Langmuir 2006, 22, 10575-10583
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Electrochemical Preparation and Structural Characterization of Co Thin Films and Their Anomalous IR Properties† Qing-Song Chen, Shi-Gang Sun,* Jia-Wei Yan, Jun-Tao Li, and Zhi-You Zhou State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed May 26, 2006. In Final Form: August 1, 2006 Nanometer scale cobalt thin films of different structures and thicknesses supported on glassy carbon were prepared by electrochemical deposition under cyclic voltammetric conditions (denoted nm-Co/GC(n)). The thickness of Co thin films was altered systematically by varying the number (n) of potential cycling within a defined potential range in electrodeposition. Electrochemical in situ scanning tunneling microscopy (STM) and ex situ scanning electron microscopy (SEM) were employed to characterize the surface structure of Co thin films. It has been illustrated that the Co thin films were uniformly composed of Co nanoparticles, whose structure and size varied with increasing n. The structure of nanoparticles inside the Co thin films underwent a transition from bearded nanoparticles to multiform nanoparticles and finally to hexagonal nanosheets, accompanying with an increase of average size. In situ FTIR reflection spectroscopic studies employing CO adsorption as probe reaction revealed that the Co thin films all exhibited anomalous IR properties; that is, along with their different nanostructures they presented abnormal IR effects, Fano-like IR effects, and surfaceenhanced IR absorption effects. CO adsorbed on Co thin films dominated by bearded nanoparticles yielded abnormal IR absorption bands; that is, the direction of the bands is inverted completely, with enhanced intensity in comparison with those of CO adsorbed on a bulk Co electrode. The enhancement of abnormal IR absorption has reached a maximal value of 26.2 on the nm-Co/GC(2) electrode. Fano-like IR features, which describe the bipolar IR bands with their positive-going peak on the low wavenumbers side, were observed in cases of CO adsorbed on Co thin films composed mainly of multiform nanoparticles, typically on the nm-Co/GC(8) electrode. IR features were finally changed into surface-enhanced IR absorption as CO adsorbed on the nm-Co/GC(30) electrode, on which the Co thin film is dominated by Co hexagonal nanosheets.
Introduction Nanostructured thin film materials attracted, in recent years, considerable attentions because of their novel properties in diverse fields such as optics,1 electronics,2 catalysis,3 magnetic data storage,4 etc. The nanostructured thin film materials have also played important roles in corrosion, electrocatalysis, fuel cells, and photochemistry thanks to their ease of preparation, high catalytic activities, and low cost.5-8 Nanostructured Co thin films exhibit also unusual physicochemical properties arising from the quantum confinement and surface effects. Thin Co layers and clusters possess pronounced magnetic characteristics such as giant magnetoresistance effect (GMR),9 superparamagnetism,10 high coercivity,11 strong anisotropy,12 and high-density recording,13 which are of significant importance in diverse technological †
Part of the Electrochemistry special issue. * To whom correspondence should be addressed. Fax: +86 592 2183047; E-mail:
[email protected]. (1) Shalaev, V. M.; Sarychev, A. K. Phys. ReV. B 1998, 57, 13265. (2) Palermo, V.; Palma, M.; Samori, P. AdV. Mater. 2006, 18, 145. (3) St, Clair, T. P.; Goodman, D. W. Top. Catal. 2000, 13, 5. (4) Krusin-Elbaum, L.; Shibauchi, T.; Argyle, B.; Gignac, L.; Weller, D. Nature 2001, 410, 444. (5) Balbyshev, V. N.; King, D. J.; Khramov, A.; Kasten, L. S.; Donley, M. S. Thin Solid Films 2004, 447, 558. (6) Chen, S. M. Electrochim. Acta 1998, 43, 3359. (7) Park, K. W.; Choi, J. H.; Sung, Y. E. J. Phys. Chem. B 2003, 107, 5851. (8) Kamat, P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem. Eur. J. 2000, 6, 3914. (9) Baibich, M. N.; Broto, J. M.; Fert, A.; Nguyen, F.; Dau, Van; and Petroff, F. Phys. ReV. Lett. 1988, 61, 2472. (10) Watanabe, N.; Morais, J.; Accione, S. B. B.; Morrone, A.; Schmidt, J. E.; Alves, M. C. M. J. Phys. Chem. B 2004, 108, 4013. (11) Wang, Y. C.; Ding, J.; Yi, J. B.; Liu, B. H.; Yu, T.; Shen, Z. X. Appl. Phys. Lett. 2004, 84, 2596. (12) Madurga, V.; Vergara, J.; Favieres, C. J. Magn. Magn. Mater. 2004, 272, 1681. (13) Osaka, T.; Asahi, T.; Kawaji, J.; Yokoshima, T. Electrochim. Acta 2005, 50, 4576.
applications such as electronic, magnetic, electrical, mechanical, petrochemical, and medical devices. In 1996, Sun and co-workers14 discovered first that nanostructured platinum thin films exhibited abnormal IR effects (AIREs);15 that is, CO adsorbed on electrodes of these materials gave rise to three abnormal IR characteristics in comparison with normal IR spectra for CO adsorbed on a bulk platinum electrodes: (1) the direction of COad bands is completely inverted (abnormal IR absorption), (2) the IR absorption of COad is significantly enhanced (enhanced IR absorption), and (3) the full width at half-maximum (fwhm) of IR bands is considerably increased (inhomogeneous broadening). Further investigations15-29 (14) 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., 1996, Proceedings PV 96, 136. (15) Lu, G. Q.; Sun, S. G.; Cai, L. R.; Chen, S. P.; Tian, Z. W.; Shiu, K. K. Langmuir 2000, 16, 778. (16) Lu, G. Q.; Sun, S. G.; Chen, S. P.; Cai, L. R. J. Electroanal. Chem. 1997, 421, 19. (17) Lu, G. Q.; Sun, S. G.; Chen, S. P.; Cai, L. R.; Tian, Z. W. Chem. J. Chin. UniVersity 1997, 18, 1491. (18) Cai, L. R.; Sun, S. G.; Xia, S. Q.; Chen, F.; Zheng, M. S.; Chen, S. P.; Lu, G. Q. Acta Phys.-Chim. Sin. 1999, 15, 1023. (19) Lu, G. Q.; Cai, L. R.; Sun, S. G.; He, J. X. Chin. Sci. Bull. 1999, 44, 1470. (20) Zheng, M. S.; Sun, S. G. J. Electroanal. Chem. 2001, 500, 223. (21) Zheng, M. S.; Sun, S. G.; Chen, S. P. J. Appl. Electrochem. 2001, 31, 749. (22) Chen, Z.; Sun, S. G.; Zhou, Z. Y.; Ding, N. Chin. Sci. Bull. 2001, 46, 1439. (23) Lin, W. G.; Sun, S. G.; Zhou, Z. Y.; Chen, S. P.; Wang, H. C. J. Phys. Chem. B 2002, 106, 11778. (24) Gong, H.; Sun, S. G.; Li, J. T.; Chen, Y. J.; Chen, S. P. Electrochim Acta 2003, 48, 2933. (25) Chen, W.; Sun, S. G.; Zhou, Z. Y.; Chen, S. P. J. Phys. Chem. B 2003, 107, 9808. (26) Chen, Y. J.; Sun, S. G.; Chen, S. P.; Li, J. T.; Gong, H. Langmuir 2004, 20, 9920. (27) Wang, H. C.; Sun, S. G.; Yan, J. W.; Yang, H. Z.; Zhou, Z. Y. J. Phys. Chem. B 2005, 109, 4309.
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revealed that CO or other species (e.g., SCN-, poly-o-phenylenediamine (POPD), etc.) adsorbed on electrodes of nanostructured thin films of different platinum-group metals (Pd, Ru, and Rh) and their alloys (PtPd and PtRu) as well as on nanostructure Ni films, a member of iron-triad group, also yield anomalous IR properties including the AIREs. Similar anomalous IR features on nanometer-scale thin films of Ir,30 Os,31 and platinized platinum32 were also reported. It has also been found that the IR features significantly depend on the size, structure, and agglomeration states of nanoparticles.24-27 Wang et al.27 have prepared Ni thin films under cyclic voltammetric conditions with varying potential cycling numbers, and obtained three types of nanostructures including layer, island, and lumpish arris. They observed interestingly that the IR bands of CO adsorbed on these nanostructures of Ni thin films exhibited abnormal IR features, Fano-like IR features, and enhanced IR features, respectively. Since the AIREs together with other enhanced optical phenomena such as surface-enhanced Raman scattering (SERS),33 surfaceenhanced infrared absorption (SEIRA),34 surface-enhanced second harmonic generation (SESHG),35 and surface-enhanced sum frequency generation (SESFG)36 are all closely related to nanostructure of materials, the study of anomalous IR properties is of great importance to understand the fundamentals of nanomaterials and to develop their applications in fields concerning electrocatalysis, surface analysis, optical senor technology, etc. It is known that the species of CO chemisorbed on bulk Co in alkaline solutions are mainly linear-bonded CO (COL, 19901956 cm-1) with a small portion of multi-bonded CO (COM, 1810 cm-1).37 IR properties of CO chemisorbed on nanometer scale Co thin films have not been reported yet. In this article, nanometer scale thin films of Co supported on glassy carbon were prepared by electrochemical deposition. In situ STM and ex situ SEM were employed to monitor the growth and to inspect the structure of Co thin films. Employing CO adsorption (COad) as probe reaction in studies of in situ FTIR spectroscopy, anomalous IR properties of nanometer scale Co thin films were revealed for the first time. Experimental Section Preparation of Nanostructured Co Thin Films. Glassy carbon substrate (GC, 6 mm in diameter, geometric area: 0.28 cm2) was sealed into a Teflon holder and polished mechanically using successively sand paper (6#) and alumina powder of size 5, 1, 0.3, and 0.05 µm before metal deposition. A polycrystalline Co rod of 5 mm in diameter (99.95%, Alfa) was polished mechanically following the same procedure and has served as a reference surface of bulk Co electrode in the study. The Co thin films were deposited electrochemically onto GC substrate under cyclic voltammetric conditions in 0.02 mol‚L-1 CoSO4 + 0.1 mol‚L-1 Na2SO4 solution. The lower and upper potential limits in potential cycling were respectively -1.05 and -0.675 V (vs SCE), and the sweep rate was 50 mV‚s-1. A series of Co thin films with different structure and (28) Wang, H. C.; Zhou, Z. Y.; Tang, W.; Yan, J. W.; Sun, S. G. Chin. Sci. Bull. 2004, 49, 442. (29) Li, J. T.; Chen, Y. J.; Su, Z. F.; Zhou, Z. Y.; Chen, S. P.; Sun. S. G. Chem. J. Chin. UniVersity 2005, 26, 710. (30) Ortiz, R.; Cuesta, A.; Marquez, O. P.; Marguez, J.; Meadez, J. A.; Gutierrez, C. J. Electroanal. Chem. 1999, 465, 234. (31) Orazco, G.; Gutirrez, C. J. Electroanal. Chem. 1999, 484, 64. (32) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967. (33) Maxwell, D. J.; Emory, S. R.; Nie, S. M. Chem. Mater. 2001, 13, 1082 (34) Osawa, M. In Near-Field Optics and Surface Plasmon Polaritons; Kawata, S., Ed.; Springer-Verlag: Berlin, Germany, 2001; Vol. 81, p 163. (35) Leskova, T. A.; Leyva-Lucero, M.; Mendez, E. R.; Maradudin, A. A.; Novikov, I. V Opt. Commun. 2000, 183, 529. (36) Baldelli, S.; Epple, A. S.; Anderson, E. J. Chem. Phys. 2000, 113, 5432. (37) Cuesta, A.; Gutierrez, C. Langmuir 1998, 14, 3390.
Chen et al. thickness were prepared by varying the number (n) of potential cycles and denoted as nm-Co/GC(n). An EG&G potentiostat/ galvanostat (model 263A) was used in electrochemical studies. A commercial saturated calomel electrode (SCE) and a platinized platinum foil were served as reference electrode and counter electrode, respectively. All potentials reported in this paper are referred to the SCE scale. Solutions were prepared by using Millipore water (18 MΩ‚cm) provided by a Milli-Labo apparatus (Nihon Millipore Ltd.), analytical grade Na2SO4 and super pure NaOH. All solutions were deaerated by bubbling high-purity N2 before measurements. Experiments were carried out at room temperature around 22 °C. Electrochemical in Situ STM and ex Situ SEM Measurements. Both electrochemical in situ STM and ex situ SEM were used to investigate the structure of Co thin films. The advantage of using an electrochemical in situ STM consists of monitoring the growth of Co thin film. A Nanoscope IIIa (Digital Instruments) operated under the constant current mode was employed in the in situ STM measurements. Tungsten tips were electrochemically etched in aqueous NaOH and coated with polymethylstyrene to reduce the surface area exposed to solution. A Pt wire and a freshly made Ag/AgCl wire served in the STM cell as counter and reference electrodes, respectively. To facilitate the discussion, electrode potentials reported thereafter were converted into the SCE scale. The nm-Co/GC(n) electrodes prepared in the STM cell were done using the same parameters of cyclic voltammetry (CV) as those made in a conventional electrochemical cell. In situ STM images were acquired in 0.02 mol‚L-1 CoSO4 + 0.1 mol‚L-1 Na2SO4 solution at -0.6 V. The surface structures of freshly prepared nm-Co/GC(n) electrodes were also investigated by LEO l530 scanning electron microscope (SEM) that operated at 20 kV. Electrochemical in Situ FTIR Spectroscopy Measurements. In situ FTIR spectra were recorded on a Nexus 870 spectrometer (Nicolet) equipped with an EverGlo IR source. A CaF2 disk was used as the IR cell window, and the nm-Co/GC(n) electrode was pushed against the IR window to create a thin layer of a few micrometers in thickness for in situ FTIR measurements. The incident IR beam, which was aligned at about 60° to the normal of electrode surface, traverses the CaF2 window and the thin layer solution and irradiates on the surface of nm-Co/GC(n) electrode. The reflection IR beam is totally detected by a liquid nitrogen cooled MCT-A detector. In situ FTIR spectra were collected using both MSFTIRS (multistep FTIR spectroscopy)38 and SNIFTIRS (subtractively normalized interfacial FTIR spectroscopy)39 procedures. The resulting spectra were reported as the relative change in reflectivity and calculated by eq 1 ∆R R(ES) - R(ER) ) R R(ER)
(1)
where R(ES) and R(ER) are single-beam spectra of the reflection collected at sample potential ES and reference potential ER, respectively. Each single-beam spectrum was co-added with 400 interferograms at a spectral resolution of 8 cm-1.
Results and Discussion 1. Preparation of nm-Co/GC(n) Electrodes. Cyclic voltammograms of Co electrodeposition on GC in 0.02 mol‚L-1 CoSO4 + 0.1 mol‚L-1 Na2SO4 solution are displayed in Figure 1. We observe from the first cycle of the voltammograms a sharply rising cathodic current, as the electrode potential is scanned in the negative direction to -0.88 V. In the second cycle, the cathodic current peak is significantly decreased, and the Co has begun to deposit at the upper limit potential (-0.675 V) that is much higher than that in the first cycle. This result suggests that a significant overpotential is required for the formation of Co nuclei on the GC electrode. With the increase of the potential cycle (n), (38) Lin, W. F.; Sun, S. G. Electrochim. Acta 1996, 41, 803. (39) Pons, S.; Davison, T.; Bewick, A. J. Electroanal. Chem. 1984, 160, 63.
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Figure 1. Cyclic voltammograms of electrodeposition of Co onto GC electrode, 0.02 mol‚L-1CoSO4 + 0‚1 mol‚L-1 Na2SO4, scan rate 50 mV‚s-1.
Figure 2. Relationship between the applied potential cycles (n) in electrodeposition and the estimated Co thin films thickness (de).
the cathodic current peak decreases gradually and tends to be constant after n g 4. Co deposited on GC under different conditions has also been reported in the literature.40,41 Co thin films prepared in the current study can be considered as a kind of 2D nanomaterial, and their thicknesses (de) are hence estimated by using the following equation (eq 2):23
de )
QdN0VCo 2FAGC74.05%
(2)
where Qd is electric charge consumed in electrodeposition of Co onto GC (i.e., Co2+ + 2e- f Co) and has been measured through integration of j-E curves recorded in electrodeposition. F is the Faraday constant, N0 is Avogadro’s number, VCo is the volume of one Co atom (VCo ) 4/3πrCo3), and AGC is the geometric area of the GC substrate. The estimation of de using eq 2 is based on the assumption that the efficiency of electrodeposition of Co is 100% and that Co atoms are hexagonally closest packed to yield space occupancy of 74.05%. The relationship between de and n is plotted in Figure 2. We can see that de increases linearly with increasing n. It is worthwhile to mention that, though the efficiency of electrodeposition of Co could not reach 100%, the real thickness of electrodeposited Co thin films might be much larger than de, (40) Gomez, E.; Marin, M.; Sanz, F.; Valles, E. J. Electroanal. Chem. 1997, 422, 139. (41) Floate, S.; Hyde, M.; Compton, R. G. J. Electroanal. Chem. 2002, 523, 49.
because the Co atoms electrodeposited onto GC substrates could not be perfectly closest packed in a hexagonal way in long range as shown in SEM images (see Figure 4). In the present study, both n and de can be used as relative parameters that are in direct proportion to the thickness of nanostructured Co thin films. 2. In Situ STM and ex Situ SEM Studies. It is well-known that the nanostructure of Co thin films prepared under electrochemical conditions can be controlled by solution composition, electrochemical parameters, temperature, etc.42 In the current study, the nanostructure of Co thin films on GC is controllable, which is achieved by varying the number (n) of potential cycling in electrodeposition. Both in situ STM and ex situ SEM studies were employed to investigate the surface structure of nm-Co/ GC(n) electrodes, whereas the monitoring of the growth of Co thin films was conducted by in situ STM. In situ STM images of nm-Co/GC(n) were scanned at -0.6 V immediately after electrodeposition of the Co thin films. Figure 3 shows three typical in situ STM images together with their cross sections and corresponding 3D illustrations. In the case of the nm-Co/GC(1) electrode, the surface is composed of irregular nanoparticles with a mean size of ca. 80-110 nm in length and ca. 7 nm in height. When n is increased to 4 (i.e., the nm-Co/ GC(4) electrode), the surface is formed of multiform nanoparticles with more obvious grain boundaries in comparison with that of the nm-Co/GC(1) electrode. The average dimension of nanoparticles is ca. 110-130 nm in length and ca. 8 nm in height. Finally, the nanoparticles formed in the nm-Co/GC(30) are more regular and clear in boundary with a hexagonal shape. The nanoparticles are measured ca. 170-200 nm in length and ca. 30 nm in height. The roughnesses of root-mean-square average (Rq),43 which represents the standard deviation of the z value in the measured area determined from in situ STM images, are 3.1, 5.9 and 8.8 nm, respectively, on these nm-Co/GC(1, 4, and 30) electrodes. The in situ STM results demonstrated clearly that along with n increasing the Co thin films have undergone a change in their structures, not only the increase of film thickness and size of nanoparticles but also the shape of nanoparticles inside the films. The structural change of Co thin films can be also visualized clearly by ex situ SEM studies. Freshly prepared Co thin films were dipped into ethanol to avoid exposing to the air before SEM measurement. Typical SEM patterns of nm-Co/GC(n) are shown in Figure 4, which confirms evidently that the structure of Co thin films depends on n. The prepared Co thin films contain Co nanoparticles with random size distribution when n is small. The nanoparticles can be clearly differentiated into three types: bearded nanoparticles (n ) 1 and 2), multiform nanoparticles (n ) 4, 8, and 16), and hexagonal nanosheets (n ) 30). We observe obviously that the Co thin films are mostly composed of bearded nanoparticles with a mean dimension ranging from ca. 110 to 190 nm as n < 4, and a few large grains ranging from 400 up to 500 nm aggregated by Co crystallites can be also found in these electrodes. When n is increased from 4 to 16, the surfaces of nm-Co/GC(n) electrodes are mostly composed of multiform Co nanoparticles with an average size ranging from ca. 200 to 220 nm, and a few hexagonal nanosheets appear on these Co thin films. Finally, as n increasing up to 30, the surface of the nmCo/GC(n) electrode is dominated by smooth hexagonal nanosheets with relatively regular distribution in their dimension (i.e., ca. 60-150 nm in height and ca. 150-350 nm in length of the side of hexagonal nanosheets). (42) Hoshino, K.; Hitsuoka, Y. Electrochem. Comm. 2005, 7, 821. (43) Gong, H.; Sun, S. G.; Chen, Y. J.; Chen, S. P. J. Phys. Chem. B 2004, 108, 11583
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Figure 3. (a) In situ STM images of nm-Co/GC (n ) 1, 4, and 30), recorded immediately after electrodeposition; (b) Cross section of line A-A marked in (a); (c) 3D structure of (a). Scan size: 500 × 500 nm; Solution: 0.02 mol‚L-1 CoSO4 + 0.1 mol‚L-1 Na2SO4; Eb ) -0.21 V.
Figure 4. SEM patterns of nm-Co/GC(n) electrodes.
It is worthwhile pointing out that similar results are demonstrated in both in situ STM and ex situ SEM studies, though it exists deviation in measuring the average size of Co nanoparticles from in situ STM images and corresponding ex situ SEM patterns, which may be caused under different electrodeposition circumstances (i.e., in different cells). We demonstrated clearly that, along with increasing n, the average size of Co nanoparticles and the thickness of thin films increase gradually, and the shape of Co nanoparticles undergoes a change from rough, irregular nanoparticles to hexagonal nanosheets with a relatively smooth surface. The in situ STM and ex situ SEM results suggest that the electrodeposition of Co is followed by three-dimensional growth on glassy carbon.41 Although the shape of the deposited
Co nanoparticles is not well defined in early stages of electrodeposition, the hexagonal nanosheets are the most preferable and stable nanoparticles formed under present cyclic voltammetric conditions. 3. Cyclic Voltammetry Studies. Before recording any cyclic voltammogram (CV), the nm-Co/GC(n) electrode was subjected to potential cycling between -1.1 and -0.9 V for 10 min in a N2 saturation solution, to reduce electrochemically possible oxides formed on the nm-Co/GC(n) electrode surface. We focus initially on electrochemical properties of nm-Co/GC(n) in 0.1 mol‚L-1 NaOH aqueous solution. The cyclic voltammetric curves recorded in the first cycle on nm-Co/GC(n) electrodes with n ) 1, 2, 4, 8, 16, and 30 are displayed in Figure 5a. A broad anodic peak
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Langmuir, Vol. 22, No. 25, 2006 10579 Table 2. Variations of CV Parameters of CO Adsorbed in Saturation on nm-Co/GC(n) Electrodes
Figure 5. Cyclic voltammograms of nm-Co/GC(n) electrodes with n ) 1, 2, 4, 8, 16, and 30, in 0.1 mol‚L-1 NaOH. (a) Native nmCo/GC(n) alone. (b) CO adsorbed on nm-Co/GC(n) in saturation, sweep rate 50 mV‚s-1. Table 1. Variations of CV Parameters of nm-Co/GC(n) Electrodes n
EP/V (SCE)
jp/mA‚cm-2
fwhm/V
QCo/mC‚cm-2
1 2 4 8 16 30
-0.53 -0.52 -0.54 -0.57 -0.52 -0.50
0.55 0.77 1.69 2.14 3.18 3.28
0.09 0.09 0.29 0.25 0.30 0.28
1.85 2.83 10.83 13.19 21.53 21.38
appears around -0.52 V in the positive-going potential scan (PGPS) due to Co passivation, which yields an oxidation electric charge corresponding to electrooxidation of Co into Co(II).37 The intensity of peak current (jp), the peak potential (Ep), the oxidation electric charge (QCo), and the full width at halfmaximum (fwhm) of the current peak corresponding to the passivation of nm-Co/GC (n) electrodes are listed in Table 1. It is apparent that the Ep remains almost constant around -0.52 V as n increases. However, the jp, fwhm, and QCo all increase significantly as n augments till 4 and then increase gradually or slightly when n is increased above 4. This result may be attributed to the increasing amount of Co deposits on GC and the transition of surface structure of nm-Co/GC(n) electrodes, which have been discussed in previous sections. The cathodic current corresponding to electroreduction of Co(II) to Co in the negative-gonging potential scan (NGPS) is relatively small, due to the passivation of Co occurring in the PGPS. Our attention next focuses on CV studies of CO adsorbed in saturation on freshly prepared nm-Co/GC(n) electrode in 0.1mol‚L-1 NaOH. The cyclic voltammograms shown in Figure
n
EP/V (SCE)
jp/mA‚cm-2
fwhm/V
QCO-Co/mC‚cm-2
1 2 4 8 16 30
-0.45 -0.45 -0.47 -0.45 -0.46 -0.45
0.98 1.84 2.44 3.36 4.17 4.70
0.08 0.10 0.08 0.12 0.12 0.15
2.30 5.21 5.89 9.72 13.04 16.60
5b were recorded after the following procedures: (1) CO was bubbled through the solution for 10 min as potential cycling between -1.1 and -0.9 V to ensure the saturation adsorption of CO. (2) CO dissolved in the solution was removed completely by purging N2; thus, only adsorbed CO is subjected to CV investigations. In comparison with Figure 5a, the small cathodic current peak around -1.0 V, which corresponds to the electroreduction of Co(II) to Co, and the increased jp observed in Figure 5b suggest that the anodic current peak in the PGPS may be attributed to co-oxidation of CO and Co on nm-Co/GC(n) electrode. The key features of the cyclic voltammograms for CO adsorbed on nm-Co/GC(n) in saturation are listed in Table 2. The Ep remains almost constant at -0.45 V as n increases. However, the jp, and QCO-Co all increase sharply when n is increased from 1 to 4, and then increase slowly for n > 4. As compared with the data listed in Table 1, it is interesting to see that the Ep has shifted positively from -0.52 V to -0.45 V, the fwhm has significantly decreased and the QCO-Co becomes even much smaller than QCo measured on corresponding nm-Co/GC(n) electrodes without CO adsorption when n>4. All these results undoubtedly demonstrate that the chemisorbed CO has inhibited the electrooxidation of Co on nm-Co/GC(n) electrodes. 4. In Situ FTIR Spectroscopic Studies. 4.1 Results of MSFTIRS. The adsorption of CO was employed as a molecule probe reaction in the MSFTIRS experiment. A series of single beam spectra (R(ES)) were collected at different sample potentials (ES) where COad is stable on nm-Co/GC(n) electrodes. The singlebeam spectrum of the reference potential (R(ER)) was taken after COad was removed completely by electrooxidation. According to CV results, the nm-Co/GC(n) electrode had been polarized at -0.5 V for 15 s prior to collecting R(ER) in order to ensure the completion of COad oxidation into CO32-. In the case of the bulk Co electrode, R(ER) was collected at -0.95 V after a polarization at -0.5 V for 15 s to avoid significant decrease in surface reflectivity caused by dissolution of Co in the solution. Under these conditions, negative-going COad bands and positive-gonging CO32- bands would be expected to appear in the resulting spectra according to eq 1. Figure 6 shows the comparison of in situ MSFTIR spectra of CO adsorbed in saturation on bulk Co and nm-Co/GC(2) electrodes. In the spectrum of the bulk Co electrode, we do observe clearly a negative-going band near 1970 cm-1, which shifts linearly to higher wavenumbers with increasing ES, yielding a Stark tuning rate of 72.5 cm-1 V-1. This band is assigned to IR absorption of linearly bonded CO (COL) to bulk Co electrode surface. The positive-going bands near 1400 and 1650 cm-1 in this spectrum are ascribed respectively to IR absorption of CO32and H2O species, which are derived from electrooxidation of COad at ER in NaOH aqueous solutions (CO + 4OH- f CO32+ 2H2O + 2e-). Since the CO32- species under the present experimental conditions were formed solely from the oxidation of COad at ER, similar to CO2 in acid solution mentioned in the literature, 15 the integral intensity of the CO32- band (ICO32-) may be taken as a measure of the quantity of COad. In the case of the nm-Co/GC(2) electrode, however, it is interesting to observe two positive-going peaks near 1980 and
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Figure 6. Comparison of in situ MSFTIR spectra of CO adsorbed in saturation on nm-Co/GC(2) and bulk Co electrodes, ES ) -0.9 ∼ -0.8 V, ER ) -0.5 V, 0.1 mol‚L-1 NaOH solution.
1820 cm-1, which both shift to higher wavenumbers with increasing ES, yielding Stark tuning rates of 77.5 and 69.5 cm-1 V-1, respectively. These two IR bands centered at around 1980 and 1820 cm-1 may be assigned respectively to IR absorption of linearly bonded CO (COL) and multibonded CO (COM) to the surface of the nm-Co/GC(2) electrode. In comparison to the case of the bulk Co electrode, the direction of the COL band is obviously inverted. Moreover, the appearance of the COM band and the weaker CO32- band suggest that the IR absorption of COad on nm-Co/GC(2) electrodes has been significantly enhanced. The results illustrated that the nanostructured Co thin film exhibits abnormal infrared effects (AIREs). Such features will be described in detail in the following paragraphs of this paper. Figure 7 compares in situ MSFTIR spectra of CO adsorbed on nm-Co/GC(n ) 1, 2, 4, 8, 16 , and 30) and bulk Co electrodes for ES ) -0.9 V. For clarity, the spectrum of bulk Co has been multiplied by a factor of 2 in ∆R/R. It is interesting to observe three types of anomalous IR features for CO adsorbed on nm-Co/GC(n) as illustrated in Figure 7 and described below. (1) Adsorption of CO on nm-Co/GC(n ) 1 and 2) electrodes yields abnormal IR absorption; that is, the direction of IR bands (COL near 1980 cm-1 and COM around 1810 cm-1) is completely inverted in comparison with those presented in the spectrum of bulk Co electrode. This is the most prominent character of the AIREs distinguishing, from the SEIRA. We can also see that the intensity of the positive-going COad bands has been significantly enhanced in both electrodes, though the COad bands are much weaker on nm-Co/GC(1) than those on nm-Co/GC(2) due to the low coverage of Co nanoparticles on GC, as can be seen from the SEM images (Figure 4). The fact that the appearance of the COM band near 1810 cm-1 in spectra recorded on nm-Co/GC(n) also confirms the enhancement of IR absorption. As mentioned previously, the integral intensity of the CO32- band (ICO32-) can be used as a measure of the quantity of COad; thus, the enhancement factor (∆IR) defined by the ratio of the IR band intensity of the same amount of CO adsorbed on a nm-Co/GC(n) electrode to that adsorbed on a bulk Co electrode can be calculated by eq 3.20
∆IR )
(ICO/ICO32-)nm-Co/GC (ICO/ICO32-)Bulk-Co
(3)
Figure 7. In situ MSFTIR spectra of CO adsorbed in saturation on nm-Co/GC(n) and bulk Co electrodes. ES ) -0.9 V, ER ) -0.5 V, 0.1 mol‚L-1 NaOH solution.
where ICO represents the sum of the integral intensity of the COL and COM bands and ICO32- is the integral intensity of the CO32band measured from spectra of nm-Co/GC(n) and bulk Co electrodes, respectively. ∆IR has been determined to be 18.9 and 26.2 for n ) 1 and 2, respectively. Furthermore, the values of fwhm of the COad bands in spectra of nm-Co/GC(n ) 1 and 2) electrodes are broadened. The values of fwhm of the COL band are measured on nm-Co/GC(n ) 1, 2) electrodes ca. 39 and 43 cm-1, which are broader than that measured on the bulk Co electrode (35 cm-1) by 4 and 8 cm-1, respectively. The above results demonstrated clearly that the Co thin films composed of bearded Co nanoparticles [i.e., nm-Co/GC(n ) 1 and 2) electrodes (see Figure 4)] exhibit AIREs. Similar results have been observed in our previous studies on nanostructured metal thin films (Pt,14 Ru,20 Rh,23 and Ni28) and their alloys (PtRu21 and PtPd22) prepared under cyclic voltammetric conditions. (2) For the case of nm-Co/GC(n ) 4, 8, and 16) electrodes, the IR absorption of COL and COM appears as bipolar bands, showing that the negative peak lies in high wavenumbers and the positive peak locates in low wavenumbers. This IR feature is obviously different from both normal absorption on the bulk Co electrode and the AIREs on nm-Co/GC(n ) 1 and 2) electrodes. We can observe that both the positive and negative peaks of the COad bipolar bands are blue-shifted with increasing ES. To illustrate clearly the potential dependence of the COad bands, Figure 8a displays typical in situ MSFTIR spectra of CO adsorbed in saturation on the nm-Co/GC(8) electrode collected at different ES. We observe clearly that either the positive peak (V˜ COadv) or the negative peak (V˜ COadV) of the bipolar COad bands shifts positively and linearly with increasing ES due to stark effect.55 The stark tuning rate (i.e., dV˜ COad/dE) can be then determined from the plot of the variation of V˜ COad with ES shown in Figure 8b. The values of Stark tuning rate were measured as 83.7 and 40.8 and 52.3 and 145.4 cm-1 V-1 respectively for the positive and negative peaks of the COL and COM bipolar bands. Apart from the bipolar line shape, IR absorption of COad is
Preparation and Characterization of Co Thin Films
Langmuir, Vol. 22, No. 25, 2006 10581 Table 3. Variations of IR Features of the COad Bands in MSFTIR Spectra Recorded on nm-Co/GC(n) and Bulk Co Electrodes n
1 -1
-1
dCOLv/dE/cm V dCOLV/dE/cm-1 V-1 dCOMv/dE/cm-1 V-1 dCOMV/dE/cm-1 V-1 fwhm (COL) ∆IR
Figure 8. (a) In situ MSFTIR spectra of CO adsorbed in saturation on surface of nm-Co/GC(8), ES ) -1.1 ∼ -0.8 V, ER ) -0.5 V, 0.1 mol‚L-1 NaOH solution; (b) variations of v˜COLV(2), v˜COLv(9), v˜COMV(1)and v˜COMv([) with ES.
obviously enhanced. The values of ∆IR determined from the spectra in Figure 7 are respectively 20.2, 30.9, and 29.7 for n ) 4, 8, and 16. The asymmetric COad IR bipolar band has been denoted as “Fano-like IR features” in the literature because of the similarity to the Fano spectral line shape.24,44 Fano resonance, which first theoretically investigated by Fano,45 is interpreted as a quantum mechanical coupling of a discrete energy state with a degenerate continuum and is widely investigated in various fields from both theoretical and experimental points of view.46-49 This Fano-like IR features were observed also in reflection or transmission IR studies of thin films of other metals such as Fe,50 (44) Krauth, O.; Fahsold, G.; Pucci-Lehmann A. J. Mol. Struct. 1999, 483, 237. (45) Fano, U. Phys. ReV. 1961, 124, 1866. (46) Dvid, C. Langreth Phys. ReV. Lett. 1985, 54, 126. (47) Patthey, F.; Schaffner, M. H.; Schneider, W. D.; Delley, B. Phys. ReV. Lett. 1999, 82, 2971. (48) Bulka, B. R.; Stefanski, P. Phys. ReV. Lett. 2001, 86, 5128. (49) Zhu, Y. M.; Uchida, H.; Watanabe, M. Langmuir 1999, 15, 8757. (50) Krauth, O.; Fahsold, G.; Magg, N.; Pucci. A. J. Chem. Phys. 2000, 113, 6330.
2
4
8
16
30
bulk
70.2 77.5
83.2 83.7 127.1 40.3 40.8 51.5 44.3 72.5 85.8 69.5 90.1 52.3 74.9 116.7 145.4 144.4 94.7 39 43 37 35 18.9 26.2 20.2 30.9 29.7 28.2 1
Pt,26 Ru,24 ZnSe,51 and NaV2O5 single crystal.52 It is worthwhile to point out that the bipolar band observed in the current studies shows opposite direction in the positive and negative peaks in comparison to our previous studies of nanostructured Pt thin films,24,26 in which the positive peak of the bipolar band lies in the high wavenumbers and the negative peak of the bipolar band locates in the low wavenumbers. The present results demonstrate that the Fano-like spectral line shape of the COad bands is a particular IR property of nanostructured Co thin films. It is interesting to notice that the Stark tuning rate of the positive peak and that of the negative peak of the bipolar COad bands are not equal. The dV˜ COad/dE of the positive peak is much larger than that of the negative peak for the COL bipolar band, whereas the inverse situation is encountered for the COM bipolar band. Such an asymmetrical phenomenon has also been observed on Pt thin films of plumelike nanostructure,26 whereas in this case, the dV˜ COad/dE of the positive peak of the COL bipolar band is smaller than that of the negative one. However, an equal Stark tuning rate of the positive peak to that of the negative one of the COL bipolar band has been also observed on Pt thin films of island nanostructure.43 The results of the current paper and those reported in refs 26 and 43 may confirm the asymmetrical character of the Fano-like IR effects and illustrate the strong dependence of anomalous IR properties on the nanostructure of thin films. (3) In the case of the nm-Co/GC(30) electrode whose surface is mostly composed of hexagonal nanosheets, two negative IR bands appear (COL near 1980 cm-1 and COM around 1830 cm-1) and shift positively with increasing ES, yielding Stark tuning rates of 44.3 and 94.7 cm-1 V-1, respectively. It is interesting to see that the direction of the COad bands in the spectrum is turned to the same direction of the COad bands appearing in a spectrum acquired with the bulk Co electrode. However, the intensity of the COad bands is significantly enhanced by considering that the appearance of the COM band. The enhancement factor (∆IR) measured from the spectrum of nm-Co/GC(30) is 28.2. The values of fwhm of the COL and COM bands are measured on nm-Co/GC(30) electrodes ca. 37 and 54 cm-1 that are both broader than those on the bulk Co electrode. These IR features correspond to typical surface-enhanced infrared absorption (SEIRA).53 The SEIRA was found to depend strongly on adsorbed species, metal film material, and morphology of substrate surface studied by transmission, attenuated total reflection (ATR), and external reflection IR techniques. Osawa et al.54 referred that the strong electromagnetic field as well as chemical effects may contribute independently to the enhancement in SEIRA. Although theoretical and experimental efforts have been both undertaken to reveal the origin of the SEIRA, the mechanism of SEIRA is still not completely understood so far. Table 3 lists parameters of IR features of the COad bands measured in MSFTIR spectra recorded on different nm-Co/GC(51) Nakata, H.; Yamada, K.; Ohyama, T. J. Cryst. Growth 2000, 214, 533. (52) Popova, M. N.; Sushkov, A. B.; Vasilev, A. N.; Isobe, M.; Ueda, Y. JETP Lett. 1997, 65, 743. (53) Hartstein, A.; Kirley, J. R.; Tsang, J. C. Phys. ReV. Lett. 1980, 45, 201. (54) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914.
10582 Langmuir, Vol. 22, No. 25, 2006
Figure 9. In situ SNIFTIR spectra of CO adsorbed saturation on nm-Co/GC(n) and bulk Co electrodes. ES ) -0.9 V and ER ) -1.1 V, 0.1 mol‚L-1 NaOH solution.
(n) and bulk Co electrodes. It is obvious that the Stark tuning rates of COL bands all increases gradually with increasing n and reach the maximal value at 127.1 cm-1 V-1 in positive peak and 51.5 cm-1 V-1 in negative peak (i.e., on nm-Co/GC(16) electrode). The Stark tuning rates of the COL positive peaks are all lager and even more than twice of those in negative peaks. On the contrary, the Stark tuning rates of the COM positive peaks are all much smaller than those of the COM negative peaks. The fwhm of COL bands (either positive peak or negative peak) of nm-Co/GC(n) electrodes are all broader than that on bulk Co electrode. From the values of ∆IR, we can conclude that all nmCo/GC(n) electrodes exhibit enhanced IR absorption in comparison with the bulk Co electrode. It is interesting to note that nm-Co/GC(8) electrode which exhibits Fano-like IR features possesses the maximum enhancement factor (∆IR) 30.9) coinciding with the high Stark tuning rate. These results indicate that the IR characteristics depend strongly on the Co thin film structures. 4.2. Results of SNIFTIRS. In the SNIFTIRS experiments, the reference potential (ER) and sample potential (ES) were set at -1.1 and -0.9 V, respectively. At both ES and ER CO can adsorb stably on Co surface as demonstrated directly by CV studies. The electrode potential was stepped alternately from ER to ES for 4 times, and in each step 100 interferograms were collected respectively at ER and ES, The total 400 interferograms that collected at each potential were co-added and Fourier transformed into single-beam spectrum R(ER) or R(ES). Finally, the result spectra were calculated using eq 1. The SNIFTIR spectra of CO adsorbed on bulk Co and nm-Co/GC(n) electrodes are displayed in Figure 9. For clarity of observation, the IR spectrum of bulk Co was expanded by 8-fold in ∆R/R. Due to Stark shift,55 the IR absorption of COad in spectra collected by in situ SNIFTIRS procedures will give rise to bipolar bands with their positive peak in low wavenumbers and negative peak in high wavenumbers. This is exactly the case of CO adsorbed on the bulk Co electrode. We observe a bipolar COL band around 1970 cm-1 (55) Lambert, D. K. Electrochim. Acta 1996, 41, 623.
Chen et al.
with its positive peak in low wavenumbers and negative peak in high wavenumbers. This observation is in agreement with IR features of CO adsorbed on a Co disk electrode reported by Cuesta et al.37 However, the IR features of CO adsorbed on nm-Co/GC(n ) 1 and 2) electrodes are quite different from those of CO adsorbed on bulk Co electrode. The direction of the positive and negative peaks of the bipolar COL band in spectra of nmCo/GC(n ) 1 and 2) electrodes is in the opposite direction to that in the spectrum of bulk Co electrode. It is evident that the bipolar COL band in spectra of nm-Co/GC(n ) 1 and 2) electrodes is completely inverted. Furthermore, the intensity of COad bands is significantly enhanced. These abnormal IR features confirm that the nm-Co/GC(n ) 1 and 2) electrodes yield AIREs. With n increasing from 4 to 16, the COad bipolar bands are turned into positive-going monopolar COad bands, i.e., COL band near 1980 cm-1 and COM band around 1820 cm-1. These kinds of monopolar bands appearing in SNIFTIR spectra can be resulted from subtraction of two bipolar bands.56 Therefore, the monopolar COad bands can be ascribed to Fano-like IR features. When n is increased further (i.e., on the nm-Co/GC(30) electrode), the spectral line shape becomes similar to that obtained on the bulk Co electrode (i.e., bipolar COad bands with their negative peak in high wavenumbers and positive peak in low wavenumbers), except the significant enhancement (SEIRA) of COad bands. The dramatic changes in IR features observed in the in situ SNIFTIRS studies are in accordance with the results obtained in the in situ MSFTIRS investigations as demonstrated previously. By comparing the surface structure (obtained from in situ STM and ex situ SEM) and the spectra (both MSFTIRS and SNIFTIRS) recorded on nm-Co/GC(n) electrodes, we could draw the conclusion that the spectral IR features have undergone a transformation from abnormal IR features (AIREs) to surfaceenhanced IR features (SEIRA) via transitional Fano-like IR features along with increasing n, i.e., with variation of structure of nm-Co/GC(n) electrodes. Similar transformation of COad IR bands via a transitional bipolar pattern has been reported by Wang et al.,27 they observed the IR features of bridge bonded CO(COB) on Ni thin films from AIREs to Fano-like IR features, and finally to enhanced IR features with the film structure varying from layer nanostructure to island nanostructure. However, Sun and co-workers26,43 and Bjerke et al.32 have observed a transformation of IR features of the COL band from enhanced IR absorption to Fano-like IR features, and finally to AIREs line shapes on various nanostructured Pt thin films prepared under different conditions. Similar transformation from enhanced IR absorption to Fano-like IR features, and finally to AIREs line shapes has been also found on Ru thin films along with variation of film structure.24 The results in this paper and those of previous studies revealed that the IR properties of nanostructured metal thin film materials are strongly dependent on the thickness and structure of the film. Recently, Wu and co-workers57 have theoretically simulated the transformation of these IR properties by considering the interparticle interaction and electron-hole damping between Pt nanoislands and CO molecules. It may be nevertheless pointed out that it is still far away from completely understanding the mechanism of the transformation of anomalous IR properties of nanostructured metal thin films. Further experimental and theoretical studies are still required to gain more insight into the origin of the anomalous IR properties of nanostructured thin film materials. (56) Gong, H. Ph.D. Thesis, Xiamen University, 2003. (57) Wu, C. X.; Lin, H.; Chen, Y. J.; Li, W. X.; Sun, S. G. J. Chem. Phys. 2004, 121, 1553.
Preparation and Characterization of Co Thin Films
Conclusions Different nanostructure and thickness of Co thin films on GC (nm-Co/GC(n)) were prepared in this paper by electrodeposition under cyclic voltammetric conditions. Studies of in situ STM and ex situ SEM demonstrated that the prepared Co thin films are uniformly composed of Co nanoparticles. Along with the increase of n, these nanoparticles in the Co thin films were varied from bearded nanoparticles to multiform nanoparticles and finally to hexagonal nanosheets accompanying with the increase of average size. Cyclic voltammetric studies illustrated that the chemisorbed CO on nm-Co/GC(n) electrodes strongly inhibited the electrooxidation of Co thin films. Employing CO adsorption as a molecular probe reaction, both MSFTIRS and SNIFTIRS studies of CO adsorption on nm-Co/GC(n) electrodes revealed that the IR properties are strongly dependent on the thickness and structure of Co thin films. When COad adsorbed on nanostructured Co thin films, the IR absorption of COad has been significantly enhanced and the bands of COad are widely broaden. It has illustrated that, with the increase of n, the IR properties
Langmuir, Vol. 22, No. 25, 2006 10583
of nm-Co/GC(n) electrodes appeared a transformation from abnormal IR features (n ) 1 and 2) to Fano-like IR features (n ) 4, 8, and 16) and finally to enhanced IR absorption features (n ) 30). The transition of IR features is an anomalous IR property peculiar to nanostructured materials. It may involve composition, structure, and the size of the nanomaterials, as well as interaction between surface and adsorbed molecules, surface plasmon resonance, etc. This study has extended the investigation of anomalous IR properties, which were initially discovered on platinum group metals, to nanostructured Co thin film, and thrown a new insight into understanding the origin of the anomalous IR properties. Acknowledgment. This study was supported by National Natural Science Foundation of China (Grant Nos. 90206039, 20021002, and 20433060) and National Key Basic Research Program (“973” project: Grant No. 2002CB211804). LA0615037
