Nanoparticulate copper - Routes towards oxidative stability

Article abstract of DOI:10.1039/c0dt00134a

A modified polyol-based reduction method in ethylene glycol that incorporates poly(N-vinylpyrrolidone) (PVP, M_av = 10000; 40000; 55000) as polymeric anti-agglomerant alongside a reducing additive (N ₂H₄·H₂O, NaBH₄, NaH ₂PO₂·H₂O) has been employed to investigate the influence of synthetic parameters on the purity, morphology and stability of an array of polymer-coated copper nanoparticles. While data point to ethylene glycol being capable of acting as a reductant in this system, the use of NaH₂PO₂·H₂O as co-reductant in tandem with the presence of PVP (M_av 40000) has rendered nanoparticles with a mean size distribution of 9.6 ± 1.0 nm that exhibit stability towards oxidation for several months. These data allow us to probe fundamentally how oxidatively stable nano-copper might be achieved. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00134a

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www.rsc.org/dalton | Dalton Transactions
Nanoparticulate copper – routes towards oxidative stability
Volker Engels,a Faysal Benaskar,b David A. Jefferson,a Brian F. G. Johnsona and Andrew E. H. Wheatley*a
Received 14th March 2010, Accepted 10th May 2010
First published as an Advance Article on the web 21st June 2010
DOI: 10.1039/c0dt00134a
A modified polyol-based reduction method in ethylene glycol that incorporates
poly(N-vinylpyrrolidone) (PVP, Mav = 10 000; 40 000; 55 000) as polymeric anti-agglomerant alongside
a reducing additive (N2H4·H2O, NaBH4, NaH2PO2·H2O) has been employed to investigate the influence
of synthetic parameters on the purity, morphology and stability of an array of polymer-coated copper
nanoparticles. While data point to ethylene glycol being capable of acting as a reductant in this system,
the use of NaH2PO2·H2O as co-reductant in tandem with the presence of PVP (Mav 40 000) has
rendered nanoparticles with a mean size distribution of 9.6 1.0 nm that exhibit stability towards
oxidation for several months. These data allow us to probe fundamentally how oxidatively stable
nano-copper might be achieved.
recently, systematic investigations have sought to elucidate the
underlying principles innate to nanoparticulate systems by seeking
Introduction
Considerable efforts have been directed towards research into
to establish rules of scalability, e.g. between size and electronic
metallic nanoparticles in recent decades,1 with one of the most
promising fields of application resulting from their favorable
properties in catalytic processes.2 Within this field, nanoparticulate
copper has been specifically focused on because, in comparison to
its noble metal counterparts, it offers low processing costs and a
desirable combination of beneficial properties which have led to
its widespread use, e.g. in optics, electronics, sensor technology
and medical applications.3 From a synthetic point of view, a
plethora of commonly employed routes have been applied to
copper systems. These include thermal decomposition,4,5 sono-
chemical reduction,6 metal vapour deposition,7 laser ablation,8,9
radiation methods,10,11 microemulsion techniques,12–15 and chem-
ical reduction.16–19 Amongst these, wet-chemical methods have
received particular attention by virtue of their striking synthetic
simplicity and the fact that the need for complex and demanding
apparatus can be avoided. Simultaneously, such methods offer
the ability to modulate both product composition and mor-
phology flexibly by manipulating various synthetic parameters
in order to affect control over the reduction kinetics of the
cationic metal precursors in solution. That said, issues such as
the stabilization of particles against agglomeration, the achieve-
ment of monodisperse size distributions and oxidative stability
have presented considerable obstacles for chemical reduction
methods, especially in the cases of non-noble metal particles.20
Moreover, it has been recognised for some years that increased
surface energies in the nanodimensional size regime crucially
influence the kinetics not only of catalytic reactions but also of
surface oxidation processes. However, the precise prediction of
the interrelationship between particle morphology and surface
chemistry has, thus far, proved to be inherently difficult. Only
properties.21–23
To the best of our knowledge, few publications exist that address
the issue of oxidative stability in nanoparticulate copper systems.
This dearth of evidence originates from the inherent instability
of zerovalent Cu surfaces in the nanosize regime.24 In most
cases this leads to the formation of Cu(core),CuO/Cu2O(shell)
systems.20,25 Amongst the relevant studies that have appeared, most
are either restricted to the PXRD investigation of particle identity
(a technique not necessarily sensitive enough for the detection of
sub-nanometre oxide shells),26 or else they report the employment
of nanoskin capping systems. The latter can be considered to limit
substrate access considerably and therefore the particles’ action as
nanocatalysts, with an employed gelatine agent having been shown
to densely coat the particle surface.27–29
In the present study, we systematically extend the polyol-based
chemical reduction method that has been previously employed for
various metals,30 focussing on the influence of the chain length
of the steric capping agent poly(N-vinylpyrrolidone) (PVP), the
effect of varying the metal precursor on nanoparticle formation,
and the effect on the system of introducing a secondary reducing
agent. We subsequently outline routes towards morphological
control over the resulting copper nanoparticles and are able
to report conditions that allow the fabrication of particles
that display significant and long-term resistance to surface
oxidation.
Experimental details
Reagents and materials
Chemicals were purchased from Sigma-Aldrich (reagents: ana-
lytical grade; solvents: HPLC grade) and used without further
purification. All aqueous solutions were prepared using deionized
water (Millipore, specific resistivity ≥18.2 MX cm). Prior to
synthesis, flasks were flushed and all solutions were degassed with
argon.
aThe University Chemical Laboratories, Lensfield Road, Cambridge CB2
1EW, UK. E-mail: aehw2@cam.ac.uk; Fax: +44 1223 336362; Tel: +44
1223 763122
bLaboratory of Chemical Reactor Engineering, Eindhoven University of
Technology, P. O. Box 513, 5600 MB, Eindhoven, The Netherlands
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Particle syntheses
2000 analytical system. Spectra revealed substantial Ni Ka and
Kb emission lines, these arising from scattered electrons impinging
on the bars of the nickel support grid. Some spectra also showed
Fe Ka and Co Ka emission lines due to parasitic scattering from
the lens pole-piece, but a previous study31 had established that no
appreciable Cu Ka emission could be generated in this way if a
nickel support grid was used.
Cu nanoparticles were synthesized by dissolving 1 mmol of
Cu(II)X (X = OAc-, SO42-, Cl-) in 120 ml of anhydrous ethylene
glycol to which varying amounts of PVP (Mav = 10 000; 40 000;
55 000) had previously been added. The resulting mixture was
heated to 80 ◦C and stirred for two hours. The solution was
cooled to 0 ◦C and the pH adjusted to 9–11 (5 ml 1 M NaOH
solution). After the addition of either 0.40 ml (2 mmol) of a 25%
aqueous solution of hydrazine hydrate, 212.0 mg (2 mmol) of
sodium hypophosphite monohydrate in 0.40 ml water or 378.3 mg
(10 mmol) of sodium borohydride in 1 ml water, the reaction was
stirred for 1 h at 100 ◦C (N2H4·H2O, NaBH4, NaH2PO2·H2O) or
140 ◦C (NaH2PO2·H2O) to yield a colloidal suspension. Aliquots
of the product were purified prior to analysis by HRTEM,
EDX, PXRD and XPS by extracting 50 ml suspensions from
ethylene glycol and surplus polymer using excess acetone. After
sedimentation, ca. 90% of the supernatant was decanted and the
remaining suspension was centrifuged for 5 min. The acetone layer
was removed and the colloidal precipitate resuspended in 50 ml of
freshly distilled, anhydrous methanol.
Powder X-ray diffraction
Powder XRD data were collected on a Roentgen PW3040/60
XPert PRO powder X-ray diffractometer with a high resolution
˚
PW3373/00 Cu LFF (unmonochromated) tube at l = 1.5404 A
(Cu Ka). The powder sample was prepared by solvent evaporation
from the colloidal suspensions deposited on the 0.5 mm deep
ground area of a glass flatplate sample holder using a microscope
slide so that the powder sample was smooth, flat and flush with the
sample holder surface. The sample holder was inserted onto the
sample stage (PW3071/60 Bracket) such that the sample material
was just free of the reference plane of the sample stage.
X-Ray photoelectron spectroscopy
Transmission electron microscopy
XPS spectra were measured on a Kratos Axis Ultra-DLD XPS
system using a dual Al–Mg X-ray anode with a circular spatial
resolution of 15 mm.
Microscopic samples were prepared by droplet coating of
methanol suspensions on Ni grids (Agar Scientific, holey carbon
film, 300 mesh) and examined using a JEOL JEM-3011 high-
resolution transmission electron microscope at nominal magni-
fications between ¥10 000 and ¥800 000. The electron optical
parameters were CS = 0.6 mm, CC = 1.2 mm, electron energy
spread = 1.5 eV and beam divergence semi-angle = 1 mrad.
From these images, first indications of particle structure were
obtained. Particle size distributions were calculated by counting
the diameters of 100 particles in the lower magnification images,
defining size intervals of 0.2 nm between dmin ≤ d ≤ dmax and
counting the number of particles falling into these intervals. The
exact magnification was previously established using images of
lattice fringes in large (>10 nm) particles of colloidal gold.
Results and discussion
Synthetic conditions, resulting mean particle size and morphology
(measured from HRTEM images, Fig. 2) are shown in Table 1
and also in Fig. 1c, Fig. 1f, Fig. 1i and Fig. 1l. In the left-
hand and middle columns of Fig. 1, the corresponding particle
size distributions are shown as a function of the different
synthetic parameters. Consistent with the LaMer model of particle
formation,32 the relative strength of the reducing agent (Fig. 1j–
Fig. 1l) is a decisive parameter through which to affect control over
the dispersity of the particle size distribution. Initial tests, which
compare mean particle dimensions achieved with and without the
addition of NaH2PO2·H2O as a co-reductant show that although
ethylene glycol may act as a reducing agent by itself (sample
10), the classic polyol reaction did not lead to full reduction to
zerovalent Cu. Instead, even at temperatures of up to 160 ◦C,
mixtures of Cu(I) and Cu(II) oxides were obtained in the absence
Energy dispersive X-ray spectroscopy
Elemental compositions were qualitatively elucidated by energy
dispersive X-ray emission spectroscopy (EDX, nominal beam
width = 4 nm) using a PGT prism Si/Li detector and an Avalon
Table 1 Summary of synthetic parameters and results
Sample
Metal precursor
Mav (PVP)
Reductant
Particle mean size
Morphology
1
2
3
4
5
6
7
8
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
Cu(OAc)2
55 000
55 000
55 000
40 000
10 000
55 000
55 000
25 000
25 000
25 000
40 000b
40 000c
NaH2PO2·H2O
N2H4·H2O
NaBH4
NaBH4
NaBH4
N2H4·H2O
N2H4·H2O
NaH2PO2·H2Oa
NaH2PO2·H2O
—
66.3 8.6
13.7 1.8
22.8 2.6
18.9 2.1
5.0 0.5
Cubic
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical, cubic
Spherical
Cuboctahedral
34.3 16.2
21.2 4.2
46.2 13.4
34.0 22.2
55.9 45.7
9.6 1.0
CuCl
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
CuSO4·5H2O
9
10
11
12
NaH2PO2·H2O
d
NaH2PO2·H2O
—
a pH = 7 b PVP/metal = 12.6 c PVP/metal = 1.6 d Agglomerated
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Fig. 1 Particle size distributions (left column), Voigt curves of the frequency distributions (middle column) and the corresponding mean sizes (right
column) obtained for the use of (1) various PVP chain lengths ((a-c, samples 3–5); precursor: CuSO4·5H2O, reducing agent: NaBH4), (2) reducing agents
((d–f, samples 1–3); precursor: CuSO4·5H2O, capping agent: PVP-55), (3) metal precursors ((g–i, samples 2, 6–7); capping agent: PVP-55, reducing agent:
N2H4·H2O) and, (4) pH regimes ((j–l, samples 8–10); precursor: CuSO4·5H2O, capping agent: PVP-25, reducing agent (RA): NaH2PO2·H2O).
of the hypophosphite additive. Interestingly, if co-reductants were
omitted we observed the formation of particles that exhibit a wide
size distribution and have a mean diameter of around 60 nm
(sample 10, Fig. 2o). This observation points to a bimodal growth
process that is likely to result from there being a reduction rate that
is relatively low compared with that taking place in experiments
in which a secondary reducing agent is present (cf. Fig. 2m–
Fig. 2p). As a result, the growth of seeds formed in the initial
nucleation stage may be envisaged as taking place while new
particles simultaneously form. A similar logic can be used to
explain the widening of the size distribution and the fact that
in sample 10 (as in sample 1, see below), the formation of a cubic
morphology was preferred over the spherical shapes found in other
syntheses. Hence, this cubic morphology of copper represents
a thermodynamically favoured morphology, whereby for face-
centered cubic (fcc) metals, truncated nanocubes, cubooctahedra
and multiple twinned particles are known to have the lowest free
energy.33 It also underlines the basic potential of the chemical
reduction approach to yield crystallographic control by enabling
both nucleation and growth to take place within the kinetic regime
through appropriate modification of the synthetic parameters rele-
vant to these stages of particle formation. Interestingly, variations
in the molar weights (chain length) of PVP (samples 3–5; Fig. 1a–
Fig. 1c) significantly effect resulting particle size. Notably, this
variable appears to influence the synthesis of zerovalent Cu much
more profoundly than has been observed in comparable particle
syntheses involving at least one noble metal.34 For monometallic
palladium particles, for example, mean sizes have been found to
vary between 3.0 nm (Mav (PVP) = 10 000)and3.3 nm (Mav (PVP) =
55 000).35 In this work we report size variations of between 5.0
0.5 nm (Mav = 10 000) and 22.8 2.6 nm (Mav = 55 000), there
being a correlating increase in standard deviations.
Regarding variations in metal substrate, it can be seen that upon
using either the acetate, chloride or sulfate precursors, a gradual
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Fig. 2 Selected HRTEM images of samples 1–12. (a,b): 1, (c,d): 2, (e): 3, (f,g): 4, (h,i): 5, (j): 6, (k,l): 7, (m): 8, (n): 9, (o,p): 10, (q,r): 11, (s,t): 12.
decrease in mean particle size from approximately 34 to 13 nm
is observable (Fig. 1g–Fig. 1i), there being a concomitant decline
in standard deviation ( 16.2, 4.2, 1.8). Similar influences of
the metal counterion have previously been noted in experiments
with reverse micelles,36 with counterions having been argued to
influence the solute free energies of the colloids as the result of
adsorption at the solvent–solute interface. We therefore consider
that the influence of the counterion on the conformational
arrangements exhibited by the PVP capping agent is likely to be
responsible for the observed size variations.
Lastly, the influence of the choice of reducing agent (Fig. 1d–1f)
has been tested. Whereas hypophosphite reduction led to signif-
icantly larger particle sizes accompanied by a relative maximum
in standard deviation indicating slower reduction kinetics, data
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reveal that, under like conditions, hydrazine hydrate gives the
best results in terms of both mean particle size and breadth
of distribution. This result is surprising as the higher reaction
rate associated with the stronger reductant, sodium borohydride,
would normally be expected to have a decisive effect on particle
fabrication at the nucleation stage. However, the documented
ability of hydrazine to coordinate to low-indexed copper surfaces
during the growth stage may inhibit growth and so be responsible
for this effect.37
Oxidative stability
When CuSO4·5H2O was employed in conjunction with
NaH2PO2·H2O and PVP (Mav 40 000), it proved possible to
form a nano-sol that, even after three months, showed no
traces of particle surface oxidation. This was initially revealed
by PXRD measurements (Fig. 3), with confirmation coming
from XPS analysis (Fig. 5). Hence, PXRD measurements on
samples, such as 2 and 4, that were prepared using N2H4·H2O
and NaBH4, respectively, revealed significant levels of CuO,
with contamination (including by CuS and Cu2SO4) also
suggested (Fig. 3a, Fig. 3b). In contrast, the deployment of
sodium hypophosphite as a co-reducing additive was successful,
with PXRD analysis of sample 11 (Fig. 3c) revealing both an
excellent level of purity and an fcc copper phase. The unchanging
PXRD signature (over a period of 3 months) could also be used to
demonstrate the long term stability of the colloids prepared by this
last route. In a similar vein, XPS analysis revealed that attempts to
reduce CuSO4·5H2O using either NaBH4 or N2H4 yielded impure
nanomaterials that contain traces of Cu(I). These likely originate
Fig. 4 EDX spectra of samples 1–12. X-Ray emission energies [keV]: C
Ka1: 0.277, O Ka1: 0.525, Cu La1: 0.930, P Ka1: 2.015, Ti Ka1: 4.510,
Ti Kb1: 4.931, Ni Ka1: 7.471, Cu Ka1: 8.048, Ni Kb1: 8.263, Cu Kb1:
8.905. Ti and Ni from grid material. Fe Ka and Co Ka lines from the lens
pole-piece are seen in spectra 1–3 to the left of the Ni Ka line.
from the particle surface (2p3/2 = 933.7 eV; Fig. 5a, Fig. 5b).38
In contrast, reduction of copper substrate by NaH2PO2·H2O
allowed the observation of symmetric peaks in sample 11 (Cu
2p1/2 at 952.2 eV and 2p3/2 at 932.0 eV; Fig. 5c), which is consistent
with colloids prepared from the sulfate using hypophosphite
co-reductant and PVP (Mav 40 000) comprising Cu(0) at the
surface and demonstrating significant oxidative stability.
During sample synthesis the rapid formation of black colloids
was observed when the stronger reducing agents hydrazine or
sodium borohydride were deployed. In contrast, the use of
hypophosphite reductant resulted in the formation of an initially
dark-blue solution of Cu(OH)2 that turned yellow at around
100 ◦C, this colour change probably originating from the for-
mation of intermediate Cu–PVP complexes.39 Subsequently, a
green colloid developed and, at 140 ◦C, the formation of a dark-
red nano-sol resulted. We believe that the comparatively slow
reduction kinetics associated with the use of hypophosphite (in
place of hydrazine or borohydride) allows the initial formation
of copper oxide intermediates through reoxidation by water, the
formation of which species should not be observed if stronger
reductants are applied. These oxides might play a role in the
subsequent formation of a surface crystallographic structure that
is energetically unfavourable for particle oxidation after further
reduction by hypophosphite (known to react to give dihydrogen
phosphite, see Fig. 640). Additionally, although it is clearly not the
only factor, because samples prepared using each of the three PVP
chain lengths tested here showed the formation of oxide shells, the
steric density with which PVP covers the particle surface can be
Fig. 3 PXRD spectra of samples 4 (a), 2 (b) and 11 (c; 3 months after
preparation).
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Fig. 5 XPS spectra of samples 4 (a), 2 (b) and 11 (c); (insets: Cu 2p1/2
(a–c) and 2p3/2 (c) peaks with envelope curves (Cu(0) red, Cu(I) green)).
considered to inhibit surface oxidation processes.27 Interestingly,
EDX spectra (Fig. 4) reveal traces of phosphorus in samples 9, 11
(minute) and 12, but not in the other syntheses. While this could
be because the dihydrogen phosphite plays a role as an additional
stabilizing ligand, it is more likely that it is attributable to residues
that remain from imperfect washing of the specimen.
Fig. 6 Proposed particle formation mechanism.
to the formation of larger and cuboctahedral particles. This, of
course, indicates the role the polymer plays in retarding overall
particle growth. The small spherical particles obtained in most of
the specimens given in Table 1 can be regarded as approximately
cubeoctahedral, displaying both (111) and (100) facets, but where
the average particle size is greater, as in samples 1 and 10, there
is a definite tendency to adopt a cubic morphology. This could
indicate a preferential co-ordination of the polymer to the (100)
face of the fcc metal, leading to accelerated growth on the (111)
facets and their consequent disappearance.
Previous investigations have found the oxidative stability of
copper to depend on the surface stress exhibited by (111)
lattice planes,41 suggesting that increased compression may have
an inhibitory effect on oxidation processes. Preliminary fringe
measurements from HRTEM images indicate considerable lattice
The data suggests that a powerful element of the presently
developed synthesis is that it incorporates multiple steps, each
of which can be modulated in order to further manipulate particle
formation. (e.g., injection rate of reducing agent or metal precursor
solution, injection sequence and pH value). In contrast, previously
reported syntheses have used the stepwise injection of metal pre-
cursor without the addition of NaOH.37 In such cases, cation con-
centrations are likely to remain below the critical supersaturation
level required during the growth step, with polydisperse particle
distributions necessarily resulting. The present addition of NaOH
should provide a metal ion concentration that is supercritical until
the precursor is fully depleted. In a similar vein, previous reports
have revealed oxide coverage of synthesised Cu nanoparticles.
Clearly, from a mechanistic point of view (cf . Fig. 6 and samples 8
and 9), the use of hypophosphite offers the advantage that reduc-
tion kinetics are straightforwardly controlled through the adjust-
ment of pH. Thus, in applying an alkaline medium it is intended
that the reaction of the copper cations with hypophosphite is accel-
erated. Overall, our data clearly reveal that the employment of an
intrinsically alkaline medium (based on the choice of co-reductant)
can accelerate the reaction and decrease particle mean size.
A comparison of samples 11 and 12 (Table 1 and Fig. 2), syn-
thesized using PVP : metal mass ratios of 12.6 and 1.6 respectively,
reveals that a drastic decrease in the polymer : metal ratio leads
˚
compression as compared to the corresponding value of 3.61 A
for bulk copper.42 However, (111) spacings appeared to be lower
in sample 12 than in sample 11. In comparing samples 11 and 12,
therefore, we can see that at least part of the remarkable stability
of the former must arise from the additional protective effect of
PVP, likely through electron donation from N,O lone pairs into
Cu sp hybrid orbitals.27
Conclusions
In conclusion, we have demonstrated that it is possible to employ
the reagents hydrazine hydrate and sodium borohydride along with
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less frequently used sodium hypophosphite to synthesize copper
nanoparticles with different morphologies and mean particle
sizes and distributions through the employment of a chemical
reduction method. Previous reports of zerovalent nanocopper
preparation have either failed to show long-term surface stability
or have required the introduction of particles likely to result in
diffusion limitations in catalytic processes. However, in this work
we attempt colloid synthesis in alcohol treated with various co-
reducing agents. While syntheses using N2H4·H2O and NaBH4
meet with only limited success, we are able to demonstrate that by
deploying NaH2PO2·H2O in the presence of PVP (Mav 40 000) it
is possible to achieve pure Cu nanoparticles that demonstrate a
mean size distribution of 9.6 1.0 nm and which exhibit stability
(according to PXRD and XPS studies) towards oxidation for
several months (sample 11 in Table 1). These data promise to be
significant to the future development of novel catalysts for a variety
of applications. Hence, for example, PVP-coated Cu(0) particles
prepared using hypophosphite demonstrate potential as catalysts
in the Ullmann-type coupling of 4-chloropyridine with phenol
to yield 4-phenoxypyridine, with excellent results having been
achieved in terms of product yield and selectivity and, importantly,
catalyst stability.43 Accordingly, nanoparticles prepared as per
sample 11 could be reused without a significant decline in
activity over at least five catalytic cycles, before, it is assumed,
they eventually underwent surface oxide formation. XPS studies
which aim to verify this view by clarifying how the nanocatalyst
evolves during testing are presently being initiated and will be
reported in due course. Further refinements to the etherification
reactions to which oxidatively stable nanoscopic copper have been
applied are envisaged, aimed at the demonstration of continuous
flow processes. Moreover, we are seeking to develop the use
of oxidatively stable nanocopper as catalysts in selective C–H
bond activation processes (e.g., the Michael addition of active
methylenes or terminal alkyne activation for selective addition
to electron-poor alkenes). In conjunction with DFT and Monte
Carlo modelling of copper-support interactions, long-term studies
will target the heterogenization of stable copper catalysts and
copper-based bimetallic colloids.
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Acknowledgements
We thank the European Commission (NOE EXCELL NMP3-
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Project 2008/R4) for financial support. We also acknowledge
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