pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of TiO2 Arrays Using Solvent-Assisted Soft Lithography Gang Shi,† Nan Lu,*,† Liguo Gao,† Hongbo Xu,† Bingjie Yang,† Ying Li,† Ying Wu,† and Lifeng Chi*,†,‡ †
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130023, P. R. China, and ‡Physikalisches Institut and Center for Nanotechnology (CeNTech), Westf€ alische Wilhelms-Universit€ at, 48149 M€ unster, Germany Received May 10, 2009. Revised Manuscript Received June 18, 2009
We present a simple solvent-assisted soft lithography method to fabricate titania (TiO2) patterns. The dimensions of the TiO2 features can be controlled by adjusting the concentration of the solution, the solvent evaporation duration, and temperature. This method may provide a facile route for fabricating large area patterns of metal oxide.
Patterned metal oxides, such as TiO2, ZnO, Fe3O4, and so on, with feature sizes from submicrometer to nanometer dimensions have attracted considerable attention because of their applications in magnetic,1,2 optical,3,4 electronic,5,6 and catalytic properties.7,8 For these applications, it is desirable to develop a simple and costefficient fabrication process. Photolithography is the common and reliable patterning technique; however, to further scale down the feather size requires novel approaches such as dip-pen nanolitography,9 nanoimprint lithography,10 ion-beam lithography,11 and deep UV and extreme UV photolithography.12,13 Although the aforementioned techniques are satisfactory for some applications, they are far from optimum and still require specific facilities and complicated fabrication processes for patterning. Recently, some unconventional patterning techniques were introduced that utilized the edges of patterns to define the created features, including photolithography with phase-shifting masks,14 topograhically directed etching,15 template-assisted self-assembly, 16-18 nanosphere lithography,19,20 micromolding in capillaries21,22
(MIMIC), lithographically controlled wetting (LCW),23,24 gridassisted LCW (GA-LCW),25-27 and so on. These techniques can be applied for patterning a wide variety of materials with feature size ranging from nanometers to micrometers. For the solventassisted methods,19-27 solvent evaporation is an important issue for controlling the dimension of the patterns; however, it has been less addressed systematically. Soft lithography,28,29 usually utilizing elastomer poly(dimethylsiloxane) (PDMS) molds, has been extensively applied to pattern photoresists, biological macromolecules, semiconducting polymers, and metal oxide precursors. In this communication, we report a simple approach for patterning metal oxide based on solventassisted soft lithography (SASL), which results in structures smaller than the lateral spacing of the stamp. TiO2 was chosen as a model material because of its good photocatalytic, optical, gas-sensing, and electronic properties. Different TiO2 arrays of feature sizes ranging from nanometers to submicrometers were generated with SASL, which may have applications in optoelectronic devices, photocatalytic systems, and gas sensors.
*Corresponding author. E-mail:
[email protected] (N.L.); chi@ uni-muenster.de (L.C.).
Chemicals and Materials. The experiments were carried out
Introduction
Experimental Section (1) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. H. Nano Lett. 2004, 4, 187. (2) Goya, G. F.; Berquo, T. S.; Fonseca, F. C. J. Appl. Phys. 2003, 94, 3520. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Yang, P. D. Science 2001, 292, 1897. (4) Yoldas, B. E.; O’Keeffe, T. W. Appl. Opt. 1979, 18, 3133. (5) Burns, G. P. J. Appl. Phys. 1989, 65, 2095. (6) Shin, H.; De Guire, M. R.; Heuer, A. H. J. Appl. Phys. 1998, 83, 3311. (7) Carlson, T.; Griffin, G. L. J. Phys. Chem. 1986, 90, 5896. (8) Lee, J. P.; Sung, M. M. J. Am. Chem. Soc. 2004, 126, 28. (9) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661. (10) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (11) Tandon, U. S. Vacuum 1992, 43, 241. (12) Holmes, S. J.; Michel, P. H.; Hakey, M. C. IBM J. Res. Dev. 1997, 41, 7. (13) Stulen, R. H.; Sweeney, D. W. IEEE J. Quantum Elect. 1999, 35, 694. (14) Rogers, J. A.; Paul, K. E.; Jackman, R. J.; Whitesides, G. M. J. Vac. Sci. Technol. B 1998, 16, 59. (15) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1998, 394, 868. (16) Cherniavskaya, O.; Adzic, A.; Knutson, C.; Gross, B. J.; Zang, L.; Liu, C. C.; Adams, D. M. Langmuir 2002, 18, 7029. (17) Yin, Y. D.; Lu, Y.; Gates, B.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 8718. (18) McLellan, J. M.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10830. (19) Sun, F. Q.; Yu, J. C.; Wang, X. C. Chem. Mater. 2006, 18, 3774. (20) Chen, J. X.; Liao, W. S.; Son, D. H.; Batteas, J. D.; Cremer, P. S. ACS Nano 2009, 3, 173. (21) Bystrenova, E.; Facchini, M.; Cavallini, M.; Cacace, M. G.; Biscarini, F. Angew. Chem., Int. Ed. 2006, 45, 4779. (22) Kim, E.; Xia, Y. N.; Whitesides, G. M. Nature 1995, 376, 581.
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in a standard chemistry laboratory. Titanium n-butoxide [Ti(OC4H9)4], ethanol, hydrochloric acid, chloroform, and acetone were purchased from Beijing Chemical Reagent Plant in the highest available purity and used without further purification. The silicon wafers [n type (100)] were obtained from Youyan Guigu Beijing, China. A kit of PDMS prepolymer (Sylgard 184 silicone elastomer curing agent) was purchased from Dow Corning Corporation. Poly(methyl methacrylate) (PMMA) (molecular weight Mw = 75 kDa) microresist was purchased from Microresist Technology GmbH, Germany. Photoresist was purchased from the Beijing Institute of Chemical Reagents (positive photoresist BP212, BP212 positive photoresist developer). Synthesis of the Colloid TiO2. Precursor of TiO2 (P-TiO2) was prepared with the sol-gel method.30 In a typical process, (23) Cavallini, M.; Biscarini, F. Nano Lett. 2003, 3, 1269. (24) Cavallini, M.; Murgia, M.; Biscarini, F. Nano Lett. 2001, 1, 193. (25) Cavallini, M.; Biscarini, F.; Farran-Morales, F.; Massi, M.; Leigh, D. A.; Zerbetto, F. Nano Lett. 2002, 2, 635. (26) Viola, I.; Mazzeo, M.; Passabi, A.; D’Amone, S.; Cingolani, R.; Gigli, G. Adv. Mater. 2005, 17, 2935. (27) Cavallini, M.; Albonetti, C.; Biscarini, F. Adv. Mater. 2009, 27, 1043. (28) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (29) Xia, Y. N.; Whitesides, G. M. Angew. Annu. Rev. Mater. Sci. 1998, 28, 153. (30) Pang, S.; Xie, T. F.; Wang, D. J. J. Phys. Chem. C 2007, 111, 18417.
Published on Web 07/28/2009
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Figure 1. AFM images and section-analysis of the TiO2 ring arrays produced with different evaporation durations: (a) 10 min, (b) 60 min, (c) 120 min, and (d) 240 min. (e) The average feature height and (f ) fwhm versus the evaporation duration. Each data was calculated from 30 randomly selected rings from several locations on a sample. Scheme 1. Schematic Illustration of the Procedure for Fabricating TiO2 Arrays by SASLa
a (A) Imprinting PDMS stamp in the P-TiO2 thin film. (B) Evaporating solvent. (C) Heating. (D) Lifting-off the stamp and calcinations. (E) A top view of the TiO2 ring arrays.
2 mL hydrochloric acid (37%), 10 mL ethanol, and 4 mL H2O were added into the mixed solution of 20 mL Ti(OC4H9)4 and 25 mL ethanol in a dropwise manner. The concentration of P-TiO2 was 0.68 mol/L. Then, the prepared solution was thermostatted at 40 °C for 4 h. Finally, it was diluted to 0.17, 0.085, 0.043, and 0.021 mol/L, respectively, with ethanol. Fabrication of TiO2 Patterns. The PDMS stamps were fabricated by molding the silicon stamps. There are three different patterns on the PDMS stamps. Stamp 1 is 5 μm discs in diameter 9640 DOI: 10.1021/la901662z
separated by 5 μm spacing, 1.5 μm in height; stamp 2 is 8 μm wide stripes separated by 5 μm spacing, 1.5 μm in height; and stamp 3 is 10 μm wide lattices separated by 10 μm spacing, 0.2 μm in height. Glass slides were cut into 2 2 cm2 pieces, and subsequently cleaned by sonication in acetone and ethanol for 3 min. Then, the glass pieces were rinsed with deionized water and dried under nitrogen gas flow before use. As shown in Scheme 1, 0.1 mL P-TiO2 was first dropped on the surface of a glass substrate. Then a PDMS stamp was placed onto the glass substrate covered with P-TiO2 under 8000 N/m2 to promote the capillary molding. After keeping the sample under a certain temperature (5 °C, 25 and 45 °C) for a period of evaporation time (10, 60, 120, and 240 min), it was put into the oven and heated at 110 °C for 30 min. Finally, the patterned TiO2 was sintered at 450 °C for 30 min after peeling off the PDMS stamp. Characterizations. The optical images were recorded on a XSP-BM optical microscope (in reflection mode) and were captured with a Panasonic color charge-coupled device (CCD) and digitized with a frame grabber. X-ray diffraction (XRD) pattern was recorded by a Rigaku D/Max-2550 diffractometer with Cu KR radiation (λ = 1.54056 A˚) (40 kV, 350 mA) in the range of 20 - 80° (2θ) at a scanning rate of 10°/min. Atomic force microscopy (AFM) analysis was carried out with a NanoScope III multimode atomic force microscope (Digital Instruments, Santa Barbara, CA) operating in tapping mode by using silicon cantilevers (Nanosensors, Digital Instruments) with a resonant frequency 250-350 kHz. Transmission electron microscopy (TEM) was conducted on a H-8100 electron microscope at an acceleration voltage of 200 kV.
Results and Discussion The experimental procedures are illustrated in Scheme 1, and details are provided in the Experimental Section. In the first step, Langmuir 2009, 25(17), 9639–9643
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Figure 2. AFM images and section-analysis of the TiO2 ring arrays generated with different concentrations: (a) 0.021 mol/L, (b) 0.043 mol/L, (c) 0.085 mol/L, and (d) 0.17 mol/L, and the correlation of the average feature height (e) and fwhm (f) with the concentration of P-TiO2, respectively. All data was calculated from 30 randomly selected rings at several locations on a sample.
P-TiO2 was dropped on the glass substrate, and a PDMS stamp was placed onto the glass substrate under a slight pressure (Scheme 1A). The ethanol solution wetted the surface of the PDMS stamp and filled up the space between the stamp and the substrate. In the second step, the ethanol in P-TiO2 gradually evaporated (Scheme 1B). During this process, the capillary force drove the solution to form menisci between the stamp protrusions upon solvent evaporation. At the same time, the solute moved to the edges of the stamp protrusions due to the capillary flow,31 which resulted in isolated structures smaller than the lateral spacing between the stamp’s protrusions. In the third step, the sample was heated in the oven (Scheme 1C). In the final step, the PDMS stamp was peeled off, and the sample was sintered (Scheme 1D). The TEM image and XRD pattern are respectively presented in Supporting Information Figures S1 and S2, which confirm that nanoparticles of the anatase TiO2 were synthesized successfully. In the process, the solvent evaporation is crucial for the formation of TiO2 patterns. By varying the concentration (C) of P-TiO2, the evaporation time (t), and the temperature (T) of solvent, the dimension of the TiO2 pattern can be readily controlled. Keeping the evaporation temperature at 25 °C and the concentration of 0.021 mol/L, the solvent evaporation duration was controlled by changing the duration of keeping the sample in ambient circumstance from 10 to 240 min to investigate its effect on the dimensions of the created patterns. Figure 1 shows that a longer evaporation time results in thicker and higher ring (31) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827.
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structures, while a shorter time leads to thinner rings with lower height. The AFM images and section-analysis of the samples with different evaporation durations (10, 60, 120, and 240 min) are shown in Figure 1a-d, and it can be observed that the feature width (full width at half-maximum, fwhm) of the sample after 10 min of evaporation is around 76 nm. The height and fwhm of the TiO2 rings in Figure 1c,d are similar, which implies that the solvent had entirely evaporated after 120 min. Figure S3 shows that the dimensions of the patterns produced with different evaporation durations are repeatable. Figure 1e,f presents the correlation of the TiO2 ring dimension and the evaporation duration. The average height of TiO2 rings gradually increases from 21 ( 5.7 to 75.2 ( 8.0 nm by extending the evaporation time from 10 to 120 min. Simultaneously, the average fwhm width increases from 79.2 ( 5.8 to 134.9 ( 4.7 nm. However, when the solvent is allowed to dry for longer than 120 min, no increase in either height or fwhm width can be observed. Figure 2a-d demonstrates the section-analysis of patterns obtained with different concentrations of P-TiO2 (0.021-0.17 mol/L), keeping the temperature at 25 °C and an evaporation duration of 120 min. The correlation of the average height and fwhm of the TiO2 features versus concentration is plotted in Figure 2e,f, respectively. Increasing the concentration increases both the height and fwhm of the TiO2 features. The effect of evaporation temperature on the size of the ring was studied by varying the ambient temperature from 5 to 45 °C (see Figure 3) with the concentration and evaporation duration constant (C=0.085 mol/L, t=24 h). The height and fwhm of the rings can be estimated from cross-sectional analyses as shown in the inset of Figure 3a-c. Figure 3a shows that the height and DOI: 10.1021/la901662z
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Figure 3. AFM images of TiO2 arrays obtained at different temperatures: (a) 45 °C, (b) 25 °C, and (c) 5 °C. The insets show section-analysis of the structures.
Figure 4. Optical images of TiO2 arrays fabricated with different stamps: (a) 8 μm PDMS stripes with 5 μm spacing and 1.5 μm in height, and (b) 10 μm wide PDMS lattices with 10 μm spacing and 0.2 μm in height. Scheme 2. Formation Mechanism of the Created Ringsa
a ev represents the solvent evaporation, fs represents the solvent flow, Fc and G are the capillary force and gravitation, respectively.
fwhm of the TiO2 rings fabricated at 45 °C are 170 and 185 nm. The height and fwhm of TiO2 rings derived from 25 °C are 223 and 227 nm (see Figure 3b). Figure 3c presents that the height and fwhm of the TiO2 rings achieved at 5 °C are 279 and 261 nm. This indicates the possibility to control the feature dimension by changing the evaporation temperature. The mechanism of the pattern formation is shown in Scheme 2. There are two competing processes during the evaporation: one is the macro-scale capillary flow that drives the solution to the edge of the PDMS mold. This is a typical coffee stain effect,31 and the 9642 DOI: 10.1021/la901662z
materials are pinned outside the pillars of the PDMS mold. The other is the pinning of materials around each individual pillar of the PDMS stamp, which favors slower evaporation rate and longer evaporation time. The atmosphere outside the PDMS is unsaturated with ethanol vapor, leading to a pressure gradient (ΔP)32 from inside to outside the PDMS, which results in the ethanol gradually evaporating through the PDMS. During the evaporation, the ethanol flow brings TiO2 nanoparticles to the side walls of the PDMS. At the same time, the sol is driven by the capillary force (Fc)33 to rise along the side walls of the PDMS and is affected by the downward gravitation (G), although it may be ignored in the micro/nanometer scale comparing with the capillary force. Additionally, van der Waals forces, which are mainly the adhesion forces between the nanoparticles and the substrate, contribute to the quality of TiO2 patterns (as shown in Figure S4), and we will investigate it in our future work. The vertical and lateral dimensions increase with extending the evaporation duration until the solvent evaporated completely in 2 h (as shown in Figure 1). This can be explained by the fact that longer evaporation duration allows more TiO2 nanoparticles to move with the ethanol to the side walls of the PDMS stamps. A higher precursor concentration results in more TiO2 nanoparticles moving to the PDMS walls, leading to rings with higher height and thicker walls (as seen in Figure 2). One part of the ethanol trapped in the center of the spacing of PDMS stamp evaporates, and the other part of it flows toward the edges. Higher temperature increases the solvent evaporation rate, so more ethanol in the center of the spacing of PDMS stamp (32) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (33) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surface; Wiley: New York, 1997.
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evaporates, and less ethanol with TiO2 nanoparticles flows and accumulates on the boundaries of the PDMS protrusions (as presented in Figure 3). Although the microscale rods stamp was used to create TiO2 rings in this work, it should be easy to extend this method to other patterns (as shown in Figure 4). Figure 4 shows the optical micrographs of TiO2 arrays obtained under the same conditions (C=0.085 mol/L, t=120 min, T=25 °C), which were patterned with the stamps of 8 μm PDMS stripes with 5 μm spacing, 1.5 μm in height, and 10 μm PDMS lattices with 10 μm spacing, 0.2 μm in height, respectively. This technique could also be extended to pattern other materials besides TiO2 nanoparticles.
than that of the stamp. Based on SASL, the dimension of the TiO2 pattern can be tuned from submicrometer to nanometer scale by varying the concentration of P-TiO2 and the evaporation time and temperature. These patterns fabricated by SASL may have potential applications in photoelectric devices, photocatalytic surfaces, magnetic elements, and gas sensors.
Conclusions In conclusion, the unconventional SASL technique has been used to fabricate TiO2 patterns with feature sizes much smaller
Supporting Information Available: Additional experimental material as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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Acknowledgment. This work was supported by the National Natural Science Foundation of China (20373019, and 20773052), the Program for New Century Excellent Talents in University, the National Basic Research Program (2007CB808003 and 2009CB939701), and Program 111.
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