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Detection of sulfur from Ni(1 1 0) surface using by electron-stimulated desorption spectroscopy
Applied Surface Science 254 (2008) 7996–7998
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Detection of sulfur from Ni(1 1 0) surface using by electron-stimulated desorption spectroscopy Kazuyuki Ueda *, Takeshi Yagami Nano-High-Tech Research Center, Graduate School of Engineering, Toyota Technological Institute, Hisakata 2-Chome, Tempaku-Ku, Nagoya 468-8511, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
On the Ni(1 1 0) surface, H+ and O+ are normally detected by electron-stimulated desorption (ESD). However, C+, N+, S+ are normally insensitive to ESD measurement. In this report S+ signal has been significantly detected by the ESD from the Ni(1 1 0) surface. Behavior of S+ has been observed in the variation of time-of-flight (TOF)–ESD spectra during heating. ß 2008 Elsevier B.V. All rights reserved.
Available online 23 April 2008 Keywords: Sulfur Ni(1 1 0) Electron-simulated desorption ESD
1. Introduction Nickel surface has been intensively investigated as a catalytic material so far [1,2]. In the processes of production of hydrogen from methanol, carbon monoxide (CO) gas appears on the catalyst as catalytic poisoning. When the CO dissociates on the Ni surface, only the C remains on the active sites on the catalyst surface. In this situation, Raney-Nickel has been expected to reduce CO contamination on Ni surface. In this sense Raney-NiSn alloy produces hydrogen gas from glucose effectively, and it has been expected to take the place of platinum catalyst [3,4]. Sn-covered Ni surface prevents CO adsorption by side-block effect as reported by Xu and Koel [5]. However, in the cleaning process of nickel surface in ultra high vacuum (UHV) sulfur (S) atoms segregate on the Ni surface after annealing at higher temperatures. The sulfur should be removed from the catalytic surface in order to prevent sulfur poisoning. Sometimes many repetitions of Ar ion bombardment and annealing treatments are needed to obtain a clean Ni surface. So far Auger electron spectroscopy (AES) has been commonly used to detect the surface sulfur as described in previous reports [6,7]. While an electron-stimulated desorption (ESD) spectroscopy is very useful to detect hydrogen as H+ from the surface. The ESD detects also O+ from CO molecule on the surface as previously published by Takano and Ueda [8,9]. Thus time-of-flight (TOF) type ESD has been used as powerful tool to investigate adsorption processes of O associating with CO adsorption on the nickel
* Corresponding author. Tel.: +81 52 809 1850; fax: +81 52 809 1850. E-mail address: [email protected] (K. Ueda). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.063
surface. In the process of investigation of hydrogen (H) on the Ni(1 1 0) surface prior to the Sn deposition, sulfur layer segregates on the surface on which S+ has been detected for the first time by TOF–ESD measurements. In the ESD spectroscopy the S+ has not been detected so far even though sulfur layer has been detected on the surface by in situ measurements of AES on Pt and Zr surfaces [10]. Therefore detection of the S+ is very rare in ESD spectroscopy. In this report, we would like to just appeal that the S+ signal in ESD has been detected for the first time, and some description about behavior of the S+ depending on the specimen temperatures. 2. Experimental Experimental apparatus is similar to one which has been published previously [11]. It is composed by two chamber made of stainless steel separated by the gate valve, base pressure of main chamber is 8 109 Pa evacuated by 400 l/s ion pump with liquid nitrogen shroud for Ti-getter sublimation pump. AES is cylindrical mirror analyzer (ANELVA, AAS-200V) and laboratory-made TOF– ESD which is consisting of 3-grid meshes and microchannel plates (MCPs) assembly with a fluorescent screen (effective diameter is 70 mm). An effective flight length of desorbed ions is 145 mm. An incidence angle of electron beam is 458 from the surface normal of specimen. Primary electron energy for TOF–ESD is normally used from 10 to 600 eV with a specimen bias between 0 and 20 V. An electron beam with this energy range is easily pulsed by a highspeed pulse generator with duration of 120 ns. A typical TOF–ESD spectra are shown in Fig. 1, where P is photon signal generated by the primary electron irradiation, H+ is proton, O+ is from CO and/or residual oxide layers in local defects, S+ from sulfur layer. The flight-times of all spectra excepting P signal are
K. Ueda, T. Yagami / Applied Surface Science 254 (2008) 7996–7998
7997
Fig. 1. A series of TOF–ESD spectrum depending on the flight time shift to shorter times. Simulation curve of Eq. (1) with a suitable kinetic energy of ion fits on peak positions.
shortened against the specimen bias, because all of positive ions are accelerated by positive specimen bias. This variation obeys an equation of flight time for desorbed ions as shown in Eq. (1) pffiffiffiffiffiffiffiffiffiffi 2mL t ¼ pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ek þ Ek þ qV s
(1)
where t is flight time of ions, Ek is kinetic energy of desorbed ion, q is electric charge, Vs is a positive specimen bias, m is mass of desorbed ion and L is flight length of desorbed ions between specimen surface and detector. The kinetic energy of desorbed ions is easily found from Fig. 1 by fitting to simulation curve of Eq. (1) with the peak energies measured by different specimen biases. The ratio of flight time of the O+ against the S+ is corresponding to the difference of mass of the O+ and the S+ if their kinetic energies are almost equivalent. Signal intensity of the S+ in ESD depends on signal intensity of peak-to-peak height of the S-peak in AES spectrum. Actually the kinetic energy of desorbed ions can be measured by retarding field method using grid meshes, namely, high pass filter method. And also energy distribution curve of the desorbed ions can be transformed numerically from a TOF-spectrum obtained at Vs = 0 by using Eq. (1). 3. Results and discussion Fig. 2(a) and (b) shows an AES spectrum and TOF–ESD spectrum, respectively, from Ni(1 1 0) surface after heating at 800 K. In the AES spectrum, a fine doubled-peak at the bottom of Speak means to be metal sulfide. Because TOF–ESD intensity is obtained from only the top layer of substrate and also depends on strongly chemical situation of sulfur layer which exists as a single flat layer or multi-layer. While AES detects S signal not only from the top layer but also under layers. Furthermore, amount of Ssegregation onto the surface depends on heating conditions and concentration of carbon contamination. Signal intensity of the S+ in ESD is roughly related to the signal intensity of peak-to-peak height of the S-peak in AES spectrum. However, electron beam in AES gives damage to the sulfur coverage due to continuous ESD. Therefore, a relationship between intensity of ESD and that of AES has not yet been obtained completely. Reason why S+ has been detected in our ESD experiment is still unclear. As described before, there was no S+ signal in ESD in the case of Zr surface [10], and also we could not recognize S+ from MoS2 crystal in ESD experiments. Therefore, in the case of Scovered Ni surface might be ESD-active surface which might be
Fig. 2. (a) Auger spectrum from S-segregated Ni(1 1 0) surface. Primary energy is 2 keV. (b) TOF–ESD spectra from the same surface. There exists H+, O+ and S+ signals.
attributed to hydrogen-stimulated segregation of sulfur as described by Romanenko et al. [6]. After step-wise heating, AES measurements were performed at around room temperatures successively TOF–ESD measurements. During heating the AES intensity of S increases gradually from 650 K and takes maximum at 850 K. Therefore heating-up to 750 K in TOF– ESD measurements, no further increment of S is induced. Fig. 3 shows integrated intensities of TOF spectra of the S+, O+, and H+, where the intensity of the O+ signal is not changed so much, but the S+ has a peak maxima in intensity curve at 440 K, and gradually decreased to the initial level in intensity. This phenomenon has
Fig. 3. Changes of integrated intensity plots of desorbed ions against elevated specimen temperature from S-segregated Ni(1 1 0) surface.
7998
K. Ueda, T. Yagami / Applied Surface Science 254 (2008) 7996–7998
reproducible even though with different the O+ intensity. In the beginning of heating stage, the S+ is gradually vibrated thermally, this vibrating might help desorption of S+. At the higher temperatures, desorption probability is decreased again as normal manners because thermally excited S atoms bond with H atoms to make SHn molecules. Therefore S atoms may desorb as SHn+ with consuming H atoms into the vacuum as shown in Fig. 3. However, in this case sulfur may also desorb as SHn with consuming hydrogen. In conclusion, (1) S+ signal has been detected for the first time in TOF ESD spectrum from sulfur segregated Ni(1 1 0) surface. (2) The ESD yield of S+ increased in elevating temperature and after peak value at 450 K it decreased at higher temperatures. Acknowledgements This experiment is supported by the ‘‘High Tech Research Center’’ Project for Private Universities: matching fund subsidy
from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2006–2008.
References [1] J.E. Demuth, T.N. Rhodin, Surf. Sci. 45 (1974) 249. [2] M.J. Dresser, Ann-M. Lanzillotto, M.D. Alvey, J.T. Yates Jr., Surf. Sci. 191 (1987) 1. [3] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B 43 (2003) 1. [4] J.W. Shabaker, R.R. Davda, G.W. Huber, R.D. Cortright, J.A. Dumesic, J. Catal. 215 (2003) 344. [5] C. Xu, B.E. Koel, Surf. Sci. 327 (1995) 38. [6] O.G. Romanenko, V.P. Shestakov, I.L. Tazhivaeva, Phys. Rev. B 61 (2000) 4934. [7] D.A. Bonnel, T. Yeh, J.D. Kiely, Surf. Sci. 393 (1997) 126. [8] A. Takano, K. Ueda, Jpn. J. Appl. Phys. 30 (1991) 1847. [9] A. Takano, K. Ueda, Appl. Surf. Sci. 60/61 (1992) 693. [10] K. Ojima, K. Ueda, Appl. Surf. Sci. 165 (2000) 141. [11] K. Ueda, A. Takano, Jpn. J. Appl. Phys. 27 (1988) L2029.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Detection of sulfur from Ni(1 1 0) surface using by electron-stimulated desorption spectroscopy Kazuyuki Ueda *, Takeshi Yagami Nano-High-Tech Research Center, Graduate School of Engineering, Toyota Technological Institute, Hisakata 2-Chome, Tempaku-Ku, Nagoya 468-8511, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
On the Ni(1 1 0) surface, H+ and O+ are normally detected by electron-stimulated desorption (ESD). However, C+, N+, S+ are normally insensitive to ESD measurement. In this report S+ signal has been significantly detected by the ESD from the Ni(1 1 0) surface. Behavior of S+ has been observed in the variation of time-of-flight (TOF)–ESD spectra during heating. ß 2008 Elsevier B.V. All rights reserved.
Available online 23 April 2008 Keywords: Sulfur Ni(1 1 0) Electron-simulated desorption ESD
1. Introduction Nickel surface has been intensively investigated as a catalytic material so far [1,2]. In the processes of production of hydrogen from methanol, carbon monoxide (CO) gas appears on the catalyst as catalytic poisoning. When the CO dissociates on the Ni surface, only the C remains on the active sites on the catalyst surface. In this situation, Raney-Nickel has been expected to reduce CO contamination on Ni surface. In this sense Raney-NiSn alloy produces hydrogen gas from glucose effectively, and it has been expected to take the place of platinum catalyst [3,4]. Sn-covered Ni surface prevents CO adsorption by side-block effect as reported by Xu and Koel [5]. However, in the cleaning process of nickel surface in ultra high vacuum (UHV) sulfur (S) atoms segregate on the Ni surface after annealing at higher temperatures. The sulfur should be removed from the catalytic surface in order to prevent sulfur poisoning. Sometimes many repetitions of Ar ion bombardment and annealing treatments are needed to obtain a clean Ni surface. So far Auger electron spectroscopy (AES) has been commonly used to detect the surface sulfur as described in previous reports [6,7]. While an electron-stimulated desorption (ESD) spectroscopy is very useful to detect hydrogen as H+ from the surface. The ESD detects also O+ from CO molecule on the surface as previously published by Takano and Ueda [8,9]. Thus time-of-flight (TOF) type ESD has been used as powerful tool to investigate adsorption processes of O associating with CO adsorption on the nickel
* Corresponding author. Tel.: +81 52 809 1850; fax: +81 52 809 1850. E-mail address: [email protected] (K. Ueda). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.063
surface. In the process of investigation of hydrogen (H) on the Ni(1 1 0) surface prior to the Sn deposition, sulfur layer segregates on the surface on which S+ has been detected for the first time by TOF–ESD measurements. In the ESD spectroscopy the S+ has not been detected so far even though sulfur layer has been detected on the surface by in situ measurements of AES on Pt and Zr surfaces [10]. Therefore detection of the S+ is very rare in ESD spectroscopy. In this report, we would like to just appeal that the S+ signal in ESD has been detected for the first time, and some description about behavior of the S+ depending on the specimen temperatures. 2. Experimental Experimental apparatus is similar to one which has been published previously [11]. It is composed by two chamber made of stainless steel separated by the gate valve, base pressure of main chamber is 8 109 Pa evacuated by 400 l/s ion pump with liquid nitrogen shroud for Ti-getter sublimation pump. AES is cylindrical mirror analyzer (ANELVA, AAS-200V) and laboratory-made TOF– ESD which is consisting of 3-grid meshes and microchannel plates (MCPs) assembly with a fluorescent screen (effective diameter is 70 mm). An effective flight length of desorbed ions is 145 mm. An incidence angle of electron beam is 458 from the surface normal of specimen. Primary electron energy for TOF–ESD is normally used from 10 to 600 eV with a specimen bias between 0 and 20 V. An electron beam with this energy range is easily pulsed by a highspeed pulse generator with duration of 120 ns. A typical TOF–ESD spectra are shown in Fig. 1, where P is photon signal generated by the primary electron irradiation, H+ is proton, O+ is from CO and/or residual oxide layers in local defects, S+ from sulfur layer. The flight-times of all spectra excepting P signal are
K. Ueda, T. Yagami / Applied Surface Science 254 (2008) 7996–7998
7997
Fig. 1. A series of TOF–ESD spectrum depending on the flight time shift to shorter times. Simulation curve of Eq. (1) with a suitable kinetic energy of ion fits on peak positions.
shortened against the specimen bias, because all of positive ions are accelerated by positive specimen bias. This variation obeys an equation of flight time for desorbed ions as shown in Eq. (1) pffiffiffiffiffiffiffiffiffiffi 2mL t ¼ pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ek þ Ek þ qV s
(1)
where t is flight time of ions, Ek is kinetic energy of desorbed ion, q is electric charge, Vs is a positive specimen bias, m is mass of desorbed ion and L is flight length of desorbed ions between specimen surface and detector. The kinetic energy of desorbed ions is easily found from Fig. 1 by fitting to simulation curve of Eq. (1) with the peak energies measured by different specimen biases. The ratio of flight time of the O+ against the S+ is corresponding to the difference of mass of the O+ and the S+ if their kinetic energies are almost equivalent. Signal intensity of the S+ in ESD depends on signal intensity of peak-to-peak height of the S-peak in AES spectrum. Actually the kinetic energy of desorbed ions can be measured by retarding field method using grid meshes, namely, high pass filter method. And also energy distribution curve of the desorbed ions can be transformed numerically from a TOF-spectrum obtained at Vs = 0 by using Eq. (1). 3. Results and discussion Fig. 2(a) and (b) shows an AES spectrum and TOF–ESD spectrum, respectively, from Ni(1 1 0) surface after heating at 800 K. In the AES spectrum, a fine doubled-peak at the bottom of Speak means to be metal sulfide. Because TOF–ESD intensity is obtained from only the top layer of substrate and also depends on strongly chemical situation of sulfur layer which exists as a single flat layer or multi-layer. While AES detects S signal not only from the top layer but also under layers. Furthermore, amount of Ssegregation onto the surface depends on heating conditions and concentration of carbon contamination. Signal intensity of the S+ in ESD is roughly related to the signal intensity of peak-to-peak height of the S-peak in AES spectrum. However, electron beam in AES gives damage to the sulfur coverage due to continuous ESD. Therefore, a relationship between intensity of ESD and that of AES has not yet been obtained completely. Reason why S+ has been detected in our ESD experiment is still unclear. As described before, there was no S+ signal in ESD in the case of Zr surface [10], and also we could not recognize S+ from MoS2 crystal in ESD experiments. Therefore, in the case of Scovered Ni surface might be ESD-active surface which might be
Fig. 2. (a) Auger spectrum from S-segregated Ni(1 1 0) surface. Primary energy is 2 keV. (b) TOF–ESD spectra from the same surface. There exists H+, O+ and S+ signals.
attributed to hydrogen-stimulated segregation of sulfur as described by Romanenko et al. [6]. After step-wise heating, AES measurements were performed at around room temperatures successively TOF–ESD measurements. During heating the AES intensity of S increases gradually from 650 K and takes maximum at 850 K. Therefore heating-up to 750 K in TOF– ESD measurements, no further increment of S is induced. Fig. 3 shows integrated intensities of TOF spectra of the S+, O+, and H+, where the intensity of the O+ signal is not changed so much, but the S+ has a peak maxima in intensity curve at 440 K, and gradually decreased to the initial level in intensity. This phenomenon has
Fig. 3. Changes of integrated intensity plots of desorbed ions against elevated specimen temperature from S-segregated Ni(1 1 0) surface.
7998
K. Ueda, T. Yagami / Applied Surface Science 254 (2008) 7996–7998
reproducible even though with different the O+ intensity. In the beginning of heating stage, the S+ is gradually vibrated thermally, this vibrating might help desorption of S+. At the higher temperatures, desorption probability is decreased again as normal manners because thermally excited S atoms bond with H atoms to make SHn molecules. Therefore S atoms may desorb as SHn+ with consuming H atoms into the vacuum as shown in Fig. 3. However, in this case sulfur may also desorb as SHn with consuming hydrogen. In conclusion, (1) S+ signal has been detected for the first time in TOF ESD spectrum from sulfur segregated Ni(1 1 0) surface. (2) The ESD yield of S+ increased in elevating temperature and after peak value at 450 K it decreased at higher temperatures. Acknowledgements This experiment is supported by the ‘‘High Tech Research Center’’ Project for Private Universities: matching fund subsidy
from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2006–2008.
References [1] J.E. Demuth, T.N. Rhodin, Surf. Sci. 45 (1974) 249. [2] M.J. Dresser, Ann-M. Lanzillotto, M.D. Alvey, J.T. Yates Jr., Surf. Sci. 191 (1987) 1. [3] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B 43 (2003) 1. [4] J.W. Shabaker, R.R. Davda, G.W. Huber, R.D. Cortright, J.A. Dumesic, J. Catal. 215 (2003) 344. [5] C. Xu, B.E. Koel, Surf. Sci. 327 (1995) 38. [6] O.G. Romanenko, V.P. Shestakov, I.L. Tazhivaeva, Phys. Rev. B 61 (2000) 4934. [7] D.A. Bonnel, T. Yeh, J.D. Kiely, Surf. Sci. 393 (1997) 126. [8] A. Takano, K. Ueda, Jpn. J. Appl. Phys. 30 (1991) 1847. [9] A. Takano, K. Ueda, Appl. Surf. Sci. 60/61 (1992) 693. [10] K. Ojima, K. Ueda, Appl. Surf. Sci. 165 (2000) 141. [11] K. Ueda, A. Takano, Jpn. J. Appl. Phys. 27 (1988) L2029.