THE ADSORPTION OF VARIOUS GASES ON LEAD TELLURIDE

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Submitted on 1 Jan 1968

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THE ADSORPTION OF VARIOUS GASES ON LEAD TELLURIDE

Mino Green, M. Lee

To cite this version:

Mino Green, M. Lee. THE ADSORPTION OF VARIOUS GASES ON LEAD TELLURIDE. Journal de Physique Colloques, 1968, 29 (C4), pp.C4-140-C4-141. �10.1051/jphyscol:1968421�. �jpa-00213625�

JOURNAL DE PHYSIQUE Colloque C 4, supple'ment au no 11-12, Tome 29, Novembre-Dkcembre 1968, page C 4 - 140

THE ADSORPTION OF VARIOUS GASES ON LEAD TELLURIDE

Mino GREEN and M. J. LEE

Zenith Radio Research Corporation (U. K) Ltd., Stanmore, Middlesex

Rhumb. - L'utilisation de molCcules polaires pour sonder la distribution de charges B la surface d'un solide est discutk et illustrke en se r6fCrant aux rCsultats r6cents sur Padsorption dans le systkme (PbTe) (Eau, MCthanol ou Wher dikthyle). La conclusion est que Ies atomes de surface de PbTe ne sont pas essentiellement ioniques.

Abstract. - The use of polar molecules as a probe of the charge distribution at the surface of a solid is discussed and illustrated with reference to recent adsorption data for the system PbTe (water, methanol or diethyl ether). It is concluded that PbTe surface atoms are not essen- tially ionic in character.

Introduction. - This paper is concerned with the kind of information that can, and might, be obtained from a careful analysis of the heat of adsorption of various molecules on the surfaces of solids, particularly semiconductors. We are espe- cially concerned with polar molecules of known (or nearly known !) charge distributions, which adsorb reversibly at low temperatures and so come under the general heading of physical adsorption.

When the heat of adsorption considerably exceeds the binding energy that one can expect from van der Waals interaction then one is presented with a system where the binding energy is dominantly electrostatic in origin. In this case if one has knowledge of the size, shape and charge distribution of the adsorbed molecule, and one knows the heat of adsorption, then it should be possjble to learn a considerable amount about the charge distribution at the surface of the solid. I t is information about the charge distribution at the surface of the solid that we are seeking. We could reverse the argument, and are seriously considering the problem, whereby knowing the surface charge distribution (the alkali halide surfaces, for instance) we could determine some of the multipole moments of an adsorbed molecule.

Some preliminary considerations and approxima- tions. - Let us consider the water molecule adsorbed on a solid. The van der Waals binding energy, this means we ignore the charge distribution on ,water, would be much the same as that for neon or a ) C H ~ group, about 0.1 eV, or a bit less. The adsorption energy versus distance normal to the surface can

be represented by a modified Lennard-Jones 6 - 12 potential or any of the-, more sophisticated 6- (something) potentials. The essential point is that the water molecule is attracted to the atoms of the solid by a sum of van der Waals pair interactions, and is repelled from the surface by rapidly varying short range forces which have their origins in the overlap of the outer regions of the electron clouds of the molecule and the surface atoms. This gives rise to the concept of a collision radius for an atom as that distance at which the energy of attraction is zero.

When fairly strong electrostatic binding occurs we can ignore the van der Waals contribution and assume that the adsorbent and adsorbate (the gas) are in contact right up to their collision radii.

The shape of a water molecule is shown in figure 1 and the calculated effective point charge distribution is shown in figure 2 [I, 21. This is the kind of infor- mation that we need for our molecular probe.

As for the solid we consider a particular crystal plane and assign collision radii to the surface atoms by adding 0.8 A to their covalent radii. Or in the case of ionic solids this comes to : collision radius of negative ions is equal to the ionic radius and for posi- tive ions is equal to the atomic radius.

Inductive effects are usually negligibly small.

Experimental results on PbTe. - The heat of adsorption of water, methanol [CH,OH] and diethyl ether [(C,H,),O] have been measured on the (100) surface of PbTe. The results are given in Table 1.

The value we have determined for krypton is included just to illustrate that there is nothing peculiar about PbTe : Kr-Ge is highly comparable being 0.15 eV3.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1968421

THE ADSORPTION OF VARIOUS GASES ON LEAD TELLURIDE C 4 - ¹⁴¹

0-collisio (1.38&;"d'

0-co-va nt rad. ^(0.584 a

H-collision rad. H-co-valent rad.

(0.608) (0.274 a

FIG. 1. - Size and shape of the Water Molecule

FIG. 2. - Duncan and Pople's Theoretical Model of a water molecule (1). Lone pair electrons, charge - 2 e at (- 0.158, f 0.275) in the x-z plane ; oxygen nucleus of charge

+ 6 e at the origin of the co-ordinate system; binding electrons of charge -2 e at (0.355, f 0.463) and protons of charge +

1 e at (0.586, f 0.764) in the x-y plane. (Distance in PI.)

Discussion of results. - We have calculated the maximum heat of adsorption (most favourable posi- tion on the surface) for water, methanol and diethyl

TABLE 1

Differential heat of adsorption at zero coverage on clean PbTe surfaces

ether, assuming an ionic model for PbTe (i. e.

Pb2'Te=). These results are given in Table 11. Compar- ing the calculated and experimental values would seem to rule out the ionic model for the reasons that : the required percentage ionic charge on lead telluride is unreasonably large and the ratios between the calculated heats are not in accord with experiment.

Substance

TABLE I1

Results of calculated adsorption energies on PbTe (Fully ionic model)

Heat of adsorption (eV per mole)

Electrostatic Van der Waals Total

1

1

1

1

1

We are part of the way through our calculations of a covalent PbTe. model and here we find much -better agreement between theory and experiment.

Preliminary indications are in favour of a model where the electrons are distributed around the surface atoms in octahedral symmetry with about 50%

of the electron in the bond and about 50% about the nucleus.

References

[I] DUNCAN (A.) and POPLE (J.), Trans. Faraday Soc., 1953, 49, 217.

[2] GLAESER (R. M.) and COULSEN (C. A.), Trans. Faraday Soc., 1965, 61, 389.

[~]_ROSENBERG ( A . J.), J. P h p . Chem., 1958, 62, 1112.