J Sol-Gel Sci Technol
DOI 10.1007/s10971-016-4009-0
ORIGINAL PAPER: FUNCTIONAL COATINGS, THIN FILMS AND MEMBRANES (INCLUDING DEPOSITION TECHNIQUES)
Deposition of (Ti, Ru)O2 and (Ti, Ru, Ir)O2 oxide coatings
prepared by solgel method on titanium
Mona Goudarzi1 Mohammad Ghorbani1,2
Received: 29 September 2015 / Accepted: 4 March 2016
Springer Science+Business Media New York 2016
Abstract Titanium anodes activated by noble metal Graphical Abstract
oxides possess a wide range of advantages and applica-
tions. Actually, coating of titanium anodes by highly con-
ductive oxides of noble metals (Ru, Ir) dramatically
increases the lifetime of these anodes. In this study, the
binary coating consisting of Ti and Ru and the ternary
coating consisting of Ti, Ru and Ir were prepared through
solgel method. After coating of the titanium substrate, the
corrosion behavior of coatings was studied by anodic
polarization and cyclic voltammetry tests. The lifetime of
anodes was determined using accelerated corrosion test.
The morphology of coatings was examined by field emis-
sion scanning electron microscopy and atomic force
microscopy. Phase analysis was done using X-ray diffrac-
tion. Results indicated that the microstructure of both
coatings is similar to cracked mud consisting of islands
separated by cracks, while the distribution of cracks in the
ternary coating is more uniform in terms of morphology.
The ternary coating is also more active in low overpoten-
tials, while ability of load transfer in the ternary coating is
reduced in high overpotentials.
Keywords Solgel Titanium Mixed metal oxide
(Ti, Ru)O2 (Ti, Ru, Ir)O2 Accelerated corrosion test
1 Introduction
& Mohammad Ghorbani
Production of mixed metal oxide (MMO) anodes has been
ghorbani@sharif.edu optimized from 1970, in both terms, technology and
economy, for productions in chlorinealkali industry such
1
Department of Materials Science and Engineering, Sharif as sodium chlorate and sodium hypochlorite [14]. Opti-
University of Technology, P.O. Box 11155-9466, Tehran,
Iran
mizing this type of coatings reduces the price and increases
2
the lifetime of anodes. Successful development of cell
Institute for Nanoscience and Nanotechnology, Sharif
University of Technology, P.O. Box 11155-8639, Tehran,
membrane manufacturing technology in a larger scale leads
Iran to more optimized coatings in production of chlorine gas
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and sodium hydroxide. Replacing RuO2 with IrO2 usually 2.2 Sol preparation
neutralizes the dissolution of ruthenium oxide in alkaline
environments at the anodes surface and can form cell In order to provide the TiO2 sol, wateracid mixture was
membrane [5, 6]. Selection of suitable materials for cor- stabilized at 70 C and this temperature was kept
rosion resistant is currently one of the important goals of throughout all the experiment, together with continuous
researches. These materials must meet the corrosion stirring. TTIP was added next, which formed a clear sol
problems in various industries, such as industrial chlorine after 2 h. The clear sol was cooled to room temperature.
alkali, chemical processes, electrolysis processes and fuel The RuO2 sol was prepared by the addition of solid
cells. A lot of researches had been done on this case. Most RuCl33H2O to stirred boiling water to form dispersed solid
of them focusing on applied coatings on anode surface [7 phase. This solution was kept at the boiling temperature for
11]. The use of titanium-based anodes in chlorinealkali 6 h. The IrO2 sol was prepared by dissolving IrCl33H2O in
industry is inevitable in order to provide good electrocat- boiling water for 6 h, too. The obtained TiO2, RuO2 and
alytic activity for reduction reaction of chlorine (low IrO2 sols were mixed with continuous stirring for 6 h at
overpotential) and chemical and mechanical stability [12, ambient temperature. Then, the sol (Ru/Ir/Ti molar ratio of
13]. This type of anodes usually contains a titanium sub- 40:20:40) was ready for running the solgel process.
strate with a noble metal oxide coating [14]. Much atten-
tion has been paid to Ti/RuO2 system because of the 2.3 Solgel process
excellent stability of electrocatalytic properties and also
prevention from passivation of titanium by the oxide The dipping process was conducted for 1 min and at
coating in this system [15, 16]. ambient temperature. Afterward, the coatings were dried at
The microstructure of MMO anodes is similar to that of 100 C in oven for 30 min. The dipping and drying were
cracked dried clay [1217]. Cracks play three primary repeated in six steps and finally annealed at 450 C for 2 h
roles in these coatings: a place to release chlorine, with the heating rate of 5 C/min. After annealing was
increasing the active surface area and a channel for dif- finished, samples cooled with the rate of 50 C/min in the
fusing oxygen and inactivating titanium substrate. The furnace.
first two factors increase the efficiency and performance of
anodes. But the third factor reduces the electrochemical 2.4 Evaluation of coatings
performance of anodes since the effect of third factor is
more significant than other factors. The number of cracks The surface morphology of coatings was observed by
should be optimal to maintain a stable active surface [17]. means of a field emission scanning electron microscope
Therefore, the presence of crack at the used anodes for (FESEM, S-4160 scanning electron microscope) and
release of chlorine gas and oxygen gas is essential, but it atomic force microscopy (AFM, DUALSCPETM C26).
is of great importance to increase compactness and uni- Then, the thickness of coatings was observed by means of a
formity of the coating to prevent the penetration of oxy- field emission scanning electron microscope (FESEM,
gen into the substrate surface because cracks can provide S-4160 Scanning Electron Microscope).
the channel for oxygen to result in the passivation of Phase analysis was performed using X-ray diffraction
titanium substrate. (XRD), and the adhesion of coatings to titanium substrate
The aim of this work is to investigate (Ti, Ru)O2 and was evaluated in accordance with ASTM D3359.
(Ti, Ru, Ir)O2 oxide coatings. For this purpose, the oxide Electrochemical behavior of the samples was studied
coatings were applied on titanium by solgel method. using polarization and cyclic voltammetry tests in 1M
Then, the coatings were investigated. H2SO4, 0.5M NaCl and 0.5M NaCl ? 1M H2SO4 solutions
at ambient temperature with a scanning rate of 5 and
20 mV/s, respectively. For this purpose, a platinum elec-
2 Experimental procedures trode and a saturated calomel electrode (SCE) were used as
counter and reference electrodes, respectively. All poten-
2.1 Materials tials were measured against a reference electrode.
The samples were subjected to an accelerated corro-
Commercial pure titanium foils (grade 2) were used as sion test (ACT) with a constant high current density in a
substrate. Titanium tetraisopropoxide (TTIP), RuCl33H2O dilute chloride solution in order to provide information
and IrCl33H2O (Merck) were used as starting materials for about the stability and the lifetime of electrode. To
solgel process. Hydrochloric acid (HCl) 37 % (Merck) perform this test, an electrochemical cell containing a
solution was used for chemical cleaning of titanium foils platinum counter electrode and a reference electrode
prior to solgel process. (SCE) was used. 0.5M NaCl solution was used at
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ambient temperature. The constant current density of and 18 %, respectively. Furthermore, the most surface
1.2 A/cm2 was applied. The potential-time curves were fraction was observed in (Ti, Ru, Ir)O2 coating.
recorded by a digital multimeter (Fluke 187 and 189). The cross section of the binary and ternary coatings
displays a uniform thickness of roughly 1.86 and 1.94 lm,
respectively (Fig. 1c, d).
3 Results and discussion The surface appearance of the oxide coatings on titanium
is shown in AFM 3D surface micrographs (Fig. 2). The
Figure 1 shows the surface morphology and the thickness topography of the binary coating surface shows that surface
of (Ti, Ru)O2 and (Ti, Ru, Ir)O2 oxide coatings on titanium of coating is not uniform and shows a high surface rough-
substrate. In fact, the structure is dependent on the coating ness (Fig. 2a). It was also observed that in the ternary
composition [18]. The microstructure of both coatings is coating, the topography of coating surface becomes more
similar to cracked mud consisting of islands separated by uniform. The average surface roughness (Ra) at the binary
cracks, with a uniform crack (Fig. 1a, b). The island border and ternary coatings was 125 and 98.2 nm, respectively.
zone appears brighter than the bulk of islands, which is The adhesion of coatings to the substrate was also minimum
usually assigned to edge segregation of titanium oxide. rating 4 A when tested in accordance with ASTM D3359.
This effect is more pronounced for (Ti, Ru)O2 than for (Ti, Figure 3 shows the results of X-ray diffraction spectrum
Ru, Ir)O2 coating, which indicates more uniform compo- from the coated samples surface. As shown in Fig. 3, the
sition of the ternary coating. Indeed, the binary coating is binary coating is formed by metallic titanium and two
heterogeneous, containing agglomerates while the ternary phases of rutile TiO2 and RuO2, while the ternary coating is
coating possesses more compact and smooth structure. In formed by metallic titanium and two phases of rutile TiO2
the other words, the distribution of cracks in the ternary and (Ru, Ir)O2. In other words, RuO2 and IrO2 exist in the
coating is more uniform. ternary coating as (Ru, Ir)O2 solid solution due to the high
The surface fraction of crack was calculated using solubility of these two oxides.
Clemex Vision software. The surface fraction of crack at Polarization curves for (Ti, Ru)O2 and (Ti, Ru, Ir)O2
(Ti, Ru)O2 and (Ti, Ru, Ir)O2 oxide coatings was about 10 coatings on titanium in 0.5M NaCl, 1M H2SO4 and 0.5M
Fig. 1 Scanning electron
micrographs and the thickness
of (Ti, Ru)O2 coating (a, c) and
(Ti, Ru, Ir)O2 coating (b, d) on
titanium
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Fig. 2 AFM micrographs on titanium for: a the binary coating and b the ternary coating
Fig. 3 X-ray diffraction spectrum for titanium with the binary and
Fig. 4 Polarization curves for (Ti, Ru, Ir)O2 coating on titanium in
ternary oxide coatings
(1) 1M H2SO4, (2) 0.5M NaCl ? 1M H2SO4 and (3) 0.5M NaCl
solutions
NaCl ? 1M H2SO4 solutions are shown in Figs. 4 and 5.
As shown in Fig. 4 and Table 1, the emission of oxygen chlorine gas emission in such circumstances occurs in less
gas is begun at more positive potentials in 1M H2SO4 overpotentials as compared with NaCl solution [19].
solution and the Tafel slope is about 0.18 V in this region. Comparing Figs. 4, 5 and Table 1, it is observed that the
With the entry of chlorine into the 1M H2SO4 solution, the polarization curve changes are similar in the binary and
reaction of chlorine gas emissions is initiated in fewer ternary coatings. The ternary coatings are more active in
potentials and the Tafel slope is about 0.10 V indicating low overpotentials, and currents for the ternary coating are
high activity of the oxide coatings for this reaction. The higher in 0.5M NaCl ? 1M H2SO4. They are prone to
curve slope in 0.5M NaCl solution is calculated 0.12 V, decrease in ability to load transfer compared with the
compared with the curve slope in 0.5M NaCl ? 1M H2SO4 binary coating in high overpotentials. This reduction is also
solution, which is the result of different reaction mecha- more visible in favorable solution for oxygen emission due
nisms. In solutions containing proton and chlorine, protons to low ability of oxygen compared with chlorine [18, 19]
are involved in active sites oxidation which create suit- since overpotentials for oxygen emission tend to be more
able sites for adsorption of chlorine gas. The reaction of than overpotentials for chlorine. Their electrocatalytic
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Fig. 6 Cyclic voltammetry curves for (Ti, Ru)O2 and (Ti, Ru, Ir)O2
Fig. 5 Polarization curves for (Ti, Ru)O2 coating on titanium in (1) oxide coatings on titanium in (1) 1M H2SO4, (2) 1M H2SO4 ? 0.5M
1M H2SO4, (2) 0.5M NaCl ? 1M H2SO4 and (3) 0.5M NaCl NaCl and (3) 0.5M NaCl
solutions
Table 1 Potential of chlorine release and Tafel slope for (Ti, Ru)O2 and (Ti, Ru, Ir)O2 oxide coatings on titanium in different solutions
Solution (Ti, Ru, Ir)O2 oxide coating (Ti, Ru)O2 oxide coating
Ba (V/decade) Potential of chlorine release (V) Ba (V/decade) Potential of chlorine release (V)
1M H2SO4 0.18 1.24 0.09 0.57
1M H2SO4 ? 0.5 M NaCl 0.10 1.12 0.07 0.44
0.5M NaCl 0.12 0.98 0.08 0.29
property in such electrodes for oxygen emission is lower potentials more positive than 1.147 V, the oxidation of
than chlorine gas, meaning that oxygen is emitted more RuO2 to RuO3 and RuO4 was took place. The oxidation of
slowly than chlorine gas. RuO2 to RuO3 is the first step in the dissolution of the
Cyclic voltammetry curves of the binary and ternary active element, assuming that the amount of titanium in the
oxide coatings on titanium in 0.5M NaCl, 1M H2SO4 and coating exists in oxidation state \?4 (non-stoichiometry
1M H2SO4 ? 0.5M NaCl solutions are shown in Fig. 6. TiO2) and titanium with capacity \?4 can revitalize RuO3
Capacitive behavior is the same for the binary and ternary in the form of stable RuO2 [21].
coatings in solutions containing proton. A broad reversible Because of less capacitive ability of IrO2 than RuO2,
peak is observed surrounding 0.60 V potential in solutions voltammetric currents for the ternary coatings are lower in
containing protons (1M H2SO4 and 1M H2SO4 ? 0.5M NaCl electrolyte. Oxidationreduction evolution on surface
NaCl), which is because of oxidationreduction evolution with the participation of protons is prevented because of
of ruthenium with the participation of protons. In fact, a the lack of them. An increase in the anodic current in
broad peak is visible in the potential ranging from 0.30 to voltagrams relating electrolytes containing chlorine ions
0.60 V and a small peak is visible in the potential rang- can be observed in potentials more positive than 1.15 V,
ing from 0.60 to 0.80 V. According to researches, the which is related to the initiation of chlorine gas emission
observed peaks can be attributed to oxidationreduction reaction. Increase is followed by the appearance of a
processes Ru2O3/RuO2(Ru3?/Ru4?) and Ru2O3/Ru(Ru3?/Ru), cathodic species which is dedicated to chlorine species
respectively. The broad peak in the potential range of disposal. Potential increases with increasing the chlorine
0.300.60 V is likely related to surface reactions sur- ion concentration. The parts of the coating containing
rounding the entire surface of electrode [20]. In fact, the segregation TiO2 (island edges) are areas with weak con-
electrode in potential range between hydrogen and oxygen ductivity, hence considered for semiconductor or insulating
reactions is acting like a capacitor that load carrier protons properties. The flow is preferably carried by ions fill in
with change of the potential into cathodic values are enter cavities in these areas. The total resistance of coating is
and by reversing the direction of potential are exit [20, 21]. dependent on the electrolyte resistance. TiO2 existing in the
The oxygen release begins in 1.10 V potential, and in coating layer provides properties for the release of chlorine
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(3)(5) stages and or produce oxygen or RuO4 that are
soluble in acidic solutions. Thus, with the presence of IrO2
in the coating, oxygen emission takes place mainly in IrO2
active sites preventing decomposition reactions on RuO2
active sites and causing significant increase in the coating
lifetime (Fig. 7).
4 Conclusions
The binary and ternary coatings of (Ti, Ru)O2 and (Ti, Ru,
Ir)O2 were prepared on the surface of titanium through sol
gel method, and their electrochemical properties were
Fig. 7 Potential-time curves versus time for the binary and ternary studied in 1M H2SO4, 0.5M NaCl and 1M H2SO4 ? 0.5M
coatings on titanium in 0.5M NaCl solution
NaCl solutions. Results indicated that when iridium oxide
is present in the coating, oxygen emission significantly
and increases the electrode lifetime and corrosion resis- takes place in active sites of iridium oxide. This prevents
tance [11, 18, 22]. decomposition reactions on active sites of ruthenium oxide
ACT results for the binary and ternary coatings in 0.5M and significantly increases the lifetime of the coating. The
NaCl solution are indicated the time dependence on the ternary coating is also more active in low overpotentials
potential at constant current density in Fig. 7. As there is and more prone to disability of load transfer in high
little amount of oxygen produced in anodes and insignifi- overpotentials.
cant compared with produced chlorine, the anodes lifetime
Acknowledgments The authors would like to express the appreci-
will increase by reducing of the amount of oxygen in ation to the Iran National Science Foundation for financial support.
chlorine gas [23]. Two reasons are suggested for the anode
electrocatalytic activity to diminish [24]:
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