M. Dolata

SURFACE-ENHANCED RAMAN SCATTERING (SERS) AT COPPER(I) OXIDE

Surface-enhanced Raman scattering spectra of pyridine at a copper(I) oxide hydrosol and at Cu2O-covered copper electrode produced by ‘oscillating reaction roughening’ are reported. It is shown that on such surfaces pyridine molecules adsorb as pyridinium cations. The ‘oscillating reaction’ pretreatment yields a passivated Cu surface which does not undergo any reduction during the cathodic scan, producing a SERS spectrum of pyridinium cations even after decreasing the electrode potential to strongly negative values.

Comparative impedance spectroscopy study of rutile and anatase TiO2 film electrodes

The purpose of this work was a comparative electrochemical impedance analysis of the titanium dioxide-solution interface for two kinds of thin film samples consisting respectively of anatase and rutile. These kind of films, frequently used in photoelectrochemical experiments, have recently been the subject of detailed physical studies showing important differences in electrical properties between those two forms of TiO2. The EIS measurements were carried out over a wide frequency range (from 0.03 Hz to 10 kHz), analysing the whole spectrum.

Photoelectrochemical studies pertaining to the activity of TiO2 towards photodegradation of organic compounds

Abstract Photoanodic (water and methanol oxidation) and cathodic (oxygen reduction) processes occurring at illuminated (with near - UV light) bulk and particulate anatase and rutile TiO 2 films were analysed. (Photo) current-potential curves recorded for both anodic and cathodic processes point to large differences in activity between the two forms of TiO 2 . In particular, in the presence of various organic compounds, the photocurrent onset potential of anatase tends to shift negatively (sometimes as much as 0.3 V), thus increasing drastically the difference with respect to the potential of oxygen reduction. The situation is quite different for the rutile samples, the photocurrent onset potential of which appears practically unaffected by the addition of organic species to the solution and remains close to the onset potential of oxygen reduction. The reverse reaction with respect to the photoanodic process, involving species generated by the photo-oxidation of water and OH − ions, is shown to play often an essential role in determining the photocurrent onset potential (i.e. the operating mixed potential) of the photocatalyst. This is the prevailing situation for rutile but is much less frequent for anatase. In fact, a number of species, especially among organic compounds, appear able to compete efficiently for positive holes with OH groups adsorbed at the anatase surface, leading to a virtual suppression of the reverse reaction. In such a case, the reduction of oxygen present in the solution becomes the main cathodic process. This description is consistent with the fact that, in most cases of real photodegradation processes, the anatase form of TiO 2 exhibits much greater activity than the rutile form.

Characterization of the copper surface optimized for use as a substrate for surface-enhanced Raman scattering

Abstract A new electrochemical procedure of roughening of a copper surface was developed, producing a surface with a high surface-enhanced Raman scattering (SERS) activity. The surface morphology and composition was characterized by energy dispersive/wavelength dispersive/X-ray microprobe analysis and compared to the Cu electrodes roughened by the commonly used oxidation–reduction reactions. The morphology of such a surface was completely different from those produced by traditional electrochemical procedures. Moreover, X-ray microprobe analysis revealed that our new surface remained unoxidized in contrast to the other SERS active Cu surfaces. The potential profiles of SERS intensity of the ring breathing mode of pyridine adsorbed on these surfaces were discussed and interpreted in terms of charge-transfer (CT) effect.

Kramers-Kronig Transforms as Validation of Electrochemical Immittance Data Near Discontinuity

Immittance data was recorded for copper rotating disk in concentrated copper sulphate/sulphuric acid electrolyte, and its evolution under potential control (PC) was analyzed starting from the active state at rest potential, through active/passive transition up to the stable passivity. In the potential range corresponding to the passivity under PC, the transition was observed from the nonminimum phase (nmp)-type of immittance to the minimum phase (mp) one which corresponded to Hopf bifurcation under current control. This transition was manifested by a resonance-like peak on the amplitude characteristic and the phase change from apparently discontinuous as displayed in [-180°, +180°] range (nonminimum) to the continuous (minimum) one. In complex coordinates this was featured by scattered impedance points. Validation by Kramers-Kronig (KK) transformation of nmp-type immittance data failed for impedance representation used in transformation but was successful for admittance representation as the latter was the form actually recorded under PC. This finding validates both nmp and mp immittance data in agreement with earlier suggestions of other authors. [See, Gabrielli et al., in Electrochemical Impedance: Analysis and Interpretation, p. 140, ASTM, Philadelphia, PA (1993).] Transition from mnp to mp type of electrode dynamics can be attributed to appearance of conduction channels representing local depassivation of the electrode.

SURFACE-ENHANCED RAMAN SCATTERING (SERS) ON COPPER ELECTRODEPOSITED UNDER NONEQUILIBRIUM CONDITIONS

Abstract Rough Cu surfaces, with pattern formation, were obtained by a nonequilibrium electrodeposition from acidic electrolyte. Wavelength dispersive spectrometer (WDS)/energy-dispersive spectrometer (EDS) investigations revealed that such surfaces are highly oxidized. Surface enhanced Raman scattering (SERS) investigations confirmed a complex structure and composition of such a substrate; a distinct difference in spectral characteristics of an adsorbate on Cu 2 O, pure metallic Cu clusters, and some other, not yet well characterized, metallic Cu clusters, which predominate on the copper surfaces activated by standard electrochemical methods, were identified and distinguished.

Modification of surface activity of Cu-based amorphous alloys by chemical processes of metal degradation

Abstract Changes in morphology, local chemical composition, catalytic activity and local surface-enhanced Raman scattering (SERS) activity due to various methods of modification of surfaces of Cu–Zr, Cu–Hf and Cu–Ti amorphous alloys were investigated. Our recent investigations have shown that otherwise detrimental processes of material degradation may help to segregate clusters of Cu and to develop a large specific surface area of Cu on ZrOx or HfOx supports. This transforms the original Cu-based amorphous ribbons of low surface area into efficient and stable catalysts. The following methods of activation based on the degradation processes, and their combinations are presented: ageing/oxidation in air, anodic activation, and pre-treatment with hydrogen (hydrogenation under high hydrogen pressure, or cathodic hydrogen charging). Structural changes in the bulk of the amorphous alloys introduced by the above pre-treatments were examined by XRD. Morphological and chemical changes on the surface were followed microscopically (optical microscopy and high resolution SEM) and by using a high resolution scanning Auger microprobe (SAM) and an X-ray electron microprobe. The amount of segregated Cu on the surface was estimated by an electrochemical method. A test reaction of dehydrogenation of 2-propanol was used to compare the catalytic activity of the materials after different chemical pre-treatments. Proposed mechanisms of changes in the local adsorption properties, local enhancement of Raman spectra and the catalytic activities due to the above combined pre-treatments are discussed.

Effect of electrochemical pretreatment on SERS and catalytic activity of Cu-Zr amorphous alloys

Abstract This paper presents results of electrochemical, microscopic, spectroelectrochemical (SERS) and catalytic investigations of Cu–Zr amorphous alloy ribbons aimed at increasing their catalytic activity for the dehydrogenation reaction of aliphatic alcohols. Rapid dissolution in sulphuric acid/copper sulphate solution of partially devitrified Cu–Zr ribbons resulted in a nearly twofold enhancement of their catalytic activity for dehydrogenation of 2-propanol. However, their local SERS activity on the Cu rich areas was not distinctly altered by this electrochemical pretreatment. Yet, the SERS heterogeneity of the devitrified material vanished: the SERS intensity increased considerably outside the Cu rich areas. A tentative mechanism of the effects of various electrochemical pretreatments on the surface activity is discussed.

Local characterisation of inhomogeneous Cu surfaces by surface-enhanced Raman scattering

Abstract An attempt was made to compare surface-enhanced Raman spectra (SERS) of the probe molecules adsorbed on highly dispersed Cu surfaces of both real catalyst and various model systems produced from pure Cu. In particular, we searched for the Cu clusters with adsorption properties significantly differing from the properties of the clusters produced from pure copper on the surface of highly active copper-containing catalysts. The SERS spectra of pyridine used as a probe molecule were collected with a Raman spectrometer connected to a microscope having a focal radius of less than 10−6 m. It was found, for example, that for the very active catalyst produced from 50Cu–50Zr amorphous precursor by high-pressure hydrogen charging, the relative intensities of pyridine bands are considerably different at some places to those observed for copper. At other areas on the sample, however, the spectrum is similar to that observed for pure Cu. This indicates that the local SERS technique has the potential to investigate adsorbate properties that are usually hidden by averaging if the signal is collected from a larger surface area.

Electrochemical modification of Cu-Zr amorphous alloys for catalysts

Abstract This paper presents the authors’ attempts to use some strictly controlled electrochemical processes of dissolution/redeposition of Cu (including disproportionation of Cu + to Cu metal and Cu 2+ ) to modify its surface, as well as surfaces of Cu base amorphous alloys (AA), to produce active substrates for various phenomena of adsorption and catalytic reactions. New methods of activation of the Cu substrate have been developed for in situ investigations of adsorbates with surface enhanced Raman spectroscopy (SERS). The method consisting in fast dissolution/redeposition produces a clean and large specific surface area substrate, which results in high quality SERS spectra. This method appeared also useful for a modification of the surface activity of partially devitrified Cu–Zr amorphous alloys. Energy dispersive spectrometry (EDS)/X-ray electron microprobe investigations of both AA ribbon surface and its cross-section revealed a peculiar structure of the devitrified material: some multilayer sandwiches of Cu/ZrO 2 were formed locally. Their fractal edges formed certain ‘flowers’ of Cu clusters supported by ZrO 2 . The electrochemical pre-treatment was eventually applied after this partial devitrification. The catalytic efficiency of dehydrogenation of 2-propanol at such a pre-treated Cu–Zr substrate increased up to a factor of 2. It has been found that the increase in the SERS activity of an electrochemically pretreated Cu–Zr substrate and the enhancement of its catalytic efficiency are of the same morphological and compositional origin. A tentative mechanism of the surface activation is discussed.