Surface segregation in Ni0.96Sb0.04

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<ul><li><p>Applications of Surface Science 5 (1980) 9297 North-Holland Publishing Company</p><p>SURFACE SEGREGATION IN Ni096 Sb0~</p><p>George D. PARKSPhillips Petroleum Company, Research and Development,Bartlesville, Oklahoma 74004, USA</p><p>Received 7 May 1979</p><p>An alloy of composition Ni096Sb004 has been studied using X-ray photoelectron spectros-copy (XPS) and Auger electron spectroscopy (AES). Annealing in vacuum at 725Cgave a stable,reproducible antimony enriched surface. Surface layers were removed by Ar+ sputtering andtop monolayer composition was calculated from changes in Auger signal intensities. The firstmonolayer of the annealed alloy was found to have an antimony atom fraction of about 0.5.This suggests that an ordered monolayer of NiSb may be formed at the alloy surface. Exper-imental photoionization cross sections for Ni and Sb were also determined.</p><p>Dreiling and Schaffer [1] have shown that solid solutions of antimony in nickelappear to be important in the passivation of nickel deposits on cracking catalysts byantimony containing compounds. Their calculations of surface composition of nick-el-antimony solid solutions predict a surface layer of antimony over a second layerof pure nickel. These calculations assume a regular solution, and therefore may notbe appropriate for the nickel antimony system where large volume effects have beenobserved [21 and surface ordering may occur [3]. Latanision and Opperhauser [4]have shown that antimony and tin both segregate at nickel grain boundaries underconditions which produce hydrogen embrittlement, although no attempt was madeto measure surface concentrations quantitatively. This study was undertaken to de-termine the extent of segregation at the alloyvacuum interface.</p><p>An alloy of composition Ni096Sb004 was prepared and studied by Auger spec-troscopy (AES) and X-ray photoelectron spectroscopy (XPS). The alloy was madeby melting a mixture of high puritypowdersunder helium. A I X 1 X 0.1 cm samplewas polished to a mirror finish and mounted on a sample stage which could beheated to 1000C by electron bombardment of a piece of molybdenum foil behindthe sample. Temperature was measured using a chromelalumel thermocouple heldagainst the front surface of the alloy. All data were taken on a Model 548 PhysicalElectronics Industries surface analysis system operated under computer control.Auger data were obtained with a 3 kV electron beam and MgK~radiation was usedfor XPS.</p><p>After initial removal of impurities by argon ion sputtering, a clean, stable anti-</p></li><li><p>G.D. Parks / Surface segregation in Ni0 96Sb0 04 93</p><p> I I I I I I I I I</p><p>0 L~~Xl ixoKINETIC ENERGY (EV)</p><p>Fig. I. Auger spectrum of Ni096Sb004 heated to 725Cin vacuum.</p><p>mony enriched surface could be created by heating the sample to 725Cfor a fewminutes. The Auger spectrum of the sample following such a treatment (fig. 1)showed only slight amounts of surface contamination. The surface was quite inerttoward adsorption and showed very little increase in carbon or oxygen signals afterseveral days in the vacuum system (p 1 X I 0~Torr). To determine the extent ofsurface segregation, the annealed sample was sputtered with a defocused beam ofI kV argon ions (Ar pressure = 5 X l0~Torr) for five second intervals; after everysputtering interval Auger spectra of the nickel and antimony regions were takenand the data were stored by the computer. Fig. 2 shows the depth profile for thesample using Auger peak-to-peak heights. The antimony signal drops dramaticallywhile the nickel peaks increase in intensity, with the Ni MMM line increasing morethan the Ni LMM line.</p><p>The data were analyzed usin~the following model to determine the extent ofsurface segregation [5], where I~is the intensity of an Auger peak from element Aat energy E:</p><p>~ (I)</p><p>K contains incident electron current, instrumental detection function, atomic</p></li><li><p>94 G.D. Parks / Surface segregation in Ni0 96Sb0 04</p><p>35</p><p>3025</p><p>0000vvvvv VVV~</p><p>~2O o N: ~ (61EV)V N:U44(8148EV)~ SB~N(1494EV)</p><p>o~150~o~</p><p>10 00</p><p>05 0~</p><p>000000 0 000 0000 00 000 DO 00 00 000 00 000 0 0</p><p>I I ~ I I I I0 1 2 14 5 6 7SPLITTER TIME (SIINLIrES)</p><p>Fig. 2. Depth profile for Ni0 96Sb004.</p><p>density (all assumed to be constant), and the backscattering factor (assumed to beunity for all layers). The variable Xf~is the atom fraction of A in layer i, d is theaverage interlayer spacing, XE is the electron mean free path in the alloy at energyE, and B is the analyzer acceptance angle. For the alloy with no surface segregation</p><p>~ (2)</p><p>where X~is the atom fraction of A in the bulk alloy. Assuming that only the com-position of the top monolayer is affected by segregation, we have for the alloy withsurface segregation</p><p>~ _X~+x~]. (3)</p><p>Experimentally, we have assumed that the surface of the sputtered alloy showsno segregation (i.e., no selective sputtering takes place). This assumption is supportedby a simple analysis of the sputtered alloy using tabulated Auger sensitivity factors</p></li><li><p>G.D. Parks / Surface segregation in Ni0 96Sh 0.04 95[6] which indicates a composition of Ni095 Sb005. Given an experimentally deter-mined value of R, the ratio of the intensity ofa given Auger line from A on the an-nealed sample to that on the sputtered sample, the atom fraction of A in the firstmonolayer of the annealed alloy is</p><p>(4)</p><p>Therefore, by measuring the increase (or decrease) in individual peak intensitiesupon sputtering, the composition of the first monolayer can be determined indepen-dently for each peak measured. The increase in the NiMMM/NiLMM intensity ratiowhich occurs upon sputtering can also be used to compute first monolayer compo-sition; the use of this ratio makes the solution less dependent on the alloys bulkcomposition.</p><p>The cube root of the atomic volume of polycrystalline nickel (0.22 nm) was usedas the average layer spacing. First monolayer composition was calculated using twosets of mean free path values. The first set, X~,was determined using theoreticalparameters tabulated by Penn [7] and a value for the NiMMM line (Penns valuesare not valid below 200 eV) from a universal curve plot by Palmberg [8]. A sec-ond set, X5, was determined using an empirical equation published by Seah andDench [9]. Table 1 gives line intensity ratios of annealed to sputtered samples de-termined from both peak-to-peak heights and integrated peak areas. Mean free pathvalues are also given along with calculated values of X1</p><p>51~,the concentration of anti-mony in the first monolayer. The data identified with NiMMM/NiLMM give val-ues of X~calculated from the change in the intensity ratio of the NiMMM line tothe NiLMM line upon scattering.</p><p>Table 1Alloy R values and corresponding first monolayer Sb concentrations</p><p>Line R x~pp heights(peak areas) a) b)</p><p>NiMMM 0.756 0.49 0.45(0.747) (0.51) (0.46)</p><p>NiLMM 0.866 0.54 0.57(0.822) (0.71) (0.74)</p><p>SbMNN 5.19 0.50 0.58(5.78) (0.56) (0.66)</p><p>NiMMM/NiLMM 0.873 0.44 0.36(0.909) (0.36) (0.28)</p><p>a) 0.41 nm, 1.01 nm and 0.66 nm. b) = 0.37 nm, 1.25 nm and 0.92 nm.</p></li><li><p>96 G.D. Parks / Surface segregation in Ni0 96Sb0 04</p><p>With the exception of the peak areas from the NiLMM line, data are centeredaround a first monolayer composition of about Ni05 Sb05. The inconsistent datafor the NiLMM peak areas may be due to the problems in setting a linearbackgroundfor the peak area determination, The integration limits were set before the experi-ment was begun, and a change in the shape or the slope of the background duringsputtering could have artificially affected the integrated area. Peak-to-peak heightsin the derivative spectrum are affected less by such changes.</p><p>XPS peak positions and areas for sputtered and annealed samples are listed intable 2, along with relative values of the experimentally measured photoionizationcross sections. Peak areas were numerically integrated after subtraction of a linearbackground drawn to include the main peak and any satellite lines at higher bind-ing energies. Cross sections were determined using Penns mean free path data [7],assuming that the Ni096Sb004 is covered by a monolayer of Ni05 Sb0,5. Ni 2p~was arbitrarily assigned a cross section of 13.92 for comparison with Scofields val-ues [101. Data for nickel show good internal agreement with the calculated crosssections. Calculated antimony cross sections do not agree so closely, and differ withthe measured values by as much as 33% relative to the nickel values. It should benoted that the validity of these measured cross sections is dependent upon the cor-rectness of our interpretation of the Auger sputtering data.</p><p>Differences in atomic radii and surface free energies (~GNi= 61.7 kJ X mol~.</p><p>Table 2XPS data for annealed and sputtered samples</p><p>Line Binding energy a) Cross-section</p><p>(Intensity) b)Sputtered Annealed Measured Calculated</p><p>Ni2p3,2 852.6 852.4 13.92 13.92(100) (100)</p><p>Ni3s NM c) 110.4 0.768 0.753(3.88)</p><p>Ni3p 66.5 66.3 1.97 2.06(10.9) (9.85)</p><p>Sb3d512 528.0 527.8 19.22 16.13(5.0) (22.7)</p><p>Sb4d 32.7 32.3 4.45 2.98(1.0) (3.4)</p><p>CIs 283.9 ND c)(2.0)</p><p>a) Cu2p3,2 = 932.5, Cu3p3,2 = 75.0. b) Ni2p312 = 100.c) NM = not measured, ND = not detected.</p></li><li><p>GD. Parks / Surface segregation in Ni0 96Sb0 ~ 97</p><p>~Gsb = 24.6 kJ X moI~ [11]) both tend to cause antimony to segregate to thesurface of nickel. Our data indicate that antimony segregates to the surface andreaches a saturation level of about one half monolayer. It is possible that the strongbonding between nickel and antimony (iXFJ~.= 32 kJ X mol~(12)) leads to a stablesurface nickel antimonide type compound so that no more antimony segregates.Guttmann [3] has predicted that metals capable of forming compounds withB8(NiAs) structures are likely to show bidimensional compound formation at sur-faces. Overbury and Somorjai [13] observed similar behavior for the goldtin sys-tem where first monolayer compositions close to Au0 5Sn05 were found for threedifferent bulk phases. The CuSn and Ni(lOO)-chalcogen systems have been shownto form ordered overlayers at maximum segregation [14,15]. All of these systemsform bulk phases with B8 structure. The half monolayer saturation coverage onnickel-antimony solid solution suggests that this system may also form an orderedoverlayer structure, although LEED experiments will be needed to fully describethe surface structure.</p><p>References</p><p>[1] M.J. Dreiing and A.M. Schaffer, J. Catalysis 56 (1979) 130.[21 J. Hudis, M.L. Perlman and R.E. Watson, Am. Inst. Phys. Conf. Proc. 18 (1973) 267.[31M. Guttmann, Surface Sd. 53 (1975) 213.[41R.M. Latanision and H. Opperhauser Jr., Met. Trans. 5 (1974) 483.[5] J.M. McDavid and S.C. Fain, Surface Sci. 52 (1975) 161.[61L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and RE. Weber, Handbook of</p><p>Auger Spectroscopy (Physical Electronics Industries, Eden Prairie, Minn., 1976).[7] DR. Penn, J. Electron Spectry. 9 (1976) 29.[8] P.W. Palmberg, Anal. Chem. 45 (1973) 549A.[91M.P. Seah and W.A. Dench, Natl. Physics Lab. Report, Chem. 82 (April, 1978).</p><p>[101 J.H. Scofield, J. Electron Spectry. 8(1976)129.[11] S.H. Overbury, P.A. Bertrand and GA. Somorjai, Chem. Rev. 75 (1975) 547.[12] R. Hultgren, PD. Desai, D.T. Hawkins, M. Gleiser and K.K. Kelley, Selected Values of the</p><p>Thermodynamic Properties of Binary Alloys (Am. Soc. for Metals, Metals Park, Ohio,1973).</p><p>[13] S.H. Overbury and G.A. Somorjai, J. Chem. Phys. 66 (1977) 3181.[14] J. Erlwein and S. Hofmann, Surface Sci. 68 (1977) 71.[15] J.E. Demuth, D.W. Jepsen and P.W. Marcus, Phys. Rev. Letters 31(1973) 540.</p></li></ul>