Fe 2 O 3 -SiO 2 Multi Coating of TiO 2 Powder Treated in Atmospheric Pressure Glow Plasma

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1 Extended Summary 本文は pp Fe 2 O 3 -SiO 2 Multi Coating of TiO 2 Powder Treated in Atmospheric Pressure Glow Plasma Masuhiro Kogoma Member (Sophia University, m-kogoma@sophia.ac.jp) Atsushi Takeda Non-member (ISI Corporation, nanobiojj@yahoo.co.jp) Kunihito Tanaka Non-member (Sophia University, tanaka@ch.sophia.ac.jp) Keywords : TiO 2, sub-micron powder, plasma treatment, UV-cosmetics, atmospheric pressure glow plasma Surface modification of fine particles has been receiving much interest in recent years, especially in the field of the synthesis of advanced materials. This is also very important in actual processes of powder handling such as dispersion, mixing, collection and recycling. Recently, some works about surface modifications of powders using plasma technology have been reported. However, in all these studies, the glow discharge was generated in the low pressure reactors. Because the powder treatments in the low pressure plasma system have some difficulties such as with the powder handling and contamination in the pumping oil of the pumping system, powder treatments in low pressure plasma systems are not suitable for large industry. On the other hand, a new technology: the atmospheric pressure glow plasma, has been developed to attain homogeneous treatment in the atmospheric pressure using He-diluted-gas mixing system. The atmospheric pressure glow plasma method may reduce the cost of production of the plasma surface treatment. In this paper, we will demonstrate this technology for powder treatment as utilized for production of cosmetics. Some kinds of pigments that are used for cosmetics give rise to a skin irritation problem when they contact the skin directly. TiO 2 is an UV-reflective white pigment powder for cosmetics. But naked TiO 2 is a kind of photosensitive catalyst, so the powder easily attacks a human skin surface and oxidizes oil in the sweat as a squalene to produce peroxy-organic compounds that may cause an allergy or cancer. So we need to avoid such effects caused by the catalytic ability of TiO 2 particles. In this experiment, we will try to develop some SiO 2 coating on the TiO 2 powder pigments by means of atmospheric pressure glow discharge using TEOS adsorbing and plasma oxidation to prevent the oxidation of squalene oil in the UV irradiation. On the other hand, the SiO 2 -coated TiO 2 powders will have not only a very high refractive index for the UV region but also high transparency for the visible and IR regions. For such reasons, silica-treated powders will have too white a tint to use for the cosmetics and cannot prevent IR penetration through the powder, which is also a cause of human skin irritation. We can expect that some metal oxide coating can achieve wide band light absorption in TiO 2. At the second stage, we will try to obtain a thin deposition layer of iron between the TiO 2 surface and the silica layer to eliminate the photo-catalytic ability and to suppress the wide band light transparency of bulk TiO 2. Finally, we measure the catalytic effect and light absorption of the new material; iron oxide-coated silicacoated TiO 2. Figure 1 shows the schematic of discharge apparatus to treat powder. The mean diameter of TiO 2 particles is two hundred nm; Fig. 1. Schematic of discharge apparatus they contain 80% of anatase and 20% of rutile (Toho Titanium Corporation, HT-170R). For iron oxide and silica coating, we used iron acethyl acetonato(iaa) and tetra ethoxysilane(teos). In this study, we employed the Adsorb and Dry method to achieve the coated surface. The discharge conditions are as follows: discharge frequency, MHz; power, kw; O 2 /He gas flow rate, 100 ml min -1 /10 l min -1. Discharge conditios were fixed to obtain maximum oxidation ability of the powder. Treating rate is about 2 g min -1 for the powder. To examine the photo catalyst ability of the powders, we irradiated the squalene oil that contained the powder samples (0.4 g in 4 ml of squalene) by UV in a Pyrex glass bottle. After 1 hour UV-irradiation by Xe lamp, the gas products in the bottle were measured by GC-MS (Shimazu QP5050). In the squalene, untreated TiO 2 produces many kinds of organic oxides such as ethanol, acetone and long chain oxidant compounds that should be harmful materials for human skin. Also, the oil emits a foul odor. The product concentrations decreased with increasing of the concentration of the iron in the protection layer.. However even for 2.5% IAA adsorbed on TiO 2, we still have a small amount of alcohol in the spectrum. But using silica-coated iron-doped TiO 2, we have almost no signal assignable as an organic oxide in the GC-MS spectra of the UV irradiated squalene. The multi- coated powder shows very fine and flesh like-color with pearl-like glowing. So we obtained very usefull materials for the UV protection cosmetics using new plasma coating technology. -9-

2 Paper Fe 2 O 3 -SiO 2 Multi Coating of TiO 2 Powder Treated in Atmospheric Pressure Glow Plasma Masuhiro Kogoma * Atsushi Takeda ** Kunihito Tanaka * Member Non-member Non-member Fe oxide-coated TiO 2 particles for UV protection cosmetics were produced by means of atmospheric pressure glow discharge using IAA(Iron acethyl acetonato). The particles were also coated by a silica protection layer. To examine the UV catalytic ability of Fe oxide -coated TiO 2, we measured a squalene that contained the powders by GCMS for the gas products after the squalene had been irradiated by a Xe lamp for one hour. Using silica-coated TiO 2 that was coated by Fe oxide, we find almost no signals from any harmful organic oxides in the GC-MS spectra of the UV irradiated squalene.the multi- coated powder shows very fine and flesh like-color with pearl-like glowing. Keywords : TiO 2, sub-micron powder, plasma treatment, UV-cosmetics atmospheric pressure glow plasma 1. Introduction Surface modification of fine particles has been receiving much interest in recent years, especially in the field of the synthesis of advanced materials. This is also very important in actual processes of powder handling such as dispersion, mixing, collection and recycling. Recently, some works about surface modifications of powders using plasma technology have been reported. Tsutsui et al. studied surface modifications of pigments using a rotary plasma reactor and reported that oxygen plasma generated acidic groups on the particle surfaces (1). In the case of NH 3 plasma, however, they succeeded in generating basic groups on the surfaces. Wierenga et al. used a microwave plasma fluidized bed reactor for the surface treatment of glass beads and ilmenites (TiFeO 3 ), and demonstrated the characteristics of this reactor as a fluidized-bed and as a plasma reactor (2). Mochizuki et al. studied the plasma oxidation of glass beads and carbon particles by the RF plasma reactor using a Ar-O 2 system. However, in all these studies, the glow discharge was generated in the low pressure reactors (3). Because the powder treatments in the low pressure plasma system have some difficulties such as with the powder handling and contamination in the pumping oil of the pumping system, powder treatments in low pressure plasma systems are not suitable for large industry. On the other hand, a new technology: the atmospheric pressure glow plasma, has been developed to attain homogeneous treatment in the atmospheric pressure using a He-diluted-gas mixing system in dielectric barrier discharge (DBD) (4). The DBD at atmospheric pressure is a simple system to obtain a low temperature non-equilibrium plasma. Thus DBD has been used for ozone synthesis, although DBD using air or oxygen is not a homogeneous discharge but a filamentary discharge. The electron charges on the dielectric surfaces of the opposite electrode limit the increasing of the electric field and avoid the * Department of Chemistry, Faculty of Science and Technology, Sophia University 7-1, Kioi-cho,Chiyoda-ku,Tokyo ** ISI Corporation , Motobuto, Saitama transition to an arc discharge. On the other hands, in He-DBD it is considered that some active species always remain in the plasma and that they construct the easy space to generate a next discharge even in weak strength. The metastable He (He m ) behavior in He DBD plasma is supposed to play an important role in the atmospheric pressure glow plasma generation mechanism because of its high energy level and long life time. Therefore a DBD system using He gas can realize homogeneous discharge in atmospheric pressure. Particularly for the powder treatments, the atmospheric pressure glow plasma method may reduce the cost not only for the production but also on the maintenance of the plasma system lower than those costs in low pressure plasma system, because all of the system parts are in the atmosphere. In this paper, we will demonstrate this technology for powder treatment as utilized for production of new material for cosmetics. Some kinds of pigments that are used for cosmetics give rise to a skin irritation problem when they contact the skin directly. TiO 2 is an UV-reflective white pigment powder for cosmetics. But naked TiO 2 is a kind of photosensitive catalyst, so the powder easily attacks a human skin surface and oxidizes oil in the sweat such as squalene. Oxidized squalene molecules will produce many kinds of peroxy-organic compounds that may cause an allergy or cancer. So we need to avoid such effects caused by the catalytic ability of TiO 2 particles. Some previous studies reported the possibility of powder handling with atmospheric pressure glow discharge (5)-(8). Then, at the first stage in this experiment, we will try to develop some SiO 2 coating on the TiO 2 powder pigments by means of atmospheric pressure glow discharge using TEOS(Tetraethoxysilane) adsorbing and plasma oxidation to prevent the oxidation of squalene oil in the UV irradiation. On the other hand, the SiO 2 -coated TiO 2 powders will have not only a very high refractive index for the UV region but also high transparency for the visible and IR regions. For such reasons, silica-treated powders will have too white a tint to use for the cosmetics and cannot prevent light penetration through the powder, which is also a cause of human skin irritation. We can expect that 電学論 A,127 巻 7 号,2007 年 417

3 some metal oxide coating can achieve wide band light absorption in TiO 2. At the second stage, we will try to obtain a thin deposition layer of iron between the TiO 2 surface and the silica layer to eliminate the photo-catalytic ability and to suppress the wide band light transparency of bulk TiO 2. Finally, we measured the catalytic effect and light absorption of the new material: iron oxide-coated, silica- coated TiO Experimental The schematic of the discharge reactor is shown in Fig. 1. The mean diameter of TiO 2 particles is two hundred nm; they contain 80% of anatase and 20% of rutile (Toho Titanium Corporation, HT-170R). For iron oxide and silica coating, we used iron acethyl acetonato(iaa) and TEOS. In this study, we employed the Adsorb and Dry method to achieve the coated surface (7). Oxgen which mixed in He carrier gas was chosen as the most efficient oxidizer for thin layer on the particles because it can easily dissociate and produce enough oxygen atoms. The discharge conditions are as follows: discharge frequency, MHz; power, kw; He gas flow rate, 10 l min -1 ; O 2 gas flow rate, 100 ml min -1. Treating rate is about 2 g min -1 for the powder. Plasma treatment conditions such as power, oxygen concentration and flow rates were set to obtain maximum oxidation ability in the plasma system (7). If we increase the power beyond 2.5kW, the powder is aggregated in the discharge zone by thermal aggregation phenomenon. When we increased oxygen flow rate higher than 100 ml min -1, oxygen atom concentration which is produced by Penning reaction of He m and oxygen molecule is decreased with increasing of flow rate. Because He m concentration is also decreased with deactivation reaction by corrision with oxygen molecules. To examine the photo catalyst ability of the powders, we irradiated the squalene oil (2, 6, 10, 15, 19, 23 - hexamethyl - 2, 6, 10, 14, 18, 22 - tetracosahexaene) that contained treated powders (0.4 g in 4 ml of squalene) by UV in a Pyrex glass bottle. After 1 hour UV-irradiation by Xe lamp, the gas products in the bottle were measured by gas chromatography-mass spectrometry (GC-MS, Shimazu QP5050). 2.1 Process 1) : Silica Coating on TiO 2 At first we tried to make sure of the possibility of using the Adsorb and Dry method with TEOS to coat SiO 2 on the bulk TiO 2. In this treatment, adsorbed TEOS is partly reacted with naturally adsorbed water on the TiO 2 surfaces to deposit SiO 2. The percentages of SiO 2 on the TiO 2 powders are calculated using the next equation: Y=Xa/100 W/M... (1) Y : SiO 2 wt % on TiO 2 powders X : powder weight/g a : wt % of SiO 2 on the sample powder W : Molecular weight of SiO 2 (60) M : Molecular weight of TEOS (208) TEOS is weighed to make 10% of ethanol solution. The powder Fig. 1. The schematic of the discharge reactor Fig. 2. Schematic of the apparatus to measure the gas reaction products from UV irradiated mixtures of powder and oil 418 IEEJ Trans. FM, Vol. 127, No.7, 2007

4 TiO2 Nano Powder Treatment in Atmospheric Pressure Plasma Fig. 3. Si/Ti atomic concentration ratios on the treated TiO 2 powder surfaces is immersed in the solution, then dried in air ambience. Adsorbed powder is treated in the atmospheric pressure glow plasma in a He-O 2 system to eliminate the organic part from the adsorbed TEOS molecules. 2.2 Process 2) : Iron Oxide Coating TiO 2 powders were pre-treated with an IAA-ethanol mixture to adsorb organic metal molecules on the surfaces of the particles. The amounts of IAA in ethanol are calculated using the same Eq. (1) as that used in TEOS adsorption. After the evaporation of the ethanol, the IAA-adsorbed powders are treated to obtain iron oxide -coated TiO 2 using atmospheric pressure glow plasma in a He-O 2 system to eliminate the organic part from the adsorbed IAA molecules. 2.3 Process 3) : Silica-Iron-TiO 2 Multi Coating After process 2, the Fe oxide coated TiO 2 powders were pre-treated with TEOS (tetraethoxysilane) - ethanol mixture aero - sol to adsorb TEOS on the surfaces of the particles. After the ethanol evaporation, the powders are oxidized in atmospheric pressure glow plasma. Using the plasma treatment, we deposited SiO 2 on the iron oxide coated TiO 2 surface as an inorganic fine layer. The thickness of SiO 2 deposited can be controlled by the concentration of TEOS in the solvent ethanol during the adsorbing process. 3. Results and Discussion 3.1 Process 1) Figure 3 shows relative intensity of Si 2p and Ti 2p spectra on the particles, measured using XPS analysis: (a)untreated, (b)tio 2 that adsorbed TEOS, (c)tio 2 that adsorbed TEOS and then was oxidized in the He-O 2 plasma. We found Si 2p and Ti 2p spectra in (b), but after plasma oxidation in (c) the Si atomic concentration is increased. This result shows that the surface layers of TEOS-adsorbed TiO 2 particles are mixed with silica which was produced by hydrization reactions with water and with residual organic substances in the layer. Normally, such hydrization reactions with water can not occur completely in the solid layer. Figure 4 shows O 1s spectra of the powders. In Figure 4, sharp peaks of O 1s were obtained in the spectra of (a), (b) and (c). In the spectrum of (a) only one peak (529.9 ev) assigned to TiO 2 existed. On the other hand, in the spectrum of (b), the peak (532.5 ev) assigned to SiO 2 newly appeared while the peak assigned to TiO 2 decreased. Therefore, we concluded that the surface of (b) was coated with SiO 2 but the coating coverage was not enough. The surface of TiO 2 is still partly covered by half-dissociated TEOS molecules that are reacted with adsorbed water on the surface of TiO 2. In the spectrum of (c), the peak assigned to TiO 2 has almost Fig. 4. XPS spectra of O 1s: (a) untreated TiO 2, (b) TiO 2 that adsorbed 10wt%TEOS and (c) Plasma oxidation of TiO 2 that adsorbed 10wt%TEOS Fig. 5. TEM photograph of TiO 2 treated by SiO 2 in plasma oxidation 電学論 A,127 巻 7 号,2007 年 419

5 disappeared and the shape of the peak looked like that of pure SiO 2. So, we could confirm that the surface of (c) was almost completely coated with an inorganic SiO 2 layer. As one can see in Fig. 5, a homogeneous thin SiO 2 layer (1-2 nm in thickness) is found without any pinholes on the particles. So we attained a very homogeneous SiO 2 layer with the TEOS Adsorb and Dry method on the particles using one path plasma reactor. 3.2 Process 2) Figure 6 shows the XPS Fe 2p spectra of IAA-treated TiO 2 particles oxidized in the O 2 /He plasma. In the figure, we can find Fe 2p 1/2 and Fe 2p 3/2 peaks assigned as Fe ion, but the binding energies of Fe 2+ and Fe 3+ ions are very close to each other. So it is difficult to define which ion is on the TiO 2. Because the particle color was changed from white to beige, we suppose that Fe 2+ should be deposited on the surface. The amount of deposited Fe ion can be controlled by the concentration of IAA in the solvent ethanol during the adsorbing process. Elemental concentrations of Fe and Ti that were measured by XPS were about the same order. So the Fe oxide layer should be very thin. But more thick Fe oxide layer is not desirable as the color of the cosmetic material. Figure 7 shows the UV-visible reflection spectra of naked TiO 2 ; IAA 1% adsorbed TiO 2 ; adsorbed, then plasma -treated TiO 2 ; and pure IAA powder. Naked TiO 2 shows a high and flat reflectivity from near IR region to UV region. Light incident to the surface partly penetrates into the bulk and is absorbed in the solid, then it reflects to the surface boundary, so the reflection spectra contain the absorption characteristics. IAA-adsorbed TiO 2 shows small peaks attributed to adsorbed IAA on the TiO 2, but such peaks are lost in the plasma-irradiated IAA itself. It seemed that plasma-treated IAA on the TiO 2 was completely oxidized to inorganic Fe ions. Figure 8 shows transmittance of the UV-visible light through the powders that are dispersed in KBr disc. For all wave lengths from 300 nm to 600 nm, naked or untreated TiO 2 curves show relatively higher transmission than that of plasma-treated powder coated by Fe oxide. Paticularly, the treated one shows almost no transmittance under 400 nm in the UV zone, because Fe ions on the TiO 2 particles may cut off the UV zone effectively. On the other hand, naked TiO 2 can easily transmit the UV light that causes the sun burn or skin irritation on the human skin. UV light irradiation of the treated powders has been done to verify the human oil catalytic oxidation effect in the mixture of the sample powder and squalene oil. Figure 9 shows the GCMS spectra of gas products in squalene which contained the powders after it had been irradiated by an Xe lamp for one hour. As shown in Fig. 9, untreated TiO 2 produces many kinds of organic oxides such as ethanol, acetone and long chain oxidant compounds that should be harmful materials for Reflection / % IAA TiO 2 TiO 2 +IAA TiO 2 +IAA after Plasma treatment Wavelength / nm Fig. 7. Reflection spectra of TiO 2; IAA adsorbed TiO 2; adsorbed, then plasma-treated TiO 2; and pure IAA powder TiO 2 12 Transmittance / % TiO 2 +IAA after plasma treatment Wavelength / nm Fig. 6. XPS Fe 2p spectra of IAA-treated TiO 2 particles oxidized in the O 2/He plasma Fig. 8. Transmittance of the visible light through the powders that are dispersed in KBr discs 420 IEEJ Trans. FM, Vol. 127, No.7, 2007

6 TiO2 Nano Powder Treatment in Atmospheric Pressure Plasma Intensity (a.u.) Ethanol Acetone Untreated TiO 2 IAA 1.0 % IAA 2.5 % IAA 2.5 % + SiO 2 Squaren only C 5 H 10 O Retention time / min Fig. 9. GC-MS spectra of gas products in squalene which contained the powders after one hour of irradiation by Xe lamp Fig. 10. Molecular structure of squalene human skin. Also, the oil emits a foul odor. The product concentrations decreased with increasing of the concentration of the iron in the layer. However even for 2.5% IAA adsorbed on TiO 2, we still have a small amount of alcohol in the spectrum. But using silica- iron-multi coated TiO 2 which is produced in process 3), we have almost no signal assignable as an organic oxide in the GC-MS spectra of the UV irradiated squalene, except for a very small amount of ethanol signal. Figure 10 shows the molecular structure of the squalene. If the particle surface is exposed to naked TiO 2, squalene molecules will be oxidized, because UVirradiated TiO 2 has strong ability to oxidize organic material, but if the surface is covered by some other material such as Fe 2 O 3, the reaction will be inhibited. When we increased the adsorption concentration of IAA on the particle, the occupied surface area ratio of TiO 2 to Fe 2 O 3 will be decreased. This is a reason why all the peaks in GC-MS are decreased with increasing of the adsorption concentration of IAA. In the molecular structure of squalene, the top of the structure has two methyl groups. Because of the existence of the small-sized holes in the layer, whose size is about the same size as the top of the molecule, only the tops of the molecules can penetrate to the TiO 2 surfaces, where they oxidize to produce ethanol or acetone. So the layer which was treated with high concentrations of IAA makes low reactivity on the surfaces. But still we have a small catalytic effect even on the material with 2.5% IAA condition. It is well known that the Fe ion is a kind of metal catalyst for some chemical reactions, such as nitrogen conversion reaction. So we need a more inactive material, such as SiO 2, on the Fe oxide-coated surface to prevent oxidation reactivity. We obtained almost no signal in the GCMS spectra when the particles were covered with a silica layer. Normally the anatase-type TiO 2 crystals have small rectangular open spaces, a few 10-1 nm in size, in the structure. It seemed that the iron ions are partly inserted into such open spaces of the TiO 2 lattice. However, the crystal lattice of iron-doped TiO 2 is the same as that of untreated one as measured by XRD. So we could not infer whether the iron ions are in solid solution in TiO 2. The treated powder shows very fine and flesh like-color with pearl-like glowing, that should be caused from a multi-reflection effect from the thin layer boundaries of silica-iron oxide-tio 2. Especially, the powder can cover the skin furrows and prevent easy drop-off of the powder from the skin surface due to sweat because of the high reflection for the incident light in the skin furrows and the strong ability of the nano powder to absorb the skin oil. 4. Conclusion Fe-Si oxide-multi coatings of TiO 2 particles were attained by means of atmospheric pressure glow discharge using IAA, TEOS adsorbing and plasma oxidation method. The powder surface oxidation were successfully done in the atmospheric glow plasma using O 2 /He system. The particles which are double coated by the metal oxides such as the Fe 2 O 3 -SiO 2 layer shows elimination of the UV catalytic ability of TiO 2. Moreover, the treated powder shows very fine and flesh colour with pearl-like glowing. Such effects will be attributed to the multi reflection of the incident light. The double-coated TiO 2 powders will provide many functional contributions for the future cosmetics. (Manuscript received Aug. 7, 2006, revised Jan. 19, 2007) References (1) K. Tsutsui, K. Nishizawa, and S. Ikeda : J. Coatings Technol.,Vol. 60, pp (1988) (2) C. R. Wierenga and T. J. Morin : AIChE J., Vol.35, pp (1989) (3) Y. Mochizuki, S. Ono, S. Teii, and J. S. Chang : Adv. Powder Technol.,Vol.4, pp (1993) (4) S. Kanazawa, M. Kogoma, T. Moriwaki, and S. Okazaki : J. Phys. D: Appl. Phys., Vol.21, pp (1988) (5) T. Yokoyama, M. Kogoma, S. Kanazawa, T. Moriwaki, and S. Okazaki : J. Phys. D: Appl. Phys., Vol.23, pp (1990) (6) T. Yokoyama, M. Kogoma. T. Moriwaki, and S. Okazaki : J. Phys. D: Appl. Phys., Vol.23, pp (1990) (7) T. Mori, K. Tanaka, T. Inomata, A. Takeda, and M. Kogoma : Thin Solid Films, Vol.316, pp (1998) (8) S. Ogawa, K. Kiuchi, K. Tanaka, and M. Kogoma : Proc. 18th Symp. Plasma Process., pp (2001) Masuhiro Kogoma (Member) was born on August 3 rd, He received the doctoral degree in applied chemistry from Sophia University in He is a Professor of the Faculty of Science and Technology of Sophia University. His major field of study is on the plasma chemistry at atmospheric pressure. 電学論 A,127 巻 7 号,2007 年 421

7 Atushi Takeda (Non-member) was born on April 10 th,1948 Graduated from the Institute of Mining Geology, Akita University, Japan. He is President of ISI Inc.(Integrated Surface Interaction Inc.) He is working in powder and cosmetics technology. Kunihito Tanaka (Non-member) was born on June 17 th, He received the M.S and Ph. D degree in applied chemistry from Sophia Unibersity in 1995 and 2004, respectively. He is presently a assistant professor of Sophia University. 422 IEEJ Trans. FM, Vol. 127, No.7, 2007