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The surface potential of water vapour on epitaxially grown (110) oriented films of platinum
SURFACE
SCIENCE
17 (1969) 474-481
THE SURFACE EPITAXIALLY
0 North-Holland
POTENTIAL
Publishing
OF WATER
GROWN (110) ORIENTED
Co., Amsterdam
VAPOUR
ON
FILMS OF
PLATINUM Received
16 February
1969
This note describes the preparation of an atomically clean (I 10) surface of platinum, and the measurement of the surface potential changes brought about by the introduction of water vapour. Previous experiments with platinum have most commonly been made either on macroscopic single crystal surfaces or on films vapour deposited onto amorphous substrates. In the latter case the resulting deposits were almost certainly polycrystallinel) though the final surface may have been clean. For the single crystals, although the crystallographic orientation was certain (excluding perhaps the outermost few layers where LEED shows - not unambiguously - some peculiarities) there is considerable doubt about the surface chemical composition. Common impurities in platinum, such as iron or silicon, are unlikely to be removed by maintaining the crystal at a temperature a few degrees below its melting point for extended periods”). Nor is heating in oxygen or hydrogen likely to improve matters although it could successfully remove carbon. Extended ion bombardment may be more effective though care then has to be taken that any subsequent anneal does not cause the reappearance of surface impurities by diffusion from the bulk. Distillation of the platinum from a suitable vapour source can give a high purity deposit and a useful surface, provided that the vapour can be condensed as a single crystal film. In the present work a tungsten substrate was used and the final platinum film was (110) oriented. It was convenient in these experiments to use a ribbon of polycrystalline foil, which had been thoroughly aged in an ultra-high vacuum at temperatures exceeding 3000”Ks) as the crystalline substrate. The mean crystallite size was 0.01 mm and X-ray examination showed the texture to be almost entirely (100) though the crystallites were oriented randomly with respect to one another. Clearly, it was to be expected that if epitaxy did occur then a textured deposit rather than a single crystal film would be formed. From the point of view of surface potential measurements, provided that the grain boundaries can be ignored (this is reasonable since the Kelvin technique used gives an area averaged result and the fractional area of the boundaries is very small) the results should be very similar 474
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
475
to those obtained from a single crystal film. We are currently studying the growth of platinum on several differently oriented tungsten single crystals. The surface potential measurements involved following the contact potential difference (c.p.d.) between a tungsten (110) oriented single crystal, used as a reference, and the platinum coated polycrystalline foil. We had previously4) determined the surface potential changes on the (110) tungsten surface as a function of the number of incident water molecules and hence it was possible to convert the c.p.d. measurements into surface potential changes on the platinum deposit. By assuming the work function of the polycrystalline tungsten foils) at 4.55 eV the final results could be expressed as work function change as a function of the number of incident molecules. The water vapour was introduced from prepared glass ampoules which could be broken magnetically once the pressure conditions were satisfactory. Initially the empty glass ampoules were sealed to a separate system and evacuated to about 5 x lo-’ Torr. Pure nitrogen was then introduced at atmospheric pressure and the vacuum system opened so that distilled water could be introduced. The vacuum was then restored and the water in the ampoules heated gently to remove dissolved gases. Finally the water in the ampoule was frozen in liquid nitrogen and, with the system evacuated to better than 10e6 Torr, the ampoule sealed-off. Control of the water vapour pressure, once the ampoule has been broken in the system, was by surrounding the side arm, containing the ampoule, in liquid nitrogen and also by adjusting a bakeable stopcock between the side arm and the main chamber. The experimental chamber and vacuum system are rather similar to those previously used in an experiment with uraniumc). The evaporator consisted of two co-axial tungsten coils and a magnetically operated loader. Following the attainment of an ultra-high vacuum with both of these coils maintained at about 28OO”K, the platinum wire on the end of the nickel loader was “touched” onto the central coil while it was hot. The metal deposited in this way could be outgassed and finally evaporated by electron bombardment from the outer tungsten coil. The maximum pressure in the chamber, recorded during the evaporation of the platinum films used in the measurements, was about 5 x lo-’ Torr. Both of the electrodes could be heated by bombardment on their reverse side, by focussed electrons. The crystal could be vibrated through a stainless steel bellows in a plane perpendicular to its surface for the c.p.d. measurements. The outgassing of the foil and the crystal was such that at completion either surface could be flashed to 3000°K without a pressure rise to greater than about 5 x lo-” Torr. A most important precaution in the use of water vapour was in the removal of carbon from all hot filaments and samples.
476
C. W. JOWElT,
P. J. DOBSON
AND B. J. HOPKINS
During our early work we observed that the partial pressure of any water introduced into the vacuum system very rapidly fell while at the same time the partial pressures of hydrogen and carbon monoxide increased in rough proportion. After extended heat treatment of the metal components and gauge filaments the degree of contamination from gases, formed in the reaction of the water with carbon dissolved in the metals, could be reduced to a negligible amount. In the present measurements this heating was performed in about 10e6 Torr of water since this was obviously very convenient. The final surface potential measurements described in this paper were always repeated with all the gauge filaments switched off as a check that impurities from reactions with the water were not influencing the results. The processing of the electronic signal from the vibrating capacitor in order to give a continuous recorder trace of the c.p.d. has been described previously7). Under the optimum conditions in the present experiments individual c.p.d. measurements could be made to better than f 10 mV. The error bars shown in the results below are, however, slightly larger than this because of the uncertainty in reproducing the position on the “incident molecules” axis. The observed c.p.d. between the clean foil and the (110) oriented crystal was 0.58+0.01 V, in good agreement with many previous determinations from this laboratory 8). Seven thin films of platinum were then deposited and the c.p.d. measured after each deposition at -0.25f0.02 V (where the negative sign indicates that the platinum had the higher work function) giving a platinum work function of 5.38kO.03 eV for the deposit formed at room temperature. Heating the foil to a minimum temperature of about 900°K caused a slight reduction in this value to 5.23 +0.02 eV which then remained constant with further heating up to 1800°K. Beyond this temperature the platinum began to re-evaporate and the work function fell back towards that of the clean foil. The adsorption measurements were made only on the annealed film and the structure of this film was examined after completion of these measurements using the techniques of X-ray diffractometry and reflection electron diffraction. Since the film thickness was estimated to be only 1000 A the intensity of X-ray reflections were very weak. The X-ray method was used therefore only to confirm the main features of the electron diffraction results. The electron diffraction was performed in an A.E.I. EM6G electron microscope using a special crystal manipulator*. An electron energy of 80 kV was found to be convenient and gave rise to the diffraction features shown in fig. 1 for two directions separated by an angle of 55”. Scanning the electron beam over the sample surface did not appreciably alter * Kindly loaned by Dr. R. M. Hill.
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
477
these patterns. The main spots in fig. la are characteristic of a platinum (110) plane viewed along a (110) azimuth. There are, however some addition spots, which can be seen most clearly in the top left hand corner, which indicate that the beam is parallel to a (114) azimuth of another crystallite. Rotation of the sample about an axis perpendicular to its surface by 55” should now view both sets of crystallites along a (211) azimuth as shown in fig. lb. The streaking in fig. 1b is probably a consequence of reflections taking place from two sets of crystallites such that the ordering parallel to the surface is much less perfect than that perpendicular to the surface. Also on fig. lb there are extra spots arising from a third orientation of the crystallites. In this case the electron beam is parallel to the (100) direction. Along the azimuth of fig. la these crystallites, provided that the electron beam was still irradiating them, ought to have given 111 reflections. These may, however, have been too faint to observe. In all of these crystallites the (110) plane was parallel to the
Fig. la.
478
C. W. IOWETT,
P. J. DOBSON AND B. J. HOPKINS
Fig. lb. Fig. 1. Reflection electron diffraction patterns of the platinum deposit at an electron energy of 80 kV. The direction of the electron beam differs in the two photographs by 55 ‘.
surface. The lattice constant of the platinum was determined from the 220, 111 and 002 reflections and found to be 3.95kO.10 A, in good agreement with the accepted value of 3.92 A. Thus, the final surface used in the adsorption measurements was a well textured one in which all of the crystallites presented a (110) platinum face to the vacuum. Clearly this implies that it will be possible to grow, by epitaxy, a (1 IO) oriented single crystal film of platinum on a tungsten (100) oriented single crystal. Since platinum and tungsten alloy, the purity of the platinum vapour stream, falling on a glass plate, from the tungsten evaporator, was analysed using an electron probe microanalyser. This showed the proportion of tungsten to be less than O.Ol%, the lower detection limit of the instrument. The possibility that an alloy was formed on the tungsten substrate, after heating the film to 900”K,
WATER
cannot
VAPOUR
be entirely
ON EPITAXIALLY
ruled out, though
GROWN
(110)
419
FILMS OF PLATINUM
the good agreement
of the measured
lattice constant suggests that this is not serious. The introduction of water vapour to the platinum and tungsten (110) surfaces gave rise to the c.p.d. changes shown in fig. 2. The water was usually introduced such that the pressure increased to a maximum of 5 x 10m4 Torr over a period of about 40 min. To achieve very high exposures the time was extended since pressure rises beyond 5 x 10e4 Torr led to difficulty in obtaining an ultra-high vacuum on re-evacuation. Removal of the water, on reaching the saturation point after an exposure of some 1O22 molecules cm- ‘, did not alter the final c.p.d. It is possible, of course, that there might have been exactly equal changes on the two surfaces during pump-out, but in view of the very different sticking probabilities of water on the two surfaces subsequently observed (fig. 2) this must be regarded as very unlikely. Also shown in fig. 2 is the work function versus exposure curve for the (1 IO) tungsten crysta14) and the final computed curve for water on platinum. The writers know of no other measurements involving the adsorption of water vapour onto platinum though such measurements have been reported
I
1
I
,
6.41
I
6.2 6.0
0.6
&O Fig. 2.
moleculesincident per cm’
The dashed curve is taken from a chart recording of the variation in the contact potential difference between the (110) platinum deposit and the (110) tungsten single crystal as water vapour was introduced. The full curves show the variation in work function of the tungsten (110) surface from ref. 3 and the changes deduced for the (110) platinum surface by combining the previous two curves.
480
C. W. JOWETT,
P. J. DOBSON
AND
B. J. HOPKINS
for tungsteng-r4). There it appears that dissociation of the water takes place with the intial possibility of both hydroxyl and hydrogen adsorbed species. Subsequent exposure leads to desorption of the hydrogen and a final covering of hydroxyl ions bonded through oxygen. From the limited measurements we have made on platinum it appears that this model might equally well apply. The electronegativity r5,r6) of the OH species is higher, 3.9, compared with that of oxygen, 3.5, so that on the basis of a simple dipole model it is to be expected that the final work function of the Pt-OH surface would be higher than that of Pt-0. Previous surface potential measurements have been reported for oxygen on various platinum samples (see ref. 2). The values observed range from 1 to 1.2 V, though the more recent measurements17*“) indicate a value close to 1 V. Thus, the difference between this value for oxygen and our surface potential for water, 1.13 +0.04 V, is very close to that expected solely on the basis of the electronegativity differences of the platinum-hydroxyl and the platinum-oxygen bonds. Conclusions The following conclusions can be drawn from this work: 1) A (110) oriented platinum film can be formed on a (100) oriented tungsten surface by vapour deposition followed by annealing to about 900°K. The work function of this deposit is 5.23 eV. 2) The adsorption of water probably takes place by dissociation into OH and H with the final surface comprising only OH ions and giving rise to an electronegative state comparable with that of oxygen on platinum but having a slightly higher surface potential, 1.13 kO.04 V compared with about I V measured for uncharacterised platinum surfaces. 3) The sticking probability of water on platinum (110) is very much less than that on tungsten (110). Acknowledgements Two of us, C.W.J. and P.J.D., would like to thank Council for the provision of maintenance grants.
the Science Research
C. W. JOWETT, P. J. DOBSON* and B. J. HOPKINS
Surface Physics, The University, Southampton, England * Now at Physics Department, Imperial College, London.
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
481
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
P. J. Dobson, Ph.D. Thesis, University of Southampton (1968). R. Lewis, and R. Gomer, Surface Sci. 12 (1968) 157. P. J. Dobson and B. J. Hopkins, Brit. J. Appl. Phys. [2] 1 (1968) 1241. B. J. Hopkins and C. W. Jowett, to be published. B. J. Hopkins and J. C. Riviere, Brit. J. Appl. Phys. [l] 15 (1964) 941. B. J. Hopkins and A. J. Sargood, Nuovo Cimento Suppl. 5 (1967) 459. T. J. Lee, Ph.D. Thesis, University of Southampton (1967). T. J. Lee, B. H. Blott and B. J. Hopkins, Appl. Phys. Letters 11 (1967) 361. H. Imai and C. Kemball, Proc. Roy. Sot. (London) A 302 (1968) 399. R. J. Hill and P. W. M. Jacobs, Nature (London) 180 (1957) 1117. R. J. Hill, Vacuum 11 (1961) 260. E. W. Mtiller, Ergeb. Exact. Naturw. 27 (1953) 290. Yu. K. Ustinov and N. I. Ionov, Soviet Phys.-Tech. Phys. 12 (1968) 1506. V. N. Ageer, N. I. Ionov and Yu. K. Ustinov, Soviet Phys-Tech. Phys. 9 (1965) 1 ;81. W. Gordy, Phys. Rev. 69 (1946) 604. J. K. Wilmshurst, J. Chem. Phys. (1957) 1129. H. Heyne and F. C. Tompkins, Proc. Roy. Sot. (London) A 292 (1966) 460.
SCIENCE
17 (1969) 474-481
THE SURFACE EPITAXIALLY
0 North-Holland
POTENTIAL
Publishing
OF WATER
GROWN (110) ORIENTED
Co., Amsterdam
VAPOUR
ON
FILMS OF
PLATINUM Received
16 February
1969
This note describes the preparation of an atomically clean (I 10) surface of platinum, and the measurement of the surface potential changes brought about by the introduction of water vapour. Previous experiments with platinum have most commonly been made either on macroscopic single crystal surfaces or on films vapour deposited onto amorphous substrates. In the latter case the resulting deposits were almost certainly polycrystallinel) though the final surface may have been clean. For the single crystals, although the crystallographic orientation was certain (excluding perhaps the outermost few layers where LEED shows - not unambiguously - some peculiarities) there is considerable doubt about the surface chemical composition. Common impurities in platinum, such as iron or silicon, are unlikely to be removed by maintaining the crystal at a temperature a few degrees below its melting point for extended periods”). Nor is heating in oxygen or hydrogen likely to improve matters although it could successfully remove carbon. Extended ion bombardment may be more effective though care then has to be taken that any subsequent anneal does not cause the reappearance of surface impurities by diffusion from the bulk. Distillation of the platinum from a suitable vapour source can give a high purity deposit and a useful surface, provided that the vapour can be condensed as a single crystal film. In the present work a tungsten substrate was used and the final platinum film was (110) oriented. It was convenient in these experiments to use a ribbon of polycrystalline foil, which had been thoroughly aged in an ultra-high vacuum at temperatures exceeding 3000”Ks) as the crystalline substrate. The mean crystallite size was 0.01 mm and X-ray examination showed the texture to be almost entirely (100) though the crystallites were oriented randomly with respect to one another. Clearly, it was to be expected that if epitaxy did occur then a textured deposit rather than a single crystal film would be formed. From the point of view of surface potential measurements, provided that the grain boundaries can be ignored (this is reasonable since the Kelvin technique used gives an area averaged result and the fractional area of the boundaries is very small) the results should be very similar 474
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
475
to those obtained from a single crystal film. We are currently studying the growth of platinum on several differently oriented tungsten single crystals. The surface potential measurements involved following the contact potential difference (c.p.d.) between a tungsten (110) oriented single crystal, used as a reference, and the platinum coated polycrystalline foil. We had previously4) determined the surface potential changes on the (110) tungsten surface as a function of the number of incident water molecules and hence it was possible to convert the c.p.d. measurements into surface potential changes on the platinum deposit. By assuming the work function of the polycrystalline tungsten foils) at 4.55 eV the final results could be expressed as work function change as a function of the number of incident molecules. The water vapour was introduced from prepared glass ampoules which could be broken magnetically once the pressure conditions were satisfactory. Initially the empty glass ampoules were sealed to a separate system and evacuated to about 5 x lo-’ Torr. Pure nitrogen was then introduced at atmospheric pressure and the vacuum system opened so that distilled water could be introduced. The vacuum was then restored and the water in the ampoules heated gently to remove dissolved gases. Finally the water in the ampoule was frozen in liquid nitrogen and, with the system evacuated to better than 10e6 Torr, the ampoule sealed-off. Control of the water vapour pressure, once the ampoule has been broken in the system, was by surrounding the side arm, containing the ampoule, in liquid nitrogen and also by adjusting a bakeable stopcock between the side arm and the main chamber. The experimental chamber and vacuum system are rather similar to those previously used in an experiment with uraniumc). The evaporator consisted of two co-axial tungsten coils and a magnetically operated loader. Following the attainment of an ultra-high vacuum with both of these coils maintained at about 28OO”K, the platinum wire on the end of the nickel loader was “touched” onto the central coil while it was hot. The metal deposited in this way could be outgassed and finally evaporated by electron bombardment from the outer tungsten coil. The maximum pressure in the chamber, recorded during the evaporation of the platinum films used in the measurements, was about 5 x lo-’ Torr. Both of the electrodes could be heated by bombardment on their reverse side, by focussed electrons. The crystal could be vibrated through a stainless steel bellows in a plane perpendicular to its surface for the c.p.d. measurements. The outgassing of the foil and the crystal was such that at completion either surface could be flashed to 3000°K without a pressure rise to greater than about 5 x lo-” Torr. A most important precaution in the use of water vapour was in the removal of carbon from all hot filaments and samples.
476
C. W. JOWElT,
P. J. DOBSON
AND B. J. HOPKINS
During our early work we observed that the partial pressure of any water introduced into the vacuum system very rapidly fell while at the same time the partial pressures of hydrogen and carbon monoxide increased in rough proportion. After extended heat treatment of the metal components and gauge filaments the degree of contamination from gases, formed in the reaction of the water with carbon dissolved in the metals, could be reduced to a negligible amount. In the present measurements this heating was performed in about 10e6 Torr of water since this was obviously very convenient. The final surface potential measurements described in this paper were always repeated with all the gauge filaments switched off as a check that impurities from reactions with the water were not influencing the results. The processing of the electronic signal from the vibrating capacitor in order to give a continuous recorder trace of the c.p.d. has been described previously7). Under the optimum conditions in the present experiments individual c.p.d. measurements could be made to better than f 10 mV. The error bars shown in the results below are, however, slightly larger than this because of the uncertainty in reproducing the position on the “incident molecules” axis. The observed c.p.d. between the clean foil and the (110) oriented crystal was 0.58+0.01 V, in good agreement with many previous determinations from this laboratory 8). Seven thin films of platinum were then deposited and the c.p.d. measured after each deposition at -0.25f0.02 V (where the negative sign indicates that the platinum had the higher work function) giving a platinum work function of 5.38kO.03 eV for the deposit formed at room temperature. Heating the foil to a minimum temperature of about 900°K caused a slight reduction in this value to 5.23 +0.02 eV which then remained constant with further heating up to 1800°K. Beyond this temperature the platinum began to re-evaporate and the work function fell back towards that of the clean foil. The adsorption measurements were made only on the annealed film and the structure of this film was examined after completion of these measurements using the techniques of X-ray diffractometry and reflection electron diffraction. Since the film thickness was estimated to be only 1000 A the intensity of X-ray reflections were very weak. The X-ray method was used therefore only to confirm the main features of the electron diffraction results. The electron diffraction was performed in an A.E.I. EM6G electron microscope using a special crystal manipulator*. An electron energy of 80 kV was found to be convenient and gave rise to the diffraction features shown in fig. 1 for two directions separated by an angle of 55”. Scanning the electron beam over the sample surface did not appreciably alter * Kindly loaned by Dr. R. M. Hill.
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
477
these patterns. The main spots in fig. la are characteristic of a platinum (110) plane viewed along a (110) azimuth. There are, however some addition spots, which can be seen most clearly in the top left hand corner, which indicate that the beam is parallel to a (114) azimuth of another crystallite. Rotation of the sample about an axis perpendicular to its surface by 55” should now view both sets of crystallites along a (211) azimuth as shown in fig. lb. The streaking in fig. 1b is probably a consequence of reflections taking place from two sets of crystallites such that the ordering parallel to the surface is much less perfect than that perpendicular to the surface. Also on fig. lb there are extra spots arising from a third orientation of the crystallites. In this case the electron beam is parallel to the (100) direction. Along the azimuth of fig. la these crystallites, provided that the electron beam was still irradiating them, ought to have given 111 reflections. These may, however, have been too faint to observe. In all of these crystallites the (110) plane was parallel to the
Fig. la.
478
C. W. IOWETT,
P. J. DOBSON AND B. J. HOPKINS
Fig. lb. Fig. 1. Reflection electron diffraction patterns of the platinum deposit at an electron energy of 80 kV. The direction of the electron beam differs in the two photographs by 55 ‘.
surface. The lattice constant of the platinum was determined from the 220, 111 and 002 reflections and found to be 3.95kO.10 A, in good agreement with the accepted value of 3.92 A. Thus, the final surface used in the adsorption measurements was a well textured one in which all of the crystallites presented a (110) platinum face to the vacuum. Clearly this implies that it will be possible to grow, by epitaxy, a (1 IO) oriented single crystal film of platinum on a tungsten (100) oriented single crystal. Since platinum and tungsten alloy, the purity of the platinum vapour stream, falling on a glass plate, from the tungsten evaporator, was analysed using an electron probe microanalyser. This showed the proportion of tungsten to be less than O.Ol%, the lower detection limit of the instrument. The possibility that an alloy was formed on the tungsten substrate, after heating the film to 900”K,
WATER
cannot
VAPOUR
be entirely
ON EPITAXIALLY
ruled out, though
GROWN
(110)
419
FILMS OF PLATINUM
the good agreement
of the measured
lattice constant suggests that this is not serious. The introduction of water vapour to the platinum and tungsten (110) surfaces gave rise to the c.p.d. changes shown in fig. 2. The water was usually introduced such that the pressure increased to a maximum of 5 x 10m4 Torr over a period of about 40 min. To achieve very high exposures the time was extended since pressure rises beyond 5 x 10e4 Torr led to difficulty in obtaining an ultra-high vacuum on re-evacuation. Removal of the water, on reaching the saturation point after an exposure of some 1O22 molecules cm- ‘, did not alter the final c.p.d. It is possible, of course, that there might have been exactly equal changes on the two surfaces during pump-out, but in view of the very different sticking probabilities of water on the two surfaces subsequently observed (fig. 2) this must be regarded as very unlikely. Also shown in fig. 2 is the work function versus exposure curve for the (1 IO) tungsten crysta14) and the final computed curve for water on platinum. The writers know of no other measurements involving the adsorption of water vapour onto platinum though such measurements have been reported
I
1
I
,
6.41
I
6.2 6.0
0.6
&O Fig. 2.
moleculesincident per cm’
The dashed curve is taken from a chart recording of the variation in the contact potential difference between the (110) platinum deposit and the (110) tungsten single crystal as water vapour was introduced. The full curves show the variation in work function of the tungsten (110) surface from ref. 3 and the changes deduced for the (110) platinum surface by combining the previous two curves.
480
C. W. JOWETT,
P. J. DOBSON
AND
B. J. HOPKINS
for tungsteng-r4). There it appears that dissociation of the water takes place with the intial possibility of both hydroxyl and hydrogen adsorbed species. Subsequent exposure leads to desorption of the hydrogen and a final covering of hydroxyl ions bonded through oxygen. From the limited measurements we have made on platinum it appears that this model might equally well apply. The electronegativity r5,r6) of the OH species is higher, 3.9, compared with that of oxygen, 3.5, so that on the basis of a simple dipole model it is to be expected that the final work function of the Pt-OH surface would be higher than that of Pt-0. Previous surface potential measurements have been reported for oxygen on various platinum samples (see ref. 2). The values observed range from 1 to 1.2 V, though the more recent measurements17*“) indicate a value close to 1 V. Thus, the difference between this value for oxygen and our surface potential for water, 1.13 +0.04 V, is very close to that expected solely on the basis of the electronegativity differences of the platinum-hydroxyl and the platinum-oxygen bonds. Conclusions The following conclusions can be drawn from this work: 1) A (110) oriented platinum film can be formed on a (100) oriented tungsten surface by vapour deposition followed by annealing to about 900°K. The work function of this deposit is 5.23 eV. 2) The adsorption of water probably takes place by dissociation into OH and H with the final surface comprising only OH ions and giving rise to an electronegative state comparable with that of oxygen on platinum but having a slightly higher surface potential, 1.13 kO.04 V compared with about I V measured for uncharacterised platinum surfaces. 3) The sticking probability of water on platinum (110) is very much less than that on tungsten (110). Acknowledgements Two of us, C.W.J. and P.J.D., would like to thank Council for the provision of maintenance grants.
the Science Research
C. W. JOWETT, P. J. DOBSON* and B. J. HOPKINS
Surface Physics, The University, Southampton, England * Now at Physics Department, Imperial College, London.
WATER
VAPOUR
ON EPITAXIALLY
GROWN
(110)
FILMS OF PLATINUM
481
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
P. J. Dobson, Ph.D. Thesis, University of Southampton (1968). R. Lewis, and R. Gomer, Surface Sci. 12 (1968) 157. P. J. Dobson and B. J. Hopkins, Brit. J. Appl. Phys. [2] 1 (1968) 1241. B. J. Hopkins and C. W. Jowett, to be published. B. J. Hopkins and J. C. Riviere, Brit. J. Appl. Phys. [l] 15 (1964) 941. B. J. Hopkins and A. J. Sargood, Nuovo Cimento Suppl. 5 (1967) 459. T. J. Lee, Ph.D. Thesis, University of Southampton (1967). T. J. Lee, B. H. Blott and B. J. Hopkins, Appl. Phys. Letters 11 (1967) 361. H. Imai and C. Kemball, Proc. Roy. Sot. (London) A 302 (1968) 399. R. J. Hill and P. W. M. Jacobs, Nature (London) 180 (1957) 1117. R. J. Hill, Vacuum 11 (1961) 260. E. W. Mtiller, Ergeb. Exact. Naturw. 27 (1953) 290. Yu. K. Ustinov and N. I. Ionov, Soviet Phys.-Tech. Phys. 12 (1968) 1506. V. N. Ageer, N. I. Ionov and Yu. K. Ustinov, Soviet Phys-Tech. Phys. 9 (1965) 1 ;81. W. Gordy, Phys. Rev. 69 (1946) 604. J. K. Wilmshurst, J. Chem. Phys. (1957) 1129. H. Heyne and F. C. Tompkins, Proc. Roy. Sot. (London) A 292 (1966) 460.