2444
Langmuir 2007, 23, 2444-2452
Adsorption and Desorption of Stearic Acid Self-Assembled Monolayers on Aluminum Oxide Min Soo Lim,† Ke Feng,‡ Xinqi Chen,§ Nianqiang Wu,| Aparna Raman,⊥ Joshua Nightingale,‡ Ellen S. Gawalt,⊥ Dimitris Korakakis,‡ Larry A. Hornak,‡ and Aaron T. Timperman*,† C. Eugene Bennett Department of Chemistry, Lane Department of Computer Science and Electrical Engineering, and Department of Mechanical and Aerospace Engineering, West Virginia UniVersity, Morgantown, West Virginia 26506, NUANCE Center, Northwestern UniVersity, EVanston, Illinois 60208, and Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282 ReceiVed July 3, 2006. In Final Form: October 31, 2006 Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.
1. Introduction A self-assembled monolayer (SAM) is a two-dimensional molecular array that is spontaneously organized by adsorption of amphiphilic organic molecules on a solid inorganic surface. Several good examples include alkanethiols on noble metals such as gold1-5 and silver,6,7 alkyl carboxylic acids on metal oxides,8-11 alkyl phosphonic acids on mica,12,13 and alkyl silanes * Corresponding author: phone, 304-293-3435 ext. 6455; e-mail,
[email protected]. † C. Eugene Bennett Department of Chemistry, West Virginia University. ‡ Lane Department of Computer Science and Electrical Engineering, West Virginia University. § NUANCE Center, Northwestern University. | Department of Mechanical and Aerospace Engineering, West Virginia University. ⊥ Department of Chemistry and Biochemistry, Duquesne University. (1) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Systematic Studies of the Frictional Properties of Fluorinated Monolayers with Atomic Force Microscopy: Comparison of CF3- and CH3-Terminated Films. Langmuir 1997, 13, 71927196. (2) Shon, Y.-S.; Lee, T. R. Desorption and Exchange of Self-Assembled Monolayers (SAMs) on Gold Generated from Chelating Alkanedithiols. J. Phys. Chem. B 2000, 104, 8192-8200. (3) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, T. R.; Perry, S. S. Molecularly Specific Studies of the Frictional Properties of Monolayer Films: A Systematic Comparison of CF3-, (CH3)2CH-, and CH3-Terminated Films. Langmuir 1999, 15, 3179-3185. (4) Lee, S.; Shon, Y.-S.; Ramon Colorado, J.; Guenard, R. L.; Lee, T. R.; Perry, S. S. The Influence of Packing Densities and Surface Order on the Frictional Properties of Alkanethiol Self-Assembled Monolayers (SAMs) on Gold: A Comparison of SAMs Derived from Normal and Spiroalkanedithiols. Langmuir 2000, 16, 2220-2224. (5) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Molecular contribution to the frictional properties of fluorinated self-assembled monolayers. Tribol. Lett. 1998, 4, 137-140.
on SiO2 surfaces.14,15 An adsorbing molecule consists of a head group that enables the molecule to anchor to a surface, a carbon backbone chain whose length significantly influences the packing density of the SAM,8,11 and a tail group that determines interfacial properties of the SAM.1,3-5 The growth of a SAM relies on subtle balance between substrate-head group interactions7 and (6) Rieley, H.; Kendall, G. K.; Jones, R. G.; Woodruff, D. P. X-ray Studies of Self-Assembled Monolayers on Coinage Metals. 2. Surface Adsorption Structures in 1-Octanethiol on Cu(111) and Ag(111) and Their Determination by the Normal Incidence X-ray Standing Wave Technique. Langmuir 1999, 15, 8856-8866. (7) Rieley, H.; Kendall, G. K. X-ray Studies of Self-Assembled Monolayers on Coinage Metals. 3. Angularly Resolved Near Edge X-ray Absorption Fine Structure Determination of the Orientation in 1-Octanethiol SAMs on Ag(111) and Cu(111). Langmuir 1999, 15, 8867-8875. (8) Allara, D. L.; Nuzzo, R. G. Spontaneously Organized Molecular Assemblies. 1.Formation, Dynamics, and Physical Properties of n -Alkanoic Acids Adsorbed from Solution on an Oxidized Aluminum Surface. Langmuir 1985, 1, 45-52. (9) Taylor, C. E.; Schwartz, D. K. Octadecanoic Acid Self-Assembled Monolayer Growth at Sapphire Surfaces. Langmuir 2003, 19, 2665-2672. (10) Allara, D. L.; Nuzzo, R. G. Spontaneously Organized Molecular Assemblies. 2. Quantitative Infrared Spectroscopic Determination of Equilibrium Structures of Solution-Adsorbed n-Alkanoic Acids on an Oxidized Aluminum Surface. Langmuir 1985, 1, 52-66. (11) Tao, Y.-T. Structural Comparison of Self-Assembled, Monolayers of n-Alkanoic Acids on the Surfaces of Silver, Copper, and Aluminum. J. Am. Chem. Soc. 1993, 115, 4350-4358. (12) Nie, H.-Y.; Miller, D. J.; Francis, J. T.; Walzak, M. J.; McIntyre, N. S. Robust Self-Assembled Octadecylphosphonic Acid Monolayers on a Mica Substrate. Langmuir 2005, 21, 2773-2778. (13) Doudevski, I.; Schwartz, D. K. Mechanisms of Self-Assembled Monolayer Desorption Determined Using in Situ Atomic Force Microscopy. Langmuir 2000, 16, 9381-9384. (14) Ru¨he, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Structure and Tribological Properties of Ultrathin Alkylsilane Films Chemisorbed to Solid Surfaces. Langmuir 1993, 9, 2383-2388. (15) Grange, J. D. L.; Markham, J. L. Effects of Surface Hydration on the Deposition of Silane Monolayers on Silica. Langmuir 1993, 9, 1749-1763.
10.1021/la061914n CCC: $37.00 © 2007 American Chemical Society Published on Web 01/30/2007
Adsorption and Desorption of Stearic Acid
chain-length-dependent intermolecular interactions.8,10,11,16,17 Therefore, the substrate-head group interactions should not be too strong, allowing molecules to rearrange laterally after initial adsorption at the surface, thereby leading to the production of a SAM with high packing density through intermolecular interactions. The growth of a SAM is also influenced by various experimental variables such as solvent,9,18 temperature,2 concentration,3 growth time,9 degree of surface dehydration,15,19 and contamination.19 The intermolecular interactions of the SAM monomers can provide highly ordered monolayers with nearly defect free coverage under the appropriate conditions while the head group can provide strong binding with the surface. The uniform high coverage and stability make SAMs ideal for many technological applications,20 such as lubrication of the underlying substrate surface, corrosion inhibition by passivating the surface, surface patterning as photoresists, diffusion barrier, and prevention of nonspecific adsorption and biofouling. The ability to vary both head groups2,4,5 and tail groups1,3,5 makes it possible to tailor SAMs for specific applications. In this study, both amorphous alumina and single crystalline sapphire are used as substrates to study the desorption of SAMs. The desorption and stability of SAMs on alumina surfaces are of particular interest because alumina forms the surface of newly developed waveguide sensors, such as the stacked planar affinity regulated resonant optical waveguide (SPARROW).21 This sensor responds to changes in the interfacial refractive index, and without modification of the alumina surface it exhibits little selectivity. In order for this device to be used as a selective biosensor, nonspecific adsorption must be minimized and biomolecular recognition imparted on the surface. The layer that prevents nonspecific adsorption must provide nearly full coverage, be thin, be stable in an aqueous environment, and be removable. Thus, SAMs are excellent for this application, but little is known about their stability on alumina in an aqueous environment. Therefore, the focus of this work is to investigate the desorption and stability of SAM layers formed on alumina surfaces in water. The sapphire substrates are used to provide a homogeneous surface with atomic steps and terraces that can be reproduced by high-temperature thermal annealing. As a consequence, sapphire provides a controlled aluminum oxide surface to grow a SAM of n-octadecanoic acid or stearic acid allowing fundamental studies on the desorption kinetics of a SAM in water. X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy are used to probe the bonding nature of selforganized stearic acid on the sapphire and alumina surfaces, leading to the conclusion that stearic acid adsorbs on sapphire surfaces through a bidentate interaction of carboxylate with two (16) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Chain Length Dependence of the Frictional Properties of Alkylsilane Molecules Self-Assembled on Mica Studied by Atomic Force Microscopy. Langmuir 1996, 12, 235-237. (17) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Scanning Force Microscopic Exploration of the Lubrication Capabilities of n-Alkanethiolate Monolayers Chemisorbed at Gold: Structural Basis of Microscopic Friction and Wear. Langmuir 1997, 13, 2504-2510. (18) Anderson, M. R.; Evaniak, M. N.; Zhang, M. Influence of Solvent on the Interfacial Structure of Self-Assembled Alkanethiol Monolayers. Langmuir 1996, 12 (10), 2327-2331. (19) Pertays, K. M.; Thompson, G. E.; Alexander, M. R. Self-assembly of stearic acid on aluminum: the importance of oxide surface chemistry. Surf. Interface Anal. 2004, 36, 1361-1366. (20) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Molecular Monolayers and Films. Langmuir 1987, 3, 932950. (21) Lloyd, D.; Hornak, L.; Pathak, S.; Morton, D.; Stevenson, I. Application of Ion Beam Assisted Thin Film Deposition Techniques to the Fabrication of a Biosensor Chip With Fieldability Potential for Important Biohazard Detection Applications. Soc. Vac. Coaters, Annu. Tech. Conf., 47th 2004, 334-339.
Langmuir, Vol. 23, No. 5, 2007 2445
oxygen atoms while it adsorbs on alumina surfaces through both bidentate and monodentate interactions. Long-chain alkanoic acids were used to form the SAMs, because they have been shown previously to form close-packed monolayer films with alkyl chains highly oriented on an alumina surface.8,10,11 Following growth of the SAMs on the sapphire and alumina surfaces, desorption of the monolayer was followed over a period of 0-240 min. On the sapphire, the changes of surface topography, hydrophobic nature and packing order of the coating during desorption were monitored with atomic force microscopy (AFM), water contact angles, and IR spectroscopy. A series of measurements led to the conclusions that (1) highly organized stearic acid-alkyl layers are not stable in water because they are continuously lost through molecular desorption and (2) there is a large difference between desorption rates from atomically flat terrace sites that have lower surface energies, and step edges, kinks, and atomic vacancies that have higher surface energies. Dependence of monolayer desorption on substrate surface crystallinity was investigated with amorphous alumina and single crystalline sapphire. Water contact angles measured from amorphous alumina decreased monotonically without any regional distinction as desorption time increased. This monotonic desorption profile is consistent with desorption from amorphous (or polycrystalline) alumina with different types of adsorption sites originated from irregularly distributed surface defects that are energetically and crystallographically inhomogeneous. 2. Experimental Section 2.1. Materials. Stearic acid or n-octadecanoic acid (CH3(CH2)16COOH, 98+%) and hexadecane (CH3(CH2)14CH3, 99%) were purchased from Sigma-Aldrich, Inc. (St. Louis, MO), and were used as received without further purification. Hydrochloric acid (HCl, 37.3%), sulfuric acid (H2SO4, 95.7%), and hydrogen peroxide (H2O2, 30%) were purchased from Fisher Scientific (Fair Lawn, NJ). HPLC grade toluene, acetone, hexane, and 2-propanol were also purchased from Fisher. Absolute ethanol (200 proof) was purchased from AAPER Alcohol & Chemical Co. (Shelbyville, KY). Two inch DIAXO 43MMT sapphire wafer (C-plane) was purchased from Kyocera Corp. (Kyoto, Japan). The alumina film was produced using ion beam assisted electron beam evaporation on a 3 in. silicon wafer (University Wafer, South Boston, MA) or a 3 in. borofloat wafer (Bes Optics) with oxygen ions produced via a cold cathode gridless ion source. During the deposition, the oxygen flow rate and ion source drive current were maintained at 18.0 sccm and 1.3 A, respectively. The film was deposited at a rate of 3.5 A/s, and the final thickness was measured to be approximately 204 nm, as determined by spectroscopic ellipsometry. 2.2. Sample Preparation and Treatment. Water used for sample rinse, sample immersion, and contact angle measurements was purified with a NANOpure Infinity System (Barnstead/Thermolyne Co., Dubuque, IA) equipped with Type D8952 Remote Dispenser and NANOpure Infinity Base Units. Small pieces of sapphire were cut from a 2 in. wafer and cleaned by ultrasonication in a series of organic solvents (toluene, acetone, hexane, 2-propanol, and ethanol) for 1 h and dried in the oven. Sapphire samples were etched for another hour in either piranha solution (H2SO4:H2O2 ) 3:1) or a solution that is made of deionized water, phosphoric acid (H3PO4), and sulfuric acid (H2SO4) with a volumetric ratio of 5:1:1. The temperature for the etching process was maintained at 70 °C in a water bath. They were then rinsed extensively with deionized water and dried with Ar gas. Caution: Etching solution is Very corrosiVe. Always wear heaVy acid-proof gloVes, wear a lab coat, wear proper shoes, use double containment, and use the solution in a hood when handling an etching solution. Cleaned samples have been annealed at 800, 980, 1050, and 1500 °C in a time range between 1 h and 2 days. A box furnace (Isotemp Muffle Furnace, model 550-126; maximum temperature, 1125 °C) purchased from Fisher Scientific (Hampton, NH) was used to anneal sapphire samples up to 1050 °C.
2446 Langmuir, Vol. 23, No. 5, 2007 A tube furnace (model, STF54434C; maximum temperature, 1700 °C) purchased from Lindberg/BlueM (Asheville, NC) was used to anneal sapphire samples at 1500 °C. Surface topography was obtained for samples annealed at each temperature with AFM to monitor the reconstruction process of the sapphire surface as annealing temperature increased. Postannealed sapphire samples were cooled extensively with Ar gas and incubated immediately in a solution of SAM to minimize the risk of surface contamination from the air. Adsorption of stearic acid on sapphire surfaces was completed at room temperature with a 1.5 mM solution in n-hexadecane. Growth time of 165.5 h was applied to produce a SAM of full coverage. Completion of SAM surfaces was verified with AFM. A SAM surface grown for 165.5 h on sapphire annealed at 1050 °C was used to explore the binding state of stearic acid on the sapphire surface with XPS and IR spectroscopy. The SAMs prepared by the same condition have been immersed for designated times (3, 30, 60, 120, 180, and 240 min) in deionized water that was continuously agitated to maintain dynamic flow of water. They were then used to investigate the desorption kinetics of the SAM in water with both AFM and contact angle measurements. IR measurements were also taken of the surfaces of the SAMs before and after 240 min of immersion in water. Two groups of SAMs of stearic acid have been separately grown for 165.5 h on the sapphire surfaces previously annealed at two different temperatures, 1050 and 1500 °C. They were then immersed in water for the designated times (3, 30, 60, 120, 180, and 240 min). A series of contact angle measurements were performed on the SAMs at the designated immersion times. This was to investigate desorption kinetics of SAMs formed on two sapphire surfaces discriminated by the relative amount of surface defects. Small pieces of alumina film were cut from the wafers and cleaned by ultrasonication in a series of the same organic solvents that were used for cleaning the sapphire substrates. Alumina films were dried by Ar gas and incubated in the furnace at 250 °C for 30 min to dehydrate the surface. They were cooled with Ar gas, incubated immediately in 1.5 mM solution of stearic acid, and remained in the solution for 165.5 h. SAMs have also been grown on the sapphire surfaces previously annealed at 1050 °C. SAMs formed on alumina and sapphire surfaces have been immersed for the designated times in deionized water that was continuously agitated. Water contact angles were then measured for the SAMs grown on both alumina and sapphire. This was to investigate desorption kinetics of SAMs formed on homogeneous (sapphire) and heterogeneous (alumina) surfaces. 2.3. Topography Acquisition and Image Analysis. Surface topography of bare and SAM-coated sapphires was obtained at ambient conditions with a Veeco Metrology Nanoscope IIIa Multimode AFM and electronics (Santa Barbara, CA) equipped with an E scanner. All images were collected in tapping mode with silicon probes purchased from Veeco Metrology, LLC (Santa Barbara, CA). The probe has a sharp tip underneath the cantilever (radius,
