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Novel catalysts: Indium implanted SiO2 thin films
Applied Surface Science 257 (2010) 192–196
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Novel catalysts: Indium implanted SiO2 thin films S. Yoshimura a,∗ , K. Hine a , M. Kiuchi a,b , Y. Nishimoto c , M. Yasuda a,c , A. Baba c , S. Hamaguchi a a
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan c Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 21 June 2010 Accepted 23 June 2010 Available online 30 June 2010 Keywords: Indium Ion beam injection SiO2 thin film Catalyst
a b s t r a c t Interactions of Indium (In) and silicon (Si) atoms are known to catalyze certain organic chemical reactions with high efficiency. In an attempt of creating a material that manifests the interactions, In implanted SiO2 thin films were prepared by ion beam injection and their catalytic abilities for organic chemical reactions were examined. It has been found that, with an injection energy of approximately 0.5 keV, a thin In film is formed on a SiO2 substrate surface and the In implanted SiO2 thin film can catalyze an organic chemical reaction. It has been also shown that there is an optimal ion dose for the highest catalytic ability in the film preparation process. Thin-film-type catalyzing materials such as the one proposed here may open a new way to enhance surface chemical reaction rates. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indium (In) has been of much interest in various research fields such as semiconductor technologies and catalytic applications [1–3]. Recently it has been pointed out that interaction between In and silicon (Si) shows high catalytic ability [1,2]. However, it is difficult for a conventional chemical method to synthesize such catalyzers. In this study, we use a more “physical” approach to prepare materials that contain both In and Si atoms in close proximity as potential candidates for catalyzers, i.e., In implantation into a Si containing material by ion beam injection. In conventional experiments of In ion beam injection [4–6], the solid-state In is heated to produce liquid or gaseous In atoms and then In ions are produced [7]. However, in this study, we modified a Freeman-type ion source so that a solid-state material can be set inside the ion source chamber as a sputtering target. In this experiment, we used In2 O3 as the target in the ion source. The melting temperature of In2 O3 is much higher than that of In metal. In the ion source, In ions can be obtained from sputtering of the In2 O3 target by Ar ions generated from an Ar plasma. In ions thus obtained were injected into a SiO2 thin film that had been formed thermally on a Si substrate (SiO2 /Si substrate). The typical thickness of the SiO2 layer used in this study is 0.1 m. After In implanted SiO2 /Si substrates were prepared under different implantation conditions, we analyzed the substrate surfaces
∗ Corresponding author. Tel.: +88 6 6879 7915; fax: +88 6 6879 7916. E-mail addresses: [email protected], [email protected] (S. Yoshimura). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.063
by X-ray diffraction (XRD) and atomic force microscopy (AFM). Catalytic abilities of the In implanted SiO2 /Si substrates were also examined for a particular organic chemical reaction.
2. Experimental setup The In ion production experiment was carried out in a lowenergy mass-selected ion beam system. The system consists of an ion source, an extractor electrode, a mass selector, a decelerator, and a process chamber [8]. The configuration of the Freeman-type hybrid ion source was shown in Figs. 1 and 2 of Ref. [9]. In this experiment, an In2 O3 target was set in the ion source as a sputtering target. The procedure of In ion beam production is as follows: The arc chamber (25.4 mm in diameter, 50.8 mm in length) of the ion source is filled with Ar gas with a fixed gas flow rate. An Ar plasma is then generated by a hot tungsten wire in the arc chamber and In atoms are sputtered from the In2 O3 target by Ar ions of the plasma. The In2 O3 target was biased at −500 V for efficient formation of In ions. In the arc chamber, we have Ar ions, sputtered In ions, and impurity ions. These ions are extracted by a high voltage of −15 kV applied to the extractor electrode. The In ions are selected by the magnetic-field based mass selector. Then In ions are directed to the process chamber and decelerated to the desired energy by the decelerator. The degree of vacuum in the process chamber is about 10−9 Torr. In the process chamber, a substrate holder and a mass-energy analyzer (balzers, PPM-421) are installed and either of them can be set on the direct path of the ion beam. The substrate holder is connected with an ammeter (KEITHLEY, 485) for current measurement. In our typical experiments, the beam characteristics
S. Yoshimura et al. / Applied Surface Science 257 (2010) 192–196
Fig. 1. Ion beam intensities measured by the Faraday cup for each ion species selected by the mass selector. The horizontal axis represents the current to produce the magnetic field at the mass selector. The figure shows the mass spectrum of ions produced in the ion source.
are measured carefully by the mass-energy analyzer and then a SiO2 /Si substrate set on the substrate holder is exposed to the ion beam. The temperature of the SiO2 /Si substrate is initially set at room temperature but not actively controlled. The experiment was carried out in the following manner. Firstly, we measured the mass spectrum of the plasma in the ion source to identify ions present in the plasma by varying the current to generate the magnetic field of the magnetic-field based mass selector. Secondly, with an appropriate adjustment of the magnetic-field based mass selector, an In ion beam was selected and its mass and energy distributions were then measured by the mass-energy analyzer. Then In ion beams were injected into several SiO2 /Si substrates with different ion doses. The ion dose was evaluated from the ion current and the duration of ion injection. The surface condition of each SiO2 /Si substrate thus prepared was analyzed by XRD and AFM. Finally, catalytic abilities of In implanted SiO2 /Si substrates were examined, as will be discussed in Section 3.2. 3. Experimental results and discussion 3.1. Preparation of In implanted SiO2 thin films By changing the electric current to generate the magnetic field of the mass selector, the mass spectrum of ions extracted from the ion source was measured by a Faraday cup installed just after the mass selector. Fig. 1 shows the dependence of ion beam intensity (i.e., Faraday cup current) on the mass selector current. In Fig. 1,
Fig. 2. Mass spectrum of the (mass-selected) In ion beam used in this study. The peak mass number is 115, which corresponds to the mass number of In.
193
Fig. 3. Energy spectrum of the In ion beam used in this study. The full width at half maximum (FWHM) of the energy distribution is about 6 eV.
several ion peaks are seen. The mass numbers of some peaks were identified by the mass-energy analyzer PPM-421. It was found that N+ , O+ , Ar2+ , Ar+ , In+ , and W+ ions were present in the ion source, as shown in Fig. 1. Identification of the other peaks was difficult because the ion currents for these peaks were too low. With an appropriate adjustment of the current in the magneticfield based mass selector, we selected a specific ion beam. The mass spectrum of this mass-selected ion beam was measured by the mass-energy analyzer, which is shown in Fig. 2. The figure indicates that only a single peak appears at the mass number of 115, that is, the mass-selected ion beam is identified to be that of pure In with no impurity. The energy spectrum measured by the massenergy analyzer is given in Fig. 3, which shows that the peak energy is about 470 eV and the full width at half maximum (FWHM) of the energy distribution is about 6 eV. These results show that the massselected In ion beam used in this study was nearly monochromatic. Six In implanted SiO2 /Si substrates with different In ion doses were prepared for surface analyses. The In ion beam current used in the experiments was about 1 A and the size of the substrate is approximately 1 cm × 1 cm and the area that is exposed to the ion beam is a disk with a diameter of about 0.8 cm. The ion doses of 6 substrates are (a) 5 × 1015 , (b) 1 × 1016 , (c) 2 × 1016 , (d) 2 × 1017 , (e) 1 × 1018 , and (f) 2 × 1018 ions/cm2 . These SiO2 /Si substrates were analyzed by XRD. Fig. 4 shows the XRD measurement results of the substrate (d), where a peak that corresponds to the In (1 1 1) orientation is clearly seen. Although the data is not shown here, the In (1 1 1) peak in XRD measurement was also observed for the substrates (c), (e), and (f) while no clear
Fig. 4. X-ray diffraction spectrum (–2 method) measured for an In implanted SiO2 /Si surface. The In dose is 2.2 × 1017 ions/cm2 . The peak angle is about 32.9◦ , which corresponds to the In (1 1 1) peak. The results indicate that the surface is, partially or fully, covered by metallic In.
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Fig. 5. Atomic force microscopy (AFM) images of In injected SiO2 /Si substrate surfaces. The doses of In injection are (a) 5 × 1015 , (b) 1 × 1016 , (c) 2 × 1016 , (d) 2 × 1017 , (e) 1 × 1018 , and (f) 2 × 1018 ions/cm2 . It is seen that the substrate surface morphology varies depending on the dose.
peak for In was observed in (a) and (b), i.e., the cases with the In ion dose being less than 2 × 1016 ions/cm2 . After the XRD measurements, the surfaces of the same substrates (a)–(f) were analyzed by AFM, the photographs of which are given in Fig. 5. Fig. 5 shows that the substrate surface morphology after In ion injection depends on the ion dose. In (a), no clear structure is observed on the surface whereas in (b)–(f), particle-like structures are seen. The sizes of nano-particles observed here are about 10 nm and 20 nm in (b) and (c). In (d) and (e), the particle sizes are obviously larger. The particles seem to have agglutinated to form an almost flat surface in (f). These results suggest that not only the amount of In implanted into or deposited on the SiO2 /Si substrate but also the nano-meter scale morphologies of deposited In on the SiO2 /Si substrate may affect catalytic reactions. 3.2. Evaluation of catalytic reactions by In implanted SiO2 Catalytic ability of the In implanted SiO2 thin films were examined for a reaction of benzhydrol with acetylacetone, which is a
typical reaction that is known to be enhanced by catalytic effects of In and Si interactions. Although the SiO2 film irradiated by In ions was initially set on a crystalline Si substrate in this study, the SiO2 film is so thick that In atoms injected at around 0.5 kV hardly reach the Si substrate; injected In atoms are expected to be imbedded only in SiO2 or diffused to the SiO2 surface. In other words, catalytic ability we examine here is that of In implanted SiO2 , rather than In implanted Si. It should be noted that, unlike SiO2 [Si(IV)], Si(0) has no Lewis acidity and does not interact with In. Therefore, In embedded in a pure Si substrate does not show catalytic activity. The organic chemical reactions that we are interested in were observed in a 10-mL, flame-dried two-necked, round-bottomed flask equipped with a stop cock, a Teflon-coated magnetic stirring bar, and a reflux condenser fitted with a nitrogen inlet adapter was used as the container for the chemical reactions. The In implanted SiO2 /Si substrate was broken into approximately 5–10 small pieces and placed in the flask, together with benzhydrol (1 mmol), acetylacetone (1.5 mmol), and toluene (2 mL) [10]. The resulting mixture
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Fig. 6. Yields of the benzhydrol and acetylacetone reactions with In implanted SiO2 /Si substrate catalysts under different conditions.
in the flask was then heated at 80 ◦ C in an oil bath for various reaction periods ranging from 10 to 26 hours. After the reaction, the mixture was cooled to room temperature and treated with H2 O (10 mL) and diethyl ether (10 mL). The organic layer was separated and the aqueous layer was extracted with two 10-mL portions of diethyl ether. The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure to give the crude product. Yields were determined by 1 H NMR using the internal standard. Fig. 6 shows the reaction results for SiO2 /Si substrates with different doses of In implantation and reaction time periods. The percentage shown here is the molar distribution. In Fig. 6, the reaction of benzhydrol [denoted by] (1) and acetylacetone (2) produces the coupling product (3) and bis(diphenylmethyl) ether (4). From (a) to (e), different reaction conditions are described. Under each condition, a single SiO2 /Si substrate in pieces (1 cm × 1 cm in the original size) was placed in the flask except for (d), where pieces from two substrates of the same ion dose were placed in the flask. The ion doses were (a) null, (b) 2 × 1016 , (c) 1 × 1017 , (d) 1 × 1017 (two substrates), and (e) 1 × 1018 ions/cm2 . It is seen that, if the dose is less than 1 × 1017 cm−2 [i.e., (a) and (b)], no reaction took place and the original benzhydrol was recovered 100%. As the dose increases, in the case (c), the coupling product and bis(diphenylmethyl) ether were obtained in 6% and 38% yields, which clearly shows the catalyzing effect by the In implanted SiO2 /Si substrate. Furthermore, as shown in the case (d), the increase in the amount of In implanted SiO2 /Si substrate increases the reaction yields. Note that, in (c) and (d), no other reaction products were obtained. However, further increase in dose has an negative effect as shown in (e), where the yields of the coupling product and bis(diphenylmethyl) ether were null and nearly 100% of benzhydrol was recovered. These results show that the catalytic effect is strongly dependent on the In dose and possibly the surface morphology of deposited In thin film. The substrates used in (c) and (d), whose surface morphology is expected be similar to that of Fig. 5(c) or (d), have uneven surfaces full of mounds and therefore have larger surface areas than the area of a flat surface. With higher deposition doses, the surface may be more fully covered by In. As separate experiments, we have confirmed that In solid alone does not catalyze the reaction, which is consistent with our observation
of Fig. 6(e). As shown in Fig. 6(f), with the use of In(0) metal as a catalyst, no reaction proceeds. 4. Conclusions We have used In ion beam injection into a SiO2 thin film to create a material that has both In and Si in close proximity, which exhibits a catalytic effect on an organic chemical reaction. It has been known that a Si compound/Indium catalyst system shows strong catalytic effects on various organic chemical reactions. With the use of conventional chemical reactions, it is known to be difficult to synthesize single-molecule catalysts containing both Si and In. By the ion injection method, In can be imbedded in a Si containing material via physical (rather than chemical) means. In ion beams were produced in a mass-selected ion beam system, where an In2 O3 was used as a target set in the Freeman-type ion source. Ions generated in the ion source were accelerated to 15 kV and mass selected to produce a pure In ion beam. The In ion beam is then decelerated to 470 eV and injected into the SiO2 /Si substrate. The substrate surface conditions were examined by XRD and AFM. Our study on the catalytic abilities summarized in Fig. 6 shows that, under the conditions that we examined, the maximum catalytic effect on the benzhydrol-acetylacetone reaction was obtained when the In ion dose was 1 × 1017 ions/cm2 . It has been known that In is hardly soluble in solid-state Si. In atoms injected into solid-state Si often readily diffuse to the surface, forming two separate phases of In and Si. In this work, we use SiO2 thin films as a material for In to be implanted to because In also forms an oxide (i.e., In2 O3 ) and SiO2 is a stable material. The injection of In ions into a SiO2 substrate under the conditions we used in this work still forms a metallic In surface layer. However, as long as the surface layer is sufficiently thin, the catalytic reaction is shown to proceed. Although we have had no direct evidence so far that interaction between In and Si causes the catalytic reaction shown in Fig. 6, it is conceivable that a +3 valence indium compound formed on the substrate has the catalytic effect [10]. Identification of such a compound is a subject of future study. As shown in Fig. 6, the yield of catalytic reaction by the In implanted SiO2 thin film is still too low to be of any practical use as a catalyst. This is because, first, we have not yet optimized the
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In implantation method for practical use, and, second, the catalyst that can be made by ion beam injection is two dimensional after all. The latter nature may not be necessarily a disadvantage in different types of reactors. For example, a solution of reactants flowing in a capillary whose wall is made of In implanted SiO2 may promote such catalytic reactions efficiently. Increase of reaction yields catalyzed by In implanted Si containing materials is another subject of future study. Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8]
The work is partially supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan. The authors would like to thank Messrs. K. Ikuse and Y. Tsukazaki for their assistance in the ion beam experiments.
[9] [10]
Y. Onishi, T. Ito, M. Yasuda, A. Baba, Eur. J. Org. Chem. (2002) 1578–1581. T. Saito, M. Yasuda, A. Baba, Synlett (2005) 1737–1739. U. Schneider, S. Kobayashi, Angew. Chem. Int. Ed. 46 (2007) 5909–5912. S. Solmi, A. Parisini, M. Bersani, D. Giubertoni, V. Sonicini, G. Carnevale, A. Benvenuti, A. Marmiroli, J. Appl. Phys. 92 (2002) 1361–1366. S. Gennaro, D. Giubertoni, M. Bersani, J. Foggiato, W.S. Yoo, R. Gwilliam, J. Vac. Sci. Technol. B 24 (2006) 473–477. T.S. Anderson, R.H. Magruder III, R.A. Weeks, R.A. Zuhr, J. Non-Cryst. Solids 203 (1996) 114–119. M.-A. Hasan, J. Knall, S.A. Barnett, A. Rockett, J.–E. Sundgren, J.E. Greene, J. Vac. Sci. Technol. B 5 (1987) 1332–1339. S. Yoshimura, A. Toh, S. Sugimoto, M. Kiuchi, S. Hamaguchi, Jpn. J. Appl. Phys. 45 (2006) 8204–8207. T. Matsumoto, K. Mimoto, S. Goto, M. Ohba, Y. Agawa, M. Kiuchi, Rev. Sci. Instrum. 71 (2000) 1168–1170. M. Yasuda, T. Somyo, A. Baba, Angew. Chem. Int. Ed. 45 (2006) 793–796.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Novel catalysts: Indium implanted SiO2 thin films S. Yoshimura a,∗ , K. Hine a , M. Kiuchi a,b , Y. Nishimoto c , M. Yasuda a,c , A. Baba c , S. Hamaguchi a a
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan c Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 21 June 2010 Accepted 23 June 2010 Available online 30 June 2010 Keywords: Indium Ion beam injection SiO2 thin film Catalyst
a b s t r a c t Interactions of Indium (In) and silicon (Si) atoms are known to catalyze certain organic chemical reactions with high efficiency. In an attempt of creating a material that manifests the interactions, In implanted SiO2 thin films were prepared by ion beam injection and their catalytic abilities for organic chemical reactions were examined. It has been found that, with an injection energy of approximately 0.5 keV, a thin In film is formed on a SiO2 substrate surface and the In implanted SiO2 thin film can catalyze an organic chemical reaction. It has been also shown that there is an optimal ion dose for the highest catalytic ability in the film preparation process. Thin-film-type catalyzing materials such as the one proposed here may open a new way to enhance surface chemical reaction rates. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indium (In) has been of much interest in various research fields such as semiconductor technologies and catalytic applications [1–3]. Recently it has been pointed out that interaction between In and silicon (Si) shows high catalytic ability [1,2]. However, it is difficult for a conventional chemical method to synthesize such catalyzers. In this study, we use a more “physical” approach to prepare materials that contain both In and Si atoms in close proximity as potential candidates for catalyzers, i.e., In implantation into a Si containing material by ion beam injection. In conventional experiments of In ion beam injection [4–6], the solid-state In is heated to produce liquid or gaseous In atoms and then In ions are produced [7]. However, in this study, we modified a Freeman-type ion source so that a solid-state material can be set inside the ion source chamber as a sputtering target. In this experiment, we used In2 O3 as the target in the ion source. The melting temperature of In2 O3 is much higher than that of In metal. In the ion source, In ions can be obtained from sputtering of the In2 O3 target by Ar ions generated from an Ar plasma. In ions thus obtained were injected into a SiO2 thin film that had been formed thermally on a Si substrate (SiO2 /Si substrate). The typical thickness of the SiO2 layer used in this study is 0.1 m. After In implanted SiO2 /Si substrates were prepared under different implantation conditions, we analyzed the substrate surfaces
∗ Corresponding author. Tel.: +88 6 6879 7915; fax: +88 6 6879 7916. E-mail addresses: [email protected], [email protected] (S. Yoshimura). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.063
by X-ray diffraction (XRD) and atomic force microscopy (AFM). Catalytic abilities of the In implanted SiO2 /Si substrates were also examined for a particular organic chemical reaction.
2. Experimental setup The In ion production experiment was carried out in a lowenergy mass-selected ion beam system. The system consists of an ion source, an extractor electrode, a mass selector, a decelerator, and a process chamber [8]. The configuration of the Freeman-type hybrid ion source was shown in Figs. 1 and 2 of Ref. [9]. In this experiment, an In2 O3 target was set in the ion source as a sputtering target. The procedure of In ion beam production is as follows: The arc chamber (25.4 mm in diameter, 50.8 mm in length) of the ion source is filled with Ar gas with a fixed gas flow rate. An Ar plasma is then generated by a hot tungsten wire in the arc chamber and In atoms are sputtered from the In2 O3 target by Ar ions of the plasma. The In2 O3 target was biased at −500 V for efficient formation of In ions. In the arc chamber, we have Ar ions, sputtered In ions, and impurity ions. These ions are extracted by a high voltage of −15 kV applied to the extractor electrode. The In ions are selected by the magnetic-field based mass selector. Then In ions are directed to the process chamber and decelerated to the desired energy by the decelerator. The degree of vacuum in the process chamber is about 10−9 Torr. In the process chamber, a substrate holder and a mass-energy analyzer (balzers, PPM-421) are installed and either of them can be set on the direct path of the ion beam. The substrate holder is connected with an ammeter (KEITHLEY, 485) for current measurement. In our typical experiments, the beam characteristics
S. Yoshimura et al. / Applied Surface Science 257 (2010) 192–196
Fig. 1. Ion beam intensities measured by the Faraday cup for each ion species selected by the mass selector. The horizontal axis represents the current to produce the magnetic field at the mass selector. The figure shows the mass spectrum of ions produced in the ion source.
are measured carefully by the mass-energy analyzer and then a SiO2 /Si substrate set on the substrate holder is exposed to the ion beam. The temperature of the SiO2 /Si substrate is initially set at room temperature but not actively controlled. The experiment was carried out in the following manner. Firstly, we measured the mass spectrum of the plasma in the ion source to identify ions present in the plasma by varying the current to generate the magnetic field of the magnetic-field based mass selector. Secondly, with an appropriate adjustment of the magnetic-field based mass selector, an In ion beam was selected and its mass and energy distributions were then measured by the mass-energy analyzer. Then In ion beams were injected into several SiO2 /Si substrates with different ion doses. The ion dose was evaluated from the ion current and the duration of ion injection. The surface condition of each SiO2 /Si substrate thus prepared was analyzed by XRD and AFM. Finally, catalytic abilities of In implanted SiO2 /Si substrates were examined, as will be discussed in Section 3.2. 3. Experimental results and discussion 3.1. Preparation of In implanted SiO2 thin films By changing the electric current to generate the magnetic field of the mass selector, the mass spectrum of ions extracted from the ion source was measured by a Faraday cup installed just after the mass selector. Fig. 1 shows the dependence of ion beam intensity (i.e., Faraday cup current) on the mass selector current. In Fig. 1,
Fig. 2. Mass spectrum of the (mass-selected) In ion beam used in this study. The peak mass number is 115, which corresponds to the mass number of In.
193
Fig. 3. Energy spectrum of the In ion beam used in this study. The full width at half maximum (FWHM) of the energy distribution is about 6 eV.
several ion peaks are seen. The mass numbers of some peaks were identified by the mass-energy analyzer PPM-421. It was found that N+ , O+ , Ar2+ , Ar+ , In+ , and W+ ions were present in the ion source, as shown in Fig. 1. Identification of the other peaks was difficult because the ion currents for these peaks were too low. With an appropriate adjustment of the current in the magneticfield based mass selector, we selected a specific ion beam. The mass spectrum of this mass-selected ion beam was measured by the mass-energy analyzer, which is shown in Fig. 2. The figure indicates that only a single peak appears at the mass number of 115, that is, the mass-selected ion beam is identified to be that of pure In with no impurity. The energy spectrum measured by the massenergy analyzer is given in Fig. 3, which shows that the peak energy is about 470 eV and the full width at half maximum (FWHM) of the energy distribution is about 6 eV. These results show that the massselected In ion beam used in this study was nearly monochromatic. Six In implanted SiO2 /Si substrates with different In ion doses were prepared for surface analyses. The In ion beam current used in the experiments was about 1 A and the size of the substrate is approximately 1 cm × 1 cm and the area that is exposed to the ion beam is a disk with a diameter of about 0.8 cm. The ion doses of 6 substrates are (a) 5 × 1015 , (b) 1 × 1016 , (c) 2 × 1016 , (d) 2 × 1017 , (e) 1 × 1018 , and (f) 2 × 1018 ions/cm2 . These SiO2 /Si substrates were analyzed by XRD. Fig. 4 shows the XRD measurement results of the substrate (d), where a peak that corresponds to the In (1 1 1) orientation is clearly seen. Although the data is not shown here, the In (1 1 1) peak in XRD measurement was also observed for the substrates (c), (e), and (f) while no clear
Fig. 4. X-ray diffraction spectrum (–2 method) measured for an In implanted SiO2 /Si surface. The In dose is 2.2 × 1017 ions/cm2 . The peak angle is about 32.9◦ , which corresponds to the In (1 1 1) peak. The results indicate that the surface is, partially or fully, covered by metallic In.
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Fig. 5. Atomic force microscopy (AFM) images of In injected SiO2 /Si substrate surfaces. The doses of In injection are (a) 5 × 1015 , (b) 1 × 1016 , (c) 2 × 1016 , (d) 2 × 1017 , (e) 1 × 1018 , and (f) 2 × 1018 ions/cm2 . It is seen that the substrate surface morphology varies depending on the dose.
peak for In was observed in (a) and (b), i.e., the cases with the In ion dose being less than 2 × 1016 ions/cm2 . After the XRD measurements, the surfaces of the same substrates (a)–(f) were analyzed by AFM, the photographs of which are given in Fig. 5. Fig. 5 shows that the substrate surface morphology after In ion injection depends on the ion dose. In (a), no clear structure is observed on the surface whereas in (b)–(f), particle-like structures are seen. The sizes of nano-particles observed here are about 10 nm and 20 nm in (b) and (c). In (d) and (e), the particle sizes are obviously larger. The particles seem to have agglutinated to form an almost flat surface in (f). These results suggest that not only the amount of In implanted into or deposited on the SiO2 /Si substrate but also the nano-meter scale morphologies of deposited In on the SiO2 /Si substrate may affect catalytic reactions. 3.2. Evaluation of catalytic reactions by In implanted SiO2 Catalytic ability of the In implanted SiO2 thin films were examined for a reaction of benzhydrol with acetylacetone, which is a
typical reaction that is known to be enhanced by catalytic effects of In and Si interactions. Although the SiO2 film irradiated by In ions was initially set on a crystalline Si substrate in this study, the SiO2 film is so thick that In atoms injected at around 0.5 kV hardly reach the Si substrate; injected In atoms are expected to be imbedded only in SiO2 or diffused to the SiO2 surface. In other words, catalytic ability we examine here is that of In implanted SiO2 , rather than In implanted Si. It should be noted that, unlike SiO2 [Si(IV)], Si(0) has no Lewis acidity and does not interact with In. Therefore, In embedded in a pure Si substrate does not show catalytic activity. The organic chemical reactions that we are interested in were observed in a 10-mL, flame-dried two-necked, round-bottomed flask equipped with a stop cock, a Teflon-coated magnetic stirring bar, and a reflux condenser fitted with a nitrogen inlet adapter was used as the container for the chemical reactions. The In implanted SiO2 /Si substrate was broken into approximately 5–10 small pieces and placed in the flask, together with benzhydrol (1 mmol), acetylacetone (1.5 mmol), and toluene (2 mL) [10]. The resulting mixture
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Fig. 6. Yields of the benzhydrol and acetylacetone reactions with In implanted SiO2 /Si substrate catalysts under different conditions.
in the flask was then heated at 80 ◦ C in an oil bath for various reaction periods ranging from 10 to 26 hours. After the reaction, the mixture was cooled to room temperature and treated with H2 O (10 mL) and diethyl ether (10 mL). The organic layer was separated and the aqueous layer was extracted with two 10-mL portions of diethyl ether. The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure to give the crude product. Yields were determined by 1 H NMR using the internal standard. Fig. 6 shows the reaction results for SiO2 /Si substrates with different doses of In implantation and reaction time periods. The percentage shown here is the molar distribution. In Fig. 6, the reaction of benzhydrol [denoted by] (1) and acetylacetone (2) produces the coupling product (3) and bis(diphenylmethyl) ether (4). From (a) to (e), different reaction conditions are described. Under each condition, a single SiO2 /Si substrate in pieces (1 cm × 1 cm in the original size) was placed in the flask except for (d), where pieces from two substrates of the same ion dose were placed in the flask. The ion doses were (a) null, (b) 2 × 1016 , (c) 1 × 1017 , (d) 1 × 1017 (two substrates), and (e) 1 × 1018 ions/cm2 . It is seen that, if the dose is less than 1 × 1017 cm−2 [i.e., (a) and (b)], no reaction took place and the original benzhydrol was recovered 100%. As the dose increases, in the case (c), the coupling product and bis(diphenylmethyl) ether were obtained in 6% and 38% yields, which clearly shows the catalyzing effect by the In implanted SiO2 /Si substrate. Furthermore, as shown in the case (d), the increase in the amount of In implanted SiO2 /Si substrate increases the reaction yields. Note that, in (c) and (d), no other reaction products were obtained. However, further increase in dose has an negative effect as shown in (e), where the yields of the coupling product and bis(diphenylmethyl) ether were null and nearly 100% of benzhydrol was recovered. These results show that the catalytic effect is strongly dependent on the In dose and possibly the surface morphology of deposited In thin film. The substrates used in (c) and (d), whose surface morphology is expected be similar to that of Fig. 5(c) or (d), have uneven surfaces full of mounds and therefore have larger surface areas than the area of a flat surface. With higher deposition doses, the surface may be more fully covered by In. As separate experiments, we have confirmed that In solid alone does not catalyze the reaction, which is consistent with our observation
of Fig. 6(e). As shown in Fig. 6(f), with the use of In(0) metal as a catalyst, no reaction proceeds. 4. Conclusions We have used In ion beam injection into a SiO2 thin film to create a material that has both In and Si in close proximity, which exhibits a catalytic effect on an organic chemical reaction. It has been known that a Si compound/Indium catalyst system shows strong catalytic effects on various organic chemical reactions. With the use of conventional chemical reactions, it is known to be difficult to synthesize single-molecule catalysts containing both Si and In. By the ion injection method, In can be imbedded in a Si containing material via physical (rather than chemical) means. In ion beams were produced in a mass-selected ion beam system, where an In2 O3 was used as a target set in the Freeman-type ion source. Ions generated in the ion source were accelerated to 15 kV and mass selected to produce a pure In ion beam. The In ion beam is then decelerated to 470 eV and injected into the SiO2 /Si substrate. The substrate surface conditions were examined by XRD and AFM. Our study on the catalytic abilities summarized in Fig. 6 shows that, under the conditions that we examined, the maximum catalytic effect on the benzhydrol-acetylacetone reaction was obtained when the In ion dose was 1 × 1017 ions/cm2 . It has been known that In is hardly soluble in solid-state Si. In atoms injected into solid-state Si often readily diffuse to the surface, forming two separate phases of In and Si. In this work, we use SiO2 thin films as a material for In to be implanted to because In also forms an oxide (i.e., In2 O3 ) and SiO2 is a stable material. The injection of In ions into a SiO2 substrate under the conditions we used in this work still forms a metallic In surface layer. However, as long as the surface layer is sufficiently thin, the catalytic reaction is shown to proceed. Although we have had no direct evidence so far that interaction between In and Si causes the catalytic reaction shown in Fig. 6, it is conceivable that a +3 valence indium compound formed on the substrate has the catalytic effect [10]. Identification of such a compound is a subject of future study. As shown in Fig. 6, the yield of catalytic reaction by the In implanted SiO2 thin film is still too low to be of any practical use as a catalyst. This is because, first, we have not yet optimized the
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In implantation method for practical use, and, second, the catalyst that can be made by ion beam injection is two dimensional after all. The latter nature may not be necessarily a disadvantage in different types of reactors. For example, a solution of reactants flowing in a capillary whose wall is made of In implanted SiO2 may promote such catalytic reactions efficiently. Increase of reaction yields catalyzed by In implanted Si containing materials is another subject of future study. Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8]
The work is partially supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan. The authors would like to thank Messrs. K. Ikuse and Y. Tsukazaki for their assistance in the ion beam experiments.
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