Nanoparticle Formation in Surface Layer of Oxide Materials and Improvement of Photocatalytic Properties of Rutile Titanium Dioxide Junzo Ishikawa, Hiroshi Tsuji, Hiromitsu Sugahara and Yasuhito Gotoh Department of Electronic Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Email: [email protected] Abstract - Negative-ion implantation could be used to create nanoparticles in oxide insulators with finely controlled accuracy for both depth and size. For 50-nm-thick SiO2 film on Si, Ag nanoparticles with 3 nm in diameter were created in the center of the film with distribution thickness of 17 nm. Cu negative-ion implanted silica glass and soda-lime glass showed a high nonlinear susceptance of the 3rd order in nonlinear optical property. Cu and Ag double-implanted silica glass showed an absorption peak between two absorption peaks of surface plasmon resonance (SPR) for Cu and Ag nanoparticles. The optical absorption peak due to SPR of nanoparticle in oxide could be changed by forming nanoparticles with different kinds of elements and alloy. For application of metal nanoparticle to photocatalyst, Ag negative ions were implanted into rutile TiO2. The Ag-implanted rutile samples showed improved photocatalytic efficiency after proper annealing in a decolorization test of methylene blue solution under fluorescent light. The better one was the Ag-implanted rutile TiO2 (Ag: 65 keV, 5x1016 ions/cm2, 500oC annealed), which showed a photocatalytic efficiency higher by 2.2 times than that of unimplanted rutile TiO2. In the evaluation under fluorescent light through UV-cut filter for 19 h, the Ag-implanted rutile showed 6.7 times higher efficiency. Oxide materials including nanoparticles were expected to be used in many fields for developing nonlinear optical devices[1,2], single electron devices[3,4], and photocatalyst[5], because of showing fast optical response, high nonlinear property, and Coulomb blockade phenomena and electron acceptor. Ion implantation method for creation of nanoparticles in oxide is very attractive because of accurate controllability of distribution depth and size of the nanoparticles. However, charge-up problems by implantation due to insulating property of oxides could result in uncertainty in the implantation profile of atoms. Therefore, the authors used negative ion implantation of "charge-up free" technique[6,7] for creation of nanoparticles in oxide. In this paper, we showed formation of metal nanoparticles in a thin oxide film, nonlinear optical property of metal ion implanted glass and tuning of absorption peak of surface plasmon resonance by double metal negative-ion implantation. In addition, application of metal nanoparticles for improvement of photocatalytic property of titanium dioxide (TiO2) was investigated. IMPLANTED PROFILE AND CREATED NANOPARICLES Silver negative ions were implanted at 30 keV to a thermally grown 50-nm-thick SiO2 film on a n-type Si substrate with various doses of 1x1015, 1x1016 and 1x1017 ions/cm2 by a negative ion implanter with an RF plasma sputter-type heavy negative ion source [8,9]. The current density and residual gas pressure during the implantation were about 2µA/cm2 and less than 1x10-4 Pa, respectively. The projected range of Ag atoms for the implantation energy of 30keV is calculated to be 25 nm in SiO2 (amorphous, 2.2 g/cm2) by TRIM and it corresponds to a half of thickness of the SiO2 film. The calculated depth profiles of implanted Ag atoms by TRIM-DYN [10] were shown in Fig.1. The projected range for relatively low dose cases is almost the same as the value calculated by the simple TRIM and the depth profiles showed a Gaussian distribution. The Ag concentration at the peak is 0.8 at. % and 8 at. % in SiO2 for doses of 1x1015 and 1x1016 ions/cm2, respectively. For the high dose of 1x1017 ions/cm2, the implanted Ag atoms are expected to be almost uniformly distributed with 20 30 at. % from the surface to over 20 nm, and this 10 Concentration Atomic (arb. ratiounit) INTRODUCTION 0 10 –1 10 –2 10 –3 10 –4 1x10 TRIM–DYN 30 keV–Ag → SiO2 17 1x10 16 1x10 0 15 20 40 Depth ( nm ) Depth (nm) 60 FIGURE 1. Depth profile of implanted Ag atoms in amorphous SiO2, simulated by TRIM-DYN. CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan © 2003 American Institute of Physics 0-7354-0149-7/03/$20.00 639 Al-layer Al-layer Al-layer SiO2 SiO2 SiO2 Si sub. - 15 (a) Ag : 1x10 2 ions/cm 25 nm - Si sub. Si sub. 16 (b) Ag : 1x10 2 - ions/cm 17 (c) Ag : 1x10 ions/cm2 FIGURE 2. Cross-sectional TEM images of Ag-implanted 50-nm-thick SiO2 film after annealing at 500oC. profile different from the Gaussian distribution is due to sputtering effects. OPTICAL PROPERTIES OF NANOPARTICLE-EMBEDDED OXIDES Oxides, especially transparent glass in visible light region, including metal nanoparticles showed characteristic absorption due to surface plasmon resonance of nanoparticle. Therefore, one can estimate the existence of nanoparticles, their element and particle size from comparison of measured and calculated optical absorption properties. Fig.3 (a) shows optical density, i.e., absorption property, calculated based on Mie theory [11] for silica glass including spherical silver nanoparticles with different radius. In this case, a clear absorption peak appears at 640 Ag 40keV 5E16 Silica–glass Anneal in Ar Anneal Temp. 600℃ at 150℃~800℃ 300℃ 450℃ 0.5 Optical density Optical density 2.0 – Mie–scattering Theory Ag Particle radius 1~15nm r = 10nm nglass=1.45 r =15nm 1.0 r = 1nm 0.4 150℃ 0.3 0.0 1.5 2.0 2.5 3.0 3.5 4.0 700℃ as impla. 0.2 0.1 800℃ 0 1.5 Photon enegy (eV) 2.0 2.5 3.0 3.5 4.0 Photon energy ( eV ) (a) Calculated absorption (b) Measured Absorption FIGURE 3. Optical absorption properties of silica glass including Ag nanoparticles and Ag- implanted silica glass. 0.4 0.05 Mie–scattering Theory Particle Radius 1~10nm nglass=1.50 Cu OPTICAL DENSITY 0.04 Optical Density Fig. 2 shows cross-sectional TEM image after annealing at 500oC for Ag-implanted 50-nm-thick SiO2 film with various doses. In the low dose case of Fig.2(a), the Ag nanoparticles with diameter of 2 - 3 nm appeared in the center region of 16 - 34 nm in depth. For the sample of 1x1016 in Fig. 2(b), Ag nanoparticles with various sizes in 2 - 6 nm in diameter located at region from 24 to 37 nm in depth. In the high dose case of Fig. 2(c), Ag nanoparticles with various sizes in 4 - 10 nm were observed from the surface to 24 nm in depth. The location and size of formed Ag nanoparticles were considered to be well related to the calculated profiles. From the cross-sectional TEM images of the low dose sample after annealing at various temperatures, the followings were found. Ag nanoparticles were formed even in as implanted and the number of particles increased together with their sizes as increasing in the annealing temperature. After annealing at 800oC, the Ag particles diffused in the whole films. As a result, the location and size of nanoparticles changed with well accuracy according to the implantation energy, dose and subsequent annealing temperature. Thus, the distribution of created nanoparticles agreed well with the estimated profile of implanted atoms, and the size was almost determined by the concentration. 0.6 3.0 0.03 r=10 nm 0.02 0.01 0.3 – 2 Cu 30keV 1E17 ions/cm Soda–lime glass plate Anneal in Ar As impla. 150℃ 0.2 200℃ 300℃ 0.1 400℃ r=1 nm 0 1 2 3 4 Photon Enegy (eV) (a) Estimated profiles 5 0 1.5 2.0 2.5 3.0 3.5 4.0 PHOTON ENERGY ( eV ) (b) Measured Absorption FIGURE 4. Optical absorption properties of soda-lime glass including Cu nanoparticles and Cuimplanted soda-lime glass. photon energy of 3.1 eV (about 400 nm in wavelength). As increasing in radius of nanoparticle, the absorption peak becomes narrower. Fig. 3(b) show the measured absorption for Ag negative ion implanted silica glass at 40 keV and 5x1016 ions/cm2; for various annealing temperatures. The absorption peak appeared near 3.1 eV even in as-implanted samples. The peak became narrower as increasing in annealing temperature. In the case of Cu negative ion implantation into soda-lime glass, calculated and measured optical properties are shown in Figs. 4(a) and 4(b), respectively. The implantation conditions were 30 keV and 1x1017 ions/cm2. The soda-lime glass implanted Ag ions showed surface plasmon resonance (SPR) absorption 10 1 – REFLECTIVITY (arb.unit) Cu 30keV into soda glass 17 2 1× 10 ions/cm 10 10 0 –1 10 –1 10 0 2 PUMP INTENSITY (MW/cm ) FIGURE 5. Reflectivity of incident light as a function of intensity of pump light in the degenerated four wave mixing method with a laser of 532 nm. peak near 2.1 - 2.2 eV. This means the soda-lime glass included Ag nanoparticles. The nonlinear properties of Cu negative-ion implanted soda-lime and silica glasses were measured by using the degenerated four wave mixing method [12] with 532 nm (2.33eV) laser. The conditions of Cu negative ion implantation were 30 keV, 1x1017 ions/cm2 and no anneal. The measured reflection intensity as a function of pump intensity is shown in Fig. 5 for Cu-implanted soda-lime glass as an example. This sample was found to have the 3rd order nonlinear susceptibility because the reflectivity was proportional to the square of pump intensity. Then, the large 3rd order susceptibilities, χ(3), were calculated to be 1.3x10-7 and 1.9x10-7 esu for the Cu-implanted soda-lime glass and silica glass, respectively. These values are much larger than that of CdS2, 2.0x10-12 esu. The strong nonlinear property of nanoparticles oxide composite occurs near the wavelength that shows SPR absorption. This requires the tuning of SPR wavelength. We implanted two kinds of metals in silica glass and investigated absorption properties. Cu negative ions, at first, were implanted into a silica glass at 90 keV with 8x1016 ions/cm2. Then, Ag negative ions were subsequently implanted into the same sample Atomic ratio 0.3 Calculated by TRIM–DYN – – Ag 40keV5E16/Cu 90keV10E16 SiO2 : Cu 90keV : Cu of Ag/Cu Optical density 0.4 : Ag of Ag/Cu 0.2 0.1 0 0 50 100 Depth ( nm ) (a) Estimated profiles 150 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 at 40 keV with 5x1016 ions/cm2. Fig. 6(a) shows estimated profiles of implanted Cu and Ag atoms by TRIM-DYN. It predicts that a Cu-Ag mixed layer is formed in surface layer and that Cu rich layer is located in its deeper depth. Fig 6(b) shows the optical absorption properties of the sample at various annealing temperatures. The absorption peak of SPR appeared at 2.7 eV and 2.8 eV in as-implanted and after annealing at 300oC, respectively. Thus, we obtained SPR absorption peak between 2.2eV for Cu nanoparticle and 3.1 eV for Ag nanoparticles. After annealing at 600oC, the SPR peak shifted to 3.1 eV. The nanoparticles seemed to be covered by Ag atoms on their surfaces as shell-core structure to result the SPR peak at the same position as Ag nanoparticles. Two SPR peaks appeared at 2.2 eV and 3.1 eV for 800oC. This reason is considered as follows. The Ag atoms thermally diffused from such shell-core structure nanoparticles, Cu nanoparticles of the core then appeared. We concluded from this experiment that the wavelength of SPR absorption is tunable by the multi-element ion implantation. Nonlinear property is important in developing optical devices. Negative-ion implantation technique for oxide materials was found to be useful. Besides, by using nonlinear property, the 3rd harmonic wave can be generated. This conversion from low energy wave to high energy one is a useful property for activation of photocatalytic materials as well as existence of metal nanoparticles. IMPROVEMENT OF PHOTOCATALYTIC PROPERTY Titanium oxide is a well known photocatalytic material and the improvement of its photocatalytic efficiency and its activation by visible light are desirable. When metal nanoparticles are formed in a surface layer of rutile TiO2, the 3rd order harmonic wave of SPR light is generated around particles. This high-energy light is expected to activate surrounding rutile to form hole and electron pairs. Besides, the metal nanoparticles serve as electron acceptors to reduce recombination probability of holes and electrons. – Ag (40keV 5E16) – / Cu (90keV 8E16) → Silica Anneal in Ar 600℃ 300℃ as impla. 800℃ 1.5 2.0 2.5 3.0 3.5 4.0 Photon energy ( eV ) (b) Measured Absorption FIGURE 6. Depth profiles and absorption properties for Cu and Ag double negative-ion implanted silica glass. 641 In order to investigate the possibility of improving the photocatalytic property of rutile TiO2, we implanted silver negative ions into rutile TiO2 at 65 keV. Fig. 7 shows optical absorption properties measured for Ag-implanted rutile TiO2 samples; (a) for as-implanted sample with various doses and (b) for after annealing of Ag-implanted rutile at 5 x 1016 ions/cm2, respectively. All Ag-implanted titania samples showed absorption peaks near 2.6 eV, while the background optical density in the whole range of 1.5 - 3 eV was apparently increased with an increase in the dose. For annealing effect, the absorption peak 0.5 0.5 – 17 1x10 ions/cm 2 16 7x10 16 5x10 0.3 16 3x10 0.2 1.5 2.0 2.5 3.0 0.3 16 The irradiation conditions through the UV filter were 10,000 LUX in visible light and 0.0 µW/cm2 in a UV flux meter over 365-nm wave. After irradiation for 19 hours, the Ag-implanted sample of 3x1016 ions/cm2 annealed at 500oC showed better property by 6.7 times than that of the original rutile. 2 Ag 65keV 5x10 ions/cm o Annealed in Ar at 300 – 500 C 0.4 Optical density Optical density 0.4 – 2 Ag 65keV 2 uA/cm as–implanted with does of as implanted o 300 C o 0.2 400 C 0.1 500 C o 0.0 1.5 Photon energy ( eV ) 2.0 2.5 3.0 Photon energy (eV) (a) Implanted dose (b) Annealing temperature FIGURE 7. Optical density spectra of Ag-implanted rutiles for various doses and annealing temperatures. shifted to the low photon energy, i.e., from 2.6 eV to 2.0 eV with an increase in annealing temperature to 500oC, while the background absorption gradually decreased. The largest absorption peak was obtained at 2.30 eV at 300 oC. Although as-implanted samples showed the background absorption due to implantation damage, the absorption due to damage was decreased in the annealed sample. The characteristic absorption peak at 2.1 eV well agreed with the calculated SPR peak near 2.1 eV by Ag nanosphere. Therefore, the Ag nanoparticles were formed in the surface region of the rutile. The size of Ag nanoparticles is considered to be very small about 1 - 2 nm in radius from comparison between measured and calculated absorption profiles. We evaluated photocatalytic efficiency of Ag-implanted rutiles by using decolorization method of methylene-blue under irradiation of a fluorescent light (9600 LUX in visible light and 2.7 µW/cm2 in UV components) in a dark box. The photocatalytic efficiencies of Ag-implanted samples are shown in Fig. 8, where the relative efficiency indicates the efficiency that is normalized by that of unimplanted rutile. The as-implanted and annealed samples at below 300 oC showed worse photocatalytic properties than the original rutile due to implantation damage. The Ag-implanted rutile with 3 x 1016 ions/cm2 had the best efficiency of 2.2 after annealing at 500oC. Relative efficiency (arb. units) The fluorescent light includes UV components. We used a UV cut filter with cut-off wavelength of 400 nm. 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 TiO2 – 65keV–Ag implanted Anneal in Ar 3x10 16 1x10 17 16 5x10 unimplanted TiO2 0 100 200 300 400 500 o Anneal temperature ( C ) FIGURE 8. Relative photocatalytic efficiencies of Ag-implanted rutiles in decolorization test of methylene blue by irradiation of fluorescent light, where the efficiency is normalized by the efficiency of the unimplanted rutile. 642 CONCLUSIONS Negative-ion implantation could implant metal atoms into insulators of oxides with precise controls of depth and concentration. The size of particles can be changed by the concentration. The glasses including nanoparticles showed large nonlinear susceptibility. The resonant wavelength could be tuned by subsequent implantation with two different ions. Besides, in the ion beam modification of rutile TiO2 by Ag negative-ion implantation, Ag nanoparticles were formed in the surface layer of rutile. The improved photocatalytic efficiencies with 2.2 times larger was obtained by Ag negative-ion implantation and subsequent anneal at 500oC. Under UV-cut light, the Ag-implanted rutile showed 6.7 times larger photocatalytic efficiency. ACKNOWLEDGMENT The authors are grateful to Dr. Masayoshi Nagao and Miss Hiromi Yamauchi, National Institute of Advanced Industrial Science and Technology (AIST) at Tsukuba, for their assistance in the TEM observation, and thank Dr. Naoki Kishimoto and Dr. Yoshihiko Takeda, Nanomaterials Laboratory of National Institute for Materials Science at Tsukuba, for the measurement of nonlinear optical property by degenerated four wave mixing method. REFERENCES 1. R.F. Haglund Jr., Mater. Sci. Eng. A253 (1998) 275. 2. Y. 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