pubs.acs.org/Langmuir © 2009 American Chemical Society
Highly Stable Molecular Layers on Nanocrystalline Anatase TiO2 through Photochemical Grafting Ryan A. Franking, Elizabeth C. Landis, and Robert J. Hamers* Department of Chemistry, University of Wisconsin;Madison, 1101 University Avenue, Madison, Wisconsin 53706 Received March 31, 2009. Revised Manuscript Received July 13, 2009 Well-defined molecular layers can be formed on the surface of nanocrystalline anatase TiO2 by photochemically grafting organic molecules bearing a terminal vinyl group. The molecular layers produced are shown to have minimal oxidation and are able to be patterned and uniformly grafted through optically thick nanocrystalline films. Stability tests show that the layers have excellent stability in deionized water at 80 °C, aqueous solutions at pH = 1.0 and pH = 10.3 at 65 °C, and acetonitrile for time scales approaching 1200 h. Degradation of the films in deionized water occurs using a AM1.5 full-spectrum solar simulator as an illumination source but is partially suppressed by filtering with a 400 nm UV blocking filter which blocks the above-bandgap light. A mechanism is proposed for the grafting reaction in which the surface hydroxyl groups trap photoexcited holes, facilitating reaction with the vinyl group.
Introduction Titanium dioxide finds widespread use in chemistry and materials science because of its catalytic behavior,1 its high stability under acidic and basic conditions,2 its biocompatibility,3 and its favorable optical and electronic properties.4-6 Recently, TiO2 has been heavily investigated for renewable energy applications such as photocatalysis7,8 and solar energy conversion.6,9,10 While for many applications the bare TiO2 surface is useful, there is increasing interest in conveying improved specificity by covalently linking different functional groups such as lightharvesting molecules,6 selective molecular catalysts,11,12 or biomolecules13-17 to TiO2 surfaces. *Corresponding author. E-mail:
[email protected]. (1) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier Press: Amsterdam, 1989. (2) Ziemniak, S. E. J. Solution Chem. 1992, 21, 745. (3) Schuler, M.; Trentin, D.; Textor, M.; Tosatti, S. G. P. Nanomedicine 2006, 1, 449. (4) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (5) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (6) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (7) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (8) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428. (9) Nozik, A. J. Annu. Rev. Phys. Chem. 1978, 29, 189. (10) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (11) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802. (12) Ebitani, K.; Fujie, Y.; Kaneda, K. Langmuir 1999, 15, 3557. (13) Nanci, A.; Wuest, J. D.; Peru, L.; Brunet, P.; Sharma, V.; Zalzal, S.; McKee, M. D. J. Biomed. Mater. Res. 1998, 40, 324. (14) Heysel, S.; Vogel, H.; Sanger, M.; Sigrist, H. Protein Sci. 1995, 4, 2532. (15) Lukosz, W. Sens. Actuators, B 1995, 29, 37. (16) Topoglidis, E.; Cass, A. E. G.; Gilardi, G.; Sadeghi, S.; Beaumont, N.; Durrant, J. R. Anal. Chem. 1998, 70, 5111. (17) Prieto-Simon, B.; Campas, M.; Marty, J. L. Protein Pept. Lett. 2008, 15, 757. (18) Galoppini, E. Coord. Chem. Rev. 2004, 248, 1283. (19) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (20) Bai, Y.; Cao, Y. M.; Zhang, J.; Wang, M.; Li, R. Z.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. Nat. Mater. 2008, 7, 626. (21) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto, M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27. (22) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Gratzel, M. J. Phys. Chem. B 2003, 107, 8981. (23) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. J. Phys. Chem. B 2000, 104, 1300.
10676 DOI: 10.1021/la901116c
Several approaches have been taken toward the formation of molecular layers on titanium dioxide surfaces.18-24 Carboxylate groups have been used to link molecules to TiO2 surfaces and are widely used in dye-sensitized solar cells.19,20 However, the resulting layers show poor stability in aqueous environments due to protonation of the carboxylate group at low pH and hydrolysis at pH > 9.21-23 Phosphonates24-27 and silanes27-29 also have been used to link molecules to TiO2 and have been the preferred chemistries for biological interfaces.13,17 Covalent grafting of phosphonates typically involves a slow evaporation of solvent over several days at temperatures of 120 °C or higher,24 producing covalently grafted layers that exhibit good stability in aqueous environments between pH = 1 and pH = 10.30,31 Organosilanes and organohalosilane react readily with the TiO2 surface when dosed in ultrahigh vacuum.28,29 The more common solution-phase method of grafting has a complex mechanism involving aggregation in solution and formation of a relatively small number of direct bonds of the aggregates to the TiO2 surface.27 The stability of the layers can be good in aqueous environments31 but is poor in acidic solutions due to protonation of the surface ligands.30 Recently, an alternative method of grafting organic molecules has emerged in which organic alkenes are covalently grafted to surfaces of hydrogen-terminated semiconductors such as silicon,32 germanium,33 and carbon-based materials34-36 when illuminated with ultraviolet (UV) light at 254 nm. (24) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Langmuir 2001, 17, 5736. (25) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.; HumphryBaker, R.; Kalyanasundaram, K.; Gr€atzel, M.; Shklover, V.; Haibach, T. Inorg. Chem. 1997, 36, 5937. (26) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014. (27) Helmy, R.; Fadeev, A. Y. Langmuir 2002, 18, 8924. (28) Gamble, L.; Jung, L. S.; Campbell, C. T. Langmuir 1995, 11, 4505. (29) Gamble, L.; Hugenschmidt, M. B.; Campbell, C. T.; Jurgens, T. A.; Rogers, J. W. J. Am. Chem. Soc. 1993, 115, 12096. (30) Marcinko, S.; Fadeev, A. Y. Langmuir 2004, 20, 2270. (31) Mani, G.; Johnson, D. M.; Marton, D.; Dougherty, V. L.; Feldman, M. D.; Patel, D.; Ayon, A. A.; Agrawal, C. M. Langmuir 2008, 24, 6774. (32) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (33) Choi, K.; Buriak, J. M. Langmuir 2000, 16, 7737. (34) Baker, S. E.; Tse, K. Y.; Hindin, E.; Nichols, B. M.; Clare, T. L.; Hamers, R. J. Chem. Mater. 2005, 17, 4971. (35) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968.
Published on Web 08/11/2009
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This photochemical grafting process is an attractive way to link functional molecules to surfaces because it does not require high temperature or ultrahigh vacuum, can give a direct bond between organic molecules and inorganic surfaces, and offers a facile method for patterning the functionalization by simple photomasking procedures. Yet, it has not been established whether similar grafting processes can be extended to more ionic materials such as transition metal oxides. Here, we investigate the photochemical grafting of organic alkenes to the surface of nanocrystalline anatase TiO2 films similar to those widely used in dye-sensitized solar cells10 and characterize the molecular layers using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The high surface area and three-dimensional nature of the nanostructured films allows the grafted layers to be easily probed by FTIR measurements with much greater sensitivity than planar substrates. Our results demonstrate that the resulting layers are extraordinarily stable in hot aqueous media even at very low pH and are also stable in acetonitrile. These results suggest that photochemical grafting may be a useful method for linking novel types of molecular functionalities to the surface of TiO2 for applications in renewable energy such as solar conversion and/or photocatalysis.
Experimental Section Nanoporous Anatase Film Preparation. TiO2 films were prepared by screen-printing a 0.5 cm diameter circle of a paste containing 20 nm anatase nanoparticles (Ti-Nanoxide T20/SP, Solaronix) onto a relevant substrate. For substrates, fluorinedoped tin oxide (FTO) treated in aqueous 50 mM TiCl4 at 70 °C for 30 min was used for FTIR and stability studies, and 20 nm of titanium evaporated onto highly conductive n-Si was used for XPS measurements. The general procedure for deposition of the films was adapted from Ito et al.37 Briefly, three layers of the Solaronix paste were screen-printed onto the substrates. Each layer was immediately dried at 125 °C after printing to maintain some surface roughness and to help mitigate interference patterns in FTIR measurements. The dried films were then gradually heated to 500 °C over 30 min and kept at 500 °C for an additional 15 min to remove the organic binders used in the paste. Finally, the substrates were cleaned under UV illumination (λ = 254 nm, 10 mW/cm2) in air overnight until no C-H related peaks could be observed by FTIR measurements. The thickness of the prepared films was observed by SEM to be between 7 and 9 μm. The number of layers could be adjusted to increase or decrease the thickness of the film. Carboxylate Chemisorption. Before reaction, the TiO2 films were heated to 500 °C for 15 min to drive off water and any remaining surface contaminates. After cooling to ∼80 °C, the films were immersed in a 1 mM solution of sodium dodecanoate (Sigma grade, Sigma) in anhydrous acetonitrile for 16 h. The reacted films were rinsed thoroughly with fresh acetonitrile and blown dry using dry nitrogen. Alkene Photochemical Grafting. The relevant neat alkene was purged with argon for 15-30 min to reduce oxygen impurities. After UV cleaning, the TiO2 films were placed in a nitrogenpurged, Teflon reaction cell with a UV grade fused silica window. A drop of the purged alkene was placed on the films to completely cover them in the liquid. A UV grade fused silica cover glass was placed on top to reduce evaporation. A low-pressure mercury quartz grid lamp (λ = 254 nm, ∼10 mW/cm2) was used to illuminate the film. Typical illumination times were 16 h for dodecene and 18 h for 10-undecylenic acid methyl ester. After (36) Sun, B.; Colavita, P. E.; Kim, H.; Lockett, M.; Marcus, M. S.; Smith, L. M.; Hamers, R. J. Langmuir 2006, 22, 9598. (37) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M. Thin Solid Films 2008, 516, 4613.
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illumination the films were rinsed thoroughly with chloroform and methanol and blown dry under nitrogen flow. Photopatterning of Alkene Layers. A photomask with a variety of feature sizes on the 1-100 μm length scale was fabricated using electron-beam lithography to pattern a photoresist on a UV transparent fused silica substrate. A 70 nm thick layer of chromium was then deposited followed by standard liftoff procedures.38 This photomask was used in place of the fused silica cover glass. All other steps in the reaction remained unaltered. Desorption of Organic Layers. We carried out the thermal stability experiments by placing the films in screw-top glass vials containing deionized (DI) water, an acidic solution at pH = 1 (KCl/HCl Fixanal, Riedel-de Haen), a basic solution at pH = 11 (H3BO3/NaOH/KCl Fixanal buffer, Riedel-de Haen), or anhydrous acetonitrile. For the deionized water and acetonitrile, we preheated the vials and stored the samples immersed in the fluids in an 80 °C oven. The stability of the functionalized surfaces under acidic and basic solutions was measured after storage in a 65 °C oven. In all cases the films were rinsed with fresh deionized water or acetonitrile and dried in a flow of dry nitrogen before the FTIR spectrum was measured, and the sample was returned to the same vial. We investigated the photoinduced desorption of the organic layers using a standardized air mass 1.5 (AM1.5) direct light source (Newport Full Spectrum Solar Simulator, 100 mW/cm2). The films were placed in 100 mL beaker and covered with 20 mL of deionized water during illumination. In a separate set of experiments, a 400 nm cut-on filter (FSQ-GG400, Newport) was used to block light at wavelengths shorter than the bandgap. Before FTIR measurements the films were blown dry under nitrogen flow. Characterization. Infrared reflection-absorption spectra were collected on a Bruker Vertex 70 FTIR spectrometer using a VeeMaxII variable angle specular reflectance accessory with a wire grid polarizer. All spectra were collected with a resolution of 4 cm-1 with p-polarized light. Unless otherwise stated, an incident illumination angle of 50° off normal was used. The background used in calculating the absorbance was the single beam spectrum of the relevant film taken immediately after UV cleaning. A second background spectrum was also taken after the measured spectrum to identify instrumental or environmental drift that may have occurred over time. Sloping baselines were removed to improve the clarity of the spectra. X-ray photoelectron spectroscopy (XPS) was performed using a monochromatic Al KR source (nominally 1486.6 eV photon energy) with an analyzer resolution of between 0.1 and 0.2 eV and an electron takeoff angle of 45°. Peak positions and area ratios were calculated by fitting the raw data to Voigt functions after a baseline correction and normalization of the areas with their corresponding atomic sensitivity factors (C= 0.296; O = 0.711; Ti(2p) = 1.798).39,40 Scanning electron microscopy (SEM) of the photopatterned films was carried out using a Leo Supra55 VP microscope. All images were taken with a 5 keV electron accelerating voltage and a 7 mm working distance using the standard in-lens secondary electron detector.
Results Infrared Characterization of Photochemical Grafting of 1-Dodecene and of 10-Undecylenenic Acid Methyl Ester. We first demonstrated the grafting of 1-dodecene to the TiO2 film substrates. Figure 1a shows three FTIR spectra: a reflectance (38) Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era; Lattice Press: Sunset Beach, CA, 1999; Vol. 1. (39) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of Xray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (40) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
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Figure 1. FTIR of nanocrystalline anatase films grafted with various molecules. Spectra of the pure liquids/solid are included for reference. (a) 1-Dodecene and dodecane control, (b) 10-undecylenic acid methyl ester, and (c) sodium dodecanoate.
spectrum of a sample reacted with the neat 1-dodecene for 16 h using 254 nm light (10 mW/cm2), a spectrum obtained after a similar procedure using the analogous fully saturated molecule dodecane, and a transmission spectrum of a thin film of pure (neat) liquid 1-dodecene. Comparing the spectra of the 1-dodecene grafted surface and the pure 1-dodecene liquid shows a number of similarities but also important differences. Both spectra show peaks at 2856, 2927, and 2958 cm-1 and a sharp peak at 1464 cm-1. The neat liquid shows a clear peak at 3078 cm-1 that is absent after grafting and a sharp peak at 1641 cm-1 that appears to be either absent or shifted to 1676 cm-1 and significantly broadened after grafting. Finally, the dodecene-grafted surface shows downward (negative) features near 3675 cm-1. The 2856 and 2927 cm-1 features are attributed to the symmetric and asymmetric CH2 stretching modes of the alkyl chain, while the 2958 cm-1 feature arises from the CH3 asymmetric stretching mode of the terminal CH3 group. The ratio of intensity of the CH3 peak at 2958 cm-1 to that of the CH2 peak at 2927 cm-1 is 0.33, within 10% of the ratio (0.29) observed in the neat liquid transmission spectrum. The peak at 1464 cm-1 in the neat liquid and the surface-bound molecules correlates well with the anticipated scissor and deformation vibrational modes of the alkyl chain.41 These peaks all suggest that surface-grafted molecules (41) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.
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bond to the surface with little or no change to the alkyl region of the molecule. The peak at 3078 cm-1 in neat 1-dodecene mode arises from the vinylic =CH2 stretching mode; after grafting this peak is reduced in intensity by at least 100-fold. While FTIR is difficult to quantify, this large reduction in intensity implies that the vast majority of the molecules (greater than 99% assuming intensity is linear with concentration) have reacted with the surface via the vinyl group. The sharp negative peaks near 3675 cm-1 match the known frequencies of surface Ti-OH groups;42,43 the downward peaks imply that grafting is accompanied by the loss of some surface hydroxyl groups that were on the surface before reaction. The origin of the 1676 cm-1 peak remains unclear; while this could correspond to a CdC vinyl stretching mode, the broadening and higher frequency are also in the range of carbonyl CdO stretching modes. Two control samples were also measured: one in which the UV grafting reaction was attempted using the saturated molecule dodecane (shown in Figure 1a) and a second in which the sample was exposed to 1-dodecene but not illuminated. No significant infrared absorption was observed after either of these controls, except for some loss of Ti-OH groups; the decrease in Ti-OH intensity from dodecane is only ∼20% of that observed with 1dodecene. These infrared measurements establish that the grafting reaction is photoinitiated and that the molecules bind to the surface using the vinyl group. The remainder of the alkyl chain is left largely unchanged, but a small amount of oxidation cannot be ruled out. We also invested grafting of 10-undecylenic acid methyl ester (UAME) to TiO2 surfaces. The UAME was used in order to determine whether the stability of the grafted layers depends on the hydrophobicity/hydrophilicity of the molecules; the dodecene-modified surface is hydrophobic with a water contact angle of >60°, while the methyl ester yields a hydrophilic surface with a water contact angle of 0.99 that passes nearly through the origin. This fitting is consistent with a uniform grafting throughout the entire film independent of film thickness up to at least d = 10 μm. Direct Photopatterning. By controlling the spatial distribution of light using a photomask, it is possible to directly pattern the spatial distribution of molecules on the surface. Using a photomask as described above, we patterned the distribution of 1-dodecene on TiO2 films and then used SEM to image the resulting patterns. Because different surface terminations can provide different secondary electron yields, a clear contrast can be seen between grafted and bare surfaces.57-59 Figure 5a shows a SEM image of a photopatterned film in which the fused silica (57) Mack, N. H.; Dong, R.; Nuzzo, R. G. J. Am. Chem. Soc. 2006, 128, 7871. (58) Nichols, B. M.; Metz, K. M.; Tse, K. Y.; Butler, J. E.; Russell, J. N.; Hamers, R. J. J. Phys. Chem. B 2006, 110, 16535. (59) Wang, X.; Colavita, P. E.; Metz, K. M.; Butler, J. E.; Hamers, R. J. Langmuir 2007, 23, 11623.
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Figure 6. (a) FTIR-derived desorption kinetics of UAME in deionized (DI) water at 80 °C (red squares), pH = 1.0 solution at 65 °C (green circles), pH = 10.3 at 65 °C (blue triangles), and sodium dodecanoate in DI water at 80 °C (purple diamonds). Inset is a blow-up of the sodium dodecanoate. (b) Final FTIR spectra of the UAME grafted films after immersion in the solutions for nearly 50 days above a representative initial spectrum.
mask contained a 150 μm square of chromium. The light region around the outside corresponds to areas illuminated by UV light and reacted with 1-dodecene and the darker region in the center to unreacted, bare film. Figure 5b graphs a horizontal linescan taken from the image. The sharpest transition from reacted area to unreacted occurs over ∼6 μm. This edge acuity is slightly larger than observed with other nanocrystalline substrates such as nanocrystalline diamond, which shows a resolution approaching the diffraction limit or grain size of the nanocrystals.59 Aqueous Stability. In order to test the stability of the attached molecules in aqueous environments, we immersed grafted films in 80 °C deionized water, 65 °C water at a pH of 1.01 (KCl/HCl), and 65 °C water buffered at a pH of 10.3 (H3BO3/NaOH/KCl). Because 1-dedecene exhibits poor wetting with polar solvents and even physisorbed material can remain adsorbed on the surface when immersed in water, we used films grafted with 10-undecylenic acid methyl ester. The water contact angle of a UAME film is
