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The reaction of copper and calcium dipivaloylmethanates (Cu(DPM)2 and Ca(DPM)2) with hydroxyls on oxide surface
508
Surface
The reaction (Cu(
DPW
2
Rika Sekine,
of copper and calcium dipivaloylmethanates and Ca( DPM) J with hydroxyls on oxide surface
Maki Kawai, Tokihisa
Research L.uhorutory
Sctence 242 ( 1YY1) 50X-5 12 North-Holland
of Engineering
Materrals,
Hikita
and Takashi
Tokyo Institute
of Technokop.
Hanada 4259 Nagatsutu.
Mldorr-ku,
Yokohama.
,?.?7. .Jqm~
and Institute
Received
of Ph.ysicaf and Chemicul
16 May 1990; accepted
Research.
Wako-shi,
for publication
Suitarm
351.01.
Jupm
29 June 1990
Selective and stoichiometric reactions between surface hydroxyl groups (OH) on SiO, and calcium dipivaloylmethanatc (Ca(DPM),) were investigated by infrared (IR) and photoelectron (XPS) spectroscopies. The stoichiometric ratio of initial surface OH and adsorbed Ca(DPM)2 is estimated from IR absorbance to be CG. (2-3): 1. Introduction of water vapor at 673 K to this surface results in the removal of ligand DPM from the adsorbed Ca(DPM),, leaving the Ca on the surface. The reactive property of CafDPM), is identical to that of Cu(DPM),. previously reported by us. A similar reaction on the surface of single crystal SrTiO,(lOO) with the CU(DPM)~ is also carried out.
1. Introduction
The reaction between surface functional groups and organometallics or complexes is useful for depositing metals on oxide surfaces. One of the advantages of this method is that the amount of deposited metals can be restricted by the amount of the surface functional groups controlled in advance [I]. The combination of surface hydroxyl (OH) groups and dipivaloylmethanate (DPM, i.e., 2,2,6, 6-tetramethyl-3,%heptadione, see fig. la) complex is one of the most promising systems because the OH group is known to exist on various oxides and
M (CH3’3C\&c(cH& *; - ‘)* ‘G: 2*
various kinds of metals are known to give complexes coordinated by DPM. Recently, we have discovered that Cu(DPM), adsorbed stoichiometrically on surface OH on 50, and by the reaction with HzO, the adsorbed Cu(DPM), decomposes to Cu oxide (the valence state of Cu after the reaction is estimated to be + 1 by X-ray photoelectron spectroscopy [XPSjf and figand DPM [2]. In the previous work we proposed that any kind of P-diketonate complex and OH groups on various oxides could exhibit similar reactivity. In this study. we have examined the reaction of Ca(DPM), with OH on SiO,, and found that this system shows a reactivity similar
‘Ct$$C.C~C-acH& a
6
..,_ (a) of (a) Ca(DPM),.
0039~6028/91/$03.50
0 1991 - Elsevier Science Publishers
0
S’i A
4 m (Cl
(b)
Fig. I. The structures
0
(b) the liquid DPM, and (c) the adsorption model of Ca(DPM)Z between Si-0 and Cu(DPM), is ambiguous.
B.V. (North-Holland)
and Yamada
on SiO,. The bonding
Science Foundation
state
R. Sekine et al. / Reacrion of Cu(DPM),
and Ca(DPM),
509
with OH
to that of Cu(DPM),. Not only the surface OH but also the adsorbed H,O on the oxide surface is considered to show reactivity similar to DPM complex. Here we also examined the reaction between adsorbed water on the SrTiO,(lOO) surface and Cu(DPM),.
2. Experimental The SiO, sample was prepared by spraying a suspension of SiO, (Cab-0-Sil, HS-5) in ethanol onto the surface of NaCl crystal. Details of the preparation have been described elsewhere [3]. The SiO, layers thus prepared were mounted in a glass infrared (IR) reaction cell and a pretreatment was carried out at 673 K under 1 X lo3 Pa of oxygen for 4 h, followed by evacuation at room temperature. After this pretreatment, the seal of the glass ample containing Cu(DPM), or Ca (DPM),, purified in advance, was broken in the reaction cell and the DPM complexes were introduced onto the SiO, surface by heating the P-diketonates to 353 K [CufDPM),] or to 483 K [Ca(DPM)2]. Ca(DPM), and Cu(DPM), (Toso Akuzo Co. or Triche~cal Lab.) were purified in advance as follows: the rare complexes were evacuated by a conventional diffusion pump [base or pressure is - 10m3 Pa] at 300 K [Cu(DPM),] 393 K [Ca(DPM),] for - 24 h to remove the unreacted ligand DPM and the solvents, H,O and alcohol, then enclosed in glass ample with a breakable seal in vacua. The amount of surface OH on SiO, and that of adsorbed Cu(DPM), or Ca(DPM), were determined by infrared transmission spectroscopy (NICOLET FT-IR 510), together with the structure of adsorbed species. The electronic states of Cu and Ca on SiO, were investigated by XPS (JEOL JPS-80). The binding energy of the emission peaks in XPS were calibrated by assuming the Si 2p in SiO, to be 103.4 eV and C Is in CH to be 285.1 eV, respectively [4]. The SrTiO,(lOO) single crystal (Earth Jewelry Co., Ltd.) was pretreated in 5 x lo5 Pa of 0, gas at 673 K in a conventional closed glass circulation system, then after introducing the sample into the UHV chamber (VG ADES, base pressure < 1x
al Wavenumber/cm-1 Fig. 2. Infrared spectra for the Ca(DPM),/SiO, system in the OH stretching region. (a) SiO, after the pretreatment. (b) Ca(DPM), was introduced on the SiO, surface at room temperature for 20 h, evacuating at the same time. (c) The surface of(b) was treated with 2 x lo3 Pa of H,O at 673 K for 20 min.
1O-7 Pa), it was oxidized at 623 K with 1 X 10m4 Pa of oxygen. The angle dependence of the emitted electrons in XPS was studied. Changes in the intensity ratio of Ti2p to Sr3d indicate that the SrTiO,(lOO) surface thus treated has mainly the TiO, cleaved surface [5]. The observed binding energy for Ti 3d (- 1 eV) exhibits that Ti3+ did not exist on the surface after above-mentioned treatment.
3. Results and discussions In our previous paper [2], we reported that Cu(DPM), reacts with OH groups on the SiO, surface stoichiometrically. In order to clarify the general reactivity of DPM complex with surface OH, the reactivity of Ca(DPM), is examined. Figs. 2 and 3 show the IR spectra of the OH stretching region and that of the CH stretching region, respectively, during the reaction of
510
Wavenumber/cm-’ Fig. 3. Infrared spectra of the CH stretching region for the system Ca(DPM), adsorbed on SiO?. (b) Ca(DPM),was introduced onto the SiO, surface at room temperature for 20 h. evacuating at the same time. (c) Surface (b) treated with H,O (2 X lo3 Pa) at 673 K for 20 min.
Ca(DPM), and surface OH. After the pretreatment of SiO, in 1 x lo3 Pa of 0, at 673 K, only the isolated OH group is left on the SiO, surface. which shows a sharp absorption band at 3750 cm- ’ (fig. 2a). This OH peak disappeared after the deposition of Ca(DPM), onto the surface (fig. 2b). accompanied by the appearance of the typical absorption bands of CH, and CH, due to adsorbed Ca(DPM) (fig. 3b). The absorption bands observed are quite similar to those observed for the Cu(DPM),/SiO, system [2]. The absorption bands at 2963, 2932, 2911, and 2870 cm-’ are assigned to an antisymmetric stretching vibration of CH, (v.,,(CH3)). that of CH2 (v,,,(CHZ)), a combination of stretching vibrations of CH, and an overlapping of symmetric (v(CH,)). stretching vibrations of CH, (v,(CH~)) and CH, ( v~(CH~)), respectively. The absorption bands of the CH stretching region and the OH regions indicate that upon adsorption of Ca(DPM),, H atoms from surface OH moved into the DPM ligand, resulting in changes in the absorption bands in the CH stretching region. On the basis of the ionizing tendency of the metal, the bonding character of Cu . O=C and
that of Ca . . . O=C can be predicted to be different. In spite of the differing coordinated metal. the structure around the metal seems to be similar for Ca( DPM), and CU(DPM)~ in the adsorbed state on SiO,. Figs. 4a and 4b show the IR spectra in the C=O and C-C stretching regions for Ca(DPM), on SiO, and Cu(DPM), on SiO,, respectively. The spectra in this region are too complicated to be fully assigned but they show valuable information on the bonding state around the metal. [6] Infrared spectra of adsorbed Ca(DPM), or Cu(DPM), are similar to those of free Ca (DPM), or Cu(DPM),. The infrared spectrum of Cu(DPM), on NaCl, i.e., isolated Cu(DPM), is shown in fig. 4c. That of Ca(DPM), on NaCl is similar to this, though it is not displayed here. In contrast to the DPM complex, the liquid DPMH gives a spectrum with a hydrogen-bond structure around 1600 cm- ’ shown in fig. 4d. The similarity between the IR spectra of adsorbed DPM complex and that of the isolated complex suggests that
I
I
1700
1600
1500
1400
Wavenumber I cm’ Fig. 4. Infrared spectra of C-C and C‘=O stretching rrgwns for (a) C‘a(DPM), adsorbed on SiO,. (b) Cu(DPM), ;ldsorhcxj on SiO,. (c) C’u(DPM), adsorbed on NaC‘I. and Cd) the liquid DPM.
the structure around the metaf coordinated by the ligand DPM is not changed after adsorption. Even though the nature of the coordinated metals, i.e., Ca and Cu, is quite different, the nature of the adsorption on OH is similar. As a result, the adsorption of DPM complex on surface hydroxyl groups is not affected by the atomic properties of the centered metal. This fact suggests that wide variety of DPM complexes may exhibit similar reactivity with surface hydroxyls. Taking above evidence into account, an adsorption model for CafDPM), (which is the same as in the case for Cu(DPM)~) is described as shown in fig. lc. This model satisfies the fact that (1) H has moved From the surface OH to the adsorbed CafDPM),, (This is because the stretching modes of both surface OH and the CH group of the Ca(DPM), disappeared together, while that of CH, in Ca(DPM), increased.) (2) Ca(DPM), was not decomposed but only strongly adsorbed on the SiO, surface holding its fourfold-coordinated oxygen structure around Ca. (This is supported by the IR band structure around 1600 ctW’.) The stoichiometric ratio of surface OH and adsorbed Ca(DPM)~ was estimated from the intensity of IR absorbance of OH and CH. Fig. 5 shows that the decrease in the amount of surface OH on SiO, and the increase in the amount of CH, groups of adsorbed CafDPM), are propor-
tionat, suggesting that they react stoichiometritally. The broken line indicates the reaction ratio between Cu(DPM), and the OH; the lines differ 80%. The reaction stoichiometry between by the initial surface OH and SiO, and adsorbed Ca(DPM), or Cu(DPM), is estimated to be (2-3) : 1. Here, the absorption coefficient for OH and CH are independently obtained [2]. A study of the reaction between water vapor and Ca(DPM), adsorbed on SiOz was also carried out. Figs. Zc and 3c show the IR spectra of the adsorbed species after treatment with H&I vapor at 673 IS for 20 min. The absorption bands due to CH stretching almost disappeared and those for OH were reproduced. The amount of reproduced OH was almost the same as the initial amount. After decomposition of the adsorbate, the emission peaks m XPS due to Ca2p,,, and Ca 2~,,~ were observed at 347.8 and 351.2 eV, respectively, showing that the Ca is in a divalent state. Therefore, by the reaction with water vapor only the DPM ligand was removed from the adsorbate leaving Ca on the surface of SQ. This reactivity is identical to the case of Cu(DPM),. The reaction between adsorbed H,O on a SrTiO~(lO0) surface and Cu(DPM)~ was studied. After sputtering of the clean surface of SrTi~~(lQO) by Ar+ in the UHV chamber, H,O is known to adsorb on the surface defects of Ti3* (7,X]. Onto this surface, Cu(DPM), was introduced. Changes in the intensity of the C Is emission peak from the surface of SrTiO,(lOO) is shown in fig. 6. After deposition of Cu(DPM), on the surface, a clear increase in the Cls intensity is observed. This is due to the adsorption of Cu(DPM), on this surface. The reaction between water vapor (H,O) and this surface results in a decrease in the Cls intensity. showing that the elimination of ligand DPM is realized on the single crystal surface, as in the case of the SiO, surface.
4. Canclusions OH Absorbance Fig. 5. Relation between the absorbance of v,,(CH,) (2963 cm‘ ’ f from adsorbed Ca(DPMf, and that of v(OH) on SQ. The broken fine indicates a similar relation observed for adsorbed Cu(DPM)~ and OH on SK&.
(1) Using selective reaction, a controlled amount of Ca(DPM), can be attached on the SiO, surface proportional to the amount of surface OH, identical to in the case of Cu(DPM),.
R. Sekine et al. / Reaction of Cu(DPM),
512
md Cu(DPM),
with ON
(5) The adsorption and decomposition of the DPM complex is also realized on the SrTiQ(100) surface.
0
02
ArBom.
EmAd.
Treatment Fig. 6. Normalized intensity of the C Is emission peak to Ti 2p as observed by X-ray photoelectron spectroscopy. [Ox.]: SrTiO~(l~) surface was oxidized in the UHV chamber. [Ar Born.]: [Ox.] was sputtered by Ar+. [H20 Ad.]: Water was adsorbed on [Ar Born.]. [Cu(DPM), Ad.]: Cu(DPM), was adsorbed on [Hz0 Ad.]. [Hz0 Treat.]: [Cu(DPM), Ad.] was treated with water vapor. The solid line and the dotted line correspond to emission measured in the normal direction and inclined by 70’ with respect to the normal to the surface, respectively.
(2) The stoichiometry of the adsorption is similar in Ca(DPM), and Cu(DPM),. The ratio between initial OH on the surface and Ca(DPM), or Cu(DPM)~ is (2-3) : 1. (3) Ca(DPM), is decomposed by the reaction with water leaving the Ca atoms on the SiO, substrate. This reactivity is similar to Cu(DPM),. (4) A similar reactivity is observed for quite different metal DPM complexes (i.e., Ca(DPM), and Cu(DPM),,) suggesting that this reactivity of DPM complexes with surface hydroxyl groups can be seen in a variety of metal-DPM complexes.
A part of this works was supported by a Grantin-Aid for Scientific Research on Chemistry of New Superconductors, from the Ministry of Education, Science and Culture of Japan. The authors are grateful to Mr. Gonda and Professor Koinuma of the Tokyo Institute of Technology for the XPS measurements.
References 111Y. Iwasawa, Adv. Catal. 35 (1987) 187. I21 R. Sekine and M. Kawai, Appl. Phys. Lett. 56 (1990) 1466. (31 M. Kawai, Y. Tsuboi, K. Tanaka, S. Teratani and K. Taya. Surf. Sci. 207 (1989) 354. [41 C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. ~uilenberg, Handbook of X-ray Photoeiectron Spectroscopy (Perkin-Elmer. Eden Prairie. MN, 197X). [51 V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Phys. Rev. B 17 (1978) 4908. and T. Shimanouchi. SpectroI61 M. Mikami, 1. Nakagawa chim. Acta 23A (1967) 1037. [71 R.G. &dell and P.D. Naylor. Chem. Phyo. Lett. 91 (1982) 200. 181 P.A. Cox, R.G. Egdell and P.D. Naylor. J. Electron Spectrosc. Relat. Phenom. 29 (1983) 247.
Surface
The reaction (Cu(
DPW
2
Rika Sekine,
of copper and calcium dipivaloylmethanates and Ca( DPM) J with hydroxyls on oxide surface
Maki Kawai, Tokihisa
Research L.uhorutory
Sctence 242 ( 1YY1) 50X-5 12 North-Holland
of Engineering
Materrals,
Hikita
and Takashi
Tokyo Institute
of Technokop.
Hanada 4259 Nagatsutu.
Mldorr-ku,
Yokohama.
,?.?7. .Jqm~
and Institute
Received
of Ph.ysicaf and Chemicul
16 May 1990; accepted
Research.
Wako-shi,
for publication
Suitarm
351.01.
Jupm
29 June 1990
Selective and stoichiometric reactions between surface hydroxyl groups (OH) on SiO, and calcium dipivaloylmethanatc (Ca(DPM),) were investigated by infrared (IR) and photoelectron (XPS) spectroscopies. The stoichiometric ratio of initial surface OH and adsorbed Ca(DPM)2 is estimated from IR absorbance to be CG. (2-3): 1. Introduction of water vapor at 673 K to this surface results in the removal of ligand DPM from the adsorbed Ca(DPM),, leaving the Ca on the surface. The reactive property of CafDPM), is identical to that of Cu(DPM),. previously reported by us. A similar reaction on the surface of single crystal SrTiO,(lOO) with the CU(DPM)~ is also carried out.
1. Introduction
The reaction between surface functional groups and organometallics or complexes is useful for depositing metals on oxide surfaces. One of the advantages of this method is that the amount of deposited metals can be restricted by the amount of the surface functional groups controlled in advance [I]. The combination of surface hydroxyl (OH) groups and dipivaloylmethanate (DPM, i.e., 2,2,6, 6-tetramethyl-3,%heptadione, see fig. la) complex is one of the most promising systems because the OH group is known to exist on various oxides and
M (CH3’3C\&c(cH& *; - ‘)* ‘G: 2*
various kinds of metals are known to give complexes coordinated by DPM. Recently, we have discovered that Cu(DPM), adsorbed stoichiometrically on surface OH on 50, and by the reaction with HzO, the adsorbed Cu(DPM), decomposes to Cu oxide (the valence state of Cu after the reaction is estimated to be + 1 by X-ray photoelectron spectroscopy [XPSjf and figand DPM [2]. In the previous work we proposed that any kind of P-diketonate complex and OH groups on various oxides could exhibit similar reactivity. In this study. we have examined the reaction of Ca(DPM), with OH on SiO,, and found that this system shows a reactivity similar
‘Ct$$C.C~C-acH& a
6
..,_ (a) of (a) Ca(DPM),.
0039~6028/91/$03.50
0 1991 - Elsevier Science Publishers
0
S’i A
4 m (Cl
(b)
Fig. I. The structures
0
(b) the liquid DPM, and (c) the adsorption model of Ca(DPM)Z between Si-0 and Cu(DPM), is ambiguous.
B.V. (North-Holland)
and Yamada
on SiO,. The bonding
Science Foundation
state
R. Sekine et al. / Reacrion of Cu(DPM),
and Ca(DPM),
509
with OH
to that of Cu(DPM),. Not only the surface OH but also the adsorbed H,O on the oxide surface is considered to show reactivity similar to DPM complex. Here we also examined the reaction between adsorbed water on the SrTiO,(lOO) surface and Cu(DPM),.
2. Experimental The SiO, sample was prepared by spraying a suspension of SiO, (Cab-0-Sil, HS-5) in ethanol onto the surface of NaCl crystal. Details of the preparation have been described elsewhere [3]. The SiO, layers thus prepared were mounted in a glass infrared (IR) reaction cell and a pretreatment was carried out at 673 K under 1 X lo3 Pa of oxygen for 4 h, followed by evacuation at room temperature. After this pretreatment, the seal of the glass ample containing Cu(DPM), or Ca (DPM),, purified in advance, was broken in the reaction cell and the DPM complexes were introduced onto the SiO, surface by heating the P-diketonates to 353 K [CufDPM),] or to 483 K [Ca(DPM)2]. Ca(DPM), and Cu(DPM), (Toso Akuzo Co. or Triche~cal Lab.) were purified in advance as follows: the rare complexes were evacuated by a conventional diffusion pump [base or pressure is - 10m3 Pa] at 300 K [Cu(DPM),] 393 K [Ca(DPM),] for - 24 h to remove the unreacted ligand DPM and the solvents, H,O and alcohol, then enclosed in glass ample with a breakable seal in vacua. The amount of surface OH on SiO, and that of adsorbed Cu(DPM), or Ca(DPM), were determined by infrared transmission spectroscopy (NICOLET FT-IR 510), together with the structure of adsorbed species. The electronic states of Cu and Ca on SiO, were investigated by XPS (JEOL JPS-80). The binding energy of the emission peaks in XPS were calibrated by assuming the Si 2p in SiO, to be 103.4 eV and C Is in CH to be 285.1 eV, respectively [4]. The SrTiO,(lOO) single crystal (Earth Jewelry Co., Ltd.) was pretreated in 5 x lo5 Pa of 0, gas at 673 K in a conventional closed glass circulation system, then after introducing the sample into the UHV chamber (VG ADES, base pressure < 1x
al Wavenumber/cm-1 Fig. 2. Infrared spectra for the Ca(DPM),/SiO, system in the OH stretching region. (a) SiO, after the pretreatment. (b) Ca(DPM), was introduced on the SiO, surface at room temperature for 20 h, evacuating at the same time. (c) The surface of(b) was treated with 2 x lo3 Pa of H,O at 673 K for 20 min.
1O-7 Pa), it was oxidized at 623 K with 1 X 10m4 Pa of oxygen. The angle dependence of the emitted electrons in XPS was studied. Changes in the intensity ratio of Ti2p to Sr3d indicate that the SrTiO,(lOO) surface thus treated has mainly the TiO, cleaved surface [5]. The observed binding energy for Ti 3d (- 1 eV) exhibits that Ti3+ did not exist on the surface after above-mentioned treatment.
3. Results and discussions In our previous paper [2], we reported that Cu(DPM), reacts with OH groups on the SiO, surface stoichiometrically. In order to clarify the general reactivity of DPM complex with surface OH, the reactivity of Ca(DPM), is examined. Figs. 2 and 3 show the IR spectra of the OH stretching region and that of the CH stretching region, respectively, during the reaction of
510
Wavenumber/cm-’ Fig. 3. Infrared spectra of the CH stretching region for the system Ca(DPM), adsorbed on SiO?. (b) Ca(DPM),was introduced onto the SiO, surface at room temperature for 20 h. evacuating at the same time. (c) Surface (b) treated with H,O (2 X lo3 Pa) at 673 K for 20 min.
Ca(DPM), and surface OH. After the pretreatment of SiO, in 1 x lo3 Pa of 0, at 673 K, only the isolated OH group is left on the SiO, surface. which shows a sharp absorption band at 3750 cm- ’ (fig. 2a). This OH peak disappeared after the deposition of Ca(DPM), onto the surface (fig. 2b). accompanied by the appearance of the typical absorption bands of CH, and CH, due to adsorbed Ca(DPM) (fig. 3b). The absorption bands observed are quite similar to those observed for the Cu(DPM),/SiO, system [2]. The absorption bands at 2963, 2932, 2911, and 2870 cm-’ are assigned to an antisymmetric stretching vibration of CH, (v.,,(CH3)). that of CH2 (v,,,(CHZ)), a combination of stretching vibrations of CH, and an overlapping of symmetric (v(CH,)). stretching vibrations of CH, (v,(CH~)) and CH, ( v~(CH~)), respectively. The absorption bands of the CH stretching region and the OH regions indicate that upon adsorption of Ca(DPM),, H atoms from surface OH moved into the DPM ligand, resulting in changes in the absorption bands in the CH stretching region. On the basis of the ionizing tendency of the metal, the bonding character of Cu . O=C and
that of Ca . . . O=C can be predicted to be different. In spite of the differing coordinated metal. the structure around the metal seems to be similar for Ca( DPM), and CU(DPM)~ in the adsorbed state on SiO,. Figs. 4a and 4b show the IR spectra in the C=O and C-C stretching regions for Ca(DPM), on SiO, and Cu(DPM), on SiO,, respectively. The spectra in this region are too complicated to be fully assigned but they show valuable information on the bonding state around the metal. [6] Infrared spectra of adsorbed Ca(DPM), or Cu(DPM), are similar to those of free Ca (DPM), or Cu(DPM),. The infrared spectrum of Cu(DPM), on NaCl, i.e., isolated Cu(DPM), is shown in fig. 4c. That of Ca(DPM), on NaCl is similar to this, though it is not displayed here. In contrast to the DPM complex, the liquid DPMH gives a spectrum with a hydrogen-bond structure around 1600 cm- ’ shown in fig. 4d. The similarity between the IR spectra of adsorbed DPM complex and that of the isolated complex suggests that
I
I
1700
1600
1500
1400
Wavenumber I cm’ Fig. 4. Infrared spectra of C-C and C‘=O stretching rrgwns for (a) C‘a(DPM), adsorbed on SiO,. (b) Cu(DPM), ;ldsorhcxj on SiO,. (c) C’u(DPM), adsorbed on NaC‘I. and Cd) the liquid DPM.
the structure around the metaf coordinated by the ligand DPM is not changed after adsorption. Even though the nature of the coordinated metals, i.e., Ca and Cu, is quite different, the nature of the adsorption on OH is similar. As a result, the adsorption of DPM complex on surface hydroxyl groups is not affected by the atomic properties of the centered metal. This fact suggests that wide variety of DPM complexes may exhibit similar reactivity with surface hydroxyls. Taking above evidence into account, an adsorption model for CafDPM), (which is the same as in the case for Cu(DPM)~) is described as shown in fig. lc. This model satisfies the fact that (1) H has moved From the surface OH to the adsorbed CafDPM),, (This is because the stretching modes of both surface OH and the CH group of the Ca(DPM), disappeared together, while that of CH, in Ca(DPM), increased.) (2) Ca(DPM), was not decomposed but only strongly adsorbed on the SiO, surface holding its fourfold-coordinated oxygen structure around Ca. (This is supported by the IR band structure around 1600 ctW’.) The stoichiometric ratio of surface OH and adsorbed Ca(DPM)~ was estimated from the intensity of IR absorbance of OH and CH. Fig. 5 shows that the decrease in the amount of surface OH on SiO, and the increase in the amount of CH, groups of adsorbed CafDPM), are propor-
tionat, suggesting that they react stoichiometritally. The broken line indicates the reaction ratio between Cu(DPM), and the OH; the lines differ 80%. The reaction stoichiometry between by the initial surface OH and SiO, and adsorbed Ca(DPM), or Cu(DPM), is estimated to be (2-3) : 1. Here, the absorption coefficient for OH and CH are independently obtained [2]. A study of the reaction between water vapor and Ca(DPM), adsorbed on SiOz was also carried out. Figs. Zc and 3c show the IR spectra of the adsorbed species after treatment with H&I vapor at 673 IS for 20 min. The absorption bands due to CH stretching almost disappeared and those for OH were reproduced. The amount of reproduced OH was almost the same as the initial amount. After decomposition of the adsorbate, the emission peaks m XPS due to Ca2p,,, and Ca 2~,,~ were observed at 347.8 and 351.2 eV, respectively, showing that the Ca is in a divalent state. Therefore, by the reaction with water vapor only the DPM ligand was removed from the adsorbate leaving Ca on the surface of SQ. This reactivity is identical to the case of Cu(DPM),. The reaction between adsorbed H,O on a SrTiO~(lO0) surface and Cu(DPM)~ was studied. After sputtering of the clean surface of SrTi~~(lQO) by Ar+ in the UHV chamber, H,O is known to adsorb on the surface defects of Ti3* (7,X]. Onto this surface, Cu(DPM), was introduced. Changes in the intensity of the C Is emission peak from the surface of SrTiO,(lOO) is shown in fig. 6. After deposition of Cu(DPM), on the surface, a clear increase in the Cls intensity is observed. This is due to the adsorption of Cu(DPM), on this surface. The reaction between water vapor (H,O) and this surface results in a decrease in the Cls intensity. showing that the elimination of ligand DPM is realized on the single crystal surface, as in the case of the SiO, surface.
4. Canclusions OH Absorbance Fig. 5. Relation between the absorbance of v,,(CH,) (2963 cm‘ ’ f from adsorbed Ca(DPMf, and that of v(OH) on SQ. The broken fine indicates a similar relation observed for adsorbed Cu(DPM)~ and OH on SK&.
(1) Using selective reaction, a controlled amount of Ca(DPM), can be attached on the SiO, surface proportional to the amount of surface OH, identical to in the case of Cu(DPM),.
R. Sekine et al. / Reaction of Cu(DPM),
512
md Cu(DPM),
with ON
(5) The adsorption and decomposition of the DPM complex is also realized on the SrTiQ(100) surface.
0
02
ArBom.
EmAd.
Treatment Fig. 6. Normalized intensity of the C Is emission peak to Ti 2p as observed by X-ray photoelectron spectroscopy. [Ox.]: SrTiO~(l~) surface was oxidized in the UHV chamber. [Ar Born.]: [Ox.] was sputtered by Ar+. [H20 Ad.]: Water was adsorbed on [Ar Born.]. [Cu(DPM), Ad.]: Cu(DPM), was adsorbed on [Hz0 Ad.]. [Hz0 Treat.]: [Cu(DPM), Ad.] was treated with water vapor. The solid line and the dotted line correspond to emission measured in the normal direction and inclined by 70’ with respect to the normal to the surface, respectively.
(2) The stoichiometry of the adsorption is similar in Ca(DPM), and Cu(DPM),. The ratio between initial OH on the surface and Ca(DPM), or Cu(DPM)~ is (2-3) : 1. (3) Ca(DPM), is decomposed by the reaction with water leaving the Ca atoms on the SiO, substrate. This reactivity is similar to Cu(DPM),. (4) A similar reactivity is observed for quite different metal DPM complexes (i.e., Ca(DPM), and Cu(DPM),,) suggesting that this reactivity of DPM complexes with surface hydroxyl groups can be seen in a variety of metal-DPM complexes.
A part of this works was supported by a Grantin-Aid for Scientific Research on Chemistry of New Superconductors, from the Ministry of Education, Science and Culture of Japan. The authors are grateful to Mr. Gonda and Professor Koinuma of the Tokyo Institute of Technology for the XPS measurements.
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