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R. M. DELL
tains contributions from the amplitudes of all three of the periodic curves for &, Q’ and &” against p. It is a rather large al’ which causes the heat capacity to fall outside the range in which it was a t first expected to appear. It has been stated that the methods discussed and referred to here separate the problems of the external and internal rotations. It is true that the properties and factors assignable to the external rotation in the limit for an infinite barrier are separated out. The rest of the thermodynamic property as calculated for both degrees of freedom is then assigned to the internal rotation and included in a general table. It should be kept in mind,
Vol. 61
however, that the separation is arbitrary as well as convenient. All solutions of the problem sum up for both degrees of freedom and subsequently divide the property between them in an arbitrary but convenient manner. Actually, when two degrees of freedom are coupled as in the internal rotation problem, there is no device by which a physically meaningful separation can be made. The character of the “separation” effected here is well emphasized by the column of positive free energies obtained by Li and Pitzer. For a truly separated independent degree of freedom a positive free energy would represent statistical nonsense.
THE ADSORPTION OF GASES ON GERMANIUM POWDER BY R. M. DELL’ Admiralty Research Laboratory, Teddington, Middlesex, England Received March 21, 1967
A study has been made of the adsorption of hydrogen, carbon monoxide and oxygen on germanium powder prepared by reduction of pure germanium dioxide. At 25”, no chemisorption of H) or GO was observed on either reduced or oxidized germanium surfaces. Oxygen, however, was adsorbed readily a t this temperature, the uptake obeying a logarithmic rate law. The extent of oxygen adsorption appears to be independent of the semi-conductor type of germanium but to vary with the detailed preparative procedure, indicating a dependence upon the crystal structure of the germanium surface. with a resistivity of about 15 ohm-cm. Four different Introduction samples were used ranging in specific surface area from 0.45A knowledge of the adsorptive properties of 1.37 sq. m./g. according to the previous thermal treatment. germanium for various common gases is of interest Surface areas were determined by krypton adsorption a t from two points of view. First, the presence of a -195”. For quantitative comparison with the york of chemisorbed film of gas will, in general, change both Green, Kafalas and Robinson, a value of 19.4 A.2 was for the cross-sectional area of the adsorbed krypton the surface barrier height and the surface recom- chosen atom.6 Adsorption measurements were carried out in a bination velocity of holes and electrons. The elec- constant volume high-vacuum system of conventional detrical properties of germanium are thereby modi- sign, a multi-stage, high speed, mercury diffusion pump of fied. Secondly, recent work on the chemisorption the type described by Gray’ being employed. The gercontained in a silica vessel, was protected from of gases on metallic oxides has shown this to be, in manium, mercury vapor by a cold trap kept permanently a t -195”. many cases, an electron transfer process, and that a Between adsorption experiments the germanium was heated relationship exists between the semi-conductor and in hydrogen, usually at 650°, for periods of 21 to 100 hours the adsorptive properties of a solid. Since the and evacuated a t the same temperature for 10-24 hours. oxygen adsorptions, the amount of hydrogen taken up semi-conductor characteristics of germanium have After corresponded quantitatively to the removal of oxygen as been so well established, its surface chemistry is water. At higher temperatures of reduction, the amount worthy of study. of hydrogen used was rather less than the theoretical beThe present paper reports some studies of the cause some GeO evaporated to a cool part of the reaction interaction of oxygen, hydrogen and carbon monox- tube before reduction could be effected. ide with reduced germanium powder. Since this Results work was undertaken, a number of papers have A series of experiments was carried out to deterbeen published dealing with the adsorption of gases mine whether hydrogen is adsorbed on reduced on germanium films,2 filaments3 and crushed single germanium At 25”, no adsorption could ~ r y s t a l s . ~A comparison of these results with be measured,powder. the limit being 5 X 1O’O those for germanium powder reveals several in- molecules/cm.2 a t 5 x of detection mm. pressure and 2 X teresting features. 1013 molecules/cm.2 a t 0.2-0.4 mm. pressure. At Experimental - 195”, however, slight adsorption occurred (just Germanium waR prepared by reduction of Johnsondetectable at 0.2 mm. pressure) ; this appeared to Matthey “Spectroscopically Standardized” germanium di- be pressure sensitive and was probably physical oxide in pure hydrogen a t 600-650°.6 The stated impurities were copper and calcium, and a fused sample was n-type adsorption, No hydrogen uptake was found (< 6 X 10l2 molecules/cm.2) a t either 25 or 100” on (1) Houdry Process Corporation, Marcus Hook, Pa. germanium surfaces which had been pre-saturated (2) K. Tamaru, THISJOURNAL, 61, 647 (1957). with oxygen a t 25’ and 0.5-1 mm. pressure. (3) J. T. Law and E. E. Francois. Ann. N . Y . Acad. Sei., 68, 925 Similar experiments were performed with car(1954); J. T . Law, THISJOURNAL, 59, 543 (1955). (4) M. Green, J. A. Kafalas and P. H. Robinson, “Semiconductor bon monoxide. At - 195O, reversible, pressure deSurface Physics,” University of Pennsylvania, 1957, p. 349. (5) L. M. Dennis, K. M. Tressler and F. E. Hance, J . A m . Chem. Soc., 46, 2033 (1923).
(6) A. J. Rosenberg, ibid., 78, 2929 (1956). (7) T. J. Gray, Disc. Faraday Soc., 8, 331 (1950).
ADSORPTION OF GASESON GERMANIUM POWDER
Dec., 1957
pendent adsorption occurred on reduced germanium, a surface coverage of 60-70y0 being reached only a t 2.5 mm. pressure. No uptake was observor 0.3 mm. on able a t 25" and pressures of 4 X either reduced or oxygen-treated germanium. Adsorption of oxygen on reduced germanium occurred readily at 25", an initial fast adsorption being followed by a slow process; even after five hours gas still was being taken up. The amount of oxygen adsorbed by a given sample in successive experiments was quite reproducible and almost independent of pressure in the range studied (0.1-0.6 mm.). However, the volume adsorbed per unit surface area a t 25" was not constant from sample t o sample but varied according t o the detailed preparative procedure and thermal history of the specimen. I n agreement with Green, Kafalas and Robinson4 the number of atoms of oxygen adsorbed ( N ) increased with time according to the law: N = le log t constant, Figure 1 shows this law for several different samples of germanium and the results are summarized in Table I.
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TABLE I1 ADSORPTION OF OXYGEN ON GERMANIUM (SAMPLE B, 2:08 G.) AT 25" Expt.
Temp; of redn., C.
Vol. adsorbed in 30 min., ml. STP.
Surface area, m.2
650 650 650 750 650 750 800 700 800 850 650
0.14 .14 .14 .13 .13 .12
1.42
Bi Bl3 B16 Big B*6
B26 €328
B30 &I
B32
.ll .ll
.12
.08 .06
0.94
tion of oxygen usually occurs with transfer of electrons from the solid to the adsorbate. It might be expected that this process would be accompanied by an increase in conductivity for p-type germanium, and a decrease for n-type germanium.
TABLE I
%?i Temo. sufface of rearea, duction, Sample m.*/g. 'C. B C D
Eq. of graph
0.68 650 (5.0 X l O I 3 log,, t ) 1.37 600 (7.0 X lOl31oglot) 0.69 650 (8.9 X 1013log,, t ) Eq. of Green, (14.3 x 1013 log,, t ) et ale4
+ (4.60 X + (7.42 X 1014) + (9.65 X 1014) + (8.4 x 1014) lOI4)
2
0.5 - SAMPLE
B
I
, I The surface coverage after one minute varies 0.0 0.2 0.4 0.G 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 from 4.6 X 1014to 9.6 X 1014atoms/cm.2 compared Log,, time (minutes). t o a reproducible value of 8.4 X l O I 4 atoms/cm.2 Fig. 1.-Kinetics of oxygen adsorption a t 25". reported by Green for single crystals crushed in vctcuo. (The number of germanium atoms per Such effects have, in fact, already been observed cm.2 of surface has been calculated by Green as 7.7 by Clarkeg for bulk germanium, despite the fact X l O I 4 ) . None of the adsorbed oxygen was de- that the volatility of GeO suggests the surfacesorbed on pumping a t room temperature indicating oxygen bond to be partly covalent in character. a moderately strong heat of adsorption. I n order to measure the direction of resistance When sample C was treated with oxygen a t 25" change of powdered germanium on adsorbing for 2.5 hours and the temperature then raised t o oxygen, the silica vessel containing the sample was loo", there was a marked increase in the rate of inserted between the plates of a capacitor which oxygen uptake, as might be expected for an ac- was tuned in a resonant circuit a t 56-64 Mc./s. tivated process. From the change in the Q factor of the circuit Sintering of germanium, by reduction at tem- when oxygen was adsorbed a t room temperature, peratures above 650" , resulted in a progressive the direction in which the sample conductance aldecrease in the amount of oxygen adsorbed. Table tered could be deduced. When oxygen was adI1 shows a sequence of experiments for a single mitted to a specimen reduced and evacuated a t sample (B), in which the temperature of hydrogen 650-700", there was an immediate fall in Q, correreduction between adsorption measurements was sponding, with the present circuit, to a decrease in raised gradually to 850". It is seen that the loss in sample conductance. If, on the other hand, the adsorptive capacity on sintering in hydrogen more temperature of evacuation was 800-850", Q rose than parallels the loss in area and is irreversible, on adsorbing oxygen, indicating an increase of suggesting a permanent change in the germanium sample conductance. These effects were quite surface. A similar result was reported previously marked; typically, the Q value of the resonant cirfor adsorption of other gases on copper powder.8 cuit changed by 20-30y0 immediately oxygen was It is of interest to inquire whether the extent of admitted. The volume of gas adsorbed was indeoxygen adsorption on germanium is related to its pendent of the direction in which Q changed (cf. electrical properties, in particular, to the sign and Table 11,experiments B28, B29, B30). Finally, two experiments were carried out to deconcentration of the charge carriers. The adsorptermine whether the catalytic oxidation of hydro( 8 ) R. M. Dell, F. S . Stone and P. F. Tiley, Trans. Faraday Soc.,
49, 195 (1953),
(9)
E. N. Clarke, Phgs. R E V . 96, , 284 (1954).
R. M. DELL
1586
gen or carbon monoxide would proceed a t 25" over germanium powder pre-saturated with oxygen a t this temperature. Using stoichiometric mixtures of CO ' / a 0 2 (0.26 mm. total pressure) and Hz -I- ' / z On (0.5G mm. total pressure), no measurable reaction was observed in either case. Discussion The extent of adsorption of hydrogen and carbon monoxide on germanium single crystals has been studied by Law, using the desorption technique. For hydrogen, a mixed physical and chemical adsorption was reported; from the isobars, one may estimate the chemisorption at room temperature and mm. pressure to represent 1-5% surface coverage. In the present experiments, any adsorption occurring on germanium powder a t this temperature and pressure amounts to less than 0.01% coverage. The origin of this discrepancy is uncertain; it is considered likely that germanium surfaces prepared by "flashing" in vacuo (that is, evaporation of GeO) have different physical structures and therefore adsorptive properties to surfaces prepared by reduction in hydrogen. The experiments with carbon monoxide confirm Law's observation that chemisorption of this gas does not occur on a clean germanium surface a t 25". However, a t -195" where physical adsorption takes place, Law finds a complete monolayer to be formed a t mm. pressure, compared with only 60-70% coverage a t 2.5 mm. pressure in the present study. Such a divergence in order of magnitude hardly may be attributed to small differences in surface structure. Physical adsorption of carbon monoxide or nitrogen to monolayer coverage a t -195" and a pressure of to loM4 mm. is not paralleled in any previous adsorption studies. Usually, a monolayer is formed a t a relative pressure PIP0 > 0.001 corresponding to an absolute pressure > 0.7 mm. in the present instance. No appreciable physical adsorption would be expected a t a pressure of mm. In contrast to hydrogen and carbon monoxide, oxygen is chemisorbed rapidly at 25" to an appreciable fraction of a monolayer. The variation in coverage from sample to sample, and the decrease in coverage per unit area after sintering in hydrogen, both suggest that oxygen adsorption is a structure sensitive phenomenon. Support for this view is provided by the results for samples C and D (Fig. 1). D was prepared from C by partial oxidation in air for several hours a t 600" followed by reduction in hydrogen a t (350"; this treatment, which may be expected to increase the concentration of lattice defects and imperfections in the surface region, resulted in a reduction in specific surface area to one half the initial value, while the oxygen adsorption per cm.2 increased by 30%. It is suggested that on sintering in hydrogen, the surface structure of the powder is changed by migration of germanium atoms in such a way as to anneal surface strains in the crystal. These strains arise from lattice imperfections introduced during reduction of the oxide and from the unsaturated valence orbitals of the resulting surface atoms.
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Vol. G I
Green, et ~ l . observed , ~ that for germanium surfaces formed by fracture, the chemisorption of oxygen proceeds to the same coverage for both nand p-type material. In the experiments reported here, samples quenched from 650-700" are believed to be n-type (decrease in conductivity on adsorbing oxygen), while those quenched from 800-850" would be p-type (increase in conductivity on adsorbing oxygen). Again, no difference in the extent of oxygen uptake is observed. The change over t o p-type conductivity on raising the temperature of quenching above 800" has been explained by impurity copper atoms entering the germanium lattice to form acceptor centers.10 The absence of dependence of the extent of oxygen chemisorption on the semi-conductor type of germanium indicates that the adsorption process, although involving electron transfer from solid to gas, is not determined primarily by the electronic properties of the solid. This result, although not foreseen, readily may be interpreted in terms of the band theory of solids. Electron transfer between a solid and a gas at its surface usually involves a single band only, either the conduction band or the valence band of the solid. I n the case of semi-conductors where the energy gap is too large for intrinsic conduction to occur, the extent of chemisorption may be limited by the availability of charge carriers in the appropriate band. This is true for "depletive chemisorption" (e.g., oxygen on n-type ZnO, hydrogen on p-type NiO) which takes place only to a small extent before the surface potential barrier reaches a height where the activation energy for further adsorption is too large.l1~lZ "Cumulative chemisorption," however, in which additional charge carriers are produced in the solid (e.g., oxygen on NiO, CuzO, hydrogen and carbon monoxide on ZnO) is not subject to this restriction and proceeds readily to high surface coverages. For germanium and other semi-conductors which have a relatively small energy gap width, intrinsic conduction is possible a t room temperature and majority and minority carriers are in equilibrium. When the concentration of electrons in the conduction band of n-type germanium is depleted by chemisorption of oxygen, equilibrium will be rapidly restored by promotion of electrons from the valence band. Since the concentration of electrons (n) is initially many orders of magnitude larger than the concentration of holes ( p ) and the equilibrium condition is n.p = constant, it follows that the over-all conductivity, given by (T = nep, pep,,, will decrease on adsorbing oxygen. The extent of oxygen chemisorption is not limited by the initial concentration of electrons in the conduction band but by other factors. Among these will be the physical structure of the germanium surface. The author wishes to thank Dr. W. N. Reynolds and Mr. M. S. Wills for their interest and encouragement and the Admiralty for permission to publish this paper.
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( 1 0 ) C. 8. Fuller, J. D. Strothers, J. 9.Ditzenberger and K. 5. Wolfstirn, Pkys. Reu., 93, 1182 (1954). (11) F. S. Stone, "Chemistry of the Solid State," ed. W. E. Garner, Butterworths, 1955, p. 379. (12) S, 8. Morrison, Advances i n Catalpsis, 7, 2bE (1955).
