Schlapbach, L.

Yu, Xin-Nan

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Zcilschrift fur Physikalische Chemie Neue Folge, Bd. 164, S. 1171-1176(1989)© by R. Oldenbourg Verlag, Miinchen 1989-0044-3336/89 $3.00 + 0.00Cu Precipitations at the Surfaceof Crystalline Cu — Zr Alloysafter Annealing in Hydrogen *Xin-Nan Yu, Laboratorium fur Festkorperphysik, ETH Zurich, CH-8093 ZurichandL.Schlapbach, Institut de Physique, Universite de Fribourg. CH-1700 FribourgCrystalline Cu-Zr alloys of various composition were annealed (200-400C) inultrahigh vacuum and in hydrogen, and their surface composition wasanalyzed by X-ray photoelectron spectroscopy. Contrary to the mostlyobserved oxygen induced surface segregation of Zr we found strong Cusegregation after annealing Cu-rich samples in hydrogen atmosphere.Scanning electron micrographs show surface Cu precipitates, apparentlyformed under the effect of the heat of chemisorption of hydrogen on Cu.IntroductionAmorphous and also crystalline alloys of Cu with the d-transition metals Zr, Niand Pd have been studied quite intensively for the last few years among otherreasons because of their special dynamical and diffusional properties in thebulk and at the surface which results in interesting catalytic and hydrogensorption phenomena [1-15].Studies of the electronic structure of amorphous CuxZr-|-x alloys reveal that inthe middle of the glass forming range (20-75 at% Cu), i.e. around 50 at% Cu,the Fermi level Ef is situated in the Zr 4d band and there is no tendency forordering. At Cu concentrations > 80 at %Cu Ef shifts to the edge of the Zr 4dband so that the density of states N(Ef) gets drastically reduced to s-bandvalues, in agreement with a sign reversal of the Hall coefficient [1] andincreased tendency for local order. These drastic changes of electronicproperties are interrelated to total energy changes and structural instabilitiesand relaxation effects [2]. Activation energies for metal atom diffusion inamorphous Cu-Zr alloys are rather small, by far smaller than forrecrystallization, implying that recrystallization is limited by nucleation ratherthan surface diffusion [3]. The enthalpy variation upon crystallization amountsto 5 kJ/mol [4].As many other intermetallic compounds also alloys of Cu with d-transitionmetals show surface segregation effects. The rather high mobility of the metalatoms guarantees to reach thermodynamic equilibrium composition at the* Presented at the International Symposium on Metal—Hydrogen Systems, Fundamen-tals and Applications, Stuttgart, FRG, September 4-9,1988.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AM1172 Xin-Nan Yu and L. Schlapbachsurface more easily than in other alloys. Indeed, the solid solution systemCuxNh-x is the best studied surface segregation alloy [6-8]. On the cleansurface segregation of Cu is observed for compositions 0 < x < 0.75 and Nisegregation is observed at the Cu-rich end of the compositional range.Electronic theory based on a tight binding Hartree approximation is capable ofexplaining this crossover by preserving charge neurality [6] in a comparablemodel by Mukeherjee et al. [7], in which crossover is absent. Cu enrichmentwas also observed in the topmost surface layers of single crystalline Cu3f\li97and Cu24Ni76 after annealing at 700 to 800 K and after sputtering [8].Oxygen chemisorption leads to the "normal" surface segregation by selectiveoxydation of the more reactive component (Zr). Activated bimetallic Zr-Nicatalysts thus consist of active Ni metal clusters dispersed on Zr oxide support,both of which are supported on the underlying bulk intermetallic alloy [9]. Attemperatures around 400° surface enrichment of Ni in the form of NiO and Niis found after annealing of ZrNi3 in O2 and H2, resp. [9]. Preferential oxidationof Zr and Zr enrichment at the surface was also observed upon oxygenexposure of amorphous Cu -Zr alloys of various compositions [10].Whereas surface segregation effects induced by oxygen are known to bestrong and important e.g. for bimetallic catalyst, activation of hydrogen storagematerials and getters [9,11-13], surface effects induced by the adsorption orabsorption of hydrogen are usually rather weak or absent [12,13,14]. In thecase of Cu-d transition metal alloys, however, the results on hydrogen inducedsurface phenomena are rather controversial [14]. Hydrogen adsorptionreduces the Cu segregation of Cu-Ni alloys as compared to clean surfaces, inagreement with theory [15]. Strong surface enrichment of Cu and Pd wasfound upon hydrogen exposure of amorphous Cu-Zr and Pd-Zr, resp., around200C, but on the Pd and Cu rich side only [14,16]. It was argued recently thatthis unusual Cu segregation is characteristic of amorphous alloys and doesnot exist on crystalline alloys. In view of the importance of these segregationeffects and their potential application,e..g. in the preparation of bimetalliccatalyst, we have studied hydrogen adsorption phenomena on crystalline Cu-Zr alloys.Photoelectron spectroscopy was used to investigate the surface compositionafter various treatments of the alloys in an ultrahigh vacuum system. Theformation of Cu precipitates was observed after annealing of Cu rich samplesin hydrogen. Scanning electron micrographs show that the precipitates reachurn diameter.ExperimentalCu forms with Zr many intermetallic compounds. The congruently meltingcompounds are CuZr2 (MoSi2 type structure [17]) and Cu3Zr or Cu5-|Zr-|4(Ag5-|Gd-|4 type structure [17]) according to older [18] and newer [19] phasediagrams, respectively. We have prepared alloys of the composition Cu3Zr,Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AMCu Precipitations at Crystalline Cu —Zr Alloys 1173Cu3ZrOn.04. Cu5lZr-|4 and CuZr2 by RF levitation melting in a water cooledCu crucible at 5.10"7 mbar the constituants Cu (99.99%, Johnson-Matthey),Zr (99,9%, Johnson-Matthey) and in the case of Cu3ZrOn.o4 some CuO(99.9%, Merck). Guinier X-ray patterns of CuZr2 and Cu5-|Zr-|4 agree withthose reported in the literature [17], the patterns of Cu3Zr and Cu3ZrOo.o4show additional lines probably due to the presence of Zr richer compounds.The lines of Zr02 and of Cu are absent.Heat treatment and surface analysis of the samples were performed on a VGEscalab 5 spectrometer at base pressurel .10~1u mbar using MgKa radiationof 1254 eV (Au 4f at 84.0 eV, 1.2 eV FWHM). The spectrometer is equippedwith a high pressure cell, fracture stage, heating and sample cleaning(diamond file, Ar+ bombardement) facilities. Thus all sample treatment wasperformed inside the ultrahigh vacuum (UHV) spectrometer avoiding thetransfer of the sample at air between treatment and analysis.Micrographs of some samples were taken by a scanning electron microscope.Results and DiscussionTypical precipitates observed after 10 to 15n annealing at 200C and 1 bar H2of previously UHV-cleaned samples of Cu3Zr, Cu3ZrOrj.04 and Cu5-|Zr-|4 areshown in Fig. 1. The alloy surface is covered to a high degree with precipitatesof 0.1 to 1 mm diameter. Analysis of the characteristic X-rays by EDAX andXPS data reveal that the precipitates are of pure Cu. Those samples are Cucoloured, contrary to the grey-metallic colour of the fractured samples.Fig. 1: SEM micrograph ot a Cu3Zr alloy (UHV fractured, 10 hours annealed at 200C at 1 barH2) showing the spherically shaped surface Cu precipitates.It is questionable whether hydrogen chemisorption is responsible for this Cusegregation and it is interesting to learn what happened to Zr. Surfaceanalysis by XPS should give the answer.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AM1174 Xin-Nan Yu and L. SchlapbachTypical Zr3d photoelectron spectra and Cu : Zr ratios are shown in Fig.2. Themore bulk or subsurface (B) and the more top surface (S) sensitive ratios asevaluated from Cu3p and Cu2p core level peaks, respectively, are indicatedas well as the colour. Air exposed samples show heavily oxidized Zr and arather strong Cu enrichment at the surface (Cu : Zr (S) = 6.22 :1). The Zr3dbinding energy (BE) of 182.5 eV is smaller than that of ZrC>2 pointing to asuboxide.UHV clean surfaces are Zr rich. Annealing in UHV (200° for 10n) causes someoxidation of Zr due to oxygen diffusion from bulk to surface and due to thereaction with residual gas. A mixture of Zr suboxide and Zr metal is found.Furthermore annealing brings along a strong Zr enrichment in the near surfaceregion and relative to the latter one a Cu enrichment on top of the surface.No Cu precipitates and no Cu colour were found on the air exposed, UHVcleaned and UHV annealed samples, respectively.I i i i i—i—i—i-1XPS_* I_^ Zr metal'Zr o>ide y UHV I .Oil B200 °C 1.8:1 S10 HOURS_l_I_I_I_I I l_188 184 180 176 eV— BINDING ENERGYFig 2: Photoelectron spectra of the Zr3d core levels of crystalline CU51 Zri 4 after varioussample treatments. The Cu : Zr atomic ratios S and B were evaluated using the largelytop surface sensitive Cu2p peak (S) and the largely bulk sensitive Cu3p peak (B),respectively.Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AMCu Precipitations at Crystalline Cu —Zr Alloys 1175Annealing in hydrogen (1 bar H2, 200C, 10n) causes full oxidation ofall surface and near surface Zr into Zr02 aand does not significantly affect theCu : Zr ratio, although the sample changes completely to Cu colour in asurface layer of several tenth of a millimeter thickness. On some samples asplitting of the Cu core level peaks was observed apparently due to theformation of some electrically isolated Cu precipitates which get charged uponphotoelectron emission.Apparently annealing in UHV leads to the normal, fast, oxygen inducedsurface segregation of Zr with the formation of Zr02 at the surface, whereasannealing in hydrogen ends up in a surface disproportionation of Cu-Zr alloysinto Zr02 and Cu precipitates. In order to analyse the influence of time andtemperature, hydrogen and oxygen partial pressure as well as of the alloycomposition on the formation and size of the Cu precipitates annealing ofvarious samples at temperatures up to 400C at 0..1 to 30 bar H2 duringg 0.5 to50n was studied. Detailed results will be published elsewhere; they can besummarized as follows:-no surface Cu precipitates are formed on the Zr richcompound CuZr2.-minor differences were only observed between Cu5-|Zr-|4and Cu3Zr.-size and number of particles per unit surface area dependon the annealing parameters time, temperature, and pressure.-the unannealed air exposed samples show a rather highoxygen content down to 100 nm depth.-samples with additional oxygen (Cu3ZrOo.04) show similar Cuprecipitates.Based on the above results we suggest the following mechanism:The Cu rich alloys have a rather high oxygen content. The oxygen is dissolvedin the metal matrix or present in a ternary suboxide. Residual gas andhydrogen gas are further sources of oxygen. Elevated temperature favours theformation of the thermodynamically more stable Zr02 by a disproportionationreaction of the typeCu-Zr-Ox + O2 Zr02 + Cu + Cu-ZrUpon annealing in UHV Zr is selectively oxidized and strongly enriched on topof the surface, i.e. Cu is covered by Zr02- The presence of hydrogen blocksthe Zr enrichment and the free surface Cu forms precipitates, possiblyfavoured by the heat of hydrogen chemisorption on Cu. Similardisproportionation of FeTi with the formation of surface Fe precipitates wasobserved earlier [20].Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AM1176 Xin-Nan Yu and L. SchlapbachWe gratefully acknowledge P.Wagli, ETHZ, for taking the SEM micrographs, S.BOchler, M. Erbudak and F. Vanini for discussions and the Swiss NationalEnergy Research Foundation (NEFF) for financial support.1. D.Nguyen Mann, D.Pavuna, F.Cyrot-Lackmann, D.Mayou, and A.Pasturel,Phys.Rev. B 33, 5920 (1986)2. Alfredo L.Yeyati and M.Weissmann, Phys. Rev.. B 35, 2714 (1987)3. E.C.Stelter and D.Lazarus, Phys.Rev. B 36, 9545 (1987)4. J.Chevrier, Solid State Commun. 65, 1461 (1988)5. W.Wagner, R.Poerschke and H.Wollenberger, Europhys. Lett, 2, 379 (1986)6. Yi-Chen Cheng, Phys. Rev. B 34, 7400 (1986)7. S.Mukherjee, J.L.Moran-Lopez, V.Kumar, and K.H.Bennemann, Phys.Rev.B 25, 730 (1982)8. Th.Berghaus, Ch.Lunau, H.Neddermeyer and V.Rogge, Surface Science182, 133 (1987)9. R.B.Wright, M.R.Hankins, M.S.Owens and D.LCocke.J.Vac.Sci.Technol. A5, 593 (1987)10. P.Sen, D.D.Sarma, R.C.Budhani, K.L.Chopra, and ON.R.Rao, J.Phys.F:Met.Phys. 14, 565 (1984)11. L.Schlapbach, A.Seiler, F.Stucki, H.C.Siegmann, J.Less-Common Met. 73,145 (1980)12. L.Schlapbach, NATO ASI B 136, 397 (1986)13. L.Schlapbach, in Hydrogen In Intermetalllc Compounds II, ed.L.Schlapbach, Springer TAP 64. chapt. 2, in press14. F.Vanini, St.Buchler, X.N.Yu, M.Erbudak, L.Schlapbach, and A.Baiker,Surface Science 189, 1117 (1987)15. S.Modak and B.C.Khanra, Phys. Rev. B, 34 , 5909 (1986)16. P.Oelhafen, R.Lapka, U.Gubler, J.Krieg, A.DasGupta, H.J.Guntherodt,T.Mizoguchi, C.Hague, J.Kubler and S.R.Nagel, Rapidly QuenchedMetals IV, eds. T.Masumoto and K.Suzuki (Japan Institute of Metals,Sendai), 1259 (1982)17. P.Villars and L.D.Calvert, Pearson's Handbook of Crystallographic Datefor Intermetallic Phases, Am. Soc. for Metals, 198518. T.B.Massalski, ed., Binary Alloy Phase Diagrams Vol I, American Soc.for Metals, Ohio, 198619. E.Keller, Y.Khan, U.Gorres, Z.Metallkde. 77, 43 (1986)20. G.Kirch, R.Hampelmann, E.Schwab, H.Zuechner, Z. Physik. Chemie NF,126, 109 (1981)Brought to you by | Universitaetsbibliothek BaselAuthenticatedDownload Date | 7/11/17 11:25 AM

Cu Precipitations at the Surface of Crystalline Cu — Zr Alloys after Annealing in Hydrogen*

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