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Infra-red spectra of carbon monoxide adsorbed on vacuum deposited metal films
SURFACE
SCIENCE
INFRA-RED
12 (1968) 426-436 0 North-Holland
SPECTRA VACUUM
F. S. BAKER, Department
OF CARBON DEPOSITED
A. M. BRADSHAW,
Publishing Co., Amsterdam
MONOXIDE METAL
J. PRITCHARD
qf Chemistry,
Queen Mary College, London, E. I., England
ADSORBED
ON
FILMS and K. W. SYKES Mile End Road,
Received 28 May 1968 Infra-red absorption spectra of adsorbed carbon monoxide have been obtained using single layer evaporated metal films as adsorbents. The films were deposited at low temperatures and were sufficiently thin to transmit approximately 70 per cent of the incident radiation at 2000 cm-l. Nickel, cobalt, iron and iridium films were deposited in high vacuum, but useful spectra with manganese, chromium and tungsten were obtained only with films deposited in carbon monoxide.
1. Introduction Absorption spectroscopy of adsorbed molecules by transmission in the infra-red region of the spectrum is complicated by the loss of energy due to absorption and scattering by the metal adsorbent. The problem can be eased by reducing the particle size of the metal to well below the wavelength of the radiation. Absorption and scattering are reduced while the surface area, and therefore the quantity of adsorbate in the light path, is increased. In most work to dater-v) a small particle size has been achieved by the reduction of metal oxides or salts supported on silica or alumina. Although this technique provides a well-dispersed metal sample it suffers from some disadvantages. Reduction may not be complete and residual adsorbed species may be present. The support material may possess absorption bands which limit the measurable spectrum. Furthermore, it has been shown3) that the spectrum obtained from a given adsorbate-metal system is dependent upon the support material. In attempting to overcome these disadvantages some workers have used thin evaporated metal films. Spectra of carbon monoxide have been obtainedr,*) with films deposited in vacuum but exposed to air before adsorption of carbon monoxide. Garlands) has reported that metal films evaporated and kept in a high vacuum fail to give spectra with carbon monoxide, but that good spectra are obtained when the metal is evaporated in a pressure of carbon monoxide. 426
INFRA-RED
The latter
technique
had been
421
SPECTRA
used previously
by Haywardis).
Recently
spectra have been reportedrl) for carbon monoxide on some platinum metals by transmission through a stack of twenty very thin metal films deposited under ultra-high vacuum conditions. It is probable that the failure to obtain spectra of carbon monoxide on single films deposited in a high vacuum was due to the combined effects of insufficient surface area and too large a particle size. It is well known that small particle sizes and large surface areas are obtained when films are deposited on substrates at low temperatures. Therefore we have investigated the possibility of using low substrate temperatures to give metal films with areas adequate to yield spectra with adsorbed carbon monoxide. In the work described here spectra were obtained as soon as carbon monoxide was added to films of nickel, cobalt, iron and iridium evaporated in vacuum. Barely detectable changes occurred with vacuum deposited films of manganese, chromium and tungsten, and it was found necessary to evaporate these metals in a low pressure (lo-’ Torr) of carbon monoxide to give spectra comparable with those obtained from the first group of metals*. More detailed studies of the nickel-carbon monoxide system under ultra-high vacuum conditions will be described in a following paper. 2. Experimental 2.1. APPARATUS The infra-red cell is shown diagrammatically in fig. 1. It was constructed in two Pyrex glass sections with a B40 cone and socket joint. The outer section had periclase windows attached with picein wax, and tungsten leads on which the evaporation filament could be mounted. The inner section carried a glass re-entrant to which a copper block was attached through an intermediate glass to metal seal. The film substrate was a fluorite plate which was clipped to a bracket on the block, and which could be cooled by conduction when the re-entrant was filled with liquid nitrogen. The films were evaporated either from hairpin filaments of the pure metals (Ni, Co, Fe, W, Ir) or from chips of manganese or chromium in a tungsten spiral. During evaporation the window adjacent to the evaporation source was shielded with a shutter of foil which could be moved away magnetically afterwards. Nickel foil was used at first, but with high pressures of carbon monoxide an absorption band attributable to nickel carbonyl was often observed. A mild steel shutter was substituted later. The cell was attached to an all glass vacuum system evacuated by a
* A preliminary account of these results was given at the Chemical Nottingham, September, 1965.
Society Meeting,
428
F. S. BAKER,
A.M.
BRADSHAW,
J. PRITCHARD
AND K. W. SYKES
mercury diffusion pump and equipped with mercury cut-offsl2) for dosing gases into the infra-red cell. The whole vacuum system could be raised or lowered so that the cell could be inserted into the cell well of a Unicam SP 100 spectrophotometer.
Fig. 1.. The infra-red cell with cooled re-entrant.
2.2.
PR~CEOURE
Because of the waxed joints the cell could not be baked, and the vacuum conditions were poor compared with those normally attained in work with evaporated films. The cell was evacuated for several hours via a trap at 77 “K. The metal to be evaporated was then heated in hydrogen at about 1 Torr for 30 min to reduce surface oxides, and finally it was outgassed for one hour. During outgassing the pressure was in the range lo-’ to 10m6 Torr. The re-entrant was cooled with liquid nitrogen. After one hour the Guorite plate reached a steady temperature of about 170°K as measured in separate experiments with a thermocouple attached to the front surface of the plate. For the vacuum evaporated metals the film was then deposited. It was found that the best spectra could be obtained with films giving about 70 per cent was below 50 per cent the transmission near 2000 cm- ‘. If the transmission absorption bands were small and di~cult to observe against the spectrometer noise. One nickel film was deposited on a plate coored to only 208°K. No spectra were obtainable. After evaporation the cell was lowered into the sample beam cell well of the spectrometer and a variable attenuator in the reference beam was used to adjust the recorder output to 95 per cent of the full scale at 1800 cm-‘. The spectrum of the bare film was recorded while the substrate was still cold.
INFRA-RED
429
SPECTRA
Doses of carbon monoxide were then admitted until the pressure reached 10e3 Torr. The spectrum was again recorded on the same chart. After pumping, the film was allowed to warm to room temperature, and spectra were recorded as before. Finally oxygen was admitted to a pressure of about 1 Torr, and its effect on the carbon monoxide spectrum was observed. For those metals (Mn, Cr, W) which did not give spectra after being deposited in vacuum some films were deposited in a pressure of lo-’ Torr of carbon monoxide and subsequently treated exactly as the vacuum deposited films. In this preliminary work no compensating cell was used in the reference beam. As a result bands due to atmospheric water vapour appeared below 1900 cm-‘. Flushing with dry nitrogen removed the interfering bands between 1800 and 1900 cm- r. The spectrometer was therefore set to scan over the range 1800 to 2150 cm-‘. Any number of spectra could be superimposed on the recorder chart to enable the detection of small changes.
3. Results The spectra which were obtained are reproduced in figs. 2-9. The base-lines, corresponding to the bare films, are shown for iron, cobalt, nickel and iridium. 3.1. NICKEL The admission of carbon monoxide to a pressure of 10m3 Torr at 170°K gave the spectrum shown in fig. 2b. Two distinct bands were obtained centred at 2050) 10 cm-r and 1900+ 10 cm-‘. The band at 2050 cm-’ is accompanied by a transmission peak on its high frequency side. The spectrum was unchanged by pumping, but on warming to room temperature a considerable reduction took place in the absorption near 2060 cm-’ resulting in spectrum 2c.
;
go-
.-
.-2
E
2
,m
I-
1o02100 cm-’
Fig. 2.
Nickel film. (a) bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
430
F.S. BAKER,
A.M.BRADSHAW,
I. PRITCHARD
AND
K. W. SYKES
The transmission peak is not as marked as before. In addition an overall loss of transmission of about 3 to 4 per cent occurred, and the spectrum was recorded after adjusting the attenuator in the reference beam to compensate for this and to allow exact comparison of the spectra. The band at 1900 cm-’ remained unchanged, but the higher frequency band was centred at 2020 cm- ‘. With another nickel film a similar spectrum was observed after pumping and warming to room temperature, but the further addition of carbon monoxide to give a pressure of 24 Torr yielded the spectrum shown in fig. 3.
2100
2000
1900
cm-’ Fig. 3.
Nickel film showing band due to nickel carbonyl with 24 Torr of CO.
It was suspected that the sharp band at 2060+5 cm-’ was due to gaseous nickel carbonyl, and this was confirmed as follows. A further film was evaporated in the cell without a substrate plate mounted on the re-entrant. Both windows were shielded during this evaporation. After increasing the pressure to 24 Torr a similar band appeared together with the P-branch of gaseous
carbon
monoxide,
although
there was no nickel film in the beam.
3.2. COBALT Fig. 4b shows
the spectrum
obtained
when carbon
monoxide
was admitted
(a) 100,
) 2100
I 2000
1900
cm-’
Fig. 4.
Cobalt film. (a) bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
INFRA-RED
431
SPECTRA
to the bare cobalt film. There is a maximum
in the absorption
at 1990 cm-r
and a broad tail extending to below 1800 cm-r. As with nickel no change occurred on pumping, but after warming to room temperature a similar overall loss of transmission took place and the higher frequency absorption was considerably reduced causing the maximum to shift to 1970 cm-’ (fig. 4~). The tail remained unchanged, like the lower frequency band on nickel. 3.3. IRON The behaviour was very similar to that of cobalt. Fig. 5b shows the low temperature spectrum at 10m3 Tort-, and fig. 5c the result of warming to room temperature. The band maximum shifted from 1950 to 1900 cm-‘. A similar low frequency tail was observed, and also an overall loss of transmission after warming.
1001 2100 Fig. 5.
2000 cm-’
1900
Iron film. (4 bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
3.4. IRIDIUM Admission of carbon monoxide to the bare film at low temperature gave the very weak band shown in fig. 6b. After standing at room temperature
1oo%7tr--2000
1900
cm -1
Fig. 6.
Iridium film. (a) bare film, (b) CO spectrum at 170”K, (c) after four hours at room temperature with 52 Torr, and pumped.
432
F.S. BAKER,
A.M.
BRADSHAW,
J. PRITCHARD
AND
K. W. SYKES
for four hours with a pressure of 52 Torr and then pumping appeared (fig. 6c) in the same position, 1970+ 10 cm-‘.
a stronger
band
3.5. TUNGSTEN, MANGANESE,CHROMIUM The spectra obtained with films of tungsten, manganese and chromium after deposition in carbon monoxide at low3 Torr are shown in figs. 7, 8a
100
2100
2000
1900
cm-’
Fig. 7. Tungsten film deposited in CO.
IOOL
Fig. 8.
Manganese
’ 2100
I
to
I
2 000 1900 cm-l film deposited in CO. (a) at 170”K, (b) after warming to room temperature. 2100
Chromium
1900
film deposited in CO. (a) at 170”K, (b) after warming room temperature.
100 Fig. 9.
I
2000 c m-l
INFRA-RED
and 9a respectively.
Broad
SPECTRA
single bands
were obtained
433
with tungsten
and
chromium, but manganese gave two bands at 2000f 10 cm-’ and 208Ok 5 cm-‘. The tungsten band at 1970+ 10 cm-’ was lost after pumping and warming to room temperature. With chromium the band at 1950 cm-’ lost some intensity and shifted to 1930 cm- 1 (fig. 9b) after warming to room temperature. In fig. 8b a similar effect is shown for manganese; the band at 2080 cm- ’ has been lost and the 2000 cm- ’ band has shifted to 1980 + 10 cm- ‘. 3.6. THE EFFECT OF OXYGEN In all cases except iridium the admission of 2-3 Torr of oxygen at room temperature led to the rapid disappearance of the absorption bands. This effect was accompanied by a slight overall increase of transmission. The single band observed with iridium at 1970 cm-l was much less easily affected by oxygen. After 3 hours exposure the band was reduced by about 25 per cent. 4. Discussion 4.1. CONDITIONS FOR OBTAINING SPECTRA The results show that clearly distinguishable absorption bands may be obtained from carbon monoxide adsorbed on single layer vacuum deposited films provided that the temperature of deposition of the films is kept low. It is also necessary to limit the film thickness. This limitation is not set by the available infra-red beam intensity. An overall transmission of 10 per cent would provide adequate energy to observe bands of the magnitudes which have been found, but in practice films with an overall transmission of this order failed to yield any spectra. The maximum absorbance of the bands reported here is about 0.02, which may be compared with about 0.005 per film in the 20 film stacks used by Harrod et al.ll). The structure of these thin films is likely to control the magnitude of the absorption spectrum. Very thin films have an island structure consisting largely of discrete nucleils) which join and aggregate as the film grows in thickness. The aggregation sintering processes may well be responsible for the disappearance of the spectra when thicker films are used. For example, with a simple model structure in which the film consists of discrete cubic crystallites with the cube faces parallel or perpendicular to the substrate 80 per cent of the available surface is perpendicular to the substrate, and this fraction would be lost on coalescence to form a continuous film. Moreover, if the C-O dipole is normal to the metal surface it is only on the perpendicular faces that the dipoles are suitably oriented to absorb radiation at normal incidence to the substrate. Although a real metal film would be more complex in structure similar effects should occur.
434
F. S.BAKER,
A. hi. BRADSHAW,
3. PRITCHARD
AND K. W. SYKES
The failure to observe measurable absorption bands with vacuum deposited films of tungsten, manganese and chromium may have been due to the structural effects. However, it is equally probable that heavy contamination played an important part. This is particularly likely for tungsten in view of the high temperature of the evaporation source and the ease with which all residual gases are chemisorbed by tungsten. Furthermore manganese and chromium were evaporated from chips which could not be properly outgassed without depositing thick films whereas nickel, cobalt, iron and iridium could be evaporated from outgassed filaments. It is probable that all these metals would yield spectra if very thin films were deposited in ultra-high vacuum conditions. When films were evaporated in a low pressure of carbon monoxide the surfaces of the growing crystallites would have been covered immediately with adsorbed carbon monoxide. In addition to inhibiting contamination by other gases the adsorbed carbon monoxide may have affected the growth of the film. Contamination and film structure are not independent factors. It should be emphasized however that the pressure of carbon monoxide used in this work was such that little or no scattering of metal vapour occurred during the deposition of the films. This is to be contrasted with the higher pressure used by Garland9) at which metal atoms make many collisions with carbon monoxide molecules before condensing on the substrate. Gas phase and heterogeneous reactions may then be involved in the mechanism of film growth. 4.2. THE NATURE OF THE SPECTRA The absorption bands are generally broad, as predictedr4) for carbon monoxide coupled with a crystal. In several cases, however, this broadness is extremely pronounced. The broad bands with iron and cobalt show long low frequency tails similar to those reported by Harrod et al.ll) on evaporated films of platinum and iridium. These results are to be contrasted with the much sharper bands found on supported platinum’), iron’) and cobalt4). The only fairly narrow bands which we have observed are the 2080 cm-’ band with manganese deposited in carbon monoxide, and the 2060 cm-’ band with nickel. The latter has been shown to be due to nickel carbonyl, and the former may be a consequence of some reaction with carbon monoxide during the film deposition. Both bands were removed by pumping at room temperature. It seems probable that the sharpness of the bands is related to the size of the crystallites on which the carbon monoxide is adsorbed. The average size of the crystallites in a supported catalyst is usually much smaller than in an evaporated film of the same metal. This is shown by volumetric adsorption
INFRA-RED
data as well as by electron
microscopy.
SPECTRA
However,
435
there will be some overlap
in the size distributions, especially for the very thin low temperature films used in this work. The spectra obtained with supported metals may therefore often show similar features to those reported here, but overshadowed by the more intense contributions from smaller particles. For example, the spectrum of carbon monoxide on supported cobalta) is broadly similar to our low temperature result but with a more pronounced maximum, particularly when the reversibly adsorbed carbon monoxide is present. In general it is possible to select spectra for iron, cobalt, and nickel, from the wide range of published data for supported metals which are similar to those described here for films. In the case of iridium we can compare the present result with anotherlr) for vacuum evaporated films, and in the case of nickel with films deposited in carbon monoxide or argong,lO). The spectrum with iridium is very similar to that obtained by Harrod l1) after oxidising an iridium film. It suggests that under the relatively poor vacuum conditions some oxidation may have occurred. On the other hand the results for nickel are in good accord with those of Haywardlo) for films deposited in lo-’ Torr of carbon monoxide, and of Garlandg) for films deposited at 2 Torr, especially for the broad band around 1900 cm-l. The effect of warming the films to room temperature is to lose some absorption at the high frequency end of the spectrum leaving the lower frequency band or tail unchanged. At the same time the film structure appears to be slightly altered because there is an overall increase of transmission. The loss of high frequency absorption may be due to desorption of some carbon monoxide or its conversion to a less strongly absorbing state. It is probable that sintering takes place and that it is accompanied by the removal of adsorption sites of special geometry. It has not been possible to estimate the amount of carbon monoxide adsorbed or desorbed in these experiments, but we suspect that the appreciable room temperature involve species ficients.
intensity changes with particularly
caused by warming to high extinction coef-
The appearance of a transmission peak at the high frequency end of the nickel spectrum is surprising, but it may be a marked case of the Christiansen filter effect15) which has been thought responsible for similar effects on platinumg). It is to be emphasized that the results presented here are of a preliminary nature only, but they illustrate the usefulness of vacuum deposited films for spectroscopic work. Because of the relatively poor vacuum conditions the films must have been appreciably contaminated and more detailed comparison of these spectra with other results is not at present worthwhile It is clearly desirable to carry out similar experiments under ultra high vacuum
436
F.S. BAKER,
A. MBRADSHAW,
.I. PRITCHARD
conditions, and to investigate the relationships metal films and the kind of spectra obtained.
AND K. W. SYKES
between
the structure
of the
Acknowledgements We are grateful to the D.S.I.R. and the Science Research Council for a grant for equipment and for maintenance awards (F.S.B. and A.M.B.).
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
R. P. Eischens and W. A. Pliskin, Advan. Catalysis 10 (1958) 1. J. T. Yates and C. W. Garland, J. Phys. Chem. 65 (1961) 617. C. E. O’Neill and D. J. C. Yates, J. Phys. Chem. 65 (1961) 901. J. S. Cho and J. H. Schulman, Surface Sci. 2 (1964) 245. N. N. Kavtaradze and V. I. Lygin, Dokl. Akad. Nauk SSSR 138 (1961) 616. A. W. Smith and J. M. Quets, J. Catalysis 4 (1965) 163. C. R. Guerra and J. H. Schulman, Surface Sci. 7 (1967) 229. J. B. Sardisco, Perkin-Elmer Instr. News 15 (1963) 13. C. W. Garland, R. C. Lord and P. F. Troiano, J. Phys. Chem. 69 (1965) 1188, 1195. D. 0. Hayward, described by L. H. Little, Znfia-red Spectra of Adsorbed Species (Academic Press, London, 1966) p. 59. J. F. Harrod, R. W. Roberts and E. F. Rissman, J. Phys. Chem. 71 (1967) 343. F. S. Baker and J. Pritchard, J. Sci. Instr. 44 (1967) 652. D. W. Pashley, Advan. Phys. 14 (1965) 327. W. H. Smith and H. C. Eckstrom, J. Chem. Phys. 46 (1967) 3657. W. C. Price and A. N. Tetlow, J. Chem. Phys. 16 (1948) 1157.
SCIENCE
INFRA-RED
12 (1968) 426-436 0 North-Holland
SPECTRA VACUUM
F. S. BAKER, Department
OF CARBON DEPOSITED
A. M. BRADSHAW,
Publishing Co., Amsterdam
MONOXIDE METAL
J. PRITCHARD
qf Chemistry,
Queen Mary College, London, E. I., England
ADSORBED
ON
FILMS and K. W. SYKES Mile End Road,
Received 28 May 1968 Infra-red absorption spectra of adsorbed carbon monoxide have been obtained using single layer evaporated metal films as adsorbents. The films were deposited at low temperatures and were sufficiently thin to transmit approximately 70 per cent of the incident radiation at 2000 cm-l. Nickel, cobalt, iron and iridium films were deposited in high vacuum, but useful spectra with manganese, chromium and tungsten were obtained only with films deposited in carbon monoxide.
1. Introduction Absorption spectroscopy of adsorbed molecules by transmission in the infra-red region of the spectrum is complicated by the loss of energy due to absorption and scattering by the metal adsorbent. The problem can be eased by reducing the particle size of the metal to well below the wavelength of the radiation. Absorption and scattering are reduced while the surface area, and therefore the quantity of adsorbate in the light path, is increased. In most work to dater-v) a small particle size has been achieved by the reduction of metal oxides or salts supported on silica or alumina. Although this technique provides a well-dispersed metal sample it suffers from some disadvantages. Reduction may not be complete and residual adsorbed species may be present. The support material may possess absorption bands which limit the measurable spectrum. Furthermore, it has been shown3) that the spectrum obtained from a given adsorbate-metal system is dependent upon the support material. In attempting to overcome these disadvantages some workers have used thin evaporated metal films. Spectra of carbon monoxide have been obtainedr,*) with films deposited in vacuum but exposed to air before adsorption of carbon monoxide. Garlands) has reported that metal films evaporated and kept in a high vacuum fail to give spectra with carbon monoxide, but that good spectra are obtained when the metal is evaporated in a pressure of carbon monoxide. 426
INFRA-RED
The latter
technique
had been
421
SPECTRA
used previously
by Haywardis).
Recently
spectra have been reportedrl) for carbon monoxide on some platinum metals by transmission through a stack of twenty very thin metal films deposited under ultra-high vacuum conditions. It is probable that the failure to obtain spectra of carbon monoxide on single films deposited in a high vacuum was due to the combined effects of insufficient surface area and too large a particle size. It is well known that small particle sizes and large surface areas are obtained when films are deposited on substrates at low temperatures. Therefore we have investigated the possibility of using low substrate temperatures to give metal films with areas adequate to yield spectra with adsorbed carbon monoxide. In the work described here spectra were obtained as soon as carbon monoxide was added to films of nickel, cobalt, iron and iridium evaporated in vacuum. Barely detectable changes occurred with vacuum deposited films of manganese, chromium and tungsten, and it was found necessary to evaporate these metals in a low pressure (lo-’ Torr) of carbon monoxide to give spectra comparable with those obtained from the first group of metals*. More detailed studies of the nickel-carbon monoxide system under ultra-high vacuum conditions will be described in a following paper. 2. Experimental 2.1. APPARATUS The infra-red cell is shown diagrammatically in fig. 1. It was constructed in two Pyrex glass sections with a B40 cone and socket joint. The outer section had periclase windows attached with picein wax, and tungsten leads on which the evaporation filament could be mounted. The inner section carried a glass re-entrant to which a copper block was attached through an intermediate glass to metal seal. The film substrate was a fluorite plate which was clipped to a bracket on the block, and which could be cooled by conduction when the re-entrant was filled with liquid nitrogen. The films were evaporated either from hairpin filaments of the pure metals (Ni, Co, Fe, W, Ir) or from chips of manganese or chromium in a tungsten spiral. During evaporation the window adjacent to the evaporation source was shielded with a shutter of foil which could be moved away magnetically afterwards. Nickel foil was used at first, but with high pressures of carbon monoxide an absorption band attributable to nickel carbonyl was often observed. A mild steel shutter was substituted later. The cell was attached to an all glass vacuum system evacuated by a
* A preliminary account of these results was given at the Chemical Nottingham, September, 1965.
Society Meeting,
428
F. S. BAKER,
A.M.
BRADSHAW,
J. PRITCHARD
AND K. W. SYKES
mercury diffusion pump and equipped with mercury cut-offsl2) for dosing gases into the infra-red cell. The whole vacuum system could be raised or lowered so that the cell could be inserted into the cell well of a Unicam SP 100 spectrophotometer.
Fig. 1.. The infra-red cell with cooled re-entrant.
2.2.
PR~CEOURE
Because of the waxed joints the cell could not be baked, and the vacuum conditions were poor compared with those normally attained in work with evaporated films. The cell was evacuated for several hours via a trap at 77 “K. The metal to be evaporated was then heated in hydrogen at about 1 Torr for 30 min to reduce surface oxides, and finally it was outgassed for one hour. During outgassing the pressure was in the range lo-’ to 10m6 Torr. The re-entrant was cooled with liquid nitrogen. After one hour the Guorite plate reached a steady temperature of about 170°K as measured in separate experiments with a thermocouple attached to the front surface of the plate. For the vacuum evaporated metals the film was then deposited. It was found that the best spectra could be obtained with films giving about 70 per cent was below 50 per cent the transmission near 2000 cm- ‘. If the transmission absorption bands were small and di~cult to observe against the spectrometer noise. One nickel film was deposited on a plate coored to only 208°K. No spectra were obtainable. After evaporation the cell was lowered into the sample beam cell well of the spectrometer and a variable attenuator in the reference beam was used to adjust the recorder output to 95 per cent of the full scale at 1800 cm-‘. The spectrum of the bare film was recorded while the substrate was still cold.
INFRA-RED
429
SPECTRA
Doses of carbon monoxide were then admitted until the pressure reached 10e3 Torr. The spectrum was again recorded on the same chart. After pumping, the film was allowed to warm to room temperature, and spectra were recorded as before. Finally oxygen was admitted to a pressure of about 1 Torr, and its effect on the carbon monoxide spectrum was observed. For those metals (Mn, Cr, W) which did not give spectra after being deposited in vacuum some films were deposited in a pressure of lo-’ Torr of carbon monoxide and subsequently treated exactly as the vacuum deposited films. In this preliminary work no compensating cell was used in the reference beam. As a result bands due to atmospheric water vapour appeared below 1900 cm-‘. Flushing with dry nitrogen removed the interfering bands between 1800 and 1900 cm- r. The spectrometer was therefore set to scan over the range 1800 to 2150 cm-‘. Any number of spectra could be superimposed on the recorder chart to enable the detection of small changes.
3. Results The spectra which were obtained are reproduced in figs. 2-9. The base-lines, corresponding to the bare films, are shown for iron, cobalt, nickel and iridium. 3.1. NICKEL The admission of carbon monoxide to a pressure of 10m3 Torr at 170°K gave the spectrum shown in fig. 2b. Two distinct bands were obtained centred at 2050) 10 cm-r and 1900+ 10 cm-‘. The band at 2050 cm-’ is accompanied by a transmission peak on its high frequency side. The spectrum was unchanged by pumping, but on warming to room temperature a considerable reduction took place in the absorption near 2060 cm-’ resulting in spectrum 2c.
;
go-
.-
.-2
E
2
,m
I-
1o02100 cm-’
Fig. 2.
Nickel film. (a) bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
430
F.S. BAKER,
A.M.BRADSHAW,
I. PRITCHARD
AND
K. W. SYKES
The transmission peak is not as marked as before. In addition an overall loss of transmission of about 3 to 4 per cent occurred, and the spectrum was recorded after adjusting the attenuator in the reference beam to compensate for this and to allow exact comparison of the spectra. The band at 1900 cm-’ remained unchanged, but the higher frequency band was centred at 2020 cm- ‘. With another nickel film a similar spectrum was observed after pumping and warming to room temperature, but the further addition of carbon monoxide to give a pressure of 24 Torr yielded the spectrum shown in fig. 3.
2100
2000
1900
cm-’ Fig. 3.
Nickel film showing band due to nickel carbonyl with 24 Torr of CO.
It was suspected that the sharp band at 2060+5 cm-’ was due to gaseous nickel carbonyl, and this was confirmed as follows. A further film was evaporated in the cell without a substrate plate mounted on the re-entrant. Both windows were shielded during this evaporation. After increasing the pressure to 24 Torr a similar band appeared together with the P-branch of gaseous
carbon
monoxide,
although
there was no nickel film in the beam.
3.2. COBALT Fig. 4b shows
the spectrum
obtained
when carbon
monoxide
was admitted
(a) 100,
) 2100
I 2000
1900
cm-’
Fig. 4.
Cobalt film. (a) bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
INFRA-RED
431
SPECTRA
to the bare cobalt film. There is a maximum
in the absorption
at 1990 cm-r
and a broad tail extending to below 1800 cm-r. As with nickel no change occurred on pumping, but after warming to room temperature a similar overall loss of transmission took place and the higher frequency absorption was considerably reduced causing the maximum to shift to 1970 cm-’ (fig. 4~). The tail remained unchanged, like the lower frequency band on nickel. 3.3. IRON The behaviour was very similar to that of cobalt. Fig. 5b shows the low temperature spectrum at 10m3 Tort-, and fig. 5c the result of warming to room temperature. The band maximum shifted from 1950 to 1900 cm-‘. A similar low frequency tail was observed, and also an overall loss of transmission after warming.
1001 2100 Fig. 5.
2000 cm-’
1900
Iron film. (4 bare film, (b) CO spectrum room temperature.
at 170”K, (c) after warming
to
3.4. IRIDIUM Admission of carbon monoxide to the bare film at low temperature gave the very weak band shown in fig. 6b. After standing at room temperature
1oo%7tr--2000
1900
cm -1
Fig. 6.
Iridium film. (a) bare film, (b) CO spectrum at 170”K, (c) after four hours at room temperature with 52 Torr, and pumped.
432
F.S. BAKER,
A.M.
BRADSHAW,
J. PRITCHARD
AND
K. W. SYKES
for four hours with a pressure of 52 Torr and then pumping appeared (fig. 6c) in the same position, 1970+ 10 cm-‘.
a stronger
band
3.5. TUNGSTEN, MANGANESE,CHROMIUM The spectra obtained with films of tungsten, manganese and chromium after deposition in carbon monoxide at low3 Torr are shown in figs. 7, 8a
100
2100
2000
1900
cm-’
Fig. 7. Tungsten film deposited in CO.
IOOL
Fig. 8.
Manganese
’ 2100
I
to
I
2 000 1900 cm-l film deposited in CO. (a) at 170”K, (b) after warming to room temperature. 2100
Chromium
1900
film deposited in CO. (a) at 170”K, (b) after warming room temperature.
100 Fig. 9.
I
2000 c m-l
INFRA-RED
and 9a respectively.
Broad
SPECTRA
single bands
were obtained
433
with tungsten
and
chromium, but manganese gave two bands at 2000f 10 cm-’ and 208Ok 5 cm-‘. The tungsten band at 1970+ 10 cm-’ was lost after pumping and warming to room temperature. With chromium the band at 1950 cm-’ lost some intensity and shifted to 1930 cm- 1 (fig. 9b) after warming to room temperature. In fig. 8b a similar effect is shown for manganese; the band at 2080 cm- ’ has been lost and the 2000 cm- ’ band has shifted to 1980 + 10 cm- ‘. 3.6. THE EFFECT OF OXYGEN In all cases except iridium the admission of 2-3 Torr of oxygen at room temperature led to the rapid disappearance of the absorption bands. This effect was accompanied by a slight overall increase of transmission. The single band observed with iridium at 1970 cm-l was much less easily affected by oxygen. After 3 hours exposure the band was reduced by about 25 per cent. 4. Discussion 4.1. CONDITIONS FOR OBTAINING SPECTRA The results show that clearly distinguishable absorption bands may be obtained from carbon monoxide adsorbed on single layer vacuum deposited films provided that the temperature of deposition of the films is kept low. It is also necessary to limit the film thickness. This limitation is not set by the available infra-red beam intensity. An overall transmission of 10 per cent would provide adequate energy to observe bands of the magnitudes which have been found, but in practice films with an overall transmission of this order failed to yield any spectra. The maximum absorbance of the bands reported here is about 0.02, which may be compared with about 0.005 per film in the 20 film stacks used by Harrod et al.ll). The structure of these thin films is likely to control the magnitude of the absorption spectrum. Very thin films have an island structure consisting largely of discrete nucleils) which join and aggregate as the film grows in thickness. The aggregation sintering processes may well be responsible for the disappearance of the spectra when thicker films are used. For example, with a simple model structure in which the film consists of discrete cubic crystallites with the cube faces parallel or perpendicular to the substrate 80 per cent of the available surface is perpendicular to the substrate, and this fraction would be lost on coalescence to form a continuous film. Moreover, if the C-O dipole is normal to the metal surface it is only on the perpendicular faces that the dipoles are suitably oriented to absorb radiation at normal incidence to the substrate. Although a real metal film would be more complex in structure similar effects should occur.
434
F. S.BAKER,
A. hi. BRADSHAW,
3. PRITCHARD
AND K. W. SYKES
The failure to observe measurable absorption bands with vacuum deposited films of tungsten, manganese and chromium may have been due to the structural effects. However, it is equally probable that heavy contamination played an important part. This is particularly likely for tungsten in view of the high temperature of the evaporation source and the ease with which all residual gases are chemisorbed by tungsten. Furthermore manganese and chromium were evaporated from chips which could not be properly outgassed without depositing thick films whereas nickel, cobalt, iron and iridium could be evaporated from outgassed filaments. It is probable that all these metals would yield spectra if very thin films were deposited in ultra-high vacuum conditions. When films were evaporated in a low pressure of carbon monoxide the surfaces of the growing crystallites would have been covered immediately with adsorbed carbon monoxide. In addition to inhibiting contamination by other gases the adsorbed carbon monoxide may have affected the growth of the film. Contamination and film structure are not independent factors. It should be emphasized however that the pressure of carbon monoxide used in this work was such that little or no scattering of metal vapour occurred during the deposition of the films. This is to be contrasted with the higher pressure used by Garland9) at which metal atoms make many collisions with carbon monoxide molecules before condensing on the substrate. Gas phase and heterogeneous reactions may then be involved in the mechanism of film growth. 4.2. THE NATURE OF THE SPECTRA The absorption bands are generally broad, as predictedr4) for carbon monoxide coupled with a crystal. In several cases, however, this broadness is extremely pronounced. The broad bands with iron and cobalt show long low frequency tails similar to those reported by Harrod et al.ll) on evaporated films of platinum and iridium. These results are to be contrasted with the much sharper bands found on supported platinum’), iron’) and cobalt4). The only fairly narrow bands which we have observed are the 2080 cm-’ band with manganese deposited in carbon monoxide, and the 2060 cm-’ band with nickel. The latter has been shown to be due to nickel carbonyl, and the former may be a consequence of some reaction with carbon monoxide during the film deposition. Both bands were removed by pumping at room temperature. It seems probable that the sharpness of the bands is related to the size of the crystallites on which the carbon monoxide is adsorbed. The average size of the crystallites in a supported catalyst is usually much smaller than in an evaporated film of the same metal. This is shown by volumetric adsorption
INFRA-RED
data as well as by electron
microscopy.
SPECTRA
However,
435
there will be some overlap
in the size distributions, especially for the very thin low temperature films used in this work. The spectra obtained with supported metals may therefore often show similar features to those reported here, but overshadowed by the more intense contributions from smaller particles. For example, the spectrum of carbon monoxide on supported cobalta) is broadly similar to our low temperature result but with a more pronounced maximum, particularly when the reversibly adsorbed carbon monoxide is present. In general it is possible to select spectra for iron, cobalt, and nickel, from the wide range of published data for supported metals which are similar to those described here for films. In the case of iridium we can compare the present result with anotherlr) for vacuum evaporated films, and in the case of nickel with films deposited in carbon monoxide or argong,lO). The spectrum with iridium is very similar to that obtained by Harrod l1) after oxidising an iridium film. It suggests that under the relatively poor vacuum conditions some oxidation may have occurred. On the other hand the results for nickel are in good accord with those of Haywardlo) for films deposited in lo-’ Torr of carbon monoxide, and of Garlandg) for films deposited at 2 Torr, especially for the broad band around 1900 cm-l. The effect of warming the films to room temperature is to lose some absorption at the high frequency end of the spectrum leaving the lower frequency band or tail unchanged. At the same time the film structure appears to be slightly altered because there is an overall increase of transmission. The loss of high frequency absorption may be due to desorption of some carbon monoxide or its conversion to a less strongly absorbing state. It is probable that sintering takes place and that it is accompanied by the removal of adsorption sites of special geometry. It has not been possible to estimate the amount of carbon monoxide adsorbed or desorbed in these experiments, but we suspect that the appreciable room temperature involve species ficients.
intensity changes with particularly
caused by warming to high extinction coef-
The appearance of a transmission peak at the high frequency end of the nickel spectrum is surprising, but it may be a marked case of the Christiansen filter effect15) which has been thought responsible for similar effects on platinumg). It is to be emphasized that the results presented here are of a preliminary nature only, but they illustrate the usefulness of vacuum deposited films for spectroscopic work. Because of the relatively poor vacuum conditions the films must have been appreciably contaminated and more detailed comparison of these spectra with other results is not at present worthwhile It is clearly desirable to carry out similar experiments under ultra high vacuum
436
F.S. BAKER,
A. MBRADSHAW,
.I. PRITCHARD
conditions, and to investigate the relationships metal films and the kind of spectra obtained.
AND K. W. SYKES
between
the structure
of the
Acknowledgements We are grateful to the D.S.I.R. and the Science Research Council for a grant for equipment and for maintenance awards (F.S.B. and A.M.B.).
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
R. P. Eischens and W. A. Pliskin, Advan. Catalysis 10 (1958) 1. J. T. Yates and C. W. Garland, J. Phys. Chem. 65 (1961) 617. C. E. O’Neill and D. J. C. Yates, J. Phys. Chem. 65 (1961) 901. J. S. Cho and J. H. Schulman, Surface Sci. 2 (1964) 245. N. N. Kavtaradze and V. I. Lygin, Dokl. Akad. Nauk SSSR 138 (1961) 616. A. W. Smith and J. M. Quets, J. Catalysis 4 (1965) 163. C. R. Guerra and J. H. Schulman, Surface Sci. 7 (1967) 229. J. B. Sardisco, Perkin-Elmer Instr. News 15 (1963) 13. C. W. Garland, R. C. Lord and P. F. Troiano, J. Phys. Chem. 69 (1965) 1188, 1195. D. 0. Hayward, described by L. H. Little, Znfia-red Spectra of Adsorbed Species (Academic Press, London, 1966) p. 59. J. F. Harrod, R. W. Roberts and E. F. Rissman, J. Phys. Chem. 71 (1967) 343. F. S. Baker and J. Pritchard, J. Sci. Instr. 44 (1967) 652. D. W. Pashley, Advan. Phys. 14 (1965) 327. W. H. Smith and H. C. Eckstrom, J. Chem. Phys. 46 (1967) 3657. W. C. Price and A. N. Tetlow, J. Chem. Phys. 16 (1948) 1157.