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Na adsorption sites on TiO2(110)−1 × 2 and its 2 × 2 superlattice
surface science
iii ELSEVIER
Surface Science323 (1995) L281-L286
Surface Science Letters
Na adsorption sites on TiO2(110)-1 X 2 and its 2 x 2 superlattice P.W. Murray, N.G. Condon, G. Thornton
1
Interdisciplinary Research Centre in Surface Science, University of Liverpool, Liverpool L69 3BX, UK
Received 14 September1994;acceptedfor publication4 November1994
Abstract
Scanning tunnelling microscopy (STM) has been used to investigate tt3e structures formed by a low coverage (< 0.01 ML) of Na on reduced 1 x 2 surfaces of TiO2(ll0). The STM images recorded at positive sample bias indicate that Na adsorbs in three-fold coordinated sites adjacent to the bridging-O row, forming a p(4 x 2) overlayer. Adsorption on areas of the clean surface which contain the 2 x 2 superlattice suggest that Na adsorbs in a site with a four-fold coordination to oxygen. This is accompanied by Na-induced substrate restructuring.
Despite the well-known catalytic promotion ef4'ects of alkali metals, relatively little is known about taeir interaction with metal oxide surfaces [1]. Of the information which is available, most is associated with rutile TiO 2 substrates [2-6]. In part this derives from the significant database on the clean substrates, and in part because on TiO 2 it is relatively straightforward to monitor charge-transfer to the substrate. Previous studies of alkali metal adsorption on TiO 2 have investigated the accompanying changes to the electronic structure [2,4-6] and reactivity [3] as well as the crystallography [2,4]. In this Letter we focus on the structural changes associated with Na adsorption on reduced surfaces of TiO2(ll0). The structures associated with TiO2(ll0) have previously been characterised by LEED [1], medium energy electron diffraction [7] and scanning tunnelling microscopy (STM) [8-10]. Two principal
1Also at: Chemistry Department, Manchester University, Manchester M13 9PL, UK.
phases are formed, the stoichiometric 1 x 1 termination and the O-deficient 1 x 2 reconstruction. The 1 x 2 phase coexists with a structure having 2 x 2 local symmetry. Structural models of these surfaces derived from earlier work are shown in Figs. l a - l c . As for Na adsorption, previous work has established that a c(4 X 2) overlayer is formed on the 1 × 1 substrate at 0.5 monolayer (ML) coverage [2], the structural model for which is shown in Fig. ld. Here we describe STM results for Na on TiO2(ll0)-I × 2 and its associated 2 × 2 superlatrice. To an extent, this work serves to examine the potential of the technique in the study of an alkali metal overlayer on an oxide surface. Although there have been numerous S]'M studies of alkali metal adsorption on metal [12] and classical semiconductor surfaces [13], there do not appear to be any previous reports relating to oxides. Our results are consistent with Na bonding to both tha, bridging-O row and in-plane O atoms in the 1 × 2 substrate. The interaction with the 2 × 2 phase appears to involve Na bonding to four O atoms, with an accompanying
0039-6028/95/$09.50 © 1995 ElsevierScienceB.V. All rightsreserved SSDI 0039-6028(94)00751-9
P.W. Murray et aL / Surface Science 323 (1995) L281-L286
,,,,.,,,.,,.,,,,,,,, to the registry ¢f the adjacent bridging-O rows.
(a)
STM mea~,urements employed a commercial UHV instrument (Omicron GmbH), the base pressure during this wf~rk being < 10-zo rnbar. This instrument is desigmd for room temperature operation, with the tip held at ground potential and the sample biased. The it~ages presented below were acquired in the const~at current mode with the sample held at positive ~Jias. Under these conditions Ti atoms are imago~J on the clean surface [8-10,14]. Vertical and h(,dzontal distances were calibrated using images of Cu(110)2 × 1-O, for which the dimensions arc known
ts]. The TiO 2 sample (Commercial Crystals Inc.) was cut and polished (0.25 /~m) to within 0.5 ° of the (ll0) plane, as checked by Laue diffraction. It was vacuum reduced to introduce n-type conductivity
5.folcl Tt4, coordinated
4-fold =, coordinated1'1"
2.5A~
Fig. 2. 500 x 200 ~2 STM images recorded before (a) ( + 2 V, 0.3 hA) and (b) after ( + 1.5 V, 0.5 hA) dosing TiO2(110)1 x 2 with Na. In (a), a double strand of the 2 X 2 superlattice is indicated (A), along with an area which contains a single strand (B). In (b), a clean area (C) is indicated, along with areas which contain a 1)(4X 2) overlayer (D). One of these areas is shown expanded in the inset, with the p(4X2) unit cell outlined. The two lines marked on the image are discusse0 in the text.
ol1
(at
(b)
(o)
(e)
lo)
,4[t~0]
i J
(f)
Fig, 1. Structural models of Ti02(llO)substrates and associated Na overlayers. (a) The stoichiometric 1 x I termination [1,8,9]; (b) the reduced 1 × 2 phase, in which in-plane Ti atoms relax by 0.5 /~ towards the missing bridging-O tow [10]; (c) the 2 x 2 superlatrice, which forms a double strand a[ong [1]0], along with a portion of the analogous single strand on the lower right [10]; (d) the c(4 × 2)-Na overlayer on the ] × 1 surface [2]; (e) a model of TiO2(ll0)l×2-p(4×2)-Na based on the model in (d); (f) the model of TiO2(110)! × 2-1)(4× 2)-Na derived from our STM data. Large shaded and open circles represent in-plane and bridging oxygen atoms, rCSl~ctively, while small black and open circles represent Ti and Na, respectively, scaled to their ionic radii [11]. [n (c), lightly-shaded small and large cir,:ies represent Ti and O atoms associated with the double anO single strands of the 2 × 2 sup~r]attice. The corresponding unit cells are indicated. The line marked on (b) is discussed in the Fig. 3 caption.
(-- l0 ts cm-3). Cleaning of the sample in situ employed cycles of 500 eV Ar + bombardment and annealing to 1200 K. Cycles were repeated until TiO,(ll0)l x 2 STM images were obtained which were of the same quality as those previously published [10]. At this point, Auger spectra were consistent with a clean surface and the LEED pattern showed sharp integral order spots with streaking along [110]. Sodium was deposited from a weU-degassed getter source (SAES) to yield a coverage of < 0.01 ML, as estimated from the STM images. The chamber pressure remained below 10 -9 mbar during evaporation. Auger spectra recorded after STM measurements on the Na-dosed surfaces showed no sign of contamination. Fig. 2 compares an STM image of TiO2(ll0)l x 2 with an image recorded after exposure to Na. In Fig. 2a, the 1 x 2 reconstructed surface is evidenced by the bright ~'~ rows which have a [110] periodicity of 13 ~. These rows arise from tunnelling into the triple Ti row seen in the model in Fig. lb [10]. The bridging-O rows comained in this model give rise to
P.W. Murray et al./ Surface Science 323 (1995) L281-L~.8(~ I
3.0-
I
~ ' ~ -
I
Na
'< 2.0 ~ 1.0
s
0.0
o
[001] between these features is 11.9 4-0.2 ~k, as estimated from a height profile recorded from fl~e [001]-direction line drawn on the image in Fig. 21o. This distance is consistent with four bulk-terminated surface unit cells in this direction (11.8 .~). At t~ae coverage examined these [001]-direction rows are not well ordered in the perpendicular [110] direction, the minimum separation observed being 13 A. This corresponds to two bulk-terminated surface unit cells along [110], being the distance between pa~lel bridging-O rows on the 1 x 2 surface. Although the Na-induced [001J-direction rows are not generally ordered along [110], some ordered areas with p(4 x 2) symmetry are apparent. One such area is picked out in Fig. 2b. The p(4 x 2) symmetry evidenced here is not inconsistent with the model of TiO2(ll0)c(4 ;~<2)-Na shown in Fig. ld; the loss of bridging-O rows from the 1 x 1 phase to form the 1 X 2 termination would remove the centred Na-O cluster. An appropriate modification of the c(4 ,'< 2) model is shown in Fig. le to illustrate this point. While the symmetry of the Na overlayer observed here is in line with that expected from the c(4 x 2),-Na overlayer model of Onishi et al. [2], details of the Na positions are less consistent. A height profile recorded along the [l'i0]-Iine shown on the image in Fig. 2b illustrates this point. This is shown in Fig. 3, where it is compared with a profile from a corresponding line of the clean surface. The double peaks in the
;o
Horizontal Distance(A)
40
Fig. 3. A height profile recorded from the [110] line marked on Fig. 2b, compared with a corresponding line from the clean surface. On the basis of our clean surface study [10], the latter is along the line marked on the model in Fig. lb. The clean surface profile has been vertically off-set by - 1 ,~. The origin of the features is indicated.
the dark areas which lie between the Ti rows in the STM images. In addition to the 1 × 2 structure, a [110] direction row is observed in Fig. 2a, which represents the 2 × 2 superlattice [10]. Areas of the clean surface are still visible in the image shown in Fig. 2b. In addition, bright features of atomic dimensions are observed, which we associate with Na. They are periodically aligned along [001] in close proximity to the dark areas corresponding to the bridging-O rows. The spacing along
Fig. 4. A 450 x 250 ~2 ~,TM image ( + 1.5 V, 0.5 hA) of an area of TiO2(110)-1 x 2 containing a high density of the 2 × 2 superlattice, following exposure to Na. Areas of the clean surface (C) and a p(4 x 2)-Na overlayer (B) can be observed in addition to more complex structures (A). In the latter regions there appears to be a discontinuity in the dark areas associated with the bridging-O rows, as highlighted by the line marked on the image.
P. W. Murray et al. / Surface Science 323 (1995) L281-L286
clean surface profile represent the position of the triple Ti row of the 1 × 2 reconstruction [10], while the minima correspond to the position of the bridging-O rows. We first note that the Na-induced feature does not lie symmetrically about a bridging-O
row, ruling Out the local N a - O clustering proposed by Onishi et al. [2] for the c(4 × 2)Na overlayer. The comparison of height profiles in Fig. 3 indicates tha~ the Na induced feature lies 2.2 + 0.3 ~, away from the centre of the bridging-O row along
(a)
(b)
\
[001]
Fig. 5. (a) A higher resolution 100 × 200/'~'~ STM image ( + 1.5 V, 0.5 hA) recorded from a similar area to that imaged in Fig. 4, compared to (b) a structural model of the area marked on the image, in (a), a singl© unit of the 2 × 2 superlattice is indicated (A) and the positions of two pairs of Ti atoms which arc adjacent to Na are highlighted. These are discussed in the text. The large box in the image corresponds to the area covered by the model in (b). In both (a) and (b) the first inner rectangle connects Na atoms, while the smallest rectangle connects two pairs of Ti atoms which form prt of the 2 x 2 superlattice. Large shaded and open circles represent in-plane and bridging oxygen atoms, respectively, while small black and open circles represent Ti and Na, with lightly-shaded small and large circles representing Ti and O atoms associated with the 2 × 2 superlattice. Atom sizes are scaled to their ionic radii [11].
P.W. Murray et al. /Surface Science 323 (1995) L281-L286 m
o
[110]. On the basis of the ionic radii (O; 1.4 A, Na; 1.0 .~) [11], this is consistent with the model shown in Fig. If, which implies that the bright Na-induced features arise from single Na atomsu This model is based on the [l'i0]-direction height profile and the assumption that Na adsorption does not relax Ti and O atoms from the positions derived from an earlier STM study of the 1 × 2 surface [10]. The [001]-direction height profile along the line shown in ~"ig. 2b is consistent with this model in showing only s'ngle maxima at the position of the Na-induced feature~ rather than two, expected on the basis of the model in Fig. ld. We infer bonding to the in-plane O atoms from the lateral position of Na along [1]0], leading to a three-fold coordination, the maximum that can be achieved on the 1 x 2 surface. We next briefly describe scanning tunnelling spectroscopy (STS) results from the p(4 × 2)Na overlayer. Previous work has shown that it is possible to distinguish between Ti and O on TiO 2 surfaces using STS [16], which relies on the ionic character of the substrate. This leads to a low local density of occupied states at Ti sites compared with that at O sites. STS data recorded from the bright features in Fig. 2b indicate a low density of occupied states. This is consistent with the results of a photoemission study of Na adsorption on TiO2(ll0)l × 1, which evidenced charge transfer to the substrate [2]. The corollary, that Na will have a relatively high density of unoccupied states, is in line with the appearance of the associated features in STM images recorded at positive sample bias. Turning now to the Na-induced structures formed on the 2 × 2 superlattice. Fig. 4 contains an image recorded from an area with a relatively high density of the 2 × 2 phase, following exposure to Na. This image contains p(4 × 2)-Na areas, as well as areas where the bridging-O rows appear to be discontinuous, with a shift along [110]. The latter effect is highlighted by reference to a line drawn on the image. Overall, the bright features, which we again associate with Na, appear to be less well ordered than in the image shown in Fig. 2b. In the higher resolution image shown in Fig. 5, units of the double strand 2 × 2 structure as well as the single strand analogue can clearly be seen. The bright Na features appear to have some degree of alignment along both principal azimuths. Most of the
Na atoms are adjacent to two other features which form part of a five-fold Ti row, being associated with the 2 × 2 superlattice. To simplify the discussion we have chosen an area of the image and constructed a structural model to explain the features observed within it. We base the model on that shown in Fig. lc. The separation along [110] between the Na features within the highlighted area of the image corresponds to about three bulk unit cells, with a separation of four unit cells along [001]. The model shown in Fig. 5 accommodates Na in a four-fold coordinated site, which are uniquely available in the 2 x 2 phaseo. On the basis of the ionic radii [11], there is a 1 A gap between two top-layer O atoms along [110], which is just sufficient to accommodate Na +, allowing it to bond to two O atoms in the second layer. This position of Na leads to the correct location in the image relative to the two adjacent Ti features. That the corresponding Ti atoms closer to the Na adsorption site are not imaged may be due to the loss of contrast introduced by their proximity to Na. It is apparent from the image in Fig. 5 that there is a shift in the position of 1 × 2 bridgi~,g-O rows associated with the Na adsorption site, which is represented in the model. This shift appears to result from tile interaction between Na and an O atom in the bridging-row site which has been re-occupied by formation of the 2 × 2 superiattice. The resulting local re-occupation of this bridging-O row produces the discontinuities in the images noted above. It seems likely that this reconstruction serves to avoid charge transfer from Na into what would have been the vicinity of four-fold coordinated Ti 3+ ato~ls in the 2 × 2 superlattice. In summary, we have used STM to study the structures formed by Na on reduced surfaces of TIO2(110). The results indicate that at low coverage Na occupies a site on TiO2(110)-1 × 2 in which it is three-fold coordinated to oxygen, forming a p(4 × 2) overlayer. Adsorption onto a 1 × 2 surface having a high concentration of the 2 x 2 superlattice results in a complex reconstruction which involves a shift in the position of bridging O-rows in the [110] direction. As far as we are aware, this is the first observation of alkali metal-induced restructuring of an oxide surface. This restructuring accompanies adsorption into a site in which Na is four-fold coordinated to E
P.W. Murray et al. / Surface Science 323 (1995) L281-L286
uz,y~;~., n~,lce, Na maximises its coordination to O on both the 1 × 2 and 2 × 2 areas of the reconstructed TiO2(ll0) surface.
Acknowledgement This work was funded by the UK EPSRC.
References [1] V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994). [2] H,Onishi, T. Aruga, C. Egawa and Y. Iwasawa, Surf. Sci. 199 (1988) 54. [3] H.Onishi, T. Atuga, C. Egawa and Y. Iwasawa, J. Chem. Soc. Faraday Trans. 85 (1989) 2597. [4] K. Prabhakaran, D. Purdie, R. Casanova, C.A. Muryn, P.J.
Hardman, P.L. Wincott and G. Thornton, Phys. Rev. B 45 (1992) 6969. [5] PJ. Hardman, R. Casanova, K. Prabhakaran, C.A. Muryn, P.L. Wincott and G. Thornton, Surf. Sci. 269/270 (1992) 677. [6] B.E. Hayden and G.P. Nicholson, Surf. Sci. 274 (1992) 277. [7] B.L. Maschhoff, J.M. Pan and T.E. Madey, Surf. Sci. 259 (1991) 190. [8] M. Sander and T. Engel, Surf. Sci. 302 (1994) L263. [9] H. Onishi and Y. lwasawa, Surf. SCi. 313 (1994) L783. [10] P.W. Murray, N.G. Condon and G. Thornton, Phys. Rev. B, submitted for publication. [11] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [12] J.V. Batth, R. Schuster, R.J. Behm and O. Ertl, Surf. Sci. 302 (1994) 158. [13] Y. Hasegawa, I. Kamiya, T. Hashizume, T. Sakurai, H. Tochihara, M. Kubota and Y. Murata, Phys. Rev. B 41 (1990) 9688. [14] P.W. Murray, F.M. Leibsle, C.A. Muryn, H.J. Fisher, C.F.J. Flipse and G. Thornton, Phys. Rev. Lett. 72 (1994) 689. [15] D.J. Coulman, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett. 64 (1990) 1761. [16] P.W. Murray, F.M. Leibsle, H.J. Fisher, C.F.J. Flipse, C.A. Muryn and G. Thornton, Phys. Rev. B. 46 (1992) 12877.
iii ELSEVIER
Surface Science323 (1995) L281-L286
Surface Science Letters
Na adsorption sites on TiO2(110)-1 X 2 and its 2 x 2 superlattice P.W. Murray, N.G. Condon, G. Thornton
1
Interdisciplinary Research Centre in Surface Science, University of Liverpool, Liverpool L69 3BX, UK
Received 14 September1994;acceptedfor publication4 November1994
Abstract
Scanning tunnelling microscopy (STM) has been used to investigate tt3e structures formed by a low coverage (< 0.01 ML) of Na on reduced 1 x 2 surfaces of TiO2(ll0). The STM images recorded at positive sample bias indicate that Na adsorbs in three-fold coordinated sites adjacent to the bridging-O row, forming a p(4 x 2) overlayer. Adsorption on areas of the clean surface which contain the 2 x 2 superlattice suggest that Na adsorbs in a site with a four-fold coordination to oxygen. This is accompanied by Na-induced substrate restructuring.
Despite the well-known catalytic promotion ef4'ects of alkali metals, relatively little is known about taeir interaction with metal oxide surfaces [1]. Of the information which is available, most is associated with rutile TiO 2 substrates [2-6]. In part this derives from the significant database on the clean substrates, and in part because on TiO 2 it is relatively straightforward to monitor charge-transfer to the substrate. Previous studies of alkali metal adsorption on TiO 2 have investigated the accompanying changes to the electronic structure [2,4-6] and reactivity [3] as well as the crystallography [2,4]. In this Letter we focus on the structural changes associated with Na adsorption on reduced surfaces of TiO2(ll0). The structures associated with TiO2(ll0) have previously been characterised by LEED [1], medium energy electron diffraction [7] and scanning tunnelling microscopy (STM) [8-10]. Two principal
1Also at: Chemistry Department, Manchester University, Manchester M13 9PL, UK.
phases are formed, the stoichiometric 1 x 1 termination and the O-deficient 1 x 2 reconstruction. The 1 x 2 phase coexists with a structure having 2 x 2 local symmetry. Structural models of these surfaces derived from earlier work are shown in Figs. l a - l c . As for Na adsorption, previous work has established that a c(4 X 2) overlayer is formed on the 1 × 1 substrate at 0.5 monolayer (ML) coverage [2], the structural model for which is shown in Fig. ld. Here we describe STM results for Na on TiO2(ll0)-I × 2 and its associated 2 × 2 superlatrice. To an extent, this work serves to examine the potential of the technique in the study of an alkali metal overlayer on an oxide surface. Although there have been numerous S]'M studies of alkali metal adsorption on metal [12] and classical semiconductor surfaces [13], there do not appear to be any previous reports relating to oxides. Our results are consistent with Na bonding to both tha, bridging-O row and in-plane O atoms in the 1 × 2 substrate. The interaction with the 2 × 2 phase appears to involve Na bonding to four O atoms, with an accompanying
0039-6028/95/$09.50 © 1995 ElsevierScienceB.V. All rightsreserved SSDI 0039-6028(94)00751-9
P.W. Murray et aL / Surface Science 323 (1995) L281-L286
,,,,.,,,.,,.,,,,,,,, to the registry ¢f the adjacent bridging-O rows.
(a)
STM mea~,urements employed a commercial UHV instrument (Omicron GmbH), the base pressure during this wf~rk being < 10-zo rnbar. This instrument is desigmd for room temperature operation, with the tip held at ground potential and the sample biased. The it~ages presented below were acquired in the const~at current mode with the sample held at positive ~Jias. Under these conditions Ti atoms are imago~J on the clean surface [8-10,14]. Vertical and h(,dzontal distances were calibrated using images of Cu(110)2 × 1-O, for which the dimensions arc known
ts]. The TiO 2 sample (Commercial Crystals Inc.) was cut and polished (0.25 /~m) to within 0.5 ° of the (ll0) plane, as checked by Laue diffraction. It was vacuum reduced to introduce n-type conductivity
5.folcl Tt4, coordinated
4-fold =, coordinated1'1"
2.5A~
Fig. 2. 500 x 200 ~2 STM images recorded before (a) ( + 2 V, 0.3 hA) and (b) after ( + 1.5 V, 0.5 hA) dosing TiO2(110)1 x 2 with Na. In (a), a double strand of the 2 X 2 superlattice is indicated (A), along with an area which contains a single strand (B). In (b), a clean area (C) is indicated, along with areas which contain a 1)(4X 2) overlayer (D). One of these areas is shown expanded in the inset, with the p(4X2) unit cell outlined. The two lines marked on the image are discusse0 in the text.
ol1
(at
(b)
(o)
(e)
lo)
,4[t~0]
i J
(f)
Fig, 1. Structural models of Ti02(llO)substrates and associated Na overlayers. (a) The stoichiometric 1 x I termination [1,8,9]; (b) the reduced 1 × 2 phase, in which in-plane Ti atoms relax by 0.5 /~ towards the missing bridging-O tow [10]; (c) the 2 x 2 superlatrice, which forms a double strand a[ong [1]0], along with a portion of the analogous single strand on the lower right [10]; (d) the c(4 × 2)-Na overlayer on the ] × 1 surface [2]; (e) a model of TiO2(ll0)l×2-p(4×2)-Na based on the model in (d); (f) the model of TiO2(110)! × 2-1)(4× 2)-Na derived from our STM data. Large shaded and open circles represent in-plane and bridging oxygen atoms, rCSl~ctively, while small black and open circles represent Ti and Na, respectively, scaled to their ionic radii [11]. [n (c), lightly-shaded small and large cir,:ies represent Ti and O atoms associated with the double anO single strands of the 2 × 2 sup~r]attice. The corresponding unit cells are indicated. The line marked on (b) is discussed in the Fig. 3 caption.
(-- l0 ts cm-3). Cleaning of the sample in situ employed cycles of 500 eV Ar + bombardment and annealing to 1200 K. Cycles were repeated until TiO,(ll0)l x 2 STM images were obtained which were of the same quality as those previously published [10]. At this point, Auger spectra were consistent with a clean surface and the LEED pattern showed sharp integral order spots with streaking along [110]. Sodium was deposited from a weU-degassed getter source (SAES) to yield a coverage of < 0.01 ML, as estimated from the STM images. The chamber pressure remained below 10 -9 mbar during evaporation. Auger spectra recorded after STM measurements on the Na-dosed surfaces showed no sign of contamination. Fig. 2 compares an STM image of TiO2(ll0)l x 2 with an image recorded after exposure to Na. In Fig. 2a, the 1 x 2 reconstructed surface is evidenced by the bright ~'~ rows which have a [110] periodicity of 13 ~. These rows arise from tunnelling into the triple Ti row seen in the model in Fig. lb [10]. The bridging-O rows comained in this model give rise to
P.W. Murray et al./ Surface Science 323 (1995) L281-L~.8(~ I
3.0-
I
~ ' ~ -
I
Na
'< 2.0 ~ 1.0
s
0.0
o
[001] between these features is 11.9 4-0.2 ~k, as estimated from a height profile recorded from fl~e [001]-direction line drawn on the image in Fig. 21o. This distance is consistent with four bulk-terminated surface unit cells in this direction (11.8 .~). At t~ae coverage examined these [001]-direction rows are not well ordered in the perpendicular [110] direction, the minimum separation observed being 13 A. This corresponds to two bulk-terminated surface unit cells along [110], being the distance between pa~lel bridging-O rows on the 1 x 2 surface. Although the Na-induced [001J-direction rows are not generally ordered along [110], some ordered areas with p(4 x 2) symmetry are apparent. One such area is picked out in Fig. 2b. The p(4 x 2) symmetry evidenced here is not inconsistent with the model of TiO2(ll0)c(4 ;~<2)-Na shown in Fig. ld; the loss of bridging-O rows from the 1 x 1 phase to form the 1 X 2 termination would remove the centred Na-O cluster. An appropriate modification of the c(4 ,'< 2) model is shown in Fig. le to illustrate this point. While the symmetry of the Na overlayer observed here is in line with that expected from the c(4 x 2),-Na overlayer model of Onishi et al. [2], details of the Na positions are less consistent. A height profile recorded along the [l'i0]-Iine shown on the image in Fig. 2b illustrates this point. This is shown in Fig. 3, where it is compared with a profile from a corresponding line of the clean surface. The double peaks in the
;o
Horizontal Distance(A)
40
Fig. 3. A height profile recorded from the [110] line marked on Fig. 2b, compared with a corresponding line from the clean surface. On the basis of our clean surface study [10], the latter is along the line marked on the model in Fig. lb. The clean surface profile has been vertically off-set by - 1 ,~. The origin of the features is indicated.
the dark areas which lie between the Ti rows in the STM images. In addition to the 1 × 2 structure, a [110] direction row is observed in Fig. 2a, which represents the 2 × 2 superlattice [10]. Areas of the clean surface are still visible in the image shown in Fig. 2b. In addition, bright features of atomic dimensions are observed, which we associate with Na. They are periodically aligned along [001] in close proximity to the dark areas corresponding to the bridging-O rows. The spacing along
Fig. 4. A 450 x 250 ~2 ~,TM image ( + 1.5 V, 0.5 hA) of an area of TiO2(110)-1 x 2 containing a high density of the 2 × 2 superlattice, following exposure to Na. Areas of the clean surface (C) and a p(4 x 2)-Na overlayer (B) can be observed in addition to more complex structures (A). In the latter regions there appears to be a discontinuity in the dark areas associated with the bridging-O rows, as highlighted by the line marked on the image.
P. W. Murray et al. / Surface Science 323 (1995) L281-L286
clean surface profile represent the position of the triple Ti row of the 1 × 2 reconstruction [10], while the minima correspond to the position of the bridging-O rows. We first note that the Na-induced feature does not lie symmetrically about a bridging-O
row, ruling Out the local N a - O clustering proposed by Onishi et al. [2] for the c(4 × 2)Na overlayer. The comparison of height profiles in Fig. 3 indicates tha~ the Na induced feature lies 2.2 + 0.3 ~, away from the centre of the bridging-O row along
(a)
(b)
\
[001]
Fig. 5. (a) A higher resolution 100 × 200/'~'~ STM image ( + 1.5 V, 0.5 hA) recorded from a similar area to that imaged in Fig. 4, compared to (b) a structural model of the area marked on the image, in (a), a singl© unit of the 2 × 2 superlattice is indicated (A) and the positions of two pairs of Ti atoms which arc adjacent to Na are highlighted. These are discussed in the text. The large box in the image corresponds to the area covered by the model in (b). In both (a) and (b) the first inner rectangle connects Na atoms, while the smallest rectangle connects two pairs of Ti atoms which form prt of the 2 x 2 superlattice. Large shaded and open circles represent in-plane and bridging oxygen atoms, respectively, while small black and open circles represent Ti and Na, with lightly-shaded small and large circles representing Ti and O atoms associated with the 2 × 2 superlattice. Atom sizes are scaled to their ionic radii [11].
P.W. Murray et al. /Surface Science 323 (1995) L281-L286 m
o
[110]. On the basis of the ionic radii (O; 1.4 A, Na; 1.0 .~) [11], this is consistent with the model shown in Fig. If, which implies that the bright Na-induced features arise from single Na atomsu This model is based on the [l'i0]-direction height profile and the assumption that Na adsorption does not relax Ti and O atoms from the positions derived from an earlier STM study of the 1 × 2 surface [10]. The [001]-direction height profile along the line shown in ~"ig. 2b is consistent with this model in showing only s'ngle maxima at the position of the Na-induced feature~ rather than two, expected on the basis of the model in Fig. ld. We infer bonding to the in-plane O atoms from the lateral position of Na along [1]0], leading to a three-fold coordination, the maximum that can be achieved on the 1 x 2 surface. We next briefly describe scanning tunnelling spectroscopy (STS) results from the p(4 × 2)Na overlayer. Previous work has shown that it is possible to distinguish between Ti and O on TiO 2 surfaces using STS [16], which relies on the ionic character of the substrate. This leads to a low local density of occupied states at Ti sites compared with that at O sites. STS data recorded from the bright features in Fig. 2b indicate a low density of occupied states. This is consistent with the results of a photoemission study of Na adsorption on TiO2(ll0)l × 1, which evidenced charge transfer to the substrate [2]. The corollary, that Na will have a relatively high density of unoccupied states, is in line with the appearance of the associated features in STM images recorded at positive sample bias. Turning now to the Na-induced structures formed on the 2 × 2 superlattice. Fig. 4 contains an image recorded from an area with a relatively high density of the 2 × 2 phase, following exposure to Na. This image contains p(4 × 2)-Na areas, as well as areas where the bridging-O rows appear to be discontinuous, with a shift along [110]. The latter effect is highlighted by reference to a line drawn on the image. Overall, the bright features, which we again associate with Na, appear to be less well ordered than in the image shown in Fig. 2b. In the higher resolution image shown in Fig. 5, units of the double strand 2 × 2 structure as well as the single strand analogue can clearly be seen. The bright Na features appear to have some degree of alignment along both principal azimuths. Most of the
Na atoms are adjacent to two other features which form part of a five-fold Ti row, being associated with the 2 × 2 superlattice. To simplify the discussion we have chosen an area of the image and constructed a structural model to explain the features observed within it. We base the model on that shown in Fig. lc. The separation along [110] between the Na features within the highlighted area of the image corresponds to about three bulk unit cells, with a separation of four unit cells along [001]. The model shown in Fig. 5 accommodates Na in a four-fold coordinated site, which are uniquely available in the 2 x 2 phaseo. On the basis of the ionic radii [11], there is a 1 A gap between two top-layer O atoms along [110], which is just sufficient to accommodate Na +, allowing it to bond to two O atoms in the second layer. This position of Na leads to the correct location in the image relative to the two adjacent Ti features. That the corresponding Ti atoms closer to the Na adsorption site are not imaged may be due to the loss of contrast introduced by their proximity to Na. It is apparent from the image in Fig. 5 that there is a shift in the position of 1 × 2 bridgi~,g-O rows associated with the Na adsorption site, which is represented in the model. This shift appears to result from tile interaction between Na and an O atom in the bridging-row site which has been re-occupied by formation of the 2 × 2 superiattice. The resulting local re-occupation of this bridging-O row produces the discontinuities in the images noted above. It seems likely that this reconstruction serves to avoid charge transfer from Na into what would have been the vicinity of four-fold coordinated Ti 3+ ato~ls in the 2 × 2 superlattice. In summary, we have used STM to study the structures formed by Na on reduced surfaces of TIO2(110). The results indicate that at low coverage Na occupies a site on TiO2(110)-1 × 2 in which it is three-fold coordinated to oxygen, forming a p(4 × 2) overlayer. Adsorption onto a 1 × 2 surface having a high concentration of the 2 x 2 superlattice results in a complex reconstruction which involves a shift in the position of bridging O-rows in the [110] direction. As far as we are aware, this is the first observation of alkali metal-induced restructuring of an oxide surface. This restructuring accompanies adsorption into a site in which Na is four-fold coordinated to E
P.W. Murray et al. / Surface Science 323 (1995) L281-L286
uz,y~;~., n~,lce, Na maximises its coordination to O on both the 1 × 2 and 2 × 2 areas of the reconstructed TiO2(ll0) surface.
Acknowledgement This work was funded by the UK EPSRC.
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