Room-temperature preparation of crystallized luminescent Sr1-xCaxWO4 solid-solution films by an electrochemical method

Article abstract of DOI:10.1063/1.116781

A complete series of well-crystallized solid-solution oxide films, Sr1-XCaXWO4 (0≤X≤1), has been prepared on a tungsten substrate in the electrolytic solution containing Sr2+ and Ca2+ ions by an electrochemical method at room temperature (25°C). The composition of solid-solution oxide films could easily be controlled by the concentrations of Sr and Ca species in the starting solutions. The films showed only single blue emission at liquid nitrogen temperature (-196°C), strongly suggesting that they consisted of well-crystallized defect-free crystals.

Full text of DOI:10.1063/1.116781

Roomtemperature preparation of crystallized luminescent Sr1−x Ca x WO4 solid
solution films by an electrochemical method
WooSeok Cho, Masatomo Yashima, Masato Kakihana, Akihiko Kudo, Tadayoshi Sakata, and Masahiro
Yoshimura
Citation: Applied Physics Letters 68, 137 (1996); doi: 10.1063/1.116781
View online: http://dx.doi.org/10.1063/1.116781
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Room-temperature preparation of crystallized luminescent Sr1؊xCaxWO4
solid-solution films by an electrochemical method
Woo-Seok Cho,a) Masatomo Yashima, and Masato Kakihana
Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226, Japan
Akihiko Kudob) and Tadayoshi Sakata
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Masahiro Yoshimura
Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226, Japan
͑Received 9 August 1995; accepted for publication 20 October 1995͒
A complete series of well-crystallized solid-solution oxide films, Sr1ϪXCaXWO4 ͑0рXр1͒, has
been prepared on a tungsten substrate in the electrolytic solution containing Sr2ϩ and Ca2ϩ ions by
an electrochemical method at room temperature ͑25 °C͒. The composition of solid-solution oxide
films could easily be controlled by the concentrations of Sr and Ca species in the starting solutions.
The films showed only single blue emission at liquid nitrogen temperature ͑Ϫ196 °C͒, strongly
suggesting that they consisted of well-crystallized defect-free crystals. © 1996 American Institute
of Physics. ͓S0003-6951͑96͒01101-3͔
Scheelite-type alkaline earth tungstates have been exten-
sively investigated since they have been proven to be good
laser host materials.1–8 Their films are more important for
better performance in industrial applications. In order to fully
realize the potential of the films, it is necessary to synthesize
the solid-solution films and to investigate systematically their
physical properties such as luminescence, lattice parameters,
and so on, because the composition of the solid solution
affects such physical properties. Nevertheless, even for
CaWO4, only limited investigations have been performed on
the preparation of the film.9,10 Some problems remain in
these studies.9,10 CaWO4 films9 prepared by sputtering con-
tained impurity phases such as Ca3WO6, Ca6WO9, WO3 ,
and WO3•0.33H2O because of the high vapor pressure of
WO3 . CaWO4 films10 prepared by the vacuum evaporation
showed no luminescence because of their poor crystallinity.
In order to increase the crystallinity of the films, heat treat-
ments ͑500 to 1100 °C͒ were necessary. This led to cracking
and/or peeling of the film due to shrinkage during crystalli-
zation and/or thermal treatment. These problems make the
preparation of solid-solution films more difficult.
from the solution during the electrochemical treatments. In
these previous works, as-deposited films were amorphous;
therefore, an additional heat treatment ͑400–1000 °C͒ was
necessary for their crystallization. Moreover, impurity phases
existed in these films. To our knowledge, there have been no
reports on a complete series of well-crystallized solid-
solution oxide films at room temperature. On the other hand,
our research effort has been devoted to low-temperature
preparation of crystalline oxide films directly from solutions
without firing or heat treatments after/during film
shaping.13–15 Our work in a previous letter15 has confirmed
the synthesis of CaWO4 by electrochemically reacting a
tungsten substrate with alkaline Ca͑OH͒ solution at room
2
temperature ͑25 °C͒. In the CaWO4 preparation, a growth
model has been proposed as follows:15
Wϩ4H2O→WO24Ϫϩ8Hϩϩ6eϪ,
WO24ϪϩCa2ϩ→CaWO4.
͑1͒
͑2͒
In alkaline solution containing both Ca and Sr species, solid-
solution films of Sr1ϪXCaXWO4 could be prepared as fol-
lows:
Recently, electrodeposition methods have been thought
to have potential for the economical production of thin films
because they require simple instrumentation and can produce
large-area films. While most of the studies so far have been
restricted to the electrodeposition of a single oxide or a
double oxide, the deposition of defined solid-solution films
has been the subject of only a few investigations.11,12 All
components of the oxides were codeposited on the electrode
WO24Ϫϩ 1ϪX͒Sr2ϩϩXCa2ϩ→Sr1ϪXCaXWO4 .
The synthesis of solid-solution films with multicomponents
has more significant meaning for the general applicability of
this film technique. In the present study, we describe the
͑3͒
͑
successful
synthesis
of
luminescent
crystallized
Sr1ϪXCaXWO4 ͑0рXр1͒ solid-solution films at room tem-
perature.
Tungsten metal substrates with 99.9 wt % in purity and
dimensions of 10ϫ40ϫ0.2 mm were used as counterelec-
trode ͑i.e., cathode͒ and working electrode ͑i.e., anode͒. Prior
to the electrochemical treatment, the tungsten substrates
were degreased in acetone with an ultrasonic cleaner. Elec-
a͒
Present address: Takayanagi particle surface project, Research Develop-
ment Corporation of Japan, 3-1-2 Musashino, Akishima, Tokyo 196, Japan.
b͒
Present address: Department of Applied Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162,
Japan.
Appl. Phys. Lett. 68 (1), 1 January 1996
0003-6951/96/68(1)/137/3/$6.00
© 1996 American Institute of Physics
137
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FIG. 1. X-ray diffraction patterns of the Sr1ϪXCaXWO4 films prepared on
the anodic tungsten substrates by the electrochemical treatment. The com-
positional formula SrxCay denotes the concentrations in the starting mixed
solution: xM for Sr and yM for Ca.
FIG. 2. Variation of the lattice parameters of the Sr1ϪXCaXWO4 films with
the Ca/͑SrϩCa͒ ratio of the starting solution.
Sr0.31Ca0.69WO4 from the c axis͒ calculated from the unit-cell
parameters agrees with that (Sr0.36Ca 0.64WO4͒ obtained by
x-ray photoelectron spectroscopy. These results indicate that
crystallized solid solutions form in the whole compositional
range ͑0рXр1͒ and that their compositions are controlled by
the concentrations of the starting solutions. The x-ray diffrac-
tion patterns also show a large decrease in peak intensities
and a large increase in full width at half maximum ͑FWHM͒
of peaks going from the Sr to the Ca tungstate while the
intensity and FWHM of the tungsten substrate remain about
the same. The decrease in peak intensities is due to both a
decrease in the film thickness and the difference of atomic
scattering factor18 between Ca ͓14.32 for ͑sin ␪͒/␭ϭ0.2
trolytic solutions were prepared by preheating the redistilled
water to remove dissolved CO2 and then adding
Sr͑OH͒2•8H2O and Ca͑OH͒ with a purity of 96.0 wt. %. All
2
experiments were performed with stirring of 200 rpm at
room temperature ͑25 °C͒ under galvanostatic conditions
͑i.e., constant current͒ with a current density of 1 mA/cm2
generated by a commercial power source ͑Type PAD
1K-0.2L, Kikusui Electronics Co.͒. The microstructures of
the films were investigated by scanning electron microscopy
͑JSM-T200, JEOL͒. The phases thus obtained were analyzed
by x-ray diffractometry ͑MXP3VA, Cu K␣, graphite mono-
chrometer, 40 kV–40 mA, MAC Science͒. Raman spectros-
copy ͑T64000, Atago-Jobin Yvon͒ was also employed to in-
vestigate the existing phase and the homogeneity of the
prepared film. Raman spectra were excited with a 514.5 nm
line of an argon ion laser using a micro-Raman system ͑spot
size ϳ1␮m͒. Luminescence spectra were measured using a
spectrofluorometer ͑Fluoromax™, Spex͒ at liquid nitrogen
temperature ͑Ϫ196 °C͒. The composition of the film pre-
pared in a solution with a Ca/͑CaϩSr͒ ratio of 0.5 was con-
firmed by x-ray photoelectron spectroscopy ͑ESCA-850, Shi-
madzu͒.
Å
Ϫ1] and Sr ͓29.83 for ͑sin ␪͒/␭ϭ0.2 ÅϪ1]. Scanning elec-
tron micrographs showed the decrease of film thickness from
about 6 ␮m for SrWO4 to about 3 ␮m for CaWO4. The peak
broadening with increasing of the Ca/͑SrϩCa͒ ratio in the
solution is ascribed to a decrease in crystallite size. The crys-
tallite size has been estimated using Scherrer’s formula from
the FWHM values of the 100 and 001 reflections. The size
Figure 1 shows the x-ray diffraction patterns of
Sr1ϪXCaXWO4 ͑0рXр1͒ films prepared on the tungsten
substrates by the electrochemical treatment. Each peak of the
Sr1ϪXCaXWO4 film shifted to a higher 2␪ position with an
increase of Ca concentration of the starting aqueous solution,
although the peak positions of the tungsten substrate were
almost unchanged. The unit-cell parameters of tetragonal
Sr1ϪXCaXWO4 changed continuously with the Ca/͑CaϩSr͒
ratio in solution from aϭ5.428 Å, cϭ11.989 Å for Xϭ0 to
aϭ5.245 Å, cϭ11.399 Å for Xϭ1 ͑Fig. 2͒. These values for
Xϭ0 and 1 agree well with those in the literature.16,17 The
composition of film prepared in a solution of 0.01 M
FIG. 3. Raman spectrum of film prepared on the anodic tungsten substrate
by the electrochemical treatment of 4.5 h at room temperature in the mixed
solution of Sr͑OH͒2•8H2O and Ca͑OH͒2 of 0.01 M, respectively. Each num-
ber denotes the Raman shift of a peak top.
Ca͑OH͒ and 0.01 M Sr͑OH͒2•8H2O has been investigated
2
by both the unit-cell parameter and x-ray photoelectron spec-
troscopy. The composition (Sr0.33Ca0.67WO4 from the a axis,
138
Appl. Phys. Lett., Vol. 68, No. 1, 1 January 1996
Cho et al.
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of the Sr1ϪXCaXWO4 solid-solution films were independent
of the Ca content: 250Ϯ2 nm for excitation and 463Ϯ2 nm
for emission. Almost all the researchers working in this field
have shared the opinion that the blue band in CaWO4 is due
to an electron transition within undisturbed WO42Ϫ
complexes.22 Our results suggest that the energy gap related
to the blue emission in WO24Ϫ complexes is almost unaf-
fected by the change in Ca content. However, there were
variations in the intensities of the emissions: The films in the
Ca-rich region showed an extremely strong luminescence,
while the films in the Sr-rich region showed a much fainter
emission. A theoretical approach is necessary for a successful
interpretation of the optical properties of these films. How-
ever, theoretical band calculations have not been carried out
yet, due to the fact that these solids have a large number of
atoms in the unit cell.22
It is rather surprising that a complete series of crystal-
lized Sr1ϪXCaxWO4 ͑0рxр1͒ solid-solution films can be
formed even at room temperature ͑25 °C͒. Furthermore, it is
noteworthy that the present films are defect-free showing a
single blue emission and do not contain any impurity phases,
contrary to the films prepared by sputtering at higher tem-
peratures.
FIG. 4. Scanning electron micrographs of the surface ͑a͒ and cross section
͑b͒ of Sr1ϪXCaXWO4 film prepared on an anodic tungsten substrate at room
temperature in the mixed solution of Sr͑OH͒2•8H2O and Ca͑OH͒ of 0.01
2
M, respectively.
decreased with an increase of the Ca/͑SrϩCa͒ ratio in the
solution. For CaWO4 film, the values were about 15 nm from
a 100 reflection and about 55 nm from a 001 reflection,
which agree with the average size ͑about 40 nm͒ observed by
scanning electron microscopy. A typical Raman spectrum of
the Sr1ϪXCaXWO4 film is shown in Fig. 3. All samples ex-
hibited Raman spectra of the tetragonal scheelite-type
structure19 without any impurity phases. Scanning electron
micrographs in Fig. 4 show the surface and cross section of
the Sr1ϪXCaXWO4 solid-solution film. The grain size is
larger than the film thickness. This means that the initially
formed crystals grow preferentially in the horizontal direc-
tion of the substrate, which provides a better supply of the
tungsten species. As shown in Fig. 5, the important feature is
that the films exhibited only a blue emission. Such a single
luminescent feature strongly suggests that the films are
defect-free.20,21 Grasser and Scharmann observed a green
emission in addition to the blue emission at liquid nitrogen
temperature ͑Ϫ196 °C͒ in the alkaline earth tungstates with a
scheelite-type structure that contained defects. This green
emission was interpreted to be ascribed to the existence of
Schottky-defect WO3 .21 In the present film, such green
emission was not observed, but there existed only single blue
emission. The peak positions of the excitation and emission
This work was supported by a Grant-in-Aid for Scien-
tific Research No. 07405028, NEDO International Joint Re-
search Grant. The authors would like to thank M. Osada for
help with Raman spectroscopy, and M. Ishiyama for assis-
tance with x-ray photoelectron spectroscopy.
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FIG. 5. Excitation ͑a͒ and emission ͑b͒ spectra of the Sr1ϪXCaXWO4 films
at liquid nitrogen temperature. The intensities of the spectra were normal-
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starting mixed solution: xM for Sr and yM for Ca.
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Cho et al.
139
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