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. Takeda, C.G. Lee, N. Kishimoto, Nucl. Instr. and Meth.,
B191 (2002) 422.
3. K. Yano, T. Ishii, T. Hashimoto, T. Kobayashi, F. Murai, K. Seki,
IEEE Trans., ED 41 (1994) 1628.
4. A. Nakajima, T. Futatsugi, N. Horiguchi, H. Nakao,
N. Yokoyama, Tech. Dig., IEDM (1997) 159.
5. H. Tsuji, T. Sagimori, K. Kurita, Y. Gotoh, J. Ishikawa,
(to be published in Surface Coatings and Technology in 2002).
6. H. Tsuji, Y. Toyota, J. Ishikawa, S. Sasaki, Y. Okayama,
S. Nagumo, Y. Gotoh, K. Matsuda, Ion Implantation
Technology-94, Elsevier, New York, 1995, p. 612.
7. H. Tsuji, Y. Gotoh, J. Ishikawa, Nucl. Instr. and Meth., B 141
(1998) 645.
8. H. Tsuji, J. Ishikawa, Rev. Sci. Instrum., 63 (1992) 2488.
9. H. Tsuji, J. Ishikawa, Y. Gotoh, Y. Okada, AIP Conf. Proc., 287
(1994) 530.
10. J.P. Bierzack, Nucl. Instr. and Meth., B 27 (1987) 21.
(TRIM-DYN ver. 4. 0 was provided by I. G. Brown, LBNL,
USA.)
11. R. Ruppin, J. Appl. Phys., 59 (1986) 1355.
12. R.K. Jain, R.C. Lind, J. Opt. Soc. Am., 73 (1983) 647.