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Thermal decomposition of acetylene on Pt(1 1 1) studied by scanning tunneling microscopy
Surface Science 514 (2002) 414–419 www.elsevier.com/locate/susc
Thermal decomposition of acetylene on Pt(1 1 1) studied by scanning tunneling microscopy Osamu Nakagoe, Noriaki Takagi *, Yoshiyasu Matsumoto Department of Photoscience, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan Received 1 October 2001; accepted for publication 2 February 2002
Abstract The adsorption and the thermal decomposition processes of C2 H2 on Pt(1 1 1) have been investigated by the use of scanning tunneling microscopy. For a high C2 H2 coverage, a (2 2) structure is observed locally after the C2 H2 -covered Pt(1 1 1) surface is heated to 120 K. By heating to 220 K, the sample surface is covered with the (2 2) structure separated with brighter lines. After heating up to 370 K, (2 2) domains related to ethylidyne species appear with larger and brighter protrusions. For a low C2 H2 coverage, adsorbates are located randomly and no ordered islands are observed after heating to 120 K. In contrast, heating to 220 K leads to the formation of (2 2) islands. This indicates that the adsorbate–adsorbate interaction in the (2 2) structure observed by 120 K heating is different from that in the (2 2) structure observed by 220 K heating; the former is repulsive, but the latter attractive. Therefore, it is suggested that different surface species are formed by the decomposition of C2 H2 at 220 K. The detail of the decomposition processes of C2 H2 and the relation between the adsorbate–adsorbate interaction and the surface species are discussed. The results are also reported measured by heating up to higher temperatures above 400 K. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Chemisorption; Surface chemical reaction; Alkynes; Platinum
1. Introduction The interaction of transition metal surfaces with hydrocarbons is of importance, not only from the scientific viewpoint, but also from the technological applications in connection with heterogeneous catalysis. The adsorption and the thermal reactions of C2 H2 on Pt(1 1 1) have been extensively investigated as a prototype of catalytic reactions
*
Corresponding author. Fax: +81-468-58-1544. E-mail address: [email protected] (N. Takagi).
[1–12]. Annealing a Pt(1 1 1) surface covered with C2 H2 at 400 K leads to an ordered overlayer structure showing a (2 2) pattern in low energy electron diffraction (LEED) [1,2,6]. In this overlayer structure, ethylidyne (CCH3 ) species are formed by the thermal decomposition of C2 H2 [5,6]. Surface vibrational spectroscopy has been applied to investigate the thermal decomposition processes of C2 H2 [4,10,12]. From high resolution electron energy loss spectroscopy (HREELS) studies, adsorbed C2 H2 species are thermally intact below 300 K and are converted to CCH3 at 330– 400 K [4,10]. On the other hand, from the infrared–visible sum frequency generation (SFG) work
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O. Nakagoe et al. / Surface Science 514 (2002) 414–419
of Cremer et al. [12], C2 H2 rearranges by intramolecular 1–2 shift of H to form vinylidene (CCH2 ) even at 125 K, and the reaction pathway for the decomposition of C2 H2 is proposed as follows: CCH2 is converted to vinyl (CHCH2 ) at 190 K, then ethylidene (CHCH3 ) is formed by H addition to CHCH2 at 339 K, and finally CCH3 is formed by the dehydrogenation of CHCH3 in 356– 381 K. Since a disproportionation reaction is required for the formation of CCH3 from C2 H2 , it is also proposed that hydrogen poor species, probably ethynyl (CCH), must be formed simultaneously to provide a hydrogen necessary to produce CCH3 . However, CCH species has not been identified yet. For these disproportionation reactions, the possibilities of the carbon atoms desolving into bulk [13] and the hydrogenation from the residual H2 [4] instead of the ethynyl formation are ruled out. In this work, we have investigated the decomposition processes of C2 H2 on the Pt(1 1 1) surface by using variable temperature scanning tunneling microscopy (STM). We show that (2 2) ordered overlayer structures of decomposed species of C2 H2 are formed by heating to 120 and 220 K. Although the (2 2) periodicity is common in the two structures, the nature of adsorbate–adsorbate interactions responsible for them is different: repulsive interaction for the (2 2) structure observed by 120 K heating while attractive interaction for the (2 2) structure observed by 220 K heating.
2. Experimental All the experiments were carried out in an ultrahigh-vacuum (UHV) chamber equipped with a variable temperature STM (Omicron VT STM). The base pressure of the UHV system was below 1 1010 Torr. For STM measurements, sample temperature can be changed in the STM stage by adjusting a liquid-He flow rate and balancing with the use of a heater. Chemically etched W wires were used for the tips that were cleaned by Arþ -ion sputtering and annealing. All STM images were acquired in a constant current mode with tunneling current of
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0.1–5 nA and typical bias voltages of 5 mV to 1 V applied to the tip. A clean Pt(1 1 1) surface was prepared by many cycles of Arþ -ion sputtering, annealing at 1000 K, oxidation, and flashing at 1000 K. Sample cleanness was checked by STM. Research grade C2 H2 gas was introduced through a variable leak valve. The fractional coverage of C2 H2 , hC2 H2 , was estimated by counting the number of protrusions in STM images after exposing the Pt(1 1 1) surface to C2 H2 at 30 K (hC2 H2 ¼ 1 is defined as 1:505 1015 molecules/cm2 ). The saturation coverage of C2 H2 is estimated to be 0.25 [11].
3. Results and discussion Fig. 1(a) shows an STM image of the Pt(1 1 1) surface exposed to C2 H2 at 30 K (hC2 H2 ¼ 0.2), and Fig. 1(b) and (c) show STM images of the same surface subsequently heated to 120 and 220 K, respectively. These STM images are acquired at 30 K. Each image is not obtained for the same region because of the thermal drift during the heating–cooling process. In Fig. 1(a), protrusions of 0.01–0.02 nm in height and about 0.5 nm in diameter are distributed randomly. HREELS studies for C2 H2 on Pt(1 1 1) report that C2 H2 is adsorbed molecularly by the di-r=p bonding interaction, and is located in a threefold hollow site [4,10]. In STM measurements for C2 H2 on Pd(1 1 1) of Dunphy et al. [14], the di-r=p bonded C2 H2 species is observed as a protrusion of 0.01–0.02 nm in height and 0.4– 0.5 nm in diameter. Thus, these protrusions are associated with di-r=p bonded C2 H2 . When the surface is heated to 120 K, the ordered (2 2) periodicity appears locally as shown in Fig. 1(b). By further heating to 220 K, the surface is covered with the (2 2) structure, together with brighter lines (Fig. 1(c)). For larger scanned area such as 50 50 nm2 , bear surface regions are observed (not shown). The (2 2) structures observed by heating to 120 and 220 K are denoted as a1 and a2 , respectively. The brighter lines are domain boundaries where adsorbates are more compressed than in the regular (2 2) region. Since the compression of the adsorbates
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Fig. 1. (a) An STM image obtained at 30 K after Pt(1 1 1) is exposed to C2 H2 at 30 K (hC2 H2 ¼ 0.2) (22:5 22:5 nm2 , 0.4 V, 0.2 nA). STM images (b) (22:5 22:5 nm2 , 0.9 V, 0.2 nA) and (c) (20 20 nm2 , 0.5 V, 0.2 nA) taken at 30 K after the same surface as (a) was heated to 120 and 220 K, respectively. (d) An STM image taken at room temperature after the Pt(1 1 1) surface was saturated with C2 H2 at room temperature and then was heated to 370 K (25 25 nm2 , 1.0 V, 0.5 nA).
leads to the increase of the electronic density and thus the increase of the total electronic density at the boundaries, they are imaged as brighter lines. Note that the total electronic density determines STM images to the first order [15]. A similarly increased imaging height has been observed in STM images of Au on Ru(0 0 0 1), where a Au(1 1) layer is compressed [16]. Not only the electronic factor but also the geometrical factor may be involved. The adsorbates in the compressed region are pushed up slightly to reduce the lateral adsorbate–adsorbate interaction, and thus the boundaries are imaged more brightly.
The STM image of the CCH3 -induced (2 2) structure is shown in Fig. 1(d), which is acquired at room temperature after the sample is exposed to C2 H2 at room temperature and is subsequently heated to 370 K. Not only the (2 2) regions but also larger and brighter structures are observed. Since the formation of CCH3 species is accompanied with dehydrogenated hydrocarbon species as described above, the larger and brighter structures may be attributed to these hydrocarbons or their clusters. In order to elucidate the adsorbate–adsorbate interaction for the a1 and a2 phases, STM
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Fig. 2. STM images (a) (18 18 nm2 , 90 mV, 0.6 nA) and (b) (20 20 nm2 , 90 mV, 0.5 nA) taken at 30 K after the C2 H2 covered Pt(1 1 1) surface (hC2 H2 ¼ 0.04) was heated to 120 and 220 K, respectively.
measurements were made in the low coverage region (hC2 H2 ¼ 0.04), and results are shown in Fig. 2. For 120 K heating, adsorbates are located randomly and no islands with ordered structures are observed as shown in Fig. 2(a). On the other hand, islands with the (2 2) structure appear for 220 K heating (Fig. 2(b)). These results suggest that the driving force of the a1 phase is repulsive interaction between adsorbates and that of the a2 phase is attractive interaction. There is a possibility that
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the surface does not reach the equilibrium if the mobility of the adsorbate is very low at 120 K. For higher coverage, the disordered structure (Fig. 1(a)) is converted to the structure with the local (2 2) regions (Fig. 1(b)) after heating to 120 K, which is accompanied by the migration of the adsorbate. Although the STM measurements are not performed at 120 K, this suggests that the adsorbate is mobile at 120 K. For C2 H2 on Pd(1 1 1), the diffusion barrier of 180 meV is obtained [14]. Assuming that the barrier for the adsorbate is similar to this value and that the preexponential factor is the order of 1012 Hz, the lifetime at one site is estimated to be the order of 103 s. Thus, it is considered that the STM image shown in Fig. 2(a) reflects the mutual interaction between the adsorbates rather than the kinetics effects. According to a number of theoretical models, the adsorbate–adsorbate interaction is classified into two groups: one is the direct interaction (i.e., electrostatic interaction and overlap of adsorbate electronic orbitals), and the other is the indirect interaction through metal surfaces mediated by the lattice deformation and the valence electrons [17,18]. For most adsorption systems, these interactions contribute to the total adsorbate– adsorbate interaction in a complicated manner, and it is difficult to separate them. Furthermore, the indirect interaction changes the nature from repulsive to attractive depending on the mutual distance between adsorbates. Thus, we discuss favorable candidates of the interactions (especially operative between nearest neighbors) for the a1 and a2 phases below. According to the reaction pathway proposed by Cremer et al., the dominant surface species is vinylidene in the a1 phase. From the work function change of 1.42 eV after the adsorption of C2 H2 at 120 K, the dipole moment of the vinylidene species is estimated to be 1.45 D [9]. Thus, it is considered that the dipole–dipole repulsive interaction between the vinylidene species is the driving force of the a1 phase observed only for high coverages. The formation of islands is energetically unfavorable, and thus there exist no islands for low coverages. For the a2 phase, it is probable that vinyl and ethynyl are dominant surface species based on the reaction scheme proposed by the SFG
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measurements [12]. There are two models for the a2 phase. One is that vinyl and ethynyl species form their own (2 2) domains which are separated with the domain boundaries. In this case, the attractive interaction is operative between the identical surface species. The other is that two surface species form a mixed layer where the attractive interaction is operative mainly between the vinyl and ethynyl species. For the first model, a (2 2) LEED pattern should be observable, but no LEED patterns are observed below 300 K [11]. Thus, the first model is not suitable, which suggests that the interactions between the identical surface species are repulsive. No clear LEED patterns are expected for the second model if the two species are distributed randomly without the formation of their sublattices and the electron scattering factors of two species are different. Therefore, the second model is not unreasonable to explain the outcomes of the LEED and STM observations. What is the origin of the attractive interaction? The vinyl species is formed by the addition of a H atom to acetylene, while the ethynyl species is created by the removal of a H atom from acetylene. From the reduction–oxidation reaction points of view, vinyl and ethynyl are reduced and oxidized species, respectively, so that the vinyl species may tend to donate more electrons than the ethynyl species. Thus, the origin of the attractive interaction in the a2 phase seems likely to be indirect interaction of the donor–acceptor type through substrate electrons. The vinyl and ethynyl species play as a donor and an acceptor of electrons, respectively. Assuming that the interactions between the identical adsorbed species are repulsive and they are weaker than the attractive interaction between the vinyl and ethynyl species, the formation of islands can be rationalized as follows: A vinyl and an ethynyl are paired to form an island. Then, the island starts to grow twodimensionally to increase the number of the vinyl– ethynyl pair in the island. In this growth mode, we can find the islands or domains where vinyl and ethynyl species are mixed without the formation of their own sublattices. However, the problem still remains for the STM image of the a2 phase; the protrusions seem iden-
tical as shown in Fig. 1(c) except for the domain boundaries. If the vinyl and ethynyl species behave as a donor and an acceptor as discussed above, these species should appear differently in STM images reflecting the difference in their electronic structures. However, we observed that the acquired images did not show any voltage dependence on tip voltages from 1 to 1 V. This apparent contradiction may be due to the fact that the STM images reflect the electronic structure around the substrate Fermi level rather than the positions of the atomic nuclei. Thus, small molecules often appear as a protrusion or depression with no clear internal structure related to the constituent atoms [19]. In fact, adsorbed acetylene, vinylidene, vinyl and ethynyl species appear as similar protrusions in our STM images. Moreover, there is no experimental work to our knowledge that different kinds of C2 hydrocarbon adsorbates are distinguished in STM images. In order to rationalize this contradiction, and in addition, to understand the natures of the adsorbate– adsorbate interactions for both a1 and a2 phases, more detailed experimental and theoretical analyses of the electronic states of these species will be needed. Finally, we mention briefly the STM observations acquired after the C2 H2 -covered Pt(1 1 1) surface is heated to high temperatures where surface hydrocarbon species are fully dehydrogenated. When the surface is heated to 600 K, larger protrusions of about 0.5–1 nm in diameter are observed. By further heating to 900 K, graphite islands are observed as shown in Fig. 3. The average diameter of the graphite islands is estimated to be about 1 nm. For the larger graphite islands that often grow at step edges, moire superstructures are observed as a result of the lattice mismatch between the graphite and Pt, and the rotation of the graphite lattice relative to the Pt lattice. Land et al. [20] have investigated the thermal decomposition processes of C2 H4 on Pt(1 1 1) by using STM. When the C2 H4 -covered Pt(1 1 1) surface is heated to 430 K, carbonacious clusters of various diameters up to 1.3 nm are observed. The average size of these clusters becomes larger by heating to 700 K. After heating to 900 K, graphite islands of about 1.5–3 nm indiameter are
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Acknowledgements This work was supported in part by Grantsin-Aid for Scientific Research (11304041 and 13874068) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 3. An STM image obtained at room temperature after the C2 H2 -saturated Pt(1 1 1) surface was heated to 900 K (20 20 nm2 , 0.2 V, 0.7 nA).
observed which show the moire superstructures of various periodicities up to 2.2 nm. Our results agree well with those of Land et al. In conclusion, we have investigated the thermal decomposition processes of C2 H2 on Pt(1 1 1) by using STM. It is found that there are two ordered phases of the same (2 2) periodicity after the C2 H2 -covered Pt(1 1 1) surface is heated to 120 and 220 K. The (2 2) phases observed at 120 and 220 K are formed by the repulsive and attractive interactions between adsorbates, respectively. The origin of the different interaction is associated with the different surface species formed by the thermal decomposition of C2 H2 . Heating to 900 K, graphite islands of average diameter of about 1 nm are formed.
[1] W.H. Weinberg, H.A. Deans, R.P. Merill, Surf. Sci. 41 (1974) 312. [2] L.L. Kesmodel, P.C. Stair, R.C. Baetzold, G.A. Somorjai, Phys. Rev. Lett. 36 (1976) 1316. [3] J.E. Demuth, Chem. Phys. Lett. 45 (1977) 12. [4] H. Ibach, S. Lehwald, J. Vac. Sci. Technol. 15 (1978) 407. [5] P. Skinner, M. Howard, I. Oxton, S. Kettle, D. Powell, N. Sheppard, J. Chem. Soc. Faraday Trans. 15 (1978) 407. [6] L.L. Kesmodel, L.H. Dubois, G.A. Somorjai, J. Chem. Phys. 70 (1979) 2180. [7] D.B. Kang, A.B. Anderson, Surf. Sci. 155 (1985) 639. [8] C.E. Megiris, P. Berlowitz, J.B. Butt, H.H. Kung, Surf. Sci. 159 (1985) 184. [9] M. Abon, J. Billy, J.C. Bertolini, Surf. Sci. 171 (1986) L387. [10] N.R. Avery, Langmuir 4 (1988) 445. [11] C. Xu, J.W. Peck, B.E. Koel, J. Am. Chem. Soc. 115 (1993) 751. [12] P.S. Cremmer, X. Su, Y.R. Chen, G.A. Somorjai, J. Phys. Chem. B 101 (1997) 6474. [13] N. Freyer, G. Pirug, H.P. Bonzel, Surf. Sci. 126 (1983) 487. [14] J.C. Dunphy, M. Rose, S. Behler, D.F. Ogletree, M. Salmeron, P. Sautet, Phys. Rev. B 57 (1998) 12705. [15] J. Tersoff, D.R. Hamann, Phys. Rev. B 31 (1985) 805. [16] J. Schr€ oder, C. Gunther, R.Q. Hwang, Ultramicroscopy 42–44 (1992) 475. [17] J.M. White, S. Akhter, CRC Reviews 14 (1988) 131. [18] J.K. Nørskov, in: D.A. King, D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces, vol. 6, Elsevier, Amsterdam, 1993, p. 1. [19] M.-L. Bocquet, P. Sautet, Surf. Sci. 415 (1998) 148. [20] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger, G. Comsa, Appl. Phys. A 53 (1991) 414.
Thermal decomposition of acetylene on Pt(1 1 1) studied by scanning tunneling microscopy Osamu Nakagoe, Noriaki Takagi *, Yoshiyasu Matsumoto Department of Photoscience, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan Received 1 October 2001; accepted for publication 2 February 2002
Abstract The adsorption and the thermal decomposition processes of C2 H2 on Pt(1 1 1) have been investigated by the use of scanning tunneling microscopy. For a high C2 H2 coverage, a (2 2) structure is observed locally after the C2 H2 -covered Pt(1 1 1) surface is heated to 120 K. By heating to 220 K, the sample surface is covered with the (2 2) structure separated with brighter lines. After heating up to 370 K, (2 2) domains related to ethylidyne species appear with larger and brighter protrusions. For a low C2 H2 coverage, adsorbates are located randomly and no ordered islands are observed after heating to 120 K. In contrast, heating to 220 K leads to the formation of (2 2) islands. This indicates that the adsorbate–adsorbate interaction in the (2 2) structure observed by 120 K heating is different from that in the (2 2) structure observed by 220 K heating; the former is repulsive, but the latter attractive. Therefore, it is suggested that different surface species are formed by the decomposition of C2 H2 at 220 K. The detail of the decomposition processes of C2 H2 and the relation between the adsorbate–adsorbate interaction and the surface species are discussed. The results are also reported measured by heating up to higher temperatures above 400 K. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Chemisorption; Surface chemical reaction; Alkynes; Platinum
1. Introduction The interaction of transition metal surfaces with hydrocarbons is of importance, not only from the scientific viewpoint, but also from the technological applications in connection with heterogeneous catalysis. The adsorption and the thermal reactions of C2 H2 on Pt(1 1 1) have been extensively investigated as a prototype of catalytic reactions
*
Corresponding author. Fax: +81-468-58-1544. E-mail address: [email protected] (N. Takagi).
[1–12]. Annealing a Pt(1 1 1) surface covered with C2 H2 at 400 K leads to an ordered overlayer structure showing a (2 2) pattern in low energy electron diffraction (LEED) [1,2,6]. In this overlayer structure, ethylidyne (CCH3 ) species are formed by the thermal decomposition of C2 H2 [5,6]. Surface vibrational spectroscopy has been applied to investigate the thermal decomposition processes of C2 H2 [4,10,12]. From high resolution electron energy loss spectroscopy (HREELS) studies, adsorbed C2 H2 species are thermally intact below 300 K and are converted to CCH3 at 330– 400 K [4,10]. On the other hand, from the infrared–visible sum frequency generation (SFG) work
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 6 6 1 - 8
O. Nakagoe et al. / Surface Science 514 (2002) 414–419
of Cremer et al. [12], C2 H2 rearranges by intramolecular 1–2 shift of H to form vinylidene (CCH2 ) even at 125 K, and the reaction pathway for the decomposition of C2 H2 is proposed as follows: CCH2 is converted to vinyl (CHCH2 ) at 190 K, then ethylidene (CHCH3 ) is formed by H addition to CHCH2 at 339 K, and finally CCH3 is formed by the dehydrogenation of CHCH3 in 356– 381 K. Since a disproportionation reaction is required for the formation of CCH3 from C2 H2 , it is also proposed that hydrogen poor species, probably ethynyl (CCH), must be formed simultaneously to provide a hydrogen necessary to produce CCH3 . However, CCH species has not been identified yet. For these disproportionation reactions, the possibilities of the carbon atoms desolving into bulk [13] and the hydrogenation from the residual H2 [4] instead of the ethynyl formation are ruled out. In this work, we have investigated the decomposition processes of C2 H2 on the Pt(1 1 1) surface by using variable temperature scanning tunneling microscopy (STM). We show that (2 2) ordered overlayer structures of decomposed species of C2 H2 are formed by heating to 120 and 220 K. Although the (2 2) periodicity is common in the two structures, the nature of adsorbate–adsorbate interactions responsible for them is different: repulsive interaction for the (2 2) structure observed by 120 K heating while attractive interaction for the (2 2) structure observed by 220 K heating.
2. Experimental All the experiments were carried out in an ultrahigh-vacuum (UHV) chamber equipped with a variable temperature STM (Omicron VT STM). The base pressure of the UHV system was below 1 1010 Torr. For STM measurements, sample temperature can be changed in the STM stage by adjusting a liquid-He flow rate and balancing with the use of a heater. Chemically etched W wires were used for the tips that were cleaned by Arþ -ion sputtering and annealing. All STM images were acquired in a constant current mode with tunneling current of
415
0.1–5 nA and typical bias voltages of 5 mV to 1 V applied to the tip. A clean Pt(1 1 1) surface was prepared by many cycles of Arþ -ion sputtering, annealing at 1000 K, oxidation, and flashing at 1000 K. Sample cleanness was checked by STM. Research grade C2 H2 gas was introduced through a variable leak valve. The fractional coverage of C2 H2 , hC2 H2 , was estimated by counting the number of protrusions in STM images after exposing the Pt(1 1 1) surface to C2 H2 at 30 K (hC2 H2 ¼ 1 is defined as 1:505 1015 molecules/cm2 ). The saturation coverage of C2 H2 is estimated to be 0.25 [11].
3. Results and discussion Fig. 1(a) shows an STM image of the Pt(1 1 1) surface exposed to C2 H2 at 30 K (hC2 H2 ¼ 0.2), and Fig. 1(b) and (c) show STM images of the same surface subsequently heated to 120 and 220 K, respectively. These STM images are acquired at 30 K. Each image is not obtained for the same region because of the thermal drift during the heating–cooling process. In Fig. 1(a), protrusions of 0.01–0.02 nm in height and about 0.5 nm in diameter are distributed randomly. HREELS studies for C2 H2 on Pt(1 1 1) report that C2 H2 is adsorbed molecularly by the di-r=p bonding interaction, and is located in a threefold hollow site [4,10]. In STM measurements for C2 H2 on Pd(1 1 1) of Dunphy et al. [14], the di-r=p bonded C2 H2 species is observed as a protrusion of 0.01–0.02 nm in height and 0.4– 0.5 nm in diameter. Thus, these protrusions are associated with di-r=p bonded C2 H2 . When the surface is heated to 120 K, the ordered (2 2) periodicity appears locally as shown in Fig. 1(b). By further heating to 220 K, the surface is covered with the (2 2) structure, together with brighter lines (Fig. 1(c)). For larger scanned area such as 50 50 nm2 , bear surface regions are observed (not shown). The (2 2) structures observed by heating to 120 and 220 K are denoted as a1 and a2 , respectively. The brighter lines are domain boundaries where adsorbates are more compressed than in the regular (2 2) region. Since the compression of the adsorbates
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Fig. 1. (a) An STM image obtained at 30 K after Pt(1 1 1) is exposed to C2 H2 at 30 K (hC2 H2 ¼ 0.2) (22:5 22:5 nm2 , 0.4 V, 0.2 nA). STM images (b) (22:5 22:5 nm2 , 0.9 V, 0.2 nA) and (c) (20 20 nm2 , 0.5 V, 0.2 nA) taken at 30 K after the same surface as (a) was heated to 120 and 220 K, respectively. (d) An STM image taken at room temperature after the Pt(1 1 1) surface was saturated with C2 H2 at room temperature and then was heated to 370 K (25 25 nm2 , 1.0 V, 0.5 nA).
leads to the increase of the electronic density and thus the increase of the total electronic density at the boundaries, they are imaged as brighter lines. Note that the total electronic density determines STM images to the first order [15]. A similarly increased imaging height has been observed in STM images of Au on Ru(0 0 0 1), where a Au(1 1) layer is compressed [16]. Not only the electronic factor but also the geometrical factor may be involved. The adsorbates in the compressed region are pushed up slightly to reduce the lateral adsorbate–adsorbate interaction, and thus the boundaries are imaged more brightly.
The STM image of the CCH3 -induced (2 2) structure is shown in Fig. 1(d), which is acquired at room temperature after the sample is exposed to C2 H2 at room temperature and is subsequently heated to 370 K. Not only the (2 2) regions but also larger and brighter structures are observed. Since the formation of CCH3 species is accompanied with dehydrogenated hydrocarbon species as described above, the larger and brighter structures may be attributed to these hydrocarbons or their clusters. In order to elucidate the adsorbate–adsorbate interaction for the a1 and a2 phases, STM
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Fig. 2. STM images (a) (18 18 nm2 , 90 mV, 0.6 nA) and (b) (20 20 nm2 , 90 mV, 0.5 nA) taken at 30 K after the C2 H2 covered Pt(1 1 1) surface (hC2 H2 ¼ 0.04) was heated to 120 and 220 K, respectively.
measurements were made in the low coverage region (hC2 H2 ¼ 0.04), and results are shown in Fig. 2. For 120 K heating, adsorbates are located randomly and no islands with ordered structures are observed as shown in Fig. 2(a). On the other hand, islands with the (2 2) structure appear for 220 K heating (Fig. 2(b)). These results suggest that the driving force of the a1 phase is repulsive interaction between adsorbates and that of the a2 phase is attractive interaction. There is a possibility that
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the surface does not reach the equilibrium if the mobility of the adsorbate is very low at 120 K. For higher coverage, the disordered structure (Fig. 1(a)) is converted to the structure with the local (2 2) regions (Fig. 1(b)) after heating to 120 K, which is accompanied by the migration of the adsorbate. Although the STM measurements are not performed at 120 K, this suggests that the adsorbate is mobile at 120 K. For C2 H2 on Pd(1 1 1), the diffusion barrier of 180 meV is obtained [14]. Assuming that the barrier for the adsorbate is similar to this value and that the preexponential factor is the order of 1012 Hz, the lifetime at one site is estimated to be the order of 103 s. Thus, it is considered that the STM image shown in Fig. 2(a) reflects the mutual interaction between the adsorbates rather than the kinetics effects. According to a number of theoretical models, the adsorbate–adsorbate interaction is classified into two groups: one is the direct interaction (i.e., electrostatic interaction and overlap of adsorbate electronic orbitals), and the other is the indirect interaction through metal surfaces mediated by the lattice deformation and the valence electrons [17,18]. For most adsorption systems, these interactions contribute to the total adsorbate– adsorbate interaction in a complicated manner, and it is difficult to separate them. Furthermore, the indirect interaction changes the nature from repulsive to attractive depending on the mutual distance between adsorbates. Thus, we discuss favorable candidates of the interactions (especially operative between nearest neighbors) for the a1 and a2 phases below. According to the reaction pathway proposed by Cremer et al., the dominant surface species is vinylidene in the a1 phase. From the work function change of 1.42 eV after the adsorption of C2 H2 at 120 K, the dipole moment of the vinylidene species is estimated to be 1.45 D [9]. Thus, it is considered that the dipole–dipole repulsive interaction between the vinylidene species is the driving force of the a1 phase observed only for high coverages. The formation of islands is energetically unfavorable, and thus there exist no islands for low coverages. For the a2 phase, it is probable that vinyl and ethynyl are dominant surface species based on the reaction scheme proposed by the SFG
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measurements [12]. There are two models for the a2 phase. One is that vinyl and ethynyl species form their own (2 2) domains which are separated with the domain boundaries. In this case, the attractive interaction is operative between the identical surface species. The other is that two surface species form a mixed layer where the attractive interaction is operative mainly between the vinyl and ethynyl species. For the first model, a (2 2) LEED pattern should be observable, but no LEED patterns are observed below 300 K [11]. Thus, the first model is not suitable, which suggests that the interactions between the identical surface species are repulsive. No clear LEED patterns are expected for the second model if the two species are distributed randomly without the formation of their sublattices and the electron scattering factors of two species are different. Therefore, the second model is not unreasonable to explain the outcomes of the LEED and STM observations. What is the origin of the attractive interaction? The vinyl species is formed by the addition of a H atom to acetylene, while the ethynyl species is created by the removal of a H atom from acetylene. From the reduction–oxidation reaction points of view, vinyl and ethynyl are reduced and oxidized species, respectively, so that the vinyl species may tend to donate more electrons than the ethynyl species. Thus, the origin of the attractive interaction in the a2 phase seems likely to be indirect interaction of the donor–acceptor type through substrate electrons. The vinyl and ethynyl species play as a donor and an acceptor of electrons, respectively. Assuming that the interactions between the identical adsorbed species are repulsive and they are weaker than the attractive interaction between the vinyl and ethynyl species, the formation of islands can be rationalized as follows: A vinyl and an ethynyl are paired to form an island. Then, the island starts to grow twodimensionally to increase the number of the vinyl– ethynyl pair in the island. In this growth mode, we can find the islands or domains where vinyl and ethynyl species are mixed without the formation of their own sublattices. However, the problem still remains for the STM image of the a2 phase; the protrusions seem iden-
tical as shown in Fig. 1(c) except for the domain boundaries. If the vinyl and ethynyl species behave as a donor and an acceptor as discussed above, these species should appear differently in STM images reflecting the difference in their electronic structures. However, we observed that the acquired images did not show any voltage dependence on tip voltages from 1 to 1 V. This apparent contradiction may be due to the fact that the STM images reflect the electronic structure around the substrate Fermi level rather than the positions of the atomic nuclei. Thus, small molecules often appear as a protrusion or depression with no clear internal structure related to the constituent atoms [19]. In fact, adsorbed acetylene, vinylidene, vinyl and ethynyl species appear as similar protrusions in our STM images. Moreover, there is no experimental work to our knowledge that different kinds of C2 hydrocarbon adsorbates are distinguished in STM images. In order to rationalize this contradiction, and in addition, to understand the natures of the adsorbate– adsorbate interactions for both a1 and a2 phases, more detailed experimental and theoretical analyses of the electronic states of these species will be needed. Finally, we mention briefly the STM observations acquired after the C2 H2 -covered Pt(1 1 1) surface is heated to high temperatures where surface hydrocarbon species are fully dehydrogenated. When the surface is heated to 600 K, larger protrusions of about 0.5–1 nm in diameter are observed. By further heating to 900 K, graphite islands are observed as shown in Fig. 3. The average diameter of the graphite islands is estimated to be about 1 nm. For the larger graphite islands that often grow at step edges, moire superstructures are observed as a result of the lattice mismatch between the graphite and Pt, and the rotation of the graphite lattice relative to the Pt lattice. Land et al. [20] have investigated the thermal decomposition processes of C2 H4 on Pt(1 1 1) by using STM. When the C2 H4 -covered Pt(1 1 1) surface is heated to 430 K, carbonacious clusters of various diameters up to 1.3 nm are observed. The average size of these clusters becomes larger by heating to 700 K. After heating to 900 K, graphite islands of about 1.5–3 nm indiameter are
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Acknowledgements This work was supported in part by Grantsin-Aid for Scientific Research (11304041 and 13874068) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 3. An STM image obtained at room temperature after the C2 H2 -saturated Pt(1 1 1) surface was heated to 900 K (20 20 nm2 , 0.2 V, 0.7 nA).
observed which show the moire superstructures of various periodicities up to 2.2 nm. Our results agree well with those of Land et al. In conclusion, we have investigated the thermal decomposition processes of C2 H2 on Pt(1 1 1) by using STM. It is found that there are two ordered phases of the same (2 2) periodicity after the C2 H2 -covered Pt(1 1 1) surface is heated to 120 and 220 K. The (2 2) phases observed at 120 and 220 K are formed by the repulsive and attractive interactions between adsorbates, respectively. The origin of the different interaction is associated with the different surface species formed by the thermal decomposition of C2 H2 . Heating to 900 K, graphite islands of average diameter of about 1 nm are formed.
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