Morphology Control in AgCu Nanoalloy Synthesis by Molecular Cu(I) Precursors

Article abstract of DOI:10.1021/acs.inorgchem.9b02172

As nanoparticle preparation methods employing bottom-up procedures rely on the use of molecular precursors, the chemical composition and bonding of these precursors have a decisive effect on nanoparticle formation and their resulting morphology and properties. We synthesized the Cu(I) complexes [Cu(PPh₃)₂(bea)] (1, bea = benzoate) and [Cu(PPh₃)₃(Hphta)] (2, phta = phthalate) by reducing the corresponding Cu(II) mono- A nd dicarboxylates with triphenylphosphine. We characterized 1 and 2 by single-crystal X-ray diffraction analysis, elemental analyses, infrared and nuclear magnetic resonance spectroscopy, and mass spectrometry and obtained complete information about their structures in the solid state and in solution. Also, we examined their thermal stability in oleylamine and determined their decomposition temperatures to be used as the minimal reaction temperature in metal nanoparticle synthesis. The complexes 1 and 2 differ in the number of reducing PPh₃ ligands and the strength of carboxylate bonding to the Cu(I) center. Therefore, we employed them in combination with [Ag(NH₂C₁₂H₂₅)₂]NO₃ as molecular precursors in the solvothermal hot injection synthesis of AgCu nanoalloys in oleylamine and demonstrated their influence on the elemental distribution, phase composition, particle size distribution, shape, morphology, and optical properties of the resulting nanoparticles. The nanoalloy particles from the benzoate complex 1 were oblate and polydisperse and exhibited two surface plasmons at 393 and 569 nm, which is caused by their Janus-type structure. The nanoparticles prepared from the phthalate complex 2 were round and monodisperse and exhibited one plasmon at 413 nm, as they formed an AgCu solid solution with a random distribution of the elements in a particle.

Full text of DOI:10.1021/acs.inorgchem.9b02172

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Morphology Control in AgCu Nanoalloy Synthesis by Molecular Cu(I)
Precursors
Vit Vykoukal,†,‡ Vitezslav Halasta,† Michal Babiak,†,‡ Jiri Bursik,§ and Jiri Pinkas*,†,‡
†Masaryk University, Faculty of Science, Department of Chemistry, Kotlarska 2, 611 37 Brno, Czech Republic
‡Masaryk University, CEITEC MU, Kamenice 5, 625 00 Brno, Czech Republic
§Institute of Physics of Materials, ASCR, Zizkova 22, 616 62 Brno, Czech Republic
S
* Supporting Information
ABSTRACT: As nanoparticle preparation methods employing bottom-up procedures rely on the use of molecular precursors,
the chemical composition and bonding of these precursors have a decisive effect on nanoparticle formation and their resulting
morphology and properties. We synthesized the Cu(I) complexes [Cu(PPh3)2(bea)] (1, bea = benzoate) and
[Cu(PPh3)3(Hphta)] (2, phta = phthalate) by reducing the corresponding Cu(II) mono- and dicarboxylates with
triphenylphosphine. We characterized 1 and 2 by single-crystal X-ray diffraction analysis, elemental analyses, infrared and
nuclear magnetic resonance spectroscopy, and mass spectrometry and obtained complete information about their structures in
the solid state and in solution. Also, we examined their thermal stability in oleylamine and determined their decomposition
temperatures to be used as the minimal reaction temperature in metal nanoparticle synthesis. The complexes 1 and 2 differ in
the number of reducing PPh3 ligands and the strength of carboxylate bonding to the Cu(I) center. Therefore, we employed
them in combination with [Ag(NH2C12H25)2]NO3 as molecular precursors in the solvothermal hot injection synthesis of AgCu
nanoalloys in oleylamine and demonstrated their influence on the elemental distribution, phase composition, particle size
distribution, shape, morphology, and optical properties of the resulting nanoparticles. The nanoalloy particles from the benzoate
complex 1 were oblate and polydisperse and exhibited two surface plasmons at 393 and 569 nm, which is caused by their Janus-
type structure. The nanoparticles prepared from the phthalate complex 2 were round and monodisperse and exhibited one
plasmon at 413 nm, as they formed an AgCu solid solution with a random distribution of the elements in a particle.
distribution of alloy nanoparticles are determined by many
reaction parameters; one of the most important is the chemical
nature of the reactants. This fact is a strong motivation for the
preparation and study of new molecular metal complexes that
can serve as effective precursors.
An opportunity to influence the final nanoparticle properties
is offered by employing carefully selected metal sources. New
molecular and single-source precursors were applied in
INTRODUCTION
■
The combination of inorganic and material chemistry
approaches enables the design and synthesis of novel
precursors and their application in the preparation of new
materials. Particularly noteworthy is the development of
advanced molecular precursors to metal and alloy nano-
particles. Bimetallic nanoalloys exhibit different properties
from their macroscopic (bulk) counterparts, such as surface
plasmon resonance (SPR),1 spinodal decomposition,2 depres-
sion of melting point,3,4 increased component miscibility,5 and
higher catalytic activity.6−8 The resulting size, shape, and size
Received: July 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02172
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603.1063 [Cu(PPh3)2O]+, 100%; 667.0443 [Cu2(PPh3)2OH]+, 5%;
6 8 1 . 0 5 9 9 [ C u 2 ( P P h 3 ) 2 ( O C H 3 ) ]+ , 1 0 % ; 7 0 9 . 0 9 0 9
[Cu2(PPh3)2(OC3H7)]+, 20%; 771.0699 [M + Cu]+, 25%. TG/
DSC: thermal degradation in oleylamine at 227 °C (Figure S3).
31P{1H} NMR (300 MHz, CDCl3): δ −2.5 ppm. Anal. Found: Cu,
9.28; C, 73.03; H, 4.85. Calcd for C43H35O2P2Cu: Cu, 8.96; C, 72.82;
H, 4.97.
Synthesis of [Cu(PPh3)3(Hphta)] (2). Copper(II) phthalate
hydrate (1.228 g, 5.000 mmol) was reduced with PPh3 (4.590 g,
17.5 mmol, a molar ratio of 1:3.5) for 56 h in boiling methanol (30
cm3). A light blue precipitate was obtained. The solid product was
filtered off, washed with diethyl ether, and dried in the ambient
atmosphere. Yield: 2.678 g, 52.74%.
gold9−11 and AuAg nanoalloy12 synthesis by microwave-
assisted reactions. Another example is the CuNi nanoalloy
synthesis by hydrazine reduction of double complex salts.13
Molecular precursors based on triphenylphosphine were used
for the preparation of silver and copper nanoparticles.
Triphenylphosphine complexes were studied in the 1960s14
and 1970s,15,16 but they have only recently been used as
molecular precursors for nanoparticle synthesis. Although
bis(triphenylphosphine)silver(I) myristate17 and Cu(I) triphe-
nylphosphine derivatives of ethylene glycol carboxylates18,19
have been used for the preparation of nanoparticles of
individual metals, triphenylphosphine carboxylate complexes
have not been previously used in the synthesis of bimetallic
nanoalloys.
FTIR (ATR, cm−1): ν 414 w, 447 m, 495 s, 515 vs, 638 m, 692 vs,
726 m, 740 s, 794 w, 998 w, 1027 m, 1072 m, 1090 s, 1159 w, 1217 m,
1365 s (νs COO), 1433 s, 1456 s, 1473 m, 1558 m (νas COO), 1684
w, 2901 w, 2988 m, 3055 w. ESI-MS (+): m/z 587.1102
[Cu(PPh3)2]+, 70%; 603.1049 [Cu(PPh3)2O]+, 100%; 815.0586
[Cu2(PPh3)2(C8H5O4)]+, 15%; 1139.0709 [Cu3(PPh3)3(C8H4O4)]+,
15%; 1695.0302 [Cu4(PPh3)3(C8H4O4)4]+, 15%; 1957.1214
[Cu4(PPh3)4(C8H4O4)4]+, 20%. TG/DSC: thermal degradation in
oleylamine at 221 °C (Figure S4). 31P{1H} NMR (300 MHz,
CDCl3): δ −1.9 ppm. Anal. Found: Cu, 6.30; C, 73.29; H, 4.88. Calcd
for C62H50O4P3Cu: Cu, 6.26; C, 73.33; H, 4.96.
Synthesis of [Ag(NH2C12H25)2]NO3. The synthesis of [Ag-
(NH2C12H25)2]NO3 has been described by Wakuda et al.33 AgNO3
(11.05 g, 65.03 mmol) was mixed with NH2C12H25 (24.10 g, 130.0
mmol, a 1:2 molar ratio). Acetonitrile (350 cm3) was added, and the
mixture was stirred for 3 h at ambient temperature. Then a white
precipitate was filtered off, washed with acetonitrile and diethyl ether,
and dried at ambient temperature, avoiding light. TG/DSC: thermal
degradation in oleylamine at 199 °C.
Synthesis of AgCu Nanoparticles. In a typical synthesis,
[Cu(PPh3)2(bea)] or [Cu(PPh3)3(Hphta)] and [Ag(NH2C12H25)2]-
NO3 were combined in the eutectic Ag:Cu ratio of 60.1:39.9 mol % in
a total amount of 0.4 mmol and placed into a Schlenk flask, and the
flask was evacuated/refilled with dry N2 three times. Then dry
oleylamine (4 cm3) was added via a syringe. The mixture was heated
to 85 °C to dissolve the precursors to a clear colorless solution.
Oleylamine (16 cm3) and 1-octadecene (20 cm3) were preheated in a
three-neck Schlenk flask to 120 °C under vacuum to remove residual
water and oxygen. After 20 min, the vacuum was replaced with a dry
N2 atmosphere, and the temperature was increased to 230 °C. The
precursor solution was rapidly injected into the hot mixture of
solvents. After 10 min, the black reaction mixture was cooled to room
temperature in a water bath, 20 cm3 of acetone was added, and the
suspension was centrifuged. Acetone (20 cm3) was added to increase
the yield by aggregating nanoparticles that could then be easily
separated by centrifugation (6000 rpm, 10 min). The precipitate was
washed twice with a mixture of hexane and acetone (a 1:3 volume
ratio), and then it was dispersed in hexane and characterized.
Oleylamine acts as a reducing and capping agent,36−39 and 1-
octadecene dilutes oleylamine to give the most regular shape.7
Characterization Methods. Single-crystal X-ray diffraction data
were collected on a Rigaku diffraction system equipped with a rotating
anode X-ray source with multilayer optics (Mo Kα, 7.107 nm). The
final structures were adjusted by Mercury40 and Olex241 software. See
Table S1 in the Supporting Information. IR spectra were recorded on
a Bruker Tensor T27 spectrometer (4000−400 cm−1). The ATR
(Bruker Platinum ATR) technique was used. Electrospray ionization
mass spectrometry (ESI-MS) was performed on an Agilent 6224
Accurate-Mass TOF mass spectrometer. Positive and negative modes
were measured under the following conditions: nitrogen flow 5 dm3
min−1, gas temperature 325 °C, nebulizer pressure 45 psi, capillary
voltage −2500 V in positive mode and 2500 V in negative mode.
Samples were dissolved in methanol and injected directly into the
spectrometer using a syringe pump. The thermal stability of
nanoparticle precursors was studied by the thermogravimetry−
differential scanning calorimetry (TG-DSC) technique on a Netzsch
STA 449C Jupiter apparatus from 25 to 500 °C under flowing
A silver/copper nanoalloy was prepared by the reduction of
22
a solution of metal precursors with hydrazine20,21 or NaBH4
in the presence of polyvinylpyrrolidone (PVP) or by thermal
decomposition in long-chain alkyl amines, which act as high-
boiling-point solvents, reducing agents, and surfactants.23
Oleylamine is the most frequently used long-chain alkyl
amine in nanoalloy synthesis (e.g., CuAg,2,24,25 CuNi,7 CoNi,26
AuAg,27 and AuCu28). Silver/copper bimetallic nanoalloys are
currently being studied for their thermal29 and optical
properties and potential industrial applications in catalysis,22,30
electronics,31 and medicine.32
In this work, we compared two triphenylphosphine Cu(I)
complexes of aromatic carboxylic acids, 1 and 2, as copper
precursors in the AgCu nanoparticle synthesis. They differ in
the number of reducing phosphine ligands and also in the
strength of carboxylate binding (chelating vs terminal). These
structural factors may influence their reactivity in solvothermal
decomposition into alloy nanoparticles. We combined them
with [Ag(NH2C12H25)2]NO333 as a precursor of silver in a hot
mixture of oleylamine and 1-octadecene and prepared the
AgCu nanoalloys. We determined the influence of precursor
type on the course of the reaction and different properties of
the prepared nanoparticles, such as elemental distribution,
particle shape, size and size distribution, and surface plasmon
resonance maxima.
EXPERIMENTAL SECTION
■
Materials. Benzoic acid (Hbea), phthalic acid (H2phta), copper-
(II) hydroxide, and silver(I) nitrate were of house stock,
triphenylphosphine (99%) was purchased from Acros Organics, and
methanol (p.a.) was purchased from Lach-Ner. Oleylamine (80−
90%) and 1-octadecene (90%) were purchased from Sigma-Aldrich,
dried over sodium, distilled under reduced pressure, and stored in
Schlenk flasks over molecular sieves.
Synthesis of Copper(II) Carboxylate Precursors. Cu(bea)2·
3H2O and Cu(phta)·H2O were prepared according to literature
procedures34,35 and characterized by TG/DSC in a synthetic air
atmosphere (Figures S1 and S2). The content of Cu was determined
by ICP-OES, and for both compounds the contents found matched
the theoretical values. For details, see the Supporting Information.
Synthesis of [Cu(PPh3)2(bea)] (1). Copper(II) benzoate (1.799
g, 5.000 mmol) was reduced with PPh3 (4.590 g, 17.5 mmol, a molar
ratio of 1:3.5) in boiling methanol (30 cm3). When the reaction was
performed for 1.5 h, the final solution was colorless. The reaction
mixture was cooled to 25 °C, and colorless crystals formed. The solid
was filtered off, washed with diethyl ether, and dried in the ambient
atmosphere. Yield: 1.565 g, 44.13%.
FTIR (ATR, cm−1): ν 420 m, 488 s, 502 vs, 513 s, 530 m, 679 s,
694 vs, 722 s, 746 s, 842 m, 997 w, 1026 m, 1067 m, 1096 s, 1394 s
(νs COO), 1435 s, 1479 m, 1539 s (νas COO), 1591 m, 2901 w, 2988
w, 3049 w, 3067 w. ESI-MS (+): m/z 587.1115 [Cu(PPh3)2]+, 40%;
B
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nitrogen (70 cm3 min−1) with a heating rate of 5 °C min−1.
Complexes 1 and 2 were placed into alumina crucibles and heated
without and with an excess of oleylamine. The hydrodynamic
nanoparticle diameter was determined by the dynamic light scattering
(DLS) technique on a Zetasizer Nano ZS (Malvern) instrument in a
hexane solution at 25 °C. Samples were diluted and filtered by a
syringe filter (pore size 450 nm). Small-angle X-ray scattering (SAXS)
measurements were carried out on a Biosaxs 1000 (Rigaku) system at
25 °C with λ = 14 nm for 5 min. Samples were sealed into 1.5 mm
borosilicate glass capillaries (WJM-Glas). Data were analyzed by the
Primus42 and GNOM43 software. The UV−vis spectra were obtained
on a Unican UV4 (Chromspec) instrument. Samples were prepared
by the same procedure as for DLS, but they were diluted to one-
fourth. The TEM measurements were carried out on a Philips CM 12
TEM/STEM microscope with an EDAX Phoenix EDS detector and
on JEOL JEM2010 and 3010 microscopes equipped with an EDX
detector and a CCD camera with a resolution of 1024 × 1024 pixels.
The samples for TEM measurements were dispersed in hexane, and
one drop of colloidal solution was placed on a carbon-coated copper
grid and allowed to dry by evaporation at ambient temperature. Image
analyses were performed by the ImageJ software.44 STEM-EDS
measurements were performed on a FEI Titan Themis instrument
with a combination of a spherical aberration image (Cs) corrector, a
monochromator system, sensitive ChemiSTEM technology, and a
high-end GATAN GIF Quantum energy filter for EELS and EFTEM
with a new enhanced piezo stage, FEI and GATAN software, and a
FEI Ceta 16-megapixel CMOS camera. Powder X-ray diffraction
(PXRD) measurements were carried out on a SmartLab diffrac-
tometer (Rigaku), with a Cu lamp, fine focus λ(Kα1) = 15.4060 nm,
λ(Kα2) = 15.4443 nm, and λ(Kβ) = 13.9225 nm. Samples were
measured in parallel beam order in transmission mode between two
foils. The metal contents were analyzed by inductively coupled plasma
optical emission spectroscopy (ICP-OES) on an iCAP 6500 Duo
(Thermo) spectrometer. Dried nanoparticles were completely
dissolved in HNO3, diluted, and characterized. Elemental analysis
(C, H, and N) was performed on a Flash 2000 CHNS Elemental
Analyzer (Thermo Scientific).
Table 1. Selected Bond Lengths and Bond Angles of
[Cu(PPh3)2(bea)] (1) and [Cu(PPh3)3(Hphta)] (2)
1
2
Bond Lengths (Å)
Cu1−P1
Cu1−P2
2.2228(7)
2.2321(6)
Cu1−P1
Cu1−P2
Cu1−P3
Cu1−O1
2.3325(6)
2.3289(6)
2.3415(6)
2.126(1)
Cu1−O1
Cu1−O2
2.194(2)
2.187(2)
Bond Angles (deg)
P1−Cu1−P2
126.35(3)
P1−Cu1−P2
P1−Cu1−P3
P2−Cu1−P3
P1−Cu1−O1
P2−Cu1−O1
P3−Cu1−O1
116.60(2)
119.38(2)
110.38(2)
92.85(4)
115.65(4)
99.51(4)
P1−Cu1−O1
P2−Cu1−O1
117.32(5)
110.56(5)
P1−Cu1−O2
P2−Cu1−O2
O1−Cu1−O2
110.33(5)
113.25(5)
60.74(6)
one free carboxylic group in the molecule. Refinement
converged to R1(observed) = 3.04%.
The complex 1 contains two molecules of PPh3 and a
bidentate carboxylic group, which is symmetrically bound. The
Cu−O distances are the same on the 3σ criteria. The Cu−P
bond lengths are between 2.223 and 2.233 Å. In contrast,
complex 2 contains three molecules of PPh3 and a
monodentate carboxylic group. The Cu−P bond lengths are
substantially longer, between 2.329 and 2.342 Å. Both
structure types are known in the literature, but Adner et al.19
reported that complexes recrystallized from dichlomethane by
the addition of n-hexane contained two PPh3 molecules and a
bidentate carboxylic group. The molecular structure of 1 agrees
with this observation, but 2 differs because after the same
recrystallization it contains three molecules of PPh3, which are
more weakly bonded to the central Cu atom and also the Cu−
P bonds are longer than in 1. These structural differences may
influence the relative reactivities of 1 and 2 in thermolysis
decomposition reactions (vide supra), as there are more
equivalents of reducing PPh3 per Cu in 2 in comparison to 1.
Infrared spectroscopy can be a reliable method for
establishing the binding mode of carboxylato groups to a
metal.46 Regarding ionic carboxylates, the bridging and
bidentate carboxylate complexes show a small difference in
the value of Δν(CO2) (Δν(CO2) = ν(CO2,asym) − ν-
(CO2,sym)), while the parameter significantly increases for
monodentate complexes. The values of Δν(CO2) are 145 cm−1
(141 cm−1 for Na-bea) and 193 cm−1 (173 cm−1 for Na2-
terphta) for complexes 1 and 2, respectively. These values are
in agreement with the crystal structure, as complex 1 contains a
bidentate benzoate ligand while complex 2 possesses a
monodentate phthalate.
RESULTS AND DISCUSSION
■
Copper(I) Triphenylphosphine Carboxylate Com-
plexes. The complexes [Cu(PPh3)2(bea)] (1) and [Cu-
(PPh3)3(Hphta)] (2) were synthesized from the correspond-
ing Cu(II) carboxylates Cu(bea)2·3H2O and Cu(phta)·H2O
by reduction with PPh3 in boiling methanol and isolated as
white solids in 44.13% and 52.74% yields, respectively. For a
precise structural investigation of the prepared precursors, we
performed single-crystal X-ray diffraction analyses. Suitable
single crystals were grown from a chloroform solution by
hexane diffusion. The complexes 1 and 2 crystallize in the
P212121 and Pbca space groups, respectively, without any
additional solvent molecules. Selected crystallographic data
and structure refinement parameters are gathered in Table S1
in the Supporting Information. Relevant bond distances and
angles are summarized in Table 1. The metal center of
complex 1 is formed by a Cu(I) ion in a distorted-tetrahedral
arrangement (Figure 1). The central atom is connected to both
oxygen atoms of a chelating carboxylate group and two
molecules of PPh3. The crystal structure of 1 has already been
reported.45 Our refinement converged to a substantially better
value of R1(observed) = 2.21%.
For additional characterization of the prepared complexes,
ESI-MS was used. Molecular peaks were not observed for 1
and 2, but other fragments of both complexes could be
assigned. Peaks at m/z 587 and 603 were observed in the
spectra of both complexes, and they were assigned to
[Cu(PPh3)2]+ and [Cu(PPh3)2O]+, respectively. Fragments
with multiple Cu atoms were also present, such as [M + Cu]+
for 1 and [M + 2Cu − H]+ for 2.
The metal center of 2 is formed by a Cu(I) ion in a trigonal-
pyramidal arrangement (Figure 1). The copper ion is
connected to one oxygen atom of a monodentate carboxylic
group and phosphorus atoms of three PPh3 ligands. There is a
nonconnected oxygen atom (O2) of the carboxylic group and
C
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Figure 1. Molecular structures of [Cu(PPh3)2(bea)] (1, left) and [Cu(PPh3)3(Hphta)] (2, right). Thermal ellipsoids are drawn at the 50%
probability level.
The stabilities of the synthesized compounds in solution
were characterized by NMR spectroscopy. Only one singlet
was observed in the 31P NMR spectrum of both 1 and 2 in
CDCl3. Complex 1 displayed a resonance at −2.5 ppm (Δν1/2
= 53 Hz) and 2 at −1.9 ppm (Δν1/2 = 18 Hz). These singlets
were broader than the singlet of free PPh3 (δ −5.2 ppm, Δν1/2
= 2.9 Hz). The downfield chemical shift of phosphorus
confirmed that a coordination reaction took place. The
broadening of the signals is probably caused by the dynamic
exchange of bound and free PPh3. When oleylamine was added
in excess to the solutions of 1 and 2, the resonances shifted
toward the value of free PPh3 in oleylamine (δ −5.5 ppm,
Δν1/2 = 1.8 Hz) and broadened, displaying values of −4.2 ppm
(Δν1/2 = 115 Hz) and −4.9 ppm (Δν1/2 = 42 Hz),
respectively.
The thermal behavior of both precursors was studied by
TG/DSC in a nitrogen atmosphere. The melting points are
221.3 and 169.9 °C for 1 and 2, respectively. These melting
events are not accompanied by decomposition, in contrast to
their corresponding parent Cu(II) carboxylate complexes
without the PPh3 ligands. Thermal stability was determined
in excess oleylamine under a nitrogen atmosphere. This
arrangement simulated the reaction conditions of nanoparticle
preparation in order to observe the thermal degradation of the
metal precursors in the presence of alkylamine. The
decomposition of 1 and 2 occurred at 227 and 221 °C,
respectively, forming elemental copper (Figures S3 and S4).
The measured data show that 1 is more thermally stable in the
presence of alkylamine, and its melting point is higher than
that of 2.
characterization methods. Their elemental composition,
particle size, and SPR maxima are summarized in Table 2.
Table 2. Summary of AgCu Nanoalloy Characterization
Results
[Cu(PPh3)2(bea)] [Cu(PPh3)3(Hphta)]
(1)
(2)
Ag/Cu nominal content
(mol %)
60.17/39.83
60.67/39.33
Ag/Cu exptl ICP-OES
(mol %)
50.83/49.17
59.69/40.31
DLS
x (nm)
24
24
Z average (nm)
24
22
PdI
Dmax (nm)
s
x (nm)
s
SPR (nm)
0.262
19.2
5.50
13.3
5.41
393/569
0.070
15.7
4.50
14.1
0.90
413
SAXS
TEM
UV−
vis
The starting eutectic Ag:Cu molar ratio (60:40) in the
precursor mixture is well maintained in the nanoparticle
elemental composition for the reaction of 2, while the product
of 1 contains less silver (50:50) (Table 2).
The size and dispersity of nanoparticles were determined by
the DLS method. Their weighted-average diameters are the
same in both cases, 24 nm. The AgCu nanoalloy prepared from
2 exhibits a slightly smaller Z average (the difference is 2 nm);
however, its polydispersive index (PdI) is more than 3 times
lower. This difference shows that the size distribution of
nanoparticles prepared from 2 is narrower than that of 1.
These results are confirmed by the TEM and SAXS analysis.
Figure 2 shows the TEM images and size distribution
histograms. Nanoparticles made from complex 2 are regular
and almost monodisperse, and their average diameter is 14.1
nm with standard deviation s = 0.90. Nanoparticles from 1
have an oblate shape with broader size distribution and an
average diameter of 13.3 nm and s = 5.41. The smaller average
diameter is caused by the presence of smaller particles, but the
most abundant diameter is in the range 15−17 nm, larger than
AgCu Nanoparticles. Colloidal solutions of AgCu nano-
alloys were prepared by the hot injection technique in a
preheated mixture of oleylamine and 1-octadecene. Oleylamine
acts as a high-boiling-point solvent, reducing agent, and
surfactant.23 [Ag(NH2C12H25)2]NO3 served as the silver
precursor because of its good solubility and reducing nature
of its dodecylamine ligands. As a copper source, we used the
two complexes [Cu(PPh3)2(bea)] (1) and [Cu-
(PPh3)3(Hphta)] (2) with the aim to compare their
performance in producing AgCu alloy nanoparticles. The
resulting colloids were investigated by physical/chemical
D
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Figure 2. TEM images and size distribution histograms of AgCu prepared from 1, x = 13.3 nm (A), and 2, x = 14.1 nm (B).
for the nanoparticles from 2. Larger diameter and a less regular
shape were also determined by a SAXS analysis, where the
most represented diameters of AgCu nanoparticles synthesized
from 1 and 2 were 19.2 nm (s = 5.50) and 15.7 nm (s = 4.50),
respectively. The less regular shape of nanoparticles from 1 is
illustrated by the model calculated from the SAXS data, and
Figure 3 displays average AgCu nanoparticles and higher values
of standard deviation. While nanoparticles from 2 are spherical,
those obtained from 1 are oblate with some irregularities in
their shape. These models are in good agreement with the
TEM observations, but the SAXS method, by its principle,
provides information on a much larger set of particles. The
graphs of the distribution function and plots of the measured
data are shown in Figures S5 and S6 in the Supporting
Information.
The results of powder X-ray diffraction measurements of
nanoparticles prepared from 1 revealed diffractions of phase-
separated silver and copper. Nanoparticles synthesized from 2
display broad diffractions assignable to an fcc AgCu solid
solution. The lattice parameter of 4.070 Å, calculated by
Vegard’s law, corresponds to 3.4% of Cu in Ag.47,48 However,
broad diffractions of small AgCu crystallites can be composed
of contributions from several metastable phases, as was
observed in samples prepared by arc discharge,49 ball milling,50
solution reduction,51 rapid quenching,52,53 or sputtering54
methods. The presence of an amorphous phase can also be
considered.55
The spatial distribution of Ag and Cu elements in individual
nanoparticles was examined by high-resolution STEM-EDS.
Figurse 4 and 5 show the Ag and Cu elemental distribution in
nanoparticles synthesized from 1 and 2, respectively. These
images demonstrate differences in the morphology of the
prepared nanoparticles. Some of the nanoparticles synthesized
from 1 have a Janus-type morphology, and silver and copper
phases are separated within a selected particle. There are also
particles composed of only silver, which was in excess in the
Figure 3. Models of average AgCu nanoparticles prepared from 1, d =
19.2 nm (A), and 2, d = 15.7 nm (B), obtained from the SAXS data
analysis.
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Figure 4. STEM-EDS of AgCu nanoparticles prepared from 1: (A) bright field; (B) high angle annular dark field; (C) silver distribution; (D)
copper distribution; (E) silver/copper distribution.
Figure 5. STEM-EDS of AgCu nanoparticles prepared from 2: (A) bright field; (B) high angle annular dark field; (C) silver distribution; (D)
copper distribution; (E) silver/copper distribution.
reaction mixture due to the eutectic Ag:Cu ratio (60:40 mol
%) used for the synthesis. On the other hand, nanoparticles
prepared from 2 exhibit homogeneous distribution of both
metals within one particle. This phenomenon demonstrates the
crucial role of employed Cu precursor complexes, because
different elemental distributions may influence nanoparticle
properties, such as a less regular shape (Figure 3) and biphasic
nature of AgCu nanoparticles prepared from 1 observed in
HRTEM (Figure 6). Additional results from STEM-EDS
analysis are shown in the Supporting Information (Figures S7−
S10).
equal to 400 pm. The formation of phase-separated (Janus)
and solid solution nanoparticles from 1 and 2, respectively,
could be tentatively explained by the different thermal
stabilities of the copper precursors. Complex 2 decomposes
at 221 °C, which is closer to the temperature degradation of
[Ag(NH2C12H25)2]NO3 in oleylamine (199 °C) in compar-
ison to 1, which reacts at a slightly higher temperature of 227
°C. Silver and copper atoms are formed from 2 simultaneously
to precipitate randomly mixed in one particle. In contrast,
silver is formed more easily from its precursor than copper
from 1, which results in particles with the Ag/Cu Janus
morphology.
A comparison of the optical properties of the AgCu
nanoalloys prepared from different precursors is shown in
Figure 7. The AgCu nanoalloy prepared from 1 exhibited two
local absorption maxima in the UV−vis spectrum (Figure 7A).
The first band at 393 nm was assigned to the surface plasmon
resonance (SPR) of neat silver and the second band at 569 nm
to the SPR of neat copper. Two local maxima are caused by the
Janus-type morphology of these nanoparticles56 and are
consistent with SPR of neat Cu and Ag metal nanoparticles
prepared separately from the same precursors by an identical
procedure. The AgCu nanoalloy prepared from 2 exhibited one
absorption maximum in its UV−vis spectrum at 413 nm
(Figure 7B). This SPR absorption maximum is slightly shifted
to longer wavelengths in comparison to neat silver nano-
particles prepared from the [Ag(NH2C12H25)2]NO3 precursor
by the same route (393−395 nm). This red shift points to the
formation of the solid solution of these metals with limited
solubility of Cu in Ag. The SPR of copper occurs typically at
longer wavelengths (around 570 nm), and its maximum is
dependent on composition if alloy nanoparticles exhibit
measurable SPR and particular metals are miscible. This
assignment to SPR of AgCu is in accordance with the literature
data.24 These results are additional proof of the importance of
the precursor in nanoalloy synthesis, because by changing the
molecular precursors, we can influence the final optical
properties of the nanoalloy.
Figure 6. HRTEM of AgCu nanoalloys prepared from 1 (A, left) and
2 (B, right).
The results from STEM-EDS were confirmed by the
HRTEM technique. Figure 6A shows a Janus structure of
nanoparticles prepared from 1. Analysis of lattice distances
showed that one particle is composed of two separated phases.
A measurement of lattice distances demonstrates the presence
of both Cu (111) and Ag (111) lattices in one single particle.
The Cu (111) lattice constant was measured and calculated
from the lattice distance, and its value is 358 pm, comparable
to the theoretical value of 361 pm. Similarly, the Ag (111)
lattice constant was 406 pm and is also close to the theoretical
value of 408 pm. On the other hand, Figure 6B shows a AgCu
solid solution with the composition 78 mol % of Ag and 22
mol % of Cu with a theoretical lattice constant of 398 pm.47,48
The lattice constant for AgCu (111) was determined to be
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Figure 7. UV−vis spectra of the AgCu nanoalloys prepared from 1 (A) and 2 (B). The intensity values for absorbance are plotted relative to the
intensity of the oleylamine peak at 220 nm, which is considered to remain constant.
CONCLUSION
Synthesis of starting compounds, TG/DSC curves,
crystallographic data and structure refinement parame-
ters, SAXS analyses, and STEM-EDS images and
elemental maps (PDF)
■
The two copper(I) molecular carboxylate complexes [Cu-
(PPh3)2(bea)] (1) and [Cu(PPh3)3(Hphta)] (2) were
prepared and structurally characterized, and their thermal
stability was determined. They differ in the coordination mode
of the carboxylate ligands and also in the number of bound
triphenylphosphine ligands. We used them as precursors for
the AgCu nanoalloy synthesis by the hot injection method
together with the [Ag(NH2C12H25)2]NO3 silver source. We
have shown the effects of the precursor nature on the resulting
nanoparticle size distribution, morphology, and optical proper-
ties. The AgCu nanoalloy particles prepared from 1 are phase-
separated featuring Janus-type morphology, a less regular
shape, and broader size distribution. Their UV−vis absorption
spectrum exhibited two local maxima assigned to separate
surface plasmon resonances of Ag and Cu.
Accession Codes
CCDC 1859625−1859626 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.P.: jpinkas@chemi.muni.cz.
ORCID
■
In contrast, the nanoalloy synthesized from 2 exhibited an
increased Ag/Cu mixing, resulting in a AgCu solid-solution
phase composition, a narrow size distribution, and regular
spherical shape. The absorption spectrum exhibits only one
SPR maximum, corresponding to the homogeneous AgCu
alloy. One can speculate that complex 2 with its weaker
association of the phthalate ligand to the Cu(I) center and the
high number of coordinated phosphine molecules presents a
more reactive precursor and, therefore, leads to a nucleation
pathway that results in AgCu nanoparticles possessing a
metastable solid-solution structure. The reactions of metal
clusters and nanoparticles may be strongly influenced by small
changes in the nature and arrangement of ligands, it was shown
in the case of the destabilizing role of the triphenylphosphine
shell in Au11 clusters.57 The presence of specific ligands has
been demonstrated as a decisive factor in the formation of
prenucleation species, which in fact control the final metal
nanoparticle composition and architecture in the AuCu alloy.58
To elucidate further this structure−property relationship, we
continue our studies of other Ag/Cu carboxylate precursors.
Jiri Pinkas: 0000-0002-6234-850X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research has been financially supported by the MEYS of
the Czech Republic under the project CEITEC 2020
(LQ1601) and by the Czech Science Foundation under the
grant GA 17-15405S. The CIISB research infrastructure
project LM2015043 funded by the MEYS CR is gratefully
acknowledged for the financial support of the measurements at
the CEITEC MU CF X-ray Diffraction and Bio-SAXS, the CF
Cryo-electron Microscopy and Tomography, and the Josef
Dadok National NMR Centre. The TEM part of this work was
carried out with the support ofthe CEITEC Nano Research
Infrastructure (ID LM2015041, MEYS CR, 2016−2019),
CEITEC Brno University of Technology. The authors also
thank L. Simonikova for ICP-OES analyses, Dr. M. Bittova for
ESI-MS analysis, and Dr. Z. Moravec for TG/DSC analysis.
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02172.
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