Chemical Microscopy of Surfaces by Sum Frequency Generation

Her research interests are in the areas of surface chemistry/reactions, nonlinear spectroscopy/microscopy, imaging, and bio-physical chemistry studies...
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J. Phys. Chem. C 2009, 113, 16575–16588

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FEATURE ARTICLE Chemical Microscopy of Surfaces by Sum Frequency Generation Imaging Katherine A. Cimatu and Steven Baldelli* ReceiVed: April 30, 2009; ReVised Manuscript ReceiVed: June 13, 2009

Sum frequency generation imaging microscopy has been developed and implemented in different systems from self-assembled monolayer on metal surfaces to reaction of a metal surface upon exposure to a corrosive material (cyanide solution). It also helped the fundamental issues of heterogeneity to be understood by using mapping analysis that considers an acquired spectrum in a chosen region-of-interest to be independent of its neighboring ROIs. The construction of the microscope has been utilized and modified to accommodate the improvement of its spatial resolution, signal-to-noise ratio, and faster accumulation time. The latter aspect is more dependent toward the kind of sample to be analyzed. Finally, the current resolution of this diffractionlimited microscope is ∼2 µm and there is still some room for improvement to reach its ultimate resolution of ∼800 nm. The main and final goal of this study is to provide chemical sensitivity, interfacial specificity, and a two-dimensional image based on the chemical properties of the adsorbed molecules on the surface and also the substrate itself. I. Introduction As the study of interfaces has become more sophisticated in the past few decades, it has been realized that even wellprepared surfaces may contain inhomogeneities from Ångstrom to millimeter length scale. Further, as surface chemistry has developed into a highly molecular science, it has also progressed in terms of microscopic analysis. While several techniques possess high spatial resolution, others high chemical selectivity, and others with the ability to study the interface in situ, few techniques are capable of all three.1-5 Due to this realization, there is an imminent need to develop surface spectroscopic techniques that are able to address the spatial inhomogeneities that exist on all solid surfaces.6 This article focuses on the recent progress in the development and implementation of an imaging microscope based on sum frequency generation vibrational spectroscopy. Sum frequency generation (SFG) is a nonlinear vibrational spectroscopy where the signal is due to the inherent vibrational spectrum of the molecules at the surface. Due to the symmetry properties of the technique, it is only sensitive to the molecules present at the surface.7-10 Since the SFG signal is coupled to an imaging system,11 the contrast in the images is from the spectral properties of the monolayer and thus, provides a chemical map of the surface. The spatial resolution is diffraction limited and on the order of one micrometer, which is appropriate to the study of SAMs in microcontact printing, spatial effects in heterogeneous catalysis, and electrochemistry including corrosion and corrosion inhibition. The imaging microscope has been used recently to address some systems that contribute to the fundamental views of surface chemistry. These studies and experiments are as follows:

(1) The patterned microcontact printed self-assembled monolayers12-14 on gold with variation in different terminal functional groups, chain length, stamping, and backfilling times, mixing between two defined regions at the boundary that affected the interpretation of chemical contrast (two-dimensionally); (2) the initial catalysis reaction/corrosion (carbon monoxide chemisorbed on Pt and cyanide ions reacting on gold forming linearly bound CN- ions and complex compounds); (3) the adsorbed octadecanethiol on mild steel and zinc surfaces to study the orientation of the molecules on the surface which also includes the identification of the heterogeneity of the surface.15-20 II. SFG Theory Since the contrast in the SFG images is critically dependent on the local SFG spectrum, the most relevant properties of the spectroscopy are provided. SFG vibrational spectroscopy is a second-order nonlinear process that, in the present setup, involves the overlap of a fixed 1064 nm beam and a tunable IR beam on the surface at the same time and space that generates the coherent SFG signal at the sum of the two input frequencies. The basic geometry is shown in Figure 1. The SFG intensity is directly proportional to the square of the second-order nonlinear susceptibility and the incident beams (eq 1). This nonlinear susceptibility is composed of two components which are the resonant susceptibility due to the vibrational transitions of the adsorbed molecules and the nonresonant background from the metal substrate (eq 2). The resonant susceptibility contains the chemical information of the molecules at the interface and which is also equal to the total number of modes, the orientational averaged hyperpolarizability that contains the product of the IR and Raman transition

10.1021/jp904015s CCC: $40.75  2009 American Chemical Society Published on Web 08/26/2009

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Cimatu and Baldelli cos(ε-δ(ωIR)), between the resonant and the nonresonant susceptibility results in a variety of line shapes as seen in the spectrum (Figure 2) and as demonstrated in Figure 3 for a variety of relative phase differences. The consequence of this effect is that all spectra must be curve fit, according to Equation 3, in order to extract useful information or interpretation from the images.10 Finally since SFG intensity is proportional to the second order susceptibility, χ(2), only molecules in a noncentrosymmetric environment, such as the surface, will produce SFG signal.22,26 III. Experimental Section

Katherine A. Cimatu graduated from University of the Philippines-Diliman and received her Ph.D. from University of Houston. She is currently working as a postdoctoral research associate under Dr. Paul Cremer at Texas A&M UniversitysCollege Station. Her research interests are in the areas of surface chemistry/reactions, nonlinear spectroscopy/microscopy, imaging, and biophysical chemistry studies.

Steven Baldelli received his B.S. degree from Framingham State College in Massachusetts in 1992 and his Ph.D. from Tufts University in 1998 under the direction of Mary Shultz. After spending three years at the University of California, Berkeley, with Gabor Somorjai and Phil Ross, he moved to University of Houston, where he is now an Associate Professor of chemistry. He is also a visiting professor at the Royal Institute of Technology in Stockholm, Sweden. His interests center on using linear and nonlinear spectroscopic and microscopic methods to study surface chemistry problems including liquid and solid interfaces, SAMs, electrochemical interfaces, and problems in corrosion.

moments. If the IR frequency matches any of the vibrational transition frequency of the adsorbed molecules on the surface, the difference will approach zero and thus enhancing the SFG intensity (eq 3). The change in SFG intensity signal is plotted as a function of the IR frequency as shown in Figure 2. The plot displays the resonant modes by the dips from the baseline of the nonresonant background according to eq 4.21-25

ISF ∝ |χ(2) | 2IIRI1064nm

(1)

(2) χ(2) ) χNR + χR(2)

(2)

(2) (2) χ(2) ) χNR + χR(2) ) χNR +

∑ ωIR -N〈βωq 〉+ iΓ (2)

(3)

q

(2) 2 (2) 2 I ∝ |χR(2) + χNR | ) |χR(2) | 2 + |χNR | + (2) |cos[ε - δ(ωIR)] (4) 2|χR(2) ||χNR

It is important to note that the cross-term in eq 4 affects the line shape of the vibrational spectrum. The phase difference,

General Overview of the Sum Frequency Generation Imaging Microscopy. The SFG imaging microscope involves the same ‘front-end’ as normal SFG spectroscopy, the difference being the treatment of the signal beam.27,28 In normal SFG (IR scanning) spectroscopy, the signal is usually detected by a single channel element such as a photomultiplier tube, PMT. A photograph and the schematic diagram of the imaging microscope are shown in Figures 4 and 5, respectively. The letter labels in Figure 4 correspond to the combination of optical parts used in our SFG imaging microscope. In this system laser pulses of ∼20 ps are used with a energy density of 10 and 50 mJ/cm2 for the 1064 nm and infrared beams, respectively. SFG is capable of detecting sub monolayer coverage and as low as 1% of a monolayer in certain systems.28 As shown in Figures 4 and 5, the SFG beam is generated at an angle of ∼62.1°. The coherent SFG beam generated at the interface passes through several different optical components such as the 1:1 telescope system, the diffraction grating, the microscope objective, and the intensified CCD camera. These components are described in the following figures and paragraphs. Figure 5 illustrates the parts of the telescope which functions to maintain the size of the SFG beam as generated at a specific angle. The telescope is composed of lenses that image the beam onto the plane of the grating. These set of optics are constructed by combining plano-convex and -concave lenses to help reduce any chromatic or spherical aberrations that may occur. The grating is used to project the intermediate focused image perpendicularly onto the focal plane of the objective. The SFG beam impinges the grating at the first order diffraction angle and is then diffracted perpendicular to the grating plane. This configuration keeps the entire field of view in focus. A microscope is placed after the grating and is composed of 2 optical components, an objective and a tube lens. The objective lens magnifies the beam ten times its initial size (a 20× or 50× objective are also used) while the tube lens collimates the intermediate SFG image. A 1064 nm holographic notch filter is placed after the microscope to cut or filter any reflected 1064 nm beam. The collimated SFG beam is detected by a chargedcoupled device (CCD) camera that can detect the low signals of sum frequency generated signal. As the IR wavelength is scanned, the collected SFG beam is developed into a spectroscopic image of the surface. The resolution and calibration of the microscope was performed by the use of USAF 1951 test pattern and application of the Rayleigh criterion to the acquired SFG images. The Rayleigh criterion determines whether two points are resolvable from each other, this criteria is applied to a line pattern etched Au surface as shown in Figure 6a. A region of interest (ROI) was chosen as depicted by the red box in the figure. From which a line profile was extracted. The intensity profile is plotted as a function of the number of pixels as shown in Figure 6c. Then, its first derivative was taken and fitted using the Gaussian fitting

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Figure 1. Schematic diagram of two beams overlapping on the surface and their descriptions is as follows: (A) Surface plane, (B) plane of incidence, (C) fixed 1064 nm beam, (D) tunable infrared beam, and (E) sum frequency generation beam.

Figure 2. SFG vibrational spectrum which is obtained from a solutiondeposited octadecanethiol on gold substrate.15

Figure 3. Simulation of an SFG spectrum, with a single resonance at 2900 cm-1, that shows a constructive, destructive and asymmetric line shapes based on phase. If the value of the term is equal to (+1) then constructive interference gives rise to a peak (-90°). On the other hand, if the value is equal to (-1) then a destructive interference is expected of the line shape, out-of-phase (90°) which gives rise to dips. While, between the constructive and destructive interferences will result to asymmetric lineshapes. Line shapes in SFG are dependent on the phases of the two susceptibilities thus if the phase for both does not coincide with each other, then the resultant line shape is a combination of a peak and a dip (asymmetric line shape).

function, Figure 6d. In Figure 6e, the concept of Rayleigh criterion was applied to find 85% of the sum of the two maximum peaks and then determine the resolution of the image. The edge resolution of this area was determined to be approximately 5 pixels which are determined to have an equivalent size of approximately 5 µm.

The second method that we used for determining the resolution of our current SFG imaging microscopy setup is by using a 1951 U.S. Air Force Test Resolution pattern, as shown in Figure 7a. This is used to evaluate and determine the size of two clearly resolved line spaces. As shown in Figure 7b, this test resolution pattern was used to determine the dimensions of the lines of the AF pattern (spacing for resolution) which are shown as 100-, 20-, 5-, 2-, 1-, 8-, and 50-µm-sized line patterns. This AF pattern was also used as a master pattern to prepare the polydimethylsiloxane pattern for lithography so that a sample with known dimension can be utilized for comparison purposes. Figure 7c shows an SFG image of an etched Au (using the PDMS to pattern on the gold surface), and then an SFG image of the pattern was obtained to determine the resolution of the microscope. As shown in Figure 7d, an SFG image of a microcontact printed pattern of a thiol monolayer was also obtained and used to confirm the resolution of the recent SFG imaging microscope. After comparison, it was verified that the microscope is able to resolve the 2 µm spaced lines but not the 1 µm lines. SFGIM Data Analysis. SFG image analysis serves at least two functions. First, the images provide local information on the surface chemistry. Second, the local spectra are analyzed statistically to provide information on the distribution of surface species at the interface. This section describes the process of analyzing the imaging data, Figure 8. The first step is the acquisition of the images where a set of images are collected for every 5 cm-1. The images collected are acquired for 1000-5000 accumulations using the CCD camera. This setting is dependent on which sample is analyzed. If a sample has high SFG signal, then the accumulation time can also be shortened. A scan in the CH range of 2800-3150 cm-1 is performed, a total of 71 SFG images are then acquired. These images are stacked together in increasing wavenumber position using the ImageJ program.29 Then, the next step is to choose a regionof-interest (ROI). The ROI might be the entire 1024 × 1024 pixel array of the CCD or it can also be an (n × n) pixel array. The local spectra are extracted and curvefit to obtain the spectral quantities that are then used to supply information on the local molecular species, orientation, or conformation. Similarly, the ensemble data is used to establish an average and distribution on the monolayer properties when each ROI is considered as a delta-function orientation. Sample descriptions and peak assignments are further discussed in the next few sections of this article. The next few sections will describe how the images and results are interpreted based on the chemical contrast. These

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Figure 4. Photograph of a working SFG imaging microscope: (A) sample stage, (B) 1:1 telescope, (C) grating, (D and E) microscope, (F) filter, and (G) intensified CCD camera. Note: orange and bright red lines are the two incidents beams necessary to generate the SFG beam (maroon line).

Figure 5. Schematic diagram of the SFG imaging microscope.

experiments were performed to test the capability of the imaging microscope. Microcontact printed thiol monolayers on gold surface were used in this section to provide a more defined area for the imaging experiment where the thiol molecules can be easily recognized based upon their chemical nature. The projects on microcontact printed samples were critical in the development of the SFG imaging microscopy by interpreting the images based on the inherent vibrational spectrum of the adsorbed molecules in defined and confined

regions such as striped and checkered patterns. Using this knowledge, real samples without the patterns, whether homogeneous or heterogeneous, were investigated within the limit of the technique’s resolution. IV. Results Based on Different Chemical Contrast Interpretation This section describes the factors that affect the SFG signal and demonstrate the factors’ effects in the contrast of the SFG

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Figure 6. Etched Au with line patterns and used the microscope to collect SFG images: (a) A region-of-interest (ROI) was chosen as depicted by the red box in the figure. (b) A zoom image of the ROI. In this red box, an intensity profile or line profile was extracted from the image. (c) The extracted intensity profile is plotted as a function of the number of pixels. (d) A first derivative was taken and fitted using the Gaussian fitting function. (e) The concept of Rayleigh criterion was applied to find 85% of the sum of the two maximum peaks and then, determine the resolution of the image.

images. This also includes the effects of disorder in selfassembled monolayers. Chemical Contrast Based on Organic Functional Groups. Microcontact printing (µ-CP) is a convenient and effective method to prepare controlled heterogeneous systems by modifying the identity of surface molecules such as the end group of n-alkanethiols. It is a unique soft lithography method that utilizes a polydimethylsiloxane (PDMS) stamp that is saturated with specific thiol molecules which are transferred onto a substrate in a patterned and controlled fashion.14 The stamped pattern on gold, as an example, may contain areas without the adsorbed molecules due to the pitch and space in between each pattern. A second functional group is introduced by backfilling in between the pattern with another alkanethiol. The microcontact printing process was used to prepare methyl- and methoxy-terminated SAM pattern. A 1 mM C16 dithiocarboxylic acid was used as a stamping solution. This solution was used to saturate the surface of a PDMS surface containing line patterns with different sizes before printing it on a gold surface. Then, a 1 mM of 14-methoxy-tetradecane1-thiol was used as a backfilling solution which introduced methoxy functional groups onto the open spaces that were not covered by the stamping solution. Thus, as expected, there will be two regions with different functionalized self-assembled thiol monolayers.

The SFG images were taken from 2750-3050 cm-1 obtaining a total of 61 images. From the stack of images, two images are shown that correspond to 2810 and 2875 cm-1, respectively in Figure 9, panels a and b. 2810 and 2875 cm-1 are the A′ outof-plane-OCH3 stretch, doublet, and methyl (CH3) symmetric stretch.30-37 These two resonances are chosen to illustrate where the chemical contrast is observed. At 2810 cm-1, a contrast is observed (Figure 9a), the dark region corresponds to the methoxy-terminated C16 thiol adsorbed molecules which is further demonstrated by tuning to 2875 cm-1, the methyl resonance, the image contrast inverts clearly indicating the contrast in SFG imaging is primarily due to the chemical functionality present on the surface. The SFG image is directly related to the SFG spectrum, the negative contrast in this system, where “dark” is the location of the molecule or area of interest, and is where the resonances appear as dips in the spectrum (2) of gold with opposite phase (see eq 4 and against a large χNR Figure 3). If the IR frequency is not in resonance with any of the vibrational modes of the adsorbed surface molecules, the SFG image remains a bright region. For example, in an image taken at IR position 2900 cm-1, as shown in Figure 9e, almost no contrast is observed between the two regions. By examining the two vibrational spectra, Figure 9, panels c and d, at 2900 cm-1, there is no presence of a dip. The SFG intensity at this

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Figure 7. (a) 1951 U.S. Air Force Test Resolution pattern, (b) an optical image of an AF pattern using a regular microscope, (c) an SFG image of an etched gold pattern, and (d) an SFG image of a microcontact printed thiol monolayer obtained at a certain wavenumber.

Figure 8. SFGIM data analysis where SFG images are collected for every 5 cm-1 from 2800-3150 cm-1 to create an image stack. A region of interest (ROI) is chosen, and the SFG intensity values are extracted at a specified region and plot every point as a function of IR wavenumber. The data is presented in a histogram to obtain the statistical information on the monolayer.

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Figure 9. Two SFG images were taken at (a) 2810 and (b) 2875 cm-1. SFG spectra were also extracted from these two images as shown in (c) the spectrum obtained at 2810 cm-1 and (d) the spectrum obtained at 2875 cm-1. Another image was also taken at 2900 cm-1.30

Figure 10. Demonstration of the odd-even effect on the orientation of the terminal methyl group of SAMs. (a) Even number of carbon atoms has the C3 axis of the methyl oriented more along the surface normal; (b) odd number of carbon atoms has the C3 axis more tilted with respect to the surface normal. (c) The direction of the normal-mode vibrations for the symmetric and antisymmetric CH3 stretching vibration.

IR position has remained along the red line. The bright signal of the image is coming from the gold SFG nonresonant background. Metal substrates such as gold and copper have a large nonresonant signal. The nonresonant signal indicates that this signal is not dependent on the IR frequency or any vibrational mode of the adsorbed molecules. This section has demonstrated SFG imaging microcopy is capable of differentiating between two functional groups based upon the unique vibrational modes of the adsorbed molecules on the surface. However, since intensity in the SFG spectrum depends on orientation and concentration on the surface, these features are also exploited to obtain image contrast and thus local chemical information. This next section will demonstrate the capability of the microscope to differentiate and obtain contrast based upon the orientation of the adsorbed molecules on the surface with the same terminal functional group. The SFG analysis on chemical contrast is now based on the orientation of the terminal methyl group with difference in chain lengths (odd/even effect) where a chemical contrast is evident on the images.

Chemical Contrast Based on Orientation (Odd-Even Effect). In this experiment, the microcontact printed monolayer on gold was prepared by using a checkered board pattern with dimentions of 100 × 100 µm2.17 A 1 mM solution of C16 dithiocarboxylic acid was utilized to saturate the PDMS stamp (with the pattern) before the thiol molecules were transferred onto the gold surface. Then, 1 mM C17 dithiocarboxylic acid was used as a backfilling solution to fill in the areas without the C16 dithiocarboxylic acid molecules. As shown in Figure 10a, the terminal group of the even C16 dithiol molecule is oriented approximately 30° along the surface normal compared to the odd C17 dithiol molecules where its terminal group is more tilted away from the surface normal (≈60°), Figure 10b. This difference in orientation of both molecules affects the SFG signal in the vibrational spectrum. Based upon the IR selection rule, only vibrational transitions with normal mode components that are positioned along the surface normal are observed in the spectrum.38-40 Therefore, if the chain is tilted farther away from the surface normal, the CH3(asym) mode has a greater component along the surface

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Figure 11. SFG images and extracted spectra are shown to demonstrate the odd-even effect dithiol carboxylic acid molecules on gold substrate. The shown images are taken at (a) 2875 and (b) 2975 cm-1. (c) Vibrational spectra of the C16 (0) and C17 (b) dithiol carboxylic acid molecules are shown.17

normal, and consequently the SFG signal increases. The odd and even numbers of methylene groups present in the chain affect the orientation of the terminal methyl group, as illustrated in Figure 10, panels a and b. In Figure 10c, the methyl group orientation is shown where also the symmetric (2875 cm-1) and asymmetric (2965 cm-1) vibrational modes are labeled. This difference in the SFG intensity for both of the spectra obtained at 2975 cm-1 (methyl asymmetric stretch), as shown in Figure 11c, gives good contrast in the SFG image, Figure 11b. If there is no difference in the SFG output signal for a specific vibrational mode then not much contrast is observed; for example, the image taken at 2875 cm-1 (symmetric stretch) as shown in Figure 11a. Thus, SFG imaging is able to obtain local identification of the monolayer based upon the orientation of the adsorbed molecules even with the same functional groups.17 The next section describes a system where the chemical contrast is affected by mixing of the molecules across the border of two regions through (a) a diffusion process, (b) a backfilling process, and (c) the chemical nature of the adsorbed molecules. Chemical Contrast Being Affected by Mixing between Two Regions. The samples are prepared by utilizing a 1 mM solution of C16 dithiol and 1 mM solution of C16 phenylterminated thiol, as stamping and backfilling solutions, respectively. Two structures of the thiol molecules are illustrated in Figure 12, panels a and b, and corresponding SFG images of the µ-CP patterns are also presented in Figure 12c-e that correspond to images taken at IR positions 2875, 3065, and 2900 cm-1, respectively. To show that there is significant mixing between two regions, a detailed analysis by extracting the spectrum data across an area for every 20 × 20 pixel array as shown in Figure 12, panels c and f (zoom), was performed. The red box in Figure 12g, the spectra labeled from 1 to 15, the intensity of the peak positioned at 2875 cm-1 for the methyl symmetric stretch varies and at the same time, the intensity of the peak at 3065 cm-1 (blue box) for the aromatic CH stretch mode of the phenyl ring from positions 1 to 15.

For example, for the transitions from spectra positions 1-7, at the spectra labeled 1-3, the dominant species at this region is the methyl-terminated thiol from the C16 dithiocarboxylic acid monolayer, stamped region. However, under close examination, at spectrum 3, there is already a significant contribution from the 3065 cm-1 peak. Then, from positions 4-6, the 3065 cm-1 species is more dominant over this area. However, on the sixth spectrum, the methyl group peak from the C16 dithicarboxylic acid is now more evident compared to the aromatic CH stretch peak from the phenyl-terminated C16 thiol molecule. The result is that backfilled molecules (phenyl-terminated) appear into the stamp pattern of the surface (methyl-terminated), especially at the boundary regions. Thus, this mixing across the border of two regions affects the quality of the chemical contrast of the obtained SFG images in terms of extracting the spectrum and also how it affects the resolution of the border. Since the contrast is maximum for perfect segregated molecules and zero for randomly mixed system, partial mixing blurs the boundary between domains in the SFG image. The next few sections provide the information on surfaces without any defined regions such as occurrence of corrosion, corrosion-inhibition, and catalysis. V. Application Results of SFGIM on Heterogeneous Samples Based on Corrosion, Corrosion Inhibition, and Catalysis Concepts Corrosion of Gold Surface upon Exposure to Cyanide Solution. Corrosion is a very important and ubiquitous process. Often corrosion process of metal is not uniform, but develops in a localized region. Thus, while the majority of the sample is initially passive or inactive, the reaction is initiated at certain local spot. By imaging the sample, these local corrosion spots can be identified and studied to determine the local corrosive species that are present and help explain how the reaction initiates. The following results were shown to test the capability

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Figure 12. (a) Methyl-terminated C16 dithiol carboxylic acid molecule, (b) phenyl-terminated C16 thiol molecule; SFG images taken at (c) 2875, (d) 3070, and (e) 3105 cm-1. (f) Enlargment of the area with a blue circle in Figure 12a; (g) extracted SFG vibrational spectra of the area in Figure 12f; (h) The areas where the spectra were extracted.

Figure 13. (A and B) SFG images of gold film’s initial reaction to 0.5 M CN- solution acquired at 2105 and 2225 cm-1, respectively. (C) Average spectrum of the first scan acquisition upon exposure of gold film to cyanide solution. (D) A 300 × 300 pixel area portion of the whole (a representative of the whole area). (E and F) SFG images acquired approximately after 8 h of exposure to the reactive cyanide solution. (G) Average spectrum of the last scan acquisition (after 8 h).41

of the imaging microscope in a heterogeneous system and also to analyze a solid-liquid interface. In this experiment, a 0.5 M NaCN solution was prepared and exposed the 1 × 1 cm2 gold wafer inside the SFG cell; this concentration allowed the reaction to proceed slow enough to image with SFG.41 The image in Figure 13A was taken at IR position 2105 cm-1, whereas Figure 13B was taken at 2225 cm-1. As observed, only a minor difference in contrast is observed from changing the IR position. The average spectrum of the initial reaction of gold to cyanide is shown at Figure 13C, where the dominant peak is at 2105 cm-1 and a strong band near 2200 cm-1 exists. After 8 h of exposure to cyanide solution, the reaction progresses and the resulting images are shown in Figure 13, panels D and E. The average spectrum that corresponds to this set of images taken after 8 h has a dominant peak at 2225 cm-1.42 The peaks in the spectra are assigned to specific Au-CN species; that is, 2105 cm-1 is the linear bound Au(0)-CN, whereas the 2140

and 2170 cm-1 are due to the Au(I)-(CN)2 complex and the >2200 cm-1 is the tetracyanogold complexes of the Au(III) species. The high oxidation states of gold are indications of the extent of the corrosion of the gold metal surface. As the reaction has progressed, the surface has more of the tetracyano-gold complexes, instead of the linear gold-cyanide (Au-CN), and they are distributed in a nonuniform way across the surface.41,43-50 The distribution of the gold cyanide complexes are visualized using the mapping analysis. A 50 × 50 µm2 pixel area extraction of the data was performed. Then, the intensities were plotted as a function of IR wavenumber and fitted using eq 3. The raw fitting amplitude values were taken and mapped back into a contour plot (two-dimensional plot). Examples of contour plots are shown in Figures 14 and 15. Panels a and b in Figure 14 are the contour plots taken at 2105 and 2225 cm-1, respectively. As seen, smaller ROI were chosen at the contour plots to represent the entire area. Two spectra

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Figure 14. Chemical maps of the C≡N peaks on the gold surface at early stage of the reaction (a) 2105 and (b) 2225 cm-1(labeled areas are ROI A-F); (c and d) Extracted SFG spectra from selected 50 × 50 µm regions to show the inhomogeneities on the surface.41

Figure 15. Chemical maps of the C≡N peaks on the gold surface after 8 h of reaction. (a) 2105 and (b) 2225 cm-1 (labeled areas are ROI A-F); (c and d) Extracted SFG spectra from selected 50 × 50 µm regions to show the inhomogeneities on the surface.41

are shown in Figure 14, panels c and d. In a comparison of both spectra, the spectrum shown in Figure 14c has the 2225 cm-1 species more dominant over the rest of the species, whereas in Figure 14d, the 2105 cm-1, the linear Au-CN species, is more dominant compared to higher frequencies, 2140, 2170, and 2225 cm-1 species. Thus, from the spectra of two smaller regions, the distribution of the cyanide species across the whole area differs at different positions in the image, and it is also clearly seen at the contour plot that the distribution varies from point-to-point. In Figure 15, panels a and b, contour plots are shown for 2105 and 2225 cm-1. These spectra were taken after 8 h of exposing the gold surface to the cyanide solution. Smaller ROIs were chosen, and two spectra are shown in Figure 15, panels c

and d. From the two spectra, the contribution of the different species varies from the two ROIs, though the most dominant evident species is the tetracyano gold complex, 2225 cm-1. As the reaction progressed, the linearly attached cyanide on gold led to the formation of cyano-complexes, especially the tetracyano gold complex, the final corrosion product. This corrosion experiment demonstrates that the imaging microscope can be used as a tool to analyze and perform in situ and solid-liquid interface experiments. Specifically, the SFG microscopy demonstrates the local nature of this corrosion reaction. While the entire surface appears to be covered by the linear CN species only in this specific location are the higher CN complexes observed. Thus while most of the

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Figure 16. (a)An average spectrum of an octadecanethiol deposited on a mild steel surface and (b) a set of SFGIM spectra obtained from the stack of images. The image size is 1 × 1 mm.

Figure 17. SFG imaging analysis for ODT/MS (A) contour plots CH3 (sym/asym) intensity ratios (scale is a.u.); (B) CH2/CH3 (sym) intensity ratios; and (C and D) histogram plots of the same ratios, respectively.18

surface is passive, there are specific regions active in the reaction. The reaction proceeds according to the following reaction scheme: +CN-

+CN-

Au(0) + CN- a Au(0)CN {\} [Au(I)(CN)2]- {\} Linear

Dicyanide +CN-

[Au(I)(CN)3]-2 {\} [Au(III)(CN)4]Tetracynide

Without the ability to spatially resolve the spectra, we interpret only the average change in the surface species

not realizing that the progress from linear to tetracyanide species occurs in local regions not average over the surface. While other studies have observed the evolution in the surface as the oxidation reaction progress,51 this is the first to show the local evolution of the chemical species directly. Corrosion Inhibition of Mild Steel Surface by Forming Self-Assembled Octadecanethiol Monolayer. In this experiment, the polished and cleaned surface of the mild steel was used and upon application of reducing potential in a 1 mM solution of octadecanethiol (ODT) solution, the ODT monolayer assembled on mild steel.18,52

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Figure 18. SFG imaging analysis for ODT/Au (A) contour plots CH3 (sym/asym) intensity ratios; (B) CH2/CH3 (sym) intensity ratios; and (C and D) histogram plots of the same ratios, respectively.18

As shown in Figure 15b (labeled as a, b, c, d, and e) each ROI produces a different spectra; further these spectra are different from the average spectrum, Figure 15a. The spectra from these smaller ROIs have the same number of peaks but are different in terms of their ratio with each other. From the figure, a 20 × 20 pixel area analysis was performed. The intensity data values were plotted as a function of IR wavenumber. The spectra for each ROI were fitted using eq 3. Then, the raw values obtained from the fitting were used for mapping analysis. For example, the ratio of the CH3 symmetric/asymmetric stretch (intensity ratio) corresponds to tilt angle; the higher the value for the ratio means the smaller tilt angle from the surface normal. Thus, as shown in Figure 16a, the interpretation of the map shows the distribution differs across the surface. By analyzing the histogram, Figure 16c, the most dominant ratio value is ∼1.3 which means that on average the molecules are tilted at ∼35°. Then, the intensity ratio of the CH2sym/CH3sym was also obtained which pertains qualitatively to the number of conformational gauche defects across the area, as shown in Figure 16b. An increase in the ratio value means an increase in the number of defects. The average ratio value obtained the histogram plot is ∼0.6 in Figure 16d. The results from the ODT/MS system were also compared to ODT/Au since gold is known to be a stable, well-ordered monolayer. The same analysis was performed to obtain the distribution of molecules in terms of tilt angle and gauche defects. The maps and histogram plots are shown in Figure 17, and based on these results, the ODT/Au is more homogeneous compared to ODT/MS.18 The results show that SAMs on the gold surface form well packed dense monolayers with a narrow range of orientations, while the same SAMs on mild steel have a wider range of orientations and a greater amount of defects. This is understood to be due to the chemical reactivity of the substrate. While the gold is inert at ambient conditions the mild steel is active and will oxidize to form iron oxides. There are no SAMs on these oxidized patches and thus the SAMs adjacent to these are more

Figure 19. (A-E) False-color, unprocessed SFG image of CO on platinum at PCO ) 1 atm. Images are taken with pp polarization. (F) Spectra are extracted from the images obtained at each wavenumber interval. The bar in image B represents the position where the spectra are extracted, right-to-left.16

disordered than the SAMs on the smooth metal surface where they are densely packed. In addition, statistical analysis of SFG image provides important information on the monolayer that is not easily obtained by the normal, spatially averaged, signal. By treating

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Figure 20. SFG image maps of the spatial distribution of CO on a Pt surface (zoom in on top half of image). (a) A spectrum presenting the low frequency peak centered at 2062 cm-1, (b) a spectrum presenting the high frequency peak centered at 2083 cm-1, (c) the low frequency peak centered at 2062 cm-1, (d) the high frequency peak centered at 2083 cm-1, and (e) histogram of the number of occurrence of a CO peak at a specific wavenumber.16

each ROI interest as an independent spectrum both the average and deviation of the spectroscopic properties are obtained and thus sample uniformity is evaluated quantitatively. Heterogeneous Sample That Exhibits Catalysis Reaction. This section describes an example related to heterogeneous catalytic reactions wherein a polished polycrystalline platinum surface is exposed to carbon monoxide. The carbon monoxide was chemisorbed on the Pt surface at 1 atm pressure. As shown in Figure 18, a set of SFG images were obtained from 1900-2150 cm-1. The images shown have different chemical contrast. Five images were chosen as sample images from Figure 18a-e to show the changes in chemical contrast. For example, in Figure 18b, a ROI was chosen to indicate the area where intensity values were extracted from the set of acquired images. These values are plotted as a function of IR wavenumber. As shown in Figure 18f, from left-to-right, top-to-bottom, the top site CO on platinum has different IR positions which might be due to the electronic nature of the substrate or presence of substrate defects that affected the CO attachment on the platinum surface. To further explore the imaging results, a 20 × 20 µm2 pixel area was performed for mapping analysis, Figure 19. The values were extracted, plotted, and fitted. The raw values from the fitting

are remapped onto the surface. The maps are presented to show the distribution of peaks in the chosen ROI. For example, at low frequency peak (Figure 19a spectrum) centered at 2065 cm-1, CO species are more dominant at the area as indicated by the red color in the map (Figure 20c). As shown in Figure 20d, on the other hand, when the higher frequency peak was chosen for the mapping centered at 2080 cm-1, the map shows that these species are more present in the area surrounding the lower frequency peak area, as indicated by the red color (higher raw intensity value). The histogram plot in Figure 20e presents the number of occurrences of a CO peak at a specific number, and as analyzed, the species are fairly distributed across the area. This experiment was performed to further exemplify the heterogeneity of a surface. The presence of different CO species on a polycrystalline surface was determined using SFGIM at the interface where vibrational spectra and images were obtained. SFGIM allowed the surface to be visualized for a better understanding of interfacial chemistry, and the results presented demonstrated the importance of considering the defects and inhomogeneities that exist on the surface that also influence the interpretation of SFG data and images. The heterogeneity of a surface clearly depends on several factors which are as follows: (a) quality of the

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substrate (polished or not cleaned), (b) nature of the attaching species on the surface, (c) exposure time, and (d) reaction time. Or the heterogeneity can also be explained by a change in temperature, humidity, pressure, and amount/concentration of the attaching species, in this case, CO species.16 VI. Conclusions The significance of all of these experiments will soon lead to more complicated systems to learn more about the surface chemistry of the molecules adsorbed on the surface and how their stability is affected by a change in their environment by applying heat, potential, substrate, and chemical environment (exposure of the substrate with adsorbed molecules to different corroding solvents). The ability to locate the molecules spatially and spectrally has opened an opportunity to study the adsorbed molecules in a two-dimensional domain and understand the nature of the adsorbed molecules (chemical and physical properties) separately. A functional sum frequency generation imaging microscope was constructed and built with a ∼2 µm resolution. The microcontact printing process provided the method in testing the capability of the SFGIM with the variation in analyzing the chemical contrast observed on the images with the following factors: (1) The difference in the SFG spectra of the two molecules (2) Degree of orientational ordering (3) Instrumental factors (4) Microcontact printing process, stamp, gold, and solution qualities Corrosion: the gold film when exposed to cyanide solution was another breakthrough because the experiment itself was in a solid-liquid chemical interface. The gold film etched with time and other complex species of the cyanide on gold existed beside the linearly attached CN on gold atoms as the reaction progresses. These experiments showed that at certain ROIs detection of other species were possible and were spatially distributed across the region by mapping. Corrosion Inhibition: Octadecanethiol on different metal substrates were performed and the nature of the both adsorbed thiol molecules and metal substrate were studied. The results presented demonstrated the importance of considering the defects and inhomogeneities existing on the surface that also influence the interpretation of SFG data and images. Catalysis: the chemisorbed carbon monoxide (CO) on platinum surface showed that depending on the nature of the platinum substrate, there are other CO-Pt species that existed. Acknowledgment. We are grateful to the NSF and PRF for support of the SFG imaging projects. We also thank Ekspla for initial funds to help build the new SFG microscope. We all greatly appreciate Prof. T. Randy Lee and his group for making all of these interesting alkanethiols for our studies. References and Notes (1) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13. (2) Zhou, Y.; Fan, H.; Fong, T.; Lopez, G. P. Langmuir 1998, 14, 660. (3) Klauser, R.; Hong, L.-H.; Wang, S.-C.; Zharnikov, M.; Paul, A.; Golzhauser, A.; Terfort, A.; Chuang, T. J. J. Phys. Chem. B 2003, 107, 13133–13142.

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