ARTICLE IN PRESS
Physica E 27 (2005) 457–461 www.elsevier.com/locate/physe
The dispersion study of TiO2 nanoparticles surface modified through plasma polymerization Feng Zhua,, Jing Zhangb, Zhongxue Yanga, Ying Guob, Hua Lia, Yafei Zhanga a
Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, Research Institute of Micro/Nano Technology, Jiaotong University, 1954 Huashan-Road, Shanghai 20030, PR China b College of Science, Donghua University, 1882 West Yan-an Road, Shanghai 200051, PR China Received 5 January 2005; accepted 10 January 2005 Available online 24 March 2005
Abstract In this study, the surface of TiO2 nanoparticles was modified through plasma polymerization, which is a dry coating method at room temperature. The surface morphology of TiO2 nanoparticles was characterized by high-resolution transmission electron microscope (HRTEM). The dispersion behavior of TiO2 nanoparticles in water and ethyl glycol was investigated by laser size distribution and ultraviolet–visible absorption spectrum. TiO2 nanoparticles were coated with a thin film through plasma polymerization, which prevents the agglomeration and improves the dispersion behavior of TiO2 nanoparticles. r 2005 Elsevier B.V. All rights reserved. PACS: 52.80.Vp; 52.75. Rx; 61.46.+w; 81.15. z Keywords: Dispersion; Plasma polymerization; TiO2 nanoparticles; Coating
1. Introduction Titania nanoparticles are one of the most researched transition metal oxides for various applications including ultraviolet light absorbers, photocatalysts, gas sensors, etc [1–3]. Due to the small dimension, it makes the nanoparticles Corresponding author. Tel.: +86 21 62933527;
fax: +86 21 62823631. E-mail address:
[email protected] (F. Zhu).
agglomerate severely. Therefore, it is a challenge to make nanoparticles disperse at nanoscale in the background material. Surface modification of inorganic powders coated with polymer film could be seen in many areas because of their excellent properties, including improved particle dispersion, better colloidal stability, improved compatibility with the dispersion medium and enhanced mechanical properties [4]. Many ways including solvent evaporation, emulsion polymerization, dispersion polymerization,
1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.01.004
ARTICLE IN PRESS F. Zhu et al. / Physica E 27 (2005) 457–461
graft polymerization and plasma polymerization were often used to achieve the effect of encapsulation [5,6]. Surface modification through plasma polymerization is attractive in these days because of the benign reaction environment and the easily controlled reaction processes [7,8]. In this process, the reactive species created by the high-energy electrons and ions collide with the nanoparticle surface and form a depositing film through the energy loss of the reactive species or chemical bonding of the reactive species with reactive species on the nanoparticle surface. Because the coating film forms a space barrier and is compatible with the background material, the dispersion behavior of nanoparticle is improved. Many cheap monomers can be chosen to do the coating according to the requirement of the substrate material. Besides, if the radio frequency (RF) plasma discharge is pulsely modulated to make the plasma polymerization proceed between ‘‘on’’ and ‘‘off’’ status, the surface chemical structure and the surface energy of the nanoparticles could be changed in a more flexile way to meet the different needs for their surface or interface. In this paper, pulsed RF plasma polymerization was applied to coat a thin film on TiO2 nanoparticles [9,10] and the dispersion behavior of coated TiO2 nanoparticles was studied in different plasma parameters. The surface morphology and chemical structure of the coating film on TiO2 nanoparticles were investigated by HRTEM, and the dispersion behavior of the coated TiO2 nanoparticles was studied by laser size distribution test and ultraviolet–visible light absorption spectrum analysis.
2. Experiment Nanoscale anatase TiO2 nanoparticles with diameters ranging from 10 to 30 nm were selected. The coated TiO2 nanoparticle was performed in a reactor (Fig. 1). RF power (13.56 MHz) is applied through the impedance matching network to one of the concentric ring electrodes around the Pyrex glass reactor, while the other electrode is grounded. The RF power source was supplied by American Advanced Energy Inc., which an automatic matching network used to minimize the
Throttle Valve
458
Dust Trap
x cm Pressure Gauge
Roots Blower Rotary Pump
Pyrex Tube
13.56 MHz RF Generator stirrer Impedance Matching Mass Flow Controllers Porous Plate Motor
Monomer,Gas Supply
Fig. 1. Schematic diagram of fluidized bed plasma reactor for coating TiO2 nanoparticles.
reflection power of the system and to supply in a continuous mode or pulsed mode. The TiO2 nanoparticles were put into the reactor before the plasma treatment, and then pumped down to a base pressure less than 3 Pa. The mixture of monomer acrylic acid (AA) and carrier gas (argon) is introduced into the reactor through a mass flow controller. Under the continuous shearing of the stirrer by stirrer, TiO2 nanoparticles were easily separated and renewed to be exposed to plasma atmosphere for an efficient coating. Finishing the treatment the nanoparticles are collected at the bottom of the reactor. The coating conditions on TiO2 nanoparticles are as follows: input power of 55 W the system pressure of 16 pa, treatment time of 120 min, pulse duty cycle of 10%, pulse treat time of 10 ms.
3. Result and discussion Fig. 2(a) shows the HRTEM image of the original uncoated TiO2 nanoparticles. It was
ARTICLE IN PRESS F. Zhu et al. / Physica E 27 (2005) 457–461
459
Fig. 2. HRTEM image of TiO2 nanoparticles: (a) uncoated and (b) AA plasma polymer coated.
Fig. 3. LS particle size distribution of TiO2 nanoparticles: (a) uncoated and (b) AA plasma polymer coated.
shown that in Fig. 2(a) the crystal TiO2 lattice is quite apparent, with no polymer film coating on the TiO2 nanoparticles. Fig. 2(b) is the HRTEM image of the coated nanoparticles, where an ultra thin film of AA plasma polymer is clearly seen around TiO2 nanoparticles (about 3–5 nm) and the dense film is tightly bound to the nanoparticles. It is shown that a typical amorphous structure uniform film of AA plasma polymer was indeed achieved using the technique. The dispersion behavior of TiO2 nanoparticles was examined by laser size distribution test and ultraviolet–visible light absorption spectrum analysis. Fig. 3 shows the result of the laser size distribution test. It is demonstrated that the mean
particle size of the nanoparticles is 3.824 mm and the particle size is mainly distributed among 0.3–15 mm because of the coagulation of TiO2 nanoparticles in water in Fig. 3(a). After 2 h AA plasma polymer coating, the mean particle size of coagulated TiO2 nanoparticles decreases to 2.267 mm, and the distribution area is 0.04–0.4 mm and 0.4–6 mm in Fig. 3(b). From the laser particle size distribution analysis, it is summarized that the mean particle size of coagulated TiO2 nanoparticles was lowered effectively and dispersion was improved through AA plasma polymer coating. Spectrophotometry was applied to determine the relation between absorbency of TiO2 nanoparticles
ARTICLE IN PRESS F. Zhu et al. / Physica E 27 (2005) 457–461
460
0.8
0.8 (a)
absorbency (%)
0.6 0.5 (b) 0.4 0.3
0.6 0.5 0.4
0.2
0.1
0.1 300
400 500 600 wavelength (nm)
700
800
Fig. 4. 0 h Ultraviolet–visible spectrum of TiO2 nanoparticles: (a) uncoated and (b) AA plasma polymer coated.
in ethyl glycol solution and wavelength. The absorbency of dispersive solution reflects particle size and properties of the dispersed phase. The maximal absorbed wavelength of TiO2 nanoparticles (o50 nm) belongs to the UV region, along with particle size becoming larger, and the maximal wavelength shifts to low wavelength, namely ‘‘Einstein shift’’. The higher value of absorbency of dispersive solution in the UV region (o300 nm) demonstrated the higher value of content of nanoscale particles, which showed that the dispersion of TiO2 nanoparticles improved. It is shown that the absorbency of coated TiO2 nanoparticles was more improved than uncoated TiO2 nanoparticles (o300 nm) in Figs. 4 and 5, which showed that the dispersion behavior of coated TiO2 nanoparticles was improved. Different plasma parameters affected the dispersion of coated TiO2 nanoparticles in ethyl glycol. Fig. 6 showed the ultraviolet–visible absorption spectrum under different discharge conditions and different discharge power (Fig. 6(a) was pulsed discharge and Fig. 6(b) was continuous discharge). The results indicated that the absorbency (o300 nm) of coated TiO2 nanoparticles was improved in pulsed discharge under similar conditions, which demonstrated that the dispersion behavior of coated TiO2 nanoparticles in pulsed discharge was more improved than that of coated TiO2 nanoparticles in continuous dis-
(b)
0.3
0.2
200
(a)
0.7
200
300
400 500 600 wavelength (nm)
700
800
Fig. 5. 24 h Ultraviolet–visible spectrum of TiO2 nanoparticles: (a) uncoated and (b) AA plasma polymer coated.
1.1 1.0
(a)
0.9 absorbency (%)
absorbency (%)
0.7
0.8 0.7 0.6 0.5
(b)
0.4 0.3 0.2 0.1 200
300
400 500 600 wavelength (nm)
700
800
Fig. 6. Ultraviolet–visible spectrum of coated TiO2 nanoparticles in different discharge conditions: (a) pulsed discharge, and (b) continuous discharge.
charge. The discharge power of Fig. 7(a) was 20 W and Fig. 7(b) was 10 W. Fig. 7(a) showed that the absorbency (o300 nm) of coated TiO2 nanoparticles was improved under similar conditions, which indicated that the higher the discharge power, the more improved the dispersion behavior of coated TiO2 nanoparticles. The deposit rate of film on TiO2 nanoparticles surface in pulsed discharge was higher than that in
ARTICLE IN PRESS F. Zhu et al. / Physica E 27 (2005) 457–461
4. Conclusion
0.9 (a)
0.8
In summary, we have deposited AA thin film on the surface of TiO2 nanoparticles through plasma polymerization, which is an effective surface coating method for nanoparticles. It can improve TiO2 nanoparticle dispersion behavior in water and glycol. The optimal condition is the pulsed discharge and high power.
0.7 absorbency (%)
461
0.6 0.5 (b) 0.4 0.3 0.2 200
300
400 500 600 wavelength (nm)
700
800
Fig. 7. Ultraviolet–visible spectrum of coated TiO2 nanoparticles in different discharge powers: (a) 20 W and (b) 10 W.
continuous discharge, because the life span of the reactive species was only several milliseconds in the plasma atmosphere and therefore the film on TiO2 nanoparticles was destroyed in continuous discharge atmosphere all the time, If the discharge interval is equal to the life span of the reactive species, the film was hardly destroyed in the plasma atmosphere. Along with the discharge power being higher, the electron concentration is also higher, which results in the deposit rate of the film becoming high. So the higher the discharge power, the more improved the dispersion of the coated nanoparticles.
References [1] W. Li, S.I. Shah, M. Sung, C.P. Huang, J. Vac. Sci. Technol. B 20 (2002) 2303. [2] I. Ilisz, A. Dombi, K. Mogyorosi, I. Dekany, Colloids and Surfaces A: Physicochem. Eng. Aspects 230 (2003) 89. [3] M. Niederberger, G. Garnweitner, F. Krumeich, R. Nesper, H. Colfen, M. Antonietti, Chem. Mater. 16 (2004) 1202. [4] R. Templeton-Knight, Chem. Ind. 16 (1990) 512. [5] A.W.G. Janssen, A.M. van Herk, A.L. German, Polymer Preprints, 34 (1) (1993) 532. [6] E. Bourgeat-Lami, J. Lang, J. Colloid Interface Sci. 197 (1998) 293. [7] S. Morooka, T. Okubo, K. Kusakabe, Powder Technol. 63 (1990) 105. [8] Y. Luting, S. Wenjie, M. Hezhuo, H. Wei, G. Zhigang, P. Yikang, Key Eng. Mater. 264–268 (2004) 109. [9] D. Shi, S.X. Wang, W.J. Van Ooij, L.M. Wang, J.G. Zhao, Y. Zhou, Appl. Phys. Lett. 78 (2001) 243. [10] W. He, Z. Guo, Y. Pu, Appl. Phys. Lett. 85 (2004) 896.
