Shape-and size-controlled Ag nanoparticles stabilized by in situ generated secondary amines

Article abstract of DOI:10.1016/j.jallcom.2015.01.035

Silver amides such as AgNⁱPr₂ and AgN(SiMe₃)₂ have been employed successfully as precursors for the yield synthesis of silver nanoparticles under mild conditions of dihydrogen gas reduction (2 atm) in organic media. Transmission electron microscopy (TEM) showed the formation of silver nanoparticles with FCC structure, variously sized from 26 to 35 nm for AgNⁱPr₂ and from 14 to 86 nm for AgN(SiMe₃)₂, the synthesis could take place in absence of added stabilizers due to the in situ formation of secondary amines from the reaction of dihydrogen gas with the amide ligands of the silver precursor. Indeed, the presence of HNR₂ (R = iPr₂, N(SiMe₃)₂) on the surface of the nanoparticle was confirmed by spectroscopic means. Finally, the addition of ethylenediamine as additional capping agent allowed not only the control of the structural characteristics of the resulting Ag nanoparticles (well-dispersed with spherical shape), but that regarding the nanoparticle size as it inhibited overgrowth, limiting it to ca. 25 nm.

Full text of DOI:10.1016/j.jallcom.2015.01.035

Journal of Alloys and Compounds xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Shape-and size-controlled Ag nanoparticles stabilized by in situ
generated secondary amines
b
c
c
E. Ramírez-Meneses a, , V. Montiel-Palma , M.A. Domínguez-Crespo , M.G. Izaguirre-López ,
⇑
E. Palacios-Gonzalez d, H. Dorantes-Rosales e
a Departamento de Ingeniería y Ciencias Químicas, Universidad Iberoamericana, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe, Distrito Federal C.P. 01219, Mexico
b Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001 Col. Chamilpa, Cuernavaca, Morelos C.P. 62209, Mexico
c Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada-IPN, Unidad Altamira. Km 14.5 Carretera Tampico-Puerto Industrial, 89600 Altamira, Tamaulipas, Mexico
d Laboratorio de Microscopia de Ultra alta Resolución, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas No. 152, C.P. 07730 México D.F., Mexico
e Departamento de Metalurgia, E.S.I.Q.I.E.-I.P.N., Unidad Profesional Adolfo López Mateos, Zacatenco, Delegación. Gustavo A. Madero, C.P. 07738 México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Silver amides such as AgNiPr2 and AgN(SiMe3)2 have been employed successfully as precursors for the
yield synthesis of silver nanoparticles under mild conditions of dihydrogen gas reduction (2 atm) in
organic media. Transmission electron microscopy (TEM) showed the formation of silver nanoparticles
with FCC structure, variously sized from 26 to 35 nm for AgNiPr2 and from 14 to 86 nm for AgN(SiMe3)2,
the synthesis could take place in absence of added stabilizers due to the in situ formation of secondary
amines from the reaction of dihydrogen gas with the amide ligands of the silver precursor. Indeed, the
presence of HNR2 (R = iPr2, N(SiMe3)2) on the surface of the nanoparticle was confirmed by spectroscopic
means.
Article history:
Available online xxxx
Keywords:
Ag nanoparticles
Chemical synthesis
Amines
Stabilizers
Finally, the addition of ethylenediamine as additional capping agent allowed not only the control of the
structural characteristics of the resulting Ag nanoparticles (well-dispersed with spherical shape), but that
regarding the nanoparticle size as it inhibited overgrowth, limiting it to ca. 25 nm.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
lotions, etc. For the production of metallic silver from the cationic
species, a variety of different reduction methods have been pro-
Metal nanoparticles (MNPs) with unusual chemical physical
properties, often significantly different from those of their bulk
counterparts, are undoubtedly the synthetic targets for nanosci-
ence and engineering technology [1–8]. Furthermore, the focus of
attention has not only been the synthesis of monodispersed nano-
particles, but also their self-organization into 2-dimensional (2-D)
arrays [9–15]. Thus, one of the principal objectives of various syn-
thetic strategies concerning metallic nanomaterials is to achieve a
precise control of their size, shape and dispersion [16]. Among all
metals, silver and gold are promising materials for their application
in nonlinear optics as chemical sensors and optoelectronic nanode-
vices [17]. Ionic or metallic silver features low toxicity to human
cells, high thermal stability and low volatility; such properties
can be exploited in medicine for burning treatment and dental
materials, and in the manufacturing industry as coatings for stain-
less steel materials, textile fabrics, water treatment, sunscreen
posed such as c-rays [18,19], ultraviolet or visible light [20], micro-
waves [21,22], ultrasonic radiation [23], electrochemical
deposition [24], laser irradiation [25], thermal decomposition
[26] and recently, hydrothermal methods [27]. As for the last
methods, numerous reducing agents such as sodium borohydride
(NaBH4), formaldehyde, sodium citrate, hydrazine, ascorbic acid,
glucose polysaccharides and even biological microorganisms [28–
35] have been employed. To prevent the agglomeration and precip-
itation of silver nanoparticles, capping agents, either in organic or
aqueous media, such as poly(vinylpyrrolidone) (PVP), poly(ethyl-
ene glycol) (PEG), oleic acid, dodecanoic acid, sodium citrate dehy-
drate, some surfactants and amines have been used [36–40].
In some cases, amines can serve as both reducing and capping
agents. For instance, Mishra et al. [41] reported the synthesis of
Ag nanomaterials with elongated structures in a two-phase system
using hexadecylamine, whereas Chen et al. [42] obtained monodi-
spersed silver nanoparticles (ꢀ12 nm) on a large scale in a simple
oleylamine–liquid paraffin system. Oleylamine was also used as
stabilizer by Hiramatsu and co-workers to obtain nearly monodi-
spersed silver nanoparticles with variable size in the mixture of
⇑
Corresponding author. Tel.: +52 (55) 9177 4400, +52 5950 4000x4057; fax: +52
(55) 5950 4279.
E-mail address: esther.ramirez@ibero.mx (E. Ramírez-Meneses).
http://dx.doi.org/10.1016/j.jallcom.2015.01.035
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
synthesis reaction of AgNR2 precursors is presented in Fig. 1. The resulting com-
pounds were insoluble in most common organic solvents, but gave satisfactory
microanalytical data and FT-IR analysis.
oleylamine and toluene or hexane or dichlorobenzene [43]. Kash-
iwagi et al. [44] reported the synthesis of monodispersed silver
NPs by heating a suspension of insoluble silver myristate in tertiary
alkylamines at 80 °C. Alternatively, hexamethylenetetramine has
been used as an efficient reducing agent [45]. More recently, trieth-
ylamine have been used as a promoted and directing agent for sil-
ver nanoparticles [46]. Others works have employed amines such
as tetraethylenepentamine [8] and poly-amino compounds [47]
as stabilizers.
2.2. Synthesis of Ag nanoparticles from AgNiPr2 and AgN(SiMe3)2
The synthesis of silver NPs was carried out in the darkness in a Fischer–Porter
bottle either at room temperature or at 60 °C. A typical procedure is described
below. AgNiPr2 or AgN(SiMe3)2 (40 mg) were introduced in a Fisher–Porter bottle
under argon atmosphere and a mixture of freshly distilled and degassed tetrahydro-
furan, THF (20 mL), and toluene (20 mL) was added. When an additional capping
agent was needed, either 1 or 5 equivalents of it (either ethylenediamine or hexam-
ethyldisilizane) were added at this point by means of a syringe. The obtained dark
gray solutions were then pressurized under dihydrogen atmosphere (2 bar) for 16 h
under stirring. After this time, homogeneous brown colloidal solutions were
obtained. Schematic illustrations of the proposed stabilization of the silver NPs
obtained from the AgNiPr2 and AgN(SiMe3)2 precursors are shown in Figs. 2 and
3, respectively.
Despite these advances regarding the accessibility of silver NPs,
the use of alternative precursors to AgCl and AgNO3 is much less
developed with only a handful of reports using AgClO4 [48],
Ag(CO2C6H5CO2) [18], Ag(CH3COO) [49], Ag(acac) [50], [Ag(l-mesi-
tyl)]4 [51] and more recently the organometallic [Ag(C6F5)] [52].
However, it is evident that the selection of starting molecular pre-
cursors is crucial and often the most difficult task when targeting
the controlled synthesis of nanoparticles. As for precursors,
imposed requirements such as thermal stability, chemical selectiv-
ity and even solubility in non-polar organic media are often diffi-
cult to achieve from commercial or easily available precursors.
Hence, apart from using organometallic compounds as nanoparti-
cle sources, simple metal and metalloid amides, and in general ele-
ment-nitrogen precursors, have very recently been proposed as
alternative metal sources. Indeed, during the submission of this
manuscript, a comprehensive review on the synthesis of colloidal
nanocrystals and nanoparticles from metal and metalloid amides
was published [53].
2.3. Characterization of the as-synthesized silver nanoparticles
TEM specimens were prepared by slow evaporation of a drop of each crude col-
loidal solution deposited onto a holey carbon covered copper grid. Then, the colloi-
dal solutions were purified by hexane washings (to eliminate the impurities).
Finally, the resulting gray solution was evaporated in vacuum until the residue
was completely dry. Size and morphology of the as-synthesized silver NPs were
investigated by means of a JEOL-2000 FX II electron microscope, operating at
200 kV. The presence and bonding mode of the capping molecules after the purifi-
cation step were studied through Fourier Transform-infrared spectroscopy (FT-IR,
Spectrum One Perkin Elmer). KBr pellets (Aldrich, 99% IR grade) were employed
to carry out this analysis.
In this work, AgNiPr2 and AgN(SiMe3)2 have been proposed as
practical alternative precursors to conventional AgNO3 to form sil-
ver NPs under dihydrogen atmosphere either at room temperature
or at 60 °C. The agglomeration of the nanoparticles was prevented
by the in situ formation of the corresponding amines. The effect of
the additional presence of capping agents such as ethylenediamine,
NH2(CH2)2NH2 or hexamethyldisilazane and HN(SiMe3)2 on the
size, shape and dispersion of the attained nanostructures was also
studied. Although this is the first time silver amide precursors are
used for this purpose, Chaudret et al. [54] employed previously
analogous Co[N(SiMe3)2]2 for the synthesis of Co nanoparticles.
3. Results and discussion
3.1. Ag nanoparticles from AgNiPr2 reduction
Fig.
4 illustrates typical transmission electron microscopy
(TEM) images and their corresponding selected area electron dif-
fraction (SAED) pattern of the silver nanoparticles obtained from
the reaction of AgNiPr2 under H2 atmosphere (2 bar) in the absence
of additional capping agents either at room temperature or at
60 °C. From the TEM images, it can be observed spherical particles
with no elongated or rod-shape. Regardless of the reaction temper-
ature, the particle size distribution ranged from 20 to 50 nm. It is
well documented that high reaction temperatures provoke impor-
tant effects on the shape and size of nanoparticles, generally
increasing the latter [42,56]. In our system, small Ag nanoparticles
(<20 nm) were produced, displaying a very slight tendency to
agglomerate at higher temperatures, i.e. the TEM micrograph in
Fig. 4a shows the dispersion obtained when the synthesis was car-
ried out at room temperature, which is slightly better than that
obtained at 60 °C, where some aggregates of particles are formed
(Fig. 4c). This behavior is in agreement with the stabilization of
the silver NPs likely resulting from the coordination of solvent
(THF) as well as of the in situ generated HNiPr2, both of which
are volatile and will therefore be less efficient in preventing aggre-
gation at higher temperatures. The corresponding SAED of the par-
ticles was identified and confirmed the silver FCC (face centered
cubic) structure (JCPDS 04-0783), Fig. 4b. The average distances
between the fringes and the corresponding crystallographic planes
are presented in Table 1.
The Ag particles obtained at 60 °C show a different crystallo-
graphic plane (420) that is not observed in the case of the particles
obtained at room temperature, Fig. 4d. These different lattice planes
of the Ag crystals in the Ag nanoparticles could be attributed to alter-
native growth mechanisms dependent on temperature and the pres-
ence of the in situ generated HNiPr2. Thus, according to previous
works, reaction temperature variations affect the particle growth
mechanisms [57]. In our case, the synthesis carried out at 60 °C
showed that the initial solution exhibited a color change from white
to gray during the first 30 min and at room temperature, change
2. Experimental
2.1. Synthesis of AgNiPr2 and AgN(SiMe3)2 precursors
The synthesis of AgNiPr2 was first reported by Lappert et al. [55] to proceed
from the reaction of AgNCO and M0[N(SiMe3)2] (were M0 = Sn, Pb, Yb). As for the
present research work, AgNiPr2 and AgN(SiMe3)2 were prepared from either AgCl
or AgNO3 (Aldrich) using standard Schlenk and glove box techniques. Although
LiNR2 (R = SiMe3) is commercially available, its fresh preparation (R = iPr) from
the corresponding secondary amine (either diisopropilamine or hexamethyldisilaz-
ane) and stoichiometric amounts of a titrated n-BuLi (n-BuLi = C4H9Li) in hexanes
was preferred. The white precipitate (either LiNiPr2 or LiN(SiMe3)2) was then fil-
tered and dried under vacuum. Subsequently,
a
suspension of LiNR2 (R = iPr,
N(SiMe3)) and one equivalent mol of AgCl in THF were vigorously stirred for 24 h
at room temperature in darkness. The solution was filtered off the residue, concen-
trated to eliminate the remaining LiCl and recrystallized from THF. The general
exane
-n u
B H
h
LiNR2
+ n u
B L
HNR2
i
+
-
A
Li l
l
C
C
g
THF
i
w ere
h
=
e
iM
S
r
P
R
,
3
A NR
g
2
Fig. 1. General reaction of synthesis of AgNR2 precursors.
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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
3
H
3C
H
C
3
H
3C
H
C
H
H
C
3C
H2N
H
C
H
C
3
3
H
NH
H
3C
H
3C
In absence
of additional
capping agent
C
NH
H
H
C
3
H
C
NH2
H
C
H
3C
H
3C
H
3C
H
N
H2N
H2N
H
C
C
3
-
-
H H NH
2
-
H N
C
C
2
H
C
A
H
g
C
N
N
H
C
H
3
THF/toluene
ar
THF/toluene
H
3C
H
C
H
C
3
H
C
NH
H
3C
H2
2
b
º
,
)
(
N
H2
r. t. or
ar
2
b
,
)
(
H
3C
H
C
H
C
H
3C
H
C
60 C
H
60 ºC
3
H
C
H
H
C
H
3C
H
3C
H
C
33C
H
C
3
H
H
3C
H
C
3
Stabilized Ag nanoparticle
Stabilized Ag nanoparticle
Fig. 2. Schematic illustration of the proposed stabilization of Ag nanoparticles obtained from AgNiPr2 precursor in absence of additional capping agent (a) or in the presence
of ethylenediamine as capping agent (b).
Fig. 3. Schematic illustration of the proposed formation of Ag nanoparticles obtained from AgN(SiMe3)2 precursor in absence of additional capping agent or in the presence of
ethylenediamine or hexamethyldisilizane.
color was observed after 1 h. These observations seems to indicate
that the rate of nucleation process and subsequent stage of nanopar-
ticle growth are influenced by temperature [58,59].
dodecanethiol capped Ag nanocrystals [10] and nonanethiol passiv-
ated Ag NPs [61]. Some of these works state that the displacement is
the consequence of the constraint of the capping molecular motions
most likely resultant from the formation of a relatively close-packed
amine layer on the surface of the Ag NPs [42,61].
FT-IR spectroscopy was used to validate both the formation of
the amine ligand from the amide precursor and its presence after
the purification step. It is expected that the amino group of the
molecule acts as an electron donor group to the Ag particle surface,
thus surrounding the particle, exposing the HNiPr2 groups to the
outside and constituting a steric hindrance for the metal particle
growth. Fig. 5 shows FT-IR spectra corresponding to Ag nanoparti-
cles synthesized at room temperature and 60 °C. The FT-IR spec-
trum of AgNiPr2 is also shown as a reference. In the spectra of
the synthesized nanoparticles, a band at ca. 3400 cmꢁ1 is observed
and it is attributed to the stretching mode of the amino functional
group (–NH). A slight shift in the position of this band is observed
when comparing the samples obtained at the two different reac-
tion temperatures. The N–H bending scissoring absorption band
was observed at around 1600 cmꢁ1 in all the spectra of the NPs.
Additionally, the band corresponding to the methyl asymmetric
The use of organic capping agents (also known as stabilizing or
protecting agents) in the bottom-up approach is usually required
during the synthesis of Ag nanopowders to control the material
particle size, agglomeration, and morphology [62–64]. Previous
works employed ethylenediamine as an efficient stabilizer of metal
nanoparticles or even in quantum dots to preserve their surface
properties acting as an assisted ligand exchange [65–69]. Thus, to
evaluate the effect of additional capping agents during the synthe-
sis of Ag nanoparticles, a series of experiments using AgNiPr2 at
60 °C in presence of 1 or 5 equivalents of ethylenediamine
(NH2(CH2)2NH2) was also performed. The morphology, particle size
distribution and structure of the as-prepared Ag nanoparticles in
presence of capping agents were analyzed by TEM with the corre-
sponding SAED, and the results are shown in Fig. 6a–d. The micro-
graphs show that the addition of capping agents exerts some
effects on the size, dispersion and shape of the obtained Ag NPs
in comparison with the samples synthesized in absence of addi-
tional capping agents. A noticeable change in shape from semi-
spherical to spherical nanoparticles with some additional nanorods
was observed. It is well known that a capping agent plays a twofold
role: it acts as a stabilizing agent, preventing aggregation of metal
particles and as a uniform colloidal dispersion keeper [70]. The
average particle size using different amounts of ethylenediamine
m
as(CH3) mode is observed at ca. 2961 cmꢁ1 in the spectrum of free
AgNiPr2, while it is shifted to 2950 cmꢁ1 and 2963 cmꢁ1 for silver
NPs synthesized at room temperature and at 60 °C, respectively.
All of these findings are in agreement with the in situ formation
of HNiPr2 in the reaction media and account for the previously
described stabilization.
Relatively small shifts in the IR bands of stabilizers upon
coordination of the NPs surfaces have also been reported in systems
such as oleylamine capped Ag NPs [42], PVP capped Ag NPs [60],
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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
Fig. 4. TEM images and corresponding electron diffraction patterns of Ag nanoparticles obtained under dihydrogen (2 bars) from AgNiPr2 in organic media in the absence of
additional capping agents synthesized at room temperature (a,b) and 60 °C (c,d).
Table 1
Interplanar distances and planes of Ag nanoparticles obtained from AgNiPr2.
Ag Nps obtained at r.t. SAED from Fig. 1b
in absence of additional capping agent
Ag Nps obtained at 60 °C SAED from Fig. 1d
d (Å)
d (Å) JCPDS file 04-0783
Planes
d (Å)
d (Å) JCPDS file 04-0783
Planes
2.33
2.04
1.42
1.19
0.902
2.3833
2.0640
1.4594
1.1916
0.9470
(111)
(200)
(220)
(222)
(331)
2.34
2.02
1.41
1.21
0.91
2.3833
2.0640
1.4594
1.1916
0.9230
(111)
(200)
(220)
(222)
(420)
Ag Nps at 60 °C
in the presence of additional capping agent
Ag Nps/1 equiv. of NH2(CH2)2NH2
Ag Nps /5 equiv. of NH2(CH2)2NH2
d (Å)
d (Å) JCPDS file 04-0783
Planes
d (Å)
d (Å) JCPDS file 04-0783
Planes
2.33
2.02
1.42
1.17
0.91
2.3833
2.0640
1.4594
1.1916
0.9230
(111)
(200)
(220)
(222)
(420)
2.36
2.03
1.42
1.22
0.92
2.3833
2.0640
1.4594
1.2446
0.9230
(111)
(200)
(220)
(311)
(420)
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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
5
ranged from 10 to 30 nm, which are smaller than those obtained in
the absence of additional capping agent (20–50 nm). It is impor-
tant to notice that the amount of capping agent exerts an effect
on the shape of the nanostructures; i.e. whereas mainly spherical
shapes were observed when using 5 equivalents of ethylenedia-
mine (Fig. 6c), nanorods and spherical nanostructures could be
obtained with 1 equivalent (Fig. 6a). The corresponding SAED pat-
terns (Fig. 6b–d) for the as-synthesized particles are consistent
with the reported FCC silver structure (Table 1).
AgN'Pr2
Ag Nps/HN'Pr2 (25 °C)
Ag Nps/HN'Pr2 (60 °C)
The corresponding FT-IR spectra of the Ag NPs synthesized in
the presence of 1 and 5 equivalents of added ethylendiamine exhi-
bit absorbing peaks at ꢀ2900 cmꢁ1, corresponding to
mN–H, Fig. 7.
Once again, it seems that the broad band attributed to the amino
group shifts to lower wavenumbers (ꢀ3200 cmꢁ1) as a result of
the electron donation from the amino group to the silver surface.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
3.2. Ag nanoparticles from AgN(SiMe3)2 as precursor
Fig. 5. FT-IR spectrum of AgNiPr2 precursor and spectra of Ag nanoparticles
obtained from AgNiPr2 in organic media in the absence of additional capping agents
synthesized at room temperature (25 °C) and 60 °C.
In order to evaluate the effect of the precursor on the shape and
size of the Ag nanoparticles, a similar set of experiments was per-
(a)
(b)
(c)
(d)
Fig. 6. TEM images of Ag nanoparticles obtained under dihydrogen (2 bars) from AgNiPr2 in organic media at 60 °C in the presence of (a) 1 equiv. and (c) 5 equiv. of
ethylenediamine as capping agent and their corresponding typical electron diffraction patterns (b and d).
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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
formed using AgN(SiMe3)2 as a precursor. Fig. 8a–d shows TEM
Ethylenediamine
images of Ag nanoparticles obtained from AgN(SiMe3)2 without
any additional capping agent at room temperature, under H2 atmo-
sphere (3 bar) and with different reaction times (16 and 240 h).
After 16 h, polydispersed and agglomerated semispherical particles
are observed (14 8 nm, Fig. 8a and b). Under these conditions, the
formed HN(SiMe3)2 molecules surround the particle, enabling a
steric stabilization [71,72]. When the reaction mixture is left to
react for longer times (240 h), the TEM images show a significant
increase in the average particle size (86 23 nm, Fig. 8c and d),
characteristic of Ostwald ripening phenomena during the crystal
growth process. Thus, polydispersed and spherical (or semi-spher-
ical) Ag particles were obtained, Fig. 8c. From the similar shape and
surface appearance observed in the micrographs of the Ag NPs
obtained from either AgNiPr2 or AgN(SiMe3)2, it can be inferred
that the amine group functionality is responsible of the stabiliza-
tion; on the other hand, the presence of either HNiPr2 or
HN(SiMe3)2 on the surface of the synthesized Ag NPs does not
seem to exert a significant effect on their shape. Tao et al. [70]
reported that fluxional structures have been considered to behave
as reactors or templates during the synthesis. However, in this
work, the relatively low concentration of ligands in the colloidal
solution seems to limit the possibility of template formation.
C-H
N-H
N-H
Ag Nps/1 equiv. ethylenediamine
N-H
N-H
O-H
N-H
Ag Nps/5 equiv. ethylenediamine
N-H
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1
)
Fig. 7. FT-IR spectra for of Ag nanoparticles obtained under dihydrogen (2 bars)
from AgN0 Pr in organic media at 60 °C in the presence of ethylenediamine (1 and 5
equiv.) as additional capping agent.
i
Fig. 8. TEM images and size distributions of Ag nanoparticles obtained from AgN(SiMe3)2 without additional capping agent at room temperature and its corresponding
histograms of size distribution (inset typical electron diffraction pattern of Ag nanoparticles) with reaction time of 16 h (a and b) and 10 days (c and d).
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7
The synthesis of Ag NPs from AgN(SiMe3)2 was also carried out
by adding a certain amount of HN(SiMe3)2 as a supplementary cap-
ping agent. The addition of it was expected to provoke a better dis-
persion or even a particle size reduction. Fig. 10 shows TEM images
of the as-synthesized Ag nanoparticles obtained after 16 and 240 h
of reaction time in the presence of 1 equivalent of HN(SiMe3)2 (see
Table 2). In both cases, agglomerates of Ag structures with a wide
size distribution were obtained, most of them having undefined
shapes. Ag structures obtained after 16 h presented an average par-
ticle size of 46 17 nm, larger than those obtained in absence of
added HN(SiMe3)2. Once again, longer reaction times (240 h) pro-
duced increased average sizes of silver structures of 58 27 nm.
Thus, the presence of added HN(SiMe3)2 added to the reaction
media is not as effective as it is the addition of HNiPr2, probably
due to larger steric hindrance in the latter and/or to different
growth mechanisms. Although it is proposed that the particle sur-
face is covered by either amine, in line with the proposal made for
analogous Co systems [54], the presence of additional HN(SiMe3)2
from the beginning of the reaction seems to disrupt the organiza-
tion of reaction media, letting the formation of Ag structures that
are bigger than those obtained in the absence of additional amine.
These series of experiments demonstrate the predominant role of
chemical equilibrium between coordinating agents such as the
ligands resulting from the precursor and additional amines for sta-
bilizing Ag structures.
Fig. 9. FT-IR spectra of Ag nanoparticles obtained from AgN(SiMe3)2 without
additional capping agent at r.t. with reaction times of 16 h and 10 days. Addition-
ally, AgN(SiMe3)2 precursor and HN(SiMe3)2 as references.
Significant differences are observed when the syntheses are
performed in the presence of additional 5 equivalents of a supple-
mentary stabilizer such as ethylenediamine. Indeed, the TEM anal-
yses demonstrate that the obtained nanoparticles have a defined
shape (spherical) and are well dispersed. The effect of the supple-
mentary stabilizing agent can also be noted in the diminished par-
ticle size of the Ag nanostructures (25 10 nm) with respect to the
systems of added HN(SiMe3)2, Fig. 11. Although the size could be
considered to be slightly larger than those obtained in the absence
of any added stabilizer (14 8 nm, Fig. 8a and b), it should be noted
that in this case, dispersion is improved and no agglomeration is
present, Fig. 11. Thus, the addition of ethylenediamine allows the
enhanced control of the properties of the particles. Moreover, the
morphology results (spherical) are comparable with the nature of
the particles when AgNiPr2 was used as a precursor in the presence
of 1 equivalent of ethylendiamine (Fig. 6). The ethylendiamine role
preventing nanoparticle agglomeration has also been observed in
Ru systems in aqueous environment [73].
Fig. 9 shows the FT-IR spectra of the Ag NPs obtained from
AgN(SiMe3)2 without additional capping agent. Comparisons with
the AgN(SiMe3)2 precursor and HN(SiMe3)2 were also analyzed,
Fig. 9. The spectra corresponding to the Ag NPs/HN(SiMe3)2 sys-
tems for 16 and 240 h were qualitatively similar. The FT-IR spectra
of HN(SiMe3)2 and AgN(SiMe3)2, displayed, in both cases,
at c.a. 2962 cmꢁ1. In the Ag NPs/HN(SiMe3)2 systems, however, this
peak is slightly shifted by 4 and 6 cmꢁ1 as(CH3) = 2966 cmꢁ1 and
as(CH3) = 2968 cmꢁ1. This relatively small shift has also been
mas(CH3)
:
m
m
reported in the case of oleylamine-capped silver colloids, dode-
canethiol-capped Ag nanocrystals and nonanethiol-passivated Ag
nanoparticles and can be correlated to the constraint of the cap-
ping molecular motions [42]. Additionally, the peak at
ꢀ1580 cmꢁ1, corresponding to N–H in the AgN(SiMe3)2 spectra,
also appeared shifted in the Ag NPs/HN(SiMe3)2 systems at 1578
and 1540 cmꢁ1, in agreement with the coordination of the ligand
on the silver surface [54]. The band positions of Si–CH3 bending
and stretching modes of Ag NPs/HN(SiMe3)2 systems also appeared
shifted towards higher wavenumbers (1–2 cmꢁ1) in comparison
with the corresponding AgN(SiMe3)2 precursor and HN(SiMe3)2
(ꢀ800 and ꢀ1260 cmꢁ1), which is once more in agreement with
capping molecular motions of these ligands on the surface of the
Ag NPs.
The presence of nanoparticle-coordinated ethylendiamine is
supported by the IR spectra of the obtained nanoparticles which
present similar but shifted bands with respect to those obtained
with pure ethylenediamine, Fig. 12. Indeed, slightly higher wave-
numbers in the bands corresponding to N–H vibrations suggest
the amine coordination to the Ag nanoparticles on the surface
Table 2
Interplanar distances and planes of Ag nanoparticles obtained from AgN(SiMe3)2 in the presence of hexamethyldisilazane as additional capping agent.
Ag Nps obtained at r.t. and t = 16 h SAED from Fig. 10b Ag Nps obtained at r.t and t = 10 days SAED from Fig. 10d
d (Å)
d (Å) JCPDS file 04-0783
Planes
d (Å)
d (Å) JCPDS file 04-0783
Planes
2.43
2.04
1.43
1.23
2.3833
2.0640
1.4594
1.2446
(111)
(200)
(220)
(311)
–
2.40
2.00
1.44
1.22
2.3833
2.0640
1.4594
1.2446
(111)
(200)
(220)
(311)
–
–
–
0.93
0.9400
(331)
–
–
–
0.91
0.9230
(420)
–
Please cite this article in press as: E. Ramírez-Meneses et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2015.01.035

8
E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
(d)
(a)
(b)
(e)
(c)
(f)
Fig. 10. TEM images, corresponding electron diffraction patterns and histograms of size distribution of Ag nanoparticles obtained under dihydrogen (2 bars) from
AgN(SiMe3)2 precursor in the presence of 1 equivalent of hexamethyldisilazane as additional capping agent synthesized at 16 h (a–c) and 10 days (d–f).
Please cite this article in press as: E. Ramírez-Meneses et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2015.01.035

E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
9
Fig. 11. TEM images of Ag nanoparticles obtained from AgN(SiMe3)2 at room temperature in the presence of 5 equiv. of ethylenediamine as capping agent (a and b) and its
corresponding histogram of size distribution of Ag nanoparticles.
temperature did not affect significantly the size distribution or
shape of the Ag nanostructures.
The addition of 1 or 5 equivalents of ethylenediamine as addi-
tional capping agent notably decreased the average size of the par-
ticles with respect to that obtained from the Ag NPs/HNiPr2 system.
On the other hand, Ag nanostructures obtained from the
AgN(SiMe3)2 precursor, polydispersed and agglomerated semi-
spherical particles from ꢀ14 to 86 nm, are observed in the absence
of additional capping agent. Nevertheless, when 1 equivalent of
HN(SiMe3)2 is added as supplementary capping agent, agglomer-
ates of Ag structures with undefined shape and wide size distribu-
tion were obtained. The presence of additional HN(SiMe3)2 must
disrupt the organization of the reaction media, enabling the forma-
tion of Ag structures that are bigger in size than those obtained in
the absence of additional amine.
Fig. 12. FT-IR spectra of ethylenediamine and stabilized Ag nanoparticles obtained
from AgN(SiMe3)2 at room temperature in the presence of
ethylenediamine.
5 equivalents of
The interaction of the amine molecules with the silver nanopar-
ticle surface is quite strong, but the exact nature of the interaction
is not fully understood at this time. However its incorporation to
the reaction media helped obtain well dispersed spherical particles
with ca. 25 nm in size. Based on these observations, ethylenedia-
mine must favor the formation of spherical particles due to the
coordination between amine groups and the silver surface.
[72]. It both spectra, the correspondence between the bands due to
the N–H bond vibration at ca. at 3360 and 3280 cmꢁ1 as well as the
band at 1598 cmꢁ1 is evident. The same is true for the stretching
C–H vibrations of CH2 groups at ca. 2920 and 2850 cmꢁ1
.
4. Conclusions
Silver nanoparticles were synthesized successfully from silver
amide complexes such as AgNiPr2 and AgN(SiMe3)2 by reduction
with dihydrogen (2 bar) using two different temperatures: room
temperature and 60 °C. FT-IR spectra showed that the as-prepared
silver nanoparticles are stabilized by in situ generated HNiPr2 or
HN(SiMe3)2. The Ag nanoparticles synthesized from AgNiPr2 dis-
played sizes ranging from 26 to 35 nm, however, the synthesis
Acknowledgments
The authors wish to acknowledge the financial support pro-
vided by CONACyT through the 105762 and 157613 projects, Uni-
versidad Iberoamericana 0053 project and SNI-CONACyT. The
research was conducted in the framework of the ‘‘French-Mexican
International Laboratory (LIA) LCMMC
Please cite this article in press as: E. Ramírez-Meneses et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2015.01.035

10
E. Ramírez-Meneses et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
[29] D.G. Shchukin, I.L. Radtchenko, G.B. Sukhorukov, Photoinduced reduction of
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