Raman Investigations of Adsorbate-Substrate Interactions on Composite Ag (or Cu)/TiO 2 Nanotubes/Ti Substrates

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1 WDS'09 Proceedings of Contributed Papers, Part III, , ISBN MATFYZPRESS Raman Investigations of Adsorbate-Substrate Interactions on Composite Ag (or Cu)/TiO 2 Nanotubes/Ti Substrates A. Roguska, M. Pisarek Warsaw University of Technology, Faculty of Materials Science and Engineering, Warsaw, Poland. Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. A. Kudelski University of Warsaw, Department of Chemistry, Warsaw, Poland. M. Lewandowska, K.J. Kurzydłowski Warsaw University of Technology, Faculty of Materials Science and Engineering, Warsaw, Poland. M. Janik-Czachor Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. Abstract. A tubular array of TiO 2 nanotubes on Ti matrix was used as a support for Ag or Cu sputter-deposited layers intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples of Ag/TiO 2 -nanotube/ti and Cu/TiO 2 - nanotube/ti were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES and SAM) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured after it had been adsorbed on the TiO 2 -nanotube/ti substrates covered with thin Ag or Cu deposit as well as on the bulk electrochemically roughened Ag or Cu reference substrates. The Ag/TiO 2 nanotubes/ti samples appeared very Raman active, whereas these with a Cu deposit, were less active, resulting in weaker Raman signals. The SERS activity of the Ag/TiO 2 nanotubes/ti composite samples was strongly dependent on the amount of the Ag deposit. The SERS intensity vs. electrode potential dependences measured for the these samples were interpreted in terms of the modified electronic structure of the Ag deposits due to the interaction of the Ag clusters with the TiO 2 nanotube/ti substrate. Introduction In various fields of materials science it is of great importance to identify adsorbed species, to determine their orientation with respect to the surface and to observe how the strength of different chemical bonds changes upon adsorption. Information of this sort can be obtained by surface vibrational spectroscopies, e.g. Raman scattering. Application of normal Raman scattering, however, is limited due to its low Raman scattering cross section (typical Raman scattering cross-section is cm 2 per molecule). The efficiency of Raman scattering can be significantly increased, permitting the observation of Raman spectra even of a single molecule [Fleishmann et al., 1974; Pettinger et al., 1977]. When the wavelength of the excitation radiation is tuned to the absorption band of the scattering molecules, the intensity of Raman signal is increased in so-called resonance Raman effect. The other possibility is creation of an electromagnetic resonator (which locally enhances the electromagnetic field) on the surface under study. Utilising both effects, the Raman scattering crosssection can be sometimes increased even to cm 2 per molecule [Nie et al., 1997]. The above described resonator-induced effect is called surface-enhanced Raman scattering (SERS). The high enhancement factors necessary to measure the SERS spectra are achieved only for those molecules which are in close contact with the metallic clusters of SERS active metals (Ag, Cu) [Pettinger et al., 1981] at the surface. Notably, the molecule metallic particle distance should not exceed several atomic dimensions if high enhancement factors are to be observed. The SERS spectrum reflects the changes in the electronic structure of the molecule due to adsorption and an interaction with the metal substrate. Hence, the unusual spectra may result from the fact that the probe molecule is sensing some local differences in the properties of the metal clusters; the difference in the site orientation, and in the 136

2 spatial and the energy distribution of electrons by an interaction with the oxide supported sublayer, etc. Thus, high sensitivity of SERS to a physical and chemical state of the substrate may be used for identifying various types of clusters present at a metal surface when a well known probe molecule is used in spectroelectrochemical measurements. Moreover, a SERS spectrum may be measured even for molecules adsorbed on the surface of a SERS-inactive substrate when clusters/particles of a SERSactive metal are present in the immediate vicinity [Pettinger et al., 2003]. It is known from the literature that the SERS activity of a SERS-active substrate critically depends on the morphology of the surface [Schultz et al., 1981 Van Duyne et al., 1993]. One may expect large SERS enhancements when clusters of a SERS-active metal are deposited on surfaces having a developed morphology like e.g. self organized arrangement of nanotubes, and notably on TiO 2 nanotubes (which have been successfully grown on Ti in electrolytes containing fluoride ions [Macak et al., 2006]). Such nanotubes have attracted the attention of scientists and engineers for their unique properties, utilized in applications as functionalized material [Macak et al., 2007]. Literature data have shown that Ag clusters deposited on TiO 2 nanotubes are very promising systems for photochromic switching [Kawahara et al., 2005]. Those systems may also be considered as mechanically stable catalysts with a large specific surface area. The aim of this work was to examine the possibility of using SERS to study the differences between adsorption properties of bulk silver or copper and clusters of these metals deposited on TiO 2 /Ti selforganized nanotubes. In order to gain more insight into the character of the active sites on the composite Ag/TiO 2 nanotube/ti materials the effect of electrode potential on the SERS spectrum of pyridine (a probe molecule) adsorbed on composite Ag/TiO 2 nanotubes/ti substrate was measured in the spectroelectrochemical experiments. Experimental The material used as a substrate was Ti foil of 0.25 mm thickness (99.5 % purity, Alfa Aesar, USA). Titanium oxide nanotube layers were fabricated by electrochemical anodization of Ti samples in an optimized mixture of NH 4 F + deionized (DI) water + glycerol electrolyte [Macak et al., 2006]. The anodization process consisted of a scan voltage from an open circuit potential up to V max = 25 V with a scan rate of 200 mv s -1, followed by holding the applied voltage at 25 V for 800 s. After anodization, the samples were annealed at 600ºC for 1 h to improve their mechanical stability. The mechanically stable nanotubes were covered with a thin Ag or Cu layer by the sputter deposition technique in a vacuum (p = 3 x 10-3 Pa) using a standard JEE-4X JEOL equipment, in a configuration perpendicular to the surface of the samples. Spectral purity Ag and Cu metals were used. The average thickness of the deposited layer was estimated from the mass gain of the samples during the metal deposition process, divided by the geometric surface area. Raman spectra were recorded with a Jobin Yvon-Spex T64000 Raman microscope equipped with a Kaiser holographic notch filter, 600 grooves/mm holographic grating and 1024 x 256 pixels nitrogen-cooled CCD detector. A Laser-Tech model LJ-800 mixed argon/krypton ion laser provided excitation radiation of nm and the spectra were acquired at laser power of 30 mw. Spectroelectrochemical measurements were carried out in a typical three-electrode potentiostatic system in a 0.05 M pyridine M KCl electrolyte, usually at cathodic potentials within a range of from 0.2 V down to 1.2 V, to ensure that adsorption of the molecules of interest was followed. Pt wire was used as an counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. All potentials are reported with respect to SCE. A Microlab 350 (Thermo VG Scientific) was used to examine the morphology and composition of the surface of the TiO 2 nanotube/ti substrate before and after Ag or Cu sputter deposition, utilizing the Auger electron spectroscopy (AES). In addition, high resolution SEM examinations were performed with a high resolution scanning electron microscope equipped with a Duo-STEM detector (STEM, Hitachi S- 5500, 15 kv) to reveal the characteristic morphological features of the composite samples. Results and Discussion The optimized anodization conditions applied have resulted in the formation of TiO 2 nanotubes (hollow cylinders) perpendicular to the substrate and separated from each other. SEM examinations revealed that the nanotubes, with an average diameter of 100 nm and a height of 1 μm, were open 137

3 at the top and closed at the bottom. Figures 1a and b shows SEM images of typical TiO 2 nanotube layers covered with 0.01 mg/cm 2 and 0.09 mg/cm 2 of Ag, respectively. The sputter-deposited Ag formed nanoparticles, which diameter varies from 5 to 20 nm. For the sample with 0.01 mg/cm 2 of Ag the particles are located at the top edges of the nanotubes, and on their side walls. TiO 2 nanotubes covered with 0.09 mg/cm 2 Ag exhibit quite a different morphology. The Ag particles are agglomerated and in the form of rings at the top of the nanotubes. The amount of Ag is here so high, that it results in a visible reduction of their internal diameter. Figure 2 shows AES spectra for pure Ag (a) and for two different composite samples Ag/TiO 2 nanotube/ti with 0.09 mg/cm 2 Ag (b) and 0.01 mg/cm 2 Ag (c). The AES survey spectrum for the Ag/TiO 2 nanotube/ti sample with 0.09 mg/cm 2 Ag suggests that the Ti is almost completely covered with Ag, and the corresponding Ag MNN signals are distinctly broadened at the lower kinetic energy site. This suggests that the electronic structure of Ag is modified by an interaction of the deposited Ag with TiO 2. The AES survey spectrum for the Ag/TiO 2 nanotube/ti sample with 0.01 mg/cm 2 Ag shows distinct Ag, Ti and O signals due to incomplete coverage by the Ag deposit. Figure 3 shows Raman spectra from the Ag/TiO 2 -nanotubes/ti samples as compared with those of Cu/TiO 2 -nanotubes/ti samples. Two different amounts of Raman active metals were sputterdeposited onto the TiO 2 -nanotubes/ti substrate, as indicated. The Ag/TiO 2 -nanotubes/ti electrodes appeared very Raman active, whereas these with a Cu deposit, were less active, resulting in weaker Raman signals. The bands in the low wavenumber region (520 and 630 cm -1 ) are due to the vibrations of the TiO 2 oxide layers and are also observed in the Raman spectra of the Ti/TiO 2 -nanotubes before deposition of the Ag or Cu layer [Pal et al., 2007]. Our investigations have revealed that annealing procedure at 600 C results in crystallization of Ti0 2 nanotubes into rutile, as the Raman spectra suggest (data not shown). The most interesting feature of the spectra presented in Fig. 3 is the band at about 900 cm -1, which is well visible, irrespective of the kind of the metal deposited. This signal probably arose from the Ti=O stretch of the oxide directly interacting with the Raman active metal clusters (Ag or Cu). Figure 1. SEM image of a TiO 2 nanotube layer covered with (a) 0.01 mg/cm 2 Ag; (b) 0.09 mg/cm 2 Ag. Figure 2. Left: Typical Auger survey spectra taken at the surface of (a) pure Ag, (b) 0.09 mg/cm 2 Ag/TiO 2 -nanotube/ti substrate and (c) 0.01 mg/cm 2 Ag/TiO 2 -nanotube/ti substrate. Right: Auger signals from silver ( ev) taken at a TiO 2 -nanotube/ti substrate surface covered with sputterdeposited Ag of (b) 0.09 mg/cm 2 and (c) 0.01 mg/cm 2. For comparison, an Ag MNN reference spectrum for pure Ag is given (a). 138

4 Figure 3. Typical Raman spectra of TiO 2 -nanotubes/ti substrate covered with sputter-deposited (a) Ag of 0.01 mg/cm 2 ; (b) Ag of 0.09 mg/cm 2, (c) Cu of 0.02 mg/cm 2 and (d) Cu of 0.08 mg/cm 2. All spectra were measured in air. The SERS spectra of pyridine adsorbed on Ag/TiO 2 nanotubes/ti composite samples at an open circuit potential resembles only partly that recorded on rough Ag. We suppose that the differences between the spectra of pyridine adsorbed on the rough bulk silver surface and on the silver clusters deposited on TiO 2 nanotubes are due to different types of pyridine molecules preferentially adsorbed at various locations. We decided to verify whether the local properties of various materials containing silver clusters could be more effectively investigated by analysing how locally measured SERS spectra change with electrode potential for a model adsorbate. Figure 4 illustrates the effect of electrode potential on the normalized Raman intensity of the ring breathing mode of pyridine adsorbed on two different samples of TiO 2 nanotubes/ti substrates covered with approximately the same amount of Ag (0.09 mg/cm 2 ), and on a standard roughened silver surface. The shape of the reference plot of Raman intensity vs. electrode potential, E, measured on a standard Ag electrode exhibits a well know single maximum at 0.6 V. The corresponding plots measured on Ag/TiO 2 nanotubes/ti composite electrodes are very different and, to some degree, vary from sample to sample. A maximum at 0.6 V is always observed, and yet is always lower than that for the reference Ag sample. Two new maxima appear at 1 V - a high one, and for some samples, one also at 0.25 V. This suggests three types of highly SERS-active adsorption sites on the surface of the composite electrodes. Such adsorption sites can be identified as: standard silver clusters, small silver clusters of modified electronic structure, and the interphase boundaries between silver clusters and TiO 2. Some of these sites are probably acidic in nature (the presence of an observable amount of acidic sites on such silver clusters has been noticed earlier by Akcay [Akcay et al., 2005]. The data for the reference Ag sample suggest that the local maximum at 0.6 V is apparently due to a chargetransfer enhancement for pyridine molecules adsorbed on large standard silver clusters. The maximum at 1.0 V is probably due to pyridine adsorbed on so-called acidic sites present on this composite substrate (the lowering of the energy of electronic levels in the metal clusters may be compensated for by the lowering of the applied electrode potential). The actual character of acidic sites is, however, not clear for us at this stage of research. As mentioned in the introduction, the enhancement factors on various roughened metallic surfaces strongly depend on the size and shape of the metal nano-clusters formed on the surface. Our experiments revealed that the intensity of the spectrum of pyridine adsorbed on the TiO 2 -nanotubes/ti covered with a silver deposit of 0.09 mg/cm 2 is significantly higher than the average intensity of the 139

5 Figure 4. Left: Effect of electrode potential on Raman spectra of pyridine adsorbed on Ag-covered TiO 2 nanotubes/ti substrate polarized cathodically in 0.1 M KCl M pyridine. Right: Effect of electrode potential on the normalized Raman intensity of the ring breathing mode (at 1005 cm 1 ) of pyridine adsorbed on two different of Ag/TiO 2 nanotubes/ti samples. Electrolyte: 0.1 M KCl M pyridine. Lines are eye guidelines only. Normalized intensity values were calculated by dividing the SERS intensity by the highest value in the corresponding plot. A plot of Raman intensity vs. electrode potential for a standard roughened Ag electrode is also given. spectra measured on the TiO 2 -nanotubes/ti substrate covered with a silver deposit of 0.01 mg/cm 2. We established that the 1005 cm 1 band of pyridine adsorbed on the substrates covered with 0.09 mg/cm 2 of Ag is about times more intense than this band measured in experiments on substrates covered with only 0.01 mg/cm 2 of Ag. The significantly higher SERS-activity of the TiO 2 -nanotube substrate covered with a thicker layer of silver may be explained by: (i) the higher enhancement factors achievable on larger clusters (for example, values of the optimum sizes of metal clusters for SERS measurements can be found in [Wu et al., 2008]), (ii) the dominant contribution in the recorded SERS spectrum of pyridine molecules adsorbed in the narrow slits between silver clusters. Some previous theoretical calculations suggest that, frequently, the measured SERS signal is dominated by the contribution from molecules adsorbed in the slits between metal grains [Xu et al., 1999; Kudelski, 2009]. Since, the silver clusters present in the thinner deposit are relatively isolated, whereas those for the thicker deposit provide a substantial amount of relatively narrow slits between silver clusters, see in Figures 1 and 2, in our opinion, the increase in the number of slits between silver clusters is the key factor explaining significantly higher SERS-activity of a TiO 2 -nanotube substrate covered with a thicker deposit of silver. Summary and Conclusions A TiO 2 nanotubular array on a Ti metal matrix covered with 0.09 mg/cm 2 Ag deposit proved to be a very active SERS substrate. The intensity of adsorbed pyridine (probe molecule) ring breathing mode band (1005 cm -1 ) was times larger than that for a similar substrate with only 0.01 mg/cm 2 Ag deposit. SEM examinations revealed that, the thinner deposit consists of well separated Ag particles. The thicker one consists of aggregated Ag particles with relatively narrow slits between them. A significant number of the narrow slits between the silver particles and/or the clusters may be the dominant factor responsible for a very high increase in the SERS activity of a TiO 2 -nanotube substrate covered with a thicker deposit of silver. 140

6 AES spectra taken from the surface of a Ag/TiO 2 nanotube/ti composite exhibit a distinct broadening of the Ag (MNN) signal and a different intensity ratio in the silver doublet. This suggests a modification of the electronic structure of the Ag particles due to an interaction with TiO 2 substrate. Indeed, Raman intensity vs. electrode potential for Ag/TiO 2 nanotube/ti exhibits three distinct maxima (suggesting three different types of adsorption sites) as opposed to only one for pure Ag. This finding bolsters the above conclusion. Experiments similar to those with Ag deposits have been carried out on TiO 2 -nanotubes/ti substrate covered with Cu clusters. Unfortunately, these substrates were not highly SERS-active under the above experimental conditions, and we were unable to carry out a reliable comparison of different substrates. This is probably due to the surface oxidation of Cu. Copper oxide increases the distance of pyridine molecules to Cu metal, so that the giant SERS effect is not possible there. More research is required to develop a procedure of covering TiO 2 -nanotubes/ti substrate with non-oxidized Cu and to study adsorption there. Acknowledgments. The authors are grateful to Prof. Patrik Schmuki at the Chair of Surface Science and Corrosion, University of Erlangen-Nueremberg and his co-workers for their technical guidance offered on the production of nanotubes fabrication. Thanks are also due to Dr. Mirosław Dolata for helpful discussions. Surface characterizations were performed using a Microlab 350 located at the Physical Chemistry of Materials Center of the Institute of Physical Chemistry, PAS, and of the Faculty of Materials Science and Engineering, WUT. This work was financially supported by the Polish Ministry of Science and Higher Education (grant No. N N ), and by the Institute of Physical Chemistry PAS. References Fleishmann M., Hendra P.J., McQuillan A.J., Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett. 26, 163, Pettinger B., Wenning U., Raman spectra of pyridine adsorbed on silver (100) and (111) electrode surfaces, Chem. Phys. Lett. 56, 253; Nie S., Emory S.R., Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering, Science 275, 1102, Pettinger B., Wetzel H., Surface enhanced Raman scattering from pyridine, water, and halide ions on Au, Ag and Cu electrodes, Ber. Bunsenges Phys. Chem. 85, 473, Pettinger B., Picardi G., Schuster R., Ertl G., Surface-enhanced and tip-enhanced Raman spectroscopy of CN ions at gold surfaces, J. Electroanal. Chem. 554, 293, Schultz S.G., Janik-Czachor M., Van Duyne R.P., Surface enhanced Ramam spectroscopy: a re-examination of the role of the surface roughness and electrochemical anodization, Surf. Sci. 104, 419, Van Duyne R.P., Hulteen J.C., Treichel D.A., Atomic Force Microscopy and Surface-Enhanced Raman Spectroscopy. I. Ag Island Films and Ag Film Over Polymer Nanosphere Surfaces Supported on Glass, J. Chem. Phys. 99, 2101, Macak J.M., Schmuki P., Anodic growth of self-organized anodic TiO 2 naotubes in viscous electrolytes, Electrochim. Acta 52, 1258, Macak J.M., Tsuchiya H., Ghicov A., Yasuda K., Hahn R., Bauer S., Schmuki P., TiO 2 nanotubes: Selforganized electrochemical formation, properties and applications, Curr. Opin. Solid St. M. 11, 3, Kawahara K., Suzuki K., Ohko Y., Tatsuma T., Electron Transport in Silver-Semiconductor Nanocomposite Films Exhibiting Multicolor Photochromism, Phys. Chem. Chem. Phys. 7, 3851, Pal M., Serrano J. G., Santiago P., Pal U., Size-Controlled Synthesis of Spherical TiO 2 Nanoparticles: Morphology, Crystallization, and Phase Transition, J. Phys. Chem. C 111, 96, Akcay M., The surface acidity and characterization of Fe-montmorillonite probed by in situ FT-IR spectroscopy of adsorbed pyridine, Appl. Catal. A-Gen. 294, 156, Wu D.Y., Li J.F., Ren B., Tian Z.Q., Electrochemical surface-enhanced Raman spectroscopy of nanostructures, Chem. Soc. Rev. 37, 1025, Kudelski A., Raman spectroscopy of surfaces, Surf. Sci. 603, 1328, Xu H., Bjerneld E.J., Käll M., Börjesson L., Spectroscopy of single hemoglobin molecules by surface enhanced Raman scattering, Phys. Rev. Lett. 83, 4357,