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>
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Submitted on 1 Jan 1968
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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 GREENand 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 essentially 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 especially 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 approximations. - 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 612 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 information 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 positive 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
0-collisio
(1.38&;"d'
a
TABLE1
Differential heat of adsorption at zero
coverage on clean PbTe surfaces
0-co-va nt rad.
(0.584
Heat of adsorption
(eV per mole)
Substance
H-collision rad.
(0.608)
ether, assuming an ionic model for PbTe (i. e.
Pb2'Te=). These results are given in Table 11. Comparing 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.
H-co-valent rad.
(0.274
a
FIG. 1. - Size and shape of the Water Molecule
1
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
6e 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.)
+
-
C 4 141
+
TABLEI1
Results of calculated adsorption energies on PbTe
(Fully ionic model)
substance
1
1
Electrostatic Van der Waals
binding
(ev)
energy (eV)
1 1
Total
ew
;: y
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.,
Discussion of results. - We have calculated the
maximum heat of adsorption (most favourable position on the surface) for water, methanol and diethyl
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.