Anomalous IR Properties of Nanostructured Films Created by Square

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Anomalous IR Properties of Nanostructured Films Created by Square Wave Potential on an Array of Pt Microelectrodes: An In Situ Microscope FTIRS Study of CO Adsorption You-Jiang Chen, Shi-Gang Sun,* Sheng-Pei Chen, Jun-Tao Li, and Hui Gong State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China Received June 19, 2004. In Final Form: September 30, 2004 Thin films of different nanostructures on an array of nine Pt microelectrodes were prepared by applying a square wave potential treatment for different times (τ). It has been measured from the cyclic voltammetric studies that the relative surface roughness of the films was increased slightly and reached a maximal value of about 2.5. SEM studies demonstrated that with the increase of τ, the growth of island-shaped Pt crystallites on the films led to the formation of plumelike crystallites that can reach about 2-3.5 µm in length when τ exceeded 70 min. In situ microscope FTIR reflection spectroscopic studies illuminated that CO adsorbed on the array yielded different anomalous IR features. With the increase of τ, the direction of the COL band (linearly bonded CO) was transformed from the negative-going direction (normal IR adsorption) to bipolar (Fano-like spectral line shape) and finally to the positive-going direction (abnormal IR adsorption). The intensity of the COL band was enhanced significantly and a maximal enhancement factor of about 33 was measured when τ was 40 min; the center of the COL band and the Stark tuning rate also showed regular changes. This study demonstrated that specific nanostructures of Pt thin films can be prepared through a square wave potential treatment for different times and revealed the intrinsic relationship between anomalous IR properties and surface nanostructures of the thin films.

Introduction In recent years, nanomaterials have been the subject of innumerable studies in chemistry due, first, to their intriguing optical, electrical, magnetic, thermal, and chemical properties that are remarkably different from their bulk counterparts and also to their high potentials for technical applications ranging from electronic devices to catalysts to medicines.1-5 In particular, nanometer thin films play an important role in the studies of corrosion, electrocatalysis, fuel cells, and photochemistry because of their ease of preparation, high catalytic activities, stability, and low cost.6-9 Also, unique optical properties of nanometer thin films have long been recognized as being crucial to the development of chemistry at interfacial surfaces. The infrared spectroscopy of adsorbates on nanometer thin films has experienced great progress and been widely used in the in situ mode to investigate electrochemical interfaces,10-12 from which several novel IR optical * To whom correspondence should be addressed. Fax: + 86 592 2183047. E-mail: [email protected]. (1) Ashoori, R. C. Nature 1996, 379, 413. (2) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (3) Zhong, C. J.; Zheng, W. X.; Leibowitz, F. L. Electrochem. Commun. 1999, 1, 72. (4) Pileni, M. P.; Lalatonne, Y.; Ingert, D.; Lisiecki, I.; Courty, A. Faraday Discuss. 2004, 125, 251. (5) Wang, X. H.; Liu, J. F.; Pope, M. T. Dalton Trans. 2003, 5, 957. (6) Aramaki, K. Corros. Sci. 2000, 42, 2023. (7) Moss, J. A.; Leasure, R. M.; Meyer, T. J. Inorg. Chem. 2000, 39, 1052. (8) Chun, Y. G.; Kim, C. S.; Peck, D. H.; Shin, D. R. J. Power Sources 1998, 71, 174. (9) Kamat, P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem.sEur. J. 2000, 6, 3914. (10) Lin, W. F.; Sun, S. G. Electrochim. Acta 1996, 41, 803. (11) Sun, S. G. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; Chapter 6, p 243. (12) Park, S.; Tong, Y. Y.; Wieckowski, A.; Weaver, M. J. Langmuir 2002, 18, 3233.

phenomena have been observed so far. Surface-enhanced infrared absorption (SEIRA), first observed by Hartstein in 1980,13 is a phenomenon in which the IR absorption intensity of a molecule can be dramatically enhanced when the molecule is adsorbed on thin films of coinage metals with island structure.14,15 Since its discovery, the experimental and theoretical approaches of SEIRA have been advanced by many groups. Merklin et al. have observed that the roughness of the metal surface makes the selection rules of SEIRA not applicable in all cases.16 Zhang et al. reported that SEIRA highly depends on structures and vibration modes of adsorbates.17 Osawa and co-worker18 proposed an explanation that SEIRA enhancement arises from the combination of electromagnetic and chemical effects with the former playing a predominant role, in analogy to surface-enhanced Raman scattering (SERS).19 Another related IR phenomenon observed on nanoscale thin films of platinum group metals and alloys is called abnormal infrared effects (AIREs).20 Somewhat similar to SEIRA, AIREs is also characterized by enhanced IR bands of adsorbates. However, the main difference between them is that the direction of the IR bands in AIREs is inverted in contrast to that in SEIRA. Sun’s group systematically studied the AIREs of adsorbates on nanometer thin films of platinum group metals, such as Pt, Pd, Ru, and their alloys.20-24 They have pointed out that (13) Hartstein, A.; Kirley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201. (14) Sato, S.; Kamada, K.; Osawa, M. Chem. Lett. 1999, 1, 15. (15) Atsushi, M.; Shen, Y.; Osawa, M. Chem. Commun. 2002, 14, 1500. (16) Merklin, G. T.; Griffiths, P. R. J. Phys. Chem. B 1997, 101, 5810. (17) Zhang, Z. J.; Imae, T. J. Colloid Interface Sci. 2001, 233, 99. (18) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (19) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463. (20) Lu, G. Q.; Sun, S. G.; Cai, L. R.; Chen, S. P.; Tian, Z. W. Langmuir 2000, 16, 778. (21) Sun, S. G.; Cai, W. B.; Wan, L. J.; Osawa, M. J. Phys. Chem. B 1999, 103, 2460. (22) Zheng, M. S.; Sun, S. G. J. Electroanal. Chem. 2001, 500, 223.

10.1021/la048484q CCC: $27.50 © 2004 American Chemical Society Published on Web 10/16/2004

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the interparticle interaction and electron-hole damping contribute predominantly to these AIREs features.25 Bjerke et al. reported that the enhancement and distortion of IR bands can be explained by using the Bergman representation of effective dielectric functions.26 Besides SEIRA and AIREs, some asymmetrical and bipolar IR line shapes have also been observed. Zhu et al.27 proposed these asymmetrical shapes as characteristics for Fano resonance28 that usually appears when a discrete phonon excitation interacts with a continuous excitation. Priebe et al. attributed the asymmetrical lines and their enhancement to the interaction of adsorbate vibrations with surface plasma of metal islands.29 Despite these experimental successes and tentative interpretations, the nature and mechanism of these phenomena are far from completely understood. The above-mentioned studies illuminated that the enhancement and direction of IR bands of adsorbates contain rich information about the properties of the underlying thin films. Not only substrate materials but also the average thickness and roughness of metal films, as well as scale and proximity of the metal islands, are universally critical to the IR optical features.23,26,30-32 To gain a better understanding of the observed IR phenomena, it is indispensable to systematically study the intrinsic relationship between these IR optical features and the surface nanostructures of thin films. However, most studies have been based only on an individual surface structure as related to its IR spectrum, obviously lacking a systematic study up to this point, and are unable to provide enough information to illuminate the intrinsic mechanism. Fortunately, the combinatorial method was introduced to surface electrochemistry, offering an effective approach for such studies.33 The idea of the combinatorial method is based essentially on the principle of parallelism and allows searching a fairly large phase space rapidly and exhaustively.33,34 According to this idea, an individually addressable array of microelectrodes was designed recently in Sun’s group.30,35 This shows impressive advantages in that it can produce different nanostructured thin films simultaneously on an array, and the corresponding IR spectra can be acquired under the same experimental conditions, greatly facilitating the comparison and analysis of IR data on different nanostructured films. For the purpose of systematically studying the intrinsic relationship between IR optical features and surface nanostructures, nanometer thin films with different nanostructures on an array of nine Pt microelectrodes were prepared by using the repetitive square wave (23) Chen, W.; Sun, S. G.; Zhou, Z. Y.; Chen, S. P. J. Phys. Chem. B 2003, 107, 9808. (24) Sun, S. G. In Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Ed.; Marcel Dekker: New York, 2003; Chapter 21, p 785. (25) Wu, C. X.; Lin, H.; Chen, Y. J.; Li, W. X.; Sun, S. G. J. Chem. Phys. 2004, 121, 1553. (26) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967. (27) Zhu, Y.; Uchida, H.; Watanabe, M. Langmuir 1999, 15, 8757. (28) Fano, U. Phys. Rev. 1961, 124, 1866. (29) Priebe, A.; Sinther, M.; Fahsold, G.; Pucci, A. J. Chem. Phys. 2003, 119, 4887. (30) Gong, H.; Sun, S. G.; Li, J. T.; Chen, Y. J.; Chen, S. P. Electrochim. Acta 2003, 48, 2933. (31) Chen, Y. J.; Sun, S. G.; Gong, H.; Chen, S. P.; Zhou, Z. Y.; Li, J. T. Acta Phys.-Chim. Sin. 2004, 20, 129. (32) Burgi, T. Phys. Chem. Chem. Phys. 2001, 3, 2124. (33) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735. (34) Pirrung, M. C. Chem. Rev. 1997, 97, 473. (35) Gong, H.; Sun, S. G.; Chen, Y. J.; Chen, S. P. J. Phys. Chem. B 2004, 108, 11575.

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potential treatment36-38 in the present paper. The electrochemical features of the films were characterized by cyclic voltammetry. Scanning electron microscopy (SEM) was used to study the nanostructures of the thin films. Employing CO adsorption as a molecule-probe reaction, anomalous IR properties of nanostructured Pt films were investigated by using in situ microscope FTIR reflection spectroscopy. This paper illuminates the intrinsic connection between the nanostructured films prepared through repetitive square wave potential treatment and anomalous IR properties. Experimental Section Preparation of Individually Addressable Arrays of Pt Microelectrodes. Nine Pt wires of 200 µm in diameter were hosted in an order in a Teflon template that ensures the arrangement of 9 Pt microelectrodes (denoted as PtMEs thereafter) as a 3 × 3 matrix. A homemade switch box was designed to connect any individual or a group of PtMEs to potentiostat.30,35 A Pt wire and a Pd wire saturated with hydrogen (Pd|H) served as the auxiliary and the reference electrodes, respectively. All potentials presented in this paper were quoted versus the Pd|H scale. Prior to electrochemical treatment, the array of PtMEs was polished mechanically with sandpaper and alumina powder of size 5, 1, and 0.05 µm to obtain a mirror finish. After being cleaned in an ultrasonic bath, the array of PtMEs was subjected to potential cycles between 0.02 and 1.52 V at 0.10 V s-1 in 0.1 M H2SO4 solution to remove any possible impurity on the surface. Afterward, the array of PtMEs was treated by a square wave potential between 0.02 and 1.57 V at 10 Hz. The treatment time (τ) applied to each individual PtME of the array varied sequentially from 0 to 80 min with an interval of 10 min. In such a way, thin films of different nanostructure and thickness were created on PtMEs of the array. Finally, voltammetric cycling was applied to the array in a fresh 0.1 M H2SO4 solution until a reproducible cyclic voltammogram of Pt was obtained. A PtME subjected to different τ is denoted subsequently as PtME(τ). The advantage of such an array was clearly witnessed by time savings in the treatment process of PtMEs. At the start, all PtMEs except PtME(0) were subjected to the treatment of the square wave potential for 10 min. Then one of the PtMEs was disconnected with potentiostat, but the remaining PtMEs continued with the treatment for another 10 min. The sequence was repeated, and so on. Electrochemical In Situ Microscope FTIR Reflection Spectroscopic Measurements. A Nexus 870 FTIR spectrometer (Nicolet) equipped with a liquid-nitrogen-cooled MCT detector and an infrared microscope (IR-plan Advantage, SpectraTech, Inc.) were employed in these studies. A more detailed description of the apparatus and the configuration of the microscope IR cell were given in the literature.30,35,39 A series of single-beam spectra were obtained by Fourier transform processing of the coadded interferograms at each potential, and the resulting spectrum was reported as the relative change in reflectivity that was defined as

∆R R(ES) - R(ER) ) R R(ER)

(1)

where R(ES) and R(ER) represent the single-beam spectra of reflection collected at sample potential ES and reference potential ER, respectively. One hundred interferograms were collected and coadded into each single-beam spectrum, and the spectral resolution was 8 cm-1. (36) Chialvo, A. C.; Triaca, W. E.; Arvia, A. J. J. Electroanal. Chem. 1983, 146, 93. (37) Vazquez, J.; Gomez, J.; Baro, A. M.; Garcia, N.; Marcos, M. L.; Velasco, J. G.; Vara, J. M.; Arvia, A. J.; Presa, J.; Garcia, A.; Aguilar, M. J. Am. Chem. Soc. 1987, 109, 1730. (38) Visintin, A.; Canullo, J. C.; Triaca, W. E.; Arvia, A. J. J. Electroanal. Chem. 1988, 239, 67. (39) Sun, S. G.; Hong, S. J.; Chen, S. P.; Lu, G. Q.; Dai, H. P.; Xiao, X. Y. Sci. China B 1999, 42, 261.

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Figure 1. Cyclic voltammograms of the array in 0.1 M H2SO4 solution at a scan rate of 0.1 V s-1.

Figure 3. Cyclic voltammograms of CO adsorbed on the array in CO-saturated 0.1 M H2SO4 solution at a scan rate of 0.1 V s-1; τ is (a) 0, (b) 20, and (c) 70 min. Table 1. Variations of the Potential and the Current of CO Oxidation Peaks with τ τ/min 0

10

20

30

40

50

60

70

80

Ep/V 0.865 0.880 0.881 0.889 0.892 0.892 0.893 0.893 0.894 Ip/µA 1.35 2.32 2.46 2.74 2.99 3.70 4.59 4.90 5.02

Figure 2. Variation of Rr with τ. Other Conditions. The surface nanostructures of PtMEs were investigated by scanning electron microscopy (LEO l530). Electrolytes used in this study were prepared from superpure H2SO4 and Millipore water supplied by Milli-Q lab equipment (Nihon Millipore Ltd.). The solutions were deaerated by bubbling N2 before measurement. All experiments were carried out at room temperature around 20 °C.

Results and Discussion 1. Studies of Cyclic Voltammetry. Figure 1 shows the cyclic voltammograms (CVs) of PtME(τ) in 0.1 M H2SO4 solution at a scan rate of 0.1 V s-1. It can be seen easily that all curves of PtME(τ) display similar CV features regardless of the difference in τ. They all have two distinct pairs of current peaks around 0.09 and 0.25 V ascribed to the adsorption/desorption of hydrogen on PtME(τ). It can be observed from CVs that the peak current of hydrogen adsorption/desorption and the current of double layer charging in the potential range 0.35-0.55 V actually share the same increasing trend, that is, they are increased with increasing τ, and reach an almost constant value when τ > 50 min. The charge of hydrogen adsorption integrated from CV curves is often used to evaluate the relative surface roughness factor (Rr) of PtME(τ), which is defined as the ratio of the charge of PtME(τ) to that of PtME(0):

Rr )

Qτ Q0

(2)

where Qτ and Q0 represent the charge of hydrogen adsorption on PtME(τ) and PtME(0), respectively. The variation of Rr is plotted against τ in Figure 2. It can be seen that the Rr value increased nearly linearly with the increase of τ from 0 to 40 min, but the increasing trend

slows down gradually and finally approaches a constant value at about 2.5 when τ > 50 min. The change of the real surface area, that is, the electrochemical active surface area, in PtME(τ) can be estimated from the surface roughness; that is, it is increased to the maximal value at about 2.5, although they all have an identical geometric area. Figure 3 displays the comparison of three typical cyclic voltammograms for the oxidation of CO adsorbed on PtME(0), PtME(20), and PtME(70). The saturated adsorption of CO on PtME was conducted by potential cycling between 0.0 and 0.3 V in CO-saturated 0.1 M H2SO4 solution. In the case of PtME(0), the inhibition of adsorption/desorption of hydrogen by saturated CO adsorption and a sharp oxidation peak around 0.86 V ascribed to the oxidation of adsorbed CO species can be observed apparently. The CV curves for PtME(20) and PtME(70) share similar characteristics as those of the PtME(0) except the positive potential shift and current augmentation of the CO oxidation peak. The positive shift of the CO oxidation peak potential is certainly correlated to the variation of nanostructures on the array.31,35 Table 1 summarizes the variations of potential and current of CO oxidation peaks with the increase of τ from 0 to 80 min. 2. SEM Studies of Surface Structures of PtME(τ). The surface structures of the array of nine PtME(τ) with τ varying from 0 to 80 min were investigated systematically by SEM (Figure 4). Only a few irregular scratches and defects are observed on the PtME(0) surface and were caused by the mechanical polishing process; otherwise the whole surface was relatively smooth. There were no apparent crystallites on the surface of PtME(10), but the scratches and defects were wider and deeper than those on the surface of PtME(0). After 20 min of treatment, a Pt nanostructured film was formed and composed of layers of small Pt crystallites on the surface of PtME(20). On close examination, these crystallites consist of two different types of crystallites. The majority are island-shaped crystallites that are mostly less than 80 nm in dimension; the others are plumelike crystallites of 150-300 nm in length. It could be noticed that the scratches and defects are still seen on the surface of the PtME(20), even wider and deeper than those on the surface of the PtME(0). Around and inside these defects, the crystallites distribute.

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Figure 5. In situ microscope FTIR spectra for CO adsorbed on PtME(0) in CO-saturated 0.1 M H2SO4 solution. ER ) 1.0 V; ES values are indicated in the spectra.

Figure 4. SEM images of the array.

This phenomenon implies that the growth of Pt nanostructured films involves a process of platinum electrodissolution and electrodeposition during the square wave treatment. Hydrated platinum oxides were formed at 1.57 V and electroreduced to platinum crystallites at 0.02 V.38 Since the defect sites have relatively high electrochemical activity, they provide the priority locales for platinum electrodissolution and electrodeposition. This postulation may be responsible for the modification of the surface structure and also helps to explain the increase of real surface areas. With the increase of τ above 20 min, the proportion of island-shaped crystallites gradually decreased, and the plumelike crystallites grew larger and occupied the major proportion on the nanostructured film. For τ > 40 min, no apparent island-shaped crystallites but only the plumelike crystallites were found on the surface. The surface density and the lengths of the plumelike crystallites increased gradually. When τ increased to 70 min, the lengths of plumelike structure crystallites grew up to 2-3.5 µm. It is possible that the growth of island-shaped crystallites leads to the formation of plumelike crystallites through electrodissolution and electrodeposition processes in the square wave potential treatment. This result indicates that the plumelike structure is more preferable and stable in the crystallite’s growth. 3. In Situ Microscope FTIR Reflection Spectroscopic Studies of CO Adsorbed on PtME(τ). Figure 5 shows the in situ MFTIR spectra of CO adsorbed on PtME(0) at different sample potentials (ES) varying from 0.00 to 0.50 V with an interval of 0.05 V. The reference potential (ER) was set at 1.0 V where CO has been oxidized completely on the surface of PtME(0) as demonstrated directly from the CV result. A negative-going band around 2070 cm-1 at 0.0 V was assigned to linearly bonded CO species (COL). The intensity of the COL band remained almost constant in the potential range from 0.00 to 0.50 V, which meant that CO was stable in this potential range. This band center (ν˜ COL) shifts positively and almost linearly with the increase of ES, giving a slope (dν˜ COL/dES) of 30.3 cm-1 V-1 that is well-known as the electrochemical Stark

Figure 6. In situ FTIR microscope spectra for CO adsorbed on PtME(20) in CO-saturated 0.1 M H2SO4 solution. ER ) 1.0 V; ES values are indicated in the spectra.

tuning rate. This value is nearly the same as that reported in the literature.40 After a 20 min square wave potential treatment, the IR feature of COL exhibits surprisingly a bipolar band of COL (Figure 6), showing that the center of the positive-going peak (ν˜ COLv) is more positive than that of the negativegoing one (ν˜ COLV). In contrast to the IR features of COL adsorbed on PtME(0), the ν˜ COLv and ν˜ COLV are more positive and negative, respectively, than the ν˜ COL of the corresponding negative-going bands on PtME(0) at each potential. The Stark tuning rates of positive-going and negative-going peaks are calculated from the slope of ν˜ COL versus ES to be 22.8 and 17.8 cm-1 V-1, respectively; both are smaller than that of PtME(0). It is known that the bipolar IR features are the typical characteristics of the Fano-like spectral line shape.27-30 The present results indicate that the CO adsorbed on PtME(20) yields Fanolike IR features. In the case of CO adsorbed on PtME(70), the COL band in the spectra (Figure 7) exhibits a positive-going direction. (40) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R. Surf. Sci. 1985, 158, 596.

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Figure 7. In situ microscope FTIR spectra for CO adsorbed on PtME(70) in CO-saturated 0.1 M H2SO4 solution. ER ) 1.0 V; ES values are indicated in the spectra.

Its difference with respect to that of the COL adsorption on PtME(0) consists of not only the inverted direction of the COL band but also a dramatic enhancement of the COL adsorption peak. To determine quantitatively the effect of the enhancement of IR adsorption, an enhancement factor (∆IR) is defined as the ratio of the IR band intensity on PtME(τ) to that of the same amount of COL on PtME(0):30,35 τ ACO L 1 ∆IR ) 0 Rr A

(3)

Figure 8. Comparison of MFTIR spectra for CO adsorbed on the array in CO-saturated 0.1 M H2SO4 solution. ER ) 1.0 V; ES ) 0.0 V. Table 2. Variation of the IR Parameters with τ τ/min 0 ∆IR fwhm/cm-1 ν˜ COLV/cm-1 (0.0 V) ν˜ COLv/cm-1 (0.0 V) dν˜ COLV/dES/ cm-1 V-1 dν˜ COLv/dES/ cm-1 V-1

10

20

30

40

50

60

70

80

1 33.2 15 20 2070 2068 2066 2054

16.8 17

14.3 17

14.2 17

14.6 18

2084 2083 2073 2072 2071 2072 2072 2072 30.3

24.2

22.8

21.4

17.3

17.8

21.3

23.5

23.2

22.5

21.9

21.0

COL

τ 0 where ACO and ACO represent the integrated intensity L L of the COL band in the spectra of PtME(τ) and PtME(0), respectively, under the same experimental conditions; Rr is the surface roughness factor of PtME(τ). The calculation from eq 3 gives ∆IR ) 14.2 which indicates that the IR adsorption of CO has been enhanced 14.2 times on PtME(70). Besides the inversion and enhancement of the COL band, the full width at half-maximum (fwhm) of the COL band is slightly broadened to 20 cm-1, that is, about 5 cm-1 larger than that measured from the corresponding peak on PtME(0). These results demonstrate that CO adsorbed on PtME(70) yields AIREs.20 Figure 8 shows the comparison of FTIR spectra of CO adsorbed on PtME(τ) in the array at Es ) 0.00 V, which shows that the initially negative-going direction (for τ ) 0 min) changes to bipolar (for τ ) 10-30 min) and then eventually to the positive-going direction (for τ ) 40-80 min). With the increase of τ from 0 to 30 min, the negativegoing peak is broadened and its relative intensity is decreased gradually, while that of the positive-going one is increased, thus giving an asymmetric band. When τ is 20 min, the integrated intensity of the two peaks is nearly equal. When τ is longer than 40 min, the proportion of the positive-going peak becomes dominant, that is, the bipolar band is completely transformed to a positive-going monopolar band. Accompanied by the transformation processes, several IR features measured from the IR spectra also experience steady variations, which are listed in Table 2. The overall trends can be summarized as below: (1) The ∆IR increased to a maximal value of about 33 on PtME(40) and then decreased with the increase of τ, finally reaching a constant value of about 14 when τ g 60 min.

(2) The fwhm is also slightly broadened and shares a variation similar to that of ∆IR. (3) Both ν˜ COLV and ν˜ COLv of the bipolar band of COL are red-shifted. Concretely, ν˜ COLV is shifted from 2068 to 2054 cm-1 and ν˜ COLv is shifted from 2084 to 2073 cm-1 with τ from 10 to 30 min. When τ is longer than 40 min, only the positive-going peak is observed and the ν˜ COLv is almost invariable at 2072 cm-1. (4) The dν˜ COL/dES also shows the regular changes, while the dν˜ COLV/dES is decreased gradually from 30.3 to 21.4 cm-1 V-1 with τ from 0 to 30 min, dν˜ COLv/dES is increased initially then decreased with τ from 10 to 80 min, giving a maximum value 23.5 cm-1 V-1 at τ ) 40 min. It is interesting to see that the Stark tuning rates for PtME(τ) are all smaller than that for PtME(0), which may be a special property of nanostructured Pt films. This result indicates that the influence of the electric field across the electrolyte/electrode interface on the shift of the COL band center is less significant on nanostructured Pt films than on a bulk Pt electrode. The similar transformation of IR bands from a negativegoing direction (normal IR adsorption) to a positive-going direction (abnormal IR adsorption) via a transitional bipolar pattern (Fano-like spectral line shape) has been reported before. Bjerke et al.26 and Gong et al.30,35 have observed this transformation of a COL band on platinized platinum electrodes,26 on nanostructured Ru films electrodeposited on Pt microelectrodes,30 and on nanostructured Pt films prepared by a fast potential cycling treatment.35 In these studies, the different metal films all acquire island-shaped crystallites, the dimension of which increases when IR bands are inverted to the positivegoing direction. Our results about the surface nanostruc-

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tures of Pt thin films are much different from those of previous works even though we have obtained a similar IR band transformation. We observed that crystallites on Pt thin films also experience growth from an island shape to a plumelike structure concurrent with the transformation of IR bands. Besides the difference of surface nanostructures, a more intense surface enhancement was also acquired on this array. Conclusion In this paper, an individually addressable array of nine Pt microelectrodes (PtMEs) of 200 µm in diameter was designed and fabricated under the instruction of the combinatorial method. Thin films of different nanostructures were acquired by using square wave potential treatment times (τ) from 0 to 80 min with an interval of 10 min. Cyclic voltammetric studies showed that the relative surface roughness of the films increased slightly and reached a stable value of about 2.5 with the increase of τ. The positive potential shift and current augmentation of the CO oxidation peak on the array were also observed with the increase of τ. SEM images illustrated that the nanostructured films were made of layers of crystallites. When τ was 20 min, the crystallites consisted of two types of different structure crystallites. One was an islandshaped crystallite with a scale of less than 80 nm, and the other was a plumelike crystallite of 150-300 nm in length. When τ > 40 min, the plumelike structure crystallites grew larger and the island-shaped ones were hardly observed on the surface, so the growth of island-shaped crystallites may lead to the formation of plumelike structure crystallites during the square wave potential treatment process. Employing CO adsorption as a molecule-probe reaction, IR properties of the individually addressable array were investigated by using in situ

microscope FTIR reflection spectroscopy. On the PtME(0), that is, the bulk Pt microelectrode without the square wave potential treatment process, the linearly adsorbed CO (COL) yielded a normal IR absorption band appearing in a negative-going direction under the present conditions. The IR features of COL on the PtME with τ from 10 to 30 min exhibited a bipolar band of COL, which represents Fano-like spectral line shapes. When τ > 40 min, the bipolar band of COL was transformed to an enhanced positive-going band that designates the abnormal IR effects of the nanostructured Pt surfaces of PtME(τ). With the increase of τ, the ∆IR increased to a maximal value of 33 on PtME(40) and then decreased. The fwhm is also slightly broadened. Both ν˜ COLV and ν˜ COLv of the bipolar band of COL are red-shifted. The Stark tuning rate dν˜ COL/dES measured on nanostructured PtME(τ g 10) electrodes is always smaller than that determined on a bulk Pt electrode and also shows the regular changes, indicating a special property of nanostructured Pt films. This study has extended further the application of the combinatorial analysis method in electrochemistry and confirms that the transformation of the IR band is a general phenomenon and a special optical property of thin films characterized with successively and regularly changing nanostructures. It also provides new insights for understanding the relationship between anomalous IR features and surface nanostructures of thin films. Acknowledgment. This work was supported by grants from the National Natural Science Foundation of China (90206039, 20021002) and by the “973” program (2002CB211804). We are grateful to Dr. Yuanl. Chow, Professor Emeritus from Simon Fraser University, for valuable discussion. LA048484Q