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Analytical and chemical techniques in the study of surface species in atomic layer epitaxy
280
Thin Solid Films, 225 (1993) 280 283
Analytical and chemical techniques in the study of surface species in atomic layer epitaxy S. Haukka Microchemistry Ltd., P.O. Box 45, 02151 Espoo, and University of Helsinki, Department of Chemistry, Analytical Chemistry Division, Vuorikatu 20, 00100 Helsinki (Finland)
E.-L. Lakomaa and T. Suntola Microchemistry Ltd., P.O. Box 45, 02151 Espoo (Finland)
Abstract An extension of atomic layer epitaxy (ALE) to a porous, high-surface-area substrate commonly used in catalysis is presented. Because of the high surface area, even a sublayer of species bound to the substrate in an ALE sequence can be determined quantitatively. Thus, various analytical and chemical techniques, in addition to the high vacuum' techniques, can be applied in the study of surface reactions and surface species in ALE after a single reaction step. Use of Fourier transform IR spectroscopy, nuclear magnetic resonance, X-ray diffraction, chemical analysis and etching experiments in the characterization of different titanium species on porous silica processed using TiCI4 and H20 is presented.
I. Introduction Atomic layer epitaxy (ALE) has been used for the growth of various single-crystal and polycrystalline materials, mainly on small-area substrates. In our previous paper [1] A L E was used to grow TiO2 on a porous, high-surface-area silica substrate. The first four reaction cycles of TiCI4 and H 2 0 on silica were studied, but the more thorough study of the reaction mechanisms called for the development and application of different analysis methods [2]. Earlier only reaction temperatures of 175 and 450 °C were used. The objective of this study was to find out how A L E can be used to bind titanium species to porous SiO2 using TIC14 and H 2 0 vapors as reactants, and to introduce wet-chemical methods together with Fourier transform I R spectroscopy ( F T I R ) , nuclear magnetic resonance ( N M R ) and X-ray diffraction ( X R D ) to study the reaction mechanisms involved. The use of these various analysis methods in the study of a single A L E sequence in the temperature range 175 550 °C with silica preheated at 560 °C is dealt with in the following.
pressure and for 3 h in nitrogen flow at 6 - 1 0 k P a before the reaction. TiC14 (Merck) without further purification, vaporized at 25 °C, and deionized water vaporized at 25 "C were used as reactants. 2.2. Equipment
A modified MC 120 reactor (Microchemistry Ltd.) with a reaction chamber made of quartz was used in the experiments (Fig. 1). The reaction chamber can hold up to 10 g of silica powder. The reactions were carried out at a pressure of 6 - 1 0 kPa in nitrogen atmosphere. The sampling was made in an inert glove box in nitrogen atmosphere (MBraun). 2.3. A L E procedure
Silica preheated at 560 °C (5 8 g) was stabilized to the reaction temperature. A pulse of TIC14 vapor was fed into the reaction chamber through a solid silica bed supported on a sinter. Pumping of excess reactant and the HC1 released during the reactions took place from the bottom of the silica bed (Fig. 1). Reaction temperatures of 175, 250, 350, 450 and 550°C were used. Reaction times were 1 - 2 h followed by a nitrogen purge of same length at the reaction temperature concerned.
2. Experimental details 2.4. Analysis' methods" 2. I, Reagents SiO 2 (EP 10, Crosfield Ltd.) with a surface area of
3 0 0 m 2 g l, pore volume of 1.75cm 3 g - l and mean particle size of 100 ~tm was used as the substrate. SiO 2 was preheated for 16 h at 560 °C in air at atmospheric
0040-6090/93/$6.00
The total titanium concentration was determined by UV-visible spectrophotometry or neutron activation analysis as described in ref. 3. Etching tests in connection with element determinations were developed for distinguishing titanium species [2, 3]. Amorphous tita-
c~, 1993
ElsevierSequoia. All rights reserved
S. Haukka et al. / Surface species in A L E
281
SURFACE SPECIES (TiCI 4 175 and 250°C) H. /H Oi. "'OI. OH OH I, \ ( OH ^./O\ 7St/~TSi/~t S l ~ S1~,/77 bi//,7Si77 + TiCI 4 + HCI l Fig. 1. A L E equipment and sampling for surface saturation studies. Reactants are fed through the silica bed in a flow of nitrogen. Reaction products and excess reactants are pumped from the bottom of the bed. Samples are taken from the surface (1) and the bottom (2): A is the vessel for TiCI 4 and B is the reaction chamber.
cl\cl/cl
c L ,cl
Ti Ti 0I OH 0I 0I /0\ . I /77TSi//,// Si//// Si,-777Si,~TzSiT7~-SI/~7+ H20
nium species were etched with 3.5 M H 2 S O 4 and the amount of titanium determined in the solutions by UV-visible spectrophotometry. Samples for determination of chloride content were weighed immediately in 3 M H2 SO4 after the sample was removed from the reaction chamber so that the air moisture could not release chloride as HC1. Potentiometric titration was used to quantify the chloride. The type of bonding sites and the sites remaining after the ALE reaction on silica were analyzed with an F T I R spectrometer (Galaxy Series 6020) installed in a glove box, into which the samples were transferred inertly. The samples were loaded on the sample holder, and the spectra were recorded using a spectral resolution of 2 c m -]. The accumulation time was 4.5min, corresponding to 1000 scans. X R D spectra were measured with a Siemens 500 diffractometer with Cu K~ excitation. The 1H- and 29Si-NMR measurement procedure was as described in ref. 2.
3. Results and discussion
3. I. Surface species of silica Porous, high-surface-area silica is a complicated substrate, which consists of different reactive sites: isolated (including single and geminal) and hydrogen-bonded O H groups and siloxane bridges [2, 4]. A simplified picture of these reactive sites is shown in Fig. 2. The groups serving as bonding sites for a reagent depend on the chemical characteristics of the reactant vapor, the chemical state of the substrate and the reaction temperature. Characterization of the support and its bonding sites is vital when studying the reaction mechanisms in ALE. In this work silica preheated at 560 °C was used. This silica contains 1.6 isolated and 0.5 hydrogenbonded O H groups per square nanometer, measured by ~H N M R and reported in ref. 2. The hydrogen-bonded O H groups on 560 °C silica are inaccessible (bulk OH groups) and unreactive towards any reactant.
OH ,OiH/.CI
OH\Ti/OH
+ HCIT
6I OH 0 / \ O OR OH L I //H S i / , / / , , , Si,~TT Si,~7- Si/,,H SiT/TTSi77 Fig. 2. Bonding sites of silica and titanium surface species at reaction temperatures of 175 and 250 °C, when first TiCI 4 and then H 2 0 is brought through the silica bed.
3.2. Surface saturation Quantitative determination of titanium and chloride bound from the single reaction step of TiC14 is not reliable using instrumental surface analysis techniques, but the amount of titanium can be determined accurately by dissolution of the species concerned and analysis of the solutions with conventional methods such as UV-visible spectrophotometry, or neutron activation analysis as described in ref. 3. A proper sample handling technique assured the quantitation of chloride in the samples. Surface saturation in the silica bed after a single pulse of TiCL was confirmed by determining titanium and chloride concentrations in samples taken from the surface and the bottom part of the bed. Saturation was achieved when the titanium and chloride concentrations at the surface corresponded to those at the bottom, meaning that the bonding sites inside the pores were also reached by the reactant. The disappearance of bonding sites could be followed by F T I R in both samples. The reproducibility of the ALE process was good as evaluated by titanium saturation levels and the amount of etchable titanium species from parallel runs.
3.3. Surface species after a single pulse of TiCl4 Two different reaction temperature ranges could be distinguished: a lower reaction temperature range from 175 to 250 °C and a higher range from 350 to 550 °C. Both were studied first by determining titanium, chloride and etchable titanium species from samples prepared at different temperatures. F T I R was used to follow the type of bonding sites used in the reactions.
S. Haukka et al. / Surface species in A L E
282
Lower reaction temperature. Figure 2 shows the surface species present at 175 and 250 °C. Earlier [2] it was confirmed by ~ H N M R measurements that at 175 "C TIC14 reacts directly with O H groups and no siloxane bridges are involved in the reactions. N o w this was shown to be true for the reaction temperature of 250 °C as well. The results of the [CI]/[Ti] ratio indicated that TIC14 reacted both mono- and bifunctionally. The binding took place with the isolated O H groups of silica, but some O H groups remained intact (Fig. 3). Either the activation energy of chemisorption to those O H groups is not yet exceeded at the lower reaction temperatures, or the chlorides bound to titanium species hinder the penetration of TIC14 into these groups. No interaction of the HCI released in the reactions with O H groups of silica was observed at the lower reaction temperatures [2]. Etching with sulfuric acid removed all titanium bound at 175 and 250 °C from the surface. A m o r p h o u s titanium was present on the surface as verified by X R D measurements. Treatment of the TiClx-SiO2 surface with water vapor at 175 or 250 °C could not remove all the chloride, but 8 4 % - 8 7 % of the chloride present was released. Water vapor did not remove any titanium [2, 3]. Higher reaction temperature. The possible surface species present on silica at higher reaction temperatures
450 °C1 i
3 5 0 °C \
0
2 5 0 °C \
4J 4J
'\
t //s"ica
5 6 0 °C
t~ # B
si 4000
3750
i
i
3500
3250
3000
Wavenumbers/cm Fig. 3. F T I R spectra of silica preheated at 560 °C and after being treated with TiCI 4 at 250, 350 and 450 °C. The band of isolated OH groups is shown between 3741 and 3746 cm 1.
SURFACE SPECIES (TiCI 4 > 3 5 0 ° C )
jo,
~p.
o.
7Si777Si257-Si5,,,/
I
Si,,,,,,
o.
o.
I
Si
Si///.
,,//;,,
+ TiCl 4 _
TiO 2
_
CI \ / CI /Ti
•
'
nuc,eus:'
. . . . . . .
7 Si~qTSi ~
9'
9i
9
0 L
\0 I
Si 7~7 Si ~ 7 S i / , 7 Si 777- Si +
H20
OH / O H ['- TiO 2 '. Ti ', nucleus .' OH OH OH 0/ \0 7 Si;vv- Si-,' / / , ' 7 7 S i / ~ 7 Si/~5-, Si//7Si / F Si/77 Fig. 4. Bonding sites of silica and titanium surface species at reaction temperatures over 350 'C, when first TiCI 4 and then H 2 0 vapor is brought through the silica bed.
are shown in Fig. 4. Earlier we noticed that X R D peaks could be detected after only a single pulse of TiCI4 at 450 '~C without any high temperature water vapor treatment [2]. Both anatase and rutile forms were found at 450 ~'C. This phenomenon was studied further using different reaction temperatures. Crystalline TiO2 could be measured by X R D first when the reaction temperature was raised to 350 °C and above. In spite of the agglomeration, we found that surface saturation was valid, and isolated O H groups disappeared during the reaction (Fig. 3). Only some of the titanium was etched with sulfuric acid. The amount of these etchable species decreased with increasing reaction temperature from 30% to 18%. The X R D spectra recorded before and after etching were the same, which confirmed that the titanium species released was of amorphous character. It is unclear whether the amorphous species is directly bound to the O H groups of silica. The calculated [CI]/[Ti] ratio was 2 at 450 °C, which could lead to the conclusion that only bifunctional binding occurred. 29Si-NMR measurements showed indirectly, however, that most of the O H groups of silica must have been occupied by the chloride, since the water-treated titanium-silica sample gave an identical 2 9 8 i - N M R spectrum to the spectrum recorded from untreated silica [2]. Furthermore, the F T I R spectrum was similar to that of untreated silica. At higher temperatures the interaction of HC1 was evident [2]. Therefore, the interaction of HC1 evolved during the main reaction could be responsible for the chloride bound to the surface. This interaction of HC1 could lead to release of one water molecule per HC1 molecule reacted with an O H group, and water
S. Haukka et al. / Surface species in A L E
is thus present during the TiCI 4 pulsing. This could further cause molecular-scale growth of TiO2, which is reproducible on a macroscopic scale and stops when the bonding sites of silica are occupied either by TiClx or chloride. The water vapor treatment of TiClx-SiO2 at higher reaction temperatures releases chloride below the detection limit of the determination method used in this study, namely less than 0.006 atoms nm -2. After the first reaction cycle of TiC14 + H20, new O H groups are also formed from Si-C1 and Ti-C1 groups. As verified by IH-NMR measurements [2], only O H groups of titanium bound to amorphous titanium species were present, as illustrated in Fig. 4. These new O H groups of silica and amorphous titanium species serve as bonding sites for the second pulse of TIC14.
283
determinations. Proper sample handling before chloride determinations and accurate titanium determinations were essential so that the analytical error of the [CI]/[Ti] ratio could be decreased. Wet-chemical methods such as the etching test used in this work are of good value for the speciation of different forms present on the surface. The combination of wet-chemical methods and bulk analysis methods such as F T I R and N M R can produce information on the surface species not attainable with ultrahigh vacuum surface analysis techniques. The study of ALE reaction mechanisms of the first reaction cycles are possible with the analysis methods described here in the case when a high-surface-area substrate is used. Characterization of the bonding sites of the substrate is important, and because of the different character of porous substrates, the reaction mechanisms may differ from those in thin film and single-crystal applications.
4. Conclusions ALE was used for binding titanium species on a porous, high-surface-area substrate. The use of saturated surface reactions ensured homogeneous distribution of titanium species of SiO2, as verified by measuring the disappearance of reactive O H groups by F T I R and 1H-NMR. X R D measurements and additional measurements on water-treated samples by 298iN M R showed that at reaction temperatures above 350 °C another reaction mechanism leading to Si-C1 groups and release of water takes place. The effect of the side reaction is the formation of agglomerated TiO2 on silica. The agglomeration is also controlled by the number of O H groups of silica present at the surface. The use of several analytical methods is needed in studying the reaction mechanisms on porous, high-surface-area substrates. Analysis of both the support itself and the ALE prepared samples is necessary. Special emphasis must be placed on good accuracy of element
Acknowledgments The authors thank M. Rissanen (Microchemistry Ltd.) for the ALE processing, J. Vilhunen and A. Root (Neste Co., Analytical Research) for the X R D and N M R measurements respectively. The University of Joensuu provided us with use of the F T I R equipment. This study was funded in part by the Academy of Finland.
References 1 E.-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci., 60-61 (1992) 742. 2 S. Haukka, E.-L. Lakomaa and A. Root, submitted to J. Phys. Chem. 3 S. Haukka and A. Saastamoinen, Analyst, 117 (1992) 1381. 4 D.W. Sindorf and G. E. Maciel, J. Am. Chem. Soc., 105(1983) 1487.
Thin Solid Films, 225 (1993) 280 283
Analytical and chemical techniques in the study of surface species in atomic layer epitaxy S. Haukka Microchemistry Ltd., P.O. Box 45, 02151 Espoo, and University of Helsinki, Department of Chemistry, Analytical Chemistry Division, Vuorikatu 20, 00100 Helsinki (Finland)
E.-L. Lakomaa and T. Suntola Microchemistry Ltd., P.O. Box 45, 02151 Espoo (Finland)
Abstract An extension of atomic layer epitaxy (ALE) to a porous, high-surface-area substrate commonly used in catalysis is presented. Because of the high surface area, even a sublayer of species bound to the substrate in an ALE sequence can be determined quantitatively. Thus, various analytical and chemical techniques, in addition to the high vacuum' techniques, can be applied in the study of surface reactions and surface species in ALE after a single reaction step. Use of Fourier transform IR spectroscopy, nuclear magnetic resonance, X-ray diffraction, chemical analysis and etching experiments in the characterization of different titanium species on porous silica processed using TiCI4 and H20 is presented.
I. Introduction Atomic layer epitaxy (ALE) has been used for the growth of various single-crystal and polycrystalline materials, mainly on small-area substrates. In our previous paper [1] A L E was used to grow TiO2 on a porous, high-surface-area silica substrate. The first four reaction cycles of TiCI4 and H 2 0 on silica were studied, but the more thorough study of the reaction mechanisms called for the development and application of different analysis methods [2]. Earlier only reaction temperatures of 175 and 450 °C were used. The objective of this study was to find out how A L E can be used to bind titanium species to porous SiO2 using TIC14 and H 2 0 vapors as reactants, and to introduce wet-chemical methods together with Fourier transform I R spectroscopy ( F T I R ) , nuclear magnetic resonance ( N M R ) and X-ray diffraction ( X R D ) to study the reaction mechanisms involved. The use of these various analysis methods in the study of a single A L E sequence in the temperature range 175 550 °C with silica preheated at 560 °C is dealt with in the following.
pressure and for 3 h in nitrogen flow at 6 - 1 0 k P a before the reaction. TiC14 (Merck) without further purification, vaporized at 25 °C, and deionized water vaporized at 25 "C were used as reactants. 2.2. Equipment
A modified MC 120 reactor (Microchemistry Ltd.) with a reaction chamber made of quartz was used in the experiments (Fig. 1). The reaction chamber can hold up to 10 g of silica powder. The reactions were carried out at a pressure of 6 - 1 0 kPa in nitrogen atmosphere. The sampling was made in an inert glove box in nitrogen atmosphere (MBraun). 2.3. A L E procedure
Silica preheated at 560 °C (5 8 g) was stabilized to the reaction temperature. A pulse of TIC14 vapor was fed into the reaction chamber through a solid silica bed supported on a sinter. Pumping of excess reactant and the HC1 released during the reactions took place from the bottom of the silica bed (Fig. 1). Reaction temperatures of 175, 250, 350, 450 and 550°C were used. Reaction times were 1 - 2 h followed by a nitrogen purge of same length at the reaction temperature concerned.
2. Experimental details 2.4. Analysis' methods" 2. I, Reagents SiO 2 (EP 10, Crosfield Ltd.) with a surface area of
3 0 0 m 2 g l, pore volume of 1.75cm 3 g - l and mean particle size of 100 ~tm was used as the substrate. SiO 2 was preheated for 16 h at 560 °C in air at atmospheric
0040-6090/93/$6.00
The total titanium concentration was determined by UV-visible spectrophotometry or neutron activation analysis as described in ref. 3. Etching tests in connection with element determinations were developed for distinguishing titanium species [2, 3]. Amorphous tita-
c~, 1993
ElsevierSequoia. All rights reserved
S. Haukka et al. / Surface species in A L E
281
SURFACE SPECIES (TiCI 4 175 and 250°C) H. /H Oi. "'OI. OH OH I, \ ( OH ^./O\ 7St/~TSi/~t S l ~ S1~,/77 bi//,7Si77 + TiCI 4 + HCI l Fig. 1. A L E equipment and sampling for surface saturation studies. Reactants are fed through the silica bed in a flow of nitrogen. Reaction products and excess reactants are pumped from the bottom of the bed. Samples are taken from the surface (1) and the bottom (2): A is the vessel for TiCI 4 and B is the reaction chamber.
cl\cl/cl
c L ,cl
Ti Ti 0I OH 0I 0I /0\ . I /77TSi//,// Si//// Si,-777Si,~TzSiT7~-SI/~7+ H20
nium species were etched with 3.5 M H 2 S O 4 and the amount of titanium determined in the solutions by UV-visible spectrophotometry. Samples for determination of chloride content were weighed immediately in 3 M H2 SO4 after the sample was removed from the reaction chamber so that the air moisture could not release chloride as HC1. Potentiometric titration was used to quantify the chloride. The type of bonding sites and the sites remaining after the ALE reaction on silica were analyzed with an F T I R spectrometer (Galaxy Series 6020) installed in a glove box, into which the samples were transferred inertly. The samples were loaded on the sample holder, and the spectra were recorded using a spectral resolution of 2 c m -]. The accumulation time was 4.5min, corresponding to 1000 scans. X R D spectra were measured with a Siemens 500 diffractometer with Cu K~ excitation. The 1H- and 29Si-NMR measurement procedure was as described in ref. 2.
3. Results and discussion
3. I. Surface species of silica Porous, high-surface-area silica is a complicated substrate, which consists of different reactive sites: isolated (including single and geminal) and hydrogen-bonded O H groups and siloxane bridges [2, 4]. A simplified picture of these reactive sites is shown in Fig. 2. The groups serving as bonding sites for a reagent depend on the chemical characteristics of the reactant vapor, the chemical state of the substrate and the reaction temperature. Characterization of the support and its bonding sites is vital when studying the reaction mechanisms in ALE. In this work silica preheated at 560 °C was used. This silica contains 1.6 isolated and 0.5 hydrogenbonded O H groups per square nanometer, measured by ~H N M R and reported in ref. 2. The hydrogen-bonded O H groups on 560 °C silica are inaccessible (bulk OH groups) and unreactive towards any reactant.
OH ,OiH/.CI
OH\Ti/OH
+ HCIT
6I OH 0 / \ O OR OH L I //H S i / , / / , , , Si,~TT Si,~7- Si/,,H SiT/TTSi77 Fig. 2. Bonding sites of silica and titanium surface species at reaction temperatures of 175 and 250 °C, when first TiCI 4 and then H 2 0 is brought through the silica bed.
3.2. Surface saturation Quantitative determination of titanium and chloride bound from the single reaction step of TiC14 is not reliable using instrumental surface analysis techniques, but the amount of titanium can be determined accurately by dissolution of the species concerned and analysis of the solutions with conventional methods such as UV-visible spectrophotometry, or neutron activation analysis as described in ref. 3. A proper sample handling technique assured the quantitation of chloride in the samples. Surface saturation in the silica bed after a single pulse of TiCL was confirmed by determining titanium and chloride concentrations in samples taken from the surface and the bottom part of the bed. Saturation was achieved when the titanium and chloride concentrations at the surface corresponded to those at the bottom, meaning that the bonding sites inside the pores were also reached by the reactant. The disappearance of bonding sites could be followed by F T I R in both samples. The reproducibility of the ALE process was good as evaluated by titanium saturation levels and the amount of etchable titanium species from parallel runs.
3.3. Surface species after a single pulse of TiCl4 Two different reaction temperature ranges could be distinguished: a lower reaction temperature range from 175 to 250 °C and a higher range from 350 to 550 °C. Both were studied first by determining titanium, chloride and etchable titanium species from samples prepared at different temperatures. F T I R was used to follow the type of bonding sites used in the reactions.
S. Haukka et al. / Surface species in A L E
282
Lower reaction temperature. Figure 2 shows the surface species present at 175 and 250 °C. Earlier [2] it was confirmed by ~ H N M R measurements that at 175 "C TIC14 reacts directly with O H groups and no siloxane bridges are involved in the reactions. N o w this was shown to be true for the reaction temperature of 250 °C as well. The results of the [CI]/[Ti] ratio indicated that TIC14 reacted both mono- and bifunctionally. The binding took place with the isolated O H groups of silica, but some O H groups remained intact (Fig. 3). Either the activation energy of chemisorption to those O H groups is not yet exceeded at the lower reaction temperatures, or the chlorides bound to titanium species hinder the penetration of TIC14 into these groups. No interaction of the HCI released in the reactions with O H groups of silica was observed at the lower reaction temperatures [2]. Etching with sulfuric acid removed all titanium bound at 175 and 250 °C from the surface. A m o r p h o u s titanium was present on the surface as verified by X R D measurements. Treatment of the TiClx-SiO2 surface with water vapor at 175 or 250 °C could not remove all the chloride, but 8 4 % - 8 7 % of the chloride present was released. Water vapor did not remove any titanium [2, 3]. Higher reaction temperature. The possible surface species present on silica at higher reaction temperatures
450 °C1 i
3 5 0 °C \
0
2 5 0 °C \
4J 4J
'\
t //s"ica
5 6 0 °C
t~ # B
si 4000
3750
i
i
3500
3250
3000
Wavenumbers/cm Fig. 3. F T I R spectra of silica preheated at 560 °C and after being treated with TiCI 4 at 250, 350 and 450 °C. The band of isolated OH groups is shown between 3741 and 3746 cm 1.
SURFACE SPECIES (TiCI 4 > 3 5 0 ° C )
jo,
~p.
o.
7Si777Si257-Si5,,,/
I
Si,,,,,,
o.
o.
I
Si
Si///.
,,//;,,
+ TiCl 4 _
TiO 2
_
CI \ / CI /Ti
•
'
nuc,eus:'
. . . . . . .
7 Si~qTSi ~
9'
9i
9
0 L
\0 I
Si 7~7 Si ~ 7 S i / , 7 Si 777- Si +
H20
OH / O H ['- TiO 2 '. Ti ', nucleus .' OH OH OH 0/ \0 7 Si;vv- Si-,' / / , ' 7 7 S i / ~ 7 Si/~5-, Si//7Si / F Si/77 Fig. 4. Bonding sites of silica and titanium surface species at reaction temperatures over 350 'C, when first TiCI 4 and then H 2 0 vapor is brought through the silica bed.
are shown in Fig. 4. Earlier we noticed that X R D peaks could be detected after only a single pulse of TiCI4 at 450 '~C without any high temperature water vapor treatment [2]. Both anatase and rutile forms were found at 450 ~'C. This phenomenon was studied further using different reaction temperatures. Crystalline TiO2 could be measured by X R D first when the reaction temperature was raised to 350 °C and above. In spite of the agglomeration, we found that surface saturation was valid, and isolated O H groups disappeared during the reaction (Fig. 3). Only some of the titanium was etched with sulfuric acid. The amount of these etchable species decreased with increasing reaction temperature from 30% to 18%. The X R D spectra recorded before and after etching were the same, which confirmed that the titanium species released was of amorphous character. It is unclear whether the amorphous species is directly bound to the O H groups of silica. The calculated [CI]/[Ti] ratio was 2 at 450 °C, which could lead to the conclusion that only bifunctional binding occurred. 29Si-NMR measurements showed indirectly, however, that most of the O H groups of silica must have been occupied by the chloride, since the water-treated titanium-silica sample gave an identical 2 9 8 i - N M R spectrum to the spectrum recorded from untreated silica [2]. Furthermore, the F T I R spectrum was similar to that of untreated silica. At higher temperatures the interaction of HC1 was evident [2]. Therefore, the interaction of HC1 evolved during the main reaction could be responsible for the chloride bound to the surface. This interaction of HC1 could lead to release of one water molecule per HC1 molecule reacted with an O H group, and water
S. Haukka et al. / Surface species in A L E
is thus present during the TiCI 4 pulsing. This could further cause molecular-scale growth of TiO2, which is reproducible on a macroscopic scale and stops when the bonding sites of silica are occupied either by TiClx or chloride. The water vapor treatment of TiClx-SiO2 at higher reaction temperatures releases chloride below the detection limit of the determination method used in this study, namely less than 0.006 atoms nm -2. After the first reaction cycle of TiC14 + H20, new O H groups are also formed from Si-C1 and Ti-C1 groups. As verified by IH-NMR measurements [2], only O H groups of titanium bound to amorphous titanium species were present, as illustrated in Fig. 4. These new O H groups of silica and amorphous titanium species serve as bonding sites for the second pulse of TIC14.
283
determinations. Proper sample handling before chloride determinations and accurate titanium determinations were essential so that the analytical error of the [CI]/[Ti] ratio could be decreased. Wet-chemical methods such as the etching test used in this work are of good value for the speciation of different forms present on the surface. The combination of wet-chemical methods and bulk analysis methods such as F T I R and N M R can produce information on the surface species not attainable with ultrahigh vacuum surface analysis techniques. The study of ALE reaction mechanisms of the first reaction cycles are possible with the analysis methods described here in the case when a high-surface-area substrate is used. Characterization of the bonding sites of the substrate is important, and because of the different character of porous substrates, the reaction mechanisms may differ from those in thin film and single-crystal applications.
4. Conclusions ALE was used for binding titanium species on a porous, high-surface-area substrate. The use of saturated surface reactions ensured homogeneous distribution of titanium species of SiO2, as verified by measuring the disappearance of reactive O H groups by F T I R and 1H-NMR. X R D measurements and additional measurements on water-treated samples by 298iN M R showed that at reaction temperatures above 350 °C another reaction mechanism leading to Si-C1 groups and release of water takes place. The effect of the side reaction is the formation of agglomerated TiO2 on silica. The agglomeration is also controlled by the number of O H groups of silica present at the surface. The use of several analytical methods is needed in studying the reaction mechanisms on porous, high-surface-area substrates. Analysis of both the support itself and the ALE prepared samples is necessary. Special emphasis must be placed on good accuracy of element
Acknowledgments The authors thank M. Rissanen (Microchemistry Ltd.) for the ALE processing, J. Vilhunen and A. Root (Neste Co., Analytical Research) for the X R D and N M R measurements respectively. The University of Joensuu provided us with use of the F T I R equipment. This study was funded in part by the Academy of Finland.
References 1 E.-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci., 60-61 (1992) 742. 2 S. Haukka, E.-L. Lakomaa and A. Root, submitted to J. Phys. Chem. 3 S. Haukka and A. Saastamoinen, Analyst, 117 (1992) 1381. 4 D.W. Sindorf and G. E. Maciel, J. Am. Chem. Soc., 105(1983) 1487.