Study of the photochemical reaction of sensitizing dyes adsorbed on semiconductor powder by means of photoacoustic spectroscopy

Abstract

trolyte-plus-salt solution.2 This leads indeed to a negligible value of 6, less than 1%. If, instead of (2), we propose following values, estimated from experimental data on the investigated polyelectrolyte4J k, = 0.14 kl = 0.40(3) then one obtains 6 = -0.23. Hence, we agree with Dr. Manning to claim that, for the present analysis, a departure from the Debye-Huckel approximation cannot alone explain the massive discrepancy between apparent and effective polyion valences.Another important aspect of our Discussion1 concerns the polymer concentrations used in our experiments. Let us clearly state that, in our work, both polyion transport parameters (u3 and D3) were not obtained in the same concentration regime. Self-diffusion data belong to the dilute regime and electrophoresis data to the semidilute regime (C, = 0.5%), because we were not able to attain, in electrophoretic light scattering, concentrations below Cp*, the concentration at which polymer chains start to overlap. For high-molecular-weight polyelectrolytes (M,,, k lo6), Cp* is very low, typically less or much less than 0.1% (weight percent) depending on polyion ionization and salt concentration. This has two consequences.(i) In the field of high-molecular-weight polyelectrolytes almost all published results on electrophoresis were obtained in the semidilute regime, where transport coefficients are molecular-weight independent, but become concentration dependent,'j a result which reasonably explains the different power laws we observed for the polyion friction coefficients.(ii) The experimental u3 values might be underestimated with respect to their dilute regime values, because of increased friction with the environment, if we refer to experiments performed on neutral polymer^.^ For example, with the polystyrene/toluerie system at 25 "C (M, = 110000) the polymer friction coefficient is increased by a factor of 1.5, when the concentration rises from 0 to 0.5% .'This effect probably contributes to lower u3 and thus z~~P .Finally, let us point out that the observation of low apparent valences could also be due to overestimated polyion self-diffusion coefficients. The spectrum of light scattered by charged macromolecules is indeed complex, and the first theoretical analysis of this difficult subject has been given by Stephen,@ for the model of a rigid rod. However, Stephen's theory is only valid if the Debye shielding length is much larger than the interparticle spacing, a hypothesis which does not apply under excess of salt conditions, for a dilute polymer solution. Thus, a complete theory of the charge effects on light scattering is not yet available. In any case, if quasi-elastic light scattering only measures an apparent self-diffusion coefficient of the polyion, this cannot provide the key explanation for our results, for we also observed very low 2sap values with data published by Nagasawa et ala1O9l1 who used quite different experimental techniques.At the outset of this comment we could agree with Dr. Manning in stating that there is yet no clear-cut ...

Detection of transient intermediates of a sensitizing dye adsorbed on a semiconductor powder by means of photoacoustic spectroscopy

“…This kind of the photochemical reaction of sensitizing dyes at the surface of a semiconductor powder was observed in our previous investigation with other combinations of sensitizing dyes and semiconductors. 13 In addition to the change in the photoacoustic spectra, interesting transient signals were observed in different wavelength regions during irradiation with excitation light. Dependence of the photoacoustic signals around 450 nm on exposure to excitation light is shown in Figure 3.…”

Photoacoustic spectroscopy of solids and surfaces

“…Normalized PAS of (1) Khodamine B solution 0.0012 M, (2) Khodamine B solution (0.0012 M + 1 M BaCI~), (3) Rhodaminc B on AltOs and (4) ghodamine B on GdA10,. Iwasaki et al (1980) have employed photoacoustic spectroscopy to investigate the photochemical reaction of sensitized dyes absorbed on semiconductor powders. 1, l'-diethyl-2, 2'-cyanine and Rhodamine B adsorbed on ZnO from an aqueous solution were irradiated with light from a Xe lamp with wavelengths longer than 500 nm to eliminate the intrinsic absorption of the ZnO powder 9 With the 1, l'diethyl-2, 2'-eyanine dye, the photoaeoustie signal decreases with exposure to the excitation light without any appreciable shift in the spectrum which may be related to a rather drastic decomposition of the dye chromopkores.…”

Electron Exchanges at the Interface, Porphyrin/Semi Conductor or Insulator Oxides, Studied by Pas and "Mirage Effect" Detection

“…One p o s s i b i l i t y c o n s i s t s i n t h e a d s o r p t i o n of v i s i b l e l i p h t absorbing o r g a n i c m a t e r i a l a t t h e s u r f a c e o f t h e semi conductor. Most o f t h e compounds which have been used i n such an aim a r e dyes ( 4 ) . However p w p h y r i n s a r e o t h e r o r g a n i c compounds which a r e good candidates f o r s e n s i t i z a t i o n o f semi conductors.…”