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Langmuir 1999, 15, 5790-5794
Mechanism of the Initial Stage of the Oxidation of the Clean and Precovered with Nonmetals Iron Surface† U. Narkiewicz* and W. Arabczyk Institute of Inorganic Chemical Technology, Technical University of Szczecin, Pulaskiego 10, 70-322 Szczecin, Poland Received August 31, 1998. In Final Form: May 12, 1999 The oxidation of the iron surface has been studied using the Auger electron spectroscopy method, particularly the intermediate state of the process proceedings between a monolayer of oxygen and an oxide layer. The oxidation of the clean iron surface has been compared to that of the iron surface precovered with carbon, sulfur, nitrogen, and phosphorus. The monolayer of carbon or nitrogen does not affect the rate of the oxidation process; however the monolayer of sulfur or phosphorus inhibits the oxidation. The mechanism of the oxidation stage between the monolayer of oxygen and the oxide layer has been elucidated taking into account the incorporation of oxygen atoms into the first iron layer. The concentration of defects in the bidimensional adsorbate layer has been taken into consideration. The number of surface defects in the 2D lattice of sulfur or phosphorus is 20 times lower than in the layer of oxygen.
Introduction The initial stage of the oxidation of a clean iron surface has been frequently studied by surface techniques and different results concerning the kinetics of this process has been obtained.1-18 The rate of the oxygen adsorption decreases with the surface coverage with oxygen. The characteristic minimum on the line illustrating the dependence of the sticking coefficient of oxygen versus the surface coverage was frequently found for different metal surfaces and interpreted as the result of the oxide phase nucleation.18 This minimum of the sticking coefficient of oxygen on the Fe(111) surface (obtained on the basis of the Auger peak intensity) observed for the surface coverage θO = 1 (the Fe(111)-p(1×1)-O structure) has been interpreted in another way in our earlier works19,20 taking into account the real structure of the monocrystalline sample and the diffusion of the chemisorbed oxygen † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998.
(1) Wandelt, K. Surf. Sci. Rep. 1982, 2, 1. (2) Lu, J.-P.; Albert, M. R.; Bernasek, S. L. Surf. Sci. 1989, 215, 348. (3) Leibbrandt, G. W. R.; Spiekman, L. H.; Habraken, F. H. P. M. Surf. Sci. 1993, 287-288, 250. (4) Smentkowski, V. S.; Yates, J. T., Jr. Surf. Sci. 1990, 232, 113. (5) Erley, W.; Ibach, H. Solid State Commun. 1981, 37, 937. (6) Viefhaus, H.; Grabke, H. J. Surf. Sci. 1981, 109, 1. (7) Sakisaka,Y.; Komeda, T.; Miyano, T.; Onchi, M.; Masuda, S.; Harada, Y.; Yagi, K.; Kato, H. Surf. Sci. 1985, 164, 220. (8) Miyano, T.; Sakisaka, Y.; Komeda, T.; Onchi, M. Surf. Sci. 1986, 169, 197. (9) Masuda, S.; Harada,Y.; Kato, H.; Komeda, T.; Miyano, T.; Onchi, M.; Sakisaka, Y. Phys. Rev. B 1988, 37, 8088. (10) Borgmann, D.; Kiessling, W.; Stadelman, M.; Wedler, G. Surf. Sci. 1991, 251/252, 831. (11) Jansson, C.; Morgan, P. Surf. Sci. 1990, 233, 84. (12) Daitzchman, C.; Aharoni, C.; Ungarisch, M. Surf. Sci. 1991, 244, 362. (13) Colaianni, M. L.; Chen, J. G.; Weinberg, W. H.; Yates, J. T., Jr. Surf. Sci. 1992, 279, 211. (14) Arabczyk, W.; Narkiewicz, U.; Kałucki, K.; Freidenberg, E. Appl. Surf. Sci. 1993, 72, 45. (15) Arabczyk, W.; Mu¨ssig, H.-J. Thin Solid Films 1976, 34, 102. (16) Stambouli, V.; Palacio, C.; Mathieu, H. J.; Landolt, D. Appl. Surf. Sci. 1993, 70/71, 240. (17) Hodgson, A.; Wight, A.; Worthy, G. Surf. Sci. 1994, 319, 119. (18) Wight, A.; Condon, N. G.; Leibsle, F. M.; Worthy, G.; Hodgson, A. Surf. Sci. 1995, 331-333, 133. (19) Arabczyk, W.; Narkiewicz, U. Czech. J. Phys. 1993, 43, 869. (20) Arabczyk, W. Appl. Surf. Sci. 1993, 68, 369.
atoms into the defects opened to the surface. The number of adsorption sites localized in the defects has been found to be about 50% of the number of the adsorption sites on the surface. Taking into account the real structure of the monocrystalline iron sample, the adsorption of oxygen molecule has been determined as the limiting step of the oxidation process until a monolayer is formed. The formation of the thick oxide layer could be described by the logarithmic, linear, or parabolic law,21 and for the case of the initial stage of the oxidation of metal surfaces at lower temperatures, the logarithmic law is the most appropriate. The mechanism of the transition stage between monolayer of oxygen and an oxide layer is not clear enough. The aim of this work is an attempt to explain this stage of the oxidation process, using the Auger electron spectroscopy (AES) method for the study of the oxidation of the clean iron surface and of the iron surface covered with nonmetals. Experimental Section The experiments were performed under ultrahigh vacuum (UHV) using a SIA 100 system of CAMECA (France). The basic pressure in the analysis chamber was maintained on the level of 10-8 Pa. The MAC 3 analyzer used in the experiments is a modified cylindrical mirror analyzer. The Auger electron spectra were registered with the primary electron energy of 2.5 keV and the primary beam current of 10 µA/cm2. The work function has been determined using the retarding field method22,23 (modified diode method). The monocrystalline Fe(111) samples were used in the experiments. The samples were small and thin (4 × 4 × 0.3 mm). The coverage of the sample with nonmetal atoms was obtained by various methods. Carbon and sulfur are present in the bulk as impurities and were segregated by annealing. After the annealing at 750 °C the segregation of sulfur occurred and carbon was completely dissolved in the bulk. The sample was cooled rapidly, and the segregation of carbon was negligible under these conditions. The surface concentration of carbon was very low. Therefore the (21) Schmalzried, H. Solid State Reactions; Verlag Chemie GmbH: Weinheim/Bergstr., 1974. (22) Chang, C. C. Ph.D. Dissertation, Cornell University, Ithaca, NY, 1967. (23) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 157.
10.1021/la981133p CCC: $18.00 © 1999 American Chemical Society Published on Web 08/07/1999
Oxidation on Iron surface was completely covered with sulfur. In order to obtain a clean surface, the sequence of sputtering with argon ions and annealing was performed. Each sputtering was followed by a short annealing at 200 °C to obtain the ordered surface structure with a small surface carbon concentration. The procedure of sulfur segregation at 460 °C, sputtering, and annealing at 200 °C was repeated in order to eliminate sulfur from the surface as well as from the defects opened to the surface (the details of these procedure are given in ref 24). The surface coverage with carbon was obtained by a long annealing time at 250-300 °C. The segregation of sulfur does not occur at this temperature. In order to obtain the N precovered surface, the annealing of the sample was carried out under the NH3/N2 flow at 500 °C (to increase the nitrogen concentration in the sample). Because of the excess of nitrogen in the sample, the segregation of carbon is negligible under these conditions. The segregated sulfur was removed from the surface by sputtering with N2+ ions. The segregation of nitrogen was carried out at 300 °C to obtain the surface coverage with atomic nitrogen, while the surface coverage with molecular nitrogen required a long segregation at 100 °C. The coverage with phosphorus was achieved by enrichment of the sample with phosphorus through the reaction between iron and phosphine (PH3) formed during phosphorus combustion in flowing hydrogen at 500 °C and atmospheric pressure in a glass tube reactor. One side of the sample after cooling was mechanically polished with abrasive paper followed by a treatment with a suspension of Al2O3 in ethanol, to remove the phosphorus. A relatively homogeneous distribution of phosphorus at a low concentration was achieved after annealing at 800 °C during 5 h in the UHV chamber. The segregation of sulfur occurred at this temperature. After the sample was cooled, sulfur was removed by sputtering with Ar. During the subsequent heating, the segregation of phosphorus began above 300 °C and the maximum phosphorus surface concentration was observed at 450 °C. At this temperature the segregation of sulfur began; hence a consegregation of these two elements, phosphorus and sulfur, took place. The determined ratio of P/S is obtained for a given temperature.25,26 It is possible to find the experimental conditions (temperature and heating rate) to have only one element (P or S) on the surface. At lower temperatures it is possible to achieve the coverage only with phosphorus; however at higher temperatures sulfur eliminates phosphorus. The surface generation process is fully reproduciblesafter sputtering with Ar followed by the segregation under similar conditions the similar results can be obtained. The oxygen exposure was performed under the pressure of 2.6 × 10-6 Pa (2 orders of magnitude higher than the basic pressure) at ambient temperature. Oxygen was supplied directly to the analysis chamber, where the sample under the study was located together with the pressure gauge. During the oxygen exposure the UHV chamber was pumped by means of the ion pump. The leak valve was located far from the ion pump.
Results and Discussion The intermediate state of the oxidation process has been studied using the AES method. The oxidation of the clean Fe(111) surface as well as of that surface precovered with carbon, sulfur, nitrogen, and phosphorus has also been studied. The degree of the iron surface coverage with nonmetal under the study, θX, is defined as the ratio of the number of nonmetal species (atoms or molecules) to the number of iron atoms in the monolayer, which corresponds to 7 × 1018 atoms/m2 on the Fe(111) surface. Thd detection limit for carbon, oxygen, and nitrogen was 0.1 monolayer (ML), whereas for sulfur and phosphorus the detection limit was 0.005 ML. (24) Narkiewicz, U. Appl. Surf. Sci. 1998, 134, 6. (25) Arabczyk, W.; Mu¨ssig, H. J.; Storbeck, F. In Studies in Surface Science and Catalysis; Koukal, J., Ed.; Elsevier: Amsterdam-OxfordNew York-Tokyo, 1988; Vol. 36, p 197. (26) Arabczyk, W.; Militzer, M.; Mu¨ssig, H. J.; Wieting, J. Surf. Sci. 1988, 198, 167.
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Figure 1. Oxygen-sticking coefficient on clean and covered iron surface versus oxygen coverage. Table 1 C
N
O
S
P
EA ) eV 273 381 510 148 120 APPH(X)θ)1/APPH(Fe703) 1.5 0.7 0.34 5.6 8.2 X
EX(WXY),
The sulfur coverage on the Fe(111) surface was calibrated on the basis of p(1×1)-S structure (confirmed by low energy electron diffraction (LEED) studies). This structure corresponds to the surface coverage θS ) 1,27,28 and on the Fe(111) surface it corresponds to 7 × 1018 atoms/ m2. Phosphorus29 and oxygen28 form the same p(1×1) structure on the Fe(111) surface as sulfur. On the basis of AES results for sulfur, the Auger intensities for carbon and nitrogen at the surface coverage θ ) 1 were estimated, assuming that the back scattering is the same for all elements under consideration, by the corresponding ratios of the ionization cross sections, φ, and the Auger electron yields, γ, according to:
APPH(X)θ)1 APPH(Fe)703
≈
(
APPH(S)1×1
)
APPH(Fe)703
φXγX exp φSγS
(1)
The estimated values were then confirmed through the segregation experiments for the nonmetals under the study, and similar values were found. The experimentally obtained relative Auger intensities of different elements (referred to the AES peak of iron) at the surface coverage θX ) 1 are given in Table 1. The dependence of the sticking coefficient of oxygen versus the oxygen exposure is shown in Figure 1. The sticking coefficient is defined as a ratio of the chemisorption rate to the frequency of all the collisions of the gas molecules wiith the surface. The sticking coefficient can be determined experimentally based on the rate of the surface coverage changes
s)
1 dθ nZ dt
(2)
where Z is the number of molecules colliding with the surface during the exposure time, t, and n is the number of atoms created after a dissociation of one molecule. Thus the sticking coefficient can be determined as a tangent to the curve representing the dependence of the surface coverage on the exposure time. (27) Storbeck, F.; Mu¨ssig, H.-J.; Arabczyk, W. Acta Univ. Vratisl. 1988, 561, 164. (28) Arabczyk, W.; Mu¨ssig, H.-J.; Storbeck, F. Phys. Status Solidi A 1979, 55, 437. (29) Arabczyk, W.; Baumann, T.; Storbeck, F.; Mu¨ssig, H. J. Surf. Sci. 1987, 189/190, 190.
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The changes of the sticking coefficient of oxygen on the clean iron surface have been previously described and interpreted.19 There are some differences between the oxidation of the iron surface with filled defect (line a) and that with empty defects (line b). The infinite periodicity of the crystal structure elements is possible only in the hypothetical ideal crystal; however, in reality the arrangement of atoms in the monocrystal sample is disturbed by the presence of defects. The symptom of the defects existence is a “mosaic structure”, composed with well-ordered blocks of the size of 10-7 m, turned one to another through a small angle (1-2°). The existence of the mosaic structure should influence the adsorption and segregation phenomena, because the sites for adsorption can be present not only on the surface but also in the defects (in the bulk of the sample or opened to the surface) accessible to the chemisorbed atoms. An empty defect is a defect in which the adsorption sites are not occupied. In the filled defects the sites are occupied by chemisorbed atoms. The effect of the mosaic structure on the segregation and adsorption phenomena has been reported by us previously.20,30 Line a in Figure 1 corresponds to the oxidation of the clean (or precovered with nitrogen or with carbon) surface when the defects opened to the surface are filled. A defect is considered as a space between mosaic blocks, as described above. The oxidation experiments were performed with the sample previously heated to 1070 K in order to achieve a uniform distribution of sulfur in the bulk and obtain the sulfur surface coverage θS ) 1. The sulfur was then removed from the surface using argon ion sputtering, but the adsorption sites in the defects opened to the surface were still occupied by the sulfur atoms, blocking the access to the oxygen atoms. Line b in Figure 1 corresponds to the oxidation of the clean (or precovered with nitrogen or with carbon) surface when the defects opened to the surface are empty. The empty defects were created by the cautious segregation of sulfur at lower temperature (460 °C). At this temperature only the sulfur atoms adsorbed in the defects opened to the surface undergo segregation onto the surface, since the temperature was not high enough for the segregation of sulfur atoms from the bulk. The experimental procedure was described in ref 24. The sulfur atoms adsorbed on the surface were removed through argon ion sputtering, as in a previous experiment. As in the case of the sample with filled defects, the preadsorption of nitrogen or carbon does not inhibit the rate of the oxidation. The monolayer of oxygen corresponds to the following ratio of the Auger peak to peak height [APPH(O)/APPH(Fe)] ) 0.34. The quantity of the adsorbed oxygen was determined based on the APPH values, and the sticking coefficient was calculated as the ratio of the quantity of adsorbed oxygen to the given oxygen exposure. The initial sticking coefficient obtained by the extrapolation of the experimental line to the coverage θO ) 0 equals unity, which is consistent with the previous literature information about the initial sticking coefficient of oxygen on iron surfaces.4,19,31,32 The oxidation of the iron surface precovered with a monolayer of nitrogen or carbon (lines a and b) occurs similarly as that of a clean iron surface and can be described (only at the beginning of the oxidation, up to θO (30) Arabczyk, W.; Narkiewicz, U. Surf. Sci. 1998, 402, 502. (31) Horgan, A. M.; King, D. A. Surf. Sci. 1970, 23, 259. (32) Margoninski, Y. Solid State Commun. 1975, 17, 373.
Narkiewicz and Arabczyk
Figure 2. Relation between oxygen-sticking coefficient and sulfur coverage.
= 0.9) by the change of the sticking coefficient expressed as follows
s ) s0(1 - θO)
(3)
where s0 ) 1. The monolayer of carbon or nitrogen does not affect the rate of the oxidation process at the initial stage. This is consistent with the recent results published by Roosendaal et al.,33 reporting the lack of influence of nitrogen adsorbed on the iron surface on the rate of oxidation and the localization of nitrogen atoms under the oxide layer. Contrary to carbon or nitrogen behavior, the surface coverage by sulfur or phosphorus lower than monolayer (θS,P ) 0-1) inhibits the oxidation. Line c in Figure 1 shows the dependence of the sticking coefficient of oxygen on the iron surface precovered with sulfur (θS ) 0.5). The sticking coefficient of the oxidation of the iron surface precovered with sulfur and/or phosphorus can be described by the equation
s ) s0(1 - θS,P - θO)
(4)
The minimum of the apparent sticking coefficient of oxygen is observed for θO = 1 in the case of the oxidation of the clean Fe(111) or covered with nitrogen or carbon surface (line b in Figure 1) and for θO + θS + θP = 1 for the surface precovered with sulfur and/or phosphorus when the defects opened to the surface are empty (this case is not shown in Figure 1). The oxidation in the range of 1 < θO < 2 results in decline of the oxidation rate, and with increasing oxygen coverage it becomes practically constant (no dependency on the oxygen coverage). For surface coverage by oxygen 1 < θO < 2 the oxidation rate of iron surface precovered with nonmetals decreases with increasing surface coverage with nonmetal. The dependence of the sticking coefficient of oxygen on the sulfur coverage is shown in Figure 2.14 When the iron surface is saturated with sulfur (θS ) 1), the sticking coefficient of oxygen equals 0.005. Similar results have been obtained for an iron surface saturated with phosphorus. The values of the sticking coefficients of oxygen obtained in the range 1 < θO < 2 were very small; therefore the important oxygen exposures were applied. The difference between S and P in this area is in the limit of the measurement accuracy. There is a significant difference between the behavior of the couples (S or P) and (C or N). When the concentration of carbon or nitrogen in the bulk of the iron sample is high enough, as a result of the segregation, the surface coverage with C234 or with N235 (33) Roosendaal, S. J.; Vredenberg, A. M.; Habraken, F. H. P. M. Surf. Sci. 1998, 402-404, 135.
Oxidation on Iron
may be obtained. The coverage with the molecular forms of N2 and C2 is possible to obtain at lower segregation temperature, while atomic forms are present at higher segregation temperatures. According to our former studies, the atomic-like states were found at lower nonmetallic concentrations whereas the molecular-like states were found at higher concentrations. Recently we described the molecular-like states and the corresponding changes in the AES spectra.34,36 The spectroscopic evidence for carbon-carbon binding in “carbidic” layers on metals was reported by Hutson et al.37 In the state of the surface saturation with nonmetal atoms one adsorption site might be occupied by one atom of sulfur or phosphorus (θS,θP ) 1) or by one molecule of carbon or nitrogen (θC2,θN2 ) 1). The oxidation of the iron surface covered with a monolayer of C2 or N2 (θC2,θN2 ) 1) occurs similarly as in the case of sulfur or phosphorus (θS,θP ) 1) precovered iron surface. The question is the reason of a significant difference in the oxidation process occurring up to θO ) 1 on the different surfaces (carbon and nitrogen does not inhibit the oxidation while sulphur and phosphorus inhibit this process). According to Pauling38 the electronegativity of sulfur is the same as that of carbon and equals 2.5 eV. If the difference in the oxidation is not affected by the electronegativity, it might be due to the geometrical arrangement of atoms. There are not direct experimental results confirming the existence of the subsurface nitrogen and carbon for the Fe(111) surface, but such a possibility may be supported by the results reported in the paper. Roosendaal et al. have demonstrated a lack of nitrogen effect on the oxidation of the Fe(110) surface and attributed this fact to the location of nitrogen under the oxide layer, on the oxide/Fe interface.33 The incorporation of nitrogen under the first layer of iron atoms should be still more probable on a less dense packed surface Fe(111). The EHT (extended Hu¨ckel theory) calculations performed for the underlayer and overlayer adsorption of oxygen, nitrogen, and carbon on the Fe(111) surface39 show that a hollow site in the non-reconstructed Fe(111) surface is the most preferential site for oxygen adsorption, whereas for nitrogen or carbon a hollow site below the iron atom is the preferential site. The transition from the adsorption on the non-reconstructed iron surface to such an adsorption state requires no activation. On the contrary the preferential sites for oxygen atoms are located on the surface. On the basis of this work we can assume that the atoms of carbon and nitrogen are located under the first layer of iron atoms and do not influence on the rate of the oxygen adsorption. Generally, there is an adsorption-induced surface reconstruction, and as a result of this reconstruction the sites for adsorption are not more blocked. Sulfur and phosphorus atoms in the iron lattice might be located as substitutional atoms but not as interstitial ones, as smaller carbon or nitrogen atoms. The heating of the iron crystal causes the segregation of sulfur and phosphorus, and at a coverage θS,P ) 1 the well-ordered surface structures are formed. According to LEED studies,28 the model of the surface structure Fe(111)p(1×1)-S (34) Arabczyk, W.; Storbeck, F.; Mu¨ssig, H.-J. Appl. Surf. Sci. 1993, 65/66, 94. (35) Arabczyk, W.; Mu¨ssig, H.-J. Vacuum 1987, 37, 137. (36) Arabczyk, W.; Moszyn´ski, D.; Narkiewicz, U. Vacuum, in press. (37) Hutson, F. L.; Ramaker, D. E.; Koel, B.E. Surf. Sci. 1991, 248, 104. (38) Pauling, L.; Pauling, P. Chemistry; W.H. Freeman and Co: San Francisco, CA, 1975. (39) Arabczyk, W.; Rausche, E.; Storbeck, F. Surf. Sci. 1991, 247, 264.
Langmuir, Vol. 15, No. 18, 1999 5793
has been proposed, in which sulfur atoms were directly bonded on the surface. The similar p(1×1) structures were proposed for phosphorus or oxygen adsorbed on the Fe(111) surface. In this way, the atoms of sulfur and phosphorus are localized on the Fe(111) surface and the atoms of nitrogen and carbon are incorporated under the first layer of the iron atoms. The mechanism of the oxidation process could be explained on the grounds of the surface geometry (arrangement of different atoms). The oxidation rate is limited by the formation of the adsorption states O2* on the Fe atom in the top layer. The nitrogen and carbon atoms do not influence on the oxidation rate due to their incorporation. The oxidation of the iron surface precovered with sulfur (θS ) 1) or with phosphorus (θP ) 1) with molecules of nitrogen (θN2 ) 1) or of carbon (θC2 ) 1), as well as the surface covered with a mixture of these species (θS + θP + θN2 + θC2 ) 1) occurs in the same manner. At the initial stage of the oxidation of the iron surface precovered with a monolayer of nonmetal, the sticking coefficient of oxygen equals 0.005. The intensities of the Auger peak of iron, carbon, nitrogen, phosphorus, and sulfur are damped as a the result of the oxidation. The dependence of the intensity of the AES Fe703 on the oxygen coverage can be described (in the range of low coverage) by the following linear expression:14
APPH(FeθO)x) APPH(FeθO)0)
)
1 1 + 0.26θO
(5)
The oxidation of the iron surface precovered with sulfur, phosphorus, nitrogen, or carbon causes the damping of the Auger peaks of these elements. For a thick layer of oxygen the AES peaks of all the nonmetal elements under the study disappear. For example, the Auger signal of sulfur disappears at an exposure of 105.38 The example of the coverage with nitrogen is reported in ref 35, and the discussion concerning the effect of damping is given in ref 40. The nonmetal atoms remaining at the oxide/metal interface might be an excellent marker for study of mass transport across the interface.33 The interactions between oxygen and nonmetal atoms have not been obseved at ambient temperature. If the geometrical aspect is responsible for the oxidation process, the sites for the adsorption of oxygen are different than the adsites occupied by the other nonmetal atoms. The coefficient s0 in eq 3 equals unity; in this way every collision of an oxygen molecule with a free adsorption site on the surface results in adsorption of this molecule. When the oxygen molecule strikes an occupied adsite, the adsorption does not occur. It can be assumed that the rate of adsorption at a constant pressure is determined by the number of the free adsites (Fe atoms). On the contrary, the number of the free adsites could be determined on the grounds of the adsorption rate measurements. When the iron surface is almost completely covered with nonmetals atom, the ordered bidimensional iron-nonmetal structure is created. The number of the free sites on the precovered iron surface is limited when the degree of surface coverage is close to unity; therefore every free adsite can be considered as the defects in the bidimensional crystalline lattice. There are the 2D defects, on the (40) Arabczyk, W.; Narkiewicz, U. Appl. Surf. Sci. 1997, 108, 379.
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Narkiewicz and Arabczyk
Figure 4. Relation between work function change and oxygen coverage. Figure 3. Schematic representation of the oxidation process mechanism.
contrary, with the 3D defects opened to the surface, which were described previously. When the monolayer of oxygen or sulfur or phosphorus is adsorbed on the iron surface, the subsequent oxidation may occur only through the exchange between iron atom and oxygen atom, which is shown schematically in Figure 3. As a result of the “switch” between an iron atom and an oxygen atom, the new adsorption site is created on the surface and this site can be considered as a defect in the bidimensionnal crystalline lattice of oxygen on the iron surface. If the oxygen molecule colliding with the surface strikes the 2D defects, the adsorption occurs. The total oxygen coverage θO can be expressed as a sum of the surface (θOo) and subsurface (θOu) oxygen coverage
θO ) θOo + θOu
(6)
The sticking coefficient of oxygen depending on the degree of free adsorption sites can be expressed as follows u
s ) s0(1 - θO + θO )
(7)
In the proximity of θO ) 1, the experimentally obtained sticking coefficient of oxygen on the clean iron surface s ) 0.1. Close to θO ) 1 the coverage of the adsorption sites in the defects, θOu becomes important, then the rate of the oxidation process is determined by the number of defects
created in the 2D lattice. Taking into account the above assumption about the estimation of the number of defects in the 2D lattice based on the oxidation rate measurements, hence the concentration of defects is about 10%. The sticking coefficient of oxygen (in the range of the oxygen surface coverage 0 < θO < 1) on the iron surface covered by sulfur (θS ) 1) equals 0.005 and is 20 times lower than on the iron surface covered with one monolayer of oxygen (s ) 0.1). The number of defects in the 2D lattice of sulfur should be in that case 20 times lower than in the layer of oxygen. The proposed mechanism of the oxidation of the iron surface might be supported by the measurements of changes of the work function, presented in Figure 4. The change in the work function increases linearly with increasing oxygen coverage up to θO ≈ 0.9, then an increase becomes less important, and when θO > 1, the decrease of the work function change is observed, as a consequence of the replacement of the oxygen atoms in the top layer by the iron atoms. Conclusions The mechanism of the oxidation stage between the monolayer of oxygen and the oxide layer can be postulated based on the number of defects in the two-dimensional crystalline lattice of nonmetals. The number of defects in the oxygen lattice is 20 times greater than that in other nonmetal lattices. LA981133P
