Synthesis of Bi-O- and Bi-V-O-containing nanolayers by ionic deposition

Article abstract of DOI:10.1023/A:1012356717650

A possibility of preparing Bi₂O₃ and BiVO₄ nanolayers by ionic deposition on the silica surface was shown for the first time. The influence of the concentration and pH of reactant solutions and of the number of ionic depos

Full text of DOI:10.1023/A:1012356717650

Russian Journal of General Chemistry, Vol. 71, No. 1, 2001, pp. 4 7. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 1, 2001,
pp. 6 9.
Original Russian Text Copyright
2001 by Tolstobrov, Tolstoi, Murin.
Synthesis of Bi O- and Bi V O-containing Nanolayers
by Ionic Deposition
E. V. Tolstobrov, V. P. Tolstoi, and I. V. Murin
St. Petersburg State University, St. Petersburg, Russia
Received November 29, 1999
Abstract A possibility of preparing Bi2O3 and BiVO4 nanolayers by ionic deposition on the silica surface
was shown for the first time. The influence of the concentration and pH of reactant solutions and of the num-
ber of ionic deposition cycles on the kinetics of the nanolayer growth was studied. The conditions were de-
termined for the formation of a nanolayer by a reaction in a layer of ions sorbed on the surface. The thickness
of the synthesized layers was determined by ellipsometry. The layer compositions were studied by X-ray
spectral microanalysis, X-ray photoelectron spectroscopy, and UV and IR Fourier transform spectroscopy.
The structure of the layers was studied by powder X-ray diffraction.
Bismuth oxide and a number of vanadates are solid
electrolytes with exceptionally high oxygen conduc-
tivity. Several methods are used to synthesize thin-
layer structures based on them, but each of these
methods involves high-temperature annealing, neces-
sary to ensure diffusion of reagents, or their spraying
in a vacuum. However, these compounds can also be
prepared by the mild chemistry route using reac-
tions of ionic deposition, i.e., by successive and
multiple sorption of cations and anions on the surface
followed by heating of the samples in air to remove
water and to decompose aqua and hydroxo complexes.
Clearly the temperature of such heating will be lower
than the temperature required for the diffusion of re-
agents in the solid-phase synthesis. Such a route
resembles the sol gel process in the sequence of treat-
ments. However, an essential difference consists in
the fact that, owing to successive sorption, ionic de-
position allows very precise control of the thickness
of a layer synthesized on the surface and preparation
of a multilayer consisting of two or more components
located across the layer thickness in the sequence
preset by the synthesis program.
droxide. Aqueous solutions of BiOClO4 and NaVO3
served as reagents in the synthesis of Bi V O-con-
taining nanolayers with water as the washing liquid. It
was suggested that in the case of Bi O-containing
layers irreversible sorption of bismuth cations on the
support surface, similar to the synthesis of Sn O- [1],
Ce O- [2], and Fe O-containing layers [3], could be
ensured by redox reactions Bi3+
Bi5+ in the layer of
sorbed ions, and in the case of Bi V O-containing
layers, by the formation of a difficultly soluble bis-
muth vanadate, similar to the synthesis of ZnS [4].
We carried out reactions of ionic deposition on
the surface of KU brand melted quartz, KEF-7.5 single
crystal silicon with 111 orientation, and KSKG
brand silica gel. Prior to the synthesis, the supports
were subjected to a standard treatment as described in
detail in [5]. Then we successively immersed the sup-
ports in solutions of reagents, intermediately washing
off their excess (for example, in the synthesis of bis-
muth oxide nanolayers, in a bismuth salt solution,
a washing liquid, a slightly alkaline solution of hy-
drogen peroxide, and again the washing liquid). Such
a sequence of treatments affording a nanolayer of hy-
drated bismuth oxide represented one cycle of ionic
deposition. Thicker nanolayers were prepared by mul-
tiple repetition of the ionic deposition cycles.
The aim of this work was to determine conditions
for the synthesis of Bi O- and Bi V O-containing
nanolayers by ionic deposition on the surface of a
silica support.
The nanolayers prepared on the silicon surface
were studied by ellipsometry and IR Fourier trans-
mission spectroscopy; those on the quartz surface, by
UV-visible spectroscopy; and those on the silica gel
surface, by diffuse reflection UV-visible spectroscopy,
X-ray spectral microanalysis, and powder X-ray dif-
fraction.
We used aqueous solutions of BiOClO4 H2O and
H2O2 as reagents in the synthesis of bismuth oxide
nanolayers. Water served as a washing liquid. We
varied pH of BiOClO4 solutions within the range
3 5.5 by adding sodium acetate, and pH of H2O2
solutions in the range 5 9 by adding potassium hy-
1070-3632/01/7101-0004$25.00 2001 MAIK Nauka/Interperiodica

SYNTHESIS OF Bi O- AND Bi V O-CONTAINING NANOLAYERS
The thickness of layers on the surface of single
5
crystal silicon was measured on an ellipsometer with
the light wavelength of 632.8 nm and light incidence
angle of 45 . The layer thickness was calculated from
the experimental angles
by the Fresnel’s equation
in Archer’s approximation.
We varied the concentration and pH of solutions of
the bismuth salt to determine the optimal conditions
for the synthesis of bismuth oxide nanolayers. We
found that the layer grows only if pH of the solution
is in the range 5 5.5, and its optimal concentration
is 0.01 M. Under these conditions the thickness of
the synthesized oxide layer increases linearly, its in-
crease per one cycle of ionic deposition being 0.6 nm.
We also found that the optical density D at 300 nm
in the transmission spectra of bismuth oxide nano-
layers synthesized on the surface of melted quartz in-
creases in proportion to the number of ionic deposi-
tion cycles.
Fig. 1. X-ray photoelectron spectrum of bismuth oxide
nanolayer prepared by 30 cycles of ionic deposition on
the surface of melted quartz.
The chemical composition of the layers was studied
by the X-ray photoelectron and IR Fourier transform
spectroscopy. The X-ray photoelectron spectrum
(Fig. 1) contains a peak at 158.8 eV corresponding
to Bi3+ ions (4f7/2) in the Bi2O3 composition. The data
of the IR Fourier transform spectroscopy point to
the presence of water molecules in the layer (3450
Fig. 2. UV-visible transmission spectrum of a Bi V O-
containing nanolayer prepared by 25 cycles of ionic de-
position on the surface of melted quartz.
1
and 1640 cm ), which are removed after heating in
air at 320 C.
Later, when placed in a slightly alkaline solution of
hydrogen peroxide, Bi3+ is oxidized to Bi5+:
Our experimental data suggest the following
scheme of reactions occurring on the silica surface in
the course of the synthesis of bismuth oxide nano-
layers. The silica surface is known to be negatively
charged in neutral and slightly alkaline solutions
owing to the dissociation of hydroxy groups:
Si OBix(OH)y,aq + H2O2(OH )
Si OBix(OH)z,aq.
On the repeated treatment in a BiOClO4 solution,
BiOH groups dissociate by the following scheme:
Bix(OH)z
Bix(OH)z 1O + H+.
Si OH
Si O + H+.
The surface again becomes negatively charged,
5+
and, consequently, the sorption of Bi9(OH) ions
5+
22
22
Positively charged Bi9(OH) ions are sorbed on
the surface. According to [6], such ions occur in
the bismuth perchlorate solution at pH 5.
and the formation of the next Bi5+ oxyhydrate layer
become possible.
However, according to the data of X-ray photo-
electron spectroscopy, Bi5+ ions are absent from the
layer. Apparently, drying of the sample in air before
recording the X-ray photoelectron spectrum or its
deep evacuation necessary to obtain the spectrum is
5+
Si O + Bi9(OH)
22 aq
4+
22 aq
5+
Si OBi9(OH)
(Bi9(OH)
)
.
22 aq x,ads
accompanied by the Bi5+
Bi3+ transition.
Then, in the stage of washing off the excess of
bismuth salt with water, this sorption complex de-
composes to give a new complex:
In the synthesis of Bi V O-containing layers we
also varied the concentration and pH of the reagent
solutions. The thickness of the layer synthesized on
the surface of melted quartz was estimated from
the UV-visible transmission spectra. In one of such
spectra (Fig. 2) an intense absorption band in the range
4+
22 aq
5+
Si OBi9(OH)
(Bi9(OH)
)
22 aq x,ads
Si OBix(OH)y,aq
.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

6
TOLSTOBROV et al.
sideration is close to 1 at pH 3 and to 3 at pH 6 8.
At pH 2 this ratio is also close to 1, but, as already
noted, the concentration of such vanadium anions is
decreased at the expense of the formation of VO+2 cat-
ions in solution, and this can decrease the amount of
vanadium sorbed on the surface. It seems that, to
understand the surface reactions, it is necessary to take
into account not only variation of the composition of
vanadium oxohydroxide complexes, but also participa-
tion of hydroxide ions in these reactions. In particular,
it can be assumed that a decreased vanadium concen-
tration at its sorption from solutions with pH 6 8 is
associated with competing sorption of OH ions on
the surface.
Fig. 3. Plot of the optical density D ( 400 nm) in the trans-
mission spectra of Bi V O-containing nanolayers synthe-
sized on the surface of melted quartz vs. concentration of
a BiOClO solution (n = 15, pH
3, C
0.01 M,
4
BiOClO4
NaVO3
The Bi V O-containing samples prepared by
10 cycles of ionic deposition and dried in air were
studied by X-ray diffraction. They appeared to be
X-ray amorphous. However, in the X-ray pattern of
a sample synthesized with an NaVO3 solution (pH 3)
a number of peaks can be distinguished against the
background of amorphous halo, which are typical for
BiVO4 (ASTM no. 34-133). The intensity of these
peaks increases on heating in air at 300 C.
pH
3).
NaVO3
260 500 nm is observed. The examination of the in-
tensity of this band as a function of concentration and
pH of the reagents allowed us to choose the optimal
synthesis conditions. In particular, using the de-
pendence of the optical density at 400 nm (Fig. 3),
we chose the concentration of 0.01 M for the bismuth
salt as optimal. We found that the concentration of
sodium vanadate solution should also be 0.01 M. At
higher concentrations the layer thickness did not in-
crease proportionally, and this, in our opinion, sug-
gests that the synthesized layer forms within the layer
of ions sorbed on the surface, rather than in the bulk
of solution with subsequent precipitation on the sur-
face.
Taking into account the fact that at pH 3 bismuth
6+
12
exists in solution mainly as Bi6(OH) cations [6],
we can suggest on the basis of our experimental data
the following scheme of the synthesis of Bi V O-
containing nanolayers on the surface of a silica sup-
port:
SiOH
SiO + H+,
The layers containing Bi V O were synthesized at
various pH (2.0, 3.0, 6.0, and 8.0) of an NaVO3 solu-
tion. At pH 2, a light yellow nanolayer is formed on
the silica gel surface. According to the results of
X-ray spectral microanalysis of a sample prepared
by 10 cycles of ionic deposition, the ratio Bi : V in
the layer is 1 : 0.79.
6+
12
5+
6+
SiO + Bi6(OH)
SiOBi6(OH) Bi6(OH) (excess),
12 12
5+
12
6+
12
SiOBi6(OH)
Bi6(OH) (excess) + H2O(washing)
SiOBix(OH)y,
SiOBix(OH)y + V10O26(OH)24
At pH 3.0, we obtained a bright yellow layer with
Bi : V = 1 : 1. At pH 6.0 or 8.0, the color was less
bright, and the Bi : V ratio in the layer was 1 : 0.3.
SiOBixVyOz(OH)k V10O26(OH)42 (excess),
SiOBixVyOz(OH)k V10O26(OH)42 (excess)
These results can be accounted for by different com-
positions of vanadium(V) oxohydroxide complexes in
solutions at the above-mentioned pH values. Thus,
according to [6], at pH 2 a part of vanadium ions is
present in solution in a cationic form and probably
does not react with positively charged bismuth ions
sorbed on the surface of silica support, and another
part exists as decavanadate ions V10O26(OH)42 , which
take part in the reaction. At pH 3 vanadium mainly
exists as V10O26(OH)42 , and at pH 6.0 and 8.0, as
V3O39 anions. It is evident that the Bi : V ratio for
the bismuth cation and vanadium anions under con-
+ H2O(washing)
SiOBixVyOz(OH)k.
As already noted, the ratio x : y for samples ob-
tained from NaVO3 solutions with pH 3 appeared to
be 1 : 1. When interpreting this result, account must
be taken also of the fact that, according to the X-ray
data, bismuth and vanadium in this layer mutually
alternate in a way differing from the alternation
assumed for the polymeric cationic and anionic com-
plexes sorbed on the surface. To explain this effect,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

SYNTHESIS OF Bi O- AND Bi V O-CONTAINING NANOLAYERS
7
we suggest that the compositions of both cationic and
anionic complexes sorbed on the surface essentially
change after washing off excess reagent with water
and subsequently treating the sample with a solution
containing a counterion. A similar effect was noted
earlier, e.g., at sorption of sodium polymolybdate on
the surface of aluminum oxide [7]. In spite of the fact
that molybdenum exists in solution as a polymeric
complex, the experiments showed that it is sorbed on
the surface as a monomer, since the Al O Mo chem-
ical bond is stronger than the Mo O Mo bond in
the range of pH under study.
a DRON-3.0 diffractometer with CuK radiation at
the tape velocity of 2400 cm h , sample rotation rate
1
of 2 rpm, current of 18 mA, and voltage of 36 kV.
The X-ray photoelectron spectra were recorded on
a Perkin Elmer-5400 spectrometer with MgK excita-
tion. The X-ray spectral microanalysis was carried out
at an accelerating voltage of 20 kV on a Camscan-4
scanning electron microscope combined with an AN-
10 000 semiconductor spectrometer of characteristic
X-rays. The data were processed by the ZAF-4-FLS
program.
ACKNOWLEDGMENTS
Thus, we found the conditions and performed the
synthesis of nanolayers of bismuth oxide and vanadate
on the silica surface by ionic deposition. The Bi : V
ratio in the layer can be controlled by varying pH of
sodium vanadate solutions used in the synthesis. Crys-
talline BiVO4 was found in the layers synthesized
using an NaVO3 solution with pH 3.
This work was financially supported by the Basic
Research Universities of Russia program.
REFERENCES
1. Tolstoi, V.P., Zh. Neorg. Khim., 1993, vol. 38, no. 7,
pp. 1146 1148.
2. Tolstoy, V.P. and Ehrlich, A.G., Thin Solid Films,
1997, vol. 307, nos. 1 2, p. 60.
3. Tolstoi, V.P. Zh. Prikl. Khim., 1999, vol. 72, no. 8,
pp. 1259 1261.
4. Nikolau, Y.F. and Menard, J.C., J. Cryst. Growth,
1988, vol. 92, p. 128.
5. Tolstobrov, E.V. and Tolstoi, V.P., Zh. Obshch. Khim.,
1999, vol. 69, no. 6, pp. 890 895.
6. Baes, C.F. and Mesmer, R.E., The Hydrolysis of Cat-
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EXPERIMENTAL
The IR Fourier transmission spectra were taken on
a Perkin Elmer 1760x spectrophotometer. The num-
1
ber of scans was 300, the resolution was 4 cm .
The UV-visible diffuse reflection spectra were re-
corded on a Perkin Elmer Lambda-9 spectrometer at
a scanning rate of 50 nm min 1 and 2 nm slit program.
The transmission spectra were recorded on an SF-26
spectrophotometer. The reference samples were quartz
plates used as supports in the synthesis. The X-ray
patterns of the synthesized samples were recorded on
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001