Pd(100) Ultrathin Films toward H2

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19066

J. Phys. Chem. C 2007, 111, 19066-19077

Enhanced Reactivity of NiO/Pd(100) Ultrathin Films toward H2: Experimental and Theoretical Evidence for the Role of Polar Borders Stefano Agnoli, Andrea Barolo, and Gaetano Granozzi* Dipartimento di Scienze Chimiche and Unita` di Ricerca INFM-CNR, UniVersita` di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy

Anna Maria Ferrari and Cesare Pisani Dipartimento di Chimica IFM and Centre of Excellence NIS (Nanostructured Interfaces and Surfaces), UniVersita` di Torino, Italy ReceiVed: August 7, 2007; In Final Form: October 10, 2007

NiO layers in the ultrathin regime exhibit an enhanced reactivity toward hydrogen with respect to the typical chemical inertness of bulk-like thicker samples. Such a behavior has been studied by means of photoemission (from both core and valence band levels) and quantum mechanical calculations. It is found that after H2 dosing in mild conditions (from PH2 ) 6.5 × 10-7 Pa and T ) 330 K) ultrathin films (thickness e6MLE, MLE ) monolayer equivalent) quickly react forming metal nickel and water. The kinetic of the reaction has been followed in situ recording the intensity of the O 1s and Ni 2p photoemission spectra under different reaction conditions (PH2 ranging from 5 × 10-7 to 2 × 10-5 Pa and T from 330 up to 453 K), and a firstorder dependence of the reaction rate on the PH2 and the activation energy of the rate determining step (0.16 ( 0.02 eV) have been determined. A 8 MLE thick film recovers the behavior of bulk-like NiO(100) surfaces, where more drastic reduction conditions are needed, and the kinetic implies an induction period followed by autocatalysis. The enhanced reactivity has been explained assuming the presence of NiO(100) islands exposing polar borders, whose existence was evidenced by previous scanning tunneling microscopy investigations. Such a scenario is confirmed by ab initio quantum mechanical calculations carried out employing polar and nonpolar terminated stepped surface epitaxially strained in order to account for the presence of the metal support which has not been explicitly included. Reported calculations indicate that the polar border can easily dissociate H2 without any activation barrier. The rate determining step of the reaction has been associated to the stage of the reaction where the previously formed hydroxyl groups react with a second hydrogen molecule (an Eley-Rideal-like mechanism) to form metal Ni islands and water, which readily desorbs.

I. Introduction One of the most relevant and fascinating issues in science is the understanding of the origin of the unusual properties exhibited by matter confined to nanoscale, such as nanoclusters, nanotubes, nanowires, and ultrathin films. Their peculiar properties have been demonstrated to be of great impact for many different application fields, e.g., microelectronics and photonics,1 energetics,2 magnetism,3 and catalysis.4 Besides the factors associated with the relevance that surfaces have in nanodimensional materials, another feature which can play a significant role is the interfacial strain, which can be particularly effective in the case of heteroepitaxial ultrathin films or supported nanoparticles.5 In this respect, some important studies have been focused on the relationships between strain and catalytic properties:6-8 as an example, it has been suggested that the observed enhanced reactivity of Au nanoparticles can be associated with the presence of highly strained sites.6,9 Morphology and defectivity represent other important factors that can directly influence the reactivity. As a matter of fact, it is well-known that the chemical reactivity is critically dependent on the type and number of surface sites (especially undercoor* To whom correspondence [email protected].

should

be

addressed.

E-mail:

dinated) or more generally on the crystallographic plane which is active for a certain reaction.10,11 NiO ultrathin films supported on Pd(100) represent an appropriate system where the interplay between strain and chemical reactivity can be investigated, as well as the relevance of morphological features. Actually, this epitaxial system is characterized by a large lattice mismatch (7.4%), and it has been recently studied in great detail from the monolayer (ML) regime up to its complete relaxation. In particular, it has been proved by scanning tunneling microscopy12 (STM), spot profile analysis (SPA)-low-energy electron diffraction13 (LEED), and anglescanned X-ray photoelectron diffraction14 (XPD) that NiO(100) ultrathin films on Pd(100) grow following a Stransky-Krastanov scheme,15 where strained NiO(100) 3D islands grow on top of a c(4 × 2) wetting ML. The relaxation of the NiO to the bulk lattice constant is progressive and is complete after 1012 ML equivalents (MLE; see section II for its definition). In addition, a detailed high-resolution X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) studies of the core and valence band (VB) electronic structure of such NiO ultrathin films have been recently reported,16 which, however, did not show straightforward and unequivocal trends to be directly associated with the evolution of their structure and morphology.

10.1021/jp0763174 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2007

Reactivity of NiO/Pd(100) Ultrathin Films In the present paper, we address the issue of clarifying how the strain and/or morphology can influence the chemical reactivity of such films. The reaction of NiO with hydrogen has been the object of many fundamental studies, assuming the status of a model system concerning the reduction of transition metal oxides. Up to now, several papers have been published17-22 giving the general idea that the NiO(100) surface in its stoichiometric form is a rather unreactive material: actually, Furstenau et al.18 have observed that the exposure of a UHV cleaved NiO(100) surface to 10-4 Pa of H2 at 420 K does not induce changes, neither in LEED nor in XPS and Auger electron spectroscopy (AES) signals, no matter what the actual hydrogen dose employed. Relatively severe conditions are needed in order to observe the reduction (e.g., 10-5 Pa of H2 at 620 K18 or 10-2 Pa at 625 K19). There is now a general consensus23,24 on the interpretation of the NiO reduction by hydrogen as a direct NiO f Ni transformation without the formation of any intermediate phase. An induction period is needed to trigger the onset of the reaction, when surface defect sites are thermally created which eventually promote the dissociation of H2: O vacancies produce an increase of the H2 adsorption energy and a lowering of the activation energy associated with the cleavage of the H-H bond. A direct correlation was observed between the concentration of O vacancies in the NiO lattice and the rate of the oxide reduction. In this paper, we will show that, according to both core and VB photoemission data, an enhanced reactivity is observed on NiO ultrathin films grown on Pd(100), even in very mild experimental conditions, which is mainly attributed to the presence of NiO(100) islands exposing polar borders. The interpretation of such enhanced reactivity is based on the comparison of the reactivity for ultrathin films having different thickness and by ab initio quantum mechanical calculations. II. Experimental and Computational Models and Techniques II.1. Experimental Procedures and Equipment. Sample treatments were performed in an UHV preparation chamber at a base pressure of 5 × 10-9 Pa. The Pd(100) crystal was cleaned by repeated cycles of argon ion sputtering (E ) 2 keV) and annealing at T ) 970 K with a final flash in 1 × 10-4 Pa O2 (uncorrected ion gauge reading). The cleanliness of the substrate surface prior to the experiments was always checked by using XPS and the order of the surface by means of LEED. The procedures for growing epitaxial NiO layers on the Pd(100) substrate have been extensively described in previous reports, and the influence of the different routes on the morphology and defectivity has been studied in detail by STM and SPA-LEED.12,13 The samples investigated in the present paper were obtained following a reactive deposition (RD) procedure: Ni was deposited at 523 K by means of an electron beam evaporator (Omicron EFM) in an oxygen partial pressure of 2 × 10-4 Pa. The deposition rate of NiO was estimated to be ∼0.5 ( 0.1 Å/min, as determined by angle resolved XPS. Thickness values, expressed in MLE, are calculated assuming an interlayer distance between adjacent planes in NiO(100) equal to 2.09 Å. The conditions chosen for the growth are those which, according to previous experience, maximize the occurrence of polar borders in the NiO(100) islands for thicknesses up to 6 MLE.12 The hydrogen dose is reported in Langmuir (L, 1 L ) 10-4 Pa·s exposure). The conditions used to study the reactivity toward hydrogen will be detailed in section III. To prepare the films and to carry out photoemission experiments (XPS and UPS measurements), UHV equipment (a

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19067

Figure 1. Schematic representation of the model systems adopted. The top panel depicts the NP border, the bottom panel the O-terminated PO polar border; Ni-terminated PNi polar borderis derived from the PO model by removing edge oxygens. OB and NiB label ions at the border of the edge, OA and NiA ions in the row adjacent to the border.

modified VG ESCALAB MK II ,vacuum generators, Hastings, England) has been employed where a four grids rear view LEED, an electron beam evaporator with an integrated flux monitor, a mass quadrupole, a twin (Mg/Al) anode X-ray source, a discharge lamp for noble gas ionization (VUV HIS 13 Omicron discharge lamp), a sputter gun, and a hemispherical electrostatic analyzer ending with a five channeltrons detector are incorporated. The angular acceptance of the analyzer can be varied between 1.5° and 8° (the latter used for UPS experiments). The binding energy (BE) calibration was determined using the Fermi edge (EF) and 4f peaks of a gold sample. II.2. Computational Models and Techniques. All ab initio calculations have been performed by means of the CRYSTAL06 periodic code,25 by adopting a periodic slab model and a hybridexchange density functional theory (DFT) Hamiltonian and using a localized basis set of Gaussian type functions (GTF). The reactivity of polar (P) and nonpolar (NP) borders of the NiO(100) islands growing at the Pd(100) surface has been considered. Borders have been described employing models derived from stepped surfaces, the for a NP model and the surface for a P model, Figure 1. Note that the two P model can be distinguished, i.e., the one whose border is O-terminated, PO, or Ni-terminated, PNi. In Figure 1 and in the text, the following labels are assumed: OB and NiB indicate ions at the border of the edge and OA and NiA ions in the row adjacent to the border. The presence of the Pd substrate has not been explicitly taken into account. The justification for that rests on the consideration based on previous theoretical investigations that the influence of the metal support is strictly limited to the oxide layer of the interface, whereas it quickly vanishes starting from the second layer.26,27 Moreover, in the case of silver supported MgO, the chemical activity of the oxide borders has been recently found to be qualitatively unchanged by the explicit introduction of the metal substrate in DFT calculations.28

19068 J. Phys. Chem. C, Vol. 111, No. 51, 2007 However, the influence of the metal support on the strain of the ultrathin film has not been ignored: the NiO lattice parameter has been varied in order to test the role of the strain on the chemical reactivity of the islands borders. To mimic the situation without a strain release, the NiO parameters have been fixed to match the dimensions of the Pd substrate (computed lattice parameter a0 ) 3.93 Å), whereas two cases with a total (at the NiO bulk lattice parameter a0 ) 4.19 Å) or partial (a0 ) 4.09 Å) strain relief have been also explored. It is to be noted that the experimental data tell us that in the range of 3-4 MLE thick NiO films the residual strain is almost reduced to a half.12,13 Geometry optimizations have been carried out employing analytical gradients as recently implemented in CRYSTAL06,29 and have concerned, apart from the atoms from the hydrogen molecule, all O and Ni ions at the edge of the step and Ni and O ions in the row adjacent to the edge. Within a DFT approach, the use of hybrid functionals is required in order to account for the localized nature of the unpaired electrons in magnetic systems. The B3PW functional has been selected that includes a certain amount (20%, in the present case) of exact Hartree-Fock exchange in the Becke exchange functional30 and uses the correlation functional of Perdew-Wang31 since it was found to be a good compromise between the need, from one side, to account for the localized nature of the unpaired electrons in magnetic systems32-34 and, from the other side, to furnish an accurate description of the energetics and of the electronic and structural properties of the system. Ni has been described with a Hay-Wadt small core pseudopotential with the associated basis set derived from the standard Hay-Wadt double zeta basis (HW-DZ),35 where the s and p most diffuse functions have been removed and the d and p exponents of the outermost GTF have been optimized for the Ni+ ion (Rp ) 0.25 Rd ) 0.2836). The basis set of O ions (an all-electron 8-411G basis function) is the same as previously employed.36-38 However, in order to furnish a better description of the energetics of the bond, the oxygen in the hydroxyl and Ni in the hydride groups has been augmented by a d polarized function (Rd ) 0.8) and by a p function (Rp ) 0.2), respectively. This also favors charge localization and change in the electron configuration for Ni atoms: the same augmented basis set has been also employed for O and Ni ions directly connected to the hydroxyl or to the hydride groups. In all cases, symmetric adsorption on both sides of the slab has been considered. All spin-unrestricted calculations have been performed by considering the ferromagnetic (FM) arrangement of the NiO film. With the present computational scheme, the properties of bulk NiO and of the perfect NiO(100) surface are well reproduced (see previous papers for a detailed discussion).34,36 III. Experimental Results In order to investigate the interplay between strain and morphology, ultrathin films of different thickness have been investigated, namely 3.5, 4.5, and 8 MLE. In a first set of experiments (sections III.1, III.2, and III.3), these films have been exposed to hydrogen (PH2 ) 2 × 10-5 Pa, mass quadrupole reading) at 390 K for a variable time and, after each dose, the photoemission spectra have been taken at room temperature (RT). It is worth mentioning that the NiO films are stable at 390 K in UHV for days, so no spurious effects (i.e., annealing induced reduction) affect our data. In a subsequent set of experiments (see section III.4), kinetic data of the reduction reaction in a wide range of hydrogen pressures and temperatures have been obtained.

Agnoli et al. III.1. Films of 3.5 MLE Thickness. The LEED image of the as deposited 3.5 MLE NiO ultrathin film is reported in Figure 2a: the (1 × 1) pattern is clearly visible, but the diffraction spots are not sharp and present some typical halos pointing toward the center of the Brillouin zone which indicate the presence of residual strain.13 It can also be noted that there is no trace of the c(4 × 2) spots (not even a more intense background) indicating that the interfacial layer is completely buried, as expected in correspondence of this coverage. The changes induced in such a film after hydrogen dosing (from 50 to 1200 L; PH2 ) 2 × 10-5 Pa, T ) 390 K) have been monitored by photoemission spectroscopy (both XPS and VB, see Figure 2b-d). The evolution of the O 1s spectrum is quite informative (Figure 2b): even though a detailed analysis is hampered by the overlap with the Pd 3p3/2 peak, it can be seen that even after the exposure of a small quantity of hydrogen (50 L) there is a rapid reduction of the O 1s signal, which further decreases as the dose increases. The overlap with the Pd 3p3/2 peak does not allow us to give hints about the possible formation of hydroxyl groups which, according to literature data, should be situated at about 531.4 ( 0.1 eV.39 However, we can safely exclude the residual presence of chemisorbed water whose O 1s states should be found at about 533.2 ( 0.4 eV. The Ni 2p spectrum just after the deposition is presented at the bottom of Figure 2c: the line shape is typical of bulk-like NiO, with the only exception being the lack of the shoulder at 1.2 eV from the Ni 2p3/2 maximum, characteristic of a stoichiometric and ordered NiO. For a detailed explanation of all of the features, we redirect the reader to our previous paper.16 As a function of the hydrogen dose, some distinct changes are visible: in particular, we observe the formation of a small shoulder on the lower BE side of the Ni 2p3/2 peak which can be ascribed to the formation of a metallic component, which progressively becomes dominant over the component due to the oxidized fraction. The UPS photoemission data are in tune with the core level data. The spectrum reported at the bottom of Figure 2d shows the VB region of the NiO ultrathin film prior to hydrogen dosing: the peaks at 1.9 and 3.7 eV can be assigned to the multiplet structure of the Ni 3d8 states, whereas the features centered at 5.3 and 7.15 eV are related to the O 2p π and σ bands, respectively.16 The broad low-intensity feature between 9.5 and 11.5 eV corresponds to the Ni 3d satellite, usually attributed to emission from Ni 3d8 states characterized by an unscreened 3d7 final state.40 The only remarkable difference with respect to the VB data of the NiO(100) surface is the intense component just below EF, which, as already discussed elsewhere16 is not due to the presence of defects (i.e., metallic Ni) but to the 4d states of the underlying Pd substrate. As a consequence of the hydrogen dosing, an intense peak just below EF is rapidly increasing, becoming eventually the most prominent feature, whereas the two peaks related to the O 2p slightly decrease in intensity. As a final comment, it is worth mentioning that the BE position of the different peaks does not change during the hydrogen exposure. III.2. Films of 4.5 MLE Thickness. In Figure 3a, we report the LEED pattern of the ultrathin film prior to the hydrogen dosing: it shows a (1 × 1) pattern similar to the one reported for the 3.5 MLE film. Figure 3b-d reports the photoemission spectra of a 4.5 MLE NiO film as a function of different hydrogen exposure, namely 50, 100, 200, 400, 800, 1600, 2400, and 3400 L, at 390 K. The photoemission data are very similar to the ones obtained in correspondence to the lower coverage film: the O 1s photoemission line smoothly decreases for increasing hydrogen dose,

Reactivity of NiO/Pd(100) Ultrathin Films

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19069

Figure 2. Reactivity of a 3.5 MLE NiO(100) ultrathin film on Pd(100) for different hydrogen exposures: (a) LEED pattern (Ek ) 74 eV) of the as deposited film; (b) O 1s and Pd 3p3/2 photoemission region (source: Al KR; take off angle 65° from surface normal); (c) Ni 2p photoemission region (source: Mg KR; take off angle 65° from surface normal); (d) VB photoemission spectra (source: He II line; take off angle 45°). The spectrum relative to the clean substrate is reported at the bottom of panels b and d; all the remaining spectra have been labeled according to the hydrogen dose.

pointing to a progressive removal of O atoms, whereas in the Ni 2p spectra, it is possible to observe the formation of a small shoulder on the lower BE side of the Ni 2p3/2 peak which can be ascribed to the formation of a metallic component. The VB data evidence the reactivity of the NiO layer as well: just below EF, the peak related to the presence of metallic Ni rapidly rises as function of hydrogen dosing and the O derived features slightly decrease in intensity. With respect to the data taken with the film of the lower thickness, the fraction of the film which has undergone the reaction is lower, as can be qualitatively seen by the lower intensity of the features associated with the metallic phase. III.3. Films of 8 MLE Thickness. The LEED pattern of the 8 MLE film (Figure 4a) has a low contrast and presents diffraction spots characterized by a diffuse quincunx (i.e., a dot

at the center of a square) shape, which indicates the presence of some mosaics and a thickness dependent strain, in agreement with the previous analysis done by SPA-LEED.13 The photoemission data as a function of hydrogen dosing are reported in Figure 4b-d. It can be seen that, for the as deposited 8 MLE film, the fingerprint of all photoemission data is typical of the bulk-like NiO(100) surface; that is, the fine structure of the Ni 2p3/2 peak is clearly resolved, in perfect agreement with the data reported in literature for NiO stoichiometric single crystal;41 similarly, the VB spectra reproduce very well the electronic structures of the NiO(100) surface with the pertinent band gap.42 As it can be seen by the entire set of the experimental data reported in Figure 4b-d, no change is detectable by photoemission, no matter the amount of gas dosed, so we can with

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Figure 3. Reactivity of a 4.5 MLE NiO ultrathin film on Pd(100) for different hydrogen exposures: (a) LEED pattern (Ek ) 74 eV) of the as deposited film; (b) O 1s and Pd 3p3/2 photoemission region (source: Al KR; take off angle 65° from surface normal); (c) Ni 2p photoemission region (source: Mg KR; take off angle 65°); (d) VB photoemission spectra (source: He II line; take off angle 45°). The spectrum relative to the clean substrate is reported at the bottom of panels b and d; all the remaining spectra have been labeled according to the hydrogen dose.

certainty conclude that under the mild explored conditions the NiO ultrathin films in this thickness region, despite the presence of mosaics, residual strain,13 and epitaxial defects, are completely inert to hydrogen. III.4. In Situ Kinetic Data. The reaction of the 3.5 MLE films in a wide range of pressure and temperature has been followed in situ in order to extract kinetic data. As an example, we report in Figure 5 the data of the Ni 2p3/2 (Figure 5a) and O 1s (Figure 5b) photoemission peaks as a function of dosing (hence time) at the following experimental conditions: PH2 ) 6.5 × 10-7 Pa at 393 K. The spectral changes are shown in a 2D plot where the intensity of the photoelectron signal is given by a color scale, where the yellow represents the maximum value and the black the minimum one. From the whole set of data, we can extract relevant kinetic information. Let us consider first the O 1s peak: the problems with the overlap between the Pd 3d3/2 peak and a possible contribution due to the presence of hydroxyls can be partially circumvented by renormalizing all of the spectra to the same

value of the Pd 3p1/2 peak (which has no overlap problem) and taking the difference spectrum with respect to the as deposited film.43 Similarly, if we normalize the Ni 2p spectra to the same integral intensity (no Ni atom is removed from the surface, but we must consider the same caveat reported before43 connected to the change in the electron attenuation length due to the oxygen removal) and we take the difference spectrum with respect to the as deposited film, we can put in evidence the decrease of the oxide component and the formation of the metallic one. Plotting the intensity of the difference spectra peaks as a function of the hydrogen dose (i.e., time; see for example the data reported in Figure 5c, obtained in the following conditions: PH2 ) 6.5 × 10-7 Pa, T ) 393 K), one can obtain the kinetic profile of the reaction. The important outcome is the following: the two determinations obtained from the Ni 2p and the O 1s data are well in agreement since they both show a fast reaction rate in the early stage of dosing which progressively slows down as the reaction proceeds. This trend can be better observed in the data related to an experiment where hydrogen dosing was carried

Reactivity of NiO/Pd(100) Ultrathin Films

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19071

Figure 4. Reactivity of a 8 MLE NiO ultrathin film on Pd(100) for different hydrogen exposures: (a) LEED pattern (Ek ) 77 eV) of the as deposited film; (b) O 1s and Pd 3p3/2 photoemission region (source: Al KR; take off angle 65° from surface normal); (c) Ni 2p photoemission region (source: Mg KR; take off angle 65°); (d) VB photoemission spectra (source: He II line; take off angle 45°). The spectra reported are labeled with the total hydrogen dose.

out at 453 K with PH2 ) 10-6 Pa, reported in Figure 5d. Here in the early stage of the reduction, a fast linear regime is present followed by a progressive decrease of the reaction rate and finally by a smooth plateau. This behavior is completely different (almost perfectly reversed) from what was observed in the case of the bulk-like stochiometric NiO(100) surface, where an induction period is required to start the reaction and the transformation rate increases as the metallic phase is nucleated (autocatalysis).23,24 From the systematic analysis of several experiments, it has been possible to study the effect of hydrogen pressure on the reaction rate. The reaction rate was determined from the XPS signal of the metallic component of Ni 2p peak and from the O1s peak in the initial stages of the reaction where a linear fit of dI/dt vs time is possible, as is evident from the previous

results (Figure 5c,d). This value represents also the maximum value of the reaction rate at the respective conditions. The use of the reaction rate in the initial stages in addition avoids problems deriving from other reduction channels which can be active when the oxide is highly reduced and a relevant Ni metallic phase is nucleated. Figure 6a shows the behavior of the reduction rate for different hydrogen pressures at the same temperature (393 K). Ni (black triangle) and O (red triangle) derived determinations are well in agreement between them and can be nicely fitted by a line whose slope results to be 0.95 ( 0.1; therefore, the reaction appears to have a first-order dependence with respect to the hydrogen pressure. The other important parameter describing the reaction is the activation energy, which can be obtained by a plot of the reaction

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Figure 5. 2D intensity plot of the (a) Ni 2p3/2 and (b) O1s XPS peaks as a function of hydrogen dosing. The intensity of the photoelectron signal is given by a color scale where the yellow represents the maximum value and the black the minimum one. (c) reaction profile determined using XPS (black triangle, Ni 2p; red triangle, O1s) at PH2 ) 6.5 × 10-7 Pa, T ) 393 K (see text for the procedure); and (d) at PH2 ) 10-6 Pa, T ) 453 K.

rate vs 1/kT while keeping constant the hydrogen pressure (in our case PH2 ) 10-6 Pa). The data were taken in a temperature range between 330 and 453 K, the upper value being limited by the intrinsic stability of the film, which at temperatures higher than 473 K starts to undergo interface diffusion and spontaneous oxygen removal. In Figure 6b, the experimental data are reported; also in this case they can be fitted by a line and the activation energy results to be 0.16 ( 0.02 eV (3.7 kcal mol-1). This is an extremely low value as compared to the energy barrier calculated for the reaction of hydrogen dissociation on the NiO(100) surface (16 kcal mol-1)23 or determined experimentally on the NiO/Ni(100) system (23 kcal mol-1).22 IV. Computational Results The results of several numerical experiments aimed at exploring the chemical reactivity of the borders of NiO(100) islands toward hydrogen, described with strained models, are reported in Table 1. The structural parameters and the energetics of each model are reported in the table and described in the following. We first consider the interaction of H2 with a completely strained PO border. Due to the low coordination of the OB ions, this system is expected to exhibit a high reactivity, as recently demonstrated in the case of a MgO submonolayer grown at a silver surface.28 The electronic structure at the border is very

different from that of the regular NiO(100) slab;34 in particular, the ionic character is considerably decreased: according to the Mulliken analysis,44 the net charges of OB and NiA are reduced from (0.98 to -0.43 and +0.71 au, respectively, and also, the spin population on NiA is lowered from 1.76 to 1.15 au. According to the present results, homolytic hydrogen dissociation can easily occur at a pair of adjacent OB ions (see the following reaction R1a)

H2 + 2NiA-OB f 2NiA(OBH)

(R1a)

which produces two adjacent hydroxyl groups -OBH: the resulting product compound has been named PO-R1a (the name is composed by the label of border and by the actual reaction occurred), and it is schematically illustrated in Figure 7. The process is highly exothermic (∆Ediss ) -3.66 eV per H2 molecule) and occurs without any energy barrier. This is not surprising since hydroxylation has been reported as an efficient way to stabilize a polar surface.45 On the contrary on bulk NiO, the dissociation of hydrogen is highly endothermic and has been calculated to have an activation barrier of 16 kcal mol-1 for a stoichiometric surface and of 8 kcal mol-1 if it involves oxygen vacancies.23 The d(OB-H) bonding distance (0.957 Å) is close to the value in the water molecule and to that of hydroxyls derived from water dissociation at the border of silver supported MgO islands.28

Reactivity of NiO/Pd(100) Ultrathin Films

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19073 TABLE 1: Main Features of the Systems Resulting from the Interaction of Hydrogen with the P or NP Bordersa PO-R1a d(NiB-H) d(NiA-H) d(OB-H) ∆Ediss q(NiB) q(NiA) q(OB) q(OA) µ (NiB) µ (NiA)

PO-R1b

0.957 -3.66

-1.424 0.960 -0.81

0.87 -0.80

0.60 -0.63

PNi-R1c 1.835 -2.32 0.68 -0.82 1.71

1.74

NP-R1d 1.567 0.975 0.15 0.60 -0.67 1.64

1.064

a

The columns refer to a model whose name indicates the specific border on which H2 reacts (PO, PNi, and NP) and the reaction through which the final system is produced (R1a-d, see text). The dissociation energy ∆Ediss is expressed in eV, with reference to the PO, PNi, NP plus H2 subunits; q (Mulliken net charges) and µ (spin populations) are in au; distances in Å. OB or NiB indicate ions at the edge of the step, and OA or NiA, ions in the adjacent row, see also Figure 1. Data refer to fully strained models (a0 ) 3.93 Å).

However PNi-R1c is not the most stable product. The system can further evolve forming -OBH hydroxyls and metallic Ni adsorbed (Ni)met,ads at the NiO surface (PNi-R1c′ in Figure 7) according to the reaction

Figure 6. (a) Plot of the reaction rate (dINi2p/dt, black triangle; dIO1s/ dt, red triangle) vs log PH2 (T ) 393 K) (b) Arrhenius plot of the of the reaction rate (dINi2p/dt, black triangle; dIO1s/dt, red triangle) vs 1/kT (PH2 ) 10-6 Pa). The slopes obtained by the linear fit are reported in the graphs.

It can be observed that after reaction the ionic character at the border is almost completely restored. The PO edge can also dissociate H2 through a heterolytic route involving the OB and the NiA ions resulting in a product labeled in Figure 7 as PO-R1b, according to the corresponding reaction

H2 + NiA-OB f (NiAH)-(OBH)

(R1b)

The process produces -OBH hydroxyl groups and terminal NiAH hydrides. The reaction is also exothermic (∆Ediss ) -0.81 eV per H2 molecule) but considerably less with respect to the homolytic path. The reason is the scarce reactivity of NiA ions: indeed calculations indicate that a hypothetical hydrogen dissociation producing only NiAH hydride (with the same geometry as in PO-R1b) would be highly endothermic, by more than 3 eV. The PNi model is also characterized by a reduced ionicity at its border (the net Mulliken charge of NiB is reduced from +0.98 in the perfect slab to +0.50 au) but is not as reactive as PO. It can dissociate exothermally hydrogen (∆Ediss ) -2.32 eV per H2 molecule) producing a bridging hydride (Hb) according to the following reaction:

The study of this process is not feasible with the present models because of the limited width of the terrace of the step that in this case determines spurious interactions between adsorbates on adjacent terraces. Work is in progress to investigate this process using more suitable models. A rough estimate of the reaction energy, based on the assumption that (Ni)met,ads has about the same energy as bulk Ni, gives a ∆E value of -0.4 eV. The activation energy is expected to be rather low, because Hb is close to the oxygen with which it will eventually form the hydroxyl bond. We shall come back in the following to this crucial reaction step. From the results so far reported, it is clear that on both the PO and PNi borders the channel for the hydrogen dissociation follows a homolytic path and the reactivity is strictly limited to the ions of the edge. The results so far discussed have been obtained on a completely strained border. However, a partial release of the strain has been experimentally observed, and it may have a non-negligible effect on the reactivity of the surface species. An accurate analysis of the relation strain reactivity is not a simple task since it depends on the thickness of the film and therefore on the number of the NiO layers included in the model. However, results from a preliminary analysis are summarized in Figure 8. For the bare models PO and PNi, the optimized lattice constant is a 4.09 Å intermediate between the Pd (a0 ) 3.93 Å) and the NiO bulk (a0 ) 4.19 Å) values. Notice that the optimized lattice constant depends on the thickness of the model: it increases with the number of the NiO layers included in the model up to the bulk value. In the condition of a0 ranging between 3.93 and 4.09 Å, the present models can be adequately employed to mimic NiO thin films from a situation in which they are completely strained up to situations in which the strain has been partially released. The lattice parameter affects significantly the stability of the models: at a0 ) 4.09 Å both PO and PNi are stabilized by 1.0 and 0.8 eV, respectively.

19074 J. Phys. Chem. C, Vol. 111, No. 51, 2007

Agnoli et al.

Figure 7. Schematic representation of the energetics and of the structure of the reaction products resulting from the interaction between the H2 molecule and the P and NP borders. The name indicating the products is composed by the specific border on which H2 reacts (PO, PNi, and NP) and the reaction through which the final system is produced (R1a-d, see text). Dissociation energies are computed with respect to the non interacting subunits (horizontal line). In the picture of PNi-R1c′, a larger Ni atom has been displayed in order to outline its metallic nature.

Figure 8. Schematic description of the dependence of the total energy of the stepped model described in Figure 1 on the lattice constant adopted in the calculations. Solid lines are for the bare PO (blue) and PNi (red) models. Dotted lines described the corresponding hydrogenated products, PO-R1a and PNi-R1c. The double vertical arrows indicates the hydrogen dissociation energies per O-H (or Ni-H) bond.

However, the formation of the hydride species in reaction R1c is only marginally affected by the strain release: the two curves PNi and PNi-R1c run parallel and the corresponding ∆Ediss is almost unchanged with a0 (at a0 ) 4.09 Å, it is reduced by 0.16 eV); on the contrary, ∆Ediss of PO-R1a is slightly favored by the elongation of the lattice constant (by 0.2 eV at a0 ) 4.09 Å). Finally, it is worthwhile to note that also ∆Ediss depends on the thickness of the models that has been evaluated to account for 0.3 eV in the energy estimates. Finally, we turn to the NP borders in order to compare the data with those so far reported. The low coordination of the border ions also in this case is reflected in a reduced ionicity (the charge is reduced by 0.1 and by 0.15 au for NiB and OB) even though less pronounced than in polar cases.46,47 Heterolytic hydrogen dissociation can occur at a pair of adjacent border ions according to the reaction

H2 + NiB-OB f (NiBH)-(OBH)

(R1d)

which produces a hydroxyl -OBH and a -NiBH hydride (see NP-R1d in Figure 7 and Table 1). The dissociation is slightly endothermic (∆Ediss ) 0.15 eV). To be noted are the NiB-H and OB-H distances, appreciably longer than in the case of the

heterolytic dissociation at the PO borders. Taking into account the possible energy gain from a partial strain release, it cannot be excluded with certainty that H2 dissociation at NP borders could be exothermic. However, from the whole set of the reported calculations, it is evident that hydrogen dissociation occurs preferentially on P than on NP borders. We now discuss the possible fate of the dissociated hydrogen (present as hydroxyls). From the experimental data reported in the previous section, we cannot exclude the presence of hydroxyls, but we can exclude that molecularly adsorbed water is remaining at the surface. Because we detect clearly an oxygen removal, we assume that some easy water desorption process should follow the hydrogen dissociation. The progressive removal of oxygen is accompanied by the formation of metallic Ni. Let us start by discussing a possible scenario starting from the highly improbable dissociation on the NP border. Assuming the presence of dissociated hydrogen on NP borders, water formation would involve condensation between two adjacent hydroxyl and hydride groups according to a LangmuirHinshelwood-like mechanism,48 that is independent of hydrogen pressure, contrary to the experimental evidence. The process would lead to the disruption of the lattice network with formation of oxygen vacancies. Hypothesizing that one of two OB leaves the surface to form water, as sketched in Figure 9d, the reaction is highly endothermic, ∆Eform ) 1.6 eV. Again, this result excludes that NP borders can have an important role in the processes observed. As concerns P borders, we have considered a few mechanisms as possible follow up of the very stable PO-R1a intermediate

NiA-OBH + NiA-OBH f H2O(g) + NiB-O-NiB

(R2a)

2NiA-OBH + H2(g) f 2H2O(g) + (NiB)2

(R2b)

NiA-OBH + H2(g) f H2O(g) + NiB Hb

(R2c)

According to the first one (R2a) (Langmuir-Hinshelwood-like), two vicinal NiA-OBH groups would couple with the formation of H2O(g) and restoring the PO border, Figure 9a. The process is found to be highly endothermic, ∆Eform ) 2.7 eV per molecule, since it requires the breaking of the strong OB-H bond. Moreover, the process should not depend on the H2 pressure, in disagreement with the observations (see section III.4).

Reactivity of NiO/Pd(100) Ultrathin Films

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19075

(Ni-O)nNiA-OBH + H2(g) f H2O(g) + (Ni-O)n-1NiA-OBH + (Ni)met,ads (R2c′) That is, reaction with hydrogen produces gas-phase water and metallic Ni, while the polar terraces are progressively eroded; their hydroxylated PO character is preserved, so that the reaction can go on indefinitely until the whole polar terrace is consumed. As anticipated, the simulation of reaction R1c′ is not feasible with the present models. However, the global energetic balance of R2c′ can be obtained from the simplified expression

(NiO)ads + H2(g) f H2O(g) + (Ni)met,ads If one approximates the thermodynamic properties of (NiO)ads and of (Ni)met,ads with those of the corresponding crystals, (NiO)cry and (Ni)cry, respectively, it comes out that the standard enthalpy is practically zero, whereas the free energy of reaction is slightly negative (∆G ) -0.17 eV at 300 K, -0.22 eV at 400 K). V. Discussion

Figure 9. Schematic representation of reactions R2x. (a) Two adjacent hydroxyls condense to form water. The PO border undergoes a sort of nonpolar reconstruction. (b) Two adjacent hydroxyls react with a H2(g) molecule to form water and a PNi bare border. The water is bound to the surface by almost 1 eV. (see the transformation c f c′) A hydroxyl reacts with H2(g) to form water and a bridging hydride PNi, evolving to hydroxylated PO plus adsorbed metallic Ni. (d) A hydroxyl and the adjacent hydride condense to form water. An oxygen vacancy is created in the lattice. ∆E (and ∆G) values per water molecule produced in the process are expressed in eV.

The two mechanisms R2b and R2c (Eley-Rideal-like)48 involve the direct interaction of one or two adjacent NiA-OBH groups directly with a H2(g) molecule leading to the formation of water. In R2b, the interaction requires the homolytic splitting of a H2 molecule on a pair of adjacent hydroxyls giving rise to the formation of two water molecules (one molecule per surface hydroxyl) plus a bare PNi border (NiB)2, Figure 9b. The process is globally slightly endothermic, ∆Eform ) 0.4 eV and produces water molecules strongly bound to the surface (their desorption requires about 1 eV per molecule); moreover, any attempt to identify a reasonable reaction path starting from the homolytic splitting of H2 on adjacent hydroxyls failed because of the scarce tendency of -OBH groups to further receive electron charge. According to the other reaction (R2c), the interaction of a NiAOBH group with a H2(g) molecule results in heterolytic splitting and in the formation of the bridged hydride (NiB)2Hb, Figure 9c. The reaction is slightly endothermic, ∆Eform) 0.2 eV, and in any case, it is much more favored with respect to R2b. From the data in Figure 8, it is possible to estimate the ∆Eform value for a different lattice constant. For instance for a0 ) 4.05 Å (corresponding to a strain release of 50%) ∆Eform ) 0.4 eV. It is important to observe that the product of reaction R2c is equivalent to PNi-R1c of Table 1 and Figure 7, so that the step R1c′ already discussed can follow the R2c. Combination of R2c with R1c′ then gives (see Figure 9c)

The abundance of small islands (a byproduct of nanodimensional confinement) is intrinsically a source of an enhanced chemical reactivity, as a consequence of the large quantity of low coordination sites present at the borders. This has been recently explored with numerical experiments for reactions at borders of MgO(100) ultrathin islands supported on a Ag(100) substrate:28 it has been clearly indicated that the enhanced reactivity of the borders is not related to a variation in the electronic structure induced by the interaction with the metal support, but it is intrinsically connected to the nanodimensionality of the islands. This is even more relevant when considering 3D islands of three-four atomic layers having a pyramidal shape, since the variation in the electron properties induced by the metal support are strictly confined at the vicinity of the interface. Among the possible borders of nanoislands, the P ones are intrinsically less stable than the NP ones. The P borders imply the presence of rows of ions of the same type with a resulting high dipole moment across them, with a destabilizing effect with respect to islands exhibiting NP borders. However, their presence has been theoretically justified considering the interaction with the metal substrate that provides a way to redistribute and to screen the electron charge and largely contributes to the reduction of the instability of the P borders.46,47 This stabilization mechanism, active when the islands are in the ML regime, might be active for kinetic reasons even in ultrathin films of higher thickness, as demonstrated by their appearance which is strongly dependent on the actual preparative route. As a matter of fact, in a previous extensive investigation, we have demonstrated that NiO(100)/Pd(100) ultrathin films having a nominal coverage up to 5-6 MLE present small oxide islands, that, depending on the preparative routes, may exhibit a high density of borders aligned along the (100) or along the (110) crystallographic directions (P borders). This has been also reported in the case of MgO(100) ultrathin films on Ag(100),49 and similarly for the NiO(100)/Ag(100) system.50,51 It is then of great interest to find an experimental way to test the relative reactivity of the P borders with respect to the NP ones. We retain that the NiO(100)/Pd(100) films can represent an optimal system where we can speculate on such a topic. What comes out immediately from the inspection of the experimental data reported in section III is the strong dependence of the ultrathin film reactivity on the NiO film thickness. At the origin of this phenomenon, many factors could in principle

19076 J. Phys. Chem. C, Vol. 111, No. 51, 2007 contribute: the film is subject to a large strain and, at the lowest thicknesses, its electronic structure is rather modified by the hybridization with the substrate and by its metal screening. Moreover, the abundance of point defects could be dependent on the actual preparation procedure. Following a widely accepted interpretation,6-8 the thickness dependent strain should be the favorite candidate in order to explain the link between thickness and reactivity. However, despite of the large strain, the photoelectron VB data do not show large differences of the electronic fingerprinting of the films with respect to bulk like unstrained samples.16 On the other hand, the reactivity data on the 8 MLE thick film demonstrate that, even in the presence of residual strain as well as of an extended formation of mosaics, epitaxial defects, and 3D islands,12,13 the film is totally unreactive under the experimental conditions. In this respect, it must be considered that the total number of intrinsic defects of the films (namely island borders) is larger for thicker films: this trend has been demonstrated by STM measurements which have allowed an evaluation of the root-mean-square (rms) surface roughness at different film thickness. This value increases from 1.6 Å for the 3 MLE films to 1.9 Å for 6 MLE arriving at a value of 3.8 Å in correspondence of 20 MLE thick films.12 These data rule out the possibility that the enhanced reactivity observed in our films can be related just to the number (regardless the type) of the unsaturated sites originated by the morphology of the growth. An alternative explanation for the observed enhanced chemical reactivity can be traced back to another peculiarity of the NiO(100)/Pd(100) layers. As thoroughly documented by the STM data reported in ref 12, the NiO ultrathin films grown at 523 K in the ultrathin regime are made up of relatively small islands with a well developed rectangular shape whose edges are parallel to the 011 and equivalent directions; that is, the islands grow by exposing P borders. For thicker films, the number of islands with P borders is strongly decreased. In the present paper, we propose an interpretation of our experimental data based on the assumption that the P borders can be more efficient in the dissociation of hydrogen with respect to the flat surface and NP borders, so bursting the NiO reduction reaction in mild conditions. This hypothesis is substantiated by the reported computational results (section IV). Actually, putting together both theoretical and experimental results, a detailed picture of the reaction mechanism of hydrogen reduction of the NiO(100)/Pd(100) layers in the medium coverage range can be proposed. We suggest to associate the first step to a facile homolytic dissociation of molecular hydrogen on a polar border (either PO or PNi) with the consequent formation of -OBH hydroxyl groups (see reactions R1a and R1c + R1c′). As already reported in section IV, such a process has no activation barrier (in the former case) and a low one (in the latter) and leads to the formation of very stable byproducts. As a second step, two different pathways can be guessed: a Langmuir-Hinshelwood-like process where two vicinal hydroxyls condense forming NiB-OA-NiB + H2O(g) and restoring an indented PO border (reaction R2a) or an Eley-Rideallike mechanism where the -OBH group reacts directly with an H2(g) molecule leading to the formation of (NiB)2Hb (a bridging hydride PNi) + H2O(g), followed by an internal process leading to the restoration of the -OBH border and to the formation of metallic Ni (reaction R2c′). Since this second step must be the rate determining one (see section III.4), the first-order dependence of the reaction rate from the hydrogen pressure allows us to reject the first hypothesis, since it would require 0th order

Agnoli et al. kinetics; reaction R2a is also thermodynamically unfavorable. On the contrary, the second reaction path appears in agreement with the experimental evidence. According to this mechanism, the water molecules would be readily desorbed, in agreement with Schulze and Reissner,52 who demonstrated that water easily desorbs from epitaxial NiO/Ag(100) even at about 220 K. Consistently, our photoemission data show no sign of water molecules neither as a component in the O1s peak nor in the VB data. The mechanism outlined above is also in agreement with the absence of an induction period; actually the surface already exposes P borders that are ready for an immediate reaction without the need to nucleate an active phase. It is interesting to note that the experimentally proved absence of an induction period as well as the progressive decrease of oxygen removal are totally dissimilar from what is observed in the case of bulk NiO, where the progressive formation of oxygen vacancies leads to an autocatalytic mechanism: the P borders must be much more efficient than the Ni metal itself with respect to the hydrogen dissociation. Therefore, according to the observed kinetics, we can conclude that, in our experimental conditions, the metal Ni phase has just a minor role along the reduction reaction on P borders. Once the terraces with P borders are consumed, the reaction slows down and eventually stops or goes on by adopting a different slower channel (for example as in the case of bulk NiO using oxygen vacancies and the metal Ni phase). Finally, we want to comment on the activation barrier experimentally obtained in relation with the computational results. From the Arrhenius plot reported in Figure 6b, an activation energy of 0.16 eV has been determined. From the present simulations, the first step of reaction R2c′, that is reaction R2c, can be estimated to have an activation energy of about 0.4 eV, and an approximately equal one can be expected for the second step. With all of the approximations inherent in the present models, this agreement seems acceptable. In conclusion, we have demonstrated that nanostructured NiO ultrathin films exhibit a surprisingly high reactivity toward hydrogen, whose origin is totally different from the mechanism taking place on conventional bulk material. Our contribution clearly traces back the origin of such behavior to the presence of islands with polar borders that are extremely effective in the dissociation of molecular hydrogen, so that the oxide reduction process occurs in two successive stages, the first being barrierless. We believe that this finding is not only of specific interest in the study of ultrathin oxide films but also of great relevance for heterogeneous catalysis in general. As a matter of fact, the microscopic mechanism unraveled by our work can be potentially active in many catalytic systems, especially oxide supported nanoparticles, and underline the pivotal importance of morphology and its tailoring to determine the performances of real catalyst. Acknowledgment. This work has been funded by European Community through the STRP project (Growth and Supraorganization of Transition and Noble Metal Nanoclusters) and by the Italian Ministry of Instruction, University and Research (MIUR) through the fund “Programs of national relevance” (PRIN-2003, PRIN-2005). References and Notes (1) (a) Scholes, G. D.; Rumbles, G. Nat. Mat. 2006, 5, 683. (b) Ozbay, E. Science 2006, 311, 189. (2) Za¨ch, M.; Ha¨gglund, C.; Chakarov, D.; Kasemo, B. Curr. Opin. Solid State Mater. Sci. 2006, 10, 132.

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