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A coverage-dependent reaction of Li adatoms on Cu(001) with H2O
Vohune 194, number I ,2
CHEMICAL PHYSICS LETTERS
A coverage-dependent with Hz0 Hiroshi Tochihara
19 June 1992
reaction of Li adatoms on Cu( 001)
and Seigi Mizuno
Catalysis Research Center, Hokk-aido University, Sapporo 060, Japan Received 2 1 November 1991; in final form 14 February 1992
A reaction of Hz0 with Li adatoms on Cu(OO1) at 300 K has been studied with X-ray photoelectron spectroscopy, high-resolution electron energy toss spectroscopy and work function measu~ment. While surface hydroxide (LiOH) forms on the surface at low coverages of Li, surface monoxide (L&O) forms first, followed by the formation of hydroxide on the monoxide, at high covetages of Li. The origin of this coverage-dependent reaction is discussed. Structures of the reaction products on the surface are proposed.
1. I~~uc~on It is known that alkali-metal (AM) adatoms affect considerably the chemisorption and reaction of molecules on metal surfaces [ 1,2 1. In particular, the promoter effect on the rate of catalytic reactions by the presence of AM impounds [ 21 is impo~ant in connection with practical applications. Therefore, the study on the coadsorption of AM atoms and molecules is one of the most interesting fields in surface chemistry. Another aspect of the coadsorption is the direct pa~icipation of AM adatoms in reactions as a reactant. Direct reactions of AM adatoms with HZ0 [3-l 1 ] and CHJOH [ 3,12,13 ] have been investigated. Furthermore, it is interesting to study the coverage dependence of AM adatoms on these reactions, because electronic properties of AM adatoms change drastically as a function of coverage, as typically seen in the work function change [ 1,2 1. Previously, Hoffmann and co-workers found a coverage-dependent reaction of K adatoms with CH30H onRu(001) ~11,12].AtlowcoveragesofKtheydid not observe the decomposition of CH,OH, but at high coverages (monolayer) CH30K was formed. In reactions of Na, K and Cs adatoms with Hz0 similar coverage-dependent reactions have been reported [ 5Correspondence to: H.Tochihara, Catalysis Research Center, Hokkaido University, Sapporo 060, Japan.
111. At low coverages of AM atoms reactions do not take place, but at high coverages of AM atoms hydroxides are formed on metal surfaces. These results indicate that there are critical coverages of AM adatoms for the formation of methoxides and hydroxides. On the other hand, in the present study we have found a distinct coverage-dependent reaction of Li adatoms with Hz0 on Cu( 001 ), that is, a reaction product formed on a surface with high coverage of Li is different from that on a surface with low coverage of Li.
2. Experimental Details of the equipment and experiment will appear elsewhere f 14 ] . Briefly, the ex~~ment has been performed in a three-level UHV system equipped with a high-resolution electron energy loss spectrometer (HREELS) , X-ray photoelectron spectrometer (XPS), Auger electron spectrometer (AES), angleresolved ultraviolet phot~l~tron spectrometer ( ARUPS ) , low-energy electron diffraction (LEED ) and a mass spectrometer for partial pressure measurement. The Cu (00 1) surface was cleaned by the standard method. Li atoms were dosed from a SAES Getter source at 300 K. For dosing of water we used residual HZ0 molecules in the UHV chamber of total pressure in the 10W8Pa range at 300 K. The work
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function change was measured using the derivative mode, and cutoff of secondary electron emission was used with sample bias.
3. Results of Li adsorption on Cu(OO1) Adsorption of Li on Cu (00 1) will be described in detail elsewhere [ 151. Briefly, the work function change against Li deposition time showed a typical change for AM adsorption on metals as shown in fig. 1. A vertical broken line corresponds to the completion of the first Li monolayer. Coverages of Li adatoms were determined by using LEED and AES. The vibration of Li adatoms was studied with HREELS, and a Cu-Li stretching mode was observed at 300 cm- ’ [ 14,15 1. The behaviour of this peak against Li coverage is similar to that of Na on Cu ( 111) [ 16 1. It is generally recognized that the character of AM overlayers on metal surfaces changes substantially at about a half of the full monolayer [ 1,2]; at low coverages AM adatoms are adsorbed on surfaces singly, forming polarized bonds with substrate atoms. The adsorbate is ionic in nature. On the other hand at high coverages in the monolayer, AM adatoms form a kind of two-dimensional metal on metal surfaces
19 June 1992
as a result of mutual overlapping of wavefunctions of the AM valence electrons. Therefore, the Li monolayer can be divided into two characteristic regions as shown at the top of fig. 1 in terms of ionic and metallic overlayers (OL ) . Two deposition times are chosen in this study to represent ionic and metallic overlayers as indicated in fig. 1 by arrows with L and H, respectively. Henceforth, these overlayers are called L and H surfaces in this Letter, and Li coverages for L and H surfaces are 0.125 and 0.5, respectively.
4. Results of reactions of Hz0 with Li adatoms on cu(oo1) 4.1. Work function change (A@) Work function changes were measured with increasing Hz0 exposure for L and H surfaces. The changes after HZ0 exposure are completely different for the two surfaces, particularly at low exposures, as seen in fig. 2. For the H surface the work function is decreased initially with increasing HZ0 exposure, while for the L surface the work function is increased. In the case of the H surface the work function change showed a minimum at the exposure of
0’
0 0
0
20
40
60
80
100
Li DEPOSITION TIME (s)
Fig. 1. Work function change as a function of Li deposition time. A vertical broken line shows the completion of the Li monolayer. Arrows with L and H indicate the deposition times for two surfaces studied in the present experiment. On top, the nature of the monolayer is shown together with layer number of Li.
52
0.0
0.1 0.2 0.3 WATER EXPOSURE(L)
014
Fig. 2. Work function changes of the Li covered Cu(OO1) surfaces as a function of water exposure. Closed and open circles are for H and L surfaces, respectively. A broken curve indicates a further change obtained from a different run.
19 June 1992
CHEMICAL PHYSICS LETTERS
Volume 194, number 1,2
4.3. Stoichiometry
0.1 langmuir (L). A similar coverage-dependent change of work function was found for oxygen adsorption on K-preadsorbed Ru (00 1) [ 171.
We dosed oxygen molecules onto Cu( 001) at 300 K to estimate coverages of oxygen atoms on the reacted surfaces. The ($x2$)-R45”O/Cu(OOl) structure formed was studied well and the coverage of oxygen atom is 0.5 [21,22]. Areas of O,, peaks are used for determination of stoichiometry. First, we determine the stoichiometry of the oxide formed on the H surface at low Hz0 exposures. As seen in fig. 3c, the height of the 529 eV peak shows a maximum at 0.2 L of Hz0 exposure, and we obtain the area of this peak at 0.2 L. Thus the ratio of Li to oxygen atoms is 2.5, and this oxide is reasonably assigned to LizO, because other possible oxides are L&O2 or LiOz. Second, the 532 eV peak of OH species on the H surface shows saturation at 0.5 L, where the ratio of Li to OH is found to be 1. Third, for the L surface the 531.5 eV peak of hydroxide shows saturation at 0.3 L, where the ratio is also found to be 1. LiOH is formed in the latter two cases.
4.2. XPS XPS spectra of oxygen 1s level (O,,) from L and H surfaces were obtained with various Hz0 exposures using Al Ku source. For the L surface a single O,, peak was observed as shown in lig. 3a and its binding energy remained constant at 53 1.5 eV. It is known that this value is for hydroxides at surfaces [ 18,191. On the other hand, XPS spectra of the H surface showed different development as seen in fig. 3b; at low exposures a peak at 529 eV grew rapidly, but another peak at 532 eV started growing at 0.1 L. The peak heights of the two O,, peaks are plotted as a function of Hz0 exposure in fig. 3c. The 529 eV peak was confirmed to be oxide from a measurement of oxygen adsorption on Li-preadsorbed Cu( 001). The 532 eV peak was assigned to hydroxide. Previously Hz0 adsorption on Li films was studied with XPS, and two Or, peaks of binding energies at 530.7 and 533.6 eV were assigned to oxide and hydroxide, respectively [ 20 1. The binding energies are different from those in the present study, but the energy separation of the two peaks is the same. Probably this is caused by a difference in specimen, that is, films or monolayers.
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We have observed almost the same HREELS spectra for L and H surfaces as shown in fig. 4, after Li adatoms are completely reacted with water. This indicates that the outermost species on both surfaces are the same. Species underneath the outermost surface, however, are not identified because of insensitivity of HREELS to the subsurface region. For the
I
530 BINDING
4.4. HREELS
(eV)
540
530
540 BINDING
ENERGY
(eV)
WATBRBXPGSURE
Q
Fig. 3. (a) Growth of 0 Is XPS peak on the L surface with increasing Hz0 exposure . A vertical dotted line corresponds to the binding energy of 531.5 eV. (b) Growth of 0 Is peaks on the L surface. Two dotted lines indicate the binding energies of 529 and 532 eV. (c) 0 Is peak-height changes as a function of Hz0 exposure from (b). Closed and open circles correspond to the peaks of 532 and 529 eV, respectively.
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19 June 1992
_-L-_
0
500
1500 1000 ENERGY LOSS (cm-l)
3500
Fig. 4 . HREELS spectra after complete reaction of Hz0 with (a) Land (b) H surfaces.
L surface an intense peak at 600 cm-’ was assigned to the Li-OH stretching mode [ 141. Therefore, the corresponding peak for the H surface can also be assigned to the same mode, and this implies that LiOH exists on the outermost surface. Other small peaks at 3600, 1300 and 1100 cm-’ are assigned to OH stretching, LiOH bending and the overtone of Li-OH stretching, respectively [ 14 1.
(a) Li low coveruge
(b) Li high covemge
5. Discussion
The results mentioned above demonstrate that the reaction product on the L surface is different from that on the H surface, at least at low Hz0 exposures. For the L surface we discussed the structure of the product in detail [ 141. A linear triatomic molecule of LiOH on Cu (001) fulfils all our experimental findings (HREELS, XPS and A@) and an experiment of electron stimulated desorption ion angular distribution by Madey and co-workers [ 71. Briefly, this structure explains the increase of the work function, because a LiOH admolecule is polarized like Li6+-0H6- with OH6- pointing toward the vacuum. This polarized bond leads to the intense losspeak of the Li-OH stretching mode in HREELS spectra at 600 cm-’ in fig. 4. A schematic illustration of the product is shown in fig. Sa. For the H surface the reaction appears to take place in two successive stages because of the following two observations. First, the work function is decreased at low Hz0 exposures, then it is increased at high ex54
Fig. 5 . Schematic illustrations for (a) a reaction of Hz0 with low coverage of Li adatoms and (b ) reaction processes of Li adatoms at high coverages with Hz0 on Cu(OO1). Broken curves in the top panel represent the delocalization of the valence electrons of Li adatoms.
posures as seen in fig. 2. Second, fig. 3c shows that lithium oxide (LizO) is formed at low H20 exposures and that lithium hydroxide (LiOH) is formed at high exposures. These two observations are consistent if we assume that oxygen atoms penetrate the Li monolayer and stay between the outermost Cu layer and the Li overlayer. That is, the initial reaction-product of Liz0 accounts for the initial decrease
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CHEMICAL PHYSICS LETTERS
of the work function, and the second reaction-product of LiOH explains the final increase of the work function. Hence, the reaction processes can be illustrated as in fig. 5b. As seen in fig. 5b, Li atoms are shared by the oxide and hydroxide, so this surface compound can be represented as OH/Li/O/ Cu (001). In these reactions we could not identify hydrogen atoms, and they would be either present on the copper surface or desorbed from the surface as hydrogen molecules. One might consider the possibility of a mixture of Cu-Li-OH and Cu-0-Li as the end product on the H surface instead of the present model, because the Li-OH stretching frequency on the H surface is identical with that on the L surface, as shown in fig. 4. However, this mixture model is not likely because of the following two experimental results. First, at low HZ0 exposure on the H surface a peak at 420 cm-’ grows initially, then the 600 cm-i peak develops with increasing exposure [ 23 1. The 420 cm- ’ peak is assigned to the Li-0 stretching mode of L&O on Cu( 001) from an experiment of O2 adsorption on the Li covered Cu (001) [ 23 1. At saturation, however, only the 600 cm-’ peak is observed and this indicates that Cu-0-Li is converted to Cu-O-LiOH. Identical frequencies of Li-OH stretching modes in Cu-Li-OH and Cu-0-Li-OH would be accidental. Second, as mentioned in section 4.3, a ratio of concentration of oxygen atoms (oxide+OH) to Li atoms is 1.4 for the H surface from XPS measurements, when Li atoms are completely reacted. The ratio expected from the present model, 1.5, is in good agreement with the experimental result, while the ratio expected from the mixture model should be less than 1. Having obtained the structural models for the reaction products at low and high coverages of Li adatoms, we discuss why the reactions are dependent on the coverage of Li. The literature, however, indicates that the reactions of AM adatoms with HZ0 are also dependent on AM atoms [5-l 11. Other AM adatoms, i.e. Na, K and Cs, do not show such coveragedependent reactions with Hz0 on metal surfaces, but they form only hydroxides as a result of reaction with HzO. Therefore, the interaction of AM adatoms with Hz0 is dependent on AM atoms as well as on the coverage of AM adatoms. Here, we separate the discussion into two parts.
19 June 1992
First, why do only Li adatoms form oxide as a result of reaction with HzO? Previously, the formation of Liz0 was also reported for Hz0 adsorption on Li films, together with LiOH [ 201. At the present stage we point out that the prominent stability of Liz0 is related to its exceptional formation. The heats of reaction for the formation of LiZO, Na,O, KzO and CsZO solids from AM solids and water vapour at 298 K are -356, - 173, - 119 and -75 kJ/mol, respectively [ 241. On the other hand, the heats of reaction for the formation of LiOH, NaOH, KOH and CsOH solids from AM solids and water vapour are - 245, - 184, - 183 and - 164 kJ/mol, respectively [ 24 1. Formation of peroxides and superoxides from water vapour is not likely #I. These values are for bulk solids, but we can use these in a rough estimation of the heats of reaction for the formation of the surface oxides and hydroxides. These values above indicate that Liz0 is exceptionally stable among the oxides and hydroxides. Thus we conclude that the exceptional formation of the surface oxide Liz0 is caused by its prominent thermodynamic stability. For Na the literature shows that the surface monoxide is not formed even at the full monolayer, although the heats of reaction for the formation of Na,O and NaOH solids are almost the same [ 8 1. This indicates that the surface hydroxides are more stabilized on metal surfaces than the surface oxides. The stabilization of the surface hydroxides is confirmed by the formation of LiOH in the present study. Second, why do Li adatoms show coverage-dependent reactions With H20? The argument above allows formation of both the surface lithium monoxide and lithium hydroxide on Cu (00 1) as a result of reaction of Li adatoms with H20. In the present study we classify the reactions into two groups. First, surface hydroxides are always formed when Li adatoms have an ionic character; at low coverages of Li adatoms LiOH is formed on the copper surface, and at high coverages of Li atoms OH is formed on the ionic Li atoms which consist of the surface oxide. Second, on the other hand, HZ0 molecules are completely dissociated to form L&O on the H surface where Li adatoms are considered to be almost neutral [ 14,15 1. Therefore, the reactions of Li atoms with Hz0 on *’ These heats of reaction from AM solids and water vapour are derived from the heats of formation listed in ref. [ 241. 55
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CHEMICAL PHYSICS LETTERS
Cu( 001) are dependent on the ionicity of Li adatoms, that is, the coverage-dependent reaction in the present study is caused by the coverage-dependent ionicity of Li adatoms. Therefore, the coverage-dependent reaction is not related to the thermadynamic stability of the product but originates from a difference in interaction of valence electrons of Li adatoms with unoccupied molecular orbitals of HzO. The detailed mechanism of this coverage-dependent reaction, however, is still unclear, so further studies, including theoretical calculations, are required.
6. Conclusion A direct reaction of Li adatoms with Hz0 on Cu( 00 1) has been studied with XPS, HREELS and A@at room temperature. We have found a coveragedependent reaction of Li adatoms with H20, that is, hydroxide (LiOH) forms on Cu( 00 1) at low coverge of Li, while monoxide (Li,O) forms at high coverages (monolayer) followed by the formation of hydroxide on the monoxide. For other alkali-metal atoms, i.e. Na, K and Cs, such coverage-dependent reaction is not reported in the literature, but they form only hydroxides. The exceptional formation of Liz0 is caused by its prominent stability compared to those of other oxides. It is concluded that the observed coverage-dependent reaction is related to the change of the ionicity of Li adatoms.
References [ 1] H.P. Bonzel, Surface Sci. Rept. 8 ( 1987) 43. [ 21 G. Ertl, in: Physics and chemistry of akali metal adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl (Elsevier, Amsterdam, 1989) p. 1.
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[ 3 ] J. Paul, R.A. dePaola and F.M. Hoffmann, in: Physics and chemistry of alkali metal adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl (Elsevier, Amsterdam, 1989) p.2 13. [4] P.A. Thiel and T.E. Madey, Surface Sci. Rept. 7 (1987) 211. [5] P.A. Thiel, J. Hrbek, R.A. dePaola and F.M. Hoffmann, Chem. Phys. Letters 108 (1984) 25. [6] M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. I50 (1985) 319. [7] S. Semancik, D.L. Doering and T.E. Madey, Surface Sci. 176 (1986) 165. [8] D.L. Doering, S. Semancik and T.E. Madey, Surface Sci. 133 (1983) 49. [9] W.D. Clendening, J.A. Rodriguez, J.M. Campbell and C.T. Campbell, Surface Sci. 2 I6 ( 1989) 429. [lo] D. Lackey, J. Schott, B. Straehler and J.K. Saas, J. Chem. Phys. 91 (1989) 1365. [ 111 T. Bomemann, H.-P. Steiniick, W. Huber, K. Eberle, M. Glanz and D. Menzel, Surface Sci. 254 ( 199 1) 105. [ 121 J. Hrbek, R. dePaola and F.M. Hoffmann, Surface Sci. 166 (1986) 361. [ 131 R.A. dePaola, J. Hrbek and EM. Hoffmann, Surface Sci. 169 (1986) L348. [ 141 S. Mizuno, H. Tochihara, T. Kadowaki, H. Minagawa, K. Hayakawa, I. Toyoshima and C. Oshima, Surface Sci. (1992), in press. [ 151 S. Mizuno and H. Tochihara, to be published. [ 161 S.-A. Lindgren, C. Svensson and L. Walldtn, Phys. Rev. B 42 (1990) 1467. [ 171 R.A. dePaola, F.M. Hoffmann, D. Heskett and E.W. Plummer, J. Chem. Phys. 87 (1987) 1361. [ 181 G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 ( 1980) 683. [ 19 ] A.F. Carley, P.R. Davies, M. W. Roberts and K.K. Thomas, Surface Sci. 238 ( 1990) L467. [ 201 J.R. Hoenigman and R.G. Keil, Appl. Surface Sci. 18 ( 1984) 207. [21] Ch. Wijll, R.J. Wilson, S. Chiang, H.C. Zeng and K.A.R. Mitchell, Phys. Rev. B 42 ( 1990) 11926. [22] H.C. Zeng and K.A.R. Mitchell, Surface Sci. 239 (1990) L571. [23] S. Mizuno and H. Tochihara, unpublished. [24] J.C. Bailar, H.J. Emeltus, R. Nyholm and A.F. TrotmanDickenson, eds., Comprehensive inorganic chemistry, (Pergamon Press, Oxford, 1973) chs. 7 and 8.
CHEMICAL PHYSICS LETTERS
A coverage-dependent with Hz0 Hiroshi Tochihara
19 June 1992
reaction of Li adatoms on Cu( 001)
and Seigi Mizuno
Catalysis Research Center, Hokk-aido University, Sapporo 060, Japan Received 2 1 November 1991; in final form 14 February 1992
A reaction of Hz0 with Li adatoms on Cu(OO1) at 300 K has been studied with X-ray photoelectron spectroscopy, high-resolution electron energy toss spectroscopy and work function measu~ment. While surface hydroxide (LiOH) forms on the surface at low coverages of Li, surface monoxide (L&O) forms first, followed by the formation of hydroxide on the monoxide, at high covetages of Li. The origin of this coverage-dependent reaction is discussed. Structures of the reaction products on the surface are proposed.
1. I~~uc~on It is known that alkali-metal (AM) adatoms affect considerably the chemisorption and reaction of molecules on metal surfaces [ 1,2 1. In particular, the promoter effect on the rate of catalytic reactions by the presence of AM impounds [ 21 is impo~ant in connection with practical applications. Therefore, the study on the coadsorption of AM atoms and molecules is one of the most interesting fields in surface chemistry. Another aspect of the coadsorption is the direct pa~icipation of AM adatoms in reactions as a reactant. Direct reactions of AM adatoms with HZ0 [3-l 1 ] and CHJOH [ 3,12,13 ] have been investigated. Furthermore, it is interesting to study the coverage dependence of AM adatoms on these reactions, because electronic properties of AM adatoms change drastically as a function of coverage, as typically seen in the work function change [ 1,2 1. Previously, Hoffmann and co-workers found a coverage-dependent reaction of K adatoms with CH30H onRu(001) ~11,12].AtlowcoveragesofKtheydid not observe the decomposition of CH,OH, but at high coverages (monolayer) CH30K was formed. In reactions of Na, K and Cs adatoms with Hz0 similar coverage-dependent reactions have been reported [ 5Correspondence to: H.Tochihara, Catalysis Research Center, Hokkaido University, Sapporo 060, Japan.
111. At low coverages of AM atoms reactions do not take place, but at high coverages of AM atoms hydroxides are formed on metal surfaces. These results indicate that there are critical coverages of AM adatoms for the formation of methoxides and hydroxides. On the other hand, in the present study we have found a distinct coverage-dependent reaction of Li adatoms with Hz0 on Cu( 001 ), that is, a reaction product formed on a surface with high coverage of Li is different from that on a surface with low coverage of Li.
2. Experimental Details of the equipment and experiment will appear elsewhere f 14 ] . Briefly, the ex~~ment has been performed in a three-level UHV system equipped with a high-resolution electron energy loss spectrometer (HREELS) , X-ray photoelectron spectrometer (XPS), Auger electron spectrometer (AES), angleresolved ultraviolet phot~l~tron spectrometer ( ARUPS ) , low-energy electron diffraction (LEED ) and a mass spectrometer for partial pressure measurement. The Cu (00 1) surface was cleaned by the standard method. Li atoms were dosed from a SAES Getter source at 300 K. For dosing of water we used residual HZ0 molecules in the UHV chamber of total pressure in the 10W8Pa range at 300 K. The work
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function change was measured using the derivative mode, and cutoff of secondary electron emission was used with sample bias.
3. Results of Li adsorption on Cu(OO1) Adsorption of Li on Cu (00 1) will be described in detail elsewhere [ 151. Briefly, the work function change against Li deposition time showed a typical change for AM adsorption on metals as shown in fig. 1. A vertical broken line corresponds to the completion of the first Li monolayer. Coverages of Li adatoms were determined by using LEED and AES. The vibration of Li adatoms was studied with HREELS, and a Cu-Li stretching mode was observed at 300 cm- ’ [ 14,15 1. The behaviour of this peak against Li coverage is similar to that of Na on Cu ( 111) [ 16 1. It is generally recognized that the character of AM overlayers on metal surfaces changes substantially at about a half of the full monolayer [ 1,2]; at low coverages AM adatoms are adsorbed on surfaces singly, forming polarized bonds with substrate atoms. The adsorbate is ionic in nature. On the other hand at high coverages in the monolayer, AM adatoms form a kind of two-dimensional metal on metal surfaces
19 June 1992
as a result of mutual overlapping of wavefunctions of the AM valence electrons. Therefore, the Li monolayer can be divided into two characteristic regions as shown at the top of fig. 1 in terms of ionic and metallic overlayers (OL ) . Two deposition times are chosen in this study to represent ionic and metallic overlayers as indicated in fig. 1 by arrows with L and H, respectively. Henceforth, these overlayers are called L and H surfaces in this Letter, and Li coverages for L and H surfaces are 0.125 and 0.5, respectively.
4. Results of reactions of Hz0 with Li adatoms on cu(oo1) 4.1. Work function change (A@) Work function changes were measured with increasing Hz0 exposure for L and H surfaces. The changes after HZ0 exposure are completely different for the two surfaces, particularly at low exposures, as seen in fig. 2. For the H surface the work function is decreased initially with increasing HZ0 exposure, while for the L surface the work function is increased. In the case of the H surface the work function change showed a minimum at the exposure of
0’
0 0
0
20
40
60
80
100
Li DEPOSITION TIME (s)
Fig. 1. Work function change as a function of Li deposition time. A vertical broken line shows the completion of the Li monolayer. Arrows with L and H indicate the deposition times for two surfaces studied in the present experiment. On top, the nature of the monolayer is shown together with layer number of Li.
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0.0
0.1 0.2 0.3 WATER EXPOSURE(L)
014
Fig. 2. Work function changes of the Li covered Cu(OO1) surfaces as a function of water exposure. Closed and open circles are for H and L surfaces, respectively. A broken curve indicates a further change obtained from a different run.
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Volume 194, number 1,2
4.3. Stoichiometry
0.1 langmuir (L). A similar coverage-dependent change of work function was found for oxygen adsorption on K-preadsorbed Ru (00 1) [ 171.
We dosed oxygen molecules onto Cu( 001) at 300 K to estimate coverages of oxygen atoms on the reacted surfaces. The ($x2$)-R45”O/Cu(OOl) structure formed was studied well and the coverage of oxygen atom is 0.5 [21,22]. Areas of O,, peaks are used for determination of stoichiometry. First, we determine the stoichiometry of the oxide formed on the H surface at low Hz0 exposures. As seen in fig. 3c, the height of the 529 eV peak shows a maximum at 0.2 L of Hz0 exposure, and we obtain the area of this peak at 0.2 L. Thus the ratio of Li to oxygen atoms is 2.5, and this oxide is reasonably assigned to LizO, because other possible oxides are L&O2 or LiOz. Second, the 532 eV peak of OH species on the H surface shows saturation at 0.5 L, where the ratio of Li to OH is found to be 1. Third, for the L surface the 531.5 eV peak of hydroxide shows saturation at 0.3 L, where the ratio is also found to be 1. LiOH is formed in the latter two cases.
4.2. XPS XPS spectra of oxygen 1s level (O,,) from L and H surfaces were obtained with various Hz0 exposures using Al Ku source. For the L surface a single O,, peak was observed as shown in lig. 3a and its binding energy remained constant at 53 1.5 eV. It is known that this value is for hydroxides at surfaces [ 18,191. On the other hand, XPS spectra of the H surface showed different development as seen in fig. 3b; at low exposures a peak at 529 eV grew rapidly, but another peak at 532 eV started growing at 0.1 L. The peak heights of the two O,, peaks are plotted as a function of Hz0 exposure in fig. 3c. The 529 eV peak was confirmed to be oxide from a measurement of oxygen adsorption on Li-preadsorbed Cu( 001). The 532 eV peak was assigned to hydroxide. Previously Hz0 adsorption on Li films was studied with XPS, and two Or, peaks of binding energies at 530.7 and 533.6 eV were assigned to oxide and hydroxide, respectively [ 20 1. The binding energies are different from those in the present study, but the energy separation of the two peaks is the same. Probably this is caused by a difference in specimen, that is, films or monolayers.
/
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ENERGY
We have observed almost the same HREELS spectra for L and H surfaces as shown in fig. 4, after Li adatoms are completely reacted with water. This indicates that the outermost species on both surfaces are the same. Species underneath the outermost surface, however, are not identified because of insensitivity of HREELS to the subsurface region. For the
I
530 BINDING
4.4. HREELS
(eV)
540
530
540 BINDING
ENERGY
(eV)
WATBRBXPGSURE
Q
Fig. 3. (a) Growth of 0 Is XPS peak on the L surface with increasing Hz0 exposure . A vertical dotted line corresponds to the binding energy of 531.5 eV. (b) Growth of 0 Is peaks on the L surface. Two dotted lines indicate the binding energies of 529 and 532 eV. (c) 0 Is peak-height changes as a function of Hz0 exposure from (b). Closed and open circles correspond to the peaks of 532 and 529 eV, respectively.
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_-L-_
0
500
1500 1000 ENERGY LOSS (cm-l)
3500
Fig. 4 . HREELS spectra after complete reaction of Hz0 with (a) Land (b) H surfaces.
L surface an intense peak at 600 cm-’ was assigned to the Li-OH stretching mode [ 141. Therefore, the corresponding peak for the H surface can also be assigned to the same mode, and this implies that LiOH exists on the outermost surface. Other small peaks at 3600, 1300 and 1100 cm-’ are assigned to OH stretching, LiOH bending and the overtone of Li-OH stretching, respectively [ 14 1.
(a) Li low coveruge
(b) Li high covemge
5. Discussion
The results mentioned above demonstrate that the reaction product on the L surface is different from that on the H surface, at least at low Hz0 exposures. For the L surface we discussed the structure of the product in detail [ 141. A linear triatomic molecule of LiOH on Cu (001) fulfils all our experimental findings (HREELS, XPS and A@) and an experiment of electron stimulated desorption ion angular distribution by Madey and co-workers [ 71. Briefly, this structure explains the increase of the work function, because a LiOH admolecule is polarized like Li6+-0H6- with OH6- pointing toward the vacuum. This polarized bond leads to the intense losspeak of the Li-OH stretching mode in HREELS spectra at 600 cm-’ in fig. 4. A schematic illustration of the product is shown in fig. Sa. For the H surface the reaction appears to take place in two successive stages because of the following two observations. First, the work function is decreased at low Hz0 exposures, then it is increased at high ex54
Fig. 5 . Schematic illustrations for (a) a reaction of Hz0 with low coverage of Li adatoms and (b ) reaction processes of Li adatoms at high coverages with Hz0 on Cu(OO1). Broken curves in the top panel represent the delocalization of the valence electrons of Li adatoms.
posures as seen in fig. 2. Second, fig. 3c shows that lithium oxide (LizO) is formed at low H20 exposures and that lithium hydroxide (LiOH) is formed at high exposures. These two observations are consistent if we assume that oxygen atoms penetrate the Li monolayer and stay between the outermost Cu layer and the Li overlayer. That is, the initial reaction-product of Liz0 accounts for the initial decrease
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of the work function, and the second reaction-product of LiOH explains the final increase of the work function. Hence, the reaction processes can be illustrated as in fig. 5b. As seen in fig. 5b, Li atoms are shared by the oxide and hydroxide, so this surface compound can be represented as OH/Li/O/ Cu (001). In these reactions we could not identify hydrogen atoms, and they would be either present on the copper surface or desorbed from the surface as hydrogen molecules. One might consider the possibility of a mixture of Cu-Li-OH and Cu-0-Li as the end product on the H surface instead of the present model, because the Li-OH stretching frequency on the H surface is identical with that on the L surface, as shown in fig. 4. However, this mixture model is not likely because of the following two experimental results. First, at low HZ0 exposure on the H surface a peak at 420 cm-’ grows initially, then the 600 cm-i peak develops with increasing exposure [ 23 1. The 420 cm- ’ peak is assigned to the Li-0 stretching mode of L&O on Cu( 001) from an experiment of O2 adsorption on the Li covered Cu (001) [ 23 1. At saturation, however, only the 600 cm-’ peak is observed and this indicates that Cu-0-Li is converted to Cu-O-LiOH. Identical frequencies of Li-OH stretching modes in Cu-Li-OH and Cu-0-Li-OH would be accidental. Second, as mentioned in section 4.3, a ratio of concentration of oxygen atoms (oxide+OH) to Li atoms is 1.4 for the H surface from XPS measurements, when Li atoms are completely reacted. The ratio expected from the present model, 1.5, is in good agreement with the experimental result, while the ratio expected from the mixture model should be less than 1. Having obtained the structural models for the reaction products at low and high coverages of Li adatoms, we discuss why the reactions are dependent on the coverage of Li. The literature, however, indicates that the reactions of AM adatoms with HZ0 are also dependent on AM atoms [5-l 11. Other AM adatoms, i.e. Na, K and Cs, do not show such coveragedependent reactions with Hz0 on metal surfaces, but they form only hydroxides as a result of reaction with HzO. Therefore, the interaction of AM adatoms with Hz0 is dependent on AM atoms as well as on the coverage of AM adatoms. Here, we separate the discussion into two parts.
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First, why do only Li adatoms form oxide as a result of reaction with HzO? Previously, the formation of Liz0 was also reported for Hz0 adsorption on Li films, together with LiOH [ 201. At the present stage we point out that the prominent stability of Liz0 is related to its exceptional formation. The heats of reaction for the formation of LiZO, Na,O, KzO and CsZO solids from AM solids and water vapour at 298 K are -356, - 173, - 119 and -75 kJ/mol, respectively [ 241. On the other hand, the heats of reaction for the formation of LiOH, NaOH, KOH and CsOH solids from AM solids and water vapour are - 245, - 184, - 183 and - 164 kJ/mol, respectively [ 24 1. Formation of peroxides and superoxides from water vapour is not likely #I. These values are for bulk solids, but we can use these in a rough estimation of the heats of reaction for the formation of the surface oxides and hydroxides. These values above indicate that Liz0 is exceptionally stable among the oxides and hydroxides. Thus we conclude that the exceptional formation of the surface oxide Liz0 is caused by its prominent thermodynamic stability. For Na the literature shows that the surface monoxide is not formed even at the full monolayer, although the heats of reaction for the formation of Na,O and NaOH solids are almost the same [ 8 1. This indicates that the surface hydroxides are more stabilized on metal surfaces than the surface oxides. The stabilization of the surface hydroxides is confirmed by the formation of LiOH in the present study. Second, why do Li adatoms show coverage-dependent reactions With H20? The argument above allows formation of both the surface lithium monoxide and lithium hydroxide on Cu (00 1) as a result of reaction of Li adatoms with H20. In the present study we classify the reactions into two groups. First, surface hydroxides are always formed when Li adatoms have an ionic character; at low coverages of Li adatoms LiOH is formed on the copper surface, and at high coverages of Li atoms OH is formed on the ionic Li atoms which consist of the surface oxide. Second, on the other hand, HZ0 molecules are completely dissociated to form L&O on the H surface where Li adatoms are considered to be almost neutral [ 14,15 1. Therefore, the reactions of Li atoms with Hz0 on *’ These heats of reaction from AM solids and water vapour are derived from the heats of formation listed in ref. [ 241. 55
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Cu( 001) are dependent on the ionicity of Li adatoms, that is, the coverage-dependent reaction in the present study is caused by the coverage-dependent ionicity of Li adatoms. Therefore, the coverage-dependent reaction is not related to the thermadynamic stability of the product but originates from a difference in interaction of valence electrons of Li adatoms with unoccupied molecular orbitals of HzO. The detailed mechanism of this coverage-dependent reaction, however, is still unclear, so further studies, including theoretical calculations, are required.
6. Conclusion A direct reaction of Li adatoms with Hz0 on Cu( 00 1) has been studied with XPS, HREELS and A@at room temperature. We have found a coveragedependent reaction of Li adatoms with H20, that is, hydroxide (LiOH) forms on Cu( 00 1) at low coverge of Li, while monoxide (Li,O) forms at high coverages (monolayer) followed by the formation of hydroxide on the monoxide. For other alkali-metal atoms, i.e. Na, K and Cs, such coverage-dependent reaction is not reported in the literature, but they form only hydroxides. The exceptional formation of Liz0 is caused by its prominent stability compared to those of other oxides. It is concluded that the observed coverage-dependent reaction is related to the change of the ionicity of Li adatoms.
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[ 3 ] J. Paul, R.A. dePaola and F.M. Hoffmann, in: Physics and chemistry of alkali metal adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl (Elsevier, Amsterdam, 1989) p.2 13. [4] P.A. Thiel and T.E. Madey, Surface Sci. Rept. 7 (1987) 211. [5] P.A. Thiel, J. Hrbek, R.A. dePaola and F.M. Hoffmann, Chem. Phys. Letters 108 (1984) 25. [6] M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. I50 (1985) 319. [7] S. Semancik, D.L. Doering and T.E. Madey, Surface Sci. 176 (1986) 165. [8] D.L. Doering, S. Semancik and T.E. Madey, Surface Sci. 133 (1983) 49. [9] W.D. Clendening, J.A. Rodriguez, J.M. Campbell and C.T. Campbell, Surface Sci. 2 I6 ( 1989) 429. [lo] D. Lackey, J. Schott, B. Straehler and J.K. Saas, J. Chem. Phys. 91 (1989) 1365. [ 111 T. Bomemann, H.-P. Steiniick, W. Huber, K. Eberle, M. Glanz and D. Menzel, Surface Sci. 254 ( 199 1) 105. [ 121 J. Hrbek, R. dePaola and F.M. Hoffmann, Surface Sci. 166 (1986) 361. [ 131 R.A. dePaola, J. Hrbek and EM. Hoffmann, Surface Sci. 169 (1986) L348. [ 141 S. Mizuno, H. Tochihara, T. Kadowaki, H. Minagawa, K. Hayakawa, I. Toyoshima and C. Oshima, Surface Sci. (1992), in press. [ 151 S. Mizuno and H. Tochihara, to be published. [ 161 S.-A. Lindgren, C. Svensson and L. Walldtn, Phys. Rev. B 42 (1990) 1467. [ 171 R.A. dePaola, F.M. Hoffmann, D. Heskett and E.W. Plummer, J. Chem. Phys. 87 (1987) 1361. [ 181 G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 ( 1980) 683. [ 19 ] A.F. Carley, P.R. Davies, M. W. Roberts and K.K. Thomas, Surface Sci. 238 ( 1990) L467. [ 201 J.R. Hoenigman and R.G. Keil, Appl. Surface Sci. 18 ( 1984) 207. [21] Ch. Wijll, R.J. Wilson, S. Chiang, H.C. Zeng and K.A.R. Mitchell, Phys. Rev. B 42 ( 1990) 11926. [22] H.C. Zeng and K.A.R. Mitchell, Surface Sci. 239 (1990) L571. [23] S. Mizuno and H. Tochihara, unpublished. [24] J.C. Bailar, H.J. Emeltus, R. Nyholm and A.F. TrotmanDickenson, eds., Comprehensive inorganic chemistry, (Pergamon Press, Oxford, 1973) chs. 7 and 8.