Practically Modified Attenuated Total Reflection Surface-Enhanced IR

Nov 28, 2007 - Jian-Yun Xu , Ti-Wei Chen , Wen-Jing Bao , Kang Wang , and Xing-Hua Xia. Langmuir 2012 28 (50), 17564-17570. Abstract | Full Text HTML ...
0 downloads 0 Views 166KB Size
Download PDF
Anal. Chem. 2008, 80, 166-171

Practically Modified Attenuated Total Reflection Surface-Enhanced IR Absorption Spectroscopy for High-Quality Frequency-Extended Detection of Surface Species at Electrodes Xiao-Kang Xue,† Jin-Yi Wang,† Qiao-Xia Li,† Yan-Gang Yan,† Jian-Hua Liu,*,‡ and Wen-Bin Cai*,†

Shanghai Key Laboratory for Molecular Catalysis and Innovative Materials, and Department of Chemistry, and Department of Optics Science and Engineering, Fudan University, Shanghai 200433, China

A practically modified ATR configuration has been proposed for in situ electrochemical surface-enhanced IR absorption spectroscopy (SEIRAS) by sandwiching an ultrathin water interlayer between a hemicylindrical ZnSe prism and a Si wafer as an integrated window. This new ATR optics significantly enhances the throughput of an effective IR beam across the ZnSe/gap/Si/metal film, enabling high-quality spectral fingerprints down to 700 cm-1 to be readily detected at larger incidence angles without compromising the electrochemical feasibility and stability of metallic films deposited on Si. The advantages of this modified ATR-SEIRAS have been initially applied to explore two selected systems: wide-ranged in situ ATRSEIRA spectra provided strong evidence in support of the formate intermediate pathway for methanol electrooxidation at the Pt electrode in an acid solution; in addition, new spectral fingerprints revealed comprehensive orientational information about of the p-nitrobenzoate species at Pt electrode as a result of the dissociative adsorption of p-nitrobenzoic acid molecules from an acid solution. Providing chemical and structural information on electrode surfaces at the molecular level belongs to the most challenging goals in surface electrochemistry and analytical electrochemistry. Infrared spectroscopy, with either external or internal reflection mode, is a popular surface-sensitive technique for such purposes. IR spectroscopy with external reflection can be applied to most bulk (single and polycrystalline)1,2 or nanostructure-modified bulk electrodes.3,4Nevertheless, the thin-layer structure of the spectral cell may cause problems of inhomogeneous current distribution * To whom correspondence should be addressed. Fax: +86-21-65641740. E-mail: [email protected]; [email protected]. † Shanghai Key Laboratory for Molecular Catalysis and Innovative Materials, and Department of Chemistry. ‡ Department of Optics Science and Engineering. (1) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; pp 347-389. (2) (a) Hoon-Khosla, M.; Fawcett, W. R.; Chen, A.; Lipkowski, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611-621. (b) Zawisza, I.; Bin, X. M.; Lipkowski, J. Langmuir 2007, 23, 5180-5194. (c) Bin, X. M.; Lipkowski, J. J. Phys. Chem. B 2006, 110, 26430-26441. (3) Sun, S. G. In Catalysis and Electrocatalysis at Nanoparticle Surface; Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; Marcel Dekker, Inc.: New York, 2003; pp 785-826.

166 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

and stalled mass transport and, thus, may not be good for realtime spectroelectrochemical measurement. The one with internal reflection, in particular surface-enhanced IR absorption spectroscopy with the attenuate total-reflection configuration (ATR-SEIRAS) has merits of higher surface sensitivity and unrestricted mass transport.5 Therefore, it is regarded as a powerful analytical tool for probing electrochemical adsorption and reaction,5-14 as demonstrated in identifying orientations of adsorbates6-8 and intermediates in electrocatalysis9-13 and in real-time monitoring irreversible reactions at electrodes.9-13,14a Surface-enhanced IR absorption (SEIRA) originates from the interactions of IR photons with the metal and adsorbed molecules; thin metal films consisting of nanoparticles in particular facilitate the SEIRA effect.4,5,15,16 The prerequisite for applying electrochemical ATR-SEIRAS is the appropriate fabrication of nanoparticle film electrodes on IR-transparent windows. For in situ ATR-SEIRAS application, Si is the most frequently used IR window to support the working electrodes as compared to ZnSe and Ge owing to its (4) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 19671974. (5) (a) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861-2880. (b) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, U.K., 2002; Vol. 1, pp 785-799. (6) (a) Sato, Y.; Noda, H.; Mizutani, F.; Yamakata, A.; Osawa, M. Anal. Chem. 2004, 76, 5564-5569. (b) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992-6998. (c) Noda, H.; Wan, L. J.; Osawa, M. Phys. Chem. Chem. Phys. 2001, 3, 3336-3342. (7) Ataka, K.; Heberle, J. Anal. Bioanal. Chem. 2007, 388, 47-54. (8) Futamata, M.; Luo, L. Q.; Nishihara, C. Surf. Sci. 2005, 590, 196-211. (9) Pronkin, S.; Wandlowski, T. Surf. Sci. 2004, 573, 109-127. (10) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680-3681. (11) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. J. Angew. Chem., Int. Ed. 2006, 45, 981-985. (12) Shao, M. H.; Liu, P.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128, 7408-7409. (13) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 26542659. (14) (a) Yan, Y. G.; Li, Q. X.; Huo, S. J.; Ma, M.; Cai, W. B.; Osawa, M. J. Phys. Chem. B 2005, 109, 7900-7906. (b) Wang, H. F.; Yan, Y. G.; Huo, S. J.; Cai, W. B.; Xu, Q. J.; Osawa, M. Electrochim. Acta 2007, 52, 5950-5957. (c) Huo, S. J.; Li, Q. X.; Yan, Y. G.; Chen, Y.; Cai, W. B.; Xu, Q. J.; Osawa, M. J. Phys. Chem. B 2005, 109, 15985-15991. (d) Huo, S. J.; Xue, X. K.; Yan, Y. G.; Li, Q. X.; Ma, M.; Cai, W. B.; Xu, Q. J.; Osawa, M. J. Phys. Chem. B 2006, 110, 4162-4169. (e) Huo, S. J.; Xue, X. K.; Li, Q. X.; Xu, S. F.; Cai, W. B. J. Phys. Chem. B 2006, 110, 25721-25728. (15) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201-204. (16) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58, 324A338A. 10.1021/ac7017487 CCC: $40.75

© 2008 American Chemical Society Published on Web 11/28/2007

Figure 1. Schematic diagram of the practically modified ATR-SEIRAS configuration containing an ultrathin H2O interlayer in between two solid surfaces of a ZnSe hemicylindrical prism and a Si wafer.

stability in acidic electrolytes, as well as readily available mature and economical wet processes for fabricating various metal electrodes on Si (see part I of the Supporting Information).14,17-21 However, a Si prism per se suffers from strong IR absorption at frequencies lower than 1000 cm-1, preventing the observation of important spectral fingerprints in this region, and hence compromises the structural and mechanistic elucidation at electrodes. To overcome this limitation, combining a thin Si wafer with a ZnSe prism for the ATR window has been reported.12 However, this configuration requires a critical control of overall flatness and local smoothness in order for two mechanically polished solid surfaces to be integrated into an optically efficient interface, keeping it from general use in surface and analytical electrochemistry. Notably, an apparent incidence angle of 36° was adopted, and the resultant spectral quality below 900 cm-1 was rather poor.12 In practice, depending upon the gap width and surface roughness, the apparent incidence angle in the ZnSe prism is limited to be relatively low due to the increasing reflection from the ZnSe/air interface at larger angles (total reflection in the extreme). Although smaller incidence angles benefit the transmission of the IR beam into the Si wafer, they are unfavorable to surface enhancement (vide infra). To address this issue, primarily based on the theory of prismfilm coupler for plane wave,22 we present here a practically modified ATR-SEIRAS configuration by sandwiching an ultrathin water interlayer (submicrometers thick) between a Si wafer and a ZnSe prism (Figure 1). The water interlayer, acting as an effective interface coupler, minimizes the strict requirements for surface flatness and roughness to integrate two different solids, enabling a greater portion of the IR beam to penetrate into Si at wide-ranged incidence angles and allowing repeated laboratory use of window materials. Obviously, larger incidence angles (17) Miyake, H.; Ye, S.; Osawa, M. Electrochem. Commun. 2002, 4, 973-977. (18) Shao, M. H.; Adzic, R. R. Electrochim. Acta 2005, 50, 2415-2422. (19) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500-1501. (20) Miyake, H.; Osawa, M. Chem. Lett. 2004, 33, 278-279. (21) (a) Delgado, J. M.; Orts, J. M.; Rodes, A. Electrochim. Acta 2007, 52, 46054613. (b) Delgado, J. M.; Orts, J. M.; Rodes, A. Langmuir 2005, 21, 88098816. (c) Rodes, A.; Orts, J. M.; Perez, J. M.; Feliu, J. M.; Aldaz, A. Electrochem. Commun. 2003, 5, 56-60. (22) Ulrich, R. J. Opt. Soc. Am. 1970, 60, 1337-1350.

further increase the surface sensitivity due to the enhanced surface electromagnetic fields.5,23a In addition to the greatly reduced cost of window materials, this simply modified ATR-SEIRAS enables one to detect spectral fingerprints practicably down to 700 cm-1, ensuring a reliable characterization of chemical species on electrodes supported on Si without sacrificing their electrochemical feature and stability for metal films otherwise deposited on Ge or ZnSe ATR elements. The advantages of this modified ATRSEIRAS have been applied to investigate two selected important systems, i.e., the reactive intermediate for methanol oxidation and the molecular orientation of p-nitrobenzoate (or the PNBA anion) on Pt electrode surfaces in acid solutions. We are seeing the spectral fingerprints that were never seen on electrode surfaces. EXPERIMENTAL SECTION An n-type Si (111) wafer cut in 20 mm × 25 mm (23-28 Ω cm, 200 µm thick) was polished and thoroughly cleaned. The working electrode (apparent area 1.54 cm2) used in this work is either a Au nanoparticle film chemically deposited on Si or a virtually pinhole-free Pt film further electrochemically deposited on the Au underlayer on Si following the recipes and procedures reported by Yan et al.14a and Miyake et al.17 Nevertheless, any other metal films deposited appropriately (wet or dry process) on an Si wafer can be also used as the working electrodes in the future application. A saturated calomel electrode (SCE) and a Pt mesh serve the reference and counter electrodes, respectively. The Pt film electrode was electrochemically cleaned by cycling potential from -0.22 to 1.0 V (vs SCE) in 0.1 M HClO4, until stable cyclic voltammograms were achieved. Solutions were prepared with suprapure chemicals and Milli-Q water and deaerated with high-purity Ar for all measurements at room temperature. A hemicylindrical prism ZnSe crystal was used as the primary ATR element. The metal film-coated Si wafer was pressed with its uncoated side against the water-wetted reflecting plane of the (23) (a) Beden, B.; Leger, M.; Lamy, J. C. In Modern Aspects of Electrochemistry; Bockris, J. O. M., et al., Eds.; Plenum Press: New York, 1992; Vol. 22, pp 97-264. (b) Hamnett, A. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker, Inc.: New York, 1999; pp 843. (c) Markovic, N. M.; Ross, J. P. N. Surf. Sci. Rep. 2002, 45, 117-229. (d) Iwasita, T. Electrochim. Acta 2002, 47, 3663-3674.

Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

167

Figure 2. (A) ATR-SEIRA spectra for a Pt film electrode in CO-saturated 0.1 M HClO4 at -0.2 V. Spectra 1a and 2a were recorded at the incidence angle θ )20°, and spectra 1b and 2b at θ )70° with ZnSe/air/Si (1a and 1b) and ZnSe/water/Si (2a and 2b) windows, respectively. (B) SEIRA band intensity for COL on Pt electrode as a function of θ, measured with the ZnSe /water /Si window.

ZnSe prism and served as a combined ATR element (see Figure 1). The thickness of the water interlayer may not be always even but can be controlled to be less than 1 µm by tightening four screws of a clamping apparatus against the glassware cell with the silicone circular gasket sealing the electrolyte. After 1-2-h incubation, stable light interference rings appear on the interface of Si wafer/ZnSe crystal. At this moment, liquid olefin (consisting mainly of mixed alkanes C22H46 and C28H58) was used to seal the circumambient rim between the Si wafer and the reflecting plane of the ZnSe prism. With this configuration, the water interlayer remains stable for at least 1 day. For comparison, we designate the combined ATR optics with and without the ultrathin water interlayer as ZnSe/water/Si and ZnSe/air/Si. The apparently smoothly polished Si wafer and the ZnSe prism were purchased from two companies in Shanghai, but the overall flatness across the area of 20 mm × 25 mm and the local roughness is not critically matched in general. A Varian 3100 FT-IR Excalibur Series spectrometer equipped with an MCT detector was used for ATR-SEIRAS measurements at a resolution of 4 cm-1 with unpolarized IR radiation. A total of 256 interferograms were coadded to each single-beam spectrum, except where otherwise indicated. All the spectra are shown in the absorbance unit as -log (I/I0), where I and I0 represent the intensities of the reflected radiation of the sample and reference spectra, respectively. A CHI 660B electrochemistry workstation was employed for potential/current control. RESULTS AND DISCUSSION SEIRA Signals with the ZnSe/Air/Si and ZnSe/Water/Si ATR Windows Effect of Incidence cangles. Figure 2A shows in situ SEIRA spectra for Pt film electrodes in 0.1 M HClO4 solution saturated with CO at -0.2 V with the reference spectrum taken at 0.8 V, at which CO on the Pt surface can be oxidized entirely. Spectra 1a and 2a were recorded at the apparent incidence angle (θ) of 20°, and spectra 1b and 2b at 70° without (1a and 1b) and with (2a and 2b) introducing a H2O interlayer in the combined ATR window, respectively. The 2072- and 1857-cm-1 bands correspond to linearly bonded CO (COL) and bridge-bonded CO (COB) at the Pt electrode and the 3656- and 1634-cm-1 sharp bands to ν(OH) and δ(HOH) of coexisted interfacial free H2O, respec168 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

tively.14a From Figure 2A, it can be seen that with the ZnSe/air/ Si window only at smaller θ can the COL band be detected (under our conditions, the surface signal is totally gone at θ ∼g40°), in contrast to the wide-ranged practicable θ with the ZnSe/water/ Si window. The COL band intensity as a function of θ with the ZnSe/water/Si ATR window was measured and plotted in Figure 2B. It is noted that a higher surface signal an be obtained at larger incidence angles with this configuration, similar to that found with a normal ATR Si prism.5 The modified ATR-SEIRAS can also be extended for real-time measurement with high spectral quality (see part II of the Supporting Information). For gap widths comparable to or smaller than the incidence IR wavelengths, a simplified model calculation based on “the theory of prism-film coupler for plane wave”22 reveals qualitatively that the gap (including gap width d and interlayer refractive index n2) between these two solid surfaces (here ZnSe (n3 ) 2.43) and Si (n1 ) 3.4)) as well as θ greatly influences effective optical coupling (see part III of the Supporting Information). The larger gap, the more reflection from the ZnSe/gap interface, and the water gap (n2 ) 1.33) significantly increases the transmission of the IR beam into the Si wafer as compared to the air gap (n2 ) 1.0), the factor of which increases with increasing θ. The simplified model calculation can at least partly explain that ATR-SEIRAS with the ZnSe/water/Si window can be operated at wide-ranged incidence angles. On one hand, for IR reflection-absorption spectroscopy (either external or internal mode) on metal surfaces, larger incidence angles benefit the surface signals owing to larger electric fields formed along the metal surface normal.5,24 On the other hand, a larger incidence angle causes a smaller portion of the IR beam to arrive at the metal film, i.e., less effective IR radiation (p-polarized) for detecting surface signals, especially in the case of ZnSe/air/Si window. In practice, the water interlayer may also function to “smooth” the two imperfectly smooth solid surfaces, decreasing the diffuse lights and further increasing the transmission, somewhat similar to the enhanced transmission of visible light through wetted ground glass. Application of the Modified ATR-SEIRAS To Characterize Reactive Intermediates on Electrodes. Owing to the long-term (24) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315.

Figure 3. Cyclic voltammograms for a Pt electrode in 0.1 M HClO4 (dotted line) and in 0.1 M HClO4 + 0.5 M CH3OH (dashed line) at 50 mV s-1. Potential-dependent IR band intensities for ν(COL), νs(OCO), and δ(OCO) adapted from Figure 4.

worldwide interest in a direct methanol fuel cell, the electrooxidation of methanol on Pt-based electrodes has been extensively investigated in the past decades. It was postulated that methanol can be oxidized to CO2 via a dual-path mechanism, that is, via adsorbed CO (COad) or non-CO reactive intermediates.23 The formation of the poisoning intermediate CO has been confirmed. However, diverse species including (CHxOH)ad, (COH)ad, (HCO)ad, and (COOH)ad were claimed to be the reactive intermediates.23 Recently, with in situ ATR-SEIRAS and carbon isotope tagging, Osawa’s group proposed the adsorbed formate species as the reactive intermediate with the 1320-cm-1 band assignable to νs(OCO).10 Nevertheless, they missed the other important band, i.e., the scissoring vibration δ(OCO), for convincing spectroscopic characterization of the adsorbed formate species. Figure 3 shows the cyclic voltammograms for a Pt film electrode in 0.1 M HClO4 in the absence (dotted line) and the presence of 0.5 M CH3OH (dashed line) with electrochemical features close to those observed for a polycrystalline Pt bulk electrode. The electrooxidation of CH3OH occurs at E > 0.2 V with a current peak around 0.7 V in the positive-going sweep. While in the negative-going scan, the anodic current increases from ∼0.7 V with a peak at 0.55 V and then drops to nearly zero at E < 0.2 V. Figure 4 presents series of in situ multistep SEIRA spectra for the Pt electrode in 0.1 M HClO4 + 0.5 M CH3OH, with the reference spectrum taken at 1.0 V. The band around 1230 cm-1 (marked with asterisk) may be assigned to SiOx species formed at the Si/electrolyte interface.14b The small downward band at ∼995 cm-1 increases with potential, possibly due to the adsorption of trace oxyanions from solution. At 0.0 V, two bands were clearly observed at ∼2050 and ∼1810 cm-1, attributable to COL and COB poisoning species on the Pt electrode surface, respectively, as a result of the dissociative adsorption of methanol. Accompanied with the decrease of the two CO bands, two other bands show up around 1320 and 778 cm-1, with their intensities initially increased with potential and peaked at ∼0.6-0.7 V, in accordance with the potential dependence of the anodic current observed in the

Figure 4. Multistep SEIRA spectra for a Pt electrode in 0.1 M HClO4 + 0.5 M CH3OH with the reference spectrum later taken at 1.0 V.

positive-going sweep. Furthermore, their peak positions were found to shift with potential from 1320 and 778 cm-1 (0.4 V) to 1324 and 781 cm-1 (0.7 V), indicating that the bands originate from a surface species. Of particular interest is the newly detected 778-cm-1 band. Although several groups identified the 1320-cm-1 band and assigned it to the symmetric stretch of adsorbed formate [νs(OCO)],10,11,14a,25 from the spectral viewpoint, the detection of the scissoring vibration for this intermediate species is essential for a conclusive assignment. The simultaneous response of the 1320- and 778-cm-1 bands with potential suggests that they are from the same intermediate species. Further evidence for this argument comes from the comparison with a transmission IR spectrum of sodium formate hydrate (see part IV in the Supporting Information), where three bands related to the formate species are 774 [δ(OCO), a1 mode], 1363 [νs(OCO), a1 mode], and 1605 cm-1 [νas(OCO), b1 mode], respectively. Notably, the integrated intensity of the 1320-cm-1 band is ∼4.5 times as that of the 778cm-1 band, close to the intensity ratio of the νs(OCO) band versus the δ(OCO) band in the transmission spectrum. (25) Endo, M.; Matsumoto, T.; Kubota, J.; Domen, K.; Hirose, C. J. Phys. Chem. B 2000, 104, 4916-4922.

Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

169

The above results strongly support the reactive pathway proposed by Osawa et al.10 that formate species is the reactive intermediate for the methanol oxidation at the Pt electrode in acid solution, with the 1320- and 778-cm-1 bands assignable respectively to νs(OCO) and δ(OCO). The initially strongest νas(OCO) band for bulk formate is virtually not detected, which can be explained by the surface selection rule for formate bridge-bonded to Pt surface through its two oxygen atoms. Application of the Modified ATR-SEIRAS To Characterize Molecular Orientation of Aromatic Compounds on Electrodes. Studying the molecular orientation on electrode surfaces is a long-lasting fundamental topic for surface electrochemists, and recent interest in molecular devices may call for a deep insight into the orientations of aromatic molecules on metal surfaces. Detecting out-of-plane vibrations located around 700-850 cm-1 is very important for understanding molecular orientation at electrodes.26 Characterization of adsorption geometry of an aromatic compound at electrodes with Si prism-based ATR-SEIRA spectroscopy has long been puzzled over because of the lack of outof-plane vibrations. Considering a π-electrons-delocalized aromatic adsorbate with the C2v symmetry (like the PNBA anion in the present work), its orientational configuration can be more conveniently represented by the dihedral angle R between the molecular plane and the local surface and the edge-tilted angle β of the C2 axis in its molecular plane,14d and the two parameters can be determined experimentally according to the following equations:

tan2 R )

2

tan β )

I0(a1) I(b1) I0(b1) I(a1) I0(a1) I(b2) I0(b2) I(a1)

(I)

(III)

The above equations indicate that the band intensity of outof-plane vibration, i.e., I(b1), is essential for determining the dihedral angle R and other important parameters relevant to the molecular orientation. Having its merits in mind, we extend our modified ATR-SEIRAS to characterize the orientational geometry of p-nitrobenzoate adsorbed at the Pt electrode. Figure 5 shows the ATR-SEIRA spectrum for a Pt electrode in saturated pnitrobenzoic acid (PNBA, ∼2 mM) 0.1 M HClO4 at 0.6 V with the reference spectrum taken at -0.1 V. All bands measured are inplane vibrations with the a1 modes predominant.27 The strong bands at 1357 and 1398 cm-1 correspond to the symmetric (26) Li, N. H.; Zamlynny, V.; Lipkowki, J.; Henglein, F.; Pettinger, B. J. Electroanal. Chem. 2002, 524-525, 43-53.

170 Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

Table 1. Assignments of the SEIRA Spectrum for a Pt (or Au) Electrode in 0.1 M HClO4 Saturated with PNBA at 0.6 V wavenumbers/cm-1

assignmenta

1530 (1526) 1398 (1389) 1357 (1358) 1145 (1145) 1113 (1113) 1016 (1018) 866 (865) 835 (830)

ν20(b2), νas(ONO) ν5(a1), νs(OCO) ν6(a1), νs(ONO) ν8(a1) ν9(a1) ν10(a1) ν11(a1), δ(OCO) ν12(a1), δ(ONO)

a Assignments based on ref 27. a and b modes correspond to in1 2 plane vibrations, b1 mode out-of-plane vibrations.

(II)

where I0 and I represent the band intensities measured for the bulk and surface species, respectively. In-plane a1 and b2 modes have dipole moment derivatives along the z- (given for the C2 axis) and y-axes, respectively. On the other hand, out-of-plane b1 modes have dipole moment derivatives perpendicular to the molecular plane (along the x-axis). With known R and β, the angle γ between the C2 axis of the molecule and the surface normal can be further derived with the following expression:

cos γ ) cos β sin R

Figure 5. SEIRA spectrum for a Pt electrode in 0.1 M HClO4 saturated with PNBA at 0.6 V with the reference spectrum later taken at -0.1 V.

stretching modes of nitro and carboxylate groups, that is, νs(ONO) and νs(OCO), respectively. Detailed assignment for the observed characteristic bands is given in Table 1. In particular, the bands at 835 and 866 cm-1 assignable to δ(ONO) and δ(OCO) can be readily observed. In addition, the out-of-plane vibrations at 802 and 881 cm-1 (b1 modes) were not be detected, indicating that the PNBA anion vertically binds to the Pt local surface with a bridged coordination via its carboxylate group as a result of PNBA losing its carboxyl proton upon adsorption.6c It should be noted the above bands at 800-870 cm-1 cannot be seen with a traditional Si ATR window (see part V of the Supporting Information). The absence of b1 modes in Figure 5, i.e., I(b1) ≈ 0, at least with the above experimental sensitivity, yields R ≈ 90°. Moreover, β can be obtained by comparison of the relative intensities of the νs(ONO) (a1 mode) and νas(ONO) (b2 mode) bands measured for bulk PNBA salts and adsorbed PNBA anions on the Pt electrode surface. In the former, the two bands are nearly equal, and in the later, the ratio of these two bands, i.e., I(a1)/I(b2) ≈ 15, is calculated from Figure 5, yielding β ≈ γ ≈ 14°. In a word, the technical advance enables us to reveal comprehensive and new orientational information for adlayers at metal electrodes. CONCLUSIONS ATR-SEIRAS with practicably modified optics, i.e., the ZnSe/ water/Si combined ATR window, enhances greatly the transmis(27) Ernstbrunner, E. E.; Girling, R. B.; Hester, R. E. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1540-1549.

sion of an effective IR probing beam from ZnSe to metal films deposited on Si. Electrochemical ATR-SEIRAS with this configuration enables spectral fingerprints down to 700 cm-1 to be readily detected without compromising the electrochemical feasibility and stability of metallic films. Wider-ranged ATR-SEIRA spectra containing new fingerprint bands strongly supported the adsorbed formate as the intermediate moiety in the electrooxidation of methanol, and revealed comprehensively the orientational configuration of p-nitrobenzoate species on Pt electrode surfaces in acid solutions. The successful application of this practically modified ATR-SEIRAS may trigger in-depth mechanistic and orientational studies at electrode/electrolyte interfaces.

SUPPORTING INFORMATION AVAILABLE

ACKNOWLEDGMENT We thank Prof. Osawa for partial technical help and NSFC (2067302720473025),SRFDP(20040246008)andSNPC(0652nm028) for financial support.

Received for review August 17, 2007. Accepted October 6, 2007.

The advantages of depositing metal film electrodes on Si (part I), the technical checking of the modified ATR-SEIRAS for realtime measurement (part II), the simplified model calculation on the transmission of the p-component of IR beam through the ZnSe/gap/Si (part III), the IR spectra of solid formate salts (part IV), the comparison of the modified ATR-SEIRAS with the traditional one (part V) and a few examples of simplified model calculation results (the appendix). These materials are available free of charge via the Internet at http://pubs.acs.org.

AC7017487

Analytical Chemistry, Vol. 80, No. 1, January 1, 2008

171