J. Phys. Chem. B 1998, 102, 4933-4943
4933
Examination of the Photoreaction of p-Nitrobenzoic Acid on Electrochemically Roughened Silver Using Surface-Enhanced Raman Imaging (SERI) X. M. Yang,†,§ D. A. Tryk,† K. Hashimoto,‡ and A. Fujishima*,† Department of Applied Chemistry, Faculty of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan, and Research Center for AdVanced Science and Technology, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku 153, Japan ReceiVed: February 17, 1998
The present work is the first report of the use of surface-enhanced Raman scattering (SERS) for two-dimensionally spatially resolved, chemically selective monitoring of the course of a surface-catalyzed photoreaction. We have recently developed the surface-enhanced Raman imaging (SERI) technique, which is chemically selective, has monolayer sensitivity, and can be used under ambient conditions. p-Nitrobenzoic acid (PNBA) deposited on an electrochemically roughened silver surface was used as a model system. In order to examine the dependence of the photoreaction on the state of aggregation of the molecules, two types of samples were prepared, one in which the compound was uniformly adsorbed as a monolayer and another in which there was a combination of monolayer plus crystallites. In the form of a uniformly adsorbed monolayer, the SERS image (based on the 1348 cm-1 band) exhibited a relatively uniform intensity level over the sample surface, due to uniform roughness, but there were slight variations that are related to small differences in the surface roughness, as measured with atomic force microscopy (AFM). A photoreaction, induced by the Ar+ illumination (514.5 nm), involving PNBA is known to be strongly catalyzed by metallic silver and is strongly suspected to produce azodibenzoate. The extent of this photoreaction, which was successfully mapped using SERI, was found to depend upon whether the PNBA was in the form of a monolayer or multilayers (i.e., crystallites). The time dependence of the SERS intensity clearly shows that the photocatalytic reaction, as monitored by both the 1348 cm-1 reactant peak and the 1437 cm-1 product peak, takes place to a much larger extent (∼80%) when PNBA is adsorbed as a monolayer, while the reaction proceeds to a much lesser extent (10-20%) when the compound is present as small crystallites on the surface, even for the molecules that are in direct contact with the Ag surface, due to steric limitations. However, work of others (Bercegol, H.; Boerio, F. J. J. Phys. Chem. 1995, 99, 8763-8767) has shown that PNBA molecules in the form of an adsorbed layer but separated from the Ag surface by ∼1.8 nm can still undergo photoreaction, showing that an electromagnetic-type mechanism may also be operative for surface-catalyzed photoreactions.
1. Introduction Various types of photochemical reactions have been found to be catalyzed on silver and gold surfaces, particularly roughened surfaces, and this phenomenon is recognized to be related to that of surface-enhanced Raman scattering (SERS).1-12 As in the case of SERS itself, this relationship may involve either a short-range, chemical-type enhancement mechanism such as charge transfer (CT) or a long-range electromagnetictype (EM) mechanism, as predicted initially by Nitzan and Brus13 or, in most cases, a combination of both. Subsequently, a number of different types of reactions have been examined, including the photofragmentation of several different aromatic and cyclic compounds,1 the photodecomposition of fluorobenzene,4 the photodimerization of pyridyl-substituted ethylenes,14 the photodimerization of substituted stilbenes,15 the photoreduction of methylviologen,16 and photoinducedl charge transfer from adsorbed flavin mononucleotide (FMN).12 In addition, there have been a large number of studies dealing with nitrobenzene and its derivatives, which were much used †
Department of Applied Chemistry. Current address: Department of Chemical Engineering, University of WisconsinsMadison, 1415 Engineering Dr., Madison, WI 53706. ‡ Research Center for Advanced Science and Technology. §
for a number of years because of their strong SERS signals.6,7,911,17-23 For much of this period of time, it was not recognized that such compounds can readily undergo photochemical reactions on rough silver surfaces; however, in recent years, with the advent of more sensitive detection systems and the use of lower laser power illumination, this has become increasingly well recognized.6,7,911 Also during the same period, there has been additional theoretical work carried out on the subject of surface-catalyzed photochemistry in terms of EM enhancement.2,3,24 Very recent experimental studies have also examined photoinduced charge-transfer reactions on roughened silver surfaces with adsorbed molecular monolayers.12,16 The present work has sought to examine the surface-catalyzed photochemistry of p-nitrobenzoic acid (PNBA) on roughened silver using the recently developed technique of surfaceenhanced Raman imaging (SERI)25-29 in order to gain further insight into the factors that control the photoreaction and the question of whether the reaction involves either short-range or long-range enhancement. SERI is useful in this regard, because it can detect spatial inhomogeneities in the SERS signal, and these can be related back to morphological features in the silver surface and, more importantly, to the presence of either a monolayer or crystallites on the same surface. This makes it possible to compare the enhancements directly in terms of the
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4934 J. Phys. Chem. B, Vol. 102, No. 25, 1998 SERS signals and the degree of photoreaction. During the past several years, micro-Raman imaging has emerged as a powerful technique.30-37 Because Raman peaks are often quite narrow, they are ideal as a means of producing chemically selective images. In addition, Raman in general is highly versatile, being usable in a variety of types of environments, as long as the surface or interface is accessible to light of the wavelength of the excitation laser (UV to IR). Thus the combination of the SERS technique with micro-Raman imaging offers high sensitivity together with spatially resolved chemical information and versatility. The first report of the combined use of SERS and Raman imaging was by Evans et al.;38 they examined the difference in appearance of a self-assembled monolayer on roughened and smooth regions of a gold surface. We have realized the power of this technique and have used it in the study of a number of different types of surfaces.25-28 2. Experimental Section 2.1. Preparation of Electrochemically Roughened Silver Films. Silver films were freshly prepared by vacuum evaporation of silver onto nonheated, ultrasonically cleaned ITO glass slides (Asahi Glass Co., Ltd) using a model EX-200 evaporator (Ulvac, Inc., Japan) with an initial pressure of 4 × 10-6 Torr. The evaporation rate was approximately 2.0 Å s-1, and the thickness was l µm. The films were roughened prior to use, so as to yield SERS activity, by means of the electrochemical oxidation-reduction cycle (ORC) procedure, using 15 cycles at 50 mV s-1 between -300 and 500 mV in an electrochemical cell. The supporting electrolyte was air-saturated aqueous 0.1 M KCl, which was prepared from triply distilled water. The three electrodes used in the ORC treatment were a silver working electrode, a Pt counter electrode, and a saturated calomel electrode (SCE) as a reference. All electrode potentials are quoted versus the SCE. From the cyclic voltammogram, the onset of silver dissolution was found to occur at ∼+50 mV, and the anodic peak was at 200 mV, corresponding to an anodic charge density Qa ) 26 mC cm-2. The cathodic peak was at -230 mV. The silver films were then rinsed thoroughly with triply distilled water to remove residual chloride prior to the deposition of PNBA. 2.2. PNBA Adsorption and Photochemical Reaction. PNBA (reagent grade, purity 99%) was purchased from Tokyo Kasei Co. and used without further purification. Absolute ethanol was used to prepare the l mM PNBA solution. The roughened silver films were immersed in the 1 mM PNBA solutions for at least 10 h in order to allow the PNBA to completely reach adsorption equilibrium. The samples were then rinsed thoroughly with ethanol and dried under a stream of dry Ar. For comparison, some of the samples were purposefully not thoroughly rinsed, and thus some crystallites remained. By use of the optical microscope, we were able to observe the surface density of crystallites and thus to control it by varying the amount of washing. The photochemical reaction of PNBA adsorbed on the silver surface was induced by means of visible light illumination using the same laser line (514.5 nm) as that used for the Raman imaging. Laser light with a power of approximately 5 mW was used to uniformly illuminate the sample surface, with a spot size of approximately 5 mm in diameter, yielding a power density of 25 mW cm-2. 2.3. Raman Measurements. The SERS spectra and images were recorded in ambient air with a Renishaw Raman Imaging Microscope System 2000 (Renishaw plc, U.K.), which has been described earlier.25,34,39 This high-throughput system uses a single monochromator together with a Raman holographic filter
Yang et al.
Figure 1. Low-resolution AFM image of an electrochemically roughened silver film used as a SERS substrate. The scanning area was 50 µm × 50 µm.
and has the capability of obtaining full spectra in less than 1 s, which is necessary in the present mode of operation, in which the images are constructed from the full spectra taken at each spatial position. The 514.5 nm line of an Ar+ laser (ILT 5000, Ion Laser Technology) was used as the excitation source. The laser power was controlled from 0.05 to 25 mW using neutraldensity filters. The laser light was focused onto the sample using a 50× objective lens mounted on an Olympus BH-2 microscope, with a spot size of approximately 2 µm. The Raman spectrometer was physically connected to the optical microscope, which was equipped with a video monitor through a CCD camera (TK-870, Victor Inc., Japan). This enables us to examine the surface morphology of the sample and subsequently to carry out the Raman measurements. The SER imaging was performed with a low laser power (50 µW) and short exposure time (0.1 s) in order to suppress the photoinduced reaction. The sample was placed below the objective lens on an XYZ stage (Newport M-462-XYZ-M) equipped with stepper motors (Newport 850B), which were controlled by a motion controller (Newport PMC400). The sample surface was sequentially scanned in 2.0 µm steps. Integrated Raman intensities at each point in the 2D image were obtained by integrating the intensities over a 10 cm-1 range on either side of the wavenumber of interest. All of the SERS images presented here were background-corrected by subtracting an image obtained in an off-peak portion of the spectrum close to the peak of interest. The raw images were smoothed and then remapped onto an array of pixels that were approximately 30% of the size of the original pixels, e.g., 0.3 µm versus 1 µm. No significant distortion was observed when comparing the smoothed versus unsmoothed images, as shown in a separate publication.28 2.4. AFM Measurements. The surface roughness and nanometer scale structure of electrochemically roughened silver films used for SERS were characterized using atomic force microscopy (AFM). AFM measurements was performed in the contact mode with a SPA-300 System (Seiko Company, Japan). A triangular cantilever with integral pyramidal Si3N4 tip was used, and the typical imaging force used was on the order of 10-9 N. 3. Results and Discussion 3.1. AFM Imaging of Electrochemically Roughened Silver Films. Prior to the SER imaging, we used AFM to examine the surface morphology of the electrochemically roughened silver films. Figures 1-4 show AFM images on four different scales, ranging from a scale of tens of micrometers down to a
Photoreaction of p-Nitrobenzoic Acid
Figure 2. Medium-resolution AFM image of an electrochemically roughened silver film used as a SERS substrate. The scanning area was 20 µm × 20 µm. In the center of the image, a region of closerpacked particles can be observed, which corresponds to one of the brighter areas in Figure 1. Surrounding this region are regions of higher porosity.
scale of several hundreds of nanometers, at which the individual silver particles are observable. From the low-resolution AFM image (50 µm × 50 µm; see Figure 1), we observe that the electrochemically roughened silver surfaces exhibited brighter, higher-elevation features on the 5-10 µm scale, consisting of silver aggregates, in which the particles are more closely packed. These can be seen more clearly from the medium-resolution AFM images (20 µm × 20 µm, see Figure 2). Neighboring these closer-packed areas are other areas in which the particles are arranged in a more porous type morphology. As will be
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4935 shown later, the SERS enhancement factors are slightly different comparing these two types of areas, and this can be seen in the SER image. The type of morphology shown in Figure 1 was typical of the whole sample surface and was also found to be reproducible from sample to sample. The samples roughened in this way were found to exhibit reasonably similar SERS enhancements on the micrometer scale, as disussed later. Further observation shows that the closer-packed aggregates are composed of a number of interconnected silver particles, as also seen in Figure 2. In Figure 3a, taken from one of the more closely packed areas, much of the area is at a similar height, whereas in Figure 3b, taken from one of the more porous areas, relatively deep holes can be seen. In order to examine the morphology of the actual particles that make up the surfaces, high-resolution AFM images (500 nm × 500 nm, see Figure 4) were obtained, also for a more closely packed region (a) and a more porous region (b), and interestingly, the particle sizes and shapes in these two regions are quite similar. For both, the morphology on the nanometer scale is characterized by “random substructure roughness” (RSR),40 with particles in the range 50-150 nm in diameter. According to Van Duyne et al., roughness in this size range leads to an enhancement on the order of 105, while roughness on the order of 1 µm leads to an additional factor of 102.40 Thus, the enhancements observed in the close-packed versus porous regions, for which the differences in roughness occur mainly in the range of 1-2 µm (see Figure 4b), are expected to be less than 102. The most important aspect of the ORC-treated silver surface is that the roughness is rather uniform, with the exception of this type of small difference,
Figure 3. Medium-high-resolution AFM image of an electrochemically roughened silver film used as a SERS substrate. The scanning area was 5 µm × 5 µm. In (a), a more closely packed region is shown, and in (b), a more porous region is shown.
Figure 4. High-resolution AFM image of an electrochemically roughened silver film used as a SERS substrate. The scanning area was 500 nm × 500 nm. In (a), a more closely packed region is shown, and in (b) a more porous region is shown.
4936 J. Phys. Chem. B, Vol. 102, No. 25, 1998
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Figure 5. Raman spectra for PNBA: (a) powder, and (b) adsorbed on a silver surface prior to illumination.
over the whole sample surface, and thus the SERS enhancement should also be similar over the whole surface, as observed in the actual SER images (see below). This uniform roughness allows us to effectively ignore the possibility that the 2D spatial variations in the Raman signal (i.e., the SER image) are due to variations in the roughness. 3.2. SERS Spectra of PNBA Adsorbed on Silver Surfaces. Figure 5 shows the normal Raman spectrum of PNBA powder, with two intense peaks at 1348 and 1102 cm-1 and two mediumintensity bands at 860 and 1600 cm-1 (see Table 1). The major band at 1348 cm-1 has been assigned to the NO2 symmetric stretch.23,41 Essentially the same peaks were observed for the SERS spectrum for PNBA adsorbed on the electrochemically roughened silver surface, with the exception of several very small peaks, for example, at 1390 and 1437 cm-1. These new peaks are due to a very slight but unavoidable extent of the photoreaction induced by the 514.5 nm laser illumination used to record the spectrum. However, Bercegol and Boerio have pointed out that a peak at 1390 cm-1 in the SER spectrum for PNBA can be assigned to COO-.11,42 This is also consistent with our initial spectrum (discussed later in detail).
Figure 6. Time dependence of SERS spectra for PNBA adsorbed on an electrochemically roughened silver film, which was illuminated with 514.5 nm laser light at 25 mW cm-2. The illumination times were (a) 10 s; (b) 60 s; (c) 120 s; (d) 180 s; (e) 240 s; (f) 300 s; and (g) 600 s.
In order to examine the course of the photochemical reaction, the sample was illuminated with the same laser line at 25 mW cm-2 power density for various lengths of time from 10 to 600 s (Figure 6). The changing spectra as a function of time clearly indicate that a surface photochemical reaction is occurring. With increasing illumination time, the bands assigned to PNBA gradually decreased in intensity, while, alternatively, several new
TABLE 1: Raman Peaks (cm-1) and Assignments for p-Nitrobenzoic Acid (PNBA) and Azobenzene (AB) PNBA powder
PNBA SAM on Ag
photolyzed PNBA SAM on Aga
864
860
860b
AB adsorbed on Ag 987
1109
1102
1168h
1352
1437h
1597 a
1600
AB powder 988
mode 10a 12
1076
1067
1
1142 1172h
1132 1170
1131 1170
C-N, 13/9a 9b
1298
1300
14
1348
1348h
1390h
1390
1385
N-N
1437
1428 1458
1428 1460
N-N 19b
1480
1478
19a
1577
1578
8a/b
1578
N-O
Exposed to 514.5 nm laser light at 25 mW cm-2 for 600 s. b Low intensity peak.
comments CH wag aromatic sextant stretch aromatic whole ring stretch aromatic modes aromatic in-plane bending aromatic in-plane bending NO2 symmetric stretch N-N adsorbed on Ag N-N, 19a/b mixed aromatic semicircle stretch aromatic semicircle stretch aromatic quadrant stretch
reference 23, 62 41, 63 23, 62 63-65 63-65 63 23,41 this work 63-65 63-65 63-65 23, 41, 62
Photoreaction of p-Nitrobenzoic Acid
Figure 7. Raman spectra for (a) azobenzene powder; (b) azobenzene adsorbed on silver; and (c) PNBA after illumination for 600 s with 514.5 nm laser light at 25 mW cm-2.
peaks increased in intensity, indicating the formation of photoproducts on the silver surface. Specifically, the five very small peaks that were observed, even with no illumination, at 1076, 1142, 1172, 1390, and 1437 cm-1, grew relatively rapidly until the 120 s point was reached, and then they increased more slowly to the 600 s point. As already mentioned, the process of recording the spectrum itself can also induce the photoreaction, but in the present work, using relatively low laser power and very short acquisition times, this effect, even with repeated spectral scans, was very slight. It should also be noted that the photochemical reaction rate was found to depend significantly on the degree of roughness, with the rate being much slower on an evaporated Ag film without roughening.39 The photoreaction of PNBA adsorbed on silver surfaces has been discussed in a number of previous studies.6,7,11 All of these reports agree that the reaction involves a photoinduced reduction, but there has been a lack of consensus concerning the actual photoproduct. Sun et al., citing the similarity of the SERS spectrum of the photoproduct to that for p-aminobenzoic acid (PABA), have assumed that the product is in fact PABA.7 Other authors have proposed that the photoproduct is rather azodibenzoate.6,11 This is plausible because PABA adsorbed on silver films can also undergo hydrolysis and dimerization to form azodibenzoate by an oxidative route when exposed to atmospheric moisture and laser light.8 In a separate report, we will show that the spectra for the photoproducts obtained from PNBA and PABA are very similar to each other and also to the SERS spectrum for azobenzene on silver.39 The SERS spectrum for azobenzene is almost identical to that for azobenzene powder (compare Figure 7, curves a and b), except for a new, medium-intensity peak at 1390 cm-1 and a very small peak at 1067 cm-1. Vibrational mode assignments are given in Table 1. Azobenzene itself appears to be quite stable on the silver surface even under illumination, with no changes being observed with time. We have proposed that the 1390 cm-1 peak is due to the same NdN stretch as that of the 1460 cm-1 peak but has been shifted to lower frequency due to
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4937 an interaction of the NdN double bond with the silver surface.28 This idea is based on a similar observation for CdC and CtC bonds.43 For comparison, the SERS spectrum for the photoproduct of PNBA after 600 s is shown as curve c. There is a reasonably good correspondence between the spectra, particularly the two main signature peaks for azobenzene, i.e., the 1428 cm-1 peak for the NdN stretch and the 1132 cm-1 peak for the C-N stretch. In curve c, the corresponding peaks are found at 1437 and 1142 cm-1, respectively. Finally, the 1390 cm-1 peak observed in the photoproduct spectrum cannot be due entirely to COO-, because a similar feature was observed in the spectrum for azobenzene, which has no COO- groups. Moreover, the ratio of the 1390 and 1437 cm-1 peak intensities, which was close to unity in the initial spectrum (Figure 8, curve a), becomes less than unity with later spectra (curve b and succeeding spectra) and appeared to reach a constant value that is characteristic of the photoproduct. A more detailed discussion of the arguments will be presented in a separate publication.39 For the present work, the importance is that this reaction rate can be controlled by adjusting the reaction conditions, i.e., laser power, illumination time, and the degree of surface roughness. Also, we have found that these spectral changes are completely reproducible, having been repeated many times in the present work. In addition, we have found that the spectral differences before and after photoreaction are sufficient for obtaining contrast in the SER imaging.26,28 We selected the SERS bands at 1348 and 1437 cm-1 to represent the surface concentrations of PNBA and its photoproduct, respectively, and thus as the most appropriate for the production of images. The latter can be based either on (i) the disappearance of the PNBA species or (ii) the appearance of the photoproduct on the silver surface. These two bands are relatively intense and have the least degree of interference, from adjoining peaks. Due to the nature of Raman peaks in general, i.e., the relatively small peak widths, the choice of other peaks could also have resulted in high-quality images. The general rule for maximizing the contrast in SER images is to select the two frequencies so as to maximize the differences in the spectral intensities. 3.3. SER Imaging of PNBA Adsorbed on Silver Surfaces. A strong surface enhancement effect was obtained from the roughened silver samples used in the present work. For the case of thoroughly rinsed surfaces, i.e., those on which the PNBA is present as an adsorbed monolayer, the enhancement factor (EF) ranged from 1.0 × 104 to 1.3 × 104, depending upon the region. The EF values were calculated according to a standard formula (see, for example, ref 44). It should be noted that these EF values are lower limits, because it has been assumed that the molecules in the monolayer are close-packed vertically and are therefore at a maximum coverage on the Ag surface. In the calculation, it is necessary to correct for the roughness factors, which were determined by analyzing the AFM images, as discussed later in this section. This relatively narrow range of EF values is consistent with the uniform roughness observed with AFM. As observed with AFM, however, there were some variations on the micrometer scale. These variations, although relatively small, can be seen clearly in the SER image (Figure 8). When the z-axis (Reman intensity) scale was decreased in sensitivity in order to match that for Figure 9 (see Figure 8, inset, lower left corner), the uniformity of the SERS intensity can be better appreciated. On the basis of many careful comparisons between results of optical microscopy, AFM, and SERI, it was found that the more intense SER signals arose from those regions with a more porous
4938 J. Phys. Chem. B, Vol. 102, No. 25, 1998
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Figure 8. SERS image for PNBA adsorbed on a roughened silver surface, after thorough rinsing, over a 50 µm × 50 µm region, obtained at 1348 cm-1. Images (a) and (b) are 2D and 3D representations, respectively. In the lower left corner, a 20 µm × 20 µm region is shown with the same vertical (Raman intensity) axis as that in Figure 9.
structure. For example, more porous regions appear darker in the optical miscroscopy due to the higher degree of roughness, but the corresponding regions appear brighter in the SER images and vice versa. The latter can be ascribed to surface plasmon polaritons arising from the long-range, regular undulations of the surface.40,45 Initially we postulated that the reason for the difference in Raman intensity from the closer-packed and more porous regions was that the surface areas were different. However, based on AFM measurements of images obtained over an area of 2 µm × 2 µm (not shown), which is similar to that sampled with the focused laser beam during the Raman measurements, corresponding to one pixel in the SER images, the surface areas were measured from the AFM images, and the average values were 5.76 and 6.41 µm2, respectively, for the closer-packed and more porous areas, yielding roughness factors of 1.4 and 1.6, respectively. These values are rather similar, and thus we calculated the root-mean-square surface roughness Rrms values,
which were 70 and 142 nm, respectively, for these two types of areas. Thus it can be seen that the roughness is more important than the surface area in determining the surface enhancement. One of the main objectives of the present work was to probe the effect of monolayers versus multilayers on the photochemical reaction, both on an identical Ag surface, in order to eliminate variations due to different degrees of roughness and other experimental conditions. A convenient method for obtaining a surface with both monolayer and multilayer material was to use a sample that had purposefully been only rinsed a short time and therefore exhibited small crystallites of PNBA. In this type of experiment, it was also possible to examine semiquantitatively the effect of distance from the silver surface on the SERS intensity. The SER image is shown in Figure 9, which clearly shows bright areas (corresponding to the 1348 cm-1 peak, O-N-O stretch) that are due to the PNBA crystallites.This was also confirmed using optical microscopy (see later). An AFM
Photoreaction of p-Nitrobenzoic Acid
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4939
Figure 9. SERS image for PNBA adsorbed on a roughened silver surface, without thorough rinsing, over a 50 µm × 50 µm region, obtained at 1348 cm-1. Images (a) and (b) are 2D and 3D representations, respectively. The bright regions correspond to crystallites, as shown in the optical micrograph in Figure 10.
Figure 10. AFM image for a typical PNBA crystallite on the roughened Ag surface: (a) 2D image; (b) line scan; and (c) 3D image.
image of a typical crystallite is shown in Figure 10, from which it can be seen that the thickness is 240 nm. The range of thicknesses observed was 200-300 nm. The sides are sloping, probably due to the effect of partial dissolution during the light
washing procedure. In the SER image, the appearance of the high-intensity structures, which correspond to these particles, is pointed at the top. This is simply due to the fact that the resolution is insufficient to observe a flat top. Comparing the background-corrected, integrated Raman signals from these small crystallites with those from either the surface of the same sample (Figure 9) or the monolayer-adsorbed surface (Figure 8), there was approximately a factor of 5-6 difference. The question of the effect of distance has been a crucial one in the development of the theoretical foundations of the SERS effect. For example, in terms of the CT model and EM-based models, as well as others, there are varying types of distance dependence of the enhancement factor, all the way from the equivalent of one monolayer (CT)9,46-50 to a relatively slowly decaying exponential function (EM).48,51-53 In the examination of the Raman intensity from a multilayer slab of material, the enhancement factor is integrated through the thickness of the layer in order to compare with theory. This type of experiment
4940 J. Phys. Chem. B, Vol. 102, No. 25, 1998 was carried out, for example, by Sanda et al.54 in order to compare with the combined EM-polarizability model of Jha et al.45 They also found approximately a factor of 6 in comparing a single monolayer with a much thicker layer, for which the integrated intensity approached a plateau. In more recent work, the distance dependence continues to be examined, for example, using Langmuir-Blodgett films as spacers52,55 and self-assembled monolayers with various chain lengths.42,56 The experimentally determined half-lengths include values such as 3.552,55 and 0.35 nm.56 In the present work, the thicknesses of the microcrystallites were too large to probe this dependence, however. This explains why all of the bright features in Figure 9 exhibit essentially the same Raman intensity, even though the thicknesses vary over a range of 200-300 nm. 3.4. SER Imaging of PNBA Photoreduction on Silver Surfaces. The combination of SERS plus 2D mapping is particularly powerful in examining the characteristics of a photochemical reaction (or electrochemical reaction) in situ with both spatial resolution and chemical selectivity. This is the first report that we are aware of that takes this approach. In Figure 11 (panel a), an optical image is shown for a partially washed surface, with a number of small crystallites present, as in Figure 9. The SER images in Figure 11, however, were obtained after 600 s of laser illumination (514.5 nm) at 25 mW cm-2. As will be shown later, this length of time corresponds to essentially complete reaction. In panel b, in which the 1348 cm-1 (ON-O stretch) is monitored, the images of the crystallites can be seen clearly, corresponding exactly to the locations of the crystallites in the optical image (panel a). It can be seen that the NO2 group remains intact for the PNBA contained in the crystallites. Comparing this image with that in Figure 9, for the unreacted compound, it is clear that the image intensity has decreased somewhat. This is due to the photoreaction, which probably only involves about 20% of the the layer of molecules actually in contact with the silver surface (see later). Turning to the product image (Figure 11, panel c), which was obtained using the 1437 cm-1 (NdN stretch for azodibenzoate), it is clear that the product intensity is strong and highly uniform in the areas between the crystallites. This is due to the fact that the PNBA in these areas is predominantly in the form of an adsorbed monolayer. In this case, the PNBA molecules are directly in contact with the highly catalytic Ag surface, and in addition, they have sufficient space in which to react. It appears that the reaction proceeds essentially to completion up to the edge of the crystallite; that is, the yellow areas extend very precisely to the boundaries as seen in the optical image. In contrast, under the crystallites, there was much less photoreaction. There are two possible explanations for this behavior: (1) the crystallite might absorb the excitation laser light; or (2) there might be steric problems, which could impede the reaction. The first possibility is not appropriate, however, because the absorption of PNBA in the region of 514.5 nm is negligible. Therefore, the second reason appears to be more appropriate. It is virtually impossible for molecules, in the bulk of a crystalline solid to undergo the structural rearrangement necessary for the formation of the cis form of azobenzene, but either those on the surface of the solid and very close to the Ag surface or those in the adsorbed monolayer have sufficient freedom of movement to do so (Figure 12). Moreover, based on the SERS spectra, it is possible to conclude that essentially all of the azobenzene moieties are in the trans form, because the spectrum for the cis form simply does not exhibit a strong peak in the 1437 cm-1 region; in fact, there are only weak bands in the region from 1150-1600 cm-1.57 Again, this isomeriza-
Yang et al.
Figure 11. Images from the same 50 × 50 µm region of an electrochemically roughened Ag surface with PNBA crystallites and monolayer-adsorbed PNBA: (a) optical microscopic image; (b) SERS image obtained at 1348 cm-1, corresponding to unreacted PNBA; and (c) SERS image obtained at 1437 cm-1, corresponding to the photoproduct. For (b) and (c), the images were obtained after 600 s of 514.5 nm laser illumination at 25 mW cm-2.
tion, which, incidentally, is strongly induced by visible light illumination,58 is possible on the surface of the crystal or in the adsorbed monolayer. However, there is a possible difference between the behavior of these two forms, in that, in the adsorbed monolayer, it is easier for one molecule to move laterally and encounter another molecule so that the dimerization reaction can take place. It is a well-accepted idea that the mechanisms for SERS and surface enhancement of photochemical reactions are similar.2,3,13,24 In the present results, this appears to be quite valid,
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Figure 13. Time dependence of the SERS intensities during photoreaction of PNBA adsorbed on roughened Ag: (a) 1437 cm-1, corresponding to the photoproduct; and (b) 1348 cm-1, correspoding to the unreacted PNBA, obtained on monolayer-adsorbed regions (O) and on crystallites (4).
Figure 12. Schematic diagram of (a) a monolayer of adsorbed PNBA molecules; (b) after photoreaction to produce cis-azodibenzoate moieties; and (c) after visible light-induced isomerization to the trans form. The hydrogen ions that are shown on the COOH groups are expected to be ionized under most conditions.
in that there is a strong enhancement for both processes for the monolayer adjacent to the Ag surface. This is most likely because of a CT-type mechanism, in which electron density fluctuates rapidly back and forth between the Ag surface and the adsorbed molecule.46 However, it has also been shown in the present work that there is a significant additional enhancement of the Raman signal through multiple layers, due to an EM-type effect. It is not possible to conclude anything about an EM effect for the surface-catalyzed photoreaction due to the steric limitations mentioned above, however. It should be mentioned that, in work of Bercegol and Boerio, in which PNBA molecules were adsorbed on top of a self-assembled monolayer (∼1.8 rim thick) deposited on a silver surface, evidence was observed for the photoreaction,11 showing that an EM enhancement mechanism is perhaps also possible for PNBA molecules not in direct contact with the Ag surface. 3.5. Time Dependence of the Photocatalytic Reaction. The time dependence of the photoreaction of PNBA was examined in detail for both a monolayer area and a crystallite on the surface shown in Figure 11. For the monolayer area, the reaction proceeded relatively quickly, reaching a plateau of 80% relative intensity after approximately 300 s, as monitored in terms of the product peak at 1437 cm-1 (Figure 13a). In contrast, for the PNBA within the crystallite, the reaction quickly (∼40 s) reached a plateau at less than 20% relative intensity.
In terms of the PNBA itself, the results were consistent (Figure 13b). In the monolayer area, the 1348 cm-1 NO2 peak decreased and reached a plateau of ∼15% at ∼300 s, while that for the crystallite reached a plateau of ∼80% after ∼150 s. The time dependence is consistent with the model already described (Figure 12). The PNBA molecules within the adsorbed monolayer can react to a much greater extent, but the time involved is greater due to the fact that the molecules that are not initially lined up for reaction must undergo some degree of lateral movement in order to react. Conversely, the molecules within the crystallite, since they are not free to move laterally, react either quickly or not at all. 4. Conclusions To the best of our knowledge, this is the first report of the two-dimensionally spatially resolved, chemically selective monitoring of the course of a surface-catalyzed photoreaction using SERS. In the course of this work, we have been able to reach a number of conclusions. 1. The electrochemical roughening procedure for the Ag surface provided a reasonably uniform degree of roughness as observed with AFM, which is beneficial for obtaining strong, uniform (1.0 × 104 to 1.3 × 104) surface enhancement of Raman signals and thus high-quality images, with resolution down to 2 µm, which can be significantly improved, for example, using near field optics.59-61 2. As also found by several previous groups,6,7,23,42 using low-intensity laser illumination and/or short acquisition times, it is possible to obtain the spectrum for PNBA without significant photoreaction, and this spectrum is quite similar to that for PNBA powder. 3. By examining a surface with both adsorbed PNBA molecules and small crystallites using the SER 2D imaging technique, it was possible to compare the enhancements directly and unambiguously.
4942 J. Phys. Chem. B, Vol. 102, No. 25, 1998 4. For crystallites that ranged in thickness from 200 to 300 nm, the Raman signal (1348 cm-1, O-N-O stretch) was remarkably constant and was approximately a factor of 6 larger than that for the adjacent monolayer-adsorbed regions, demonstrating that the increase in the SERS signal due to integration of the exponentially decreasing EM-type enhancement had already reached a limiting value. 5. After only 25 s of continuous higher intensity laser illumination, a substantial extent of photoreaction was found for molecules in the monolayer-adsorbed regions, and after 600 s, the spectrum matched that for azobenzene adsorbed on a roughened Ag surface, indicating that the photoreaction involves head-to-head dimerization to form the cis form of the azodibenzoate, followed by visible light-induced isomerization to the trans form. 6. With the SERI technique, it is possible to very conveniently monitor the progress of the photoreaction in a 2D spatially resolved fashion; the time resolution can be greatly improved in the near future, using direct imaging techniques,36 making it possible to examine much faster reactions. 7. The results show that the PNBA molecules in the monolayer regions react to a high degree of completion, while those within the small crystallites react to a much lesser degree. 8. This is mostly consistent with previous literature, which has pointed out that the catalytic effect of the Ag surface requires that the molecule be in actual contact with the surface,6,42 although work of Bercegol and Boerio11 has shown that PNBA molecules not in direct contact with the surface, but in the form of an adsorbed monolayer or so, can also react. 9. The reason for this difference in reactivity is proposed to involve the fact that the PNBA molecules trapped within the solid lattice are simply not free to undergo the geometric rearrangements necessary for the reaction. 10. Thus, it is impossible to make firm conclusions concerning the distance dependence of the surface-enhanced photochemical reaction, except to say that there is a major enhancement for the layer of molecules adjacent to the Ag surface, and the mechanism may involve a combination of CT and EM effects. Acknowledgment. The authors wish to acknowledge Dr. K. Ajito of NTT Basic Research Laboratories, Kanagawa, Japan, for his work in developing the scanning Raman system. Also, we thank Dr. J. J. Kim for experimental assistance. This research work was partially supported by the Ministry of Education, Science and Culture of Japan. One of the authors (X.M.Y) gratefully acknowledges the Japan Society for the Promotion of Science (JSPS) for a postdoctoral fellowship. References and Notes (1) Goncher, G. M.; Harris, C. B. J. Chem. Phys. 1982, 77, 37673768. (2) Goncher, G. M.; Parsons, C. A.; Harris, C. B. J. Phys. Chem. 1984, 88, 4200-4209. (3) Das, P.; Metiu, H. J. Phys. Chem. 1985, 89, 4680-4687. (4) DiLella, D. P.; Smardzewski, R. R.; Guha, S.; Lund, P. A. Surf. Sci. 1985, 158, 295-306. (5) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307-313. (6) Roth, P. G.; Venkatachalam, R. S.; Boerio, F. J. J. Chem. Phys. 1986, 85, 1150-1155. (7) Sun, S.; Birke, R. L.; Lombardi, J. R.; Leung, K. P.; Genack, A. Z. J. Phys. Chem. 1988, 92, 5965-5972. (8) Venkatachalam, R. S.; Berio, F. J.; Roth, P. G. J. Raman Spectrosc. 1988, 19, 281-287. (9) Mrozek, I.; Otto, A. Appl. Phys. A 1989, 49, 389-391.
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