UV-visible reflection measurement

The Basis of UV-visible Reflection Measurement at an Electrode Surface... 

Instrumentation of the Potential-modulated UV-visible Reflection Measurement... 

Fig. 2.7 (a) Experimental set-up for a nearnormal incidence UV-visible reflectance measurement at constant potential for a basal plane pyrolytic graphite (BPG) electrode, (b) Plot of the change of dc reflectance as a function of electrode potential at A=737 nm at a BPG electrode... 

The electrode substrate for the reflection measurement is not necessarily in the form of a flat plate in a quiescent electrolyte solution but sometimes in the form of a sphere or in a hydrodynamic condition. We here consider three typical examples of in-situ UV-visible reflectance measurement in special electrode configurations. 

The UV-visible reflectance measurement for a mercury electrode not in contact with polymer but solely with electrolyte solution may be still in demand. The challenge to make the measurement with a mercury drop possible may be quite valuable. 

Scope for Future Development of UV-visible Reflection Measurements 91... 

Fig. 230 Schematic diagram of a set-up of optics for doublebeam UV-visible reflectance measurement at the two spots on a single facet of a small single crystal metal electrode.

Infrared spectroscopy can provide a great deal of information on molecular identity and orientation at the electrode surface [51-53]. Molecular vibrational modes can also be sensitive to the presence of ionic species and variations in electrode potential [51,52]. In situ reflectance measurements in the infrared spectrum engender the same considerations of polarization and incident angles as in UV/visible reflectance. However, since water and other solvents employed in electrochemistry are strong IR absorbers, there is the additional problem of reduced throughput. This problem is alleviated with thin-layer spectroelectro-chemical cells [53]. 

In this chapter, we are concerned with UV-visible reflectance spectroscopy for an electrode covered with a thin organic film. The UV-visible reflectance spectroscopy is a simple optical measurement What one needs is a sensitive UV-... 

This chapter is devoted to describing the basic aspects of the measurement, instmmentation, measurement techniques, and practical applications of potential-modulated UV-visible spectroscopy as a representative spectroelectrochemi-cal tool to characterize thin organic films on electrode surfaces and to track the kinetics of the electrode surface processes. At the same time, miscellaneous features of the measurement, which may be important for those who intend to apply for the first time the potential-modulated UV-visible spectroscopic method in their experiments, will also be included. However, because of the Hmit to the chapter length as well as the existence of superior review articles on UV-visible reflectance spectroscopy at electrode/solution interfaces [2,6-9], detailed comprehensive description is minimized. With the intention of overviewing the UV-visible spectroscopic method for the benefit of experimental electrochemists, optical issues, especially optical reflection theory, are not detailed. 

The third example is the reflection measurement at a rotating disk electrode (RDE). Scherson and his coworkers have developed near-normal incidence UV-visible reflection-absorption spectroscopy at RDEs [50-52]. Both (AR/R)dc and (AR/R)er have been measured under hydrodynamic conditions. The use of an RDE enables them to quantitatively control the diffusion layer concentration profile of the solution phase species, especially the species generated electro-... 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

This image has similarities to that presented CroU in his modd of the drying process. CroU envisions the top l er to be transparent and essentially dry, but with a percolation network of tir pores. This sits atop the flocculated phase, which in turn rests upon tire aqueous dispeisian. Tent s results require the flocculated phase to be wet, witii die particles separated by a water-swollen hydrophilic membrane. If, howevo-, there were a diin transparmt layer at the top which grew steadUy in thickness, the changing interference between reflections at the upiper and lower interfaces would almost certainly be doected in die UV-visible transmission measurements. 

L. Jiang, Q. J. Xie, L. Yang, X. Y. Yang, S. Z. Yao, Simultaneous EQCM and diffuse reflectance UV-Visible spectroelectrochemical measurements poly(aniline-co-o-anthranilic acid) growth and property characterization, Journal of Colloid and Interface Science 2004, 274,150. 

The PLM can be used in a reflection or a transmission mode. With either mode, light of various wavelengths from ultraviolet to infrared, polarized or unpolarized, is used to yield a wide variety of physical measurements. With just ordinary white light, a particle or any object detail down to about 0.5 p.m (500 nm) in diameter can be observed to detect shape, size, color, refractive index, melting point, and solubiUty in a group of solvents, all nondestmetively. Somewhat larger particles yield UV, visible, or IR absorption spectra. 

Ultraviolet-visible (UV-vis) diffuse reflectance spectra of supported WOx samples and standard W compounds were obtained with a Varian (Cary 5E) spectrophotometer using polytetrafluoroethylene as a reference. The Kubelka-Munk function was used to convert reflectance measurements into equivalent absorption spectra [12]. Spectral features of surface WOx species were isolated by subtracting from the W0x-Zr02 spectra that of pure Z1O2 with equivalent tetragonal content. All samples were equilibrated with atmospheric humidity before UV-vis measurements. 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

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