Synthesis and characterization of monodisperse zinc and zinc oxide nanoparticles from the organometallic precursor [Zn(C6H11)2]

Article abstract of DOI:10.1016/S0022-328X(01)01378-X

Decomposition of the organometallic precursor [Zn(C₆H₁₁)₂] in wet anisole leads to the formation of monodisperse spherical Zn particles of 6 nm mean diameter. High resolution electron microscopy (HRTEM) indicates the crystalline nature of these particles and photoelectron studies (XPS) are consistent with the presence of both zinc and zinc oxide. In the presence of polyvinylpyrolidone (PVP), the decomposition leads to well dispersed nanoparticles for which HRTEM studies evidenced the presence of hexagonal zinc (0) surrounded by a thin layer of hexagonal zinc oxide of wurtzite type. The thermal oxidation of these zinc nanoparticles yields well-crystallized nanoparticles of ZnO without coalescence or size change. An X-ray diffraction pattern shows that the powder consists of pure hexagonal wurtzite-type phase.

Full text of DOI:10.1016/S0022-328X(01)01378-X

Journal of Organometallic Chemistry 643–644 (2002) 307–312
www.elsevier.com/locate/jorganchem
Synthesis and characterization of monodisperse zinc and zinc
oxide nanoparticles from the organometallic precursor [Zn(C6H11)2]
Franck Rataboul a, Ce´line Nayral a, Marie-Jose´ Casanove b, Andre´ Maisonnat a,*,
Bruno Chaudret a,*
a Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 route de Narbonne, F-31077 Toulouse, Cedex 04, France
b Centre d’Elaboration des Mate´riaux et d’Etudes structurales du CNRS, UPR 8011, 29, rue Jeanne Mar6ig BP 4347, F-31055 Toulouse,
Cedex 04, France
Received 13 July 2001; accepted 9 October 2001
Dedicated to Franc¸ois Mathey on his 60th birthday in recognition of his pioneering work in main group and organometallic chemistry
Abstract
Decomposition of the organometallic precursor [Zn(C6H11)2] in wet anisole leads to the formation of monodisperse spherical Zn
particles of 6 nm mean diameter. High resolution electron microscopy (HRTEM) indicates the crystalline nature of these particles
and photoelectron studies (XPS) are consistent with the presence of both zinc and zinc oxide. In the presence of polyvinylpy-
rolidone (PVP), the decomposition leads to well dispersed nanoparticles for which HRTEM studies evidenced the presence of
hexagonal zinc (0) surrounded by a thin layer of hexagonal zinc oxide of wurtzite type. The thermal oxidation of these zinc
nanoparticles yields well-crystallized nanoparticles of ZnO without coalescence or size change. An X-ray diffraction pattern shows
that the powder consists of pure hexagonal wurtzite-type phase. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Zinc; Zinc oxide; Nanoparticles; Organometallic precursor
ZnO-based varistors decreases linearly as the average
size of the ZnO grains decreases [4]. Similarly, the
sensitivity of SnO2-based gas sensors is expected to
increase as the average size of the individual grains
decreases [5–7].
1. Introduction
The precise control of the morphology of semi-con-
ducting oxide materials at the nanometric scale is of
basic importance for a fine-tuning of their physical
properties such as electrical conductivity or magnetic,
optical and mechanical characteristics, and for the wide
range of applications in which these properties are
implemented, e.g. gas sensors, varistors, transducers,
photocells, UV absorbing materials or pigment
additives.
For example, the optical properties of nano-sized
luminescent semi-conducting particles are known to be
substantially different from those of bulk materials
[1–3]. The electrical properties of polycrystalline metal-
oxide materials depend strongly on their morphology,
and it has been reported that the nominal voltage of
Concerning zinc oxide itself, its industrial applica-
tions are of particular importance and the gas sensing
properties [8–11] as well as the optical properties
[1,2,12,13] and varistor applications [14–17] are sub-
jects of extensive studies. In these applications, the
sol–gel route yielding zinc oxide colloids is widely used.
This leads generally to polycrystalline materials that
require subsequent thermal treatments in order to im-
prove their stability and the reproducibility of their
properties. However, thermal treatments lead to struc-
tural modifications (increase of the mean grain size,
spreading of the grain size distribution, etc.) with in-
creasing annealing temperature. Synthesis of zinc parti-
cles in a first step followed by their oxidation can be
viewed as an alternative process to prepare ZnO parti-
cles displaying a low size dispersion. There are, how-
ever, few reports dealing with the synthesis of
* Corresponding authors. Tel.: +33-561-333179; fax: +33-561-
553003 (A.M.); Tel.: +33-561-333181; fax: +33-561-553003 (B.C.).
E-mail addresses: maisonat@lcc-toulouse.fr (A. Maisonnat),
chaudret@lcc-toulouse.fr (B. Chaudret).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01378-X

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F. Rataboul et al. / Journal of Organometallic Chemistry 643–644 (2002) 307–312
2. Results and discussion
zerovalent zinc particles. The most common methods
involve physical processes such as thermal evaporation
of bulk zinc and condensation of vapors in an inert gas
atmosphere [11,18–21] or co-condensation with organic
solvent vapors [22,23]. This includes also the photo-
chemical decomposition of dimethyl zinc in a PVD
process [24]. Alternative chemical methods like homo-
geneous reduction of metal salts producing metal col-
loids in non-aqueous solvents have also been reported
[25].
We have previously shown that the synthesis of metal
nanoparticles of controlled size, composition and sur-
face state could be achieved by decomposition of
organometallic precursors in mild conditions by ther-
mal activation or by a chemical process involving the
presence of a reactive gas. This method has been suc-
cessfully applied to the preparation of monodisperse
nanoparticles of noble and magnetic metals [26–30]. It
has been extended to the preparation of alloys [31] and
of monodisperse tin and indium nanoparticles. Tin
nanoparticles, easily oxidize into well-crystallized tin
dioxide nanoparticles without change in the size, and
constitute a convenient starting material to generate
sensitive semiconducting layers in miniaturized gas sen-
sors [32–34]. Indium nanoparticles can be sponta-
neously organized in super lattices in the presence of
trioctylphosphine oxide [35].
Homoleptic amido zinc complexes as well as
organozincic complexes are potential precursors for the
synthesis of metal nanoparticles, owing to thermochem-
ical data which suggest the low thermodynamic stability
of the ZnꢀN and ZnꢀC bonds. In fact, in contrast with
tin amido complexes from which tin–tin oxide
nanoparticles were easily obtained upon thermal activa-
tion, the homoleptic amido zinc complexes
{Zn[N(SiMe3)2]2} and {Zn[NiPr2]2} do not lead to de-
composition products. However, interesting results
were obtained starting from the bis(cyclohexyl) zinc
complex. [Zn(C6H11)2] was prepared according to pub-
lished method starting from ZnCl2 and (C6H11)MgCl
[36]. Crystals of [Zn(C6H11)2] decompose in a single step
near 180 °C under flowing helium in a thermogravimet-
ric equipment with a weight loss of 72% consistent with
the formation of pure metal residues.
In toluene or anisole solutions containing ca 200 ppm
of water ([H2O]/[Zn]=0.1), [Zn(C6H11)2] decomposes
slowly upon heating to yield a black suspension. The
black solid which can be isolated from this suspension
is immediately oxidized into a white powder upon
exposure to air. Transmission electron microscopy
(TEM) studies of the black product resulting from the
thermolysis of [Zn(C6H11)2] in anisole (0.27 mol l−1) at
130 °C during 1 h indicate the presence of agglomer-
ated particles with sizes centered near 6 nm and with a
narrow size distribution. A typical example of such
particles is given in Fig. 1. High resolution transmission
electron microscopy (HRTEM) images display fringe
patterns as shown in Fig. 2, indicating the crystalline
nature of these particles. Surface analysis of the materi-
als by photoemission spectroscopy reveals the presence
of both metallic and oxide Auger Zn 2p3/2 electrons
with energies of, respectively 991.2 and 988.1 eV, the
XPS Zn 2p3/2 electrons being observed as a single peak
with a binding energy of 1021.8 eV. Brief exposure of
the sample to air prior to introduction into the spec-
trometer and the presence of traces of water in the
initial solutions are assumed to be partially at the origin
of the presence of zinc oxide at the surface of the
particles. The mean size and size distribution is highly
dependent upon the reaction time. An important
growth of the particles may be observed when the
solution is heated for a further 16 h (from 6 to 15 nm)
together with a broadening of the size distribution
(standard deviation from 0.7 to 2 nm).
In this paper, we demonstrate that the organometal-
lic approach can be extended to the case of zinc and we
describe hereafter the synthesis of well crystallized zinc
and zinc oxide nanoparticles together with their charac-
terization by electron microscopy, photoelectron spec-
troscopy and X-ray diffraction studies.
Decomposition of [Zn(C6H11)2] in wet anisole (0.12
mol l−1) containing polyvinylpyrolidone (PVP) with a
weight ratio of zinc to PVP fixed at 30%, proceeds
slowly at 130 °C to produce, after 3 h, a black colloidal
solution of well dispersed nanoparticles as shown in
Fig. 3. In contrast to the particles prepared in the
absence of PVP, these particles do not appear spherical
Fig. 1. TEM image of Zn nanoparticles prepared by thermolysis of
ZnCy2 in anisole at 130 °C.

F. Rataboul et al. / Journal of Organometallic Chemistry 643–644 (2002) 307–312
309
coming from the two different phases and lead to the
formation of moire´ fringes in the HRTEM image. For
instance the horizontal moire´ fringes spaced by 2.4 nm
(
correspond to the interference of Zn and ZnO 1010
reflections. The moire´ fringes are only observed in the
core of the particle, while the surface region clearly
displays the hexagonal lattice of ZnO, the facets being
(
parallel of (1010) type plane. These different features
point to Zn particles surrounded by a ZnO crystalline
shell, which has grown coherently with the core of the
particles. The synthesis and sampling experiments being
performed under rigorous anaerobic conditions, the
presence of a ZnO surrounding shell is attributed to the
water content of the solvents, as previously stated in the
case of tin [32–34]. The moisture sensitivity of organoz-
incic compounds is well known and the formation of
zinc oxide protected nanoparticles of zerovalent zinc
are expected to result from competitive processes in-
volving thermolysis and hydrolysis of the precursor in a
similar way to that observed for the growth of [Sn–
SnOx] nanocomposite [34].
Fig. 2. HRTEM micrograph of zinc nanoparticles evidencing their
crystalline structure.
The zinc nanoparticles obtained in the absence of
PVP were oxidized in air for 3 h at 300 °C and were
further oxidized at 600 °C for 3 h. The micrograph
displayed in Fig. 5 shows the same type of arrangement
of individual particles as in the non-oxidized particles
of Fig. 1 as well as no sign of coalescence (average
particle size of 8 (0.8) nm). An X-ray diffraction pattern
(Fig. 6) indicates the presence of a pure zinc oxide
phase adopting the typical hexagonal wurtzite
structure.
Fig. 3. TEM image showing well-dispersed crystalline small particles
of zinc stabilized by PVP.
but display a faceted crystalline aspect with a mean size
near 17 nm and a broad size distribution. Fig. 4 shows
a typical HRTEM micrograph of these particles and its
corresponding numerical diffractogram (Fourier trans-
form). The diffractogram is consistent with the [0001]
zone axis pattern of two superimposed hexagonal
phases with well-aligned crystalline axes but slightly
different parameters. The measurement of the radial
distances of the different spots gave the inverse of the
(
,
(1010) plane spacing in ZnO (d=2.815 A; inner spots)
,
and in Zn (d=2.3036 A; outer spots in oblique charac-
Fig. 4. HRTEM micrograph and corresponding diffractogram (inset)
of a Zn nanoparticle stabilized by PVP. Both the image and the
numerical spot pattern reveal the presence of a ZnO surface shell
surrounding a Zn core. The oblique numbers refer to Zn reflections.
ter in the figure). The sets of spots corresponding to
large lattice distances and located close to the center of
the pattern originate from the interference of reflections

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F. Rataboul et al. / Journal of Organometallic Chemistry 643–644 (2002) 307–312
cence was observed but full oxidation of the particles
without changes in their size or morphology. These
particles are of interest for applications in microelec-
tronic devices and are presently studied in our
laboratory.
4. Experimental
4.1. Materials and reagents
All operations were carried out using standard
Shlenk tube or Fischer–Porter bottle techniques under
Ar. The preparation of the zinc precursor [Zn(C6H11)2]
follows the route described earlier [36]. It was purified
by sublimation, and the resulting highly sensitive white
crystals were stored under Ar at −40 °C in a glove-
box. Anisole was purchased from Aldrich and its water
content was measured by Karl–Fischer coulometric
titration, using a Metrohm equipment. Toluene was
heated under reflux over sodium in nitrogen atmo-
sphere and distilled just before use.
Fig. 5. TEM image of nanoparticles of ZnO prepared by thermal
oxidation of Zn nanoparticles.
3. Conclusions
In conclusion, we report in this paper the first synthe-
sis of monodisperse crystalline zinc nanoparticles in
mild conditions, using the organometallic complex
[Zn(C6H11)2] as precursor. The particles can be pre-
pared in the presence or in the absence of a polymer.
Whatever the synthesis and stabilization modes, the
particles display a uniform size and a narrow size
distribution. The presence of a zinc oxide layer at the
surface of the particles, evidenced by HREM studies,
explained that upon thermal treatment in air no coales-
4.2. Synthesis of zinc nanoparticles
[Zn(C6H11)2] (1.529 g, 6.6 mmol) in anisole (30 ml)
containing traces of water (400 ppm; 0.7 mmol; [H2O]/
[Zn]=0.1) was heated at 130 °C under magnetic stir-
ring in a Fischer–Porter bottle. The initial light yellow
solution darkened within 30 min and a black solid
Fig. 6. XRD pattern (Cu–Ka radiation) of nanoparticles of ZnO prepared by thermal oxidation of the nano-Zn particles.

F. Rataboul et al. / Journal of Organometallic Chemistry 643–644 (2002) 307–312
311
progressively precipitated. After 3 h, the solution was
removed by filtration, and the black precipitate was
washed with anisole (2×30 ml) and dried under vac-
uum (135 mg). TEM analysis: Zn nanoparticles, size: 60
Senocq for the measures of X-ray diffraction (ENSI-
ACET-INP), and to CNRS for financial support.
,
(7) A; yield: 30%.
References
Thermogravimetric analysis (TGA) of the zinc pre-
cursor was obtained on a SETARAM TG-DTA 92
instrument.
[1] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826.
[2] S. Sakohara, M. Ishida, M.A. Anderson, J. Phys. Chem. Sect. B
102 (1998) 10169.
In the presence of PVP: [Zn(C6H11)2] (0.227g; 0.98
mmol) in wet anisole (8 ml; [H2O]/[Zn]=0.2) was
added to PVP (213.5 mg; Zn/PVP=30/100 mass). The
resulting light yellow limpid solution was heated at
130 °C under magnetic stirring in a Fischer–Porter
bottle. The solution darkened very slowly and the black
limpid solution obtained after 3 h was sampled for
TEM and HRTEM experiments.
[3] H. Henglein, Chem. Rev. 89 (1989) 1861.
[4] L. Hozer, Semiconductor ceramics: grain boundary effects, Ellis
Horwood and Polish Scientific Publishers PWN, Warsaw, New
York, 1994.
[5] X. Wang, S.S. Yee, W.P. Carey, Sens. Actuators Sect. B 24–25
(1995) 454.
[6] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators Sect.
B 3 (1991) 147.
[7] N. Yamazoe, Sens. Actuators Sect. B 5 (1991) 7.
[8] P.T. Moseley, A.M. Stoneham, D.E. Williams, in: P.T. Moseley,
J.O.W. Norris, D.E. Williams (Eds.), Techniques and Mecha-
nisms in Gas Sensing, Adam Hilger, Bristol, Philadelphia and
New York, 1991, pp. 108.
4.3. Electron microscopy experiments
Samples for TEM and HRTEM studies were pre-
pared in a glove-box by slow evaporation of a drop of
suitably diluted colloidal suspensions deposited on
holey carbon-covered copper grids. The TEM experi-
ments were performed at the ‘Service Commun de
Microscopies de l’Universite´ Paul Sabatier’ on a JEOL
JEM 200CXT-T electron microscope working at 200
kV and a Philips CM12 electron microscope working at
120 kV while the high resolution images were obtained
with a JEOL JEM 2010 electron microscope working at
200 kV. The size distribution of the particles was
determined from enlarged photographs by measuring a
minimum of 200 particles for each sample.
[9] D. Kohl, in: G. Sberveglieri (Ed.), Gas Sensors, Principles,
Operation and Developments, Kluwer Academic, Dordrecht,
Boston, London, 1992, p. 43.
[10] A.V. Chadwick, N.V. Russell, A.R. Whitham, A. Wilson, Sens.
Actuators Sect. B 18–19 (1994) 99.
[11] L.F. Dong, Z.L. Cui, Z.K. Zhang, Nanostructured Mater. 8
(1997) 815.
[12] S. Sakohara, L.D. Tickanen, M.A. Anderson, J. Phys. Chem. 96
(1992) 11086.
[13] B.S. Zou, V.V. Volkov, Z.L. Wang, Chem. Mater. 11 (1999)
3037.
[14] S. Fujitsu, H. Toyoda, H. Yanagida, J. Am. Ceram. Soc. 70
(1987) C71.
[15] T.K. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817.
[16] D.F.K. Hennings, R. Hartung, P.J.L. Reijnen, J. Am. Ceram.
Soc. 73 (1990) 645.
[17] N. Raghu, T.R.N. Kutty, Appl. Phys. Lett. 60 (1992) 100.
[18] K. Recknagle, Q. Xia, J.N. Chung, C.T. Crowe, H. Hamilton,
G.S. Collins, Nanostructured Materials, vol. 4, Elsevier Science
Ltd, Amsterdam, New York, 1994, p. 103.
[19] J. Xu, J. Zhang, Y. Du, X. Zhang, Y. Li, Mater. Lett. 29 (1996)
131.
[20] O. Daub, W. Langel, C. Reiner, L. Kienle, Ber. Bunsenges. Phys.
Chem. 101 (1997) 1753.
[21] J. Xu, W. Ji, X.B. Wang, H. Shu, Z.X. Shen, S.H. Tang, J.
Raman Spectrosc. 29 (1998) 613.
4.4. X-ray photoelectron spectroscopy experiments
Samples for XPS/Auger studies were prepared in a
glove-box by slow evaporation of a nanoparticle sus-
pension deposited on a 1 cm2 silicon support. The
spectra were recorded on a VG ESCALAB MKII spec-
trometer with Mg–Ka radiation.
[22] T.G. Cardenas, C.E. Salgado, Polym. Bull. 38 (3) (1997) 279.
[23] T.G. Cardenas, C.R. Oliva, H.H. Carbacho, Bol. Soc. Chil.
Quim. 40 (1995) 261.
[24] V.V. Polonski, Y. Yamamoto, M. Kourogi, H. Fukuda, M.
Ohtsu, J. Microsc. 194 (1999) 545.
[25] K.-L. Tsai, L. Dye, Chem. Mat. 5 (1993) 540.
[26] M. Respaud, J.M. Broto, H. Rakoto, A.R. Fert, L. Thomas, B.
Barbara, M. Verelst, E. Snoeck, P. Lecante, A. Mosset, J.
Osuna, T. Ould Ely, C. Amiens, B. Chaudret, Phys. Rev. Sect. B
57 (1998) 2925.
4.5. X-ray powder diffraction
XRD profile of the oxidized particles deposited on
silicon support were obtained with a SEIFERT XRD
3000 TT X-ray diffractometer with Cu–Ka radiation.
The data were collected in the grazing incidence
configuration.
[27] A. Rodriguez, C. Amiens, B. Chaudret, M.J. Casanove, P.
Lecante, J.S. Bradley, Chem. Mat. 8 (1996) 1978.
[28] C. Pan, F. Dassenoy, M.-J. Casanove, K. Philippot, C. Amiens,
P. Lecante, A. Mosset, B. Chaudret, J. Phys. Chem. Sect. B 103
(1999) 10098.
[29] F. Dassenoy, K. Philippot, T. Ould Ely, C. Amiens, P. Lecante,
E. Snoeck, A. Mosset, M.-J. Casanove, B. Chaudret, N. J.
Chem. 22 (1998) 703.
Acknowledgements
The authors are grateful to V. Collie`re for assistance
with the electron microscopy (Service Commun de Mi-
croscopie Electronique de l’Universite´ Paul Sabatier),
to G. Chatainier for XPS and Auger spectra to Dr F.

312
F. Rataboul et al. / Journal of Organometallic Chemistry 643–644 (2002) 307–312
[30] O. Vidoni, K. Philippot, C. Amiens, B. Chaudret, O. Balmes,
J.-O. Malm, J.-O. Bovin, F. Senocq, J. Casanove, Angew. Chem.
Int. Ed. 38 (1999) 3736.
[31] T. Ould Ely, C. Pan, C. Amiens, B. Chaudret, F. Dassenoy, P.
Lecante, M.-J. Casanove, A. Mosset, M. Respaud, J.-M. Broto,
J. Phys. Chem. Sect. B 104 (2000) 695.
[33] C. Nayral, E. Viala, V. Collie`re, P. Fau, F. Senocq, A. Maison-
nat, B. Chaudret, Appl. Surf. Sci. 164 (2000) 219.
[34] C. Nayral, E. Viala, P. Fau, F. Senocq, J.-C. Jumas, A. Maison-
nat, B. Chaudret, Chem. Eur. J. 6 (2000) 4082.
[35] K. Soulantica, A. Maisonnat, M.-C. Fromen, M.-J. Casanove, P.
Lecante, B. Chaudret, Angew. Chem. Int. Ed. 40 (2001) 448.
[36] K.H. Thiele, S. Wilcke, M. Ehrhardt, J. Organomet. Chem. 14
(1968) 13.
[32] C. Nayral, T. Ould-Ely, A. Maisonnat, B. Chaudret, P. Fau, L.
Lescouzeres, A. Peyre-Lavigne, Adv. Mater. 11 (1999) 61.