24P00424P004-O
Control of Crystalline Structures of Titanium Dioxide
in a Low-Pressure O2/Ar plasma

低圧Ar/O プラズマによる二酸化チタンの結晶構造制御
2Yousuke Yamazaki, Satoru Iizuka
山崎陽介，飯塚哲
Department of Electrical Engineering, Graduate School of Engineering, Tohoku University,
Aoba 6-6-05, Aramaki, Aoba-ku, Sendai 980-8579, Japan
980-8579
6-6-05東北大学大学院工学研究科，〒

仙台市青葉区荒巻字青葉

In this study, we investigated a production of titanium dioxide (TiO2) on a surface of Si substrate by using
a low-pressure O2/Ar discharge with coaxial electrodes. We observed depositions of thin films made ofTiO2 with anatase and rutile, the property of which was controlled by the discharge power, mixing
flow-rate ratio, and total pressure of the working gases. We observed also spherical particles with a scale ofa few 100 nm, attached on the Si surface. Shape of nanoparticles could be changed from baroque tospherical by controlling the discharge condition. From the electron refraction patterns it turned out that thenanoparticles with a size of 20-30 nm were made of anatase.

1. Introduction
Titanium dioxide TiO2 is a wide band-gapsemiconductor with superior chemical stability. For
this reason, TiO2 has been used in many fields suchas, for instance, sunscreen agent, white paints, solarcell, and photocatalyst. Today, TiO2 has attracted
much attention for application to energy andenvironmental problem. TiO2 acts as photocatalyst
for cleaning environments by exploiting sunlightenergy. Superior photochemical properties of TiO2
are also applied to photochemical reaction of anelectrode of dye-sensitized solar cells. The
application to medical treatments such as
self-cleaning effect, purification, sterilization, anddeodorization, has also been considered.
TiO2 has typically two crystallites, i.e., anataseand rutile. The former is useful for photoactivities,
and the latter is widely used as white dye powder.
Several types of TiO2 nanostructures have beenreported, such as, for instance, baroque andspherical structures. If the solar cell electrodeconsists of superficial TiO2 nanoparticles, surface
electrode area becomes larger for the reaction,
which may lead to an increase of electric power
generation. For this reason, spherical TiO2 particles
are paid attention for applying to the electrode ofdye-sensitized solar cells.
In this study, we report a control of crystalline
structure of thin TiO2 films, together with a creationof nanostructures, containing spherical particles, bychanging the experimental condition such asdischarge power, mixing ratio, and total pressure ofworking gases O2/Ar.
2. Experimental apparatus and method

Figure 1 shows a schematic drawing of theexperimental apparatus used for plasma sputtering[1]. As a source of Ti, a Ti rod electrode was usedand installed in a ceramic pipe. The flow rates of Ar
and O2 gases were adjusted by mass flow
controllers independently and the mixed gas wasintroduced into a stainless tube. Rectified dc
voltage was supplied to the titanium rod.
Using this system we can change the dischargeparameters such as O2/Ar flow rates, pressure in thedischarge region, and applied voltage. The
discharge took place between the tip of the titanium
electrode and a titanium plate electrode with anopen hole at the center.
We selected the partial pressure ratio andsupplied voltage as a parameter. Typically, O2/Ar=1/1 and 1500 [V] for sample 1, and O2/Ar =1/3
and 4500 [V] for sample 2. Other parameters were
fixed constant through the experiment. The totalpressure of O2/Ar was kept at 0.50 [Torr].

Fig.1. Schematic drawing of experimental apparatus.

3. Experimental results
Deposition experiment was carried out bychanging the discharge parameters. From thedetailed evaluation of the crystallites, it turned outthat anatase seemed to be deposited in outer region
apart from the main discharge. Here, weconcentrated mainly on the case of anataseformation.
Scanning electron microscopy (SEM) images ofthe surface of sample 1 is shown in Fig. 2(a). It wasobserved that many spherical particles of 100~1500 [nm] in diameter were deposited. In addition,

the surface of sample 2. Distribution of the particles
on the substrate is shown in Fig. 4. Almost noparticles was observed in the center region of thesubstrate. The position of the peak was around 4mm, which was a little bit outer than the edge of theanode hole. The particle deposition position wasshifted outward by the discharge power.

Fig.2. (a)SEM and (b) TEM images of sample1.

4. Discussion
In the case of sample 1, it was found thatspherical particulate was consisted of many
nanoparticles, made of anatase, with a size of 10~20nm. It is considered that these larger particles havegrown like snowballs. Deposited thin films werealso considered to be anatase. From the result ofsample 2, it was shown that the larger particles
were distributed in the outer region. As an origin of
the spherical particles, the growth around the sheaththe particles with a distorted shape also existed atthe periphery. Even on the Si substrate we observeda thin film, which was quite similar to that on thesurface of the spherical particles.
In order to clarify the crystallite of the particles,
transmission electron microscopy (TEM) image ofsample 1 was taken as shown in Fig. 2(b), where
many coagulated nanoparticles with a size of about
10~20 nm were found. This meant that the particles
of a few 100 nm were composed by small
nanoparticles with a size of 10 ~20 nm.

Fig.4 Distribution of particles on thesubstrate of sample 2.

Fig.3 (a) Electron refraction pattern and(b) EDX spectrum of sample1.

edge was considered. They were transported to thesubstrate by a gas flow along the periphery of theplasma column. The particles may grow via
coagulation during the trapping around the sheath.
When the deposition condition was changed, thecomposition rate, the size, and the number ofparticles were changed. The transition between
rutile and anatase was dependent on the discharge
condition. TiO2 anatase seemed to be deposited in alow power discharge. During the growth ofcrystallites, anatase might turn to rutile in the highpower plasma.

Figure 3(a) shows the electron refraction patternof the particles of sample 1. We find several
refraction spots, which are well distributed on thepositions of TiO2 anatase. The atomic components
were also analyzed by EDX spectrum as shown in
Fig. 3(b), where typical peaks corresponding to Ti
and O were observed. Any other atomic
components were not found. Therefore, it wasconcluded that the particles were consisted of TiO2
and the crystalline structure was anatase.
We also found many spherical nanoparticles on

5. ConclusionIt was verified that the crystalline structure of
TiO2 , i.e., anatase and rutile, could be controlled by
the discharge parameters, which also gave an effecton the shape and distribution. We observed
spherical particles of 100-1500 nm, formed by
20-30 nm nanoparticles made of anatase. Particle
distribution depended on the particle size.
Reference[1] T. Muraoka et al., Thin Solid Films, 518, 1012(2009).