Electrodes based on nanodispersed titanium and tungsten oxides for a sensor of dissolved oxygen

Article abstract of DOI:10.1134/S1070427206040173

It is suggested to use films of nanodispersed Ti and W oxides as electrodes in a sensor for dissolved oxygen. It is shown that films of this kind have stable characteristics in electroreduction of O₂ in saline. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070427206040173

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 4, pp. 596 601.
Original Russian Text G. Ya. Kolbasov, V. S. Vorobets, A. M. Korduban, A. P. Shpak, M. M. Medvedskii, I. G. Kolbasova, O. V. Linyucheva, 2006,
published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 4, pp. 605 610.
Pleiades Publishing, Inc., 2006.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrodes Based on Nanodispersed Titanium and Tungsten
Oxides for a Sensor of Dissolved Oxygen
G. Ya. Kolbasov, V. S. Vorobets, A. M. Korduban, A. P. Shpak,
M. M. Medvedskii, I. G. Kolbasova, and O. V. Linyucheva
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of the Ukraine, Kiev, Ukraine
Kurdyumov Institute of Metal Physics, National Academy of Sciences of the Ukraine, Kiev, Ukraine
Kiev Polytechnic Institute National Technical University, Kiev, Ukraine
Received December 13, 2005
Abstract It is suggested to use films of nanodispersed Ti and W oxides as electrodes in a sensor for dis-
solved oxygen. It is shown that films of this kind have stable characteristics in electroreduction of O2 in
saline.
DOI: 10.1134/S1070427206040173
Oxygen dissolved in biologically active liquids
plays an important role in redox processes. Therefore,
development of sensitive methods for determining
its concentration is a topical problem. Oxygen sensors
are now used to control biological processes in
fermenters and at plants for treatment and processing
of biological wastes, to monitor the content of dis-
solved oxygen in water used at thermal power sta-
tions, and to make various technological analyses.
However, the most important is application of such
sensors in medical practice.
O2 + 4H+ + 4e
for an acid medium and
O2 + 2H2O + 4e
2H2O,
4OH ,
(I)
(II)
for an alkaline medium.
In this case, the process goes via an intermediate
stage of hydrogen peroxide formation. For a number
of metallic electrodes, a two-electron overall process
without formation of hydrogen peroxide is also pos-
sible [1, 3 5]:
For example, in oxygen-inhalation hypoxia therapy,
it is necessary to monitor that part of oxygen in blood
plasma, which is not bound to hemoglobin; a similar
analysis is required in hyperbaric-oxygenation therapy
of lungs, in hypoxytherapy of cardiac and gastroen-
teric illnesses, in determining the effect of fetal hemo-
globin on the blood pathology in oncology, in therapy
of pathological pregnancy, etc.
O2
2Oads
,
(III)
(IV)
(V)
Oads + 2H+ + 2e
Oads + H2O + 2e
H2O,
2OH ;
reaction (IV) proceeds in an acid medium, and reac-
tion (V), in an alkaline medium.
The concentration of dissolved oxygen should also
be monitored in preparation and storage of physiolog-
ical solutions, blood plasma, and other biologically
active fluids.
The specificity of electrochemical reactions on
semiconductor electrodes primarily consists, in con-
trast to reactions on metallic electrodes, in a poten-
tial redistribution between the ionic double layer and
the space charge region near the semiconductor sur-
face, which can affect the kinetics of these reactions
[5, 6]. For example, it has been shown for mixed ox-
ides SiO2/Nb2O5 [7] that oxygen reduction involves
Because of its high sensitivity the electrochemical
method is promising for analyzing oxygen concentra-
tion in biologically active fluids [1 4]. The operation
of a sensor is based on electroreduction of oxygen,
whose total concentration is described, for a number
of metals, by the equations [1, 3, 4]
596

ELECTRODES BASED ON NANODISPERSED TITANIUM AND TUNGSTEN OXIDES
597
two electrons to form hydrogen peroxide. At the same
time, for a TiO2 electrode fabricated by anodic oxi-
dation of titanium, the total number of electrons in-
volved in the reaction may vary from ns = 4 to ns = 2
in passing from alkaline to acid solutions [8]. It is
noteworthy that ns affects the limiting sensitivity of
an oxygen sensor. Electrodes based on ultradispersed
semiconducting materials and, in particular, on transi-
tion metal oxides, exhibit a high catalytic activity
[9 11] and are being extensively studied now.
In this study, nanodispersed Ti and W oxides,
which are stable in many biologically active liquids,
on Ti and InP substrates were examined as semi-
conducting electrode materials for oxygen sensors.
The oxygen reduction in a 0.9% isotonic physiological
and 7.5% hypertonic NaCl solutions was analyzed.
EXPERIMENTAL
TiO2 nanoparticles were obtained by the sol gel
Fig. 1. TEM micrograph of WO
nanoparticles.
method from TiCl4. The procedure used included syn-
thesis of a sol of amorphous titanium peroxide from
titanium hydroxide and its subsequent heat treatment
[12]. Titanium hydroxide was synthesized by hydro-
thermal decomposition of Ti(IV) chloride. To com-
pletely shift the equilibrium toward formation of
the target product, an NH4OH solution was introduced
into the reaction mixture. After the neutralization
was complete, the pH of the medium was adjusted
to 6.5 6.8. The precipitate formed was washed
with distilled water to remove chloride ions. A sol of
amorphous titanium peroxide was obtained by adding
a solution of hydrogen peroxide to a titanium hy-
droxide solution. The subsequent heating of the sol
of amorphous titanium peroxide at 100 C yielded
a TiO2 sol, which was converted to TiO2 (anatase) at
400 430 C. Titanium served as a substrate for de-
position of TiO2 nanoparticles. The size of the semi-
conducting TiO2 nanoparticles (d = 2.9 4.0 nm) was
determined from the band gap of the semiconductor
[9 11], found by measuring spectra of the photoelec-
trochemical current [6], using the method described
in [13, 14]. The average thickness of the TiO2 films
(500 to 1000 nm) was determined from the change in
the substrate mass after their deposition. The TiO2
films were modified with Pt nanoparticles from a so-
lution of chloroplatinic acid by exposure of the semi-
conductor to light in the spectral range of fundamental
absorption [15].
3 x
tions. The deposition was performed in a quartz tube
containing a heated TiO2 powder and InP single crys-
tals at its opposite ends. An inert gas (purified Ar)
was passed through the working chamber. The InP
samples were placed in the temperature zone 400
480 C. The average size d of the TiO2 particles on
InP was determined with a scanning electron micro-
scope (d = 100 300 nm).
Nanoparticles of a nonstoichiometric tungsten ox-
ide WO3
were obtained by the method of electric
x
explosion of conductors (EEC). The procedure in-
cluded purification of a tungsten wire, formation
of an oxygen-deficient atmosphere in the explosion
chamber, and choice of the optimal energy range [16].
The average size of the nanoparticles, d, was deter-
mined by transmission electron microscopy (TEM).
According to the TEM data, d varies within 5 50 nm,
the nanoparticles are anisotropic, have the form of
a sphere inscribed in a parallelepiped, and are not
agglomerated (Fig. 1). The WO3
films were ob-
tained by sedimentation from a solx of nanoparticles
in water. Titanium served as a substrate onto which
WO3
nanoparticles were deposited. The average
x
thickness of the WO3
films (about 2000 nm) was
x
determined from the change in the substrate mass after
the deposition.
The electronic structure of the nanodispersed tita-
nium and tungsten oxides was studied by X-ray pho-
toelectron spectroscopy (XPS) on an ES-2402 spec-
trometer equipped with a PHOIBOS-100-SPECS en-
TiO2 particles were obtained on an InP substrate
by the method used in semiconductor industry to pro-
duce protective TiO2 films on crystals with p n junc-
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598
KOLBASOV et al.
measured relative to a silver chloride reference elec-
trode. The oxygen concentration in solution was varied
by adding a deaerated NaCl solution. For this purpose,
argon was bubbled through the solution for 2 3 h.
The solution was mixed in a certain proportion with
the working NaCl solution containing dissolved ox-
ygen. This was done in a box filled with Ar.
The polarization curves measured in oxygen reduc-
tion on the TiO2 electrodes are shown in Fig. 2. As
is known, the titanium foil used as a support for de-
position of TiO2 nanoparticles is covered with an ox-
ide film under natural conditions and upon its anneal-
ing and anodizing. Oxygen is also reduced on this film,
but its activity is lower as compared with the activ-
ity of films based on TiO2 nanoparticles produced by
the sol gel method. This is also indicated by the I U
curves measured in oxygen reduction on films of TiO2
nanoparticles (Fig. 2, curves 1 4) and on TiO2 films
formed upon annealing of Ti (Fig. 2, curve 5). As
in the case of platinum metals, all the polarization
curves show a single wave at potentials from 0.45
to 1.2 V, which may correspond to the overall
process of oxygen reduction, (III) (V), without for-
mation of hydrogen peroxide. As can be seen from
Fig. 2, the hydrogen evolution is clearly pronounced
at U < 1.3 V.
Fig. 2. I U curves measured in oxygen reduction in a 0.9%
NaCl solution at a potential sweep rate v = 10 mV s in
1
relation to a cycle number. Curves: (1 5) TiO /Ti elec-
2
trodes; (6, 7) TiO /Ti electrodes modified with Pt nanopar-
2
ticles, (6) cycle 1, and (7) cycle 4. (I) Current density and
(U) potential; the same for Fig. 3.
ergy analyzer. As the excitation source served an X-ray
gun with a magnesium anode (E MgK = 1253.6 eV,
P = 200 W). The absolute resolution, measured for
the Au4f7/2 peak of gold, was 0.9 eV, and the ac-
curacy in determining the peak of the W4f7/2 line,
0.05 eV. The procedure used to calibrate the spec-
trometer was described in [15]. The residual pressure
in the vacuum chamber was 2 10 7 Pa. As substrates
served 10 10-mm Al plates.
The W4f and Ti2p spectra were resolved into
the interconnected component pairs, 4f7/2/4f5/2 and
2p3/2/2p1/2 ( E = 2.1 eV, I1/I2 = 0.77 and E =
5.7 eV, I1/I2 = 0.5, respectively), to take into account
the spin-orbit splitting. We employed the Gauss
Newton method for coupled parameters. The intensity
of the components and their binding energies were
varied. The width of the corresponding components
and the relative contributions of the Gauss Lorentz
distributions in spectral resolution were fixed.
The areas of the components were determined after
the background was subtracted by the Shirley method
[17]. The integral intensities of the components, ob-
tained in this way, are proportional to the concentra-
tion of nonequivalent titanium and tungsten ions in
the samples.
The reduction of oxygen on TiO2 films obtained
upon annealing of Ti occurred at a potential shifted
to the cathodic region by 0.23 V, compared with
the electrodes based on TiO2 nanoparticles, which
points to a lower electrocatalytic activity of these
films. The high overvoltage of oxygen reduction may
cause difficulties in analysis for the oxygen con-
tent of, because in this case side reactions may pro-
ceed at the cathode in a number of biologically active
solutions. To make the potential of oxygen reduction
lower, the TiO2 films were modified with Pt nanopar-
ticles, because the potentials of O2 reduction at the Pt
electrode are more positive than those at the TiO2
electrode [4]. However, the modification of the elec-
trodes with Pt considerably deteriorated their stability,
which was manifested in that their I U characteristics
were irreproducible in cycling (Fig. 2, curves 6, 7).
In addition, the hydrogen evolution potential for such
electrodes was markedly shifted to the anodic region.
As a result, the region of potentials at which analysis
for the content of oxygen in solution is possible be-
came narrower. For TiO2/InP electrodes, the repro-
ducibility of the characteristics in cycling was better
(Fig. 3, curves 1 3). At the same time, the potentials
of oxygen reduction at these electrodes were more
positive than those at electrodes based on TiO2 nano-
particles. The differences in current density between
The current voltage (I U) dependences of the elec-
trodes were measured in the potentiodynamic mode,
using a special computerized electrochemical instal-
lation. The installation has the following character-
istics: measurable currents in the range from 2
9
1
10 A to 10 A, potential sweep rate of 0.01
1
50 mV s , and potential of the working electrode
varying from 4 to 4 V. The electrochemical studies
were performed in a cell with separated cathode and
anode compartments. The electrode potential was
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ELECTRODES BASED ON NANODISPERSED TITANIUM AND TUNGSTEN OXIDES
599
TiO2/Ti and TiO2/InP electrodes based on nanopar-
ticles are related both to a surface macroporosity and
to nonuniformity of film deposition on the electrode
surface, with these parameters affecting the current
density in the opposite directions.
The most stable characteristics were exhibited by
WO3 x /Ti electrodes. Their I U characteristics re-
mained virtually unchanged already after the third
cycle (Fig. 3, curves 4 7).
Analysis of the structure of XPS spectra of the O1s
Fig. 3. I U curves measured in oxygen reduction in a 7.5%
NaCl solution at a potential sweep rate v = 10 mV s in
level in these samples shows that, along with the sig-
nal related to the lattice oxygen (O2 , Eb = 530.6 eV
for tungsten oxide and Eb = 530.0 eV for titanium
oxide), there are a strong signal due to the oxygen
of OH groups (Eb = 531.0 531.8 eV) and a weaker
signal associated with the oxygen from the H2O
molecule (533.0 533.8 eV). The results of resolu-
tion into components of normalized XPS spectra of
the W4f and Ti2p levels of the samples studied are
presented in Figs. 4a, 4b, and 4c and in the table. As
can be seen from Fig 4a, the surface of nanoparticles
from which the electrodes were fabricated is mainly
formed by the W6+ states of tungsten ions with binding
energies Eb W4f7/2 = 36.1 eV and Eb W4f7/2 = 35.7 eV
and by W5+ states (Eb W4f7/2 = 34.8 eV [17 18].
The components in the range Eb = 35.7 eV (W6+)
and Eb = 34.8 eV (W5+) correspond to the main
1
relation to a cycle number. Curves: (1 3) TiO /InP elec-
2
trodes; (4 7) WO
(5 7) cycles 2 4.
/Ti electrodes, (4) cycle 1, and
3
x
phase of protonated oxide W5x+W61+ xO3
(OH)x.
x
The contributions from the W5+ and W6+ states (11.7
and 36.5%, respectively, see table) can be used to de-
termine the nonstoichiometry coefficient (x = 0.243)
and the composition of the main phase, WO2.76
(OH)0.24. The signal at Eb = 36.1 eV can be attributed
to formation on the surface of nanoparticles of a hy-
droxide phase, WO3 nH2O, which was possibly formed
from the main phase upon its contact with water.
The results obtained in resolution of the Ti2p
spectra of the sample of titanium oxide nanopar-
ticles are shown in Fig. 4. The components at
EbTi2p3/2 = 458.5 eV (Ti4+, 69.7%, see table) and
EbTi2p3/2 = 457.5 eV (Ti3+, 7.2%, see table) [18, 19]
can be assigned to the phase of a protonated oxide
Ti3x+ Ti41+ xO2
(OH)x. The nonstoichiometry coef-
x
ficient determined from the contributions of the Ti3+
and Ti4+ ions to the oxide phase (7.2% and 69.7%,
respectively) can be used to find the nonstoichiom-
etry coefficient (x = 0.093), which corresponds to
a composition TiO1.91 (OH)0.09. The component at
EbTi2p3/2 = 459.4 eV (Ti4+, 23.1 0.7%, see table)
can be attributed to formation of the phase TiO2 nH2O
on the surface of nanoparticles.
Fig. 4. Resolution of the XPS spectra of (a) W4f levels
of WO and (b) Ti2p levels of TiO into compo-
3
x
2
x
nents. (E ) Binding energy.
b
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600
KOLBASOV et al.
Results* of decomposition of the Ti2p and W4f spectra into
conductor electrodes under study is the limiting dif-
fusion current. This assumption was verified by
studying how the maximum limiting current varies
with the potential sweep rate v at potentials in
the region of limiting currents. It was found that
the plot of this dependence in the coordinates id =
components for nanodispersed oxides WO3 x and TiO2
x
I, %
Eb, eV
WO3
TiO2
x
x
f( v ) [22] is a straight line, which shows that the ox-
ygen reduction in this region is controlled by diffu-
sion. In the region of potentials corresponding to cur-
rents lower than the limiting current, the polarization
curve of oxygen reduction was described by the Tafel
W3+, 34.8
W6+, 35.7
W6+, 36.1
Ti3+, 457.5
Ti4+, 458.5
Ti4+, 459.3
11.7 0.7
36.5 0.7
51.8 0.7
7.2 0.9
69.7 0.7
23.1 0.7
equation
= ak + bk log i [23], where is the over-
voltage ofc the reaction; ak, Tafel constant; bk
=
0.059/ n and , transfer coefficient ( = 0.5 [23]).
The relation n = 0.5 was obtained, i.e., the number
of electrons involved in oxygen reduction at the dis-
charge stage is n = 1. The value n = 1 has been re-
ported for platinum and silver in an acid medium and
n = 2 for alkaline solutions, and also n = 1 for a num-
ber of other metals [1, 3, 4]. In the given case, the be-
havior of the semiconductor electrodes studied is sim-
ilar to that of metallic electrodes, in which the applied
potential drops across the ionic double layer. Such
a behavior of the semiconductor electrodes is due to
the fact that oxygen reduction occurs at cathodic po-
tentials in the region of a pronounced accumulation
of electrons at the surface and the shielding of the sur-
face charge by these electrons is important. In this
case, as also in the case of metals, the potential mostly
drops across the ionic double layer [5, 6].
*
E , binding energies of W4f , Ti2p electrons; I, integral
intensities of the components.
b
7/2
3/2
Thus, according to the XPS data, the electrode sur-
face is composed of nonstoichiometric oxides of tung-
sten, WO2.76 (OH)0.24, and titanium, TiO1.91 (OH)0.09
,
and of their hydroxide phases. The high catalytic ac-
tivity of the nanoparticles under study in oxygen re-
duction can be attributed to formation of catalytically
active W5+ and Ti3+ centers and to presence of OH
groups in the oxide matrix. The comparatively high
stability of tungsten oxide electrodes in redox pro-
cesses, compared with titanium oxide electrodes, may
be due to an increase in the contribution of the va-
lence d states to the W W bond. For example,
a subband absent in the spectrum of stoichiometric
oxides [20] is formed in the XPS spectra of the va-
lence band of nonstoichiometric tungsten and mo-
We determined the dependence of the limiting
current on the concentration of dissolved oxygen in
a NaCl solution at potentials from 0.6 to 1.0 V, when
the catalytic activity of the electrodes studied was at
a maximum (Fig. 5). The current remained stable dur-
ing several hours. From the results presented in Fig. 5,
the total amount ns of electrons involved in oxygen
reduction was determined. For this purpose, we meas-
ured the dependence of the current of the working
electrode on the time of the electrolysis in the poten-
tiostatic mode at a potential in the range of diffusion
currents of oxygen reduction and, using the data of
Fig. 5, calculated the concentration of reduced oxy-
gen. We found that ns = 2 in NaCl solutions.
lybdenum oxides. The integral intensity of this sub-
band grows as the number of the M(n
states in-
1)+
creases. The formation of this subband is mainly due
to contributions from the W5d states involved in the
formation of W W bonds in the oxides WO2 x < 3 [21].
The oxygen reduction at the electrodes studied
is characterized by mixed kinetics. It was shown that
the limiting current of oxygen reduction for the semi-
CONCLUSIONS
(1) The maximum sensitivity of electrodes based
on nanodispersed Ti and W oxides to dissolved ox-
ygen is reached at cathode potentials from 0.6 to
1.0 V (relative to a silver chloride reference elec-
trode), at which the limiting diffusion current of
oxygen reduction is observed.
Fig. 5. Current I of oxygen reduction vs. oxygen concen-
tration c for a TiO /InP electrode in a 0.9% NaCl solution.
2
1
v = 2 mV s
.
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ELECTRODES BASED ON NANODISPERSED TITANIUM AND TUNGSTEN OXIDES
601
(2) A single electron is involved at
= 0.5 in
9. Hagfeld, A. and Grotzel, M., Chem. Rev., 1995,
vol. 95, no. 1, pp. 49 68.
the oxygen reduction at TiO2 electrodes at the dis-
charge stage. The total number of electrons involved
in the oxygen reduction was found to be ns = 2.
10. Hoffmann, M.R., Martin, S.T., and Choi, W., Chem.
Rev., 1995, vol. 95, no. 1, pp. 69 96.
11. Khairutdinov, R.F., Usp. Khim., 1998, vol. 67, no. 2,
(3) The high catalytic activity of the electrodes
based on nanodispersed tungsten and titanium ox-
pp. 125 139.
12. Seok, Sang Il, Kim, Mi Sun, and Suh, Tae Soo, J. Am.
Ceram. Soc., 2002, vol. 85, no. 7, pp. 1888 1890.
ides, WO3
and TiO2, may be due to formation
x
of W5+ and Ti3+ states and presence of OH groups
in the oxide matrix.
13. Kublanovsky, V.S., Kolbasov, G.Ya., and Litovchen-
ko, K.I., Polish J. Chem., 1996, vol. 70, no. 11,
pp. 1453 1458.
(4) The stability of the TiO2
and WO3
elec-
x
x
trodes in the course of oxygen reduction was studied.
These electrodes have highly renewable characteristics
in prolonged cycling, which makes them promising
for manufacture of electrochemical sensors for dis-
solved oxygen. The higher stability of the electrodes
14. Kolbasov, G.Ya., Krasnov, Yu.S., and Volkov, S.V.,
Abstracts of Papers, 55th ISE Annual Meet., Thes-
saloniki (Greece), September 19 24, 2004, p. 541.
15. Shpak, A.P., Korduban, A.M., Mel’nikova, V.A.,
and Medvedskii, M.M., Metallofiz. Noveishie Tekhn.,
2003, vol. 25, no. 11, pp. 1409 1415.
based on WO3
nanoparticles in redox processes
x
may be due to the increased strength of the W W
bond upon formation of W5+ states.
16. Practical Surface Analysis by Auger and X-ray Photo-
electron Spectroscopy, Biggs, D. and Seah, M.P.,
Eds., Chichester: Wiley, 1983.
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