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The structure of sulfur overlayers on stepped Mo(910) and kinked Mo(28,4,1) single crystal surfaces
Surface Science 272 (1992)326-333 North-Holland
surface science
The structure of sulfur overlayers on stepped Mo(910) and kinked Mo(28,4,1) single crystal surfaces Colette C. Knight a n d G a b o r A. Somorjai Center for Adcanced Materials, Materials Sciences Dicision, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA and Department of Chemistry, University of California, Berkeley., CA 94720, USA Received 14 October 1991; accepted for publication 4 December 1991
We have studied the ordered surface structures that develop upon sulfur adsorpt|on on the stepped Mo(910) and kinked Mo(28,4,1) surfaces. The influence of adsorbed sulfur on the terrace width and step height has also been investigated. While the clean Mo(910) and Mo(28,4,1) surfaces have predominantly monatomic height steps, sulfur adsorption le~,ds to a restructuring of the surface through the formation of two atom height steps. On both surfaces, c(2 x 2) and then p(2 x 1) structures form with increasing sulfm coverage. The steps are domain selective for the p(2 x 1) structure. Only those domains with the longer side of the unit cell perpendicular to the step edges a, c well ordered. This suggests a particularly strong adsorbate-step interaction.
1. Introduction
Clean and sulfur covered molybdenum single crystal surfaces with low Miller indices have been successfully used as model surfaces for important industrial catalysts [1,2]. These include molybdenum sulfide catalysts which are extensively used to remove sulfur from sulfur containing molecules in the hydrodesulfurization 0rocess. Single crystals have the advantage over bulk catalysts in that the structure and composition of their surfaces are more readily characterized. As a result, the relationship between the catalytic activity and the initial surface composit.,on and structure can be determined. Complex heterogeneous catalysts are composed not only of low Miller index surfaces of closed packed structure, but also of high Miller index surfaces that may have high concentrations of atomic steps and kinks. Therefore it is important to understand the structure of clean and adsorbate covered stepped and kinked surfaces.
Stepped surfaces have recently received considerable attention. An example is the observation of transitions between double and single atomic steps on vicinal Si(001) surfaces. Using low energy electron diffraction (LEED), Aumann et al. [3]. have found that on vicinal Si(001) surfaces, the structure of the surface changes from having double to single height steps at high temperatures. Adsorbates have also been shown to have a pronounced effect on the step structure of vicinal surfaces. Lanzillotto and Benasek [4] and Comsa et al. [5] have shown that both sulfur and oxygen lead to doubling of the step height on vicinal Pt(ll 1)surfaces. On the other hand, Dowben et al. [6] have reported that dissociatively adsorbed nitrogen induces reconstruction that leads to an increase in the step density on the Fe(S)-[11(100) × 2(110)] surface. Their results indicate that with the smallest surface concentration of nitrogen, terraces six to seven atoms wide and monatomic height steps are formed, indicat-
0039-602bI/92/$05.00 ~; 1992 - Elsevier Science Publishers B.V. All rights reserved
C.C. Knight, G.A. Somorjai ,/S overlayers on Mo(910) and Mo(28,4,1)
ing that nitrogen pins the steps. At higher nitrogen coverages, a c(2 × 2) structure attributed to the surface nitride Fe4N is seen. The authors suggested that by decreasing the step height and the terrace width, the strain caused by the lattice mismatch between the nitride overlayer and (100) terraces of the substrate is reduced. This paper presents results of investigations of sulfur overlayers on the stepped (910) and kinked (28,4,1) molybdenum surfaces. We have found that sulfur forms ordered overlayers and induces step-doubling on these surfaces. Following a description of the experiment, we discuss the structure of the clean vicinal Mo(100) surfaces. Then the surface structures and surface rearrangements induced by sulfur chemisorption are presented. The remainder of the manuscript is devoted to a discussion and summary of the results.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system described previously [2,7]. The chamber is equipped with a four grid retarding field energy analyzer for Auger electron spectroscopy (AES) and LEED and a quadrupole mass spectrometer for temperature programmed desorption (TPD) and residual gas analysis. The single crystals used were approximately one centimeter in diameter and less than one millimeter thick. They were oriented to within less than 1° of the desired face and polished using standard metallurgical techniques. The major impurity was carbon which was removed from the near surface region by repeated heating in oxygen (5 × 10 -7 Torr at 1600 K). Remaining oxygen was removed by heating the crystals in UHV above 1900 K. Sulfur was deposited using an electrochemical source (Ag/AgI/AgzS) described elsewhere [8]. Coverages of one, three-quarters, two-thirds and half of a monolayer of sulfur were ordered on the Mo(100) surface by annealing at approximately 970, 1220, 1370 and 1620 K, respectively. The coverages were determined by AES in conjunction with the appearance of ordered overlayers observed by LEED.
327
3. Results
3.1. Structure of the clean vicinal Mo(lO0) surfaces The structures of the different molybdenum crystal surfaces were determined using LEED. Diffraction experiments ~i!<,v the determination of the terrace width, st~.p height and direction, enabling a description of the structure of stepped surfaces. Regular atomic steps on single crystal surfaces cause L E E D beams to have spot profiles that oscillate between doublets and singlets as the primary energy is varied. Each non-equivalent set of diffraction beams has characteristic voltages at which the beams appear as doublets. The direction of spot splitting indicates the direction of the step edge periodicity. Since the diffraction pattern is a map of the reciprocal lattice of the surface, the steps run perpendicular to the observed direction of spot splitting. The relative separation between the spots in a doublet depend on t.he average terrace width. This relationship is shown beio'.~.,:
AKol/Kol =a/w, where AK01 = the distance between split spots in the (01) direction, Ko~ = the distance between nearest integer spots in the (01) direction, a = the terrace lattice constant, w = the widtk of the terrace. From the energy dependence on the spot splitting, the step height can be determined. The voltages at which singlets are seen, correspond to constructive interference of electrons scattered from atoms located on adjacent terraces. Doublets are seen at energies where electrons from atoms on neighboring terraces scatter out of phase. These energies can be calculated within the kinematic approximation using an equation first derived by Henzler [9,10]. These energies are independent of the step orientation and terrace width. The calculated and experimental energies of singlets and doublets for the (01) beam are given in table 1 for the (910) and (28,4,1) molybdenum crystal surfaces. By comparison with the frequency of oscillations between singlets and doublets, we have determined that monatomic height steps are present on the clean surfaces. Since monatomic height steps are present on
C C Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1)
328
Table 1 Calculated and experimental energies at which sing'ets and doublets are seen for monatomic and diatomic steps on vicinal Mo(100) surfaces for the symmetrically equivalent (01), (01), (10) and (]0) beams s
Calculated energies (eV)
1.5 2 2.6 3 3.5 4 4.5 5 5.5
"
Monatomic steps
Diatomie steps
Singlets
Singlets
Doublets 24
Experimental energies (eV) Mo(910)
Doublets
Singlets
Mo(28,4,1) Doublets
Singlets
Doublets
18
42
40
24 70
45 69
32
103
42 144
100 130
54
194
70
99
128
69 251
85 105 123 146
J.5 7 7.5 8 8.5
168 193 221 251 282
both clean surfaces, spot splitting is observed at similar energies. From the diffraction pattern, it was determined that AKo~/Kol is ~ 1 / 4 . 7 for the Mo(910) surface. This means that there are four molybdenum atoms on an average terrace, and a shift of half an atomic distance between
neighboring terraces. The steps run along the [001] direction of the crystal. For the Mo(28,4,1) surface, AKm/Ko~ is ~ 1/3.6. This indicates that the average terrace is composed of three molybdenum atoms, and there is a shift of half an atomic distance between neighboring terraces.
Table 2 Experimental energies at which sirtglets and doublets a~e ~ee.n ior the (0]) and equivalent beams for sulfur covered surfaces. The data given was obtained for the Mo(28,4,1) surface, the calculated energies listed for comparison were obtained by assuming that two atom height steps were present Calculated energies (eV)
Experimental energies (eV)
Clean surface
c(2 x 2)S
Singlets 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
Doublets
Singlets
(1 x 1)S Doublets
Singlets
Doublets
18 24 32 42
30 45
54 69
42 60
72 85
105
96 125
123 146
58 72 86 115
135
125
153
150
180
200
168 193 221 251
160 220
220 250
CC. Knight, G.A. Somorjai / S ocerlayers on Mo(910) and Mo(28,4,1) (Ol)
Kol
I t
*
(~)
I
•
* (10)
(a)
(01)
329
not be detected. On the vicinal surfaces, c(2 × 2) diffraction features are seen at lower coverages (below 0.1 ML) than on the flat Mo(100) surface. In addition to this, the steps are domain selective. Only the domains of the p(2 × 1) structure with the longer side of the unit cell perpendicular to the step edges are well ordered. Diffuse streaking is observed in the regions of k-space where sharp spots from the second domain would be expected. For the well ordered domain, the diffraction spots are as sharp as the sharpest integer order spots. This indicates the presence of rather large domains, ~ 100 .~, or more, while the domains of the second orientation are definitely much smaller. Typical diffraction patterns of the
* AK°II"]Kol I
I l
e
(oo)
•
(tO)
O •
• O
(b) Fig. 1. Schematic representations of the diffraction patterns observed for the clean Mo(910) (a) and Mo(28,4,1) (b) surfaces for an incident beam energy of 70 eV. The (00), (01) and (10) beams, AKoi and K01 are labelled on the figures.
Consecutive terraces are separated by monatomic steps running along the [0]4] direction. The diffraction patterns observed for the vicinal surfaces are schematically presented in fig. 1. 3.2. Chemisorption of sulfur on flat and vicinal
molybdenum (100) surfaces On Mo(lO0), a variety of sulfur overlayers are fr~rn~,~rl oe o f l l n o t i ~ n ~ f t h ~ elmlfllr ¢,r~x;orsoo oncl
annealing temperature [7,11,12]. At coverages in the range of 0.2-0.5, 0.6-0.7, 0.75-0.9 and 0.951.0 ML, c(2 x 2), (12}), c(4 × 2) and p(2 x 1) ordered structures are observed. On both the stepped Mo(9!0) and kinked Mo(28,4,1)surfaces, only the c(2 × 2) and p(2 x 1) structures are observed, while the (2 i) and c(4 x 2) structures can-
~i~i~!~ii~!~i
¸~i~!~':~!~:~:~i::~!~::~ili~i~i:i~i~!iiii~ii~i~ill ¸i!i!~!~i~i!~!;~ii~i!i~i~i!i~ili!!~i!~i~!~i~
Fig. 2. Diffraction patterns of (a) Mo(910)-p(2x 1)S (beam voltage = 125 eV) and (b) Mo(28,4,1)-p(2 x 1)S surfaces (beam voltage = 130 eV). In both cases, only the domains with the longer side of the unit cell perpendicular to the step edges are well ordered. Diffuse streaking is obse.wed where sharp spots from the second domain would be expected.
330
C.C. Knight, G.A. Somorjai / S ocerlayers on Mo(910) and Mo(28,4,1)
4. Discussion
Monatomic height steps are present on the clean Mo(910) and Mo(28,4,1)surfaces. The structure of a stepped surface can be determined once the size, direction and energies of diffraction beam splitting due to periodic steps are known. These quantities have been determined for both the clean and adsorbate covered stepped surfaces. For the clean (910) surface, spots are split in the (01) [13] direction and therefore the steps run in the [001] direction. In the case of the kinked (28,4,1) surface, the spots are split along the direction that is ~ 14° away from the (01) direction. Thus steps run along the [014] direction. From the size of spot splitting, the average terrace width has been determined. On the Mo(910) surface, the average terrace width is four atoms and on the Mo(28,4,1) surface, the average terrace width is three atoms. The spot splitting for both these surfaces indicates the presence of monatomic steps and kinks. Consecutive terraces are displaced by half of a lattice constant relative to each other [14]. The structures of these vicinal ~v~u~ ~'~r~r~n~,,,,, surfaces are presented in fig. 4. Gn the sulfur covered surfaces, noticeable changes were observed. Both the size of the spot splitting and th,: energies at which spots appear split are changed. This has been shown explicitly for the sulfur covered Mo(28,4,1) surface in fig. 3 and table 2 and is similar for the other sulfur covered vicinal s~;rfaces. This shows that the adsorption and subsequent annealing of the sulfur overlayer leads to a restructuring of these vicinal (100) surfaces. We have found that on the sulfur covered surfaces, double height steps and kinks are present. This is different from results reported on other vicinal body centered cubic (100) surfaces. Dowben et al. [6] found that nitrogen induced a double step to single step transition on vicinal Fe(100) surfaces. In the enti:e coverage range studied ( < 0.1-1 ML), salfur induced step-doublir~g. In this respect, our results are also different from results of Lanzillotto and Bernasek [4] on sulfur induces step-doubling of the Pt(S)[6(111 ) × (11)0)] surface. While these authors have found that annealing surfaces with low sulfur w
Fig. 3. Diffraction pattern of the Mo(28,4,1)-(1 x 1)S structure formed on the vicinal surfaces between sulfur coverages of 0.5 and 0.75 ML (beam voltage = 86 eV). AK01/Kol is ~ 1/7 as compared to approximately 1/3.5 for the clean surface. This surface is composed of two atom height steps and seven atom wide terraces.
p(2 × 1) structures are shown in fig. 2. At sulfur coverages between one-half and three-quarters of a monolayer, a poorly ordered (1 × 1) diffraction pattern was observed on the vicinal surfaces. A diffraction pattern of this surface showing the split spots in a pronounced background which indicates the disorder is shown in fig. 3. The energies at which the (01) and other symmetrically equivalent integer beams appeared as singlets and doublets for the c(2 × 2)S and (1 × 1)S structures on the kinked Mo(28,4,1) surface are given in table 2. The agreement in the frequency of oscillations between the experimental values and those calculated for double height steps is evidence that the adsorption of sulfur induces the formation of diatomic steps. Observation of the diffraction pattern of the (1 x 1) sulfur covered kinked surface shows that AKo~/Kol is reduced to 1/7. This spot splitting distance is consistent with the presence of diatomic steps separating terraces that are on the average composed of seven molybdenum atoms This sulfur induced step-doubling is completely reversible. Heating the crystal above 1900 K to desorb the sulfur restores the monatomic steps characteristic of the clean surfaces.
CC. Knight, G.A. Somorjai / S orerlayers on Mo(910) and Mo(28,4,1)
331
l O~Ol [i9o]
1q [oox]
L
I q q q q ¢
(a)
q
[oi4]
! U
/ [o101 [~70] (b) Fig. 4. Diagrammatic representations of real-space Mo(910) (a) and Mo(28,4,!) (b) surfaces. The upper arrow in (a) indicates both the [010] direction in the plane of the terrace and the [190] direction which is parallel to the macroscopic surface. The lower arrow in (b) indicates both the [010] direction in the plane of the terrace and the [770] direction which is parallel to the macroscopic sur[ace.
332
C.C. Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1)
coverages ( < 0.05 ML) induce step-doubling, they report that surface coverages exceeding 0.25 ML leave the platinum surface unreconstructed. The presence of steps can result in preferential orientation of one ordered domain. For the p(2 × 1) structures on the vicinal Mo(100) surfaces, the domains with the longer side of the unit cell perpendicular to the step edges are well ordered. Within these domains, sulfur atoms can be located on all of the metal atoms on or close to the step edges. Preferential ordering of these domains maximizes the number of sulfur-step atom bonds, suggesting a strong interaction between sulfur atoms and metal step edges. This is in agreement w~th the explanation of Lanzillotto and Bernasek [4] of why sulfur induced step-doubling occurred at very low sulfur coverages on vicinal Pt(lll)surfaces. These authors attributed this adsorbate-induced restructuring to the preferential adsorption of sulfur at the (100) platinum step edges. We have observed c(2 × 2) diffraction patterns on the vicinal Mo(100) surfaces at sulfur coverages less than 0.1 ML. Similar diffraction patterns are not observed on the flat Mo(100) surface at comparable sulfur coverages. Tl.~s indicates that at these low coverages, sulfur atoms coalesce and order along the step edges. This further implicates step edges as important sites for sulfur nuc!eation and ordering on the vicinal ~urfa.ces. The (12i) and c(4 x 2) structures that are observed on Mo(100) are not formed on these vicinal surfaces. It is not possible to arrange the (2 ~) overlayer on the terraces, and still decorate all of the metal step atoms with adsorbate atoms. Instead, at these coverages a poorly ordered (1 × 1) diffraction pattern is observed. The ordering of sulfur atoms along the step edges determines which ordered structures are formed on the terraces and which ordered domain grows. atlt ~ u l l t . , l u s l o n , U l l e l l l l S U l p t l U l ? o f ~u~tu~ _..1£.._ w a s s e e n
to restructure vicinal Mo(100) surfaces, leading to the formation of two atom height steps and terraces of doubled width. Several studies have shown that the chemisorption of oxygen on stepped platinum surfaces leads to the formation of two atom height steps [5,15]. Several explanations have been advanced to explain step restruc-
turing. One explanation is that the surface free energy is lowered by decreasing the step density. Another, proposed by Dowben [6], is that adsorbate induced surface restructuring of steps occurs to relieve the strain induced by the lattice mismatch between the overlayer and substrate. A possible explanation for the restructuring observed in this work is that the number of most strongly bound chemisorption sites is increased by such a restructuring. This is similar to the explanation advanced by Lanzillotto and Bernasek [4].
5. Summary Using LEED, we have shown that sulfur forms a variety of ordered overlayers on as well as restructures stepped and kinked Mo(100) surfaces. We have also shown that for ordered sulfur structures, the domains that maximize adsorbate-step interactions are preferentially ordered.
Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the US Department of Energy under Contract No. DEAC03-76SF00098. Travel support by A T & T Bell Laboratories is gratefully acknowledged.
References [1] M.E. Bussell and G.A. Somorjai, J. Catal. 106 (1987) 93. [2] M.E. Busseli, A.J. Gellman and G.A. Somorjai, Catal. Lett. 1 (1988) 195. [3] C.E. Aumann, J.J. de Miguei, R. Kariotis and M.G. Lagally, presented at the 38th National Symposium of the American Vacuum Society, Seattle, Washington, November 11-15, 1991. [4] A.-M. Lanzillotto and S.L. Bernasek, J. Chem. Phys. 84 (1986) 3553. [5] G. Comsa, G. Mechtersheimer and B. Poelsema, Surf. Sci. 119 (1982) 159, 172. [6] P.A. Dowben, M. Grunze and R.G. Jones, Surf. Sci. 109 (1981) L519. [7] C.C. Knight and G,A. Somorjai, Surf. Sci. 240 (1990) 101.
C.C. Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1) [8] [9] [10] [11]
C. Wagner, J. Chem. Phys. 21 (1953) 1819. M. Henzler, Surf. Sci. 22 (1970) 12. M. Henzler, Appl. Phys. 9 (1976) 11. M. Salmeron, G.A. Somorjai and R.R. Chianelli, Surf. Sci. 127 (1983) 526. [12] V. Maurice, L. Peralta, Y. Berthier and J. Oudar, Surf. Sci. 148 (1984) 623. [13] The (01) reciprocal space direction given is related to the terrace periodicity, i.e., this vector is identical to (01) vector on the flat (100) surface. [14] Strictly speaking, the clean unrestructured surfaces have the same periodicity as the restructured surfaces with two
[151
333
atom height steps. This periodicity, 9-fold and 7-fold perpendicular to the step edge of the Mo(910) and Mo(28,4,1) surfaces, respectively, arises from the fact that consecutive terraces are shifted by half an atomic distance relative to each other on the unrestructured surfaces. This can be seen in fig. 4. However, calculations in the kinematic approximation show that for the unrestructured surfaces with monatomic steps, the intensities of the n / 9 and n / 7 order spots with odd n are very low. D. Biakely, PhD Thesis, University of California, Berkeley, 1976.
surface science
The structure of sulfur overlayers on stepped Mo(910) and kinked Mo(28,4,1) single crystal surfaces Colette C. Knight a n d G a b o r A. Somorjai Center for Adcanced Materials, Materials Sciences Dicision, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA and Department of Chemistry, University of California, Berkeley., CA 94720, USA Received 14 October 1991; accepted for publication 4 December 1991
We have studied the ordered surface structures that develop upon sulfur adsorpt|on on the stepped Mo(910) and kinked Mo(28,4,1) surfaces. The influence of adsorbed sulfur on the terrace width and step height has also been investigated. While the clean Mo(910) and Mo(28,4,1) surfaces have predominantly monatomic height steps, sulfur adsorption le~,ds to a restructuring of the surface through the formation of two atom height steps. On both surfaces, c(2 x 2) and then p(2 x 1) structures form with increasing sulfm coverage. The steps are domain selective for the p(2 x 1) structure. Only those domains with the longer side of the unit cell perpendicular to the step edges a, c well ordered. This suggests a particularly strong adsorbate-step interaction.
1. Introduction
Clean and sulfur covered molybdenum single crystal surfaces with low Miller indices have been successfully used as model surfaces for important industrial catalysts [1,2]. These include molybdenum sulfide catalysts which are extensively used to remove sulfur from sulfur containing molecules in the hydrodesulfurization 0rocess. Single crystals have the advantage over bulk catalysts in that the structure and composition of their surfaces are more readily characterized. As a result, the relationship between the catalytic activity and the initial surface composit.,on and structure can be determined. Complex heterogeneous catalysts are composed not only of low Miller index surfaces of closed packed structure, but also of high Miller index surfaces that may have high concentrations of atomic steps and kinks. Therefore it is important to understand the structure of clean and adsorbate covered stepped and kinked surfaces.
Stepped surfaces have recently received considerable attention. An example is the observation of transitions between double and single atomic steps on vicinal Si(001) surfaces. Using low energy electron diffraction (LEED), Aumann et al. [3]. have found that on vicinal Si(001) surfaces, the structure of the surface changes from having double to single height steps at high temperatures. Adsorbates have also been shown to have a pronounced effect on the step structure of vicinal surfaces. Lanzillotto and Benasek [4] and Comsa et al. [5] have shown that both sulfur and oxygen lead to doubling of the step height on vicinal Pt(ll 1)surfaces. On the other hand, Dowben et al. [6] have reported that dissociatively adsorbed nitrogen induces reconstruction that leads to an increase in the step density on the Fe(S)-[11(100) × 2(110)] surface. Their results indicate that with the smallest surface concentration of nitrogen, terraces six to seven atoms wide and monatomic height steps are formed, indicat-
0039-602bI/92/$05.00 ~; 1992 - Elsevier Science Publishers B.V. All rights reserved
C.C. Knight, G.A. Somorjai ,/S overlayers on Mo(910) and Mo(28,4,1)
ing that nitrogen pins the steps. At higher nitrogen coverages, a c(2 × 2) structure attributed to the surface nitride Fe4N is seen. The authors suggested that by decreasing the step height and the terrace width, the strain caused by the lattice mismatch between the nitride overlayer and (100) terraces of the substrate is reduced. This paper presents results of investigations of sulfur overlayers on the stepped (910) and kinked (28,4,1) molybdenum surfaces. We have found that sulfur forms ordered overlayers and induces step-doubling on these surfaces. Following a description of the experiment, we discuss the structure of the clean vicinal Mo(100) surfaces. Then the surface structures and surface rearrangements induced by sulfur chemisorption are presented. The remainder of the manuscript is devoted to a discussion and summary of the results.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system described previously [2,7]. The chamber is equipped with a four grid retarding field energy analyzer for Auger electron spectroscopy (AES) and LEED and a quadrupole mass spectrometer for temperature programmed desorption (TPD) and residual gas analysis. The single crystals used were approximately one centimeter in diameter and less than one millimeter thick. They were oriented to within less than 1° of the desired face and polished using standard metallurgical techniques. The major impurity was carbon which was removed from the near surface region by repeated heating in oxygen (5 × 10 -7 Torr at 1600 K). Remaining oxygen was removed by heating the crystals in UHV above 1900 K. Sulfur was deposited using an electrochemical source (Ag/AgI/AgzS) described elsewhere [8]. Coverages of one, three-quarters, two-thirds and half of a monolayer of sulfur were ordered on the Mo(100) surface by annealing at approximately 970, 1220, 1370 and 1620 K, respectively. The coverages were determined by AES in conjunction with the appearance of ordered overlayers observed by LEED.
327
3. Results
3.1. Structure of the clean vicinal Mo(lO0) surfaces The structures of the different molybdenum crystal surfaces were determined using LEED. Diffraction experiments ~i!<,v the determination of the terrace width, st~.p height and direction, enabling a description of the structure of stepped surfaces. Regular atomic steps on single crystal surfaces cause L E E D beams to have spot profiles that oscillate between doublets and singlets as the primary energy is varied. Each non-equivalent set of diffraction beams has characteristic voltages at which the beams appear as doublets. The direction of spot splitting indicates the direction of the step edge periodicity. Since the diffraction pattern is a map of the reciprocal lattice of the surface, the steps run perpendicular to the observed direction of spot splitting. The relative separation between the spots in a doublet depend on t.he average terrace width. This relationship is shown beio'.~.,:
AKol/Kol =a/w, where AK01 = the distance between split spots in the (01) direction, Ko~ = the distance between nearest integer spots in the (01) direction, a = the terrace lattice constant, w = the widtk of the terrace. From the energy dependence on the spot splitting, the step height can be determined. The voltages at which singlets are seen, correspond to constructive interference of electrons scattered from atoms located on adjacent terraces. Doublets are seen at energies where electrons from atoms on neighboring terraces scatter out of phase. These energies can be calculated within the kinematic approximation using an equation first derived by Henzler [9,10]. These energies are independent of the step orientation and terrace width. The calculated and experimental energies of singlets and doublets for the (01) beam are given in table 1 for the (910) and (28,4,1) molybdenum crystal surfaces. By comparison with the frequency of oscillations between singlets and doublets, we have determined that monatomic height steps are present on the clean surfaces. Since monatomic height steps are present on
C C Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1)
328
Table 1 Calculated and experimental energies at which sing'ets and doublets are seen for monatomic and diatomic steps on vicinal Mo(100) surfaces for the symmetrically equivalent (01), (01), (10) and (]0) beams s
Calculated energies (eV)
1.5 2 2.6 3 3.5 4 4.5 5 5.5
"
Monatomic steps
Diatomie steps
Singlets
Singlets
Doublets 24
Experimental energies (eV) Mo(910)
Doublets
Singlets
Mo(28,4,1) Doublets
Singlets
Doublets
18
42
40
24 70
45 69
32
103
42 144
100 130
54
194
70
99
128
69 251
85 105 123 146
J.5 7 7.5 8 8.5
168 193 221 251 282
both clean surfaces, spot splitting is observed at similar energies. From the diffraction pattern, it was determined that AKo~/Kol is ~ 1 / 4 . 7 for the Mo(910) surface. This means that there are four molybdenum atoms on an average terrace, and a shift of half an atomic distance between
neighboring terraces. The steps run along the [001] direction of the crystal. For the Mo(28,4,1) surface, AKm/Ko~ is ~ 1/3.6. This indicates that the average terrace is composed of three molybdenum atoms, and there is a shift of half an atomic distance between neighboring terraces.
Table 2 Experimental energies at which sirtglets and doublets a~e ~ee.n ior the (0]) and equivalent beams for sulfur covered surfaces. The data given was obtained for the Mo(28,4,1) surface, the calculated energies listed for comparison were obtained by assuming that two atom height steps were present Calculated energies (eV)
Experimental energies (eV)
Clean surface
c(2 x 2)S
Singlets 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
Doublets
Singlets
(1 x 1)S Doublets
Singlets
Doublets
18 24 32 42
30 45
54 69
42 60
72 85
105
96 125
123 146
58 72 86 115
135
125
153
150
180
200
168 193 221 251
160 220
220 250
CC. Knight, G.A. Somorjai / S ocerlayers on Mo(910) and Mo(28,4,1) (Ol)
Kol
I t
*
(~)
I
•
* (10)
(a)
(01)
329
not be detected. On the vicinal surfaces, c(2 × 2) diffraction features are seen at lower coverages (below 0.1 ML) than on the flat Mo(100) surface. In addition to this, the steps are domain selective. Only the domains of the p(2 × 1) structure with the longer side of the unit cell perpendicular to the step edges are well ordered. Diffuse streaking is observed in the regions of k-space where sharp spots from the second domain would be expected. For the well ordered domain, the diffraction spots are as sharp as the sharpest integer order spots. This indicates the presence of rather large domains, ~ 100 .~, or more, while the domains of the second orientation are definitely much smaller. Typical diffraction patterns of the
* AK°II"]Kol I
I l
e
(oo)
•
(tO)
O •
• O
(b) Fig. 1. Schematic representations of the diffraction patterns observed for the clean Mo(910) (a) and Mo(28,4,1) (b) surfaces for an incident beam energy of 70 eV. The (00), (01) and (10) beams, AKoi and K01 are labelled on the figures.
Consecutive terraces are separated by monatomic steps running along the [0]4] direction. The diffraction patterns observed for the vicinal surfaces are schematically presented in fig. 1. 3.2. Chemisorption of sulfur on flat and vicinal
molybdenum (100) surfaces On Mo(lO0), a variety of sulfur overlayers are fr~rn~,~rl oe o f l l n o t i ~ n ~ f t h ~ elmlfllr ¢,r~x;orsoo oncl
annealing temperature [7,11,12]. At coverages in the range of 0.2-0.5, 0.6-0.7, 0.75-0.9 and 0.951.0 ML, c(2 x 2), (12}), c(4 × 2) and p(2 x 1) ordered structures are observed. On both the stepped Mo(9!0) and kinked Mo(28,4,1)surfaces, only the c(2 × 2) and p(2 x 1) structures are observed, while the (2 i) and c(4 x 2) structures can-
~i~i~!~ii~!~i
¸~i~!~':~!~:~:~i::~!~::~ili~i~i:i~i~!iiii~ii~i~ill ¸i!i!~!~i~i!~!;~ii~i!i~i~i!i~ili!!~i!~i~!~i~
Fig. 2. Diffraction patterns of (a) Mo(910)-p(2x 1)S (beam voltage = 125 eV) and (b) Mo(28,4,1)-p(2 x 1)S surfaces (beam voltage = 130 eV). In both cases, only the domains with the longer side of the unit cell perpendicular to the step edges are well ordered. Diffuse streaking is obse.wed where sharp spots from the second domain would be expected.
330
C.C. Knight, G.A. Somorjai / S ocerlayers on Mo(910) and Mo(28,4,1)
4. Discussion
Monatomic height steps are present on the clean Mo(910) and Mo(28,4,1)surfaces. The structure of a stepped surface can be determined once the size, direction and energies of diffraction beam splitting due to periodic steps are known. These quantities have been determined for both the clean and adsorbate covered stepped surfaces. For the clean (910) surface, spots are split in the (01) [13] direction and therefore the steps run in the [001] direction. In the case of the kinked (28,4,1) surface, the spots are split along the direction that is ~ 14° away from the (01) direction. Thus steps run along the [014] direction. From the size of spot splitting, the average terrace width has been determined. On the Mo(910) surface, the average terrace width is four atoms and on the Mo(28,4,1) surface, the average terrace width is three atoms. The spot splitting for both these surfaces indicates the presence of monatomic steps and kinks. Consecutive terraces are displaced by half of a lattice constant relative to each other [14]. The structures of these vicinal ~v~u~ ~'~r~r~n~,,,,, surfaces are presented in fig. 4. Gn the sulfur covered surfaces, noticeable changes were observed. Both the size of the spot splitting and th,: energies at which spots appear split are changed. This has been shown explicitly for the sulfur covered Mo(28,4,1) surface in fig. 3 and table 2 and is similar for the other sulfur covered vicinal s~;rfaces. This shows that the adsorption and subsequent annealing of the sulfur overlayer leads to a restructuring of these vicinal (100) surfaces. We have found that on the sulfur covered surfaces, double height steps and kinks are present. This is different from results reported on other vicinal body centered cubic (100) surfaces. Dowben et al. [6] found that nitrogen induced a double step to single step transition on vicinal Fe(100) surfaces. In the enti:e coverage range studied ( < 0.1-1 ML), salfur induced step-doublir~g. In this respect, our results are also different from results of Lanzillotto and Bernasek [4] on sulfur induces step-doubling of the Pt(S)[6(111 ) × (11)0)] surface. While these authors have found that annealing surfaces with low sulfur w
Fig. 3. Diffraction pattern of the Mo(28,4,1)-(1 x 1)S structure formed on the vicinal surfaces between sulfur coverages of 0.5 and 0.75 ML (beam voltage = 86 eV). AK01/Kol is ~ 1/7 as compared to approximately 1/3.5 for the clean surface. This surface is composed of two atom height steps and seven atom wide terraces.
p(2 × 1) structures are shown in fig. 2. At sulfur coverages between one-half and three-quarters of a monolayer, a poorly ordered (1 × 1) diffraction pattern was observed on the vicinal surfaces. A diffraction pattern of this surface showing the split spots in a pronounced background which indicates the disorder is shown in fig. 3. The energies at which the (01) and other symmetrically equivalent integer beams appeared as singlets and doublets for the c(2 × 2)S and (1 × 1)S structures on the kinked Mo(28,4,1) surface are given in table 2. The agreement in the frequency of oscillations between the experimental values and those calculated for double height steps is evidence that the adsorption of sulfur induces the formation of diatomic steps. Observation of the diffraction pattern of the (1 x 1) sulfur covered kinked surface shows that AKo~/Kol is reduced to 1/7. This spot splitting distance is consistent with the presence of diatomic steps separating terraces that are on the average composed of seven molybdenum atoms This sulfur induced step-doubling is completely reversible. Heating the crystal above 1900 K to desorb the sulfur restores the monatomic steps characteristic of the clean surfaces.
CC. Knight, G.A. Somorjai / S orerlayers on Mo(910) and Mo(28,4,1)
331
l O~Ol [i9o]
1q [oox]
L
I q q q q ¢
(a)
q
[oi4]
! U
/ [o101 [~70] (b) Fig. 4. Diagrammatic representations of real-space Mo(910) (a) and Mo(28,4,!) (b) surfaces. The upper arrow in (a) indicates both the [010] direction in the plane of the terrace and the [190] direction which is parallel to the macroscopic surface. The lower arrow in (b) indicates both the [010] direction in the plane of the terrace and the [770] direction which is parallel to the macroscopic sur[ace.
332
C.C. Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1)
coverages ( < 0.05 ML) induce step-doubling, they report that surface coverages exceeding 0.25 ML leave the platinum surface unreconstructed. The presence of steps can result in preferential orientation of one ordered domain. For the p(2 × 1) structures on the vicinal Mo(100) surfaces, the domains with the longer side of the unit cell perpendicular to the step edges are well ordered. Within these domains, sulfur atoms can be located on all of the metal atoms on or close to the step edges. Preferential ordering of these domains maximizes the number of sulfur-step atom bonds, suggesting a strong interaction between sulfur atoms and metal step edges. This is in agreement w~th the explanation of Lanzillotto and Bernasek [4] of why sulfur induced step-doubling occurred at very low sulfur coverages on vicinal Pt(lll)surfaces. These authors attributed this adsorbate-induced restructuring to the preferential adsorption of sulfur at the (100) platinum step edges. We have observed c(2 × 2) diffraction patterns on the vicinal Mo(100) surfaces at sulfur coverages less than 0.1 ML. Similar diffraction patterns are not observed on the flat Mo(100) surface at comparable sulfur coverages. Tl.~s indicates that at these low coverages, sulfur atoms coalesce and order along the step edges. This further implicates step edges as important sites for sulfur nuc!eation and ordering on the vicinal ~urfa.ces. The (12i) and c(4 x 2) structures that are observed on Mo(100) are not formed on these vicinal surfaces. It is not possible to arrange the (2 ~) overlayer on the terraces, and still decorate all of the metal step atoms with adsorbate atoms. Instead, at these coverages a poorly ordered (1 × 1) diffraction pattern is observed. The ordering of sulfur atoms along the step edges determines which ordered structures are formed on the terraces and which ordered domain grows. atlt ~ u l l t . , l u s l o n , U l l e l l l l S U l p t l U l ? o f ~u~tu~ _..1£.._ w a s s e e n
to restructure vicinal Mo(100) surfaces, leading to the formation of two atom height steps and terraces of doubled width. Several studies have shown that the chemisorption of oxygen on stepped platinum surfaces leads to the formation of two atom height steps [5,15]. Several explanations have been advanced to explain step restruc-
turing. One explanation is that the surface free energy is lowered by decreasing the step density. Another, proposed by Dowben [6], is that adsorbate induced surface restructuring of steps occurs to relieve the strain induced by the lattice mismatch between the overlayer and substrate. A possible explanation for the restructuring observed in this work is that the number of most strongly bound chemisorption sites is increased by such a restructuring. This is similar to the explanation advanced by Lanzillotto and Bernasek [4].
5. Summary Using LEED, we have shown that sulfur forms a variety of ordered overlayers on as well as restructures stepped and kinked Mo(100) surfaces. We have also shown that for ordered sulfur structures, the domains that maximize adsorbate-step interactions are preferentially ordered.
Acknowledgements This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the US Department of Energy under Contract No. DEAC03-76SF00098. Travel support by A T & T Bell Laboratories is gratefully acknowledged.
References [1] M.E. Bussell and G.A. Somorjai, J. Catal. 106 (1987) 93. [2] M.E. Busseli, A.J. Gellman and G.A. Somorjai, Catal. Lett. 1 (1988) 195. [3] C.E. Aumann, J.J. de Miguei, R. Kariotis and M.G. Lagally, presented at the 38th National Symposium of the American Vacuum Society, Seattle, Washington, November 11-15, 1991. [4] A.-M. Lanzillotto and S.L. Bernasek, J. Chem. Phys. 84 (1986) 3553. [5] G. Comsa, G. Mechtersheimer and B. Poelsema, Surf. Sci. 119 (1982) 159, 172. [6] P.A. Dowben, M. Grunze and R.G. Jones, Surf. Sci. 109 (1981) L519. [7] C.C. Knight and G,A. Somorjai, Surf. Sci. 240 (1990) 101.
C.C. Knight, G.A. Somorjai / S overlayers on Mo(910) and Mo(28,4,1) [8] [9] [10] [11]
C. Wagner, J. Chem. Phys. 21 (1953) 1819. M. Henzler, Surf. Sci. 22 (1970) 12. M. Henzler, Appl. Phys. 9 (1976) 11. M. Salmeron, G.A. Somorjai and R.R. Chianelli, Surf. Sci. 127 (1983) 526. [12] V. Maurice, L. Peralta, Y. Berthier and J. Oudar, Surf. Sci. 148 (1984) 623. [13] The (01) reciprocal space direction given is related to the terrace periodicity, i.e., this vector is identical to (01) vector on the flat (100) surface. [14] Strictly speaking, the clean unrestructured surfaces have the same periodicity as the restructured surfaces with two
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atom height steps. This periodicity, 9-fold and 7-fold perpendicular to the step edge of the Mo(910) and Mo(28,4,1) surfaces, respectively, arises from the fact that consecutive terraces are shifted by half an atomic distance relative to each other on the unrestructured surfaces. This can be seen in fig. 4. However, calculations in the kinematic approximation show that for the unrestructured surfaces with monatomic steps, the intensities of the n / 9 and n / 7 order spots with odd n are very low. D. Biakely, PhD Thesis, University of California, Berkeley, 1976.