FOCUSED ON SURFACE AND

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1 A SUITE OF UNDERGRADUATE LABORATORIES FOCUSED ON SURFACE AND INTERFACE SCIENCE K.G. Severin, G.J. Blanchard and M.L. Bruening, Department of Chemistry, Michigan State University, East Lansing, MI 48824

2 Abstract Although many breakthroughs in modern interdisciplinary science occur at the interface between two phases, most undergraduate curricula offer little to enhance students understanding of interfaces. To address this need, we have developed an undergraduate laboratory course in surface and interface science that will be offered for the first time this fall. In this semester-long course, students will meet for two 1-hour lectures and one 3- hour lab session each week. In lab they will perform a suite of experiments on a single well-characterized interfacial system: alkanethiols on gold. Students will monitor monolayer growth in real time using a quartz crystal microbalance; monolayer thickness and organization will be measured using optical ellipsometry and reflectance FTIR; and cyclic voltammetry and contact angle measurements will be used to probe the physical properties of the monolayers. Taken as a whole, this course and lab experience will provide our students with a solid background in an important but often ignored area of chemistry.

3 Why teach this course? Introduction Interfaces are important in many applications including separations, catalysis, chemical sensors, and tribology. Students gain experience in simple material synthesis and materials characterization. Using surface sensitive techniques allows students to see the connection between microscopic structure and the macroscopic properties. By applying a cohesive suite of experiments to one chemical problem, students get a realistic idea of how a materials chemist might study a new material and rationally approach the tailoring of its properties. The methods used in this course demonstrate the remarkable fact that, even under ambient conditions, tools exist for characterizing films that are only one molecule thick: Ellipsometry has a resolution of a few angstroms Grazing incidence external reflectance FTIR can provide a spectrum of a monolayer Electrochemical measurements are sensitive to defects in a monolayer Contact angles reflect molecular level disorder at an interface Quartz crystal microbalances respond to the uptake of nanogram quantities of molecules adsorbing at their surfaces

4 Laboratory Experiments and Lecture Topics I. Overview Model System: Alkanethiols on Gold II. III. IV. Optical Ellipsometry Laws of refraction and reflection Polarization of light Fresnel Equations Ellipsometric quantities and Ψ and determination of layer thickness Contact Angle Measurements Basis of measurement (Young s Equation) Surface roughness effects Surface tension Zisman plots and critical surface tension Surface free energy determination Quartz Crystal Microbalance (QCM) Gravimetry History of the technique Bulk acoustic wave devices Operation of QCM/Sauerbrey equation Vapor vs. liquid phase operation (Liquids-equivalent circuits) Langmuir adsorption isotherms: assumptions Determination of G ads, H ads, and S ads V. Electrochemistry Fundamentals of electrode processes Cyclic Voltammetry AC Impediance Electrochemical equivalent circuits VI. Fourier Transform Infrared Spectroscopy (FTIR) Fundamentals of infrared spectroscopy Interferometry Fourier transforms Reflectance FTIR Considerations in measuring monolayers Determination of monolayer organization and aliphatic chain tilt angle

5 Model Interfacial System Alkanethiols on Gold Formed readily Highly reproducible Exhibit a high degree of order Properties (i.e. surface wettability, aliphatic chain ordering, electrochemical activity) are well-characterized Surface properties can be modified by using thiol adsorbates with different terminal functionalities Thiol adsorption is much faster than aliphatic chain organization so the two processes can be studied independently Potential for these materials to have technological applications in the near term. Reaction: Au + RSH Au-SR + ½ H 2 Annealed Monolayer Structure: 30 Tail Group Head Group

6 Preparation of Self-Assembled Monolayers Substrates: Glass slides: ½ x ½ coated with a 2000 Å layer of vacuum evaporated 99.99% gold on a 50 Å chromium adhesion layer Quartz crystal microbalances: AT-cut crystalline quartz with polished, vapor-deposited gold surfaces Thiol adsorbates: Octadecanethiol 11-mercaptoundecanoic acid (MUD) CH 3 (CH 2 ) 17 SH HOOC(CH 2 ) 10 SH Monolayer formation: Immerse cleaned* substrate in a 0.1 mm thiol solution in n- hexane for 24 h. *Substrate Cleaning: Plasma cleaning or Pirhana solution (3:1 conc. sulfuric acid : 30% H 2 O 2 ) followed by rinsing in distilled water and immediate drying. Caution: pirhana solution is extremely corrosive and should be used only in a hood. Annealing Monolayer structural formation which is mediated by aliphatic chain organization. Annealing can be done under an inert atmosphere or in the thiol solution and can require from several hours to days, depending upon the conditions.

7 Optical Ellipsometry Purpose To demonstrate the effect of surface contamination on the formation of monolayers. To study the structural evolution of the monolayers by measuring the monolayer thickness as a function of annealing time. To demonstrate the formation of the monolayer and estimate its thickness. Theory The polarization condition of an electromagnetic wave is altered by reflection from a surface. In general, both the phase and the relative amplitude of the Cartesian components of light are changed. These changes are reflected in the ellipsometric quantities and Ψ which can be used to determine the complex dielectric response (refractive index) and thickness of overlayers on a reflective surface.

8 Optical Ellipsometry (continued) Experimental Setup Light Source PMT Polarizer Quarter Wave Plate Null Analyzer Ellipsometer (adapted from H.G. Tompkins, A User s Guide to Ellipsometry Experiments (1) SiO 2 /Si standards will be analyzed to familiarize the students with the technique and to show the wavelength dependence of the refractive index. (2) Freshly prepared monolayers will be compared with those stored in air, under N 2, in solution, or in hexane for a week. This will illustrate the effects of surface contamination, annealing and dissolution on the measured thickness. (3) Thickness changes will be measured as a function of in-solution annealing time. Since the mass adsorption is essentially complete within minutes of immersion, decreases in thickness will reflect the extent of organization of the aliphatic chains. (4) Compare the apparent thickness of monolayers prepared on aged and cleaned gold surfaces with those prepared on freshly evaporated gold surfaces.

9 Contact Angle Measurements Purpose To demonstrate the relationship between the properties and chemistry of a surface: Wettability (ability of a fluid to cover a surface) varies with both the completeness of the monolayer and its degree of order. Wettability also varies with the polarity of the monolayer surface functional groups. To determine the surface free energy of the monolayers by measuring contact angles as a function of surface tension of a series of liquids. Theory Tangent to drop profile θ = Contact Angle θ Liquid drop Substrate Young s Equation: γ sg = γ sl + γ lg cos θ γ = surface tension which can be thought of as the energy required to create a unit area of an interface Energy minimization: If the energy required to create the solid-liquid (sl) interface is greater than that required for creation of a solid-gas (sg) interface, then the critical angle will be greater than 90. In other words, the liquid will bead up on the surface to minimize the solid-liquid interfacial area.

10 Contact Angle Measurements (continued) Determination of surface free energy cos θ θ Zisman plot for monolayer of CH 3 (CH 2 ) 21 SH on gold 1 Critical surface tension γ c γ lg (mn/m) For a particular surface, liquids with a surface tension this value will wet the surface. A fluid with θ 10 wets the surface. Surface free energy γ c so the lower γ c, the lower free energy of the surface. Critical Surface Tensions γ c 1 Surface γ c, mn/m CF 3 (CF 2 ) 10 CO 2 H/Pt 6 Teflon 18 CH 3 (CH 2 ) 21 SH/Au 19 CH 3 (CH2) 17 SiCl 3 /Si 20 Hexatriacontane 22 CH 3 (CH 2 ) 17 NH 2 /Pt 24 polyethylene 31

11 Contact Angle Measurements (continued) Experimental Setup: Syringe Fiber Optic Illumination Sample Camera Computer Experiments (1) Contact angles on cleaned and uncleaned gold surfaces will be measured. This will to show the effect of surface contamination on the wettability. (2) Contact angles on an octadecanethiol monolayer will be measured as a function of the annealing time. This will allow correlation between the degree of organization of the alkyl chain and the wettability. (3) The surface free energy of gold, octadecanethiol monolayers, and MUD monolayers will be determined by measuring the contact angles of different fluids on these surfaces and constructing Zisman plots.

12 Purpose Quartz Crystal Microbalance (QCM) Gravimetry To study in situ solution phase kinetics of adsorption/desorption of alkanethiols, graphically illustrating the reversible nature of the adsorption process. To determine the free energy, enthalpy, and entropy of adsorption. Theory Monolayer formation follows Langmuir adsorption isotherm kinetics: dθ = ka( 1 θ ) C k d θ (1) dt where θ is the fraction of available sites, C is the thiol concentration, and k a and k d are the rate constants for the adsorption and desorption processes, respectively. Integration of equation 1: Setting: C ( kac+ kd ) t θ ( t) = (1 e ) (2) kd C + k a k = k C + k obs a d (3) and gives: k = C k C + k d a (4) k obs t θ ( t) = k (1 e ) (5) By fitting experimental data to equation 5, k obs and k can be determined.

13 QCM Gravimetry (continued) For this experiment, the monolayers are assembled on gold-coated quartz crystal microbalances (QCM s). The oscillation frequency of a QCM is a function of its mass. A change in oscillation frequency f, as the thiol is adsorbed on the QCM, is directly proportional to θ, the fraction of the surface that is covered Raw QCM data and best-fit curve to equation 5 for 10 µm thiol solution at 288 K. k obs is determined from the fitted curve. 9 - f (Hz) time (s) A plot of k obs vs. thiol concentration has k a as the slope and k d as the y- intercept (equation 3). The equilibrium constant K eq is equal to the ratio of k a /k d. The free energy of adsorption G ads at a given temperature is determined from: G = RT ln (6) abs K eq Since: G = H T S (7) abs abs abs both H ads and S ads can be determined by carrying out this experiment over a range of temperatures. H ads is the y-intercept and - S ads is the slope of a plot of G ads vs T.

14 Experimental Setup QCM Gravimetry (continued) +5V DC Oscillator Circuit Frequency Counter 5,923, Temperature Controlled Waterbath Jacketed Beaker QCM Stirrer Computer Once the QCM oscillation frequency has stabilized, the stock thiol solution is injected into the beaker with a syringe. Experiments (1) The kinetics of the adsorption reaction will be studied for a range of solution concentrations: 300 to 3µM (Langmuir adsorption isotherm kinetics are followed over this concentration range). Students then use curving fit to determine the rate constants k obs vs. thiol concentration. From this data, K eq for this reaction and the free energy of adsorption G ads are determined. (2) By repeating this experiment at a number of temperatures between 283 K and 303 K, H ads and S ads are determined.

15 Electrochemistry Purpose To show that electrochemistry is more sensitive than bulk methods (FTIR and ellipsometry) at revealing monolayer defects To detemine the capacitance and dielectric constant of the monolayer interface To estimate the defect density of the monolayers and study them as a function of time Theory The capacitance C of the monolayer can be determined using cyclic voltammetry in the supporting electrolyte: i = Cυ where i is the current and υ is the scanning rate. From this, the dielectric constant of the monolayer ε can be determined using: εε C = 0 d where ε 0 is the permittivity of free space and d is the thickness of the monolayer. In AC Impedence spectroscopy, impedence is measured as a function of frequency. Using the equivalent circuit shown below: R soln Capacitance Z w R ct Diffusional Capacitance the charge transfer resistance R ct can be estimated. Surface coverage can be estimated from: R R bare ct coated ct = 1 θ where θ is the fraction of the surface covered.

16 Experimental Setup Electrochemisty (continued) N 2 purge gas CH Instruments Electrochemical Bench Ag/AgCl reference electrode Gold working electrode Pt counter electrode Computer Experiments (1) Cyclic voltammetry with only supporting electrolyte using both bare gold and monolayer-coated electrodes to determine the capacitance of the monolayer. (2) Cyclic voltammetry using Fe(CN) 6 3- as the redox couple with both bare and monolayer-coated electrodes. The shape of the current may suggest an assembly of pinholes (microelectrodes) in the monolayer. In poorly organized monolayers, peaks will appear and rate constants can be estimated. (3) AC impedance using Fe(CN) 6 3- /Fe(CN) 6 4- with both bare gold and monolayer coated electrodes to estimate the surface coverage. Using electrodes with new and aged monolayers, defects can be studied as a function of time.

17 External Reflectance FTIR Purpose To study the conformational distribution of the constituent molecules within the monolayer (are they ordered or disordered?). To estimate the average tilt angle of the aliphatic chains with respect to the surface. To verify experimentally the surface selection rule governing adsorption at metallic interfaces by measuring the signal as a function of polarization angle. Theory Absorbance FTIR spectrum of the C-H stretch region of an alkanethiol monolayer on gold 8 Frequency (cm -1 ) The absorbance frequency is dependent upon the degree of organization as shown in the following table: Comparison Of Liquid And Crystalline State -CH 2 - Absorbance Frequencies With Annealed Monolayer Values 8 Peak positons (cm -1 ) Group mode Crystalline Liquid CH 3 (CH 2 ) 17 SH on Gold -CH 2 - ν a ν s As the monolayers anneal, the CH 2 - absorption frequency decreases from a value characteristic of a disordered liquid state to one indicative of a more ordered crystalline-like state.

18 Orientation of the aliphatic chains Reflectance FTIR (continued) Using the Fresnel equations previously developed in ellipsometry, we can show that absorbance A at a metal interface depends upon the orientation of the dipole moment of the absorber. Only components of the dipole that are normal to the surface are active. Using this fact, we can determine the orientation of the dipole using the equation: A A isotropic 2 = 3cos θ Estimation of the tilt angle of aliphatic chains requires comparison of the measured absorbance with that calculated for a hypothetical isotropic monolayer 10. Experimental Setup Polarizer Sample 80 MCT Detector Source Fixed 80 Grazing Angle Accessory Experiments (1) Measure crystalline and liquid alkanethiols (transmittance) to determine the location of the C-H stretching frequencies for each. (2) Compare the location of the absorbances in freshly prepared and annealed monolayers. (3) Determine the tilt angle of the alkyl chains.

19 References 1. C. D. Bain, E. B. Troughton, Y.-T. Tao, J. Evall, G. M. Whitesides, R. G. Nuzzo, Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold J. Am. Chem. Soc., 111, 321 (1989). 2. A. Ulman, Formation and Structure of Self-Assembled Monolayers, Chem. Rev., 96, (1996). 3. D. S. Karpovich and G. J. Blanchard, An Undergraduate Laboratory Experiment for the Direct Measurement of Monolayer Formation, J. Chem. Ed., 72, (1995). 4. D. S. Karpovich and G. J. Blanchard, Direct Measurement of Adsorption Kinetics of Self-Assembled Monolayer Films on a Microcrystalline Gold Surface, Langmuir, 10, (1994). 5. E. B. Troughton, C. D. Bain, G. M. Whitesides, Monolayer Films Prepared by the Spontaneous Self-Assembly of Symmetrical and Unsymmetrical Dialkyl Sulfides from Solution onto Gold Substrates: Structure, Properties, and Reactivity of Constituent Functional Groups Langmuir, 4, (1988). 6. C. D. Bain, G. M. Whitesides, Correlations between Wettability and Structure in Monolayers of Alkanethiols Adsorbed on Gold J. Am. Chem. Soc., 110, (1988). 7. L. H. Dubois and R. G. Nuzzo, Synthesis, Structure and Properties of Model Organic Surfaces, Annu. Rev. Phys. Chem., 43, (1992). 8. M. D. Porter, T. B. Bright, D. L. Allara, C. E. D. Chidsey, Spontaneously Organized Molecular Assemblies. 4. Structural Characterization of n-alkyl Thiol Monolayers on Gold by Optical Ellipsometry, Infrared Spectroscopy, and Electrochemistry J. Am. Chem. Soc., 109, 3559 (1987). 9. H. M. Shessler, D. S. Karpovich and G. J. Blanchard, Quantitating the Balance Between Enthalpic and Entropic Forces in Alkanethiol/Gold Monolayer Self Assembly, J. Am. Chem. Soc., 118, (1996). 10. D. L. Allara and R. G. Nuzzo, Spontaneously Organized Molecular Assemblies. 2. Quantitative Infrared Spectroscopic Determination ofequilibrium Structures of Solution-Adsorbed n-alkanoic Acids on an Oxidized Aluminum Surface, Langmuir, 1, (1985).

20 Acknowledgment We are grateful to The National Science Foundation for their support in the development and implementation of this course through grant NSF DUE