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Chemical properties of anion vacancies on zinc oxide
Surface Science 0 North-Holland
102 (1981) Publishing
L21 --L28 Company
SURFACE SCIENCE LETTERS CHEMICAL PROPERTIES OF ANION VACANCIES ON ZINC OXIDE
W.H. CHENG and H.H. KUNG * Department of Chemical Engineering, the Materials Research Center, and the Ipatieff Catalytic Laboratory, Northwestern University, Evanston, Illinois 60201, USA Received
5 June 1980;accepted
for publication
4 August
1980
A ZnO(4041) surface, which is a stepped [4(1070) X (OOOl)] surface containing a high density of anion vacancies was prepared. When compared with the nonpolar (1070) surface, the (4041) surface adsorbed 02 and methanol more strongly, but CO2 more weakly. The decomposition products of methanol were different on these two surfaces.
It has long been postulated that surface defects play important roles in the surface properties of materials. Such a belief has recently been strongly substantiated when it was demonstrated that the chemisorption and catalytic properties of high index metal surfaces are quite different from those of low index planes. A high index plane is one containing a high density of steps and kinks, which can be regarded as defects on a low index plane. Atoms at the steps and kinks are in general much more reactive. For example, it has been reported that dissociation on H-H, C E 0, and C-C bonds occur much more readily on stepped surfaces of Pt [l-3],Ni [4],andRh [.5]. In spite of the relatively large volume of work performed on defect metallic single crystal surfaces, similar studies involving preparation and characterization of oxide surfaces of high densities of defects using ultra high vacuum techniques are rare. Unlike the simplest metal which has only one element, the simplest oxide has two elements that are arranged in a regular spatial order. It should then be possible to generate defect oxide surfaces of high index planes in which the defects are only cation vacancies or only anion vacancies. We report here our first results in the preparation and characterization of a stepped ZnO(4041) (or 4(1OiO) X (0001)) surface. This surface corresponds to a plane that makes an angle of 7.7” with the (1010) plane in the direction of positive c axis. Assuming that a surface is formed by breaking the smallest possible number of bonds, it would seem that a (4041) surface is most likely formed by breaking a surface Zn-0 bond as shown in fig. lb. An ideal (4041) surface as shown is stoichiometric, and there is an equal number of
* To whom
correspondence
should
be sent. L21
L22
W.H. Cheng, H.H. Kung /Anion vacancieson ZnO
(a)
[loTo]
lr, l
[OOOIJ Zn
00
( b)
Fig. 1. Models of stepped
ZnO surfaces.
surface zinc and oxygen atoms. However, the steps would have a Zn edge atom, and the region at the step (point B in fig. lb) can then be viewed as a row of anion vacancies. We have studied the chemical properties of these vacancies with respect to the interaction with 02, CO* and methanol by temperature programmed desorption. The results are compared with those on a nonpolar ZnO(lOi0) surface. Experiments were performed in a conventional stainless steel chamber evacuated by a 200 l/s ion pump. The chamber is equipped with a PHI low energy electron diffractometer, a CMA Auger spectrometer, and a UT1 quadrupole mass spectrometer. The base pressure is typically in the 10-lo Torr range. The ZnO single crystals were oriented by back reflection Laue X-ray technique to within 0.5”. The well ordered planes were obtained by mechanical polishing, which was followed by several cycles of argon ion-sputtering and annealing at 773 K for the (1010) surface, or at 923 K and then at 873 K for the (4041) surface. Once an ordered surface was obtained, repeated sputtering and annealing always reproduced the LEED pattern. These surfaces appeared to be stable at room temperature in vacua for at least a day. The pretreatment procedure also removed the typical impurities of K, S, and
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
L23
C, and resulted in clean surfaces as judged by AES. Details of the preparation procedure will be reported in future publications. The electrical conductivities of the ZnO samples used here were very low. Thus the samples were subjected to electrical charging when studied by electron spectroscopies. This did not cause any problem in the qualitative Auger electron measurements. However, surface charging makes difficult quantitative LEED measurements. In particular, it prevented the observation of LEED patterns at low electron beam energies. At high beam energies where LEED patterns were observed, sporadic changes in the degree of charging resulted in drift of the diffraction spots, making it difficult to make representative photographic records. Such behavior appears to be common among insulators [6]. Similar to published results, a (1010) surface showed a 1 X 1 LEED pattern (fig. 2a), indicating that there is no surface reconstruction [7]. Indeed, analysis of LEED intensities suggested that except for a small inward relaxation of the surface ions, this surface is almost like an abrupt truncation of the bulk [8]. Figs. 2b-2d show the LEED patterns of a (4041) surface. To minimize charging and any resulting beam damage of the sample, the LEED patterns were taken with very low emission current (
W.H. Cheng, f1.H. Kung /Anion
vacancies
on ZnO
Fig. 2. LEED patterns gf 2110 surfaces at various incident electron beam en:rgies: (a) (1070) E, = 153.7 eV; (b) (4041) E, = 81.3 eV; (c) (4041) Ep = 129.7 eV; (d) (4041) Ep = 47.5 eV; (e) (5031) Ep = 67.7 eV.
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 3. Temperature rate was lO”C/s.
programmed
desorption
profiles
L25
(“C)
of oxygen
from ZnO surfaces.
The heating
cycles of freeze-thaw and vacuum distillation was introduced via a doser which points at the crystal surface in the manner similar to that of Madix [ 1 I]. The crystals were always at room temperature during adsorption. After adsorption, the chamber was evacuated for two minutes. The sample was positioned in front of the mass spectrometer, and thermal desorption was then achieved by a linear increase in the sample temperature. Desorption from the ceramic sample holder and the tungsten filament that was used to heat the sample radiatively was determined by placing a clean gold foil in place of ZnO and performing an otherwise identical experiment. This background desorption has been subtracted from the data reported here. There was no observable carbon residues left on the surfaces after desorption, and successive adsorption and desorption experiments were reproducible. Fig. 3 shows the thermal desorption profde of oxygen from surfaces previously adsorbed with oxygen. From the nonpolar (lOjO) surface, a single peak with a peak maximum at about 100°C was observed. From the (4041) surface, two desorption peaks were observed. The intensity of the 100°C peak was suppressed and a new peak with peak maximum at about 28O’C appeared. The latter peak had an area about four times that of the lower temperature peak and can reasonably be assumed to be originated from the step site. Since the region at the steps of the (4041) plane can be thought of as a row of anion vacancies, it is expected that oxygen would bind more strongly there. Taking in account that the step area is one quarter the terrace area, the coverage at the steps is more than ten times that on the
L26
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 4. Temperature ingrate was 4”C/s.
programmed
desorption
profiles
(“C)
of methanol
from ZnO surfaces.
The hcat-
terrace. The overall coverage, however, is only a few percent, as has been reported on (lOi0) [12,13], and is due to the fact that oxygen adsorption is accompanied by a net charge transfer. It has been reported that high temperature annealing in vacua of ZnO(1010) results in the generation of anion vacancies on the surface the concentration of which increases with increasing temperature [ 121. However, even for a 923 K treatment, the concentration of such defect is less than low4 of a monolayer. Therefore, the effect of this type of vacancies on our measurements should be minimal. The thermal desorption profiles of methanol from adsorbed methanol are shown in fig. 4. Similar to the case of oxygen, only a low temperature desorption peak was observed from the (1010) surface. However, an additional high temperature peak was observed from the (4041) surface. Again this suggests that a second form of methanol which interacts more strongly with the surface exists on the stepped surface. Methanol adsorption on ZnO(lOi0) has been studied by UPS and the result suggested that the molecule interacts with the surface via the oxygen lone pair [14]. If methanol also interacts with the step sites via the oxygen, it is not surprising that such interaction should be stronger than that on the terrace sites. The difference in the adsorption of CO2 from these two surfaces, however, is quite different from those of O2 and methanol. Fig. 5 shows the desorption profiles of CO?. A single desorption peak was observed from (lOjO), yet little or no desorp-
W.H. Cfzeng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 5. Temperature programmed heating rate was lO”C/s.
desorption
profiles
I.27
(“C)
of carbon
dioxide
from ZnO surfaces.
The
tion was observed from (4041). This indicates that either CO2 does not adsorb on the (4041) surface, or it adsorbs so weakly that all of it is removed during the brief period of pumping prior to desorption. This observation also implies that the presence of steps inhibits the adsorption of CO2 on the terrace sites. Hotan et al. studied the adsorption of CO2 on (lOi0) with electrical conductivity, work function and other measurements [ 151. Their thermal desorption results are in general agreement with ours. Based on their results, these authors suggested that CO2 is covalently adsorbed, and the adsorption results in a slight relaxation of the surface atom positions. If this relaxation is a major factor in the adsorption process, our result would indicate that the presence of steps inhibits such relaxation. Of course, the presence of surface steps could also very likely change the density and energy of the surface states such that adsorption of CO2 becomes very weak. In addition to the strength of adsorption of molecules as studied by thermal desorption, in the case of methanol, heating of the crystal also resulted in desorption of some decomposition products. On the (lOjO) surface, only CO2 was detected as a decomposition product. However, on the (4041) surface, CO and Hz were detected but no COs. The formation of CO2 on (IOiO) must be accompanied by other products. The relatively high Hz back~ound during the exper~ent did not permit the detection of a small amount of Hz_ We looked for but did not detect any dimethyl ether, water or methane. Further characterization of this decomposition reaction will be conducted using isotopically labeled molecules.
W.H. Cheap, H.H. Kung /Anion
L28
vacancies on ZnO
Our results reported here demonstrate that it is possible to prepare and study an ordered oxide surface of high densities of anion vacancies. These anion vacancies are shown to bind oxygen and the oxygen lone pair in methanol more strongly than the terrace sites. However, their presence much weakens the adsorption of carbon dioxide. They are also shown to possess different catalytic properties than the terrace sites. In addition to generating surfaces of anion vacancies, we also postulate that it should be possible to prepare surfaces of a high density of cation vacancy, such as the one shown in fig. la. Experiments on such a surface will be part of our future work. This work was supported by the Division of Basic Energy Sciences, Departtnent of Energy. W.H.C. acknowledges support from the Materials Research Center of Northwestern University (NSF-DMR 76-80847). Part of the ultra-high vacuum system was purchased through a grant from the National Science Foundation, ENG78-18997, and a grant from the Research Corporation.
References [l] [2] [3] (41 [5] [6] [7] [8] [9]
[lo] [ll] [12] [13] [ 141 [ 151
G.A. Somorjai, Act. Chem. Res. 9 (1976) 248. M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys. 67 (1977) 5324. D.W. Blakely, and G.A. Somorjai, .I. Catalysis 42 (1976) 181. W. Erley and H. Wagner, Surface Sci. 74 (1978) 333; W. Erley, H. Ibach, S. Lehwald and I-1.Wagner, Surface Sci. 83 (1979) 585. D. Castner and G.A. Somorjai, Surface Sci. 83 (1979) 60. K. Miiller and C.C. Chang, Surface Sci. 9 (1968) 455. S.C. Chang and P. Mark, Surface Sci. 45 (1974) 721. C. Duke, A. Lubinsky, S. Chang, B. Lee and P. Mark, Phys. Rev. 1315 (1977) 4865. M. Henzler, Surface Sci. 19 (1970) 195. M. Prutton, J.A. Walker, M.R. Welton-Cook, R.C. Felton and J.A. Ramsey, Surface (1979) 95. J. Falconer and R. Madix, J. Catalysis 30 (1973) 235. W. GGpel, Surface Sci. 62 (1977) 165. W. Gapel, J. Vacuum Sci. Technol. 15 (1978) 1298. C.W. Rubloff, H. Luth and W.D. Grobman, Chem. Phys. Letters 39 (1976) 493. W. Hotan, W. GBpel and R. Haul, Surface Sci. 83 (1979) 162.
Sci. 89
102 (1981) Publishing
L21 --L28 Company
SURFACE SCIENCE LETTERS CHEMICAL PROPERTIES OF ANION VACANCIES ON ZINC OXIDE
W.H. CHENG and H.H. KUNG * Department of Chemical Engineering, the Materials Research Center, and the Ipatieff Catalytic Laboratory, Northwestern University, Evanston, Illinois 60201, USA Received
5 June 1980;accepted
for publication
4 August
1980
A ZnO(4041) surface, which is a stepped [4(1070) X (OOOl)] surface containing a high density of anion vacancies was prepared. When compared with the nonpolar (1070) surface, the (4041) surface adsorbed 02 and methanol more strongly, but CO2 more weakly. The decomposition products of methanol were different on these two surfaces.
It has long been postulated that surface defects play important roles in the surface properties of materials. Such a belief has recently been strongly substantiated when it was demonstrated that the chemisorption and catalytic properties of high index metal surfaces are quite different from those of low index planes. A high index plane is one containing a high density of steps and kinks, which can be regarded as defects on a low index plane. Atoms at the steps and kinks are in general much more reactive. For example, it has been reported that dissociation on H-H, C E 0, and C-C bonds occur much more readily on stepped surfaces of Pt [l-3],Ni [4],andRh [.5]. In spite of the relatively large volume of work performed on defect metallic single crystal surfaces, similar studies involving preparation and characterization of oxide surfaces of high densities of defects using ultra high vacuum techniques are rare. Unlike the simplest metal which has only one element, the simplest oxide has two elements that are arranged in a regular spatial order. It should then be possible to generate defect oxide surfaces of high index planes in which the defects are only cation vacancies or only anion vacancies. We report here our first results in the preparation and characterization of a stepped ZnO(4041) (or 4(1OiO) X (0001)) surface. This surface corresponds to a plane that makes an angle of 7.7” with the (1010) plane in the direction of positive c axis. Assuming that a surface is formed by breaking the smallest possible number of bonds, it would seem that a (4041) surface is most likely formed by breaking a surface Zn-0 bond as shown in fig. lb. An ideal (4041) surface as shown is stoichiometric, and there is an equal number of
* To whom
correspondence
should
be sent. L21
L22
W.H. Cheng, H.H. Kung /Anion vacancieson ZnO
(a)
[loTo]
lr, l
[OOOIJ Zn
00
( b)
Fig. 1. Models of stepped
ZnO surfaces.
surface zinc and oxygen atoms. However, the steps would have a Zn edge atom, and the region at the step (point B in fig. lb) can then be viewed as a row of anion vacancies. We have studied the chemical properties of these vacancies with respect to the interaction with 02, CO* and methanol by temperature programmed desorption. The results are compared with those on a nonpolar ZnO(lOi0) surface. Experiments were performed in a conventional stainless steel chamber evacuated by a 200 l/s ion pump. The chamber is equipped with a PHI low energy electron diffractometer, a CMA Auger spectrometer, and a UT1 quadrupole mass spectrometer. The base pressure is typically in the 10-lo Torr range. The ZnO single crystals were oriented by back reflection Laue X-ray technique to within 0.5”. The well ordered planes were obtained by mechanical polishing, which was followed by several cycles of argon ion-sputtering and annealing at 773 K for the (1010) surface, or at 923 K and then at 873 K for the (4041) surface. Once an ordered surface was obtained, repeated sputtering and annealing always reproduced the LEED pattern. These surfaces appeared to be stable at room temperature in vacua for at least a day. The pretreatment procedure also removed the typical impurities of K, S, and
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
L23
C, and resulted in clean surfaces as judged by AES. Details of the preparation procedure will be reported in future publications. The electrical conductivities of the ZnO samples used here were very low. Thus the samples were subjected to electrical charging when studied by electron spectroscopies. This did not cause any problem in the qualitative Auger electron measurements. However, surface charging makes difficult quantitative LEED measurements. In particular, it prevented the observation of LEED patterns at low electron beam energies. At high beam energies where LEED patterns were observed, sporadic changes in the degree of charging resulted in drift of the diffraction spots, making it difficult to make representative photographic records. Such behavior appears to be common among insulators [6]. Similar to published results, a (1010) surface showed a 1 X 1 LEED pattern (fig. 2a), indicating that there is no surface reconstruction [7]. Indeed, analysis of LEED intensities suggested that except for a small inward relaxation of the surface ions, this surface is almost like an abrupt truncation of the bulk [8]. Figs. 2b-2d show the LEED patterns of a (4041) surface. To minimize charging and any resulting beam damage of the sample, the LEED patterns were taken with very low emission current (
W.H. Cheng, f1.H. Kung /Anion
vacancies
on ZnO
Fig. 2. LEED patterns gf 2110 surfaces at various incident electron beam en:rgies: (a) (1070) E, = 153.7 eV; (b) (4041) E, = 81.3 eV; (c) (4041) Ep = 129.7 eV; (d) (4041) Ep = 47.5 eV; (e) (5031) Ep = 67.7 eV.
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 3. Temperature rate was lO”C/s.
programmed
desorption
profiles
L25
(“C)
of oxygen
from ZnO surfaces.
The heating
cycles of freeze-thaw and vacuum distillation was introduced via a doser which points at the crystal surface in the manner similar to that of Madix [ 1 I]. The crystals were always at room temperature during adsorption. After adsorption, the chamber was evacuated for two minutes. The sample was positioned in front of the mass spectrometer, and thermal desorption was then achieved by a linear increase in the sample temperature. Desorption from the ceramic sample holder and the tungsten filament that was used to heat the sample radiatively was determined by placing a clean gold foil in place of ZnO and performing an otherwise identical experiment. This background desorption has been subtracted from the data reported here. There was no observable carbon residues left on the surfaces after desorption, and successive adsorption and desorption experiments were reproducible. Fig. 3 shows the thermal desorption profde of oxygen from surfaces previously adsorbed with oxygen. From the nonpolar (lOjO) surface, a single peak with a peak maximum at about 100°C was observed. From the (4041) surface, two desorption peaks were observed. The intensity of the 100°C peak was suppressed and a new peak with peak maximum at about 28O’C appeared. The latter peak had an area about four times that of the lower temperature peak and can reasonably be assumed to be originated from the step site. Since the region at the steps of the (4041) plane can be thought of as a row of anion vacancies, it is expected that oxygen would bind more strongly there. Taking in account that the step area is one quarter the terrace area, the coverage at the steps is more than ten times that on the
L26
W.H. Cheng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 4. Temperature ingrate was 4”C/s.
programmed
desorption
profiles
(“C)
of methanol
from ZnO surfaces.
The hcat-
terrace. The overall coverage, however, is only a few percent, as has been reported on (lOi0) [12,13], and is due to the fact that oxygen adsorption is accompanied by a net charge transfer. It has been reported that high temperature annealing in vacua of ZnO(1010) results in the generation of anion vacancies on the surface the concentration of which increases with increasing temperature [ 121. However, even for a 923 K treatment, the concentration of such defect is less than low4 of a monolayer. Therefore, the effect of this type of vacancies on our measurements should be minimal. The thermal desorption profiles of methanol from adsorbed methanol are shown in fig. 4. Similar to the case of oxygen, only a low temperature desorption peak was observed from the (1010) surface. However, an additional high temperature peak was observed from the (4041) surface. Again this suggests that a second form of methanol which interacts more strongly with the surface exists on the stepped surface. Methanol adsorption on ZnO(lOi0) has been studied by UPS and the result suggested that the molecule interacts with the surface via the oxygen lone pair [14]. If methanol also interacts with the step sites via the oxygen, it is not surprising that such interaction should be stronger than that on the terrace sites. The difference in the adsorption of CO2 from these two surfaces, however, is quite different from those of O2 and methanol. Fig. 5 shows the desorption profiles of CO?. A single desorption peak was observed from (lOjO), yet little or no desorp-
W.H. Cfzeng, H.H. Kung /Anion
vacancies on ZnO
TEMPERATURE Fig. 5. Temperature programmed heating rate was lO”C/s.
desorption
profiles
I.27
(“C)
of carbon
dioxide
from ZnO surfaces.
The
tion was observed from (4041). This indicates that either CO2 does not adsorb on the (4041) surface, or it adsorbs so weakly that all of it is removed during the brief period of pumping prior to desorption. This observation also implies that the presence of steps inhibits the adsorption of CO2 on the terrace sites. Hotan et al. studied the adsorption of CO2 on (lOi0) with electrical conductivity, work function and other measurements [ 151. Their thermal desorption results are in general agreement with ours. Based on their results, these authors suggested that CO2 is covalently adsorbed, and the adsorption results in a slight relaxation of the surface atom positions. If this relaxation is a major factor in the adsorption process, our result would indicate that the presence of steps inhibits such relaxation. Of course, the presence of surface steps could also very likely change the density and energy of the surface states such that adsorption of CO2 becomes very weak. In addition to the strength of adsorption of molecules as studied by thermal desorption, in the case of methanol, heating of the crystal also resulted in desorption of some decomposition products. On the (lOjO) surface, only CO2 was detected as a decomposition product. However, on the (4041) surface, CO and Hz were detected but no COs. The formation of CO2 on (IOiO) must be accompanied by other products. The relatively high Hz back~ound during the exper~ent did not permit the detection of a small amount of Hz_ We looked for but did not detect any dimethyl ether, water or methane. Further characterization of this decomposition reaction will be conducted using isotopically labeled molecules.
W.H. Cheap, H.H. Kung /Anion
L28
vacancies on ZnO
Our results reported here demonstrate that it is possible to prepare and study an ordered oxide surface of high densities of anion vacancies. These anion vacancies are shown to bind oxygen and the oxygen lone pair in methanol more strongly than the terrace sites. However, their presence much weakens the adsorption of carbon dioxide. They are also shown to possess different catalytic properties than the terrace sites. In addition to generating surfaces of anion vacancies, we also postulate that it should be possible to prepare surfaces of a high density of cation vacancy, such as the one shown in fig. la. Experiments on such a surface will be part of our future work. This work was supported by the Division of Basic Energy Sciences, Departtnent of Energy. W.H.C. acknowledges support from the Materials Research Center of Northwestern University (NSF-DMR 76-80847). Part of the ultra-high vacuum system was purchased through a grant from the National Science Foundation, ENG78-18997, and a grant from the Research Corporation.
References [l] [2] [3] (41 [5] [6] [7] [8] [9]
[lo] [ll] [12] [13] [ 141 [ 151
G.A. Somorjai, Act. Chem. Res. 9 (1976) 248. M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys. 67 (1977) 5324. D.W. Blakely, and G.A. Somorjai, .I. Catalysis 42 (1976) 181. W. Erley and H. Wagner, Surface Sci. 74 (1978) 333; W. Erley, H. Ibach, S. Lehwald and I-1.Wagner, Surface Sci. 83 (1979) 585. D. Castner and G.A. Somorjai, Surface Sci. 83 (1979) 60. K. Miiller and C.C. Chang, Surface Sci. 9 (1968) 455. S.C. Chang and P. Mark, Surface Sci. 45 (1974) 721. C. Duke, A. Lubinsky, S. Chang, B. Lee and P. Mark, Phys. Rev. 1315 (1977) 4865. M. Henzler, Surface Sci. 19 (1970) 195. M. Prutton, J.A. Walker, M.R. Welton-Cook, R.C. Felton and J.A. Ramsey, Surface (1979) 95. J. Falconer and R. Madix, J. Catalysis 30 (1973) 235. W. GGpel, Surface Sci. 62 (1977) 165. W. Gapel, J. Vacuum Sci. Technol. 15 (1978) 1298. C.W. Rubloff, H. Luth and W.D. Grobman, Chem. Phys. Letters 39 (1976) 493. W. Hotan, W. GBpel and R. Haul, Surface Sci. 83 (1979) 162.
Sci. 89