Dyes, surface concentrations

Orientation of porphyrin molecules has not much influence on efficiency of the DSSCs. The PCE of the 3-AB-type porphyrin was higher than that of the trans-2A2B-type porphyrin and the ZnO semiconductor was superior to Ti02. However, the PCE values of all cells were similar, and raffier low, being 0.27 and 0.23 % for the cell 1 and 2, respectively (Scheme 29). This difference might originate from different surface concentrations of dyes on the MO surface. The 1 2 dye surface concentrations ratio was 1.25. 

Tt is the purpose of this paper to describe methods for determining and A interpreting dye spectra in aqueous dispersions of silver halides and other substrates. Such spectra can be utilized for the direct measurement of surface concentrations of dyes from which, in turn, the surface area of the substrate can be derived. The techniques involved are not limited to a specific dye class but will be illustrated in this paper by the behavior of cyanine dyes. 

The applicability of this in situ method for the determination of surface areas depends not only on knowledge of the dye s molecular area in the adsorbed state but also on the assumption that the chosen spectral parameter measures the surface concentration of the dye. In order to test the relation between adsorption of dye to silver halide and its spectral characteristics in the bound state, the behavior of Pseudocyanine in a coarse silver halide suspension (Dispersion D) was studied. This particular dispersion was chosen because some of its relevant adsorption characteristics had already been examined (22, 23). Moreover, observations by Boyer and Cappelaere with Pseudocyanine adsorbed on AgBr powders (5) indicated that /-band intensity varied with the amount of adsorbed dye and was not sensitive to the concentration of Ag+ or Br" ions in the range pAg 3.3-8.7. 

Whereas the optical method indicates attainment of saturation coverage by the dye in its /-state, results obtained with the phase-separation technique show that, after reaching apparent saturation, further adsorption occurs as the dye concentration in solution is increased. The horizontal step observed with the phase separation measurement is in approximate agreement with the maximum surface concentration of dye in its /-state as determined by the optical method. Hence, it is concluded that, contrary to earlier suppositions (70), only the first layer of this dye is adsorbed in its /-state subsequent dye layers must be adsorbed in different states. 

It remains to be determined to what extent the dye adsorption technique is applicable to other substrates. No evidence was obtained for Pseudocyanine adsorption to Mn02, Fe2Os or to pure silver surfaces, although this dye can be bound to mica, lead halides, and mercury salts with formation of a /-band (61). Not only cyanines but other dye classes can yield surface spectra which may be similarly analyzed. This is specifically the case with the phthalein and azine dyes which were recommended by Fajans and by Kolthoff as adsorption indicators in potentio-metric titrations (15, 30). The techniques described are also convenient for determining rates and heats of adsorption and surface concentrations of dyes they have already found application in studies of luminescence (18) and electrophoresis (68) of silver halides as a function of dye coverage. 

Fig. 4.9. (a, top) The 8iph/iph vs. v 1 dependence for W03 electrode sensitized by Dye II in monomeric form ( ) partially aggregated by coprecipitation with PD IV (O). The excitation wavelength 560 nm. / = 20 s. The total surface concentration of Dye II 10 8 mol cm 2. Electrolyte 0.25 M Na2S04. (b, bottom) The potential-time programme and corresponding photocurrent-time curves used for x evaluation. Hatched areas indicate the exposure periods. 

Measurements of surfactant concentrations on travelling capillary waves is complicated by the rapid decay rate of these waves, necessitating measurements close to the source of wave generation. To avoid this complication, we utilized a field of standing capillary waves. The wave tank was a circular (6.99 cm, inner diameter) glass vessel. The inner wall was coated with paraffin to avoid loss of the surfactant to the tank side walls. Triply distilled water was used as the substrate. The tank was overflowed to clean the surface prior to spreading the insoluble hemicyanine surfactant mono-layer at a surface concentration of 0.288 pg cm"2. Hemicyanine, 4-[4-(dimethylamino)styrl]-l-docosyl-pyridinium bromide, is a stilbazolium dye molecule to which is attached on one end a saturated twenty two car-... 

The choice of donor and acceptor dyes used in this study was determined by several criteria Surface concentrations had to be varied over as large a range as possible. For this reason only cationic dyes could be employed. The spectral overlap between donor emission and acceptor absorption also had to be as large as possible. The donor dyes should have a sufficiently high fluorescence quantum yield while the deactivation of the acceptor dyes should be predominantly radiationless. 

The dyes were adsorbed from aqueous solution onto silica having a specific surface area of 300 m g and a mean pore diameter of 35 A. Surface concentrations were determined by measuring the absorptions of the solutions before and after the adsorption. 

The enhancement of the photoacoustic signal by energy transfer is illustrated in Fig. 2 with RHB as donor and THI as acceptor. Here the total surface concentration was kept constant at 10 mole m (except for Curve 1). The donor concentration was varied between 10 and 10 mole m" the acceptor concentration was varied accordingly. In this Figure Curve 1 is the intensity of the pure donor dye as a function of surface coverage. In this curve the donor concentration corresponds to the donor mole fractions in the mixtures. Curve 2 is the observed intensity when acceptor molecules are added. This curve can be described by the empirical formula... 

Discrepancies between nonsorptive dye and radionuclide movement were observed. In some experiments where 90-100% of Rhodamine B passed through the flume after one day from release, only 5-25% of 65Zn, 58Co, 137Cs, and 85Sr added at the inlet was detected in the exit flume water. The retention of radionuclides was primarily owing to surface concentration by sediments and biomass (1, 2, 3, 4, 5, 6, 7, 8). 

Spectra of benzeneazodiphenylamine adsorbed on DSA-1 were determined over a range of surface concentrations between 0 and 1 x 1011 dye molecules/cma by exposing the same catalyst specimen to several concentrations of the dye. Figure 15 shows adherence to Beer s... 

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