Volume 52, Number 8, 1998 APPLIED SPECTROSCOPY 10470003-7028 / 98 / 5208-1047$2.00 / 0

q 1998 Society for Applied Spectroscopy

Diffuse Re¯ ectance Infrared Spectra of 4-NitrobenzoicAcid and 4-Cyanobenzoic Acid Self-Assembled onFine Silver Particles

HYOUK SOO HAN, CHANG HWAN KIM, and KWAN KIM *Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea

Adsorption of 4-nitrobenzoic acid and 4-cyanobenzoic acid on ® ne

silver powders was investigated by diffuse re¯ ectance infrared Fou-rier transform spectroscopy. It was concluded that both molecules

were adsorbed on the surfaces of silver powders as carboxylate,

after deprotonation, assuming a perpendicular orientation with re-spect to the silver surface. In a comparison with the re¯ ection-ab-

sorption infrared spectra taken for the same molecules on vacuum-

evaporated thick silver ® lms, the usual surface selection rule thatapplied to ¯ at metal surfaces also seemed applicable to the surface

of ® ne metal particles, as long as the diameter of the particles was

near 2 m m. Metal powders with diameters greater than 5 m m ap-peared inappropriate as an adsorbent, however, probably because

of particle sizes quite close to the wavelength of the infrared light

used.

Index Headings: Infrared; Diffuse re¯ ectance; DRIFT; Silver par-ticle; 4-Nitrobenzoic acid; 4-Cyanobenzoic acid.

INTRODUCTION

In the past decade, adsorption of molecular monolayerson metal surfaces has attracted tremendous research in-terest.1,2 In addition to the fundamental interest in suchmetal adsorbate systems, practical considerations such asthe modi® cation of metal surfaces and the preparation oforganic thin ® lms has increased research activity in thisarea.

For a better understanding of the adsorption behaviorof organic molecules on metal surfaces, numerous spec-troscopic techniques have been developed. Infrared spec-troscopy is one of these techniques; the transmissionmethod has been applied to molecules adsorbed on ® nemetal particles dispersed on high-surface-area dielectricmaterials,3±6 and the re¯ ection-absorption method hasbeen applied to molecules adsorbed on ¯ at metal sur-faces.7±9 Recently, molecules adsorbed on thin metal ® lmshave been known to be probed also by transmission orattenuated total re¯ ection methods;10±17 this approach isbased on the phenomenon of surface-enhanced infraredabsorption occurring at certain speci® cally prepared met-al ® lms.

Diffuse re¯ ectance infrared Fourier transform (DRIFT)spectrometry is a relatively unproven tool for the studyof adsorbed species. Nonetheless, it is generally regardedas a very useful technique for studying the chemistry ofprocesses taking place on the surface of powdered cata-lysts of high surface area. In fact, DRIFT spectra of COadsorbed on supported metal or metal oxide catalysts arefrequently reported in the literature.18±21 DRIFT spectro-

Received 6 March 1998; accepted 14 April 1998.* Author to whom correspondence should be sent.

scopic studies on unsupported ® ne metal particles are,however, still very scarce.

It is our hope to enlarge the capability of DRIFT spec-trometry in the area of surface chemistry on ® ne metalparticles. We are trying to obtain the DRIFT spectra ofvarious organic molecules on ® ne metal particles. In thispaper, we present the DRIFT spectra of 4-nitrobenzoicacid (4-NBA) and 4-cyanobenzoic acid (4-CBA) self-as-sembled on ® ne silver particles. Those compounds arechosen as either an adsorbate or an adsorber by recallingthat both molecules, 4-NBA and 4-CBA, are known tochemisorb on a silver surface as carboxylate in a nearlyperpendicular orientation with respect to the silver sur-face; i.e., the long molecular axis is perpendicular to themetal substrate.15,22 Accordingly, we wish to discoverwhether the usual infrared surface selection rule,23,24

which states that only the vibrational modes whose dipolemoment derivatives have components normal to the metalsurface are exclusively infrared active, is applicable evenwith respect to DRIFT spectra on ® ne metal particles.

EXPERIMENTAL

Silver powders with nominal particle sizes of 2±3.5m m ( . 99.9% purity), 5±8 m m ( . 99.9% purity), and ; 11m m ( . 99.99% purity) were purchased from Aldrich. Be-fore use, these powders were sonicated in absolute eth-anol (Hayman, . 99.9%); this procedure was performedonly to clean the silver powders, and the sonication itselfdid not change the particle size. 4-Nitrobenzoic acid(TCI, reagent grade) and 4-cyanobenzoic acid (Aldrich,99%) were used as received. Stock solutions of 1 mM 4-NBA and 4-CBA in ethanol, prepared for the self-assem-bly of the molecules on silver particles, were bubbledwith nitrogen before use; all self-assembly described be-low was performed in ambient conditions.

For the self-assembly of 4-NBA (or 4-CBA) on pow-dered silver, 0.050 g of appropriately sized silver powderswas placed in a cleaned small vial into which 0.5 mL ofethanol, 1.0 mL of the stock solution, and 0.5 mL ofethanol were added consecutively. After 3 h, the solutionphase was decanted. The remaining solid particles werewashed with excess ethanol and left to dry in ambientconditions for 2 h. These powdered samples were trans-ferred to a 4 mm diameter cup (Harrick microsamplingcup) without compression. After leveling by tapping thecup gently, the samples were subjected to infrared anal-yses.

For comparative study, 4-NBA and 4-CBA monolayersself-assembled on ¯ at silver substrates were also pre-pared. The silver substrates were prepared by the resistive

1048 Volume 52, Number 8, 1998

FIG. 1. DRIFT spectra of neat powdered silver with particle sizes of(a) 2, (b) 5, and (c) 11 m m.

evaporation of titanium (Aldrich, . 99.99%) and silver(Aldrich, . 99.99%) at 1 3 10 2 6 torr on batches of glasssildes, cleaned previously by sequentially sonicating inisopropyl alcohol, hot 1:3 H2O2(30%)/H2SO4, and triplydistilled H2O. Deposition of titanium prior to that of sil-ver was performed to enhance adhesion to the substrate.After a deposition of approximately 200 nm of silver, theevaporator was back-® lled with nitrogen. The silver sub-strates were immersed subsequently in a 1 mM solutionof 4-NBA or 4-CBA in ethanol for 3 h. After the sub-strates were taken out, they were rinsed with excess eth-anol to remove any trace of physisorbed adsorbates andthen subjected to a strong nitrogen gas jet to blow offany remaining liquid droplets on the surface or the edgesof the substrates. Thereafter, the re¯ ection-absorption in-frared (RAIR) spectra were recorded.

Infrared spectra were measured with a Bruker IFS113v Fourier transform IR spectrometer equipped with aglobar light source and a liquid N2-cooled wide-bandmercury cadmium telluride detector. The method for ob-taining the RAIR spectra has been reported previous-ly.22,25 Each RAIR spectrum was obtained by averaging512 interferograms at 4 cm 2 1 resolution, with p-polarizedlight incident on the silver substrate at 80 8 . To reduce theeffect of water vapor rotational lines, we alternately re-corded the sample and reference interferograms after ev-ery 32 scans. To record the DRIFT spectra, we used theHarrick Model DRA 3CI diffuse re¯ ectance accessory.A total of 128 scans were measured in the range 4000±600 cm 2 1 at a resolution of 4 cm 2 1 with the use of pre-viously scanned pure Ag powder as the background. TheHapp±Genzel apodization function was used in Fouriertransforming all the interferograms. The RAIR spectraare reported as 2 log(R/R0), where R and R0 are the re-¯ ectivities of the sample and the bare clean metal sub-strates, respectively. Considering that the infrared lightwill not penetrate into the metal particles, and thus theusual Kubelka±Munk (KM)26 equation cannot be appli-cable to the present system, the DRIFT spectra are alsoreported in 2 log(R/R0); surprisingly, the DRIFT spectralpattern was, however, barely dependent on the spectralunits of KM and 2 log(R/R0).

Separately, the shape of the powdered silver was ex-amined by using a scanning tunneling microsope (DigitalInstruments, Model Nanoscope IIIA). All the samplesotherwise speci® ed were reagent grade.

RESULTS AND DISCUSSION

The diffuse re¯ ectance spectrum of a solid sample isusually measured by using a nonabsorbing powder as areference and diluent. Among various powders, ® nelyground KCl was reported by Fuller and Grif® ths27 to bethe best material for those purposes since it exhibited thelowest interference and the highest re¯ ectance. Makingthe use of powdered gold as a diluent was claimed to beimpracticable since the infrared beam did not penetratethe sample at all. Nonetheless, using a powdered gold asa reference, Angelici and co-workers28,29 could identifythe n (NC) bands of aryl and alkyl isocyanide adsorbedon gold powder from methanol solution. On thesegrounds, at ® rst we examined the feasibility of using

powdered silver as a reference material for diffuse re-¯ ectance.

Figure 1 shows the DRIFT spectra of neat powderedsilver. The spectra are plotted by dividing the spectra of® nely ground KBr by the spectra of silver; the DRIFTspectrum of ® nely ground KCl was almost same as thatof KBr. Evidence for the presence of water on KBr isgiven by the upward-going bands at 3300 and 1640 cm 2 1.Otherwise, the diffuse re¯ ectance spectra of powderedsilver are essentially featureless, although their intensitiesare about 75±95% of that of KBr in the mid-infraredregion. The oxide layers that might be present on thesurface of the silver powder seem not to absorb in thespectral region shown in Fig. 1 (at least within the de-tection limit of X-ray diffractometry, we could not iden-tify any oxides on the powdered silver).

Before we present the DRIFT spectra of 4-NBA onsilver particles, it may be better ® rst to recall the infraredabsorption spectral features of sodium and copper saltsof 4-NBA, reported recently by Merklin and Grif® ths.15

For the sodium salt, three bands corresponding to thesymmetric and antisymmetric stretching modes of theNO 2 group [n s(NO2) and n as(NO2)] and the antisymmetricstretching mode of the COO 2 group [n as(COO 2 )] areequally the most intense, while the next intense one isthe band corresponding to the symmetric stretching modeof the COO 2 group [n s(COO 2 )]. For the copper salt, theabsorption intensities of those modes are on the order ofn s(NO2) . n s(COO 2 ) . n as(NO 2)±n as(COO 2 ). Locations ofthe symmetric and antisymmetric NO2 and COO 2 stretch-ing bands are hardly different between the two salts, how-ever. The n s(NO 2), n as(NO2), n s(COO 2 ), and n as(COO 2 )bands are observed, respectively, at 1354, 1524, 1410,

APPLIED SPECTROSCOPY 1049

FIG. 2. (a) DRIFT spectrum of 4-NBA on 2 m m sized Ag powder. (b)DRIFT spectrum of neat 4-NBA diluted with KBr powders. (c) RAIRspectrum of 4-NBA on thick Ag ® lm.

and 1592 cm 2 1 for the Na salt and at 1352, 1528, 1413,and 1592 cm 2 1 for the Cu salt. The same authors alsoreported, with the use of surface-enhanced infrared ab-sorption (SEIRA) spectroscopy, that bands correspondingto the above four modes were observed at 1343, 1521,1384, and 1570 cm 2 1 when 4-NBA was anchored on avacuum-evaporated thin Ag ® lm. The band correspond-ing to the n s(NO2) mode was the most intense, and theband corresponding to the n as(NO2) mode was the leastintense. The bands due to the n s(COO 2 ) and n as(COO 2 )modes were 2±3 times less intense than the n s(NO 2) band.When the Ag ® lm was rinsed throughly with methanol,the antisymmetric NO2 and COO 2 stretching bands dis-appeared, however, while the intensity of the bands re-sulting from the NO2 and COO 2 symmetric stretchingmodes was reduced by about one-half. On this ground,Merklin and Grif® ths15 proposed that two somewhat dif-ferent chemical species would be present on the silversubstrate simultaneously: one that only gives rise to thesymmetric bands and does not easily rinse off, and an-other, in which both sets of bands are visible, that is suf-® ciently labile to allow it to be removed with a methanolrinse. Along with the roughness of the metal surface, dueto the different surface sites or the different adsorptiongeometries, the usual infrared surface selection rule wasthus claimed to be nonapplicable to SEIRA under somecircumstances.

Figure 2a shows the DRIFT spectrum of 2 m m sized

Ag powders onto which 4-NBA had been previously self-assembled. In taking the spectrum, we used the samesized neat Ag powders as a reference material. TheDRIFT spectral pattern of 4-NBA on other sized Ag pow-ders will be described later. For comparative purposes,the DRIFT spectrum of neat 4-NBA diluted with KBrpowders and the RAIR spectrum of 4-NBA self-assem-bled on a vacuum-evaporated thick silver ® lm are shownin Fig. 2b and 2c, respectively. It is remarkable that onlya few peaks are identi® ed in Fig. 2a. Nonetheless, thosepeaks are correlated well with those in the DRIFT spec-trum of sodium p-nitrobenzoate15 as well as in the RAIRspectrum. The two strong peaks at 1356 and 1390 cm 2 1

in Fig. 2a can be correlated, respectively, with the peaksat 1354 [n s(NO2)] and 1410 [n s(COO 2 )] cm 2 1 in the so-dium salt spectrum (vide supra ) as well as the peaks at1343 and 1384 cm 2 1 in the RAIR spectrum (Fig. 2c). Inboth the DRIFT and RAIR spectra shown in Fig. 2a and2c, the peak corresponding to the C 5 O stretching modeof the carboxyl group that results in the most intense peakat 1700 cm 2 1 in the neat 4-NBA spectrum (Fig. 2b) iscompletely absent. This observation implies that, regard-less of the detailed nature of the metal surface, 4-NBAis adsorbed on silver as a p-nitrobenzoate species.

In the RAIR spectrum, the antisymmetric NO 2 andCOO 2 stretching bands are completely absent, while theirsymmetr ic stretch ing counterpar ts appear d istinctly.These spectral features can be understood by virtue ofthe usual surface selection rule, which states that only thevibrational modes whose dipole moment derivatives havecomponents normal to the metal surface are exclusivelyinfrared active. Cited frequently, p-nitrobenzoate shouldbe bound to silver symmetrically via the two oxygen at-oms of the carboxylate group, assuming a nearly verticalorientation with respect to the silver surface. The ap-pearance of an absorption band at 827 cm 2 1 in Fig. 2ccan be understood similarly, since the band can be as-signed to a combination mode of d (CO2) and d (NO 2)aligned along the long molecular axis.30

It is intriguing that the antisymmetric NO2 and COO 2

stretching bands are also completely absent in the DRIFTspectrum in Fig. 2a. Nonetheless, as mentioned above,the symmetric NO 2 and COO 2 stretching bands are ob-served very distinctly. Being comparatively less intense,other peaks are also identi® ed clearly in the DRIFT spec-trum, i.e., at 826, 864, 1015, and 1107 cm 2 1. All thesepeaks can be attributed to ring or combination modes[ d (CO2)/ d (NO2), d (CO2)/ d (NO 2), n 12, and n 19a] alignedalong the long molecular axis of p-nitrobenzoate. Thepresent observation dictates that the usual surface selec-tion rule is applicable even to the surfaces of metal pow-ders.

As mentioned previously, Merklin and Grif® ths15 pro-posed that two different p-nitrobenzoate species could bepresent on the silver surface simultaneously: one that isbound to silver via the two oxygen atoms of the carbox-ylate group so that the molecular plane is perpendicularwith respect to the metal surface, and another that isbound to silver less strongly by forming only one Ag±Obond so that it has a tilted orientation. The latter type ofadsorbate might have been formed even on the surfacesof powdered silver. However, considering that such anadsorbate could be removed easily with an ethanol rinse,

1050 Volume 52, Number 8, 1998

FIG. 3. (a) DRIFT spectrum of 4-CBA on 2 m m sized Ag powder. (b)DRIFT spectrum of neat 4-CBA diluted with KBr powders. (c) RAIRspectrum of 4-CBA on thick Ag ® lm.

the absence of the antisymmetric NO 2 and COO 2 stretch-ing bands in the DRIFT spectrum (Fig. 2a) would not beunreasonable.

With respect to the nature of the silver powders, wehave mentioned previously that we could not identify anyoxides on the powdered silver from X-ray diffraction andinfrared spectroscopy. Nonetheless, we speculate that thepowdered silver is covered with oxides; in this respect,we plan in the near future to perform an extended X-rayabsorption ® ne structure (EXAFS) study. We believe,however, that the oxide layers are removed upon the self-assembly of the carboxylic acid, and that any carboxylicacid that is bound to the oxide layers via H bonding isremoved easily with an ethanol rinse.

It is seen in Fig. 2 that the signal-to-noise ratio (SNR)of the DRIFT spectrum is far greater than that of theRAIR spectrum. In particular, the low wavenumber ab-sorption bands are more clearly identi® ed in the DRIFTspectrum. This observation may suggest that, at least forthe identi® cation of the low-frequency bands of adsor-bates on a metal surface, DRIFT spectroscopy applied to® ne metal particles would be more advantageous thanRAIR spectroscopy applied to ¯ at metal surfaces.

Recalling our previous RAIR spectroscopic studyshowing that 4-cyanobenzoic acid (4-CBA) is adsorbedon a vacuum-evaporated thick silver ® lm as 4-cyanoben-zoate with a perpendicular orientation,22 we have also at-tempted to compare the DRIFT spectrum of 4-CBA on apowdered silver with the RAIR spectrum. Figure 3ashows the DRIFT spectrum of 4-CBA self-assembled on2 m m sized silver powders. Once again, the same sizedneat Ag powders were used as a reference material. Fig-ure 3b and 3c show, respectively, the DRIFT spectrumof neat 4-CBA diluted with KBr powders and the RAIRspectrum of 4CBA self-assembled on an evaporated sil-ver ® lm. The distinct peak at 1700 cm 2 1 in Fig. 3b arisingfrom the C 5 O stretching mode of the carboxyl group in4-CBA is not seen in Fig. 3a and 3c. This observationimplies that 4-CBA should be bound to silver as 4-cy-anobenzoate after deprotonation. Further, it is noteworthythat the DRIFT spectral pattern in Fig. 3a is almost thesame as the RAIR spectral pattern in Fig. 3c. The anti-symmetric stretching band of the COO 2 group is com-pletely absent in both spectra. Instead, the symmetricCOO 2 stretching and the C [ N stretching bands are seenclearly at 1385 and 2235 cm 2 1 in Fig. 3a and at 1388 and2233 cm 2 1 in Fig. 3c, respectively. In addition, the totallysymmetric vibrational bands such as d (COO 2 ) and n 18a

(ring mode) are also seen in both spectra at 846 and 1020cm 2 1, respectively. The present observation dictates that4-CBA is as likely to bind to powdered silver as to anevaporated silver ® lm through the carboxylate groupsymmetrically with a perpendicular stance. This obser-vation implies that the usual surface selection rule is, infact, applicable to the surfaces of metal powders.

Greenler et al.31 have performed both classical andquantum mechanical calculations for the interaction ofthe electromagnetic ® eld with the metal surface for par-ticles of varying sizes and arrived at the conclusion thatsurface selection rules should apply for particles largerthan about 1.5 nm. For particles smaller than 0.2±0.6 nm,the surface selection rule based on electromagnetic ® eldorientation was determined to break down. In their cal-

culations, Greenler et al.31 assumed that the particles weresmooth and regular. In this respect, we have attempted tosee the atomic scale roughness of the powdered silver,used in this work, by scanning tunneling microscopy(STM). Figure 4 shows the STM image for the 2 m msized silver powder. Although the image taken in a 500nm scan size appears to be somewhat rugged and irreg-ular, the image taken in a 20 nm scan size reveals clearlythat the surface of silver powders is, in fact, smooth and¯ at at the atomic scale. In this sense, it seems not unrea-sonable to observe only a limited kind of peak in theDRIFT spectra of 4-NBA and 4-CBA adsorbed on thesurfaces of powdered silver.

Finally, it has to be mentioned that the particle sizeseems to be a very important factor in getting the DRIFTspectra of adsorbates on ® ne metal particles. As shownin Figs. 5 and 6, spectral peaks due to 4-NBA or 4-CBAadsorbed on silver could not be identi® ed at all whensilver powders with sizes greater than 5 m m were em-ployed as the adsorbent. Instead, a very broad and strongabsorption band was seen at ; 1100 cm 2 1. The latter bandseems to have nothing to do with the presence of oxidelayers. Its exact cause is not certain, but as reported ear-lier by Fuller and Grif® ths,27 the band is supposed to arisemainly from the close match between the diameter of thesilver particles and the wavelength of the infrared radia-tion emitted from a light source with a maximum inten-sity.

APPLIED SPECTROSCOPY 1051

FIG. 4. STM images of 2 m m sized Ag particle with scanning sizes of (a) 500 and (b) 20 nm.

FIG. 6. DRIFT spectra of 4-CBA on Ag powders with particles sizesof (a) 2, (b) 5, and (c) 11 m m.

FIG. 5. DRIFT spectra of 4-NBA on Ag powders with particles sizesof (a) 2, (b) 5, and (c) 11 m m.

In summary, we have attempted to record the infraredspectra of 4-NBA and 4-CBA adsorbed on ® ne silverparticles by diffuse re¯ ectance spectroscopy. When 2 m msized silver powders were used as the adsorbent, veryhigh SNR DRIFT spectra could be obtained reproduciblyfor the adsorbed species. The DRIFT spectral pattern was

found to show very little difference from the RAIR spec-tral pattern taken for the same molecules on vacuum-evaporated thick silver ® lms. The usual surface selectionrule thus seemed applicable even to the surfaces of ® nemetal particles. Although metal powders with diametersgreater than 5 m m would be inappropriate as an adsor-bent, probably due to their particle sizes being quite close

1052 Volume 52, Number 8, 1998

to the wavelength of the source light, the observationmade on the 2 m m sized silver powders dictated thatDRIFT spectroscopy should be as important as otherspectroscopic means, especially in the characterization ofthe vibrational structure of adsorbate on ® ne metal par-ticles.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and EngineeringFoundation through the Center for Molecular Catalysis at Seoul Na-tional University and by the Ministry of Education, Republic of Korea,through the Basic Science Research Fund. K.K. acknowledges the Re-search Institute of Molecular Science in SNU for providing an instru-ment fund to purchase a scanning probe microscope.

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