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Hydrogen Electro-Insertion into Pd/Pt(111) Nanofilms: An in Situ Surface X-ray Diffraction Study Y. Soldo-Olivier,*,† M. C. Lafouresse,†,|| M. De Santis,‡ C. Lebouin,†,^ M. de Boissieu,§ and E. Sibert† †
LEPMI, CNRS-Grenoble INP-UJF, BP 75, 38402 Saint Martin d’Heres cedex, France Institut Neel, CNRS-UJF, 25 avenue des Martyrs, BP 166, 38042 Grenoble cedex 9, France § SIMAP, CNRS-Grenoble INP-UJF, BP 75, 38402 Saint Martin d’Heres cedex, France ‡
ABSTRACT: Metallic hydrides represent a very interesting solution for hydrogen storage thanks to their capability to absorb hydrogen in a reversible way. Light metallic alloys recently developed may need the use of metallic films as protective barriers, and palladium nanofilms represent a very relevant example. In the present study we analyze with in situ surface X-ray diffraction the structure of about 20 monolayer thick Pd nanofilms electrodeposited onto Pt(111) before and after hydrogen electro-insertion and after hydrogen electrodesorption. The influence of the substrate on the pseudomorphic character of the palladium deposit over about 10 palladium layers and on the structure evolution during the hydrogen insertion/desorption processes is demonstrated. After hydrogen insertion, an asymmetric expansion of the film, almost four times smaller in the surface plane with respect to the out of plane direction, is observed. The presence of two differently ordered regions in the hydride film and of a morphological irreversibility after hydrogen desorption is also shown.
1. INTRODUCTION Metallic hydrides are a very interesting solution for hydrogen storing because of their ability to absorb hydrogen in a reversible way. Light metallic alloys like LaNi5, TiFe, LiMg2Ru, NaAlH4, ...1,2 have been recently developed and are characterized by a large hydrogen storage capacity. Such materials, and in particular Mg-based metal hydrides, may present the disadvantage of a high reactivity toward air and oxygen. Moreover, metal hydride electrodes may undergo a degradation of their performances upon cycling due to the oxidation of the alloy surface. In this context, the use of metallic films as protective barriers has been considered, and palladium nanofilms represent a very relevant example.1,2 Indeed, palladium presents a catalytic role for hydrogen dissociation and a high insertion/desorption kinetic. Pd coatings are easily permeable by hydrogen atoms but not by oxygen atoms: a palladium-coated metal alloy surface is therefore resistant to oxidation during the charge/discharge cycles. The nanometric dimensions of these protecting films raise some questions that are specific to the size reduction, like the influence of thickness on the catalytic properties as well as the effect of the film’s support on the hydrogen insertion/desorption processes. In this context, the palladium deposit on platinum singlecrystal Pt(111) represents an interesting model system. Indeed, due to the close lattice parameter of both metals, it is possible to obtain well-ordered palladium monolayers with (111) structure r 2011 American Chemical Society
through pseudomorphic deposition.3,4 Furthermore, the use of chlorocomplexes of palladium strongly favors 2D deposition.3,5 In a recent in situ surface X-ray diffraction (SXRD) work,6 we presented some evidence for the substrate effect on hydrogen insertion into two monolayers (ML) as equivalent thickness palladium nanofilms. No hydride formation was observed close to the platinumpalladium interface, in particular, in the first two Pd deposited layers. Electronic and/or structural effects7,8 must be considered to understand this behavior for the very first palladium monolayers. Mechanical stress due to the slight lattice parameter difference between Pt(111) and Pd(111) may also play a role in the insertion/desorption properties of the deposited film. Also, and contrarily to bulk Pd for which hydrogen insertion/ desorption irreversibly induces defects in the fcc structure,9 we could observe a good reversibility of the thin film structure after one insertion/desorption cycle. To better understand the influence of the Pd film thickness on its structure reversibility upon hydrogen insertion/desorption and the effect of the substrate on the Pd deposited film, we consider in the present study thicker Pd/Pt(111) films, with an equivalent thickness of about 20 ML. We present in situ SXRD Received: February 11, 2011 Revised: April 28, 2011 Published: May 25, 2011 12041
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2. EXPERIMENTAL SETUP AND PROCEDURE The Pd was electrodeposited on a platinum (111) single crystal characterized by a mosaic spread of about 0.1 and a diameter of ca. 10 mm. Before each experiment, the Pt(111) surface was flame annealed H2/air and cooled in a reducing atmosphere (Ar þ H2, 90:10), as described previously.3 The quality and cleanness of the surface were checked by cyclic voltammetry in 0.1 M H2SO4. A saturated calomel electrode (SCE) was used as a reference electrode. In the following, all potentials are expressed versus the reversible hydrogen electrode (RHE) potential (0 VSCE = þ0.301 VRHE). Pd deposition was carried out in a PdCl2 104 M þ HCl 3 3 10 M þ H2SO4 0.1 M electrolyte using a VMP2 Biologic potentiostat. The Pt(111) electrode was introduced at 1.05 V vs RHE, scanned negatively to 0.701 V vs RHE in the Pd bulk deposition region, and maintained at this potential value until the required equivalent number of monolayers was obtained. The amount of deposited Pd was monitored through the coulometric charge, assuming one palladium atom for each surface platinum atom and a two-electron charge transfer. Transfer to the dedicated electrochemical PTFE cell for in situ experiment was done with an ultrapure water drop protection on the crystal surface. Before SXRD experiments, deposits were checked by cyclic voltammetry in 0.1 M H2SO4.5 The thin-layer configuration of the cell allows an electrolyte thickness of only a few tens of micrometers in front of the crystal surface, minimizing the beam absorption by the solution. The experiment was carried out on the five-circle diffractometer of the bending magnet D2AM French CRG beamline at the European Synchrotron Radiation Facility (ESRF, France), with a monochromatic incident beam of 23.5 keV. A unit cell was chosen to index the reflections with A1 and A2 defining a hexagonal mesh in the surface plane. A3 is perpendicular to the surface, and its length equals the period of the fcc close packing stacking: A1 = (aPt/2) [110], A2 = (aPt/2) [011], and A3 = aPt [111], aPt being the bulk lattice constant of Pt. The diffracted intensity was measured by fixing the (HK) Miller indices and scanning the momentum transfer perpendicular to the surface, describing the so-called crystal truncation rod (CTR).10 The more general term rod applies to a diffraction signal showing a peak in (HK), with a characteristic intensity distribution in L. For nonspecular measurements, we worked in grazing incidence (1), keeping the normal to the surface in the plane defined by the incident beam and the direction perpendicular to the plane defined by its linear polarization. A rocking scan of the sample azimuth is required for a proper measurement of each (HKL) reflection; however, such a procedure is timeconsuming when a small L step is required, as is the case for nanometric thick films. A good compromise between the need to correctly follow the oscillations of the signals and the limited lifetime of the electrochemical cell was reached performing L scans along the rods. This method gives equivalent results, when the proper intensity corrections are applied.11 Rocking scans have been nevertheless recorded with a large L step, allowing the evaluation of the background signal, which has been subtracted to the measured CTR’s diffraction intensities. Only for the specular measurements the background signal has not been estimated and subtracted.
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Previous to the extraction of the modulus of the structure factors, standard area and polarization corrections adapted to the diffractometer geometry were applied. The Lorentz correction for the stationary (L scan) case11 was used, modified to take into account the detector acceptance, using a Gaussian profile, which better fits our rod shape in the (HK) plane. For the specular (00) reflection, the intensities have been corrected for the standard area, polarization, and rod factors.12 SXRD measurements were carried out under potential control using a PAR 273A potentiostat at different stages: before insertion (0.2 V vs RHE), after insertion (0.02 V vs RHE), and after desorption (0.2 V vs RHE) of hydrogen. We strictly controlled the potential applied to the crystal during the data acquisition. For each potential value, we measured the specular (00) reflection around L = 3 and the nonspecular (10), (01), and (11) CTRs, as well as the equivalent (21) CTR. Error bars for the structure factors are based on the agreement factor ε = 0.089 between equivalent reflections and on the error bar at each given (HKL).10 The last one is the sum of the statistical error on the L-scan intensities and on the subtracted background signal, which results in quite a large noise/signal ratio in the region far from the Bragg peaks. The same agreement factor has been used to estimate the error bar on the (00) signal. The data analysis was performed with the ROD package.13 It was restricted to the region L > 3 [5] for the (10) [(11)] CTR to minimize the X-ray absorption of the diffracted beam by the cell. The palladium layers occupancy is described by a suitable profile function. A few occupancy models were tested, including the β one,14 and finally the complementary error function ERFC, better fitting our data, was retained. Only the first deposited Pd layer, noted in the following as Pd1, is assumed to be complete, as indicated by the electrochemical characterization of the palladium film.3 The distance between the two upper layers of the Pt crystal, dPtPt, has been fixed equal to the Pt bulk value, 2.265 Å, in all the fitting procedures.6 For the analysis before hydrogen insertion, the distance dPtPd1 has been adjusted separately, while the following Pd interplanar distances have been modeled using the ERFC error function. For the deposited film structure modeling of the nonspecular reflection, the Pd atomic layers follow the same ABC stacking as the underlying platinum single crystal. After hydrogen insertion, the measured scans across the CTRs show (see Section 3.2) that the film is no more pseudomorphic, with the presence of a misfit of about 1% with respect to the substrate and the formation of domains about 50 Å wide. This results in the overlapping of the nonspecular Pt CTR with the broad hydride rod. In this case, the Pd contribution to the Pt CTR was modeled multiplying the occupancy by a DebyeWallerlike factor parallel to the surface involving the domain width and the misfit.15A unique parameter has been used for the description of the Pd interlayer distances: the details on the fitted distance parameters will be given in the corresponding paragraph. The other fitted parameters are the DebyeWaller (DW) factors for the last Pt layer and for Pd, as well as the Pt roughness parameter βPt. For nonspecular background corrected data, βPt has been set equal to zero, corresponding to a flat Pt surface, as higher values do not improve the quality of the obtained fits. We recall that DW factors obtained by the fitting of specular reflection only concern the component perpendicular to the crystal surface. The structural parameters were optimized through a fit of the structure factors, employing a χ2 minimization for the refinement, and the error bars are obtained with the covariant matrix method.10 12042
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Figure 1. K scan across the (01) CTR at L = 4.75 (a) and L = 5.15 (b) before hydrogen insertion (continuous line) and after hydrogen desorption (dashed line). We also reported in (a) the position corresponding to the Pd(111) CTR (continuous vertical line). The gray rectangle centered on the Pt rod represents the portion of the diffracted signal seen by the detector through the horizontal slits during the L scans.
All electrolytes were prepared from H2SO4 (Merck, suprapur), HCl (Merck, suprapur), PdCl2 (Alfa Aesar, 99.99%), and Milli-Q grade ultrapure water (18.2 MΩcm, 3 ppb Total Organic Carbon). The solutions were deaerated under argon flow (Air Liquide N45). All experiments were carried out at room temperature.
3. RESULTS 3.1. Before Hydrogen Insertion. The diffraction signals were measured at 0.2 V vs RHE, corresponding to a potential value where no hydrogen is inserted in the palladium nanofilm. The measurements across the (01) CTR (K scan), shown in Figure 1, reveal the pseudomorphic structure of the palladium deposit: only the diffraction peak corresponding to the underlying Pt crystal is present. The continuous vertical line at K = 1.0086 corresponds to the position of a Pd(111) rod with bulk lattice constant. The pseudomorphism is confirmed by the presence of the Kiessig fringes in the (10) and (11) CTR signals (Figure 2). The quantitative results for the interlayer distances d111 and the occupancies obtained from the best fitting procedure on the CTRs are shown in Figure 3. A schematic representation of the film structure is given in Figure 4. The first about 17 layers are fully occupied. The occupancy value then rapidly decreases, and the obtained equivalent thickness for the Pd film is 22 layers. The Pd deposit reveals to be quite flat, as the presence of the Kiessig fringes already suggested, with a large amount of fully occupied layers.
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The dPtPd1 distance, 2.25 ( 0.01 Å, is close to the Pt bulk value. The d111 value of the following 12 Pd layers is equal to 2.218 ( 0.001 Å, corresponding to an almost complete preservation of the Pd bulk unitary cell volume V in the pseudomorphic growth (interplanar compression is compensated by lateral expansion, here (VPd_film VPd)/VPd = 0.0045 ( 0.0005). Above 12 layers, a gradient of the interlayer distances extending over about 8 layers is present; d111 reaches the asymptotic value of 2.238 ( 0.003 Å, close to the Pd bulk value of 2.245 Å. This corresponds likely to an in plane Pd relaxation of these last incomplete layers, which induces misfit dislocations and hence the presence of domains. These small displacements close to the solidliquid interface with respect to the pseudomorphic layer cannot be treated with the DebyeWaller like factor introduced in the last section because their contribution could not be resolved in the rod profile. The Pd film has been considered pseudomorphic, inducing a slight underestimation of the Pd occupancy in the last layers. The DW parameter for the last Pt layer corresponds to the bulk value, within the error bar, while the DW parameter for the Pd film is equal to 1.2 Å2. To have better insight into the thin Pd film structure, we measured the specular signal (Figure 5) on a sample Pd14ML/Pt(111), which is characterized by a similar, even if slightly smaller, thickness. The specular reflection around L = 3 shows the Kiessig fringes as well, but these measurements only depend on the structural parameters perpendicular to the surface. Hence, the presence of fringes only indicates that the deposited film is atomically flat. It is interesting to note that the same Kiessig fringes frequency is observed in the specular and nonspecular CTRs of this sample (Figure 6). From a qualitative point of view, it can already be deduced that the Pd film is pseudomorphic over almost its full thickness. Fitting the data within the same model used for nonspecular signals, the first about 10 layers are fully occupied (Figure 7). The occupancy then rapidly decreases, and the number of equivalent layers is equal to 14.3. The dPtPd1 distance has been fixed equal to 2.25 Å, according to the results obtained for Pd22ML/Pt(111) (see Table 1). Contrarily to the results obtained with nonspecular measurements, one single d111 value for the following Pd layers is enough to fit the data. Within the error bar, this value, 2.215 ( 0.005 Å, is the same obtained for the 10 first PdPd interlayer spacing of Pd22ML/Pt(111). The introduction of a gradient does not improve the quality of our fit. This is because the specular signal has been measured at low L values (near L = 3) and over a small L range. The fitted bulk roughness results in a βPt value equal to 0.357. Such a high value is an artifact, compensating for the (unknown) background signal. It affects the calculated CTR intensity far from the Bragg peak only, improving the fit agreement, but does not modify the occupancies and interlayer spacing within the error bars. The DW factor for palladium is equal to 3.96 Å2, but the uncertainty is very large due to the low transferred momentum Q and to the limited L extent. The comparison between the quantitative analysis of two different Pd film thicknesses seems to show that, independently of the thickness of the film, the Pd deposits are quite flat and have a pseudomorphic character preserving the Pd unit cell volume over the first about 10 layers. This is a clear indication that the influence of the Pt(111) substrate on the structure of the deposited Pd film extends over a quite high number of layers, 12043
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Figure 2. Pd22 ML/Pt(111), H2SO4 0.1 M, before (0.2 V vs RHE) and after (0,02 V vs RHE) hydrogen insertion. Experimental (10) and (11) CTRs (points) and best fit (continuous line). χred2 = 0.85 and χred2 = 0.55 for the fitting procedure before and after hydrogen insertion, respectively. The error bars are plotted every 10 points.
Figure 3. Pd22ML/Pt(111), H2SO4 0.1 M, 0.2 V vs RHE (before hydrogen insertion). Occupation rate (full circles) and interplanar distances d111 (full triangles) obtained by the quantitative analysis of the nonspecular signals. Interplanar distances are described by the ERFC function, and continuous lines are a representation of their error bars. The x axis represents the atomic layers concerned: x = n corresponds to the nth deposited palladium layer. For the interlayer distances, the reported value for x = n corresponds to the dPdnPd(n1) distance, where x = 0 is the outermost Pt layer.
allowing the preparation of thick crystalline Pd films with (111) orientation. Moreover, this result confirms the role of chloride in the deposit solution, as has already been shown for thinner Pd films.6 For thicknesses higher than 10 layers, the occupation number rapidly decreases, and the interplanar distance undergoes a gradient, reaching a final value close to the bulk Pd parameter. We believe that this is due to an at least partial loss of pseudomorphism in islands at the surface. 3.2. After Hydrogen Insertion. The surface diffraction signal was measured at 0.02 V vs RHE, the potential value ensuring that any further hydrogen insertion is negligible. As a first result, we observe that the film keeps the same orientation as the substrate in plane cell, but hydrogen insertion induces a loss of
the pseudomorphism, as shown in Figure 1a. The shape of the scan at L = 4.75 after hydrogen insertion is due to the convolution of the Pt CTR with the broad palladium hydride rod shifted by about 1% toward lower K values, corresponding to the larger in plane lattice constant of the palladium hydride. The width of the hydride rod reveals the in plane poor correlation length, corresponding to about 50 Å wide domains. On the (01) CTR, the hydride contribution is much stronger for L < 5 (Figure 1a) than for L > 5 (Figure 1b), L = 5 corresponding to the Pt Bragg peak position. This is a clear indication of a larger interlayer distance in the hydride compared to Pt(111). Nevertheless, Figure 1b is very interesting, as it shows a symmetric decrease of the CTR intensity after hydrogen insertion, a clear signature of the pseudomorphic growth of palladium before hydrogen insertion and of its relaxation after it. Hydride formation is clearly visible in nonspecular CTR measurements (Figure 2): the Pd shoulder at larger L values than the Pt Bragg peaks disappear, as well as the Kiessig fringes, confirming the loss of pseudomorphism with hydrogen insertion. The poor in plane correlation length results in the overlapping between the hydride rod and the Pt CTR. A new shoulder is hence observed at L values lower than the Pt Bragg peaks, related to the larger (out of plane) lattice parameter characteristic of the palladium hydride. The misfit associated to the high structural disorder induces an important decrease in the intensity of the surface diffracted signal, when measured with respect to the Pt CTRs. The gray rectangle shown in Figure 1a and centered on the Bragg peak represents the portion of diffracted signal seen by the detector through the horizontal slits during the L scans. Fitting the film structure with the same method used for the pseudomorphic Pd film results in an underestimation of the occupation values. Instead of more than 15 complete and pseudomorphic layers, as was the case before hydrogen insertion, the occupancy rapidly decreases, giving an equivalent thickness of 7.3 layers spread over about 25 layers. 12044
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Figure 4. Out-of-plane schematic representation of the Pd film structure before (left) and after (right) hydrogen insertion, where the in plane “disordered” region in the hydride Pd film is indicated by the gray zone.
Figure 5. Pd14ML/Pt(111), H2SO4 0.1 M, 0.2 V vs RHE (before hydrogen insertion). Modulus of the structure factor for the specular reflection (full circles) and best fit (continuous line), χ2red = 2.5.
Figure 7. Pd14ML/Pt(111), H2SO4 0.1 M. Occupation rates obtained from the specular reflection signal before hydrogen insertion (circles) and after hydrogen insertion (squares). The x-axis represents the atomic layers concerned: x = n corresponds to the nth deposited palladium layer.
Table 1. Interlayer Distances Obtained after the Best Fitting Procedure for Pd22ML/Pt(111) (Nonspecular Reflexion) and for Pd14ML/Pt(111) (Specular Reflexion) before and after Hydrogen Insertion Pd22ML/Pt(111) nonspecular reflection dPd1Pt dPdnPdn1, 2 e n < 13 dPdnPdn1, 13 < n < 21 dPdnPdn1, n > 21 Pd14ML/Pt(111) specular reflection dPd1Pt
Figure 6. Pd14ML/Pt(111), H2SO4 0.1 M, 0.2 V vs RHE (before hydrogen insertion). Comparison between specular (00) reflection (continuous line) and (01) CTR (dashed line) measured in the same experimental conditions. (01) CTR is shifted by þ2.0 on the x-axis.
To explain this result, one can consider that only a fraction of interfacial Pd atoms is close to the adsorption site of the nonhydrided film. Dislocations are induced once the atomic displacement is too large with respect to this original site, resulting in domain boundaries. Only the atoms that are close to the initial (before hydrogen insertion) adsorption sites are correlated through the substrate and contribute to the intensity of the measured nonspecular CTRs. As explained above, this is
dPd2Pd1 dPdnPdn1, 3 e n
before insertion
after insertion
2.25 ( 0.01
2.25 ( 0.014
2.218 ( 0.001
2.40 ( 0.02
gradient 2.238 ( 0.003 before insertion
after insertion
2.25 fixed
2.26 ( 0.01
2.215 ( 0.005
2.25 ( 0.01 2.356 ( 0.007
modeled with a DebyeWaller like factor.15 The lateral correlation is progressively lost going from the interface with the substrate to the solidliquid interface which results in the decrease of the observed occupancy. The smaller the coherence length parallel to the surface, the broader is the scattering in the reciprocal space and the lower is the contribution in the Pt CTRs. This phenomenon is observed in Figure 1 as an increase of the background signal. The only partial contribution of the hydride deposit to the nonspecular CTRs prevents a detailed description of the film structure. 12045
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Figure 8. Pd14ML/Pt(111), H2SO4 0.1 M. Specular reflection measured before (full circles) and after (crosses) hydrogen insertion. The inserted figure shows the modulus of the structure factor after insertion (crosses) and the best fit obtained (continuous line, χ2red = 0.55).
Figure 2 shows the best fit obtained considering a unique Pd interlayer distance, equal to 2.40 ( 0.02 Å. This value is higher than for PdH0.6 (2.327 Å), but we recall that the measured in plane expansion is smaller, resulting in a distorted hydride having, within the error bars, the same unitary cell volume (49.0 ( 0.4 Å3) compared to PdH0,6 (49.09 Å3). Such a distortion is not surprising, considering that hydrogen intercalates between Pd(111) planes, and it is surely the effect of the Pt substrate, which constrains the lateral expansion of the Pd film. Moreover, the calculated PdH bond length in the hydride film (2.01 Å) has the same value as the PdH distance in the bulk Pd hydride, suggesting that in both systems the hydrogen insertion rate is the same. The palladium DW parameter increases by a factor of three, being equal to 3.9 ( 0.3 Å2, which is a clear indication that hydrogen insertion induces an important structural disorder in the system. We can conclude that the part of the deposited film characterized by an in plane order and hence contributing to the nonspecular signal is limited to a small region (about 7 layers equivalent thickness) near the PtPd interface. The nonspecular signal having a lack of information on the structure over all the thicknesses of the hydride Pd film, it is interesting to consider the specular reflection, which is independent of the in plane order. Figure 8 shows the (00) rod of the Pd14ML/Pt(111) film before and after insertion. The hydride rod still presents the Kiessig fringes, but they are now visible mainly at L values lower than the Pt Bragg peak, as expected for a film with larger lattice constant. Their presence and their period is a clear indication that the film remains atomically flat, even after hydrogen insertion. These qualitative observations are confirmed by the quantitative results of the fit. The occupancies obtained are very close to their value before hydrogen insertion (Figure 7). Contrarily to the nonspecular measurements, data are sensitive to the dPd2Pd1 distance, which can now be fitted as an independent parameter (see Table 1). It results that no interplanar distance expansion is present for the two first deposited Pd layers, signature of the absence of hydride formation at the PtPd interface, in agreement with the results obtained for the very thin deposited film Pd2ML/Pt(111).6 Only one interlayer distance describes the
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structure for the following Pd layers. The resulting best fit value, dPdnPdn1 = 2.356 ( 0.007 Å, is expanded by about 4.9% compared to bulk palladium (Table 1). This value represents the mean expansion over the whole Pd film. Considering the result previously obtained for the in plane ordered region near the PtPd interface, interlayer distance expansion of about 6.9% over about 1/3 of the thickness compared to bulk Pd, an expansion of about 3.9% for the “in plane disordered” portion of the deposit can be extrapolated. It results that the interplanar distance increase induced by hydrogen insertion is higher for the Pd layers near the PtPd interface, where the substrate influence is revealed by the maintaining of in plane coherence, than for the outmost Pd layers. For this last Pd region, the measured expansion is even very near to the hydride bulk Pd. The palladium DW factor increases and is equal to about 6.6 ( 0.6 Å2. This is an indication of a significant structural disorder, but, as explained above, the DW factor is overestimated. The Pt and the Pd1 DW parameters were kept equal to their bulk values, as different solutions do not affect the obtained results for distances and occupation numbers. The fitted βPt parameter is equal to 0.388. Our measurements show that after hydrogen insertion the loss of pseudomorphism and the structural disorder strongly affect the intensity of the nonspecular reflexions and that specular measurements become mandatory to precisely describe the structure of the deposited film. The comparison between the results obtained with nonspecular rods, probing mainly a region close to the interface with Pt, and specular rods, probing the film over all its thickness, shows the presence of two differently ordered regions. There is an in plane coherent region near the PtPd surface, extending over about 1/3 of the equivalent thickness and characterized by almost twice the interlayer expansion observed in PdH0.6 compared to Pd bulk. This region is followed by an in plane “disordered” outermost zone, where the d111 increase is in agreement with the expansion observed for PdH0.6. As in the case of ultrathin films, we found that hydride formation is hindered at the PtPd interface, over the first two deposited Pd layers. A schematic representation of the hydride Pd film structure is presented in Figure 4. 3.3. After Hydrogen Desorption. After hydrogen desorption, the specular rod reveals to be completely reversible, reproducing exactly the diffracted intensity obtained before hydrogen insertion. This result indicates that the structural parameters describing the Pd/Pt(111) system in the direction perpendicular to the surface do not undergo any irreversible modification after one insertion/desorption cycle. In particular, interlayer distances and occupation numbers are the same as before insertion. This is not true for nonspecular CTRs. We observe that Kiessig fringes come back after desorption, a signature of the fact that the film finds a pseudomorphic structure again, but the diffracted intensity is smaller, as shown in Figure 9. This decrease in intensity is due to the irreversible loss of in plane coherence, with the decrease of the domain's characteristic size in the Pd layers. This results in some diffuse scattering which is not integrated by the detector and in an apparent decrease of the occupation when measured on these CTRs.16 It can be concluded that after hydrogen desorption, even if a very good reversibility of the structure in the direction perpendicular to the surface is observed and the pseudomorphic character is found again, a morphological irreversibility takes place. 12046
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)
Present Addresses
Laboratoire d’Electrochimie Moleculaire, CNRS, Universite Paris 7, 15 rue Jean-Antoine de Baïf, Paris cedex 13, FR 75205. ^ Universite d’Aix-Marseille I, II, III, Centre Saint Jer^ome, 13397 Marseille Cedex 20, France.
Figure 9. Pd22ML/Pt(111), H2SO4 0.1 M. (10) CTR recorded before insertion (continuous line) and after desorption (dotted line) of hydrogen.
This partial irreversibility of the Pd/Pt(111) system can be seen as a result of a process, where, under desorption, the Pd layer accommodates back to the Pt(111) surface mesh but with a large amount of defects. 3.4. Conclusions. Our results on Pd/Pt(111) suggest a structural model where the lattice misfit is accommodated, above a given thickness, by a misfit strain gradient. The pseudomorphic character of the deposited Pd film is preserved over about 10 layers. Hence, the Pt single crystal influence extends over at least 10 layers of the deposited palladium film, allowing the preparation of quite thick single crystal Pd deposits. The following layers are described by higher interplanar distances, which undergo a gradient and reach a final value close to the Pd bulk one. However, in our experiment, this happens close to the limit of island formation, and a direct connection between strain gradient and islands cannot be excluded. After hydrogen insertion, the pseudomorphism is lost. Two regions are evidenced in the Pd film. Near the PtPd interface, over about 1/3 of the equivalent thickness of the film, there is an in plane ordered phase, having a large interplanar expansion, almost twice the expansion observed in PdH0.6. The out of plane expansion is also higher by almost a factor of four compared to the in plane one. This is a clear effect of the constraints induced by the Pt(111) substrate. This region seems to have the same maximal hydrogen insertion rate found in hydride bulk Pd. The rest of the deposit is characterized by domains with in plane coherence length of only a few nanometers and by an interlayer expansion comparable to the one observed in PdH0.6. The specular signal confirms the absence of hydride formation at the PtPd interface over the two first Pd layers. After hydrogen desorption, a very good reversibility of the interlayer spacing in the direction perpendicular to the surface is found, whereas a morphological irreversibility takes place in the plane. Even if pseudomorphism is found again, we observe that the loss of the lateral spatial coherence induced by hydriding is maintained. Nevertheless, this anisotropic reversibility being induced by the Pt substrate constraints, a complete structural reversibility for unsupported nanometric Pd systems cannot be excluded.
’ ACKNOWLEDGMENT We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities, and we would like to thank the D2AM beamline staff for assistance. We are also grateful to Marian Chatenet for experimental support. The present work was financed by the French ANR, grant # 06-JCJC-0111. ’ REFERENCES (1) Z€uttel, A. Mater. Today 2003, 6 (9), 24. (2) Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Int. J. Hydrogen Energy 2007, 32, 1121. (3) Lebouin, C.; Soldo-Olivier, Y.; Sibert, E.; Millet, P.; Maret, M.; Faure, R. J. Electroanal. Chem. 2009, 626, 59. (4) Ball, M. J.; Lucas, C. A.; Markovich, N. M.; Stamenkovic, V.; Ross, P. N. Surf. Sci. 2002, 518, 201. (5) Hoyer, R.; Kibler, L. A.; Kolb, D. M. Surf. Sci. 2004, 562, 275. (6) Lebouin, C.; Soldo-Olivier, Y.; Sibert, E.; De Santis, M.; Maillard, F.; Faure, R. Langmuir 2009, 25 (8), 4251. (7) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. J. Mol. Catal. 1997, 115, 421. (8) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. Rev. Lett. 2004, 93, 156801. (9) Flanagan, T. B.; Balasubramaniam, R.; Kirchheim, R. Platinum Met. Rev. 2001, 45 (3), 114. (10) Robinson, I. K. In Handbook on synchrotron radiation; Brown, G. S., Moncton, D. E., Eds.;North-Holland: Amsterdam, 1991; Vol. 3. (11) Torelles, J. J. Appl. Crystallogr. 2004, 37, 395. (12) Vlieg, E. J. Appl. Crystallogr. 1997, 30, 532. (13) Vlieg, E. J. Appl. Crystallogr. 2000, 33, 401. (14) Robinson, I. K. Phys. Rev. B 1986, 33 (6), 3830. (15) Renaud, G.; Robach, O.; Barbier, A. Faraday Discuss. 1999, 114, 157. (16) Vlieg, E.; Van Der Veen, J. F.; Gurman, S. J.; Norris, C.; Macdonald, J. E. Surf. Sci. 1989, 210, 301.
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dx.doi.org/10.1021/jp201376d |J. Phys. Chem. C 2011, 115, 12041–12047
