Investigation of RuO2-IrO2-SnO2 thin film evolution: A thermoanalytical and spectroscopic study

Article abstract of DOI:10.1007/s10973-006-7578-2

The thermal evolution process of RuO₂-IrO₂-SnO ₂ mixed oxide thin films of varying noble metal contents has been investigated under in situ conditions by thermogravimetry-mass spectrometry (TG-MS), infrared emission spectroscopy (IR) and cyclic voltammetry (CV). The gel-like films prepared from aqueous solutions of the precursor compounds RuOHCl₃, H₂IrCl₆ and Sn(OH)₂(CH ₃COO)_2-xCl_x on titanium metal support were heated in an atmosphere containing 20% O₂ and 80% Ar up to 600°C. Chlorine evolution takes place in a single step between 320 and 500°C accompanied with the decomposition of the acetate ligand. The decomposition of surface species formed like carbonyls, carboxylates and carbonates occurs in two stages between 200 and 500°C. The temperature of chlorine evolution and that of the final film formation increases with the increase of the iridium content in the films. The anodic peak charge shows a maximum value at 18% iridium content. Springer-Verlag 2006.

Full text of DOI:10.1007/s10973-006-7578-2

Journal of Thermal Analysis and Calorimetry, Vol. 86 (2006) 1, 141–146
INVESTIGATION OF RuO2-IrO2-SnO2 THIN FILM EVOLUTION
A thermoanalytical and spectroscopic study
Erzsébet Horváth1*, J. Kristóf 2, L. Vázquez-Gómez3, Á. Rédey1 and Veronika Vágvölgyi2
1University of Veszprém, Department of Environmental Engineering and Chemical Technology, 8201 Veszprém, P.O. Box 158
Hungary
2University of Veszprém, Department of Analytical Chemistry, 8201 Veszprém, P.O. Box 158, Hungary
3Department of Chemistry, University of Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
The thermal evolution process of RuO2–IrO2–SnO2 mixed oxide thin films of varying noble metal contents has been investigated
under in situ conditions by thermogravimetry-mass spectrometry (TG-MS), infrared emission spectroscopy (IR) and cyclic
voltammetry (CV). The gel-like films prepared from aqueous solutions of the precursor compounds RuOHCl3, H2IrCl6 and
Sn(OH)2(CH3COO)2–xClx on titanium metal support were heated in an atmosphere containing 20% O2 and 80% Ar up to 600°C.
Chlorine evolution takes place in a single step between 320 and 500°C accompanied with the decomposition of the acetate ligand.
The decomposition of surface species formed like carbonyls, carboxylates and carbonates occurs in two stages between 200
and 500°C. The temperature of chlorine evolution and that of the final film formation increases with the increase of the iridium con-
tent in the films. The anodic peak charge shows a maximum value at 18% iridium content.
Keywords: electrocatalysis, IrO2, RuO2, SnO2, sol–gel process, thin films
Introduction
the thermal decomposition of precursor salt mixtures
(sol–gel method) [6]. Electrochemical and surface
properties of thermally prepared oxide anodes are
widely dependent on the conditions of preparation
(e.g. precursor solution composition, substrate
pretreatment and firing temperature) [7].
Mixed oxide coatings on titanium metal support are
extensively used in industrial electrochemical pro-
cesses as anodes, e.g. for chlorine production [1]. Re-
cently these types of electrochemical devices are being
used to convert low-grade water into drinkable water
based on faradaic effects in specially designed electro-
chemical cells installed at the site of consumption. For
this purpose two types of treatment exist. During the
‘direct’ electrochemical treatment water is passed
through cathodic and anodic compartments in one or
more electrochemical cells. The treatment process con-
sisting of the application of an electric field, a
‘pH shock’ and faradaic processes (cathodic reduction
and anodic oxidation) results in the elimination of bio-
logical and chemical impurities (e.g. traces of heavy
metals). The second way is an ‘indirect’ process in
which ‘neutral analytes’ are produced by electrolyzing
diluted brines. These analytes are then added to water
allowing a complete elimination of biological and
chemical impurities [2, 3]. The above technology has
already found practical application but it requires im-
provements in cell engineering and electrode materials
(especially anodic materials) [4, 5].
RuO2-based systems suffer from low stability,
while the main disadvantage of the IrO2–SnO2 binary
mixtures is their high cost. The stability of RuO2 can
be increased with the addition of IrO2 and a synergic
effect of the two oxides on oxygen evolution reac-
tions was experienced [8, 9]. However, for commer-
cial applications the addition of a third, cheaper oxide
is necessary. Recently SnO2 has been used to stabilize
iridium-based electrodes with good results [10]. Tin
oxide also has a rutile structure and is widely used in
gas sensors, solar cells and opto-electronic de-
vices [11, 12]. There is only limited information
available on the properties of RuO2–IrO2–SnO2
mixed oxide coatings prepared from alcoholic solu-
tions and on their performance in phenol oxida-
tion [13]. Studies on the thermal evolution process
have been made separately on RuO2 and IrO2 films
stabilized by TiO2, SnO2 and Ta2O5 [14–18].
In the present work the thermal evolution pro-
cess of the RuO2–IrO2–SnO2 ternary system from
aqueous precursor solutions was investigated by
thermogravimetry-mass spectrometry, infrared emis-
sion spectroscopy and cyclic voltammetry.
The improvement of these devices requires new
coating materials for dimensionally stable an-
odes (DSAs) representing low cost and adequate ser-
vice life. The most efficient method used for the prep-
aration of mixed oxide films on metallic substrates is
*
Author for correspondence: elizabet@almos.vein.hu
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

HORVÁTH et al.
Experimental
ing 19.8% oxygen and 80.2% argon (Messer
Griesheim, Hungary). The purity of the gas mixture
was 99.995%, and the heating rate was 10°C min–1. In
order to follow simultaneously the evolution of the
gaseous decomposition products over the temperature
range from ambient to 600°C, the thermobalance was
connected to a Balzers MSC 200 Thermo-Cube type
mass spectrometer (Balzers AG, Lichtenstein). The
transfer line to introduce gaseous decomposition
products into the mass spectrometer was a deactivated
fused silica capillary (Infochroma AG, Zug, Switzer-
land; 0.23 mm o.d.) temperature controlled to 150°C
to avoid condensation of high-boiling organic matter.
Thin film preparation
A Sn(IV)hydroxyacetochloride
(Sn(OH)2(CH3COO)2–xClx) solution was prepared at a
concentration of 1.65 M. For this purpose
SnCl2⋅2H2O (200 g) were dissolved in deionized wa-
ter (500 cm3) subsequently adding of acetic (ethanoic)
acid (200 cm3). Small amounts of metallic tin powder
and hydrogen peroxide (30%) were added slowly and
the mixture was maintained for 3 days at room tem-
perature. Then the density of the filtered solution was
brought to 1.28 g cm–3 corresponding to a tin concen-
tration of 1.65 M [19]. In the case of the iridium pre-
cursor (H2IrCl6), H2IrCl6⋅nH2O was dissolved in
deionized water with successive additions of acetic
acid and 30% hydrogen peroxide, following the same
procedure as for the Sn(IV) precursor, but for iridium
the final concentration was 0.9 M (172.8 g Ir dm–3).
For the ruthenium precursor (RuOHCl3), RuCl3⋅3H2O
was dissolved in deionized water with successive ad-
ditions of acetic acid and 30% hydrogen peroxide,
following the same procedure as for the Sn(IV) pre-
cursor, but the ruthenium concentration at the end
was 0.9 M (91 g Ru dm–3). All precursor solutions had
an acetic acid concentration of 10% (v/v). Mixtures of
varying composition were made of the precursor
stock solutions with noble metal contents of
around 30% (molar). The following compositions
were prepared: Ru0.3Sn0.7O2, Ru0.2Ir0.1Sn0.7O2,
Ru0.15Ir0.15Sn0.7O2, Ru0.1Ir0.2Sn0.7O2 and Ir0.3Sn0.7O2.
The precursor salt mixtures were deposited onto tita-
nium metal supports (size 4 mm×4 mm, thick-
ness 0.3 mm). The titanium plates were sandblasted,
dipped in a boiling aqueous solution of caustic soda
of 1.30 g cm–3 density for 15 min, rinsed with boiling
distilled water and dried at room temperature [20].
The coatings were prepared by applying the precursor
salt solution drop by drop onto the support and re-
moving the solvent by infrared radiation (using
a 250 W infrared lamp) keeping the temperature be-
low 50°C. This procedure was repeated until a mea-
surable quantity of the gel-like film (1–5 mg) was de-
posited. For the electrochemical investigations the
electrode coatings were prepared with a procedure ba-
sically similar to the one described above, but the tita-
nium strips were bigger. The calcination procedure
was repeated six times with smaller amounts of solu-
tion at each deposition.
FTIR spectroscopic analyses
Infrared emission spectroscopic measurements were
performed with a Bruker Equinox 55 type FTIR spec-
trometer using a factory-made emission adapter. The
titanium sheet with the coating on it was arranged in a
vertical position and the emitted radiation from ther-
mally excited vibrational levels was sent directly to the
interferometer. The sample temperature was controlled
to 0.5°C. The emission spectra were acquired by
co-addition of 1024 scans at a resolution of 4 cm–1 us-
ing a Peltier cooled room temperature DTGS detector.
Cyclic voltammetry measurements
An Autolab PGSTAT 20 system, controlled by GPES
EcoChimie software was employed for the voltam-
metric measurements. A platinum net was used as coun-
ter electrode and a double-walled, saturated calomel
electrode (SCE), with an intermediate saturated NaNO3
solution, was used as the reference in a conventional
three-electrode cell. Test solutions were prepared with
deionized water using HClO4 (1 M) as support electro-
lyte. The electrodes were cycled for at least five times in
the range between 0.15 and 1.15 V (vs. SCE).
Results and discussion
The thermogravimetric (TG) and mass spectrometric
ion intensity curves of the m/z=43 (CH3CO+),
m/z=44 (CO+ ) and m/z=70 (Cl+ ) fragment/molecular
2
2
ions of the 20% Ru–10% Ir–70% Sn ternary system
are shown in Fig. 1. Interpretation of the thermal de-
composition process is possible only with the contin-
uous monitoring of the gas phase composition. By
comparing the TG and MS curves it can be concluded
that thermal decomposition takes place in three major
steps. In the first step of decomposition water is re-
leased until 250°C (not shown for clarity). Be-
tween 110 and 190°C acetic acid fragments are de-
tected due to the liberation of acetic acid trapped in
Thermoanalytical investigations
Thermoanalytical investigations of the coatings were
carried out in a Netzsch (Selb, Germany) TG 209 type
thermobalance in a flowing gas atmosphere contain-
142
J. Therm. Anal. Cal., 86, 2006

RuO2–IrO2–SnO2 THIN FILM EVOLUTION
Fig. 1 TG and MS curves of the 20% Ru–10% Ir–70% Sn system
Fig. 2 TG and MS curves of the 15% Ru–15% Ir–70% Sn system
the film and to the partial decomposition of the tin
precursor. The liberation of a small amount of CO2 in
the 60–160°C temperature range (with maximum rate
at 120°C) indicates an oxidative cracking process.
The absence of chlorine-containing species in the gas
phase at low temperatures indicates that, unlike in al-
coholic precursor solutions, intramolecular hydrolytic
reactions do not occur. In the 200–320°C temperature
range carbon dioxide only is formed. This is due to
the decomposition (and combustion) of organic sur-
face species connected to the noble metals. Be-
tween 320 and 480°C the final stage of decomposition
(and film solidification) occurs. In this temperature
range the decomposition of the acetate ligand takes
place along with chlorine evolution and carbon
dioxide formation. All the three processes take place
simultaneously with maximum rate at 376°C.
Fig. 3 TG and MS curves of the 10% Ru–20% Ir–70% Sn system
Changing the Ru to Ir ratio from 1:1 to 1:2
(10% Ru–20% Ir–70% Sn system) resulted in signifi-
cant changes in the thermoanalytical curves, as well
(Fig. 3). Liberation of water was observed up to
180°C with maximum rate at 98°C (not shown for
clarity). Residual acetic acid was liberated at 110°C,
while a small amount of chlorine formation can be ob-
served between 70 and 130°C due to intramolecular
hydrolytic reactions. It is interesting to observe that
carbon dioxide is formed in two overlapping steps at
approximately 99 and 140°C. Low temperature CO2
formation is due to catalytic cracking reactions of
complicated nature with the involvement of the noble
metal(s) as catalyst(s). As to the second stage of film
evolution, the decomposition of the noble metal-con-
taining surface species takes place at higher tempera-
The thermal decomposition curves of the
15% Ru–15% Ir–70% Sn system are given in Fig. 2.
Changing the Ru to Ir ratio from 2:1 to 1:1 resulted in
significant changes in the thermal decomposition pat-
tern. During the first stage of decomposition, water
was liberated until 190°C with maximum rate at 94°C
(not shown). Trapped acetic acid is released in a
broad temperature range between 48 and 193°C. High
intensity carbon dioxide formation can be observed
between 50 and 170°C indicating a low-temperature
combustion process catalyzed by the noble metal(s).
In the second stage, similarly to the previous compo-
sition, carbon dioxide is liberated only between 230
and 310°C. Since this carbon dioxide formation is due
to the decomposition of organic surface species con-
nected to the noble metal(s), the decrease of the CO2
peak temperature by 34°C indicates a significant de-
crease in thermal stability (or a change in the nature)
of the surface complexes. The chlorine evolution
peak temperature in the third stage increased
from 376 to 400°C. The decomposition of the acetate
ligand (and the formation of CO2 as a combustion
product) takes place simultaneously with the
formation of chlorine (between 310 and 480°C).
tures (the peak temperature of the m/z=44 (CO+ ) ion
2
intensity curve increased from 265 to 339°C). At the
same time, the decomposition of the acetate ligand
started at 270°C (i.e. a temperature lower by 40°C
than in the case of the previous composition). In the
third stage, similar to the previous compositions,
chlorine formation takes place simultaneously with
the decomposition of the acetate ligand and the for-
mation of carbon dioxide as decomposition/combus-
tion product with maximum rate at 410°C. The gen-
eral tendency regarding the third stage of decomposi-
J. Therm. Anal. Cal., 86, 2006
143

HORVÁTH et al.
tion is that while the temperature range of decomposi-
bonate-type surface species can be identified. CO and
CO2 are the oxidation products of the acetate ligand
and can react with water as follows:
tion is broadening, the peak temperature is increasing
with the increase of the iridium content.
The overall pattern of the thermal evolution pro-
cess for the above compositions can be evaluated by
the comparison of the TG and DTG curves in Fig. 4.
Below 200°C decomposition/gas (vapour) evolution
reactions take place in two overlapping stages. The
second stage of decomposition takes place be-
tween 200 and 350°C. The most significant differ-
ences in the thermal behaviour of the coating compo-
sitions can be observed in the third stage between 350
and 500°C. The DTG peak maxima show a gradual
shift to higher temperatures with the increase of the
iridium content. A similar tendency can be observed
in the final film solidification temperature, as well.
The emission infrared (IRES) spectra of the pre-
cursors as well as of the 20% Ru+10% Ir+70% Sn and
the 10% Ru+20% Ir+70% Sn ternary systems at
200°C are shown in Fig. 5. Assignment of the bands is
given in Table 1. In the 2200–1300 cm–1 spectral
range the presence of carbonyl-, carboxyl- and car-
CO2+H2O↔CO2– +2H+
(1)
3
CO+H2O↔HCOOH↔H++HCOO–
H++H2O↔H3O+
(2)
(3)
(4)
H++2H2O↔H O+ , etc.
5
2
The spectra of the two ternary systems show close
similarity. At lower iridium content, however, the rela-
tive intensities of the carbonyl and carboxylate bands
and those of the Zundel-type complexes (e.g. H O+ )
5
2
are higher. The C–H stretching vibration bands of the
organic species above 200°C are rather weak, while
those of the bending vibrations overlap with the car-
bonate and carboxylate bands.
The amount of the anodic peak charge determined
by cyclic voltammetry (CV) is proportional to the
number of surface active sites available in the electro-
chemical reaction. Figure 6 shows the change of the
Table 1 Assignment of IR bands (cm–1) and the schematics of the proposed surface structures
Spectral band/cm–1
Type of surface species
Structure [22–26]
2120
1916
1832
Me–O⋅⋅⋅C≡O
νC≡O
linear
coordinatively bonded CO
νC=O
acetic acid
1708
νC
bridging bidentate
as = O
1612
1521
bidentates
νas(O–C–O)
bicarbonates
1488
1430
carboxylates
1430–1404
βC–H
organic species
1354
1302
δs(CH
)
3
acetic acid fragments
νs(O–C–O)
νs(C=O)
C–H bending
bridging bidentate
organic species
1257
bidentates
Zunder structure
1105–909
800–600
H5O+2
Me-oxides
144
J. Therm. Anal. Cal., 86, 2006

RuO2–IrO2–SnO2 THIN FILM EVOLUTION
Fig. 6 Dependence of the voltammetric charges on electrodes
¢
composition in 1 N HClO4; – total anodic charge,
¿
– external anodic charge, – internal anodic charge
*
was estimated directly from CV curves obtained
at 600 mV s–1. The internal anodic charge was deter-
mined by the difference of the two sets of figures [21].
Figure 6 shows that the anodic charge is maximum
at 18% iridium content. At this composition the rate of
gas evolution through the solidifying surface layer is
high, resulting in cracks and pores in the outermost
part of the film leading to a higher surface area. The
broader range of thermal decomposition as well as an
increase of the gas evolution peak temperature can also
contribute to the increase of the number of the active
sites at the surface. The fact that the internal anodic
charge can also contribute (to a little extent) to the total
anodic charge indicates that not only the outermost, but
also some of the inner layers can take part in the elec-
trode processes.
Fig. 4 TG and DTG curves for the prepared mixtures;
a – 20% Ru–10% Ir–70% Sn
b – 15% Ru–15% Ir–70% Sn and
c – 10% Ru–20% Ir–70% Sn
Conclusions
The complicated decomposition reactions of the pre-
cursor coatings prepared onto titanium support can
only be followed with the continuous monitoring of the
gas phase composition by thermogravimetry combined
with mass spectrometry. In addition, the vibrational
spectroscopic identification of surface species formed
on heating is indispensable to completely characterize
the subtle process of thin film evolution. The informa-
tion obtained on the nature of decomposition reactions,
the temperature ranges of formation/decomposition of
the surface species as well as on the final temperature
of mixed oxide film formation can be advantageously
used in the design and preparation of real electrodes.
Since the electrocatalytic activity of the final oxide
films depends strongly on the surface properties, the in
situ study of film evolution can be advantageously
used to find correlations between surface characteris-
tics and preparation conditions.
Fig. 5 IR spectra of the coatings heated at 200°C;
a – 100% Ir, b – 100% Ru, c – 100% Sn,
d – 20% Ru+10% Ir+70% Sn and
e – 10% Ru+20% Ir+70% Sn
anodic charge – as coulomb per mmol noble metal
(C mmol M–1) – as a function of the film composition.
The total charge capacity was estimated from CV
curves recorded at a potential sweep rate of 10 mV s–1.
The contribution due to the outermost part of the films
J. Therm. Anal. Cal., 86, 2006
145

HORVÁTH et al.
13 M. E. Makgae, C. C. Theron, W. J. Przybylowicz and
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DOI: 10.1007/s10973-006-7578-2
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