Characterization of thin CeO2 films electrochemically deposited on HOPG

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Characterization of thin CeO2 films electrochemically deposited on HOPG Firas Faisal a , Arafat Toghan a,b,∗ , Ivan Khalakhan c , Mykhailo Vorokhta c , Vladimír Matolin c , Jörg Libuda a,d a

Lehrstuhl f¨ ur Physikalische Chemie II, Friedrich-Alexander-Universit¨ at Erlangen-N¨ urnberg, Egerlandstrasse 3, 91058 Erlangen, Germany Chemistry Department, Faculty of Science, South Valley University, 83523 Qena, Egypt c Department of Surface and Plasma Science, Charles University in Prague, V Holeˇsoviˇckách 747/2, 180 00 Prague 8, Czech Republic d Erlangen Catalysis Resource Center, Friedrich-Alexander-Universit¨ at Erlangen-N¨ urnberg, Egerlandstrasse 3, 91058 Erlangen, Germany b

a r t i c l e

i n f o

Article history: Received 29 October 2014 Received in revised form 14 January 2015 Accepted 24 January 2015 Available online xxx Keywords: Fuel cell catalyst Electrodeposition Cerium oxide Thin films AFM Cracking

a b s t r a c t Electrodeposition is widely used for industrial applications to deposit thin films, coatings, and adhesion layers. Herein, CeO2 thin films were deposited on a highly oriented pyrolytic graphite (HOPG) substrate by cathodic electrodeposition. The influence of the deposition parameters on the yield and on the film morphology is studied and discussed. Morphology and composition of the electrodeposited films were characterized by in-situ atomic force microscopy (AFM), scanning electron microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). By AFM we show that the thickness of CeO2 films can be controlled via the Ce3+ concentration in solution and the deposition time. After exposing the films to ambient air, cracking structures are formed, which were analyzed by AFM in detail. The chemical composition of the deposits was analyzed by XPS indicating the formation of nearly stoichiometric CeO2 . © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ceria (CeO2 ) is among the most commonly used materials in catalysis, with high mechanical strength, thermal stability, and, most importantly its oxygen storage capacity [1,2]. Because of its ion conductivity, CeO2 is also of considerable interest for applications in solid oxide fuel cells (SOFC) operating at reduced temperatures [3]. A less common application is the one in proton exchange membrane fuel or polymer electrolyte membrane fuel cells (PEMFCs), which were invented in 1960s [4]. PEMFCs became a very promising route in fuel cell development as they combine large current density, fast start-up, low working temperatures and a solid non-toxic electrolyte. Among the challenge of the technology are the water management, cooling issues and, most importantly the high noble metal loading in the electrocatalytic layer. Also the sensitivity of the catalyst to carbon monoxide as a catalyst poison is an important issue [4–7]. Recently we have shown that ceria-based materials, in fact, hold a large potential as electrocatalysts in PEM

FC applications [8,9]. Using dispersed Pt in ceria films prepared by magnetron sputtering, outstandingly high moble metal efficiencies could be reached [10]. For the preparation of ceria thin films, electrodeposition is an attractive alternative to the vacuum based deposition methods. Herein, we explore electrochemical deposition of different cerium oxide from acidic solutions (pH < 6) containing Ce3+ ions. The global reaction in aqueous media to form CeO2 involves molecular oxygen and peroxide species [11]. The reaction is initiated by reduction of molecular oxygen in the electrolyte and, as this reaction predominantly occurs via a two electron pathway, Ce3+ is oxidized by the hydrogen peroxide intermediate to Ce4+ hydroxide. As a result a hydroxide gel forms, which reacts with the OH− ions that are generated by the former oxygen reaction. Finally, CeO2 is deposited on the electrode surface [12]. Thus, the overall net reaction (1) can be depicted in reactions (2–4) as: 2Ce3+ + 4OH− + O2 + 2e− → 2CeO2 + 2H2 O −

O2 + 2H2 O + 2e → H2 O2 + 2OH 3+

2Ce ∗ Corresponding author at: Chemistry Department, Faculty of Science, South Valley University, 83523 Qena, Egypt. Tel. +20965211281; fax: +20965211279. E-mail address: [email protected] (A. Toghan).

−

−

+ OH + H2 O2 → 2Ce(OH)2

2Ce(OH)2

2+ −

−

2+

+ 4OH → 2CeO2 + 4H2 O

H2 O2 + 2e → 2OH

−

(1) (2) (3) (4) (5)

http://dx.doi.org/10.1016/j.apsusc.2015.01.198 0169-4332/© 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: F. Faisal, et al., Characterization of thin CeO2 films electrochemically deposited on HOPG, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.01.198

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When hydrogen peroxide is added to the electroplating bath, as represented by reaction (5), reaction (2) is partially blocked. Therefore, the formation of Ce2 O3 is inhibited by the oxidation of the Ce3+ ions leading to deposition of CeO2 . In other words, the addition of hydrogen peroxide shifts the equilibrium towards Ce4+ ions, as represented by equation (3), thus resulting in the deposition of less oxygen deficient and more homogeneous CeO2 films. Additionally, H2 O2 provides the advantage of partially removing chlorine ions, which originate from adding HCl to adjust the pH values. Thus the risk of contamination is reduced [13,14]. 3H2 O2 + 2HCl → 4H2 O + O2 + Cl2

(6)

The main goal of this study is to prepare thin film coatings that may be suitable for use in PEM FCs. Therefore, we deposit the cerium oxide on a carbon substrate with the most well defined model substrate being HOPG (highly oriented pyrolytic graphite). The HOPG substrate furthermore provides the advantage that the quality, morphology and stoichiometry can be straightforwardly investigated by AFM, EDX and XPS. 2. Experimental The electrochemical deposition of CeO2 films were carried out in a three-electrode setup. All electrochemical deposition experiments were performed at room temperature in a glass cell with a volume of approximately 0.2 cm3 . A square-shaped plate of HOPG (1 cm2 , 1 mm thick, Alfa Aesar) served as working electrode (WE). The HOPG has flat terraces of 100 nm to 1 ␮m width. This allows measuring the grain sizes in a straightforward fashion as the substrate can be considered as flat. A silver wire with a thickness of 0.1 mm was used as reference electrode (RE), and a platinum wire (0.25 mm thick) in the shape of a circular coil of the radius r = 1.5 cm was used as a counter electrode (CE) as shown in Fig. 1b. The HOPG WE was renewed before each experiment by using a scotch tape to remove the first layers of the substrate. The roughness of the clean HOPG surface was checked by AFM. It reveals mostly flat surface areas with few atomic steps (0.2–0.3 nm) and few steps bunches of several 10 to 100 atomic layers (not shown) with an average height varying around 20 nm. Before each run, the cell was cleaned with a sulfuric acid solution followed by rinsing in deionized water. The electrolytes employed for electrodeposition of CeO2 films have been freshly prepared by using analytical grade chemicals without any further purification, dissolved in an appropriate volume of deionized water. Five different baths were prepared by dissolving different relative amounts of CeCl3 ·7H2 O (99% purity, Alfa Aesar) and H2 O2 (35%, Alfa Aesar) at pH 2 (see Table 1). The pH value of 2 was adjusted before the electrodeposition process

Table 1 Composition of plating baths employed fort he electrodeposition of various CeO2 films. Bath composition

CeCl3 ·7H2 O H2 O2

Concentration (mmol l−1 ) Bath (1)

Bath (2)

Bath (3)

Bath (4)

Bath (5)

9.40 0.40

4.70 0.40

2.40 0.40

0.024 0.40

64.50 1.30

with hydrochloric acid (35%, Ing. Petr Svec - PENTA). All deposition experiments were performed without stirring and at room temperature. The desired amount of the electroplating bath (≈0.2 cm3 ) was injected via a syringe to the electrochemical cell as shown in Fig. 1a. The electrochemical AFM experiments were performed with a MultiMode 8 electrochemical atomic force microscope (EC-AFM) connected to a Universal Bipotentiostat from Bruker Corporation. With this setup, in-situ EC-AFM was performed in order to provide information on the surface topography, film growth and corrosion with high spatial resolution as shown in Fig. 1a [14,15]. Additionally, the deposition process was monitored by a light optical camera in the EC-AFM during the deposition process. The CeO2 thin films were electrodeposited potentiostatically by applying a fixed negative potential of −1.3 V between the WE and the RE. To measure the corrosion resistance of the deposited films, after coating the HOPG potentistatically with CeO2 , a positive anodic potential of 2 V was applied in the same electroplating bath. The dissolution of the film was monitored by in-situ AFM as a function of the time of the applied potential. The surface morphology of the films was characterized by exand in-situ atomic force microscopy (AFM) and by scanning electron microscopy (SEM, MIRA TESCAN, 30 keV). Surface impurities were analyzed using energy dispersive x-ray (EDX) spectroscopy. X-ray photoelectron spectroscopy (XPS) was used to study the surface composition. The XPS system we used in this work was equipped with an energy analyzer SPECS Phoibos MCD 9 and a dual Mg/Al X-ray source with total energy resolution E = 1 eV. In our case only Al K␣ X-ray source (1486.6 eV) was used because lower photon energy of the Mg K␣ X-ray source (1253.6 eV) would lead in principle to higher and more inclined nonlinear Ce 3d spectrum background and consequently to lower peak fitting precision. All XPS experiments were performed ex-situ in an ultra-high vacuum (UHV) chamber operating at base pressures of about 5 × 10−9 mbar [16]. All binding energies (BE) were referenced to the energy of the C 1s peak at 284.6 eV (Ce 3d, O 1s). For excitation an Al-K␣ radiation was used with a photon energy of 1486.7 eV. The Ce 3d envelopes were fitted with Voigt profiles (sum of Gaussian and Lorentzian)

Fig. 1. Bruker EC fluid cell: (a) cell mounted into MultiMode 8 EC-AFM head, and (b) exemplary setup of the three electrodes in the fluid cell. WE = working electrode, CE = Counter electrode, and RE = reference electrode.

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using the KolXPD software [17] after subtraction of a Shirley background [18,19].

e) 1800 s

3. Results and discussion 3.1. Effect of deposition time

d) 30 s

Probability

Fig. 2 shows ex-situ AFM images taken at different deposition times. Altogether five different samples were used. All samples were prepared by using the same electrolyte (bath 4 in Table 1), but with different deposition times of 5, 10, 20, 30, and 1800 s. After preparation, the films were first cleaned with deionized water and then exposed to air for 10 min. Already after deposition times of 5 s evidence for the formation of small cerium oxide grains were observed. Fig. 2a shows that the deposit not only consists of small isolated grains but that these already started to agglomerate. The height of those particles and agglomerated structures appear rather homogeneous and fall in the range of 4–6 nm as shown in Fig. 3a. For the isolated grains a width of approximately 30 nm was observed. After a deposition time of 10 s (sample 2) AFM reveals that the substrate is still partially covered with CeO2 , but the grain size increased to a value of about 6–8 nm (Fig. 2b). By further extending the deposition time the film morphology becomes more complex. In addition to a flat film we observe the formation of small particlelike structures. After 20 s of deposition most of these structures are in the height range from 6 to 10 nm. They reside on a flat film which is formed slowly. As shown in Fig. 3 the height of the structures, i.e. the roughness, increases to a maximum of about 13 nm already after 20 s without growing further at longer deposition times. Also the profile indicate that the film formation is not fully completed after 20 s as there still are areas which can be assigned to the substrate. The height distribution shows several maxima which we

c) 20 s

b) 10 s

a) 5 s 0

3

5

8

10

13

15

18

20

Height (nm) Fig. 3. Height distribution for CeO2 films prepared on HOPG from bath 4 at different deposition times: (a) 5, (b) 10, (c) 20, (d) 30, and (e) 1800 s.

Fig. 2. Ex-Situ AFM images for CeO2 films prepared on HOPG from bath 4 recording at different deposition times: (a) 5, (b) 10, (c) 30, and (d) 1800 s. All images are taken at the center of the sample.

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assign to the above described morphology with flat films and growing particles. For 30 s of deposition, the height distribution shifts towards larger values and shows no multiple maxima anymore. We attribute this to the fact that the substrate is now almost fully covered with a polycrystalline film of cerium oxide (Fig. 2c). After an extended deposition period of 1800 s, the film finally appears rather homogeneous and the measured roughness of the grain structure decrease to an average value of around 5 nm (see Fig. 2d). The morphology suggests that the deposition is taking place through a nucleation and growth mechanism, indicated by the fact

that granular structures appear. The microscopic nuclei begin to form at the electrode surface once a sufficiently high concentration of cerium ions is established in the solution near the electrode to initiate the nucleation process. After the nuclei have reached a certain size, they become thermodynamically stable and new deposits are preferentially added onto these nuclei leading to their rapid growth. The smallest observed grains are in the range of 2 nm, which suggest that smaller grains are either not thermodynamically stable or cannot be resolved by the AFM. Metal deposition processes often lead to formation of nanometer sized crystallites,

Fig. 4. In-situ AFM micrographs for CeO2 films prepared on HOPG recording after 240 s deposition time and at different Ce3+ concentrations: (a) bath 1, (b) bath 2. All images are taken at the center of the sample.

Fig. 5. Ex-situ 3D AFM images for CeO2 films prepared on HOPG with deposition time of 240 s at different Ce3+ concentrations and after exposing the films to ambient air, cracking structures are formed with cavities: (a) bath 1, (b) bath 2, (c) bath 3. (d) Dependence of the average cavities depth (film thickness) on the Ce3+ concentrations in the electrolyte in a reciprocal scale. All images are taken at the edge of the sample.

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which was also observed for CeO2 [20]. These studies together with the fact that small nuclei appear similar and sharp in the AFM image, may be interpreted as a hint that crystalline nanoparticles are formed in the nucleation step [21]. After the film formation the deposited CeO2 can be considered as rough and disordered [13]. The interaction between the HOPG substrate and the deposit appear to be weaker. This is indicated by the fact that the nuclei form aggregates on the HOPG. On the growing cerium film, however, the interaction appears to be stronger as indicated by the decreased size of the nucleating particles on the substrate. This more homogeneous distribution of growing particles is helpful towards the growth of a flatter film and leads to the decrease in roughness that we observe for cerium oxide film after the long deposition time (1800 s). 3.2. Effect of cerium concentration To investigate in-situ the morphology of CeO2 films at constant deposition time of 240 s we used three different baths with different Ce3+ concentrations, which are labeled as 1, 2, and 3 (see Table 1). The AFM results are shown in Fig. 4. In addition, the morphology of the dried films was investigated ex-situ by AFM after drying in air for 600 s. It was found that after exposure of the

5

films to ambient air, cracking structures are formed. The resulting cavities revealed the carbon substrate as displayed in Fig. 5. This cracking behavior allows us to measure the film thickness. The cavities depth and width was found to increase with increasing concentration of Ce3+ . Empirically we find that the film thickness increases linearly with the reciprocal value of the Ce3+ concentration (see Fig. 5d). This finding has two important implications: Firstly, the film deposition stops or becomes very slow on the timescale of our experiment, if the Ce3+ concentration decreases below 2 mmol/L. Secondly, the film growth stops or becomes very slow at film thicknesses above 800 nm. In contrast to the cavity height, the concentration dependence of the cavity width appears to be more complex. In comparison to the higher concentration regime (bath 1 and 2, cavity width 5 of ␮m) we observe a strong decrease of the cavity width for the lowest Ce3+ concentration to a value of 0.8 ␮m, i.e. 1/6 of the former value. This fact indicates that the cracking behavior, characterized by the width of the cavities, does not show a simple dependence on the concentration or on film thickness. Not surprisingly it depends on other parameters, such as the defect density, strain or the gas evolution during the deposition process. Additionally, we found that the deposited films were thicker at the edge than in the center of the cell. This effect is a result of the shape of the CE and the fact that the distance to the

Fig. 6. EDX spectra for cerium oxide films deposited on HOPG from bath 5 at different deposition times: (a) 5, and (b) 40 s.

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WE varies. We investigated the film morphology close to the center and close to the edge region in another sample prepared from bath 1 at a deposition time of 10 s. The growing front as represented by the cracking behavior shown in Fig. S1 in the SI. The AFM images show that the film is not growing homogeneously, as indicated by the different cavity depths as a function of position. Returning to the AFM images in Fig. 4 the flat areas in the center of the sample indicate that with decreasing Ce3+ concentration the film roughness decreases and the ceria structures become smaller. The height of the film decreased significantly with decreasing the Ce3+ concentration from 85 nm (bath 1) to approximately 20 nm (bath 2). By further decreasing the Ce3+ concentration (bath 3), a very homogeneous smoothly structured film was obtained. With high concentrations of Ce3+ in the electrolyte, the equilibrium is pushed towards the adsorbed state. As a result desorption of ceria from the surface is suppressed. Desorption and readsorption is essential for the formation of smooth and compact film. It is noteworthy that the concentration is also a crucial factor for the initial nucleation step, since a higher concentration should lead to formation of smaller nuclei and a higher nucleation density. Therefore, we conclude that the growth kinetics results in the formation of the smoother and more homogeneous morphology for lower concentrations [22,23]. Next, EDX was performed on cerium oxide films from bath 5 (see Table 1) after 5 and 40 s of deposition time (Fig. 6). Both spectra show an intense peak, which we assign to the carbon substrate. We allocate four cerium peaks in the 5–6 keV region from the Lseries and one peak at 0.8 keV from the M-series [24]. The intensity of oxygen, cerium, and also the bremsstrahlung is increasing with the deposition time (see Fig. 6). A negligible amount of chlorine was detected after a deposition time of 40 s. We assume that it originates from remaining solution and the fact that the sample was not dried perfectly. With increasing deposition time the cerium and oxygen signals increase in intensity, indicating an increase of the film thickness. Also the continuous bremsstrahlung increases as an effect of deceleration by cerium (Z = 58) compared to carbon (Z = 12). Composition mapping by EDX shows that the cavities can be assigned to carbon and the flat areas to ceria. As the EDX resolution is in the ␮m and the high penetration depth it can, however not be proven strictly whether the cavities are completely free of CeO2 . Due to the weak interaction of ceria with HOPG (see above) it appears unlikely that a thin ceria film remains adsorbed on the substrate. Fig. 7 shows XP spectra of the Ce 3d and O 1s core levels of theCeO2 film prepared electrochemically from bath 5 with a deposition time of 40 s (the same film was used for EDX). The Ce 3d spectrum shown in Fig. 7a reveals three 3d3/2 -3d5/2 spin-orbit-split doublets that are characteristic for the Ce4+ states of stoichiometric CeO2 [16,25], and two very small 3d3/2 -3d5/2 spin-orbit-split doublets which are characteristic for Ce3+ . The different doublets arise from different 4f configurations in the photoemission final state and result from the Ce 4f hybridization in both initial and final states [26]. The O 1s spectrum consists of three species (O1–O3), as can be seen in Fig. 8b. The component O1 is observed at a binding energy (BE) of 529.7 eV, which is characteristic for CeO2 [16]. The main component O2 at 531.8 eV can be assigned to OH groups. An additional oxygen component O3 at higher BE (533.0 eV) is assigned to molecular water. We can conclude that a nearly stoichiometric CeO2 film is formed. Finally, we investigated corrosion of the electrodeposited films. As cerium oxide is well known as a good corrosion inhibitor, the corrosion is decelerated. In order to explore the corrosion behavior of the deposited CeO2 films, in-situ measurements by AFM and an optical camera (not shown) were applied. We used the deposited CeO2 film grown from bath 5 with deposition time of 10 s. For the corrosion process a positive potential of +2.0 V was applied in the same electroplating bath and AFM images were taken in-situ to

Fig. 7. XPS spectra for CeO2 films coated on HOPG from bath 5 after 40 s of deposition times: (a) Ce 3d, and (b) O 1s. The photon excitation energy = 1486 eV. The black dashed and dotted lines represent fitted peaks for Ce in the oxidation states (III) and (IV), respectively.

follow the anodic dissolution behavior (see Fig. 8a). The corrosion process propagates from the center of the sample where the film thickness is lower to the edge where the AFM images were recorded (Fig. 8a). The AFM images show that the film roughness decreases as long as the potential is applied. The initial structures with heights up to 600 nm directly after deposition decreased to 300 nm and 100 nm after 300 and 420 s, respectively. The height of the structures decreases nearly linearly with the corrosion time as shown in Fig. 8b. In fact, the deposition and dissolution behavior of cerium compounds from aqueous solution containing hydrogen peroxide depends not only on the applied potential, but also on the pH value [27]. It should be noted that the pH value was adjusted only at the beginning of the experiment and could not be controlled during the corrosion experiment processes itself. Also the applied potential had to be measured against a silver pseudo-reference electrode without a fixed reference potential. Nevertheless we can assume that the corrosion reaction takes place at pH greater than 2, as the pH increases due to cathodic reactions of water (7–9) [28]; O2 + 4H3 O + 4e−  6H2 O E 0 = +1.23 V −

−

O2 + 2H2 O + 4e  4OH

0

E = +0.40 V

H2 O2 + 2H+ + 2e−  2H2 O E 0 = +1.78 V

(7) (8) (9)

During the corrosion process insoluble cerium compounds, e.g. Ce(OH)4 , are formed, which forms a relative stable protective layer on the CeO2 film, thus reducing the corrosion rate. The cerium hydroxide formation is described by reactions (10–12) [28,29]: Ce3+ + 3OH– → Ce(OH)3

(10)

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that the films provide acceptable corrosion-resistance and that the corrosion rate is nearly independent of the film thickness. This opens up the possibility to prepare ceria films with well-defined thickness by electrodeposition of thick films and subsequent controlled corrosion. Acknowledgments This project was financially supported by the European Commission (“chipCAT”, FP7-NMP-2012-SMALL-6, Grant Agreement no. 310191). We gratefully acknowledge additional support by the COST Action CM1104 “Reducible oxide chemistry, structure and functions”, specifically the travel support (STSM) for F.F. The authors gratefully acknowledge further financial support by the Ministry of Education of the Czech Republic (LG12003 and LD11047) and from the Deutsche Forschungsgemeinschaft (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the Excellence Initiative. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.01. 198 References

Fig. 8. In-situ 3D AFM images showing the reaction front of the corrosion process recorded during applying anodic potential of 2 V to the CeO2 film prepared from bath 5 with deposition time of 10 s.

2Ce3+ + 2OH– + H2 O2 → 2Ce(OH)2 2+ Ce(OH)2

2+

+ 2OH– → Ce(OH)4

(11) (12)

It is noteworthy that in spite of the complex corrosion process dissolution of the film proceeds with a nearly constant rate. This is in sharp contrast to the electrodeposition step, which is strongly nonlinear as a function of time and concentration, but becomes very slow at large film thickness. Interestingly, this opens up the possibility to prepare ceria films with variable thickness by electrodeposition of thick films and a subsequent corrosion step. 4. Conclusions In this study, we followed in-situ the deposition and the corrosion behavior of different CeO2 thin films prepared electrochemically on a HOPG substrate at room temperature. The influence of the deposition parameters on the film growth and the morphology were investigated in detail. The morphology and composition of the electrodeposited films were characterized by in-situ AFM, EDX, and XPS. Exposing the films to ambient air, cracking structures are formed. These structures allow straightforward measurements of the formation kinetics. The results of AFM on these structures indicate that the thickness of the CeO2 films depends on the Ce3+ concentration and the deposition time in a complex fashion. Electrodeposition is suppressed at short times and low concentrations. In addition, the film roughness decreases with increasing film thickness and decreasing Ce3+ concentration. XPS data indicate the formation of nearly stoichiometric CeO2 . A corrosion test shows

[1] A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, 2002. [2] D.H. Lima, W.-D. Leea, D.-H. Choia, H.H. Kwonb, H.I. Lee, Electrochem. Commun. 10 (2008) 592. [3] A.B. Stambouli, E. Traversa, Renew Sustain. Energy Rev. 6 (2002) 433. [4] P. Costamagna, S. Srinivasan, J. Power Sources 102 (2001) 242. [5] B.J. Holland, J.G. Zhu, L. Jamet, Fuel Cell Technol. Appl. 13 (2007) 361. [6] K. Kordesch, G. Simader, Fuel Cells Appl., VCH (1996). [7] D. Stolten, B. Emonts, Fuel Cell Sci. Eng. Mater. Process. Syst. Technol., VCH (2012). [8] V. Matolín, I. Matolínová, M. Vaclavu, I. Khalakhan, M. Vorokhta, R. Fiala, I. Pis, Z. Sofer, J. Poltierova-Vejpravova, T. Mori, V. Potin, H. Yoshikawa, S. Ueda, K. Kobayashi, Langmuir 26 (2010) 12824. [9] V. Matolin, R. Fiala, I. Khalakhan, J. Lavkova, M.V. Vaclavu, M. Vorokhta, Int. J. Nanotechnol. 9 (2012) 680. [10] A. Bruix, Y. Lykhach, I. Matolínová, A. Neitzel, T. Skála, N. Tsud, M. Vorokhta, ˇ cíková, J. Mysliveˇcek, K.C. Prince, S. Bruyère, V. Potin, V. Stetsovych, K. Sevˇ F. Illas, V. Matolín, J. Libuda, K.M. Neyman, Angew. Chem. Int. Ed. 53 (2014) 10525. [11] V. Fernandes, J.J. Klein, W.H. Schreiner, N. Mattoso, D.H. Mosca, J. Electrochem. Soc. 156 (2009) 199. [12] F.-B. Li, R.C. Newman, G.E. Thompson, Electrochim. Acta 42 (1997) 2455. [13] I. Zhitomirsky, A. Petric, Ceram. Int. 27 (2001) 149. [14] M. Koinuma, K. Uosaki, Electrochim. Acta 40 (1995) 1345. [15] S.V. Kalinin, A. Gruverman, Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale Band 1, Springer, 2007. [16] R. Fiala, I. Matolínová, V. Matolín, K. Sevcikova, H. Yoshikawa, A. Tapan, Int. J. Electrochem. Sci. 8 (2013) 10204. [17] http://www.kolibrik.net/science/kolxpd; 17.10.2014. [18] D.A. Shirley, Phys. Rev. 5 (1972) 4709. [19] N. Fairley, A. Carrick, The Casa Cookbook–Part 1: Recipes for XPS Data Processing, Acolyte Science, Kinderton Close, High Legh, Knutsford, Cheshire, UK (2005). [20] D. Banham, S. Ye, T. Cheng, S. Knights, M. Stewart, M. Wilson, F. Garzonc, J. Electrochem. Soc. 161 (2014) F1075. [21] F.-B. Li, R.C. Newman, G.E. Thompson, Electrochim. Act 42 (1997) 2455. [22] E. Budevski, G. Staikov, W.J. Lorenz, Electrochem. Phase Fomation Growth, VCH (1996). [23] W.Schindler. T. Koop, D. Hoffmann, Trans. Magnet. 34 (1998) 963. [24] J. Kirz, X-ray Data Booklet, Lawrence Berkeley Laboratory, University of California, 1985. [25] A. Fujimori, Phys. Rev. B 28 (1983) 2281. ˇ ˇ [26] V. Matolín, M. Cabala, V. Cháb, I. Matolínová, K.C. Skoda, M. Sutara, F. Prince, T. Skála, K. Veltruská, Surf. Interf. Anal. 40 (2008) 225. [27] P. Yu, et al., J. Electrochem. Soc. 153 (2006) C74–C79. [28] G. Milazzo, S. Caroli, V.K. Sharma, Tables of Standard Electrode Potentials, Wiley, 1978. [29] C. Evan, Brown, Electrochemically Deposited Ceria Structures for Advanced Solid Oxide Fuel Cells, California Institute of Technology Pasadena, California, 2011.

Please cite this article in press as: F. Faisal, et al., Characterization of thin CeO2 films electrochemically deposited on HOPG, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.01.198