900
NORMAN HACICERMAN AND EMERSON H. LEE
Vol. 59
carbon surface tend to reduce the capacity of a carbon adsorbent for metanil yellow, and probably for anionic adsorbates in general, roughly in proportion to the concentration of these groups in the surface.
groups, in the carbon surface, hinder the adsorption of either dye. I n the single case observed, the effect was greatly reduced by heating in nitrogen a t 900". (4) The nominal limiting pore diameter for summary methylene blue (and presumably also for metanil (1) The areas occupied by individual molecules yellow) is approximately 13 8. of methy1en.e blue and metanil yellow in mono(5) Acidic substituents in the carbon surface layers adsorbed on Graphon have been determined reduce the surface area accessible to the anionic and used to measure the surface areas of activated adsorbate, metanil yellow, roughly in proportion to their concentration in the surface. carbons accessible to these molecules. (2) The surface area of an activated carbon Acknowledgment.-The author wishes to thank accessible to Methylene Blue is usually limited Dr. F. C. Chromey of this Laboratory for assistonly by the size of the pores. ance in the statistical treatment of experimental (3) I n some cases, unidentified substituent data.
THE EFFECT OF GASES ON THE CONTACT POTENTIALS OF EVAPORATED METAL FILMS BY NORMAN HACKERMAN AND EMERSON H. LEE Department of Chemistry, University of Twxs, Austin 11,Texas Received February 86, 1966
The effect with time of oxygen, nitrogen, water vapor and air on the contact potential differences between aged bulk platinum and evaporated metal films was studied by the vibrating condenser method. The metals used were aluminum, lead, nickel, chromium and iron. Both reversible and irreversible effects were observed. An explanation is offered based on sorption of the gases which provides either a dipole barrier or an ion barrier to the emission of electrons.
Introduction
If two unlike metals are brought into contact in
cleavage of a metal crystal,1° out-gassing of a pure metal in a vacuum,11v12and evaporation of metal films.13 Use of evaporated films offers some advantages since occluded gases are boiled out prior to and during evaporation. The disadvantage in using metal films is that of correlating the properties of the films with those of bulk metals.'4~~~ Two important factors to be considered are the effect of crystal orientation and film thickness on the work function of the films. I n this work, contact potentials between evaporated films and an aged platinum reference were measured in a vacuum system a t room temperature as a function of time after evaporation. Potentials were measured by the vibrating condenser method.16 The effects of air, oxygen, nitrogen and water vapor on evaporated films of aluminum, chromium, nickel, iron and lead were studied. The stability of the platinum reference was also studied for the experimental conditions used.
air and then separated slightly, a static electrical potential difference exists between them. This potential difference is called' the contact potential, or Volta potential difference. Contact potentials are a function of the electronic work functions of the two metals involved; the difference of the two work functions, in electron volts, is numerically equal to the contact potential difference, in volts. The work function of a metal is changed by adsorbed gas films or chemical films'; therefore contact potentials can be used to study adsorption and reactions of a gas with a metal. This may be done by measuring the potentials between a freshly formed surface and an aged surface. Since the aged metal is relatively stable, any change in contact potentials is ascribable to a change in work function of the fresh metal surface as it adsorbs a gas or reacts with it. (10) F. B. Daniels and M. Y. Colby, Phys. Rev., 52, 1200 (1937). The work functions of pure metals have been (11) J. R. Anderson and A. E. Alexander, Australian J . Ch.. 6 , 109 found to be anisotropic2-4 and to vary with allo- (1953). (12) C. W. Oatley, PTOC.Phys. Soc., 6 1 , 318 (1939). tropic modifications.6J Therefore crystal strucP. A. Anderson, Phys. Rev., 47, 958 (1953); 48, 320 (1936); ture and orientation in metals affect contact po- 57,(13) 122 (1940); 76, 388 (1949); R. Kh. Burstein, M. D. Surova and tential measurements. I. A. Zuidenman, Zhur. Fiz. K h i m . , 24, 214 (1950) [C. A , , 44, 6743 Methods for obtaining a bare metal surface (1950)l; E. W.J. Mitchell and J. W. Mitchell, Proc.Roy.Soc. (London), include abrasion in air1 or in a vacuum s y ~ t e m , ~AalO, , ~ 70 (1952); I. Ogawa, T.Doke and I. Nakada, Oyo Butsure, aa, 101 (1953) [C.A., 47, 10380 (1953)l; T. V. Kalish and R. Kh. Bur(1) R. Suhrman, Z . Elektrochem., 66, 351 (1952). (2) R. Smoluchowski, Phys. Rev., 60, 661 (1941). (3) 8. T. Martin, ibid., 6 6 , 947 (1939). (4) P.A. Anderson, ibid., 59, 1034 (1941). (5) A. Goetz, ibid., 33, 373 (1929). (6) H.B. Wahlin, ibid., 61, 509 (1942). (7) H.II. Uhlig. J . Applied Phys., 28, 1399 (1951). (8) F. Fianda and E. Lange, Z. Elektrochem., 5 6 , 237 (1951). (9) J. Giuer and E. Lange, Nalurwissenschajlen. 40, 506 (1953).
stein, Dolclady Acad. Nauk SSSR., 81, 1093 (1951) [C.A . , 46, 3829 (1952)l; C. F. Ying and H. E. Farmworth, Phys. Rea., 8 6 , 485 (1952); Y.Yashiro, Bull. Nagoya Insl. Technol., 8 , 333 (1951); J. C. P. Mignolet, J. Chem. Phys.. ao. 341 (1952); Disc. Faraday Soc., 185 (1950). (14) J. -4. Allen, Rev. PUTS A p p . Ch., 4, 133 (1954). This is a comprehensive review on evaporated metal films. (15) R. A. Sennett, T. A. McLaughlin and G. D. Scott, Can. J . Phys., 30, 370 (1952). (16) W. A. Zisman, Rev. Sei. Instr., 3, 367 (1932).
Sept., 1955
EFFECT OF GASESON CONTACT POTENTIALS OF EVAPORATED METALFILMS
90 1
Experimental The vacuum system included a Megovac forepump and B two-stage oil-diffusion pump, employing Octoil as the pump fluid. The ultimate pumping pressure was about 7 X lo-’ mm.; the system could be pumped down readily to the 10-6 mm. range. With the vacuum system sealed off from the pumps, the pressure stayed in this range for several hours. The system contained a Pirani gage, a Phillips gage and a calibrated McLeod gage. All stopcocks and sources of mercury vapor were separated from the contact potential unit by two cold traps in series. These traps were usually maintained with liquid nitrogen, although the use of Dry Ice and alcohol instead did not noticeably affect the experimental results. A magnetically operated, dry,.ground-glass joint was used to separate the contact potential unit from the cold traps when water vapor was admitted. The contact potential unit was outgassed for several hours at 300’ before each ex eriment. The system was then flushed several times wit! dry, oxygen-free nitrogen before pumping down. A mercury cut-off opened and closed the system to the pumps; metering stopcocks were used to admit gases. Thus any number of samples could be quick!y admitted and withdrawn from the system thereby providing a means for studying the reversibility of adsorption and contact potentials. The contact potential unit is shown in Fig. 1. Metal-toglass joints on each of the three necks of the flask were softsoldered to the various units indicated. The metals were evaporated from tungsten or other filaments according to recommendations of other workers.14 The metals were outgassed and a small amount was evaporated into the flask to provide some gettering action. However, pressures still rose to 1 X 10-4 to 5 X lo-* mm., during subsequent evaporations; thus no completely bare metal surfaces were obtained. The metal vapor was directed downward by a glass tube and was condensed on microscope cover-glasses. These were outgassed at 300” for several hours in the contact potential unit before evaporation of the metal samples. Each cover-glass was coated on one edge with evaporated metal prior to installation. This gave good electrical contact with a brass spring clip which also held the cover-glasses in place. After evaporation, the cover-glass and support was moved under the platinum reference by means of an external magnet. The distance between the film and the reference was adjusted through a monel bellows, which supported the reference electrode, by a screw adjustment whlch compressed the bellows. Contact potential measurements began about one minute after evaporation of the films; the method of measurement was that described earlier,” except that an oscilloscope was used as the null indicator. Sensitivity of the measurements was A 3 mv. Contact potentials were measured as the gases were admitted to the system. After each run the films were weighed by difference, and their thickness was estimated by the weight, area and density of the metal. The evaporated films were checked spectrographically for tungsten and other impurities. These data, along with data on the gases used, are shown in Table I. Two non-transition metals, aluminum and lead, were studied for comparison with the transition metals, chromium, nickel and iron. The work functions of these metals are 3.38, 3.94, 4.38, 4.32 and 4.40 e.v., respectively.la The effects of air, oxygen and nitrogen were measured by exposing the freshly evaporated films to the gases at pressures of 0.03 to 0.05 mm. Water vapor was used at lower pressures, 5 X 10-4 to 3 X 10-3 mm., because of the difficulty in manipulating it in the vacuum system.
Results Nitrogen had no permanent effect on the work function of the films. However, temporary changes were observed when nitrogen first entered the system, apparently because of the temperature drop of the gas when it was admitted. This reversibly increased the work function of the metals studied, and was apparently caused by (17) L. Antes and N. Hackerman, J . A p p . Phgs., 22, 1395 (1951). (18) B. E. Conway. “Electrochemical Data,” Elsevier Publishing Co., Houston. Texas, 1952, p. 31.
Fig. 1.
temporary adsorption of the gas before it warmed to the temperature of the apparatus. On standing in the evacuated system the work functions decreased continuously and irreversibly for several hours, undoubtedly the result of oxidation by adsorbed oxygen. After the films reached a stable potential, exposure to oxygen caused a reversible increase in work function, just as with nitrogen. The final potentials between oxidized films and platinum compared favorably with potentials found for abraded metals vs. platinum in air.7 The work functions of the transition metals increased when first exposed to oxygen, becoming greater than that of the platinum reference. The values remained a t these noble levels until oxidation, with corresponding decreases in work function, took place. Aluminum and lead did not go through these “passive” states, instead the work functions steadily decreased until a stable potential was reached. Water vapor had a noticeable effect only on the lead films. The work functions of the platinum reference and other aged bulk metals were stable under the experimental conditions used, as shown in Fig. 2. A small change in contact potential was observed when the distance between two unlike metal surfaces was varied as shown in Fig. 3. With like metals, the potential passed through zero a t one point, and the potential-distance relation was essentially linear. This effect has been observed before with this type of a p p a r a t ~ s . ~ Some variation of potential was observed on rotating one metal surface a few degrees with respect to the other. There was also some effect
902
NORMAN HACKERMAN AND EMERSON H. LEE
VOl. 59
TABLE I MATERIALS USED Materia
Original purity,
Form
%
Purification treatment
99.5 99.5
Oxygen Nitrogen
Commercial Commercial
Water
Distilled
Aluminum
Wire
99.7
Dried Oxygen removed with vanadyl sulfate soln.; dried Redistilled from alk. permanganate, boiled to remove gases Outgassed before evaporation
Iron
Wire
99.8
Outgassed before evaporation
Chromium Nickel
Electro-deposited Wire
... 99
Outgassed before evaporation Outgassed before evaporation
Lead
Sheet
..
...
Outgassed before evaporation
because of change in amplitude of vibration. These variations were on the order of a few hundreths of a volt. No variation with frequency of vibration was observed. 0.6 OS4
.
AL V S . P T
?L IA
’”,
b\.
- --p
AU V S . AL
,I
Spectrographic anal. of films approx. impurities
.. .... I
Si, Fe, 0.10 Cu, trace; W, none Cr, Mg, Mn, Si, heavy trace; W, Al, Cu, Sn, trace Fe, Cu, trace; W, none Fe, Mn, Mg, Cu, 0.10; B, Cr, Ca, Ag, W, trace Ni, Mn, Ag, 0.01; Cu, Fe, B, trace
during any one run and therefore even the small effects mentioned above did not enter into the observed potential changes of the metal films with time. Aluminum Films.-As shown in Fig. 4, nitrogen did not. affect the initial potential trend, which was caused by oxidation of the aluminum film by residual oxygen in the vacuum system. The reversal of the potential changes due to nitrogen which became evident on the lightly oxidized film took place before the gas was pumped out, indicating that adsorption was caused by a temporary drop in temperature of the gas. Slower admission of the nitrogen through a capillary leak eliminated the cooling effect and the potential changes, as shown.
---
u 6 0.0 o.2=7ys 0.4
,
0
,
,
40 60 80 Time, min. Fig. 2.-Effect of aging on the contact potential difference between the aged bulk metals shown. Air admitted to 0.04 mm. at AI, to 0.10 mm. a t As; system pumped at P. 20
u 4 6 8 23 25 27 Time, hr. Fig. 4.-Conta~t~potential difference between platinum reference and 300 A. aluminum film. Nitrogen admitted instantaneously to 0.04 mm. at N1; same but 1 minute for admission at Nz, same but 4 minutes at N8, same but 50 minutes at Ng; system pumped at P. 0
I 0.04 0.06 0.08 R, mm. Fig. 3.-Effect of minimum distance of separation on the contact potential difference between bulk aluminum and brass. 0
0.02
These facts indicate that the measured contact potentials are in fact a difference between modified work functions. In these experiments the position and amplitude of vibration remained constant
2
Admission of air (or oxygen) to a fresh aluminum film gave the effects shown in Fig. 5 . The peak values of potential were about the same after the films had been exposed to the atmosphere and pumped again. The reversible potential changes of the oxidized film are believed to be due to reversible, physical adsorption. No correlation was found between film thicknesses of aluminum and contact potentials for
EFFECT OF GASESON CONTACT POTENTIALS OF EVAPORATED METALFILMS
Sept., 1955
903
ported by M i g n ~ l e t . ' ~The drop in potential that occurred on pumping at 42 hours was not observed inlother experiments. 0.6 0
1.01
'\P '
"
20
1
'
1
1
'
'
I
1
'
"
"
'
I
/
60 80 100 120 140 160 180 Time, min. Fig. 5.-As for Fig. 4. Film thickness a t 700 b. Air admitted to 0.04 mm. at AI, to 0.10 mm. at Az; system pumped a t P. 0
40
films thicker than about 250 A. Data on several films are shown in Table 11. Some of the differences seen resulted from the different ages of the films and some from the positional effects mentioned previously. TABLEI1 EVAPORATED ALUMINUM FILMS Thickness,
A. 280 620 695 710
Hr.
Thickness,
Final C.P.
elapsed
R.
1.514 1.400 1.024 1.424
24 24 1 2
792 1400 2130
Final C.P.
Hr. elapsed
1.484 1.362 1.301
7 8 3
Chromium Films.-Chromium films exhibited fairly active, stable potentials until air or oxygen was admitted, then the potentials immediately dropped by a volt or more, as shown in Fig. 6. The effect of air on the potentials decreased continously as the film oxidized. The over-all potential changes involved an increase in the work function, although a decrease in work function took place during the subsequent oxidation process, as it did with aluminum.
d +
0.4
o r>
I
0.4
I
i
1
W-
0.61 0
I
I
2
17
19 41 43 45 Time, hr. Fig. 7.-Contact potential difference between platinum reference and 250 b. nickel film. Nitrogen to 0.05 mm. at N ; oxygen to 0.06 mm. a t 0; water vapor to 0.001 mm. a t W; cold traps warmed to -78" at H; system pumped a t P.
Iron Films.-The initial potentials of the iron films were more noble than that of the platinum references, as shown in Figs. 8 and 9. Comparison with data on chromium and nickel indicates that the iron films were already partly covered with 0.4 0.2 v G
O
3
g- 0.2 4
NPNP ;I;
I
&
0.4 O.GF;b, 0.8 0 2
PA
, I
5
7
19 21 23 2529 3 1 4 8 7 0 Time, hr. Fig. 8.-Contact potential difference between platinum reference and 250 A. iron film. Air to 0.04 mm. at A; nitrogen to 0.04 mm. at N ; water vapor to 0.002 mm. a t W ; system pumped a t P.
0.4
3
g- 0.2 6
0
0
0.2
u9 11 30 32 52 Time, hr. Fig. 6.-Contact potential difference between platinum reference and 250 A. chromium film, Nitrogen to 0;04 mm. at N ; air to 0.05 mm. at A ; system pumped a t P. 0.4
0
2
4
Nickel Films.-Nickel films reacted much as did chromium films (Fig. 7). The increase in work function due to physical adsorption and decrease for the oxidation process is opposite to that re-
0.6 0
c-- P
,tu
2
4 6 8 22200 Time, hr. Fig. 9.-As for Fig. 8 except water vapor t o 1 X mm. a t W and system pumped at PI after exposure to atmosphere. (19) J. C. P. Mignolet, Disc. Faraday Soc.. 8, 105, 326 (1950); J . Cham. P h y s . , 20, 341 (1952).
NORMAN HACKERMAN AND EMERSON H. LEE
904
oxygen. DushmanZ0has shown that all types of iron contain significant amounts of dissolved oxygen; therefore the iron sample itself was a source of oxygen contamination. The iron films differed from those of chromium and nickel in that the first addition of oxygen caused a rapid rise in potential. This indicated that the iron oxidized much more rapidly than the nickel and chromium. After the initial exposure to oxygen, the effect of this gas was reversible, and potential changes were in the same direction as with other metals. Burstein, Surova and Zuidenman2* also found that the work function of iron films decreased during the initial oxidation process. Subsequent heating of these films caused the work function to increase again as more massive oxidation took place. I n the present work, the later increase in work function was not observed. At room temperature, the films showed an insignificant oxidat,ion rate; the films retained a bright lustre in the vacuum system, and the work function approached a constant value. Lead Films.-The lead films showed the general characteristics of the aluminum films; no passive states were observed. After the films were lightly oxidized, reversible adsorption was measurable as with other films, as shown in Fig. 10. Comparison of Figs. 10 and 11 shows that the combination of oxygen and water vapor quickly oxidized the lead. This is in agreement with the known properties of freshly distilled lead. 2 2 1.o
0.8 cd
0.6
42
P P
e
g- 0.4
0
NPo
6 820 22 24 Time, hr. Fig. 10.-Contact potential difference between platinum reference and lead film. Nitrogen to 0.04 mm. at N; oxygen to 0.05 a t 0; system pumped at P. 0
2
P
IC--
I
i 1
0
0 and x = 0 dr/dl = D(bC/bz),,o (5) This method of electrolysis is also very valuable in the kinetic study of electrode processes (Ershler, Rendles, Gerischer, Grahame and others). For a detailed review see P. Delahay “New Instrumental Methods in Electrochemistry-Theory, Instrumentation, and Applications to Analytical and Physical Chemistry.” Interscience Publishers, New York, N . Y., 1954, pp. 146-178.
L
