A study of physicochemical processes in film-forming ethanol solutions of zirconium and cobalt salts and properties of thin films obtained on their base

Article abstract of DOI:10.1023/B:RJAC.0000030346.70645.12

The main physicochemical processes that occur in film-forming solutions based on zirconium oxochloride ZrOCl₂·8H₂O, cobalt salts CoCl₂·6H₂O and Co(NO₃) ₂·6H₂O, and ethanol were studied. The intermolecular interaction in film-forming solutions was analyzed qualitatively and quantitatively.

Full text of DOI:10.1023/B:RJAC.0000030346.70645.12

Russian Journal of Applied Chemistry, Vol. 77, No. 2, 2004, pp. 182 186. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 2, 2004,
pp. 188 192.
Original Russian Text Copyright
2004 by Kozik, Chernov, Borilo, Turetskova, Shul’pekov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
A Study of Physicochemical Processes in Film-forming Ethanol
Solutions of Zirconium and Cobalt Salts and Properties
of Thin Films Obtained on Their Base
V. V. Kozik, E. B. Chernov, L. P. Borilo, O. V. Turetskova, and A. M. Shul’pekov
Tomsk State University, Tomsk, Russia
Received March 26, 2003
Abstract The main physicochemical processes that occur in film-forming solutions based on zirconium
oxochloride ZrOCl2 8H O, cobalt salts CoCl2 6H O and Co(NO ) 6H O, and ethanol were studied.
2
2
3 2
2
The intermolecular interaction in film-forming solutions was analyzed qualitatively and quantitatively.
Thin films find wide use as light-redistributing,
protective, and decorative coatings [1]. Extensive
studies intended to obtain new compositions of films,
analyze their structure and properties, and extend ap-
plication fields are being carried out in this country
and abroad [2, 3]. Particular attention is paid to com-
posite thin-film materials based on binary and ternary
systems constituted by oxides of Group-IV elements
99% ethanol, zirconium oxochloride ZrOCl2 8H O,
2
and cobalt salts CoCl2 6H O or Co(NO ) 6H O.
2
3 2
2
Ethanol was preliminarily dehydrated with calcium
oxide, using the known procedure [6]. This is neces-
sary for removing excess water, since the salts used
contain crystallization water and its amount is suffi-
cient to ensure a minor degree of hydrolysis in FFS.
The excess amount of water, introduced with nonde-
hydrated ethanol, enhances the hydrolysis, with the
result that the working life of FFS becomes substan-
tially shorter and a polymer is formed rapidly. The
total concentration of the salts in the solutions was
0.4 M, the content of zirconium oxochloride was var-
ied within the range 20 100 mol %.
[
4, 5]. The properties of coatings of this kind are de-
termined by the properties of the constituent oxides
and their relative amounts. Films based on zirconium
dioxide are promising because of their practically
valuable properties: transparency and high refractive
index, adhesion, thermal resistance, and chemical
stability. However, the use of thin films based on zir-
conium dioxide is limited by the occurrence of poly-
morphic transformations, which affects the service
characteristics and quality of films. Introduction of
cobalt oxide into thin-film materials based on zirco-
nium dioxide makes it possible to stabilize its struc-
ture and properties and vary widely the optical char-
acteristics of the films.
The processes that occur in FFS were studied using
viscometry (Ostwald viscometer), and the optical den-
sity of the solutions was measured with a KFK-2 pho-
tocolorimeter. The concentration dependence of re-
duced viscosity sp/c of an FFS based on zirconium
oxochloride and cobalt nitrate was measured by dilut-
ing the initial FFS with ethanol, in the zirconium oxo-
1
chloride concentration range 0.5 3.3 g dl . The res-
This study is concerned with physicochemical
processes that occur in film-forming solutions (FFS),
qualitative and quantitative analysis of intermolecular
interactions in solution, formation of thin films of
ulting concentration dependences of sp/c were pro-
cessed using the method described in [7, 8].
Films were obtained on glass [GOST (State Stan-
dard) III-2001] and quartz substrates by centrifugation
(rate of substrate rotation 1000 4000 rpm) and pull-
the system ZrO CoO, and examination of their prop-
2
erties.
1
ing from solution (pulling velocity 5 mm min ) at a
EXPERIMENTAL
temperature of 290 300 K. A thermal treatment of the
films was done in two stages at 330 and 770 870 K.
The optical characteristics of the films were studied
on an LEF-3M laser ellipsometer and Specord-M40
Thin films of double oxides of zirconium and co-
balt were obtained by deposition from FFS based on
1
070-4272/04/7702-0182 2004 MAIK Nauka/Interperiodica

A STUDY OF PHYSICOCHEMICAL PROCESSES IN FILM-FORMING ETHANOL SOLUTIONS
183
spectrophotometer; the adhesion was determined with
a PMT-3 device.
The film-forming solutions based on ZrOCl2 8H O
2
are colorless; addition of cobalt chloride leads to ap-
pearance of a blue coloration, and that of cobalt nitrate
makes the solution blue and violet. The composition
of cobalt complexes in the FFS based on zirconi-
um(IV) oxochloride and cobalt(II) nitrate was deter-
mined using data on the varied coloration of complex
compounds of cobalt [9] and the Ostromyslenskii
Zhoba method [10]. Figure 1 shows how the optical
density D of FFS at a wavelength of 670 nm varies
with the content of cobalt nitrate. Probably, the exis-
tence of a varied coloration of FFS is associated
with changes in the coordination of H O molecules
and Cl ions around a Co ion, with the blue color
2
2
+
Fig. 1. Optical density D of an FFS based on zirconium
oxochloride vs. the content of cobalt nitrate, [Co(NO ) ].
pointing to the presence of chloride complex anions
3 2
2
[
CoCl ] , and violet color, to that of a mixed com-
4
plex [Co(H O) Cl ] [9]. It may be assumed that chlo-
2
4
2
2
ride complex species of composition [CoCl4] pre-
dominate at a cobalt nitrate content in FFS lower than
33 mol %, and mixed-ligand complexes of composi-
tion [Co(H O) Cl ], at a higher content of cobalt ni-
2
4
2
trate (Fig. 1). The presence of cobalt(II)chloride com-
plex species in FFS based on zirconium oxochloride
and cobalt nitrate is accounted for by the high content
of chloride ions formed in dissociation of zirconium
oxochloride and also by the affinity of Co(II) for chlo-
ride ions.
The stability of the FFS viscosity with time is
technologically important for obtaining films with
reproducible properties. Therefore, the viscosity can
serve as a criterion of suitability of a solution for film
formation. The viscosity of FFS commonly varies
with time in three stages, which correspond to ripen-
ing, region of stable properties, and aging through
gelation and polymerization [11]. For the FFS based
on zirconium oxochloride the three stages are pre-
served; the variation of the specific viscosity of this
solution is shown in Fig. 2, curve 1. The solution rip-
ening takes 4 days, during which alcoholysis and hy-
drolysis occur, and oxozirconium(IV) ions, which
exhibit high affinity for oxygen, are bound together
by the alcohol molecules by ol ( OH) and oxol ( O )
bonds. The film-forming capacity of alcoholic solu-
tions based on zirconium oxochloride is due to the
presence of hydroxo complex of zirconium of the
composition [Zr(OH) (C H OH) (H O) ]Cl [11].
Fig. 2. Specific viscosity
um oxochloride and cobalt chloride vs. the storage time .
of an FFS based on zirconi-
sp
CoCl content (mol %): (1) 0, (2) 50, (3) 80, and (4) 100.
2
Introduction of cobalt(II) chloride into an FFS
based on zirconium oxochloride in amount of up to
0 mol % has no pronounced effect on the variation
of viscosity with time (Fig. 2, curve 2). At CoCl2
content in FFS of 80 or 90%, the specific viscosity
is anomalously high, and these solutions turn into gels
already after 1.5 months (Fig. 2, curve 3). In an FFS
7
with 80 mol % CoCl , there are four cobalt atoms
2
per zirconium atom, and, therefore, a complex tetra-
meric particle, in which each zirconium atom is bound
to four cobalt atoms, can serve as a polymerization
center [12]. The alcoholysis processes that occur in
parallel lead to formation of larger macromolecules.
2
2
5
x
2
4
x
2
Since Zr(IV) shows strong tendency toward formation
of polynuclear compounds, particle condensation oc-
curs in FFS to give more complex macromolecules,
and, consequently, the viscosity changes.
The affinity of cobalt for oxygen is low, and the
probability of formation of ol and oxol bonds is con-
siderably lower as compared with zirconium. There-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004

184
KOZIK et al.
sp/c = [ ]a + k [ ]2c
(1)
and nonlinear extrapolation equations
sp/c = a + bexp( dc),
(2)
(3)
sp/c = a + b exp( d c) + b exp( d c),
1
1
2
2
where a, b , and d are empirical solvation coefficients,
i
i
which depend on the dipole momentum and donor and
acceptor numbers of a solvent and have a dimension-
ality of molar volume.
Various functions applicable to description of em-
pirical concentration dependences of viscosity make
it possible to determine the numerical values of the
characteristic viscosity [ ].
Fig. 3. Reduced viscosity of FFS,
/c, vs. its con-
sp
1
centration c. Co(NO ) concentration (g dl ): (1) 0,
3
2
The variation of the reduced viscosity of an FFS
based on zirconium(IV) oxochloride (Fig. 3, curve 1)
can be described using Eqs. (1) (3). The Huggins
constants k and the characteristic viscosities, calcu-
lated for this solution by three equations, are listed
in Table 1. The Huggins constant has an unrealistic
value (k = 2.41), which is probably due to the pres-
ence of hydrogen bonds in solution. As indicated
by variances and correlation coefficients, Eq. (3) de-
scribes the given experimental dependence the most
precisely, and, consequently, the significant value of
(
2) 10, and (3) 20.
fore, the specific viscosity is lower, too (Fig. 2, cur-
ve 4). The aging of a solution of this kind begins
after 14 months.
It should be noted that processes in FFS with zir-
conium(IV) oxochloride and cobalt(II) nitrate are sim-
ilar to those in film-forming solutions with zirconi-
um(IV) oxochloride and cobalt(IV) chloride salts.
1
Taking into account the fact that the FFS based on
zirconium oxochloride are polymer solutions, the ap-
proach described in [7] can be used to obtain quantita-
tive characteristics of the interaction of macromole-
cules in solution. To describe the concentration de-
pendence of the reduced viscosity sp/c, the authors
of [7] employed a linear extrapolation equation (Hug-
gins equation)
the characteristic viscosity is 0.102 dl g . The varia-
tion of the reduced viscosity of an FFS based on zir-
conium oxochloride with addition of cobalt nitrate
(Fig. 3, curves 2 and 3) is also described more pre-
cisely by Eq. (3). The Gibbs energies of solvation
1
(kJ mol ), calculated using the values of [ sp] for
the FFS under study, are listed in Table 2. The inter-
action of the forming hydroxo complex of zirconi-
Table 1. Viscometric parameters of FFS based on zirconium(IV) oxochloride
Equation
1)
, dl g 1
0.059
k
Equation
(2)
, dl g 1
0.137
Equation
(3)
, dl g 1
0.102
(
2.41
Table 2. Effect of the composition of an FFS based on zirconium(IV) oxochloride and cobalt(II) nitrate on the visco-
metric parameters
Ga
Gb1
Gb2
Co(NO ) ,
mol %
3
2
[ ], dl g 1
a
b1
b2
d1
d2
kJ mol 1
0
0
0
0.145
0.310
0.114
0.150
0.301
0.222
0.107
0.018
0.129
3.5
28.4
13.4
3.64
27.60
26.18
2.60
1.65
15.21
0.063
0.050
0.160
0.686
0.046
0.050
0.102
0.027
0.021
1
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004

A STUDY OF PHYSICOCHEMICAL PROCESSES IN FILM-FORMING ETHANOL SOLUTIONS
185
um(IV) with the solvent occurs by both the donor
acceptor and ionic mechanisms. In an FFS that con-
Table 3. Physical properties of films obtained from FFS
based on zirconium(IV) oxochloride and cobalt(II) nitrate
substrate, glass)
(
tains only zirconium(IV) oxochloride, there are insig-
1
nificant electrostatic ( G = 3.5 kJ mol ) and specif-
a
CoO,
mol %
Refractive
index n
Film thickness Adhesion,
ic donor acceptor interactions ( G = 3.61, Gb2
=
b1
2
1
d, nm
kg mm
2
.60 kJ mol ). This circumstance is accounted for
by the fact that zirconium(IV) tends to form complex
D polymeric structures to a greater extent and has
0
2.1600
2.0875
2.0211
1.9907
1.8916
105.0
102.5
100.2
80.9
69.3
56.5
56.5
31.0
28.2
3
1
2
6
0
0
0
high characteristic viscosity. Introduction of cobalt(II)
nitrate strongly affects the Gibbs energies (Table 2):
G , G , and Gb2 increase and the characteristic
a
b
1
80
63.1
viscosity decreases, which is due to the lower ability
of cobalt(II) to form polymeric structures and to the
enhancement of intermolecular interaction forces in
solution. Thus, the film-forming capacity is character-
ized by formation of a microheterogeneous system in
the FFS, and this may affect the thickness of the films
obtained and their adhesion to a substrate.
in solution lead to a poor adhesion of cobalt oxide
films to glass substrates.
The absorption edge of zirconium oxide films lies
at 220 nm, which allows their use as cutoff filters that
eliminate the hard UV radiation. By changing the rela-
tive amounts of zirconium and cobalt oxides, the trans-
mission coefficient of the films in the UV and in the
visible can be varied widely (Fig. 4). Introduction of
cobalt(II) oxide makes stronger the absorption not
only in the UV range, but also in the visible, with
the films remaining uniform and homogeneous. In this
case, the color of a glass substrate with a deposited
film of zirconium(IV) and cobalt(II) oxides changes
from transparent to dark brown, which makes it pos-
sible to control the intensity of sunlight, obtain coat-
ings with sun-protective effect, and adjust the spectral
composition of light sources in the UV range and in
the visible [13. 14].
Applying an FFS to a substrate and carrying out
a thermal treatment yields thin films of zirconium(IV)
and cobalt(II) oxides [12]. It has been established in
previous studies (X-ray phase analysis) that films with
cobalt(II) oxide content below 10 mol % are solid
solutions based on the cubic modification of zirconium
dioxide, and those with >10 mol % cobalt(II) oxide,
a mixture of zirconium(IV) and cobalt(II) oxides [5].
The physicochemical properties of the films obtained
are listed in Table 3. The tendency for zirconium to
produce 3D polymeric structures leads to formation
of thicker films. Raising the content of Co(II) oxide
yields thinner films and makes lower their adhesion to
a substrate, in agreement with the change in the struc-
ture of the hydroxo complex and the decrease in vis-
cosity. The less pronounced affinity of cobalt for oxy-
gen and the high extent of intermolecular interactions
Figure 5 shows how the refractive index of the films
depends on their thickness. Films of thickness 50
100 nm have a higher refractive index. Probably, this
Fig. 4. Transmission spectra of films on quartz substrates.
(
) Wavelength. (1) Quartz substrate; films obtained from
FFS with varied content of cobalt(II) nitrate (mol %):
2) 0, (3) 20, (4) 30, and (5) 50.
Fig. 5. Refractive index n of films obtained from FFS
with cobalt chloride content of 10 mol % vs. the film
thickness.
(
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004

186
KOZIK et al.
Table 4. Influence exerted by the content of cobalt(II) chloride and nitrate on the refractive index of films obtained
from FFS based on zirconium(IV) oxochloride (substrate, glass)
Refractive index n obtained with a salt indicated
Refractive index n obtained with a salt indicated
CoO,
mol %
CoO,
mol %
CoCl2 6H O
Co(NO3)2 6H O
CoCl2 6H O
Co(NO3)2 6H O
2
2
2
2
2
3
0
0
2.075
2.089
2.021
1.950
40
50
2.025
1.959
1.881
1.930
occurs because of the formation of a metastable cubic
modification of zirconium dioxide, which may be
due to a manifestation of a confinement effect, which
leads to stabilization of high-temperature phases in
nanosystems [15].
3. Borilo, L.P., Tonkoplenochnye neorganicheskie nano-
sistemy (Thin-Film Inorganic Nanosystems), Tomsk:
Tomsk. Univ., 2003.
4. Mal’chik, A.G., Borilo, L.P., and Kozik, V.V.,
Zh. Prikl. Khim., 1996, vol. 69, no. 2, pp. 224 227.
5
6
. Borilo, L.P., Shul’pekov, A.M., and Turetskova, O.V.,
Steklo Kerami., 2002, no. 4, pp. 30 31.
. Voskresenskii, P.I., Tekhnika laboratornykh rabot
The choice of the starting cobalt(II) salt (chloride,
nitrate) has no effect on the chemical stability and
color of a film. However, use of cobalt(II) chloride
makes the refractive index of the films approximately
(
1
Manual of Laboratory Works), Moscow: Khimiya,
973.
4
.5% higher (Table 4), which indicates that the films
7
8
9
. Safronov, S.M., Berezina, E.M., Terent’eva, G.A.,
et al., Vysokomol. Soedin., 2001, vol. 43B, no. 4,
pp. 751 754.
. Bogoslovskii, A.Yu., Pribytkov, E.G., Terent’eva, G.A.,
et al., Zh. Prikl. Khim., 1998, vol. 71, no. 2,
pp. 294 300.
. Nekrasov, B.V., Osnovy obshchei khimii (Fiundations
of General Chemistry), Moscow: Khimiya, 1973,
vol. 2.
obtained have higher density. This may be due to dif-
ferent compositions of cobalt complexes in the FFS
(
chloride, mixed-ligand), whose decomposition under
thermal treatment affects the structure of a forming
oxide film.
CONCLUSIONS
(
1) Thin films of the system ZrO CoO were ob-
2
1
0. Kumok, V.N. and Skorik, N.A., Laboratornye ra-
boty po khimii kompleksnykh soedinenii (Laboratory
Works on Chemistry of Complex Compounds), Tomsk:
Tomsk. Univ., 1983.
tained by deposition from film-forming solutions and
their physicochemical properties (refractive index,
thickness, transmission spectra, adhesion to substrate)
were determined.
1
1
1. Kozik, V.V., Borilo, L.P., and Shul’pekov, A.M.,
Zh. Prikl. Khim., 2000, vol. 73, no. 11, pp. 1872 1876.
(2) It was shown that the composition of the start-
2. Khimiya i tekhnologiya redkikh i rasseyannykh ele-
mentov (Chemistry and Technology of Rare and Trace
Elements), Bol’shakov, K.A., Ed., Moscow: Vysshaya
Shkola, 1976, part 2.
3. Suikovskaya, N.V., Khimicheskie metody polucheniya
tonkikh prozrachnykh pokrytii (Chemical Methods for
Obtaining Thin Transparent Coatings), Leningrad:
Khimiya, 1971.
ing cobalt salt (chloride, nitrate) and the film thick-
ness affect the refractive index. Films of the system
ZrO CoO are promising as coatings protecting bio-
2
logical objects from the UV radiation and as decora-
tive and sunlight-protecting coatings.
1
REFERENCES
1
2
. Saifulin, R.S., Neorganicheskie kompozitsionnye ma-
terialy (Inorganic Composite Materials), Moscow:
Khimiya, 1983.
14. Physics of Thin Films. Advances in Research and
Development, vol. 5, Hass, G. and Thun, R.E., Eds.,
New York: Academic, 1967.
. Min’ko, N.I., Mikhal’chuk, I.N., and Lipko, M.Yu.,
15. Uvarov, N.F. and Boldyrev, V.V., Usp. Khim., 2001,
Steklo Keram., 2000, no. 4, pp. 3 7.
vol. 70, no. 4, pp. 307 329.
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