Matching the organic and inorganic counterparts during nucleation and growth of copper-based nanoparticles - in situ spectroscopic studies

Article abstract of DOI:10.1039/c5ce00454c

Although syntheses in organic solvents provide access to a wide range of copper-based nanoparticles, the correlation between organic reactions in solution and nucleation and growth of nanoparticles with defined properties is not well understood. Here, we utilize the Multivariate Curve Resolution-Alternative Least Squares (MCR-ALS) methodology to examine spectroscopic data recorded in situ during the synthesis of copper-based nanoparticles. While earlier studies showed that depending on the temperature copper(ii) acetylacetonate reacts with benzyl alcohol and forms either copper oxides or copper nanoparticles, we link the inorganic reaction with their organic counterparts. From X-ray Absorption Near Edge Spectroscopy (XANES) and Ultraviolet-visible spectroscopy (UV-vis) data we learn that copper(i) oxide forms directly from the solution and is the final product at low temperature of 140 °C. We observe in Fourier Transformed Infrared (FTIR) spectra an increasing concentration of benzyl acetate that co-occurs with the formation of a copper enolate and evolution of benzaldehyde, which accompanies the reduction of copper ions. We also record the interaction of organic species at the Cu₂O surface, which inhibits a further reduction to metallic copper. When we raise the synthesis temperature to 170 °C it turns out that the Cu₂O is just an intermediate species. It subsequently transforms by solid-state reduction to metallic copper accompanied by oxidation of benzyl alcohol to benzaldehyde.

Full text of DOI:10.1039/c5ce00454c

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Matching the organic and inorganic counterparts
during nucleation and growth of copper-based
nanoparticles – in situ spectroscopic studies†
Cite this: DOI: 10.1039/c5ce00454c
a
b
c
a
a
Malwina Staniuk, Daniel Zindel, Wouter van Beek, Ofer Hirsch, Niklaus Kränzlin,
a
a
Markus Niederberger and Dorota Koziej*
Although syntheses in organic solvents provide access to a wide range of copper-based nanoparticles, the
correlation between organic reactions in solution and nucleation and growth of nanoparticles with defined
properties is not well understood. Here, we utilize the Multivariate Curve Resolution-Alternative Least
Squares (MCR-ALS) methodology to examine spectroscopic data recorded in situ during the synthesis of
copper-based nanoparticles. While earlier studies showed that depending on the temperature copperIJII)
acetylacetonate reacts with benzyl alcohol and forms either copper oxides or copper nanoparticles, we link
the inorganic reaction with their organic counterparts. From X-ray Absorption Near Edge Spectroscopy
(XANES) and Ultraviolet-visible spectroscopy (UV-vis) data we learn that copperIJI) oxide forms directly from
the solution and is the final product at low temperature of 140 °C. We observe in Fourier Transformed
Infrared (FTIR) spectra an increasing concentration of benzyl acetate that co-occurs with the formation of
a copper enolate and evolution of benzaldehyde, which accompanies the reduction of copper ions. We
Received 4th March 2015,
Accepted 26th May 2015
also record the interaction of organic species at the Cu
2
O surface, which inhibits a further reduction to
metallic copper. When we raise the synthesis temperature to 170 °C it turns out that the Cu
2
O is just an
DOI: 10.1039/c5ce00454c
intermediate species. It subsequently transforms by solid-state reduction to metallic copper accompanied
by oxidation of benzyl alcohol to benzaldehyde.
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nanoparticles, the organic reactions do not terminate, and
1
. Introduction
1
7–21
in fact, new reactions are initiated.
By far the most
1
Twenty years after the initial findings the nonaqueous sol–
gel process is now an established method used to synthesize
crystalline nanoparticles of customizable size, composition
and functionalities. Specially for metal oxides, the advance-
ment has been made possible due to the understanding that
solvents are not only reaction medium but may also be an oxy-
gen source and act as stabilizing agent. That knowledge has
been gained by studying the organic compounds forming dur-
intriguing example is the formation of monolithic tungsten
oxide-polybenzylene hybrids induced by the nucleation of
20
tungsten oxide nanoparticles and their catalytic activity.
2–6
Moreover, in the last decade, the non-aqueous method has
been further adopted to synthesize metal phosphates, -sulfides,
1
8,22–25
-diimides and even metallic nanoparticles.
With the
increased complexity of the inorganic product also the
linkage of the inorganic species to their respective organic
counterparts became more challenging.
7
–13
ing synthesis.
When benzyl alcohol is used as a solvent, the
most common routes are: alkyl halide elimination, ester elimi-
Here, our main focus is the determination of organic and
inorganic species formed during the nucleation and growth
of copper-based nanoparticles and their interdependence
by utilizing different spectroscopic techniques. The latest
nation, ether elimination, aldol condensation and C–C bond
5
,12,14–16
formation between benzylic alcohols and alkoxides.
Remarkably, for some metal oxides with the occurrence of
development
of
synchrotron-
and
laboratory-based
a
equipment and analysis tools allow for in situ monitoring
Laboratory for Multifunctional Materials, Department of Materials, ETH Zurich,
7
,26–36
the complex reactions in solution.
However, the speci-
Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.
E-mail: dorota.koziej@mat.ethz.ch
ficity of the individual methods often makes a quantitative
analysis or a direct comparison of results difficult. We
overcome this problem by selecting two spectroscopic
methods, X-ray Absorption Near Edge Spectroscopy (XANES)
and Ultraviolet–visible spectroscopy (UV-vis), which even
though they probe fundamentally different phenomena,
b
Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, 8093
Zurich, Switzerland
c
Swiss-Norwegian Beamlines at European Synchrotron Research Facility, 71
Avenue des Martyrs, 38043 Grenoble, France
†
Electronic supplementary information (ESI) available: See DOI:
0.1039/c5ce00454c
1
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provide information on the chemical composition. In
order to determine the changes of the composition of the
(170 °C). The precipitate was centrifuged and washed three
times with ethanol.
2
reaction solution during nucleation and growth of Cu O and
Cu nanoparticles we apply Multivariate Curve Resolution-
Alternative Least Squares (MCR-ALS) method. It has been
previously successfully used to determine the concentration
profile and pure XANES spectra of individual species during
in situ studies of Cu-catalysts, Cu-doped V O batteries and to
Ex situ characterization
X-ray Absorption Spectroscopy (XAS) data of the correspond-
ing powder pellets were measured at the Swiss Norwegian
Beamline (SNBL) BM01B at the European Synchrotron
Research Facility (ESRF) from 8.850 to 9.800 keV in continu-
ous scanning mode with two ion chambers, one before the
sample and one after the sample. The powders were diluted
in cellulose to optimize the edge-step and pressed into pel-
lets. Powder X-ray diffraction (PXRD) patterns of synthesized
powders were collected on an Empyrean powder diffractome-
ter (PANalytical B.V., The Netherlands), operating in reflec-
tion mode under constant irradiated area conditions with Cu
Kα radiation (45 kV, 40 mA). Scanning electron microscopy
2
3
follow the synthesis of Co, TiO
nanoparticles.
2 2
and Ni-doped MoO
1
8,37–45
Additionally, we track the formation of organic com-
pounds by Fourier Transform Infrared spectroscopy (FTIR)
studies that are recorded simultaneously with UV-vis mea-
surements. In the control experiments without copper precur-
sor, we: (a) evaluate the decomposition of benzyl alcohol
upon heating, (b) determine the lower detection limit and (c)
calibrate the curve for benzaldehyde, benzyl acetate and
dibenzyl ether, compounds that are considered the main
organic counterparts of copper-based nanoparticle nucle-
ation. This lays the foundation for a quantitative analysis of
in situ FTIR studies. We find that only the formation of ben-
zyl acetate correlates directly with the occurrence of copper-
based nanoparticles in benzyl alcohol. At low reaction tem-
perature, the formation of benzaldehyde as a product of
(
SEM) images were taken on a Magellan 400 FEG (FEI, USA)
to show the shape and morphology of the nanoparticles. Gas
chromatography-mass spectrometry (GC-MS) analysis of the
filtered reaction liquids was performed with a Thermo Scien-
tific Trace 1300 (GC) coupled with an ISQ single quad (MS).
The samples were diluted in tetrahydrofuran (ratio 1 : 20).
Attenuated total reflection (ATR)-FTIR spectra of organic sam-
ples in preheated benzyl alcohol were measured as references
on a ReactIR15 (Mettler Toledo AutoChem) with a 6.5 mm
AgX DiComp Fiber Conduit probe. 60 mL of benzyl alcohol
were heated to 140 or 170 °C and then the corresponding
organic species were added. The analyzed range was 800 to
Cu O synthesis makes up approximately thirty percent of the
2
total amount of benzaldehyde. The remaining seventy per-
cent are a product of catalytic activity of Cu O nanoparticles.
2
At high reaction temperature, Cu O nanoparticles are further
2
reduced to Cu. Interestingly, the formation of copper nano-
particles terminates the catalytic formation of benzaldehyde,
and instead triggers the catalytic formation of dibenzyl ether.
−
1
−1
1
800 cm and the resolution was 4 cm .
In situ characterization
2
. Experimental
XAS. The measurements were recorded with a Vortex EM
fluorescence detector equipped with Xia digital electronics.
The Cu K-edge XANES spectra were recorded in the range
between 8.949 to 9.049 keV with a resolution of 0.5 eV. We
averaged three consecutive spectra in ATHENA software to
Chemicals
Benzyl alcohol (99–100.5%,), benzaldehyde (≥99%), dibenzyl
ether (99%), and copper hydroxide IJCuIJOH) ) (technical
2
grade) were supplied by Sigma Aldrich, copperIJII)
acetylacetonate (98%) by Acros Organics, benzyl acetate
47
increase signal to noise ratio.
ATR-FTIR. ReactIR15 with a 6.5 mm AgX DiComp Fiber
Conduit probe was used for in situ ATR-FTIR measurements.
The spectrum of preheated benzyl alcohol was used as back-
ground. The spectra were recorded every 2 minutes for the
synthesis at 140 °C and every 15 seconds for the synthesis at
(99%), ethanol (99.8%) by Fluka and tetrahydrofuran by
Riedel-de-Haen (99.9%, stabilized by butylhydroxytoluol
(BHT)). The chemicals were used without any further purifi-
4
6
2
cation. CuO was obtained by thermal oxidation of Cu O.
1
70 °C. Due to deposition of particles on the probe, the mea-
Synthesis procedure
surements were paused to clean the probe: (a) at 140 °C, only
in the time period between 2996 and 3067 minutes; (b) at
170 °C, starting after 30 minutes of the reaction and then
every 15 minutes.
Control experiment. The decomposition of pure benzyl
alcohol upon heating was measured every 5 minutes (140 °C)
or every 15 seconds (170 °C). To mimic the conditions during
the synthesis at 170 °C, the reactor was opened every 15
minutes. The final products were additionally investigated by
GC-MS.
Synthesis at the synchrotron. CuIJacac)2 (1 mmol) was
added to 20 mL of benzyl alcohol. Then 1 mL of the reaction
mixture was transferred to a reactor with a total volume of
1
8
1
3
.66 mL. The solution was heated to 180 °C and kept for
43 minutes. The reaction reached 180 °C after around 3–4
minutes.
Synthesis in the laboratory. The synthesis was performed
in a Mettler Toledo EasyMax Synthesis Workstation. 60 mL of
benzyl alcohol were heated to 140 or 170 °C in a 100 mL reac-
tor. CuIJacac) (4.2 mmol) was added to the preheated benzyl
alcohol and kept for 4050 minutes (140 °C) or 326 minutes
ATR-UV-vis. The spectra were recorded by a Cary 60
spectrophotometer (Agilent Technology) with ATR-probe with
2
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sapphire crystal (Hellma). Spectra in the range from 800 to
negativity of spectra and concentration (XANES), non-
negativity of concentration (XANES and ATR-FTIR),
unimodality of concentration (XANES and ATR-FTIR), conver-
gence criterion: 0.1 (XAS and ATR-FTIR). For more informa-
tion please refer to ESI† Fig. S2–S4, Tables S2–S5.
3
00 nm were recorded every 30 seconds (140 °C) or every 36
seconds (170 °C). At 140 °C after around 7.5 hours a film of
particles grew on the ATR crystal and almost fully blocked
the light. In the case of synthesis at 170 °C, after 29 minutes
the probe was taken out of the solution, cleaned and placed
back into the reaction solution. The measurement was con- 3. Results and discussion
tinued after 40 minutes.
3.1. Synthesis
In a previous study we found that the reaction between
copper(II) acetylacetonate and benzyl alcohol resulted in the
Data handling
formation of a metallic copper foil with Cu O nanoparticles
2
Extended X-ray Absorption Fine Structure (EXAFS). We
performed the EXAFS data reduction using the ATHENA soft-
ware. The normalization parameters (pre-edge ranges, post-
2
5
as intermediates. Now we adopt this synthesis to a reactor
equipped with in situ ATR-UV-vis and -FTIR probes to collect
information about the formation mechanism. In order to be
able to probe the organic species in solution we prevent for-
mation of the Cu foil at the surface of ATR crystals by vigor-
ous stirring. Similar to the previous report at low reaction
temperature cuprous oxide and at high temperature metallic
0
edge ranges and E ) are given in ESI† Table S1. The resulting
3
χIJk)-function was k weighted and Fourier transformed (FT)
using a Hanning window function.
ATR-FTIR data. We applied a baseline offset correction
and shifted the in situ spectra so that the absorbance value
2
5
−
1
copper are formed, respectively.
Obviously, the time
was always 0 at 1800 cm . In addition, due to the particle
deposition, we divided the analyzed spectra into two sets. At
required for the reduction of cuprous oxide to copper varies
because of the differences in reactor volume, stirring velocity
and heating rate. Therefore, for the in situ studies we choose
temperature conditions at which the product in the Mettler
Toledo reactor is phase pure. At 140 °C and 170 °C we obtain
1
40 °C the first and second sets contained spectra before and
after particle deposition on the probe, respectively. For more
information please refer to ESI† Fig. S1. At 170 °C the first
and second sets contained spectra recorded before and after
the cleaning of the probe, respectively.
Cu
2
O or Cu nanoparticles, respectively. The corresponding
PXRD patterns and SEM images are shown in Fig. 1.
Multivariate Curve Resolution-Alternative Least Squares
(
MCR-ALS). In situ ATR-FTIR and XANES data were analyzed
3
.2. In situ studies of inorganic species in solution
with the MCR-ALS, implemented under MATLAB environ-
ment and discussed by De Juan and Tauler.
3
7,48
The underly-
(a) In situ XANES studies. To elucidate the mechanism of
ing idea behind MCR-ALS is the recovery of spectral compo-
nents and their correlated fraction profiles from time
evolving data by bilinear decomposition of experimental data,
represented by matrix D, according to eqn (1):
the reduction of CuIJacac)2 to Cu, we measure in situ X-ray
absorption at the Cu-K edge and analyze the oxidation state
of Cu and the short-range ordering around the Cu ions as
0
+
2+
shown in Fig. 2–4. In general Cu , Cu , and Cu exhibit the
absorption edge at 8.979, 8.981 and 8.984 keV,
T
49–51
D = CS + E
(1)
respectively.
During synthesis we observe shifting of the
absorption edge from 8.984 to 8.979 keV, which indicates the
2
+
0
Rows of matrix D are spectra acquired during measure-
ment, columns of matrix C and rows in matrix S are concen-
reduction of Cu to Cu as shown in Fig. 2a. Furthermore,
we compare the in situ XANES spectra recorded in the reac-
tion solution with the reference powders: copper oxides
(CuO, Cu O), copper hydroxide IJCuIJOH) ), metallic copper
T
tration profiles and spectra of resolved components, respec-
tively. Matrix E contains residuals not explained by the
model. Eqn (1), generally valid for spectroscopic data
governed by the Lambert–Beer law like XANES or FTIR, is
solved by ALS algorithm, which iteratively calculates the C
2
2
(Cu) foil and nanoparticles, and copper(II) acetylacetonate
IJCuIJacac) ) as shown in Fig. 2b. However, in the case of parti-
cles smaller than a few nanometers the position and relative
intensities of pre-edge and post-edge features in XANES spec-
trum are also sensitive to their size and to the species
2
T
and S matrices that fits best the experimental data. The opti-
T
mization of C and S is carried out for a proposed number of
T
41
components. Initial estimates of C and S are obtained by
adsorbed at their surface. Here, the spectra of CuIJacac)2,
using SIMPLe-to-use Interactive Self-Modeling Algorithm
2
Cu O and Cu, which were recorded during in situ experi-
(
SIMPLISMA), which finds the most different spectra within
ments vary from their bulk powder counterparts and thus the
utilization of the linear combination analysis (LCA) to deter-
mine their relative concentration during synthesis is not pos-
the dataset that are further used as an input for ALS
optimization.
The number of components was selected on the basis of
Singular Value Decomposition (SVD). The initial spectra of
the components were estimated with the PURE algorithm
with the noise level set to 3% (XANES) and 4% (ATR-FTIR).
Constraints applied for ALS calculation were as follows: non-
sible. For the sake of completeness in Fig. 2c we compare the
3
k
weighted Fourier Transform (FT) of the EXAFS spectra of
the final product and the reference compounds. This further
underlines the differences between the nanoparticles in solu-
tion and their bulk counterparts.
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2
Fig. 1 (a) PXRD patterns of the nanoparticles synthesized at 140 (black pattern) and 170 °C (red pattern). Reference PXRD patterns of Cu O (ICDD
no. 00-005-0667) and Cu (ICDD no. 01-071-4610). SEM images of (b) Cu O synthesized at 140 °C and (c) Cu synthesized at 170 °C.
2
We first determine the initial oxidation state of copper
species upon dissolution in benzyl alcohol at 180 °C as
shown in Fig. 3. Then, we utilize the MCR-ALS method to
analyze the in situ XANES spectra during nucleation of copper
nanoparticles. This solely variance based method allows to
determine, without a priori knowledge, the number of com-
ponents, their spectra and relative concentration in a solu-
tion as shown in Fig. 4.
species adsorbed at their surface and temperature at which
the spectrum was measured. Thus, the recovered spectra are
the spectra of a given compound at the particular reaction
conditions. The first recovered spectrum is the precursor
dissolved in benzyl alcohol at 180 °C. The second and third
spectrum show characteristic features observed in the refer-
ence spectra of Cu O and Cu, respectively. Therefore, the
2
appearance of the second and third component indicates the
Precursor. Here, a careful comparison of the XANES
spectra of the precursor as pellet (red curve), dissolved in
benzyl alcohol at RT (blue curve) and at 180 °C (green curve),
shown in Fig. 3, reveals the subtle differences in the pre- and
post-edge regions. In general, such differences are related to
the changes of geometry of the first coordination sphere of
nucleation of Cu
concentration dependence of the three recovered compo-
nents proves that Cu O and Cu are not formed concurrently.
First, exclusively Cu O forms and only after 83 minutes it gets
reduced to metallic copper, in line with previously reported
2
O and its consecutive reduction to Cu. The
2
2
2
5
2
PXRD studies. The direct formation of Cu O confirms the
5
2,53
2+
+
the absorbing element.
Above the edge, the spectrum of
reduction of Cu to Cu in the solution and solid intermedi-
solid CuIJacac) shows more pronounced features in compari-
ate species like CuO and CuIJOH) are excluded.
2
2
son to the CuIJacac)
2
in benzyl alcohol. The decrease of their
(b) UV-vis studies. To connect the synchrotron and the
laboratory studies, we investigate the inorganic side of the
reaction with UV-vis spectroscopy. Even though UV-vis and
XAS spectroscopy measure different physical phenomena, the
information about changes in the chemical composition that
can be derived from the in situ studies are basically the same.
This enables us to pin down the time scale of the consecutive
formation of cuprous oxide and copper, and later to match
them with the changes of organic species tracked with FTIR
spectroscopy. Based on UV-vis spectra, shown in ESI† Fig. S5
we learn that at 140 °C the cuprous oxide nanoparticles form
between 209 and 330 minutes. At 170 °C the reaction is much
faster. Already after 17 minutes we observe the formation of
cuprous oxide nanoparticles, followed by their consecutive
growth and after 52 minutes their transformation to metallic
copper nanoparticles.
intensity indicates a decrease of multiple scattering that
involves second nearest neighbor or higher shells. We assign
it to the loss of long-range order due to the dissolution of the
precursor in benzyl alcohol. In addition, the deviation in the
1
s → 4s transition indicates changes in the coordination of
the copper ion, suggesting complexation with benzyl alcohol.
The spectra of CuIJacac) in benzyl alcohol at 180 °C shows,
with respect to RT, an increase of the intensity of the 1s →
p transition. We do not observe characteristic features either
of the CuO or of the CuIJOH) . Thus, we can conclude that no
solid CuO or the CuOH species are formed before formation
2
4
2
2
2
of Cu O.
Cu reduction at 180 °C in benzyl alcohol. To track the
changes from CuIJacac) to Cu during the reaction at 180 °C
2
we analyze the in situ XANES data by MCR-ALS as shown in
Fig. 4. According to SVD results, three components suffice to
fit the data. We use SIMPLISMA based algorithm to find the
spectra that are used as initial estimates for MCR-ALS. We
show the spectra of powder references recorded at room tem-
perature in transmission mode only as guidance, since the
XANES spectra of nanoparticles strongly depend on their size,
3.3. Organic species forming during synthesis –
complementary in situ ATR-FTIR and GC-MS studies
Simultaneously with UV-vis, we measure in situ FTIR to track
the organic species forming in solution during the synthesis
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Fig. 3 Comparison of Cu K-edge XANES spectra of CuIJacac)
2
powder
measured in transmission mode (red curve) and CuIJacac) dissolved in
2
benzyl alcohol at RT (blue curve) and 180 °C (green curve) measured
in fluorescence mode.
give complementary information about species forming in
the solution below the detection limit of FTIR. Finally, we
determine the lower detection limit and calibration curve for
the main organic species, which allows us to estimate their
quantities and correlate the organic and the inorganic
compounds.
(
a) Qualitative results. Differently than for the XANES data,
MCR-ALS analysis of the FTIR data does not recover the spec-
tra of individual chemical species. Nevertheless it enables the
identification of a combination of the compounds, which
evolve together/at the same time or at the same rate. The
MCR-ALS recovered spectra of components (Fig. S2c–d†),
their concentration profiles (Fig. S3†) and the assignments of
individual vibrations are given in the ESI.
At both reaction temperatures, as long as some of the pre-
cursor is present in the solution, the same organic com-
pounds, benzyl acetate and benzaldehyde, form. At 140 °C
the reaction terminates with the reduction of CuIJacac)
Cu O. Only at that point the bands assigned to the species
adsorbed at the surface of Cu O become clearly visible as
2
to
2
2
2
shown in ESI† Fig. S4a–b. At 170 °C the nucleation of Cu O is
only an intermediate step towards the formation of Cu nano-
particles and we do not observe adsorption of organic species
2
at the Cu O surface. Instead, we observe the formation of
dibenzyl ether as the product of benzyl alcohol condensation,
which is a result of the catalytic activity of metallic copper.
Fig. 2 Cu K-edge XAS spectra; (a) XANES spectra taken during the syn-
(
b) GC-MS studies. To complete our in situ experiments
thesis of copper nanoparticles from CuIJacac)
C; (b) comparison of XANES spectra of powder references (pellets) and
powders in benzyl alcohol; Note: the spectrum of Cu O in benzyl alcohol
2
in benzyl alcohol at 180
we analyze the organic compounds, which are present
in benzyl alcohol after the synthesis with GC-MS. At 140 °C,
in addition to benzaldehyde and benzyl acetate already
observed with FTIR, we detect acetone and benzylacetone
°
2
was recorded at 160 °C; (c) FT EXAFS spectra of powder pellets. The
spectra were shifted on the ordinate for better representation.
(
ESI† Fig. S6a). Whereas at 170 °C, besides benzaldehyde,
benzyl acetate and dibenzyl ether already observed with FTIR,
we detect trace amounts of acetone and acetylacetone as
shown in ESI† Fig. S6b. Moreover, we analyze the organic
products of the control experiment and detect benzaldehyde
of Cu-based nanoparticles at 140 and 170 °C as shown in
ESI† Fig. S2a–b. In the following, we first qualitatively
describe the in situ FTIR results. Then, the GC-MS studies
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Fig. 4 Results of the MCR-ALS analysis of in situ Cu K-edge XANES spectra recorded in fluorescence mode; (a), (b) and (c) recovered spectra of
individual components and corresponding reference spectra; The reference compounds were measured as pellets in transmission mode and are
only given here as guidance; (d) concentration profiles of the three recovered components during the synthesis at 180 °C.
as well as trace amounts of dibenzyl ether and benzyl benzo-
ate as shown in ESI† Fig. S6c–d.
In summary, comparing the organic species from these
three experiments, benzyl acetate is the only compound that
exclusively forms when copper-based nanoparticles
crystallize. All other organic compounds may also form as
products of parallel, side reactions that are not directly
related to the nucleation of the copper-based nanoparticles.
(c) Towards quantitative analysis – calibration, detection
limit and side reactions. Based on the in situ FTIR studies,
Fig. 5 Molar concentration of copperIJII) acetylacetonate, benzyl acetate and benzaldehyde during the synthesis of copper-based particles as well
as molar concentration of benzaldehyde from the control experiments calculated from in situ FTIR data; (a) reaction at 140 °C and (b) at 170 °C.
We cannot fully avoid the deposition of copper oxide and copper on the surface of the ATR crystals. In a) the vertical, dashed lines indicate the
start of “particle deposition” and “measurements reestablished after probe cleaning”, respectively. The measurements in this time span were
background-corrected.
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we identify benzaldehyde, benzyl acetate and dibenzyl ether
as the three main organic compounds which are formed
from benzyl alcohol during crystallization of the Cu-based
nanoparticles. Although, as mentioned before, not all three
compounds are necessarily related to Cu formation. Their
lower detection limits, when measured in heated benzyl
alcohol, were determined to be 0.003, 0.007 and 0.008 M,
respectively. The calibration curves are shown in ESI†
Fig. S7.
Scheme
3
Oxidation of benzyl alcohol to benzaldehyde and
Additionally, we heat benzyl alcohol without CuIJacac) at
2+
+
+
0
2
reduction of Cu to Cu (a) and Cu to Cu (b).
1
40 and 170 °C, respectively, and in both cases with FTIR we
exclusively detect benzaldehyde as shown in Fig. 5 (gray
curves). It is the product of the oxidation of benzyl alcohol
5
4
according to the reaction shown in Scheme 1. The amounts
of dibenzyl ether and benzyl benzoate detected by GC-MS are
obviously below the detection limit of FTIR.
Now we discuss the quantities of the two compounds ben-
zyl acetate and benzaldehyde we measured in comparison to
what would be expected from the proposed chemical reac-
tions. The chemical reactions themselves will be further
discussed in section (d).
Scheme 4 Formation of Cu
2
O, benzyl acetate and benzaldehyde from
CuIJacac) and benzyl alcohol at 140 °C.
2
We assume that benzyl acetate is formed by the reaction
of the acetylacetonate ligand with benzyl alcohol, as shown
in Scheme 2.
Benzaldehyde is the oxidation product of benzyl alcohol
2
+
+
0
as a result of the reduction of Cu to Cu and finally Cu
according to Scheme 3.
Having these reactions in mind and just considering the
organic species detected, we can formulate an overall chemi-
Scheme 5 Formation of Cu, benzyl acetate and benzaldehyde from
CuIJacac) and benzyl alcohol at 170 °C.
2
cal reaction for the formation of Cu O, as shown in Scheme 4,
2
0
and for Cu as displayed in Scheme 5.
From Scheme 4 it is evident that the formation of 1 mmol
+
Cu
2
O produces 4 mmol of benzyl acetate and 1 mmol of
acetate, 0.035 M benzaldehyde and 0.070 M Cu (or 0.035
M Cu O) as summarized in eqn (2):
benzaldehyde. The initial concentration of CuIJacac) in ben-
2
2
zyl alcohol is 0.070 M, which means that 0.175 M benzyl
alcohol (BnOH) are required, resulting in 0.140 M benzyl
2
+
0.070 M Cu + 0.175 M BnOH → 0.140 M BnAc
+
+
0.035 M BnAd + 0.070 M Cu
(2)
According to Scheme 5, the formation of Cu at 170 °C fol-
lows eqn (3):
2
+
0
.070 M Cu + 0.210 M BnOH → 0.140 M BnAc
0
+
0.070 BnAd + 0.070 M Cu
(3)
54–57
Scheme 1 Oxidation of benzyl alcohol to benzaldehyde.
For the reaction at 140 °C, the measured concentration of
benzyl acetate matches with the nominal value of 0.137 M
within the resolution limit of our method, as shown in
Fig. 5a and Table 1. At the same time, the sum of the concen-
trations of benzaldehyde in the control experiment (0.052 M)
and the nominal value based on eqn (2) equals to 0.087 M,
which is much lower than the actually measured concentra-
tion of just 0.115 M. Evidently, the Cu
able to catalyze the oxidation of benzyl alcohol
increase the benzaldehyde concentration beyond the expected
value. The high coverage of the surface of the Cu O nano-
2
O nanoparticles are
5
8,59
to
Scheme 2 Formation of benzyl acetate by nucleophilic attack of
benzyl alcohol on one of the carbonyl groups of the acetylacetonate
ligand.
2
particles as mentioned before terminates the reaction at this
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Table 1 Quantitative analysis of compounds formed during synthesis
Synthesis at 140 °C
Synthesis at 170 °C
Compound
Lower detection limit (M)
Expected value (M)
Measured value (M)
Expected value (M)
Measured value (M)
Benzyl acetate
Benzaldehyde
Dibenzyl ether
Control experiment
Benzaldehyde
0.007
0.003
0.008
0.140
0.035
0
140 °C
—
0.137
0.115
0
0.140
0.070
0
170 °C
—
0.112
0.133
0.047
0.008
0.052
0.062
point without further reduction and we observe a saturation
of the benzaldehyde concentration as evidenced in Fig. 5a.
For the reaction at 170 °C, based on the shape of the con-
centration profile of the organic compounds, the mechanism
seems to be even more complex as shown in Fig. 5b. The
final concentration of benzyl acetate equals to 0.112 M,
which is lower than the nominal value of 0.140 M expected
from eqn (3). This can be explained by the observation of ace-
tone and acetylacetonate in the GC-MS chromatogram. There-
fore, we assume that ligand exchange reactions, as shown in
Scheme 6, are competing for CuIJacac)2 with the reaction
shown in Scheme 2.
In contrast to the synthesis at 140 °C, at 170 °C the mea-
sured concentration of benzaldehyde of 0.133 M agrees well
with the expected value of 0.132 M, which is the sum of benz-
aldehyde from the control experiment (0.062 M) and from
the reaction shown in eqn (3). This result is not surprising,
benzyl acetate and an enolate ligand form (Scheme 2). In the
next step, benzyl alcohol coordinates to the copper ion. While
2
+
the benzyl alcohol gets oxidized to benzaldehyde, Cu is
+
reduced to Cu as shown in Scheme 3a.
Up to here, we did not discuss, where the oxygen for the
formation of the copper oxide comes from. Based on the
results described above, we did not find any organic com-
pounds that are typically ascribed to condensation reactions
responsible for the formation of a metal–oxygen–metal
1
5
bond. Since the synthesis is performed under ambient con-
ditions, we assume that water impurities in solvent can influ-
ence the formation of nanoparticles. In addition hydrogen
ion formed during benzyl alcohol oxidation might react with
5
7,61,62
oxygen to form water as proposed in Scheme 7a.
Once
water is present, two possible scenarios for the formation of
a Cu–O bond are possible. A water molecule directly coordi-
nates to the enolate ligand, which leads to the formation of
acetone and CuIJI)–OH species as shown in Scheme 7b. Alter-
natively, a benzyl alcohol molecules reacts with the enolate
ligand, which leads to the formation of acetone and a copper
benzyl alcoholate. These scenarios are both plausible,
because traces of acetone were detected by GC-MS. A consec-
utive reaction of the copper alkoxide with water leads to a
2
because with the transformation of Cu O to Cu also the cata-
lytically active nanoparticles disappear. However, in the very
short time frame between 120–150 min we observed a rapid
increase of the concentration of benzaldehyde and a decrease
of the precursor concentration. It is interesting to note that
only at this point of the reaction, the concentration of benzal-
dehyde does not correlate with the concentration of benzyl
acetate, which indicates that the reactions shown in Scheme 6
might become more pronounced.
(d) Overview on chemical reactions taking place in solu-
tion during synthesis of copper nanoparticles. After the quan-
titative discussion of the different chemical reactions, we will
have now a closer look at the reaction mechanisms. Benzyl
acetate is exclusively forming due to the reaction between
benzyl alcohol and acetylacetonate ligand as shown in
1
5
60
Scheme 2 and as already reported for FeIJacac)
3
,
InIJacac)
3
9
or ZnIJacac)2. This step starts with the nucleophilic
attack of the hydroxyl group of benzyl alcohol on one of the
carbonyl groups of the acetylacetonate ligand. As a result,
Scheme 7 (a) Formation of water, (b) reaction of the copper enolate
with water to acetone and copper hydroxide, and (c) reaction of the
copper enolate with benzyl alcohol to acetone and copper benzyl
alcoholate, which further reacts to copper(I) hydroxide and benzyl
alcohol.
Scheme 6 Ligand exchange between acetylacetonate and benzyl
alcohol as competition reaction.
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Fig. 6 Overview on organic and inorganic reactions taking place during synthesis of copper-based nanoparticles.
CuIJI)–OH species, as displayed in Scheme 7c. Finally, the
CuIJI)–OH species condense with each other under release of
water molecules and formation of Cu–O–Cu bonds.
thanks Dr. I. Bilecka and Dr. F. Heiligtag for their help in the
initial phase of the project.
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