TiO2 nano-composite coatings for long-term electrodeposition on disk electrodes

Surface & Coatings Technology 363 (2019) 128–134

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Growth mechanisms of Co-Mo/TiO2 nano-composite coatings for long-term electrodeposition on disk electrodes

T

⁎

H. Krawieca, , V. Vignalb, A. Krystianiakb, O. Heintzb, M. Latkiewicza,b a b

AGH - University of Science and Technology, Faculty of Foundry Engineering, ul. Reymonta 23, 30-059 Krakow, Poland ICB, UMR 6303 CNRS, Université Bourgogne - Franche Comté, BP 47870, 21078 Dijon Cedex, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Coating Cobalt Molybdenum Microstructure Corrosion

The structure of Co-Mo/TiO2 nano-composite coatings electrodeposited for long-term periods (between 1.5 and 31 h) on pure cobalt is investigated by means of electrochemical measurements and field-emission scanning electron microscopy (FE-SEM) with an integrated electron dispersion spectrometer (EDS). From the obtained results, two stages are identified during electrodeposition. Microstructure and growth mechanisms associated to each stage are presented and discussed.

1. Introduction Co-Mo nano-crystalline coatings obtained by electrodeposition are characterized by high hardness, high thermal resistance and good magnetic properties [1–14]. They are also good catalytic electrodes for hydrogen evolution reaction. These properties make these coatings promising materials for various applications (in biology, energy, nanotechnologies, aerospace… [15]). For short-term electrodeposition (times generally < 1 h), Co-Mo coatings have a nodular structure. This structure was previously studied by means of FE-SEM/EDS, AFM and XRD [14]. A model was also developed to explain its formation and to predict the current density vs time plots for the very first moments (up to 300 s) [16,17]. This model was based on the initiation and growth of semi-hemispherical crystallites (nodules). It was assumed that Co deposition is charge-transfer controlled whereas Mo deposition is charge-transfer controlled for the very first seconds and then is mass-transfer controlled. For long-term electrodeposition, almost no data are available in the literature about the growth mechanisms and the structure of Co/Mo nano-crystalline coatings. Under potentiostatic conditions, the current generally reaches a quasi-stationary value corresponding to the limiting current imposed by ions diffusion through the solution. Mo and Co deposition is then mass-transfer control. For other coatings, it has however been suggested that the local diffusion field may be an important parameter driving the growth rate and morphology of coatings. For example, this is the case of Pb coatings [18,19]. Residual stresses almost systematically exceed a critical value in the Co-Mo nano-crystalline coatings and micro-cracks are quickly formed

⁎

during electrodeposition [1,3,5–7,9–12]. Peeling off can be also observed (due to low adhesion). A previous study [14] showed that the greatest thickness of the Co-Mo nano-crystalline coatings reached without cracks is about 2 μm. It was found that cracking is promoted when the molybdenum content is high (> 20% on graphite substrate [11], for example), the electrodeposition rate is high (corresponding to applied potentials lower than −1000 mV vs. Ag/AgCl on copper and graphite substrates [1,12], for example) or the electrodeposition time is long (thick coating). The presence of a certain quantity of saccharine [9] or citrates [5] tends to decrease the density of cracks. However, they are still observed in the Co-Mo nano-crystalline coatings. Recently, it has been shown that depositing Co-Mo/TiO2 nanocomposite coatings may be an efficient solution to reduce significantly the level of residual stresses, to have good adhesion and to form compact (crack-free) coatings [20,21]. In this case, thick coating can be elaborated. The corrosion behavior of these coatings after long-term immersion in NaCl-based media (up to 455 h) has been studied [21]. In the present paper, the structure of Co-Mo/TiO2 nano-composite coatings electrodeposited for long-term periods (between 1.5 and 31 h) on pure cobalt is investigated. This was done using field-emission scanning electron microscopy (FE-SEM) with an integrated electron dispersion spectrometer (EDS). From obtained results, growth mechanisms are discussed.

Corresponding author. E-mail address: [email protected] (H. Krawiec).

https://doi.org/10.1016/j.surfcoat.2019.02.028 Received 18 October 2018; Received in revised form 21 January 2019; Accepted 9 February 2019 Available online 11 February 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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2. Materials and methods 2.1. Specimens and surface preparation The substrate was made of ultra-pure Co (Co: 99.99+ wt%, Al < 1 ppm, Ag < 2 ppm, Cr < 1 ppm, Cu < 3 ppm, Fe < 7 ppm, Mg < 1 ppm and Si < 5 ppm). It was delivered in the form of as drawn rods (diameter: 12 mm and length: 100 mm) by Goodfellow. Rods were cut (height of 10 mm) and embedded in an epoxy resin. The surface (surface area of 1.13 cm2) was then mechanically ground with emery papers (down to 4000 grit), cleaned in ethanol under ultrasonics for 5 min and dried in air. Co-Mo/TiO2 nano-composite coatings were electrodeposited at −1.2 V vs. SCE for different times (within the range 5 min–31 h) under stirring conditions (260 rpm, using a magnetic stirrer). The aqueous solution (volume of 150 mL) used for electrodeposition is 0.2 M CoSO4 7H2O + 0.02 M Na2MoO4 2H2O + 0.5 M H3BO3 + 0.3 M Na3C6H5O7 + 20 g/L TiO2 (primary particle size of 21 nm, provided by Sigma-Aldrich). All chemicals were mixed in a flask and the aqueous solution was then prepared by adding distilled water under stirring conditions. The solution pH and temperature are 5.8 and 25 °C, respectively. A Pt counter electrode (Pt foil) and a silver/silver chloride reference electrode (Ag/AgCl, 3.5 M) were used. The potentiostat/ Galvanostat/EIS Analyzer Parstat 4000 (AMTEK Princeton Applied Research, USA) was used. Electrodeposited samples were rinsed under gentle flow of distilled water for a few seconds and ultrasonically cleaned in acetone for 1 min.

Fig. 1. (a–b) Evolution of the current density vs. time during electrodeposition in different solutions. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

2.2. Characterization techniques FE-SEM/EDS (JEOL JSM 7600F) equipment was used to determine the chemical composition and morphology of Co-Mo/TiO2 nano-composite coatings. Images were acquired in the LABE (low angle backscatter electron image) mode. This detector is capable of producing qualitative compositional images with a very high degree of atomic number contrast. It was already noticed [21] that Co-Mo/TiO2 nanocomposite coatings are very beam sensitive. Cracks quickly appear upon exposure to the FE-SEM electron beam. Cracking is observed even at low accelerating voltage (1 kV). 3D images of the coating surface were obtained using AFM (Q-Scope 350 from Quesant Instrument Corporation). AFM images were acquired in the tapping mode, and they were analysed using the Q-analysis 4.0 software package. 3. Results and discussion 3.1. Electrochemical measurements Fig. 1 shows the evolution of the current density vs. time at the applied potential of −1.2 V vs. SCE in different solutions. Experiments were performed for 250 min. In the absence of Co2+ and MoO42− ions in the solution (0.5 M H3BO3 + 0.3 M Na3C6H5O7 + 20 g/L TiO2), a steady state defined by a value of −0.5 mA cm−2 is quickly reached, black curve in Fig. 1. Under these conditions, no coating is observed at the substrate surface. Only side reactions occur, namely the oxygen reduction reaction (ORR, reaction (1)), the protons reduction reaction (reaction (2)) and the water reduction reaction (reaction (3)). Side reactions are the reactions that occur during electrodeposition, but that do not lead to the formation of metallic cobalt and molybdenum.

O2 + 2H2 O + 4e− → 4OH−, E0 = 0.4 V vs. NHE

(1)

2H+ + 2e− → H2 , E0 = 0 V vs. NHE

(2)

2H2 O + 2e− → H2 + 2OH−, E0 = −0.83 V vs. NHE

(3)

Fig. 2. Evolution vs time of (a) the current density measured during electrodeposition experiments and (b) the thickness of coatings.

potential of reaction (3)), the main side reaction is reaction (1). Kinetics of reactions (2) and (3) are low. The low release of hydrogen observed during electrodeposition confirms this statement. The current density evolution visible in Fig. 1 (black curve) is mainly related to reaction (1). In a previous work on Co-Mo nanocrystalline coatings [22], it has been shown that reactions (2) and (3) has significant impact on the

As the pH of the solution used for electrodeposition is close to 6 and the potential is set at −1.2 V vs. SCE (very close to the standard 129

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(a)

10 µm

(b) 2.2 µm

Fig. 3. (a) Cross-sectional FE-SEM (LABE mode) and (b) 3D-view AFM images of the coating electrodeposited for 30 min.

(reactions (1)–(3), (6a) and (6c)), one may assume that the pH of the solution near the surface is slightly > 6 (and therefore molybdates ions are stable in the solution). When Co salts (red curve in Fig. 1) and then cobalt and molybdate salts (blue curve in Fig. 1) are added, the current densities increase progressively. We can suppose that the increase of surface roughness (nodular shape) induces an increase of current density. In the presence of cobalt and molybdate salts in the solution (blue curve in Fig. 1), current oscillations are observed. These fluctuations may be due to local changes in the surface topography induced by electrodeposition. The faradic efficiency was estimated by comparing the coating thickness obtained experimentally to that calculated from the Faraday law (Eq. (7)).

electrodeposition mechanisms for greater applied potentials (−1.3 V vs SCE, for example). The adsorption of some species from the solution (citrates for example) on the cobalt substrate may explain the decrease observed from −4 mA cm−2 down to −0.5 mA cm−2. The presence of these adsorbed species on the substrate can block some active sites where side reactions occur. When Co2+ ions are added in the solution (0.2 M CoSO4 7H2O + 0.5 M H3BO3 + 0.3 M Na3C6H5O7 + 20 g/L TiO2), the current density is significantly greater than previously (values around −12 mA cm−2, red curve in Fig. 1). The cathodic reactions correspond again to the side reactions (like in the previous case), but also to reactions (4) (reduction of free cobalt ions to cobalt) and (5) (reduction of citrate complexes to cobalt) [23,24].

Co2 + + 2e− → Co E0 = −0.28 V vs SHE

(4)

CoCit− + 2e− → Co + Cit3 −

(5) 2+

δ=

MoO42− −2

ions in the solution, the In the presence of both Co and current density is about −18 mA cm , blue curve in Fig. 1. The cathodic reactions include reactions (1)–(5) (like in the previous case) and the reduction of MoO42− ions to molybdenum. According to [24] this is a three-step reaction (reactions (6a)–(6c)).

MoO4 2 − + 2H2 O + 2e− → MoO2 + 4OH− E0 = 0.6 V vs SHE

(6a)

MoO2 + CoCit− → [MoO2 + CoCit−]ads

(6b)

[MoO2 +

CoCit−]ads

+ 2H2 O +

4e−

→ Mo +

CoCit−

+

4OH−

M nFρS

∫ i dt

(7)

where δ is the coating thickness (expressed in cm), M is the atomic weight of the coating (M = 66.39 g mol−1), n is the number of electrons (n = 2), F is the Faraday constant (F = 96,485 C mol−1), ρ is the coating density (ρ = 8.9 g cm−3), S is the surface area (A = 0.785 cm2), i is the current measured experimentally (blue curve in Fig. 1, expressed in A). The coating thickness derived from Eq. (7) is 98 μm whereas the experimental value (measured on cross-sectional FE-SEM/EDS images) is 83 μm. Therefore, a value of the faradic efficiency of 90% was found. Electrodeposition was carried out for different times (from 5 min up to 31 h) under potentiostatic control (at −1.2 V vs. SCE). It was found that the bulk pH does not change, even after long electrodeposition. The evolution of the current density vs time was recorded during electrodeposition. Fig. 2(a) shows these plots for eight experiments. The structure of the Co-Mo/TiO2 nano-composite coatings was then studied

(6c)

(MoO42−)

are stable in the solution. For pH > 6, molybdates ions They do not become protonated as it is when the pH is lowered. The initial pH of the solution is 5.8. Considering that OH− ions are produced and H+ ions are consumed during the electrodeposition process 130

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(a)

2

+

1

+

10 µm (b)

10 µm

(c2) (c1)

Fig. 4. (a–b) Cross-sectional FE-SEM images (LABE mode) of the coating electrodeposited for 1.5 h. (c1–c2) Schematic representation of the formation of protuberances. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

The chemical composition of nodular coatings is very homogeneous. EDS analyses performed in the coating electrodeposited for 30 min reveal that the molybdenum and cobalt contents are in average (30 measurement points located randomly in the coating, error = standard deviation) 13.6 ± 1.1 at.% and 63.1 ± 1.5 at.%. A small amount of oxygen (7.2 ± 0.3 at.%) was also found. This is due to the agglomeration of the TiO2 nanoparticles and the absorption of citrates in the coating. A small amount of titanium (from nanoparticles) was also found (1.3 ± 0.2 at.%). The ratio Mo/Co in the coating is then 0.21. This value is very close to that predicted from the potentiostatic experiments presented in Section 3.1. FE-SEM images (Fig. 3(a)) show that aggregates of TiO2 nano-particles are uniformly distributed in the coating. In aqueous solutions, TiO2 nanoparticles aggregate together at pH values near the point of zero charge (PZC) to reduce surface energy. In aqueous solutions, the pH of the PZC is in the range of 5.6–6.8 [25]. As the pH of the electrodeposition bath is within this range (pH = 5.8), aggregates are visible in the coating. As it was already mentioned in Section 1, this nodular structure was previously studied by means of FE-SEM/EDS, AFM and XRD [21] and a model was developed to explain its formation [16,17].

by means of FE-SEM/EDS. From these results, two stages (stages I and II in Fig. 2(a)) were identified. Each stage was associated with a specific structure of the coating and therefore a growth mechanism.

3.2. Short term electrodeposition: nodular structure (stage I) For electrodeposition times up to roughly 1h (thickness < 15–20 μm, stage I), the current density is almost constant over time (Fig. 2(a)). This means that the growth rate is constant. A quasi steady state exists during electrodeposition within this stage. However, dispersion in the current density value is observed between experiments (from −14 to −20 mA cm−2). As no significant dispersion was found in the value of the coating thickness and composition, it is suggested that such dispersion is related to the kinetics of side reactions. Within this stage, the thickness increases very slowly with time (Fig. 2(b)). Under these conditions, a nodular structure is observed. Fig. 3(a–b) shows cross-sectional FE-SEM and 3D-view AFM images of the coating electrodeposited for 30 min (within stage I). This structure can also be observed in the inner part of the coatings electrodeposited for longer times (24 h in Fig. 5(b)). 131

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Fig. 5. (a1–a3) Schematic representation of the growth of the columnar structure. (b) Cross-sectional FE-SEM image (LABE mode) of the coating electrodeposited for 24 h. Numbers indicate the Mo content in the coating (at.%).

3.3. Long term electrodeposition: intermediary structure (stage II) From roughly 1 h of electrodeposition, the current density increases significantly (Fig. 2(a)). Therefore, the growth rate increases (associated with a different growth mechanism). The thickness of the CoMo/TiO2 nano-composite coating then increases sharply with time, Fig. 2(b). A power law was found between thickness and time within the period of 5 min–24 h. FE-SEM observations after electrodeposition interrupted at the beginning of stage II (1.5 h) reveal the presence of protuberances (Fig. 4(a–b)). Numerous protuberances were broken during embedding in resin and polishing. Similar observations were performed on the coating electrodeposited for 4.2 h. The transition between stage I (nodules) and stage II (protuberances) may be explained by the influence of the surface topography (and therefore surface roughness) on the type of local diffusion. Nodules with different sizes are formed during stage I. Some nodules become much bigger than others. The condition of spherical diffusion (SD) is fulfilled around big nodules (blue arrows in Fig. 4(c1)). On the contrary, the existence of small nodules causes a planar (or cylindrical) type of diffusion (P/CD in Fig. 4(c1)). The limiting current density is

200 µm Fig. 6. Top-view FE-SEM image of the coating electrodeposited for 24 h.

132

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Ti2p TiO2 blanc Ti2p TiO2 rose

458.7 eV: TiO2

100

both perpendicular (preferential direction due to spherical diffusion, indicated by bold and black arrows) and parallel to the surface (planar/ cylindrical diffusion, indicated by thin and black arrows), Fig. 4(c2). Growth occurs preferentially along the axis perpendicular to the surface. Similar mechanisms have been proposed for electrodeposited Pb coatings [18,19]. By contrast to the coatings with a nodular structure, the coating electrodeposited for 1.5 h has heterogeneous composition. Different levels of grey corresponding to different Co/Mo ratio values are visible in Fig. 4(a–b). FE-SEM/EDS analyses show that bright zones (like site 1 in Fig. 4(a): 14.7 at.% Mo, 82.5 at.% Co and 2.8 at.% O) contain 11–15 at.% of molybdenum whereas dark zones (like site 2 in Fig. 4(a): 2.5 at.% Mo, 94.8 at.% Co and 2.8 at.% O) only contain 1–4 at.%. The appearance of a heterogeneous composition coincides with the very rapid increase of the growth rate. As mentioned in Section 1, for longterm electrodeposition periods, Co and Mo deposition is certainly masstransfer controlled. Considering that the Co content is 100 times greater than that of Mo in the bath, Co deposition is promoted. This leads to the heterogeneous composition observed in Fig. 4(a–b) and the presence of zones depleted in Mo.

as-received TiO2 freeze-dried TiO2

80

(a) 60

40

456.9 eV: TiO2-x

20

3.4. Very-long term electrodeposition: columnar structure (stage II)

0 468 468

464

460

464

460

456

Binding Energy (eV)

While protuberances grow, channels are generated between them, Figs. 5(a1)–(a2) and 6. These channels become deep and very narrow (Fig. 5(a2)). Mo42− ions present in these channels are quickly consumed (low content in the solution: 0.02 M Na2MoO4 2H2O). The diffusion of Mo in the solution is very low compared to the growth rate of protuberances. Then cobalt is only deposited (Fig. 5(a2)). At the end of the process, channels are closed (growth also occurs parallel to the surface) and a region poor in molybdenum is observed, Fig. 5(a3). The morphology visible in Fig. 5(a3) was observed experimentally. Indeed, for very long term electrodeposition (24 h in Fig. 5(b)), a columnar structure is clearly identified in the outer part of the coating by means of FE-SEM. The regions at the interface between columns contain an extremely small amount of Mo, in the range of 1–2 at.% (images insert in Fig. 5(b)). No cracks were observed on top-view SEM images. The crack was generated during the diamond cutting operation (to generate the cross-section surface).

452

456

452

Binding energy (eV)

Co2p CoSO4 Co2p TiO2 rose

781.1 eV (Co(OH)2)

(b)

785.6 eV

778.1 eV (Comet)

100

80

3.5. Role of TiO2 nanoparticles in the growth of the Co-Mo/TiO2 nanocomposite coatings

60

To determine if interactions exist between TiO2 nano-particles and ions from the bath, TiO2 nanoparticles were first immersed in the electrodeposition bath for 1 month under continuous stirring (800 rpm). FE-SEM/EDS and XPS analyses were then performed on freeze-dried nano-particles. FE-SEM/EDS reveal the presence of carbon/sodium (from sodium citrate) and a very small amount of cobalt/sulfur (from cobalt sulfate): 12.5 at.% C, 8 at.% Na, 1.6 at.% Co, 1.8 at.% S, 58.46 at.% O and 17.7 at.% Ti. These FE-SEM/EDS results indicate that citrates and a small amount of Co react with TiO2 nanoparticles in the electrodeposition bath. Fig. 7(a) (green curve) shows the XPS Ti(2p) spectrum of freezedried TiO2 nano-particles. This spectrum is compared to that of as-received nano-particles (red curve). The two peaks related to TiO2 overlap perfectly at 458.7 eV, indicating that the large majority of TiO2 nanoparticles do not react with ions from the bath. However, a second XPS peak was detected at 456.9 eV (with very low intensity) on the freeze-dried nano-particles. It corresponds to TiO2−x (reduction of titanium). In addition, a peak related to metallic cobalt (at 778.1 eV) was observed in the XPS Co(2p) spectrum of freeze-dried nanoparticles, green curve in Fig. 7(b). This peak was not observed in the spectrum of as-received CoSO4 (red curve in Fig. 7(b)). Therefore, FE-SEM/EDS and XPS analyses on TiO2 nano-particles show that a very small quantity of nanoparticles interacts with species

40

20

0 815

as-received CoSO4 (powder) freeze-dried TiO2 810

810

805

805

800

800

795

790

785

780

795 790 785 780 Binding energy (eV)

775

775

770

770

765

765

Fig. 7. XPS spectra of (a) titanium (Ti(2p) level) in as-received (red curve) and freeze-dried (green curve) TiO2 nanoparticles and (b) Cobalt (Co(2p) level) in CoSO4 (red curve) and in freeze-dried TiO2 nanoparticles (green curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

larger around big nodules (spherical diffusion) than elsewhere (planar or cylindrical diffusion). For this reason, protuberances grow locally. As protuberances grow, the condition of spherical diffusion is maintained at their top (Fig. 4(c2)) while the condition of planar/cylindrical diffusion is fulfilled at their walls. Then protuberances grow with time 133

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from the electrodeposition bath and that there is probably an interaction between those nanoparticles and cobalt. It was however not possible to identify the new compound by XPS. These analyses reveal that most of nano-particles only aggregate together. The aggregates may be incorporated in the coating by an electrophoretic deposition process [26,27] or a mechanical incorporation process due to the stirring conditions. To determine if nanoparticles are incorporated by a mechanical process due to the stirring conditions, a Co-Mo/TiO2 nano-composite coating was electrodeposited on pure cobalt using the same experimental conditions than previously, expect the stirring conditions (100 rpm instead of 260 rpm during the previous experiments). EDS analyses performed in the coating gives in average (6 measurements, error = standard deviation): 18.5 ± 1.8 at.% O, 1.5 ± 0.3 at.% Ti, 64.6 ± 1.8 at.% Co and 15.4 ± 0.9 at.% Mo. Therefore, the quantity of TiO2 nanoparticles incorporated in the coating does not depend on the stirring conditions (1.3 ± 0.2 at.% at 260 rpm and 1.5 ± 0.3 at.% at 100 rpm). The aggregates are then incorporated in the coating by an electrophoretic deposition process [26,27].

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4. Conclusions The following conclusions can be drawn. Within stage I (up to roughly 1 h), the growth rate is low. The coating is then chemically homogeneous and has a nodular structure. Within stage II (electrodeposition > 1 h), coatings with protuberances and then a columnar structure grow. These coatings have heterogeneous composition (large fluctuations in the Co/Mo ratio). The transition between the different morphologies observed over time of electrodeposition is proposed to be governed by the type of local diffusion. There is almost no interaction between TiO2 nanoparticles and ions in the electrodeposition bath. They aggregate together in the bath and are certainly incorporated in the coating by an electrophoretic deposition process. The addition of TiO2 allows to deposit very thick (few hundreds micrometers) and compact Co-Mo nano-composite coating at high electrodeposition rates. Acknowledgments This work is supported by the bilateral programme PHC POLONIUM (under the project #35214UK). The French Embassy in Poland and the Ministry of Foreign Affairs (France) are warmly acknowledged for providing a cotutelle PhD grant to M.L. References [1] E. Gomez, E. Pellicer, X. Alcobé, Properties of Co-Mo coatings obtained by electrodeposition at pH 6.6, J. Solid State Electrochem. 8 (2004) 497–504. [2] V.S. Kublanovskii, Y.S. Yapontseva, Y.N. Troshchenkov, V.A. Gromova, Corrosion and magnetic properties of electrolytic Co-Mo alloys, Russ. J. Appl. Chem. 83 (2010) 440–444. [3] Q.F. Zhou, L.Y. Lu, L.N. Yu, X.G. Xu, Y. Jiang, Multifunctional Co–Mo films fabricated by electrochemical deposition, Electrochim. Acta 106 (2013) 258–263. [4] Y. Messaoudi, N. Fenineche, A. Guittoum, A. Azizi, G. Schmerber, A. Dinia, A study on electrodeposited Co–Mo alloys thin films, J. Mater. Sci. Mater. Electron. 24

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