In Situ FTIR Spectroscopic Studies of Adsorption of CO, SCN-, and

A polycrystalline Pt disk was polished mechanically following the same procedure and ... All solutions were prepared from super pure H2SO4, Millipore ...
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Langmuir 2000, 16, 778-786

In Situ FTIR Spectroscopic Studies of Adsorption of CO, SCN-, and Poly(o-phenylenediamine) on Electrodes of Nanometer Thin Films of Pt, Pd, and Rh: Abnormal Infrared Effects (AIREs) Guo-Qiang Lu, Shi-Gang Sun,* Li-Rong Cai, Sheng-Pei Chen, and Zhao-Wu Tian State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China

Kwok-Keung Shiu Department of Chemistry, Hong Kong Baptist University, Hong Kong Received March 9, 1999. In Final Form: September 16, 1999 Pt, Pd, and Rh films of a few nanometers in thickness supported on glassy carbon (GC) and other substrates were prepared by electrochemical voltammetry. STM patterns illustrated that the prepared thin films are composed of crystallites of layer structure and exhibit a low surface roughness. Studies of in situ FTIR spectroscopy on chemisorption of CO and SCN- and formation of a polymer of ophenylenediamine (POPD) on electrodes of nanometer thin films have been conducted to explore the abnormal infrared effects (AIREs), which consist of two main characteristics: (1) inversion of IR bands; (2) the enhancement of IR absorption of adsorbates. The results demonstrated that the AIREs depend mainly on the structure and the chemical nature of nanometer thin films. In all cases of chemisorption on thin films of platinum-group metals supported on GC or supported on polymer-covered GC, the direction of IR bands of adsorbates is inverted in comparison with the direction of IR bands of the same adsorbates on corresponding massive metal electrodes. The IR absorption of adsorbed CO species (COad) on nanometer thin films of Rh, Pt, and Pd supported on GC has been enhanced respectively by a factor of 11, 20, and 26. The fact that the IR absorption of adsorbed CO and SCN- has been enhanced but the IR absorption of POPD has not suggested that the IR absorption enhancement in AIREs is related to an effect of shortrange domain of surface. The results in the present paper demonstrated also that the AIREs belong to a new phenomenon of IR reflection spectroscopy and relate to effects of material at the nanometer scale. The present study manifests remarkable advantages of AIREs for studying surface processes and may contribute considerably to fundamental studies of electrocatalysis and reflection spectroscopy.

1. Introduction Materials at a nanometer scale often exhibit unusual chemical properties, which can be of great interest in revealing new principles of chemical processes. The study of materials at nanometer scales presents an active subject for multidisciplinary research and consists of not only the preparation and characterization of different nanostructured materials but also the application of these materials for various purposes.1,2 Nanostructured materials are also of vital importance in electrochemistry, especially for electrocatalytic applications. In recent years, the electrocatalytic properties of nanostructured materials were extensively studied and particular efforts were made on dispersing metallic nanoparticles on conductive substrates of low cost.3-5 In the fundamental study of electrocatalysis, it is very important to determine the intrinsic relationship between * To whom correspondence should be addressed. Fax: +86 592 2085349. E-mail: [email protected]. (1) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (2) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 97, 11951230. (3) Munk, J.; Christensen, P. A.; Hamnett, A.; Skou, E. J. Electroanal. Chem. 1996, 401, 215-222. (4) Yang, H.; Lu, T.-H.; Xue, K.-H.; Sun, S.-G.; Lu, G.-Q.; Chen, S.-P. J. Electrochem. Soc. 1997, 144, 2302-2307. (5) Jacobs, P. W.; Wind, S. J.; Ribeiro, F.H.; Somorjai, G. A. Surf. Sci. 1997, 372, 249-253

surface structure and electrocatalytic properties. The in situ investigation of the electrode/electrolyte interface by using different spectroscopies is a key step for this approach. In situ infrared spectroscopy, which can provide information at the molecular level and possesses a high sensitivity to surface structure, has been considered as one of the most powerful techniques to study the reaction mechanism and the inherent principle of electrocatalytic system. The knowledge gained from in situ IR investigations may serve further to guide the preparation of electrocatalysts of high performance.6-8 Nevertheless, the success of applying infrared spectroscopy to in situ electrochemical studies carried out on solid/liquid interfaces was not achieved until early 1980s9,10 because of three main obstacles: (1) IR energy is strongly absorbed by solution species of electrolyte, which are in most cases water molecules and ions. (2) IR energy is partially lost during its reflection at electrode surface. (3) The quantity of molecules on electrode surface to be determined (e.g., monolayer or submonolayer of adsorbates) is too small to (6) Friedrich, K. A.; Geyzers, K. P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123-128. (7) Xia, X.-H.; Iwasita, T.; Ge, F.; Vielstich, W. Electrochim. Acta 1996, 41, 711-718. (8) Pastor, E.; Gonzalez, S.; Arvia, A. J. J. Electroanal. Chem. 1995, 395, 233-242. (9) Bewick, A.; Kunimatsu, K.; Pons, B. S. Electrochim. Acta 1980, 25, 465-468. (10) Bewick, A.; Kunimatsu, K.; Pons, B. S. Surf. Sci. 1980, 101, 131-138.

10.1021/la990282k CCC: $19.00 © 2000 American Chemical Society Published on Web 11/24/1999

Adsorption on Electrodes of Nanometer Thin Films

yield a reasonably strong IR absorption. To overcome these obstacles, some specific experimental considerations were necessary,11 including the employment of an IR cell of thin layer to minimize the IR absorption of solution species, the use of an electrode surface of high reflectivity with an appropriate angle of incidence, and the definition of the resulting signal as a potential-difference spectrum. As a consequence, most studies up to now using in situ IR spectroscopy were carried out on well-polished surfaces, such as electrodes of polycrystal or single crystals of metals or other materials.9-12 Although the study employing in situ IR spectroscopy has been extended recently to the surfaces of dispersed materials supported on conductive substrate,3 the spectra obtained on these “rough” surfaces were of relatively poor quality, especially for the determination of adsorbed species that play the most important role in electrocatalysis. The chemisorption of compounds on surfaces is an essential subject with respect to surface coordination, heterogeneous catalysis, and spectroscopy and has been intensively studied.11-15 Studies of chemisorption of simple molecules or ions such as CO, NO, CN-, SCN-, etc., on transition metal surfaces were extremely attractive for decades for two reasons. On one hand, molecules or ions of simple structure may be used as probe species, and an understanding of the nature of their interaction with metal surfaces is very helpful in elucidating the mechanisms of surface reactions involving larger molecules. On the other hand, the transition metal surfaces are particularly active toward chemisorption and frequently exhibit an irreversible character, both of which are related directly to the high catalytic activities of these metals. Among simple adsorbates, CO has been the most popular.9-12,16-29 The CO molecule interacts strongly with transition metal surfaces, gives rise to a pronounced sensitivity to the local physicochemical environments, and provides a model system for surface science and related disciplines. The adsorption of CO on surface of transition metals has been also proved invaluable in infrared spectroscopic studies since the C-O stretching vibration has a large absorptivity. (11) Sun, S.-G. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; Chapter 6 pp 243-290. (12) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271-340. (13) Katsanos, N. A.; Thede, R.; Roubanikalantzopoulou, F. J. Chromatogr. A 1998, 795, 133-184. (14) Minot, C.; Markovits, A. THEOCHEM 1998, 424, 119-134. (15) Ueba, H. Prog. Surf. Sci. 1997, 55, 115-179. (16) Markovic, N. M.; Widelov, A.; Ross, P. N.; Monteiro, O. R.; Brown, I. G. Catal. Lett. 1997, 43, 161-166. (17) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8290-8301. (18) Zou, S. Z.; Weaver, M. J. J. Phys. Chem. 1996, 100, 4237-4242. (19) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, 11, 2777-2786. (20) Weaver, M. J.; Chang, S. C.; Leung, L. W. H.; Jiang, X.; Rubel, M.; Szklarczyk, M.; Zurawski, D.; Wieckowski, A. J. Electroanal. Chem. 1992, 327, 247-260. (21) Sung, Y. E.; Thomas, S.; Wieckowski, A. J. Phys. Chem. 1995, 99, 13513-13521. (22) Wieckowski, A.; Rubel, M.; Gutierrez, C. J. Electroanal. Chem. 1995, 382, 97-101. (23) Rhee, C. K.; Feliu, J. M.; Herrero, E.; Mrozek, P.; Wieckowski, A. J. Phys. Chem. 1993, 97, 9730-9735. (24) Gomez, R.; Orts, J. M.; Feliu, J. M.; Clavilier, J.; Klein, L. H. J. Electroanal. Chem. 1997, 432, 1-5. (25) Herrero, E.; Feliu, J. M.; Aldaz, A. J. Catal. 1995, 152, 264-274. (26) Sun, S.-G.; Cai, W.-B.; Wan, L.-J.; Osawa, M. J. Phys. Chem. B 1999, 103, 2460-2466. (27) Lu, G.-Q.; Sun, S.-G.; Chen, S.-P.; Cai, L.-R. J. Electroanal. Chem. 1997, 421, 19-21. (28) Lu, G.-Q.; Sun, S.-G.; Chen, S.-P.; Li, N.-H.; Yang, Y.-Y.; Tian, Z.-W. In Electrode Processes VI; Wieckowski, A., Itaya, K., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1996; pp 436-445. (29) Lu, G.-Q.; Sun, S.-G.; Chen, S.-P.; Cai, L.-R.; Li, N.-H.; Tian, Z.-W. Chem J. Chin. Univ. 1997, 18, 1491-1495.

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We have investigated recently the electrooxidation of C1 molecules (CH3OH, HCHO, HCOOH, and CO) on supported Pt electrocatalysts and observed that the IR features of adsorbed CO species (derived either from direct adsorption of CO molecule or from dissociative adsorption of these C1 molecules) on dispersed Pt surfaces were different from those obtained on massive Pt electrodes.27-31 During these observations, we discovered that the abnormal IR features for CO adsorption on dispersed Pt electrode are related to a new phenomenon of surface reflection spectroscopy, named abnormal infrared effects (AIREs). Excitingly, our discovery is also attracting following-up research from other groups.32,33 In the present paper, the chemisorption of CO, SCN-, and poly(ophenylenediamine) (POPD) on surfaces of nanometer thinfilm of platinum-group metals (Pt, Pd and Rh) was studied systematically, and the AIREs have been further explored as the general effects of reflection IR spectroscopy conducted on surfaces of nanometer thin-films. 2. Experimental Section Films of platinum-group metals (Pt, Pd, Rh) a few nanometers in thickness were deposited electrochemically on glassy carbon (denoted as nm-M/GC thereafter) or other substrate materials. The deposition of nanometer thin films was carried out in 0.5 M H2SO4 solutions containing different metal ions by applying cyclic voltammetry. The upper and lower limits of the cyclic voltammetry used in the deposition were -0.24 and 0.40 V (vs SCE), respectively. The GC substrate of 6 mm diameter is sealed into a Teflon support and polished mechanically using sandpaper and alumina powder of size 5, 1 down to 0.3 µm to obtain a mirror finish before metal deposition. A polycrystalline Pt disk was polished mechanically following the same procedure and has served as a reference surface of the massive Pt electrode in the study. Electrochemical in situ FTIR spectroscopic measurements were carried out on a Nicolet 730 FTIR spectrometer equipped with a liquid-nitrogen-cooled wide-band MCT detector. A CaF2 disk was used as IR window, and an IR cell of thin layer configuration between electrode and IR window was approached by pushing the electrode against the window during FTIR measurements. The incident infrared beam was aligned at about 60° to the normal of electrode surface. In any measurement the potential difference technique was employed, and the resulting spectrum was reported as the relative change in reflectivity that is calculated as

∆R R(Es) - R(ER) ) R R(ER)

(1)

The R(ES) and R(ER) are the single-beam spectra of reflection collected at sample potential ES and reference potential ER, respectively. The 400 interferograms were collected and coadded into each single-beam spectrum. The adsorption of CO on electrodes was conducted by introducing CO gas of high purity into 0.1 M H2SO4 solution at open circuit. The saturation adsorption of CO was confirmed subsequently by potential cycling between -0.25 and 0.1 V, for which the hydrogen adsorption current has been suppressed completely. The free CO species in solution were removed completely after CO adsorption procedure by bubbling N2 gas into the solution while holding the electrode potential at 0.0 V. In such a way only adsorbed CO species were investigated by in situ FTIR spectroscopy. For all IR studies of CO adsorption in this paper the reference potential was fixed at 0.70 V (vs SCE), (30) Chen, S.-P.; Sun, S.-G.; Huang, T.-S. Chin. Sci. Bull. 1995, 40, 377-381 (31) Lu, G.-Q.; Sun, S.-G.; Chen, S.-P.; Tian, Z.-W.; Yang, H.; Xue, K.-H. Abstracts of the 189th ECS Meeting, Los Angeles, May, 1996; 1115-1116. (32) Ortiz, R.; Cuesta, A.; Ma´rquez, O. P.; Ma´rquez, J.; Me´ndez, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1999, 465, 234-238. (33) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967-1974.

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Figure 1. In situ FTIR spectra for CO adsorbed on Pt (a) and nm-Pt/GC (b) electrodes (ES ) 0.0 V, ER ) 0.7 V, 0.1 M H2SO4). at which the adsorbed CO can be oxidized immediately and completely into CO2. As a consequence, the adsorbed CO species existed below 0.40 V. According to the definitions represented by eq 1 and under above experimental conditions, the IR absorption by CO adsorbed on a metal substrate at ES will yield negative-going bands in the spectra, and positive-going bands are expected for IR absorption of CO2 at ER. In considering that the resulting spectra will contain mainly COad and CO2 features under present experimental conditions, the spectral resolution of IR measurements was generally at 16 cm-1. However, spectra of higher resolutions (8, 4, 2 cm-1) were also presented for the sake of comparison. The observations of surface structure of electrodeposited thin films of Pt and Pd were carried out ex situ (i.e., in ambient environment) using a scanning tunneling microscope (STM) (Nanoscope IIIa, Digital Instruments). All solutions were prepared from super pure H2SO4, Millipore water supplied from a Milli-Q lab equipment (Nihon Millipore Ltd.), and other chemicals of analytical grade. The solutions were deaerated by bubbling N2 before measurement. The reference electrode was a saturated calomel electrode (SCE), and all potentials reported in this paper were quoted versus this electrode scale. All experiments were performed at room temperature around 20 °C.

3. Results and Discussion 3.1. Abnormal IR effects of Nanometer Thin Films of Pt, Pd, and Rh Supported on Glassy Carbon for CO Adsorption. 3.1.1. CO Adsorption on the Surface of the nm-Pt/GC Electrode. Figure 1 shows the comparison of FTIR spectra for CO adsorbed on nm-Pt/GC and on massive Pt electrodes in 0.1 M H2SO4 solution. The sample potential ES was set at 0.0 V at which the adsorbed CO (COad) is stable on electrode surface, and the reference potential ER was chosen at 0.7 V where COad is oxidized immediately to CO2. It is well-known that two IR bands appear normally in the spectrum obtained on a Pt electrode (Figure 1a), i.e., a negative-going band around 2070 cm-1 assigned to the IR absorption of linearly bonded CO (COL) at ES and a positive-going band near 2345 cm-1 ascribed to the IR absorption at ER by CO2 species. The results indicate that COad has been removed completely through its oxidation at ER, and that the CO2 species determined at ER was derived exclusively from COad oxidation because no CO2 species was presented initially at ES in solution of the thin layer between the electrode and IR window. It is necessary to note that, for CO adsorption on a Pt surface under the present experimental

Lu et al.

conditions, the COL band (∼2070 cm-1) appears always in the opposite direction of the CO2 band (2345 cm-1, positivegoing), just as those reported by Kunimatsu et al.34 Dramatic changes in IR features were observed for CO adsorption on a nm-Pt/GC surface. We can see that the spectrum (Figure 1b) displays also a positive-going CO2 band near 2345 cm-1 whose intensity is slightly larger than that of the CO2 band appeared in Figure 1a, but manifests abnormal IR features for the COL band near 2065 cm-1. First, the COL band becomes a positive-going band; i.e., it appears in the same direction as that of the CO2 band. Second, the intensity of the COL band is increased significantly, and an enhancement factor at ca. 20 has been determined in comparison with the intensity of COL band appeared in the spectrum recorded on a massive Pt surface (Figure 1a), by taking the intensity of the CO2 band in the normalization.27,29 The IR features of CO adsorbed on a nm-Pt/GC electrode surface, i.e., the inversion of the direction of COad bands and the enhancement of IR absorption of COad species, have been denoted as the abnormal IR effects (AIREs) of a nm-Pt/GC surface with respect to a massive Pt surface for CO adsorption and have been reported, for the first time, in our previous papers.27-29 It is interesting to recognize that the appearance of IR bands of COL and CO2 in the same direction in the spectrum recorded on a nm-Pt/GC surface yields a distinct indication of the abnormal IR effects. We can observe also from the spectrum recorded on a nm-Pt/GC electrode, besides the IR bands of CO2 and COL, a small positive-going band near 1860 cm-1 that is assignable to an IR absorption of a bridge-bonded CO species (COB). This band cannot be determined from the spectrum recorded on Pt surface under the same conditions. It may be worthy to mention that the COB band exhibits also abnormal IR features since it appears in the same direction of the CO2 band. It should be pointed out that the nm-Pt/GC surface, which possesses very interesting infrared properties, was prepared conveniently by electrodeposition of Pt onto GC substrate under potential cycling conditions. The average thickness of the deposited Pt film has been estimated at 6.3 nm from the quantity of electric charge (26.7 mC cm-2) passed for the deposition. Physically, the thin film of Pt sticks firmly on a GC substrate, and the surface of the thin film looks shiny, which indicates effectively a low surface roughness. Figure 2 shows the comparison of cyclic voltammograms of nm-Pt/GC and massive Pt electrodes in 0.5 M H2SO4. The voltammogram recorded on the nmPt/GC electrode displays two distinct pairs of current peaks around -0.02 and -0.16 V, which are characteristic of hydrogen adsorption-desorption on a polycrystalline Pt surface. Nevertheless the current in the potential region between 0.1 and 0.5 V due to double layer charging is much larger than that recorded on a Pt surface, signifying the influence of the GC substrate. The charge of hydrogen adsorption per geometric area of GC substrate (712 µC cm-2) integrated from the voltammogram recorded on the nm-Pt/GC electrode is close to the value obtained on the Pt surface (663 µC cm-2), indicating that the two electrodes have a comparable number of surface sites for hydrogen adsorption. The roughness of the nm-Pt/GC and the mechanically polished Pt surfaces can be estimated from the comparison of the above electric charge densities with the value of 210 µC cm-2 measured for hydrogen adsorption on a perfectly smooth polycrystalline Pt electrode; the values of 3.39 and 3.15 have been obtained respectively (34) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gorden, J. G., II; Philpott, M. R. Langmuir 1986, 2, 464-468.

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Figure 2. Cyclic voltammograms of Pt and nm-Pt/GC electrodes in 0.5 M H2SO4, with sweep rate at 100 mV s-1.

Figure 4. In situ FTIR spectra for CO adsorbed on Pd (a) and nm-Pd/GC (b-e) electrodes. The spectral resolution is 16 (b), 8 (a, c), 4 (d), and 2 (e) cm-1 (ES ) 0.0 V, ER ) 0.7 V, 0.1 M H2SO4).

Figure 3. STM patterns of the nm-Pt/GC surface: (a) 400 × 400 nm, It ) 2 nA, Vb ) 50 mV; (b) 8 nm × 8 nm, It ) 10 nA, Vb ) 10 mV.

for the roughness of nm-Pt/GC and mechanically polished Pt surfaces. To study further the surface structure of nm-Pt/GC, a scanning tunneling microscope was employed. As shown by the STM image in Figure 3a, the surface of nm-Pt/GC consists of localized flat layers. Usually, for metal electrodeposition in the absence of additives, the crystallites

grew into three-dimensional hemispheres or islands and resulted in a rather nonuniform deposit over the substrate surface. Consequently, it produced a surface of great roughness.35,36 Since the average thickness of the deposited Pt film is a few nanometers, the rise and the fall on the microscopic scale observed in Figure 3a may be initiated by the topography of the GC substrate. The STM pattern shown in Figure 3a suggests accordingly that the topography of the nm-Pt/GC surface on the microscopic scale keeps similar morphology of a polished GC substrate. When the scanning area of STM is zoomed down to an area of a few nanometers over the flat layers, we can clearly obtain STM images at atomic resolution, as shown by the STM pattern of Figure 3b. The terraces of the nm-Pt/GC surface with a layered structure yield, in most cases, ordered an atomic arrangement of (111) or (100) symmetry. 3.1.2. CO Adsorption on the Surface of nm-Pd/GC. The in situ FTIR spectra for CO adsorption on surfaces of nm-Pd/GC and massive palladium (Pd) electrodes in 0.1 M H2SO4 saturated with CO are compared in Figure 4. To investigate the band distortions in spectra recorded on the nm-Pd/GC electrode, spectra of different spectral resolutions (2, 4, 8, and 16 cm-1) are compared in the same figure. For all spectra displayed in Figure 5 the sample and reference potentials were at 0.0 and 0.7 V, respectively. We can observe from the spectrum recorded on Pd electrode (Figure 4a) a weak negative-going band near 1962 cm-1 and a much stronger positive-going band around 2345 cm-1. These two bands can be assigned respectively to IR absorption of bridge-bonded CO (COB) at 0.0 V and IR absorption of CO2 species derived from COB oxidation at 0.7 V. The IR features for CO adsorption on a Pd electrode are similar to those observed on a Pt electrode in Figure 1a. In both cases only one adsorbed CO species is determined (COB on Pd and COL on Pt), and the COad band appears in the direction opposite to that (35) Nichols, R. J.; Beckmann, W.; Meyer, H.; Batina, N.; Kolb, D. M. J. Electroanal. Chem. 1992, 330, 381-394. (36) Pletcher, R. I. Vrbina, J. Electroanal. Chem. 1997, 421, 137144.

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Figure 5. STM pattern of the nm-Pd/GC surface (It ) 2 nA, Vb ) 50 mV). Figure 6. In situ FTIR spectra for CO adsorbed on the nmRh/GC electrode, (ER ) 0.7 V, ES indicated for each spectrum, 0.1 M H2SO4).

of the CO2 band. In contrast to IR features of CO adsorbed on a Pd electrode, CO adsorbed on a nm-Pd/GC electrode yields positive-going bands that are in the same direction of the CO2 band. We can observe from spectra of different spectral resolutions (Figure 4b-e) two positive-going COad bands, one near 1950 cm-1 due to IR absorption of bridgebonded CO and another small band around 2061 cm-1 that can be ascribed to linearly bonded CO. Furthermore, the intensity of the COB band has been enhanced by a factor of about 26 in comparison to that of the COB band in the spectrum recorded on the Pd electrode.27 It is worthy of note that the COL band cannot be determined at all from the spectrum recorded on the surface of bulk Pd under the same experimental conditions. The appearance of the COL band is assigned to the enhancement of IR absorption of a small quantity of COL species on the nmPd/GC surface. The above results demonstrated that, like nm-Pt/GC, the nm-Pd/GC also exhibited abnormal infrared effects for CO adsorption. Recently, Lipert et al. have studied theoretically the specular reflection spectroscopy of thin films on different substrates by applying classical electromagnetic theory.37 They predicted that the maximum in the z-component of the mean-square electric field at surface of glassy carbon occurs at 59° angle of incidence and that the IR bands in spectra of thin films on GC substrates will be distorted. They demonstrated also that the distortion of IR bands depends on both the angle of incidence and the thickness of the film. We observe in Figure 5 that the COB band appearing in all spectra of different spectral resolutions recorded on the nm-Pd/GC electrode is not defined symmetrically. It can be measured that, by taking the band center as reference, the COB band spreads more widely in the lower wavenumber side (∼100 cm-1) than in the higher wavenumber side (∼20 cm-1). We observe that following the increase of spectral resolution from 16 to 2 cm-1, the CO2 band becomes narrow, but no significant changes in the appearance of the COB band can be observed, which confirmed our experimental considerations concerning the relatively simple IR features presented in spectra of CO adsorption. It is noteworthy that the shapes of the COL and COB bands observed respectively in Figures 1 and 5 are similar, from the point of view of their asymmetry, to the shape of the IR band for a film on glassy carbon substrate as illustrated in ref 37 signifying the influence of the GC substrate. However,

the direction of the COL and COB bands observed in Figures 1 and 4 has been inverted, because the direction of these bands is expected to be negative-going according to the definition of eq 1, i.e., the relative change in reflectivity (∆R/R) that is different from the definition of absorbance (-log(R/R0)). The STM pattern of a nm-Pd/GC surface is shown in Figure 5. We observe that the nm-Pd/GC is composed of localized flat layers of size ranging from a few tens of nanometers to 100 nm. Although the layered structure of nm-Pd/GC is slightly different from that of nm-Pt/GC (Figure 3), the surface of nm-Pd/GC still possesses a low degree of roughness such as for the nm-Pt/GC surface. 3.1.3. CO Adsorption on the Surface of nm-Rh/GC. It is known38,39 that the adsorption of CO on a polycrystalline Rh electrode will produce both COL and COB and that the intensity of the COL band is slightly larger than that of COB band. In considering that the main CO adsorbate on nm-Pt/GC is COL and that on nm-Pd/GC is COB, the adsorption of CO on a nm-Rh/GC electrode may offer an excellent system of study for the two types of CO adsorbates coexisting on the same surface. A series of in situ FTIR spectra for CO adsorption on a nm-Rh/GC electrode at different ES and with ER at 0.7 V are displayed in Figure 6. Two well-determined positivegoing bands at around 2030 and 1900 cm-1 appear in all spectra. These two bands can be assigned to IR absorption of COL and COB species, respectively. It is interesting to see that the two COad bands appeared in the same positivegoing direction of the CO2 band. The peak value of ∆R/R for the COL and COB bands is at 8.2 × 10-3 and 6.8 × 10-3, respectively, which demonstrates significant enhancement of IR absorption of both COL and COB species. It may be convenient to compare the intensities of COL and COB bands measured from Figure 6 with those acquired for CO adsorption on a massive polycrystalline Rh electrode under similar FTIR spectroscopic conditions.39 The intensity of relative change in reflectivity, i.e., ∆R/R, of the COL band was measured at 7.0 × 10-4, and that of COB band was evaluated at 6.0 × 10-4 for CO adsorption on a Rh electrode. It has been determined that, by taking the intensity of the CO2 band in the normalization as those calculated for nm-Pt/GC and nm-Pd/GC electrodes,27,29

(37) Lipert, R. J.; Lamp, B. D.; Porter, M. D. In Modern Techniques in Applied Molecular Spectroscopy; Mirabella, F. M., Ed.; John Wiley & Sons: New York, 1999; Chapter 3, pp 83-126.

(38) Kunimatsu, K.; Lezna, R. O.; Enyo, M. J. Electroanal. Chem. 1989, 258, 115-126 (39) Lin, W.-F.; Sun, S.-G. Electrochim. Acta 1996, 41, 803-809.

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the IR absorption of both COL and COB species on the nm-Rh/GC electrode has been enhanced to about 11 times. These results demonstrated clearly that the nm-Rh/GC electrode also demonstrates the abnormal IR effects for CO adsorption. We can observe from the spectra displayed in Figure 6 that the C-O stretching frequency of both COL and COB species is shifted to higher wavenumbers when ES is increased. From the linear variation of the band center (υCO) versus ES we have obtained the electrochemical Stark shift (dυCO/dE) as 40 cm-1 V-1 for COL and 39 cm-1 V-1 for COB. These values are in good agreement with the Stark shifts of COL and COB adsorbed on a massive polycrystalline Rh electrode. 38,39 As we have described previously,27-29 similar Stark shifts have been obtained for CO adsorption on Pt and nm-Pt/GC and also for CO adsorption on Pd and nm-Pd/GC electrodes. These results imply that the AIREs of nanometer thin films of platinumgroup metals do not change the Stark shift, which represents the influence of electrode potential on the bonding of CO with electrode surface. 3.2. Abnormal IR Effects of nm-Pt/GC Surface for Adsorption of Different Molecules Other Than CO. 3.2.1. SCN- Adsorption on the Surface of nm-Pt/GC. It should be pointed out that CO adsorption is not the sole example concerning the abnormal IR effects of nanometer thin films of platinum-group metals. Adsorption of SCNon the surface of nm-Pt/GC was chosen as another test in this study. Because of the particular adsorption properties of SCN-, the in situ FTIR spectroscopic measurements were carried out with a procedure slightly different from that employed previously in the studies of CO adsorption. A reference single-beam spectrum was collected first at ER ) 0.0 V in a 0.1 M Na2SO4 solution free of NaSCN. Next, a defined quantity of NaSCN was introduced into the IR cell so as to form a solution with the concentration of SCN- at ca. 1 mM. After waiting enough time (ca. 30 min) for SCN- to diffuse into the thin layer between the electrode and the IR window and establishing an equilibrium adsorption on the nm-Pt/GC surface, a set of singlebeam spectra were recorded at the sample potentials (Eis). ES was varied progressively by an interval of 0.2 V from -0.8 to 0.4 V. The single-beam spectra collected at Eis (R(Eis)) and ER(R(ER)) were used to calculate the resulting spectra according to eq 1. In this in situ IR experimental procedure, there was no SCN- species at ER, and the surface and solution species of SCN- existed only at Eis. As a consequence, the IR absorption by adsorbed SCN- and by solution SCN- species are all expected to yield negative-going bands in the FTIR spectra in the absence of AIREs. We observed a positive-going band located near 2085 cm-1 for ES at -0.8 V; see Figure 7a. This band may be assigned to adsorbed SCN- species on nm-Pt/GC surface, since its center is shifted significantly to higher wavenumber with the increase of electrode potential. At 0.4 V, the center of this band has been shifted positively to 2133 cm-1, manifesting a Stark shift of 40 cm-1 V-1. We also observe a small negative-going band around 2063 cm-1 that may be ascribed to solution SCN- species in the thin layer, since the center of this small band is independent of electrode potential. The assignment of this negativegoing band was confirmed by adding additional NaSCN to the solution. As the concentration of SCN- increases, the negative-going band near 2063 cm-1 becomes dominant in the spectra (Figure 7b) and does not shift with the variation of electrode potential. The appearance of a positive-going band due to IR absorption of adsorbed SCN-

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Figure 7. (a) In situ FTIR spectra for SCN- adsorbed on the nm-Pt/GC electrode, (ER is chosen at 0.0 V without SCN- species, ES is indicated for each spectrum, 0.1 M Na2SO4). (b) FTIR spectra for SCN- in solution (see text for detail).

species confirmed the abnormal IR effects of nm-Pt/GC for SCN- adsorption. To illustrate further the AIREs of nm-Pt/GC for SCN- adsorption, a parallel experiment using a bulk Pt electrode under the same conditions was carried out. The results demonstrated that on the Pt surface the IR absorption of adsorbed SCN- yielded a negative-going band, the intensity of which was too small to be well determined from the spectra. From this result, it can be deduced that the intensity of the band due to adsorbed SCN- on the nm-Pt/GC surface seen in Figure 7a is significantly enhanced. It is generally agreed that SCN- ions can adsorb on Pt via either the nitrogen atom or the sulfur atom. The former species are dominant at negative potentials, while the latter species are the main adsorbates at more positive potentials.40-42 It is reported also that the N-bound SCN(40) Ashley, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1989, 270, 349-364.

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Figure 8. In situ FTIR spectra for POPD film on Pt and nmPt/GC surfaces (ES ) -0.25 V, ER ) 0.4 V, 0.1 M HClO4).

species possess a Stark shift (dυCN/dE) much larger than that of S-bound SCN- species.41,42 As stated above, the Stark shift at ca. 40 cm-1 V-1 was measured from the slope of the plot of υCN versus E. This value is in a good agreement with the results obtained from surface Raman studies42 and supports the assignment of this band to N-bound SCN- species. It is interesting to observe the broadening of the positive-going band from spectra recorded at potentials above 0.2 V in Figure 7a. A shoulder peak is growing to the high wavenumber side of the positive-going band. These observations suggest that some of the N-bound SCN- species have changed their orientation to form S-bound SCN- species when the electrode potential is increased to above 0.2 V. 3.2.2. Poly(o-phenylenediamine) on the Surface of nm-Pt/GC. Besides the simple species, CO and SCN-, we have also examined the characteristic IR features of poly(o-phenylenediamine) (POPD) film electropolymerized on surfaces of nm-Pt/GC and Pt electrodes. The sample and reference potentials were chosen at 0.4 and -0.25 V, respectively. The following equation shows the structure of POPD and the probable oxidation-reduction mechanism:43-45

The potential difference spectra are compared in Figure 8. From the spectrum of the POPD film on the Pt surface, three main bands are observed. The negative-going band around 1550 cm-1 can be assigned to CdN stretching43,44 (41) Tadjeddine, A.; Guyot-Sionnest, P. Electrochim. Acta 1991, 36, 1849-1854. (42) Tian, Z.-Q.; Ren, B.; Mao, B.-W. J. Phys. Chem. B 1997, 101, 1338-1346. (43) Chiba, K.; Ohsaka, T.; Ohnuki, Y.; Oyama, N. J. Electroanal. Chem. 1987, 219, 117-124. (44) Lin, X.-Q.; Zhang, H.-Q. Electrochim. Acta 1996, 41, 2019-2024. (45) Habib, M. A.; Maheswari, S. P. J. Electrochim. Soc. 1989, 136, 1050-1053.

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(the oxidized state II), the positive-going band near 1488 cm-1 is associated with the skeletal breathing of aromatic rings,43,44 and another positive-going band around 1308 cm-1 is ascribed to C-N stretching of the C-N-H group43-45 (the reduced state I). In the case of POPD film on a nm-Pt/GC electrode (the thickness of the POPD film is estimated to be ca. 20 nm), the spectrum recorded under the same experimental condition also displays three bands but all in opposite direction to the corresponding bands appearing in the spectrum recorded on the Pt surface. Nevertheless the bands near 1110 cm-1 due to IR absorption of solution ClO4- species in both spectra are in the same negative-going direction. These results demonstrated that the nm-Pt/GC also displays abnormal IR effects for a polymer film. It should be nevertheless pointed out that no enhancement in band intensity can be observed for any of the three bands, which may imply that the enhancement of IR absorption in the AIREs is an effect of short-range domain of surface. Since the POPD film spreads continuously over the GC substrate, the enhancement of IR absorption in the AIREs may not be present on such films of long-range domain. In considering that, the nanometer thin films of transition metals are composed of individual crystallites as illustrated by STM images, the domain is localized to a short range on this kind of film. As a consequence, the enhancement of IR absorption in the AIREs may be similar to that in the SEIRA (surface enhancement IR absorption) for which Osawa has shown50-53 that the effect of IR absorption enhancement is localized to about 5 nm of island crystallites on a substrate. 3.3. Influence of Substrate Materials on the AIREs of nm-Pt for CO Adsorption. According to our studies so far, glassy carbon served as the typical substrate for the AIREs of nanometer thin film of transition metals. To investigate the influence of substrate materials on the AIREs, we studied in our previous paper28 the influence of different substrates other than GC, such as graphite, smooth Pt, and Au for CO adsorption. On all these substrate materials the AIREs can be confirmed. It is more interesting to report in this section another material, the conducting polymers of pyrrole (PPy) and o-phenylenediamine (POPD) for the present investigations. Figure 9 shows the in situ FTIR spectra for CO adsorbed on the surface of nanometer thin films of Pt deposited on conducting polymers, PPy or POPD, which are supported on glassy carbon (denoted as nm-Pt/PPy/GC and nm-Pt/ POPD/GC). Experimental conditions were the same as those in Figure 1, i.e., the ES at 0.0 V and the ER at 0.7 V. Each spectrum in Figure 9 displays two positive-going bands at around 2070 and 2345 cm-1 corresponding to COL at 0.0 V and CO2 at 0.7 V, respectively. It is evident that the abnormal IR effects are clearly presented, since the COL band appears in the same direction as the CO2 band. Although the intensities of the COL band in both spectra displayed in Figure 9 are smaller than that (46) Hartstein, A.; Kirtly, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201-204 (47) Kamata, T.; Kato, A.; Umemura, J.; Takenaka, T. Langmuir 1987, 3, 1150 (48) Badilescu, S.; Ashrit, P. V.; Truong, V.; Badilescu, I. I. Appl. Spectrosc. 1989, 43, 549-552. (49) Nishikawa, Y.; Fujiwara, K.; Shima. T. Appl. Spectrosc. 1990, 44, 691-694. (50) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914-9919. (51) Nishikawa, Y.; Nagasawa, T.; Fujiwara, K.; Osawa, M. Vib. Spectrosc. 1993, 6, 43-53. (52) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. Anal. Chem. 1993, 65, 556-562. (53) Osawa, M.; Yoshii, K.; Ataka, K.; Yotsuyanagi, T. Langmuir 1994, 10, 640-642.

Adsorption on Electrodes of Nanometer Thin Films

Figure 9. In situ FTIR spectra for CO adsorbed on surfaces of nm-Pt/PPy/GC and nm-Pt/POPD/GC (ES ) 0.0 V, ER ) 0.7 V, 0.1 M H2SO4).

recorded on a nm-Pt/GC surface (Figure 1b), they are still about 5 times larger than that recorded on a Pt surface (Figure 1a). It is noted that the thickness of the PPy or POPD films has no noticeable influence on the features of spectra when it varies from a few nanometers to a few tens of nanometers. This results confirms once again that the enhancement of IR absorption is a localized effect, since the nanometer thin films of Pt over polymer are composed of individual crystallites. In terms of applications, a Pt/polymer system is of great interest as a potential electrocatalyst in direct fuel cells. As discussed in the introduction, in situ IR spectroscopy is particularly important to the understanding of oxidation processes of fuels on catalytic surface. However, the spectra obtained on the surfaces of real catalysts (e.g. carbon-supported catalysts)3 were of relatively poor quality, especially for the determination of adsorbed species which play the most important role in electrocatalysis. In this sense, the above observations that the nm-M/polymer/GC system exhibited high sensitivity for IR determination of surface species may provide a bright prospect to take advantage of the AIREs for fundamental studies of electrocatalysis toward fuel cell applications. 3.4. Comparison of the AIREs with the Phenomenon of Surface-Enhanced IR Absorption (SEIRA). Since enhancement of IR absorption is one of the main important characters of abnormal IR effects observed in previous sections, it is essential to compare our findings with the phenomenon of so-called surface enhanced infrared absorption (SEIRA) reported in the literature. SEIRA was reported for the first time by Hartstein et al.46 in the early 1980s. Since then, a number of groups have explored and expanded the application of this technique.46-57 However, up to now, the measurements were carried out mostly in the attenuated total reflection (ATR) mode and transmission mode. Vacuum evaporated metal island films on IR-transparent substrates were employed in all studies. p-Nitrobenzoic acid (PNBA) and other molecules of similar structure are typical compounds used in the investigations. In SEIRA, an enhancement factor of 1-3 orders of magnitude for IR absorption of (54) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664-10672. (55) Osawa, M.; Yoshii, K. Appl. Spectrosc. 1997, 51, 512-518. (56) Johnson, E.; Aroca, R. J. Phys. Chem. 1995, 99, 9325-9330. (57) Merklin, G. T.; Griffiths, P. R. Langmuir 1997, 13, 6159-6163.

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some particular vibrational modes has been reported. The adsorbate of organic molecules over the island metal surface were prepared by depositing an aliquot of solution containing the organic molecules and allowing the solvent to be evaporated. The origin of the surface enhancement has not been completely understood yet and has been discussed by a similar enhancement mechanism as that for surface-enhanced Raman spectroscopy (SERS), i.e., the electromagnetic (EM) effect and the chemical effects.51,52 As for the EM mechanism, excitation of surface plasmon polariton (SPP) caused by metal islands or rough metal surfaces is introduced for the interpretation of the SEIRA phenomenon. Although SEIRA with tin island films51 has been reported, almost all observations were conducted on island films of coinage-group metals (Cu, Ag, Au) as those employed in the SERS studies, where the significant SPP effect is involved. Obviously, for the investigation of molecules adsorbed on coinage-group metals, SEIRA is much less popular and also much less significant than its counterpart SERS, especially in the in situ electrochemical studies where infrared spectroscopy is facing the additional difficulties as described in the introduction of this paper. In fact, there are only very few in situ electrochemical SEIRA investigations on electrode/ electrolyte interfaces. Osawa and co-workers have reported SEIRA for the water molecule on gold54 and the heptyl viologen on silver55 thin films. And more recently, Sun and Osawa et al. reported the first SEIRA study of CO adsorption on gold films.26 It is evident that our discovery on the AIREs may be different from SEIRA in the following aspects: (1) For not only the enhancement of IR absorption, the AIREs reported here exhibit also other distinct character, such as the direction inversion of IR bands that was not observed in SEIRA. (2) In the AIREs studies the nanometer thin film supported on glassy carbon substrate was prepared by the convenient electrodeposition method of cyclic voltammetry, and a layered structure of the thin-film was observed. (3) The present AIREs were observed with platinum-group metals, which are of particular interest in many applications especially in electrocatalysis. It may be worthy to point out that the layered structure of nanometer thin films of transition metals on a glassy carbon substrate prepared by the electrodeposition method of cyclic voltammetry may be one of origins of the AIREs. This point has been demonstrated by the present results and also by Ortiz et al. in their recent paper32 concerning CO adsorption on thin films of iridium electrodeposited on GC by applying cyclic voltammetry. In contrast to this, the island structure of Pt thin films on a Pt substrate (i.e., the platinized platinum) prepared by applying constant potential or constant current with lead additive in electroplating solution33 yielded mainly enhanced IR absorption that is similar to SEIRA. 4. Conclusions In the present paper, nanometer thin films of platinumgroup metals (Pt, Pd, and Rh) were prepared by electrodeposition on glassy carbon and other substrates. The surface structure of the prepared thin films was observed by scanning tunneling microscopy. It was found that the nanometer thin films of platinum-group metals are composed of crystallites of layer structure and exhibit a low degree of surface roughness. The chemisorption of CO, SCN-, and poly(o-phenylenediamine) (POPD) on surfaces of the thin films was studied via in situ FTIR spectroelectrochemistry. We have systematically explored the abnormal infrared effects (AIREs) of the nanometer

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thin films for chemisorption at catalytically significant solid/liquid interfaces. The results illustrated that the AIREs consist of two important characteristics in comparison with adsorption of the same species on surface of corresponding massive metal electrode, i.e., (1) the inversion of IR band direction and (2) the IR absorption enhancement of adsorbates. It has been revealed that the abnormal IR effects depend mainly on the nature and the structure of nanometer thin films of platinum-group metals and can be observed using a series of substrate materials. The inversion of IR band direction has been observed for CO, SCN-, and POPD adsorbed on nanometer thin films of Pt supported on either GC or GC substrates covered with conducting polymers (PPy and POPD). Nevertheless the IR absorption enhancement has been confirmed only with molecules of small size and simple structure (CO and SCN-) adsorbed on nanometer thin films of transition metals, and no enhancement of IR absorption of a polymer film spreading continuously over GC was observed. These results suggest that the IR absorption enhancement in AIREs is an effect of shortrange domain of surface. It was measured that the IR absorption of CO adsorbed on nanometer thin films of Rh, Pt, and Pd has been enhanced respectively to 11, 20, and 26 times. The AIREs reported in this paper related directly to reflection of IR radiation from the electrode surface. Thus the information gained may be helpful in obtaining a better understanding of the fundamental theory of reflection spectroscopy. In the cases of CO adsorption on different surfaces of nanometer thin films (nm-Pt/GC, nm-Pd/GC, nm-Rh/GC, nm-Pt/PPy, nm-Pt/POPD, etc.) and SCNadsorption on nm-Pt/GC surface, the essential information concerning the chemisorption (the vibration mode, the bonding configuration, the Stark shift, etc.) gained from

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IR studies is the same as that acquired on surfaces of corresponding massive metal electrodes and is in good agreement with those reported in the literature. These results signify that the surfaces of nanometer thin films may be employed as alternative surfaces for spectroelectrochemical studies. However, the IR characteristics (direction and intensity of IR band) of species chemisorbed on these surfaces changed dramatically, and the sensitivity of the determination was increased significantly. Obviously, the AIREs manifest a remarkable advantage for studying surface processes and may provide a further testament to the virtues of in situ infrared spectroscopy as a powerful surface-sensitive technique. Since the AIREs have been discovered on nanometer thin films of platinumgroup metals which are good catalyst materials, the application of AIREs in determining the adsorbed intermediate species involving in electrocatalytic reactions and in revealing the detail of surface process of chemisorption will be invaluable. The results demonstrated also that our findings on the AIREs are different from those of surface-enhanced infrared absorption (SEIRA). In summary, the AIREs described in the present paper may relate to a general surface phenomenon concerning materials at the nanometer scale and initiate interests in different disciplines. Acknowledgment. This work is supported financially by the National Natural Science Foundation of China (NNSFC). K.-K.S. acknowledges the Faculty Research Grant (FRG/96-97/II-04) from Hong Kong. The assistance of Mr. Xiong-Wei Cai and Mr. Cai-Hui Shi on the STM measurements and helpful discussions with Prof. BingWei Mao are acknowledged. LA990282K