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Nanostructured Pt–CeO2 thin film catalyst grown on graphite foil by magnetron sputtering
Applied Surface Science 267 (2013) 119–123
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nanostructured Pt–CeO2 thin film catalyst grown on graphite foil by magnetron sputtering Mykhailo Vorokhta a,b,∗ , Ivan Khalakhan a , Iwa Matolínová a , Masaaki Kobata b , Hideki Yoshikawa b , Keisuke Kobayashi b , Vladimir Matolín a a b
Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsoviˇckách 2, 18000 Prague 8, Czech Republic NIMS Beamline Station at SPring-8, National Institute for Materials Science, Sayo-cho, Sayo, Hyogo 679-5148, Japan
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 4 August 2012 Accepted 11 August 2012 Available online 28 August 2012 Keywords: Cerium oxide Graphite foil Magnetron sputtering Nanoporous Photoelectron spectroscopy HAXPES
a b s t r a c t Layers of different thickness of CeO2 doped by Pt were prepared by magnetron sputtering on different substrates: Si (1 0 0) and a graphite foil. The structure and chemical composition of the Pt–CeO2 catalysts have been investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAXPES). SEM showed that the layers prepared on different substrates had very different morphology. XPS and HAXPES studies demonstrated that Pt was dispersed only in Pt2+ and Pt4+ oxidation states in CeO2 . Intensity of Pt2+ - and Pt4+ -peaks was affected by the plasma substrate interaction effects showing that carbon substrate played an active role by determining the film structure. The Pt2+ /Pt4+ and Ce3+ /Ce4+ ratios depend on the layer thickness and increases in the case of the graphite substrate. The reduced character of porous layer was explained by a general effect of formation of defects and oxygen vacancies at oxide edges and steps, and oxygen reaction with carbon which is responsible of formation of oxygen deficient cerium oxide at the interface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceria has a fluorite structure whose cations can switch between Ce3+ and Ce4+ oxidation states and it has the ability to act as an oxygen buffer [1]. The high storage capacity and the oxygen transport [2] give rise to its wide industrial application. Cerium oxide doped by noble metals is widely used as a catalyst in different oxidation processes like oxidation of CO [3], elimination of CO and NOx contaminants from automotive exhaust gases [4], water-gas shift reactions [5], oxidation of ethanol [6], and decomposition of methanol [7]. Recently, high activity of Pt–CeO2 oxidation catalyst was explained by two types of oxidative metal–oxide interaction: electron transfer from the Pt nanoparticles to the support and oxygen transfer from ceria to Pt [8]. Properties of Pt–CeO2 have also been investigated for development of proton exchange membrane fuel cell anode (PEMFC) [9–12]. It was shown that relatively cheap Pt–CeO2 catalyst, due to very low Pt loading, could be a potential candidate for replacing widely used expensive platinum catalyst. Carbon possesses unique electrical and structural properties that make it an ideal material for use in fuel cell construction. Various forms of carbon – from graphite and carbon blacks to composite
∗ Corresponding author at: Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsoviˇckách 2, 18000 Prague 8, Czech Republic. Tel.: +420 221 912 733; fax: +420 284 685 095. E-mail address: [email protected] (M. Vorokhta). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.036
materials – are frequently used as fuel cell components. In recent years, development of carbon nanotubes (CNTs) has opened up new avenues of materials development for the low-temperature proton membrane fuel cells. CNTs exhibit high electrical conductivity, excellent corrosion resistances, high electrochemical stability and perfect hydrophobicity; therefore, they have been explored as the catalyst supports to replace widely used carbon blacks in recent PEMFC. Recently we have reported platinum-doped CeO2 catalyst deposited on the CNTs by magnetron sputtering [13–16]. By using the catalysts in the PEMFC anode it was found that the layers containing nearly 100% concentration of cationic platinum Pt2+ and Pt4+ revealed very high specific power [13] which was explained by high activity of ionic Pt [9]. It was also shown that Pt doped cerium oxide films prepared by this method exhibited highly porous columnar structure assuming a large active surface of the catalyst. It opened a new way for using such systems as highly active thin film catalysts, e.g. in planar fuel cell systems where planar technology and catalyst thin film deposition would be considered as main fabrication processes. All our results, published or unpublished yet, showed that structure of the catalyst film substantially depends on type of substrate. In this work we show that films sputtered on flat graphite substrate exhibit similar columnar high porous structure as those deposited on the CNTs. We report scanning electron microscopy (SEM), hard X-ray photoelectron spectroscopy (HAXPES) and XPS study of the Pt–CeO2 catalyst thin film deposited at normal angle of deposition on the graphite foil.
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Fig. 1. AFM (a) and SEM (b) images of clean graphite foil.
2. Experimental Commercial graphite foil (Alfa Aesar, 0.254 mm thick) and Si (1 0 0) wafer were used as catalyst substrates. Roughness of the graphite substrate was checked by Atomic Force Microscopy (AFM). The Pt–CeO2 catalyst was deposited by means of two magnetrons working simultaneously at room temperature in argon atmosphere at total pressure 6 × 10−1 Pa. The CeO2 sputtering was performed perpendicularly by using 2 inch diameter CeO2 target at distance of 90 mm from the substrates with rf power of 100 W. Pt was added by using a second DC magnetron tilted by 45◦ relative to the CeO2 target. The film deposition rate was about 1 nm/min. It was determined from AFM measurement of the thickness of the reference Pt–CeO2 films deposited on the silicon substrate. Hence, by knowing the film deposition rate, the equivalent film thickness was easily determined from time of deposition for both the silicon and the carbon substrates. The atomic concentration ratio Pt/Ce of the Pt–CeO2 catalysts was calculated from areas of Ce 3d and Pt 4f XPS peaks by using XPS sensitive factors. It has been found to be close to 0.15. It should be noted that the obtained values corresponded to the surface concentration due to surface sensitivity of XPS. XPS system 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. For our measurements only Al K␣ X-ray source (1486.6 eV) was chosen because lower photon energy of the Mg K␣ X-ray source (1253.6 eV) would lead in principle to higher and more inclined non linear Ce 3d spectrum background and consequently to lower peak fitting precision. All XPS experiments were performed in an ultrahigh vacuum (UHV) experimental chamber operating at base pressures <10−8 Pa. Morphology of thin films was examined by means of field emission scanning electron microscopy (FE-SEM) by using a HITACHI S-4800 microscope at 10 keV electron beam energy.
The HAXPES measurements were performed at the undulator beamline BL15XU of the SPring-8 synchrotron facility. The X-ray was monochromatized at 5950.3 eV by using a Si 333 channel-cut post-monochromator. The total energy resolution E = 280 meV was determined from the Fermi cutoff of the Au reference sample. All HAXPES experiments were performed ex situ in ultra-high vacuum (UHV) experimental chamber operating at base pressures around 5 × 10−7 Pa, the spectra were taken at the grazing photon incidence and normal emission geometry. 3. Results and discussion In order to study the support-catalyst interaction, 10 nm Pt–CeO2 thin films were deposited on the graphite foil and the native oxide-covered Si (1 0 0) wafer simultaneously. The oxidecovered Si (1 0 0) was considered to be a truly chemically inert reference substrate because neither the Ce 3d or Pt 4f XPS and HAXPES spectra indicate any film-Si substrate chemical interaction. Graphite foil is composed of graphite sheets with a very flat surface. Fig. 1b presents an SEM image giving a low magnification picture of the surface morphology. A High resolution AFM image in Fig. 1a clearly shows hexagonal shallow holes of the hexagonal graphite structure. The depth of the craters was estimated to be ∼2 nm with roughness of ∼0.5 nm, on average. The SEM images of the silicon substrate coated by the Pt–CeO2 catalyst is presented in Fig. 2a. It reveals a rather homogeneous surface structure, with fine structural features showing the polycrystalline character of the film. Contrarily, Fig. 2b shows a porous surface morphology of the catalyst film sputtered on the graphite foil. High porosity of the film is given by growth of well separated catalyst islands that have fractal like structure. The size of these islands is about 15 nm across and more than 50 nm lengthways, with the islands separated by gaps of about 25 nm wide. A detailed
Fig. 2. SEM images of the 10 nm Pt–CeO2 films deposited on Si wafer (a), graphite (b) and 30 nm Pt–CeO2 films deposited on graphite (c).
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Fig. 3. HAXPES Ce 3d (a) and XPS Ce 3d (b) spectra of 10 nm Pt–CeO2 films deposited on Si wafer (A), graphite (B) and 30 nm Pt–CeO2 films deposited on graphite (C).
view shows that the islands are composed of fine, well dispersed nanowires at the catalyst film–graphite interface, which agglomerate with increasing film thickness giving pyramidal form of the islands. We showed in our previous works that the same structures were obtained on the CNTs coated by the Pt–CeO2 catalyst [16]. In [15] we speculated about influence of the grazing deposition angle on the film porosity. Comparison of the CNTs (from [15]) to the flat graphite substrate (in this work), where the Pt–CeO2 layers are prepared at normal deposition, leads to the conclusion that the carbon support is playing the main role in the growing mechanism. It is reasonable to suppose that the nanowire island formation originates from individual nucleus formed at an early stage of the growth. In order to study the dependence of morphology changes on the film thickness, a 30 nm thick Pt–CeO2 film was deposited on the graphite foil (Fig. 2c). The change in the surface morphology after increasing the deposited thickness from 10 to 30 nm is clear by comparing Fig. 2b and c. Fig. 2c inset illustrates the film morphology obtained by SEM of the film edge of the tilted sample and Fig. 2c shows again the columnar character of the catalyst deposition. The vertical structures are composed of bonded vertical nanorods forming together into a characteristic island structure. It can be seen that the heights of elemental nanorods are slightly dispersed and more than 30 nm in length. It should be noted that the thickness of porous films is always higher than the effective thickness of the deposits as determined from the deposition rate obtained for non porous layers. Chemical state and composition of the catalyst were investigated by HAXPES and XPS. The HAXPES technique is suitable for porous structure studies due to its large information depth of analysis. The kinetic energy of Ce 3d photoelectrons was about 5 keV corresponding to an inelastic mean free path (IMFP) in CeO2 about 7 nm [17]. Such a large inelastic mean free path (IMFP) allows us to take information from the depth more than 20 nm because about 95% of the photoelectron signal comes from the surface region of 3 IMFP thick. The high-resolution Ce 3d core level HAXPES spectrum of the reference 10 nm Pt–CeO2 films deposited on the Si wafer is presented in Fig. 3a (spectrum A). It consists of three 3d5/2 –3d3/2 spin–orbit-split doublets characteristic of Ce4+ states [18] and two 3d5/2 –3d3/2 spin–orbit-split doublets characteristic of Ce3+ states at 885.6 and 881.1 eV. The doublets represent different 4f configurations in the photoemission final state and arise from the Ce 4f hybridization in both the initial and the final states [19]. The Ce 3d5/2 4f2 peak at 882 eV is fitted by an asymmetric feature according to [20].
Table 1 Ce3+ /Ce4+ and Pt2+ /Pt4+ concentration ratios. HAXPES 2+
10 nm-Si 10 nm-graphite 20 nm-graphite
4+
XPS 3+
4+
Pt /Pt
Ce /Ce
Pt2+ /Pt4+
Ce3+ /Ce4+
0.47 7.86 2.72
0.05 0.15 0.11
0 1.12 0.16
0 0.11 0.09
Ce 3d spectra obtained from the film deposited on graphite foil are shown in Fig. 3a (B and C). The increase of Ce3+ state intensity corresponding to a partial reduction of cerium oxide can be seen. In the case of 30 nm films the relative concentration of Ce3+ states is less pronounced (spectrum C) than in the case of the 10 nm film. Ce 3d XPS spectra shown in Fig. 3b reveal the same tendency but with lower relative concentration of Ce3+ atoms. In the case of XPS, IMFP for Ce 3d electrons in CeO2 was 1.5 nm [17]. Using the areas of HAXPES and XPS peaks, the Ce3+ /Ce4+ ratios were calculated and are presented in Table 1. The Pt 4f HAXPES spectra obtained for the Pt-doped ceria film deposited on the Si (1 0 0) surface are plotted in Fig. 4a (spectrum A). The spectrum reveals Pt 4f7/2 –4f5/2 doublets at energies 72.5/75.8 eV and 74.2/77.5 eV, respectively. The first one can be associated with Pt2+ content while the second corresponds to Pt4+ [21]. The Pt–CeO2 film with thickness of 10 nm deposited on graphite (spectrum B) shows the same doublets but one can see a considerable increase of Pt2+ state concentration. The increase of film thickness causes relative increase of Pt4+ peak intensity, nevertheless ratio of Pt2+ /Pt4+ is still high (spectrum C). The Pt2+ /Pt4+ ratios of the sputtered films are also presented in Table 1. The Pt 4f XPS spectra obtained from the same Pt–CeO2 films are plotted in Fig. 4b. The spectra also exhibit only completely oxidized Pt2+ and Pt4+ states with same tendency of the concentration ratios Pt2+ /Pt4+ . Ratios obtained by XPS are, however, different from those of HAXPES (Table 1). In the case of the silicon substrate only Pt4+ species are presented while graphite substrate reveals both oxide states of platinum Pt4+ and Pt2+ . It can be seen that on graphite the Pt2+ peak intensity decreases with increasing film thickness (spectrum C in Fig. 4b). The observed influence of the substrate on cerium oxide stoichiometry as well as the relative occurrence of Pt2+ and Pt4+ cations is surprising because both graphite and SiO2 covered Si wafer are generally considered as inert supports. The HAXPES and XPS study of the samples reveals that nonporous film consists almost of Pt4+ and Ce4+ ions while Pt2+ and Ce3+ ions appear when the film
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Fig. 4. HAXPES Pt 4f (a) and XPS Pt 4f (b) spectra of 10 nm Pt–CeO2 films deposited on Si wafer (A), graphite (B) and 30 nm Pt–CeO2 films deposited on graphite (C).
porosity is increasing. Thus we can conclude that an increase of the Pt2+ state concentration is accompanied by an increase of the Ce3+ state concentration showing overall oxide reduction and this reduction is connected to the layer porosity. This effect can be tentatively explained by formation of two qualitatively different mixed oxides: reduced mixed oxide Ce3+ 2−x Pt2+ x O3−ı and stoichiometric mixed oxide Ce4+ 1−x Pt4+ x O2 . The PES results showing dependence of Pt2+ /Pt4+ and Ce3+ /Ce4+ ratios on the film thickness point out that the films are more reduced in deeper parts, i.e. closer to the interface. We propose that sputtered particles impinging the substrate are composed mainly of Ce, O and Pt atoms. In addition some CeOx species could be probably formed in the oxygen containing plasma above the target. Adsorbed Ce atoms are oxidized on the surface by very active atomic oxygen from gaseous or adsorbed phase. This mechanism can well explain the cerium oxide film growth on the Si substrate covered by the silicon oxide layer. On the other hand the mechanism of the sputtered cerium oxide film growth on graphite seems to be more complicated. Beside the film, morphology depends on choice of the substrate and on the film thickness, stoichiometry is another variable parameter. There are many different growth mechanisms of columnar structures reported in literature, e.g. [21–26]. In most of them the initial surface roughness develops in fractal geometry [27] which, due to atomic shadowing and limited surface diffusion, leads to extensive porosity. We propose that the carbon surface is etched by oxygen plasma generated by the magnetron. The oxygen atoms or ions interact with the carbon surface forming CO or CO2 molecules desorbing from the surface. To test this hypothesis the Pt–CeO2 film covering the carbon substrate was dissolved out in HCl and it was found that the bare graphite surface (where no CeO2 was seen by XPS) revealed very similar surface morphology to that of Pt–CeO2 coating structure. This means that the deposit is somehow copying the substrate structure (form of islands). We are therefore proposing an explanation of such porous growth based on initial formation of a Ce rich 3 dimensional nucleus separated by holes etched in the graphite substrate. Consequently, due to higher accessibility of 3D structures
impinging cerium, pillars of cerium oxides are grown while oxygen is still etching non covered graphite. The reduced character of porous layer can be explained by a general effect of formation of defects and oxygen vacancies at oxide edges and steps. Moreover, oxygen reaction with carbon is responsible of formation of oxygen deficient cerium oxide at the interface. Therefore the phase formed on the carbon surface contains reduced species Ce3+ 2−x Pt2+ x O3−ı . We suggest that during layer growth further particles adsorb on the partially reduced islands and are oxidized to stoichiometric mixed oxide Ce4+ 1−x Pt4+ x O2 . This mechanism supports the XPS and HAXPES results which show the presence of Ce3+ and Pt2+ in the interface region. 4. Conclusion The Pt–CeO2 catalyst thin films deposited at normal deposition angle on the graphite foil and Si wafer were investigated by means of photoelectron spectroscopy and scanning electron microscopy. The main conclusions of this investigation are: (1) the catalyst sputtered on the graphite shows highly porous surface morphology; (2) the porous films are partially reduced; (3) the Ce3+ /Ce4+ and Pt2+ /Pt4+ ratios show that the films are more reduced in deeper parts, i.e. closer to the interface. Acknowledgments The authors would like to thank Dr. Robert George Acres for his careful reading of the paper and his helpful comments. The present work was supported by the projects P204/10/1169 and 202/09/H041 financed by the Czech Grant Agency. References [1] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935. [2] A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002. [3] W. Liu, M. Flytzani-Stephanopoulos, Journal of Catalysis 153 (1995) 317.
M. Vorokhta et al. / Applied Surface Science 267 (2013) 119–123 [4] I. Manuel, C. Thomas, H. Colas, N. Matthess, G. Diega-Mariadassou, Topics in Catalysis 311 (2004) 30. [5] D. Pierre, W. Deng, M. Flytzani-Stephanopoulos, Topics in Catalysis 46 (2007) 363. [6] X. Tang, B. Zhang, Y. Li, Y. Xu, Q. Xin, W. Shen, Journal of Molecular Catalysis A 235 (2005) 122. [7] S. Imamura, T. Higashihara, Y. Saito, H. Aritani, H. Kanai, Y. Matsumura, N. Tsuda, Catalysis Today 50 (1999) 369. [8] G.N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G.P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas, K.C. Prince, V. Matolín, K.M. Neyman, J. Libuda, Nature Materials 10 (2011) 310. [9] P. Bera, K.C. Patil, V. Jayaram, G.N. Subbana, M.S. Hegde, Journal of Catalysis 196 (2000) 293. [10] C.W. Xu, R. Zeng, P.K. Shen, Z.D. Wei, Electrochimica Acta 51 (2005) 1031. [11] C.L. Campos, C. Roldan, M. Aponte, Y. Ishikawa, C.R. Cabrera, Journal of Electroanalytical Chemistry 581 (2005) 206. [12] M. Takahashi, T. Mori, F. Ye, A. Vinu, H. Kobayashi, J. Drennan, Journal of the American Ceramic Society 90 (2007) 1291. ˚ I. Matolínová, J. Mysliveˇcek, R. Fiala, V. Matolín, Journal of the [13] M. Václavu, Electrochemical Society 156 (2009) B938. ˇ ˚ K.C. Prince, T. Skála, M. Václavu, [14] V. Matolín, M. Cabala, I. Matolínová, M. Skoda, T. Mori, H. Yoshikawa, Y. Yamashita, S. Ueda, K. Kobayashi, Fuel Cells 10 (2010) 139.
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˚ I. Khalakhan, M. Vorokhta, R. Fiala, I. Piˇs, [15] V. Matolín, I. Matolínová, M. Václavu, Z. Sofer, J. Poltierová-Vejpravová, T. Mori, V. Potin, H. Yoshikawa, S. Ueda, K. Kobayashi, Langmuir 26 (2010) 12824. ˚ M. Vorokhta, Z. Sofer, S. Huber, [16] R. Fiala, I. Khalakhan, I. Matolínová, M. Václavu, V. Potin, V. Matolín, Journal of Nanoscience and Nanotechnology 11 (2011) 5062. [17] S. Tanuma, C.J. Powell, D.R. Penn, Surface and Interface Analysis 21 (1993) 165. [18] A. Fujimori, Physical Review B 28 (1983) 2281. ˇ ˇ F. Sutara, T. [19] V. Matolín, M. Cabala, V. Cháb, I. Matolínová, K.C. Prince, M. Skoda, Skála, K. Veltruská, Surface and Interface Analysis 40 (2008) 225. ˇ K.C. Prince, V. Matolín, Journal of Electron Spectroscopy and [20] T. Skála, F. Sutara, Related Phenomena 169 (2009) 20. [21] L. Osterlund, S. Kielbassa, C. Werdinius, B. Kasemo, Journal of Catalysis 215 (2003) 94. [22] R.P.U. Karunasiri, R. Bruinsma, J. Rudnick, Physical Review Letters 62 (1989) 788. [23] G.S. Bales, A. Zangwill, Journal of Vacuum Science and Technology A 9 (1991) 145. [24] C.V. Thompson, R. Carel, Materials Science and Engineering B 32 (1995) 211. [25] I. Petrov, P.B. Barna, J.E. Greene, Journal of Vacuum Science and Technology A 21 (2003) 117. [26] A.A. Turkin, Y.T. Pei, K.P. Shasha, C.Q. Chen, D.I. Vainshtein, J.T. De Hosson, Journal of Applied Physics 108 (2010) 094330. [27] R. Messeir, Journal of Vacuum Science and Technology A 4 (1986) 490.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nanostructured Pt–CeO2 thin film catalyst grown on graphite foil by magnetron sputtering Mykhailo Vorokhta a,b,∗ , Ivan Khalakhan a , Iwa Matolínová a , Masaaki Kobata b , Hideki Yoshikawa b , Keisuke Kobayashi b , Vladimir Matolín a a b
Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsoviˇckách 2, 18000 Prague 8, Czech Republic NIMS Beamline Station at SPring-8, National Institute for Materials Science, Sayo-cho, Sayo, Hyogo 679-5148, Japan
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 4 August 2012 Accepted 11 August 2012 Available online 28 August 2012 Keywords: Cerium oxide Graphite foil Magnetron sputtering Nanoporous Photoelectron spectroscopy HAXPES
a b s t r a c t Layers of different thickness of CeO2 doped by Pt were prepared by magnetron sputtering on different substrates: Si (1 0 0) and a graphite foil. The structure and chemical composition of the Pt–CeO2 catalysts have been investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAXPES). SEM showed that the layers prepared on different substrates had very different morphology. XPS and HAXPES studies demonstrated that Pt was dispersed only in Pt2+ and Pt4+ oxidation states in CeO2 . Intensity of Pt2+ - and Pt4+ -peaks was affected by the plasma substrate interaction effects showing that carbon substrate played an active role by determining the film structure. The Pt2+ /Pt4+ and Ce3+ /Ce4+ ratios depend on the layer thickness and increases in the case of the graphite substrate. The reduced character of porous layer was explained by a general effect of formation of defects and oxygen vacancies at oxide edges and steps, and oxygen reaction with carbon which is responsible of formation of oxygen deficient cerium oxide at the interface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceria has a fluorite structure whose cations can switch between Ce3+ and Ce4+ oxidation states and it has the ability to act as an oxygen buffer [1]. The high storage capacity and the oxygen transport [2] give rise to its wide industrial application. Cerium oxide doped by noble metals is widely used as a catalyst in different oxidation processes like oxidation of CO [3], elimination of CO and NOx contaminants from automotive exhaust gases [4], water-gas shift reactions [5], oxidation of ethanol [6], and decomposition of methanol [7]. Recently, high activity of Pt–CeO2 oxidation catalyst was explained by two types of oxidative metal–oxide interaction: electron transfer from the Pt nanoparticles to the support and oxygen transfer from ceria to Pt [8]. Properties of Pt–CeO2 have also been investigated for development of proton exchange membrane fuel cell anode (PEMFC) [9–12]. It was shown that relatively cheap Pt–CeO2 catalyst, due to very low Pt loading, could be a potential candidate for replacing widely used expensive platinum catalyst. Carbon possesses unique electrical and structural properties that make it an ideal material for use in fuel cell construction. Various forms of carbon – from graphite and carbon blacks to composite
∗ Corresponding author at: Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsoviˇckách 2, 18000 Prague 8, Czech Republic. Tel.: +420 221 912 733; fax: +420 284 685 095. E-mail address: [email protected] (M. Vorokhta). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.036
materials – are frequently used as fuel cell components. In recent years, development of carbon nanotubes (CNTs) has opened up new avenues of materials development for the low-temperature proton membrane fuel cells. CNTs exhibit high electrical conductivity, excellent corrosion resistances, high electrochemical stability and perfect hydrophobicity; therefore, they have been explored as the catalyst supports to replace widely used carbon blacks in recent PEMFC. Recently we have reported platinum-doped CeO2 catalyst deposited on the CNTs by magnetron sputtering [13–16]. By using the catalysts in the PEMFC anode it was found that the layers containing nearly 100% concentration of cationic platinum Pt2+ and Pt4+ revealed very high specific power [13] which was explained by high activity of ionic Pt [9]. It was also shown that Pt doped cerium oxide films prepared by this method exhibited highly porous columnar structure assuming a large active surface of the catalyst. It opened a new way for using such systems as highly active thin film catalysts, e.g. in planar fuel cell systems where planar technology and catalyst thin film deposition would be considered as main fabrication processes. All our results, published or unpublished yet, showed that structure of the catalyst film substantially depends on type of substrate. In this work we show that films sputtered on flat graphite substrate exhibit similar columnar high porous structure as those deposited on the CNTs. We report scanning electron microscopy (SEM), hard X-ray photoelectron spectroscopy (HAXPES) and XPS study of the Pt–CeO2 catalyst thin film deposited at normal angle of deposition on the graphite foil.
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Fig. 1. AFM (a) and SEM (b) images of clean graphite foil.
2. Experimental Commercial graphite foil (Alfa Aesar, 0.254 mm thick) and Si (1 0 0) wafer were used as catalyst substrates. Roughness of the graphite substrate was checked by Atomic Force Microscopy (AFM). The Pt–CeO2 catalyst was deposited by means of two magnetrons working simultaneously at room temperature in argon atmosphere at total pressure 6 × 10−1 Pa. The CeO2 sputtering was performed perpendicularly by using 2 inch diameter CeO2 target at distance of 90 mm from the substrates with rf power of 100 W. Pt was added by using a second DC magnetron tilted by 45◦ relative to the CeO2 target. The film deposition rate was about 1 nm/min. It was determined from AFM measurement of the thickness of the reference Pt–CeO2 films deposited on the silicon substrate. Hence, by knowing the film deposition rate, the equivalent film thickness was easily determined from time of deposition for both the silicon and the carbon substrates. The atomic concentration ratio Pt/Ce of the Pt–CeO2 catalysts was calculated from areas of Ce 3d and Pt 4f XPS peaks by using XPS sensitive factors. It has been found to be close to 0.15. It should be noted that the obtained values corresponded to the surface concentration due to surface sensitivity of XPS. XPS system 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. For our measurements only Al K␣ X-ray source (1486.6 eV) was chosen because lower photon energy of the Mg K␣ X-ray source (1253.6 eV) would lead in principle to higher and more inclined non linear Ce 3d spectrum background and consequently to lower peak fitting precision. All XPS experiments were performed in an ultrahigh vacuum (UHV) experimental chamber operating at base pressures <10−8 Pa. Morphology of thin films was examined by means of field emission scanning electron microscopy (FE-SEM) by using a HITACHI S-4800 microscope at 10 keV electron beam energy.
The HAXPES measurements were performed at the undulator beamline BL15XU of the SPring-8 synchrotron facility. The X-ray was monochromatized at 5950.3 eV by using a Si 333 channel-cut post-monochromator. The total energy resolution E = 280 meV was determined from the Fermi cutoff of the Au reference sample. All HAXPES experiments were performed ex situ in ultra-high vacuum (UHV) experimental chamber operating at base pressures around 5 × 10−7 Pa, the spectra were taken at the grazing photon incidence and normal emission geometry. 3. Results and discussion In order to study the support-catalyst interaction, 10 nm Pt–CeO2 thin films were deposited on the graphite foil and the native oxide-covered Si (1 0 0) wafer simultaneously. The oxidecovered Si (1 0 0) was considered to be a truly chemically inert reference substrate because neither the Ce 3d or Pt 4f XPS and HAXPES spectra indicate any film-Si substrate chemical interaction. Graphite foil is composed of graphite sheets with a very flat surface. Fig. 1b presents an SEM image giving a low magnification picture of the surface morphology. A High resolution AFM image in Fig. 1a clearly shows hexagonal shallow holes of the hexagonal graphite structure. The depth of the craters was estimated to be ∼2 nm with roughness of ∼0.5 nm, on average. The SEM images of the silicon substrate coated by the Pt–CeO2 catalyst is presented in Fig. 2a. It reveals a rather homogeneous surface structure, with fine structural features showing the polycrystalline character of the film. Contrarily, Fig. 2b shows a porous surface morphology of the catalyst film sputtered on the graphite foil. High porosity of the film is given by growth of well separated catalyst islands that have fractal like structure. The size of these islands is about 15 nm across and more than 50 nm lengthways, with the islands separated by gaps of about 25 nm wide. A detailed
Fig. 2. SEM images of the 10 nm Pt–CeO2 films deposited on Si wafer (a), graphite (b) and 30 nm Pt–CeO2 films deposited on graphite (c).
M. Vorokhta et al. / Applied Surface Science 267 (2013) 119–123
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Fig. 3. HAXPES Ce 3d (a) and XPS Ce 3d (b) spectra of 10 nm Pt–CeO2 films deposited on Si wafer (A), graphite (B) and 30 nm Pt–CeO2 films deposited on graphite (C).
view shows that the islands are composed of fine, well dispersed nanowires at the catalyst film–graphite interface, which agglomerate with increasing film thickness giving pyramidal form of the islands. We showed in our previous works that the same structures were obtained on the CNTs coated by the Pt–CeO2 catalyst [16]. In [15] we speculated about influence of the grazing deposition angle on the film porosity. Comparison of the CNTs (from [15]) to the flat graphite substrate (in this work), where the Pt–CeO2 layers are prepared at normal deposition, leads to the conclusion that the carbon support is playing the main role in the growing mechanism. It is reasonable to suppose that the nanowire island formation originates from individual nucleus formed at an early stage of the growth. In order to study the dependence of morphology changes on the film thickness, a 30 nm thick Pt–CeO2 film was deposited on the graphite foil (Fig. 2c). The change in the surface morphology after increasing the deposited thickness from 10 to 30 nm is clear by comparing Fig. 2b and c. Fig. 2c inset illustrates the film morphology obtained by SEM of the film edge of the tilted sample and Fig. 2c shows again the columnar character of the catalyst deposition. The vertical structures are composed of bonded vertical nanorods forming together into a characteristic island structure. It can be seen that the heights of elemental nanorods are slightly dispersed and more than 30 nm in length. It should be noted that the thickness of porous films is always higher than the effective thickness of the deposits as determined from the deposition rate obtained for non porous layers. Chemical state and composition of the catalyst were investigated by HAXPES and XPS. The HAXPES technique is suitable for porous structure studies due to its large information depth of analysis. The kinetic energy of Ce 3d photoelectrons was about 5 keV corresponding to an inelastic mean free path (IMFP) in CeO2 about 7 nm [17]. Such a large inelastic mean free path (IMFP) allows us to take information from the depth more than 20 nm because about 95% of the photoelectron signal comes from the surface region of 3 IMFP thick. The high-resolution Ce 3d core level HAXPES spectrum of the reference 10 nm Pt–CeO2 films deposited on the Si wafer is presented in Fig. 3a (spectrum A). It consists of three 3d5/2 –3d3/2 spin–orbit-split doublets characteristic of Ce4+ states [18] and two 3d5/2 –3d3/2 spin–orbit-split doublets characteristic of Ce3+ states at 885.6 and 881.1 eV. The doublets represent different 4f configurations in the photoemission final state and arise from the Ce 4f hybridization in both the initial and the final states [19]. The Ce 3d5/2 4f2 peak at 882 eV is fitted by an asymmetric feature according to [20].
Table 1 Ce3+ /Ce4+ and Pt2+ /Pt4+ concentration ratios. HAXPES 2+
10 nm-Si 10 nm-graphite 20 nm-graphite
4+
XPS 3+
4+
Pt /Pt
Ce /Ce
Pt2+ /Pt4+
Ce3+ /Ce4+
0.47 7.86 2.72
0.05 0.15 0.11
0 1.12 0.16
0 0.11 0.09
Ce 3d spectra obtained from the film deposited on graphite foil are shown in Fig. 3a (B and C). The increase of Ce3+ state intensity corresponding to a partial reduction of cerium oxide can be seen. In the case of 30 nm films the relative concentration of Ce3+ states is less pronounced (spectrum C) than in the case of the 10 nm film. Ce 3d XPS spectra shown in Fig. 3b reveal the same tendency but with lower relative concentration of Ce3+ atoms. In the case of XPS, IMFP for Ce 3d electrons in CeO2 was 1.5 nm [17]. Using the areas of HAXPES and XPS peaks, the Ce3+ /Ce4+ ratios were calculated and are presented in Table 1. The Pt 4f HAXPES spectra obtained for the Pt-doped ceria film deposited on the Si (1 0 0) surface are plotted in Fig. 4a (spectrum A). The spectrum reveals Pt 4f7/2 –4f5/2 doublets at energies 72.5/75.8 eV and 74.2/77.5 eV, respectively. The first one can be associated with Pt2+ content while the second corresponds to Pt4+ [21]. The Pt–CeO2 film with thickness of 10 nm deposited on graphite (spectrum B) shows the same doublets but one can see a considerable increase of Pt2+ state concentration. The increase of film thickness causes relative increase of Pt4+ peak intensity, nevertheless ratio of Pt2+ /Pt4+ is still high (spectrum C). The Pt2+ /Pt4+ ratios of the sputtered films are also presented in Table 1. The Pt 4f XPS spectra obtained from the same Pt–CeO2 films are plotted in Fig. 4b. The spectra also exhibit only completely oxidized Pt2+ and Pt4+ states with same tendency of the concentration ratios Pt2+ /Pt4+ . Ratios obtained by XPS are, however, different from those of HAXPES (Table 1). In the case of the silicon substrate only Pt4+ species are presented while graphite substrate reveals both oxide states of platinum Pt4+ and Pt2+ . It can be seen that on graphite the Pt2+ peak intensity decreases with increasing film thickness (spectrum C in Fig. 4b). The observed influence of the substrate on cerium oxide stoichiometry as well as the relative occurrence of Pt2+ and Pt4+ cations is surprising because both graphite and SiO2 covered Si wafer are generally considered as inert supports. The HAXPES and XPS study of the samples reveals that nonporous film consists almost of Pt4+ and Ce4+ ions while Pt2+ and Ce3+ ions appear when the film
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Fig. 4. HAXPES Pt 4f (a) and XPS Pt 4f (b) spectra of 10 nm Pt–CeO2 films deposited on Si wafer (A), graphite (B) and 30 nm Pt–CeO2 films deposited on graphite (C).
porosity is increasing. Thus we can conclude that an increase of the Pt2+ state concentration is accompanied by an increase of the Ce3+ state concentration showing overall oxide reduction and this reduction is connected to the layer porosity. This effect can be tentatively explained by formation of two qualitatively different mixed oxides: reduced mixed oxide Ce3+ 2−x Pt2+ x O3−ı and stoichiometric mixed oxide Ce4+ 1−x Pt4+ x O2 . The PES results showing dependence of Pt2+ /Pt4+ and Ce3+ /Ce4+ ratios on the film thickness point out that the films are more reduced in deeper parts, i.e. closer to the interface. We propose that sputtered particles impinging the substrate are composed mainly of Ce, O and Pt atoms. In addition some CeOx species could be probably formed in the oxygen containing plasma above the target. Adsorbed Ce atoms are oxidized on the surface by very active atomic oxygen from gaseous or adsorbed phase. This mechanism can well explain the cerium oxide film growth on the Si substrate covered by the silicon oxide layer. On the other hand the mechanism of the sputtered cerium oxide film growth on graphite seems to be more complicated. Beside the film, morphology depends on choice of the substrate and on the film thickness, stoichiometry is another variable parameter. There are many different growth mechanisms of columnar structures reported in literature, e.g. [21–26]. In most of them the initial surface roughness develops in fractal geometry [27] which, due to atomic shadowing and limited surface diffusion, leads to extensive porosity. We propose that the carbon surface is etched by oxygen plasma generated by the magnetron. The oxygen atoms or ions interact with the carbon surface forming CO or CO2 molecules desorbing from the surface. To test this hypothesis the Pt–CeO2 film covering the carbon substrate was dissolved out in HCl and it was found that the bare graphite surface (where no CeO2 was seen by XPS) revealed very similar surface morphology to that of Pt–CeO2 coating structure. This means that the deposit is somehow copying the substrate structure (form of islands). We are therefore proposing an explanation of such porous growth based on initial formation of a Ce rich 3 dimensional nucleus separated by holes etched in the graphite substrate. Consequently, due to higher accessibility of 3D structures
impinging cerium, pillars of cerium oxides are grown while oxygen is still etching non covered graphite. The reduced character of porous layer can be explained by a general effect of formation of defects and oxygen vacancies at oxide edges and steps. Moreover, oxygen reaction with carbon is responsible of formation of oxygen deficient cerium oxide at the interface. Therefore the phase formed on the carbon surface contains reduced species Ce3+ 2−x Pt2+ x O3−ı . We suggest that during layer growth further particles adsorb on the partially reduced islands and are oxidized to stoichiometric mixed oxide Ce4+ 1−x Pt4+ x O2 . This mechanism supports the XPS and HAXPES results which show the presence of Ce3+ and Pt2+ in the interface region. 4. Conclusion The Pt–CeO2 catalyst thin films deposited at normal deposition angle on the graphite foil and Si wafer were investigated by means of photoelectron spectroscopy and scanning electron microscopy. The main conclusions of this investigation are: (1) the catalyst sputtered on the graphite shows highly porous surface morphology; (2) the porous films are partially reduced; (3) the Ce3+ /Ce4+ and Pt2+ /Pt4+ ratios show that the films are more reduced in deeper parts, i.e. closer to the interface. Acknowledgments The authors would like to thank Dr. Robert George Acres for his careful reading of the paper and his helpful comments. The present work was supported by the projects P204/10/1169 and 202/09/H041 financed by the Czech Grant Agency. References [1] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935. [2] A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002. [3] W. Liu, M. Flytzani-Stephanopoulos, Journal of Catalysis 153 (1995) 317.
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