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Overlayer formation and inter-diffusion of Rh atoms at the Pt(100) surface
ELSEWIER
Surface Science 318 (1994) 358-362
Overlayer formation and inter-diffusion of Rh atoms at the Pt(lOO! surface Hao-tse WI, Tien T. Tsong * Inst~~te of Physics, Academia Sinicia, Taipei 11529, Taiwan, ROC
Received 23 February 1994; accepted for publication 29 June 1994
Abstract The kinetics of Rh inter-diffusion into a Pt(100) surface was investigated on abrupt interfaces of Rh/Pt(lOO) which were artificially created by thermal evaporation of 0.8 and 1.5 monolayers of Rh on a (5 x 20) reconstructed Pt(100) substrate at 110 IL Deposition of a small amount of Rh on a clean and reconstructed Pt(lOO) surface destroys the reconstruction and yields a (1 X 1) structure. The kinetics of Rb diffusion from the overlayer into the bulk was monitored by Auger electron intensities of the Rh and Pt signals as a function of time and annealing tempera~re. The activation energies of inter-diffusion are derived to be 1.63 + 0.4 and 1.68 + 0.2 eV for 0.8 and 1.5 monolayer coverages, respectivefy.
1. Introduction
the bulk. Applications which depend on the diffnsion
Growth of thin-films of metals on transition metal surfaces by vapor deposition has been studied exten-
sively during the last decade. The interfacial properties of ultrathin heteroepitaxial films are of fundamental and practical importance as evidenced from the vast amount of existing literature in surface science and materials research. In most studies the motivation has been to determine the influence of the substrate on the properties of the overlayer film; some questions have been experimentally investigated such as the growth modes, and diffusion of adatoms, etc. Diffusion of atoms and molecules on a metal surface is also a technologically important phenomenon. Diffusion is the primary mechanism for the transport of matter across surfaces and into
* Co~es~nding ~39-6028/94/$07.~
author. Fax: 886 2 7899601.
of adsorbates include the growth of thin films, the formation of alloy thin-films and crystals, the formation and the stability of epitaxial layers, and heterogeneous catalysis [l], etc. Alloys formed from metals in group VIII in the periodic table are used increasingly as catalysts for hydrocarbon reactions. The Pt-Rh catalyst is a typical bimetallic or alloy catalyst used to remove CO, hydro~bon, and NO, from automotive exhaust gases [z]. For these reasons we undertook an analysis of inter-diffusion of Rh films on the Pt(100) surface. Deposition of Rh atoms onto a (5 X 20) reconstructed Pt(100) surface should exhibit interesting features since the substrate itself is reconstructed by the formation of a quasi-hexagonal layer [3]. To the best of our knowledge, until now only one study about inter-diffusion in the near surface layers of Rb into Pt(ll0) [7] has been carried out. In that study by He et al., the following results were obtained. The barrier height for Rh di~sion into Pt has been
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0039-6028(94)00424-Z
H. Wu, T.T. Tsong/Surface
determined to be 2.69 f 0.55 eV from short annealing time data. There was about 5% carbon contamination during the deposition. Various equilibrium structural phases, c(2 X 21, (1 X 3) and (1 X 21, associated with certain Rh concentrations during diffusion were observed by using high-resolution LEED. At 670 K the diffusion constant determined from the data points at short annealing times is (4.32 f 0.75) X 10e2’ cm2/s. At long annealing times the value of the diffusion constant is two orders of magnitude higher than this value. In this work we investigate the kinetics of Rh atom inter-diffusion at an abrupt interface of Rh on the (5 X 20) reconstructed Pt(100) surface using Auger electron spectroscopy @ES). We study the AES signal intensity as a function of time and annealing temperature. With 0.8 and 1.5 Rh overlayers, the diffusion constants and activation energies were determined for inter-diffusion of Rh atoms into the near surface layers of the Pt(100) surface. We derive the diffusion parameters from a least-squares fit of our experimental data over the entire time span of our measurements with a theoretical equation.
2. Experimental
procedures
In this experiment, low energy electron diffraction (LEED) was used for identifying the atomic structure of the clean and (5 x 20) reconstructed Pt(100) substrate and the Rh overlayers, and AES was used for determining the surface coverages and elemental concentrations of the surface as a function of time at a given annealing temperature. Two types of experiment were done: (a) After deposition of Rh on the clean and reconstructed Pt surface, the diffraction pattern was investigated by LEED. (b) The kinetics of inter-diffusion was analysed by monitoring the Pt and Rh peak-to-peak heights of the AES signals as a function of annealing time at a fixed annealing temperature. The substrate was prepared according to conventional procedures. The Pt crystal ( < 10 ppm impurities, Technisch Co.) was oriented and spark cut to within 0.5” of the (100) plane; it was then polished to a 1 pm alumina grit. The base pressure of the chamber was about 2 X lo-” Torr and cleaning [3] in situ involved tens of cycles of a combination of
359
Science 318 (1994) 358-362
argon-ion bombardment (1 X 10e6 Torr, 1.2 keV, for 30 min at room temperature) and light oxygen treatment (3 X lop7 Torr, for 20 min at 500°C) followed by annealing at about 550°C for 1 h until the AES signals showed no detectable impurities and the LEED pattern showed the sharp spot pattern of the well-known (5 X 20) reconstruction. Cleaning of the sample took well over a month. The Rh source consisted of a 0.25 mm Rh coil spot-welded on Ni feedthroughs that were electrically heated. The distance between the Rh source and the Pt substrate was about 6 cm. The surface coverage was determined from the between the intensity of the ratio R = &h(300&64) Rh AES line at 300 eV and the intensity of the Pt AES line at 64 eV, using the formula [4] 1 -e-d/A,oo
R=R 0
e-d/b
'
(1)
where R, = l&30,,~/Z&64~ = 2.33 [5]; P” denotes the intensity of the AES line for a very thick sample of Rh at 300 eV or Pt at 64 eV; the h’s are the inelastic mean free paths of electrons with 300 or 64 eV energy (A,,, = 0.665 mn, A,, = 0.410 nm, data obtained from the formula given by Tanuma et al. [6]); and d is the thickness of the Rh film which is assumed to be uniform over the substrate surface. Taking d to be about 0.2 nm, we will quote surface coverages below in units of layer equivalent (abbreviated LE) [4], with the understanding that 1 LE is the amount of adsorbate that produces the same value of R as would be produced by a uniform monolayer of the adsorbate on the substrate surface. For 1 and 2 LE of Rh overlayers, the intensity ratios are 0.986 and 2.7928, respectively. After each deposition of Rh on the clean (5 x 20) reconstructed Pt(100) surface to the desired coverage of about 0.8 or 1.5 monolayers, the sample was subsequently heated to a preselected annealing temperature where diffusion of Rh into the bulk occurs at a certain rate. The degree of coverage of the deposited Rh overlayer was monitored by AES at a substrate temperature of N 110 K. At this temperature Rh atoms will not diffuse into the bulk even under the radiation heating from the nearby Rh evaporation source. We measured the peak intensity (the differential peak-to-peak height) ratio of the
H. Wu, T.T. Tsong /Surface Science 318 (I 994) 358-362
360
Auger lines of the Pt substrate at 64 eV and the Rh overlayer at 300 eV to determine the Rh coverage. The kinetics of inter-diffusion was then studied by detecting the Pt and Rh Auger peak intensities as a function of annealing time at various up-quenching temperatures as the Rh atoms diffuse into the bulk of the Pt(100).
3. Results and dissusions For the clean sample after the elaborate cleaning and annealing procedures described above, the LEED pattern exhibited (5 X 20) with a very high signalto-background ratio. After deposition of Rh, to an estimated surface coverage of 0.3 LE, the LEED pattern transformed into (1 X 1) with a smaller signal-to-background ratio. This result is very similar to those of Rh and Fe on Au(100) [4] where the (5 X 20) resconstructed pattern shifts to (1 X 1) after about 0.2 LE deposition of Rh or Fe, and electrochemical deposition of Rh ions on PtilOO) [2] where the deposited Pt surface gives a (1 X 1) LEED pattern for about 0.4 monolayer of Rh ions. We find that after the up-quenching, the Rh concentration decreases rapidly at the beginning but the rate of decrease slows down gradually. In the model of Ref. [7], where the initial condition of having an overlayer of Rh on a semi-infinite slab of Pt is used, the normalized concentration of Rh as a function of time and depth is described by
where C,(t) is the Rh concentration at the surface after an annealing time t, C,(O) is the Rh concentration before annealing, and S = (D(T>t/d2>“‘. For D(T>t/d2 -=K1, or for short annealing times, the normalized Rh surface concentration is +2s
J;; *
(4)
It is implied in Eq. (4) that the normalized Rh surface concentration is a function of fi at short annealing times. The yielded slope is proportional to /D(T) at the corresponding temperature. The normalized Rh surface concentrations, CL(t>/Cb(O), were obtained from the experiments. The values of D(T) and D’(T), can be determined by substituting the thickness of the overlayer and the corresponding annealing time and the normalized Rh surface concentration into Eq. (4). In this work we first got a rough estimate of D(T) for short annealing times by averaging about the first eight diffusion constants, D’(T), at a given annealing temperature. Then the value of D(T) is adjusted until the best fit, least-
C( x, t> C( x, 0) =exp(a+T)
erfc[&+(%)“i].
(2)
where C(x, t) is the Rh concentration at depth x at annealing time t; C(x, 0) is the Rh concentration at depth x before annealing; erfc is the complementary error function; d is the thickness of the deposited Rh overlayer; and D is the diffusion constant at a given annealing temperature, i.e. D = D(T), where T is the annealing temperature. At the surface (i.e. x = 01, Eq. (2) can be rewritten as C,( t)/C,(O)
= exp(S2)
erfc(Q,
(3)
: 0.2
-c,(t)/c,(o)=exp(sE)sric(s), SB=Dt/dE
: D=6.77~10-%?/r
: T=767K 0.0 0.0
d=O.BLE
‘,..‘....‘....‘.““.‘..“,..‘,“““‘. 0.1 0.2 0.3
0.4 s
0.5
0.6
0.7
0.6
Fig. 1. Shows the normalized Rh surface concentration as a function of S with 0.8 LE of Rh overlayer. The solid curve represents the time dependent concentration from Eq. (3); the annealing temperature is 767 K; the diffusion constant for this best least-squares fit is 8.77X 10-20 cm’/%
361
H. Wu, T. T. Tsong / Surface Science 318 (I 994) 358-362
squares fit, is achieved between the experimental data and Eq. (3) for all the data points. Figs. 1 and 2 show the normalized Rh surface concentration, C,(t>/C,(O), as a function of S at 767 and 791 K for different Rh coverages. The horizontal scale of S has been obtained for the adjusted value of D(T) using S =(D(T)t/d ’ ) ‘I2 . In Figs. 1 and 2 the circular spots are the experimental data and the solid curve is deriveded from Eq. (3) using D(T) = 8.77 X 10p2’ cm2/s for Fig. 1 and 2.35 X lo-l9 cm2/s for Fig. 2. Fig. 1 is for inter-diffusion with 0.8 LE of Rh at 767 K, and Fig. 2 is for 1.5 LE of Rh at 791 K. For bulk diffusion, the diffusion constants are found to vary with temperature as [8] D(T) = Do exp( -E/&J), (5) with Do = ua2, a the lattice constant, Y the atomic vibrational frequency, T the annealing temperature, Do the diffusivity, and E the activation energy. Thus a logarithmic plot of the diffusion constants against inverse annealing temperature, known as Arrhenius plot, should give a straight line. The slope of this straight line should give the activation energy E and the intercept of the line should give the diffusivity
‘*O* 0.6
G -,0.6
-
-
Y t:
‘%I J
-
. . . . . . . 44
-42
.I.. For
. . . . . . I.. O.BLE.
. . . . . . .
E,=1.63&0.4eV
.
D,,=1.4xlO-afa’aom*/r 1
1
-43
E; -44 ?i -45
-45
-47 1 .2
1.3
lOOO/T(K
1.14
1.5
)
Fig. 3. Shows the diffusion constants of Rh atoms into the (5 X 20) reconstructed Pt(100) substrate versus inverse annealing temperature. The slopes and intercepts give the inter-diffusion activation energies and diffusivities to be 1.63 + 0.4 eV and 1.4 X 10m8 * 2.8 cm*/s for the 0.8 LE of Rh overlayer and 1.68k 0.2 eV and 0.6 X lo-* * 1.4 cm*/s for the 1.5 LE of Rh overlayer.
Do. From the slopes and intercepts of the two plots
2..
oOo.4
-41
-C,(t)/C,(o)=erp(Sa)erfo(s), #=Dt/dQ
shown in Fig. 3, we obtain activation energies of 1.63 f 0.4 and 1.68 f 0.2 eV, and diffusivities of 1.4 X lo-* * 2.8 and 0.6 X 10e8 * 1.4 cm2/s for 0.8 and 1.5 LE Rh coverages, respectively. Within the experimental uncertainties, we find that the activation energy of interdiffusion of Rh along the (100) direction in the near surface layers of the Pt(100) is independent of the surface coverage of deposited Rh.
4. Conclusions 0.2
T=781K
0.0 * 0.0
0.1
d=l.6LE
0.2
0.3
0.4
0.5
0.6
s Fig. 2. Shows the function of S with represents the time annealing temperature best least-squares fit
normalized Rh surface concentration as a 1.5 L,E of Rh overlayer. The solid curve dependent concentration from Eq. (3); the is 791 K, the diffusion constant for this is 2.35 X lo-r9 c&/s.
(1) The deposition of a small amount of Rh on a clean and (5 X 20) reconstructed Pt(100) surface destroys the reconstruction and the surface structure returns to (1 X 1). (2) For a surface coverage of 0.8 LE of Rh on the (5 X 20) reconstructed Pt(100) surface, the parameters of inter-diffusion are found to be E = 1.63 f 0.4 eV, Do = 1.4 X 10p8* 2.8 cm2/s, and v= 3.6 X 107f
2.8 s-1
362
H. Wy TX Tsong/~~ace
(3) For a surface coverage of 1.5 LE of Rh on the (5 X 20) reconstructed Pt(100) surface, the parameters of inter-diffusion are found to be E = 1.68 + 0.2 eV, D, = 0.6 X 1O-s*‘.4 cm2/s, and v- 1.6 X 107Etl.4
s-1
(4) The activation energy of surface diffusion of single Pt adatoms on Pt(100) is 0.47 eV [9], for Rh on Rh(100) it is 0.88 eV [lo] from FIM experiments, and for Rh on Pt(100) it is 0.57 eV [ll] from a theoretical calculation. We are not able to find data for the activation energy of bulk diffusion of Rh in a Pt crystal. The closest systems we can find are Cu and Pt diffusion in Pt. Their activation energies are as high as 2.6 eV 1123and 3.0 eV [13], respectively. Near the surface, the surface layers are slightly relaxed. The activation energy of diffusion in the near surface layers should therefore be somewhere between those of surface diffusion and bulk diffusion. Our result of 1.63 and 1.68 eV indeed substantiates this simple argument. (5) The techniques we use do not allow us to study whether the Rh film we created on the Pt surface is atomic~ly flat or not. As our results obtained with 0.8 and 1.5 monolayer coverages do not differ appreciably, we believe the uniformity of the deposited film is good enough not to affect our measurement of the inter-diffusion parameters.
Acknowledgement This work was supported by the National Science Council of ROC under grant NSC 83-0208-M-001002
Science 318 (1994~358-362
References Ill D.E. Sanders and A.E. DePristo, Surf. Sci. 260 (1992) 116; L.A. Ray and R.C. Baetzold, J. Chem. Phys. 93 (1990) 2871; L. Barbier, B. Salanon, J. Sprosser and J. Lapujoulade, Surf. Sci. 2a7/2aa (1993) 930; M. AInot and J. Fusy, Appl. Surf. Sci. 55 (1992) 209; Y.-F. Liew, Y.-L. He, A. Chan and G.-C. Wang, Surf. Sci. 273 (1992) L461; P.J. S&nits, W.-Y. Leung, G.W. Graham and PA. Tbiel, Phys. Rev. B 40 (1989) 11477; J.W.A. SachtIer, M.A. Van Hove, J.P. Biberian and Gh. Somorjai, Phys. Rev. Lett. 45 (1980) 1601. I21M. Tar&u&i, E.K. Kuzembaeu and K.-I. Tanaka, Surf. Sci. 290 (1993) L711. t31 M.A. Van Hove, R.J. Koestner, P.C. Stair, J.P. Biberian, L.L. Kesmodel, I. Bartos and G.A. Somorjai, Surf. Sci. 103 (1981) 189. [41 H. Li, SC. Wu, D. Tian, Y.S. Li, J. Quinn and F. Jona, Phys. Rev. B 44 (1991) 1438; J.F. Wendelken and D.M. Zehner, Surf. Sci. 71 (1978) 178. [51LE. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Perk&Elmer, Eden Prarie, MN, 1976). f61 S. Tamrma, C.J. Powell and D.R. Penn, J. Vat. Sci. Technol. A 8 (1990) 2213. 171 Y.-L. He, J.-K. Zuo and G.-C. Wang, Surf. Sci. 255 (1991) ‘269. @I C. Kittel, Introduction to Solid State Physics, 6th. ed. (Wiley, New York, 1986) Ch. 18. t91 G.L. Kellogg and P.J. Feibelman, Phys. Rev. Len. 64, 3143 (1990). [lOI G. Ayrault and G. Ehrlich, J. Chem. Phys. 60 (1974) 281. [ill D.E. Sanders and A.E. DePristo, Surf. Sci. 260 (1992) 116. WI R.D. Johnson and B.H. FauIkenberry, ASD-TDR-63-625 (July 1963). 1131 G.V. Kidson and R. Ross, Proc. 1st UNESCO Int. Conf. Radioisotopes in Scientific Research (1958) p. 185.
Surface Science 318 (1994) 358-362
Overlayer formation and inter-diffusion of Rh atoms at the Pt(lOO! surface Hao-tse WI, Tien T. Tsong * Inst~~te of Physics, Academia Sinicia, Taipei 11529, Taiwan, ROC
Received 23 February 1994; accepted for publication 29 June 1994
Abstract The kinetics of Rh inter-diffusion into a Pt(100) surface was investigated on abrupt interfaces of Rh/Pt(lOO) which were artificially created by thermal evaporation of 0.8 and 1.5 monolayers of Rh on a (5 x 20) reconstructed Pt(100) substrate at 110 IL Deposition of a small amount of Rh on a clean and reconstructed Pt(lOO) surface destroys the reconstruction and yields a (1 X 1) structure. The kinetics of Rb diffusion from the overlayer into the bulk was monitored by Auger electron intensities of the Rh and Pt signals as a function of time and annealing tempera~re. The activation energies of inter-diffusion are derived to be 1.63 + 0.4 and 1.68 + 0.2 eV for 0.8 and 1.5 monolayer coverages, respectivefy.
1. Introduction
the bulk. Applications which depend on the diffnsion
Growth of thin-films of metals on transition metal surfaces by vapor deposition has been studied exten-
sively during the last decade. The interfacial properties of ultrathin heteroepitaxial films are of fundamental and practical importance as evidenced from the vast amount of existing literature in surface science and materials research. In most studies the motivation has been to determine the influence of the substrate on the properties of the overlayer film; some questions have been experimentally investigated such as the growth modes, and diffusion of adatoms, etc. Diffusion of atoms and molecules on a metal surface is also a technologically important phenomenon. Diffusion is the primary mechanism for the transport of matter across surfaces and into
* Co~es~nding ~39-6028/94/$07.~
author. Fax: 886 2 7899601.
of adsorbates include the growth of thin films, the formation of alloy thin-films and crystals, the formation and the stability of epitaxial layers, and heterogeneous catalysis [l], etc. Alloys formed from metals in group VIII in the periodic table are used increasingly as catalysts for hydrocarbon reactions. The Pt-Rh catalyst is a typical bimetallic or alloy catalyst used to remove CO, hydro~bon, and NO, from automotive exhaust gases [z]. For these reasons we undertook an analysis of inter-diffusion of Rh films on the Pt(100) surface. Deposition of Rh atoms onto a (5 X 20) reconstructed Pt(100) surface should exhibit interesting features since the substrate itself is reconstructed by the formation of a quasi-hexagonal layer [3]. To the best of our knowledge, until now only one study about inter-diffusion in the near surface layers of Rb into Pt(ll0) [7] has been carried out. In that study by He et al., the following results were obtained. The barrier height for Rh di~sion into Pt has been
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0039-6028(94)00424-Z
H. Wu, T.T. Tsong/Surface
determined to be 2.69 f 0.55 eV from short annealing time data. There was about 5% carbon contamination during the deposition. Various equilibrium structural phases, c(2 X 21, (1 X 3) and (1 X 21, associated with certain Rh concentrations during diffusion were observed by using high-resolution LEED. At 670 K the diffusion constant determined from the data points at short annealing times is (4.32 f 0.75) X 10e2’ cm2/s. At long annealing times the value of the diffusion constant is two orders of magnitude higher than this value. In this work we investigate the kinetics of Rh atom inter-diffusion at an abrupt interface of Rh on the (5 X 20) reconstructed Pt(100) surface using Auger electron spectroscopy @ES). We study the AES signal intensity as a function of time and annealing temperature. With 0.8 and 1.5 Rh overlayers, the diffusion constants and activation energies were determined for inter-diffusion of Rh atoms into the near surface layers of the Pt(100) surface. We derive the diffusion parameters from a least-squares fit of our experimental data over the entire time span of our measurements with a theoretical equation.
2. Experimental
procedures
In this experiment, low energy electron diffraction (LEED) was used for identifying the atomic structure of the clean and (5 x 20) reconstructed Pt(100) substrate and the Rh overlayers, and AES was used for determining the surface coverages and elemental concentrations of the surface as a function of time at a given annealing temperature. Two types of experiment were done: (a) After deposition of Rh on the clean and reconstructed Pt surface, the diffraction pattern was investigated by LEED. (b) The kinetics of inter-diffusion was analysed by monitoring the Pt and Rh peak-to-peak heights of the AES signals as a function of annealing time at a fixed annealing temperature. The substrate was prepared according to conventional procedures. The Pt crystal ( < 10 ppm impurities, Technisch Co.) was oriented and spark cut to within 0.5” of the (100) plane; it was then polished to a 1 pm alumina grit. The base pressure of the chamber was about 2 X lo-” Torr and cleaning [3] in situ involved tens of cycles of a combination of
359
Science 318 (1994) 358-362
argon-ion bombardment (1 X 10e6 Torr, 1.2 keV, for 30 min at room temperature) and light oxygen treatment (3 X lop7 Torr, for 20 min at 500°C) followed by annealing at about 550°C for 1 h until the AES signals showed no detectable impurities and the LEED pattern showed the sharp spot pattern of the well-known (5 X 20) reconstruction. Cleaning of the sample took well over a month. The Rh source consisted of a 0.25 mm Rh coil spot-welded on Ni feedthroughs that were electrically heated. The distance between the Rh source and the Pt substrate was about 6 cm. The surface coverage was determined from the between the intensity of the ratio R = &h(300&64) Rh AES line at 300 eV and the intensity of the Pt AES line at 64 eV, using the formula [4] 1 -e-d/A,oo
R=R 0
e-d/b
'
(1)
where R, = l&30,,~/Z&64~ = 2.33 [5]; P” denotes the intensity of the AES line for a very thick sample of Rh at 300 eV or Pt at 64 eV; the h’s are the inelastic mean free paths of electrons with 300 or 64 eV energy (A,,, = 0.665 mn, A,, = 0.410 nm, data obtained from the formula given by Tanuma et al. [6]); and d is the thickness of the Rh film which is assumed to be uniform over the substrate surface. Taking d to be about 0.2 nm, we will quote surface coverages below in units of layer equivalent (abbreviated LE) [4], with the understanding that 1 LE is the amount of adsorbate that produces the same value of R as would be produced by a uniform monolayer of the adsorbate on the substrate surface. For 1 and 2 LE of Rh overlayers, the intensity ratios are 0.986 and 2.7928, respectively. After each deposition of Rh on the clean (5 x 20) reconstructed Pt(100) surface to the desired coverage of about 0.8 or 1.5 monolayers, the sample was subsequently heated to a preselected annealing temperature where diffusion of Rh into the bulk occurs at a certain rate. The degree of coverage of the deposited Rh overlayer was monitored by AES at a substrate temperature of N 110 K. At this temperature Rh atoms will not diffuse into the bulk even under the radiation heating from the nearby Rh evaporation source. We measured the peak intensity (the differential peak-to-peak height) ratio of the
H. Wu, T.T. Tsong /Surface Science 318 (I 994) 358-362
360
Auger lines of the Pt substrate at 64 eV and the Rh overlayer at 300 eV to determine the Rh coverage. The kinetics of inter-diffusion was then studied by detecting the Pt and Rh Auger peak intensities as a function of annealing time at various up-quenching temperatures as the Rh atoms diffuse into the bulk of the Pt(100).
3. Results and dissusions For the clean sample after the elaborate cleaning and annealing procedures described above, the LEED pattern exhibited (5 X 20) with a very high signalto-background ratio. After deposition of Rh, to an estimated surface coverage of 0.3 LE, the LEED pattern transformed into (1 X 1) with a smaller signal-to-background ratio. This result is very similar to those of Rh and Fe on Au(100) [4] where the (5 X 20) resconstructed pattern shifts to (1 X 1) after about 0.2 LE deposition of Rh or Fe, and electrochemical deposition of Rh ions on PtilOO) [2] where the deposited Pt surface gives a (1 X 1) LEED pattern for about 0.4 monolayer of Rh ions. We find that after the up-quenching, the Rh concentration decreases rapidly at the beginning but the rate of decrease slows down gradually. In the model of Ref. [7], where the initial condition of having an overlayer of Rh on a semi-infinite slab of Pt is used, the normalized concentration of Rh as a function of time and depth is described by
where C,(t) is the Rh concentration at the surface after an annealing time t, C,(O) is the Rh concentration before annealing, and S = (D(T>t/d2>“‘. For D(T>t/d2 -=K1, or for short annealing times, the normalized Rh surface concentration is +2s
J;; *
(4)
It is implied in Eq. (4) that the normalized Rh surface concentration is a function of fi at short annealing times. The yielded slope is proportional to /D(T) at the corresponding temperature. The normalized Rh surface concentrations, CL(t>/Cb(O), were obtained from the experiments. The values of D(T) and D’(T), can be determined by substituting the thickness of the overlayer and the corresponding annealing time and the normalized Rh surface concentration into Eq. (4). In this work we first got a rough estimate of D(T) for short annealing times by averaging about the first eight diffusion constants, D’(T), at a given annealing temperature. Then the value of D(T) is adjusted until the best fit, least-
C( x, t> C( x, 0) =exp(a+T)
erfc[&+(%)“i].
(2)
where C(x, t) is the Rh concentration at depth x at annealing time t; C(x, 0) is the Rh concentration at depth x before annealing; erfc is the complementary error function; d is the thickness of the deposited Rh overlayer; and D is the diffusion constant at a given annealing temperature, i.e. D = D(T), where T is the annealing temperature. At the surface (i.e. x = 01, Eq. (2) can be rewritten as C,( t)/C,(O)
= exp(S2)
erfc(Q,
(3)
: 0.2
-c,(t)/c,(o)=exp(sE)sric(s), SB=Dt/dE
: D=6.77~10-%?/r
: T=767K 0.0 0.0
d=O.BLE
‘,..‘....‘....‘.““.‘..“,..‘,“““‘. 0.1 0.2 0.3
0.4 s
0.5
0.6
0.7
0.6
Fig. 1. Shows the normalized Rh surface concentration as a function of S with 0.8 LE of Rh overlayer. The solid curve represents the time dependent concentration from Eq. (3); the annealing temperature is 767 K; the diffusion constant for this best least-squares fit is 8.77X 10-20 cm’/%
361
H. Wu, T. T. Tsong / Surface Science 318 (I 994) 358-362
squares fit, is achieved between the experimental data and Eq. (3) for all the data points. Figs. 1 and 2 show the normalized Rh surface concentration, C,(t>/C,(O), as a function of S at 767 and 791 K for different Rh coverages. The horizontal scale of S has been obtained for the adjusted value of D(T) using S =(D(T)t/d ’ ) ‘I2 . In Figs. 1 and 2 the circular spots are the experimental data and the solid curve is deriveded from Eq. (3) using D(T) = 8.77 X 10p2’ cm2/s for Fig. 1 and 2.35 X lo-l9 cm2/s for Fig. 2. Fig. 1 is for inter-diffusion with 0.8 LE of Rh at 767 K, and Fig. 2 is for 1.5 LE of Rh at 791 K. For bulk diffusion, the diffusion constants are found to vary with temperature as [8] D(T) = Do exp( -E/&J), (5) with Do = ua2, a the lattice constant, Y the atomic vibrational frequency, T the annealing temperature, Do the diffusivity, and E the activation energy. Thus a logarithmic plot of the diffusion constants against inverse annealing temperature, known as Arrhenius plot, should give a straight line. The slope of this straight line should give the activation energy E and the intercept of the line should give the diffusivity
‘*O* 0.6
G -,0.6
-
-
Y t:
‘%I J
-
. . . . . . . 44
-42
.I.. For
. . . . . . I.. O.BLE.
. . . . . . .
E,=1.63&0.4eV
.
D,,=1.4xlO-afa’aom*/r 1
1
-43
E; -44 ?i -45
-45
-47 1 .2
1.3
lOOO/T(K
1.14
1.5
)
Fig. 3. Shows the diffusion constants of Rh atoms into the (5 X 20) reconstructed Pt(100) substrate versus inverse annealing temperature. The slopes and intercepts give the inter-diffusion activation energies and diffusivities to be 1.63 + 0.4 eV and 1.4 X 10m8 * 2.8 cm*/s for the 0.8 LE of Rh overlayer and 1.68k 0.2 eV and 0.6 X lo-* * 1.4 cm*/s for the 1.5 LE of Rh overlayer.
Do. From the slopes and intercepts of the two plots
2..
oOo.4
-41
-C,(t)/C,(o)=erp(Sa)erfo(s), #=Dt/dQ
shown in Fig. 3, we obtain activation energies of 1.63 f 0.4 and 1.68 f 0.2 eV, and diffusivities of 1.4 X lo-* * 2.8 and 0.6 X 10e8 * 1.4 cm2/s for 0.8 and 1.5 LE Rh coverages, respectively. Within the experimental uncertainties, we find that the activation energy of interdiffusion of Rh along the (100) direction in the near surface layers of the Pt(100) is independent of the surface coverage of deposited Rh.
4. Conclusions 0.2
T=781K
0.0 * 0.0
0.1
d=l.6LE
0.2
0.3
0.4
0.5
0.6
s Fig. 2. Shows the function of S with represents the time annealing temperature best least-squares fit
normalized Rh surface concentration as a 1.5 L,E of Rh overlayer. The solid curve dependent concentration from Eq. (3); the is 791 K, the diffusion constant for this is 2.35 X lo-r9 c&/s.
(1) The deposition of a small amount of Rh on a clean and (5 X 20) reconstructed Pt(100) surface destroys the reconstruction and the surface structure returns to (1 X 1). (2) For a surface coverage of 0.8 LE of Rh on the (5 X 20) reconstructed Pt(100) surface, the parameters of inter-diffusion are found to be E = 1.63 f 0.4 eV, Do = 1.4 X 10p8* 2.8 cm2/s, and v= 3.6 X 107f
2.8 s-1
362
H. Wy TX Tsong/~~ace
(3) For a surface coverage of 1.5 LE of Rh on the (5 X 20) reconstructed Pt(100) surface, the parameters of inter-diffusion are found to be E = 1.68 + 0.2 eV, D, = 0.6 X 1O-s*‘.4 cm2/s, and v- 1.6 X 107Etl.4
s-1
(4) The activation energy of surface diffusion of single Pt adatoms on Pt(100) is 0.47 eV [9], for Rh on Rh(100) it is 0.88 eV [lo] from FIM experiments, and for Rh on Pt(100) it is 0.57 eV [ll] from a theoretical calculation. We are not able to find data for the activation energy of bulk diffusion of Rh in a Pt crystal. The closest systems we can find are Cu and Pt diffusion in Pt. Their activation energies are as high as 2.6 eV 1123and 3.0 eV [13], respectively. Near the surface, the surface layers are slightly relaxed. The activation energy of diffusion in the near surface layers should therefore be somewhere between those of surface diffusion and bulk diffusion. Our result of 1.63 and 1.68 eV indeed substantiates this simple argument. (5) The techniques we use do not allow us to study whether the Rh film we created on the Pt surface is atomic~ly flat or not. As our results obtained with 0.8 and 1.5 monolayer coverages do not differ appreciably, we believe the uniformity of the deposited film is good enough not to affect our measurement of the inter-diffusion parameters.
Acknowledgement This work was supported by the National Science Council of ROC under grant NSC 83-0208-M-001002
Science 318 (1994~358-362
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