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Adsorption of water on clean aluminum by measurement of work function changes
S U R F A C E SCIENCE 32 (1972) 543-553 © North-Holland Publishing Co.
A D S O R P T I O N OF W A T E R O N CLEAN A L U M I N U M BY M E A S U R E M E N T OF W O R K F U N C T I O N C H A N G E S
TOMLINSON FORT, Jr. and ROBERT L. WELLS
Division o f Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A.
Received 23 February 1972; revised manuscript received 14 April 1972 Clean surfaces were produced on bulk aluminum samples by a new cutting procedure inside an ultra-high vacuum system. This cutting technique made it possible to stabilize the reference electrode of a vibrating capacitor by equilibration with the gas. Adsorption of water on the clean surfaces was studied by following changes in the work function of the surface at various temperatures and water vapor pressures. Water chemisorbed on the aluminum causing a work function change of -- 1.4 V at one monolayer. The rate and amount of chemisorption were negatively dependent on temperature. The characteristics of the work function changes lead to the conclusion that the water chemisorbs by a precursor mechanism.
I. Introduction Very few studies have been made of the adsorption of water on aluminum. Huber and Kirk 1) followed the adsorption of water on evaporated aluminum films by gravimetric and work function measurements. Their study was made only at room temperature and its purpose was to show that water contamination could cause the lowered work functions obtained by other authors for the oxygen-aluminum system. They showed that exposure of a fresh aluminum film to water at 25 °C caused the work function to decrease by 1.1 V after an exposure of about 5 x 10 -5 torr-min. It then remained constant. Batt and Mee 2) obtained similar results with photoelectric work function measurements on evaporated aluminum films. In addition, they found that with much larger exposures the work function dropped further to a minimum value of about 1.4 V below the clean metal work function and then increased by about 0.5 V. This report describes an investigaion of the adsorption of water vapor on clean bulk aluminum by measurements of work function changes. It is shown that at room temperature the work function change observed for chemisorption of water on a bulk aluminum surface is the same as these other authors obtained on evaporated films. Experiments at other tempera543
544
T. F O R T , J R . A N D R. L. W E L L S
tures lead to the conclusions that the water chemisorbs by a precursor mechanism, and that the work function change for a complete monolayer of water on aluminum is about - 1.4 V. 2. Experimental 2.1 MATERIALS AND EQUIPMENT
The adsorption studies were carried out in an ultra-high vacuum system (fig. l) which could be baked out at 300 °C. Pumping was accomplished by a molecular sieve sorption pump, 1401/sec ion pump, and a titanium sublimation pump (Varian Associates, Palo Alto, California). The base pressure of the system was 0.5 to 1.0 x 10 -1° torr. Pressure measurements were made by a nude ion gauge for the range of 10 -11 torr to 10 -3 torr (reduced emission), by a Magnevac GMA-140 gauge (Consolidated Vacuum Corp., Rochester, N e w York) for the range of 10-3 torr to 4.58 torr and by conversion from the temperature of a water reservoir for the range of 4.58 torr to saturation. The gas pressure was controlled automatically in the range of 10-4 to 10-10 torr by means of a Granville-Phillips automatic pressure controlling leak valve (Granville-Phillips Co., Boulder, Colorado).
MASS S PECTROMETE..~=~
8 L/S
\\ ~ /
.o,,. VACION PUMP
ION
UHV VALVE
,
J
;ELLOWS
VIBRATING REFERENCE
BELLOWS
CROSS SLIDE TABLE
COOLANT HEATER THERMOCOU PLES
AUTOMATIC
~ LEAK
VALVE
Fig. 1.
Diagram of adsorption system.
5c~.le /": ~'t
t
A D S O R P T I O N OF W A T E R O N C L E A N A L U M I N U M
545
For higher pressures manual control was used. A Veeco SPI-10 monopole residual gas analyzer (Veeco Instruments, Plainview, New York) was used to monitor gas compositions at all pressures. At pressures above 10 - s torr the analysis was made by leaking gas from the main system into a tee containing the mass spectrometer and an 8 1/sec ion p u m p (Varian Assoc., Palo Alto, California). The sample was a polycrystalline 99.999~ A1 rod, 2½in. long x ¼in. in diameter. This was mounted in an insulated (split mullite tubing) clamp as shown in fig. 1. Its temperature could be manually controlled between - 30 °C and + 300 °C. A cutting tool was fixed in a stainless steel rod which extended through a flexible stainless steel bellows to the outside of the vacuum system where it was firmly attached to a heavy cross-slide milling table (Troyke Mfg. Co., Cincinnati, Ohio). By movement of this cross-slide table in the two horizontal directions thin slices ( ~ 2 rail thick) could be cut from the end of the aluminum sample. Similarly the reference electrode (99.95~ Au disk, i in. diameter x ~ in. thick) was attached to a rod which extended through a stainless steel bellows to the outside of the vacuum system where it was clamped into a mount which could be moved in any of the three perpendicular directions. Thus, the reference electrode alignment and seperation could be adjusted from outside the vacuum system. Work function changes were followed by means of the vibrating capacitor technique. A short rod attached to the cone of a 3 inch radio speaker pressed against the reference electrode rod between the bellows and the clamp on the outside of the vacuum system. This made it possible to vibrate the reference electrode inside the vacuum system. The alternating current generated by this "vibrating capacitor" was observed on a General Radio tuned amplifier and null detector (General Radio Co., West Concord, Massachusetts). Contact potential difference measurements were then made by manuall3~ adjusting a bias voltage until the null point was found. These manual measurements could be made at the rate of about one every 15 sec. With this arrangement, work function changes could be measured with an accuracy of __ 0.002 V. However, the reproducibility of CPD valves for experimental runs under seemingly identical conditions was about + 0.03 V. The water used as an adsorbate in these experiments was first distilled in a tin lined still and then distilled from KMnO4 solution. The water was then put into a reservoir below a spherical trap where it was degassed by repeated freezing in vacuum. 2.2 PROCEDURES
Before each set of experiments the vacuum chamber was baked and pumped down to its base pressure of less than 10- i 0 torr. Then the adsorbate
546
T. FORT, JR. AND R. L. WELLS
gas was admitted to the system. The gas was held at the pressure at which the experiment would be done and the work function difference (samplereference) was followed until it became constant. At that time both the sample and reference electrodes had become equilibrated with the gas. Then, the reference was moved back and a thin slice was cut off the end of the sample, thus exposing a clean* surface. After cutting a clean surface the reference was placed back in position and the work function difference was measured as a function of time after the cut was made. Nothing had been done either before cutting or during the experiment to affect the equilibrium that was established between the reference electrode and the gas environment. Thus the reference's work function should remain constant during the experiment. This cutting technique eliminates the common problem of reference electrode stability in contact potential difference measurements. The work function difference changes that occur in these adsorption experiments are thus completely due to the reequilibration of the freshly cut sample surface with its environment and are thus at times referred to as work function changes of the sample. 3. Results and discussion
3.1. ULTRA-HIGHVACUUM AS a reference for following adsorption on fleshly cut aluminum surfaces by work function measurements it was necessary to establish whether the work function of an aluminum surface produced by this cutting technique would remain constant when no adsorption occurred on it. Fig. 2 shows that the work function difference between the two electrodes after the vacuum bakeout was about - 0 . 6 V. Cutting a new surface on the aluminum sample resulted in a surface with a work function about 0.7 to 0.8 V higher. At both 25 °C and 200 °C the work function of this new surface was constant in UHV. The work function increase caused by the cutting process is primarily due to the change from a contaminated to a clean aluminum surface. 3.2. ADSORPTION OF WATER VAPOR AT LOW PRESSURES
Fig. 3 shows the change of work function (work function change is zero at zero time) of a freshly cut aluminum surface at room temperature in different pressures of water vapor. Huber and Kirk 1) showed that the water first reacts to form an oxide layer with very little work function change and * Electron microprobe investigations and duplicate adsorption experiments with steel and carbide cutting tools proved that no significant a m o u n t of metal was transferred from the cutting tool to the freshly cut sample surface.
ADSORPTION
OF WATER
ON
CLEAN
ALUMINUM
547
.4 210 "C
.2
o 0'<2C)'-0-'(2~0
0 0
C. 0
4XIo-IOToRR
T,P.__2 5 °C 1 X 10-10 TORR
(,~
h
.~
0
o
o
~. ".2
-4
,,c-~W.F.D. AFTER VACUUM BAKE -.6
0
I
I
I
20
40
60
1
I
80 100 TIME , MINUTES
I
I
I
120
140
160
180
Fig. 2. Work function change of aluminum cut in ultra-high vacuum.
.0 25 °C
-.2 _4 0 ;>
P,TORR
[]
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-1.2 -1.4
,
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,
,
,
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.
.
.
.
10- 4
Fig. 3. Effect of pressure on adsorption of water on aluminum• then a d s o r b s on t o p o f this oxide as H 2 0 o r O H causing a large w o r k function decrease. Thus the w o r k function changes in o u r experiments are p r o b a b l y also due to a d s o r p t i o n o f water on an oxide m o n o l a y e r . The abscissa o f o u r fig. 3 is exposure (pressure times time). This m e t h o d o f plotting the results was chosen to show the effect o f pressure on the rate o f a d s o r p t i o n . The results show that the rate o f a d s o r p t i o n is p r o p o r t i o n a l to pl.0 a n d t h a t the a m o u n t o f a d s o r p t i o n at equilibrium is i n d e p e n d e n t o f pressure in the
548
T. F O R T , J R . A N D R. L. W E L L S
range from 10 - 6 to 10 -8 torr. The work function change of l.l to 1.2 V at r o o m temperature is in g o o d agreement with the results o f other investigators. This adsorption is irreversible, i.e., the work function difference does not change from its equilibrium value when the water vapor is p u m p e d out. The temperature dependence of the adsorption of water on aluminum (fig. 4) was quite surprising. As the temperature is increased, there is a decrease in both the rate of work function change and the equilibrium a m o u n t o f work function change. According to Langmuir's 4) classical
~
.0 -:2
200°C
0
>
_6
u~
-1.0 -1.2
-1.4 0.1
1.0
10. TIME
100.
1000.
, MINUTES
Fig. 4. Effect of temperature on adsorption of water on aluminum.
theory for chemisorption, as the temperature is increased the rate of chemisorption should increase by an a m o u n t dependent on the size o f the activation energy. There is no way in which a decrease in rate with increasing temperature could occur by this classical mechanism. The sticking coefficient (S) was calculated from these results using the equation N O dAq~ -
=
VPS,
AqS0= 1 dt where No is the number of molecules/cm 2 at a coverage of one monolayer (No=7.1 ×10agmolecules/cm 2 for H20)*, AqSo=l is the work function * This approximate value for No is obtained by assuming that Am of water is 14/~.Z/molecule.
ADSORPTION OF WATER ON CLEAN ALUMINUM
549
change of the aluminum surface at a coverage of one monolayer (A~b0= 1 = 1.4 V), dA~/dt is the rate of work function change, V is the number of molecules/cm2sec that strike the surface when P = 1 torr [-V= ( 2 n m k T ) - ~: ~-0.82 × 1022 T -½ for H 2 0 ] and P is the pressure in torr. The dependence of sticking coefficient on the amount of work function change (approximately proportional to coverage) and the temperature is shown in fig. 5. "[he sticking coefficient is constant for about the first 0.3 V change in work function or the first 25 percent coverage. This was also surprising. Again according to
20 141618 ~27 °C +o~,12108 ~x 4
6 7 ~C
0
~
0
Fig. 5.
P=10-7TORR
i
2
-'~,~
~
4
I
~-
r
6 8 10 A W.F.D. , V O L T S
I
I
12
14
Variation of sticking coefficient of water on a l u m i n u m with coverage.
Langmuir's classical theory the rate of chemisorption is determined by the rate at which molecules from the gas phase have favorable collisions with vacant sites on the metal surface. Thus the rate should decrease as the surface is covered instead of remaining constant*. A chemisorption mechanism which fits these results satisfactorily utilizes the role of physically adsorbed molecules as precursors. This mechanism was first suggested in 1929 by Langmuir 5) but even today only a few systems 6, v) have been found which fit it. According to this mechanism, molecules from * A constant rate could however exist if the activation energy decreased with increasing coverage in such a way as to compensate for the decrease vacant sites.
550
T. FORT, JR. AND R. L. WELLS
the gas phase impinging on covered or vacant sites are physically adsorbed and can diffuse over the surface. These physically adsorbed molecules will eventually return to the gas phase unless during their migration on the surface they have a favorable collision with a vacant site and become chemisorbed. The general expression for the sticking coefficient according to the precursor mechanism as derived by Ehrlich 8) is S=
1 dn 2
k_ 1
=l-
p k 1 dt
k _ 1 + k 2'
where kl
k2
MIg) ~ Mr1 ) (physically a d s o r b e d ) ~ Mt2) (chemically adsorbed). k-1
The sticking coefficient is a function of both the rate constant for chemisorption of the physically adsorbed species and the rate constant for desorption of the physically adsorbed species. Thus the temperature dependence of the sticking coefficient is a function of the difference between the activation energies for desorption and chemisorption of the precursor and can be either positive or negative. From an Arrhenius plot of the sticking coefficient at zero coverage, it was found that this activation energy difference (EA(2)- EA(_ ~)) was equal to --2.6 kcal/mole. The constancy of the sticking coefficient with increasing chemisorption occurs because molecules, which strike covered areas, physically adsorb and have a chance of diffusing to and reacting with a vacant site. In addition, as Ehrlich 9) explained, if the surface is heterogeneous with some patches which will more easily chemisorb the adsorbate than others, then the initial sticking coefficient would be less than 1.0 since only those molecules which hit the surface on or near active patches would chemisorb. This initial sticking coefficient would decrease with increasing temperature because the mean distance traveled by a physically adsorbed molecule would decrease. The low initial sticking coefficient for adsorption of water on aluminum and its decrease with an increase in temperature indicates that the aluminum surface is heterogeneous to adsorption of water. 3.3.
A D S O R P T I O N OF W A T E R V A P O R AT H I G H PRESSURES
Fig. 6 shows the effect on the work function difference of the following series of experiments" (1) A1 cut in PH2o= l0 -7 torr at room temperature, (2) 1-t20 let in to 4.58 torr*, * Separate experiments in our laboratory x°) showed that water does not adsorb on the gold reference electrode at 25 °C for Pi~2o~ 4.58 torr. Thus, the work function difference changes in these experiments are due only to work function changes of the sample.
ADSORPTION
OF
WATER
ON
%¢
CLEAN
551
ALUMINUM
+1 0 -1 -2 == -3 o
o
.2
/
0
PRESSURE
O3
-4
0-.2
-5
.,..,; -.4
-6 q -7
a.
u_"
-.8
-1.0 -1.2 0
Fig. 6.
I
I
I
I
i
I
i
t
[
i
{
I
40
80
120
160
200
240
0
40
80
120
160
200
TIME, MN I UTES
A d s o r p t i o n of water on a l u m i n u m at 4.58 torr a nd de s orpt i on by p u m p i n g
(3) AI cut in PH2O=4.58 torr and waiting 19 hr, (4) A1 cut again in P,2o=4.58 torr, (5) H 2 0 pumped out to ~ 10 - 7 torr. As the water vapor was slowly let in to 4.58 torr the change of work function difference (where AWFD = 0 for clean A1) first decreased from its equilibrium value of - 1 . 1 9 0 V to a minimum value of - 1 . 4 2 5 V and then began to rise. Cutting the aluminum at Pn2o =4.58 torr gave only an increasing work function difference which after 19 hr was 0.525 V above the minim u m value of - 1.425 V. Pumping out the water caused the work function difference to return to - 1 . 2 0 5 V which is very close to the equilibrium value ( - 1 . 1 9 0 V) attained after cutting at 10 - 7 torr. These high pressure results are very similar to those obtained by Batt and Mee on evaporated aluminum films. The experiment at 10 - 7 torr and - 2 7 °C indicates that the 1.2 V work function decrease obtained at r o o m temperature and 10 - 6 t o 10 - 8 torr H 2 0 is not, as Huber and Kirk 1) had stated, a complete monolayer. Rather, it appears that since at high pressure the maximum work function decrease is about - 1.4 V, this change corresponds to the monolayer point. Because of the surface heterogeneity, a complete monolayer can only be attained by either lowering the temperature below room temperature or by raising the pressure enough to complete the monolayer with physically adsorbed water. The work function increase above this monolayer value which occurred when the pressure was 4.58 torr was apparently also due to physically adsorbed water since pumping out the water reversed it. It is possible that
552
T. F O R T , J R . A N D R. L. W E L L S
this work function increase is reflecting physical adsorption of a second layer of water on top of the first chemisorbed layer. Culver, Pritchard, and Tompkins 11) obtained a similar reversal of work function change due to second layer formation for the system of CO on Cu. However, the kinetics of this work function increase at high water pressures is much slower than would be expected for physical adsorption. No explanation for these slow kinetics could be determined. Linke and Meyer 12) suggested that exoelectron emission from abraded aluminum surfaces was due to the work function decrease caused by adsorption of water on the resulting clean surfaces. Our results lend support to this theory. Assuming a clean aluminum work function of 4.2 eV12), a change of - 1.4 eV due to adsorption of H 2 0 would leave the work function ( A I - H 2 0 ) = 2 . 8 eV. A surface of this work function would have a photoelectric threshold of 2=4400 ~ which is in the visible light range. Thus, room lighting would cause electron emission from this surface. 4. Conclusions
A new cutting technique was developed which produces clean metal surfaces on which adsorption could be studied through measurement of work function changes by the vibrating capacitor technique. 3-he procedure allowed the reference electrode of the vibrating capacitor to be stabilized by equilibration. Results of a study of the adsorption of water on freshly cut bulk aluminum surfaces at room temperature are in good agreement with results obtained by other investigators on evaporated films. It was shown that the work function decrease associated with the adsorption of a complete monolayer of water on aluminum is about 1.4 V. The characteristics of the adsorption (a sticking coefficient which decreases with increasing temperature and is relatively independent of coverage at low coverage) indicate that the water adsorbs by a precursor mechanism. At high water vapor pressure an increase in work function above the minimum occurs, possibly reflecting adsorption of a second layer of water. The work function decrease caused by chemisorption of water on aluminum is sufficient to allow photostimulated exoelectron emission to occur in visible light.
References
1) 2) 3) 4) 5)
E. E. Huber, Jr. and C. T. Kirk, Jr., Surface Sci. 5 (1966) 447. R. J. Batt and C. H. B. Mee, Appl. Opt. 9 (1970) 79. W. A. Zisman, Rev. Sci. Instr. 3 (1932) 367. I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. I. Langmuir, Chem. Rev. 6 (1929) 451.
ADSORPTION OF WATER ON CLEAN ALUMINUM
6) 7) 8) 9) 10) 11)
553
J. A. Becker and C. D. Hartman, J. Phys. Chem. 57 (1953) 153. M. P. Hill and B. A. Pethica, Trans. Faraday Soc. 65 (1969) 876. G. Ehrlich, J. Phys. Chem. 59 (1955) 473. G. Ehrlich, J. Phys. Chem. Solids 1 (1956) 1. R. L. Wells and T. Fort, Jr., Surface Sci. 32 (1972) 554. R. Culver, J. Pritchard and F. C. Tompkins, in: Proc. 2nd Intern. Congr. of Surface Activity, Vol. 2 (Butterworths, London, 1975) p. 243. 12) E. Linke and K. Meyer, Surface Sci. 20 (1970) 304.
A D S O R P T I O N OF W A T E R O N CLEAN A L U M I N U M BY M E A S U R E M E N T OF W O R K F U N C T I O N C H A N G E S
TOMLINSON FORT, Jr. and ROBERT L. WELLS
Division o f Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A.
Received 23 February 1972; revised manuscript received 14 April 1972 Clean surfaces were produced on bulk aluminum samples by a new cutting procedure inside an ultra-high vacuum system. This cutting technique made it possible to stabilize the reference electrode of a vibrating capacitor by equilibration with the gas. Adsorption of water on the clean surfaces was studied by following changes in the work function of the surface at various temperatures and water vapor pressures. Water chemisorbed on the aluminum causing a work function change of -- 1.4 V at one monolayer. The rate and amount of chemisorption were negatively dependent on temperature. The characteristics of the work function changes lead to the conclusion that the water chemisorbs by a precursor mechanism.
I. Introduction Very few studies have been made of the adsorption of water on aluminum. Huber and Kirk 1) followed the adsorption of water on evaporated aluminum films by gravimetric and work function measurements. Their study was made only at room temperature and its purpose was to show that water contamination could cause the lowered work functions obtained by other authors for the oxygen-aluminum system. They showed that exposure of a fresh aluminum film to water at 25 °C caused the work function to decrease by 1.1 V after an exposure of about 5 x 10 -5 torr-min. It then remained constant. Batt and Mee 2) obtained similar results with photoelectric work function measurements on evaporated aluminum films. In addition, they found that with much larger exposures the work function dropped further to a minimum value of about 1.4 V below the clean metal work function and then increased by about 0.5 V. This report describes an investigaion of the adsorption of water vapor on clean bulk aluminum by measurements of work function changes. It is shown that at room temperature the work function change observed for chemisorption of water on a bulk aluminum surface is the same as these other authors obtained on evaporated films. Experiments at other tempera543
544
T. F O R T , J R . A N D R. L. W E L L S
tures lead to the conclusions that the water chemisorbs by a precursor mechanism, and that the work function change for a complete monolayer of water on aluminum is about - 1.4 V. 2. Experimental 2.1 MATERIALS AND EQUIPMENT
The adsorption studies were carried out in an ultra-high vacuum system (fig. l) which could be baked out at 300 °C. Pumping was accomplished by a molecular sieve sorption pump, 1401/sec ion pump, and a titanium sublimation pump (Varian Associates, Palo Alto, California). The base pressure of the system was 0.5 to 1.0 x 10 -1° torr. Pressure measurements were made by a nude ion gauge for the range of 10 -11 torr to 10 -3 torr (reduced emission), by a Magnevac GMA-140 gauge (Consolidated Vacuum Corp., Rochester, N e w York) for the range of 10-3 torr to 4.58 torr and by conversion from the temperature of a water reservoir for the range of 4.58 torr to saturation. The gas pressure was controlled automatically in the range of 10-4 to 10-10 torr by means of a Granville-Phillips automatic pressure controlling leak valve (Granville-Phillips Co., Boulder, Colorado).
MASS S PECTROMETE..~=~
8 L/S
\\ ~ /
.o,,. VACION PUMP
ION
UHV VALVE
,
J
;ELLOWS
VIBRATING REFERENCE
BELLOWS
CROSS SLIDE TABLE
COOLANT HEATER THERMOCOU PLES
AUTOMATIC
~ LEAK
VALVE
Fig. 1.
Diagram of adsorption system.
5c~.le /": ~'t
t
A D S O R P T I O N OF W A T E R O N C L E A N A L U M I N U M
545
For higher pressures manual control was used. A Veeco SPI-10 monopole residual gas analyzer (Veeco Instruments, Plainview, New York) was used to monitor gas compositions at all pressures. At pressures above 10 - s torr the analysis was made by leaking gas from the main system into a tee containing the mass spectrometer and an 8 1/sec ion p u m p (Varian Assoc., Palo Alto, California). The sample was a polycrystalline 99.999~ A1 rod, 2½in. long x ¼in. in diameter. This was mounted in an insulated (split mullite tubing) clamp as shown in fig. 1. Its temperature could be manually controlled between - 30 °C and + 300 °C. A cutting tool was fixed in a stainless steel rod which extended through a flexible stainless steel bellows to the outside of the vacuum system where it was firmly attached to a heavy cross-slide milling table (Troyke Mfg. Co., Cincinnati, Ohio). By movement of this cross-slide table in the two horizontal directions thin slices ( ~ 2 rail thick) could be cut from the end of the aluminum sample. Similarly the reference electrode (99.95~ Au disk, i in. diameter x ~ in. thick) was attached to a rod which extended through a stainless steel bellows to the outside of the vacuum system where it was clamped into a mount which could be moved in any of the three perpendicular directions. Thus, the reference electrode alignment and seperation could be adjusted from outside the vacuum system. Work function changes were followed by means of the vibrating capacitor technique. A short rod attached to the cone of a 3 inch radio speaker pressed against the reference electrode rod between the bellows and the clamp on the outside of the vacuum system. This made it possible to vibrate the reference electrode inside the vacuum system. The alternating current generated by this "vibrating capacitor" was observed on a General Radio tuned amplifier and null detector (General Radio Co., West Concord, Massachusetts). Contact potential difference measurements were then made by manuall3~ adjusting a bias voltage until the null point was found. These manual measurements could be made at the rate of about one every 15 sec. With this arrangement, work function changes could be measured with an accuracy of __ 0.002 V. However, the reproducibility of CPD valves for experimental runs under seemingly identical conditions was about + 0.03 V. The water used as an adsorbate in these experiments was first distilled in a tin lined still and then distilled from KMnO4 solution. The water was then put into a reservoir below a spherical trap where it was degassed by repeated freezing in vacuum. 2.2 PROCEDURES
Before each set of experiments the vacuum chamber was baked and pumped down to its base pressure of less than 10- i 0 torr. Then the adsorbate
546
T. FORT, JR. AND R. L. WELLS
gas was admitted to the system. The gas was held at the pressure at which the experiment would be done and the work function difference (samplereference) was followed until it became constant. At that time both the sample and reference electrodes had become equilibrated with the gas. Then, the reference was moved back and a thin slice was cut off the end of the sample, thus exposing a clean* surface. After cutting a clean surface the reference was placed back in position and the work function difference was measured as a function of time after the cut was made. Nothing had been done either before cutting or during the experiment to affect the equilibrium that was established between the reference electrode and the gas environment. Thus the reference's work function should remain constant during the experiment. This cutting technique eliminates the common problem of reference electrode stability in contact potential difference measurements. The work function difference changes that occur in these adsorption experiments are thus completely due to the reequilibration of the freshly cut sample surface with its environment and are thus at times referred to as work function changes of the sample. 3. Results and discussion
3.1. ULTRA-HIGHVACUUM AS a reference for following adsorption on fleshly cut aluminum surfaces by work function measurements it was necessary to establish whether the work function of an aluminum surface produced by this cutting technique would remain constant when no adsorption occurred on it. Fig. 2 shows that the work function difference between the two electrodes after the vacuum bakeout was about - 0 . 6 V. Cutting a new surface on the aluminum sample resulted in a surface with a work function about 0.7 to 0.8 V higher. At both 25 °C and 200 °C the work function of this new surface was constant in UHV. The work function increase caused by the cutting process is primarily due to the change from a contaminated to a clean aluminum surface. 3.2. ADSORPTION OF WATER VAPOR AT LOW PRESSURES
Fig. 3 shows the change of work function (work function change is zero at zero time) of a freshly cut aluminum surface at room temperature in different pressures of water vapor. Huber and Kirk 1) showed that the water first reacts to form an oxide layer with very little work function change and * Electron microprobe investigations and duplicate adsorption experiments with steel and carbide cutting tools proved that no significant a m o u n t of metal was transferred from the cutting tool to the freshly cut sample surface.
ADSORPTION
OF WATER
ON
CLEAN
ALUMINUM
547
.4 210 "C
.2
o 0'<2C)'-0-'(2~0
0 0
C. 0
4XIo-IOToRR
T,P.__2 5 °C 1 X 10-10 TORR
(,~
h
.~
0
o
o
~. ".2
-4
,,c-~W.F.D. AFTER VACUUM BAKE -.6
0
I
I
I
20
40
60
1
I
80 100 TIME , MINUTES
I
I
I
120
140
160
180
Fig. 2. Work function change of aluminum cut in ultra-high vacuum.
.0 25 °C
-.2 _4 0 ;>
P,TORR
[]
16 8
-.8 -1.0
-1.2 -1.4
,
1(~ 8
,i
,
,
,
I
,
,
,i
1() 7 10- 6 10- 5 EXPOSURE , TORR°MINUTES
.
.
.
.
10- 4
Fig. 3. Effect of pressure on adsorption of water on aluminum• then a d s o r b s on t o p o f this oxide as H 2 0 o r O H causing a large w o r k function decrease. Thus the w o r k function changes in o u r experiments are p r o b a b l y also due to a d s o r p t i o n o f water on an oxide m o n o l a y e r . The abscissa o f o u r fig. 3 is exposure (pressure times time). This m e t h o d o f plotting the results was chosen to show the effect o f pressure on the rate o f a d s o r p t i o n . The results show that the rate o f a d s o r p t i o n is p r o p o r t i o n a l to pl.0 a n d t h a t the a m o u n t o f a d s o r p t i o n at equilibrium is i n d e p e n d e n t o f pressure in the
548
T. F O R T , J R . A N D R. L. W E L L S
range from 10 - 6 to 10 -8 torr. The work function change of l.l to 1.2 V at r o o m temperature is in g o o d agreement with the results o f other investigators. This adsorption is irreversible, i.e., the work function difference does not change from its equilibrium value when the water vapor is p u m p e d out. The temperature dependence of the adsorption of water on aluminum (fig. 4) was quite surprising. As the temperature is increased, there is a decrease in both the rate of work function change and the equilibrium a m o u n t o f work function change. According to Langmuir's 4) classical
~
.0 -:2
200°C
0
>
_6
u~
-1.0 -1.2
-1.4 0.1
1.0
10. TIME
100.
1000.
, MINUTES
Fig. 4. Effect of temperature on adsorption of water on aluminum.
theory for chemisorption, as the temperature is increased the rate of chemisorption should increase by an a m o u n t dependent on the size o f the activation energy. There is no way in which a decrease in rate with increasing temperature could occur by this classical mechanism. The sticking coefficient (S) was calculated from these results using the equation N O dAq~ -
=
VPS,
AqS0= 1 dt where No is the number of molecules/cm 2 at a coverage of one monolayer (No=7.1 ×10agmolecules/cm 2 for H20)*, AqSo=l is the work function * This approximate value for No is obtained by assuming that Am of water is 14/~.Z/molecule.
ADSORPTION OF WATER ON CLEAN ALUMINUM
549
change of the aluminum surface at a coverage of one monolayer (A~b0= 1 = 1.4 V), dA~/dt is the rate of work function change, V is the number of molecules/cm2sec that strike the surface when P = 1 torr [-V= ( 2 n m k T ) - ~: ~-0.82 × 1022 T -½ for H 2 0 ] and P is the pressure in torr. The dependence of sticking coefficient on the amount of work function change (approximately proportional to coverage) and the temperature is shown in fig. 5. "[he sticking coefficient is constant for about the first 0.3 V change in work function or the first 25 percent coverage. This was also surprising. Again according to
20 141618 ~27 °C +o~,12108 ~x 4
6 7 ~C
0
~
0
Fig. 5.
P=10-7TORR
i
2
-'~,~
~
4
I
~-
r
6 8 10 A W.F.D. , V O L T S
I
I
12
14
Variation of sticking coefficient of water on a l u m i n u m with coverage.
Langmuir's classical theory the rate of chemisorption is determined by the rate at which molecules from the gas phase have favorable collisions with vacant sites on the metal surface. Thus the rate should decrease as the surface is covered instead of remaining constant*. A chemisorption mechanism which fits these results satisfactorily utilizes the role of physically adsorbed molecules as precursors. This mechanism was first suggested in 1929 by Langmuir 5) but even today only a few systems 6, v) have been found which fit it. According to this mechanism, molecules from * A constant rate could however exist if the activation energy decreased with increasing coverage in such a way as to compensate for the decrease vacant sites.
550
T. FORT, JR. AND R. L. WELLS
the gas phase impinging on covered or vacant sites are physically adsorbed and can diffuse over the surface. These physically adsorbed molecules will eventually return to the gas phase unless during their migration on the surface they have a favorable collision with a vacant site and become chemisorbed. The general expression for the sticking coefficient according to the precursor mechanism as derived by Ehrlich 8) is S=
1 dn 2
k_ 1
=l-
p k 1 dt
k _ 1 + k 2'
where kl
k2
MIg) ~ Mr1 ) (physically a d s o r b e d ) ~ Mt2) (chemically adsorbed). k-1
The sticking coefficient is a function of both the rate constant for chemisorption of the physically adsorbed species and the rate constant for desorption of the physically adsorbed species. Thus the temperature dependence of the sticking coefficient is a function of the difference between the activation energies for desorption and chemisorption of the precursor and can be either positive or negative. From an Arrhenius plot of the sticking coefficient at zero coverage, it was found that this activation energy difference (EA(2)- EA(_ ~)) was equal to --2.6 kcal/mole. The constancy of the sticking coefficient with increasing chemisorption occurs because molecules, which strike covered areas, physically adsorb and have a chance of diffusing to and reacting with a vacant site. In addition, as Ehrlich 9) explained, if the surface is heterogeneous with some patches which will more easily chemisorb the adsorbate than others, then the initial sticking coefficient would be less than 1.0 since only those molecules which hit the surface on or near active patches would chemisorb. This initial sticking coefficient would decrease with increasing temperature because the mean distance traveled by a physically adsorbed molecule would decrease. The low initial sticking coefficient for adsorption of water on aluminum and its decrease with an increase in temperature indicates that the aluminum surface is heterogeneous to adsorption of water. 3.3.
A D S O R P T I O N OF W A T E R V A P O R AT H I G H PRESSURES
Fig. 6 shows the effect on the work function difference of the following series of experiments" (1) A1 cut in PH2o= l0 -7 torr at room temperature, (2) 1-t20 let in to 4.58 torr*, * Separate experiments in our laboratory x°) showed that water does not adsorb on the gold reference electrode at 25 °C for Pi~2o~ 4.58 torr. Thus, the work function difference changes in these experiments are due only to work function changes of the sample.
ADSORPTION
OF
WATER
ON
%¢
CLEAN
551
ALUMINUM
+1 0 -1 -2 == -3 o
o
.2
/
0
PRESSURE
O3
-4
0-.2
-5
.,..,; -.4
-6 q -7
a.
u_"
-.8
-1.0 -1.2 0
Fig. 6.
I
I
I
I
i
I
i
t
[
i
{
I
40
80
120
160
200
240
0
40
80
120
160
200
TIME, MN I UTES
A d s o r p t i o n of water on a l u m i n u m at 4.58 torr a nd de s orpt i on by p u m p i n g
(3) AI cut in PH2O=4.58 torr and waiting 19 hr, (4) A1 cut again in P,2o=4.58 torr, (5) H 2 0 pumped out to ~ 10 - 7 torr. As the water vapor was slowly let in to 4.58 torr the change of work function difference (where AWFD = 0 for clean A1) first decreased from its equilibrium value of - 1 . 1 9 0 V to a minimum value of - 1 . 4 2 5 V and then began to rise. Cutting the aluminum at Pn2o =4.58 torr gave only an increasing work function difference which after 19 hr was 0.525 V above the minim u m value of - 1.425 V. Pumping out the water caused the work function difference to return to - 1 . 2 0 5 V which is very close to the equilibrium value ( - 1 . 1 9 0 V) attained after cutting at 10 - 7 torr. These high pressure results are very similar to those obtained by Batt and Mee on evaporated aluminum films. The experiment at 10 - 7 torr and - 2 7 °C indicates that the 1.2 V work function decrease obtained at r o o m temperature and 10 - 6 t o 10 - 8 torr H 2 0 is not, as Huber and Kirk 1) had stated, a complete monolayer. Rather, it appears that since at high pressure the maximum work function decrease is about - 1.4 V, this change corresponds to the monolayer point. Because of the surface heterogeneity, a complete monolayer can only be attained by either lowering the temperature below room temperature or by raising the pressure enough to complete the monolayer with physically adsorbed water. The work function increase above this monolayer value which occurred when the pressure was 4.58 torr was apparently also due to physically adsorbed water since pumping out the water reversed it. It is possible that
552
T. F O R T , J R . A N D R. L. W E L L S
this work function increase is reflecting physical adsorption of a second layer of water on top of the first chemisorbed layer. Culver, Pritchard, and Tompkins 11) obtained a similar reversal of work function change due to second layer formation for the system of CO on Cu. However, the kinetics of this work function increase at high water pressures is much slower than would be expected for physical adsorption. No explanation for these slow kinetics could be determined. Linke and Meyer 12) suggested that exoelectron emission from abraded aluminum surfaces was due to the work function decrease caused by adsorption of water on the resulting clean surfaces. Our results lend support to this theory. Assuming a clean aluminum work function of 4.2 eV12), a change of - 1.4 eV due to adsorption of H 2 0 would leave the work function ( A I - H 2 0 ) = 2 . 8 eV. A surface of this work function would have a photoelectric threshold of 2=4400 ~ which is in the visible light range. Thus, room lighting would cause electron emission from this surface. 4. Conclusions
A new cutting technique was developed which produces clean metal surfaces on which adsorption could be studied through measurement of work function changes by the vibrating capacitor technique. 3-he procedure allowed the reference electrode of the vibrating capacitor to be stabilized by equilibration. Results of a study of the adsorption of water on freshly cut bulk aluminum surfaces at room temperature are in good agreement with results obtained by other investigators on evaporated films. It was shown that the work function decrease associated with the adsorption of a complete monolayer of water on aluminum is about 1.4 V. The characteristics of the adsorption (a sticking coefficient which decreases with increasing temperature and is relatively independent of coverage at low coverage) indicate that the water adsorbs by a precursor mechanism. At high water vapor pressure an increase in work function above the minimum occurs, possibly reflecting adsorption of a second layer of water. The work function decrease caused by chemisorption of water on aluminum is sufficient to allow photostimulated exoelectron emission to occur in visible light.
References
1) 2) 3) 4) 5)
E. E. Huber, Jr. and C. T. Kirk, Jr., Surface Sci. 5 (1966) 447. R. J. Batt and C. H. B. Mee, Appl. Opt. 9 (1970) 79. W. A. Zisman, Rev. Sci. Instr. 3 (1932) 367. I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. I. Langmuir, Chem. Rev. 6 (1929) 451.
ADSORPTION OF WATER ON CLEAN ALUMINUM
6) 7) 8) 9) 10) 11)
553
J. A. Becker and C. D. Hartman, J. Phys. Chem. 57 (1953) 153. M. P. Hill and B. A. Pethica, Trans. Faraday Soc. 65 (1969) 876. G. Ehrlich, J. Phys. Chem. 59 (1955) 473. G. Ehrlich, J. Phys. Chem. Solids 1 (1956) 1. R. L. Wells and T. Fort, Jr., Surface Sci. 32 (1972) 554. R. Culver, J. Pritchard and F. C. Tompkins, in: Proc. 2nd Intern. Congr. of Surface Activity, Vol. 2 (Butterworths, London, 1975) p. 243. 12) E. Linke and K. Meyer, Surface Sci. 20 (1970) 304.