Chemisorption of Electronegative Gases on Refractory MetalsMarion L. Shaw and N. P. Carleton Citation: The Journal of Chemical Physics 44, 3387 (1966); doi: 10.1063/1.1727242 View online: http://dx.doi.org/10.1063/1.1727242 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/44/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Laser welding of refractory metals J. Laser Appl. 13, 199 (2001); 10.2351/1.1404416 Chemisorption of Gases on Metals by F. C. Tompkins J. Vac. Sci. Technol. 16, 1077 (1979); 10.1116/1.570164 Instability of Refractory Metal Thermocouples Rev. Sci. Instrum. 36, 816 (1965); 10.1063/1.1719710 Thermocouples of the Refractory Metals J. Appl. Phys. 21, 112 (1950); 10.1063/1.1699609 Chemisorption of Gases J. Chem. Phys. 15, 336 (1947); 10.1063/1.1746508

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C, AND CF OSCILLATOR STRENGTHS 3387

values, the oscillator strengths obtained in these ex­periments may be considered as plausible upper and lower limits for the C2 Swan and CF (A-+X) band systems. It is seen from Table VI, that the C2 oscillator strength f (0, 0) of this work is more in line with the shock-tube results of Refs. 7 and 8. Better agreement in f number is noticed for the data analyzed with the shock-tube llHr (CF2) and llHr (CF). This comparison

THE JOURNAL OF CHEMICAL PHYSICS

tends to indicate that the lower values ofthe CF (A-+X) band system oscillator strengths would be quantitatively more preferable.

ACKNOWLEDGMENT

The support of the U.S. Air Force under Contract AF04(694)-498 REST Program, is gratefully acknowl­edged in the present research.

VOLUME 44. NUMBER 9 1 MAY 1966

Chemisorption of Electronegative Gases on Refractory Metals

MARION L. SHAW AND N. P. CARLETON

Smithsonian Astrophysical Observatory and Lyman Laboratory of Physics, Harvard University, Cambridge, Massachusetts (Received 4 August 1965)

This experiment is a study of some aspects of the chemisorption of diatomic gases on metal surfaces for temperatures of 1800o-2200oK and pressures of 4.D-6.0XlO-6 torr. The systems studied were chlorine and carbon on tungsten where the carbon was present as a contaminant, and chlorine and hydrogen chloride on hafnium. We observed the flux of atomic negative ions formed from the dissociated gases on the surfaces as functions of temperature concurrently with the thermionic electron flux from the same surfaces. From these data we calculated work functions and sticking probabilities, using the Saha-Langmuir equation. For tungsten, we find that the sticking probability of chlorine was about 4 X 10-3 and tended to decrease with increase in temperature. The increment in work function was about the order of the experimental un­certainty, 0.50 eV. For hafnium the sticking probability was about 1 X 10-2 for both chlorine and hydrogen chloride and tended to increase with temperature, especially in the presence of oxygen at a partial pressure of 5XlO-6 torr. This temperature dependence we have interpreted as an indication of a contaminated surface. The work function was not noticeably changed by chlorine or hydrogen chloride. Introduction of oxygen into the system caused the work function to increase noticeably.

I. INTRODUCTION

THIS experiment is a study of some aspects of the chemisorption of diatomic gases on metal surfaces

at high temperatures. We observed the flux of atomic negative ions formed from the dissociated gases on the surfaces as functions of temperature concurrently with the thermionic electron flux from the same surfaces. The surface-ionization technique represents a new method for obtaining chemisorption sticking prob­abilities at high temperature; heretofore, the principal means has been the flash-filament technique, which was used by Becker and Hartmann1 to study the chemi­sorption of nitrogen on tungsten to temperatures of about Hoo°K.

The gas-metal systems here chosen were such that negative ions are readily formed from the gases on the surfaces. Data were taken for chlorine on tungsten and carbon on tungsten, where the carbon was present as a contaminant, and for chlorine, hydrogen chloride, and a 50-50 mixture of chlorine and oxygen, all on hafnium. From the Cl- and electron fluxes as functions of tem-

1 J. A. Becker and C. D. Hartmann, J. Phys. Chern. 57, 157 (1953) .

perature and pressure and the previously known elec­tron affinity of chlorine,2 the sticking probabilities for chemisorption and the apparent thermionic constants of the Richardson equation were calculated. The approximate temperature range for the runs taken on tungsten was 1970°< T<2220oK and that for hafnium was 17200 <T<1890°K.

We define the sticking probability for chemisorption to be that fraction of the incident molecular flux which becomes bound to the surface with an energy com­parable to the energies involved in chemical reactions (say of the order of 10 kcal/mole or more). Obser­vations of Cl- when the gas incident on either tungsten or hafnium is hydrogen chloride or chlorine are an indication that the chemisorption is dissociative, as is the case for oxygen, nitrogen, and hydrogen on tung­sten.a We assume that the chemisorption is wholly dissociative. Furthermore, we assume that (1) detailed balance exists between the incident molecular flux which chemisorbs dissociatively and the total flux of atoms

2 R. S. Berry, C. W. Riemann, and G. N. Spokes, J. Chern. Phys. 37,2278 (1962).

3 G. Ehrlich, Struct. Properties Thin Films, Proc. Intern. Conf., Bolton Landing, N.Y., 1959, 546 (1959).

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3388 M. L. SHAW AND N. P. CARLETON

TO BUTTeRFLY VALVE LIOUID-NlT,EN ;::"'A:JP,:":PU'f",,-,,PS ___ ~-r _____ -,

/ J

/ /

/ /

/ /

/ "'--. / ~

/ .~

ION

GAUGE

DETECTOR ASSEMBLY BOLTED

TO THIS FLANGE (OJ

/ ~. / '~®

CD POSITION OF IMAGE FORMED BY ELECTROSTATIC LENSES.

® ~:GGNEE~ CD FORMED BY

Sco~ in incMs

... ",--

FIG. 1. Diagram of system showing arrangement of electrostatic lenses, magnet, detector, and vacuum pumps. T: voltage feed through; HS, VI, V2: three aperture lens; V2, Vs: double cylinder. Electrode spacings: BP, 0.072 in.; F-HS, 0.028 in.; HS-VI, 0.027 in.; VI-V2, 0.196 in.; V2-VS, 0.500 in.

and atomic ions evolving from the surface:

2Sn2=n+n_, (1)

and (2) the Saha-Langmuir equation is valid in this situation (see Sec. II). This equation states that

n_/n= (Q-/Qo) exp[(A-¢)/kT]. (2)

In Eqs. (1) and (2): S is the sticking probability for chemisorption of the diatomic molecule, n2 is the incident molecular flux, n is the flux of atoms off the surface, n_ is the flux of atomic negative ions off the surface, Qo, Q_ are the partition functions for atoms and ions, A is the electron affinity of the atomic species, T is absolute temperature of the surface, and ¢ is the average work function of the surface. For a poly­crystalline surface we would write ¢= 'L,iWi¢i, where Wi

is the fraction of the surface having a work function ¢i, and we would replace (2) by

n_ Q- {A [" (¢i)J} - = - exp - ,,-,Wi exp - - . n Qo kT i kT

Our tungsten sample was approximately uniform; the hafnium was polycrystalline (see Sec. III). However, over the limited temperature range of the experiment we have been able to fit the data for n-/n with Ex­pression (2) and the thermionic data with a Richardson equation just as for the tungsten (see Figs. 4 and 5). In the results it is the energies derived from the slopes of the data for n-/n and the thermionic electron current which we have defined as A - ¢ and ¢ with refer­ence to Expression (2). From (1) and (2) we have that

S= (n_/2n2) {1 + (Qo/Q-) exp[(¢- A)/kT]}. (3)

The expression is set in terms of measured quantities in Sec. II.

II. APPARATUS

The main components of the apparatus are the filament or surface on which the chemisorption occurs; a system of electrostatic lenses which form an image of the filament suitable for focusing by an electromagnet which resolves the various charged species emitted from the filament and focuses them at the detector; and the detector assembly, which consists of an electron multiplier, an electrometer, and a recorder (see Fig. 1).

The experimental procedure consisted of baking out the system and pumping down until the background pressure remained below 2 X 10-7 torr at the highest temperature of the filament and no pressure burst over the background occurred on flashing the filament to 2400 oK, then admitting the test gas into the source chamber to a pressure of about 5 X 10-6 torr" and at temperature intervals of 15°-20oK over a range of about 1800o-2200oK scanning the magnet auto­matically over two ranges: 0-20 G for observing the electron beam and 1500-3600 G for negative ions. The lower temperature limit was set by the requirement that the nature of the surface not change due to ad­sorption of gases, which have a longer lifetime at lower temperatures, and the higher limit by increased evolution of background gases from the hot surfaces. The pressure in the vacuum system between the pole pieces and at the detector was at all times < 2.5 X 10-7

torr. The ion gauge (Type RG-75P, Veeco Company) was

calibrated against a Knudsen gauge (Type lA, Edwards

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CHEMISORPTION OF ELECTRONEGATIVE GASES 3389

~ I

., ~~o FIG. 2. Schematic diagram 1~~ -

of voltages on source electrodes. - - - -, voltages with respect to filament; --, voltages with respect to ground.

BP

Company) for use in chlorine.4 It was found to be 10% more sensitive to chlorine than to nitrogen, linear, and reproducible for pressures up to about 1.5 X 10-5 torr.

Some dissociation of chlorine probably occurred at the filament of the gauge; however, there were several baffles between the gauge and the filament or cathode F (see Fig. 1), which we estimate would make it unlikely for these atoms to ionize on the cathode. The most likely direction for a pressure gradient is per­pendicular to the length of F due to pumping through the aperture in Vs so that the gauge pressure should still approximate that in the source chamber at the cathode.

For the tungsten data the filament consisted of a piece of tungsten ribbon, 0.040 in.XO.002 in.X 1i in., which was spring loaded at one end. The area at the center which contributed to the beam was estimated by conventional ray-tracing techniques to have a radius of about 2X10-a in., ensuring that this area was essentially isothermal, since the change in temperature over a length of about i in. of the central portion of the filament was observed by an optical pyrometer not to exceed 1500 K when the center was at 2200oK. The filament was heated by dc and calibrated in terms of the heating current by an optical pyrometer.

For the hafnium data a piece of cold-rolled hafnium ribbon, 0.040 in.XO.OO6 in., was spot welded to the central section of a tungsten filament. It was not possible to construct the entire filament from hafnium owing to the plasticity of hafnium at high temperatures.

The electrostatic lens system consisted of the plate BP behind the filament, the three apertured electrodes. HS, VI, and V2, in front and a double-cylinder lens Va and V4 (see Fig. 1). The electrode HS shielded VI from the heat radiated from the filament; VI provided a field at the filament strong enough to overcome space-charge effects, and these electrodes in conjunction with V2 and the double-cylinder lens formed an image of the filament at approximately Position 1 in Fig. 1. The operating voltages are shown schematically in Fig. 2.

The magnetic field was produced by a 50° sector magnet whose water-cooled coils have an equivalent dc resistance of 125 n. The limits of its resolution are is < flM/M <T~' The width of the beam at the de-

4 M. L. Shaw, Rev. Sci. Instr. 37,113 (1966).

-2355

"r

V2 E~ECTROOE SPACI~I

.IOOIICH

V3

v, IS flOATlII 2355 VOLTS BElOI mUIo; V,IIOT SHOllilS AT mUll]

tector is about -fi in., which is the width of the slit in front of the detector.

The data for hafnium were taken using a DuMont SPM 01401 multiplier, and those for tungsten with a Bendix M306 multiplier. The signal from the multiplier was amplified by a Keithley 600A electrometer and recorded by a Sanborn 350 six-channel recorder which also monitored the pressure in the source chamber and the voltage applied across the coils of the electromagnet.

The system was evacuated by a Consolidated MGH 180 4-in. mercury diffusion pump backed by a Welch forepump and trapped by a Consolidated 4-in. Freon baffle and a 6-in. liquid-nitrogen trap manufactured by the Edwards Company. Between the liquid-nitrogen trap and the main vacuum system is a 4-in. butterfly valve (see Fig. 1). Beyond the liquid-nitrogen trap, the vacuum system is all stainless steel and has aluminum gaskets except for four Viton 0 rings in the butterfly valve. The insulators in the source are boron nitride and glass; the voltage throughputs were made by Ceramaseal Company. The test gas was bled into the source chamber through a high-vacuum valve (manu­factured by the Granville-Phillips Company) from an intermediate pressure chamber.

We now rewrite the expression for the sticking probability (derived in the introduction) in terms of empirical quantities. In this experiment <P- A ~ 1 eV, and kT-v O.2 eV. Therefore,

S~ (n-Qo/2n2Q_) exp[ (<p- A) /kTJ. (4)

The observed quantities are the currents of thermionic electrons and negative ions, the pressure in the source chamber, and the temperature of the emitter. From the temperature and the electron current, we calculate <p. The incident flux n2 is related to the ambient pressure P and the temperature To of the walls of the vacuum system according to 1t2=P(211"mkTo)-!, where m is the mass of the incident molecule.

The flux of ions n_ is ~related to the output signal I at the anode of the multiplier, whose gain is M, ac­cording to n_= (I/eM) (1/aa') , where e is the charge on the electron, a' is the emitting area, and a is the efficiency with which the system conveys to the de­tector the charge emitted from a'.

The term 1/aa' is evaluated by comparing the current in the electron beam at the detector with the

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3390 M. L. SHAW AND N. P. CARLETON

T ABLE I. Chemisorption sticking probabilities and Richardson parameters.

Thermionic emission Sticking probability

i/aP=B exp(-q,/kT) A/cm2(OK)2 S=So exp(E/kT)

Surface No test gas Test gas

B q,(eV) B q,(eV) So E(eV) Type of P (torr) Temperature range test gas

Hf 2.56 3.17±0.02 1720o-1890oK

8.8 3.20±0.30 0.021 -0.39±0.39 Cb 8.5XlO-j! 1720o-1890oK

0.57 3.16±0.30 0.010 0.04±0.04 HCI 4.8X10-j! 1720o-1890oK

9.4 3.36±0.40 4S.2 1.49±0.20 02+Ch 1.0XlO-5 1720o-1890oK

W 221 4.70±0.40 1970o-2220oK

1.9X 106 S.24±0.SO 0.192XlO-2 0.20±0.20 CI2 4.7XlO-1 1970o-2220oK

current from a known area on the filament, that defined by HS, to the electrode Vl. The quantity (1laa') was seen to have a reproducible temperature dependence, decreasing by a factor of about 2 as the temperature increased from its minimum to its maximum value. Since the area over which the temperature is uniform is considerably larger than the emitter area defined by the electrostatic focusing system, and since we are able to verify that the product of the emission area and the Richardson pre-exponential was constant over the temperature range of the experiment, it would seem that this effect represents a variation of the efficiency, a, probably due either to space charge perturbing the off-axial potential or to a warping of the electrodes as they heat up. The gains of the mUltipliers were meas­ured directly by comparing the beam current at the cathode with the signal from the anode.

FIG, 3, Back rell<:ction Laue pattern of tungsten emitter.

Making these substitutions, we have

S= {[QoI(211"mkTa)!J/(2Q-MPaa'e)}

exp[(cf>-A)lkTJ. (5)

III. DATA AND CALCULATIONS

A. Results for Tungsten

Back reflection Laue patterns showed that the central portion of the tungsten filament had recrystal­lized into a single crystal about 1 cm long with the [211J direction about 10° from the normal to the surface (see Fig. 3). Also visible were reflections from the (111), (110), and (100) planes. The thermionic parameters calculated from a semilogarithmic plot of liP- vs liT (the Richardson plot) are given in Table I. The value calculated for the work function is within the range of values obtained by others for tungsten surfaces of similar orientation. The pre-exponential appears to be slightly high; however, this statement is only tentative due to the experimental uncertainty in the slope and the limited temperature range of the measurements.

I ]1

-11 "0

-11 "0 L-;-~;-!;-~

.6 .1 .8 .9 4.5 5.0

I/TIIO'

FIG. 4. l/P vs 1/TX1<J4, electron beam from tungsten surface. Chlorine pressure is 4.7Xl~ torr. Relative scale for current.

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CHEMISORPTION OF ELECTRONEGATIVE GASES 3391

Also shown in the table are the thermionic param­eters describing the Richardson plot in the presence of chlorine and the sticking probability for chlorine on tungsten. Samples of the data are shown in Figs. 4 and S. The sticking probability is shown as a function of 1/ T in Fig. 6. The errors inherent in this experiment are such that the absolute value of the sticking probability is known only to within a factor of about 2, and the exponential form in which the results are displayed in the table cannot be taken with extreme seriousness. This is due to the limited temperature range of the experiment. Data were taken over a range of about 200 oK centered about a temperature of approximately 20000K. Hence any uncertainty in the slope of a semi­logarithmic plot of I/Tl for thermionic emission or 1/ P for negative ions vs 1/T will be reflected in a relatively large uncertainty in the intercept of the curve on the I/T2 or 1/ P axis, and this uncertainty will be cor­respondingly reflected in the value of the sticking probability, as can be seen from Eq. (5). These results show that the slope of the Richardson plot is increased by the presence of chlorine, as one expects for an electronegative gas, and that the sticking probability tends to decrease with increase in temperature, an effect which has been observed for nitrogen (300°< T< 1100 OK) , although not for hydrogen (77°< T<3000K). In the absence of either a desorption lifetime or data on the variation of work function with coverage, it is not possible to derive a surface coverage from our results. We can make a crude estimate of the coverage by assuming that the desorption lifetime for chlorine on tungsten is approximately equal to that of oxygen on tungsten, which has been observed to be about 5 sec at 22000K by Johnson and Vick.o Choosing the value

FIG. 5. liP vs 11TX10', chlorine negative-ion beam from tungsten sur­face. Chlorine pressure is 4.7 X 10--6 torr. Relative scale for liP.

x 10' r-,------~

1.. p

x 103 L--!-+-+-+-....L..-! 4.5.6 .1 .8 .9 5.0 .I

6 M. C. Johnson and F. A. Vick, Proc. Roy. Soc. (London) A151,296 (1935).

s

XI0-3.,-::-_~_----!:: __ !:-_~_---::":::--_.L-_ 4.5 .6 .7 .8 .9 5.0

liT x 104

FIG. 6. Sticking probability vs 11TX104, chlorine on tungsten. The line represents the best fit [of the form S= So exp( -ElkT) ] to our data. The uncertainty in S is about a factor of 2, so the temperature dependence is not significant.

of 8X1014 for the surface density of tungsten atoms derived for the (211) plane by Stranski6 and the sticking probability of 4X 10-3 from our results, a chlorine pressure of 4X 10-6 torr, and a desorption lifetime of 5 sec, we arrive at an estimated fractional coverage of 0.05. This estimate for the coverage may be a safe upper limit since oxygen does bind relatively strongly to tungsten.

By means of the magnet calibration, negative ions with mass numbers 24, 14, and 12 were identified independent of the presence of test gas in the system. Numbers 12 and 14 were present in such minute amounts (less than 10-11 A at the multiplier anode at the highest filament temperatures) that it was not possi­ble to make reliable electron affinity measurements for them. Number 24 was the main contaminant ion pres­ent. We have tentatively identified No. 24 as C2-,

since carbon is often present in tungsten filaments, does not affect the work function appreciably, and has the relatively large electron affinity of 4.0 eV.7 The currents associated with this ion were about 5% of those associ­ated with CI- at the highest temperatures of the filament when the pressure due to chlorine was about SXlO-6 torr. It is possible to relate the observed current of this ion, assuming it to be C2-, to a popu­lation of carbon on the surface in two cases: (1) that in which the adsorption of a hydrocarbon molecule from the gas phase produces a carbon molecule which immediately evaporates as C2 or C2-j and (2) that in which the probability of a carbon molecule dissociating before evaporating is large enough that it is reasonable to discuss an equilibrium reaction on the surface (C+ C = C2) • The source of the carbon population migh t be either a solution of carbon within the lattice or back­ground gases evolving from the hot surfaces and walls. In either of these cases, it is estimated that, for any reasonable value (i.e., less than 0.1 sec) chosen for the desorption lifetime for the molecular carbon, the total carbon surface population is surely less than 0.1% of a monolayer.

6 I. N. Stranski and R. Suhrmann, Ann. Physik I, 153 (1947). 7 L.rM. Branscomb, Advan. Electron. Electron Phys. 9. 43

(1957).

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3392 M. L. SHAW AND N. P. CARLETON

Hel

00 ' ----'----'----'----'----'---1---1---1---l .4 .5 .6 .I .S .9 6.0 .1 .1 .3

I/rxl04

FIG. 7. Sticking probability vs 1/TXlO', chlorine, hydro­gen chloride, 02+Ch on haf­nium. (See note)n caption to Fig. 6.)

B. Results for Hafnium

The Cl- and thermionic electron fluxes off a hafnium surface were observed as functions of temperature in a series of experiments in which the source of Cl- was varied the source gases being chlorine, hydrogen chloride, and an approximately SO-SO mixture of oxygen and chlorine.

At the conclusion of the experiments the hafnium surface was examined by means of x-ray diffraction to the extent that its size and mounting would allow. It was seen that the surface had fairly large grains, at least 104 A in diameter, and that the grains were oriented with the (110) plane perpendicular to the rolling direction.

Itemized in the table are the thermionic parameters both in the absence and in the presence of various test gases and the respective sticking probabilities. It should be noted that the comparison of the thermionic param­eters of hafnium obtained by different experimenters is somewhat risky due to the almost unavoidable presence of some zirconium impurity in the lattice.s

8 D. Thomas and E. Hayes, The Metallurgy of Hafnium (U.S. Atomic Energy Commission, U.S. Government Printing Office, Washington, D.C., 1960).

It can be seen from the data in the table that, to within the experimental uncertainty, the thermionic characteristics of the surface are not appreciably altered by the presence of hydrogen chloride or chlorine at pressures from 5 to 9X 10-6 torr, although the sticking probabilities are several times larger than that for chlorine on tungsten, for which the work-function change was of the order of 0.50 eV. Hence, it would seem that there is a real difference between the states of polarization of the adsorbed layer formed by these gases on tungsten and on hafnium. The sticking probabilities, calculated from our data and shown. in Fig. 7, indicate that chlorine and hydrogen chlOrIde have similar behavior on hafnium. It should be noted that their sticking probabilities do not tend to decrease with temperature as markedly as that of chlorine on tungsten although this remark must be tentative due to the experimental uncertainty.

To investigate this effect, a SO-SO mixture of oxygen and chlorine at total pressure of 1 X 10-5 torr was introduced in the source, and it was found that the above type of temperature dependence of the sticking probability was enhanced. Our interpretation of these results is based on experimental work, such as flash­desorption studies of nitrogen on tungsten, wh~ch indicates that sticking probabilities decrease WIth increasing coverage.9 In other words, the increase in sticking probability which we observe is likely due to a cleaning up of the surface as its temperature increased. That the effect was noticeably enhanced in the presence of oxygen may indicate something of the nature of t.he contaminant. Hafnium is known to adsorb readIly gases such as oxygen, nitrogen, and hydrogen. -

ACKNOWLEDGMENT

The authors wish to thank Professor D. Turnbull of Harvard University for many valuable discussions.

9 G. Ehrlich, J. Phys. Chern. Solids 1, 1 (1956).

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