ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Pt-Rh ALLOYS

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ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Pt-Rh ALLOYS

N. Sano, T. Sakurai

To cite this version:

N. Sano, T. Sakurai. ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Pt-Rh ALLOYS.

Journal de Physique Colloques, 1989, 50 (C8), pp.C8-321-C8-325. �10.1051/jphyscol:1989854�. �jpa-

00229952�

COLLOQUE. DE

PHYSIQUE

Colloque C8, Suppl6ment au noll, Tome 50, novembre 1989

ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Pt-Rh ALLOYS N. SAN0 and T. SAKURAI

The Institute for Solid State Physics, The University of Tokyo, 7 - 2 2 - 1 , Roppongi , Tokyo 106, Japan

Abstract

An atom-probe field ion microscope(AP-FIM) has been employed to determine the surface composition and its depth profile of the (100) plane of Pt-20, 40wt.%(32.1, 55.8at.%)Rh alloys. The enrichment of Pt at the top surface layer on annealing at 700 C( 1000K) was found, agreeing with previous studies by AES, ISS, and AP-FIM. On annealing below 600 C, however, a reversed surface segregation, Rh enrichment at the first

layer, has been observed without any impurity atoms such as S and P.

An oscillatory behavior was observed in composition of both Pt and Rh over ten atomic layers. Annealing up to 700 C in the presence of oxygen, we found no appreciable oxidation of the alloy samples, instead,there appeared chemisorbed oxygen forming an overlayer.

Introduction

Surface segregation of Pt-Rh alloys has been extensively studied with many surface analytical techniques, such as AES[1,2], ISS[3], and AP-FIM[4]. The interest in Pt-Rh binary alloys has been enhanced by the fact that; they are industrially important autocatalysts, and their surface segregation behavior cannot be accounted for by the simple ideal solution model. It predicts Rh enrichment at the surface layer, but many studies have revealed Pt segregation at the top layer, instead of Rh. The contribution of lattice vibration entropy to the surface segregation has been proposed[5,6] and applied to the Pt-Rh systeml71.

Tsong et al.181 have reported Rh segregation at the (100) top surface of Pt-44.8at.BRh alloys equilibrated at 700 C, but they attributed this to CO-segregation with S, which formed an overlayer even with a very small amount(100 ppm). The influence of traces of impurities has been discussed by van Delft et al.[2].

At high temperatures, an entropy term may play the dominant role for the surface segregation of Pt-Rh alloys, as suggested, thus, it might be natural to expect that below certain temperature an enthalpy term overcomes the entropy effect and that Rh would surface-segregate as predicted with ideal solution models. We report here the surface segregation study of the (100) plane of Pt-20, 40wt.%(32.1, 55.8at.%)Rh alloys a ~ e a l e d at 700 C and below 600 C using a time of flight atom-probe field ion microscope[9].

High purity commercial Pt-20, 4Owt.%Rh wires 0.2mm in diameter were used in this study. To remove impurities, the wires were pre-annealed by Joule heating at 1000 C for 1 hour in 1x10-4 Torr high purity 02 flow. FIM tips were prepared from these wires by electrochemical etching in molten salt bath of NaN03:NaCl = 4:l. FIM tips were once again pr -annealed around 800 C for 30 seconds in a main chamber of the AP-FIM(~x~o-~% Torr) to smooth the tip surface in prior to observing a field ion image. A F1 imaye was observed with 2x10-~ Torr helium as an imaging gas at 40 K and the (100) pole was adjusted to a probe-hole. He gas was evacuated \mile keeping the DC high voltage applied to the FIM tip unchanged, and it was reduced to the 50% of the imaging voltage when the tip temperature was raised up to 140 K in order to pump out ansorbed gas on a cold refrigerator and the sample holder. After cooling the tip down to 40 K, the DC voltage was turned down to zero and followed by annealing by Joule heating under 4x10-10 Torr UHV. The tip temperature

during annealing at 700 C was measured with an optical pyrometer, and below 600 C, at which the FIM tip was not seen in red, it was estimated by an

extrapolation. Quenching from the annealing temperature was done by merely

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989854

turning off the current for Joule heating. Since the sample holder was kept at cryogenic temperature and the volume of the FIM tip was very small, the

quenching rate was considered to be fast enough to freeze equilibrated phase.

On finishing the anneal, we soon started atom-probe analyses. It should be noted here that the FIM tip was not imaged after the annealing, thus, %he tip surface was free from gas contamination or gas-induced preferential field evaporation.

The conditions of atom-pr be analyses were; pulse fraction, 0.15-0.2, tip tem~erature. 40 K. UHV. 4x10-18 Torr. These conditions had been confirmed by a pgelimine study to-give correct bulk composition within the accuracy of 2%.

Results

In the present atom-probe analyses, both platinum and rhodium ions were detected doubly or triply charged. No sulfur was detected either at the top surface layer or in the bulk, in contrast to the AP data by Tsong et al.[8], who reported that S formed an overlayer even with traces of S as 100 ppm. From the so-called ladder-step diagram(the number of detected ions was plotted against the number of high voltage trigger pulses),the layer-by-layer composition depth profiles were determined. Fig.1 shows the ladder-step diagrams obtained for the (100) plane of a Pt-4Owt.%Rh annealed at 700 C and below 600 C. As is shown in Fig.1, approximately 40 to 70 ions were collected from each layer of the (100) plane field-evaporated. Thus, the statistical error of Rh concentration at individual (100) plane is +8at.% ( 1 st. dev.) Fig.2 is composition depth profiles with atomic layer resolution obtained from Fig.1. On annealing at 700 C, Pt is enriched at the first layer and depleted at the second layer. Annealing below 600 C reverses the surface segregation, namely, Rh enrichment is observed at the top surface layer and its depletion at the second layer. At both annealing temperatures,an oscillatory behaviour of composition was found over ten atomic layers. Similar oscillation has been found for Pt-Rh alloys by an AP-FIM study[10]. Composition depth profiles of the (100) plane of Pt-20wt.%Rh is shown in Fig.3. Similar to the result of Pt-4Owt.%Rh, the reversion of the surface-segregant with decreasing temperature was observed.

Discussion

It has been reported so far that annealing at 1000 K of Pt-Rh alloys with any Rh concentration resulted in Pt segregation at the top surface. Present AP study agrees with it, but upon annealing below 6b0 C surface enrichment of Rh was observed. Similar tendency has been obtained for Pt-lOwt.%Rh alloys, which is now under investigation. It may be a question whether the tip surface composition was equilibrated at 600 C for 20 min, but Rh concentration at the top layer substantially increased, impossible to explain within the statistical error, compared with non-annealed samples.

Low temperature surface segregation of Pt-Rh alloys has seemed to be open to question. Below 1000 K, the possibility of Rh enrichment at the top layer has been suggested by van Delft and Neiuwenhuys[l], and Langeveld and

Niemantsverdriet[ll]. Langeveld and Niemantsverdriet[7] have assumed that the contribution of the lattice vibration entropy to the surface segregation, A&ib,can be expressed using Debye temperature,8,as follows,

where R is the gas constant. Therefore, ASvib is rewritten as

Substituting the corresponding 8 values of 6(Ptbulk)=230 K, 8(Ptsurf)

=l18 K [121, 8(Rhbulk)=350 K, 8(~h,~,f)=200 K [13], into eq.(3), we obtain Svib = 2.7 J/mol K.

On the other hand, the enthalpy term for surface segregation, AHsegrr is generally written as

in which Z is the number of neighboring atoms in the bulk, Zv is the number of missing atoms at the surface, and Hsub is the enthalpy for sublimation.

Zv/Z is 1/3 for the (100) plane of fcc metals, and dHsd!Pt)=565.7+1.3 kJ/mol, AHsd(Rh) =557+4 kJ/mol [14], thus by substituting those values into eq.(4), we obtain AHsqgr = 2.921.4 kJ/mol. If the value 2.9 kJ/mol is used for the enthalpy term In the free energy for surface segregation, AGsegr,the entropy term, T ASvib, counterbalances the enthalpy term at 1074

K, at which temperature dGsegr becomes zero. This suggests that at low

temperatures Rh would surface-segregate due to the dominant enthalpy term and that the segregant would be changed to Pt with increasing temperature, where the entropy term becomes dominant.

Our results are consistent with the predictions[l, 111 and are in good agreement with the above-mentioned simple calculation incorporating the lattice vibration entropy effect. However, there are some shortcomings in this simple calculation, that is, the degree of surface segregation should be considerably reduced because of near-zero free energy for segregation.

Conclusions

The surface composition and its depth profile of the (100) plane of Pt-20, 40wt%(32.1, 55.8at.%)Rh alloys were investigated using the AP-FIM. Upon

annealing at 700 C, Pt is enriched at the top surface layer, agreeing wlth previous studies, but below 600 C Rh has been found to segregate at the surf ace.

References

[l] F.C.M.J.M van Delft and B.E.Nieuwenhuys, Surf.Sci. 162(1985)538.

[2] F.C.M.J.M van Delft, A.D van Langeveld, and B.E.Nieuwenhuys, Surf-Sci.

189/190(1987)1129.

C31 F.L.Williams and G.C.Nelson, Appl.Surf.Sci. 3(1979)409.

[4] T.T.Tsong, D.M.Ren, and M.Ahmad, Phys-Rev. =(1988)7428.

[5] K-Hoshino, J.Phys.Soc.Japan =(1981)577.

[6] A-Sakai and T-Sakurai, Surf-Sci. m(1984)159.

[7] A.D van Langeveld and J.W.Niemantsverdriet, J.Vac.Sci.Techno1.

A5(1987)558.

C81 D.Ren and T.T.Tsong, Surf.Sci. u(1987)L439.

[g] T-Sakurai, A.Sakai, and H.W.Pickering, Adv.Electronics & Electron Phvs.

Su~~1.20, Academic Press(1988).

[l01 M.Ahmad and T.T.Tsong, Surf-Sci. M(1985)L7.

[ll] A.D van Langeveld and J.W.Niemantsverdriet, Surf-Sci. 178(1986)880.

[l21 H.B.Lyon and G.A.Somorjai, J-Chem-Phys. &(1966)3707.

[l31 D.G.Castner, G.A.Somorjai, J.E.Black, D-Castiel, and R.F.Wallis, Phys.Rev.

B24(1981)1616.

[l41 Handbook of Chemistrv and Physics. 64th ed.(Chemical Rubber, Cleveland, 1983-84)

l l

0 Number of pulses 2.7

106

P t

-

'o

600°C 20

0 Number of pulses 1.8

106

Fig.1 Ladder-step diagrams of the (100) plane of ~t-40wt.%(55.8at.%)Rh alloys annealed; (a) at 700 C for 2 min, (b) at(be1ow) 600 C for 20 min. Vertical lines separate individual (100) planes.

Number of layers

Fig. 2 Composition depth profiles of the (100) plane of ~t-40wt. % (55.8at. %)Rh alloys. Statistical error of each datum point is +8 at.% (1 standard dev.)

I

Number of layers

Fig.3 Composition depth profiles of the (100) Plane of Pt-20wt.%(32.lat.%)m alloys.