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A Cobalt(II) Hexafluoroacetylacetonate Ethylenediamine Complex
As a CVD Molecular Source of Cobalt Oxide Nanostructures
Giuliano Bandoli, Davide Barreca, Alberto Gasparotto, Chiara Maccato, Roberta
Seraglia, Eugenio Tondello, Anjana Devi, Roland A. Fischer, and Manuela Winter
Inorg. Chem., 2009, 48 (1), 82-89 • DOI: 10.1021/ic801212v • Publication Date (Web): 02 December 2008
Downloaded from http://pubs.acs.org on January 7, 2009
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A Cobalt(II) Hexafluoroacetylacetonate Ethylenediamine Complex As a
CVD Molecular Source of Cobalt Oxide Nanostructures
Giuliano Bandoli,†Davide Barreca,*,‡Alberto Gasparotto,§Chiara Maccato,§Roberta Seraglia,‡
Eugenio Tondello,§Anjana Devi,|Roland A. Fischer,|and Manuela Winter|
Department of Pharmaceutical Sciences, PadoVa UniVersity, 35131 PadoVa, Italy, ISTM-CNR and
INSTM, Department of Chemistry, PadoVa UniVersity, 35131 PadoVa, Italy, Department of
Chemistry, PadoVa UniVersity and INSTM, Via Marzolo, 1, 35131 PadoVa, Italy, Inorganic
Materials Chemistry Group, Lehrstuhl fu ¨r Anorganische Chemie II, Ruhr-UniVersity Bochum,
D-44780 Bochum, Germany
Received July 1, 2008
by a simple procedure and, for the first time, thoroughly characterized by several analytical methods in order to
elucidate its structure (single-crystal X-ray diffraction), chemical composition (elemental analyses, FT-IR), optical
andahighvolatilityat moderatetemperatures. Preliminarychemical vapor deposition(CVD) experimentshighlight
its very promising features as a CVD/atomic layer deposition molecular source for cobalt oxide nanosystems.
The attractive properties of cobalt oxide (CoO, Co3O4)
nanosystems, such as high catalytic activity,1,2antiferro-
magnetism,3and electrochromism,4offer significant potential
in view of various technological applications. As a matter
of fact, both CoO and Co3O4nanosystems are promising
candidates for use in thermal solar energy conversion
devices,5-7electrochemical capacitors,8,9solid-state gas
sensors,10,11magnetic materials,12,13and negative electrodes
for lithium-ion batteries.14-17
In the past decade, this broad spectrum of attractive
utilization has stimulated widespread research efforts aimed
at the synthesis of Co-O thin films, nanotubes, and ordered
nanoarrays, as summarized by various reports.6,7,10,12,18
Among the different proposed routes, chemical vapor
deposition (CVD) and atomic layer deposition (ALD)
* Author to whom correspondence should be addressed. Tel.: + 39 049
8275170. Fax: + 39 049 8275161. E-mail: firstname.lastname@example.org.
†Department of Pharmaceutical Sciences, Padova University.
‡ISTM-CNR and INSTM.
§Department of Chemistry, Padova University and INSTM.
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82 Inorganic Chemistry, Vol. 48, No. 1, 2009 10.1021/ic801212v CCC: $40.75
2009 American Chemical Society
Published on Web 12/02/2008
represent preferred techniques for the growth of high-
quality nanoarchitectures.19Yet, the success of a CVD/
ALD process depends critically on the availability of
volatile and thermally stable precursors, enabling uniform
and reproducible growth processes along with a tailoring
of the system properties. In this regard, several cobalt-
(II) and (III) salts have been proposed, such as CoI2,
Co(OCOCH3)2, and Co(NO3)3,10-12,20but they do not
present a sufficiently controlled and reproducible mass
supply. Alternatively, cobalt complexes such as Co2(CO)8,
Co(C5H5)2, and related organometallic species, including
Co(CO)3NO, HCo(CO)4, and Co(C5H5)(CO)2, have also been
adopted as CVD precursors, but their use presents serious
drawbacks, such as undesired gas-phase reactions, low
thermal stability, and a high deposition temperature.21-27
Other works have focused on the use of conventional Co
?-diketonates as CVD/ALD precursors, such as Co(acac)2
(acac ) 2,4-pentanedionate) and Co(dpm)2(dpm ) 2,2,6,6-
former is characterized by the formation of oligomeric
structures.11Although such phenomena can be limited by
increasing the steric hindrance of the ligand, for instance,
upon going to Co(dpm)2, the latter compound suffers from
sintering at elevated temperatures and presents a limited shelf
life. In fact, the structure of bis(?-diketonato)Co(II) chelates
was long clouded by ambiguity, due to the pronounced
tendency of Co(II) to achieve six-coordination through adduct
formation.30As a consequence, the development of improved
Co(II) ?-diketonate sources actually remains an open chal-
lenge in this field.
Co(hfa)2·2H2O·L adducts [L ) bis(2-methoxyethyl)ether,
2,5,8,11-tetraoxadodecane, and 2,5,8,11,14-pentaoxapenta-
decane] have been reported.5,6,31Although the properties of
these precursors compare favorably with those of conven-
tional ?-diketonates, water molecules in the metal coordina-
tion sphere might induce uncontrolled decomposition or
premature reactions in CVD/ALD processes.
An attractive alternative is the formation of adducts
between Co(II) ?-diketonates and various N-donor Lewis
bases,11,30,32which enables a complete saturation of the
Co(II) coordination sphere, thus resulting in stable adducts
with improved properties. Recently, Co(acac)2and Co(dpm)2
adducts with N,N,N′,N′-tetramethylethylenediamine (TME-
DA) and 1-dimethylamino-2-propanol (DMAPH) have been
proposed as CVD precursors.11,12While the latter is dimeric,
Co(II)-TMEDA adducts present better resistance to air
oxidation than the parent Co(II) ?-diketonates,11implying
higher stability and a longer shelf life, significant advantages
for large-scale CVD/ALD applications.
In the past decade, various M(hfa)x·TMEDA complexes
(x ) 1, 2; e.g., M ) Mg(II),33Ag(I),34Zn(II)35-37and
Cd(II)38,39compounds) have come under intense scrutiny
thanks to their favorable properties as CVD/ALD precursors
to metal and metal oxide films. In fact, the introduction of
fluorinated substituents (e.g., hfa versus acac) effectively
increases both the precursor volatility and the stability of
ancillary amine ligand binding via enhanced Lewis acidity.35
On this basis, in the present work, our attention was focused
on an innovative Co(II) compound, Co(hfa)2·TMEDA,
appealing as a potential CVD/ALD precursor of Co-O
nanosystems. To the best of our knowledge, only a previous
patent quoting the use of such a compound and of its Mn,
Fe, and Ni homologues as gasoline additives is available,40
but no reports on its synthesis, characterization, and use are
available in the literature to date.
In this paper, the target compound was prepared and fully
characterized with particular attention to its mass-transport
properties and reactivity, two critical issues for CVD/ALD
applications. The functional validation of the precursor has
also been established by performing preliminary Co-O CVD
depositions on Si(100) substrates.
Reagents. CoCl2·6H2O (98%) was purchased from Prolabo-
Rectapur. The chemical reagents Hhfa (98+%, d ) 1.47 g mL-1)
and TMEDA (99%, d ) 0.77 g mL-1) were commercial products
obtained from Alfa Aesar and Janssen, respectively. All of the above
products were used as received without any further purification.
General Procedures. Elemental microanalyses were performed
using a Fisons Carlo Erba EA1108 instrument, CHNS version. The
(19) Hitchman, M. L.; Jensen, K. F. Chemical Vapor Deposition: Principles
and Applications; Academic Press: London, 1993.
(20) Rooth, M.; Lindahl, E.; Harsta, A. Chem. Vapor Deposition 2006,
(21) Crawford, N. R. M.; Knutsen, J. S.; Yang, K. A.; Haugstad, G.;
McKernan, S.; McCormick, F. B.; Gladfelter, W. L. Chem. Vapor
Deposition 1998, 4, 181.
(22) Lane, P. A.; Oliver, P. E.; Wright, P. J.; Reeves, C. L.; Pitt, A. D.;
Cockayne, B. Chem. Vapor Deposition 1998, 4, 183.
(23) Holgado, J. P.; Caballero, A.; Espino ´s, J. P.; Morales, J.; Jime ´nez,
V. M.; Justo, A.; Gonza ´lez-Elipe, A. R. Thin Solid Films 2000,
(24) Choi, H.; Park, S. Chem. Mater. 2003, 15, 3121.
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(27) Li, Z.; Lee, D. K.; Coulter, M.; Rodriguez, L. N. J.; Gordon, R. G.
Dalton Trans. 2008, 19, 2592.
(28) Barison, S.; Barreca, D.; Daolio, S.; Fabrizio, M.; Tondello, E. Rapid
Commun. Mass Spectrom. 2001, 15, 1621.
(29) Klepper, K. B.; Nilsen, O.; Fjellvåg, H. J. Cryst. Growth 2007, 307,
(30) Tzavellas, L. C.; Tsiamis, C.; Kavounis, C. A.; Cardin, C. J. Inorg.
Chim. Acta 1997, 262, 53.
(31) Gulino, A.; Fragala `, I. Inorg. Chim. Acta 2005, 358, 4466.
(32) Colborn, R. E.; Garbauskas, M. F.; Hejna, C. I. Inorg. Chem. 1988,
(33) Wang, L.; Yang, Y.; Ni, J.; Stern, C. L.; Marks, T. J. Chem. Mater.
2005, 17, 5697.
(34) Zanotto, L.; Benetollo, F.; Natali, M.; Rossetto, G.; Zanella, P.;
Kaciulis, S.; Mezzi, A. Chem. Vapor Deposition 2004, 10, 207.
(35) Ni, J.; Yan, H.; Wang, A.; Yang, Y.; Stern, C. L.; Metz, A. W.; Jin,
S.; Wang, L.; Marks, T. J.; Ireland, J. R.; Kannewurf, C. R. J. Am.
Chem. Soc. 2005, 127, 5613.
(36) Barreca, D.; Ferrucci, A. P.; Gasparotto, A.; Maccato, C.; Maragno,
C.; Tondello, E. Chem. Vapor Deposition 2007, 13, 618.
(37) Barreca, D.; Comini, E.; Ferrucci, A. P.; Gasparotto, A.; Maccato,
C.; Maragno, C.; Sberveglieri, G.; Tondello, E. Chem. Mater. 2007,
(38) Babcock, J. R.; Wang, A. C.; Metz, A. W.; Edleman, N. L.; Metz,
M. V.; Lane, M. A.; Kannewurf, C. R.; Marks, T. J. Chem. Vapor
Deposition 2001, 7, 239.
(39) Metz, A. W.; Lane, M. A.; Kannewurf, C. R.; Poeppelmeier, K. R.;
Marks, T. J. Chem. Vapor Deposition 2004, 10, 207.
(40) McCormack, W. B.; Sandy, C. A. Patent Ger. Offen. DE 292095,
1979; U.S. 78-907875, 1978.
Co(II) Hexafluoroacetylacetonate Ethylenediamine Complex
Inorganic Chemistry, Vol. 48, No. 1, 2009 83
complex melting point was measured in the air by means of a
Koffler microscope. FT-IR analyses were performed on KBr pellets
in transmittance mode by means of a Thermo-Nicolet Nexus 860
spectrophotometer, using a spectral resolution of 4 cm-1. Optical
absorption analyses were performed in 2 × 10-4M ethanolic
solutions using a Cary 5000 UV-vis-NIR spectrophotometer
(Varian) with a spectral bandwidth of 1 nm. Measurements were
carried out on ethanolic solutions using quartz cuvettes (optical path
) 1 cm). Thermal analyses were performed using an SDT 2960
apparatus from TA Instruments, which allows performance of
simultaneous DSC-TGA measurements. The weight of the used
sample was in the range 6-7 mg. The traces were recorded under
both N2and synthetic air (N2/O2) 80:20) flows, with a heating
rate of 10 °C/min. Isothermal investigations were carried out in
The ESI mass spectra were obtained using an LCQ instrument
(Finnigan), operating in both positive and negative ion modes. The
entrance capillary temperature and voltage were set at 200 °C and
(5 kV, respectively. 10-6M solution of the target compound in
methanol, methanol/water (50:50 v/v), and chloroform were
introduced by direct infusion using a syringe pump at a flow rate
of 8 µL/min. Accurate mass measurements were performed using
an Accurate-Mass 6520 Q-TOF instrument (Agilent Technologies,
Inc.), operating under ESI negative ion conditions. The solutions
of Co(hfa)2·TMEDA were directly injected into the Dual ESI source
at a flow rate of 5 µL/min. Instrument calibration was achieved by
injection of a TOF ESI Tune Mix solution (Agilent Technologies,
Inc.). The mass resolution was >13.000 at m/z 2722.
Synthesis. 0.94 g of NaOH (23.5 mmol) were dissolved in 10
mL of deionized H2O. Subsequently, 3.3 mL of Hhfa (23.3 mmol)
was introduced in the above solution kept under stirring. To the
resulting liquid was added dropwise CoCl2·6H2O (2.79 g, 11.73
mmol) dissolved in 50 mL of H2O, under vigorous stirring,
producing a color change from violet to orange. After 1 h, TMEDA
(1.8 mL, 11.93 mmol) was dropped stepwise into the obtained
solution, resulting in a color change to a brownish color. After
reaction for 2.5 h, the complex was repeatedly extracted in 1,2-
dichloroethane until a colorless aqueous phase was obtained. Solvent
removal at reduced pressure yielded, finally, a light brown solid
(yield ) 60%).
Melting point: 92-94 °C at 1 atm. Anal. calcd. for
C16H18O4N2F12Co: C, 32.61%; H, 3.08%; N, 4.75%. Found: C,
32.66%; H, 3.00%; N, 4.71%.
X-Ray Cystallographic Study. A crystal having the dimensions
0.60 × 0.50 × 0.41 mm3was used for the data collection.
Crystallographic data were obtained by means of an Xcalibur 2
Oxford apparatus using graphite monochromated Mo KR radiation
(λ ) 0.71073 Å, T ) 107 K). The structure was solved using the
SHELXL-97R software package and refined by full-matrix least-
squares methods based on F2with all of the observed reflections.
CVD Deposition and Film Characterization. Cobalt oxide
based specimens were deposited on HF-etched Si(100) substrates
in an O2atmosphere using a cold-wall low-pressure horizontal CVD
reactor equipped with a quartz tubular chamber and a resistively
heated susceptor. In the present study, the precursor vaporization
temperature and the deposition temperature were set at 60 and 400
°C, respectively. The pressure and gas flow rates (10 mbar and
200 sccm, respectively) were measured by a capacitance manometer
(BOC Edwards) and mass-flow controllers (MKS Instruments),
respectively. The deposition time was 120 min.
Glancing Incidence X-Ray Diffraction (GIXRD) patterns were
recorded using a Bruker D8 Advance instrument equipped with a
Go ¨bel mirror and a Cu KR source (40 kV, 40 mA), at an incidence
84 InorganicChemistry, Vol. 48, No. 1, 2009
angle of 0.5°. The average crystallite size was estimated by means
of the Scherrer equation. X-ray photoelectron spectroscopy (XPS)
and X-ray Excited Auger Electron Spectroscopy were run by a
Perkin-Elmer Φ 5600ci spectrometer with a nonmonochromatized
Al KR (1486.6 eV) source, at a working pressure lower than 2 ×
10-9mbar. The reported binding energies (BEs; standard deviation)
( 0.2 eV) were corrected for charging effects, assigning to the
C1s line of adventitious carbon a BE of 284.8 eV. The atomic
compositions were evaluated using sensitivity factors provided by
Φ V5.4A software. Ar+sputtering was carried out for 5 min at 3.5
kV, with an argon partial pressure of 5 × 10-8mbar. Field emission-
scanning electron microscopy (FE-SEM) and energy dispersive
X-ray spectroscopy (EDXS) measurements were carried out by
means of a Field Emission Zeiss SUPRA 40VP instrument,
equipped with an Oxford INCA x-sight X-ray detector.
Results and Discussion
Synthesis. The preparation of Co(hfa)2·TMEDA was
made according to the following scheme:
The above reaction is simple, straightforward, and low-cost
and can be easily carried out on open benches, without the
need of controlled atmospheres, thus resulting in more
amenable routes than the previous ones proposed for Co(II)
?-diketonate diamine adducts.32The Co complex can be
readily dissolved in a great variety of liquids, including
ethanol, acetone, CH2Cl2, 1,2-dichloroethane, and n-pentane,
whereas it is insoluble in H2O. In view of CVD applications,
it is important to highlight that the resulting compound
possesses a relatively low melting point (92-94 °C), is
moisture- and light-stable, and has a shelf life of several
months, implying that its manipulation can be readily
performed in the air without inducing any undesired prema-
X-Ray Crystallographic Study of Co(hfa)2·TMEDA.
An ORTEP drawing of the obtained compound structure is
proposed in Figure 1, while relevant crystallographic data
and structural refinement parameters are summarized in Table
1. As can be seen, the target product is monomeric and
Figure 1. ORTEP view of the molecular structure of Co(hfa)2·TMEDA.
Bandoli et al.
possesses a cis geometry, with a hexacoordinated Co(II)
center. The coordinating donor atoms are four oxygen atoms
belonging to hfa ligands, plus two nitrogen atoms of the
TMEDA molecule, resulting in a CoO4N2pseudo-octahedral
environment around Co. In fact, O-Co-O, N-Co-N, and
O-Co-N bond angles (Table 2) point out to a certain
distortion, within 4°, from an idealized octahedron.
The present structure is appreciably different from
Co(hfa)2·2H2O·CH3(OCH2CH2)4OCH3, in which the poly-
ether moiety is not directly bonded to the Co(II) center but
rather interacts with coordinated water through hydrogen
bonds involving its O atoms, thus bridging the Co(hfa)2·
2H2O units.5Conversely, the target product reported herein
is completely solvent-free, despite its synthesis in an aqueous
medium. This difference, which anticipates improved mass
transport properties (see below), can be attributed to the fact
that a diamine is a stronger Lewis base and competes more
favorably with water than the polyether in the coordination
of the metal center.35
In the crystal lattice of Co(hfa)2·TMEDA, no intermo-
lecular hydrogen bonds are present, implying an appreciable
volatility for the adduct, a relevant feature in view of CVD/
ALD application.19This characteristic can be traced back
to the absence of water molecules in the crystalline structure
and to the presence of fluorinated ligands, which are well-
known to prevent H-bridge intermolecular interaction.35In
a different way, for other Co(II) ?-diketonates like the
adducts of Co(acac)2 with DMAPH, the occurrence of
hydrogen bonding has been evidenced.11
The present bond lengths (see Table 2 for representative
values) are consistent with the occurrence of strong
metal-ligand interactions. The Co-O bond distances are
in agreement with those found for Co(II)-hfa complexes
(mean d(Co-O) ) 2.050 Å) in the Cambridge Structural
Database (CSD, version 5.25),41as well as with those
reported for Co(II)-hfa polyether derivatives like Co(hfa)2·
2H2O·CH3(OCH2CH2)4OCH3(d(Co-O) ) 2.074 Å)5and
Co(hfa)2·2H2O·CH3OCH2CH2OCH3 (d(Co-O) ) 2.014
It is also interesting to compare the metrical parameters
of Co(hfa)2·TMEDA reported herein to those of the structur-
ally similar compounds Co(acac)2·TMEDA and Co(acac)2·
TEEDA (TEEDA) N,N,N′,N′-tetraethylethylenediamine), as
well as with those of analogous M(hfa)2·TMEDA adducts
previously characterized. Very similar d(Co-O) values have
been reported (average ) 2.064 and 2.054 Å) for
Co(acac)2·TMEDA and Co(acac)2·TEEDA adducts, respec-
tively, although the d(Co-N) values are higher in these cases
with respect to Co(hfa)2·TMEDA (average ) 2.228 and
2.262 Å, respectively, versus 2.162 Å in Co(hfa)2·TMEDA;
compare Table 2).11,32These differences highlight the role
of the fluorinated hfa ligands with respect to the acac ones,
increasing the Lewis acidity of the Co(II) center by electron-
withdrawing effects and, in turn, strengthening its bonds with
the ancillary diamine ligand, as already reported for Zn(?-
In addition, in the present case, the Co-O bonds trans to
the nitrogen atoms [Co-O(2), 2.080 Å, and Co-O(3), 2.087
Å] are slightly longer than those trans to the oxygen
[Co-O(1), 2.061 Å, and Co-O(4), 2.060 Å]. A similar trans
effect is in agreement with the results reported for other
Mg(hfa)2·TMEDA [d(Mg-O)trans-O ) 2.043 Å; d(Mg-
O)trans-N) 2.061 Å],33and Zn(hfa)2·TMEDA [d(Zn-O)trans-
O ) 2.100 Å; d(Zn-O)trans-N) 2.119 Å],35while it has not
been reported for Cd(hfa)2·TMEDA38and Cu(hfa)2·
TMEDA.42Among these compounds, Mg(hfa)2·TMEDA
presents metal-oxygen distances very similar to those of
Co(hfa)2·TMEDA (compare the above values with the ones
in Table 2). As regards the O-Co-O an O-Co-N angular
values (Table 2), they are in general agreement with those
of Co(acac)2·TMEDA11and Zn(hfa)2·TMEDA.35
Chemical, Physical, and Mass Transport Properties
of Co(hfa)2·TMEDA. The complex FT-IR spectrum (Figure
S1, Supporting Information) shows signals between 2800 and
3300 cm-1(N-H, C-H, and N-CH3stretching),5,7,30,43and
a broad band at ca. 3400 cm-1due to adsorbed H2O.11,31
Peaks at 1645 and 1597 cm-1(CdO and CdC stretching)
and at 1410 and 1346 cm-1(C-C and CF3stretching) are
typical of coordinated hfa moieties.5,7,11,30,31,43,44Conversely,
signals at 1260, 1188, and 1145 cm-1can be ascribed to C-N
as well as for
(41) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380.
(42) Delgado, S.; Mun ˜oz, A.; Medina, M. E.; Pastor, C. J. Inorg. Chim.
Acta 2006, 359, 109.
(43) Rao, C. N. R. Chemical Applications of Infrared Spectroscopy;
Academic Press: London, 1963.
(44) Morris, M. L.; Moshier, R. W.; Sievers, R. E. Inorg. Chem. 1963, 2,
Table 1. Crystal Data and Structure Refinement for Complex
cryst syst, space group
Z; calculated density (g × cm-3)
ϑ range for data collection (deg)
final R indices [I > 2σ(I)]
R indices (all data)
goodness of fit on F2
41371/5134 [R(int) ) 0.031]
R1) 0.0273, wR2) 0.0663
R1) 0.0351, wR2) 0.0677
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Co(II) Hexafluoroacetylacetonate Ethylenediamine Complex
Inorganic Chemistry, Vol. 48, No. 1, 2009 85
stretching in TMEDA,43,44whereas the peaks at 667 and 586
cm-1can be related to Co-O and Co-N vibrations.11
The optical absorption spectrum of Co(hfa)2·TMEDA
(Figure S2, Supporting Information) further supports the
octahedral Co(II) coordination. The low-intensity vis band
between 500 and 600 nm arises from the overlap of the
4T1 g(F) f4T1g(P) and4T1g(F) f4A2g(F) excitations,30,45
while the signal at 420 nm can be ascribed to ligand-metal
charge transfer.10In addition, the intense UV band at 303
nm is assigned to π f π* intraligand transitions, without
any significant metal center contribution.46,47
In order to examine the thermal behavior of the complex,
thermal analyses were carried out under both inert (N2) and
oxidizing (synthetic air) flows. Irrespective of the adopted
atmosphere, very similar results were obtained, thus indicat-
ing the occurrence of clean vaporization processes free from
undesired side reactions in both cases. Figure 2 illustrates
the TGA-DSC data for Co(hfa)2·TMEDA in the presence
of oxygen, which is employed as a reactant gas even in CVD
processes (see below). As can be noticed, the compound
remained stable up to 120 °C and subsequently underwent a
remarkable weight loss, associated with the powder vapor-
ization in a single step. Remarkably, an almost zero constant
weight residual was observed for T > 190 °C, suggesting a
quantitative sublimation of the complex. In a different way,
Co(hfa)2·2H2O, Co(hfa)2·2H2O·L with L ) polyether, and
Co(acac)2·TMEDA adducts possess a less favorable thermal
behavior, being characterized by a broader weight loss at
higher temperatures (i.e., a lower volatility),5,6,31along with
a nonquantitative sublimation for the latter precursor.11The
present favorable features can be related to the absence of
H-bonding in Co(hfa)2·TMEDA, as already discussed.
In agreement with the above findings, the DSC curve
(Figure 2, dotted line) displayed two endothermic peaks
located at 95 and 187 °C, respectively, associated with the
adduct melting (see above) and subsequent vaporization,
Isothermal TG studies were carried out on Co(hfa)2·
TMEDA at ambient pressure, and the results are displayed
in Figure 3. The recorded curves revealed the compound
sublimation at a constant and appreciable rate for long
periods of time (>100 min in the temperature range 50-100
°C). Remarkably, the linear weight losses throughout the
investigated temperature range are indicative of a pure
vaporization, with no evidence of premature decomposition
phenomena. This result is of considerable relevance for a
CVD/ALD precursor, since it ensures reproducibility in
constant vapor supply throughout the deposition process.
Figure 4 displays the logarithmic dependence of the
vaporization rate, obtained by the TG profile derivative, on
the inverse absolute temperature in the air. As can be noticed,
a linear relationship is obtained, confirming the occurrence
of clean vaporization processes with minimal side decom-
position pathways. On the basis of the proportionality
between the vaporization rate and the vapor pressure,48the
logarithmic plot of the former versus the inverse absolute
temperature enables evaluation of the apparent molar va-
porization enthalpy from the curve slope on the basis of the
Clausius-Clapeyron equation.49Irrespective of the adopted
atmosphere (nitrogen or synthetic air), the calculation yielded
∆H ) 60 ( 1 kJ mol-1, a typical value for volatile CVD
Overall, thermal analyses indicate that Co(hfa)2·TMEDA
is thermally stable and volatile, thus opening interesting
perspectives for its subsequent application in CVD routes
to cobalt oxide nanostructures.
TMEDA. Further information on the adduct behavior were
gained by MS analyses and, in particular, by the use of ESI-
MS. In fact, despite conventional electron ionization (EI)-
MS seeming more appropriate for the investigation of CVD
precursor reactivity, the more drastic ionization conditions
might result in the destruction of particular ions diagnostic
of the compound fragmentation. As a consequence, a softer
ionization method like ESI represents the preferred choice.
(45) Bencini, A.; Benelli, C.; Gatteschi, D.; Zanchini, C. Inorg. Chem. 1983,
(46) Nagashima, N.; Kudoh, S.; Nakata, M. Chem. Phys. Lett. 2003, 374,
(47) Naik, M. B.; Gill, W. N.; Wentorf, R. H.; Reeves, R. R. Thin Solid
Films 1995, 262, 60.
(48) Ashcroft, S. J. Thermochim. Acta 1971, 2, 512.
(49) Atkins, P. Physical Chemistry, 6thed.; W. H. Freeman & Co. and
Sumanas, Inc.: New York, 1998.
Figure 2. TG (solid line) and DSC (dotted line) profiles of the compound
Co(hfa)2·TMEDA recorded under an air flow.
Figure 3. Isothermal weight loss studies for Co(hfa)2·TMEDA carried out
at different temperatures.
Figure 4. Arrhenius plot for the vaporization of Co(hfa)2·TMEDA under
an air flow.
Bandoli et al.
86 InorganicChemistry, Vol. 48, No. 1, 2009
The positive ESI-MS spectrum from a chloroform solution
(Figure 5a) was characterized by the ion at m/z 382,
corresponding to [Co(hfa)·TMEDA]+. The related MS/MS
spectrum (Figure 5b) revealed a very simple fragmentation
of the above ion, since only the signal at m/z 214 was
detected. On the basis of previous EI-MS studies on Co(hfa)2
adducts reporting ligand-to-metal fluorine transfer,6,7,31a
possible formula for this ion could be [CoF2·TMEDA +
H]+. In negative ion mode (spectrum not reported), the only
detected peak at m/z 207 was due to [hfa]-species.
Conversely, in the ESI-MS analyses of Co(hfa)2·TMEDA
solutions in CH3OH and H2O/CH3OH, TMEDA-related
signals were never detected, thus indicating a very rapid loss
of the ancillary diamine ligand. In positive ion mode, no
signals related to the complex were revealed, whereas in
negative ion mode, three peaks at m/z 645, 661, and 680
could be detected. While the ion at m/z 680 corresponded to
[Co(hfa)3]-, the identification of the negative ions at m/z 645
and 661 was not straightforward. In order to obtain their exact
mass values and get further insight into their chemical nature,
the methanolic solution of Co(hfa)2·TMEDA was analyzed
by a Q-TOF instrument equipped with an ESI source. The
resulting negative ion spectrum (Figure 6) enabled accurate
measurment of the m/z values of the above ions as 644.9502,
660.9283, and 679.8975. As regards the latter, the calculated
elemental formula was C15H3O6F18Co (theoretical mass value,
679.8969; mass accuracy, 0.93 ppm), enabling its unambigu-
ous attribution to [Co(hfa)3]-. In the MS/MS spectrum of
ions at m/z 679.8975 (Figure 7a), only the signal at m/z
206.9886 was detected, corresponding to [hfa]-(theoretical
mass value, 206.9875; mass accuracy, 5.3 ppm). Conse-
quently, the ions at m/z 660.9283 and 644.9502 could not
be considered as deriving from “in-source” fragmentation
processes of [Co(hfa)3]-(m/z 679.8975). The MS/MS spectra
of these ions are shown in Figure 7b and c, respectively.
Interestingly, in Figure 7b a low-abundance fragment at
m/z472.9113 was detected (calculated formula, C10H2-
O4F12Co; theoretical mass value, 472.9088; mass accuracy,
5.3 ppm). This composition corresponded to a [Co(hfa)2]-
fragment, whose existence could be explained only by
Figure 5. (a) Positive ion ESI-MS spectrum of a chloroform solution of Co(hfa)2·TMEDA. (b) MS/MS spectra of the ions at m/z 382.43 in (a).
Figure 6. Negative ion ESI-MS spectrum of a methanolic solution of Co(hfa)2·TMEDA recorded using an Accurate-Mass 6520 Q-TOF instrument.
Co(II) Hexafluoroacetylacetonate Ethylenediamine Complex
Inorganic Chemistry, Vol. 48, No. 1, 2009 87
supposing the presence of a cobalt(I) center. As a conse-
quence, a reduction from Co(II) (the original oxidation state
in the precursor) to Co(I) seemed to take place under the
adopted ESI conditions. This process could have occurred
at the metal-solution interface of the ESI capillary in negative
ion mode, owing to the high voltage differences existing
between the ESI capillary and the counter-electrode in an
ESI ion source.50-52It is worthwhile recalling that the
electrospray source can be considered as a particular
electrolytic cell, in which electrolysis maintains the charge
balance, allowing the continuous production of charged
droplets.53,54In addition, note that the Co(I) oxidation state
is well-known and that various Co(I) complexes have been
reported.55On this basis, a possible structure for the ion at
m/z 660.9283 is [Co(I)(hfa)2(Hhfa-HF)]-(elemental formula,
C15H3O6F17Co; theoretical mass, 660.8985; mass accuracy,
41 ppm). Finally, the low-intensity peak at m/z 136.9850
was tentatively assigned to [CF3-CO-CdCdO]-, arising
from the hfa moiety by means of a CHF3loss. The calculated
elemental formula for the m/z 644.9502 ion is C15H6O6F16Co
(theoretical mass value, 644.92354; mass accuracy, 41 ppm),
and a possible structure is [Co(I)(hfa)2(CF2d(COH)-
CH2-(COH)dCF2)]-, where the fragment (CF2d(COH)-
CH2-(COH)dCF2) is likely derived from an hfa moiety. In
the MS/MS spectrum of the ion at m/z 644.9502 (Figure 7c),
apart from the signal at m/z 136.9855 (see above), the main
peak at m/z 206.9886 was due to [hfa]-, similarly to the case
reported in Figure 7a.
Despite the above data pointing out to a sort of solVent
effect in the ionization process, a common feature to all of
the obtained ESI-MS and MS/MS spectra of Co(hfa)2·
TMEDA solutions was the presence of very few peaks, thus
suggesting relatively simple fragmentation patterns. In ad-
dition, it is worth highlighting that no polynuclear species
were ever detected, a promising feature for CVD/ALD
CVD Depositions from Co(hfa)2·TMEDA. A key point
in the present study has been the precursor validation in CVD
experiments aimed at the production of cobalt oxide nano-
systems. The use of a vaporization temperature as low as
60 °C resulted in a reproducible and uniform mass supply,
thus confirming the precursor volatility, as already high-
lighted by thermal analyses (see above). Note that the
conventional Co(dpm)2precursor required a higher vaporiza-
tion temperature under analogous CVD conditions.10Pre-
liminary depositions in O2atmospheres on Si(100) substrates
yielded uniform and crack-free samples, characterized by a
bluish-gray color. GIXRD analyses (Figure 8) confirmed the
presence of cubic Co3O4as the only crystalline phase, as
evidenced by the reflections at 2ϑ ) 31.2°, 36.7°, and
(50) Kebarle, P.; Tang, L. Anal. Chem. 1993, 65, 972A.
(51) Van Berkel, G. J. In Electrospray Ionization Mass Spectrometry:
Fundamentals, Instrumentation and Applications; Cole, R. B., Ed.;
John Wiley & Sons: New York, 1997; pp 65-105.
(52) Van Berkel, G. J.; Zhou, F.; Aronson, J. T. Int. J. Mass Spectrom.
Ion Processes 1997, 162, 55.
(53) Kebarle, P.; Ho, Y. In Electrospray Ionization Mass Spectrometry:
Fundamentals, Instrumentation and Applications; Cole R. B., Ed.; John
Wiley & Sons: New York, 1997; pp 3-63.
(54) Blades, A. T.; Ikonomou, M. G.; Kebarle, P. Anal. Chem. 1991, 63,
(55) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; John
Wiley & Sons: New York, 1980, and references therein.
Figure 7. MS/MS spectra of the ions at m/z 679.8975 (a), m/z 660.9283 (b), and m/z 644.9502 (c), detected in the negative ion mode ESI-MS analysis of
methanolic solutions of Co(hfa)2·TMEDA. Spectra have been recorded by means of an Accurate-Mass 6520 Q-TOF instrument.
Bandoli et al.
88 InorganicChemistry, Vol. 48, No. 1, 2009
44.8°,56with no appreciable preferential orientations and a Download full-text
mean nanocrystal size of 25 nm. In agreement with structural
results, the XPS Co 2p3/2BE (780.5 eV, full width at half
maximum ) 3.8 eV), which did not show intense shake-up
peaks, and the Auger parameter (1552.5 eV, calculated by
the sum of the Co 2p3/2BE and the CoLMM Auger peak
kinetic energy) confirmed the presence of Co3O4,10,57with
no appreciable fluorine contamination. The high purity of
the obtained systems was testified by the disappearance of
the carbon signals (≈20 atom % on the sample surface) upon
a mild Ar+sputtering.
The high purity of the synthesized Co3O4films has been
further confirmed by EDXS analysis. The EDX spectrum of
Figure 9a was characterized by peaks located at 0.528, 0.788,
and 6.937 keV, attributable to OKR, CoLR, and CoKR
transitions, respectively. The more intense signal at 1.748
keV was ascribed to the SiKR line of the substrate. The
absence of contamination peaks pertaining to carbon or
fluorine confirmed the clean decomposition pathway of the
precursor, in agreement with the above analyses. Spectra
recorded on different surface areas indicated an almost
constant O/Co ratio of 1.32 ( 0.02, confirming the formation
of Co3O4with a uniform chemical composition. Finally, the
FE-SEM micrograph reported in Figure 9b revealed a
compact and very uniform morphology, characterized by
well-interconnected triangular lamellar features with mean
sizes ranging from 20 to 50 nm. Cross-sectional analyses
allowed estimation of an average film thickness of 200 nm.
This study has proposed an efficient and low-cost synthetic
strategy for the synthesis of Co(hfa)2·TMEDA, a Co(II)
?-diketonate adduct whose structure and chemico-physical
characterization had never been reported previously. The
target complex, obtained from commercially available prod-
ucts, is monomeric with a cis pseudo-octahedral arrangement
of the ligands around a CoO4N2 core. In addition, it is
characterized by a remarkable long-term stability and much
lower air and moisture sensitivity compared to both Co(II)
?-diketonate systems and homologous Co-hfa adducts.
Thermal analyses have indicated that Co(hfa)2·TMEDA
possesses an appreciable volatility and gives rise to vaporiza-
tion processes free from premature decomposition. These
favorable mass transport properties, along with the simple
and relatively clean fragmentation pattern evidenced by MS
analyses, demonstrate the great potential possessed by
Co(hfa)2·TMEDA as a CVD/ALD precursor for cobalt oxide
nanosystems, as confirmed by a preliminary CVD functional
validation. Further experiments devoted to the interrelation
between the Co-O nanosystem properties and the synthesis
conditions are already under way, and the pertaining results
will be the object of future investigation.
Acknowledgment. This work was financially supported
by CNR-INSTM PROMO and the CARIPARO Foundation
within the project “Multi-layer optical devices based on
inorganic and hybrid materials by innovative synthetic
strategies”. Thanks are due to Mr. Loris Calore and Dr.
Roberta Saini (Padova University) for elemental microanaly-
ses and thermal analyses, respectively. Mr. Andrian Milanov
(Bochum University) and Mr. Antonio Ravazzolo (Padova
University) are also acknowledged for skilful technical
Supporting Information Available: X-ray crystallographic data
of Co(hfa)2·TMEDA in the form of CIF file data. FT-IR and optical
absorption spectra of Co(hfa)2·TMEDA. This material is available
free of charge via the Internet at http://pubs.acs.org.
(56) Pattern No. 36-1451, Joint Committee on Powder Diffraction Standards
(57) Armelao, L.; Barreca, D.; Gross, S.; Tondello, E. Surf. Sci. Spectra
2001, 8, 14.
Figure 8. GIXRD patterns of a cobalt oxide sample deposited on Si(100)
from Co(hfa)2·TMEDA at 400 °C.
Figure 9. (a) EDX spectrum of a cobalt oxide specimen deposited on
Si(100) at 400 °C. (b) Representative plane-view FE-SEM micrograph for
the same system.
Co(II) Hexafluoroacetylacetonate Ethylenediamine Complex
Inorganic Chemistry, Vol. 48, No. 1, 2009 89