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A mechanism for surface reconstruction at room temperature
SURFACE SCIENCE 18 (1969) 431-436 0 North-Holland Publishing Co., Amsterdam
A MECHANISM
FOR SURFACE
AT ROOM
RECONSTRUCTION
TEMPERATURE
Received 29 July 1969 According to the hypothesis of reconstructive adsorptionl), reactive foreign atoms adsorbing on a clean metal surface at 300°K can be spontaneously incorporated into a mixed superficial layer containing both foreign atoms as well as substrate host atoms. Some of the surface atoms of the substrate lattice become disconnected and are transported to new locations elsewhere on the surface. The displaced atoms are replaced by the reactive adsorbate atoms, giving rise to a 2-dimensional corrosion monolayer. This kind of 2-dimensional structure at a gas-metal interface need bear no resemblance at all to any of the well-known 3-dimensional bulk compounds of the same atoms, because of the quite different character of the bonding at low concentration of adsorbate. At high coverage, however, and following the development of simpler structures, recognizable 3-dimensional compounds certainly can form at room temperature, e.g. an oxide after adsorption of 0, on a Ni(ll0) surfacea). The purpose of this letter is to suggest a way by which an initially clean metal surface can be reconstructed at 300°K by strongly exothermic adsorption of very reactive gas molecules. Although many indirect arguments have been put forward to support the idea of surface reconstruction at room temperature, and considerable experimental evidence accumulated, the reconstruction point of view for reactive gas adsorption has not been supported by any proposal of how the metal shift their locations to other distant sites.
atoms
of the substrate
can
The new idea offered here is specifically suggested for highly exothermic adsorption, such as oxygen adsorption on metal surfaces. It is proposed that not all of the heat of adsorption of oxygen molecules is immediately carried away by the substrate lattice at the instant of dissociation into firmly bound atoms. Rather, and by analogy with gas phase reactions, a considerable fraction of the heat of adsorption is available to create excited “hot” oxide molecules which temporarily have high kinetic energy paralell to the surface. Traverse of these “hot” particles away from the site of their formation can account for transport of metal atoms over comparatively large distances. As they travel along, the “hot” molecules gradually leak 431
432
J. W.
MAY
away th.eir excess energy to the substrate, and come to rest.
until eventually
they are captured
In the case of a reconstructed half-monolayer we visualize formation of a hypothetical surface chain -M-O-M-O-M-Oin which every second metal atom M is replaced by an oxygen atom 0 at half coverage. Clearly, the replaced M atoms must be relocated if, as is assumed, the reconstructed film is only a single monolayer in thickness. How this could happen is indicated by two plausible mechanisms illustrated in fig. 1. These are shown in one dimension for simplicity, and are not intended to represent actual atomic sizes, actual atomic locations, actual repositionings or actual atomic motions. Rather they are to serve as general illustrations. Actual mechanisms will certainly be more complicated, and the crude sketches indicate only two of many possibilities. In the left hand example of fig. 1, a physisorbed 0, molecule first becomes
Fig. 1. (Left) Initiation of reconstruction at a step. A physisorbed oxygen molecule, after arrival at a position at the head of a line of atoms at the edge of a step, i.e. at a jog in the step (frame 1) becomes chemisorbed by reacting with two metal atoms. At the instant of changeover from the physisorbed molecular state to the chemisorb~d atomic state, a considerable fraction of the heat of adsorption is carried away from the site of dissociation by “hot” oxide molecules temporariiy having excess translational kinetic energy parallel to the surfaces. These “hot” molecules gradually leak away their excess energy and come to rest some distance away, where they create a region of a (2 x 1) reconstructed monolayer film (sequence l-3). Repetition (sequence 4-6) extends the film and the jog advances along the ledge. This sequence is illustrated in one dimension for simplicity and can easily be extended to two dimensions. (Right) Extension of a reconstructed island by place exchange. One atom of a physisorbed precursor 02 molecule exchanges with the underlying metal lattice and the other combines with this displaced metal atom and moves off as a “hot” oxide molecule (sequence l-3). Repetition extends the island and also creates a new island some distance away (sequence 4-6). The mechanism sketched corresponds to fig. 2~.
A MECHANISM
positioned
FOR SURFACE
RECONSTRUCTION
AT
ROOM TEMPERATURE
433
at the edge of a step one atom high. On a real surface this would
probably be a jog (or kink) position along a ledge. One notes that such steps occur reasonably frequently even on relatively perfect metal surfaces. At the moment of dissociative chemisorption, (MO)” hot oxide molecules are formed which move rapidly off, parallel to the surface and away from the ledge. If such species have translational kinetic energy E measured in eV, and are of grammolecular weight m, then their velocity u is equal to 1.37 x lo6 (E/m)* cmjsec. Taking as a typical example E= 1 eV and m= 100 g one sees that the translational speed parallel to the surface is comparable with that of H atoms having energy kT in a 2-dimensional atomic hydrogen gas at 300°K. Since H atoms adsorbed on surfaces at room temperature are often as mobile as a 2-dimensional gas, we can expect a prolonged motion of the ““hot” (MO)* molecules. This expectation is reinforced by the likelihood that dissipative coupling of the excess energy E to substrate phonons propagating into the crystal is rather difficult for such motion along the surface at right angles to the surface normal. In the right hand example of fig. 1, a physisorbed 0, molecule becomes chemisorbed at the edge of a (2 x 1) island. One of the 0 atoms is captured by the island after place exchange involving extraction of only one metal atom. This metal atom might well be more reactive than usual owing to its immediate proximity to the island edge. The other 0 atom moves off, again as hot (MO)*, in a direction towards a clean surface region. We next consider if the above proposals can be applied to adsorption of oxygen on tungsten. Tracy and Blakelys) have recently shown that 0, molecules enter into chemisorption on the clean surfaces of W(ll0) and W(112) by growth of islands of chemisorbed atoms. Oxygen molecules from the gas phase land on the surface and become bound in a precursor physisorbed state. The physisorbed O2 molecules diffuse rapidly over the surface. They become dissociated and che~z~~o~bed as atoms only when they contact the edge of a growing island of half monolayer structure (a half monolayer is the first structure to form by oxygen adsorption on both clean W(112) and W(110)). According to Tracy and Blakelya) these islands are unreconstructed and consist of a lattice of 0 atoms resting upon undisturbed substrate. However, it is important to realize that an island growth mechanism is not at all inconsistent with formation of reconstructed islands. Indeed, all of Tracy and Blakely’s data can agree with such a scheme. The plausible mechanisms illustrated in fig. 1 are certainly compatible with island growth. In the left hand mechanism, the islands are formed entirely by displaced “hot” oxide molecules, perhaps WO molecules in this case. In the right hand mechanism, original islands grow as well as new islands from displaced WO. There is an important distinction between these
434
J. W.
MAY
ideas and those of Tracy and Blakelys). They have assumed that atoms from a precursor physisorbed molecule become bound to the same island as the island that induced its dissociation. Their assumption is now seen to be too restrictive, and in the models of fig. 1, oxygen atoms end up quite far away from the dissociation site. One also notes that according to our second model in fig. 1, only one quarter of the metal substrate atoms need to be displaced to create a half monolayer over the entire surface. That the surface monolayer produced in this way rests on steps at different levels is immaterial, since steps are already present on the clean surface. Tracy and Blakelys) have objected to the reconstruction of W (110) by oxygen, as originally proposed by Germer, Stern and MacRaed), Germer and May5), and Gorodetskii, Melnik and YaskoG). They based their reasoning on the fact that atomic 0 is preponderantly desorbed at high temperature from W( 110) up to coverage of half a monolayer7). However, this fact is insufficient to rule out reconstructions). In support of the reconstruction view, we can cite the experiments of Brennan, Hayward and Trapnellg), who showed that adsorption of 0, on an evaporated W film is exothermic by 8.2 eV per molecule and that the heat of adsorption is essentially constant up to completion of a monolayer. These facts, plus other measurements of Tracy and Blakelylo) demonstrating a linear relationship between oxygen coverage and change of work function on W (112), show that as each 0 atom is bound, it enters into an environment identical to that of previously chemisorbed 0 atoms already inside islands. This deduction, of course, is equally compatible with reconstruction or no reconstruction. However, the additional fact that Tracy and Blakelyla) observed no depolarizing effect as the coverage of electro-negative oxygen increased seems to favor reconstruction, because of shielding of neighboring negatively charged 0 atoms by interposed W atoms. Finally we note that both the (110) and (112) surfaces of W are “smooth” for surface diffusion, and therefore the “hot” molecule mechanism can be acceptable. But the fifth densest (I 11) surface of W is “rough”. No ordered structures form at 300”K, and the LEED evidence of Taylorli) strongly suggests disordered reconstruction for 0, on W (111). This disorder can be a direct consequence of atomic roughness with high probability of de-excitation of “hot” oxide molecules, so that an ordered structure does not develop. Actually there is even some indication of the beginning of facetting into { 112) planes after exposure of W(111) to oxygen at room temperatureli). We close with a few general suggestions about the act of dissociation itself. In fig. 2 are shown some hypothetical “activated complexes” that could lead to the formation of the proposed “hot” oxide molecules. The first of these corresponds to the left hand mechanism of fig. 1. The second involves a
A MECHANISM
FOR SURFACE
RECONSTRUCTION
AT ROOM TEMPERATURE
435
(b)
W Fig. 2. Suggested activation mechanisms to translational kinetic energy along the surface. on a real surface). (b) simple place exchange. activated complex (4-member ring). (d) rotation of a
give “hot” oxide molecules having excess (a) reaction along a step (at a jog position (c) rotation of a cyclic, ring-like molecular Compare right hand part of fig. 1. 3-member ring.
simple place exchange, and the other two envisage rotation of a 4-member or 3-member ring (compare the right hand mechanism of fig. 1 with the sketch of fig. 2~). The activation energy for such processes can be rather small, perhaps negligible, if there is tunnelling across the potential barrier to the much lower energy of the chemisorbed state. JOHN W. MAY
Bartol Research Swarthmore,
Foundation
Pennsylvania,
of the Franklin Institute, U.S.A.
References 1) L. H. Germer and A. U. MacRae, Robt. A. Welch Foundation Res. Bull. No. 11 November (1961); H. E. Farnsworth and H. H. Madden, Jr., J. Appl. Phys. 32 (1961) 1933; L. H. Germer and A. U. MacRae, Proc. Natl. Acad. Sci. US 48 (1962) 997. 2) J. W. May and L. H. Germer, Surface Sci. 11 (1968) 443. 3) J. C. Tracy and J. M. Blakely, Surface Sci. 15 (1969) 257. 4) L. H. Germer, R. M. Stem and A. U. MacRae, in: Metal Surfaces (Amer. Sot. Metals, 1963). 5) L. H. Germer and J. W. May, Surface Sci. 4 (1966) 452. 6) D. A. Gorodetskii, Yu. P. Melnik and A. A. Yasko, Ukr. Fiz. Zh. 12 (1967) 970.
436
J.W.MAY
7) Yu. G. Ptushinskii and B. A. Chuikov, Fiz. Tver. Tela 10 (1968) 722; translated in Soviet Phys-Solid State 10 (I 968) 565. Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 6 (1967) 42. 8) N. P. Vas’ko, Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 14 (1969) 448. 9) D. Brennan, D. 0. Hayward and B. M. W. Trapnell, Proc. Roy. Sot. (London) A 256 (1960) 81. 10) J. C. Tracy and J. M. Blakely, in: The Structure and Chemistry of’ Solid Surfaces, Proc. Fourth Intern. Materials Symp., Berkeley, Calif., 1968, Ed. G. A. Somorjai (Wiley, New York, to be published). 11) N. J. Taylor, Surface Sci. 2 (1964) 544.
A MECHANISM
FOR SURFACE
AT ROOM
RECONSTRUCTION
TEMPERATURE
Received 29 July 1969 According to the hypothesis of reconstructive adsorptionl), reactive foreign atoms adsorbing on a clean metal surface at 300°K can be spontaneously incorporated into a mixed superficial layer containing both foreign atoms as well as substrate host atoms. Some of the surface atoms of the substrate lattice become disconnected and are transported to new locations elsewhere on the surface. The displaced atoms are replaced by the reactive adsorbate atoms, giving rise to a 2-dimensional corrosion monolayer. This kind of 2-dimensional structure at a gas-metal interface need bear no resemblance at all to any of the well-known 3-dimensional bulk compounds of the same atoms, because of the quite different character of the bonding at low concentration of adsorbate. At high coverage, however, and following the development of simpler structures, recognizable 3-dimensional compounds certainly can form at room temperature, e.g. an oxide after adsorption of 0, on a Ni(ll0) surfacea). The purpose of this letter is to suggest a way by which an initially clean metal surface can be reconstructed at 300°K by strongly exothermic adsorption of very reactive gas molecules. Although many indirect arguments have been put forward to support the idea of surface reconstruction at room temperature, and considerable experimental evidence accumulated, the reconstruction point of view for reactive gas adsorption has not been supported by any proposal of how the metal shift their locations to other distant sites.
atoms
of the substrate
can
The new idea offered here is specifically suggested for highly exothermic adsorption, such as oxygen adsorption on metal surfaces. It is proposed that not all of the heat of adsorption of oxygen molecules is immediately carried away by the substrate lattice at the instant of dissociation into firmly bound atoms. Rather, and by analogy with gas phase reactions, a considerable fraction of the heat of adsorption is available to create excited “hot” oxide molecules which temporarily have high kinetic energy paralell to the surface. Traverse of these “hot” particles away from the site of their formation can account for transport of metal atoms over comparatively large distances. As they travel along, the “hot” molecules gradually leak 431
432
J. W.
MAY
away th.eir excess energy to the substrate, and come to rest.
until eventually
they are captured
In the case of a reconstructed half-monolayer we visualize formation of a hypothetical surface chain -M-O-M-O-M-Oin which every second metal atom M is replaced by an oxygen atom 0 at half coverage. Clearly, the replaced M atoms must be relocated if, as is assumed, the reconstructed film is only a single monolayer in thickness. How this could happen is indicated by two plausible mechanisms illustrated in fig. 1. These are shown in one dimension for simplicity, and are not intended to represent actual atomic sizes, actual atomic locations, actual repositionings or actual atomic motions. Rather they are to serve as general illustrations. Actual mechanisms will certainly be more complicated, and the crude sketches indicate only two of many possibilities. In the left hand example of fig. 1, a physisorbed 0, molecule first becomes
Fig. 1. (Left) Initiation of reconstruction at a step. A physisorbed oxygen molecule, after arrival at a position at the head of a line of atoms at the edge of a step, i.e. at a jog in the step (frame 1) becomes chemisorbed by reacting with two metal atoms. At the instant of changeover from the physisorbed molecular state to the chemisorb~d atomic state, a considerable fraction of the heat of adsorption is carried away from the site of dissociation by “hot” oxide molecules temporariiy having excess translational kinetic energy parallel to the surfaces. These “hot” molecules gradually leak away their excess energy and come to rest some distance away, where they create a region of a (2 x 1) reconstructed monolayer film (sequence l-3). Repetition (sequence 4-6) extends the film and the jog advances along the ledge. This sequence is illustrated in one dimension for simplicity and can easily be extended to two dimensions. (Right) Extension of a reconstructed island by place exchange. One atom of a physisorbed precursor 02 molecule exchanges with the underlying metal lattice and the other combines with this displaced metal atom and moves off as a “hot” oxide molecule (sequence l-3). Repetition extends the island and also creates a new island some distance away (sequence 4-6). The mechanism sketched corresponds to fig. 2~.
A MECHANISM
positioned
FOR SURFACE
RECONSTRUCTION
AT
ROOM TEMPERATURE
433
at the edge of a step one atom high. On a real surface this would
probably be a jog (or kink) position along a ledge. One notes that such steps occur reasonably frequently even on relatively perfect metal surfaces. At the moment of dissociative chemisorption, (MO)” hot oxide molecules are formed which move rapidly off, parallel to the surface and away from the ledge. If such species have translational kinetic energy E measured in eV, and are of grammolecular weight m, then their velocity u is equal to 1.37 x lo6 (E/m)* cmjsec. Taking as a typical example E= 1 eV and m= 100 g one sees that the translational speed parallel to the surface is comparable with that of H atoms having energy kT in a 2-dimensional atomic hydrogen gas at 300°K. Since H atoms adsorbed on surfaces at room temperature are often as mobile as a 2-dimensional gas, we can expect a prolonged motion of the ““hot” (MO)* molecules. This expectation is reinforced by the likelihood that dissipative coupling of the excess energy E to substrate phonons propagating into the crystal is rather difficult for such motion along the surface at right angles to the surface normal. In the right hand example of fig. 1, a physisorbed 0, molecule becomes chemisorbed at the edge of a (2 x 1) island. One of the 0 atoms is captured by the island after place exchange involving extraction of only one metal atom. This metal atom might well be more reactive than usual owing to its immediate proximity to the island edge. The other 0 atom moves off, again as hot (MO)*, in a direction towards a clean surface region. We next consider if the above proposals can be applied to adsorption of oxygen on tungsten. Tracy and Blakelys) have recently shown that 0, molecules enter into chemisorption on the clean surfaces of W(ll0) and W(112) by growth of islands of chemisorbed atoms. Oxygen molecules from the gas phase land on the surface and become bound in a precursor physisorbed state. The physisorbed O2 molecules diffuse rapidly over the surface. They become dissociated and che~z~~o~bed as atoms only when they contact the edge of a growing island of half monolayer structure (a half monolayer is the first structure to form by oxygen adsorption on both clean W(112) and W(110)). According to Tracy and Blakelya) these islands are unreconstructed and consist of a lattice of 0 atoms resting upon undisturbed substrate. However, it is important to realize that an island growth mechanism is not at all inconsistent with formation of reconstructed islands. Indeed, all of Tracy and Blakely’s data can agree with such a scheme. The plausible mechanisms illustrated in fig. 1 are certainly compatible with island growth. In the left hand mechanism, the islands are formed entirely by displaced “hot” oxide molecules, perhaps WO molecules in this case. In the right hand mechanism, original islands grow as well as new islands from displaced WO. There is an important distinction between these
434
J. W.
MAY
ideas and those of Tracy and Blakelys). They have assumed that atoms from a precursor physisorbed molecule become bound to the same island as the island that induced its dissociation. Their assumption is now seen to be too restrictive, and in the models of fig. 1, oxygen atoms end up quite far away from the dissociation site. One also notes that according to our second model in fig. 1, only one quarter of the metal substrate atoms need to be displaced to create a half monolayer over the entire surface. That the surface monolayer produced in this way rests on steps at different levels is immaterial, since steps are already present on the clean surface. Tracy and Blakelys) have objected to the reconstruction of W (110) by oxygen, as originally proposed by Germer, Stern and MacRaed), Germer and May5), and Gorodetskii, Melnik and YaskoG). They based their reasoning on the fact that atomic 0 is preponderantly desorbed at high temperature from W( 110) up to coverage of half a monolayer7). However, this fact is insufficient to rule out reconstructions). In support of the reconstruction view, we can cite the experiments of Brennan, Hayward and Trapnellg), who showed that adsorption of 0, on an evaporated W film is exothermic by 8.2 eV per molecule and that the heat of adsorption is essentially constant up to completion of a monolayer. These facts, plus other measurements of Tracy and Blakelylo) demonstrating a linear relationship between oxygen coverage and change of work function on W (112), show that as each 0 atom is bound, it enters into an environment identical to that of previously chemisorbed 0 atoms already inside islands. This deduction, of course, is equally compatible with reconstruction or no reconstruction. However, the additional fact that Tracy and Blakelyla) observed no depolarizing effect as the coverage of electro-negative oxygen increased seems to favor reconstruction, because of shielding of neighboring negatively charged 0 atoms by interposed W atoms. Finally we note that both the (110) and (112) surfaces of W are “smooth” for surface diffusion, and therefore the “hot” molecule mechanism can be acceptable. But the fifth densest (I 11) surface of W is “rough”. No ordered structures form at 300”K, and the LEED evidence of Taylorli) strongly suggests disordered reconstruction for 0, on W (111). This disorder can be a direct consequence of atomic roughness with high probability of de-excitation of “hot” oxide molecules, so that an ordered structure does not develop. Actually there is even some indication of the beginning of facetting into { 112) planes after exposure of W(111) to oxygen at room temperatureli). We close with a few general suggestions about the act of dissociation itself. In fig. 2 are shown some hypothetical “activated complexes” that could lead to the formation of the proposed “hot” oxide molecules. The first of these corresponds to the left hand mechanism of fig. 1. The second involves a
A MECHANISM
FOR SURFACE
RECONSTRUCTION
AT ROOM TEMPERATURE
435
(b)
W Fig. 2. Suggested activation mechanisms to translational kinetic energy along the surface. on a real surface). (b) simple place exchange. activated complex (4-member ring). (d) rotation of a
give “hot” oxide molecules having excess (a) reaction along a step (at a jog position (c) rotation of a cyclic, ring-like molecular Compare right hand part of fig. 1. 3-member ring.
simple place exchange, and the other two envisage rotation of a 4-member or 3-member ring (compare the right hand mechanism of fig. 1 with the sketch of fig. 2~). The activation energy for such processes can be rather small, perhaps negligible, if there is tunnelling across the potential barrier to the much lower energy of the chemisorbed state. JOHN W. MAY
Bartol Research Swarthmore,
Foundation
Pennsylvania,
of the Franklin Institute, U.S.A.
References 1) L. H. Germer and A. U. MacRae, Robt. A. Welch Foundation Res. Bull. No. 11 November (1961); H. E. Farnsworth and H. H. Madden, Jr., J. Appl. Phys. 32 (1961) 1933; L. H. Germer and A. U. MacRae, Proc. Natl. Acad. Sci. US 48 (1962) 997. 2) J. W. May and L. H. Germer, Surface Sci. 11 (1968) 443. 3) J. C. Tracy and J. M. Blakely, Surface Sci. 15 (1969) 257. 4) L. H. Germer, R. M. Stem and A. U. MacRae, in: Metal Surfaces (Amer. Sot. Metals, 1963). 5) L. H. Germer and J. W. May, Surface Sci. 4 (1966) 452. 6) D. A. Gorodetskii, Yu. P. Melnik and A. A. Yasko, Ukr. Fiz. Zh. 12 (1967) 970.
436
J.W.MAY
7) Yu. G. Ptushinskii and B. A. Chuikov, Fiz. Tver. Tela 10 (1968) 722; translated in Soviet Phys-Solid State 10 (I 968) 565. Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 6 (1967) 42. 8) N. P. Vas’ko, Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci. 14 (1969) 448. 9) D. Brennan, D. 0. Hayward and B. M. W. Trapnell, Proc. Roy. Sot. (London) A 256 (1960) 81. 10) J. C. Tracy and J. M. Blakely, in: The Structure and Chemistry of’ Solid Surfaces, Proc. Fourth Intern. Materials Symp., Berkeley, Calif., 1968, Ed. G. A. Somorjai (Wiley, New York, to be published). 11) N. J. Taylor, Surface Sci. 2 (1964) 544.