Aqueous solution-based ceramic thin film deposition using organic polymers with amide groups

Article abstract of DOI:10.1246/cl.2001.1150

An aqueous solution-based dip-coating technique was presented for ceramic thin film deposition. Organic polymers with amide groups such as polyvinylpyrrolidone, polyvinylacetamide and polyacrylamide, incorporated in aqueous solutions of metal salts, were

Full text of DOI:10.1246/cl.2001.1150

1150
Chemistry Letters 2001
Aqueous Solution-Based Ceramic Thin Film Deposition
Using Organic Polymers with Amide Groups
Hiromitsu Kozuka* and Tomoko Kishimoto
Department of Materials Science and Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680
(Received August 8, 2001; CL-010763)
An aqueous solution-based dip-coating technique was pre-
sented for ceramic thin film deposition. Organic polymers with
amide groups such as polyvinylpyrrolidone, polyvinyl-
acetamide and polyacrylamide, incorporated in aqueous solu-
tions of metal salts, were demonstrated to offer much better
wetting of the substrates and gel film formability. Transparent,
crack-free TiO2 and ZrO2 thin films of 0.07–0.1 µm in thick-
ness could be obtained from aqueous solutions of Ti(SO4)2 and
ZrCl2O, respectively, containing these organic polymers.
Gel films were deposited on silica glass substrates (20 × 40
× 1.2 mm3) by dip-coating at a substrate withdrawal speed of 3
cm min–1, immediately heated at 100 °C for 10 min and then
fired at 700 °C for 10 min. In the absence of the organic poly-
mers, continuous films failed to be formed due to the poor wet-
ting. The inclusion of the organic polymers in solutions, on the
other hand, greatly enhanced the wettability and film formabili-
ty, resulting in crack-free, transparent films with traces of tiny
dots. The films stuck strongly to the substrate as in the case of
alkoxide-derived coating films, and were not delaminated with
Scotch tape or by ultrasonic wave irradiation in water.
As summarized in Table 1, the thickness of the fired films
ranged from 0.07–0.1 µm, which was measured using a contact
probe surface profilometer as in the same way as that described
elsewhere.4 Figure 1 shows the spectra of the fired films, where
a bare silica glass substrate was used as the reference. The
spectra indicate that the films obtained are highly transparent in
the visible range. The ripples in the spectra result from the
interference of the light. It might be the smooth surface and
dense microstructure that brought about the large amplitude of
the ripples, because both of them provide high reflectivity at the
air/film and film/substrate interfaces, which causes effective
interference of the light.
Dip- or spin-coating solutions for ceramic thin film deposi-
tion are prepared mostly through hydrolysis and polycondensa-
tion of metal alkoxides. Since metal alkoxides are hydrophobic
and immiscible with water, alcohols are used as mutual solvents
for homogenizing alkoxides and water.1 Alcohols, however,
are volatile, inflammable, and hence not favorable solvents to
be handled in manufacturers. Therefore, replacement of alco-
hols by water, which is not inflammable, is strongly demanded
in industries. However, water has significantly much higher
surface tension (72 mN m–1) than alcohols (20–23 mN m–1),
causing poor wettability of the substrates and film formability.
In fact, even when water-soluble metal salts like nitrates and
acetates are used as the starting materials, water is never used
as the major solvent but alcohols.2,3 In the present paper, it is
demonstrated that aqueous solutions of water-soluble metal
salts can be used as the coating solutions when organic poly-
mers with amide groups are contained.
The starting materials employed in preparing solutions
were ZrCl2O (Wako Pure Chemical Industries), aqueous solu-
tion of Ti(SO4)2 (30 wt%, Wako Pure Chemical Industries) con-
taining 13 wt% H2SO4, polyvinylpyrrolidone (PVP, 1, 1 × 104
in average molecular weight, Tokyo Kasei Kogyo), aqueous
solution of polyvinylacetamide (PVAcAm, 2, 40 wt% , 3 × 104
in average molecular weight, Showa Denko, GE-191LL), aque-
ous solution of polyacrylamide (PAAm, 3, 50 wt%, 1 × 104 in
average molecular weight, Aldrich) and ion-exchanged water.
The starting solutions of the batch compositions shown in Table
1 were prepared at room temperature. All the solutions were
clear, transparent and slightly colored.
Figure 2 shows the X-ray diffraction patterns of the fired
films measured using an X-ray diffractometer with a thin film
attachment and with Cu Kα radiation. Rutile and anatase phas-
es were detected in the TiO2 film prepared from the PVP-con-
taining solution, and only rutile in that from the PVAcAm-con-
taining solution. Cubic and monoclinic ZrO2 phases were
found in the ZrO2 film prepared from the PVP-containing solu-
tion, and only cubic ZrO2 in that from the PAAm-containing
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1151
solution, though in both cases the precipitation of orthorhombic
ZrO2 cannot be ruled out. Figure 3 shows the SEM picture of
the TiO2 film prepared from the PVP-containing solution,
where smooth surface and relatively dense microstructure are
seen.
Aqueous solutions of metal salts containing organic poly-
mers with amide groups have been demonstrated to work as the
precursor coating solutions for transparent, crack-free ceramic
thin films. Another benefit of the technique is the stability of
viscosity and the long lifetime of the solutions. Alkoxide-
derived sols generally increase in viscosity with time due to the
progress of polycondensation reaction towards metalloxane
polymers. In contrast, the present metal salts are stable as com-
plexes in aqueous media, and do not undergo polymerization
towards metalloxane polymers except at high pH. Although
analytical study has not been carried out, probably the major part
of the polymeric species are built up with the metal atoms or
complexes linked with the organic polymers. The O and/or N
atoms of the amide groups could coordinate with the metal
atoms, and the C=O groups could make hydrogen bonds with
the OH groups of the inorganic species,5 possibly constructing
organic–inorganic polymeric networks.
This work is financially supported by Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research (B)
and (C)), the Institute of Industrial Technology, Kansai
University, and the Kansai University Research Grants (Grant-
in-Aid for Encouragement of Scientists, 2001).
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C. J. Brinker and G. W. Scherer, “Sol-Gel Science, The
Physics and Chemistry of Sol–Gel Processing,” Academic
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H. Kozuka and M. Kajimura, J. Am. Ceram. Soc., 83, 1056
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