structure

of bismuth, according to the standard ICDD PDF

(Card. No. 05–0519), apart only one peak corre-

sponding to Bi2S3 (240). No other foreign phases can

be seen within the apparatus detection limits.

The good purity of the Bi(0) product was con-

firmed by SEM–EDS measurements. The EDS

pattern (Fig. 5) of the gray powder resulted from

the mercaptide thermolysis shows only peaks related

to Bi (apart one peak corresponding to carbon C,

which comes from the graphite substrate of the

sample or from the graphitization process).

The obtained structural data allow us to assume

that the thermolysis process of Bi(SC12H25)3 is based

on the homolytic dissociation of the Bi–S bonds with

0 200 400 600 800

20

40

60

80

100

)%( thgie

W

Temperature (°C)

Fig. 2 TGA curve (heating rate: 10 C/min) of bismuth

dodecyl-mercaptide under fluxing nitrogen

Fig. 3 2DCOSY spectrum, with mono-dimensional 1H-NMR

experiments on the two F1 and F2 axes, of the organic by-

product of bismuth dodecyl-mercaptide thermal decomposition

20 30 40 50 60 70 800

1000

2000

3000

4000

5000

6000

) 5 2 1 ( ) 7 2 0 (

) 5 0 2 ( ) 7 0 1 (

) 8 1 0 (

) 0 0 3 ( ) 4 1 2 (

) 2 2 1 ( ) 6 1 1 ( i

B 2 S

3

) 4 2 0 (

) 2 0 2 ( ) 3 1 1 ( ) 5 1 0 (

) 0 1 1 (

) 3 0 0 ( ) 1 0 1 (

) . u . a ( y t i s n e t n I

2 θ (degrees )

) 2 1 0 (

) 4 0 1 (

Fig. 4 Typical XRD spectrum of the gray powder resulting

from the bismuth dodecyl-mercaptide thermolysis

B

BB

B

C

Energy (keV) 2.00 6.00 10.00 14.00 18.00

B

Fig. 5 Typical SEM–EDS pattern of the gray powder obtained

from the thermal decomposition of the bismuth dodecyl-

mercaptide

J Nanopart Res (2009) 11:1729–1738 1733

123

the formation of Bi atoms and H25C12S• radicals,

which combine together leading to disulfide mole-

cules (SC12H25)2. The Bi phase separation and atomic

clustering take place following a temperature-depen-

dent mechanism that is further described below. The

Bi clusters are an electrophilic species due to the

presence of 6p empty orbitals, while the disulfide

molecules are strongly nucleophilic owing to the high

polarizable lone-pair electrons on the sulfur atoms.

Consequently, the nucleophilic disulfide molecule

bonds the electrophilic surface of the Bi clusters,

leading to the formation of an effective efficient steric

barrier which prevent the particles from aggregation

and limit their growth (Larsen et al. 2003). Therefore,

the Bi(SC12H25)3 thermolysis leads to nanosized Bi

powder due to the disulfide capping layer on the

metallic particles’ surface.

The temperature of the mercaptide thermolysis

was varied in the range 140–180 C, in order to

investigate the temperature influence on the mor-

phology of the resulting Bi(0) particles. The

reproducibility of the samples morphology as a

function of the processing temperature has been

verified by several sample preparations for each

decomposition temperature. The representative SEM

images of the Bi(0) powders prepared at 140, 160 and

180 C (Fig. 6a–c, respectively) reveal the formation

of well defined, regular spherical shape nanoparticles

for all performed thermolyses. The Bi particles’ size

and the size distribution (Fig. 6 insets) were evalu-

ated examining the SEM micrographs by means of an

image analysis software. The determined average

diameter D and the related standard deviation r of the

Bi particles obtained from the mercaptide thermal

decomposition at 140, 160 and 180 C are

D = 601 nm and r = 202 nm, D = 202 nm and

r = 32 nm, and D = 170 nm and r = 31 nm,

respectively. On increasing the thermal decomposi-

tion temperature, a decrease in the particles size can

be seen together with an increase in shape regularity

and a narrower size distribution. The mechanism

governing the nucleation and growth of the Bi

particles in the used thermolysis conditions is the

following: during the thermal treatment, the mercap-

tide decomposes producing a large amount of Bi

atoms and the phase separation takes place at a high

supersaturation level for the whole duration of the

thermal treatment. The metal nuclei are continuously

Fig. 6 Representative

SEM micrographs of the Bi

nanopowder obtained by

thermal decomposition of

bismuth dodecyl-

mercaptide at 140 C (a),

160 C (b) and 180 C,

(c) and of the Bi pills (d)

prepared by uniaxial

compression at 590 MPa,

for 15 min of the Bi

nanopowder obtained at

180 C. The related size

histograms of the Bi

nanoparticles are presented

in the insets

1734 J Nanopart Res (2009) 11:1729–1738

123

generated by atomic clustering, at a nucleation rate

increasing with the thermolysis temperature, but with

a low growth rate due to the short treatment time and

limited Bi atomic diffusion into the mercaptide/di-

sulfide mixture.

The SEM analysis of the as-synthesised Bi pills

morphology (Fig. 6d) shows the preservation of the

nanostructure, with no essential grains deformation

from their spherical shape, but with the increase in

the average particles size (e.g., D = 272 nm,

r = 54 nm for the pills obtained by the compression

of Bi nanopowder with the average size of 170 nm

prepared from 180 C mercaptide thermolysis) with

respect to the precursor Bi nanopowder, due to grain

aggregation events induced by the compression

process.

The first thermoelectric investigation of the as-

synthesised Bi pills was focused on the measurement

of the current–voltage curves I = f(V) at both room

temperature and %77 K. In the case of the pill

prepared by pressing the 170 nm Bi nanopowder, a

non-linear current dependence on the applied voltage

and a higher electrical resistance at %77 K with

respect to room temperature for the same values of

applied voltage can be seen in Fig. 7a. These results

indicate a semiconductive behavior. A better visual-

ized evidence of the resistance non-linearity is

furnished by the non-linear conductance (1/R) depen-

dence on the applied voltage V at room temperature

and at %77 K (Fig. 7b). The current–voltage curves

for the pills obtained by pressing the 601 and 202 nm

Bi nanopowders exhibit a typical metallic behavior,

similar to bulk Bi. Until now, semimetal–semicon-

ductor transition in Bi nanostructures, induced by

quantum confinement effects, has been reported for

nanoparticles with diameter less than 40 nm (Wang

et al. 2006), for nanowire arrays and thin films below

50 nm (Lin et al. 2000; Heremans 2005; Wang et al.

2006), and for nanowires in the range 100–60 nm

(Yonghui and Jingying 2005).

A further evidence for the semiconductor behavior

of the pills prepared by pressing the 170 nm Bi

nanopowder was obtained from the temperature-

dependent resistance measurements performed by

cooling the Bi pill from 280 to 80 K at 1 V constant

voltage (Fig. 8a). As shown in the figure, the

electrical resistance decreases with increase in tem-

perature. This inverse trend with respect to the bulk

Bi semimetal behavior is typical of a semiconductor.

The measured resistance R values were further

plotted versus (1/T) (Fig. 8b) in order to evaluate

the energy gap Eg. It is well known that the resistance

of a semiconductive material is strongly dependent

on the width of the band gap, following the equation

(Seeger 1985):

R ¼ R0 expEg

2kBT

ð2Þ

where R0, Eg, kB, and T are pre-exponential constant,

the energy gap, the Boltzmann constant

(kB = 8.617 9 10-5 eV), and the temperature,

respectively. Therefore, the energy gap of the inves-

tigated Bi pill can be found from a non-linear curve

fit of R ¼ f 1T

in two temperature ranges, 280–117 K

and 100–82 K, from the relation:

1

t1

¼ Eg

2kB

ð3Þ

where t1 was determined to be t1 = 0.00102 ±

0.00006 for 100–82 K and t1 = 0.00294 ± 0.00003

for 280–117 K. The energy gap evaluated using Eq. 3

is Eg % 0.06 eV in 280–117 K temperature range

and Eg % 0.17 eV for 100–82 K.

-12 -8 -4 0 4 8 12

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

a

Room temperature 77 K

( I×

01 4-

)A

V (V)

-6 -4 -2 0 2 4 6

-40

0

40

80

120

160

200b

Room temperature 77 K PolyFit_Smoothed1InterEx PolyFit_Smoothed2InterEx

)S( ecnatcu dno

C

Voltage (V)

Fig. 7 DC current intensity

I versus applied voltage V at

room temperature (d) and

at 77 K (s) (a) and

conductance versus applied

voltage (b) for the Bi pill

synthesised by uniaxial

pressing the Bi powder

obtained from 180 C

mercaptide thermolysis

J Nanopart Res (2009) 11:1729–1738 1735

123

The effective Seebeck coefficient S ¼ DVDT of the

prepared nanostructured Bi pills was estimated from

the linear fitting of the Seebeck voltage, as a function

of the temperature difference in the range 318–373 K.

Figure 9 reports the effective Seebeck coefficient as a

function of temperature for the Bi pill prepared by

pressing the 170 nm Bi nanopowder. It can be seen

that the average absolute value of the Seebeck

coefficient S % -146 lV K-1 is higher than that of

bulk Bi [-72 lV K-1 (Hostler et al. 2007)] over the

whole range of temperatures investigated. The nega-

tive sign of the determined Seebeck coefficients shows

an n-type semiconductive behavior. The pills obtained

by the consolidation of the higher size Bi powders

obtained after 140 C (601 nm) and 160 C (202 nm)

mercaptide thermolysis exhibit no improvement in

Seebeck coefficient, which has similar values with

bulk Bi. These results could be interpreted in the frame

of the theory which predicts the largest enhancement

in the Seebeck coefficient in comparison to bulk Bi in

the case of quantum dots structures, with respect to

nanowires and thin film systems (Sun et al. 1999). It is

well known that the S value for bulk Bi is quite small

due to the equal concentrations of electrons and holes,

which lead to a nearly complete cancelation between

the positive and negative contribution to the Seebeck

coefficient, while the increase in the S value of

nanostructures is determined by the quantum confine-

ment effects on the electrical charge that result in an

enhanced electronic density of states near the Fermi

energy. In any case, until now, none of the investigated

Bi nanostructures (nanowires and thin films) with the

dimension of the constituent units similar to the size of

the prepared semiconductive nanopowder (D =

272 nm, r = 54 nm) have shown such improvement

in the Seebeck coefficient value. For example, liter-

ature reports S values similar to bulk Bi, in the range

70–80 lV K-1 at room temperature, for Bi nanowires

with the diameter of 240 and 480 nm (Nikolaeva et al.

2008; Lin et al. 2000).

Conclusions

The thermal decomposition of Bi(SC12H25)3 repre-

sents an effective preparation route of Bi

nanoparticles, with spherical shape and rhombohedral

crystal structure, offering the possibility to control the

grains size through: (1) the formation of an efficient

50 100 150 200 250 300

0

30

60

90

120

150

180

R (

× 0 1

3 Ω

)

T (K)

a

0,003 0,006 0,009 0,012

0

30

60

90

120

150

1/T ( k -1 )

( R

Ω

)

b

Fig. 8 Resistance R versus temperature T for the nanostruc-

tured Bi pill (prepared by room temperature uniaxial

compression of the Bi powder obtained from 180 C thermol-

ysis of the Bi mercaptide) at 1 V constant voltage (a);

Resistance R versus inverse temperature 1/T for the same

sample and linear curve fit in the 280–117 and 100–82 K

temperature range (b)

25 30 35 40 45 50 55 60 65 70-200

-180

-160

-140

-120

-100

-80

(S

µK

V1 -)

Temperature (°C)

Fig. 9 Variation in the Seebeck coefficient S of nanostruc-

tured Bi pills (prepared by room temperature uniaxial

compression of the Bi powder obtained from 180 C thermol-

ysis of the Bi mercaptide) with the temperature

1736 J Nanopart Res (2009) 11:1729–1738

123

steric barrier made of (SC12H25)2 molecules (capping

agent), which prevents the aggregation of particles

and reduce their growth (ii) the control of the

thermolysis temperature.

The investigation of the transport properties of the

Bi pills made of nanoparticles with the average

diameter of 270 nm shows an n-type semiconductor

behavior, with a direct band gap energy

Eg % 0.06 eV in 280–117 K temperature range and

Eg % 0.17 eV in 100–82 K, while the pills sintered

from the higher dimension nanoparticles exhibit a

semimetal behavior typical to bulk Bi. To the best of

our knowledge, the observed semimetal–semicon-

ductor transition for Bi grains size of about 270 nm is

the first report on quantum-like confinement effects in

Bi nanosystems with constituent units higher than

100 nm. Consistent with these results, the preli-

minary study on the thermoelectric characteristics of

the prepared Bi nanopowders shows: (1) larger value

for the effective Seebeck coefficient in the investi-

gated temperature range with respect to bulk Bi in the

case of the Bi pills made of 270 nm nanoparticles; (2)

no enhancement in S magnitude with respect to bulk

Bi for higher dimension systems.

The obtained improvement in the thermoelectric

characteristics with respect to bulk Bi even at grain

dimensions of hundreds of nanometres shows a great

potentiality of the Bi nanoparticles in thermoelec-

tricity, but much research is still needed before their

full potential is realized. Further research on tailoring

the process parameters for decreasing the nanoparti-

cles size and on the control of uniform morphologies

formation is currently underway. Further investiga-

tion on the thermoelectric characteristics, on

the origin of the semimetal–semiconductor transition

and of the Seebeck coefficient increment in abso-

lute value will be done to gain insight and

understanding about the real possibilities of Bi

nanopowders in reaching, by itself and in com-

pounds, the efficiency level required for

thermoelectric applications.

Acknowledgment The technical assistance of Dr. Manlio

Colella for SEM investigations is gratefully acknowledged.

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