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Comments on the dynamical properties of iron pentacarbonyl, Fe(CO)5, adsorbed on (0001) graphite
L 10
SURFACE
SCIENCE
LETTERS
COMMENTS ON THE DYNAMICAL PROPERTIES OF IRON PENTACARBONYL, Fe(CO),, ADSORBED ON (Oool) GRAPHITE *
and
Received
14 July 1981: accepted
for publication
16 September
1981
A fraction of a monolayer of Fe(CO)S was deposited on a clean Papvex stack following adsorption vapor pressure isotherm. Mbssbauer spectra for k, and perpendicular surface yield evidence spectrum suggests possible list of the molecular axis of -50” to the
The
dynamical
an
properties
studied and under interest. Monolayers ultimate two-dimensional structure, and Mossbauer spectroscopy powerful microscopic effects of on hyperfine interactions dynamics of molecules [l-5]. The monolayers are generally prepared by condensation from a gas phase. In this note, we report some preliminary results for an adsorbed film with a surface density of 0.8 monolayer of iron pentacarbonyl, Fe(CO),. The film was adsorbed on the (0001) plane of graphite in an exfoliated form called Papyex (Carbone Lorraine, France). The bulk Fe(CO), and its properties are well covered in the current literature 16-91, including a few Mossbauer studies. The motivation for this work is a recent report on Fe(CO), adsorbed on Grafoil, and its decomposition into a superparamagnetic Fe,O, [lo]. The Mossbauer results in this report indicate that for a one-monolayer coverage the Fe(CO), molecules show a considerably higher mean squared displacement along the substrate surface than perpendicular to it; above 180 K, the mobility of the surface molecules * Supported 1022.
in part by the Israel Academy
~39-6028/82/0000-~~/$02.75
of Sciences Foundation
For Rasic Research,
0 1982 North-Holland
Cirant Ko.
reduces the Mossbauer fraction to background level. The authors report however that no sharp transition is observed (which would be followed by a sharp reduction in the line intensity at a certain r,). We carefully checked this point in particular for a lower surface density, namely 0.8 monolayer. Other surface densities will be reported in detail elsewhere. The main experimental course we apply is detailed in ref. [3]; the Fe(CO), film was prepared by following a vapor pressure isotherm on an area-calibrated Papyex stack, after vacuum cleaning at 950°C. The iron pentacarbonyl is known to be sensitive to ambient conditions. It suffers, for example, of some degree of photolysis and radiolysis [9,11]. For this reason, stainless steel tubing with metallic valves and gaskets was used; the entire system was cleaned of grease, and it kept the Fe(CO), in complete darkness, For the same reason, the previous Mylar windows in the Mossbauer cell described in ref. [3] were replaced by beryllium windows. After the completion of the room temperature charging of the film, the sample was slowly cooled down to liquid nitrogen temperature, and only after this low temperature was reached, the sample was exposed to gamma rays. We used for the Mossbauer spectra a 25 mCi of 57Co: Pd source (Amersham). Generally the spectrum displays a doublet pattern (fig. 1). We ran spectra for this surface density for k, (the gamma wave vector) perpendicular and parallel to the Papyex sheets’surface, for various ambient temperatures.
-6
-4
-2
0
2
4
6
VELOCITY Fig. I. The k, follows due to
-6
-4
-2
0
2
4
6
(mm/set)
Miissbauer spectrum of iron pentacarbonyl adsorbed on Papyex. Density: 0.8 monolayer. was perpendicular (I) and parallel (11) to the surface of the Papyex sheets. The solid line a 3-Lorentzian computer best fit. The small peak between the components of the doublet is iron impurities in the beryllium windows. This part was not counted in the determination.
There are four apparent features of these spectra: (a) The doublet shows only a slight temperature-dependent observable anisotropy in parallel and perpendicular directions of observation, This fact indicates that the molecules may be settled in a particular preferred orientational order [9,13]: The Fe(CO), molecule has a large electric field gradient V_; as a result of the charge distribution arising from the bonds of the D?!, symmetry. It was shown [9] that this electric field gradient (EFG) is predominantly determined by the electronic states and symmetry of the isolated molecule. Disregarding the partial misalignment of the Papyex, we roughly estimated the general direction of the molecular (long) z-axis from the observed spectral asymmetry. It results in a list of the molecule from the normal of some (50 i- 5)“. A qualitative view is shown in fig. 2. (b) The quadrupole splitting is slightly smaller than that observed in the bulk. This effect was pointed out recently [I 11. It happens to arise from the displacement of the equatorial CO groups out of the equatorial plane. This results in a more nearly tetrahedral arrangement of the four CQ groups. The quadrupole splitting (T= 83 K) is 2.49 mm/s (as compared to - 2.57 mm/s for the bulk), thus somewhere between the trigonal bipyramid value and the smaller tetrahedral value [14]. This change is caused by the small Van der Waals interaction between the Fe(CO), molecule and the graphite which leaves the pentacarbonyl only slightly altered. (c) The spectral intensity in the direction of k, perpendicular to the surface is consistently larger than that for k, parallel to the surface. Points (a), (b) and (c) indicate that we have a physisorbed film: The adsorption energy co = mo2zi/27 (z. 2 R + c/2, R is the molecular radius of the Fe(CO), molecule, and c the graphite axis); w was obtained from the slope of the temperature-dependent logarithm of the spectral intensity [ 11. R _- 3.76 A (from bulk liquid density and from vapor pressure isotherms). We obtain co - 7.86 kcal/mole. This value is close to the available heat of vaporization AH = 9.59 kcal/mole [ 171. Previously studied samples such as butadiene iron tricarbonyl and tetramethyl tin on graphite showed co - 20 and 7.1 kcal/mole respectively. (d) The spectral intensity versus temperature and direction: fig. 2 illustrates IS,“=,,,(T) (the on resonance cross section for k, perpendicular to the Papyex sheets), and ud’-,,,( T) (for the k vector parallel to these sheets). It is apparent that there is a substantial and sharp drop in the spectral intensity at T, - 120 K: Au I’/u. - 30%. Based on a theory published elsewhere [3], one could think that these irregularities are the result of a two-dimensional melting, and that T,,,(bulk) is known to be 252 K Gl - 120 K is the surface melting temperature. and T,(2D)/T,(3D)-0.5 is a reasonable ratio found in other 2D melting processes [15]. However, if such were the case, one would expect the value of k, parallel to go to almost zero above the transition temperature and in fig. 3, it clearly does not. A more satisfactory explanation would be the occurrence of a commensurate-incommensurate transition which may affect the line intensity through the DebyeeWalIer factor.
H.
Shechfer
er nl. / Qsnumicul proper&s
of Fe(CO).,
Fe( CO), Fe-0 c -* 0 -0
1 60
Fig. 2. Qualitative view of iron pentacarbonyl: The average R c_o = I. 13 A, The details are given in ref. [ I8].
I
80
,
100
distances
120 T(K)
140
160
180
are: RFe_C 2- 1.82 A and
Fig. 3. On resonance intensity of the Miissbauer lines obtained from the normalized area under the Lorentzians for surface density 0.8. for k, perpendicular (_L) and parallel (II) to the planes of the Papyex sheets.
The fact that no gradual broadening is observed brings us to believe that the transition is of the first order. It may be possible that partial decomposition and contaminated surface could impede the sharp 2D transition and this prevented its observation in the reported experiment in ref. [IO]. It may also well be that at the coverage studied in this reference (= one monolayer) such a transition does not exist any more. We further study surface concentration, to be reported soon [ 161, and intend to perform neutron diffraction experiments in order to determine the nature of the transition as well as the structure of the 2D solid(s). We wish to thank D. Katmor for his assistance in performing the reported measurements. We are also indebted to N. Lupu for his design of the electronic hardware and software.
References [I ] (21 [3] [4]
H. H. H. B.
Shechter, J.G. Dash, M. Mor, R. Ingalls and S. Rukshpan, Phys. Rev. B14 (1976) Shechter. J. Physique 38, C4 (1977) 38. Shechter. J. Suzanne arid J.G. Dash, J. Physique 40 (1979) 467. Bukshpan, T. Sonnino and J.G. Dash, Surface Sci. 52 (1975)466.
1876.
[5] [h] [7] [X]
H. Shechter. R. Brener and J. Suzanne, Phys. Rev. Letters 45 (19X0) 1639. M. Kalvius, V. Zahn. P. Kienle and H. Either. 2. Naturforsch. I7a ( 1962) 494 E. Fluck. W. Kerler and W. Neuwirth. Angew. Chem (Engl. Ed.) 2, 277. T.G. Gibb, R. Greatrex. N.N. Greenwood and D.T. Thomson, J. Chcm. Sot. A (1967) 1663: (196X) X40. [9] P. Kuhn, U. Haufer and N Neuwirth. Z. Physik 264 (1973) 2X7. [IO] D.G. Howard and R.H. Nussbaum. Surface Sci. 93 (19X0) LlO5. [I I] J. Phillips. B. Clausen and J.A. Dumeaic. J. Phys. Chem X4 (19X0) 1X14. [I21 H.E. Carlton and J.H. Oxley. AIChE J. I I (1965) 79. [ 131R.S. Bukshpan and G.J. Kemerink. J. Chem. Phys. 72 (19X0) 4767. [I41 R.H. Herber, W.R. Ringaton and G.K. Wertheim. Inorg. Chcm. 2 (1963) 153. [I51 J.G. Dash, in: Film on Solid Surfaces (Academic Press. New York. 1975). [I61 H. Shechter. R. Brener and J. Suzanne, to be published. [ 171 A.G. Gilbert and K.G.P. Sulzman, J. Electrochem. Sot. 121 (1974) X32. [IX] L.H. Jones, R.S. MC Dowell. M. Goldblatt and B.I. Swanson. J. Chem. Phys. 57 (1972) 2050
SURFACE
SCIENCE
LETTERS
COMMENTS ON THE DYNAMICAL PROPERTIES OF IRON PENTACARBONYL, Fe(CO),, ADSORBED ON (Oool) GRAPHITE *
and
Received
14 July 1981: accepted
for publication
16 September
1981
A fraction of a monolayer of Fe(CO)S was deposited on a clean Papvex stack following adsorption vapor pressure isotherm. Mbssbauer spectra for k, and perpendicular surface yield evidence spectrum suggests possible list of the molecular axis of -50” to the
The
dynamical
an
properties
studied and under interest. Monolayers ultimate two-dimensional structure, and Mossbauer spectroscopy powerful microscopic effects of on hyperfine interactions dynamics of molecules [l-5]. The monolayers are generally prepared by condensation from a gas phase. In this note, we report some preliminary results for an adsorbed film with a surface density of 0.8 monolayer of iron pentacarbonyl, Fe(CO),. The film was adsorbed on the (0001) plane of graphite in an exfoliated form called Papyex (Carbone Lorraine, France). The bulk Fe(CO), and its properties are well covered in the current literature 16-91, including a few Mossbauer studies. The motivation for this work is a recent report on Fe(CO), adsorbed on Grafoil, and its decomposition into a superparamagnetic Fe,O, [lo]. The Mossbauer results in this report indicate that for a one-monolayer coverage the Fe(CO), molecules show a considerably higher mean squared displacement along the substrate surface than perpendicular to it; above 180 K, the mobility of the surface molecules * Supported 1022.
in part by the Israel Academy
~39-6028/82/0000-~~/$02.75
of Sciences Foundation
For Rasic Research,
0 1982 North-Holland
Cirant Ko.
reduces the Mossbauer fraction to background level. The authors report however that no sharp transition is observed (which would be followed by a sharp reduction in the line intensity at a certain r,). We carefully checked this point in particular for a lower surface density, namely 0.8 monolayer. Other surface densities will be reported in detail elsewhere. The main experimental course we apply is detailed in ref. [3]; the Fe(CO), film was prepared by following a vapor pressure isotherm on an area-calibrated Papyex stack, after vacuum cleaning at 950°C. The iron pentacarbonyl is known to be sensitive to ambient conditions. It suffers, for example, of some degree of photolysis and radiolysis [9,11]. For this reason, stainless steel tubing with metallic valves and gaskets was used; the entire system was cleaned of grease, and it kept the Fe(CO), in complete darkness, For the same reason, the previous Mylar windows in the Mossbauer cell described in ref. [3] were replaced by beryllium windows. After the completion of the room temperature charging of the film, the sample was slowly cooled down to liquid nitrogen temperature, and only after this low temperature was reached, the sample was exposed to gamma rays. We used for the Mossbauer spectra a 25 mCi of 57Co: Pd source (Amersham). Generally the spectrum displays a doublet pattern (fig. 1). We ran spectra for this surface density for k, (the gamma wave vector) perpendicular and parallel to the Papyex sheets’surface, for various ambient temperatures.
-6
-4
-2
0
2
4
6
VELOCITY Fig. I. The k, follows due to
-6
-4
-2
0
2
4
6
(mm/set)
Miissbauer spectrum of iron pentacarbonyl adsorbed on Papyex. Density: 0.8 monolayer. was perpendicular (I) and parallel (11) to the surface of the Papyex sheets. The solid line a 3-Lorentzian computer best fit. The small peak between the components of the doublet is iron impurities in the beryllium windows. This part was not counted in the determination.
There are four apparent features of these spectra: (a) The doublet shows only a slight temperature-dependent observable anisotropy in parallel and perpendicular directions of observation, This fact indicates that the molecules may be settled in a particular preferred orientational order [9,13]: The Fe(CO), molecule has a large electric field gradient V_; as a result of the charge distribution arising from the bonds of the D?!, symmetry. It was shown [9] that this electric field gradient (EFG) is predominantly determined by the electronic states and symmetry of the isolated molecule. Disregarding the partial misalignment of the Papyex, we roughly estimated the general direction of the molecular (long) z-axis from the observed spectral asymmetry. It results in a list of the molecule from the normal of some (50 i- 5)“. A qualitative view is shown in fig. 2. (b) The quadrupole splitting is slightly smaller than that observed in the bulk. This effect was pointed out recently [I 11. It happens to arise from the displacement of the equatorial CO groups out of the equatorial plane. This results in a more nearly tetrahedral arrangement of the four CQ groups. The quadrupole splitting (T= 83 K) is 2.49 mm/s (as compared to - 2.57 mm/s for the bulk), thus somewhere between the trigonal bipyramid value and the smaller tetrahedral value [14]. This change is caused by the small Van der Waals interaction between the Fe(CO), molecule and the graphite which leaves the pentacarbonyl only slightly altered. (c) The spectral intensity in the direction of k, perpendicular to the surface is consistently larger than that for k, parallel to the surface. Points (a), (b) and (c) indicate that we have a physisorbed film: The adsorption energy co = mo2zi/27 (z. 2 R + c/2, R is the molecular radius of the Fe(CO), molecule, and c the graphite axis); w was obtained from the slope of the temperature-dependent logarithm of the spectral intensity [ 11. R _- 3.76 A (from bulk liquid density and from vapor pressure isotherms). We obtain co - 7.86 kcal/mole. This value is close to the available heat of vaporization AH = 9.59 kcal/mole [ 171. Previously studied samples such as butadiene iron tricarbonyl and tetramethyl tin on graphite showed co - 20 and 7.1 kcal/mole respectively. (d) The spectral intensity versus temperature and direction: fig. 2 illustrates IS,“=,,,(T) (the on resonance cross section for k, perpendicular to the Papyex sheets), and ud’-,,,( T) (for the k vector parallel to these sheets). It is apparent that there is a substantial and sharp drop in the spectral intensity at T, - 120 K: Au I’/u. - 30%. Based on a theory published elsewhere [3], one could think that these irregularities are the result of a two-dimensional melting, and that T,,,(bulk) is known to be 252 K Gl - 120 K is the surface melting temperature. and T,(2D)/T,(3D)-0.5 is a reasonable ratio found in other 2D melting processes [15]. However, if such were the case, one would expect the value of k, parallel to go to almost zero above the transition temperature and in fig. 3, it clearly does not. A more satisfactory explanation would be the occurrence of a commensurate-incommensurate transition which may affect the line intensity through the DebyeeWalIer factor.
H.
Shechfer
er nl. / Qsnumicul proper&s
of Fe(CO).,
Fe( CO), Fe-0 c -* 0 -0
1 60
Fig. 2. Qualitative view of iron pentacarbonyl: The average R c_o = I. 13 A, The details are given in ref. [ I8].
I
80
,
100
distances
120 T(K)
140
160
180
are: RFe_C 2- 1.82 A and
Fig. 3. On resonance intensity of the Miissbauer lines obtained from the normalized area under the Lorentzians for surface density 0.8. for k, perpendicular (_L) and parallel (II) to the planes of the Papyex sheets.
The fact that no gradual broadening is observed brings us to believe that the transition is of the first order. It may be possible that partial decomposition and contaminated surface could impede the sharp 2D transition and this prevented its observation in the reported experiment in ref. [IO]. It may also well be that at the coverage studied in this reference (= one monolayer) such a transition does not exist any more. We further study surface concentration, to be reported soon [ 161, and intend to perform neutron diffraction experiments in order to determine the nature of the transition as well as the structure of the 2D solid(s). We wish to thank D. Katmor for his assistance in performing the reported measurements. We are also indebted to N. Lupu for his design of the electronic hardware and software.
References [I ] (21 [3] [4]
H. H. H. B.
Shechter, J.G. Dash, M. Mor, R. Ingalls and S. Rukshpan, Phys. Rev. B14 (1976) Shechter. J. Physique 38, C4 (1977) 38. Shechter. J. Suzanne arid J.G. Dash, J. Physique 40 (1979) 467. Bukshpan, T. Sonnino and J.G. Dash, Surface Sci. 52 (1975)466.
1876.
[5] [h] [7] [X]
H. Shechter. R. Brener and J. Suzanne, Phys. Rev. Letters 45 (19X0) 1639. M. Kalvius, V. Zahn. P. Kienle and H. Either. 2. Naturforsch. I7a ( 1962) 494 E. Fluck. W. Kerler and W. Neuwirth. Angew. Chem (Engl. Ed.) 2, 277. T.G. Gibb, R. Greatrex. N.N. Greenwood and D.T. Thomson, J. Chcm. Sot. A (1967) 1663: (196X) X40. [9] P. Kuhn, U. Haufer and N Neuwirth. Z. Physik 264 (1973) 2X7. [IO] D.G. Howard and R.H. Nussbaum. Surface Sci. 93 (19X0) LlO5. [I I] J. Phillips. B. Clausen and J.A. Dumeaic. J. Phys. Chem X4 (19X0) 1X14. [I21 H.E. Carlton and J.H. Oxley. AIChE J. I I (1965) 79. [ 131R.S. Bukshpan and G.J. Kemerink. J. Chem. Phys. 72 (19X0) 4767. [I41 R.H. Herber, W.R. Ringaton and G.K. Wertheim. Inorg. Chcm. 2 (1963) 153. [I51 J.G. Dash, in: Film on Solid Surfaces (Academic Press. New York. 1975). [I61 H. Shechter. R. Brener and J. Suzanne, to be published. [ 171 A.G. Gilbert and K.G.P. Sulzman, J. Electrochem. Sot. 121 (1974) X32. [IX] L.H. Jones, R.S. MC Dowell. M. Goldblatt and B.I. Swanson. J. Chem. Phys. 57 (1972) 2050