Synthesis and characterization of ternary AgInS2 nanocrystals by dual- and multiple-source methods

Article abstract of DOI:10.1039/b707960e

The precursors [In(bipy)(SC{O}Ph)₃] and [Ag(SC{O}Ph)] have been used to prepare monodispersed polyhedral shaped AgInS₂ nanocrystals (NCs). The NCs were fully characterized by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis. The synthesis of AgInS₂ NCs was further investigated by varying the dual- and multiple-source precursors and other parameters such as surfactants, surfactant ratio, temperature and time duration, to optimize the conditions to get better quality NCs. In all these reactions, a heterobimetallic thiobenzoate intermediate similar to the single-source precursor [(Ph₃P)₂AgIn(SC{O}Ph)₄] has been proposed to have formed which decompose subsequently to AgInS₂ NCs. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b707960e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and characterization of ternary AgInS2 nanocrystals by
dual- and multiple-source methodsw
Lu Tian and Jagadese J. Vittal*
Received (in Durham, UK) 25th May 2007, Accepted 19th July 2007
First published as an Advance Article on the web 8th August 2007
DOI: 10.1039/b707960e
The precursors [In(bipy)(SC{O}Ph)3] and [Ag(SC{O}Ph)] have been used to prepare
monodispersed polyhedral shaped AgInS2 nanocrystals (NCs). The NCs were fully characterized
by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction
and energy dispersive X-ray analysis. The synthesis of AgInS2 NCs was further investigated by
varying the dual- and multiple-source precursors and other parameters such as surfactants,
surfactant ratio, temperature and time duration, to optimize the conditions to get better quality
NCs. In all these reactions, a heterobimetallic thiobenzoate intermediate similar to the single-
source precursor [(Ph3P)2AgIn(SC{O}Ph)4] has been proposed to have formed which decompose
subsequently to AgInS2 NCs.
multiple-source methods using a number of simple inorganic
Introduction
compounds. The details are discussed below.
Ternary compounds possess a wide variety of physical proper-
ties, including electroluminescent displays,1 superconductiv-
ity,2,3 and magnetic properties,4,5 that are important for many
Experimental
scientific advances and technological applications. Tradition-
Synthesis of precursors
ally, high-temperature arc melting or powder metallurgy
techniques have been used to produce ternary compounds.
They generally yield thermodynamically stable structures and
offer little control over nanostructure and morphology.6,7
Nowadays scientists have been exploiting this challenging field
to synthesize ternary compounds at low temperatures and
control their size in the nano-range.8 Beside the gas-phase
deposition methods (e.g., sputtering,9 MBE,10 MOCVD11),
some solution routes, such as solvothermal method,12 photo-
chemical decomposition13 or wet chemical process14 have also
been developed. Low-temperature solution synthesis may be
more attractive because the solvent and the surface stabilizing
agents can play a key role in kinetically trapping metastable
phases with more complex structures and compositions.15,16
I–III–VI2 ternary compounds are attractive materials as
active semiconductors in photovoltaic and optoelectronic
applications.17,18 Ternary AgInS2 has a distinctive absorption
in the visible and near-infrared region because of its large
absorption coefficient and its band gap energy between 1.87
and 2.03 eV.19,20 In this work, our interest in this system
prompted us to explore diverse ways to synthesize AgInS2
NCs. Recently we have reported one-pot colloidal synthesis of
high-quality NCs of AgInS2 by direct thermal decomposition
of the single-source precursor [(Ph3P)2AgIn(SC{O}Ph)4], 1 in
bisurfactant system.21 In order to expand the synthetic
methodology of AgInS2, we have now investigated dual- and
All the solvents and chemicals, unless stated otherwise, were
commercially available and used as received. [(Ph3P)2AgIn-
(SC{O}Ph)4], 1, [Ag(SC{O}Ph)], 2, [Et3NH][In(SC{O}Ph)4], 3,
and (PPh3)2AgCl were synthesized according to reported
methods.22–24
[In(bipy)(SC{O}Ph)3], 4, was synthesized as follows: sodium
thiobenzoate, PhC{O}SꢀNa+, was obtained by reacting
NaOH (0.06 g, 1.5 mmol) with 176 mL of PhC{O}SH in
15 mL of EtOH. To this clear yellow solution, 2,20-bipyridyl
(bipy, 0.08 g, 0.5 mmol) and then InCl3 (0.11 g, 0.5 mmol) in
10 mL of EtOH were added. A creamy yellow precipitate was
formed and the contents were stirred for 30 min. Light yellow
[In(bipy)(SC{O}Ph)3], 4, was separated by filtration and the
product was rinsed with EtOH, H2O and dried in vacuo. Yield:
0.28 g (83%). Anal (%). Calc. for [In(bipy)(SC{O}Ph)3]
(Mol wt. 682.55): C, 54.55; H, 3.40; N, 4.10; S, 14.09. Found:
1
C, 54.04; H, 3.51; N, 4.01; S, 14.19. H NMR (d, CDCl3): for
2,20-bipyridyl: 7.62–7.66 (2H, 4,40), 8.25–8.28 (2H, 6,60),
9.48–9.50 (2H, 3,30); for thiobenzoate anion: 7.26–7.32
(6H, m), 7.40–7.44 (3H, p), 8.04–8.11 (8H, overlapped with
6H from o of thiobenzoic ligand and 2H from 5,50 of
bipyridyl). 13C{1H} NMR (d, CDCl3): for thiobenzoate anion:
128.0 (C2/6 or C3/5), 128.9 (C2/6 or C3/5), 139.9 (C4), 143.5 (C1);
for 2,20-bipyridyl: 121.4 (6,60), 126.6 (4,40), 132.3 (5,50), 149.1
(3,30), 156.5 (1,10).
Molecular structure diagrams of 1, 3 and 4 are shown in
Scheme 1.
Department of Chemistry, National University of Singapore,
Singapore 117543. E-mail: chmjjv@nus.edu.sg
Synthesis of nanocrystals
w Electronic supplementary information (ESI) available: Experimental
details, EDX analysis spectrum and XRPD patterns. See DOI:
10.1039/b707960e
The precursors [Ag(SC{O}Ph)], 2, (13 mg, 0.051 mmol) and
[In(bipy)(SC{O}Ph)3], 4, (35 mg, 0.051 mmol) were added to
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1
salt. H and 13C{1H} NMR were recorded on a Bruker ACF
300 MHz instrument with TMS as internal reference at 25 1C.
X-Ray powder diffraction (XRPD) patterns were obtained
using a D5005 Bruker X-ray diffractometer equipped with
Cu-Ka radiation. An accelerating voltage and current of
40 kV and 40 mA, respectively, and scan speed of 0.0121 sꢀ1
were employed. Transmission electron microscopy (TEM) was
performed on a Philips CM 10 microscope operating at
100 kV. High resolution transmission electron microscopy
(HRTEM) images and electron diffraction patterns were
obtained from a JEOL JSM-3010 instrument.
Results and discussion
Scheme 1 Molecular structure diagrams of 1, 3 and 4.
As-synthesized AgInS2 NCs from precursors, 2 and 4, were
characterized by TEM, as shown in Fig. 1. Fig. 1(a) reveals
that NCs have polyhedral shapes and narrow size distribution
(see histogram in Fig. 1(d)), which are comparable with
AgInS2 NCs by single-source method.21 The mean diameter
of the individual NCs is measured to be 10.9 ꢂ 1.3 nm (based
on 300 particles from the TEM image). The clear lattice fringes
of NCs in Fig. 1(b) show that individual NCs are single
crystals. The width of 0.356 nm from neighboring lattice
fringes corresponds to plane (120) of AgInS2. The strong ring
patterns from selected area electron diffraction (SAED) given
in Fig. 1(c) can be well indexed to the orthorhombic phase
AgInS2 (JCPDS 00-025-1328). The XRPD data is given in the
spectrum of Fig. 2(a). Energy dispersive X-ray (EDX) analysis
of NCs in the electron microscope showed the ratio of Ag, In
and S is 1 : 1.05 : 2.18.25 The morphology and quality of NCs
are very similar to that obtained from 1.21
dodecanethiol (DT, C12H25SH, Aldrich, 2.55 mmol, 0.61 mL)
and oleic acid (OA, C17H33COOH, Aldrich, 1.83 mL) at room
temperature (molar ratio of precursor : DT = 1 : 50; the
volume ratio of DT : OA = 1 : 3) and the contents were heated
at 200 1C for 2 h with gentle stirring under nitrogen atmo-
sphere. The solution was cooled to B70 1C and then an excess
of EtOH was added, and a dark red flocculent precipitate was
formed which was separated by centrifugation, washed with
EtOH and dried in vacuo. This can be easily redispersed in
solvents such as toluene, chloroform and hexane. For multi-
ple-source methods, a similar synthetic strategy was applied.25
Measurements
The microanalytical laboratory at the Department of Chem-
istry, National University of Singapore performed the micro-
analysis. The yields were calculated with respect to the indium
Fig. 1 (a) Typical TEM images of AgInS2 NCs produced from 2 and 4 in DT and OA at 200 1C for 2 h (molar ratio of precursor : DT = 1 : 50;
volume ratio of DT : OA = 1 : 3), (b) HRTEM images, (c) SAED pattern and (d) histogram of the AgInS2 NCs.
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Fig. 2 Typical XRPD patterns of AgInS2 prepared from (a) 2 and 4
in DT and OA at 200 1C for 2 h (molar ratio of precursor : DT =
1 : 50; volume ratio of DT : OA = 1 : 3); (b) 2 and 3; (c) (PPh3)2AgCl,
InCl3, PhCOSH and NaOH; (d) (PPh3)2AgCl, InCl3 and PhCOSH; (e)
AgCl, InCl3 and S in oleylamine; (f) AgCl, InCl3 and thiourea in
oleylamine. The experimental conditions for (b)–(f) are otherwise
similar to that for (a). * Corresponds to peak of Ag (JCPDS 00-001-
1167) and # corresponds the peak of Ag2S (JCPDS 00-009-0422).
Fig. 3 Typical TEM images of AgInS2 NCs prepared in DT and OA
at 200 1C for 2 h (molar ratio of precursor : DT = 1 : 50; volume ratio
of DT : OA = 1 : 3) from (a) 2 and 3; (b) (PPh3)2AgCl, InCl3,
PhCOSH and NaOH; (c) AgCl, InCl3 and S in oleylamine; (d) AgCl,
InCl3 and thiourea in oleylamine.
XRPD patterns of AgInS2 NCs prepared by dual and
multiple sources are shown in Fig. 2. Fig. 2(a) shows
the XRPD patterns of AgInS2 NCs prepared from 2 and 4
in DT and OA at 200 1C for 2 h. All the observed peaks can
be indexed to the orthorhombic phase of AgInS2 (JCPDS
00-025-1328). Peaks are slightly broadened with decreasing
nanocrystal size. Fig. 2(b) shows the XRPD patterns of
AgInS2 NCs prepared from 2 and 3 revealing pure ortho-
rhombic phase AgInS2 NCs. The TEM image of these NCs as
shown in Fig. 3(a) reveals that the nanosize particles are
agglomerated. The ionic precursor 3 may have affected the
capping by the surfactants on the formed NCs. We propose
that the dual- or multiple-source precursors react to form an
intermediate similar to the single molecular precursor 1 and
then decomposed to the corresponding AgInS2 NCs. However
Qian and co-workers26 have proposed that nucleophilic attack
by ethylenediamine at the thione carbon atoms of metal
diethyldithiocarbamates generates compounds with inorganic
cores [InS1.5] and [Cu2S2], and then they combine stoichiome-
trically with each other producing CuInS2 grains. The dual-
source method has prompted us to widen the synthetic route
to multiple-source methods. Experimental results have shown
that when mixtures of (PPh3)2AgCl, InCl3, PhCOSH and
NaOH were heated with bisurfactants, AgInS2 NCs were
obtained as major product; Ag was an impurity according to
the XRPD pattern as shown in Fig. 2(c). A small amount of
Ag may come from direct decomposition of (PPh3)2AgCl. Fig.
3(b) shows that the obtained NCs have a diameter of 18.0 ꢂ
2.7 nm. On the contrary, mixtures of (PPh3)2AgCl, InCl3 and
PhCOSH without NaOH can only produce bulk AgInS2 with
Ag2S impurity (XRPD pattern in Fig. 2(d)). It is presumed
that in the absence of NaOH, both Clꢀ and PhCOSꢀ may
compete to bridge Ag to form [(Ph3P)2Ag(m-Clx)-
(m-SCOPh)2ꢀxIn(SC{O}Ph)2] which can easily dissociate to
form [(Ph3P)2Ag(SC{O}Ph)] and further decompose to Ag2S,
thus accounting for Ag2S as impurity. On the other hand facile
formation of a heterobimetallic thiobenzoate intermediate
similar to
1 can occur from the precursors [In(bipy)-
(SC{O}Ph)3] or [Et3NH][In(SC{O}Ph)4 and [Ag(SC{O}Ph)]
at the first stage of reaction, compared to the multiple-source
methods.
According to the papers published by Qian and co-workers,
mixtures of In(S2CNEt2)3 and Cu(S2CNEt2)2 or Ag(S2CNEt2)
have been used for CuInS2 and AgInS2 nanorods in solvo-
thermal processes.26 This group has also developed the
solvothermal synthesis of AgGaS2 and AgInS2 using AgCl,
Ga (or In) and S as the reactants27 and hydrothermal synthesis
of AgInS2 by using AgCl, InCl3 and thiourea as starting
materials.28 while our results show that under normal atmo-
spheric pressure, a mixture of AgCl, InCl3 and S upon heating
with oleylamine produced AgInS2 NCs as well as Ag2S
impurity (Fig. 2(e)). Fig. 3(c) shows that the obtained NCs
are more spherical with diameter of 22.0 ꢂ 2.9 nm. When
AgCl, InCl3 and thiourea were mixed under similar experi-
mental conditions, the major product is AgInS2 NCs with Ag
impurity (Fig. 2(f)). Fig. 3(d) shows that the obtained NCs
have diameter of 32.1 ꢂ 3.5 nm. On the contrary, when
bisurfactants (DT and OA) were employed for this multi-
source synthesis, there is no formation of AgInS2.25 Obviously
oleylamine plays a key role in this synthesis. It was found that
amine could lower reaction temperature and facilitate the
formation of nanoparticles.29 In conclusion, among all the
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Fig. 4 Typical TEM images of AgInS2 NCs prepared from (a) 2 and 4 in DT at 200 1C for 2 h (molar ratio of precursor : DT = 1 : 50); (b) 1 in DT
at 200 1C for 2 h; (c) 2 and 4 in OA at 200 1C for 2 h (molar ratio of precursor : OA = 1 : 50); (d) 1 in OA at 200 1C for 2 h; (e) 1 in DT and OA at
250 1C for 2 h (molar ratio of precursor : DT = 1 : 50; volume ratio of DT : OA = 1 : 3); (f) 2 and 4 in DT and OA at 250 1C for 2 h.
above dual- or multiple-source methods, 2 and 4 are the best
combination to synthesize AgInS2 NCs.
is no obvious effect on the size and morphology of the
obtained AgInS2 NCs.
Guided by these results, [In(bipy)(SC{O}Ph)3] and
[Ag(SC{O}Ph)] have been chosen to prepare AgInS2 NCs
and the effect of the reaction conditions on the size and
morphology of NCs have been investigated. When the ratio
of DT to OA was varied from 2 : 1 to 1 : 1, 1 : 3 and 1 : 6, there
AgInS2 NCs
When DT alone was used as the capping agent, AgInS2 NCs
with diameter 8.4 ꢂ 1.2 nm were formed, which are different
Fig. 5 Typical TEM images of AgInS2 NCs prepared from 2 and 4 at 200 1C for 2 h (molar ratio of precursor : surfactant = 1 : 50 : 50) in
(a) DT + TOPO (20.5 ꢂ 5.6 nm); (b) oleylamine + TOPO (13.7 ꢂ 3.0 nm); (c) oleylamine + DT (8.3 ꢂ 0.9 nm); (d) oleylamine + OA
(16.5 ꢂ 4.8 nm); (e) oleylamine (14.1 ꢂ 1.2 nm); (f) HDA + TOPO (12.0 ꢂ 1.7 nm).
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from agglomerated product obtained from 1.21 The AgInS2
NCs prepared by the above two different methods are shown
in Fig. 4(a) and (b), respectively. They indicate that dual-
source method does not rely on the bisurfactant system so
much as compared to the single-source precursor method. The
use of OA alone in both cases, however, resulted in micro-
meter sized AgInS2 particles (Fig. 4(c) and (d)). A reaction
temperature range from 125–250 1C was found to be suitable
for the production of uniform AgInS2 NCs in both cases. The
AgInS2 NCs prepared from 1 at 250 1C were of polyhedral
shape with wider size distribution (Fig. 4(e)) while the NCs
prepared from 2 and 4 at 250 1C were of irregular shape with
size of 11.4 ꢂ 3.7 nm (Fig. 4(f)). These results infer that the
NCs prepared from 1 at 250 1C may have undergone Ostwald
ripening. Larger nanocrystals (44.4 ꢂ 8.4 nm) were formed
when many small nanocrystals (12.0 ꢂ 2.6 nm) dissolved back
and slowly disappeared, while the NCs prepared from 2 and 4
at 250 1C seem to be a kinetically trapped product. This may
be due to the two-step formation of AgInS2 NCs from the
dual-source precursor system. For all the samples discussed
above, the XRPD patterns show orthorhombic phase AgInS2
(JCPDS 00-025-1328).25
Acknowledgements
We would like to thank NUS (Grant No. R-143-000-283-112)
for their generous financial support. We thank Mr Liu Binghai
for the HRTEM images.
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Conclusions
We have successfully synthesized high-quality NCs of AgInS2
from dual-source precursors [In(bipy)(SC{O}Ph)3] or
[Et3NH][In(SC{O}Ph)4], and [Ag(SC{O}Ph)]. The synthetic
routes to the ternary system have been extended and the
quality of AgInS2 NCs obtained from dual- or multiple-source
methods has been compared. In other words, low-temperature
synthetic routes for AgInS2 NCs have been successfully
employed and size control has been accomplished by using
dual- and multi-source molecular precursors. All these NCs
can be dispersed freely in toluene, chloroform or hexane,
which enable investigation of physical properties in solution.
In all the dual- and multiple-source methods, a heterobi-
metallic thiobenzoate intermediate similar to single molecular
precursor 1 is proposed to have formed and decompose to
AgInS2 NCs.
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