Materials Sciences and Applicatio ns, 2011, 2, 258-264
doi:10.4236/msa.2011.24033 Published Online April 2011 (
Copyright © 2011 SciRes. MSA
NiOx Nanoparticle Synthesis by Chemical Vapor
Deposition from Nickel Acetylacetonate
Pavel Moravec1, Jiří Smolík1, Helmi Keskinen2,3, Jyrki M. Mäkelä2, Snejana Bakardjieva4,
Valeri V. Levdansky5
1Institute of Chemical Process Fundamentals AS CR, Prague, Czech Republic; 2Department of Physics, Tampere University of
Technology, Tampere, Finland; 3Department of Physics and Mathematics, University of Eastern Finland, Kuopio, Finland; 4Institute
of Inorganic Chemistry AS CR, Husinec-Řež, Czech Republic; 5A. V. Luikov Heat and Mass Transfer Institute, National Academy
of Sciences of Belarus, Minsk, Belarus
Received November 4th, 2010; revised January 17th, 2011; accepted March 9th, 2011.
Ni/NiO nanoparticles were synthesized by metal organics chemical vapor deposition of nickel acetylacetonate in an
externally heated tube flow reactor at moderate temperatures, up to 500˚C. Particle production and characteristics
were studied by evaluating the effects of reactor temp erature, precursor concentratio n, and flow rate through the reac-
tor. In addition, two precursor decomposition methods were examined: thermal decomposition and reduction by hy-
drogen. Particle production was monitored with a scanning mobility particle sizer, and particle characteristics were
studied using transmission electron microscopy, high resolution transmission electron microscopy, selected area elec-
tron diffraction, and energy dispersive spectroscopy. The presence of hydrogen in the reaction mixture influenced sig-
nificantly both particle production and their characteristics.
Keywords: Nickel Nanostructures, MOCVD, Hot Wall Tube Reactor, Electron Diffraction
1. Introduction
Nickel and nickel oxide nanoparticles show many unique
optical, magnetic, electrical and chemical properties [1,2],
and thus, these nanoparticles have great potential in ap-
plications such as ceramic materials, electronic compo-
nents, sensors, magnetic data storage materials and cata-
lysts [3,4]. However, most physical and chemical proper-
ties depend on the size and shape of the nanoparticles [5].
Moreover, nickel nanoparticles are unstable in air, and
their surface can oxidize to NiO even at room tempera-
ture [6].
In recent years, several methods of Ni/NiO nanopartic-
le synthesis have been reported and, of course, each dif-
ferent method of synthesis imparts the final product with
different properties. Liquid phase synthesis is one subset
of methods for synthesizing Ni/NiO [7-9], but wet chem-
istry methods can be quite expensive, particularly in the
steps that involve solid-liquid separation, washing and
drying [10].
Ni/NiO nanoparticles also can be synthesized in the
gas phase, and several variants of this method have been
used for the preparation of nickel or nickel oxide nano-
particles. In one case, Ni/NiO particles were synthesized
by spray pyrolysis of water solutions of nickel precursors
(e.g., nitrate, chloride, formate or acetate) but the result-
ing particles typically were in the submicron size range
[11] or were polydisperse mixtures of nanosized particles,
submicron-sized particles and, under some conditions,
even supermicron-sized particles [12]. However, Leng-
goro et al. [3] developed conditions for synthesizing NiO
nanoparticles with controlled morphology by using
low-pressure spray pyrolysis with a filter expansion aero-
sol generator [3]. Nickel nanoparticles also have been
synthesized by a DC sputtering process in an argon at-
mosphere [13] or by a laser-assisted photonucleation
process [1]. Suh et al. synthesized nickel nanoparticles in
a tubular furnace reactor reducing NiCl2 with hydrogen
(present in a carrier gas) [2]. Another method for prepar-
ing Ni/NiO nanostructures is chemical vapor deposition
(CVD) of metal organic precursors (MOCVD). MOCVD
has a number of advantages over other methods: the
process is relatively simple, it uses inexpensive equip-
ment, and particle formation can be controlled by a vari-
ety of process parameters like reactor temperature, pre-
NiO Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate 259
cursor concentration, or residence time in the reactor. An
overview of Ni precursors used for CVD was presented
by Brissonneau and Vahlas [14]. Promising Ni precur-
sors for MOCVD include nickelocene [15] or nickel
acetylacetonate [16].
In this work, we studied Ni/NiO nanoparticle synthesis
in an externally heated tube flow reactor at moderate
temperatures using nickel acetylacetonate (NiAA) as a
precursor and two different methods of precursor de-
compositions. The goals of this study were to identify an
experimental set-up suitable for the production of nickel
or nickel oxide particles with valuable properties for pos-
sible applications, and to investigate the influence of
process parameters on the rate of particle production and
particle characteristics.
2. Experimental
Particle production was studied by two different methods
of NiAA decomposition processes (Figure 1) [17]:
1) thermal decomposition in an inert atmosphere (py-
Ni(C5H7O2)2 Ni + 2C5H7O2, (1)
2) reduction with hydrogen (present in the carrier gas)
Ni(C5H7O2)2 + H2 Ni + 2HC5H7O2. (2)
Experiments were performed using the apparatus
shown in Figure 2. Particles were synthesized in an ex-
ternally heated tube flow reactor 55 cm in length and an
inner diameter (i.d.) of 2.7 cm. The inlet nozzle that in-
troduced the reaction mixture was 15 cm in length and
had an i.d. of 1.5 cm. Deoxygenated (1), dry (2) and par-
ticle-free nitrogen (F) was used as the carrier gas and was
saturated with NiAA vapor in an externally heated satu-
rator (S). The precursor concentration was controlled
both by the flow rate through the saturator and by the
saturator temperature (TS). The partial pressure of the
precursor vapor was calculated on the basis of the ex-
perimental data of Götze et al. [18] from the equation
10.01316 K
NiAA Pa133.322 10
 T
Saturated carrier gas was fed axially through a nozzle
into the center of the reactor, which was surrounded by a
coaxial stream of nitrogen. In reductive decomposition
experiments, the stream was composed of a mixture of
nitrogen and hydrogen. The mixture of gas and particles
leaving the reactor was cooled in a diluter (D) by mixing
it with a stream of nitrogen gas. The saturator-reactor-
diluter system was arranged vertically in this work. Flow
rates of individual gas streams were controlled with elec-
tronic mass flow meters (Tesla 306 KA/RA), and the
temperatures of the saturator and reactor were controlled
with electronic temperature controllers (RLC T48).
Figure 1. Scheme of the inlet arrangements for used de-
composition techniques of NiAA precursor.
Particle production was monitored with a scanning
mobility particle sizer (SMPS), which consisted of a TSI
model 3080 electrostatic classifier (EC) and a TSI model
3025 condensation particle counter (CPC). Samples for
particle characterization were deposited onto carbon-co-
ated Cu grids by point-to-plate electrostatic precipitator
or onto polytetrafluoroethylene (PTFE) or Ag filters.
Samples for transmission electron microscopy (TEM)
from particles deposited on filters were prepared by dis-
persion in an organic solvent. The resulting suspension
was dropped onto a carbon-coated Cu grid. The mor-
phology of particles was studied using two TEM instru-
ments with different resolutions. Images from the JEOL
2010 were appropriate for characterizing overall mor-
phology; images from the high resolution TEM (HR-
Figure 2. Scheme of the apparatus. (1) Deoxygenator, (2)
Dryer, (CPC) condensation particle counter, (D) Diluter,
(EC) Electrostatic classifier, (F) Filter, (M) Electronic mass
flowmeter, (P) Pressure reducing valve, (S) Saturator, (SF)
STERLITECH Ag filter or electrostatic precipitator, (VP)
Vacuum pump.
opyright © 2011 SciRes. MSA
NiO Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate
260 x
TEM) JEOL 3010 could also distinguish details on the
particle surface. The crystallinity of the particles was
studied by selected area electron diffraction (SAED) and
by analysis of lattice fringes detected on particles. The
composition of the particles was analyzed by energy dis-
persive spectroscopy (EDS; Noran Vantage connected to
JEOL 2010 and/or INCA/Oxford connected to JEOL
Particle production and particle characteristics were
also studied by varying reactor temperature (TR) (400˚C -
500˚C; the upper limit is set by the construction material
of the reactor and furnace), reactor flow rate (QR, 400 -
1000 cm3/min), hydrogen concentration (cH, 0 vol% - 10
vol%), saturator temperature (TS, 150˚C - 190˚C), and
central nozzle flow rate (QCF, 10% - 20% of QR). It
should be noted that QCF and TS control the precursor
concentration (PNiAA). Axial temperature profile in the
reactor for TR set to 500˚C and QR = 0 is shown in Fig-
ure 3. Experimental conditions for sample preparation
are summarized in Table 1.
3. Results
3.1. Particle Production
Particle generation was monitored by SMPS in the form
of particle size distribution (PSD) curves, including the
following statistics: total number concentration (Nt),
geometric mean diameter (GMD), geometric standard
deviation (GSD), etc. Several examples of PSD curves
are shown in Figure 4 and Figure 5. dN/dlogdP repre-
sents differential number concentration, normalized to
one decade of particle size. The influence of reactor
temperature on particle production was studied for re-
ductive decomposition of NiAA. Particle generation was
observed at temperatures of 400˚C and higher. No parti-
cles were detected at TR = 300˚C. For thermal decompo-
sition, experiments were performed only at TR = 500˚C.
Figure 3. Axial temperature profile in the axis of the reactor
for TR set to 500˚C.
Table 1. Process parameters of the samples for particle
[vol%] Rem.
NiAA4 500180900 2.9 0 PYROLYSIS
NiAA7 400180800 2.9 10 REDUCTION
NiAA8 500180800 2.9 7 REDUCTION
NiAA9 500180600 2.9 7 REDUCTION
NiAA10500180800 2.9 10 REDUCTION
NiAAF1500150800 0.48 0 PYROLYSIS
NiAAF2500180800 2.9 0 PYROLYSIS
Figure 4. Influence of TS on PSD at TR = 500˚C, QR = 800
cm3/min, QCF = 20% QR, pyrolysis.
Generally, particle production (mean particle size and
number concentration) increases with increasing satura-
tor temperature (TS) (Figure 4). The particle concentra-
tion increased from 4.92 × 106 cm–3 at TS = 150˚C to 2.85
× 107 cm–3 at 180˚C. Values of GMD at these TS values
were 32 and 156 nm, respectively, and the GSD values
were 1.37 and 1.52, respectively. Figure 5 shows the
Figure 5. Particle size distributions corresponding to TEM
samples NiAA4, NiAA7 and NiAA8. Experimental condi-
tions are shown in Table 1.
opyright © 2011 SciRes. MSA
NiO Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate 261
influences of the precursor decomposition method and
the reactor temperature; the remaining conditions in these
experiments were nearly identical (Table 1). It is evident
that the rate of particle formation by reduction of NiAA
at 400˚C (NiAA7) is much lower (Nt = 7.48 × 106 cm3,
GMD = 26 nm, GSD = 1.38) than that at 500˚C (NiAA8,
Nt = 2.96 × 107 cm3, GMD = 59 nm, GSD = 1.58). At
500˚C, the mean particle size for reductive precursor
decomposition (NiAA8) was smaller than those for
thermal decomposition (NiAA4, Nt = 2.58 × 107 cm3,
GMD = 185 nm, GSD = 1.51) (Figure 5). The other pa-
rameters in Table 1, i.e. reactor flow rate and/or hydro-
gen concentration variation from 7 to 10 vol%, had not
remarkable effect on particle production.
3.2. Particle Characteristics
3.2.1. Pyrolysis
The morphology of particles prepared by pyrolysis of
NiAA can be seen in Figure 6. The particles have
shell-like structures, the mean size of the primary parti-
cles is well below 50 nm and they are arranged into clus-
ters and/or chains. The samples (NiAAF1, NiAAF2) de-
posited on PTFE filter are dark in color, suggesting that
they may be contaminated by carbon from incomplete
decomposition of the precursor. Formation of carbon-
coated Ni nanoparticles by thermal decomposition of
solid nickel acetate in a closed Swagelok reactor was
observed by Pol et al. [19]. Electron diffraction pattern
Figure 6. Bright field TEM image and SAED pattern of
particles prepared by pyrolysis of NiAA, sample NiAA4: TR
= 500˚C, TS = 180˚C, QR = 900 cm3/min.
(EDP) in Figure 6 is due to small particle size rather
weak. Nevertheless, there are two recognizable rings in
the EDP that correspond to interplanar spacings of 0.205
nm and 0.189 nm, respectively. There are also less-visi-
ble dots that correspond to spacings of 0.176 nm, 0.123
nm and 0.106 nm. These values may represent d111 =
0.203 nm, d200 = 0.176 nm, d220 = 0.125 nm, d311 = 0.106
nm of face centred cubic (FCC) Ni (PDF ICDD 4-0850).
The spacing 0.205 nm is also consistent with d200 = 0.208
nm FCC NiO (4-0835) and 0.123 nm is consistent with
d311 = 0.126 or d222 = 0.121 nm of FCC NiO, but there
are missing rings or dots corresponding to d111 and d220
NiO. The spacing 0.189 nm does not fit with any inter-
planar spacing of either FCC Ni or FCC NiO. HRTEM
images enabled a more detailed analysis of particle mor-
phology, crystallinity and, therefore, particle composition.
Figure 7 shows HRTEM images of particles of sample
NiAAF1. We can see lattice fringes d111 of FCC Ni
(4-0850), see area A and corresponding fast Fourier
transformation (FFT) pattern in Figure 7(a) and d105 of
hexagonal NiOOH (6-0075) (see area B and correspond-
ing FFT in Figure 7(a). The crystalline structure of hex-
agonal NiOOH (6-0075) and FCC NiO (22-1189) can be
seen in Figure 7(b). As inset, there is FFT pattern of
cubic NiO crystalline structure along [–300]c zone axis.
EDS analysis (INCA Oxford) was applied to the samples
NiAAF1 and NiAAF2, deposited on filters. The diameter
of the spot area varied from ~5 nm to ~30 nm, and O/Ni
(atomic%) ratios of 0.38 to 1.86 were obtained. Because
the sensitivity of EDS to individual elements of the peri-
odic table depends on the atomic number, the values of
the O/Ni ratio do not reflect stoichiometry. Furthermore,
the O/Ni ratio could be affected (increased) during the
preparation of TEM samples from the particles deposited
on filters.
3.2.2. Reduction of NiAA
The morphology of particles obtained by reduction dif-
fers significantly from those produced by thermal de-
composition and depends also on the reactor temperature.
At TR = 400˚C, primary particles 10 - 15 nm in size were
agglomerated into clusters and/or chains, and they had
Figure 7. HRTEM images and FFT patterns of particles
prepared by pyrolysis of NiAA, sample NiAAF1: TR =
500˚C, TS = 150˚C, QR = 800 cm3/min.
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NiO Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate
262 x
also faintly shell-like structures (Figure 8). Particles
produced at 500˚C had slightly broader size distribution
(from 10 nm to ca. 50 nm) and samples consisted of
separated nanoparticles of various sizes from the afore-
mentioned range as well as from clusters of small
nanoparticles (~10 nm) (Figure 9). The shell-like struc-
ture almost disappeared. Particles produced at 400˚C
were amorphous, while those synthesized at 500˚C were
crystalline. Two EDPs taken by the JEOL 2010 are
shown in Figure 9. The EDP of the cluster of nanoparti-
cles is visible on the left-hand side of Figure 9. The in-
nermost ring corresponds to d111 of FCC NiO (0.241 nm),
the third to d220 FCC NiO (0.145 nm), while the middle
resolvable ring (0.206 nm) can be attributed both to d111
FCC Ni and to d200 FCC NiO, so it is probable that both
crystalline structures are present in the sample. The EDP
of a single crystal of FCC Ni from approximately 50 nm
nanoparticle is apparent on the right-hand side of Figure
Results obtained by SAED are in qualitative agree-
ment with those obtained by EDS. EDS analyses showed
an oxygen to nickel ratio (atom%) of 3.0 in the cluster of
nanoparticles from Figure 9(a), while the ratio was only
0.23 in the large particle of Figure 9(b). The value of
O/Ni ratio in the sample NiAA7 (400˚C) was 0.89.
Several samples of particles prepared by reduction
were also analyzed using HRTEM, which enabled detec-
tion of the lattice fringes of several of Ni/NiOx crystalline
structures. Figures 10(a-c) show the lattice fringes of
cubic NiO (4-0835) with an interplanar spacing of d200 =
0.208 nm, as well as lattice fringes of cubic Ni (4-0850)
with an interplanar distance of d111 = 0.203 nm and the
hexagonal lattice structure of NiOOH (6-0075), con-
firmed by FFT pattern along the [006]c zone axis. A de-
tailed analysis of the crystalline particle of the sample
NiAA10 (Figure 10(d)) shows FCC Ni (4-0850) in the
bulk and a NiO/NiOOH crystalline structures on the sur-
face. As inset, there is a FFT pattern of a typical cubic Ni
along the [02-2] axis.
Comparing lattice fringe images and SAED patterns,
we can conclude that the particles consist of a metallic Ni
core and a thin surface layer composed of NiO/NiOOH
crystalline structures. These observations are in agree-
ment with those of Uchikoshi et al. [20]. Ni core and
NiO shell nanoparticles produced by DC sputtering were
reported by Rellinghaus et al. [13].
4. Discussion
From a comparison of TEM images with PSD curves, it
is evident that SMPS detected already agglomerated pri-
mary particles. The size of primary particles on TEM
images is much smaller than indicated by the PSD curves
in Figure 5. In spite of this, SMPS still provides useful
Figure 8. Brigth field TEM image of particles produced by
reduction of NiAA, sample NiAA7: TR = 400˚C, TS = 180˚C,
QR = 800 cm3/min, cH = 10 vol%.
information about the particle formation process. The
curves in Figure 5 are in good qualitative agreement
with the particle morphology shown in the TEM images.
Particles synthesized by reduction at 400˚C (Figure 8)
are the smallest, and their clusters are much smaller than
those of the other samples (PSD curve NiAA7 in Figure
Figure 9. Bright field TEM images and SAED patterns of
particles produced by reduction of NiAA, sample NiAA8:
TR = 500˚C, TS = 180˚C, QR = 800 cm3/min, cH = 7 vol%.
opyright © 2011 SciRes. MSA
NiO Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate 263
Figure 10. HRTEM images and FFT patterns of particles
prepared by reduction of NiAA, a-c; sample NiAA9: TR =
500˚C, TS = 180˚C, QR = 600 cm3/min, cH = 7 vol%, d;
sample NiAA10: TR = 500˚C, TS = 180˚C, QR = 800 cm3/min,
cH = 10 vol%.
5). And particles prepared by reduction of NiAA at
500˚C (Figure 9) have slightly broader size distribution
than those prepared by pyrolysis (Figure 6), but they are
less aggregated, which corresponds to the much smaller
mean particle size indicated by PSD curve NiAA8 in
Figure 5.
The shell-like structure of particles seen in samples
NiAA4 (Figure 6), and somewhat in NiAA7 (Figure 8)
seems to be formed from incomplete decomposition of
the precursor by pyrolysis TR = 500˚C, and also by re-
duction at TR = 400˚C, respectively. This incomplete
decomposition is suggested by the fact that deposits of
particles from pyrolysis on the PTFE filter were dark in
color, while those from reduction at 500˚C were silver in
color. Furthermore, it seems that the shell layer also pro-
tects particles against oxidation. The O/Ni ratio was
lower in the samples of particles with shell-like struc-
tures: 0.38 - 1.86 in sample NiAA4 (pyrolysis) and 0.89
in sample NiAA7 (reduction, 400˚C), while in the parti-
cles without a shell (NiAA8; reduction, 500˚C), it was
3.0. However, the O/Ni ratio also depends on particle
size. The lowest value of the O/Ni ratio (0.23) was de-
tected in spot analysis of the large particle in sample
NiAA8, and this low value can be attributed to the small
surface-to-volume ratio of the relatively large particle.
5. Conclusions
Ni/NiOx nanoparticles were synthesized by pyrolysis and
reduction of NiAA in an externally heated tube flow re-
actor. Particle production was observed at TR = 400˚C
and above, and depends mainly on TR, PNiAA and the type
of precursor decomposition method. The size range of
primary particles varied from ~10 nm to ~50 nm, and
they typically were somewhat agglomerated. Particles
prepared by pyrolysis had shell-like structure, and were
aggregated into clusters and/or branched chains. EDP
consists predominantly of rings or dots characteristic of
FCC Ni, while HRTEM lattice images showed lattice
fringes of both FCC Ni and NiO, and also of NiOOH.
Results obtained by SAED are in qualitative agreement
with those obtained by EDS (O to Ni ratio).
Particles produced by reduction differ significantly in
morphology, crystallinity and composition. These parti-
cles had slightly broader size distribution, they were less
agglomerated and the shell-like structure was almost
nonexistent. EDPs vary with particle size. SAED, EDS
and HRTEM lattice fringes images suggest that particles
consist of a metallic core with a surface layer composed
of various oxide forms of Ni (NiO, NiOOH).
6. Acknowledgements
This work was supported by the Grant Agency of the
Czech Republic No. 104/07/1093 and by the Finnish
Academy of Sciences and Letters. Some TEM/EDS
analyses were performed by Tomi Kanerva, Institute of
Material Science, Tampere University of Technology.
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