# New Thiophene Monolayer-Protected Copper Nanoparticles: Synthesis and Chemical-Physical Characterization

Authors: Elisabetta Foresti; Guido Fracasso; Massimiliano Lanzi; Isidoro Giorgio Lesci; Luisa Paganin; Tommaso Zuccheri; Norberto Roveri
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

For the first time copper 3-(6-mercaptohexyl)thiophene-protected nanoparticles (Cu T6SH) have been synthesized by a one-phase system, utilizing anNaBH4/LiCl mixture in diglyme as the reducing reagent and avoiding water medium dissolving copper salts. The prepared nanoclusters, characterized by transmission electron microscopy (TEM), have shown a constant spherical morphology with a size dimension of 5-6 nm in diameter. After their synthesis, no morphological evolution and irreversible aggregation process has been observed after a storage in CH2Cl2 at low temperature for a period up to six months long. Cu T6SH nanoparticles have been investigated by UV-Visible (UV-Vis) and Fourier transmission infrared (FTIR) spectroscopes to characterize the alkylthiophenes monolayer conformations and the particles optoelectronic properties. The UV-Vis reveals the lack of the surface plasmonic band, previously observed in Cu-nanosized clusters at about 556–570 nm, and shows a wide-band centered at 293 nm, probably due to the high-conformational surface ordering of thiophene rings on the Cu core. The results highlight the importance of the modifications ported to the well-known one-phase synthetic reactions to obtain a clear lack, even after a storage of six months, of any irreversible aggregation that has always characterized chain thiophene-protected metallic nanoparticles.


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## Body

## 1. Introduction

The properties of
materials, which can be tailored by controlling their size and morphology,
recently have been engineered by nanotechnology [1]. Nanostructured and
nanosized materials have allowed recently innovated technological applications
in catalysis [2, 3], biomedical [4], optic, photoresponsive, and electronic
devices [5]. Nanoparticles are very interesting for the quantum effect by which
their properties are tunable by the particle nanosize and surface
chemical-physical characteristics [6]. Nanoparticle
science and technology have received an important advancement by Schmid et al., they formerly synthesized ligand-stabilized gold nanoparticles [7]
opening the right way to Brust et al. [8] to prepare monolayer-protected
gold nanoparticles [8]. The organic monolayers on the surface of metal
nanoparticles prevent aggregation and oxidation [9]. Many organic compounds
with reactive head groups, such as thiol [10], sulphide [11], thiosulfate [12],
xanthate [13], amine [14], selenide [15], and isocyanide [16], have been
utilized to protect by self-assembly different metal nanoparticles including Au
[10–16], Ag [13], Cu [13, 17], Pt [18], Pd [19], Ni [20], and bimetallics [21].
The synthesis of metal nanoparticles protected with various monolayers
containing different reactive head and functional tail groups is of high
significance to improve their chemical and biochemical technological
applications. Because of the expected marked electronic and optical properties
of copper nanoparticles, there has been an increased interest in the
preparation of stable monolayer-protected copper nanoparticles. Alkyl xanthates
[13], alkylamines [22], tetraalkyl ammonium complexes [23], and alkanethiols
[17, 24] ligands have been utilized to protect copper nanoparticles. However, a
limited progress has been obtained because of the high instability of copper
nanoparticles. In order to contrast copper nanoparticles, irreversible
aggregation and oxidation,  Chen and  Sommers [17] have prepared copper nanoparticles
protected by alkanethiolate in one-phase system. Their prepared spherical (1-2 nm in diameter) Cu particle bound hexanethiolate monolayers undergo to the
oxidation of copper core, morphological evolution, and irreversible aggregation
process. Long-chain alkanethiols, which have been found to
self-assemble into compact monolayers (i.e., three-dimensional self-assembled
monolayers) on metallic nanoparticles [25, 26], cannot only
induce metallic nanoparticles to be readily isolated, but also reduce their
conductivity. The aim of this study is to synthesize 3-(6-mercaptohexyl)thiophene-protected
copper nanoparticles which,
in absence of long-chain alkanethiols and in absence of irreversible
aggregation, could represent new high-conductive metallic nanoparticles with
improved potentialities of tuning their optical and electronic properties.


## 2. Experimental Section

### 2.1. Chemicals

Copper chloride(CuCl2, Sigma Aldrich, Milano, Italy), 1-butanethiol (C4H9SH, 97,0%, Fluka), sodium borohydride (NaBH4, 99%, Aldrich), lithium
chloride (LiCl, 98%, Fluka) were used as received. The solvents were purchased
from Fluka (diethyl ether), Riedel de Haen (tetrahydrofuran, THF), and Acros
(2-methoxyetylether 99%, diglyme) and used as received.Water was supplied by a
Millipore A 10 system (18.50  MΩ).


### 2.2. Synthesis of 3-(6-mercaptohexyl)thiophene

The
preparation of the monomer 3-(6-mercaptohexyl)thiophene
has been performed according to the guidelines described by Ahn et al. [27]. 1.65 g (6.67 mmol) of 3-(6-bromohexyl)thiophene were added to a solution of 0.61 g (8.00 mmol) of
thiourea dissolved in 36 mL of anhydrous ethanol. The reaction was refluxed for
3 hours under stirring and in inert atmosphere. 0.46 g (11.10 mmol) of NaOH
dissolved in 15 mL of distilled water were then added and, after refluxing for
1 hour, the reaction mixture was poured into 150 mL of an aqueous solution of
HCl (pH=4) and extracted several times with diethyl ether. The collected
organic phases were washed with distilled water, dried over MgSO4, and
after evaporation of the solvent at reduced pressure 1.25 g (6.24 mmol) of crude
monomer were obtained (94% yield). The latter was then purified by column
chromatography (silica gel; n-heptane/CHCl3 8:2) giving 1.09 g (5.45 mmol) of pure
monomer (82% yield).H1 and C13-NMR were run on a
Varian Gemini 300 FT-NMR spectrometer (300 MHz) using TMS as reference. Mass spectrometry (MS) was carried out
on a VG7070E spectrometer at 70 eV.H1-NMR (CDCl3, ppm): δ7.22 (m, 1H), 6.98 (m, 2H), 2.62 (t, 2H), 2,50
(t, 2H), 1.60 (m, 4H), 1.40 (m, 4H).C13-NMR (CDCl3, ppm): δ143.71, 128.92, 125.78, 120.52, 32.83, 31.09, 30.87, 30.30, 29.57, 29.40.FT-IR-ATR
(Ge crystal): 3102, 3050, 2928, 2855, 2673 (-SH), 1537, 1462, 1439,
1410, 1293, 1279, 1264, 1238, 1152, 1079, 939, 856, 833, 771, 684, 661, 633,
595,577cm−1.Mass (m/z, relative intensity): 200 (11.4,M+), 98 (100.0, C5H6S+).


### 2.3. Synthesis of 3-(6-mercaptohexyl)thiophene
Monolayer-Coated Copper Nanoparticles

The
particles were synthesized according to a modification of the one-phase system
method of Chen and Sommers [17], avoiding any aqueous solvent
during the nanoparticles formation.In
a round-bottomed flask 0.24 mmol (41 mg) of CuCl2 dissolved in 20 mL
of tetrahydrofuran (THF) were dropped in 0.71 mmol (143 mg) of 3-(6-mercaptohexyl)thiophene (T6SH) under stirring until the solution
remained transparent becoming yellowish. In this solution, 247 mg of NaBH4/LiCl
mixture (2/3 molar ratio—92 mg NaBH4/155 mg LiCl)  suspended in 40 mL of diglyme (CH3OCH2CH2)O2
were dropped under stirring. The rapid turn of solution to dark brown reveals
the 3-(6-mercaptohexyl)thiophene-coated Cu
nanoparticles
formation. Diethyl ether (150 mL)
was added to the reaction mixture before the coated nanoparticles extraction
with an NaCl saturated solution. The extraction was performed using 300 mL of
saline solution and was repeated three times. The diethyl ether solvent was
removed under reduced pressure. The dark brownish precipitate was washed twice
with cooled methanol and was then separated by centrifugation at 3000 rpm.


### 2.4. Morphological Characterization

Transmission
electron microscopy (TEM) investigations were carried out using a Philips CM
100 electron microscope at an accelerating voltage of 100 kV. The powdered
samples were ultrasonically dispersed in ultrapure water and then deposited by
dropcasting on conventional Formvar/Carbon 200 mesh copper microgrids.


### 2.5. Spectroscopic Characterization

FTIR spectra were collected with a Thermo Nicolet
FTIR 380 spectrometer equipped with ATR accessory and the spectra resolution
was4cm−1. The spectrum of the capping agent
3-(6-mercaptohexyl)thiophene was obtained by dropcasting the monomer thiophene
onto germanium crystal and the spectrum of the Cu thiophene-capped
nanoparticles was acquired by placing an aliquot of 5–10 mg onto
germanium crystal without any additional sample preparation. The spectra were the result of 32 scans.UV-Vis spectroscopy has been carried out using a Varian Cary 300
Bio spectrophotometer with a 1 cm quartz cuvette. Solutions of Cu T6SH nanoparticles
were prepared at concentration of 0.1 mg/mL in dichloromethane(CH2Cl2).


## 2.1. Chemicals

Copper chloride(CuCl2, Sigma Aldrich, Milano, Italy), 1-butanethiol (C4H9SH, 97,0%, Fluka), sodium borohydride (NaBH4, 99%, Aldrich), lithium
chloride (LiCl, 98%, Fluka) were used as received. The solvents were purchased
from Fluka (diethyl ether), Riedel de Haen (tetrahydrofuran, THF), and Acros
(2-methoxyetylether 99%, diglyme) and used as received.Water was supplied by a
Millipore A 10 system (18.50  MΩ).


## 2.2. Synthesis of 3-(6-mercaptohexyl)thiophene

The
preparation of the monomer 3-(6-mercaptohexyl)thiophene
has been performed according to the guidelines described by Ahn et al. [27]. 1.65 g (6.67 mmol) of 3-(6-bromohexyl)thiophene were added to a solution of 0.61 g (8.00 mmol) of
thiourea dissolved in 36 mL of anhydrous ethanol. The reaction was refluxed for
3 hours under stirring and in inert atmosphere. 0.46 g (11.10 mmol) of NaOH
dissolved in 15 mL of distilled water were then added and, after refluxing for
1 hour, the reaction mixture was poured into 150 mL of an aqueous solution of
HCl (pH=4) and extracted several times with diethyl ether. The collected
organic phases were washed with distilled water, dried over MgSO4, and
after evaporation of the solvent at reduced pressure 1.25 g (6.24 mmol) of crude
monomer were obtained (94% yield). The latter was then purified by column
chromatography (silica gel; n-heptane/CHCl3 8:2) giving 1.09 g (5.45 mmol) of pure
monomer (82% yield).H1 and C13-NMR were run on a
Varian Gemini 300 FT-NMR spectrometer (300 MHz) using TMS as reference. Mass spectrometry (MS) was carried out
on a VG7070E spectrometer at 70 eV.H1-NMR (CDCl3, ppm): δ7.22 (m, 1H), 6.98 (m, 2H), 2.62 (t, 2H), 2,50
(t, 2H), 1.60 (m, 4H), 1.40 (m, 4H).C13-NMR (CDCl3, ppm): δ143.71, 128.92, 125.78, 120.52, 32.83, 31.09, 30.87, 30.30, 29.57, 29.40.FT-IR-ATR
(Ge crystal): 3102, 3050, 2928, 2855, 2673 (-SH), 1537, 1462, 1439,
1410, 1293, 1279, 1264, 1238, 1152, 1079, 939, 856, 833, 771, 684, 661, 633,
595,577cm−1.Mass (m/z, relative intensity): 200 (11.4,M+), 98 (100.0, C5H6S+).


## 2.3. Synthesis of 3-(6-mercaptohexyl)thiophene
Monolayer-Coated Copper Nanoparticles

The
particles were synthesized according to a modification of the one-phase system
method of Chen and Sommers [17], avoiding any aqueous solvent
during the nanoparticles formation.In
a round-bottomed flask 0.24 mmol (41 mg) of CuCl2 dissolved in 20 mL
of tetrahydrofuran (THF) were dropped in 0.71 mmol (143 mg) of 3-(6-mercaptohexyl)thiophene (T6SH) under stirring until the solution
remained transparent becoming yellowish. In this solution, 247 mg of NaBH4/LiCl
mixture (2/3 molar ratio—92 mg NaBH4/155 mg LiCl)  suspended in 40 mL of diglyme (CH3OCH2CH2)O2
were dropped under stirring. The rapid turn of solution to dark brown reveals
the 3-(6-mercaptohexyl)thiophene-coated Cu
nanoparticles
formation. Diethyl ether (150 mL)
was added to the reaction mixture before the coated nanoparticles extraction
with an NaCl saturated solution. The extraction was performed using 300 mL of
saline solution and was repeated three times. The diethyl ether solvent was
removed under reduced pressure. The dark brownish precipitate was washed twice
with cooled methanol and was then separated by centrifugation at 3000 rpm.


## 2.4. Morphological Characterization

Transmission
electron microscopy (TEM) investigations were carried out using a Philips CM
100 electron microscope at an accelerating voltage of 100 kV. The powdered
samples were ultrasonically dispersed in ultrapure water and then deposited by
dropcasting on conventional Formvar/Carbon 200 mesh copper microgrids.


## 2.5. Spectroscopic Characterization

FTIR spectra were collected with a Thermo Nicolet
FTIR 380 spectrometer equipped with ATR accessory and the spectra resolution
was4cm−1. The spectrum of the capping agent
3-(6-mercaptohexyl)thiophene was obtained by dropcasting the monomer thiophene
onto germanium crystal and the spectrum of the Cu thiophene-capped
nanoparticles was acquired by placing an aliquot of 5–10 mg onto
germanium crystal without any additional sample preparation. The spectra were the result of 32 scans.UV-Vis spectroscopy has been carried out using a Varian Cary 300
Bio spectrophotometer with a 1 cm quartz cuvette. Solutions of Cu T6SH nanoparticles
were prepared at concentration of 0.1 mg/mL in dichloromethane(CH2Cl2).


## 3. Results and Discussion

3-(6-bromohexyl)thiophene was first synthesized as previously described
[28] and reacted with thiourea giving 3-(6-mercaptohexyl)thiophene (MDT) with a
good yield. Monolayer-protected Cu nanoparticles were prepared in one-phase
system by modified Brust reaction [8] using as reducing reagent a mixture of
finely grounded LiCl and NaBH4 (3/2 molar ratio) suspended in
diglyme (CH3OCH2CH2)O2,
instead of LiBH4 (super hydride) dissolved in tetrahydrofuran (THF)
according to Chen and  Sommers (2001) [17]. These authors have
dissolved Cu(NO3)2 in water-THF mixture, while we have used pure THF to
suspend CuCl2⋅2⋅H2O, in order to avoid any
oxidation process and enhance the LiBH4 reducing capacity.
Furthermore, diglyme acts not only as chelating agent of the Li cations, but
also as dissolvent. The MDT/CuCl2⋅2⋅H2O reactants ratio
was 3:1.


### 3.1. Spectroscopic Studies

The conformational structure of the Cu particle-organic surfactant
monolayer has been examined by FTIR  spectroscopy. In Figure1, infrared spectra
in the range of 2750cm−1 and 2550cm−1 of the free
thiophene (a) and Cu thiophene on protected nanoparticles (b) are compared. The
lack of the SH stretching of the free ligand at about 2580cm−1 is
due to the chemisorptions on the Cu nanoparticles. The CH2 antisymmetric (d−) and symmetric (d+) stretching recorded
for thiophene ligand have been repeatedly used as a sensitive diagnostic
indicator for the conformational ordering of the metal-linked alkyl chains [29–31]. In fact,
these two characteristic vibrational stretching modes for the
3-(6-mercaptohexyl)thiophene have been found at 2928 and 2856cm−1 in
solution [17, 32, 33], while in crystalline state, for all zigzag trans ordered conformations the νas(d−) and νs(d+) are usually found at 2920 and
2850 cm-1, respectively [20]. The lyophilized Cu thiophene-protected
nanoparticles FTIR spectrum reported in Figure [2] shows the characteristic vas(d−) and νs(d+) vibrational stretching modes
of CH2 at 2924cm−1 and at 2852cm−1, respectively,
revealing a marked shift toward lower wave numbers than those recorded for the
free thiophene ligand. This finding highlights that the capping agent for
synthesized Cu T6SH has an enhanced ordered structure close to the Cu cluster
surface [32]. The FTIR spectrum reported in Figure [2] has been obtained for Cu
T6SH particles immediately after synthesis, but it does not differ appreciably from
the FTIR spectra recorded for the same Cu thiophene-protected nanoparticles
after a long period of storage at low temperature (+4°C), suspended in CH2Cl2 up to six months long.Figure 1
Comparison
of the infrared spectra in the range from 2750 to2550cm−1of the
free thiophene (a) and Cu T6SH-protected nanoparticles (b).Figure 2
Infrared
spectrum of Cu T6SH-protected nanoparticles.Usually, the optical properties of alkanethiolate-capped
transition-metal nanoparticles analyzed by UV-Vis characterization display the
presence of a remarkable surface plasmon band (SP) having an intensity and an
energy that are strongly dependent on the size and the chemical composition of
the system we are studying. The typical absorption peak of the characteristic
surface plasmon Cu-nanosized cluster is expected at about 556 nm [21], 566 nm
[17], and 570 nm [33, 34]. On the contrary, a UV-Vis absorption profiles exhibiting an
exponential decay Mie scattering [35] with decreasing photon energy, without
the surface-plasmon band, is a characteristic of nanosized particles and has been
observed for a copper very small core size diameter of the particles (<2 nm) [17, 36]. The spectrum of the Cu T6SH particles, reported in Figure 3, does
not reveal any evidence of the surface plasmonic band at about 556–570 nm, but shows
only a wideband centered at 293 nm due according to Mie’s theory is the
summation of all electronic and magnetic multipole oscillations, contributing
to the absorption and scattering of the interacting electromagnetic field [37].
A comparable adsorption band at about 340 nm has been observed for dodecanethiol-protected
Cu nanoclusters of about 3 nm in diameter [21]. The lowest position of the band
at 293 nm, never observed before also for <2 nm sized nanoparticles, could
be attributed to the shorter (C6) alkyl chain of thiophene ring surface ordered
on the Cu core. Furthermore, a possible interaction between the thiophene rings
and the metallic core cannot be left out considering the short alkyl chain
length.Figure 3
UV-Vis spectrum of the Cu
T6SH-protected nanoparticles dissolved inCH2Cl2.After a storing period of six months long, the same Cu T6SH particles
exhibit a UV-Vis absorption profile very close to that previously recorded for
fresh nanoparticles, but reveal a slight decrease to 291 nm of the adsorption
band. This data support s the lack of irreversible aggregation previously
observed for metallic thiophene-protected nanoparticles [17].


### 3.2. Transmission Electron Microscopic Investigation

TEM has been utilized in
investigating the Cu T6SH particles dimension.
Figure4(a)
shows a TEM image of the Cu T6SH nanoparticles recorded after a
period up to six months long of storage at low temperature (+4°C) suspended in
CH2Cl2 after their synthesis. Nanoparticles exhibit
spherical shape while particle size histogram inserted in Figure 4(a) puts in
evidence that the median size is 5.65 nm in diameter. The Cu T6SH nanoparticles
shape is homogenously spherical without exhibiting the irregular shapes and the
well-defined faceted morphologies observed for other polythiophene-coated
transition-metal nanoparticles [38] and especially for copper nanoparticles
dodecanethiol protected [17, 21]. The Cu T6SH nanoparticles morphology and
dimensions observed immediately after synthesis (Figure 4(b)) were
appreciably unchanged after six months of storage at low temperature (+4°C)
and suspended in CH2Cl2 after their synthesis. This finding
reveals the absence of irreversible nanoparticles
aggregation reported for long-chain alkanethiols capped metallic nanoparticles
[17].TEM image (a) of the fresh Cu T6SH nanoparticles synthesized by (Li diglyme)BH4 as a reducing agent and the depict of the nanoparticle
size histogram is reported. (b) TEM image of the Cu T6SH nanoparticles
synthesized by (Li diglyme)BH4 as
a reducing agent after a period of six months long of storage at low temperature
(+4°C)
were suspended in CH2Cl2 after their synthesis.
(a)(b)This result is due to the
modifications produced to the one-phase synthetic process of alkanethiolate-protected
copper nanoparticles [17] which undergo to the oxidation of copper core,
morphological evolution, and irreversible aggregation process.


## 3.1. Spectroscopic Studies

The conformational structure of the Cu particle-organic surfactant
monolayer has been examined by FTIR  spectroscopy. In Figure1, infrared spectra
in the range of 2750cm−1 and 2550cm−1 of the free
thiophene (a) and Cu thiophene on protected nanoparticles (b) are compared. The
lack of the SH stretching of the free ligand at about 2580cm−1 is
due to the chemisorptions on the Cu nanoparticles. The CH2 antisymmetric (d−) and symmetric (d+) stretching recorded
for thiophene ligand have been repeatedly used as a sensitive diagnostic
indicator for the conformational ordering of the metal-linked alkyl chains [29–31]. In fact,
these two characteristic vibrational stretching modes for the
3-(6-mercaptohexyl)thiophene have been found at 2928 and 2856cm−1 in
solution [17, 32, 33], while in crystalline state, for all zigzag trans ordered conformations the νas(d−) and νs(d+) are usually found at 2920 and
2850 cm-1, respectively [20]. The lyophilized Cu thiophene-protected
nanoparticles FTIR spectrum reported in Figure [2] shows the characteristic vas(d−) and νs(d+) vibrational stretching modes
of CH2 at 2924cm−1 and at 2852cm−1, respectively,
revealing a marked shift toward lower wave numbers than those recorded for the
free thiophene ligand. This finding highlights that the capping agent for
synthesized Cu T6SH has an enhanced ordered structure close to the Cu cluster
surface [32]. The FTIR spectrum reported in Figure [2] has been obtained for Cu
T6SH particles immediately after synthesis, but it does not differ appreciably from
the FTIR spectra recorded for the same Cu thiophene-protected nanoparticles
after a long period of storage at low temperature (+4°C), suspended in CH2Cl2 up to six months long.Figure 1
Comparison
of the infrared spectra in the range from 2750 to2550cm−1of the
free thiophene (a) and Cu T6SH-protected nanoparticles (b).Figure 2
Infrared
spectrum of Cu T6SH-protected nanoparticles.Usually, the optical properties of alkanethiolate-capped
transition-metal nanoparticles analyzed by UV-Vis characterization display the
presence of a remarkable surface plasmon band (SP) having an intensity and an
energy that are strongly dependent on the size and the chemical composition of
the system we are studying. The typical absorption peak of the characteristic
surface plasmon Cu-nanosized cluster is expected at about 556 nm [21], 566 nm
[17], and 570 nm [33, 34]. On the contrary, a UV-Vis absorption profiles exhibiting an
exponential decay Mie scattering [35] with decreasing photon energy, without
the surface-plasmon band, is a characteristic of nanosized particles and has been
observed for a copper very small core size diameter of the particles (<2 nm) [17, 36]. The spectrum of the Cu T6SH particles, reported in Figure 3, does
not reveal any evidence of the surface plasmonic band at about 556–570 nm, but shows
only a wideband centered at 293 nm due according to Mie’s theory is the
summation of all electronic and magnetic multipole oscillations, contributing
to the absorption and scattering of the interacting electromagnetic field [37].
A comparable adsorption band at about 340 nm has been observed for dodecanethiol-protected
Cu nanoclusters of about 3 nm in diameter [21]. The lowest position of the band
at 293 nm, never observed before also for <2 nm sized nanoparticles, could
be attributed to the shorter (C6) alkyl chain of thiophene ring surface ordered
on the Cu core. Furthermore, a possible interaction between the thiophene rings
and the metallic core cannot be left out considering the short alkyl chain
length.Figure 3
UV-Vis spectrum of the Cu
T6SH-protected nanoparticles dissolved inCH2Cl2.After a storing period of six months long, the same Cu T6SH particles
exhibit a UV-Vis absorption profile very close to that previously recorded for
fresh nanoparticles, but reveal a slight decrease to 291 nm of the adsorption
band. This data support s the lack of irreversible aggregation previously
observed for metallic thiophene-protected nanoparticles [17].


## 3.2. Transmission Electron Microscopic Investigation

TEM has been utilized in
investigating the Cu T6SH particles dimension.
Figure4(a)
shows a TEM image of the Cu T6SH nanoparticles recorded after a
period up to six months long of storage at low temperature (+4°C) suspended in
CH2Cl2 after their synthesis. Nanoparticles exhibit
spherical shape while particle size histogram inserted in Figure 4(a) puts in
evidence that the median size is 5.65 nm in diameter. The Cu T6SH nanoparticles
shape is homogenously spherical without exhibiting the irregular shapes and the
well-defined faceted morphologies observed for other polythiophene-coated
transition-metal nanoparticles [38] and especially for copper nanoparticles
dodecanethiol protected [17, 21]. The Cu T6SH nanoparticles morphology and
dimensions observed immediately after synthesis (Figure 4(b)) were
appreciably unchanged after six months of storage at low temperature (+4°C)
and suspended in CH2Cl2 after their synthesis. This finding
reveals the absence of irreversible nanoparticles
aggregation reported for long-chain alkanethiols capped metallic nanoparticles
[17].TEM image (a) of the fresh Cu T6SH nanoparticles synthesized by (Li diglyme)BH4 as a reducing agent and the depict of the nanoparticle
size histogram is reported. (b) TEM image of the Cu T6SH nanoparticles
synthesized by (Li diglyme)BH4 as
a reducing agent after a period of six months long of storage at low temperature
(+4°C)
were suspended in CH2Cl2 after their synthesis.
(a)(b)This result is due to the
modifications produced to the one-phase synthetic process of alkanethiolate-protected
copper nanoparticles [17] which undergo to the oxidation of copper core,
morphological evolution, and irreversible aggregation process.


## 4. Conclusions

Copper 3-(6-mercaptohexyl)thiophene-protected nanoparticles have been synthesized in
a one-phase system by a modified Brust reaction [8] utilizing an NaBH4/LiCl
mixture in diglyme as the reducing reagent and avoiding water medium in
dissolving copper salts. TEM investigation carried out on the Cu T6SH
nanoparticles immediately after synthesis and after different periods up to six
months long of storage at low temperature has put in evidence that their spherical
shape of about 5-6 nm in diameter does
not vary appreciably. The same invariability related to the storage period has
been observed for the Cu T6SH nanoparticles by FTIR and UV-Vis spectroscopic
characterizations. The characteristic FTIR adsorption bands related to νas(d−) and νs(d+) vibrational stretching modes
of CH2 have been recorded for synthesized Cu T6SH at lower wave
numbers than those recorded for the free thiophene ligand, revealing an
enhanced ordered structure for the capping agent.The UV-Vis spectrum of the Cu T6SH nanoparticles does not reveal
any evidence of the surface plasmonic band previously observed in Cu-nanosized
clusters at about 556–570 nm, but shows
only a wideband centered at 293 nm probably due to the shorter (C6) alkyl chain
of thiophene ring surface ordered on the Cu core.TEM, FTIR, and UV-Vis results have pointed out the lack of
irreversible aggregation previously observed for long-chain thiophene-protected
metallic nanoparticles [17] revealing the importance of the modifications
ported to the well-known one-phase synthetic Brust reaction [8]. The
presence of short-chain alkanethiols around the
metallic core and the observed absence
of irreversible aggregation could improve their conductivity and potentialities
of tuning their optical and electronic properties which are under
investigation.



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*Source: 649130-2008-11-09.xml*