pubs.acs.org/Langmuir © 2009 American Chemical Society
ToF-SIMS and XPS Studies of the Adsorption Characteristics of a Zn-Porphyrin on TiO2 Manuela S. Killian,† Jan-Frederik Gnichwitz,‡ Andreas Hirsch,‡ Patrik Schmuki,† and Julia Kunze*,† † Department of Materials Science and Engineering 4, Chair for Surface Science and Corrosion, Friedrich-Alexander-University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany, and ‡ Department of Chemistry and Pharmacy and Interdisciplinary Center of Molecular Materials (ICMM), Friedrich-Alexander-University of Erlangen-Nuremberg, Henkestrasse 42, 91054 Erlangen, Germany
Received August 27, 2009. Revised Manuscript Received September 22, 2009 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) were used to study monolayers (ML) and thick films of porphyrin Zn-TESP (C67H75N5O5SiZn) attached to titanium dioxide (TiO2) substrates via silanization. Films on ideal hydroxyl-terminated silicon (SiO2) surfaces were used for comparison. ToF-SIMS and XPS spectra show that the type of adsorption varies depending on the thickness of the organic film, the preparation temperature, and the adsorption time. We show that the intensity of a molecular peak at mass 1121.5 u in ToF-SIMS can be used as a direct measure of the ratio of chemisorption/physisorption of Zn-TESP. On TiO2, the amount of chemisorbed porphyrin can be increased by increasing the reaction temperature and time during the silanization process. On the SiO2 reference, only chemisorbed species were detected under all investigated preparation conditions. The present work thus not only gives information on the Zn-TESP linkage to TiO2 but provides a direct tool for generally determining the type of adsorption of monolayers.
Introduction Self-assembled monolayers (SAMs) are highly ordered structures to which molecules can arrange upon adsorption on or reaction with a substrate. Because of the modularity of the organic building blocks, SAMs can be used to alter the surface properties of various kinds of substrates relatively independently of the characteristics of the bulk. The formation of self-assembled monolayers (SAMs) on metal or metal oxide surfaces1 is commonly employed in the fabrication of model surfaces with highly controlled chemical properties. To date, the best-studied SAM systems are self-assembled alkanethioles on gold,1-6 alkyl phosphates and phosphonates on metal oxide surfaces,1,7-9 and SAMs formed via silanization on *Corresponding author. Current address: Technical University of Munich (TUM), Institute for Advanced Study (IAS), Physics Department E19, JamesFranck Strasse 1, D-85748 Garching, Germany. Phone: þ49 89 28912526. E-mail:
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silicon and other oxides.10-16 The orientation of the SAMs on the surface can in many cases determine the surface properties.17-19 Titanium dioxide20-22 modified by self-assembled monolayers is technologically of great interest due to the possibility to adjust surface properties such as wetting behavior,23,24 protein adsorption,25,26 cell interaction in biomedical applications,27,28 and controlled drug release.29-31 (14) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98. (15) Silverman, B. M.; Wieghaus, K. A.; Schwartz, J. Langmuir 2005, 21, 225– 228. (16) Ohlhausen, J. A.; Zavadil, K. R. J. Vac. Sci. Technol., A 2006, 24, 1172– 1178. (17) Allara, D. L. Nanoscale Structures Engineered by Molecular Self-Assembly of Functionalized Monolayers. In Nanofabrication and Biosystems; Hoch, H. C., Jelinski, L. W., Craighead, H. G., Eds.; Cambridge University Press: Cambridge, U.K., 1996; pp 180-201. (18) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; H€ahner, G.; Spencer, N. D. Langmuir 2000, 16, 3257–3271. (19) Hofer, R. Surface Modification for Optical Biosensor Applications. Ph.D. Dissertation, ETH Zurich, Zurich, Switzerland, 2000. (20) Dyer, C. K.; Leach, J. S. L. J. Electrochem. Soc. 1978, 125, 1032–1038. (21) Schultze, J. W.; Lohrengel, M. M.; Ross, D. Electrochim. Acta 1983, 28, 973–984. (22) Schmuki, P. J. Solid State Electrochem. 2002, 6, 145–164. (23) Balaur, E.; Macak, J. M.; Tsuchiya, H.; Schmuki, P. J. Mater. Chem. 2005, 15, 4488–4491. (24) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7, 1066–1070. (25) Shumaker-Parry, J. S.; Campbell, C. T.; Stormo, G. D.; Silbaq, F. S.; Aebersold, R. H. Probing Protein: DNA Interactions Using a Uniform Monolayer of DNA and Surface Plasmon Resonance;Proceedings of Scanning and Force Microscopies for Biomedical Applications II, SPIE; Nie, S., Tamiya, E., Yeung, E. S., Eds.; SPIE: San Jose, CA, 2000; Vol. 3922, pp 158-166. (26) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426–436. (27) Kingshott, P.; Griesser, H. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 403–412. (28) Bauer, S.; Park, J.; v.d. Mark, K.; Schmuki, P. Acta Biomaterialia 2008, 4, 1576–1582. (29) Song, Y.-Y.; Schmidt-Stein, F.; Bauer, S.; Schmuki, P. J. Am. Chem. Soc. 2009, 131, 4230–4231. (30) Song, Y. Y.; Hildebrand, H.; Schmuki, P. To be submitted for publication. (31) Shrestha, N. K.; Macak, J. M.; Schmidt-Stein, F.; Hahn, R.; Mierke, C. T.; Fabry, B.; Schmuki, P. Angew. Chem. 2008, 120, 1–5.
Published on Web 10/07/2009
DOI: 10.1021/la9032139
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SAMs consisting of porphyrin molecules provide ideal archetypes of 2D structures for fundamental studies on one hand, and they can be useful in a host of technology applications on the other hand. They are employed in photovoltaic devices32-37 and for specific catalytical processes.38 Furthermore, porphyrins occupy an important role in natural processes because they are the main building blocks of hemoglobin, chlorophyll, vitamin B12, and various enzymes.39 They can be synthesized in a large variety and can easily be manipulated by changing the central metal atom or by introducing different ligands into the porphyrin skeleton.40 In the present study, a porphyrin with a triethoxysilane anchor group was synthesized prior to its assembly on the oxide substrates and examined with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Because of its high surface sensitivity, which lies in the monolayer range,41-43 ToF-SIMS is a well-suited tool for the analysis of ultrathin organic layers. For example, Hagenhoff et al. observed the occurrence of defects in Langmuir-Blodgett films,44 Houssiau and co-workers investigated the formation of siloxane monolayers on hydroxyl-terminated surfaces,45,46 and several other SAM systems have been examined by ToF-SIMS.18,47-49 Surface-attached thin coatings of proteins can also be investigated with ToF-SIMS,50-52 and even attempts to determine their surface orientation have been undertaken.53 Porphyrins have only scarcely been examined with ToF-SIMS. For the current investigation, however, it is interesting that they were reported to often clearly display a molecular peak in their mass spectra.54-56 In this work, a zinc porphyrin containing a triethoxysilane functional group, Zn(II)-5-(tris-ethoxy-silane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrin (ZnTESP, C67H75N5O5SiZn) with a mass of 1121.5 u, was studied (32) Wamser, C. C.; Kim, H.-S.; Lee, J.-K. Opt. Mater. 2003, 21, 221–224. (33) Kay, A.; Gr€atzel, M. J. Phys. Chem. 1993, 97, 6272–6277. (34) Kay, A.; Humphry-Baker, R.; Gr€atzel, M. J. Phys. Chem. 1994, 98, 952– 959. (35) Liu, G.; Jaegermann, W.; He, J.; Sundstr€om, V.; Sun, L. J. Phys. Chem. B 2002, 106, 5814–5819. (36) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev. 2004, 248, 1363–1379. (37) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655–4665. (38) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bampos, N.; Lambert, R. M. Chem. Commun. 2004, 15, 1688–1689. (39) W€ohrle, D. J. Porphyrins Phthalocyanines 2000, 4, 418. (40) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 1, p 3. (41) Benninghoven, A. Surf. Sci. 1994, 299/300, 246–260. (42) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F., Jr. Anal. Chem. 1994, 66, 2170–2174. (43) Chilkoti, A.; Ratner, B. D.; Briggs, D. Anal. Chem. 1993, 65, 1736–1745. (44) Hagenhoff, B.; Deimel, M.; Benninghoven, A.; Siegmund, H.-U.; Holtkamp, D. J. Phys. D: Appl. Phys. 1992, 25, 818–832. (45) Houssiau, L.; Bertrand, P. Appl. Surf. Sci. 2001, 175-176, 351–356. (46) Houssiau, L.; Bertrand, P. Appl. Surf. Sci. 2003, 203-204, 580–585. (47) Ohlhausen, J. A.; Zavadil, K. R. J. Vac. Sci. Technol., A 2006, 24, 1172– 1178. (48) Houssiau, L.; Bertrand, P. Appl. Surf. Sci. 2001, 175-176, 399–406. (49) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Langmuir 2005, 21, 280–286. (50) Mantus, D. S.; Ratner, B. D.; Carlson, B. A.; Moulder, J. F. Anal. Chem. 1993, 65, 1431–1438. (51) Wagner, M. S.; Castner, D. G. Langmuir 2001, 17, 4649–4660. (52) Henry, M.; Bertrand, P. Surf. Interface Anal. 2009, 41, 105–113. (53) Leufgen, K.; Mutter, M.; Vogel, H.; Szymczak, W. J. Am. Chem. Soc. 2003, 125, 8911–8915. (54) Spiro, M.; Blais, J. C.; Bolbach, G.; Fournier, F.; Tabet, J. C.; Driaf, K.; Gaud, O.; Granet, R.; Krausz, P. Int. J. Mass Spectrom. Ion Processes 1994, 134, 229–238. (55) Quirke, J. M. E. Mass Spectrometry of Porphyrins and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 7, pp 371-423. (56) Suo, Z.; Avci, R.; Higby Schweitzer, M.; Deliorman, M. Astrobiology 2007, 7, 605–615.
3532 DOI: 10.1021/la9032139
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in regard to understand its adsorption behavior on native TiO2 layers on titanium metal and thin silicon dioxide on silicon for comparison.
Experimental Section Synthesis. Chemicals. All chemicals were purchased by chemical suppliers and used without further purification. All analytical reagent-grade solvents were purified by distillation. Dry solvents were prepared using customary literature procedures.57 Thin layer chromatography (TLC): Riedel-de Ha€en silica gel F254 and Merck silica gel 60 F254. Detection: UV lamp and iodine chamber. Flash chromatography (FC): Merck silica gel 60 (230-400 mesh, 0.04-0.063 nm). Solvents were purified by distillation prior to use. UV/vis spectroscopy: Shimadzu UV-3102 PC UV/vis/NIR scanning spectrophotometer; absorption maxima λmax are given in nanometers. Mass spectrometry: Micromass Zabspec, FAB (LSIMS) mode, matrix 3-nitrobenzyl alcohol. NMR spectroscopy: JEOL JNM EX 400 and JEOL JNM GX 400 and Bruker Avance 300. The chemical shifts are given in ppm relative to TMS. The resonance multiplicities are indicated as s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), and m (multiplet), and nonresolved and broad resonances are indicated as br. Elemental analysis (C, H, N): Succeeded by combustion and gas chromatographical analysis with an EA 1110 CHNS analyzer (CE Instruments). Synthesis of the Porphyrin Zn-TESP. 5-(Tris-ethoxy-silane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyltriphenyl)-porphyrin. A solution of 500 mg (0.55 mmol) of 5-(p-tert-butyl-acetato-phenoxy)-10,15,20-(p-tert-butyl-triphenyl)-porphyrin58 in 200 mL of formic acid was stirred for 8 h to obtain the deprotected acid. The progress of the reaction was followed via TLC. The solvent was removed on a rotary evaporator. Subsequently, the product was transferred to toluene and evaporated twice to remove any residual formic acid. The product was finally dried under reduced pressure. The dried product was dissolved in 50 mL of dry dichloromethane at 0 °C. EDC (105 mg, 0.55 mmol) and 75 mg HOBT (0.55 mmol) were added, and the solution was stirred for 1 h at 0 °C. After that period of time, 0.2 mL of 3-aminopropyltriethoxysilane (APS) was added to the solution, and the mixture was stirred at room temperature for 48 h. The reaction mixture was purified directly by column chromatography on silica gel without concentration on a rotary evaporator to avoid any polymerization of the silane anchoring group. A mixture of dichloromethane/ethyl acetate (4:1) as the eluent was used. The yield of the product after evaporation of the solvent was 380 mg (65% yield). 1H NMR (400 MHz, 25 °C, CDCl3): δ -2.76 (s, 2H, pyrr.-NH), 0.76 (m, 2H, CH2Si), 1.26 (t, 3J = 7.3, 9H, CH3CH2OSi), 1.61 (s, 27H, C(CH3)3), 1.81 (m, 2H, SiCH2CH2CH2), 3.51 (dd, 3J = 6.9, 13.4, 2H), 3.88 (q, 3J = 7.02, 6H, SiOCH2CH3), 4.80 (s, 2H, CH2CdO) 7.02 (t, 3J = 5.9, 1H, Cd ONH), 7.32 (d, 3J = 8.7 Hz, 2H, m-Ar-H), 7.77 (d, 3J = 8.3 Hz, 6H, m-Ar-H), 8.16 (m, 8H, o-Ar-H), 8.82 (d, 3J = 4.6 Hz, 2H, βpyrr.-H), 8.88 (m, 6H, β-pyrr.-H). 13C NMR (100.5 MHz, 25 °C, CDCl3): δ 7.67 (1C, CH2-Si), 18.27 (3C, SiOCH2CH3), 23.08 (1C, CH2CH2CH2), 31.63 (9C, C(CH3)3), 34.83 (3C, C(CH3)3), 41.39 (1C, NH-CH2), 58.50 (3C, Si-O-CH2-CH3), 67.71 (1C, CH2CdO), 112.97 (2C, m-Ar-C), 120.30 (3C, m-C), 120.38 (1C, m-C), 123.65 (6C, o-Ar-C), 131.08 (8C, β-pyrr.-C), 134.54 (6C, o-Ar-C), 135.88 (2C, m-Ar-C), 136.27 (1C, i-Ar-C), 139.23 (3C, i-Ar-C), 150.58 (3C, p-Ar-C), 157.14 (1C, p-Ar-C), 168.19 (1C, CdO). UV/vis (CH2Cl2) λmax (log ε): 419 (5.30), 516 (4.27), 553 (4.06), 593 (3.83), 648 nm (3.82). EA calculated for C67H77N5O5Si (1060.4): C 75.88; H 7.32; N 6.60; found: C 76.10, H 7.23, N 6.73%. MS (FAB): m/z (%) 1060 [M]þ. (57) Perrin, D. D.; Amarego, W. L. F. Purification of Laboratory Chemicals; 3rd ed.; Pergamon Press: Oxford, U.K., 1988. (58) Wessendorf, F.; Gnichwitz, J.-F.; Sarova, G. H.; Hager, K.; Hartnagel, U.; Guldi, D. M.; Hirsch, A. J. Am. Chem. Soc. 2007, 129, 16057–16071.
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Scheme 1. Schematic Drawing of the Last Steps of the Zn-TESP Synthesis and Possible Fragmentation of Zn-TESP in ToF-SIMS
5-(Tris-ethoxy-silane-propyl-amide-acetato-phenoxy)-10,15, 20-(p-tert-butyl-triphenyl)-porphyrinato-zinc. 5-(Tris-ethoxysilane-propyl-amide-acetato-phenoxy)-10,15,20-(p-tert-butyltriphenyl)-porphyrin (300 mg, 0.28 mmol) was dissolved in 200 mL of THF, and 120 mg of zinc-acetate (0.72 mmol) was added. The mixture was heated to reflux overnight. The reaction was monitored by TLC by the disappearance of the free porphyrin base. The solvent was removed on a rotary evaporator, and the residue was dissolved again in dichloromethane and filtered to remove any unreacted zinc-acetate. Evaporation of the solvent yielded 305 mg of product (95% yield). 1H NMR (400 MHz, 25 °C, CDCl3): δ 0.57 (m, 2H, CH2Si), 1.23 (t, 3J = 7.3, 9H, CH3CH2OSi), 1.54 (m, 2H, SiCH2CH2CH2), 1.62 (s, 27H, C(CH3)3), 2.93 (dd, 3J = 6.9, 13.4, 2H), 3.82 (q, 3J = 7.02, 6H, SiOCH2CH3), 3.94 (s, 2H, CH2CdO), 6.65 (t, 3J = 6.3, 1H, Cd ONH), 7.12 (d, 3J = 8.5 Hz, 2H, m-Ar-H), 7.77 (d, 3J = 8.3 Hz, 6H, m-Ar-H), 8.16 (m, 8H, o-Ar-H), 8.99 (m, 8H, β-pyrr.-H). 13C NMR (100.5 MHz, 25 °C, CDCl3): δ 7.48 (1C, CH2-Si), 18.23 (3C, SiOCH2CH3), 22.71 (1C, CH2CH2CH2), 31.65 (9C, C(CH3)3), 34.81 (3C, C(CH3)3), 40.91 (1C, NH-CH2), 58.44 (3C, Si-O-CH2-CH3), 66.88 (1C, CH2CdO), 112.77 (2C, m-Ar-C), 119.85 (1C, m-C), 121.20 (3C, meso-C), 123.46 (6C, o-Ar-C), 131.53 (2C, β-pyrr.-C), 132.08 (6C, β-pyrr.-C), 134.40 (6C, mAr-C), 135.73 (2C, o-Ar-C), 136.99 (1C, i-Ar-C), 139.99 (3C, i-Ar-C), 150.35 (11C, p-Ar-C, R-pyrr.-C), 156.73 (1C, p-Ar-C), 167.78 (1C, CdO). UV/vis (CH2Cl2) λmax (log ε): 421 (5.37), 549 (4.30), 588 nm (3.81). EA calculated for C67H75N5O5SiZn *3.5CH2Cl2 *3THF (1631): C 60.51; H 6.53; N 4.28; found: C 60.45, H 6.44, N 4.10. MS (FAB) m/z (%): 1121 [M]þ. The yields of the coupling reaction are good (65%) when ethyldiaminoisopropylcarbodiimide (EDC) is used as a coupling agent. Dry solvents are used and acids and bases are avoided for the reaction and during the purification on silica gel by column chromatography to prevent the functionalization of the silica gel or the polymerization of the molecule. The subsequent metalation with zinc works nearly quantitatively. Scheme 1 shows the last steps in the synthesis of Zn-TESP. Sample Preparation. Thin TiO2. Titanium foils (99.6% purity, Advent Ltd., 0.1 mm thickness, (1 1) cm2 pieces) were ultrasonically cleaned in ethanol (Merck) and deionized (DI) water, dip-etched in a solution consisting of 5 wt % HF, 50 wt % HNO3 (all Merck), and 45 wt % DI water, and thoroughly rinsed with DI water (Millipore) to grow an oxide film of ∼6 nm thickness. The oxide thickness was determined with XPS. Thin SiO2. Si wafers (Si(100), both sides polished, p-doped, thickness 485 ( 25 μm, 10 nm oxide) were cut and cleaned in acetone, isopropanol, and DI water in an ultrasonic bath. Subsequently, they were dipped into ammonium fluoride solution (40%, Merck) for 10 min in order to remove the native oxide, rinsed thoroughly, and immersed in piranha solution (1:3 H2O2 (30%)/ H2SO4 (97%), Merck) for 90 min at 120 °C to grow a defined oxide film of ∼2 nm thickness, as determined with XPS. The prepared wafers were thoroughly washed and stored in DI water. Langmuir 2010, 26(5), 3531–3538
Functionalization. The pretreated samples were blown dry in a nitrogen stream and immersed in a 0.05 mM solution of Zn-TESP in water-free toluene (Aldrich, 99.8% purity) for up to 24 h (1440 min) at 8 °C, room temperature (RT, 20 °C), and 70 °C. After self-assembly, the samples were rinsed with the solvent for 15 min to remove loosely bound material and to receive a monolayer coating of Zn-TESP. The samples were dried at 70 °C for 30 min and stored in the dark in a refrigerator. For the preparation of thick films, a droplet of Zn-TESP solution was allowed to evaporate on the substrates at 70 °C. All samples were stored in glass vessels covered with Al foil to prevent contamination. They are stable for more than 1 week of storage but were introduced into the UHV system quickly after preparation. ToF-SIMS and XPS Analysis. Positive and negative static SIMS measurements were performed on a Tof.SIMS 5 spectrometer (ION-TOF, M€ unster). The samples were irradiated with a pulsed 25 keV Bi3þ liquid-metal ion beam. Spectra were recorded in high mass-resolution mode (m/Δm > 8000 at 29Si). The beam was electrodynamically bunched down to 25 ns to increase the mass resolution and rastered over a 500 500 μm2 area. The primary ion dose density (PIDD) was kept at ∼5 1011 ions cm-2, ensuring static conditions. Signals were identified using the accurate mass as well as their isotopic pattern. In this article, only the positive ToF-SIMS spectra are presented because they contain a greater amount of information regarding our area of interest. XPS measurements were conducted on a high-resolution X-ray photoelectron spectrometer (PHI 5600) using monochromoated Al KR radiation (1486.6 eV, 300 W) for excitation. The binding energy of the target elements was determined at a pass energy of 23.5 eV and a total energy resolution of
