SERS, SEIRA, TPD, and DFT Study of Cyanobenzoic Acid Isomer Film

J. Phys. Chem. C 2010, 114, 14953–14961

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SERS, SEIRA, TPD, and DFT Study of Cyanobenzoic Acid Isomer Film Growth on Silver Nanostructured Films and Powder Donald A. Perry,* James S. Cordova, Whitney D. Spencer, and Lauren G. Smith Department of Chemistry, UniVersity of Central Arkansas, Conway, Arkansas 72035

Alexandru S. Biris Department of Applied Sciences, Nanotechnology Center, UniVersity of Arkansas, Little Rock, Arkansas 72204 ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: July 22, 2010

A combination of surface-enhanced infrared absorption (SEIRA), surface-enhanced Raman spectroscopy (SERS), temperature programmed desorption (TPD), and density functional theory (DFT) calculations were used to explore the adsorption of the cyanobenzoic acid (CBA) isomers on evaporated silver films and a silver powder. All substrates exhibited the nanoscale roughness appropriate for SERS and SEIRA enhancement. SERS and DFT demonstrated that each of the CBA isomers adsorbed to a silver film as a carboxylate ion. The impact of resonance effects in SERS spectra among the different CBA isomers was discussed. For 2-cyanobenzoic acid (2CBA), the DFT calculated dipole moment was 5.87 D, for 3-cyanobenzoic acid (3CBA), the dipole moment was 5.31 D, and for 4-cyanobenzoic acid (4CBA), the dipole moment was 3.57 D. It was shown with SEIRA that 3CBA and 2CBA underwent significant ionization in the multilayer when deposition occurred using an alkane solvent with nonpolar bonds. 4CBA adsorption was not impacted by the polar properties of the deposition solvent because 4CBA had a smaller dipole moment than 2CBA and 3CBA which induced less attraction to the underlying silver nanostructures. A comparison of the SEIRA results on silver films versus silver powder highlighted the importance of the nanoscale nature of the silver substrate on film growth during deposition of the CBA isomers. Details of the TPD spectra of each of the CBA isomers on the silver powder were also outlined. It is anticipated that this work will have a significant impact in areas of nanotechnology, applied physics, biochemistry, and organic synthesis where benzonitrile chemistry is important. Introduction There has been a lot of attention given to metal-nitrile complexes owing to the facts that the RCtN: group is isoelectronic with molecular nitrogen, organonitriles are precursors for a range of different compounds, and organonitriles are important in some biochemical processes.1 In accordance, the adsorption of substituted benzonitrile isomers has been extensively examined using SERS and other vibrational spectroscopies.2-8 When considering the importance of substituted benzonitriles in a range of applications, it is surprising that only 4-cyanobenzoic acid (4CBA) adsorption, and neither 3-cyanobenzoic acid (3CBA) nor 2-cyanobenzoic acid (2CBA), has been investigated with surface-enhanced Raman spectroscopy (SERS), surface-enhanced infrared absorption (SEIRA), or some other type of surface sensitive vibrational spectroscopy.9-14 It is also interesting that none of the previous 4CBA adsorption studies looked at the growth of a cyanobenzoic (CBA) film. SEIRA and SERS derive their vibrational enhancement from the electromagnetic fields produced by the presence of surface plasmons associated with metal nanoparticles (NPs). These NPs are typically smaller than 100 nm with spacing between the metal NPs of less than about 10 nm.15-18 SERS has been shown to be most sensitive to the first adsorbed monolayer, while SEIRA can be sensitive to both the monolayer and multilayer.18,19 A number of studies have combined SEIRA and SERS to explore adsorbate interactions with metal NPs.20-34 It has also been shown that on rough silver films in a transmission

experiment the surface selection rules hold for a SEIRA experiment.35 4CBA ionizes in the monolayer during adsorption to a rough silver surface and, depending on the nanoscale nature of the metal substrate and other adsorption conditions, it can adsorb in an upright orientation through the benzoate group, upright through the cyano group, or in a flat orientation so that both the cyano and benzoate groups interact with the metal substrate.9-14 This not only highlights the importance of the metal nanostructure and other conditions on 4CBA adsorption, but it also suggests that CBA adsorption may be altered by the polar properties of the deposition solvent, as observed previously for other substituted, aromatic compounds.29-34 The work presented here used a combination of SERS, SEIRA, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations to study the adsorption of the CBA isomers on SERS and SEIRA active substrates, which include vacuum-evaporated silver films and a commercial 2-3 µm silver powder. The impact of solvent on CBA film growth on silver nanostructures (SNS) was also explored. SERS was used to study the direct interaction of the CBA adsorbates (the monolayer) with the SNS. SEIRA was used to investigate film growth of each CBA isomer from the submonolayer through a multilayer coverage. To our best knowledge, such a comprehensive approach has not been used to study adsorption of the CBA isomers. This work will have a significant impact in nanotechnology, industrial, energy, and biochemical applications where the chemistry of benzonitiles is important.

10.1021/jp104256h  2010 American Chemical Society Published on Web 08/13/2010

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Experimental Section The CBA isomers were purchased from ACROS, 99.9% pure silver wire was purchased from Myron Toback, and the 2-3 µm silver powder was purchased from Aldrich. All solvents used for the preparation of solutions were HPLC or Optima grade (Fisher Scientific or Aldrich). 25 × 4 mm polished CaF2 windows were purchased from International Crystal Laboratories. SEIRA spectra were obtained in transmission mode (2-4 cm-1 resolution and 16 scans were averaged) with a ThermoNicolet IR100 FTIR spectrometer. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were obtained on a Nicolet Magna 560 FTIR spectrometer using an International Crystal Laboratories DRIFTS attachment (2-4 cm-1 resolution and 16 scans were averaged). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected on a Thermo Foundation Series Performer ATR Attachment with a diamond ATR crystal (single-bounce design) on an IR100 spectrometer. SERS and Raman spectra were taken in a back scattering geometry with a Horiba Jobin Yvon HR00UV Raman spectrometer at 18 mW with a 632.81 nm laser line. TPD experiments were performed with a Varian Chromatoprobe inserted into a Varian 1070 temperature programmable injector equipped with liquid CO2 cooling capacity attached to a Saturn 2200 ion trap mass spectrometer. All TPD experiments began at 293 K and were ramped at 40 K/min to a maximum temperature of 593 K. SEIRA and SERS substrates were prepared by vacuum deposition of silver on CaF2 substrates in a home-built vacuum chamber with a base pressure of 1 × 10-6 Torr equipped with an Infinicon quartz crystal microbalance. CaF2 windows were polished with a Buehler Mastermet 2 colloidal silica suspension, rinsed and sonicated in nanopure water, and air-dried before use. It has previously been shown that a 7 nm silver film is optimal for SEIRA, and these silver films have been fully characterized using UV-vis/NIR, AFM, and SEM.30,33 Henceforth, the phrase “a silver film” is with reference to a 7 nm silver film evaporated on a CaF2 substrate. The approximate monolayer CBA coverage on a silver film was prepared by pipeting 25 µL of a 50 ppm AP solution and allowing the solvent to evaporate. An average spot size of 4 cm2 for 25 µL of a 50 ppm solution of all solvents on a silver film was determined to result in reasonably uniform CBA films of about 200 ng/cm2.33 For the DRIFTS and TPD experiments, the appropriate aliquot of a 1000 ppm solution of a CBA isomer was exposed to 0.01-0.05 g of 2-3 µm silver powder which resulted in an adsorbate multilayer on the silver powder.33 It has been demonstrated with SEM and AFM that the nanoscale roughness on the 2-3 µm silver powder enables high quality SERS and SEIRA experiments.33 Throughout the manuscript, the phrase “silver powder” refers to the 2-3 µm silver powder. Density functional theory (DFT) calculations were performed using the Gaussian 2003 suite at the B3LYP level of theory to simulate infrared and Raman spectra. A LANL2DZ basis set was used for calculations of each cyanobenzoate ion (CBI) complexed with silver ion(s), and frequencies are reported without correction in accordance with the method of Aroca et al.20 For each CBI isomer, separate calculations were performed with one silver ion in proximity to the oxygen of the carboxylate group, a calculation with one silver ion close to the cyano group, and a calculation with two silver ions associated with the cyano substituent and the oxygen of the carboxylate group. Although the DFT simulation of the Raman spectrum of possible CBI silver salts is not a perfect model of CBI adsorption, this approach has been demonstrated to yield good qualitative

Figure 1. The DFT optimized structures of each of the CBA isomers with one silver ion in proximity to the carboxylate group and with two silver ions in proximity to the carboxylate and cyano groups.

representations of expected SERS, and sometimes SEIRA, spectra.20,31-34 Calculations were also performed for each CBA isomer with a 6-311G(d,p) basis set and scaled by 0.9613. Figure 1 displays the DFT optimized structures for all of the CBI isomers for one silver ion optimized in proximity to the carboxylate group and with two silver ions next to the cyano and carboxylate groups. Results and Discussion In Figure 2a from bottom to top are the SERS spectrum from a monolayer of 3CBA deposited from acetone on a silver film,

Cyanobenzoic Acid Isomer Film Growth

Figure 2. (a) From bottom to top are SERS of a monolayer of 3CBA on a silver film, SERS of 3CBA on silver powder, and DFT Raman simulations of 2CBI with silver ion interacting with the carboxylate group, 2CBI with silver ion interacting with the cyano group, and 2CBA with two silver ions interacting with the carboxylate and cyano groups. (b) From bottom to top are SERS of a monolayer of 2CBA on a silver film, SERS of 2CBA on silver powder, and DFT Raman simulations of 3CBI with silver ion interacting with the carboxylate group and 2CBI with silver ion interacting with the cyano group. (c) From bottom to top are SERS of a monolayer of 4CBA on a silver film, SERS of 4CBA on silver powder, and a DFT Raman simulation of 4CBI with silver ion interacting with the carboxylate group.

the SERS spectrum of a monolayer of 3CBA deposited from acetone on silver powder, the Raman DFT simulation with one silver ion on the carboxylate group of 3-cyanobenzoate ion (3CBI), the Raman DFT simulation with one silver ion on the cyano group of 3CBI, and the Raman DFT simulation with two silver ions associated with both the carboxylate and cyano groups of 3CBI. Observe that the SERS spectra of 3CBA deposited on the silver film and powder are comparable in Figure 2a, which suggests that a monolayer of 3CBA adsorbs in a similar fashion

J. Phys. Chem. C, Vol. 114, No. 35, 2010 14955 on both the silver film and powder in the monolayer. Note the strong band at 1382 cm-1 in the SERS spectrum of 3CBA adsorbed on a silver film and the corresponding band at 1376 cm-1 for 3CBA adsorbed on silver powder. The band is wellknown as the carboxylate symmetric stretch mode which appears when a carboxylic acid dissociates and adsorbs as a carboxylate ion in the presence of metal NPs.9-14,16,18-20,31,33,35 When comparing in Figure 2a the SERS spectra of 3CBI to the three Raman simulations, the SERS spectra match well qualitatively to the simulated Raman spectrum representing one silver ion on the carboxylate group of 3CBI. Moreover, the Raman simulation with one silver ion close to the CBI cyano group is significantly different than the CBI SERS spectra. This result immediately implies that 3CBI adsorbs to the silver NPs through the carboxylate group. However, the Raman simulation of one silver ion on the carboxylate group of CBI and the Raman simulation with two silver ions on the cyano and carboxylate groups are quite similar. Because the interaction of a silver ion with the cyano group of 3CBI does not alter to a large degree the dominant 3CBI carboxylate interaction with a silver ion, it is difficult to determine if 3CBI adsorbs in a predominantly upright, tilted, or flat orientation with respect to the substrate. In Table 1, the first column has the SERS frequencies for a 3CBA monolayer adsorbed on a silver film; the values in parentheses in the column are the SERS frequencies for 3CBA adsorbed on silver powder. Columns 2 and 3 in Table 1 are the DFT derived vibrational frequencies and mode assignments for the 3CBI silver salt with the metal/anion interaction through the carboxylate group. Modes in column 2 serve as assignments for the SERS bands in column 1 of Table 1. Figure 2b displays from bottom to top the SERS spectrum of a monolayer of 2CBA adsorbed on a silver film and on silver powder, a Raman simulation of the 2CBI carboxylate interaction with one silver ion, and a Raman simulation of one silver ion interacting with the carboxylate group and a second silver ion in proximity to the cyano group of 2CBI. Bands at 1370 and 1371 cm-1 for 2CBA adsorbed on a silver film or powder are from the carboxylate symmetric stretch mode and confirm that 2CBA adsorbs as 2-cyanobenzoate (2CBI) ion in the monolayer. The Raman simulations reveal for 2CBI that a silver ion in proximity to the cyano group can significantly alter the 2CBI carboxylate interaction with a silver ion, which was not the case for 3CBI. A cyano group in an ortho- (or para-) position with respect to an aromatic carboxylate group can resonantly stabilize the 2CBI (or 4-cyanobenzoate) ion, thus altering the carboxylate/ silver ion interaction. Cyano groups in a meta-position to an aromatic carboxylate group cannot resonantly stabilize the CBI anion. In particular, the Raman simulation of the 2CBI carboxylate group interacting with one silver ion predicts a strong doublet of ring deformation modes above 1600 cm-1 which is observed in the SERS spectra of 2CBI adsorbed on a silver film (1569 and 1598 cm-1) and silver powder (1576 and 1597 cm-1). The presence of these two modes suggests that 2CBI adsorbs to SNS in mainly an upright orientation through the carboxylate group. According to Varsanyi,36 these are the υ8a and υ8b modes. It has been previously shown that the frequencies of these bands are a good indication of the strength of a carboxylate ion interaction with a metal ion in a salt or with a metal film.37 The fact that the frequencies of these bands are different in the SERS spectra of 2CBI adsorbed on a silver film and powder as well as the Raman simulation of the 2CBI/Ag+ complex suggests that the degree of 2CBI interaction with the corresponding SNS or silver ion is different in each case.

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TABLE 1: Frequencies for 3CBA Adsorbed on Silver Powdera

SERS 3CBA 656 (681) 774 (771) 829 (809) 930 (926) 1001 (999) 1040 (1046) 1091 (1088) 1168 (1170) 1207 (1206) 1280 (1285)

1382 (1376) 1428 (1428)

DFT 3CBI/Ag+ complex modes

DFT 3CBI/Ag+ (COO) frequencies

ring def./COO scissor ring def./COO scissor CH bend ring def./COO scissor CC bend ring def. CC bend CH bend CH bend CH bend ring def. CH bend

685 775 862 905 1002 1015 1054 1115 1141 1213 1230 1334

ring ring ring ring

def./COO def./COO def./COO def./COO

str. str. str. str.

1376 1387 1427 1473

1502 (1502) 1578 (1582) 1601 (1599)

ring def./COO str. ring def. ring def.

1513 1629 1652

2231 (2233)

CN str.

2265

a

SEIRA 3CBA/n-pentane

1020 1066 1112 1173 1208 1224

1365 1385 1425 1462 1503 1552 1597 1705 2226

SEIRA 3CBA in CH2Cl2 DRIFTS

DFT 3CBA modes

DFT 3CBA freq.

ATR 3CBA

1086 1113 1182 1203 1281

CH bend/CsOH str. CH/OH bend CH/OH bend ring def./OH bend ring def.

1077 1127 1153 1191 1260

1086 1107 1180 1201 1278

1302 1313

ring def. CH/OH bend

1294 1323

1296

1390 (1394) 1419 (1425)

ring def.

1404

1414

1443 1481

ring def.

1453

1441 1483

1583 (1574) 1603 (1618) 1686 (1700) 2231 (2231)

ring def. ring def. CdO str. CN str.

1557 1582 1740 2252

1583 1603 1678 2231

Acetone was the solvent in these experiments.

In Table 2, the first column has the SERS frequencies for a 2CBA monolayer adsorbed on a silver film, and the values in parentheses in column 1 are the SERS frequencies for 2CBI adsorbed on silver powder. Columns 2 and 3 in Table 1 are the DFT derived vibrational frequencies and mode assignments for the 2CBI silver salt with the metal/anion interaction through the carboxylate group. Modes in column 2 serve as assignments for the SERS bands in column 1 of Table 2. Figure 2c from bottom to top has the Raman spectra of a 4CBA monolayer adsorbed on a silver film and on silver powder and the Raman DFT simulation of 4-cyanobenzoate (4CBI) interacting with a silver ion through the carboxylate group. Carboxylate symmetric stretch modes at 1386 and 1392 cm-1 in the SERS spectra for 4CBA adsorbed on a silver film and powder, respectively, imply that 4CBA adsorbs as the 4CBI ion in the monolayer just like 3CBA and 2CBA. Only the Raman simulation of 4CBI interacting with a silver through the carboxylate group is shown in Figure 2c because it best matches the 4CBI SERS spectra for the same reasons that the Raman simulation of 2CBI interacting with a silver through the carboxylate group best mimics the 2CBI SERS spectra. The υ8a mode is the strongest band in the 4CBI SERS spectra (1606 cm-1 on a silver film and 1605 cm-1 on silver powder) and silver salt simulation, while the υ8b mode is weak or absent. This has been shown to be quite common for para- substituted benzoic acid isomers.32,33,35-37 In Table 3, the first column has the SERS frequencies for a 4CBA monolayer adsorbed on a silver film, while the values in parentheses in the column are the SERS frequencies for 4CBA adsorbed on silver powder. Columns 2 and 3 in Table 1 are the DFT-derived mode assignments and vibrational frequencies (respectively) for the 4CBI silver ion interaction through the carboxylate group. Modes in column 2 serve as assignments for the SERS bands in column 1 of Table 3.

In Figure 3 from bottom to top are SEIRA spectra representing 25, 50, 100, and 200 µL exposures of a 50 ppm 2CBA solution deposited onto a silver film from methanol. It is important to remember in this work that a 25 µL exposure of a 50 ppm solution of an adsorbate leaves about a monolayer after solvent evaporation. The top spectrum in Figure 3 is the ATRFTIR spectrum of 2CBA powder. Table 2 lists the SEIRA frequencies for 2CBA deposited from methanol in column 5, columns 6 and 7 have the mode assignments and frequencies derived from an infrared simulation of one gas phase 2CBA molecule, and column 8 has the frequencies from the ATRFTIR spectrum of 2CBA powder. In the SEIRA spectrum of a monolayer of 2CBA (25 µL exposure), the carboxylate symmetric mode is present at 1376 cm-1, and there is a weak ring deformation centered at about 1713 cm-1. As the exposure of the 2CBA is increased, bands in the SEIRA spectra grow in from 1000-1600 cm-1 that are consistent with the ATR-FTIR of 2CBA powder. This implies that 2CBA does not undergo much ionization to 2CBI in the multilayer. Also, a ring deformation mode does slowly shift in the SEIRA spectra from a low coverage (25 µL exposure) value of 1717 cm-1 to 1698 cm-1 (200 µL exposure) at a higher coverage. It is expected that at a high enough exposure this band would shift to the value found in the ATR-FTIR spectrum of 2CBA of 1680 cm-1. There is also a small shift in the cyano stretch from about 2225 to 2229 cm-1 as a function of increasing 2CBA coverage in the SEIRA spectra. These frequency shifts are probably the byproduct of a longer range adsorbate interaction with the underlying SNS. All of the SEIRA bands in column 5 for 2CBA deposited from methanol match the mode assignments for 2CBA in column 6 spare the strong 2CBI carboxylate stretch mode at 1376 cm-1. A number of other solvents were used to deposit 2CBA on silver films including CCl4, acetone, and dichloromethane with


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SERS 2CBA 663 (685) 703 762 (763) 825 (810) 845 (844) 931 (931) 1002 (1000) 1036 (1037) 1089

DFT 2CBI/Ag+ (COO) modes


COO scissor CC bend COO scissor CC bend CC bend CC bend CC bend CC bend ring def. ring def. CH bend CH bend

674 706 772 803 822 944 1027 1055 1059 1105 1172 1208

ring def.

1222

1371 (1370)

CH bend COO str./ring def. COO str./ring def.

1307 1367 1385

1458 (1444) 1478 (1480) 1515 (1527)

COO str./ring def. COO str./ring def. ring def. ring def.

1437 1481 1521 1521

1569 (1576) 1598 (1597)

ring def. ring def.

1617 1646

1156 (1160) 1192 (1193) 1265 (1269) 1298 (1298)

2227 (2226) a

CN str.

SEIRA 2CBA/npentane

1022 1128 1149 1223 1223 1236 1290 1309 1354 1379 1392 1423 1458 1506 1520 1539 1564 1599 1686 1744 1774

2263

SEIRA 2CBA in methanol/ DRIFTS

DFT 2CBA modes

DFT 4CBA freq.

ATR 4CBA

1044 1082 (1086)

CC bend ring breath ring def.; CsOH str.

1018 1066 1081

1022 1048 1080

1153 (1154) 1193

CH/OH bend CH/OH bend

1154 1192

1151 1190

1260 1296 (1296) 1311 1326 1376 (1382)

OH bend CH bend CH bend ring def. OH bend

1205 1219 1298 1335 1374

1255 1296 1308 1326 1389

1421 1446 1493 1552 1578 1591 (1600)

1416 ring def. ring def.

1474 1519

ring def.

1611

1703 1772

ring def. CdO str. (combination band?)

1637 1816

2229 (2227)

CN str.

2339

1491 1552 1574 1589 1680 1743 1773 2229



SERS 4CBA

DFT 4CBI/Ag+ (COO) modes


SEIRA 4CBA/ DRIFTS

DFT 4CBA modes

DFT 4CBA freq.

575 (577) 643 (630) (687) 758 (816) (846) 1001 (1063) 1135 (1134)

COO scissor ring def. CC bend COO scissor CC bend COO scissor/ring breath CH bend ring def. CH bend

579 659 709 764 794 838 1040 1043 1140

1022 (1024) 1049 1126

CH bend COO bend (out plane) CH/bend

997 1067 1091

1179 (1184)

COO str.

1142

1182 (1178)

CH/OH bend

1144

CH bend COO str./ring def. COO str./ring def.

1344 1365 1388

1246 1288 (1288) 1304 1323 (1323) 1370 1398 (1389)

CH/OH bend ring def. ring def. ring def./OH bend

1173 1273 1286 1325

ring def.

1380

COO str./ring def. COO str./ring def.

1421 1453

1408 1431

1527 (1546)

ring def.

1538

ring def. ring def.

1477 1538

1506 1566

1606 (1605)

ring def. ring def. CN str.

1611 1659 2262

ring def. CdO str. CN str.

1589 1738 2250

1612 1686 2231

(1328) 1386 (1392)

2233 (2233) a


1431 1464 1506 1541 1560 (1558) 1608 (1608) 1699 (1707) 2231 (2233)

ATR 4CBA

1020 1117 1126 1182 1192 1246 1286 1303 1323

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Figure 3. From bottom to top are SEIRA of 25, 50, 100, and 200 µL of 2CBA deposited on silver film from 50 ppm methanol and the ATRFTIR spectrum of 2CBA powder.

Figure 4. SEIRA of 5, 15, 25, 50, 100, 200, 400, and 800 µL exposures of a 50 ppm 2CBA/n-pentane on a silver film.

results similar to those presented in methanol in Figure 3. All of these solvents have polar bonds. Figure 4 shows a series of SEIRA spectra where the exposure of 2CBA to a silver film was increased using n-pentane, a nonpolar, alkane solvent with virtually nonpolar bonds. Exposures in Figure 4 from bottom to top include 5, 15, 25, 50, 100, 200, 400, and 800 µL of a 50

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Figure 5. A series of TPD spectra as a function of increasing 3CBA exposure.

ppm 2CBA/n-pentane solution. The spectra are dominated by a series of ring deformation/carboxylate stretch modes from 1350 to 1500 cm-1 due to 2CBI formation that grows with increasing coverage. Column 4 in Table 2 shows the frequencies of modes associated with 2CBA adsorption on a silver film. It is apparent that modes corresponding to protonated 2CBA begin to appear around a 200 µL exposure. In particular, note the bands between 1650 and 1800 cm-1 that are similar to bands in the ATR-FTIR of 2CBA from Figure 3. Still, the dominant intensity of the ring deformation/carboxylate ion stretch modes in all SEIRA spectra in Figure 4 between 1350 and 1500 cm-1 demonstrate that, when deposition of 2CBA occurs on a silver film via a solvent with nonpolar bonds, 2CBA ionizes to 2CBI to a much larger degree than when 2CBA is deposited using a deposition solvent with polar bonds such as methanol. It has previously been shown that the deposition of substituted benzoic acids such as the hydroxybenzoic or aminobenzoic acid isomers using deposition solvents with nonpolar bonds often results in more adsorbate ionization in the multilayer.31-33 The substituted benzoic acid isomers ionize more in the multilayer using an alkane deposition solvent with nonpolar bonds because of a lower degree of solvation that allows for more attraction to the underlying SNS in oxidized silver films.33 However, for 2-hydroxybenzoic acid (2HBA) and 2-aminobenzoic acid (2ABA), strong intramolecular hydrogen bonding between the hydroxy or amino group and either the carboxylate or carboxylic acid group served to limit the ionization of 2HBA and 2ABA to mostly the monolayer. Such was not the case for 2CBA because the 2CBI cyano group cannot hydrogen-bond to a 2CBI carboxylate group. TPD results involving 2CBA adsorbed on silver powder are not presented here because 2CBA desorption was not observed until an extremely large amount of 2CBA was adsorbed. This suggests that either the 2CBA is strongly chemisorbed to the silver powder and/or there are strong intermolecular attractions induced in 2CBA layers close to the SNS of the silver powder. Figure 5 shows TPD spectra of a series of increasing multilayer exposures of 3CBA from a 1000 ppm acetone solution on silver


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Figure 6. From bottom to top are the ATR-FTIR spectra of 3CBA powder followed by SEIRA spectra of 5, 15, 25, 50, 100, 200, and 400 µL exposures of 3CBA from 50 ppm CH2Cl2.

powder. First, see the high temperature shoulders centered at about 465 and 480 K attributed to 3CBA desorption from the most strongly adsorbed layers. The low temperature shoulder shifts from a maximum of about 435 K for the lowest 3CBA exposure to about 455 K for the largest 3CBA exposure and is attributed to increased levels of intermolecular attraction (most likely hydrogen-bonding) as the number of 3CBA molecules in the multilayer is increased. In Figure 6, the bottom spectrum is the ATR-FTIR of 3CBA powder followed by SEIRA spectra of 5, 15, 25, 50, 100, 200, and 400 µL exposures of a 50 ppm 3CBA solution in CH2Cl2 on a silver film. Band frequencies for the ATR-FTIR spectrum of 3CBA powder are in column 8 of Table 1. Columns 6 and 7 of Table 1 contain DFT mode assignments and frequencies from the simulation of one gas phase 3CBA molecule. Column 5 in Table 1 lists the band frequencies of the SEIRA spectra. A carboxylate stretch mode at 1390 cm-1 in the SEIRA spectra in Figure 6 is from 3CBI formation in the first few layers. As the exposure of the 3CBA is increased, bands appear and grow in intensity that are consistent with the formation of a 3CBA film virtually identical to that seen in the ATR-FTIR spectrum of 3CBA powder. These results suggest that there is not much 3CBA ionization in the multilayer. Unlike the SEIRA spectra of 2CBA deposited in methanol from Figure 3, there were no frequency shifts of any of the bands with increasing 3CBA coverage in Figure 6. SEIRA spectra obtained from a 3CBA multilayer using deposition solvents such as acetone, methanol, and CCl4, which all have polar bonds, were similar to that obtained with CH2Cl2. Figure 7 from bottom to top has SEIRA spectra from the deposition of 5, 15, 25, 50, 100, 200, 400, and 800 µL of a 50 ppm 3CBA solution in n-pentane on a silver film. Column 4 of Table 1 has the vibrational frequencies for the 3CBA/n-pentane

Figure 7. From bottom to top are SEIRA spectra of 5, 15, 25, 50, 100, 200, 400, and 800 µL exposures of 50 ppm 3CBA in n-pentane and an infrared DFT simulation of 3CBA interacting with one silver ion through the carboxylate group.

SEIRA spectra. The top spectrum is the DFT infrared simulation of 3CBI complexed to a silver ion through the carboxylate group whose mode assignments and band frequencies are in columns 2 and 3 of Table 1. In the simulation, a series of carboxylate stretch/ring deformation modes dominates the spectrum in the range 1370-1520 cm-1. These same bands grow with increasing 3CBA exposure, suggesting significant 3CBI formation in the multilayer at exposures as high as 800 µL. Figure 8 shows a series of TPD spectra from an increasing multilayer coverage of 4CBA deposited from a 1000 ppm acetone solution. 4CBA, like 3CBA (see Figure 5), has a number of different desorption states that appear with increasing coverage. Maxima can be found in the TPD spectra at 492, 478, and 465 K with a low temperature peak that shifts from 435 to 445 K due to increased intermolecular attractions in the multilayer as the 4CBA coverage is increased. The highest 4CBA exposure in Figure 8 is not as large as that for 3CBA in Figure 5 so as to have an ordinate scale that emphasizes the three high-temperature 4CBA desorption states. The bottom spectrum in Figure 9 is the ATR-FTIR spectrum of 4CBA powder. Red SEIRA spectra in Figure 9 are from 5, 15, 25, 50, 100, 200, 400, and 800 µL exposures of a 50 ppm 4CBA solution in n-pentane. Column 4 in Table 3 has the SEIRA frequencies from Figure 9, columns 5 and 6 are the mode assignments and frequencies derived from the DFT simulation of one gas phase 4CBA molecule, and column 7 has the frequencies from ATR-FTIR of 4CBA powder. In the SEIRA spectra, bands at 1370 and 1398 cm-1 from carboxylate stretch/ ring deformation modes from 4CBI grow until a 25 µL exposure

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Figure 10. DRIFTS of a multilayer of 2CBA, 3CBA, and 4CBA adsorbed on silver powder. Figure 8. A series of TPD spectra as a function of increasing 4CBA exposure.

Figure 9. From bottom to top are ATR-FTIR spectra of 5, 15, 25, 50, 100, 200, 400, and 800 µL exposures of 4CBA in n-pentane.

is reached at which point upon increasing 4CBA exposure bands from 4CBA begin to appear matching those in the ATR-FTIR spectrum of 4CBA powder. Unlike 2CBA and 3CBA, the polar properties of the deposition solvent did not have much of an impact on 4CBA film growth. SEIRA spectra obtained from a 4CBA multilayer grown from acetone, methanol, CCl4, and CH2Cl2 were virtually identical to those obtained from an n-pentane solution. DFT-derived dipole moments are 5.87 D for 2CBA, 5.31 D for 3CBA, and 3.57 D for 4CBA. It has been shown in previous

work that the dipole moment of a substituted benzoic acid impacts the degree of ionization in the multilayer that occurs when deposition occurs using an alkane solvent without polar bonds.31,32 4CBA, with the lower dipole moment, feels less attraction to the underlying SNS and does not ionize as much as 2CBA and 3CBA in the multilayer when deposited on a silver film from n-pentane. The calculated binding energies of the CBI ions to a silver ion also mimic the dipole moment trend: 2CBI (-652.5 kJ/mol) > 3CBI (-647.6 kJ/mol) > 4CBI (-642.9 kJ/ mol). Figure 10 has the DRIFTS spectrum for a multilayer of 4CBA (bottom), 3CBA (middle), and 2CBA (top) adsorbed on silver powder from 1000 ppm of acetone. In Tables 1, 2, and 3, the numbers in parentheses under the columns with SEIRA frequencies are from the DRIFTS experiments. Each DRIFTS spectrum in Figure 10 has a large carboxylate symmetric stretch/ring deformation mode around 1380-1395 cm-1 which suggests that significant ionization of all the CBA isomers has occurred in the multilayer even when using acetone as the deposition solvent. The result is quite different than that obtained on silver films, and the outcome implies that the nanoscale nature of the underlying silver substrate has a profound impact on the degree of ionization of the CBA isomers, as observed in prior investigations of substituted benzoic acid systems.31-33 This is in spite of the fact that the Raman spectra in Figure 2 show that the adsorption of all the CBA isomers on a silver film and powder is similar in the monolayer. In conclusion, we have used a combination of SERS, SEIRA, TPD, and DFT spectral simulations to investigate the adsorption and layer growth of the CBA isomers adsorbed on silver films and powder. SERS and DFT simulations revealed that each of the CBA isomers ionize in the monolayer and adsorb through the carboxylate group on SNS. It was shown that the dipole moment of the CBA isomers has a profound impact on subsequent ionization in the multilayer. 2CBA and 3CBA are more polar than 4CBA and ionize more in the multilayer when deposition occurs via an alkane solvent with nonpolar bonds. A comparison of SEIRA results from CBA adsorption on silver films versus DRIFTS work on silver powder demonstrated the importance of the nanoscale nature of the SNS on CBA

Cyanobenzoic Acid Isomer Film Growth ionization in the multilayer. 3CBA and 4CBA both have TPD spectra with multiple peaks, and the low temperature shoulders in the TPD spectra of 3CBA and 4CBA shift to higher temperatures as the multilayer grows and the strength of the intermolecular forces within the films increases. Acknowledgment. We acknowledge Dr. Jerry Manion and Dr. Pat Desrochers for discussions on this work. References and Notes (1) Park, S. H.; Kim, K.; Kim, M. S. J. Mol. Struct. 1993, 301, 57. (2) Holz, R. Electrochim. Acta 1991, 36, 1523. (3) Solomon, T.; Christmann, K.; Baumgartel, H. J. Phys. Chem. 1989, 93, 7199. (4) Gao, X.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858. (5) Leung, L. W. H.; Gosztola, D. Langmuir 1987, 3, 45. (6) Joo, T. H.; Kim, K.; Kim, H.; Kim, M. S. Chem. Phys. Lett. 1985, 117, 518. (7) Chen, A.; Richer, J.; Roscoe, S. G.; Lipkowski, J. Langmuir 1997, 13, 4737. (8) Lee, E.; Yi, S.; Kim, S.; Kim, M. S.; Kim, K. J. Mol. Struct. 1993, 298, 47. (9) Beegum, M. F.; Kumari, L. U.; Harikumar, B.; Varghese, H. T.; Panicker, C. Y. Rasayan J. Chem. 2008, 1, 258. (10) Kitamura, F.; Ohsaka, T.; Tokuda, D. Denki Kagaku 1994, 62, 532. (11) Han, S. W.; Han, H. S.; Kim, K. Vib. Spectrosc. 1999, 21, 133. (12) Kim, S. H.; Ahn, S. J.; Kim, K. J. Phys. Chem. 1996, 100, 7174. (13) Han, H. S.; Kim, C. H.; Kim, K. Appl. Spectrosc. 1998, 52, 1047. (14) Wadayama, T.; Suzuki, O.; Suzuki, Y.; Hatta, A. Appl. Phys. 1997, 64, 501. (15) Otto, A. Phys. Status Solidi A 2001, 188, 1455. (16) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (17) Osawa, M. Near-Field Optics and Surface Plasmon Polaritons. Top. Appl. Phys. 2001, 81, 163. (18) Leverette, C. L.; Jacobs, S. A.; Shanmukh, S.; Chaney, S. B.; Dluhy, R. A.; Zhao, Y. P. Appl. Spectrosc. 2006, 60, 196A.

J. Phys. Chem. C, Vol. 114, No. 35, 2010 14961 (19) He, L.; Griffiths, P. R. In Fourier Transform Spectroscopy: 11th International Conference; de Haseth, J. A., Ed.; American Institute of Physics: New York, 1998. (20) Goulet, P. J. G.; Aroca, R. F. Can. J. Chem. 2004, 82, 987. (21) Aroca, R.; Ross, D. Appl. Spectrosc. 2004, 58, 324A. (22) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. J. Phys. Chem. 1991, 95, 9914. (23) Aroca, R.; Bujalski, R. Vib. Spectrosc. 1999, 19, 11. (24) Carrasco-Flores, E. A.; Clavijo, R. E.; Campos-Valiette, M. M.; Aroca, R. F. Spectrochim. Acta, Part A 2005, 61, 509. (25) Carrasco-Flores, E. A.; Campos-Valiette, M. M.; Clavijo, R. E. Spectrochim. Acta, Part A 2007, 66, 474. (26) Kellner, R.; Mizaikoff, B.; Jakusch, M.; Wanzenbo¨ck, H. D.; Weissenbacher, N. Appl. Spectrosc. 1997, 51, 495. (27) Leyton, P.; Domingo, C.; Sanchez-Cortes, S.; Campos-Vallette, M.; Garcia-Ramos, J. V. Langmuir 2005, 21, 11814. (28) Sanchez-Cortes, S.; Domingo, C.; Garcia-Ramos, J. V.; Azna´rez, J. A. Langmuir 2001, 17, 1157. (29) Le, F.; Brandi, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Norlander, P. ACS Nano 2008, 2, 707. (30) Posey, K. L.; Viegas, M. G.; Boucher, A. J.; Wang, C.; Stambaugh, K. R.; Smith, M. M.; Carpenter, B. G.; Bridges, B. L.; Baker, S. L.; Perry, D. A. J. Phys. Chem. C 2007, 111, 12352. (31) Smith, M.; Perry, D.; Stambaugh, K.; Smith, L.; Son, H.; Garner, A.; Cordova, S.; Posey, K.; Biris, A. S. Vib. Spectrosc. 2009, 49, 288. (32) Perry, D.; Boucher, J.; Posey, K.; Cordova, S.; Smith, L.; Son, H. J.; Pandey, R.; Biris, A. S. Spectrochim. Acta, Part A 2009, 79, 104. (33) Perry, D. A.; Cordova, J. S.; Smith, L. G.; Son, H. J.; Schiefer, E. M.; Dervishi, E.; Watanabe, F.; Biris, A. S. J. Phys. Chem. C 2009, 113, 18304. (34) Perry, D. A.; Son, H. J.; Cordova, J. S.; Smith, L. G.; Biris, A. S. J. Colloid Interface Sci. 2010, 342, 311. (35) Merklin, G. T.; Griffiths, P. R. J. Phys. Chem. B 1997, 101, 5810. (36) Varsanyi, G. Assignments for Vibrational Spectra of SeVen Hundred Benzene DeriVatiVes; Elesevier: New York, 1974. (37) Fleger, Y.; Mastai, Y.; Rosenbluh, M.; Dressler, D. H. J. Raman Spectrosc. 2009, 40, 1572.

JP104256H

SERS, SEIRA, TPD, and DFT Study of Cyanobenzoic Acid Isomer Film

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