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Oxidation process of Mo2C(0 0 0 1) studied by photoelectron spectroscopy
Applied Surface Science 237 (2004) 498–502
Oxidation process of Mo2C(0 0 0 1) studied by photoelectron spectroscopy K. Edamotoa,*, M. Sugiharaa, K. Ozawaa, S. Otanib a
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-0033, Japan b National Institute for Materials Science, 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan Available online 29 July 2004
Abstract Oxygen adsorption on the a-Mo2C(0 0 0 1) surface has been investigated with valence photoelectron spectroscopy (PES) utilizing synchrotron radiation and X-ray photoelectron spectroscopy (XPS). It is found that the adsorbed oxygen atoms interact both with Mo and C atoms forming an oxycarbide layer on the Mo2C(0 0 0 1) surface. Valence PES study shows that the oxygen adsorption induces a state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra. # 2004 Published by Elsevier B.V. PACS: 73.20.r; 79.60.Dp Keywords: Photoelectron spectroscopy; Adsorption; Oxidation; Mo2C
1. Introduction The early transition metal carbides (TMCs) have attracted much attention because they have an interesting combination of useful properties such as extreme hardness, a high melting point and metallic conductivity [1]. In practical applications, they have been used as stable field electron emitters, coating materials, catalysts, etc. Because the surface properties play an essential role in all these applications, it has become important to investigate the surface properties of TMCs.
*
Corresponding author. Tel.: þ81-3-5734-2238; fax: þ81-3-5734-2655. E-mail address: [email protected] (K. Edamoto). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.06.074
The surface properties of Mo2C, which is one of group 6 TMCs, are in particular of interest, because Mo2C is known to have high catalytic activities for a variety of reactions, such as hydrogenation of benzene, ethylene, and carbon monoxide [2]. In addition, it has been reported that the catalytic activities of Mo2C and other group 6 TMCs are often improved by slight oxidation of the surfaces [3–5]. The activation of their surfaces has been explained to be due to the oxycarbide layer formation [3–5], however, the microscopic understanding as to the activation mechanism of the oxygen-modified group 6 TMC surfaces has not been obtained yet. In this work, the oxidation process of Mo2C(0 0 0 1) and the effect of the oxidation on the surface electronic structure have been investigated with valence photoelectron spectroscopy (PES) and X-ray photoelectron spectroscopy (XPS).
K. Edamoto et al. / Applied Surface Science 237 (2004) 498–502
2. Experimental The valence PES measurements using synchrotron radiation were conducted on the Beam Line 3B of the Photon Factory, High Energy Accelerator Research Organization (KEK), where the synchrotron light was dispersed by a grazing incidence monochromator. An electron energy analyzer of 1508 hemispherical sector type with an acceptance angle of 18 was used for all valence PES measurements. Photoelectrons emitted in the surface normal direction are collected for all valence PES measurements. The valence PES spectra presented below are normalized by photon flux estimated from the photocurrent of the final stage mirror. The base pressure in the vacuum system was 8 1011 Torr. The XPS measurements were performed in a separate chamber equipped with a dual anode Xray source, a He I discharge lamp, an electron energy analyzer of 1808 hemispherical sector type (VG 100AX), and a LEED optics. A Mg Ka radiation source was used for all XPS measurements. The base pressure in the vacuum system was 1 1010 Torr. In this paper, the background drawn by the Shirley procedure has been subtracted from each raw data. An a-Mo2C single crystal was grown by the floating zone method at the National Institute for Materials Science [6]. The crystal was cut at an orientation of (0 0 0 1) by spark erosion into a disk of 1 mm thickness and subsequently polished mechanically. The Mo2C(0 0 0 1) surface was cleaned in the vacuum chamber by several cycles of Arþ ion bombardment (1 kV for 60 min) and annealing (<1600 K). The
499
temperature of the sample was monitored using a Pt/Pt–Rh (13%) thermocouple and an optical pyrometer. The Mo2C(0 0 0 1) should be terminated with either a Mo layer or a C layer, and our previous study showed that the clean surface prepared with the procedure described above is terminated with a C layer in which the number of the C atoms is nearly equal to that of the Mo atoms in the second layer (a MoC-like surface) [7].
3. Results and discussion The Mo2C(0 0 0 1) clean surface gives a complex LEED pattern composed of (1 1) orthorhombic spots, (2 2)R158 spots and (4/3 4/3)R58spots [7]. As the surface is exposed to O2 at room temperature, the (2 2)R158 spots and (4/3 4/3)R58spots are weakened. However, the background is decreased in intensity and the (1 1) orthorhombic spots are sharpened by O2 exposure, indicating that some ordering of the surface atomic arrangement to form a (1 1) periodicity is induced by oxygen adsorption. Fig. 1(a) shows the change in the O 1s core-level spectrum of the O/Mo2C(0 0 0 1) system as a function of O2 exposure at room temperature. The O 1s peak appears at 530.3 eV at 0.3 L exposure. The peak slightly shifts to the lower binding energy side with increasing coverage, and is observed at 530.1 eV at 1.0 L. The integrated O 1 s peak intensity is plotted as a function of exposure in Fig. 1(b). The plot shows that the adsorption nearly saturates at 2–3 L. The
Fig. 1. (a) Change in the O 1s spectrum of the Mo2C(0 0 0 1) surface as a function of O2 exposure; (b) the O 1s peak intensities as a function of exposure.
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oxygen coverage, which is defined as the ratio of the number of adsorbed O atoms (NO) to that of the Mo atoms in the second layer, can be estimated from the intensities of O 1s and Mo 3d peaks in XPS spectra using the calculation involving layer-by-layer summation of exponentially decayed photoemission signals [7]. Assuming the mean free path of the Mo 3d photoelectrons to be 1.73 nm [8], the saturation coverage is estimated to be 1.3 (NO 1.6 1015 cm2). Since the saturation is reached at 2–3 L, this result means that the sticking probability is nearly unity in the initial stage of adsorption, indicating that the Mo2C(0 0 0 1) surface is very reactive towards oxygen adsorption. The coverage estimation given above is based on the assumption that all the adsorbed O atoms sit above the second Mo layer, and thus the coverage may be higher than the above value in case some of the oxygen atoms penetrate into the substrate. In Fig. 2(a), the Mo 3d spectrum of the Mo2C(0 0 0 1) surface exposed to 20 L of O2 is shown (upper spectrum). As the surface is exposed to O2, the Mo 3d5/2 and 3d3/2 peaks are broadened and seem to shift to the higher binding energy side, indicating the appearance of oxidized components. To extract the oxidized components, we obtain the difference spectrum between the spectrum of O/Mo2C(0 0 0 1) and that of Mo2C(0 0 0 1). The result is shown in Fig. 2(a) (upper spectrum), where the raw spectrum is divided into oxidized and non-oxidized components on the assumption that the raw spectrum has the same spectral shape as that of the non-oxidized component in the onset region of the 3d5/2 peak (<227.5 eV). This procedure is based on the assumption that the oxidized components should be formed in the higher binding energy side of the non-oxidized component, and thus the onset region of the 3d5/2 peak should be composed of the non-oxidized component only. The lower spectrum in Fig. 2(a) is the obtained difference spectrum, which corresponds to the 3d levels of oxidized Mo atoms. We tried to deconvolute the oxidized component of Mo 3d band using Gaussian functions, and the best-fitted result is also shown in Fig. 2(a). The result shows that the oxidized component is composed of three peaks, which are labeled as P1, P2 and P3 in the order of the binding energies. We tentatively attribute the P1, P2 and P3 peaks to the 3d states of Mo atoms bound to one, two and three adsorbed O atoms, respectively.
Fig. 2. (a) Mo 3d spectra of the Mo2C(0 0 0 1) clean surface (solid line) and of the surface exposed to 20 L of O2 (dotted line), and the difference between the dotted line and solid line (gray line). The difference spectrum is fit with Gaussian components. (b) C 1s spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 20 L of O2.
The comparison of C 1s spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 20 L of O2 is shown in Fig. 2(b). For the clean surface, the C 1s peak is observed at 282.9 eV, which is agreement with the result of our previous XPS study [7]. It is found that the C 1s peak intensity is not changed by O2 exposure, indicating that the substrate’s C atoms are not depleted during O2 exposure. This is contrary to the case for O/TiC(1 0 0) [9] and O/ZrC(1 0 0) [10] systems, where it has been found that the substrate’s C atoms are depleted probably due to their desorption as CO or CO2 molecules and TiOx (1.5 < x < 2.0) and ZrOx (1.0 < x <2.0) oxide layers are formed upon O2 exposure at room temperature, respectively. Thus, the oxidation process of
K. Edamoto et al. / Applied Surface Science 237 (2004) 498–502
Mo2C(0 0 0 1) is substantially different from that of group 4 TMC(1 0 0) surfaces. Fig. 2(b) shows that the oxygen adsorption induces a shoulder in the higher binding energy side (283.5–284.0 eV). The shoulder is attributed to the oxidized C atoms, indicating that the adsorbed O atoms interact with the surface C atoms as well as the Mo atoms. It is deduced from Mo 3d and C 1s measurements that both the Mo and C atoms interact with adsorbed O atoms, indicating that a sort of oxycarbide layer is formed on Mo2C(0 0 0 1) upon oxygen exposure at room temperature. This result also means that, though the Mo2C(0 0 0 1) surface is covered with a monolayer of C atoms, some or all adsorbed O atoms penetrate the surface C layer and directly interact with the Mo atoms in the second layer. This is probably due to the fact that the Mo atoms in the second layer are very reactive towards adsorption because the highest occupied level on Mo2C(0 0 0 1) is mostly composed of Mo 4d orbitals, as has been proved by our previous studies [7,11]. Fig. 3 shows valence PES spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 10 L of O2. In the spectrum of the clean surface, the valence band is observed at 0–7 eV. The spectrum shows clear cut-off at the Fermi level (EF), indicating the metallic nature of Mo2C. Our previous resonant PES studies have revealed that the valence band is mostly composed of Mo 4d–C 2p hybrid band except for the region near EF (0–1 eV) where the band is mostly composed of Mo 4d orbitals [7,11]. As the surface is exposed to O2, O-induced bands are formed at 4–8 eV and at the region just below EF. The band formed at 4–8 eV is well ascribed to the O 2p-induced bonding states. The spectrum shows clear cut-off at EF, indicating that Mo2C(0 0 0 1) remains the metallic nature even after oxygen adsorption. Furthermore, the density of states at the Fermi edge is enhanced by oxygen adsorption due to the formation of O-induced states around EF. These results suggest that an O-induced half-filled state is formed upon oxygen adsorption on Mo2C(0 0 0 1). In the inset of Fig. 3, we plot the intensities of the Oinduced bands as a function of photon energy. The intensities are estimated by taking the difference between the spectra of the 10 L O2-exposed surface and of the clean surface. Our previous PES study on
501
Fig. 3. Normal-emission spectra of the Mo2C(0 0 0 1) clean surface (dashed line) and of the surface exposed to 10 L of O2 (solid line). The intensities of the O-induced bands at 4–8 eV (open circles) and at just below EF (solid triangles) are plotted as a function of photon energy in the inset.
Mo2C(0 0 0 1) has shown that the emission intensities of the bands including Mo 4d contribution are prominently maximized at hn 46 eV due to the resonance associated with the Mo 4p ! 4d excitation [10]. Neither O-induced band shows the maximum at 46 eV, indicating that the bands include little contribution of Mo 4d orbitals. This result denies the possibility that the O-induced band around EF is the antibonding part of the Mo 4d–O 2p hybrid states, because the antibonding part is expected to include dominant contribution of Mo 4d orbitals. The details of the origin of this band is unknown at present, and the resonant PES results suggest that the state should be ascribed to a Mo 5sp–O 2p hybrid state or to a C 2p–O 2p hybrid state. It is known that the catalytic activities of Mo2C are often improved by slight oxidation of the surface [3–5]. The O2 adsorption saturates
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at 2–3 L, and thus the formation of the O-induced state around EF does not activate the surface towards O2 adsorption. However, since the state around EF on the surface can act as a frontier orbital in many surface reactions, the O-induced state around EF may have some contribution to the improvement of the catalytic activities of Mo2C.
Acknowledgements We are pleased to thank the staff of the Photon Factory, High Energy Accelerator Research Organization, particularly Prof. Y. Azuma, for their excellent support. Part of this work has been performed with the approval of the Photon Factory Advisory Committee (Proposal No. 2002G036).
References [1] Y. Kumashiro, Electric Refractory Materials, Marcel Dekker, New York, 2000. [2] S.T. Oyama, Catal. Today 15 (1992) 1979. [3] M. Orita, I. Kojima, E. Miyazaki, Bull. Chem. Soc. Jpn. 59 (1986) 689. [4] C. Pham-Huu, M.J. Ledoux, J. Guille, J. Catal. 143 (1993) 249. [5] N. Liu, S.A. Rykov, J.G. Chen, Surf. Sci. 487 (2001) 107. [6] S. Otani, Y. Ishizawa, J. Cryst. Growth 154 (1995) 202. [7] M. Sugihara, K. Ozawa, K. Edamoto, S. Otani, Solid State Commun. 121 (2002) 1. [8] P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [9] Y. Shirotori, K. Sawada, K. Ozawa, K. Edamoto, S. Otani, Jpn. J. Appl. Phys. 42 (2003) 1725. [10] S. Kato, K. Ozawa, K. Edamoto, S. Otani, Jpn. J. Appl. Phys. 39 (2000) 5217. [11] M. Sugihara, K. Ozawa, K. Edamoto, S. Otani, Solid State Commun., in press.
Oxidation process of Mo2C(0 0 0 1) studied by photoelectron spectroscopy K. Edamotoa,*, M. Sugiharaa, K. Ozawaa, S. Otanib a
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-0033, Japan b National Institute for Materials Science, 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan Available online 29 July 2004
Abstract Oxygen adsorption on the a-Mo2C(0 0 0 1) surface has been investigated with valence photoelectron spectroscopy (PES) utilizing synchrotron radiation and X-ray photoelectron spectroscopy (XPS). It is found that the adsorbed oxygen atoms interact both with Mo and C atoms forming an oxycarbide layer on the Mo2C(0 0 0 1) surface. Valence PES study shows that the oxygen adsorption induces a state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra. # 2004 Published by Elsevier B.V. PACS: 73.20.r; 79.60.Dp Keywords: Photoelectron spectroscopy; Adsorption; Oxidation; Mo2C
1. Introduction The early transition metal carbides (TMCs) have attracted much attention because they have an interesting combination of useful properties such as extreme hardness, a high melting point and metallic conductivity [1]. In practical applications, they have been used as stable field electron emitters, coating materials, catalysts, etc. Because the surface properties play an essential role in all these applications, it has become important to investigate the surface properties of TMCs.
*
Corresponding author. Tel.: þ81-3-5734-2238; fax: þ81-3-5734-2655. E-mail address: [email protected] (K. Edamoto). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.06.074
The surface properties of Mo2C, which is one of group 6 TMCs, are in particular of interest, because Mo2C is known to have high catalytic activities for a variety of reactions, such as hydrogenation of benzene, ethylene, and carbon monoxide [2]. In addition, it has been reported that the catalytic activities of Mo2C and other group 6 TMCs are often improved by slight oxidation of the surfaces [3–5]. The activation of their surfaces has been explained to be due to the oxycarbide layer formation [3–5], however, the microscopic understanding as to the activation mechanism of the oxygen-modified group 6 TMC surfaces has not been obtained yet. In this work, the oxidation process of Mo2C(0 0 0 1) and the effect of the oxidation on the surface electronic structure have been investigated with valence photoelectron spectroscopy (PES) and X-ray photoelectron spectroscopy (XPS).
K. Edamoto et al. / Applied Surface Science 237 (2004) 498–502
2. Experimental The valence PES measurements using synchrotron radiation were conducted on the Beam Line 3B of the Photon Factory, High Energy Accelerator Research Organization (KEK), where the synchrotron light was dispersed by a grazing incidence monochromator. An electron energy analyzer of 1508 hemispherical sector type with an acceptance angle of 18 was used for all valence PES measurements. Photoelectrons emitted in the surface normal direction are collected for all valence PES measurements. The valence PES spectra presented below are normalized by photon flux estimated from the photocurrent of the final stage mirror. The base pressure in the vacuum system was 8 1011 Torr. The XPS measurements were performed in a separate chamber equipped with a dual anode Xray source, a He I discharge lamp, an electron energy analyzer of 1808 hemispherical sector type (VG 100AX), and a LEED optics. A Mg Ka radiation source was used for all XPS measurements. The base pressure in the vacuum system was 1 1010 Torr. In this paper, the background drawn by the Shirley procedure has been subtracted from each raw data. An a-Mo2C single crystal was grown by the floating zone method at the National Institute for Materials Science [6]. The crystal was cut at an orientation of (0 0 0 1) by spark erosion into a disk of 1 mm thickness and subsequently polished mechanically. The Mo2C(0 0 0 1) surface was cleaned in the vacuum chamber by several cycles of Arþ ion bombardment (1 kV for 60 min) and annealing (<1600 K). The
499
temperature of the sample was monitored using a Pt/Pt–Rh (13%) thermocouple and an optical pyrometer. The Mo2C(0 0 0 1) should be terminated with either a Mo layer or a C layer, and our previous study showed that the clean surface prepared with the procedure described above is terminated with a C layer in which the number of the C atoms is nearly equal to that of the Mo atoms in the second layer (a MoC-like surface) [7].
3. Results and discussion The Mo2C(0 0 0 1) clean surface gives a complex LEED pattern composed of (1 1) orthorhombic spots, (2 2)R158 spots and (4/3 4/3)R58spots [7]. As the surface is exposed to O2 at room temperature, the (2 2)R158 spots and (4/3 4/3)R58spots are weakened. However, the background is decreased in intensity and the (1 1) orthorhombic spots are sharpened by O2 exposure, indicating that some ordering of the surface atomic arrangement to form a (1 1) periodicity is induced by oxygen adsorption. Fig. 1(a) shows the change in the O 1s core-level spectrum of the O/Mo2C(0 0 0 1) system as a function of O2 exposure at room temperature. The O 1s peak appears at 530.3 eV at 0.3 L exposure. The peak slightly shifts to the lower binding energy side with increasing coverage, and is observed at 530.1 eV at 1.0 L. The integrated O 1 s peak intensity is plotted as a function of exposure in Fig. 1(b). The plot shows that the adsorption nearly saturates at 2–3 L. The
Fig. 1. (a) Change in the O 1s spectrum of the Mo2C(0 0 0 1) surface as a function of O2 exposure; (b) the O 1s peak intensities as a function of exposure.
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oxygen coverage, which is defined as the ratio of the number of adsorbed O atoms (NO) to that of the Mo atoms in the second layer, can be estimated from the intensities of O 1s and Mo 3d peaks in XPS spectra using the calculation involving layer-by-layer summation of exponentially decayed photoemission signals [7]. Assuming the mean free path of the Mo 3d photoelectrons to be 1.73 nm [8], the saturation coverage is estimated to be 1.3 (NO 1.6 1015 cm2). Since the saturation is reached at 2–3 L, this result means that the sticking probability is nearly unity in the initial stage of adsorption, indicating that the Mo2C(0 0 0 1) surface is very reactive towards oxygen adsorption. The coverage estimation given above is based on the assumption that all the adsorbed O atoms sit above the second Mo layer, and thus the coverage may be higher than the above value in case some of the oxygen atoms penetrate into the substrate. In Fig. 2(a), the Mo 3d spectrum of the Mo2C(0 0 0 1) surface exposed to 20 L of O2 is shown (upper spectrum). As the surface is exposed to O2, the Mo 3d5/2 and 3d3/2 peaks are broadened and seem to shift to the higher binding energy side, indicating the appearance of oxidized components. To extract the oxidized components, we obtain the difference spectrum between the spectrum of O/Mo2C(0 0 0 1) and that of Mo2C(0 0 0 1). The result is shown in Fig. 2(a) (upper spectrum), where the raw spectrum is divided into oxidized and non-oxidized components on the assumption that the raw spectrum has the same spectral shape as that of the non-oxidized component in the onset region of the 3d5/2 peak (<227.5 eV). This procedure is based on the assumption that the oxidized components should be formed in the higher binding energy side of the non-oxidized component, and thus the onset region of the 3d5/2 peak should be composed of the non-oxidized component only. The lower spectrum in Fig. 2(a) is the obtained difference spectrum, which corresponds to the 3d levels of oxidized Mo atoms. We tried to deconvolute the oxidized component of Mo 3d band using Gaussian functions, and the best-fitted result is also shown in Fig. 2(a). The result shows that the oxidized component is composed of three peaks, which are labeled as P1, P2 and P3 in the order of the binding energies. We tentatively attribute the P1, P2 and P3 peaks to the 3d states of Mo atoms bound to one, two and three adsorbed O atoms, respectively.
Fig. 2. (a) Mo 3d spectra of the Mo2C(0 0 0 1) clean surface (solid line) and of the surface exposed to 20 L of O2 (dotted line), and the difference between the dotted line and solid line (gray line). The difference spectrum is fit with Gaussian components. (b) C 1s spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 20 L of O2.
The comparison of C 1s spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 20 L of O2 is shown in Fig. 2(b). For the clean surface, the C 1s peak is observed at 282.9 eV, which is agreement with the result of our previous XPS study [7]. It is found that the C 1s peak intensity is not changed by O2 exposure, indicating that the substrate’s C atoms are not depleted during O2 exposure. This is contrary to the case for O/TiC(1 0 0) [9] and O/ZrC(1 0 0) [10] systems, where it has been found that the substrate’s C atoms are depleted probably due to their desorption as CO or CO2 molecules and TiOx (1.5 < x < 2.0) and ZrOx (1.0 < x <2.0) oxide layers are formed upon O2 exposure at room temperature, respectively. Thus, the oxidation process of
K. Edamoto et al. / Applied Surface Science 237 (2004) 498–502
Mo2C(0 0 0 1) is substantially different from that of group 4 TMC(1 0 0) surfaces. Fig. 2(b) shows that the oxygen adsorption induces a shoulder in the higher binding energy side (283.5–284.0 eV). The shoulder is attributed to the oxidized C atoms, indicating that the adsorbed O atoms interact with the surface C atoms as well as the Mo atoms. It is deduced from Mo 3d and C 1s measurements that both the Mo and C atoms interact with adsorbed O atoms, indicating that a sort of oxycarbide layer is formed on Mo2C(0 0 0 1) upon oxygen exposure at room temperature. This result also means that, though the Mo2C(0 0 0 1) surface is covered with a monolayer of C atoms, some or all adsorbed O atoms penetrate the surface C layer and directly interact with the Mo atoms in the second layer. This is probably due to the fact that the Mo atoms in the second layer are very reactive towards adsorption because the highest occupied level on Mo2C(0 0 0 1) is mostly composed of Mo 4d orbitals, as has been proved by our previous studies [7,11]. Fig. 3 shows valence PES spectra of the Mo2C(0 0 0 1) clean surface and of the surface exposed to 10 L of O2. In the spectrum of the clean surface, the valence band is observed at 0–7 eV. The spectrum shows clear cut-off at the Fermi level (EF), indicating the metallic nature of Mo2C. Our previous resonant PES studies have revealed that the valence band is mostly composed of Mo 4d–C 2p hybrid band except for the region near EF (0–1 eV) where the band is mostly composed of Mo 4d orbitals [7,11]. As the surface is exposed to O2, O-induced bands are formed at 4–8 eV and at the region just below EF. The band formed at 4–8 eV is well ascribed to the O 2p-induced bonding states. The spectrum shows clear cut-off at EF, indicating that Mo2C(0 0 0 1) remains the metallic nature even after oxygen adsorption. Furthermore, the density of states at the Fermi edge is enhanced by oxygen adsorption due to the formation of O-induced states around EF. These results suggest that an O-induced half-filled state is formed upon oxygen adsorption on Mo2C(0 0 0 1). In the inset of Fig. 3, we plot the intensities of the Oinduced bands as a function of photon energy. The intensities are estimated by taking the difference between the spectra of the 10 L O2-exposed surface and of the clean surface. Our previous PES study on
501
Fig. 3. Normal-emission spectra of the Mo2C(0 0 0 1) clean surface (dashed line) and of the surface exposed to 10 L of O2 (solid line). The intensities of the O-induced bands at 4–8 eV (open circles) and at just below EF (solid triangles) are plotted as a function of photon energy in the inset.
Mo2C(0 0 0 1) has shown that the emission intensities of the bands including Mo 4d contribution are prominently maximized at hn 46 eV due to the resonance associated with the Mo 4p ! 4d excitation [10]. Neither O-induced band shows the maximum at 46 eV, indicating that the bands include little contribution of Mo 4d orbitals. This result denies the possibility that the O-induced band around EF is the antibonding part of the Mo 4d–O 2p hybrid states, because the antibonding part is expected to include dominant contribution of Mo 4d orbitals. The details of the origin of this band is unknown at present, and the resonant PES results suggest that the state should be ascribed to a Mo 5sp–O 2p hybrid state or to a C 2p–O 2p hybrid state. It is known that the catalytic activities of Mo2C are often improved by slight oxidation of the surface [3–5]. The O2 adsorption saturates
502
K. Edamoto et al. / Applied Surface Science 237 (2004) 498–502
at 2–3 L, and thus the formation of the O-induced state around EF does not activate the surface towards O2 adsorption. However, since the state around EF on the surface can act as a frontier orbital in many surface reactions, the O-induced state around EF may have some contribution to the improvement of the catalytic activities of Mo2C.
Acknowledgements We are pleased to thank the staff of the Photon Factory, High Energy Accelerator Research Organization, particularly Prof. Y. Azuma, for their excellent support. Part of this work has been performed with the approval of the Photon Factory Advisory Committee (Proposal No. 2002G036).
References [1] Y. Kumashiro, Electric Refractory Materials, Marcel Dekker, New York, 2000. [2] S.T. Oyama, Catal. Today 15 (1992) 1979. [3] M. Orita, I. Kojima, E. Miyazaki, Bull. Chem. Soc. Jpn. 59 (1986) 689. [4] C. Pham-Huu, M.J. Ledoux, J. Guille, J. Catal. 143 (1993) 249. [5] N. Liu, S.A. Rykov, J.G. Chen, Surf. Sci. 487 (2001) 107. [6] S. Otani, Y. Ishizawa, J. Cryst. Growth 154 (1995) 202. [7] M. Sugihara, K. Ozawa, K. Edamoto, S. Otani, Solid State Commun. 121 (2002) 1. [8] P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [9] Y. Shirotori, K. Sawada, K. Ozawa, K. Edamoto, S. Otani, Jpn. J. Appl. Phys. 42 (2003) 1725. [10] S. Kato, K. Ozawa, K. Edamoto, S. Otani, Jpn. J. Appl. Phys. 39 (2000) 5217. [11] M. Sugihara, K. Ozawa, K. Edamoto, S. Otani, Solid State Commun., in press.