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On the mechanism of subsurface hydrogen formation on Ni(111)
Volume 175, number I ,2
CHEMICAL PHYSICS LETTERS
30 November 1990
On the mechanism of subsurface hydrogen formation on Ni (
111)
A. Golchet ’ and J.M. White Department of
Chemistryand Centerfor Materials Chemistry, University of Texas at Austin, Austin, TX 78712, USA
Received I2 June 1990; in final form 7 September 1990
By combining ion implantation and thermal dosing of hydrogen and deuterium, we present evidence for population of a subsurface site on Ni( I 11) by activated migration from surface sites.
1. Introduction One of the most interesting features of the interaction of hydrogen with group VIII metals, and one which has been the source of several publications, is the formation of subsurface hydrogen. The presence and concentration of these species is still a subject of considerable debate (see, for example refs. [ l-3 ] ). In this paper, we deal with this subject using Ni( 111) as a substrate. The chemisorption of hydrogen on Ni( 111) generates a temperature-programmed desorption (TPD) peak that saturates at half monolayer (the BZpeak) and a second peak (the PI peak) that saturates at full monolayer coverage. In a recent publication [ 41, we reported additional desorption at the high-temperature side of f12.These sites can be weakly populated using common thermal adsorption procedure but are enhanced considerably by ion implantation, which also gives large amounts of bulk H. The high-temperature signal was attributed to hydrogen from subsurface layers. These subsurface species also stabilize a new surface state desorbing below p, at around 3 10 K. Thus in Ni ( 111) there are four kinds of hydrogen: ( 1) bulk H, (2) subsurface H, (3) normal chemisorbed PI- and P2-H, and (4) surface H stabilized only in the presence of subsurface H. Evidence exists that the subsurface species are potentially important for surface processes. For ex’ On sabbatical leave from The University of Isfahan, Isfahan, Iran. ooO9-2614/90/$
03.50 0 1990 - Elsevier Science Publishers B.V.
ample, Stacy et al. [ 51 report that formation of hydrogen in subsurface sites of group VIII metals affects the binding energy and reactivity of adsorbates on the surface. Thus, it is important to establish connections between the surface and subsurface H species. This point is addressed in the present work. We also discuss the mechanism of formation of these species using ion implantation and TPD. We describe our results with a model which asserts that the channel for H entrance to subsurface sites is activated migration of surface species, consistent with other work [ 2 1, and that only a small portion of the surface hydrogen becomes involved in the subsurface sites.
2. Experimental Experiments were carried out in a turbomolecularpumped ultra-high vacuum chamber which has been described previously [ 4,6]. Briefly, a single-pass cylindrical mirror electron energy analyzer and a quadrupole mass analyzer were used for Auger electron spectroscopy and TPD, respectively. The hydrogen and deuterium were implanted with an ion gun operated at 3 keV. The H2 and Dz were purified by passage through a liquid nitrogen trap and were injected directly into the ionizing region of the ion gun. Thermal dosing was done by backfilling the chamber (continuously pumped) through a leak valve. In the TPD figures, relative mass spectrometer sensitivities (North-Holland)
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for the three isotopic forms of hydrogen are taken into account.
3. Results and discussion After thermally dosing D2 at 100 K ( 83 langmuir, L) and then dosing 3 keV H: (1 x 10” H: cm-*) the thermal desorption spectra of HZ, HD and D2 were obtained, fig. 1. The ion dose was calculated as the measured current at the sample (2 PA) multiplied by the implantation time and divided by the electron charge. Fig. 1, heating rate 12 K/s, shows two maxima. The first occurs near 270 K and is the major feature of implantation. It is ascribed to H in the bulk as reported previously [ 4,7 1. In this region much more Hz and HD desorb than D2. The relatively large HD peak is attributed either to bulk H recombining with surface D or to the formation, during implantation, of some bulk D by momentum transfer from hydrogen ions. But the very weak Dz signal indicates that the 3 keV H: ions do not drive large amounts of D into the bulk. Since the thermal D2 dose at 100 K was not sufficient to saturate the
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Fig. 1.Thermal desorption spectra of Hz, HD and D2 after thermally dosing 83 L Dz followed by 1x 10” Hf cm-* all at 100 K. The coverages are 0.92, 0.37 and 0.09 ML, respectively. The heating rate was 12 K/s and the ion energy in all the implantation experiments reported here was 3 keV.
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surface, the remaining surface sites can be populated during the hydrogen ion dose either by H: collisions or by normal thermally activated dissociative adsorption. These TPD results are consistent with hydrogen atoms diffusing through the bulk and recombining and desorbing without equilibrating in the chemisorption well, in agreement with the suggestion by Comsa et al. [8,9]. These authors studied the angular distribution of molecular hydrogen derived from the bulk of a Ni( 111) crystal and found a strongly forward-peaked desorption. This was interpreted in terms of recombination of hydrogen atoms in a region beneath the surface of elevated potential energy followed by desorption without thermalization. Since, in fig. 1, most of the Dz appears in the normal p1 and fi2desorption peaks, and appears after the bulk (mostly H) has been depopulated, the recombination of bulk atoms does not directly involve PI and pZ sites, even transiently. Referenced to saturation of surface H ( 1 ML), the coverages of HZ, HD and D2 in fig. 1 are 0.92, 0.37 and 0.09 ML, respectively. A thermal 83 L D2 dose gives 0.33 ML of Dz TPD whereas, the total D coverage (reported as D2) in fig. 1 is 0.27 ML. Therefore, some of the surface D desorbed during the implantation [4] of HZ. When compared to spectra from thermal dosing, the desorption curves in fig. 1 show a shoulder around 425 K. This small feature, which is often indistinguishably superimposed on the high-temperature tail of p2, has been ascribed to desorption from subsurface H [ 41. It also appears in the D2 desorption, even though deuterium was thermally dosed prior to the hydrogen implantation. Since this shoulder does not appear when D2 is thermally dosed at 100 K and is not followed by ion implantation, we conclude that the incident ions induce some migration of the chemisorbed surface species into the subsurface. The energy barrier for this penetration is overcome by the collision between surface species and impinging ions (momentum transfer). That the subsurface species desorb at higher temperatures than bulk species implies that the rate of desorption is controlled by activated subsurface-tosurface migration and that the energy barrier is higher than for diffusion, recombination and desorption from the bulk. These observations are consistent with
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the theoretical results reported for Ni, Pd, Pt and Nb by Lagos [3] in which the subsurface bonding is stronger than surface chemisorption. Semiquantitative agreement is also realized with the theoretical and experimental results for H/Pd( 111), reported in refs. [ 10,111, respectively. In the former, using the embedded atom method that provides an approximate method for calculating total energies, it is found that there are three preferred adsorption sites for H on Pd( 111). Two are the usual threefold hollow sites above the surface. The third is an octahedral-like site between the first and second layers of Pd atoms. A small high-temperature TPD state, ascribed to subsurface H, has also been reported on Ni( 110) [ 121. Fig. 2 shows the TPD results (5 K/s) of dosing 277LD2followedby 1.5~10’~H~ cm-‘allat 100 K. Comparison of figs. 1 and 2 and the results of similar experiments for normal doses between 87 and 277 L (not shown here) reveals that the contribution of the H2 desorption in the high-temperature peak is inversely related to the exposure (coverage) of deuterium, before the ion dose. We interpret this as follows: As the D2 exposure increases, the surface sites are increasingly saturated with D and less sur-
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face H accumulates during ion implantation. Thus, the surface contribution of H2 and HD TPD decreases. In fig. 2, the coverages of Hz, HD and D2 molecules are 0.97, 0.51 and 0.55 ML respectively. We now turn to modified Ni( 111). By depositing C, desorption from the bulk of the Ni( 111) crystal persists (though to a lower extent) after ion implantation, while no desorption from surface and/or subsurface is observed (i.e. no pi, p2, high-T shoulder at 425 K or low-T shoulder at 3 16 R). Carbon (0.5 ML) was deposited by prolonged dose of ethylene followed by annealing at 420 K, where there is no diffusion to the bulk [ 131. In fig. 3, the TPD of a 0.55~ lOi D: cm-* dose (no carbon) is plotted in curve (a), and 2 x 10” D: cmm2 dose (with carbon) in curve (b). For comparison, the TPD of 400 L normal thermal dose of D2 (saturation exposure) is plotted in curve (c). The heating rate was 9.5 K/ s. Curves (a) and (b) reveal that surface carbon reduces the penetration of impinging ions, but does not prevent the implantation, recombination and desorption. Considering the ion dose and the intensity of curves (a) and (b) at around 260 K (the bulk desorption signal), fig. 3 reveals that the carbon-cov-
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Fig. 2. Thermal desorption spectra (5 K/s) of Hz, HD and D2 after thermally dosing 277 L D2 followed by 1.5~ 1OrSHz cm-*, all at 100 K. The coverages are 0.97, 0.51 and 0.55 ML, respectively.
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Fig. 3. (a) Thermal desorption spectra of D2 after dosing 0.55~ IO” D: cm-* at 100 K (no carbon). (b) Thermal de sorption spectra of D2 after dosing 2X 10” D: cm-’ at 100 K on carbon-covered surface. (c) TPD of 400 L normal dose D1. The heating rate was 9.5 K/s.
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ered surface accepts 46Ohless of the impinging ions per unit dose than the clean surface (the bulk peak grows linearly with dose on both surfaces). Moreover, because there is no peak at 425 K, fig. 3 shows that the subsurface species do not accumulate in the presence of surface carbon. The effect of carbon on hydrogen desorption has also been studied on Ni( 100) using TPD and temperature-programmed static secondary ion mass spectroscopy (TPSSIMS) [ 141. A comparison of the TPSSIMS and the integrated TPD showed that the two normalized spectra overlapped below 280 K, but the TPSSIMS fell well below the integrated TPD above 280 K. By depositing carbon on the crystal face, the difference decreased and was negligible at carbon coverages higher than 0.14 ML. One proposed explanation was the penetration of hydrogen species to subsurface sites around room temperature when the coverage of surface carbon is low enough (below 0.14 ML) not to preclude this penetration. By analogy it is not surprising that 0.5 ML carbon on Ni ( 111) prevented both surface and subsurface formation in the experiment represented by fig. 3. Our data can be understood in terms of the following model. Assume, in thermal uptake, that hydrogen first dissociatively adsorbs on the surface and then moves to subsurface sites. These processes require adjacent empty sites on the surface and activation energy for penetration to subsurface layers. Adsorption is prevented when the carbon coverage is 0.5 ML. In view of the C-covered experiments, we assume that the subsurface sites are populated only by migration from surface sites and the activation energy for migration is supplied either during the temperature ramp of TPD (thermal energy) or during ion dose (impinging ion energy). Within the framework of this model, desorption from subsurface sites involves two steps: leaving the potential well of the subsurface and migrating back to the surface. These two will probably have different frequency factors and activation energies, and one expects a variation in the peak temperature of the subsurface signal as a function of heating rate. Of course, other activated processes would show similar behavior. The most obvious, surface defects created during implantation, is not important as shown in our previous work [4]. Fig. 4 shows differences of 146
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Fig. 4. Differences of two TPD spectra: that following implanting 2 X 10” H2+cm-’ at 245 K followed by a normal dose of hydrogen minus that of the thermally saturated surface. A variety of heating rates (5,9.5 and 12 K/s) was used. The normal thermal dose following the implantation is 0, 100 and 150 L hydrogen in curves (a) through (c) respectively.
two TPD spectra (that following implanting 2x 10” H: cm-’ at 245 K followed by a normal dose of hydrogen minus that of the thermally saturated surface) for a variety of heating rates (5,9.5 and 12 K/ s). The normal dose following the implantation is 0, 100 and 150 L in curves (a) through (c ), respectively. The desorption signals peaking at 306-317 (depending on the heating rate) are attributed to the new surface, not the subsurface, state we reported earlier [ 41. A comparison of curves (a)-(c) reveals that as the normal dose increases the intensity (coverage) of this surface state also increases. Population of this state requires that hydrogen be present in subsurface sites. Two other peaks in fig. 4 are located in the temperature regions of surface (p, and pz) and subsurface (440 K) H, respectively. Curves (a) and (b) show that the subsurface desorption signals for the heating rates of 5 and 9.5 K/s are weak. This is because the formation and desorption of subsurface H is kinetically limited. Slowing the temperature ramp provides sufficient time for most of the subsurface species to migrate back to the surface before tem-
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perature passes the point where it becomes largely depleted of hydrogen. Under these conditions, the subsurface species emerge within the desorption region of the g2 state. Fig. 4c shows that at a heating rate of 12 K/s, the subsurface signal is better resolved from the surface features. This is consistent with the observation of subsurface sites on silicasupported Pd [ 51, where similar changes were observed as the carrier gas flow rate and heating rate were varied. The observed increase in the peak temperature (4 1Cl-430 K) with heating rate is compatible with involvement of an activated migration process. At least two reasons could be proposed for the high desorption temperatures of the subsurface species. The first, due to the potential well associated with the subsurface sites, and the second, migration of adsorbate from the subsurface to the surface is delayed until the surface is substantially depleted of surface hydrogen by desorption [ 15 1. From this point of view, recombination of the subsurface species occurs on the surface sites transiently using the p2 state sites. Implanting HZ and D: in two separate experiments shows a difference in the relative rate of isotope desorption on the high-temperature trailing edge of the TPD; subsurface HZ desorbing later than subsurface Dz. Similar “inverse isotope effects” are observed on Pt(ll1) [16], Ru(O001) [17] and Pd( 111) [ 181 and have been ascribed to preferential accommodation of hydrogen. On the other hand, TPD of bulk hydrogen from Ni( 111) shows the opposite isotope effect; the onset for D, is about 30 K higher. Thus according to our proposed description, migration of surface species to subsurface layers and vice versa is quite different and kinetically disconnected from diffusion from the bulk, We expect other qualitative differences in the nature of hydrogen populating the subsurface and bulk sites. For the bulk (the activation energy for diffusion of 4 kcal/mol [ 19]), the interaction is rather delocalized, whereas for the subsurface sites we expect the chemisorption bond to be localized with bonding energy comparable with that of surface (23.0? 0.75 kcal/mol for desorption of p2 [ 201). Our model implies that a small fraction of the surface hydrogen converts to subsurface sites during the TPD ramp. This fraction increases using implantation. This is consistent with a partial overlayer/un-
30 November 1990
derlayer conversion of hydrogen upon heating suggested for the H/Ru(OOl) system [17]. These authors suggested that the subsurface binding sites are located between the first and second atomic layers (subsurface sites). This geometry resembles the subsurface octahedral sites below the threefold hollow sites described above [ 10 1. These subsurface sites have the same probability to be populated as the threefold hollow sites above them according to the embedded atom method calculations [ 10 1. On the other hand, the theoretical calculations of Lagos et al. [ 2 1 ] suggest that “limited layers” are available for the subsurface species, from which they concluded that the tightly bonded subsurface species inhibit further thermally driven penetration of hydrogen to other layers. Their calculation is based on an exact solution for coherent quantum diffusion of H into a host lattice of Nb [ 22 1. Coherent diffusion was defined as the propagation in a band narrowed by the lattice distribution surrounding the hydrogen atom. Brief qualitative angular dependence studies of the subsurface desorption spectra after dosing 0.5 x lOI DJ cmT2 show that the subsurface TPD signal is more strongly peaked along the surface normal than the surface desorption. This is qualitatively in agreement with large velocity and high rotational excitation of H2 permeating out of Cu( 110) [23].
4. Summary In summary, we report a desorption state for the H/Ni( 111) system which desorbs on the high-temperature edge ( > 4 10 K) of P2-H2 desorption. This state is readily populated by implantation and is attributed to subsurface hydrogen. It is eliminated if the Ni ( 111) surface is.covered with carbon. This and other kinetic properties indicate that this new state is populated by activated migration from the p, or p2 surface states. We do not draw definite conclusions regarding the location and nature of the subsurface sites. However, we hope that this report will motivate careful assessment of the location of these sites due to their importance in inducing new surface states [4] and 147
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due to their potential for change of reactivity of adsorbates on the surface [ 51.
Acknowledgement Interesting suggestions from and discussions with Dr. X.-Y. Zhu, Dr. S.J. Gravelle, and G.E. Poirier are gratefully acknowledged. This work was supported in part by the Texas Advanced Technology Research Program and by the Robert A. Welch Foundation.
References [ 1 ] P. Feulner, H. Pfnur, P. Hofmann and D. Menzel, Surface Sci. 173 (1986) L576; 184 (1987) L411. [ 21 C.H.F. Peden, D.W. Goodman, J.E. Houston and J. T. Yates Jr., Surface Sci. I84 (1987) L405. [3]M. Lagos, SurfaceSci. 122 (1982) L601. [4] A. Golchet, G.E. Poirier and J.M. White, Surface Sci., in press. [S] K.J. L.eary, J.N. Michaels and A.M. Stacy, Langmuir 4 (1988) 1251. [6] 8. Roop, S.A. Costello, D.R. Mullins and J.M. White, J. Chem. Phys. 86 (1987) 3003.
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[7] I. Chorkendorff, J.N. Russell Jr. and J.T. Yates Jr., Surface Sci. 182 (1987) 375. [ 81 G. Comsa, R. David and B.J. Schumacher, Surface Sci. 85 (1979) 45. 191 G. Comsa and R. David, Chem. Phys. Letters 49 (1977) 412. [lo] C.T. Chan andS.G. Louie, Phys. Rev. B 30 (1985) 1565. [ 111 T.E. Felter and R.H. Stulen, J. Vacuum Sci. Technol. A 3 (1985) 1566. [ 121 D.A. Hanington and P.R. Norton, Surface Sci. 195 ( 1988) L135. [ 131 M.G. Cattania, M. Simonetta and M. Tescari, Surface Sci. 82 (1979) L615. [ 141 X.-Y. Zhu, Ph.D. Dissertation, The University of Texas at Austin (July 1989). [ 151 K.J. Leary, J.N. Michaels and A.M. Stacy, AlChE J. 34 (1988) 263. [ 161 C.M. Greenlief, S. Akhter and J.M. White, J. Phys. Chem. 90 (1986) 4080. [17] J.T. Yates Jr., C.H.F. Peden, J.E. Houston and D.W. Goodman, Surface Sci. 160 (1985) 37. [ 181 G.E. Gdowski and T.E. Felter, J. Vacuum Sci. Technol. A 4 (1986) 1409. [ 191 G. Comsa, R. David and B.J. Schumacher, Surface Sci. 95 (1980) L210. [20] K. Christmann, 0. Schoher, G. Ertl and M. Neumann, SurfaceSci. Rept. 5 (1985) 145. [21] M. Lagos and 1-K. Schuller, Surface Sci. 138 (1984) L161. [ 221 T. McMullen, Solid State Commun. 35 ( 1980) 221. [ 231 G. Comsa and R. David, Surface Sci. 117 (1982) 77.
CHEMICAL PHYSICS LETTERS
30 November 1990
On the mechanism of subsurface hydrogen formation on Ni (
111)
A. Golchet ’ and J.M. White Department of
Chemistryand Centerfor Materials Chemistry, University of Texas at Austin, Austin, TX 78712, USA
Received I2 June 1990; in final form 7 September 1990
By combining ion implantation and thermal dosing of hydrogen and deuterium, we present evidence for population of a subsurface site on Ni( I 11) by activated migration from surface sites.
1. Introduction One of the most interesting features of the interaction of hydrogen with group VIII metals, and one which has been the source of several publications, is the formation of subsurface hydrogen. The presence and concentration of these species is still a subject of considerable debate (see, for example refs. [ l-3 ] ). In this paper, we deal with this subject using Ni( 111) as a substrate. The chemisorption of hydrogen on Ni( 111) generates a temperature-programmed desorption (TPD) peak that saturates at half monolayer (the BZpeak) and a second peak (the PI peak) that saturates at full monolayer coverage. In a recent publication [ 41, we reported additional desorption at the high-temperature side of f12.These sites can be weakly populated using common thermal adsorption procedure but are enhanced considerably by ion implantation, which also gives large amounts of bulk H. The high-temperature signal was attributed to hydrogen from subsurface layers. These subsurface species also stabilize a new surface state desorbing below p, at around 3 10 K. Thus in Ni ( 111) there are four kinds of hydrogen: ( 1) bulk H, (2) subsurface H, (3) normal chemisorbed PI- and P2-H, and (4) surface H stabilized only in the presence of subsurface H. Evidence exists that the subsurface species are potentially important for surface processes. For ex’ On sabbatical leave from The University of Isfahan, Isfahan, Iran. ooO9-2614/90/$
03.50 0 1990 - Elsevier Science Publishers B.V.
ample, Stacy et al. [ 51 report that formation of hydrogen in subsurface sites of group VIII metals affects the binding energy and reactivity of adsorbates on the surface. Thus, it is important to establish connections between the surface and subsurface H species. This point is addressed in the present work. We also discuss the mechanism of formation of these species using ion implantation and TPD. We describe our results with a model which asserts that the channel for H entrance to subsurface sites is activated migration of surface species, consistent with other work [ 2 1, and that only a small portion of the surface hydrogen becomes involved in the subsurface sites.
2. Experimental Experiments were carried out in a turbomolecularpumped ultra-high vacuum chamber which has been described previously [ 4,6]. Briefly, a single-pass cylindrical mirror electron energy analyzer and a quadrupole mass analyzer were used for Auger electron spectroscopy and TPD, respectively. The hydrogen and deuterium were implanted with an ion gun operated at 3 keV. The H2 and Dz were purified by passage through a liquid nitrogen trap and were injected directly into the ionizing region of the ion gun. Thermal dosing was done by backfilling the chamber (continuously pumped) through a leak valve. In the TPD figures, relative mass spectrometer sensitivities (North-Holland)
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for the three isotopic forms of hydrogen are taken into account.
3. Results and discussion After thermally dosing D2 at 100 K ( 83 langmuir, L) and then dosing 3 keV H: (1 x 10” H: cm-*) the thermal desorption spectra of HZ, HD and D2 were obtained, fig. 1. The ion dose was calculated as the measured current at the sample (2 PA) multiplied by the implantation time and divided by the electron charge. Fig. 1, heating rate 12 K/s, shows two maxima. The first occurs near 270 K and is the major feature of implantation. It is ascribed to H in the bulk as reported previously [ 4,7 1. In this region much more Hz and HD desorb than D2. The relatively large HD peak is attributed either to bulk H recombining with surface D or to the formation, during implantation, of some bulk D by momentum transfer from hydrogen ions. But the very weak Dz signal indicates that the 3 keV H: ions do not drive large amounts of D into the bulk. Since the thermal D2 dose at 100 K was not sufficient to saturate the
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Fig. 1.Thermal desorption spectra of Hz, HD and D2 after thermally dosing 83 L Dz followed by 1x 10” Hf cm-* all at 100 K. The coverages are 0.92, 0.37 and 0.09 ML, respectively. The heating rate was 12 K/s and the ion energy in all the implantation experiments reported here was 3 keV.
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surface, the remaining surface sites can be populated during the hydrogen ion dose either by H: collisions or by normal thermally activated dissociative adsorption. These TPD results are consistent with hydrogen atoms diffusing through the bulk and recombining and desorbing without equilibrating in the chemisorption well, in agreement with the suggestion by Comsa et al. [8,9]. These authors studied the angular distribution of molecular hydrogen derived from the bulk of a Ni( 111) crystal and found a strongly forward-peaked desorption. This was interpreted in terms of recombination of hydrogen atoms in a region beneath the surface of elevated potential energy followed by desorption without thermalization. Since, in fig. 1, most of the Dz appears in the normal p1 and fi2desorption peaks, and appears after the bulk (mostly H) has been depopulated, the recombination of bulk atoms does not directly involve PI and pZ sites, even transiently. Referenced to saturation of surface H ( 1 ML), the coverages of HZ, HD and D2 in fig. 1 are 0.92, 0.37 and 0.09 ML, respectively. A thermal 83 L D2 dose gives 0.33 ML of Dz TPD whereas, the total D coverage (reported as D2) in fig. 1 is 0.27 ML. Therefore, some of the surface D desorbed during the implantation [4] of HZ. When compared to spectra from thermal dosing, the desorption curves in fig. 1 show a shoulder around 425 K. This small feature, which is often indistinguishably superimposed on the high-temperature tail of p2, has been ascribed to desorption from subsurface H [ 41. It also appears in the D2 desorption, even though deuterium was thermally dosed prior to the hydrogen implantation. Since this shoulder does not appear when D2 is thermally dosed at 100 K and is not followed by ion implantation, we conclude that the incident ions induce some migration of the chemisorbed surface species into the subsurface. The energy barrier for this penetration is overcome by the collision between surface species and impinging ions (momentum transfer). That the subsurface species desorb at higher temperatures than bulk species implies that the rate of desorption is controlled by activated subsurface-tosurface migration and that the energy barrier is higher than for diffusion, recombination and desorption from the bulk. These observations are consistent with
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the theoretical results reported for Ni, Pd, Pt and Nb by Lagos [3] in which the subsurface bonding is stronger than surface chemisorption. Semiquantitative agreement is also realized with the theoretical and experimental results for H/Pd( 111), reported in refs. [ 10,111, respectively. In the former, using the embedded atom method that provides an approximate method for calculating total energies, it is found that there are three preferred adsorption sites for H on Pd( 111). Two are the usual threefold hollow sites above the surface. The third is an octahedral-like site between the first and second layers of Pd atoms. A small high-temperature TPD state, ascribed to subsurface H, has also been reported on Ni( 110) [ 121. Fig. 2 shows the TPD results (5 K/s) of dosing 277LD2followedby 1.5~10’~H~ cm-‘allat 100 K. Comparison of figs. 1 and 2 and the results of similar experiments for normal doses between 87 and 277 L (not shown here) reveals that the contribution of the H2 desorption in the high-temperature peak is inversely related to the exposure (coverage) of deuterium, before the ion dose. We interpret this as follows: As the D2 exposure increases, the surface sites are increasingly saturated with D and less sur-
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face H accumulates during ion implantation. Thus, the surface contribution of H2 and HD TPD decreases. In fig. 2, the coverages of Hz, HD and D2 molecules are 0.97, 0.51 and 0.55 ML respectively. We now turn to modified Ni( 111). By depositing C, desorption from the bulk of the Ni( 111) crystal persists (though to a lower extent) after ion implantation, while no desorption from surface and/or subsurface is observed (i.e. no pi, p2, high-T shoulder at 425 K or low-T shoulder at 3 16 R). Carbon (0.5 ML) was deposited by prolonged dose of ethylene followed by annealing at 420 K, where there is no diffusion to the bulk [ 131. In fig. 3, the TPD of a 0.55~ lOi D: cm-* dose (no carbon) is plotted in curve (a), and 2 x 10” D: cmm2 dose (with carbon) in curve (b). For comparison, the TPD of 400 L normal thermal dose of D2 (saturation exposure) is plotted in curve (c). The heating rate was 9.5 K/ s. Curves (a) and (b) reveal that surface carbon reduces the penetration of impinging ions, but does not prevent the implantation, recombination and desorption. Considering the ion dose and the intensity of curves (a) and (b) at around 260 K (the bulk desorption signal), fig. 3 reveals that the carbon-cov-
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Fig. 2. Thermal desorption spectra (5 K/s) of Hz, HD and D2 after thermally dosing 277 L D2 followed by 1.5~ 1OrSHz cm-*, all at 100 K. The coverages are 0.97, 0.51 and 0.55 ML, respectively.
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Fig. 3. (a) Thermal desorption spectra of D2 after dosing 0.55~ IO” D: cm-* at 100 K (no carbon). (b) Thermal de sorption spectra of D2 after dosing 2X 10” D: cm-’ at 100 K on carbon-covered surface. (c) TPD of 400 L normal dose D1. The heating rate was 9.5 K/s.
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ered surface accepts 46Ohless of the impinging ions per unit dose than the clean surface (the bulk peak grows linearly with dose on both surfaces). Moreover, because there is no peak at 425 K, fig. 3 shows that the subsurface species do not accumulate in the presence of surface carbon. The effect of carbon on hydrogen desorption has also been studied on Ni( 100) using TPD and temperature-programmed static secondary ion mass spectroscopy (TPSSIMS) [ 141. A comparison of the TPSSIMS and the integrated TPD showed that the two normalized spectra overlapped below 280 K, but the TPSSIMS fell well below the integrated TPD above 280 K. By depositing carbon on the crystal face, the difference decreased and was negligible at carbon coverages higher than 0.14 ML. One proposed explanation was the penetration of hydrogen species to subsurface sites around room temperature when the coverage of surface carbon is low enough (below 0.14 ML) not to preclude this penetration. By analogy it is not surprising that 0.5 ML carbon on Ni ( 111) prevented both surface and subsurface formation in the experiment represented by fig. 3. Our data can be understood in terms of the following model. Assume, in thermal uptake, that hydrogen first dissociatively adsorbs on the surface and then moves to subsurface sites. These processes require adjacent empty sites on the surface and activation energy for penetration to subsurface layers. Adsorption is prevented when the carbon coverage is 0.5 ML. In view of the C-covered experiments, we assume that the subsurface sites are populated only by migration from surface sites and the activation energy for migration is supplied either during the temperature ramp of TPD (thermal energy) or during ion dose (impinging ion energy). Within the framework of this model, desorption from subsurface sites involves two steps: leaving the potential well of the subsurface and migrating back to the surface. These two will probably have different frequency factors and activation energies, and one expects a variation in the peak temperature of the subsurface signal as a function of heating rate. Of course, other activated processes would show similar behavior. The most obvious, surface defects created during implantation, is not important as shown in our previous work [4]. Fig. 4 shows differences of 146
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,
400 500 TEMPERATURE (K)
Fig. 4. Differences of two TPD spectra: that following implanting 2 X 10” H2+cm-’ at 245 K followed by a normal dose of hydrogen minus that of the thermally saturated surface. A variety of heating rates (5,9.5 and 12 K/s) was used. The normal thermal dose following the implantation is 0, 100 and 150 L hydrogen in curves (a) through (c) respectively.
two TPD spectra (that following implanting 2x 10” H: cm-’ at 245 K followed by a normal dose of hydrogen minus that of the thermally saturated surface) for a variety of heating rates (5,9.5 and 12 K/ s). The normal dose following the implantation is 0, 100 and 150 L in curves (a) through (c ), respectively. The desorption signals peaking at 306-317 (depending on the heating rate) are attributed to the new surface, not the subsurface, state we reported earlier [ 41. A comparison of curves (a)-(c) reveals that as the normal dose increases the intensity (coverage) of this surface state also increases. Population of this state requires that hydrogen be present in subsurface sites. Two other peaks in fig. 4 are located in the temperature regions of surface (p, and pz) and subsurface (440 K) H, respectively. Curves (a) and (b) show that the subsurface desorption signals for the heating rates of 5 and 9.5 K/s are weak. This is because the formation and desorption of subsurface H is kinetically limited. Slowing the temperature ramp provides sufficient time for most of the subsurface species to migrate back to the surface before tem-
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perature passes the point where it becomes largely depleted of hydrogen. Under these conditions, the subsurface species emerge within the desorption region of the g2 state. Fig. 4c shows that at a heating rate of 12 K/s, the subsurface signal is better resolved from the surface features. This is consistent with the observation of subsurface sites on silicasupported Pd [ 51, where similar changes were observed as the carrier gas flow rate and heating rate were varied. The observed increase in the peak temperature (4 1Cl-430 K) with heating rate is compatible with involvement of an activated migration process. At least two reasons could be proposed for the high desorption temperatures of the subsurface species. The first, due to the potential well associated with the subsurface sites, and the second, migration of adsorbate from the subsurface to the surface is delayed until the surface is substantially depleted of surface hydrogen by desorption [ 15 1. From this point of view, recombination of the subsurface species occurs on the surface sites transiently using the p2 state sites. Implanting HZ and D: in two separate experiments shows a difference in the relative rate of isotope desorption on the high-temperature trailing edge of the TPD; subsurface HZ desorbing later than subsurface Dz. Similar “inverse isotope effects” are observed on Pt(ll1) [16], Ru(O001) [17] and Pd( 111) [ 181 and have been ascribed to preferential accommodation of hydrogen. On the other hand, TPD of bulk hydrogen from Ni( 111) shows the opposite isotope effect; the onset for D, is about 30 K higher. Thus according to our proposed description, migration of surface species to subsurface layers and vice versa is quite different and kinetically disconnected from diffusion from the bulk, We expect other qualitative differences in the nature of hydrogen populating the subsurface and bulk sites. For the bulk (the activation energy for diffusion of 4 kcal/mol [ 19]), the interaction is rather delocalized, whereas for the subsurface sites we expect the chemisorption bond to be localized with bonding energy comparable with that of surface (23.0? 0.75 kcal/mol for desorption of p2 [ 201). Our model implies that a small fraction of the surface hydrogen converts to subsurface sites during the TPD ramp. This fraction increases using implantation. This is consistent with a partial overlayer/un-
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derlayer conversion of hydrogen upon heating suggested for the H/Ru(OOl) system [17]. These authors suggested that the subsurface binding sites are located between the first and second atomic layers (subsurface sites). This geometry resembles the subsurface octahedral sites below the threefold hollow sites described above [ 10 1. These subsurface sites have the same probability to be populated as the threefold hollow sites above them according to the embedded atom method calculations [ 10 1. On the other hand, the theoretical calculations of Lagos et al. [ 2 1 ] suggest that “limited layers” are available for the subsurface species, from which they concluded that the tightly bonded subsurface species inhibit further thermally driven penetration of hydrogen to other layers. Their calculation is based on an exact solution for coherent quantum diffusion of H into a host lattice of Nb [ 22 1. Coherent diffusion was defined as the propagation in a band narrowed by the lattice distribution surrounding the hydrogen atom. Brief qualitative angular dependence studies of the subsurface desorption spectra after dosing 0.5 x lOI DJ cmT2 show that the subsurface TPD signal is more strongly peaked along the surface normal than the surface desorption. This is qualitatively in agreement with large velocity and high rotational excitation of H2 permeating out of Cu( 110) [23].
4. Summary In summary, we report a desorption state for the H/Ni( 111) system which desorbs on the high-temperature edge ( > 4 10 K) of P2-H2 desorption. This state is readily populated by implantation and is attributed to subsurface hydrogen. It is eliminated if the Ni ( 111) surface is.covered with carbon. This and other kinetic properties indicate that this new state is populated by activated migration from the p, or p2 surface states. We do not draw definite conclusions regarding the location and nature of the subsurface sites. However, we hope that this report will motivate careful assessment of the location of these sites due to their importance in inducing new surface states [4] and 147
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due to their potential for change of reactivity of adsorbates on the surface [ 51.
Acknowledgement Interesting suggestions from and discussions with Dr. X.-Y. Zhu, Dr. S.J. Gravelle, and G.E. Poirier are gratefully acknowledged. This work was supported in part by the Texas Advanced Technology Research Program and by the Robert A. Welch Foundation.
References [ 1 ] P. Feulner, H. Pfnur, P. Hofmann and D. Menzel, Surface Sci. 173 (1986) L576; 184 (1987) L411. [ 21 C.H.F. Peden, D.W. Goodman, J.E. Houston and J. T. Yates Jr., Surface Sci. I84 (1987) L405. [3]M. Lagos, SurfaceSci. 122 (1982) L601. [4] A. Golchet, G.E. Poirier and J.M. White, Surface Sci., in press. [S] K.J. L.eary, J.N. Michaels and A.M. Stacy, Langmuir 4 (1988) 1251. [6] 8. Roop, S.A. Costello, D.R. Mullins and J.M. White, J. Chem. Phys. 86 (1987) 3003.
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[7] I. Chorkendorff, J.N. Russell Jr. and J.T. Yates Jr., Surface Sci. 182 (1987) 375. [ 81 G. Comsa, R. David and B.J. Schumacher, Surface Sci. 85 (1979) 45. 191 G. Comsa and R. David, Chem. Phys. Letters 49 (1977) 412. [lo] C.T. Chan andS.G. Louie, Phys. Rev. B 30 (1985) 1565. [ 111 T.E. Felter and R.H. Stulen, J. Vacuum Sci. Technol. A 3 (1985) 1566. [ 121 D.A. Hanington and P.R. Norton, Surface Sci. 195 ( 1988) L135. [ 131 M.G. Cattania, M. Simonetta and M. Tescari, Surface Sci. 82 (1979) L615. [ 141 X.-Y. Zhu, Ph.D. Dissertation, The University of Texas at Austin (July 1989). [ 151 K.J. Leary, J.N. Michaels and A.M. Stacy, AlChE J. 34 (1988) 263. [ 161 C.M. Greenlief, S. Akhter and J.M. White, J. Phys. Chem. 90 (1986) 4080. [17] J.T. Yates Jr., C.H.F. Peden, J.E. Houston and D.W. Goodman, Surface Sci. 160 (1985) 37. [ 181 G.E. Gdowski and T.E. Felter, J. Vacuum Sci. Technol. A 4 (1986) 1409. [ 191 G. Comsa, R. David and B.J. Schumacher, Surface Sci. 95 (1980) L210. [20] K. Christmann, 0. Schoher, G. Ertl and M. Neumann, SurfaceSci. Rept. 5 (1985) 145. [21] M. Lagos and 1-K. Schuller, Surface Sci. 138 (1984) L161. [ 221 T. McMullen, Solid State Commun. 35 ( 1980) 221. [ 231 G. Comsa and R. David, Surface Sci. 117 (1982) 77.