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Photoemission from small Pd clusters on Al2O3 and SiO2 substrates
Applied Surface Science 25 (1986) 81-94 North-Holland, Amsterdam
PHOTOEMISSION SUBSTRATES Shigemi Mutsushita
Received
FROM
SMALL
Pd CLUSTERS
ON AI,O, AND SiO,
KOHIKI Technoresearch,
Inc., Morigwhl,
1 July 1985; accepted
Osakrr 570, Japan
for publication
4 September
1985
Positive core-electron binding energy shifts in small palladium clusters supported on AlzO, and SiO, substrates are shown to arise predominantly from an initial-state effect that is more sensitive to cluster size than a final-state effect in lower coverage regions (Pd 5 1 X IO” atoms relaxation cm -’ for Al,O, and 2 1~10’~ atoms cm-* for SiO,). The change of the extra-atomic energy in larger coverage regions directly correlates to the polarizability of the substrates.
1. Introduction Small metal clusters on substrates are presently a subject of great interest in the transition of electronic states from the isolated atoms to the bulk metal. This activity has been motivated primarily by the tremendous technological importance of metal clusters, particularly in heterogeneous catalysis [l-5]. The metal nuclei formed in the earliest stages of vapor deposition on well-characterized substrates are ideal systems for study by X-ray photoelectron spectroscopy (XPS). Egelhoff and Tibbetts [6] reported that the core-level electron binding energies (BE) of Cu, Ni and Pd changed by larger amounts for amorphous carbon substrate than for crystalline carbon substrate. Amorphous carbon is the most widely used substrate, and the noble metals and group VIII metals are the most thoroughly studied metals. It has been reported that the electron BEs for small metal clusters supported on poorly conducting substrates generally diminish with an increase in the number of cluster atoms [6-121. It is possible to consider two different origins for interpreting the BE shift. One of the origins is that the shift is a result of a size dependence of the initial-state electronic structure. The alternative one is that it is due to variations in final-state relaxation processes. In this experiment Pd clusters on sputtered Al,O, and SiO, substrates were investigated. The bond ionicity (f,) of Al,O, is unknown and that of SiO, is reported as 0.61 [13]. It is ionic contrary to the bond of carbon (.f, = 0) [14].
0169-4332/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
In the photoemission final state, the hole state of the Pd core-level in the cluster should be screened by the valence electrons of the Pd cluster and the conduction electrons of the substrate. This relaxation shift depends on the relative magnitude of the polarizability of the substrate and the Pd metal. The final-state extra-atomic relaxation energy observed in an ionic solid is smaller than that observed in a covalent solid. It is expected that the polarization energy of the substrate, screening the localized core-hole state in the cluster. is very small for small Pd clusters on AllO, and SiOZ substrates in contrast with the case of an amorphous carbon substrate. This paper presents experimental results which suggest that the photoemission initial-state effect is primarily responsible for the measured BE shifts in small Pd clusters. The extra-atomic relaxation energy observed in Pd clusters on AI,O, and SiOZ substrates was different for each substrate and correlated to the difference of the polarizability of the substrate and Pd metal.
2. Experimental The photoemission spectra measurements were made on a VG ESCALAB-5 electron spectrometer using unmonochromatized Al Kcu radiation. The linewidth (FWHM) for the Ag 3d,, photopeak was 1.15 eV. No attempt has been made to remove the instrumental broadening. The spectrometer was calibrated by utilizing the energy difference (233.0 eV) between Al Ka and Mg Ka radiation. Then, the core-level BEs of Pd. Ag and Au foils were measured. The Pd 3d,/,, Ag 3d, ,? and Au 4f,,,,> BEs were, respectively, 335.4. 368.3 and 84.0 eV relative to the Fermi level. The probable electron energy uncertainty amounts to 0.1 eV. The normal operating vacuum pressure was less than 3X10mxPa. Pd M,VV Auger electrons could be excited by Al Ka X-rays. The Auger electron spectra were also recorded on the instrument. The Auger energies were taken as the peak in the second-derivative spectra. Firstly, the single crystalline Al?O, (sapphire) and SiO, (silica) surface was sputtered with 7 keV Ar+ ions in the sample preparation chamber of the spectrometer at room temperature. The sputtered substrate was not annealed produced a clean to maintain the amorphous surface. Art ion sputtering substrate surface. Spectra of the valence-band (VB), Al 2p, Al 2s Si 2p, Pd 3d, Pd M,VV. Ar 2p, 0 Is and C Is regions were recorded to monitor the condition of the substrate. No carbon contamination could be detected. The atomic concentration of implanted Ar was 3.5%, for Al,O, and 0.6% for SiO,. The composition of sputtered Al,O, and SiO, surface before Pd deposition was measured by varying the photoelectron take-off angle (0 = 10, 25. 35, 50. 90”). The effect of preferential sputtering was negligible in this experiment. The effect of preferential sputtering reported is serious for relatively low
energy ( < 1.5 keV) and for small atomic number (He, Ne) primary ions. In this experiment relatively high energy (7 keV) and a heavier (Ar) primary ion was used for sputtering. The Pd was deposited by resistive evaporation in the sample preparation chamber at room temperature. The sample was transferred between the analyzer chamber and the preparation chamber under a vacuum below 3 x 10 ' Pa. The coverage of the Pd was determined from the Pd 3ds,? peak intensity [15]. BEs were referenced to the 2p,,, line of the Ar implanted into the substrates, which was assigned a value of 242.3 eV [16]. VGSlOOO data system was used for data acquisition and data processing. Determination of core-level peak positions and spectral intensities (peak areas) was accomplished after smoothing and subtracting a smooth background. The subtraction of background in the VB region induced by the satellite X-ray (Al was performed. The Pd clusters contribution to VB spectra K (Y.4 ) irradiation was obtained by simply subtracting the spectrum obtained on the clean SiO, substrate.
3. Results and discussion All spectral
features
of the substrates
are unchanged
by the adsorption
of
Pd. It is generally stated that the actual photon absorption process occurs nearly instantaneously ( 5 lo- ” s) and the hole switching occurs in a time very much to lO_” s) is very less than lo- ” s. The localized screening response (lo-” fast in contrast to the delocalized screening response (lo-” to lo-” s). Delocalized screening is accompanied with coreevalence-valence (CVV) Auger transitions [17]. The BE of a level j. BE(j), is the difference in the total energy of the system in its ground state and in the state with one electron missing in orbital j. For most situations encountered in photoemission. the approximation. BE(j)=
-c(j)-R(j).
(I)
is close enough to discuss the chemical shift [lg]. Here -c(j) is the orbital energy term (initial-state effect) calculated by solving the Hartree-Fock equations by Koopmans’ theorem. R(j) is the relaxation energy term. which is the result of a flow of negative charge towards the hole created in the photoemission process in order to screen the suddenly appearing positive charge. The screening lowers the energy of the hole state left behind and therefore lowers the measured BE as well. The relaxation energy (R) can be partitioned into two terms: intra-atomic relaxation energy (R,,)and extra-atomic relaxation energy (R,,). The former is constant for the core-level electrons of a given atom. The latter varies with changes in chemical and physical states.
x4
S. KohiX, / Smull Pd clu.vten on .41,0., uml StO,
The kinetic energy (KE) referenced to the Fermi level of an Auger electron emitted from a transition jkl is given [19] by KE(jkl.
X)=BE(j)-BE(k)-BE(I)-F(k/;
X)+R”(kl),
(2)
where BE(j), BE(k) and BE( 1) are the BEs of the atomic core-level electrons j. k and 1, respectively. F( kl; X) is the interaction energy of the two holes k and 1 in the final-state multiplet X. And R”( kl) is the static relaxation energy describing the polarization energy (final-state effect). Here RS(kl)=RT(k/)-RD(k)-RI’(/). RT( kl) denotes the total two-hole relaxation energy. If the two final-state holes have the same main quantum number n and angular momentum quantum number I, the total two-hole relaxation energy RT( kk) is four times the one-hole relaxation energy RD( k ). So, we have the following relation RS( kk) = 2R”( k). According to eqs. (1) and (2), the photoelectron KE shift can be written as ABE(j)=
BE shift and Auger electron
-Ac(.j)-ARyx(,j).
(3)
and AKE(jkl:
X)=ABE(j)-ABE(k)-ABE(I)-AF(kl;
X)+AR&(k/).
(4)
and M,VV Auger In this experiment we use Pd 3d,,, p hotoelectrons electrons. The two-hole Coulomb interaction energy in the final-state valence band, F(W; X). is reasonably assumed to be independent of the number of Pd cluster atoms on each substrate. A F( kl; X) in eq. (4) can thus be omitted. Furthermore, identical final-state levels k = I are involved in the M,VV Auger process, and the approximate relationship Rs,(VV)= 2RK(V) may be used [20]. In the simplest approximation, the change in extra-atomic relaxation energy can be derived from the combination of Pd 3d,,, BE and M,VV Auger KE referenced to the Fermi level. These quantities define the modified Auger + KE(M,VV). The difference in the modified parameter [21] (Y= BE(3d,,,) Auger parameters for a given element in two different environments is twice the difference in dynamic extra-atomic relaxation energies [22], Acr = 2AR’,‘,(V). From these derivations, eqs. (3) and (4) can be written as ABE(3d,,,)
= -Ac(3d,,,)
-AR:(3d&
(5)
and AKE(M,VV)
= ABE(3d,,,)
- 2ABE(V)
where BE(V) is the mean valence
+ 2ARFx(V),
band electron
BE.
(6)
S. Kohlki / Small Pd clusters on AI,O,
and SiO,
85
If the assumption ARz(V)=ARFX(3d,,,), which has been found approximately for various levels [23], is valid, eqs. (5) and (6) yield the following result: AKE(M,VV)
= -Ar(3d,,,)
-2ABE(V)
+ARg(V).
(7)
A variation in size of the work function for small metallic spheres has been reported [24]. The work function change contains two terms: an increase due to the attraction of the unit charge left behind by photoemission, and a reduction due to the weaker image potential of a sphere compared to that of a plane, giving a net increase of 5.40(eV)/r(A) in the work function [25]. r is the radius of the metallic sphere. It is well known that when a sample is grounded to the spectrometer, changes in the work function do not affect the measured photoelectron BE and Auger electron KE relative to the Fermi level. It is not necessary to consider the effect of size variation of the work function in this experiment.
3.1. Al,03 experiment
Fig. 1 shows the Pd 3d,,, BE versus the Pd coverage. The Pd 3d,,, BE increases by 0.9 eV with decreasing coverage in the low-coverage region (less than = 1 X lOi atoms cm-* ). In the region of intermediate coverage ((l-3) x lOI atoms cm-*), the Pd 3d,,, BE decreased by 0.3 eV with increasing coverage. The Pd 3d,,, BE for Pd clusters in the high-coverage region (more than = 3 X lOI atoms cm-* ) is constant and identical to that obtained for bulk Pd metal.
1 bulk
AVERAGE
COVERAGE
value
btorns/crn2)
Fig. 1. Coverage dependence of the Pd 3d,,, electron binding energy (BE)
w
0
y326.0
0 0
t
db 10 1:” AVERAGE
10
COVERAGEbtomsicm2)
Fig. 2. Coverage dependence of the Pd M
Fig. 2 shows the Pd M,VV Auger KE versus the Pd coverage. The Pd M,VV Auger KE shifts almost linearly with the coverage increase. Fig. 3 shows the modified Auger parameter versus the Pd coverage. CI is almost constant in the low-coverage region (below = 1 x lOI atoms cm -‘). Therefore, the extra-atomic relaxation energy did not change. In the higher(Y increases by 1.3 eV with coverage region (Pd 2 1 X 1015 atoms cm-*).
663.0 O(D
> .!? t z E 2 6620 aa
C
xdk
value
0 0 0
00 -
00 00
0
0
00
i-+c-k++ 10
AVERAGE
10
COVERAGE
(atoms/cm’)
Fig. 3. Coverage dependence of the modified Auger parameter (a)
87
S. Kohiki / Small Pd clusters on AlJO, and SiO,
Pd =6X1015 (dtomscm-2)
5
10
BE(eV) Fig. 4. Valence band spectra of evaporated Pd/Al,O, substrate for various Pd coverages. satellite X-ray contribution to the valence band spectrum has been subtracted.
The
increasing coverage. In this coverage region the change in valence band dynamic extra-atomic relaxation energy, ARE(V), amounts to 0.65 eV. In region I (Pd < 1 x lOI atoms cm-*), AKE(M,W) is +l.l eV, ABE(3d,,,) is -0.9 eV and ARE(V) is zero with increasing coverage. From eq. (7), ABE(V) amounts to - 1.0 eV. In region II (Pd = (1-3)X lOI atoms cm-*), AKE(M,VV) is + 1.1 eV, ABE(3d,,,) is -0.3 eV and ARE(V) is +0.45 eV with increasing coverage. A BE(V) amounts to - 0.25 eV. In region III (Pd >- 3 X 1015 atoms cm-*), AKE(M,VV) is +0.4 eV, ABE(3d,,,) is zero and A R:(V) is +0.2 eV with increasing coverage. ABE(V) amounts to zero eV.
Table 1 Changes of extra-atomic relaxation energy, mean valence band electron binding energy, initial-state potential and 3d,,, electron binding energy for Pd clusters on Al,O, substrate with increasing coverage; energies are in eV Region
Coverage (atoms/cm2)
ARE(V)
A BE(V)
W3d,/,)
A BE(3d,,,)
I II III
(1x10’5 - (lL3)X10’5 2 3 x lOI5
0 + 0.45 +0.2
-1.0 -0.25 0
+ 0.9 -0.15 - 0.2
- 0.9 - 0.3 0
xx
S. Kohiki / Small Pd clusters on Al_d,
md SLO,
In this experiment it was possible to observe the VB shift directly. The ABE(V) shift in regions I, II and III is obvious as shown in fig. 4. Ac(3d,,,) can be obtained from eq. (7). In table 1 AR:(V), ABE(V), Ae(3ds,,z) and ABE(3d,,,,) are listed. In region I the - 0.9 eV shift for the Pd 3d,,, BE is ascribed to the change ( + 0.9 eV) in initial-state effect, Ae(3d,,,). In regions II and III. the sum of the change in the initial-state effect and the change in the final-state effect is in agreement with the change in the Pd 3d,,? BE. 3.2. SiO, experiment BE versus the Pd coverage. The Pd 3ds,,, BE Fig. 5 shows the Pd 3d,,, increases by 0.8 eV with decreasing coverage in the low-coverage region (less than = 1 X 1Or4 atoms cmp2). In the region of intermediate coverage ((0.1-l) x 1015 atoms cm-‘), the Pd 3d,,, BE decreased by 1.0 eV with increasing coverage. The Pd 3d,,, BE for Pd clusters in the high-coverage region (more than = 1 X 10” atoms cmp2 ) is constant and identical to that obtained for bulk Pd metal. Fig. 6 shows the Pd M,VV Auger KE versus the Pd coverage. The Pd M,VV Auger KE shifts almost linearly with the coverage increase. Fig. 7 shows the modified Auger parameter versus the Pd coverage. (Y is almost constant at coverages below = 1 X 1014 atoms cm 2. Therefore, the extra-atomic relaxation energy did not change with Pd coverage. In the higher-coverage region (Pd 2 1 X 1014 atoms crn~.‘), (Yincreases by 2.2 eV with the increase of coverage. In this coverage region the change in dynamic extra-atomic relaxation energy amounts to 1 .l eV. II 0
337.0
-
0 0
m 0 00
> ??
0 0
W
0
336.0
m
u0
-
mo
Ll?
0
00 0
B
0 0
335.0 I_I 13 10
Ill
14 10
Pd average
111 Id" coverage
00
111
metal value
co-
1,
lP (atoms
crri’)
Fig. 5. Coverage dependence of the Pd 3d,,? electron binding energy (BE).
S. Kohikr / Small Pd clusiers on Al,O_, and SIOz
89
, oo
327.0 -
o” -
metal
value
0 00°
1 aJ W li
0
325.0 -
0 0
k ?
0
Q
2 B Q 08
3230
’ Id”
’
’
’ 1014
Pd average
’
III
’ ’ Id”
coverage
I, 1016
(atoms
1
cm/)
Fig. 6. Coverage dependence of the Pd M,VV Auger electron kinetic energy (KE)
In region I (Pd < 1 X 1014 atoms cm-‘), AKE(M,W) is +0.9 eV, ABE(3d,,,) is -0.8 eV and AR:(V) is zero with increasing coverage. From eq. (7), ABE(V) amounts to -0.85 eV. In region II (Pd = (0.1-l) X lOI atoms cm-‘), AKE(M,VV) is +2.1 eV,
0 0
metal value
c
0
0 0
0
8
0
02000 0
o”Eo I
13
10
II 14 10
Pd average
I
II 15 10
coverage
III
I,
Id” (atoms
? cm-?
Fig. 7. Coverage dependence of the modified Auger parameter (a)
Table 2 Changes of extra-atomic relaxation energy. mean valence hand electron hindlng energy. initial-state potential and 3d,,.: electron binding energy for Pd clusters on SO, auhstrate with ~ncreahlng coverage; energies are in eV Region I II
III
Coverage (atoms/cm’
ARK(V)
ABE(V)
Ac(3d,
0 +o.ci +o.s
- 0.8.5 - 0.95 ~ 0.05
+ 0.8 + 0.4 -0.5
2)
JBE(3d,
-)
) ~ 0.X + 1.o 0
ABE(3d,,z) is -1.0 eV and ARFx(V) is +0.6 eV with increasing coverage. ABE(V) amounts to -0.95 eV. In region III (Pd 2 1 x lOI atoms cm-‘). AKE(M,VV) is +l.l eV. ABE(3d,,,,) is zero and A R:(V) is +0.5 eV with increasing coverage. ABE(V) amounts to -0.05 eV. Ac(3d,,,) can be obtained from eq. (7). In table 2 ARK(V), ABE(V), Ac(3d,,,) and ABE(3d,,,,) are listed. In the low-coverage region the -0.8 eV shift for the Pd 3d,,? BE is ascribed to the change (+0.8 eV) in initial-state effect, Ac(3d 5,,L). In the higher-coverage region, the sum of the change in the initial-state effect and the change in the final-state effect corresponds to the change in the Pd 3d 5,,Z BE. In this XPS experiment it was possible to observe the VB shift directly as shown in fig. 8. In region I only isolated Pd atoms are believed to be present. Assuming that spectrum (b) in fig. 8 corresponds almost entirely to Pd atoms on the surface, it is found that the peak which is accompanied by a shoulder (5.0 eV) lies 4.3 eV below the Fermi level. The ground state of the free Pd atom is 4d’O5d” (‘S,,) which would give a 4d’5s0 configuration with the two spinorbit terms 2D5,2 and ‘D3,z after the removal of a d electron. The ’ Dj ,7 state is also the ionic ground state. Thus, these are the first (lowest ionization energy) features expected in the spectrum occurring at 8.3 and 8.8 eV relative to the vacuum level [26]. Hybridization of the d-band with the empty s-p conduction states will cause the counts of true d-electrons to be lower. The lower BE side of the Pd VB spectrum grew with increasing coverage and the maximum point shifted toward to the Fermi level. The (n + 1)s orbitals are much more diffuse than nd orbitals. and consequently more energy would be required to form a solid with higher s-electron occupation such as adsorbed Pd. Spectrum (c) in fig. 8 is believed to correspond to adsorbed Pd atoms within the surface. It is found that the d-levels lie 2.3 and 3.9 eV below the Fermi level. The crystal field will split the d-level into a doublet with spacing on the order of 1.5 eV for adsorbed Pd [27,28]. Ionization of a d-electron from the d’s’ configuration gives the ‘S
S. Kohiki / Small Pd clusters on Al,O,
and s1.0,
91
Fig. 8. Valence band spectra of evaporated Pd with various coverages. The substrate background contribution to the valence band spectrum has been subtracted. (a) SO, substrate VB; (b) 3.9 x IO”; (c) 1.6 X 1014; (d) 4.0 X 1014; (e) 5.8 X 1015; (f) 1.8 X 10lh Pd atoms/cm*.
state of the d’s’ configuration. Prediction of the resulting XPS spectrum complicated by the lack of optical data on the d’s’ *S state [29]. 3.3. Interaction
is
between clusters and substrate
In the photoemission final state, the hole state of the Pd core-level in the cluster should be screened by the valence electrons of the Pd cluster and the conduction electron of the substrate. This relaxation shift depends on the relative magnitude of the polarizability of the substrate and the Pd metal. Pd clusters on Al,O, and SiO, substrates have been investigated. Such clusters represent a close facsimile of the important small-metal particle catalysis [l-5]. The reactivity of Pd catalysis was investigated concerning the interaction with supports such as Al,O,, TiO, and SiO, [5]. In this experiment it was shown that small Pd clusters contain fewer d-electrons relative to the bulk metal. The result may suggest a correlation between the density of empty d-electron states and the observed variation in catalytic activity with cluster size. In ionic solids the available mechanisms for screening the incremental positive charge in photoemission final state are relaxation of neighboring ions
and polarization of the electronic charge on these ions. The former is too slow to affect the active electrons BE, so only the latter is effective. The BE will be reduced by a corresponding polarization energy (Er,,,). The polarization contribution must therefore be regarded as a form of static extra-atomic relaxation. E po, of the simple hole state was derived by Fadley et al. [30] from a relationship, due to Mott and Gurney [31]. which includes the dielectric constant. The one-hole relaxation cannot be measured directly whereas that due to Auger ionization can. The modified Auger parameter is a measure of Epc,, due to the substrate. The substrate conduction band electron contribution in extra-atomic relaxation for the Pd core-level is constant in small clusters for each substrate (Pd 2 1 x 10” atoms cm-’ for AllO, substrate and < 1 x lOI atoms cm ’ for SiO, substrate) and decreases with increasing cluster atoms. In small supported clusters the screening by the substrate is less effective because the average distance of the screening charge is greater than in the metal. In the higher-coverage region the core-level BE is almost constant because the bulk metal like initial-state electronic structure in the cluster, as the effective source of shifts, has been obtained almost completely. The extra-atomic relaxation energy increases with the effective source of extra-atomic relaxation energy varying from the poorly conducting substrates to the Pd metallic film. The change in modified Auger parameter is equal to the change of static extra-atomic relaxation energy and the term of (E - l)/( E + 2) is the scale of polarizability of the substrate. The change of static extra-atomic relaxation energy of Pd and the polarizability of the substrate calculated by the Clausius-Mossotti relationship is shown in table 3. Both the physical and the electronic properties of A120, and SiOz are dominated by the 0 ion. The Al ion (radius 0.6 A) and the Si ion (radius 0.4 A) are both much smaller than the 0 ion (radius 1.3 A.). The 0 ion accounts for the major part of the polarizability of the substrate. The direct correlation between the change of modified Auger parameter of Pd and the bulk polarizaTable 3 Correlation polarizability
of the change in Pd modified of the substrates
Substrate
Al,% SiO z ‘) Ref. [14]. hJ Ref. [32]. ” Pd modified respectively,
(E-l)/(F+2)
0.79 0.32
a)
Auger
parameter
Bulk polarizahility
h,
with
the dielectric
constant
and
Difference of Pd ‘) modified Auger
(A’)
parameter
3.0 4.3
1.3 2.2
Auger parameter observed in Pd bulk metal. small cluster 662.7. 661.4 and 660.5 eV.
(eV)
on AlzOJ
and SiOz is.
S. Kohiki
/ Small Pd clusterr ON AI,O.,
und SIO,
93
bility of the substrate calculated by the ClausiussMossotti relationship is also shown in table 3. The change of dynamic extra-atomic relaxation energy for the core-hole in the Pd cluster supported on the Al ,O? substrate is + 0.65 eV and that observed on the SiO, substrate is + 1.1 eV with increasing coverage. The difference in the change of the dynamic extra-atomic relaxation energy between Al,O, substrate and SiO, substrate may suggest that the R,, for SiOZ is smaller than that for Al 2O,. The screening should be weaker for a cluster evaporated onto a more insulating, highly polarizable substrate.
4. Concluding remarks The present observation of BE shift with cluster size may be interpreted as a result of photoemission initial-state effect for the small metal clusters (Pd 2 1 X 1015 atoms cm-’ on Al,O, and < 1 X 1014 atoms cme2 on SiO,). Increased d-electron density in larger clusters is seen by an enhanced intensity at the top of the VB as the coverage increases as shown in figs. 4 and 8. Hence, the number of occupied d-orbitals is dependent on cluster size; such increased d-orbital occupation qualitatively accounts for the changes in corelevel BE and mean valence band electron BE. In the higher-coverage region, the change in Pd 3d,,, BE is connected with the sum of the changes in initial- and final-state effects in this experiment as shown in tables 1 and 2. The polarizability of the substrates directly correlates to the difference in extra-atomic relaxation energy of Pd between small clusters and bulk metal. The screening due to substrate conduction electrons was weaker for small clusters evaporated onto highly polarizable substrates. The result suggests a correlation between the density of empty d-electron states and the observed variation in catalytic activity with cluster size. Sample charging should cause shifts which have opposite sign and identical value of Pd 3d,,, BE and Pd M,VV KE at the same coverage on each substrate. The values of the shifts observed in Pd 3d5,z BE and Pd M,VV KE were different from each other at the same coverage on an identical substrate. It is obvious that sample charging cannot predict this difference.
Acknowledgements The author thanks Professor S. Ikeda, Osaka University, for helpful discussions and suggestions, Miss K. Oki for assistance and Dr. F. Konishi for support of this work.
94
References 111 P.N. Ross. K. Minoshita and P. Stonehart. J. C‘atalqvs 32 (1974) 163. [2] S. Ladaa. R.A. Dab Betta and M. Boudart. J. Catalyls 53 (197X) 356. [3] J.C. Vedvine. M. Dufaux. C. Naccache and B. Imelik. _I.Chem. Sec. Faraday Trans. 74 (IY7X) 440. [4] H. Arai and T. Kunugi. J. Catalysis 39 (1975) 294. (51 F. Buon-Verduraz. A. Omar. J. Fxard and B. Pont\inne. J. C‘atal>sls 53 (197X) 126. [6] W.F. Egelhoff. Jr. and G.G. Tihhetts. Phys. Rr\. B19 (1979) 502X. [7] G.K. Wertheim. S.B. Dicenu? and SE. Youngquist. Ph!s. Reb. Letters 51 (19X3) 2310. [X] M.G. Mason. Whys. Rev. B27 (1983) 74X. [9] Y. Takasu. R. Unuln. B. Teache. A.M. Brndbhau and M. Grunx. Surface Sci. 77 (lY7X) 21Y. [IO] H. Roulct. J.-M. Mariot. G. Dufour and C‘.F. Hague. J. Phys. FlO (19X0) 1025. 11l] L. Oherli. R. Monet. H.J. Mathleu. I>. Landolt and J. Buttet. Surface Sci. 106 (19X1) 301 [12] K.S. Liang. W.R. Salaneck and I.A. .Aksay. Solid State Commun. 19 (1976) 329. 1131 K. Huhner. Phys. Status Solidi a40 (1977) 4X7. K. Huhner and A. Lehmann. Phps. Status Solidi a46 (1978) 451. [14] J.C. Phillips, Bonds and Bands in Semlconductorb (Acadcmlc. Nem York, 1973). 1151 S. Kohiki. Appl. Surface Sci. 17 (19X4) 497. [16] S. Kohiki, T. Ohmura and K. Kusao. J. Electron Spectrosc. Related Phenomenn 7X (19X3) 229: 31 (1983) X5. [ 171 J.W. Gadruk, in: Photoemission and The Electronic Properties of Surfaces. Eds. B. Fcuerhacher. B. Fitton and R.F. WIllis (Wiley. New York. 197X). [1X] D.A. Shirley, J. F.lectron Spectrox. Related Phenomena 5 (1974) 135: Phys. Rec. A7 (lY73) 1520: Chem. Phvs. Letters. 17 (1972) 312. 1191 S.P. Kowalczyk,~R.A. Pollak. F.R. McFwly. L. Ixy and D.A. Shirley. Phys. Rev. BX (lY73) 23x7. 1201 S.P. Kowaluyk. I.. Ley. F.R. McFzely. R.A. Pollak and D.A. Shirley. Phy. Rev. B9 (1Y74) 3x1. [Zl] C.D. Wagner. Faraday Disc. Chem. Sot. 60 (197.5) 291: C.D. Wagner. L.H. Gale and R.H. Raynond. Anal. Chem. 51 (1979) 466. 1221 T.D. Thomas. J. Electron Spectrosc. Related Phenomena 20 (1980) 117. [23] F. Bechwzdt, R. Enderlein. R. Fellenherg. P. Streuhel and A. Meisel. J. tlectron Spectrosc. Related Phenomena 31 (19X3) 131. [24] A. Schmidt-Ott. P. Schurtenberger and H.C‘. Siegmann. Phys. Rev. Letters 45 (1980) 1284. [2S] D.M. Wood. Phys. Rev. Letters 46 (1981) 749. [26] C.E. Moore, Atomic Energy Levels. Natl. Bur. Std. Circ No. 467 (US Government Prlntlng Office. Washington. DC. 1949). [27] C.Y. Fang. J. Phys. F4 (1974) 775. [2X] N.J. Dianne and T.N. Rhodin. Phys. Rev. 814 (1976) 322. [?Y] C.F.. Moore. Atomic Energy Levels as Derived from Analysis of Opical Spectra. Natl. Stand. Ref. Data Series No. 35 (US Gowrnment Printing Office, Washington, DC, 1971). [iO] <‘IS. Fadley. S.B.M. Hagstrom. M.P. Klein and D.A. Shirlev, .I. Chern. Phys. 4X (196X) 3779. [31] N.F. Mott and R.W. Gurney. Electromc Procesw in Ionic Crystals (Clarendon. Oxford. 194X). [32] R.H. West and J.F.. Castle, Surface Interface Anal. 4 (19X2) 6X.
PHOTOEMISSION SUBSTRATES Shigemi Mutsushita
Received
FROM
SMALL
Pd CLUSTERS
ON AI,O, AND SiO,
KOHIKI Technoresearch,
Inc., Morigwhl,
1 July 1985; accepted
Osakrr 570, Japan
for publication
4 September
1985
Positive core-electron binding energy shifts in small palladium clusters supported on AlzO, and SiO, substrates are shown to arise predominantly from an initial-state effect that is more sensitive to cluster size than a final-state effect in lower coverage regions (Pd 5 1 X IO” atoms relaxation cm -’ for Al,O, and 2 1~10’~ atoms cm-* for SiO,). The change of the extra-atomic energy in larger coverage regions directly correlates to the polarizability of the substrates.
1. Introduction Small metal clusters on substrates are presently a subject of great interest in the transition of electronic states from the isolated atoms to the bulk metal. This activity has been motivated primarily by the tremendous technological importance of metal clusters, particularly in heterogeneous catalysis [l-5]. The metal nuclei formed in the earliest stages of vapor deposition on well-characterized substrates are ideal systems for study by X-ray photoelectron spectroscopy (XPS). Egelhoff and Tibbetts [6] reported that the core-level electron binding energies (BE) of Cu, Ni and Pd changed by larger amounts for amorphous carbon substrate than for crystalline carbon substrate. Amorphous carbon is the most widely used substrate, and the noble metals and group VIII metals are the most thoroughly studied metals. It has been reported that the electron BEs for small metal clusters supported on poorly conducting substrates generally diminish with an increase in the number of cluster atoms [6-121. It is possible to consider two different origins for interpreting the BE shift. One of the origins is that the shift is a result of a size dependence of the initial-state electronic structure. The alternative one is that it is due to variations in final-state relaxation processes. In this experiment Pd clusters on sputtered Al,O, and SiO, substrates were investigated. The bond ionicity (f,) of Al,O, is unknown and that of SiO, is reported as 0.61 [13]. It is ionic contrary to the bond of carbon (.f, = 0) [14].
0169-4332/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
In the photoemission final state, the hole state of the Pd core-level in the cluster should be screened by the valence electrons of the Pd cluster and the conduction electrons of the substrate. This relaxation shift depends on the relative magnitude of the polarizability of the substrate and the Pd metal. The final-state extra-atomic relaxation energy observed in an ionic solid is smaller than that observed in a covalent solid. It is expected that the polarization energy of the substrate, screening the localized core-hole state in the cluster. is very small for small Pd clusters on AllO, and SiOZ substrates in contrast with the case of an amorphous carbon substrate. This paper presents experimental results which suggest that the photoemission initial-state effect is primarily responsible for the measured BE shifts in small Pd clusters. The extra-atomic relaxation energy observed in Pd clusters on AI,O, and SiOZ substrates was different for each substrate and correlated to the difference of the polarizability of the substrate and Pd metal.
2. Experimental The photoemission spectra measurements were made on a VG ESCALAB-5 electron spectrometer using unmonochromatized Al Kcu radiation. The linewidth (FWHM) for the Ag 3d,, photopeak was 1.15 eV. No attempt has been made to remove the instrumental broadening. The spectrometer was calibrated by utilizing the energy difference (233.0 eV) between Al Ka and Mg Ka radiation. Then, the core-level BEs of Pd. Ag and Au foils were measured. The Pd 3d,/,, Ag 3d, ,? and Au 4f,,,,> BEs were, respectively, 335.4. 368.3 and 84.0 eV relative to the Fermi level. The probable electron energy uncertainty amounts to 0.1 eV. The normal operating vacuum pressure was less than 3X10mxPa. Pd M,VV Auger electrons could be excited by Al Ka X-rays. The Auger electron spectra were also recorded on the instrument. The Auger energies were taken as the peak in the second-derivative spectra. Firstly, the single crystalline Al?O, (sapphire) and SiO, (silica) surface was sputtered with 7 keV Ar+ ions in the sample preparation chamber of the spectrometer at room temperature. The sputtered substrate was not annealed produced a clean to maintain the amorphous surface. Art ion sputtering substrate surface. Spectra of the valence-band (VB), Al 2p, Al 2s Si 2p, Pd 3d, Pd M,VV. Ar 2p, 0 Is and C Is regions were recorded to monitor the condition of the substrate. No carbon contamination could be detected. The atomic concentration of implanted Ar was 3.5%, for Al,O, and 0.6% for SiO,. The composition of sputtered Al,O, and SiO, surface before Pd deposition was measured by varying the photoelectron take-off angle (0 = 10, 25. 35, 50. 90”). The effect of preferential sputtering was negligible in this experiment. The effect of preferential sputtering reported is serious for relatively low
energy ( < 1.5 keV) and for small atomic number (He, Ne) primary ions. In this experiment relatively high energy (7 keV) and a heavier (Ar) primary ion was used for sputtering. The Pd was deposited by resistive evaporation in the sample preparation chamber at room temperature. The sample was transferred between the analyzer chamber and the preparation chamber under a vacuum below 3 x 10 ' Pa. The coverage of the Pd was determined from the Pd 3ds,? peak intensity [15]. BEs were referenced to the 2p,,, line of the Ar implanted into the substrates, which was assigned a value of 242.3 eV [16]. VGSlOOO data system was used for data acquisition and data processing. Determination of core-level peak positions and spectral intensities (peak areas) was accomplished after smoothing and subtracting a smooth background. The subtraction of background in the VB region induced by the satellite X-ray (Al was performed. The Pd clusters contribution to VB spectra K (Y.4 ) irradiation was obtained by simply subtracting the spectrum obtained on the clean SiO, substrate.
3. Results and discussion All spectral
features
of the substrates
are unchanged
by the adsorption
of
Pd. It is generally stated that the actual photon absorption process occurs nearly instantaneously ( 5 lo- ” s) and the hole switching occurs in a time very much to lO_” s) is very less than lo- ” s. The localized screening response (lo-” fast in contrast to the delocalized screening response (lo-” to lo-” s). Delocalized screening is accompanied with coreevalence-valence (CVV) Auger transitions [17]. The BE of a level j. BE(j), is the difference in the total energy of the system in its ground state and in the state with one electron missing in orbital j. For most situations encountered in photoemission. the approximation. BE(j)=
-c(j)-R(j).
(I)
is close enough to discuss the chemical shift [lg]. Here -c(j) is the orbital energy term (initial-state effect) calculated by solving the Hartree-Fock equations by Koopmans’ theorem. R(j) is the relaxation energy term. which is the result of a flow of negative charge towards the hole created in the photoemission process in order to screen the suddenly appearing positive charge. The screening lowers the energy of the hole state left behind and therefore lowers the measured BE as well. The relaxation energy (R) can be partitioned into two terms: intra-atomic relaxation energy (R,,)and extra-atomic relaxation energy (R,,). The former is constant for the core-level electrons of a given atom. The latter varies with changes in chemical and physical states.
x4
S. KohiX, / Smull Pd clu.vten on .41,0., uml StO,
The kinetic energy (KE) referenced to the Fermi level of an Auger electron emitted from a transition jkl is given [19] by KE(jkl.
X)=BE(j)-BE(k)-BE(I)-F(k/;
X)+R”(kl),
(2)
where BE(j), BE(k) and BE( 1) are the BEs of the atomic core-level electrons j. k and 1, respectively. F( kl; X) is the interaction energy of the two holes k and 1 in the final-state multiplet X. And R”( kl) is the static relaxation energy describing the polarization energy (final-state effect). Here RS(kl)=RT(k/)-RD(k)-RI’(/). RT( kl) denotes the total two-hole relaxation energy. If the two final-state holes have the same main quantum number n and angular momentum quantum number I, the total two-hole relaxation energy RT( kk) is four times the one-hole relaxation energy RD( k ). So, we have the following relation RS( kk) = 2R”( k). According to eqs. (1) and (2), the photoelectron KE shift can be written as ABE(j)=
BE shift and Auger electron
-Ac(.j)-ARyx(,j).
(3)
and AKE(jkl:
X)=ABE(j)-ABE(k)-ABE(I)-AF(kl;
X)+AR&(k/).
(4)
and M,VV Auger In this experiment we use Pd 3d,,, p hotoelectrons electrons. The two-hole Coulomb interaction energy in the final-state valence band, F(W; X). is reasonably assumed to be independent of the number of Pd cluster atoms on each substrate. A F( kl; X) in eq. (4) can thus be omitted. Furthermore, identical final-state levels k = I are involved in the M,VV Auger process, and the approximate relationship Rs,(VV)= 2RK(V) may be used [20]. In the simplest approximation, the change in extra-atomic relaxation energy can be derived from the combination of Pd 3d,,, BE and M,VV Auger KE referenced to the Fermi level. These quantities define the modified Auger + KE(M,VV). The difference in the modified parameter [21] (Y= BE(3d,,,) Auger parameters for a given element in two different environments is twice the difference in dynamic extra-atomic relaxation energies [22], Acr = 2AR’,‘,(V). From these derivations, eqs. (3) and (4) can be written as ABE(3d,,,)
= -Ac(3d,,,)
-AR:(3d&
(5)
and AKE(M,VV)
= ABE(3d,,,)
- 2ABE(V)
where BE(V) is the mean valence
+ 2ARFx(V),
band electron
BE.
(6)
S. Kohlki / Small Pd clusters on AI,O,
and SiO,
85
If the assumption ARz(V)=ARFX(3d,,,), which has been found approximately for various levels [23], is valid, eqs. (5) and (6) yield the following result: AKE(M,VV)
= -Ar(3d,,,)
-2ABE(V)
+ARg(V).
(7)
A variation in size of the work function for small metallic spheres has been reported [24]. The work function change contains two terms: an increase due to the attraction of the unit charge left behind by photoemission, and a reduction due to the weaker image potential of a sphere compared to that of a plane, giving a net increase of 5.40(eV)/r(A) in the work function [25]. r is the radius of the metallic sphere. It is well known that when a sample is grounded to the spectrometer, changes in the work function do not affect the measured photoelectron BE and Auger electron KE relative to the Fermi level. It is not necessary to consider the effect of size variation of the work function in this experiment.
3.1. Al,03 experiment
Fig. 1 shows the Pd 3d,,, BE versus the Pd coverage. The Pd 3d,,, BE increases by 0.9 eV with decreasing coverage in the low-coverage region (less than = 1 X lOi atoms cm-* ). In the region of intermediate coverage ((l-3) x lOI atoms cm-*), the Pd 3d,,, BE decreased by 0.3 eV with increasing coverage. The Pd 3d,,, BE for Pd clusters in the high-coverage region (more than = 3 X lOI atoms cm-* ) is constant and identical to that obtained for bulk Pd metal.
1 bulk
AVERAGE
COVERAGE
value
btorns/crn2)
Fig. 1. Coverage dependence of the Pd 3d,,, electron binding energy (BE)
w
0
y326.0
0 0
t
db 10 1:” AVERAGE
10
COVERAGEbtomsicm2)
Fig. 2. Coverage dependence of the Pd M
Fig. 2 shows the Pd M,VV Auger KE versus the Pd coverage. The Pd M,VV Auger KE shifts almost linearly with the coverage increase. Fig. 3 shows the modified Auger parameter versus the Pd coverage. CI is almost constant in the low-coverage region (below = 1 x lOI atoms cm -‘). Therefore, the extra-atomic relaxation energy did not change. In the higher(Y increases by 1.3 eV with coverage region (Pd 2 1 X 1015 atoms cm-*).
663.0 O(D
> .!? t z E 2 6620 aa
C
xdk
value
0 0 0
00 -
00 00
0
0
00
i-+c-k++ 10
AVERAGE
10
COVERAGE
(atoms/cm’)
Fig. 3. Coverage dependence of the modified Auger parameter (a)
87
S. Kohiki / Small Pd clusters on AlJO, and SiO,
Pd =6X1015 (dtomscm-2)
5
10
BE(eV) Fig. 4. Valence band spectra of evaporated Pd/Al,O, substrate for various Pd coverages. satellite X-ray contribution to the valence band spectrum has been subtracted.
The
increasing coverage. In this coverage region the change in valence band dynamic extra-atomic relaxation energy, ARE(V), amounts to 0.65 eV. In region I (Pd < 1 x lOI atoms cm-*), AKE(M,W) is +l.l eV, ABE(3d,,,) is -0.9 eV and ARE(V) is zero with increasing coverage. From eq. (7), ABE(V) amounts to - 1.0 eV. In region II (Pd = (1-3)X lOI atoms cm-*), AKE(M,VV) is + 1.1 eV, ABE(3d,,,) is -0.3 eV and ARE(V) is +0.45 eV with increasing coverage. A BE(V) amounts to - 0.25 eV. In region III (Pd >- 3 X 1015 atoms cm-*), AKE(M,VV) is +0.4 eV, ABE(3d,,,) is zero and A R:(V) is +0.2 eV with increasing coverage. ABE(V) amounts to zero eV.
Table 1 Changes of extra-atomic relaxation energy, mean valence band electron binding energy, initial-state potential and 3d,,, electron binding energy for Pd clusters on Al,O, substrate with increasing coverage; energies are in eV Region
Coverage (atoms/cm2)
ARE(V)
A BE(V)
W3d,/,)
A BE(3d,,,)
I II III
(1x10’5 - (lL3)X10’5 2 3 x lOI5
0 + 0.45 +0.2
-1.0 -0.25 0
+ 0.9 -0.15 - 0.2
- 0.9 - 0.3 0
xx
S. Kohiki / Small Pd clusters on Al_d,
md SLO,
In this experiment it was possible to observe the VB shift directly. The ABE(V) shift in regions I, II and III is obvious as shown in fig. 4. Ac(3d,,,) can be obtained from eq. (7). In table 1 AR:(V), ABE(V), Ae(3ds,,z) and ABE(3d,,,,) are listed. In region I the - 0.9 eV shift for the Pd 3d,,, BE is ascribed to the change ( + 0.9 eV) in initial-state effect, Ae(3d,,,). In regions II and III. the sum of the change in the initial-state effect and the change in the final-state effect is in agreement with the change in the Pd 3d,,? BE. 3.2. SiO, experiment BE versus the Pd coverage. The Pd 3ds,,, BE Fig. 5 shows the Pd 3d,,, increases by 0.8 eV with decreasing coverage in the low-coverage region (less than = 1 X 1Or4 atoms cmp2). In the region of intermediate coverage ((0.1-l) x 1015 atoms cm-‘), the Pd 3d,,, BE decreased by 1.0 eV with increasing coverage. The Pd 3d,,, BE for Pd clusters in the high-coverage region (more than = 1 X 10” atoms cmp2 ) is constant and identical to that obtained for bulk Pd metal. Fig. 6 shows the Pd M,VV Auger KE versus the Pd coverage. The Pd M,VV Auger KE shifts almost linearly with the coverage increase. Fig. 7 shows the modified Auger parameter versus the Pd coverage. (Y is almost constant at coverages below = 1 X 1014 atoms cm 2. Therefore, the extra-atomic relaxation energy did not change with Pd coverage. In the higher-coverage region (Pd 2 1 X 1014 atoms crn~.‘), (Yincreases by 2.2 eV with the increase of coverage. In this coverage region the change in dynamic extra-atomic relaxation energy amounts to 1 .l eV. II 0
337.0
-
0 0
m 0 00
> ??
0 0
W
0
336.0
m
u0
-
mo
Ll?
0
00 0
B
0 0
335.0 I_I 13 10
Ill
14 10
Pd average
111 Id" coverage
00
111
metal value
co-
1,
lP (atoms
crri’)
Fig. 5. Coverage dependence of the Pd 3d,,? electron binding energy (BE).
S. Kohikr / Small Pd clusiers on Al,O_, and SIOz
89
, oo
327.0 -
o” -
metal
value
0 00°
1 aJ W li
0
325.0 -
0 0
k ?
0
Q
2 B Q 08
3230
’ Id”
’
’
’ 1014
Pd average
’
III
’ ’ Id”
coverage
I, 1016
(atoms
1
cm/)
Fig. 6. Coverage dependence of the Pd M,VV Auger electron kinetic energy (KE)
In region I (Pd < 1 X 1014 atoms cm-‘), AKE(M,W) is +0.9 eV, ABE(3d,,,) is -0.8 eV and AR:(V) is zero with increasing coverage. From eq. (7), ABE(V) amounts to -0.85 eV. In region II (Pd = (0.1-l) X lOI atoms cm-‘), AKE(M,VV) is +2.1 eV,
0 0
metal value
c
0
0 0
0
8
0
02000 0
o”Eo I
13
10
II 14 10
Pd average
I
II 15 10
coverage
III
I,
Id” (atoms
? cm-?
Fig. 7. Coverage dependence of the modified Auger parameter (a)
Table 2 Changes of extra-atomic relaxation energy. mean valence hand electron hindlng energy. initial-state potential and 3d,,.: electron binding energy for Pd clusters on SO, auhstrate with ~ncreahlng coverage; energies are in eV Region I II
III
Coverage (atoms/cm’
ARK(V)
ABE(V)
Ac(3d,
0 +o.ci +o.s
- 0.8.5 - 0.95 ~ 0.05
+ 0.8 + 0.4 -0.5
2)
JBE(3d,
-)
) ~ 0.X + 1.o 0
ABE(3d,,z) is -1.0 eV and ARFx(V) is +0.6 eV with increasing coverage. ABE(V) amounts to -0.95 eV. In region III (Pd 2 1 x lOI atoms cm-‘). AKE(M,VV) is +l.l eV. ABE(3d,,,,) is zero and A R:(V) is +0.5 eV with increasing coverage. ABE(V) amounts to -0.05 eV. Ac(3d,,,) can be obtained from eq. (7). In table 2 ARK(V), ABE(V), Ac(3d,,,) and ABE(3d,,,,) are listed. In the low-coverage region the -0.8 eV shift for the Pd 3d,,? BE is ascribed to the change (+0.8 eV) in initial-state effect, Ac(3d 5,,L). In the higher-coverage region, the sum of the change in the initial-state effect and the change in the final-state effect corresponds to the change in the Pd 3d 5,,Z BE. In this XPS experiment it was possible to observe the VB shift directly as shown in fig. 8. In region I only isolated Pd atoms are believed to be present. Assuming that spectrum (b) in fig. 8 corresponds almost entirely to Pd atoms on the surface, it is found that the peak which is accompanied by a shoulder (5.0 eV) lies 4.3 eV below the Fermi level. The ground state of the free Pd atom is 4d’O5d” (‘S,,) which would give a 4d’5s0 configuration with the two spinorbit terms 2D5,2 and ‘D3,z after the removal of a d electron. The ’ Dj ,7 state is also the ionic ground state. Thus, these are the first (lowest ionization energy) features expected in the spectrum occurring at 8.3 and 8.8 eV relative to the vacuum level [26]. Hybridization of the d-band with the empty s-p conduction states will cause the counts of true d-electrons to be lower. The lower BE side of the Pd VB spectrum grew with increasing coverage and the maximum point shifted toward to the Fermi level. The (n + 1)s orbitals are much more diffuse than nd orbitals. and consequently more energy would be required to form a solid with higher s-electron occupation such as adsorbed Pd. Spectrum (c) in fig. 8 is believed to correspond to adsorbed Pd atoms within the surface. It is found that the d-levels lie 2.3 and 3.9 eV below the Fermi level. The crystal field will split the d-level into a doublet with spacing on the order of 1.5 eV for adsorbed Pd [27,28]. Ionization of a d-electron from the d’s’ configuration gives the ‘S
S. Kohiki / Small Pd clusters on Al,O,
and s1.0,
91
Fig. 8. Valence band spectra of evaporated Pd with various coverages. The substrate background contribution to the valence band spectrum has been subtracted. (a) SO, substrate VB; (b) 3.9 x IO”; (c) 1.6 X 1014; (d) 4.0 X 1014; (e) 5.8 X 1015; (f) 1.8 X 10lh Pd atoms/cm*.
state of the d’s’ configuration. Prediction of the resulting XPS spectrum complicated by the lack of optical data on the d’s’ *S state [29]. 3.3. Interaction
is
between clusters and substrate
In the photoemission final state, the hole state of the Pd core-level in the cluster should be screened by the valence electrons of the Pd cluster and the conduction electron of the substrate. This relaxation shift depends on the relative magnitude of the polarizability of the substrate and the Pd metal. Pd clusters on Al,O, and SiO, substrates have been investigated. Such clusters represent a close facsimile of the important small-metal particle catalysis [l-5]. The reactivity of Pd catalysis was investigated concerning the interaction with supports such as Al,O,, TiO, and SiO, [5]. In this experiment it was shown that small Pd clusters contain fewer d-electrons relative to the bulk metal. The result may suggest a correlation between the density of empty d-electron states and the observed variation in catalytic activity with cluster size. In ionic solids the available mechanisms for screening the incremental positive charge in photoemission final state are relaxation of neighboring ions
and polarization of the electronic charge on these ions. The former is too slow to affect the active electrons BE, so only the latter is effective. The BE will be reduced by a corresponding polarization energy (Er,,,). The polarization contribution must therefore be regarded as a form of static extra-atomic relaxation. E po, of the simple hole state was derived by Fadley et al. [30] from a relationship, due to Mott and Gurney [31]. which includes the dielectric constant. The one-hole relaxation cannot be measured directly whereas that due to Auger ionization can. The modified Auger parameter is a measure of Epc,, due to the substrate. The substrate conduction band electron contribution in extra-atomic relaxation for the Pd core-level is constant in small clusters for each substrate (Pd 2 1 x 10” atoms cm-’ for AllO, substrate and < 1 x lOI atoms cm ’ for SiO, substrate) and decreases with increasing cluster atoms. In small supported clusters the screening by the substrate is less effective because the average distance of the screening charge is greater than in the metal. In the higher-coverage region the core-level BE is almost constant because the bulk metal like initial-state electronic structure in the cluster, as the effective source of shifts, has been obtained almost completely. The extra-atomic relaxation energy increases with the effective source of extra-atomic relaxation energy varying from the poorly conducting substrates to the Pd metallic film. The change in modified Auger parameter is equal to the change of static extra-atomic relaxation energy and the term of (E - l)/( E + 2) is the scale of polarizability of the substrate. The change of static extra-atomic relaxation energy of Pd and the polarizability of the substrate calculated by the Clausius-Mossotti relationship is shown in table 3. Both the physical and the electronic properties of A120, and SiOz are dominated by the 0 ion. The Al ion (radius 0.6 A) and the Si ion (radius 0.4 A) are both much smaller than the 0 ion (radius 1.3 A.). The 0 ion accounts for the major part of the polarizability of the substrate. The direct correlation between the change of modified Auger parameter of Pd and the bulk polarizaTable 3 Correlation polarizability
of the change in Pd modified of the substrates
Substrate
Al,% SiO z ‘) Ref. [14]. hJ Ref. [32]. ” Pd modified respectively,
(E-l)/(F+2)
0.79 0.32
a)
Auger
parameter
Bulk polarizahility
h,
with
the dielectric
constant
and
Difference of Pd ‘) modified Auger
(A’)
parameter
3.0 4.3
1.3 2.2
Auger parameter observed in Pd bulk metal. small cluster 662.7. 661.4 and 660.5 eV.
(eV)
on AlzOJ
and SiOz is.
S. Kohiki
/ Small Pd clusterr ON AI,O.,
und SIO,
93
bility of the substrate calculated by the ClausiussMossotti relationship is also shown in table 3. The change of dynamic extra-atomic relaxation energy for the core-hole in the Pd cluster supported on the Al ,O? substrate is + 0.65 eV and that observed on the SiO, substrate is + 1.1 eV with increasing coverage. The difference in the change of the dynamic extra-atomic relaxation energy between Al,O, substrate and SiO, substrate may suggest that the R,, for SiOZ is smaller than that for Al 2O,. The screening should be weaker for a cluster evaporated onto a more insulating, highly polarizable substrate.
4. Concluding remarks The present observation of BE shift with cluster size may be interpreted as a result of photoemission initial-state effect for the small metal clusters (Pd 2 1 X 1015 atoms cm-’ on Al,O, and < 1 X 1014 atoms cme2 on SiO,). Increased d-electron density in larger clusters is seen by an enhanced intensity at the top of the VB as the coverage increases as shown in figs. 4 and 8. Hence, the number of occupied d-orbitals is dependent on cluster size; such increased d-orbital occupation qualitatively accounts for the changes in corelevel BE and mean valence band electron BE. In the higher-coverage region, the change in Pd 3d,,, BE is connected with the sum of the changes in initial- and final-state effects in this experiment as shown in tables 1 and 2. The polarizability of the substrates directly correlates to the difference in extra-atomic relaxation energy of Pd between small clusters and bulk metal. The screening due to substrate conduction electrons was weaker for small clusters evaporated onto highly polarizable substrates. The result suggests a correlation between the density of empty d-electron states and the observed variation in catalytic activity with cluster size. Sample charging should cause shifts which have opposite sign and identical value of Pd 3d,,, BE and Pd M,VV KE at the same coverage on each substrate. The values of the shifts observed in Pd 3d5,z BE and Pd M,VV KE were different from each other at the same coverage on an identical substrate. It is obvious that sample charging cannot predict this difference.
Acknowledgements The author thanks Professor S. Ikeda, Osaka University, for helpful discussions and suggestions, Miss K. Oki for assistance and Dr. F. Konishi for support of this work.
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