pubs.acs.org/Langmuir © 2010 American Chemical Society
Directed Adhesion and Patterning by Ultraviolet Irradiation of TiO2(110) Jagdeep Singh and James E. Whitten* Department of Chemistry and Center for High-Rate Nanomanufacturing, The University of Massachusetts, Lowell, Lowell, Massachusetts 01854, United States Received August 9, 2010. Revised Manuscript Received October 26, 2010 Forces of adhesion between a hydroxylated silicon oxide tip and a TiO2(110) surface, before and after irradiation of the surface with 254 nm light, were measured using atomic force microscopy. The work of adhesion before and after irradiation was 32 and 166 mJ/m2, respectively, but a difference was observed only if ultraviolet light exposure was used in the presence of oxygen. The change in adhesion correlated strongly with decreasing water contact angle, which changed from ca. 70 to 0° because of irradiation. The contrast in adhesion between irradiated and nonirradiated regions of the surface makes possible a simple method of patterning molecules with micrometer, and potentially nanoscale, resolution. As an example, fluorescein was selectively adsorbed onto hydrophilic regions of the surface by spin coating an ethanolic fluorescein solution onto TiO2(110) that had been irradiated through a photomask.
Introduction Titanium dioxide surfaces have been studied for applications that include heterogeneous photocatalytic environmental cleanup,1 phototherapy,2 solar cells,3 antifogging and self-cleaning windows,4 and antibacterial construction materials.5 As first shown by Fujishima, Watanabe, and co-workers,6-8 they have the interesting property that they become significantly more hydrophilic upon exposure to UV light. Storage in the dark6-8 or irradiation with visible light9 causes the surface to revert to its original hydrophobic condition. Although the mechanism is still not fully resolved, Yates and co-workers10 have shown that the photocatalytic decomposition of adsorbed hydrocarbon contamination plays a key role in UV-induced hydrophilicity. Whereas titanium dioxide has been successfully patterned onto various substrates, including silicon surfaces11 and polymer films,11-13 few studies have attempted to use light-induced hydrophilicity to pattern subsequently deposited molecules onto TiO2 surfaces. Examples of the limited research that has been carried out include using UV light to initiate the grafting of 1-alkenes onto a *Corresponding author. Phone: (978) 934-3666. Fax: (978) 934-3013. E-mail:
[email protected]. (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. (2) Wamer, W. G.; Yin, J. J.; Wei, R. R. Free Radical Biol. Med. 1997, 23, 851– 858. (3) Oregan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (4) Takagi, K.; Makimoto, T.; Hiraiwa, H.; Negishi, T. J. Vac. Sci. Technol., A 2001, 19, 2931–2935. (5) Wolfrum, E. J.; Huang, J.; Blake, D. M.; Maness, P. C.; Huang, Z.; Fiest, J.; Jacoby, W. A. Environ. Sci. Technol. 2002, 36, 3412–3419. (6) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431–432. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135–138. (8) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188–2194. (9) Miyauchi, M.; Kieda, N.; Hishita, S.; Mitsuhashi, T.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Surf. Sci. 2002, 511, 401–407. (10) Zubkov, T.; Stahl, D.; Thomson, T. L.; Panayotov, D.; Diwald, O.; Yates, J. T., Jr. J. Phys. Chem. B 2005, 109, 15454–15462. (11) Triani, G.; Campbell, J. A.; Evans, P. J.; Davis, J.; Latella, B. A.; Burford, R. P. Thin Solid Films 2010, 518, 3182–3189. (12) Park, O.-H.; Cheng, J. Y.; Kim, H. S.; Rice, P. M.; Topuria, T.; Miller, R. D.; Kim, H.-C. Appl. Phys. Lett. 2007, 90, 233102/1–233102/3. (13) Kim, M.; Kang, B.; Yang, S.; Drew, S.; Samuelson, L. A.; Kumar, J. Adv. Mater. 2006, 18, 1622–1626. (14) Li, B.; Franking, R.; Landis, E. C.; Kim, H.; Hamers, R. J. ACS Appl. Mater. Interfaces 2009, 1, 1013–1022.
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TiO2 surface14 and UV-induced photodegradation, via irradiation through a photomask, of adsorbed phosphonic acid monolayers.15,16 In this letter, a simple method of using UV light to pattern molecules onto a single-crystal titanium dioxide surface is demonstrated. This work differs from previous studies in that it is shown that radiation-induced hydrophilicity on a bare TiO2 surface is adequately stable to serve, without modification, as a template for spin-coated molecules such as fluorescein. Atomic force microscope (AFM) measurements between hydroxylated silicon oxide tips and TiO2(110) demonstrate that light-induced patterning is driven by the adhesion of the material onto irradiated regions of the surface. The effect of oxygen on the process has been studied by comparing force measurements after irradiation in pure nitrogen to those after irradiation in an oxygen/nitrogen mixture.
Experimental Section Silicon oxide colloidal AFM tips used for force measurements were purchased from Novascan Technologies. The tips were rinsed in ethanol, dried in a stream of N2, and then cleaned and hydroxylated by UV ozone treatment for 30 min in a 100 cc/min flow of oxygen. Similarly treated SiO2/Si(111) wafers resulted in a water contact angle of 0°, as measured with a Kr€ uss model DSA-100 sessile drop instrument. All force measurements were performed with a Veeco Nanoscope IIIa AFM instrument. The spring constant of the cantilever was determined using the Sader method.17 All force curves were processed by SPIP software to determine adhesive forces. A homemade gas-purging chamber was used to cover the AFM head, and all force measurements were performed at a relative humidity of less than 1%, as determined with a Vaisala M170, in order to avoid capillary effects. A total flow rate of 100 cc/min was used. For experiments in which it was desirable to flow a mixture of oxygen and nitrogen, flow meters were used to control the gas ratio. A pen ray mercury lamp (Ultraviolet Products Corporation, model no. 90-0020-01), emitting mainly 254 nm light, was used to irradiate TiO2 surfaces immediately prior to force measurements. The light intensity was 2.5 μW/cm2, as measured with an optical power meter. The pen lamp was inserted into the cover of the AFM purging chamber and positioned 1 cm away from an as-received polished TiO2(110) substrate (MTI Corporation). (15) Tizazu, G.; Adawi, A. M.; Leggett, G. J.; Lidzey, D. G. Langmuir 2009, 25, 10746–10753. (16) Zhang, X.; Jin, M.; Liu, Z.; Tryk, D. A.; Nishimoto, S.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2007, 111, 14521–14529. (17) Sader, J. E.; Chon, J. W.; Mulvaney, P. Rev. Sci. Instrum. 1999, 70, 3967–3969.
Published on Web 11/09/2010
DOI: 10.1021/la103174f
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Figure 1. Water contact angle of a TiO2(110) surface as a function of UV exposure in pure nitrogen and in a 1:1 (v/v) mixture of oxygen and nitrogen. The error bars were determined from repeated measurements.
The AFM force measurement procedure consisted of the following steps. First, 50 force curves were collected prior to UV exposure on a sample spot, then the AFM tip was retracted, and UV irradiation was performed for 60 min. Immediately following irradiation, force measurements were performed on the irradiated TiO2(110) surface. During these measurements, horizontal movement of the AFM tip was avoided to ensure that (as close as possible) the same region was interrogated before and after irradiation. For patterning, equally spaced hydrophilic lines on a TiO2 surface were generated with a 27 μW/cm2 UV flood lamp through a photomask in air. The samples were placed 50 mm away from a 500 W Hg lamp for 1 h, and then fluorescein was spin coated onto the sample from a 2.0 mg/mL ethanolic solution at 2000 rpm for 60 s. Fluorescence microscopy was performed on the fluoresceinpatterned TiO2(110) surface with an Olympus FV300 confocal fluorescence microscope. The excitation source used was an Ar ion laser emitting at 488 nm; 505-525 nm band pass and 510 long pass filters were used to isolate the fluorescence emission from the excitation wavelength.
Results and Discussion The UV-induced hydrophilicity of a rutile TiO2(110) substrate was confirmed by irradiating the polished side of the substrate with a 254 nm emitting pen ray lamp under either flowing nitrogen or a 1:1 (v/v) mixture of nitrogen and oxygen. UV exposure was performed in a flow chamber such that the sample remained in the chamber during the contact angle measurements; the relative humidity during UV exposure was less than 1%. Water contact angle data versus exposure time are shown in Figure 1. A dramatic decrease in the water contact angle from ca. 70 to 0 °C was observed in less than 30 min in the presence of oxygen and nitrogen. In contrast, the contact angle of a separately irradiated TiO2(110) surface decreased only to ca. 60° after exposure for an hour when pure nitrogen gas was used. These results confirm literature reports10,18,19 that oxygen is necessary (18) Yates, J. T., Jr. Surf. Sci. 2009, 603, 1605–1612. (19) Ohtsu, N.; Masahashi, N.; Mizukoshi, Y.; Wagatsuma, K. Langmuir 2009, 25, 11586–11591.
17796 DOI: 10.1021/la103174f
Figure 2. Typical pull-off force curves using a hydroxylated SiO2 colloidal probe on a TiO2(110) substrate before and after irradiating the surface with UV light (254 nm). Irradiation was conducted in (a) a N2 atmosphere and (b) a N2/O2 atmosphere. Table 1. Average of 50 Pull-Off Forces before and after UV Irradiation of TiO2(110) in Nitrogen and in a Nitrogen/Oxygen Mixture
N2 N2/O2 (1:1)
before UV exposure (nN)
after UV exposure (nN)
42.4 ( 2.7 25.1 ( 4.1
43.7 ( 12.9 131 ( 7.4
to induce the hydrophilicity of titanium dioxide surfaces, with the mechanism likely involving the decomposition of hydrophobic contaminants by O2δ- species that are strongly bound to the surface as a result of electronic activation by UV-lightinduced electron-hole pairs.20 To measure the effect of UV irradiation on adhesive forces between a hydrophilic tip and a TiO2(110) surface, force measurements were performed using an AFM. The measurements were carried out in a dry nitrogen environment, with a relative humidity of less than 1%, to avoid capillary effects.21 Hydroxylated SiO2 AFM colloidal probes with a diameter of 2.5 μm and a force constant of 7.5 N/m were used to probe adhesive forces between the tip and surface before and after 1 h of UV irradiation. Typical pull-off force curves are shown in Figure 2a for irradiation in the presence of nitrogen gas, with no significant difference observed for UV exposure. However, in the case of exposure in the nitrogen/oxygen atmosphere (Figure 2b), more than 5-foldgreater adhesive forces were observed because of irradiation (131 vs 25 nN). Table 1 summarizes the results for 50 measurements on (20) Green, I. X.; Yates, J. T., Jr. J. Phys. Chem. C 2010, 114, 11924–11930. (21) Cappella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34, 1–104.
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the same spot on the sample. Variation in the measured forces before UV exposure in the two sets of experiments is likely due to differing initial degrees of surface contamination. Using DMT theory,22 the work of adhesion (W12) between a spherical tip and a flat surface is given by F=R ð1Þ W 12 ¼ 2π where R is the radius of curvature of the colloidal probe and F is the pull-off or adhesion force. Whereas the colloidal probe used for these studies has a nominal radius of curvature of 1.25 μm, the actual active radius may be substantially less because of asperities.21,23,24 The effective radius, determined by scanning a silicon calibration grating (Mikromasch TGT01) composed of tips having radii of less than 10 nm, was determined to be 126 nm from the scanned images. Using this value, the calculated work of adhesion before and after UV exposure of the TiO2(110) surface in the presence of oxygen is 31.7 ( 4.8 and 165.5 ( 9.6 mJ/m2, respectively. The latter value indicates strong intermolecular forces between the tip and surface, consistent with dipole-dipole and hydrogen bond interactions. For example, the work of adhesion between a poly(acrylic acid)-covered surface and an OH-terminated AFM tip is 197 ( 22 mJ/m2, as determined in a previous study by our group.25 A control experiment was performed to verify that the effects of UV irradiation, in the presence of oxygen, are not due to ozone cleaning of the surface. In this case, similar AFM force measurements were carried out using a silicon oxide surface consisting of the native oxide on a Si(111) wafer instead of TiO2(110). Force measurements between this surface and a hydroxylated SiO2 AFM tip revealed no differences in adhesive force due to UV irradiation. To demonstrate the use of UV-stimulated hydrophilicity to pattern materials, an experiment was performed with fluorescein. This molecule was chosen because of its ease of detection via fluorescence microscopy. A TiO2(110) surface was irradiated in air through a photomask containing equally spaced lines and was then immediately spin coated with an ethanolic fluorescein solution. A fluorescence microscope image of the resulting surface is shown in Figure 3. The bright green lines consist of fluorescein, and the darker regions on the surface are bare TiO2, as confirmed by AFM measurements and X-ray photoelectron spectroscopy. AFM indicates that the dye-covered regions of the surface are relatively inhomogeneous, with an average height of 380 A˚. Fluorescein patterns prepared by the aforementioned methodology were found to be stable, even after storage for at least 1 month under ambient conditions, as confirmed by fluorescence microscopy. The only detectable difference after long-term storage was a weaker fluorescence intensity, which is probably due to some oxidation/ decomposition of the fluorescein molecules. The adhesion of fluorescein to the irradiated regions of TiO2(110) is likely driven by hydrogen bonding between the hydroxyl and carboxylic acid functional groups of the molecule, consistent with the force measurement results (Figure 2) between a hydroxylated AFM tip and an irradiated surface. It is also possible that covalent bonds via the formation of -COO-Ti bonds may form, as is known to occur between TiO2 and carboxylic acid-containing molecules.26 Fujishima and co-workers16 prepared superhydrophobic-superhydrophilic surfaces by treating titanium dioxide sol-gel films with (22) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, 314–325. (23) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1–152. (24) Burnham, N. A.; Dominguez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Rev. Lett. 1990, 64, 1931–1934. (25) Singh, J.; Whitten, J. E. J. Macromol. Sci., Part A 2008, 45, 885–889. (26) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229.
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Figure 3. Laser scanning confocal microscopy image of fluorescein patterned on a TiO2(110) substrate. The fluorescein-covered regions are bright green. The excitation and emission wavelengths were 488 and 520 nm, respectively. The inset shows the chemical structure of fluorescein.
octadecylphosphonic acid (ODP). This rendered the films superhydrophobic, and UV light was then used to pattern hydrophilic regions onto the surface by the photocatalytic decomposition of ODP. The authors demonstrated the selective condensation of water droplets and polystyrene microspheres onto the hydrophilic patterns. Interestingly, it was found that the surface morphology of the TiO2 sol-gel film greatly influenced the results. The present study demonstrates that the deposition of a superhydrophobic film is not necessary, at least in the case of fluorescein. It further suggests that the major driving force for selective adsorption is the contrast in adhesive forces between the hydrophilic and hydrophobic regions of the surface and the deposited molecule.
Conclusions In previous studies, our group used the chemical functionalization of surfaces (e.g., self-assembled thiols or silanes) prepared via microcontact printing or dip pen nanolithography to direct the patterning of polymers, conjugated oligomers, and biomaterials onto surfaces. In these cases, patterning was possible because of contrasting wettability between different regions of the functionalized surface. This letter demonstrates that similar patterning may be achieved via light-induced hydrophilicity and opens up the possibility of lithographically guided nanoscale adsorption. This method could serve as an alternative to soft lithographic techniques31-33 that are presently being used for nanoscale patterning. 27-30
Acknowledgment. This work was supported by the Center for High-Rate Nanomanufacturing as part of National Science Foundation grant no. NSF-0832785. (27) Chandekar, A.; Sengupta, S. K.; Barry, C. M. F.; Mead, J. L.; Whitten, J. E. Langmuir 2006, 22, 8071–8077. (28) Chandekar, A.; Sengupta, S. K.; Whitten, J. E. Microsc. Res. Tech. 2007, 70, 506–512. (29) Chandekar, A.; Whitten, J. E. Appl. Phys. Lett. 2007, 91, 113103–113106. (30) Chandekar, A.; Sengupta, S. K.; Whitten, J. E. Appl. Surf. Sci. 2010, 256, 2742–2749. (31) Roger, J. A.; Nuzzo, R. G. Mater. Today 2005, 8, 50–56. (32) Liu, J.-F.; Miller, G. P. Nanotechnology 2009, 20, 055303/1–055303/6. (33) Mirkin, C. A. ACS Nano 2007, 1, 79–83.
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