Oxidation of Cobalt by Oxygen Bombardment at Room Temperature

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Oxidation of Cobalt by Oxygen Bombardment at Room Temperature Iva Saric, Robert Peter, and Mladen Petravic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07139 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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The Journal of Physical Chemistry

Oxidation of Cobalt by Oxygen Bombardment at Room Temperature

Iva Saric*, Robert Peter, and Mladen Petravic* Department of Physics and Center for Micro- and Nanosciences and Technologies, University of Rijeka, R. Matejcic 2, 51000 Rijeka, Croatia

Telephone: +385-51-584622; Fax: +385-51-584649; E-mail: [email protected] Telephone: +385-51-584628; Fax: +385-51-584649; E-mail: [email protected]

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Abstract

Most experiments on thermal oxidation of cobalt have shown the parabolic growth rate consistent with the diffusion of cations through neutral cation vacancies. Here, we present experimental results on the oxidation of Co metal by low-energy oxygen bombardment at room temperature, which scales with the dose of implanted oxygen, Φ, as Φ1/6. This type of oxide growth, predicted theoretically for diffusion of cobalt cations by doubly charged cation vacancies, has not been observed previously in thermal oxidation of Co.

Our results

demonstrate that oxidation of Co, which involves formation of both monoxide, CoO, and spinel, Co3O4, oxide structures, can be indeed driven by doubly charged vacancies, as predicted theoretically, when oxidation conditions enhance both the production of point defects and mobilities of cobalt cations and oxygen anions within cobalt oxide.


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Introduction Cobalt oxides have attracted large interest due to their diverse chemical and physical properties and the variety of possible applications, ranging from fuel cells, energy storage in lithium ion batteries or ferromagnetic materials to water splitting, catalysis or gas sensing.

1-7

The mechanism of the high-temperature formation of cobalt oxides (above 500 K) is well understood and supported by experiments.8 CoO is a cation-deficient p-type semiconductor for which the oxidation process occurs by the outward diffusion of cobalt cations.9 Co2+ cations diffuse through lattice defect sites in oxide, such as cation vacancies, to the surface where they react with adsorbed oxygen. The oxide growth follows a parabolic rate law according to Wagner’s oxidation theory,10,8 with the parabolic rate constant directly related to the diffusion coefficients of cations and anions. In the p-type oxides the concentration of vacancies depends critically on the partial pressure of oxygen, pO2. Therefore, the oxide growth rate also depends on pO2, leading to more rapid oxidation at larger oxygen pressures. The cation vacancies in cobalt oxide can hold a double or a single charge.8 The concentration of doubly charged vacancies, predominantly formed at low partial pressures, is found theoretically to scale with pO21/6, while at higher oxygen pressures the concentration of singly charged vacancies scales with pO21/4.11 However, no experiments have shown strictly this type of scaling. For example, the growth rate of CoO at low pO2 was found to scale with pO21/2.5,12 in contrast to the formation of either singly or doubly charged cation vacancies. To account for this discrepancy, the formation of neutral vacancies has been considered9 as they give rise to the parabolic growth rate that scales with pO21/2. On the other hand, only a few studies have been dedicated to the initial oxidation of pure Co metal to monoxide, CoO, or spinel, Co3O4, oxide structures at low temperatures



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(ranging from 100 K to 300 K).13-15 All Co samples, thermally oxidized at room temperature (RT) in oxygen environment, have shown the formation of CoO films followed by the oxidation of CoO to Co3O4 at higher oxygen pressures.13,14 However, no quantification of growth rates at or below RT is available in the literature. In the present study, we have employed x-ray photoemission spectroscopy (XPS) in order to characterize the initial stages of Co oxidation by low-energy oxygen bombardment at RT. In contrast to thermal processes, the ion bombardment, which places the growing oxide layer away from the thermodynamic equilibrium, is known to enhance the production of point defects, such as vacancies and interstitials,16 that determine cation and anion diffusion. In addition, one can finely control the composition and thickness of oxides on metallic surfaces, even at low implantation temperatures, simply by tuning the implantation parameters.17-19

Experimental Section The 0.1 mm-thick cobalt foil (Alfa Aesar, 99.95 wt.% Co) was abraded with the SiC paper of 2500 grit and ultrasonically cleaned with ethanol and redistilled water. Oxidation was performed in situ, within the analytical ultrahigh vacuum (UHV) chamber at RT (25 oC, as measured by the thermocouple within the UHV chamber). Before any oxidation step, the surface was cleaned in situ by several Ar+ bombardment/annealing cycles. The XPS survey scans on the cleaned samples have shown no carbon signal, while a small oxygen signal, corresponding to about 4 at. % of oxygen, was detected. Samples were oxidized by a broad beam of 500 eV O2+ (every 500 eV O2+ ion, after collision with the surface, breaks into two 250 eV oxygen atoms) with a typical current density of 2 µA/cm2. The implantation dose (Φ, in O atoms/cm2), is related to the bombardment time (t, in seconds), as 1.25x1013 x t. For the bombardment times used in our


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experiments (2-7200 s), the corresponding implanted dose covers more than three orders of magnitude, from 2.5x1013 to 9x1016 O atoms/cm2. All samples were characterized by a SPECS XPS spectrometer equipped with the Phoibos MCD 100 hemispherical electron analyser and a monochromatized source of Al Kα X-rays of 1486.74 eV. The typical pressure in the UHV chamber during analysis was in the 10−7 Pa range. For the electron pass energy of 10 eV used in the present study, the overall energy resolution was better than 0.8 eV. Several implantation experiments were repeated in order to determine the reproducibility of XPS results (found to be better than 5%). The photoemission spectra were simulated with several sets of mixed Gaussian-Lorentzian functions (Voight profiles) with Shirley background subtraction, using Unifit software20 and taking into account peak asymmetries and multiplet splitting effects.21

Results and Discussion The formation of different Co-O bonds on oxidized surfaces can be extracted from chemical shifts in photoemission spectra around Co 2p core levels. In Figure 1 we show several characteristic Co 2p spectra taken from a cleaned Co surface and surfaces bombarded with 500 eV O2+ ions at RT for different times. The Co 2p emission from the clean surface is characterized by two peaks, showing the large spin-orbit splitting of 2p3/2 and 2p1/2 levels, at binding energies (BE) of 778.5 eV and 793.5 eV, respectively (peaks 1 and 1' of spectrum a in Figure1), representing emission from metallic Co, Co(0). After O2+ bombardment, Co 2p emission shows typical fingerprints of Co-O bonds, as evident from spectra b-e in Figure 1. Peaks around BE of 779.6 eV and 780.4 eV (peaks 3 and 2, respectively, around 2p3/2 level, or the broad peak 2' around 2p1/2 level in Figure 1) are assigned to the emission from Co oxides, while broad features P2 and P3 (or a broad peak P4) represent corresponding satellite peaks.21



The oxide region of the Co 2p photoemission is dominated by peak 2 for short bombardment times, while for the longer bombardment times peak 3 becomes dominant. The prolonged bombardment affects the other peaks of Figure 1 as well: the intensity of Co(0) decreases dramatically, the intensity of satellite P2 decreases, the intensity of satellite P3

Normalized Intensity (arb.units)

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Co 2p

2p

3/2

2p

1/2

a b c d e P4 810

805

800

2' 1'

P3

795

790

P2

2

785

31

780

775

Binding Energy (eV) Figure 1. Co 2p core-level photoemission spectra from a) a clean Co surface and surfaces exposed to 500 eV O2+ ions at RT for b) 10 s, c) 120 s, d) 300 s and e) 1200 s.

increases, while the position of satellite P4 shifts towards higher binding energies. Spectra similar to our spectra b-e in Figure 1 have been observed previously in photoemission from CoO (with characteristic emission at BE corresponding to peaks 2 and P2 in Figure 1)21 and Co3O4 (with characteristic peaks at BE corresponding to our peaks 3, P3 and P4).21,22 We supplement our XPS core-level measurements with the valence band photoemission measurements. In Figure 2 we compare the valence band spectra taken from


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the clean Co surface and a surface bombarded with 500 eV O2+ ions for 60 minutes (spectrum a in Figure 2). The valence band from the clean surface is characterized by a sharp peak 1 at the top of the valence band at about 1.0 eV, which has been assigned previously to the emission from cobalt 3d orbitals in Co metal.13,14 The valence band emission from the oxidized

RT


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2 3 4

a

1

a clean 20

15

10

5

0

Binding Energy (eV) Figure 2. Comparison of valence band photoemission spectra from a clean Co surface and the surface bombarded by 500 eV O2+ ions at RT for 3600 s (spectrum a).

surface is characterized by a sharp peak 2 at the top of the valence band at around 1.5 eV and a broad, multi-peak structure 3 between 4.5 eV and 8.5 eV, in full agreement with the valence-band photoemission data from the literature.13,14 The broad structure 4 around 10.5 eV, characteristic for the emission from CoO,14 indicates the presence of CoO in the ion bombarded surface, in full agreement with our XPS measurements from Figure 1.



Our photoemission and valence band measurements strongly support a two step oxidation process of pure Co metal under oxygen-ion bombardment: the dominant formation of CoO for the low oxygen doses (with strong photoemission from Co(II) oxidation states) and the preferential formation of a spinel structure for the higher oxygen doses, with the characteristic emission from Co(III) and Co(II) oxidation states (see Figure 1). To quantify the oxidation sequence Co + (O2+ bombardment) CoO + Co3O4, a concentration fraction of Co, CoO and Co3O4 should be determined from the clean and oxidized Co samples. In general, the relative amount of various chemical states can be determined from a standard fitting of XPS emission curves with several sets of mixed Gaussian-Lorentzian functions with Shirley background subtraction.20 However, the fitting of the 2p photoemission spectra of


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Co 2p3/2 + 2

500 eV O

3 2 1

P3

6 5

P2

4

1

clean 792

788

784

780

776

Binding Energy (eV) Figure 3. Fitting (solid lines, representing convolution of Gaussians and Lorentzians) of Co 2p3/2 photoemission peaks, obtained from a clean Co surface and the surface bombarded by 500 eV O2+ ions at RT for 3600 s (closed circles).


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transition metals is quite complex, due to the peak asymmetries, complex multiplet splitting, shake-up and plasmon loss structures and overlapping of peaks with similar binding energies.21,22 We have followed the fitting procedure that takes into account all the above additional features in the photoemission of transition metals.21 For the fitting purposes, we have considered only the more intense Co 2p3/2 component, as the large Co 2p spin-orbit splitting of around 15 eV prevents any mixing of contributions from Co 2p3/2 and Co 2p1/2 components. All results can be found in the Supporting Information (Figures S1-S16), while in Figure 3 we show only one example. Here, the spectrum from a clean surface is fitted with an asymmetric peak at BE of 778.5 eV (curve 1), representing the emission from metallic Co, Co(0), and two plasmon loss peaks at 3.0 eV and 5.0 eV above the main peak, representing contributions from the surface and bulk plasmons, respectively, in full agreement with the fitting of metallic Co 2p peaks from the literature.21 However, a good fit was obtained only by introducing two additional small peaks at BE of 779.6 eV and 780.4 eV, respectively. We have found this contribution in all cleaned samples. It could not be removed by any further sputtering/annealing cleaning cycles. We argue that this additional contribution originates from the intrinsic oxygen, present within the Co metal foil in the form of CoO and Co3O4, that could not be removed by any standard cleaning procedure. A good fit of the more complex photoemission structure from the ion bombarded surface in Figure 3 (bombardment for 3600 s, corresponding to 4.5x1016 O atoms/cm2) requires at least eight fitting components: the contribution from Co(0), at 778.5 eV (peak 1), two contributions from the Co2+ emission from CoO at 780.4 and 782.3 eV with an associate satellite at 786.5 eV (peaks 2, 6 and P2, respectively) and three contributions from the Co3O4 emission at 779.6, 780.7 and 782.5eV with an associate satellite at 789.9 eV (peaks 3, 4, 5 and P3, respectively). The relative concentration fraction of each chemical state of Co was then



determined from the area of corresponding fitting curves, normalized to the total area of the Co 2p3/2 peak. The results are listed in Table S1, provided in the Supporting Information, and plotted in Figure 4 on a log(t) vs. concentration percentage scale.8 A Co(0) peak at BE of 778.5 eV is visible in all ion-bombarded samples. It indicates the formation of a very thin oxide film on the surface, thus XPS signal includes a contribution from the underlying metallic cobalt.

10

Concentration fraction (%)

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2

I

II

III

CoO slope 1/2

10

1/4

1

1/6

Co3O4 -1/6 0

1

2

3

4

Log t (s) Figure 4. Concentration fractions of CoO and Co3O4 as function of bombardment time on a log scale. Symbols represent experimental results.

We note in Table S1 that the relative fraction of oxides in clean sample is about 11% (4.2% of CoO and 6.8% of Co3O4). A dramatic increase in the concentration fraction of CoO takes place immediately after surface exposure to oxygen ion beam under the UHV conditions (the shortest reliable ion-bombardment time in our UHV chamber is about 2 seconds), while the increase of the spinel component is moderate. We also distinguish three different time periods, I, II and III, in Figure 4. During the period I (2-100 s, corresponding to 2.5x1013 10 Environment ACS Paragon Plus

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1.25x1015 O atoms/cm2) both oxide structures grow simultaneously with the same rate. Further bombardment during a short period II (100-500 s or 1.25x1015 - 6.25x1015 O atoms/cm2) shows a rapid increase of spinel formation with a corresponding decrease of monoxide component. During the final period III, the spinel growth rate returns back to the same rate as in I, while the CoO component shows the reciprocal decrease. In Figure 4 we also show several curves with slopes of 1/2, 1/4 and 1/6, characteristic of the parabolic rate law determined by neutral, singly charged and doubly charged cation vacancies, respectively. The slope of Co3O4 curve during periods I and III is very close to 1/6, while the slope of CoO curve changes from 1/6 within the period I to -1/6 within III. A similar three period sequence has been observed previously for thermally oxidized Co at 700-800 o

C.12 However, in reference 12 the growth rate of CoO and Co3O4 follows the parabolic rate

laws as pO21/2 during the initial stage of oxidation I, then changes slightly during a short period II as the oxygen potential gradient within the CoO layer levels out, while during the final period III, Co3O4 layer grows at the expense of CoO, again following the pO21/2 rate. In contrast to thermal processes, the ion-induced oxidation eliminates the need for elevated temperatures and bypasses several thermally activated processes, such as absorption, dissociation, nucleation, diffusion or bond braking.23 However, the ion-induced oxidation is a complex process that involves both oxygen implantation, beam-induced mixing, swelling of lattice, diffusion and chemical reactions, together with desorption and sputtering.16,24 The implantation kinetics determines the thickness of oxide films, while atomic collisions lead to decomposition of O2+ into two atoms and creation of vacancy/interstitial pairs. The presence of these defects is known to enhance mobility and reactivity of oxygen anions and metal cations in different materials. For example, the oxidation of Si by energetic oxygen ions is driven by the radiation-enhanced diffusion of oxygen in SiO2.24 On the other hand, the irradiation of Ni with energetic electrons or ions accelerates the formation of nucleation sites



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for oxide formation and increases diffusivity through the radiation-enhanced diffusion of Ni in NiO.9,19 To explain our experimental results, we first note that the oxidation of Co during the bombardment period I (see Figure 4) starts below the surface, at the penetration depth of impinging oxygen ions. One can estimate the oxide thickness from SRIM simulations.25 For 500 eV O2+ bombardment, the ion range (Rp) in Co, CoO or Co3O4 changes only slightly from 0.6-0.8 nm with the range straggling (∆Rp) of 0.7-0.9 nm, giving the total oxide thickness of 1.3-1.7 nm. The oxide thickness is limited by the sputtering of Co surface, the swelling of lattice and the enhanced diffusion of oxygen. We argue that oxygen in excess of the amount consumed in the formation of a shallow buried cobalt oxide layer around Rp rapidly diffuses toward the surface (oxygen is highly mobile in CoO during the bombardment) where it oxidizes the available Co to form a continuous CoO layer to the surface. Such oxygen does not diffuse into the underlying Co layer as diffusion is enhanced only over the depth Rp+∆Rp, where impinging ions deposit energy and create vacancy/interstitial defects.26 At the same time, the outward, radiation-enhanced diffusion of metal cations leads to further reaction of Co with impinging oxygen. When a Co2+ vacancy is created, two neighbouring Co2+ atoms, in order to balance charge, may each lose an electron forming two Co3+ ions.27 Therefore, in addition to the Co (II) oxidation states of CoO, some Co (III) oxidation states of Co3O4 may also be present in CoO. As the concentration fraction of both oxides scales with (time)1/6 (i.e. Φ1/6), the oxidation during the period I is driven by the creation of doubly charged cation vacancies within oxides, enhanced by the ion irradiation, as expected theoretically for the low oxygen partial pressures achieved under our low energy oxygen bombardment. A continuous oxide layer to the surface is formed at the end of the bombardment period I, with all available Co-O bonds saturated. The further oxidation during periods II and III proceeds through the reaction CoO + (O2+ bombardment) Co3O4, and the Co3O4 layer grows at the expense of


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CoO. As the concentration fraction of Co3O4 during the period III scales again with (time)1/6 (i.e. Φ1/6), the oxidation here is also driven by the creation of doubly charged cation vacancies.

Conclusion In summary, we present the XPS analysis of the oxidation process on the pure Co surface by low-energy oxygen bombardment at RT. The two-step oxidation process starts below the surface, at the penetration depth of impinging oxygen ions with the simultaneous formation of CoO and Co3O4. When a continuous oxide layer to the surface is formed, the further oxidation proceeds by the formation of the Co3O4 layer at the expense of CoO. The both oxidation steps are characterized by the same parabolic growth rate, which scales with the amount of implanted oxygen as Φ1/6, in contrast to the thermal oxidation processes, but in full agreement with the theoretical prediction for the oxidation driven by the diffusion of doubly charged cation vacancies.

Acknowledgment This work was supported by the European Fund for Regional Development and the Ministry of Science, Education and Sports of the Republic of Croatia under the project Research Infrastructure for Campus-based Laboratories at the University of Rijeka (grant number RC.2.2.06-0001); and the European Social Fund for Human Resources Development under the project SIZIF (grant number HR.3.2.01-0310).

Supporting Information Available All experimental XPS spectra (Figures S1-S16) fitted with mixed Gaussian-Lorentzian functions and Table S1.



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References

(1) Berger, A.; Pechan, M. J.; Compton, R.; Jiang, J. S.; Pearson, J. E.; Bader, S. D. Disorder-Tuning of Hysteresis-Loop Properties in Co/CoO-Film Structures. Physica B 2001, 306, 235-239. (2) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized Transition-Metaloxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496-499. (3) Wöllenstein, J.; Burgmair, M.; Plescher, G.; Sulima, T.; Hildenbrand, J.; Böttner, H.; Eisele, I. Cobalt Oxide Based Gas Sensors on Silicon Substrate for Operation at Low Temperatures. Sensor. Actuat. B-Chem. 2003, 93, 442-448. (4) Fontaíña-Troitiño, N.; Liébana-Viñas, S.; Rodríguez-González, B.; Li, Z.-A.; Spasova, M.; Farle, M.; Salgueiriño, V. Room-Temperature Ferromagnetism in Antiferromagnetic Cobalt Oxide Nanooctahedra. Nano Lett. 2014, 14, 640-647. (5) Li, Z.-A.; Fontaíña-Troitiño, N.; Kovács, A.; Liébana-Viñas, S.; Spasova, M.; DuninBorkowski, R. E.; Müller, M.; Doennig, D.; Pentcheva, R.; Farle, M.; Salgueiriño, V. Electrostatic Doping as a Source for Robust Ferromagnetism at the Interface Between Antiferromagnetic Cobalt Oxides. Sci. Rep. 2015, 5, 7997. (6) Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Partially Oxidized Atomic Cobalt Layers for Carbon Dioxide Electroreduction to Liquid Fuel. Nature 2016, 529, 68-71. (7) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. (8) Birks, N.; Meier, G. H.; Pettit, F. S. Introduction to the High-Temperature Oxidation of Metals; Cambridge University Press: Cambridge, U.K., 2006. (9) Koel, G. J.; Gellings, P. J. Contribution of Different Types of Point-Defects to Diffusion in CoO and NiO during Oxidation of Metals. Oxid. Met. 1972, 5, 185-203. (10) Wagner, C. Beitrag zur Theorie des Anlaufvorgangs Z. Phys. Chem. B 1933, 21, 2541. (11) Lakshmi, N.; Martin, M. Texture Evolution in Oxides During High Temperature Oxidation of Cobalt Studied by in situ X-ray Diffraction. Electrochem. Soc. Proc. 2003, 2003, 210-220. (12) Martin, M.; Koops, U.; Lakshmi, N. Reactivity of Solids Studied by in situ XAS and XRD. Solid State Ionics 2004, 172, 357-363. (13) Castro, G. R.; Küppers, J. Interaction of Oxygen with hcp (0001) Recrystallized Cobalt Surfaces. Surf. Sci. 1982, 123, 456-470. (14) Clemens, W.; Vescovo, E.; Kachel, T.; Carbone, C.; Eberhardt, W. Spin-Resolved Photoemission-Study of the Reaction of O2 with fcc Co(100). Phys. Rev. B 1992, 46, 4198-4204. (15) Zimmermann, J.; Ciacchi, L. C. Mechanisms of Initial Oxidation of the Co(0001) and Cr(110) Surfaces. J. Phys. Chem. C 2010, 114, 6614-6623. (16) Williams, J. S.; Poate, J. M. Ion Implantation and Beam Processing; Academic Press: Sydney, 1984. (17) López-Carreño, L. D.; Benitez, G.; Viscido, L.; Heras, J. M.; Yubero, F.; Espinos, J. P.; Gonzalez-Elipe, A. R. Oxidation of Molybdenum Surfaces by Reactive Oxygen Plasma and O2+ Bombardment: an Auger and XPS Study. Surf. Interface Anal. 1998, 26, 235-241.


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Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Alov, N. V. Surface Oxidation of Metals by Oxygen Ion Bombardment. Nucl. Instrum. Meth. B 2007, 256, 337-340. (19) Saric, I.; Peter, R.; Kavre, I.; Jelovica Badovinac, I.; Petravic, M. Oxidation of Nickel Surfaces by Low Energy Ion Bombardment. Nucl. Instrum. Meth. B 2016, 371, 286289. (20) Hesse, R.; Chassé, T.; Szargan, R. Peak Shape Analysis of Core Level Photoelectron Spectra Using UNIFIT for WINDOWS. Fresen. J. Anal. Chem. 1999, 365, 48-54. (21) Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717-2730. (22) Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. J. Phys. Chem. C 2010, 114, 111-119. (23) Picone, A.; Riva, M.; Brambilla, A.; Giannotti, D.; Ivashko, O.; Bussetti, G.; Finazzi, M.; Ciccacci, F.; Duò, L. Atomic Scale Insights into the Early Stages of Metal Oxidation: A Scanning Tunneling Microscopy and Spectroscopy Study of Cobalt Oxidation. J. Phys. Chem. C 2016, 120, 5233-5241. (24) Williams, J. S.; Petravic, M.; Svensson, B. G.; Conway, M. Oxidation of Silicon by Low-Energy Oxygen Bombardment. J. Appl. Phys. 1994, 76, 1840-1846. (25) Zigler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range of Ions in Solids; Pergamon: New York, 1986. (26) Todorov, S. S.; Fossum, E. R. Growth-Mechanism of Thin Oxide-Films under LowEnergy Oxygen-Ion Bombardment. J. Vac. Sci. Technol. B 1988, 6, 466-469. (27) Kim, D. S.; Lee, H. C. Nickel Vacancy Behavior in the Electrical Conductance of Nonstoichiometric Nickel Oxide Film. J. Appl. Phys. 2012, 112, 034504.



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Co 2p core-level photoemission spectra from a) a clean Co surface and surfaces exposed to 500 eV O2+ ions at RT for b) 10 s, c) 120 s, d) 300 s and e) 1200 s. 190x190mm (300 x 300 DPI)

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Comparison of valence band photoemission spectra from a clean Co surface and the surface bombarded by 500 eV O2+ ions at RT for 3600 s (spectrum a). 190x190mm (300 x 300 DPI)


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Fitting (solid lines, representing convolution of Gaussians and Lorentzians) of Co 2p3/2 photoemission peaks, obtained from a clean Co surface and the surface bombarded by 500 eV O2+ ions at RT for 3600 s (closed circles). 190x190mm (300 x 300 DPI)



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Concentration fractions of CoO and Co3O4 as function of bombardment time on a log scale. Symbols represent experimental results. 190x190mm (300 x 300 DPI)


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Oxidation of Cobalt by Oxygen Bombardment at Room Temperature

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