Nanostructure Dependent Surface Energy of Silica Nanorod Arrays through Block. Copolymer Templating Processes. Yongbin Zhao, Aihua Chen, Tomokazu ...
Mater. Res. Soc. Symp. Proc. Vol. 1312 © 2011 Materials Research Society DOI: 10.1557/opl.2011.508
Nanostructure Dependent Surface Energy of Silica Nanorod Arrays through Block Copolymer Templating Processes Yongbin Zhao, Aihua Chen, Tomokazu Iyoda* Division of Integrated Molecular Engineering, Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan. ABSTRACT The self-assembled block copolymer films with poly (ethylene oxide) (PEO) and hydrophobic polymetharylate (PMA) with azobenzene mesogen in the side chain, denoted as PEOm-b-PMA(Az)n, were used as the template to prepare hexagonally ordered silica nanorod arrays by immersing the template films in the silicate precursor containing tetraetoxysilane (TEOS). The diameter and the center-to-center distance of the SiO2 nanorod arrays were controlled by selecting the block copolymer with different PEO volume fraction.In addition, the contact angles of different kinds’ solvents for the SiO2 nanorod arrays were characterized. We further found, the diameter and the period distance of silica nanorods are very important factors for controlling the contact angle of different kind’s solvents on the surface of the SiO2 nanorod arrays. INTRODUCTION Self-organization,as a powerful route to the ‘bottom-up’ fabrication of nanostructures for use in optical, optoelectronic, and magnetic storage devices,has been attracted in the world[1-2]. Recently, the self-assembly of block-copolymeric system into aligned, highly ordered arrays that cover the range between approximately 10 and 100nm, makes them ideal candidates as template and scaffolds for the fabrication of nanostructured materials. For example, by using perpendicular PS-b-PMMA cylinder nanopatterns as template, the ferromagnetic Co nanowires with high-density vertical arrays has been fabricated through subsequent direct current electrodeposition[4]. In addition, using the similar process, it also has been demonstrated the replication of nanoscale feature into a variety of materials [5-6], including the formation of metal nanorods, nanoporous metal films, nanoelectrode arrays, etc. Recently, our group has developed a new series of amphiphilic liquid crystalline diblock copolymers consisting of poly(ethylene oxide) (PEO) and poly(methacrylate) bearing an azobenzene mesogen in the side chain as template to prepare high ordered nanoarrays, such as Ag nanodot arrays on the flexible and rigid substrated[7], mesoporous silica nanorods arrays with tunable aspect ratios[8], etc. In this work, the self-assembled block copolymer, denoted as PEOm-b-PMA(Az)n films with normal PEO cylindrical domains was used as the template to prepare hexagonally ordered silica nanorod array by immersing the template films in the silicate precursor containing tetraetoxysilane (TEOS). The diameter and the center-to-center distance of the SiO2 nanorod
449
arrays were further controlled by selecting the block copolymer thin films with different PEO volume fraction.In addition, the contact angles of different kinds’ solvents for the SiO2 nanorod arrays were characterized. We found, the diameter and the period distance of silica nanorods were very important factors for controlling the contact angle of different kind’s solvents on the surface of the different SiO2 nanorod arrays. EXPERIMENTAL DETAILS Preparation of block copolymer films with perpendicularPEO cylindrical orientation The amphiphilic liquid crystalline diblock copolymers PEOm-b-PMA(Az)n, was synthesized by atom transfer radical polymerization (ATRP). The details in synthesis and characterization of the block copolymer were described in our previous publication [9]. The solutions f PEOm-PMA(Az)n block copolymers in toluene or chloroform with 1~5 wt% were spin-coated on the Si wafer with the rotation rate of 2000 rpm and 30 sec, subsequently annealed at 140oC under vacuum for 12 h, the block copolymer films with perpendicular PEO cylindrical orientation were obtained. The thickness of block copolymer films between 100 nm to 500 nm could be controlled by adjusting the condition of spinning-coated and concentration of solution. Preparation of perpendicular aligned silica nanoarrays using block copolymer films with perpendicular PEO cylindrical orientation as template To form the silica nanoarrays on the surface of Si wafer, a silica sol was firstly prepared in the following typical procedure: a mixture of ethanol (5.5 g), TEOS (2.08 g), H2O (0.5 g) and 0.4 g of HCl aqueous solution (0.1 M) was heated at 70 oC for 1 h, and cooling to room temperature to form the silica sol. And then, the PEOm-b-PMA(Az)n films with perpendicular PEO cylindrical orientation as template, were immersed in the silica sol solution at room temperature for 2 h, and washed by de-ionized water, formed the silica/block copolymer nanocomposite films. After dried at 50 oC for overnight, the silica nanoarrays with perpendicular orientation were obtained by calcinated in air at rate of 1oC/min to 550 oC for 6h. Measurement The AFM images were taken with a Digital Instrument Dimension 3000 Scanning Force Microscope. The imaging was conducted in tapping mode using a silicon cantilever with a resonance frequency of 300 kHz [10]. Prior to the recording of FE-SEM images on a Hitachi S-5200 field-emission scanning electron microscope a Pt/Au layer (thickness ~ 2 nm) was deposited onto the surface of the samples using a HITACHI E-1010 ion sputter. Water contact angles were measured by the sessile drop method using a conventional drop shape analysis technique (Cruss DSA 100, Hamburg, Germany). Deionized regent grade water was used for contact angle measurements. Liquid droplets (3ul) were dropped carefully onto the sample’s surface. RESULTS AND DISCUSSION
450
As we all know, the simple process of solvent evaporation could produce highly orderly arrays of cylindrical microdomains oriented normal to the surface in block copolymers with long-range lateral order. In our lab, microphase-separated diblock copolymer films of amphiphilic PEOm-b-PMA(Az)n, consisting of ploy (ethylene oxide) and Poly(methacrylate) bearing an azobenzene segment in the side chain, could be prepared after annealing at 140oC for 12h in vacuum through self-assembling process. Fig. 1 shows the AFM top-images of different kinds of BCP films on the silicon wafer. It can be seen that the PEO cylindrical domains are perpendicular to the substrate and form highly ordered hexagonal structure. In addition, the diameter of the PEO cylindrical domains in the PEO114-PM(Az)45, PEO272-PM(Az)78 and PEO272-PM(Az)116 films is about 11 nm, 20nm, 20nm, respectively. In addition, the period distance between the two PEO cylinder is about 22nm, 34nm and 35nm, respectively.
Fig.1 The AFM images of BCP films: (a) PEO114-PM(Az)45; (b) PEO272-PM(Az)78; (c) PEO272PM(Az)116; (Insert: Relative FFT images, respectively; Scale: 250nm). In addition, both out-of-plane orientations of the hexagonal cylinder phase were applied for various transcription processes. The three kinds of BCP films with different perpendicular PEO cylinders’ diameters and periods as templates were immersed in a tetraetoxysilane (TEOS) precursor solution containing acidic solution for sol-gel reaction, the silicate sol would penetrate into the PEO cylindrical domains with the immersing time increasing. And then, washed by water in order to remove the overlayered sol on the surface of BCP films and dried in oven overnight. After removal of the templates by calcination, silica nanorod arrays with perfect perpendicular orientation were obtained. Fig. 2 shows the FE-SEM images of Silica nanorod arrays with different template. The nanorods’ diameter of silica nanorods from the PEO114-PM(Az)45 (Fig.2a), PEO272-PM(Az)78 (Fig.2b) and PEO272-PM(Az)116 (Fig.2c) films is about 10 nm, 19nm and 20nm and the distance between the nanorods is about 21nm, 35 nm and 38 nm, respectively, which were corresponding to the PEO domain’s diameter and the distance between the PEO cylinders, further indicating that the PEO structure can be easy to transcript to silica nanorod arrays by this method.
451
Fig.2 The FE-SEM images of Silica nanorod arrays with different tmeplate: (a) PEO114PM(Az)45; (b) PEO272-PM(Az)78; (c) PEO272-PM(Az)116; (Scale: 250nm). Fig. 3a shows that the diameter and periodicity of silica nanorod arrays increased slowly by using the different block copolymer as templates. Furthermore, it can be seen from Fig. 3b, all of the water contact angles of the perpendicular aligned silica nanorod arrays were higher than that of silica films (CA. 72.5o), showed hydrophobic properties. Duo to the nanosacle structure of one-dimensional arrays consists of silica nanorod and air, the proportion of water/air interfaces can greatly intensify the water repellency, as shown in the left inserted Cassie’s model of Fig.4a. Based on this Cassie’s equation [11], the theoretical value of the water contact angles were calculated as show in Fig. 3b. With the diameter and the period distance of silica nanorod arrays increasing, the theoretical water contact angles were also decreasing slowly from 134o to 109.3o, with corresponding in the regulation of experimental values. However, duo to the existence of the -OH functional groups on the surface of silica nanorods, the experimental water contact angles of silica nanorod arrays was lower than the theoretical values.
Fig. 3 (a) The diameter and periodicity of silica nanorod arrays by different block copolymer templates; (b) the theoretical and experimental water contact angles of silica nanorod arrays based on the cassie’s equations. Interestingly, Fig.4a shows that the contact angles of ethanol for different silica nanorod arrays were lower than that of silica films (CA. 20o), showed the superhydrophilicity. However, when non-polar solvents, such as toluene and hexane instead of water were used to measure the
452
contact angles of the silica nanrod arrays, both of the contact angles of silica nanorod arrays obtained from different BCP films was lower than the silica films in Fig. 4a, showed superlipophilicity. Toluene and hexane, as the sedimentary solvents were easier to immerse into the gap in the silica nanrods than water, so the wettability could be further improve by the roughness of substrate [12]. Based on the above deduction, the wenzel model [13] was proposed to describe the toluene absorbance of silica nanorods, as shown in the right inserted model of Fig.4a. Moreover, base on the surface tensions and contact angles of four kinds of solvents on the surface of silica nanrod arrays, the surface free energy of the silica nanrod arrays from the different block copolymer films can be calculated by the Fowks’s equation [14], as shown in Fig. 4b. With the diameter and the period distance of silica nanorod arrays from the PEO114-PM(Az)45, PEO272-PM(Az)78 and PEO272-PM(Az)116, respectively, increasing, the surface free energy of the silica nanorod arrays also grew up slowly, so the water contact angles decreased slowly, indicating that the diameter and the period distance of silica nanorods were very important factors for controlling the contact angle of different kinds solvents on the surface of the different materials.
Fig. 4 (a) The contact angles of silica nanorod arrays from the different BCP films (Insert figure is the models of contact angles); (b) the surface free energy of the silica nanorods based on the Fowks’s equation. CONCLUSION In this study, the amphiphilic liquid crystalline block copolymers films with perpendicular cylindrical domains were prepared by spin-coating toluene or chloroform solutions on the surface of silicon wafers, followed by annealing at 140oC for 2h in vacuum. In addition, the perpendicular aligned silica nanorod arrays were fabricated by applying the sol-gel method for the orientation-controlled template films. Furthermore, the diameter of silica nanorods and distance of inter-rods also could be controlled by adjusting the molecular weight of PEO and PMA (Az) segment in block copolymer.In addition, with the diameter and the center-to-center distance of the SiO2 nanorod arrays were adjusted, the contact angles of different solvents also
453
cloud be controlled, showed the hydrophobic properties. ACKNOWLEDGMENTS Y. Zhao (Ph.D) acknowledges the support of the postdoctoral fellowship for foreign researchers from the Japan Society for the Promotion of Science (JSPS). REFERENCES 1. K.J. Vahalar, nature 424, 839 (2003). 2. J. Y. Cheng, C. A. Ross, V. Z. H. Chan, E. L. Thomas, R. G. H. Lammertink, G. J. Vancso, Adv. Mater. 13, 1174 (2001). 3. S. Krishnammoorthy, Y. Ferbig, C. Hibert, R. Pugin, C. Hinderling, J. Brugger, H. Heinzelmann, Nanotech. 19, 285301 (2008). 4. T. Thurn-Albrecht, J. Schotter, G. A. Kästle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C. T. Black, M. T. Tuominen, T. P. Russell, Science 290 , 2126 (2000). 5. H. C. Kim, X. Q. Jia, C. M. Stafford, D. H. Kim, T. J. McCarthy, M. Tuominen, C. J. Hawker, T. P. Russell, Adv. Mater. 13, 795 (2001). 6. K. Shin, K. A. Leach, J. T. Goldbach, D. H. Kim, T. P. Russell, Nano Lett. 2, 933(2002). 7. J. Li, K. Kamata, S. Watanabe, T. Iyoda, Adv. Mater. 19, 1267 (2007). 8. A.Chen, K. Kamata, M. Komura, T. Iyoda, Adv. Mater. 20, 763 (2008). 9. Y. Q. Tian, K. Watanabe, X. X. Kong, J. Abe, T. Iyoda, Macromolecules 35, 373 (2002). 10. M. Komura, T. Iyoda, Macromolecules 40, 4106 (2007). 11. A. B. D. Cassie, Baxter, Trans. Faraday Soc. 40, 546 (1944). 12. R. Blossey. Nature Mater. 2, 301 (2003). 13. R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1964). 14. F. M. Fowks, J. Phys. Chem. 66, 382 (1962).
454
