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Studies of the Initial Stages of the Adsorption of Pd on an Extensively Oxidised Zn(0001) Support
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
881
STUDIES OF THE INITIAL STAGES OF THE ADSORPTION OF Pd ON AN EXTENSIVELY OXIDISED Zn(OOO1) SUPPORT
A.J.SWIFT and J.C.VICKERMAN Department of Chemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 1QD (United Kingaom)
ABSTRACT Thick oxide overlayers have been prepared from Zn(0001) surfaces by oxidation of the base crystal under high conditions of temperature and pressure. The so called ZnOX surfaces have been characterised by XPS, AFS, Static and Dynamic SIMS and by examining surface reactivity. Initial stages of the adsorption of metalic Pd at ZnOX has been monitored using these techniques. In each case the growth mode of Pd was found to be layer by layer (Franck-van der Merwe). No chemical reaction of the adsorbate with either host element or diff’usion effects of Zn or 0 have been detected. However, diffusion of Pd into and across the support is obsetved. INTRODUCTTON Research into the applications of metal oxides as catalysts offers not only a wealth of interesting surface chemistry, but also, is of tremendous economic importance to the chemistry industry. The incorporation of a second component at the surface of mtal oxide catalysts has been shown to exert a pronounced and varied effect on catalytic properties of these supports, for example in the fields of methanol synthesis (ref. 1) or s&nal generation in gas sensors (ref. 2). However, relatively little work has been performed modelling doped metal oxide systems and hence the role of support and dopant I n these cases is far from well understood. As part of a programne of work to investigate such catalysts, the ZnO/F’d system has recently been examined using a range of Instrumental techniques. The ZnO surface has been well studied in the past (refs. 3-7) as have the initial stages of Zn(0001) oxidation (ref. 8) and much is known of its electronic structure and physical properties. ‘he preparation and characterisation of the support used I n these experiments will be described elsewhere (ref. 9 ) . In short, the support is understood to consist of a heterogeneous ZnO surface, slightly rich in zinc and increasing in zinc concentration into the bulk. ’he objective of this experiment was to characterise adsorption of metallic Pd at the ‘ZnOX’ surface under well controlled conditions. A few salient points emerge fm a recent review of related studies (ref.
882
10) and these are: (I) Pd deposition by evaporation fm a hot wire is a well used and controllable process. (11) On ZnO substrates, Pd (with other mighbourlng metals; Ag, Au, N i (refs. 11-13) displays a noc-face specific growth mode. For polar and non-polar faces, layer by layer mechanisms are reported for low Pd coverages. (111) It i s hportant t o be aware of dynamic diffusion processes that can occur i n these experiments. Vertical' (Into surface) and 'lateral' (across surface) have been reported prevlously (refs. llr-16) f o r Pd a t a substrate surface. Mffusion of zinc (ref. 16) or oxygen through the metal overlayer may also be important. Diffusion of oxygen through N i overlayers on NiO(0001) has been reported previously (ref. 13). (IV) Chemical reactiqn of adsorbed Pd With either host elanent of ZnO has not been reported In any Pd/ZnO study. However, I n recent work by Huck e t 61. (ref. 1 4 ) , surface oxidation of adsorbed Pd t o PdO i s proposed following Pd deposition on Sn02(110). Lattice Sn is observed to reduce t o metallic form in this process. (This can have serious consequences upon the diffusion properties of Pd a t t h e surface, since the ionic form of Pd has a much enhanced diffusion r a t e i n semiconducting oxides compared with t h a t f o r metallic W (ref. 17)). Pd2SI has been detected a t PdSi phase boundaries when Pd is adsorbed on S i and SI/SI02. In the case of SI/SI02 thls occurs after d i f f u s i o n of Pd through the oxide i t s e l f ( r e f . 15) and i s a subsurface reaction, whereas Pd2SI is directly formed on Si(ll1) (ref. 18).
Two separate UHV systems were used i n these experiments, one for studies by Secondary Ion Mass Spectrometry (SIMS) and the other f o r analysis by electron spectroscopy.
XPS/AEs Experiments 'Ihese were performed In a modified Vcf ESCA mUII, details of which are published elsewhere ( r e f . 8). After mounting a f r e s h l y c u t and polished Zn(OOO1) crystal, the Instrument was baked (base pressure < 5 X 10-l' mbar) and the sample further cleaned using etch/anneal cycles. It was then extensively oxidised (ref. 9 ) to ZnOX by the crystal t o 573K I n mbar of oxygen (5N Messer Greshiem) for 30 mins. X ray photoelectron (XPS), X ray Induced Auger electron (XAES) and nanoamp Auger electron (nAES) spectra were recorded following each Pd deposition. For XPS analysis, a hi@ s e n s i t i v i t y (CAE = 50 eV) multiple scan of the Pd 3d region was used t o evaluate surface coverage. 0 Is, Zn XAES and Zn 2p regions were also multiply scanned at higher resolution (CAE = 20eV). I n the Auger analysis, the spectrometer was repeat scanned (CRR = 2 ) between 50 and 1050 eV (KE),
883
encompassing the Pd MNN, 0 KLL and Zn IMM transitions. This spectrum was also used to monltor surface cleanliness. SIMS Experiments These were performed in a custom built UHV system also described in hll detail elsewhere (ref. 19). After mounting, the Zn(OOO1) crystal was sMlarly cleaned by etch/anneal cycles, until mlnlmal impurity levels were detected by Static SIMS (SSIMS). A &OX surface was then generated as for the X P S / m experiments (ref. 9). SSIMS analyses were performed using Art primary ions at 3 kV and an ion current of 3-4 nAcn~-~(measured on target). !he spot size was 2-3 mn2 SSIMS spectra were recorded following each Pd deposition at high (for qualitative determination from isotopic distribution analysis) and low resolution (for quantitatlve analysis from peak area meaurements).
.
Palladim Deposition Pd (5N, Ooodfellows metals, 0.125 mn dim.) was deposited from a hot wire evaporation source (ref. 9) using a stabillsed power supply (0-10 A). Accurate sample positioning for deposition was calibrated during installation and optimdl degassing and operatlng conditions were evaluated from a series of trial experiments. ?he experimental configuration was reproduced exactly for each instmanent. RESULTS
Chemical Effects of Deposition Initial Inspection of the behaviour of the Pd source Is shown In Fig. 1, where the Pd 3d doublet and the Zn 2p3/2 peak intensities are plotted against exposure time. Although this type of plot is not the most useful f o r interpretation of growth mode characteristics, the smooth curve clearly demonstrates good reproducibility and stability I n the performance of the Pd evaporator. Fig. 2 shows the development of the Pd 3d doublet following sequential dosing of the surface. 'Ihe growth of Pd 3dgI2 and Pd 3d3/2 peaks at 334.8 eV and 340.0 eY respectively Is clear and these binding energies cmpare favourably with previous WS analyses of clean Pd surfaces (refs. 20-22). No significant change in binding energy of these peaks is observed throughout deposition. SMlarly, no chemical shlftlng is detected in the 0 Is spectra at 530.5eV, although this is not clear f r o m first inspection due to the influence of the devloplng Pd 3p3,2 intensity (see Fig. 3) upto saturation. At'this level of exposure the Zn 2p3/2 W S peak at 1022.9 eV is barely detectable, indicating that coverage at this exposure approaches the escape depth for the
884
XPS Peak Intensity vs Exposure
I
4.
Fig. 1. A plot of XPS peak area against exposure time for; + = Pd 3d Doublet, and 0 = Zn 2p 3,2 i n t e n s i t i e s
*
Zn 2p3/2 electron through Pd (" 7 1?he low resolution positive ion SSIW spectra f o r sequential Pd exposures are shown i n Fig. 4. The gradual developnent of new peaks around 100 and 165 t t mu upon Pd deposition is clear and these have been assigned t o Pd and PdZn species frm t h e i r isotope s p l i t t i n g patterns. No evidence i s seen i n the positive o r negative ion spectra f o r the formation of PdO ions. It can be seen from these spectra that the e f f e c t of Pd deposition is t o cover the ZnOX t t surface, suppressing the Zn and ZnO/OH s l g ~ 2 . st o a similar extent. The Pd' peak appears t o grow consistently with exposure, u n t i l , at saturation, it is the only significant spectral feature. In t h e negative ion spectra the ZnO/OK and Zn2/02K are suppressed as Pd is deposited. Growth Mode Evaluation ?he growth mechanism can be most readily deduced f o r t h i s system, using the XPS results, when the modelling plots of Biberian and Somorjal (ref. 24) are applied. Fig. 5 shows a plot of Pd 3d intensity against Zn 2p intensity 3/2 (based on peak area measurement using instrument software). From t h i s two breaks i n gradient can be clearly seen and the curve represents that f o r Franqk-van der Merwe (FM) growth, with each curve break corresponding t o the
* Calculated from the hanogemus overlayer model calculations of Seah and Dench (ref. 23) for an inorganic system.
885
-
CleanZnoX
53
535
-Binding EnergyIeV Fig. 2. Sequential XPS spectra of the Pd 3d doublet upon exposure of ZnOX t o Pd.
Flg. 3. Sequential XPS spectra of the 0 Is region upon exposure of ZnOX t o Pd.
formation of the first and second monolayers ( CJ1 and 9 2). me Inelastic mean free path f o r t h e Zn 2p 32, e l e c t r o n c a l c u l a t e d from t h e s e p o i n t s is comparable with t h a t predicted In the qUantitative plots of &ah and k n c h (see refs 10,241. For growth mode evaluation f m n nAEs and SSlMs data, plots of n o w i s e d intensity against exposure t h e have been used and these are presented a o r g wlth the XPS data In Figs. 6 (a)b(b). Auger Intensities could readily be deduced from peak t o Peak height measurement and consideration of the oxygen Intensity is now possible since, in the Auger spectra, the 0 KLL transition is Intense and well resolved (see Fig. 7). Changes In Intenslty, for the SSIMs spectra have been determined by triangulation area measurement.
886
DISCUSSION
It can be seen from these results that Pd overlayers have been successfully prepared on the ZnOX surf'ace by Pd evaporation. Ihe useflilness of XPS, SINS and nAES for detectchanical changes d Intensity ratio modelling plots following Pd deposition have been demonstrated. In all cases layer by layer growth (FM) has been deduced in accord with the obsemmtions of Gaebler e t al. (ref. 111, who have monitored Pd growth on both polar faces of ZnO a t similar substrate temperatures. All spectra indicate that no surface c h d c a l reaction has occured following adsorption and there is no spectral evidence for the oxidation of adsorbed Pd. mrther, it is concluded that diffusion of surface oxygen through the Pd adlayer(s) does not occur. Scheisser and Jacob1 (ref. 13) c
exposure /sec
3
1
300
520
840 2000
3320
1 ) 1 ' 50 75 10 -mass/arrm-
F&.
4. Sequential low resolution SSIMS s p e c t r a for the deposition of Pd on ZnOX.
Growth Mode Plot (XPS,
Zn
F i g . 5. Growth mode plot for the depostion of Pd on ZnOX fm XPS data.
887
Comparison of Gnmth Mode Data 6) s=sims x= xp
species ratio
I
0
10 -exjmure/min
-
Fig. 6(a). A comparison of growth mode from all three analyses for low coverages of Pd. have reported an increase in the 0 KLL:Zn lvQJN Auger intensity ratio for the adsorption of Ni on ZnO(OOO1) and thls has been attributed to oxygen diffuslan through the overlayer. No significant variance in the 0 KLL:Zn MNN Auger intensity ratio can be seen in these results. Similarly, in the SSIplls analysis the ZnO/OH+ : Zn' does not vary slgnlflcantly with exposure.
I
Comparison of Gronth Mode Data (b)
Fig. 6 ( b ) . A canparison of growth mode data from all three analyses over coverages (symbols as for Rtg. 6(a>.>
888
I " " ' " " 1 However, reexamination of the final surface, at maximum Pd coverage after 64hrs i n vacuum, revealed that the Pd Auger yield had fallen significantly and that the 0 and Z n signal Intensities had Increased concmnltantly and by shllar amounts. The spectra clearly Indicate less effective coverage of the ZnOX surface and this is thought t o be due t o migration of Pd Into or across the substrate surface. Determlnation of the dominant mechanism (ie; 'vertical' o r tlaterdll diff'usion) is impossible from these results. Jacobs e t al. (ref. 25) deduce that lateral Pd diffusion is the dominating mechanism i n this temperature regime f o r Pd adsorbed on ZnO(OOO1). However, i n these experiments, Art etching of this sample Indicated the presence of a significant level of Pd In the subsurface of ZnOX and it was clear , , , that vertical diffusion had occurred. meed Pd diffusion into ZnOX canpared with ZnO(OOO1) is not unexpected since the defect F&. 7. Sequential nA Auger spectra density a t ZnOX i s l i k e l y t o be for the deposition of Pd on considerably greater than ZnO(O001) ZnOX and defects such as grain boundaries are expected t o provide favourable tvertical' diffusion paths f o r the migrant. Excellent agreement i s seen i n the growth mode p l o t s f o r a l l of the techniques used (see Figs. 6 (a)&(b)). Best correlation is noted at e1 (=lML), e2 i s better defined and more evident after a shorter exposure t h e in the SSIlrIs analysis compared with the results from the electron spectroscopies. This can be a t t r i b u t e d t o t h e g r e a t e r surface s p e c i f i c i t y of the SSIMS analysis. As van Delft e t a l . (refs. 26,271 note, AES and XPS measure a
:v2
,M,MNN
.
889
weighted average of several layers near the surface. Therefore, despite "2ML
coverage of Pd a t the ZnOX surface, the effect on these plots can be modified by t h e presence of Zn (and 0) i n t e n s i t y i n t h e e l e c t r o n s p e c t r a from
underlying Zn (and 0) atans. Hence i n these plots the turning point at e2 is somewhat less w e l l resolved. Under the conditions used f o r the SSIMS experiments, it is proposed that the features observed are derived fm the first 2-3 atcanic layers. Consequently after e2s the Znt s i g n a l at 64 amu is t t much less intense and the Pd /Zn r a t i o rises sharply. Further evidence f o r enhanced surface specificity i n the SSIMS data can be learnt from a closer t inspection of the ZnPd cluster ion behaviour.
Mixed Positive ion Emission vs Exposure
00 10,O
250
50,O
F i g . 8. ZnPd' cluster ion r a t i o (nomalised t o the t o t a l Pd' intensity) plotted against exposure time.
t Znt
For a sampling depth of "2ML the statistical maximum secondary ion yield predicted for t h i s ion (relative t o the t o t a l Znt t Pdt ion Intensity) w i l l occur at el and this i s seen in F i g . 8. !lhe ZnPdt/[Znt + Pd'] ion intensity rises sharply t o a maxirman a t an exposure equivalent of "1ML ("15min). The mixed ion intensity then falls as the second layer develops. This is seen, not only as evidence of a shallower SSIMS sampling depth ("2ML), but also as further evidence f o r layer by layer growth of Pd. SUMMARY AND CONCLUSION Pd overlayers have been deposited a t the surface of an extensively oxidised Zn(0001) s u r f a c e (ZnOX), i n a well controlled and reproducible manner. For every experiment using XPS, nAES and SSm; (i)the growth mode of
890 Pd has been found to be layer by layer
(m),(11) no
chemical reaction between
adsorbate with e i t h e r host element has been detected and (111) preferential migration of Zn or 0 has not been detected. Evidence of diff'usion of Pd i n t o and across %OX
is forwarded, although the dominating mechanism has yet t o be
determined. The high s u r f a c e s p e c i f i c i t y of t h e SIMS a n a l y s i s has been demonstrated.
ACKNOWLEIXEMENl'S 'Ihe Health and Safety Executive and The Science and hgineering Research Council are acknowledged for t h e i r financial support. REFwENcEs
1
G. Natta, Catalysis, 3, P.H hmett (Editor), b i n h o l d , New York, 1955, p
2
3
f o r example see; T.A. Jones, Sensor Review, Jan 1982, pp. 14-19. A. Jones, T.A. Jones, B. Mann and J.G. Firth, Sensors and Actuators, 5 (19841, PP* 75-88. B. Bott, T.A. Jones and B. Mann, Sensors and Actuators, 5 (1984), pp. 65-73. T.A. Jones, P. Moseley and B. Tofield, Chemistry in Britain (Aug. 1987), PP. 749-754. G.Heiland, E. Mollwo and F. Stoclanann, Sol. State Physics, 8 (19591, p
4
H.E. Brown, "Zinc Oxide
5
W. Hirschwald, Kaldis (Editor), Curr. Top. Mat. Sci., 7 (1981), pp. 143-482. G. Heiland and H. Luth, 'Adsorption on Oxides', The Chemical Physics of Solid Surfaces and Heterogeneous Catalysts", Vol. 3, D.A. King and D.P. Woodruff (Editors), Elsevier, Amsterdam. C.S. John, 'Catalysis by Zinc Oxide' , !he Chemical Society, Specialist Periodical Reports, 3, "Catalysis", pp. 169189. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdom, C h s . 4 and 5, pp. 94-204 and references therein. A.J. Swift and J.C. Vickerman, i n preparation. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdcan, Ch. 6, pp. 205-210. W. Gaebler, K. Jacobi and W. M e , Surf. Sci., 75 (1978), pp. 35-67, E.F. Wassermann and K. Polacek, Appl. Phys. Lett., 16 (1970), p 259. D. Schneisser and K. Jacobi, Surf. Sci., 88 (1979), pp. 138-152. R. h c k , P. Kohl and G. Heiland, Proc. Int. Symp. 'Trends and New Applications I n Thim Films', Strasbourg, March 1987, pp. 1-5. B. Schleich, D. S c h e i s s e r and W. Gopel, Surf. Sci. in press, (1987). A. Fasana and L. Braicovich, Surf. Sci., 120 (1982), pp. 239250. H.J. de Bruin and M. "antreeratana, J. Phys. Chem. Solids, 42 (1981), pp.
6
7 8
9
10 11 12
13 14
15 16 17
349.
191.
(1976).
- Properties and Applications", (ILZRO), New York 'Zinc Oxide - Properties and Behawlour of the Bulk', E.
333-3340 18 S. Nishigaki, T. Komatsu, M. Arimoto and M. Suglhara, Surf. Sci., 167 (19861, PP- 27-38.
19 A. Brown, PhD. 'Ihesis (1980), UMIST, PO Box 88, Sackville Street, 0 United rUngaan, Ch. 2, pp. 47-53. Manchester ~ 6 lQD,
20 C.D. Wagner, W.M. Riggs, L.E. Wries, J.F. Moulder and G.E. Muilenberg, 'Handbook of X Ray Photoelectron Spectroscopy', Perkirr-Elmer Corporation, Physkal Electronics Division, Eden Prairie, Minnesota (1979).
891
21 L. Hilaire, P. Legare, Y. Holl and G. Marie, Sol. St. Comn., 32 (1979), p. 157. 22 P. Weightman and P.T. Andrews, J. Phys. C. Sol. St. Phys., 13, L815, L821 (1980)23 M.P. %ah and W.A. Dench, Surf. and Int. Anal., l(1) (1979), pp. 1-11. 24 J.P. Biberlan and G.A. Scmorjai, Appl. Surf. Scl., 2 (1979), pp. 352-358. o m , D. Kohl and G. Heiland, Surf. &I 160 ., (1985), pp. 25 H. Jacobs, W. M 217-234. 26 F.C.M.J.M. van Delft, A.D. van Langeveld and B.E. Nleuwenhuys, lhin Solid Films, 123 (19851, PP. 333-351. van Delft and B.E. Nleuwenhuys, Solid State Ionlcs, 16 (1985), 27 F.C.M.J.M. pp. 233-240.
881
STUDIES OF THE INITIAL STAGES OF THE ADSORPTION OF Pd ON AN EXTENSIVELY OXIDISED Zn(OOO1) SUPPORT
A.J.SWIFT and J.C.VICKERMAN Department of Chemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 1QD (United Kingaom)
ABSTRACT Thick oxide overlayers have been prepared from Zn(0001) surfaces by oxidation of the base crystal under high conditions of temperature and pressure. The so called ZnOX surfaces have been characterised by XPS, AFS, Static and Dynamic SIMS and by examining surface reactivity. Initial stages of the adsorption of metalic Pd at ZnOX has been monitored using these techniques. In each case the growth mode of Pd was found to be layer by layer (Franck-van der Merwe). No chemical reaction of the adsorbate with either host element or diff’usion effects of Zn or 0 have been detected. However, diffusion of Pd into and across the support is obsetved. INTRODUCTTON Research into the applications of metal oxides as catalysts offers not only a wealth of interesting surface chemistry, but also, is of tremendous economic importance to the chemistry industry. The incorporation of a second component at the surface of mtal oxide catalysts has been shown to exert a pronounced and varied effect on catalytic properties of these supports, for example in the fields of methanol synthesis (ref. 1) or s&nal generation in gas sensors (ref. 2). However, relatively little work has been performed modelling doped metal oxide systems and hence the role of support and dopant I n these cases is far from well understood. As part of a programne of work to investigate such catalysts, the ZnO/F’d system has recently been examined using a range of Instrumental techniques. The ZnO surface has been well studied in the past (refs. 3-7) as have the initial stages of Zn(0001) oxidation (ref. 8) and much is known of its electronic structure and physical properties. ‘he preparation and characterisation of the support used I n these experiments will be described elsewhere (ref. 9 ) . In short, the support is understood to consist of a heterogeneous ZnO surface, slightly rich in zinc and increasing in zinc concentration into the bulk. ’he objective of this experiment was to characterise adsorption of metallic Pd at the ‘ZnOX’ surface under well controlled conditions. A few salient points emerge fm a recent review of related studies (ref.
882
10) and these are: (I) Pd deposition by evaporation fm a hot wire is a well used and controllable process. (11) On ZnO substrates, Pd (with other mighbourlng metals; Ag, Au, N i (refs. 11-13) displays a noc-face specific growth mode. For polar and non-polar faces, layer by layer mechanisms are reported for low Pd coverages. (111) It i s hportant t o be aware of dynamic diffusion processes that can occur i n these experiments. Vertical' (Into surface) and 'lateral' (across surface) have been reported prevlously (refs. llr-16) f o r Pd a t a substrate surface. Mffusion of zinc (ref. 16) or oxygen through the metal overlayer may also be important. Diffusion of oxygen through N i overlayers on NiO(0001) has been reported previously (ref. 13). (IV) Chemical reactiqn of adsorbed Pd With either host elanent of ZnO has not been reported In any Pd/ZnO study. However, I n recent work by Huck e t 61. (ref. 1 4 ) , surface oxidation of adsorbed Pd t o PdO i s proposed following Pd deposition on Sn02(110). Lattice Sn is observed to reduce t o metallic form in this process. (This can have serious consequences upon the diffusion properties of Pd a t t h e surface, since the ionic form of Pd has a much enhanced diffusion r a t e i n semiconducting oxides compared with t h a t f o r metallic W (ref. 17)). Pd2SI has been detected a t PdSi phase boundaries when Pd is adsorbed on S i and SI/SI02. In the case of SI/SI02 thls occurs after d i f f u s i o n of Pd through the oxide i t s e l f ( r e f . 15) and i s a subsurface reaction, whereas Pd2SI is directly formed on Si(ll1) (ref. 18).
Two separate UHV systems were used i n these experiments, one for studies by Secondary Ion Mass Spectrometry (SIMS) and the other f o r analysis by electron spectroscopy.
XPS/AEs Experiments 'Ihese were performed In a modified Vcf ESCA mUII, details of which are published elsewhere ( r e f . 8). After mounting a f r e s h l y c u t and polished Zn(OOO1) crystal, the Instrument was baked (base pressure < 5 X 10-l' mbar) and the sample further cleaned using etch/anneal cycles. It was then extensively oxidised (ref. 9 ) to ZnOX by the crystal t o 573K I n mbar of oxygen (5N Messer Greshiem) for 30 mins. X ray photoelectron (XPS), X ray Induced Auger electron (XAES) and nanoamp Auger electron (nAES) spectra were recorded following each Pd deposition. For XPS analysis, a hi@ s e n s i t i v i t y (CAE = 50 eV) multiple scan of the Pd 3d region was used t o evaluate surface coverage. 0 Is, Zn XAES and Zn 2p regions were also multiply scanned at higher resolution (CAE = 20eV). I n the Auger analysis, the spectrometer was repeat scanned (CRR = 2 ) between 50 and 1050 eV (KE),
883
encompassing the Pd MNN, 0 KLL and Zn IMM transitions. This spectrum was also used to monltor surface cleanliness. SIMS Experiments These were performed in a custom built UHV system also described in hll detail elsewhere (ref. 19). After mounting, the Zn(OOO1) crystal was sMlarly cleaned by etch/anneal cycles, until mlnlmal impurity levels were detected by Static SIMS (SSIMS). A &OX surface was then generated as for the X P S / m experiments (ref. 9). SSIMS analyses were performed using Art primary ions at 3 kV and an ion current of 3-4 nAcn~-~(measured on target). !he spot size was 2-3 mn2 SSIMS spectra were recorded following each Pd deposition at high (for qualitative determination from isotopic distribution analysis) and low resolution (for quantitatlve analysis from peak area meaurements).
.
Palladim Deposition Pd (5N, Ooodfellows metals, 0.125 mn dim.) was deposited from a hot wire evaporation source (ref. 9) using a stabillsed power supply (0-10 A). Accurate sample positioning for deposition was calibrated during installation and optimdl degassing and operatlng conditions were evaluated from a series of trial experiments. ?he experimental configuration was reproduced exactly for each instmanent. RESULTS
Chemical Effects of Deposition Initial Inspection of the behaviour of the Pd source Is shown In Fig. 1, where the Pd 3d doublet and the Zn 2p3/2 peak intensities are plotted against exposure time. Although this type of plot is not the most useful f o r interpretation of growth mode characteristics, the smooth curve clearly demonstrates good reproducibility and stability I n the performance of the Pd evaporator. Fig. 2 shows the development of the Pd 3d doublet following sequential dosing of the surface. 'Ihe growth of Pd 3dgI2 and Pd 3d3/2 peaks at 334.8 eV and 340.0 eY respectively Is clear and these binding energies cmpare favourably with previous WS analyses of clean Pd surfaces (refs. 20-22). No significant change in binding energy of these peaks is observed throughout deposition. SMlarly, no chemical shlftlng is detected in the 0 Is spectra at 530.5eV, although this is not clear f r o m first inspection due to the influence of the devloplng Pd 3p3,2 intensity (see Fig. 3) upto saturation. At'this level of exposure the Zn 2p3/2 W S peak at 1022.9 eV is barely detectable, indicating that coverage at this exposure approaches the escape depth for the
884
XPS Peak Intensity vs Exposure
I
4.
Fig. 1. A plot of XPS peak area against exposure time for; + = Pd 3d Doublet, and 0 = Zn 2p 3,2 i n t e n s i t i e s
*
Zn 2p3/2 electron through Pd (" 7 1?he low resolution positive ion SSIW spectra f o r sequential Pd exposures are shown i n Fig. 4. The gradual developnent of new peaks around 100 and 165 t t mu upon Pd deposition is clear and these have been assigned t o Pd and PdZn species frm t h e i r isotope s p l i t t i n g patterns. No evidence i s seen i n the positive o r negative ion spectra f o r the formation of PdO ions. It can be seen from these spectra that the e f f e c t of Pd deposition is t o cover the ZnOX t t surface, suppressing the Zn and ZnO/OH s l g ~ 2 . st o a similar extent. The Pd' peak appears t o grow consistently with exposure, u n t i l , at saturation, it is the only significant spectral feature. In t h e negative ion spectra the ZnO/OK and Zn2/02K are suppressed as Pd is deposited. Growth Mode Evaluation ?he growth mechanism can be most readily deduced f o r t h i s system, using the XPS results, when the modelling plots of Biberian and Somorjal (ref. 24) are applied. Fig. 5 shows a plot of Pd 3d intensity against Zn 2p intensity 3/2 (based on peak area measurement using instrument software). From t h i s two breaks i n gradient can be clearly seen and the curve represents that f o r Franqk-van der Merwe (FM) growth, with each curve break corresponding t o the
* Calculated from the hanogemus overlayer model calculations of Seah and Dench (ref. 23) for an inorganic system.
885
-
CleanZnoX
53
535
-Binding EnergyIeV Fig. 2. Sequential XPS spectra of the Pd 3d doublet upon exposure of ZnOX t o Pd.
Flg. 3. Sequential XPS spectra of the 0 Is region upon exposure of ZnOX t o Pd.
formation of the first and second monolayers ( CJ1 and 9 2). me Inelastic mean free path f o r t h e Zn 2p 32, e l e c t r o n c a l c u l a t e d from t h e s e p o i n t s is comparable with t h a t predicted In the qUantitative plots of &ah and k n c h (see refs 10,241. For growth mode evaluation f m n nAEs and SSlMs data, plots of n o w i s e d intensity against exposure t h e have been used and these are presented a o r g wlth the XPS data In Figs. 6 (a)b(b). Auger Intensities could readily be deduced from peak t o Peak height measurement and consideration of the oxygen Intensity is now possible since, in the Auger spectra, the 0 KLL transition is Intense and well resolved (see Fig. 7). Changes In Intenslty, for the SSIMs spectra have been determined by triangulation area measurement.
886
DISCUSSION
It can be seen from these results that Pd overlayers have been successfully prepared on the ZnOX surf'ace by Pd evaporation. Ihe useflilness of XPS, SINS and nAES for detectchanical changes d Intensity ratio modelling plots following Pd deposition have been demonstrated. In all cases layer by layer growth (FM) has been deduced in accord with the obsemmtions of Gaebler e t al. (ref. 111, who have monitored Pd growth on both polar faces of ZnO a t similar substrate temperatures. All spectra indicate that no surface c h d c a l reaction has occured following adsorption and there is no spectral evidence for the oxidation of adsorbed Pd. mrther, it is concluded that diffusion of surface oxygen through the Pd adlayer(s) does not occur. Scheisser and Jacob1 (ref. 13) c
exposure /sec
3
1
300
520
840 2000
3320
1 ) 1 ' 50 75 10 -mass/arrm-
F&.
4. Sequential low resolution SSIMS s p e c t r a for the deposition of Pd on ZnOX.
Growth Mode Plot (XPS,
Zn
F i g . 5. Growth mode plot for the depostion of Pd on ZnOX fm XPS data.
887
Comparison of Gnmth Mode Data 6) s=sims x= xp
species ratio
I
0
10 -exjmure/min
-
Fig. 6(a). A comparison of growth mode from all three analyses for low coverages of Pd. have reported an increase in the 0 KLL:Zn lvQJN Auger intensity ratio for the adsorption of Ni on ZnO(OOO1) and thls has been attributed to oxygen diffuslan through the overlayer. No significant variance in the 0 KLL:Zn MNN Auger intensity ratio can be seen in these results. Similarly, in the SSIplls analysis the ZnO/OH+ : Zn' does not vary slgnlflcantly with exposure.
I
Comparison of Gronth Mode Data (b)
Fig. 6 ( b ) . A canparison of growth mode data from all three analyses over coverages (symbols as for Rtg. 6(a>.>
888
I " " ' " " 1 However, reexamination of the final surface, at maximum Pd coverage after 64hrs i n vacuum, revealed that the Pd Auger yield had fallen significantly and that the 0 and Z n signal Intensities had Increased concmnltantly and by shllar amounts. The spectra clearly Indicate less effective coverage of the ZnOX surface and this is thought t o be due t o migration of Pd Into or across the substrate surface. Determlnation of the dominant mechanism (ie; 'vertical' o r tlaterdll diff'usion) is impossible from these results. Jacobs e t al. (ref. 25) deduce that lateral Pd diffusion is the dominating mechanism i n this temperature regime f o r Pd adsorbed on ZnO(OOO1). However, i n these experiments, Art etching of this sample Indicated the presence of a significant level of Pd In the subsurface of ZnOX and it was clear , , , that vertical diffusion had occurred. meed Pd diffusion into ZnOX canpared with ZnO(OOO1) is not unexpected since the defect F&. 7. Sequential nA Auger spectra density a t ZnOX i s l i k e l y t o be for the deposition of Pd on considerably greater than ZnO(O001) ZnOX and defects such as grain boundaries are expected t o provide favourable tvertical' diffusion paths f o r the migrant. Excellent agreement i s seen i n the growth mode p l o t s f o r a l l of the techniques used (see Figs. 6 (a)&(b)). Best correlation is noted at e1 (=lML), e2 i s better defined and more evident after a shorter exposure t h e in the SSIlrIs analysis compared with the results from the electron spectroscopies. This can be a t t r i b u t e d t o t h e g r e a t e r surface s p e c i f i c i t y of the SSIMS analysis. As van Delft e t a l . (refs. 26,271 note, AES and XPS measure a
:v2
,M,MNN
.
889
weighted average of several layers near the surface. Therefore, despite "2ML
coverage of Pd a t the ZnOX surface, the effect on these plots can be modified by t h e presence of Zn (and 0) i n t e n s i t y i n t h e e l e c t r o n s p e c t r a from
underlying Zn (and 0) atans. Hence i n these plots the turning point at e2 is somewhat less w e l l resolved. Under the conditions used f o r the SSIMS experiments, it is proposed that the features observed are derived fm the first 2-3 atcanic layers. Consequently after e2s the Znt s i g n a l at 64 amu is t t much less intense and the Pd /Zn r a t i o rises sharply. Further evidence f o r enhanced surface specificity i n the SSIMS data can be learnt from a closer t inspection of the ZnPd cluster ion behaviour.
Mixed Positive ion Emission vs Exposure
00 10,O
250
50,O
F i g . 8. ZnPd' cluster ion r a t i o (nomalised t o the t o t a l Pd' intensity) plotted against exposure time.
t Znt
For a sampling depth of "2ML the statistical maximum secondary ion yield predicted for t h i s ion (relative t o the t o t a l Znt t Pdt ion Intensity) w i l l occur at el and this i s seen in F i g . 8. !lhe ZnPdt/[Znt + Pd'] ion intensity rises sharply t o a maxirman a t an exposure equivalent of "1ML ("15min). The mixed ion intensity then falls as the second layer develops. This is seen, not only as evidence of a shallower SSIMS sampling depth ("2ML), but also as further evidence f o r layer by layer growth of Pd. SUMMARY AND CONCLUSION Pd overlayers have been deposited a t the surface of an extensively oxidised Zn(0001) s u r f a c e (ZnOX), i n a well controlled and reproducible manner. For every experiment using XPS, nAES and SSm; (i)the growth mode of
890 Pd has been found to be layer by layer
(m),(11) no
chemical reaction between
adsorbate with e i t h e r host element has been detected and (111) preferential migration of Zn or 0 has not been detected. Evidence of diff'usion of Pd i n t o and across %OX
is forwarded, although the dominating mechanism has yet t o be
determined. The high s u r f a c e s p e c i f i c i t y of t h e SIMS a n a l y s i s has been demonstrated.
ACKNOWLEIXEMENl'S 'Ihe Health and Safety Executive and The Science and hgineering Research Council are acknowledged for t h e i r financial support. REFwENcEs
1
G. Natta, Catalysis, 3, P.H hmett (Editor), b i n h o l d , New York, 1955, p
2
3
f o r example see; T.A. Jones, Sensor Review, Jan 1982, pp. 14-19. A. Jones, T.A. Jones, B. Mann and J.G. Firth, Sensors and Actuators, 5 (19841, PP* 75-88. B. Bott, T.A. Jones and B. Mann, Sensors and Actuators, 5 (1984), pp. 65-73. T.A. Jones, P. Moseley and B. Tofield, Chemistry in Britain (Aug. 1987), PP. 749-754. G.Heiland, E. Mollwo and F. Stoclanann, Sol. State Physics, 8 (19591, p
4
H.E. Brown, "Zinc Oxide
5
W. Hirschwald, Kaldis (Editor), Curr. Top. Mat. Sci., 7 (1981), pp. 143-482. G. Heiland and H. Luth, 'Adsorption on Oxides', The Chemical Physics of Solid Surfaces and Heterogeneous Catalysts", Vol. 3, D.A. King and D.P. Woodruff (Editors), Elsevier, Amsterdam. C.S. John, 'Catalysis by Zinc Oxide' , !he Chemical Society, Specialist Periodical Reports, 3, "Catalysis", pp. 169189. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdom, C h s . 4 and 5, pp. 94-204 and references therein. A.J. Swift and J.C. Vickerman, i n preparation. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdcan, Ch. 6, pp. 205-210. W. Gaebler, K. Jacobi and W. M e , Surf. Sci., 75 (1978), pp. 35-67, E.F. Wassermann and K. Polacek, Appl. Phys. Lett., 16 (1970), p 259. D. Schneisser and K. Jacobi, Surf. Sci., 88 (1979), pp. 138-152. R. h c k , P. Kohl and G. Heiland, Proc. Int. Symp. 'Trends and New Applications I n Thim Films', Strasbourg, March 1987, pp. 1-5. B. Schleich, D. S c h e i s s e r and W. Gopel, Surf. Sci. in press, (1987). A. Fasana and L. Braicovich, Surf. Sci., 120 (1982), pp. 239250. H.J. de Bruin and M. "antreeratana, J. Phys. Chem. Solids, 42 (1981), pp.
6
7 8
9
10 11 12
13 14
15 16 17
349.
191.
(1976).
- Properties and Applications", (ILZRO), New York 'Zinc Oxide - Properties and Behawlour of the Bulk', E.
333-3340 18 S. Nishigaki, T. Komatsu, M. Arimoto and M. Suglhara, Surf. Sci., 167 (19861, PP- 27-38.
19 A. Brown, PhD. 'Ihesis (1980), UMIST, PO Box 88, Sackville Street, 0 United rUngaan, Ch. 2, pp. 47-53. Manchester ~ 6 lQD,
20 C.D. Wagner, W.M. Riggs, L.E. Wries, J.F. Moulder and G.E. Muilenberg, 'Handbook of X Ray Photoelectron Spectroscopy', Perkirr-Elmer Corporation, Physkal Electronics Division, Eden Prairie, Minnesota (1979).
891
21 L. Hilaire, P. Legare, Y. Holl and G. Marie, Sol. St. Comn., 32 (1979), p. 157. 22 P. Weightman and P.T. Andrews, J. Phys. C. Sol. St. Phys., 13, L815, L821 (1980)23 M.P. %ah and W.A. Dench, Surf. and Int. Anal., l(1) (1979), pp. 1-11. 24 J.P. Biberlan and G.A. Scmorjai, Appl. Surf. Scl., 2 (1979), pp. 352-358. o m , D. Kohl and G. Heiland, Surf. &I 160 ., (1985), pp. 25 H. Jacobs, W. M 217-234. 26 F.C.M.J.M. van Delft, A.D. van Langeveld and B.E. Nleuwenhuys, lhin Solid Films, 123 (19851, PP. 333-351. van Delft and B.E. Nleuwenhuys, Solid State Ionlcs, 16 (1985), 27 F.C.M.J.M. pp. 233-240.