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J. Phys. Chem. B 2005, 109, 17197-17204

17197

Experimental and Theoretical Study of a Surface Stabilized Monolayer Phase of Nickel Oxide on Pd(100) S. Agnoli,† M. Sambi,† G. Granozzi,*,† J. Schoiswohl,‡ S. Surnev,‡ F. P. Netzer,‡ M. Ferrero,§ A. M. Ferrari,§ and C. Pisani§ Dipartimento di Scienze Chimiche and INFM Research Unit, UniVersita` di PadoVa, Italy, Institut fu¨r Experimentalphysik, Karl-Franzens-UniVersita¨t Graz, Austria, and Dipartimento di Chimica IFM, and Centre of Excellence NIS (Nanostructured Interfaces and Surfaces), UniVersita` di Torino, Italy ReceiVed: May 9, 2005; In Final Form: July 19, 2005

A surface stabilized monolayer phase of nickel oxide, c(4 × 2)-Ni3O4, has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybridexchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.

1. Introduction Ultrathin NiO films deposited on several metallic substrates have been extensively investigated in recent years,1-4 primarily because of the peculiar electronic, magnetic, and chemical properties that may be expected from epitaxially strained, vertically confined, and possibly defective NiO layers in the subnanometer thickness range. Particular attention has been devoted to the growth of NiO (and of the structurally similar closed-shell oxide MgO) on Ag(100), since the rather low overlayer/substrate lattice mismatch (+2.2%) in this case is reasonably expected to lead to good-quality epitaxial films. Layer-by-layer growth and pseudomorphism up to a critical coverage of approximately 5 monolayers (ML) was initially demonstrated by means of spot profile analysis of low energy electron diffraction (SPA-LEED) patterns.5 Relaxation was shown to occur via the injection of misfit dislocations with {110} glide planes, leading to the formation of mosaics on the film surface. Polarization-dependent X-ray absorption (XAS) measurements at the Ni K edge gave a quantitative determination of the tetragonal epitaxial strain in the pseudomorphic thickness range.6,7 Primary beam diffraction modulated electron emission (PDMEE) experiments determined that the NiO films grow with O on top of Ag and that the oxide/metal interface distance is rather expanded (2.3 ( 0.1 Å) with respect to both the overlayer and the substrate bulk interlayer distance.6 The registry and the interfacial distance have been subsequently confirmed and refined by means of LEED I-V analysis.8 Furthermore, the oxygen dosage effect on the structure and composition of the growing layer and on the stoichiometry of Ni oxide was investigated,9 leading to the conclusion that there is a strong dependence of structure and composition on the oxygen-tonickel flux ratio. In particular, low oxygen dosage induces a (2 × 1) reconstruction in 1-ML films that evolves to a (1 × 1) * Corresponding author. E-mail: [email protected]. † Universita ` di Padova. ‡ Karl-Franzens-Universita ¨ t Graz. § Universita ` di Torino.

phase as the dosage and/or the film thickness increases. The (2 × 1) reconstruction has been further investigated by means of LEED intensity analysis, leading to a structural model similar to the (111) surface of NiO.10 Finally, from the point of view of the magnetic properties, it has been shown recently that tetragonal strain, derived from pseudomorphic matching of the NiO film to a lattice-mismatched substrate such as Ag(100) has consequences on the magnetic linear dichroism of the L2 edge, leading to the reversal of the dichroic signal.11 In a series of recent papers,12-15 we have tested Pd(100) as an alternative metal substrate for NiO epitaxy. It differs from Ag(100) both by the significantly larger lattice mismatch (+7.3%) and by the valence band of predominantly 4d character near the Fermi level, compared to the 5sp character of the silver valence band. This is important to assess the role and relevance of the hybridization of overlayer and metal states at the interface in determining the properties and the reactivity of the system. Hybridization has been proven to be of limited importance in the NiO/Ag(100) case, as shown by theoretical investigations carried out in our group,6,16 but it is expected to be more relevant in the present case, due to the d character of the substrate valence band. A general conclusion of our previous work is that the structure and the composition of the overlayer are strongly dependent on the deposition procedure. Both properties are driven by the surface chemistry of the substrate toward oxygen (which shows a rather complex phase diagram for oxygen adsorption) and by the competition between Pd and Ni toward oxidation during the first stages of deposition. In fact, four distinct oxygen chemisorption phases on Pd(100) are known as a function of coverage,17 ranging from p(2 × 2)-O through c(2 × 2)-O and (5 × 5)-O to the saturation phase (x5 × x5)R27°-O. We have explored both the so-called postoxidation (PO) and the reactive deposition (RD) growth procedures, by combining LEED, X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), and scanning tunneling microscopy (STM). PO consists of depositing small amounts of metallic Ni under

10.1021/jp052394s CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005

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Figure 1. Structure of the different phases considered in the computer simulations. (a) Epitaxial stoichiometric NiO monolayer (NiO-ML); (b) p(2 × 2)-O; (c) c(4 × 2)-Ni3O4 (RH, rhombic distribution of vacancies in epitaxial NiO-ML); (d) p(2 × 2)-Ni3O4 (SQ, square distribution of vacancies in epitaxial NiO-ML). Pd atoms in the surface and subsurface layers are drawn as large white circles with black and gray contours, respectively. Ni and O atoms are drawn as small black and gray circles, respectively. In the Ni3O4 phases, (1) and (2) indicate Ni atoms with 1 or 2 multiplicity. The 2-D unit cell is shown in all cases.

ultrahigh vacuum (UHV) conditions on the Pd substrate, followed by an annealing treatment in oxygen to produce the NiO overlayers. The results highlight the dependence of the ultrathin film composition and structure on the Ni dose deposited at each step of the PO procedure.12 In particular, it has been shown that there is a critical Ni dose that is able to initiate substantial substrate oxidation during the PO treatment, which ultimately leads to amorphous overlayers. Substrate oxidation can be avoided by preventing the formation of a direct metalmetal interface. This can be accomplished by presaturating the Pd surface with oxygen, to obtain the (x5 × x5)R27°-O structure.18,19 When the “x5” structure is used as the starting point for the PO growth procedure, NiO(100) ultrathin films with moderate long-range order and with good compositional homogeneity as a function of thickness can be obtained, with no substrate oxidation. In RD Ni is dosed on the Pd substrate at room temperature (RT) in the presence of an oxygen background pressure. NiO films up to several tens of monolayers thick can be obtained following this route, and they are characterized by a better developed long-range order compared to the PO procedure and with negligible intermixing at the interface.13 Compressive strain develops in the NiO overlayer in the first few monolayers. The strain is progressively relieved between 3 and 10 ML, after which the NiO film adopts the bulk lattice constant.20 In the first stages of growth, a peculiar interface-stabilized nickel oxide monolayer phase with a c(4 × 2) periodicity has been detected by means of LEED measurements following both PO and RD routes. Through a combination of different experimental techniques (LEED I-V, XPS, XPD, STM),14,15 the phase has been identified as c(4 × 2)-Ni3O4, a wetting twodimensional (2-D) layer characterized by a regular distribution of nickel vacancies (see Figure 1c). Some intriguing open questions concerning the formation of the c(4 × 2) phase are still pending. One major question is related to the thermodynamic or kinetic stabilization of the phase, which does not have a “bulk” counterpart. Moreover, the observed pattern of the Ni defects (c(4 × 2)-Ni3O4, rhombic, hereafter referred as RH; see Figure 1c) is rather unexpected on a substrate with a 4-fold symmetry. Actually, a square array of Ni defects p(2 × 2)-Ni3O4, hereafter referred to as SQ, (see Figure 1d) would also be expected to give a similar stoichiom-

Agnoli et al. etry. Therefore, ab initio computer simulations have been set up to provide some clues for answering these questions, in particular concerning the energetics of the different phases involved, to support or to discard hypothetical growth models. The results of this joint investigation are presented in this paper, which is organized as follows. The experimental and computational setups are presented in section 2. In section 3, the structural experimental evidence, as provided by STM, is reported and the results of the quantum mechanical calculations are introduced. To justify the formation of RH with respect to possible competing structures, the epitaxial (1 × 1)-NiO monolayer and p(2 × 2)-O phases have also been computationally investigated (Figure 1a,b). In section 4 we propose a plausible growth mechanism that rationalizes the STM experimental data, including the peculiar observed defects. 2. Experimental and Computational Models and Techniques STM Experiments. The experiments were carried out in a custom-designed UHV system at the University of Graz, operating at a base pressure of 5 × 10-9 Pa, equipped with a variable-temperature STM (Oxford Instruments), LEED, Auger electron spectroscopy (AES), an electron beam evaporator, and crystal cleaning facilities.21 The STM images were recorded in a constant-current mode at RT using electrochemically etched W tips cleaned in situ by electron bombardment. The typical tunneling conditions employed were sample bias 1-2 V and tunneling current 0.1-0.5 nA. The Pd(100) substrate was cleaned by repeated cycles of 2 kV Ar+ sputtering followed by annealing at 973 K, until a sharp (1 × 1) LEED pattern was obtained and no contaminants were detected in the Auger spectra. The NiO films were prepared following two different routes: in the first case metallic Ni was deposited in 2 × 10-4 Pa of oxygen at RT and then annealed in 5 × 10-5 Pa of oxygen at 570 K (recipe I). In the second case, the deposition was performed in 2 × 10-4 Pa of oxygen pressure, but the substrate was kept at 523 K (recipe II). In both procedures a deposition rate of about 0.8 MLE/min was used, as determined by quartz microbalance measurements (1 monolayer equivalent, MLE, corresponds to 1.3 × 1015 Ni atoms/cm2). Computations. The choice of a suitable computational approach to the problem here considered is far from simple, because of the complexity of the systems to be described. The metallic character of the support, the presence of the surface, the magnetic character of some of the supported phases and their unusual stoichiometry, along with the large size of the unit cell, require an accurate calibration of the computational setup. A more detailed description and validation of the procedure adopted is provided in a parallel paper;22 here we limit ourselves to indicating the main aspects. Our previous experience has encouraged us to adopt for our simulations the ab initio code CRYSTAL23 in the frame of density functional theory (DFT), and to use in all cases a slab, 2-D periodic model. The constraint of 2-D periodicity requires describing separately the various phases, first of all the bare Pd(100) surface, which is simulated by a thin slab cut from the bulk parallel to the exposed face. The same slab, covered symmetrically on both sides by the selected distribution of atoms, is used for simulating the regular adsorbed phases. The cost of the calculations increases rapidly with the number of layers; therefore, the thickness of the slab must be kept to a minimum compatible with a satisfactory reproduction of the properties of the surface: while five-layer slabs appear adequate for these

Study of c(4 × 2)-Ni3O4 on Pd(100)

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purposes, most of the calculations of the c(4 × 2) phase were performed with a three-layer Pd slab due to computational limitations. CRYSTAL adopts a basis set of contracted Gaussian type functions (GTF), which must be carefully selected for each atom, by exploiting the rich experience gained with a variety of periodic systems.24 The basis set here adopted for the Ni and O atoms in the c(4 × 2) and NiO-ML is the same as that used for studying the deposition of NiO ultrathin film at the Ag(100) surface.16 It consists of an all-electron 8-411G set for oxygen, and of 3sp/2d GTFs for the 16 outer electrons of nickel (corresponding to the electron configuration 3s23p63d8), with the inner electrons being described by the Hay-Wadt smallcore pseudopotential.25 The O atom of the adsorbed p(2 × 2)-O phase has been described by a 6-311G* basis set.26 For the 18 outer electrons of Pd (electron configuration 4s24p64d10), a basis set was used consisting of 4sp/2d GTFs with a Hay-Wadt small-core pseudopotential to describe the innermost electrons. Concerning the choice of a suitable Hamiltonian apt at adequately describing the properties of the different subsystems, we have adopted the same hybrid-exchange approach27 as in our previous study of NiO/Ag(100).16 It consists of including 35% of the exact Hartree-Fock exchange in the exchange expression of the generalized gradient approximation as proposed by Perdew and Wang.28 The resulting Hamiltonian is indicated as F35PW. To compare among themselves energy data referring to quite different systems, we will report in the following ∆E′ values for the different phases considered, obtained according to the formula

∆E′(π) ) [E(π|Pd|π) - E(Pd)]/2 -

∑Eat

(1)

That is, reference is made to the formation of the phase considered (the Pd slab sandwiched between the two adsorbed structures) starting from the bare Pd slab and from the isolated atoms of the adsorbed phase (the same reference atomic energies are always used: EO ) -75.065653EH; ENi ) -169.062767EH). For isolated phases, that is, not interacting with Pd, the formula simplifies to ∆E′(π) ) E(π) - ∑Eat. All energy data reported in this work have been corrected for the basis set superposition error (BSSE) by applying the standard counterpoise method,29 and are referred to the unit cell. 3. Results It is expedient for the following discussion to schematically recall the experimental data of the most relevant phases that are formed during the RD of Ni on Pd: (a) p(2 × 2)-O. In the oxidative ambient of the Ni deposition, the substrate is predominantly covered by this p(2 × 2)-O chemisorption phase, corresponding to a square array of oxygen atoms, one every four hollow surface sites (see Figure 1b). This phase is well-known from the literature as the main O/Pd structure that is formed at RT (see, for example, refs 30 and 31). (b) (x5 × x5)R27°-O. In the subsequent stages of the process, especially when deposition occurs at the higher temperature, this becomes progressively the dominant O/Pd structure. With respect to the previous one, its density per unit area is larger by a factor of 3.2, and its formation is believed to entail extensive reconstruction of the metal substrate (see, for instance, ref 19). The other O/Pd phases of intermediate density, c(2 × 2)-O and (5 × 5)-O,31 have never been detected in significant amounts in the present experiments.

(c) c(4 × 2)-Ni3O4 (RH). Small patches of this phase cover parts of the surface in the early stages of Ni deposition at RT. With increasing coverage they coalesce and eventually wet the whole substrate after annealing in the range between 523 and 573 K. (d) Ordered, stoichiometric NiO. Above 1 MLE coverage, three-dimensionally ordered but strained nickel oxide appears and progressively becomes the dominant phase; the strain is relaxed as the thickness is increased.20 A simple epitaxial monolayer of stoichiometric NiO (hereafter referred to as NiOML; see Figure 1a) has never been identified with certainty in the STM images. In the following we will first report the STM experimental evidence, focusing in particular on highly resolved images that give details on the structure and defects of the RH phase. Subsequently, we will introduce the results of the ab initio computations, which set the basis for the discussion of a possible growth model. STM Results. Figure 2 shows two STM images (500 × 500 Å2) obtained after RD of 0.50 MLE of Ni according to recipes I and II (see Experimental Section). In Figure 2a, an irregular network with internal order is evident, characterized by an array of bright protrusions with a c(4 × 2) periodicity with respect to the substrate unit cell. The uncovered regions of the substrate show predominantly the p(2 × 2)-O chemisorption phase. The RH layer is flat and grows in a pure 2-D habit; its apparent height is about 1.3-1.5 Å, slightly depending on tunneling bias, indicating the presence of a single monolayer structure. If the deposition of the oxide is performed at 523 K (recipe II), the topography is different and rougher than the one observed with recipe I (see Figure 2b). Patches of the (x5 × x5)R27°-O chemisorption phase are present both on the bare substrate regions and on Pd ad-islands, which are trapped in the middle of the growing RH layer. This is attributed to the higher growth temperature. In Figure 3 we report a (500 × 500 Å2) STM image obtained after RD of 1.25 MLE of Ni according to recipe I, which demonstrates that a complete wetting RH layer is obtained under appropriate conditions. Figure 4 presents a closer view of the two different situations for 0.25 MLE deposits according to the different recipes. Using recipe I (Figure 4a), large well-ordered RH oxide islands (∼100 Å wide) are evident beside regions of the p(2 × 2)-O phase. The edges of the RH islands are mainly oriented along the 〈011〉 equivalent substrate directions. According to the model for the c(4 × 2) structure obtained by LEED (see Figure 1c),14 these are polar directions in the overlayer lattice. It is noteworthy that the island borders show a peculiar contrast in the STM images. As clearly seen in Figure 4a, they show an enhanced corrugation (a series of closely spaced bright spots) located at the island edges parallel to the main diagonal of the rhombic unit cell, while for the edges in the perpendicular direction this phenomenon is less evident. The bright decorations of the island boundaries may be associated with an electronic effect. As a consequence, it is conceivable that different boundaries may have different reactivity and different growth rates. In the case of the STM image obtained after the high-temperature growth (recipe II, Figure 4b), it is evident that a p(2 × 2)-O covered palladium ad-island is totally surrounded by a perfectly matching RH layer that covers a large Pd terrace. In addition, next to the Pd ad-island an embedded oxide island is observed as well (sometimes, when the oxide patch is close to a (x5 × x5)R27°-O domain, the boundary is more disordered and [012] edges are also frequently seen). Even if the high-temperature

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Figure 2. STM images (500 × 500 Å2) obtained after reactive deposition of 0.50 MLE of Ni according to recipes I (a) and II (b), respectively. Rectangles in (a) highlight p(2 × 2)-O patches (see text for details).

Figure 3. STM images (500 × 500 Å2) obtained after reactive deposition of 1.25 MLE of Ni according to recipe I.

recipe II results in a rougher surface than the one obtained by RT deposition, the structure of the oxide phase remains the same: the oxide islands are rectangular, flat, and with a preferred edge orientation along the 〈011〉 equivalent directions. The quality of the reported images allows us to discuss details of the defects of the RH islands. The islands can be single domain or constituted by different domains with the same or a different orientation. In the latter case some interesting boundaries between the subdomains can be observed (see Figure 5). Figure 5a shows a line defect between two domains rotated by 90°; it is formed by an array of very bright maxima common to both domains separated by dark minima, forming a defect with a periodicity of 4 times the substrate lattice constant and aligned along the [011] direction (note that in this image the contrast is reversed as compared to the other images). Another

possible defect structure between orthogonal domains is presented in Figure 5d: in this case a line defect oriented along the [010] direction is formed. On the other hand, when the domains have the same orientation, two different kinds of defects can be observed: a square line and a diagonal line defect (see Figure 5b and Figure 5c, respectively). In the former case the two domains are simply shifted by two lattice constants in the direction of the main diagonal of the rhombic unit cell, so the bright spots typical of the RH structure define a square mesh at the connection between the domains. In the latter case (Figure 5c) the two domains are shifted by one lattice constant along both diagonals; hence the structure appears as a zigzag defect along the [012] direction, i.e., along the side of the rhombic unit cell. The nature of these defects will be discussed in section 4 on the basis of the theoretical model of the growth. Ab Initio Computational Results. Reference Systems. In the parallel computational study,22 it is shown that the present computational setup provides a satisfactory description of the structural, energetic, and electronic properties of Pd bulk. The same Hamiltonian/basis set combination has been considered in the case of Pd(100) slabs, while following the changes of their electronic and energetic properties with increasing thickness. A five-layer slab was found to be thick enough to reach convergence in the surface formation energy, amounting to 6.57 J m-2. No particular structural or electronic features were found to be associated with the formation of the surface. To reduce computational costs, most of the simulations were performed with the three-layer model of the surface, with five-layer slabs being used only for checking the most important results. The description of bulk NiO is also adequate, as discussed in previous work.16 In particular, the calculated equilibrium lattice constant is a0 ) 4.19 Å (experimental value is 4.1705 Å). For the sake of reference, we report some data for i-NiOML, the isolated, relaxed monolayer, in its ferromagnetic structure (the results for the antiferromagnetic configuration are very similar): a0 ) 3.91 Å; ∆E′ ) -13.80 eV (less stable than NiO bulk by 1.94 eV per NiO unit); q(Ni) ) -q(O) ) 1.75 au; µ(Ni) ) 1.94, µ(O) ) 0.16 au. Here and in the following, net charges and magnetic moments of atoms are obtained according to a Mulliken population analysis.

Study of c(4 × 2)-Ni3O4 on Pd(100)

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Figure 4. STM images (200 × 200 Å2) obtained after reactive deposition of 0.25 MLE of Ni according to recipes I (a) and II (b), respectively (see text for details).

Figure 5. High-resolution STM images of c(4 × 2)-Ni3O4 (RH) islands, showing antiphase domain boundaries with different defect structures. The different antiphase relations between the domains in (a)-(d) are given in the text. In (a), the contrast is reversed compared to the other images. The positions of some atoms of the RH phase (Ni, small circle; O, large circle) are indicated, following the model discussed in section 4. The rectangle in (a) makes reference to a corresponding one drawn in Figure 7.

Let us briefly comment on the mismatch between NiO and Pd. The calculated misfit between the two bulk structures is 6.6%, which is close to the experimental value of 7.2%. However, i-NiO-ML exhibits a considerable contraction associated with an energy gain of 0.3 eV per NiO unit, resulting in practically perfect matching with Pd, within 0.5%. Of course, when multilayer epitaxial NiO structures are formed, considerable strains are produced that must be released somehow; this problem has been discussed for the parallel case of NiO/Ag(100),6 and is not treated here any further.

p(2 × 2)-O Phase. As it is well-known,30 the “hollow” configuration (O at a hollow site) is the most stable one: ∆E′ ) -2.81 eV. The adsorption energy per pair of O atoms (5.62 eV) is slightly larger than the dissociation energy of the gasphase O2 molecule (experimental and calculated values are 5.21 and 4.81 eV, respectively), which justifies the spontaneous formation of this phase in an O2 atmosphere. The “bridge” configuration is less stable than the “hollow” one by only 0.16 eV, which suggests large mobility of adsorbed O atoms, favoring the formation of very regular patterns. The calculated equilibrium distance of O from the plane of surface Pd (0.70 Å) is in reasonable agreement with the experimental LEED result (0.83 ( 0.02 Å).30 The Mulliken net charge of O is q(O) ) -0.64 au, almost exactly balanced by those of the four nearby Pd atoms at the surface, q(Pd) ) 0.18 au. Nickel Oxide Adsorbed Phases. Because it is experimentally observed for the case of NiO/Ag(100), one would expect NiOML to be the preferred oxide structure formed at the Pd(100) surface. This would also be expected because of the practical coincidence of the two lattice constants (see above). The present computational results are very similar to those for the parallel case of silver. O atoms preferably sit on top of Pd atoms, at a distance of 2.71 Å and a value ∆E′ ) -14.20 eV. The electronic configuration of the two subsystems (the slab and i-NiO-ML) does not change significantly when they are allowed to interact, consistently with the very small binding energy of 0.40 eV. While i-NiO-ML is relatively stable, the i-c(4 × 2) structure (obtained from i-NiO-ML by simply removing one of four Ni atoms) is totally artificial. We have nonetheless considered it in order to have some information about the importance of its interaction with Pd, and we compared i-RH to i-SQ, the two isolated Ni3O4 monolayers with either a rhombic or a square distribution of the vacancies (see the Introduction and Figure 1). In both cases, the 2-D lattice parameters have been constrained to correspond to epitaxy with Pd. The two ∆E′ values are -39.83 and -38.14 eV, and they become -41.83 and -39.94 eV for i-RH and i-SQ, respectively, when internal coordinates are relaxed. The substantially higher stability (1.89 eV per cell) of the i-RH with respect to the i-SQ structure can be explained by electrostatics, due to the higher average distance between Ni vacancies in the former structure.

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TABLE 1: Optimized Calculated Geometry of c(4 × 2)-Ni3O4 (RH) Phase, and Comparison with Experimental LEED Data14 a coordinates (Å) calculated z ∆x ∆y experimental z ∆x ∆y

Ni(1)

Ni(2)

O

2.22 0.0 0.0

2.19 -0.17 0.0

2.09 0.07 0.12

2.099 ( 0.05 0.0 0.0

2.14 ( 0.03 0.1 ( 0.1 0.0

2.099 ( 0.05 0.16 ( 0.1 0.12 ( 0.1

a z is the distance from the surface plane of the substrate; ∆x and ∆y are the displacements with respect to the ideal positions (see Figure 1, which also provides the labeling of atoms).

When the two i-Ni3O4 monolayers are allowed to interact with the surface (but not to relax their internal coordinates), their ∆E′ values are changed to -44.44 and -43.42 eV, respectively. These data show the essential role of the surface in stabilizing these structures: the interaction energy of i-RH and i-SQ with the surface is 4.61 and 5.28 eV per cell, as compared to 1.6 eV for four NiO units of the full ML. Second, the Pd substrate appreciably reduces the difference in stability between RH and SQ, in agreement with the known fact that electrostatic interactions are partially screened at metallic surfaces. However, the RH arrangement is still favored over the SQ one by 1.0 eV per cell. We will come back to this point in section 4; in the rest of this section, only the RH structure is further considered. The internal coordinates of RH were optimized, with a gain in energy of 0.71 eV. Table 1 shows the main results concerning the equilibrium geometry. For comparison, data from LEED experiments are also reported.14 Consider first the O sublattice, consisting of symmetry equivalent atoms. Their distance from the surface is much smaller with respect to that of oxygens in the NiO-ML, 2.09 and 2.71 Å, respectively, consistent with the very different ionicity of the two species (see below). Oxygens are moving outward from the vacancy by 0.14 Å, approximately, to reduce their mutual electrostatic repulsion. These data are close to their experimental counterpart. For Ni atoms the agreement is not as good. In particular, LEED data indicate that Ni(2) is the farthest from the surface, while the other is at about the same height as O, while according to the calculations both Ni atoms remain higher. Also, the calculated shift of Ni(2) toward the vacancy seems to have no experimental counterpart. As concerns the electronic structure, the Mulliken net charges are q(Ni(1)) ) 1.75, q(Ni(2)) ) 1.71, and q(O) ) -1.38 au. As anticipated, the oxygen species in RH is less ionic than in NiO-ML [q(Ni) ) 1.73, q(O) ) -1.70 au]. On the whole, the epilayer is negatively charged [1.75 + (2 × 1.71) - (4 × 1.38) ) -0.35 au per unit cell], which is approximately compensated for by the positive charge (0.12 au) of the four Pd atoms in the surface layer. The Fermi energy is therefore slightly lower (by -0.1 eV) with respect to the bare slab, while it is appreciably higher (by almost 1 eV) than in p(2 × 2)-O. Other aspects of the electronic structure of the RH phase will be discussed in a specific forthcoming paper.22 4. Discussion The results presented in the previous section have allowed us to characterize the geometry and the electronic structure of the RH phase and have shown that it interacts strongly with the Pd surface, which may contribute to its stabilization. However, they also permit us to gain information about the

energetic balance of hypothetical surface reactions leading to its formation. The ∆E′ values previously reported can be used for this purpose. Consider first the tentative reaction (R1) describing the formation of RH starting from the two standard structures that encompass its stoichiometry, namely NiO-ML and p(2 × 2)-O:

3NiO[NiO-ML] + O[p(2×2)-O] f Ni3O4[RH] ∆E ) +0.26 eV (R1) The reaction is endothermic, which shows that the RH phase is intrinsically unstable, although by a small amount (2 kcal/ mol of NiO), with respect to the standard structures. The opposite process can then be thought to take place, whereby a patch of area A covered with RH reorganizes itself into two patches, one of the same area, covered with p(2 × 2)-O, and another of area (3/4)A, covered with NiO-ML. However, this cannot happen, precisely because such dissociation requires almost twice the free Pd surface as that occupied by RH, while in the initial phases of deposition the whole exposed Pd surface is covered by p(2 × 2)-O. It could be hypothesized, instead, that Ni atoms deposited on the oxidized surface (or on preexisting patches of RH) promptly react with the molecular oxygen in the chamber to form small, probably very defective quasi-stoichiometric NiO clusters, which are present only as a transient species. They should be very mobile at the annealing temperatures, and could be the source of oxide for a reaction (R2) formally similar to the previous one:

3NiO[cluster] + O[p(2×2)-O] f Ni3O4[RH] ∆E ) -0.96 eV (R2) It is important to observe that the surface area occupied by reactants and products is the same in this case. To guess the energetics of (R2), it has been assumed that the energy of NiO[cluster] is about the same as that of NiO[i-NiO-ML] (the isolated stoichiometric monolayer), because interaction of the clusters with the oxidized surface is expected to be very weak; in fact, they could be even less stable due to their high defectivity. With this assumption and using the results of the previous section, (R2) turns out to be exothermic by more than 7 kcal/mol of NiO; that is, the formation of RH from p(2 × 2)-O and NiO[cluster] at the surface is energetically favored. It is tempting to formulate a reaction mechanism according to which (R2) could occur. The growth model is described in Figure 6, which is now commented upon. RH is believed to grow at the expense of a preexisting p(2 × 2)-O phase (Figure 6a), by rows parallel to the short diagonal of the rhombus. According to our model, reaction R2 takes place at the border B between the two phases, a segment parallel to the x axis of Figure 6 (see Figure 6c,d): the quasi-stoichiometric NiO[cluster] (not shown in Figure 6), very mobile and loosely bound to the surface, provides Ni-O pairs that interact at B with surface Pd and with the oxygen atoms of p(2 × 2)-O. As more RH phase is produced, B progressively moves in the y direction. In RH the following sequence of rows can be recognized:

... β′′, R, β′, R, β′′, ... R is formed from parallel O-Ni pairs, with O on top of Pd and Ni at a hollow site nearby. In β′′, O-Ni pairs alternate with O atoms: all surface Pd atoms have an O on top, while nickel atoms occupy the hollow sites previously occupied by oxygens. β′ is similar, but Ni atoms are at those hollow sites that were previously unoccupied. Note that all these rows are

Study of c(4 × 2)-Ni3O4 on Pd(100)

Figure 6. Hypothetical reaction mechanism for reaction R2. The different panels show the progress of the formation of RH at the expense of p(2 × 2)-O phase. The Pd atoms of the surface are at the intersection points of the grid. (a) Preexisting p(2 × 2)-O phase. Open circles indicate oxygen atoms. (b) Beginning of a new RH patch. Gray circles are oxygen ions, small black circles are Ni ions, and the short lines are only a graphical help to recognize O-Ni pairs. (c) Growth of RH. The “inactive” border (B0) and “active” border (B) are indicated by dashed and dotted lines, respectively. (d) Further growth of RH. The lattice of Ni vacancies is indicated. For other explanations, and for the meaning of the Greek letters labeling the rows, see text.

polar, with the positive side oriented toward B. As suggested in Figure 6b, a new patch might start with a β′′ row. This is surely an activated process, and we are trying to set up simulations intended to evaluate its energetics. Once started, the growth of the new patch can go on more naturally, however. In Figure 6c, the formation of an R row and of a β′ row are shown. The former is made possible by the availability of a free strip of Pd surface, and by the possibility of exploiting a favorable balance of electrostatic interactions. In front of R, oxygens from the p(2 × 2)-O phase assume an ionic character and jump on top of Pd, to partially screen the positive end of the patch. In the interstices, O-Ni pairs from the cluster can enter, with Ni in front of the vacancies of the previously formed β′′ row, to improve their electrostatic stability: a β′ row is formed. The process can go on indefinitely (Figure 6d). Note that the growth is asymmetric since it cannot occur at the “inactive border” B0 for electrostatic reasons. Part of the model is also the hypothesis that starting a new row is an activated process, while its completion is relatively fast. While highly speculative, this model can justify quite a lot of experimental facts: (i) It is energetically justified. (ii) It explains the unusual stoichiometry of RH. (iii) It explains why in the STM images the borders between p(2 × 2)-O and RH are mainly straight lines oriented in a polar direction. The fact that NiO[cluster] is not visible in STM images taken after RD is in agreement with the present hypothesis that this is a reaction intermediate that rapidly disappears as soon as it is formed, and as long as there is p(2 × 2)-O available at the surface. The presence of extended B borders suggests that it is precisely there that RH is expanding at the expense of the

J. Phys. Chem. B, Vol. 109, No. 36, 2005 17203 oxidized surface. The straight character of B seems to confirm that reaction R2 proceeds much more rapidly parallel than perpendicular to the border; that is, when a new row of RH is started parallel to B, it goes on until completion before a new one is formed. The rhombic rather than square distribution of the Ni vacancies has been justified in the previous section on the basis of energetic considerations. The fact that the long diagonal of the rhombuses is consistently aligned perpendicular to the closest B line is a natural consequence of the hypothesized reaction mechanism, as shown in Figure 6. A few comments are finally appropriate concerning the defects that have been shown to be present inside or at the border of the otherwise very regular patches of RH (see Figures 4 and 5 and related comments under the section STM Results). Some of them correspond to the local presence of a square rather than rhombic arrangement of vacancies; more specifically, they have the appearance of a line defect parallel to the border, as for example in Figure 5b. While our calculations unambiguously indicate the higher stability of RH with respect to SQ, it might happen that, when a new β row begins to be formed, it starts with the “wrong” alternation of Ni atoms and vacancies: if so, it must continue like that until its completion. Healing this type of extended defect is of course very difficult for kinetic reasons. The “diagonal” line defect shown in Figure 5c presents the same orientation of the rhombuses at the two sides, and corresponds to the fact that β rows on one side are continued with R rows on the other. An analysis of its possible origin, performed according to the present growth model, shows that it is associated with a preexisting antiphase defect of the same type in the underlying p(2 × 2)-O phase: two patches of this phase are formed where the lines of the hollow sites containing the oxygens are continued on the other side of the defect by a line of free hollow sites. This perfectly explains the shape and size of the zigzag line along the structure. Another type of diagonal line defect is visible at the bottom left of Figure 4b, and more clearly in Figure 5d, and separates two patches with orthogonal orientation of the rhombuses. This is the case of a competition between the two patches that are expanding at right angle with respect to each other. Figure 5a shows still another case, a 4 × 1 line defect, where again the rhombuses are oriented at right angles at the two sides of the border B, but this is limited on one side (the right one, in this case), by a complete β row. Again, an interpretation of such a defect can be formulated in the light of the proposed growth model (see Figure 7). The patch on the right is the first formed one, starting precisely from B and growing toward the right. The patch on the left has started somewhere from a line perpendicular to B, and occupies all the available space still covered by p(2 × 2)-O. 5. Conclusions and Prospects The present study has tried to clarify some characteristics of the c(4 × 2)-Ni3O4/Pd(100) monolayer phase and to provide justifications for its formation, compatible with experimental evidence and computational results. According to the present model, Pd takes no active part in the growth of that phase, but its role is important for two aspects: (i) provision of an oxygen chemisorption structure with a precise distribution and density of surface oxygens, i.e., p(2 × 2)-O; (ii) interaction with the adsorbed species to make the formation reaction energetically feasible. Work is in progress to obtain a more complete description of the electronic features of this new phase, by using again a combined experimental and theoretical approach. Special at-

17204 J. Phys. Chem. B, Vol. 109, No. 36, 2005

Figure 7. Model of the line defect shown in Figure 5a. The rectangle in that figure corresponds to the one here. Symbols for O and Ni are as in Figure 6; for the sake of clarity some Ni vacancies are shaded. The direction of growth of the two patches is indicated by the two arrows. The one at the right has started first.

tention is being devoted to calibrate the computational tools that will permit us to accurately simulate STM images of such complex structures as the ones here described. Using the c(4 × 2) as the template for the creation of new ordered phases of oxides and metals is also envisaged in view of possible applications. Acknowledgment. This work has been funded by the European Community through the STRP GSOMEN (Growth and Supra-organization of Transition and Noble Metal Nanoclusters, Contract No. NMP4-CT-2004-001594) and by the Italian Ministry of Instruction, University and Research (MIUR) (Difettualita` e proprieta` catalitiche di film e di cluster superficiali, No. 031153). The experimental STM work has been supported by the Austrian Science Funds. References and Notes (1) Ventrice, C. A., Jr.; Bertrams, T.; Hannemann, H.; Brodde, A.; Neddermeyer, H. Phys. ReV. B 1994, 49, 5773. (2) Hildebrandt, S.; Hagendorf, C.; Doege, T.; Jeckstiess, C.; Kulla, R.; Neddermeyer, H. J. Vac. Sci. Technol., A 2000, 18, 1010. (3) Barbier, A.; Stanescu, S.; Boeglin, C.; Deville, J. P. Phys. ReV. B 2003, 68, 245418.

Agnoli et al. (4) Stanescu, S.; Boeglin, C.; Barbier, A.; Deville, J. P. Surf. Sci. 2004, 549, 172. (5) Wollschla¨ger, J.; Erdoes, D.; Goldbach, H.; Hopken, R.; Schroeder, K. M. Thin Solid Films 2001, 400, 1. (6) Lamberti, C.; Groppo, E.; Prestipino, C.; Casassa, S.; Ferrari, A. M.; Pisani, C. Phys. ReV. Lett. 2003, 91, 46101. (7) Luches, P.; Groppo, E.; D’Addato, S.; Lamberti, C.; Prestipino, C.; Valeri, S.; Boscherini, F. Surf. Sci. 2004, 566-568, 84. (8) Caffio, M.; Cortigiani, B.; Rovida, G.; Atrei, A.; Giovanardi, C.; di Bona, A.; Valeri, S. Surf. Sci. 2003, 531, 368. (9) Giovanardi, C.; di Bona, A.; Valeri, S. Phys. ReV. B 2004, 69, 75418. (10) Caffio, M.; Cortigiani, B.; Rovida, G. J. Phys. Chem. B 2004, 108, 9919. (11) Altieri, S.; Finazzi, M.; Hsieh, H. H.; Lin, H. J.; Chen, C. T.; Hibma, T.; Valeri, S.; Sawatzky, G. A. Phys. ReV. Lett. 2003, 91, 137201. (12) Sambi, M.; Sensolo, R.; Rizzi, G. A.; Petukhov, M.; Granozzi, G. Surf. Sci. 2003, 537, 36. (13) Orzali, T.; Agnoli, S.; Sambi, M.; Granozzi, G. Surf. Sci. 2004, 569, 105. (14) Agnoli, S.; Sambi, M.; Granozzi, G.; Atrei, A.; Caffio, M.; Rovida, G. Surf. Sci. 2005, 576, 1. (15) Agnoli, S.; Orzali, T.; Sambi, M.; Granozzi, G.; Schoiswohl, J.; Surnev, S.; Netzer, F. P. J. Electron Spectrosc. Relat. Phenom. 2005, 144147, 465. (16) Casassa, S.; Ferrari, A. M.; Busso, M.; Pisani, C. J. Phys. Chem. B 2002, 106, 12978. (17) Simmons, G. W.; Wang, Y. N.; Marcos, J.; Klier, K. J. Phys. Chem. B 1991, 95, 4522. (18) Saidy, M.; Warren, O. L.; Thiel, P. A.; Mitchell, K. A. R. Surf. Sci. 2001, 494, L799. (19) Todorova, M.; Lundgren, E.; Blum, V.; Mikkelsen, A.; Gray, S.; Gustafson, J.; Borg, M.; Rogal, J.; Reuter, K.; Anderse, J. N.; Scheffler, M. Surf. Sci. 2003, 541, 101. (20) Schoiswohl, J.; Agnoli, S.; Xu, B.; Surnev, S.; Sambi, M.; Ramsey, M. G.; Granozzi, G.; Netzer, F. P. Submitted for publication. (21) Surnev, S.; Vitali, L.; Ramsey, M. G.; Netzer, F. P.; Kresse, G.; Hafner, J. Phys. ReV. B 2000, 61, 13945. (22) Ferrero, M.; Ferrari, A. M.; Pisani, C. Manuscript in preparation. (23) Saunders, V. R.; Dovesi, R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Harrison, N. M.; Doll, K.; Civalleri, B.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL03 User’s Manual; Universita` di Torino: Torino, 2003. (24) http://www.chimifm.unito.it/teorica/crystal/Basis_Sets. (25) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (26) http://www.emsl.pnl.gov/forms/basisform.html. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (28) Perdew, J. P. Electronic structure of solids; Akademie Verlag: Berlin, 1991. (29) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (30) Kolthoff, D.; Jurgens, D.; Schwennicke, C.; Pfnur, H. Surf Sci. 1996, 365, 374. (31) Zheng, G.; Altman, E. I. Surf. Sci. 2002, 504, 253.