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Reaction between copper dipivaloylmethanate Cu(DPM)2 and H2O adsorbed on SrTiO3(100)
Surface Science Letters 262 (1992) L139-L143 North-Holland
surface science letters
Surface Science Letters
Reaction between copper dipiva~oy~~~thanat~ adsorbed on SrTiO,( 100) Tokihisa
Hikita,
Research Laboratory
Rika Sekine,
Takashi
of Engineer&g Materials,
Cu( DPM) 2 and H,O
Hanada
Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku,
Yokohama
227, &an
and Maki Kawai The Institute of Physical and Chemical Research (HKEN~,
2-1 Hirosawa,
Wako-shi, Saitama
351-01, Japan
Received 17 June 1.991; accepted for publication 30 October 1991
Selective reaction of copper dipivaloylmethanate (Cu(DPM),, P-diketonate complex) with H,O adsorbed on the SrTiO,(lOO) surface is investigated. Adsorption of Ha0 on SrTiOs(100) occurs on Ti3+ sites produced by prior Arf ion bombardment. It is found that the ~-diketonate complex, Cu(DPM),, reacted selectivety with the adsorbed Ha0 at room temperature. Further reaction of the adsorbed Cu(DPM)a with Ha0 vapor results in the removal of the ligand, DPM, leaving elemental copper on the surface.
1. Introduction Selective adsorption or reaction on surfaces helps us to realize the atomically dispersed and controlled deposition of thin films. Selective reactions are also indispensable in designing selfterminating deposition systems. In the latter application the reaction between surface functional groups and organometallic compounds or complexes is expected to give us information about the atomically controlled deposition mechanism itself. Finally surface reaction has a great advantage in that the mount of deposited metals can be controlled by adjusting the amount of surface functional groups. On fine particles of SiO,, a selective and stoichiometric adsorption of P-diketonate compounds, copper dipivaloylmethanates (Cu(DPM),; 2,2,6,6-tetramethyl-3,5-heptadione) with surface hydroxyl group (OH) is reported [1,21. In this reaction, H atoms from the surface OH groups 0039-6028/92/$0500
move into the DPM ligand, and the interaction between the adsorbed ligand and the surface 0 is considered to be the cause of the stoichiometrical adsorption of Cu(DPM), and the surface hydroxyl group [1,2]. The removal of the ligand from the adsorbed CufDPM), is realized by H,O vapor reacting with the adsorbate. A simifar reaction is expected to occur on the surface of the oxide single crystal, where the OH groups also exist or subsequently adsorb. It has been reported that the SrTiO, surface is active for the adsorption of H,O when it has Ti3+ sites on the surface [3-51. The SrTiO~(l~) surface with adsorbed H,O is considered to show a similar reactivity with Cu(DPM), to that of the SiO, surface with OH groups. In this Letter, the reactivity of Cu(DPM), with H,O adsorbed on the SrTiO,(lOO) surface is reported. We also discuss on the decomposition of Cu(~PM)~ adsorbed on the SrTiO,(lOO) surface via the reaction with H,O vapor.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
i? Hikita et al. / Reaction between Cu(DPM),
2. Experimental
and HI0 adsorbed on SrTiO_JlOO) XPS
Ti
The SrTiO,(lOO) single crystal (Earth Jewelry Co., Ltd.) was cleaned by oxidization in 0, of 8 x lo4 Pa at 673 K for 120 min prior to transforming into ultrahigh vacuum (UHV) chamber (VG ADESSOO, base pressure < 1 x lo-’ Pa). In order to obtain a clean and oxygen rich surface, it was reoxidized with 0, of 1 x lop4 Pa at 673 K for 120 min. An Art bombardment was carried out with the acceleration energy of 200 eV for 10 min in order to form Ti”+ sites on the SrTiO,(lOO) surface. H,O and Cu(DPM), (Toso Aquzo Co., Ltd. and TRI Chemical Laboratory Co., Ltd.) were purified by distillation at 194 and 393 K, respectively. In order to avoid a reaction between H,O and Cu(DPM), in the introduction system, H,O and Cu(DPM), were supplied from different leak valves. The reservoir and the introduction system for Cu(DPM), were heated up to 393 K during the deposition. Gaseous H,O and Cu(DPM), were supplied for 20 min with a pressure of 1 X lop4 and 1 X lop5 Pa, respectively. The elemental composition and the chemical states of Sr, Ti, 0, Cu and C were experimentally confirmed by Auger electron, X-ray photoelectron and ultraviolet photoelectron spectroscopies (AES, XPS, and UPS). In the XPS measurement, Mg Ka was used, and in the UPS measurement, He1 was used. In the XPS measurement, the emitted electron was detected in the direction of 70” to the surface normal. The binding energy observed in the XPS was calibrated by taking Ti 2~ 3,2 of Ti4+ on SrTiO,(lOO) as 458.6 eV [6,7]. The electron energy loss spectrum (EELS) was taken using an incident energy of 5 eV, where the resolution was 20 meV in FWHM, and an incident angle of 45” to the surface.
3. Results
and discussion
After the oxidation treatment, the SrTiO,(lOO) surface became clean and oxygen rich, where only Sr2+ Ti4+ and 02- were observed by XPS. The emisiion peaks of Ti2p,,, (458.6 eV) and 2p,,*
UPS 2p
456.6.9 m
I
ik 456.3&l
464.6eV
462.3eu
b
C
EUNDING ENERGY
ev
Fig. 1. The MgKa excited XPS of Ti 2p, and He I excited UPS at the valence band of the SrTiO, surface. (a) The clean and oxygen rich surface; (b) and (d) the oxygen deficient surface after bombarded by Ar + ions and (c) and (e) the surface of (b) with H,O adsorbed.
(464.6 eV) due to Ti4+ are shown in fig. la. After the Ar+ bombardment the decrease in the intensity of the photoelectron peak of 0 1s was observed, whereas those for Sr 3d or Ti 2p remained unchanged. The surface of SrTiO,(lOO) became oxygen deficient by the Art ion bombardment. Due to the desorption of oxygen from the surface, Ti”+ began to appear. The changes in the XPS spectrum of Ti2p are shown in fig. lb. In the emission peak of Ti2p, a shoulder began to appear in the low binding energy region of the peak at 456.3 and 462.3 eV. The observed spectra are fitted using the binding energies of Ti4+, Ti”+ and Ti2+ shown by Carley et al. [6]. It has been found that the observed Ti2p spectrum after the Ar+ ion bombardment consisted of Ti4+ and Ti”+, and a small trace of Ti2+, suggesting that the shoulder peak can be assigned to the Ti”+ state [3-61. In the above reduction condition, Ti on SrTiO,(lOO) can only be reduced from Ti4+ to Ti”+ and hardly Ti 2-c. Together wi th the changes in the binding energy of Ti2p, the emission from Ti3d began to appear near the Fermi edge. The UPS spectrum after Ar+ bombardment is shown in fig. Id. These facts show that by the Art bombardment, surface oxygen defects
T. Hikita et al. / Reaction between Cu(DPM),
I 0
I
100
I
I
a0
30
ENERGY
I
I
400
500
and H,O a d:Forbed on SrTiO,(lOO)
600
LOSS I meV
Fig. 2. EELS of SrTiO,(lOO) surface. Incident electron beam of 5 eV with FWHM of 20 meV. The incident beam was introduced at 45” to the surface normal. (a) Loss spectrum of the Art ion bombarded, clean surface; (b) H,O adsorbed on (a) by H,O pressure of 1 x low4 Pa at 473 K for 20 min; (c) Cu(DPM), was introduced to the H,O adsorbed surface at 1 x lo-’ Pa at 300 K for 20 min.
are created and Ti changed from Ti4+ to Ti3+ in the vicinity of the defects [5,8,9]. When H,O vapor is supplied to the oxygen deficient surface at 473 K, the emission peaks due to the Ti3 ‘- state observed in Ti 2p (XPS) and those of Ti3d (UPS) decreased in intensity (figs. lc and le>. At the same time, the emission peak of 0 1s began to show a shoulder at 532.2 eV in the high binding energy region (fig. 3b). The main emission peak of 0 1s at 530.4 eV is due to oxygen in SrTiO,(lOO). Changes of the EELS during the above treatment are shown in fig. 2. The loss peak of the clean surface shows peaks originated from surface phonons (fig. 2a). EELS of the clean surface of SrTiO, has already been reported previously [lo]. This result is consistent with previous work. The H,O vapor exposed surface showed a loss peak at 450 meV (fig. 2b). This peak originated from the OH stretching mode of adsorbed H,O on the surface [5]. From the above evidence, it is concluded that the peak at high binding energy of 532.2 eV in 0 Is (fig. 3b) is assigned to the oxygen in the adsorbed H,O on surface Ti3+ sites [3-51. The adsorbed H,O thus formed is expected to show a similar reactivity to OH on SiO,, where the selec-
Fig. 3. The XPS of 01s of SrTiO,(lOO). (a) Ar+ ion bombarded surface; (b) H,O adsorbed on (a) by H,O pressure of 1 x 10e4 Pa, at 473 K for 20 min; (c) Cu(DPM), was introduced to the H,O adsorbed surface at 1 X 10m5 Pa at 300 K for 20 min; (d) after the H,O treatment of the Cu(DPM), adsorbed surface at H,O pressure of 1 x 1O-4 Pa at 473 K for 20 min.
01
...
’
ox.
Ar
Born.
HZ0
Ad.
Cu(DPY)2 Ad.
El20 Treat.
Treatment Fig. 4. Changes in the intensity of C 1s and Cu2p,,, peaks normalized to that of Ti2p,,, observed by XPS. The circles (0) indicate C 1s and the triangles ( A ) indicate Cu 2p,,, peak intensities. [Ox.]: The clean and oxygen rich surface of SrTiO,(lOO); [Ar born.]: Ar+ ion bombarded surface; [H,O ad.]: H,O adsorbed on the bombarded surface with H,O pressure of 1 x 1O-4 Pa at 473 K for 20 min; [Cu(DPM)’ ad.]: Cu(DPM), was introduced on the H,O adsorbed surface at 1 X 10W5 Pa at 300 K and for 20 mitt; [Hz0 treat.]: After the H,O treatment of the Cu(DPM), adsorbed surface at 1 x 10m4 Pa at 473 K for 20 min.
T. Hikiia et al. / Reaction between C~(~P~~~
tive adsorption and reaction of Cu(DPM), is reported [1,21. The SrTiO, surface with adsorbed H,O was prepared first. Then Cu(DPM), vapor was supplied at room temperature and the adsorption of Cu(DPM), on the surface was observed by increase in the intensity of Auger electrons in the C KLL and Cu LMM spectra and by increase in the intensity of the loss peak in EELS due to the CH stretching mode on the surface. As shown in fig. 3c, when Cu(DPM), was adsorbed on the surface with adsorbed H,O, the higher binding energy peak of 0 1s assigned to OH species disappeared. At the same time, the energy loss due to the OH stretching mode observed in EELS disappeared and a new loss peak assigned to the CH stretching mode of the ligand of adsorbed Cu(DPM), appeared (fig. 2~). On a clean surface of SrTiO,(lOOI without defects or OH species, the adsorption of Cu(DPM), did not take place. It is clear that Cu(DPM), perfectly adsorbs on adsorbed H,O sites on the SrTiO,(lOO) surface. Decomposition of adsorbed Cu(DPM)~ by reaction with H,O vapor was examined. The changes in the peak intensity for C Is and Cu 2p,,,
I-
I 930
I 940 BINDING
I 950 ENERGY / eV
and Hz0 adsorbed on SrTi0,~100~
observed are shown in fig. 4. When Cu(DPM), was supplied to the surface, the intensities of C 1s and Cu 2p,,, increased because of adsorption of Cu(DPM), on the adsorbed H,O. After H,O treatment the intensity of C 1s decreased, whereas that of Cu 2p,,, did not or only slightly increased. This can be expiained as follows: DPM, the ligand of Cu(DPM),, was removed by reaction with H,O vapor. The small increase in the intensity of Cu2p is due to the decrease in the amount of the DPM Iigand, which absorbed a photoelectron from a copper atom. After treatment with H,O, the shoulder peak in 0 1s assigned to OH species appeared again at high binding energy (fig. 3d). In the case of adsorption of Cu(DPM), on OH on SiOz, the H atom from the surface OH is known to move into the DPM ligand changing the vibrational spectrum of the CH stretching mode for adsorbed Cu(DPM), from that of the Cu(DPM), molecule [l]. If we assume that a similar reaction takes place on the surface of the SrTiO~(lOO), the H atom from the adsorbed H,O is extracted into the ligand of the adsorbate. As H,O was adsorbed on the Ti3+ site to give Ti4’,
I 96
I 325
I I 915 920 KINFTIC ENERGY f eV
I
910
Fig. 5. The XPS of Cu2p and AEZSexcited by X-ray. (a) Adsorbed CU(DPM)~; (b) after the Hz0 treatment; (c) CLImetal foil.
T Hikita et al. / Reaction between Cu(DPM),
adsorbed H,O should have a similar character to OH species. Considering the fact that the oxygen species after releasing H was not observed at the same binding energy as that for OH and that Ti3+ did not appear, it is reasonable to think that oxygen after releasing the H atom is in the same state as bulk oxygen in the 02- state. From the above evidence, it is judged that when Cu(DPM), is adsorbed by OH on the SrTiO,(lOO) surface, H+ from the surface moves into the DPM ligand and 02- is left on the surface. 02- on the surface and (DPMH)+ in the ligand interact and are a cause of a force for the adsorption of Cu(DPM), on the surface. After reaction with H,O vapor, the DPM ligand was removed but the metal copper center still remained on the surface, and the adsorption of H,O was reproduced. The spectra of Cu2p and Cu LMM Auger electrons are shown in fig. 5. The oxidation state of the remained copper observed in the photoelectron spectroscopies was proved to be + 1, because the emission peak for Cu2p in the XPS had no satellite peaks (figs. 5a and 5b), due to Cu2+, and the observed kinetic energy for Cu L,M,,SM,,, (917.9 eV) (figs. 5a and 5b) was lower than that for the Cu metal (919 eV> and its peak shape was different from that of the Cu metal (fig. 5~). These results were similar to those of copper(I)oxi [ll-131. The reproduced OH species after the decomposition of adsorbed Cu(DPM), was used for the further adsorption-decomposition cycle of Cu(DPM),. Then the intensity of Cu2p increased further, however, that for the carbon did not. It was suggested that this adsorption-decomposition occurred on the adsorbed H,O on the SrTiO, surface. In agreement with prior work the reaction observed here is similar to that observed for the SiO, surface [1,2].
and Hz0 adsorbed on SrTiOJlOO)
4. Conclusion The OH species is produced on the SrTiO, (100) surface by reaction of H,O with the Ti3+ defects prepared by Ar+ ion bombardment. On the surface of SrTiO,(lOO) with OH species, Cu(DPM), is selectively adsorbed on the O-H sites. The interaction between the adsorbed Cu(DPM), and the surface is proved to be the interaction between the (DPMH)+ ligand and the surface 02-. Adsorbed Cu(DPM), was decomposed by reaction with H,O vapor leaving the copper atoms in a + 1 oxidation state on the SrTiO, surface.
References [l] R. Sekine and M. Kawai, Appl. Phys. Lett. 56 (1990) 1466. [2] R.Sekine, M. Kawai, T. Hikita and T. Hanada, Surf. Sci. 242 (1991) 508. [3] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Solid State Commun. 24 (1977) 623. [4] S. Ferrer and G.A. Somotjai, Surf. Sci. 94 (1980) 41. [5] R.G. Egdell and P.D. Naylor, Chem. Phys. Lett. 91 (1982) 200. [6] A.F. Carley, P.R. Chalker, J.C. Riviere and M.W. Roberts, J. Chem. Sot. Faraday Trans. 83 (1987) 351. [7] M. Murata and K. Wakino, J. Electron. Spectrosc. Relat. Phenom. 6 (1975) 459. [8] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Phys. Rev. B 17 (1978) 4908. [9] W.J. Lo and G.A. Somotjai, Phys. Rev. B 17 (1978) 494. [lo] A.D. Baden, P.A. Cox, R.G. Egdell, A.F. Orchard and R.J.D. Willmer, J. Phys. C (Solid State Phys.) 14 (1981) L1081. [ll]
G. van der Phys. Rev. [12] G. Shoen, 1131 T. Robert 277.
Laan, C. Westra, C. Haas and G.A. Sawatzky, B 23 (1981) 4369. Surf. Sci. 35 (1973) 96. and G. Offergeld, Phys. Status Solidi 14 (1972)
surface science letters
Surface Science Letters
Reaction between copper dipiva~oy~~~thanat~ adsorbed on SrTiO,( 100) Tokihisa
Hikita,
Research Laboratory
Rika Sekine,
Takashi
of Engineer&g Materials,
Cu( DPM) 2 and H,O
Hanada
Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku,
Yokohama
227, &an
and Maki Kawai The Institute of Physical and Chemical Research (HKEN~,
2-1 Hirosawa,
Wako-shi, Saitama
351-01, Japan
Received 17 June 1.991; accepted for publication 30 October 1991
Selective reaction of copper dipivaloylmethanate (Cu(DPM),, P-diketonate complex) with H,O adsorbed on the SrTiO,(lOO) surface is investigated. Adsorption of Ha0 on SrTiOs(100) occurs on Ti3+ sites produced by prior Arf ion bombardment. It is found that the ~-diketonate complex, Cu(DPM),, reacted selectivety with the adsorbed Ha0 at room temperature. Further reaction of the adsorbed Cu(DPM)a with Ha0 vapor results in the removal of the ligand, DPM, leaving elemental copper on the surface.
1. Introduction Selective adsorption or reaction on surfaces helps us to realize the atomically dispersed and controlled deposition of thin films. Selective reactions are also indispensable in designing selfterminating deposition systems. In the latter application the reaction between surface functional groups and organometallic compounds or complexes is expected to give us information about the atomically controlled deposition mechanism itself. Finally surface reaction has a great advantage in that the mount of deposited metals can be controlled by adjusting the amount of surface functional groups. On fine particles of SiO,, a selective and stoichiometric adsorption of P-diketonate compounds, copper dipivaloylmethanates (Cu(DPM),; 2,2,6,6-tetramethyl-3,5-heptadione) with surface hydroxyl group (OH) is reported [1,21. In this reaction, H atoms from the surface OH groups 0039-6028/92/$0500
move into the DPM ligand, and the interaction between the adsorbed ligand and the surface 0 is considered to be the cause of the stoichiometrical adsorption of Cu(DPM), and the surface hydroxyl group [1,2]. The removal of the ligand from the adsorbed CufDPM), is realized by H,O vapor reacting with the adsorbate. A simifar reaction is expected to occur on the surface of the oxide single crystal, where the OH groups also exist or subsequently adsorb. It has been reported that the SrTiO, surface is active for the adsorption of H,O when it has Ti3+ sites on the surface [3-51. The SrTiO~(l~) surface with adsorbed H,O is considered to show a similar reactivity with Cu(DPM), to that of the SiO, surface with OH groups. In this Letter, the reactivity of Cu(DPM), with H,O adsorbed on the SrTiO,(lOO) surface is reported. We also discuss on the decomposition of Cu(~PM)~ adsorbed on the SrTiO,(lOO) surface via the reaction with H,O vapor.
0 1992 - Elsevier Science Publishers B.V. All rights reserved
i? Hikita et al. / Reaction between Cu(DPM),
2. Experimental
and HI0 adsorbed on SrTiO_JlOO) XPS
Ti
The SrTiO,(lOO) single crystal (Earth Jewelry Co., Ltd.) was cleaned by oxidization in 0, of 8 x lo4 Pa at 673 K for 120 min prior to transforming into ultrahigh vacuum (UHV) chamber (VG ADESSOO, base pressure < 1 x lo-’ Pa). In order to obtain a clean and oxygen rich surface, it was reoxidized with 0, of 1 x lop4 Pa at 673 K for 120 min. An Art bombardment was carried out with the acceleration energy of 200 eV for 10 min in order to form Ti”+ sites on the SrTiO,(lOO) surface. H,O and Cu(DPM), (Toso Aquzo Co., Ltd. and TRI Chemical Laboratory Co., Ltd.) were purified by distillation at 194 and 393 K, respectively. In order to avoid a reaction between H,O and Cu(DPM), in the introduction system, H,O and Cu(DPM), were supplied from different leak valves. The reservoir and the introduction system for Cu(DPM), were heated up to 393 K during the deposition. Gaseous H,O and Cu(DPM), were supplied for 20 min with a pressure of 1 X lop4 and 1 X lop5 Pa, respectively. The elemental composition and the chemical states of Sr, Ti, 0, Cu and C were experimentally confirmed by Auger electron, X-ray photoelectron and ultraviolet photoelectron spectroscopies (AES, XPS, and UPS). In the XPS measurement, Mg Ka was used, and in the UPS measurement, He1 was used. In the XPS measurement, the emitted electron was detected in the direction of 70” to the surface normal. The binding energy observed in the XPS was calibrated by taking Ti 2~ 3,2 of Ti4+ on SrTiO,(lOO) as 458.6 eV [6,7]. The electron energy loss spectrum (EELS) was taken using an incident energy of 5 eV, where the resolution was 20 meV in FWHM, and an incident angle of 45” to the surface.
3. Results
and discussion
After the oxidation treatment, the SrTiO,(lOO) surface became clean and oxygen rich, where only Sr2+ Ti4+ and 02- were observed by XPS. The emisiion peaks of Ti2p,,, (458.6 eV) and 2p,,*
UPS 2p
456.6.9 m
I
ik 456.3&l
464.6eV
462.3eu
b
C
EUNDING ENERGY
ev
Fig. 1. The MgKa excited XPS of Ti 2p, and He I excited UPS at the valence band of the SrTiO, surface. (a) The clean and oxygen rich surface; (b) and (d) the oxygen deficient surface after bombarded by Ar + ions and (c) and (e) the surface of (b) with H,O adsorbed.
(464.6 eV) due to Ti4+ are shown in fig. la. After the Ar+ bombardment the decrease in the intensity of the photoelectron peak of 0 1s was observed, whereas those for Sr 3d or Ti 2p remained unchanged. The surface of SrTiO,(lOO) became oxygen deficient by the Art ion bombardment. Due to the desorption of oxygen from the surface, Ti”+ began to appear. The changes in the XPS spectrum of Ti2p are shown in fig. lb. In the emission peak of Ti2p, a shoulder began to appear in the low binding energy region of the peak at 456.3 and 462.3 eV. The observed spectra are fitted using the binding energies of Ti4+, Ti”+ and Ti2+ shown by Carley et al. [6]. It has been found that the observed Ti2p spectrum after the Ar+ ion bombardment consisted of Ti4+ and Ti”+, and a small trace of Ti2+, suggesting that the shoulder peak can be assigned to the Ti”+ state [3-61. In the above reduction condition, Ti on SrTiO,(lOO) can only be reduced from Ti4+ to Ti”+ and hardly Ti 2-c. Together wi th the changes in the binding energy of Ti2p, the emission from Ti3d began to appear near the Fermi edge. The UPS spectrum after Ar+ bombardment is shown in fig. Id. These facts show that by the Art bombardment, surface oxygen defects
T. Hikita et al. / Reaction between Cu(DPM),
I 0
I
100
I
I
a0
30
ENERGY
I
I
400
500
and H,O a d:Forbed on SrTiO,(lOO)
600
LOSS I meV
Fig. 2. EELS of SrTiO,(lOO) surface. Incident electron beam of 5 eV with FWHM of 20 meV. The incident beam was introduced at 45” to the surface normal. (a) Loss spectrum of the Art ion bombarded, clean surface; (b) H,O adsorbed on (a) by H,O pressure of 1 x low4 Pa at 473 K for 20 min; (c) Cu(DPM), was introduced to the H,O adsorbed surface at 1 x lo-’ Pa at 300 K for 20 min.
are created and Ti changed from Ti4+ to Ti3+ in the vicinity of the defects [5,8,9]. When H,O vapor is supplied to the oxygen deficient surface at 473 K, the emission peaks due to the Ti3 ‘- state observed in Ti 2p (XPS) and those of Ti3d (UPS) decreased in intensity (figs. lc and le>. At the same time, the emission peak of 0 1s began to show a shoulder at 532.2 eV in the high binding energy region (fig. 3b). The main emission peak of 0 1s at 530.4 eV is due to oxygen in SrTiO,(lOO). Changes of the EELS during the above treatment are shown in fig. 2. The loss peak of the clean surface shows peaks originated from surface phonons (fig. 2a). EELS of the clean surface of SrTiO, has already been reported previously [lo]. This result is consistent with previous work. The H,O vapor exposed surface showed a loss peak at 450 meV (fig. 2b). This peak originated from the OH stretching mode of adsorbed H,O on the surface [5]. From the above evidence, it is concluded that the peak at high binding energy of 532.2 eV in 0 Is (fig. 3b) is assigned to the oxygen in the adsorbed H,O on surface Ti3+ sites [3-51. The adsorbed H,O thus formed is expected to show a similar reactivity to OH on SiO,, where the selec-
Fig. 3. The XPS of 01s of SrTiO,(lOO). (a) Ar+ ion bombarded surface; (b) H,O adsorbed on (a) by H,O pressure of 1 x 10e4 Pa, at 473 K for 20 min; (c) Cu(DPM), was introduced to the H,O adsorbed surface at 1 X 10m5 Pa at 300 K for 20 min; (d) after the H,O treatment of the Cu(DPM), adsorbed surface at H,O pressure of 1 x 1O-4 Pa at 473 K for 20 min.
01
...
’
ox.
Ar
Born.
HZ0
Ad.
Cu(DPY)2 Ad.
El20 Treat.
Treatment Fig. 4. Changes in the intensity of C 1s and Cu2p,,, peaks normalized to that of Ti2p,,, observed by XPS. The circles (0) indicate C 1s and the triangles ( A ) indicate Cu 2p,,, peak intensities. [Ox.]: The clean and oxygen rich surface of SrTiO,(lOO); [Ar born.]: Ar+ ion bombarded surface; [H,O ad.]: H,O adsorbed on the bombarded surface with H,O pressure of 1 x 1O-4 Pa at 473 K for 20 min; [Cu(DPM)’ ad.]: Cu(DPM), was introduced on the H,O adsorbed surface at 1 X 10W5 Pa at 300 K and for 20 mitt; [Hz0 treat.]: After the H,O treatment of the Cu(DPM), adsorbed surface at 1 x 10m4 Pa at 473 K for 20 min.
T. Hikiia et al. / Reaction between C~(~P~~~
tive adsorption and reaction of Cu(DPM), is reported [1,21. The SrTiO, surface with adsorbed H,O was prepared first. Then Cu(DPM), vapor was supplied at room temperature and the adsorption of Cu(DPM), on the surface was observed by increase in the intensity of Auger electrons in the C KLL and Cu LMM spectra and by increase in the intensity of the loss peak in EELS due to the CH stretching mode on the surface. As shown in fig. 3c, when Cu(DPM), was adsorbed on the surface with adsorbed H,O, the higher binding energy peak of 0 1s assigned to OH species disappeared. At the same time, the energy loss due to the OH stretching mode observed in EELS disappeared and a new loss peak assigned to the CH stretching mode of the ligand of adsorbed Cu(DPM), appeared (fig. 2~). On a clean surface of SrTiO,(lOOI without defects or OH species, the adsorption of Cu(DPM), did not take place. It is clear that Cu(DPM), perfectly adsorbs on adsorbed H,O sites on the SrTiO,(lOO) surface. Decomposition of adsorbed Cu(DPM)~ by reaction with H,O vapor was examined. The changes in the peak intensity for C Is and Cu 2p,,,
I-
I 930
I 940 BINDING
I 950 ENERGY / eV
and Hz0 adsorbed on SrTi0,~100~
observed are shown in fig. 4. When Cu(DPM), was supplied to the surface, the intensities of C 1s and Cu 2p,,, increased because of adsorption of Cu(DPM), on the adsorbed H,O. After H,O treatment the intensity of C 1s decreased, whereas that of Cu 2p,,, did not or only slightly increased. This can be expiained as follows: DPM, the ligand of Cu(DPM),, was removed by reaction with H,O vapor. The small increase in the intensity of Cu2p is due to the decrease in the amount of the DPM Iigand, which absorbed a photoelectron from a copper atom. After treatment with H,O, the shoulder peak in 0 1s assigned to OH species appeared again at high binding energy (fig. 3d). In the case of adsorption of Cu(DPM), on OH on SiOz, the H atom from the surface OH is known to move into the DPM ligand changing the vibrational spectrum of the CH stretching mode for adsorbed Cu(DPM), from that of the Cu(DPM), molecule [l]. If we assume that a similar reaction takes place on the surface of the SrTiO~(lOO), the H atom from the adsorbed H,O is extracted into the ligand of the adsorbate. As H,O was adsorbed on the Ti3+ site to give Ti4’,
I 96
I 325
I I 915 920 KINFTIC ENERGY f eV
I
910
Fig. 5. The XPS of Cu2p and AEZSexcited by X-ray. (a) Adsorbed CU(DPM)~; (b) after the Hz0 treatment; (c) CLImetal foil.
T Hikita et al. / Reaction between Cu(DPM),
adsorbed H,O should have a similar character to OH species. Considering the fact that the oxygen species after releasing H was not observed at the same binding energy as that for OH and that Ti3+ did not appear, it is reasonable to think that oxygen after releasing the H atom is in the same state as bulk oxygen in the 02- state. From the above evidence, it is judged that when Cu(DPM), is adsorbed by OH on the SrTiO,(lOO) surface, H+ from the surface moves into the DPM ligand and 02- is left on the surface. 02- on the surface and (DPMH)+ in the ligand interact and are a cause of a force for the adsorption of Cu(DPM), on the surface. After reaction with H,O vapor, the DPM ligand was removed but the metal copper center still remained on the surface, and the adsorption of H,O was reproduced. The spectra of Cu2p and Cu LMM Auger electrons are shown in fig. 5. The oxidation state of the remained copper observed in the photoelectron spectroscopies was proved to be + 1, because the emission peak for Cu2p in the XPS had no satellite peaks (figs. 5a and 5b), due to Cu2+, and the observed kinetic energy for Cu L,M,,SM,,, (917.9 eV) (figs. 5a and 5b) was lower than that for the Cu metal (919 eV> and its peak shape was different from that of the Cu metal (fig. 5~). These results were similar to those of copper(I)oxi [ll-131. The reproduced OH species after the decomposition of adsorbed Cu(DPM), was used for the further adsorption-decomposition cycle of Cu(DPM),. Then the intensity of Cu2p increased further, however, that for the carbon did not. It was suggested that this adsorption-decomposition occurred on the adsorbed H,O on the SrTiO, surface. In agreement with prior work the reaction observed here is similar to that observed for the SiO, surface [1,2].
and Hz0 adsorbed on SrTiOJlOO)
4. Conclusion The OH species is produced on the SrTiO, (100) surface by reaction of H,O with the Ti3+ defects prepared by Ar+ ion bombardment. On the surface of SrTiO,(lOO) with OH species, Cu(DPM), is selectively adsorbed on the O-H sites. The interaction between the adsorbed Cu(DPM), and the surface is proved to be the interaction between the (DPMH)+ ligand and the surface 02-. Adsorbed Cu(DPM), was decomposed by reaction with H,O vapor leaving the copper atoms in a + 1 oxidation state on the SrTiO, surface.
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