Langmuir 1988, 4 , 521-526 enhanced fluorescence spectra, the larger efficiency difference can be simply explained in terms of the electromagnetic particle plasmon model wherein the metal island films are represented as a collection of slightly prolate hemispheroids. We have previously found' that a model of the Ag islands as prolate spheroids could quantitatively explain the distance dependence of the SERS EF for a (~-BU)~H,PC monolayer. In our experimental geometry S-polarized light has no electric field component perpendicular to the plane of the film,while P-polarized light does have an electric field component normal to the film's surface. Therefore, P-polarized light couples to both the axially symmetric plasma oscillations and also to the azimuthal plasma o~cillations.~J~ S-polarized light couples only to the azimuthally excited m o d e ~ . ~The J ~ axially symmetric plasma oscillations produce larger electromagnetic enhancements.15 Therefore, the greater efficiency of P-polarized light in producing enhanced fluorescence can be readily understood from the spheroid model. In conclusion, we have shown that LB films could be used as spacer layers to determine the distance dependence of fluorescence enhancement/quenching from a distinct LB monolayer of chromophore near a rough metal surface. The results reported here corroborate the theoretical predictions that in surface-enhanced luminescence there (15)Geraten, J. I.; Nitzan, A. In Surface Enhanced R a m n Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982;p 89.
521
are at least two competing processes: local field enhancement and decay by radiationless energy transfer to the metal. The competitive relaxation effect from the excited molecule to the nearby metallic surface has been mediated by using LB spacer layers of arachidic acid. For the Pc monolayer in direct contact with the Ag island film, the fluorescence intensity is enhanced by a factor of about 200 for P-polarized light. The fluorescence is also considerably broadened and red-shifted. With intervening spacer layers, the fluorescence profile is again enhanced and is nearly identical with the unenhanced spectrum except that it is slightly blue-shifted. The EF of the fluorescence initially decreases from the direct contact value. It then increases monotonically with spacer layers of one, three, and five monolayers of arachidic acid to a value of almost 400 for the five-monolayer spacer and P-polarized excitation. The initial decrease of the enhancement factor on breaking contact with the metal surface is not explained by current electromagnetic enhancement theories and needs further investigation. The EF for P-polarized incident light with or without spacer layers is consistently greater by at least a factor of 2 than the EF for S-polarization. This can be understood because only P-polarized light can excite the axially symmetric plasma oscillations of the islands, which produce a larger electromagnetic EF than the azimuthal oscillations. Registry No. (~-Bu)~H~Pc, 55025-11-1;Ag,7440-22-4; arachidic acid, 506-30-9.
Interaction of Ni with SiO, or SiOz Formed on Si(111) and CO Adsorption Inhibition in Ni/SiO,/n-Si( 11 1) Studied by XPS and AES Tetsuo Asakawa, Katsumi Tanaka, and Isamu Toyoshima* Research Institute for Catalysis, Hokkaido University, Sapporo, 060 Japan Received May 19, 1987. In Final Form: September 14, 1987 An interaction of Ni with SiO, or SiOz formed on Si(ll1) and CO adsorption behavior on Ni in these systems were studied with X-ray photoelectron spectroscopy (XPS)and Auger electron spectroscopy (AES). Binding energy (BE) of Ni 2p3I2and kinetic energy (KE) of Ni LMM transition shifted as a function of Ni coverage. These shifts were the same on SiOJn-Si(lll), SiO,/p-Si(lll), and Si02/n-Si(lll). However, peak to peak distance between Ni 2~312.and 2pl did not change as a function of Ni coverage and was the same to Ni metal. Satellite peaks of mckel oxide were not observed. From these results,it was concluded that (1)Ni in these systems is in the metallic state and (2) charge transfer, even if it occurs, cannot be detected only by the BE shift. The coverage of Ni was estimated by decrease of Si intensity and increase of Ni intensity in conjunction with a layer by layer growth model, and they are consistent with each other within experimentalerror. However, decrease of oxygen intensity was too small to estimate Ni coverage. From these results, it was verified that Ni is implanted in the SiO, layer. Molecular CO adsorption was inhibited on Ni/SiOJn-Si(lll) at Ni coverage below 2, although it occurred on Ni/SiOJp-Si(lll) and Ni/Si02/n-Si(lll)at the same Ni coverage. It was deduced that the suppression of CO adsorption is responsible for charge transfer occurring at low Ni coverage; that is, electron transfer occurs from the donor level of n-Si to the Ni d orbital and results in retardation of o-donation from CO to Ni.
Introduction Regulation of gas adsorption on a transition metal by using the interaction with a semiconductor has been considered significant for the modification of catalytic performance on the metal. In this sense the interaction of group VI11 metals on titania and other reducible oxides has been studied.'s2 A common feature in such systems (1) Tauster, S.J.; Fung, S.C.; Garten, R. L. J. Am. Chem. SOC.1978, 100, 170.
(2)Tauster, S. J.; Fung, S. C. J. Catal. 1978,55, 29.
0743-7463/88/2404-0521$01.50/0
is the supression of H2and CO adsorption following reduction at high temperature (-800 K). This phenomenon has been termed a strong metal-support interaction (SMSI). Many studies on metal/metal oxide systems have been carried out to elucidate the cause of SMSI.3-11 I t (3) Resasco, D. E.; Haller, G. L. J. Catal. 1983,82, 279. (4) Belton, D. N.; Sun, Y. M.; White, J. M. J. Phys. Chem. 1984,88, 5172 (5) Sadeghi, H. R.; Henrich, V. E. J. Catal. 1984,87, 279. (6) Huizinga, T.;Van't Blick, H. F. J.; Vis, J. C.; Prins, R. Surf. Sci. 1983, 135, 580.
0 1988 American Chemical Society
522 Langmuir, Vol. 4, No. 3, 1988 was inferred that such interaction is caused by encapsulation of the metal particles with a thin layer of reduced which was shown by depth profiling analysis with AES. On the other hand, an electronic effect, charge transfer between metal and the support, was considered to be the cause of SMSI,'-14 which has been studied by XPS and AES. However, these results were too controversial to interpret. I t may be based on the difficulty in explaining an electron core level shift since peak shift observed is in general influenced by the metal particle size effect,15J6metalaupport interaction,1°J1J3J4and final relaxation effect."J' In our model system Ni/SiO,/Si(lll), molecular CO adsorption occurred on Ni/SiO,/p-Si(111) but was retarded on Ni/SiOJn-Si(lll) at low Ni coverage.12 It was concluded that charge transfer from n-Si to Ni is responsible for the retardation of CO adsorption. In the present study, electronic effects induced by an impurity level in n- or p-type silicon and thickness as well as chemical state of silicon oxide formed on Si were examined to elucidate the interaction between Ni and the supports and the effect of such interaction on CO adsorption. It is reported that charge transfer is not determined simply by BE shift of Ni 2p and that electron density of Ni and molecular CO adsorption on Ni are influenced by not only n- or p s i but also by electron tunneling thickness of SiO, or Si02 and Ni coverage.
Asakawa et al.
-z W
I
I
855
850
I
I
I
865 870 Binding energy /eV
860
875
Figure 1. Ni 2p peak on Ni/SiOJn-Si(lll) as a function of Ni coverage ONi.
Experimental Section In this study, a VG ESCA3 Mark I1 photoelectron spectrometer was used. XPS and AES spectra were recorded at room temperature by using a VG dual-anode X-ray source operating the Mg anode (hu = 1253.6eV)under a pressure of less than 1X Torr. Silicon(ll1) single crystals of n- and p-type obtained by doping with P or B to the extent of 1 X 10l6atom/cm3 of Si (commerciallyavailable,Komatsu Densi, Japan; specificresistivity of n- and p-type crystals is 4500 and 16 Q cm, respectively)were held on the tantalum sample holder with tantalum wire and were treated in the preparation chamber. Carbon-frw Si(ll1) samples were obtained by successive heating in O2 atmosphere (1X lo4 Torr) at 1173 K and Ar ion sputtering at room temperature. Silicon oxide (SiOJSi or SiOz/Si) on silicon was prepared by oxidizingSi(ll1)with an O2pressure of 1 X lo* or 1X lo-' Torr at 1173K for 25 min, respectively. Controlledamountsof Ni were deposited on these substrates with a resistively heated W f i i e n t wrapped with high-purityNi wire. To avoid temperature increase of the substrate, evaporated Ni samples were cooled to liquid nitrogen temperature during the Ni deposition. The amount of Ni deposited was represented as Ni coverage ONi, which was evaluated by peak intensity of Ni 2p and Si 2p assuming their mean free paths to be kNi2 = 12.8 8, and Asizp= 24 8, and by a layer by layer growth mode!.18 Ten langmuirsof CO (1langmuir = 1 X lo4 Torr 8 ) was dosed to the samplesat room temperature, and the amount of CO adsorbed was evaluated from the C 1s intensity on XPS spectra at various Ni coverage. Binding energy (7) Raupp, G. B.; Dumesic, J. A. J. Catal. 1986,97, 85. (8) Belton, D.N.;Sun, Y. M.; White, J. M. J.Phys. Chem. 1984, 88, 1690. (9) Vannice, M. A.;Hasselbring, L. C.; Sen, B. J. Phys. Chem. 1985, 89,2972. (10)Bahl, M. K.; Tsai, S.C.; Chung, Y. M. Phys. Rev. B Condens. Matter 1980, B21, 1334. (11) Kao, C. C.; Tsai, S. C.; Chung, Y. M. J. CataZ. 1982, 73, 136. (12) Tanaka, Ka.; Viswanathan, B.; Toyoshima, I. J. Chem. SOC.. Chem. Commun. 1985, 481. (13) Viswanathan, B.; Tanaka, Ka.; Toyoshima, I. Chem. Phys. Lett. 1985, 113,294. (14) Viswanathan, B.; Tanaka, Ka.; Toyoshima, I. Langmuir 1986,2, 113. (15) Baetzold, R. C.; Mason, M. G.; Hamilton, J. F. J. Chem. Phys. 1980, 72,366; 1980, 72,6820. (16) Mason, M. G. Phys. Rev. B: Condens. Matter 1983, B27, 748. (17) Egelhoff, F.W., Jr.; Tibetts, G. G. Phys. Rev. B Condens. Matter 1979, B19, 5028. (18) Rhead, G . E. J . Vac. Sci. Technol. 1983, 13, 603.
-z W
I1
1
716
1
I
712
1
1
I
I
704 Kinetic energy /eV
708
I
I
700
1
6 6
Figure 2. Ni L3Mz3Mt8AES peak on Ni/SiOJn-Si(lll) as a
function of Ni coverage ONi.
(BE) of Ni 2p, 0 ls, and C 1s and kinetic energy (KE) of Ni L3MZ3Mz3 were all referred to Si 2p (99.5 eV).
Results and Discussion 1. Characterizationof Oxides Formed on Si(ll1). It is well-known that nickel silicide is formed easily at the Ni-Si i n t e r f a ~ e . ~Therefore ~ , ~ ~ a thin silicon oxide layer was produced on Si crystal to prevent silicide formation on which interaction of Ni with silicon oxide/silicon was studied. Two types of silicon oxides were prepared by exposure to 1 X lo* or 1 X Torr of O2at 1173 K for 25 min. The former condition gave silicon oxide on Si substrate with significant Si 2p peaks at 99.5 and 102.3 eV (broad), which are assigned to Si substrate and a mixed state of SizO, SiO, and Si203(denoted as SiO,), respectively. The BE values of these peaks are well in accord with the reported values, that is, 99.5 eV for Si and 102.6 eV for Si203.21The oxide formed by the latter condition showed Si 2p peaks at 99.5 eV due to Si substrate and at 103.5 eV assigned to Si02 The thickness of SiO, and Si02 layers was estimated to be 3 and 6 A, respectively, by using the equation (19) Hiraki, A.;Shuto, K.; Kim, S.; Kammura, W.; Iwami, M. Appl. Phys. Lett. 1977, 31, 611. (20) Culbertaon, R. J.; Feldman, L. C.; Silverman, P. J.; West, K. W.; Mayer, J. W. Phys. Rev. Lett. 1980,45, 120. (21) Grunthaner, F. J.; Grunthaner, P. J.; Vasquez, R. P.; Lewis, B. F.; Maserjian, J.; Madhukar, A. Phys. Rev. Lett. 1979, 43, 1683.
Interaction of Ni with SiO, or Si02
I = Io exp(-d/0.74X) Here Ioand I are Auger signal intensities memured before and after oxidation, respectively, and d is the thickness of the oxide layer. The mean free path X of the Auger electron at 91 eV (Si L23W) was taken as 6.5 A,which is a mean value of 5.5 AB for thin oxide layer and 7.5 AB for thick Si02. It is significant to note that the nonstoichiometric SiO, layer with 3-A thickness and the stoichiometric Si02layer with 6-A thickness were formed at 1173 K by controlling the dose pressure of 02,and the behavior of these oxide formations was the same on nSi(ll1) and p-Si(ll1). 2. Deposition of Nickel on SiOJSi(111). Figures 1 and 2 show Ni 2p XPS spectra and Ni L3M23M23 AES spectra as a function of Ni coverage on Ni/SiO,/n-Si(ll1) (0.5 < x < 1.5), respectively. Estimation of Ni coverage eNiis discussed in detail in the next section. I t is noted that Ni coverage was varied up to 8Ni = 30, and the BE value of Ni 2p as well as that of Ni LMM spectra did not change a t all when 8~~ exceeded 30. Consequently we conclude that the state of Ni at eNi = 30 can be referred to as bulk Ni. It is clear that Ni 2p peaks shifted to lower BE by 1.2 eV as Ni coverage increased from 0.25 to 30 (Figure 1). The peak separation between Ni2p3/, and 2~112 did not change at all and was 17.4 eV, which agrees with that of bulk Ni. Nickel oxide should give wider separation between those Ni 2p peaks, 18.4 eV, with intense satellite peaks adjacent to Ni 2p peaks>* From these results, it can be concluded that nickel deposited on SiOJn-Si(111) is not oxidized at the Ni/SiO, interface but stays in metallic state. The same results were obtained for Ni deposition on SiOJp-Si(111) (0.5 < x C 1.5) and Si02/nSi(ll1). It is noted that a metal deposited on metal oxide with surface hydroxyl groups is in general oxidized by the reaction M OH M+ 0- 1/2H2;26however, in our system the surface is entirely OH free so that Ni can be present on silicon oxide in metallic state. It was found that the two peaks of Ni L3MBM23shifted to higher kinetic energy by 3 and 8 eV as shown in Figure 2. The KE shifts were also observed for Ni deposited on SiOJp-Si(111) and Si02/n-Si(lll)to the same extent as for the SiO,/n-Si(lll) system. With respect to BE and KE shifts of supported metal as a function of the metal coverage, many factors have been considered so far, and the following three explanations have gathered much attention. (1)The shifts of BE and KE values on a metal with metal coverage are interpreted as resulting from a metal-support interaction which affects metal properties strongly at low metal coverage.lOJ1 (2) The shifts are due to size dependence of initial-state electronic factor of metal cluster.16J6 Specifically, changes in the number of valence d electrons with size are responsible for the observed shifts. (3) The shifts on XPS and AES spectra are not due to the initial-state properties but to variation in final-state relaxation processes of photoemission."~17 Kao et al. observed that BE of Ni 2p3/2and KE of Ni L3M23M23 shift by 0.4 and -2.0 eV, respectively, as coverage of Ni increases to bulk state of Ni in Ni/Ti02(lll).11 It was concluded that these BE and KE sh3t.a are explained by factors 1and 3 abo6e, which are based on the extents of relaxation en-
+
-
+ +
(22) Graner, C. M.; Lindau, 1.; Su, C. Y.; Pianetta, P.; Spicer, W. E. Phys. Reo. B Condens. Matter 1979,B19,3944. (23) Chang, C. C.; Baulin, D. M. Surf. Sci. 1977,69,385. (24) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp., Physical Electronics Division: Eden Prairie, MN, 1979; p 80. (25) Howe, R. F. In Tailored Metal Catalysts; Iwasawa, Y., Eds.; Reidel: Holland, 1986;p 172.
Langmuir, Vol. 4, No. 3, 1988 523 ergy (a) and chemical shift (a). They were calculated by using the relations ABE u - hR h E b e n d
+
AKE = -AE + 3AR - e E b e n d Here ABE, AKE, and @bend are the changes of Ni 2p3/2 binding energy, Ni L3MBMBkinetic energy, and measured energy due to band bending. However, Huizinga et al. proposed the opposite interpretation.6 They studied hydrogen adsorption and X P S shifts of Pt and Rh supported on y'Al2O3 and T O 2 with different metal dispersion. They concluded that the BE shift is caused by differences in the extraatomic relaxation of metal particles with different sizes and that electronic configurations in the SMSI and non-SMSI state are indistinguishable by XPS. The peak shift of Ni 2 ~ 3 1 2was reported to be 0.6 eV in a Ni/graphite s ~ t e m , and 1 ~ it was concluded to be due to reason 3 above. In our previous papers, it was reported that CO adsorption is suppressed in Ni/SiO,/n-Si(111); this phenomenon can be interpreted by charge transfer between the donor level of n-Si and the LUMO level of Nil4 and that the charge transfer can be detected by evaluating the AE value obtained by Kao's assumption.13 It is noted that in our previous experiments Ni was deposited on SiO,/ n-,p-Si at room temperature. Such an experimental condition may induce a different extent of agglomeration of Ni particles on supports of different electronic properties, which may be responsible for the hE value difference between n- and p-Si. In spite of different adsorption behaviors (discussed in part 4 of this section), AE values are the same in both n- and p-Si. These results imply that charge transfer, even if it occurs, cannot be detected by BE or KE shifts. 3. Coverage and State of Nickel Deposited. As it is not easy to evaluate Ni coverage (ONi) by XPS intensity, eNiwas estimated by following four methods: increase of Ni 2p intensity, decrease of Si 2p intensity, layer by layer growth model, and the ratio of Ni 2p and Si 2p intensity with Ni deposition. From increase of Ni 2p intensity, coverage of Ni (ONi(Ni)) is estimated as 8 ~ i ( ~= i ) d / d , = -0.74X~iIn [ ( I b - I / I b ) / 2 . 2 ] Here I,I b , X N ~ ,and d, are Ni 2pSI2intensity of deposited Ni, that of bulk Ni, a mean free path of Ni 2p, and a mean thickness of Ni monolayer, respectively, while XNi = 12.8 A and d, = 2.2 A. With respect to Si 2p intensity decrease 8Ni(Si) = d / d , = -0.74Xsi In [(I/Io)/2.2] Here Ioand I are Si 2p intensity before and after Ni deposition, Asi = 24 A. Figure 3 shows the relation between intensity of Ni 2 ~ 3 1 2 and deposition time in a fixed condition. It is noted that linear increase of Ni 2p312intensity declines at 240 s. This result indicatea that 8 ~ =i 1is obtained when Ni deposition time is performed for 240 s if the layer by layer growth modells can be applied to estimate coverage of Ni on SiO,/Si(lll). The layer by layer growth model can be used when intensity growth line declines sharply with coverage corresponding to 2 as well as 1. Unfortunately, this point (550 s) is equivocal in Figure 3 due to lack of experimental points. However, Ni coverage estimated from the layer by layer growth model (&i(L)) seems to make sense since it has a linear relation to 8 ~ i ( ~and i ) 6 ~ i ( ~evaluated i) from Ni 2p and Si 2p intensity, respectively, as shown in Figure 4. The ratio of Ni 2p3l2intensity to Si 2p intensity at 240 s,1.31, b o supports the accuracy for the estimation of 8, = 1 since the monolayer of Ni gives the ratio 1.35 by assuming that and the ratio of atomic sensitivity factors
Asakawa et al.
524 Langmuir, Vol. 4, No. 3, 1988
'0
2
4
6 8 NI coverage & , l ~ l
IO
12
Figure 5. Relation between Ni coverage 8Ni 0)and ONi(SiO (calculated from 0 ls and Si 2p(oxide)intensities) and 8") (dotted line) on Ni/SiOz/n-Si(lll)and Ni/Si02/n-Si(lll).
400 600 Ni deposit time /set
200
U
SO0
Figure 3. Ni 2p3 intensity as a function of Ni deposition time on Ni/SiO,/n-Si(ll1). A BNIINII
lo-
A
BNt(SI1
8-
-
,'
0
8
IO
6-
Q
-
,*A
,
g 4u lz
0'
&','A 2-
O
d" O
;
;
6
also deposited Ni atoms or clusters are partially implanted. The mixture state of Ni with SiOz is restricted and occupies narrower region than the mixture state of Ni with SiO,, and it is estimated to be about 2 A thick from the surface by XPS intensity. It is difficult to classify whether a chemical reaction occws between nickel and oxide or Ni is trapped by defects formed in Si oxide, because the Ni 2p peak shifts as much as 1.2 eV with Ni coverage both on SiO, and on SiO, and Si 2p and 0 Is do not shift. From the Ni 2p peak shape, satellite peak, and peak separation between Ni 2p3/, and 2plj2, Ni would not react at least with oxygen (see part 2 of this section). It is significant that deposited Ni can penetrate the SiO, layer, while such implantation of Ni does not occur appreciably in SiO,. The difference may be attributed to the assumption that SiO, has an imperfect and ambiguous structure, which induces lack of a stable covalent bond as in SiOP It is significant to note that the implantation of Ni particles occurred in SiO, formed on n-Si(ll1) and p-Si(ll1) to same extent. 4. CO Adsorption. Carbon 1s spectra were recorded following Ni/SiO,/p-Si(lll) and Ni/SiO,/n-Si(ll1) exposure to 10 langmuirs of CO at room temperature. The results are shown in Figure 6 as a function of Ni coverage BNi. When CO was dosed to Ni/SiO,/p-Si(ll1) at dNi = 0.25, a new peak appeared at 285.5 eV (b) on the background shown in spectrum a of Figure 6. It is reported that C 1s BE values of molecular adsorbed CO are 285.8 eV on Ni(100) and 285.4 eV on Ni(111).26 The value observed in our experiment (285.5 eV) is very close to these values. In addition, the presence of molecular adsorbed CO has been proved by UPS in our system.12 As a result, it is concluded that CO can be adsorbed in molecular state on Ni/SiO,/p-Si(lll) at Ni coverage as low as 0.25. It should be mentioned that molecular CO adsorption occurred on Ni/Si02/n-Si(lll) even at low Ni coverage (spectra not shown). When CO was added to Ni/SiO,/n-Si(lll) at BNi = 0.25, no appreciable change was observed (d) compared to the background (c) in Figure 6. Even in the background, a small C 1s peak was found at 284.8 eV on Ni/SiO,/nSi(ll1) at dNi = 0.25. The peak can be assigned as free carbon, and it could not be removed entirely under the present experimental conditions. No molecular CO adsorption was observed, and the C 1s level at 284.8 eV did not increase at all on Ni/SiO,/n-Si(ll1) at BNi = 1 (spectrum e, Figure 6). A t dNi = 3, a c 1s peak was newly formed at 286 eV (spectrum f, Figure 6), which is assigned (26) Norton, 1976, 41, 247.
P.R.;Tapping, R.T.;Goodale, J. W. Chem. Phys. Lett.
Langmuir, Vol. 4, No. 3, 1988 525
Interaction of Ni with SiO, or SiO, 5 1
- 30
-20
. 8
.z
-
$
Ob'
6
4
6
Ni coverage
z 8-0.25
ih' ' 30 0
IO
8 ~ i
Figure 7. C 1s peak intensity in CO adsorption and the Ni/CO ratio on Ni/SiOJSi(lll) as a function of Ni coverage BN6 ( 0 , A) n-si(111);(0,A) p-si(111).
-__.___.._________..--...
8.0.25 8-0.25
7 I .
a 4 t
p-SI(II1) I
10
420
I
I
I
8
I
O
' 0/
2
4
6
Ni coverage
8
10
12
BNI
Figure 8. C 1s peak intensity in CO adsorption and the Ni/CO ratio on Ni/Si02/n-Si(lll)as a function of Ni coverage BNi.
any Ni coverage (A in Figure 7; A in Figure 8) may be retardation of CO adsorption by implantation of Ni atom in SiO, or SOa. The increase of Ni/CO ratio (Figure 8) is undoubtedly attributed to the fact that the number of surface Ni atoms is constant; however, total Ni atoms increase. The ratio was infinite at low Ni coverage and was 14, 11,and 12.5 at 8Ni = 3, 5, and 10, respectively, on Ni/SiO,/n-Si( 111). It was reported in our previous papers but adsorption of CO is inhibited in Ni/SiO,/n-Si(ll1) at Ni coverage as low as 0.3,and it is suggested this is due to charge transfer between the donor level of n-Si and the LUMO level of Ni clusters.12 In the present work, it was ascertained strictly that molecular CO adsorption is retarded in Ni/ SiOJn-Si(ll1) at low Ni coverage but that BE and KE shifts of Ni are nevertheless the same both in p- and n-Si. Therefore it is concluded that charge transfer cannot be evaluated by BE and KE shifts of Ni. It is also noted that Ni is implanted in SiO,, and it may be a reason for inhibition of CO adsorption. This, however, is not the case, because molecular CO adsorption is observed on Ni/ SiO,/p-Si(ll1) in which the same encapsulation by SiO, takes place. We would like to interpret the CO adsorption inhibition by charge transfer from n-Si to Ni. The concentration of P in the oxide layer is much lower than n-Si substrate (1 X 1OI6 atom/cm3 of Si),27so the interaction between P and Ni would not affect CO inhibition behavior. Adsorption of CO on transition-metal surface is explained by so called a-donation and .Ir-back-donation,that is, the mixing of electrons of the highest occupied molecular orbital (HOMO) of the CO 5a levels with the unoccupied molecular orbital of the metal (t1J and alternative mixing of electrons in the highest occupied d orbital (tl,) (27) Wada, Y.;Sato,
K.Japan J. Appl. Phys. 1979, 15, 2289.
Langmuir 1988, 4 , 526-532
526
with the lowest unoccupied molecular orbital (LUMO) of If the unoccupied molecular orbital of the CO 2a Ni was filled with electrons supplied from the donor level of n-Si (charge transfer), a-donation from the CO molecule could no longer be expected. a-Back-donation plays a role in weakening the C-0 bond since the 2a level of CO is antibonding between C and 0.2sIt is difficult to estimate the proportion of a-donation to a-back-donation which contributes to the m e t a l 4 0 bond by electron spectroscopy directly. By HREELS, frequencies of C-0 and Ni-CO in CO / Ni( 100) were measured29
w-0, meV CO/ Ni ( 100)
256.5 239.5
meV 59.5 44.5
VNi-CO,
linear CO bridge CO
These indicate that the weaker C-0 bond (large a-backdonation) is not equivalent to the stronger Ni-CO bond. If it is assumed that the bond between Ni and CO is evaluated by wavenumbers of Ni-C, these results imply that contribution of a-back-donation to molecular adsorption of CO may be smaller. When charge transfer is concerned in our system, it may occur by tunneling of electrons through the thin SiO, layer. Recently, an IBM group succeeded in surveying the Pt/SiO,/Si interface by transmission electron m i c r o ~ c o p y . ~According ~ to their result, Pt clusters seem to migrate through the SiO, layer and contact Si. If this holds in our Ni/SiO,/n-Si(ll1) system, tunneling of electrons through thin SiO,, which has an imperfect covalent bond and imperfect performance as an insulator, is conceivable.
The relative location of the unoccupied molecular level of Ni and the donor level of n-Si is a matter to be solved. According to Xa calculations based on Ni13cluster,31the lowest unoccupied level of the cluster is below the donor level of n-Si. The contact of semiconductor with a metal undergoes Fermi-level pinning by the surface state and forms a potential barrier at the interface.32 However, the SiO, layer, which has low electron density of state, would inhibit the Fermi-level pinning. Therefore it is concluded that CO adsorption inhibition in Ni/SiO,/n-Si(ll1) at low Ni coverage can be explained by charge transfer from n-Si to Ni.
Conclusion Interactions of Ni with SiO, and Si02and CO adsorption were studied in model systems, Ni/SiOJSi(lll) and Ni/Si02/Si(lll), by XPS and AES. The following conclusions were obtained. (1)Charge transfer is not determined by peak shifts of Ni in XPS or AES spectra. (2) Nickel deposited is not oxidized but is kept in the metallic state. (3) Nickel is implanted in the SiO, layer and forms a mixture state with SiO,. (4)CO adsorption is suppressed in Ni/SiO,/n-Si(ll1) at BNi < 3, which is caused by electron transfer from the donor level of n-Si to nickel. Acknowledgment. We thank Dr.Yatsurughi, Komatsu Denshi Kinzoku, Japan, for kindly supplying silicon single crystals. This work was partially supported by Grant in Aid for Scientific Research (No. 62609502) from the Ministry of Education of Japan. Registry No. Ni, 7440-02-0; Si02,7631-86-9; Si, 7440-21-3; CO, 630-08-0.
(28) Sung, S. S.; Hoffman, R. J. Am. Chem. Soc. 1985, 107, 578. (29) Anderson, S. Solid State Commun. 1977,21, 75. (30) Liehr,M.; LeGoues, F. K.; Rubloff, G. W.; Ho, P. H. J. Vac. Sci. Technol. 1985, A3,983.
(31) Mesemer, R. P.; Knudson, S. K.; Johnson, K. H.; Diamond, J. B.; Yang, C. Y. Phys. Reu. E . Condens. Matter 1976, B13, 1396. (32) Bardeen, J. Phys. Rev. 1947, 71, 717.
Reaction Thermodynamics of Hydrocarbons with Ni( 100) Gregory R. Schoofs and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received March 5, 1987. In Final Form: November 10, 1987 The reactions of simple hydrocarbons with a Ni(100) surface were studied by temperature-programmed reaction and Auger electron spectroscopy. All the hydrocarbons were found to desorb molecularly below a critical temperature dependent on the hydrocarbon. Above the critical temperature dehydrogenqtion/decomposition reactions occur. Simple alkenes, ethene, propene, butene, and benzene, all decomposed to adsorbed hydrogen and carbon above this critical temperature. Cyclohexene was found to dehydrogenate to benzene circa 200 K. Butadiene was hydrogenated to butenes circa 270 K. Many of these findings are predicted by simple thermodynamic analysis of the Ni-hydrocarbon-hydrogen system. Thermodynamic analysis also predicta that a variety of hydrogenation and hydrogenolysis reactions are feasible, and these reactions were not observed. The comparison of thermodynamicpredictions with experimentalfindings suggests C-H bond activation of unsaturated hydrocarbons on Ni is facile but the hydrogenation of alkyl intermediates to alkanes is difficult. The results also suggest C-C bond activation is difficult on Ni(100).
Introduction Nickel catalysts are commonly employed in a variety of industrial hydrogenationprocesses, including methanation, saturation of alkenes and aromatics, and removal of heteroatoms.lV2 In addition to commercial process design and
* Author to whom correspondence should be addressed.
development, these reactions have been the subject of numerous studies aimed at elucidating the kinetics and selectivities of model compound^.^-^^ (1) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill: New York, 1980. (2) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979.
0 1988 American Chemical Society
