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J. Phys. Chem. C 2007, 111, 12352-12360
Surface-Enhanced Vibrational and TPD Study of Nitroaniline Isomers Krystal L. Posey, Mark G. Viegas, A. Jacob Boucher, Chen Wang, Kaitlyn R. Stambaugh, Merritt M. Smith, Brittany G. Carpenter, Bridget L. Bridges, Steven E. Baker, and Donald A. Perry* Department of Chemistry, UniVersity of Central Arkansas, Conway, Arkansas 72035 ReceiVed: March 6, 2007; In Final Form: June 7, 2007
Ortho-, meta-, and para-nitroaniline isomers (ONA, MNA, and PNA, respectively) were investigated with surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS) via adsorption of the individual isomers on vacuum-deposited silver films and on silver powders of varying physical dimensions using temperature-programmed desorption (TPD). PNA exhibited a strong SERS signal, whereas the ONA and MNA isomers showed more moderate enhancement. Only the MNA isomer exhibited SEIRA. Varying the polarity of the deposition solvent changed the MNA adsorbate orientation and the degree of vibrational enhancement. Solvent experiments with MNA were the first to show SEIRA enhancement of the -NH2 symmetric and antisymmetric stretch modes. TPD and vibrational experiments show evidence for nitroaniline intermolecular hydrogen bonding on 100-µm silver powder while suggesting stronger nitroaniline adsorption to 100-nm silver powder. Resonance effects, adsorbate geometries, and hydrogen-bonding variations combined with changes in the strength of adsorption explain the differences in SEIRA versus SERS enhancement of the individual isomers, solvent effects, and desorption characteristics.
1. Introduction Surface-enhanced infrared absorption (SEIRA) was first observed by Hartstein et al.1 in 1980 during a study of p-nitrobenzoic acid adsorbed on roughened silver films. SEIRA is analogous in many ways to the more widely known surfaceenhanced Raman spectroscopy (SERS) discovered by Fleischman et al. in 1974.2 The origins of SEIRA and SERS have been investigated by numerous researchers over the years, and at least in a fundamental sense, similar theories based on electromagnetic and/or charge-transfer models are now widely proposed explanations for the surface-enhancement phenomena in SERS and SEIRA.3-6 These electromagnetic/charge-transfer models apply to both enhancement techniques, even though SEIRA is a photon absorption process that generally requires some polarity with respect to the adsorbed molecule to obtain the largest enhancement factors and SERS is a photon scattering process that depends on molecular polarizability. Unfortunately, it remains largely unexplained why some molecules undergo SEIRA and/or SERS and other molecules do not. Among the molecules that do undergo surface enhancement, it is difficult to predict which vibrational modes will be enhanced in SEIRA or SERS and which will not. One observation with many exceptions is that vibrational modes with more-polar bonds are often the most strongly enhanced in SEIRA experiments. Merklin and Griffiths7 have shown that surface-selection rules for vibrational activity such as those observed in reflection absorption spectrometry (RAS) and electron energy loss spectroscopy (EELS) usually apply in SEIRA transmission experiments, but the selection rules sometimes break down in SEIRA because of the effects of microscopic surface roughness inherent in the thin metal films necessary for surface enhancement. * To whom correspondence should be addressed. E-mail: donp@ uca.edu.
All vibrational spectroscopies are inherently most sensitive to the first adsorbate layer.3 In a SERS experiment, only the monolayer exhibits enhancement of the vibrational modes.5 SEIRA is often a more long-range effect, and it is not uncommon to see infrared enhancement from several adsorbate layers.3 These facts suggest the power of the dual application of SEIRA and SERS in the investigation of adsorbed organic layers. In the investigation of a multilayer system, SEIRA can be used to explore the interactions among the different adsorbate layers as well as adsorbate interactions with the surface, and SERS can be used to extract data solely from the first adsorbate layer. The unpredictable nature of surface-vibrational spectroscopies makes the dual application of SERS and SEIRA attractive because one technique often provides critical information on an adsorbate system when the other technique fails. To date, only two sets of aromatic isomers have been completely investigated with SEIRA. Badilescu and co-workers8 studied the o-, m-, and p-nitrobenzoic acid isomers, whereas Griffiths and co-workers9 investigated the nitrophenol isomers. The majority of the aromatic molecules used to study the origin of the SEIRA effect have involved molecules with one or more electron-withdrawing groups. No systematic SEIRA study of a complete aromatic isomer set has been performed with an electron-donating group such as -NH2 opposite an electronwithdrawing group such as a -COOH or -NO2. In contrast, adsorbed o-, m-, and p-nitroaniline (ONA, MNA, and PNA, respectively) have been studied extensively using conventional Raman spectroscopy,10 resonant Raman spectroscopy,11 and SERS.12-16 Nitroaniline isomers are used in the manufacture of pesticides, pharmaceuticals, dyes, anti-oxidants, and semiconductor chips. The nitroanilines have also received attention because of their unique nonlinear optical behavior.13 ONA, MNA, and PNA comprise an intriguing isomer system for SERS and SEIRA studies because they have greatly different
10.1021/jp071833f CCC: $37.00 © 2007 American Chemical Society Published on Web 07/27/2007
Vibrational and TPD Study of Nitroaniline Isomers electrostatic properties and unique intermolecular and intramolecular hydrogen-bonding properties. The easiest way to form a monolayer of a molecule adsorbed to a solid without an expensive experimental apparatus is to use what is labeled here the “drop method”. In this method, the adsorbate is dissolved in a dilute solution of a volatile solvent. The concentration of the solution is adjusted to deposit very close to a monolayer of the adsorbate after solvent evaporation by accounting for the total surface area to which a given volume of the solution will spread. He and Griffiths showed that it is likely that the adsorbates are more concentrated in the vicinity of the drop point in the substrate center during the pipeting procedure.17 Because this deposition method has been used in hundreds of publications, the problem of the nonuniformity of deposited films is an issue faced by many researchers using the drop method. This study demonstrates SERS and SEIRA spectra of the nitroaniline isomers on vacuum-deposited silver films. Nitroaniline spectral mode assignments are modeled comparatively using density functional theory (DFT) calculations. DFT calculations help verify the adsorption orientation of the molecules, highlight effects due to intermolecular hydrogen bonding, and help explain the origin of the observed surfaceenhancement effects. Various solvent phenomena pertaining to the SERS and SEIRA spectra of the adsorbed nitroaniline isomers are discussed. Thermal desorption spectroscopy (TDS) studies were used to explore the differences in the ways the nitroaniline isomers adsorbed to vacuum-evaporated silver films versus silver powders of different average particle sizes. 2. Experimental Details 2.1. Methodology. The nitroanilines and 100-nm silver powder (surface area ) 2-4 g/m2) were obtained from Aldrich. ONA was used as received, but it was necessary to recrystallize MNA and PNA in water/charcoal prior to preparation of the solutions to minimize effects from nitroaniline oxidation. Silver wire and 99.999% pure 100-µm silver powder (0.0205 g/m2) were purchased from Myron Toback. All solvents used for the preparation of solutions were HPLC or Optima grade (Fisher Scientific or Aldrich). Polished CaF2 windows (25 × 4 mm) were purchased from International Crystal Laboratories. Most of the SEIRA spectra were obtained in transmission mode (2-4 cm-1 resolution, typically with averaging of 32128 scans) on a Nicolet Magna 560 FTIR spectrometer or a Thermo-Nicolet IR100 FTIR spectrometer. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data were collected on a Nicolet Magna 560 FTIR spectrometer using an International Crystal Laboratories DRIFTS attachment. Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectra were taken with a Thermo Foundation Series Performer ATR artachment with a diamond ATR crystal (single-bounce design) on an IR100 spectrometer. Raman spectra were obtained in a back-scattering geometry with a Horiba Jobin Yvon HR00UV Raman spectrometer at 18 mW with the 632.81 nm laser line. TDS experiments were carried out with a Varian Chromatoprobe inserted into a Varian 1070 temperatureprogrammable injector equipped with liquid CO2 cooling capacity attached to a Saturn 2200 ion-trap mass spectrometer. Helium was used as the carrier gas with a flow rate of 0.4 mL/ min. Ionization mode was 70 eV electron impact. All TDS experiments began at 40 °C, with the temperature ramped at 5-20 °C/min to a maximum of 300 °C. Typically, about 0.030.09 g of the Ag powder was used per TDS experiment. Imaging of the 100-µm silver powder was accomplished with an Aspex
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12353 Instruments Personal scanning electron microscope (SEM) 2000 equipped with an energy-dispersive spectrometer. The vacuumevaporated silver films were imaged with a Digital Instruments VICO Dimension 3100 atomic force microscope (AFM). Electronic spectra of the vacuum-evaporated silver films were recorded on a Varian UV-vis Cary 50 instrument in transmission mode. Silver films for SEIRA and SERS studies were prepared by vapor deposition on CaF2 substrates in a home-built vacuum chamber with a base pressure of 1 × 10-5 Torr. Film thickness was monitored with a quartz crystal microbalance (Infinicon). The CaF2 substrates were polished with a Buehler Mastermet 2 colloidal silica suspension, sonicated and rinsed with nanopure water, and air-dried prior to usage. Nitroaniline films of approximately 1 ML for SEIRA and SERS studies were formed on vacuum-evaporated silver films via the drop method by pipeting 25 µL of a 50 ppm nitroaniline solution and allowing the solvent to evaporate. A nitroaniline film was deposited onto the silver powders for FTIR-ATR, DRIFTS, SERS, and TDS studies by dispensing the appropriate aliquot of a 50-1000 ppm nitroaniline solution onto an accurately measured mass of silver powder and allowing the solvent to evaporate. Solvents of varying polarity were used to dissolve MNA and PNA in these experiments. It was necessary always to use a nonpolar solvent for ONA deposition because more polar solvents complex strongly with ONA. Experiments were also performed whereby surface films prepared via the drop method were rinsed with a solvent to observe the impact of the solvent rinse; this method is called the “rinse method”. Density functional theory (DFT) calculations were performed using the Gaussian 03 suite at the B3LYP level of theory with a 6-311G(p,d) basis set.18 The molecular geometries were optimized using analytical derivatives. Frequencies were then calculated and scaled by 0.9613 to confirm that the geometries were at a minimum and for comparison against experimental vibrational spectra.18 2.2. Preliminary Work. para-Nitrobenzoic acid (PNBA) was used to calibrate SEIRA experiments against those found in the literature.2 PNBA is known to adsorb to many metal films when deposited from polar solvents as a benzoate ion.3 By monitoring various PNBA vibrational modes that displayed strong SEIRA, it was determined that a 7-nm thickness of silver deposited on substrates including KBr, CaF2, and ZnSe gave the largest SEIRA enhancement from a monolayer of adsorbed PNBA while still maintaining high spectral quality and resolution. An AFM image of a 7-nm silver film evaporated on a polished CaF2 window is provided in Figure 1. AFM images of the 7-nm silver films revealed elliptical silver islands with dimensions on the 80-100-nm scale. Electronic spectra of the 7-nm silver films show a broad surface plasmon that extends into the nearinfrared region, and the surface plasmon continues into the midinfrared region where about 30% of the radiation is absorbed at 2400 cm-1. This 30% loss of intensity is duly compensated by the SEIRA enhancement of about a factor of 50 for the PNBA benzoate ion. Henceforth, for all non-powder experiments presented in the Results and Discussion, it is understood that CaF2 is the base substrate. Likewise, reference to a silver film is understood to mean 7 nm of vacuum-evaporated silver on CaF2. 3. Results and Discussion 3.1. Nitroaniline Adsorption on Vacuum-Evaporated Ag Films. Figure 2 consists of six infrared spectra in the range of 1000-1800 cm-1 depicting approximately 1 ML of ONA
12354 J. Phys. Chem. C, Vol. 111, No. 33, 2007
Figure 1. AFM image of a 7-nm silver film.
Figure 2. Pairs of infrared spectra representing 1 ML coverage of MNA deposited from acetone (solid spectra), PNA deposited from acetone (dotted spectra), and ONA deposited from heptane (dashed spectra). The bottom spectrum of each pair is without silver, and the top spectrum of each pair is with a silver film (or SEIRA spectra).
deposited from heptane solution and PNA and MNA each individually adsorbed from acetone solution with and without a silver film. These six spectra are separated into three sets, one for each isomer, with the bottom spectrum of each set being the infrared spectrum of the isomer adsorbed on just CaF2 and the top spectrum of the set representing the isomer adsorbed on a silver film. ONA can be seen in the bottom, dotted spectral set. No vibrational features of ONA can be seen in Figure 2. This indicates that ONA does not undergo SEIRA enhancement, and the lack of SEIRA enhancement of ONA was further confirmed by comparison of infrared spectra as a function of ONA coverage with and without a silver film. PNA can be detected in both spectra in the middle spectral set of Figure 2. There are no dramatic peak shifts between the PNA spectra adsorbed with or without a silver film, which indicates that PNA adsorbs in a similar fashion independent of the presence of a silver film. As expected of an adsorbate deposited separately on two chemically different surfaces, the relative intensities of individual peaks are slightly altered between the spectra of PNA adsorbed with and without a silver film. However, the overall
Posey et al. magnitude of the PNA peaks is about the same with or without a silver film, which suggests that PNA, like ONA, does not undergo infrared enhancement. Actually, one of the reasons PNA appears to exhibit less absorbance when a silver film is present is because 30% of the incoming IR radiation is scattered by the silver film. MNA clearly demonstrates SEIRA enhancement in the top set of spectra in Figure 2. Many MNA vibrational modes can be detected on a silver film, whereas no modes are present without the silver film for the same MNA coverage. The observed enhancement factor is approximately 50 as determined by comparison to multilayers of MNA adsorbed directly onto CaF2. The presence of modes due to PNA without a silver film (meaning that approximately 1 ML of PNA can be seen without SEIRA) in Figure 2 warrants a number of comments. PNA is a more polar molecule than the other two nitroaniline isomers. Several experiments were performed showing that nitro group stretching modes of PNA dispersed in a KBr matrix were a factor of 2-3 times greater than those of a corresponding mixture of ONA; this factor of 2-3 goes a long way in explaining why a PNA monolayer can be observed in an infrared transmission experiment whereas an ONA (and MNA) monolayer cannot be detected without a silver film. It could also be that the absorptivity of PNA is altered more than those of ONA and MNA during the adsorption process because PNA is a more polar molecule. Because the PNA monolayer was formed using the drop method, the concentration of PNA at the center of the drop could be slightly higher than that in regions farther from the drop point. To clearly elucidate the PNA modes, it was decided to include a PNA spectrum in Figure 2 that was collected at the drop point because the observed infrared absorbance of a PNA monolayer is just above the detection limit. For PNA adsorbed on CaF2 and MNA adsorbed on a silver film, a small but consistent drop in peak intensity was observed when SEIRA spectra were repeatedly collected moving toward the outer edges of the crystal from the drop point. However, the difference in infrared absorbance within the area of the drop was never observed to be more than a factor of 2 for all of the PNA and SEIRA MNA vibrational modes, and the same vibrational modes were always present in each experiment. When considering the MNA SEIRA spectrum, this factor of 2 is small in comparison to the overall enhancement factor of about 50. Because there is a drop of less than a factor of 2 in the MNA SEIRA intensity across the entire drop area, it is safe to conclude that the drop method used for the PNA spectrum in Figure 2 and all MNA SEIRA data reported in these experiments are quite close to 1 ML. A series of SEIRA spectra obtained as a function of increasing MNA coverage is presented below that further confirms that a nitroaniline film formed from the application of the drop method is close to 1 ML coverage. Figure 3 summarizes SERS spectra in the range of 8001800 cm-1 for samples similar to those of Figure 2 showing about 1 ML coverage of MNA and PNA adsorbed from acetone and ONA adsorbed from heptane on a silver film. Although SERS spectra were accumulated from 100 to 4000 cm-1, no vibrational bands were present for any of the nitroaniline isomers outside the range 800-1800 cm-1 under the adsorption conditions described for Figures 2 and 3. MNA is the bottom, least intense spectrum; the middle spectrum is ONA; and the top, most intense spectrum is PNA. All spectra were obtained under identical experimental conditions. PNA has a much stronger SERS spectrum than ONA, and ONA shows more Raman enhancement than MNA. This trend agrees with the order of Raman activities of the nitroaniline isomers reported in resonant
Vibrational and TPD Study of Nitroaniline Isomers
Figure 3. SERS spectra of 1 ML coverage of MNA, ONA, and PNA deposited on a silver film.
TABLE 1: SERS Frequencies and Mode Assignments for Adsorbed MNA, ONA, and PNA MNA (cm-1) ONA (cm-1) PNA (cm-1) 999 1326, 1355 1600
857 930 1003 1142 1275 1345, 1390 1449 1535 1602
860 929 963 1007 1112 1127 1186 1280 1307 1337, 1402 1450 1535 1594
mode CH wag CH wag CH wag ring breathing C-NO2 stretch CH bend CH bend CH bend/C-NH2 stretch CH bend/C-NH2 stretch NO2 symmetric stretch CH bend/CC stretch NO2 asymmetric stretch CC stretch
Raman experiments.11 The enhancement factor is about 105 as determined by comparison of Raman spectra of PNA multilayers on CaF2 to the PNA SERS spectrum. Under the most rigorous collection conditions, all SERS peaks for PNA and most for ONA are easily resolved, whereas the MNA spectrum is always of lesser resolution because of the low MNA Raman activity. Table 1 lists the vibrational frequencies and mode assignments of the SERS spectra associated with Figure 3, and a portion of Table 2 contains the reported frequencies and mode assignments of the SEIRA spectrum of MNA adsorbed from acetone in Figure 2. A brief inspection of Table 2 shows that, for the adsorbed nitroaniline isomers, the reported frequencies for MNA adsorbed from acetone are reasonably close to the values for MNA in KBr. However, many of the modes associated with the -NO2 and -NH2 modes are significantly altered from the single-molecule DFT calculation because of hydrogen-bonding interactions. Szostak10 assigned modes around 1330-1335 cm-1 associated with MNA in CCl4 and KBr to a C-NO2 stretch and a broad band between 1350 and 1400 cm-1 in CCl4/KBr to the -NO2 symmetric stretch mode. However, Muniz-Miranda,13 in a SERS analysis of PNA adsorbed on silver sols, noted that two bands for PNA are commonly present: a normal -NO2 symmetric stretch above 1330 cm-1 and a second -NO2 symmetric stretch band around 1310-1320 cm-1 due to -NO2 hydrogen-bonded with -NH2 on an adjacent PNA molecule. It is conceivable that a similar situation is arising during MNA adsorption. This hypothesis is supported by the fact that the two peaks commonly seen for adsorbed MNA between 1320 and 1410 cm-1 always mirror each other in relative intensity under a myriad of adsorption conditions. Henceforth, MNA modes in the range 1320-1410 cm-1 will be reported as -NO2 symmetrical stretch modes. The higher-wavenumber peak is from the normal nitro-group symmetric stretch, and the lower-
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12355 wavenumber peak is assigned to the nitro symmetrical stretch mode altered by hydrogen bonding. Aside from changes in the -NO2 symmetric stretch region, in general, there are no new vibrational modes, major peak shifts, or other discrepancies upon MNA adsorption as compared to MNA dispersed in KBr or CCl4. This fact might lead one to conclude that the nitroanilines are weakly bound (or physisorbed) to the surface. Such a conclusion would make it difficult to assign an orientation to a monolayer of adsorbed MNA because physisorbed molecules often have a higher degree of randomization than more chemisorbed molecules. However, SEIRA/SERS data are presented below that provide evidence for intermolecular hydrogen bonding that should lead to a high level of molecular ordering of adsorbed MNA. Additional TPD experiments are also presented that support the presence of intermolecular hydrogen bonding. Taking all of the previous considerations into account and assuming, in general, that the surface selection rules7 hold, the presence of numerous in-plane modes including strong -NO2 symmetric/antisymmetric stretches and C-NO2/C-NH2 stretches as well as the lack of out-of-symmetry-plane modes for the SEIRA spectrum of MNA in Figure 2 and the SERS spectrum of PNA in Figure 3 suggest that PNA and MNA are adsorbed in an upright position through the -NO2 group when adsorbed from a polar solvent such as acetone. This assignment is consistent with colloidal SERS studies in the literature.12 The same authors predict that a monolayer of ONA adsorbs in a flat position in a colloidal gold solution.12 Results summarized in Table 1 are more consistent with ONA adsorbed in an upright position through the -NO2 group for the same reasons that PNA and MNA were assigned an upright orientation. Figure 4 is a pictorial representation of the adsorbed nitroaniline isomers on Ag films. The fact that nitro-group symmetric and antisymmetric stretch modes are simultaneously present in the spectra in Figures 2 and 3 for each of the nitroaniline isomers might alter the picture described in Figure 4. There are two possibilities for this observation. First, the surface selection rule that states that the nitro-group antisymmetric stretch mode should be absent when a nitroaniline molecule is adsorbed in an upright orientation through the nitro group might not hold. This is not a strong selection rule if the surface consists of smaller particles or if charge transfer occurs between the molecule and the surface. Another possibility is that the isomers adsorb in an upright, slightly tilted orientation. Evidence is presented below that shows that the MNA molecule tilts to varying degrees when adsorbed on the surface depending on the polarity of the deposition solvent. Based on this MNA solvent effect and the fact that no other out-of-plane modes can observed for any of the nitraoniline isomers, a nitroaniline adsorption geometry is suggested that is slightly tiled from a perpendicular position with respect to the surface. Regarding the intensity of the Raman bands in Figure 3, other researchers have hypothesized that the intensities of the Raman bands associated with MNA, in particular the nitraoniline isomers, are weak because the nitro group of MNA cannot efficiently accept the electron density donated by the amino group.12 In fact, no structure exists where the amino group in MNA resonantly shares its lone-pair electrons with the nitro group.12 The same resonance phenomena that weakens the SERS spectra for MNA might actually lead to the SEIRA enhancement. The frequency of the -NO2 symmetric stretch mode of MNA not associated with hydrogen bonding appears to be sensitive
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Figure 4. Preferred orientation of 1 ML coverage of ONA deposited from heptane (left), PNA adsorbed from acetone (center), and MNA deposited from acetone (right).
Figure 5. SEIRA spectra of MNA deposited from solvents of decreasing polarity.
to small changes in coverage (note variations in Table 1 and 2). However, when a monolayer of MNA is deposited from acetone, the MNA -NO2 symmetric stretch mode ranges anywhere from 1350 to 1385 cm-1. For adsorbed ONA and PNA, the same -NO2 symmetric stretch mode is always greater than 1390 cm-1 on a silver film. These variations in frequency for the -NO2 symmetric stretch mode suggest that the bonding interactions of ONA and PNA with the surface are different from the interaction of MNA with the surface, especially when considering that all three nitroaniline isomers bind to the surface predominantly through the nitro group. A possible reason for the differences in -NO2 frequencies between the adsorbed nitroanilines again relates to the difference in resonance
Figure 6. -NH2 stretch region representing MNA deposited from acetone, ether, and heptane.
structures of ONA and PNA versus MNA. For ONA and PNA, the lone pair of electrons on the amino group can be more easily distributed through resonance, which suggests that charge density from the -NH2 group can be exchanged with the surface more efficiently when ONA or PNA is adsorbed through the -NO2 group; lone pairs of electrons cannot be shared through resonance in MNA.12 This suggests that MNA is bound to the surface differently, and the uneven charge distribution might be, at least in part, what leads to the MNA SEIRA enhancement. 3.2. Impact of Solvent Polarity on MNA Adsorption. He and Griffiths17 have shown that hydrogen-bonding interactions between the adsorbate and the deposition solvent can alter how molecules adsorb to a surface and, consequently, the corresponding surface-vibrational spectrum obtained from such an altered organic layer. Here, this idea is expanded to an investigation of the impact of the dipole moment of the deposition solvent on the formation of nitroaniline monolayers. Figure 5 summarizes SEIRA spectra displaying the fingerprint region of monolayer coverage of MNA deposited on a silver film from a series of solutions varying from an extremely polar to a nonpolar solvent on silver films. Next, Figure 6 highlights the -NH2 stretch region of a subset of the spectra in Figure 5. Table 2 lists frequencies and mode assignments for a portion of Figures 5 and 6 from 1000 to 4000 cm-1 for MNA adsorbed from solutions in acetone, 1-hexene, and heptane, as well as MNA in KBr pellets and a DFT calculation of a single gasphase MNA molecule. A polar solvent such as acetone interacts strongly with the -NH2 group of MNA and binds the amino group during the MNA adsorption process, resulting in an MNA molecule that stands upright and is adsorbed through the -NO2 group. When MNA is adsorbed from a nonpolar solvent such as heptane or CCl4, the -NH2 group is free to interact with the surface. Note in Figure 5 the gradual loss of intensity of the nitro-group symmetric stretch modes in going from a polar solvent such as acetone to a slightly polar solvent such as 1-hexene. There are three possibilities for the decreased absorbance. First, the change
TABLE 2: SEIRA Frequencies/Mode Assignments for MNA/Solvent Plus Solid MNA/KBr and DFT mode
MNA/ acetone
MNA/ 1-hexene
MNA/ n-heptane
MNA/ KBr
C-NO2 stretch CH bend C-NH2 stretch CH-NH2 bend NO2 symmetric stretch CC stretch NO2 asymmetric CC stretch NH2 scissor NH2 scissor overtone NH2 symmetric NH2 asymmetric
1164 1273 1344, 1385 1483 1525 1587 1620 -
1090 1263 1347, 1397 1453 1528 1590 1623 3211 3331 3436
1110 1178 1246 1312 1331, 1350 1498 1525 1597 1630 3209 3336 3445
1060 1161 1275 1308 1330, broad 1350-1405 1470 1526 1580 1626 3205 3320 3425
DFT 1062 1144 1243 1314 1327 1461 1541 1578 1613 3432 3528
Vibrational and TPD Study of Nitroaniline Isomers
Figure 7. Depiction of MNA adsorbing in an upright position through the -NO2 group when MNA is deposited from a polar solvent and slowly tilting to a more horizontal orientation as the polarity of the deposition solvent decreases.
in polarity of the solvent interacting with the -NH2 group could alter how the nitro group interacts electronically with the surface. Second, the solvent could interact with the silver surface during the adsorption process and alter the surface free energy. As the polarity of the solvent changed, so would the surface free energy. This change in surface free energy would alter the interaction of MNA with the surface and might directly account for the gradual loss of MNA SEIRA intensity. The data do not support either of these possibilities (although the data do not eliminate the possibilities either) because there is no discernible change in the position of the nitro-group symmetric stretch modes as the solvent polarity decreases. A third and what is believed to be a more likely possibility is that MNA begins to tilt downward as the solvent polarity decreases. Figure 7 depicts how this would occur. It might be that, when MNA is deposited from a nonpolar solvent, both the -NO2 and -NH2 groups interact with the surface. According to surface selection rules, the absorption intensity for modes such as the in-plane -NO2 modes in the range of 1320-1400 cm-1 will decrease as the modes go from a perpendicular to a parallel orientation with respect to the surface. In fact, this tilting of MNA from an upright to a flat orientation as the polarity of the solvent drops could result from several possibilities. First, the amino group on an MNA molecule has more of an opportunity to interact with the surface as the polarity of the solvent that is binding up the amino group decreases. Similarly, it could be that adjacent MNA molecules can interact more through the amino group in a less polar solvent, thus leading to MNA molecules adsorbed in a progressively more tilted orientation as the solvent polarity decreases. It could also be the change in surface free energy suggested earlier as a function of changing solvent polarity that is leading to the altered MNA adsorbate orientation. Identical SERS experiments were also performed under the conditions of Figure 5. For all adsorption solvents, no bands were present below 1000 cm-1 except for MNA adsorbed to a silver film using heptane as the solvent. When MNA was adsorbed using a heptane solvent, bands were present at 650 and 480 cm-1 that were assigned to C-H out-of-plane and N-H out-of-plane bend modes, respectively. The presence of these two bands further confirms that MNA adsorbs in a close-toparallel orientation with respect to the surface in a nonpolar solvent. Furthermore, the absence of these C-H and N-H outof-plane bend modes in any of the SERS spectra when both PNA and MNA are adsorbed on a silver film using a polar solvent and when ONA is adsorbed from a heptane solvent adds more credence to the suggestion that all of these molecules adsorb to a silver in an upright, slightly tilted orientation under the described conditions.
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12357
Figure 8. SERS spectra of PNA deposited from acetone and heptane.
Figure 6 shows the increasing intensity of the -NH2 symmetric and asymmetric MNA stretch modes from an MNA monolayer deposited from solvents of decreasing polarity. Again, the surface selection rules allow for an explanation of this occurrence. As shown in Figure 7, MNA is not a completely flat molecule, as the -NH bonds point out of the molecular plane of symmetry. When an MNA molecule tilts from a vertical to a more horizontal orientation on a surface, the -NH2 modes begin to align perpendicular to the surface, and the surface selection rules state that the infrared absorption intensity of these modes should increase. Thus, data in Figures 5 and 6 support an adsorption process seen in Figure 7 whereby MNA adsorbs upright in a polar solvent and slowly begins to tilt as the solvent polarity decreases until MNA adsorbs in a virtually flat orientation in a nonpolar solvent. MNA is almost flat when adsorbed from a nonpolar solvent such as heptane (see Table 2) because the amino stretch modes are present and the -NH2 bend mode around 1310 cm-1 is strong whereas the -NO2 symmetric stretch modes have almost completely disappeared. This set of spectra represents the first unequivocal example of SEIRA enhancement of -NH2 stretch modes. Figure 6 also contains direct evidence for intermolecular hydrogen bonding between -NH2 modes on adjacent adsorbed MNA molecules. Observe in Table 2 the -NH2 stretch frequencies of MNA adsorbed in heptane versus MNA in KBr and DFT calculations of one gas-phase MNA molecule. There is a large decrease in frequency of MNA amino stretch frequencies in KBr, 60-120 cm-1, in comparison to the DFT calculations of one MNA gas-phase molecule, because of the intermolecular hydrogen bonding found in MNA crystals. Amino-group stretching frequencies for MNA adsorbed to a silver film in nonpolar solvents are close to MNA -NH2 stretch frequencies in KBr because of intermolecular hydrogen bonding between adsorbed MNA molecules. Because there are only small changes in -NO2 stretch modes due to MNA adsorption and a large change in MNA frequency in -NH2 modes due to hydrogen bonding, it is reasonable to conclude that the hydrogen-bonding effects are much stronger energetically than the weaker MNA-surface interactions. Hence, it is also reasonable to assume that intermolecular hydrogen bonding plays a crucial role in the MNA adsorption process. SERS spectra for PNA adsorbed from acetone and heptane on silver films are shown in Figure 8. These spectra are similar, meaning that the polarity of the solvent does not alter the orientation of a PNA monolayer as a change in solvent polarity alters the adsorbate orientation for MNA. The reasons for this observation result from the two most fundamental differences between MNA and PNA monolayers. First, there is the difference between resonance structures involving the lone pair of electrons on the -NH2 group. Second, if PNA does in fact adsorb upright through the -NO2 group, the intermolecular hydrogen bonding between -NH2 groups on adjacent PNA
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Posey et al.
Figure 9. SEIRA spectra of 1 ML of MNA deposited from solutions of varying polarity following a thorough solvent rinse.
molecules in the first monolayer might be somewhat limited by the fact that the -NH2 group is directly on top of the PNA molecule. In contrast, the amino group is off to the side of an MNA molecule when MNA is adsorbed through the -NO2 group. Figure 9 illustrates 1 ML coverage of MNA deposited from solvents for samples that have undergone the rinse method. Bjerke and Griiffiths19 have suggested that only the molecules strongly bound to the surface will persist during the rinse. What is observed is that, when MNA is deposited from a polar solvent such as acetone, almost all of the adsorbed MNA molecules survive the rinse. The drop in adsorbance for the fingerprint modes in the MNA/acetone spectrum is less than a factor of 1.5 in all cases after a solvent rinse. As the polarity of the deposition solvent decreases, fewer MNA molecules survive the rinse. For MNA molecules deposited from a nonpolar solvent such as heptane, no MNA molecules survive the rinse. It is important to mention here that MNA deposited from acetone was rinsed with a range of solvents of varying polarity and the results did not change. Hence, the trend seen in Figure 9 is completely dependent on the deposition solvent and totally independent of the rinse solvent. The data in Figure 9 indicate that the change in polarity of the deposition solvent is altering more than just the orientation of the adsorbed MNA molecules. The strength of some interaction on the surface involving MNA molecules is also changing. At first glance, these data might appear to illustrate how the strength of the MNA-surface interaction changes as the changing solvent polarity alters the surface free energy. However, previous results suggest that intermolecular hydrogen bonding between adsorbed MNA molecules has a significant impact on MNA adsorption. It is reasonable to propose that changes in the interaction of MNA molecules, specifically some type of intermolecular attraction, also dictate how easy it is to remove adsorbed MNA from a surface. Unfortunately, it is difficult to elaborate on this idea because no molecules with visible modes in the -NH2 stretch region survive a solvent rinse because the MNA molecules that show the most intense -NH2 stretch frequencies are those deposited from nonpolar solvents, which are the easiest molecules to rinse off with a solvent. As suggested previously, there is always the possibility when the drop method is utilized that a spectrum is collected in a region of the drop area where slightly more than 1 ML was deposited. If the adsorbate in question is strongly bound to the surface, it is possible to use the rinse method to eliminate more weakly bound multilayers, leaving only the monolayer. In Figure
Figure 10. SEIRA spectra of increasing coverage of MNA deposited from acetone.
Figure 11. TPD spectra of increasing coverage of MNA deposited from acetone on 100-µm silver powder.
9, it is clear that the polarity of the deposition solvent determines the nature of the MNA adsorption, so that it is difficult to safely use the rinse method to selectively eliminate MNA multilayers. The data in Figure 10 represent increasing exposures of a solution of MNA in acetone to a silver film. Figure 10 has two purposes. It clearly differentiates the MNA monolayer from a multilayer and is useful in elucidating the nature of the interactions within an MNA multilayer. Up to 50-µL exposure, only the nitro-group symmetric and antisymmetric stretch modes in the range 1320-1600 cm-1 are present and increase in intensity. There are virtually no peaks present up to 50 µL for the different modes associated with the amino group in the range of 1150-1320 cm-1 or above 1600 cm-1. This is consistent with the formation of a monolayer of MNA adsorbed upright through a nitro group. As the MNA exposure increases above 50 µL, the various -NH2 modes begin to appear and grow in intensity as a result of the onset of multilayer growth. Included is a blue-shifted -NH2 deformation mode centered around 1718 cm-1 that has been seen only in multilayers of adsorbed molecules exhibiting hydrogen bonding involving an amino group.20 The simultaneous presence of modes associated with both the -NH2 and -NO2 groups is indicative of multilayer MNA coverage with molecules in more randomized orientations. 3.3. TPD Spectra of Adsorbed Nitroaniline on Ag Powders. Figure 11 presents a series of TPD spectra representing desorption of an increasing coverage of MNA deposited from
Vibrational and TPD Study of Nitroaniline Isomers
Figure 12. TPD spectra of approximately 1 ML coverage of MNA adsorbed on 100-µm silver powder from acetone (dashed line) and heptane (dotted line), as well as MNA deposited from acetone on 100nm silver powder (solid line).
acetone on 100-µm silver powder. As the MNA coverage increases, the desorption maximum shifts from about 125 °C to a maximum desorption temperature of about 180 °C. This is indicative of zeroth-order kinetics for the desorption event caused by increased hydrogen bonding between amino groups because of the growth of MNA multilayers. Although the data are not included here, ONA desorption characteristics closely mimic those of MNA desorption. PNA, on the other hand, does not desorb from 100-µm silver powder at low to moderate PNA coverage. The reason for the behavior might relate to the fact that PNA, when adsorbed upright through the -NO2 group, does not form as strong of a hydrogen-bonding network in the monolayer because the -NH2 groups are not in an advantageous geometric position to interact. This does not directly imply that the presence of intermolecular hydrogen bonding weakens the metal-PNA interaction (although this possibility exists), but the lack of PNA desorption from the 100µm silver powder does suggest that PNA interacts more strongly with the metal surface. A stronger PNA interaction with the surface in comparison to intermolecular hydrogen bonding allows for the possibility that PNA might not desorb from the surface upon heating and might eventually decompose at high enough temperatures. Preliminary data suggest that the majority of the PNA is decomposing, but the decomposition chemistry of the nitroaniline isomers on silver powders is a topic of current investigation and beyond the scope of this discussion. Figure 12 shows a series of TPD spectra for a nearly identical coverage of MNA deposited from both acetone (dashed line) and heptane (dotted line) on 100-µm silver powder, as well as MNA deposited from acetone on 100-nm silver powder (solid line). The desorption maxima are at the same temperature (∼128 °C) for MNA deposited out of heptane and acetone on 100-µm silver powder, and the areas under the TPD spectra are nearly identical. These TPD experiments suggest that MNA adsorption is independent of solvent on 100-µm silver powder. ATR-FTIR and DRIFTS spectra (not included here) involving MNA adsorption on the 100-µm silver powder confirmed this observation. For MNA deposited on 100-nm silver powder, the desorption maximum is close to 180 °C, and the maximum MNA desorption temperature for 100-nm silver powder is about 50 °C higher than that for the corresponding monolayer coverage of MNA on the 100-µm silver powder. As one might also expect, the small silver nanoparticles appeared to catalyze the decomposition of some of the MNA molecules in lieu of desorption. Again, further studies are under way to explore these catalytic properties of the silver “nanopowder”. There was no shift in desorption maximum with increasing MNA coverage on the 100-nm silver particles. The lack of a shift toward a higher desorption temperature for increasing MNA coverage on the 100-nm silver powder does not mean there that is no
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12359 intermolecular hydrogen bonding between adsorbed MNA molecules because many of the MNA molecules are decomposing on the small particles. It is necessary to mention here that a number of SERS and ATR experiments involving the nitroaniline isomers adsorbed on the 100-nm silver powder were performed and will be the focus of a future publication. These experiments confirmed two important facts. The first is that the 100-nm silver powder exhibits surface enhancement. A second important observation is that nitroaniline isomers adsorb to the 100-nm silver powder and to the silver films in similar fashions. Monolayers of each isomer adsorb in an upright orientation on silver film and on 100-nm silver powder, and MNA exhibits a solvent dependence on the 100-nm silver powder that mimics its behavior on the silver films. TPD experiments were also performed in which approximately 1 ML of each nitroaniline isomer was adsorbed on CaF2 powder. Each nitroaniline isomer exhibited behavior similar to that observed on the 100-µm silver powder. MNA and ONA desorbed from the CaF2 powder, whereas PNA decomposed at high temperatures. This suggests that PNA adsorption on CaF2 is stronger than ONA and MNA adsorption. The fact that the chemical interaction of PNA with CaF2 is stronger than those of the other two isomers might also explain in part why a PNA monolayer adsorbed on CaF2 results in more intense infrared adsorption than ONA and MNA monolayers. Gao, Hahn, and Ho recently discovered that, under the right conditions, a new type of intermolecular bond can exist between the C-H group of one molecule and an oxygen atom on another molecule that is comparable in strength to conventional hydrogen bonds.21 This interaction has been demonstrated for a number of different systems.22-25 Not only has the C-H‚‚‚O intermolecular bond been confirmed, but strong intermolecular interactions of the C-H‚‚‚N type have also been implied for several different systems over the past year or two.26 It is hypothesized that intermolecular interactions of the C-H‚‚‚N type involving either the -NO2 or -NH2 group might be contributing to the observations highlighted here. For example, an increased C-H‚‚‚N interaction for adsorbed MNA versus PNA or ONA could be contributing in some fashion to the dramatic differences in MNA SEIRA enhancement. AFM studies are currently being carried out to explore the possibility that a C-H‚‚‚N interaction is present in an adsorbed nitroaniline monolayer. 4. Conclusions SEIRA and SERS were used to study the adsorption behaviors of the nitroaniline isomers on both silver films and silver powders of varying dimensions. MNA was the only isomer to exhibit SEIRA. The polarity of the adsorption solvent had a dramatic impact on the adsorption orientation of the MNA. MNA went from an upright, slightly tilted orientation when adsorbed from a polar solvent to a nearly flat orientation when adsorbed from a nonpolar solvent. Solvent experiments presented here showed evidence for intermolecular hydrogen bonding and are also the first to exhibit true SEIRA enhancement of amino-group stretch modes. PNA exhibits the strongest SERS spectrum of the three nitroaniline isomers, and SERS experiments confirmed that a monolayer of each of the three isomers adsorbs in an upright, slightly tilted orientation through the nitro group on a silver film, assuming that a polar deposition solvent is used for MNA. SERS experiments also verified that PNA, unlike MNA, adsorbs in an upright, slightly tilted fashion independent of the adsorption solvent. TPD experiments involv-
12360 J. Phys. Chem. C, Vol. 111, No. 33, 2007 ing the nitroaniline isomers adsorbed on silver powders were performed that confirmed intermolecular hydrogen bonding within MNA multilayers and highlighted the impact of metalparticle dimensions on the adsorption and desorption characteristics of the nitroaniline isomers. Resonance effects, adsorbate geometries, and variations in intermolecular hydrogen bonding combined with changes in the strength of adsorption explain the differences in SEIRA versus SERS enhancement of the individual nitroaniline isomers, the effects of solvent polarity on nitroaniline adsorption, and the nitroaniline desorption characteristics. Acknowledgment. This work was supported in part by the NASA Arkansas Space Grant Consortium and an NSF CCLI Type I grant. We acknowledge the Arkansas Nanotechnology Center for use of the AFM, Raman, and BET isotherm equipment. We thank Dr. Peter Griffiths, Dr. Steve Lavoie, Dr. Pat Desrochers, Dr. Micah Abrams, Dr. Jerry Manion, Dr. Bill Taylor, and Dr. Karen Weaver for many fruitful discussions. We also acknowledge a reviewer of this manuscript who suggested using nitroaniline/KBr mixtures to compare the relative absorbances of the nitroanilines. References and Notes (1) Harstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. ReV. Lett. 1980, 45, 201-4. (2) Fleischman, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 74. (3) Osawa, M. Surface-Enhanced Infrared Absorption. In Near-Field Optics and Surface Plasmon Polaritons; Topics in Applied Physics; Springer: Berlin, 2001; Vol. 81, pp 163-187. (4) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58, 324A-338A. (5) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783-826. (6) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. J. Phys. Chem. 1991, 95, 9914-9919. (7) Merklin, G. T.; Griffiths, P. R. J. Phys. Chem. B 1997, 101, 58105813.
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