The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as
O and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputter
profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these
bi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 150 °C.  At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for
the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection against
oxidation

Boerma, D.O.

Heller, E.M.B.

Suyver, J.F.

Vredenberg, A.M.

Boerma, D.O

University of Groningen

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O-2 and H2O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 MI) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300 degrees C in vacuum, air and 10(-5) mbar O-2. The layered structure remains intact up to 150 degrees C. At 200 degrees C in air and O-2, Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150 degrees C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200 degrees C, 20 nm Pd may still provide sufficient protection against oxidation. (C) 1999 Elsevier Science B.V. All rights reserved.</p

ARTS repository - University of Groningen

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O-2 and H2O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 MI) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300 degrees C in vacuum, air and 10(-5) mbar O-2. The layered structure remains intact up to 150 degrees C. At 200 degrees C in air and O-2, Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150 degrees C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200 degrees C, 20 nm Pd may still provide sufficient protection against oxidation. (C) 1999 Elsevier Science B.V. All rights reserved

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University of Groningen Research Database

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as

O and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputter

profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these

bi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 150 °C.  At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for

the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection against

oxidation

Utrecht University Repository

 .Applied Surface Science 150 1999 227–234www.elsevier.nlrlocaterapsuscOxidation and annealing of thin FeTi layers covered with PdE.M.B. Heller a,), J.F. Suyver a, A.M. Vredenberg a, D.O. Boerma a,ba Debye Institute, Section Interface Physics, Utrecht Uni˝ersity, P.O. Box 80.000, 3508 TA Utrecht, Netherlandsb Department of Nuclear Solid State Physics, Materials Science Center, Groningen Uni˝ersity, Nijenborgh 4, 9747 AG Groningen,NetherlandsReceived 16 February 1999; accepted 27 April 1999AbstractThe hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such asO and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for2 2 .hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputterprofiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of thesebi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 1508C.2 .At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher2temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced forthe case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd issufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection againstoxidation. q 1999 Elsevier Science B.V. All rights reserved.PACS: 68.35.Fx; 81.05.Bx; 81.65.Mq; 89.30.xfKeywords: FeTi; Pd; Oxidation; Thin films; Ion beam analysis; Energy storage1. IntroductionThe intermetallic compound FeTi is a promisingmaterial for reversible hydrogen storage. In the FeTimatrix, 37% more hydrogen per unit volume can bew xstored than is contained in liquid hydrogen 1 . Incomparison with other hydrogen storage materials,FeTi has the advantage to store and release hydrogen .at useful pressures around atmospheric pressure) Corresponding author. Fax: q31-30-254-3165; E-mail:e.m.b.heller@phys.uu.nl .and temperatures around room temperature . How-ever, this material is highly corrosive. In addition,embrittlement and a necessary activation treatmentprior to hydrogen charging are major drawbacks.The oxidation of FeTi due to previous air expo-sure or impurities of H O and O during cycling2 2with hydrogen is seen as the reason for the necessaryw xactivation 2 . The activation treatment normally con-sists of a vacuum or hydrogen anneal at high temper-atures. As a result of this treatment the surface oxidelayer, which hinders hydrogen absorption, is re-solved in metallic Fe and TiO . During subsequent20169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0169-4332 99 00248-2( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234228loading, the atomic hydrogen can be absorbed afterw xdissociation at Fe 3 . In order to prevent impurities .H O and O to reach the reactive surface we2 2applied a coating of Pd, as was suggested by Sandersw x4 . A possible additional advantage is that Pd coat-w xing 5 or alloying is beneficial for hydrogen uptakew x6 . Embrittlement is the result of the swelling andshrinking processes that are associated with thecharging and discharging cycles. In principle, theproblem of embrittlement can be overcome by usingFeTi in the form of a powder with grain sizes of afew mm diameter. As a storage system, one couldthink of a structure composed of finely dispersedFeTi particles which are attached to an inert supportand covered by a thin Pd coating.In this paper, we present results on the oxidationbehavior of FeTi covered with a layer of Pd in thetemperature range of the hydrogen loading and re-lease. We used a layered structure as a model systemfor a storage system based on small particles. Next toseveral ion beam techniques to monitor composi-tional changes in the film and to determine theamount of O uptake, we used X-ray Photoelectron .Spectroscopy XPS to characterize the chemical stateof the elements present in the first few nm of thesurface. We studied the properties of the protectivelayer for two different thicknesses of the Pd layer, 4and 20 nm, in order to determine its usefulness as aprotective layer against oxidation.2. ExperimentalBi-layers of FeTirPd on Si wafers were preparedwith e-beam physical vapour deposition at a back-ground pressure of 6=10y8 mbar. The FeTi layerwas evaporated from a mixture of 99.7% FeTi .  .TiMet combined with 99.9% Ti Balzers in aweight ratio of 1:6. The admixture of Ti was neededto obtain an FerTi film composition close to 50r50.The Pd layer was evaporated from 99.99% Pd but- .tons Balzers . During all evaporations the pressurein the preparation chamber stayed below 5=10y7mbar. The layer thickness and composition weredetermined with Rutherford Backscattering Spec- . qtrometry RBS using a 2.7 MeV He beam. Weused a scattering angle of 1708, with the beammaking an angle of 608 with the surface normal.From RBS on ‘as deposited’ samples, it followedthat the FeTi layer had an FerTi atomic ratio of55r45 and a thickness of 90 nm. RBS spectra werew xanalyzed using the computer code RUMP 7 .Depth profiles of C, N, O were measured with .Elastic Recoil Detection ERD , using an incomingbeam of 72 MeV Ag ions, at an incident angle of 258with respect to the surface plane. Outcoming parti-w xcles were detected in a dE–E telescope 8 at arecoil angle of 508. The scattering plane was perpen-dicular to the surface. Contamination levels of C, Nand O in the FeTirPd layer were below 0.5 at.%.The H content of ‘as deposited’ layers was 1 and 3at.% in the FeTi and Pd layer, respectively. .X-ray Diffraction XRD of the ‘as deposited’FeTirPd showed Pd lines, indicating the presence ofpolycrystalline Pd. No FeTi, Fe Ti, Ti, or Fe lines2were observed. The FeTi layer is most probablyamorphous or nano-crystalline.XPS was used to analyze the chemical state of theelements present in the first 10 nm of the surface.XPS was done using a CLAM-2 hemispherical sec-tor analyzer and an Al K source. On the surface ofa‘as deposited’ layers, only metallic Pd, as well as Cand O were observed. This implies that the Pd layerfully covers the FeTi. To further analyze the depthprofiles, sputtering Auger Spectroscopy was per-formed at Groningen University using a JEOL JAMP7800 F with a primary beam of 10 keV and aprobing diameter of 100 nm. Sputtering was donewith Ar.The surface morphology of the samples was in- .spected with Atomic Force Microscopy AFM and .Scanning Electron Microscopy SEM after variousannealing steps. With SEM, the surface of the ‘asdeposited’ sample appeared featureless, while withAFM no structure down to the scale of 1 nm couldbe discerned.FeTirPd bi-layers were subjected to controlledheating in 1=10y5 mbar, in air, and in vacuum y9 .1=10 mbar in the temperature range of 150–3008C during 1 h. The anneals in 1 atm air wereperformed in an open tube-furnace. All XPS scanswere done in the same system as used for thevacuum and O anneals. No interdiffusion was ob-2served with RBS before or after the treatments be-tween the FeTi layer and the Si substrate.( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234 2293. Results3.1. 20 nm Pd coating: ˝acuum annealingSamples consisting of 20 nm Pd on FeTi wereannealed at 150, 200 and 3008C in vacuum for 1 h.Fig. 1 shows the RBS spectra of the FeTi and the Pdlayer. The surface energies of the elements are indi-cated in the picture. No difference was observed inspectra taken before and after annealing at 1508C. Inthese spectra we observe at the surface a clear Pdsignal and no Fe signal. This implies that the Pdlayer fully covers the FeTi. This is confirmed byXPS measurements, in which we observe no Fe .signal after the 1508C anneal Fig. 2 . After anneal-ing at 2008C, in the depth range of the Pd, an Feconcentration with a mean value of 1 at.% anddecreasing towards the surface is inferred from the .RBS spectra see Fig. 1 . At 3008C, the near-surfaceFe signal is flat with a corresponding concentration.of 5 at.% . In both cases, this is an indication that Fediffuses into the Pd layer, probably along grainboundaries. In this process, the Pd layer may breakup in grains separated by FeTi, such that the FeTi isvisible for the ion beam.In situ XPS measurements in Fig. 2 show thatafter vacuum annealing at 200 and 3008C, Fe is nearthe surface. Comparison with reference spectra leads .Fig. 2. XPS Fe 2p spectra of 20 nm Pd on FeTi annealed invacuum.to the conclusion that most of the Fe is metallic andonly a small fraction of the Fe signal is due toFe-oxide. ERD spectra of all vacuum annealed sam-ples show no increase in the O content in the Pd orFeTi layer. The O contents in the total layer aftereach treatment is given in Fig. 3.AFM images of the FeTirPd layer annealed at .2008C see Fig. 4a indicate that the roughness of thelayer increases with temperature. After annealing at2008C, spherical bumps with a diameter of around 15 .  .Fig. 1. RBS profiles of 20 nm Pd a and b and 4 nm c on FeTi ‘as deposited’ and after several anneals. The spectrum of FeTirPdannealed at 1508C has full overlap with the spectrum of the ‘as deposited’ layer. Note that a logarithmic scale is used to make visible smallchanges. The energies corresponding to scattering from the elements at the surface are indicated with arrows.( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234230Fig. 3. O contents in the entire FeTirPd layer from ERD and RBSmeasurements on 4 and 20 nm Pd on FeTi annealed in vacuum,1=10y5 mbar O and air.2nm and a 4 to 5 nm height are observed on a smoothsurface.3.2. 20 nm Pd coating: annealing in O and air2FeTirPd samples were annealed at 150, 200 and3008C in 1=10y5 mbar O or 1 atm air for 1 h. The2RBS spectra, shown in Fig. 1, do not show anobservable change between the ‘as deposited’ sample .and the sample annealed at 1508C Fig. 1 . Thisimplies that the composition and layered structure ofthe samples remain intact up to 1508C, the same aswith vacuum annealing. XPS measurements confirm .this see Fig. 5 . From Fig. 1, it is clear that startingfrom 2008C again Fe appears at the surface. At2008C, the corresponding fraction is 3 at.% Fe forO -annealed samples. After annealing at 3008C more2Fe is found near the surface: for O anneals the2Fig. 5. XPS spectra of samples annealed in 1=10y5 mbar O2 .  .  .  .with 20 nm Pd on FeTi. The Fe 2p left and Ti 2p rightregions are shown. Similar spectra were obtained after annealingin air.corresponding Fe fraction in Pd is 5 at.%; for airanneals a large Gaussian shaped Fe peak is found atthe surface. In the latter case the Pd peak has de-creased and has become much broader, now peakingat larger depths. From comparison of the spectra tosimulations one can derive that, in contrast with theO -annealed sample, the layer structure has changed2over at least 45 nm.From the O region in the RBS spectrum not.shown , it appears that a large amount of O isincorporated in the sample. From the XPS measure- .ments Fig. 5 it can be derived that at and above2008C Fe O forms near the surface. Also TiO is2 3 2detected. In the left hand side of Fig. 6, the depth . y5  .Fig. 4. AFM picture of 20 nm Pd on FeTi annealed at 2008C: in vacuum a and in 1=10 mbar O b .2( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234 231 .  .Fig. 6. ERD O profiles of 20 left and 4 nm Pd right on FeTi‘as deposited’ and annealed in 1=10y5 mbar O . For 20 nm Pd,2the O profile of the ‘as deposited’ layer is essentially the same asthe profile of the layer annealed at 1508C.profiles of O measured with ERD are shown forO -anneals. The peak centered around 90 nm is2present in all samples and is due to a thin nativeSi-oxide at the interface. The O content in the FeTiis approximately 0.5 at.% after deposition. Afterannealing at 2008C in O some extra O is observed2as a peak near the surface position. Because thesample may have become rough, it is not possible todecide whether this O is at or near the surface.Roughness is indeed observed in AFM images takenfrom the same sample, as shown in Fig. 4b. Thesurface has changed drastically from smooth beforeannealing to granular after annealing. The grain sizesare in the order of 10 nm in diameter and 6–7 nm inheight.The presence of O at the surface was confirmedby Auger sputter profiling. Fig. 7 shows the Augersputter profile of the sample annealed at 2008C inO , taken with a beam with a diameter of 100 nm.2Besides Pd, Fe, Ti and O are present at the surface.The O profile decreases to zero after 3 min ofsputtering and remains zero, indicating the presenceof a surface oxide. Deeper in the sample a low levelof Fe and Ti can be seen next to a high level of Pd.When the Pd decreases both the Fe and Ti increase,marking the end of the Pd top layer.3.3. 4 nm Pd coating: ˝acuum annealingFeTi samples coated with 4 nm Pd showed thesame trend in compositional change after vacuumFig. 7. Auger peak-to-peak values of the differentiated spectrumduring sputtering of 20 nm Pd on FeTi annealed in 1=10y5 mbarO at 2008C. The sputtering rate was 1.5 nmrmin.2annealing at temperatures of 150, 200 or 3008C, asthe ones coated with 20 nm Pd. Both RBS and XPSmeasurements showed this. The O content of theselayers remained low, as can be seen in Fig. 3.3.4. 4 nm Pd coating: annealing in O and air2In Fig. 1c, the RBS spectra of annealing in O2and air are shown. Up to 2008C, the trend is verysimilar to the 20 nm case. Again, some Fe is seen atthe surface, although it is more than with 20 nm Pd.Annealing at 3008C in O or air changes the near-2surface composition drastically. An Fe peak is ob-Fig. 8. XPS spectra of 4 nm Pd on FeTi annealed in air. From left .  .  .to right, the Fe 2p , Ti 2p and Pd 3d regions are shown.( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234232Fig. 9. SEM image of 4 nm Pd on FeTi annealed at 3008C in 1=10y5 mbar O .2Fig. 10. Schematic representation of the proposed model for oxidation of FeTirPd bilayers. Top: 20 nm Pd, bottom: 4 nm Pd.( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234 233served at the surface, while the Pd peak broadensand shifts to a larger depth. This is similar to what isobserved in the 20 nm air-annealed case. Apparently,with 4 nm Pd the same occurs also during O2annealing, although not as markedly as during an-nealing in air. Comparison to simulations indicatethat on the 3008C air-annealed sample an FeTi-richsurface layer of around 2 nm has formed on top.Above 1508C, the O content increases drasticallywith annealing in O , the effect being more pro-2nounced at higher temperatures, as can be seen in theright hand side of Fig. 6. Annealing in air shows the .same trend see Fig. 3 . XPS measurements, asshown in Fig. 8, clearly indicate the presence ofFe O and TiO near the surface. With higher an-2 3 2nealing temperatures the Pd signal decreases, indicat-ing either surface roughening or coverage of Pd byFe or Ti. After annealing at 3008C in air, there is no .Pd near the surface anymore see Fig. 8 , which .confirms the RBS results in Fig. 1 c . The SEMimage in Fig. 9, taken from the 3008C O -annealed2sample, shows strong roughening, suggesting theformation of small particles with a diameter of 40–70nm.4. Discussion and conclusionIn this section, we try to explain the observationsas described in Sections 2 and 3. Our explanation isinspired by observations and the description of theoxidation of a Fe layer covered with an epitaxial Agw xlayer with a thickness of 20 nm 9 . In Fig. 10, ourinterpretation of the observations is drawn schemati-cally. At temperatures of 150–2008C, transport of Feand some Ti along the grain boundaries in the Pdlayer takes place. In the presence of O or air2oxidation occurs at the surface and a small amountof Fe and Ti oxidizes into Fe O and TiO . At the2 3 2same time the surface roughens. Upon annealing inO at higher temperatures 2008C for 4 nm Pd and2.3008C for a 20-nm Pd coating , further oxidationtakes place through the same channels and regions ofFe- and Ti-oxide appear at the surface. In the case of4 nm Pd, FeTi also oxidizes deeper. This processcontinues progressively. Finally, at 3008C in air for.20 nm Pd, in O or air for 4 nm Pd , thick oxide2regions are formed along the Pd grains. In the caseof 4 nm, the oxide starts to encapsulate the grains,covering them with a thin oxide layer.This model explains the roughness we observedwith AFM in samples annealed at temperatures above2008C. The roughening and the appearance of Fe andTi at the surface may be related phenomena. Due tothe diffusion of Fe and Ti along grain boundaries inthe Pd layer roughening may be enhanced. It shouldbe noted that due to the resolution of the techniquesused, the structures we describe in our model were tosmall to observe directly. The model is also consis-tent with the appearance of Fe- and Ti-oxide at thesurface after annealing, which we observed withXPS and Auger depth profiling, as well as with thedisappearance of the Pd signal in XPS measure-ments, and with the concentration profiles followingfrom the RBS and ERD spectra.In conclusion, up to 1508C, 4 nm of Pd is suffi-cient to act as a protective layer. For a temperatureof 2008C, 20 nm Pd may still provide sufficientprotection against oxidation, regardless the smallamount of Fe- and Ti-oxide formed at the surface.AcknowledgementsThe authors would like to thank Jeike Wallingafrom Utrecht University for performing the SEMexperiments, Hilde Rinia from Utrecht University forthe AFM measurements and Dimitri van Agterveldand George Palasantzas from Groningen Universityfor the Auger depth profiles. This research wasperformed with financial support from the Stichting .voor Fundamenteel Onderzoek der Materie FOM .Referencesw x1 J. Reilly, R. Wiswall Jr., Formation and properties of iron .titanium hydride, Inorg. Chem. 13 1974 218.w x2 J. Sanders, B. Tatarchuk, Buried-interfacial reactivity of palla-dium coated Fe O rFeTi thin films during vacuum or hydro-2 3 .gen annealing, J. Phys.: Condens. Matter 2 1990 5809.w x3 L. Schlapbach, T. Riesterer, The activation of FeTi for hydro- .gen absorption, Appl. Phys. A 32 1983 169.( )E.M.B. Heller et al.rApplied Surface Science 150 1999 227–234234w x4 J. Sanders, B. Tatarchuk, Protection of FeTi thin films using .palladium coatings, J. Phys. F: Met. Phys. 18 1988 L67.w x5 L. Zaluski, A. Zaluska, P. Tessier, J. Strom-Olsen, R. Schultz,Catalytic effect of Pd on hydrogen absorption in mechanically .alloyed Mg Ni, LaNi and FeTi, J. Alloys Comp. 217 19952 5295.w x6 S. Kulshreshtha, O. Jayakumar, K. Bhatt, Hydriding character-istics of palladium and platinum alloyed FeTi, J. Mater. Sci. .28 1993 4229.w x7 L. Doolittle, A semiautomatic algorithm for Rutherford .backscattering analysis, Nucl. Inst. Meth. B 1986 .w x8 W.M. Arnold Bik, F.H.P.M. Habraken, Elastic recoil detec- .tion, Rep. Prog. Phys. 56 1993 859.w x9 A.V. Mijiritskii, unpublished.

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Oxidation and annealing of thin FeTi layers covered with Pd

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