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NiO(0 0 1) interface studied by X-ray photoelectron spectroscopy and molecular dynamics simulations
Applied Surface Science 217 (2003) 239–249
Ni/NiO(0 0 1) interface studied by X-ray photoelectron spectroscopy and molecular dynamics simulations E. Symianakisa, S. Ladasb, G.A. Evangelakisa,* a
b
Department of Physics, University of Ioannina, P.O. Box 1186 Physics, Ioannina 45110, Greece Department of Chemical Engineering, University of Patras and ICE/HT-FORTH, 26500 Rion, Patras, Greece Received 7 February 2003; received in revised form 24 March 2003; accepted 24 March 2003
Abstract X-ray photoelectron spectroscopy (XPS) results referring to the interface formed upon vapor deposition of Ni atoms on NiO(0 0 1) indicate, that upon annealing the deposited Ni is oxidized forming a NiO overlayer. The oxidation process is activated and becomes very fast above 900 K, resulting in a surface that exhibits identical photoelectron spectrum with the clean substrate. In addition, it was found that during the oxidation process two extra components in the O 1s XPS peak spectra appear, with binding energies above and below the main lattice oxygen peak, manifesting the existence of oxygen atoms carrying charges that deviate from their formal ionic value. Moreover, by molecular dynamics simulations it was found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming a series of NixOy oxides, most of which eventually coalesce into small NiO islands. Consequently, the lower binding energy component of the O 1s XPS peak may be attributed to intermediate oxides with x > y, while the higher binding energy component to intermediate oxides with x < y. # 2003 Elsevier Science B.V. All rights reserved. PACS: 82.80.Pv; 68.35.Fx; 71.15.Pd; 79.60.Jv Keywords: Ionic value; Gas sensors; Transition metal
1. Introduction Metal particles and adlayers on oxide surfaces are related to many technologically important applications like metal-oxide contacts, microelectronic and photovoltaic devices, coatings for corrosion passivation, gas sensors and oxide supported transition metal catalysts. Focusing on the deposition of Ni atoms on oxide surfaces, it is found that the reactivity of the *
Corresponding author. Tel.: þ30-26510-98590; fax: þ30-26510-98683. E-mail address: [email protected] (G.A. Evangelakis).
substrate plays a key role for the evolution of the deposited Ni atoms. Specifically, in the TiO2(1 1 0) surface there is formation of crystalline islands [1], while in the case of Yttria-stabilized ZrO2 threedimensional Ni crystallites are created, accompanied by charge transfer between Ni and the O2 anions of the surface [2]. In addition, it is reported that thin metallic Ni film above a NiO substrate can be effectively created by reducing the NiO surface either using C [3] or H [4]. It is found that for reduction up to 80% of the lattice oxygen, the adlayer that is formed adopts the structural character of the NiO(1 0 0) surface, while for smaller oxygen concentrations formation
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00553-1
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of Ni(1 0 0) islands is observed [4]. Interestingly, high temperature annealing results in a surface that appears to have fully restored its original composition and structure with oxygen supplied from the bulk [4]. Concerning Ni deposition on the NiO(0 0 1) sur˚ , a Ni face, it is reported that for a coverage up to 145 A thin film on the NiO(1 0 0) substrate can be epitaxially grown along with Ni(1 0 0) islands [5]. However, systematic studies concerning the behavior of small metallic particles and islands formed over the surface of their own oxides at very low coverage are rather scarce. From ab initio calculations, it is predicted that when Ni atoms reach the surface, charge transfer occurs that ionises them, effectively turning them into adcations [6]. In addition, recent molecular dynamics simulations have studied the diffusion properties of both adanion and adcation particles on the NiO(1 0 0) surface [7–11]. A combination of experimental studies with molecular dynamics simulations could broaden our understanding of phenomena like growth, oxidation, sintering and surface roughening. The aim of the present work is the experimental study of the interaction processes taking place in the case of near-monolayer deposition of Ni atoms on the NiO(1 0 0) surface, in conjunction with molecular dynamic simulations that intend to help in understanding the underlying mechanisms at microscopic scale.
2. Experimental setup Nickel atom deposition was carried out in an ultra high vacuum chamber (base pressure 5 1010 mbar), using a thermal evaporation source activated by electron bombardment. The chemical state of the deposit was monitored by X-ray photoelectron spectroscopy (XPS) using un-monochromatized Al Ka radiation (1486.6 eV) and a hemispherical electron energy analyzer with a pass energy of 100 eV. Prior to Ni deposition the NiO(1 0 0) surface was cleaned from carbon contamination, adopting methods for similar systems from the literature [4,12–14]. The cleaning procedure involved cycles of exposure to 3000 L of oxygen and annealing at 920 K. The efficiency of this method was established from both the XPS characteristic spectra of carbon-free NiO and the (1 1) LEED patterns of the (1 0 0) surface.
Nickel depositions where carried out at 480 K (0.21 Tm, Tm being the melting temperature), followed by repeated annealing at various temperatures between 500 and 920 K for a certain period of time. After each annealing step, the crystal was allowed to cool down to 480 K, temperature at which it remained for about 1 h with the X-ray gun on, so that the electrostatic charging of the NiO crystal was stabilized. When this condition was met, the XPS spectra of the Ni 2p and O 1s regions were obtained aiming in the investigation of NixOy-type structures, formed via possible charge transfer that could ionize the Ni particles. As binding energy reference, we used the value of 530.0 eV for the O 1s peak corresponding to NiO lattice oxygen. The amount, in monolayers (ML), of atomic nickel (Ni0) present at any time on the surface following deposition was estimated from the XPS 2p3/2 peak area ratio, R ¼ AðNi0 Þ/A(NiNiO) (with the Ni0 2p3/2 peak appearing at a binding energy around 853 eV and the NiNiO 2p3/2 peak around 855 eV). Assuming a layer-like deposition, the equivalent deposited Ni film thickness in nm was obtained, as dequiv ¼ l ln½1 þ 0:5R, where the inelastic mean-freepath l for Ni 2p photoelectrons in Ni was taken as 0.1 nm and the factor 0.5 in the brackets corresponds to an experimentally obtained ratio of the Ni 2p3/2 peak areas for two reference samples, a clean NiO and a very thick Ni0 film [15]. Taking 0.22 nm as the metallic Ni monolayer thickness, which corresponds to 2 1015 Ni atoms/cm2, the amount of Ni0 in ML, dequiv/0.22, was obtained. The above calculation underestimates the Ni coverage to the extent that three-dimensional clusters are formed, but this effect is small for nearmonolayer amounts of deposited nickel.
3. Computational details The simulations were carried out in the constant temperature canonical ensemble using the Nose scheme. The simulation cell we used consisted of a slab containing 1728 ions arranged in 12 planes with 72 cations and 72 anions in each plane. In order to have free surfaces in our simulations, we left an empty space of four times the length of the slab in the [0 0 1] direction while applying periodic boundary conditions in the three directions. This resulted in an infinite slab with two free surfaces normal to the z-direction.
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The equations of motion were integrated by means of Verlet’s algorithm and a time step of 1015 s. For the atomic interactions, we adopted a rigid ion potential developed for NiO [16] that gave good results concerning the diffusive properties of the material [17,18], while the Coulombic contributions were evaluated with the use of the Ewald summation. These technical details have been reported elsewhere [18]. The system with the clean surfaces was initially equilibrated for a short period of time, before proceeding to the deposition of Ni ions at adcation positions. In order to maintain the charge neutrality of the system, these ions were extracted from the one side of the slab and they were put on its other surface. It should be noted at this point that in our simulations, we consider Ni adatoms carrying their formal charges (þ2). This is of course a simplification since Ni adatoms can have other charges and can be associated with charged vacancies or more complex defects. However, this approximation seams to be reasonable when dealing with surface properties and it can be adopted for the study of deposition of Ni on the NiO surface [6,7].
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We studied three cases corresponding to coverage of 0.03, 0.06 and 0.11 ML by depositing simultaneously 4, 8 and 16 Ni adcations. This is a usual approximation that is due to the inherent time limitations of the method that does not give access to the experimental time scale that would correspond to a realistic deposition rate. However, for the cases of submonolayer coverage we are interested in, this simplification is not expected to affect seriously our results. The systems were simulated at three different temperatures, 0.37, 0.4 and 0.57 Tm, at which we followed their evolution for 40,000 time steps.
4. Results and discussion 4.1. Experimental results After a near-monolayer Ni deposition at 480 K, the XPS spectra exhibited the presence of an almost metallic Ni film (Ni 2p3/2 binding energies around 853 eV) on the NiO surface (Ni 2p3/2 binding energies around 855 eV) that remained stable for several hours
Fig. 1. (a) Ni 2p3/2 region of the clean NiO(1 0 0) surface, (b) Ni 2p3/2 region after Ni deposition on the clean NiO(1 0 0) surface.
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at 480 K. This observation shows that at least upon deposition of more than 0.5 ML of atomic Ni on NiO at 480 K, the deposited atoms are not immediately oxidized, but they can assemble into stable elemental Ni aggregates. Unfortunately, it was not possible to study deposits with much less initial Ni coverage because the additional Ni signal in XPS was too small to be accurately deconvoluted from the large substrate background. By employing a standard XPS peak fitting program [19], the region of the Ni 2p3/2 peak was fitted with two asymmetric Gaussian/Lorentzian components for the clean NiO surface (Fig. 1a). In the case of the Ni deposit, the previous components were used to fit the NiO substrate contribution, whereas two additional components, with characteristics obtained from reference thin Ni films, were employed to fit the deposited Ni0 (Fig. 1b). The O 1s peak of the clean substrate was fitted using two s-type peaks, the main one of which corresponds to the NiO lattice oxygen, whereas the small component at higher binding energy is most probably due to surface hydroxyl groups (Fig. 2). Following the Ni deposition at 480 K, the O 1s peak shape did not change and could be fitted in the same manner as in Fig. 2.
In order to investigate the interaction between the Ni deposit and the substrate upon annealing, the sample was heated for 13 min periods at increasing temperatures from 520 K up to a maximum of 920 K. After each heating step, XPS measurements at 480 K were employed to obtain the residual amount of Ni0 in the deposit as a function of annealing temperature. The obtained results at two different deposits exhibiting initial amounts of Ni0 (at 520 K) of about 1.4 and 0.5 ML, respectively, are shown in Fig. 3. The main observation is that, upon heating, the amount of Ni0 gradually decreases and after the final heating step at 920 K the Ni deposit is completely oxidized and indistinguishable from the NiO substrate. The shape of the curves in Fig. 3 shows that the oxidation process is clearly activated, as the rate increases drastically with increasing temperature. Furthermore, the fact that the progress of the oxidation is observable even at low temperature for the sub-monolayer amount of deposited Ni, whereas it exhibits an initial delay for the larger amount, followed by a more steep behavior at high temperatures, suggests that the oxidation process takes place through the interface and not via the gas phase.
Fig. 2. The O 1s region of the clean NiO(1 0 0) surface.
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Fig. 3. Variation of the residual Ni0 upon 13 min annealing at increasing temperatures for two different amounts of deposited Ni: more than one monolayer (*) and less than one monolayer (&).
In order to follow the oxidation process as a function of time at constant temperature, a separate experiment was carried out in which the temperature of a new deposit was raised to 820 K and kept constant for increasing time intervals, with intermediate XPS measurements at 480 K. The amount of residual Ni0 in ML as a function of the time square root is plotted in Fig. 4. A good straight line fits the data suggesting that a diffusive process supplying the necessary oxygen
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from the interior of the NiO crystal governs the oxidation. The slope of the straight line in Fig. 4 is proportional to D1/2, where D corresponds to the interlayer diffusion coefficient, if a simple model is adopted for the diffusion process, whereby oxygen ions are supplied by diffusion from a bulk source of constant concentration across an interface barrier [20]. In order to obtain an estimate of the activation energy for diffusion, an Arrhenius diagram is constructed (Fig. 5), in which the logarithm of the slope of straight lines as that in Fig. 4 is plotted as a function of 1/T. The experimental points of Fig. 5 are obtained from the slope of the straight line in Fig. 4 and from the data of Fig. 3 above 800 K, where the calculations are more accurate due to the larger slope of the curves. For the latter points, the successive incremental decrease of the Ni0 coverage at each temperature along the drawn curves in Fig. 3 is divided by (13 min)1/2 to obtain slopes analogous to that of the line in Fig. 4. Despite the relatively large scatter of the data, a straight line fits the experimental points, the slope of which is given by E/2k, where k is the Boltzmann’s constant. The activation energy of 1:1 0:3 eV is deduced from Fig. 5 that is in the range of values expected for bulk oxygen ion diffusion in oxides, especially in NiO at low partial pressures of oxygen [21].
Fig. 4. Variation of the residual Ni0 with the square root of time upon constant temperature annealing at 820 K.
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Fig. 5. Arrhenius plot for the Ni0 oxidation rates.
Upon annealing of the deposits, an interesting behavior was observed for the width of the O 1s peak. In most cases, the full-width at half-maximum (FWHM) of the O 1s peak at various stages of the Ni deposit oxidation was around 2 eV, very close to its value for the clean substrate. However, in few cases, corresponding to a residual Ni0 coverage around 0.5 ML, the FWHM exhibited a large increase up to about 3 eV. When the samples were heated at increasing temperatures for 13 min (see Fig. 3), a broadening up to an O 1s FWHM of 2.5 eV occurred only at the annealing temperature of 880 K (corresponding to about 0.4 ML of residual Ni0) for the deposit with 1.4 ML initial Ni0, whereas no broadening was observed for the initial deposit with less than 0.5 ML Ni0, Fig. 6. During the constant temperature (820 K) measurements of Fig. 4, the O 1s FWHM started to increase when the residual Ni0 fell below 0.8 ML and reached a value of 3.1 eV near 0.5 ML, Fig. 7. The changes of the O 1s peak were not accompanied by any detectable changes in the broad Ni 2p region. Fitting the broadened O 1s peak, required, in addition to the reference substrate peaks shown in Fig. 2, two small additional components, one at a lower and one at a higher binding energy, compared to the lattice oxygen of the NiO, as shown
in Fig. 8. These components could represent surface oxygen species, more and less negatively charged, respectively, than lattice oxygen and can be associated with NixOy metastable, transient entities appearing during the oxidation of the deposit. The transient nature of these entities was experimentally verified, as the O 1s FWHM of the broadened state
Fig. 6. Variation of the O 1s FWHM as a function of annealing temperature corresponding to the measurements in Fig. 3: more than one monolayer of deposited Ni (*) and less than one monolayer of deposited Ni (&).
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4.2. Molecular dynamics simulations
Fig. 7. Variation of the O 1s FWHM as a function of the square root of time upon annealing at 820 K, corresponding to the measurements in Fig. 4.
returns slowly to its lower value when the sample is kept at 480 K for several hours in UHV. Although the nature of such entities cannot be deduced from the experiments, their presence does demonstrate that the gradual depletion of the deposited Ni0 upon annealing is related indeed to a surface oxidation process and not, for example to an in-diffusion of the Ni adatoms, which in any case is not favored for the insulating NiO crystal.
In our simulations the deposition of Ni ions resulted very quickly in the formation of small clusters, four types of which attracted our interest due to their diffusive activity and their role in the structural evolution of the formed islands: the single Ni adatom that is associated with two surface oxygens that are raised from their normal lattice sites Ni(O2) [9], the Ni2O, the Ni3O2, and the Ni4O4 structures. The first one is the simplest structure and the main superstoichiometric cluster we can find on the surface, playing a very important role in the structure of the adlayer that is formed in the presence of more adcations. In particular, we found that it is fast diffusing (mainly via exchange type mechanisms), while they can combine between them to form Ni2O molecules or they can be incorporated into neighbouring larger islands. The Ni2O molecule is stable and does not diffuse, except in the case where a larger cluster is in its neighbourhood. In this case, the Ni2O molecule diffuses via exchange type mechanisms to combine with the larger structure. The process is dominated by the collective movements of the involved ions and can be described as a twin triple exchange
Fig. 8. The O 1s region corresponding to the most broadened state in Fig. 7.
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Fig. 9. Sequence of snapshots showing the twin triple exchange mechanism of the Ni2O that results in the coalesce of the Ni2O and the Ni3O2 molecules into one island, at 0.36 Tm.
mechanism of diffusion. The result of this process is the creation of a single entity on the surface that remained stable and became an attracting point of other NixOy molecules existing on the surface. A representative example of such a process, in which the three-ions molecule is associated with a neighbouring five-particles cluster, is given in Fig. 9 in a sequence of snapshots. In the starting configuration only a part of the simulation slab is shown on which the deposition of a certain amount of Ni ions resulted in the formation of clusters of various sizes. In this figure, grey and black circles represent surface Ni and O atoms, respectively, while white and striped circles stand for the deposited Ni adcations and O adanions. We have to point out here that these O adatoms have been supplied from the bulk during the equilibration of the system at the simulation temperature, 0.36 Tm. The particles A1 and B1 are nickel adcations that are associated with oxygen atoms (striped particle) forming a typical three
particles molecule usually observed after the depositions of 8 and 16 Ni ions. In the same figure, we can also see another three Ni adcations cluster aligned with two oxygen adanions forming a Ni3O2 molecule. The particles A2, A3 and B2, B3 are Ni surface cations involved in the diffusion mechanism. Beneath the B3 nickel we can distinguish a second layer O (black particle) also involved in the diffusion process. We note also the lattice distortion present in the starting configuration in the region between and close to the two clusters due to the interaction of the adlayer nickels with the surface oxygen. After 0.4 ps the A1 Ni enters into the surface by pushing the A2 surface Ni that in turn pops the A3 Ni in an adatom position (0.8 ps). In the meantime, a similar scenario is played with the B1 Ni that pushes the B2 Ni, while the B3 is popped at adatom site. The oxygen atom of the original three-ions molecule gains a surface site and finally, after 1.3 ps, the A1, A2, B1, B2 particles occupy regular surface Ni sites restoring the surface
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order, while the A3, B3 nickels are connected to the five ions cluster with the incorporation of two more surface oxygens at adatom positions. The three particles diffusion process is completed resulting in an extra oxygen adatom reducing the negatively charged character of the surface. We also note that the oxygen atom beneath the B3 nickel atom entered into the surface layer filling the vacancy that was created from the second oxygen atom that was popped to adatom position.
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At higher temperatures Ni2O becomes more active and diffusing can be associated with other entities to form more complex structures. Turning our attention to the Ni3O2 particle, we found that it is also stable adopting either a chain like form or a compact shape, Fig. 9. When chain shaped, the molecule is diffusing in a snakelike way to recover its compact shape or to be incorporated into another molecule, as shown in Fig. 9. The compact shaped molecule does not diffuse, acting rather as
Fig. 10. Sequence of snapshots showing a surface oxygen vacancy associated with a Ni4O4 molecule that diffusing combines with a larger NixOy island, at 0.36 Tm.
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nucleus for the creation of higher oxides by attracting the neighboring smaller molecules. The Ni4O4 cluster is found to be stable taking a rather rounded shape enveloping almost always a surface vacancy. It comes out that this configuration is very important since it can spontaneously change its composition and shape into a Ni4O3 molecule by the insertion of an oxygen ion into the surface filling the oxygen vacancy of the surface, Fig. 10. In fact, we could describe the process of the two islands coalescence as the result of simple vacancy hop that is realised through the shape change of the Ni4O4 cluster. Fig. 10 illustrates a representative example of the process. In the starting configuration the Ni4O4 molecule is next to a larger NixOy island. Grey and black circles represent nickel and oxygen surface ions, while the deposited Ni adcations are represented as white circles and the oxygen adanions by striped circles. The Ni4O4 molecule surrounds a vacancy forming a kind of well; the particle numbered 1 is an oxygen ion that is part of the well, while particle labelled 2 is surface oxygen. After 2.5 ps the well is collapsing with the oxygen 1 filling the vacancy. At this stage, the molecule has changed into Ni4O3. After 6.4 ps the oxygen 2 is popped at adatom position connecting the two molecules and creating a new oxygen vacancy between them. Events of this type are quit often and it turns out that the presence of the vacancy stabilizes this structure giving in the surface layer a shallow rough aspect. It is worth noting that in all cases the formation of a sub-stoichiometric NixOytype island requires a certain amount of O anions and it results in an average Ni coordination number around 1.5. Interestingly, the necessary O anions are provided by the surface layer that is initially left with an equal number of O vacancies that subsequently diffuse into the bulk, expect in the case of well formation. The present findings are in agreement with results reported for a similar system employing H for the reduction of NiO(0 0 1) surface [4] and can be used to explain the experimental finding of the XPS spectra of the present study.
5. Concluding remarks In the present study, we present results referring to the Ni/NiO(0 0 1) interface formed upon vapor
deposition of Ni atoms. The experimental findings indicate that upon annealing the deposited Ni is oxidized by oxygen supplied from the substrate forming a NiO overlayer. The oxidation process is fast (of the order of minutes) at elevated temperatures (above 900 K) resulting in a surface that is indistinguishable (within the limits of XPS) from the clean substrate. An unexpected finding was the presence of two extra components in the XPS spectra located on both sides of the lattice O 1s peak, manifesting the existence of oxygen atoms with charges that deviate from their formal value, 2. The simulations provided satisfactory explanations of these observations. Indeed, we found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming NixOy oxides (x; y 4) most of which eventually coalesce into small NiO islands. At the same time the vacancies left behind diffuse into the bulk, which is equivalent to oxygen ions diffusing out of the bulk. Furthermore, the more negatively charged component of the O 1s peak may be attributed to species like Ni2O, Ni3O2, and Ni4O4, while the less negatively charged component to species like NiO2. Acknowledgements E.S. acknowledges stimulating discussions with Ch. Lekka, D. Pantelios, L. Sygellou and E. Vamvakopoulos.
References [1] S.K. Sharma, F.J. Vastola, P.L. Walker Jr., Carbon 35 (1997) 535. [2] D. Sotiropoulou, S. Ladas, Surf. Sci. 408 (1998) 182. [3] R.E. Tanner, I. Goldfarb, M.R. Castell, G.A.D. Briggs, Surf. Sci. 486 (2001) 167. [4] R.P. Furstenau, G. McDougall, M.A. Langell, Surf. Sci. 150 (1985) 55. [5] Ming-Cheng Wu, P.J. Møller, Surf. Sci. 279 (1992) 23. [6] L.N. Kantorovich, M.J. Gillan, Surf. Sci. 374 (1997) 373. [7] P.M. Oliver, G.W. Watson, S.C. Parker, Phys. Rev. B 52 (1995) 5323. [8] D.G. Papageorgiou, T.E. Karakasidis, G.A. Evangelakis, Phys. B 318 (2002) 211. [9] T.E. Karakasidis, D.G. Papageorgiou, G.A. Evangelakis, Surf. Sci. 486 (2001) 46. [10] M. Lelsie, M.J. Gillan, Solid State Phys. 18 (1985) 973.
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[16] C. Massobrio, M. Meyer, J. Phys.: Condens. Matter 3 (1991) 279. [17] T.E. Karakasidis, M. Meyer, Phys. Rev. B55 (1997) 13853. [18] T.E. Karakasidis, G.A. Evangelakis, Surf. Sci. 436 (1999) 193. [19] XPS Peak, Version 4.1, http://www.uksaf.org/software.html. [20] J.F. Mojica, L.L. Levenson, Surf. Sci. 59 (1976) 447. [21] C. Dubois, C. Monty, J. Philibert, Solid State Ion. 12 (1984) 75.
Ni/NiO(0 0 1) interface studied by X-ray photoelectron spectroscopy and molecular dynamics simulations E. Symianakisa, S. Ladasb, G.A. Evangelakisa,* a
b
Department of Physics, University of Ioannina, P.O. Box 1186 Physics, Ioannina 45110, Greece Department of Chemical Engineering, University of Patras and ICE/HT-FORTH, 26500 Rion, Patras, Greece Received 7 February 2003; received in revised form 24 March 2003; accepted 24 March 2003
Abstract X-ray photoelectron spectroscopy (XPS) results referring to the interface formed upon vapor deposition of Ni atoms on NiO(0 0 1) indicate, that upon annealing the deposited Ni is oxidized forming a NiO overlayer. The oxidation process is activated and becomes very fast above 900 K, resulting in a surface that exhibits identical photoelectron spectrum with the clean substrate. In addition, it was found that during the oxidation process two extra components in the O 1s XPS peak spectra appear, with binding energies above and below the main lattice oxygen peak, manifesting the existence of oxygen atoms carrying charges that deviate from their formal ionic value. Moreover, by molecular dynamics simulations it was found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming a series of NixOy oxides, most of which eventually coalesce into small NiO islands. Consequently, the lower binding energy component of the O 1s XPS peak may be attributed to intermediate oxides with x > y, while the higher binding energy component to intermediate oxides with x < y. # 2003 Elsevier Science B.V. All rights reserved. PACS: 82.80.Pv; 68.35.Fx; 71.15.Pd; 79.60.Jv Keywords: Ionic value; Gas sensors; Transition metal
1. Introduction Metal particles and adlayers on oxide surfaces are related to many technologically important applications like metal-oxide contacts, microelectronic and photovoltaic devices, coatings for corrosion passivation, gas sensors and oxide supported transition metal catalysts. Focusing on the deposition of Ni atoms on oxide surfaces, it is found that the reactivity of the *
Corresponding author. Tel.: þ30-26510-98590; fax: þ30-26510-98683. E-mail address: [email protected] (G.A. Evangelakis).
substrate plays a key role for the evolution of the deposited Ni atoms. Specifically, in the TiO2(1 1 0) surface there is formation of crystalline islands [1], while in the case of Yttria-stabilized ZrO2 threedimensional Ni crystallites are created, accompanied by charge transfer between Ni and the O2 anions of the surface [2]. In addition, it is reported that thin metallic Ni film above a NiO substrate can be effectively created by reducing the NiO surface either using C [3] or H [4]. It is found that for reduction up to 80% of the lattice oxygen, the adlayer that is formed adopts the structural character of the NiO(1 0 0) surface, while for smaller oxygen concentrations formation
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00553-1
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of Ni(1 0 0) islands is observed [4]. Interestingly, high temperature annealing results in a surface that appears to have fully restored its original composition and structure with oxygen supplied from the bulk [4]. Concerning Ni deposition on the NiO(0 0 1) sur˚ , a Ni face, it is reported that for a coverage up to 145 A thin film on the NiO(1 0 0) substrate can be epitaxially grown along with Ni(1 0 0) islands [5]. However, systematic studies concerning the behavior of small metallic particles and islands formed over the surface of their own oxides at very low coverage are rather scarce. From ab initio calculations, it is predicted that when Ni atoms reach the surface, charge transfer occurs that ionises them, effectively turning them into adcations [6]. In addition, recent molecular dynamics simulations have studied the diffusion properties of both adanion and adcation particles on the NiO(1 0 0) surface [7–11]. A combination of experimental studies with molecular dynamics simulations could broaden our understanding of phenomena like growth, oxidation, sintering and surface roughening. The aim of the present work is the experimental study of the interaction processes taking place in the case of near-monolayer deposition of Ni atoms on the NiO(1 0 0) surface, in conjunction with molecular dynamic simulations that intend to help in understanding the underlying mechanisms at microscopic scale.
2. Experimental setup Nickel atom deposition was carried out in an ultra high vacuum chamber (base pressure 5 1010 mbar), using a thermal evaporation source activated by electron bombardment. The chemical state of the deposit was monitored by X-ray photoelectron spectroscopy (XPS) using un-monochromatized Al Ka radiation (1486.6 eV) and a hemispherical electron energy analyzer with a pass energy of 100 eV. Prior to Ni deposition the NiO(1 0 0) surface was cleaned from carbon contamination, adopting methods for similar systems from the literature [4,12–14]. The cleaning procedure involved cycles of exposure to 3000 L of oxygen and annealing at 920 K. The efficiency of this method was established from both the XPS characteristic spectra of carbon-free NiO and the (1 1) LEED patterns of the (1 0 0) surface.
Nickel depositions where carried out at 480 K (0.21 Tm, Tm being the melting temperature), followed by repeated annealing at various temperatures between 500 and 920 K for a certain period of time. After each annealing step, the crystal was allowed to cool down to 480 K, temperature at which it remained for about 1 h with the X-ray gun on, so that the electrostatic charging of the NiO crystal was stabilized. When this condition was met, the XPS spectra of the Ni 2p and O 1s regions were obtained aiming in the investigation of NixOy-type structures, formed via possible charge transfer that could ionize the Ni particles. As binding energy reference, we used the value of 530.0 eV for the O 1s peak corresponding to NiO lattice oxygen. The amount, in monolayers (ML), of atomic nickel (Ni0) present at any time on the surface following deposition was estimated from the XPS 2p3/2 peak area ratio, R ¼ AðNi0 Þ/A(NiNiO) (with the Ni0 2p3/2 peak appearing at a binding energy around 853 eV and the NiNiO 2p3/2 peak around 855 eV). Assuming a layer-like deposition, the equivalent deposited Ni film thickness in nm was obtained, as dequiv ¼ l ln½1 þ 0:5R, where the inelastic mean-freepath l for Ni 2p photoelectrons in Ni was taken as 0.1 nm and the factor 0.5 in the brackets corresponds to an experimentally obtained ratio of the Ni 2p3/2 peak areas for two reference samples, a clean NiO and a very thick Ni0 film [15]. Taking 0.22 nm as the metallic Ni monolayer thickness, which corresponds to 2 1015 Ni atoms/cm2, the amount of Ni0 in ML, dequiv/0.22, was obtained. The above calculation underestimates the Ni coverage to the extent that three-dimensional clusters are formed, but this effect is small for nearmonolayer amounts of deposited nickel.
3. Computational details The simulations were carried out in the constant temperature canonical ensemble using the Nose scheme. The simulation cell we used consisted of a slab containing 1728 ions arranged in 12 planes with 72 cations and 72 anions in each plane. In order to have free surfaces in our simulations, we left an empty space of four times the length of the slab in the [0 0 1] direction while applying periodic boundary conditions in the three directions. This resulted in an infinite slab with two free surfaces normal to the z-direction.
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The equations of motion were integrated by means of Verlet’s algorithm and a time step of 1015 s. For the atomic interactions, we adopted a rigid ion potential developed for NiO [16] that gave good results concerning the diffusive properties of the material [17,18], while the Coulombic contributions were evaluated with the use of the Ewald summation. These technical details have been reported elsewhere [18]. The system with the clean surfaces was initially equilibrated for a short period of time, before proceeding to the deposition of Ni ions at adcation positions. In order to maintain the charge neutrality of the system, these ions were extracted from the one side of the slab and they were put on its other surface. It should be noted at this point that in our simulations, we consider Ni adatoms carrying their formal charges (þ2). This is of course a simplification since Ni adatoms can have other charges and can be associated with charged vacancies or more complex defects. However, this approximation seams to be reasonable when dealing with surface properties and it can be adopted for the study of deposition of Ni on the NiO surface [6,7].
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We studied three cases corresponding to coverage of 0.03, 0.06 and 0.11 ML by depositing simultaneously 4, 8 and 16 Ni adcations. This is a usual approximation that is due to the inherent time limitations of the method that does not give access to the experimental time scale that would correspond to a realistic deposition rate. However, for the cases of submonolayer coverage we are interested in, this simplification is not expected to affect seriously our results. The systems were simulated at three different temperatures, 0.37, 0.4 and 0.57 Tm, at which we followed their evolution for 40,000 time steps.
4. Results and discussion 4.1. Experimental results After a near-monolayer Ni deposition at 480 K, the XPS spectra exhibited the presence of an almost metallic Ni film (Ni 2p3/2 binding energies around 853 eV) on the NiO surface (Ni 2p3/2 binding energies around 855 eV) that remained stable for several hours
Fig. 1. (a) Ni 2p3/2 region of the clean NiO(1 0 0) surface, (b) Ni 2p3/2 region after Ni deposition on the clean NiO(1 0 0) surface.
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at 480 K. This observation shows that at least upon deposition of more than 0.5 ML of atomic Ni on NiO at 480 K, the deposited atoms are not immediately oxidized, but they can assemble into stable elemental Ni aggregates. Unfortunately, it was not possible to study deposits with much less initial Ni coverage because the additional Ni signal in XPS was too small to be accurately deconvoluted from the large substrate background. By employing a standard XPS peak fitting program [19], the region of the Ni 2p3/2 peak was fitted with two asymmetric Gaussian/Lorentzian components for the clean NiO surface (Fig. 1a). In the case of the Ni deposit, the previous components were used to fit the NiO substrate contribution, whereas two additional components, with characteristics obtained from reference thin Ni films, were employed to fit the deposited Ni0 (Fig. 1b). The O 1s peak of the clean substrate was fitted using two s-type peaks, the main one of which corresponds to the NiO lattice oxygen, whereas the small component at higher binding energy is most probably due to surface hydroxyl groups (Fig. 2). Following the Ni deposition at 480 K, the O 1s peak shape did not change and could be fitted in the same manner as in Fig. 2.
In order to investigate the interaction between the Ni deposit and the substrate upon annealing, the sample was heated for 13 min periods at increasing temperatures from 520 K up to a maximum of 920 K. After each heating step, XPS measurements at 480 K were employed to obtain the residual amount of Ni0 in the deposit as a function of annealing temperature. The obtained results at two different deposits exhibiting initial amounts of Ni0 (at 520 K) of about 1.4 and 0.5 ML, respectively, are shown in Fig. 3. The main observation is that, upon heating, the amount of Ni0 gradually decreases and after the final heating step at 920 K the Ni deposit is completely oxidized and indistinguishable from the NiO substrate. The shape of the curves in Fig. 3 shows that the oxidation process is clearly activated, as the rate increases drastically with increasing temperature. Furthermore, the fact that the progress of the oxidation is observable even at low temperature for the sub-monolayer amount of deposited Ni, whereas it exhibits an initial delay for the larger amount, followed by a more steep behavior at high temperatures, suggests that the oxidation process takes place through the interface and not via the gas phase.
Fig. 2. The O 1s region of the clean NiO(1 0 0) surface.
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Fig. 3. Variation of the residual Ni0 upon 13 min annealing at increasing temperatures for two different amounts of deposited Ni: more than one monolayer (*) and less than one monolayer (&).
In order to follow the oxidation process as a function of time at constant temperature, a separate experiment was carried out in which the temperature of a new deposit was raised to 820 K and kept constant for increasing time intervals, with intermediate XPS measurements at 480 K. The amount of residual Ni0 in ML as a function of the time square root is plotted in Fig. 4. A good straight line fits the data suggesting that a diffusive process supplying the necessary oxygen
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from the interior of the NiO crystal governs the oxidation. The slope of the straight line in Fig. 4 is proportional to D1/2, where D corresponds to the interlayer diffusion coefficient, if a simple model is adopted for the diffusion process, whereby oxygen ions are supplied by diffusion from a bulk source of constant concentration across an interface barrier [20]. In order to obtain an estimate of the activation energy for diffusion, an Arrhenius diagram is constructed (Fig. 5), in which the logarithm of the slope of straight lines as that in Fig. 4 is plotted as a function of 1/T. The experimental points of Fig. 5 are obtained from the slope of the straight line in Fig. 4 and from the data of Fig. 3 above 800 K, where the calculations are more accurate due to the larger slope of the curves. For the latter points, the successive incremental decrease of the Ni0 coverage at each temperature along the drawn curves in Fig. 3 is divided by (13 min)1/2 to obtain slopes analogous to that of the line in Fig. 4. Despite the relatively large scatter of the data, a straight line fits the experimental points, the slope of which is given by E/2k, where k is the Boltzmann’s constant. The activation energy of 1:1 0:3 eV is deduced from Fig. 5 that is in the range of values expected for bulk oxygen ion diffusion in oxides, especially in NiO at low partial pressures of oxygen [21].
Fig. 4. Variation of the residual Ni0 with the square root of time upon constant temperature annealing at 820 K.
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Fig. 5. Arrhenius plot for the Ni0 oxidation rates.
Upon annealing of the deposits, an interesting behavior was observed for the width of the O 1s peak. In most cases, the full-width at half-maximum (FWHM) of the O 1s peak at various stages of the Ni deposit oxidation was around 2 eV, very close to its value for the clean substrate. However, in few cases, corresponding to a residual Ni0 coverage around 0.5 ML, the FWHM exhibited a large increase up to about 3 eV. When the samples were heated at increasing temperatures for 13 min (see Fig. 3), a broadening up to an O 1s FWHM of 2.5 eV occurred only at the annealing temperature of 880 K (corresponding to about 0.4 ML of residual Ni0) for the deposit with 1.4 ML initial Ni0, whereas no broadening was observed for the initial deposit with less than 0.5 ML Ni0, Fig. 6. During the constant temperature (820 K) measurements of Fig. 4, the O 1s FWHM started to increase when the residual Ni0 fell below 0.8 ML and reached a value of 3.1 eV near 0.5 ML, Fig. 7. The changes of the O 1s peak were not accompanied by any detectable changes in the broad Ni 2p region. Fitting the broadened O 1s peak, required, in addition to the reference substrate peaks shown in Fig. 2, two small additional components, one at a lower and one at a higher binding energy, compared to the lattice oxygen of the NiO, as shown
in Fig. 8. These components could represent surface oxygen species, more and less negatively charged, respectively, than lattice oxygen and can be associated with NixOy metastable, transient entities appearing during the oxidation of the deposit. The transient nature of these entities was experimentally verified, as the O 1s FWHM of the broadened state
Fig. 6. Variation of the O 1s FWHM as a function of annealing temperature corresponding to the measurements in Fig. 3: more than one monolayer of deposited Ni (*) and less than one monolayer of deposited Ni (&).
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4.2. Molecular dynamics simulations
Fig. 7. Variation of the O 1s FWHM as a function of the square root of time upon annealing at 820 K, corresponding to the measurements in Fig. 4.
returns slowly to its lower value when the sample is kept at 480 K for several hours in UHV. Although the nature of such entities cannot be deduced from the experiments, their presence does demonstrate that the gradual depletion of the deposited Ni0 upon annealing is related indeed to a surface oxidation process and not, for example to an in-diffusion of the Ni adatoms, which in any case is not favored for the insulating NiO crystal.
In our simulations the deposition of Ni ions resulted very quickly in the formation of small clusters, four types of which attracted our interest due to their diffusive activity and their role in the structural evolution of the formed islands: the single Ni adatom that is associated with two surface oxygens that are raised from their normal lattice sites Ni(O2) [9], the Ni2O, the Ni3O2, and the Ni4O4 structures. The first one is the simplest structure and the main superstoichiometric cluster we can find on the surface, playing a very important role in the structure of the adlayer that is formed in the presence of more adcations. In particular, we found that it is fast diffusing (mainly via exchange type mechanisms), while they can combine between them to form Ni2O molecules or they can be incorporated into neighbouring larger islands. The Ni2O molecule is stable and does not diffuse, except in the case where a larger cluster is in its neighbourhood. In this case, the Ni2O molecule diffuses via exchange type mechanisms to combine with the larger structure. The process is dominated by the collective movements of the involved ions and can be described as a twin triple exchange
Fig. 8. The O 1s region corresponding to the most broadened state in Fig. 7.
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Fig. 9. Sequence of snapshots showing the twin triple exchange mechanism of the Ni2O that results in the coalesce of the Ni2O and the Ni3O2 molecules into one island, at 0.36 Tm.
mechanism of diffusion. The result of this process is the creation of a single entity on the surface that remained stable and became an attracting point of other NixOy molecules existing on the surface. A representative example of such a process, in which the three-ions molecule is associated with a neighbouring five-particles cluster, is given in Fig. 9 in a sequence of snapshots. In the starting configuration only a part of the simulation slab is shown on which the deposition of a certain amount of Ni ions resulted in the formation of clusters of various sizes. In this figure, grey and black circles represent surface Ni and O atoms, respectively, while white and striped circles stand for the deposited Ni adcations and O adanions. We have to point out here that these O adatoms have been supplied from the bulk during the equilibration of the system at the simulation temperature, 0.36 Tm. The particles A1 and B1 are nickel adcations that are associated with oxygen atoms (striped particle) forming a typical three
particles molecule usually observed after the depositions of 8 and 16 Ni ions. In the same figure, we can also see another three Ni adcations cluster aligned with two oxygen adanions forming a Ni3O2 molecule. The particles A2, A3 and B2, B3 are Ni surface cations involved in the diffusion mechanism. Beneath the B3 nickel we can distinguish a second layer O (black particle) also involved in the diffusion process. We note also the lattice distortion present in the starting configuration in the region between and close to the two clusters due to the interaction of the adlayer nickels with the surface oxygen. After 0.4 ps the A1 Ni enters into the surface by pushing the A2 surface Ni that in turn pops the A3 Ni in an adatom position (0.8 ps). In the meantime, a similar scenario is played with the B1 Ni that pushes the B2 Ni, while the B3 is popped at adatom site. The oxygen atom of the original three-ions molecule gains a surface site and finally, after 1.3 ps, the A1, A2, B1, B2 particles occupy regular surface Ni sites restoring the surface
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order, while the A3, B3 nickels are connected to the five ions cluster with the incorporation of two more surface oxygens at adatom positions. The three particles diffusion process is completed resulting in an extra oxygen adatom reducing the negatively charged character of the surface. We also note that the oxygen atom beneath the B3 nickel atom entered into the surface layer filling the vacancy that was created from the second oxygen atom that was popped to adatom position.
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At higher temperatures Ni2O becomes more active and diffusing can be associated with other entities to form more complex structures. Turning our attention to the Ni3O2 particle, we found that it is also stable adopting either a chain like form or a compact shape, Fig. 9. When chain shaped, the molecule is diffusing in a snakelike way to recover its compact shape or to be incorporated into another molecule, as shown in Fig. 9. The compact shaped molecule does not diffuse, acting rather as
Fig. 10. Sequence of snapshots showing a surface oxygen vacancy associated with a Ni4O4 molecule that diffusing combines with a larger NixOy island, at 0.36 Tm.
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nucleus for the creation of higher oxides by attracting the neighboring smaller molecules. The Ni4O4 cluster is found to be stable taking a rather rounded shape enveloping almost always a surface vacancy. It comes out that this configuration is very important since it can spontaneously change its composition and shape into a Ni4O3 molecule by the insertion of an oxygen ion into the surface filling the oxygen vacancy of the surface, Fig. 10. In fact, we could describe the process of the two islands coalescence as the result of simple vacancy hop that is realised through the shape change of the Ni4O4 cluster. Fig. 10 illustrates a representative example of the process. In the starting configuration the Ni4O4 molecule is next to a larger NixOy island. Grey and black circles represent nickel and oxygen surface ions, while the deposited Ni adcations are represented as white circles and the oxygen adanions by striped circles. The Ni4O4 molecule surrounds a vacancy forming a kind of well; the particle numbered 1 is an oxygen ion that is part of the well, while particle labelled 2 is surface oxygen. After 2.5 ps the well is collapsing with the oxygen 1 filling the vacancy. At this stage, the molecule has changed into Ni4O3. After 6.4 ps the oxygen 2 is popped at adatom position connecting the two molecules and creating a new oxygen vacancy between them. Events of this type are quit often and it turns out that the presence of the vacancy stabilizes this structure giving in the surface layer a shallow rough aspect. It is worth noting that in all cases the formation of a sub-stoichiometric NixOytype island requires a certain amount of O anions and it results in an average Ni coordination number around 1.5. Interestingly, the necessary O anions are provided by the surface layer that is initially left with an equal number of O vacancies that subsequently diffuse into the bulk, expect in the case of well formation. The present findings are in agreement with results reported for a similar system employing H for the reduction of NiO(0 0 1) surface [4] and can be used to explain the experimental finding of the XPS spectra of the present study.
5. Concluding remarks In the present study, we present results referring to the Ni/NiO(0 0 1) interface formed upon vapor
deposition of Ni atoms. The experimental findings indicate that upon annealing the deposited Ni is oxidized by oxygen supplied from the substrate forming a NiO overlayer. The oxidation process is fast (of the order of minutes) at elevated temperatures (above 900 K) resulting in a surface that is indistinguishable (within the limits of XPS) from the clean substrate. An unexpected finding was the presence of two extra components in the XPS spectra located on both sides of the lattice O 1s peak, manifesting the existence of oxygen atoms with charges that deviate from their formal value, 2. The simulations provided satisfactory explanations of these observations. Indeed, we found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming NixOy oxides (x; y 4) most of which eventually coalesce into small NiO islands. At the same time the vacancies left behind diffuse into the bulk, which is equivalent to oxygen ions diffusing out of the bulk. Furthermore, the more negatively charged component of the O 1s peak may be attributed to species like Ni2O, Ni3O2, and Ni4O4, while the less negatively charged component to species like NiO2. Acknowledgements E.S. acknowledges stimulating discussions with Ch. Lekka, D. Pantelios, L. Sygellou and E. Vamvakopoulos.
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